b i o m a s s a n d b i o e n e r g y ( ) e4 Available online at www.sciencedirect.com ScienceDirect http://www.elsevier.com/locate/biombioe Effect of operating parameters on performance of an integrated biomass gasifier, solid oxide fuel cells and micro gas turbine system Junxi Jia a,*, Abuliti Abudula b, Liming Wei c, Baozhi Sun a, Yue Shi a a College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, China North Japan Research Institute for Sustainable Energy, Hirosaki University, Aomori 030-0813, Japan c School of Electric and Electronic Information Engineering, Jilin Jianzhu University, Changchun 130118, China b article info abstract Article history: An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and Received 13 October 2014 micro gas turbine has been investigated by thermodynamic model A zero-dimensional Received in revised form electrochemical model of SOFC and one-dimensional chemical kinetics model of down- February 2015 draft biomass gasifier have been developed to analyze overall performance of the power Accepted February 2015 system Effects of various parameters such as moisture content in biomass, equivalence Available online ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis Keywords: It is found that char in the biomass tends to be converted with decreasing of moisture Biomass gasification content and increasing of equivalence ratio due to higher temperature in reduction zone of Solid oxide fuel cell gasifier Electric and combined heat and power efficiencies of the power system increase Chemical equilibrium with decreasing of moisture content and increasing of equivalence ratio, the electrical Kinetics model efficiency of this system could reach a level of approximately 56%.Regarding entire Combined heat and power conversion of char in gasifier and acceptable electrical efficiency above 45%, operating condition in this study is suggested to be in the range of moisture content less than 0.2, equivalence ratio more than 0.46 and mass flow rate of biomass less than 20 kg hÀ1 © 2015 Elsevier Ltd All rights reserved Introduction Biomass is supposed to be one of the most common renewable sources used for power generation [1] Biomass gasification (BG) technology has been used to produce syngas and electricity, from laboratory scale test to some demonstration scale plants Although low energy density and seasonal availability of biomass lead to both the high transport cost and high capital cost of biomass plants, it has potential of being * Corresponding author E-mail address: jiajunxi99@sohu.com (J Jia) http://dx.doi.org/10.1016/j.biombioe.2015.02.004 0961-9534/© 2015 Elsevier Ltd All rights reserved commercialized to produce hydrogen in the future [2] Solid oxide fuel cell (SOFC) is considered one of the most important energy technologies for its high efficiency and low environmental impact It is ideal for syngas conversion due to its high operation temperature [3e5] Integration of BG with SOFC has received more attention as a potential substitute for fossil fuels in electric power production since it combines the merits of renewable energy sources and hydrogen energy systems 36 b i o m a s s a n d b i o e n e r g y ( ) e4 Thermodynamic analysis of BG and SOFC hybrid systems have been reported by many researchers [6e14] These studies mainly focus on effect of operating conditions on overall performance of the power systems Athanasiou et al [8] and Cordiner et al [9] investigated an integrated process of biomass gasification and solid oxide fuel cells system, the overall electrical efficiency could reach very high level of more than 40%.Fryda et al [10] assessed the combination of BG with SOFCs and micro gas turbine (MGT) Their results show that an electrical efficiency of 40.6% could be achieved at elevated pressures A hybrid plant consisting of gasification system, solid oxide fuel cells and organic Rankine cycle has been presented by Pierobon et al [11] The results show that efficiencies over 54% can be achieved Colpan et al [12] studied the effect of gasification agent (air, enriched oxygen and steam) on the performance