Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production

Parametric analysis of a circulating ﬂuidized bed biomass gasiﬁer forhydrogen production Bhawasut Chutichaia, Yaneeporn Patcharavorachotb, Suttichai Assabumrungratc, a Computational Proc

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Parametric analysis of a circulating ﬂuidized bed biomass gasiﬁer for

hydrogen production

Bhawasut Chutichaia, Yaneeporn Patcharavorachotb, Suttichai Assabumrungratc,

a Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University,

Bangkok 10330, Thailand

b School of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

c Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering,

Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:

Received 16 September 2014

Received in revised form

12 January 2015

Accepted 16 January 2015

Available online xxx

Keywords:

Biomass

Circulating ﬂuidized bed gasiﬁer

H 2 -rich synthesis gas

Performance analysis

a b s t r a c t Biomass is considered a potential energy source which can be efficiently converted to useful gaseous products via a gasification process Circulating fluidized bed (CFB) gasifiers have attracted significant attention due to their high reaction rates and thermal efficiency This study aims to investigate the CFB biomass gasification process to generate H2-rich synthesis gas A process simulator is used to analyze the gasifier performance by assuming that the gasification is fast and reach equilibrium Parametric analysis

of the CFB gasiﬁer shows that steam gasiﬁcation generates the synthesis gas attained the highest H2

content (50e65 vol.%) and the highest product gas quality (higher heating value, HHV ¼ 10e13 MJ/Nm3)

at operating temperatures approximately 650e700C High-temperature steam cannot provide enough energy for the gasifier, reducing the gross cold gas efficiency of this process to only 16% The biomass air-steam gasification process is investigated while avoiding high energy consumption, but less H2is pro-duced under these conditions

1 Introduction

Energy security becomes the most important issue because

energy demand continuously increases while fossil fuel supply

declines Presently, renewable energy sources have been explored

to reduce the global dependence on fossil fuels and the emission of

greenhouse gases Consequently, future energy solutions should

provide sufﬁcient amounts of sustainable energy with minimal

environmental impact

Hydrogen has been widely discussed as a promising energy

carrier because it provides clean and highly efﬁcient energy

con-version This gas can also be used to drive fuel cells for power

generation Currently, many technologies have been developed to

produce hydrogen from various sources[1e4] Agricultural residue

is a major resource for renewable energy; it can be converted into

various forms of energy through thermochemical or biological processes [5] The thermochemical processes, including combus-tion, pyrolysis and gasification, have some advantages over the biological methods because they are moreflexible when selecting a feedstock, faster and more efficient[6]

Currently, combustion-based processes are the conventional methods used to convert biomass into heat and electricity; how-ever, the energy efﬁciency of this process is quite low (20e40%)

[7] Pyrolysis is based on cracking biomass in the absence of ox-ygen, and the major products are in the liquid phase (“bio-oil”)[8] The commercial application of bio-oil is restricted by their limited use and difficulty during downstream processing [9] Alterna-tively, gasification is an attractive means to convert solid fuels (e.g., biomass and coal) to a combustible or synthesis gas[10,11] This process involves drying, devolatilization and a gasification/ combustion process Currently, different designs for gasification reactors or gasifiers have been proposed A circulating fluidized bed (CFB) gasifier is a type of gasifier that is currently undergoing rapid commercialization for biomass[12] This apparatus exhibits

* Corresponding author Tel.: þ66 2 218 6878; fax: þ66 2 218 6877.

E-mail address: Amornchai.A@chula.ac.th (A Arpornwichanop).

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Energy

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / e n e r g y

http://dx.doi.org/10.1016/j.energy.2015.01.051

Energy xxx (2015) 1e8

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many advantages during biomass gasiﬁcation, including a high

degree of solid mixing, a high thermal efﬁciency and good

scal-ability[13]

When operating the gasiﬁer, the quality of synthesis gas is

strongly affected by types of used gasifying agents, such as air,

oxygen and steam Air gasiﬁcation is feasible for industrial

ap-plications; however, the synthesis gas produced using this

tech-nology has a low H2content, which ranges from 8 to 14 vol.%, and

a low higher heating value (HHV) approximately 4e6 MJ/m3[14]

