FOSSIL FREE TISSUE DRYING
FOSSIL FREE TISSUE DRYING
REPORT 2016:231
Fossil free tissue drying
Feasibility study
LARS NILSSON, ROY ANDREASSON, BENGT AXELSSON, CHRISTER GUSTAVSSON,
RAFFAELE MALUTTA, ANDERS OTTOSSON, FREDRIC PAULSON, CARL ZOTTERMAN
ISBN 978-91-7673-231-1 | © 2016 ENERGIFORSK
Energiforsk AB | Phone: 08-677 25 30 | E-mail: kontakt@energiforsk.se | www.energiforsk.se
FOSSIL FREE TISSUE DRYING
Authors’ foreword
This project was carried out mainly by the authors of this report. Valuable
contributions were made by a number of suppliers of gasification
technology.
This project was co-funded by the Swedish Energy Agency through
SGC/Energiforsk. In-kind contributions were provided by all project
partners; Rexcell Tissue & Airlaid AB, Valmet AB, Pöyry AB, Södra
Skogsägarna, BAxPTC, The Paper Province and Karlstad University.
The authors would like to thank Anna-Karin Jannasch at SGC/Energiforsk
for good support during the application phase as well as during the project
period.
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FOSSIL FREE TISSUE DRYING
Sammanfattning
Energianvändningen vid torkning av mjukpapper är betydande. I moderna
mjukpappersprocesser utnyttjas direkteldade torkkåpor där rökgaserna från
gasol- eller naturgasförbränning blåses mot det våta mjukpapperet. I denna
studie har bytet från gasol till gas från termiskt förgasad biomassa
undersökts för ett mjukpappersbruk med en maximal gasolförbrukning
motsvarande 7 MW.
Effekten av att ersätta gasol med syntesgas vid mjukpapperstorkning
undersöktes genom användning av matematiska modeller. Resultaten från
de genomförda simuleringarna visar att torkkapaciteten sannolikt kommer
att upprätthållas om gasol ersätts med syntesgas. Om man antar att de
undersökta gassammansättningarna är icke-förvärmda vid förbränning
beräknas energianvändningen öka med mindre än 3,5% jämfört med
referensbränslet gasol.
Den mindre undersökning av bränsleutbytbarhet som gjorts i den aktuella
studien visar att de befintliga gasbrännarna sannolikt behöver bytas ut för
att kunna bibehålla nuvarande torkkapacitet. För att syntesgas ska vara ett
hållbart alternativ i en mjukpapperstork så måste förbränningssystemet
utformas för att uppfylla gällande emissionskrav.
Fem kommersiellt tillgängliga förgasningskoncept med efterföljande
gasrening har studerats och deras användbarhet och energieffektivitet har
analyserats på en bruksövergripande nivå. Alla förgasningstekniker har
bedömts vara tillämpliga för att omvandla de planerade bränslena (flisat
trämaterial) till gas av tillräcklig kvalitet. Potentiella bränslen i form av slam
kan emellertid vara svåra att hantera med fastbäddsteknik på grund av
risken för bildandet av täta, ogenomträngliga sektioner i bädden.
Motströmsförgasning har funnits vara mindre lämplig ur
effektivitetssynpunkt beroende på den höga tjärhalten i produktgasen och
de höga energiförluster som därmed uppstår vid kall gasrening. De tre
återstående teknikerna skiljer sig endast marginellt åt avseende den totala
energieffektiviteten och den totala förbrukningen av biomassa.
Bytet från gasol till biobaserad syntesgas har generellt sett funnits vara
genomförbart. Snabba variationer i gasförbrukningen kan dock utgöra en
utmaning för en del av förgasningsteknikerna. Baserat på kostnaden för
tjärrening å ena sidan och vikten av sotfria, icke-luktande rökgaser för
denna applikation å andra sidan, har det identifierats ett behov av
experimentell testning för att fastställa sambandet mellan syntesgasens
tjärinnehåll och lukt/sot-påverkan på mjukpapper. Sådan testning
rekommenderas som ett nästa steg i konceptutvecklingen.
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FOSSIL FREE TISSUE DRYING
Summary
In tissue production, the energy use for drying is considerable. Modern
tissue drying processes utilize direct fired, high temperature, drying hoods
where the flue gases from LPG or natural gas combustion are blown towards
the wet tissue paper. In this study the exchange of LPG with biomass
derived syngas has been examined for a tissue mill with a total LPG
consumption corresponding to 7 MW peak load.
The effects of replacing LPG with syngas in the impingement drying of
tissue were investigated by use of mathematical models. The results from
the simulation study made show that the drying capacity is likely to be
preserved if replacing LPG with syngas. Assuming that all investigated gas
compositions are non-preheated, the use of heat derived from combustion of
the studied syngases was calculated to increase by less than 3.5 % compared
to the reference case of LPG.
The minor investigation of fuel interchangeability made within the present
study shows that the existing gas burners probably need to be replaced in
order to maintain the current drying capacity. Moreover, for syngas to form a
viable option in a tissue drying application, the combustion system needs to
be designed for compliance with existing emission legislations and to avoid
concerns that are shown to occasionally arise during combustion of syngas.
Five commercially available gasification concepts with subsequent gas
cleaning have been studied, and their applicability and energy efficiency on
a mill-scale level have been analyzed. All gasification technologies have
been deemed applicable to convert the foreseen fuels (chipped woody
material) to gas of sufficient quality. Potential feedstock in form of sludge
might however be difficult to handle with fixed bed technologies due to the
risk for formation of dense, impermeable sections in the bed. The fixed-bed
updraft gasification technology is deemed less suitable from an efficiency
point-of-view due to the high tar content in the producer gas and related
energy loss with cold tar cleaning. The three remaining technologies differ
only slightly as to the overall energy efficiency and the overall biomass
consumption.
The exchange of LPG with biomass-derived syngas is generally found
feasible. Rapid variations in gas consumption might form a challenge for
some of the gasification technologies. Given the cost for tar cleaning on one
hand and the importance of soot-free, non-odorous flue gases for this tissue
drying application on the other hand, a need for further experimental testing
has been identified in which the correlation between syngas tar content and
smell/soot impact on the tissue paper could be determined. Such testing is
recommended as a next step in the concept development.
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FOSSIL FREE TISSUE DRYING
List of content
1
2
3
4
Introduction
8
1.1
9
Design criteria
10
2.1
Fuels
10
2.2
Gas quality
12
2.3
Design capacity and gas consumption characteristics
12
2.4
Concluding remarks
15
Gasification and gas cleaning
16
3.1
Assessment of gasification technologies
16
3.1.1 Fixed bed gasification
17
3.1.2 Suspension gasifier
18
3.1.3 Dual bed, steam blown
19
3.1.4 Two stage
20
3.2
Gas cleaning technologies
21
3.3
DynamicS
22
3.4
Concluding remarks
22
Use of syngas in direct heated impingement drying of tissue
4.1
4.2
4.3
5
Scope and goals
23
Effects on the heat and mass balances of Yankee drying – A simulation
study
23
4.1.1 System description
23
4.1.2 Mathematical models
24
4.1.3 Properties of considered energy gases
25
4.1.4 Simulation results
27
Replacing LPG with syngas – effects on the combustion system
32
4.2.1 Assessment of fuel interchangeability of the energy gas burners
32
4.2.2 Syngas combustion in Yankee Hood Burners: operability issues
and implications on combustion system design
33
Concluding remarks
35
System studies
36
5.1
Assumptions and method
36
5.2
Integrated systems
39
5.3
Standalone systems
42
5.4
Discussion
43
5.5
Concluding remarks
47
6
Conclusions
48
7
Future work
49
7.1
8
Verification of the applicability of syngas as LPG substitute for tissue
drying.
References
49
50
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FOSSIL FREE TISSUE DRYING
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FOSSIL FREE TISSUE DRYING
1
Introduction
In production of paper, the energy use for drying is considerable. In some product
segments, for instance tissue paper and coated paper, LPG (Liquefied Petroleum Gas),
natural gas or electricity is used for drying. In 2011, the total use of LPG for pulp and
paper production in Sweden was estimated to be 743 GWh (skogsindustrierna/ÅF). In
Sweden, the pulp and paper industry strive to reduce or in some cases even eliminate
usage of fossil energy until 2020.
The majority of tissue paper produced is dried on a circulating yankee cylinder. This
process combines two types of heat transfer to the wet web. Part of the heat needed for
evaporating the water is provided by conduction from the metal cylinder, which is
internally heated by condensing process steam (contact drying). The other part of the
heat is provided by blowing hot gases onto the wet web (impingement drying). In
Sweden, the process steam condensing inside the yankee cylinder is normally
produced in a bioboiler. The impinging gases, on the other hand, are usually produced
by combustion of natural gas or LPG. For production of fossil free tissue, an alternative
to using natural gas or LPG needs to be developed.
For the full-scale production of fossil free tissue, the LPG used for heating the
impingement gas would have to be exchanged for a renewable fuel. The fuel chosen
within this project is syngas produced by thermal gasification of biomass. The novelty
of the project is the combination of two proceses. One of them, drying of tissue paper, is
well established in full industrial scale and the production capacity and product quality
must be retained even after the change. The other process, thermal gasification of
biomass, has been demonstrated successfully in several pilot-scale reactors in reseach
projects (for instance at Chalmers university of technology [1]) and has also been
carried out in full-scale plants. The novelty will be the knowledge, on unit operationand system- level, on the matching of these two technologies in order to phase out
fossil fuels. The results might well be applicable for other drying applications as well.
The combination of production of biomass-based energy gas with the unit operation
drying is of immediate interest in paper drying processes where fossil energy gas or
electricity is used today. There are advantages of gas heated drying, however, that
might make the technology interesting also for paper drying processes where
renewable energy is used today. The introduction of impingement hoods will lead to
an increased drying capacity in a multi-cylinder dryer or, alternatively, make it
possible to shorten the drying section of the paper machine, for instance compared to
OptiDry concept from Valmet [2].
