HEFAT2008 6 International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

HEFAT2008 6 International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
HEFAT2008
6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
30 June to 2 July 2008
Pretoria, South Africa
Paper number: K9
EFFECTIVE UTILIZATION OF HEAT IN WASTE AND BIOMASS PROCESSING
P. Stehlík.
Institute of Process and Environmental Engineering
Brno University of Technology,
Brno, 616 69,
Czech Republic,
E-mail: [email protected]
ABSTRACT
Effective utilization of heat in thermal processing of waste
and biomass plays an important role since it contributes to
environmental and economic optimization of the processes and
equipment. In case of utilizing energy released during thermal
oxidization (incineration) of municipal solid waste (MSW) or
of hazardous waste for generation of process steam or for cogeneration (combined heat and power systems - CHP) we can
consider the thermal processing as a certain kind of recycling.
Since waste has sufficient heating value, it belongs to
renewable energy sources which enable to save fossil fuel as a
primary energy source. Then we speak about waste to energy
systems (WTE). In addition to environmental benefit, effective
utilization of released energy has a positive impact on
economics of the process including reduced operating costs.
WTE can partially or completely compensate costs of waste
treatment (costs of auxiliary fuel for incineration of low
calorific industrial and/or hazardous waste) and it can even
bring profit to the operator in case of waste with high calorific
value.
Typical examples of units for the thermal processing of both
MSW and hazardous waste are shown with the objective to
evaluate main factors influencing energy balance of the
processes, while taking into account various regimes of
operation. Basic rules of selection of the systems for efficient
heat utilization including CHP are summarized and illustrated
on concrete industrial examples. Conventional methods of
energy availability are discussed and analyzed. Heat flows in
the incineration plant are evaluated as well as factors like plant
efficiency and/or energy utilization rate.
A novel and original technology for combustion of various
types of biomass and fytomass consisting of a feeding system,
boiler, heat recovery system and flue gas cleaning system (in
case of contaminated biomass) is presented.
Moreover, it is necessary to take into account specific
features of flue gas (and/or off-gas) as a process fluid. For an
optimum design of heat exchangers as equipment and
integrated items it is necessary to follow a top-down approach
“process – heat recovery system – heat exchanger” while
respecting specific features of the concerned process. A
combination of intuitive design, know how and sophisticated
approach based on up-to-date computational tools with
emphasis on computational fluid dynamics (CFD) is shown in
the paper.
After selecting a convenient process for the given type of
waste and/or biomass, the available energy for heat recovery is
evaluated and a heat recovery system is designed. Novel design
of air pre-heaters, heat recovery steam generators and special
heat exchangers (e.g. those for sludge pre-heating) is shown.
This approach always respects the primary role of the process,
while stressing also the importance of analysis aimed at
selection of heat exchangers and their design including specific
features and fouling problems.
INTRODUCTION
Heat recovery in units for the thermal processing of various
types of waste i.e. waste to energy systems as well as in those
for biomass combustion can be without any doubt considered as
one of the most important parts of these processes. Design of
equipment for utilization of energy contained in flue gas
(and/or off-gas) from the thermal treatment of waste i.e.
incineration and the placement in the process is one of key
factors in these technologies. Heat recovery represents one of
subsystems which enables to consider incinerators not only as
units for the treatment of waste but as energy sources. This is
generally supported in valid environmental regulation within
European Union. In the design and operation of heat recovery
systems it is necessary to take into account the characteristics
of heat transfer equipment and/or heat exchangers and their
specific features as well as those of process fluids.
A unit for the thermal processing of waste has primarily to
be considered as a process. This is the first step in plant design
(see Figure 1) based on selecting an optimum technology using
heat and mass balance calculations. In the second step we can
divide the process into subsystems like the combustion
chamber (kiln), heat recovery system and off-gas cleaning
system and optimize these subsystems. Heat recovery
subsystem usually consists of air pre-heaters, heat recovery
steam generators (HRSG) or various types of heaters. In some
cases HRSG can be substituted by heat exchangers with heating
fluids (e.g. thermo-oil). Only afterwards we can make the third
step and design those pieces of equipment.
Optimum design of both the equipment and the process is a
combination of intuitive design, know how, practical
experience and sophisticated approach based on modeling,
simulation and CFD approach.
1st STEP
Process as a whole
2nd
STEP
Subsystems
UNIT FOR THERMAL PROCESSING OF WASTES
HEAT RECOVERY SYSTEM
INCINERATION
AIR-PREHEATERS
HRSG
HEATERS
OFF-GAS
CLEANING
SYSTEM
3rd STEP
Equipment
Figure 1 Hierarchy in design of unit for thermal processing of
waste
Specific features of process fluid
Combustion of waste (incineration) produces both gas
products and solid residues. Solid residues are divided into ash
(slag, cinder and/or sinter) and fly-ash. Off-gas from waste
incineration is a multi-component mixture of chemical species.
It contains harmless components like nitrogen, carbon dioxide
and water vapor, but also harmful components like nitrogen
oxides and sulfur oxides, carbon monoxide, hydrogen chloride
and fluoride, dust, heavy metals and their chemical compounds,
phosphoric compounds and organic compounds like
hydrocarbons. Note that harmful compounds of acid character
are present in the gas phase. Hydrogen chloride produced by
thermal decomposition of chlorinated plastics is the dominating
acid gas. Sulfur dioxide, heavy metals (Cd, Hg, Cr, Zn, Cu, Pb)
and dust also belong among the main harmful pollutants.
Further to that, the group of persistent organic pollutants (POP),
like polycyclic aromatic hydrocarbons (PAH), polychlorinated
biphenyls (PCB) and polychlorinated dibenzodioxins and
dibenzofurans (PCDD/F) are extremely harmful compounds
contained in emissions. However, the most important primary
criterion of combustion efficiency is given by CO and NOx
concentrations.
