Energy xxx (2015) 1e8 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production Bhawasut Chutichai a, Yaneeporn Patcharavorachot b, Suttichai Assabumrungrat c, Amornchai Arpornwichanop a, * a Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand 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 b a r t i c l e i n f o a b s t r a c t Article history: Received 16 September 2014 Received in revised form 12 January 2015 Accepted 16 January 2015 Available online xxx 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 650e700  C High-temperature steam cannot provide enough energy for the gasifier, reducing the gross cold gas efficiency of this process to only 16% The biomass airsteam gasification process is investigated while avoiding high energy consumption, but less H2 is produced under these conditions © 2015 Elsevier Ltd All rights reserved Keywords: Biomass Circulating fluidized bed gasifier H2-rich synthesis gas Performance analysis 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 conversion 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 * Corresponding author Tel.: þ66 218 6878; fax: þ66 218 6877 E-mail address: Amornchai.A@chula.ac.th (A Arpornwichanop) various forms of energy through thermochemical or biological processes [5] The thermochemical processes, including combustion, pyrolysis and gasification, have some advantages over the biological methods because they are more flexible 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; however, the energy efficiency of this process is quite low (20e40%) [7] Pyrolysis is based on cracking biomass in the absence of oxygen, 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] Alternatively, 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 http://dx.doi.org/10.1016/j.energy.2015.01.051 0360-5442/© 2015 Elsevier Ltd All rights reserved Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 many advantages during biomass gasification, including a high degree of solid mixing, a high thermal efficiency and good scalability [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 applications; however, the synthesis gas produced using this technology has a low H2 content, which ranges from 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 generation, steam gasification might be an interesting alternative [16e19] because this process can produce synthesis gas with high H2 contents (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 process 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] performed 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 H2 and CO production while decreasing the CO2 and 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 gasification 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 H2 and 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 Methods 2.1 Model of a circulating fluidized bed (CFB) gasifier The fluidized bed reactor has been broadly utilized for coal and biomass combustion and gasification A traditional bubbling fluidized bed gasifier has a lower carbon conversion efficiency; therefore, the design of fluidized bed gasifiers has shifted from low velocity bubbling beds to high velocity circulation-based designs because a circulating fluidized 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 material and char with a cyclone separator A schematic diagram of a CFB biomass gasifier is shown in Fig 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 associated with the fluid 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] In Fig 1(b), the ‘BIOMASS’ stream was treated as a nonconventional stream whose proximate and ultimate analyses are defined in Table (pine sawdust) The standard operating conditions of this study are shown in Table 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 materials 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 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, H2 and CH4 Reaction (1) is the partial combustion of C The generated heat from first reaction is supplied to the endothermic 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] Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 Fig (a) Schematic diagram of a circulating fluidized bed (CFB) biomass gasifier and (b) biomass CFB gasifier process workflow C þ 0:5O2 ⇔CO C þ CO2 4CO DH0298 ¼ À111 kJ=mol DH0298 ¼ þ172 kJ=mol C þ H2 O4CO þ H2 C þ 2H2 4CH4 DH0298 ¼ þ131 kJ=mol DH0298 ¼ À75 kJ=mol (1) partial combustion reaction of combustible gases (CO, H2), the wateregas shift reaction and the steamemethane reforming reaction (2) CO þ 0:5O2 ⇔CO2 DH0298 ¼ À283 kJ=mol (5) (3) H2 þ 0:5O2 ⇔H2 O DH0298 ¼ À242 kJ=mol (6) (4) CO þ H2 O4CO2 þ H2 Reactions (5)e(8) are gas phase reactions that occur during the gasification of char particles Those reactions are, respectively, the CH4 þ H2 O4CO þ 3H2 DH0298 ¼ À41 kJ=mol DH0298 ¼ þ206 kJ=mol (7) (8) Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 Table Proximate and ultimate analyses of the biomass Biomass Pine sawdusta Japan cedarb Proximate analysis (dry basis, wt.%) Volatile matter 79.5 Fixed carbon 16.8 Ash 3.7 Moisture content (wt.%) 52.8 33.7 13.5 6.1 and Zhong [28] which the gasifer was run at the temperature of 700  C, S/B of one and steam temperature of 400  C Table presents 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 reference data with small percentages of deviation because more complications may arise during the experimental processes Results and discussion 5.0 3.1 Effect of the gasifier temperature Ultimate analysis (dry basis, wt.%) Carbon Hydrogen Oxygen Nitrogen Sulfur 45.8 6.7 47.4 0.1 0.0 39.2 5.0 52.4 1.9 1.5 HHV (dry basis, MJ/kg)c 18.5 12.9 a b c Tan and Zhong [28] Keawpanha et al [29] Calculated by modified Dulong's equation [30] Table Standard operating conditions during the biomass gasification process Gasification operating condition Biomass input condition Steam input condition Air input condition T T T T ¼ ¼ ¼ ¼ 700  C 25  C 400  C 25  C P P P P ¼ ¼ ¼ ¼ 1 1 bar bar bar bar 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 700  C, S/B of one and steam temperature of 250  C The proximate and ultimate analysis of Japan cedar is reported in Table The gasification model was also compared with the simulation model of Tan The temperature of the gasifier is crucial for producing H2-rich synthesis gas from biomass The gasifier temperature varies from 500 to 1000  C 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 fluidized bed system is changed to a fixed-bed system with the increase of bed temperature In this study, the fluidized bed biomass gasifier should not be operated above 1000  C 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 in Fig The H2 content increases significantly as the gasifier temperature