Waste Incineration
EUROPEAN COMMISSION
Integrated Pollution Prevention and Control
Reference Document on the Best Available
Techniques for
Waste Incineration
August 2006
This document is one of a series of foreseen documents as below (at the time of writing, not all
documents have been drafted):
Reference Document on Best Available Techniques . . .
Code
Large Combustion Plants
LCP
Mineral Oil and Gas Refineries
REF
Production of Iron and Steel
I&S
Ferrous Metals Processing Industry
FMP
Non Ferrous Metals Industries
NFM
Smitheries and Foundries Industry
SF
Surface Treatment of Metals and Plastics
STM
Cement and Lime Manufacturing Industries
CL
Glass Manufacturing Industry
GLS
Ceramic Manufacturing Industry
CER
Large Volume Organic Chemical Industry
LVOC
Manufacture of Organic Fine Chemicals
OFC
Production of Polymers
POL
Chlor – Alkali Manufacturing Industry
CAK
Large Volume Inorganic Chemicals - Ammonia, Acids and Fertilisers Industries
LVIC-AAF
Large Volume Inorganic Chemicals - Solid and Others industry
LVIC-S
Production of Speciality Inorganic Chemicals
SIC
Common Waste Water and Waste Gas Treatment/Management Systems in the Chemical Sector
CWW
Waste Treatments Industries
WT
Waste Incineration
WI
Management of Tailings and Waste-Rock in Mining Activities
MTWR
Pulp and Paper Industry
PP
Textiles Industry
TXT
Tanning of Hides and Skins
TAN
Slaughterhouses and Animals By-products Industries
SA
Food, Drink and Milk Industries
FDM
Intensive Rearing of Poultry and Pigs
ILF
Surface Treatment Using Organic Solvents
STS
Industrial Cooling Systems
CV
Emissions from Storage
ESB
Reference Document . . .
General Principles of Monitoring
MON
Economics and Cross-Media Effects
ECM
Energy Efficiency Techniques
ENE
Executive Summary
EXECUTIVE SUMMARY
The BAT (Best Available Techniques) Reference Document (BREF) entitled Waste
Incineration (WI) reflects an information exchange carried out under Article 16(2) of Council
Directive 96/61/EC (IPPC Directive). This executive summary describes the main findings, a
summary of the principal BAT conclusions and the associated consumption and emission levels.
It should be read in conjunction with the preface, which explains this document’s objectives;
how it is intended to be used and legal terms. It can be read and understood as a standalone
document but, as a summary, it does not present all the complexities of this full document. It is
therefore not intended as a substitute for this full document as a tool in BAT decision making.
Scope of this document
The scope of this document is based on Sections 5.1 and 5.2 of Annex 1 of the IPPC Directive
96/61/EC, in so far as they deal with incineration of waste. The scope chosen for the work was
not restricted by the installation size limitations in the IPPC Directive, nor by the definitions of
waste, recovery or disposal included therein. The selected scope therefore intended to provide a
pragmatic view across the incineration sector as a whole, with a particular focus upon those
installation and waste types that are most common. The scope of the Waste Incineration
Directive was also a factor taken into account when deciding on the scope of the BREF
document. The final contents of the BREF reflect the information that was submitted during the
information exchange by the TWG.
The document deals only with the dedicated incineration of waste and not with other situations
where waste is thermally treated, e.g. co-incineration processes such as cement kilns and large
combustion plants.
Although incineration provides the main focus of the document, it also includes some
information on waste pyrolysis and gasification systems.
This BREF document does not:
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deal with decisions concerning the selection of incineration as a waste treatment option
compare waste incineration with other waste treatment options.
Waste Incineration (WI)
Incineration is used as a treatment for a very wide range of wastes. Incineration itself is
commonly only one part of a complex waste treatment system that altogether, provides for the
overall management of the broad range of wastes that arise in society.
The incineration sector has undergone rapid technological development over the last 10 to
15 years. Much of this change has been driven by legislation specific to the industry and this
has, in particular, reduced emissions to air from individual installations. Continual process
development is ongoing, with the sector now developing techniques which limit costs, whilst
maintaining or improving environmental performance.
The objective of waste incineration, in common with most waste treatments, is to treat waste so
as to reduce its volume and hazard, whilst capturing (and thus concentrating) or destroying
potentially harmful substances. Incineration processes can also provide a means to enable
recovery of the energy, mineral and/or chemical content from waste.
Waste Incineration
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Executive Summary
Basically, waste incineration is the oxidation of the combustible materials contained in the
waste. Waste is generally a highly heterogeneous material, consisting essentially of organic
substances, minerals, metals and water. During incineration, flue-gases are created that will
contain the majority of the available fuel energy as heat. The organic substances in the waste
will burn when they have reached the necessary ignition temperature and come into contact with
oxygen. The actual combustion process takes place in the gas phase in fractions of seconds and
simultaneously releases energy. Where the calorific value of the waste and oxygen supply is
sufficient, this can lead to a thermal chain reaction and self-supporting combustion, i.e. there is
no need for the addition of other fuels.
Although approaches vary greatly, the incineration sector may approximately be divided into
the following main sub-sectors:
i.
Mixed municipal waste incineration – treating typically mixed and largely untreated
household and domestic wastes but may sometimes including certain industrial and
commercial wastes (industrial and commercial wastes are also separately incinerated in
dedicated industrial or commercial non-hazardous waste incinerators).
ii.
Pretreated municipal or other pretreated waste incineration – installations that treat
wastes that have been selectively collected, pretreated, or prepared in some way, such
that the characteristics of the waste differ from mixed waste. Specifically prepared
refuse derived fuel incinerators fall in this sub-sector
iii.
Hazardous waste incineration - this includes incineration on industrial sites and
incineration at merchant plants (that usually receive a very wide variety of wastes)
iv.
Sewage sludge incineration – in some locations sewage sludges are incinerated
separately from other wastes in dedicated installations, in others such waste is combined
with other wastes (e.g. municipal wastes) for its incineration
v.
Clinical waste incineration – dedicated installations for the treatment of clinical wastes,
typically those arising at hospitals and other healthcare institutions, exist as centralised
facilities or on the site of individual hospital etc. In some cases certain clinical wastes
are treated in other installations, for example with mixed municipal or hazardous
wastes.
Data in this document shows that, at the time of its compilation:
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Around 20 - 25 % of the municipal solid waste (MSW) produced in the EU-15 is treated by
incineration (total MSW production is close to 200 million tonnes per year)
The percentage of MSW treated by incineration in individual Member States of the EU-15
varies from 0 % to 62 %
The total number of MSW installations in the EU-15 is over 400
Annual MSW incineration capacity in individual European countries varies from 0 kg to
over 550 kg per capita
In Europe the average MSW incinerator capacity is just under 200000 tonnes per year.
The average throughput capacity of the MSWI installations in each MS also varies. The
smallest plant size average seen is 60000 tonnes per year and the largest close to
500000 tonnes per year
Around 12 % of the hazardous waste produced in EU-15 is incinerated (total production
close to 22 million tonnes per year).
Expansion of the MSW incineration sector is anticipated in Europe over the next 10 – 15 years
as alternatives are sought for the management of wastes diverted from landfill by the Landfill
Directive and both existing and new Member States examine and implement their waste
management strategies in the light of this legislation.
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Waste Incineration
Executive Summary
Key environmental issues
Waste and its management are a significant environmental issue. The thermal treatment of waste
may therefore be seen as a response to the environmental threats posed by poorly or unmanaged
waste streams. The target of thermal treatment is to provide for an overall reduction in the
environmental impact that might otherwise arise from the waste. However, in the course of the
operation of incineration installations, emissions and consumptions arise, whose existence or
magnitude is influenced by the installation design and operation.
The potential impacts of waste incineration installations themselves fall into the following main
categories:
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overall process emissions to air and water (including odour)
overall process residue production
process noise and vibration
energy consumption and production
raw material (reagent) consumption
fugitive emissions – mainly from waste storage
reduction of the storage/handling/processing risks of hazardous wastes.
Other impacts beyond the scope of this BREF document (but which can significantly impact
upon the overall environmental impact of the whole chain of waste management) arise from the
following operations:
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transport of incoming waste and outgoing residues
extensive waste pretreatment (e.g. preparation of waste derived fuels).
The application and enforcement of modern emission standards, and the use of modern
pollution control technologies, has reduced emissions to air to levels at which pollution risks
from waste incinerators are now generally considered to be very low. The continued and
effective use of such techniques to control emissions to air represents a key environmental issue.
Other than its role in ensuring effective treatment of otherwise potentially polluting unmanaged
wastes, many waste incineration installations have a particular role as an energy-from-waste
recovery process. Where policies have been implemented to increase the ability of, (most
commonly municipal) waste incineration installations to recover the energy value of the waste,
this increases the exploitation of this positive environmental contribution. A significant
environmental opportunity for the industry is therefore to increase its potential as an energy
supplier.
Applied processes and techniques
Chapter 2 of this document provides a description of the processes and techniques that are
applied in the waste incineration industry. It focuses upon the most commonly applied thermal
treatment of incineration, but also includes information on gasification and pyrolysis. The
following main activities and areas are described to varying degrees of detail:
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incoming waste reception
storage of waste and raw materials
pretreatment of waste (mainly on-site treatments and blending operations)
loading of waste into the furnace
techniques applied at the thermal treatment stage (furnace design etc.)
the energy recovery stage (e.g. boiler and energy supply options)
flue-gas cleaning techniques (grouped by substance)
flue-gas cleaning residue management
emissions monitoring and control
waste water control and treatment (e.g. from site drainage, flue-gas treatment, storage)
ash/bottom ash management and treatment (arising from the combustion stage).
Waste Incineration
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Executive Summary
Where techniques are specific to certain types of wastes, relevant sections are subdivided
according to waste type.
Consumptions and emissions
The emissions, and material and energy consumptions, that arise from waste incineration
installations are described in Chapter 3. Available data are presented on installation emissions to
air and water, noise, and residues. Information on raw material consumptions is also provided,
along with a section that focuses upon energy consumption and output. Most of the data are
whole installation data arising from industrial surveys. Some information about the techniques
applied in order to achieve these emission levels is also included.
Although some European installations have yet to be upgraded, the industry is generally
achieving operational levels that meet or improve upon the air emission limit values set in
Directive 2000/76/EC.
In circumstances where CHP or heat (as heat or steam) can be supplied, it is possible for very
large percentages of the energy value of the waste (approx. 80 % in some cases) to be
recovered.
Techniques to consider in the determination of BAT
Each technique described in Chapter 4 includes the available relevant information, on: the
consumption and emission levels achievable using the technique; some idea of the costs and the
cross-media issues associated with the technique, and; information on the extent to which the
technique is applicable to the range of installations requiring IPPC permits - for example new,
existing, large or small installations, and to various waste types. Management systems, processintegrated techniques and end-of-pipe measures are included.
The techniques that are included are those that are considered to have the potential to achieve,
or contribute to, a high level of environmental protection in the waste incineration industry. The
final BAT, as agreed by the TWG, is not covered in Chapter 4, but in Chapter 5. The inclusion
of a technique in Chapter 4, but not in Chapter 5 should not be taken as an indication that the
technique is not and cannot be BAT - the rationale for excluding the technique from Chapter 5
could, for example, be that the TWG felt that the technique not sufficiently widely applicable
for it to be described as BAT in general. Furthermore, because it is not possible to be exhaustive
and because the situation is dynamic, Chapter 4 cannot be considered to be entirely
comprehensive. Other techniques may also provide for levels of performance that meet or
exceed the BAT criteria later established in Chapter 5, and when applied locally those
techniques may provide particular advantages in the situation in which they are used.
The techniques included are grouped in approximately the order in which they would appear in
the majority of waste incineration installations. The table below gives the title of the chapter
subsections and indicates the grouping to which the techniques are listed.
Chapter 4 section number
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Title of section
General practices applied before thermal treatment
Thermal processing
Energy recovery
Flue-gas treatment
Process water treatment and control
Treatment techniques for solid residues
Noise
Environmental management tools
Good practice for public awareness and communication
Table: Organisation chart for the information in Chapter 4
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Waste Incineration
Executive Summary
Chapter 4 concentrates on techniques that provide particular advantages at each of the main
stages generally seen in waste incineration installations. Dividing the techniques in this way
does however mean that, although mentioned in some cases, the important aspect of the overall
integration of all of the techniques in an installation (sometimes referred to in the BREF as their
“inter-process compatibility”) is something which requires careful consideration when reading
the individual sections of Chapter 4. The subsections on operational data and applicability are
generally where such matters are given consideration. Overall compatibility was also been given
further consideration when finally deriving the BAT conclusions in Chapter 5.
Chapter 4 does not generally describe in detail those techniques that, whilst they provide, or
contribute to, a high level of environmental performance, are so common that their use may
already be considered as standard. An example of this is that, because the applicability of the
main combustor designs to the main waste streams is relatively well established, the techniques
considered at this stage concentrate mainly on:
a)
b)
the general issue of ensuring the combustion system selected is properly matched to the
wastes fed to it, and
on some aspects relating to improving combustion performance e.g. waste preparation, air
supply control, etc.
BAT for the incineration of waste
The BAT chapter (Chapter 5) identifies those techniques that the TWG considered to be BAT in
a general sense, based on the information in Chapter 4, taking into account the Article 2(11)
definition of best available techniques and the considerations listed in Annex IV of the
Directive.
The BAT chapter does not set or propose emission limit values but suggests the operational
consumption and emission values that are associated with the use of BAT. The introduction to
Chapter 5 included in this BREF is specifically extended to clarify certain issues that were
considered to be of particular relevance to the waste incineration industry, including the links
between the Waste Incineration Directive (WID) and IPPC (see the PREFACE of the BREF).
These additional specific issues include:
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the difference between WID emission limit values and BAT performance
the relationship between BAT and site selection
how to understand and use the BAT described in Chapter 5.
The following paragraphs summarise the key BAT conclusions but reference must be made to
the BAT chapter itself to be comprehensive. The generic BAT are intended to apply to the
whole sector (i.e. waste incineration, waste gasification and waste pyrolysis of whatever type of
waste). Other BAT are given that apply to sub-sectors dealing primarily with specific waste
streams. It is therefore anticipated that a specific installation would apply a combination of the
generic and waste specific BAT, and that installations treating mixtures of waste, or wastes not
specifically mentioned, would apply the generic BAT plus a suitable selection of the waste
specific BAT. Further comment on the combining of the BAT is included in the introduction to
Chapter 5.
Generic BAT
A fundamental BAT stresses the importance of the selecting an installation design that is suited
to the characteristics of the waste received at the installation in terms of both its physical and
chemical characteristics. This BAT is fundamental to ensuring the installation may treat the
waste received with a minimum of process disturbances – which themselves may give rise to
additional environmental impacts. To this end there is also a BAT about the minimisation of
planned and unplanned shutdowns.
Waste Incineration
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Executive Summary
BAT includes establishing and maintaining quality controls over the waste input. This aims to
ensure that the waste characteristics remain suited to the design of the receiving installation.
Such quality control procedures are compatible with the application of an environmental
management system, which is also considered BAT.
There are several BAT regarding the conditions and management of the storage of incoming
wastes prior to their treatment, so that this does not give rise to pollution and odour releases.
Some specific techniques and conditions of storage are noted. A risk based approach that takes
into account the properties of the waste concerned is considered BAT.
Consideration of the demonstrated ability of some installation designs to very efficiently treat
highly heterogeneous wastes (e.g. mixed MSW), and the risks and cross-media effects
associated with pretreatment, results in a conclusion that it is BAT to pretreat incoming wastes
to the degree required to meet the design specification for the receiving installation, noting that
to treat wastes beyond this requires balanced consideration of (possibly limited) benefits,
operational factors and cross-media effects.
The design and operation of the combustion stage is identified as an important primary pollution
prevention aspect, and therefore of great relevance to achieving the aims of the IPPC Directive.
It is noted in the BAT chapter that flow modelling at the design stage may assist in ensuring that
certain key design decisions are well informed. In operation, it is considered BAT to use various
techniques (e.g. control of air supply and distribution) to control combustion. The BAT
regarding the selection of a design that suits the waste received is of particular relevance here.
In general the use of the combustion operating conditions specified in Article 6 of Directive
2000/76/EC (WID) are considered to be compatible with BAT. However the TWG noted, that
the use of conditions in excess of these (e.g. higher temperatures) could result in an overall
deterioration in environmental performance, and that there were several examples of hazardous
waste installations that had demonstrated an overall improvement in environmental performance
when using lower operational temperatures than the 1100 oC specified in WID for certain
hazardous wastes. The general BAT conclusion was that the combustion conditions (e.g.
temperature) should be sufficient to achieve the destruction of the waste but, in order to limit
potential cross-media impacts, generally not significantly in excess of those conditions. The
provision of auxiliary burner(s) for achieving and maintaining operational conditions is
considered to be BAT when waste is being burned.
When gasification or pyrolysis is used, in order to prevent the generation of waste by disposal of
the reaction products of these techniques, it is BAT either, to recover the energy value from the
products using a combustion stage, or to supply them for use. The BAT associated emission
levels for releases to air from the combustion stage of such installations are the same as those
established for incineration installations.
The recovery of the energy value of the waste is a key environmental issue for the sector,
presenting an area where the sector may make a significant positive contribution. Several BAT
cover this aspect, dealing with:
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specific techniques that are considered to be BAT
the heat transfer efficiencies expected of boilers
the use of CHP, district heating, industrial steam supply and electricity production
the recovery efficiencies that may be anticipated.
Waste Incineration
Executive Summary
With CHP and steam/heat supply generally offering the greatest opportunity for increasing
energy recovery rates, policies affecting the availability of suitable customers for steam/heat
generally play a far greater role in determining the efficiency achievable at an installation than
the detail of its design. For mainly policy and economic reasons, electricity generation and
supply is often the energy recovery option selected at individual installations. Options for CHP,
district heating and industrial steam supply are only well exploited in a few European Member
States – generally those that have high heat prices and/or that have adopted particular policies.
The supply of energy for the operation of cooling systems and desalination plants is something
that is done, but is in general poorly exploited – such an option may be of particular interest in
warmer climate zones, and in general expands the options for the supply of waste derived
energy.
The flue-gas treatments applied at waste incineration installations have been developed over
many years in order to meet stringent regulatory standards and are now highly technically
advanced. Their design and operation are critical to ensure that all emissions to air are well
controlled. The BAT that are included:
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cover the process of selection of FGT systems
describe several specific techniques which are considered to be BAT
describe the performance levels that are anticipated from the application of BAT.
The performance ranges agreed by the wider TWG resulted in some split views. These were
mainly from one Member State and the Environmental NGO, who believed that lower emission
values than the ranges agreed by the remainder of the TWG could also be considered to be
BAT.
The BAT regarding waste water control include:
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the in-process recirculation of certain effluents
the separation of drainage for certain effluents
the use of on-site effluent treatment for wet scrubber effluents
BAT associated performance levels for emissions from scrubber effluent treatment
the use of specific techniques.
The performance ranges agreed by the wider TWG resulted in some split views from one
Member State and the Environmental NGO, who believed that lower emission values than the
ranges given could also be considered to be BAT.
BAT regarding residue management include:
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a bottom ash burnout TOC level of below 3 %, with typical values falling between 1 and
2%
a list of techniques, which when suitably combined may attain these burnout levels
the separate management of bottom ash from fly ash and a requirement to assess each
stream produced
the extraction of ferrous and non-ferrous metals from ash for their recovery (where present
in ash to sufficient degree to make this viable)
the treatment of bottom ashes and other residues using certain techniques - to the extent
required for them to meet the acceptance criteria at the receiving recovery or disposal site.
In addition to these generic BAT, more specific BAT are identified for those sub-sectors of the
industry treating mainly the following wastes:
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municipal wastes
pretreated or selected municipal wastes
hazardous wastes
sewage sludge
clinical waste.
Waste Incineration
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Executive Summary
The specific BAT provide, where it has been possible, more detailed BAT conclusions. These
conclusions deal with the following waste stream specific issues:
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in-coming waste management, storage and pretreatment
combustion techniques
energy recovery performance.
Emerging techniques
The section on emerging techniques is not comprehensive. A number of the techniques supplied
by the TWG and included in earlier drafts of this document were transferred into this section. In
the majority of cases the techniques included have only been demonstrated on a pilot or trial
scale.
The degree of demonstration (as measured by overall throughput and operational hours) of
pyrolysis and gasification on the main European waste streams is low compared with
incineration and operational difficulties are reported at some installations. However, both
gasification and pyrolysis are applied in the sector and therefore, according to the BREF
definition, cannot be considered to be emerging techniques. For this reason the information
concerning these techniques is included in Chapter 4.
Concluding remarks
Information exchange
This BREF is based on several hundred sources of information, and over 7000 consultation
comments supplied by a very large working group. Some of the information was overlapping
and therefore, not all of the documents supplied are referenced in the BREF. Both industry and
Member States supplied important information. Data quality was generally good, particularly
for emissions to air, allowing valid comparisons to be made in some cases. This was not
however uniformly the case, and data regarding costs was difficult to compare owing to
inconsistencies in data compilation and reporting. The consumption and emissions data given
are predominantly for whole installations or groups of techniques, rather than individual ones.
This has lead to some important BAT conclusions being expressed as quantitative overall
performance targets, with certain technical options presented that when suitably combined, may
give rise to that performance.
Level of consensus
There was a very good general level of consensus. There was full agreement, and no split views,
in relation to the technique related BAT. There was also generally good consensus upon the
quantitative BAT, although the operational emission levels associated with the use of BAT did
give rise to some split views, with one Member State and the Environmental NGO recording
split views in relation to many of the BAT associated emission levels for releases to both air and
water.
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Waste Incineration
Executive Summary
Recommendations for future work and R&D projects
The information exchange and its result, i.e. this BREF, provide a step forward in achieving the
integrated prevention and control of pollution from waste incineration. Further work could
continue the process by providing:
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information regarding the techniques used to, and costs of, upgrading existing installations –
such information may be derived from experience of implementing WID in Member States
and might usefully be compared with the costs/performance at new installations
the more detailed cost information that is required to undertake a more precise assessment
of variations in technique affordability with plant size and waste type
information regarding smaller installations – very little information was provided regarding
small installations
information regarding installations that treat industrial non-hazardous wastes and the impact
on installations of treating mixtures of wastes e.g. sewage sludge or clinical waste with
MSW
a more detailed evaluation of the impact on pollution prevention of detailed combustion
design features e.g. grate design
further information on emerging techniques.
ammonia consumption and emission (mainly to air and water) levels for different FGT
systems (mainly wet, semi-wet and dry) and their relative NOX reduction efficiency
the impact of the dust removal temperature range upon PCDD/F releases to air and residues
further experiences with continuous emissions monitoring for Hg (to air and water).
Other important recommendations for further work beyond the scope of this BREF but arising
from the information exchange are:
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the need for consideration of the overall impact of competition for waste treatment, in
particular competition from industries co-incinerating wastes – a study of such might
usefully include consideration of: relative reliability of, and risks to, the supply of the total
waste management service; overall emissions and energy recovery according to various
degrees of diversion, and; consider and identify key risk factors e.g. waste fuel quality
assurance.
it may be useful to assess the impact on adopted waste strategies (i.e. the balance of
technologies used on a national scale), and on achieved thermal treatment installation
efficiencies, of the degree of integration of energy and waste management policy in EU
Member States (and other countries). Such studies may identify how policy on energy and
waste interact and give examples, both positive and negative.
the need to understand in more detail of the impact of absolute and relative energy prices
(for electricity and heat) upon the typically achieved energy efficiency of installations, and
the role and impact of subsidies and taxation schemes
the identification of the typical barriers to developing new installations and the approaches
that have proved successful
the development of suitable standards for the use of bottom ash – such standards have
proved helpful in improving markets for the use of bottom ash
the costs and benefits of further reducing emissions from the waste incineration industry
when compared to reductions at other industrial and anthropogenic sources of pollution.
The EC is launching and supporting, through its RTD programmes, a series of projects dealing
with clean technologies, emerging effluent treatment and recycling technologies and
management strategies. Potentially these projects could provide a useful contribution to future
BREF reviews. Readers are therefore invited to inform the EIPPCB of any research results
which are relevant to the scope of this document (see also the preface of this document).
Waste Incineration
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Preface
PREFACE
1.
Status of this document
Unless otherwise stated, references to “the Directive” in this document means the Council
Directive 96/61/EC on integrated pollution prevention and control. As the Directive applies
without prejudice to Community provisions on health and safety at the workplace, so does this
document.
This document forms part of a series presenting the results of an exchange of information
between EU Member States and industries concerned on best available technique (BAT),
associated monitoring, and developments in them. It is published by the European Commission
pursuant to Article 16(2) of the Directive, and must therefore be taken into account in
accordance with Annex IV of the Directive when determining “best available techniques”.
2.
Relevant legal obligations of the IPPC Directive and the definition of BAT
In order to help the reader understand the legal context in which this document has been drafted,
some of the most relevant provisions of the IPPC Directive, including the definition of the term
“best available techniques”, are described in this preface. This description is inevitably
incomplete and is given for information only. It has no legal value and does not in any way alter
or prejudice the actual provisions of the Directive.
The purpose of the Directive is to achieve integrated prevention and control of pollution arising
from the activities listed in its Annex I, leading to a high level of protection of the environment
as a whole. The legal basis of the Directive relates to environmental protection. Its
implementation should also take account of other Community objectives such as the
competitiveness of the Community’s industry thereby contributing to sustainable development.
More specifically, it provides for a permitting system for certain categories of industrial
installations requiring both operators and regulators to take an integrated, overall look at the
polluting and consuming potential of the installation. The overall aim of such an integrated
approach must be to improve the management and control of industrial processes so as to ensure
a high level of protection for the environment as a whole. Central to this approach is the general
principle given in Article 3 that operators should take all appropriate preventative measures
against pollution, in particular through the application of best available techniques enabling
them to improve their environmental performance.
The term “best available techniques” is defined in Article 2(11) of the Directive as “the most
effective and advanced stage in the development of activities and their methods of operation
which indicate the practical suitability of particular techniques for providing in principle the
basis for emission limit values designed to prevent and, where that is not practicable, generally
to reduce emissions and the impact on the environment as a whole.” Article 2(11) goes on to
clarify further this definition as follows:
“techniques” includes both the technology used and the way in which the installation is
designed, built, maintained, operated and decommissioned;
“available” techniques are those developed on a scale which allows implementation in the
relevant industrial sector, under economically and technically viable conditions, taking into
consideration the costs and advantages, whether or not the techniques are used or produced
inside the Member State in question, as long as they are reasonably accessible to the operator;
“best” means most effective in achieving a high general level of protection of the environment
as a whole.
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Preface
Furthermore, Annex IV of the Directive contains a list of “considerations to be taken into
account generally or in specific cases when determining best available techniques... bearing in
mind the likely costs and benefits of a measure and the principles of precaution and prevention”.
These considerations include the information published by the Commission pursuant to
Article 16(2).
Competent authorities responsible for issuing permits are required to take account of the general
principles set out in Article 3 when determining the conditions of the permit. These conditions
must include emission limit values, supplemented or replaced where appropriate by equivalent
parameters or technical measures. According to Article 9(4) of the Directive, these emission
limit values, equivalent parameters and technical measures must, without prejudice to
compliance with environmental quality standards, be based on the best available techniques,
without prescribing the use of any technique or specific technology, but taking into account the
technical characteristics of the installation concerned, its geographical location and the local
environmental conditions. In all circumstances, the conditions of the permit must include
provisions on the minimisation of long-distance or transboundary pollution and must ensure a
high level of protection for the environment as a whole.
Member States have the obligation, according to Article 11 of the Directive, to ensure that
competent authorities follow or are informed of developments in best available techniques.
3.
Objective of this Document
Article 16(2) of the Directive requires the Commission to organise “an exchange of information
between Member States and the industries concerned on best available techniques, associated
monitoring and developments in them”, and to publish the results of the exchange.
The purpose of the information exchange is given in recital 25 of the Directive, which states that
“the development and exchange of information at Community level about best available
techniques will help to redress the technological imbalances in the Community, will promote
the worldwide dissemination of limit values and techniques used in the Community and will
help the Member States in the efficient implementation of this Directive.”
The Commission (Environment DG) established an information exchange forum (IEF) to assist
the work under Article 16(2) and a number of technical working groups have been established
under the umbrella of the IEF. Both IEF and the technical working groups include
representation from Member States and industry as required in Article 16(2).
The aim of this series of documents is to reflect accurately the exchange of information which
has taken place as required by Article 16(2) and to provide reference information for the
permitting authority to take into account when determining permit conditions. By providing
relevant information concerning best available techniques, these documents should act as
valuable tools to drive environmental performance.
4.
Information Sources
This document represents a summary of information collected from a number of sources,
including in particular the expertise of the groups established to assist the Commission in its
work, and verified by the Commission services. All contributions are gratefully acknowledged.
5.
How to understand and use this document
The information provided in this document is intended to be used as an input to the
determination of BAT in specific cases. When determining BAT and setting BAT-based permit
conditions, account should always be taken of the overall goal to achieve a high level of
protection for the environment as a whole.
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Waste Incineration
Preface
Chapters 1 and 2 provide general information on the industrial sector concerned and on the
industrial processes used within the sector.
Chapter 3 provides data and information concerning current emission and consumption levels,
reflecting the situation in existing installations at the time of writing.
Chapter 4 describes in more detail the emission reduction and other techniques that are
considered to be most relevant for determining BAT and BAT-based permit conditions. This
information includes the consumption and emission levels considered achievable by using the
technique, some idea of the costs and the cross-media issues associated with the technique, and
the extent to which the technique is applicable to the range of installations requiring IPPC
permits, for example new, existing, large or small installations. Techniques that are generally
seen as obsolete are not included.
Chapter 5 presents the techniques and the emission and consumption levels that are considered
to be compatible with BAT in a general sense. The purpose is thus to provide general
indications regarding the emission and consumption levels that can be considered as an
appropriate reference point to assist in the determination of BAT-based permit conditions or for
the establishment of general binding rules under Article 9(8). It should be stressed, however,
that this document does not propose emission limit values. The determination of appropriate
permit conditions will involve taking account of local, site-specific factors such as the technical
characteristics of the installation concerned, its geographical location and the local
environmental conditions. In the case of existing installations, the economic and technical
viability of upgrading them also needs to be taken into account. Even the single objective of
ensuring a high level of protection for the environment as a whole will often involve making
trade-off judgements between different types of environmental impact, and these judgements
will often be influenced by local considerations.
Although an attempt is made to address some of these issues, it is not possible for them to be
considered fully in this document. The techniques and levels presented in the (BAT) chapter(s)
to be added will therefore not necessarily be appropriate for all installations. On the other hand,
the obligation to ensure a high level of environmental protection including the minimisation of
long-distance or transboundary pollution implies that permit conditions cannot be set on the
basis of purely local considerations. It is therefore of the utmost importance that the information
contained in this document is fully taken into account by permitting authorities.
Since the best available techniques change over time, this document will be reviewed and
updated as appropriate. All comments and suggestions should be made to the European IPPC
Bureau at the Institute for Prospective Technological Studies at the following address:
Edificio Expo, c/Inca Garcilaso, s/n, E-41092 Seville, Spain
Telephone: +34 95 4488 284
Fax: +34 95 4488 426
e-mail: [email protected]
Internet: http://eippcb.jrc.es
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Preface
6.
Interface between the IPPC and the Waste Incineration Directives
The following presentation of the issues is related to the interface between Directive
2000/76/EC of 4 December 2000 on the incineration of waste (WI Directive) and Directive
96/61/EC of 24 September 1996 concerning integrated pollution prevention and control (IPPC
Directive).
It should be noted that the ultimate interpretation of Community law is a matter for the
European Court of Justice and therefore it cannot be excluded that interpretation by the Court
may raise new issues in the future.
The WI Directive contains, among others, the following explicit reference to the IPPC
Directive:
Recital 13 of the WI Directive states that “Compliance with the emission limit values laid down
by this Directive should be regarded as a necessary but not sufficient condition for compliance
with the requirements of Directive 96/61/EC. Such compliance may involve more stringent
emission limit values for the pollutants envisaged by this Directive, emission limit values for
other substances and other media, and other appropriate conditions.”
The recital makes clear that compliance with the emissions limit values laid down in the WI
Directive does not remove the obligation to operate in compliance with all the provisions of the
IPPC Directive, including a permit containing emission limit values or equivalent parameters
and technical measures determined according to the provisions of Article 9(4) or Article 9(8) of
the latter. As presented in the standard BREF preface, a certain flexibility is anchored in the
provisions of Article 9(4) of the IPPC Directive as well as in the definition of BAT. However, if
stricter conditions, compared to the conditions of the WI Directive, are determined by a
competent authority or through general binding rules to be necessary to fulfil the requirements
of the IPPC Directive for a particular permit, these stricter conditions shall apply.
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Waste Incineration
Best Available Techniques Reference Document on
Waste Incineration
EXECUTIVE SUMMARY.........................................................................................................................I
PREFACE.................................................................................................................................................XI
SCOPE .............................................................................................................................................XXXIII
1
GENERAL INFORMATION ON WASTE INCINERATION ..................................................... 1
1.1 Purpose of incineration and basic theory ....................................................................................... 1
1.2 Overview of waste incineration in Europe ..................................................................................... 2
1.3 Plant sizes....................................................................................................................................... 5
1.4 Overview of legislation .................................................................................................................. 6
1.5 Waste composition and process design .......................................................................................... 6
1.6 Key environmental issues .............................................................................................................. 9
1.6.1
Process emissions to air and water...................................................................................... 9
1.6.2
Installation residues production ........................................................................................ 10
1.6.3
Process noise and vibration............................................................................................... 11
1.6.4
Energy production and consumption ................................................................................ 12
1.6.5
Consumption of raw materials and energy by the installation .......................................... 13
1.7 Economic information.................................................................................................................. 13
2
APPLIED TECHNIQUES .............................................................................................................. 19
2.1 Overview and introduction........................................................................................................... 19
2.2 Pretreatment, storage and handling techniques ............................................................................ 20
2.2.1
Municipal solid wastes (MSW) ........................................................................................ 21
2.2.1.1
Collection and pretreatment outside the MSW incineration plant............................... 21
2.2.1.2
Municipal solid waste pretreatment within the incineration plant............................... 22
2.2.1.3
Waste delivery and storage ......................................................................................... 22
2.2.1.3.1 Waste control ......................................................................................................... 22
2.2.1.3.2 Bunker.................................................................................................................... 22
2.2.2
Hazardous wastes.............................................................................................................. 23
2.2.2.1
Brief description of the sector ..................................................................................... 23
2.2.2.2
Waste acceptance ........................................................................................................ 24
2.2.2.3
Storage ........................................................................................................................ 25
2.2.2.3.1 Storage of solid hazardous waste ........................................................................... 26
2.2.2.3.2 Storage of pumpable hazardous waste ................................................................... 26
2.2.2.3.3 Storage for containers and tank containers............................................................. 27
2.2.2.4
Feeding and pretreatment ............................................................................................ 27
2.2.3
Sewage sludge .................................................................................................................. 28
2.2.3.1
Composition of sewage sludge.................................................................................... 28
2.2.3.2
Pretreatment of sewage sludge .................................................................................... 29
2.2.3.2.1 Physical dewatering ............................................................................................... 29
2.2.3.2.2 Drying .................................................................................................................... 30
2.2.3.2.3 Sludge digestion..................................................................................................... 31
2.2.4
Clinical waste.................................................................................................................... 31
2.2.4.1
Nature and composition of clinical wastes .................................................................. 31
2.2.4.2
Handling, pretreatment and storage of clinical waste.................................................. 32
2.3 The thermal treatment stage ......................................................................................................... 32
2.3.1
Grate incinerators.............................................................................................................. 35
2.3.1.1
Waste feeder................................................................................................................ 36
2.3.1.2
Incineration grate ........................................................................................................ 36
2.3.1.2.1 Rocking grates........................................................................................................ 37
2.3.1.2.2 Reciprocating grates............................................................................................... 37
2.3.1.2.3 Travelling grates .................................................................................................... 38
2.3.1.2.4 Roller grates ........................................................................................................... 38
2.3.1.2.5 Cooled grates ......................................................................................................... 38
2.3.1.3
Bottom ash discharger................................................................................................. 38
2.3.1.4
Incineration chamber and boiler.................................................................................. 39
2.3.1.5
Incineration air feeding ............................................................................................... 41
2.3.1.6
Auxiliary burner .......................................................................................................... 42
2.3.1.7
Incineration temperature, residence time, minimum oxygen content.......................... 42
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2.3.1.8
Sewage sludge incineration in MSWI plants ...............................................................42
2.3.1.9
Addition of clinical waste to a municipal waste incinerator ........................................43
2.3.2
Rotary kilns .......................................................................................................................44
2.3.2.1
Kilns and post combustion chambers for hazardous waste incineration......................45
2.3.2.2
Drum kiln with post-combustion chamber for hazardous waste incineration..............45
2.3.3
Fluidised beds....................................................................................................................47
2.3.3.1
Stationary (or bubbling) fluidised bed incineration .....................................................49
2.3.3.2
Circulating fluidised bed (CFB) for sewage sludge.....................................................51
2.3.3.3
Spreader-stoker furnace ...............................................................................................51
2.3.3.4
Rotating fluidised bed..................................................................................................52
2.3.4
Pyrolysis and gasification systems ....................................................................................52
2.3.4.1
Introduction to gasification and pyrolysis....................................................................52
2.3.4.2
Gasification..................................................................................................................53
2.3.4.2.1 Examples of gasification processes ........................................................................55
2.3.4.3
Pyrolysis ......................................................................................................................56
2.3.4.3.1 Example of a pyrolysis process ..............................................................................58
2.3.4.3.2 Example of pyrolysis in combination with a power plant ......................................59
2.3.4.4
Combination processes ................................................................................................61
2.3.4.4.1 Pyrolysis – incineration ..........................................................................................61
2.3.4.4.2 Pyrolysis – gasification...........................................................................................64
2.3.4.4.3 Gasification – combustion ......................................................................................66
2.3.5
Other techniques................................................................................................................67
2.3.5.1
Stepped and static hearth furnaces...............................................................................67
2.3.5.2
Multiple hearth furnaces ..............................................................................................67
2.3.5.3
Multiple hearth fluidised bed furnace ..........................................................................70
2.3.5.4
Modular systems..........................................................................................................70
2.3.5.5
Incineration chambers for liquid and gaseous wastes ..................................................71
2.3.5.6
Cycloid incineration chamber for sewage sludge ........................................................72
2.3.5.7
Example of process for the incineration of liquid and gaseous chlorinated wastes with
HCl recovery................................................................................................................72
2.3.5.8
Example of a process for the incineration of highly chlorinated liquid wastes with
chlorine recycling ........................................................................................................74
2.3.5.9
Waste water incineration .............................................................................................75
2.3.5.10
Plasma technologies.....................................................................................................77
2.3.5.11
Various techniques for sewage sludge incineration .....................................................79
2.4 The energy recovery stage ............................................................................................................81
2.4.1
Introduction and general principles ...................................................................................81
2.4.2
External factors affecting energy efficiency......................................................................82
2.4.2.1
Waste type and nature..................................................................................................82
2.4.2.2
Influence of plant location on energy recovery ...........................................................84
2.4.2.3
Factors taken into account when selecting the design of the energy cycle ..................86
2.4.3
Energy efficiency of waste incinerators ............................................................................87
2.4.3.1
Energy inputs to waste incinerators .............................................................................87
2.4.3.2
Energy outputs from waste incinerators ......................................................................88
2.4.4
Applied techniques for improving energy recovery ..........................................................88
2.4.4.1
Waste feed pretreatment ..............................................................................................88
2.4.4.2
Boilers and heat transfer ..............................................................................................89
2.4.4.2.1 Corrosion in boilers ................................................................................................91
2.4.4.3
Combustion air preheating...........................................................................................93
2.4.4.4
Water cooled grates .....................................................................................................93
2.4.4.5
Flue-gas condensation .................................................................................................93
2.4.4.6
Heat pumps ..................................................................................................................95
2.4.4.6.1 Compressor driven heat pumps ..............................................................................95
2.4.4.6.2 Absorption heat pumps ...........................................................................................96
2.4.4.6.3 Open heat pumps ....................................................................................................96
2.4.4.6.4 Example data of different heat pumps ....................................................................96
2.4.4.7
Flue-gas re-circulation .................................................................................................97
2.4.4.8
Reheating of flue-gases to the operation temperature FGT devices ............................97
2.4.4.9
Plume visibility reduction............................................................................................97
2.4.4.10
Steam-water cycle improvements: effect on efficiency and other aspects...................97
2.4.5
Steam generators and quench cooling for hazardous waste incinerators...........................98
2.4.6
Examples of energy recovery from fluidised bed incinerators ..........................................99
2.5 Applied flue-gas treatment and control systems .........................................................................100
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2.5.1
Summary of the application of FGT techniques ............................................................. 100
2.5.2
Overview of overall combined FGT system options ...................................................... 102
2.5.3
Techniques for reducing particulate emissions ............................................................... 102
2.5.3.1
Electrostatic precipitators.......................................................................................... 103
2.5.3.2
Wet electrostatic precipitators ................................................................................... 103
2.5.3.3
Condensation electrostatic precipitators.................................................................... 104
2.5.3.4
Ionisation wet scrubbers............................................................................................ 105
2.5.3.5
Fabric filters .............................................................................................................. 105
2.5.3.6
Cyclones and multi-cyclones..................................................................................... 106
2.5.4
Techniques for the reduction of acid gases (e.g. HCl, HF and SOX emissions).............. 107
2.5.4.1
Removal of sulphur dioxide and halogens ................................................................ 107
2.5.4.2
Direct desulphurisation ............................................................................................. 110
2.5.5
Techniques for the reduction of emissions of oxides of nitrogen ................................... 111
2.5.5.1
Primary techniques for NOX reduction...................................................................... 111
2.5.5.1.1 Air supply, gas mixing and temperature control .................................................. 111
2.5.5.1.2 Flue-Gas Recirculation (FGR) ............................................................................. 112
2.5.5.1.3 Oxygen injection .................................................................................................. 112
2.5.5.1.4 Staged combustion ............................................................................................... 112
2.5.5.1.5 Natural gas injection (re-burn) ............................................................................. 112
2.5.5.1.6 Injection of water into furnace/flame ................................................................... 112
2.5.5.2
Secondary techniques for NOX reduction.................................................................. 112
2.5.5.2.1 Selective Non-Catalytic Reduction (SNCR) process ........................................... 113
2.5.5.2.2 Selective Catalytic Reduction (SCR) process ...................................................... 115
2.5.6
Techniques for the reduction of mercury emissions ....................................................... 116
2.5.6.1
Primary techniques.................................................................................................... 116
2.5.6.2
Secondary techniques................................................................................................ 116
2.5.7
Techniques for the reduction of other emissions of heavy metals .................................. 117
2.5.8
Techniques for the reduction of emissions of organic carbon compounds ..................... 117
2.5.8.1
Adsorption on activated carbon reagents in an entrained flow system...................... 118
2.5.8.2
SCR systems.............................................................................................................. 118
2.5.8.3
Catalytic bag filters ................................................................................................... 118
2.5.8.4
Re-burn of carbon adsorbents.................................................................................... 119
2.5.8.5
Use of carbon impregnated plastics for PCDD/F adsorption .................................... 119
2.5.8.6
Static bed filters......................................................................................................... 120
2.5.8.7
Rapid quenching of flue-gases .................................................................................. 120
2.5.9
Reduction of greenhouse gases (CO2, N2O) ................................................................... 121
2.5.9.1
Prevention of nitrous oxide emissions....................................................................... 121
2.5.10
Overview of flue-gas treatments applied at hazardous waste incinerators...................... 121
2.5.11
Flue-gas treatment for sludge incinerators...................................................................... 122
2.6 Waste water treatment and control techniques........................................................................... 123
2.6.1
Potential sources of waste water ..................................................................................... 123
2.6.2
Basic design principles for waste water control.............................................................. 124
2.6.3
Influence of flue-gas treatment systems on waste water................................................. 125
2.6.4
Processing of waste water from wet flue-gas treatment systems .................................... 126
2.6.4.1
Physico-chemical treatment ...................................................................................... 126
2.6.4.2
Application of sulphides............................................................................................ 127
2.6.4.3
Application of membrane technology ....................................................................... 128
2.6.4.4
Stripping of ammonia................................................................................................ 128
2.6.4.5
Separate treatment of waste water from the first and the last steps of the scrubber
system........................................................................................................................ 128
2.6.4.6
Anaerobic biological treatment (conversion of sulphates into elementary sulphur) . 129
2.6.4.7
Evaporation systems for process waste water ........................................................... 129
2.6.4.7.1 In-line evaporation ............................................................................................... 129
2.6.4.7.2 Separate evaporation ............................................................................................ 130
2.6.4.8
Example of process producing hydrochloric acid with downstream cleaning .......... 131
2.6.5
Waste water treatment at hazardous waste incinerators.................................................. 131
2.7 Solid residue treatment and control techniques.......................................................................... 133
2.7.1
Types of solid residues ................................................................................................... 133
2.7.2
Treatment and re-cycling of solid residues ..................................................................... 135
2.7.3
Treatments applied to Flue-gas treatment residues ......................................................... 136
2.7.3.1
Solidification and chemical stabilisation of FGT residues ........................................ 136
2.7.3.2
Thermal treatment of FGT residues .......................................................................... 136
2.7.3.3
Extraction and separation of FGT residues ............................................................... 137
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xvii
2.7.3.4
Chemical stabilisation of FGT residues .....................................................................138
2.7.3.5
Other methods or practices for FGT residues ............................................................138
2.8 Monitoring and control techniques .............................................................................................138
2.8.1
Incineration control systems............................................................................................138
2.8.2
Overview of emissions monitoring carried out ...............................................................139
2.8.3
Experiences with continuous sampling of dioxin emissions ...........................................140
2.8.4
Experiences with continuous measurement of mercury emissions..................................141
2.8.5
Overview of safety devices and measures .......................................................................142
3
EMISSIONS AND CONSUMPTIONS.........................................................................................143
3.1 Introduction ................................................................................................................................143
3.1.1
Substance partitioning in waste incineration ...................................................................144
3.1.2
Examples of the dioxin balance for MSWI .....................................................................146
3.1.3
Composition of crude flue-gas in waste incineration plants............................................147
3.1.4
Emissions of gases relevant to climate change................................................................149
3.2 Emissions to air ..........................................................................................................................150
3.2.1
Substances emitted to air .................................................................................................150
3.2.2
Municipal waste incineration plants ................................................................................156
3.2.2.1
Summary data for emissions to air from MSWI ........................................................156
3.2.2.2
European air emissions survey data for MSWI .........................................................157
3.2.2.3
Emissions to air from fluidised bed incinerators .......................................................162
3.2.3
Hazardous waste incineration plants ...............................................................................162
3.2.3.1
Summary data of the emissions to air from HWI ......................................................162
3.2.3.2
European air emissions survey data for HWI ............................................................163
3.3 Emissions to water......................................................................................................................174
3.3.1
Volumes of waste water arising from flue-gas treatment ................................................174
3.3.2
Other potential sources of waste water from waste incineration plants...........................175
3.3.3
Installations free of process water releases......................................................................175
3.3.4
Plants with physico–chemical waste water treatment......................................................175
3.3.5
Hazardous waste incineration plants - European survey data..........................................179
3.3.5.1
General overview of emissions to water from European HWI ..................................179
3.3.5.2
Overview by parameter of emissions to water from European HWI .........................180
3.4 Solid residues..............................................................................................................................186
3.4.1
Mass streams of solid residues in MSWI ........................................................................186
3.4.2
Bottom ash composition and leachability........................................................................187
3.5 Energy consumption and production ..........................................................................................192
3.5.1
Energy efficiency calculation for waste incineration installations ..................................193
3.5.2
Waste net calorific value calculation...............................................................................193
3.5.3
Equivalence factors .........................................................................................................194
3.5.4
Data on the recovery of energy from waste.....................................................................194
3.5.4.1
Electricity recovery data ............................................................................................195
3.5.4.2
Heat recovery data .....................................................................................................196
3.5.4.3
Combined heat and power data..................................................................................197
3.5.4.4
Boiler conversion efficiency data ..............................................................................197
3.5.5
Data on the consumption of energy by the process .........................................................198
3.5.6
Data comparing energy required by, and output from, the installation ...........................199
3.6 Noise...........................................................................................................................................201
3.7 Other operating resources ...........................................................................................................202
3.7.1
Water ...............................................................................................................................202
3.7.2
Other operating resources................................................................................................203
3.7.2.1
Neutralisers ................................................................................................................203
3.7.2.2
NOX removal agents ..................................................................................................203
3.7.2.3
Fuel oil and natural gas..............................................................................................204
3.7.2.4
Merchant hazardous waste incinerator plant survey data...........................................204
4
TECHNIQUES TO CONSIDER IN THE DETERMINATION OF BAT ................................205
4.1 General practices applied before the thermal treatment stage ....................................................207
4.1.1
Suitability of process design for the waste(s) received....................................................207
4.1.2
General housekeeping measures......................................................................................208
4.1.3
Quality control of incoming wastes.................................................................................208
4.1.3.1
Establishing installation input limitations and identifying key risks .........................208
4.1.3.2
Communication with waste suppliers to improve incoming waste quality control....210
4.1.3.3
Controlling waste feed quality on the incinerator site ...............................................211
4.1.3.4
Checking, sampling and testing incoming wastes......................................................212
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4.1.3.5
Detectors for radioactive materials............................................................................ 214
4.1.4
Waste storage.................................................................................................................. 215
4.1.4.1
Sealed surfaces, controlled drainage and weatherproofing ....................................... 215
4.1.4.2
Management of storage times.................................................................................... 217
4.1.4.3
Baling or other containment of solid waste ............................................................... 218
4.1.4.4
Extraction of incineration air from storage areas for odour, dust and fugitive release
control ....................................................................................................................... 219
4.1.4.5
Segregation of waste types for safe processing ......................................................... 221
4.1.4.6
Individual labelling of contained waste loads ........................................................... 222
4.1.4.7
The use of fire detection and control systems ........................................................... 223
4.1.5
Pretreatment of incoming waste...................................................................................... 224
4.1.5.1
Pretreatment and mixing of wastes ........................................................................... 224
4.1.5.2
Shredding of mixed municipal wastes....................................................................... 227
4.1.5.3
Shredding of drummed and packaged hazardous wastes .......................................... 228
4.1.5.4
Feed equalising control system for solid hazardous wastes ...................................... 229
4.1.5.5
Pre-combustion removal of recyclable metals........................................................... 231
4.1.5.6
Pretreatment and targeted preparation of solid waste for combustion....................... 232
4.1.6
Waste transfer and loading.............................................................................................. 233
4.1.6.1
Positioning and view of operator............................................................................... 233
4.1.6.2
Provision of storage space for items removed from the waste .................................. 233
4.1.6.3
Direct injection of liquid and gaseous hazardous wastes in rotary kilns ................... 233
4.1.6.4
Reduction of air ingress into the combustion chamber during loading ..................... 234
4.2 Thermal processing .................................................................................................................... 235
4.2.1
Combustion technology selection ................................................................................... 235
4.2.2
Use of flow modelling .................................................................................................... 240
4.2.3
Combustion chamber design features ............................................................................. 241
4.2.4
Design to increase turbulence in the secondary combustion chamber ............................ 243
4.2.5
Use of continuous rather than batch operation................................................................ 244
4.2.6
Selection and use of suitable combustion control systems and parameters .................... 245
4.2.7
Use of infrared cameras for combustion monitoring and control ................................... 247
4.2.8
Optimisation of air supply stoichiometry........................................................................ 249
4.2.9
Primary air supply optimisation and distribution............................................................ 250
4.2.10
Preheating of primary and secondary air ........................................................................ 252
4.2.11
Secondary air injection, optimisation and distribution.................................................... 253
4.2.12
Replacement of part of the secondary air with re-circulated flue-gas............................. 254
4.2.13
Application of oxygen enriched air................................................................................. 256
4.2.14
Cooling of grates............................................................................................................. 258
4.2.15
Water cooling of rotary kilns .......................................................................................... 260
4.2.16
Higher temperature incineration (slagging) .................................................................... 261
4.2.17
Increased agitation and residence time of the waste in the furnace ................................ 263
4.2.18
Adjustment of throughput to maintain good burnout and combustion conditions .......... 265
4.2.19
Optimisation of time, temperature, turbulence of gases in the combustion zone, and
oxygen concentrations .................................................................................................... 266
4.2.20
Use of automatically operated auxiliary burners ............................................................ 269
4.2.21
Reduction of grate riddling rate and/or return of cooled riddlings to the combustion
chamber........................................................................................................................... 270
4.2.22
Protection of furnace membrane walls and boiler first pass with refractory or other
materials.......................................................................................................................... 272
4.2.23
Use of low gas velocities in the furnace and the inclusion of empty passes before the
boiler convection section ................................................................................................ 274
4.2.24
Determination of calorific value of the waste and its use as a combustion control
parameter ........................................................................................................................ 275
4.2.25
Low-NOX burners for liquid wastes................................................................................ 276
4.2.26
Fluidised bed gasification ............................................................................................... 276
4.2.27
High temperature combustion of gasification syngas with ash melting.......................... 279
4.3 Energy recovery ......................................................................................................................... 281
4.3.1
Optimisation of overall energy efficiency and energy recovery ..................................... 281
4.3.2
Energy loss reduction: flue-gas losses ............................................................................ 288
4.3.3
Increasing burnout of the waste ...................................................................................... 290
4.3.4
Reducing excess air volumes .......................................................................................... 290
4.3.5
Other energy loss reduction measures ............................................................................ 291
4.3.6
Reduction of overall process energy consumption ......................................................... 292
4.3.7
Selection of turbine ......................................................................................................... 294
Waste Incineration
xix
4.3.8
Increased steam parameters and application of special materials to decrease corrosion in
boilers..............................................................................................................................296
4.3.9
Reduction of condenser pressure (i.e. improve vacuum).................................................299
4.3.10
Selection of cooling system.............................................................................................301
4.3.11
Optimisation of boiler architecture..................................................................................302
4.3.12
Use of an integral furnace - boiler ...................................................................................304
4.3.13
Use of water walls in the first (empty) pass ....................................................................305
4.3.14
Use of a platten type superheater.....................................................................................305
4.3.15
Reduction of flue-gas temperatures after the boiler.........................................................306
4.3.16
Use of flue-gas condensation scrubbers ..........................................................................308
4.3.17
Use of heat pumps to increase heat recovery...................................................................309
4.3.18
Special configurations of the water/steam cycle with external power plants ..................311
4.3.19
Efficient cleaning of the convection bundles...................................................................313
4.4 Flue-gas treatment ......................................................................................................................315
4.4.1
Factors to consider when selecting flue-gas treatment systems.......................................315
4.4.1.1
General factors...........................................................................................................315
4.4.1.2
Energy optimisation...................................................................................................316
4.4.1.3
Overall optimisation and the “whole system” approach ............................................316
4.4.1.4
Technique selection for existing or new installations ................................................316
4.4.2
Reduction of dust emissions............................................................................................317
4.4.2.1
Application of a pre-dedusting stage before other flue-gas treatments......................317
4.4.2.2
Application of an additional flue-gas polishing system .............................................321
4.4.2.3
Application of double bag filtration...........................................................................324
4.4.2.4
Selection of bag filter materials .................................................................................326
4.4.3
Reduction of acid gas emissions......................................................................................328
4.4.3.1
Wet scrubbing systems ..............................................................................................328
4.4.3.2
Semi-wet scrubbing systems......................................................................................332
4.4.3.3
Intermediate systems with some water addition and residue recirculation (flash dry
systems) .....................................................................................................................336
4.4.3.4
Dry FGT systems .......................................................................................................339
4.4.3.5
Selection of alkaline reagent......................................................................................342
4.4.3.6
Addition of wet scrubbing as a flue-gas polishing system after other FGT processes
...................................................................................................................................344
4.4.3.7
Recirculation of FGT residues in the FGT system.....................................................345
4.4.3.8
Direct addition of alkaline reagents to the waste (direct desulphurisation) ...............347
4.4.3.9
Use of acid gas monitoring for FGT process optimisation ........................................348
4.4.4
Reduction in the emissions of nitrogen oxides ................................................................349
4.4.4.1
Selective catalytic reduction (SCR) ...........................................................................349
4.4.4.2
Selective non-catalytic reduction (SNCR).................................................................355
4.4.4.3
Optimisation of reagent selection for SNCR NOX reduction.....................................359
4.4.4.4
Replacement of secondary air with re-circulated flue-gas .........................................360
4.4.5
Reduction of PCDD/F emissions.....................................................................................360
4.4.5.1
Primary techniques for prevention of PCDD/F..........................................................361
4.4.5.2
Prevention of reformation of PCDD/F in the FGT system ........................................361
4.4.5.3
Destruction of PCDD/F using Selective Catalytic Reduction (SCR).........................363
4.4.5.4
Destruction of PCDD/F using catalytic filter bags ....................................................365
4.4.5.5
Destruction of PCDD/F by re-burn of adsorbents......................................................367
4.4.5.6
Adsorption of PCDD/F by activated carbon injection or other reagents ...................368
4.4.5.7
Adsorption of PCDD/F in static beds ........................................................................369
4.4.5.8
Use of carbon impregnated materials for PCDD/F adsorption in wet scrubbers .......372
4.4.5.9
Use of carbon slurries in wet scrubbers .....................................................................373
4.4.6
Reduction of mercury emissions .....................................................................................374
4.4.6.1
Low pH wet scrubbing and additive addition ............................................................374
4.4.6.2
Activated carbon injection for Hg adsorption............................................................376
4.4.6.3
Use of condensing scrubbers for flue-gas polishing ..................................................378
4.4.6.4
Separation of mercury using a resin filter..................................................................380
4.4.6.5
Chlorite injection for elemental Hg control ...............................................................380
4.4.6.6
Addition of hydrogen peroxide to wet scrubbers.......................................................381
4.4.6.7
Use of static activated carbon or coke filters .............................................................382
4.4.7
Other techniques and substances .....................................................................................382
4.4.7.1
Use of specific reagents for iodine and bromine reduction........................................382
4.5 Waste water treatment and control .............................................................................................384
4.5.1
General ............................................................................................................................384
xx
Waste Incineration
4.5.2
4.5.3
4.5.4
4.5.5
4.5.6
4.5.7
4.5.8
4.5.9
4.5.10
4.5.11
Application of optimal incineration technology ............................................................. 384
Application of waste water free gas cleaning technology............................................... 384
Re-circulation of polluted waste water in wet gas cleaning systems .............................. 386
Additional cooling of feed water of wet gas cleaning systems ....................................... 386
Use of boiler drain water as a water supply for scrubbers .............................................. 387
Treatment of laboratory waste water in the scrubber...................................................... 387
Re-circulation of effluents to the process in place of their discharge ............................. 388
Separate discharge of rainwater from roofs and other clean surfaces ............................. 389
Provision of storage/buffering capacity for waste water................................................. 390
Application of physico-chemical treatment to wet scrubber effluents and other
contaminated waste water from the plant........................................................................ 391
4.5.12
Ammonia removal from effluents................................................................................... 392
4.5.13
Separate treatment of effluents arising from different wet scrubbing stages .................. 393
4.5.14
Evaporation of wet scrubber effluent in the incineration process ................................... 394
4.5.15
Separate evaporation of wet scrubber effluent................................................................ 394
4.5.16
Recovery of hydrochloric acid from wet scrubber effluents........................................... 394
4.5.17
Recovery of gypsum from wet scrubber effluent............................................................ 395
4.6 Treatment techniques for solid residues..................................................................................... 397
4.6.1
Improving the burnout of bottom ash ............................................................................. 397
4.6.2
Segregation of the bottom ash from flue-gas treatment residues .................................... 399
4.6.3
Separation of the dust removal stage from other flue-gas treatments ............................. 400
4.6.4
Bottom ash - separation of metals................................................................................... 401
4.6.5
Bottom ash screening and crushing ................................................................................ 402
4.6.6
Bottom ash treatment using ageing ................................................................................. 403
4.6.7
Bottom ash treatment using dry treatment systems......................................................... 405
4.6.8
Bottom ash treatment using wet treatment systems ........................................................ 408
4.6.9
Bottom ash treatment using thermal systems.................................................................. 410
4.6.10
High temperature (slagging) rotary kiln.......................................................................... 412
4.6.11
FGT residue treatments................................................................................................... 412
4.6.11.1
Cement solidification of FGT residues...................................................................... 412
4.6.11.2
Vitrification and melting of FGT residues ................................................................ 414
4.6.11.3
Acid extraction of boiler and fly ash ......................................................................... 417
4.6.11.4
Treatment of FGT residues arising from dry sodium bicarbonate FGT process for use
in the soda ash industry ............................................................................................. 418
4.6.11.5
Treatment of FGT residues arising from dry sodium bicarbonate FGT process using
hydraulic binders ....................................................................................................... 420
4.7 Noise .......................................................................................................................................... 421
4.8 Environmental management tools.............................................................................................. 422
4.9 Good practice for public awareness and communication........................................................... 429
5
BEST AVAILABLE TECHNIQUES ........................................................................................... 431
5.1 Generic BAT for all waste incineration ..................................................................................... 434
5.2 Specific BAT for municipal waste incineration ......................................................................... 450
5.3 Specific BAT for pretreated or selected municipal waste incineration ...................................... 451
5.4 Specific BAT for hazardous waste incineration ......................................................................... 452
5.5 Specific BAT for sewage sludge incineration ............................................................................ 453
5.6 Specific BAT for clinical waste incineration ............................................................................. 453
6
EMERGING TECHNIQUES ....................................................................................................... 455
6.1 Use of steam as a spraying agent in post combustion chamber burners instead of air ............... 455
6.2 Application involving the reheating of turbine steam ................................................................ 455
6.3 Other measures in the crude flue-gas area for reducing dioxin emissions ................................. 456
6.4 Oil scrubber for the reduction of polyhalogenated aromatics and polyaromatic hydrocarbons
(PAHs) in the flue-gases from incineration plants ..................................................................... 456
6.5 Use of CO2 in flue-gases for the production of sodium carbonate ............................................. 457
6.6 Increased bed temperature, combustion control and oxygen addition in a grate incinerator...... 458
6.7 The PECK combination process for MSW treatment ................................................................ 459
6.8 FeSO4 stabilisation of FGT residues .......................................................................................... 463
6.9 CO2 stabilisation of FGT residues.............................................................................................. 464
6.10 Overview of some other emerging FGT residue treatment techniques ...................................... 465
6.11 Application of membrane technology for use in waste water treatment plants for wet scrubber
effluents...................................................................................................................................... 466
6.12 Combined dry sodium bicarbonate + SCR + scrubber FGT systems......................................... 466
Waste Incineration
xxi
7
CONCLUDING REMARKS.........................................................................................................471
7.1 Timing of the work .....................................................................................................................471
7.2 Sources of information and information gaps.............................................................................471
7.3 Degree of Consensus reached .....................................................................................................474
7.4 Other specific notes and issues ...................................................................................................474
7.4.1
Existence of installations with operational emission levels below those concluded as BAT
.........................................................................................................................................474
7.4.2
Comprehensiveness of Table 5.3 on selection criteria for FGT systems (BAT 37) ........474
7.4.3
Use of dry FGT systems at certain hazardous waste incinerators (BAT75)....................475
7.4.4
Impacts of energy pricing and policies on energy efficiency ..........................................475
7.4.5
Competition and regulatory impacts across waste treating industrial sectors .................475
7.4.6
Development and implementation of waste strategies ....................................................476
7.4.7
Markets and standards for bottom ash and other residues ...............................................476
7.4.8
Co-ordinated education and demonstration of health/environmental impacts.................477
7.5 Suggested topics for future R&D projects ..................................................................................477
8
REFERENCES ...............................................................................................................................479
9
GLOSSARY....................................................................................................................................483
10 ANNEXES.......................................................................................................................................489
10.1 Economic overview of MSWI - Member State information.......................................................489
10.2 Economic overview – some technological aspects of MSWI .....................................................503
10.2.1
Discharge and storage costs for MSWI ...........................................................................505
10.2.2
Firing system and boiler costs for MSWI........................................................................506
10.2.3
Water-steam cycle costs for MSWI.................................................................................507
10.2.4
Costs for some flue-gas treatment combinations used in MSWI.....................................513
10.2.4.1
Dry flue-gas cleaning.................................................................................................514
10.2.4.2
Absorption and adsorption plants for the separation of HCl, HF and SO2 ................516
10.2.4.3
NaOH scrubber ..........................................................................................................518
10.2.4.4
Secondary NOX reduction using SCR or SNCR ........................................................519
10.2.4.5
Post treatment flue-gas polishing systems .................................................................520
10.2.5
Cost estimations for some complete MSWI plants..........................................................521
10.2.6
Costs of fluidised bed combustion for MSW ..................................................................526
10.2.7
Gasification and pyrolysis system costs for MSW ..........................................................528
10.3 Example installation descriptions ...............................................................................................530
10.3.1
Examples of municipal waste incineration ......................................................................531
10.3.1.1
Grate incinerator with SCR and steam distribution ...................................................531
10.3.1.2
Grate incinerator with SCR and CHP ........................................................................536
10.3.1.3
Grate incinerator with SCR, CHP and bottom ash treatment.....................................542
10.3.1.4
Grate incinerator with SNCR de-NOX, combined double filtration and wet scrubbing
...................................................................................................................................546
10.3.1.5
Grate incinerator with semi-wet FGT, active carbon injection, ash recirculation,
bottom ash treatment and (mainly) electricity generation..........................................549
10.3.1.6
Grate incinerator with SNCR de-NOX, semi-wet FGT, active carbon injection and
high steam parameters (60 bar/380 °C) electricity generation...................................550
10.3.1.7
Grate incinerator with SNCR (NH3), semi-wet lime, active carbon and electricity
generation ..................................................................................................................551
10.3.1.8
Grate incinerator with SNCR (NH3), semi-wet lime, active carbon and electricity
generation ..................................................................................................................552
10.3.2
Examples of the hazardous wastes installations ..............................................................554
10.3.2.1
Rotary kiln with heat recovery, SNCR, EP, wet scrubber and static coke filter ........554
10.3.2.2
Rotary kiln with SCR, EP, wet scrubber and static carbon filter ...............................559
10.3.2.3
Rotary kiln with SNCR (urea), dry lime FGT, double bag filtration and dioxin
absorption ..................................................................................................................561
10.3.3
Examples of sewage sludge installations.........................................................................562
10.3.3.1
Bubbling fluidised bed with heat recovery, SNCR, EP, wet scrubbing and static coke
filter ...........................................................................................................................562
10.3.3.2
Bubbling FB with CHP, SNCR, flue-gas re-circulation, EP, wet scrubbing and bag
filters with coke/calcium carbonate injection ............................................................565
10.3.3.3
Bubbling FB Plant with CHP, EP and wet scrubbing................................................567
10.3.4
Examples of combined incineration of various wastes....................................................568
10.3.4.1
Circulating FB for selected/pretreated wastes with heat recovery, dry and wet FGT,
SCR and ash treatment...............................................................................................568
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Waste Incineration
10.3.4.2
Fluidized bed plant for selected hazardous and non-hazardous wastes with heat
recovery, EP, fabric filter, wet scrubber and SCR..................................................... 573
10.3.4.3
Water cooled grate furnace with CHP, cyclone de-dusting, SNCR and high dust SCR
de-NOX, and dry fabric filter ..................................................................................... 577
10.3.4.4
Grate incinerator treating MSW, SS & CW with SNCR (urea), dry Na bicarbonate
FGT, activated C injection and electricity generation ............................................... 580
10.3.4.5
Grate incinerator treating MSW and industrial waste with EP, wet scrubbing, effluent
evaporation, SCR and high pressure steam electricity generation............................. 582
10.3.4.6
Grate incinerator treating MW, IW, SS and waste sorting refuse with SNCR, EP, wet
scrubbing (on-line evaporation of effluent), bag filters and CHP ............................. 583
10.3.4.7
Grate incinerator treating MSW, industrial and commercial waste with SNCR and
semi-wet FGT and 20 bar 260 °C to district heating network................................... 585
10.3.4.8
Grate incinerator treating MSW, IW and clinical waste with SNCR, dry FGT and
electricity generation ................................................................................................. 587
10.3.4.9
Grate incinerator treating MSW, waste sorting residues and sludges with SNCR, dry
FGT and heat supply to DH and local electricity plant ............................................. 588
10.4 Energy calculation methodology and example calculation ........................................................ 590
10.4.1
General explanations of terms and system boundary of the energy calculation ............. 590
10.4.2
Example of NCV calculation used by energy sub-group ................................................ 591
10.4.3
Basic operational data for three examples of the energy calculation .............................. 592
10.4.4
Energy calculation formulas with basic operational data for three examples of the energy
calculation....................................................................................................................... 595
10.4.5
Equations to calculate the plant efficiency (Pl ef) .......................................................... 599
10.5 Example of a multi-criteria assessment used for the selection of FGT systems......................... 601
Waste Incineration
xxiii
List of figures
Figure 1.1: Municipal waste incineration capacity per capita.......................................................................5
Figure 1.2: Bottom ash recycled and deposited from MSWI in 1999.........................................................11
Figure 1.3: Energy production by municipal waste incinerators in Europe (1999) ....................................12
Figure 2.1: Example layout of a municipal solid waste incineration plant .................................................19
Figure 2.2: Example of some hazardous waste pretreatment systems used at some merchant HWI ..........28
Figure 2.3: Grate, furnace and heat recovery stages of an example municipal waste incineration plant....35
Figure 2.4: Different grate types.................................................................................................................37
Figure 2.5: Example of a type of ash remover used at a grate incinerator ..................................................39
Figure 2.6: Example of an incineration chamber ........................................................................................40
Figure 2.7: Various furnace designs with differing direction of the flue-gas and waste flow ....................41
Figure 2.8: Examples of the stages of a clinical waste loading systems used at a municipal waste
incinerator ...............................................................................................................................43
Figure 2.9: Schematic of a rotary kiln incineration system ........................................................................44
Figure 2.10: Drum-type kiln with post-combustion chamber .....................................................................45
Figure 2.11: Example of a drum-type kiln plant for hazardous waste incineration ....................................47
Figure 2.12: Schematic diagram showing pretreatment of MSW prior to fluidised bed combustion .........48
Figure 2.13: Main components of a stationary/bubbling fluidised bed.......................................................49
Figure 2.14: Main components of a circulating fluidised bed.....................................................................51
Figure 2.15: Representation of a packed bed and current flow gasifier......................................................54
Figure 2.16: Slag bath gasifier ....................................................................................................................55
Figure 2.17: Fluidised bed gasifier with high temperature slagging furnace..............................................56
Figure 2.18: Structure of a pyrolysis plant for municipal waste treatment .................................................57
Figure 2.19: Process scheme of ATM’s ‘pyrolysis’-unit ............................................................................59
Figure 2.20: Energy balance and weight assessment of the ConTherm plant.............................................60
Figure 2.21: Pyrolysis on a grate with directly connected high-temperature incineration..........................62
Figure 2.22: The RCP process ....................................................................................................................62
Figure 2.23: Example of a clinical waste pyrolysis-incineration plant, ZAVIN, Netherlands....................64
Figure 2.24: Schematic diagram of a push pyrolyser (example shown operated by Thermoselect) ...........65
Figure 2.25:Combined fluidised bed gasification and high temperature combustion process....................66
Figure 2.26: Principle function of a multiple hearth furnace ......................................................................68
Figure 2.27: Example of a sewage sludge incineration plant with a multiple hearth furnace.....................69
Figure 2.28: Principle function of a multiple hearth fluidised bed furnace ................................................70
Figure 2.29: Principle of an incineration chamber for liquid and gaseous wastes ......................................71
Figure 2.30: Illustration of a cycloid furnace..............................................................................................72
Figure 2.31: Diagram of a plant for HCl-extraction from residual gases and liquid halogenated wastes...73
Figure 2.32: Process scheme of a chlorine recycling unit operated by Akzo Nobel...................................75
Figure 2.33: Example of a waste water incinerator with a waste water evaporation (concentration) unit. .76
Figure 2.34: Process scheme of a caustic water treatment plant operated by AVR....................................77
Figure 2.35: Graph showing recorded variation in waste NCV at a MSWI over 4 years ...........................83
Figure 2.36: Illustration of individual heat surface areas in a steam generator...........................................89
Figure 2.37: Basic boiler flow systems.......................................................................................................90
Figure 2.38: Overview of various boiler systems: horizontal, combination, and, vertical..........................91
Figure 2.39: Pollution control and additional heat recovery by condensation of flue-gas water vapour at
the Stockholm/Hogdalen waste-fired CHP plant ....................................................................95
Figure 2.40: Overview of potential combinations of FGT systems ..........................................................102
Figure 2.41: Operating principle of an electrostatic precipitator ..............................................................103
Figure 2.42: Condensation electrostatic precipitator ................................................................................104
Figure 2.43: An example of a fabric filter ................................................................................................106
Figure 2.44: Schematic diagram of a dry FGT system with reagent injection to the FG pipe and
downstream bag filtration .....................................................................................................108
Figure 2.45: Operating principle of a spray absorber ...............................................................................108
Figure 2.46: Diagram of a 2 stage wet scrubber with upstream de-dusting ..............................................110
Figure 2.47: Temperature dependence of various NOX formation mechanisms in waste incineration.....111
Figure 2.48: SNCR operating principle ....................................................................................................113
Figure 2.49: Relationship between NOX reduction, production, ammonia slip and reaction temperature for
the SNCR process .................................................................................................................114
Figure 2.50: SCR operating principle .......................................................................................................115
Figure 2.51: Relationship between Hg emissions and the raw gas chloride content at a hazardous waste
incineration plant...................................................................................................................117
Figure 2.52: Process scheme for physico-chemical treatment of waste water from a wet flue-gas treatment
system ...................................................................................................................................126
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Waste Incineration
Figure 2.53: In-line evaporation of waste water from wet scrubbing....................................................... 129
Figure 2.54: Separate evaporation of scrubber effluent from wet scrubbing ........................................... 130
Figure 2.55: Overview of applied waste water treatment systems at merchant HWIs ............................. 132
Figure 2.56: Example of a waste water treatment facility in the merchant HWI sector........................... 133
Figure 3.1: Graph of NOX annual average emissions to air and applied abatement technique at European
HWIs .................................................................................................................................... 165
Figure 3.2: Graph of annual average dust emissions to air and applied abatement technique at European
HWIs .................................................................................................................................... 166
Figure 3.3: Graph of HCl annual average emissions to air and applied abatement technique at European
HWIs .................................................................................................................................... 167
Figure 3.4: Graph of annual average sulphur dioxide emissions to air and applied abatement technique at
European HWIs .................................................................................................................... 168
Figure 3.5: Graph of Hg annual average emissions to air and applied abatement technique at European
HWIs .................................................................................................................................... 170
Figure 3.6: Annual average emissions to air of other metals and applied abatement technique at European
HWIs .................................................................................................................................... 171
Figure 3.7: Graph of Cd and Tl annual average emissions to air and applied abatement technique at
European HWIs .................................................................................................................... 172
Figure 3.8: Graph of PCDD/F annual average emissions to air and applied abatement technique at
European HWIs .................................................................................................................... 173
Figure 3.9: CO emission reductions achieved following introduction of pretreatment techniques at a
hazardous waste incinerator.................................................................................................. 174
Figure 3.10: Graph of annual average suspended solid discharges to water and applied abatement
technique at European HWIs ................................................................................................ 181
Figure 3.11: Graph of annual average mercury discharges to water and applied abatement technique at
European HWIs .................................................................................................................... 182
Figure 3.12: Graph of annual average discharges of various metals to water at European HWIs............ 183
Figure 3.13: Graph of annual average Arsenic discharges to water at European HWIs........................... 183
Figure 3.14: Graph of annual average lead discharges to water at European HWIs ................................ 184
Figure 3.15: Graph of annual average Cadmium discharges to water at European HWIs ....................... 184
Figure 3.16: Graph of annual average Chromium discharges to water at European HWIs...................... 184
Figure 3.17: Graph of annual average Copper discharges to water European HWIs ............................... 185
Figure 3.18: Graph of annual average Nickel discharges to water at European HWIs ............................ 185
Figure 3.19: Graph of annual average Zinc discharges to water at European HWIs................................ 185
Figure 3.20: Graph showing increase in installation electrical consumption with increasing waste NCV
.............................................................................................................................................. 199
Figure 4.1: An example of the components of furnace control system .................................................... 247
Figure 4.2: Basic components of a cyclonic high temperature syngas ash melting furnace..................... 279
Figure 4.3: Schematic diagram of a “platen” type superheater ................................................................ 305
Figure 4.4: Combination of a waste incineration plant with a gas turbine power plant ........................... 312
Figure 4.5: Municipal waste incineration plant in combination with a coal power plant......................... 312
Figure 4.6: Diagram showing typical design of a semi-wet FGT system................................................. 332
Figure 4.7: Diagram of an SCR system downstream of non-wet FGT showing typical heat exchange and
temperature profiles.............................................................................................................. 351
Figure 4.8: Diagram of an SCR system downstream of a wet FGT system showing additional heat
exchange and temperature profiles ....................................................................................... 351
Figure 4.9: Effect of ageing on the leachability of selected metals: (left) effect on pH; (right) leaching as a
function of pH ...................................................................................................................... 405
Figure 6.1: Example of the reheating of steam......................................................................................... 455
Figure 6.2: Schematic of a waste incineration plant with a downstream oil scrubber for dioxin deposition
.............................................................................................................................................. 457
Figure 6.3: Basic components of the PECK process ................................................................................ 459
Figure 6.4: Fly ash treatment in the PECK process.................................................................................. 460
Figure 6.5: Bottom ash treatment in the PECK process ........................................................................... 460
Figure 6.6: Comparison of metals partitioning between a conventional grate MSWI and the PECK process
.............................................................................................................................................. 461
Figure 6.7: Material flow mass balance for the PECK process................................................................ 462
Figure 10.1: Water-steam cycle, option 1 ................................................................................................ 508
Figure 10.2: Water-steam cycle, option 2 and 6....................................................................................... 509
Figure 10.3: Water-steam cycle, option 3 ................................................................................................ 510
Figure 10.4: Water-steam cycle, options 4, 5, 7 and 8 ............................................................................. 510
Figure 10.5: The impact of plant size and energy utilisation on the specific waste treatment costs of new
MSWI installations ............................................................................................................... 522
Waste Incineration
xxv
Figure 10.6: The impact of varying FGT systems and plant sizes on the treatment costs of new MSWI
installations using the same energy utilisation techniques ....................................................522
Figure 10.7: Process flow scheme of the waste incineration plant Flötzersteig........................................531
Figure 10.8: Process flow scheme of the waste incineration plant Spittelau ............................................537
Figure 10.9: Process flow scheme of the waste incineration plant Wels – line 1 .....................................543
Figure 10.10: Process flow scheme of the rotary kilns of the Plant Simmeringer Haide..........................555
Figure 10.11: Process flow scheme of the fluidised bed reactors of the Plant Simmeringer Haide..........562
Figure 10.12: Process flow scheme of AVE-Reststoffverwertung Lenzing .............................................570
Figure 10.13: Process flow scheme of the Fluidised bed reactors at Arnoldstein.....................................574
Figure 10.14: Summary of the energy system inputs and outputs used by BREF ESG............................590
xxvi
Waste Incineration
List of tables
Table 1.1: Purpose of various components of a waste incinerator ............................................................... 2
Table 1.2: Amounts of municipal waste (MSW), hazardous waste (HW) and sewage sludge (SS) in EU-15
MSs, and their treatment........................................................................................................... 3
Table 1.3: Annual quantities of municipal and hazardous waste arising and the number of incineration
plants in some Accession Countries ......................................................................................... 4
Table 1.4: Geographical distribution of incineration plants for municipal, hazardous and sewage sludge
waste......................................................................................................................................... 4
Table 1.5: Average MSW incineration plant capacity by country ............................................................... 5
Table 1.6: Typical throughput ranges of thermal treatment technologies .................................................... 6
Table 1.7: Typical composition of waste in Germany.................................................................................. 8
Table 1.8: Gate fees in European MSW and HW incineration plants ........................................................ 14
Table 1.9: Comparative costs of MSW incineration in different MSs ....................................................... 15
Table 1.10: Specific investment costs for a new MSWI installation related to the annual capacity and
some types of FGT in Germany ............................................................................................. 15
Table 1.11: Example of the comparative individual cost elements for MSW and HW incineration plants16
Table 2.1: Typical reaction conditions and products from pyrolysis, gasification and incineration
processes................................................................................................................................. 20
Table 2.2: Prime impact of waste selection and pretreatment on residual waste ....................................... 21
Table 2.3: Summary of the differences between operators in the HWI market ......................................... 24
Table 2.4: Average composition of dewatered communal sewage sludge after dewatering ...................... 29
Table 2.5: Summary of the current successful application of thermal treatment techniques to the main
waste types at dedicated installations ..................................................................................... 34
Table 2.6: Properties of various RDF (Refuse Derived Fuel) fractions treated in fluidised beds. ............. 48
Table 2.7: Main operational criteria for stationary fluidised beds.............................................................. 50
Table 2.8: Operational criteria for a multiple hearth furnace ..................................................................... 69
Table 2.9: Comparison of furnace systems for sewage sludge incineration............................................... 80
Table 2.10: Ranges and typical net calorific values for some incinerator input wastes ............................. 82
Table 2.11: Calculated NCV values for waste treated at 50 European MSWI plants................................ 83
Table 2.12: Energy potential conversion efficiencies for different types of waste incineration plants ...... 84
Table 2.13: Factors taken into account when selecting the design of the energy cycle for waste
incineration plants .................................................................................................................. 86
Table 2.14: Example data showing the variation in heat and electricity output when using various
different types of heat pumps ................................................................................................. 96
Table 2.15: Steam-water cycle improvements: effect on efficiency and other aspects. ............................. 98
Table 2.16: Summary of the main differences between quench cooling and heat recovery....................... 99
Table 2.17: Summary of the main applied FGT systems for MSWIs in Europe in 2000/2001 ............... 101
Table 2.18: Tested continuous working measuring devices for emission measurements of mercury ...... 141
Table 3.1: Distribution of various substances in an example MSWI installation (in mass %)................. 145
Table 3.2: Percentage (%) distribution of heavy metals in a hazardous waste incineration process ........ 145
Table 3.3: Average operational conditions during partitioning tests on a HWI installation .................... 146
Table 3.4: PCDD/PCDF balance for a municipal waste incineration plant in Germany.......................... 146
Table 3.5: Example PCDD/F load data for an MSWI in France .............................................................. 147
Table 3.6: Flue-gas concentrations after the boiler (crude flue-gas) at various waste incineration plants
(O2 reference value 11 %) .................................................................................................... 148
Table 3.7: Total emissions relevant to climate change in Germany in the year 1999 compared with those
arising form waste incineration ............................................................................................ 150
Table 3.8: Range of clean gas operation emissions levels reported from some European MSWI plants. 156
Table 3.9: Operational emission levels to air from MSWI expressed per tonne of MSW incinerated..... 157
Table 3.10: HCl emissions survey of European MSWIs.......................................................................... 157
Table 3.11: HF emissions survey of European MSWIs ........................................................................... 158
Table 3.12: Sulphur dioxide emissions survey of European MSWIs ....................................................... 158
Table 3.13: Dust emissions survey of European MSWIs ......................................................................... 159
Table 3.14 Nitrogen oxides emissions survey of European MSWIs ........................................................ 159
Table 3.15: Total organic carbon emissions survey of European MSWIs ............................................... 159
Table 3.16: PCDD/F (TEQ) emissions survey of European MSWIs ....................................................... 160
Table 3.17: Mercury emissions survey of European MSWIs................................................................... 160
Table 3.18: Combined Cd and Hg emissions of selected MSWIs in France............................................ 161
Table 3.19: Emission results and techniques applied for Hg control at European MSWIs...................... 161
Table 3.20: Typical range of clean gas emissions to air from hazardous waste incineration plants ........ 162
Waste Incineration
xxvii
Table 3.21: Survey data of the annual average emissions to air from hazardous waste incinerators in
Europe...................................................................................................................................163
Table 3.22: Survey data of mass flow and annual sector emissions to air from merchant hazardous waste
incinerators in Europe ...........................................................................................................164
Table 3.23: Typical values of the amount of scrubbing water arising from FGT at waste incineration
plants treating low chlorine content wastes...........................................................................174
Table 3.24: Other possible waste water sources, and their approximate quantities, from waste incineration
plants .....................................................................................................................................175
Table 3.25: Typical contamination of waste water from wet FGT facilities of waste incineration plants
before treatment ....................................................................................................................176
Table 3.26: Releases to surface water and sewers from Dutch waste incinerators in 1999 ......................177
Table 3.27: Waste water quality (after treatment with Trimercaptotriazine) - Comparison between raw
and treated waste water and various standards......................................................................178
Table 3.28: Annual average range of concentrations of the emissions to water after treatment from
merchant hazardous waste installations that discharge waste water .....................................179
Table 3.29: Mass flows of the emissions to water from surveyed merchant HWIs in Europe .................180
Table 3.30: Typical data on the quantities of residues arising from municipal waste incineration plants.
..............................................................................................................................................186
Table 3.31: Mass streams of solid residues from MSWI expressed per tonne of MSW incinerated ........187
Table 3.32: Concentration ranges of organic compounds in bottom, boiler and filter ashes ....................187
Table 3.33: PCDD/F concentrations in various MSWI incineration residues in NL (data 2000 – 2004) .187
Table 3.34: Range of PCDD/F concentrations in MSWI residues (excluding peak high and low values)
..............................................................................................................................................188
Table 3.35: Leaching properties of mechanically treated bottom ash, measured using NEN7343...........189
Table 3.36: Quantities of the main waste streams produced by HWI (European survey data).................190
Table 3.37: Typical leaching values of bottom ash from hazardous waste incineration plants, measured
using DIN-S4 ........................................................................................................................190
Table 3.38: Some factors and their influence on energy recovery options ...............................................192
Table 3.39: Energy equivalence conversion factors .................................................................................194
Table 3.40: Electricity production and export rates per tonne of MSW ...................................................196
Table 3.41: Electricity production and export data per tonne of MSW for MSWI in France ...................196
Table 3.42: Heat production and export rates per tonne of MSW ............................................................196
Table 3.43: Heat production and export rates per tonnes of MSW for MSWI in France..........................196
Table 3.44: Average CHP percentage efficiency (calculated as energy equivalents) for 50 MSWI plants
..............................................................................................................................................197
Table 3.45: Average CHP recovery values per tonne of MSW in MSWI in France ................................197
Table 3.46: Survey data of MSWI boiler efficiencies...............................................................................197
Table 3.47: Electricity, heat and total energy demand data for 50 surveyed European MSWI per tonne of
waste treated..........................................................................................................................198
Table 3.48: Ratio of exported and consumed energy for various waste incinerators................................200
Table 3.49: Sources of noise at waste incineration plants........................................................................201
Table 3.50: Stoichiometric calculation of amounts of lime used for absorption during flue-gas cleaning
(reactants expressed at 100 % concentration and purity) ......................................................203
Table 3.51: Amount of additives used by merchant hazardous waste incineration processes ..................204
Table 4.1: Organisation chart for the information in Chapter 4................................................................205
Table 4.2: Information breakdown for each technique described in this Chapter 4..................................206
Table 4.3: Some checking and sampling techniques applied to various waste types................................213
Table 4.4: Some examples of applied storage techniques for various waste types...................................216
Table 4.5: Main techniques for reducing fugitive releases of odour, and GHG emissions.......................220
Table 4.6: Some segregation techniques applied for various waste types ................................................221
Table 4.7: A comparison of combustion and thermal treatment technologies and factors affecting their
applicability and operational suitability (table 1/3)...............................................................236
Table 4.8: A comparison of combustion and thermal treatment technologies and factors affecting their
applicability and operational suitability (table 2/3)...............................................................237
Table 4.9: A comparison of combustion and thermal treatment technologies and factors affecting their
applicability and operational suitability (table 3/3)...............................................................239
Table 4.10: A comparison of the features of some different furnace geometries .....................................242
Table 4.11: Crude flue-gas measurements at a test plant under normal operation, with IR camera and O2
conditioning ..........................................................................................................................249
Table 4.12: Some combustion specifications applied to incineration .......................................................266
Table 4.13: Relationship between nitrous oxide emissions and process temperatures for a bubbling
fluidised bed plant burning sewage sludge ...........................................................................267
Table 4.14: Estimated cost impacts of some alterations to combustion parameters .................................269
xxviii
Waste Incineration
Table 4.15: TWG energy sub-group survey data for specific energy flows at some European MSWIs per
tonne of waste treated ........................................................................................................... 285
Table 4.16: Techniques for the reduction of various energy losses at WI plants ..................................... 291
Table 4.17: Plant throughput and total process energy demand for MSWI in Germany.......................... 293
Table 4.18: Example energy outputs and income at various steam pressures for a CHP MSWI using
elevated steam pressures....................................................................................................... 298
Table 4.19: Relationship between the additional energy efficiency and the cooling medium (district
heating) return temperature .................................................................................................. 308
Table 4.20: Cross-media effects associated with the use of various pre-dedusters .................................. 318
Table 4.21: Operational data associated with the use of pre-dedusting systems ...................................... 319
Table 4.22: A comparison of dust removal systems................................................................................. 320
Table 4.23: Assessment of the applicability of pre-dedusting.................................................................. 320
Table 4.24: Emission levels associated with the use of BF flue-gas polishing systems........................... 322
Table 4.25: Cross-media effects associated with the use of additional flue-gas polishing....................... 322
Table 4.26: Operational data associated with the use of flue-gas polishing............................................. 323
Table 4.27: Assessment of the applicability of flue-gas polishing ........................................................... 323
Table 4.28: Cross-media effects associated with the use of double filtration .......................................... 325
Table 4.29: Operational data associated with the use of double filtration................................................ 325
Table 4.30: Assessment of the applicability of double filtration.............................................................. 326
Table 4.31: Operational information for different bag filter materials..................................................... 327
Table 4.32: Emission levels associated with the use of wet scrubbers..................................................... 328
Table 4.33: Cross-media effects associated with the use of wet scrubber FGT ....................................... 329
Table 4.34: Operational data associated with the use of wet FGT ........................................................... 330
Table 4.35: Assessment of the applicability of wet FGT ......................................................................... 331
Table 4.36: Estimated investment costs of selected components of wet FGT systems ............................ 331
Table 4.37: Emission levels associated with the use of wet scrubbers..................................................... 332
Table 4.38: Cross-media effects associated with the use of semi-wet acid gas treatment........................ 333
Table 4.39: Operational data associated with the use of semi-wet FGT .................................................. 334
Table 4.40: Assessment of the applicability of semi-wet FGT ................................................................ 335
Table 4.41: Estimated investment costs of selected components of typical semi-wet FGT systems....... 335
Table 4.42: Emission levels associated with the use of flash dry FGT.................................................... 336
Table 4.43: Cross-media effects associated with the use of flash dry systems ........................................ 337
Table 4.44: Operational data associated with the use of flash dry FGT .................................................. 338
Table 4.45: Assessment of the applicability of flash dry FGT ................................................................ 338
Table 4.46: Emission levels associated with the use of dry lime FGT .................................................... 339
Table 4.47: Emission levels associated with the use of dry sodium bicarbonate FGT ............................ 339
Table 4.48: Cross-media effects associated with the use of dry FGT...................................................... 340
Table 4.49: Operational data associated with the use of dry FGT............................................................ 340
Table 4.50: Assessment of the applicability of dry FGT ......................................................................... 341
Table 4.51: Comparison of features of various alkaline reagents ............................................................ 342
Table 4.52: Assessment of the applicability of various alkaline reagents .............................................. 343
Table 4.53: Operational data associated with the use of residue re-circulation ....................................... 345
Table 4.54: Assessment of the applicability of residue re-circulation ..................................................... 346
Table 4.55: Assessment of the applicability of raw gas monitoring for optimisation of FGT................. 348
Table 4.56: Emission levels associated with the use of SCR................................................................... 350
Table 4.57: Cross-media effects associated with the use of SCR ............................................................ 352
Table 4.58: Operational data associated with the use of SCR.................................................................. 352
Table 4.59: Assessment of the applicability of SCR ................................................................................ 354
Table 4.60: Estimated investment costs of selected components of typical semi-wet FGT systems using
SCR and SNCR .................................................................................................................... 354
Table 4.61: Emission levels associated with the use of SNCR ................................................................ 356
Table 4.62: Cross-media effects associated with the use of SNCR.......................................................... 356
Table 4.63: Operational data associated with the use of SNCR .............................................................. 357
Table 4.64: Assessment of the applicability of SNCR ............................................................................. 358
Table 4.65: Advantages and disadvantages of urea and ammonia use for SNCR.................................... 359
Table 4.66: Assessment of the applicability of PCDD/F reformation prevention techniques .................. 362
Table 4.67: Assessment of the applicability of SCR for PCDD/F removal ............................................. 364
Table 4.68: Destruction efficiency data for catalytic filter bags over 21 months of operation ................ 366
Table 4.69: Assessment of the applicability of catalytic bag filters ......................................................... 366
Table 4.70: Assessment of the applicability of re-burn of absorbers ....................................................... 367
Table 4.71: Assessment of the applicability of carbon injection for PCDD/F removal ........................... 369
Table 4.72: Cross-media effects associated with the use of static filters ................................................. 369
Table 4.73: Operational data associated with the use of static coke filters .............................................. 370
Waste Incineration
xxix
Table 4.74: Assessment of the applicability of static coke filters ............................................................371
Table 4.75: Operational data associated with the use of carbon impregnated materials in wet scrubbers372
Table 4.76: Assessment of the applicability of the use of carbon impregnated materials in wet scrubbers.
..............................................................................................................................................373
Table 4.77: Assessment of the applicability of wet scrubbing for Hg control ..........................................376
Table 4.78: Assessment of the applicability of carbon injection for Hg removal.....................................377
Table 4.79: Assessment of the applicability of Na2S2O3 for halogen removal..........................................383
Table 4.80: Quantity of HCl (30 %) recovered per tonne of waste ..........................................................395
Table 4.81: Quantities of Gypsum recovered per tonne of waste treated .................................................396
Table 4.82: Slag output concentration (mg/kg) data reported for an example slag treatment facility ......406
Table 4.83: Slag output eluate (ug/l) data reported for an example slag treatment...................................407
Table 4.84: Relative yield of various output fractions of wet bottom ash treatment ................................408
Table 4.85: Example of leaching results of the produced granulates........................................................409
Table 4.86: Slag output concentration (mg/kg) data reported for an example slag treatment facility ......409
Table 4.87: Slag output eluate (ug/l) data reported for an example slag treatment...................................409
Table 4.88: Relative costs of some ash treatment techniques ...................................................................411
Table 4.89: Variations in solidification treatments for FGT residues between some countries................414
Table 4.90: FGT vitrification processes used in the US and Japan ..........................................................416
Table 4.91: Examples of plants using the acid extraction process for FGT residue treatment .................418
Table 4.92: Characteristics of some acid extraction processes used for FGT residue treatment..............418
Table 5.1: How to combine the BAT described for a specific case ..........................................................434
Table 5.2 Operational emission level ranges associated with the use of BAT for releases to air from waste
incinerators............................................................................................................................441
Table 5.3: An example assessment of some IPPC relevant criteria that may be taken into account when
selecting between wet/semi-wet/dry FGT options ................................................................443
Table 5.4: BAT associated operational emission levels for discharges of waste water from effluent
treatment plant receiving FGT scrubber effluent ..................................................................446
Table 6.1: Residue quality using SYNCOM system.................................................................................458
Table 6.2: Emission levels associated with the use of combined dry sodium bicarbonate and SCR FGT
system ...................................................................................................................................467
Table 6.3: Cross-media effects associated with the use of combined dry sodium bicarbonate and SCR
FGT system...........................................................................................................................467
Table 6.4: Operational data associated with the use of combined dry sodium bicarbonate and SCR FGT
system ...................................................................................................................................468
Table 6.5: Assessment of the applicability of the combined dry sodium bicarbonate and SCR FGT system
..............................................................................................................................................468
Table 9.1: Country codes and currencies ...............................................................................................488
Table 10.1: Treatment costs for a MSW Grate Incinerator with varying capacity ...................................491
Table 10.2: Grate MSW incinerator costs 200000 t/yr Germany .............................................................492
Table 10.3: Estimated cost to build and operate a mass-burn MSW incineration plant of 200000 tonne
capacity in Ireland .................................................................................................................493
Table 10.4: Incinerator costs in Italy based on model calculations ..........................................................495
Table 10.5: Fees and expected amount of waste to be incinerated at SIDOR facility, Luxembourg in 1999
..............................................................................................................................................496
Table 10.6: Specific operational costs of the SIDOR MSWI in Luxembourg 1998 and 1999 .................497
Table 10.7: Extract from the budget Of SIDOR for the years 1998 and 1999..........................................498
Table 10.8: Capital investment and treatment costs for MSWI in NL......................................................499
Table 10.9: Cost breakdown for various incinerators in Sweden .............................................................500
Table 10.10: UK gate fees for different incinerator capacities and energy outputs ..................................500
Table 10.11: Breakdown of estimated United Kingdom incinerator costs ...............................................502
Table 10.12: Specific costs for discharge and storage facilities as a function of throughput when waste is
delivered by refuse collection vehicles .................................................................................505
Table 10.13: Specific costs for discharge and storage facilities as a function of throughput when waste is
delivered by train ..................................................................................................................505
Table 10.14: Specific costs for a grate firing system and the boiler of waste incineration plants as a
function of throughput ..........................................................................................................506
Table 10.15: Specific costs of a water-steam cycle with pure heat decoupling and feeding into district
heating systems as a function of waste throughput ...............................................................508
Table 10.16: Specific costs of a water-steam cycle comprising a steam extraction turbine as a function of
waste throughput ...................................................................................................................509
Table 10.17: Specific costs of a water-steam cycle comprising a steam extraction turbine in combination
with the steam system of an adjacent power plant as a function of waste throughput ..........510
xxx
Waste Incineration
Table 10.18: Specific costs of a water-steam cycle comprising cogeneration (CHP) and low steam
parameters as a function of waste throughput ...................................................................... 511
Table 10.19: Specific costs of a water-steam cycle comprising cogeneration (CHP) and high steam
parameters as a function of waste throughput ...................................................................... 511
Table 10.20: Specific costs of a water-steam cycle comprising a steam extraction turbine (normal steam
parameters) as a function of waste throughput when energy can be substituted .................. 511
Table 10.21: Specific costs of a water-steam cycle comprising cogeneration (CHP - normal steam
parameters) as a function of waste throughput when energy can be substituted .................. 512
Table 10.22: Specific costs of a water-steam cycle comprising cogeneration (CHP - high steam
parameters) as a function of waste throughput when energy can be substituted .................. 512
Table 10.23: Survey of specific income from different options of the water-steam cycle as a function of
waste throughput .................................................................................................................. 513
Table 10.24: Specific costs for dedusting with an electrostatic precipitator as a function of waste
throughput ............................................................................................................................ 515
Table 10.25: Specific costs for wet dedusting as a function of waste throughput.................................... 515
Table 10.26: Specific costs of a dry flue-gas cleaning system with fabric filters as a function of waste
throughput ............................................................................................................................ 516
Table 10.27: Specific costs of a dry flue-gas cleaning system with adsorption as a function of waste
throughput ............................................................................................................................ 517
Table 10.28: Specific costs of a gypsum scrubber as a function of waste throughput ............................. 517
Table 10.29: Specific costs of a scrubber with precipitation as a function of waste throughput.............. 518
Table 10.30: Specific costs of a NaOH scrubber as a function of waste throughput ............................... 518
Table 10.31: Specific costs of SCR as a function of waste throughput.................................................... 519
Table 10.32: Specific costs of SNCR as a function of waste throughput................................................. 520
Table 10.33: Specific costs of a flow injection absorber as a function of waste throughput.................... 520
Table 10.34: Specific costs of an activated coke plant as a function of waste throughput....................... 521
Table 10.35: Option 1: Costs of a grate firing system incorporating delivery by train, dry, wet and
catalytic flue-gas treatment and with the steam cycle connected to that of an adjacent power
plant as a function of throughput .......................................................................................... 523
Table 10.36: Option 2: Costs of a grate firing system incorporating delivery by train, dry, wet and
catalytic flue-gas treatment with power generation as a function of throughput .................. 524
Table 10.37: Option 3: Costs of a grate firing system incorporating delivery by train, dry, wet and
catalytic flue-gas treatment with cogeneration (CHP) as a function of throughput.............. 524
Table 10.38: Option 4: Costs of a grate firing system incorporating delivery by train, electrostatic
precipitator, NaOH scrubber, flow injection absorber and catalytic plant with power
generation as a function of throughput ................................................................................. 525
Table 10.39: Option 5: Costs of a grate firing system incorporating delivery by train, electrostatic
precipitator, precipitation, activated coke absorber and catalytic plant with power generation
as a function of throughput................................................................................................... 525
Table 10.40: Option 6: Costs of a grate firing system incorporating delivery by train, dry adsorption,
activated coke absorber and catalytic plant with power generation as a function of throughput
.............................................................................................................................................. 526
Table 10.41: Costs for the firing system and the boiler of waste incineration plants with fluidised bed
combustion as a function of throughput (not including waste pretreatment costs)............... 527
Table 10.42: Specific costs of a water-steam cycle comprising a steam extraction turbine (normal steam
parameters) as a function of waste throughput ..................................................................... 528
Table 10.43: Specific costs and income of waste treatment, firing, boiler and energy utilisation ........... 528
Table 10.44: Capital and operating costs of the Lahti RDF gasification plant, Finland........................... 529
Table 10.45: Hypothetical cost calculations for a pyrolysis plant in the Flanders Region of Belgium.... 530
Table 10.46: General data of the waste incineration plant Flötzersteig (reference year: 2000) ............... 531
Table 10.47: Input and output flows of the waste incineration plant Flötzersteig (reference year: 2000)532
Table 10.48: Emissions to air from the waste incineration plant Flötzersteig (reference year: 2000) ..... 533
Table 10.49: Waste water parameters of the waste incineration plant Flötzersteig after the waste water
treatment (reference year: 2000)........................................................................................... 534
Table 10.50: Chemical data of wastes from the waste incineration plant Flötzersteig (reference year:
2000)..................................................................................................................................... 535
Table 10.51: Leaching tests; waste incineration plant Flötzersteig (reference year: 2000)...................... 536
Table 10.52: General data of the waste incineration plant Spittelau (reference year: 2000) .................... 536
Table 10.53: Input-output flows of the waste incineration plant Spittelau (reference year: 2000) .......... 537
Table 10.54: Emissions to air from the waste incineration plant Spittelau (reference year: 2000) .......... 539
Table 10.55: Waste water parameters of the waste incineration plant Spittelau after treatment (reference
year: 2000)............................................................................................................................ 540
Waste Incineration
xxxi
Table 10.56: Chemical data of waste fractions from the waste incineration plant Spittelau (reference year:
2000) .....................................................................................................................................541
Table 10.57: Leaching tests; waste incineration plant Spittelau (reference year: 2000)...........................542
Table 10.58: General data of the waste incineration plant Wels (reference year: 2000) ..........................542
Table 10.59: Input and output of the waste incineration plant Wels (reference year: 2000) ....................543
Table 10.60: Emissions to air from the waste incineration plant Wels (reference year: 2000).................545
Table 10.61: Waste water parameters of the waste incineration plant Wels after waste water treatment
(reference year: 2000) ...........................................................................................................546
Table 10.62: Average values measured in clean gas (operating values) ..............................................547
Table 10.63: Slag quality......................................................................................................................548
Table 10.64: Energy efficiency ratio (assumed average calorific value 9500 kJ/kg)........................548
Table 10.65: Types of waste and waste quantities incinerated in the rotary kilns of Plant Simmeringer
Haide (reference year: 2000).................................................................................................554
Table 10.66: General data of the rotary kilns of the Plant Simmeringer Haide (reference year: 2000)....554
Table 10.67: Input and output flows of the rotary kilns of the Plant Simmeringer Haide (reference year:
2000) .....................................................................................................................................555
Table 10.68: Emissions to air from the rotary kilns of the Plant Simmeringer Haide (reference year: 2000)
..............................................................................................................................................557
Table 10.69: Waste water parameters of the rotary kilns of the Plant Simmeringer Haide after waste water
treatment (reference year: 2000) ...........................................................................................558
Table 10.70: Chemical data of wastes from the rotary kilns (reference year: 2000) ................................559
Table 10.71: Leaching tests (according to ÖNORM S 2115) rotary kilns of the Plant Simmeringer Haide
(reference year: 2000) ...........................................................................................................559
Table 10.72: Average values measured in clean gas (operating values) ..............................................560
Table 10.73: General data of the fluidised bed reactors of the Plant Simmeringer Haide (2000) ............562
Table 10.74: Input and output flows of the fluidised bed reactors (reference year: 2000) .......................563
Table 10.75: Emissions to air from the fluidised bed reactors (reference year: 2000) .............................564
Table 10.76: Chemical data of wastes from the fluidised bed reactors (reference year: 2000) ................565
Table 10.77: Leaching tests according to ÖNORM S 2115 – fluidised bed reactors (reference year: 2000)
..............................................................................................................................................565
Table 10.78: Characterization of the incineration materials.................................................................566
Table 10.79: Average values measured in clean gas (operating values) – BAT5.................................566
Table 10.80: Characterization of the incineration material ..................................................................567
Table 10.81: Average values measured in clean gas (operating values) – BAT6.................................567
Table 10.82: Emission values of the waste water from the waste gas cleaning system before mixing
(BAT6)..................................................................................................................................568
Table 10.83: Types of waste and waste quantities treated at AVE - Reststoffverwertung Lenzing
(reference year: 2000) ...........................................................................................................569
Table 10.84: General data of the fluidised bed reactor of AVE-RVL Lenzing ........................................569
Table 10.85: Emissions to air from the fluidised bed reactor of AVE - Reststoffverwertung Lenzing ....572
Table 10.86: General data of the fluidised bed reactor of the waste incineration plant Arnoldstein
(reference year: 2001) ...........................................................................................................573
Table 10.87: Output flows of the fluidised bed reactors of the waste incineration plant Arnoldstein
(reference year: 2001) ...........................................................................................................574
Table 10.88: Emissions to air from the waste incineration plant Arnoldstein (reference year: 2001) ......576
Table 10.89: Waste water parameters (composite sample) of the waste incineration plant Arnoldstein after
waste water treatment (reference year: 2001) .......................................................................576
Table 10.90: Chemical data of ash from the fluidised bed combustion of the waste incineration plant
Arnoldstein (reference year: 2001) .......................................................................................577
Table 10.91: Concentration of pollutants in the eluate of ash from the waste incineration plant Arnoldstein
(reference year: 2001) ...........................................................................................................577
Table 10.92: Average values measured in clean gas (operating values) ..............................................578
Table 10.93: Measuring devices used for continuous measuring .........................................................578
Table 10.94: Deposition degrees in waste gas cleaning .......................................................................579
Table 10.95: Energy efficiency ratio (supposed average calorific value Hu 11500 kJ/kg)...............579
Table 10.96: Slag quality – BAT2........................................................................................................580
Table 10.97: Energy efficiency calculation data checklist 1.....................................................................593
Table 10.98: Energy efficiency calculation data checklist 2.....................................................................594
Table 10.99: Energy efficiency calculation data checklist 3.....................................................................595
Table 10.100: Example of a multi-criteria assessment of FGT system selection .....................................601
Table 10.101: Example of a multi-criteria cost assessment used for comparing FGT system options.....602
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Waste Incineration
Scope
SCOPE
The following comments are relevant to the scope of this document:
1. The scope of this document is mainly influenced by the scope of the information provided
by, and decisions of, the members of the Technical Working Group (TWG) on waste
incineration, and the time and resources available.