From the characteristic features of off-gas described above
it follows that the off-gas composition and properties differ
significantly from those of flue gas in conventional combustion
chambers, furnaces, boilers etc. Therefore in units for the
thermal processing of waste it is necessary to apply a specific
arrangement. Fouling obviously represents a serious problem.
HEAT RECOVERY IN THERMAL PROCESSING OF
WASTE AND BIOMASS
Waste incineration in up-to-date facilities is a complex
process. From the time when first incineration plants were put
into operation, the technologies have undergone a large
progress. They have developed from simple units designed only
for waste disposal into complex systems, where waste is
thermally destroyed and energy is efficiently utilized, while the
negative impact on the environment is minimized. Waste
processing is described in many papers and monographs (e.g.
[1-3])
Let us only briefly mention typical waste processing
technologies according to the waste type and origin, namely
municipal solid waste incinerators (MSWI) and hazardous
waste incinerators (HWI).
Municipal solid waste incineration
A typical unit for municipal solid waste (MSW) incineration
is displayed schematically in Figure 2. Waste incineration is
performed in a combustion chamber equipped with a moving
grate and followed by a secondary combustion chamber (SCC)
under temperatures ranging between 850 and 1000°C. The heat
released in this process is utilised in a heat recovery steam
generator (HRSG), most often for the production of
superheated steam. The flue gas is cooled down to
approximately 250 to 280°C in the boiler together, while
separating a major part of fine fly ash particles (carried by flue
gas from the combustion chamber). Mechanical cleaning of flue
gas that collects the remaining particulates is performed in
electrostatic precipitator (ESP).
A part of flue gas (and/or off-gas in terms of incineration
terminology) leaving this equipment may be recycled back to
the combustion chamber. This is a feature of up to date
incinerators since recycling contributes to decreasing exhaust
emissions. Thus based on the lower off-gas flow rate, size of
equipment forming the final part of the off-gas cleaning system
is decreased as well. The remaining part of flue gas enters the
off-gas cleaning system, comprising a wet scrubber, and a
system for final gas cleaning. This step may include the
catalytic reduction of nitrogen oxides (SCR) and destruction or
removing the persistent organic pollutants (POP), especially
polychlorinated dibenzodioxins and dibenzofurans (PCDD/F).
Our experience shows that a very efficient way for the
destruction of dioxins and furans is catalytic filtration in a
dioxin filter.
Q exp
Power
I circ,el
I circ,el
Internal
power
demand
G
Q prod
Steam
SCC
I circ,th
ESP
Steam
HRSG
FAN
Waste
Ew
DeNOx
I circ,th
Supplementary
fuel
Ef
SCRUBBER
STACK
I circ,th
Combustion
air
I imp
Condensate
I imp
Natural gas
Figure 2 Main energy streams in case of municipal solid waste
incineration
Hazardous waste incineration
In order to achieve a perfect combustion in thermal
treatment of industrial and hazardous waste, two-stage
incineration is the most common approach. A typical
arrangement of unit for the thermal treatment of solid and
liquid waste is shown in Figure 3.
Rotary kiln is usually used in the first stage of incineration.
Combustible portion of the waste burns in oxidizing
atmosphere and the heat releasing process is virtually
completed in the kiln. Thermal decomposition and oxidation of
harmful compounds however continues in the secondary
combustion (after-burner) chamber, where temperature of flue
gas is increased to the required level by means of burners firing
auxiliary (gaseous) fuel.
Cooling of flue gas is partially carried out in the air preheater, and particularly in the waste heat boiler and/or heat
recovery steam generator (HRSG). Steam can be used for the
incineration plant itself, for power generation, heating purposes
etc.
Cooled off-gas is mechanically and chemically cleaned so
that fine solid particles and harmful products of the thermal
decomposition (namely HCl, sulphur oxides, emissions of
heavy metals etc.) are removed down to levels required by
emission regulations.
Combustion
(incineration)
Heat utilisation
(heat recovery)
Flue gas
cleaning
Figure 3 Typical configuration of industrial waste incinerator
with rotary kiln
Apart from this common technology we can meet with a
number of other special incinerators. An example is an
incineration plant of industrial and hazardous waste discussed
in [4].
Rules for the design of efficient WTE systems
It can be stated that in most of the applied technologies
there is a number of similar items. It can be clearly seen that the
above described MSW and IHW incinerators differ only by the
thermal system – the remaining parts are more or less identical.
However, in most cases it is necessary to “hand-tailor” the
technology from case to case. These specific features as well as
different position of every piece of equipment integrated within
the technology have strong influence on the process energy
demand and the way of energy utilization, which is performed
in the heat recovery system. The more and more sweeping
environmental limits mean that the number of devices (not just
in the off-gas cleaning systems) is rising. Energy demand
increases (power for fan driving, plant heating up to the
operating temperature during start-up, etc.). The integration of
every single apparatus in the unit thus becomes increasingly
important. For better understanding to the highly efficient waste
to energy systems it is vital to analyze heat fluxes in the unit
(see Figure 2).
Regarding energy distribution it is obvious that heat
recovery system represents the “heart” of the unit, i.e. the place
where energy is produced (usually in the form of steam) and
afterwards supplied as circulated energy (Icirc) for on-site
consumption or exported (Qexp) to other consumers. Therefore
sophisticated design of heat recovery systems plays a key role.
To compare the effectiveness of energy production in
different incineration plants several criteria have been
developed by several institutions [5-7]. Their names are very
similar – plant efficiency factor, plant efficiency, energy
efficiency, energy utilization rate. Their common feature is
their purpose – to describe the relationship between energy
outputs (produced or exported energy) on one side and energy
demand on the other.
As an example the criterion called Plant Efficiency Factor
defined in the CEWEP can be used. It defines the ratio between
energy produced by incinerating the waste and energy
consumed by the process itself:
Q prod − ( E f + I imp ) .