Although using pure oxygen during gasiﬁcation can produce gas

with a higher heating value (10e18 MJ/m3), the high cost of pure

oxygen generated using current technology, such as a cryogenic

air separation, makes the gasiﬁcation process impractical

[9,14,15] To obtain H2-rich gas for internal combustion engines,

gas turbine systems or fuel cells for electricity and heat

genera-tion, steam gasiﬁcation might be an interesting alternative

[16e19]because this process can produce synthesis gas with high

H2contents (30e60 vol.%) and higher heating values (10e16 MJ/

m3)[14] However, steam gasiﬁcation reactions are endothermic,

requiring large amounts of energy for the gasiﬁer[20] Adding air

to steam gasiﬁcation, which is called air-steam gasiﬁcation, is an

alternative for supplying energy based on the partial combustion

of biomass with air; however, the quality of the product gas may

be lower[11,21]

In general, the composition of the synthesis gas is the major

parameter affecting the performance during biomass gasiﬁcation

because it directly affects the heating value of the product gas and

the gasiﬁcation efﬁciency[6,9] However, making exact predictions

of synthesis gas compositions is not easy because these models

depend on many parameters, such as the biomass composition,

operating conditions and gasifying agent Umeki et al.[20]studied

on the performance of a high temperature steam gasiﬁcation

pro-cess for woody biomass and found that the obtained synthesis gas,

which contained 35e55 vol.% H2, was generated by wateregas and

steam reforming reactions The cold gas efﬁciency was 60.4%, but

the gross cold gas efﬁciency was 35% due to the heat supplied by

high-temperature steam Mehrdokht and Mahinpey [22]

per-formed a sensitivity analysis of a biomass ﬂuidized bed gasiﬁer,

ﬁnding that the H2 content in the product gas increased when

increasing the operating temperature Adding more steam to the

gasiﬁer increases the H2and CO production while decreasing the

CO2and carbon conversion Kumar et al.[23]also studied the effect

of operating parameters of ﬂuidized bed gasiﬁcation, such as

gasiﬁcation temperatures and gasifying agent feed rates, on the

energy conversion efﬁciencies The results showed that the

gasiﬁ-cation temperature is the most inﬂuential parameter while the

gasifying agent feed rates has the strong effect on the carbon

conversion and energy efﬁciencies The balance between air and

steam feed rates was the way to achieve H2-rich gas production

Doherty et al.[15]developed a model of a CFB biomass gasiﬁer to

predict its performance under various operating conditions The

heating value of the synthesis gas increased with the equivalent

ratio of the air supply Preheating the air increased the H2and CO

contents Steam was introduced to promote H2-rich synthesis gas

production

The aim of this study is focused on improving the CFB biomass

gasiﬁcation process to produce a H2-rich synthesis gas A model of

the CFB gasiﬁer is developed using a commercial process simulator

to investigate the effect of key operating parameters, such as the

gasiﬁer temperature, steam temperature, steam-to-biomass ratio

(S/B), equivalent ratio (ER) and type of gasifying agents, on the

performance of the CFB gasiﬁer The synthesis gas composition and

heating value, as well as the biomass gasiﬁcation process efﬁciency,

are the criteria used to determine suitable operating conditions for

the CFB gasiﬁer

2 Methods 2.1 Model of a circulatingfluidized bed (CFB) gasifier Thefluidized bed reactor has been broadly utilized for coal and biomass combustion and gasification A traditional bubbling fluid-ized bed gasifier has a lower carbon conversion efficiency; there-fore, the design of fluidized bed gasifiers has shifted from low velocity bubbling beds to high velocity circulation-based designs because a circulatingfluidized bed gasifier (CFB) has a higher char circulation rate, improving the overall efficiency[24]

Circulating fluidized bed gasifiers might improve biomass gasification by using higher gasifying agent flow rates to entrain and move the bed material, which can be either sand or char; in addition, these apparatuses recirculate nearly all of the bed mate-rial and char with a cyclone separator A schematic diagram of a CFB biomass gasifier is shown inFig 1(a) When the biomass is added to the gasifier, it is rapidly dried and pyrolyzed, releasing all of the gaseous portions of the biomass at a relatively low temperature The remaining char is oxidized within the bed to provide a heat source for the drying and gasification processes The large thermal capacity of the inert bed material plus the intense mixing associ-ated with thefluid bed allow this system to handle a much greater quantity of material with a much lower quality fuel

2.2 Process workflow The CFB gasifier is modeled using a commercial process simulator (Aspen Plus) The model is divided into three stages including devolatilization, gasification and solid recirculation, as shown in

Fig 1(b) The main assumptions made to develop the CFB model are

as follows: the process is operated under steady state conditions; the gases are treated as ideal gases; the ash is treated as an inert solid, and tar formation is ignored because of the relatively high operating temperature [25]; the syngas is produced by the gasiﬁer at the chemical equilibrium; heat losses are ignored, the cyclone separation

efﬁciency is 90%[26], and 2% of the carbon is lost to the ash[27]

InFig 1(b), the‘BIOMASS’ stream was treated as a nonconven-tional stream whose proximate and ultimate analyses are deﬁned

inTable 1(pine sawdust) The standard operating conditions of this study are shown inTable 2 The‘DECOMP’ block is used to represent the devolatilization process, which is a thermal decomposition process for the biomass; the biomass is converted to volatile ma-terials and solids, such as H2, N2, O2, C (carbon), S (sulfur), and ash The RYield module is ASPEN Plus is used for modeling at this stage after specifying the yield distribution, which is determined based

on the ultimate analysis of the pine sawdust (Table 1) The enthalpy

of the‘DECOMP’ product stream does not equal that of the feed stream Consequently, the‘Q-DECOMP’ heat stream is inserted to balance the enthalpy of the biomass stream

The product of the thermal decomposition process (‘DECOMP’ stream) and the recirculating solid carbon (‘CRECYCLE’ stream) reacts with steam (‘STEAM’ stream) in the gasiﬁcation reaction block, which

is called‘GASIF1’ The gasification mechanism involves a complex collection of various reactions during a real gasification process; however, the gasification reactions are simplified to 8 major reactions

in the present model These reactions are summarized in Eqs.(1)e(8) [15] Reactions(1)e(4)are the gasiﬁcation processes for char particles that produce CO, H2and CH4 Reaction(1)is the partial combustion of

C The generated heat fromﬁrst reaction is supplied to the endo-thermic reaction(2), which is the Boudouard reaction, and reaction

(3), which is the heterogenous shift reaction Reaction(4)describes the equilibration of the hydro gasiﬁcation reaction process, which depends on the volatile matter in the feedstock The reaction rates of

(2)e(4)are known to be slower than that of reaction(1) [31]

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Cþ 0:5O2⇔CO DH0

Reactions (5)e(8)are gas phase reactions that occur during the

gasiﬁcation of char particles Those reactions are, respectively, the

partial combustion reaction of combustible gases (CO, H2), the wateregas shift reaction and the steamemethane reforming reaction

H2þ 0:5O2⇔H2O DH0

Fig 1 (a) Schematic diagram of a circulating fluidized bed (CFB) biomass gasifier and (b) biomass CFB gasifier process workflow.

B Chutichai et al / Energy xxx (2015) 1e8 3

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These reactions are simulated by minimizing the Gibbs free

energy in the RGibbs blocks:‘GASIF1’ and ‘GASIF’2 The ‘GASIF2’

block is added to control the temperature of the system

The‘ASHSEP’ block accounts for ash removal through an SEP

block in which all ash is removed The‘PROD-GAS’ stream is fed to

the‘CYCLONE’ block, which represents a cyclone separator with a

90% efﬁciency In addition, 90% of the solid carbon from the gas

stream is removed as the ‘SOLID’ stream, which is fed into the

separator block (‘CSEP’) The remainder makes up the product-gas

stream, which is‘SYNGAS’ The separator block ‘CSEP’ is set using

a calculator block while assuming that 2% of the solid carbon in

biomass is lost with the ash The‘CRECYCLE’ stream circulates the

solid carbon back to block‘GASIF1’ The ‘CWASTE’ stream carries 2%

of the solid carbon from the biomass, mixing it with ash at a mixer

block called‘ASH-C’ to generate the ‘ASH’ stream

The gasiﬁcation model used in this study was validated against

the experimental data of Keawpanha et al.[29]using Japan cedar as

a biomass The gasiﬁer was operated at the temperature of 700C,

S/B of one and steam temperature of 250C The proximate and

ultimate analysis of Japan cedar is reported inTable 1 The gasi

ﬁ-cation model was also compared with the simulation model of Tan

and Zhong[28]which the gasifer was run at the temperature of

700C, S/B of one and steam temperature of 400C.Table 3 pre-sents the synthesis gas compositions obtained from the simulation model as well as from the references with the same feedstock and operating conditions The model predictions agree with the refer-ence data with small percentages of deviation because more complications may arise during the experimental processes

