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Performance evaluation of an integrated small-scale SOFC-biomass gasification power generation system
Journal of Power Sources 216 (2012) 314e322
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Performance evaluation of an integrated small-scale SOFC-biomass gasification power generation system Suranat Wongchanapai*, Hiroshi Iwai, Motohiro Saito, Hideo Yoshida Department of Aeronautics and Astronautics, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
h i g h l i g h t s < We evaluate an integrated small-scale SOFC-biomass gasification power generation system through energy and exergy. < We examine the effects of steam-to-biomass ratio, SOFC inlet stream temperatures, fuel utilization factor and anode off-gas recycle ration on system performance. < The results show the optimal operating parameters and their effects on SOFC stack, gasifier and system performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2012 Received in revised form 14 May 2012 Accepted 28 May 2012 Available online 5 June 2012
The combination of biomass gasification and high-temperature solid oxide fuel cells (SOFCs) offers great potential as a future sustainable power generation system. In order to provide insights into an integrated small-scale SOFC-biomass gasification power generation system, system simulation was performed under diverse operating conditions. A detailed anode-supported planar SOFC model under co-flow operation and a thermodynamic equilibrium for biomass gasification model were developed and verified by reliable experimental and simulation data. The other peripheral components include three gasto-gas heat exchangers (HXs), heat recovery steam generator (HRSG), burner, fuel and air compressors. To determine safe operating conditions with high system efficiency, energy and exergy analysis was performed to investigate the influence through detailed sensitivity analysis of four key parameters, e.g. steam-to-biomass ratio (STBR), SOFC inlet stream temperatures, fuel utilization factor (Uf) and anode offgas recycle ratio (AGR) on system performance. Due to the fact that SOFC stack is accounted for the most expensive part of the initial investment cost, the number of cells required for SOFC stack is economically optimized as well. Through the detailed sensitivity analysis, it shows that the increase of STBR positively affects SOFC while gasifier performance drops. The most preferable operating STBR is 1.5 when the highest system efficiencies and the smallest number of cells. The increase in SOFC inlet temperature shows negative impact on system and gasifier performances while SOFC efficiencies are slightly increased. The number of cells required for SOFC is reduced with the increase of SOFC inlet temperature. The system performance is optimized for Uf of 0.75 while SOFC and system efficiencies are the highest with the smallest number of cells. The result also shows the optimal anode off-gas recycle ratio of 0.6. Regarding with the increase of anode off-gas recycle ratio, there is a trade-off between overall efficiencies and the number of SOFC cells. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Biomass gasification System analysis Power generation system Exergy analysis
1. Introduction In pursuing of green power generation, biomass is recently receiving increased attention as an alternative to fossil energy. As
* Corresponding author. Tel./fax: þ81 75 753 5203. E-mail addresses: [email protected], (S. Wongchanapai).
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0378-7753/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2012.05.098
one of the various biomass conversion processes, biomass gasification is a prominent conversion route for producing a renewable clean feedstock for power generation. Unlike conventional energy conversion devices, fuel cells are attracting significant attention as promising choices in the future owing to its high efficiency even at very small scale. Among fuel cells, high-temperature solid oxide fuel cell (SOFC) is the most suitable candidate for biomass power conversion technologies because of its high electricity conversion efficiency, fuel flexibility, tolerance to fuel contaminants, and its
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operating temperature close to that in biomass gasification process. SOFC operation on biomass-derived gas is also proven to be feasible by several experimental studies [1e4], while the experimental work on the combination of SOFC and biomass gasification system is still limited. Therefore, in order to investigate integrated SOFC-biomass gasification systems, numerical simulation are very helpful in providing theoretical guidance for optimization of design. As several numerical studies focused on biomass-fueled integration of SOFC with gas turbine (GT) systems [5e9], it seems to be very attractive for intermediate- and large-scale (electrical output range of several hundred kWe to several hundred MWe) electricity production. Sucipta et al. [5] conducted a performance analysis of a tubular SOFC and recuperated micro GT hybrid system fueled with syngas coming from air gasification, oxygen gasification and steam gasification, though the actual performance of the gasification and gas cleaning systems was not included. Fryda et al. [6] investigated biomass air gasification integrated with an SOFC stack and micro GT systems; the combined system operated under pressure ratio of approximately 4 showed the highest efficiency of 40.6%. Bang-Møller et al. [7] evaluated three different CHP plant concepts based on biomass gasification using thermodynamic modeling combining zero-dimensional component models. Biomass gasifiers were coupled to a micro GT, an SOFC and a combined SOFC micro GT system. The biomass gasification system combined with the SOFC micro GT was found to be the most efficient owing to the utilization of excess fuel and heat from the SOFC stack in the micro GT. Furthermore, Bang-Møller et al. [8] later presented results from the energy and exergy analyses which indicate potential improvements of the CHP plant combining gasification, SOFC and micro GT. The optimized hybrid plant produced approximately 290 kWe at an electrical efficiency of 58.2% based on lower heating value (LHV). In a study preformed by Toonssen et al. [9], four different systems based on integrated SOFCGT and biomass gasification were modeled with the thermodynamic flow-sheeting program Cycle-Tempo to compare cold gas cleaning with hot gas cleaning processes and 100 kWe-scale with 30 MWe-scale systems. The study showed that the large-scale system based on high-temperature gas cleaning has the highest electrical exergy efficiency of 49.9%. Athanasiou et al. [10] compared the production of electricity from the gasifier through a conventional steam turbine to the integrated process of gasifierSOFC-steam turbine. The system electrical efficiency was found to be 43.3%. In a study by Fryda et al. [11], a gasifier is coupled to an SOFC through heat pipes; the heat pipes supply the heat extracted from the SOFC to the gasification system. On this system, they performed an exergy analysis and found an exergetic electrical efficiency of 36% for a net electrical power production of 170 kW. Recently, with improved sealing materials and sealing concepts, small-scale 1e25 kW size planar SOFC stacks have been successfully developed by various organizations. In this scale of electricity generation, SOFCs produce the highest electrical efficiency compared to other energy conversion devices. This makes an integrated small-scale SOFC-biomass gasification power generation system very attractive for residential applications. In such smallscale SOFC based system, exhaust fuel and heat of SOFC can be effectively utilized for gasification process to enhance system performance. Colpan et al. [12] studied the effect of the gasification agent (air, enriched oxygen and steam) on the performance of an integrated biomass gasification and SOFC system. The studied system includes biomass gasification, hot gas clean up and a 10 kWclass SOFC. The study indicated that steam as the gasification agent yields the highest electrical and efficiencies of 41.8% and 39.1%, respectively.
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The aim of this study is to determine the optimum performance of an integrated small-scale SOFC-biomass gasification power generation system for safe and efficient system operations as well as economic solution. In this paper, the integration of a biomass gasification and 5 kW-class SOFC power system is evaluated through numerical simulation. The biomass fuel considered in this work is represented by ash-free typical wood fuel formula of CH1.4O0.59N0.0017 [13]. A sensitivity analysis was carried out to achieve a better understanding of the influence of key parameters e.g. steam-to-biomass ratio (STBR), SOFC inlet stream temperatures, fuel utilization factor (Uf) and anode off-gas recycle ratio (AGR) on the performance of key system components. By performing energy and exergy analysis, the causes of exergy losses were revealed to identify the areas of improvement of the combined system. Since SOFC stack is accounted for the most expensive part of the initial investment cost, the number of cells required for SOFC stack is also taken into consideration as well. 2. System configuration and description The schematic of the integrated SOFC-biomass gasification power generation system in this study is shown in Fig. 1. A 1D model of direct internal reforming planar SOFC, capable of internal reforming of the methane in the syngas into hydrogen, was developed in the previous work by the authors [14] and applied to this study. On the other hand, thermodynamic equilibrium model is used for biomass gasification. The other peripheral components include three gas-to-gas heat exchangers (HX1, HX2 and HX3), heat recovery steam generator (HRSG), burner, pump, fuel and air compressors are thermodynamically modeled under steady-state operational conditions. In the integrated system, depleted fuel and air from the cell stack were combusted in the burner and supplied heat to wet biomass drying process to upgrade the heating value of the produced gas called “syngas”. The steam from drying process is mixed with additional steam and air and then directed to gasification process. The syngas produced by gasification generally contains some amount of tar, sulfur and other contaminants, which may lead to degradation of SOFCs. Consequently, a hot gas clean up is facilitated. After syngas is cleaned through hot gas cleaning unit, the fuel stream is cooled down in HX1 by preheating the air, then entering a fuel compressor served as suction blower to overcome pressure drops in the gasifier and SOFC systems. To prevent the carbon deposition in the SOFC, before clean syngas entering the cell, the steam-to-carbon ratio is set at 2 [15] by adjusting the external steam from HRSG and anode off-gas recycle ratio (AGR). The SOFC air and fuel streams are preheated by the cathode and anode off-gas in HX2 and HX3 and are heated up to 700 C. An inverter is also used in the system to convert the DC power output of the SOFC stack into AC power output. The HRSG uses the heat from the flue gas to generate steam for the gasification and SOFC anode gas moistening. The flue gas is released to the environment at atmospheric pressure and cooled down to 100 C. For all of the HXs and the HRSG, 2% heat losses of heat transferred are assumed. Possible variations in pressure drops across each unit operation are assumed 2%, except in SOFC is assumed 3%. 3. System modeling The integrated system components, e.g. SOFC, Gasifier, HXs, burner, HRSG, air and fuel compressors, were thermodynamically modeled under steady-state operations. Mass and energy balances were applied for each component using lumped models, which consider each component as a control volume with the exception of
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Fig. 1. Schematic flow diagram of an integrated SOFC-biomass gasification power generation systems.