2. Annexe 1 of the IPPC Directive (96/61/EC) provided the starting point for the scope of this
BAT reference document, where it includes sections as follows:
5.1. Installations for the disposal or recovery of hazardous waste as defined in the list
referred to in Article 1 (4) of Directive 91/689/EEC, as defined in Annexes II A and II B
(operations R1, R5, R6, R8 and R9) to Directive 75/442/EEC and in Council Directive
75/439/EEC of 16 June 1975 on the disposal of waste oils (2), with a capacity exceeding
10 tonnes per day
5.2. Installations for the incineration of municipal waste as defined in Council Directive
89/369/EEC of 8 June 1989 on the prevention of air pollution from new municipal waste
incineration plants (3) and Council Directive 89/429/EEC of 21 June 1989 on the
reduction of air pollution from existing municipal waste-incineration plants (4) with a
capacity exceeding 3 tonnes per hour
The TWG working on this document decided at an early stage that the document should
not be restricted by the size limitations in these sections of the IPPC Directive, nor by the
definitions of waste, and recovery or disposal included therein. This being the case, the
selected scope of the document aimed to reflect a pragmatic view across the incineration
sector as a whole, with a particular focus upon those installations and waste types that are
most common. The scope of the Waste Incineration Directive 76/2000/EC was also a factor
taken into account by the TWG when deciding on the scope for the document.
3. The document seeks to provide information concerning dedicated waste incineration
installations. It does not cover other situations where waste is thermally treated, e.g. coincineration processes such as some cement kilns and large combustion plants - these
situations are (or will be) covered by the BREF that deals specifically with those industries.
While some of the techniques that are included here may be technically applicable to other
industries (i.e. those that are not dedicated incinerators) that incinerate waste, or a
proportion of waste, whether the techniques identified here, or the performance levels they
give rise to, are BAT for those sectors, has not been a part of the scope of this work.
4. Although incineration provides the main focus of this document, three main thermal
treatment techniques are described, in general as they relate to some common waste
streams. These are:
•
•
•
incineration
pyrolysis
gasification.
Various incineration techniques are covered. Incineration is applied to the treatment of a
very wide variety of wastes. Pyrolysis and gasification are less widely applied to wastes,
and generally to a narrower range of wastes.
Combinations of incineration, pyrolysis and gasification are also referred to. Each of the
techniques and combinations of techniques are covered in this document within the context
of their application to the treatment of various wastes (although this does not imply any
definition of the meaning of waste - see also comment 5 below)
Waste Incineration
xxxiii
Scope
5. In addition to the thermal treatment stage of the installation this document also covers (to
varying degrees):
•
•
•
•
•
•
waste reception, handling and storage
the effect of waste pretreatment on the selection and operation of waste incineration
processes (in some cases this includes a description of the techniques applied)
applied flue-gas treatment techniques
applied residue treatments techniques (for the main residues commonly produced)
applied waste water treatment techniques
some aspects of energy recovery, the performance achieved and techniques used (details of
electrical generation equipment etc. are not included).
6. If an installation is referred to or included in this document this does not have any legal
consequence. It does not mean that the installation is legally classed as an incinerator nor
does it imply that the material being treated is legally classed as waste
7. No size threshold has been applied when gathering information. However, it is noted that, to
date, limited information has been supplied concerning smaller incineration processes
8. This document does not deal with decisions concerning the selection of incineration as a
waste treatment option. Neither, does it compare incineration with other waste treatments.
9. There is another BREF that deals with “Waste Treatments”. It has a wide scope and covers
many other installations and techniques that are applied to the treatment of waste.
xxxiv
Waste Incineration
Chapter 1
1 GENERAL INFORMATION ON WASTE INCINERATION
1.1 Purpose of incineration and basic theory
[1, UBA, 2001], [64, TWGComments, 2003]
Incineration is used as a treatment for a very wide range of wastes. Incineration itself is
commonly only one part of a complex waste treatment system that altogether, provides for the
overall management of the broad range of wastes that arise in society.
The incineration sector has undergone rapid technological development over the last 10 to 15
years. Much of this change has been driven by legislation specific to the industry and this has, in
particular, reduced emissions to air from individual installations. Continual process
development is ongoing, with the sector now developing techniques which limit costs, whilst
maintaining or improving environmental performance.
The objective of waste incineration is to treat wastes so as to reduce their volume and hazard,
whilst capturing (and thus concentrating) or destroying potentially harmful substances that are,
or may be, released during incineration. Incineration processes can also provide a means to
enable recovery of the energy, mineral and/or chemical content from waste.
Basically, waste incineration is the oxidation of the combustible materials contained in the
waste. Waste is generally a highly heterogeneous material, consisting essentially of organic
substances, minerals, metals and water. During incineration, flue-gases are created that will
contain the majority of the available fuel energy as heat.
The organic fuel substances in the waste will burn when they have reached the necessary
ignition temperature and come into contact with oxygen. The actual combustion process takes
place in the gas phase in fractions of seconds and simultaneously releases energy where the
calorific value of the waste and oxygen supply is sufficient, this can lead to a thermal chain
reaction and self-supporting combustion, i.e. there is no need for the addition of other fuels.
The main stages of incineration process are:
1. drying and degassing – here, volatile content is evolved (e.g. hydrocarbons and water) at
temperatures generally between 100 and 300 °C. The drying and degassing process do not
require any oxidising agent and are only dependent on the supplied heat
2. pyrolysis and gasification - pyrolysis is the further decomposition of organic substances in
the absence of an oxidising agent at approx. 250 – 700 °C. Gasification of the carbonaceous
residues is the reaction of the residues with water vapour and CO2 at temperatures, typically
between 500 and 1000 °C, but can occur at temperatures up to 1600 °C. Thus, solid organic
matter is transferred to the gaseous phase. In addition to the temperature, water, steam and
oxygen support this reaction
3. oxidation - the combustible gases created in the previous stages are oxidised, depending on
the selected incineration method, at flue-gas temperatures generally between 800 and 1450 °C.
These individual stages generally overlap, meaning that spatial and temporal separation of these
stages during waste incineration may only be possible to a limited extent. Indeed the processes
partly occur in parallel and influence each other. Nevertheless it is possible, using in-furnace
technical measures, to influence these processes so as to reduce polluting emissions. Such
measures include furnace design, air distribution and control engineering.
Waste Incineration
1
Chapter 1
In fully oxidative incineration the main constituents of the flue-gas are: water vapour, nitrogen,
carbon dioxide and oxygen. Depending on the composition of the material incinerated and on
the operating conditions, smaller amounts of CO, HCl, HF, HBr, HI, NOX SO2, VOCs,
PCDD/F, PCBs and heavy metal compounds (among others) are formed or remain. Depending
on the combustion temperatures during the main stages of incineration, volatile heavy metals
and inorganic compounds (e.g. salts) are totally or partly evaporated. These substances are
transferred from the input waste to both the flue-gas and the fly ash it contains. A mineral
residue fly ash (dust) and heavier solid ash (bottom ash) are created. In municipal waste
incinerators, bottom ash is approximately 10 % by volume and approximately 20 to 30 % by
weight of the solid waste input. Fly ash quantities are much lower, generally only a few per cent
of input. The proportions of solid residue vary greatly according to the waste type and detailed
process design.
For effective oxidative combustion, a sufficient oxygen supply is essential. The air ratio number
"n" of the supplied incineration air to the chemically required (or stoichiometric) incineration
air, usually ranges from 1.2 to 2.5, depending on whether the fuel is gas, liquid or solid, and the
furnace system.
The combustion stage is only one stage of the overall incineration installation. Incinerators
usually comprise a complex set of interacting technical components which, when considered
together, effect an overall treatment of the waste. Each of these components has a slightly
different main purpose, as described in Table 1.1 below:
•
•
•
•
•
•
•
•
Objective
destruction of organic substances
evaporation of water
evaporation of volatile heavy metals and inorganic salts
production of potentially exploitable slag
volume reduction of residues
recovery of useable energy
removal and concentration of volatile heavy metals and
inorganic matter into solid residues e.g. flue-gas cleaning
residues, sludge from waste water treatment
minimising emissions to all media
Responsibility of
Furnace
Energy recovery system
Flue-gas cleaning
Table 1.1: Purpose of various components of a waste incinerator
Source [1, UBA, 2001], [64, TWGComments, 2003]
1.2 Overview of waste incineration in Europe
The scale of use of incineration as a waste management technique varies greatly from location
to location. For example, in European Member States the variation of incineration in municipal
waste treatments ranges from zero to 62 per cent.
[9, VDI, 2002] In EU-15 Member States (MS) an annual quantity of approximately 200 million
tonnes of waste may be considered suitable for thermal waste treatment. However, the total
installed capacity of thermal waste treatment plants is only in the order of 50 million tonnes.
Table 1.2 below gives an estimate of the treatment of the waste arising in each MS for
municipal waste, hazardous waste and sewage sludge. Deposited waste is included because a
considerable proportion of these wastes may, in future, be diverted to other waste treatment
methods, including incineration.
Note: as definitions and waste categories differ from one country to another, some of the values
given may not be directly comparable.
2
Waste Incineration
Chapter 1
Municipal Solid Waste (MSW)
Country
Hazardous Waste (HW)
Total estimated
% landfilled % incinerated
MSWYear of
(or amount in (or amount in
production
data source
106 tonnes)
106 tonnes)
(in 106 tonnes)
1.32
1999
51
35
Total estimated
HW-production
(in 106 tonnes)
Year of data
source
Amount
landfilled
(in 106 tonnes)
Sewage Sludge (SS)
Amount
incinerated
(in 106 tonnes)
Total estimated
SS-production
Year of
(in 106 tonnes as data source
dry solids)
0.39
1999
0.97
1999
Not supplied
0.11
Belgium
4.85
1997
42
35
2.03
1997
0.79
0.14
0.85
1997
Denmark
2.77
1996
15
56
0.27
1996
0.09
0.1
0.15
1997
Finland
0.98
1997
77
2
0.57
1997
0.28
0.1
0.14
1997
France
48.5
2000
55
26
Not supplied
Not supplied
0.77 (note 5)
0.82
1997
2.7
0.85
2.48
1998
Not supplied
Not supplied
Not supplied
Not supplied
Austria
Germany
Greece
45
2000
30
29
9.17
1997
2001
2000
3.20
1993
93
0
Not supplied
1993
Ireland
1.80
1998
100
0
0.23
1995
0.03
0.03
0.39
1997
Italy
25.40
1995
85
8
Not supplied
1995
Not supplied
Not supplied
Not supplied
Not supplied
Luxembourg
0.30
1995
24
48
0.14
1995
Not supplied
Not supplied
Not supplied
Not supplied
Portugal
4.6
2002
71
20
0.25
2001
Not supplied
Not supplied
0.24
2000
Not supplied
Not supplied
17
1997
85
10
2
1997
Not supplied
0.03
3.80
1999 (note 1)
1999
Not supplied
0.1
10.2
2002
38
(1.44)
76
0.27
Netherlands
24
(0.92)
11
2.7
2002
0.6
0.28
0.69
1999
United
Kingdom
EU-15 Totals
(note 6)
27.20
1999
85
6
2.37
1996
0.86
0.24
1.2
1999
(note 3)
5.35
2.72
7.58
Spain
Sweden
196.92
21.92
1997
Notes
1 Swedish Waste Management 2000 (RVF)
2
3 ENDS Report 312 January 2001 (figures include co-incineration (50 %/50 %)
4 The balance to 100 % for the treatment methodologies is e.g. due to recovery and recycling
5 Hazardous waste incinerated in external dedicated units
6 Totals given are a simple addition of figures provided and therefore are of mixed years. Percentages landfilled etc not averaged as figures have little meaning without actual mass data.
Table 1.2: Amounts of municipal waste (MSW), hazardous waste (HW) and sewage sludge (SS) in EU-15 MSs, and their treatment
[1, UBA, 2001], [64, TWGComments, 2003]
Waste Incineration
3
Chapter 1
Table 1.3 shows the quantities of some wastes arising and number of waste incinerator plants in
other European Countries:
Country
Bulgaria
Czech
Republic
Estonia
Hungary
Latvia
Lithuania
Poland
Romania
Slovakia
Slovenia
Totals
Data
year
1998
1999
Municipal
waste in 106
tonnes
3.199
4.199
1999
1998
0.569
1998
1999
1999
1999
1999
1995
0.597
1.211
12.317
7.631
3.721
1.024
39.468
Total
number of
MSWI
0
3
5
0
3
Hazardous
waste
in 106 tonnes
0.548
3.011
0
1
0
1
0.06
3.915
0
0
4
0
2
0
10
0
0
1
0
2
0
7
0.0411
0.2449
1.34
2.323
1.7376
0.025
13.2456
MSWI
(>3 t/h)
Total
HWI
number of
(>10 t/d)
HWI
0
0
72
14
1
7
0
Not
supplied
0
0
0
0
13
4
3
3
Not supplied
1
0
0
96
22
Note: Totals are simple column totals and therefore include mixed year data
Table 1.3: Annual quantities of municipal and hazardous waste arising and the number of
incineration plants in some Accession Countries
[1, UBA, 2001], [64, TWGComments, 2003]
Table 1.4 shows the number and total capacity of existing incineration plants (not including
planned sites) for various waste types:
Country
Austria
Belgium
Denmark
Finland
France
Germany
Greece
Ireland
Italy
Luxembourg
Portugal
Spain
Sweden
Netherlands
United
Kingdom
Norway
Switzerland
Totals
Total number
Capacity
of MSWMt/yr
incinerators
5
17
32
1
2101
59
0
0
32
1
3
9
30
11
17
0.5
2.4
2.7
0.07
11.748
13.4
11
29
467
0.65
3.29
49.7
1.71
0.15
1.2
1.13
2.5
5.3
2.97
Total number Capacity
Mt/yr
Total number of Capacity of dedicated
Mt/yr sewage sludge
(dry
HW-incinerators
solids)
incinerators
2
0.1
1
3
0.3
1
0.02
2
0.1
5
0.3
1
0.1
203
1.0
312
1.23
23
0.63
0
11
6
0.1
0
0
1
0.03
1
0.1
1
0.1
2
0.19
3
0.12
11
0.42
11
93
2
5.28
14
57
0.1
1.66
1 On 6 Jan 2003 123 MSW incinerators were operating with combined capacity of 2000t/h
2 Figure includes installations used in the chemical industry
3 Dedicated commercial sites only (i.e. not including in-house plants)
Table 1.4: Geographical distribution of incineration plants for municipal, hazardous and sewage
sludge waste
[1, UBA, 2001], [64, TWGComments, 2003]
4
Waste Incineration
Chapter 1
Figure 1.1 shows the variation in per capita capacity for municipal waste incineration:
Figure 1.1: Municipal waste incineration capacity per capita
* means incomplete data [42, ISWA, 2002], [64, TWGComments, 2003]
1.3 Plant sizes
The size of installations varies greatly across Europe. Variations in size can be seen within and
between technology and waste types. The largest MSW plant in Europe has a capacity in excess
of 1 million tonnes of waste per year. Table 1.5 below shows the variation in average MSW
incinerator capacity by country:
Country
Austria
Belgium
Denmark
France
Germany
Italy
Netherlands
Portugal
Spain
Sweden
United Kingdom
Norway
Switzerland
AVERAGE
Average MSW incinerator capacity
(k tonnes/yr)
178
141
114
132
257
91
488
390
166
136
246
60
110
193
Table 1.5: Average MSW incineration plant capacity by country
[11, Assure, 2001], [64, TWGComments, 2003]
Waste Incineration
5
Chapter 1
Table 1.6 below shows the typical application range of the main different incineration
technologies:
Technology
Moving grate (mass burn)
Fluidised bed
Rotary kiln
Modular (starved air)
Pyrolysis
Gasification
Typical application range (tonnes/day)
120 - 720
36 – 200
10 – 350
1 – 75
10 - 100
250 - 500
Note: values are for typical applied ranges –each is also applied outside the range shown.
Table 1.6: Typical throughput ranges of thermal treatment technologies
[10, Juniper, 1997], [64, TWGComments, 2003]
1.4 Overview of legislation
The waste incineration sector has been the subject of extensive legislative requirements at
regional, national and European level for many years.
In addition to the requirements of the IPPC Directive, the incineration (and associated) sector is
also subject to the requirements of specific legislation. At present, the following EU-directives
are in force for waste incineration plants:
•
•
•
•
•
89/369/EEC for new municipal waste incineration plants
89/429/EEC for existing municipal waste incineration plants
94/67/EC for the incineration of hazardous waste (including co-incineration)
2000/76/EC for the incineration of waste (including co-incineration).
Regulation (EC) No. 1774/2002 of the European Parliament and of the Council of
3 October 2002, laying down health rules concerning animal by-products not intended for
human consumption.
It should be noted that Directive 2000/76/EC progressively repeals the first three directives.
This directive sets the minimum requirements in respect of permissible emissions, monitoring
and certain operational conditions. The scope of 2000/76/EC is broad (certain exclusions are
specifically listed in Article 2) and does not have a lower capacity limit.
Directive 2000/76/EC requires that its standards are adopted as follows:
•
•
new waste incineration plants, from 28 December 2002
existing waste incineration plants, by 28 December 2005 at the latest.
In the meantime, existing waste incineration plants have to comply with Directives 89/369/EEC,
89/429/EEC and 94/67/EC. [2, infomil, 2002]
1.5 Waste composition and process design
The precise design of a waste incineration plant will change according to the type of waste that
is being treated. The following parameters and their variability are key drivers:
•
•
•
6
waste chemical composition
waste physical composition, e.g. particle size
waste thermal characteristics, e.g. calorific value, moisture levels, etc.
Waste Incineration
Chapter 1
Processes designed for a narrow range of specific inputs can usually be optimised more than
those that receive wastes with greater variability. This in turn can allow improvements to be
made in process stability and environmental performance, and may allow a simplifying of
downstream operations such as flue-gas cleaning. As flue-gas cleaning is often a significant
contributor to overall incineration costs (i.e. approx 15 to 35 % of the total capital investment)
this can then lead to reduced treatment costs at the incinerator. The external costs
(i.e. those generally beyond the IPPC permit assessment boundary) of pretreatment, or the
selective collection of certain wastes can however add significantly to the overall costs of waste
management and to emissions from the entire waste management system. Often, decisions
concerning the wider management of waste (i.e. the complete waste arising, collection,
transportation, treatment, disposal etc.) take into account a very large number of factors. The
selection of the incineration process can form a part of this wider process.
The waste collection and pretreatment systems utilised can have a great impact on the type and
nature of waste that will finally be received at the incinerator (e.g. mixed municipal waste or
RDF) and hence on the type of incinerator that is best suited to this waste. Provision for the
separate collection of various fractions of household waste can have a large influence over the
average composition of the waste received at the MSWI. For example, the separate collection of
some batteries and dental amalgam can significantly reduce mercury inputs to the incineration
plant. [64, TWGComments, 2003]
The cost of the processes used for the management of residues arising at the incinerator, and for
the distribution and use of the energy recovered also play a role in the overall process selection.
In many cases, waste incinerators may have only limited control over the precise content of the
wastes they receive. This then results in the need for some installations to be designed so that
they are sufficiently flexible to cope with the wide range of waste inputs they could receive.
This applies to both the combustion stage and the subsequent flue-gas cleaning stages.
The main types of waste to which incineration is applied as a treatment are:
•
•
•
•
•
•
municipal wastes (residual wastes - not pretreated)
pretreated municipal wastes (e.g. selected fractions or RDF)
non-hazardous industrial wastes and packaging
hazardous wastes
sewage sludges
clinical wastes.
[64, TWGComments, 2003] Many incineration plants accept several of these types of waste.
Waste itself is commonly classified in a number different ways:
•
•
•
by origin, e.g. household, commercial, industrial, etc.
by its nature, e.g. putrescible, hazardous, etc.
by the method used for its management, e.g. separately collected, recovered material, etc.
These different classes overlap. For example, wastes of various origins may contain putrescible
or hazardous fractions.
Table 1.7 below provides data concerning the content of waste arising in Germany. The term
hazardous waste refers to those wastes classified as hazardous under Directive 91/689/EC.
Sewage sludge includes sludge from the waste water treatment of communities and industries:
Waste Incineration
7
Chapter 1
Parameter
Calorific value (upper) (MJ/kg)
Water (%)
Ash
Carbon (% d.s.)
Hydrogen (% d.s.)
Nitrogen (% d.s.)
Oxygen (% d.s.)
Sulphur (% d.s.)
Fluorine (% d.s.)
Chlorine (% d.s.)
Bromine (% d.s.)
Iodine (% d.s.)
Lead mg/kg d.s.)
Cadmium mg/kg d.s.)
Copper mg/kg d.s.)
Zinc mg/kg d.s.)
Mercury mg/kg d.s.)
Thallium mg/kg d.s.)
Manganese mg/kg d.s.)
Vanadium mg/kg d.s.)
Nickel mg/kg d.s.)
Cobalt mg/kg d.s.)
Arsenic mg/kg d.s.)
Chrome mg/kg d.s.)
Selenium mg/kg d.s.)
PCB mg/kg d.s.)
PCDD/PCDF (ng I-TE/kg)
Municipal waste
7 – 15
15 – 40
20 – 35
18 – 40
1–5
0.2 – 1.5
15 - 22
0.1 - 0.5
0.01 – 0.035
0.1 – 1
not supplied
100 – 2000
1 – 15
200 – 700
400 – 1400
1–5
<0.1
250
4 – 11
30 – 50
3 – 10
2–5
40 - 200
0.21 - 15
0.2 – 0.4
50 – 250
Hazardous waste
1 – 42
0 – 100
0 – 100
5 – 99
1 – 20
0 – 15
not supplied
not supplied
0 - 50
0 - 80
0 - 80
0 - 50
0 - 200000
0 – 10000
not supplied
not supplied
0 – 40000
not supplied
not supplied
not supplied
not supplied
not supplied
not supplied
not supplied
not supplied
Up to 60 %
10 – 10000
Sewage sludge
2 – 14
3 – 97
1 – 60
30 – 35
2–5
1–4
10 – 25
0.2 - 1.5
0.1 - 1
0.05 - 4
No data
No data
4 - 1000
0.1 – 50
10 – 1800
10 – 5700
0.05 – 10
0.1 – 5
300 – 1800
10 – 150
3 – 500
8 – 35
1 – 35
1 – 800
0.1 – 8
0.01 – 0.13
8.5 – 73
Notes:
% d.s. means percentage dry solids
the calorific value for sewage sludge relates to raw sludge of>97 % d.s.
Sub-fractions of HW can show variations outside these ranges
Table 1.7: Typical composition of waste in Germany
[1, UBA, 2001], [64, TWGComments, 2003]
The range of installation designs is almost as wide as the range of waste compositions.
New plants have the advantage that a specific technological solution can be designed to meet
the specific nature of the waste to be treated in the plant. They also benefit from years of
industry development and knowledge of the practical application of techniques and may
therefore be designed for high environmental standards, whilst containing costs.
Existing plants have significantly less flexibility when selecting upgrade options. Their design
may be the product of 10 to 20 years of process evolution. Often in Europe this will have been
motivated by requirements to reduce emissions to air. The next stage of process development
will often then be highly (or even totally) dependent upon the existing design. Many sitespecific local solutions can be seen in the sector. Many of these would probably be constructed
in a different way if completely rebuilt. [6, EGTEI, 2002]
8
Waste Incineration
Chapter 1
1.6 Key environmental issues
Waste itself, and its management, are themselves a significant environmental issue. The thermal
treatment of waste may therefore be seen as a response to the environmental threats posed by
poorly or unmanaged waste streams.
The target of thermal treatment (see also Section 1.1) is to provide for an overall reduction in
the environmental impact that might otherwise arise from the waste. However, in the course of
the operation of incineration installations, emissions and consumptions arise, whose existence or
magnitude are influenced by the installation design and operation. This section therefore,
briefly, summarises the main environmental issues that arise directly from incineration
installations (i.e. it does not include the wider impacts or benefits of incineration). Essentially
these direct impacts fall into the following main categories:
•
•
•
•
•
•
•
overall process emissions to air and water (including odour)
overall process residue production
process noise and vibration
energy consumption and production
raw material (reagent) consumption
fugitive emissions – mainly from waste storage
reduction of the storage/handling/processing risks of hazardous wastes.
Other impacts beyond the scope of this BREF document (but which can significantly impact
upon the overall environmental impact of an entire project) arise from the following operations:
•
•
transport of incoming waste and outgoing residues
extensive waste pretreatment (e.g. preparation of waste derived fuels and the associated
refuse treatment).
1.6.1 Process emissions to air and water
Emissions to air have long been the focus of attention for waste incineration plants. Significant
advances in technologies for the cleaning of flue-gases in particular have lead to major
reductions in the emissions to air.
However, the control of emissions to air remains an important issue for the sector. As the entire
incineration process is usually under slightly negative pressure (because of the common
inclusion of an induced draught extraction fan), routine emissions to air generally take place
exclusively from the stack. [2, infomil, 2002]
A summary of the main emissions to air from stack releases (these are described in more detail
in Section 3.2.1) is shown below:
•
•
•
•
particulate matter,
acid and other gases,
heavy metals,
carbon comp. (non-GHG),
others.
–particulate matter - various particle sizes
–including HCl, HF, HBr, HI, SO2, NOX, NH3 amongst others
–including Hg, Cd, Tl, As, Ni, Pb, amongst others
–including, CO, hydrocarbons (VOCs), PCDD/F, PCB amongst
Other releases to air may include, if there is no measure to reduce them:
•
•
•
odour,
–from handling and storage of untreated waste
green house gases (GHGs) –from decomposition of stored wastes e.g. methane, CO2
dusts,
–from dry reagent handling and waste storage areas.
Waste Incineration
9
Chapter 1
The principle potential sources of releases to water (process dependent) are:
•
•
•
•
•
•
•
•
effluents from air pollution control devices,
final effluent discharges from waste water treatment plants,
boiler water
- blowdown bleeds,
cooling water
- from wet cooling systems,
road and other surface drainage,
incoming waste storage, handling and transfer areas,
raw material storage areas,
residue handling, treatment and storage areas,
e.g. salts, heavy metals (HMs)
e.g. salts, heavy metals
e.g. salts
e.g. salts, biocides
e.g. diluted waste leachates
e.g. diluted incoming wastes
e.g. treatment chemicals
e.g. salts, HMs, organics.
The waste water produced at the installation can contain a wide range of potentially polluting
substances depending upon its actual source. The actual release made will be highly dependent
on the treatment and control systems applied.
1.6.2 Installation residues production
The nature and quantity of residues produced are a key issue for the sector. This is because they
provide both: (1) a measure of the completeness of the incineration process, and (2) generally
represent the largest potential waste arising at the installation.
[64, TWGComments, 2003], [1, UBA, 2001] Although the types and quantities of residue
arising varies greatly according to the installation design, its operation and waste input, the
following main waste streams are commonly produced during the incineration process:
•
•
•
•
•
ashes and/or slag
boiler ashes
filter dust
other residues from the flue-gas cleaning (e.g. calcium or sodium chlorides)
sludge from waste water treatment.
In some cases, the above waste streams are segregated; in other cases, they are combined within
or outside the process.
Some thermal treatment residues (most commonly vitrified slags from very high temperature
processes) can be used directly without treatment. Substances which can be obtained after the
treatment of the bottom ashes are:
•
•
•
construction materials
ferrous metals
non ferrous metals.
In addition, some plants using wet FGC processes with additional specific equipment recover:
•
•
•
•
calcium sulphate (Gypsum)
hydrochloric acid
sodium carbonate
sodium chloride.
Of these outputs, although very dependent upon the waste type, bottom ashes are generally
produced in the largest quantities. In many locations, often depending on local legislation and
practice, bottom ash is treated for re-cycling as an aggregate replacement.
10
Waste Incineration
Chapter 1
Figure 1.2: Bottom ash recycled and deposited from MSWI in 1999
*means incomplete data [42, ISWA, 2002]
Residues produced from the flue-gas cleaning are an important source of waste production. The
amount and nature of these varies, mainly according to the types of waste being incinerated and
the technology that is employed.
1.6.3 Process noise and vibration
[2, infomil, 2002] The noise aspects of waste incineration are comparable with other heavy
industries and with power generation plants. It is common practice for new municipal waste
incineration plants to be installed in completely closed building(s), as far as possible. This
normally includes operations such as the unloading of waste, mechanical pretreatment, flue-gas
treatment, and the treatment of residues. Usually, only some parts of flue-gas cleaning systems
(pipes, tubes, SCR, heat exchangers, etc.), cooling facilities and the long-term storage of bottom
ash are carried out directly in the open air.
The most important sources of external noise are:
•
•
•
•
•
•
•
•
vehicles used for the transport of waste, chemicals and residues
mechanical pretreatment of waste, e.g. shredding, baling, etc.
exhaust fans, extracting flue-gases from the incineration process and causing noise at the
outlet of the stack
noise, related to the cooling system (from evaporative cooling, especially air cooling)
turbine generation noise (high level so usually placed in specific sound-proofed buildings)
boiler pressure emergency blowdowns (these require direct release to atmosphere for boiler
safety reasons)
compressors for compressed air
noise related to the transport and treatment of bottom ash (if on the same site).
SCR systems and flue-gas ducts give rise to little noise and are often not inside buildings. Other
installation parts are not usually significant for external noise production but may contribute to a
general external noise production by the plant buildings.
Waste Incineration
11
Chapter 1
1.6.4 Energy production and consumption
Waste incinerators both produce and consume energy. In a large majority of cases, the energetic
value of the waste exceeds the process requirements. This may result in the net export of energy.
This is often the case with municipal waste incinerators in particular.
Given the total quantities of waste arising, and its growth over many years, the incineration of
waste can be seen to offer a large potential source of energy. In some MSs this energy source is
already well exploited. This is particularly the case where the use of CHP is used. Energy issues
are discussed in more detail later in this document (see Sections 3.5 and 4.3).
[64, TWGComments, 2003]
Figure 1.3 below shows the production of heat and electricity from municipal waste incineration
plants for various countries in 1999:
Figure 1.3: Energy production by municipal waste incinerators in Europe (1999)
* means incomplete data [42, ISWA, 2002]
Most wastes contain biomass (to differing degrees). In such cases, the energy derived from the
biomass fraction may be considered to substitute for fossil fuel and therefore the recovery of
energy from that fraction be considered to contribute to a reduction in the overall carbon dioxide
emissions from energy production. In some countries, this attracts subsidies and tax reductions.
[64, TWGComments, 2003]
Energy inputs to the incineration process can include:
•
•
•
waste
support fuels, (e.g. diesel, natural gas):
for start-up and shutdown
to maintain required temperatures with lower CV wastes
for flue-gas reheating before treatment or release
imported electricity:
for start-up and shutdown phases when all lines are stopped and for plants without
electricity generation.
(Note: some of the above energy inputs contribute to steam/heat production where boilers are
used and are therefore the energy is partially recovered in the process.)
12
Waste Incineration
Chapter 1
Energy production, self-consumption and export can include:
•
•
•
electricity
heat (as steam or hot water)
syngas (for pyrolysis and gasification plants that do not burn the syngas on site).
The efficient recovery of the energy content of the waste is generally considered to be a key
issue for the industry.
[74, TWGComments, 2004]
1.6.5 Consumption of raw materials and energy by the installation
Waste incineration plants (process dependent) may consume the following:
•
•
•
•
•
•
•
electricity, for process plant operation
heat, for specific process needs
fuels, support fuels (e.g. gas, light oils, coal, char)
water, for flue-gas treatment, cooling and boiler operation
flue-gas treatment reagents, e.g. caustic soda, lime, sodium bicarbonate, sodium sulphite
hydrogen peroxide, activated carbon, ammonia, and urea
water treatment reagents, e.g. acids, alkalis, tri-mercapto tri-azine, sodium sulphite, etc.
high pressure air, for compressors.
[74, TWGComments, 2004]
1.7 Economic information
[43, Eunomia, 2001] [64, TWGComments, 2003]
The economic aspects of incineration vary greatly between regions and countries, not only due
to technical aspects but also depending on waste treatment policies. A study [43, Eunomia,
2001] of these aspects provided to the TWG gives information on the situation in EU MSs –
some information from this study has been included in the annexe to this document.
The costs of incineration are generally affected by the following factors:
•
•
•
•
•
•
•
•
•
•
costs of land acquisition
scale (there may often be significant disadvantages for small scale operation)
plant utilisation rate
the actual requirements for the treatment of flue-gases/effluents, e.g. the imposed emission
limit values can drive the selection of particular technologies that in some circumstances
impose significant additional capital and operational costs
the treatment and disposal/recovery of ash residues, e.g. bottom ash may often be used for
construction purposes, in which case, the landfilling cost is avoided. The costs of treatment
for fly ash varies significantly, owing to the different approaches and regulations applied
regarding the need for treatment prior to recovery or disposal, and the nature of the disposal
site
the efficiency of energy recovery, and the revenue received for the energy delivered. The
unit price of energy delivered, and whether revenues are received for just heat or electricity
us for both are both important determinants of net costs
the recovery of metals and the revenues received from this
taxes or subsidies received for incineration and/or levied on emissions - direct and indirect
subsidies can influence gate fees significantly i.e. in the range of 10 – 75 %
architectural requirements
development of the surrounding area for waste delivery access, and other infrastructure
Waste Incineration
13
Chapter 1
•
•
•
•
availability requirements, e.g. availability may be increased by doubling each pump but this
imposes additional capital costs
planning and building cost/ depreciation periods, taxes and subsidies, capital cost market
insurance costs
administration, personnel, salary costs.
The owners and operators of incineration plants may be municipal bodies, as well as private
companies. Public/private partnerships are also common. The finance cost of capital
investments may vary depending upon the ownership.
Waste incineration plants receive fees for the treatment of the waste. They can also produce and
sell electricity, steam, and heat, and recover other products, such as bottom ashes for use as civil
construction material, iron scrap and non-ferrous scrap for use in the metal industry, HCl, salt or
gypsum. The price paid for these commodities, and the investment required to produce them,
has a significant impact on the operational cost of the installation. It can also be decisive when
considering specific technical investments and process designs (e.g. whether heat can be sold at
a price that justifies the investment required for its supply). The prices paid for these
commodities vary from one MS to another or even from one location to another.
In addition, significant differences occur due to the variations in emission requirements, salary
costs and depreciation periods, etc. For these reasons, the gate fees in Table 1.8 are only
comparable to a limited extent:
Member states
Belgium
Denmark
France
Germany
Italy
Netherlands
Sweden
United Kingdom
Gate fees in EUR/t incineration plants
Municipal waste
Hazardous waste
56 – 130
100 – 1500
40 – 70
100 – 1500
50 – 120
100 – 1500
100 – 350
50 – 1500
40 – 80
100 – 1000
90 – 180
50 – 5000
20 – 50
50 - 2500
20 – 40
Not available
Table 1.8: Gate fees in European MSW and HW incineration plants
[1, UBA, 2001]
It is important not to confuse the real cost of the gate fee 'needed' in order to pay for the
investment and operation, and the market price that is adopted in order to deal with competition.
Competition with alternative methods of waste management (e.g. landfills, fuel production, etc.)
as well as investment costs and operational expenses have an effect on the final gate fee at
incineration plants. Competition prices vary greatly from one MS or location to another.
Table 1.9 shows (except where noted) the variation in municipal waste incineration costs across
MSs. Note that the costs presented in Table 1.9 are different to those in Table 1.8 above (which
presents data on gate fees):
14
Waste Incineration
Chapter 1
Pre-tax2 costs net of
revenues in EUR per
tonne waste input
A
Tax (for
plant with
energy
recovery)
B
326 @ 60 kt/yr
159 @150 kt/yr
97 @ 300 kt/yr
72 average
DK
30 – 45
FIN
None
F
86 - 101 @ 37.5 kt/yr
80 - 90 @ 75 kt/yr
67 - 80 @ 150 kt/yr
250 (50 kt/yr and below)1
105 (200 kt/yr) 1
65 @ 600 kt/yr1
D
EL
IRL
I
L
NL
P
E
S
UK
Revenues from energy
supply
(EUR per kWh)
Electricity: 0.036
Heat: 0.018
EUR 12.7/t
(Flanders)
EUR 44/t
None
None
41.3 – 93
(350 kt, depends on
revenues for energy and
packaging recovery)
97 (120 kt)
Electricity: 0.025
Electricity: 0.05
Bottom ash: 34
Flue-gas treatment
residues: 80
For gasification,
Electricity 0.034
Heat 0.017
Electricity 0.033 - 0.046
Heat: 0.0076 - 0.023
Electricity 0.015 – 0.025
Not known
Not known
Electricity: 0.14 (old)
0.04 (market)
0.05 (green cert.)
Electricity: 0.025
(estimated)
71 – 1101
70 – 1341
46 – 76 (est.)
34 – 56
21 – 53
Costs of ash
treatment (EUR per
tonne of ash unless
otherwise specified)
Bottom ash: 63
Flue-gas residues:
363
Not available
Bottom ash:
EUR 13 – 18 per
tonne input
Bottom ash:25 - 30
Fly ash/air pollution
control residues:
100 - 250
Not known
Not known
Bottom ash: 75
Fly ash and air
pollution control
residues: 29
Bottom ash EUR 16/t
input waste
Flue-gas residues:
EUR 8/t input waste
Electricity: 0.027 - 0.04
(estimated)
No data
Electricity: 0.036
Electricity: 0.03
Heat: 0.02
Electricity: 0.032
69 @ 100kt/yr
47 @ 200kt/yr
Bottom ash recycled
(net cost to operator)
fly ash circa 90
Notes:
1. These figures are gate fees, not costs
2. Pre-tax cost refers to gross costs without any tax.
Table 1.9: Comparative costs of MSW incineration in different MSs
[43, Eunomia, 2001, 64, TWGComments, 2003]
The following table illustrates how the capital costs of an entire new MSWI installation can
vary with the flue-gas and residue treatment processes applied:
Type of flue-gas
cleaning
Dry
Dry plus wet
Dry plus wet with
residue processing
Specific investment costs (EUR/tonne waste input/yr)
100 ktonnes/yr 200 ktonnes/yr 300 ktonnes/yr 600 ktonnes/yr
670
532
442
347
745
596
501
394
902
701
587
457
Table 1.10: Specific investment costs for a new MSWI installation related to the annual capacity
and some types of FGT in Germany
[1, UBA, 2001], [64, TWGComments, 2003]
Waste Incineration
15
Chapter 1
Table 1.11 shows some examples of average specific incineration costs (1999) for municipal
waste and hazardous waste incineration plants (all new plants). The data indicates that the
specific costs for incineration are heavily dependent on the financing costs of the capital and,
therefore, by the investment costs and the plant capacity. Significant cost changes can occur and
depend on the set-up, such as the depreciation period, interest costs, etc. Plant utilisation can
also have a significant influence on the incineration costs.
Incineration plant for
Municipal waste with a
Cost structure
Hazardous waste with a capacity of
capacity of 250 ktonnes/yr in
70 ktonnes/yr in EUR 106
6
EUR 10
Planning/approval
3.5
6
Machine parts
70
32
Other components
28
28
Electrical works
18
20
Infrastructure works
14
13
Construction time
7
7
Total investment costs
140
105
Capital financing cost
14
10
Personnel
4
6
Maintenance
3
8
Administration
0.5
0.5
Operating resources/energy
3
2.5
Waste disposal
3.5
1.5
Other
1
0.5
Total operational costs
29
12.5
Specific incineration costs
Approx EUR 115/tonne
Approx EUR 350/tonne
(without revenues)
Note: The data provides an example in order to illustrate differences between MSWI and HWI. Costs of each and
the differential between them vary
Table 1.11: Example of the comparative individual cost elements for MSW and HW incineration
plants
[1, UBA, 2001], [64, TWGComments, 2003]
Energy prices:
[43, Eunomia, 2001] Revenues are received for energy sales. The level of support per kWh for
electricity and/or heat generation varies greatly. For example, in Sweden and Denmark, gate
fees are lower, at least in part because of the revenue gained from the sales of thermal energy as
well as electricity. Indeed, in Sweden, the generation of electricity is often not implemented in
the face of considerable revenues for heat recovery.
In some other countries, support for electricity production has encouraged electrical recovery
ahead of heat recovery. The UK, Italy, and Spain, amongst others, have at some stage,
supported incineration through elevated prices for electricity generated from incinerators.
In other MSs, the structure of incentives available for supporting renewable energy may also
affect the relative prices of alternative waste treatments and hence competition prices.
The potential revenues from energy sales at waste incineration facilities constitute an incentive
for all concerned parties to include energy outlets in the planning phase for incineration
facilities [64, TWGComments, 2003].
Revenues received for recovery of packaging materials:
[43, Eunomia, 2001] These have also influenced relative prices. For example, in Italy and the
UK, incinerators have received revenues associated with the recovery of packaging material.
It should be noted that, legislative judgements concerning recovery and disposal may influence
whether incinerators can legally benefit from these revenues [64, TWGComments, 2003]
16
Waste Incineration
Chapter 1
Taxes on incineration:
[43, Eunomia, 2001] In Denmark, the tax on incineration is especially high. Hence, although
underlying costs tend to be low (owing primarily to scale, and the prices received for energy),
the costs net of tax are of the same order as that of several other countries where no tax is in
place. This tax along with a landfill tax were adopted in Denmark to promote waste treatment in
compliance with the waste hierarchy. This has resulted in a large shift from landfill to recycling,
but with the percentage of waste being incinerated remaining constant [64, TWGComments,
2003].
Waste Incineration
17
Chapter 2
2 APPLIED TECHNIQUES
2.1 Overview and introduction
The basic linear structure of a waste incineration plant may include the following operations.
Information describing these stages is included later in this chapter:
•
•
•
•
•
•
•
•
•
•
•
•
•
incoming waste reception
storage of waste and raw materials
pretreatment of waste (where required, on-site or off-site)
loading of waste into the process
thermal treatment of the waste
energy recovery (e.g. boiler) and conversion
flue-gas cleaning
flue-gas cleaning residue management (from flue-gas treatment)
flue-gas discharge
emissions monitoring and control
waste water control and treatment (e.g. from site drainage, flue-gas treatment, storage)
ash/bottom ash management and treatment (arising from the combustion stage)
solid residue discharge/disposal.
Each of these stages is generally adapted in terms of design, for the type(s) of waste that are
treated at the installation.
Many installations operate 24h/day, nearly 365 days/yr. Control systems and maintenance
programmes play an important role in securing the availability of the plant. [74,
TWGComments, 2004]
Figure 2.1: Example layout of a municipal solid waste incineration plant
[1, UBA, 2001]
In the example shown above (Figure 2.1), the incoming waste storage and handling stages are
on the left of the diagram, before the incineration stage. The flue-gas cleaning system is shown
labelled as waste gas cleaning, to the right of the furnace and boiler. The example shown is a
wet FGT system with several unit operations. Other modern installations use FGT systems with
fewer process units.
Waste Incineration
19
Chapter 2
Although incineration is by far the most widely applied, there are three main types of thermal
waste treatment relevant to this BREF:
•
•
•
pyrolysis
gasification
incineration
- thermal degradation of organic material in the absence of oxygen
- partial oxidation
- full oxidative combustion.
The reaction conditions for these thermal treatments vary, but may be differentiated
approximately as follows:
Reaction
temperature (ºC)
Pressure (bar)
Atmosphere
Stoichiometric ratio
Products from the
process
Pyrolysis
Gasification
Combustion
250 – 700
500 – 1600
800 - 1450
1
1 – 45
Gasification agent:
O2, H20
<1
1
Air
Inert/nitrogen
0
>1
Gas phase:
H2, CO,
hydrocarbons, H2O,
N2
H2, CO, CO2, CH4,
H2O, N2
CO2, H2O, O2, N2
Solid phase:
Ash, coke
Slag, ash
Ash, slag
Liquid phase:
Pyrolysis oil and
water
Table 2.1: Typical reaction conditions and products from pyrolysis, gasification and incineration
processes
Adapted from[9, VDI, 2002]
Pyrolysis and gasification plants follow a similar basic structure to waste incineration
installations, but differ significantly in detail. The main differences are as follows:
•
•
•
•
•
pretreatment, may be more extensive to provide a narrow profile feedstock.
Additional equipment is required for handling/treating/storing the rejected material
loading, greater attention required to sealing
thermal reactor, to replace (or in addition to) the combustion stage
product handling, gaseous and solid products require handling, storage and possible
further treatments
product combustion, may be a separate stage and include energy recovery by combustion
of the products and subsequent gas/water/solid treatments and management.
2.2 Pretreatment, storage and handling techniques
The different types of wastes that are incinerated may need different types of pretreatment,
storage and handling operations. This section is organised in such a way that it describes in
order the most relevant of these operations for each type of waste, in particular for:
•
•
•
•
20
municipal solid wastes
hazardous wastes
sewage sludge
clinical wastes.
Waste Incineration
Chapter 2
2.2.1 Municipal solid wastes (MSW)
2.2.1.1 Collection and pretreatment outside the MSW incineration plant
Although beyond the immediate scope of this BREF document, it is important to recognise that
the local collection and pretreatment applied to MSW can influence the nature of the material
received at the incineration plant. The requirements concerning the pretreatment and other
operations should therefore be consistent with the collection system in place.
Recycling schemes may mean that some fractions have been removed. Their effect will be
approximately as follows:
Fraction removed
Glass and metals
Paper, card and plastic
Organic wastes, e.g. food and
garden wastes
Bulky wastes
Hazardous wastes
•
•
•
•
•
•
•
•
•
Prime impacts on remaining waste
increase in calorific value
decrease in quantity of recoverable metals in slag
decrease in calorific value
possible reduction in chlorine loads if PVC common
reduction in moisture loads (particularly of peak loads)
increase in net calorific value
reduced need for removal/shredding of such wastes
reduction in hazardous metal loading
reduction in some other substances e.g. Cl, Br, Hg
Table 2.2: Prime impact of waste selection and pretreatment on residual waste
[74, TWGComments, 2004]
One study assessing the effect of selective collection on the remaining household (called "grey
waste") gave the following conclusions:
•
•
•
•
•
glass collection decreased the throughput (-13 %) and increased the NCV (+15 %) of the
residual "grey waste"
packaging and paper collection decreased the throughput (-21 %) and decreased the NCV (16 %) of the "grey waste"
in general, throughput and NCV of the "grey waste" decreased when the efficiency of the
selective collection increased. The maximum impact of selective collection was -42 % for
the throughput and -3 % for the NCV of the "grey waste"
selective collection had an effect on the grey waste quality - it increased significantly the
content of the fine element, which can be particularly rich in heavy metals (fines increased
from 16 % to 33 %)
bottom ash ratio decreased due to selective collection (-3 %).
[74, TWGComments, 2004]
The degree to which separate collection and similar schemes effect the final waste delivered to
the installation are seen depends on the effectiveness of the separation and pretreatment systems
employed. This varies greatly. Some residual fractions are always likely to remain in the
delivered waste.
Reject materials from recycling plants, monofractions of waste, commercial and industrial
wastes, and some hazardous wastes may also be found in the delivered waste.
Waste Incineration
21
Chapter 2
2.2.1.2 Municipal solid waste pretreatment within the incineration plant
In-bunker mixing is commonly used to blend MSW. This usually consists of using the same
waste grab that is also used for hopper loading. Most commonly, the pretreatment of MSW is
limited to the shredding of pressed bales, bulky waste, etc, although sometimes more extensive
shredding is used. The following equipment is used:
•
•
•
•
crocodile shears
shredder
mills
rotor shears.
For fire-safety reasons, the following arrangements may be used:
•
•
•
•
separation of the dumping areas from the storage in the bunker
separation of hydraulic plants (oil supply, pump- and supply equipment) from the cutting
tools
collection devices for leaked oil
decompression release in the housings to reduce explosion damage.
It is generally necessary to pretreat (i.e. crush) bulky waste when its size is greater than that of
the feed equipment to the furnace. Another reason for pretreatment is to homogenise the waste
so that it has more consistent combustion characteristics (e.g. for some wastes with high NCVs).
This may be achieved by mixing, crushing or shredding the waste.
Additional waste pretreatment is unusual for grate furnace plants, but may be essential for other
furnace designs.
2.2.1.3 Waste delivery and storage
2.2.1.3.1
Waste control
The waste delivery area is the location where the delivery trucks, trains, or containers arrive in
order to dump the waste into the bunker, usually after visual control and weighing. The
dumping occurs through openings between the delivery area and the bunker. Tilting and sliding
beds may be used to help waste transfer to the bunker. The openings can be locked, and
therefore also serve as odour and seal locks, as well as fire and crash-protecting devices.
Enclosure of the delivery area can be one effective means of avoiding odour, noise and emission
problems from the waste.
2.2.1.3.2
Bunker
The bunker is usually a waterproof, concrete bed. The waste is piled and mixed in the bunker
using cranes equipped with grapples. The mixing of wastes helps to achieve a balanced heat
value, size, structure, composition, etc. of the material dumped into the incinerator filling
hoppers.
Fire protection equipment is used in the bunker area and feeder system. For example:
•
•
•
•
22
fire proofed cabling for the cranes
safety design for the crane cabs
fire detectors
automatic water cannon sprays, with or without foam.
Waste Incineration
Chapter 2
Crane cabs are designed in such a way that the crane operator has a good overview of the entire
bunker. The cab has its own ventilation system, independent from the bunker.
In order to avoid excessive dust development and gas formation (e.g. methane) from fermenting
processes, as well as the accumulation of odour and dust emissions, the primary incineration air
for the furnace plants is often extracted from the bunker area. Depending on the calorific value
of the waste as well as the layout and the concept of the plant, preference is most often given to
supplying the bunker air to either the primary or secondary air. [74, TWGComments, 2004]
The bunker usually has a storage capacity of several days (commonly 3 - 5 days) of plant
operational throughput. This is very dependent on local factors and the specific nature of the
waste.
Additional safety devices may be implemented such as: dry standpipe at the waste hopper level,
foam nozzle above waste hopper, fire detection for the hydraulic group, fire resistant walls
between the bunker and the furnace hall, fire resistant walls between the furnace hall and the
control room, water curtains on the window between the control room and the furnace, smoke
and fire extraction (5 - 10 % of the surface of the roof) etc.
[74, TWGComments, 2004]
2.2.2 Hazardous wastes
2.2.2.1 Brief description of the sector
The hazardous waste incineration sector comprises two main sub-sectors:
•
•
merchant incineration plants
dedicated incineration plants.
Waste Incineration
23
Chapter 2
The main differences between these are summarised in the table below:
Criteria
Ownership
Characteristics of
wastes treated
Combustion
technologies
applied
Operational and
design
considerations
Merchant plants
Private companies,
municipalities or partnerships
• very wide range of wastes
• knowledge of exact waste
composition may be limited
in some cases.
•
•
•
•
Flue-gas
treatment
•
•
•
Cost/market
considerations
•
predominantly rotary kilns
some dedicated
technologies for dedicated
or restricted specification
wastes.
Flexibility and wide range
of performance required to
ensure good process
control.
wet scrubbing often applied
to give flexibility of
performance, as well as
a range of FGT techniques
applied in combination.
operators usually compete
in an open (global) market
for business
some plants benefit from
national/regional policies
regarding the destination of
wastes arising in that
country/region.
Movement of hazardous
waste in the EU is
controlled by Transfrontier
Shipment Regulations
which limits the scope of
open global market.
Dedicated plants
Usually private companies (used for
their own wastes)
• wide range of wastes
• often only the waste arising
within one company or even
from one process
• knowledge of waste composition
generally higher.
• rotary kilns plus
• a wide variety of specific
techniques for dedicated or
restricted specification wastes.
•
•
•
•
•
Process can be more closely
designed for a narrower
specification of feed in some
cases.
wet scrubbing often applied to
give flexibility of performance,
as well as
a range of FGT techniques
applied in combination.
competition more limited or in
some cases non-existent
higher disposal costs tolerated
by users in some cases for
reasons of waste producer policy
on in-house disposal.
Table 2.3: Summary of the differences between operators in the HWI market
Source: discussions with TWG
[EURITS, 2002 #41]. The individual incineration capacity of rotary kilns used in the merchant
sector varies between 30000 and 100000 tonnes a year. The mass capacity for an individual
design varies considerably with the average calorific value of the waste, with the principal
factor being thermal capacity.
The following sections refer mainly to the delivery, storage and pretreatment of hazardous waste
for the merchant sector.
2.2.2.2 Waste acceptance
Due to the very wide variety of wastes encountered, their high potential hazard, and elevated
uncertainties over the precise knowledge of the waste composition, significant effort is required
to assess, characterise and trace incoming wastes through the entire process. The systems
adopted need to provide a clear audit trail that allows the tracing of any incidents to their source.
This then enables procedures to be adapted to prevent incidents.
24
Waste Incineration
Chapter 2
The exact procedures required for waste acceptance and storage depends on the chemical and
physical characteristics of the waste.
Identification and analysis of wastes:
[1, UBA, 2001]For each type of hazardous waste, a declaration of the nature of the waste made
by the waste producer is submitted so that the waste manager can then decide whether the
treatment of each specific type of waste is possible. Such a declaration may include:
•
•
•
•
•
•
•
•
data on the waste producer and responsible persons
data on the waste code and other designations for the waste
data on the origin of the waste
analytical data on particular toxic materials
general characteristics, including combustion parameters, such as: Cl, S, calorific value,
water content, etc.
other safety/environmental information
legally-binding signature
additional data upon request of the accepting plant.
Some types of waste require additional measures. Homogeneous, production-specific waste can
often be adequately described in general terms. Additional measures are usually required for
waste of less well-known composition (e.g. waste from refuse dumps or from the collections of
hazardous household waste), including the investigation of each individual waste container.
When the waste composition cannot be described in detail (e.g. small amounts of pesticides or
laboratory chemicals), the waste management company may agree with the waste producer on
specific packaging requirements, making sure that the waste will not react during transport,
when it is accepted for incineration, or within containers. For example, risks may arise from:
• wastes with phosphides
• wastes with isocyanates
• wastes with alkaline metals (e.g., or other reactive metals)
• cyanide with acids
• wastes forming acid gases during combustion
• wastes with mercury.
[74, TWGComments, 2004]
Delivered wastes generally undergo specific admission controls, whereby the previously
received declaration by the waste producer provides the starting point. After comparison by
visual and analytical investigations with the data contained in the declaration, the waste is either
accepted, allocated to the appropriate storage area, or rejected in the case of significant
deviations.
2.2.2.3 Storage
The general principles of storage are described in the BREF on emissions from storage.
However, this section serves to outline some issues that are specific to the hazardous waste
industry.
In general, the storage of wastes needs, additionally, to take into account the unknown nature
and composition of wastes, as this gives rise to additional risks and uncertainties. In many cases,
this uncertainty means that higher specification storage systems are applied for wastes than for
well-characterised raw materials.
Waste Incineration
25
Chapter 2
A common practice is to ensure, as far as possible, that hazardous wastes are stored in the same
containers (drums) that are used for transport; thus avoiding the need for additional handling
and transfer. Good communication between the waste producer and the waste manager help to
ensure wastes are stored, transferred, etc, such that risks all along the chain are well managed. It
is also important that only well characterised and compatible wastes are stored in tanks or
bunkers.
For hazardous waste incineration, storage arrangements for some substances may need to be
consistent with the COMAH/(Seveso II) requirements, as well as BAT described in the storage
BREF. There may be circumstances where the major accident and hazard (MAH)
prevention/mitigation measures take precedence.
[EURITS, 2002 #41] Appropriate waste assessment is an essential element in the selection of
storage and loading options. Some issues to note are:
•
•
•
•
for the storage of solid hazardous waste, many incinerators are equipped with a bunker
(500 to 2000 m³) from where the waste is fed into the installation by cranes or feed hoppers
liquid hazardous waste and sludges, these are usually stored in a tank farm. Some tanks
have storage under an inert (e.g. N2) atmosphere. Liquid waste is pumped via pipelines to
the burners and introduced into the rotary kiln and/or the post combustion chamber (PCC).
Sludges can be fed to rotary kilns using special “viscous-matter” pumps
some incinerators are able to feed certain substances, such as toxic, odorous, reactive and
corrosive liquids, by means of a direct injection device, directly from the transport
container into either the kiln or PCC
almost half of the merchant incinerators in Europe are equipped with conveyors and
elevators to transport and introduce drums and/or small packages (e.g. lab packs) directly
into the rotary kiln. These may be via air locks systems, and can use inert gas flood systems.
2.2.2.3.1
Storage of solid hazardous waste
[1, UBA, 2001]Solid and unpumpable pasty hazardous waste that has not been degassed and
does not smell, is stored temporarily in bunkers. Storage and mixing areas can be separated in
the bunker. This can be achieved through several design segments. Cranes feed both solid and
pasty waste products. The bunker must be designed in such a way that ground emissions can be
prevented.
The bunker and container storage must be enclosed unless health and safety reasons (danger of
explosion and fire) exist. The air in the bunker is usually removed and used as incineration air.
In anticipating fires, monitors such as heat-detecting cameras are used, in addition to constant
monitoring by personnel (control room, crane operator).
2.2.2.3.2
Storage of pumpable hazardous waste
[1, UBA, 2001] Larger amounts of fluid and pumpable pasty wastes are temporarily stored in
tanks that must be available in sufficient numbers and sizes to accommodate reacting liquids
separately (danger of explosion, polymerisation).
Tanks, pipelines, valves, and seals must be adapted to the waste characteristics in terms of
construction, material selection, and design. They must be sufficiently corrosion-proof, and
offer the option of cleaning and sampling. Flat bed tanks are generally only deployed for large
loads.
It may be necessary to homogenise the tank contents with mechanical or hydraulic agitators.
Depending on the waste characteristics, the tanks must be heated indirectly and insulated. Tanks
are set in catch basins that must be designed for the stored material, with bund volumes chosen
so that they can hold the liquid waste in the event of leakage.
26
Waste Incineration
Chapter 2
2.2.2.3.3
Storage for containers and tank containers
[1, UBA, 2001]For safety reasons, hazardous waste is most often accumulated in special
containers. These containers are then delivered to the incineration plant. Delivery is also taken
of bulk liquids.
The delivered containers may be stored or the contents transferred. In some cases, according to
a risk assessment, the waste may be directly injected via a separate pipeline into the furnace.
Heated transfer lines may be used for wastes that are only liquid at higher temperatures.
Storage areas for containers and tank containers are usually located outside, with or without
roofs. Drainage from these areas is generally controlled, as contamination may arise.
2.2.2.4 Feeding and pretreatment
Because of the wide range of chemical and physical specification of some hazardous wastes,
difficulties may occur in the incineration process. Some degree of waste blending or specific
pretreatment is thus often carried out in order to achieve more even loads.
[2, infomil, 2002] It is also necessary for acceptance criteria to be developed for each
installation. Such a recipe will describe the range of concentrations within which key
combustion and chemical waste characteristics should be maintained, in order to ensure the
process runs predictably, to prevent exceeding the process capacity, and thus to comply with
operational and environmental (e.g. permit conditions) requirements.
Factors that set such ranges include:
•
•
•
•
•
the flue-gas cleaning technology capacity for individual pollutants (e.g. scrubber flowrates,
etc)
the existence or absence of a particular flue-gas cleaning technique
emission limit values required
heat throughput rating of the furnace
design of the waste feed mechanism and the physical suitability of the waste received.
[EURITS, 2002 #41] Some incinerators have dedicated and integrated homogenisation
installations for the pretreatment of waste. These include:
•
•
•
a shredder for bulky solids (e.g. contaminated packages) [74, TWGComments, 2004]
a dedicated shredder purely for drums. Depending on the installation, drums containing
solid and/or liquid waste can be treated. The shredded residues are then fed via the bunker
and/or tanks
a shredder combined with a mechanical mixing device. This results in a homogenised
fraction which is pumped directly into the kiln by means of a thick-matter pump. Some
shredders can deal with both drums and/or solid waste in packages of up to one tonne.
Depending on the waste composition and the individual characteristics of the incineration plant,
together with the availability of other treatment means for any wastes produced, other
pretreatment may also be carried out. For example [1, UBA, 2001]:
•
•
•
neutralisation (for waste acceptance, pH-values from 4 - 12 are normal)
sludge drainage
solidification of sludge with binding agents.
Waste Incineration
27
Chapter 2
The following figure shows an example of some hazardous waste pretreatment systems used at
some merchant HWI:
Figure 2.2: Example of some hazardous waste pretreatment systems used at some merchant HWI
[25, Kommunikemi, 2002]
2.2.3
Sewage sludge
2.2.3.1 Composition of sewage sludge
The composition of sewage sludge varies according to many factors, including:
•
•
•
•
28
system connections, e.g. industrial inputs can increase heavy metal loads
coastal locations, e.g. for salt water inclusion
treatments carried out at the treatment works, e.g. crude screening only, anaerobic sludge
digestion, aerobic sludge digestion, addition of treatment chemicals
weather/time of year, e.g. rainfall can dilute the sludge.
Waste Incineration
Chapter 2
The composition of sewage sludge varies greatly. Typical composition ranges for dewatered
communal and industrial sewage sludge are given below:
Component
Dry solids (%)
Organic material (% of dry solids)
Heavy metals mg/kg d.s.):
Cr
Cu
Pb
Ni
Sb
Zn
As
Hg
Cd
Mo
Communal sewage sludge
10 – 45
45 – 85
20 – 77
200 – 600
100 – 700
15 – 50
1–5
500 – 1500
5 – 20
0.5 – 4.6
1–5
4 – 20
Industrial sewage sludge
170
1800
40
170
<10
280
<10
1
<1
-
Table 2.4: Average composition of dewatered communal sewage sludge after dewatering
[2, infomil, 2002], [64, TWGComments, 2003]
Particularly important factors to take into account when incinerating sewage sludges are:
•
•
•
•
•
the dry solids content (typically this varies from 10 % up to 45 % - this can have a major
impact on the incineration process)
whether the sludge is digested or not
the lime, limestone and other conditioning contents of the sludge
The composition of the sludge as primary-, secondary-, bio-sludge, etc.
Odour problems, especially during sludge feeding in the storage.
[64, TWGComments, 2003] [74, TWGComments, 2004]
2.2.3.2 Pretreatment of sewage sludge
2.2.3.2.1
Physical dewatering
[1, UBA, 2001, 64, TWGComments, 2003]
Mechanical drainage before incineration reduces the volume of the sludge mixture, by the
reduction of the water content. An increase in the heat value is associated with this process. This
allows independent and economical incineration. The success of mechanical drainage depends
on the selected machines, the conditioning carried out, and the type and composition of the
sludge.
Through mechanical drainage of the sewage sludge in decanters, centrifuges, belt filter presses
and chamber filter presses, a dry solids (DS) level of between 10 and 45 % can be achieved.
Often the sludge is conditioned before the mechanical drainage to improve its drainage. This is
achieved with the help of additives that contain flock building materials. It is necessary to
differentiate between inorganic flocking substances (iron and aluminium salts, lime, coal, etc.)
and organic flocking substances (organic polymers). Inorganic substances not only act as
flocking substances but are also builders, i.e. they increase the inorganic content substantially,
and hence the unburned proportion of the drained sludge (ash). For this reason, mostly organic
conditioning substances are used in the treatment of sewage sludge.
Waste Incineration
29
Chapter 2
2.2.3.2.2
Drying
[1, UBA, 2001, 64, TWGComments, 2003]
Often a substance that has been dried by mechanical drainage is still insufficiently dry for auto
thermal incineration. In this case, a thermal drying plant for additional drying can be used
before the incineration furnace. In this case, the sewage sludge is further reduced in volume and
the heat value is further increased.
The drying/dewatering of sewage sludge is carried out in separate or connected drying plants.
The following dryer plants are utilised:
•
•
•
•
•
•
•
•
•
•
disk dryer
drum dryer
fluidised bed dryer
belt dryer
thin film dryer/disk dryer
cold air dryer
thin film dryer
centrifugal dryer
solar dryer
combinations of different types.
Drying processes can be divided, in principal, into two groups:
• partial drying, up to approximately 60 - 80 % d.s.
• complete drying, up to approximately 80 - 90 % d.s.
[74, TWGComments, 2004]
An alternative to external drying is the in-situ drying of sludge by incineration together with
higher calorific waste. In such cases, the water from the dewatered sludge helps to prevent the
otherwise possible high temperature peaks that can be seen if only high CV waste were
incinerated.
For auto thermal incineration in mono-sewage sludge incineration plants, the drainage of raw
sewage up to a dry substance content of 35 per cent is generally sufficient. This can be achieved
by mechanical dewatering and may not require thermal drying.
The required dry substance content for auto thermal incineration in a given installation will
depend on the composition of the sludge (energy content of the dry solids, largely related to the
content of organic material). This is influenced by the nature of the sludge as such, but also by
the applied pretreatment, e.g. by sludge digestion, or by the use of organic or inorganic sludge
conditioners.
For the simultaneous incineration of sewage sludge with other waste streams in municipal waste
incineration plants (typically with a mixture ratio of drained sewage sludge to municipal waste
of max. 10 % weight of drained sewage sludge (i.e. dryness of 20 – 30 %), additional sludge
drying may be required. [74, TWGComments, 2004]
The heat required for the drying process is usually extracted from the incineration process. In
some drying processes, the sewage sludge to be dried comes into direct contact with the thermal
carrier, e.g. in convection dryers or direct dryers (e.g. belt, double-deck, fluidised bed dryers).
During the drying process, vapour is produced that is a mixture of steam, air, and released gases
from the sludge; and hot gases are produced in the direct drying process. The vapour and gas
mixture must be cleaned. Generally, the steam from the drying process is injected in the furnace.
Direct dryers can be used in an indirect system by the recirculation of evaporation vapours. This
system has clear advantages and is often used (but hardly or not in combination with sludge
incineration).
30
Waste Incineration
Chapter 2
In indirect drying systems (e.g. worm, disk, thin film dryers), the heat is injected via steam
generators or thermal oil plants and the heating fluid is not in contact with the sludge. Heat
transfer occurs between the wall and the sludge.
Contact dryers generally achieve a dry solids level of 35 - 40 %. The evaporated water produced
through the drying process is only contaminated with leaking air and small amounts of volatile
gases. The steam can be condensed almost totally from the vapour and the remaining inert gases
can be deodorised in the boiler furnace. The treatment of the condensate may be complicated
due to the presence of NH4OH, TOC, etc.
2.2.3.2.3
Sludge digestion
Sludge digestion decreases the content of organic material in the sludge and produces biogas (at
least in the case of anaerobic digestion). Digested sludge can generally be dewatered more
easily than non-digested sludge, thus allowing a slightly higher dry solids content after
mechanical dewatering.
[64, TWGComments, 2003]
2.2.4 Clinical waste
2.2.4.1 Nature and composition of clinical wastes
Special attention is required when dealing with clinical wastes to manage the specific risks of
these wastes (e.g. infectious contamination, needles, etc.), the aesthetic standards (residues of
operations etc.) and their incineration behaviour (very variable calorific value and moisture
contents).
Specific clinical waste often contains materials with very high NCVs (plastics, etc.), but also
residues with very high water contents (e.g. blood, etc.). Clinical waste therefore usually
requires long incineration times to ensure thorough waste burnout and that the residue quality is
good.
Similar to hazardous wastes, the composition of specific clinical wastes varies greatly. Clinical
waste may include (to varying degrees):
•
•
•
•
•
•
•
•
•
•
infectious agents
contaminated clothing/wipes and swabs
pharmaceutical substances
sharp materials, e.g. hypodermic needles
veterinary wastes
body parts
used medical equipment
packaging materials
laboratory wastes
radioactive contaminated materials.
In some cases a distinction is made between the incineration routes for pathological (potentially
infectious waste) and non-pathological waste. The treatment of pathological waste is sometimes
restricted to dedicated incinerators, while non-pathological waste is, in some cases, incinerated
with other wastes in non-dedicated incinerators e.g. MSWI.
Waste Incineration
31
Chapter 2
2.2.4.2 Handling, pretreatment and storage of clinical waste
The risks associated with the handling of clinical waste can generally be reduced by limiting the
contact with the waste and by ensuring good storage, e.g. through the use of:
•
•
•
•
•
dedicated containers and the provision of washing/disinfection facilities
sealed and robust combustible containers, e.g. for sharps and biological hazard materials
automatic furnace loading systems, e.g. dedicated bin lifts
segregated storage and transfer areas (especially where co-incineration with other wastes
takes place)
refrigerated or freezer storage, if required.
Pretreatment may be carried out using:
•
•
steam disinfection, e.g. autoclaving at elevated temperature and pressure
boiling with water.
Each of these may allow the waste to be sufficiently sterilised to permit its subsequent handling
in a similar manner to municipal wastes. Work and storage areas are usually designed to
facilitate disinfection.
Appropriate cleaning and disinfection equipment are usually installed for the cleaning of
returnable containers. The solid wastes from disinfection are collected for disposal. The waste
water from disinfection are collected, and are then recycled in the incineration process (e.g. in
the FGT or with the fed waste) or treated and discharged. [74, TWGComments, 2004]
Pretreatment may be applied to improve the homogeneity of the waste, such as shredding or
maceration, although safety aspects require careful consideration with some clinical wastes.
Clinical waste is also incinerated in hazardous waste and other incineration plants with other
types of waste. If incineration does not take place immediately, the wastes require temporary
storage. In some cases, where it is necessary for clinical waste to be stored for longer than
48 hours, the waste is kept in cooled storage areas with a restricted maximum temperature (e.g.
+10 °C).
2.3 The thermal treatment stage
Different types of thermal treatments are applied to the different types of wastes, however not
all thermal treatments are suited to all wastes. This chapter and Table 2.5 review the concepts
and applications behind the most common technologies, in particular:
•
•
•
•
grate incinerators
rotary kilns
fluidised beds
pyrolysis and gasification systems.
As well as some other more specific technologies.
[EGTEI, 2002 #6]
Municipal solid waste - can be incinerated in several combustion systems including travelling
grate, rotary kilns, and fluidised beds. Fluidised bed technology requires MSW to be of a certain
particle size range– this usually requires some degree of pretreatment and/or the selective
collection of the waste.
32
Waste Incineration
Chapter 2
Incineration of sewage sludge - this takes place in rotary kilns, multiple hearth, or fluidised
bed incinerators. Co-combustion in grate-firing systems, coal combustion plants and industrial
processes is also applied. Sewage sludge often has a high water content and therefore usually
requires drying, or the addition of supplementary fuels to ensure stable and efficient
combustion.
Incineration of hazardous and medical waste - rotary kilns are most commonly used, but
grate incinerators (including co-firing with other wastes) are also sometimes applied to solid
wastes, and fluidised bed incinerators to some pretreated materials. Static furnaces are also
widely applied at on-site facilities at chemical plants.
Other processes have been developed that are based on the de-coupling of the phases which
also take place in an incinerator: drying, volatilisation, pyrolysis, carbonisation and oxidation of
the waste. Gasification using gasifying agents such as, steam, air, carbon-oxides or oxygen is
also applied. These processes aim to reduce flue-gas volumes and associated flue-gas treatment
costs. Some of these developments met technical and economical problems when they were
scaled-up to commercial, industrial sizes, and are therefore pursued no longer. Some are used on
a commercial basis (e.g. in Japan) and others are being tested in demonstration plants
throughout Europe, but still have only a small share of the overall treatment capacity when
compared to incineration.
Waste Incineration
33
Chapter 2
Untreated Municipal
waste
Pretreated MSW and
RDF
Grate - reciprocating
Widely applied
Widely Applied
Not normally applied
Not normally applied
Applied
Grate - travelling
Applied
Applied
Rarely applied
Not normally applied
Applied
Grate - rocking
Applied
Applied
Rarely applied
Not normally applied
Applied
Grate - roller
Applied
Widely Applied
Rarely applied
Not normally applied
Applied
Grate - water cooled
Applied
Applied
Rarely applied
Not normally applied
Applied
Grate plus rotary kiln
Applied
Not normally applied
Rarely applied
Not normally applied
Applied
Rotary kiln
Not normally applied
Applied
Widely applied
Applied
Widely applied
Rotary kiln - water cooled Not normally applied
Applied
applied
Applied
applied
Technique
Hazardous waste
Sewage sludge
Clinical waste
Static hearth
Not normally applied
Not normally applied
Applied
Not normally applied
Widely applied
Static furnace
Not normally applied
Not normally applied
Widely applied
Not normally applied
Applied
Rarely applied
Applied
Not normally applied
applied
Not normally
applied
Rarely applied
Applied
Not normally applied
Widely applied
Not normally
applied
Fluid bed - rotating
Applied
Applied
Not normally applied
Applied
Applied
Pyrolysis
Rarely applied
Rarely applied
Rarely applied
Rarely applied
Rarely applied
Gasification
Rarely Applied
Rarely applied
Rarely applied
Rarely applied
Rarely applied
Fluid bed - bubbling
Fluid bed - circulating
Note: This table only considers the application of the technologies described at dedicated installations. It does not therefore include detailed consideration of the situations
where more than one type of waste is processed.
Table 2.5: Summary of the current successful application of thermal treatment techniques to the main waste types at dedicated installations
[64, TWGComments, 2003]
34
Waste Incineration
Chapter 2
2.3.1 Grate incinerators
Grate incinerators are widely applied for the incineration of mixed municipal wastes. In Europe
approximately 90 % of installations treating MSW use grates. Other wastes commonly treated
in grate incinerators, often as additions with MSW, include: commercial and industrial nonhazardous wastes, sewage sludges and certain clinical wastes.
Grate incinerators usually have the following components:
•
•
•
•
•
•
waste feeder
incineration grate
bottom ash discharger
incineration air duct system
incineration chamber
auxiliary burners.
Figure 2.3 shows an example of a grate incinerator with a heat recovery boiler:
Figure 2.3: Grate, furnace and heat recovery stages of an example municipal waste incineration
plant
Source [1, UBA, 2001]
Waste Incineration
35
Chapter 2
2.3.1.1 Waste feeder
The waste is discharged from the storage bunker into the feeding chute by an overhead crane,
and then fed into the grate system by a hydraulic ramp or another conveying system. The grate
moves the waste through the various zones of the combustion chamber in a tumbling motion.
The filling hopper is used as a continuous waste supplier. It is filled in batches by the overhead
crane. As the filling hopper surface is exposed to great stress, materials with high friction
resistance are selected (e.g. boilerplates or wear-resistant cast iron). The material must survive
occasional hopper fires unscathed.
The waste hopper may sometimes be fed by a conveyor. In that case, the overhead crane
discharges waste into an intermediate hopper that feeds the conveyor. [74, TWGComments,
2004]
If the delivered waste has not been pretreated, it is generally very heterogeneous in both size
and nature. The feed hopper is therefore dimensioned in such a way that bulky materials fall
through and bridge formations and blockages are avoided. These blockages must be avoided as
they can result in uneven feeding to the furnace and uncontrolled air ingress to the furnace.
Feeder chute walls can be protected from heat using:
•
•
•
•
water-cooled double shell construction
membrane wall construction
water-cooled stop valves
fireproof brick lining.
If the feed chute is empty, stop valve equipment (e.g. door seals) can be used to avoid
flashbacks and for the prevention of uncontrolled air infiltration into the furnaces. A uniform
amount of waste in the filling chute is recommended for uniform furnace management.
The junction between the lower end of the filling chute and the furnace consists of a dosing
mechanism. The dosing mechanism may be driven either mechanically or hydraulically. Its
feeding rate is generally adjustable. Different construction methods have been developed for the
various types of feeder systems, such as:
• chain grates/plate bands
• feeder grates
• variable taper feed chutes
• RAM feeders
• hydraulic ramp
• feed screws.
[74, TWGComments, 2004]
2.3.1.2 Incineration grate
The incineration grate accomplishes the following functions:
•
•
•
36
transport of materials to be incinerated through the furnace
stoking and loosening of the materials to be incinerated
positioning of the main incineration zone in the incineration chamber, possibly in
combination with furnace performance control measures.
Waste Incineration
Chapter 2
A target of the incineration grate is a good distribution of the incineration air into the furnace,
according to combustion requirements. A primary air blower forces incineration air through
small grate layer openings into the fuel layer. More air is generally added above the waste bed
to complete combustion.
It is common for some fine material (sometimes called riddlings or siftings) to fall through the
grate. This material is recovered in the bottom ash remover. Sometimes it is recovered
separately and may be recycled to the grate for repeated incineration or removed directly for
disposal. When the sifting is recirculated in the hopper, care should be taken not to ignite the
waste in the hopper. [74, TWGComments, 2004]
Normally, the residence time of the wastes on the grates is not more than 60 minutes. [74,
TWGComments, 2004]
In general, one can differentiate between continuous (roller and chain grates) and discontinuous
feeder principles (push grates). Figure 2.4 shows some types of grates:
Figure 2.4: Different grate types
Source [1, UBA, 2001]
Different grate systems can be distinguished by the way the waste is conveyed through the
different zones in the combustion chamber. Each has to fulfil requirements regarding primary
air feeding, conveying velocity and raking, as well as mixing of the waste. Other features may
include additional controls, or a more robust construction to withstand the severe conditions in
the combustion chamber.
2.3.1.2.1
Rocking grates
[4, IAWG, 1997] The grate sections are placed across the width of the furnace. Alternate rows
are mechanically pivoted or rocked to produce an upward and forward motion, advancing and
agitating the waste.
2.3.1.2.2
Reciprocating grates
[4, IAWG, 1997] Many modern facilities (for municipal wastes) use reciprocating grates. The
quality of burnout achieved is generally good.
This design consists of sections that span the width of the furnace but are stacked above each
other. Alternate grate sections slide back and forth, while the adjacent sections remain fixed.
Waste tumbles off the fixed portion and is agitated and mixed as it moves along the grate.
Numerous variations of this type of grate exist, some with alternating fixed and moving
sections, others with combinations of several moving sections to each fixed section. In the latter
case, the sections can either move together or at different times in the cycle.
Waste Incineration
37
Chapter 2
There are essentially two main reciprocating grate variations:
1. Reverse reciprocating grate:
The grate bars oscillate back and forth in the reverse direction to the flow of the waste. The
grate is sloped from the feed end to the ash discharge end and is comprised of fixed and moving
grate steps.
2. Push forward grate:
The grate bars form a series of many steps that oscillate horizontally and push the waste in the
direction of the ash discharge.
2.3.1.2.3
Travelling grates
This consists of a continuous metal belt conveyor or interlocking linkages that move along the
length of the furnace. The reduced potential to agitate the waste (it is only mixed when it
transfers from one belt to another) means that it is seldom used in modern facilities. [IAWG,
1997 #4]
2.3.1.2.4
Roller grates
This consists of a perforated roller that traverses the width of the grate area. Several rollers are
installed in series and a stirring action occurs at the transition when the material tumbles off the
rollers. [4, IAWG, 1997]
2.3.1.2.5
Cooled grates
Most grates are cooled, most often with air. In some cases a liquid cooling medium (usually
water) is passed through the inside of the grate. The flow of the cooling medium is from colder
zones to progressively hotter ones in order to maximise the heat transfer. The heat absorbed by
the cooling medium may be transferred for use in the process or for external supply.
Water cooling is most often applied where the calorific value of the waste is higher
e.g.>12 - 15 MJ/kg for MSW. The design of the water cooled system is slightly more complex
than air cooled systems.
The addition of water cooling may allow grate metal temperature and local combustion
temperature to be controlled with greater independence from the primary air supply (normally
between the grate bars). This may then allow temperature and air (oxygen) supply to be
optimised to suit specific on-grate combustion requirements and thereby improve combustion
performance. Greater control of grate temperature can allow incineration of higher calorific
value wastes without the normally increased operational and maintenance problems.
2.3.1.3 Bottom ash discharger
The bottom ash discharger is used for cooling and removal of the solid residue that accumulates
on the grate. It also serves as an air seal for the furnace and cools and humidifies the ash.
Water-filled pressure pistons and drag constructions are commonly used to extract the bottom
ash. Other bottom ash discharges, such as belt conveyors are also commonly used. Grate ashes,
as well as any bulky objects are thus conveyed.
38
Waste Incineration
Chapter 2
The water used for cooling is separated from the grate ash at the exit, and may be re-circulated
to the ash discharger. A water top-up feed is usually required to maintain an adequate water
level in the discharger. The top-up water replaces losses with the removed ash and evaporation
losses. In addition a water drain may be needed to prevent the build up of salts – such bleed
systems can help to reduce the salt content of the residues if the flowrates are adjusted
specifically for this purpose. The bottom ash removal shaft is usually fireproof and is
constructed in such a way that bottom ash caking is avoided.
Figure 2.5: Example of a type of ash remover used at a grate incinerator
Source [1, UBA, 2001]
2.3.1.4 Incineration chamber and boiler
Combustion takes place above the grate in the incineration chamber (see Figure 2.6). As a
whole, the incineration chamber typically consists of a grate situated at the bottom, cooled and
non-cooled walls on the furnace sides, and a ceiling or boiler surface heater at the top. As
municipal waste generally has a high volatile content, the volatile gases are driven off and only
a small part of the actual incineration takes place on or near the grate.
The following requirements influence the design of the incineration chamber:
•
•
•
•
form and size of the incineration grate - the size of the grate determines the size of the
cross-section of the incineration chamber
vortexing and homogeneity of flue-gas flow - complete mixing of the flue-gases is essential
for good flue-gas incineration
sufficient residence time for the flue-gases in the hot furnace - sufficient reaction time at
high temperatures must be assured for complete incineration
partial cooling of flue-gases - in order to avoid fusion of hot fly ash at the boiler, the fluegas temperature must not exceed an upper limit at the incineration chamber exit.
Waste Incineration
39
Chapter 2
Waste feeding
Sewage
sludge
feeder
Secondary
air
Secondary air
Feeder table
Waste
Flue gas
Primary air
Drying
De-gassing
Exhaust
Incineration
Slag discharge
Figure 2.6: Example of an incineration chamber
Source [1, UBA, 2001]
The detailed design of a combustion chamber is usually linked to the grate type. Its precise
design demands certain compromises as the process requirements change with the fuel
characteristics. Each supplier has their own combination of grate and combustion chamber, the
precise design of which is based on the individual performance of their system and their specific
experiences. European operators of MSW have found no fundamental advantage or
disadvantage for the different designs of the combustion chamber.
In general, three different designs can be distinguished. The nomenclature comes from the flow
direction of the flue-gases in relation to the waste flow: unidirectional current; countercurrent
and medium current (see Figure 2.7).
Unidirectional current, co-current, or parallel flow furnace:
In a co-current combustion arrangement, primary combustion air and waste are guided in a
co-current flow through the combustion chamber. Accordingly, the flue-gas outlet is located at
the end of the grate. Only a comparatively low amount of energy is exchanged between the
combustion gases and the waste on the grate.
The advantage of unidirectional current concepts is that the flue-gas has the longest residence
time in the ignition area and that it must pass through the maximum temperature. To facilitate
ignition, the primary air must be pre-warmed with very low heat values.
Counter-flow or countercurrent furnace:
In this case, primary combustion air and waste are guided in a countercurrent flow arrangement
through the combustion chamber and the flue-gas outlet is located at the front end of the grate.
The hot flue-gases facilitate drying and ignition of the waste
Special attention must be paid to avoid the passage of unburned gas streams. As a rule, counterflow current concepts require higher secondary or upper air additions.
Medium-current or centre-flow furnace:
The composition of municipal solid waste varies considerably and the medium current concept
is a compromise for a wide feed value spectrum. A good mixture of all partial flue-gas currents
must be considered through mixture-promoting contours and/or secondary air injections. In this
case, the flue-gas outlet is located in the middle of the grate.
40
Waste Incineration
Chapter 2
Figure 2.7: Various furnace designs with differing direction of the flue-gas and waste flow
[1, UBA, 2001]
2.3.1.5 Incineration air feeding
The incineration air fulfils the following objectives:
•
•
•
•
provision of oxidant
cooling
avoidance of slag formation in the furnace
mixing of flue-gas.
Air is added at various places in the combustion chamber. It is usually described as primary and
secondary, although tertiary air, and re-circulated flue-gases are also used.
The primary air is generally taken from the waste bunker. This lowers the air pressure in the
bunker hall and eliminates most odour emissions from the bunker area. Primary air is blown by
fans into the areas below the grate, where its distribution can be closely controlled using
multiple wind boxes, and distribution valves.
The air can be preheated if the value of the waste degenerates to such a degree that it becomes
necessary to pre-dry the waste. The primary air will be forced through the grate layer into the
fuel bed. It cools the grate bar and carries oxygen into the incineration bed.
Secondary air is blown into the incineration chamber at high speeds via, for example, injection
lances or from internal structures. This is carried out to secure complete incineration and is
responsible for the intensive mixing of flue-gases, and prevention of the free passage of
unburned gas streams.
Waste Incineration
41
Chapter 2
2.3.1.6 Auxiliary burner
At start-up, auxiliary burners are commonly used to heat up the furnace to a specified
temperature through which the flue-gases can pass. This is the main use of auxiliary burners.
These burners are usually switched on automatically if the temperature falls below the specified
value during operation. During shut down, the burners are often only used if there is waste in
the furnace. [74, TWGComments, 2004]
2.3.1.7 Incineration temperature, residence time, minimum oxygen content
To achieve good burn out of the combustion gases, a minimum gas phase combustion
temperature of 850 °C (1100 °C for some hazardous wastes) and a minimum residence time of
the flue-gases, above this temperature, of two seconds after the last incineration air supply have
been established in legislation (Directive 2000/76/EC and earlier legislation). Derogations from
these conditions are allowed in legislation if they provide for a similar level of overall
environmental performance. [74, TWGComments, 2004]
A minimum oxygen content of 6 % was required by earlier legislation but removed from the
most recent EC Directive on incineration.
Operational experiences have in some cases shown that lower temperatures, shorter residence
times and lower oxygen levels can, in some situations, still result in good combustion and may
result in overall improved environmental performance. However, low oxygen content may lead
to significant corrosion risk and therefore require specific material protection. [74,
TWGComments, 2004]
The carbon monoxide content of the flue-gas is a key indicator of the quality of combustion.
2.3.1.8 Sewage sludge incineration in MSWI plants
Sewage sludge is sometimes incinerated with other wastes in grate municipal waste incineration
plants (see Section 2.3.3, for information regarding the use of fluid beds and other technologies)
Where added to MSWI it is often the feeding techniques that represent a significant proportion
of the additional investment costs.
The following three supply technologies are used:
•
•
•
42
dried sewage sludge (~90 % d.s) is blown as dust into the furnace
drained sewage sludge (~20 - 30 % d.s) is supplied separately through sprinklers into the
incineration chamber and distributed on a grate. The sludge is integrated into the bed
material by overturning the waste on the grates. Operational experiences show up to 20
mass-% sludge (at 25 % d.s.). Other experiences have shown that if the sludge ratio is too
high (e.g.>10 %.), high fly ash content or unburnt material in bottom ash may occur.
drained, dried or semi-dried (~50 - 60 % d.s.) sludge is mixed with the remaining waste or
fed together into the incineration chamber. This can occur in the waste bunker through
targeted doses by the crane operator, or controlled in a feeding hopper by pumping
dewatered sludge into the hopper or by spreading systems into the bunker. [74,
TWGComments, 2004]
Waste Incineration
Chapter 2
2.3.1.9 Addition of clinical waste to a municipal waste incinerator
(Denmark 2002) Clinical waste is sometimes added into an existing municipal waste
incinerator. In some cases the waste is loaded into the same hopper as the MSW.
Separate loading systems, with airlocks are also used. The airlock helps to prevent the entry of
uncontrolled combustion air and the possibility of fugitive emissions at the loading area.
Combustion takes place in the same furnace as the MSW.
The combined incineration of clinical waste with municipal solid waste can be also carried out
without a separate loading. For example, automatic loading systems are implemented in order to
put the clinical waste directly in the feed hopper with MSW.
National regulations sometimes limit the ratio of clinical waste that may be treated in combined
incineration (e.g. in France <10 % thermal load)
Note that Article 6.7 of Waste Incineration Directive requires that infectious clinical waste
should be placed straight in the furnace, without first being mixed with other categories of waste
and without direct handling. [74, TWGComments, 2004]
Flue-gases from the different wastes are then treated in common FGT systems.
In Figure 2.8 below the order of the stages for a separate loading system are shown:
Figure 2.8: Examples of the stages of a clinical waste loading systems used at a municipal waste
incinerator
Source [49, Denmark, 2002]
Waste Incineration
43
Chapter 2
2.3.2 Rotary kilns
Rotary kilns are very robust and almost any waste, regardless of type and composition, can be
incinerated. Rotary kilns are, in particular, very widely applied for the incineration of hazardous
wastes. The technology is also commonly used for clinical wastes (most hazardous clinical
waste is incinerated in high temperature rotary kiln incinerators [64, TWGComments, 2003],
but less so for municipal wastes.
Operating temperatures of rotary kilns used for wastes range from around 500 °C (as a gasifier)
to 1450 °C (as a high temperature ash melting kiln). Higher temperatures are sometimes
encountered, but usually in non-waste applications.
When used for conventional oxidative combustion, the temperature is generally above 850 °C.
Temperatures in the range 900 - 1200 °C are typical when incinerating hazardous wastes.
Generally, and depending on the waste input, the higher the operating temperature, the greater
the risk of fouling and thermal stress damage to the refractory kiln lining. Some kilns have a
cooling jacket (using air or water) that helps to extend refractory life, and therefore the time
between maintenance shut-downs.
A schematic drawing of a rotary kiln incineration system is shown below.
Figure 2.9: Schematic of a rotary kiln incineration system
Source [EGTEI, 2002 #6]
The rotary kiln consists of a cylindrical vessel slightly inclined on its horizontal axis. The vessel
is usually located on rollers, allowing the kiln to rotate or oscillate around its axis (reciprocating
motion). The waste is conveyed through the kiln by gravity as it rotates. Direct injection is used
particularly for liquid, gaseous or pasty (pumpable) wastes – especially where they have safety
risks and require particular care to reduce operator exposure.
The residence time of the solid material in the kiln is determined by the horizontal angle of the
vessel and the rotation speed: a residence time of between 30 to 90 minutes is normally
sufficient to achieve good waste burnout.
Solid waste, liquid waste, gaseous waste, and sludges can be incinerated in rotary kilns. Solid
materials are usually fed through a non-rotating hopper; liquid waste may be injected into the
kiln through burner nozzles; pumpable waste and sludges may be injected into the kiln via a
water cooled tube.
In order to increase the destruction of toxic compounds, a post-combustion chamber is usually
added. Additional firing using liquid waste or additional fuel may be carried out to maintain the
temperatures required to ensure the destruction of the waste being incinerated.
44
Waste Incineration
Chapter 2
2.3.2.1 Kilns and post combustion chambers for hazardous waste incineration
The operational kiln temperature of installations for incineration usually varies from 850 °C
up to 1300 °C. The temperature may be maintained by burning higher calorific (e.g. liquid)
waste, waste oils, heating oil or gas. Higher-temperature kilns may be fitted with water-based
kiln cooling systems, which are preferred for operation at higher temperatures. The operation at
higher temperatures may result in molten (vitrified) bottom ash (slag); at lower temperatures the
bottom ashes are sintered.
The temperatures in the post combustion chamber (PCC) typically vary between
900 - 1200 °C depending on the installation and the waste feed. Most installations have the
ability to inject secondary air into the post combustion chamber. Due to the high temperatures
and the secondary air introduction, the combustion of the exhaust gases is completed and
organic compounds (e.g. PAHs, PCBs and dioxins) including low molecular weight
hydrocarbons, are destroyed. In several countries exemptions from the 1100 °C rule are granted,
on the basis of studies demonstrating that lowering the temperature in the PCC does not
influence the quality of air emissions.
2.3.2.2 Drum kiln with post-combustion chamber
incineration
for hazardous waste
For the incineration of hazardous waste, a combination of drum-type kilns and post-combustion
chambers has proven successful, as this combination can treat solid, pasty, liquid, and gaseous
wastes uniformly (see Figure 2.10).
Figure 2.10: Drum-type kiln with post-combustion chamber
Source [1, UBA, 2001]
Drum-type kilns between 10 and 15 metres in length, and with a length to diameter ratio usually
in the range of 3 to 6, and with an inner diameter between one and five metres are usually
deployed for hazardous waste incineration.
Some drum-type kilns have throughputs of up to 70000 tonnes/yr each. In correlation to the
average heat value of the waste, where heat recovery is carried out steam generation increases
correspondingly.
Waste Incineration
45
Chapter 2
Drum-type kiln plants are highly flexible in terms of waste input characteristics. The following
range is usual in the composition of the waste input menu:
•
•
•
•
solid wastes :
liquid wastes:
pasty wastes:
barrels:
10 – 70 %
25 – 70 %
5 – 30 %
up to 15 %.
To protect the drum-type kilns from temperatures of up to 1200 °C, it is equipped with
refractory bricks. Bricks with a high content of Al2O3 and SiO2 are used. The decision regarding
the selection of bricks appropriate for each application is a function of the waste composition.
The bricks can be attacked by alkaline metal compounds (formation of low melting eutectic
alloys), as well as by HF. (formation of SiF4). To protect refractory bricks form chemical attacks
and from the mechanical impact of falling barrels, a hardened slag layer will usually be formed
at the beginning of the operation with the help of good slag forming wastes or materials as
mixtures of glass or sand and glass. Later on the kiln temperature is usually managed so as to
keep this slag layer, based on the mineral matter of the wastes and perhaps some additives as
e.g. sand. [74, TWGComments, 2004]
There have been tests with other surfacing systems but neither injected nor stamped refractory
masses have proved successful. The surfacing of the drum-type kiln with special alloyed steels
was only successful in some special applications. The durability of the fireproof surfacing
remains dependent upon the waste input. Service life of between 4000 and 16000 hours is
normal.
Cooling the drum-type kilns is a means of lengthening their service life. Several positive
experiences have been noted at various plants.
Drum-type kilns are tilted towards the post combustion chamber. This, along with the slow
rotation (approx. 3 – 40 rotations per hour) facilitates the transport of solid hazardous wastes
that are fed from the front side, as well as the bottom ash produced during incineration, in the
direction of the post combustion chamber. These are then removed together with the ash from
the post combustion chamber via a wet bottom ash remover. The residence time for solid wastes
generally amounts to more than 30 minutes.
The post combustion chamber, provides residence time for the incineration of the flue-gases
produced during incineration, as well as for the incineration of directly injected liquid and
gaseous wastes. Minimum residence times in excess of two seconds are the basic requirement of
EC Directive 2000/76/EC. The size of the post-combustion chamber and gas flows predict the
actual residence times achieved. Reducing residence times can increase risks of incomplete gas
burnout.
Operational experiences have in some cases shown that lower temperatures, shorter residence
times and lower oxygen levels can, in some situations, still result in good combustion and may
result in lower overall emissions to air. [74, TWGComments, 2004]
A drum-type kiln incineration plant with an incineration capacity of 45000 tonnes/yr is shown
in Figure 2.11. The plant is divided into three main areas:
•
•
•
drum-type kiln with post combustion chamber
waste heat boiler for steam generation
multi-step flue-gas cleaning.
There is, in addition, the infrastructure for the storage, feed system, and disposal for the waste
and waste waters (from wet gas scrubbing) produced during incineration.
46
Waste Incineration
Chapter 2
RG von
VA 2
Bunker
Drum-type kiln
Afterburner
chamber
Slag remover
Process gas cooler
Ash remover Quench
Rotation
washer
Condensation
EGR
SCR-plant
suction draft
(for VA 1 and VA 2)
Chimney
Suction draft
Stand: April 1997
Figure 2.11: Example of a drum-type kiln plant for hazardous waste incineration
Source [1, UBA, 2001]
2.3.3 Fluidised beds
Fluidised bed incinerators are widely applied to the incineration of finely divided wastes e.g.
RDF and sewage sludge. It has been used for decades, mainly for the combustion of
homogeneous fuels. Among these are coal, raw lignite, sewage sludge, and biomass (e.g. wood).
The fluidised bed incinerator is a lined combustion chamber in the form of a vertical cylinder. In
the lower section, a bed of inert material, (e.g., sand or ash) on a grate or distribution plate is
fluidised with air. The waste for incineration is continuously fed into the fluidised sand bed
from the top or side [66, UllmansEncyclopaedia, 2001].
Preheated air is introduced into the combustion chamber via openings in the bed-plate, forming
a fluidised bed with the sand contained in the combustion chamber. The waste is fed to the
reactor via a pump, a star feeder or a screw-tube conveyor.
In the fluidised bed, drying, volatilisation, ignition, and combustion take place. The temperature
in the free space above the bed (the freeboard) is generally between 850 and 950 °C. Above the
fluidised bed material, the free board is designed to allow retention of the gases in a combustion
zone. In the bed itself the temperature of is lower, and may be around 650 °C or higher.
Because of the well-mixed nature of the reactor, fluidised bed incineration systems generally
have a uniform distribution of temperatures and oxygen, which results in stable operation. For
heterogeneous wastes, fluidised bed combustion requires a preparatory process step for the
waste so that it conforms with size specifications. For some waste this may be achieved by a
combination of selective collection of wastes and/or pretreatment e.g. shredding. Some types of
fluidised beds (e.g. the rotating fluidised bed) can receive larger particle size wastes than others.
Where this is the case the waste may only require only a rough size reduction.
[64, TWGComments, 2003] [74, TWGComments, 2004]
Pretreatment usually consists of sorting out and crushing larger inert particles, and shredding.
Removal of ferrous and non-ferrous materials may also be required. The particle size of the
waste must be small, often with a maximum diameter of 50 mm. However, it is reported that
average acceptable diameters for rotating fluidised beds are 200 - 300 mm. [74,
TWGComments, 2004]
Waste Incineration
47
Chapter 2
The schematic diagram below shows an installation that pretreats mixed MSW for incineration
in a fluidised bed incineration plant. Several pretreatment stages are shown including
mechanical pulverisation and pneumatic separation, along with the final stages of incineration,
FGT and residue storage:
Figure 2.12: Schematic diagram showing pretreatment of MSW prior to fluidised bed combustion
During incineration the fluidised bed contains the unburned waste and the ash produced. The
ash surplus is usually removed at the bottom of the furnace. [1, UBA, 2001, 33, Finland, 2002]
The heat produced by the combustion can be recovered by devices either integrated inside the
fluidised bed or at the exit of the combustion gases or a mixture of layouts.
The relatively high cost of pretreatment processes required for some wastes has restricted the
economic use of these systems to larger scale projects. This has been overcome in some cases
by the selective collection of some wastes, and the development of quality standards for waste
derived fuels (WDF). Such quality systems have provided a means of producing a more suitable
feedstock for this technology. The combination of a prepared quality controlled waste (instead
of mixed untreated waste) and fluidised bed combustion can allow improvements in the control
of the combustion process, and the potential for a simplified, and therefore reduced cost, fluegas cleaning stage.
