i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Biomass fueling of a SOFC by integrated gasifier: Study of the effect of operating conditions on system performance5 Gennaro Campitelli b, Stefano Cordiner a, Mridul Gautam b, Alessandro Mariani a, Vincenzo Mulone a,* a b Dipartimento di Ingegneria Industriale, Universita` di Roma “Tor Vergata” via del Politecnico 1, 00133 Roma, Italy Mechanical and Aerospace Engineering, West Virginia University ESB, Evansdale Drive, Morgantown, WV 26506-6106, USA article info abstract Article history: Biomass gasification can be efficiently integrated with Solid Oxide Fuel Cells (SOFCs) to Received 14 May 2012 properly deploy the energy content of this renewable source and increasing the ratio of Received in revised form electric to thermal converted energy The key objective of this work is to analyze in 21 September 2012 a systematic and wide process the integration of a biomass gasifier process with the SOFC Accepted October 2012 operation In particular the work aims at identifying the role of SOFC H2 utilization as Available online 30 October 2012 a basic parameter to maximize the system output and avoid gasifier bad operation issues such as tar production and carbon deposition An efficient simulation framework is used to Keywords: that purpose allowing for a detailed analysis of the influence of key driving parameters Solid oxide fuel cells The performance of the integrated system is thoroughly analyzed in the range of 1e2 kW Biomass gasification electric power by also varying the input biomass characteristics in terms of Moisture Fuel cell modeling Content (MC) Results show how a variation of the SOFC H2 utilization, a parameter whose Thermal integration effects are also correlated with the gasifier air requirement, affects electrical power output also depending on the biomass Moisture Content Copyright ª 2012, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights reserved Introduction Power production from biomass may represent a significant way to limit CO2 emissions and deploy the local availability of energy sources Biomass is, in fact, the fourth largest source of energy in the world, accounting for 15% of world’s primary energy consumption; this number is consistently higher, reaching 38%, in the developing countries [1,2] However, due to the limited volumetric energy content, the high potential of biomass is more suitable within the Distributed Generation (DG) power production concept that foresees the use of small size (from few to about 500 kWe) power plants In such a characteristic size, the combined generation of heat and power is of utmost importance to guarantee the best fuel exploitation, and reach the maximum as possible total efficiency htot ¼ hel þ hth which accounts for both thermal and electric use of the converted energy, commonly in excess of 0.8e0.9 Depending on the specific application, different values of the electrical to thermal power ratio may correspond to same total efficiencies htot, depending on both technology and system characteristic power size Several integration strategies are currently available, allowing for the use of biomass for energy conversion, which may either be based on traditional technology (i.e microturbines, internal combustion engines or Organic Rankine Cycles ORC turbines) having electric efficiency in the range of Presented at the ASME 4th European Fuel Cell Technology and Applications Conference, December 14e16, 2011 * Corresponding author Mechanical Engineering Dept., University of Rome “Tor Vergata”, via del Politecnico 1, 00133 Rome, Italy Tel.: þ39 06 72597170; fax: þ39 06 2021351 E-mail address: mulone@uniroma2.it (V Mulone) 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.ijhydene.2012.10.012 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 15e20% [3,4] or use H2 as an energy vector [5] Among the thermo-chemical conversion technologies, biomass gasification is a popular option giving high overall electric efficiency [2,6e8], especially as far as more complicated layouts are concerned, such as microturbine-fuel cell hybrid [9] An accurate system integration would nevertheless be required in this case to guarantee a better deployment of the biomass energy potential The combination of highly efficient Solid Oxide Fuel Cells (SOFCs) and gasification systems is an effective technology for reaching both energy and economic feasibility of combined heat and power production [10] In this case, in fact, the syngas can directly feed the fuel cell, which is tolerant to CO and able to directly or indirectly