Pl ef =
E f + I imp + I circ
The meaning of particular symbols is evident from Figure 2.
A comparison of the efficiency of energy utilization according
to this criterion gives [8]. In this comparative study were
included 97 European waste-to energy units. The resulting
weighted average, which accounts not only for the number of
the incinerators but also heating value of the waste and the
plant capacity is 4.08 , with minimum value of 0.04 and
maximum equal to 21.08. The threshold (minimum) value that
identifies a plant as a waste-to-energy system is set at 1.0.
The basic rule for efficient operation of a process with a
high degree of energy utilization is based on the principle of
self sufficiency and may be formulated as follows: To
minimize the amount of imported energy while maximizing
the amount of exported energy.
Concrete ways of achieving this objective are summarized
below:
1. Process optimization and the selection of technology
aiming to decrease or to completely eliminate consumption
of external imported energy, which is however generally hard
to be substituted by energy generated in the heat utilization
section. This concerns mainly additional fuel (Ef) consumed
in secondary combustion chamber.
The combustion system has to be designed and operated in
a way that minimizes the temperature of gaseous products
downstream the last air inlet. This temperature is specified by
regulatory limits and differs according to the type of
combusted waste (see Table 1).
Type of waste
Required
temperature
Tmin [°C]
Hazardous waste containing more
than 1 % of halogenated organic substances
1,100
Other waste
850
Table 1 Minimum temperature required in combustion
chamber [9]
If heat released by waste combustion is not sufficient to
meet the threshold temperature, additional energy has to be
supplied into the system (pre-heating of combustion air Icirc,
possibly combustion of auxiliary fuel Ef). This situation
occurs usually during combustion of low-caloric waste which
is for example industrial sludge and waste water treatment
sludge [3].
2. Identification of energy-saving measures leading to the
decrease of energy consumption directly in the process. This
consumption is covered by the so-called circulating energy
(Icirc) and in case of need also by imported energy (Iimp).
One of the possibilities is the efficient utilization of waste
heat and low-grade heat. Widely used, simple and popular
tool that allows to analyze energy flows inside the process
and to identify the ways of maximum waste heat utilization is
Pinch analysis. The results must however always be checked
for realizability constraints of technological and space nature.
3. Design of the heat utilization section in a way that secures
internal plant energy requirements while maximizing energy
export. Example using an air preheater is provided in the
following chapter.
4. The highest degree of energy utilization is achieved by
applying the cogeneration approach, i.e. production of heat
and electricity. Systems with a steam turbine are the most
frequently used methods of excess steam utilization for heat
or power production. However it is possible to design various
combinations and arrangements for specific applications with
the aim of maximum energy utilization.
5. The highest efficiency expressed in saving of primary
energy sources is reached when backpressure turbine is
applied. In this case all the steam going through the turbine
and used for power generation is then utilized for heating
purposes, which results in higher efficiency. However the
disadvantage of this system is the direct dependence of the
incinerator on the grid conditions, to which steam is
distributed. Suitable consumers of heat for this application
are for example industrial processes with steady heat
consumption.
6. When stable demand for heat throughout the year is not
secured, the owner must seek other solutions. A very flexible
solution for such unsteady heat consumption is application of
bleeding condensing turbine, where excess steam is utilized
for power production. The turbine can then accommodate
varying demand for heat by working either in backpressure
mode or condensing mode.
7. However the consequences of so-called summer operation
mode with a reduced heat output and increased power output
on plant performance is evident - the efficiency of the cycle
as well as primary energy savings are considerably lower.
8. When the structure of heat consumers allows it, it is
advantageous to perform expansion in the turbine with
minimized outlet pressure and to export the steam of the
lowest possible grade, or to export hot water.
Lower grade of the exported steam enables to increase the
enthalpy gradient in the turbine and so to achieve maximized
electrical power output. Similar effect is obtained by
decreasing pressure losses of the steam system after the
turbine.
9. When the steam bleeding from the turbine matches internal
energy consumption, it is important to check parameters of
the low-pressure steam, whether they are sufficient for
heating of internal flows (e.g. air preheating). When the
required steam parameters are higher than the threshold
values of steam for export, it is advantageous to consider a
turbine with several bleeding stages. The application of a bypass for these purposes may negatively influence the energy
utilization rate.
10. In the absence of heat consumers, it is necessary to
generate only electricity and to waste the remaining heat in a
condenser. The efficiency of electricity generation is
however relatively low even though several approaches are
available to increase it (heat regeneration, re-heating of steam
etc.).
Biomass combustion
A considerable decrease of greenhouse gases (GHG)
released to the atmosphere is not possible without a substitution
of fossil fuels by renewables. Biomass has the largest potential
from all the presently available renewable energy sources.
Figure 4 Novel technology for biomass combustion
Figure 4 shows a realistic model of a technology for
combustion of various types of biomass and phytomass. In spite
of the fact that similar technologies are commonly used it is
necessary (like in the case of incinerators’ heat recovery
systems – e.g. flue gas recycle, air pre-heating) to solve heat
transfer problems. In this case the heat exchanger for air pre-
heating represents the main concern because its design has to
be resistant to fouling caused by tarry products on flue gas side.
Characteristic features of the developed prototype unit are
as follows:
- An effort to integrate field proven state-of-the-art
technologies into one unit. This includes measures
contributing to high performance and operation flexibility
(e.g. flue gas recirculation or air preheating).
- The possibility to combust various types of biomass from
saw dust and wood chips to fast-growing energy crops (e.g.
amaranth). Naturally, not all types of fuel can be processed in
one unit. Especially straw fuels require a different boiler
design or co-firing in a mixture.