The following table shows the properties of various waste fractions that are treated in fluidised
beds [33, Finland, 2002]:
Lower heating
value as received
Moisture
Ash
Sulphur
Chlorine
Storage properties
MJ/kg
MWh/t
Wt %
Wt %
Wt %
Wt %
Wt %
Commercial
waste
16 – 20
4.4 – 5.6
10 – 20
5–7
<0.1
<0.1 – 0.2
Good
Pretreated
construction waste
14 – 15
3.8 – 4.2
15 – 25
1–5
<0.1
<0.1
Good
Sorted and pretreated
household waste
13 – 16
3.6 – 4.4
25 – 35
5 – 10
0.1 – 0.2
0.3 – 1.0
Good as pellets
Table 2.6: Properties of various RDF (Refuse Derived Fuel) fractions treated in fluidised beds.
[33, Finland, 2002]
48
Waste Incineration
Chapter 2
The following fluidised bed furnace technologies can be differentiated according to the gas
speeds and design of the nozzle plate:
•
•
•
stationary (or bubbling) fluidised bed (atmospheric and pressurised): The inert material
is mixed, but the resulting upwards movement of solids is not significant (see Figure 2.13)
a version of bubbling fluidised bed is the rotating fluidised bed: Here, the fluidised bed is
rotated in the incineration chamber. This results in longer residence time in the incineration
chamber. Rotating fluidised bed incinerators have been used for mixed municipal waste for
about ten years
circulating fluidised bed: The higher gas speeds in the combustion chamber are
responsible for partial removal of the fuel and bed material, which is fed back into the
incineration chamber by a re-circulation duct (see diagram Figure 2.14).
In order to start-up the incineration process, the fluidised bed must be heated to at least the
minimum ignition temperature of the added waste (or higher where required by legislation).
This may be accomplished by preheating the air with oil or gas burners, which remain operative
until incineration can occur independently. The waste falls into the fluidised bed, where it is
crushed through abrasion and incineration. Usually, the major part of the ash is transported with
the flue-gas flow and requires separation in FGT equipment, although the actual proportion of
bottom ash (removed from the base of the bed) and the fly ash depends on the fluidised bed
technology and waste itself. [1, UBA, 2001].
Fouling problems, common in waste incineration boilers can be managed by controlling waste
quality (mostly keeping Cl, K, Na and Al low) and by boiler and furnace design. Some boiler
and furnace designs can be used in fluidised beds (but not in mixed waste grate boilers) because
of the more stable temperatures and the presence of the bed material.
2.3.3.1 Stationary (or bubbling) fluidised bed incineration
This type of fluidised bed is commonly used for sewage sludge, as well as for other industrial
sludges e.g. petrochemical and chemical industry sludges.
The stationary, or bubbling fluidised bed (see Figure 2.13), consists of a cylindrical or
rectangular lined incineration chamber, a nozzle bed, and a start-up burner located below.
1
2
3
4
5
6
7
8
9
Sludge feed with disintegration/spraying
Additional fuel
Atmospheric oxygen
Waste gas
Fluidized bed
After-burner chamber
Start-up incineration chamber
Inspection glass
1
Air preheater
9
4
6
8
3
5
2
7
Figure 2.13: Main components of a stationary/bubbling fluidised bed
Source [1, UBA, 2001]
Waste Incineration
49
Chapter 2
Preheated airflows up through a distribution plate and fluidises the bed material. According to
the application, various bed materials (silica sand, basalt, mullite, etc.) and bed material particle
sizes (approx 0.5 – 3 mm) can be used.
[2, infomil, 2002], [64, TWGComments, 2003]
The waste can be loaded via the head, on the sides with belt-charging machines, or directly
injected into the fluidised bed. In the bed, the waste is crushed and mixed with hot bed material,
dried and partially incinerated. The remaining fractions (volatile and fine particles) are
incinerated above the fluidised bed in the freeboard. The remaining ash is removed with the
flue-gas at the head of the furnace.
Drainage and drying pretreatment stages can be used so that the waste burns without the need
for additional fuels. Recovered heat from the incineration process may be used to provide the
energy for waste drying.
At start-up, or when sludge quality is low, (e.g. with old sludge or a high share of secondary
sludge) additional fuel (oil, gas, and/or waste fuel) can be used to reach the prescribed furnace
temperature (typically 850 °C). Water can be injected into the furnace to control the
temperature.
The furnace is usually preheated to its operating temperature before waste feeding starts. For
this purpose a start-up incineration chamber (see Figure 2.13) may be located below the nozzle
bed. This has an advantage over an overhead burner, as the heat is introduced directly into the
fluidised bed. Additional preheating may be provided by fuel lances that protrude over the
nozzle bed into the sand bed. The sewage sludge is supplied when the furnace temperature
reaches the operating temperature, e.g. 850 °C.
The size of the furnace is largely determined by the required evaporation (furnace crosssection), the heat turnover in the furnace (furnace volume) and the required amount of air.
Example operational parameters for a fluidised bed sewage sludge incinerator are shown in
Table 2.7:
Parameter
Steam load
Feed air amount
Heat turnover
Final incineration temperature
Residence time, open space and afterburner zone
Preheating of atmospheric oxygen
Units
kg/m2h
Nm³/m2h
GJ/m³h
°C
sec.
°C
Value
300 – 600
1000 – 1600
3–5
850 – 950
min. 2
400 – 600
Table 2.7: Main operational criteria for stationary fluidised beds
Source [1, UBA, 2001]
The preheating of air can be eliminated completely with higher caloric fuels (e.g. dried sewage
sludge, wood, animal by-products). The heat can be removed via membrane walls and/or
immersed heat exchange systems.
Some processes incorporate drying as a first step. Steam for the drying may be produced by a
boiler and then used as the heating medium with no direct contact between the steam and the
sludge. Sludge vapours can be extracted from the dryer and condensed. The condensed water
typically has a high COD (approx. 2000 mg/l) and N-content (approx. 600 - 2000 mg/l) and may
contain other pollutants (e.g. heavy metals) from the sewage sludge, and therefore will often
require treatment before final discharge. The remaining non-condensates may be incinerated.
After incineration, the flue-gases can be cooled in a heat exchanger in order to preheat the
incineration air to temperatures of approximately 300°C and in some cases over 500°C. The
remaining heat in the steam boiler can be recovered and used for the production of saturated
steam (pressure level approx. 10 bar), which in turn can be used for the partial pre-drying of
sludge. [64, TWGComments, 2003]
50
Waste Incineration
Chapter 2
2.3.3.2 Circulating fluidised bed (CFB) for sewage sludge
The circulating fluidised bed (CFB see Figure 2.14 below) is especially appropriate for the
incineration of dried sewage sludge with a high heat value. It works with fine bed material and
at high gas speeds that remove the greater part of the solid material particles from the fluidised
bed chamber with the flue-gas. The particles are then separated in a downstream cyclone and
returned to the incineration chamber.
Sewage sludge
Lime bunker
Fluidized bed
Incineration chamber
Recycling
cyclone
Flue gas to
the boiler
Secondary air
Primary
air
Coarse
ash
Air
Fluidized bed
condenser
Figure 2.14: Main components of a circulating fluidised bed
Source [1, UBA, 2001]
The advantage of this process is that high heat turnovers and more uniform temperature along
the height can be reached with low reaction volume. Plant size is generally larger than BFB and
a wider range of waste inputs can be treated. The waste is injected at the side into the
incineration chamber and is incinerated at 850 - 950 °C. The surplus heat is removed through
membrane walls and via heat exchangers. The fluid bed condenser is placed between recycling
cyclones and the CFB, and cools the returned ash. Using this method, the heat removal can be
controlled.
2.3.3.3 Spreader-stoker furnace
[64, TWGComments, 2003]
This system may be considered as an intermediate system between grate and fluidised bed
incineration.
The waste (e.g. RDF, sludge etc) is blown into the furnace pneumatically at a height of several
metres. Fine particles participate directly in the incineration process, while the larger particles
fall on the travelling grate, which is moving in the opposite direction to the waste injection. As
the largest particles are spread over the greatest distance, they spend the longest time on the
grate in order to complete the incineration process. Secondary air is injected to ensure that the
flue-gases are adequately mixed in the incineration zone.
Compared to grate incineration the grate is of less complicated construction due to the relatively
smaller thermal and mechanical load. When compared to fluidised bed systems the uniformity
of particle size is less important and that there is a lower risk of clogging.
Waste Incineration
51
Chapter 2
2.3.3.4 Rotating fluidised bed
[74, TWGComments, 2004]
This system is a development of bubbling bed for waste incineration. Inclined nozzle plates,
wide bed ash extraction chutes and upsized feeding and extraction screws are specific features
to ensure reliable handling of solid waste. Temperature control within the refractory lined
combustion chamber (bed and freeboard) is by flue-gas recirculation. This allows a wide range
of calorific value of fuels, e.g. co-combustion of sludges and pretreated wastes.
2.3.4 Pyrolysis and gasification systems
2.3.4.1 Introduction to gasification and pyrolysis
[9, VDI, 2002] Alternative technologies for thermal waste treatment have been developed since
the 1970s. In general these have been applied to selected waste streams and on a smaller scale
than incineration.
These technologies attempt to separate the components of the reactions that occur in
conventional waste incineration plants by controlling process temperatures and pressures in
specially designed reactors (see Table 2.1).
As well as specifically developed pyrolysis/gasification technologies, standard incineration
technologies (i.e. grates, fluidised beds, rotary kilns, etc) may be adapted to be operated under
pyrolytic or gasifying conditions i.e. with reduced oxygen levels (sub-stoichiometric), or at
lower temperatures. Often pyrolysis and gasification systems are coupled with downstream
combustion of the syngas generated (see Section 2.3.4.4 on combination processes).
As well as the normal targets of waste incineration (i.e. effective treatment of the waste), the
additional aims of gasification and pyrolysis processes are to:
•
•
convert certain fractions of the waste into process gas (called syngas)
reduce gas cleaning requirements by reducing flue-gas volumes.
Both pyrolysis and gasification differ from incineration in that they may be used for recovering
the chemical value from the waste (rather than its energetic value). The chemical products
derived may in some cases then be used as feedstock for other processes. However, when
applied to wastes, it is more common for the pyrolysis, gasification and a combustion based
process to be combined, often on the same site as part of an integrated process. When this is the
case the installation is, in total, generally recovering the energy value rather than the chemical
value of the waste, as would a normal incinerator
In some cases the solid residues arising from such processes contain pollutants that would, in an
incineration system, be transferred to the gas phase, and then with efficient flue-gas cleaning, be
removed with the FGT residue. [64, TWGComments, 2003]
The following systems and concepts have been developed (with different levels of proven
success on an industrial scale):
Pyrolysis - incineration systems for wastes:
System 1
Pyrolysis in a rotary kiln - coke and inorganic matter separation - incineration
of pyrolysis gas
System 2
Pyrolysis in a rotary kiln - separation of inert materials - combustion of the
solid carbon rich fraction and the pyrolysis gas
52
Waste Incineration
Chapter 2
System 3
Pyrolysis in a rotary kiln - condensation of pyrolysis gas components incineration of gas, oil and coke
System 4
Pyrolysis on a grate - directly connected incineration
System 5
Pyrolysis on a grate (with subsequent melting furnace for low metal content
molten bottom ash production) - circulating fluidised bed (burnout of particles
and gas).
Gasification systems for wastes:
System 1
Fixed bed gasifier - pretreatment drying required for lumpy material
System 2
Slag bath gasifier - as fixed bed but with molten bottom ash discharge
System 3
Entrained flow gasifier - for liquid, pasty and fine granular material that may be
injected to the reactor by nozzles
System 4
Fluidised bed gasifier - circulating fluid bed gasifier for pretreated municipal
waste, dehydrated sewage sludge and some hazardous wastes
System 5
Bubbling bed gasifier - similar to bubbling fluidised bed combustors, but
operated at a lower temperature and as a gasifier.
Pyrolysis - gasification systems for wastes:
System 1
Conversion process - pyrolysis in a rotary kiln - withdrawal and treatment of
solid phase - condensation of gas phase - subsequent entrained flow gasifier for
pyrolysis gas, oil and coke
System 2
Combined gasification-pyrolysis and melting - partial pyrolysis in a push
furnace with directly connected gasification in packed bed reactor with oxygen
addition (e.g. Thermoselect).
Other systems have been developed for the purpose of pretreating wastes that are then
combusted in other industrial plants. These co-incineration processes do not fall within the
scope of this BREF.
2.3.4.2 Gasification
[64, TWGComments, 2003] Gasification is a partial combustion of organic substances to
produce gases that can be used as feedstock (through some reforming processes), or as a fuel.
[1, UBA, 2001] There are several different gasification processes available or being developed
which are in principle suited for the treatment of municipal wastes, certain hazardous wastes
and dried sewage sludge.
It is important that the nature (size, consistency) of the wastes fed keeps within certain
predefined limits. This often requires special pretreatment of municipal waste, for example.
Waste Incineration
53
Chapter 2
The special features of the gasification process are:
•
•
•
•
•
•
•
smaller gas volume compared to the flue-gas volume in incineration (by up to a factor of 10
by using pure oxygen)
predominant formation of CO rather than CO2
high operating pressures (in some processes)
accumulation of solid residues as slag (in high temperature slagging gasifiers)
small and compact aggregates (especially in pressure gasification)
material and energetic utilisation of the synthesis gas
smaller waste water flows from synthesis gas cleaning.
The following gasification reactors are used:
•
•
•
•
fluidised bed gasifier (see Figure 2.17)
current flow gasifier
cyclone gasifier
packed bed gasifier.
Figure 2.15: Representation of a packed bed and current flow gasifier
Source [1, UBA, 2001]
For utilisation in entrained flow, fluidised bed or cyclone gasifiers, the feeding material must be
finely granulated. Therefore pretreatment is necessary, especially for municipal wastes.
Hazardous wastes, on the other hand, may be gasified directly if they are liquid, pasty or finely
granulated.
54
Waste Incineration
Chapter 2
2.3.4.2.1
Examples of gasification processes
[1, UBA, 2001]
In Germany, a entrained flow gasifier is at present in use for the gasification of fluid hazardous
wastes at Sekundärrohstoffverwertungszentrum (SVZ; Centre for Secondary Raw Materials
Utilisation) at Schwarze Pumpe.
The fluid wastes enter into the reactor via the burner system and are transformed into synthesis
gas at temperatures of 1600 – 1800 °C. Since 1995, approx. 31000 tonnes of waste oil have
been disposed of in this plant.
Lumpy charging material is required for the packed bed gasifier, but drying is sufficient as a
pretreatment process. SVZ Schwarze Pumpe GmbH runs six packed bed gasifiers for
gasification of coal waste mixtures. The feed rate proportion for waste is up to 85 %. In the
reactors, each with a throughput of 8 - 14 tonnes per hour, mainly compacted waste plastics,
dehydrated sewage sludge and contaminated soils are treated. The waste enters into the reactor
through the entry lock and is transformed into synthesis gas at approx. 800 – 1300 °C and 25 bar
with the help of steam and oxygen (the gasification agent).
A development from these packed bed gasifiers is the slag bath gasifier shown in Figure 2.16
below. One such plant is currently operating on a trial basis, receiving up to 70 % waste, at a
throughput rate of 30 t/hr. The gasifier operates at a temperature of up to 1600 °C and the slag is
discharged as a liquid.
Figure 2.16: Slag bath gasifier
Source [1, UBA, 2001]
A waste gasification process based on fluidised bed in combination with current flow
gasification is used in Japan (see Figure 2.17 below).
Waste Incineration
55
Chapter 2
Figure 2.17: Fluidised bed gasifier with high temperature slagging furnace
Source [68, Ebara, 2003]
This process is designed to generate syngas from plastic packaging waste or other high calorific
waste material. The main components of the process are a fluidised bed gasifier and a second
stage high temperature gasifier. The fluidised bed enables rapid gasification of comparatively
heterogeneous materials, which are pelletised for smooth feeding. Several per cent of noncombustible components, even metal pieces, are acceptable, as the ash is continuously
discharged from the fluidised bed. The high temperature gasifier is designed as cyclone, to
collect the fine ash particles on the wall. After vitrification the slag is discharged though a water
seal. Both reactors are operated under elevated pressure, typically 8 bar.
A first plant of this technology was under commercial operation in year 2001 to treat plastic
packaging waste. The capacity of this demonstration plant is 30 tonnes per day. An additional
plant of 65 tonnes per day started operation in 2002. The syngas produced is fed to an adjacent
ammonia production plant. Other similar plants are under construction. [68, Ebara, 2003]
Other variations on gasification processes have been tried and are being developed, for a variety
of waste stream.
2.3.4.3 Pyrolysis
[1, UBA, 2001] Pyrolysis is the degassing of wastes in the absence of oxygen, during which
pyrolysis gas and a solid coke are formed. The heat values of pyrolysis gas typically lies
between 5 and 15 MJ/m³ based on municipal waste and between 15 and 30 MJ/m³ based on
RDF. In a broader sense, “pyrolysis” is a generic term including a number of different
technology combinations that constitute, in general, the following technological steps:
•
•
•
•
56
smouldering process: Formation of gas from volatile waste particles at temperatures
between 400 and 600 °C
pyrolysis: Thermal decomposition of the organic molecules of the waste between 500 and
800 °C resulting in formation of gas and a solid fraction
gasification: Conversion of the carbon share remaining in the pyrolysis coke at 800 to
1000 °C with the help of a gasification substance (e.g. air or steam) in a process gas (CO,
H2)
incineration: Depending on the technology combination, the gas and pyrolysis coke are
combusted in a incineration chamber.
Waste Incineration
Chapter 2
A pyrolysis plant for municipal waste treatment is operational in Germany, and another was due
to start up at the end of 2003 in France. Other pyrolysis projects exist in Europe and elsewhere
(notably in Japan) receiving certain specific types or fractions of waste, often after pretreatment.
Pyrolysis plants for waste treatment usually include the following basic process stages:
1
preparation and grinding: the grinder improves and standardises the quality of the waste
presented for processing, and so promotes heat transfer
2
drying (depends on process): a separated drying step improves the LHV of the raw
process gases and increase efficiency of gas-solid reactions within the rotary kiln
3
pyrolysis of wastes, where in addition to the pyrolysis gas a solid carbon-containing
residue accumulates which also contains mineral and metallic portions
4
secondary treatment of pyrolysis gas and pyrolysis coke, through condensation of the
gases for the extraction of energetically usable oil mixtures and/or incineration of gas
and coke for the destruction of the organic ingredients and simultaneous utilisation of
energy.
Figure 2.18: Structure of a pyrolysis plant for municipal waste treatment
Source [1, UBA, 2001]
In general, the temperature of the pyrolysis stage is between 400 °C and 700 °C. At lower
temperatures (approx. 250 °C) other reactions occur to some extent. This process is sometimes
called conversion (e.g. conversion of sewage sludge).
In addition to the thermal treatment of some municipal wastes and sewage sludge, pyrolysis
processes are also used for:
•
•
•
decontamination of soil
treatment of synthetic waste and used tyres
treatment of cable tails as well as metal and plastic compound materials for substance
recovery.
Waste Incineration
57
Chapter 2
The potential advantages of pyrolysis processes may include:
•
•
•
•
possibility of recovering the material value of the organic fraction e.g. as methanol
possibility of increased electrical generation using gas engines or gas turbines for generation
(in place of steam boilers)
reduced flue-gas volumes after combustion, which may reduce the FGT capital costs to
some degree
the possibility of meeting specifications for external use of the produced char by washing
(e.g. chlorine content).
[64, TWGComments, 2003] [74, TWGComments, 2004]
2.3.4.3.1
Example of a pyrolysis process
[2, infomil, 2002]
In this example, solid industrial sludges and shredded paint waste/chemical packaging are
treated.
The ‘pyrolysis’ unit is combined with a thermal treatment plant for polluted soil, in which
synthesis gas (syngas) from the pyrolysis unit is used as fuel. The pyrolysis unit consists of two
parallel reactors. Both are equipped with screws, which transport the feed material through the
reactors. Feed materials include the filter cake and sediment of other on-site process waste water
treatment facilities, as well as paint waste. The average organic material content varies between
25 – 85 %, and the average water content is approx. 25 %.
At start-up, the reactors are heated up with natural gas to approx. 500oC. Then feeding starts and
the use of natural gas is stopped. The amount of air is kept below stoichiometric demand,
resulting in a gasification process. Gasification temperature is approx. 900 – 1200oC. The
capacity of the reactors is approx. 2 x 4 tonnes/hour.
The syngas is cooled down in a quench condenser. Remaining syngas (LHV approx. 7 MJ/Nm³)
is used as fuel in another unit for the thermal treatment of polluted soil. Incineration and fluegas treatment takes place according to Dutch emission standards. The condensed water of the
quench is treated in a decanter for the separation of carbon. The water fraction is used for
moisturising the reactor residues.
The residue of the reactor (temperature level approx. 500oC) passes a magnetic separation
system for removal of the iron from the paint waste and the packaging fraction. The remaining
fraction is cooled down and moisturised with condensed water, for disposal to landfill.
58
Waste Incineration
Chapter 2
A general process scheme, including the main mass flows is given in the Figure below:
Kiln TRI
Afterburner
TRI
Water
Quench
Waste water
Decanter
Decanter
residue
Overflow
tank
Pyrogas
Air
Start up
burner
Natural gas
Residue
Waste
input
Mixer
Cooler/
mixer
Dryer
Magnet
Cooler
Waste
input
Metal
scrap
Shredder
Figure 2.19: Process scheme of ATM’s ‘pyrolysis’-unit
Source [2, infomil, 2002]
The main advantage of this pyrolysis unit is, that the surplus LHV, present in the treated filter
cake, sediment and paint waste can be directly used in the thermal treatment unit for polluted
soil. Energy efficiency, therefore, is at least comparable with waste incineration. Furthermore,
the iron scrap fraction (15 %) is removed for recycling, while the volume of the treated waste is
reduced by approx. 50 %. The remaining residues can partly be treated in ATM’s own facilities.
Overhead costs are reduced by the fact that it uses the incinerator and flue-gas treatment of a
large polluted soil and waste treatment plant.
2.3.4.3.2
Example of pyrolysis in combination with a power plant
[1, UBA, 2001]
In this example the pyrolysis unit is designed to be added to an existing power plant. It consists
of two lines of drum-type kilns with a scheduled annual municipal waste throughput of
50000 tonnes each. The existing boiler unit will be supplied at full load with up to 10 % of the
furnace thermal output from pyrolysed substitute fuels.
Specification of the ConTherm plant:
Heating in the absence of oxygen, to approx. 500 °C in an indirectly heated drum-type kiln
plant, thermally decomposes the prepared waste fuels. The organic components are broken
down into gaseous carbohydrates. Coke, pyrolysis gas, metals and inert materials are produced.
Waste Incineration
59
Chapter 2
The metals in the fed waste, are now present in their metal form and can be withdrawn in a state
of high purity. For this purpose there is a reutilisation plant at the end of drum-type kilns where
the solid residue is separated into individual fractions. The residue is separated into a coarse
fraction (metals, inerts) and a fine fraction. 99 % of the carbon is contained as coke in the fine
fraction. After sifting, the coarse fraction is supplied to a wet ash remover, cooled and separated
into ferrous and non-ferrous metals in a reprocessing plant.
The thermal energy is emitted through the furnace shell by radiation and to a lesser degree by
convection to the waste within the drum-type kiln. The pyrolysis drum-type kiln is designed for
the waste to be heated to approx. 450 to 550 °C and gasified within one hour.
The resulting pyrolysis gas consists of:
•
•
•
•
•
vaporised water
carbon monoxide
hydrogen
methane
high-order carbohydrates.
A cyclone de-dusts the pyrolysis gas. The deposited dusts and carbon particles are added to the
pyrolysis coke.
Integration of the ConTherm plant into the power plant:
The power plant has a maximum furnace thermal output of 790 MW. In addition to the regular
fuels: coal, coke and petroleum coke, pyrolysis coke and pyrolysis gas can also be used.
The coke is first fed into the coal bunkers, ground together with the coal and then blown into the
boiler with dust burners. The incineration of the pyrolysis product runs at temperatures of
approx. 1600 °C. During the incineration, the organic agents are transformed into CO2 and
water. Due to the high ratio of sulphur to chlorine in the crude flue-gas, and because of the
cooling to approx. 120 °C, any new formation of dioxins is prevented. All toxic agents that have
not changed into their gaseous phase are bound into the melting chamber granulate together
with the recycled airborne dust and the ground inert material.
Energy balance and weight assessment:
The energy and mass balance of the ConTherm plant are illustrated in the following diagram:
18.0 GJ
1000 kg Substitute fuel mixture
(waste material rich in
calorific value)
- Schredder light fraction
- BRAM
- DSD
- Industrial waste
700 kg Pyrolyse gas to
boiler firing system
17.7 GJ
Heat loss
0.2 GJ
Cyclone
Pyrolyse drum-type kiln
60 kg Inert materials (stones,
glass) to reprocessing
50 kg Natural gas
2.0 GJ
1200 kg Combustion air
2250 kg Sum total
60 kg Metals to reprocessing
Air preheating
180 kg Pyrolyse coke to
boiler firing system
1.8 GJ
1250 kg Waste gas drum-type
kiln heating to chimney
0.3 GJ
2250 kg Sum total
20.0 GJ
20.0 GJ
Figure 2.20: Energy balance and weight assessment of the ConTherm plant
Source [1, UBA, 2001]
60
Waste Incineration
Chapter 2
Depending on the calorific value of the RDF (e.g. 15 - 30 MJ/m³) it is possible to reduce
primary fuel such as coal in the range of 0.5 to 1.0 tonne of hard coal per tonne of RDF.
Data on emissions to air were not supplied.
Costs:
Due to the connection of the pyrolysis plant to a coal-fired power station and the utilisation of
the pyrolysis products in the power station, new installations (and hence capital costs) are
limited to:
•
•
•
waste reception and storage (bunker)
the drum-type kiln system with the required heating installations, and
the reprocessing system for valuable substances.
The power plant shares the incineration unit, waste heat utilisation system, flue-gas cleaning
system and the chimney. Using the process equipment, machinery and infrastructure of the
power plant results in reduced investment costs and hence reduced interest payments. In
addition, staff, operation and maintenance costs are also reduced. Thus, disposal costs per tonne
of waste are also reduced, and may be below those of standalone incineration plants.
[1, UBA, 2001]
2.3.4.4 Combination processes
This term is used for processes consisting of a combination of different thermal processes
(pyrolysis, incineration, gasification).
2.3.4.4.1
Pyrolysis – incineration
[1, UBA, 2001]
The following techniques are at various stages of development:
1. Pyrolysis in a drum-type kiln with subsequent high temperature incineration of pyrolysis
gas and pyrolysis coke. In Germany, the full commissioning of a plant of this type was not
completed.
2. Pyrolysis in a drum-type kiln, followed by condensation of the gaseous tars and oils,
subsequent high-temperature incineration of pyrolysis gas, pyrolysis oil and pyrolysis coke.
3. Pyrolysis on a grate with directly connected high-temperature incineration.
The solid residues from these processes are granular, which can be advantageous for later
reutilisation or disposal. Sewage sludge (dehydrated or dried) may be co-treated with the
municipal waste fractions.
Process number 2 (above) is similar to process number 1 in principle, but differs in two main
aspects:
•
•
the pyrolysis gases are cooled on leaving the drum-type kiln, to deposit oil, dust and water
this is followed by oxidative high-temperature treatment in a special aggregate furnace,
where the pyrolysis products, oil-water-dust mixture, pyrolysis coke and pyrolysis gas are
combusted, and the solid residues are transformed into a liquid melt.
Waste Incineration
61
Chapter 2
Figure 2.21: Pyrolysis on a grate with directly connected high-temperature incineration
Source [1, UBA, 2001]
Pyrolysis on a grate with directly connected high-temperature incineration (see Figure 2.21) was
developed from conventional grate incineration but with the objective of producing a liquid
melt. The wastes are first pyrolysed on a grate by direct heating. This heat originates from a
partial incineration of the pyrolysis gases with pure oxygen. In a second step, the products,
pyrolysis gas, coke and inert substances are combusted or melted, respectively, at high
temperatures in a directly connected drum-type kiln. The accumulating melt residue contains
glass, stones, metals and other inert materials and is different from the corresponding product of
process 1 above.
Figure 2.22: The RCP process
Source [1, UBA, 2001]
62
Waste Incineration
Chapter 2
The RCP process (see Figure 2.22) is a development of the pyrolysis on a grate with directly
connected high-temperature incineration process. The molten bottom ash is depleted of metallic
components and upgraded to a cement additive in a special secondary treatment stage. In
Germany, the RCP process concept is now being applied for the first time on an industrial scale
at a plant with a throughput of 90000 tonnes/yr (investment costs approx. EUR 88 million)
connected to an existing incineration plant for municipal wastes at Bremerhaven.
The flue-gas cleaning techniques applied for the three pyrolysis combination processes named
above do not, in principle, differ from the systems used in municipal waste incineration plants.
The same residues and reaction products accumulate. Their type and composition mainly
depend upon the system of flue-gas cleaning selected. However, in contrast to municipal waste
incineration, filter dusts can be recycled into the melting chamber.
Example pyrolysis – combustion installation for clinical wastes in the Netherlands:
[2, infomil, 2002]
The non-specific clinical waste is collected regularly from hospitals and other health care
institutes, including doctors, dentists and veterinarians. The waste is collected in special 30 or
60 litre bins, which have been filled at the institutions and which do not need to be opened
again. The waste is then incinerated, including the bins, which also act as an auxiliary fuel.
The non-clinical waste from hospitals and health care institutions is collected and treated as
normal municipal waste.
The collected waste is stored in closed transport containers on-site. The bins are collected and
transported semi-automatically to the incineration unit, which is located in a closed building.
Feeding the incinerator is through an air lock, in order to prevent the introduction of false
incineration air.
Incineration takes place in a two-stage process (see Figure 2.23). In the lower incineration room,
a controlled pyrolysis occurs, followed by incineration with primary air as the waste progresses
through the room. Finally, the waste ends in a water-filled ash discharger, from which the ash is
removed by a chain conveyer system.
The flue-gases are incinerated with secondary air and, if required, with auxiliary fuel at a
temperature level of approx. 1000 °C. Subsequently, they are cooled in a saturated steam boiler
(steam temperature 225 °C, pressure 10 bar), a heat-exchanger, and a scrubber. Steam is
supplied to the adjacent municipal waste incineration plant which uses the steam and returns the
related boiler feed-water.
The scrubber is a two-stage system for removing acid compounds. The treated flue-gas is heated
up (in a heat-exchanger and in a steam-flue-gas heat-exchanger) before passing a dust bag filter
with adsorbent injection (activated carbon and lime), for removal of dioxins, and an SCR-De
NOX unit. Emission concentrations of the emitted flue-gases are according to Dutch standards.
The flue-gas is emitted through a 55-metre high stack.
Waste Incineration
63
Chapter 2
Figure 2.23: Example of a clinical waste pyrolysis-incineration plant, ZAVIN, Netherlands
Source [2, infomil, 2002]
2.3.4.4.2
Pyrolysis – gasification
[1, UBA, 2001]
Two different types of pyrolysis-gasification processes can be distinguished:
•
•
disconnected (pyrolysis with subsequent gasification = conversion process) and
directly connected processes.
Conversion process:
In the conversion process, metals and, if required, inert material may be removed after the
pyrolysis step. As pyrolysis gas and pyrolysis coke require reheating in the gasification process,
the technical and energetic requirements are higher than with connected processes. The
condensed exhaust vapour is treated as waste water and discharged.
In the conversion process, the waste needs to be shredded and dried before it can be used in the
first thermal stage. This stage more or less corresponds with that of the Smoulder-burn process.
The subsequent stages are:
•
•
•
•
pyrolysis in the drum
withdrawal of solid residues
separation of the fine fraction enriched with carbon
sorting of the metal and inert fraction.
The pyrolysis gas is cooled to condense exhaust vapour and pyrolysis oil. It is then supplied,
together with the pyrolysis oil and the fine fraction, to the second thermal stage, which is a
current flow gasifying reactor. The oil and the fine fraction are gasified in the current flow at
high pressure and at a temperature of 1300 °C. The resulting synthesis gas is cleaned and then
combusted for energy recovery. Solid residues are withdrawn as melted granulate through a
water bath. They correspond in type and quantity with those from the Smoulder-burn process.
A conversion plant for the treatment of 100000 tonnes/yr of municipal wastes and
16000 tonnes/yr of dehydrated sewage sludge was approved at Northeim, Lower Saxony (D).
With direct connection, there may be improved electrical generation rates, but the metals and
inert material go into a melt for which no use has been found to date.
64
Waste Incineration
Chapter 2
Combined gasification-pyrolysis and melting process:
In such processes, (see Figure 2.24) the un-shredded wastes are dried in a push furnace and
partially pyrolysed. From this furnace they are transferred directly and without interruption into
a standing packed-bed gasifier. Here they are gasified (in the lower part) at temperatures of up
to 2000 °C with the addition of oxygen. Pure oxygen is also added in the upper part of the
gasification reactor to destroy the remaining organic components in the generated synthesis gas,
through oxidation, gasification and cracking reactions.
Although reported to be capable of treating a wider range for wastes, this process is mainly used
for municipal and non-hazardous industrial wastes. Wastes of LCV 6 - 18 MJ/kg and moisture
content up to 60 % may be treated. Automotive shredder residues with a chlorine content of up
to 3.5 % have been treated with approximately equal amounts of MSW [69, Thermoselect,
2003].
The synthesised gas is subjected to a gas cleaning process and then combusted to utilise the
energy value. The originally solid residues leave the reactor molten. During test operations,
approx. 220 kg of bottom ash with approx. 30 kg metal accumulated per tonne of waste input.
Figure 2.24: Schematic diagram of a push pyrolyser (example shown operated by Thermoselect)
Source [1, UBA, 2001]
A plant of this type with a municipal waste throughput of 108000 tonnes/yr is currently under
construction at Ansbach. Another plant with a throughput of 225000 tonnes/yr has been built at
Karlsruhe (D), but has not yet achieved the design throughput. Two plants of this type are
operated in Japan (2003).
Waste Incineration
65
Chapter 2
2.3.4.4.3
Gasification – combustion
An example for the combination of gasification with combustion for ash melting is shown in
Figure 2.25 below:
Figure 2.25:Combined fluidised bed gasification and high temperature combustion process
Source [68, Ebara, 2003]
Shredding residues, waste plastics or shredded MSW is gasified in an internally circulating
bubbling fluidised bed, which is operated at about 580 °C. Larger inert particles and metals are
discharged at the bottom and separated from the bed material. The bed material is returned to
the gasifier. Fine ash, small char particles and combustible gas is transferred to the cyclonic ash
melting chamber, where air is added to achieve the desired temperature for ash melting
(normally 1350 - 1450º C).
The ash melting chamber is an integrated part of the steam boiler, for energy recovery.
Products from this process – besides power or steam – are metals in pieces, a vitrified slag (low
leaching and stable) and metal concentrates derived from the secondary ash.
Different from other gasification processes, this process is operated at atmospheric pressure and
with air rather than oxygen. Pretreatment of MSW by shredding is necessary to reduce particle
size to 300 mm diameter. Wastes already within this specification can be treated without
shredding. In the various plants in operation, other wastes like sewage sludge, bone meal,
clinical waste and industrial slags and sludges are treated in addition to MSW. [68, Ebara, 2003]
66
Waste Incineration
Chapter 2
2.3.5 Other techniques
2.3.5.1 Stepped and static hearth furnaces
Static hearth furnaces consist of a refractory lined box in which the wastes burned on the base of
the furnace, often with the injection of support fuels above the burning waste to help maintain
temperatures. In some cases the waste loading mechanism is a simple door opening (although
this is not common in modern plants due to the instability caused to the incineration process by
the uncontrolled ingress of air that results) or is provided by a hydraulically operated ram,
which also provides a measure of waste agitation. Such processes often operate on a batch basis,
with de-ashing carried out in between batch loading. De-ash mechanisms are usually fairly
simple drag systems – in older, smaller units de-ashing was carried out manually using scrapers,
although this causes difficulties with air ingress to the furnace. Such, very basic technology has
been widely applied, particularly to small incineration units (<250 kg/hr) but is less widely
applied owing to the application of new air emission, ash burn-out etc legislation, which such
systems cannot meet in the majority of circumstances. Such systems have been used in some
cases to provide a means for the disposal of dead animals, animal parts, packaging wastes and
some clinical wastes – but generally only at the low throughput rates noted above.
Stepped hearth systems are a development from static hearths. They consist of a usually 2 to
4 static hearths arranged as a series of steps. The waste is generally pushed forward through the
furnace and over the steps using hydraulic rams. The pushing and tumbling of the waste
provided agitation and allows improved burnout. Such systems continue to be applied,
particularly at plants of below 1 t/hr. Loading mechanisms are generally air sealed hoppers or
hydraulic batch loaders. De-ashing is generally continuous, and maybe via a water batch to
provide an air seal and prevent air ingress to the furnace. Such systems are capable of reaching
modern legislative requirements with some waste types. Burnout of the waste may be variable
and highly dependent of the waste type – pretreatment of the waste by shredding usually assists
in reaching required burnout standards.
2.3.5.2 Multiple hearth furnaces
Multiple hearth incinerators are mainly applied to the incineration of sludges (e.g. sewage
sludge).
The multiple hearth furnace (see Figure 2.26) consists of a cylindrical lined steel jacket,
horizontal layers, and a rotating sleeve shaft with attached agitating arms. The furnace is lined
with refractory bricks. The number of trays for drying, incineration, and cooling is determined
based on the residual material characteristics. The multiple hearth furnace is also equipped with
a start-up burner, sludge dosing mechanism, circulation-, sleeve shaft- and fresh air - blowers.
Sewage sludge is fed at the top of the furnace and moves downwards through the different
hearths countercurrent to the combustion air, which is fed at the bottom of the furnace. The
upper hearths of the furnace provide a drying zone, where the sludge gives up moisture while
the hot flue-gases are cooled.
Waste Incineration
67
Chapter 2
1
2
3
4
5
6
7
8
9
10
11
Sludge supply
Auxiliary fuel
Atmospheric oxygen
Waste gas
Ash cooling air
Cool air
Ash
Multiple hearth furnace
After-burn chamber
Start-up incineration chamber
Circulation blower
6
1
9
4
11
8
3
10
3
7
5
6
M
7
Figure 2.26: Principle function of a multiple hearth furnace
Source [1, UBA, 2001]
The material to be incinerated is supplied at the highest furnace layer. It is captured by agitator
sprockets, divided, and forced through the furnace layers through constant rotation. In a counterdirection to the sludge, hot flue-gas is conducted from the highest incineration layer via the
drying layers. The sludge is dried by the flue-gas and heated to ignition temperature. The
circulating air is augmented with steam and volatile particles during the drying process. It is
then lead towards the lowest incineration layer.
The incineration mainly takes place on the central hearths. The incineration temperature is
limited to 980 °C, as above this temperature the sludge ash fusion temperature will be reached
and clinker will be formed. In order to prevent leakage of hot toxic flue-gases, multiple hearth
furnaces are always operated at a slight vacuum pressure.
The conversion of organic sludge particles into CO2 and H2O occurs at temperatures of between
850 and 950 °C. If the desired incineration temperature cannot be reached independently, a
start-up burner is used for support incineration. As an alternative, solid auxiliary fuel can be
added to the sludge. The ash is cooled to approximately 150 °C at the lower layers of the
furnace with counter-flowing cool air and the ash is removed via the ash system. The flue-gas
that is produced is fed through a post-reaction chamber with a guaranteed residence time of two
seconds. Carbon compounds that have not been converted are oxidised here.
The multiple hearth furnace is employed with sludge where the ash forms such low eutectics
with the fluidised bed material that it would cause operational problems in the fluidised bed
furnace.
Multiple hearth furnaces can be operated by removing the flue-gases at the highest drying level
and then feeding them to an post-combustion (e.g. in an incineration chamber). This is
advantageous at such locations where boiler plants are already available, facilitating the feeding
of flue-gases into those plants. The after-burning process and the flue-gas cleaning occur at
those plants.
68
Waste Incineration
Chapter 2
The essential operational parameters are shown in the following table:
Operational parameters
Evaporation capacity
Heat conversion in incineration layers
Incineration end temperature
Residence time, free space, and after-burn zone
Atmospheric oxygen preheating
Units
kg/m2h
GJ/m2h
°C
sec.
°C
Values
25 – 45
0.4 – 0.6
850 – 950
min. 2
max. 600
Table 2.8: Operational criteria for a multiple hearth furnace
Source [1, UBA, 2001]
Figure 2.27 below shows a practical example of a sewage sludge incineration plant with a
capacity of 80000 tonnes/yr.
VA1,2,4
M
Sludge
bunker
Sludge transport
Liquid
residuals
Doubledeck
Process cooler
furnace
After-burner
chamber
Ash discharge
Rotation
washer
Quench
Flow absorber
Jet washer
Chimney
Suction draft
Dust discharge
Figure 2.27: Example of a sewage sludge incineration plant with a multiple hearth furnace
Source [1, UBA, 2001]
The above plant essentially consists of the following parts:
•
•
•
•
multiple hearth furnace
post-combustion chamber
waste water boiler for heat utilisation
multiple stage flue-gas cleaning.
The accumulated sewage sludge is conditioned, meaning that it is converted into a form suitable
for filtering using additives or other measures. The sludge is drained as much as possible in
chamber filter presses and then temporarily stored in a bunker. From there, the press cake is
deposited in buckets via a bucket loader. These buckets have a capacity of approximately
1.5 tonnes each. The sludge is loaded from the buckets into a filler container at the highest layer
of the incineration plant and continuously fed into the furnace. Up to 12 tonnes of sewage
sludge can be processed per hour. This represents the contents of eight buckets.
Waste Incineration
69
Chapter 2
2.3.5.3 Multiple hearth fluidised bed furnace
Several layers are installed into the freeboard of a stationary fluidised bed, enabling the sludge
to be pre-dried with flue-gas. Using this pre-drying process, only a small amount of water must
be evaporated in the actual fluidised bed, meaning that the grate surface and entire furnace can
be reduced.
Uniform incineration is promoted in the multiple hearth fluidised bed furnace by optimising air
supply, sand addition, and evaporation in the layers and in the fluidised bed. Higher
temperatures (temperature differences between the furnace head and foot) can be avoided
leading to a lower formation of NOX.
1
2
3
4
5
6
7
8
9
10
11
12
13
Sludge supply
Auxiliary fuel
Atmospheric oxygen
Waste gas
Cool air
Pre-dried zone
Incineration zone
Fluidized bed
After-burner chamber
Start-up incineration chamber
Circulation blower
Inspection glass
Air preheater
5
1
5
13
4
6
9
11
3
7
12
8
2
10
Figure 2.28: Principle function of a multiple hearth fluidised bed furnace
Source [1, UBA, 2001]
2.3.5.4
Modular systems
[Bontoux, 1999 #7]
Waste incineration can occur in a selective manner in smaller facilities that are dedicated to:
•
•
specific kinds of wastes, or
specifically pretreated wastes.
These specialised forms of waste incineration are often performed in commercial or industrial
tailor-made facilities that usually receive consistent waste streams. As a result, they usually
benefit from optimised operating conditions and treat a much smaller tonnage of waste than
mass burn facilities.
One of the designs used is the “starved air” or “two-stage” incinerator in which wastes are
partially burned and partially pyrolysed at the front end of a hearth with the resulting char being
fully burned out at the back end.
70
Waste Incineration
Chapter 2
Depending on the furnace design, various wastes are treated in such systems. (Energos 2002) As
well as dealing with specific industrial non-hazardous waste streams (e.g. packaging and paper
wastes, fish industry) modular semi-pyrolytic processes are also successfully applied to
pretreated (shredded) municipal wastes. Plants in the range of 35000 - 70000 tonnes per year are
operational in Europe. It is reported that these achieve NOX emissions below 100 mg/m³,
without specific NOX abatement, mainly through careful attention to combustion design and
control. Whilst costs per unit disposal for mass burn facilities of this size are generally very
high, the cost of systems dealing with specific waste streams is greatly reduced through a
combination of:
•
•
simple small scale gas cleaning systems may be used as flue-gas variation is reduced
positioning of plants adjacent to heat users to increase energy supply and income which can
then offset incoming disposal costs.
2.3.5.5 Incineration chambers for liquid and gaseous wastes
Incineration chambers are designed specifically for the incineration of liquid and gaseous
wastes, as well as solids dispersed in liquids (see Figure 2.29) A common application of
incineration chambers is in the chemical industry for the incineration of liquid and process offgas. With chloride-containing wastes, HCl may be recovered for use.
All post-combustion chambers in hazardous waste incineration plants are essentially
incineration chambers. In one plant (Ravenna, Italy) the post-combustion chamber is so large
that the total thermal process can occur there.
Operational temperatures are usually chosen to ensure good destruction of the wastes fed to the
chamber. In some cases catalytic systems are used for specific waste streams, these run at
reduced temperatures of 400 – 600 °C. In general, temperatures in excess of 850 °C are selected
for non-catalytic chambers. Support fuels are frequently used to maintain steady combustion
conditions. Heat recovery may be used to supply hot water/steam via a boiler system.
Vapor vents
Vapor vents
Incineration air (secondary)
Air purge
TE TE
Camera
Burner
Liquids, vapors, support fuel
Atomizing media
Windbox
Sight
port
Pilot fuel
Air
Igniter
aspirator
900 - 1500 C
Incineration
chamber
I
To
boiler
or
quench
Refractory
200 C
Incineration air (primary)
Figure 2.29: Principle of an incineration chamber for liquid and gaseous wastes
Source [1, UBA, 2001]
Waste Incineration
71
Chapter 2
2.3.5.6 Cycloid incineration chamber for sewage sludge
The cycloid incineration chamber was originally developed for incinerating old coke derived
from flue-gas cleaning at waste incineration plants but is now also used for the thermal disposal
of sewage sludge. The optimal particle size for fuel ignition lies between 1 and 5 mm.
Therefore, only dried sewage sludge granules can be used.
The fuel granules are supplied gravimetrically via a radial chute into the lower part of the
incineration chamber, which is designed as a metallic air-cooled hopper. Atmospheric oxygen is
blown into the incineration chamber at various air levels: The primary air enters the furnace at
an angle through the lower part of the hopper, and the secondary air is injected on different
levels through tangentially placed jets above the fuel feed. The distribution of primary and
secondary air varies according to the specific fuel characteristics.
The incineration of sewage sludge requires an even temperature distribution of between 900 and
1000 °C throughout the entire incineration chamber. Using this method, the temperature of the
ash is maintained under its softening point. Flying dust is removed along with flue-gas from the
incineration chamber. The coarse kernels circulate in the tangential flow field until they are
incinerated to the point that they can be removed as fine kernels. Crude ash, remaining coke, or
metallic parts will be removed in a downward direction via a lock system.
Figure 2.30: Illustration of a cycloid furnace
Source [1, UBA, 2001]
2.3.5.7 Example of process for the incineration of liquid and gaseous
chlorinated wastes with HCl recovery
[1, UBA, 2001] The process includes:
•
•
•
•
the incineration chamber
steam generator
flue-gas cleaner combined with hydrochloric acid recovery and
the flue-gas chimney (see Figure 2.31).
The plant treats liquid and gaseous chlorinated wastes using waste heat and produces
hydrochloric acid.
Heat is converted into steam in the steam generator (212 °C, 20 bar) and transferred, for
distribution. The particulate content of the flue-gases produced during incineration is separated,
to produce the highest possible concentration of hydrochloric acid in the flue-gas cleaning plant.
The removal and utilisation of hydrochloric acid normally occurs within the plant.
72
Waste Incineration
Chapter 2
Gaseous residual substances (flue-gases) are fed to the recovery plant via transfer pipelines.
Each flue-gas flow is conducted through a separate deposit container before incineration. Liquid
particles are separated from the flue-gas flow in this deposit container. The feed lines are
equipped with the appropriate flashback safety guards, according to the classification of the
flue-gases. The number of feed lines depends on the control mechanisms. The volume flow is
collected via flow measurements that are pressure and temperature compensated. The flue-gases
are fed into the incineration chamber via a pressure regulator with a maximum pressure limit
control. In addition, all flue-gas lines to the incineration chamber are equipped with automatic
emergency shutdown valves.
BURNER
INCINERATION
CHAMBER
FURNACE
PIPE BOILER
ACID
WASHER
ALKALINE
WASHER
HCl
WASTE
WATER
SUCTION
DRAFT
CHIMNEY
WASTE
GAS
LIQUID
NATURAL
GAS
STEAM
Figure 2.31: Diagram of a plant for HCl-extraction from residual gases and liquid halogenated
wastes
Source [1, UBA, 2001]
Transfer pipelines for the liquid wastes are also equipped with automatic emergency shutdown
valves. All liquid wastes are conducted to a multi-material burner that is situated at the front
side of the incineration chamber. Vaporisation of these liquids occurs via pressured air and/or
steam that have been fed into the burner under a separate gas quantity control. In addition,
various flue-gas flows are fed into the multi-material burner through lances. Each of these
lances consists of concentric pipes. Several flue-gas flows can thus be fed separately into the
incineration chamber. For cooling and to avoid corrosion, the lances are continuously sprayed
with air through the outer circular gap.
Primary energy (natural gas) is required for the plant start-up and to maintain the desired
temperature in the incineration chamber. It is also fed to the multi-material burner by a separate
blast connection. The flow of natural gas is regulated via a quantity control and is fed into the
burner using a pressure regulator depending on the temperature in the incineration chamber.
Natural gas is also required for the ignition flame that ignites the multi-material burner. Two
automatic emergency shutdown valves with automatic gap releases can be found in the natural
gas line to the multi-material burner and to the ignition flame.
Two independent flame-failure alarms (UV and IR) are installed to monitor the burner flame. In
addition, the burner flame can be observed through inspection windows and with the help of a
television camera installed on the back wall of the waste heat boiler. The amount of air is
recorded with the appropriate gauges, as well as with pressure produced from a blower.
Waste Incineration
73
Chapter 2
The cylindrical incineration chamber is designed in such a way that the wastes will have
sufficient residence time to guarantee flawless incineration in relation to an operational
temperature higher than 1100 °C during normal operation. The incineration chamber has been
designed for a temperature of 1600 °C. The operational temperature is monitored continuously
by thermal elements. Based on this high temperature, the whole incineration chamber, up to the
entrance to the steam boiler plant, is lined exclusively with refraction bricks. The incineration
chamber shell is made of boiler plate. The wet cleaning of the flue-gases occurs in two wash
towers with a simultaneous recovery of technically re-usable hydrochloric acid with the highest
concentration. The deployment of chlorinated wastes facilitates the recovery of approximately
5 – 20 % hydrochloric acid.
2.3.5.8 Example of a process for the incineration of highly chlorinated liquid
wastes with chlorine recycling
[2, infomil, 2002]
This incineration unit for highly chlorinated liquid wastes (chlorinated hydrocarbons) is located
on an industrial site. The total plant capacity is approx. 36000 t/yr. The processed waste
originates on site, as well as from external customers. Wastes are limited in their content of
solids (<10g/kg), fluorine, sulphur and heavy metals. PCBs are also treated.
Incineration takes place in two furnaces at a temperature level of 1450 – 1550 °C (gas residence
time 0.2 – 0.3 sec). This temperature level can normally be maintained without auxiliary fuel.
Water is injected in order to suppress the formation of Cl2. After leaving the furnace, the fluegas passes through a quench section, where the temperature is lowered to approx. 100 °C.
Insoluble matter and heavy metal salts are removed from the circulating liquid in a quench tank.
The flue-gas continues through an isothermal and an adiabatic absorber. The recuperated
hydrochloric acid is distilled at elevated pressure and temperature, after which the gas is cooled
down to –15 °C in order to reduce the water content to practically zero. The recovered
anhydrous HCl is reprocessed in a vinyl-chloride-monomer plant.
Flue-gases pass through an alkaline scrubber and an activated carbon filter (for dioxin
absorption). TOC, HCl NOX, O2, CO and dust are continuously analysed. The concentration of
dioxins and PCBs in emissions is below 0.1 ng TEQ/Nm3. Other emissions to air comply with
Dutch emission limit values.
The effluent from the quench and the scrubber unit is treated in a physical/chemical unit and in
a biological waste water treatment unit. Dioxin content is <0.006 ng TEQ/l. PCBs are below the
detection limit (<10 ng/l).
74
Waste Incineration
Chapter 2
A scheme of the process is given in Figure 2.32.
Anhydrous HCI
To VCM-Plant
Water
NaOH
NaOH
C.W.
Water
C.W.
Steam
C.W.
Natural Gas
Comb. air
CHCbyproduct
UREA
C.W.
To Bio-treatment
Byproduct
acceptance
and storage
Combustion
and Quenching
Isothermal
and adiabatic
absorption
Flue gas
scrubber
Distillation
Drying and
Compression
Waste water
pretreatment
Figure 2.32: Process scheme of a chlorine recycling unit operated by Akzo Nobel
Source [2, infomil, 2002]
The main advantage of this dedicated incineration unit is that chlorine can be recovered. Also in
this case, overhead costs are reduced by the fact that it is part of a larger chemical plant.
2.3.5.9 Waste water incineration
[1, UBA, 2001]
Waste water can be cleaned through incineration of the organic content materials. This is a
special technology for the treatment of industrial waste water where organic and sometimes
inorganic waste water content material is chemically oxidised with the help of atmospheric
oxygen with the evaporation of the water, at high temperatures. The term “gas phase oxidation”
is used to differentiate this type of incineration from other technologies, such as wet oxidation.
The process of gas phase oxidation is used if the organic substances in the water cannot be reused or if their recovery is not economical or another technique is not applied.
Waste water incineration is an exothermic process. Independent incineration can only take place
if the organic load is sufficient to evaporate the water share independently and to perform
superheating. Therefore, waste water incineration plants normally require the use of support
fuels for low organic load wastes. Reduction of the requirement for additional energy can be
achieved by reducing water content. This can be achieved through deployment of a preconnected, or multi-step, condensation plant. In addition, a heat recovery part (boiler) can be
installed to recover steam for condensation from the furnace heat that is produced.
Depending on the individual organic and inorganic content of the waste water and the various
local conditions, very different plant designs result.
Waste water and fuel are injected via burners or lances at several locations within the
incineration chamber. Atmospheric oxygen is also supplied at several locations (primary air =
atmospheric oxygen combined with fuel, secondary air = mixed air).
An example of an waste water incinerator with a waste water evaporation (concentration) unit is
shown in the following figure below [74, TWGComments, 2004]
Waste Incineration
75
Chapter 2
Figure 2.33: Example of a waste water incinerator with a waste water evaporation (concentration)
unit.
Source [1, UBA, 2001]
Example of an installation for the incineration for caustic waste water:
[2, Infomil, 2002]
Caustic water is a specific waste water stream from MSPO plants (Mono-Styrene PropyleneOxide). This water is produced in several washing steps in the process. It contains
approximately 10 % to 20 % organic components and has a high sodium load (mainly NaCl).
Both the high organic fraction and the sodium make it difficult or even impossible to use
biological water treatment. The caloric value of this water is too low for unsupported
incineration, so co-incineration or the use of supporting fuel is necessary. The high sodium
content, together with the large quantities, can cause problems for co-incineration in municipal
waste incinerators.
Applicable treatment technologies are wet oxidation and incineration. For this purpose, four
static vertical incinerators (total capacity approx. 350 – 400 kt/yr) are used in this example,
which have been in operation since 1999/2000.
The incinerators are static vertical top-down incinerators. The low caloric waste (caustic water
with 10 – 20 % organics) can be led through a falling film evaporator. This evaporator operates
on excess low-pressure steam, which comes from the incinerator wall cooling, thus using less
fuel in the incinerator.
The remaining liquid and the produced vapour are incinerated with natural gas and/or high
caloric liquid fuel (waste or fuel oil). The resulting flue-gases are partially cooled by a
membrane wall, producing steam of 27 bar. Subsequently the flue-gases are quenched to clean
the gases of sodium salts and other water soluble impurities.
In the heat recovery section, re-circulation water is sprayed over the flue-gases. This recirculation water flashes out in the flash chamber, generating approximately 30 t/h of steam per
unit.
76
Waste Incineration
Chapter 2
After the heat recovery the flue-gases pass through a venture scrubber and a wet electrostatic
precipitator where aerosols and dust are removed.
The incinerators operate at a temperature of 930 – 950 °C, with low excess air (3 – 4 % O2).
Depending on the concentration of organics, the throughput of caustic water is 10 – 15 t/h per
unit.
The water from the quench is treated in ion-exchange beds to remove heavy metals. Special ionexchange beds concentrate the Molybdenum (catalyst in the MSPO process) to a re-usable
grade.
The main advantage of these incinerators is the possibility to incinerate large quantities of low
caloric waste with high salt concentrations.
The following diagram shows an example plant for this process:
Figure 2.34: Process scheme of a caustic water treatment plant operated by AVR
Source [2, Infomil, 2002]
2.3.5.10
Plasma technologies
Plasma is a mixture of electrons, ions and neutral particles (atoms and molecules). This high
temperature, ionised, conductive gas can be created by the interaction of a gas with an electric
or magnetic field. Plasmas are a source of reactive species, and the high temperatures promote
rapid chemical reactions.
Plasma processes utilise high temperatures (5000 to 15000 °C), resulting from the conversion of
electrical energy to heat, to produce a plasma. They involve passing a large electric current
though an inert gas stream.
Under these conditions, hazardous contaminants, such as PCBs, dioxins, furans, pesticides, etc.,
are broken into their atomic constituents, by injection into the plasma. The process is used to
treat organics, metals, PCBs (including small-scale equipment) and HCB. In many cases
pretreatment of wastes may be required.
Waste Incineration
77
Chapter 2
An off-gas treatment system depending on the type of wastes treated is required, and the residue
is a vitrified solid or ash. The destruction efficiencies for this technology are quite high,
>99.99 %. Plasma is an established commercial technology, however the process can be very
complex, expensive and operator intensive.
Thermal plasmas can be generated by passing a DC or AC electric current through a gas
between electrodes, by the application of a radio frequency (RF) magnetic field without
electrodes, or by application of microwaves. Different kinds of plasma technologies are
introduced below:
1. Argon plasma arc
This is an “in flight” plasma process, which means that the waste mixes directly with the argon
plasma jet. Argon was selected as the plasma gas since it is inert and does not react with the
torch components.
The destruction and removal efficiency (DRE) is reported to exceed 99.9998 % for destroying
ozone depleting substances (ODS) at 120 kg/h and with 150kW electrical power.
The advantage of this technology over some other plasma systems is that it has demonstrated
high efficiency destruction of both CFCs and halons on a commercial scale for several years. It
has also demonstrated low emissions of PCDD/F. Mass emissions of pollutants are also low
because of the relatively low volume of flue-gas produced by the process. Also, the very high
energy density results in a very compact process that may easily be transported.
2. Inductively coupled radio frequency plasma (ICRF)
In ICRF applications, inductively coupled plasma torches are used, and energy coupling to the
plasma is accomplished through the electromagnetic field of the induction coil. The absence of
electrodes allows operation with a large range of gases, including inert, reducing or oxidizing
atmospheres and better reliability than plasma arc processes.
The ICRF plasma process has demonstrated a DRE exceeding 99.99 % while destroying CFC at
a rate of 50 - 80 kg/h.
The process is reported to have been demonstrated on a commercial scale to achieve high
destruction of CFC and low emission of pollutants. The ICRF plasma does not require argon
and may therefore cost less to operate than other similar systems. In addition, the low volume of
gas produced by the process results in low levels of mass emission of pollutants.
3. AC plasma
The AC plasma is produced directly with 60 Hz high voltage power but in other respects is
similar to the inductively coupled RF plasma. The system is electrically and mechanically
simple and is thus claimed to be very reliable. The process does not require argon and can
tolerate a wide variety of working gases, including air, or steam as plasma gases and is claimed
to be tolerant of oil contamination in ODS.
4. CO2 plasma arc
A high temperature plasma is generated by sending a powerful electric discharge into an inert
atmospheric gas, such as argon. Once the plasma field has been formed, it is sustained with
ordinary compressed air or certain atmospheric gases depending on desired process outcomes.
The temperature of the plasma is well over 5000 ºC at the point of generation into which the
liquid or gaseous waste is directly injected. The temperature in the upper reactor is about
3500 ºC and decreases through the reaction zone to a precisely controlled temperature of about
1300 ºC.
78
Waste Incineration
Chapter 2
A special feature of the process is the use of CO2, which is formed from the oxidation reaction,
as the gas to sustain the plasma.
The process has demonstrated high DREs with refractory compounds at a reasonably high
demonstration rate. Mass emission rates of the pollutants of interest are low, primarily because
of the low volume of flue-gas produced by the process.
5. Microwave plasma
This process feeds microwave energy at 2.45 GHz into a specially designed coaxial cavity to
generate a thermal plasma under atmospheric pressure. Argon is used to initiate the plasma but
otherwise the process requires no gas to sustain the plasma.
The DRE for the microwave plasma process is reported to exceed 99.99 % while destroying
CFC-12 at a rate of 2 kg/h.
The process is reported to have a high destruction efficiency and to be capable of achieving the
high operating temperatures in a very short time, thus providing operating flexibility and
reduced downtime.
There is no need for an inert gas to operate the process, which improves the power efficiency,
reduces operating cost, as well as reducing the volume of flue-gas produced. In addition, the
process is very compact.
6. Nitrogen plasma arc
This process uses a DC non-transferred plasma torch operating with water cooled electrodes and
using the nitrogen as the working gas generates the thermal plasma. The process was developed
in 1995 and there are commercial systems available.
The process is reported to achieve a DRE of 99.99 % while destroying CFCs, HCFCs and HFCs
at a feed rate of 10 kg/h.
A key advantage of this technology is that the equipment is very compact in size. The system
requires only 9 m x 4.25 m area for installation, which includes space for a precipitation and
dehydration unit for the by-products (CaCl2 and CaCO3). Therefore, the system is capable of
being carried on a truck to the waste generation spot, leading to an on-site treatment.
2.3.5.11 Various techniques for sewage sludge incineration
Typical process conditions applied to sewage sludge incineration:
In addition to sewage sludge, other wastes from the waste water treatment process are often
incinerated e.g. swim scum, screenings, and extracted fats.
Plants receiving partially dried sludge require less additional fuels than raw sludges. The heat
values of the sludge for auto thermal incineration lie between 4.8 MJ/kg and 6.5 MJ/kg. Values
between 2.2 MJ/kg and 4.8 MJ/kg sludge are seen where raw sewage is treated. Approximately
3.5 MJ/kg sludge is considered the limit for auto thermal incineration. The need for additional
fuel can be reduced by the use of efficient internal energy recovery systems e.g. recovery of heat
form flue-gases to heat incineration air and/or use of heat to provide for sludge drying.
Used oil is the mainly used additional fuel in mono-sewage sludge incinerators. Heating oils,
Natural gas, coal, solvents, liquid and solid waste and contaminated air are also used.
Contaminated gas is preferred for the incineration of digested sludge.
The primary influences on the requirement for additional energy are the air preheating and
degree of drainage needed. The influence of conditioning agents is relatively low.
Waste Incineration
79
Chapter 2
Dedicated sewage sludge incinerators are generally designed and operated at temperatures
between 850 and 950 °C. Temperatures below 850 °C can result in odour emissions, while
temperatures above 950 °C may result in ash fusion. Gas residence times of in excess of
2 seconds are commonly employed.
The temperature level achieved during incineration depends mainly on the energy content and
the amount of sewage sludge to be incinerated and on the atmospheric oxygen level.
There are some examples of sewage sludge incinerators (often fluidised bed processes) that
operate at temperatures closer to 820 °C without a deterioration in incineration performance or
increased emissions.
Comparison of furnace systems for sewage sludge incineration:
The described furnace systems function according to different process technologies. The furnace
structure, design, and operational technology of the incineration plant, the resulting postconnected cleaning equipment, as well as the transport of different material flows, all have a
significant influence on the resulting emissions. The characteristics of the various furnaces are
shown in the following table:
Fluidised Bed
Multiple hearth
Furnace
Furnace
• no
• no separate premechanically
drying is necessary
Main
moveable parts • extensive furnace
features of
and low wear.
structure with
technique
moveable parts
• cooled hollow shaft
• fast start-up
• long heating time,
and shut-down continuous operation
through short
necessary
Operational
heating- and
aspects
cooling times,
intermittent
operation
possible
Possible • agglomeration,
operational
de-fluidisation.
problems
• low air surplus • incineration difficult
required
to control
• complete
• immune to
incineration
fluctuations in loads
Incineration
only above the and coarse material
stage main
fluidised bed
features
Ash content • high
in flue-gas
• via flue-gas
Ash removal flow and sand
removal
• ash
Residues • fluidised bed
material
Multiple hearth
Cycloid Furnace
Fluidised Bed Furnace
• no separate pre-drying • no mechanically
is necessary
moveable parts and
• moveable hollow shaft low wear
• no fluidised bed
• low fluidised bed
material.
volume.
• medium heating- and
cooling time
• comparable to the
fluidised bed
• deployable for a wide
range of wastes
• low
• possible emissions of • maintaining desirable
temperature
organics, movable
parts in the furnace.
• solid material shares
• low air surplus
required
• long and gaseous
• good incineration
shares
control
• short residence times
• incineration completed • variable primary and
within the fluidised bed secondary air supply
• greater immunity to
on several levels.
quality fluctuations in
the sludge than
fluidised bed furnaces.
• high
• high
• directly from the
lowest level
• via flue-gas flow and
sand removal
• ash
• ash
• fluidised bed material
• via flue-gas flow
• crude ash at the
bottom
• ash
• possibly coarse ash
Table 2.9: Comparison of furnace systems for sewage sludge incineration
Source [1, UBA, 2001]
80
Waste Incineration
Chapter 2
2.4 The energy recovery stage
2.4.1 Introduction and general principles
[28, FEAD, 2002]
Combustion is an exothermic (heat generating) process. The majority of the energy produced
during combustion is transferred to the flue-gases. Cooling of the flue-gas allows:
•
•
the recovery of the energy from the hot flue-gases and
cleaning of flue-gases before they are released to the atmosphere.
In plants without heat recovery, the gases are normally cooled by the injection of water, air, or
both. In the majority of cases a boiler is used.
In waste incineration plants, the boiler has two interconnected functions:
•
•
to cool the flue-gases
to transfer the heat from the flue-gases to another fluid, usually water which, most often, is
transformed inside the boiler into steam.
The characteristics of the steam (pressure and temperature) or of the hot water are determined
by the local energy requirements and operational limitations.
The design of the boiler will mainly depend on:
•
•
the steam characteristics
the flue-gas characteristics (corrosion, erosion and fouling potentials).
The flue-gas characteristics are themselves highly dependent upon the waste content. Hazardous
wastes for example, tend to have very wide variations in composition and, at times, very high
concentrations of corrosive substances (e.g. chlorides) in the raw gas. This has a significant
impact on the possible energy recovery techniques that may be employed. In particular, the
boiler can suffer significant corrosion, and steam pressures may need to be reduced with such
wastes.
Similarly the thermal cycle (steam-water cycle) will depend on the objective, for example:
•
•
the highest electrical outputs require the most sophisticated cycles, but
simpler cycles suit other situations e.g. supply of heat.
Water walls (the walls of the combustion chamber are made of water filled heat exchange
pipes - usually with a protective coating of some type) are widely used to cool the combustion
gases in the empty (i.e. of heat-exchange bundles) boiler passes. The first pass generally needs
to be empty as hot gases are too corrosive and particulate matter is too sticky for the effective
use of heat exchange tubes in this area.
Depending on the nature of the waste incinerated and the combustor design, sufficient heat may
be generated to make the combustion process self supporting (i.e. external fuels will not be
required).
The principal uses of the energy transferred to the boiler are:
•
•
•
production and supply of heat (as steam or hot water)
production and supply of electricity
combinations of the above.
Waste Incineration
81
Chapter 2
The energy transferred may be used on-site (thus replacing imported energy) and/or off-site.
The energy supplied may be used for a wide variety of other processes. Commonly heat and
steam are used for industrial or district heating systems, industrial process heat and steam and
occasionally as the driving force for cooling and air conditioning systems. Electricity is often
supplied to national distribution grids and/or used within the installation.
2.4.2 External factors affecting energy efficiency
2.4.2.1 Waste type and nature
The characteristics of the waste delivered to the installation will determine the techniques that
are appropriate and the degree to which energy can be effectively recovered. Both chemical and
physical characteristics are considered when selecting processes.
The chemical and physical characteristics of the waste actually arriving at plants or fed to the
incinerator can be influenced by many local factors including:
•
•
•
contracts with waste suppliers (e.g. industrial waste added to MSW)
on-site or off-site waste treatments or collection/separation regimes
market factors that divert certain streams to or from other forms of waste treatment.
In some cases the operator will have very limited scope to influence the characteristics of the
waste supplied, in other cases this is considerable.
The table below gives typical net calorific value ranges for some waste types:
Input type
Mixed municipal solid
waste (MSW)
Bulky waste
Waste similar to MSW
Residual MSW after
recycling operations
Commercial waste
Packaging waste
Comments and examples
NCV in original substance
(humidity included)
Range GJ/t
Average GJ/t
Mixed household domestic wastes
6.3 - 10.5
9
e.g. furniture etc delivered to MSWIs
Waste of a similar nature to household
waste but arising from shops, offices
etc.
Screened out fractions from composting
and materials recovery processes
Separately collected fractions from
shops and offices etc
10.5 - 16.8
13
7.6 - 12.6
11
6.3 - 11.5
10
10 - 15
12.5
17 - 25
20
11 - 26
18
18 – 23
20
0.5 - 20
9.75
See below
See below
1.7 - 2.5
2.1
0.5 - 1.2
0.8
Separately collected packaging
Pellet or floc material produced from
RDF-refuse derived fuels municipal and similar non-hazardous
waste
Product specific industrial
e.g. plastic or paper industry residues
waste
Hazardous waste
Also called chemical or special wastes
Arising from waste water treatment
works
Sewage sludges
Raw (dewatered to 25 % dry solids)
Digested (dewatered to 25 % dry solids)
Table 2.10: Ranges and typical net calorific values for some incinerator input wastes
Source (Energy sub-group 2003)
82
Waste Incineration
Chapter 2
Figure 2.35: Graph showing recorded variation in waste NCV at a MSWI over 4 years
Waste net calorific value calculation:
When considering the efficiency of any combustion process it is important take into account the
energy flows of the system. With waste incinerators it can be difficult to properly assess
efficiencies owing to uncertainties concerning the calorific value of the main energetic input i.e.
the waste.
There are several calculation methods for the calorific value. Using the example calculation
method outlined below, the following NCV results were obtained for 50 (mainly German)
investigated MSW plants (2001 data):
NCV units
MJ/kg
MWh/tonne
Minimum
8
2.2
Average
10.4
2.9
Maximum
12.6
3.5
Table 2.11: Calculated NCV values for waste treated at 50 European MSWI plants
Source [Energy subgroup, 2002 #29]
Example of a calculation method:
A method allowing a very simple but reliable calculation (+/-5 %) of the NCV of the waste is
shown in the following equation. The losses of heat etc. are taken into account. The data
required for the calculation are generally available at incineration plants and are either measured
or calculated from dimensioning figures such as steam parameters.
NCV
= (1.133 x (mst w/m) x cst x + 0.008 x Tb)/ 1.085 (GJ/tonne)
NCV
= lower calorific value (NCV) of the incinerated waste with mstw/m
where, mst w
mst w
mst x
mx
m
1 (GJ/tonne)
= mst x – (mf x(cf /cst x) x `b)
= amount of the steam produced from the waste in the same time period to mst e.g. per
year (tonne/yr)
= total amount of steam produced in a defined time period e.g. per year (tonne/yr)
= amount of supplementary fuel used in the corresponding time period e.g. per year
(tonne/yr)
= mass of waste incinerated in the defined time period e.g. per year (tonne/yr)
Waste Incineration
83
Chapter 2
cst x
= net enthalpy of steam i.e. enthalpy of steam minus enthalpy of boiler water (GJ/tonne)
cf
= net calorific value of the supplementary fuel that add to steam production (GJ/tonne)
= temperature of flue-gas after boiler at 4 – 12 % O2 in flue-gas (°C)
Tb
0.008 = specific energy content in flue-gas (GJ/tonne x °C).
1.133 and 1.085 are constants derived from regression equations
`b
= efficiency of heat exchange to the boiler (approx. 0.80)
Note: This NCV calculation is only applicable to existing plants and not for the purposes of
dimensioning new plants. It should also be noted that the formula can be applied within an
operating range of 4 – 12 % O2, when the original design point was 7 - 9 % O2. Plants designed
with O2 concentrations outside the range of 7 - 9 % would require the use of modified
coefficients to maintain accuracy.
2.4.2.2 Influence of plant location on energy recovery
In addition to waste quality and technical aspects, the possible efficiency of a waste incineration
process is influenced to a large extent by the output options for the energy produced. Processes
with the option to supply electricity, steam or heat will be able to use more of the heat generated
during the incineration for this purpose and will not be required to cool away the heat, which
otherwise results in reductions in efficiency.
The highest waste energy utilisation efficiency can usually be obtained where the heat recovered
from the incineration process can be supplied continuously as district heat, process steam etc.,
or in combination with electricity generation. However, the adoption of such systems is very
dependent on plant location, in particular the availability of a reliable user for the supplied
energy.
The generation of electricity alone (i.e. no heat supply) is common, and generally provides a
means of recovering energy from the waste that is less dependent on local circumstances. The
table below gives approximate ranges for the potential efficiencies at incineration plants in a
variety of situations. The actual figures at an individual plant will be very site-specific. The idea
of the table is therefore to provide a means to compare what might be achievable in favourable
circumstances. Doubts of calculation methods also make figures hard to compare – in this case
the figures do not account for boiler efficiencies (typical losses ~ 20 %), which explains why
figure approaching 100 % (figures exceeding 100 % are also quoted in some cases) are seen in
some circumstances:
Plant type
Electricity generation only
Combined heat and power plants (CHP)
Heating stations with sales of steam and/or
hot water
Steam sales to large chemical plants
CHP and heating plants with condensation of
humidity in flue-gas
CHP and heating plants with condensation
and heat pumps
Reported potential thermal efficiency %
((heat + electricity)/energy output from the boiler)
17 - 30
70 - 85
80 - 90
90 - 100
85 - 95
90 - 100
Note: The figures quoted in this table are derived from simple addition of the MWh of heat and MWh electricity
produced, divided by the energy output from the boiler. No detailed account is taken of other important
factors such as: process energy demand (support fuels, electrical inputs); relative CO2 value of electricity
and heat supply (i.e. generation displaced).
Table 2.12: Energy potential conversion efficiencies for different types of waste incineration plants
Source [RVF, 2002 #5]
84
Waste Incineration
Chapter 2
The potential efficiencies are dependent on self-consumption of heat and electricity. Without
taking the self-consumption into account, the calculated efficiencies of some facilities can lead
to figures quoted of over 100 %. Distortions of efficiency figures are also common when boiler
heat exchange losses are discounted (i.e. a boiler efficiency of 80 % means that 20 % of the
flue-gas heat is not transferred to the steam, sometimes efficiency is quoted in relation to the
heat transferred to the steam rather than the heat in the waste).
Where there is no external demand for the energy, a proportion is often used on-site to supply
the incineration process itself and thus to reduce the quantity of imported energy to very low
levels. For municipal plants, such internal use may be in the order of 10 % of the energy of the
waste incinerated.
Cooling systems are employed to condense boiler water for return to the boiler.
Processes that are conveniently located for connection to energy distribution networks (or
individual synergistic energy users) increase the possibility that the incineration plant will
achieve higher overall efficiencies.
Waste Incineration
85
Chapter 2
2.4.2.3 Factors taken into account when selecting the design of the energy
cycle
The following factors are reported to be taken into account when determining the local design of
a new waste incineration plant [51, CNIM, 2003]:
Factor to consider
Waste feed
Energy sales possibilities
Local conditions
Combined heat and power
Other
Detailed aspects to consider
•
Quantity and Quality
•
Availability, Regularity, Delivery variation with seasons
•
Prospect of change in both the nature and the quantity of waste
•
Effects of waste separation and recycling.
Heat
•
To communities e.g. district heating
•
To private industries
•
Heat use e.g. process use, heating use
•
Geographical constraints; delivery piping feasibility
•
Duration of the demand, duration of the supply contract
•
Obligations on the availability of the supply i.e. is there another source
of heat when the incinerator is shut down?
•
Steam/Hot water conditions: pressure (normal/minimum), temperature,
flowrate, condensate return or not?
•
Season demand curve
•
Subsidies can influence economics significantly
•
Heat customer holdings in the plant financing i.e. security of supply
contract.
Electricity
•
National grid or industrial network (rare), plant self consumption,
customer self consumption (i.e. in a sewage sludge treating plant)
•
Price of electricity significantly influences investment
•
Subsidies or loans at reduced rates can increase investment
•
Technical requirements: voltage, power, availability of distribution
network connection.
•
Cooling medium selected: air or water
•
Meteorological conditions in time: temperature, hygrometry, (min,
average, max, curves)
•
Acceptability of a "plume" of water vapour (cooling tower)
•
Availability of cold water source: river or sea
- Temperature, quality of water
- Flowrate which can be pumped according to the season
- Permitted temperature increase.
•
Apportionment according to the season
•
Evolution of the apportionment in future.
•
Choice between: Increasing energy output, reducing investment cost,
operational complexity, availability requirements, etc.
•
Acceptable noise level (air coolers)
•
Available space
•
Architectural Constraints.
Table 2.13: Factors taken into account when selecting the design of the energy cycle for waste
incineration plants
Source [51, CNIM, 2003]
86
Waste Incineration
Chapter 2
2.4.3 Energy efficiency of waste incinerators
[Energy subgroup, 2002 #29]
In order to enable a comparison of energy performance between waste incinerators, it is
necessary to ensure that these comparisons are made in a consistent way. In particular it is
necessary to standardise:
•
•
•
assessment boundaries i.e. what parts of the process are included/excluded?
calculation methods
how to deal with different energy inputs and outputs e.g. heat, steam, electricity, primary
fuels, re-circulation of energy produced by the plant, etc.
The sections that follow describe the typical inputs and outputs seen at many waste incinerators.
See also appendix 10.4 for information regarding energy efficiency calculation.
2.4.3.1 Energy inputs to waste incinerators
[Energy subgroup, 2002 #29]
In addition to the energy in the waste, there are other inputs to the incinerator that need to be
recognised when considering energy efficiency of the plant as a whole.
Electricity inputs:
Electrical consumption is usually easily calculated. In situations where economic incentives are
provided to support the production of electrical energy from incineration (e.g. as a renewable
source) there may be a price differential between purchased and exported electricity. Plants may
then choose (for economic reasons) to export all of the electricity generated by the incinerator,
and import from the grid, that which is required to run the incineration process itself. Where this
is the case, the incineration plant will often have distinct electricity flows for input and output.
Steam/heat/hot water inputs:
Steam (heat or hot water) can be used in the process. The source can be external or circulated.
Fuels:
They are required for several uses. For instance, conventional fuels are consumed in order to:
i.
ii.
iii.
iv.
v.
ensure that the required combustion chamber temperatures are maintained (this then
contributes to steam production)
increase the temperature in the combustion chamber to the required level before the
plant is fed with waste (this contributes partially to steam production)
increase the flue-gas temperature (e.g. after wet scrubbers) in order to avoid bag house
filter and stack corrosion, and to suppress plume visibility
preheat the combustion air
heat-up the flue-gas for treatment in specific devices, such as SCR or fabric filters.
When considering the overall efficiency of recovery of energy from the waste, it is important to
note that some of these primary fuel uses can contribute to steam production and others will not.
A failure to consider this may result in misleading efficiency figures due to the incorrect
attribution of energy derived from the burning of primary fuels. For example:
•
•
fuels used in auxiliary burners for i (fully) and ii (partially), will contribute to steam
production (typically around 50 – 70 % of the additional fuel usage), whereas
fuels used for items ii (the remaining 30 – 50 % auxiliary fuel use), iii and v above will not
contribute to steam production.
Waste Incineration
87
Chapter 2
Fuel (e.g. coal/coke) inputs (in addition to the waste) can also be made at gasification plants in
order to produce a syngas with a desired chemical composition and calorific value.
2.4.3.2 Energy outputs from waste incinerators
Electricity:
The electricity production is easily calculated. The incineration process itself may use some of
the produced electricity.
Fuels:
Fuel (e.g. syngas) is produced in gasification/pyrolysis plants and may be exported or
combusted on site with (usually) or without energy recovery.
Steam/hot water:
The heat released in the combustion of waste is often recovered for a beneficial purpose, e.g. to
provide steam or hot water for industrial or domestic users, for external electricity generation or
even as a driving force for cooling systems.
Combined heat and power (CHP) plants provide both heat and electricity. Steam/hot water not
used by the incineration plant can be exported.
2.4.4 Applied techniques for improving energy recovery
2.4.4.1 Waste feed pretreatment
There are two main categories of pretreatment techniques of relevance to energy recovery:
•
•
homogenisation
extraction/separation.
Homogenisation of waste feedstock mixes the wastes received at the plant using the physical
techniques (e.g. bunker mixing and sometimes shredding) outlined elsewhere in this document,
in order to supply a feed with consistent combustion qualities.
The main benefits achieved are the improved process stability that results, which thus allows
smooth downstream process operation. Steadier steam parameters result from the boiler, which
can allow for increased electricity generation. The overall energy efficiency benefits are thought
to be limited but cost savings and other operational benefits may arise.
Extraction/separation involves the removal of certain fractions from the waste before it is sent
to the combustion chamber.
Techniques range from extensive physical processes for the production of refuse derived fuels
(RDF) and the blending of liquid wastes to meet specific quality criteria, to the simple spotting
and removal by crane operators of large items that are not suitable for combustion, such as
concrete blocks or large metal objects.
The main benefits achieved are:
•
•
•
88
increased homogeneity, particularly where more elaborate pretreatment are used (see
comments above for homogeneity benefits)
removal of bulky items – thus the risks of obstruction and thus of non scheduled shut-downs
possible use of fluidised beds or other techniques that could improve combustion efficiency.
Waste Incineration
Chapter 2
Extraction, separation and homogenisation of the waste can significantly improve the energy
efficiency of the incineration plant itself. This is because these processes can significantly
change the nature of the waste that is finally delivered to the incineration process, which can
then allow the incineration process to be designed around a narrower input specification, and
lead to optimised (but less flexible) performance. However, for a wider assessment (beyond the
scope of this document) it is important to note that the techniques that are used in the
preparation of this different fuel, themselves require energy and will result in additional
emissions.
(Note: The scope of this BREF does not extend to recommending the upstream systems that can
influence the combustion characteristics and energy content of the waste received. It does
however recognise that these upstream issues have a key influence on the characteristics of the
waste finally received at the plant and hence what is achievable.)
2.4.4.2 Boilers and heat transfer
Tubular water boilers are generally used for steam and hot water generation from the energy
potential of hot flue-gases. The steam or hot water is generally produced in tube bundles in the
flue-gas path. The envelopment of the furnace, the following empty passes and the space where
evaporator and superheater tube bundles are located are generally designed with water cooled
membrane walls.
In steam generation, it is usually possible to differentiate between the three heat surface areas,
shown in Figure 2.36:
Figure 2.36: Illustration of individual heat surface areas in a steam generator
Source [1, UBA, 2001]
Waste Incineration
89
Chapter 2
Key to some of the features shown in Figure 2.36 (above):
7 Feed-water preheating (Economiser):
In this area, the boiler feed-water is heated by flue-gases to a temperature close to the boiling
point (designed as a bundled heating surface).
6 Evaporation:
In this area, the water coming from the economiser is heated until it reaches the saturated steam
temperature (designed as a bundled heating surface, envelopment wall of the incineration
chamber).
5 Superheating:
In this area, the saturated steam coming from the evaporator is superheated to the end
temperature (as a rule, bundled heating surfaces or bulkhead heating surfaces).
The following traditional evaporation systems can be differentiated (see Figure 2.37):
Figure 2.37: Basic boiler flow systems
Source [1, UBA, 2001]
•
•
•
natural circulation: The water/steam mass flow in the evaporator is maintained due to the
different density of the medium in heated and unheated pipes. The water/steam mixture
flows into a drum. Here, steam and water are separated. The saturated steam then reaches
the post-connected superheater
forced circulation: This principle corresponds with the natural circulation, but is expanded
by a circulation pump supporting the circulation in the evaporator
forced continuous flow (once through boiler): In this system, the feed-water is pressed in
a continuous flow through the economiser, the evaporator, and the superheater.
Spray coolers and surface coolers are used in circulation boilers in order to maintain the exact
required steam temperature. It is their function to balance the fluctuations of the steam
temperature, these fluctuations being the consequences of load fluctuations, changes in the
waste quality, the surplus air, as well as contamination of the heat surfaces.
The preparation of boiler feed water and make up water is essential for a effective operation and
to reduce corrosion (inside the tubes) or risk of turbine damage. The quality of boiler water must
be higher when increased steam parameters are used.
90
Waste Incineration
Chapter 2
A compromise is required when determining steam parameters from waste fired boilers. This is
because, while the selection of high temperatures and pressures better utilise the energy
contained in the waste, these higher steam parameters can lead to significantly increased
corrosion problems, especially at the superheater surfaces and the evaporator. In municipal
waste incinerators it is common to use 40 bar and 400 °C, when there is electricity production
although higher values are used, especially with pretreated MSW and prepared RDF (value of
60 Bar and 520 °C are in use with special measures to prevent corrosion). In case of heat
production, steam with lower conditions or superheated water may be produced. Based on these
rather low (compared to most primary fuel power stations) steam parameters, almost
exclusively, natural circulation steam boilers are selected.
A feature of waste incineration is the high dust load in flue-gases. Measures that can assist dust
removal in the boiler areas by gravity separation of fly ash, are:
•
•
low flue-gas speeds, and
turns in the gas flow path.
The high proportion of ash in flue-gas causes a risk of a correspondingly high contamination of
the heat transfer surfaces. This leads to a decline in heat transfer and therefore a performance
loss. Thus, heat transfer surface cleaning plays an important role. This cleaning can be
accomplished manually or automatically with lances (compressed air or water jet), with
agitators, with soot blowers using steam, with a hail of pellets (sometimes shot cleaning), with
sound and shock waves, or with tank cleaning devices.
Different boiler concepts can be used in waste incineration plants. They are from left to right
(see Figure 2.38):
•
•
•
horizontal boilers
combination of vertical and horizontal boilers
vertical boilers.
Figure 2.38: Overview of various boiler systems: horizontal, combination, and, vertical
Source [1, UBA, 2001]
In horizontal and vertical systems usually a number of empty passes with evaporation walls are
followed by an arrangement of bundles of heat transfer surfaces i.e. evaporator, superheater and
economiser. The selection of the system to be deployed depends on the given building concept,
the selected steam parameters, and the customer specifications.
2.4.4.2.1
Corrosion in boilers
[1, UBA, 2001]With the introduction of minimum temperature residence times and oxygen
content requirements, corrosion has increased in steam generators at waste incineration plants.
Waste Incineration
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Corrosion is caused by the chemical attack of flue-gas and ash particles from the furnace. The
incineration chamber, the water walls of the first blank (empty) passes, and the super heater are
the boiler components that are most in danger of corrosion.
Erosion, which is the abrasion of surface material through vertical wear-and-tear, is caused
primarily by the ash particles present in flue-gas. Erosion appears mostly in the area of gas
redirection.
Tube wear is caused by a combination of corrosion and abrasion. Corrosion appears on clean
metallic surfaces. If the corrosion products deposit themselves as film on the pipe surface (oxide
layer), they function as a protective layer and slow down corrosion. If this protective layer
wears out through erosion, and if the metallic surface reappears, the entire process starts anew.
Coherent consideration of the corrosion processes is difficult, as physical, chemical,
incineration technical, metallurgical and crystallographic parameters interact.
Various types of flue-gas corrosion exist:
•
Tinder process: High temperature corrosion
•
Initial corrosion: Time-limited ferrous chloride formation before the first oxide layer
formation at “blank” steel during start-up. This reaction occurs continuously after film
removal through erosion
•
Oxygen-deficiency corrosion: through FeCl2-formation under deoxygenated flue-gas
atmosphere, e.g. under films (such as oxides, contamination or fireproof material) and in the
furnace area. FeCl2 is sufficiently volatile in the temperatures used in WI and is therefore
mobilised. An indicator for such corrosion is the appearance of CO (this explains the often
falsely used term CO corrosion). The microscopic situation at the border between material
and film is, however, decisive. This corrosion is observed in individual cases with steam
pressures above 30 bar, but more usually above 40 bar. Corrosion rate increases with metal
temperature. The corrosion products appear in flaky layers
•
Chloride-High temperature corrosion: Corrosion by chloride, which is released during the
sulphating of alkaline chlorides, and attacks iron or lead hydroxides. This corrosion
mechanism is observed in waste incineration plants with flue-gas temperatures>700 °C and
at pipe wall temperatures above 400 °C. The corrosion products can be recognised as a
black firmly bonded cup that includes a hygroscopic red FeCl3 layer in thicker films
•
Molten salt corrosion: The flue-gas contains alkali and similar components, which can
form eutectics. Eutectic compounds have a lower melting point than the single components
which form the eutectic system. These molten systems are highly reactive and can cause
severe corrosion of steel. They can react with the refractory lining and lead to the internal
formation of compounds like kalsilite, leucite, sanidine which destroy the refractory
mechanically. It can also form low viscous melts on the surface consisting of deposited
material and refractory material (refractory corrosion).
[64, TWGComments, 2003] [74, TWGComments, 2004]
•
Electrochemical Corrosion: This is based on the electrical potential equalisation of
different metals. The conductor can be aqueous or a solid that shows sufficient electrical
conductivity at the temperatures seen. The conductivity can arise from the water dew point
to the sulphuric acid dew point to molten salt
•
Standstill corrosion: Based on its high chloride content (especially CaCl2), the deposits are
hygroscopic. The humidity in the air dissolves these compounds and causes chemical
dissolution appearances in the material
92
Waste Incineration
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•
Dew point corrosion: When the temperature falls beneath the acid dew point, wet chemical
corrosions appear on cold surfaces. This damage can be avoided by raising the temperature
or by selecting an appropriate material.
In reality, from a thermodynamic perspective, a degree of corrosion is unavoidable. Counter
measures only help to reduce corrosion damage to an acceptable level. The causes of corrosion
require constructive and operational counter-measures. Improvement possibilities are mainly
found in the steam generator. Low steam parameters, long reaction times before entry into the
heat surfaces, lowering the flue-gas speed, and levelling of the speed profile could all be
successful. Protective shells, tooling, stamping, and deflectors can also be used to safeguard
heat surfaces.
A compromise must be found in determining the boiler cleaning intensity between the best
possible heat transfer (metallic pipe surface) and optimal corrosion protection.
2.4.4.3 Combustion air preheating
Preheating the combustion air is particularly beneficial for assisting the combustion of high
moisture content wastes. The pre-warmed air supply dries the waste, thus facilitating its
ignition. The supply heat can be taken from the combustion of the waste by means of heatexchange systems.
Preheating of primary combustion air can have a positive influence on overall energy efficiency
in case of electricity production.
2.4.4.4 Water cooled grates
Water cooling of grates is used to protect the grate. Water is used as a cooling medium to
capture heat from the burning waste bed and use it elsewhere in the process. It is common that
the heat removed is fed back into the process for preheating the combustion air (primary and/or
secondary air) or heating the condensate. Another option is to directly integrate the watercooling into the boiler circuit, operating it as an evaporator.
These grates are applied where the net calorific value of the waste is higher, typically above
10MJ/kg. At lower calorific values their application is more limited. Increases in the calorific
value of municipal waste seen in Europe have increased the application of this technique.
There are other reasons for the use of water-cooled grates – these are discussed in section
2.3.1.2.5.
2.4.4.5 Flue-gas condensation
[5, RVF, 2002]
Water in the flue-gas from combustion comprises evaporated free water from the fuel and
reaction water from the oxidation of hydrogen, as well as water vapour in the combustion air.
When burning wastes, the water content in the flue-gas after the boiler and economiser normally
varies between 10 and 20 % by volume, corresponding to water dew points of about 50 – 60 °C.
During cleaning of the boiler with steam the water content in the flue-gas increases to about
25 %.
The minimum possible dry gas temperature at this point is 130 - 140 °C using normal boiler
construction material. This temperature is mostly determined in order to be above the acid dew
point, linked to the SO3 content and the H2O content in the flue-gas.
Waste Incineration
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Lower temperatures result in corrosion. The boiler thermal efficiency (steam or hot water from
waste) will, under these conditions, be about 85 %, as calculated based on the calorific value of
the waste input. However, if there is more available energy in the flue-gas, a water vapour will
result which has a latent specific energy of about 2500 kJ/kg and dry gas with a specific heat of
about 1 kJ/(kg °C).
Return water from district heating at a temperature of 40 - 70 °C (system configuration
dependent), can be used directly to cool and condense the water vapour in the flue-gas. This
system is common at plants burning bio-fuel, which normally is very wet and gives water dew
points of 60 - 70 °C in the flue-gas.
Example: Stockholm/Hogdalen (Sweden):
At the Stockholm/Hogdalen (Sweden) plant this system is used with three conventional grate
fired steam boilers and one with a circulating fluidised bed. Flue-gases from the conventional
grate fired boilers are cooled in shot cleaned waste heat boilers to about 140 °C. Return water
from district heating is used as the cooling media.
FGT starts with a dry cleaning system for each boiler in which dry hydrated lime is injected and
mixed with the flue-gas in a reactor. The acid impurities react with the lime and solid salts are
formed which are removed in a fabric filter together with fly ash and the excess of lime. The
final reaction takes place in the dust cake on the bags. The fluidised bed boiler has a slightly
different reactor as re-circulated dust from the fabric filter is slightly humidified before it is
mixed with fresh lime and injected into the flue-gases.
The second cleaning stage includes wet scrubbers, which saturate the flue-gas and remove the
rest of the acid gases, particularly hydrogen chloride (HCl) and sulphur dioxide (SO2). The
saturated gas leaving the wet scrubbers has a temperature of about 60 °C. It is sucked to a tube
condenser, which is cooled by return water from the district heating at a temperature of
40 - 50 °C. One wet system is used for all three grate boilers, although the CFB-boiler has its
own.
If the return water temperature is 40 °C (the normal case for this plant but very low in
comparison with the majority of European climates) 14 % additional energy is recovered in the
condenser. On the other hand, if the return water temperature is 50 °C only about 7 % additional
energy is recovered. For extreme cases, when the return water temperature is as high as 60 °C,
no extra heat is recovered.
In the Stockholm/Hogdalen case the flue-gas is reheated before the ID fan and stack and for this
reheating some MW of low-pressure steam is consumed. It is also possible to operate without
this reheat but with a wet fan and stack.
94
Waste Incineration
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Figure 2.39: Pollution control and additional heat recovery by condensation of flue-gas water
vapour at the Stockholm/Hogdalen waste-fired CHP plant
Source [RVF, 2002 #5]
This simplified example shows that condensation can be effective only if there is a
comparatively big temperature difference between the water dew point in the flue-gas and the
cooling water (normally district heating return water). If this condition is not fulfilled heat
pumps can be installed (see below).
It should be noted that, in this case, it is the cold district heating water return that provides the
energetic driver for the condensation of the flue-gases. This situation is only likely to exist in
regions with the lower ambient temperatures found mostly in Northern Europe.
2.4.4.6 Heat pumps
[RVF, 2002 #5]
The main purpose of heat pumps is to transform energy from one temperature level to a higher
level. There are three different types of heat pumps in operation at incineration installations.
Theses are described below with examples.
2.4.4.6.1
Compressor driven heat pumps
This is the most well known heat pump. It is, for instance, installed in refrigerators, air
conditioners, chillers, dehumidifiers, and heat pumps used for heating with energy from rock,
soil, water and air. An electrical motor normally drives the pump, but for big installations steam
turbine driven compressors can be used.
In a closed-circuit, a refrigerant substance (e.g. R134a), is circulated through a condenser,
expander, evaporator and compressor. The compressor compresses the substance, which
condenses at a higher temperature and delivers the heat to the district heating water. There the
substance is forced to expand to a low pressure, causing it to evaporate and absorb heat from the
water from the flue-gas condenser at a lower temperature. Thus the energy at low temperature in
the water from the flue-gas condenser has been transformed to the district heating system at a
higher temperature level. At typical incineration conditions, the ratio between output heat and
compressor power (heat to power ratio) can be as high as 5. The compressor driven heat pump
can utilise very much of the energy from the flue-gas.
Waste Incineration
95
Chapter 2
2.4.4.6.2
Absorption heat pumps
Similar to the compressor type pump, absorption heat pumps were originally developed for
cooling. Commercial heat pumps operate with water in a closed loop through a generator,
condenser, evaporator and absorber. Instead of compression the circulation is maintained by
water absorption in a salt solution, normally lithium bromide, in the absorber. The diluted
water/salt solution is pumped to the generator. There the water is evaporated by hot water or
low-pressure steam and is then condensed in the condenser at a higher temperature. The heat is
transferred to the district heat water. The concentrated salt solution is circulated back to the
absorber. The process is controlled by the pressure in the system, in relation to the vapour
pressure of the liquids, water and lithium bromide.
Electrical power consumption is very low, limited to a small pump between the absorber and
generator, and there are few moving parts. The ratio between the output heat and absorber
power is normally about 1.6.
2.4.4.6.3
Open heat pumps
The third heat pump is sometimes called open heat pump. The principle is to decrease the water
content of the flue-gas downstream of the condenser using a heat and humidity exchanger with
air as intermediate medium.
The higher water content in the flue-gas in the condenser means a higher water dew point, and a
bigger difference between the water dew point and the dew point of the return water from the
district heating system.
2.4.4.6.4
Example data of different heat pumps
The following table has been collated from data from three different plants in Sweden, each
using a different type of heat pump, as described above.
As it can be seen from the table, the use of heat pumps consumes electricity; therefore the net
electrical output is reduced. However, the thermal heat output is increased.
Heat pump type
Net heat output using
heat pump
Net heat output
without heat pump
Variation in heat
output
Net electricity output
using
heat pump
Net electricity output
without heat pump
Variation of
electricity
production
Example 1
Example 2
Example 3
Open heat
Compressor driven Absorption heat pump
pumps
82
80
81
60
63
70
+37 %
+28 %
+16 %
15
15
0
20
19
0
-25 %
-21 %
0
Data refer to an energy input of 100, therefore all numbers are percentages.
Example 3 does not produce electricity
Source: Data have been collated from 3 examples of plants in Sweden.
Table 2.14: Example data showing the variation in heat and electricity output when using various
different types of heat pumps
Source [5, RVF, 2002]
96
Waste Incineration
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2.4.4.7 Flue-gas re-circulation
A proportion (approx. 10 – 20 % by volume) of the (usually cleaned) flue-gases is re-circulated,
normally after pre-dedusting, to replace secondary air feeds in the combustion chamber.
This technique is reported to reduce heat losses with the flue-gas and to increase the process
energy efficiency by around 0.75 % - 2 %. Additional benefits of primary NOX reduction are
also reported.
Lagging of the re-circulation ducting is reported to provide an effective remedy for corrosion
concerns in this area.
2.4.4.8 Reheating of flue-gases to the operation temperature FGT devices
Some air pollution control equipment requires the flue-gases to be reheated to enable their
effective operation. Examples include SCR systems and bag filters that generally require
temperature in the region of 250° C and 120° C respectively.
The energy for heating the gases can be obtained from:
•
•
external energy sources (e.g. electrical heating, gas or oil burners)
use of process generated heat or power (e.g. steam bleeds from the turbine).
The use of heat-exchangers to recapture the heat after the equipment reduces the need for
external energy input. This is carried out where the next stage of the process does not require the
flue-gas temperature to be as high as that emitted from the earlier equipment.
2.4.4.9 Plume visibility reduction
In some locations sensitivity to visible plumes is high. Certain techniques (e.g. wet scrubbing)
also give rise to higher levels of moisture in the flue-gas and therefore increase the possibility of
high visibility plumes. Lower ambient temperature and higher humidity levels increase the risk
of plume condensation, and hence visibility.
Increasing the temperature of the flue-gases provides one way of reducing plume visibility, as
well as improving dispersion characteristics of the release. Dependent on flue-gas moisture
content and atmospheric conditions, plume visibility is greatly reduced above stack release
temperatures of 140 °C.
Reducing the moisture content of the flue-gases also reduces the plume visibility. This can be
achieved by selecting alternative flue-gas treatment (i.e. avoiding wet systems) or by the use of
condensing scrubbers to remove water from the flue-gas (see Section 2.4.4.5).
2.4.4.10 Steam-water cycle improvements: effect on efficiency and other aspects
The selection of the steam water cycle will generally have a much greater impact on energy
efficiency of the installation than improving individual elements of the system, and therefore
provides the greatest opportunity for increased use of the energy in the waste.
The following table provides example information concerning techniques of actions that are
used for improving energy recovery at a municipal waste to energy incinerator, along with an
estimation of their "weight". The figures given were calculated for one example plant that only
generated electricity [50, CNIM, 2003]:
Waste Incineration
97
Chapter 2
Technique
Increase steam pressure
Decrease vacuum at
turbine outlet
(e.g. a hydro-condenser
may be used to improve
vacuum)
Heating secondary air
Air heater in 2 stages
(i.e. 2 bleeds on the
turbine)
Increase deaerator
temperature
Net Power output increase
(approx.)
Disadvantages
and other advantages
3 % for 60 bars instead of 40 increase in investment cost
bars
corrosion risk slightly increased
significant increase in investment cost (air
condenser area: + 10 % between 120 and
110 mbar at air temperature=15 °C)
1 to 2 % for 20 mbar reduction
size and noise increase.
uncertainties on suppliers commitments for
very low pressure
complexity and cost increase if there are 2
0.7 % to 1.2 %
air fans
1 to 1.5 %
cost increase
space requirement increase
0.9 % for 140 °C instead of
130 °C
increase the size and the cost of the
economiser
cost of the equipment and piping
not necessarily applicable for small TG sets
Add a condensate heater
0.5 to 1.2 %
corrosion problem may occur in particular
during transitory phases (start up, shut down
etc.)
increase the investment cost
0.75 to 2 %
decreasing the O2 by other means reduces
for a decrease of 1 % of dry O2
Recycle a part of the
the interest of flue-gas recycling
-----flue-gas
corrosion problem may occur in particular
Decrease NOX level by approx.
during transitory phases (start up, shut down
100 mg/Nm³
etc.)
Reduce the flue-gas
0.4 to 0.7 %
the boiler outlet temperature is determined
temperature at boiler
for 10 °C lower between
according to the FGT system type
outlet
190 °C and 140 °C
Use SNCR de-NOX
see discussions about SCR and SNCR de3 to 6 %
instead of SCR
according to the processes used NOX
1 to 2 % instantaneous
some TG sets have higher efficiency at
Optimise the choice of
But much higher difference
nominal conditions but lower reliability,
the TG set
over a long period of time if
availability and/or flexibility at partial load
low availability
Reduction of O2-content
with lower O2 content, CO may increase
in flue-gas of 1 % (in
1 – 2 % increase
low oxygen content may increase corrosion
range 6 - 10 %)
risk.
Table 2.15: Steam-water cycle improvements: effect on efficiency and other aspects.