convert the CH4 contained in the producer gas In the integrated biomass gasifier e SOFC system, special attention must be given to the three main system components (gasifier, gas processing unit and fuel cell) while still maintaining the system complexity minimum as possible not compromising the long-term stability of the fuel cell at the same time [11] The coupling of SOFCs to gasifiers is discussed in several studies [12e16] Experimental and modeling activities are reported aiming at identifying the most efficient configuration of key operating parameters on the performances of the different components and at analyzing the influence of syngas composition on the fuel cell performance [17,18] From the modeling point of view, some papers describe the effect of basic syngas fueling on cell performance by using a 0D representation [16], whereas the influence of Fuel Cell design parameter is also evaluated in detail (either by multidimensional modeling or direct experimental testing) to understand the difference in operation when feeding the cell with hydrogen or methane or with syngas characterized by different composition Among the others, thermal integration of the different components is a key aspect in the development of sustainable solutions as it potentially allows for a considerable increase of electric efficiency hel [19,20] Depending on the design, gasification may be autothermal or allothermal whereas the SOFC may be operated under different fuel utilization conditions To the aim of sustaining the gasifier operation and producing syngas characterized by high energy content (e.g Lower Heating Value e LHV) as well as a more favorable H2/CO ratio [19e22], system optimal configuration may be different from what is required for the single component Moreover, solid carbon deposition issues may arise thus requiring the use of steam to avoid the rapid clogging of FC channels This issue may be the target of a specific optimization strategy while operating with standard biomass feedstock, which may generally be characterized by high Moisture Content (MC) and contain enough water to face with the cited issues The correct use of this water content is nevertheless a function of required energy for steam production to sustain the steam reforming regime This energy may be recovered from the SOFC off-gas cooling but requires specific design and control of the integrated systems However, in the cited papers, a key parameter, such as fuel utilization, and more specifically the utilization of H2 that is the main gaseous reactant responsible of electrochemical reactions especially at average (e.g 0.6e0.7 V) voltage operating conditions [23], is imposed as a constant [24], or the 321 impact of its variation is not even discussed [21] In this paper the role of this parameter is studied from the point of view of system efficiency and has been varied accordingly in order to evaluate the combined effects on fuel cell efficiency (which may increase if more H2 is available to the electrochemical reaction) and the corresponding losses at a system level Special focus has also been given to the analysis of the variation of feedstock characteristics, in terms of biomass MC, that may easily change during normal operation for biomass fueled power production systems To the presented aim, a 0-D model describing the power plant, including a gasifier, a SOFC and all the thermal exchangers providing integration heat, has been developed and implemented under the Matlab/Simulink environment The model has been used to make a wide screening of operating conditions and identify the most efficient ones The details of the used models for the single components and the overall system are given in the next section whereas results are illustrated in a specific section SOFC-gasifier integrated 0-D model A numerical model, that comprises two main sub-modules, has been implemented to represent the behavior of a SOFC coupled to an integrated downdraft gasifier In fact, updraft and downdraft technologies are usually preferred to fluidized bed for small size applications especially for temperature control issues [25] The downdraft technology was moreover selected for the low tar yield, that is particularly important for SOFC fueling [26,27] According to the system schematic provided in Fig 1, the downdraft gasifier is directly fed with wet biomass, and is thermally integrated by the SOFC offgases after combustion in the burner The system includes another heat exchanger to pre-heat the SOFC feeding air The 0-D approach has been selected, as it is characterized