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Mechanisms of Thermochemical Biomass ConversionProcesses. Part 2: Reactions of GasificationM. Balat aa Sila Science, University Mah , Trabzon, TurkeyPublished online: 27 Feb 2008.

To cite this article: M. Balat (2008) Mechanisms of Thermochemical Biomass Conversion Processes. Part 2: Reactionsof Gasification, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 30:7, 636-648, DOI:10.1080/10407780600817600

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Energy Sources, Part A, 30:636–648, 2008Copyright © Taylor & Francis Group, LLCISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/10407780600817600

Mechanisms of Thermochemical

Biomass Conversion Processes.

Part 2: Reactions of Gasification

M. BALAT1

1Sila Science, University Mah, Trabzon, Turkey

Abstract Gasification as a thermochemical process is defined and limited to com-bustion and pyrolysis. The gasification of biomass is a thermal treatment which results

in a high proportion of gaseous products and small quantities of char (solid product)and ash. Biomass gasification technologies have historically been based upon partial

oxidation or partial combustion principles, resulting in the production of a hot, dirty,low Btu gas that must be directly ducted into boilers or dryers. In addition to limiting

applications and often compounding environmental problems, these technologies arean inefficient source of usable energy. The main objective of the present study is

to investigate gasification mechanisms of biomass structural constituents. Completegasification of biomass involves several sequential and parallel reactions. Most of

these reactions are endothermic and must be balanced by partial combustion of gasor an external heat source.

Keywords biomass, gasification, mechanisms of gasification

Introduction

Biomass is now well recognized as a potential renewable source of energy (Çulcuogluet al., 2002). It is available as a potential resource of chemicals. The use of biomassfuels provides substantial benefits as far as the environment are concerned (Surmen,2002). Biomass can be considered as a relatively clean fuel, as it decreases or eveneliminates net carbon dioxide (CO2) emissions, has low sulfur and nitrogen oxide (NOx)contents, and has particulate emissions lower than fuels (Demirbas, 2003). Biomass cancontribute in stabilizing CO2 concentrations in the atmosphere in two ways: throughbiomass production for fossil fuel substitution and through CO2 storage in vegetationand soil (Ericsson and Nilsson, 2006).

Energy can be obtained from biomass through direct combustion, physical processes,and conversion processes. Physical processes are grinding, drying, filtration, pressing,extraction, and briquetting (Demirbas and Yazici, 2000). Direct combustion of biomassto take advantage of its heating value has been known for ages, but direct combustion ofbiomass is not favored anymore because it has too high a content of moisture to performstable combustion. Thus, it has highly changeable combustion rates. On the other hand,the density for many kinds of biomass is lower than that of coal, leading to importanteconomic limitations in transportation. In order to overcome these problems, briquetting

Address correspondence to Mustafa Balat, H. Osman Yucesan Cad. Zambak Sok. PolatogluAp. Kat 6, Besikduzu, Trabzon, Turkey. E-mail: mustafabalat@yahoo.com

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of low-density biomass species before combustion has been considered. Furthermore,it is also possible to blend biomass with coal in various proportions and then producecoal-biomass briquettes (Yaman, 2004).

Biomass conversion may be conducted on two broad pathways: chemical decom-position and biological digestion. Thermochemical decomposition can be utilized forenergy conversion of all five categories of biomass materials, but low-moisture herbaceous(small grain field residues) and woody (wood industry wastes and standing vegetationnot suitable for lumber) are the most suitable (Demirbas, 2000a, 2001, 2002a). Thermalconversion of biomass has received special attention since it leads to useful products andsimultaneously contributes to solving pollution problems arising from biomass accumu-lation (Rocha et al., 1999).

Biological processes are essentially microbic digestion and fermentation. High mois-ture herbaceous plants (vegetables, sugar cane, sugar beet, corn, sorghum, and cotton)marine crops and manure are most suitable for biological digestion. Intermediate-heatgas is methane mixed with CO and CO2. Methane (high-heat gas) can be efficientlyconverted into methanol (Güllü, 2003).