of an integrated SOFC and BG system The results show that using steam as the gasification agent yields the highest electrical efficiency of 41.8%.Rokni et al [13] reported a hybrid plant producing combined heat and power (CHP) from BG, SOFC and a MGT An electrical efficiency of 58.2% has been reported resulting from optimization efforts Recently, Campitelli et al [14] have invested the effect of operating conditions on BG-SOFC systems performance The influence of H2 utilization of SOFC and moisture content in biomass have been analyzed in details In their work, a zerodimensional chemical equilibrium model was used in gasifier The authors did not take into account any char conversion in the reduction zone of gasifier Most of gasification models adopted to analyze the performance of BG, SOFC, and GT system mentioned above [6e14] are based on thermodynamic equilibrium as those reported in Refs [15e18] These equilibrium models are developed by the thermodynamic parameters based on minimization of Gibbs free energy Although these pure equilibrium models are relatively easy to be applied with fast convergence, they have certain limitations such as considering sufficient residence time, high reaction temperature, and fast reaction rates The dying, pyrolysis and oxidant process is assumed to be lumped together in a single reaction The gas compositions and temperature remains essential uniform in gasifier rather than variable with the height of the gasifier All the char is assumed to be completely consumed before leaving the gasifier, which could not take place in actual gasification process Since few of chemical kinetic model of gasifier is available for analysis of an integrated BG, SOFC, and GT system, in this paper kinetics model of downdraft biomass gasifier is presented in order to overcome the limitations of the equilibrium model The gas composition, reaction temperature, and unreacted char are predicted along height of the reduction zone Effect of process parameters, such as moisture content, equivalence ratio and mass flow rate of dry biomass on char flow rate and overall performance of BG, SOFC and GT system is examined Energy analysis is applied by thermodynamic model Regarding entire char conversion and acceptable system efficiency, the suggested operating conditions are proposed System description A schematic of an integrated biomass gasification, SOFC and GT system is shown in Fig Biomass enters a dryer and its moisture content is reduced to a level acceptable by gasifier Air, oxygen and steam may be used as gasification agents In this work air enters a downdraft gasifier The syngas produced by gasification is cleaned up after entering a hot gas cleaning unit according to the tolerance limits of SOFC Then, the cleaned syngas enters the SOFC, where electricity is produced The depleted fuel and air enter a combustor to burn The high temperature and pressure effluent from the combustor is expanded through GT to generate mechanical power, which is used to generate electrical power The GT exhaust is used to increase the temperature of air supplied by compressor to the Fig e Integrated biomass gasifier, SOFCs and GT system b i o m a s s a n d b i o e n e r g y ( ) e4 37 SOFC Then the stream of burned gas supplies heat to a steam generator, where feed water for user takes up the heat up to its corresponding saturation temperature at pressure of 121.59 kPa Finally, the stream gives heat to the dryer and goes into the atmosphere In order to analyze the effects of air supply and moisture content of biomass on process of gasification, moisture content (MC) of biomass and equivalence ratio (ER) are defined as Model description ER ¼ 3.1 Dryer To solve the problem, equilibrium reactions are required The two equilibrium reactions in the pyrolysis-oxidant zone are In order to analyze drying of wet biomass prior to gasification, it is assumed that the initial moisture content of wet biomass is 40% After drying, wet biomass in which water mass fraction of 10%e30% enters a gasifier The chemical equation the dryer is shown as: CHa Ob Np ỵ wtotal H2 Olị ẳ CHa Ob Np ỵ wH2 Olị ỵ wv H2 Ovị (1) The enthalpy of evaporation for water is 44.011 kJ molÀ1 at 25 C 3.2 Gasifier The structure of a downdraft gasifier in this work is shown in Fig 2, the dimensions of the gasifier are similar to that from Jayah et al [19] The gasifier is divided into two parts: pyrolysis-oxidation zone where pyrolysis and oxidation reactions take place and reduction zone, where the reduction reactions occur The output data from the exit of the pyrolysis -oxidation zone are transferred as input data to entrance of the reduction zone 3.2.1 Model of pyrolysis-oxidation zone The global reaction in the pyrolysis-oxidation zone can be written as CHa Ob Np ỵ wH2 O ỵ mO2 ỵ 3:76N2 ị ẳ x1 H2 þ x2 CO þ x3 CO2 þ x4 H2 O þ x5 CH4 þ x6 N2 þ x7 C (2) MC ẳ Masswater 18w ẳ Masswaterỵbiomass 12 ỵ a ỵ 16b ỵ 14pị ỵ 18w Airactural m ẳ Airstoichiometric þ 0:25a À 0:5b (3) (4) C þ 2H2 4CH4 (5) CO ỵ H2 O4H2 ỵ CO2 (6) The equilibrium constants for them are K1 ¼ h i x5 nT ẳ exp G0T;CH4 2G0T;H2 =Rm Tị x1 P0 (7) K2 ¼ h i x1 x3 ẳ exp G0T;H2 ỵ G0T;CO2 G0T;CO À G0T;H2 O =ðRm TÞ x2 x4 (8) where nT is total mole of the syngas, P0 is total pressure The energy balance equation can be written as (assuming no heat loss and work ẳ 0) Hbiomass ỵ wHH2 O þ mHO2 þ 3:76mHN2 ¼ x1 HH2 þ x2 HCO þ x3 HCO2 þ x4 HH2 O þ x5 HCH4 þ x6 HN2 þ x7 HC (9) The values of unknownsx1, x2, x3, x4, x5, x6, x7 and the reaction temperature T are determined by eight equations These equations are four atom balances, one fixed carbon balance, two chemical equilibrium equations and one energy balance equation The values of the thermodynamic properties are adopted from Perry [20] Once tow equilibrium constants are calculated at a tentative temperature, thex1, x2, x3, x4, x5, x6 ,x7 are Fig e Schematic diagram of a downdraft gasifier and reduction zone for calculation 38 b i o m a s s a n d b i o e n e r g y ( ) e4 determined by solving the equations using NewtoneRaphson method Then, the temperature is obtained by bisection method This temperature is taken as the initial temperature for the next iteration until a specified convergence criterion is obtained 3.3 Filter and scrubber The output data from the exit of the pryo-oxidation zone is transferred as input data to entrance of the reduction zone The control volumes of reduction zone for calculation are shown in Fig The reduction reactions considered in this zone are The syngas consists impurities such as tar, sulphur and other contaminant which may cause the degradation of SOFC A gas cleanup unit should be used to clean the syngas There are two options, hot and cold gas cleanup subsystems are supplied, which can be found in Ref [24] In this study, a hot gas cleanup unit is chosen After entering filter and scrubber, the syngas are suitable to be used in SOFC To simplify the calculation, the mass balances of syngas in filter and scrubber are ignored, the mass flow rate of products is supposed to be constant R1 : C ỵ CO2 42CO (10a) n3 ẳ n4 R2 : C ỵ H2 O4CO ỵ H2 (10b) R3 : C ỵ 2H2 4CH4 (10c) 3.2.2 Model of reduction zone (10d) These four chemical reactions are considered to be reversible The specific reaction rates are expressed as kinetic rate equations [21,22] The kinetic rate parameters are obtained as reported by Wang and Kinoshita [23] Thus the volumetric reaction rate of each chemical reaction can be written as ÀER1 y2 yCO2 À CO rR1 ¼ CRF AR1 exp Rm T KR1 (11a) ÀER2 yH yCO yH2 O À rR2 ¼ CRF AR2 exp Rm T KR2 (11b) rR3 ¼ CRF AR3 rR4 ¼ CRF AR4 After leaving the filter and scrubber, the temperature of the syngas is decreased to the level of that at anode inlet Heat loss of gas cleanup unit may be written as Qẳ R4 : CH4 ỵ H2 O43H2 þ CO ÀER3 yCH4 y2H2 À exp Rm T KR3 ! yCO y3H2 ÀER4 yH2 O yCH4 À exp Rm T KR4 (14) X n3;i $H3;i À i¼1 3.4 X n4;i $H4;i (15) i¼1 Solid oxide fuel cell In general, the ideal reversible potential of H2eO2 SOFC can be determined by the Nernst equation 1=2 DG0 RT pH2 $ pO2 ln ỵ (16) E0 ¼ 2F 2F pH2 O Nernst potential is reduced to the terminal voltage by the sum of the local voltage polarizations The three polarizations are