a 1D SOFC model [14]. The assumptions made for developing the system model are given as: (i) all system components operate under steady-state conditions; (ii) all gas mixtures behave as ideal gases; (iii) heat losses from SOFC and gasifier to the environment are negligible and heat losses in HXs and the HRSG are assumed 2% of heat transferred; (iv) chemical reactions are assumed to be in equilibrium. 3.1. Gasifier modeling In this paper, a steady-state equilibrium model was developed to predict the product gas from the biomass gasification with mixed air-steam. To produce high quality syngas rich in H2, the amount of steam as oxidizing agent was varied, while the relatively small amount of air is tuned to sustain the operating temperature to the desired point by partial oxidation. Generally, at sufficiently high gasifying temperatures, tar production can be negligibly eliminated from gasification products. Since the model does not take into account tars formation in the gasifier, biomass gasifier operate at 800 C under near ambient pressure, and syngas outgoing temperature of 800 C is assumed. The carbon-hydrogen-oxygen (C-H-O) ternary diagrams have been constructed as shown in Fig. 2. The biomass fuel considered in this work is represented by ash-free typical wood fuel formula of CH1.4O0.59N0.0017. In ternary C-H-O diagram, the solid line presents, the so-called carbon deposition boundary. Since the biomass fuel is located in the carbon deposition region, oxygen, steam are used as gasifying agent to bring down chemical equilibrium below carbon deposition boundary lines. Fig. 2 shows that the increase of steam as oxidizing agent (STBR increases) in this study minimizes the risk for carbon deposition. Gasification is a partial-oxidation process for the conversion of carbonaceous feedstocks to combustible gas mixtures consisting primarily of carbon monoxide (CO), hydrogen (H2), and methane (CH4). The global gasification reaction for CHaObNc can be written as follows:
CHa Ob Nc þ mH2 O þ lO2 þ 3:76lN2 ¼ n1 H2 þ n2 CO þ n3 CH4 þ n4 CO2 þ n5 H2 O þ n6 N2
(1)
where m is number of moles of water vapor. Gasification process consists of two stages. In the first stage, pyrolysis releases the volatile components of the organic compounds and results in char. In the second stage, the carbon in the char is reacted with steam, air, or pure oxygen. It is reported that the gas compositions is dominated by the wateregas shift reaction at the higher temperature >750 C in biomass gasification process [16]. Wateregas shift reaction:
Fig. 2. CeHeO ternary diagram with carbon deposition boundary at 800 C, 1 atm.
S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322
CO þ H2 O4CO2 þ H2
(2)
Methane is formed through the following exothermic reaction. Carbon hydrogenation reaction:
C þ 2H2 4CH4
(3)
The above two reactions are the major reactions that occur in gasification process [17,18]. The equilibrium constant for wateregas shift and carbon hydrogenation reactions are:
K1 ¼
PCO2 PH2 n n ¼ 4 1 PCO PH2 O n2 n5
(4)
PCH4 n3 2 ¼ 2 ntot n1
(5)
K2 ¼
PH2
KðTÞ ¼ exp½ DGo =RT
(6)
The main operating parameters are steam-to-biomass ratio (STBR) and equivalence ratio (ER). They refer to the amount of gasifying agents affecting the performances of gasifier. The steam-to-biomass ratio (STBR) can be defined as:
STBR ¼
_ bio;moisture _ steam þ m m _ mbio;d:b:
(7)
_ steam is the mass flow rate of the steam, m _ bio;moisture is Here, m _ bio;d:b: is the the mass flow rate of the moisture in biomass and m mass flow rate of the dry biomass. The equivalence ratio (ER) can be defined as:
ER ¼
_ air m _ air;stoic m
(8)
_ air is the mass flow rate of air and m _ air;stoic is the mass Here, m flow rate of air required for stoichiometric combustion. 3.1.1. Gasifier model validation In order to evaluate the above models, simulation results are compared with experimental and numerical results from literature. The simulations were preformed by setting the same conditions as the experiments of Altafini et al. [19] and Zainal et al. [20]. The results are compared in Table 1. A good agreement exists between this model and the two models in the literature indicating that the equilibrium model predicts reasonably well the producer gas for a gasifier.
The direct internal reforming SOFC model developed in the recent work by the authors [14] is applied to the present system Table 1 The comparison between model predictions and measurements for two biomass gasification processes. Present Altafini et al. [19] Present Zainal et al. [20] model model Model Experiment Model Experiment 18.70 21.87 0.22 10.51 47.30 5.31 1.11
(9)
Wateregas shift reaction:
CO þ H2 O4CO2 þ H2
(10)
Overall cell reaction:
1 H2 þ O2 /H2 O 2
(11)
The open-circuit voltage (VOC), is described by the Nernst equation as a function of operating temperature (T) and partial pressure (p):
VOC
0 1 1=2 RT @pH2 pO2 A þ ln ¼ 2F 2F pH2 O
DG0
(12)
where Faraday constant (F) ¼ 96,485 A/mol. The terminal voltage of the single cell can be obtained as follows:
V ¼ VOC Vact Vohm Vconc
(13)
The terminal voltage of the cell is decreased from its opencircuit voltage when the current is drawn from the cell because of tree types of overpotential losses, i.e. activation (Vact), ohmic (Vohm) and concentration (Vconc) losses. However, the details of the direct internal reforming planar SOFC has been the subject of earlier work [14] and will not be described here. The SOFC stack’s power output is:
PSOFC ¼ hinv VI
(14)
where hinv is the inverter efficiency and the current (I) is I ¼ 2Fðn_ H2 ;react þ n_ CO;react þ 4n_ CH4 ;react Þcell .
3.2. SOFC modeling
H2 (vol%, d.b.) CO (vol%, d.b.) CII4 (vol%, d.b.) CO2 (vol%, d.b.) N2 (vol%, d.b.) HHV (MkJ m3) RMS Err (%)
analysis. The model is capable of capturing the distribution of the local temperatures, species concentrations, current density, and polarization losses in streamwise direction. The electrochemical reaction is considered to be attributed to only hydrogen; the electrochemical fuel value of CO is readily exchanged for hydrogen by the rapid shift reaction assuming chemical equilibrium [21]. In other words, CO is considered to take part only in the shift reaction. Whereas the electromotive force (EMF) of the SOFC is calculated according to the electrochemical oxidation of H2, the species’ consumption and production is determined collectively from the reactions (9) and (10). Mass balances are formulated for each species linking the local current with the change in the concentrations. The following three equations summarize the reactions considered in the cell. Steam reforming reaction:
CH4 þ H2 O4CO þ 3H2
The equilibrium constant is determined by Gibbs free energy as a function of temperature as follows:
317
20.06 19.70 0.00 10.15 50.10 5.04 1.14
14.00 20.14 2.31 12.06 50.79 5.28
19.82 23.42 0.29 12.66 43.80 5.60 1.04
21.06 19.61 0.64 12.01 46.68 5.42 1.54
15.23 23.04 1.58 16.42 42.31 5.49
3.3. System parameters and efficiencies Anode off-gas recycle (AGR) is a system concept where anode off-gas is recirculated to the anode inlet to provide water vapor and heat to the anode feed gas. The amount of AGR is defined on a molar basis as:
AGR ¼
n_ recycle n_ anodeoff
(15)
Since the fuel is not completely consumed by electrochemical reactions in anode channel, the excess fuel flow rate can be determined by the fuel utilization factor (Uf), a key parameter investigated in this study. The fuel utilization factor is defined here as the
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X
X
ratio of fuel consumed by anode reactions to the fuel entering anode channels, and is expressed as:
exch ¼
n_ H2 ;react þ n_ CO;react þ 4n_ CH4 ;react Uf ¼ n_ H2 ;anode þ n_ CO;anode þ 4n_ CH4 ;anode
where ex0,i represents the standard molar chemical exergy of the ith species, assuming a reference atmospheric composition given by Kotas [24]. For biomass fuel, thermodynamic properties are not available. Therefore, the statistical correlation of Szargut and Styrylska was used [25] is applied:
(16) in
To comprehensively evaluate biomass gasification, both cold gas and exergy efficiencies are introduced. In energy conversion processes, the efficiencies can be defined in many ways. In this study we defined two kinds of efficiencies. Energetic efficiency or fuel efficiency and the definition of rational efficiency for steadystate processes are as follows [22]. 3.3.1. Energetic efficiency (h) The energetic efficiency of a gasification process, generally known as the cold gas efficiency(hcg), is the ratio between the chemical energy content in the produced syngas compared to the chemical energy in the original biomass fuel. The equation can be expressed as:
hcg ¼
_ syn LHVsyn m 100% _ bio: LHVbio þ Qin m
(17)
SOFC energetic efficiency (hSOFC) is the ratio of net electrical output generated from SOFC to energy content fed to the anode channel of the SOFC. The equation can be expressed as:
hSOFC ¼
P CH4 ;CO;H2
PSOFC 100% _ anode LHVÞin ðm
(18)
The energetic efficiency for power generation system is also called overall system efficiency (hsys). It is the ratio of net output energy to energy content of biomass fed to the gasifier and can be expressed as:
hsys
PSOFC ¼ 100% _ bio: LHVbio Þin ðm
(19)
In order to gain more insight into the system, exergy analysis is used as an assessment tool to identify and locate irreversibility. Exergy associated with material stream is equal to the maximum amount of work obtainable when the stream is brought from its initial state to the dead state by processes. Exergy transfer rate associated with material stream can be divided into physical and chemical exergy components:
Ex ¼ n_ exph þ exch
(20)
Physical exergy is the work obtainable by taking the substance through reversible processes from its initial state temperature T and pressure P, to the environmental state. It can be expressed as:
exph ¼
X
xi ½ðh h0 Þ T0 ðs s0 Þi
xi ex0;i þ RT0
i
xi ln ðxi Þ
Exch;bio ¼ bLHVbio
(23)
where LHVbio is the lower heating value of the biomass and b is the ratio of the chemical exergy to the LHV of the organic fraction of biomass.