Thermochemical processes involve the pyrolysis, liquefaction, gasification, and su-percritical fluid extraction methods. The products of the thermochemical processes aredivided into a volatile fraction consisting of gases, vapors, tar components, and a carbon-rich solid residue (Yaman, 2004). Pyrolysis is the thermochemical process that convertsorganic materials into usable fuels. Pyrolysis produces energy fuels with high fuel-to-feed ratios, making it the most efficient process for biomass conversion and the methodmost capable of competing and eventually replacing non-renewable fossil fuel resources.Pyrolysis is the technique of applying high heat to lignocellulosic materials in the absenceof air or in reduced air. The process can produce charcoal, condensable organic liquids(pyrolytic fuel oil), non-condensable gases, acetic acid, acetone, and methanol (Demirbasand Güllü, 1998). The gasification of biomass is a thermal treatment which results in ahigh proportion of gaseous products and small quantities of char (solid product) and ash(Yaman, 2004). In contrast to gasification, in which all structural identity is lost in theformation of simple molecules, pyrolysis potentially offers high yields of liquid products(Sensöz and Can, 2002).

Biomass Gasification Technologies

Gasification, one thermochemical conversion route, is widely recognized at present be-cause its end product gas can find flexible application by industries or by home users,particularly in decentralized energy production coupled with micro turbine/gas, tur-bine/engine, boiler, and even fuel cell (Chen et al., 2004). Biomass gasification technolo-gies have historically been based upon partial oxidation or partial combustion principles,resulting in the production of a hot, dirty, low-Btu gas that must be directly ducted intoboilers or dryers. In addition to limiting applications and often compounding environ-mental problems, these technologies are an inefficient source of usable energy (Demirbas,2000b). Figure 1 shows a diagram of a biomass gasifier.

Gasification as a thermochemical process is defined and limited to combustion andpyrolysis. A systematic overview of reactor designs categorizes fixed bed and fluidizedbed reactors (Warnecke, 2000). Downdraft fixed-bed gasifiers are limited in scale andrequire a well-defined fuel, making them not fuel flexible. Updraft fixed-bed gasifierscan be scaled up; however, they produce a product gas with very high tar concentrations.

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Figure 1. Schematic diagram of a biomass gasifier.

This tar should be removed for the major part from the gas, creating a gas-cleaningproblem (Boerrigter and van der Drift, 2004).

Although fluidized bed (FB) gasifiers yield a product gas containing tars, whenchoosing the right gasification conditions (i.e., gasification temperature, use of steam,and special bed material), the amount of tars is limited and acceptable in a productgas that is fired to the gas turbine. FB gasifiers are typically operated at 1073–1273 K(limited by the melting properties of the bed material) and are therefore not generallysuitable for coal gasification, as due to the lower reactivity of coal compared to biomass,a higher temperature is required (>1573 K) (Boerrigter and van der Drift, 2004). Thedevelopment of the FB gasifier for small particle materials has made great progress forbiomass gasification. The productivity of the FB gasifier has increased by five times thatof the FB gasifier, and the heating value of gas increased by about 20%. However, theflying char loss makes the FB gasifier a kind of low-heat efficiency gasifier (around 60%).The circulating FB gasifier has the following features: fast fluidization which enhancesthe heat and mass transfer so as to speed up the gasification process, and the circulation ofthe char which increases the residence time of char so as to satisfy the need of reductionreaction and decrease the char loss (Bingyan et al., 1994).

Gasification may be defined as that regime in which organic materials, includingbiomass as well as solid fuels, are degraded by thermal reactions in the presence ofcontrolled amounts of oxidizing agents to provide (when carried to completion) a simple

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gaseous phase comprising hydrogen, water, CO, CO2, methane, small or trace amountsof other components, and residues from contained inorganic matter. Gasification andpyrolysis are both aspects of thermal processes that are involved in combustion. Ona practical basis, gasification in normally carried out in a higher temperature regionthan pyrolysis. To accomplish gasification, it is always necessary to pass through apyrolytic stage, i.e., pyrolysis is one aspects of gasification. However, pyrolysis differsfrom gasification (Maschio et al., 1994) in that the products of interest are the char andliquids, which as a result of the incomplete nature of the process retain much of thestructure, complexity, and signature of the raw material undergoing pyrolysis (Demirbas,2004a). Table 1 shows the range of the main operating parameters.

Char gasification is the rate-limiting step in the production of gaseous fuels frombiomass. Arrhenius kinetic parameters were determined for the reaction of chars preparedby pyrolysis of cottonwood and Douglas fir at 1273 K with steam and CO2. Resultsindicate that both reactions are approximately zero order with respect to char; the overallreaction rate is fairly constant throughout and declines only when the char is nearlydepleted. This suggests that the reaction rate depends on such factors as total availableactive surface area or interfacial area between the char and catalyst particles. Theseparameters would remain relatively constant during the gasification process. Softwoodand hardwood chars exhibited similar gasification behavior. Results indicate that themineral (ash) content and composition of the original biomass material, and pyrolysisconditions under which char is formed, significantly influence the char gasificationreactivity (Demirbas, 2000b).