Source [50, CNIM, 2003]
2.4.5 Steam generators and quench cooling for hazardous waste
incinerators
In Europe there are two main approaches adopted for cooling the combustion gases from
hazardous waste incinerators. Their principle advantages and disadvantages are described in the
table below:
98
Waste Incineration
Chapter 2
Gas cooling
system
Advantages
•
Heat recovery
boiler
Rapid quench
cooling
•
•
•
•
high energy recovery efficiency possible
(70 – 80 % can be converted to steam)
lower water consumption and water
treatment volumes.
reduced risk of dioxin re-formation
need for additional dioxin controls on
emissions to air may be reduced
it may be possible to treat wastes with a
more variable range and higher halogen
or salts loading if this technique is used.
Disadvantages
• possible increased risk of
dioxin reformation in
boiler
• additional capital and
maintenance costs of boiler
system.
• very limited energy
recovery
• water consumption may be
higher
• water treatment volumes
may be higher.
Table 2.16: Summary of the main differences between quench cooling and heat recovery
Source adapted from [Cleanaway, 2002 #46], [EURITS, 2002 #41]
Heat recovery boilers in hazardous waste incineration installations:
[EURITS, 2002 #41]
The hot combustion gases are cooled in a steam generator (or boiler) with a capacity of
between 16MW and 35MW depending on the installation. The steam that is produced has a
pressure of 13 bar to 40 bar with a temperature between 207 and 385 °C. As a guideline, a fullyequipped installation normally produces an average of 4 – 5 tonnes of steam per tonne of
incinerated waste, thereby attaining a thermal efficiency of 70 – 80 % (energy in steam versus
energy in waste). Most installations are equipped with an economiser device and a superheater if electricity is produced. A range of factors influence the efficiency of the steam
generators used in hazardous waste incinerators, including the composition of the gas and the
potential for deposition to occur on the heat-exchange surfaces. This has a significant influence
on the construction materials used and on the design, as well as on the operational life and
performance of the equipment.
For some installations, the steam is used in a turbine to produce electricity. The electricity is
used by the incineration plant for its own purposes or exported. Alternatively steam may be
transported for direct use in industrial processes, e.g. the production of chemicals, or to other
waste treatment processes or fed into a district heating system. Combinations of these are also
applied.
Rapid quench cooling:
Some installations are not equipped with a boiler, but the combustion gases are reduced in
temperature by means of very quick quench cooling (e.g. 1100 °C to 100 °C in less than
1 second). This is performed to prevent the formation of dioxins and to avoid the installation of
an extra end-of-pipe dioxin removal technique. These installations are referred to as
‘quenchers’, and have been adopted in some plants where a very wide range of highly
halogenated wastes inputs have to be treated. This limits the potential options for energy
recovery.
2.4.6 Examples of energy recovery from fluidised bed incinerators
The different designs and size of fluidised bed incinerators influence the behaviour of the boiler
and the amount and type of energy produced [33, Finland, 2002]. The following two examples
give approximate figures for different sizes of incinerators:
Waste Incineration
99
Chapter 2
1.
15 - 30 MW heat and low pressure steam producing boilers:
This size of fluidised bed boiler uses approx. 35000 - 40000 tonnes per year of ready made
recovered fuel. If it is made of commercial waste, demolition waste and separately collected
packages from households, it can use all of this kind of material generated by a city of about
150000 inhabitants. The heat produced is about 150 GWh, which could be used by industry or
for district heating.
Boilers of this size are very similar to operate to normal power plant boilers of 50 - 100 MW. Its
behaviour is steady and uniform, because of the ready made controlled fuel made of sorted
waste, and the heavy bed.
When a suitable energy user is available an energy efficiency range of 70 - 90 % can be
achieved.
Rotating fluidised bed incinerators have been designed for thermal capacities from 10 - 55 MW
(thermal) and corresponding waste throughput of 22000 - 167000 tonnes/yr per line. Energy is
recovered by steam generators and used for electricity production and/or heating purposes
depending on local requirements. Thermal efficiency is can be about 80 %, and electrical
efficiency typically around 25 %. [64, TWGComments, 2003]
2.
50 - 100 MW electricity producing power plants:
If the size of the waste to energy boiler is>30 MW, it may be more difficult to find a suitable
customer for such quantity of heat energy. Whenever electricity is also produced, the economics
of the waste to energy boiler is mostly dependent on the price of the electricity, not on the price
of the heat.
Electrical efficiency with well-defined, quality controlled feeds can be relatively high, up to
level of 30 – 35 % with typical steam temperatures from 450 – 500 ºC.
2.5 Applied flue-gas treatment and control systems
2.5.1 Summary of the application of FGT techniques
Flue-gas treatment systems are constructed from a combination of individual process units that
together provide an overall treatment system for the flue-gases. A description of the individual
process units, organised according to the substances upon which they have their primary effect,
is given in this chapter.
Table 2.17 below gives a summary of the application of some systems in the municipal waste
incineration sector. The balance of applied systems is different with different waste streams. A
description of each of the techniques listed in the table is given later in this section:
100
Waste Incineration
Chapter 2
Austria
Belgium
Denmark
France
Germany
Great
Britain
Hungary
Italy
Netherlands
Norway
Portugal
Spain
Sweden
Switzerland
Total plants
Number of MSWI plants with various flue-gas treatment systems
Electrostatic
Fabric
Wet
Dry and Wet SD and Wet
precipitator only filter only
2
8
1
6
1
1
45
19
30
2
5
1
Dry with FF
Semi-dry
with FF
2
7
13
5
9
17
25
16
1
9
26
6
1
1
3
7
1
1
7
29
2
95
138
12
SNCR de-NOX
SCR de-NOX
applied
applied
applied
applied
17
applied
applied
42
applied
1
4
1
5
64
3
5
3
8
4
4
3
applied
3
14
21
2
applied
4
23
applied
applied
43
(of 200)3
Notes:
1. All figures (except SCR data) are derived from data provided to TWG in [42, ISWA, 2002] - Tables 1 and 2 and TWG Comments
2. Other combinations of FGT unit operations are applied but not included in the table
3. Data supplied to EIPPCB by FEAD suggests 43 of around 200 surveyed MSWI use SCR
4. Belgium data only represents Flemish region and Brussels only
5. applied indicates that the technique is applied – data for blanks was not provided
Table 2.17: Summary of the main applied FGT systems for MSWIs in Europe in 2000/2001
Source adapted from [42, ISWA, 2002, 64, TWGComments, 2003]
Waste Incineration
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Chapter 2
Some flue-gas treatment techniques are also explained in detail in the horizontal BREF
“Reference Document on Best Available Techniques in the Waste Water and Waste Gas
Treatment/Management Systems in the Chemical Sector (CWW)”.
2.5.2 Overview of overall combined FGT system options
The individual components of a FGT system are combined to provide an effective overall
system for the treatment of the pollutants that are found in the flue-gases. There are many
individual components and designs, and they may be combined in many ways. The diagram
below shows an example of the options and their possible combination. It can be seen that in
this assessment there are a total of 408 different combined systems:
(Dry or wet) adsorption (de-diox, etc.)
With liquid effluents
1 stage with soda (NaOH),
tray-plate, packed
or spray column
42
Combined or without
liquid effluents
WET
PROCESSES
Catalytic baghouse (de-diox, etc)
2
14
42
2 stages, 1 water + 1 soda,
tray-plate, packed
or spray columns
Evapo-cristallisation
168
Reburning
42
Raw gas (low dust)
SCR de-NOx
SCR de-diox
3
6
(Dry or wet) adsorption (de-diox, etc)
Clean gas (tail end)
3
14
Condensation
Ammonia solution
2 stages, 1 lime + 1 soda,
spray columns
42
6
14
Catalytic baghouse (de-diox, etc.)
2
SNCR de-NOx
Urea solution
(Dry or wet) adsorption
2
Solid urea
Catalytic baghouse
2
Compressed air spraying
24
Without fly ash
recirculation
24
¾ dry
G.S.S. lime injection
Raw gas (low dust)
SCR de-NOx
2
4
8
SEMI-WET
PROCESSES
With lime milk
Mechanical atomization
Ammonia solution
3
Fly ash recirculation
24
Dry adsorption
2
SNCR de-NOx
8
Circulation fluidised bed
Clean gas (tail end)
SCR de-diox
8
Dry adsorption
Urea solution
Solid urea
Reburning
Dry adsorption
1
SCR de-diox
Lime without
recirculation
Water spray
Raw gas (low dust)
SCR de-NOx
14
56
DRY PROCESSES
With baghouse filter
Heat exchanger
56
168
Catalytic baghouse
3
14
Dry adsorption
Reburning
Sodium bicarbonate
with wimple filtration
Air dilution
I = 408 possible combinations
Clean gas (tail end)
Lime with
recirculation
Catalytic baghouse
2
14
56
Dry adsorption
3
6
Sodium bicarbonate
with double filtration
Ammonia solution
SNCR de-NOx
14
6
2
Dry adsorption
Urea solution
2
Solid urea
Catalytic baghouse
2
Figure 2.40: Overview of potential combinations of FGT systems
2.5.3 Techniques for reducing particulate emissions
[1, UBA, 2001] The selection of gas cleaning equipment for particulates from the flue-gas is
mainly determined by:
•
•
•
•
•
•
•
102
particle load in the gas stream
the average particle size
particle size distribution
flowrate of gas
flue-gas temperature
compatibility with other components of the entire FGT system (i.e. overall optimisation)
required outlet concentrations.
Waste Incineration
Chapter 2
Some parameters are rarely known (such as particle size distribution or average size) and are
empirical figures. Available treatment or disposal options for the deposited substances may also
influence FGT system selection i.e. if an outlet exists for treatment and use of fly ash, this may
be separately collected rather than the fly ash collected with FGT residues.
[74, TWGComments, 2004]
2.5.3.1 Electrostatic precipitators
[1, UBA, 2001]
Electrostatic precipitators are sometimes also called electrostatic filters. The efficiency of dust
removal of electrostatic precipitators is mostly influenced by the electrical resistivity of the dust.
If the dust layer resistivity rises to values above approx. 1011 to 1012 cm removal efficiencies
are reduced. The dust layer resistivity is influenced by waste composition. It may thus change
rapidly with a changing waste composition, particularly in hazardous waste incineration.
Sulphur in the waste (and water content at operational temperatures below 200 °C [64,
TWGComments, 2003]) often reduces the dust layer resistivity as SO2 (SO3) in the flue-gas and,
therefore facilitates deposition in the electric field.
Figure 2.41: Operating principle of an electrostatic precipitator
Source [1, UBA, 2001]
For the deposition of fine dust and aerosols, installations that maintain the effect of the electric
field by drop formation in the flue-gas (pre-installed condensation and wet electrostatic
precipitators, condensation electrostatic precipitators, electro-dynamic venture scrubbers,
ionised spray coolers) can improve removal efficiency.
Typical operational temperatures for electrostatic precipitators are 160 - 260 °C. Operation at
higher temperatures (e.g. above 250 °C) are generally avoided as this may increase the risk of
PCDD/F formation (and hence releases).
2.5.3.2 Wet electrostatic precipitators
[1, UBA, 2001]Wet electrostatic precipitators are based upon the same technological working
principle as electrostatic precipitators. With this design, however, the precipitated dust on the
collector plates is washed off using a liquid, usually water. This may be done continuously or
periodically. This technique operates satisfactorily in cases where moist or cooler flue-gas
enters the electrostatic precipitator.
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2.5.3.3 Condensation electrostatic precipitators
[1, UBA, 2001]The condensation electrostatic precipitator is used to deposit very fine, solid,
liquid or sticky particles, for example, in the flue-gas from hazardous waste incineration plants.
Unlike conventional wet electrostatic precipitators, the collecting surfaces of condensation
electrostatic precipitators consist of vertical plastic tubes arranged in bundles, which are
externally water-cooled.
The dust-containing flue-gas is first cooled down to dew-point temperature in a quench by
direct injection of water and then saturated with vapour. By cooling the gases in the collecting
pipes further down, a thin, smooth liquid layer forms on the inner surface of the tubes as a result
of condensation of the vapour. This is electrically earthed and thus serves as the passive
electrode.
Particles are deposited by the influence of the electric field between the discharge electrodes
suspended in the tube axes and the condensation layer in continuous flow. At the same time the
condensation layer also causes continuous removal of deposited particles from the deposition
area. Even water-insoluble dust and poorly wet-able soot are washed off. The constantly
renewed wetting prevents dry spots and sticking, which can cause sparking (electrical
discharges between the electrodes). Avoiding sparking allows for a higher deposition voltage,
which in turn leads to improved and consistent high deposition performance (see Figure 2.42).
Figure 2.42: Condensation electrostatic precipitator
Source [1, UBA, 2001]
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2.5.3.4 Ionisation wet scrubbers
[1, UBA, 2001] The purpose of the Ionisation Wet Scrubber (IWS) is to remove various
pollutants from the flue-gas flow. The IWS combines the principles of:
•
•
•
electrostatic charging of particles, electrostatic attraction and deposition for aerosols
(smaller than 5 bm)
vertical deposition for coarse, liquid and solid particles (larger than 5 bm), and
absorption of hazardous, corrosive and malodorous gases.
The IWS system is a combination of an electrostatic filter and a packed scrubber. It is reported
to require little energy and has a high deposition efficiency for particles in the submicron as well
as the micron range.
A high voltage zone is installed before each packed tower stage. The function of the high
voltage zone is to ionise the particles (dust, aerosols, submicron particles) contained in the fluegas. The negatively charged particles induce opposing charges on the neutral surface of the
wetted packing material and the falling water drops. Because of this they are attracted and are
then washed out in the packed section. This is referred to as Image/Force attraction (IF
attraction), i.e. attraction through electron shift. Hazardous, corrosive and malodorous gases are
also absorbed in the same scrubber fluid and chemically combined to be discharged with the
scrubber effluent.
Another type of ionization wet scrubber includes a Venturi. The pressure changes that occur
through the Venturi allows the fine particles to grow and the electrode charges them. They are
then collected by the dense layer of water droplets projected by a nozzle, serving as collecting
electrode. [74, TWGComments, 2004]
2.5.3.5 Fabric filters
Fabric filters, also called bag filters, are very widely used in waste incineration plants. Filtration
efficiencies are very high across a wide range of particle sizes. At particle sizes below
0.1 microns, efficiencies are reduced, but the fraction of these that exist in the flue-gas flow
from waste incineration plants is relatively low. Low dust emissions are achieved with this
technology. It can also be used following an ESP and wet scrubbers. [74, TWGComments,
2004]
Compatibility of the filter medium with the characteristics of the flue-gas and the dust, and the
process temperature of the filter are important for effective performance. The filter medium
should have suitable properties for thermal, physical and chemical resistance (e.g. hydrolysis,
acid, alkali, oxidation). The gas flowrate determines the appropriate filtering surface i.e.
filtering velocity.
Mechanical and thermal stress on the filter material determines service life, energy and
maintenance requirements.
In continuous operation, there is gradual loss of pressure across the filtering media due to the
deposit of particles. When dry sorption systems are used, the formation of a cake on the media
helps to provide the acid removal. In general, the differential pressure across the filter is used to
monitor the need of cleaning. Periodic replacement is required when the residual lifetime is
achieved or in the case of irreversible damage (e.g. an increasing loss of pressure may be caused
by irreversible deposit of fine dust in the filter material). Several parameters help to control the
lifetime of the bags: pressure drop drift, visual, microscopic analysis, etc. Potential leaks in the
bag filter will also be detected by the increased emissions or by some process disturbance. [64,
TWGComments, 2003]
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Chapter 2
The application of dry deposition is limited for dusts that are hygroscopic at high temperatures
(300 to 600 °C) and become sticky at these temperatures. This type of dust forms deposits in the
deposition equipment, which cannot be extracted sufficiently by conventional cleaning
techniques during operation, but may need to be removed by ultrasound vibration. These may
be dusts from complex salts e.g. from wastes containing phosphorus, sulphur or silicon.
Figure 2.43: An example of a fabric filter
Source [1, UBA, 2001]
2.5.3.6 Cyclones and multi-cyclones
[64, TWGComments, 2003]
Cyclones and multicyclones use centrifugal forces to separate particulate matter from the gas
stream. Multi-cyclones differ from single cyclones in that they consist of many small cyclone
units. The gas flow enters the separator tangentially and leaves from a central port. Solids are
forced to the outside of the cyclone and collected at the sides for removal.
In general, cyclones on their own cannot achieve the emission levels now applied to modern
waste incinerators. They can, however, have an important role to play where applied as a prededuster before other flue-gas treatment stages. Energy requirements are generally low as there
is no pressure drop across the cyclone.
Advantages of cyclones are their wide operational temperature range and robust construction.
Erosion of cyclones, particularly at the point of impingement of dirty flue-gases, can be an issue
where the flue-gas is more heavily loaded with particulate, and particularly where bed material
escapes from fluidised bed plants. Circulating fluidised beds usually incorporate a cyclone for
the removal and recirculation of the bed material to the furnace.
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2.5.4 Techniques for the reduction of acid gases (e.g. HCl, HF and SOX
emissions)
These substances are generally cleaned from the flue-gas using alkaline reagents. The following
flue-gas cleaning processes are applied:
•
dry processes:
•
semi-wet processes:
•
wet processes:
A dry sorption agent (e.g. lime, sodium bicarbonate) is added to
the flue-gas flow. The reaction product is also dry
Also called semi-dry, the sorption agent added to the flue-gas
flow is an aqueous solution (e.g. lime milk) or suspension (e.g.
as a slurry). The water solution evaporates and the reaction
products are dry. The residue may be re-circulated to improve
reagent utilisation. A sub-set of this technique are flash-dry
processes which consist of injection of water (giving fast gas
cooling) and reagent at the filter inlet
The flue-gas flow is fed into water, hydrogen peroxide, or/and a
washing solution containing part of the reagent (e.g. sodium
hydroxide solution). The reaction product is aqueous.
2.5.4.1 Removal of sulphur dioxide and halogens
[1, UBA, 2001] Sulphur dioxide and gaseous halogens are cleaned from flue-gases by the
injection of chemical or physical sorption agents, which are brought into contact with the fluegas. According to technique, the reaction products are dissolved or dry salts.
Dry systems:
In dry sorption processes the absorption agent (usually lime or sodium bicarbonate) is fed into
the reactor as a dry powder. The dose rate of reagent may depend on the temperature as well as
on reagent type. With lime this ratio is typically two or three times the stoichiometric amount of
the substance to be deposited, with sodium bicarbonate the ratio is lower. This is required to
ensure emission limits are complied with over a range of inlet concentrations. The reaction
products generated are solid and need to be deposited from the flue-gas as dust in a subsequent
stage, normally a bag filter.
The overdose of lime (or other reagent) leads to a corresponding increase in the amount of
residues, unless reagent recirculation is carried out, when the un-reacted fraction can be
recirculated and the stoichiometric ratio reduced accordingly.
If there is no pre-deposition stage (e.g. electrostatic precipitator), particles are removed with the
used reagent and reaction products. The cake of reagent that forms on fabric filters gives
effective contact between flue-gas and absorbent.
Plumes are rarely visible with this technique.
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107
Chapter 2
Figure 2.44: Schematic diagram of a dry FGT system with reagent injection to the FG pipe and
downstream bag filtration
Semi-wet systems:
These are also called semi-dry processes. In the spray absorption, the absorption agent is
injected either as suspension or solution into the hot flue-gas flow in a spray reactor (see Figure
2.45).
This type of process utilises the heat of the flue-gas for the evaporation of the solvent (water).
The reaction products generated are solid and need to be deposited from the flue-gas as dust in a
subsequent stage e.g. bag filter. These processes typically require overdoses of the sorption
agent of 1.5 to 2.5.
Here, the fabric filter is also an important part of the process. Plumes are also rarely visible with
this technique.
Figure 2.45: Operating principle of a spray absorber
[1, UBA, 2001]
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Waste Incineration
Chapter 2
A system which falls between the normal dry and semi-wet systems is also applied. This is
sometimes known as a flash-dry system. (Alstom 2003) These systems re-inject into the inlet
flue-gas a proportion of the solids collected on a bag filter. Water is added at a controlled rate to
the collected fly ash and reagent to ensure that it remains free flowing and not prone to
stickiness or scaling. No contact tower or slurry handling is required (cf. semi-wet systems) and
no effluents are produced (cf. wet systems).
The recycling of reagent reduces demand for reagent and the amount of solid residue produced.
Stoichiometric ratios in the range of 1.5 to 2 are common. Recycling of reagent can also be
applied to dry and semi-wet systems.
Wet systems:
Wet flue-gas cleaning processes use different types of scrubber design. For example:
•
•
•
•
•
•
jet scrubbers
rotation scrubbers
venturi scrubbers
dry tower scrubbers
spray scrubbers
packed tower scrubbers.
The scrubber solution is (in the case of water only injection) strongly acidic (typically pH 0 - 1)
due to acids forming in the process of deposition. HCl and HF are mainly removed in the first
stage of the wet scrubber. The effluent from the first stage is recycled many times, with small
fresh water addition and a bleed from the scrubber to maintain acid gas removal efficiency. In
this acidic medium, deposition of SO2 is low, so a second stage scrubber is required for its
removal.
Removal of sulphur dioxide is achieved in a washing stage controlled at a pH close to neutral or
alkaline (generally pH 6 - 7) in which caustic soda solution or lime milk is added. For technical
reasons this removal takes place in a separated washing stage, in which, additionally, there
occurs further removal of HCl and HF.
If the treated waste contains bromine and iodine, these elements can be deposited from the fluegas flow if waste containing sulphur is combusted simultaneously. In addition to sulphur
compounds, water-soluble salts of bromine and iodine will form, which can be deposited
through the wet SO2 flue-gas cleaning processes. Additionally, the deposition of elementary
bromine and iodine may be improved by specific employment of reductive washing stages
(sulphite solution, bisulphite solution). In any case, it is important to be aware of which wastes
contain iodine or bromine.
If lime milk or limestone is used as a neutralising agent in the wet flue-gas cleaning stages,
sulphate (as gypsum), carbonates and fluorides will accumulate as water-insoluble residues.
These substances may be removed to reduce the salt load in the waste water and hence reduce
the risk of encrustation within the scrubbing system. Residues of the cleaning process (e.g.
gypsum) can be recovered. When using a caustic soda solution there is no such risk because the
reaction products are water-soluble. If NaOH is used, CaCO3 may form (depending upon water
hardness), which will again lead to deposits within the scrubber. These deposits need to be
removed periodically by acidification.
The diagram below shows a typical 2 stages wet scrubbing system. The number of scrubbing
stages usually varies between 1 and 4 with multiple stages being incorporated in each vessel:
Waste Incineration
109
Chapter 2
Figure 2.46: Diagram of a 2 stage wet scrubber with upstream de-dusting
Waste water from wet scrubbers:
To maintain scrubbing efficiency and prevent clogging in the wet scrubber system, a portion of
the scrubber liquor must be removed from the circuit as waste water. This waste water must be
subjected to special treatment (neutralisation, precipitation of heavy metals), before discharge or
use internally. Mercury removal is given special attention. Volatile Hg compounds, such as
HgCl2, will condense when flue-gas is cooled, and dissolve in the scrubber effluent. The
addition of reagents for the specific removal of Hg provides a means for removing it from the
process.
In some plants, the waste water produced is evaporated in the incineration plant by spraying it
back into the flue-gas as a quench in combination with a dust filter.
2.5.4.2 Direct desulphurisation
[1, UBA, 2001] Desulphurisation in fluidised bed processes can be carried out by adding
absorbents (e.g. calcium or calcium/magnesium compounds) directly into the incineration
chamber. Additives such as limestone dust, calcium hydrate and dolomitic dust are used. The
system can be used in combination with downstream flue-gas desulphurisation.
The arrangement of the jets and the injection speed influence the distribution of the absorbents
and thus the degree of sulphur dioxide deposition. Part of the resulting reaction products are
removed in filter installations downstream; however, a significant proportion remains with the
bottom ashes. Therefore, direct desulphurisation may impact on bottom ash quality [64,
TWGComments, 2003].
Ideal conditions for direct desulphurisation exist in a cycloid furnace due to the constant
temperature level.
It is reported that, on its own, this techniques does not lead to compliance with the ELV
requirements of Directive 2000/76/EC. [1, UBA, 2001]. The amount of residue from the fluegas treatment system itself can be reduced, resulting in lower disposal costs.
Absorption (and adsorption) of pollutants can also be performed in a (circulating) fluid bed
reactor into which residues and reagents are recirculated in the combustor at a high rate.
Recirculation of flue-gas keeps the gas flow above a minimum level in order to maintain
fluidisation of the bed. The bed material is separated in a bag filter. Injection of water reduces
the consumption of absorbents (and hence the production of residues) significantly.
[74, TWGComments, 2004]
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Chapter 2
2.5.5 Techniques for the reduction of emissions of oxides of nitrogen
[3, Austria, 2002]
Nitrogen oxides (NOX) may be formed in three ways:
•
•
•
thermal NOX: During combustion a part of the air nitrogen is oxidised to nitrogen oxides.
This reaction only takes place significantly at temperatures above 1300 °C. The reaction
rate depends exponentially on the temperature and is directly proportional to the oxygen
content
fuel NOX: during combustion a part of the nitrogen contained in the fuel is oxidised to
nitrogen oxides
formation of NOX via radical reaction (prompt NOX): Atmospheric nitrogen can also be
oxidised by reaction with CH radicals and intermediate formation of HCN. This mechanism
of formation is of relatively low importance in waste incineration.
Figure 2.47: Temperature dependence of various NOX formation mechanisms in waste incineration
Source [3, Austria, 2002]
2.5.5.1 Primary techniques for NOX reduction
[1, UBA, 2001] NOX production can be reduced using furnace control measures that:
•
•
prevent over supply of air (i.e. prevention of the supply of additional nitrogen)
prevent the use of unnecessarily high furnace temperatures (including local hot spots).
2.5.5.1.1
Air supply, gas mixing and temperature control
The use of a well distributed primary and secondary air supply to avoid the uneven temperature
gradients that result in high temperature zones and, hence, increased NOX production is a widely
adopted and important primary measure for the reduction of NOX production.
Although sufficient oxygen is required to ensure that organic materials are oxidised (giving low
CO and VOC emissions), the over supply of air can result in additional oxidation of
atmospheric nitrogen, and the production of additional NOX.
Achieving effective gas mixing and temperature control are important elements.
Waste Incineration
111
Chapter 2
2.5.5.1.2
Flue-Gas Recirculation (FGR)
This technique involves replacement of around 10 - 20 % of the secondary combustion air with
recirculated flue-gases. NOX reduction is achieved because the supplied re-circulated flue-gases
have lower oxygen concentration and therefore lower flue-gas temperature which leads to a
decrease of the nitrogen oxide levels. [74, TWGComments, 2004]
2.5.5.1.3
Oxygen injection
The injection of either pure oxygen or oxygen enriched air provides a means to supply the
oxygen required for combustion, while reducing the supply of additional nitrogen that may
contribute to additional NOX production.
2.5.5.1.4
Staged combustion
Staged combustion has been used in some cases. This involves reducing the oxygen supply in
the primary reaction zones and then increasing the air (and hence oxygen) supply at later
combustion zones to oxidise the gases formed. Such techniques require effective air/gas mixing
in the secondary zone to ensure CO (and other products of incomplete combustion) are
maintained at low levels.
2.5.5.1.5
Natural gas injection (re-burn)
[70, USEPA, 1994]
Natural gas injection into the over-grate region of the furnace can be used to control NOX
emissions from the combustor. For MSWIs, two different natural gas based processes have been
developed:
•
•
Re-burning – a three stage process designed to convert NOX to N2 by injecting natural gas
into a distinct re-burn zone located above the primary combustion zone
Methane de-NOX – this technique injects natural gas directly into the primary combustion
unit to inhibit NOX formation.
2.5.5.1.6
Injection of water into furnace/flame
A properly designed and operated injection of water either into the furnace or directly into the
flame can be used to decrease the hot spot temperatures in the primary combustion zone. This
drop in peak temperature can reduce the formation of thermal NOX.
[74, TWGComments, 2004]
2.5.5.2 Secondary techniques for NOX reduction
[1, UBA, 2001] Directive 2000/76/EC requires a daily average NOX (as NO2) clean gas value of
200 mg/Nm³. In order to achieve compliance at this level, it is common for secondary measures
to be applied. For most processes the application of ammonia or derivatives of ammonia (e.g.
urea) as reduction agent has proved successful. The nitrogen oxides in the flue-gas basically
consist of NO and NO2 and are reduced to nitrogen N2 and water vapour by the reduction agent.
112
Waste Incineration
Chapter 2
Reaction equations:
4 NO + 4 NH3 + O2
2 NO2 + 4 NH3 + O2
4 N2 + 6 H2O
3 N2 + 6 H2O
Two processes are important for the removal of nitrogen from flue-gases - the Selective NonCatalytic Reduction (SNCR) and the Selective Catalytic Reduction (SCR).
Both NH3 and urea are applied in aqueous solutions. NH3 is normally, for safety reasons,
delivered as a 25 % solution.
2.5.5.2.1
Selective Non-Catalytic Reduction (SNCR) process
In the Selective Non-Catalytic Reduction (SNCR) process nitrogen oxides (NO + NO2) are
removed by selective non-catalytic reduction. With this type of process the reducing agent
(typically ammonia or urea) is injected into the furnace and reacts with the nitrogen oxides. The
reactions occur at temperatures between 850 and 1000 °C, with zones of higher and lower
reaction rate within this range.
Figure 2.48: SNCR operating principle
[1, UBA, 2001]
Reducing NOX by SNCR more than 60 – 80 %, requires a higher addition of the reducing agent.
This can lead to emissions of ammonia, also known as ammonia slip. The relationship between
NOX reduction, ammonia slip and reaction temperature is given in Figure 2.49 below:
Waste Incineration
113
Chapter 2
Figure 2.49: Relationship between NOX reduction, production, ammonia slip and reaction
temperature for the SNCR process
[Austria, 2002 #3] [64, TWGComments, 2003]
In Figure 2.49, it is shown that, at a reaction temperature of, for example, 1000 °C, the reduction
of NOX would be about 85 %, and there would be an ammonia slip of about 15 %. In addition,
at this temperature there would be a production of NOX, from the incineration of the injected
NH3, of about 25 %.
Figure 2.49 also shows that, at higher temperatures (with ammonia), the percentage of NOX
reduction is higher, and while the ammonia slip is lower, the NOX produced from the ammonia
rises. At high temperatures, (>1200 °C) NH3 itself oxidises and forms NOX. At lower
operational temperatures the NOX reduction is less efficient, and ammonia is slip higher
Application of urea instead of ammonia in SNCR leads to relatively higher N2O emissions in
comparison with ammonia reduction. [64, TWGComments, 2003]
In order to ensure an optimum utilisation of ammonia at varying degrees of load, which cause
varying temperatures in the combustion chamber, NH3 can be injected at several layers.
When used with wet scrubbing systems, the excess ammonia may be removed in the wet
scrubber. The ammonia can then be recovered from the scrubber effluent using an ammonia
stripper and fed back to the SNCR feed system.
Important for optimisation of the SNCR process is the effective mixing of flue-gases and NOX
reduction reagent, and sufficient gas residence time to allow the NOX reduction reactions to
occur.
In the case of pyrolysis and gasification processes, optimisation of SNCR is achieved by
injecting the reagent into the syngas combustion zones with a well controlled temperature and
effective gas mixing.
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Chapter 2
2.5.5.2.2
Selective Catalytic Reduction (SCR) process
Selective Catalytic Reduction (SCR) is a catalytic process during which ammonia mixed with
air (the reduction agent) is added to the flue-gas and passed over a catalyst, usually a mesh (e.g.
platinum, rhodium, TiO2, zeolites). [74, TWGComments, 2004] When passing through the
catalyst, ammonia reacts with NOX to give nitrogen and water vapour.
To be effective, the catalyst usually requires a temperature of between 180 and 450 °C. The
majority of systems used in waste incinerators currently operate in the range 230 - 300 °C.
Below 250 °C more Catalyst volume is necessary and there is a greater risk of fouling and
catalyst poisoning. In some cases catalyst temperature regulated bypasses are used to avoid
damage to the SCR unit. [74, TWGComments, 2004]
The SCR process gives high NOX reduction rates (typically over 90 %) at close to
stoichiometric additions of the reduction agent. For waste incineration, SCR is mainly applied in
the clean gas area i.e. after de-dusting and acid gas removal. For this reason, the flue-gases
generally require reheating to the effective reaction temperature of the SCR system. This adds to
the energy requirements of the flue-gas treatment system. However, when SOX levels in the
flue-gas have already been reduced to a very low value at the inlet of the SCR section, reheating
may be reduced substantially, or even omitted. Heat exchangers are used to reduce additional
energy demand.
After a wet FGT system, droplets may be removed to prevent salt deposits inside the catalyst.
Due to risk of ignition, safety measures are of importance, e.g. by passes, CO control, etc. [74,
TWGComments, 2004]
Low-temperature SCR requires catalyst regeneration due to salts formation (especially
ammonium chloride and ammonium sulphate). The regeneration may be critical because of the
salt sublimation may lead to exceedences of the applied ELV for releases to air for some
pollutants e.g. HCl, SO2, NOX. [74, TWGComments, 2004]
SCR is sometimes positioned directly after the ESP, to reduce or eliminate the need for
reheating in the flue-gas. When this option is used, the additional risk of PCDD/F formation in
the ESP (typically when the ESP operated at temperatures above 220 - 250 ºC) must be
considered. Such operation can result in increased PCDD/F emissions to ESP residues and
higher concentrations in the gas stream leaving the ESP and passing to the SCR unit.. SCR can
also be used for PCDD/F destruction. Multi layer SCR systems are used to provide combined
NOX and PCDD/F control.
Figure 2.50: SCR operating principle
[3, Austria, 2002]
The flue-gases discharged by the reactor may be directed through a gas-gas heat-exchanger to
preheat the entering gases in order to maintain the operating temperature of the catalyst and to
save a part of the imported energy (see diagrams in Section 4.4.4.1).
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Chapter 2
2.5.6 Techniques for the reduction of mercury emissions
2.5.6.1 Primary techniques
Mercury is highly volatile and therefore almost exclusively passes into the flue-gas stream. The
limit value set in the waste incineration directive is 0.05 mg/m3. Limit values as low as
0.03 mg/m³ as a daily average value (with continuous monitoring) have been set in some
European Member States [1, UBA, 2001]. Continuous measurement is also prescribed in some
national waste incineration legislation (e.g. Austria, Germany). The majority of installations
cannot meet these limit values, particularly for peak loads, without the addition of special gas
cleaning measures for Hg.
The only relevant primary techniques for preventing emissions of mercury to air are those which
prevent or control, if possible, the inclusion of mercury in the waste:
•
•
•
•
efficient separate collection of waste that may contain heavy metals e.g. cells, batteries,
dental amalgams, etc.
notification of waste producers of the need to segregate mercury
identification and/or restriction of receipt of potential mercury contaminated wastes
by sampling and analysis of wastes where this is possible
by targeted sampling/testing campaigns
where such wastes are known to be received - controlled addition to avoid overload of
abatement system capacity.
2.5.6.2 Secondary techniques
[1, UBA, 2001]Mercury vaporises completely at a temperature of 357 °C and remains gaseous
in the flue-gas after passing through furnace and boiler. Inorganic mercury (mainly Hg2+ as a
chloride) and elemental mercury are effected differently by FGT systems and detailed
consideration of the fate of both is required.
The selection of a process for mercury abatement depends upon the load fed in and upon the
chlorine content of the burning material. At higher chlorine contents, mercury in the crude fluegas will be increasingly in the ionic form which can be deposited in wet scrubbers. This is a
particular consideration at sewage sludge incineration plants where raw gas chlorine levels may
be quite low. If, however, the chlorine content in the (dry) sewage sludge is 0.3 % by mass or
higher, only 10 % of the mercury in the clean gas is elemental; and the elimination of only the
ionic mercury may achieve a total Hg emission level of 0.03 mg/Nm³. [74, TWGComments,
2004]
Metallic mercury can be removed from the flue-gas stream by:
•
•
transformation into ionic mercury by adding oxidants and then deposited in the scrubber the effluent can then be fed to waste water treatment plants with heavy metal deposition,
where the mercury can be converted to a more stable form (e.g. HgS), thus more suitable for
final disposal [74, TWGComments, 2004] or
direct deposition on sulphur doped activated carbon, hearth furnace coke, or zeolites.
Tests have shown that sulphur dioxide neutralisation in the furnace by adding limestone, can
reduce the proportion of metallic mercury, making overall Hg removal from the gas stream
more efficient.
In incineration plants for municipal and hazardous wastes, the chlorine content in the average
waste is usually high enough, in normal operating states, to ensure that Hg is present mainly in
the ionic form. However, specific inputs of certain waste may change the situation and metallic
mercury may need to be deposited, as mentioned above.
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High Hg wastes:
For the incineration of waste with a high mercury content in hazardous waste incineration plants
Hg deposition degrees of 99.9 % can only be ensured when highly chlorinated waste is also
incinerated in an appropriate proportion to the Hg load. Multistage wet scrubbing processes are
typical of this type of plant. High proportions of ionic Hg (e.g.>99.9 %) in the boiler crude fluegas before wet gas cleaning are caused by including highly chlorinated waste. This assists total
Hg removal from the flue-gas.
High chlorine total loads (approx. 4 % w/w input) and a therefore high interim Cl2 supply lead
to high Hg chlorination levels and Hg deposition of close to 100 %. With lower chlorine loads
the Hg deposition degree reduces rapidly.
Figure 2.51: Relationship between Hg emissions and the raw gas chloride content at a hazardous
waste incineration plant
Source [1, UBA, 2001]
2.5.7 Techniques for the reduction of other emissions of heavy metals
[1, UBA, 2001] Other heavy metals in incineration are converted mainly into non-volatile
oxides and deposited with flue ash. Thus, the main techniques of relevance are, therefore, those
applicable to dust removal (see Section 2.5.3).
Activated carbon is reported to be also used for reducing heavy metals emissions [74,
TWGComments, 2004]
2.5.8 Techniques for the reduction of emissions of organic carbon
compounds
Effective combustion provides the most important means of reducing emissions to air of organic
carbon compounds.
[1, UBA, 2001] Flue-gas from waste incineration plants can contain trace quantities of a very
wide range of organic species including:
•
•
•
•
halogenated aromatic hydrocarbons
polycyclic aromatic hydrocarbons (PAH)
benzene, toluene and xylene (BTX)
PCDD/F.
Polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) may form after the
furnace from precursor compounds. Precursor compounds are, for example, polychlorinated
biphenyls (PCB), polychlorinated diphenylmethanes (PCDM), chlorobenzenes and
chlorohydroxybenzenes.
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PCDD and PCDF may also form in catalytic reactions of carbon or carbon compounds with
inorganic chlorine compounds over metal oxides, e.g. copper. These reactions will occur
especially on fly ash or filter dust at temperatures between 200 and 450 °C.
The following three mechanisms are believed to lead to the formation of dioxin/furan in waste
incineration:
1. formation of PCDD/F from chlorinated hydrocarbons already in, or formed in the furnace,
(such as chlorohydrobenzene or chlorobenzene)
2. de-novo synthesis in the low-temperature range (typically seen in boilers, dry ESPs)
3. incomplete destruction of the PCDD/F supplied with the waste
Optimum flue-gas incineration largely destroys the precursor compounds. The formation of
PCDD/PCDF from the precursor compounds is, therefore, suppressed.
The emission limit value for the total of dioxins and furans in Directive 2000/76/EC is 0.1 ng ITEQ/m³. Adsorption processes and oxidising catalysts are available, amongst others, for
achieving this value. Oxidising catalysts are reported to also reduce emission of NH3-slip and
CO. [74, TWGComments, 2004]
Emissions of organic hydrocarbon compounds can also be reduced by further dust and aerosol
deposition, since these pollutant preferably adsorb onto the fine fraction of dust, and by
enforced flue-gas cooling (condensation).
2.5.8.1 Adsorption on activated carbon reagents in an entrained flow system
Activated carbon is injected into the gas flow. The carbon is filtered from the gas flow using
bag filters. The activated carbon shows a high absorption efficiency for mercury as well as for
PCDD/F.
Different types of activated carbon have different adsorption efficiencies. This is believed to be
related to the specific nature of the carbon particles, which are, in turn, influenced by the
manufacturing process.
2.5.8.2 SCR systems
SCR systems are used for NOX reduction (see description in Section 2.5.5.2.2). They also
destroy gaseous PCDD/F (not particle bound) through catalytic oxidation; however, in this case,
the SCR system must be designed accordingly, since it usually requires a bigger, multi-layer,
SCR system than for just the de-NOX function. Destruction efficiencies for PCDD/F of 98 to
99.9 % are seen.
The main reactions involved are: [74, TWGComments, 2004]
C12HnCl8 nO2 + (9 + 0.5 n) O2 => 12CO2 + (n-4)H2O + (8-n)HCl and
C12HnCl8 nO + (9.5 + 0.5 n) O2 => 12CO2 + (n-4)H2O + (8-n)HCl
2.5.8.3 Catalytic bag filters
(Belgium 2002) Filter bags are either impregnated with a catalyst, or the catalyst is directly
mixed with organic material in production of fibres. Such filters have been used to reduce
PCDD/F emissions.
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Gaseous PCDD/F can be destroyed on the catalyst rather than adsorbed in carbon (as with
carbon injection systems). The particle bound PCDD/F fraction can be removed by filtration.
The catalyst has no effect on mercury and therefore it is generally necessary to implement
additional techniques (such as activated carbon or sulphur reagent etc.) to remove mercury in
order to meet the modern Emission Limit Value in air. [74, TWGComments, 2004]
The gas temperature entering the filter bags should be above 190 °C in order to have effective
destruction of the PCDD/F and to prevent adsorption of PCDD/F in the media. [74,
TWGComments, 2004]
2.5.8.4 Re-burn of carbon adsorbents
[55, EIPPCBsitevisits, 2002] Carbon is used to adsorb dioxins (and mercury) at many waste
incinerators. Where processes have another outlet for the mercury that provides an adequate
removal rate, (i.e. a greater rate than the input rate to avoid circulation and hence emission
breakthrough) it is possible for the net dioxin emissions from the plant to be reduced by reburning the adsorbed PCDD/F by re-injection into the furnace. Usually the additional mercury
removal is provided by a low pH wet scrubbing system. Gas streams with low HCl
concentration may not find there are sufficient mercury removal rates to use this process.
Examples of the application of this technique include the re-burn of:
•
•
•
static coke bed adsorbents
entrained flow activated carbon adsorbents
carbon impregnated inserts used to adsorb dioxins in wet scrubbers and prevent memory
effects.
In some MSs local regulations do not allow re-burn.
2.5.8.5 Use of carbon impregnated plastics for PCDD/F adsorption
[58, Andersson, 2002] Plastics are widely used in the construction of flue-gas cleaning
equipment due to their excellent corrosion resistance. PCDD/F is adsorbed on these plastics in
wet scrubbers, where the typical operational temperature is 60 – 70 ºC. If the temperature is
increased by only a few degrees Celsius, or if the dioxin concentration in the gas is reduced the
absorbed PCDD/F can be desorbed to the gas phase and increase emissions to air. Lower
chlorinated PCDD/Fs are subject to the highest de-sorption rate increase with respect to
temperature rise. These can lead to increased TEQ values downstream of wet scrubbers.
The addition of a tower packing in the scrubber that contains polypropylene embedded with
carbon provides a means of selectively absorbing PCDD/F (Hg is not absorbed in the packing).
This material becomes saturated after a certain period of time. Therefore the charged material
can periodically be removed for disposal or, if permitted, burned in the furnace. [74,
TWGComments, 2004]
With inlet concentrations of 6 – 10 ng TEQ/Nm³, gas phase removal efficiencies in the range of
60 – 75 % are reported across a wet scrubber. This compares with 0 – 4 % without the
impregnated packing material. Absorption efficiency is reported not to decline not to have
declined over the test period (1 year). [58, Andersson, 2002]. [74, TWGComments, 2004]
An installation, like the one reported above, achieves outlet concentration of 2 – 3 ng TEQ/Nm³
which do not, on their own, comply with the 0.1 ng/Nm³ requirement of Directive 2000/76/EC.
The technique can also be used in a more extensive tower packing installation and/or in
combination with subsequent up-stream or downstream dioxin FGT to provide overall PCDD/F
compliance (also for start-up and with FGT devices in bypass). [74, TWGComments, 2004]
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2.5.8.6 Static bed filters
[1, UBA, 2001] Activated coke moving bed filters are used as a secondary cleaning process in
the flue-gas of municipal and hazardous waste incineration. Using this adsorption system, it is
possible to deposit substances contained in the flue-gas at extremely low concentrations with
high efficiency. Lignite coke produced in hearth furnace coke process is used in moving bed
absorbers.
Wet and dry coke beds are used in waste incineration. Wet systems have the addition of a
countercurrent flow of water that washes the cokes. In doing so, the reactor temperature is
lowered and some of the accumulated pollutants are washed from the filter. When activated
lignite is used in the place of cokes/coal, it does not require the preheating of the flue-gas above
the acid dew point and can even be effectively operated with “wet” or water saturated flue-gas.
For this reason the activated lignite absorber can be placed directly behind a wet flue-gas
scrubber. [64, TWGComments, 2003]
The flue-gases pass through a filling of grained Hearth Furnace Coke (HFC – a fine coke of
1.25 to 5 mm). The HFC’s depositing effect is essentially based upon mechanisms of adsorption
and filtration. It is thus possible to deposit almost all emission relevant flue-gas components, in
particular, residual contents of hydrochloric acid, hydrofluoric acid, sulphur oxides, heavy
metals (e.g. mercury), to sometimes below the detection limit.
An essential feature of the moving bed system is its high efficiency with all emissions due to the
large bulk of activated coke, so that variations from incineration and upstream flue-gas cleaning
caused by operation will not cause disadvantageous effects.
The flue-gas is guided to the activated coke filling over a distributor bed equipped with a
multitude of double funnels. The gas flows through them from the bottom to the top, while the
HFC passes through the absorber from the top to the bottom. By this, an ideal distribution of the
flue-gas over the whole cross-section of the absorber and an optimal utilisation of the absorber
capacity is achieved at a minimum consumption of activated coke.
Operating results from plants of an industrial scale (municipal and hazardous waste
incineration) have shown that the emission values, in particular for dioxins/furans, are well
below the limit values of Directive EC 2000/76/EC.
Care is required with such processes, to ensure temperature and CO are well monitored and
controlled, to prevent fires in the coke filter. This filter may become saturated after a certain
period of time and should then be disposed of and replaced.
2.5.8.7 Rapid quenching of flue-gases
This technique involves the use of a water scrubber to cool flue-gases directly from their
combustion temperature to below 100 ºC. The technique is used in some HWI. The action of
rapid quenching reduces the residence of flue-gases in temperature zones that may give rise to
additional de-novo PCDD/F synthesis.
The scrubber must be designed to cope with the high particulate (and other pollutant) loads that
will be transferred to the scrubber water.
The scrubbers used are single or multi stage, with the later stages sometimes cooled to reduce
evaporative water losses with the flue-gas.
A boiler is not used and energy recovery is limited to heat transfer from the hot scrubber
liquors.
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2.5.9 Reduction of greenhouse gases (CO2, N2O)
[1, UBA, 2001] There are essentially two ways of reducing greenhouse gas emissions:
•
•
increase the efficiency of energy recovery and supply (see Sections 2.4 and 4.3)
control CO2 emissions using flue-gas treatment.
Production of sodium carbonate by reacting CO2 in the flue-gases with NaOH is possible. This
technique is discussed further in Section 6.5 on emerging techniques.
2.5.9.1 Prevention of nitrous oxide emissions
Emissions of nitrous oxide from waste incineration can arise from:
•
•
use of lower combustion temperatures – typically this becomes of interest below 850 ºC
the use of SNCR for NOX reduction (particularly where urea is the reagent chosen).
[71, JRC(IoE), 2003] The optimum temperature for the simultaneous minimisation of both NOX
and N2O production is reported to be in the range 850 – 900 °C. Under conditions where the
temperature in the post combustion chamber is above 900 ºC the N2O emissions are reported to
be low. N2O emissions from the use of SCR are also low. Thus, provided combustion
temperatures are above 850 °C, in general, SNCR represents the only significant source of N2O
emissions at modern waste incinerators.
If not properly controlled, SNCR, especially with urea, can give rise to increased emissions of
nitrous oxide. Similarly, it is possible for nitrous oxide to be emitted from process with substoichiometric oxygen supply levels (e.g. gasification and pyrolysis process) and also from
fluidised bed furnaces operated under certain conditions. [74, TWGComments, 2004]
To avoid nitrous oxide emissions, the following techniques are used:
•
•
•
•
•
•
reduction of SNCR reagent dosing by SNCR process optimisation
selecting optimised temperature window for SNCR reagent injection
use of flow modelling methods to optimise injection nozzle locations
designing to ensure effective gas/reagent mixing in the appropriate temperature zone
over-stoichiometric burnout zones to ensure oxidation of nitrous oxide
utilization of ammonia instead of urea in SNCR.
2.5.10 Overview of flue-gas treatments applied at hazardous waste
incinerators
This section provides an overview of the flue-gas treatment techniques that are applied in the
Merchant HWI sector in Europe. For detailed descriptions of the FGT techniques themselves
see earlier in this chapter.
[EURITS, 2002 #41] After the steam generator or quench cooling, the flue-gases pass through
the flue-gas cleaning section. In almost 40 % of the installations, this section starts with a spray
dryer or a similar technique to cool the gases further, and to evaporate the waste water (in those
installations that do not have water discharges). Other installations just have an intermediate
quench step in order to reduce the flue-gas temperature for further treatment (e.g. 250 °C to
60 °C).
Different techniques are used to reduce the concentrations of polluting components in the fluegases; these are described below.
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Scrubber systems are used to reduce the acid components (e.g. as below Cl, S) in the flue-gases.
Almost 80 % of the installations are equipped with an acidic and an alkali wet scrubber system,
of which 30 % have an additional scrubber system for removal of specific components (e.g. Br,
I, Hg). The remaining 20 % use a dry scrubber with lime injection or the injection of
bicarbonate.
To decrease the amount of dust and heavy metals in the flue-gas, Electrostatic Precipitators
(ESPs) and bag-house filters are used:
•
•
•
•
54 % of the installations are equipped with a dry ESP (one installation with a wet ESP)
70 % of the installations are equipped with a bag-house filter
25 % of the installations combine these two techniques
one installation is equipped with two bag-house filters installed.
ESPs systems are normally installed in the front end of wet scrubbers to reduce the solid input
to the washing liquid, but not generally for dry or semi-dry treatment systems where bag filters
are used. The bag filters themselves provide a dust control system.
To reduce the release of dioxins to air, the following techniques are used:
•
•
•
activated carbon (or an alternative reagent such as brown coal cokes) is injected before the
bag-house filter (67 % of installations)
a fixed-bed activated carbon filter is used (17 % of installations); this can be either a dry
or wet system and alternatively brown coal cokes can be used as a reagent
one installation uses a Selective Catalytic Reduction (SCR) specifically to reduce dioxins,
as well as other organics and NOX.
Installations with very quick quench cooling and no boiler system do not use additional dioxin
abatement measures (8 %). The amount of dioxins in the flue-gases is very low due to the fast
cooling process. If the flue-gases are fed to an ESP after the quench step, the temperature must
be less than 220 °C in order to avoid dioxin reformation.
In order to reduce NOX emissions:
•
•
•
29 % of the installations use Selective Catalytic (SCR) or Selective Non Catalytic
(SNCR) Reduction (almost all in Germany)
three installations use an SNCR, and
four installation use an SCR.
58 % of installations already comply with the requirement in Directive 2000/76/EC for an ELV
of 200 mg/Nm³ without applying a specific NOX abatement technique. The remaining 42 %
installations are not currently equipped with a dedicated NOX removal system and do not yet
comply with this ELV.
2.5.11 Flue-gas treatment for sludge incinerators
[2, infomil, 2002] The type of FGT systems used depends largely upon the composition of the
waste, and will often be similar to those applied to municipal waste incinerators. However,
special attention may be required for removing nitrogen oxides (NOX) and mercury.
In two Dutch fluidised bed incineration plants, NOX emissions are reduced by the injection of
ammonia during the incineration process (SNCR). By using this system, it is possible to reduce
a normal emission concentration level of 100 – 200 mg/Nm³ to less than 70 mg/Nm³.
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During the sludge incineration process, mercury is mainly released in the metallic state. In
municipal waste incineration, due to the larger concentration of chloride in municipal waste
mercury is largely in the ionic state (mainly chloride). Metallic mercury is more difficult to
remove from the flue-gases than mercury in an ion state. Techniques for the reduction of Hg
emissions are described in Section 2.5.6.
2.6 Waste water treatment and control techniques
2.6.1 Potential sources of waste water
[2, infomil, 2002]
Potential emissions to water from waste incineration plants are as follows:
•
process waste water
Process waste water generally only arises to any significant degree from wet FGT systems.
Other types of flue-gas cleaning systems (dry and semi-dry) do not usually give rise to any
effluent. Measures can also be taken with wet systems so that the effluent arising is not
discharged from the installation (see later).
•
waste water from collection, treatment and (open air-) storage of bottom ash
This type of waste water can be used as the water supply for wet de-slaggers, and therefore,
normally it will not need to be discharged. It is, however, important to have sufficient storage
(and treatment) capacity, in order to be able to cope with fluctuations in storage levels, caused
by rainfall. Generally, the treatment options for excess water are: discharge to an available
process waste water treatment system; discharge to the local sewerage system; and/or special
disposal. This type of waste water can be re-used in the FGT system if the quality is suitable,
generally after treatment by sedimentation, filtration etc.
•
other less specific process waste water streams
For example, waste water from the water/steam cycle (resulting from the preparation of boiler
feed-water, boiler drainage, and cooling water discharge). In many practical situations, these
water flows can be re-used in the incineration and flue-gas treatment process (e.g. as make-up
water) and therefore will not lead to emissions to the environment. However, the recycling of
waste water to the FGT system is only possible in the case of semi-wet or wet system if the
quality of waste water is suitable; otherwise the waste water is discharged (mainly due to the
high salt content).
•
sanitary waste water
This originates from toilets, kitchens and cleaning. It is normally discharged to the sewerage
system, for treatment in a communal waste water treatment plant. A septic tank may be used if
there is no other possibility. As this category of waste water is not specific for waste
incineration, it is not discussed in this document.
•
clean rainwater
This arises from rain falling on non-polluted surfaces, such as roofs, service roads and parking
places, etc. Normally this water is discharged by a “clean” water collection system and is
discharged directly to the local surface water or via soakaways. Pretreatment may be required
for rainwater from roads or parking areas.
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•
polluted rainwater
This arises from rain falling on polluted surfaces (unloading activities etc). It is usually
segregated from clean water and may be treated before use or discharge.
•
used cooling water
By far, the largest cooling capacity is required where water condenser cooling is used, i.e. for
electricity production with a steam turbine. Depending on the design of the plant, various types
of cooling water streams will need to be disposed of.
These include:
•
•
•
cooling water from convection cooling of the condenser, which is connected with the steam
turbine
cooling water, drained off from an evaporation cooling water system, as used for condenser
cooling
cooling water from various other equipment parts which require cooling (waste chute,
hydraulic systems, strippers, etc.).
Because these cooling water streams are not specific for waste incineration, they are discussed
in the European “Reference document on the application of Best Available Techniques to
Industrial Cooling”.
•
condensed waste water from the partial pre-drying of sewage sludge
This type of waste water is specific to sewage sludge incineration, although it does not arise in
all cases as the steam generated during drying is sometimes evaporated with the incinerator fluegas instead of being condensed. It generally has a high Chemical Oxygen Demand (COD) and
contains substantial concentrations of N (mainly NH3), as well as other pollutants that were
originally present in the treated sludge. The high nitrogen content can form a bottleneck for
treatment; in this case stripping of nitrogen may be used, although there may be a risk of fouling
and additional energy requirements for its operation. A solution in this case may be recycling
into the furnace, when the recovered ammonia-solution (concentration approx. 10 %) can be
used for SNCR de-NOX feed.
2.6.2 Basic design principles for waste water control
[2, infomil, 2002]
The following basic principles are applied to incineration waste water control:
1. Application of optimal incineration technology
Running an optimised incineration process, important in terms of stability of the incineration
process, also provides an effective control of emissions to water where wet processes are used
(it is not relevant to other processes in respect of water releases because water releases do not
generally arise from non-wet processes). Incomplete incineration has a negative effect on the
flue-gas and fly ash composition, by increasing the presence of organic compounds with a
polluting and/or toxic character. This, in turn, can impact on the content of scrubber effluent.
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2. Reduction of water consumption and discharge of waste water
Some examples of measures which can be taken to achieve this are:
•
•
•
•
•
•
•
•
•
maximisation of re-circulation of polluted waste water in wet flue-gas treatment systems
(the scrubber), or semi-wet flue-gas treatment systems, including effective control of
process parameters, in order to reduce the amount of waste water for discharge
additional cooling of polluted waste water from wet flue-gas treatment systems (see also
condensing scrubbers 2.4.4.5), results in lower water losses to flue-gases and therefore in
reduced water consumption. This design can eliminate cooling water consumption.
application of waste water free flue-gas treatment technology (e.g. semi-dry or dry sorption
systems)
use of boiler drain water as water supply for the scrubber
treatment of laboratory waste water in the scrubber
application of waste water free de-slaggers
use of leachate of open-air bottom ash storage areas for supply of water to the de-slaggers
direct discharge of clean rainwater from roofs and other clean surfaces
use of segregated drainage and reduce the exposed surface areas used for waste storage and
handling (i.e. roofed enclosures).
3. Compliance with relevant water emission standards
Some process options will be greatly effected by local factors. An example of this, is the
discharge of salt effluent from scrubbers. While such discharges may be acceptable to marine
environments, discharges to fresh watercourses require the consideration of dilution factors etc.
Such decisions may, therefore, cause fundamental changes to incineration process design,
particularly the FGT system and effluent treatment selection.
4. Optimal operation of the water treatment systems
Discharges can only be reduced through the optimal operation of the treatment system.
Having sufficient storage capacity for the buffering of waste water storage, can allow time for
operators to react to disturbances in the process conditions.
2.6.3 Influence of flue-gas treatment systems on waste water
[2, infomil, 2002]
The production of waste water depends on the selected type of flue-gas treatment system. The
following main FGT options are used:
1. dry flue-gas treatment
2. semi-wet flue-gas treatment
3. wet treatment:
a) with physical/chemical scrubber effluent treatment
b) with in line scrubber effluent evaporation
c) with separate scrubber effluent evaporation.
Of these options, only option 3(a) has a waste water stream for discharge. Treatment options for
the scrubber effluent from system 3(a) are discussed in the following sections, along with the
techniques used to evaporate effluent (options 3b and 3c).
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2.6.4 Processing of waste water from wet flue-gas treatment systems
The process waste water resulting from wet flue-gas treatment contains a wide variety of
polluting components. The amounts of waste water and concentrations depend on the
composition of the waste and on the design of the wet flue-gas system. The re-circulation of
waste water in wet FGT systems can result in a substantial reduction in the amount of waste
water, and as a consequence, in higher concentrations of pollutants.
Three main methods are applied, for treatment of the waste water from wet flue-gas treatment
systems:
•
•
•
physico-chemical treatment based on pH-correction and sedimentation. With this system,
a treated waste water stream containing dissolved salts is produced, and if not evaporated
(see below) requires discharge
evaporation in the waste incineration process line by means of a spray drier, into a semiwet FGT system, or other system that uses a bag filter. In this case, the dissolved salts are
incorporated in the residue of the flue-gas treatment system. There is no emission of waste
water, other than that evaporated with the flue-gases (for more detail on in-line evaporation
see Section 2.6.4.7.1)
separate evaporation of waste water. In this case, the evaporated water is condensed, but
as it is generally very clean can often be discharged (or re-used) without special measures.
(for more detail on separate evaporation see Section 2.6.4.7.2).
These are discussed further in the following sections. Some of these techniques are also
described in the “Reference Document on Best Available Techniques in the Waste Water and
Waste Gas Treatment/Management Systems in the Chemical Sector” (CWW BREF).
If an SNCR is used for NOX control with a down stream wet FGT system, NH3 stripping may be
required. [74, TWGComments, 2004]
2.6.4.1 Physico-chemical treatment
A typical set-up of a physico-chemical treatment unit for process waste water is given in Figure
2.52 below:
From scrubber
Ca (OH)2
or
NaOh
Polyelectroytes sulfures
Complex builders
Ca(OH)
Waste
water
storage
Neutralisation
Flocculation
Precipitation
Endfiltration
Discharge
Filterpress
Filter cake
(landfill)
Figure 2.52: Process scheme for physico-chemical treatment of waste water from a wet flue-gas
treatment system
Source [2, infomil, 2002]
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The process shown consists of the following steps, some or all of these may be in use:
•
•
•
•
•
neutralisation of the polluted waste water
flocculation of pollutants
settlement of the formed sludge
dewatering of the sludge
filtration of the effluent (‘polishing’).
Other steps can also be included
•
•
•
precipitation (e.g. of heavy metals)
coagulation
pH and temperature control.
For the neutralisation, lime is often used. This results in the precipitation of sulphites and
sulphates (gypsum). Where discharging of sulphites/sulphates to surface water is allowed (e.g.
some marine environments), caustic soda (NaOH) can be used instead of lime, resulting in a
substantially lower production of filter cake.
Removal of heavy metal compounds is based on flocculation, followed by precipitation. Heavy
metal compounds have a very low solubility with a pH range of 9 - 11. Above a pH of 11 heavy
metals can re-dissolve again. The optimal pH is different for various heavy metal compounds.
In particular, the optimal pH for nickel and cadmium deviates from other heavy metals.
Two-step (or more) neutralisation improves the stability and control of discharge acidity (pH).
The first step is a coarse neutralisation, especially in the case of waste water from the first acid
step of the scrubber system. The second step is a fine neutralisation. The provision of sufficient
waste water storage capacity helps to reduce process variations in time, by providing a buffering
capacity.
The flocculation of heavy metal hydroxides takes place under the influence of flocculation
agents (poly-electrolytes) and FeCl3. The additional removal of mercury and other heavy metals
can be achieved if complex-builders are added.
The precipitation of fluorides requires a pH range between 8 and 9. [74, TWGComments, 2004]
Precipitation generally takes place in settling tanks or in lamellar separators.
The resulting sludge is normally dewatered in filter presses. Dry solids contents of 40 – 60 %
can be achieved, depending on the chemicals used and on other conditions.
If required, for filtration of the resulting effluent (“polishing”), sand filters and/or active carbon
filters can be used. The direct effect of sand filters is mainly a reduction of suspended solids, but
this also results in a reduction of heavy metal concentrations. Filtration with active carbon is
especially effective for a reduction of PCDD/F-compounds, PAHs, etc. The active carbon needs
to be replaced regularly. Other filtration systems are also used (e.g. disc filters).
Physico-chemical waste water treatment units require special operational attention, as they are
quite sensitive systems.
2.6.4.2 Application of sulphides
In order to carry out flocculation, organic agents (e.g. polyelectrolytes) are commonly used. The
addition of complex-builders and sulphides (e.g. Na2S, Tri-Mercaptan - TMT, etc.) allow further
reductions in mercury and other heavy metal discharges.
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The use of sulphides requires special safety regulations, because of their toxicity. One
advantage of their use is the lower costs of sulphides in comparison with other complexbuilders.
2.6.4.3 Application of membrane technology
One option for treatment of waste water polluted with salts and micro-pollutants is membranefiltration. This technique is especially efficient for large water flows with relatively low salt
concentrations. With higher salt concentrations, energy consumption increases rapidly.
The salt content of the process waste water of waste incineration is high (up to 10 w-%).
Therefore, this option usually requires significant additional energy consumption.
The remaining water with high solute concentration has to be removed in an appropriate outlet.
[74, TWGComments, 2004]
2.6.4.4 Stripping of ammonia
For the application of SNCR de-NOX, the waste water from the wet scrubber contains ammonia
compounds. The actual ammonia concentration depends on the process conditions of the SNCR
de-NOX unit. Depending on the actual ammonia concentration, stripping of ammonia from the
effluent may be an option.
An ammonia-stripping unit consists mainly of a heated distillation column. The vapours are
condensed, resulting in an ammonia solution. Though ammonia concentration is normally below
the original concentration of the trade product, the solution can be re-used in the SNCR-process.
Stripping of ammonia requires an increase in the pH to 11 - 12.5 and the use of steam. Fouling
risks are reported when used with lime neutralisation.
2.6.4.5 Separate treatment of waste water from the first and the last steps of the
scrubber system
The first step(s) of wet scrubber systems are typically operated at a very low pH-level. Under
these process conditions, specifically HCl is removed from the flue-gas stream. The removal of
SO2 takes place in the final step, at a neutral pH.
If these two effluent streams are dealt with separately the waste water treatment process can be
optimised for each stream and recyclable gypsum can be recovered from the SO2 scrubber
effluent.
The waste water from the first step of the scrubber is neutralised with lime, followed by removal
of heavy metal compounds by normal flocculation and precipitation. The treated waste water,
containing mainly CaCl2 is mixed with the waste water from the final step, mainly containing
Na2SO3/4. This results in the formation of gypsum and a liquid effluent, mainly consisting of
NaCl.
Depending on local conditions, this salty waste water is either discharged or evaporated.
Evaporation results in the production of NaCl, household salt.
Because the salt is separated from other flue-gas treatment residues contained in the effluent,
this results in a very substantial reduction in the mass of residues - the precipitated sludge of
heavy metal compounds is the only residue which remains.
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2.6.4.6 Anaerobic biological treatment (conversion of sulphates into elementary
sulphur)
One of the problems with discharging the treated waste water may be the remaining content of
sulphates. Sulphates can affect concrete sewerage systems. To solve this problem, a system has
been developed for anaerobic biological treatment of waste water from waste incineration.
The sulphates in the waste water can be reduced to sulphides in a reactor, by the activity of
anaerobic bacteria. The effluent of this reactor, which has a high content of sulphides, is treated
in a second reactor. In this second reactor, the sulphides are biologically oxidised in an aerobic
atmosphere into elemental sulphur. Care must be taken to ensure that adequate oxygen is
available in the aerobic stage, otherwise thiosulphate will be produced instead of elemental
sulphur and this will restrict disposal of the waste water.
Subsequently the sulphur is removed from the waste water in a laminated separator. The
collected sludge is dewatered in a decanter, resulting in a sulphur cake, which can be used. The
remaining waste water can be re-used in the scrubber and/or discharged.
It is reported that this technology may be difficult to apply in hazardous waste field [64,
TWGComments, 2003].
2.6.4.7 Evaporation systems for process waste water
If the discharge of soluble salts (chlorides) is not acceptable, the process waste water needs to
be evaporated. For this purpose two main options exist:
•
•
in-line evaporation
separate evaporation.
2.6.4.7.1
In-line evaporation
In this configuration, the waste water is recycled in the process by means of a spray dryer.
Figure 2.53 below gives an overview of the process configuration:
Filter cake
(optional)
(to landfill)
Incineration
Pre-removal
of dust
Waste water
Waste
water
treatment
Spraydryer
Absorbentsremoval
Scrubbersystems
Flue gas
polishing
Solid residue
(landfill)
Figure 2.53: In-line evaporation of waste water from wet scrubbing
[2, infomil, 2002]
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Chapter 2
The spray dryer is comparable with the spray absorber, used in the semi-wet FGT system. The
difference is that, in the case of semi-wet treatment, lime is injected and, for in-line evaporation,
the waste water from the scrubber is used for injection after a neutralisation step. This
neutralisation step can be combined with flocculation and the settling of pollutants, resulting in
a separate residue (filter cake). In some applications, lime is injected in the spray absorber for
gas pre-neutralisation.
The neutralised waste water, containing soluble salts, is injected in the flue-gas stream. The
water evaporates and the remaining salts and other solid pollutants are removed in a dust
removal step (e.g. ESP or bag filter). This flue-gas treatment residue consists of a mixture of fly
ash, salts and heavy metals.
Due to the application of a wet scrubbing system, the consumption of chemicals is
approximately stoichiometric and consequently residue production is lower than in semi-dry
FGT systems.
2.6.4.7.2
Separate evaporation
Separate evaporation is based on evaporation in steam heated evaporation systems. Figure 2.54
below gives an example of a process scheme.
Figure 2.54: Separate evaporation of scrubber effluent from wet scrubbing
[2, infomil, 2002]
The waste water, containing soluble salts is fed into a storage tank, containing a mixture of
waste water and already partially evaporated liquid. Subsequently, water is partly evaporated in
a reactor under low pressure. The required heat is supplied by (low-pressure) steam and
transferred to the liquid in a heat-exchanger. The surplus liquid flows back to the storage tank.
The vapours are cooled down, resulting in a clean condensate, which is then discharged.
Due to the increasing salt concentrations in the liquid, crystallisation of salts starts.
Subsequently, the salt crystals are separated in a decanter and collected in a container.
130
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Figure 2.54 shows a two-stage process, where two evaporators are installed. The input of heat
for the second evaporator is from the first evaporator, thus reducing the specific energy
consumption. Additionally, if not used for some other purpose (e.g. district heating) the
effective energy consumption may be reduced as low-pressure steam can be used.
This technique requires energy and there may be operational risks such as fouling of the
crystallisation. [64, TWGComments, 2003]
2.6.4.8 Example of process producing hydrochloric acid with downstream
cleaning
[1, UBA, 2001]
When wastes containing chlorine are combusted, hydrogen chloride is formed. Hydrogen
chloride is absorbed in water forming hydrochloric acid. The hydrochloric acid produced like
this, is a colourless liquid and free of impurities after treatment. It has a concentration of approx.
19 % by weight HCl and can be used as a raw material in different consumer installations, e.g.
for pH control in chlorine-producing plants.
In the production of hydrochloric acid, the flue-gases leaving the steam boiler are first
discharged into a quench and cooled down. The quench unit lining contains jets through which
hydrochloric acid from the downstream washing column is sprayed into the flue-gas. A portion
of the hydrochloric acid is then evaporated, which causes the flue-gases to cool down.
The hydrochloric acid is transferred from the quench to the washing column together with the
cooled flue-gas. In the washing column hydrogen chloride and other acid gases contained in the
flue-gas are absorbed. The hydrochloric acid is then transferred to a temporary storage tank. The
flue-gas, now stripped of hydrogen chloride, leaves the acid washing column via a mist
eliminator installed at the head of the column and enters the ionisation wet scrubber.
The hydrochloric acid generated in the acid washing column of the flue-gas washing system is
stripped of dissolved salts and solids in an evaporator system. This cleaning step can enable the
hydrochloric acid to be used as feedstock in a variety of production plants.
From the temporary storage tank, a pump transfers the hydrochloric acid to an evaporator. Here,
the raw acid is upgraded in a vacuum to become an azeotropic mixture. The excess water and
small amounts of hydrogen chloride pass into the vapour phase and are condensed with water in
an adsorption tower.
From the vacuum unit, the process liquid is pumped into the waste water plant together with the
excess water. The raw acid, upgraded to an azeotrope, will evaporate, and then condense again.
The remaining acid containing solids and heavy metals is drawn from the evaporator and
pumped into a mixer for neutralisation purposes.
[64, TWGComments, 2003]
2.6.5 Waste water treatment at hazardous waste incinerators
55 % of European HWI installations do not discharge waste water, they either use systems that
do not generate waste water (e.g. dry or semi-dry FGT) or evaporate the water via the stack by
means of spray dryers or in a separate evaporation plant, sometimes after treating the waste
water to remove Hg [74, TWGComments, 2004]
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Chapter 2
The remaining 45 % of the HWI installations have a waste water treatment facility. The
current situation is described in Figure 2.55 below and can be summarised as follows:
•
•
a general distinction can be made between the incinerators equipped with a boiler and the
other HWI installations equipped with a quick quench-cooling system, with the flow of
discharged effluent being greater for the latter due to technical reasons. (Note: some HWI
installations are equipped with both a quick quench-cooling and boiler) [74,
TWGComments, 2004]. Installations equipped with a boiler discharge between <1 and 5
l/kg incinerated waste. Installations with only quench-cooling systems discharge between
10 and 20 l/kg incinerated waste, although they can reduce their water flow to 5 l/kg by recirculating the effluent of the waste water treatment plant or recycling within the quench
unit itself
normally the effluents of the acidic section of the wet gas cleaning (containing NaCl, CaCl2,
Hg, CaF2 and SO3) are mixed with the effluent of the alkaline section (containing Na2SO4)
in order to precipitate part of the gypsum (and to decrease the sulphate content of the
effluent to less than 2 g/l, which is the solubility concentration of gypsum) before further
treatment. There is, however, one installation where the effluents of acidic and alkali
scrubbers are treated separately.
Waste
treatment
Quench
cooling
Boiler
Combined
treatment
Acidic/Basic
(gypsum precip.)
Single
discharge
to fresh water
Separate treatment
of Acidic/Basic
water streams
Single discharge
to the sea
Figure 2.55: Overview of applied waste water treatment systems at merchant HWIs
[EURITS, 2002 #41]
Whether an installation has an on-site waste water treatment plant or transfers the waste water to
an external treatment plant, depends on its location.
Figure 2.56 below gives a typical set-up of a waste water treatment plant for the treatment of
effluents from the wet flue-gas cleaning section from hazardous waste incineration.
The main elements of these facilities are:
•
•
•
neutralisation (e.g. addition of lime, NaOH/HCl)
the addition of reagents specifically for the precipitation of metals as hydroxides or metal
sulphides (e.g. flocculation agents, tri-mercapto-tri-azine, sulphides, polyelectrolyte)
the removal of sediment: either using sedimentation by gravity and decantation, or using
mechanical techniques such as filter press, centrifuge.
In some waste water treatment plants the waste water is polished by passing it through a sand
filter, followed by an activated carbon filter.
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Waste Incineration
Chapter 2
Filter
press
Active
carbon
filter
Sand filter
Sedimentation
Addition of
reagents
Neutralisation
Waste water
Filter cake
M
Discharge
M
= Measurement of emission parameters
Figure 2.56: Example of a waste water treatment facility in the merchant HWI sector
[EURITS, 2002 #41]
2.7 Solid residue treatment and control techniques
2.7.1 Types of solid residues
Waste incineration results in various types of solid residues, some of which have uses in
different countries to varying degrees. A distinction can be made between those residues
directly resulting from the incineration process and those resulting from the FGT system. The
FGT residues may be fine fly ash and/or reaction products and unreacted additives from the
FGT system (or associated waste water treatment system). The latter category is often called
Flue-gas Treatment (FGT) or Air Pollution Control (APC) residues. The solid residues from
(wet) scrubber effluent treatment processes are often pressed to form a solid called a filter cake
or mixed with fly ash to minimise volume or for better dewatering with gypsum from the plant.
[74, TWGComments, 2004] In addition gypsum and salt may be recovered from wet flue-gas
treatment systems if specific processes are used (see below and Section 2.6). [64,
TWGComments, 2003]
Residues arising from the combustion stage of the incinerator are:
Municipal waste incineration:
•
•
•
bottom ash, resulting from grate incineration of municipal waste. Because of its large
volume, this is an important type of residue, options for its use are discussed in Section
3.4.2
boiler ash, is collected in the boiler of municipal waste incineration plants and often treated
together with fly ash [74, TWGComments, 2004]
fly ash, is collected in a dust removal step in municipal waste incineration and is discussed
further below under FGT residues. This type of waste is generally disposed of, often after
pretreatment, but has been used as a filling material for bitumen bound applications in civil
construction, in countries where this practise is permitted. [74, TWGComments, 2004]
Treatment and disposal are further discussed below.
Hazardous waste and specific clinical waste:
•
•
slag, resulting from rotary kiln incineration of hazardous wastes. In general, this type of
residue is disposed of by landfill without further treatment, or may be recycled if locally
permitted
other ashes are similar to those from MSWI but in as they may contain higher levels of
pollutants, general the practice has been mostly for their disposal.
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Chapter 2
Sewage sludge:
•
•
fly ash, resulting from fluidised bed incineration of sewage sludge. This type of waste can
be used as a filling material for bound applications in civil construction, in countries where
this practice is permitted. It is also used as a filling material for mines in Germany, both
applications without further treatment. Fly ash which is not used, is landfilled
bed ash, resulting from fluidised bed incineration of sewage sludge. This is a relatively
small category. It is often added to the fly ash or landfilled without further treatment.
RDF:
•
•
bed ash, resulting from fluidised bed incineration of RDF. Depending on the specific
characteristics of the material, bed ash amounts may be substantially higher than for sewage
sludge incineration. There is little experience of its re-use
ash, resulting from small and medium scale incineration of waste wood. This concerns
relatively small quantities and is not further discussed.
Some installations operate at especially high temperatures (e.g.>1400 °C) with the specific aim
of melting the ash in order to form a slag. Such slags may have improved use options owing to
lower leachability etc. High temperature slagging rotary kilns and combined gasificationcombustion process provide examples of such systems. The latter is used in Japan, where very
strict leachability criteria are applied to MSWI residues, specifically to increase residue re-use
and reduce the need for landfill.
Both within and beyond Europe there are variations in policy and procedures regarding the reuse of residues from incinerators. [74, TWGComments, 2004]
The second category of residues are the FGT residues:
FGT residues contain concentrated amounts of pollutants (e.g. hazardous compounds and salts)
and therefore normally are not considered appropriate for recycling purposes. The main
objective is then to find an environmentally safe final disposal option. The following types of
flue-gas treatment residues can be distinguished:
•
•
•
•
134
residues from dry and semi-wet flue-gas treatment. These residue are a mixture of calcium
and/or sodium salts, mainly as chlorides and sulphites/sulphates. There are also some
fluorides and unreacted reagent chemicals (e.g. lime or sodium carbonate). This mixture
also includes some fly ash that has not been removed by any preceding dust removal step. It
can, therefore, also include polluting heavy metals and PCDD/F. The normal way of
disposal is landfilling as hazardous waste, (e.g. big-bags). The leachability of the residues is
an important aspect for subsequent landfill disposal, therefore treatments to lower the
leachability of these residues prior to landfilling is currently used in Europe (e.g. Austria,
the Netherlands, Portugal, France). The FGT residues coming from the dry sodium
bicarbonate process can be purified and recycled in an industrial process, e.g. as raw
material in the chemical industry; this can require segregation of fly ash and salt residues
(e.g. two stages of flue-gas filtration) in order to reduce the inert content. The transport to
the end-user can be a critical factor for economics. [74, TWGComments, 2004]
improvement of the properties for landfilling by cold solidification
filter cake from the physico/chemical treatment of waste water from wet flue-gas treatment.
This material is characterised by a very high heavy metals content, but can also include salts
of limited solubility, such as gypsum. The normal way of disposal is landfilling (as
hazardous waste). These residues may be concentrated in PCCD/F and are therefore
sometimes pretreated before landfilling
gypsum. Gypsum may also be recovered with or without cleaning depending on the process
parameters and quality requirements. Recovery of gypsum is possible when limestone or
hydrated lime is used in a two stage wet scrubber with an efficient droplet separator. [74,
TWGComments, 2004] The recovered gypsum can be re-cycled in some circumstances
Waste Incineration
Chapter 2
•
•
•
•
salts, resulting from in-line evaporation of waste water. This residue is comparable with the
residue from (semi-)dry flue-gas treatment
salts, resulting from separate evaporation of waste water. Salt use or disposal depends on
the composition of the residue. It is usually more pure than where in-line evaporation has
been carried out
residues from flue-gas polishing. Options for use depend on the adsorbent used (activated
carbon, cokes, lime, sodium bicarbonate, zeolite). The residue of (activated) carbon from
fixed bed reactors is sometimes permitted to be incinerated in the waste incineration plant
itself, if certain process conditions are fulfilled. The residue of entrained bed systems can
also be incinerated, if the applied adsorbent is activated carbon or oven cokes only. If a
mixture of other reagents and activated carbon is used, the residue is generally sent for
external treatment or disposal, since there might be risks of corrosion. If zeolite is used,
there are in principle possibilities to recover the mercury, but these techniques are not yet
available in practice. [2, infomil, 2002] [64, TWGComments, 2003]
use as filler material in salt mines – in some MSs FGT residues of various types are used as
fill material in mines.
2.7.2 Treatment and re-cycling of solid residues
The high mineral content of incineration ash residues can make them potentially suitable for use
as road or other construction material. Use is possible if the material complies with a set of
environmental and technical criteria. This requires an optimisation of the ash quality through
primary or secondary measures. The general parameters of concern are:
•
•
•
•
•
burn-out
mineral reactivity
metal leaching
salt content
particle size and particle size distribution.
Residues from many modern waste incineration plants fulfil the environmental and technical
requirements for these quality parameters. Regulatory and political barriers sometimes provide
the main barriers to the use of (in particular) bottom ashes, from suitably designed/operated
installations.
Residue treatment methods generally aim to optimise one or more of these parameters in order
to mimic primary construction material quality. Due to its large production volume, lower
hazardous character and leachability, treatment for recycling is mainly applied to MSW bottom
ash. Bottom ash use is promoted in the Netherlands (>90 % used), Denmark (90 %), Germany
(80 %), France (>70 %), Belgium and the UK (21 %).
[Vehlow, 2002 #38], [Vrancken, 2001 #39], [56, UKEnvAgency, 2002], [64, TWGComments,
2003], [74, TWGComments, 2004]
Filter and boiler ash treatment is performed in only a few installations in Europe. In The
Netherlands fly ash from MSWI and SSI plants is applied as filling material for road
construction materials (asphalt) without any pretreatment at the incineration plant. About 1/3rd
of the total fly ash from MSWI plants and 80 % of the fly ash from SSI plants (approx.
80000 tonnes total yearly) has been used in this way. [74, TWGComments, 2004]
Primary measures for controlling residue outputs involve optimising control of the combustion
process in order to [Vehlow, 2002 #38]:
•
•
•
guarantee an excellent burn-out of carbon compounds
promote the volatilisation of heavy metals such as, Hg and Cd out of the fuel bed, and
fix lithophilic elements in the bottom ash, thus reducing their leachability.
Waste Incineration
135
Chapter 2
Secondary treatment systems involve one or more of the following actions:
•
•
•
•
•
•
size reduction, to allow metal segregation and improve technical quality
segregation of ferrous and non-ferrous metals, which may be recycled in the metals industry
washing, in order to remove soluble salts
ageing, to stabilise the matrix structure and reduce the reactivity
treatment with a hydraulic or hydrocarbon binder, for re-use as road base
thermal treatment, to make and contain inert metals in a glassy matrix.
Both primary and secondary measures will be discussed in more detail in Section 4.6.
2.7.3 Treatments applied to Flue-gas treatment residues
The information in this section is taken from [48, ISWA, 2003]. Further details of the
techniques that fall within each of the categories of treatment given below can be found in
Section 4.6
2.7.3.1 Solidification and chemical stabilisation of FGT residues
The main purpose of solidification is to produce a material with physical and mechanical
properties that promote a reduction in contaminant release from the residue matrix. An addition
of cement, for example, generally decreases hydraulic conductivity and porosity of the residue,
and, on the other hand increases durability, strength and volume. In addition, it usually increases
the alkalinity of the mixture, therefore improving the leaching behaviour of the product,
although the solubility of amphoteric metals, such as lead and zinc, may result increased.
The solidified product is usually cast into blocks (e.g. 1 m³) or landfilled directly. A major
consideration here is to reduce the interaction between the water and the residue. According to
Swiss studies, this only influences the leaching behaviour of landfilled products over the first
few years of storage.
Solidification methods commonly make use of several, mostly inorganic, binder reagents:
cement, lime and other pozzolanic materials such as coal fly ash, blast furnace bottom ash or
cement kiln dust, although some organic binders such as bitumen/asphalt or paraffin and
polyethylene can also be used. Combinations of binders and various types of proprietary or nonproprietary additives are used as well. The most prevalent solidification technique is by far
cement stabilisation.
The main concept of chemical stabilisation is to bind the heavy metals in more insoluble forms
than they are present in the original untreated residues. These stabilisation methods make use of
both the precipitation of metals in new minerals as well as the binding of metals to minerals by
sorption. This process includes the solubilisation of the heavy metals in the residues and a
subsequent precipitation in, or sorption to, new minerals.
Several of the stabilisation methods incorporate an initial washing step where a major part of
soluble salts and to some extent metals are extracted before chemical binding of the remaining
metals. These methods can be completed by dewatering the stabilised product and removal of
organic compounds.
[74, TWGComments, 2004]
2.7.3.2 Thermal treatment of FGT residues
The thermal treatment of incineration residues (sometimes FGT and bottom ash are mixed
together for treatment) takes place extensively in a few countries, mainly to reduce volume of
the residues, but also to reduce its organic and heavy metal content and to improve the leaching
behaviour before landfilling. [74, TWGComments, 2004]
136
Waste Incineration
Chapter 2
Thermal treatment can be grouped into three categories: vitrification, melting and sintering. The
differences between these processes are chiefly related to the characteristics and properties of
the final product:
•
Vitrification is a process where residues are treated at high temperature (currently 1300C to
1500 °C and then quickly quenched (with air or water) to obtain an amorphous glassy
matrix. After cooling down, the melt forms a single phase product called a vitrificate. The
vitrificate can be a glass like or stone-like product depending on the melt composition.
Additives are sometimes added to the residues to favour the formation of the glassy matrix
[64, TWGComments, 2003]
•
Melting is similar to vitrifying, but the quenching step is controlled to allow crystallisation
of the melt as much as possible. It results in a multi-phase product. Temperatures and the
possible separations of specific metal phases are similar to those used in vitrifying. It is also
possible to add specific additives to favour the crystallisation of the matrix. [64,
TWGComments, 2003]
•
Sintering involves the heating of residues to a level where bonding of particles occurs and
the chemical phases in the residues reconfigure. This leads to a denser product with less
porosity and a higher strength than the original product. Typical temperatures are around
900 °C. When MSW is incinerated, some level of sintering will typically take place in the
incineration furnace. This is especially the case if a rotary kiln is used as part of the
incineration process.
Regardless of the actual process, the thermal treatment of residues in most cases results in a
more homogeneous, denser product with improved leaching properties. Vitrifying also adds the
benefits of physical containment of contaminants in the glass matrix.
The energy requirements of stand alone treatments of this type, are generally very high. The
main problem is the heat transport into the melting reactor. [74, TWGComments, 2004] In some
cases residue melting is achieved within the installation (i.e. not in a separate melting process)
using a higher temperature combustion stage (see 2.3.4.4.3). In such cases the energy demand is
partially met by the use of the flue-gas thermal energy and external energy input requirements
may be reduced.
The flue-gas issued from thermal treatment of solid residues may contains high levels of
pollutants such as NOX, TOC, SOX, dust and heavy metals etc. Therefore appropriate flue-gas
treatment is required. Sometimes the flue-gas produced is fed into the FGT of the incinerator if
nearby. [74, TWGComments, 2004]
The high salt concentrations in FGT residues can cause corrosion problems in the flue-gas
treatment from such processes. Sintering is not used as a dedicated treatment option for FGT
residues, although some combined treatments do involve this.
2.7.3.3 Extraction and separation of FGT residues
Treatment options using extraction and separation processes can, in principle, cover all types of
processes extracting specific components from the residues. However, most emphasis has been
put on processes involving an extraction of heavy metals and salts with acid.
Several techniques have been proposed both in Europe and in Japan. Most of these techniques
make use of the acidic solution from the first scrubber in wet FGT systems.
Waste Incineration
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Chapter 2
2.7.3.4 Chemical stabilisation of FGT residues
The main concept of chemical stabilisation is to bind the heavy metals in more insoluble forms
than they are present in the original untreated residues. These stabilisation methods make use of
both the precipitation of metals in new minerals as well as the binding of metals to minerals by
sorption. This process includes the solubilisation of the heavy metals in the residues and a
subsequent precipitation in, or sorption to, new minerals.
Several of the stabilisation methods incorporate an initial washing step where a major part of
soluble salts and to some extent metals are extracted before chemical binding of the remaining
metals. These methods are completed by dewatering the stabilised product.
2.7.3.5 Other methods or practices for FGT residues
A commonly used management option at incinerators with wet cleaning systems is to combine
the fly ash with the sludge produced by treating the scrubber solutions; the resulting product is
called a Bamberg cake. Sulphides in the sludge used in the waste water treatment facility to
precipitate heavy metals can further help decrease leachability of heavy metals from the
Bamberg cake in a landfill. This method has been used for more than a decade to improve
residue properties before landfilling.
It is also possible to contact the fly ash with the acidic waters of a scrubber. It is reported that
this can achieve very significant extraction of the heavy metal and organic components. [74,
TWGComments, 2004]
2.8 Monitoring and control techniques
2.8.1 Incineration control systems
[2, infomil, 2002]
One of the main challenges with waste incineration results from the often wide variation in
waste composition, including differences in some properties that have a significant effect on the
incineration process. Because of these wide differences, incineration processes have been
developed to cope with large variations in process conditions. However, when unfavourable
process conditions occur, interventions in the operation are still required.
The introduction of sophisticated control systems is, therefore, an important development.
These systems result in an incineration process that has less variations in time (improved
stability) and space (more homogeneous). The improved process control has many potential
advantages, such as (note: the main reason(s) for the improvement are given in parentheses):
•
•
•
•
•
•
•
•
•
•
138
better bottom ash quality (due to sufficient primary air distribution and a better positioning
of the incineration process on the grate)
less fly ash production (due to less variations in the amount of primary incineration air)
better fly ash quality (less unburned material, due to more stable process conditions in the
furnace)
less CO and CxHy-formation (due to more stable process conditions in the furnace; i.e. no
'cold' spots)
less NOX formation (due to more stable process conditions in the furnace; i.e. no 'hot' spots)
better utilisation of the capacity (because the loss of thermal capacity by variations is
reduced)
better energy efficiency (because the average amount of incineration air is reduced)
better boiler operation (because the temperature is more stable, there are less temperature
'peaks' and thus less risk of corrosion and clogging fly ash formations)
better operation of the flue-gas treatment system (because the amount and the composition
of the flue-gases is more stable)
the indicated advantages also result in less maintenance and better plant availability.
Waste Incineration
Chapter 2
In order to be able to control the incineration process, detailed process information is required, a
control system ('philosophy') must be designed, and it is necessary to be able to intervene in the
process. Design of the overall control system depends on the specific grate and furnace design
of each supplier. Therefore, this section only provides an overview of potential process
information, control philosophy systems and process interventions.
Process information may include:
•
•
•
•
•
•
•
grate temperatures for various positions
thickness of waste layer on the grate
pressure drop over the grate
furnace and flue-gas temperatures at various positions
determination of temperature distribution over the grate surface by optic or infrared
measurement systems
CO-, O2-, CO2- and/or H2O-measurements (at various positions)
steam production.
The control philosophy may be a classic control system, which is part of the process control
computer. Additionally, fuzzy control systems are applicable.
Control interventions include:
•
•
•
•
•
the dosing system for the waste
frequencies and speed of grate movements in various parts of the grate
amount and distribution of primary air at the various grate compartments
temperature of the primary air (if preheating facilities are available)
amount and distribution of secondary air in the furnace (and, if available, of re-circulated
flue-gas).
2.8.2 Overview of emissions monitoring carried out
General information on emissions monitoring is presented in the BREF “Reference Document
on the General Principles of Monitoring” (MON code).
[1, UBA, 2001]
The recent EU directive (2000/76/EC) on the incineration of waste includes requirements for
emissions measurement.
The following emission compounds are to be measured on a continuous basis:
•
•
•
•
•
•
•
dust
HCl
SO2
CO
CxHy
NOX (if emission standards apply)
HF (but not if the process ensures adequate HCl- removal).
Continuous measurements are not imperative for HCl, HF and SO2, where the process is such
that it is not possible that emission standards to be exceeded (Art.11 (6) of EU Directive
2000/76/EC).
Waste Incineration
139
Chapter 2
Additionally, the following process parameters need to be monitored continuously:
•
•
•
•
•
furnace temperature
O2
pressure
flue-gas outlet temperature
water vapour content (unless emission measurements are executed in dried flue-gas).
Other emission compounds to be measured on a regular base (minimum of 2 – 4 times per year)
are:
•
•
heavy metals
PCDD/F.
Measurement techniques for Mercury (Hg) and dioxins (PCDD/F’s) are relatively complicated
and expensive.
Measurements of mercury are more complicated than measurements of other heavy metals, as a
substantial part of the emitted mercury is in the gaseous state. Some analysers measure only
elemental mercury, and others can measure total mercury (e.g. ionic and elemental mercury). In
the last decade, measurement systems for mercury have become more sophisticated. Older
measurements were often unreliable, as the gaseous part of the mercury emission was neglected.
Continuous measurement of Hg has proven to be a reliable method within certain borders and it
is prescribed in some national legislations (e.g. in Germany and in Austria).
While there is not currently a continuous measurement system for dioxins. However, a
continuous sampling system is available. This system is operational in some waste incineration
plants in Austria and Belgium and has been operated for six months in a Dutch hazardous waste
incineration plant. Samples can be analysed as frequent as necessary or desirable.
In some cases actual impacts of the emissions may be assessed by bio-monitoring (e.g. with
lichen). Although it may be difficult to attribute impacts to individual sources, such monitoring
may make a useful contribution in respect of the assessment of combined impacts where there
are multiple sources.
[74, TWGComments, 2004]
2.8.3 Experiences with continuous sampling of dioxin emissions
(Belgium 2002)
According to the EN1948 standard, dioxin emissions from waste incinerators are sampled
during 6 to 8 hours. This measurement is generally carried out once or twice a year, although at
much greater frequencies in some cases.
Continuous sampling has proven to be useful for the assessment of dioxin emissions during
unfavourable process conditions. The technique has been used to demonstrate low PCDD/F
emissions over the entire range of operational conditions. The results can also be used to guide
technological improvements, revised monitoring requirements, or other changes.
Cost data for continuous sampling of dioxins (from Indaver):
Investment:
EUR 110000 - 140000
Testing of the system:
EUR 4900 (estimation)
Analysis (26 samples/yr):
EUR 20000/yr
Maintenance by the supplier (preventive):
EUR 2500/yr
140
Waste Incineration
Chapter 2
2.8.4 Experiences with continuous measurement of mercury emissions
Continuous measurement and recording of emissions of mercury and its compounds has been
required by law for waste incineration installations in Germany since 1999, except those
installations where it can be reliably proven that mercury levels are less than 20 % of the
defined limits.
Continuous monitoring of a HWI is also reported to have been carried out since 1992 using a
reduction unit and cold vapour instrument.
The standard reference method for comparative measurements during calibration is the
potassium permanganate method in accordance with EN 13211. It should be noted that this
method determines the total mercury content (i.e. metallic/elemental Hg + ionic Hg), while
some Hg analysers only detect the proportion of metallic mercury.
During the test, the instrument is calibrated using test gases. The test gases must be produced
immediately before being used (e.g. by setting the required gas pressure in the gas phase over a
mercury reactor). When using test gas, it may be necessary to take the cycle time of the
measuring device into consideration. In the same way, the sampling interval for the comparative
measurements must be adjusted to the enrichment phase for the measurement device.
Examples of suitability-tested continuous working measuring devices for emission
measurements of mercury are listed in the table below:
Suitable
measurement
devices
Type
OPSIS AR 602 Z
Manufacturer/Distribution
OPSIS AB
HG MAT II
HGMAT 2.1
HM 1400
HG 2000
MERCEM
SM 3
Quecksilbermonitor
Hg 2010
Hg-CEM
HM 1400 TR
MERCEM
Seefelder Messtechnik
Seefelder Messtechnik
VEREWA
SEMTECH AB
Bodenseewerk Perkin-Elmer
Mercury Instrument und IMT
Innovative Messtechnik
SEMTECH AB
Seefelder Messtechnik
VEREWA
SICK UPA
Announcement in the
GMBI
Year
1994
1996
1995
1998
1996
1996
1996
No.
289
42
7
20
28
28
28
Page
869
882
101
418
592
592
592
1999
33
720
2000
2000
2001
2001
60
60
19
19
1193
1193
386
386
Table 2.18: Tested continuous working measuring devices for emission measurements of mercury
[64, TWGComments, 2003]
Cost data for continuous measurement of mercury (estimated):
Investment:
EUR 30000
Testing of system:
EUR 5000
Waste Incineration
141
Chapter 2
2.8.5 Overview of safety devices and measures
This section deals with safety in the sense of preventing accidents that could give rise to
pollutant emissions.
[64, TWGComments, 2003]
Plant safety is an important aspect in the planning, establishment and operation of waste
incineration plants. To ensure a high level of plant safety and operational safety, the safetyrelevant parts of the installation are equipped with protective systems. These are to prevent, as
far as possible, the occurrence of malfunctions or accidents with the potential to cause negative
effects on the environment in the vicinity of the plant, or to reduce such effects if a malfunction
or accident occurs.
Safety-relevant parts of waste incineration plants and, therefore, potential sources of danger
include, in particular areas in which certain substances are present or can be formed in safetyrelevant quantities.
These are, in particular:
•
•
•
the waste bunker and other areas for the storage of potentially hazardous waste
the combustion and flue-gas purification plants, and
storage facilities for necessary auxiliaries (e.g. ammonia, activated carbon, etc.).
Protective systems used to control risks include:
•
•
•
•
•
•
•
•
systems for controlling the release of pollutants, such as retention systems for used firefighting water, bunding of tanks for substances constituting a hazard to water
fire protection systems and devices such as fire walls, fire detectors, fire extinguishing
systems
systems for protection against explosions, such as pressure relief systems, bypasses,
arrangements for avoiding sources of ignition, inert gas systems, earthing systems etc.
systems for protection against sabotage (e.g. building security, access control and
surveillance measures)
systems for protection against lightning strike
fire dividing walls to separate the transformers and retention devices
fire detection and protection where low voltage power distribution panels are located
pollutant detection (ammonia, gas etc.) near corresponding storage, distribution etc.
Other plant components required for operational safety:
•
•
•
machines and equipment designed to ensure input and output of energy (e.g. emergency
power generator)
components for the discharge, removal or retention of hazardous substances or mixtures of
hazardous substances, such as holding tanks, emergency relief and emptying systems
warning, alarm and safety systems, which trigger when there is a disruption of normal
operations, prevent a disruption of normal operations or restore normal operations. This
includes all instrumentation and control systems of a plant. In particular, it includes all
instrumentation and control systems for the various process parameters which are essential
to secure normal operations, on the one hand, and which, in the event of a disturbance bring
the affected plant components to a safe condition and inform the operating personnel of the
disturbance in good time, on the other.
The response of a protective device to a malfunction or an accident may cause a temporary
increase in pollutant emissions. The aim of all safety measures must be to keep this time span to
a minimum and to restore the safety of the plant.
[64, TWGComments, 2003]
142
Waste Incineration
Chapter 3
3 EMISSIONS AND CONSUMPTIONS
3.1 Introduction
Emissions and consumptions at waste incinerators are mainly influenced by:
•
•
•
waste composition and content
furnace technical measures (design and operation)
design and operation of flue-gas cleaning equipment.
Emissions to air:
Emissions of HCl, HF, SO2, NOX, and heavy metals depend mainly on the structure of the waste
and the flue-gas cleaning quality. CO and VOC emissions are determined primarily by furnace
technical parameters and the degree of waste heterogeneity when it reaches the combustion
stage. The furnace design and operation to a large extent also effect NOX. Dust emissions are
very dependent upon flue-gas treatment performance. PCDD/PCDF emissions to air depend on
waste structure, furnace (temperature and residence times) and plant operating conditions
(reformation and de-novo synthesis are possible under certain conditions) and flue-gas cleaning
performance.
Municipal waste incineration plants generally produce flue-gas volumes (at 11 % oxygen) of
between 4500 and 6000 m³ per tonne of waste. For hazardous waste incineration plants, this
value (at 11 % oxygen) is generally between 6500 and 10000 m³, depending mainly on the
average thermal value of the waste. Plants using pyrolysis, gasification or oxygen enriched air
supply results in lower flue-gas volumes per tonne of waste incinerated.
The emission levels to air noted in this document are reported over specified averaging periods
– usually annual, daily and half-hourly averages. Some installations, particularly those that treat
highly heterogeneous wastes, may experience transient conditions that give rise to instantaneous
emission concentrations that are outside of the numerical range of the averaged levels. [64,
TWGComments, 2003]
Emissions to water:
Depending on the type of flue-gas cleaning applied, emissions into the medium water may also
occur. Wet flue-gas cleaning is the main source of effluents, although in some cases this effluent
is also eliminated by evaporation.
Some other waste water streams may arise from storage, boilers etc. These have already been
described in Section 2.6.1.
Solid residues:
Solid residues that may arise are:
•
•
•
•
bottom ash or slag – mainly the incombustible fraction of the waste
boiler ash – the ash that accumulates and is removed from the boiler
fly ash – the light ash that travels with the flue-gas and is then removed by FGT equipment
air pollution control residues accumulated, reacted and un-reacted that are accumulated in
the FGT equipment
• waste water treatment residues.
[64, TWGComments, 2003]
Waste Incineration
143
Chapter 3
The production and content of these solid residues is influenced by:
•
•
•
waste content and composition, e.g. different ash contents vary the amount of bottom ash
arising, or different substances that will end up in flue-gas cleaning residues
furnace design and operation, e.g. pyrolysis plants deliberately produce a char in place of
the ash, and higher temperature furnaces may sinter or vitrify the ash and volatilise some
fractions
flue-gas treatment design and operation, e.g. some systems separate dusts from chemical
residues, wet systems produce an effluent for treatment to extract solids.
Energy output from the installation:
The major influences on the achieved export levels are:
•
•
availability of an energy user (particularly for heat/steam supply)
installation design (particularly for electrical output where the steam parameters chosen for
electrical generation have a significant influence on electrical generation rates).
The energy output system design adopted is often heavily influenced by the income to be
derived from the sales of the energy supplied. Relative and absolute prices of heat, steam and
electricity all have an influence the final design and hence the energy output and efficiency
levels achieved.
Energy consumption by the installation itself:
Main influences are:
•
•
the waste composition - some wastes require the addition of fuels to assist their treatment
others are auto thermal i.e. they generate sufficient heat to support the combustion without
additional fuel input
the design of the installation e.g. varying energy requirements of different flue-gas
treatment equipment designs in general, the lower the required emissions to air the higher
the energy consumption by FGT.
Other consumptions:
The consumption of chemical reagents is mainly associated with the design and operation of
flue-gas cleaning equipment - which, to a large degree, is dependent upon waste type and the
desired air emission levels – lower air emissions generally require higher reagent dosing rates.
3.1.1 Substance partitioning in waste incineration
[1, UBA, 2001]
As a result of their chemical properties, the different elements contained in the waste are
distributed differently in the incineration process. Table 3.1 gives an example of this distribution
on the basis of Austrian examinations at the waste incineration plant of Spittelau, Vienna.