ohmic, activation and concentration polarization Therefore the cell terminal voltage is given by V ¼ E0 À hact;a À hact;c À hohm À hcon (11c) (11d) (17) The activation polarizations of anode and cathode have been given in literature [25] Ohmic polarization is expressed by Ohm's law as shown in Ref [26] In the SOFC, the overall electrochemical is as follows, which is significantly exothermic The mass balance for the species i across the control volume k can be expressed as H2 þ O2 4H2 O nki ¼ nikÀ1 þ Rki DVk For a BG-SOFC system, usual high operating temperature of SOFC allows sustaining the reforming and shifting reactions as follows to produce hydrogen (12) where nki is molar flow rate (mol sÀ1),Rki is the net rate of production of species i (mol mÀ3 sÀ1): for exampleRkH2 ¼ rR2 À 2rR3 þ 3rR4 ,RkC ¼ ÀrR1 À rR2 À rR3 , etc., DVk is volume of the kth control volume (m3) The energy balance on the element can be expressed as X i¼1 À Á X À Á nikÀ1 Hk1 ỵ n7k1 Cp;C Tk1 T0 ẳ nki Hki þ nk7 Cp;C Tk À T0 i (18) CO þ H2 O4CO2 ỵ H2 (19) CH4 ỵ H2 O4CO ỵ 3H2 (20) The electric power produced in SOFC is given by WSOFC ¼ IV i¼1 (13) Once the equilibrium constants KR1ÀKR4 are calculated at a tentative temperature, Rki is determined and nki is given by Eq (12) Then, the gasification temperature of the kth control volume of the reduction zone is obtained by Eq (13) using bisection method This temperature is taken as the initial temperature for the next iteration until a specified criterion is satisfied (21) The equation for the energy balance of SOFC is X i Hin i ỵ X k Rk DHk ị ẳ WSOFC ỵ X Hout i (22) i The energy balance includes the electrical power WSOFC and the enthalpy changes of the chemical and electrochemical reactions, and gives the evaluation of the average temperature of the stack The detailed description of the electrochemical simulation of SOFC could be found in Refs [27,28] b i o m a s s a n d b i o e n e r g y ( ) e4 3.5 Combustor 3.7 The depleted fuel and air from SOFC enter a combustor to burn for heat recovery Enough oxygen is supplied so that all unreacted fuel from SOFC can be consumed That is to say, complete combustion occurs in the combustor The energy balance about the combustor is expressed as 0 1 ZTin ZTout X X in @ out @ A DHf ỵ ni DHf ỵ Cp dT ẳ ni Cp dTA (23) iẳ1 298 i¼1 Micro gas turbine and compressor Model of gas turbine and compressor are well described in the literature [10] To simply the study, it is assumed that the gas turbine and compressor work at their respective designed condition under steady-state operation A set of operating parameters and the assumed efficiencies are given in Table Once the pressure ratio is given, the outlet temperature of the compressor and gas turbine is given as: TCOM;out TGT;in kÀ1 ¼ ¼pk TCOM;in TGT;out (24) Then the compressor work and gas turbine output can be obtained, respectively WCOM ẳ ẵHTCOM;out ị HTCOM;out ị hCOM;s (25) WGT ẳ hGT;s H TGT;in À HðTGT;out Þ (26) where, hs is isentropic efficiency given in Table Table e Operating conditions Environmental Ambient temperature Ambient pressure Biomass data Type of biomass Ultimate analysis (wt%) Moisture content in biomass Mass flow rate of dry biomass Gasifier Gasifier operating pressure Moisture content of biomass entering gasifier (State 2) Equivalence ratio Molar fraction of air SOFC SOFC operating temperature Anode inlet temperature Fuel utilization Uf DC/AC inverter efficiency Peripheral equipment Isentropic efficiency of compressor Pressure ratio of compressor Isentropic efficiency of GT Outlet temperature of GT (State 11) Pressure ratio of water pump Exhaust temperature (State 14) Energy efficiencies The performance of BG，SOFC and GT power systems can be evaluated by energy efficiencies Energy efficiency is defined as the ratio of useful energy products to total energy inputs [29] Net electrical power output of the system is expressed as: Wnet ẳ WSOFC ỵ WGT À WCompressorÀ1 À WCompressorÀ2 À WPump (27) The heating production for user in Fig is given as: 298 Then the adiabatic combustion temperature can be determined from Eq (23) 3.6 39 25 C 101.325 