Tar formation is one of the major problems to deal with during biomass gasification(Pfeifer et al., 2004a). The use of an active bed material as a primary catalyst is the bestsolution in contrast to more expensive use of secondary catalytic reactor downstreamof the gasifier. The choice of an appropriate in-bed catalyst is then crucial for theoptimization of gasification technology (Pfeifer et al., 2004b). Sodium and potassiumcatalysts were equally effective for the gasification of wood char. The iron and nickeltransition metals provided the highest initial catalytic activity, but lost their activitywell before the char completely reacted (Demirbas, 2000b). Ni-olivine catalyst has beendeveloped (Courson et al., 2000, 2002) to enhance olivine performances in steam biomassgasification by methane and tar reforming, leading to hydrogen production and gasupgrading. The main advantage of Ni-olivine catalyst is its attrition resistance due tostrong metal support interactions permitting its direct use in the FB reactor. The mainlimitation of Ni catalyst use for hot gas conditioning of biomass gasification productgases is its sensitivity to deactivation, which leads to limited catalyst lifetimes. Ni catalystdeactivation is caused by several reasons. Sulfur, chlorine, and alkali metals present in

Table 1

Range of the main operating parameters

Parameter Pyrolysis Fast pyrolysis Steam gasification

Temperature (K) 675–875 975–1225 975–1225Heating rate (K/s) 0.1–1 250–300 300–500Solid residence time (s) 600–2000 1–3 0.5–2Water/biomass ratio 0.1–2 0.2–0.6 0.8-2

Source: Maschio et al., 1994.

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gasification product gases act as catalyst poisons. Coke formation on the catalyst surfacecan be substantial when tar levels in product gases are high. Loss of catalyst activity isapparently due to fouling by buildup of carbon, which blocks access to the catalyst pores(Pfeifer et al., 2004b). Nickel-based catalysts have proven to be very effective for hotconditioning of biomass gasification product gases above 1023 K (Pfeifer et al., 2004a).

Studies on Gasification of Biomass

Many kinds of biomass species have been subjected to gasification conditions. Someof these biomass species are as follows: agricultural residues (Natarajan et al., 1998);bagasse (Knight, 2000; Gabra et al., 2001a, 2001b; Filippis et al., 2004); black liquor(Backman et al., 1993; Demirbas, 2002b; Sricharoenchaikul, et al., 2003); cane trash(Gabra et al., 2001c); cellulose (Yoshida et al., 2004; Hanaoka et al., 2005a); hazelnutshell (Demirbas, 2004a); Japanese oak (Quercus mongolica), Japanese red pine (Pinus

densiflora), lignin (woody biomass model comp.) (Hanaoka et al., 2005a); miscanthusparticles (Chen et al., 2004); olive oil waste (Leached orujillo) (Ibanez et al., 2004);olive residue (Arvelakis et al., 2002; Ollero et al., 2003); olivine (Rapagna et al., 2000);peach stone (Arvelakis et al., 2005); rice husk (Mansaray et al., 1999; Jain and Goss,2000); sawdust (Li et al., 2004; Wander et al., 2004); softwood lignin (Yoshida et al.,2004); straw (Sadaka et al., 2002a, 2002b, 2002c); wood (Zainal et al., 2002); woodybiomass (Hanaoka et al., 2005b); wood chips (Knight, 2000; Zainal et al., 2002; Jayahet al., 2003); wood cylinders (Di Blasi et al., 2003); and xylan (woody biomass modelcomp.) (Hanaoka et al., 2005a). Table 2 shows typical characteristics of some biomassspecies for gasification.