The fuel feeding system consists of two separate paths for
wooden biomass and for phytomass. The combustion process
takes place on an inclined hydraulic grate and is completed in a
secondary combustion chamber. The products of combustion
then flow through a tube bundle in the heat exchange section of
the boiler (HE1 in Figure 4). The fly ash contained in the gas is
removed in the multi-cyclone (C1). Induced draft fan is situated
downstream the multi-cyclone (V01) and it is the only driving
equipment of the flue gas stream from the combustion chamber
to the stack (S1). Controlled amount of flue gas is extracted in a
splitter situated downstream of the fan. Using this recycled
stream, part of the flue gas is brought back to the combustion
chamber. The rest of flue gas continues to special recuperative
heat exchanger “flue gas – air” (HE2), where its sensible heat is
used for preheating primary and secondary combustion air. The
aim is to cool down the flue gas as much as possible and thus
eliminate the stack losses. On the other hand, excessive fouling
caused by condensation of tarry compounds has to be avoided.
Flue gas cooled in the exchanger than goes to the stack (S1).
Specific systems
A typical problem concerning design of heat exchangers in
the field of biomass processing is given by the fact that most
applications are custom-made and therefore we face demand for
specific types of equipment. One such application is
demonstrated below. There is shown a special compact unit for
the thermal processing of gas waste consisting of a combustion
chamber placed inside heat exchanger. Heat recovery requires
new ideas for optimum design of heat exchangers, discussed in
the following chapter.
HEAT
EXCHANGERS
AS
EQUIPMENT
AND
INTEGRATED ITEMS
The processes displayed in Figures 2 to 4 show subsystems
for heat recovery and waste heat utilization. The pieces of
equipment where heat transfer plays a predominant role
include: air pre-heater, HRSG and condenser in the steam
turbine system.
Other applications in different technologies include e.g.
utilization of waste as alternative fuel in cement or lime
production. In that case, heat exchanger “off-gas/oil” can be
inserted into the existing technology with the aim to utilize
waste heat from cement works for drying of sewage sludge
(from waste water treatment plants, WWTP) that may be used
as alternative fuel. Case study published in [11] has analyzed
the potential of such heat-exchanger application.
Air pre-heaters
Air preheating is the most frequently used method of heat
utilization for internal plant consumption. It includes
- Preheating of primary air brought to under the grate
with the aim to enhance combustion processes
- Preheating of combustion air for burners installed in the
secondary combustion chamber which leads to fuel
consumption reduction.
Figure 5 Typical temperature profiles, heat transfer between
hot and cold streams and feasible integration of air-preheaters
in waste processing technology
The required amount of heat in the air preheater (circulated
energy Icicr) is the result of heat and mass balance calculations
of the whole process as well as single pieces of equipment. A
number of possible solutions can be applied (see Figure 5).
When making the final decision it is necessary to respect the
main technological parameters related to the process and to the
heat transfer equipment (e.g. temperature of flue gas leaving the
thermal system, target temperature of air preheating, flue gas
temperature at the HRSG outlet, amount of produced steam,
parameters over the turbine). Each technique has its advantages
as well as weak points.
- Usually the air preheating exchanger is placed downstream
the waste heat boiler (heat recovery steam generator, HRSG),
i.e. where combustion gases lost most of their heat. However,
the temperature of hot stream (in this case flue gas) has to be
sufficient for the target temperature of air (to ensure adequate
temperature gradient). Conventional types of plate heat
exchangers can be used and optimized with the help of
optimization procedures [12].
- In some cases it is more advantageous to break
thermodynamic principles and place the exchanger upstream
the heat recovery steam generator. In technologies for
industrial and hazardous waste incineration, raw combustion
gases leaving the combustion chamber have a
characteristically high temperature, which often exceeds
1000°C, at which temperature they contain melted and sticky
fly ash. In such cases, solution may be a radiant air preheater.
Off-gases are there cooled down and partly cleaned of solids
at the same time, which eliminates the otherwise excessive
fouling on boiler walls, thus leading to a reduction in heat
transfer, reduced boiler output and increased frequency of
shut-downs.
- Heating by steam. In cogeneration systems, where low
pressure steam is taken from the turbine outlet, it can be used
advantageously to serve for air preheating. This solution
maximizes power production. If steam parameters are too
low, it is necessary to by-pass the turbine and utilize steam
directly from the boiler. Combination of afore mentioned, i.e.
initial heating by low-pressure (LP) steam and final heating
by higher-grade steam from the by-pass is possible in specific
cases (e.g. in Figure 5 MP, medium-pressure steam).
Air-preheating of combustion air in a biomass-unit
The following example shows a dedicated recuperative heat
exchanger designed for air preheating in an experimental unit
for biomass utilization for energy production. This device
ensures high efficiency of the process. The objective is to cool
down the flue gas as much as possible and thus eliminate the
stack losses. On the other hand excessive fouling caused by
condensation of tarry compounds has to be avoided. These
influences can be minimized by a suitable design of all parts.
The flow among the tubes as well as inside them has been
investigated with the help of modeling by computational fluid
dynamics (see Figure 6).
Figure 6 Illustration of computational support in the modeling
of inlet and outlet chambers of a recuperative heat exchanger
Waste heat boilers and heaters
Selection of a convenient type of heat recovery steam
generator (HRSG) depends on operating conditions and
capacity of incinerator (throughput of waste). In case of flue
gas with high propensity to fouling it is better to use a fire-tube
boiler which provides easy access for a mechanical cleaning.
Fire-tube HRSGs are also preferred for incinerators with lower
throughput of waste (approximately of 0.5t/h and less). When
production of steam is not required, then this equipment
actually represents a heat exchanger like e.g. that in Figure 7
(details of its application will be described later). This heat
exchanger is provided with a bellows compensator in a shell for
compensating thermal expansion.
Figure 7 Industrial fire-tube exchanger
For incinerators with higher waste treatment capacity,
conventional water-tube HRSG (see e.g. [13]) are used.