3 Results and discussion 3.1 Effect of the gasifier temperature The temperature of the gasifier is crucial for producing H2-rich synthesis gas from biomass The gasifier temperature varies from

500 to 1000C while the other parameters are maintained at the standard values During the gasification process, the alkali species contained in biomass can be melted and coated the surfaces of ash particles, which make ash particles sticky Consequently, the flu-idized bed system is changed to afixed-bed system with the in-crease of bed temperature In this study, thefluidized bed biomass gasifier should not be operated above 1000C to ensure that the ash does not melt, which would induce agglomeration and

defludization[32,33] The gas composition is shown as a function of the gasifier temperature, as indicated inFig 2 The H2content in-creases significantly as the gasifier temperature increases, peaking

at 61% H2at approximately 700C before remaining nearly con-stant Additionally, the CO content obviously increases with the gasiﬁer temperature, while the CO2and CH4 contents decreased correspondingly The gas composition of the biomass in the gasiﬁer

is generated by a series of complex and competing reactions, as shown inreactions (1)e(8) The major reactions are the Boudouard

(2)and wateregas shift reaction(7); the Boudouard reaction is an intensive endothermic process, similar to the reforming reaction

(8), while the wateregas shift reaction(7)is an exothermic reac-tion The partial combustion of the char(1)and the combustible gases(5),(6)also release heat

Higher temperatures favor the reactants during exothermic re-actions, while the same conditions favor the products in endo-thermic reactions Therefore, endoendo-thermic reactions(2), (3), and (8)

have a stronger effect when increasing the gasiﬁer temperature, increasing the H2and CO contents and decreasing the CO2and CH4

Table 1

Proximate and ultimate analyses of the biomass.

Biomass Pine sawdust a Japan cedar b

Proximate analysis (dry basis, wt.%)

Moisture content (wt.%) 6.1 5.0

Ultimate analysis (dry basis, wt.%)

HHV (dry basis, MJ/kg) c 18.5 12.9

a Tan and Zhong [28]

b Keawpanha et al [29]

c Calculated by modiﬁed Dulong's equation [30]

Table 2

Standard operating conditions during the biomass gasiﬁcation process.

Gasiﬁcation operating condition T ¼ 700 C P ¼ 1 bar

Biomass input condition T ¼ 25 C P ¼ 1 bar

Steam input condition T ¼ 400 C P ¼ 1 bar

Air input condition T ¼ 25 C P ¼ 1 bar

Table 3

Comparison of the gas compositions obtained from the predictive model and the

reference data of Keawpanha et al [29] and Tan and Zhong [28]

No Sources HHV a Gas composition (vol %, dry basis)

H 2 CO CO 2 CH 4

1 Keawpanha et al [29] 10.9 44.4 18.5 29.6 7.4

Model prediction 10.6 45.9 21.3 27.6 5.2

2 Tan and Zhong [28] 10.4 61.2 18.9 19.4 0.5

Model prediction 10.4 61.0 19.0 19.5 0.5

a Calculated in dry basis at 0C and 1 bar (MJ/Nm 3 ) Fig 2 Effect of the gasiﬁer temperature on the composition of the product gas (S/

B ¼ 1, ER ¼ 0).

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contents The presence of steam favors the wateregas shift reaction,

increasing the H2 content In addition, H2is formed through CH4

reforming Although the wateregas shift reaction also releases CO2,

the CO2content decreases as the temperature increases because the

Boudouard reaction, which consumes CO2, becomes more

domi-nant; consequently, the CO content increases while the CO2content

decreases At 700C, which is the temperature at which the highest

H2content is obtained, the composition of product gas is 61 vol.% H2,

19 vol.% CO, 19.5 vol.% CO2, and 0.5 vol.% CH4 The optimal

temper-ature for this gasiﬁcation process is approximately 650e700 C,

which generates the highest amount of H2

3.2 Effect of the steam-to-biomass ratio

The steam feed rate is another important parameter that affects

the product gas compositions This parameter is deﬁned as the

steam-to-biomass ratio (S/B), which is the ratio between rate of

steam fed into gasiﬁer to rate of biomass feeding The gas

compo-sition varies with S/B, as shown inFig 3(a) As the biomass feeding

rate remains at standard condition, increasing the S/B has no initial

effect on the gas composition Subsequently, the H2and CO2

con-tents begin to increase at a transition when S/B is approximately

0.3, especially in the range of 0.3e1.2, while CO and CH4decrease

After S/B reached 1.2, the changes in the gas composition are quite

small

Three main reactions govern the product gas composition:

hydro gasiﬁcation (4), wateregas shift reaction (7), and

steam-methane reforming reaction (8) The reforming reaction is a

gaseous reaction that can be balanced more easily, while the hydro

gasiﬁcation reaction is a relatively slow, heterogenous reaction The

reaction integration effects decrease the CH4concentration when

increasing the S/B ratio; consequently, the H2and CO contents

in-crease more The wateregas shift reaction plays important role in

determining the CO2content When more steam is introduced, the

wateregas shift reaction shifts to produce more CO2and H2 In

addition, the wateregas shift reaction has a stronger effect on the

CO content than the reforming reaction; therefore, the decreased

CO content is primarily attributed to an increase in the wateregas

shift reaction activity Numerous studies reported optimized results

for S/B; however,Fig 3(a) shows that the S/B ratio, which favors the

H2-rich synthesis gas, is approximately 0.8e1.2, which produces a

gas composition of 60e62 vol.% of H2, 16e22 vol.% of CO,

17e21 vol.% of CO2, and 0.3e0.7 vol.% of CH4

The quality of the product gas is deﬁned as its higher heating

value (HHV), which is determine by the amount of H2and CO in the

product gas, while the performance of the gasiﬁcation process is

deﬁned by its cold gas efﬁciency (CGE), which is the ratio of the

HHV values for the product gas and the biomass Moreover, the

gross cold gas efﬁciency (G-CGE), which is the ratio of the chemical

energy from the product gas and the total energy added to the

gasiﬁer, is utilized to assess the overall efﬁciency of the process The

energy added to the gasiﬁer includes the chemical energy in the

biomass and the energy required for preheating and balancing

plant CGE and G-CGE can be calculated using Eqs (9) and (10),

respectively

Total input energy to gasifierðMJ=kgÞ 100

(10)

When more steam has been introduced to the gasiﬁer, the CO

content decreases faster than the increase in the H2content The

sum of the CO and H2content decreases, decreasing the HHV of the product gas, as presented inFig 3(b) The CGE and G-CGE values show trends similar to that of the HHV For the gross cold gas ef-ficiency determination, the process performance is reduced due to the strongly endothermic effect of the steam gasification process More energy must be added to the gasifier to maintain the gasifier temperature when more steam is added The G-CGE of the steam gasification is much lower than the CGE by approximately 60e70% When the S/B ratio is approximately 0.8e1.2, the HHV ranges from 10.14 to 10.68 MJ/Nm3, while the CGE and G-CGE are 81e85%, and

16e18%, respectively

3.3 Effect of the steam temperature Steam gasification is almost an endothermic reaction Therefore, additional heat is needed to maintain the gasifier temperature However, the gasifier may operate without heat from an outside source by balancing the energy required for the gasifier with the

Fig 3 Effect of the steam-to-biomass ratio (S/B) on (a) the composition of the product gas and (b) the higher heating value (HHV), the cold gas efficiency (CGE), and the gross cold gas efficiency (G-CGE) of the product gas (gasifier temperature ¼ 700 C, ER ¼ 0).

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energy supplied during steam injection The effect of the steam

feeding rate and temperature has been investigated by varying the

S/B between 0.1 and 3.0 and the steam temperature from

approx-imately 400e2000 C, as shown in Fig 4 Without the

supple-mental heat, the gasiﬁcation process utilizes steam as a heat carrier

The additional energy comes from the higher steam feeding rate

and higher steam temperature, which increases the gasiﬁer

tem-perature and accelerates the gasiﬁcation process The H2yield also

increases when the gasiﬁer temperature increases Injecting steam

favors H2production, as previously reported

However, at S/B values of approximately 0.8e1.2, the gasiﬁer

temperature is below the optimal range, which stated in Section3.1,

even if the steam is introduced into the gasiﬁer at 2000C The

optimal gasiﬁer temperature can be achieved by introducing steam

above 1500C when the S/B exceeds 1.5, as shown in Fig 4(a)

Under these conditions, a large amount of energy is required to

produce the steam Furthermore, under the standard conditions

when the steam temperature is 400C, no H2is produced because the gasiﬁer temperature is too low, as shown inFig 4(b) Therefore, heat from an external source is necessary during an optimized biomass steam gasiﬁcation process, which will be investigated in next sections