The annual growth of the market for tissue paper grades is almost 4 % [3]. Tissue paper
produced without usage of fossil fuels will be an interesting and exciting consumer
product. Technology for fossil free tissue is also a product with a global potential. The
world-leading tissue paper machine producer Valmet has production facilities and
development in Karlstad.
The project has been carried out in close co-operation between organizations covering
the entire value chain from raw material, wood chips, to the consumer product tissue
paper:
•
•
An industrial tissue paper producer (Rexcell)
A company specializing in development and production of tissue paper machines
(Valmet)
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FOSSIL FREE TISSUE DRYING
•
•
•
A supplier of biomass (Södra)
Experienced process consultants in energy technology and paper making (Pöyry)
An academic partner with a research portfolio in paper making and energy
technology (Karlstad University)
The project has had the full support of the regional industrial cluster organization, The
Paper Province or TPP. Thanks to the support from VINNOVA and a number of
regional actors, TPP has set as its goal to create a regional demonstrator for a full-scale
bioconomy.
1.1
SCOPE AND GOALS
The scope of the project is to contribute to the development of technology for the
production of tissue paper without the usage of fossil fuels, while retaining paper
quality, production capacity and availability. (In this context, availability represents
ratio of the total time the tissue machine is capable of being used during a given
interval to the length of the interval.)
The goal of the project is the identification and evaluation of concepts for gas
generation, gas cleaning and gas firing relevant also from a financial point of view in
the interesting scale (< 10 MW).
Two sub-goals have been set up:
•
•
The identification of a robust and efficient system for the elimination of tar and
soot from the gas. The system should be characterized by good operational stability
and high mill level energy efficiency.
The employment of a detailed simulation model to be able to predict and to
compensate for any production capacity changes that arise as a consequence of the
replacement of fossil gas with an energy gas that has a reduced lower heating
value and different chemical composition.
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FOSSIL FREE TISSUE DRYING
2
Design criteria
The successful introduction of fossil-free tissue drying technology requires that a
number of design criteria must be fulfilled. Fuel for gasification must be available close
to the plant and the selected gasification technology must be well suited for those fuels
that are available in the region. It is of great importance that the flue gas produced is
clean and free from contaminations so that the superior quality of the tissue produced
is maintained. The maximum capacity of the gasification plant must match the
requirements during periods of maximum production from three paper production
lines.
This chapter specifies, as far as possible, the design criteria that the selected gasification
technology needs to fulfill.
2.1
FUELS
A growing share of the forest-based biomass that cannot be converted to sawn timber
or be used for production of pulp is used as bio fuel. All kinds of trees are used for
energy purposes; even decay-damaged trees can be used. Södra has identified an
assortment of biofuels that could be gasified:
Wood fuel chips oak
Wood fuel chips
Wood chips softwood
Whole-tree chips hardwood
Whole-tree chips oak/beech
(code 6383)
(code 6393)
(code 6493)
(code 6533)
(code 6583)
Out of the list above, the Wood fuel chips (code 6393) and Wood chips softwood (code
6493) were identified as the most interesting assortments.
The starting point of this feasibility study has been that Södra has guaranteed the
supply of biomass from the immediate surroundings. In this specific case, this means
mainly Wood fuel chips (code 6393) and Wood chips softwood (code 6493), supplying
100 % of the fuel needed for gasification, which can be estimated at approximately 65
GWh/year for the planned gasification plant.
Within a distance of 100 km from the plant, there is also yearly access to approximately
45 000 tons of fiber sludge with a moisture content of approximately 80 %. The high
moisture content currently makes transport of the sludge unrealistic. However, the
supplier at present strives to develop methods for reducing the moisture content of the
sludge.
The reduction in operational cost associated with replacing the LPG will depend on a
number of factors. The price for LPG has decreased during the last two years, cf. Figure
1. The price of the fuel substituting the LPG will be different depending on what
assortment is finally chosen. The data in Figure 2 represent fuel chips, whereas the
other alternatives such as bark are cheaper. Finally, for estimating the reduction in
operational cost, also the efficiency of gasification of the integrated system will be
needed as well as an estimation of the syngas lost due to production breaks and quick
changes in the syngas need that cannot be matched by the gasification process.
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FOSSIL FREE TISSUE DRYING
Figure 1. Costs LPG.
Figure 2. Costs biofuel for gasification.
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FOSSIL FREE TISSUE DRYING
2.2
GAS QUALITY
Modern tissue drying processes utilize direct fired high temperature drying hoods
where the flue gases are blown towards the wet tissue paper.
In this application the gas composition itself is of limited relevance as only the energy
released during combustion is utilized. But as the combustion flue gases are brought
into contact with the tissue paper it is important that no smelling or hazardous
compounds are transferred to the tissue paper during the drying process. The gas
quality requirements are mainly linked to the concentration of tar.
Ideally, substitution of LPG with biomass derived syngas should be trouble-free from
this perspective as the main combustible syngas components H2, CO, and CH4 all burn
without soot formation, yielding nothing but CO2 and H2O as reaction products.
However, in the gas from the gasifier tar is also present. Tar is often defined as organic
compounds with molecular weight greater than that of benzene [4]. Soot formation
during tar combustion is a highly complex area [5-7]. To determine a safe tar-level from
a soot formation perspective is difficult. In this study it has been preliminary foreseen
that a tar concentration below 100 mg/m3 is reached, which is considered sufficient for
gas engine applications [8] would be a realistic target. However, this concentration
should be practically verified (see chapter 7.1).
2.3
DESIGN CAPACITY AND GAS CONSUMPTION CHARACTERISTICS
At present, LPG is used for drying at three production lines. Each of the machines
produces a multiple of qualities and therefore has gas consumptions that vary over
time. In Figure 3, Figure 4 and Figure 5 below logged LPG consumption for the 1st
quarter of 2015 is shown for the three machines. Sampled values are average measured
during a 10 min period. The function of the LPG flow meter for the third production
line was unstable during the period, explaining the irregular consumption pattern in
Figure 5.
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FOSSIL FREE TISSUE DRYING
Figure 3. LPG consumption of production line 1.
Figure 4. LPG consumption of production line 2.
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FOSSIL FREE TISSUE DRYING
Figure 5. LPG consumption of production line 3.
The total gas consumption when combining the three machines is illustrated in Figure 6
below. Based on the LPG consumption for the period, a design capacity for the
gasification plant of 550 kg/h (LPG equivalent), corresponding to 7 MW (LHV) has
been set.
As can be seen in Figure 6 the gas consumption typically varies rapidly between 150
and 500 kg/h corresponding to a turndown ratio of 3,3:1. Furthermore, from the
consumption statistics it can be observed that the maximum increase and decrease rate
of gas consumption is in the region of 20 kg/(min⋅h), corresponding to almost 250
kW/min. During process disturbances even higher load change rates can occur. Such a
fast response from the gasifier may be difficult to achieve. For this reason the system
shall comprise regulatory functions in the form of: (i) A flare for combustion of excess
gas in case of a rapid decrease in gas consumption, (ii) An LPG back-up for balance of
syngas shortage due to rapid increase in gas consumption.
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FOSSIL FREE TISSUE DRYING
Figure 6. Total LPG consumption of the three production lines. The three red dotted lines indicate that the
consumption typically varies between 150 kg/h and 500 kg/h with a top consumption of 550 kg/h.
2.4
CONCLUDING REMARKS
The biomass assortment is primarily planned to consist of chipped woody material.
This fuel is a feasible feedstock for all examined gasification technologies. Potential
feedstock in form of sludge might however be difficult to handle with fixed bed
technology due to its risk for forming of dense, impermeable sections in the bed.
The gas quality requirements in this project are mainly focused on tar content.
Chlorine-, sulphur- and nitrogen content as well as H2, CO, CH4 -concentrations that all
are important parameters in syngas utilization based on catalytic conversion are less
important in this application. An acceptable tar concentration has tentatively been set
to 100 mg/m3 but this should be further investigated as the relation between tar content
and soot formation during combustion is difficult to predict.
The gas consumption variations at the studied tissue mill is significant, with a turndown ratio of >3. Furthermore, the gas consumption change rate is high. During grade
changes the gas requirement can change in the order of 250 kW/min. In connection
with production disturbances, even higher rates can be expected.
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FOSSIL FREE TISSUE DRYING
3
Gasification and gas cleaning
The project aims at evaluating different gasification technologies in terms of their
suitability for replacing LPG for tissue paper drying. Several different gasification
technologies exist, yielding different gas compositions and syngas with different
heating values. This chapter will very briefly present some possible technologies. This
review of possible technologies has a number of purposes: (i) Gas compositions are
needed for evaluating the suitability of a specific technology from the hood burner’s
perspective, (ii) Data for the heat and mass balances of the gasification process are
needed for evaluating the biomass consumption when the gasification process is
integrated in the plant, (iii) Some understanding of the dynamics of the gasification
process are needed for evaluating how well a specific gasification technology can be
controlled to match the fluctuations in the need for syngas for tissue drying.
The chapter also presents a strategy for gas cleaning to match the design criteria for gas
quality.
3.1
ASSESSMENT OF GASIFICATION TECHNOLOGIES
Data, in terms of gas compositions and heating values, have been obtained from two
sources: the scientific literature [9-15] and technology supplier information. These data
have been used when setting up mathematical models for gasification. The
mathematical models for gasification were set up in the commercial flow sheeting
software CHEMCAD 6.4.1 (Chemstations Inc., Houston, TX, USA) with the aim of
evaluating the possibilities of an advantageous integration of each technology in the
plant. The modelling of the gasification reactor was quite crude, trying to find a
stoichiometry to match the experimental data. More specific information on the
modelling process is given in the section on Process Integration. The gas characteristics
from the CHEMCAD models are included already here to provide a quick overview of
data from three types of sources: scientific literature, supplier information, and flow
sheeting model developed within this study. Three separate columns are included in
the tables, although all three types of data are not available for every technology. The
quotations obtained also differed somewhat as to the level of detail about the syngas
composition.