A large number of research projects in the field of thermochemical conversion ofbiomass, mainly on pyrolysis, carbonization, and gasification, have been carried out. Theproblems associated with the realization of the process and utilization of the productshas been recently reviewed (Baker et al., 1987; Demirbas, 2000a, 2000b, Demirbas andCaglar, 1998; Caglar and Ozmen, 2000). In the literature, a large number of experimentalstudies concerning the gasification of biomass with air and oxygen have been presented.Extensive research has been carried out on small- and medium-size air gasifiers to producelow-heating value fuel gas and power (Rapagna et al., 1992; Corella et al., 1991). In thesteaming reactor, hydrogen gas is liberated according to the following reaction at 1225 K:

2CC 3H2O D 3H2 C CO2 C CO (1)

Table 2

Typical characteristics of some biomass species for gasification

Biomass Volatilespecies, Moisture, Ash matter, Bulk density Average

wt% dry% dry% kg/m3 MJ/kg dry HHV

Charcoal 2–10 2–5 5–30 200–300 30Wood 20–40 0.1–1.0 70–80 600–800 20Rice husk 3–5 15–25 60 100 15Coconut Shell 25 0.8 79 n.a. 20

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The experiments of supercritical water gasification at the Hawaii Natural EnergyInstitute (HNEI) were carried out at 873 K, 34.5 MPa, and catalyzed by activated carbonor charcoal. Different types of feedstocks, such as glucose, sawdust, and sewage sludge,were used in their work. The gasification efficiency was near 100%. The addition ofexcess water to produce gas mixture under high pressure results in hydrogen with apurity above 90 mol% in gas phase. The experiments at the Pacific Northwest Laboratory(PNL) of the United States were carried out in a batch reactor at 623 K and 34.5 MPa.Biomass and organic waste were used as feedstocks. Methane was produced as the mainproduct gas. Cellulose and wood (Japanese oak) were gasified in the presence of waterat a temperature of 623 K and a pressure of 17 MPa using a reduced nickel catalyst andsodium carbonate as a promoter at the National Institute for Resources and Environment(NIRE) of Japan. The gas yields reached 94wt% for cellulose and 55wt% for wood.Table 3 shows state of catalytic gasification in near-critical water. Real biomass (wood assawdust, straw) and wastes (sewage, sludge, and lignin) were treated in batch experimentsat the Forschungszentrum Karlsruhe Institut für Technische Chemie (FKITC) of Germany.It was found that at 873 K and 25 MPa, all compounds were completely gasified by theaddition of KOH or K2CO3, forming an H2-rich gas containing CO2 as the main carboncompound (Hao et al., 2003).

Steam gasification is a promising technology for thermochemical hydrogen produc-tion from biomass. Hydrogen is produced from the steam gasification of beech wood,corncob, olive waste, wheat straw (Demirbas, 2006), hazelnut shell (Demirbas, 2005),woody biomass (Hanaoka et al., 2005b), and wood sawdust (Demirbas, 2004b).

Concentrated solar radiation can be used for gasification of biomass to producehydrogen. A detailed review with many references of the technology describes solargasification of carbonaceous materials to produce a syngas quality intermediate forproduction of hydrogen and other fuels (Midilli et al., 2000).

Table 3

State of catalytic gasification in near-critical water

ReactionCatalyst product

Institute Feedstock condition gas Main Reactor

HNEI Glucose, sewagesludge, and woodsawdust

Charcoal andactivated carbon

873 K34:5 MPa

Hydrogen Supercritical flowreactor

PNL High-moisturebiomass andprocessed wastes

Reduced nickelmetal,silica-alumina assupport

623 K20 MPa

Methane High-pressureautoclave

NIRE Cellulose andJapanese Oak

Reduced nickel,sodium carbonateas a promoter

623 K17 MPa

Hydrogen Stainless-steelautoclave

FKITC Glucose, catechol,vanillin, glycine,sawdust, straw,sewage, sludge,lignin

KOH, K2CO3 873 K25 MPa

Hydrogen Two batchautoclave andtwo tubular flowreactors

Source: Hao et al., 2003.

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Production of hydrogen gas by gasification of carbon with superheated steam wasreported (Lowry, 1963).The reacting carbon material is charcoal and is obtained fromwood (Taheshita et al., 1975; Wienhaus, 1979), palm stones (Rabah et al., 1983), andother domestic waste materials (Constantine, 1983).

Mechanisms of Gasification Reactions

Complete gasification of biomass involves several sequential and parallel reactions. Mostof these reactions are endothermic and must be balanced by partial combustion of gasor an external heat source (Boer and Duffie, 1985). As the biomass particle is heated,it initially pyrolyzes to form charcoal plus gases and vapors (Hoque and Bhattacharya,2001).

BiomassC heat ���! ��� charcoal C volatileC gases

After pyrolysis is completed, the charcoal can react with oxygen and steam or the productsof pyrolysis according to

CharcoalC gases ���! ��� reduced gases

In addition, the vapors formed initially from the solid may undergo cracking to formsecondary products (Boer and Duffie, 1985), either gases or other condensable species.