by affordable computational timings for optimization analyses The overall model has been implemented in MatlabSimulink computational environment Synthetic details of the two modules are given in the following two sub-sections 2.1 Gasifier module This module predicts the syngas chemical composition downstream of the gasifier in terms of gaseous species mass Fig e Schematic of the system: SOFC fed by integrated biomass gasifier 322 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 fraction and overall HHV as functions of the inlet MC of the woody biomass Chemical equilibrium is assumed for all reactions and implies that pyrolysis products are all consumed into the reduction zone before they leave it The global gasification reaction taken into account [28] may be written for woody material (CH1.44O0.66) as CH1:44 O0:66 þ wH2 O þ mO2 þ 3:76mN2 /x1 H2 þ x2 CO þ x3 CO2 Table e Volumetric heating value [MJ/m3] as a function of temperature [K]: experimental-numerical comparison in the case of MC [ 0.2 T 1023 1073 1123 1173 Model Experiment Error % 4.812 4.739 4.638 4.517 4.9 4.8 4.5 4.6 1.8 1.3 3.0 1.8 þ x4 H2 O þ x5 CH4 þ 3:76mN2 (1) where w is the amount of water per mol wood, related to the biomass MC as shown below w¼ 24MC 18ð1 À MCÞ m is the amount of oxygen per mol wood, and xi are the molar fractions of gaseous product unknowns The energy balance may be written instead as follows: dHfwood þ wdHH2 OðlÞ þ dHint ¼ x1 dHH2 þ x2 dHCO þ x3 dHCO2 þ x4 dHH2 OðvapÞ þ x5 dHCH4 þ 3:76mdHN2 (2) where dHfwood is the wood standard enthalpy of formation, while dHint is the integration heat term given by the burner under the assumption of exploiting the residual heating value of the syngas composed by H2, CO and CH4 The energy balance, along with elemental balances, equilibrium constants, and further chemical reactions such as methane formation and water gas shift reactions, constitute a three non-linear equation set Experimental data [29] in the case of adiabatic condition (dHint ¼ 0) have been used to validate this module The effect of the input biomass MC on the syngas composition in terms of dry basis volume fractions is shown in Fig First, CO decreases with MC increment as expected Hydrogen and carbon dioxide increase with MC, while methane fraction is negligible (1%) High N2 fraction is observed all over the entire MC range [30] Table shows the validation of this module against experimental data [29] This has been performed in terms of syngas heating value for different operating temperatures and MC equal to 0.2 For a wide temperature range where the SOFC is expected to operate, the heating value leads to appreciable predictive accuracy The availability of enough residence time (that is a function of the gasifier specific design) and temperature, allow chemical reactions to almost reach equilibrium, thus giving relatively low errors overall In summary, this module is capable of evaluating the amount of air required for gasification of the selected biomass and the syngas composition by only using biomass water content w, operating temperature and integration heat (dHint) as input data 2.2 SOFC module The SOFC module, that is based as mentioned on a 0-D approach, is capable of representing a planar stack under steady state operating conditions [19,20] The main assumptions are that the cell is isothermal, H2 is the only gaseous species participating to electrochemical reactions, and the SOFC H2 fuel utilization is a constant input parameter Water Gas Shift (WGS) reaction is also taken into account to calculate further H2 production via H2O in the SOFC anode This has been modeled at its equilibrium state with an operating temperature equal to the SOFC, and atmospheric pressure The equilibrium constant of reaction is evaluated by van’t Hoff isothermal assumption and according to reactants and products molar fractions Fig e MC effect on outlet gasifier composition (T [ 1073 K) 323 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 Kp ¼ Kc ¼ À Á0:25 À120$103 =RT  à J0 CAT ¼ 5:5$108 xO2 e A=m2 ½CO2 Š½H2 Š ½COŠ½H2 OŠ with Since residual moles may be computed as follows: xO2 ¼ nCO res ¼ nCO init ð1 À a1 Þ nH2 O res ¼ nH2 O init ð1 À a2 Þ l À mf ¼ nH, l À mf 0:21 xH2 O ¼ n , moles of H2 yielded with the WGS are H2 O init nH2 ¼ nCO init a1 ¼ nH2 O init a2 þn , init H2 init À mf _ TOT m _ TOT $mf m By solving this equation set and applying Hesse’s law, the new syngas composition available for the SOFC is obtained as well as enthalpy balance of the WGS reaction The cell voltage is defined as In the presented equations l is the air