b¼
1:044þ0:0160H=C0:3493O=Cð1þ0:0531H=CÞþ0:0493N=C 10:4124O=C (24)
where H/C, O/C, and N/C represent the atomic ratios in the biomass. The exergy transfer connected with the heat transfer rate Q at temperature T can be expressed as:
ExTQ ¼ Q
T T0 T0
(25)
Exergy balance can be represented in the following from considering exergy values of entering and leaving material streams of a control volume at steady state:
X
Ex þ
in
X
ExTQ ¼
X
Ex þ
X
out
in
ExTQ þ ExW þ ExD
(26)
out
where ExW and ExD represent electricity exergy and exergy destruction rate, respectively. 3.3.2. Rational efficiency (j) The rational or rational exergetic efficiency is defined by Kotas [24] as a ratio of the desired exergy output to the exergy used or consumed. It can be applied as equations below. Gasifier rational efficiency (jgas) is the ratio of the exergy of produced syngas to the sum of exergy associated with heat, biomass and gasifying agents entering the gasifier. The equation can be expressed as:
jgas ¼
Exsyn Exbio þ
T ExQgas in
þ Exair þ Exsteam
100%
(27)
SOFC rational efficiency (jSOFC) is the ratio of the produced electricity exergy to the exergy comsumed by SOFC. The equation can be expressed as:
jSOFC ¼ P
Exfuel þ Exair
(21)
PSOFC P Exfuel þ Exair
in
100% out
(28)
i
where xi represents the molar fraction of the i-th component, h is the specific enthalpy and s the specific entropy and the properties indicated with the subscript 0 refer to the environmental state. Enthalpy and entropy values are evaluated based on temperature, according to the thermodynamic property data available from the literature [23]. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environmental state, defined by the parameters T0 and P0, to the reference state by processes involving heat transfer and exchange of substances only with the environment. The chemical exergy for mixtures can be calculated as follows:
(22)
i
System rational efficiency (jsys) is the ratio of the produced electricity exergy to the exergy consumed by the system. The equation can be expressed as:
jsys ¼
PSOFC 100% Exch;bio Exexhaust
(29)
4. Results and discussion To understand the operational scenarios of the integrated SOFCbiomass gasification power generation system, an independent
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parameter analysis of a single component is not enough to assess the whole system because each component in the system affects one another. In this study, sensitivity analysis is used to quantify the effects of STBR, SOFC inlet stream temperatures, Uf and AGR on SOFC, gasifier and system performances as well as the size of SOFC stack. The required number of cells for the specified stack power is determined by the cell operating voltage, current density and fuel utilization factor assuming the cell geometry shown in Table 2. As the active electrode area of a single cell is 100 cm2, the total cell area in the stack can easily be calculated. The input operational parameter values, as presented in Table 2, are used as constant throughout the study unless mentioned specifically. In order to determine how much each component contributes to the total irreversibility of the plant, exergy analysis in every branch of the plant is performed and the results corresponding to the base case system in Table 2 are shown in Fig. 3. The results show that the largest exergy destruction rate occurring in the gasifier is 4.20 kW or 44.0% of the total exergy destruction rates mainly caused by intrinsic irreversibility. The SOFC is also responsible for large exergy destruction, which is 2.63 kW mainly due to irreversibilities associated with the electrochemical reactions. Fig. 3 exhibits that the burner, HRSG, HX1 and HX3 are responsible for 13.6, 8.5, 2.5 and 2.1% of the total exergy destruction rate, respectively, and those of the other system components account for less than 1% of the total exergy destruction rate. This implies that the exergy losses in gasifier and SOFC are two central units with larger exergy losses than the other sections. Therefore, the SOFC and gasifier are key elements to the improvement of system efficiency. 4.1. The influence of steam-to-biomass ratio The gasifier operating temperature is kept constant at 800 C in all of the case studies, while the small portion of air is adjusted to Table 2 Operational parameter values for the base case system. Parameters Biomass fuel data Biomass composition (dry. ash-free basis) Moisture content M.C. (e) LHVbio (kJ/kg. wet basis) Gasifier input data Gasifier operational temperature ( C) Steam-to-biomass ratio (kg/kg. wet basis) Air inlet temperature to gasifier ( C) Steam input temperature ( C) System input data Anode off-gas recycle ratio AGR (-) Fuel utilization factor Uf (e) Air utilization factor Ua (e) Exhaust gas temperature ( C) Pump isentropic efficiency hpump (e) Air compressor isentropic efficiency hair,com (e) Fuel compressor isentropic efficiency hfuel,com (e) Pump mechanical efficiency hpump,me (e) Air compressor mechanical efficiency hair,com,me (e) Fuel compressor mechanical efficiency hfuel, com,me (e) Inverter efficiency hinv (e) Stack input data Average current density (iave) (A m2) Number of channel per SOFC cell Air inlet temperature to the SOFC Tair,in ( C) Fuel inlet temperature to the SOFC Tfuel,in ( C) Steam-to-carbon ratio (e) Cell length (mm) Width covered by one channel (mm) Air channel height (mm) Fuel channel height (mm) Anode thickness (mm) Cathode thickness (mm) Electrolyte thickness (mm)
Value CH1.4OH0.59N0.0017 0.20 15,455 800 1.5 25 250 0 0.75 0.35 100 0.95 0.75 0.75 0.98 0.98 0.98 0.95 4000 20 700 700 2.0 100.0 5.0 1.5 0.4 500.0 50.0 10.0
319
Fig. 3. Local exergy destruction rates of the base case system.
maintain the set gasification temperature. Among various gasification agents, steam gasification is the most energy demanding process. In this study, steam and air are added, while exhaust gas supplies direct heat for biomass drying to achieve a high thermodynamic efficiency. As shown in Fig. 2, the syngas produced in four STBR study cases (0.5, 1.0, 1.5 and 2.0) locate below the carbon boundary to avoid carbon deposition. Modeling results summarized in Table 3 show that addition of steam yields steam-rich syngas, thus lowering the LHV of species and the cold gas efficiency. The increase of STBR results in a rapid decrease of CO concentration in the produced syngas due to shift reaction. It should be noted that the increasing steam content in syngas also reduces in the amount of external steam used for raising steam-to-carbon ratio in the SOFC feed gas to 2 [15]. To illustrate how STBR affects SOFC, gasifier and system performances and the number of cells, Fig. 4 is plotted and shows their sensitivity to STBR with the variation from 0.5 to 2.0. As can be seen in Fig. 4(a), the SOFC energetic and rational efficiencies change from 39.9 to 43.1% and 55.6 to 67.0%, respectively. The optimized STBR is 1.5 when SOFC energetic and rational efficiencies are the maximum of 43.5 and 67.3%, respectively. As shown in Fig. 4(b), increasing STBR deteriorates cold gas efficiency from 80.8 to 73.4% mainly due to steam dilution of the produced syngas, while gasifier rational efficiency shows insignificant change. The gasifier rational efficiencies are approximately 77%. The effects of STBR to overall system performance is shown in Fig. 4(c) that with increasing STBR, overall system and rational efficiencies varies from 35.1 to 37.9% and 33.9 to 36.8%, respectively. However, the difference of the system performance between STBR of 1.5 and 2.0 is negligibly small. Since the required number of cell corresponds to overall system efficiency, as shown in Fig. 4(d), the smallest required number of cell of 173 was found at STBR of 1.5 when the overall system efficiency was maximized.
Table 3 Gas compositions after the gas cleaning system for different steam-to-biomass ratios and heating values. Syngas compositions (vol%, w.b.) @800 C STBR ER H2 CO CO2 CH4 H2O N2 LHV (MJ Nm3)
0.5 0.089 0.355 0.263 0.107 0.008 0.132 0.135 8.576
1.0 0.088 0.332 0.162 0.135 0.009 0.255 0.108 7.960
1.5 0.096 0.291 0.106 0.141 0.008 0.357 0.098 7.414
2.0 0.095 0.263 0.076 0.136 0.008 0.434 0.084 7.203
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S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322 Table 4 SOFC performance comparison on SOFC inlet temperature effects.