kPa Wood 50% C, 6% H, 44% O 40% 10e30 kg hÀ1 253.313 kPa 10%e30% 0.39e0.5 21%O2,79%N2 800 C 750 C 0.85 95% 0.75 2.5 0.85 790 C 1.2 130 C Q ẳ n15 $H17 H15 ị (28) Therefore, the electrical efficiency, combined heat and power efficiency can be calculated by Equations (29) and (30), respectively hel ¼ Wnet nbiomass $LHVbiomass hCHP ẳ Wnet ỵ Q nbiomass $LHVbiomass (29) (30) Results and discussion The output data from the exit of gasifier are transferred as input data to entrance of the SOFC The key parameter in SOFC computation is the air utilization ratio which is dependent on various operating and design data The electrochemical model determines terminal voltage and electric power The energy balance Eq (22) accepts these results from electrochemical model and calculates a new molar flow rate of air at the cathode inlet The air utilization ratio is applied to the electrochemical model for the next calculation of cell terminal voltage and power until the convergence is obtained For the whole system model, since the calculation of heat exchanger need the heating fluid parameters such as the gas temperature at the combustor exit, which are not known at the beginning of the simulation, a set of initial parameters has to be assumed in order to run the system model until convergence is met eventually A set of operating parameters and the assumed efficiencies of the system components are given in Table The power system is simulated using Matlab 7.0 The present model has been validated against the experimental results of Jayah et al [19] Comparison of predicted and measured gas composition at gasifer exit is shown in Fig Comparison of the temperature distribution with the experimental result is shown in Fig The species concentrations at the gasifier exit are obtained from the data of the last control volume of the reduction zone This work did not take into account the heat loss in the gasifier, the molar fractions of CO, H2 and CH4 contents are slightly higher than the real values At the same time, the value of N2 is slightly less than that of experiment However, the good agreement between the model prediction and the experiment shows the present model is reliable A parametric analysis is carried out to study the effects of the process parameters (MC, ER and mass flow rate) on the overall performance of the power system 40 b i o m a s s a n d b i o e n e r g y ( ) e4 Fig e Comparison of predicted and measured gas composition at gasifier exit 4.1 Fig e Variation of char flow rate along the height of reduction zone for different moisture contents Effect of MC Moisture content is one of the important parameters since most of the biomass contains high percentage of moisture In this study the original MC is assumed to be 40% After drying, MC varies between 10% and 30% before entering gasifier As effect of MC on the performance of power system is analyzed, only the studied parameter is changed, ER, operating pressure of gasifier and mass flow rate of dry biomass are constant as _ ¼ 20 kg hÀ1, other input data ER ¼ 0.42, P ¼ 253.313 kPa andm are assumed as in Table Figs and show the effect of MC on char flow rate and temperature along the height reduction zone It is observed that the temperature decreases from the entrance to the exit of reduction zone and remains lower with higher MC As MC is higher, much heat generated in gasifier is used to evaporate the moisture and superheat the vapour, which resulting in the decreasing of gasifier temperature Accordingly, the char flow rate is lower with lower MC, because the lower temperature is unfavourable for char conversion It is seen all the char is consumed in the reduction zone as MC less than 0.2, while 24% of char is left at the exit of gasifier as MC equal to 0.3 Therefore, the biomass should be dried to the level of 10e20% for moisture before gasification Effect of MC on syngas composition is shown in Table Power input and output, combustor temperature, net power and heat output are also shown in Table Fig e Comparison of predicted and measured temperatures along the height of the reduction zone It can be seen