When comparing data from different sources, it is evident that the water vapour
content of the produced syngas might vary considerably. Early on in the project, the
high contents of water vapour of the syngas were identified as a potential problem
when burning the syngas and it was deemed necessary to reduce the water vapour
content by cooling the gas. For these reasons, the gas compositions are given only for
dry syngas and the water vapour content has been omitted.
Focus when searching for literature data for gas compositions and gas heating values as
well as during contacts with possible technology suppliers was on using woody
biomass as a fuel for gasification and syngas generation.
Gasification is a number of endothermal chemical reactions, where solid and liquid
components are transformed into syngas containing such energy rich compounds as
hydrogen, carbon monoxide and methane. Gasification is a partial oxidation of the fuel,
so that the presence of some oxygen in the fuel is necessary, although the oxygen can
be provided in several ways, for instance gasification in air, in steam or in carbon
16
FOSSIL FREE TISSUE DRYING
dioxide. The heat that is needed for gasification of the fuel is provided either through a
partial combustion of the fuel (in case the gasification medium is air) or from an
external heat source (in case the gasification medium is steam or carbon dioxide). Some
phases in the gasification process have been defined as: drying, pyrolysis, and
combustion (only when the gasification medium is air) and gasification. [16]
In case the gasification medium is air, the process is often characterized in terms of the
Equivalence Ratio or ER. ER is defined as the quotient of the actual flow of air to the
reactor and stoichiometric flow of air. For ER = 0, the process corresponds to pyrolysis
and for ER = 1, complete combustion of the fuel occurs. For gasification in superheated
steam, the process is instead defined in terms of the Steam to Biomass Ratio or SBR.
SBR is the quotient of the flow of steam to the reactor and the flow of dry biomass. [16]
3.1.1
Fixed bed gasification
In a fixed bed gasifier, the gasification medium is blown through the fuel which
remains at the bottom of the reactor. The gasification medium can flow upwards or
downwards, see Figure 7. A solid bed gasifier is well suited for the gasification of
biomass, since it can handle particles with a size up to 50 mm. As the temperature falls
in the direction of the fuel feed, an updraft arrangement will lead to higher tar
production than the downward flow arrangement. Approximate data for gas
composition, the syngas lower heating value and the tar formation are given in Table 1
for an updraft fixed bed gasifier and in Table 2 for a downdraft fixed bed gasifier
[11,12,14,16,17].
Figure 7. Two examples of fixed bed gasifiers, an updraft reactor to the left and a downdraft to the right [13].
Table 1 (representing the updraft fixed bed gasifier) contains representative literature
data and also data from the gasification model set up in the flowsheeting software
CHEMCAD. We obtained one quotation for the udraft fixed bed gasification
technology. However, it did not fully match our requirements, since the suggestion was
installing separate reactors for producing syngas at each production line rather than
one gasification reactor producing the syngas needed for all three production lines.
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FOSSIL FREE TISSUE DRYING
Still, the technology was investigated in terms of the possibilities for process
integration, since it was interesting to keep a gasification technology leading to a
syngas containing considerable amounts of tar.
Table 2 (representing the downdraft fixed bed gasifier) contains all three types of data,
literature data, technology supplier data and our own model data.
Table 1. Data representative of syngas produced in an updraft fixed bed gasifier.
Literature
data
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Nitrogen, N2
Ethylene, C2H4
Tar
Lower heating value
(dry gas)
Quotation
obtained
-----------------
20-25 Vol-%
8-12 Vol-%
15-25 Vol-%
3-6 Vol-%
40-50 Vol-%
--<200 g/m3
5-7 MJ/Nm3
CHEMCAD
model
18 Vol-%
17 Vol-%
17 Vol-%
2 Vol-%
46 Vol-%
0 Vol-%
136 g/m3
5,0 MJ/Nm3
Table 2. Data representative of syngas produced in a downdraft fixed bed gasifier.
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Nitrogen, N2
Ethylene, C2H4
Tar
Lower heating value
(dry gas)
3.1.2
Literature
data
20-25 Vol%
8-12
15-25
2-5
40-50
--<5 g/m3
5-7 MJ/Nm3
Quotation
obtained
20
12
20
1-3
45-47
--0 g/m3
5 MJ/Nm3
CHEMCAD
model
22
11
17
2
48
0
4,2 g/m3
5,4 MJ/Nm3
Suspension gasifier
In a suspension gasifier, very small fuel particles are needed. The particles are
suspended in flowing air and are gasified, see Figure 8. The temperature is often higher
than in a fixed bed gasifier. The particles need to be very small, no larger than 0.15 mm
in diameter, which makes this technology less suitable for biomass since grinding of the
fuel might be necessary.
A similar technology to the suspension gasifier, a cyclone gasifier, was offered by one technology supplier.
Table 3 compares syngas data representing the suspension gasifier taken from the
literature [12] to supplier information regarding the cyclone gasifier. The gas
composition and heating values are similar to the data for the fixed bed gasifiers and
this technology was not used as a basis for setting up a specific CHEMCAD model.
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FOSSIL FREE TISSUE DRYING
Figure 8. Schematic illustration of a suspension gasifier [13].
Table 3. Data representative of syngas produced in a suspension gasifier.
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Nitrogen, N2
Ethylene, C2H4
Tar
Lower heating value
(dry gas)
3.1.3
Literature
data
20-25
8-12
20-25
1
40-50
--<30 g/m3
5-7 MJ/Nm3
Quotation
obtained
20
12
11
3
50
2
≈10 g/m3
6,0-6,3 MJ/Nm3
CHEMCAD
model
-----------------
Dual bed, steam blown
A dual bed gasifier combines the gasification reactor with a combustion reactor, see
Figure 9. Superheated steam can be used as a gasification medium, leading to the
gasification being endothermal. Any solid residue (gasification char) is separated from
the syngas and combusted. The heat of gasification is supplied by circulating bed
material between the two reactors.
Since gasification can occur in superheated steam, it is possible to produce a syngas without excessive
amounts of nitrogen. The syngas gets a higher lower heating value, as illustrated in
Table 4, which compares literature data [9,12], data from a technology supplier and
CHEMCAD model data.
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FOSSIL FREE TISSUE DRYING
Figure 9. Schematic illustration of a steam-blown dual bed gasifier [9].
Table 4. Data representative of syngas produced in a steam-blown dual bed gasifier.
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Nitrogen, N2
Ethylene, C2H4
Tar
Lower heating value
(dry gas)
3.1.4
Literature
data
20-30
15-25
35-45
8-12
<10
--<40 g/m3
14-16 MJ/Nm3
Quotation
obtained
35
19
26
13
2
4
???
16,4 MJ/Nm3
CHEMCAD
model
32
16
34
15
0
4
32 g/m3
15,1 MJ/Nm3
Two stage
In a two stage gasification process, the fuel is pyrolysed and the pyrolysis char is separated from the pyrolysis
gas. The pyrolysis char is gasified in superheated steam and the heat needed for pyrolysing the fuel as well as
for gasification of the pyrolysis char is supplied by combustion of the pyrolysis gas, as illustrated schematically
in Figure 10. This technology is offered in a suitable scale and the great advantage is claimed to be the
production of tar-free syngas with a relatively high lower heating value.
Table 5 provides a comparison between syngas data taken from the literature [15],
technology supplier data and data from our CHEMCAD model.
20
FOSSIL FREE TISSUE DRYING
Figure 10. Schematic illustration of a two stage gasifier [15].
Table 5. Data representative of syngas produced in a two-stage gasifier.
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Nitrogen, N2
Ethylene, C2H4
Tar
Lower heating value
(dry gas)
3.2
Literature
data
(20-30)
(8-12)
(50-60)
(3)
0
--0 g/m3
10-11 MJ/Nm3
Quotation
obtained
18-30
8-17
50-60
1-3
0
--0 g/m3
10-12 MJ/Nm3
CHEMCAD
model
34
9
54
3
0
0
0 g/m3
11,2 MJ/Nm3
GAS CLEANING TECHNOLOGIES
Gas cleaning technologies can be grouped into: cold-, warm-, and hot- systems [8]. In
many applications a hot gas cleaning system is preferred as this potentially enables
higher energy efficiency due to less loss of sensible heat. Such hot gas cleaning employs
hot gas filtering and catalytic tar cracking. In-spite of extensive research and recent
achievements [18] these technologies have not found broad commercial utilization and
most realized gasification plants utilizes cold cleaning by means of textile- and
activated carbon filters and scrubbing with oil and/or water. Sometimes an ElectroStatic Precipitator (ESP) is used to capture aerosols that are formed in the scrubber
system.
In this small scale application where cold gas is preferred from a distribution point-of
view, the obvious technology for tar cleaning is oil scrubbing followed by activated
carbon filtration, which is foreseen to eliminate sufficient amounts of tar to prevent soot
formation during combustion.
A gas cleaning concept according to Figure 11 below has been considered sufficient for
the studied application.
21
FOSSIL FREE TISSUE DRYING
Figure 11. A gas cleaning set-up suitable for the studied application.
As a first step after the gasifier a particle separator is used to reduce the amount of
solids entering the heat exchanger and the subsequent gas cleaning system. An oil
scrubber is placed after the gas cooling. Meaning that gas at approximately 200 ᵒC is fed
into the scrubber where it is cooled to 30 ᵒC. In the scrubber, tar is condensed and
absorbed into the bio oil. After the oil scrubber, the gas passes an activated carbon (AC)
filter where residual tar is adsorbed, yielding a tar concentration below 50 mg/m3. After
the AC filter, the gas is filtered in a textile fabric dust filter. Such a filter is capable of
reducing the dust concentration to <1 µg/m3 (for particles larger than 1 µm) [8]. These
levels of tar and particulates are deemed sufficient for a trouble-free operation of
subsequent gas handling equipment as well as for soot- and dust free combustion.