fuel ratio, which is usually in the range < l < [36,37] for syngas fueled SOFCs, _ TOT is the minimum syngas molar flow rate at the inlet while m SOFC section to allow for H2 oxidation nH, init and nH, O init are 2 the molar flow rates, for H2 and H2O respectively Concentration losses due to mass transfer limitations are taken into account according to the 0-D modeling approach and the fuel utilization The whole phenomenon has been represented by using two different formulas E ¼ EMF À DEOHM À DEACT À DECONC DECONC ¼ and the equilibrium constant can be expressed as KC ¼ nCO init a21 nH2 O init ð1 À a1 À a2 þ a1 a2 Þ (3) where EMF is the open circuit ideal voltage according to the Gibbs free energy at the imposed temperature, whereas modeling of the three losses is implemented via electrolyte resistance and thickness (DEOHM), Tafel equation (DEACT) and as a function of H2 partial pressure via the SOFC H2 utilization (DECONC) [19,20] Ohmic losses are computed as follows: DEOHM ¼ RE $Jt where Jt is the current density and RE is the electrolyte resistance defined as RE ¼ le se with le and se equal to electrolyte thickness and conductivity respectively The last one is evaluated by using the formula se ¼ b1 eÀb2 =T ½U$mŠÀ1 where b1 and b2 are coefficients describing the YSZ electrolyte behavior [22,31e35] Overpotential losses have been modeled for anode and cathode according to Tafel’s law  DEACT ¼ ACAT $ln Jt J0 CAT    Jt þ AAN $ln J0 AN where the cathode gives the main contribution, and ACAT/AAN are defined as ACAT ¼ AAN ¼ RT 2FgCAT RT 2FgAN Exchange current density values J0 are affected by molar fractions of species evolving at the anode and cathode respectively and thus the fuel utilization mf factor plays a key role for their evaluation À100$103 =RT J0 AN ¼ 1$10 xH2 xH2 O e  à A=m2 RT À $ln PH2 2F init =PH2 final Á DECONC ¼ m$en$Jt The first equation better represents concentration losses when current density is low and the major part of the H2 is still available The second equation instead is empirical and represents fairly well the partial pressure decrease effect at high current density due to mass transfer limits Data of a SOFC modeled with 1-D approach and fed with pure H2 [21] have been used for validation Voltage and current density at different temperatures and mf’s have been investigatedand are provided in Table It can be observed how the maximum power density [W/m2] output at isothermal condition (T ¼ 1073 K) matches reasonably well with the lower mf cases, while, for fixed mf cases (at 0.8), better results have been obtained at lower operating temperatures In summary, given the satisfactory agreement with a 1-D model, the SOFC 0-D module here presented has been considered capable of evaluating cell voltage, current density and H2, CO, CH4, residual molar fractions for a unit cell unit in planar configuration once given as inputs the electrolyte characteristics, operating temperature and syngas characteristics in terms of species molar fractions Table e Comparison of the 0-D SOFC model (current work) with a literature available [21] 1-D SOFC model in terms of power density [W/m2] as a function of temperature T [K] (at fuel utilization[0.8) and fuel utilization [$] (at temperature[1073K) T 0-D model 1-D model Error % 1023 1123 1223 mf 578.4 1602 2997 0-D model 600 1720 3250 1-D model 3.6 6.92 7.78 Error % 0.85 0.95 759.8 396.3 750 430 1.29 7.84 324 2.3 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 Algorithm of the numerical model The presented numerical model is composed by several modules interacting until the steady Gasifier-Solid Oxide Fuel Cell energy system operating point is attained A flow diagram of the numerical model is provided in Fig to better understand the solution algorithm Fig shows how air pre-heating and reforming require a certain amount of heat which has to be assured for the energy system sustainability If these two requirements are not satisfied, the input setting imposed is considered by the model as invalid to run the whole energy system Conversely, with an input setting which yields extra heat at the burner, the integration process at the gasifier is considered physically consistent Analysis of results The model has been applied to the analysis of the main performance parameters of the SOFC and the integrated system, when fueled with woody biomass The influence of biomass MC by varying fuel cell operating conditions has been analyzed to define the best compromise among power density and efficiency of SOFC and thermal balance of the system This is in fact directly influenced