This distribution varies from plant to plant, depending on the flue-gas cleaning method used,
waste type and other factors, but these figures provide a guide to the percentage distribution of
various substances in a MSWI. The installation concerned uses an ESP as a pre-deduster, before
wet FGT, with an ETP treating the scrubber effluent:
144
Waste Incineration
Chapter 3
Cleaned
flue-gas
discharge
98 (+/-2)
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
Substance
Carbon %
Chlorine %
Fluorine %
Sulphur %
Phosphor %
Iron1 %
Copper %
Lead %
Zinc %
Cadmium %
Mercury %
ESP dust
Waste water
<1
35
15 (+/-1)
38 (+/-6)
17 (+/-1)
1 (+/-0.5)
6 (+/-1)
28 (+/-5)
54 (+/-3)
90 (+/-2)
30 (+/-3)
<1
54
<1
8 (+/-1)
<1
<1
<1
<1
<1
<1
<1
Filter cake from
waste water
treatment
<1
<1
<1
6 (+/-1)
<1
<1
<1
<1
<1
<1
65 (+/-5)
Bottom ash 2, 3
1.5 (+/-0.2)
11
84 (+/-1)
47 (+/-7)
83 (+/-1)
18 (+/-2)
94 (+/-1)
72 (+/-5)
46 (+/-3)
9 (+/-1)
5 (+/-1)
Note:
1. the remaining approx. 80 % are sorted out as scrap
2. the bio-availability of materials that remain in the bottom ash depends on leachability in-situ during
subsequent use/disposal
3. the risk associated with the re-use of bottom ash is not necessarily indicated by the presence or absence of
the substances indicated – the chemical and physical form of the substance as well as the nature of the
environment where the material will be used is also important. [64, TWGComments, 2003]
Table 3.1: Distribution of various substances in an example MSWI installation (in mass %)
[1, UBA, 2001, 64, TWGComments, 2003]
Additional differences result from different contents of waste, especially in the case of
hazardous waste incineration facilities.
Table 3.2 gives the percentage distribution of six heavy metals, Hg, Cd, As, Pb, Cu and Zn,
averaged over a test period in a HWI. The table also gives the mass fraction of the following
solid residues: slag, fly ash and filter-cake, related to the amount of waste incinerated during the
test.
Heavy
metal
Solid residues for disposal
Slag Fly ash
% Mass
fraction
Hg
Cd
As
Pb
Cu
Zn
30
3
Release to environment
FilterAct.
To air
Sum
cake
carbon
Water
effluent
Water
landfill
To
land
Sum
0.07
<0.01
<0.01
0.02
0.01
0.02
0
0
0
0
0
0
0
0
0
0
0
0
0.07
<0.01
<0.01
0.05
0.01
0.03
4
<0.01 <0.01 99.88
1.3
94.2
4.49
14.6 80.0
5.39
41.2 56.0
2.75
75.9 22.4
1.69
41.9 56.9
1.17
99.88
99.99
99.99
99.95
99.99
99.97
0.05
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.03
<0.01
0.01
Table 3.2: Percentage (%) distribution of heavy metals in a hazardous waste incineration process
[41, EURITS, 2002]
The most important parameters that influence the behaviour of metals are:
•
•
•
•
kiln temperature
O2 excess in the kiln
the chlorine and sulphur contents of the waste and
the mass transfer of fine particles in the flue-gas.
Waste Incineration
145
Chapter 3
The average conditions during the tests on a HWI that gave rise to the data in Table 3.2 are
given below in Table 3.3.
Parameter
Kiln temperature
PCC temperature
Oxygen content (in the kiln)
Cl-content (in the waste)
S-content (in the waste)
Test data
1120 ± 40 °C
1100 ± 20 °C
11.9 ± 1.3 %
5.1 ± 1.0 %
1.0 ± 0.2 %
Table 3.3: Average operational conditions during partitioning tests on a HWI installation
[41, EURITS, 2002]
From Table 3.2 the following observations regarding the metals studied can be made:
•
•
•
about 99.6 % of the pollutants are concentrated in the solid residues
about 70 – 80 % of the pollutants are concentrated and immobilised in the fly ash and
filter-cake fraction; both residues amount in weight to approximately 7 % of the original
waste input
the removal of Hg from the flue-gas is (in this case) mainly the result of the low pH of the
first gas-cleaning stage.
3.1.2 Examples of the dioxin balance for MSWI
[1, UBA, 2001]
PCDD/PCDF is contained in the input (municipal waste) as well as the output (outgoing air,
waste water and residues) of municipal waste incineration plants. Most of the PCDD/PCDF
input is destroyed during the incineration process but it can also be reformed.
The balance below is for a typical plant in Germany, operating free of process water releases
and complying with German emission limit values:
Output streams
Flue-gas
Bottom ash
Waste water
Filter dust and other residues
from flue-gas cleaning
Total output to all media:
Amount
per kg of waste
input
6.0 m³
0.25 kg
0
0.07 kg
Specific load
Specific account stream
per kg of waste input
0.08 ng/m³
7.0 ng/kg
n/a
220 ng/kg
0.48 ng/kg
1.75 ng/kg
0
15.40 ng/kg
17.63 ng TEQ/kg of waste.
Note: Estimated input with the waste: 50 ng TEQ/kg of waste
Table 3.4: PCDD/PCDF balance for a municipal waste incineration plant in Germany
[1, UBA, 2001], [64, TWGComments, 2003]
From Table 3.4 above it can be seen that, for the example given, the estimated output released to
air is approx. 1 % of the input (0.48 ng TEQ/kg out of 50 ng TEQ/kg). The estimated output
released to all media is 17.63 ng TEQ/kg of incoming waste. This corresponds to 35.3 % of the
estimated input (i.e. a net destruction of 64.7 % of the PCDD/F originally contained in the
waste). It can therefore be concluded that, in this case, the installation acts as a net sink for
PCDD/F. [64, TWGComments, 2003]
146
Waste Incineration
Chapter 3
Other data from a 1997 study (French Ministry of Environment/TIRU) of 8 MSWI and 2 HWI
showed significant variation in residue PCDD/F content:
•
•
•
•
•
bottom ash:
boiler ash:
fly ash:
filter cake (wet FGT):
semi-wet FGT residues:
0.3 - 300 ng I-TEQ/kg
40 - 700 ng I-TEQ/kg
60 - 5000 ng I-TEQ/kg
600 - 30000 ng I-TEQ/kg
800 ng I-TEQ/kg (approx.).
Where data shows variation to the extent indicated in the bullets above, it is more difficult to
draw conclusions regarding the overall mass balance of PCDD/F.
[64, TWGComments, 2003]
The following data is an example of an MSWI (in France) operating with a release to water:
Output stream
Flue-gas
Bottom ash
FGT residues
Waste water
Specific Load
3
0.1 ng I-TEQ/Nm
7 ng I-TEQ/kg
5200 ng I-TEQ/kg
<0.3 ng I-TEQ/l
Note: Example given is for a MSWI with FGT of ESP + wet scrubber (2 stage) + SCR
Table 3.5: Example PCDD/F load data for an MSWI in France
[64, TWGComments, 2003]
3.1.3 Composition of crude flue-gas in waste incineration plants
The composition of crude flue-gas in waste incineration plants depends on the structure of the
waste and on furnace-technical parameters.
Table 3.6 provides an overview of typical crude flue-gas concentrations after the boiler and
before the flue-gas treatment.
Waste Incineration
147
Chapter 3
Components
Dust
Carbon monoxide (CO)
TOC
PCDD/PCDF
Mercury
Cadmium + thallium
Other heavy metals
(Pb, Sb, As, Cr, Co, Cu,
Mn, Ni, V, Sn)
Inorganic chlorine
compounds (as HCl)
Inorganic fluorine
compounds (as HF)
Sulphur compounds, total
of SO2/SO3, counted as
SO2
Nitrogen oxides,
counted as NO2
Nitrous oxide
CO2
Water steam (H2O)
Units
Municipal
waste
mg/Nm³
mg/Nm³
mg/Nm³
ngTEQ/
Nm³
mg/Nm³
mg/Nm³
1000 – 5000
5 – 50
1 – 10
Incineration plants for
Industrial sewage
Hazardous
sludge
waste
(fluidised bed)
1000 – 10000
30000 – 200000
<30
5 – 50
1 – 10
1 – 10
0.5 – 10
0.5 – 10
0.1 – 10
0.05 – 0.5
<3
0.05 – 3
<5
0.2
2.5
mg/Nm³
<50
<100
800
mg/Nm³
500– 2000
3000 – 100000
mg/Nm²
5 – 20
50 – 550
mg/Nm³
200 – 1000
1500 – 50000
mg/Nm³
250 – 500
100 – 1500
<200
mg/Nm³
%
%
<40
5 – 10
10 – 20
<20
5–8
6 – 20
10 – 150
Notes:
1. Sewage sludge plants are those for the incineration of industrial sewage sludge
2. The information in this table refers to German plants. The values seen at older plants can be considerably higher,
especially in the case of emissions influenced by furnace-technical parameters e.g. CO, TOC, etc.
3. Hazardous waste values refer to mixed HW merchant plants rather than dedicated stream plants.
Table 3.6: Flue-gas concentrations after the boiler (crude flue-gas) at various waste incineration
plants (O2 reference value 11 %)
[1, UBA, 2001], [64, TWGComments, 2003]
Municipal waste:
In the case of municipal waste, the structure depends, among other things, on the systems used
for the collection of different waste fractions and on the use or absence of pretreatment. For
example, the separate collection of different municipal waste fractions can influence the thermal
value of municipal waste in the following way:
•
•
•
•
glass and metal - reduction of the ash content, resulting in an increase in the thermal value
paper - reduction of the thermal value
light packaging - reduction of the thermal value
clinical/hospital waste - increase in the thermal value.
Parameters such as the chlorine content and heavy metals content are also influenced, but the
changes remain within the typical range of variations. The provision of separate collections of
various fractions of household waste can have a significant influence over the average
composition of the waste received at MSWIs. For example, separate collection of some batteries
and dental amalgam can significantly reduce mercury inputs to the incineration plant. [64,
TWGComments, 2003]
Commercial non-hazardous waste:
In the case of non-hazardous waste from commercial enterprises, the ranges of variations can be
considerably greater than MSW. When incinerated with other MSW, mixing in the bunker and
shredding may be used to limit these variations.
148
Waste Incineration
Chapter 3
Hazardous waste:
The composition of hazardous waste may vary within a considerably greater range. In the case
of hazardous waste, fluorine, bromine, iodine and silicon can be significant. Unlike municipal
waste, however, the structure of hazardous waste is usually verified at the incineration plants by
means of a check analysis of all essential parameters. Due to the possible variations, a
hazardous waste incineration plant is designed with regard to an average waste structure (menu),
in some cases with considerable additional reserves for flue-gas cleaning.
Such an incineration menu can then be created by intentionally mixing the incoming waste in
bulk tanks or the bunker, or by individually feeding the waste to the furnace in separate pipes in
hourly amounts corresponding to the design of the plant. This is also be taken into account if
waste is fed in barrels, which can themselves exert sudden shock loads. Incineration plants
specifically designed for recovering HCl and SO2 from waste streams containing chlorine or
sulphur, respectively, may have very different raw gas structures.
Sewage Sludge:
[64, TWGComments, 2003]
Variations in the raw gas at sewage sludge incineration plants correspond to changes in the
waste composition of the incinerated waste. This, in turn, is influenced by the presence or
absence of pretreatment, and the composition of the sludge received. The composition of
sewage sludge is strongly dependent upon the nature of the drainage catchment served by the
sewage treatment works (STW) where the sludge arises, and the treatments applied at the STW.
Where sewage sludge is incinerated with other wastes, variations in sewage sludge quality may
have a less pronounced effect on raw gas quality owing to the buffering effect of the other
wastes. The water content of the sewage sludge may indeed provide benefits at some MSWI
installations as when sprayed through special nozzles in selected locations above the waste bed
(often in the gas burnout zone) it provides an additional means of controlling temperature and
may assist with primary NOX control.
Clinical waste:
[64, TWGComments, 2003]
Variations in the raw gas at clinical incineration plants correspond mainly to changes in the
waste composition of the incinerated waste. Physical pretreatment that may limit the range of
variation of raw gas composition are not often used for clinical wastes because of concerns
regarding the infectivity of the waste.
Categorising incoming waste streams according to their source and probable combustion
characteristics (mainly relating to CV, moisture content and calorimetric throughput rate) and
feeding them to the incineration process so as to comply with an appropriate input recipe, may
be used to reduce the range combustion related raw gas composition variations.
3.1.4 Emissions of gases relevant to climate change
Sources and total emissions relevant to climate change
The total emissions relevant to climate change in Germany in the year 1999 and the emissions
from waste incineration (related to the fossil portion of the waste that is considered relevant to
climate change in Germany) are summarised in Table 3.7:
Waste Incineration
149
Chapter 3
Global warming
Waste incineration
Total emissions potential (GWP) CO2
(fossil portion)
Pollutants in 1999
(kt/yr)
equivalents
of the total emissions
(kt/yr)
(kt/yr)
Carbon dioxide (CO2)
858511
858511
8685
Nitrous oxide (N2O)
141
43710
0.81 (252)*
Methane (CH4)
3271
68691
n/a
Fluorinated hydrocarbons
3284
4290
CF4 (perfluorinated hydrocarbons)
0.186
1209
C2F6 (perfluorinated hydrocarbons)
0.046
423
C3F8 (perfluorinated hydrocarbons)
0.011
77
SF6 (sulphur hexafluoride)
0.229
5473
Total GWP
982384
(c. 9000)*
Indirectly effective greenhouse gases
Nitrogen oxide (NOX as NO2)
1637
15.2 (122.24)*
Carbon monoxide (CO)
4952
3.82 (11.46)*
NMVOC (non-methane volatile
organic compound)
Ammonia (NH3)
Sulphur dioxide SO2
1651
0.76 (8.36)*
624
Aerosol formers
831
0.3
n/a
(..)* in brackets: the converted emission value in CO2 equivalents for comparison with the GWP
Table 3.7: Total emissions relevant to climate change in Germany in the year 1999 compared with
those arising form waste incineration
[1, UBA, 2001]
This table indicates that in 1999 in Germany, waste incineration accounted for approximately
1 % of GHG emissions.
3.2 Emissions to air
3.2.1 Substances emitted to air
[1, UBA, 2001] [64, TWGComments, 2003].
Carbon monoxide
CO is an odourless toxic gas. Carbon monoxide (CO) in the flue-gas of incineration plants is the
product of the incomplete combustion of carbon based compounds. CO is produced when there
is insufficient oxygen locally and/or insufficiently high temperature of combustion to carry out
full oxidation to carbon dioxide. In particular, this can occur if spontaneously evaporating or
rapid-burning substances are present, or when combustion gas mixing with the supplied oxygen
is poor. Continuous measuring of the CO level can be used to check the efficiency of the
incineration process. CO is a measure of quality of combustion. If the CO emissions are very
low then the gas burn out quality is very high and TOC emissions are also low (and vice versa).
[74, TWGComments, 2004]
After its release to the atmosphere, CO is oxidised to CO2, after some time. Particularly high
concentrations of CO (>lower explosion limit) must be avoided as they can create explosive
mixtures in the flue-gas. In particular, at hazardous waste incineration plants, increased CO
emissions can occur with some drummed wastes.
CO in the plants is measured continuously. Daily averages of CO emissions below 50 mg/Nm³
are achieved; at some plants, the daily averages are well below this figure [64, TWGComments,
2003]
150
Waste Incineration
Chapter 3
It is reported that NOX treatment with SCR may increase CO emission levels. [74,
TWGComments, 2004]
Total organic carbon (TOC)
This parameter includes a number of gaseous organic substances, the individual detection of
which is generally complex or not possible. During the incineration of organic waste, a large
number of chemical reactions take place, some of which are incomplete. This leads to an
extremely complex pattern of compounds of the trace amounts. A complete account of every
substance within the TOC parameter is not available, however incineration generally provides
high destruction efficiencies for organic substances.
TOC can be measured continuously in the flue-gas. Low TOC levels are key indicators for the
quality of combustion in an incineration process. Emissions in the range of 0.1 mg/Nm3 to
10 mg/Nm3 are seen. [64, TWGComments, 2003]
Hydrogen chloride
Many wastes contain chlorinated organic compounds or chlorides. In municipal waste typically
approximately 50 % of the chlorides come from PVC [64, TWGComments, 2003]. In the
incineration process, the organic component of these compounds is destroyed and the chlorine is
converted to HCl. Part of the HCl may react further to metal chlorides on inorganic compounds
which are also contained in the waste.
HCl is highly soluble in water and has an impact on plant growth. It is measured continuously
with emissions in the range of 0.1 - 10 mg/Nm3. [74, TWGComments, 2004]
The formation and emission of Cl2 is of minor importance under normal incineration conditions.
However it is essential for the fouling and corrosion. So it is worth while to control the
formation so that the mentioned process takes place in the gas phase and not after deposition on
boiler tubes. [74, TWGComments, 2004]
Hydrogen fluoride
The formation mechanism of HF in incineration plants corresponds to that of HCl. The main
sources of HF emissions in municipal waste incineration plants are probably fluorinated plastic
or fluorinated textiles and, in individual cases, the decomposition of CaF2 in the course of the
incineration of sludge.
HCl is highly soluble in water and can have an impact on plant growth. It can be measured
continuously with emissions in the range of 0.1 - 1 mg/Nm3. [74, TWGComments, 2004]
Various kinds of fluorinated waste are treated in hazardous waste incineration plants.
Hydrogen iodide and iodine, hydrogen bromide and bromine
Municipal waste usually contains very small quantities of bromine or iodine compounds.
Bromine or iodine emissions are, therefore, of minor importance to municipal waste
incineration plants.
In hazardous waste incineration plants, organic and inorganic wastes containing bromine or
iodine are sometimes treated. For example, bromine compounds can still be found in some
electronic devices as flame protection agents. Iodine can be contained in medicines or may be
used for the treatment of metal surfaces. On the whole, however, their quantity is small in
relation to chlorinated compounds. Bromine and Iodine help to oxidise the mercury and
decrease the mercury content in the clean gas by improving the retaining capacity of wet
scrubbers. [74, TWGComments, 2004]
Where present, the chemical properties of elementary iodine and bromine can result in
colouration of chimney plumes. Special measures can be taken for the incineration of such
waste in order to prevent the formation and release of elemental bromine or iodine. These
substances can also have toxic and irritant effects. [64, TWGComments, 2003]
Waste Incineration
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Sulphur oxides
If the waste contains sulphur compounds, mainly SO2 will be created during the incineration of
the waste. Under appropriate reaction conditions, SO3 can also be created. For MSW, the
proportion of SO3 can be around 5 % at the inlet to the FGT (note: the SO3 content is important
to determine the acid dew point). Common sources of sulphur in some waste streams are: waste
paper; plaster board (calcium sulphate), and sewage sludges. [64, TWGComments, 2003].
SO2 gives rise to acidification and can be measured continuously with emissions in the range of
1 - 50 mg/Nm3 (stp; 11 % O2). [74, TWGComments, 2004]
Nitrogen oxides
Various oxides of nitrogen are emitted from incineration plants. They can have toxic, acidic and
global warming effects depending on the oxide concerned. In many cases they are measured
using continuous emission monitors.
The NO and NO2 emitted from waste incineration plants originates from the conversion of the
nitrogen contained in the waste (so-called fuel NOX) and from the conversion of atmospheric
nitrogen from the combustion air into nitrogen oxides (thermal NOX). In municipal waste
incineration plants, the proportion of thermal NOX is usually very low due to lower temperatures
in the afterburner chamber. Production of thermal NOX generally becomes more significant at
temperatures above 1000 °C. In MSWI the amount of thermal NOX can also critically depend on
the quantity, and manner, of injection of secondary air into the afterburner chamber – with
higher NOX seen with higher nozzle temperatures (i.e. above 1400 °C).
The mechanisms for the formation of NOX from the nitrogen contained in the waste are very
complicated. Amongst other reasons, this is because nitrogen can be contained in the waste in
many different forms, which, depending on the chemical environment, can react either to NOX
or to elementary nitrogen. A conversion rate of approx. 10 - 20 % of the fuel nitrogen is usually
assumed depending on waste type. High chlorine and sulphur concentrations, O2 content and
temperature may have great influence. The proportion of NO/NO2 in the total NOX stack
emissions is usually approx. 95 % NO and 5 % NO2.
Nitrous oxide is not usually measured as a part of NOX estimation. Nitrous oxide (N2O) can be
emitted if insufficient temperature for the combustion process is applied (e.g. less than 850 °C)
and there is an insufficient oxygen concentration. The N2O emission from incineration
processes are, therefore, often correlated with CO emissions.
Where SNCR is applied for de-NOX, formation of N2O may increase, dependent upon reagent
dose rates and temperature. Values of 20 - 60 mg/m3 have been measured, but especially where
low NOX values are sought (i.e. N2O can increase when higher SNCR dose rates are used to
secure lower NOX emission targets). This is particularly the case when urea is used (ammonia is
the alternative reagent).
For municipal waste incineration, N2O emissions of 1 - 12 mg/Nm³ (for individual
measurements) and averages of 1 - 2 mg/Nm³ are seen. For the incineration of MSW in fluidised
bed plants, the measured N2O emission values (individual measurements) are usually higher.
Individual measurements in hazardous waste incineration plants have resulted in N2O emission
values of 30 to 32 mg/Nm³ [64, TWGComments, 2003]
Normal N2O emission levels for fluidised bed sludge incineration can be as low as 10 mg/Nm³,
with some values reported up to 500 mg/Nm³.
Whilst incineration is a low (in terms of anthropogenic emissions) contributor of emissions of
nitrous oxide, they add to the global warming impact of releases from incineration processes.
152
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NOX gives rise to acidification and eutrophication and can be measured continuously. Emissions
at modern plants are reported to be generally in the range between 30 and 200 mg/Nm3. (daily
average, stp, 11 % O2).. [74, TWGComments, 2004] Some installation may give rise to daily
average NOX levels of up to 400 mg/Nm3 – in general these are already in the process of closing
down or upgrading to the daily average levels of 200 mg/Nm3 required by Directive
2000/76/EC.
Dust
Dust emissions from waste incineration plants mainly consist of the fine ash from the
incineration process that are entrained in the gas flow. Depending on the reaction balance, other
elements and compounds are concentrated in this airborne dust. The separation of dust from the
flue-gas using air pollution control devices removes the majority of the dust and entrained
inorganic and organic substances (e.g. metal chlorides, PCDD/F, etc).
Air pollution control equipment greatly reduces emissions of total particulate matter from waste
incineration plants. In common with all combustion processes, the type of air pollution control
equipment used effects the particle size distribution of the emitted dust. The filtration equipment
is generally more effective on the larger particles, and therefore changes the proportion of finer
particulate in the resulting emissions to air, whilst reducing the total particulate emission.
Dust is normally measured continuously with reported emissions of between <0.05 and
15 - mg/Nm3 (stp, 11 % O2). [74, TWGComments, 2004]
Mercury and mercury compounds
Mercury can currently still be found in municipal waste, notably in the form of batteries,
thermometers, dental amalgam, fluorescent tubes or mercury switches. Separate collection of
these can help reduce overall loads in mixed MSW but collection rates of 100 % are not
achieved in practice.
Mercury is a highly toxic metal. Without adequate air pollution controls, the incineration of
mercury containing wastes can give rise to significant emissions.
Emissions can be continuously measured and abated levels have been reported to be generally in
the range between 0.0014 and 0.05 mg/Nm3 (11 % O2). [74, TWGComments, 2004] Short-term
higher emission levels are reported where inlet concentration vary greatly.
In hazardous waste incineration, there are several specific streams that may contain increased
concentrations of mercury in the received waste:
•
•
•
•
tars from coking plants
waste from chlorine alkaline electrolysis (amalgam process)
caustic oil sludge from refineries
chemicals containing mercury.
The form of the mercury emissions depends strongly on the chemical environment in the fluegas. A balance between metallic mercury (Hgo) and HgCl2 normally develops. Where there is a
sufficiently high concentration of HCl in the flue-gas (in relation to the reduction agent SO2)
mercury will mainly be contained in the flue-gas as HgCl2. This can be separated from the fluegas significantly more easily than metallic mercury. If, however, HCl is contained in the fluegas at lower concentrations (e.g. in sewage sludge incineration plants) mercury exists in the
flue-gas mainly in metallic form and is then more difficult to control.
The combustion temperature also influences HgCl2 formation.
In wet scrubbers (only) the HgCl2 removed can be reduced if SO2 is also present (the separation
of these substances is one reason why distinct wet scrubber stages are operated for the removal
of HgCl2 and SO2) The Hg2Cl2 formed when this happens can itself disproportionate to HgCl2
and Hg. These reactions can be prevented by adjusting the pH in wet scrubbers to low values
and by withdrawing Hg from the scrubber effluent.
Waste Incineration
153
Chapter 3
Metallic mercury is virtually insoluble in water (59bg/l at 25 °C). Mercuric (II) chloride is much
more soluble at 73 g/l. Mercury (II) chloride can therefore be separated in wet scrubbers,
whereas the separation of metallic mercury requires further flue-gas treatment stages (see
Section 2.5.6 for further details).
[64, TWGComments, 2003]
Cadmium and thallium compounds
Common sources of cadmium in municipal waste incineration plants are electronic devices
(including accumulators), batteries, some paints and cadmium-stabilised plastic. Thallium is
virtually non-existent in municipal waste.
Hazardous wastes may contain high concentrations of Cd and Tl compounds. Effluent treatment
sludges and drummed wastes from metal plating and treatment may be significant sources.
Cadmium is highly toxic and can accumulate in the soil. The range of emissions have been
reported to be 0.0002 to 0.2 mg/Nm3. (11 % O2). [74, TWGComments, 2004]
Other heavy metal compounds
This term comprises the heavy metals antimony, arsenic, lead, chromium, cobalt, copper,
manganese, nickel, vanadium, tin and their respective compounds. European and many national
regulations, thus, group them together for emission measurement requirements. This group
contains carcinogenic metals and metal compounds such as arsenic and chromium (VI)
compounds, as well as metals with toxicity potential.
The retention of these metals depends largely on an effective separation of dust as they are
bound in dust due to the vapour pressures of their compounds, as contained in the flue-gas
(mainly oxides and chlorides).
Polychlorinated biphenyls
Low quantities of polychlorinated biphenyls (PCBs) are found in most municipal waste streams
and also in some industrial wastes. Wastes with large proportions of PCBs, however, generally
only arise from specific PCB collection and destruction programmes, when concentrations of
PCB in such waste can be very high.
In hazardous waste incineration plants, wastes with a PCB content as high as 60 - 100 % are
combusted. The same applies to special plants for the incineration of highly chlorinated
hydrocarbons. PCBs are more efficiently destroyed if higher incineration temperature are used
(e.g. above 1200 °C); however, lower temperatures (e.g. 950 °C) together with appropriate
conditions of turbulence and residence time have also been found to be effective for PCB
incineration. [74, TWGComments, 2004] PCBs contained in the crude flue-gas of waste
incineration plants can be the result of incomplete destruction.
PCB emissions are classified as potentially toxic by some international organisations (e.g.
WHO). A toxicity potential (similar to that of dioxins and furans) is ascribed to some of the
PCBs (coplanar PCBs).
Polyaromatic hydrocarbons
Polyaromatic hydrocarbons are well known as products of incomplete combustion. They are
toxic and have carcinogenic and mutagenic properties. [74, TWGComments, 2004]
Polychlorinated dibenzo-dioxins and furans (PCDD/F)
Dioxins and furans (PCDD/F) are a group of compounds, some of which are of extreme
toxicity, and are considered to be carcinogens. Dioxins and furans have played a main part in
the debate about waste incineration for many years. Their production and release is not specific
to waste incineration but occurs in all thermal processes under certain process conditions.
154
Waste Incineration
Chapter 3
[64, TWGComments, 2003] Significant advances in PCDD/F emission control have been
achieved in recent years in the WI sector. Improvements in the design and operation of
combustion and flue-gas treatment systems have resulted in systems that can reliably achieve
very low emission limit values. National [44, RVF, 2001] and regional emissions inventories
confirm that, where compliance with Directive 2000/76/EC is secured, incineration represents a
low contributor to overall emissions to air of dioxins and furans [45, FEAD, 2002].
[64, TWGComments, 2003] In well designed and operated incineration plants, material
balances have shown that incineration effectively removes dioxins from the environment (see
Section 3.1.2). This balance is made most favourable by ensuring that:
•
•
incoming dioxins and pre-cursors are effectively destroyed using appropriate combustion
conditions
reducing the use of conditions that may give rise to PCDD/F formation and re-formation
including de-novo synthesis.
Dioxins and furans entering the process with the waste are destroyed very efficiently if
sufficiently high incineration temperatures and appropriate process conditions are used.
Standards for operating conditions are stated in existing European legislation on incineration
(i.e. Directive 2000/76/EC). The dioxins and the furans found in the crude flue-gas of waste
incineration plants are the result of a re-combination reaction of carbon, oxygen and chlorine.
Suitable precursor substances (e.g. from chlorophenols) can also react to form dioxins and
furans. In the formation of the substances, certain catalysers in the form of transitional metal
compounds (e.g. copper) also play an important part.
Ammonia
Ammonia has a significant impact on eutrophication and acidification of the environment.
Ammonia emissions can arise from the overdosing or poor control of NOX reduction reagents
that are used for NOX control. The emissions normally range from 1 to 10 mg/Nm³, with an
average of 4 mg of NH3/Nm³.
[64, TWGComments, 2003]
Carbon Dioxide (CO2)
If one tonne of municipal waste is combusted, approx. 0.7 to 1.7 tonnes of CO2 is generated.
This CO2 is released directly into the atmosphere and, as a result, the climate relevant share of
CO2, (resulting from the fossil origin) contributes to the greenhouse effect. [64,
TWGComments, 2003]
Because municipal waste is a heterogeneous mixture of biomass and fossil material, the portion
of CO2 from MSWIs of fossil origin (e.g. plastic) which is considered relevant to climate
change is generally in the range 33 to 50 %.
Methane CH4
It can be assumed that, if combustion is carried out under oxidative conditions, methane levels
in the flue-gas will be almost zero and consequently not emitted to air. Methane is measured
with the VOC component. [64, TWGComments, 2003]
Methane can also be created in the waste bunker if there are low oxygen levels and subsequent
anaerobic processes in the waste bunker. This is only the case where wastes are stored for long
periods and not well agitated. Where the storage area gases are fed to the incineration chamber
air supply they will be incinerated and emissions will be reduced to insignificant levels.
Waste Incineration
155
Chapter 3
3.2.2 Municipal waste incineration plants
3.2.2.1 Summary data for emissions to air from MSWI
Table 3.8 gives the range of values for emissions to air from some European MSWI plants.
Thirty minute, daily and annual averages are shown. It is important to note that data that are the
result of non-continuous (or spot) measurements are also included in the Table. They are
indicated (N) in the type of measurement column. Furthermore, where non-continuous
measurements appear in an averaging column, the values presented for non-continuous
measurements are not collected over the stated averaging period for that column, and should
only be interpreted as non-continuous measurements:
Half hour averages
Daily averages (where
Type of
(where continuous
Parameter
continuous measurement
Measurement
measurement used)
used) in mg/m³
in mg/m³
Limits in
Limits in
Range of
C: continuous
2000/76/ Range of values 2000/76/
N: non-cont.
values
EC
EC
Dust
C
10
0.1 – 10
20
<0.05 – 15
HCl
C
10
0.1 – 10
60
<0.1 – 80
HF
C/N
1
0.1 – 1
4
<0.02 – 1
SO2
C
50
0.5 – 50
200
0.1 – 250
NOX
C
200
30 – 200
400
20 – 450
NH3
C
n/a
<0.1 - 3
0.55 – 3.55
N 2O
n/a
VOC (as
C
10
0.1 – 10
20
0.1 – 25
TOC)
CO
C
50
1 – 100
100
1 – 150
0.0014 –
Hg
C/N
0.05
0.0005 – 0.05
n/a
0.036
Cd
N
n/a
0.0003 – 0.003
n/a
As
N
n/a
<0.0001 – 0.001
n/a
Pb
N
n/a
<0.002 – 0.044
n/a
Cr
N
n/a
0.0004 – 0.002
n/a
Co
N
n/a
<0.002
n/a
Ni
N
n/a
0.0003 – 0.002
n/a
Cd and Tl
N
0.05
n/a
other
N
0.5
n/a
metals 1
other
N
n/a
0.01 – 0.1
n/a
metals 2
Benz(a)pyr
N
n/a
n/a
ene
N
n/a
n/a
PCB
N
n/a
n/a
PAH
PCDD/F
0.1
n/a
(ng
N
(ng
TEQ/m³)
TEQ/m³)
Annual
averages
mg/m³)
Range of
values
0.1 – 4
0.1 – 6
0.01 – 0.1
0.2 – 20
20 – 180
0.1 – 5
2 – 45
0.0002 – 0.05
0.0002 – 0.03
0.0002 – 0.05
<0.001
<0.005
<0.01
0.0002 – 0.08
(ng TEQ/m³)
1
. In some cases there are no emission limit values in force for NOX. For such installations a typical range
of values is
250 - 550 mg/Nm³ (discontinuous measurement).
2. Other metals 1 = Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V
3. Other metals 2 = Sb, Pb, Cr, Cu, Mn, V, Co, Ni, Se and Te
4. Where non-continuous measurements are indicated (N) the averaging period does not apply. Sampling
periods are generally in the order of 4 – 8 hours for such measurements.
5. Data is standardised at 11 % Oxygen, dry gas, 273K and 101.3kPa.
Table 3.8: Range of clean gas operation emissions levels reported from some European MSWI
plants.
[1, UBA, 2001], [2, infomil, 2002], [3, Austria, 2002], [64, TWGComments, 2003]
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Waste Incineration
Chapter 3
Table 3.9 below gives emissions to air for various substances per tonne of MSW incinerated.
The data given is average data for 12 MSWI in the Flanders Region of Belgium in 1999 and
average data for three MSWI plants in Austria [3, Austria, 2002]:
Parameter
Dust
HCl
HF
SO2
NOX
CO
TOC
Hg
Cd + Tl
Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V, Sn
PCDD/F
Average Value (g/tonne incinerated)
3 Austrian plants
12 Belgian plants
165
7
70
4
2.2
0.36
129
24.8
2141
189
126
101
19
0.048
0.1
0.095
1.737
250 ng
44.4 ng
TEQ/tonne
TEQ/tonne
incinerated
incinerated
Table 3.9: Operational emission levels to air from MSWI expressed per tonne of MSW incinerated
[64, TWGComments, 2003] [3, Austria, 2002] [74, TWGComments, 2004]
3.2.2.2 European air emissions survey data for MSWI
The data presented here are based on the results of a survey of 142 European non-hazardous
waste incineration plants submitted to the TWG [45, FEAD, 2002], with additional information
from comments made by the TWG [64, TWGComments, 2003]
The information relates to process lines rather than individual plants. The size of the data set
may therefore, in some cases, exceed the number of plants surveyed. The data set is not a
complete survey of European MSWIs - most of the plants that were complying only with the
earlier 1999 Waste Incineration Directives, were excluded from this survey.
Hydrogen chloride and hydrogen fluoride
Different national emission limit values apply.
Most of the data presented are based on continuous measurements.
Level of annual averages
>50 mg/Nm³
>30 <50 mg/Nm³
>10 <30 mg/Nm³
>5 <10 mg/Nm³
<5 mg/Nm³
Note:
Number of plants/lines
0
10
24
35
73
For German plants only some representative examples have
been taken into account. All the other incinerators (about 50
plants) not mentioned here also operate below 10 mg/Nm³.
Table 3.10: HCl emissions survey of European MSWIs
Source [45, FEAD, 2002]
Basically three types of flue-gas cleaning systems are in use:
1. wet systems using different types of scrubbers in which the HCl is taken out by water,
working normally at a pH <1
2. semi-wet systems, which use lime in water
3. dry systems, which use lime or sodium bicarbonate (usually with activated carbon) often
combined with a bag house filter.
[74, TWGComments, 2004]
Waste Incineration
157
Chapter 3
The emissions will depend, among other factors, on the amount of additives used and the
operational/design set point of the plant.
The data on hydrogen fluoride (HF) are mainly based on discontinuous measurement. HF is
reduced by the same tools as HCl, meaning that an effective flue-gas cleaning system for HCl
will also deal with HF. The chemical behaviour of HF is not exactly the same as of HCl, so the
efficiency of HF removal will differ slightly from system to system.
Level of annual averages
>5 <10 mg/Nm³
>2 <5 mg/Nm³
>1 <2 mg/Nm³
<1 mg/Nm³
Number of plants/lines
0
1
1
53
Table 3.11: HF emissions survey of European MSWIs
[45, FEAD, 2002]
Sulphur-dioxide
Different national emission limit values are applied.
Most of the data are from continuous measurement.
Level of annual averages
>200 mg/Nm³
>100 <200 mg/Nm³
>50 <100 mg/Nm³
>25 <50 mg/Nm³
<25 mg/Nm³
Number of plants/lines
3
5
16
25
123
Table 3.12: Sulphur dioxide emissions survey of European MSWIs
[45, FEAD, 2002]
The types of flue-gas cleaning in use are the same as those mentioned for HCl, with the main
difference being that, for wet scrubbers, they are operated at a slightly basic pH (usually 7 - 8).
Dust
Most of the data are from continuous measurement. They show the values of total dust.
For dust, mainly three types of flue-gas cleaning are in use:
1. dry electrostatic precipitator (dry ESP)
2. wet electrostatic precipitator (wet ESP) (note: the wet ESP is not often used in MSWI)
3. bag house filter (BF).
In several cases (mainly in NL and D), two of these tools have been combined with each other,
for example a dry electrostatic precipitator directly after the boiler with a bag house filter
directly before the stack.
Recent new plants have been built with a bag house filter only.
Wet scrubbers can also significantly contribute to dust removal. Typically about 50 % efficiency
is observed for dust (with additional selected heavy metal) removal.
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Chapter 3
An important point to note is that, all tools are connected and generally have an influence on
each other. In the case of dry and semi-wet processes, bag filters also act as a reactor for acid
removal. In addition they can remove PCDD/F and metals (including mercury and cadmium) if
a suitable reagent is used e.g. activated carbon.
Level of annual averages
>50 mg/Nm³
>30 <50 mg/Nm³
>10 <30 mg/Nm³
>5 <10 mg/Nm³
<5 mg/Nm³
Number of plants/lines
3
1
8
29
103
Table 3.13: Dust emissions survey of European MSWIs
[45, FEAD, 2002]
Nitrogen oxides
Most of the data presented are from continuous measurements. In some countries there are
currently no limit values for NOX from municipal waste incinerators.
Many plants already achieve results below 200 mg/Nm³. In some cases emissions of less than
70 mg/Nm³ are achieved.
A variety of combustion control techniques are used to reduce NOX formation. SCR or SNCR
are the main techniques in use for the further abatement of NOX emissions in MSWIs. Emission
values below 100 mg/Nm³ normally require the use of SCR. The use of SNCR can also lead to
emissions below 150 mg/Nm³ and exceptionally below 100 mg/Nm³ (e.g. when primary NOX
reduction measures are also implemented) [74, TWGComments, 2004]
Level of annual averages
>400 mg/Nm³
>300 <400 mg/Nm³
>200 <300 mg/Nm³
>100 <200 mg/Nm³
<100 mg/Nm³
Number of plants/lines
9
35
22
48
11
Note: the 11 plants (not lines) below 100 mg/Nm³ are in NL – all
comply with applied ELVs of 70 mg/Nm³. Other plants operating
below 100 but not included here are found in Europe (commonly in D,
A, B)
Table 3.14 Nitrogen oxides emissions survey of European MSWIs
Source [45, FEAD, 2002], [64, TWGComments, 2003]
TOC (Total organic carbon)
TOC is an important measure of the efficiency of combustion. The achieved levels of the TOCemissions are mainly a result of the design of the firing system and the after-burning chamber,
as the possibilities to decrease those emissions by flue-gas cleaning are limited. The same
equipment used for dust will reduce solid organic particles. Some of the organic compounds
will be reduced by the use of activated carbon.
Level of annual averages
>10 mg/Nm³
>5 <10 mg/Nm³
<5 mg/Nm³
Number of plants/lines
4
7
79
Table 3.15: Total organic carbon emissions survey of European MSWIs
[45, FEAD, 2002]
Waste Incineration
159
Chapter 3
PCDD/PCDF
The data on PCDD/PCDF emissions from MSWI do not represent the whole range of plants
currently operating. Data from Denmark and Italy were not available. Data from France are also
not included, although the data for these showed emission above 0.1ng/m³ in many cases.
PCDD/PCDF emissions reported here are all based on discontinuous measurements, mainly
twice a year. There is experience on continuous collection of dioxin measurements especially
for MSWI in Flanders (B) and in Austria.
For reaching low levels of PCDD/PCDF emissions, primary as well as secondary measures are
important. In the firing system, effective mixing of the gases (high turbulence) improves the
destruction of PCDD/PCDF and similar compounds already present in the waste. Avoiding the
temperature window for the recombination of PCDD/PCDF and similar compounds in the boiler
and flue-gas treatment system avoids the breeding of new PCDD/Fs.
For further reduction, mainly three types of flue-gas cleaning are in use:
1. static activated carbon filter
2. bag house filter with injection of activated carbon (usually mixed with other reagents)
3. catalyst destruction for gaseous PCDD/F
Both the activated carbon systems above have the advantage of also reducing Mercury
emissions. The catalyst systems are used to reduce NOX and PCDD/F.
Level of annual averages
>2 ng/Nm³
>1 <2 ng/Nm³
>0.5 <1 ng/Nm³
>0.1 <0.5 ng/Nm³
>0.05 <0.1 ng/Nm³
<0.05 ng/Nm³
Number of plants/lines
3
11
4
7
22
72
Table 3.16: PCDD/F (TEQ) emissions survey of European MSWIs
[45, FEAD, 2002], [64, TWGComments, 2003]
Mercury
The data include results from continuous measurement (used in Germany for over two years and
Austria for over one year) and from discontinuous measurements (minimum twice a year).
Therefore, comparability of data between these two types of measurement may be not very high.
Continuous measurements will also include events with elevated emissions due to higher loads
in the waste feed, which have been reported by some plants.
Level of annual averages
>200 bg/Nm³
>100 <200 bg/Nm³
>50 <100 bg/Nm³
>30 <50 bg/Nm³
<30 bg/Nm³
Number of plants/lines
0
1
3
7
83
Table 3.17: Mercury emissions survey of European MSWIs
[45, FEAD, 2002]
160
Waste Incineration
Chapter 3
For several plants in France, mercury measurements are not given alone but in combination with
Cd (the ELV being given as a sum of the two). As the distribution of the two is not necessarily
predictable, these results are presented in the following additional table:
Level of annual averages
>200 bg/Nm³
>100 <200 bg/Nm³
>50 <100 bg/Nm³
>30 <50 bg/Nm³
<30 bg/Nm³
Number of plants/lines
0
1
5
8
18
Table 3.18: Combined Cd and Hg emissions of selected MSWIs in France
[45, FEAD, 2002]
The plants from which data are included in this report are equipped with, amongst others, the
following types of flue-gas cleaning systems. The Hg emission levels reported are also shown:
System
identifier
Dry ESP
Wet acid
scrubber
Wet ESP
Bag house
filter
Activated
carbon
injection
Activated
carbon
filter
1
2
3
4
5
6
7
Emission
of Hg
(Ug/Nm³)
0.1
0.1
1.77 and
1.93 and
3.16
3
3 and
6
2 and
7.3 and
10
22 and
50
Table 3.19: Emission results and techniques applied for Hg control at European MSWIs
[45, FEAD, 2002]
The lowest results are seen where activated carbon is used, either as a static bed system, or in an
entrained flow activated carbon injection system with a bag filter. The consumption rate as well
as the quality of activated carbon (e.g. sulphur impregnation) directly affects the emission
levels. The techniques in Table 3.19 correspond to different ELVs requirements and to different
costs.
Under certain conditions (e.g. high input rate of mercury) the removal capacity limits of a FGT
systems may be exceeded, leading to temporarily elevated Hg emissions. MSW usually contains
low quantities of Hg. However, some short-term high loads have been noted. These are
generally associated with the inclusion in the MSW of batteries, electrical switches,
thermometers, laboratory wastes, etc.
The wet acidic scrubber can serve as a sink for mercury if the mercury is present as the Hg(II),
chloride form. The mercury that has been transferred from the gas stream to the scrubber liquors
can then be removed by a waste water treatment plant or captured by spray drying of the waste
water in the flue-gas. In the second case mercury recycles can occur unless there is an adequate
rate Hg removal step.
Additional treatment may be required if mercury is present as metallic form (see Hg removal
techniques).
[74, TWGComments, 2004]
Waste Incineration
161
Chapter 3
3.2.2.3 Emissions to air from fluidised bed incinerators
Efficient heat and mass transfer allows operation at lower temperatures than other combustor
designs, but there is still a lower limit. The lower temperatures often used together with the
more uniform distribution of temperatures, which eliminates hot spots and high oxygen zones,
thermal NOX production may then be reduced and the conversion of fuel nitrogen into NOX can
also be very low. The lower combustion temperatures together with the lack of air can
sometimes lead to the formation of nitrous oxide (N2O). Normal N2O emission levels for
fluidised bed sludge incineration are approx 10 mg/Nm³, with some values reported up to
100 mg/Nm³ and above. These values are higher than with other combustion systems.
The generally lower NOX production that results from combining prepared or selected wastes
with fluidised bed combustion can lead to similar or lower emission levels using simpler FGT
than inherently high NOX combustion systems.
Due to relatively lower temperature of the fluidised bed combustion, the contents of heavy
metals in the raw flue-gas (and hence FGT residues) may be lower than from mixed waste grate
combustion. The actual emissions to air depend on the waste, and on the chosen flue-gas
cleaning system.
A combination of fluidised bed incineration at 850 - 950 °C and SNCR (ammonia) is reported to
reduce NOX emissions at Dutch sewage sludge incinerators to less than 70 mg/Nm3.
[2, infomil, 2002]
3.2.3 Hazardous waste incineration plants
3.2.3.1 Summary data of the emissions to air from HWI
Table 3.20 represents the results of a survey of European (mainly German and Dutch) operators
of plants with regard to typical emissions from plants. Thirty minute, daily and annual averages
are shown. It is important to note that data that are the result of non-continuous measurements
are also included in the table, and is indicated (N) in the type of measurement column.
Furthermore, where non-continuous measurements appear in an averaging column, the values
presented for non-continuous measurements are not collected over the stated averaging period
for that column, and should only be interpreted as non-continuous measurements:
Parameter
Type of
Daily averages (mg/Nm³)
measurement
Typical
range of
values
0.1 – 15
0.1 – 60
0.1 – 2
0.1 – 150
50 – 400
0.1 – 20
5 – 100
0.0003- 1
Annual
averages
(mg/Nm³)
Typical range
of
values
0.1 – 2
0.3 – 5
0.05 – 1
0.1 – 30
70 – 180
0.01 – 5
5 – 50
0.0004 – 0.05
0.0005 – 0.05
C
C
C/N
C
C
C
C
C/N
N
10
10
1
50
200
10
50
0.05
0.05
Typical
Limits in
range of
2000/76/EU
values
0.1 – 10
20
0.1 – 10
60
0.04 – 1
4
0.1 – 50
200
40 – 200
400
0.1 – 10
20
5 - 50
100
0.0003 – 0.03
n/a
0.0005 – 0.05
n/a
N
0.5
0.0013 – 0.5
n/a
0.004 – 0.4
N
0.1
0.002 – 0.1
n/a
0.0003 – 0.08
C: cont.
Limits in
N: non-cont. 2000/76/EU
Dust
HCl
HF
SO2
NOX
TOC
CO
Hg
Cd +Tl
other heavy
metals
PCDD/PCDF
(ng TEQ/m³)
Thirty-minute Averages
(mg/Nm³)
1. Data is standardised at 11 % Oxygen, dry gas, 273K and 101.3kPa.
2. Other metals = Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V
Table 3.20: Typical range of clean gas emissions to air from hazardous waste incineration plants
[1, UBA, 2001], [2, infomil, 2002], [64, TWGComments, 2003], [74, TWGComments, 2004]
162
Waste Incineration
Chapter 3
3.2.3.2 European air emissions survey data for HWI
[41, EURITS, 2002]
This section gives an overview of the merchant hazardous waste incineration sector in the EU.
Information is given for 24 European merchant rotary kiln installations which collectively have
a total annual capacity of 1500000 tonnes of waste (70 % of the total capacity of specialised
waste incinerators in the EU that is commercially available to third parties). On-site
installations, such as those in the chemical industry, are not considered in this overview. The
reference year for data collection is 1999 - 2000. Some specific data are more recent and refer to
the year 2001 - 2002.
There is a very high diversity of waste streams treated in these installations. Composition and
physical constitution can vary a lot from kiln to kiln and for each kiln over a period of time. For
this reason the kilns are equipped with sophisticated flue-gas cleaning systems.
General overview
Due to efficient flue-gas cleaning, the air emissions of the different installations covered in this
survey already meet the emission standards of Directive 2000/76/EC on incineration of waste.
In Table 3.21 below, an overview is given of the emissions of the waste incinerators as average
yearly concentrations. The minimum and maximum values of the individual installations, and
the average of all installations, are also given.
Parameter mg/Nm³
unless stated
HF
TOC
O2 (%)
NOX
Dust
HCl
SO2
Hg
Cd +Tl
Sum metals
PCDD/PCDF
(ngTEQ/Nm³)
CO
Minimum
0.01
0.01
8
44.4
0.075
0.25
0.1
0.0004
0.00014
<0.004
Yearly average
Maximum
<1
6
13.66
<300
9.7
8.07
22.7
0.06
0.046
0.84
Average
0.3
1.5
11.0
139
1.69
1.56
7.8
0.01
0.01
0.2
0.0003
<0.1
0.038
3
26
12.9
Table 3.21: Survey data of the annual average emissions to air from hazardous waste incinerators
in Europe
[41, EURITS, 2002]
In Table 3.22 below, the average of the mass flows (in kg/t of incinerated waste) for some
substances, together with the total amount of all the installations (if recorded) are given. The
latter demonstrates the outputs of the sector as a result of the treatment of about 1.3 to
1.5 million tonnes of waste per year.
Waste Incineration
163
Chapter 3
Parameter
Dust
SO2
NOX
Hg
Sum of metals
CO
HCl
Average mass flow
(kg/t of waste incinerated)
0.0098
0.047
0.87
0.000056
0.0013
0.07
0.0097
Total amount recorded
(t/yr)
16.2
60.6
1191
0.083
1.3
76.2
16.8
Table 3.22: Survey data of mass flow and annual sector emissions to air from merchant hazardous
waste incinerators in Europe
[41, EURITS, 2002]
Overview by each parameter
In the following paragraphs, the emissions for each parameter are discussed in more detail.
Where possible, the relationship between these emissions and the installed technology is
described.
The numbering given in the X-axes of the graphs below is not related to the specific
installations. Also, concentrations (bars, relating to the left Y-axis) and mass flows (diamonds,
relating to the right Y-axis) are given in the graphs. Mass flows based on non-absolute values
(e.g. values smaller than the determination level) are expressed as hollow diamonds.
HF is not described in detail because all the data collected for the 24 installations, which is
mostly obtained as a result of continuous monitoring, are below 1 mg/Nm³, which is the
analytical lower determination level (LDL) of this monitoring technique. Additional data
obtained by discontinuous measurements, a technique with a lower detection limit, confirm this
conclusion.
TOC is not described in detail because 95 % of the data collected, which again is mostly
obtained as a result of continuous monitoring, are below 1 - 2 mg/Nm³. Two installations have
higher yearly average emission of 4 and 6 mg/Nm³.
O2 data are given to indicate that the average concentration level is close to the standard
reference value of 11 %, to which all raw data have to be calculated.
Oxides of nitrogen
In the graph below the yearly average NOX values for all installations are given and given as:
•
•
average concentration of NOX expressed as NO2, in mg/Nm³, 11 % O2, dry and standard
conditions
average mass flow of NOX expressed as NO2 in g/t of incinerated waste.
The data are the result of continuous measurements of this parameter in the flue-gas. The
individual measurement points are in general integrated over half an hour, and then respectively
the daily, monthly and yearly averages are calculated. The analytical lower determination level
for continuous monitoring of this parameter is usually 5 - 10 mg/Nm³.
For each installation the installed NOX abatement technique is shown, which may be:
•
•
•
164
the use of selective non catalytic reduction (indicated as SNCR)
the use of selective catalytic reduction (indicated as SCR)
no specific abatement technique.
Waste Incineration
2000
350
1800
NOx(mg/Nm3)
300
1600
250
1400
1200
200
1000
150
800
600
100
400
50
0
200
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
NOx (g/tonne inc. waste)/Temp PCC (°C)
Chapter 3
Installation
No abatement technique
SNCR
SCR
NOx (g/tonne)
Temp (PCC)
Figure 3.1: Graph of NOX annual average emissions to air and applied abatement technique at
European HWIs
[41, EURITS, 2002]
General conclusions from the graph:
•
•
•
•
•
•
90 % of the installations perform below 200 mg/Nm³
50 % of the installations perform between 50 - 150 mg/Nm³. For these there is no clear
direct relationship with the abatement technique which is installed (note: some installations
operate at a set point which is not the lowest level that is technically achievable, e.g. No. 5
an SCR operating at 180 mg/Nm³)
for the four installations equipped with an SCR, the emissions are 180, 120, 72 and
59 mg/Nm³, respectively. The set point for the operation of each of these installations is
different and does not necessarily reflect the lowest level that is technically achievable. In
addition, the influence of conditions which determine the formation of NOX during
incineration cannot be deduced from the available data
for the three installations equipped with an SNCR, the emissions are 157, 118 and
93 mg/Nm³ respectively; for these results the same remark applies as that given in previous
bullet points
for the other installations not equipped with an SCR/SNCR there is a wide variation in the
emissions, mainly as a result of the different conditions for NOX formation in the individual
installations
several of the installations without SCR or SNCR but with low NOX emissions
(<120 mg/Nm³) operate at lower temperatures in the post combustion chamber (PCC):
950 - 1000 °C, in comparison with other installations operating at 1100 - 1200 °C in the
PCC.
Dust
In the graph below, the yearly average dust values for all installations are given and given as:
•
•
the average concentration of dust in mg/Nm³, 11 % O2, dry and standard conditions
the average mass flow of dust in g/tonne incinerated waste.
The data are the result of continuous measurements of this parameter in the flue-gas. The
individual measurement points are, in general, integrated over half an hour, and then
respectively the daily, monthly and yearly average is calculated. The analytical lower
determination level for continuous monitoring of this parameter is around 1 - 2 mg/Nm³.
Waste Incineration
165
Chapter 3
For each installation the dust emission technique is indicated. In this case, there is:
12
70
10
60
50
8
40
6
30
4
20
2
0
10
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Dust (g/tonne incinerated waste)
the use of electrostatic precipitators (ESPs), a dry ESP or a wet ESP
the use of a bag house filter
the use of a combination of these two techniques.
Dust(mg/Nm3)
•
•
•
0
Installation
ESP-dry
ESP-wet
Bag house filter
Dust (g/tonne)
ESP & baghouse filter
Dust (g/tonne, value <)
Figure 3.2: Graph of annual average dust emissions to air and applied abatement technique at
European HWIs
[41, EURITS, 2002]
General conclusions from the graph:
•
•
96 % of the installations perform below 5 mg/Nm³
one installation has a dust emission between 5 - 10 mg/Nm³.
HCl
In the graph below, the yearly average HCl values for all installations are given and given as:
•
•
the average concentration of HCl and volatile chloride compounds in mg/Nm³, 11 % O2, dry
and standard conditions
the average mass flow of HCl in g/t incinerated waste.
The data are the result of continuous measurement of this parameter in the flue-gas. The
individual measurement points are, in general, integrated over half an hour, and then
respectively the daily, monthly and yearly average is calculated. The analytical lower
determination level for continuous monitoring of this parameter is about 1 - 2 mg/Nm³.
For each installation the installed HCl abatement technique is shown. The techniques used are:
•
•
•
•
•
166
initial quenching of the flue-gases
the use of a wet scrubber (injection of lime-based compounds in water) with subsequent
evaporation of scrubbing water
the use of a wet scrubber with subsequent discharge of the treated scrubbing water
the use of a dry or semi-wet scrubber with the injection of lime based compounds in water
the injection of NaHCO3.
Waste Incineration
Chapter 3
Most of the HCl in raw flue-gases from hazardous waste incineration originates from organics
containing chlorine but some of it also comes from inorganic salts such as NaCl.
At the temperatures achieved during incineration the Deacon equilibrium is important to
consider:
4 HCl + O2 2 H2O + 2 Cl2 (+ 114.5 kJ)
9
40
8
35
HCl (mg/Nm3)
7
30
6
25
5
20
4
15
3
10
2
5
1
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
HCl (g/tonne incinerated waste)
During the combustion of hydrocarbon-containing waste the equilibrium is shifted to the left
side of the equation, due to the fact that during combustion an excess of H2O is formed, and as a
result, chlorine is present in the HCl form in the combustion gas. When, for example, low
hydrogen-containing waste, e.g. PCB, is incinerated this is not the case and the equilibrium is
shifted to the right side of the equation, meaning that a mixture of HCl and Cl2 will be formed.
In this case, the flue-gas cleaning has to be adapted for the de-chlorination of the combustion
gases.
0
Installation
Wet scrubber & evap. of water
Wet scrubber (or quench) &water treatment
Cooling & injection of NaHCO3
Dry scrubber
HCl (g/tonne value <)
HCl (g/tonne)
Figure 3.3: Graph of HCl annual average emissions to air and applied abatement technique at
European HWIs
[41, EURITS, 2002]
General conclusions from the graph:
•
•
•
90 % of the installations perform below 2 mg/Nm³
this data does not reveal any clear relationship between technique and annual average
emission levels
for the three other installations the emissions are 8, 4 and 3 mg/Nm³ respectively.
SO2
In the graph below, the yearly average SO2 values for all installations are given. These are given
as:
•
•
average concentrations of SO2 in mg/Nm³, 11 % O2, dry and standard conditions
average mass flow of SO2 in g/t incinerated waste.
Waste Incineration
167
Chapter 3
The data are the result of continuous measurements of this parameter in the flue-gas. The
individual measurement points are in general integrated over half an hour, and then the daily,
monthly and yearly average respectively is calculated. The analytical lower determination level
for continuous monitoring of this parameter is around 1 - 5 mg/Nm³.
For each installation the installed SO2 emission abatement technique is shown. In this case there
is:
•
•
•
•
•
initial quenching of the flue-gases
the use of a wet scrubber (injection of lime-based compounds in water) and subsequently
the evaporation of the scrubbing water
the use of a wet scrubber and subsequently the discharge of the treated scrubbing water
the use of a dry or semi-wet scrubber (injection of lime-based compounds in water)
the injection of NaHCO3 in the flue-gas transport channel.
The formation of SO2 in incineration processes originates from S-compounds in the waste e.g.
CxHyS + z O2
CO2 + SO2 + H2O
There is a direct linear relationship between the amount of SO2 in the raw flue-gases and the
amount of sulphur in the waste. Most sulphur containing compounds, also inorganic, degrade
during combustion and end up in the raw gas as SO2.
120
SO2(mg/Nm3)
20
100
15
80
60
10
40
5
0
20
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SO2 (g/tonne incinerated waste)
140
25
0
Installation
Wet scrubber & evap. of water
Wet scrubber (or quench) &water treatment
Dry scrubber
Cooling & injection of NaHCO3
SO2 (g/tonne)
SO2 (g/tonne value <)
Figure 3.4: Graph of annual average sulphur dioxide emissions to air and applied abatement
technique at European HWIs
[41, EURITS, 2002]
168
Waste Incineration
Chapter 3
General conclusions from the graph:
•
•
•
•
•
90 % of the installations perform below 20 mg/Nm³
dry systems give results in the range of 5 – 23 mg/Nm³, with a median value of approx.
12 mg/Nm³. SOX abatement is reported to be improved with dry sodium bicarbonate than
dry lime systems [74, TWGComments, 2004]
wet systems give results in the range of 2 – 22 mg/Nm³, with a median value of approx
4 mg/Nm³
about 50 % of the installations perform below 5 mg/Nm³ which is near the analytical lower
determination level for continuous monitoring of this parameter
for the two other installations the emissions are respectively 23 and 21 mg/Nm³.
Mercury
In the graph below, the yearly average mercury values for all installations are given. These are
given as:
•
•
the average concentration of mercury in mg/Nm³, 11 % O2, dry and standard conditions
the average mass flow of mercury in g/t incinerated waste.
The data of eight installations are the result of continuous measurements of this parameter in the
flue-gas. The individual measurement points are in general integrated over half an hour, and
then the daily, monthly and yearly average respectively is calculated. The analytical lower
determination level for continuous monitoring of this parameter is 1 - 2 µg/Nm³.
All of these continuously monitored installations have yearly average emission levels below
5µg/Nm³.
The data of the other installations are obtained by periodic discontinuous Hg measurements,
ranging from twice a month to twice a year. The analytical lower determination level for this
method is 1 µg/Nm³.
For each installation the installed mercury abatement technique is shown. In this case, there is:
•
•
•
the use of a wet scrubber system (the lower the pH of the scrubbing water, the higher the
removal efficiency of Hg)
the injection of activated carbon (or an alternative reagent, e.g. brown-coal cokes)
the use of a static activated carbon filter (or an alternative reagent, e.g. brown-coal cokes).
In the graph, the availability of activated carbon injection or the presence of an activated carbon
filter is not mentioned because all the installations are equipped with it, except installations
numbered 5, 6 and 11.
The mercury in the flue-gases originates from mercury-containing waste. There is a direct linear
relationship between the amount of mercury in the raw flue-gases and the amount of mercury in
the waste. For one installation equipped with wet gas scrubbing and an activated carbon filter, it
is calculated that the total mercury input via the waste amounts to 1000 kg/yr for an installation
with an incineration capacity of 50000 t/yr. Taking into account a maximum yearly-emitted Hg
flow via the flue-gases of less than 1.25 kg/yr, this means a total removal efficiency of 99.99 %.
Installations with a continuously or temporarily high Hg input are able to add sulphurcontaining reagents in the wet scrubber system to increase the removal efficiency of Hg. The
screening of the waste inputs for Hg is, therefore, important.
Waste Incineration
169
Chapter 3
0.08
0.25
0.2
Hg (mg/Nm3)
0.06
All installations have injection of AC or AC filter
except number 5, 6 and 11
0.05
0.15
0.04
0.1
0.03
0.02
0.05
Hg (g/tonne incinerated waste)
0.07
0.01
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Installation
Wet scrubber
Dry scrubber
Hg (g/tonne)
Cooling & injection of NaHCO3
Hg (g/ton value <)
Figure 3.5: Graph of Hg annual average emissions to air and applied abatement technique at
European HWIs
Source [41, EURITS, 2002]
General conclusions from the graph:
•
•
90 % of the installations perform below 0.01 mg/Nm³
for the 3 other installations the emissions are 0.06, 0.04 and 0.013 mg/Nm³ respectively.
Although not shown in these results, practical experience is that the type of activated carbon
(physical characteristics and the impregnation of the carbon) has an influence on Hg removal
efficiency.
Other metals: Sum of As, Sb, Pb, Cr, Co, Cu, Mn, Ni, V, Sn
In the graph below, the yearly average metal emissions for all installations are given. These
values are given as:
•
•
the average concentration of the sum of the metals in mg/Nm³, 11 % O2, dry and standard
conditions
the average mass flow of the sum of the metals in g/t incinerated waste.
For most installations this shows an average concentration of two to eight discontinuous
measurements a year. These measurements are performed based on the US Environmental
Protection Agency (EPA) Method 29.
Over 60 % of the installations perform under 0.2 mg/Nm³.
Detection limit reporting differences:
The key potential difference in reported values is partly a result of the manner of reporting of
undetected metals. In some countries these metals are calculated as zero, in other countries the
detection limit values of the metals are reported.
The detection limit of the analysed metals depends on the total amount of sample taken and on
the type of metal analysed (detection limits up to 0.018 mg/Nm³ for some metals are reported).
170
Waste Incineration
Chapter 3
In other countries one detection limit value (0.001 or 0.005 mg/Nm³) for all metals is reported,
independent of the type of metal or the amount of sample taken.
Taking account of the detection limit value of the undetected metals, results in the reporting of a
much higher sum value of the ten reported metals.
4.5
0.6
4
Sum metals (mg/Nm3)
0.5
3.5
3
0.4
2.5
0.3
2
0.2
1.5
1
0.1
0
0.5
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
Sum metals (g/tonne incinerated waste)
As a result only the data equal to or higher than 0.05 mg/Nm³ are shown in the graph and the
results below 0.05 mg/Nm³ are indicated as less than 0.05 mg/Nm³.
Installation
ESP-dry
ESP-wet
Bag house filter
Sum metals (g/tonne)
ESP&Bag house
Sum metals < 0.05 mg/Nm3
Sum metals (g/tonnne value <)
Figure 3.6: Annual average emissions to air of other metals and applied abatement technique at
European HWIs
Source [41, EURITS, 2002]
General conclusions from the graph:
•
•
63 % of the installations perform below 0.2 mg/Nm³ and for these installations there is no
direct relationship with the abatement technique that is installed
the other five installations, all equipped with a bag-house filter, have a higher metal
emission.
Cadmium and thallium
In the graph below, the yearly average metal emissions for all installations are given. These
values are given as:
•
the average concentration of the sum of Cd and Tl in mg/Nm³, 11 % O2, dry and standard
conditions.
For most installations this shows an average concentration of two to eight discontinuous
measurements a year. These measurements are performed based on the US Environmental
Protection Agency (EPA) Method 29.
75 % of the installations perform under 0.02 mg/Nm³. The key potential difference in reported
values is partly a result of the different way of treatment of undetected metals as discussed in
the paragraph on other metals (above). Using the detection limit value of the undetected metals
results in a higher sum value of the reported metals. As a result, only the data equal to or higher
than 0.01 mg/Nm³ are shown in the graph and the results below 0.01 mg/Nm³ are indicated as
less than 0.01 mg/Nm³.
Waste Incineration
171
Cd and Tl (mg/Nm3)
Chapter 3
0.05
0.05
0.045
0.045
0.04
0.04
0.035
0.035
0.03
0.03
0.025
0.025
0.02
0.02
0.015
0.015
0.01
0.01
0.005
0
0.005
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
Installation
ESP-dry
ESP-wet
Bag house filter
Cd+Tl < 0.01 mg/Nm³
ESP&Bag house
Figure 3.7: Graph of Cd and Tl annual average emissions to air and applied abatement technique
at European HWIs
[41, EURITS, 2002]
Dioxins and furans
In the graph below, the data for polychlorinated dibenzo-dioxins (PCDD) and polychlorinated
dibenzofurans (PCDF) for all surveyed installations are given. These are given as average
concentrations expressed as TEQ ng/Nm³, 11 % O2, dry and standard conditions. For most
installations, it shows an average of two to four discontinuous measurements a year (based on
CEN: EN1948).
Detection limit differences:
Between the official laboratories which are certified for the determination of dioxins, there is a
large difference in the reporting of the attainable detection limit of the analytical method. It
ranges from 0.01 to less than 0.001 ng TEQ/Nm³, notwithstanding the fact that comparable
sampling procedures are followed (e.g. 6 - 8 hour sampling period). The lowest detection limits
are reported by German laboratories.
Here, only the data equal or higher than 0.01 ng TEQ/Nm³ are shown in the graph and the
results below 0.01 ng TEQ/Nm³ are indicated as less than 0.01 ng TEQ/Nm³.
The key potential difference in reported values is possibly a result of the inconsistent treatment
of undetected PCDD/PCDF isomers some being calculated at the LOD (EN 1948 pt 3 refers),
others being calculated as zero. The relative influence of the variation therein, is the function
merely of the respectively assigned toxic equivalence factor (TEF) for that isomer.
From the graph, no specific conclusion can be drawn regarding the performance of the different
techniques, as the ranking of the results is not directly related to the type of abatement technique
installed. The low emission values and the variable accuracy of the analytical measurements at
this level are additional confounding factors. Monitoring results from plants using continuous
sampling show similar levels as short period monitoring.
172
Waste Incineration
0.1
0.7
Dioxin (ngTEQ/Nm3)
0.09
0.6
0.08
0.5
0.07
0.06
0.4
0.05
0.3
0.04
0.03
0.2
0.02
0.1
0.01
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
Dioxin (ug/tonne incinerated waste)
Chapter 3
Installation
AC injection
No dioxin abatement technique
PCDD/PCDF (ug/tonne)
AC filter
Dioxin emission < 0.01 ng TEQ/Nm3
PCDD/PCDF (ug/tonne value <)
Quench
AC inject. + SCR dediox
Figure 3.8: Graph of PCDD/F annual average emissions to air and applied abatement technique at
European HWIs
[41, EURITS, 2002]
PCBs and PAHs
The emission of Poly-Chlorinated-Biphenyls (PCBs) is not always monitored. The available
data show values mostly less than detection limit and ranging from <1 µg/Nm³ to <2 ng/Nm³.
Here again, a critical analytical remark has to be made about the variability of the reported
detection limits of the measurement methods.
The emission of Poly-Aromatic-Hydrocarbons (PAHs) is also not always monitored. The
available data show values range from <1 µg/Nm³ to <0.1 µg/Nm³. Here also, a critical
analytical remark has to be made about the variability of the reported detection limits of the
measurement methods.
Carbon monoxide
Combustion efficiency is partly described by CO levels, which also indicates formation of also
other Products of Incomplete Combustion (PICs).
The yearly average values for all installations surveyed, obtained as a result of continuous
measurements vary from 3 to 26 mg/Nm³.
CO is a typical parameter with a low baseline emission but which periodically shows sharp peak
emissions, due to sudden variations in local combustion conditions (e.g. variations in
temperature of parts of the kiln). The monitoring and control of these peak emissions is an
important aspect of the daily operation of an incinerator. With the pretreatment of drummed
waste and feed equalisation it is possible to decrease CO peaks.
Waste Incineration
173
Chapter 3
Figure 3.9 below shows the reductions in CO emission achieved at an HWI following the
introduction of drum shredding and other waste input blending techniques (technique described
in Section 2.2.2.4 and Figure 2.2):
Figure 3.9: CO emission reductions achieved following introduction of pretreatment techniques at a
hazardous waste incinerator
[20, EKOKEM, 2002]
3.3 Emissions to water
3.3.1 Volumes of waste water arising from flue-gas treatment
[1, UBA, 2001]
Water is used in waste incineration for various purposes. Wet flue-gas cleaning systems give
rise to waste water whereas semi-wet and dry systems generally do not. In some cases the waste
water from wet systems is evaporated and in others it is treated and discharged.
Table 3.23 shows examples of the typical quantities of scrubbing water arising from the flue-gas
cleaning of waste incineration plants.
Plant type and waste throughput
Municipal waste incineration plant with
a throughput of 250000 t/yr
Municipal waste incineration plant with
a throughput of 250000 t/yr
Hazardous waste incineration plant with
a throughput of 60000 t/yr
Hazardous waste incineration plant with
a throughput of 30000 t/yr
Type of flue-gas cleaning
2 stages, with milk of lime
Approx. quantity of
waste water
(m³/tonne waste)
0.15 (design value)
2 stages, with sodium hydroxide
(before condensation facility)
2 stages, with milk of lime
0.3 (operating value)
0.15 (annual average)
2 stages, with sodium hydroxide
0.2 (annual average)
Table 3.23: Typical values of the amount of scrubbing water arising from FGT at waste
incineration plants treating low chlorine content wastes
[1, UBA, 2001]
174
Waste Incineration
Chapter 3
3.3.2 Other potential sources of waste water from waste incineration
plants
[1, UBA, 2001]
Besides the waste water from the flue-gas cleaning, waste water can also arise from a number of
other sources. Regional rainfall variations can have a great effect. Owing mainly to differences
on installation design, not all of these waste water streams will arise at all plants and those given
here are streams which may arise:
Waste water
Chimney condensates after wet scrubbing
Wet ash removing/wet declining
Reversible flow water from ion exchanger
Boiler water
Water from the cleaning of storage containers
Other cleaning water
Contaminated rainwater
Laboratory water
Approx. amount
• 20 m³/d
• 6600 m³/yr
• 5 m³/d
•
1650 m³/yr
• 1 m³/4 weeks
• 120 m³/yr
• 500 m³/yr
• 800 m³/yr
• 300 m³/yr
• 200 m³/yr (Germany)
• 200 m³/yr
Occurrence
(c) continuous
c
(d) discontinuous
d
d
d
d
d
Data calculated on the basis of 330 operating days per year
Table 3.24: Other possible waste water sources, and their approximate quantities, from waste
incineration plants
[1, UBA, 2001]
3.3.3 Installations free of process water releases
[1, UBA, 2001]
In some incineration plants waste water arising from wet gas scrubbing is evaporated in the
incineration process using a spray dryer. This can eliminate the need for effluent releases from
the process.
In such cases the waste water is generally pretreated in an Effluent Treatment Plant (ETP),
before it is fed to the spray dryer. Treatment in an ETP can help to prevent the recirculation and
accumulation of some substances. Hg recirculation is of particular concern, and specific
reagents are usually added to provide a means for Hg removal from the system.
Salt (NaCl) can be recovered from the treated effluent for possible industrial uses, or may be
collected in the FGT residues.
3.3.4 Plants with physico–chemical waste water treatment
[1, UBA, 2001]
The treatment of waste water from the flue-gas cleaning in waste incineration plants is not
fundamentally different from the treatment of waste water from other industrial processes.
Waste water from municipal waste incineration plants mainly contains the following substances,
which require treatment:
•
•
•
heavy metals, including mercury
inorganic salts (chlorides, sulphates etc.)
organic compounds (phenols, PCDD/PCDF).
Waste Incineration
175
Chapter 3
The following table shows typical contamination levels of waste water from flue-gas cleaning
facilities of municipal and hazardous waste incineration plants before the treatment of waste
water.
Parameter
pH Value
Conductivity
(bS)
COD mg/l)
TOC mg/l)
Sulphate mg/l)
Chloride mg/l)
Fluoride mg/l)
Hg (bg/l)
Pb mg/l)
Cu mg/l)
Zn mg/l)
Cr mg/l)
Ni mg/l)
Cd mg/ l)
PCDD/PCDF
(ng/l)
Municipal waste incineration
Minimum
<1
Maximum
Average
n/a
>20000
Hazardous waste incineration for
common commercial plants
Minimum Maximum
Average
No data
No data
n/a
No data
No data
140
47
1200
85000
6
1030
0.05
0.05
0.39
<0.05
0.05
<0.005
390
105
20000
180000
170
19025
0.92
0.20
2.01
0.73
0.54
0.020
260
73
4547
115000
25
6167
0.25
0.10
0.69
0.17
0.24
0.008
No data
No data
615
No data
7
0.6
0.01
0.002
0.03
0.1
0.04
0.0009
No data
No data
4056
No data
48
10
0.68
0.5
3.7
0.5
0.5
0.5
22
No data
No data
No data
No data
No data
No data
Table 3.25: Typical contamination of waste water from wet FGT facilities of waste incineration
plants before treatment
[1, UBA, 2001]
The following two tables show:
•
•
176
Table 3.26 shows the annual specific emissions to surface water and/or sewer from various
waste incinerators in the Netherlands in 1999.
Table 3.27 shows the impact of waste water treatment on the effluent from a MSWI and
provides a comparison of this performance with various standards.
Waste Incineration
Chapter 3
Site
Incinerated
As
Cd
Cr
Cu
Hg
Pb
Ni
Zn
Chlorides
Sulphates
COD
N-Kjeldahl
(kt/yr)
mg/t)
mg/t)
mg/t)
mg/t)
mg/t)
mg/t)
mg/t)
mg/t)
(g/t)
(g/t)
(g/t)
(g/t)
Municipal Waste Incineration
Gevudo
171
23.2
9.1
17
115
3.04
72
39.9
552
4990
2070
298
46
AVR Rotterdam
386
0.5
0.3
5
6
0.10
9
8.6
4
n/a
n/a
15
1
AVR-Botlek
1106
0.6
2.7
2
4
0.72
5
2.1
20
n/a
n/a
34
4
AVR AVIRA
301
0.0
2.0
2
6
0.07
2
1.6
26
0
133
10
AVI Roosendaal
55
4.4
0.1
7
62
0.02
16
4.9
45
0
0
24
1
ARN
250
3.7
1.3
43
25
0.71
23
44.4
181
708
111
207
131
AVI Amsterdam
789
0.0
0.0
0
0
0
0
0
0
0
0
0
AVI Noord-
452
0.1
0.1
1
3
0.02
4
0.4
27
1
n/a
n/a
n/a
AVI Wijster
433
23.1
0.0
30
58
0.16
53
36.9
226
335
84
380
44
AZN
603
0.2
0.2
0
2
0.17
0
0.3
23
4602
254
18
4
AVI Twente
285
n/a
0.0
0
0
n/a
0
0.0
1
2
n/a
12
1
44
4.6
4.6
14
25
6.84
23
18.3
228
n/a
n/a
319
26
7
191.1
632.1
658
2694
4391.27
11676
459.0
72832
n/a
n/a
658
16
DRSH
368
21.4
3.5
5
79
5.97
15
3.0
92
1561
4560
1829
n/a
SNB
406
5.8
0.6
18
17
1.23
8
12.3
51
725
31
816
768
V.I.T.
89
1.9
1.5
3
14
0.51
19
6.0
56
n/a
56083
155
30
Hazardous Waste Incineration
AVR-Chemie DTs
Clinical Waste Incineration
ZAVIN
Sewage Sludge Incineration
Table 3.26: Releases to surface water and sewers from Dutch waste incinerators in 1999
[2, infomil, 2002]
Waste Incineration
177
Contaminant
Chapter 3
pH
Susp.
95 %
solids 100 %
Hg
Cd
Tl
As
Pb
Cr
Cu
Ni
Zn
Dioxin ng/l
Dioxin ng/l
Limit values
German
33Abw
EC Dir.
2000/76
mg/l
mg/l
30
45
0.03
0.05
0.05
0.15
0.1
0.5
0.5
0.5
1.0
0.3
30
45
0.03
0.05
0.05
0.15
0.2
0.5
0.5
0.5
1.5
0.3
Waste water
Treated effluent
Input
(Trimercaptotriazine
add. 150 ml/m³)
First stage wet scrubber 289 l/t waste input
Range
Range
Avg.
Avg.
Min.
Max.
Min.
Max.
Treated effluent
(Trimercaptotriazine
add. 55 ml/m³)
Second stage scrubber 55 l/t waste input
Range
Range
Avg.
Avg.
Min.
Max.
Min.
Max.
mg/l
0.3
Input
mg/l
0.6
mg/l
0.5
mg/l
6.7
mg/l
8.3
mg/l
7.6
mg/l
7.4
mg/l
8.4
mg/l
8.1
1.8
5.7
<0.01
0.76
<0.01
0.03
<0.03
0.1
1.2
24
0.46
1.3
1.9
29
1.9
4.5
4.1
67
In liquids
In solids
3.6
0.45
0.028
0.05
8.8
0.7
8.6
2.5
24
0.01
11.7
<0.001
<0.01
<0.01
<0.05
0.03
<0.02
0.1
0.23
0.17
0.013
<0.01
0.013
<0.05
1.2
<0.02
0.32
0.64
0.25
0.01
<0.01
<0.01
<0.05
0.13
<0.02
0.23
0.41
0.22
<0.01
0.25
0.04
0.1
<0.01
<0.05
0.7
0.02
0.81
0.02
6.9
1.42
0.62
0.02
0.08
9.2
0.14
3.1
0.13
36
0.82
0.37
0.016
0.06
3.5
0.06
1.4
0.08
17
<0.01
15.9
mg/l
9.4
1
1
<0.001
<0.01
<0.01
<0.03
<0.05
<0.02
0.02
<0.02
0.01
mg/l
11.1
56 (1)
56 (1)
0.013
0.02
0.023
<0.10
1.4 (2)
0.03
0.79 (3)
0.83 (4)
1.7 (5)
mg/l
10.3
23.8
23.8
<0.003
<0.01
<0.013
<0.04
<0.11
<0.02
0.10
<0.20
0.19
<0.01
0.32
Notes:
1. 1 in excess of 24 measurements in 2001
2. 5 in excess of 104 measurements 0.18 – 0.27 (1 x 1.4) mg/l in 2001
3. 2 in excess of 104 measurements 0.66 and 0.79 mg/l in 2001
4. 3 in excess of 104 measurements 0.57- 0.83 mg/l in 2001
5. 1 in excess of 104 measurements in 2001
Table 3.27: Waste water quality (after treatment with Trimercaptotriazine) - Comparison between raw and treated waste water and various standards
[52, Reimann, 2002]
178
Waste Incineration
Chapter 3
3.3.5 Hazardous waste incineration plants - European survey data
[EURITS, 2002 #41]
The data in this section describe the emissions to water arising from treated flue-gas waste water
streams. The data are taken from a survey of European merchant hazardous waste incinerators
as reported by [EURITS, 2002 #41].
3.3.5.1 General overview of emissions to water from European HWI
An overview of the yearly average minimum and maximum concentrations for the different
installations is given in Table 3.28.
The concentration of most parameters varies a lot between the different installations, as does the
water flow (expressed in litre per kilogram of waste incinerated).
Yearly average
Parameter
all mg/l
(unless stated)
Suspended solids
COD
Cd
Tl
Hg
Sb
As
Pb
Cr
Co
Cu
Mn
Ni
V
Sn
Zn
ClSO42Dioxins (ng TEQ/l)
Flow of water (l/kg waste)
Minimum
Maximum
3
<50
0.0008
0.01
0.0004
0.005
0.0012
0.001
0.001
<0.005
0.01
0.02
0.004
<0.03
<0.02
<0.02
3000
300
0.0002
0.2
60
<250
0.02
0.05
0.009
0.85
0.05
0.1
0.1
<0.05
0.21
0.2
0.11
0.5
<0.5
0.3
72000
1404
<0.05
20
Table 3.28: Annual average range of concentrations of the emissions to water after treatment from
merchant hazardous waste installations that discharge waste water
[EURITS, 2002 #41]
Table 3.29 below shows the emissions to water as the mass flow of these components in mg/kg
of waste input:
Waste Incineration
179
Chapter 3
Parameter
Suspended solids
COD
Cd
Hg
Sb
As
Pb
Cr
Co
Cu
Mn
Ni
V
Sn
Zn
Cl
SO42
mg/kg waste incinerated)
Minimum
Maximum
2.4
325
76.5
1040
0.001
0.16
0.00048
0.112
0.0325
0.72
0.001
0.325
0.0084
0.65
0.0024
2
0.045
0.325
0.0085
4.2
0.023
1
0.0042
2
0.325
0.6
0.09
0.565
0.0226
1.95
4520
60000
240
6572
Table 3.29: Mass flows of the emissions to water from surveyed merchant HWIs in Europe
[EURITS, 2002 #41]
3.3.5.2 Overview by parameter of emissions to water from European HWI
Suspended solids
In the graph below, the yearly average values for suspended solids released for all of the
surveyed installations are given as suspended solids in mg/l effluent.
For each installation the type of waste water treatment technique effective for suspended solids
is shown; these are:
•
•
•
the use of a sand filter
the separate treatment of the acidic and alkali scrubber waters - in this case no forced
precipitation, nor post precipitation of CaSO4 is performed, and higher loads of sulphate are
discharged
no additional water treatment step.
From the graph on heavy metals (Figure 3.6) it can be seen that these metals are only a marginal
constituent of the suspended solids.
For the origin of the residual suspended solids in the effluent which is discharged, three
scenarios can be given:
•
•
•
180
residual fractions of the precipitated components which are not removed by decantation or
filtration
when groundwater containing Fe(II) is used in wet flue-gas cleaning, a slow oxidation of
Fe(II) to Fe(III) and subsequent precipitation of Fe(OH)3 can result in suspended solids
where the residence time in the waste water treatment plant is shorter than the time the
reaction needs to be completed
in other cases, the suspended solid can originate from post precipitation reactions of
sulphates and carbonates with Ca2+ which is present in the effluent or in other water streams
which come into contact with the effluent before discharge and when the residence time is
shorter than the time the reaction needs to be completed.
Waste Incineration
Chapter 3
70
Suspended solids (mg/l)
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
Installation
No abatement technique
Seperate treatment acidic/alkalic water + sand filter
Quencher + sand filter
Figure 3.10: Graph of annual average suspended solid discharges to water and applied abatement
technique at European HWIs
[EURITS, 2002 #41]
General conclusions from the graph:
•
•
all the installations perform below 60 mg/l
the installations that have separate treatments for the acidic and alkali scrubber waters
achieve the lowest emissions of suspended solids (3 mg/l).
Mercury
In the graph below, the yearly average mercury values for all installations are given. And given
as:
•
•
•
the average concentration in mg/l, for 24 hour representative samples for continuous
discharge (90 % of installations) or for batch representative samples for batch discharge
(10 % of installations)
the 95 percentile in mg/l, if daily values or several values a week are available
the 99 percentile in mg/l, if daily values or several values a week are available.
For five of the waste water streams, Hg is measured daily (or several times a week) and for four
installations data are obtained weekly or monthly. It can be concluded that the data in the graph
are representative of a complete working year.
For each installation the type of waste water treatment technique is shown, so far as it has an
influence on mercury emissions. In this case, there is:
•
•
•
the precipitation of mercury as a M-sulphide or a M-trimercaptotriazine component
the precipitation as M-sulphide component and subsequently the use of an activated carbon
filter
no additional water treatment step.
Mercury in the effluent originates, of course, from mercury contained in the waste. It is common
practice that incinerators apply an input limit for mercury over a time period.
Waste Incineration
181
Chapter 3
0.05
0.05
0.045
0.045
0.04
0.04
0.035
0.035
0.03
0.03
0.025
0.025
0.02
0.02
0.015
0.015
0.01
0.01
0.005
0.005
0
1
2
3
4
5
6
7
8
9
95 and 99 Percentile (mg/l)
Hg (mg/l)
For one installation equipped with wet gas scrubbing, it is calculated that the total mercury input
via the waste, amounts to 2000 kg/yr for an installation with an incineration capacity of
100000 t/yr. Taking into account a maximum yearly emitted Hg flow via the waste water of less
than 4 kg/yr, a removal efficiency higher than 99.8 % can be reached based on
M-trimercaptotriazine precipitation and subsequent efficient removal of the precipitate.
0
Installation
M-sulphide or TMT
No abatement technique
95 Percentile
99 Percentile
M-sulphide or TMT + AC filter
Figure 3.11: Graph of annual average mercury discharges to water and applied abatement
technique at European HWIs
[EURITS, 2002 #41]
General conclusions from the graph:
•
•
•
all the installations perform below 0.01 mg/l
the 95 and 99 percentile values vary from one installation to another
in three cases occasional peak values of Hg are detected (average <P95 <P99) which
can amount to 50 µg/l and higher; the reason for this is unexpected situations, e.g.
unexpectedly high input or failures of the treatment installation
in two cases no peak values are detected (P99= P95 = average); the reason for this is
in one case that discharges are made periodically and not continuously, and in the
other case the fact that no Hg is present in the raw alkaline scrubber water
there is no direct relationship visible between the abatement technique and the annual
average emission of mercury.
Metal emissions
In the graph below the yearly average metal emissions for all installations are given and given
as:
•
•
•
average concentrations in mg/l, for 24 hour representative samples in the case of continuous
discharges (90 % of installations) or for batch representative samples in the case of batch
discharges (10 % of installations)
the 95 percentile in mg/l, if daily values or several values a week are available
the 99 percentile in mg/l, if daily values or several values a week are available.
The waste water treatment technique used to decrease the metal emissions consists of
precipitation of metals as hydroxides and/or as metal sulphide components. Flocculation
additives are used to optimise the precipitation.
182
Waste Incineration
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0.35
Metals (mg/l)
0.3
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
7
8
9
Installation
Cd
Tl As
Pb
Cr
Cu
Ni
Zn
Figure 3.12: Graph of annual average discharges of various metals to water at European HWIs
[EURITS, 2002 #41]
General conclusions from the graph:
•
•
almost all the individual metal emissions are below 0.1 mg/l
only higher values are registered for Zn and Cu in specific cases.
In the following graphs a more detailed overview is given per parameter with 95- and 99percentile values. From these graphs it can be seen that, in some cases, higher values are
sometimes registered.
0.06
Metal (mg/l)
0.05
0.04
0.03
0.02
0.01
0
1
2
3
4
5
6
7
8
9
Installation
As
95 Percentile
99 Percentile
Figure 3.13: Graph of annual average Arsenic discharges to water at European HWIs
[EURITS, 2002 #41]
Waste Incineration
183
Chapter 3
Metal (mg/l)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
1
2
3
Pb
4
5
6
Installation
95 Percentile
7
8
9
99 Percentile
Figure 3.14: Graph of annual average lead discharges to water at European HWIs
[EURITS, 2002 #41]
0.035
Metal (mg/l)
0.03
0.025
0.02
0.015
0.01
0.005
0
1
2
3
4
5
6
7
8
9
Installation
Cd
95 Percentile
99 Percentile
Figure 3.15: Graph of annual average Cadmium discharges to water at European HWIs
[EURITS, 2002 #41]
0.14
Metal (mg/l)
0.12
0.1
0.08
0.06
0.04
0.02
0
1
2
3
4
5
6
7
8
9
Installation
Cr
95 Percentile
99 Percentile
Figure 3.16: Graph of annual average Chromium discharges to water at European HWIs
[EURITS, 2002 #41]
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Waste Incineration
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1.6
Metal (mg/l)
0.2
1.4
1.2
0.15
1
0.8
0.1
0.6
0.4
0.05
0.2
0
1
2
3
4
5
6
7
8
9
95 and 99 Percentile (mg/l)
1.8
0.25
0
Installation
Cu
95 Percentile 99 Percentile
Figure 3.17: Graph of annual average Copper discharges to water European HWIs
[EURITS, 2002 #41]
Metal (mg/l)
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
7
8
9
Installation
Ni
95 Percentile
99 Percentile
Figure 3.18: Graph of annual average Nickel discharges to water at European HWIs
[EURITS, 2002 #41]
0.6
Metal (mg/l)
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
Installation
Zn
95 Percentile
99 Percentile
Figure 3.19: Graph of annual average Zinc discharges to water at European HWIs
[EURITS, 2002 #41]
Waste Incineration
185
Chapter 3
Chloride and sulphate content
[EURITS, 2002 #41] The amount of chloride in the effluent demonstrates a linear relationship
to the amount of chlorine in the waste in the input to the incinerator. Most incinerators discharge
their waste water into, or near, the sea. A concentration of 3 - 72 g/l of effluent is quoted.
One surveyed installation discharges the effluent containing salt into the fresh water of a river.
The sulphate content in the effluent is controlled in most installations by the partial
precipitation of gypsum, so the discharged concentration of SO42- is between 1 and 2 g/l.
There is one installation which treats the acidic and alkali scrubber waters separately, without
precipitation of gypsum, leading to a higher load of sulphate, discharged to the sea in this case.
3.4 Solid residues
3.4.1 Mass streams of solid residues in MSWI
In Table 3.30, some typical data on residues from municipal waste incineration plants are
summarised:
Types of waste
Slag/ash (including grate siftings/riddlings)
Dust from boiler and de-dusting
FGC residues, reaction products only:
Wet sorption
Semi-wet sorption
Dry sorption
Reaction products, and filter dust, from:
Wet sorption
Semi-wet sorption
Dry sorption
Loaded activated carbon
Specific amount (dry)
(kg/t of waste)
200 – 350
20 – 40
8 – 15
15 – 35
7 – 45
30 – 50
40 – 65
32 – 80
0.5 – 1
Note: wet sorption residue has a specific dryness (e.g. 40 – 50 % d.s.) [74, TWGComments, 2004]
Table 3.30: Typical data on the quantities of residues arising from municipal waste incineration
plants.
[1, UBA, 2001]
State-of-the-art MSWI plants typically produce between 200 and 350 kg bottom ashes per tonne
of waste treated. Most published numbers include the grate siftings, which only recently (and
only in some countries) have been kept separate from the bottom ash. The mass flow of siftings
depends on the type of grate and its time of operation. The siftings may increase the amount of
unburned matter in the bottom ash and can contribute to leaching of copper. Concerning bottom
ash re-use, ferrous and non ferrous materials (e.g. Al) may be segregated. However, the
inventory of metallic Al, which drips through the grate voids, is of higher concern (e.g. can
cause grate blockage) [74, TWGComments, 2004]
The production of boiler ash depends on the type of boiler and on the amount of dust originally
released from the grate.
[Vehlow, 2002 #38] The mass flow of flue-gas treatment residues shows the highest variation of
all residues. 10 – 12 kg/t is a mean value for wet systems, which operate close to stoichiometry.
This figure comprises the dry neutral sludge (2 – 3 kg/t) and the soluble salts (8 – 9 kg/t). In
semi-wet or dry lime systems the amount is increased because of unreacted additives, while the
dry sodium bicarbonate process gives the lower values [64, TWGComments, 2003].
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Waste Incineration
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Table 3.31 below, gives mass streams of solid residues for various substances per tonne of
MSW incinerated. The data given is average data for 12 MSWI in the Flanders Region of
Belgium in 1999:
Type of solid residue
Bottom ash
Fly ash + gas cleaning residue + sludge from wet scrubbers
Scrap recuperated from bottom ash
Percentage (%)
21
4.2
1.2
Table 3.31: Mass streams of solid residues from MSWI expressed per tonne of MSW incinerated
[64, TWGComments, 2003]
3.4.2 Bottom ash composition and leachability
Requirements concerning the quality of the residues from the incineration process are included
in European incineration legislation. Directive 2000/76/EC (Art. 6.1) includes an operational
condition requiring that incineration plants achieve a level of incineration such that, in slag and
bottom ashes, the loss on ignition is f5 % or the TOC is f3 %. In modern well-operated MSWI
plants the TOC in bottom ashes can be below 1 wt %. Combustion trials have demonstrated that
an increase in heating value of the waste feed and resulting higher bed temperatures improve the
burnout of bottom ash [Vehlow, 2002 #38]
Typical concentrations of organic compounds in the various solid residues are compiled in
Table 3.32. Only data from modern facilities have been used in this table. TOC determination in
accordance with the standard EN 13137 also detects elementary carbon as TOC, which does not
cause any problems on landfills. The TOC of bottom ashes comprises mainly elementary
carbon, but to a certain extent, organic compounds are also found (coming e.g. from sifting of
plastics). These cover the spectrum from short-chain compounds up to low volatile species such
as PAH or PCDD/F. The I-TEQ levels detected in the bottom ashes of modern incineration
plants are in the same order of magnitude as those found in some urban and industrial soils.
Parameter
PCDD/F (I-TEQ)
PCB
PCBz
PCPh
PAH
Bottom ash
<0.001 – 0.01
<5 – 50
<2 – 20
<2 – 50
<5 – 10
Boiler ash
0.02 – 0.5
4 – 50
200 – 1000
20 – 500
10 – 300
Filter ash
0.2 – 10
10 – 250
100 – 4000
50 – 10000
50 – 2000
All values in ng/g
Table 3.32: Concentration ranges of organic compounds in bottom, boiler and filter ashes
[Vehlow, 2002 #38]
Table 3.33 below shows data for PCDD/F for 10 MSWI in Netherlands over 5 years
(2000 - 2004):
Residue
Bottom ash
Fly ash
Boiler ash
Wet FGC salts
Filter cake
Average value
in ng/kg I-TEQ
46
2946
42
636
17412
Max value
in ng/kg I-TEQ
46
16900*
86
5400
66000*
Number of
samples
1
34
3
16
30
Total amount in
2003/tonnes
1100000
82200
2900
25500
8300
* This is a relatively old installation with modern FGT-equipment that prevents dioxin emissions to air. The
residue is land filled on a hazardous waste landfill site.
Table 3.33: PCDD/F concentrations in various MSWI incineration residues in NL (data 2000 –
2004)
Waste Incineration
187
Chapter 3
Table 3.34 below gives survey data of an overview of the PCDD/F content in residues from
MSWI plants. The data excludes peak high and low results:
Residue
Bottom ash
Boiler ash
Fly ash (ESP)/filter dust
Range of values
1 - 68
<40 – 600
140 - 5720
units
ng TEQ/kg dry solid
ng TEQ/kg dry solid
ng TEQ/kg dry solid
Note: In this table the peak high and low values have been removed
Table 3.34: Range of PCDD/F concentrations in MSWI residues (excluding peak high and low
values)
The relative partitioning of elements into bottom ash depends mainly on the composition of the
MSW fed to the incinerator, the volatility of the elements it contains, the type of incinerator and
grate system applied and the operation of the combustion system. [4, IAWG, 1997]
The mass and volume reduction of waste incineration causes an enrichment of a number of
heavy metals in the bottom ashes compared to their concentration in the waste feed. Some heavy
metals, e.g. As, Cd, or Hg are, to a great extent, volatilised out of the fuel bed. It is evident that,
with the exception of the mainly lithophilic Cu, all selected heavy metals are highly enriched in
filter ashes.
Note: It is important to note that the risks associated with bottom ash are not indicated only by
the presence or absence of substances – their chemical and physical form, as well as the nature
of the environment where the material will be used is also important to prevent emissions from
the ashes to the environment. [64, TWGComments, 2003] The important thing is, therefore, not
the fact that the bottom ashes contain pollutants but to check possible emissions from the ashes
to the environment.
Almost all regulations for the disposal or utilisation of waste products are based on standardised
leaching tests. However, different tests are used in different countries. Harmonisation and
standardisation of the testing procedures is under development within CEN (European
Committee for Standardisation TC 292). Hence the testing is done under country specific
conditions and the interpretation of the results of various tests has to take this into account.
Table 3.35 gives the average values for Dutch MSW incinerator bottom ash after mechanical
treatment, as measured from 1993 to 1997. Data have been taken from regular quality controls
performed by the national organisation of waste managers (VVAV) at all MSW incinerators and
from the National Institute for Environmental Protection (RIVM).
188
Waste Incineration
Chapter 3
Compound
Sb
As
Ba
Cd
Cr
Co
Cu
Hg
Pb
Mo
Ni
Se
Sn
V
Zn
BrClCN- (free)
CN- (total)
F(SO4)2-
Leaching value
(mg/kg)
0.22
0.022
0.6
0.003
0.08
0.05
3
0.001
0.07
1.52
0.13
0.01
0.04
0.23
0.09
7.6
2615
0.01
0.048
14.1
5058
Table 3.35: Leaching properties of mechanically treated bottom ash, measured using NEN7343
Leaching of bottom ashes can very significantly depending on the type of waste. Recent values
from a wide population of MSW indicates an average leaching for Cu of 5.79 mg/kg in 2001
and 6.21 mg/kg in 2002. [74, TWGComments, 2004]
As compared to stony or inert materials, the following compounds may be considered critical
for MSW bottom ash: Cu, Zn, Sb, Mo, chloride, and sulphate. Treatment techniques aim to
reduce the leachability of these critical compounds.
Residues from Hazardous waste incineration plants:
Residues from hazardous waste incineration are not fundamentally different from those of
municipal waste incineration plants. However, the following differences can be observed:
•
•
•
in the case of ash and slag: the incineration of hazardous waste in drums is usually
performed at temperatures higher than those used for municipal waste incineration. This can
result in different metal partitioning
owing to variations in waste type and content, the specific amount of bottom ash can be
subject to variations much greater than those in municipal waste incineration plants. These
variations can be seen within the same plant according to the wastes fed, as well as between
different plants and technologies
in the case of filter dust/FGT residues, as the concentration of heavy metals is normally
higher in hazardous waste, the solid residues produced may also contain considerably higher
concentrations of heavy metals.
Table 3.36 below gives data from a European survey of merchant HWI operators concerning the
total production of various residues:
Waste Incineration
189
Chapter 3
Residue production (kg/t waste input)
Bottom ash
Boiler ash + fly ash +
solid flue-gas
cleaning residue
Filter cake from ETP
(Tonnes)
Minimum
Maximum
Average
83
246
140
Total annual amount
(recorded)
193372
32
177
74
79060
9
83
30
16896
Table 3.36: Quantities of the main waste streams produced by HWI (European survey data)
[EURITS, 2002 #41]
Typical leaching values for bottom ashes from hazardous waste incineration are given in Table
3.37. It must be noted that the German DIN-S4 leaching test was used, results are therefore
given in mg/l. For comparison with the data from Table 3.35, approximate values in mg/kg may
be obtained by multiplication by a factor of 10.
Compound
Cr (VI)
Cr (total)
As
Pb
Cu
Hg
Zn
Cd
Ni
ClF(SO4)2-
Minimum
mg/l)
<0.03
<0.001
<0.01
<0.01
<0.01
0.00
<0.01
<0.001
<0.01
2
0.8
5
Maximum
mg/l)
2.87
2.87
0.08
0.18
1.50
<0.01
0.3
0.001
0.02
450
13
300
Table 3.37: Typical leaching values of bottom ash from hazardous waste incineration plants,
measured using DIN-S4
[1, UBA, 2001]
Residues from sewage sludge incinerators:
The chemical structure of sewage sludge ash is influenced considerably by the weather, in
particular the amount of rain. In the case of rainy weather, larger amounts of clay and fine sand
enter the sewerage system, pass the grit chamber, are precipitated in the preliminary
sedimentation basin and reach the sludge incineration with the primary sludge. As a result, the
silicate content of the ash is increased considerably, and the contents of other components are
diluted in periods of rainy weather.
In addition, the type of catchment and treatments carried out have a great effect on the sludge
quality. Areas with a large number of heavy industrial connections may result in higher
concentrations of heavy metals (etc) fed to the incinerator, these substances may then
accumulate in bottom and fly ashes. Rural areas, with little industry, may give rise to a cleaner
sludge and hence a lower contamination of incinerator residues.
Another point of major influence is the nature of the treatment (and therefore of the reagents:
mineral, polymeric, etc.) that is applied in order to purify the waste water.
[74, TWGComments, 2004]
190
Waste Incineration
Chapter 3
Issues with other waste types:
Clinical wastes:
•
•
•
burnout needs to be thorough to ensure destruction of infective agents and to disguise
recognisable body parts
the partitioning of radioactive isotopes used in medicines that give rise to wastes may be to
the bottom ash or fly ashes - this may result in additional disposal/re-cycling considerations
hypodermic needles and other sharp materials in the bottom ash may give rise to additional
handling risks.
Solid residue quality from fluidised beds:
Because of the difference in the process, waste properties and the combustion temperatures, the
quality of ashes is very different to the ashes of grate incinerators. Generally, the lower (but
more even) operational temperatures, nature of the fuel and process in fluidised beds mean that:
A greater proportion of volatile heavy metals remain in the bottom ash:
•
•
•
consequently concentrations of heavy metals in the flue-gas residues are reduced. However,
sometimes there are problems with CrVI levels in the soluble part of the bottom ash
the degree of vitrification of the ash may be reduced
burnout may be improved.
When recovered fuel is produced for fluidised bed boilers, the ash content is usually 1 – 10 %,
and with construction and demolition waste it is normally 1 – 7 %. [33, Finland, 2002].
Household waste burnt in rotating fluidised bed has ash content up to 30 % and with RDF up to
15 %.
Majority of solid residue from fluidised bed incineration is fly ash, which, according to
conditions and applied fluidised bed technology, can form up to 90 % of the total ash residue.
The bottom ash is also mixed with fluidised bed material (e.g. sand, additives for
desulphurisation etc.). When waste or RDF is burnt in a rotating fluidised bed the ratio of
bottom ash to fly ash is about 50:50.