3.3
DYNAMICS
The gas consumption variations as described in chapter 2.3 have been briefly discussed
with the potential gasification suppliers. All suppliers consider these variations as
possible to handle. However, based on the discussions and the supplier statements, the
variations seem to be somewhat easier to manage with suspension gasifiers than with
fixed bed- and dual fluidized bed gasifiers. Concepts with multiple gasification units
needs to shut-off one or several units during low load periods. Restart of these units at
fast increase in gas consumption will be difficult to manage and LPG backup will be
important to have.
The gas cleaning concepts foreseen in this project generally has a high tolerance for gas
flow variations. The most sensitive process section will be the multi-cyclones for
removal of coarse particles. The other process steps consisting of scrubbers and barrier
filters are less sensitive to flow variations.
3.4
CONCLUDING REMARKS
Several technologies for gasification have been identified as interesting for the current
application. These include fixed bed gasification, suspension gasification, steam-blown
dual bed gasification and two-stage gasification. Furthermore, all these technologies are
available commercially and five (or at least four) relevant quotations have been
obtained. The gasification technologies produce syngas differing in composition and
heating value.
Gas characteristics from the quotations complemented with data from the scientific
literature will be used for evaluating the different technologies in terms of tissue drying
process and regarding the possibilities for process integration.
Gasification processes are available commercially in the scale of technology relevant to
the application at the plant.
22
FOSSIL FREE TISSUE DRYING
4
Use of syngas in direct heated impingement
drying of tissue
The investigation presented in this chapter studies the effects of replacing LPG with
syngas in the process of combined contact and impingement drying used in the
manufacture of tissue.
4.1
EFFECTS ON THE HEAT AND MASS BALANCES OF YANKEE DRYING – A SIMULATION
STUDY
In processes for manufacture of tissue utilizing water-laid forming, a fiber suspension
is laid on a fabric for subsequent operations of dewatering. Valid for the production
process of the current study, post forming processes of water removal involves the
usage of pressing and thermal drying. The thermal dewatering is performed by the
usage of simultaneous contact and impingement drying in which the hot gas impinged
onto the web is directly heated by combustion of LPG. The aim of the work presented
in this chapter is to quantify the effects of replacing LPG with syngas on the local and
overall drying behavior as well as on the heat and mass balances of tissue drying. This
includes studying effects on the drying capacity, on the use of thermal and electrical
energy and on the potential for recovery of exhaust gas excess heat.
4.1.1
System description
The Yankee dryer considered in the present work comprises a steam heated cylinder
used for drying and conveying at the web. The potential for rapid water removal
allowed by the low thickness of the web is utilized by the usage of two hot gas
impingement hoods employing recirculation of the impinging gas, Figure 12. The
recirculating drying gas is directly heated by combustion of energy gas. To maintain
the anticipated moisture content of the recirculating gas of the second hood, a countercurrent flow is transferred to the first hood from which the exhaust gas stream of the
dryer system is expelled. The stream of fresh air feeding the energy gas combustors
(hereafter called combustion air) and diluting the recirculating gas (hereafter called
make-up air) is preheated using excess heat recovered from the exhaust gas stream.
23
FOSSIL FREE TISSUE DRYING
Figure 12. Flowsheet of the Yankee dryer system air and gas flows.
4.1.2
Mathematical models
A mathematical model developed to enable simulations of Yankee drying was used to
assess the effects of replacing LPG with syngas. The model used was presented by
Ottosson et al. [19] and it can be employed for simulations of both local and overall
drying behavior.
In order to quantify the influence on energy use and on the potential for excess heat
recovery, the mass and energy balances of the drying system were solved. In this work,
numerical values for temperature, humidity and impingement velocity of the drying
gas were for both hoods taken from results retrieved by employing the drying model
mentioned above. The content of dry gas of the exhaust gas stream is determined by
summing the combined fresh air inflows together with total dry energy gas flow
deducted for combustion induced water generation. The mass flow of water of the
same stream is determined from the water evaporated from the web and from the
moisture content of the used energy gas with the addition of the combustion generated
water vapor. Knowing the described relationship between in- and outgoing flows of
material as well as the properties of the impinging gas allows for iteratively solving the
mutually dependent variables of the balances for water, dry gas and enthalpy.
Heat losses from the dryer body are assumed to amount to 5 % of the total evaporation
load [20]. Enthalpy estimations made assume gases to consist of humid air with the
exception for the inflow of energy gas. It is assumed that the combustion system can
burn the investigated energy gases at the rates required to fulfill the conditions set by
the heat and mass balances of the drying operation. The stream of fresh air distributed
to the recirculating gas and to the energy gas combustors are assumed to be preheated
to 200 °C utilizing excess heat recovered from the exhaust gas stream. The composition
of the flue gas resulting from syngas combustion was calculated in the software
24
FOSSIL FREE TISSUE DRYING
CHEMCAD. The size of the flue gas mass flows were subsequently scaled to match
values predicted for the drying system use of syngas.
4.1.3
Properties of considered energy gases
Effects of use of two biomass derived energy gases of varying composition, calorific
value and combustion characteristics are considered in the present work, Table 6. The
reference gas used is LPG which for convenience is assumed to consist of 100 %
propane, C3H8. The main differences between the considered syngases are the
hydrogen and the nitrogen content as well as an accompanying change in calorific
value.
25
FOSSIL FREE TISSUE DRYING
Table 6. Assumed dry basis composition and combustion characteristics of considered energy gases.
Propane, C3H8
Hydrogen, H2
Carbon monoxide, CO
Methane, CH4
Carbon dioxide, CO2
Nitrogen, N2
Lower heating value
Lower Wobbe index
[MJ/Nm3 dry gas]
Stoichiometric air-fuel
ratio
[kg dry air/kg dry gas]
Water content
[kg water/kg dry gas]
Combustion derived
water generation
[kg water/kg dry gas]
LPG
ref. energy gas
100 Vol-%
0 Vol-%
0 Vol-%
0 Vol-%
0 Vol-%
0 Vol-%
90.0 MJ/Nm3
75
Syngas 1
low H2 content
0 Vol-%
17 Vol-%
22 Vol-%
2 Vol-%
11 Vol-%
48 Vol-%
6.2 MJ/Nm3 dry
syngas
5.7
Syngas 2
high H2 content
0 Vol-%
54 Vol-%
34 Vol-%
3 Vol-%
9 Vol-%
0 Vol-%
11.2 MJ/Nm3 dry
syngas
16
15.6
1.29
4.54
0.0
0.032
0.014
1.714
0.148
0.717
Four simulation cases were defined in order to study the effects of replacing propane
with syngas of varying composition. Case 1 is the reference where propane is used as
energy carrier. Case 2 and 3 utilize syngas 1 and syngas 2, respectively. The combustion
reactions of case 1, 2 and 3 were supplied with 40 % excess of air. However,
stoichiometric combustion of gas mixtures of carbon monoxide and hydrogen can
result in an adiabatic flame temperature significantly higher than what is reached in
stoichiometric combustion of propane [21]. The hydrogen and carbon monoxide
content of at least one of the energy gases involved in the present study might therefore
require the application of strategies to control formation of thermally formed nitrogen
oxide. Using sufficient levels of excess air can under certain conditions reduce the flame
temperature sufficiently to avoid unwanted levels of thermally formed nitrogen oxides
[22]. To investigate effects of such a strategy on the heat and mass balances of tissue
drying, a case utilizing 100 % excess of combustion air were defined for the use of
syngas 2.
The parameters describing the tissue production conditions, shown in Table 7, were
held constant for all simulated cases in order to form an appropriate basis for
comparison. To establish the conditions of the impinging gas required to dry the web to
the expected final dryness, simulations using the aforementioned drying model were
made. As there is a mutual dependency between the rate of evaporation and the
humidity of the impinging gases of the hoods, an iterative solution was used to find the
appropriate values for the moisture content of the drying gases.
26
FOSSIL FREE TISSUE DRYING
Table 7. Input parameters valid for all simulated cases.
Machine speed
Web basis weight on the Yankee cylinder
Web basis weight at reel
Post pressure roll web consistency
Final dryness
Temperature of Yankee dryer condensing steam
Impinging gas temperature of the 1st hood
Impinging gas temperature of the 2nd hood
Impinging gas velocity of the 1st hood
Impinging gas velocity of the 2nd hood
4.1.4
1485 m/min
11.65 g/m2
15.3 g/m2
42 %
94 % ± 0.1
171.4 °C
394 °C
390 °C
106 m/s
100 m/s
Simulation results
Using the numerical values presented in Table 7, a simulated final web dryness of 94 %
was, for all involved energy gases, obtained without the need for case individual
adjustments of the impinging gas temperature or velocity. However, as the
investigated energy gases have different calorific values, contain varying amounts of
water, and use different amounts of combustion air as well as produce different
quantities of water during combustion, adjustments of the make-up air flow were
necessary to maintain a constant drying capacity, see Table 8.
Table 8. Simulation results.