increases, peaking at 61% H2 at approximately 700  C before remaining nearly constant Additionally, the CO content obviously increases with the gasifier temperature, while the CO2 and 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 in reactions (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 reaction The partial combustion of the char (1) and the combustible gases (5), (6) also release heat Higher temperatures favor the reactants during exothermic reactions, while the same conditions favor the products in endothermic reactions Therefore, endothermic reactions (2), (3), and (8) have a stronger effect when increasing the gasifier temperature, increasing the H2 and CO contents and decreasing the CO2 and CH4 Table 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 HHVa Gas composition (vol %, dry basis) H2 CO CO2 CH4 Keawpanha et al [29] Model prediction Tan and Zhong [28] Model prediction 10.9 10.6 10.4 10.4 44.4 45.9 61.2 61.0 18.5 21.3 18.9 19.0 29.6 27.6 19.4 19.5 7.4 5.2 0.5 0.5 a Calculated in dry basis at  C and bar (MJ/Nm3) Fig Effect of the gasifier temperature on the composition of the product gas (S/ B ¼ 1, ER ¼ 0) Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 contents The presence of steam favors the wateregas shift reaction, increasing the H2 content In addition, H2 is formed through CH4 reforming Although the wateregas shift reaction also releases CO2, the CO2 content decreases as the temperature increases because the Boudouard reaction, which consumes CO2, becomes more dominant; consequently, the CO content increases while the CO2 content decreases At 700  C, which is the temperature at which the highest H2 content 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 temperature 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 composition varies with S/B, as shown in Fig 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 H2 and CO2 contents begin to increase at a transition when S/B is approximately 0.3, especially in the range of 0.3e1.2, while CO and CH4 decrease 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 steammethane 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 CH4 concentration when increasing the S/B ratio; consequently, the H2 and CO contents increase more The wateregas shift reaction plays important role in determining the CO2 content When more steam is introduced, the wateregas shift reaction shifts to produce more CO2 and 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 H2 and 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 CGE ð%Þ ¼ HHV of product gas ðMJ=kgÞ Â 100 HHV of biomass ðMJ=kgÞ G À CGE ð%Þ ¼ (9) Chemical energy of product gas ðMJ=kgÞ Â 100 Total input energy to gasifier ðMJ=kgÞ (10) When more steam has been introduced to the gasifier, the CO content decreases faster than the increase in the H2 content The Fig 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) sum of the CO and H2 content decreases, decreasing the HHV of the product gas, as presented in Fig 3(b) The CGE and G-CGE values show trends similar to that of the HHV For the gross cold gas efficiency 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 Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 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 approximately 400e2000  C, as shown in Fig Without the supplemental 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 temperature and accelerates the gasification process The H2 yield also increases when the gasifier temperature increases Injecting steam favors H2 production, as previously reported However, at S/B values of approximately 0.8e1.2, the gasifier temperature is below the optimal range, which stated in Section 3.1, even if the steam is introduced into the gasifier at 2000  C The optimal gasifier temperature can be achieved by introducing steam above 1500  C 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 400  C, no H2 is produced because the gasifier temperature is too low, as shown in Fig 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, H2 and CO are the two most important gas species in the gaseous product; the product composition is used to determine its quality From Fig 5, the H2 and CO contents between biomass steam, airesteam, and air gasification processes are roughly compared At the same biomass feed rate, the H2 content for steam gasification is higher than that of air or airesteam gasification 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 H2 is 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 reaction, 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 H2 and 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 Effect of the steam-to-biomass ratio (S/B) and the steam temperature on a) the gasifier temperature and b) the H2 concentration (vol.%, dry basis) (ER ¼ 0) Fig Effect of the gasifying agent on the H2 and CO contents and the higher heating value of the product gas (gasifier temperature ¼ 700  C) Please cite this article in press as: Chutichai B, et al., Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.051 B Chutichai et al / Energy xxx (2015) 1e8 and the stoichiometric amount of air needed for complete combustion 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 H2 content varies with the ER, as shown in Fig 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 CO2 and 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 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 parameters on the production of H2-rich synthesis gas The results show that when increasing the gasifier temperature, the H2 content increases significantly, peaking at approximately 700  C The CO content also increases with the gasifier temperature, while the CO2 and CH4 contents 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 steamto-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 H2 content obtained from steam gasification is higher than that from air gasification or airsteam gasification; the HHV of the product of steam gasification, which produced the most H2 and CO, is also highest Introducing air into the gasifier promotes the combustion reaction that produces energy for the gasifier; however, the H2 content 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, Chulalongkorn University References Fig Effect of the equivalence ratio (ER) on (a) the amount of heat supplied to the gasifier and (b) the H2 concentration (vol.%, dry basis) (gasifier temperature ¼ 700  C) [1] Kalinci Y, Hepbasli A, Dincer I Biomass-based hydrogen production: a review and analysis Int J Hydrog Energy 2009;34:8799e817 [2] Hajjaji N, Chahbani A, Khila Z, Pons M A comprehensive energyeexergybased assessment and parametric study of a hydrogen 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