3.4 Effect of gasifying agent

As mentioned previously, H2and CO are the two most important gas species in the gaseous product; the product composition is used

to determine its quality FromFig 5, the H2and CO contents be-tween biomass steam, airesteam, and air gasification processes are roughly compared At the same biomass feed rate, the H2content for steam gasification is higher than that of air or airesteam gasi-fication This result occurs for two reasons First, the near absence of

N2 during steam gasiﬁcation condition reduces the gas ﬂow, increasing the residence time to allow the cracking and reforming

of biomass gasiﬁcation gas to proceed further and yield more H2 Second, the presence of steam enhances the effect of the steam reforming reactions; therefore, more H2is produced

However, the injected steam intensifies the wateregas shift reaction, producing a lower CO content than during air gasification During airesteam gasification, the CO content decreases as the combustion reaction proceeds in air with the wateregas shift re-action, decreasing the CO content in the product gas The product gas quality is measured using the higher heating value (HHV) of the gas, which is defined by heating value of H2and CO If the sum of the H2 and CO contents is higher, an HHV is generated for the product gas The HHV of the product gas obtained from steam gasification, which produced the highest sum of H2 and CO, is higher than that in airesteam gasification and air gasification, respectively

3.5 Effect of equivalence ratio (ER)

To operate a gasifier under self-sustaining conditions, some air must be introduced The biomass is oxidized by the oxygen in the air The high amount of heat produced by this oxidation reaction is supplied to the gasifier, balancing the exothermic and endothermic reactions The air feed rate is represented by the equivalence ratio (ER) which is the ratio between the amount of air fed into gasifier

Fig 4 Effect of the steam-to-biomass ratio (S/B) and the steam temperature on a) the

gasiﬁer temperature and b) the H 2 concentration (vol.%, dry basis) (ER ¼ 0).

Fig 5 Effect of the gasifying agent on the H 2 and CO contents and the higher heating value of the product gas (gasiﬁer temperature ¼ 700 C).

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and the stoichiometric amount of air needed for complete

com-bustion At higher ER, more heat is released from the combustion

reaction, explaining why less energy is required from external

sources to maintain the optimal gasiﬁcation conditions.Fig 6(a)

shows the effect of the ER on the amount of heat supplied to the

gasiﬁer A completely self-sustaining gasiﬁer, which does not

require additional heat, can be produced when air is added to the

system at an ER of 0.38 In addition, a smaller amount of air, which

is when ER is approximately 0.28, can become self-sustainable

operation when the heat from the hot product gas is recovered

by air pre-heating

The H2content varies with the ER, as shown inFig 6(b) A higher

ER value promotes the combustion reaction, which releases heat

and accelerates the endothermic gasiﬁcation reactions Moreover,

the strong combustion reactions of char and combustible gases

produce more CO2and some steam while lowering the H2, content

Furthermore, the presence of air in the gasiﬁer means that product

gas quality has been decreased by dilution with N2

4 Conclusions The work presents a theoretical study on a biomass-feed circulating fluidized bed gasifier Simulations of the gasifier are performed to investigate the effect of primary operating parame-ters on the production of H2-rich synthesis gas The results show that when increasing the gasifier temperature, the H2content in-creases significantly, peaking at approximately 700 C The CO content also increases with the gasifier temperature, while the CO2 and CH4contents decreased Increasing the steam-to-biomass ratio decreases the sum of the H2 and CO contents and decreases the higher heating value (HHV) of the product gas and the cold gas

efficiency (CGE) The optimal gasification process occurs at a steam-to-biomass ratio of 0.8e1.2 For steam gasification, additional heat

is necessary because large amounts of steam or higher temperature steam cannot be used It is found that the H2content obtained from steam gasification is higher than that from air gasification or air-steam gasification; the HHV of the product of steam gasification, which produced the most H2and CO, is also highest Introducing air into the gasifier promotes the combustion reaction that produces energy for the gasifier; however, the H2content and the product gas quality decrease upon addition of N2 Self-sustaining gasifier operation can be achieved when ER equals 0.28

Acknowledgments Support from the Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES560530168-EN) is gratefully acknowledged

B Chutichai would like to acknowledge the Dutsadiphiphat Scholarship, Ratchadaphiseksomphot Endowment Fund, Chula-longkorn University

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Fig 6 Effect of the equivalence ratio (ER) on (a) the amount of heat supplied to the

gasiﬁer and (b) the H 2 concentration (vol.%, dry basis) (gasiﬁer temperature ¼ 700 C).

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