When waste originated from construction and demolition is used, a small increase can be found
in the heavy metal content of both ashes compared to wood combustion. When the recovered
fuel is made of household waste, there is a greater increase in heavy metals. The amount of the
increase depends on the type of household waste used. If all the household waste is combusted,
the increase is high. If source separation is used, and only combustible packaging material is
combusted, the increase of heavy metals is less. Recovered fuels made of industrial wastes can
be very variable and therefore result in a wide range of ash qualities.
Waste Incineration
191
Chapter 3
3.5 Energy consumption and production
Energy inputs to the incineration process may include:
•
•
•
waste (mainly)
support fuels (usually very few)
imported electricity (if any).
Production and exports may include:
•
•
heat (as steam or hot water)
electricity.
Pyrolysis and gasification processes may export some of the energetic value of the incoming
waste with the substances they export e.g. syngas, chars, oils, etc. In many cases these products
are either directly or subsequently burned as fuels to utilise their energy value, although they
may also be used for their chemical value as a raw material, after pretreatment if required.
There are a significant number of incineration plants in Europe that produce and export both
electricity and heat.
The combination of exports which is selected depends on a number of factors. Often, whether a
local demand exists for steam or heat is decisive for decisions concerning its supply. The
relative prices for the supply of the energy produced, and the duration of sales contracts are
generally seen as key factors in determining the outcome. This, in turn, has a decisive input on
technological decisions regarding the process design. Some of these factors are described in
Table 3.38 below:
Factor
•
•
High electricity price paid for
supply or reliable demand
Higher electricity price for
imported electricity than that
produced
Higher price paid for heat and
higher reliability of demand
Colder climate
Hotter climate
Base load energy supply
contract
Very low permitted air
emissions
Not permitted to discharge
treated waste water from wet
scrubbers
Vitrification of ash required
Higher incineration
temperature required
•
•
•
•
•
•
•
•
•
•
•
Influence
encourages investment to produce electricity
boiler claddings may be purchased to allow higher steam
pressures and greater electrical outputs
less heat will be available for supply
plant may import electricity to ensure own produced exports can
be maximised
encourages the use of own produced electricity for running the
incineration process
heat only plants may decide to divert some energy to supply own
electricity demands
investment in distribution networks becomes more viable
overall plant efficiency gains possible due to ability to supply
more of the recovered energy
can allow heat supply over more months of the year
less reliable heat demand for heating
may increase options to supply heat to drive chillers for air
conditioning, to feed seawater thermal desalination plants, etc.
increases reliability of sales contract and encourages investment
in techniques to utilise available energy (heat and electricity)
additional energy demand of flue-gas treatment equipment
•
reduction in available heat for export owing to need to supply
evaporation energy
•
higher plant energy demand results in increased selfconsumption and reduced outputs
possible need for additional fuels to obtain relevant temperature
•
Table 3.38: Some factors and their influence on energy recovery options
192
Waste Incineration
Chapter 3
3.5.1 Energy efficiency calculation for waste incineration installations
The energy efficiency of a waste incineration installation is often expressed in terms of a
percentage. When considering such data it is important to ensure that the calculations that
underpin these have been performed in a way that permits comparisons to be made. Failure to
do so may result in inappropriate conclusions being drawn.
Some steps that are required to avoid problems with such calculations are:
1.
Define the system/calculation boundary
If the incoming waste requires significant pretreatment (e.g. crushing, shredding, drying etc.)
this can result in very significant additional energy requirements.
2.
Account for all energy inputs
Some installations use additional fuels to maintain combustion temperatures. The energy
recovered at the installation will be partly derived from the waste, and partly derived from the
additional fuel.
3.
Account for re-circulating energy flows
In some cases electricity and/or heat that is recovered from the waste, is then used within the
installation. When this is carried out, the net result is a reduction of exported energy and an
equivalent reduction of imported energy.
4.
Decide whether to simply add energy outputs or use equivalence factors to account
for their relative value?
Simple addition of the electrical and heat outputs can create difficulties when considering the
relative efficiencies of installations that produce different quantities of these energy flows. The
use of equivalence factors can allow consideration of the relative value of these commodities i.e.
it can allow consideration of the value of the energy production that the recovered energy
displaces. The equivalence factors assigned will be dependent upon the energy mix that the
energy recovered at the incineration installation replaces.
Where equivalence factors have been used in this document, a note of the factor used is
included (see also Section 3.5.3 regarding equivalence factors).
An example an energy efficiency calculation is given in appendix 10.4. This method was
developed by members of a sub-group of the TWG, and was used to provide some of the
summary survey data reported in this chapter.
3.5.2 Waste net calorific value calculation
Information regarding the typical calorific value ranges exhibited by various waste types, NCV
survey and variation data and an example method for the calculation of net calorific value are
included in Section 2.4.2.
Waste Incineration
193
Chapter 3
3.5.3 Equivalence factors
[Energysubgroup, 2002 #29]
When comparing different incineration plants, a common unit of energy measure is needed.
Energy can be quantified in a number of ways, depending on the energy type of the resource.
Fuels are usually quantified either by their heat content (joules) or in fuel equivalence values
(usually, oil or hard coal equivalents).
The joule (J) is the common unit used in this document to convert the measuring units of
different forms of energy into a common unit. To calculate and express energy efficiencies at
WI plants it is necessary to take into account the form of the energy consumed and produced.
Taking account of the energy form, requires the comparison of different units of measurement
i.e. MWh, MWhe(electricity), MWhth(thermal energy). The following table gives conversion
factors (for externally generated sources) assuming an average of 38 % for electrical conversion
efficiency (i.e. 1MWh = 0.38 MWhe), and 91 % for external heat generation (i.e. 1MWh =
0.91MWth):
From:
To:
GJ
MWh
MWhe
MWhth
Gcal
Multiply by:
GJ
MWh
1
0.2778
3.6
1
9.4737
2.6316
3.9560
1.0989
4.1868
1.163
MWhe
0.1056
0.3800
1
0.4421
MWhth
0.2528
0.9100
1
1.0583
Table 3.39: Energy equivalence conversion factors
[29, Energysubgroup, 2002, 64, TWGComments, 2003]
It is important to understand that equivalence values are not exact coefficients or conversion
factors. They provide an estimate of the energy that is required to produce the energy externally.
3.5.4 Data on the recovery of energy from waste
[1, UBA, 2001]
The generation of electricity is limited by:
•
•
the high-temperature corrosion that may occur in the heat conversion area (boiler,
economiser etc.) due to the contents of certain materials, including chlorine, in the waste
fouling of the boiler - above approx. 600 to 800 °C the ashes are sticky due to the presence
of some smelting substances.
The steam parameters (and hence electrical efficiency) of incineration plants are therefore
limited. A steam pressure of 60 bar and a temperature of 520 °C can be considered the
maximum at present, and only then where special measures are taken to limit corrosion.
For electricity production from MSW typical superheated steam conditions are 40 to 45 bar and
380 to 400 °C. [74, TWGComments, 2004] Lower figures, generally less than 30 bar and
300 °C, are applied where electricity is generated from hazardous wastes owing to the increased
corrosion risks (leading to operational difficulties and costs) with acidic flue-gases at higher
steam parameters.
Where only heat or steam is supplied, operators tend to use lower boiler pressures and
temperatures to avoid the need for the additional investment and maintenance and the more
complex operation conditions associated with the higher parameters. In the case where heat
supply is prioritised, high pressure and temperature are not justified. Typically for heat supply,
the steam will be generated at lower values e.g. around 25 to 30 bar and 250 to 350 °C.
194
Waste Incineration
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The majority of larger waste incinerators in Europe recover energy from the waste. There are
some plants without heat utilisation, these concern generally relate to very specific designs or
older/smaller plants. For example:
•
•
hazardous waste incineration plants using flue-gas quenching in order to reduce risks of
PCDD/F reformation (e.g. UK and France). In these cases, some heat recovery may still be
made from the hot quench water that is produced by the quench scrubber
relatively small municipal waste incineration plants (particularly in France, but also some in
Italy and Belgium).
The following boiler efficiencies are reported to be achieved:
•
fluidised bed boilers with exhaust gas temperatures of about 160 °C can achieve boiler
efficiencies of about 90 %.
• grate firing furnaces have a boiler efficiency of about 80 %.
[74, TWGComments, 2004]
With such boiler efficiencies (80 – 90 %) and higher than normal steam parameters (note: actual
application depends greatly on waste type owing to increased corrosivity of flue-gases with
some waste types) the following approximate electrical efficiencies may result:
•
•
steam parameters of 60 bar and 420° about 25 % of the energy converted in the steam
generator can be recovered as electrical energy (i.e. overall electrical efficiency of 20 % in
the case of grate firing and 22.5 % in the case of FBR)
if the steam parameters are further increased to 80 bar and 500 °C an electrical efficiency of
30 % can be achieved (i.e. overall electrical efficiency of 27 % in the case of FBR).
[74, TWGComments, 2004]
If there is the possibility to connect the steam cycle of a waste incineration plant to the steam
cycle of an adjacent power plant, the overall electrical efficiency can be as high as 35 %. [74,
TWGComments, 2004]
3.5.4.1 Electricity recovery data
[1, UBA, 2001]
Although there are significant local variations, typically approx. 400 to 700 kWh of electricity
can be generated with one tonne of municipal waste in a municipal waste incineration plant.
This is dependent upon the size of the plant, steam parameters and degrees of steam utilisation
and mainly on the calorific value of the waste.
The amount of energy available for export usually depends upon the amount produced and the
degree of self consumption by the installation - which can itself vary significantly. The FGT
system consumption is often significant and varies with the type of system applied (and
emission levels required). In some cases, the energy required to run the installation is imported
from external supply, with all of that generated by the installation being exported – the local
balance usually reflects local pricing for the electricity generated compared to general grid
prices.
A survey of eight investigated MSW plants (2001 data) carried out by the TWG energy subgroup gave the following results:
Waste Incineration
195
Chapter 3
Electricity
Production
Export
1.
2.
3.
4.
5.
Units
MWhe/t waste
GJe/t waste
MWhe/t waste
GJe/t waste
Minimum
0.415 (12.9 %)
1.494
0.279 (8.7 %)
1.004
Average
0.546 (18 %)
1.966
0.396 (13 %)
1.426
Maximum
0.644 (22 %)
2.319
0.458 (18 %)
1.649
Figures are given as measured (i.e. not factored equivalents)
Percentage efficiencies are given in parenthesis (also not factored) and take account of energy derived from
imported fuels as well as from waste
Figures for production include all electricity generated
Figures for export exclude electricity produced by the process but consumed in the process
NCV average value was 2.9MWh/t
Table 3.40: Electricity production and export rates per tonne of MSW
Source [Energysubgroup, 2002 #29]
Other data supplied for French installations shows the following results:
Electricity
Units
MWhe/tonne waste
Production
GJe/tonne waste
MWhe/tonne waste
Export
GJe/tonne waste
Minimum
0.148 (4.6 %)
0.5328
For units>3t/h
Average
Maximum
0.368 (11.4 %) 0.572 (17.8 %)
1.389
1.897
0.285 (8.8 %)
1.026
New Units
Average
0.528 (16.4 %)
1.900
0.430
1.548
Table 3.41: Electricity production and export data per tonne of MSW for MSWI in France
[64, TWGComments, 2003]
3.5.4.2 Heat recovery data
A survey of fifteen investigated MSW plants (2001 data) carried out by the TWG energy subgroup gave the following results:
Heat
Units
MWhth/t waste
Production
GJth/t waste
MWhth/t waste
Export
GJth/t waste
1.
2.
3.
4.
Minimum
1.376 (45.9 %)
4.953
0.952 (29.9 %)
3.427
Average
1.992 (65.8 %)
7.172
1.786 (58.8 %)
6.600
Maximum
2.511 (74.3 %)
9.040
2.339 (72.7 %)
9.259
All figures are given as measured (i.e. not factored equivalents)
Percentage efficiencies are given in parenthesis (also not factored) and take account of energy
derived from imported fuels as well as from waste.
Figures for production include all heat produced by the boiler
Figures for export exclude heat produced by the process but consumed in the process
Table 3.42: Heat production and export rates per tonne of MSW
[Energysubgroup, 2002 #29]
Other data supplied by France show the following results:
Heat
Production
Export
Units
MWhth/t waste
GJth/t waste
MWhth/t waste
GJth/t waste
Minimum
0.292 (9 %)
1.051
For units >3t/h
Average
Maximum
0.978 (30.4 %) 1.595 (49.6 %)
3.502
5.742
0.902 (28 %)
3.247
Table 3.43: Heat production and export rates per tonnes of MSW for MSWI in France
[64, TWGComments, 2003]
196
Waste Incineration
Chapter 3
3.5.4.3 Combined heat and power data
[1, UBA, 2001]
In the case of combined electricity/heat generation, approx. 1250 kWh of additional heat per
tonne of waste can be used at full load.
If a base load supply situation exists, the gross degree of utilisation can be increased to 75 % to
76 % of the energy input (thermal value).
A survey of 50 investigated MSW plants (2001 data) carried out by the TWG energy sub-group
gave the following percentage efficiencies for CHP:
CHP
Production
Export
Note:
Average efficiency
59.4 %
49.3 %
To allow addition of heat and electricity to provide a single efficiency measure, a factor of
2.6316 is applied to electrical efficiencies. This factor takes account of the unavoidable
losses of electrical energy production and allows processes producing different balances of
heat and power to be compared (and hence averaged) with greater meaning.
Table 3.44: Average CHP percentage efficiency (calculated as energy equivalents) for 50 MSWI
plants
Source [Energysubgroup, 2002 #29]
Note: A statement about minimum and maximum efficiencies for combined heat and power
production (export) is not possible and therefore not included in Table 3.44. This is because the
summation of minimum heat and minimum electricity as well as of maximum values leads to
misleading results.
Other data provided by France are shown below. The figures show average values:
Units
Electricity production
Heat production
Electricity exported
Heat exported
MWhe/t waste
GJe/t waste
MWhe/t waste
GJe/t waste
MWhe/t waste
GJe/t waste
MWhe/t waste
GJe/t waste
For installations
>3t/h
0.168
0.604
0.647
2.329
0.107
0.385
0.546
1.965
New
installations
0.382
1.375
0.944
3.398
0.300
1.08
0.578
2.08
Table 3.45: Average CHP recovery values per tonne of MSW in MSWI in France
[64, TWGComments, 2003]
3.5.4.4 Boiler conversion efficiency data
A survey of 50 investigated MSW plants (2001 data) carried out by the TWG energy sub-group
gave the following data:
Boiler efficiency
1.
2.
3.
Minimum
75.2 %
Average
81.2 %
Maximum
84.2 %
The percentages show the efficiency of transfer of energy from the hot flue-gases to the boiler
steam
The NCV of the waste is calculated using the method given in Section 2.3.2.1
Boiler efficiency may be lower for small units [74, TWGComments, 2004]
Table 3.46: Survey data of MSWI boiler efficiencies
[64, TWGComments, 2003]
Waste Incineration
197
Chapter 3
3.5.5 Data on the consumption of energy by the process
[1, UBA, 2001]
The incineration process itself requires energy for its operation e.g. pumps and fans. The
demand varies greatly depending on the construction of the plant [1, UBA, 2001]. In particular
the process demand may be increased by:
•
mechanical pretreatment systems e.g. shredders and pumping devices or other waste
preparation
• incineration air preheating
• reheat of flue-gas (e.g. for gas treatment devices or plume suppression)
• operation of waste water evaporation plant or similar
• flue-gas treatment systems with high pressure drops (e.g. filtration systems) which require
higher powered forced draught fans
• decreases in the net heat value of the waste - as this can result in the need to add additional
fuels in order to maintain the required minimum combustion temperatures
• sludge treatment e.g. drying.
[64, TWGComments, 2003]
In some cases, these demands can be met partially or entirely through heat exchange with the
hot incineration gases.
Older plants with retrofitted flue-gas cleaning systems may consume more electricity compared
with modern plants with integrated systems. For industrial plants for hazardous waste
incineration, a range of 132 to 476 kWh/t of waste is seen [1, UBA, 2001].
Table 3.47 below shows the specific energy demand of 50 investigated MSW plants (2001
data), as carried out by the TWG energy sub-group. The table shows the electricity demand, the
heat demand and the total (as equivalents) demand for entire incineration plants, expressed per
tonne of treated waste:
Energy
demand type
Electricity
(absolute)
Heat
(absolute)
Total demand
(equivalents)
1
2
3
4
Units
MWhe/t waste
GJe/t waste
MWhth/t waste
GJth/t waste
MWheq/t waste
GJeq/t waste
Minimum
Average
Maximum
0.062
0.223
0.021
0.076
0.155
0.558
0.142
0.511
0.433
1.559
0.575
2.070
0.257
0.925
0.935
3.366
1.116
4.018
All figures are given as measured (i.e. not factored equivalents)
Percentage efficiencies are given in parenthesis (also not factored) and take account of energy
derived from imported fuels as well as from waste.
Figures for production include all heat produced by the boiler
Figures for export exclude heat produced by the process but consumed in the process
Table 3.47: Electricity, heat and total energy demand data for 50 surveyed European MSWI per
tonne of waste treated
[Energysubgroup, 2002 #29]
The energy consumption of the installation also varies according to the calorific value of the
waste. This is largely due to increased flue-gas volumes with higher NCV waste – requiring
larger FGT capacity. The relationship is shown in the graph below:
198
Waste Incineration
Chapter 3
Figure 3.20: Graph showing increase in installation electrical consumption with increasing waste
NCV
3.5.6 Data comparing energy required by, and output from, the
installation
A number of different methodologies may be used to compare installation consumption with
overall energy recovery rates. In this example, developed by the energy sub-group of the BREF
TWG the energy required to treat the waste is compare to that recovered from the waste. Other
indicators are also used that compare the ratio of output to input energy.
The plant efficiency potential (Plef) provides a figure that compares the energy exported from
the process and the energy that the process itself requires for its operation:
Plef = (Oexp-(Ef + E imp))/(Ef + E imp + E circ)
Where:
Ef
= annual energy input to the system by non-waste fuels that add to steam production (GJ/yr)
E imp = annual imported energy (Note: energy from the treated waste (Ew) is not included)
E circ = annual energy circulated (i.e. that generated by, but used in, the installation)
Oexp = annual exported energy (combined total of heat plus electricity as equivalents)
Note: Because different types of energy (electricity and heat) are added all figures calculated as
equivalents at the consumption.
The exported (e.g. sold) energy minus the imported energy is divided by the total energy
demand for the waste incineration process, including flue-gas cleaning, generation of heat and
electricity. Because the calculation does not take into account the energy content in the waste, it
only allows efficiency comparison of incinerators processing similar wastes.
Table 3.48 below shows the results of a survey by the TWG energy sub group:
Waste Incineration
199
Chapter 3
Process type
Number of plants
surveyed
Minimum
Average
Maximum
CHP
50
0.6
2.0
7.1
Pl ef (CHP)
Electricity only
8
0.6
1.2
1.6
Pl ef (electr.)
Heat only
15
1.0
2.8
7.1
Pl ef (heat)
Note:
Because the calculation does not take into account the energy content in the waste,
it only allows efficiency comparison of incinerators processing similar (CV) wastes.
Table 3.48: Ratio of exported and consumed energy for various waste incinerators
Source [Energysubgroup, 2002 #29]
Where the result is higher than 1 this shows that the plant is exporting more energy gained from
waste than that which is required to operate the waste incineration process.
Where the result is below one this shows that the plant is using more energy to operate the
waste incineration installation than it is recovering form the waste. Such a situation may be
envisaged at an installation treating very low calorific value wastes.
This calculation does not require knowledge of the energy content of the waste. However, the
result will be influenced by the waste energy content, and it can be expected that wastes with a
higher energy content can result in greater energy exports, and hence higher values of Pl ef.
200
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Chapter 3
3.6 Noise
Table 3.49 below described the sources and levels of noise, generated at waste incineration
installations, along with some of the reduction measures used:
Area relevant to noise/
main emitters
Delivery of waste i.e. noise
from lorries etc.
Shredding
Waste bunker
Boiler building
Machine building
Flue-gas cleaning:
ESP
Scrubbing
Suction draught
Chimney
Total flue-gas cleaning
system
Disposal of residues
Bottom ash discharge
Loading
Transportation from the
plant
Total waste
management residues
Air cooler
Energy transformation
facility
Total level LWA of the plant
Day
Night
Reduction measures
Noise level
LWA in dB(A)
Tipping hall closed to all sides
104 - 109
Scissors in tipping hall
Noise insulation of the building with gas
concrete, gates with tight design
Enclosure with multi-shell construction
or gas concrete, ventilation channels with
connecting link silencers, tight gates
Use of low-noise valves, noise-insulated
tubes, noise insulation of the building as
described above
95 - 99
Noise insulation, enclosure of the facility
e.g. with sheets with trapezoidal
corrugations, use of blimps for the
suction draught and silencer for the
chimney
82 - 85
82 - 85
82 - 84
84 – 85
79 - 81
78 - 91
82 - 85
89 - 95
Enclosure, loading in the bunker
71 - 72
73 – 78 (day)
92 - 96 (day)
92 – 96 (day)
71 – 72 (night)
Silencers on the suction and pressure
sides (see also BREF on cooling systems 90 – 97
for further information)
Low-noise design, within specially
71 - 80
constructed noise proofed building
105 - 110
93 - 99
Note: Day/night indicates that the operation is usually carried out during the day or night.
Table 3.49: Sources of noise at waste incineration plants
[1, UBA, 2001]
With the noise reduction measures described above, the noise emission limits, given for a
specific project based on the local conditions, can be safely met, by day and by night.
Noise is also generated during the construction phase. This may result in considerable noise
exposure in neighbouring residential areas, depending mainly on the location. Three main
construction stages are all equally relevant as noise sources:
•
•
•
digging the excavation
laying the foundations (including pile-driving) and
erecting the outer shell of the building.
Appropriate measures, such as restrictions on operating hours, particularly during the night, use
of low-noise construction machinery and temporary structural sound insulation measures, may
be taken. In some MSs, specific legislation also exists for this.
[1, UBA, 2001], [2, infomil, 2002], [64, TWGComments, 2003]
Waste Incineration
201
Chapter 3
3.7 Other operating resources
This section describes some of the substances consumed by the incineration process and gives
available data. Table 3.51 at the end of this section, provides data regarding the quantities of
various substances consumed by hazardous waste incinerators.
3.7.1 Water
The main consumption of water in waste incineration plants is for flue-gas cleaning. Dry
systems consume the least water and wet systems generally the most. Semi-wet systems fall in
between.
Typical effluent rates at a MSWI are around 250kg/t of waste treated (wet scrubbing, other FGT
technologies provide different figures).
It is possible for wet systems to reduce consumption greatly by re-circulating treated effluent as
a feed for scrubbing water. This can only be performed to a certain degree as salt can build up in
the re-circulated water.
The use of cooled condensing scrubbers provides a further means by which water can be
removed from the flue-gas stream, which then, after treatment, can be re-circulated to the
scrubbers. Salt build up remains an issue.
Processes without energy recovery boilers may have very much higher water consumption. This
is because the required flue-gas cooling is carried out using water injection. Consumption rates
of up to 3.5 tonnes water/tonne waste are seen in such cases (Belgium 2002). Installations with
a rapid quench system (such as those operated in the UK for HWI) may use up to 20 tonnes of
water per tonne of waste incinerated.
The water consumption for FGT in HWI is about 1 - 6 m3 per tonne of waste; and for sewage
sludge is about 15.5 m3 per tonne of waste.
[74, TWGComments, 2004]
202
Waste Incineration
Chapter 3
3.7.2 Other operating resources
[1, UBA, 2001]
The following consumption (and residual products) rates can be calculated for their
stoichiometric reaction during flue-gas cleaning:
Pollutant
kg
1
1
1
HCl
HF
SO2
Pollutant
HCl
HF
SO2
1
1
1
Pollutant
HCl
HF
SO2
1
1
1
Pollutant
NO
NO2
1
1
Pollutant
NO
NO2
1
1
Note:
1.
2.
Ca(OH)2
kg
1.014
1.850
1.156
NaOH
1.097
2.000
1.249
Sodium
Bicarbonate
2.301
4.200
2.625
Ammonia
0.370
0.739
Urea
0.652
1.304
Residual products
kg
CaCl2
1.521
CaF2
1.950
CaSO4
2.125
Residual product
NaCl
1.600
NaF
2.100
Na2SO4
2.217
Residual product
NaCl
1.603
NaF
2.100
Na2SO4
2.219
Residual product
Not applicable
Residual product
Not applicable
to establish accurate reagent ratios it is necessary to take into account the
initial emission level and the targeted emission level.
Reactants may be supplied at varying concentrations and this may therefore
alter overall mixed reagent consumption rates.
Table 3.50: Stoichiometric calculation of amounts of lime used for absorption during flue-gas
cleaning (reactants expressed at 100 % concentration and purity)
[1, UBA, 2001] [74, TWGComments, 2004]
3.7.2.1 Neutralisers
[1, UBA, 2001]
To neutralise the acids contained in the flue-gas, either NaOH, hydrated lime milk of lime or
sodium bicarbonate is used. Their consumption depends on the specific structure of the waste
(and hence the raw gas content) as well as the technical equipment used (contact, mixing etc).
For hydrated lime, 6 kg/t to 22 kg/t of waste are consumed depending on flue-gas cleaning type
and other factors. For NaOH, 7.5 - 33 kg/t of waste
[74, TWGComments, 2004]
3.7.2.2 NOX removal agents
Typical reagents for the removal of NOX from the flue-gas are ammonia, ammonia water (25 %
NH3) and urea solution. The latter, is, in particular, depending on the producer, often
supplemented by additional ingredients.
If upstream NOX concentrations are known this helps for a well controlled process. [74,
TWGComments, 2004]
The use of these materials must be performed in a targeted manner and well controlled to
prevent excessive formation of ammonia or the direct slippage of the excess ammonia.
For ammonia water, a consumption rate of 2.5 kg/t of waste is quoted. Research has shown a
range of 0.5 to 5 kg/t of waste.
Waste Incineration
203
Chapter 3
3.7.2.3 Fuel oil and natural gas
Light fuel oil (diesel), heavy fuel oil (about 0.03 - 0.06 m³ per tonne of waste) and natural gas
(in Austrian plants between 4.5 and 20 m³ per tonne of waste) are used for process heating and
support burners. [74, TWGComments, 2004]
Waste solvents (typically with a thermal value of >25 MJ/kg) are also used as support fuels in
some plants.
High calorific wastes (e.g. oils and solvents, typically those with a thermal value of >15 MJ/kg)
are routinely used as support fuel in rotary kiln hazardous waste incineration plants.
If the flue-gas is reheated for individual process steps (e.g. SCR) this is mainly done with
natural gas.
3.7.2.4 Merchant hazardous waste incinerator plant survey data
[EURITS, 2002 #41]
An overview is given below of the minimum and the maximum amount of additives in
kilograms per tonne of incinerated waste for surveyed merchant hazardous waste installations:
kg/t waste
Additives
CaO + Ca(OH)2 (100 %),
as CaO
NaOH (50 %)
CaCO3
HCl (33 %)
TMT-15 or other
sulphide treatment
Na2S
Na2S2O3
FeCl3
FeClSO4
Fe Al chloride
PE
Activated carbon
Urea (45 %)
NH4OH
CaCl2
Minimum
Maximum
Average
1.33
97
0.40
11.9
0.14
41.67
23.76
10
15.5
17.4
1.5
0.0085
0.98
0.23
0.008
0.08
0.049
0.15
1.75
0.01
0.3
3.1
0.50
2.36
0.83
4.2
0.50
0.96
1.75
1.30
19.31
3.1
3.33
2.36
0.44
1.7
0.27
0.55
1.75
0.3
3.7
3.1
2.1
2.36
28.6
Note: This table gives only some reference values and may not be representative for a
specific installation or technique.
Table 3.51: Amount of additives used by merchant hazardous waste incineration processes
Source [EURITS, 2002 #41]
204
Waste Incineration
Chapter 4
4 TECHNIQUES TO CONSIDER IN THE DETERMINATION OF
BAT
This chapter sets out techniques considered generally to have potential for achieving a high
level of environmental protection in the industries within the scope of the document.
Management systems, process-integrated techniques and end-of-pipe measures are included.
Prevention, control, design, management and re-cycling procedures are considered as well as
the re-use of materials and energy
Techniques may be presented singly or as combinations to achieve the objectives of IPPC.
Annex IV to the Directive lists a number of general considerations to be taken into account
when determining BAT and techniques within this chapter will address one or more of these
considerations. As far as possible a standard structure is used to outline each technique, to
enable comparison of techniques and an objective assessment against the definition of BAT
given in the Directive.
Because it is not possible to be exhaustive and because of the dynamic nature of industry, and
the momentary nature of this document, it is possible that there may be additional techniques
not described but which may also be considered BAT. These are likely to be techniques that
meet or exceed the BAT criteria established here and in Chapter 5, applied locally as thus
provide particular advantages in the situation in which they are used.
[64, TWGComments, 2003]
Organisation of Chapter 4:
This chapter groups the techniques in approximately the order in which they would appear in
the majority of waste incineration installations. Thus it highlights the specific techniques that
can be applied at each stage of the incineration process, and that can lead to improved
environmental performance or other benefits that are of relevance to determining BAT.
Table 4.1 gives the title of the sections and indicates the grouping to which the techniques have
been divided for BREF purposes.
Chapter 4 section number
(and hyperlink to section)
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Title of section
General practices applied before thermal treatment
Thermal processing
Energy recovery
Flue-gas treatment
Process water treatment and control
Treatment techniques for solid residues
Noise
Environmental management tools
Good practice for public awareness and communication
Table 4.1: Organisation chart for the information in Chapter 4
Description:
Each technique described includes relevant information, made available by the TWG, on the
consumption and emission levels considered achievable by using the technique, some idea of
the costs and the cross-media issues associated with the technique and information on the extent
to which the technique is applicable to the range of installations requiring IPPC permits, for
example new, existing, large or small installations, and to various waste types.
Waste Incineration
205
Chapter 4
As far as possible, a standard structure is used to outline each technique, as shown in the
following table, to enable comparisons of techniques and an objective assessment against the
definition of BAT given in the Directive. BAT determination itself is not covered here but is
covered in Chapter 5. Table 4.2 below shows the structure of the information that is included
where possible, for each technique in this chapter:
Type of information considered
Description
Achieved environmental benefits
Cross-media effects
Operational data
Applicability
Economics
Driving force for
implementation
Example plants
Reference literature
Type of information included
Technical description of the technique
Main environmental impact(s) to be addressed by the
technique (process or abatement), including emission values
achieved and efficiency performance (see also IPPC Directive
annexe IV). Environmental benefits of the technique in
comparison with others
Any side-effects and disadvantages caused by implementation
of the technique. Details on the environmental problems of the
technique in comparison with others
Performance data on emissions/wastes and consumption (raw
materials, additives, water and energy). Any other useful
information on how to operate, maintain and control the
technique, including safety aspects, operability constraints of
the technique, output quality, etc.
Consideration of the factors involved in applying and
retrofitting the technique (e.g. space availability, process
specific)
Information on costs (investment and operation) and any
possible savings (e.g. reduced raw material consumption,
waste charges) also as related to the capacity of the technique
Reasons for implementation of the technique (e.g. other
legislation, improvement in production quality)
Reference to a plant where the technique is reported to be used
Literature for more detailed information on the technique
Table 4.2: Information breakdown for each technique described in this Chapter 4
When possible, this chapter provides information on actual activities that are being, or can be
implemented by this sector, including actual associated costs. Where possible, the information
provided also gives the context in which the technique can be used effectively.
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4.1 General practices applied before the thermal treatment
stage
4.1.1 Suitability of process design for the waste(s) received
One of the most important decisions to be made by the waste incinerator operator relates to the
selection of a combustion (or thermal treatment) stage that is technically suited to the material
that will be fed to the process. Once that design has been selected, the operational objective then
becomes one of managing the incoming waste so that its properties remain within the range for
which the process is designed (see techniques described in 4.1.3).
In general, existing technologies have been developed in order to meet the specific waste
treatment requirements of particular waste streams. The application of a technology developed
for a different waste, of possibly unsuitable characteristics, can result in poor or unreliable
performance. Some installations are designed as “mass burn” (i.e. to treat wastes of varying
composition), others only to receive selected waste streams with narrow specifications. The
design required depends on the wastes that will be received for treatment in the incinerator.
Significant operational, safety and environmental consequences may result from attempting to
treat the wrong waste in the wrong design of installation.
In addition to the target performance (e.g. waste destruction, energy outputs, emission levels),
the choice of thermal treatment technique generally needs to take account of the following
technical criteria:
•
•
•
•
•
•
•
•
waste chemical composition and variation
waste physical composition, e.g. particle size and variation
waste thermal characteristics, e.g. calorific value, moisture levels
throughput and process availability required
required bottom ash, and other residue(s) quality and composition
possibilities for use of products of partial oxidation, such as syngas or coke
emission level targets and selected abatement system
type of energy recovery (e.g. heat, electrical power, CHP).
In addition to these technical criteria, the following may also influence the final design choice:
•
•
•
degree of technical risk
operational experience and available skill
budget.
Installations that are designed to treat a narrow range of specific wastes (or highly pretreated
and hence more homogeneous waste) operate within a narrower range of performance limits,
than those that receive wastes with greater variability. More homogenous waste can allow
improved process stability, with more even and predictable flue-gas composition. Where waste
quality can be well controlled, FGT system capacity may be narrowed to some degree without
increasing the risk of raw gas concentrations exceeding FGT capacity.
In practice, many waste incinerators may have only limited control over the precise content of
the wastes they receive. Operators receiving such wastes thus need to design their processes to
be sufficiently flexible to cope with the range of waste inputs that could be fed to the process.
[64, TWGComments, 2003]
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4.1.2 General housekeeping measures
General tidiness and cleanliness contribute to an enhanced working environment and can allow
potential operational problems to be identified in advance.
The main elements of good housekeeping are:
• the use of systems to identify and locate/store wastes received according to their risks
• the prevention of dust emissions from operating equipment
• effective waste water management, and
• effective preventive maintenance.
[64, TWGComments, 2003]
4.1.3 Quality control of incoming wastes
4.1.3.1 Establishing installation input limitations and identifying key risks
Description
Every installation has limitations on the characteristics of the wastes that can be fed to the
incinerator itself. From knowledge of the incineration process input limitations, it is possible to
derive a waste input specification that highlights the maximum and desirable system input rates.
It is then possible to identify the key risks, and procedural controls required to prevent or reduce
operation outside these limitations.
Factors that set such boundaries include:
•
•
•
•
design of waste feed mechanism and the physical suitability of waste received.
waste flowrate and heat throughput rating of the furnace
emission limit values required to be reached (i.e. % pollutant reduction required)
flue-gas cleaning technology capacity for individual pollutant removal (e.g. limit on fluegas flowrate, pollutant loading, etc.).
Examples of key risks identified can be:
•
•
•
•
•
•
•
•
high mercury input, leading to high raw flue-gas concentrations
high iodine or bromine input, leading to high raw flue-gas concentrations
high variability in moisture content or CV, leading to combustion irregularities
high chlorine loading exceeding FGT capacity
high sulphur loading exceeding FGT capacity
rapid change in flue-gas chemistry that effects FGT function
physically large items blocking feed systems - leading to an interruption of regular
operation
excessive slagging/fouling of boiler components when certain types of waste are being fed
e.g. high Zn concentration sources (contaminated wood waste) have been reported to cause
abnormal slagging in the first boiler pass.
Once the theoretical and actual (i.e. those occurring at operational plants) risks have been
established the operator can then develop a targeted control strategy to reduce these risks e.g. if
operator experience shows that the installation may experience exceedences of HCl emission
values then the operator may decide to attempt to control sources and peak concentrations of Cl
in the waste as fed to the combustion stage and/or design and operational features of the acid
gas FGT applied.
Achieved environmental benefits
The use of this technique helps ensure smooth and stable operation of the incinerator and
reduces requirement for reactive and emergency process intervention.
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Cross-media effects
The implementation of process input limitation procedures results in the removal of wastes
which fall outside the established specification. Those wastes are then diverted from the
incineration process to other waste treatment options. The type and magnitude of cross-media
effects that result are therefore dependent upon the type and performance of the alternative
treatment option.
Operational data
See description above.
Applicability
Applicable to all waste incineration plants, particularly those that receive wastes from diverse
sources and of a wide or difficult to control specification (e.g. merchant hazardous waste
plants).
Existing installations will have the advantage of experience and knowledge from previous
situations encountered during the operational lifetime of the installation. New plants may be
able to learn from the operational experience of similar existing plants and then adapt and
develop their own procedures according to their specific operational experiences.
Installations with extensive storage and pretreatment facilities may be able to accept wastes that
are initially outside the normal combustor specification and then treat the waste to meet the
combustor requirements.
While merchant HWIs are often built to be able to receive any kind of hazardous waste, this is
not the case for many other installations including MSWIs. However, some types of waste
which are similar in nature to MSW are treated in some MSWIs e.g. commercial waste, some
clinical wastes and sewage sludges. The installation may require some adaptation to be suitably
equipped to treat wastes that differ in nature from the main type received. This would generally
include the provision of adequate reception, storage and handling systems. If the waste is
significantly different then more extensive adaptations may also be required e.g. to the furnace
type, FGT, waste water treatment system, specific safety measures and laboratory/testing
equipment. [64, TWGComments, 2003]
Economics
Costs are not precisely quantifiable.
Excluding some waste sources/types may reduce income. In addition, specific investment may
be required to introduce techniques to identify and manage such wastes, e.g. analysis,
pretreatment.
Driving force for implementation
A good knowledge of process limitations is required in order to assess and select procedures to
control input and hence the overall process performance.
Example plants
Widely employed practice at hazardous waste incineration plants in particular.
The technique is also applied at many European MSWIs in order to identify and possibly
exclude undesired waste types.
Reference literature
[55, EIPPCBsitevisits, 2002] [64, TWGComments, 2003]
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4.1.3.2 Communication with waste suppliers to improve incoming waste quality
control
Description
Wastes are commonly received from a wide variety of sources over which the operator may
have only limited control. Where the operator has identified specific wastes, substances or
properties of wastes, or individual sources that can or do cause operational problems, the
communication of the operator’s concerns to those persons producing and supplying the waste
can help in the overall chain of waste management. An example is the separate collection of Hg
containing wastes such as batteries, or dental amalgam so that the Hg content of the MSW
stream is reduced.
The type of techniques used and the degree to which they are employed depends upon the
degree of risk and the frequency and nature of any operational difficulties encountered. In
general, the greater the variability of the waste types, compositions and sources, the more effort
is required in waste input control.
Achieved environmental benefits
Avoiding the receipt of unsuitable wastes or controlling the delivery of wastes that are difficult
to treat or that require special care can reduce operational difficulties and hence avoid additional
releases.
Cross-media effects
Some wastes may need to be diverted from the incinerator to other waste treatment options.
Operational data
Applicability
This technique can be applied to all waste incineration plants, but is of most use at those
receiving wastes from diverse sources and of a wide, or difficult to control, specification (e.g.
merchant hazardous waste plants).
Processes that are designed to receive a narrow range of well-defined wastes may need to take
particular care to ensure key substances are controlled.
Existing plants will have the advantage of learning from the real situations already encountered.
Economics
Savings may arise from avoiding operational difficulties.
Driving force for implementation
Procedures to control input can reduce the risks of operational upsets and associated releases.
Example plants
Widely employed practice at hazardous waste incineration plants, in particular.
SELCHP (South East London, UK) MSWI identified sources of gypsum (Calcium Sulphate)
which were disturbing the operation.
In Caen (France) a successful information campaign to reduce the Hg content in MSW was
carried out.
Reference literature
[64, TWGComments, 2003]
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4.1.3.3 Controlling waste feed quality on the incinerator site
Description
To help control the waste feed quality, and hence stabilise the combustion process within design
parameters, a set of quality requirements can be derived for the waste fed to the combustor. The
waste quality requirements can be derived from an understanding of the process operational
limitations, such as :
•
•
•
•
•
•
thermal throughput capacity of the incinerator
physical requirements of the feed (particle size)
controls used for the incineration process (e.g. using NCV, steam production, O2 content
etc.)
capacity of flue-gas treatment system and the derived maximum raw gas input
concentrations/rates
the emission limit values that need to be met
bottom ash quality requirements.
Wastes can be stored, mixed or blended (this is restricted by some national legislation) to ensure
that the final waste that is fed to the combustor falls within the derived set of quality
requirements.
The key substances/properties that will usually require particular procedures to be put in place
for their management relate to variations in the concentration and distribution in the waste of the
following:
•
•
•
•
•
•
•
mercury, alkali metals and heavy-metals
iodine and bromine
chlorine and sulphur
variations in heat values/moisture content
critical organic pollutants e.g. PCBs
physical consistency of waste e.g. sewage sludge
mixability of different kind of waste.
The results of CEN/TC 292 and CEN/TC 343 can be relevant for carrying out the sampling of
these substances in the waste.
Achieved environmental benefits
Reduced emissions to in the flue-gas through:
•
•
•
•
•
smooth process operation
effective combustion
improved energy recovery
more even raw gas concentrations and hence improved operation of flue-gas cleaning plant.
reduced fouling in boiler by reducing dust release.
Cross-media effects
The preparation and storage of wastes can give rise to fugitive emissions that themselves require
management.
Operational data
No information.
Applicability
All installations need to derive their own set of key process input limitations and then adopt
suitable receipt restrictions and possible pretreatment to ensure that these limitations are not
exceeded.
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A requirement to do so will be especially necessary where highly variable waste compositions
are encountered (e.g. merchant HWIs), and at smaller capacity plants as these have less
operational buffering capacity than larger plants.
[64, TWGComments, 2003] This technique finds its main application and benefits at hazardous
waste incinerators, although in some countries (e.g. Austria) it is performed at every waste
incineration plant.
Economics
Information not supplied.
Driving force for implementation
To help ensure that the feedstock material is suited to the processes used, and hence to allow
emissions and consumptions to be controlled within required parameters.
Example plants
Applied particularly at hazardous waste incineration plants in Europe.
Reference literature
[25, Kommunikemi, 2002] [64, TWGComments, 2003]
4.1.3.4 Checking, sampling and testing incoming wastes
Description
This technique involves the use of a suitable regime for the assessment of incoming waste. The
assessments carried out are selected to ensure:
•
•
•
that the wastes received are within the range suitable for the installation
whether the wastes need special handling/storage/treatment/removal for off-site transfer
whether the wastes are as described by the supplier (for contractual, operational or legal
reasons).
The techniques adopted vary from simple visual assessment to full chemical analysis. The
extent of the procedures adopted will depend upon:
•
•
•
•
•
•
•
nature and composition of waste
heterogeneity of the waste
known difficulties with wastes (of a certain type or from a certain source)
specific sensitivities of the installation concerned (e.g. certain substances known to cause
operational difficulties)
whether the waste is of a known or unknown origin
existence or absence of a quality controlled specification for the waste
whether the waste has been dealt with before and experiences with it.
Example procedures are provided under Operational data below.
Achieved environmental benefits
Advanced identification of unsuitable wastes, substances or properties can reduce operational
difficulties and hence to avoid additional releases.
Cross-media effects
No significant negative cross-media effects.
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Operational data
Waste type
Mixed
municipal
wastes
Pretreated
municipal
wastes and RDF
•
•
•
•
•
•
•
•
Hazardous
wastes
•
•
•
•
•
•
•
•
•
Sewage sludges
•
•
Clinical wastes
•
•
Animal byproducts
•
Example techniques applied
visual inspection in bunker
spot checking of individual deliveries
by separate off loading
weighing the waste as delivered
radioactive detection
visual inspection
periodic sampling and analysis for key
properties/substances
visual inspection
control and comparison of data in the
declaration list in comparison with
delivered waste
sampling/analysis of all bulk tankers
random checking of drummed loads
unpacking and checking of packaged
loads
assessment of combustion parameters
blending tests on liquid wastes prior to
storage
control of flashpoint for wastes in the
bunker
screening of waste input for elemental
composition e.g. by EDXRF
periodic sampling and analysis for key
properties and substances
checking for hard materials e.g.
stones/metal/wood/plastics prior to
pumping transportation, dewatering
and drying stages
process control to adapt to sludge
variation
control and comparison of data in the
declaration list in comparison with
delivered waste
screening for radioactivity
control and comparison of data in the
declaration list in comparison with
delivered waste
sampling/testing of low risk material
for fat, moisture content
Comments
Industrial and commercial loads
may have elevated risks and
require greater attention
Extensive and effective procedures
are particularly important for this
sector.
Plants receiving mono-streams
may be able to adopt more
simplified procedures
The suitability of the techniques is
dependent on the kind of sewage
sludge e.g. raw sludge, digested
sludge, oxidised sludge etc.
Infection risk makes sampling
inadvisable. Control is required by
waste producer
Sampling not advisable for high
risk material for safety reasons
Table 4.3: Some checking and sampling techniques applied to various waste types
[1, UBA, 2001, 2, infomil, 2002, 41, EURITS, 2002], [64, TWGComments, 2003]
Applicability
The most extensive sampling and analysis regimes are appropriate where waste compositions
and sources are most variable (e.g. merchant hazardous waste plants) or where there are known
difficulties e.g. history of problems with a particular waste type or source.
Economics
The cost of applying these techniques increases rapidly with the extent and complexity of the
procedures adopted.
The costs for the sampling, analysis, storage and additional processing time required, can
represent a significant proportion of operational costs at hazardous waste plants in particular,
where the most extensive sampling and analysis regimes are applied.
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Driving force for implementation
To enable better process control and for plant protection.
Example plants
Widely used throughout Europe.
Reference literature
[40, EURITS, 2003], and discussions during site visits. [64, TWGComments, 2003]
4.1.3.5 Detectors for radioactive materials
Description
Although radioactive materials are not specifically regulated by IPPC, the inclusion of
radioactive sources or substances in waste, can lead to operational and safety problems. Very
low “background” levels of radioactivity are present throughout the natural environment and are
also be found in wastes – such levels do not require specific measures for their detection and
control. However, some wastes are at risk of containing higher levels, particularly those arising
from activities that use radioactive materials. Some hospital and industrial wastes may therefore
routinely or occasionally contain specific radioactive sources or contamination, although the
inclusion of such wastes with municipal waste, and the difficulties of controlling mixed waste
collections, can lead to radioactivity in other wastes.
Radioactive materials can often be detected using specific detectors situated at, for example, the
entrance to the plant. Tests of waste loads that may have a higher risk of contamination can also
be carried out. Such tests are specifically carried out where loads are accepted on the basis of a
maximum level of contamination. Such maximum levels are derived from knowledge of the fate
of the isotopes treated, and of the particular process receiving them, and on consideration of the
limits set on the contamination levels allowed in releases to land, air and water.
Plastic scintillation detectors are one type of detector used; these measure photons from gamma
emitting radionuclides and to a lesser extent from beta emitters. Radionuclides are regularly
detected in clinical waste, laboratory waste and technically enhanced natural occurring radioactive material. Also important are the controls put in place to prevent the mixing of radioactive
waste with regular waste (sometimes done so as to avoid the high treatment cost associated with
radioactive waste).
Achieved environmental benefits
Prevention of plant contamination and release of radioactive substances. Contamination of the
installation can result in lengthy and costly shutdowns for decontamination.
Cross-media effects
The main concern is how to manage the waste that is identified as radioactive - as neither its
transport nor treatment may be permitted. Developing plans and procedures advance of such
situations for managing any radio-active wastes identified is advantageous.
Operational data
Some plants report good experiences of using gate controls for radioactive material, after they
recognised that the MSW they receive may occasionally contain radioactive materials. [64,
TWGComments, 2003]
Applicability
Applicable to incineration plants where heterogeneous wastes are received from a wide variety
of suppliers. Applied less when the sources and variability of the waste are well known and
controlled, or where the risk of receiving radioactive materials is judged to be low.
Economics
Investment cost for installing detectors is approx. EUR 25000 – 50000.
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Driving force for implementation
Reductions in the tolerable threshold for low level radioactive contamination encourage the use
of the technique. These thresholds may vary from one MS to another according to legislative
requirements. [64, TWGComments, 2003]
In some Member States, e.g. France, the regulation on MSWI enforces the implementation of
detector for radioactive materials (with a few exceptions).
Example plants
Applied at hazardous waste and some municipal waste installations.
Reference literature
[40, EURITS, 2003], and discussions during site visits. [64, TWGComments, 2003]
4.1.4 Waste storage
The basic principles of storage are outlined in the horizontal draft BREF on storage are
applicable to the storage of wastes and should be referred to for general guidance on techniques.
However, because wastes often have a less well defined or even unknown composition, it is
often the case that additional techniques are employed that further improve the security of the
storage in order to deal with these unknown risks. This section of the BREF therefore
concentrates on the specific techniques that are relevant to wastes, rather than the more general
aspects of storage.
4.1.4.1 Sealed surfaces, controlled drainage and weatherproofing
Description
The storage of wastes in areas that have sealed and resistant surfaces and controlled drainage
prevents the release of substances either directly from the waste or by leaching from the waste.
The techniques employed vary according to the type of waste, its composition and the
vulnerability or risk associated with the release of substances from the waste. In general, the
following storage techniques are appropriate:
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Waste type
•
•
•
General issues
applicable to all wastes
•
•
•
•
Solid municipal and
non hazardous
industrial wastes
Solid pretreated
MSW and RDF
Bulk liquid wastes and
sludges
Drummed liquid
wastes and sludges
Hazardous waste
Clinical/Biohazard
wastes
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Storage techniques
Odorous materials stored inside with controlled air systems using the
discharged air as combustion air (see 4.1.4.4)
designated areas for loading/offloading with controlled drainage
clearly marked (e.g. colour coded) areas for drainage from potential
areas of contamination (storage/loading/transportation)
limitation of storage times according to waste type and risks
adequate storage capacity
baling or containment of some wastes for temporary storage is
possible depending on the waste and location specific risk factors
fire protection measures, e.g. fire resisting wall between the bunker
and the furnace hall.
sealed floor bunkers or sealed level storage areas
covered and walled buildings
some bulk items with low pollution potential can be stored without
special measures
enclosed hoppers
sealed floor bunkers or level storage areas
covered and walled buildings
wrapped or containerised loads may be suitable for external storage
without special measures, depending on the nature of the waste
attack resistant bunded bulk tanks
flanges and valves within bunded areas
ducting of tank spaces to the incinerator for volatile substances
explosion control devices in ducts, etc.
storage under covered areas
bunded and resistant surfaces
segregated storage according to risk assessment
special attention to the length of storage times
automatic handling and loading devices
cleaning facilities for surfaces and containers
segregated storage
refrigerated or freezer storage for biohazard wastes
special attention to the reduction of storage times
automatic handling and loading devices
container disinfection facilities
freezer storage, if the storage period exceeds certain time periods e.g.
48 hours
Table 4.4: Some examples of applied storage techniques for various waste types
[64, TWGComments, 2003]
Achieved environmental benefits
Proper storing of wastes has many benefits:
•
•
•
•
•
•
•
•
•
216
reduction of risks of releases through secure containment
prevention of rainwater penetration of the stored waste (and thus reduction in LCV and
difficulty in combustion)
prevents wind scatter
reduces leachate production (and thus subsequent management requirements)
reduces mobilisation of pollutants
reduces deterioration of containers (corrosion and sunlight)
reduces temperature related expansion and contraction of sealed containers
reduces odour releases and allows their management
allows management of fugitive releases.
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Cross-media effects
Additional buildings and infrastructure required.
Operational data
No specific information supplied.
Applicability
The general principle of assessing the waste types received and providing appropriate (i.e. that
reduces the spread of contamination and the risks of storage and handling releases) secure
storage for them,, is applicable to all installations.
The degree and precise methods adopted depends upon the wastes received and are outlined
above. In general, liquid wastes and hazardous wastes require the most attention.
Economics
No specific data supplied.
Driving force for implementation
The application of safe storage is a fundamental technique for effective waste management and
for the prevention of releases.
Example plants
Widely applied throughout Europe. Examples seen in B, D, DK, F, FIN.
Reference literature
Discussions during site visits. [64, TWGComments, 2003]
4.1.4.2 Management of storage times
Reducing storage times can be useful for:
•
•
•
preventing the deterioration of containers (weathering, ageing, corrosion)
preventing the putrefaction of organic waste (which may otherwise lead to odour releases,
processing and handling difficulties, fire and explosion risks)
reducing the risk of labels becoming detached.
Storage times can be reduced by:
•
•
preventing the volumes of wastes stored from becoming too large
controlling and managing deliveries (where possible) by communication with waste
suppliers, etc.
In general, MSW is stored in enclosed buildings for a period of 4 to 10 days, with the storage
periods being strongly influenced by collection/delivery patterns. Because of the desire to run
installations on a continuous basis, the storage capacity and hence maximum storage times will
often be determined by the maximum amount of time when no waste is likely to be delivered to
the plant. Holiday periods in particular can result in several days when no waste is delivered.
[64, TWGComments, 2003]
A limited time for the maturation of municipal waste in the bunker may have a positive effect
on the homogeneity of the waste. Feeding fresh waste immediately after it has been delivered
may induce fluctuations in the process. [74, TWGComments, 2004]
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Where various sources and types of waste are received and added to the furnace to meet a
particular feed menu (e.g. hazardous waste installations), longer storage times for particular
substances may be beneficial, even of several months in some cases. This allows time for
difficult-to-treat wastes to be slowly fed into the system when sufficient compatible materials
are also available. Such practices are acceptable where those particular substances are stored in
a manner that the risk of substance and container deterioration is well managed.
4.1.4.3 Baling or other containment of solid waste
Description
During peak delivery times, if the rate of waste receipt is in excess of the plant throughput,
waste is wrapped in a plastic cover and stored. Waste received during maintenance or other
shutdown periods can also be stored. The technique can facilitate the longer term storage of
some wastes and effectively extend the storage capacity of the installation.
Stored wastes can be re-introduced into the main waste flow to the installation when the
delivery rate drops, or when the heat output demand is higher, or when energy (electricity or
heat) sale prices are higher.
The machinery and materials used for the baling are similar to those used in some areas for the
baling of animal feeds. Waste is compacted and wrapped with plastic film in big cylinders,
usually about 1 m high by 1 m diameter. The oxygen inside is quickly consumed and no more
becomes available as atmospheric air cannot enter a well packed bale, even if the film is torn.
The main advantage of baling and hence the longer-term storage of wastes is that variations in
the delivery rate of wastes can be accommodated, and the process can continue running at a
steady rate.
Achieved environmental benefits
There are 3 main benefits:
•
•
•
minimisation of the amount of waste to be sent elsewhere - during the shut-downs of the
plant or of one of its lines, the waste can be baled and burned later when the plant/line is
started again
Optimisation of plant design. The plant can be operated at a more consistent load conditions
all over the year.
Improvement of the valorisation of recovered energy – the stored waste can be burned when
there is greater demand/price for the supplied energy.
Cross-media effects
There is a need to adopt suitable measures to manage the following storage related risks:
•
•
•
•
•
odour
vermin
litter
fire risks
leachate arising from rainwater penetration of the baled wastes.
Operational data
City centre sites or other locations where there are adjacent sensitive receptors may mean that
the waste storage aspects (e.g. odours) may be more difficult or expensive to manage in an
adequate way.
The technique is less likely to be required where there are multiple incineration lines, as such an
arrangement can itself provide some level of flexibility of operation through the staggered
scheduling of maintenance operations, so that incineration capacity is continually available.
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Applicability
Applicable in circumstances where waste storage is carried out and it can be undertaken such
that it does not give rise to particular concerns regarding cross-media effects (see above). May
be applied to non-hazardous industrial solid wastes and either pretreated or mixed MSW,
although in practice it is not widely used.
Not suitable to high hazard wastes as the risks (direct or indirect) of longer term storage are
likely to outweigh the possible benefits.
Economics
Greater income possible from increased energy sales during high demand/high price periods.
The objective of the technique is to ensure that over the course of e.g. a year, costs are reduced
by the additional income that is provided by incinerating waste during periods that otherwise it
might not (no deliveries) or by ensuring that the waste is burned when there is a higher demand
(and hence higher price) fro the energy supplied. The technique therefore is likely to find most
economical benefit where (a) heat is sold, and (b) there is a variable spot market for energy.
Driving force for implementation
Varying energy prices can create a situation where it becomes desirable to bring on line
additional incineration capacity to meet this demand – the stored waste can then be used during
these periods.
Locations with seasonal populations (e.g. vacation areas) may produce very different amounts
of combustible waste depending on the season. The storage of waste allows flexibility, so that
the waste can be used when there is spare incineration capacity or additional energy demand.
Example plants
The technique is applied in various European MS e.g. Sweden, France.
Reference literature
[28, FEAD, 2002] [64, TWGComments, 2003]
4.1.4.4 Extraction of incineration air from storage areas for odour, dust and
fugitive release control
Description
The incinerator air supply (primary or secondary) can be taken from the waste (or chemical)
storage areas. By enclosing the waste storage areas and limiting the size of the entrances to the
waste storage areas, the whole waste storage area can be maintained under a slight negative
pressure.
This reduces the risk of odour releases and ensures fugitive releases are destroyed in the
incinerator rather than released.
It is also possible for raw material storage to be ventilated to either the combustion chamber or
to the flue-gas cleaning equipment, depending on the nature of the fumes extracted.
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The main techniques employed are:
Technique
Solid waste in enclosed buildings
from which incineration air is
drawn
Ducting tank vent to incineration
air feed
•
•
•
•
•
•
•
•
•
Application
municipal wastes
bulky solid and pasty hazardous wastes
RDF
sewage sludges
clinical wastes
other odorous wastes
odorous and volatile hazardous wastes e.g.
solvent wastes
odorous sludges e.g. sewage sludge
other odorous or volatile wastes
Table 4.5: Main techniques for reducing fugitive releases of odour, and GHG emissions.
[2, infomil, 2002] p 150, [1, UBA, 2001] p 36, [40, EURITS, 2003]
Achieved environmental benefitss
General reduction of fugitive releases, odour, GHG emissions, and sanitary risks.
Cross-media effects
Alternative air handling and treatment (e.g. for odour, VOC or other substances according to the
waste type) measures may be required when the incinerator is not running. Even for multiple
line processes where it is usual for at least one line to be running at any particular time,
provision of alternative air handling and treatment may be used since it is possible that all lines
of a waste incineration plant simultaneously have to stop their operation (e.g. in case of
accidents, maintenance of one line and simultaneous breakdown of the other line, end of
maintenance when waste is already delivered). [74, TWGComments, 2004]
Operational data
Typical air requirements for waste incineration processes are 3000 – 10000 m³/tonne of treated
waste, depending mainly on the LCV.
If air inlets (e.g. doorways, etc) to waste storage areas are smaller (in terms of their combined
total cross-sectional area), the inlet velocity of the air across these inlets will be higher and the
risk of fugitive releases via these routes consequently lower.
Care is required with extraction from hazardous waste (particularly flammable/volatile material)
storage areas in order to avoid explosion risks.
In the case of fire in the bunker, air channels must be automatically closed to prevent fire
jumping from bunker into the incineration building.
Applicability
All incinerators where there is a risk of odour or other substances being released from storage
areas.
Plants storing volatile solvents can very significantly reduce their VOC emissions using the
technique.
Where applied only for reasons of odour control, locations that are nearer to sensitive odour
receptors have a greater need for this technique.
Economics
Additional ducting costs for retrofits.
The provision of a back-up system for periods when the incinerator is not available entails the
additional cost of that system.
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Driving force for implementation
Control of fugitive releases, including odour.
A proximity to sensitive odour receptors will increase the need for this technique, including the
need for alternative measures where the incineration process is not available.
Example plants
Widely used at waste incineration plants throughout Europe.
In Germany up to 60 MSWI plants have long experience with this measure.
Reference literature
[2, infomil, 2002] p 150, [1, UBA, 2001] p 36, [40, EURITS, 2003] [64, TWGComments, 2003]
4.1.4.5 Segregation of waste types for safe processing
Description
Waste acceptance procedures and storage depend on the chemical and physical characteristics of
the waste. Appropriate waste assessment is an essential element in the selection of storage and
input operations.
This technique is strongly related to the checking, sampling and assessment of incoming wastes
outlined in Section 4.1.3.4.
The segregation techniques applied vary according to the type of wastes received at the plant,
the ability of the plant to treat those wastes, and the availability of specific alternative treatments
or incineration pretreatment. In some cases, particularly for certain reactive mixtures of
hazardous wastes, the segregation is required when the materials are packed at the production
site, so that they can be packaged, transported, offloaded, stored and handled safely. In these
cases, segregation at the incineration installation relates to maintaining the separation of these
materials so that hazardous mixtures are avoided. [64, TWGComments, 2003]
Waste type
•
Mixed municipal wastes
Pretreated municipal wastes and RDF
Hazardous wastes
Sewage sludges
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Clinical wastes
•
Segregation techniques
segregation is not routinely applied unless various
distinct waste streams are received when these can be
mixed in the bunker
bulky items requiring pretreatment can be segregated
emergency segregation areas for rejected waste
for fluidised beds, removal of metals may be
required to facilitate shredding and prevent blockage.
segregation not routinely applied
emergency segregation areas for rejected waste
extensive procedures required to separate chemically
incompatible materials (examples given as follows)
water from phosphides
water from isocyanates
water from alkaline metals
cyanide from acids
flammable materials from oxidising agents
maintain separation of pre-segregated packed
delivered wastes
wastes generally well mixed before delivery to plant
some industrial streams may be separately delivered
and require segregation for blending
moisture content and CV can vary greatly dependent
on source
segregate different containers to allow suitable
storage and controlled feeding
Table 4.6: Some segregation techniques applied for various waste types
[2, infomil, 2002] p 150, [1, UBA, 2001] p 36, [40, EURITS, 2003] [64, TWGComments, 2003]
Waste Incineration
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Achieved environmental benefits
Segregating incompatible wastes reduces risks of emissions by:
•
•
reducing accident risks (that may lead to environmental and/or health and safety relevant
releases)
allowing the balanced feeding of substances, thereby avoiding system overloads and
malfunctions and thus preventing plant shut down.
Cross-media effects
None identified.
Operational data
In France, legislation requires the storage of clean containers in a separate room from dirty ones.
Applicability
Not applicable where waste is already collected and delivered so that further segregation is not
required.
Economics
Information not supplied
Driving force for implementation
Controlling the hazards that may arise from the mixing of incompatible materials and protecting
the installation by ensuring that the waste fed to the incinerator falls within the range for which
the installation is designed.
Example plants
Information not supplied
Reference literature
[64, TWGComments, 2003]
4.1.4.6 Individual labelling of contained waste loads
The proper labelling of the wastes (e.g. in accordance with the European Waste Catalogue) that
are delivered in containers, assists their continued identification and trace-ability. Identification
of wastes, and their source, has the following benefits:
•
•
•
knowledge of waste content is required for choice of handling/processing operations
it increases the operators ability to trace sources of problems and then to take steps to
eliminate or control them
ability to demonstrate conformance with restrictions on waste types and quantities
received/processed. [64, TWGComments, 2003]
Bar code systems and scan readers can be used for packaged and liquid wastes. The costs of
such systems are low in relation to the benefits.
In general, waste delivery is accompanied by a suitable description of the waste; an appropriate
assessment of this description and the waste itself forms a basic part of waste quality control.
The existence of such a declaration is prescribed in European and other legislation.
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An indicative list of the most important parameters for labelling includes:
•
•
•
•
•
•
name and address of the deliverer
origin of the waste
volume
water and ash content
calorific value
concentration of chlorides, fluorides, sulphur and heavy metals.
An example of an adequate description of the waste was developed by the CEN/TC 343 on
"Solid Recovered Fuels".
Applicability
Mainly applicable to hazardous waste, clinical waste plants or other situations where wastes are
held in containers and have variable/distinct compositions.
Example plants
Labelling is widely applied, particularly at HWIs.
Reference literature
Site visit discussions and [64, TWGComments, 2003]
4.1.4.7 The use of fire detection and control systems
Description
Automatic fire detection systems have been used in waste storage areas as well as for fabric and
static bed coke filters, electrical and control rooms, and other identified risk areas.
Automatic fire control systems are applied in some cases, most commonly when storing
flammable liquid waste although also in other risk areas.
Foam and carbon dioxide control systems provide advantages in some circumstances e.g. for the
storage of flammable liquids. Foam nozzles are commonly used in MSW incineration plants in
the waste storage bunker. Water systems with monitors, water cannons with the option to use
water or foam, and dry powder systems are also used. Nitrogen blanketing may be used in fixed
coke filters, fabric filters, tank farms, or for the pretreatment and kiln loading facilities for
hazardous wastes. [74, TWGComments, 2004]
Continuous automatic measurement of temperature can be carried out on the surface of wastes
stored in the bunkers. Temperature variations can be used to trigger an acoustic alarm.
There are also other safety devices, such as:
•
•
•
nozzles above the waste feed hoppers,
fire resistant walls to separate transformers and retention devices under transformers
gas detection above gas distribution module.
When ammonia is used, its storage requires specific safety measures: NH3 detection and water
spray devices to absorb releases. [74, TWGComments, 2004]
Achieved environmental benefits
Reduced risk of accidental fugitive releases from fires and explosions.
Cross-media effects
Consumption of nitrogen for blanketing.
Containment is required to prevent the uncontrolled discharge of polluted fire fighting
water/chemicals.
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Operational data
Use of nitrogen blanketing requires effective operating procedures and containment to avoid
operator exposure. Asphyxiation can occur outside enclosed areas as well as inside.
Complementary visual control by operators can be an effective fire detection measure. [74,
TWGComments, 2004]
Applicability
The selection of suitable fire prevention, detection and control systems is applicable to all
installations.
Economics
Costs are for installation and maintenance. Nitrogen costs, where used.
Prevention of damage by fire can save significant cost. Installation of fire safety measures may
reduce insurance premiums.
Driving force for implementation
Safety is a significant driver.
According to a recent European regulation, equipments located in explosive atmosphere should
be explosion-proof (electrically + mechanically) [74, TWGComments, 2004]
Example plants
Many plants in Europe. [74, TWGComments, 2004]
Reference literature
[40, EURITS, 2003], [64, TWGComments, 2003]
4.1.5 Pretreatment of incoming waste
4.1.5.1 Pretreatment and mixing of wastes
Description
Techniques used for waste pretreatment and mixing are wide ranging, and may include:
•
•
•
•
•
mixing of liquid hazardous wastes to meet input requirements for the installation
shredding, crushing, and shearing of packaged wastes and bulky combustible wastes
mixing of wastes in a bunker using a grab or other machine (e.g. sprelling machines for
sewage sludge)
different grades of shredding of MSW – from
production of RDF – usually produced from source separated waste and/or other non
hazardous waste. [74, TWGComments, 2004]
Mixing of waste may serve the purpose of improving feeding and combustion behaviour.
Mixing of hazardous waste can involve risks. Mixing of different waste types may be carried
out according to a recipe [74, TWGComments, 2004]
Solid heterogeneous wastes (e.g. municipal and packaged hazardous wastes) can benefit from a
degree of mixing in the bunker prior to loading into feed mechanisms.
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In bunkers, the mixing involves the mixing of wastes using bunker cranes in the storage bunker
itself. Crane operators can identify potentially problematic loads (e.g. baled wastes, discrete
items that cannot be mixed or will cause loading/feeding problems) and ensure that these are:
removed, shredded or directly blended (as appropriate) with other wastes. The technique is
commonly applied at municipal plants and other incinerators where batch loads are delivered for
pre-incineration storage in a common bunker. Crane capacity must be designed so that it is
sufficient to allow mixing and loading at a suitable rate. Usually there are two cranes, each of
them sufficient to cope with the blending and feeding of all the incineration lines.
When special wastes are incinerated together with MSW, they may require specific pretreatment.
Clinical waste may be delivered in special packaging, sewage sludge, when not in a relatively
small proportion, may require preliminary partial or total drying, and usually specific feeding
system e.g. in the feed hopper, in the feed chute, directly in the furnace through a sidewall or
above the feeder. [74, TWGComments, 2004]
Achieved environmental benefits
The burnability of the waste is improved by making it more homogeneous, thus reducing and
stabilising emissions from the furnace, and leading to steadier steam/hot water generation in
boilers. Although greater homogeneity generally improves the “smoothness” of the operation,
the degree of treatment suitable for a given waste type depends upon the nature of the waste and
the receiving installation design (i.e. does or will the degree of heterogeneity of the waste lead
to particular problems of challenges in the installation, and will the use of additional
pretreatment provide sufficient benefit to outweigh the cross-media effects and costs?).
The resultant more even raw flue-gas compositions may allow closer optimisation of the fluegas cleaning process.
Cross-media effects
Energy consumption and emissions from the operation of the pretreatment equipment can range
widely depending on the nature of the waste, the technique used and the desired feed quality.
For example; the production of pelletised RDF from unsorted MSW can require high energy
inputs (and, hence, additional costs for the pretreatment), whereas simpler shredding and mixing
of selected waste streams can impose a relatively small burden.
Operational data
The safety of waste blending and crushing operations requires consideration when designing
such systems and procedures. This is particularly the case for flammable, toxic, odorous and
infectious waste packed in drums. Nitrogen blanketing and air locks for the pretreatment
equipment are effective in reducing risks.
Fires and explosions at mechanical sorting and blending plants are a significant risk. However,
blending of MSW in the bunker does not cause normally any particular risk. [74,
TWGComments, 2004]
The machinery required for the sorting and shredding of heterogeneous wastes is of heavy duty.
Effective management and maintenance is required to avoid breakdowns and loss of
availability. For thermal processes dealing with MSW which require more than blending, the
pretreatment (shredding, shearing, crushing, etc.) must be looked at carefully since it is often a weak
point. Special checks should be made on the shredder output because of risk of fire in the shredded
waste. [74, TWGComments, 2004]
Mixing of wastes with the objective of achieving compliance with the emission levels in permits
is forbidden in some cases (e.g. in Austria).
Applicability
All plants receiving heterogeneous solid wastes (e.g. untreated municipal and packaged
hazardous wastes) can apply the technique in principle.
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Significant and adequate pretreatment of municipal solid waste is a prerequisite for some thermal
processes. Shredding is not widely applied on MSWI, except for specific combustion designs
such as fluidised bed; sometimes double shredding (in series) may be necessary (e.g. pyrolysis
plant of Arras, France).
For MSW grate incinerators, the blending of the MSW in the bunker with the crane and grab is
considered essential and widely used. However, bulky objects may require removal or, if they are
to be incinerated, shredding. Commercial and industrial non-hazardous wastes may require size
reduction in order to homogenise the waste. [74, TWGComments, 2004]
The benefits of significant pretreatment are most likely to be realised at new plants that can
design the entire incineration installation for the post treated waste.
At existing plants, that have been specifically constructed to allow for wide feedstock
flexibility, and are already able to achieve low emission and otherwise good performance levels,
the benefits of simple pretreatment may still be seen. However, the adoption of pretreatment
techniques that effectively require wholesale changes to the waste collection and pretreatment
chain prior to the incineration installation are likely to involve very significant investment in
infrastructure and logistics. Such decisions are likely to be beyond the scope of a single
installation, and require overall consideration of the entire waste management chain in the
region from which wastes are received.
Economics
Costs vary greatly depending on the nature of the waste, the technique used and the desired feed
quality.
See also comments under applicability above.
Driving force for implementation
Improved homogeneity of the waste to be incinerated allows better process stability, improved
combustion conditions and better process optimisation. Emissions from the incineration
installation may, therefore, be reduced, or more closely controlled.
The link to the local waste strategy is important when determining to what extent pretreatment
needs to be carried out.
Example plants
All MSWI in Europe blend the MSW in the bunker. Numerous plants are equipped with shear,
shredder or crusher for bulky objects, e.g. Toulon
Sewage sludge drying prior to addition with municipal waste is carried out at a number of plants
in Europe, e.g. in Nice-Ariane and Bourg St Maurice. It is also carried out without, and fed in the
feed chute with MSW e.g. Thiverval, Thumaide, and separately fed into the furnace e.g. Monaco,
Bordeaux Bègles, Bordeaux Cenon.
[74, TWGComments, 2004]
Reference literature
[40, EURITS, 2003] and personal communications. [64, TWGComments, 2003]
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4.1.5.2 Shredding of mixed municipal wastes
Description
Untreated mixed municipal waste can be roughly shredded (i.e. not finely shredded) by passing
delivered waste through either:
•
•
•
•
•
crocodile shears
shredders
mills
rotor shears
crushers.
Achieved environmental benefits
The homogeneity of the waste is improved, resulting in more even combustion and reduced and
more stable emissions from the furnace. Having a more even raw gas composition may allow
closer optimisation of the flue-gas cleaning process. Blockages of the feeder systems to the
combustor and of bottom ash extraction and transporting systems may also be reduced, hence
resulting in reduced downtime and shutdowns.
Shredding of bulky waste added to a municipal waste incineration plant has been reported to
improve operation and burnout levels from 3 % TOC to closer to 1 % TOC.
Cross-media effects
The shredding equipment is mechanically substantial and results in:
•
•
•
•
•
increased energy consumption for shredder operation
potential noise - insulation of equipment is required
production of dusts and odour - controlled ducting of relevant air space to incinerator air
supply can be applied
additional explosion, fire and accident risks
shredder jamming may cause additional start-up/shut downs and significant periods of
unavailability.
Noise, odour and other releases from bulky waste shredding at MSWI plants may be reduced by
placing the shredder in the waste reception hall. In some cases the shredding machinery is
designed into the bunker itself so that the shredded waste falls directly into the bunker.
Operational data
Shredder systems are prone to jamming and physical damage if care is not taken to exclude
certain materials.
Operators loading the shredders require specific training to identify problem materials and
loads.
Where grate systems are used, the size of the post shredded material will need to be high enough
to avoid excessive riddling of the grate. There is, generally, not a minimum size requirement
with rotary kilns or fluidised beds. For fluidised beds oversized material tends to be the
difficulty, typically due to the blockage of bottom ash extraction or waste feeders, a maximum
size of about 50 mm is recommended by some manufacturers. For rotary kilns the size depends
on the opening from drum feeding.
Applicability
Applicable to all plants receiving heterogeneous solid waste e.g. untreated municipal and
packaged hazardous wastes.
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The potential environmental benefits of producing a more homogeneous fuel are mainly accrued
at the combustion and subsequent stages (e.g. FGT) and need to be weighed against the possible
disadvantages of the additional waste treatment (see cross-media effects above). Whether an
overall benefit is seen, is greatly dependent upon the nature of the waste received, and the
combustion technology employed. At existing installations additional pretreatment may not
provide any significant operational or environmental benefits. Grate incinerators are the least
likely to achieve major benefits from intensive shredding of mixed MSW, other than the rough
shredding of the waste, especially larger components of the waste.
Economics
Additional costs of shredding operation reported to be in the region of EUR 10 per tonne of
waste for coarse shredding [16, Energos, 2002]. A higher price of EUR 30 /t is also quoted. [64,
TWGComments, 2003]
Savings may be made through the optimisation of the operation of flue-gas treatment plants.
Such savings will most likely be possible at new plants, by allowing the selection of smaller
flue-gas treatment plants.
Driving force for implementation
Improved stability of combustion process.
Example plants
Several smaller scale (35000 tonnes/yr) municipal plants in Norway (e.g. Energos).
Reference literature
[8, Energos, 2002], [1, UBA, 2001], [64, TWGComments, 2003]
4.1.5.3 Shredding of drummed and packaged hazardous wastes
Description
The pretreatment of liquid packaged waste and packed or bulk solid waste to produce a mixture
for continuous feed to the furnace can be carried out. Suitable wastes may be treated to a pumpable state for pumped injection to the kiln or shredded for adding to the storage burner where
solids and liquids separate and are then fed to the kiln separately using grabs and pumping
respectively.
Pallets containing packaged liquid wastes of low to medium high viscosity are shredded to 5 to
10 cm. The shredded waste may then be screened before being transferred to tanks. Screened
out plastics are passed for incineration, ferrous metals are removed using magnets for washing
and recycling. In other cases the waste is not screened, and is pumped as a mixture of liquids
and shredded solids to the kiln with thinning liquids e.g. waste oils.
The liquid waste is pumped to a conditioning tank where it can be mixed with solvent waste
from bulk deliveries to meet viscosity requirements, before final pumping to the furnace.
Packed and bulk solid waste is shredded using a separate line and a heavy-duty cutter shredder.
If the power consumption of the shredder is high, this indicates that the consistency of the
mixture is becoming too solid for pumping and waste oil is added through a pipeline. If the
mixture becomes very thin (low viscosity), bulk solid waste can be added. Piston pumps are
used to transfer the mixture to the kiln.
All equipment is sealed under a nitrogen blanket to reduce fire and explosion risks. Air lock
doors are used to load the wastes.
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Achieved environmental benefits
Use of continuous feed:
•
•
•
•
•
improves the combustion performance and reduces peaks of CO and VOCs
increases average heat recovery due to stable gas flow in boilers
stabilises conditions for operation of flue-gas cleaning equipment
prevents explosions in the kiln
reduces downtime due to refractory etc. damage.
Metals removed before the combustion (see also Section 4.1.5.5) may be of superior quality to
those removed after combustion from bottom ashes. This is particularly likely to be the case for
those metals removed from higher temperature operations, because of the greater deterioration
in quality of post-combustion treated metals that results.
Reduction in consumption of furnace support fuel by 85 % has been achieved in one example
[25, Kommunikemi, 2002].
Cross-media effects
Energy consumption by shredding and pumping equipment.
Operational data
Disadvantages are the demand for better inspection and higher requirements for the quality of
the waste to prevent damage and downtime of the shredders. This downtime is compensated for
by reductions in maintenance requirements of the furnace due to reduced explosion risks.
Applicability
Applicable to incinerators receiving packaged hazardous wastes. The general principle of
increasing the homogeneity through suitable waste preparation can be applied to all incinerators
where significant variations in raw gas parameters are seen post combustion.
Economics
In one example the recycling of packaged steel from a 35 t/d plant produced an additional
income of EUR 35000/yr. The number of operators required for handling of packages was
reduced from 6 to 3.
Construction costs of two line were reported:
•
•
35 t/d packaged liquid line
= EUR 2.9 million (1990 prices)
75 t/d packaged and bulk solid line = EUR 5.4 million (1996 prices).
Driving force for implementation
Improved combustion performance leads to reduced emissions. The technique also reduces
manual handling of packaged wastes, damage to and maintenance of the kiln.
Example plants
Kommunikemi, DK; Ekokem, FIN
Reference literature
[25, Kommunikemi, 2002], [20, EKOKEM, 2002] [64, TWGComments, 2003]
4.1.5.4 Feed equalising control system for solid hazardous wastes
Description
The feed equaliser itself consists of two robust screw conveyors capable of crushing and feeding
solid waste and a tailor made feed hopper for receiving various types of waste. Safety measures
are designed according to plant requirements.
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Solid bulk waste is fed to the feed hopper with a crab crane through the horizontal feed gates.
The feed gates are normally closed to prevent gas leakage into the ambient air.
In the bottom of the feed hopper there are two hydraulically operated feed screws feeding the
waste continuously into the feed chute through fire doors. The fire doors prevent back draught
from starting fires in the feed hopper.
The feed hopper is equipped with a radioactive source level measurement for the upper and
lower fill limits of the hopper. At the upper limit this provides a signal to stop the feed into the
hopper.
The lower limit signal slows down the operation of the screws so that there will always be some
waste left at the buffer zone in the hopper to act as a barrier between the screw and the feed
hopper. The feed hopper works as a buffer zone preventing:
•
•
nitrogen from leaking into the kiln and
back-draught from causing fire in the feed hoppers.
If there is no need for barrel feeding, the feed equalising system can also feed the waste directly
through the front wall of the rotary kiln without a feed hopper.
Achieved environmental benefits
The feed equalising system provides a safe and reliable solution for the controlled continuous
feeding of solid hazardous waste, and reduces CO-peaks by ensuring uniform and stable
combustion conditions inside the rotary kiln and inside the secondary combustion chamber.
In general, the main environmental benefits are:
•
•
•
•
•
continuous feeding of solid hazardous waste improves the controllability of waste feeding
and reduces CO-peaks compared to batch feeding
optimal utilisation of the incineration capacity of the rotary kiln for low calorific solid
hazardous waste
homogenous stream of molten bottom ash is formed in the rotary kiln at high temperature
fire safety is improved in the hazardous bunker area by the use of automatic fire
extinguishing equipment
installation of video monitoring equipment enable continuous observation of waste feeding
into the rotary kiln.
Cross-media effects
Energy consumption by screw feeders.
Applicability
Applicable to hazardous waste incinerators receiving heterogeneous solid wastes.
Economics
Data not supplied.
Controlled continuous feeding of solid waste into the rotary kiln contributes to efficient use of
the maximum incineration capacity.
Driving force for implementation
See environmental benefits above.
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Example plants
Feed equalising system has been operating successfully at Ekokem in Riihimäki, Finland since
1989, at Sakab in Kumla, Sweden since 1993 and also at A.V.R.-Chemie in Rotterdam, the
Netherlands since 1996.
Reference literature
[20, EKOKEM, 2002]
4.1.5.5 Pre-combustion removal of recyclable metals
Description
Many wastes contain appreciable quantities of ferrous and non-ferrous metals. These can be an
inherent part of the waste itself (e.g. food and drink containers in MSW) or arise from the
packaging of waste in drums (e.g. hazardous wastes) or other metal containers.
Where the incoming wastes are shredded metals can be removed before incineration to allow
recycling.
Metal separation can be achieved by using:
•
•
•
over-band magnets for large ferrous materials e.g. shredded drums
drum magnets for small and heavy ferrous items such as batteries, nails, coins, etc.
eddy current separators for non-ferrous metals – mainly copper and aluminium used for
packaging and electrical components.
It may be necessary to wash the removed metals in order to remove contamination from the
wastes they have been in contact with. Whether this is necessary, depends on the type of
contamination, subsequent storage, transport and recycling process requirements.
Metal separation with reduced oxidation of the metals can also be achieved in fluidised bed
gasification plants treating shredded mixed MSW. Here the gasification temperature of 500 –
600 °C and the action of the fluidised bed can together, allow largely un-oxidised metals to be
removed from the fluidisation material (e.g. sand) using the same separation technologies
described above. The cleaned bed material is re-circulated to the fluidisation chamber.
Achieved environmental benefits
The main achieved environmental benefits are:
•
•
•
•
recovery of recyclable metal streams
improved value of metals that have not been partially oxidised at high temperatures in the
incinerator
reduction of content of volatile metals in the flue-gas leading to reduced contamination of
flue-gas cleaning residues
improved bottom ash quality by reduction of metal content (non-volatile fraction).
Cross-media effects
Energy required for shredders and operation of separation devices.
Possible consumptions and effluents may arise from washing stages (if used). It may be possible
for the contaminated washing effluent to be fed to the incineration process.
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Operational data
Operational data regarding shredders is given where available in Sections 4.1.5.3 and 4.1.5.2.
Metal removal may be an essential requirement for certain thermal processes. This process may
help preventing risk of fouling of the bed and blockage of the solid discharge due to metal
fusion.
In some case. for recovery it may be better to separate the metal after thermal treatment, as the
metal with low fusion point are removed. [74, TWGComments, 2004]
Applicability
With MSW in particular, the effective separate collection of these items may mean that greatly
reduced quantities of the recoverable metals remain in the waste – making steps to remove these
metal at the incineration plant less or not worthwhile.
Economics
There are investment and operational costs associated with the use of the shredding and
separation equipment.
With FB combustors shredding may be an essential part of the installation for many waste types
(e.g. MSW).
Local market prices determine the income from the recovered metals.
Driving force for implementation
Demand and higher prices for increased quality metal produced improve the economics of such
systems. Where outlets already exist for the recovery of post-combustion metals there is a
reduced incentive to adopt pre-combustion removal.
Example plants
Hazardous waste: shredding and removal of ferrous drums - Kommunikemi, Denmark
Municipal SW: shredding and removal of Fe and non-Fe scrap - plants in Austria
Municipal SW: example of fluidised bed with pre-shredding, metal ejection and separation –
Asahi Clean Centre, Kawaguchi City, Tokyo, Japan
Reference literature
[64, TWGComments, 2003]
4.1.5.6 Pretreatment and targeted preparation of solid waste for combustion
Description
The waste is accepted in different fractions and prepared specifically for incineration. The
appropriate crushing and removal of valuable materials (primarily metal) and the merging of the
individual fractions using conveyors allows the generation of a standardised, homogeneous fuel.
Achieved Environmental Benefits
Improved combustion through homogenisation of the waste. Reduced pollutant loads, reduced
heat value fluctuations and reduced emissions and consumptions from smoother operation.
The intensive mixing of waste before it enters the bunker can improve fuel qualities.
Cross-media effects
Odour, noise and dust emissions from the pretreatment and storage stages. Additional energy
consumption associated with the equipment used.
232
Waste Incineration
Chapter 4
Operational data
Improved process operation with the potential for longer component life, particularly for the
incineration stage. More even energy generation.
Applicability
Mainly applicable to wastes that can be delivered in various fractions or efficiently treated to
separate the fractions required.
The technique may be particularly applicable to installation designs that have narrow input
specifications e.g. fluidised beds. The benefits of applying the technique may be more limited
where an installation is already designed for “mass burn” e.g. grates and rotary kilns.
Economics
Cost of separating mixed wastes may be significant. Costs will be reduced where efficient predelivery segregation schemes, perhaps coupled with some simple pretreatment, and already in
place, allowing only storage and mixing to be carried out at the incineration installation.
Driving force for implementation
Availability of pre-selected waste streams e.g. from prior segregation of the waste before
delivery to the installation, which then do not need to be separated and may be stored separately.
Example plants
RMVA Cologne, Germany
Reference literature
[64, TWGComments, 2003]
4.1.6 Waste transfer and loading
4.1.6.1 Positioning and view of operator
The operators of waste feed systems need to have a good view of waste storage and loading
areas and their mechanisms to monitor them. This can be achieved by positioning the control
room with a view of the combustor loading areas and by the use of video monitors or other
detection systems. The former is preferable unless there are particular safety or other technical
reasons why this cannot be achieved.
[64, TWGComments, 2003]
4.1.6.2 Provision of storage space for items removed from the waste
Some waste streams commonly require the removal of certain components of the waste, usually
because they are unsuitable for processing in the facility. Suitable storage needs to be provided
for these items. See also Section 4.1.3
4.1.6.3 Direct injection of liquid and gaseous hazardous wastes in rotary kilns
Description
Liquid, pasty and gaseous wastes can be fed directly to rotary kilns via several direct feeding
lines. In 2002, almost 8.5 % of the total waste incineration in rotary kilns consisted of liquid
waste processed through direct injection lines. Each rotary kiln has several direct feeding lines.
In general, the direct injection operation is done by connecting the waste container and the
feeding line and pressurising the container with nitrogen or in case of sufficiently low viscosity
by emptying the container with appropriate pumps. In this way, the liquid waste is fed into the
processing line. Depending on the calorific value of the liquid waste, it is injected either at the
front of the rotary kiln or into the post combustion chamber.
Waste Incineration
233
Chapter 4
Depending on which direct injection line is used, after processing the line can be purged with
nitrogen, fuel, waste oil or steam.
Multi-purpose and dedicated injection lines are used, largely depending on the substances to be
incinerated.
Achieved environmental benefits
Prevention of diffuse air emissions due to the fact that the waste is fed by a complete closed
system.
Cross-media effects
Use of nitrogen and steam.
Operational data
The direct injection lines allow the incineration of liquid wastes that have properties that
exclude other processing possibilities.
Appropriate materials/linings are required for feeder lines, with heating required in some cases.
Feed rate capacity ranges depend upon incineration process factors (e.g. thermal capacity and
FGT capacity) but can range from 50 – 1500 kg/hr.
Injection can be via dedicated lance or multi-fuel burner.
Applicability
Applicable to liquid hazardous wastes, particularly those that present health and safety handling
risks that require minimal worker exposure.
Economics
An average investment price for a dedicated line amounts to EUR 100000 - 200000.
Driving force for implementation
The need to feed toxic, odorous, reactive and corrosive liquids and gases safely.
Example plants
Indaver, Antwerp plant (Belgium)
HIM, Biebesheim plant (Germany) and GSB, Ebenhausen plant (Germany).
Reference literature
[64, TWGComments, 2003]
4.1.6.4 Reduction of air ingress into the combustion chamber during loading
The use of systems that prevent air ingress to the combustion chamber helps to maintain process
stability and reduce emissions.
Such systems include:
•
•
•
•
234
maintaining a filled hopper for solid wastes
use of enclosed screw feeders
use of interlocked double doors for batch loading
use of pumped direct injection for liquid and pasty wastes.
Waste Incineration
Chapter 4
4.2 Thermal processing
4.2.1 Combustion technology selection
Description
A combustion (or thermal treatment) stage, that is technically suited to the material that will be
fed to the process, is required. The application of a technology developed for a different waste
of unsuitable characteristics can often result in poor or unreliable performance. See comments in
Section 4.1.1 regarding the need for the selection of a process suited to the waste to be received.
Tables 4.7, 4.8 and 4.9 below together provide a comparison of the main applied combustion
and thermal treatment technologies and factors affecting their applicability and operational
suitability. It is important to note that, whilst applied in the sector, the degree of demonstration
of the technologies listed varies, as does the nature of the waste to which they have been applied
successfully:
Waste Incineration
235
Chapter 4
Technique
•
•
Moving grate •
- air-cooled
•
Key waste characteristics
and suitability
Throughput
per line
low to medium heat values
(LCV 5 – 16.5 GJ/t)
municipal and other
heterogeneous solid wastes
can accept a proportion of
sewage sludge and/or
medical waste with
municipal waste
applied at most modern
MSW installations
1 to 50 t/h
with most
projects 5 to 30
t/h. Most
industrial
applications not
below 2.5 or 3
t/h.
Operational/Environmental information
Disadvantages/
Advantages
limitations of use
•
•
•
•
very widely proven at
large scales
robust - low
generally not suited to
•
maintenance cost
powders, liquids or materials
long operational history
that melt through the grate
can take heterogeneous
wastes without special
preparation
1 to 50 t/h
with most
As air-cooled grates but:
projects 5 to 30
Moving grate
• higher heat value waste
Same as air-cooled grates except: t/h. Most
treatable
- liquid
industrial
• LCV 10 – 20 GJ/t
• better combustion
cooled
applications not
control possible
below 2.5 or 3
t/h
Same as other grates except:
• can accept very
Grate plus
heterogeneous waste and
• improved burnout of
1 to 10 t/h
still achieve effective
bottom ash possible
rotary kiln
burnout
• not widely used
Static grate
with
ash/waste
transport
mechanism
•
•
As air-cooled grates but:
• risk of grate damaging
leaks
• higher complexity
•
•
•
municipal wastes require
selection or some shredding Generally low
less problems with powders <1 t/h
etc. than moving grates
•
Bottom ash
quality
lower maintenance - no
•
moving parts
•
throughput lower than
grate only
maintenance of rotary
kiln
only for
selected/pretreated
wastes
lower throughput
some static grates
require support fuel
Flue-gas volume
Cost information
4000 to 7000 Nm³/t
High capacity
TOC
waste input. Depends
reduces specific cost
0.5 % to
upon the LCV.
per tonne of waste
3%
3
Typically 5200 Nm /t.
•
4000 to 7000 Nm³/t
Slightly higher
TOC
waste input. Depends
capital cost than air0.5 % to
upon the LCV.
cooled
3%
3
Typically 5200 Nm /t.
•
4000 to 7000 Nm³/t
TOC
waste input. Depends Higher capital and
0.5 % to
upon the LCV.
revenue costs
3%
Typically 5200 Nm3/t.
•
<3 %
with
prepared
waste
Slightly lower than
other grate systems
where staged
combustion used
(higher if support fuel
used)
Competitive with
moving grates at
small scales (<100
Kt/y).
Table 4.7: A comparison of combustion and thermal treatment technologies and factors affecting their applicability and operational suitability (table 1/3)
[24, CEFIC, 2002] [2, infomil, 2002] [10, Juniper, 1997] [8, Energos, 2002] [1, UBA, 2001] [64, TWGComments, 2003]
236
Waste Incineration
Chapter 4
Technique
Key waste characteristics
and suitability
•
Rotary Kiln
•
•
can accept liquids and
pastes
solid feeds more limited
<10 t/h
than grate (owing to
refractory damage)
often applied to hazardous
wastes
Rotary kiln As rotary but:
• higher CV wastes possible
<10 t/h
(cooled
due to greater temperature
jacket)
tolerance
Fluid bed bubbling
Fluid bed Rotating
•
•
•
•
•
•
Fluid bed circulating
•
only finely divided
consistent wastes. Limited
use for raw MSW
often applied to sludges
wide range of heat values
(7 - 18 MJ/kg)
coarsely shredded MSW
may be treated
combined incineration of
sludge
only finely divided
consistent wastes. Limited
use for raw MSW
often applied to
sludges/RDF
Operational/Environmental information
Throughput
range (per
line)
1 to 10 t/h
3 to 22 t/h
1 to 20 t/h most
used above 10
t/h
Advantages
Bottom ash
quality
Disadvantages
•
•
•
very well proven
broad range of wastes Throughputs lower than
good burn out - even of grates
HW
•
•
very well proven
can use higher
combustion
temperatures (if
required)
better refractory life
than un-cooled kiln
Throughput lower than
grates
•
•
good mixing
fly ashes of good
leaching quality
- careful operation
required to avoid clogging •
bed
- Higher fly ash quantities.
•
good mixing/high
turbulence
wide range of LCV
high burnout, dry
bottom ash
•
good mixing
greater fuel flexibility
than BFB
fly ashes of good
leaching quality
•
•
•
•
•
•
•
•
•
shredding of MSW
required
higher fly ash
quantity than grates
•
•
•
•
cyclone required to
conserve bed material •
Higher Flying ashes
quantities
TOC <3 %
Flue-gas
volume
Cost
information
Higher specific
6- 10000 m³/ t cost due to
reduced
waste input
capacity
Higher specific
low leaching 6- 10000 m³/ t cost due to
reduced
vitrified slag waste input
capacity
TOC <3 %
TOC <3 %
often
0.5 - 1 %
TOC <3 %
Relatively
lower than
grates
FGT cost may
be lower.
Costs of waste
preparation
4000 to
6000Nm³/t
Relatively
lower than
grates
FGT cost may
be lower.
Costs of
preparation.
Table 4.8: A comparison of combustion and thermal treatment technologies and factors affecting their applicability and operational suitability (table 2/3)
[24, CEFIC, 2002] [2, infomil, 2002] [10, Juniper, 1997] [8, Energos, 2002] [1, UBA, 2001] [64, TWGComments, 2003]
Waste Incineration
237
Chapter 4
Technique
Key waste characteristics
Throughput
and suitability
range (per line)
Operational/Environmental information
•
Oscillating • MSW
furnace • heterogeneous wastes
Pulsed
hearth
Stepped
and static
hearths
•
•
•
•
Spreader - •
stoker
•
combustor •
•
•
Gasification
- fixed bed •
•
•
Gasification
•
- entrained
flow
•
238
only higher CV waste
(LCV >20 GJ/t)
mainly used for clinical
wastes
only higher CV waste
(LCV >20 GJ/t)
mainly used for clinical
wastes
RDF and other particle
feeds
poultry manure
wood wastes
mixed plastic wastes
other similar consistent
streams
gasification less widely
used/proven than
incineration
mixed plastic wastes
other similar consistent
streams
not suited to untreated
MSW
gasification less widely
used/proven than
incineration
1 – 10 t/h
<7 t/h
Information not
supplied
Information not
supplied
•
•
•
to 10 t/h
Bottom ash
quality
Disadvantages
•
•
higher thermal loss
than with grate
furnace
LCV under 15 G/t
Flue-gas
volume
Cost
information
•
TOC 0.5 – 3 %
Information
not supplied
Similar to other
technologies
•
can deal with liquids
and powders
•
bed agitation may be
lower
•
dependent on
waste type
Information
not supplied
Higher specific
cost due to
reduced capacity
•
can deal with liquids
and powders
•
bed agitation may be
lower
•
dependent on
waste type
Information
not supplied
Higher specific
cost due to
reduced capacity
•
simple grate
construction
less sensitive to particle
size than FB
low leaching residue
good burnout if oxygen
blown
syngas available
reduced oxidation of
recyclable metals
•
only for well defined
mono-streams
•
Information
not supplied
Information Information not
not supplied supplied
•
•
•
•
•
limited waste feed
not full combustion
high skill level
tar in raw gas
less widely proven
•
low leaching
bottom ash
good burnout
with oxygen
Lower than
straight
combustion
High operation/
maintenance costs
•
•
•
•
limited waste feed
not full combustion
high skill level
less widely proven
low leaching
slag
Lower than
straight
combustion
High operation/
maintenance costs
pretreatment costs
high
•
•
•
to 20 t/h
Advantages
robust - low
maintenance
long history
low NOX level
low LOI of bottom ash
•
•
•
•
low leaching slag
reduced oxidation of
recyclable metals
•
•
Waste Incineration
Chapter 4
Technique
Key waste characteristics
Throughput
Operational/Environmental information
Disadvantages
and suitability
range (per line) Advantages
•
•
•
•
Gasification •
- fluid bed •
•
Pyrolysis short drum •
•
•
Pyrolysis - •
medium
drum
mixed plastic wastes
shredded MSW
shredder residues
sludges
metal rich wastes
other similar consistent
streams
gasification less widely
used/proven than
incineration
•
•
5 – 20 t/h
~ 5 t/h
pretreated MSW
high metal inert streams
shredder residues/plastics
pyrolysis is less widely
used/proven than
5 – 10 t/h
incineration
•
•
•
•
•
•
can use low reactor
temperatures e.g. for Al
recovery
separation of main noncombustibles
can be efficiently
combined with ash
melting
reduced oxidation of
recyclable metals
no oxidation of metals
no combustion energy
for metals/inert
in reactor acid
neutralisation possible
syngas available
Bottom ash
quality
•
•
•
•
•
•
•
•
•
•
limited waste size
(<30cm)
tar in raw gas
higher UHV raw gas
less widely proven
limited wastes
process control and
engineering critical
high skill req.
not widely proven
need market for
syngas
•
•
•
Flue-gas
volume
if combined
with ash
melting
chamber ash is Lower than
vitrified
straight
combustion
ash quality
without ash
chamber – info.
not supplied
dependent on
process
temperature
residue
produced
requires further
processing
sometimes
combustion
Cost
information
Lower than other
gasifiers
Very low due
to low excess High pretreatment,
air required
operation and
for gas
capital costs
combustion
Table 4.9: A comparison of combustion and thermal treatment technologies and factors affecting their applicability and operational suitability (table 3/3)
[24, CEFIC, 2002] [2, infomil, 2002] [10, Juniper, 1997] [8, Energos, 2002] [1, UBA, 2001] [64, TWGComments, 2003]
Waste Incineration
239
Chapter 4
4.2.2 Use of flow modelling
Description
Physical and/or computer models may be used to investigate the effect of design features.
Various parameters may be investigated including gas velocities and temperatures inside the
furnace and boiler. Gas flow through FGT systems may also be studied with a view to
improving their efficiency e.g. SCR units.
Computerised Fluid Dynamics (CFD) is an example of modelling tool that may be used to
predict gas flows. Using such techniques can assist in the selection of a design that will allow
optimisation of the gas flows, so as to encourage effective combustion conditions and avoid
long gas residence times in those temperature zones, which may otherwise increase the risks of
PCDD/F formation. By applying the technique to FGT systems design it may be used to
improve performance e.g. by ensuring even flow across SCR catalyst mesh.
Modelling has been successfully used at both new and existing incineration plants to:
•
•
•
•
optimise furnace and boiler geometry
optimise the positioning of secondary and/or flue-gas re-circulation air (if used)
optimise the reagent injection points for SNCR NOX reduction
optimise gas flow through SCR units.
Achieved environmental benefits
The optimisation of furnace design may enhance the combustion performance and therefore
limit the formation of CO, TOC, PCDD/F and/or NOX (i.e. combustion related substances).
There is no effect on other, waste contained, pollutants. [64, TWGComments, 2003]
Reduction of fouling due to excessive local flue-gas velocities by using CFD modelling can
increase the availability of plants and improve the energy recovery over time.
Improvement in performance of abatement equipment.
Cross-media effects
Improving performance at the combustion stage may allow the selection of gas cleaning
equipment with reduced emissions and consumptions.
Operational data
The improvements of the flue-gas flow distribution along the boiler helps to reduce erosion and
fouling leading to corrosion.
Applicability
The technique is applicable to:
•
•
•
•
•
new waste incineration projects – to optimise design
existing plants where concerns exist regarding the combustion and boiler design - this will
allow the operator to investigate and prioritise optimisation possibilities
existing plants undergoing alterations in the furnace/boiler
new and existing plants investigating the positioning of secondary/flue-gas re-circulation air
injection equipment
installations installing or using SCR – to optimise the SCR unit itself.
Economics
Typically, a computer optimisation study will cost in the region of EUR 10000 to 30000,
depending on the scope of the study and the number of modelling runs required.
240
Waste Incineration
Chapter 4
Savings in investment and operational costs may arise from:
•
•
•
selection of alternative abatement system technology options
smaller/less complex abatement systems
lower consumptions by the abatement system.
The savings noted above are less likely to be realised where the key design issues for the
selection of the abatement system is the presence of heavy metals or halogens e.g. hazardous
waste plants. This is because the driver for FGT design in these cases is normally the loading of
intractable substances rather than combustion related substances.
Significant costs can be associated with modifying the furnace or boiler design of existing
installations.
Driving force for implementation
Optimisation of combustor design for low raw gas pollutant concentrations and possible
reduced emissions and consumptions.
Example plants
The technique has been used at:
•
•
•
•
the application stage in the UK to demonstrate effective combustion design of a proposed
installation
to optimise the combustion stage design at small municipal plants in Norway
for some new and existing municipal plants in Belgium
French plants: St Ouen (1989) - Nancy(1995) - Toulouse - St Germain.
Reference literature
[15, Segers, 2002], [16, Energos, 2002], [17, ONYX, 2000], [64, TWGComments, 2003]
4.2.3 Combustion chamber design features
Description
For some furnace types, including grates and static kilns, options exist regarding the positioning
and shape of the exit from the primary combustion chamber to secondary combustion zones. A
design that is not appropriate would lead to poor retention of combustible gases in the
combustion zones, poor gas phase burnout and higher emissions.
The design of the exit from the first stage of the furnace to the gas combustion and burnout zone
(the throat) should be selected to compliment the waste composition and other components of
the furnace e.g. grate type. See text in Section 2.3.1.4 and Figure 2.7.
For grate incineration, the design of the combustion chamber is closely linked to the supplier of
the grate. Suppliers can optimise the combination of grate and combustion chamber, based on
the individual performance of their system and experience. There is, generally, no overall
advantage/disadvantage from one design of combustion chamber to the other - all can be
applied. Furthermore, combustion chamber design cannot usually be chosen independently from
grate selection; together these form a clear and non-separable unit. [64, TWGComments, 2003]
CFD modelling (see 4.2.2) may be helpful in designing the combustion chamber.