Fig. 4. The influence of steam-to-biomass ratios on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
4.2. The influence of SOFC inlet stream temperatures The SOFC inlet stream temperatures are key design parameters of a system, because they affect the temperature distribution through heat transfer phenomena in the SOFC and eventually affect its performance [4]. To safely operate SOFC system and to obtain high system efficiency, the SOFC cell temperature distributions should be monitored to avoid severe operating condition. SOFC performance under co-flow operation is chosen in this study, because it generally has more uniform temperature distribution than other flow configurations [14]. The cell temperature distributions of the four cases are shown in Fig. 5. The four stream inlet temperatures are 650, 700, 750 and 800 C. In this study, the maximum allowable temperature gradient and the maximum allowable cell temperature, well known as the most important operational constraints for the planar SOFC, are set at 1127 C and 5 C/mm, respectively, following Stiller et al. [26]. The SOFC cell temperature profiles for the four different SOFC inlet stream temperature cases are depicted in Fig. 5. The electrochemical oxidation of H2 and the shift reaction are the major reactions proceed in the fuel passage and release reaction heat resulting in the cell temperature rising near the gas inlets. Fig. 5 shows that the peaks of the cell temperature profiles move toward gas inlets as the stream inlet temperatures increase. The maximum cell temperatures (TPEN,max) and the maximum cell temperature gradients (vTPEN/vx)max corresponding to Fig. 5 are listed in Table 4.
Fig. 5. The SOFC cell temperature profiles for different SOFC inlet stream temperatures.
Tfuel,in ( C)
Tair,in ( C)
TPEN.max ( C)
(vTPEN/vx)max ( C/mm)
650 700 750 800
650 700 750 800
1019 1039 1070 1115
2.22 1.41 1.62 1.76
Although, in Table 4, all case studies operate safely under the material constraints, the most favorable SOFC operating condition is at SOFC inlet stream temperatures of 700 C when maximum temperature gradient is the smallest. Fig. 6 shows the influence of SOFC inlet stream temperatures on SOFC, gasifier and system performance and the number of cells. As shown in Fig. 6(a), with the increase of SOFC inlet temperature from 650 to 800 C, the SOFC energetic efficiency increases from 42.5 to 44.3%, while SOFC rational efficiency drops from 67.4 to 64.6%. On the other hand, in Fig. 6(b), the increase of SOFC inlet stream temperatures decreases cold gas and gasifier rational efficiencies from 77.4 to 74.4% and from 76.5 to 74.5%, respectively. This reduction in gasifier performance is accounted for by the decrease of exhaust heat, which is partially used for air and fuel pre-heaters in order to raise inlet stream temperatures of the stack. In Fig. 6(c), as the SOFC inlet stream temperatures increase, the overall system and system rational efficiencies decrease monotonously from 38.2 to 36.2% and from 37.1 to 35.0%, respectively, owing to prominent losses in gasifier. The study also shows in Fig. 6(d) the highest system performance at SOFC inlet stream temperatures of 650 C required the highest number of cell of 179, when the smallest number of cells is 169 at the highest SOFC inlet temperature. 4.3. The influence of fuel utilization factors (Uf) The fuel utilization factor (Uf) is an important parameter, which is closely related to the performance of SOFC and exhaust heat reflecting the gasifier performance. The proper amount of exhaust heat and unreacted fuel from electricity conversion process results in the efficient thermochemical conversion of biomass. The influence of Uf on SOFC, gasifier and system performance and the number of cells is shown in Fig. 7. As shown in Fig. 7(a), SOFC energetic efficiency increases monotonously from 32.5 to 48.8%,
Fig. 6. The influence of SOFC inlet stream temperatures STBR on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322
321
Fig. 7. The influence of fuel utilization factor on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
Fig. 8. The influence of anode off-gas recycle ratio on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
with the increase of Uf from 0.65 to 0.90. This is due to the fact that as the Uf is enhanced, electrochemical reaction rates increase, thus raising the electrical power output per cell. However, by considering exergy in the outgoing streams, SOFC rational efficiency is the maximum of 67.3% at the fuel utilization factor of 0.75. As can be seen in Fig. 7(b), with increasing Uf from 0.65 to 0.90, cold gas and gasifier rational efficiencies decrease monotonously from 83.2 to 71.5% and from 79.7 to 72.3%, respectively. When the fuel utilization factor increases the heat generation in burner decreases and consequently the exhaust heat utilized in the gasifier becomes lower leading to dilution of syngas by N2. The reductions of gasifier efficiencies are mainly due to poor quality syngas generated. The optimum fuel utilization factor of 0.75 for the system performance is shown in Fig. 7(c), where the overall system and system rational efficiencies are the highest at 37.9 and 36.7%, respectively. As can be seen in Fig. 7(d), at the fuel utilization factor of 0.75, the number of cells is the lowest of 173 among the cases studied. This is mainly due to the compensation between quality of syngas and SOFC efficiencies.
5. Conclusions
4.4. The influence of anode off-gas recycle ratio (AGR) Anode off-gas recycle is functioned to recirculate steam from anode off-gas to reduce the amount of external steam and also to bring up the SOFC fuel inlet temperature. By reducing the amount of external steam, more heat can be used to dry and preheat a wet feedstock before entering gasifier, thus increasing gasifier efficiency. Fig. 8 demonstrates the modeling results of the influence of AGR on SOFC, gasifier, system performance and the number of cells. As can be seen in Fig. 8(a), with the variation of AGR from 0 to 0.8, the energetic efficiency of SOFC slightly decreases from 43.5 to 41.3% owing to dilution effect of the hydrogen content in fuel stream, while variation in rational efficiency of SOFC is very insignificant approximately 67.3%. In Fig. 8(b), when AGR increases from 0 to 0.8, the cold gas and gasifier rational efficiencies increase monotonously from 75.8 to 77.2% and 75.3 to 78.4%, respectively. The optimum performance for the combined system (hsys ¼ 38.9%, jsys ¼ 37.4%) is achieved at AGR of 0.6. From an economic point of view, increasing AGR leads to increasing capital cost. Fig. 8(d) shows the number of cells increasing from 173 to 182 cells by implementing AGR of 0.8, mainly due to the decrease in SOFC energetic efficiency.
In this study, an integrated 5 kW SOFC-biomass gasification power generation system was proposed and its performance was assessed through numerical simulation. In order to achieve reliable results, the SOFC model was validated in previous work [14], whereas the gasifier model was validated against published data. Sensitivity analyses were carried out to give insight into the influence of the main variables on the system. The main parameters concerning the integration of SOFC technology and thermal process of biomass gasification are STBR, SOFC inlet stream temperatures, Uf and AGR. Their effects on SOFC, gasifier and system performances are investigated. For economical competitiveness of the combined system, the number of cells required for SOFC stack was also taken into consideration. From the analysis, the following conclusions are obtained: The increase of STBR shows positive effect of the performance of SOFC and the system while at STBR higher than 1.5 the effect becomes adverse. With the minimum number of SOFC cell and the highest system performance, the STBR was optimized at 1.5. Increasing SOFC inlet stream temperatures reduce to the amount of exhaust heat used for biomass gasification process leading to rapid decline of gasifier efficiencies. In the system studied, the fuel utilization factor of 0.75 is the optimum, when the number of cells is the lowest and the system efficiencies are the highest due to the optimal balance of the plant condition. Anode-off-gas recycle can boot the combined system performance, but at the same time the higher the AGR also requires bigger SOFC stack. The optimal performance of the combined system (hsys ¼ 38.9%, jsys ¼ 37.4%) is achieved at AGR of 0.6 References [1] B. Zhu, X.Y. Bai, G.X. Chen, W.M. Yi, M. Bursell, International Journal of Energy Research 26 (1) (2002) 57e66. [2] S. Baron, N. Brandon, A. Atkinson, B. Steele, R. Rudkin, Journal of Power Sources 126 (2004) 58e66. [3] P.V. Aravind, J.P. Ouweltjes, E. de Heer, N. Woudstra, G. Rietveld, Electrochemical and Solid-State Letters 11 (2008) B24e28. [4] J. Mermelstein, M. Millan, N. Brandon, Journal of Power Sources 195 (2010) 1657e1666. [5] M. Sucipta, S. Kimijima, K. Suzuki, Journal of Power Sources 174 (2007) 124e135.
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[6] L. Fryda, K.D. Panopoulos, E. Kakaras, Energy Conversion and Management 49 (2008) 281e290. [7] C. Bang-Møller, M. Rokni, Energy Conversion and Management 51 (2010) 2330e2339. [8] C. Bang-Møller, M. Rokni, B. Elmegaard, Energy 36 (2011) 4740e4752. [9] R. Toonssen, S. Sollai, P.V. Aravind, N. Woudstra, A.H.M. Verkooijen, International Journal of Hydrogen Energy 36 (2011) 10414e10425. [10] C. Athanasiou, E. Vakouftsi, F.A. Coutelieris, G. Marnellos, A. Zabaniotou, Chemical Engineering Journal 149 (2009) 183e190. [11] L. Fryda, K.D. Panopoulos, J. Karl, E. Kakaras, Energy 33 (2008) 292e299. [12] C.O. Colpan, F. Hamdullahpur, I. Dincer, Y. Yoo, International Journal of Hydrogen Energy 35 (2010) 5001e5009. [13] R. Desrosiers, in: T.B. Reed (Ed.), A Survey of Biomass Gasification, Solar Energy Research Institute, Colorado, 1979. [14] S. Wongchanapai, H. Iwai, M. Saito, H. Yoshida, Journal of Power Sources 204 (2012) 14e24. [15] E. Achenbach, Annex II: Modeling and Evaluation of Advanced Solid Oxide Fuel Cells, International Energy Agency, Germany, 1996. [16] J. Herguido, J. Corella, J. Gonzalez-Saiz, Industrial and Engineering Chemistry Research 31 (1992) 1274e1282.
