HYDROGEN FROM BIOMASS GAS STEAM REFORMING FOR LOW TEMPERATURE FUEL CELL: ENERGY AND EXERGY ANALYSIS

Transcription

1 Brazilia Joural of Chemical Egieerig ISSN Prited i Brazil Vol. 26, No., pp , Jauary - March, 29 HYDROGEN FROM BIOMASS GAS STEAM REFORMING FOR LOW TEMPERATURE FUEL CELL: ENERGY AND EXERGY ANALYSIS A. Sordi *, E. P. Silva 2,3, L. F. Milaez 3, D. D. Lobkov 2,3 ad S. N. M. Souza 4 Federal Techology Uiversity, Paraá, Rua Alagoas 2, , Lodria - PR, Brazil. alexadresordi@utfpr.edu.br 2 Uiversity of Campias, UNICAMP, Physics Istitute, Hydroge Laboratory, P.O. Box 639, , Campias - SP, Brazil. 3 Uiversity of Campias, UNICAMP, Faculty of Mechaical Egieerig, Departmet of Eergy, P.O. Box 622, , Campias - SP, Brazil. 4 State Uiversity of West Paraa, UNIOESTE, , Cascavel - PR, Brazil. (Submitted: September 28, 27 ; Accepted: July 4, 28) Abstract - This work presets a method to aalyze hydroge productio by biomass gasificatio, as well as electric power geeratio i small scale fuel cells. The proposed methodology is the thermodyamic modelig of a reactio system for the coversio of methae ad carbo mooxide (steam reformig), as well as the eergy balace of gaseous flow purificatio i PSA (Pressure Swig Adsorptio) is used with eight types of gasificatio gases i this study. The electric power is geerated by electrochemical hydroge coversio i fuel cell type PEMFC (Proto Exchage Membrae Fuel Cell). Eergy ad exergy aalyses are applied to evaluate the performace of the system model. The simulatio demostrates that hydroge productio varies with the operatio temperature of the reformig reactor ad with the compositio of the gas mixture. The maximum H 2 mole fractio ( mol.mol - ) ad exergetic efficiecy of % for the reformig reactor are achieved whe gas mixtures of higher quality such as: GGAS2, GGAS4 ad GGAS5 are used. The use of those gas mixtures for electric power geeratio results i lower irreversibility ad higher exergetic efficiecy of 3-3.5%. Keywords: Hydroge; Fuel cell; Gasificatio gas of biomass; Steam reformig; Thermodyamic aalysis. INTRODUCTION The fuel cell is a electrochemical device that coverts the fuel chemical eergy ito electrical eergy directly. The mai classificatio of fuel cells is by type of electrolyte used ad operatio temperature. PEMFC uses NAFION membrae as the electrolyte ad gaseous diffusio electrodes with platium (Pt) as catalyst, ad the operatig temperature is uder 8ºC. Due to the low temperature, the PEMFC operates oly with hydroge of high purity (99.99%mol.mol - ), ad the cocetratio of carbo mooxide i the gaseous flux should ot exceed µmol.mol -. The mai sources for hydroge productio are: water, fossil hydrocarbos ad biomass. Techologies utilized to remove the hydroge of those sources are: water electrolysis ad hydrocarbo reformig. Hydroge ca be obtaied from biomass maily through two processes, both ivolvig the reformig of methae. The first is the aaerobic degradatio of orgaic matter, producig biogas maily composed of methae (CH 4 ) ad carbo dioxide (CO 2 ). The secod is biomass gasificatio, a thermo-chemical process i which biomass is trasformed ito fuel gas, usually called producer gas or sythesis gas (GGAS). *To whom correspodece should be addressed