by the following operating aspects:  The gasifier regime, that is primarily defined, given the operating temperature, by the air mass flow rate, whose variation has a high impact on syngas composition and yield;  The SOFC H2 fuel utilization, which is an operating parameter linked to the electrochemical exploitation of the cell active area The SOFC active area has been considered equal to m2, while the woody biomass inlet flow has been set to about kg/h (dry basis) and constant, corresponding to an input thermal power in the range of 4.5 kWth The SOFC operating voltage has been controlled in the code to lie in the range 0.6e0.65 V, that gives an optimal compromise between cell power density and efficiency A first comparison between the performance of the autothermal and thermally integrated layouts has been analyzed by means of the system model at biomass MC ¼ 0.4 Results are given in Table 3, confirming the increase in efficiency of the integrated system (hsys,integ ¼ 37.7% vs hsys,autoth ¼ 24.7%) The higher system performance has been achieved despite the lower total fuel utilization, while the SOFC efficiency has been kept in the optimal range of 50% System efficiency increase has then been observed for the better gasifier operating regime, which allows for a different syngas composition, that is reported in Table A remarkable increase in terms of H2 concentration is in fact obtained with the integrated system (48.5% vs 17.8% on a volume basis) This has been achieved by a drop in gasifier air flow rate, that leads to both the decrease in N2 concentration, i.e no related transport losses occur in the SOFC, and to a lower O2 flow rate (almost vs 0.69 kg/h) that is possible as heat is provided by the combustion of SOFC off-gas, according to the schematic of Fig This basically means that the gasifier, due to the high biomass MC (0.4), is operating under “steam reforming like” regime with no O2, that would otherwise be unfeasible without the thermal integration Thus, the drop in SOFC fuel utilization is not indicative of fuel waste, but rather of better gasifier operation that eventually leads to higher system efficiency Three different MCs have then been further analyzed for the integrated system: 0.1, 0.3 and 0.5 by further assuming constant SOFC H2 utilization (0.8) The effect of MC is such that a higher electric power is delivered by the SOFC going from MC ¼ 0.1 to MC ¼ 0.3 conditions, meaning that fuel utilization is enough to thermally sustain the gasifier and allowing for a decrease in air (i.e O2) flow rate (red curve in Fig 4) The situation radically changes going from MC ¼ 0.3 to MC ¼ 0.5 conditions: in fact, MC is so high that the integration heat dHint is not enough to support the gasifier via the off-gas combustion This is also linked to the SOFC H2 utilization, that has been thus identified as primarily important Table e Comparison of system performance parameters between thermally integrated and autothermal gasifiers System performance parameter Fig e Whole numerical model flow diagram Current [A/m2] Voltage [V] mf hSOFC hSYS O2 [kg/h] H2 [%mol] CO [%mol] CO2 [%mol] H2O [%mol] CH4 [%mol] N2 [%mol] Integrated Autothermal 3045 0.64 70 51.3 37.7 0.004 48.5 28.3 9.7 12.4 0.7 0.4 2129 0.60 95 48.0 24.7 0.690 17.8 9.4 14.0 19.8 0.1 39.0 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 parameter Results are reported in Fig 5, where the effect of the variation of SOFC H2 utilization has been added It may be observed that, as a general trend, the lower SOFC H2 utilization operating conditions have to be avoided In fact, despite the theoretically high availability of integration heat, a high amount of air (O2 flow rate in the Figure) is required to let the SOFC operate at the selected operating voltage This also leads to a poor exploitation of the SOFC off-gas that is evident by the analysis of Fig 6, where dHint is reported as a function of SOFC H2 utilization and MC dHint represents the amount of the offgases enthalpy that is actually utilized in the biomass gasifier It thus appears that low energy is required by the gasifier to operate if air flow is sufficiently high, as in the already commented operating conditions with low SOFC H2 utilization and MC An increase in the electric power output is observable with H2 fuel utilization at any of the proposed MCs This is directly related to the decrease in required O2 flow rate (again Fig 5) At the same time, the integration heat dHint (again Fig 6) is increased, as an opposite effect to the decrease of O2 flow rate: in other words, the gasifier is moving