A limited supply of oxygen, air, steam, or a combination serves as the oxidizingagent. Using steam on the composition of the feed gas, water gas reaction (C C H2O $CO C H2) and water gas shift reaction (CO C H2O $ CO2 C H2) are assumed to takeplace during gasification process. The primary water gas reaction (endothermic) becomessignificant at temperatures from 1273 K to 1373 K and upward. The secondary watergas reaction (exothermic) begins and predominates between 773 K and 873 K while thewater gas shift reaction is purely a gaseous reaction, which takes place in the presence ofundecomposed steam. In total, both reactions are net endothermic and supplemental heatmust be supplied to carry the reaction to completion. Therefore, an increase or decreaseof the temperature displaces either of the reactions in opposite directions (Blasiak et al.,2002).

The conversion process from solid biomass to fuel gas resolve around the initialcombustion reaction (Eq. (2)), which occurs in the reactor nearest the air inlet.

CC O2 C 3:76 N2 ! 3:76 N2 C CO2 (2)

The CO2 produced in reaction (Eq. (2)) is then, in the presence of glowing carbon,reduced to CO (Eq. (3)). When the gasifier is first lit, the level of CO2 is at its highest.The quality of the fuel gas increases with the increase in gasifier reactor temperature.The percentage of the CO2 reduced to CO will depend on the temperature in the gasifier.

CC CO2 C 3:76 N2 ! 3:76 N2 C 2CO (3)

The ideal reaction that should occur in the gasifier bed is (Eq. (4)):

2CC O2 C 3:76 N2 ! 3:76 N2 C 2CO (4)

Chemical reactions (Eqs. (2), (3), and (4)) are regarded as only occurring with carbon,and with wood as the feedstock, there are other elements involved. The main constituents

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of dry wood are cellulose (40-50%), hemicelluloses (20–30%), lignin (20–25%), resin,and trace mineral matter (Encinar et al., 2001; Blasiak et al., 2002).

The main secondary reactions are mentioned below:

CC H2O$ H2 C CO (5)

COC H2O$ H2 C CO2 (6)

CC 2H2 $ CH4 (7)

tarC H2O! H2 C CO (8)

tarC H2 ! light hydrocarbonsC gases (9)

In the steam gasification of carbonaceous material, several reactions take placesimultaneously; see Eqs. (5) and (7). Thus reaction (Eq. (10)) requires high temperature,and reactions (Eqs. (7) and (11)) require high pressure. Hence, reactions (Eqs. (5) and (6))are the main reactions of interest in steam gasification. Equations (5) and (10) representthe gasification reactions of carbon deposits with H2O and CO2. These reactions becomesignificant at 973 K. In addition to being endothermic, reaction (Eq. (5)) is relatively slowcompared to carbon-oxygen or carbon-CO2 (Boudouard reaction, Eq. (10)) (Blasiak et al.,2002).

Main reactions show that heat is required during the reduction process. Hence, thetemperature of gas goes down during this stage. If complete gasification occurs, all thecarbon is burned or reduced to CO, a combustible gas, and some other mineral matter isvaporized. The remains are ash and some char (unburned carbon). The following reactionsalso occur during the gasification process (Demirbas, 2002d):

CC CO2 $ 2CO (10)

CH4 C H2O$ COC 3H2 (11)

Hydrogen gas was produced on a pilot scale by steam gasification of charred lignocel-lulosic waste material. The gas was freed from moisture and CO2. The beneficial effectof some inorganic salts such as chlorides, carbonates, and chromates on the reactionrate and production cost of the hydrogen gas was investigated. When the objective isto maximize the production of H2, the stoichiometry describing the overall process is(Demirbas, 2002c, 2002d, 2004a):

CnHm C 2nH2O! nCO2 C Œ2nC .m=2/�H2 (12)

The simplicity of Eq. (12) hides the fact that, in a hydrocarbon reformer, the followingreactions take place concurrently (Demirbas, 2002c, 2002d, 2004a):

CnHm C nH2O ���! ��� nCOC Œ2nC .m=2/�H2 (13)

At normal reforming conditions, steam reforming of higher hydrocarbons (CnHm)is irreversible (Eq. (10)), whereas the methane reforming (Eq. (11)) and the water-shiftconversion (Eq. (6)) reactions approach equilibrium. A large molar ratio of steam tohydrocarbon will ensure that the equilibrium for Eqs. (12) and (13) is shifted toward H2

production. The hydrogen yields were obtained by use of three different processes, asgiven in Table 4 (Wang et al., 1997; Demirbas, 2002d).