However, various types and arrangements are preferred in
industrial practice - depending on the specific application and
know-how [7]. For a preliminary design and simulation
calculations of HRSGs, consisting of several sections
(superheater, evaporator and economizer) a computational tool
has been developed as reported by [14]. This tool enables
versatile and fast preliminary design of boiler temperature
profile and its basic geometrical characteristics.
Specific applications
As described in section 2 a novel design of heat exchangers
is necessary in some cases. Let us demonstrate it on a process
of waste to energy where sludge coming from waste water
treatment plants (WWTP) is disposed and at the same time used
as a fuel.
A potential application of heat exchangers in sludge
utilization for power production is in flue gas stream after the
sludge utilization technology. Energy contained in the flue gas
produced by combustion of WWTP sludge may be used to preheat the sludge before de-watering. The underlying idea
consists in assumption that preheating sludge before dewatering may increase the attainable water extraction rate. The
expected improved de-watering of sludge would provide a
more profitable energy balance of the combustion process,
possibly even self-sustained sludge combustion. Schematic
drawing of this technology is shown in Figure 8.
For the research and development of a new type of heat
exchganger it is necessary to select a convenient procedure. In
the first phase there has been proposed a special heat exchanger
“water-sludge” (see Figure 9), the concept of which is based on
two counter-current helical channels with rectangular crosssection. Equations for thermal and hydraulic calculation will be
obtained by corrections applied to plate-type heat exchanger,
validated on a pilot-scale model. The experimental heat
exchanger will also serve in the investigation of sludge dewatering temperature dependence. Should the investigation
confirm the expected positive influence of increased
temperature on sludge de-watering efficiency, a new heat
exchanger “flue-gas – sludge” will be implemented.
The main difference between the two systems (“watersludge” and “flue-gas – sludge”) is the heat capacity of the heat
carrier medium. Other changes include different flow velocities
and fluid properties. Due to that it is necessary to consider
modifications to the heat exchanger design. The main two
advantages of the proposed design are simple geometry,
enabling easy cleaning of heat-exchange surfaces and low
pressure drop for sludge pumping. The sludge contains
approximately 5 % of dry matter and will thus flow easily
through the helix. Therefore sludge transport will require low
investment and operating costs (we can avoid using sludge
pump).
Figure 8 Process for thermal treatment of sludge with
utilization of off-gas heat
Figure 9 Heat exchanger “water-sludge”
Fouling in heat recovery systems for the thermal treatment
of wastes applications
Fouling represents a very important and complex problem
in waste and biomass processing applications. Fouling of a
surface takes place as a result of the complex processes
(mechanisms) that cause deposits to form on process surfaces.
A quite large number of parameters influence development of
fouling, including: flow velocity, surface temperature, exposed
surface material/finish, surface geometry and fluid properties
[15]. Based on results of numerous present research studies,
fouling can be classified according to the principal process:
precipitation fouling, particulate fouling, chemical reaction
fouling, corrosion fouling, bio-fouling, freezing fouling, and
crystallization. In most thermal treatment of wastes
applications, more than one type of fouling will occur
simultaneously. Moreover, the form and structure of a fouling
deposit is influenced by type of burned fuel and incinerated
waste. Generally the most troublesome deposits are formed
when solid or liquid type of waste and fuel are processed.
Deposit thickness is difficult to predict, however, thickness is
extremely important in determining density and distribution of
the various constituents in the deposit.
Various reviews have been attempted describing the
attachment/formation process, however this is still not a very
well understood process in industrial applications of thermal
treatment of wastes. First results of our current research show
that it is possible to develop mathematical models, based on
broadly recognized method of balance of forces acting on an
elementary particle. Moreover, the model considers that particle
is drifted by flue gas and getting in the contact with heat
transfer area. Developed model allows determining so called
critical flow velocity, strictly speaking the theoretically
determined minimum flow velocity that avoids particulate
fouling (particles with given size). Obtained results were
compared with experimental data obtained from worldwide
available literature and very good agreement was found.
Mathematical model is suitable for preliminary analyzing of
fouling tendency and also for prevention in design and
operation of tubular heat transfer equipment. Designer obtains
from results of the model clear idea about interdependence
between heat exchanger arrangement and fouling propensity.
Thus, complex evaluation of optimum tubular heat transfer
equipment with respect to fouling, requires technical-economic
optimization taking into account investment, operating and
maintenance cost and based on the character and constitution of
deposit.
If fouling cannot be prevented, it is necessary to make some
provisions for periodical removal of deposits. The removal of
created deposit involves a combination of dissolution, erosion
or spalling of the deposited material. On some heat transfer
equipment (for example different types of tube banks placed in
flue gas channels) in thermal treatment plant can be applied
several methods of surface cleaning for deposits removal
during equipment operation (“on-line” cleaning), like for
example soot blowers. However, frequently removal of fouling
deposits cannot be performed online and fouling formation can
be controlled only by adhering to proper operating conditions.
In such case the periodical removal of deposits when equipment
is shut down is the only possible way. Periodic cleaning
removes deposits by chemical or mechanical means.
Mechanical methods include steam, high-pressure jets, brushes
or water guns. Chemical cleaning is designed to dissolve
deposits by a chemical reaction with the cleaning fluid. Hardly
accessible areas may be cleaned using this method. Mechanical
methods of cleaning are expensive and also tend to erode the
heat transfer surface.
It is often noted in boiler-related literature that 2mm deposit
will effectively increase fuel consumption by approximately
5%. Thus, because fouling is very important problem in heat
transfer equipment placed in thermal waste treatment
applications, it is necessary to use only smooth surfaces for the
heat transfer areas. Such configurations allow easy cleaning on
both outer and inner heat transfer surfaces. This restriction
strongly limits possible configurations of heat transfer
equipment. Any offence against this constraint can lead to large
losses. Figure 10 demonstrates unsuitable application of
extended surface (finned tube bank) in heat transfer equipment.
a)
b)
Figure 10 Fouled (a) and cleaned (b) finned tube bank
COMPUTATIONAL SUPPORT FOR EQUIPMENT
DESIGN AND OPERATION
Improved or even optimum design of heat exchangers can
be obtained most conveniently through a sophisticated
approach based on simulations and modelling. Computational
support in this sense may be divided into the following areas:
(i) simulation based on energy and mass balance,
(ii) thermal and hydraulic calculation of heat exchangers, (iii)
computational fluid dynamics (CFD) approach, (iv)
optimization, and (v) heat integration.