[17] S. Jarungthammachote, A. Dutta, Energy 32 (2007) 1660e1669. [18] A. Melgar, J.F. Pérez, H. Laget, A. Horrillo, Energy Conversion and Management 48 (2007) 59e67. [19] C.R. Altafini, P.R. Wander, R.M. Barreto, Energy Conversion and Management 44 (2003) 2763e2777. [20] Z.A. Zainal, R. Ali, C.H. Lean, K.N. Seetharamu, Energy Conversion and Management 42 (2001) 1499e1515. [21] M.A. Khaleel, Z. Lin, P. Singh, W. Surdoval, D. Collin, Journal of Power Sources 130 (2004) 136e148. [22] M. Shukuya, A. Hammache, Introduction to the Concept of Exergy for a Better Understanding of Low-Temperature-Heating and High-Temperature-Cooling Systems, Espoo 2002, VTT Tiedotteita, Research Notes 2158. [23] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, Properties of Gases and Liquids, McGraw-Hill, New York, 1977. [24] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, second ed. Krieger Publishing Company, USA, 1995. [25] M.J. Prins, K.J. Ptasinski, F.J.J.G. Janssen, Chemical Engineering Science 58 (2003) 1003e1011. [26] C. Stiller, B. Thorud, S. Seljebø, Ø Mathisen, H. Karoliussen, O. Bolland, Journal of Power Sources 141 (2005) 227e240.
Contents lists available at SciVerse ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Performance evaluation of an integrated small-scale SOFC-biomass gasification power generation system Suranat Wongchanapai*, Hiroshi Iwai, Motohiro Saito, Hideo Yoshida Department of Aeronautics and Astronautics, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
h i g h l i g h t s < We evaluate an integrated small-scale SOFC-biomass gasification power generation system through energy and exergy. < We examine the effects of steam-to-biomass ratio, SOFC inlet stream temperatures, fuel utilization factor and anode off-gas recycle ration on system performance. < The results show the optimal operating parameters and their effects on SOFC stack, gasifier and system performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2012 Received in revised form 14 May 2012 Accepted 28 May 2012 Available online 5 June 2012
The combination of biomass gasification and high-temperature solid oxide fuel cells (SOFCs) offers great potential as a future sustainable power generation system. In order to provide insights into an integrated small-scale SOFC-biomass gasification power generation system, system simulation was performed under diverse operating conditions. A detailed anode-supported planar SOFC model under co-flow operation and a thermodynamic equilibrium for biomass gasification model were developed and verified by reliable experimental and simulation data. The other peripheral components include three gasto-gas heat exchangers (HXs), heat recovery steam generator (HRSG), burner, fuel and air compressors. To determine safe operating conditions with high system efficiency, energy and exergy analysis was performed to investigate the influence through detailed sensitivity analysis of four key parameters, e.g. steam-to-biomass ratio (STBR), SOFC inlet stream temperatures, fuel utilization factor (Uf) and anode offgas recycle ratio (AGR) on system performance. Due to the fact that SOFC stack is accounted for the most expensive part of the initial investment cost, the number of cells required for SOFC stack is economically optimized as well. Through the detailed sensitivity analysis, it shows that the increase of STBR positively affects SOFC while gasifier performance drops. The most preferable operating STBR is 1.5 when the highest system efficiencies and the smallest number of cells. The increase in SOFC inlet temperature shows negative impact on system and gasifier performances while SOFC efficiencies are slightly increased. The number of cells required for SOFC is reduced with the increase of SOFC inlet temperature. The system performance is optimized for Uf of 0.75 while SOFC and system efficiencies are the highest with the smallest number of cells. The result also shows the optimal anode off-gas recycle ratio of 0.6. Regarding with the increase of anode off-gas recycle ratio, there is a trade-off between overall efficiencies and the number of SOFC cells. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Biomass gasification System analysis Power generation system Exergy analysis
1. Introduction In pursuing of green power generation, biomass is recently receiving increased attention as an alternative to fossil energy. As
* Corresponding author. Tel./fax: þ81 75 753 5203. E-mail addresses: [email protected], (S. Wongchanapai).
[email protected]
0378-7753/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2012.05.098
one of the various biomass conversion processes, biomass gasification is a prominent conversion route for producing a renewable clean feedstock for power generation. Unlike conventional energy conversion devices, fuel cells are attracting significant attention as promising choices in the future owing to its high efficiency even at very small scale. Among fuel cells, high-temperature solid oxide fuel cell (SOFC) is the most suitable candidate for biomass power conversion technologies because of its high electricity conversion efficiency, fuel flexibility, tolerance to fuel contaminants, and its
S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322
operating temperature close to that in biomass gasification process. SOFC operation on biomass-derived gas is also proven to be feasible by several experimental studies [1e4], while the experimental work on the combination of SOFC and biomass gasification system is still limited. Therefore, in order to investigate integrated SOFC-biomass gasification systems, numerical simulation are very helpful in providing theoretical guidance for optimization of design. As several numerical studies focused on biomass-fueled integration of SOFC with gas turbine (GT) systems [5e9], it seems to be very attractive for intermediate- and large-scale (electrical output range of several hundred kWe to several hundred MWe) electricity production. Sucipta et al. [5] conducted a performance analysis of a tubular SOFC and recuperated micro GT hybrid system fueled with syngas coming from air gasification, oxygen gasification and steam gasification, though the actual performance of the gasification and gas cleaning systems was not included. Fryda et al. [6] investigated biomass air gasification integrated with an SOFC stack and micro GT systems; the combined system operated under pressure ratio of approximately 4 showed the highest efficiency of 40.6%. Bang-Møller et al. [7] evaluated three different CHP plant concepts based on biomass gasification using thermodynamic modeling combining zero-dimensional component models. Biomass gasifiers were coupled to a micro GT, an SOFC and a combined SOFC micro GT system. The biomass gasification system combined with the SOFC micro GT was found to be the most efficient owing to the utilization of excess fuel and heat from the SOFC stack in the micro GT. Furthermore, Bang-Møller et al. [8] later presented results from the energy and exergy analyses which indicate potential improvements of the CHP plant combining gasification, SOFC and micro GT. The optimized hybrid plant produced approximately 290 kWe at an electrical efficiency of 58.2% based on lower heating value (LHV). In a study preformed by Toonssen et al. [9], four different systems based on integrated SOFCGT and biomass gasification were modeled with the thermodynamic flow-sheeting program Cycle-Tempo to compare cold gas cleaning with hot gas cleaning processes and 100 kWe-scale with 30 MWe-scale systems. The study showed that the large-scale system based on high-temperature gas cleaning has the highest electrical exergy efficiency of 49.9%. Athanasiou et al. [10] compared the production of electricity from the gasifier through a conventional steam turbine to the integrated process of gasifierSOFC-steam turbine. The system electrical efficiency was found to be 43.3%. In a study by Fryda et al. [11], a gasifier is coupled to an SOFC through heat pipes; the heat pipes supply the heat extracted from the SOFC to the gasification system. On this system, they performed an exergy analysis and found an exergetic electrical efficiency of 36% for a net electrical power production of 170 kW. Recently, with improved sealing materials and sealing concepts, small-scale 1e25 kW size planar SOFC stacks have been successfully developed by various organizations. In this scale of electricity generation, SOFCs produce the highest electrical efficiency compared to other energy conversion devices. This makes an integrated small-scale SOFC-biomass gasification power generation system very attractive for residential applications. In such smallscale SOFC based system, exhaust fuel and heat of SOFC can be effectively utilized for gasification process to enhance system performance. Colpan et al. [12] studied the effect of the gasification agent (air, enriched oxygen and steam) on the performance of an integrated biomass gasification and SOFC system. The studied system includes biomass gasification, hot gas clean up and a 10 kWclass SOFC. The study indicated that steam as the gasification agent yields the highest electrical and efficiencies of 41.8% and 39.1%, respectively.