Combustion air excess [%]
Impinging gas humidity of the 1st hood
[g water/kg dry air]
Impinging gas humidity of the 2nd hood
[g water/kg dry air]
Exhaust gas humidity [g water/kg dry air]
Flow of dry air for combustion [kg/s]
Flow of dry air for make-up [kg/s]
Propane mass flow [kg/h]
Propane volume flow [Nm3/h]
Dry mass flow of syngas 1 [kg dry gas/h]
Dry volume flow of syngas 1 [Nm3/h]
Dry mass flow of syngas 2 [kg dry gas/h]
Dry volume flow of syngas 2 [Nm3/h]
Mass flow of water in energy gas stream
[kg/h]
Combustion derived water generation
[kg/h]
Case 1
40
354
Case 2
40
348
Case 3
40
357
Case 4
100
341
234
238
228
245
454
1.094
0.970
180.3
92.93
0.0
0.0
0.0
0.0
0.0
448
0.785
0.883
0.0
0.0
1561
1386
0.0
0.0
49.87
457
0.919
1.095
0.0
0.0
0.0
0.0
520.1
772.6
7.059
440
1.318
0.786
0.0
0.0
0.0
0.0
521.9
775.2
7.084
294.9
230.5
372.9
374.2
If it is possible to keep the temperature, velocity and humidity of the impinging gas
constant for the simulated cases, this means that the drying conditions are identical.
However, due to the differences in the characteristics of the examined energy gases, it
is not possible to accomplish an identical impinging gas humidity for the investigated
27
FOSSIL FREE TISSUE DRYING
cases, implying that local drying conditions are to some extent varying. The time
averaged rate of drying is however identical if equal final web dryness is reached.
Valid for the syngas cases employing 40 % air excess (case 2 and 3), the need for
combustion air is reduced compared to the reference case of LPG, Figure 13. For the
case of syngas 1, the reduction is approximately 30 %. However, the energy gas flow
was increased 8.7 times in terms of mass flow and 14.9 times in terms of volume flow.
Figure 13. Total inflow of make-up air, combustion air and energy gas entering the drying system.
Figure 14 presents the mass flows of the gas components resulting from combustion of
the investigated energy gases using oxygen from atmospheric air as oxidant. The air
excess was set to 40 % with the exception of case 4, where it was set to 100 %. The
combustion reaction products of carbon dioxide and water of the syngas cases are the
components computed to deviate most compared to the reference case. Emissions of
carbon dioxide are calculated to increase by 78 % and 29 % for case 2 and case 3
respectively.
28
FOSSIL FREE TISSUE DRYING
Figure 14. Combustion flue gas components entering the recirculating drying gas.
Although the relative differences between the concentrations of combustion flue gas
components are substantial, the components of carbon dioxide and water constitute a
relatively small fraction of the combined combustion flue gas and make-up air stream,
Figure 15. Compared to the results presented in Figure 14, only atmospheric air (makeup air) is added and consequently differences are mainly experienced for the
components of nitrogen and oxygen together with the total mass flow. The relative
difference in total mass flow entering the drying gas stream is, comparing cases
utilizing 40 % air excess, less than 3 %.
Figure 15. Components of combustion flue gas and make-up air entering the recirculating drying gas.
29
FOSSIL FREE TISSUE DRYING
Aspects on energy efficiency
The predicted use of energy carriers involved in the investigated drying process are
compared in Figure 16. As mentioned earlier, the drying process simulations made for
the different cases have common values for web dryness at the start and the finish of
drying, thus the same amount of water is removed by means of thermal dewatering.
Figure 16. Use of energy for involved energy carriers and potential for recovery of exhaust gas excess heat.
The use of steam for heating of the dryer cylinder is calculated to be equal for the
compared cases. Hence, the calculated variation regarding the usage of heat produced
from energy gas implies that the thermal efficiency of the impingement hoods is
slightly dependent on the characteristics of the used gaseous fuel.
Results presented above show that a higher inflow of energy gas due to a lower
calorific value was accompanied by an expected reduced need for combustion air.
However, the total mass flow of combustion air and energy gas only changed slightly.
Therefore, as streams of combustion and make-up air are assumed to be preheated to
200 °C while the energy gas has a temperature of 30 °C prior to admission of the
combustion system, the enthalpy inflow to the dryer is lower for the syngas cases. The
lower inflow of enthalpy needs to be compensated for to preserve the drying capacity.
This compensation is achieved by an increased rate of combustion of energy gas at the
cost of a reduced thermal efficiency of the drying process. Here, enthalpy coming from
excess heat based preheating is replaced by thermal energy produced by use of energy
gas.
The amount of water added to the drying gas due to combustion varies between the
studied energy gases. This water stems from moisture content in the energy gas and/or
water generated in the combustion reaction. In order to maintain the desired drying
capacity, increased addition of water to the drying gas requires an increased outflow
and inflow of exhaust gas and make-up air respectively. Increased outflow of warm
exhaust gas is a loss of enthalpy that is compensated for by energy gas combustion for
heating of the needed supplement of dry air, leading to a reduction in thermal
efficiency of the drying process.
30
FOSSIL FREE TISSUE DRYING
For the syngas used in case 2, the calculation results presented in Figure 16 show that
the demand for thermal energy derived from combustion of energy gas increases
slightly compared to the reference case. The thermal efficiency of the dryer is reduced
due to less use of preheating as heat source. On the other hand, the amount of water
added to the drying gas by combustion is approximately 5 % lower compared to the
reference case. Combined, these effects have a negative impact on the drying efficiency,
resulting in an increased need for heat from energy gas of approximately 2 %.
Considering the calculated results for use of the hydrogen-rich syngas of case 3, also in
this case the demand for heat produced by energy gas combustion is higher than for
the reference case. This is the result of a somewhat reduced use of preheating as
thermal energy source as well as of an increased addition of combustion derived water
of around 30 %. Still, the predicted increase in use of primary energy is relatively small,
being approximately 3 %.
The potential for recovering exhaust gas excess heat was for the investigated cases
assessed by calculating the sensible and latent heat acquired when cooling the exhaust
gas stream to 55°C, Figure 16. The dependency of the energy gas used was found to be
low. However, at constant air excess the potential for heat recovery correlates well with
use of primary energy.
As previously described, data for predicted energy use presented in Figure 16 are
calculated assuming an energy gas temperature prior to combustion of 30 °C. However,
the cases of syngas combustion experience a lower contribution from preheating to the
overall heat balance. Therefore, the effects of preheating the energy gas on the need for
primary energy was also considered. Results from calculations assuming availability of
recovered excess heat to warm the energy gas are presented in Figure 17. Preheating
the syngas of case 2 to a temperature of 110 °C is predicted to result in a drying
efficiency equal to the reference case. The effect of preheating the syngas of case 3 is
less pronounced due to the lower mass flow of energy gas.
Figure 17. Effect of preheating energy gases on use of thermal energy derived from combustion.
31
FOSSIL FREE TISSUE DRYING
Effects of varying equivalence ratio
In the simulation made for the parameters of case 4, the hydrogen-rich syngas 2 is
burned at an air excess of 100 %. Compared to the results for case 3, where the same
energy gas is used but the air excess is set to 40 %, use of heat derived from combustion
of syngas is only marginally increased, see Figure 16. The somewhat reduced potential
for recovery of excess heat is a result of the decreased concentration of water vapor in
the exhaust gas stream from the dryer, see Table 8.
4.2
REPLACING LPG WITH SYNGAS – EFFECTS ON THE COMBUSTION SYSTEM
The simulation study presented in chapter 4.1 pointed out the significant increase in the
energy gas flow. The focus of the investigation presented in the current chapter will
therefore be on assessing the fuel interchangeability of the burners in the existing
combustion system and on operational issues plausible in combustion of syngas.
4.2.1
Assessment of fuel interchangeability of the energy gas burners
The viability of replacing a gaseous fuel with a replacement fuel is dependent on
several factors [23,24], however, a main issue is obviously to ensure the ability to
generate a sufficient amount of heat. A widely used fuel interchange parameter
developed to address this question is the Wobbe index, shown in Eq. (1).
(1)
√
LHV is the lower heating value and SG is the specific gravity of the investigated fuel.
For a specific burner and a given gas pressure, two fuels having equal Wobbe index
generate equal amounts of heat. Hence, this parameter indicates the rate of energy flow
of the gas flow passing through the burner nozzle at a given pressure drop. Although
the Wobbe index is a useful parameter in evaluating fuel interchangeability, it does
however not address effects on burner operability, that is, phenomena such as
flashback, blowout and autoignition.
Calculated Wobbe indices for the energy gases selected for evaluation in the current
study are presented in Table 6. As the Wobbe indices for Syngas 1 and Syngas 2 are
only 8 % and 21 % respectively of the value for propane, it is concluded that the gas
pressure or component/components of the burner or both are subjects for amendments
in order to preserve the amount of generated heat.
In a recent study [25], implications of replacing fossil fuels in industrial applications
utilising directly heated processes were investigated. No experimental work was made
and statements regarding needed combustion system component replacements or
modifications were therefore reliant upon reports from burner suppliers and from the
industrial sectors involved in the project. Use of a syngas of low calorific value,
typically produced by air blown gasification, was concluded to result in a need for
replacement of the burner assembly. The range of lower heating values of the
considered syngas was in this case 4 to 6 MJ/m3 which is relatively close to the value for
Syngas 1, Table 6. Use of a syngas of higher hydrogen content and therefore higher
calorific value and, most likely, also higher Wobbe index was concluded to implicate a
major modification to the existing burner assembly including new burner nozzles and
seals. The considered range of lower heating values for this higher calorific value
syngas was 12 to 29 MJ/m3, thus somewhat higher than the value for Syngas 2, Table 6.
32
FOSSIL FREE TISSUE DRYING
Existing burners used in the direct heated processes investigated in the cited study
were designed for combustion of natural gas.
4.2.2
Syngas combustion in Yankee Hood Burners: operability issues and implications
on combustion system design
Syngas, or synthesis gas, is a fuel mixture containing hydrogen, carbon monoxide and
carbon dioxide, in variable ratio, depending on the type of gasification process.
In the Syngas fuel mixture also nitrogen is present, and as a result of the presence on N2
and CO2, the calorific value of the mixture is usually much lower compared to LPG.