Simulation (energy and mass balance)
Design of equipment for heat exchange typically consists of
the following steps: mass and energy balances, preliminary
design, and detailed design.
The first step (process heat and mass balance calculations)
is necessary for evaluation of all process parameters for further
calculation of heat exchangers (values of process fluids’
temperatures, flow rates, properties etc.). Various software
packages for simulation exist, however, for special areas like
thermal processing of waste including energy utilization a
creation of own software packages proved to be the preferable
solution. Currently there is an ongoing development of a new
integrated simulation system WTE (Waste-to-Energy), first
introduced by Pavlas et al. in [16], which takes up on the TDW
system see [17, 4]. Besides plant design support in the area of
waste and biomass utilization, the WTE system enables also a
complex evaluation (i.e. economic and environmental analysis)
of a given problem with the objective of maximum waste heat
utilization.
calculation methods must be developed. An example of
solution approach in such kind of problem is shown in the next
chapter.
A correct selection of convenient type of heat exchanger is
important especially in case of so called hot gas applications
especially in the case of heat recovery system in waste
processing units. Energy contained in the flue gas is utilized for
air pre-heating, steam generation, water heating or
technological purposes. Without taking into account specific
features of the process fluids, serious operation problems can
occur (e.g. excessive fouling, damage caused by thermal
expansion). These and other reasons gave impetus to the
development of a multi-purpose computational system with a
database for hot gas applications (HGA) where data concerning
both the common and specific types of heat exchangers are
collected. Conventional heat exchangers are preferred; however
in some cases there it is profitable to propose a new type of heat
exchanger [18, 19].
Characteristics of the HGA approach are as follows:
− Use of elimination strategy based on AHP method [20] for
selection of potential candidates in the first stage, based on
main process characteristics (process fluids, temperatures,
pressures, fouling propensities etc.). Screenshot of the HGA
database software enabling selection of candidate heat
exchanger type is presented in Figure 11.
− Simplified preliminary design calculations to narrow the
range of possible solutions.
− Preliminary rough estimate of investment and operational
costs and selection of optimum candidate.
− Final design check and summary of design data, required
for making enquiries to heat exchanger manufacturers or
alternatively to design of a new type of heat exchanger.
− Modules for selecting, costing and design/rating
calculations of heat exchangers are independent and can be
used separately, based on actual requirements of the designer.
− HGA database is an open system prepared for further
extensions.
HGA DATABASE
Conventional types
Segmental baffles
Thermal and hydraulic calculations
A wide range of existing heat exchangers can be used for
various purposes. Let us focus on the field of thermal
processing of waste and waste to energy systems.
When the character of application allows it, it is preferable
to use conventional shell-and-tube heat exchangers or compact
(plate type) heat exchangers. Manufacturers mostly have at
their disposal a commercial software package (e.g. HTRI or
HTFS) enabling reliable design calculations of those heat
exchanger types.
However, in real applications of units for the thermal
treatment of waste, it frequently is necessary to design special
types of heat exchangers for which no reliable design methods
exist. It is the responsibility of the general contractor to provide
manufacturers with detailed documentation. Such custom-made
designs have to be carefully investigated and new design
Shell-and-Tube
Special types
Coaxial heat
exchanger
Helical baffles
Simple
With regenerative
layer
Rod baffles
Double-Pipe
Double U-tubes
Plate-Type
Sludge aplications
Orifice baffles
Twisted tubes
Water – Sludge
Flue gas - Sludge
Heat-Pipe
Radiation
recuperator
Figure 11 Main window of HGA database for selection of
suitable heat exchanger type
CFD approach
Simulations of fluid flow and heat transfer in various pieces
of equipment within waste-to-energy plants may provide very
useful information both in the design phase and in
troubleshooting. The CFD methodology itself is a very broad
field so here we rather focus on a recent specific application,
relevant to the topic of the present paper. The main candidates
for CFD modeling among devices in thermal waste treatment
units include secondary combustion chamber, flue gas ducts,
low-NOx burners, filters, wet scrubbers and heat exchangers. In
the following we briefly present a case study related to a heat
recovery system.
Flue gas duct optimization
Uniformity of flow across tube banks and/or bundles is a
common objective in many heat exchanger applications.
Methods of computational fluid dynamics are well-suited for
studying the flow inside of an exhaust duct of a waste sludge
incineration plant (see Figure 12). The flow pattern in this duct
leads to fouling in a connected heat exchanger. CFD analysis is
used to find what causes the fouling and to optimize the duct
design in order to eliminate the undesirable phenomena.
Previous work on this application has been performed using
software STAR-CD (product of CD-adapco Group), as
documented by Hájek et al. [21]. The analysis has been
however recently re-simulated in FLUENT (product of
ANSYS, Inc.) software and the optimization has been taken one
more step further, using a geometry optimization software
SCULPTOR (product of Optimal Solutions Software, LLC),
coupled with FLUENT. This enabled a rigorous optimization of
geometry that has been previously identified by an intuitive
trial-and-error approach. The number of degrees of freedom
thus could have been increased as compared to the manual
approach.