Waste Incineration
241
Chapter 4
Type
•
Co-current or
parallel flow
•
•
Countercurrent
or counter-flow
•
•
Central current
•
or central flow
•
Split flow
Design features
exit to combustion
chamber at end of
furnace
gas flow in same
direction as waste
movement
exit to combustion
chamber at start of
furnace
gas flow in opposite
direction to the waste
exit to combustion
chamber in middle of
furnace
exit from combustion
chamber in mid
position but split by
central section
Comments
•
•
•
•
•
•
•
•
•
suited to higher NCV wastes
all evolved gases must pass through maximum
temperature zone and have long retention time
primary air heating required in ignition zone
suited to low NCV/high moisture/high ash waste (as
hot gases from volatilisation zone pass over the drying
zone)
higher secondary air requirements to ensure gas
burnout
compromise of the above for wide spectrum of waste
furnace configuration/secondary air important to
ensure gas burnout.
central section aids retention of gases and allows
secondary air to be injected from additional locations
mainly used for very large dimension furnace
Table 4.10: A comparison of the features of some different furnace geometries
[1, UBA, 2001, 2, infomil, 2002, 4, IAWG, 1997, 15, Segers, 2002]
Achieved environmental benefits
Improved combustion results in lower emissions to all media and reduced consumptions.
Cross-media effects
No significant negative effects identified.
Operational data
The combustion chamber is usually supplied with the grate and optimised for the particular
grate type that is selected. Combustion chamber design is therefore dependent upon the grate
selection. Each system described in Table 4.10 can result in operational improvements when
suitably applied.
Applicability
These techniques are generally applicable to most incinerator designs, except rotary kilns where
the exit to the secondary combustion chamber is always at the end of the kiln. However with
rotary kilns, the sizing and shape of the connection to the secondary chamber and the
positioning of the secondary air injection should also be such that it provides for sufficient gas
retention and mixing, to encourage gas burnout (as indicated by low and steady PIC
concentrations).
Split flow systems are mainly applicable to larger dimension furnaces because of the additional
secondary air mixing it allows in central positions of the furnace. In smaller furnaces, adequate
mixing may be achieved using sidewall injection of the secondary air.
A balanced overall combustion chamber design ensures that gases evolved from the waste are
well mixed and retained at sufficient temperature in the combustion chamber to allow the
combustion process to be fully completed. This principle is applicable to all incineration
processes.
Economics
At new plants the combustion chamber design features can be optimised at the outset. The
additional costs of such design refinements may then be small in relation to the overall cost of
the project.
242
Waste Incineration
Chapter 4
At existing plants the cost of redesigning (usually this means replacing) the furnace is very high
and may often outweigh the benefits to be achieved unless there are very serious difficulties
with the combustion stage, or the relevant equipment is due to be replaced for other reasons.
Driving force for implementation
Reductions in emissions from effective combustion.
Example plants
All plants select one of these options.
Split flow has been applied at: Indaver, BE, AZN (Afvalverbranding Zuid-Nederland, Moerdijk,
The Netherlands) as well as the Bonn-plant (Germany) and the Mke-line of MVV (Mannheim,
Germany).
Reference literature
[1, UBA, 2001, 2, infomil, 2002, 4, IAWG, 1997, 15, Segers, 2002, 64, TWGComments, 2003]
4.2.4 Design to increase turbulence in the secondary combustion
chamber
Description
See also related techniques in sections:
4.2.11 Secondary air injection, optimisation and distribution,
4.2.12 Replacement of part of the secondary air with re-circulated flue-gas
4.2.19 Optimisation of time, temperature, turbulence of gases in the combustion zone, and
oxygen concentrations
This technique relates to design features that increase the turbulence and hence mixing of
combustion gases in the zone after the primary combustion zone, but before or at the start of the
main heat recovery areas when the gas temperatures will generally still exceed approx. 850 ºC.
After the zone being considered here, as the combustion gases may pass onwards through the
main heat recovery areas (exchangers), stable and even gas velocity and flow are required to
prevent gas counter-flow and circulation that might lead to heat exchange problems and
pollutant generation.
In some cases, special configurations of the Secondary Combustion Area (SCA) can be used to
increase turbulence in the secondary combustion chamber. Examples of designs include:
•
•
•
•
•
vortex chambers
inclusion of baffles (cooling required)
several passes and turns in the chamber
tangential secondary air input
location and position of the secondary air injection systems (nozzles, …).
Achieved environmental benefits
Improved combustion leading to lower raw gas concentrations of combustion related
parameters.
This technique can reduce the volume of secondary air required, and hence reduce overall fluegas volumes and NOX production. Effective turbulence will also result in improved burnout of
combustion gases with reduced VOC and CO levels
Cross-media effects
None identified.
Operational data
Information not supplied
Waste Incineration
243
Chapter 4
Applicability
The SCC is designed by the supplier at the design stage. Additional features might appear
necessary with some furnace designs for some type of waste. [74, TWGComments, 2004]
The use of additional physical features to increase mixing is currently mainly applied in the
HWI industry.
Economics
Information not supplied
Driving force for implementation
Information not supplied
Example plants
Hazardous wastes - Cleanaway UK.
Reference literature
[40, EURITS, 2003] [64, TWGComments, 2003]
4.2.5 Use of continuous rather than batch operation
Description
Emissions at incineration plants are easier to control during routine operation than during startup and shutdown operations. Reducing the number of start-ups and shutdowns required is,
therefore, an important operational strategy that can reduce overall emissions and consumptions.
Waste collection/delivery regime and seasonal waste generation fluctuations can cause
shutdowns through lack of wastes, although they are often avoided by running the plant at
partial load in order to deal with such fluctuations. Running at partial load normally does not
cause problems for a modern combustor. [74, TWGComments, 2004]
Factors that help to achieve continuous throughput include:
•
•
•
•
•
the process design throughput rate is similar to the rate at which waste is received
waste storage (where possible) may cover slow periods
organisation of the supply chain to prevent slow periods
supplementing waste feed with additional fuels
use of online cleaning.
Sizing and maintaining plants to maximise continuous running is, therefore, important.
Achieved environmental benefits
Consistent plant operation improves energy efficiency.
Cross-media effects
Energy efficiency can be reduced by continuous operation on a lower load, because turbine
efficiency is lower.
Operational data
Predicting and controlling waste flows to the plant are important.
Good maintenance is important for avoiding/limiting shut downs. On line maintenance
programme can be designed into the installation so that availability is maximised.
Applicability
Planning for and achieving a reduced number of shutdowns is likely to reduce the annual mass
emission levels of any plant.
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Economics
Avoiding shutdowns can reduce costs at the incineration installations by:
•
•
•
allowing continuous throughput and hence greater installation utilisation
decreasing furnace maintenance due to lower thermal stress on the process
avoiding capital costs of an unnecessarily large processes.
Where the capacity of the installation is larger than the quantity of the waste received, and the
decision is taken to supplement the throughput with other wastes or fuels, there may be costs
associated with the purchase of those fuels/wastes.
Driving force for implementation
Main driving forces are operational.
Example plants
In general all large waste incineration plants are operated continuously. MSWI plants of an
industrial size (above ~2 t/h) can be operated continuously with a minimum number of
shutdowns.
Reference literature
[28, FEAD, 2002] [64, TWGComments, 2003]
4.2.6 Selection and use of suitable combustion control systems and
parameters
Description
[2, infomil, 2002]
The incineration of wastes of variable composition requires a process that can cope with large
variations in process conditions. When unfavourable process conditions occur, interventions in
operational control are required.
In order to be able to control the incineration process, detailed process information is required, a
control system ('philosophy') must be designed, and it is necessary to be able to intervene in the
process. The details of the systems used, vary from plant to plant. The following provides an
overview of process information, control philosophy systems and process interventions that can
be used.
Process information may include:
•
•
•
•
•
grate temperatures for various positions
thickness of waste layer on the grate (visual control)
pressure drop over the grate
furnace and flue-gas temperatures at various positions
determination of temperature distribution over the grate surface by optic or infrared
measurement systems
• CO-, O2-, CO2- and/or H2O-measurements (at various positions)
• steam production data (e.g. temperature, pressure)
• openings in the combustion wall for visual observation by individuals or camera
• length and position of the fire in the furnace
• emissions data for combustion related substances (unabated levels).
[74, TWGComments, 2004]
The control philosophy may be a classic control system, which may already be included in the
process control computer. Additionally, fuzzy control systems are applicable.
Waste Incineration
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Control interventions include adjusting :
•
•
•
•
•
the dosing system for the waste
frequencies and speed of grate movements in various parts of the grate
amount and distribution of primary air
temperature of the primary air (if preheating facilities are available)
amount and distribution of secondary air in the furnace (and, if available, of re-circulation
gas)
• the ratio primary to secondary air.
[74, TWGComments, 2004]
Achieved environmental benefits
The use of sophisticated control systems can result in an incineration process that has less
variations in time (i.e. improved stability) and space (i.e. more homogeneous) thus allowing for
improved overall combustion performance and reduced emissions to all media.
Improved process control has the following specific advantages:
•
•
•
•
•
•
•
•
•
•
•
better bottom ash quality (due to sufficient primary air distribution and a better positioning
of the incineration process on grates)
less fly ash production (due to less variations in the amount of primary incineration air)
better fly ash quality (less unburned material, due to more stable process conditions in the
furnace)
less CO and VOC -formation (due to more stable process conditions in the furnace; i.e. no
'cold' spots)
less NOX formation (due to more stable process conditions in the furnace; i.e. no 'hot' spots)
less risks of formation of dioxin (and precursors) due to a more stable process in the furnace
better utilisation of the capacity (because the loss of thermal capacity by variations is
reduced)
better energy efficiency (because the average amount of incineration air is reduced)
better boiler operation (because the temperature is more stable, there are less temperature
'peaks' and thus less risk of corrosion and clogging fly ash formations)
better operation of the flue-gas treatment system (because the amount and the composition
of the flue-gases is more stable)
higher destruction potential, combined with more effective combustion of the waste. [74,
TWGComments, 2004]
The indicated advantages also result in less maintenance and thus better plant availability.
Cross-media effects
None identified.
Operational data
Grate bar temperature may be measured using thermocouples. Flue-gas temperatures are more
difficult to measure due to severe conditions - high level of dust, risk of metal melting, etc.
Measurements at the furnace outlet are also not easy to implement due to operational conditions
(dust, acid, etc.), in particular for CO and CO2 measurements. For control purposes, quick
measurements are required. It is very difficult to measure H2O accurately. [64, TWGComments,
2003]
Applicability
Selection and use of suitable combustion control systems and parameters is applicable to all
waste incineration installations. The detailed components of such a system will vary from one
process design to another. Most of the specific techniques in the description section above are
applicable to grate rather than other incinerators.
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The technique is of particular benefit where the waste fed to the furnace is highly heterogeneous
in nature, i.e. of variable composition, or its quality is difficult to predict or assure.
Economics
The indicated advantages also result in less maintenance and, therefore, better plant availability.
Driving force for implementation
The improved combustion performance results in overall improvements in environmental
performance.
Example plants
Widely employed throughout Europe particularly at modern plants.
Reference literature
[2, infomil, 2002] [64, TWGComments, 2003]
4.2.7 Use of infrared cameras for combustion monitoring and control
Description
The continuous adaptation of the distribution and amount of incineration air to match the
precise incineration reaction requirements in the individual zones of the furnace can improve the
incineration process. An infrared camera is an example of a techniques that can be used to
create a thermal image of the burning waste bed. Ultra-sound and visual cameras are also used.
The temperature distribution on the grate appears on a screen as an isothermal field graduated in
coloured areas.
For the subsequent furnace performance control, the characteristic temperatures of the
individual grate zones may be determined and passed on to the furnace performance controller
as input parameters for furnace variables. Using fuzzy logic, some variables (e.g. temperature,
CO, O2 content) and a sequence of rules can be determined to maintain the process within these
settings. In addition, flue-gas re-circulation and tertiary air addition can be controlled.
Figure 4.1: An example of the components of furnace control system
[1, UBA, 2001]
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By computer controlled image processing, the video images provided by the IR camera can be
transformed into signals which are coupled, in the furnace control system, with parameters such
as oxygen content in the flue-gas and steam quantity.
The charging of the incineration chamber can then be controlled by recording the average
temperature of the waste bed at the foremost part of the grate and evaluation of the O2 value at
the end of the boiler. With the help of camera-controlled incineration bed temperature recording
over the first three grate zones, primary air can be added according to demand (air quantity and
distribution), which helps to even out the incineration process in the main incineration zone. In
grate zone 2 (ignition zone), the air demand is controlled, as a function of incineration bed
surface temperature and a more constant temperature profile may be reached. Adapting the air
quantity in grate zones 3 and 4, and the temperature of the incineration surface leads to steady
incineration and efficient bottom ash burnout.
In a project, incineration tests were run with oxygen conditioned primary and secondary air and
additions of nitrogen in the secondary air. A favourable influence on dust, CO and the
total VOC concentrations in the flue-gas behind the steam generator was recorded, in particular
with oxygenated primary air (O2 content of supplied enriched air between 25 and 28 % by
volume). Moreover, the NOX content in the flue-gas could be reduced due to the addition of
nitrogen to secondary air.
The results from this investigation have led to the development of a system combining the
following process steps:
•
•
•
fully automatic incineration control through infrared camera and fuzzy logic
flue-gas recirculation to the furnace via a secondary air system, and
oxygenation of the primary incineration air in the main incineration zones.
Measures introduced at another existing plant with feed grate included:
•
•
•
•
graded addition of incineration air
constant dosing of waste through height of layer control
incineration monitoring by optic sensors (so-called incineration sensors) in different grate
zones
flue-gas re-circulation.
Compared to the plant’s conventional operation, the combustion related pollutants were
reduced.
Achieved environmental benefits
Improved overall combustion performance and reduced emissions to all media.
Cross-media effects
No cross-media effects identified in respect of the use of infrared cameras.
Use of oxygen and energy for its generation – where applied.
Operational data
The results from the tests with normal operation and with incineration control with the IR
camera and with oxygen addition are shown in Table 4.11 below:
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Flue-gas component
(crude flue-gas behind
generator)
Oxygen content (Vol.-%)
Carbon monoxide mg/m³)
Dust (g/m³)
Total carbon mg/m³)
Dioxins/furans (ng I-TE/m³)
steam
Normal
operation
IR camera plus fuzzy O2 conditioning
logic
9.1 – 9.3
12 – 32
0.7 – 1.7
1.1. – 2.4
1.5 – 2.7
8.9 – 9.3
9 – 26
0.6 – 1.0
0.9 – 1.0
1.0 – 1.3
6.2 – 10.9
20 – 27
0.5 – 1.0
1.0 – 1.2
2.0 – 3.5
TWG comment: The increase in PCDD/F with additional oxygen shown here, is not the theoretically expected
result.
Table 4.11: Crude flue-gas measurements at a test plant under normal operation, with IR camera
and O2 conditioning
[1, UBA, 2001]
Applicability
Mainly applicable to grate incinerators. This technique is applicable only if can be applied when
the furnace design (in particular the throat) is such that the camera can view the relevant areas
of the grate. Moreover, the application is limited to and in general with larger scale furnaces,
with several grate lines (e.g. >10 t/h). [74, TWGComments, 2004]
Economics
The order of magnitude for one camera (not installed and as a stand-alone unit, i.e. not
integrated in the control circuit of the plant) is reported at approx. EUR 50000. However it is
also reported that one supplier quotes EUR 300000 per line (information provided is not clear if
this relates to the whole system of IR plus O2 control etc) [74, TWGComments, 2004]
Driving force for implementation
Improved combustion performance results in overall improvements in environmental
performance.
Example plants
Coburg, Germany.
Ingolstadt, Fribourg, Brescia, Arnoldstein and others.
Reference literature
[1, UBA, 2001], [64, TWGComments, 2003]
4.2.8 Optimisation of air supply stoichiometry
Description
In combustion systems, sufficient oxygen (usually from air) must be supplied to ensure that the
combustion reactions go to completion.
In addition to this, the supply of air has the following roles:
•
•
•
•
cooling
avoidance of slag formation in the combustion chamber/boiler
mixing of gases to improve efficiency
influencing burnout quality.
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Chapter 4
Supplying too little, or too much air, causes difficulties. The precise amount of air required is
dependent upon:
•
•
•
waste type and characteristics (CV, moisture, heterogeneity)
type of combustor (fluidised beds have lower overall air requirements due to increased
waste agitation, which increases exposure of the waste to the air)
ensuring the air is supplied in the correct locations and quantities.
In general, the over-supply of air should be avoided, but importantly, it must still be sufficient to
ensure effective combustion (as demonstrated by low and stable CO concentrations downstream
of the furnace). The over supply of air will result in increased flue-gas volumes and hence the
increased size and associated costs of flue-gas treatment systems.
Achieved environmental benefits
Reduction of flue-gas volumes (and hence treatment requirements) whilst achieving effective
gas burnout are the aim of the optimisation.
Cross-media effects
No information supplied.
Operational data
No information supplied.
Applicability
No information supplied.
Economics
No information supplied.
Driving force for implementation
Optimisation of the incineration process
Example plants
Most of waste incineration plants in EU
Reference literature
[64, TWGComments, 2003]
4.2.9 Primary air supply optimisation and distribution
Description
Primary air is that which is supplied into, or directly above, the waste bed to provide the oxygen
necessary for the combustion. Primary air also helps drying, gasifying and cooling some of the
combustion equipments.
The manner of primary air supply is directly related to the incineration technology.
In grate systems it is supplied through the grate into the waste bed to:
•
•
250
bring the necessary air to the different zones of the grate where the reactions occur (drying,
gasification, volatilisation) and ensure homogeneous and sufficient distribution inside the
waste bed which improves bottom ash burnout
cooling of the grate bars to prevent slagging and corrosion. The cooling of fluid-cooled
grates is typically achieved by means of a separate water circuit and the effect of the
primary air on cooling is therefore irrelevant.
[74, TWGComments, 2004]
Waste Incineration
Chapter 4
In MSWI grates, the primary airflow is determined primarily by the oxygen requirement (a
sizing function) and not by the grate cooling requirements. [74, TWGComments, 2004]
In rotary kilns, stepped and static hearths, the primary air is generally introduced above the
waste bed. In some stepped hearth designs primary air can be partially introduced below the
waste bed.
In fluidised bed systems the primary air is introduced directly into the fluidisation material and
also serves to fluidise the bed itself. The primary air is blown through nozzles from the bottom
of the combustion chamber into the bed.
The balance of primary and secondary air will depend upon the waste characteristics and upon
which of the combustion technologies is utilised. Optimisation of this balance is beneficial for
process operation and emissions. In general, higher calorific value wastes allow lower primary
air ratios.
Separating the supply of the primary air (using individual wind boxes and, if suitable, multiple
or distributed supply fans) to the different zones within a grate incinerator, allows the separate
control of the air supply to each of the zones. This allows each process that occurs on the grate
(drying/pyrolysing/gasification/volatilisation/ashing out) to be optimised by provision of its own
optimised air supply.
Insufficient supply of primary air to the final (ashing out) stage can result in poor ash burnout if
residence time in the chamber is not high enough.
If the combustion air is extracted from waste storage areas this will help to reduce odour risks
from waste storage.
Achieved environmental benefits
Optimisation of air supply and distribution is beneficial for the optimisation of the combustion
stage of the incineration process and for reducing overall emissions.
Improved burnout of bottom ash.
Reduced demand of primary fuels for the support of combustion. [74, TWGComments, 2004]
Cross-media effects
No significant effects
Operational data
Easily installed. Primary air supply is essential for combustion process. Its optimisation differs
with the combustion technique.
Applicability
Applied at every plant
Economics
Provided the initial design is correct and provides systems and equipment for primary air
control, additional equipment and costs are not normally incurred. Where intervention is
required at an existing plant, additional fans and ducting may be required to control and
distribute the air supply.
Driving force for implementation
Where improved combustion and reduced emissions to all media, and in particular where
improved burn-out of bottom ash are drivers.
Example plants
All the incineration plants.
Reference literature
[64, TWGComments, 2003]
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Chapter 4
4.2.10 Preheating of primary and secondary air
Description
Heating the primary air supply can improve the combustion process by drying the waste. This is
especially important where low LCV/high moisture wastes are burned as they may require
additional drying. [2, infomil, 2002, 64, TWGComments, 2003]
Heating the secondary air supply can improve the efficiency and assist the combustion process
in case of low LCV wastes by ensuring that temperatures in the gas burnout zone are adequate
and evenly distributed.
Preheating of incineration air in grate municipal waste incineration plants is normally done with
low pressure steam and not by heat exchange from the flue-gases (complicated air ducts,
corrosion problems).
Preheating of air for bubbling fluidised bed incineration is normally done with flue-gas by
means of heat exchange, but sometimes also with steam or supporting fuel. [64,
TWGComments, 2003]
In some installations this heat is taken from the cooling air behind the refractory material.
The heat supplied with the air supply is not lost since it may be recovered later in the boiler. [74,
TWGComments, 2004]
Achieved environmental benefits
More stable combustion leads to lower emissions to air.
Upgrading of flow value steam/energy to better quality steam is possible.
Cross-media effects
Where heat is taken from the incineration process the cross-media effects will be minimal. If
external fuel sources are used the consumption of that external energy and the additional
emissions (e.g. of NOX, particulates) are a factor.
Operational data
Primary air is heated to 150 °C by mixing primary air with cooling air of refractory material in
the furnace. [74, TWGComments, 2004]
Applicability
The heating of primary and possibly secondary air is of particular benefit where low calorific
value wastes are burned. In the case of primary air, this is because it supports the drying and
ignition of the waste, with secondary air this is because it helps to maintain temperatures in the
gas burnout zone.
Plants burning high calorific value waste need the cooling effect of the air supply and will not,
therefore, benefit from this technique.
Economics
The design of the system for new plants adds the cost of a heat exchanger plus
steam/condensate circuit. The impact of the additional cost depends on the plant scale.
Retrofitting at existing plants will require specific additional investment.
Capital costs of heat-exchange equipment can be offset against the avoided cost of external
fuels.
Driving force for implementation
Improved combustion performance, especially where low LHV wastes are encountered.
252
Waste Incineration
Chapter 4
Example plants
Applied to plants throughout Europe.
Reference literature
[2, infomil, 2002], [64, TWGComments, 2003]
4.2.11 Secondary air injection, optimisation and distribution
Description
During drying, gasifying, incineration, and burnout, the combustible waste materials are
transformed into a gaseous form. These gases are a mixture of many volatile components, which
must be further oxidised. For this purpose, additional air (so-called secondary air) is introduced
into the furnace.
The incineration temperature can be raised by preheating the incineration air, and lowered by
allowing in more incineration air (note: sufficient gas residence time mainly depends on the
dimensions of the furnace). Therefore, in some cases the secondary air may provide cooling as
well.
Another main function of the secondary air is to mix the hot flue-gases, for this purpose it is
blown into the furnace through a large number of nozzles, which ensures that the furnace's
entire cross-section is sufficiently covered. Because the mixing of hot gases requires sufficient
mixing energy, the secondary air is blown in at relatively high speed. Additionally, dimensions
of the furnace are selected to ensure adequate flue-gas flow patterns and sufficient overall
residence times. For MSWI the flowrate is determined by the mixing requirements.
The injection port locations, directions and quantities can be studied and optimised for various
furnace geometries, using for example computerised flow modelling.
Temperatures at the nozzle heads can contribute significantly to NOX production. Typical
temperatures are in the range of 1300 to 1400 °C. The use of special design nozzles and of FGR
to replace some of the nitrogen can reduce nozzle temperatures and nitrogen supply that lead to
higher NOX production.
Achieved environmental benefits
•
•
•
low and stable emission of combustion related substances
improved oxidation of combustion gases produced during earlier combustion stages
reduced carry over of products of incomplete combustion and fly ash to gas cleaning stages.
The benefits are to reduce the quantity of combustion related substances (e.g. NOX, CO and/or
VOC). CO and VOC levels are not treated in the FGT.
Cross-media effects
If secondary air with normal oxygen content is injected into the afterburning zone, on top of the
nozzles while entering the afterburning zone, temperatures above 1400 °C can be measured and
by this thermal NOX is produced. [74, TWGComments, 2004]
Operational data
The amount of secondary air depends on the LCV. [74, TWGComments, 2004]
For grate technology, the amount of secondary air is normally between 20 and 40 % of the total
amount of incineration air, with the remainder being primary air.
Waste Incineration
253
Chapter 4
There is a risk of rapid corrosion of the water walls of the post combustion chamber and the
boiler, if secondary air is too low, as the CO/CO2 level could pulse between oxidising and
reducing conditions.
Applicability
All waste incineration plants.
Economics
The costs of making changes to optimise secondary air at individual existing plants will vary
greatly according to specific design features. This cost is included in the design of the process of
new plants. [74, TWGComments, 2004]
If the NOX level is reduced, it may also reduce the cost of the corresponding treatment, and
improve NOX achievable level of abatement concerning SNCR technique. The secondary air
optimisation might reduce the flue-gas volume and therefore reduce correspondingly the FGT
plant size. However the mass flowrate of the pollutants remains similar. [64, TWGComments,
2003]
Driving force for implementation
Improvements at the combustion stage result in reductions in emissions to all media.
Example plants
Employed at the design stage of the majority of new plants.
An examples of retrofit to improve this aspect are: Toulon (F), lines 1 & 2 (2 x 12 t/h), when
fans and injection nozzles were changed.
Reference literature
[2, infomil, 2002] [64, TWGComments, 2003]
4.2.12 Replacement of part of the secondary air with re-circulated flue-gas
Description
One of the purposes of the secondary air addition (apart from oxidising the combustible species
in the flue-gas), is to improve the mixing and homogeneity of flue-gas. However, the use of
more secondary air than is necessary results in higher flue-gas quantities. This reduces the
energy efficiency of the plant, leading to larger flue-gas treatment units and, therefore, to higher
costs.
By replacing part of the secondary air with re-circulated flue-gases the flue-gas volume is
reduced downstream of the extraction point and at the point of emission. The reductions in the
fresh nitrogen supply (from air) to the furnace may help to reduce NOX emissions.
In general the re-circulation extraction point is after FGT to reduce corrosion and other
operational problems caused by raw flue-gas, this involves some energy losses and FGT system
must be designed for a larger flow.
However, if the flue-gases are re-circulated from upstream of the FGT system then the size of
the FGT system can be reduced, [64, TWGComments, 2003] although it needs to be set to treat
more polluted flue-gases because of the increased concentration and there is higher risk of
erosion, corrosion and fouling. [74, TWGComments, 2004]
See also Section 4.2.11 on secondary air optimisation.
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Achieved environmental benefits
•
•
•
•
reduced flue-gas volumes and hence FGT treatment size of the FGT downstream of the
flue-gas extraction point (i.e. generally where dirty gas is re-circulated)
improved energy efficiency (approx. 0.75 % increases reported at a CHP plant)
reduction in NOX production by 10 % up to 30 % (if high NOX levels exist in the raw gas)
reduction in reagent consumption for NOX control.
At high excess air rates, approximately 50 % of the required amount of secondary air can be
replaced by re-circulating flue-gases. When the recirculated gas is raw flue-gas, this results in a
10 – 15 % reduction of the total amount of incineration air and flue-gases. The load of the fluegas treatment system may be reduced proportionally if the concentrated pollutants in the
reduced flue-gas quantity can be cleaned in the same way (resulting also in a reduction of
emission loads) and the thermal efficiency of the plant may increase by approximately 1– 3 %.
Cross-media effects
Depending on precise furnace design, at high replacement rates the effective reduction of
oxygen can result in elevated CO (and other PICs) levels. Care must, therefore, be taken to
ensure replacement rates are optimised.
There may be a negative cooling effect in the rotary kiln, in some cases, especially with lower
CV wastes, extra fuel is necessary to maintain the rotary kiln temperature.
Operational data
Corrosion in the re-circulation ducting has been reported. It is also reported that this can be
overcome by the elimination of joints, and using effective insulation of ducting to prevent cold
spots, where condensation of the flue-gas and corrosion can rapidly occur. Corrosion may also
occur in the boiler due to lower oxygen levels in the flue-gas.
[21, FNADE, 2002] If the operator is not attentive, corrosion can be very rapid. In such cases
the expected operational savings are quickly turned into higher repair costs and plant
availability loss. Corrosion risk is reduced if the hotter parts of the boiler are covered by special
claddings. However, when this cladding is installed, the O2 excess concentration at the boiler
exit can be reduced even without FGR. This, then, reduces the benefit of FGR.
In some German MSWI plants with installed recirculated flue-gas systems the recirculation is
reported to be closed or out of operation for operational reasons. The reduced flue-gas flow is in
most cases is not used in the sizing of the FGT plant; many operators choose to size the FGT
plant with flue-gas recirculation OFF, so as to cover all possible operating conditions.
[74, TWGComments, 2004]
Applicability
This technique has been applied to new waste incineration plants. Some existing plants have
retrofitted this technique, for which space is required for the ducting.
The technique has a limited applicability for HWI. In the case of rotary kilns HWI there is a
need of high O2 content and therefore the recirculation of gas has a limited applicability. [74,
TWGComments, 2004]
Economics
This technique involves additional investments for new plants and significant cost for
retrofitting existing plants. [74, TWGComments, 2004]
Driving force for implementation
Reduction of NOX using primary techniques.
Even with FGR, a de-NOX device is required for reaching, under any operational condition, a
level of 200 mg/Nm³.[21, FNADE, 2002]
Waste Incineration
255
Chapter 4
Example plants
Applied at some new and existing plants throughout Europe.
Reference literature
[2, infomil, 2002], [21, FNADE, 2002] [64, TWGComments, 2003]
4.2.13 Application of oxygen enriched air
Description
Replacement of air supply with (technically) pure oxygen or oxygen enriched air.
This technique is applied at some gasification and pyrolysis plants for the combustion of the
gases they produce, often as part of systems that are designed to raise combustion temperatures
in order to melt the incinerator ashes. In such cases the initial pyrolysis or gasification reactor is
often a physically separate unit from the subsequent combustion chamber. The fuel rich
syngases pass into the combustion zone, where the oxygen enriched air is added at a controlled
rate in order to achieve the desired combustion conditions. Depending on oxygen addition rate
and gas quality, temperatures in the combustion chamber are generally between 850 and
1500 °C, although in some specific case temperatures of up to 2000 °C (or higher) are used. At
temperatures above around 1250 °C, entrained fly ashes are melted.
This technique has also been applied on a trial basis at existing large incineration plants in order
to improve the process performance and as a specific design technology at smaller plants that
are generally dedicated to the destruction of particular (often hazardous) waste streams. In these
smaller plants (e.g. trailer mountable plants) the process may be applied on a batch basis in a
sealed reactor, with elevated pressure (8 bar) and temperatures (e.g. in the range of 2000 to
8000 °C).
Achieved environmental benefits
Rapid and efficient combustion can result in very low and controllable CO and other
combustion related emissions.
Replacement of the nitrogen in air with oxygen can reduce the potential for thermal NOX
formation. However, NOX production also depends on flame temperature so care is required to
ensure that nitrogen replacement is sufficient to prevent the combination with higher
temperatures from resulting in an overall increase in NOX.
A lower volume of waste gas is released compared to air fed combustion technologies.
However, at temperatures above 1500 °C this benefit may be reduced owing to expansion of
flue-gases. The more concentrated pollutants that result from the lower flue-gas volume can be
captured with a compact FG treatment line. However, such an adaptation would require specific
adaptations in the flue-gas treatment at existing plants. Reduced FGT size may reduce
consumptions to some degree (e.g. for NOX), but this is largely related to pollutant load (rather
than concentration) and therefore reductions may be negligible for waste contained pollutants. It
is reported that the boiler size may also be reduced using this technique.
[74, TWGComments, 2004]
The use of temperatures in excess of 1500 – 2000 ºC are reported to have only a limited
additional benefit in terms of emissions reduction. [64, TWGComments, 2003]
Cross-media effects
The production of pure oxygen or oxygen enriched air is energy consuming.
Presence of CO during transitory phases: start-up, shutdown and emergency stops.
Problems of reduced resistance of refractory materials and an increase of corrosion.
256
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Chapter 4
Operational data
Trials at an existing grate municipal incineration plant in the Netherlands encountered
difficulties with locally increased temperatures and corrosion. These difficulties are reported to
be overcome with better waste mixing and optimised injection.
At higher temperatures (above 1000 ºC) furnace and refractory maintenance is generally greatly
increased. The higher temperatures used can cause significant materials selection and use
difficulties.
In addition intensive gas cooling is required to reduce flue-gas temperature to a suitable level
for FGT.
Molten fly ash requires systems to ensure its removal (e.g. vortex gas flow) so that it does not
come into contact with downstream heat exchangers to cause clogging/erosion.
Additional safety risks result from the production, storage and use of oxygen.
Applicability
In general, installations require specific design adaptations to incorporate the use of this
technique. Attention is required to most details of plant design including particular adaptations
to the combustion chamber, heat exchange areas, and sizing of FGT systems. At low levels of
oxygen addition the design changes may be more limited, but so too then are the potential
advantages of the use of the technique.
The technique may be applicable as a retrofit option at existing plants where:
•
•
combustion related emissions are high or difficult to control and
air supply volumes are already high.
The high combustion efficiency can make this technique of use for the incineration of materials
that are very highly resistant to combustion, e.g. PCBs.
In practise oxygen enrichment is not widely applied owing to the additional costs and crossmedia impacts associated with the generation of oxygen, additional operational challenges (e.g.
higher temperatures may result leading to molten ash control issues) and the ability of air based
techniques to achieve good performance levels.
Economics
Pure oxygen is costly, oxygen enriched air is less expensive but still gives rise to additional
costs over normal air. The costs of both may be reduced if the incinerator is on a site where
oxygen is already available e.g. some industrial sites. Parasitic electrical loads for on-site
oxygen generation are significant. This demand varies according to plant size, temperature and
oxygen purity requirements but is generally in the order of 0.5 – 2 MW electrical.
The use of this technique may add significantly to capital and operational costs.
Reductions in the flue-gas volume may reduce the size of flue-gas treatment devices required.
Driving force for implementation
The technique has been reported to be used for the treatment of some types of hazardous wastes
that are otherwise expensive to dispose of.
The technique is reported to have been used as a retrofit at existing plants that have combustion
performance difficulties.
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Example plants
In Austria a municipal waste incineration plant has been commissioned at the beginning of 2004
where oxygen enriched air is applied. Annual throughput is about 80000 t/yr, average oxygen
content is about 26 %, temperature on the grate is about 1100 - 1200 °C whereas the
temperature in the combustion chamber is reduced by means of flue-gas recirculation. No
problems have been reported by the operator until now. [74, TWGComments, 2004]
Oxygen enrichment is applied at gasification and pyrolysis processes for municipal and
industrial wastes in Japan as part of systems that are designed to melt the incinerator ashes (e.g.
Asahi Clean Centre, Kawaguchi City, Tokyo).
The first full-scale unit for HW is now operating at SEABO (Municipality of Bologna). So far it
has been used for treating materials such as: hardened paints, halogenated solvents, inks,
refinery sludge, plastic packaging, polluted rags, oil containing PCBs, pesticides, expired
medicines, among others.
Reference literature
[18, italy, 2002], [2, infomil, 2002], [64, TWGComments, 2003]
4.2.14 Cooling of grates
Description
[19, Babcock, 2002] [64, TWGComments, 2003]
Grate cooling is carried out to control metal temperatures and thereby improve grate life. The
cooling medium can be air or water (other liquids may also be used, such as oils or other heat
conducting fluid).
Air is supplied below the grate and passes through the grate spacings; the main function of this
air is to provide the necessary oxygen for oxidation, and the flowrate is designed according to
this requirement. Simultaneously, this air provides cooling to the grates, which is the source of
cooling for air-cooled grates. When more excess air is introduced additional cooling is supplied,
but a larger amount of flue-gas is produced.
Liquid-cooled grates include a circuit inside the grate by which the liquid is flowing for cooling
the grate. The higher heat transfer capacity of liquids make liquid cooled grates more suitable
for situations where the cooling with air has limitations, in particular when burning high NCV
wastes (e.g. >10 MJ/kg).
The liquid flows from the cool parts of the grate to the hotter ones in order to maintain a
temperature differential. The temperature of the liquid can be used to monitor the reactions
(some are endothermic, some exothermic, and to differing degrees) occurring in the waste bed
above the grate. These reactions can then be controlled by varying the amount of air supplied
through that section of the grate to the waste above. This separation of the cooling and air
supply functions may increase the control of the process.
Achieved environmental benefits
Both air and water cooled grates can provide for effective waste burnt out.
For higher LCV wastes, using liquid cooled grates can allow slightly increased combustion
process control, as the additional cooling capacity required with such wastes can be obtained
from the cooling liquid instead of supplying more air so it is, therefore, possible to reduce the
primary air supply and hence the overall flue-gas volumes.
Cross-media effects
No significant negative effects identified.
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Operational data
Air-cooled grates are very widely used and proven for municipal wastes, and for a range of
other mainly solid wastes. They are reported to be highly reliable and provide for effective
performance and long operational use. Complexity of the air-cooled systems is somewhat lower
than liquid cooled systems and this can have operational benefits. The use of air-cooled grates
in Europe is very common, with approximately 90 % of incinerated MSW being treated in
plants using air-cooled grates.
The liquid-cooled grate system increases grate cooling efficiency as the liquid circulates directly
inside the grate. Heat damage may be reduced, and even with the waste high in calorific value, it
is possible to achieve a service life of over four years. Effective liquid-cooled grate bar
fabrication is required to prevent problems of cracking and liquid leakage, and subsequent effect
on installation availability. In order to increase grate temperature control, a sophisticated liquid
circuit is required if all the grate bars are to be fed individually with liquid. An alternative is for
zones of the whole grate system to be controlled.
In the higher temperature conditions that may arise when incinerating high heat value wastes,
the liquid cooled grates can have longer life due to the reduced corrosion they experience but
they may have a higher risk of grate damage through leaks of the cooling liquid than with aircooled systems.
Operational experience has shown that, with water cooled grates, virtually all the leaks occur at
the connections between the tiles or the connection between the tiles and the cooling circuit
collectors. Hence, the risk on cooling circuit failures can be minimised by reducing the number
of these connections. Fluid-cooled grate designs with a low number of connections are
preferred. Lifetime of a water-cooled grate tile may be in excess of 35000 – 40000 operational
hours.
Applicability
A specific feature of grates is that they are highly robust in nature and may be applied to almost
any mainly solid waste type, including highly heterogeneous wastes. Both liquid and air-cooled
grates are applied for municipal wastes, with approximately 90 % of MSWI using the air-cooled
type.
In general liquid cooled grates are applied where there is a specific need for additional grate
cooling i.e. where waste LCV is higher (e.g. above ~10 - 13 MJ/kg, depending on the grate type)
Air-cooled systems may also be used in such circumstances, sometimes with other cooling
features e.g. water walled furnaces.
Economics
Air-cooled grates are more economic to purchase than liquid cooled grates.
Risk of damage to the grate, and hence high repair costs and downtime, may be higher with
non-air-cooled systems as liquid leaks may cause damage (but see also Operational Data above).
Driving force for implementation
Selection of grate cooling systems is generally made on the balance of operational advantages
and disadvantages depending on the heat value and composition of the waste that will be
treated. Depending on the particular circumstances (i.e. notably the grate and waste type) it may
be possible to treat higher calorific value wastes with a fluid cooled grate than with an the same
air-cooled grate.
Example plants
Cooling of grates is widely used in Europe and worldwide. Water cooled systems are less
widely used but are reported to be applied at least in Denmark and Germany.
Reference literature
[19, Babcock, 2002], [64, TWGComments, 2003]
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4.2.15 Water cooling of rotary kilns
Description
[20, EKOKEM, 2002]
This technique is usually used together with higher temperatures in the kiln (see Section 4.2.16)
The rotary kiln cooling system consists of two cooling circuits. The primary cooling water
circuit delivers primary cooling water on top of the rotary kiln and distributes it evenly to
guarantee equal cooling effect all over the shell of the kiln. Water is then collected into four
water collection basins located under the kiln and it continues to flow freely into the water
collection tank. Water is circulated back through a filter and a heat-exchanger with a circulation
pump. Evaporation is compensated with additional make-up water, which is automatically
buffered with NaOH in order to avoid corrosion.
The secondary circuit removes heat from the primary circuit through heat-exchangers and
transfers it for use. If there is no need for energy recovery, a multi-sectional air cooling system
can be used for removing heat from the system. In order to avoid freezing, a water-glycol
mixture is circulated through the liquid-air heat-exchangers.
The system delivers cooling water through hundreds of spray nozzles situated all over the shell
of the kiln keeping the temperature of the shell at 80 – 100 °C, whereas, for air cooling the steel
shell temperature is typically a few hundred degrees higher. The rotary kiln cooling increases
the heat transfer through the refractory enough to reduce the rate of chemical erosion to
minimum. Higher temperatures can be used in the kiln.
Achieved environmental benefits
The main benefit of rotary kiln water cooling is that higher combustion temperatures may be
used where required (see advantages in Section 4.2.16).
The heat transfer rate through the furnace into the primary cooling fluid is increased. According
to theoretical calculations and practical measurements at example installations, the heat transfer
through the furnace into the cooling water varies between 0.5 MW and 3.0 MW, depending on
the size of the rotary kiln and the thickness of the refractory. The thickness of the refractory
includes the remaining brick lining and the solidified bottom ash layer. For example, in 1995
Kommunikemi (DK) reported an average kiln heat recovery of 2.2 MW.
Cross-media effects
No negative aspects identified.
Operational data
Operational benefits are:
•
•
•
extended refractory lifetime when operating at higher temperatures - lower maintenance
increased throughput rates possible - especially for higher calorific value wastes
better working environment - lower temperatures beside the kiln.
Applicability
This technique is applicable to rotary kiln incinerators with higher LHV inputs. It is mainly
applied at hazardous waste plants but could also have wider applications to other waste burned
in rotary kilns. The technique is especially suited to plants that require high temperatures for the
destruction of particular types of wastes.
Water cooled kilns are reported to mainly offer extended refractory life for wastes with low
melting mineral matter.
Where the system is combined with a high temperature kiln, water flowrates must be high to
achieve sufficient heat removal rates. This will result in a larger quantity of warm water, rather
than a lower quantity of hotter water. The technique is, therefore, more likely to be applicable to
processes that have a demand/use for the warmed water generated.
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Economics
Both Ekokem in Finland and Kommunekemi in Denmark, have reported a capacity of over
100000 tonnes of incinerated hazardous waste (waste water not included) at high temperatures
with the same brick lining. This equals an overall lifetime of two to three years. Both plants
typically shut down the waste incineration once a year for a planned two-week pre-maintenance
period and only twice a year for shorter inspections.
Driving force for implementation
Some operators have been required to operate high temperature slagging kilns and have,
therefore, developed the water cooling system to allow economic operation in such
circumstances. Where non-disposal outlets (e.g. re-cycling as aggregate replacement) exist for
the semi-vitrified bottom ashes produced, this may reduce disposal costs and hence increase
interest, or offset costs of the technique.
Availability of a customer for the warmed cooling water increases interest in this technique.
Availability of higher LHV wastes (e.g. solvents and oils) allows high operation temperatures
which then require the additional cooling noted here. Where such wastes are diverted to other
waste treatment options, their availability may be restricted and operation in higher temperature
mode only possible with additional fuels/costs.
Example plants
Ekokem, Finland. Kommunikemi, Denmark
Reference literature
[20, EKOKEM, 2002], [40, EURITS, 2003] [64, TWGComments, 2003]
4.2.16 Higher temperature incineration (slagging)
Description
This technique is most commonly applied in Europe to rotary kilns treating hazardous wastes.
However, the principle of operating at elevated temperatures can, to some degree, be applied to
other furnace types. For example, higher temperatures are sometimes used downstream of
fluidised beds receiving non-hazardous wastes (see Section 4.2.26 and 4.2.27).
Hazardous wastes, both solid and liquid, are fed into the process through the front wall of a
rotary kiln. Only secondary air and waste water are injected into the secondary combustion
chamber.
The incineration temperature rises to 1100 – 1400 °C for a couple of seconds, as all the high
calorific wastes are introduced through the front wall of the kiln. This means that the
temperature of the flue-gases stay over 1050 °C until they pass from the outlet of the secondary
combustion chamber. Next, they enter the waste heat boiler and finally flow through the fluegas cleaning system.
Achieved environmental benefits
High temperature incineration at 1100 – 1400 °C has been reported to provide the following
advantages compared to lower temperature (850 – 900 °C) combustion:
•
•
•
•
all organic materials are completely incinerated. Organic matter content in bottom ash after
incineration is typically less than 1 %
lower contents of hydrocarbons and CO in flue-gases
higher destruction of PCB molecules
a molten bottom ash is formed in the rotary kiln.
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The slag, when quenched in water, becomes finely granulated, vitrified and has low leaching
characteristics. It may be recovered or transported to landfill.
Iron, glass, aluminium and other inorganic solid materials form a molten bottom ash mixture in
the rotary kiln when high temperature incineration is applied. This liquefied ash then flows
slowly towards the outlet of the kiln where it falls down into a bottom ash quenching basin,
positioned under the secondary combustion chamber. Here, the bottom ash is quickly solidified
in the water and it becomes finely granulated and vitrified. Due to the granulation effect, the
total volume of bottom ash produced is smaller than with conventional incineration.
Depending on the leaching tests or other local standards applied, the bottom ash produced in
high temperature incineration may be classified as non hazardous material. This may then
reduce the cost of disposal, since the bottom ash may then be transported to a regular landfill
(under certain conditions), or even sold for utilisation in road construction. If used for
construction, the total content of heavy metals may need to be low, and impact may be judged
by comparison with that of other material used normally for that purpose. [74, TWGComments,
2004]
Cross-media effects
Higher temperatures can result in additional NOX formation and a requirement for additional
control measures.
If the LHV of the waste itself is insufficient, additional fuels may be required.
Adapted gas cleaning may be needed to deal with the higher levels of heavy metals vaporised.
[40, EURITS, 2003]
Operational data
Higher temperatures require a good quality refractory lining. Additionally a constant slag layer
should be managed during the operation.
In addition:
•
•
•
•
•
incineration air may require preheating
the amount of incineration air needs reducing
membrane boiler walls need removal or protection with refractory
high temperature corrosion may be a problem
molten slag may cause operational problems in the furnace and heat exchange areas.
Low residual hydrocarbons and CO is dependent on the combination of flue-gas mixing
(turbulence), residence time and temperature. Temperatures of 900 - 1000 °C have been
reported to achieve very high destruction levels, similar to those achieved at the higher
temperatures considered here. Homogeneous waste feeding is also considered an important
influence. Hence, very high temperatures alone are not a guarantee for high gas burnout (i.e.
low CO and VOC). Also, at higher temperatures the gas velocity is higher, thus the residence
time lower - therefore the overall combination of parameters is important.
Applicability
Mainly applicable to rotary kilns burning hazardous wastes of higher calorific values e.g. those
that include various solvents and waste oils.
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Economics
Additional costs arise from:
• the need to use a water cooled kiln in order to avoid high maintenance costs
• support fuels may be needed to maintain high temperatures
• modifications to furnace may be required to retain heat
• addition of inorganic materials (glass, sand), producing more bottom ash
• scrubbing of heavy metals which evaporate more at higher temperatures.
[74, TWGComments, 2004]
In some cases the use of higher temperatures has been abandoned on account of costs associated
with refractory maintenance.
Driving force for implementation
The technique has been implemented where:
•
•
additional assurance of very high destruction efficiencies are required
vitrified bottom ash residues are required.
Mainly employed at plants where the calorific content of the wastes are sufficiently high
(overall average typically above 15 MJ/kg) to avoid the need for support fuels.
Example plants
Ekokem, Finland. Kommunikemi, Denmark
Reference literature
[20, EKOKEM, 2002] [64, TWGComments, 2003]
4.2.17 Increased agitation and residence time of the waste in the furnace
Description
Waste fed to the furnace needs to be well mixed and allowed sufficient time to react and ensure
that effective burnout is achieved, thereby leaving a residue that is low in organic carbon. In
addition, the supply of adequate and well distributed primary air, which does not result in
excessive cooling, will assist this process.
Longer exposure of the waste to elevated temperatures in the combustion chamber, higher bed
temperatures and physical agitation of the waste all combine to ensure that ashes produced are
low in organic species.
Burnout rates may, therefore, be improved by:
•
•
•
•
•
•
•
•
•
furnaces that turn and agitate the waste effectively
use of rotary kilns
pretreating waste and then using fluidised beds (where the waste is suited to this
technology)
longer residence times in the furnace burnout zones
design of furnace to reflect radiant heat and increase burnout
optimising primary air distribution and supply
addition of other wastes/fuels to support effective combustion
disintegration of bigger pieces of waste
sifting (riddling) return for repeated incineration.
The use of these techniques can result in organic-carbon-in-ash levels of below 1 %.
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The level of burnout achievable using any technique will depend upon the characteristics of the
waste being incinerated. Waste physical characteristics will also have a critical influence on the
practicality of feeding the waste type to the different designs of combustor, e.g. mixed
municipal waste cannot be treated in a fluidised bed without pretreatment.
Typically, better burnout will be achieved where waste is finely divided and has been
homogenised (e.g. by mixing). The pretreatment of highly heterogeneous wastes can improve
burnout.
Achieved environmental benefits
Effective burnout of the waste results in:
•
•
•
effective waste destruction
improved characteristics of the solid residue for possible use
increased extraction of the energetic value from the waste.
Cross-media effects
Excessive physical agitation of the waste can result in higher quantities of unburned material
being carried into the secondary combustion chambers. This can result in additional dust and
other pollutant loading on downstream abatement processes. Additionally, excessive agitation
CAN result in more riddlings, i.e. unburnt material passing through the grate. [74,
TWGComments, 2004]
Operational data
It should be noted that, while some technologies may incorporate greater agitation to result in
better burnout and hence lower unburned matter in the residues produced, they are not generally
chosen specifically for these reasons, but primarily for the their mechanical suitability to the
physical characteristics of the waste received i.e. the waste homogeneity etc. [64,
TWGComments, 2003]
The level reported to be achieved in Austrian incineration plants for a stable process is 1 %
TOC (dry substance) and about 3 % TOC (dry substance) for start and shutdown.
Applicability
The nature of the waste received may restrict the actual combustion technology selection (i.e.
fluidised bed or grate, etc) and hence restrict the operators ability to select between technology
options. However, the principles of increased agitation and of holding the waste in the furnace
for sufficient time at sufficient temperature apply in all cases. Each chosen technology can,
therefore, review the options described here that are available to it, to improve burnout.
Economics
New projects can take account of the need to ensure effective burnout without significant
additional costs.
Major reconstruction of combustion chambers at existing plants is expensive. Retrofit may
therefore, only be possible when complete refit is planned (unless the minimum legal standard
of 3 % TOC is not achieved, in which case action is mandatory).
Driving force for implementation
The main driving forces are:
•
•
•
264
improving waste destruction
improving possibilities for residue use
extraction of full energetic value from the waste.
Waste Incineration
Chapter 4
Article 6 of EC Directive 2000/76/EC requires that slag and bottom ashes have a Total Organic
Carbon (TOC) content of less than 3 % or their loss on ignition is less than 5 % of the dry
weight of the material.
Example plants
Throughout Europe.
Reference literature
[4, IAWG, 1997] [64, TWGComments, 2003]
4.2.18 Adjustment of throughput to maintain good burnout and
combustion conditions
Description
The level of burnout achieved in the bottom ash residues is a parameter to consider when
determining the throughput limitations of the process with a particular waste input.
Levels of TOC above 3 % (5 % LOI) are prohibited by European legislation. Levels of below
1 % are achieved in some circumstances (see Section 4.2.17 above).
For a given range of waste characteristics, the thermal capacity of the combustor is the limiting
factor rather than the mass throughput. Exceeding the thermal capacity of the plant leads to a
deterioration in combustion performance and of the quality of the residues produced.
Achieved environmental benefits
Maintaining the process within the thermal throughput capacity of the process ensures wastes
are properly destroyed and that the residues produced are of better quality, with improved
possibilities of their use.
Cross-media effects
Use of the technique avoids cross-media effects.
Operational data
Burnout levels may be reduced by increasing waste residence time in the furnace. This then
results in a reduction of the throughput rate of the installation.
Applicability
Applicable to all waste incineration plants.
Economics
Restricting the waste throughput can result in lower income from waste disposal.
Driving force for implementation
•
•
ensures full waste destruction
improving ash quality.
Example plants
This is a common practice widely applied in the industry.
Reference literature
Discussions during site visits. [64, TWGComments, 2003]
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Chapter 4
4.2.19 Optimisation of time, temperature, turbulence of gases in the
combustion zone, and oxygen concentrations
Description
To achieve effective combustion of the gases produced during the incineration process, the gas
needs to be well mixed with sufficient oxygen, at a high enough temperature and for a long
enough time. Based upon these principles and practical experience of industrial scale
incineration plants, minimum criteria have been established in various European and National
legislation. The aim of these criteria is to ensure processes are designed and operated in such a
way that they ensure that the gases produced are oxidised and that organic pollutants are
destroyed, so as to reduce polluting emissions of these substances.
Table 4.12 below provides a brief summary of some specifications that have been applied to the
combustion of the gases that are produced during the incineration process:
Parameter
•
Minimum combustion
•
temperature during gas
residence time
Minimum gas
residence time
Turbulence
Oxygen concentration
(excess)
•
Specification
at least 850 °C, or
at least 1100 °C for hazardous wastes •
with greater than 1 % halogenated
organic substances (as Cl)
•
2 seconds after the last injection of
combustion air
•
•
Sufficient to ensure effective gas
mixing and combustion reaction
•
greater than 6 % (note that the precise
•
oxygen requirement was removed in
most recent EU legislation)
Purpose
adequate temperatures to
allow oxidation
sufficient residence time at
high enough temperature in
the presence of sufficient
oxygen to react and oxidise
mixing of gas to allow
reactions to proceed across
the entire gas stream
sufficient oxygen must be
supplied to allow oxidation
Table 4.12: Some combustion specifications applied to incineration
Operational experiences have shown that these criteria are generally appropriate and will
achieve good levels of destruction. However, many existing plants operate with reduced
temperatures, residence times and oxygen concentrations, and still achieve effective combustion
and low emissions to all environmental media. Indeed, at some plants, reductions in NOX
emissions (in particular) have been achieved without significant deterioration of other
performance parameters, or corrosion problems.
It is therefore possible, in some circumstances and for some waste types, that departures from
these basic criteria to represent the optimal environmental outcome. If such departures are to be
permitted the following aspects need to be examined to ensure effective overall performance:
•
•
•
•
•
low and stable CO concentrations in emissions to air (<50 mg/m³ daily average)
burnout of bottom ashes of good quality (TOC <3 %)
benefits outweigh the risks (e.g. NOX reduction achieved is significant)
is the waste (as fed) suitably homogeneous, consistent and quality assured to give
confidence that pollutant destruction will be sufficient across the operational spectrum?
is the level of turbulence in the combustion zone adequate to allow reduced
temperatures/residence time?
Residence time is highly dependent upon furnace size, and therefore, there are few options for
increasing residence time once a plant has been constructed, unless a major rebuild is
conducted, which can result in very significant expenditure. Generally, new plants are designed
to assure residence times of two seconds or more except when specific specialist and highly
controllable and homogeneous waste streams are burned, thereby allowing an increased
certainty of achieving emission levels. [64, TWGComments, 2003]
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Achieved environmental benefits
The potential benefits of reducing the minimum oxygen concentration and the minimum
temperature levels are:
•
•
•
reductions in NOX production and hence treatment requirements and/or emissions
reduced flue-gas volumes, which can result in reduced FGT requirements
improved energy efficiency.
In general, reducing the gas residence time on its own will not result in any specific
environmental benefits but will result from a smaller combustion chamber.
Increasing gas turbulence generally improves mixing and, hence, the oxidation reaction rate that
then leads to effective combustion. However, where turbulence is mainly achieved by secondary
air injection a balance is required because it is also necessary to avoid excessive air supply
which may lead to: excessive cooling or over supply of nitrogen with the air that can increase
NOX production.
Cross-media effects
Reductions in time, temperature, oxygen and turbulence can result in increased emissions of
PICs if conditions are reduced to such a degree that combustion is not completed. These risks
are greater where the wastes (as fed) are highly heterogeneous, of variable composition or
where the waste quality is difficult to assure.
N2O (nitrous oxide) emission concentrations (and hence global warming impacts) are increased
at lower combustion temperatures. CO levels may also be increased at lower temperatures.
The use of higher temperatures than are necessary for the destruction of the type of waste being
incinerated, generally results in only small reduction in the quantities of PICs in the untreated
raw flue-gas – after gas cleaning the relative benefit will be even smaller. On the other hand,
higher temperatures may lead to a significant increase in NOX production. Therefore, unless
some other specific environmental benefit is sought (e.g. improvement in residue quality using
slagging or guaranteed high destruction efficiencies for PCBs) the reduction in some emissions
to air achievable through the use of higher temperatures may be outweighed by the cross-media
effects of additional fuel consumption, NOX production etc. [64, TWGComments, 2003]
EXAMPLE: Bubbling fluidised bed burning sewage sludge:
Table 4.13 below shows the relationship found between nitrous oxide emissions and process
temperatures at a bubbling fluidised bed plant burning sewage sludge. It should be noted that
fluidised bed combustion is very different from grate combustion, and that the nitrous oxide
emissions from grate MSWI with a secondary combustion temperature over 900 ºC are
generally negligible [64, TWGComments, 2003]:
Bed temperature range (°C)
820 – 845
795 – 820
730 – 795
Free board temperature
930 °C
910 °C
890 °C
Nitrous Oxide emission
70
120
200
100
170
270
180
250
350
Data shown are for nitrous oxide concentrations in mg/m³
Table 4.13: Relationship between nitrous oxide emissions and process temperatures for a bubbling
fluidised bed plant burning sewage sludge
[22, Mineur, 2002]
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Chapter 4
Operational data
There are several different methods for the determination of the time and temperature
parameters for a given plant that use different temperature measuring devices located at
different positions in boilers with different heat transfer properties: the measurements are
conducted with the plant under different percentages of load, and at different times with respect
to plant cleaning. These methods have associated levels of accuracy. Despite these uncertainties,
modern plants generally show environmental compliance with the EU Directive 2000/76. [64,
TWGComments, 2003]
Example:
VERA Sewage sludge incinerator, Hamburg, Germany:
Studies were carried out at this fluidised bed plant to demonstrate the impact on emissions of
the use of reduced temperature and oxygen in the combustion phase. For fluidised beds both the
temperature in the bed and the freeboard are important for effective combustion.
Waste type:
Plant size:
Combustor type:
Combustion temp (T min)
Oxygen conc. (min)
Abatement systems:
Sewage sludge and screenings
79000 t/yr
bubbling fluidised bed
810º Celsius (in freeboard of combustor)
4%
ESP/HCl and SO2 scrubbers/fabric filter.
The following data/conclusions were noted from the results at this installation:
•
•
•
•
•
•
•
reducing FB temperature from 924 °C to 810 °C did not significantly change PCDD/F
emissions (reported figures were a change from 0.0005 ng/m³ to 0.0008 ng/m³)
reducing oxygen concentration from 6.8 % to 4 % resulted in a reduction of PCDD/F
(0.0015 to 0.0005 ng/m³)
at freeboard temperatures below 890 °C NOX emissions were between 30 and 40 mg/Nm³
and independent of changes in bed temperature between 730 °C and 845 °C
increasing free board temperatures above 890 °C increased NOX emissions - the effect was
most marked at higher bed temperatures
at freeboard temperatures below 900 ºC, SNCR has little impact on NOX emissions
emissions of N2O are higher at lower freeboard and bed temperatures (see data shown in
cross-media effects above)
emissions of N2O are almost unaffected by the use of SNCR.
It is reported that lowering oxygen levels can result in increased corrosion rates requiring
specific counter measures. [64, TWGComments, 2003]
Applicability
Mainly applicable at the design stage for new plants and for existing plants undergoing or
planning significant retrofitting of the combustion chamber.
Applicable where the available space limits the size of the combustion chamber.
Less applicable where wastes (as fed) are highly heterogeneous, of variable composition or
where waste quality is difficult to assure.
Existing plants may already have residence times that are below two seconds. These may be
able to justify not carrying out extensive (and expensive) retrofitting by gathering and
comparing real performance data concerning the levels of PICs in the raw gas.
Economics
The estimated cost relevant impacts of changing these combustion parameters, compared to the
normal design values, are indicated in the table below:
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Increase or Decrease in
Estimated cost impact
parameter
Additional cost of support fuels increased furnace
maintenance costs may reduce ash disposal costs
Increase
Combustion
of slagged ash if product saleable
temperature
Reduced need for support fuels may restrict waste
Decrease
types that can be burned
Larger furnace = higher cost may increase range
Increase
of wastes accepted changing design of combustor
expensive
Gas residence time
Smaller process means lower costs may restrict
Decrease
wastes that can be burned
Changing design of combustor expensive
increasing air injection increases flue-gas
Increase
Turbulence
volumes and FGT costs
Decrease
Not technically advisable/desirable
Higher oxygen excess may allow wider range of
Increase
waste to be incinerated without emission
Oxygen concentration
problems
(excess)
Reduced flue-gas volumes mean lower FGT costs
Decrease
may restrict wastes that can be burned
Parameter
Table 4.14: Estimated cost impacts of some alterations to combustion parameters
In general the most significant cost issue is for existing plants where the rebuild/retrofit costs of
upgrading an existing plant (that is already achieving effective emissions performance) to meet
the traditional temperature and residence time requirements, will be very large.
Driving force for implementation
Reductions in NOX production and, hence, a reduction in the measures required to treat fluegases may be achieved. With very consistent wastes it may be possible to reach NOX levels
compliant with EC Directive requirements without, or with very minimal need for specific NOX
controls (e.g. SNCR or SCR).
Lower gas residence times, and flue-gas volumes (reduced by reducing the air supply) mean that
smaller combustion chambers and flue-gas treatment plants can be used, resulting in cost
reductions.
Example plants
VERA, Sewage sludge incinerator, Hamburg, Germany
Reference literature
[22, Mineur, 2002]. Information supplied by UBA during site visit to VERA. [64,
TWGComments, 2003]
4.2.20 Use of automatically operated auxiliary burners
Description
During start-up, auxiliary burners are used to create a zone above the required minimum
temperature, through which the flue-gases are fed from the furnace zone to the secondary
incineration. This is the predominant operational condition for the burner design.
In order to assure a sufficient temperature under extreme conditions, auxiliary burners are
installed. These are used when the temperature falls under the required minimum temperature.
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When the plant is shutdown, the auxiliary burners are used when the temperature has fallen
below the design limit of the furnace and the temperature sinks below the required minimum
temperature. They operate until there is no waste in the incineration chamber.
Achieved environmental benefits
Ensuring that incineration temperatures are adequate by the use of automatically operated
burners ensures that the gases produced are properly combusted, reducing raw gas
concentrations of PICs at the furnace outlet and hence emissions to all media.
Cross-media effects
Consumption of fuels (usually light oils or natural gas) by the burners.
Auxiliary burners must be optimized for low CO emissions, otherwise high emissions during
start-up and shut-down are possible.
Operational data
Start-up without auxiliary burners is possible, but smoother starting with reduced soot and better
control of temperature is attained by starting with the burners. Starting without auxiliary burners
in normal MSWI plants can increase corrosion risks due to the chlorine content of the waste.
[64, TWGComments, 2003]
Applicability
Applicable to all waste incinerators, particularly those receiving wastes of low LHV and/or
wastes of inconsistent quality.
Economics
Retrofit costs may be significant owing to difficulties in positioning the burners.
Costs will be significantly higher at processes that operate on a batch basis, although this may
be considered to be commensurate with the additional risk of releases that accompanies such
operational modes.
Driving force for implementation
Ensuring emissions are reduced to all media and that start-up and shutdown operations are
controlled and do not give rise to additional pollutant emissions other than those arising from
the burning of gasoil and other fuels.
Legally required by Directive 2000/76/EC, although under certain circumstances this legislation
allows for exemptions from the use of a start up burner (2000/76/EC article 6 paragraph 4).
Example plants
Widely used in modern plants throughout Europe.
Reference literature
[1, UBA, 2001] [64, TWGComments, 2003]
4.2.21 Reduction of grate riddling rate and/or return of cooled riddlings to
the combustion chamber
Description
In grate incinerators, some of the waste may fall through the grate and be partially combusted.
These are called riddlings. Attention to grate design, particularly the reduction of spacings
between grates, can reduce the effect. The quantity and quality of these riddlings depend on the
design of the grate, on the interfaces between the moving pieces together and with the walls and
on the mechanism for keeping them tight. [74, TWGComments, 2004]
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In order to reduce the passage of riddlings through the grate, the following options are available:
•
•
design and maintain grates such that bar spacings and other factors that may increase
riddling passage are reduced
where riddling rate and type is such that gives rise to ash quality concerns – re-burn in the
combustion chamber.
Automatic conveyor systems are used to collect these riddlings. The collected material is
usually stored for cooling before being re-introduced to the bunker (to avoid fire risk). A
proportion of the re-introduced riddlings pass down the grate and are discharged with the
bottom ash. They will then have been subjected to the full incineration process and, therefore,
be more sterile in nature.
This may be a particular issue where clinical or other wastes with infectivity risks, particularly
those that are finely divided in nature, or which contain discreet objects that may fall through
the grate (e.g. hypodermic needles) are co-combusted with other wastes.
Riddlings that arise earlier in the passage of the waste through the combustion chamber have a
higher risk of retaining infectivity, or having poor burnout and should, therefore, be most
closely examined. Riddlings arising at later stages may be well treated and, therefore, less likely
to require re-burning.
Achieved environmental benefits
The main benefits are:
•
•
improved burnout of the waste
improved ash quality.
The riddling of molten, burning drops of some common plastics (found in MSW) like PE and
PET can contribute significantly to total carbon content in ash, to higher COD (Chemical
Oxygen Demand) of the bottom ash and to the significant leaching of copper. All of these
parameters are improved by the use of this technique. [64, TWGComments, 2003]
Cross-media effects
Risk of fire when riddlings are in contact with waste.
Operational data
Storage of the riddlings prior to their re-introduction with the main waste feed is very important
to prevent fire risks. Water may be added to ensure that cooling is thorough.
Regular observation to avoid clogging in the collecting areas of riddlings under the grates
necessary combined with safety risks to operators and personal [74, TWGComments, 2004]
In France, the regulations set a burnout requirement for bottom ash in MSWIs that incinerate
clinical waste of 3 % LOI, which is normally achieved without repeated incineration of
riddlings. [74, TWGComments, 2004]
Applicability
Applicable, in principle, to all grate incinerators, but particularly applicable to those grate
incinerators where:
•
•
•
particular concerns or requirements exist that require improved burnout
clinical or other infectious wastes are co-combusted that can go through the grate
grates with larger spacing between grate bars and high riddlings or in other systems where
riddling levels may be relatively higher.
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Economics
Improving ash burnout quality may reduce disposal costs where the current ash burnout does
not meet re-use/disposal requirements without this technique. [74, TWGComments, 2004]
There can be significant investment costs for retrofitting existing plants and extra operational
(handling) costs. [74, TWGComments, 2004]
Driving force for implementation
• improved and thorough waste destruction.
• improved bottom ash quality.
The technique is applied at grate incinerators in the Flanders Region of Belgium for the purpose
of ensuring burnout and improved ash quality.
Example plants
Indaver GF, Beveren, Belgium.
Reference literature
[64, TWGComments, 2003]
4.2.22 Protection of furnace membrane walls and boiler first pass with
refractory or other materials
Description
[2, infomil, 2002] The furnace is formed by membrane walls (also called “water walls”),
consisting of rows of vertical tubes, connected by strips, welded together in order to form a
closed (membrane) wall, which are part of the boiler's evaporation section. In the lower section
of the furnace especially, the membrane walls must be protected against the corrosive and
abrasive effect of the flue-gases, which are not yet fully incinerated at that position. For this
purpose, the furnace walls of the lower section are covered with a layer of ceramic refractory
material or other protective materials. An additional advantage of this wall protection for lower
calorific value wastes, is the reduction of the heat transfer to the boiler, which is beneficial
where temperatures need to be maintained.
In most modern waste incineration plants, the whole boiler, except the economiser, is made of
water tube walls. In steam boilers, these tubes are a part of the vaporiser.
When there are no water walls, cooling is sometimes made by air circulating behind the
refractory plates, often the warmed air is then used for primary air heating.
The role of the refractory is to reduce the heat transfer and to protect the tubes from excessive
heat and corrosion. It is installed inside the furnace/boiler; on the outside of the furnace/boiler
there is thermal insulation material. [74, TWGComments, 2004]
With high LCV waste, combustion temperatures are normally reached without problems. The
main purposes of the ceramic refractory layer are, therefore, the protection of the furnace walls
against high temperature corrosion and to prevent the temperature from decreasing too quickly.
With higher steam conditions and a higher LCV waste, a larger part of the furnace walls needs
to be covered with ceramic refractory material. An alternative solution is the use of specific
anti-corrosive alloy claddings, such as Nickel/Chromium, on boiler tubes for protecting the
tubes from corrosion – it has a higher heat exchange coefficient (see Section 4.3.8) than the
ceramic refractory alternatives.
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Techniques have been developed to prevent the formation of solidified ash deposits in the
furnace. These include:
•
•
cooling of the furnace side walls - which lowers the ash temperature, causing the ash to
solidify before it reaches the furnace wall
designing the furnace with a lower specific heat load (larger dimensions for the same
thermal capacity).
Achieved environmental benefits
The greater plant availability means that the additional risks of emissions associated with start
up and shutdown are reduced.
Retaining heat generated during combustion in the furnace will reduce the need to add support
fuels to lower LCV wastes.
The high heat capacity of the refractory lined furnace helps to reduce temperature fluctuations
that may arise with wastes of variable LCV, thus promoting more stable incineration and
reducing emissions from the combustion stage.
The use of water and air-cooled walls allows:
•
•
•
the heat to be recovered
gas temperature reduction, which reduces the stickiness of fly ashes and, hence, improves
boiler availability and heat transfer efficiency
reductions to be made in secondary air addition (where this has been added for gas cooling)
leading to lower flue-gas volumes, reduced flue-gas treatment equipment capacity
requirements, and possibly lower NOX in the raw gas.
Similar effects can be obtained with air-cooled refractory.
Cross-media effects
Where LCV of the waste is sufficient to maintain the combustion temperature, providing
refractory lining over too wide an area can reduce heat transfer to the boiler and hence reduce
energy recovery performance.
If support fuels are necessary because the water walls extract too much heat, and this results in
combustion temperatures dropping below those required for effective combustion performance,
then refractory walls should be used.
Operational data
Improved plant availability through reduced corrosion and bottom ash build-up.
With lower CV waste, increasing the refractory covering assists with maintaining combustion
temperature.
Applicability
This technique is mainly applied to municipal grate incinerators [64, TWGComments, 2003]
Increasing protection of the boiler tubes by using refractory lining close to the primary
combustion zone is mainly applicable where:
•
•
•
low calorific wastes require additional heat retention in the primary combustion zones
high chlorine loading give rise to additional corrosion concerns
high temperature combustion gives concerns regarding corrosion.
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Economics
Improved plant availability results in reduced operational costs.
Changing the configuration of the furnace at existing plants may not be practicable due to high
costs.
Driving force for implementation
Operational and environmental benefits. In particular, a better incineration process, less furnace
corrosion, less maintenance, longer boiler lifetime.
Example plants
Widely used at MSWIs throughout Europe.
Reference literature
[2, infomil, 2002, 64, TWGComments, 2003] [28, FEAD, 2002]
4.2.23 Use of low gas velocities in the furnace and the inclusion of empty
passes before the boiler convection section
Description
The furnaces of waste incinerators are normally designed large enough to provide low gas
velocities and long gas residence times. This allows combustion gases to be fully burned out,
and prevents boiler tube fouling by:
•
•
reducing the fly ash content of the flue-gases
allowing the temperature of the flue-gases to be reduced before coming into contact with the
heat-exchange bundles.
Heat-exchanger fouling may also be reduced by including empty passes (e.g. water walls
without obstructions in the gas path) between the main furnace area and the heat-exchange
bundles to allow gas temperature, and hence fly ash stickiness, to be reduced. Temperatures of
below 650 °C before the convective superheater are used for the reduction of adhesion of ash to
the boiler tubes and thus prevent corrosion. [2, infomil, 2002, 64, TWGComments, 2003]
Achieved environmental benefits
Reduced emissions of organic substances from the combustion stage.
Improved heat exchange in boilers owing to a reduction of deposits on boiler tubes. This can
result in improved energy recovery.
Cross-media effects
No specific effects identified, but care may be required to ensure secondary air or other
mechanisms used to support gas mixing are adequate with larger furnaces.
Operational data
Reductions in boiler tube deposits results in improved process availability and better heat
exchange.
Applicability
Mainly applicable to the design of new plants and where substantial re-fits of existing furnaces
and boilers are being carried out.
Applicable to nearly all kind of incinerators.
Economics
Larger furnaces are more expensive to construct.
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Driving force for implementation
Operational benefits from better incineration process, reduced erosion, fly ash and maintenance,
and a longer boiler lifetime.
Example plants
Commonly applied to MSWI in Europe.
Reference literature
[2, infomil, 2002], [64, TWGComments, 2003]
4.2.24 Determination of calorific value of the waste and its use as a
combustion control parameter
Description
Fluctuations in the characteristics of the waste are more pronounced with some types of wastes
(e.g. untreated MSW, some hazardous wastes) meaning that mass and volume are less reliable
control parameters. The determination of NCV on line (e.g. as described in 3.5.2) may then help
to optimise process conditions and improve combustion with such heterogeneous inputs.
Techniques have been developed based upon:
•
•
•
•
retrospective analysis of process performance parameters (not a predictive method)
mass balance calculations based on CO2, O2 and H20 concentrations in the flue-gas (i.e. not
a predictive method as based on downstream measurement) and on waste input (averaged
values using e.g. a “crane” scale) or on other smaller heat flows estimated
use of microwave equipment to assess waste moisture content in the feed chute
waste colour and dimensions linked to plant output signals by fuzzy logic.
Achieved environmental benefits
Improved combustion control results in reduced emissions from the combustion stage.
Cross-media effects
None identified.
Operational data
Knowledge of the calorific value of the fuel input is useful in that it allows optimisation of the
air supply and other critical parameters that control combustion efficiency. With heterogeneous
fuels, the mass and volume input rate of the fuel can be used as an additional control parameter.
Control of quality of the waste may be part of the delivery contract.
Applicability
This technique may be used as a diagnostic tool or (with on-line methods) for process control
e.g. MSWI burning heterogeneous waste.
Economics
No data supplied.
Driving force for implementation
Improving the combustion and energy recovery performance of MSWI’s.
Example plants
On-line techniques have been used in municipal plants in the Netherlands.
Reference literature
[23, VanKessel, 2002] [64, TWGComments, 2003]
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4.2.25 Low-NOX burners for liquid wastes
Low-NOX burners provide a means of introducing and burning liquid wastes into the
combustion chamber in such a way that nozzle head temperatures are reduced and therefore the
production of thermal NOX is reduced. Descriptions of low-NOX burners used for conventional
fuels are given in the LCP BREF (note: these may require specific modifications for use with
wastes).
There are relatively few examples where low-NOX burners are successfully used with wastes.
Particular attention is required to ensure adequate combustion efficiency (with waste). The
technique is only applicable to specific liquid waste streams. It may be suited to some liquid
hazardous wastes.
The installation of Low-NOX burners during plant construction helps reduce the production of
NOX while adding little to the capital cost. The cost of retrofitting such burners to existing
plants, however, can be high.
[64, TWGComments, 2003]
4.2.26 Fluidised bed gasification
Description
This technique involves the use of a fluidised bed (see 2.3.3) under gasifying conditions (see
2.3.4).
In the fluidised bed, an inert material, like sand, is heated to reaction temperature and kept in a
fluidised state by an up streaming gas, i.e. in general air or steam. In a circulating fluidised bed,
the bed material is carried out of the reactor due to high gas velocity, collected in a cyclone and
returned. In a bubbling fluidised bed the gas velocity is lower, so that the bed material moves
only within the reactor. The basic operating principal of an internally circulating/bubbling
fluidised bed is shown in Figure 2.25 and a more detailed description in section.
The gasification process may be carried out at high or low temperatures. In low temperature
gasification, the waste is heated in the FB to a reaction temperature of usually between 500 and
600 ºC and in high temperature systems to 800 to 1000 ºC. They are supplied with substoichiometric oxygen levels, to be gasified. The synthesis gas produced in the gasification stage
is usually then passed to a combustion stage, but in some circumstances may be purified for use
as a chemical feedstock.
In some cases the syngas is cleaned before its combustion, in others not. This depends on the
waste content and the combustion/generation options chosen. However most waste materials
contain chlorine and other contaminants, which have to be removed from the gas prior to
combustion. Syngas cleaning systems based on wet scrubbers and bag filters have been
designed.
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Achieved environmental benefits
Compared to incineration, gasification has the following features:
•
•
•
•
•
•
at the lower reaction temperature, metals (e.g. steel, copper, aluminium or brass) contained
in the waste are not sintered, oxidised or molten, so when separated from the ash/bed
material they may have higher value for recycling
a gas with heating value and chemical potential is generated, which may be used for various
recycling or recovery options
the combustible gas may itself be combusted and the energy generated used for energy
recovery
combination of the gas in special combustion conditions has been used to melt ashes
(vitrification), generating a low leaching product (see 4.2.27)
the separation of the gasification and combustion stages may allow improved process
control and result in a more stable overall combustion process
lower flue-gas volumes and hence improved FGT can result from the use of lower air ratios.
Cross–media effects
Compared with grate systems, waste particles are limited to 300 mm, which normally requires
some energy for shredding and can affect the availability of the plant. While fluidised beds
generate less bottom ash with less fine particle content, more fly ash is generated which requires
special attention if not vitrified.
Tars and other pollutants removed at any syngas cleaning stages require management. Wet
scrubbing systems result in an effluent which when treated yields a tarry residue that then
requires disposal. Dry syngas treatment will also result in an accumulation of the removed
impurities. In some cases the tarry residue, which has a calorific value, has been supplied to
other industries as a waste derived fuel (information concerning the acceptance criteria/emission
controls applied at these external industries was not supplied).
Operational data
Where shredders are required to prepare the waste these may experience some difficulties with
blockages etc. See comments in Section 4.1.5.2.
Generally, fluidised beds achieve good mixing of the waste and a very homogeneous
temperature distribution in the reactor. Due to the very turbulent reaction zone and the large heat
content of the bed material, fluidised beds have low sensitivity to changes in the physical and
thermal properties of the waste.
Bubbling fluidised beds offer the advantage of a large reaction volume without moving parts, so
comparatively large waste pieces (up to 30 cm in diameter) can be gasified.
The following additional operational comments were made by the BREF TWG:
•
•
•
if metal pieces are in the waste they can throw it out of balance and can accumulate –
resulting in operational difficulties
difficulties may be encountered finding a market for the syngas produced, limiting the
options for chemical recovery
the market may not exist or be any better for the improved ash produced (ash quality
improvements may arise from the homogenisation of the waste at the shredding stage and/or
the coupling of the techniques with a high temperature post-gasification combustion stage).
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Applicability
The main restriction on the use of FB Gasification arises from the need for the waste to be
suitable for feeding to a fluidised bed, or readily treatable so that it may be fed to the FB.
Wastes which have been fed to such systems include: pretreated (shredded) MSW, shredder
residues, waste plastics and other industrial wastes of a suitable size or where they can be
shredded to a suitable size. Oils and other liquids, bone meal, sludges and slags are suitable
wastes in mixtures with others.
In Europe, although applied to some selected waste streams, in practice the technique has (at the
time of writing) yet to be demonstrated on a full operational scale on many waste streams.
Economics
Investment costs are reported to be higher than for conventional grate incineration e.g. JPY 20
billion (approx. EUR 160 million) in 2002 for a plant treating 120000 t/yr of MSW. Cost
includes construction and plant costs of an entire plant including: six waste shredder lines, fly
ash melting (see 4.2.27), flue-gas treatment, and adjacent integrated offices, swimming pool and
spa complex. Cost excludes land purchase.
Operated with air, at atmospheric pressure and moderate temperatures, fluidised bed gasifiers
are more economic in construction and operation than other gasification technologies applied
for waste.
Revenue incomes are to be expected for materials diverted for recycling. Metals recovered from
the gasifier will be disinfected and not require cleaning (cf. removal from the shredder stage)
and may be sold at higher value than that recovered from grate incineration ash owing to a
reduced level of fusion and oxidation (where they are removed at a lower temperature). Markets
for the use of vitrified ashes may be better than for grate bottom ashes due to the improved
properties of the material produced – this is the case in Japan but is often not the case in Europe.
Electrical revenues may be reduced due to higher in-process consumption where oxygen
generation is used i.e. where the technique is coupled with oxygen enrichment.
Difficulties have been encountered with the shredding stages required to prepare municipal
wastes for fluidised beds. The provision of multiple shredding lines can help to reduce the risk
of expensive loss of availability, but in turn the provision of such additional equipment adds
significantly to costs.
Driving forces for implementation
The following factors promote the implementation of this technique:
•
•
•
•
•
waste within the size range required for the FB or waste which can relatively easily be
treated to meet the specification required
waste has a high calorific value
requirements for very low leaching residues (such residues are produced where this
technique is coupled with a secondary ash melting stage - see Section 4.2.27) either through
availability of recycling markets or restrictions on disposal sites
the higher recycling value of the metals removed from the gasifier can be realised
a favourable market exists for the use of the syngas as a chemical feedstock.
Example plants
There are several examples in Japan for MSW, sewage sludge, shredder residues, plastic wastes
and selected industrial wastes.
There is an operational example in Europe (Spain) treating selected commercial and industrial
plastic wastes. This installation uses fluidised bed gasification coupled with syngas cleaning
prior to its combustion of electricity generation in gas engines. Tars and other pollutants are
removed to a cleaning stage by the syngas cleaning.
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It is reported that some experiences in Germany (KWU- Schwelbrennverfahren, Thermoselect,
Noell -Konversionsverfahren) of the use of pyrolysis/gasification have had difficulties in
reaching their design throughput capacity, and that consequent real treatment costs have
escalated. It is also reported that this has in some cases lead to the closure of installations.
[74, TWGComments, 2004]
A 50 – 60 MW circulating fluidised bed gasifier has been successfully operated since 1998 on
waste derived fuels in Lahti, Finland. The gasification gas is co-combusted in a 350 MW hard
coal combined heat and power plant replacing about 15% of the primary fossil fuel. A 50 MW
bubbling fluidised bed gasifier for reject plastic material containing 10 – 15% aluminium foil
has been operating at Varkaus cardboard mill in Finland since 2001. Part of the plastic waste is
a reject from a pulping process recycling fibres of used liquid packaging. Metallic aluminium is
recovered from the product gas for material recycling. The gas is fired in a gas fired boiler
producing process heat and steam for energy production.
Reference literature
[64, TWGComments, 2003]
4.2.27 High temperature combustion of gasification syngas with ash
melting
Description
This technique is an extension of fluidised bed gasification. Syngas from a gasification process
and carbon-rich fly ash particles are combusted in a down flow melting chamber. Air and/or
oxygen enriched air is introduced such, that a cyclonic flow is induced, which drives ash
particles to the wall.
Figure 4.2: Basic components of a cyclonic high temperature syngas ash melting furnace
Source [68, Ebara, 2003]
The temperature is set to that at which the ash is vitrified (about 1400 °C) and will proceed
towards the slag outlet. Due to the high temperature level, halogens and volatile metals, like
lead and zinc, are evaporated.
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The walls of the ash melting chamber are refractory lined and cooled by evaporator tubes of the
boiler. In this way, the refractory is covered by a protective layer of slag, and the heat recovery
is achieved.
Achieved environmental benefits
The following benefits are reported:
•
•
•
•
very efficient destruction of organic species at the high temperatures used
vitrification of the fly ash generates a stable low leaching granulate with additional (cf. grate
ash) recycling possibilities
quantities of fly ash passed to the FGT residues are reduced and hence volumes of FGT
residues themselves are reduced
lower contamination of FGT residues with fly ash may enhance FGT treatment options.
Compared with ash melting in a separate, off-line process, integrated ash melting is far more
energy efficient, as it is an integrated part of the primary thermal process, where the high
temperature required for vitrification is still available for steam production.
Cross–media effects
Vitrification requires high temperatures and, hence, sufficient energy to generate these
temperatures. With lower calorific value fuels the temperature may be maintained by adding
additional fuels (e.g. natural gas or fuel oils) and/or by increasing oxygen supply to the
combustion chamber. The use of an oxygen generator adds an electrical requirement of
approximately 0.5 – 1 MW depending on oxygen requirements.
Operational data
Plants using this technology have been in full commercial operation since 2000 in Japan.
Cooling of the refractory walls has proven to enable long refractory lifetimes. From the current
level of experience, two annual inspections of the refractory are sufficient.
Some fly ash is re-evaporated and still needs special attention.
The flue-gas may be highly corrosive, and flue-gas treatment is required to remove those
pollutants. Energy recovery may be complicated by this corrosivity. [74, TWGComments,
2004]
Applicability
The technique is applicable for gasification plants combined with syngas production when a
vitrified fly ash is desired. The restrictions on the applicability of this technique to the various
waste types are therefore the same as those to which this technique is connected i.e. where FBs
are used many wastes require preparation before they can be treated.
Although applied in Japan, in practice the technique has yet to be demonstrated on a full
operational scale in Europe.
Economics
Ash vitrification, even when process-integrated, increases waste treatment costs due to
additional investment and operation effort. The resulting overall economics depends highly on
the savings resulting from recycling of vitrified ash.
The product is used as a construction material and will most often not generate a higher revenue
than mechanically treated bottom ash. [39, Vrancken, 2001]
Driving forces for implementation
The following factors promote the use of ash vitrification using this technique:
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•
•
•
high costs for fly ash/FGT residue treatment/disposal
high costs for treatment/disposal of bottom ashes
requirements for very low leaching residues before re-use permitted.
Example plants
Examples operational in Japan.
It is reported that some experiences in Germany (KWU- Schwelbrennverfahren, Thermoselect,
Noell -Konversionsverfahren) of the use of pyrolysis/gasification have had difficulties in
reaching their design throughput capacity, and that consequent real treatment costs have
escalated. [74, TWGComments, 2004] It is also reported that this has in some cases lead to the
closure of installations.
Reference literature
[64, TWGComments, 2003]
4.3 Energy recovery
4.3.1 Optimisation of overall energy efficiency and energy recovery
Description
Incineration plants can liberate the energy value of waste and can supply electricity, steam and
hot water. Where a plant is located so that the supply and use of these outputs can be
maximised, this will allow better use of the energy value of the waste.
Energy inputs to incineration plants are mainly from the calorific content of the waste, but may
also come from additional fuels added to support the combustion process, and also from
imported power (electricity).
The plant itself can use some of the energy generated. The difference between the inputs and the
(usable) outputs are the losses. A part of this (usable) recovered energy may be valorised (used).
In general, the whole of the electricity generated is valorised but in respect of steam or hot
water, it depends on the needs of the user; the most favourable situations being when the users
needs are greater all year long than the incineration plant output (e.g. industrial steam use or
large district heating network). [74, TWGComments, 2004]
Optimising the efficiency of the plant consists in optimising the whole process. This includes
reducing the losses and limiting the process consumption. Increasing energy output does not,
therefore, solely equate to the energy conversion efficiency, as it is also necessary to consider
the energy required by the process itself and the possibilities of valorisation.
The optimal energy efficiency technique depends, to an extent, on the particular location and on
operational factors. Examples of factors that need to be taken into account when determining the
optimal energy efficiency are:
•
•
•
•
location. Is there a user/distribution network for the energy or can one be provided?
demand for the energy recovered. There is little point in recovering energy that will not be
used. This is a particular issue with heat but generally less of an issue with electricity
variability of demand. E.g. summer/winter heat requirements will vary. Plants that export
steam as a base load can achieve higher annual supply and hence export more of the
recovered heat than those with variable output options, who will need to cool away some
heat during low demand periods
climate. In general heat will be of greater value in colder climates (although the use of heat
to drive chilling units can provide options where there is a demand for cooling/air
conditioning)
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•
•
•
•
•
reliability of fuel/power supply. Isolated plants may experience unreliable waste deliveries
(related to storage times and capacities) or even electrical supply interruptions that can
result in shut downs or greater dependence on the use of self generated power
local market price for the heat and power produced. Low heat price will result in a shift to
electricity generation and vice versa
waste composition. Higher concentrations of corrosive substances (e.g. chlorides) can result
in an increased corrosion risks, thus limiting steam parameters (and hence possibilities for
electricity generation) if process availability is to be maintained. The composition can also
change depending on the season, including for example holiday seasons which cause
population changes in some areas
waste variability. Rapid and extensive fluctuations in composition can give rise to fouling
and corrosion problems that limit steam pressures and hence electricity generation.
Variation in waste composition (itself influenced by many factors) over the plant life
favours the use of wide spectrum rather than finely optimised design
high electrical efficiency conversion plants may be attractive when electrical power prices
are high, however, frequently more sophisticated technology has to be used with a possible
negative effect on availability.
[74, TWGComments, 2004]
[28, FEAD, 2002] The optimisation of energy recovery techniques requires the incineration
installation to be designed to meet the demand requirements of the energy user. Plants that can
supply electricity only will be designed differently to plants than have the opportunity to supply
heat or those plants that supply combined heat and power.
Recovery of only heat, as hot water or steam:
The energy recovered can be supplied as hot water or steam (at various pressures). The possible
consumers are:
•
•
district heating (DH) and cooling (much less common) networks
industries, e.g. chemical plant, power plant, desalination plant.1
Most of these users require steam, except DH, which can use either steam or hot water. If no
steam user is connected to the DH network, hot water can be used as the energy conveyor. With
water, the pipes have wider diameters and pumping requires more energy, but the DH operation
and the safety measures are easier.
Most modern DH networks use hot water2. In this case, the incineration plant boiler can either
produce steam or hot water. If the plant also generates electricity (CHP see below), it will be
steam. If not, hot water is generally preferred.
Hot water will often be superheated3, pressurised and at higher temperature (e.g. 200 °C), in
order to facilitate the heat transfer in the exchangers by increasing the heat differential between
the heating and cooling mediums.
When the DH uses steam, the steam parameters (pressure and temperature) at the boiler outlet
need to be above the highest level required for the DH. In the case of plants supplying heat only,
the steam pressure is usually 2 or 3 bar above the DH pressure and the steam is superheated by 2
bar or 30 °C.
1)
2)
3)
282
There is one example in Europe (AVR Rotterdam) of coupling between MSWI and desalination plants. This is interesting
since thermal desalination plants need steam at low or medium pressure (3 bars in multi-flash, 20 bars in thermocompression) and usually operate all year.
In some cases like in Basle (Switzerland) there are two networks, one DH network with hot water and another network
supplying steam.
Superheated water is water at a temperature higher than 100 °C.
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Electricity only:
Two factors contribute to increasing the TG (Turbo-Generator) electricity output quantity:
1. the high enthalpy of the steam, i.e. its high pressure and high temperature
2. the low enthalpy at turbine outlet, which results from a low temperature of condensation of
the steam.
This latter temperature depends on the temperature of the cold source (air or water) and from the
difference between this and the turbine outlet temperature.
In addition to electricity, hot water can be produced by the use of a heat exchanger after the
turbine. [74, TWGComments, 2004]
CHP (Combined Heat and Power):
The CHP situation reduces the loss of the energy when there is a low heat demand e.g. in
summertime for DH. The electricity is generated in the best conditions if the heat bleed is well
located in the thermal cycle i.e. low pressure steam is used for heat supply, leaving higher
pressures for electricity generation.
With CHP, when the heat demand is high, the pressure at the outlet of the low pressure part of
the turbine is constant. The pressure depends only of the temperature of the cold DH water (or
other return) and variation of the heat demand can be balanced by an accumulator tank or air
cooler. [64, TWGComments, 2003]
When the heat demand is not high the low pressure section of the turbine must be very flexible
because the steam flow inside will vary according to the heat demand. For example:
•
•
from the maximum value (100 % electricity production, no heat demand) when 100 % of
the steam flow is passed to the low pressure section of the turbine
to the minimum value: maximum heat demand, when only a minimal steam flow is required
for turbine protection.
[74, TWGComments, 2004]
In the case of a CHP, the pressure and temperature of the steam sent to the heat consumer is
determined by the location of the bleed on the turbine4. The bleed is located in such a way that
the difference between the saturation temperature at this bleed and the temperature at the DH
head will be around 10 °C. This bleed supplying the DH is usually controlled. This means that
the pressure is maintained as a constant.
Priority is usually given to the heat supply but it can also be to the electricity supply. This often
depends on the sale contracts.
Achieved environmental benefits
Increasing the recovery and effective supply/use of the energy value of the waste replaces the
need for the external generation of this energy, resulting both in a saving of the resources and in
avoiding the emissions and consumptions of the avoided external energy generation plant. The
amount of energy that is available to be recovered from the waste depends on the calorific value
of the waste.
[74, TWGComments, 2004]
[30, UBA, 2002] In general, about 0.3 to 0.7 MWh of electricity can be generated in a MSW
incineration plant from one tonne of MSW, depending on steam quantity as a function of the
waste LCV, the plant size, steam parameters and steam utilisation efficiency.
4)
In the case of back pressure turbines (without low pressure section), the steam pressure and temperature are those at the outlet
of the turbine.
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For the co-generation of electricity and heat, an additional 1.25 to 1.5 MWh (full-load hours), of
heat per tonne of waste can be supplied, depending on the LCV of the waste, but the electricity
generation may be reduced. Site-dependent heat supply opportunities are very significant in
such cases. These include:
•
•
geographical location
normal (district) heat utilisation periods (e.g., in Germany, this is only 1300 1500 hrs/yr of
a possible 8760 hrs/yr but in Denmark it is between 4000 - 8760 hrs/yr of a possible 8760
hrs/yr where the large size of the DH networks enables year round supply of the whole heat
production).
Concerning heat, under favourable conditions, energy supplied after the boiler (as hot water or
steam) can be increased to about 90 % of the total energy input to the boiler (not including
internal consumption requirements) for an incineration plant operated at base load. Where high
conversion efficiency and base load demand are available (i.e. special arrangements made to
create such circumstances) with specially prepared high calorific value wastes (in excess of 20
MJ/kg) it is possible to recover a total of 4 – 5.5 MWh of heat per tonne of waste [45, FEAD,
2002]. Such outputs are not available with lower calorific value wastes e.g. untreated MSW –
which is generally of LCV between 8 and 12 MJ/kg.
Maximisation of the benefit available from the energy value of the waste is often most likely to
be achieved in situations where CHP is used, as this can allow exergy maximisation. This is
because it allows the higher pressure steam to be used for electrical generation while the
remaining steam energy (lower pressures) can still be supplied and used as heat. In individual
circumstances where CHP is not be possible, other options may give the optimal solution. [64,
TWGComments, 2003]
Cross-media effects
The recovery of energy should not impede the safe and effective destruction of the waste. For
example, high steam conditions may compromise plant availability if all precautions are not
taken. Higher energy efficiency leads to higher investment and maintenance costs and may also
lead to lower availability. [74, TWGComments, 2004]
Particular attention must be given to boiler design in the temperature range 450 – 200 °C to
ensure dioxin reformation is minimised. e.g. prevention of dust retention in such zones (see
Section 4.3.19).
Certain pollution control equipment and techniques have a high energy demand, and whilst
removing some FGT components may give rise to improved possibilities for energy recovery
(through reductions in in-process energy consumption) this can also result in unacceptably high
emissions – a balance is therefore required. Some examples of significant energy demand
techniques include:
•
•
•
•
•
bag filters
- reduce dust (and other) emissions, multiple filters in series
further increase the demand of energy
SCR
- reduces NOX and gaseous PCDD/F emissions but, as a tail end
FGT system, SCR requires energy for flue-gas heating
HCl or NaCl regeneration - external evaporation of effluent
plume reheat
- reduces plume visibility
ash melting
- improve ash quality.
In Austria the energy demand of waste incineration plant itself (also known as the “parasitic
load”) is in the range of 2 – 3 % of the thermal output (for grate firing or rotary kilns) and 3 –
4 % in the case of FBR. [74, TWGComments, 2004]
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Operational data
The following data summarises the results of surveys carried out by the TWG energy sub-group.
The data given shows the range and average values for heat and electricity production and
demand per tonne of waste treated. They are annual averages for installations in middle Europe
during 2001:
Parameter
Electricity
Heat
Installation energy
demand
Production
Exported
Production
Exported
Total
Electrical
Values in MWh/t of MSW
treated
Range
Range
0.415 – 0.644
0.546
0.279 – 0.458
0.396
1.376 – 2.511
1.922
0.952 – 2.339
1.786
0.155 – 1.116
0.575
0.062 – 0.257
0.142
Number of
plants
surveyed
8
15
50
Note:
1. All figures are absolute i.e. no conversion/equivalence factors have been used.
2. Calculations carried out consistently according to methodologies used by TWG sub-group
Table 4.15: TWG energy sub-group survey data for specific energy flows at some European
MSWIs per tonne of waste treated
Source [64, TWGComments, 2003], [29, Energysubgroup, 2002]
Applicability
The efficiency range that can be achieved, depends importantly on the chemical and physical
nature of the waste being burned (i.e. MSW, HW, SS, etc.) as well as its calorific content. In
general, higher electrical efficiencies can be achieved where the waste contains lower and/or
less variable concentrations of substances that may enhance corrosion in boilers. As high
temperature corrosion becomes an increasing problem at higher steam parameters, the need for
high plant availability can become a limiting factor.
[29, Energysubgroup, 2002] Quite often, when recovered energy is used as heat, a part of that
heat supplied is not actually used. In some cases, the losses from the system which is supplied
with the heat may be very significant because the demand is not constantly at full load.
Typically the recovered heat may be maximised in situations where:
•
•
•
the consumer is an industry with a demand for all the energy recovered
waste can be stored and then burned when heat is required (this avoids wasting fuel energy)
the needs of the district heating network are greater than the energy supplied by the WI
plant.
The last case is most commonly found in cities or other locations with extensive district-heating
networks.
Where a suitable heat distribution and use network is not available, this will limit the
opportunities and rationale for achieving high levels of heat recovery and hence limit the ability
of the process to export all of the available energy, thus making the highest levels of efficiency
difficult to reach.
The choice of the site where the plant will be built and the European (climate) zone where it is
located (e.g. colder climates are advantageous for heat export) are therefore critical in
determining the available energy outlets, and hence the attainable efficiency.
Therefore, when comparing the achieved (and achievable) efficiencies of existing plants, it is
important that the location of the installation is taken into account.
Opportunities for increasing overall energy recovery from new WI plants can therefore be seen
to be greatest at the time when the location of new plants are being selected. The decision to
locate a plant so that it may be economically connected to a suitable energy distribution network
will generally have a much greater influence on overall energy recovery than the technological
choices made in the operating plant.
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Decreasing installation energy demand may involve decisions regarding the type and extent of
FGT system used. A balanced approach to such decisions is appropriate i.e. one that takes
account of the general desire to reduce installation energy requirements and the degree to which
pollutants should be abated.
Economics
Higher relative treatment costs at smaller plants and the lack of economy of scale tend to lead to
a lower availability of capital for investment in the most sophisticated energy recovery
techniques. This, in turn, means that lower efficiencies can be expected at smaller installations
e.g. municipal waste incinerators below 100K tonnes/yr throughput.
The prices paid for supplied electricity and heat have a major influence upon the economics of
investments made to increase these outputs. In some cases, subsidies are paid that provide very
attractive prices for electricity production. In other cases, a high demand for heat can result in
favourable prices for heat. In such cases, the income to be derived from these sources can make
capital investments to increase output more favourable.
For plants supplying electricity only, increasing electrical efficiencies induce higher revenues
from the additional electricity generated/sold, but also involve higher investment costs and often
higher maintenance costs. Therefore the price of the (sold) kWh will play a key role in the
choice of a solution. [74, TWGComments, 2004]
For CHP plants, the larger the quantity of energy exported as heat, the lesser benefit will result
from improved efficiency on the electricity generation. [74, TWGComments, 2004]
Driving force for implementation
The main driving forces for increased energy efficiency are favourable economic conditions.
This in turn is affected by factors such as:
•
•
•
climate
location
prices for heat and power.
Higher income from energy sales can, in particular, allow:
•
•
•
increases to be made in electrical export efficiency to between 20 and 30 % (e.g.
0.6 - 0.9 MWh/tonne for an untreated mixed MSW of 2.9 MWh/t). The higher levels are
achieved through the use of waste pretreatment systems (note that pretreatment stages often
require energy and can use all and more of that to be gained by increased efficiency at the
incineration stage) including RDF production for fluidised bed combustion, and increased
steam parameters beyond 40 bar 400 °C
investment in heat or steam supply networks to increase the ability to use the available
energy, to give efficiencies in the order of 80 to 90 % (e.g. >2.3 MWh/tonne for an
untreated mixed MSW of 2.9 MWh/t) where there is a year-round heat demand
investment in techniques to capture the available heat from low temperature sources that
might otherwise not be economic e.g. condensing scrubbers and heat pumps (see later
sections).
In terms of optimising the energy recovery, reducing the technical risk and reducing costs, heat
supply is favourable where this is possible. However this still depends on the local conditions,
and significantly on the respective sale prices of electricity and heat. If a (substantial) part of the
heat cannot be used, then CHP might well be the right solution. If no heat can be sold, then
good practice is generally to use the available energy to create electricity.
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Heat:
The client demand is the key driving force. Therefore location is of great importance.
An important factor is the duration of the contract to receive heat. Often industrial clients cannot
commit themselves for more than one or two years. This does not fit well with incineration
plants, where a project can require several years before the plant even starts; and incineration
plants financing and operation is usually long term (15 – 25 years).
The most favourable situation is when the recovered heat can be entirely sold for heat use. This
may occur with industrial customers or with DH either in colder climates or for very large DH
with a “base” load higher than the plant output5.
Where all the recoverable heat cannot be sold, the aim is then to try to use the remaining energy
for generating electricity. The decision depends on the remaining quantity of energy and on the
capital investment and income costs to be derived from electricity sales.
CHP:
Generally CHP provides a solution for increasing overall energy output when only part of the
heat can be sold. CHP is especially effective if the temperature level of the required heat is low.
Electricity:
If there is no customer for the heat, the only option is then to generate electricity. Increasing
electrical output can be achieved by using increased steam parameters (see Section 4.3.8). The
choice of the steam parameters (high or low) is most commonly taken on an economics basis.
The technological risk is also a factor as this increases where higher steam parameters are used
(e.g. above 40 bar 400 °C for mixed municipal waste), and, if not well managed and maintained
the plant may then experience losses of availability.
Example plants
Municipal incineration plants:
•
•
•
Renova, Gothenburg and Dyvamuren (Umea), Sweden – high levels of internal energy
integration with a view to maximising heat supply to local networks
Odense, DK - use of special claddings in boilers to allow high steam parameters and
electrical output
Indaver, Beveren, B - supply of process steam directly to neighbouring industry.
Example of MSWI plants generating hot water for district heating:
•
•
•
•
•
Rungis (south of Paris, France)
Villejust (south of Paris, France)
Caen (France)
Thiverval-Grignon (West of Paris, France)
Nantes East (France).