315
The aim of this study is to determine the optimum performance of an integrated small-scale SOFC-biomass gasification power generation system for safe and efficient system operations as well as economic solution. In this paper, the integration of a biomass gasification and 5 kW-class SOFC power system is evaluated through numerical simulation. The biomass fuel considered in this work is represented by ash-free typical wood fuel formula of CH1.4O0.59N0.0017 [13]. A sensitivity analysis was carried out to achieve a better understanding of the influence of key parameters e.g. steam-to-biomass ratio (STBR), SOFC inlet stream temperatures, fuel utilization factor (Uf) and anode off-gas recycle ratio (AGR) on the performance of key system components. By performing energy and exergy analysis, the causes of exergy losses were revealed to identify the areas of improvement of the combined system. Since SOFC stack is accounted for the most expensive part of the initial investment cost, the number of cells required for SOFC stack is also taken into consideration as well. 2. System configuration and description The schematic of the integrated SOFC-biomass gasification power generation system in this study is shown in Fig. 1. A 1D model of direct internal reforming planar SOFC, capable of internal reforming of the methane in the syngas into hydrogen, was developed in the previous work by the authors [14] and applied to this study. On the other hand, thermodynamic equilibrium model is used for biomass gasification. The other peripheral components include three gas-to-gas heat exchangers (HX1, HX2 and HX3), heat recovery steam generator (HRSG), burner, pump, fuel and air compressors are thermodynamically modeled under steady-state operational conditions. In the integrated system, depleted fuel and air from the cell stack were combusted in the burner and supplied heat to wet biomass drying process to upgrade the heating value of the produced gas called “syngas”. The steam from drying process is mixed with additional steam and air and then directed to gasification process. The syngas produced by gasification generally contains some amount of tar, sulfur and other contaminants, which may lead to degradation of SOFCs. Consequently, a hot gas clean up is facilitated. After syngas is cleaned through hot gas cleaning unit, the fuel stream is cooled down in HX1 by preheating the air, then entering a fuel compressor served as suction blower to overcome pressure drops in the gasifier and SOFC systems. To prevent the carbon deposition in the SOFC, before clean syngas entering the cell, the steam-to-carbon ratio is set at 2 [15] by adjusting the external steam from HRSG and anode off-gas recycle ratio (AGR). The SOFC air and fuel streams are preheated by the cathode and anode off-gas in HX2 and HX3 and are heated up to 700 C. An inverter is also used in the system to convert the DC power output of the SOFC stack into AC power output. The HRSG uses the heat from the flue gas to generate steam for the gasification and SOFC anode gas moistening. The flue gas is released to the environment at atmospheric pressure and cooled down to 100 C. For all of the HXs and the HRSG, 2% heat losses of heat transferred are assumed. Possible variations in pressure drops across each unit operation are assumed 2%, except in SOFC is assumed 3%. 3. System modeling The integrated system components, e.g. SOFC, Gasifier, HXs, burner, HRSG, air and fuel compressors, were thermodynamically modeled under steady-state operations. Mass and energy balances were applied for each component using lumped models, which consider each component as a control volume with the exception of
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Fig. 1. Schematic flow diagram of an integrated SOFC-biomass gasification power generation systems.
a 1D SOFC model [14]. The assumptions made for developing the system model are given as: (i) all system components operate under steady-state conditions; (ii) all gas mixtures behave as ideal gases; (iii) heat losses from SOFC and gasifier to the environment are negligible and heat losses in HXs and the HRSG are assumed 2% of heat transferred; (iv) chemical reactions are assumed to be in equilibrium. 3.1. Gasifier modeling In this paper, a steady-state equilibrium model was developed to predict the product gas from the biomass gasification with mixed air-steam. To produce high quality syngas rich in H2, the amount of steam as oxidizing agent was varied, while the relatively small amount of air is tuned to sustain the operating temperature to the desired point by partial oxidation. Generally, at sufficiently high gasifying temperatures, tar production can be negligibly eliminated from gasification products. Since the model does not take into account tars formation in the gasifier, biomass gasifier operate at 800 C under near ambient pressure, and syngas outgoing temperature of 800 C is assumed. The carbon-hydrogen-oxygen (C-H-O) ternary diagrams have been constructed as shown in Fig. 2. The biomass fuel considered in this work is represented by ash-free typical wood fuel formula of CH1.4O0.59N0.0017. In ternary C-H-O diagram, the solid line presents, the so-called carbon deposition boundary. Since the biomass fuel is located in the carbon deposition region, oxygen, steam are used as gasifying agent to bring down chemical equilibrium below carbon deposition boundary lines. Fig. 2 shows that the increase of steam as oxidizing agent (STBR increases) in this study minimizes the risk for carbon deposition. Gasification is a partial-oxidation process for the conversion of carbonaceous feedstocks to combustible gas mixtures consisting primarily of carbon monoxide (CO), hydrogen (H2), and methane (CH4). The global gasification reaction for CHaObNc can be written as follows:
CHa Ob Nc þ mH2 O þ lO2 þ 3:76lN2 ¼ n1 H2 þ n2 CO þ n3 CH4 þ n4 CO2 þ n5 H2 O þ n6 N2
(1)
where m is number of moles of water vapor. Gasification process consists of two stages. In the first stage, pyrolysis releases the volatile components of the organic compounds and results in char. In the second stage, the carbon in the char is reacted with steam, air, or pure oxygen. It is reported that the gas compositions is dominated by the wateregas shift reaction at the higher temperature >750 C in biomass gasification process [16]. Wateregas shift reaction:
Fig. 2. CeHeO ternary diagram with carbon deposition boundary at 800 C, 1 atm.
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CO þ H2 O4CO2 þ H2
(2)
Methane is formed through the following exothermic reaction. Carbon hydrogenation reaction:
C þ 2H2 4CH4
(3)
The above two reactions are the major reactions that occur in gasification process [17,18]. The equilibrium constant for wateregas shift and carbon hydrogenation reactions are:
K1 ¼
PCO2 PH2 n n ¼ 4 1 PCO PH2 O n2 n5
(4)
PCH4 n3 2 ¼ 2 ntot n1
(5)
K2 ¼
PH2
KðTÞ ¼ exp½ DGo =RT
(6)
The main operating parameters are steam-to-biomass ratio (STBR) and equivalence ratio (ER). They refer to the amount of gasifying agents affecting the performances of gasifier. The steam-to-biomass ratio (STBR) can be defined as:
STBR ¼
_ bio;moisture _ steam þ m m _ mbio;d:b:
(7)
_ steam is the mass flow rate of the steam, m _ bio;moisture is Here, m _ bio;d:b: is the the mass flow rate of the moisture in biomass and m mass flow rate of the dry biomass. The equivalence ratio (ER) can be defined as:
ER ¼
_ air m _ air;stoic m
(8)
_ air is the mass flow rate of air and m _ air;stoic is the mass Here, m flow rate of air required for stoichiometric combustion. 3.1.1. Gasifier model validation In order to evaluate the above models, simulation results are compared with experimental and numerical results from literature. The simulations were preformed by setting the same conditions as the experiments of Altafini et al. [19] and Zainal et al. [20]. The results are compared in Table 1. A good agreement exists between this model and the two models in the literature indicating that the equilibrium model predicts reasonably well the producer gas for a gasifier.
The direct internal reforming SOFC model developed in the recent work by the authors [14] is applied to the present system Table 1 The comparison between model predictions and measurements for two biomass gasification processes. Present Altafini et al. [19] Present Zainal et al. [20] model model Model Experiment Model Experiment 18.70 21.87 0.22 10.51 47.30 5.31 1.11
(9)
Wateregas shift reaction:
CO þ H2 O4CO2 þ H2
(10)
Overall cell reaction:
1 H2 þ O2 /H2 O 2
(11)
The open-circuit voltage (VOC), is described by the Nernst equation as a function of operating temperature (T) and partial pressure (p):
VOC
0 1 1=2 RT @pH2 pO2 A þ ln ¼ 2F 2F pH2 O
DG0
(12)
where Faraday constant (F) ¼ 96,485 A/mol. The terminal voltage of the single cell can be obtained as follows:
V ¼ VOC Vact Vohm Vconc
(13)
The terminal voltage of the cell is decreased from its opencircuit voltage when the current is drawn from the cell because of tree types of overpotential losses, i.e. activation (Vact), ohmic (Vohm) and concentration (Vconc) losses. However, the details of the direct internal reforming planar SOFC has been the subject of earlier work [14] and will not be described here. The SOFC stack’s power output is:
PSOFC ¼ hinv VI
(14)
where hinv is the inverter efficiency and the current (I) is I ¼ 2Fðn_ H2 ;react þ n_ CO;react þ 4n_ CH4 ;react Þcell .