Manually optimized
flow homogenizing
swirl generator
Inlet (outlet from air
preheater )
Duct expansion element (with
water injection nozzles)
Heat exchanger “flue gas-water”,
Outlet (inlet into stack fan)
Figure 12 Geometry of exhaust duct with heat exchanger
To briefly introduce the previous results, let us firstly
inspect the overall geometry and setup in Figure 12. Based on
experience from operation and complex analyses it was decided
to install a convenient water-tube heat exchanger instead of the
fire-tube one. The previous work has focused on several flow
homogenizing measures in the form of vanes and swirl
generators. Figure 12 shows the selected insert as well as its
location in the overall duct geometry. It is a swirl generator,
which produced the best results of all considered alternatives.
Alternative designs were compared using two objective
functions, based on the distribution of velocity magnitude in a
horizontal cross-section of the duct located just above the heat
exchanger. The first measure of flow homogeneity was defined
as ratio of minimum to maximum velocity in the reference
cross-section. The second measure was the maximum velocity
magnitude in the same cross-section. Both objective functions
however led to the same optimum.
Examples of the geometry modifications performed during
the automatic re-shaping optimization process are displayed in
Figure 13. The optimization process is controlled by the
SCULPTOR code, which performs re-shaping of the concerned
geometry and of the computational grid, calls the CFD solver
FLUENT and after receiving results of the simulation,
evaluates the objective function.
Figure 13 Original swirl generator and two examples of
geometrical modifications (from left to right)
An improvement of about 10% in terms of maximum
velocity magnitude has been obtained with the automatic
optimization approach. The analysis has been recently
described in more detail in [22].
Optimization of plate type air pre-heater
Plate type air pre-heaters [23, 24] are widely used in WTE
applications.
The design of these heat exchangers is usually realized with
the aid of CAD methods using either commercial software
packages available at the market or in-house software products.
However, the final solution (even if technically correct) can be
sometimes far from the optimum design. It was found that the
heat exchanger configuration and geometry significantly
influences capital cost and operating costs of heat exchangers
[25, 26].
Since the costs of heat exchangers represent an important
issue [26, 27], a new optimization algorithm was developed
with the aim to achieve minimum total annual cost of air preheaters [12]. Pressure drop and heat transfer are interdependent
and both of them strongly influence capital and operating costs
of any heat transfer system. During the design process of a heat
exchanger it is necessary to determine optimum dimensions of
the equipment, connected with its operating conditions.
The total annual cost consisting of fixed and variable costs
was selected as an objective function, which is to be
minimized. If we consider a heat exchanger system for a gasgas application in general (Figure 14), we can specify the major
cost components as follows: capital, operating and maintenance
costs of gas 1 and gas 2 fans, and capital and maintenance costs
of the heat exchanger.
Capital cost of any process equipment can be estimated with
a reasonable accuracy using relations which take into account
installed cost as a function of characteristic equipment
parameters. The maintenance cost is assessed as a percentage of
capital cost. Operating cost can be predicted as a function of
power consumption for overcoming the pressure drops.
FAN1
GAS 2
PLATE TYPE
HEAT EXCHANGER
FAN 2
GAS 1
plant into the local utility networks. The specific way of
utilisation of produced heat (typically in the form of steam) will
be different according to local conditions (climatic as well as
legislative). Basic rules of selection and integration of the
systems for energy utilization including CHP (Combined Heat
& Power) ones have been specified, analyzed and illustrated by
examples in [28].
INTEGRATED EQUIPMENT FOR GAS WASTE
TREATMENT
Research and development of a novel unit for thermal
(and/or catalytic) treatment of waste gases contaminated mainly
by VOC (Volatile Organic Compounds), HOC (Halogenated
Organic Compounds) or CO was initiated by more and more
sweeping environmental regulations. This equipment has
considerable advantages compared with commonly used
arrangement (combustion chamber (catalytic reactor) - pipeline
- heat exchanger) shown in Figure 15a. It is very compact since
the combustion chamber, catalytic reactor and heat exchanger
are integrated into one unit (Figure 15b). Maximum re-use of
heat lost in the combustion chamber and catalytic reactor is
achieved.
Figure 14 Heat exchanger system for „gas/gas“ application
Considering several relations (concerning heat transfer,
pressure drop, investment costs, operating costs etc.) we obtain
the total annual cost as a function of heat transfer coefficient.
Thus it is possible to obtain optimum values of both heat
transfer coefficients and consequently optimum values of
pressure drops. By a convenient re-arrangement of equations
relating heat transfer and geometry, relations were obtained,
which provide the possibility to calculate optimum dimensions
of a heat exchanger.
For an improved and economic design of plate type heat
exchangers we recommend firstly to apply the above
optimization approach [12] and utilize the results of
calculations as input data to a commercial and/or an in-house
design calculation software package which provides the final
results.
This methodology has been developed mainly for grassroots
design of plate type heat exchangers. It can be however adapted
also for specific situations and constraints required at retrofit
cases (space limitations, pressure drop allocations).
Heat integration
In order to design a highly efficient, optimised heat
utilization process for incinerators, it does not suffice to design
highly efficient individual heat exchangers. It is always
necessary to consider heat exchangers also as elements of the
overall process. Principles of process integration reviewed by
[27] should be used wherever possible. Unfortunately there is
frequently not enough space for this approach in units for
thermal processing of waste. In this area, heat integration
usually means integration of the heat produced in incineration
a) Two independent pieces
b) Fully integrated compact unit
Figure 15 Comparison of a conventional and compact unit
As mentioned above the novel and original design is based
on integration of several pieces of equipment into one unit,
which has several advantages. It is characterized by a
cylindrical combustion chamber placed inside a heat exchanger
– polluted air preheater (see Figure 15). This cylindrical
preheater consists of several concentric stainless sheets. Both
flue gas from the combustion chamber and polluted air (which
is heated by flue gas) flow in the spaces between each couple of
cylindrical sheets. Narrow strips helically placed between the
sheets form helical rectangular ducts. This results in countercurrent flow of process fluids. In specific cases, the combustion
chamber can be extended by a catalytic reactor as shown in
Figure 16. Catalytic treatment is more suitable for waste gas
with small concentrations of pollutants.
validation of mathematical design models by measured data
such as inlet and outlet pollutants concentration in processed
gas, temperatures and pressures in various parts of unit, flow
rates of waste gas, flue gas and natural gas, etc.