more toward a “steam reforming like” operating regime However, a peak in dHint is observed, more evident for the higher MC values (e.g MC ¼ 0.5), as far as higher than 0.8 SOFC H2 utilizations are approached That peak indicates that a limit is achieved in terms of thermal integration potential that is due to two concurrent phenomena: 325 Fig e Electric power (kW) and required O2 mass flow (m3/h) as functions of SOFC H2 fuel utilization and biomass MC the gasifier air requirement are practically equivalent to optimize the SOFC H2 utilization 1) The heating value presented by the off-gas is decreased by the higher exploitation of the SOFC at higher SOFC H2 utilizations 2) Higher MCs allow the gasifier to produce syngas characterized by higher H2/CO ratio further giving much more favorable water gas shift behavior into the SOFC [19,20] This is key to both have the SOFC operating much closer to pure H2 fueling operating conditions, as well as having much lower off-gas waste 3) The peak in power is also obtained at the minimum in terms of O2 request, further testifying that criteria based on In Fig 7, the plot of current density is given, whose trend is exactly similar to the power curve plotted in Fig On the other hand efficiency, again in Fig 7, presents a slightly different trend since the biomass LHV (being the denominator of efficiency) is also affected by MC Thus, the difference between maximum efficiencies at 0.5MC and at 0.3MC is higher than the same difference in terms of power Finally, the link between air (O2) requested by the gasifier and integration heat (dHint) is extremely important, and the capability of the gasifier to work with minimum air request has to be sought, along with the control of SOFC operating voltage Among the cases reported in Fig 5, optimal operating conditions in terms of SOFC H2 utilization or O2 requested flow rate may be selected depending on the specific biomass MC, further confirming that high MC operating conditions should be preferred in terms of electric power maximization Fig e Electric power of FC (kW), H2 production (%) and required O2 mass flow (m3/h) as functions of MC at constant SOFC H2 fuel utilization equal to 0.8 Fig e Efficiency (%) and current density (A/m2) as functions of SOFC H2 fuel utilization and biomass MC 326 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( ) e3 F J K l n n_ R x Faraday coefficient current equilibrium constant electrolyte thickness moles molar flow rate electrolyte resistance molar coefficients Greek symbols b thermal conductivity coefficients h efficiency l air fuel ratio m utilization s thermal conductivity Fig e Integration heat as a function of SOFC H2 utilization and biomass MC Conclusions The operation of a SOFC fed by an integrated biomass gasifier has been demonstrated to highly depend on cell operating conditions, and specifically on SOFC H2 utilization, that is also correlated to gasifier air requirement, and the biomass Moisture Content (MC) In fact, gasifier operation, and more specifically its steam, autothermal or intermediate operating regime, determines a great difference in terms of SOFC flow rate and syngas composition Obtained results allow to draw the following main conclusions:  At constant cell operating voltage and MC operating conditions, the thermally integrated system gives much better performance in terms of power output and system electric efficiency (37.7% vs 24.7%) The MC (i.e available H2O) and integration heat are thus used to operate the gasifier more in the steam operating conditions rather than autothermal That is also indicated by the almost zero requirement in terms of gasifier air flow rate  System performances are highly dependent on SOFC H2 utilization, that has a direct impact on gasifier operating conditions via its thermal integration by the combustion of the SOFC off-gases  SOFC H2 utilization influences the system performance mainly by two effects: air flow rate is more important in the low SOFC H2 utilization end, where the selected cell voltage is achievable only with high air flow rate; the limitation in terms of integration heat dHint is instead more evident in the higher SOFC H2 utilization end where air is required to thermally sustain the gasifier Nomenclature dH E enthalpy variation cell voltage, overpotentials Subscripts and superscripts c, conc concentration e electrolyte el electric f fuel p pressure tot total th thermal references [1] Kalinci Y, Hepbasli A, Dincer I Biomass-based hydrogen production: a review and analysis 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