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Table 4

Comparison of hydrogen yields were obtained by use ofthree different processes

Hydrogen energyHydrogen contents/biomass

Processes yield, w energy content

Pyrolysis C catalytic reforming 12.6 91Gasification C shift reaction 11.5 83Biomass C steam C except heat 17.1 124

(theoretical maximum)

Source: Wang et al., 1997; Demirbas, 2002d.

As a part of the European research project AER-GAS, the Absorption EnhancedReforming (AER) technique is being utilized for the unpressurized steam gasification ofbiomass (Eq. (14)). Through simultaneous CO2 absorption (with CaO as the sorbent in theexample, Eq. (16), the equilibrium of the homogenous water gas shift reaction (Eq. (15))is shifted towards H2 and CO2, and all of the parallel reforming/gasification reactions arealso influenced in favor of the desired products. Accordingly, a hydrogen-rich product gasresults with reduced CO and CO2 concentration. Equation (17) represents the idealizedsum reaction for AER gasification—the formations of secondary products (like methane,coke, and tars) are neglected here (Möllenstedt et al., 2004).

CHxOy C .1 � y/ H2O! COC .0:5x C 1 � y/ H2; �HR > 0 (14)

COC H2O! CO2 C H2; �HR < 0 (15)

CaOC CO2 ! CaCO3; �HR < 0 (16)

CHxOy C .2 � y/H2OC CaO! CaCO3 C .0:5x C 2 � y/ H2; (17)

The CO2 absorption is exothermal so that the gasification is nearly energetically self-sufficient. Loaded sorbent is regenerated in a subsequent process step by adding heat.For continuous gas production, unloaded sorbent and solid fuels are converted in thereactor at temperatures <973 K. The carbonated bed material, together with the biomasscoke, is removed and regenerated in a separate reactor at 1023–1173 K under air supply(with additional fuel if necessary). Thus, a low-inert gas product gas flow and a CO2-richexhaust gas flow are generated in two separate steps (Möllenstedt et al., 2004).

Conclusion

Gasification, one thermochemical conversion route, is widely recognized at present be-cause its end product gas can find flexible application by industries or by home users,particularly in decentralized energy production coupled with micro turbine/gas, tur-bine/engine, boiler, even fuel cell. The gasification of biomass is a thermal treatment,which results in a high proportion of gaseous products and small quantities of char (solidproduct) and ash.

Tar formation is one of the major problems during biomass gasification. The useof an active bed material as a primary catalyst is the best solution in contrast to more

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expensive use of secondary catalytic reactor downstream of the gasifier. The choice of anappropriate in-bed catalyst is then crucial for the optimization of gasification technology.Ni-olivine catalyst has been developed to enhance olivine performances in steam biomassgasification by methane and tar reforming, leading to hydrogen production and gasupgrading. The main advantage of Ni-olivine catalyst is its attrition resistance due tostrong metal support interactions permitting its direct use in the fluidized bed reactor.The main limitation of Ni catalyst use for hot gas conditioning of biomass gasificationproduct gases is its sensitivity to deactivation, which leads to limited catalyst lifetimes.

Complete gasification of biomass involves several sequential and parallel reactions.Most of these reactions are endothermic and must be balanced by partial combustion ofgas or an external heat source. A limited supply of oxygen, air, steam, or a combinationserves as the oxidizing agent. Using steam on the composition of the feed gas, water gasreaction (C C H2O $ CO C H2) and water gas shift reaction (CO C H2O $ CO2 C

H2) are assumed to take place during gasification process.

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Arvelakis, S., Gehrmann, H., Beckmann, M., and Koukios, E. G. 2005. Preliminary results on theash behavior of peach stones during fluidized bed gasification: Evaluation of fractionation andleaching as pre-treatments. Biomass Bioenergy 28:331–338.

Backman, R., Frederic, W. J., and Hupa, M. 1993. Basic studies on black-liquor pyrolysis and chargasification. Bioresour. Technol. 46:153–158.

Baker, E. G., Mudge, L. K., and Brown, M. D. 1987. Steam gasification of biomass with nickelsecondary catalyst. Ind. Eng. Chem. Res. 26:1335–1339.

Bingyan, X., Zengfan, L., Chungzhi, W., Haitao, H., and Xiguang, Z. 1994. Circulating fluidizedbed gasifier for biomass. In: Integrated Energy Systems in China—The Cold Northeastern

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