When burning syngas one has then to take in account these two key factors:
•
•
High hydrogen content
Low calorific value
The lower calorific value impact is mainly in the higher fuel flow needed. This will
reflect both in a larger size of the burner components, starting from gas piping to
burner nozzles, and in the heat and mass balance of the burner itself due to the higher
quantity of gas that is introduced in the combustion chamber and in the heat/mass
balance need to be heated up to hood impingement temperature.
Lower calorific value requires also a burner that develops the flame in a hot reaction
area like a refractory block or reaction chamber. This will exclude the possibility to
install so-called “in-line” burners, and force to choose corner burner, with combustion
chamber and slave designed in order to keep flame temperature as high as possible.
This will also help to burn possible contaminants present in syngas even after the
cleaning process.
More complicated is the complete analysis of the issues linked to the high hydrogen
content. Those aspects are analyzed both by McDonell [23] and Lieuwen et al. [24].
As highlighted by McDonell [23], hydrogen behaves differently than a hydrocarbon in
many ways including specific heat (hydrogen has a much higher specific heat than
other gases, even if syngas mixture will have a lower one due to its inert components),
diffusivity (hydrogen has a much higher diffusivity than other gases), flammability
limits (hydrogen has a wide range of volume concentrations over which it is
flammable), and flame speed (hydrogen has a much higher laminar flame speed than
do other gases).
As mentioned before, corner burner should be used due to low LHV. Corner burners
are characterized by mixing of fuel and oxidant (combustion air) in the burner nozzle.
Given both the high flame speeds of hydrogen and wider flammable limits, the
possibility of reaction evolving into the premixing region, not designed to accept high
temperature, is a major concern that must be examined carefully during the design
phase.
Lieuwen et al. [24] summarise the most critical of these operability issues of operating a
combustor with Syngas in four categories: blowout, flashback, combustion instability
and autoignition.
Blowout refers to situations where the flame becomes detached from the location where
it is anchored and is physically ‘‘blown out’’ of the combustor. Blowout is often
referred to as the ‘‘static stability’’ limit of the combustor. Blowoff involves the
33
FOSSIL FREE TISSUE DRYING
interactions between the reaction and propagation rates of highly strained flames in a
high speed, often high shear flow. Blowoff events can require a lengthy and often
expensive system shut down, purge cycle, and restart.
A second issue is flashback, where the flame propagates upstream of the region where it
is supposed to anchor and into premixing passages that are not designed for high
temperatures. Flashback involves turbulent flame speed propagation in a highly
inhomogeneous, swirling flow. Since premix nozzles are not well cooled, flame
flashback is a serious safety risk. After flashback has occurred, flame anchoring in the
nozzle leads to a fast rise of material temperatures, with subsequent overheat and
failure.
Combustion instability refers to damaging pressure oscillations associated with
oscillations in the combustion heat release rate. These oscillations cause wear and
damage to combustor components and, in extreme cases, can cause liberation of pieces
into the hot gas path, damaging downstream components.
Autoignition refers to the homogeneous ignition of the reactive mixture upstream of
the combustion chamber. Similar to flashback, it results in chemical reactions and hot
gases in premixing sections, but its physical origins are quite different from those of
flashback. Rather than the flame propagating upstream into the premixing section,
autoignition involves spontaneous ignition of the mixture in the premixing section.
There are other possible operability issues due to water and other contaminants coming
from the gasification process.
Presence of water is problematic, not necessarily during operation but mostly during a
shut-down, when water condenses inside piping and instrumentation. Even if pipelines
could be designed to collect condensate or could be traced to avoid condensation, water
in syngas could have severe consequences on the operational reliability and increase
the cost of the instrumentation that needs to be installed, especially when combined
with acid fractions.
Dust, greasy or oily fractions must be eliminated in fuel gases. These have a terrible
effect on the pipe-train instrumentation. There are methods to fire these dirty gases but
this involves heavy ball valves and possibly steam cleaning cycles which are very
difficult to apply in Yankee Hood burners and cannot be justified.
Tar or other aromatic substances could also have a negative impact on the
papermaking process, being transferred to paper during drying. This situation
absolutely needs to be avoided and further investigations have to be done. If
investigations will give a negative response (smelling or contaminant substances
transferred to paper) there is still one possibility, i.e. the use of indirect burners such as
radiant tube burners. This system consists of a nozzle-mixing burner that is firing in a
radiant tube, that could have different shapes, and that is exchanging heat with the
process through its external surface, without any contact between combustion products
(and then possible contaminants) and tissue-making process.
Radiant Tubes exhaust fumes could be used in a heat exchanger to preheat combustion
air, in order to increase the overall burner efficiency, that will be of course lower than
direct firing.
34
FOSSIL FREE TISSUE DRYING
By proper selection of hood operating point and radiant tube burner recuperator, an
overall efficiency higher that 70% can be reached. Further amount of energy could be
recovered after fumes/combustion air heat exchanger, to be used in other part of the
tissue-making process (process water heating) or gasification process (biomass
heating).
4.3
CONCLUDING REMARKS
The effects of replacing LPG with syngas in the impingement drying of tissue were
investigated by use of mathematical models. The study focused on examining the
impact on drying capacity and thermal efficiency as well as on the influence on the
potential for recovery of excess heat.
Assuming that the combustion system can burn the investigated energy gases at the
rate required to fulfill the calculated process heat demand, the results from the
simulation study show that the drying capacity is likely to be preserved if replacing
LPG with syngas.
Assuming that all investigated gaseous fuels are non-preheated, the use of heat derived
from combustion of the studied syngases was calculated to increase by less than 3.5 %
compared to the reference case of LPG.
At 40 % excess of combustion air, the potential to recover excess heat from the exhaust
gas stream was predicted to correlate well with the use of primary energy. The
potential for recovery of excess heat is thereby predicted to be relatively unaffected by
the replacement of LPG with syngas.
The minor investigation of fuel interchangeability made within the present study
shows that the existing gas burners probably need to be replaced in order to preserve
the drying capacity. Moreover, for syngas to form a viable option in a tissue drying
application, the combustion system needs to be designed for compliance with existing
emission legislations and to avoid concerns (e.g. blowout and flashback) that are shown
to occasionally arise during combustion of syngas.
35
FOSSIL FREE TISSUE DRYING
5
System studies
Based on our literature survey of gasification and gas cleaning technologies, on
information from technology suppliers and on process data from the three production
lines, this chapter investigates the advantages and disadvantages of integrating a
biomass gasification process in the three production lines.
5.1
ASSUMPTIONS AND METHOD
The overall data regarding steam production and LPG use in the present process are
illustrated in Figure 18. The process steam at the plant is produced in a boiler powered
with biofuels. At present, no flue gas condenser is installed so that the flue gases leave
the system at a temperature of 142°C. The LPG consumption for the three production
lines can be estimated as 25,0 GJ/h – almost 7 MW – during periods with top
production. These data for the consumption of process steam and energy gas are used
as a basis for the analysis throughout the chapter even though the need for energy gas
is periodically lower.
The method used for energy system analysis involves setting up and solving the heat
and mass balances for the production system after the integration of the biomass
gasification using the commercial flowsheeting software CHEMCAD. In order to
simulate the thermochemical conversion of biomass, the component list in CHEMCAD
needs to be updated with three components representing biomass, pyrolysis char and
gasification char. The elemental compositions as well as the lower heating values of the
new components are listed in Table 9. The two new components pyrolysis char and
gasification char are similar in their heating values but differ somewhat in their
elemental compositions. The gasification char is the solid residue from a higher
temperature process than the pyrolysis char and has a higher content of carbon. [26-28]
Table 9. The new components that were added to the database in CHEMCAD in order to model thermal
gasification of biomass.
Component
wt-% C
wt-% H
wt-% O
Biomass
Pyrolysis char
Gasification char
52.0
83.0
95.0
6.0
4.0
0.6
42.0
13.0
4.4
LHV
(MJ/kg)
19.0
29.8
30.4
The concept of Lower Heating Value (LHV) assumes that the condensation enthalpy of
the flue gases cannot be utilized within the system. Normally, the energy efficiency is
based on the LHV, so that a system including flue gas condensation might reach a total
energy efficiency that is higher than 100 %. When defining the Higher Heating Value
(HHV) or calorimetric heating value, the opposite assumption is made: All water vapor
is assumed to condense within the system and leave in the form of liquid water. The
biomass component defined according to Table 9 will have a HHV of 20.3 MJ/kg. Any
moisture content of a solid fuel will act as an inert in the context of the Higher Heating
Value but will lead to a further reduction in the Lower Heating Value due to the energy
need for vaporization. For a biomass with a moisture content of 50 % as assumed in the
present study, the values for the lower and higher heating values will be 8,92 MJ/kg
and 10.14 MJ/kg respectively, when the Biomass component in Table 9 is taken to
represent the dry fuel.
36
FOSSIL FREE TISSUE DRYING
When using the flowsheeting software CHEMCAD, the material and energy flows into
the system must be defined along with the unit operations. In the models set up the
following unit operations are used:
1.
2.
3.
4.
5.
6.
7.
Stoichiometric reactors for simulating the four gasification processes. This means
that the stoichiometry was fixed to yield a syngas composition in good agreement
with scientific literature and technology supplier data. The pyrolysis process in the
two-stage gasification technology was modelled in the same way, using a
stoichiometric reactor. Once the stoichiometry has been defined, the simulation
results will provide relevant information as to the energy balance of the reaction
(heat needed or heat produced).
Gibbs free energy reactors for simulating the bio boiler and the combustion of the
syngas. As long as excess oxygen is provided to such a Gibbs free energy reactor,
combustion will be complete. The simulation result then provides relevant
information as to the energy balances of the system.
Dryers for simulating the drying of the biomass prior to gasification
Flashes for simulating cold gas cleaning
Separators for simulating the separation of solid residues from gasification
(pyrolysis char and gasification char) from the syngas as well as for simulating
removal of the final tar in an active coal filter
Heat exchangers for simulating air heaters and steam generators
Controllers (not included in the inserted process diagrams) for keeping track of any
system constraints that cannot be defined in the unit operation blocks themselves.