3.2. SOFC modeling
H2 (vol%, d.b.) CO (vol%, d.b.) CII4 (vol%, d.b.) CO2 (vol%, d.b.) N2 (vol%, d.b.) HHV (MkJ m3) RMS Err (%)
analysis. The model is capable of capturing the distribution of the local temperatures, species concentrations, current density, and polarization losses in streamwise direction. The electrochemical reaction is considered to be attributed to only hydrogen; the electrochemical fuel value of CO is readily exchanged for hydrogen by the rapid shift reaction assuming chemical equilibrium [21]. In other words, CO is considered to take part only in the shift reaction. Whereas the electromotive force (EMF) of the SOFC is calculated according to the electrochemical oxidation of H2, the species’ consumption and production is determined collectively from the reactions (9) and (10). Mass balances are formulated for each species linking the local current with the change in the concentrations. The following three equations summarize the reactions considered in the cell. Steam reforming reaction:
CH4 þ H2 O4CO þ 3H2
The equilibrium constant is determined by Gibbs free energy as a function of temperature as follows:
317
20.06 19.70 0.00 10.15 50.10 5.04 1.14
14.00 20.14 2.31 12.06 50.79 5.28
19.82 23.42 0.29 12.66 43.80 5.60 1.04
21.06 19.61 0.64 12.01 46.68 5.42 1.54
15.23 23.04 1.58 16.42 42.31 5.49
3.3. System parameters and efficiencies Anode off-gas recycle (AGR) is a system concept where anode off-gas is recirculated to the anode inlet to provide water vapor and heat to the anode feed gas. The amount of AGR is defined on a molar basis as:
AGR ¼
n_ recycle n_ anodeoff
(15)
Since the fuel is not completely consumed by electrochemical reactions in anode channel, the excess fuel flow rate can be determined by the fuel utilization factor (Uf), a key parameter investigated in this study. The fuel utilization factor is defined here as the
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X
X
ratio of fuel consumed by anode reactions to the fuel entering anode channels, and is expressed as:
exch ¼
n_ H2 ;react þ n_ CO;react þ 4n_ CH4 ;react Uf ¼ n_ H2 ;anode þ n_ CO;anode þ 4n_ CH4 ;anode
where ex0,i represents the standard molar chemical exergy of the ith species, assuming a reference atmospheric composition given by Kotas [24]. For biomass fuel, thermodynamic properties are not available. Therefore, the statistical correlation of Szargut and Styrylska was used [25] is applied:
(16) in
To comprehensively evaluate biomass gasification, both cold gas and exergy efficiencies are introduced. In energy conversion processes, the efficiencies can be defined in many ways. In this study we defined two kinds of efficiencies. Energetic efficiency or fuel efficiency and the definition of rational efficiency for steadystate processes are as follows [22]. 3.3.1. Energetic efficiency (h) The energetic efficiency of a gasification process, generally known as the cold gas efficiency(hcg), is the ratio between the chemical energy content in the produced syngas compared to the chemical energy in the original biomass fuel. The equation can be expressed as:
hcg ¼
_ syn LHVsyn m 100% _ bio: LHVbio þ Qin m
(17)
SOFC energetic efficiency (hSOFC) is the ratio of net electrical output generated from SOFC to energy content fed to the anode channel of the SOFC. The equation can be expressed as:
hSOFC ¼
P CH4 ;CO;H2
PSOFC 100% _ anode LHVÞin ðm
(18)
The energetic efficiency for power generation system is also called overall system efficiency (hsys). It is the ratio of net output energy to energy content of biomass fed to the gasifier and can be expressed as:
hsys
PSOFC ¼ 100% _ bio: LHVbio Þin ðm
(19)
In order to gain more insight into the system, exergy analysis is used as an assessment tool to identify and locate irreversibility. Exergy associated with material stream is equal to the maximum amount of work obtainable when the stream is brought from its initial state to the dead state by processes. Exergy transfer rate associated with material stream can be divided into physical and chemical exergy components:
Ex ¼ n_ exph þ exch
(20)
Physical exergy is the work obtainable by taking the substance through reversible processes from its initial state temperature T and pressure P, to the environmental state. It can be expressed as:
exph ¼
X
xi ½ðh h0 Þ T0 ðs s0 Þi
xi ex0;i þ RT0
i
xi ln ðxi Þ
Exch;bio ¼ bLHVbio
(23)
where LHVbio is the lower heating value of the biomass and b is the ratio of the chemical exergy to the LHV of the organic fraction of biomass.
b¼
1:044þ0:0160H=C0:3493O=Cð1þ0:0531H=CÞþ0:0493N=C 10:4124O=C (24)
where H/C, O/C, and N/C represent the atomic ratios in the biomass. The exergy transfer connected with the heat transfer rate Q at temperature T can be expressed as:
ExTQ ¼ Q
T T0 T0
(25)
Exergy balance can be represented in the following from considering exergy values of entering and leaving material streams of a control volume at steady state:
X
Ex þ
in
X
ExTQ ¼
X
Ex þ
X
out
in
ExTQ þ ExW þ ExD
(26)
out
where ExW and ExD represent electricity exergy and exergy destruction rate, respectively. 3.3.2. Rational efficiency (j) The rational or rational exergetic efficiency is defined by Kotas [24] as a ratio of the desired exergy output to the exergy used or consumed. It can be applied as equations below. Gasifier rational efficiency (jgas) is the ratio of the exergy of produced syngas to the sum of exergy associated with heat, biomass and gasifying agents entering the gasifier. The equation can be expressed as:
jgas ¼
Exsyn Exbio þ
T ExQgas in
þ Exair þ Exsteam
100%
(27)
SOFC rational efficiency (jSOFC) is the ratio of the produced electricity exergy to the exergy comsumed by SOFC. The equation can be expressed as:
jSOFC ¼ P
Exfuel þ Exair
(21)
PSOFC P Exfuel þ Exair
in
100% out
(28)
i
where xi represents the molar fraction of the i-th component, h is the specific enthalpy and s the specific entropy and the properties indicated with the subscript 0 refer to the environmental state. Enthalpy and entropy values are evaluated based on temperature, according to the thermodynamic property data available from the literature [23]. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environmental state, defined by the parameters T0 and P0, to the reference state by processes involving heat transfer and exchange of substances only with the environment. The chemical exergy for mixtures can be calculated as follows:
(22)
i
System rational efficiency (jsys) is the ratio of the produced electricity exergy to the exergy consumed by the system. The equation can be expressed as:
jsys ¼
PSOFC 100% Exch;bio Exexhaust
(29)
4. Results and discussion To understand the operational scenarios of the integrated SOFCbiomass gasification power generation system, an independent
S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322
parameter analysis of a single component is not enough to assess the whole system because each component in the system affects one another. In this study, sensitivity analysis is used to quantify the effects of STBR, SOFC inlet stream temperatures, Uf and AGR on SOFC, gasifier and system performances as well as the size of SOFC stack. The required number of cells for the specified stack power is determined by the cell operating voltage, current density and fuel utilization factor assuming the cell geometry shown in Table 2. As the active electrode area of a single cell is 100 cm2, the total cell area in the stack can easily be calculated. The input operational parameter values, as presented in Table 2, are used as constant throughout the study unless mentioned specifically. In order to determine how much each component contributes to the total irreversibility of the plant, exergy analysis in every branch of the plant is performed and the results corresponding to the base case system in Table 2 are shown in Fig. 3. The results show that the largest exergy destruction rate occurring in the gasifier is 4.20 kW or 44.0% of the total exergy destruction rates mainly caused by intrinsic irreversibility. The SOFC is also responsible for large exergy destruction, which is 2.63 kW mainly due to irreversibilities associated with the electrochemical reactions. Fig. 3 exhibits that the burner, HRSG, HX1 and HX3 are responsible for 13.6, 8.5, 2.5 and 2.1% of the total exergy destruction rate, respectively, and those of the other system components account for less than 1% of the total exergy destruction rate. This implies that the exergy losses in gasifier and SOFC are two central units with larger exergy losses than the other sections. Therefore, the SOFC and gasifier are key elements to the improvement of system efficiency. 4.1. The influence of steam-to-biomass ratio The gasifier operating temperature is kept constant at 800 C in all of the case studies, while the small portion of air is adjusted to Table 2 Operational parameter values for the base case system. Parameters Biomass fuel data Biomass composition (dry. ash-free basis) Moisture content M.C. (e) LHVbio (kJ/kg. wet basis) Gasifier input data Gasifier operational temperature ( C) Steam-to-biomass ratio (kg/kg. wet basis) Air inlet temperature to gasifier ( C) Steam input temperature ( C) System input data Anode off-gas recycle ratio AGR (-) Fuel utilization factor Uf (e) Air utilization factor Ua (e) Exhaust gas temperature ( C) Pump isentropic efficiency hpump (e) Air compressor isentropic efficiency hair,com (e) Fuel compressor isentropic efficiency hfuel,com (e) Pump mechanical efficiency hpump,me (e) Air compressor mechanical efficiency hair,com,me (e) Fuel compressor mechanical efficiency hfuel, com,me (e) Inverter efficiency hinv (e) Stack input data Average current density (iave) (A m2) Number of channel per SOFC cell Air inlet temperature to the SOFC Tair,in ( C) Fuel inlet temperature to the SOFC Tfuel,in ( C) Steam-to-carbon ratio (e) Cell length (mm) Width covered by one channel (mm) Air channel height (mm) Fuel channel height (mm) Anode thickness (mm) Cathode thickness (mm) Electrolyte thickness (mm)
Value CH1.4OH0.59N0.0017 0.20 15,455 800 1.5 25 250 0 0.75 0.35 100 0.95 0.75 0.75 0.98 0.98 0.98 0.95 4000 20 700 700 2.0 100.0 5.0 1.5 0.4 500.0 50.0 10.0
319
Fig. 3. Local exergy destruction rates of the base case system.
maintain the set gasification temperature. Among various gasification agents, steam gasification is the most energy demanding process. In this study, steam and air are added, while exhaust gas supplies direct heat for biomass drying to achieve a high thermodynamic efficiency. As shown in Fig. 2, the syngas produced in four STBR study cases (0.5, 1.0, 1.5 and 2.0) locate below the carbon boundary to avoid carbon deposition. Modeling results summarized in Table 3 show that addition of steam yields steam-rich syngas, thus lowering the LHV of species and the cold gas efficiency. The increase of STBR results in a rapid decrease of CO concentration in the produced syngas due to shift reaction. It should be noted that the increasing steam content in syngas also reduces in the amount of external steam used for raising steam-to-carbon ratio in the SOFC feed gas to 2 [15]. To illustrate how STBR affects SOFC, gasifier and system performances and the number of cells, Fig. 4 is plotted and shows their sensitivity to STBR with the variation from 0.5 to 2.0. As can be seen in Fig. 4(a), the SOFC energetic and rational efficiencies change from 39.9 to 43.1% and 55.6 to 67.0%, respectively. The optimized STBR is 1.5 when SOFC energetic and rational efficiencies are the maximum of 43.5 and 67.3%, respectively. As shown in Fig. 4(b), increasing STBR deteriorates cold gas efficiency from 80.8 to 73.4% mainly due to steam dilution of the produced syngas, while gasifier rational efficiency shows insignificant change. The gasifier rational efficiencies are approximately 77%. The effects of STBR to overall system performance is shown in Fig. 4(c) that with increasing STBR, overall system and rational efficiencies varies from 35.1 to 37.9% and 33.9 to 36.8%, respectively. However, the difference of the system performance between STBR of 1.5 and 2.0 is negligibly small. Since the required number of cell corresponds to overall system efficiency, as shown in Fig. 4(d), the smallest required number of cell of 173 was found at STBR of 1.5 when the overall system efficiency was maximized.