from Table that the molar fraction of H2 and CO decrease while the content of H2O and CO2 increase with the increasing of MC As seen from the Table 2, the molar fraction of the CH4 is far less than other gases at the gasifier exit, most of the carbon in the biomass is converted into CO At the anode exit of SOFC (State 5), the concentration of CO decrease according to the mildly exothermic water-gas shift reaction in SOFC, the concentration of H2 decreases due to the electrochemical reaction, the concentration of H2O increases accordingly The output power of SOFC decreases due to the decreasing of molar ratio of H2 to H2O at the anode inlet as MC increasing which leads to the lower terminal voltage of SOFC The higher the molar fraction of H2O at the exit of SOFC anode, the lower the temperature of the combustor (State 10) The temperature falls by 17 K as shown in Table as MC varying from 0.1 to 0.2 It results in the decline of the output of GT Both of the decreasing of output power of SOFC and GT determine the reduction of the system net power On the other hand, with higher MC the heat provided to dryer to evaporate the moisture is less, therefore, more heat is left to steam generator The overall performance of the BG-SOFC-GT system is also shown in Table The electrical efficiency decreases by 6% as MC increasing from 0.1 to 0.2 The electrical efficiency is above 40% as long as MC less than 0.2 The heat efficiency increases Fig e Variation of temperature along the height of reduction zone for different moisture contents 41 b i o m a s s a n d b i o e n e r g y ( ) e4 Table e Effect of moisture contents on gas composition and performance of the power system MC ¼ 0.1 H2 (%) CO (%) CH4 (%) CO2 (%) H2O (%) N2 (%) WC1 (W) WC2 (W) WSOFC (W) T(K) (State 10) WGT (W) Wnet (W) Heat (W) hel (%) hCHP (%) MC ¼ 0.15 MC ¼ 0.2 State (3) State (5) State (3) State (5) State (3) State (5) 18.63 17.96 0.003 11.48 5.02 46.91 1724.5 21,251 35,098 1204 4.58 6.06 0.01 23.38 19.07 46.90 17.21 17.95 0.003 10.55 8.30 45.98 1724.5 21,866 34,431 1195 4.48 5.33 0.01 23.17 21.03 45.98 15.88 17.15 0.003 10.09 11.73 45.14 1724.5 21,790 33,067 1187 4.28 4.50 22.74 23.34 45.14 34,066 46,189 20,799 46.54 67.49 32,842 43,682 22,456 44.01 66.63 30,739 40,291 23,953 40.59 64.73 from 21% to 24%.As a result, combined heat and power efficiency decreases from 67% to 64% as MC increasing from 0.1 to 0.2 4.2 Effect of ER Effect of equivalence ratio on char flow rate and temperature along the reduction zone are shown in Figs and 8, respectively MC and pressure of gasifier are constant as MC ¼ 0.2, _ ¼ 20 kg hÀ1, other input data are P ¼ 253.313 kPa andm assumed as in Table It is seen that conversion of char is more remarkable with higher ER, owing to higher reaction temperature, which determining the extent of carbon conversion All the char is consumed in reduction zone on the condition of ER more than 0.42 Gas composition, power input and output, combustor temperature, net power and heat output for different ER is shown in Table It shows that the molar fraction of H2, CO2 and N2 increase slightly with higher ER, whereas a significant decrease of the molar fraction of H2O occurs Therefore, the Fig e Variation of temperature along the height of reduction zone for different equivalence ratios higher molar ratio of H2 to H2O results in the increasing of output power from SOFC The increasing of output power of GT is due to the higher combustor temperature with higher ER Although more power given to compressors are required, the overall useful output power from SOFC and GT overweighs the input power for compressors Effect of ER and MC on char conversion is shown in Fig The entire conversion of char is gained if ER and MC is selected above the line Effect of ER and MC on overall performance of the BGSOFC-GT system are shown in Figs 10 and 11 Both of electrical and CHP efficiencies increase with higher ER For example, the electrical efficiency increase from 41% to 45%, the CHP efficiency from 65% to 71% as ER changing from 0.42 to 0.46 when moisture content and mass flow rate of dry _ ¼ 20 kg hÀ1 