This includes for instance controlling the air flow to the boiler so that the oxygen
content of the flue remains at a set value or controlling the flow of gasification
medium to the gasification reactor according to the stoichiometric constraints
defined.
Figure 18. The base case energy system – a bioboiler and tissue machine hoods heated with LPG.
37
FOSSIL FREE TISSUE DRYING
For comparing the technologies, a number of assumptions were made that were
applied to all four systems:
•
•
•
•
•
•
•
•
•
•
The need for process steam for the present processes will not change. The only
change in the steam consumption is the additional steam used as a gasification
medium for two of the studied technologies (steam blown dual bed and two-stage
gasification).
It is assumed that the same biomass that is used for steam production in the boiler
is also gasified. The moisture content of the biomass prior to drying is assumed to
be 50 wt%.
Prior to gasification the biomass is dried to a moisture content of 10 %.
When exchanging the LPG for green syngas, a 3 % increase in the heat needed from
the energy gas is assumed. This assumption is based on results in the chapter on
tissue drying and means that the product of the flow of syngas and its LHV should
be 25.75 GJ/h.
For representing tar in the CHEMCAD models, the component naphthalene was
used.
For removing tars and excess water, the syngas is cooled to 30°C, resulting in two
liquid fractions that are assumed to be easy to separate. The liquid fraction that is
rich in tar will be used for steam production. The water fraction leaves the system
as a condensate at 30°C.
An active carbon filter is used for removing the tar that remains in the syngas after
the condenser. The tar that is removed here represents an energy loss.
The temperature of the flue gases from the bio boiler will remain at 142°C in all
systems.
Hot gas streams will be used for steam generation. The gas temperature after each
steam generator was assumed to be 190°C.
Any pressure drops in the added process equipment will be neglected, hence the
need for electricity for pumps and fans was not taken into account.
In addition to the list of assumptions common for all technologies above, some
technology-specific assumptions were also made. Table 10 contains the assumptions for
the flow of the gasification media along with, for instance, the resulting compositions of
the syngas and the temperature of the gasification reactor. Some of the data in Table 10
were already presented in the chapter on gasification technologies to provide a
comparison of our original work in the project, literature data and technology supplier
information. In addition to the parameters presented in Table 10, it could be mentioned
that no preheating of the air for gasification was assumed for the two fixed bed
technologies where air is used as a gasification medium. This might explain (to some
degree) the rather low gasification temperature resulting for the fixed bed, downdraft.
For the two technologies where superheated steam is used as a gasification medium,
the steam is assumed to be produced within the system adding to the need for process
steam as compared to the base case presented in Figure 18.
38
FOSSIL FREE TISSUE DRYING
Table 10. Mass balances for the four modelled gasification processes.
Gasification reactor
Gasification temp
LHV syngas (dry gas)
Carbon monoxide, CO
Carbon dioxide, CO2
Hydrogen, H2
Methane, CH4
Ethylene, C2H4
Nitrogen, N2
Tar
Gasification char
(kg char/kg dry
biomass)
Pyrolysis char (kg
char/kg dry biomass)
Gasification medium
Flow of gasification
medium
5.2
.
Fixed bed,
updraft
Adiabatic
616°C
5,0 MJ/Nm3
18 Vol-%
17 Vol-%
17 Vol-%
2 Vol-%
0 Vol-%
46 Vol-%
136 g/Nm3
0,006
Fixed bed,
downdraft
Adiabatic
868°C
5,4 MJ/Nm3
22 Vol-%
11 Vol-%
17 Vol-%
2 Vol-%
0 Vol-%
48 Vol-%
4,2 g/Nm3
0,006
Dual bed
Two stage
Isothermal
800°C
15,1 MJ/Nm3
32 Vol-%
16 Vol-%
34 Vol-%
15 Vol-%
4 Vol-%
0 Vol-%
32 g/Nm3
0,128
Istothermal
1100 °C
11,4 MJ/Nm3
34 Vol-%
9 Vol-%
54 Vol-%
3 Vol-%
0 Vol-%
0 Vol-%
0 g/Nm3
0
0
0
0
0,399
Air
ER = 0,168
3850 kg/h
Air
ER = 0,339
3770 kg/h
Steam
SBR = 0,667
1270 kg/h
Steam
SBR = 0,518
890 kg/h
INTEGRATED SYSTEMS
In order to evaluate the suggested technologies from a process integration point of
view, a number of key parameters characterizing the thermodynamic performance and
the operational cost of the technologies have been defined. The base case is
characterized in terms of a boiler energy efficiency ηboiler that is defined as the quotient
between the heat for producing process steam Qsteam and the lower heating value of the
biomass LHV used in the base case, Eq. (2):
(2)
∙
The marginal energy efficiency of gasification ηgas,marginal is defined as the quotient
between the heat from the syngas produced 1.03.QLPG and the product of the increase in
biomass consumption mincrease and its lower heating value, Eq. (3). Here, the left hand
side is a thermodynamic property of the system whereas the increase in biomass
consumption is a direct measure of the additional operating cost, which should be
related to the cost of the LPG that is bought at present:
.
,
∙
(3)
∙
The total energy efficiency of the system ηtotal,integrated is defined as the sum of the heat
from the syngas and the heat from the steam divided by the product of the total
biomass consumption mbasecase + mincrease and its lower heating value, Eq. (4).
,
.
∙
∙
39
(4)
FOSSIL FREE TISSUE DRYING
The four integrated systems are illustrated in Figure 19, Figure 20, Figure 21 and Figure
22. For the two fixed bed systems, no extra steam is needed for gasification so that the
steam production from the hot syngas leads to a decrease in the steam produced in the
boiler. For the updraft system (Figure 19), the excessive tar production leads to the
situation that all the demand for process steam is covered by firing the tar that is
separated from the syngas in the condenser. For the downdraft system (Figure 20),
much less tar is separated from the syngas and the need for biomass for steam
production is far from eliminated.
The steam blown dual bed (Figure 21) and the two-stage gasification (Figure 22) are
similar in that some additional process steam is needed as a gasification medium. Here,
additional process steam can be produced from the hot syngas, but also from the hot
flue gases. The flue gases are not assumed to be mixed with the flue gases from the
boiler but rather leave the system at a temperature of 190°C after the steam generator.
Figure 19. The energy system after the integration of an updraft fixed bed gasification reactor for production
of green syngas replacing the LPG used at present.
40
FOSSIL FREE TISSUE DRYING
Figure 20. The energy system after the integration of a downdraft fixed bed gasification reactor for production
of green syngas replacing the LPG used at present.
Figure 21. The energy system after the integration of a steam blown dual bed gasification process for
production of green syngas replacing the LPG used at present.
41
FOSSIL FREE TISSUE DRYING
Figure 22. The energy system after the integration of a steam blown two stage gasification process for
production of green syngas replacing the LPG used at present.
5.3
STANDALONE SYSTEMS
For the purpose of evaluating the importance of process integration for the operational
cost of the technologies, simulations assuming instead a stand-alone gasification plant
were performed. This was done in order to evaluate the energy efficiency of the
technologies assuming instead a stand-alone operation of the gasification as opposed to
the concept of an integrated system.
Figure 23 illustrates the changes made for the downdraft fixed bed system when setting
up syngas production and process steam production as two standalone operations. The
flue gases from the boiler are no longer used for heating the dryer. Instead, heat for
drying not supplied from the hot syngas is assumed to come from a separate system for
combustion of biomass. The hot syngas is no longer used for steam generation so that
all the process steam needed is assumed to be produced in the boiler. Any tar formed
during gasification is assumed to leave the system and will be counted as energy (and
resource) lost from the system.
The benefits of process integration can be expressed as the decrease in biomass
consumption of the integrated plant as compared to a system where the process steam
and the syngas are produced in standalone units.
The standalone energy efficiency of gasification ηgas,standalone is defined as the quotient
between the heat from the produced syngas produced and the product of the biomass
consumption of a standalone gasification plant mgasification,standalone and lower heating value
of the biomass, Eq. (5):
,
.
∙
,
∙
The total energy efficiency of the system is defined as the sum of the heat from the
syngas and the heat from the steam divided by the product of the total biomass
consumption mbasecase + mgasification,standalone and its lower heating value, Eq. (6). The
42
(5)
FOSSIL FREE TISSUE DRYING
definition is analogous to the definition of the total energy efficiency of the integrated
system.
.
,
∙
∙
,
(6)
As is evident from the data presented in Table 11, the total energy efficiency of the
integrated system will be higher than the total energy efficiency of the two standalone
systems, so that process integration leads to reduced operational costs due to lower
biomass consumption. However, a highly integrated system is also somewhat
vulnerable, if the high total energy efficiency is a result of successful process
integration alone. For that reason, it is interesting also to describe the decrease in
biomass consumption that results from process integration, Eqs. (7) - (8):
,
,
,
,
.
(7)
∙
,
.
∙
(8)
Figure 23. One example of a system where gasification and steam production occur in non-integrated,
standalone units.
This example represents the downdraft fixed bed gasification technology. All process
steam is produced in the bioboiler and all energy for heating the drying air comes from
the hot syngas.
5.4
DISCUSSION
The key parameters in the form of energy efficiencies and biomass flows are
summarized in Table 11. The first two rows represent the marginal energy efficiencies
of gasification and increased biomass consumption of the integrated systems. Figure 24
illustrates the marginal gas energy efficiency and the increase in biomass consumption
for the four gasification technologies together with the analytical expression deduced in
Eq. (3). The upper theoretical limit of the marginal gas energy efficiency is also
illustrated together with the corresponding lower theoretical limit of the increase in
43
FOSSIL FREE TISSUE DRYING
biomass consumption. The theoretical limit assumes that all water vapour is condensed
before leaving the system, so that the total energy efficiency of the system equals 100 %
if it is based on the higher heating value of the biomass rather than (as convention
dictates) the lower heating value.