Table 3 Gas compositions after the gas cleaning system for different steam-to-biomass ratios and heating values. Syngas compositions (vol%, w.b.) @800 C STBR ER H2 CO CO2 CH4 H2O N2 LHV (MJ Nm3)
0.5 0.089 0.355 0.263 0.107 0.008 0.132 0.135 8.576
1.0 0.088 0.332 0.162 0.135 0.009 0.255 0.108 7.960
1.5 0.096 0.291 0.106 0.141 0.008 0.357 0.098 7.414
2.0 0.095 0.263 0.076 0.136 0.008 0.434 0.084 7.203
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S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322 Table 4 SOFC performance comparison on SOFC inlet temperature effects.
Fig. 4. The influence of steam-to-biomass ratios on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
4.2. The influence of SOFC inlet stream temperatures The SOFC inlet stream temperatures are key design parameters of a system, because they affect the temperature distribution through heat transfer phenomena in the SOFC and eventually affect its performance [4]. To safely operate SOFC system and to obtain high system efficiency, the SOFC cell temperature distributions should be monitored to avoid severe operating condition. SOFC performance under co-flow operation is chosen in this study, because it generally has more uniform temperature distribution than other flow configurations [14]. The cell temperature distributions of the four cases are shown in Fig. 5. The four stream inlet temperatures are 650, 700, 750 and 800 C. In this study, the maximum allowable temperature gradient and the maximum allowable cell temperature, well known as the most important operational constraints for the planar SOFC, are set at 1127 C and 5 C/mm, respectively, following Stiller et al. [26]. The SOFC cell temperature profiles for the four different SOFC inlet stream temperature cases are depicted in Fig. 5. The electrochemical oxidation of H2 and the shift reaction are the major reactions proceed in the fuel passage and release reaction heat resulting in the cell temperature rising near the gas inlets. Fig. 5 shows that the peaks of the cell temperature profiles move toward gas inlets as the stream inlet temperatures increase. The maximum cell temperatures (TPEN,max) and the maximum cell temperature gradients (vTPEN/vx)max corresponding to Fig. 5 are listed in Table 4.
Fig. 5. The SOFC cell temperature profiles for different SOFC inlet stream temperatures.
Tfuel,in ( C)
Tair,in ( C)
TPEN.max ( C)
(vTPEN/vx)max ( C/mm)
650 700 750 800
650 700 750 800
1019 1039 1070 1115
2.22 1.41 1.62 1.76
Although, in Table 4, all case studies operate safely under the material constraints, the most favorable SOFC operating condition is at SOFC inlet stream temperatures of 700 C when maximum temperature gradient is the smallest. Fig. 6 shows the influence of SOFC inlet stream temperatures on SOFC, gasifier and system performance and the number of cells. As shown in Fig. 6(a), with the increase of SOFC inlet temperature from 650 to 800 C, the SOFC energetic efficiency increases from 42.5 to 44.3%, while SOFC rational efficiency drops from 67.4 to 64.6%. On the other hand, in Fig. 6(b), the increase of SOFC inlet stream temperatures decreases cold gas and gasifier rational efficiencies from 77.4 to 74.4% and from 76.5 to 74.5%, respectively. This reduction in gasifier performance is accounted for by the decrease of exhaust heat, which is partially used for air and fuel pre-heaters in order to raise inlet stream temperatures of the stack. In Fig. 6(c), as the SOFC inlet stream temperatures increase, the overall system and system rational efficiencies decrease monotonously from 38.2 to 36.2% and from 37.1 to 35.0%, respectively, owing to prominent losses in gasifier. The study also shows in Fig. 6(d) the highest system performance at SOFC inlet stream temperatures of 650 C required the highest number of cell of 179, when the smallest number of cells is 169 at the highest SOFC inlet temperature. 4.3. The influence of fuel utilization factors (Uf) The fuel utilization factor (Uf) is an important parameter, which is closely related to the performance of SOFC and exhaust heat reflecting the gasifier performance. The proper amount of exhaust heat and unreacted fuel from electricity conversion process results in the efficient thermochemical conversion of biomass. The influence of Uf on SOFC, gasifier and system performance and the number of cells is shown in Fig. 7. As shown in Fig. 7(a), SOFC energetic efficiency increases monotonously from 32.5 to 48.8%,
Fig. 6. The influence of SOFC inlet stream temperatures STBR on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
S. Wongchanapai et al. / Journal of Power Sources 216 (2012) 314e322
321
Fig. 7. The influence of fuel utilization factor on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
Fig. 8. The influence of anode off-gas recycle ratio on (a) SOFC energetic and rational efficiencies, (b) cold gas and gasifier rational efficiencies, (c) overall system and system rational efficiencies, and (d) the required number of SOFC cells.
with the increase of Uf from 0.65 to 0.90. This is due to the fact that as the Uf is enhanced, electrochemical reaction rates increase, thus raising the electrical power output per cell. However, by considering exergy in the outgoing streams, SOFC rational efficiency is the maximum of 67.3% at the fuel utilization factor of 0.75. As can be seen in Fig. 7(b), with increasing Uf from 0.65 to 0.90, cold gas and gasifier rational efficiencies decrease monotonously from 83.2 to 71.5% and from 79.7 to 72.3%, respectively. When the fuel utilization factor increases the heat generation in burner decreases and consequently the exhaust heat utilized in the gasifier becomes lower leading to dilution of syngas by N2. The reductions of gasifier efficiencies are mainly due to poor quality syngas generated. The optimum fuel utilization factor of 0.75 for the system performance is shown in Fig. 7(c), where the overall system and system rational efficiencies are the highest at 37.9 and 36.7%, respectively. As can be seen in Fig. 7(d), at the fuel utilization factor of 0.75, the number of cells is the lowest of 173 among the cases studied. This is mainly due to the compensation between quality of syngas and SOFC efficiencies.
5. Conclusions
4.4. The influence of anode off-gas recycle ratio (AGR) Anode off-gas recycle is functioned to recirculate steam from anode off-gas to reduce the amount of external steam and also to bring up the SOFC fuel inlet temperature. By reducing the amount of external steam, more heat can be used to dry and preheat a wet feedstock before entering gasifier, thus increasing gasifier efficiency. Fig. 8 demonstrates the modeling results of the influence of AGR on SOFC, gasifier, system performance and the number of cells. As can be seen in Fig. 8(a), with the variation of AGR from 0 to 0.8, the energetic efficiency of SOFC slightly decreases from 43.5 to 41.3% owing to dilution effect of the hydrogen content in fuel stream, while variation in rational efficiency of SOFC is very insignificant approximately 67.3%. In Fig. 8(b), when AGR increases from 0 to 0.8, the cold gas and gasifier rational efficiencies increase monotonously from 75.8 to 77.2% and 75.3 to 78.4%, respectively. The optimum performance for the combined system (hsys ¼ 38.9%, jsys ¼ 37.4%) is achieved at AGR of 0.6. From an economic point of view, increasing AGR leads to increasing capital cost. Fig. 8(d) shows the number of cells increasing from 173 to 182 cells by implementing AGR of 0.8, mainly due to the decrease in SOFC energetic efficiency.
In this study, an integrated 5 kW SOFC-biomass gasification power generation system was proposed and its performance was assessed through numerical simulation. In order to achieve reliable results, the SOFC model was validated in previous work [14], whereas the gasifier model was validated against published data. Sensitivity analyses were carried out to give insight into the influence of the main variables on the system. The main parameters concerning the integration of SOFC technology and thermal process of biomass gasification are STBR, SOFC inlet stream temperatures, Uf and AGR. Their effects on SOFC, gasifier and system performances are investigated. For economical competitiveness of the combined system, the number of cells required for SOFC stack was also taken into consideration. From the analysis, the following conclusions are obtained: The increase of STBR shows positive effect of the performance of SOFC and the system while at STBR higher than 1.5 the effect becomes adverse. With the minimum number of SOFC cell and the highest system performance, the STBR was optimized at 1.5. Increasing SOFC inlet stream temperatures reduce to the amount of exhaust heat used for biomass gasification process leading to rapid decline of gasifier efficiencies. In the system studied, the fuel utilization factor of 0.75 is the optimum, when the number of cells is the lowest and the system efficiencies are the highest due to the optimal balance of the plant condition. Anode-off-gas recycle can boot the combined system performance, but at the same time the higher the AGR also requires bigger SOFC stack. The optimal performance of the combined system (hsys ¼ 38.9%, jsys ¼ 37.4%) is achieved at AGR of 0.6 References [1] B. Zhu, X.Y. Bai, G.X. Chen, W.M. Yi, M. Bursell, International Journal of Energy Research 26 (1) (2002) 57e66. [2] S. Baron, N. Brandon, A. Atkinson, B. Steele, R. Rudkin, Journal of Power Sources 126 (2004) 58e66. [3] P.V. Aravind, J.P. Ouweltjes, E. de Heer, N. Woudstra, G. Rietveld, Electrochemical and Solid-State Letters 11 (2008) B24e28. [4] J. Mermelstein, M. Millan, N. Brandon, Journal of Power Sources 195 (2010) 1657e1666. [5] M. Sucipta, S. Kimijima, K. Suzuki, Journal of Power Sources 174 (2007) 124e135.
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