biomass are constants as MC ¼ 0.2 and m 4.3 Effect of mass flow rate of dry biomass The char flow rate along the height of the reduction zone for different mass flow rates of dry biomass is shown in Fig 12 In Fig 12, ER and MC are constant as ER ¼ 0.42 and MC ¼ 0.2, Table e Effect of ER on syngas composition and performance of the power system Fig e Variation of char flow rate along the height of reduction zone for different equivalence ratios H2 (%) CO (%) CH4 (%) CO2 (%) H2O (%) N2 (%) WC1 (W) WC2 (W) WSOFC (W) T(K) (State 10) WGT (W) Wnet (W) Heat (W) hel (%) hCHP (%) ER ¼ 0.4 ER ¼ 0.42 ER ¼ 0.44 15.57 17.24 0.004 9.67 13.01 44.50 1642.4 21,262 31,796 1184 29,316 38,208 23,054 35.49 61.72 15.88 17.15 0.003 10.09 11.73 45.14 1724.5 21,790 33,067 1187 30,739 40,291 23,953 40.59 64.73 16.37 16.87 0.002 10.67 10.35 45.74 1806.6 22,215 34,359 1190 32,215 42,552 24,762 42.87 67.82 42 b i o m a s s a n d b i o e n e r g y ( ) e4 Fig e Effect of ER and MC on char conversion Fig 10 e Effect of ER and MC on electrical efficiency other input data are assumed as in Table Effect of mass flow rate on syngas composition and electrical and CHP efficiencies is shown in Table It is shown that char conversion is more active with smaller mass flow rate All the char get consumed completely in the range of less than 20 kg hÀ1 As mass flow rate decreasing, molar fraction of H2 and CO increase, however all these species don't show significant variation as mass flow rate changing from 15 kg hÀ1 to 25 kg hÀ1 Although the total Fig 12 e Variation of char flow rate different mass flow rate of dry biomass work from SOFC and GT and heat are enhanced as mass flow rate increasing, the mass flow rate increase significantly, the electrical and CHP efficiencies are reduced ultimately The electrical efficiency is above 41% in the range of 10 kg hÀ1 to 20 kg hÀ1 Effect of ER and mass flow rate on char conversion is shown in Fig 13 The entire conversion of char is gained if ER and mass flow rate is selected above the line Effect of ER and mass flow rate of dry biomass on overall performance of the BGeSOFCeGT system are shown in Figs 14 and 15 Both of electrical and CHP efficiencies increase with smaller mass flow rate For example, the electrical efficiency increase from 41% to 46%, the CHP efficiency from 65% to 71% as mass flow rate changing from 20 kg hÀ1 to 10 kg hÀ1 when ER and MC are constants as MC ¼ 0.2 and ER ¼ 0.42 Many variables affect the overall system's electric and CHP efficiencies The total plant performance can be compared to the results of other literature Colpan et al [12,30] studied the effect of gasification agent (air, enriched oxygen and steam) on the performance of an integrated SOFC and BG system The results show that the electrical efficiency of the system is 25% with superheated steam and pre-heated air as gasification agent and the highest electrical efficiency of 41.8% could be gained using steam as the gasification agent The electrical Table e Effect of mass flow rate on syngas composition and performance of the power system Fig 11 e Effect of ER and MC on CHP efficiency H2 (%) CO (%) CH4 (%) CO2 (%) H2O (%) N2 (%) WC1 (W) WC2 (W) WSOFC (W) WGT (W) Wnet (W) hel (%) hCHP (%) _ ¼ 15kg hÀ1 m _ ¼ 20kg hÀ1 m _ ¼ 25kg hÀ1 m 16.45 17.90 0.004 9.92 10.94 44.79 1293.4 17045 26,004 24,297 31,962 42.94 67.59 15.88 17.15 0.003 10.09 11.73 45.14 1724.5 21790 33,067 30,739 40,291 40.59 64.73 15.43 16.57 0.003 10.22 12.35 45.42 2155.6 26,316 39,805 36,923 48,256 38.89 62.56 b i o m a s s a n d b i o e n e r g y ( ) e4 Fig 13 e Effect of ER and mass flow rate on char conversion and 50.8% (LHV) A hybrid plant consisting BG, SOFC and organic Rankine cycle has been reported in Ref [11] with electrical efficiency of 54%, which close to the highest electrical efficiency of 56% in our study However, the electrical efficiency of BG-SOFC-MGT system reached a level of 58% has been shown by some researchers [13], meanwhile, the CHP efficiency of 87.5% in their study is more than the highest CHP efficiency of 85% in this study The improvement comes mainly from the optimization