The results for all four technologies summarized in Figure 24 agree with the theoretical
expression deduced in Eq. (3). This serves to support that the flow sheeting models
were successfully implemented.
It is evident from the data presented in Figure 24 that the highest marginal energy
efficiency and correspondingly lowest operational cost for biomass consumption is
reached for the downdraft fixed bed. The main reason for this is that the heat recovery
from the syngas for steam generation and drying means that little excess heat is wasted
from the system. For the two systems involving separate combustion reactors, the
steam blown dual-bed and the two stage gasification technology, the results are very
close. For these two technologies, there will be a flue gas stream leaving the system
with a temperature of 190°C representing an energy loss from the system. The flue gas
is used for steam generation, but no use of the low grade heat can be found within the
system since the flue gas from the bioboiler and the hot syngas provides the required
drying energy. The updraft fixed bed exhibits the lowest marginal energy efficiency
and thereby the highest operational cost. The tar production in this technology is
considerable, compare Table 10. In fact, the tar contains so much heat that it cannot be
utilized within the system even assuming that no biomass at all is combusted in the bio
boiler. This leads to the conclusion that this technology is not suitable for the
application.
Table 11. Some key parameters related to the energy efficiencies and operational costs and to the benefits of
process integration for the studied gasification technologies.
Marginal energy efficiency
of gasification
Increased flow of biomass
(kg/h)
Standalone energy efficiency
of gasification
Decreased biomass flow due
to process integration (kg/h)
Total energy efficiency
integrated system
Total energy efficiency
standalone units
Fixed bed,
updraft
0,685
Fixed bed,
downdraft
1,124
Dual bed
Two stage
1,157
1,007
4550
2770
2690
3090
0,408
0,867
0,817
0,907
3080
820
1120
342
0,767
1,010
1,024
0,955
0,543
0,885
0,855
0,907
44
FOSSIL FREE TISSUE DRYING
Figure 24. Two key operational parameters indicating the marginal energy efficiency and the increase in the
biomass consumption as a consequence of integrating biomass gasification in the plant.
Table 11 also contains the data for the standalone systems. Figure 25 depicts the
decrease in biomass consumption of a process integrated plant as compared to a plant
where steam and syngas are produced in standalone processes. Again, the results for
all four technologies agree with the theoretical expression deduced in Eq. (7). A system
with high standalone gasification energy efficiency will be somewhat more robust. For
a gasification process where the energy efficiency of the standalone process equals the
marginal energy efficiency of the integrated system, no biomass is saved as a
consequence of process integration.
The steam-blown dual bed and the two stage gasification have almost the same
performance in terms of the marginal energy efficiency and the biomass consumption
of the integrated systems. However, successful process integration is somewhat more
crucial for the good performance of the two-stage gasification technology than for the
other technology (Table 11).
It is also interesting to compare the magnitudes of the flowrates, compare
Table 12. For the two systems where the fluidization medium is air, considerably
higher flow rates will result as a consequence of the inert nitrogen present in the
syngas.
45
FOSSIL FREE TISSUE DRYING
Figure 25. Two key parameters indicating the importance of process integration for the investigated
technologies.
Table 12. Some additional process parameters related to the flow rates.
Flow of syngas (kg/h)
Flow of syngas (Nm3/h)
Heat in syngas (GJ/h)
Steam production (GJ/h)
Air flow to dryer (kg/h)
Fixed bed,
updraft
6220
5420
25,75
31,6
120 000
Fixed bed,
downdraft
55
4990
25,75
21,2
56 000
Dual bed
Two stage
1580
1780
25,75
21,2 + 3,5
59 000
1574
2330
25,75
21,2 + 3,0
54 000
A number of assumptions were made in the system analysis. The liquid fraction of the
syngas after condensation at 30°C will contain water as well as tar. Here, it was
assumed that those two fractions were easily separated into one tar-rich fraction and
one water-rich fraction. Combustion of the tar increased the energy efficiency of the
system whereas the water-rich fraction was taken to cleaning. (Only the tar removed in
the filter is assumed to represent an energy loss.) However, it is likely that separation of
these two fractions is not as easy as assumed here. Gasification technologies producing
low amounts of tar might have an additional advantage. Table 13 lists the flows of
condensate and the tar removed in the filter.
Table 13. Tar flows and condensate flows connected to cold gas cleaning together with the amount of tar
removed in the filter.
Tar flow gas cleaning (kg/h)
Flow of condensate (kg/h)
Tar removed in filter (kg/h)
Fixed bed,
updraft
896
7998
15
46
Fixed bed,
downdraft
9
183
13
Dual bed
113
1437
5
Two
stage
0
0
0
FOSSIL FREE TISSUE DRYING
5.5
CONCLUDING REMARKS
Four gasification technologies: updraft fixed bed, downdraft fixed bed, steam-blown
dual bed, and two stage gasification, were investigated in terms of the possibilities to
integrate these technologies in the plant. Process integration was investigated for
steady-state conditions corresponding to the maximum production of syngas foreseen
according to the data presented in section 2.3 of this report.
The flue gas from the bio boiler and the hot syngas generated provides enough heat for
drying the biomass prior to gasification. It is also possible to use excess heat from the
gasification process for steam production, so that the steam production in the bio boiler
can be somewhat reduced as compared to the base case. With the introduction of flue
gas condensation, the marginal energy efficiency of gasification for three of the
technologies is above 100%.
The technology producing a syngas with the highest tar content (the updraft fixed bed)
can be ruled out. The heat contained in the tar cannot be utilized within the system,
which leads to a poor energy efficiency. The three remaining technologies are all very
interesting from a process integration point of view. They differ slightly as to the
overall energy efficiency and the overall biomass consumption. However, other aspects
than biomass consumption alone should be considered. Such aspects include the flow
of tar-containing condensate from the cold gas cleaning.
47
FOSSIL FREE TISSUE DRYING
6
Conclusions
The main conclusion of the study is that a number of gasification technologies are
commercially available in the scale needed to supply the three production lines with
syngas, approximately 7 MW. Fixed bed updraft gasification technology is less feasible
due to the potentially high tar content of the syngas produced and the related low
overall energy efficiency obtained. Exchanging LPG for green syngas has only a limited
influence on the gas consumption in terms of the energy content (the product of the
flow of syngas and its lower heating value) which remains close to constant. Probably,
new burners will need to be installed when exchanging the LPG for a syngas with a
much reduced heating value as compared to LPG. At steady-state, there are possible
benefits in integrating the production of syngas and the production of process steam,
since the total energy efficiency of an integrated system will be considerably higher
than the total energy efficiency of two standalone units, one for syngas production and
one for steam production.
48
FOSSIL FREE TISSUE DRYING
7
Future work
7.1
VERIFICATION OF THE APPLICABILITY OF SYNGAS AS LPG SUBSTITUTE FOR TISSUE
DRYING.
Given the cost for deep tar cleaning on one hand and the importance of soot-free, nonodorous flue gases for this tissue drying application on the other hand, there has been
identified a need for experimental testing in order to determine correlation between
syngas tar content and smell/soot impact on the tissue paper.
Principally there are two possible approaches to such a test:
1.
2.
Bring syngas to a paper machine
Bring paper to a gasification plant
To our knowledge there is no portable gasification equipment available with a capacity
sufficient for actual production tests at a pilot machine for tissue production.
Furthermore, such a test would be costly and possess a high risk of fouling of valuable
equipment. For this reason, approach 1 is considered non-viable.
The other alternative (2) would be to arrange for combustion of biomass-derived
syngas with subsequent transfer of the flue gases towards a wet paper sheet at any of
the gasification plants, already in operation in Sweden or Europe. Such an approach
should have the potential to be significantly less costly and risky, compared to
alternative 1.
Practically, the test arrangement could be carried out in a test rig where a tissue paper
is supported on a wet sponge, securing a wet sheet for at least some 10-20 seconds
thereby enabling a substantial amount of flue gases to be brought in contact with the
paper.
49
FOSSIL FREE TISSUE DRYING
8
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51
FOSSILFRI TISSUETORKNING
Det går åt mycket energi vid torkning av mjukpapper. I moderna mjukpappersprocesser utnyttjas direkteldade torkkåpor där rökgaserna från gasol- eller naturgasförbränning blåser mot det våta pappret. I den här förstudien har forskarna
undersökt vad som händer vid ett byte från gasol till gas från termiskt förgasad
biomassa i ett mjukpappersbruk med en maximal gasolförbrukning motsvarande 7 MW.
Det visar sig att torkkapaciteten sannolikt kommer att upprätthållas om
gasol ersätts med biobaserad syntesgas. Resultaten visar också att de undersökta
förgasningsteknikerna alla kan tillämpas för att omvandla flisat trämaterial till
gas av tillräcklig kvalitet. Tre tekniker har visat sig vara mest lämpade avseende
den totala energieffektiviteten och den totala förbrukningen av biomassa. Det
finns dock behov av ytterligare experimentella test för att fastställa sambandet
mellan gasens tjärinnehåll och påverkan av lukt och sot på mjukpappret i nästa
steg av en konceptutveckling.
Another step forward in Swedish energy research
Energiforsk – Swedish Energy Research Centre is a research and knowledge based organization
that brings together large parts of Swedish research and development on energy. The goal is
to increase the efficiency and implementation of scientific results to meet future challenges
in the energy sector. We work in a number of research areas such as hydropower, energy gases
and liquid automotive fuels, fuel based combined heat and power generation, and energy
management in the forest industry. Our mission also includes the generation of knowledge
about resource-efficient sourcing of energy in an overall perspective, via its transformation and
transmission to its end-use. Read more: www.energiforsk.se
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