Figure 16 Compact unit
The unit can be used in various branches of industry such as
paint shops, refining plants, sewage treatment plants, food
processing industry, pharmaceutical industry, processing and
transporting of crude oil or natural gas, etc.
Basic process parameters of the experimental equipment
were evaluated with the aid of software for simulation of
processes for the thermal treatment of wastes. Additionally,
sophisticated computational support based on CFD proved to
be a very efficient approach and resulted in an optimized design
and elimination of bottlenecks [22].
Thermal and hydraulic calculations
Equations for the thermal and hydraulic calculation of a
heat exchanger and relationships for its sizing form the core of
its mathematical model. In the present specific case, general
equations for heat balancing were used as in any other case.
However, the specific design has to be reflected by the
mathematical model as well. This means that we need
equations for the evaluation of heat transfer coefficient h and
friction factor f. Unfortunately no formulae existed for the new
heat exchanger. Therefore, a tentative mathematical model
containing an approximate description of transfer phenomena
had to be created in order to provide design guidelines for
construction of an experimental equipment. Equations have
been selected from reputable literature, namely [26], [29], [23],
considering geometrical and hydraulic similarity with other
heat exchanger types. Experience from the research of
segmentally baffled shell-and-tube heat exchangers [30, 31]
were utilized..
New equations for calculation of heat transfer and pressure
drop were derived both for laminar and turbulent flow and can
be found in [32, 18]. The equations are based on measurements
at the experimental unit.
Experimental facility
In the frame of research, a full-scale experimental facility
for thermal and catalytic treatment of waste gases polluted by
VOC has been designed. The main objective of this facility is
Figure 17 Photo and simplified flow sheet of the facility
Table 2 Technical specification of the experimental facility
Flow rate of waste gas
Type of pollutants
Volume of pollutants in waste gas
Type of catalyst
Height of catalytic reactor
Diameter of catalytic reactor
Volume of catalyst
Weight of catalyst
Space velocity VHSV
Operating temperature in catalytic
reactor
Max. burner duty
Excess of combustion air
Consumption of electric power
Consumption of natural gas
Pressure drop of equipment
mN3/h
ppm
-
500 ÷ 1600
VOC (HOC)
to 5000
Pd/Al2O3
(CHEROX 4031)
m
0.2
m
0.6
m3
0.0565
kg
36
mN3/m3.h 8850 ÷ 28320
°C
250 ÷ 400
kW
kW
mN3/h
kPa
160
1.05
3÷6
0 ÷ 10
6
The experimental facility displayed in Figure 17 contains
the combustion chamber and catalytic reactor K01, burner B01,
heat exchanger E01 (these three parts are integrated into one
unit), waste gas fan V01, flue gas fan V02 and the control
panel. The unit has also automatic data acquisition and control
system, which measures various values and feeds them into a
computer. Sophisticated processing of the data enables to
investigate the operation of the whole integrated unit in a
number of operating modes. Based on evaluation of operating
data, the unit can be further optimized. Technical specifications
of the experimental unit designed for catalytic treatment of
waste gases are shown in Table 2.
Practical industrial applications
Requirements coming from the industrial practice influence
the further development of the equipment and give rise to
various modified units based on the new equipment. Two
examples of application are shown in Figures 25 and 26.
The first is a unit for the thermal treatment of waste gas
from a chemical plant (see Figure 18). Waste gas with flow-rate
of 2200 mN3/h and concentration of volatile organic compounds
(VOC) of 26.4 g/mN3 is pre-heated to a sufficient (yet safe)
temperature in the heat exchanger. Then the pre-heated waste
gas enters the combustion chamber where volatile organic
compounds are burnt so that the concentration of pollutants
falls well below the allowable limit. Flue gas produced by
combustion enters the heat exchanger only in such amount that
is required for pre-heating counter-currently flowing waste gas.
The remaining flue gas flows through a bypass. This bypass is
controlled by a pneumatically operated valve. Flue gas leaves
the unit with temperature ranging from 500 to 750 °C, which
provides a possibility for further heat recovery.
Another industrial application is a unit for thermal treatment
of waste gas having high heating value together with waste
vapours as a part of process for drying and cleaning natural gas
(see Figure 19).
ACKNOWLEDGEMENTS
A number of people have contributed either directly or
indirectly to this paper. Gratitude is expressed to Jiri Hajek,
Martin Pavlas, Radek Dvorak, Jaroslav Boran, Lucie Houdkova
and Zdenek Jegla from the Institute of Process and
Environmental Engineering for their assistance and valuable
contributions as well as to Jaroslav Oral, director of EVECO
Brno Ltd, and other specialists from this company for providing
results of practical applications.
REFERENCES
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[3] Santoleri J.J., Reynolds J., Theodore L., 2000, Introduction
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Figure 18 Compact unit for
thermal treatment of waste gas
(Courtesy of EVECO Brno
Ltd)
Figure 19 Unit for thermal
treatment of waste gases
(Courtesy of EVECO Brno
Ltd)
CONCLUSIONS
It has been shown that heat exchangers, heat recovery steam
generators, and many other heat-transferring devices play a key
role in processes for the thermal treatment of wastes and
thermal processing of biomass. Secondly, it may be concluded
that efficient application of waste-to-energy systems requires
systematic analysis of various options in the design phase and
integration of individual devices by rigorous methods like the
Pinch analysis. Specific properties of process fluids and other
constraints encourage the development of non-traditional and
innovative solutions and designs of heat transfer devices.
Several examples and case studies are included in the text to
illustrate the discussion of general principles.
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