Example of plants providing steam for district heating:
Three plants in France supply steam for DH for over 200000 flats in Paris:
•
•
•
5)
Issy-les-Moulineaux (SW of Paris, France)
Ivry (SE of Paris, France)
St Ouen (NW of Paris, France).
Some plants that report 100 % export of the heat that they recover actually supply steam to another company which then turns
it into electricity e.g. Brussels, Belgium; Metz, France; Vancouver, Canada. Among 8 German plants selling heat only, all the
plants which sell a high quantity of heat per tonne of waste, supply their heat directly to a power station and, usually,
exclusively as process steam.
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Examples of plants providing steam to industry:
•
•
Nantes (F)
Rambervilliers (F).
Examples of plants generating electricity only:
•
•
Mataro (Spain)
Chineham (UK).
Examples of MSWI plants providing steam to electricity generation installations:
• Brussels (Belgium)
• AZN Moerdijk (NL)
• Several examples in Germany
• Vancouver (Canada).
[74, TWGComments, 2004]
Examples of hazardous waste plants:
•
•
•
•
Ekokem, (FIN) - electricity generation and heat supply
Indaver, Antwerp, (B) – on-site use of steam for other processes.
HIM, Biebesheim (D) – on-site use of steam for evaporation of oil/water wastes
German chemical industry (19 plants, capacity >500.000 t/yr) with on-site use of steam for
other processes, electricity (4 plants) and additional district heating.
Reference literature
[29, Energysubgroup, 2002], [28, FEAD, 2002], [30, UBA, 2002], [5, RVF, 2002], [64,
TWGComments, 2003]
4.3.2 Energy loss reduction: flue-gas losses
Description
[28, FEAD, 2002]
Flue-gas losses correspond to the heat leaving the plant (usually considered at the boiler level)
with the flue-gas. The actual loss depends on the flue-gas flow and on its temperature
(enthalpy).
Some possibilities for reducing these losses are:
•
•
•
•
•
•
reduce the flue-gas flow; to achieve this several options are possible:
reduce excess air e.g. improve primary and/or secondary air distribution
recycle flue-gas, i.e. replace a part of the secondary air by flue-gas
enrich the combustion air in O2, i.e. increase the proportion of O2 and decrease the one of N2
by O2 injection (only in special cases is this carried out - see Section 4.2.13)
reduce the flue-gas temperature, e.g. by using flue-gas condensation or by decreasing the
temperature at the boiler exit - see techniques described later in this section.
selecting FGT facilities with decreasing temperatures from the boiler to the stack as far as
possible. [74, TWGComments, 2004]
Achieved environmental benefits
By using techniques to reduce flue-gas losses the additional energy recovered can be supplied
for use.
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Cross-media effects
Reducing the gas flow by reducing the excess air and FGR can increase corrosion risks and
therefore can require additional technical remedies. If levels are reduced too far this may
jeopardise the gas burnout and leave PICs in the flue-gas.
Oxygen injection can increase combustion temperatures. If not carefully controlled, this may
lead to clogging, and risk of destruction of refractory and ferrous materials.
[74, TWGComments, 2004]
The significant energy consumption for generating the required O2 is higher than the achieved
reduction of the energy loss making the technique not viable if the only benefit considered is the
energetic balance – other benefits of enriched oxygen (e.g. improved combustion) may,
however, provide for an overall justification for this technique. [64, TWGComments, 2003]
Reducing gas temperatures at the boiler exit below the temperatures required to operate
subsequent flue-gas treatment devices, will lead to additional energy requirements to reheat the
flue-gases in order to operate those devices, as well as increased risk corrosion in economisers.
This is a particular issue for bag filters and for SCR devices.
Lower flue-gas temperatures at the stack exit can result in:
•
•
•
highly visible condensed plumes (less of a problem if condensing scrubbers are used as they
reduce the water content of the flue-gases)
reduced plume buoyancy and, hence, dispersion
corrosion in stacks (lining with fibreglass or similar required).
Operational data
For a municipal waste incineration plant the energy losses via the flue-gases are typically in the
range of 13 - 16 % of the energy input from the waste.
Applicability
Optimisation of the boiler outlet temperature can be carried out at all plants. The extent to which
it can be reduced will need consideration of the energy requirements of downstream FGT
equipment, and the acid dew point of the flue-gas.
New installations have the most significant opportunity for designing in techniques to reduce
losses from flue-gases. Plants that have an outlet for the supply of relatively low temperature
heat (most common in colder climates) are best placed to make use of the additional heat
removed from the flue-gases. Where this heat cannot be supplied, or used within the plant, this
low grade heat may be better employed within the flue-gas to aid dispersion, etc.
Changes to the design exit temperature of the boiler must take account of the requirements of
subsequent flue-gas cleaning operations. The removal of heat that must later be re-added from
another source is likely to be counterproductive from an energy efficiency point of view, owing
to additional losses from the heat exchange process.
Economics
Information not supplied.
Driving force for implementation
Reduction of overall process energy consumption and increase of energy valorisation products.
[74, TWGComments, 2004]
Example plants
Many plants in Europe, e.g. Brescia Italy
Reference literature
[28, FEAD, 2002], [64, TWGComments, 2003]
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4.3.3 Increasing burnout of the waste
Description
Good burnout results in a high conversion of fuel to energy. Therefore, techniques that ensure
effective burnout and hence low TOC levels in ash may contribute, to some degree, to improved
energy efficiency. Combustion techniques that agitate the waste well, and retain wastes within
the combustion zone to allow unburned carbon in the bottom ash to enter the gas phase for
combustion will assist.
Techniques to increase burnout are described in Section 4.2.17
Achieved environmental benefits
Extraction of energy value from the waste for possible recovery/use.
Improvement of residue quality by decreasing the proportion of unburned material left in it.
See Section 4.2.17 for further information.
Diminishing energy returns are seen as waste is progressively burned out to a greater extent.
This is because the amount of energy remaining in reasonably well burned out waste is low. The
main benefit to be obtained from increased burnout is therefore less one of energy recovery and
more one of improving residue quality.
4.3.4 Reducing excess air volumes
See comments in Section 4.3.2 for further information.
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4.3.5 Other energy loss reduction measures
Description
In addition to other techniques described in this document (see Sections 4.3.2, 4.3.3, 4.3.12), the
following losses and reduction techniques can be used:
Description of energy loss Techniques to reduce losses
Heat radiation and
• lagging
convection - mainly from
• build plant inside building
furnace and boiler
Losses with solid residues
(bottom and fly ashes)
•
•
Boiler blowdowns and
effluent
Re-use of the energy in the water
for building heating at the plant.
• Design to lower the boiler
fouling rate
• Effective boiler cleaning - see
Section 4.3.12
Avoid SU and SD by design and
procedures for continuous
operation and good maintenance
• waste mixing and quality
assurance/control
• supply heat to buffer storage
network
Fouling of boiler reduces
heat transfer efficiency
Start-up and shutdown
procedures
Fast changes in waste
characteristics or heat
demand
Plant failures/outage
Maintenance procedures to
prevent failures
•
Reductions/variations in
external demand for energy
•
Losses by measuring
devices and instruments
good waste burnout
use heat from slag bath
•
secured contracts with user
improve possibility of
maximising supply of
recovered energy
supply of heat to buffer
storage network
Use of measuring systems
with low pressure drop and
precise results
Comments
For a municipal plant losses can be
limited to approx. 1 % of energy input.
For a municipal plant losses are in the
order of 0.5 % - 1.0 % - most with
bottom ash.
closed loop for cooling purposes
Example: design to reduce boiler
fouling may reduce SU and SD
frequency and improve availability.
Techniques that increase stability of
input and output will assist ability to
optimise for the local circumstances.
Some outages may result in partial
shut-down of energy recovery
equipment (e.g. TG set). Others may
require waste diversion from the plant
and hence loss of throughput where
storage is not an option.
External demand for the recovered
energy has a major impact on ability of
the plant to supply the energy.
Generally more of an issue for heat than
electricity.
New instruments allow e.g. for steam
measuring nearly zero losses with high
accuracy.
Notes: SU = start-up, SD = shutdown
Table 4.16: Techniques for the reduction of various energy losses at WI plants
Source adapted from [28, FEAD, 2002], [74, TWGComments, 2004]
Achieved environmental benefits
The additional energy recovered may be supplied for use.
Cross-media effects
Information not supplied.
Operational data
Furnaces, boilers and some ducting are commonly covered with rock wool lagging or other
insulating material to limit heat losses and to limit external temperatures for operator safety.
[74, TWGComments, 2004]
Applicability
The techniques described can help to reduce losses at all installations. However, site-specific
factors may mean that some techniques are not locally available or not relevant (e.g. heat
recovery from slag discharger, blowdown or heat buffer storage are considered only where heat
at low characteristics can be sold all year around) [74, TWGComments, 2004]
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Economics
Information not supplied.
Driving force for implementation
Information not supplied.
Example plants
Information not supplied.
Reference literature
[28, FEAD, 2002], [64, TWGComments, 2003]
4.3.6 Reduction of overall process energy consumption
Description
The incineration process itself requires energy for the operation of the plant. This energy can be
recovered from the waste. The amount of energy required depends upon the type of waste being
burned and the design of the plant.
The reduction of the installation energy requirement needs to be balanced against the need to
ensure effective incineration, to achieve treatment of the waste and control of emissions
(particularly to air).
Common sources of significant process energy consumption are:
•
•
•
•
•
•
•
•
•
Induced and forced draught fan to overcome pressure drops and for combustion air
waste transfer/loading equipment (e.g. pumps/cranes and grabs/screw feeders)
air-cooled condensers
waste pretreatment (shredders, etc.)
flue-gas heating for specific air pollution control devices (e.g. bag filters, SCR systems)
flue-gas reheating for reduction in plume visibility
fuels for combustion support and start-up/shut down (most common for low CV wastes)
wet flue-gas treatments, which cool flue-gases greater than semi-wet and dry systems
electricity demand from other devices.
In many cases, particularly where a step change in FGT technology is required, the lower the
ELVs the more energy is consumed by the FGT system – it is therefore important that the crossmedia impact of increased energy consumption is considered when seeking to reduce emission
levels.
The following techniques and measures can reduce process demand:
•
•
•
•
•
•
292
avoiding the use of unnecessary equipment
using an integrated approach to target overall installation energy optimisation rather than
optimising each separate process unit [74, TWGComments, 2004]
placing high temperature equipment upstream of lower temperature or high temperature
drop equipment
the use of heat-exchangers to reduce energy inputs e.g. for SCR systems
the use of energy generated by the WI plant that would otherwise not be used or supplied, to
replace the import of external energy sources
the use of frequency controlled rotating equipment for those equipment parts which operate
at variable speeds, such as fans and pumps, where they are effectively often operated at
reduced load. This will reduce their average energy consumption substantially, because
pressure variations will be realised by varying speeds and not by valves.
Waste Incineration
Chapter 4
Achieved environmental benefits
Reducing the process demand reduces the need for external energy generation or allows the
export of greater quantities of energy. The additional energy recovered may be supplied for use.
Cross-media effects
Reducing energy consumption by FGT equipment design and operation can result in increased
emissions to air.
Operational data
[28, FEAD, 2002]
For a municipal incineration plant, the electricity consumption is typically between 60 and 190
kWh/t of waste, depending on the LCV.
An average electricity self-consumption value of 75 kWh/t of waste is reported for: a MSWI
incinerating waste of LCV 9200 kJ/kg; generating electrical power only (not heat); complying
with EC/2000/76 ELVs using a semi-wet cleaning system and SNCR de-NOX; no de-plume
device. In a MSWI such as this one, without pretreatment, flue-gas reheating, or plume visibility
reduction reheating the main electrical consumptions are roughly:
• induced draught fan : 30 %
• forced draught fan : 20 %
• feed pumps and other water pumps : 20 %
• air-cooled condenser : 10 %
• others : 20 %.
[74, TWGComments, 2004]
Larger throughput plants have an economy of scale, which results in lower energy consumption
per unit of waste treated. This is shown in Table 4.17 below:
MSWI plant size range
(t/yr)
Up to 150000
150000 – 250000
More than 250000
Process energy demand
(kWh/t waste input)
300 – 700
150 – 500
60 – 200
Table 4.17: Plant throughput and total process energy demand for MSWI in Germany
Source [31, Energysub-group, 2003]
The size of such economies of scale may be less dramatic than the example data suggests above.
The generally higher parasitic load values seen in the table above, are most likely to be
explained by the application of complex, retrofitted FGT systems that have often been applied
in Germany in order to guarantee emission levels at values sometimes well below 2000/76/EC.
[64, TWGComments, 2003]
Applicability
Much of the additional energy consumed arises from the application of additional flue-gas
treatment techniques that themselves consume energy. Reducing process energy consumption
by eliminating these components is less appropriate where there are local environmental drivers
that justify further reductions in emissions.
Options for optimisation are greatest at new installations - where it will be possible to examine
and select from a variety of overall designs in order to achieve a solution that balances emission
reduction against energy consumption.
At existing installations, options may be more limited, owing to the expense (and additional
technical risk) associated with complete re-design. Plants that have been retrofitted to achieve
particular emission limit values generally have to fit tail-end gas cleaning equipment and will
therefore have higher energy consumption figures.
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Economics
Operational cost savings may be made by reducing the external process energy demand. Where
the energy saved can be exported this can result in additional income.
Capital costs of significant re-design at existing plants may, in some cases, be large in relation
to the benefits that can be achieved.
Driving force for implementation
Additional income from energy sales or reductions in operating cost due to reduced energy use.
Example plants
Information not supplied.
Reference literature
[28, FEAD, 2002], [31, Energysub-group, 2003], [64, TWGComments, 2003]
4.3.7 Selection of turbine
Description
The main types of turbine in common use in incineration plants are:
•
•
•
•
back pressure turbines
condensing turbines
extraction condensing turbines
double stage condensing turbines.
Back pressure turbines are used when a significant and possibly constant amount of heat can be
supplied to customers. The back pressure level is dependent on the required temperature level of
the supplied heat. The exhaust pressure of a back pressure turbine is above atmosphere (e.g. 4
bar abs.) [74, TWGComments, 2004]
Condensing turbines are used when there are few or no possibilities to supply heat to customers
and the recovered energy is to be converted into electricity. The efficiency of the electricity
production is influenced by the applied cooling system (see also 4.3.9). The exhaust pressure of
a condensation turbine is under vacuum (e. g. 0.2 bar abs.) and the low pressure part of a
condensation turbine is much bigger. [74, TWGComments, 2004]
The extraction condensing turbines are condensing turbines with a significant extraction of
steam at intermediate pressure for some purpose. There are nearly always some extraction(s) for
the process use on a condensing turbine. Extraction condensing turbines are used when a
significant and varying amount of heat or steam can be supplied to customers. The required
amount of (low-pressure) steam is extracted from the turbine and the remaining steam is
condensed.
Double stage condensing turbines heat up the steam between the two stages by using some of
the input steam for superheating the steam in the second stage to reach higher energy production
at low condensation temperatures, without damaging the turbine. [74, TWGComments, 2004]
Achieved environmental benefits
The type chosen has an influence on electricity production and energy output.
Steam tapping results in optimised energy use. Savings in fossil fuels lower pollutant and
greenhouse gas emissions by reduced use of additional power from external generation stations.
Cross-media effects
Low condensation temperature at the end of the turbine may cause corrosion by high humidity
in the steam. [74, TWGComments, 2004]
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Operational data
Turbines in MSWI plants are usually not very big, typically 10 MW (range from 1 MW to 60
MW). The number of bleeds is usually limited to 3 or 4 (which is different to power plants
where a turbine can have many more bleeds).
The low pressure section of the turbine needs a minimum steam flow for cooling the blades, for
avoiding vibrations and for preventing condensation.
If the remaining steam flow is too small in some operating conditions, instead of one turbogenerator with a high pressure section and a low pressure section, it is also possible to have two
turbines (one high pressure, one low pressure). The choice is made in function of the local
conditions and the optimum choice may vary with time. [74, TWGComments, 2004]
In order to improve the electrical generation output with condensation turbines, the steam
required by the incineration process (e.g. for the deaerator, air heater, soot blowers) is normally
taken from the turbine after its expansion in the high pressure part of the turbine. This is made
by "bleed(s)" (also called "extraction" or "tapping"). These "bleed(s) are said to be 'uncontrolled'
because the pressure depends on the turbine load (the pressure can be divided by two at a bleed
when the steam flow is reduced by 50 %). The bleed(s) are located in such a manner that the
pressure(s) are high enough for the process requirement whatever is the load of the turbine
Applicability
The turbine selection must be made at the same time as the rest of steam cycle characteristics
and depends more on external aspects than the incineration process. [64, TWGComments, 2003]
Economics
Information not supplied.
Driving force for implementation
Information not supplied.
Example plants
1. RMVA Köln, Germany
By steam tapping from a condensation turbine, various uses for the energy are found and overall
energy use can be optimised. The steam is carried to the turbine at a temperature of 400 °C and
at a pressure of 40 bars. The tapping of the steam at approximately 300 °C and 16 bars for local
and remote steam use and tapping at approx. 200 °C and 4.8 bars for purely local use means that
the energy can be exploited to the best possible effect.
The steam is used locally for air and water preheating for local systems and buildings, here
particularly as process heat for steam generation, and also for cleaning exhaust air. The remote
steam is primarily used to support production processes, but is also used as a remote source for
heating. [64, TWGComments, 2003]
2. Rennes plant - Retrofit
2 lines of 5 t/hr each of waste producing steam at 26 bar abs and 228 °C (saturated) for the DH.
In 1995, a third line of 8 t/hr of waste was added producing steam at the same pressure and
380 °C (150 °C of superheat).
A TG set of 9.5 MVA receives the mixed steam from the 3 lines with an intermediate superheat.
However, the TG set can run with any of the 3 lines shut down. This means that it can run with
the superheat of 150 °C when the 2 lines of 5 t/hr are stopped but also with saturated steam
when the 8 t/hr line is off. The turbine uses only saturated steam.
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295
Chapter 4
3. Three plants in Paris
These plants are incinerating a total of 1800000 t/yr and supply to the Paris District Heating
network 4000000 t/yr of steam (2900000 MWhth/yr), which corresponds to 45 % of the needs,
and also generate 290000 MWh/yr of electricity, of which 160000MWh/yr are supplied to the
national grid.
Steam is delivered at a pressure adjusted by valve between 12 and 21 bar according to the DH
requirements and is delivered directly to the DH network without heat exchangers. A varying
part of the steam comes back as condensate. The demineralisation plants are able to produce 2/3
of the steam flowrate.
The large size of the DH demand in comparison to the steam generated and the fact that the 3
plants supply the same DH, belong to the same owner and are operated by the same company
leads to these specific choices for the equipments.
[74, TWGComments, 2004]
Reference literature
[64, TWGComments, 2003]
4.3.8 Increased steam parameters and application of special materials to
decrease corrosion in boilers
Description
Higher steam parameters increase the turbine efficiency and result in higher electricity
production per tonne of waste burned. However, because of the corrosive nature of the gases
evolved from the waste when it is burned, incinerators cannot use the same temperatures and
pressures as some primary power generators e.g. 100 – 300 bar and 620 °C. For example, a
normal maximum temperature is 540 °C in coal power plants.
There is a distinction between:
•
•
the steam pressure which gives the temperature (saturation pressure) in the water walls
(which can be protected by claddings) and in the heat exchange bundles
the steam temperature (superheated steam) which gives the temperature in the superheaters.
[64, TWGComments, 2003]
In general, unless special measures are taken to avoid the effects of corrosion (that result in
reduced installation availability and increased costs), MSWI are generally limited to 40 - 45 bar
and 380 - 400 °C. Above these values there is a trade off between:
•
•
•
costs of special measures e.g. special materials to reduce corrosion
costs of lost availability where increased maintenance is required
value of any additional electricity produced.
To reduce the effects of corrosion, nickel/chromium (main components) alloy claddings or other
special materials can be used for protecting exposed heat exchange surfaces from the flue-gases.
The cladding usually starts after the refractory and can cover the first pass and the beginning of
the second pass of the boiler. The refractory wall can also be air-cooled (slight overpressure) to
reduce corrosion of the tubes behind the refractory. Ceramic tiles are also used to protect boiler
tubes.
High temperature corrosion of membrane walls and super-heaters can be reduced by lowering
the flue-gas temperature under 650 °C before the superheaters and/or protecting heat-exchange
surfaces with ceramic tiles or special alloys.
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The main advantage of special alloys over a ceramic cover on the furnace walls is the better
transfer of heat to the boiler, resulting in a lower temperature of the flue-gases before the first
convection bundles.
An alternative to cladding is to install composite boiler tubes. Composite tubes consist of two
tubes, inner and outer, metallurgically bonded together. They come in different alloy
combinations. Installations in waste incineration boilers have been made since the seventies.
[64, TWGComments, 2003]
Achieved environmental benefits
Higher electrical output per tonne of waste burned can be achieved by the increase of steam
pressure and/or temperature. This higher efficiency reduces external (e.g. in power plants) use
of fossil fuel (resource saving) and the related CO2 emissions (GHG). [74, TWGComments,
2004]
Cross-media effects
Increased steam parameters without the application of special anti-corrosion measures increases
risk of corrosion and associated maintenance costs and availability loss.
Operational data
This technique involves an increased technological risk and requires skill levels for assessment
and maintenance.
Applicability
The use of increased steam parameters is applicable to all incinerators recovering electricity
only, or where the heat proportion of CHP is low, to increase electricity outputs.
The technique has a limited applicability to processes that have reliable options for the supply of
steam or heat, since there is no need to increase electrical output, with the associated additional
technical risk and costs, and the steam/heat energy can be supplied.
The use of claddings and other special materials is applied to reduce corrosion when using
increased steam parameters and/or highly corrosive waste content.
Existing plants that have experienced rising LCV in the waste supplied may benefit from the use
of special materials and claddings because it can reduce the maintenance costs and allow
improved electrical outputs.
Economics
The cost of cladding can be discounted against reduced maintenance costs and income from
electricity sales and improved plant availability.
Cost of cladding has been reported to be approx. EUR 3000/m2.
The range of increase in income depends on the energy price obtained.
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Chapter 4
[32, Denmark, 2003] The Table below is based on actual electricity and heat prices in Denmark
and on real operational data for a modern CHP MSWI of 34 t/h capacity. The table shows the
outputs and income at various steam parameters:
Steam value
Generator output
Heat production
Change in electricity output
Change in heat output
Economy
Availability
Electricity price
Heat price
Annual income change
MW
MJ/s
MW
MJ/s
bar
50
19.6
71.2
0
0
40
18.6
72.2
-1
1
30
17.3
73.5
-2.3
2.3
hr/yr.
EUR/MWh
EUR/MWh
million EUR/yr
8000
47
18
0.0
8000 8000
47
47
18
18
-0.25 -0.54
Table 4.18: Example energy outputs and income at various steam pressures for a CHP MSWI using
elevated steam pressures
[32, Denmark, 2003]
According to this example, taking into account energy prices in Denmark, increasing steam
pressure from 40 to 50 bar results in an additional annual income approx. 250000 EUR.
Calculated over an operational period of 15 years (at 2002 inflation rates) this gives an
additional income of approx. EUR 2.5 million.
These figures do not include the additional capital cost required to increase pressures from 40 to
50 bar. In this case these additional costs were considered not to be significant (over 15 years)
compared to the additional income gained over the same period.
A change of the pressure might mean a complete change of the piping and the valves of the
vessel and also a change of the steam drum which would then lead to the need to apply for a
new permit under the Pressure Equipment Directive 97/23/EC. [74, TWGComments, 2004]
Driving force for implementation
Higher electricity prices will encourage adoption of this technique, as it will allow faster
payback of the investments required.
Example plants
High Steam Parameters:
Odense (Denmark) 50 bar 520 °C with cladding.
AVI Amsterdam, AVR Botlek, AVR AVIRA and AVI Wijster, all in the Netherlands.
AVE-RVL Lenzing, Austria: circulating fluidised bed receiving various waste fractions
including approx 60 % plastics produces steam at 78 bar and 500 °C.
Ivry, France (75 bar, 475 °C),
Mataró, Spain (60 bar, 380 °C),
Lasse Sivert Est Anjou, France (60 bar, 400 °C),
Retrofit improvement :
Rennes, France, 26 bar, 228 °C (saturation state); when 3rd line added its boiler rated at 26 bar,
380 °C; the 2 mixed steams sent to the TG.
Application of special materials:
Retrofit improvement : Toulon, Thiverval (F), Mataro (Sp), Stoke on Trent (UK);
new plant : Lasse Est Anjou (F)
[74, TWGComments, 2004]
298
Waste Incineration
Chapter 4
Reference literature
[32, Denmark, 2003], [28, FEAD, 2002], [2, infomil, 2002], [3, Austria, 2002], [64,
TWGComments, 2003]
4.3.9 Reduction of condenser pressure (i.e. improve vacuum)
Description
After leaving the low-pressure section of the steam turbine, the steam is condensed in
condensers and the heat is then passed into a cooling fluid. The condensed water from steam is
generally recirculated and used as boiler feed water [74, TWGComments, 2004]
The temperature of the cold source at the turbine outlet is important for the turbine production.
The colder the cold source the higher the enthalpy drop and, therefore, the higher the energy
generation. For reasons of climate conditions, it is obvious that it is easier to achieve this low
pressure in cooler climates. This is one reason why northern installations can give better
efficiency than they can in southern countries. [64, TWGComments, 2003]
The lowest temperatures are obtained by condensing the steam, using either air or water as
cooling fluid. These temperatures correspond to pressures below the atmospheric pressure (i.e.
vacuum).
Where a large DH network uses all year long the full load production of the MSWI, as occurs in
Sweden or Denmark, the cold source is the cold water returned by the DH which can be
sometimes very low (e.g. 40 °C or 60 °C, see Table 4.19 in Section 4.3.16)
Vacuum is not “unlimited”. As soon as the steam crosses the saturation line of the “Mollier”
diagram, it starts to be wet and the percentage of moisture increases with the expansion of the
steam in the turbine. In order to avoid damage to the end stages of the turbine (erosion by water
drops), the amount of moisture must be limited (often around 10 %).
[74, TWGComments, 2004]
Achieved environmental benefits
Increased electrical energy generation is possible by improving the vacuum.
Cross-media effects
In an open loop, i.e. a once through cooling hydro-condenser, assuming a water temperature
increase of 10 °C, the water flow needed will be around 180 m³/MWh generated.
In a closed loop with a cooling tower, the water consumption (evaporated water) is approx. 2.5
or 3 m³/MWh generated.
Both open and closed loop systems may require the addition of chemicals, or other techniques,
to reduce fouling in the heat exchange system as well as eventual process water treatment. The
impacts of discharge are much larger for open systems.
Low condenser pressure increases humidity in the steam which can increase wear in the turbine
[74, TWGComments, 2004]
Air condensers give noise emissions. Detailed design (e.g. shielding, frequency converter noise
level, etc.) is therefore important.
Cleaning of the surface of condensers is very important for its efficiency and should be carried
out at low temperatures. [74, TWGComments, 2004]
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299
Chapter 4
Operational data
Condenser pressure/condenser types:
With an ACC (Air-cooled Condenser) and air at 10 °C, typically a pressure in the condenser of
100 to 85 mbar absolute can be obtained, depending on the condenser surface. When the
ambient air temperature is 20 °C, the pressure in the same ACC will be respectively 200 to 120
mbar absolute. The design is a compromise between a reasonable heat exchange surface and
low condensation pressure. [74, TWGComments, 2004]
With hydro-condenser (once-through cooling condenser, open loop) using river water at 10 °C,
the pressure in the condenser will be around 40 – 80 mbar absolute, because the heat exchange
is easier with water.
With an atmospheric cooling tower6, the temperature of water is bound to the temperature of
the air and to its hygrometry (temperature of the humid bulb). If the wet bulb temperature is
10 °C the condenser pressure will be around 60 mbar. The steam plume above the tower can be
reduced (but not deleted) by tower design and there will be a slight increase in the condenser
pressure. This type of cooler may involve risk with respect to legionella, due to water
evaporation and direct contact. It is mainly applied for small cooling requirements (such as
turbine auxiliaries). [74, TWGComments, 2004]
According to overview calculation an increase in electrical energy generation from 24.1 % to
25.8 % (+ 7 %) if vacuum improves from 100 mbar to 40 mbar. [64, TWGComments, 2003]
Applicability
Where electrical generation is of lower priority (e.g. where heat supply is possible) the turbine
outlet pressure can be above atmospheric. In this case, the turbine is said to be operating with
back pressure and the (remaining) steam is condensed in the condenser itself.
Air-cooled condensers are often the only possible type applicable. [74, TWGComments, 2004]
Open loop hydro-condensers are only suited to locations where there is an abundant water
supply that can tolerate the heating effect of the subsequent discharge.
The gains in electrical output are greater with the reduction of condenser pressure and the
techniques are therefore, of greater benefit for condensing turbines. [64, TWGComments, 2003]
Economics
The use of higher pressure reduction techniques will be most economic where electrical energy
has a higher price.
For ACC, higher pressure drops require larger surface area equipment and higher performance
of the fan motors, which then increases cost.
Driving force for implementation
Electricity prices are a key driver.
Easier synchronisation of the TG set when the ambient temperature is high. [74,
TWGComments, 2004]
Example plants
Most of European plants got installation with ACC, e.g. Issy and Ivry plant, near Paris, and
Bellegarde France; Southampton, UK (under construction). Both have open loop hydrocondensers.
Strasbourg, and Rouen, France -both have closed loop hydro-condensers with cooling towers.
6)
300
Cooling tower or refrigeration tower. The steam condenser cooling fluid is water. This water is in a closed loop, and is cooled
itself by contact with ambient air in a cooling tower. In this tower a part of the water is evaporated. This produces a water
vapour plume above the tower.
Waste Incineration
Chapter 4
Reference literature
[28, FEAD, 2002] [64, TWGComments, 2003]
4.3.10 Selection of cooling system
Description
Choosing the cooling system that is best suited to the local environmental conditions helps to
reduce overall environmental impacts.
There are three main cooling systems:
1. Water cooling by convection:
This system uses surface water which is discharged to surface again after being heated up
several degrees. This cooling system requires much water and gives a large thermal load to local
surface water. It is mainly used if large flow rivers are available or on the coast. Noise level is
low. Cleaning facilities for water are normally necessary, which may lead to harm to fish and
other biota in water source as the water is screened/filtered. Care should be taken to reduce this
in design of water intake arrangements. [74, TWGComments, 2004]
2. Evaporation water cooling:
Water is used for condenser cooling. This is not discharged, but recycled after passing an
evaporation cooling tower, where it is cooled by the evaporation of a small part of the water. A
small water stream must be discharged to keep control of the water quality within the system.
There are three main technical variants of evaporation cooling:
•
•
•
forced draught cooling towers, where the air, required for evaporation of the water, is
provided by means of a fan, with the subsequent electricity consumption
natural convection cooling towers, where the draught of the air is caused by the (small)
temperature increase of the air temperature (the large concrete cooling towers of 100 metres
high) and
hybrid cooling towers, where part of the heat in the water is transported to air by cooling
bundles (“air cooling”), thus reducing the size of the water vapour plume.
Noise level of forced draught systems is high, and the level of convection draught systems is
medium.
This type of cooler may involve a risk with respect to legionella, due to water evaporation and
direct contact. [74, TWGComments, 2004]
3. Air cooling:
Here the steam is condensed in a heat exchanger of typical design (e.g. “camping tent model”)
with air. These condensers use larger amounts of electricity, as the required air movement is
caused by large fans.
Noise levels are higher. The surface of the condenser needs regular cleaning. [74,
TWGComments, 2004]
Influence on electricity efficiency is dependent on water temperatures, air temperatures and air
humidity (wet bulb temperature), but generally water convection cooling scores best, followed
by evaporation cooling and then followed by air cooling. Difference in efficiency between
convection cooling and air cooling is normally in the range of 2 – 3 %.
See also the BREF “Reference Document on the Application of Best Available Techniques to
Industrial Cooling Systems”. [74, TWGComments, 2004]
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Chapter 4
Achieved environmental benefits
Depending on type chosen (see description above) it is possible to:
•
•
•
•
decrease installation parasitic electricity demand
decrease thermal impacts of discharged warm water
decrease noise impacts
decrease visual impacts.
Cross-media effects
Described in description above.
Operational data
Information not supplied
Applicability
Selection of cooling system dependent upon main local environmental and health issues and
relative importance of cross-media effects associated with each system.
Water cooling by convection (type 1 above) is not applicable in dry inland situations.
Economics
Information not supplied
Driving force for implementation
Higher prices for electricity may encourage interest in convection cooling.
Example plants
Issy and Ivry plant, near Paris, France; Southampton, UK (under construction). Both have open
loop hydro-condensers.
Strasbourg, and Rouen, France -both have closed loop hydro-condensers with cooling towers.
Reference literature
[64, TWGComments, 2003]
4.3.11 Optimisation of boiler architecture
Description
[28, FEAD, 2002]
The recovered heat is the energy that is transferred from the flue-gases to the steam (or hot
water). The remaining energy of the flue-gas at the boiler exit is most commonly lost (unless
heat-exchange systems are used further downstream). So, in order to maximise the energy
recovery, it is usually favourable for the temperature of the flue-gases at the exit of the boiler to
be reduced.
Boiler fouling has two effects on energy recovery. The first one is that it decreases the heat
exchange coefficients and, therefore, leads to reduce the heat recovery. The second and major
one is that it leads to blocking up the heat-exchange bundles and, therefore, to the shut down of
the plant. Another unwanted effect of boiler fouling is that it increases the risk of corrosion
under the deposited layer. In general, the boiler is designed for a manual cleaning once a year as
a maximum to limit boiler fouling (see Section 4.3.19).
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A good boiler must have a sufficient heat-exchange surface but also a well designed geometry
in order to limit fouling. This can be achieved either in vertical, horizontal or combined verticalhorizontal boilers concepts (see Section 2.4.4.2.) [74, TWGComments, 2004] Examples of good
design practices are reported as follows:
•
gas velocities must be low (to avoid erosion) and homogeneous (to avoid high velocity
areas and to avoid stagnation, which can induce fouling) over the whole diameter (space) of
the boiler
• to maintain low gas velocities, the passes must be wide in cross-section and their geometry
must be “aerodynamic”
• the first pass(es) of the boiler should not contain heat exchangers and have sufficient
dimensions (especially height) in order to allow flue-gas temperature below 650 – 700 °C.
However, they can be cooled by water walls (working by convection). (These tube walls, in
fact, envelop the whole boiler except the economiser. In steam boilers, they are generally a
part of the vaporiser). Radiating heat exchangers may also be placed in the open passes at
higher temperatures
• the first tube bundles must not be installed at locations where the fly ashes are still sticky,
i.e. where temperatures are too high
• the gaps between the tubes of the bundles must be wide enough to avoid “building” in
between them (by fouling)
• water-steam circulation, in membrane wall and convective exchangers should be optimal in
order to prevent hot spots, inefficient flue-gas cooling, etc.
• horizontal boiler should be designed in order to avoid flue-gas preferential path, leading to
temperature stratification and ineffective heat exchange
• suitable devices for cleaning the boiler from fouling in situ should be provided
• optimisation of convective exchanger arrangement (counter-flow, co-current flow, etc.) in
order to optimise the surface according to the tube wall temperature and prevent corrosion.
[74, TWGComments, 2004]
Achieved environmental benefits
Greater plant availability and better heat exchange allows increased overall energy recovery
possibilities.
Design to reduce boiler fouling also reduces the retention of dust within temperature zones that
may increase risk of dioxin formation.
Cross-media effects
None reported.
Operational data
No additional information supplied.
Applicability
Applicable at design stage to all incineration plants with energy recovery boilers, when concerns
exist to improve the operating life and efficiency. [74, TWGComments, 2004]
Economics
Operational savings through reduced maintenance and increased energy sales can lead to very
short payback periods, and can then justify the adoption of these techniques at new installations.
Existing plants that are going to replace boilers, or where lower boiler efficiencies are seen
(generally less than 75 % heat transfer efficiency for municipal plants) may also take these
factors into account in the design of the new system.
Driving force for implementation
Decreased maintenance, increased energy recovery and possible income from energy sales.
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Example plants
Widely applied in Europe.
Reference literature
[28, FEAD, 2002], [2, infomil, 2002] [64, TWGComments, 2003]
4.3.12 Use of an integral furnace - boiler
Description
Transporting very hot gases in ducts lined with refractory can be complex. It can result in sticky
and sometimes molten deposits. In order to avoid this, the gas temperature is sometimes reduced
by increasing the air excess volume, which can result in a loss of efficiency.
In an integral furnace-boiler, the boiler directly covers the furnace without intermediate piping.
Therefore, the boilers tube can cool the furnace sides. The tubes are protected by refractory and
cool it (cross-benefit). A suitable design of the tubes and refractory allows fine control of the
cooling of the furnace. Effective cooling of the furnace is essential to avoid clogging in the
furnace, especially with higher LCV.
Achieved environmental benefits
Improves heat recovery by reducing the heat losses by radiation at the furnace outlet (to
complement the effect of the external lagging).
Allows the installation of SNCR de-NOX systems.
Reduction in the excess air requirement and hence flue-gas volumes.
Cross-media effects
None reported.
Operational data
Avoids clogging in the furnace and, therefore, the need for shut downs for manual cleaning (e.g.
with pneumatic drill).
Applicability
Suits all types of grates. Not applicable with rotary and oscillating kilns.
Indispensable for furnaces with a capacity above 10 t/h of waste.
No known lower capacity limit for integral boilers in “industrial” incinerators, i.e. capacity
above 2.5 t/h.
Economics
Usually less expensive than a separate boiler for plants above very small capacity furnaces (i.e.
1 or 2 t/h)
Driving force for implementation
Normal practice for designers today.
Example plants
Most modern plants have an integral boiler-furnace (except rotary and oscillating kilns).
Reference literature
[28, FEAD, 2002] [64, TWGComments, 2003]
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4.3.13 Use of water walls in the first (empty) pass
This technique has been described in Section 4.2.22 in the context of its benefit as a combustion
related technique.
4.3.14 Use of a platten type superheater
Description
Platten type superheaters are flat panels of folded tubes installed in parallel with wide gaps in
between and parallel to the gas flow. The inlet is protected by shells made of stainless steel, held
in place with special cement.
The heat exchange is made by radiation instead of convection; because of this, these
superheaters can be installed at hotter locations than the tube bundles (in flue-gases up to
800 °C on municipal waste), with limited fouling and reduced erosion and corrosion.
On these platten superheaters, fouling can stabilise when its thickness is around 2 cm. There is
no blocking, therefore, manual cleaning and associated shutdowns are greatly reduced.
Due to radiation exchange, the steam temperature can remain constant over a period of time of
one year of operation. Erosion and corrosion are significantly delayed.
Figure 4.3: Schematic diagram of a “platen” type superheater
Achieved environmental benefits
These platten type superheaters allow high superheated steam temperature with good
availability and stability.
Cross-media effects
None reported.
Operational data
Information not supplied.
Applicability
It can be installed in any boiler with two or three open passes.
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Economics
Less expensive than bundles for last stage superheaters (the hotter ones) when installed in areas
with hotter flue-gas temperatures (2nd or 3rd pass).
Use may increase construction cost and this needs to be considered against the additional heat
exchanger life that may be gained.
Driving force for implementation
Longer operation period with high superheated steam temperature
Example plants
• France: Toulon 3, Thiverval 3, Lons le saunier, Cergy St Ouen l’Aumône, Rennes 3,
Monthyon, Chaumont, Nice 4, Belfort, Villefranche sur Saône, Toulouse-Mirail 1 and 2,
Lasse (Saumur)
• Belgium: Thumaide
• UK: London SELCHP, Stoke-on-Trent, Dudley, Wolverhampton, Chineham, Marchwood
• Spain: Mataró
• Portugal: Maia, Loures and Santa Cruz (Madeira)
• Italy: Piacenza
• Russia: Moscow.
Reference literature
[28, FEAD, 2002] [64, TWGComments, 2003]
4.3.15 Reduction of flue-gas temperatures after the boiler
Description
[2, infomil, 2002]
Additional heat-exchange capacity in the boiler can improve possibilities for use of that heat
elsewhere and, hence, can contribute to improved energy efficiency. How much the flue-gas
temperature at the end of the boiler can be reduced depends on:
•
•
•
at temperature levels below 180 °C there is an increased risk of corrosion (as the dew point
of the various acids is progressively approached).
whether the heat in the flue-gases is required for the operation of subsequent flue-gas
cleaning equipment
whether there is a beneficial use for the additional heat recovered at low temperature.
With MSW (and other flue-gases containing these substances) flue-gases the corrosion risks
arises not only from HCl but from the SOX, which are often the first to attack steel. The dew
point depends on the concentrations of the acid gases in the flue-gases. In clean gas it can be
around 100 °C, in raw gas 130 °C or more.
The key temperature to consider in term of corrosion risk is not the temperature of the flue-gas
but the (lower) surface temperature of the (cooled) metal tubes of the exchanger (which is
necessarily colder than the flue-gas). [74, TWGComments, 2004]
Heat-exchangers, which are made of special materials (enamel, carbon) reduce low temperature
corrosion problems. An example is found at AVI Amsterdam, where a heat-exchanger is located
after the spray absorber system and the related ESP. An additional advantage of this, is the
related reduction of scrubbing temperature, which improves the efficiency of the scrubbing
system.
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Achieved environmental benefits
The recovered heat (temperature level e.g. 120 °C) can be used for heating purposes and/or
internally for preheating of the boiler feed-water, etc.
Cross-media effects
With flue-gas cleaning systems that require the flue-gases to be above a particular operational
temperature (e.g. bag filters, SCR) any heat removed will need to be re-added by some means
later in the process. Such reheating is likely to result in additional consumption of primary fuels
or external power.
Low outlet gas temperature at the boiler outlet involves risks of corrosion (internal and
external). Then specific material protection etc. can be required.
Space requirement may limit the implementation in existing plants.
Operational data
[28, FEAD, 2002]
Lowering the flue-gas temperature at the boiler outlet is limited by the acid dew point, which is
a very important limitation in many FGT systems. In addition, the flue-gas cleaning system may
require a working temperature or a temperature difference, for example:
•
with semi-wet FGT processes, the minimum temperature at the inlet is determined by the
fact that the water injection decreases the temperature of the gases. Typically, it will be 190
or 200 °C, and can be higher
•
a dry FGT process can generally accept 130 - 300 °C. With dry sodium bicarbonate the
minimum required temperature is 170 °C to obtain a fast transformation of sodium
bicarbonate into higher surface area and, hence, more efficient sodium carbonate (the socalled “pop-corn” or “diatomite” effect). The consumption of reagent is different according
to the temperature
•
Wet FGT systems do not have a theoretical minimum inlet temperature - the lower the gas
temperature at the scrubber inlet, the lower the water consumption of the scrubber.
[74, TWGComments, 2004]
It is possible to design the cycle in such a manner to avoid the corrosion conditions. In Sweden,
for example, it is quite common to install a separate "waste heat boiler" after the main boiler or
ESP. It is often cooled by a separate hot water circuit and by a heat exchanger to the district
heating net. Outlet flue-gas temperature is normally about 130 – 140 °C and water inlet
temperature should not be under 115 – 120 °C to avoid corrosion. At these temperature levels
normal carbon steel tubes can be used without corrosion problems. [64, TWGComments, 2003]
Applicability
Reducing the flue-gas temperature after the boiler is only applicable where:
•
•
the heat extracted can be supplied and put to some useful purpose
subsequent flue-gas cleaning systems are not adversely effected.
Careful consideration must be given to compatibility of this technique with subsequent flue-gas
cleaning systems. Particularly where bag filters, SCR or other systems that require particular
operational temperatures or conditions, are used.
Economics
The system is most likely to be economically viable where the value/price paid for the
additional heat recovered heat is high.
Driving force for implementation
Supply of additional heat recovered.
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Example plants
• AVI Amsterdam, Netherlands.
• Brescia, Italy.
• Many plants in Sweden and Denmark.
• Sheffield (UK)
• Rennes, Nice, St. Ouen (France)
• Monaco
Reference literature
[2, infomil, 2002] [64, TWGComments, 2003]
4.3.16 Use of flue-gas condensation scrubbers
Description
The technique has already been described in Section 2.4.4.5.
In summary, the technique involves the use of a cooled scrubber that condenses water vapour
from the flue-gas of wet, semidry and dry systems normally as tail end solution. The cooling
can be provided by heat exchange (using a heat pump system) with the returned district heating
water.
Achieved environmental benefits
The use of condensing scrubbers allows the extraction of additional energy from the flue-gases
for possible use or supply.
The amount of additional energy recovered is dependent upon the return water temperature in
the district heating system:
District heating return temperature
(°C)
40
50
60
Additional energy efficiency
14 %
7%
0%
Table 4.19: Relationship between the additional energy efficiency and the cooling medium (district
heating) return temperature
[5, RVF, 2002]
The drying effect on the flue-gases reduces the plume visibility. Where energy is used to reheat
the plume, the amount of energy required to achieve a given reduction in plume visibility will
be lower.
Emissions of ammonia to air (e.g. from SNCR) may be reduced. The ammonia is captured in the
scrubber water. By using an ammonia stripper at the water treatment plant, it is possible to
regenerate ammonia for use as a NOX reduction reagent - thus replacing the need to purchase
new ammonia, although stripping systems are reported to be complex and expensive.
The condensed water can be used to provide the majority of the scrubber feed water, therefore
reducing water consumption.
Cross-media effects
The condensed water will contain pollutants (removed from the flue-gas) that require treatment
in a water treatment facility prior to discharge. Where an upstream wet scrubbing system is
applied, the effluent from the condensing scrubber can be treated in the same facility.
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The low temperature of the stack discharge will reduce thermal buoyancy of the plume and,
hence, reduce dispersion. This can be overcome by using a taller and/or reduced diameter stack.
Operational data
The low temperature of the flue-gases can result in condensation and, hence, corrosion in the
chimney unless lined or double tube systems are used.
Applicability
Most applicable where:
•
•
•
district heating gives a reliable low temperature return (this is essential and generally only
available in colder climates)
plume visibility is a concern
prices paid for the additional energy recovered justify the additional capital investment.
The technique is less applicable where:
•
•
there is no user for the additional energy recovered
the cooling source (DH cold water return) is less reliable (i.e. warmer climates).
Waste types:
Because the technique is applied after the flue-gas cleaning stages, in principle the technique
could be applied to any waste type.
Plant size:
The technique is known to have been applied at municipal plants of 37000 (DK), 175000 (S)
and 400000 (S) tonnes per year throughput.
New/existing:
The technique is applied at/near the end of the FGT system and could, therefore, be applied to
new and existing processes alike.
Economics
The total additional investment for the condensation stage is roughly estimated to EUR 3
million for four boilers serving a MSWI CHP plant of capacity 400kt/yr.
Driving force for implementation
Additional heat sales. Water conservation in dry regions. [74, TWGComments, 2004]
Example plants
There are several examples in Sweden (see examples in Section 2.4.4.5). Possibilities are fewer
in warmer climates owing to reduced availability of cool DH water return.
Reference literature
[5, RVF, 2002], [64, TWGComments, 2003]
4.3.17 Use of heat pumps to increase heat recovery
Description
The technique has already been described in some detail in Section 2.4.4.6. The three main
types of heat pumps are also described there.
In summary, heat pumps provide a means of uniting various relatively low temperature heat and
cooling sources to provide a stream at an upgraded temperature level. This allows, for example,
the operation of condensing scrubbers (see 4.3.16) and the supply of the additional heat to users.
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Achieved environmental benefits
Enables the recovery of additional energy by the use of condensing scrubbers.
An increase in the quantity of energy recovered of 23 % was reported when using absorption
heat pumps in combination with condensing scrubbers. [35, Renova, 2002]
Based on the example of the Umea plant in Northern Sweden (175 Kt/yr) the following
estimated energy balance is seen:
The power balance including condensation and compressor heat pump:
Thermal power input, about
Electricity output from generator
Internal electricity consumption
Electricity output, net
Hot water generation incl. cond.+ heat pump
Own consumption for reheat
Hot water to district heat
Total power and heat for sale
65 MW
15.1 MW
5.4 MW
9.7 MW
54 MW
0.5 MW
53.5 MW
63 MW
Without FG condensation and heat pumps the power balance is estimated as:
Thermal power input about
Electricity output, net
Hot water to district heating, net
Total power and heat for sale
65 MW
13 MW
39 MW
52 MW
Cross-media effects
The heat pumps themselves require energy to function.
At wet ESP conditions, for compressor driven heat pumps, the ratio between output heat and
compressor power (heat to power ratio) can be about five [5, RVF, 2002].
Operational data
See detailed information given in Section 2.4.4.6.
Applicability
Most applicable where:
• district heating gives a reliable low temperature return
• district heating uses most of the available heat
• prices paid for the additional energy recovered justify the additional capital investment
• wet scrubbing is used
• plume visibility is a concern.
[74, TWGComments, 2004]
The technique is less applicable where there is no user for the additional energy recovered.
Waste types:
Because the technique is applied after the flue-gas cleaning stages, in principle the technique
could be applied to any waste type.
Plant size:
The technique is known to have been applied at municipal plants of 175000 and 400000 tonnes
per year throughput.
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New/existing:
The technique is applied at/near the end of the FGT system and could therefore be applied to
new and existing processes alike.
Economics
Heat pumps example for an MSWI in Sweden [35, Renova, 2002]:
•
•
•
EUR 4.5 million installation in 1988
EUR 5.5 million installation 2002 (12 MW capacity)
income since 1998 = EUR 24.5 million.
The additional investment at the Umea plant for the condensation stage + electric motor driven
compressor heat pump + bigger water treatment is estimated to EUR 4 million. Using the
proceeds for heat and power as given in Table 10.9, the simple pay back time for this
investment is therefore about 2.4 years (no allowance for increased maintenance and
consumables).
Driving force for implementation
Additional heat sales and income.
Example plants
Several examples in Sweden – see also Section 2.4.4.6
Reference literature
[5, RVF, 2002] [64, TWGComments, 2003]
4.3.18 Special configurations of the water/steam cycle with external power
plants
Description
[2, infomil, 2002]
Due to the corrosive nature of the flue-gases, the efficiency of electricity production of
municipal waste incineration is limited by the maximum acceptable temperature of the boiler
tube materials and by the related maximum steam temperature.
Without the use of special materials to protect boiler tubes from corrosion, steam parameters in
municipal waste incineration plants are generally less than or equal to 40 bar and 400 °C. In
hazardous waste plants (where chloride etc. loading in the raw gas is higher) lower temperatures
and pressures (e.g. 30 bar and 280 °C) are used to avoid excessive corrosion rates and
consequent high maintenance costs. The adoption of higher steam parameters allows more heat
to be transferred to the medium at higher temperature. The thermodynamic efficiency is,
therefore, increased and hence the electrical output per tonne of waste. However, the costs of the
materials required to protect the boiler tubes is generally significant in relation to the income to
be received from the additional electricity sales (see Section 4.3.8).
An option that avoids higher temperatures of boiler tube materials, is the superheating of the
steam using cleaned flue-gases, which contain much less or no chlorine. This is possible if the
municipal waste incineration plant can be combined with a power plant of sufficient capacity.
Example: Municipal waste incineration plant, AZN Moerdijk, NL
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Figure 4.4: Combination of a waste incineration plant with a gas turbine power plant
[2, infomil, 2002]
This waste incineration plant is combined with an adjacent combined cycle natural gas power
plant, as indicated above. Steam at 100 bar, slightly superheated to 400 °C is supplied to the
waste heat boilers of the gas turbine plant, where it is superheated to approximately 545 °C.
Both the municipal waste incineration plant and the gas power plant have three separate lines.
The design of both plants combined process schemes enables all incineration and gas turbine
lines to operate independently, although, under these circumstances, at a lower energy
efficiency.
Example:
A similar configuration can be used in the combination of a waste incineration plant with a coal
power plant. The coal power plant superheats the steam of the municipal incineration plant. In
order to do this, the pressure of the steam produced by the municipal waste incineration plant
has to be higher than usual.
Municipal waste incineration plant
Coal power plant
Superheater
Slightly superheated
steam
Flue gas
treatment
Flue gas
treatment
Cooling
Evaporator
Cooling
G
Coal
Condensor
Figure 4.5: Municipal waste incineration plant in combination with a coal power plant
Source [2, infomil, 2002]
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This configuration was applied in the 1970s, by the combination of a municipal waste
incineration plant in Munich with a large coal power plant. As the plant only functioned
effectively when both the coal-fired boiler and waste incineration plant were operational
simultaneously, the combination was not considered cost effective at the time.
Achieved environmental benefits
Improved overall energy efficiency by supply of heat to a synergistic user.
Cross-media effects
None reported.
Operational data
With these types of configurations, the incineration process does not need to adopt high steam
temperatures and therefore avoids corrosion and availability difficulties. However, sometimes
the pressure may be increased in order to benefit further from the integration. In such cases the
higher steam temperature in the evaporator may lead to additional maintenance costs. For
instance, at 40 bar the saturation temperature is 250 °C and at 100 bar 311 °C, a difference of
61 °C. Note that corrosion mechanisms increase exponentially with the external temperature on
the boiler tube walls when coming into contact with flue-gases.
Applicability
Only applicable where a synergistic operation is situated conveniently and suitable commercial
agreements are in place.
Mainly applicable where the focus of energy recovery is the production of electricity. Less
applicable to plants that can supply steam or heat directly to a user.
Energy efficiency will only be increased where the user has a consistent demand and makes
consistent use of the energy supplied.
Economics
High electricity prices encourage the adoption of techniques that increase electrical generation
efficiency. In this case, this has the impact of increasing the relative value of the steam/heat
supplied by the incinerator to the adjacent power plant.
Driving force for implementation
Integration of energy supply with an external user increases options for the use of energy
derived from the waste.
Example plants
See text above. Also Bilbao Zabalgarbi, ES.
Another waste incineration plant where steam is introduced in the water-steam cycle of an
adjacent power plant is located in Austria. [74, TWGComments, 2004]
Reference literature
[28, FEAD, 2002], [2, infomil, 2002] [64, TWGComments, 2003] [74, TWGComments, 2004]
4.3.19 Efficient cleaning of the convection bundles
Description
[2, infomil, 2002]
Clean boiler tubes and other heat-exchange surfaces result in better heat exchange. This may
also reduce the risk of dioxin formation in the boiler.
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Cleaning may be carried out on-line (during boiler operation) and off-line (during boiler shut
downs and maintenance periods). Dimensions of the boiler and heat exchanger design (e.g. tube
spacings) influence the cleaning regime.
Techniques for on-line cleaning include:
• mechanical rapping
• soot-blowing by steam injection
• high or low pressure water spraying (mainly on the wall in the empty passes of the boiler)
• ultra-/infra- sonic cleaning
• shot cleaning or mechanical pellet scouring
• explosive cleaning
• high pressured air injection (from 10 to 12 bar) with movable lances.
[74, TWGComments, 2004]
Off-line techniques include:
• periodic manual cleaning (in general once a year in MSWI)
• chemical cleaning.
[74, TWGComments, 2004]
In addition to these techniques, it can also be beneficial to prevent higher temperature (above
650 °C) gases (when fly ashes are more sticky and hence more likely to adhere to surfaces they
come into contact with) coming into contact with convective heat-exchange bundles by:
•
•
including empty passes with water walls only
using large furnace dimensions and hence lower gas velocities before the bundles.
Achieved environmental benefits
Improved heat-exchange increases energy recovery.
Although FGT systems can be used to absorb or destroy PCDD/F, the reformation risk may be
reduced by effective cleaning. This is because it reduces the amount of time that dusts (and
other materials that can promote their formation) are present at temperatures of between 450 and
250 °C - where reaction rates are at their highest.
By soot-blowing with self-produced steam injection most of the energy will be recovered by the
boiler itself (80 – 90 %). [74, TWGComments, 2004]
Cross-media effects
Consumption of the soot-blowing agent e.g. high pressure water, low pressure water, steam
(only partially).
Noise may be an issue with some of the techniques e.g. explosive cleaning, mechanical rapping.
Operational data
Those techniques that allow continuous on-line tube cleaning (normally operated once per 8
hours shift) normally have reduced downtime for boiler maintenance cleaning operations.
[74, TWGComments, 2004]
Usually manual cleaning is done when fouling has induced a flue-gas temperature increase of 20
to 50 °C, i.e. 1.5 to 3 % decrease in energy efficiency.
Potential mechanical damage of the boiler structure/tubes can occur, particularly with explosive
cleaning and mechanical rapping.
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Tube erosion can lead to decreasing energy efficiency and eventually tubes will require
replacement.
Applicability
All waste incinerators with boilers.
Economics
No data supplied.
Driving force for implementation
Improved availability and heat recovery, reduced corrosion, emissions, and energy
consumption. [74, TWGComments, 2004]
Example plants
All waste to energy plants. [74, TWGComments, 2004]
Specific cleaning system in several plants in NL and DK e.g. AVI ARN Beuningen (explosive
cleaning with gas), AVI Amsterdam and AVI Wijster (explosive cleaning with dynamite). [74,
TWGComments, 2004]
Reference literature
[2, infomil, 2002] p 51 – 52, [1, UBA, 2001] p 119, [64, TWGComments, 2003]
4.4 Flue-gas treatment
4.4.1 Factors to consider when selecting flue-gas treatment systems
4.4.1.1 General factors
[54, dechefdebien, 2003]
The following (non-exhaustive) list of general factors requires consideration when selecting
flue-gas cleaning (FGT) systems:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
type of waste, its composition and variation
type of combustion process, and its size
flue-gas flow and temperature
flue-gas content, size and rate of fluctuations in composition
target emission limit values
restrictions on discharge of aqueous effluents
plume visibility requirements
land and space availability
availability and cost of outlets for residues accumulated/recovered
compatibility with any existing process components (existing plants)
availability and cost of water and other reagents
energy supply possibilities (e.g. supply of heat from condensing scrubbers)
availability of subsidies for exported energy
tolerable disposal charge for the incoming waste (both market and political factors exist).
reduction of emissions by primary methods
release of noise
arrange different flue-gas cleaning devices if possible with decreasing flue-gas temperatures
from boiler to stack.
[74, TWGComments, 2004]
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4.4.1.2 Energy optimisation
Some flue-gas treatment techniques can add significantly to the overall energy requirements of
the incineration process. It is necessary to consider the additional energy requirements imposed
by applying lower ELVs. The following key observations can be made:
•
•
•
•
reducing dust emissions including boiler ash (and metals filtered with dust) generally
requires additional filtration and increases energy consumption
reducing NOX emissions to below 100 mg/m³ is most often achieved using SCR - which,
because it is typically only used as a low dust system in waste incineration, is situated at the
clean gas end of the FGT system. It therefore usually requires some additional energy for
flue-gas reheating. Very low SOX levels in the raw flue-gas may allow SCR to be used
without reheat (see 2.5.5.2.2). The energy required for the operation of additional flue-gas
cleaning (to meet very low ELVs) if supplied from that generated by the incinerator, will
result in a reduction of that available for export
boiler exit temperature has a key influence on FGT energy requirements – if below the acid
dew point additional energy input will be required to heat the flue-gas
in general, placing the FGT components so that those requiring the highest operational
temperatures, precede those that operate at lower temperatures, results in lower overall FGT
energy demand (but this cannot be achieved in some cases e.g. SCR generally requires clean
gas and must, therefore, be placed after the lower temperature gas cleaning stages).
[64, TWGComments, 2003] [74, TWGComments, 2004]
4.4.1.3 Overall optimisation and the “whole system” approach
As well as considering the energy aspects (see sections on energy above), there is benefit from
considering the FGT system as a whole unit. This is particularly relevant to the removal of
pollutants because the units often interact, providing a primary abatement for some pollutants,
and an additional effect on others. Depending on the position in the cleaning sequence different
cleaning efficiency values are obtained. [74, TWGComments, 2004] Multifunctional devices are
common, for example:
•
if a Bag-House Filter (BF), is used downstream of reagent injection, in addition to its dedusting effect, it acts as a complementary reactor. The pressure drop through the fabric
material distributes the flue-gas on the adhered cake which contains some deposited reagent
and, due to the low velocity of the gases, the residence time is long. A BF can, therefore,
contribute to the treatment of acid gases, gaseous metals such as Hg and Cd, and POPs
(Persistent Organic Pollutants) such as PAH, PCB, dioxins and furans
• in addition to acid gas treatment, wet scrubbers can help with the capture of some
particulate and, if the pH is low enough or with the use of scrubber reagents, of Hg
• SCR de-NOX has an additional destruction effect on dioxins if designed (sized) accordingly
• the adsorption by activated carbon and lignite coke has an effect on dioxins as well as on Hg
and other substances.
[64, TWGComments, 2003] [54, dechefdebien, 2003]
4.4.1.4 Technique selection for existing or new installations
Overall optimisation and the interface between FGT systems components (as well as the rest of
the incineration process) is important for both new and existing installations. With existing
installations the number of options may be more severely restricted than with new installations.
Comments regarding inter-process compatibility may be found in the sections that deal with
individual FGT techniques.
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4.4.2 Reduction of dust emissions
The application of a system to remove dust from the flue-gas is generally considered essential
for all waste incineration installations. This section considers the locating of a dust removal
stage before other subsequent FGT stages (i.e. pre-dedusting) or after other FGT systems as a
final flue-gas polishing system. In some cases double filtration is applied, this is also considered
in this section.
4.4.2.1 Application of a pre-dedusting stage before other flue-gas treatments
Description
This section considers the locating of a dust removal stage, generally after pre-dedusting in the
boiler stage, [74, TWGComments, 2004] but before other subsequent FGT stages.
The following pre-dedusting systems are used for waste incineration:
•
•
•
cyclones and multi-cyclones
electrostatic precipitators (ESPs)
bag filters (BFs).
The individual techniques themselves have already been described in Section 2.5.3
Achieved environmental benefits
Reduction of emissions to flue-gas stream by reducing particulate load on later FGT processes.
Separation of the fly ash from the FGT residues allows:
•
•
reductions in the quantity of FGT residues produced
separate treatment of fly ashes for possible re-cycling uses.
Separate collection of the flue-gas components will not be of any environmental benefit if the
separated residues are then re-mixed afterwards. Consideration of downstream aspects is
therefore required to evaluate the possibility of real benefits. [64, TWGComments, 2003]
ESPs and cyclones may have problems reaching the lower of the generally applied dust
emission standards on their own. However, they are useful as pre-dedusters and contribute to
meeting the lowest of emission levels when applied in combination with other techniques.
[2, infomil, 2002] Cyclone collection efficiency increases directly as a function of the dust load,
flue-gas-flowrate, particle size and density. As the fly ash particles are fine, the density is low
and the dust load and flue-gas flowrate changes, so the dust removal efficiency of cyclones is
limited. Normally dust concentration values no lower than 200 – 300 mg/m³ can be reached.
Multi-cyclones, which are based on the same removal principle, can reach somewhat lower
values, but values below 100 – 150 mg/m³ are very difficult to achieve.
[2, infomil, 2002] An ESP can reach substantially lower dust concentration values than (multi-)
cyclones. Depending on the design and the siting in the flue-gas treatment system (pre- or enddedusting), and the number of fields, dust emission concentration values of 15 to 25 mg/m³ can
normally be achieved. Achieving values below 5 mg/m³ are possible with larger numbers of
fields (2 or 3) and increased ESP surface (and hence increased cost and space requirement for
implementation).
A specific version of the ESP is the wet ESP. This is not generally applied to pre-dedusting on
account of the flue-gas temperatures in that area. [64, TWGComments, 2003] in general it is
more used for polishing after scrubbing. [74, TWGComments, 2004]
Waste Incineration
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Chapter 4
Bag filters are generally very efficient dust removers. Where bag filters are used, most
commonly reagents are also injected (although this is not always the case) to build pre-coat
layer over the bags to protect from corrosion and help filtration (especially for in-depth
filtration). [74, TWGComments, 2004] The reagents used are commonly lime and activated
carbon. The presence of the activated carbon reduces the dioxin loads passing on to the
subsequent flue-gas cleaning stages. For wet systems this helps to reduce memory effect dioxin
build up in the scrubber materials.
Cross-media effects
Cross-media effects are identified in the table below with available data:
Criteria
Energy requirements:
Cyclone
Units
Multi-cyclone
Dry ESP
kWh/tonne waste input
Wet ESP
Bag filter
Value
Comments
Low
Low
lower
efficiency
for
removal of particles of sizes
<5 micrometers.
Common technology
Higher (electrostatic
loading)
Higher (pressure
drop)
Highest by pressure
drop and pulsing
high pressure air
cleaning
Fly ash
Residue - type
Residue - amount
kg/t waste input
Water consumption
l/t waste input
Effluent production
l/t waste input
Plume visibility
yes/no
12 – 20 (<50)
yes
Common technology
Separate collection of fly
ash from main chemical
pollutants possible if not
operated with reagents
Without reagents (with
reagents)
• for wet ESP
• for gas cooling prior to
bag filters
• wet ESP effluent
• wet ESP highest visible
plume
Table 4.20: Cross-media effects associated with the use of various pre-dedusters
[74, TWGComments, 2004]
For this technique the most significant cross-media effects are:
•
•
•
•
•
energy consumption of bag filters is higher than other systems due to higher pressure loss
ESP requires electricity for its operation
generation of fly ash from the gas cleaning
flue-gas PCDD/F-concentrations may increase during their residence time in the ESP,
particularly when operated at temperatures above 200 to 450 °C
the FGT residues and fly ash can be separated using this pre-dedusting technique.
Operational data
Pre-dedusting reduces dust loads on subsequent FGT systems. These may then be reduced in
capacity, experience reduced clogging risks, and hence downstream units may be designed
smaller and with some degree of reduced costs.
Care should be taken concerning the level of ash in the hopper as well as cinder (especially if
bag filters are implemented directly after the boiler) to prevent risk of fire.
318
Waste Incineration
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[2, infomil, 2002] Cyclones are a relatively simple design without moving parts (except for the
transport systems used for the removal of the fly ash from the bottom) and, therefore, can have
high availability at relatively low costs. However, the pressure drop of the flue-gas stream is
relatively high, resulting in an increased power requirement for the flue-gas fan and therefore in
additional energy consumption.
[2, infomil, 2002] For the proper functioning of an ESP, it is important that the flue-gas stream
is evenly distributed over its total surface. The pressure drop of the flue-gas over an ESP is low,
reducing energy consumption. However some pre-dedusting equipment (e.g. ESPs, filters)
require electricity for their operation.. [74, TWGComments, 2004] Further information on ESP
systems can be found in Chapter 2.5.3.
ESPs can be divided into multiple compartments (usually 1 – 4 successive fields), each with
their own electrical system. This gives the advantage that, even during a breakdown of one of
the electrical systems (e.g. short-circuit by dust clogging or broken high voltage wires), [74,
TWGComments, 2004] a relatively large part of the dust removal capacity is still available.
Bag filters are often divided in compartments that may be isolated for maintenance purposes, an
even flue-gas distribution is important for optimal performance.
Description of factors
affecting criteria
Criteria
•
Complexity
•
•
Flexibility
additional process units
required
critical operational
aspects
ability of technique to
operate under range of
input conditions
Evaluation
(High/Medium/Low)
or data
M
H
Comment
The extra process units
adds complexity, but can
simplify later operations.
Each of the systems can be
applied to variable flue-gas
flows and composition
Bag filters require the most
attention, and cyclones the
least. ESPs in between.
Skill
requirements
•
Other
requirements
• Bag filters may require addition of reagents for corrosion and fire protection
notable extra training or
manning requirements
H/M
Table 4.21: Operational data associated with the use of pre-dedusting systems
Waste Incineration
319
Chapter 4
The table below provides a comparison of the various dust removal systems (used at the pre and
post de-dusting stages):
Dust removal
systems
Cyclone and
multicyclone
ESP - dry:
ESP- wet:
Bag filter
Typical emission
concentrations
- cyclones:
200 – 300 mg/m³;
- multicyclones:
100 – 150 mg/m³.
Advantages
- robust, relatively simple
and reliable.
- applied in waste
incineration.
- relatively low power
requirements.
- can use gas temperatures in
the range of 150 – 350 °C
- widely applied in waste
incineration.
<5 – 25 mg/m³
<5 – 20 mg/m³
- able to reach low emission
concentrations - sometimes
applied in waste incineration
<5 mg/m³.
- widely applied in waste
incineration
- the layer of residue acts as
an additional filter and as an
adsorption reactor
Disadvantages
- only for pre-dedusting
- relatively high energy
consumption (compared to
ESP)
- formation of PCDD/F risk
if used in range 450 - 200 °C
- little experience in waste
incineration
- mainly applied postdedusting
- generation of process waste
water
- increase of plume visibility
- relatively high energy
consumption (compared to
ESP)
- sensitive to condensation
of water and to corrosion
Table 4.22: A comparison of dust removal systems
[2, infomil, 2002]
Applicability
The applicability of the pre-dedusting technique is assessed in the following table:
Criteria
Waste type
Plant size range
New/existing
Inter-process compatibility
Key location factors
•
•
•
•
•
•
Evaluation/Comment
applicable to all waste types
may not be required for low raw gas dust concentrations
no restriction
space may be a factor for existing plants
greater temperature control required for bag filter
space required for additional process unit
Table 4.23: Assessment of the applicability of pre-dedusting
Economics
The key aspects of this technique are:
•
•
•
•
320
increased capital and investment costs - for additional process units
increased energy costs particularly for bag filtration
possible cost reductions for disposal where outlets are available for segregated fly ash
possible increased cost of handling additional residue streams (either for recovery or
disposal).
Waste Incineration
Chapter 4
Investment costs for a two line MSWI of total capacity 200000 t/yr are estimated as
[12, Achternbosch, 2002]:
•
•
•
ESP (3 field)
ESP (2 field)
fabric filter
EUR 2.2 million
EUR 1.6 million
EUR 2.2 million (not clear if this includes an upstream flue-gas cooler).
The unit operational costs of a bag filter for pre-dedusting may be higher due to the higher
energy use associated with the pressure drop and the reagent injection. However, the greater
dust and the other pollutant removal capacity of bag filters (particularly when used with reagent
injection) can result in reduced costs for subsequent components of the FGT system.
Driving force for implementation
This technique has been implemented where:
•
•
•
•
the fly ash removed can be treated and recycled
smaller capacity downstream FGT equipment is required (requirement for dust loads are
reduced)
improvements in the operation of downstream FGT systems
there exists a preference for the removal of PCDD/F before wet scrubbing to reduce
memory effects.
Example plants
Widely applied technique in many incineration plants.
Reference literature
[2, infomil, 2002], [55, EIPPCBsitevisits, 2002] [64, TWGComments, 2003]
4.4.2.2 Application of an additional flue-gas polishing system
Description
This technique relates to the application of flue-gas polishing systems for the final reduction of
dust emissions after other FGT has been applied, but before the final release of stack gases to
the atmosphere. The main systems applied are:
• bag filters
• wet-ESP
• electrodynamic venture scrubbers
• agglo-filtering modules
• ionizing wet scrubber.
[74, TWGComments, 2004]
Also, it is possible to consider that the addition of a final wet flue-gas treatment system is a
polishing treatment after other systems that deal with acid gases etc. This addition is generally
made to specifically control HCl emissions where they are highly variable. This additional
treatment is considered in Section 4.4.3.6. [64, TWGComments, 2003]
The main technical components have already been described in Section 2.5.3
Polishing devices are also implemented to remove droplets (especially fine ones). They are
generally implemented to prevent fouling in downstream devices such as SCR. [74,
TWGComments, 2004]
Waste Incineration
321
Chapter 4
Achieved environmental benefits
Further reduction of emissions to air beyond that already achieved by other FGT components
are as follows:
Substance(s)
Reduction
efficiency
range
(%)
Dust
½ hour
average
mg/Nm³)
<30
Achievable emission ranges
daily
annual
specific
average
average
emission
mg/Nm³)
mg/Nm³)
(g/tonne
waste input)
0.04 – 5
<0.5
Comments
Note: the precise final emission level realised will depend on the inlet level to the final dust removal stage (itself
depending on the performance of the earlier stages applied) and the efficiency of the final dust removal stage
used. The figures given here provide a guide to the sort of emission levels that are generally seen where a
polishing stage is added.
Table 4.24: Emission levels associated with the use of BF flue-gas polishing systems
[2, infomil, 2002], [1, UBA, 2001]
In addition to reduction of dusts, emissions to air of the following substances can also be
reduced:
•
•
•
heavy metals - as their emission concentrations are usually associated with dust removal
efficiency
mercury and PCDD/F– where carbon (usually with alkaline reagent) is added as an
absorbent on bag filters
acid gases - where alkaline reagents are added to protect bag filters.
The benefits of these additional reductions may be small where upstream techniques are already
being applied, that reduce the concentrations in the flue-gas to a low level.
Cross-media effects
Cross-media effects are identified in the following table:
Criteria
Energy requirements
Reagent consumption
Units
Range of
achieved values
kWh/t waste input
kg/t waste input
Water Consumption
Residue - type
kg/t waste input
Residue - amount
Plume visibility
+/0/-
+/0
Comments
Increased due to pressure
drop across process unit
More reagent used
Wet ESP leads to water
discharge, that may be
recycled in the process.
Fly ashes and/or other
substances removed in the
polishing filter generally
become an additional
solid waste stream
Varies according to input
loads and applied
upstream FGT techniques
but will generally be low
Non-dry systems can
increase plume visibility
Table 4.25: Cross-media effects associated with the use of additional flue-gas polishing
[74, TWGComments, 2004]
322
Waste Incineration
Chapter 4
For this technique the most significant cross-media effects are:
•
•
consumption of energy due to pressure drop across the bag filters
production of solid residues (negligible in case of polishing purpose).
Operational data
Effective maintenance of bag filters is very important to ensure their effective operation and
hence low emissions. The pressure drop across the bags is monitored in order to maintain a cake
on the filter. It can also be used as a means to detect bag damage (such as irreversible fouling).
Dust emissions can usually be controlled to a very low level, simply by more closely observing
the pressure drop and adopting stricter criteria (i.e. less latitude allowed before maintenance
action is taken) for bag replacements. Analysis of the filter media may also be used to assess the
reagent dose rate required and to assess its condition and its remaining lifetime.
Multiple chamber systems which are independently monitored for pressure drop, and bag
houses with sufficient over capacity to allow damaged areas to be shut off for bag replacement,
improve the capability to meet the lowest of emission limit values.
Bag filters are often divided into compartments that may be isolated for maintenance purposes
An even flue-gas distribution is important for optimal performance.
Description of factors
affecting criteria
Criteria
•
Complexity
•
•
Flexibility
Skill
requirements
•
additional process units
required
critical operational aspects
Evaluation
(High/Medium/Low)
or data
Comment
H
Additional process units
add complexity
ability of technique to
operate under range of
input conditions
M
significant extra training
or manning requirements
H
As a tail-end technique,
the process will be less
subject to such
variations.
Bag filters require close
maintenance
Table 4.26: Operational data associated with the use of flue-gas polishing
Applicability
The applicability of this technique is assessed in the table below:
Criteria
Waste type
•
Plant size range
•
New/existing
•
•
•
Inter-process compatibility
Key location factors
•
•
•
Evaluation/Comment
the additional abatement of heavy metals (with flue-gas) using
these techniques increases suitability where these require
further reduction
larger plants with higher gas flows can achieve greater
reductions in local contributions to emissions by applying such
techniques
applicable to new and existing plants
as and end-of-pipe technique - well suited to retrofits where
dust requires reduction
existing plants already achieving low emission levels
(<10 mg/Nm³ daily average) through other means may not
benefit greatly from the addition of this technique
temperature and acid dew point needs consideration
provides effective gas cleaning step prior to SCR
space available can be a restriction (additional process unit)
Table 4.27: Assessment of the applicability of flue-gas polishing
Waste Incineration
323
Chapter 4
Economics
Cost information for this technique is given in Section 10.2.4
The key cost aspects of this technique are:
•
•
increased capital investment costs of additional process unit
increased operational costs - mainly due to energy requirements for pressure drop, provision
of compressed air for back pulsing of BF (if used), and additional maintenance costs.
Driving force for implementation
This technique has been implemented where:
•
•
•
legislation has required low permit emission limit values for dust, (dust related) heavy
metals, or where additional dioxin and acid gas reduction potential is required
local air quality concerns exist that may be effected by the process
it acts as a de-duster for a subsequent SCR process.
Example plants
Examples in Germany, Austria and the Netherlands.
Other examples in France:
Toulouse: agglo-filtering modules after wet FGT
Tronville: tail end bag filter with lime + activated carbon injection (downstream wet FGT)
Ocreal: tail end bag filter with activated carbon injection for PCDD/F treatment
[74, TWGComments, 2004]
Reference literature
[3, Austria, 2002], [2, infomil, 2002], [64, TWGComments, 2003]
4.4.2.3 Application of double bag filtration
Description
This technique considers the application of two bag filters in series in the FGT system. The 2
bag filters may not be immediately adjacent to one another (i.e. other FGT components may be
used in between them). This technique does not refer to situations where a bag filter is
combined with a non-bag filter e.g. with and ESP, cyclone, or wet scrubber etc.
Achieved environmental benefits
Additional reduction in dust emissions to air. 24 hour average levels of below 1 mg/m3 can be
achieved in nearly all situations.
Separation of FGT residues is possible i.e. separation of fly ash from the FG neutralisation
residues. This may then allow the recovery of one or other fraction where suitable outlets exist.
324
Waste Incineration
Chapter 4
Cross-media effects
Cross-media effects are identified in the table below:
Criteria
Units
Range of
achieved values
Energy requirements
kWh/t waste input
High
Reagent consumption
kg/t waste input
3 – 15
dry
Residue - type
Residue - amount
kg/t waste input
15 – 25
Plume visibility
+/0/-
+/0
Comments
Increased due to pressure
drop across process units
Depending on the kind of
reagent
Fly ashes and salts residue
Varies according to input
loads and applied
upstream FGT techniques
Spray dry systems can
increase plume visibility
Table 4.28: Cross-media effects associated with the use of double filtration
[74, TWGComments, 2004]
For this technique the most significant cross-media effects are:
•
•
consumption of energy due to pressure drop across the FGT system as a whole, will be
significantly higher
production of solid residues (usually separated from other flue-gas residues).
It is reported that the additional energy consumption from the use of two bag filters in series
(even if separated), although providing some potential benefits in terms of additional pollutant
control, requires higher power fans to overcome the pressure drop and therefore higher electrical
consumptions.
Operational data
Description of factors
affecting criteria
Criteria
•
Complexity
•
•
Flexibility
Skill
requirements
•
Evaluation
(High/Medium/Low)
or data
additional process units
required
H
critical operational aspects
ability of technique to
operate under range of
input conditions
notable extra training or
manning requirements
H
H
Comment
Additional process units
add complexity
Input to first stage can vary
greatly without large
variation in output
emissions from second
stage.
Bag filters require close
maintenance. This is
increased because of extra
unit.
Table 4.29: Operational data associated with the use of double filtration
Bag filters are often divided into compartments that may be isolated for maintenance purposes.
An even flue-gas distribution is important for optimal performance.
Applicability
This can be applied to any incineration process, but most applicable where very low dust ELVs
are applied, or separation of FGT residue components is desired.
Waste Incineration
325
Chapter 4
The applicability of this technique is assessed in the table below:
Criteria
Waste type
Plant size range
New/existing
Evaluation/Comment
•
•
•
•
•
Inter-process compatibility
Key location factors
•
•
•
•
any
larger plants with higher gas flows can achieve greater
reductions in local contributions to emissions by applying such
techniques
applicable to new and existing plants
where the additional filtration is added as an end-of-pipe
technique it is well suited to retrofits at an existing installation
existing plants already achieving low emission levels
(<10 mg/Nm³ daily average) through other means may not
benefit greatly from the addition of this technique
temperature and acid dew point needs consideration
provides effective gas cleaning step prior to SCR
this requires larger space to implement
location of the industrial plant that can re-cycle the salts
Table 4.30: Assessment of the applicability of double filtration
[74, TWGComments, 2004]
Economics
Additional cost of an extra process units. Additional energy costs and maintenance.
Driving force for implementation
Most often applied where low emissions for dust are required (e.g. values below 2 mg/Nm3).
Possibility of salt recycling.
Example plants
Several examples in Belgium, Germany, France and others.
Reference literature
[64, TWGComments, 2003]
4.4.2.4 Selection of bag filter materials
Description
The filter material selected must be suited to the physical and chemical conditions under which
it will operate.
The key characteristics of fabrics for use in gas filtration include maximum operational
temperature and resistance to acids, alkalis and flexing (due to bag cleaning). Also gas humidity
can affect the strength and dimensional stability of the fabrics, due to hydrolysis. Several basic
fibre properties are summarised below, some may be coated or impregnated with special
chemicals (e.g. sulphur). [74, TWGComments, 2004]
326
Waste Incineration
Chapter 4
Fabric
Cotton
Polypropylene
Wool
Polyester
Nylon
PTFE
Polyimide
Fibreglass
Notes:
1.
2.
Maximum
temperature
(°C)
80
95
100
135
205
235
260
260
Resistance
Acid
Alkali
Poor
Excellent
Fair
Good
Poor to fair
Excellent
Good
Fair to good
Good
Excellent
Poor
Good
Excellent
Excellent
Good
Fair to good
Physical
flexibility
Very good
Very good
Very good
Very good
Excellent
Fair
Very good
Fair
not all of these materials are commonly used in incineration – see operational data below
Some operational experiences suggest a common maximum operational temperature to
be 200°C.
Table 4.31: Operational information for different bag filter materials
[2, infomil, 2002] [67, Inspec, 2004]
Achieved environmental benefits
Reliable filter material, suited to its application provides for reliable emissions abatement.
Cross-media effects
If the media is not appropriate, pressure drop may increase and limit waste throughput.
If high pressure air is required for cleaning the bag it can reduce the bag lifetime.
Operational data
See information provided in description above.
Increasing temperature may lead to melting of any plastic components in the fabric material,
and the potential for fires. High humidity in the flue-gas may cause the filter materials to stick
together and lead to shut downs. [74, TWGComments, 2004]PTFE covering of sheets/foils can
be used to improve the removal of such sticky salts and solid particles from the bags.
Operational improvements in semi-wet systems (see also Section 4.4.3.2) are reported to have
been obtained by using PTFE in an MSWI facility in Prague (CZ) and in Schwandorf
(Germany).
Regular bag analysis may help to assess its remaining lifetime [74, TWGComments, 2004]
Several filtration media are reported to not be commonly used in MSWI e.g. cotton, wool,
propylene. In MSWI, the main media are: polyimide (known as P84), PPS (rarely), PTFE,
fibreglass (with or without PTFE coating). Some fibres may be combined (e.g. P84+PTFE for
higher resistance at high temperature)
Chemical reactions in the absorbent media may effect operational temperature. Quality of the
scrim is also of importance, as well as fibre quality.
Applicability
Correct bag material selection is relevant to all waste incineration installations using bag filters.
Economics
Cost of the different bag filters varies.
Driving force for implementation
Main driving forces are abatement performance and engineering suitability.
Example plants
Wherever bag filters are applied i.e. very widely.
Waste Incineration
327
Chapter 4
Reference literature
[2, infomil, 2002], [64, TWGComments, 2003]
4.4.3 Reduction of acid gas emissions
The sections that follow within this section deal with the following:
•
description and assessment of the performance generally achieved by the main techniques
applied for acid gas reduction – including consideration of applicability to various situations
description and assessment of some other technological and procedural options relevant to
acid gas removal.
•
4.4.3.1 Wet scrubbing systems
Description
This technique has already been described in Section 2.5.4.
Wet scrubbers generally have at least two effective stages, the first at low pH removes mainly
HCl and HF, the second stage is dosed with lime or sodium hydroxide and operated at a pH of
6 - 8 primarily for the removal of SO2. Scrubber may sometimes be described as three or more
stages – the additional stages generally being sub-division of the first low pH stage for specific
purposes.
Achieved environmental benefits
Reduction of emissions to air as follows:
Substance(s) Reduction
efficiency
range
(%)
½ hour
average
(mg/Nm³)
Achieved emissions
daily
annual
average
average
(mg/Nm³)
(mg/Nm³)
Comments
specific
emission (g/t
waste input)
HCl
0.1 – 10
<5
0.1 - 1
1 – 10
HF
<1
<0.5
<0.1- 0.5
<0.05 – 2
SO2
<50
<20
<10
<5 – 50
Very stable outlet
concentrations
Very stable outlet
concentrations
Requires reaction stage
and absorbent (lime or
NaOH)
SO2 ½ hour averages
may fluctuate more
Table 4.32: Emission levels associated with the use of wet scrubbers
[1, UBA, 2001, 2, Infomil, 2002, 12, Achternbosch, 2002]
Wet FGT systems provide the highest removal efficiencies (for soluble acid gases) of all FGT
systems with lowest excess stoichiometric factors. [74, TWGComments, 2004]
328
Waste Incineration
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Whilst single stage filtration based FGT systems (e.g. semi-wet, dry) combine and collect
residues together, this is not generally the case with wet systems. The wet systems can treat
HCl, HF and SO2 separately from particulate, etc which is often removed before. Having noted
this, wet systems do provide some additional reductions of the following substances:
•
•
•
•
dust
- where scrubber capacity is large enough to prevent clogging (most usually a
pre-dedusting stage is used before the wet scrubber to reduce dust loads and
prevent operational problems up to 50 % of the dust input) [74,
TWGComments, 2004]
PCDD/F - if carbon impregnated packing materials are used it is possible for reductions
of 70 % to be seen across the scrubber, otherwise removal rates are negligible.
Activated carbon or coke may be added to the scrubber for a similar purpose,
with reported higher removal efficiencies [74, TWGComments, 2004]
Hg2+
- if a low pH (~1) first stage scrubber is used, and HCl concentrations in the
waste provide for acidification of this stage, then HgCl2 removal can take place,
but metallic Hg is not generally effected. [64, TWGComments, 2003]
other pollutants - when water soluble pollutants like bromine and iodine are present in the
raw gas, they may be condensed at the low temperatures in the scrubber and by
that get into the scrubber waste water.
Cross-media effects
Cross-media effects are identified in Table 4.35 below:
Criteria
Energy requirements
Units
kWh/t waste input
Reagent consumption
kg/t waste input
Reagent stoichiometry
Ratio
Range of values
19
2 – 3 (NaOH) or
~10 (CaO) or
5 – 10
(lime/limestone)
1.0 – 1.2
Kg (wet)/t waste input
Kg (dry)/t waste input
10 – 15
3–5
Water consumption
l/t waste input
100 – 500
Effluent production
l/t waste input
250 – 500
+/o/-
+
Residue - type
Residue - amount
Plume visibility
Comments
Pumps add demand
Lowest of all systems
Lowest of all systems
Effluent treatment sludge;
in some cases HCl or
gypsum may be recovered
Lowest of all systems. This
figure does not include
separately removed fly ash
approx. 16 kg/t input.
Highest of all systems but
can be reduced by treatment
and recirculation
/condensation and by low
temperatures before
scrubber inlet.
Treatment required before
discharge or re-use
High gas moisture content,
but can be reduced by
reheat/condensation
Note: the data in this table aims to provide the typical operational range. The precise amounts of residues and
effluents produced will depend on many factors including raw gas concentrations (waste related), flowrates,
reagent concentrations, etc.
Table 4.33: Cross-media effects associated with the use of wet scrubber FGT
[1, UBA, 2001, 2, infomil, 2002, 12, Achternbosch, 2002]
Waste Incineration
329
Chapter 4
For this technique, the most significant cross-media effects compared to other options are:
•
•
•
•
•
•
•
lowest reagent consumption rates
lowest solid residue production rates
higher water consumption
production of an effluent that requires management
increased plume visibility
PCDD/F build up (memory effect) on scrubber plastic components requires addressing
if input temperature is too high the material used in the wet scrubber may be destroyed. [74,
TWGComments, 2004]
Effluent production is usually considered as 300 kg/t MSW input, assuming a consumption of
water of 1000 kg/t MSW input. These figures are higher than those quoted in the table above.
[64, TWGComments, 2003]
Operational data
Description of factors
affecting criteria
Criteria
•
Complexity
•
•
Flexibility
Skill
requirements
•
Evaluation
(High/Medium/L
ow) or data
Comment
additional process units
required
critical operational aspects
ability of technique to
operate under range of
input conditions
H
The number of process units is
greater than other systems
H
notable extra training or
manning requirements
H
Very robust - highest ability of
all systems to achieve emission
reduction of HCl/HF under
fluctuating inlet concentrations.
The associated effluent treatment
plant requires a high skill input
Table 4.34: Operational data associated with the use of wet FGT
The main issues are:
PCDD/F build up in wet scrubbers can be a problem, in particular from maintenance and start
up periods, and may require specific measures to be taken.
Effluent treatment requires high skilled operation to achieve low emission levels.
For effective operation, wet scrubbers require flue-gases that have already been de-dusted using
e.g. ESP or BF. [64, TWGComments, 2003]
The flexibility of wet scrubbing in respect of variation in inlet concentration applies mainly to
HCl and HF. Sometimes additional treatment is required to meet mercury ELVs, for example:
the injection of a complex builder in the basic scrubber; injection of activated carbon in the
acidic scrubber; injection of oxidising agent or abatement in the gas phase with adsorbent. [64,
TWGComments, 2003]
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Applicability
The applicability of this technique is assessed in the table below:
Criteria
Waste type
Plant size range
New/existing
Inter-process compatibility
Key location factors
•
•
•
•
•
•
•
•
•
Evaluation/comment
can be applied in principle to any waste type
particularly suited to highly variable inlet gas compositions
(e.g. hazardous wastes)
not restricted but generally applied at medium to larger plants
where economies of scale exist
widely applied at many existing plants
low flue-gas outlet temperature (approx. 70 °C) requires
reheat for subsequent FGT systems e.g. bag filters and SCR
separate (pre-) collection of fly ash possible
increased plume visibility (unless counter measures taken)
salt water effluent (post treatment) requires discharge (or
evaporation which requires energy)
can permit recovery of HCl, salt, gypsum
Table 4.35: Assessment of the applicability of wet FGT
Economics
Capital cost information for the technique is shown in the table below:
FGT component
Two stage wet scrubber
Estimated investment
cost (M EUR)
5
Three stage wet scrubber
External scrubber effluent
evaporation plant
Spray absorber for internal
effluent evaporation
7
Comments
including waste
treatment
including waste
treatment
water
water
1.5 – 2
1.5
Cost estimate believed to
be on the low side
Costs estimated relate to a 2 line MSWI of total capacity 200 kt/yr
Table 4.36: Estimated investment costs of selected components of wet FGT systems
[12, Achternbosch, 2002] [74, TWGComments, 2004]
The key cost aspects of this technique compared to the alternatives are:
•
•
•
higher capital investment costs than other systems, mainly due to the effluent treatment
plant and the higher number of process units required
operational costs associated with disposal of residues may be lower, due to the lower
specific residue production, which are normally wet. [74, TWGComments, 2004]
labour costs higher due to increased complexity of system.
Driving force for implementation
This technique has been implemented where:
•
•
•
•
•
emission limit values have been set at or below those detailed in Directive 2000/76/EC
disposal costs for flue-gas treatment residues are high
input waste composition is particularly difficult to predict/control
input waste may contain high and variable loads of acid gases or heavy metals (i.e. ionic
mercury) [74, TWGComments, 2004]
salt containing effluent may be discharged (e.g. to the sea).
Example plants
Wet flue-gas scrubbing is widely used throughout Europe for a full range of waste types.
Waste Incineration
331
Chapter 4
Reference literature
[1, UBA, 2001, 2, infomil, 2002, 3, Austria, 2002, 12, Achternbosch, 2002] [64,
TWGComments, 2003]
4.4.3.2 Semi-wet scrubbing systems
Description
This techniques has already been described in Section 2.5.4.
The diagram below shows a typical semi-wet FGT system, with a contact tower on the left and
downstream deduster:
Figure 4.6: Diagram showing typical design of a semi-wet FGT system
Achieved environmental benefits
Reduction of emissions to air as follows:
Reduction
Substance efficiency
(s)
range
(%)
½ hour
average
mg/Nm³)
Achieved emissions
daily
Annual
Specific
average
average emission (g/t
mg/Nm³)
mg/Nm³) waste input)
HCl
<50
3 – 10
2
4 – 10
HF
<2
<1
<0.5
<2
SO2
<50
<20
<10
5 – 50
Comments
Lowest values
achieved with higher
reagent dosing and
regulation control.
Peaks can be dealt
with by upstream
HCl analyser. The
semi-wet process
can capture SO2 at
the same time as
HCl and HF in the
same scrubber.
Table 4.37: Emission levels associated with the use of wet scrubbers
[1, UBA, 2001, 2, infomil, 2002, 12, Achternbosch, 2002], [64, TWGComments, 2003]
[74, TWGComments, 2004]
There is no effluent discharge from semi-wet scrubbers as the amount of water used is generally
lower than with wet scrubbers and that used is evaporated with the flue-gases. If of suitable
quality, other site waste water (e.g. rainwater) may be sent to the FGT system.. [74,
TWGComments, 2004]
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Waste Incineration
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Semi-wet FGT systems provide high removal efficiencies (for soluble acid gases). Low ELVs
can be met by adjusting the reagent dosing rate and design point of the system, but generally at
the cost of increased reagent consumption and residue production rates.
Semi-wet systems are used with fabric filters to remove the reagents added and their reaction
products. Reagents, other than alkaline reagents, can also be added to adsorb other flue-gas
components (e.g. activated carbon for Hg and PCDD/F).
They are most commonly used as a single stage reactor/filter for the combined emission
reduction of:
•
•
•
dust
PCDD/F
Hg
- filtered by the fabric filter
- adsorbed if activated carbon is injected as well as alkaline reagent
- adsorbed if activated carbon is injected as well as alkaline reagent.
Cross-media effects
Cross-media effects are identified in the following table:
Units
Range of achieved
values
Energy requirements
kWh/t waste input
6 – 13
Reagent consumption
kg/t waste input
12 – 20 (lime)
Reagent stoichiometry
Ratio
1.4 – 2.5
kg
Not supplied
kg/t waste input
25 – 50
Water consumption
l/t waste input
Not supplied
Effluent production
l/t waste input
Not supplied
+/o/-
0
Criteria
Residue - type
Residue - amount
Plume visibility
Comments
Pressure drop across
bag filter creates
energy demand
Mid range of system
options applied
Lowest values achieved
with recirculation/with
low pollution load
wastes
Combined FGT and fly
ash
Combined FGT and fly
ash
Lowest where FGT
inlet temperature is low
otherwise water for
cooling also necessary
Mid range of applied
systems
Note: the data in this table aims to provide the typical operational range. The precise amounts of residues and
effluents produced will depend on many factors including raw gas concentrations (waste related), flowrates,
reagent concentrations etc.
Table 4.38: Cross-media effects associated with the use of semi-wet acid gas treatment
Source [3, Austria, 2002, 12, Achternbosch, 2002], [64, TWGComments,
TWGComments, 2004]
2003]
[74,
For this technique the most significant cross-media effect is:
•
higher residue production rates than wet systems.
Separate collection of fly ash is possible if this system is preceded by an ESP. This, then
increases separation of fly ash and FGT residues, which can be beneficial if separate
treatment/recycling options exist for these residues.
The semi-wet FGT system is often applied as a single stage, multi reactor. Such systems usually
have lower energy requirements than more complex multi stage FGT systems.
Waste Incineration
333
Chapter 4
Operational data
Description of factors
affecting criteria
Criteria
Evaluation
(High/Mediu
m/Low) or
data
Comment
•
•
Complexity
•
•
Flexibility
Skill
requirements
•
additional process units
required
critical operational
aspects
M
•
•
•
ability of technique to
operate under range of
input conditions
M
•
•
significant extra training
or manning requirements
M
•
the number of process units
is lower than wet systems,
but greater than dry and
flash dry.
inlet temperature requires
control
pre-dust removal may ease
semi-dry operation
can achieve low emission
levels under most
conditions
rapid inlet load changes can
be problematic
no effluent treatment
requirements
care required to optimise
reagent dosing
Table 4.39: Operational data associated with the use of semi-wet FGT
Most systems consist of only a reagent mixing unit (reagent plus water) and a spray tower, and
then a bag filter - complexity is therefore lower than with wet FGT systems.
The reagent handling and dosing require good management to ensure effective and optimised
operation, particularly where heterogeneous waste types are treated e.g. merchant HWIs.
Upstream HCl monitoring (see Section 4.4.3.9) improves optimisation of reagent dosing in
these systems and allows management of peak loads of HCl, HF, SO2 without high reagent
dosing rates.
Some installations produce the Ca(OH)2 for the FGT system by on-site by slaking of CaO.
Effective lime preparation can be critical to good operation, as can be controlling the risk of
fouling in the injection device. The injectors have to be located and designed such that they can
be easily maintained and/or replaced for cleaning. [74, TWGComments, 2004]
Bag filters require close monitoring and management to address bag damage and consequent
releases. Differential pressure monitors are commonly used to indicate bag damage and monitor
operation in general.
Temperature requirements are critical. Care is required to ensure dew point corrosion in the bag
filter is avoided - inlet gas temperatures of above 130 – 140 °C are usually used. At temperature
below 130 °C there may be problems due to the hygroscopic nature of the CaCl2 formed.
Reagents usually require a specific temperature for optimal reaction conditions.
It is reported that there may be operational problems when semi-wet FGT systems are used with
very highly acidic polluted raw gases as this can lead to increased risk of filter clogging.
334
Waste Incineration
Chapter 4
The operational complexity of reactor and bag filter used in semi-dry systems can themselves be
decreased further by the use of a degree of pre-dedusting e.g. use of one stage ESP, or by using
non-sticky bag materials (see also Section 4.4.2.4). This helps because it avoids the problems
of:
• sticking of some zinc (and similar salts with low melting temperature) and
• hygroscopic salts forming sticking layers on the surface of the reactor.
[64, TWGComments, 2003]
Applicability
The applicability of this technique is assessed in the table below:
Criteria
Waste type
•
•
Plant size range
New/existing
•
•
•
Inter-process compatibility
•
•
Key location factors
•
•
Evaluation/comment
suited to most waste types
generally less capable of dealing with very highly variable
inlet concentrations than wet scrubbers
applied at all size ranges
applied at new plants and as a retrofit
flue-gas outlet temperature (120 – 170 °C) requires reheat for
subsequent FGT systems e.g. SCR
separate (pre-) collection of fly ash possible
bag filter provides effective gas cleaning step for subsequent
SCR or wet system (if used as pre-deduster).
no effluent is produced and no discharge required
availability/cost of solid residue outlets
Table 4.40: Assessment of the applicability of semi-wet FGT
Economics
Capital cost information for the technique is shown in the table below:
FGT component(s)
Estimated investment
cost (EUR million)
2
Fabric filter
1 – 1.5
Spray dryer
Costs estimated related to a 2 line MSWI of total capacity 200 K t/yr
Comments
Table 4.41: Estimated investment costs of selected components of typical semi-wet FGT systems
Source [12, Achternbosch, 2002]
Key operational factors of this technique are:
•
•
•
•
investment costs are lower than for wet FGT systems, especially for relatively small
capacities. [2, infomil, 2002] p 119
possible higher cost of disposal of the higher quantity of residues produced (than wet
systems)
reduced labour cost (cf. wet systems) due to lower complexity, particularly because it
avoids the costs of the operation of an effluent treatment plant
increased alkaline reagent cost due to higher stoichiometric ratios.
Waste Incineration
335
Chapter 4
Driving force for implementation
This technique has been implemented where:
•
•
•
•
•
•
emission limit values have been set at those detailed in Directive 2000/76/EC
wastes treated do not result in very high and variable inlet flue-gas loads
outlets exist for solid residues produced
effluent production is not desirable
water supplies are limited because the lower water consumption
the infrastructure for waste water handling is not available, or limited, e.g. remote rural
areas in dry climates due to the lack of need for effluent treatment
lower plume visibility with non-wet systems may also be a particular advantage in areas
where there is high sensitivity to visual impacts. [64, TWGComments, 2003]
•
Example plants
Widely used in Europe e.g. UK, D, F, DK.
Reference literature
[1, UBA, 2001, 2, infomil, 2002, 3, Austria, 2002, 12, Achternbosch, 2002, 26, RSP, 1999, 54,
dechefdebien, 2003], [64, TWGComments, 2003]
4.4.3.3 Intermediate systems with some water addition and residue recirculation
(flash dry systems)
Description
This technique has already been described in Section 2.5.4. The technique has elements of both
semi-dry and dry systems, and is mainly characterised by low water addition and high residue
re-circulation rates.
This technique is reported to only be applied when lime is the reagent. [74, TWGComments,
2004]
Achieved environmental benefits
Reduction of emissions to air as follows:
Substance(s)
Reduction
efficiency
range
(%)
Achieved emission ranges
½ hour
average
(mg/Nm³)
daily
average
(mg/Nm³)
annual
average
(mg/Nm³)
specific
emission
(g/t waste
input)
HCl
>99
<10
<6
2.9
10 – 30
HF
>99.5
<2
<1
<0.5
1–5
SO2
>99
<50
<5
<1
5 – 50
Comments
Stable due
to high
circulation
rate
Stable due
to high
circulation
rate
Stable due
to high
circulation
rate
Table 4.42: Emission levels associated with the use of flash dry FGT
[57, Alstom, 2003] [64, TWGComments, 2003] [74, TWGComments, 2004]
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Waste Incineration
Chapter 4
The process, in combination with bag filters and reagent addition, also provides for the emission
reduction of:
•
•
•
dust and associated heavy metals (to 0.4 – 2 mg/Nm³)
Hg (with carbon injection to 0.002 – 0.015 mg/Nm³)
PCDD/F (with carbon injection to 0.005 – 0.1 ng/Nm³).
The re-circulation of reagents generally used with this system has the following advantages
compared with other FGT systems:
•
•
•
reduced reagent consumption (cf. dry and semi-wet systems)
reduced solid residue production (contains less un-reacted reagent)
reduced water consumption and no effluent production (cf. wet systems).
Cross-media effects
Cross-media effects are identified in the table below:
Criteria
Units
Range of achieved
values
Comments
•
Energy requirements
kWh/t waste input
Reagent consumption
Reagent stoichiometry
kg/t waste input
ratio
•
7 – 15 (lime)
1.2 – 1.8
•
Residue - type
Residue - amount
•
kg/t waste input
combined FGT and fly
ashes (if no pre-collector)
lower levels of un-reacted
reagents
12 – 25
Dependent on in-going fluegas temperature cooling
required to achieve operating
temperature
Water consumption
l/t waste input
Effluent production
l/t waste input
0
+/o/-
0
Plume visibility
pressure drop of bag filter
is main consumer
circulation system
consumes energy
•
minimal water addition
for conditioning
Note: the data in this table aims to provide the typical operational range. The precise amounts of residues and
effluents produced will depend on many factors including raw gas concentrations (waste related), flowrates,
reagent concentrations etc.
Table 4.43: Cross-media effects associated with the use of flash dry systems
[3, Austria, 2002, 12, Achternbosch, 2002, 57, Alstom, 2003] [74, TWGComments, 2004]
For this technique, the most significant cross-media effects are:
•
•
production of solid residues
energy consumption from pressure drop associated with use of the bag filter.
Waste Incineration
337
Chapter 4
Operational da