efforts in heat exchanger network and decreasing of the temperature of the exhaust gas leaving the hybrid plant in their study The temperature of the exhaust gas in their study is reduced from 120 C to 90 C, while the exhaust gas is set as 130 C in this study avoiding stack corrosion It should be noticed that all of these authors mentioned above did not take into account any char conversion in the reduction zone of gasifier as mentioned in the introduction Fig 14 e Effect of ER and mass flow rate on electrical efficiency efficiencies are below 45% due to the absence of gas turbine in their systems In this study, the electrical efficiency is always above 46% in the range of MC < 0.2, ER > 0.46 and mass flow rate less than 20 kg hÀ1 Sucipta et al [6] reported a combined SOFC and MGT system fed with syngas from biomass gasifier with electrical efficiencies between 46.4% 43 Conclusions An integrated BG, SOFC and GT system is investigated by thermodynamic model The pyrolysis-oxidant zone of the gasifier is modeled based on chemical equilibrium, reaction temperature is determined considering thermal equilibrium Meanwhile, kinetic reaction model has been adopted for onedimensional simulation of the reduction zone, the temperature and char flow rate along the height of reduction zone have been predicted Effects of process parameters (MC, ER and mass flow rate) on performance of the whole system have been studied It is found that char in the biomass tends to be converted with decreasing of MC and increasing of ER due to higher temperature in reduction zone The entire conversion of char in biomass could be expected on the condition of MC < 0.2, ER > 0.42 and m_ < 20 KghÀ1 The electric and CHP efficiencies of the power system increase with decreasing of MC and the increasing of ER, the electrical efficiency of this system could reach a level of approximately 56% Although the lower mass flow rate of biomass is favorable for char conversion and improvement of system efficiencies, it means that a larger gasifier has to be adopted and the capital cost of the equipment will increase Regarding the entire conversion of char in gasifier and acceptable electrical efficiency above 45%, the operating condition in this study is suggested to be in the range of MC less than 0.2, ER more than 0.46 and mass flow rate of biomass less than 20 kg hÀ1 The future work about this study will include a more comprehensive multi-dimensional gasifier model considering tar production and optimization study of the power system configuration Acknowledgment Fig 15 e Effect of ER and mass flow rate on CHP efficiency The authors are grateful for the support of the Centre College Primary Scientific Research Item Funds (HEUCF130311, 44 b i o m a s s a n d b i o e n e r g y ( ) e4 HEUCF140311, HEUCFZ1103), National Natural Science fund of China (51009039, 51179037) and Harbin Science and Technology Bureau (RC2013XK008002, 2013RFXXJ050) references cha F, 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J molÀ1 KÀ1 CRF: char reactivity factor ER: activation energy, J molÀ1 ER: equivalence ratio E0: reversible cell potential, V F: Faraday constant, 96,485C molÀ1 G: Gibbs function, J molÀ1 DG: change in Gibbs free energy, J molÀ1 H:: enthalpy, J molÀ1 DH:: enthalpy change of reaction, J molÀ1 I: current, A k: specific heat ratio K: equilibrium constant LHV: lower heating value, J molÀ1 _ mass flow rate of biomass, kg hÀ1 m: MC: moisture content n: Molar flow rate, mol sÀ1 P: Pressure, Pa Q: Heat, W r: volumetric reaction rate, mol mÀ3 sÀ1 R: universal gas constant, 8.314 J molÀ1 KÀ1 Ri: net rate of production of species i, mol mÀ3 sÀ1 T: temperature, K Uf: fuel utilization DVk: volume of the kth control volume, m3 V: terminal voltage of fuel cell, V y: molar fraction W: electrical power, W b i o m a s s a n d b i o e n e r g y ( ) e4 Greek Letters h: polarization, V h: efficiency, % p: pressure ratio Subscripts a: anode act: activation polarization c: cathode com: compressor con: concentration polarization ohm: ohm polarization GT: gas turbine COM: compressor CHP: combined heat and power 45