2 6 A. Sordi, E. P. Silva, L. F. Milaez, D. D. Lobkov ad S. N. M. Souza The gasificatio gas of biomass is composed of H 2, CO, CH 4, C x H y, CO 2 ad N 2. The molar fractios of these gaseous species i the GGAS compositio deped o the desig of the gasifier, the biomass compositio ad operatioal coditios. The potetial of gasificatio gas to produce hydroge depeds, i tur, o its compositio, maily o its CH 4 ad CO cotets. For electricity geeratio i a lowtemperature fuel cell, such as the PEMFC, it is ecessary a gas processig (reformig ad purificatio) util there is oly hydroge i the output gaseous flow. This work presets a methodology for the thermodyamic simulatio of hydroge productio by reformig the gasificatio gas of biomass. Gas mixtures from differet gasificatio processes were used i order to fid which had the highest potetial for hydroge ad electricity productio. The "highest potetial" mixture is uderstood as the oe i which the reformig ad purificatio of the gasificatio gas of biomass results i the miimum irreversibility for the total system. PEMFC / GASIFICATION GAS SYSTEM Figure illustrates the model of the system for hydroge ad electricity productio from gasificatio gas of biomass. The gas eters the reformig reactor at 85 C ad atmospheric pressure at positio, ad leaves at positio 2 at a temperature that depeds o its compositio at the etrace. The reform gas must be cooled before passig through the shift reactor for the reactio of carbo mooxide with steam. After the shift reactor, the shift gas must be cooled ad compressed. The last part of the system is hydroge purificatio by a PSA cycle ad electrical eergy productio i PEMFC stacks. Curretly, the PEMFC stacks are built o a power scale ragig up to 25 kw. I this work the fuel cell power was calculated based o the gas flow of GGAS pilot plat. Positio 7 represets the flow of pure hydroge, ad positio 9 represets the flow of the exhaust gas mixture at the PSA exit. Figure : PEMFC / Gasificatio gas system. Brazilia Joural of Chemical Egieerig

3 Hydroge from Biomass Gas Steam Reformig for Low Temperature Fuel Cell: Eergy ad Exergy Aalysis 6 THERMODYNAMICS OF THE FUEL CELL Accordig to Larmiie ad Dicks (23), the thermodyamics of the fuel cell is represeted by the reactio () ad equatios below. The Global reactio of hydroge with oxyge from air is: H + O H O Rc.() 2(g) 2 2(g) 2 (l) The maximum electrical work produced by the fuel cell is: δ Wel = dh TdS or: W el that ca be Wel =Δ G=ΔH TΔ S () Eq. () ca be writte o a molar basis as: Δ g =Δh T Δ s (2) The available thermal eergy is Δ h, therefore the maximum thermodyamic efficiecy of the fuel cell is defied as: Δg η TH = (3) Δ h I reactio () two electros are trasferred to a circuit exteral to the fuel cell. For each mol of hydroge ivolved i the reactio, 2N electros are trasferred, where N is the Avogadro s umber. If -e is the charge of a electro the the charge flow will be (DOE, 22): 2N e = 2F (4) Durig the operatio of the fuel cell, a differece of potetial E appears betwee the cell electrodes. The electrical work wel is calculated by the equatio: wel = 2F E (5) If there are o irreversibilities preset i the system, the the produced work will be equal to the Gibbs free eergy of the reactio: Δ g = 2F E ad Δg E = 2F (6) Equatio (6) calculates the ideal voltage or reversible potetial for a fuel cell operatig with hydroge. For stadard coditios (298.5 K ad.325 kpa), the followig applies: E Δg = (7) 2F This results i.229 V for a fuel cell based i Rc.(). However, several pheomea related to the kietics of the electrochemical coversio i the electrodes result i losses (overpotetials) i the cell potetial whe the curret itesity icreases (Matelli ad Bazzo, 25). The overpotetials are kow as: polarizatio by activatio, ohmic polarizatio ad polarizatio by cocetratio. The electrochemical efficiecy quatifies the overpotetial effects by the equatio: EOP η elq = (8) E Where E OP is the real operatig voltage of the fuel cell. Accordig to Matelli ad Bazzo (25), the practical efficiecy ca be defied by the thermodyamic efficiecy multiplied by the electrochemical efficiecy: η PRT =ηth η elq (9) For a fuel cell system operatig with hydroge from biomass gasificatio gas reformig, the overall efficiecy is associated to the total power output ad the gas lower heatig value (LHV). This efficiecy is amed first law efficiecy of the PEMFC/ gasificatio gas system: Wel W η I,FC = =.LHV Q GGAS el GGAS Where the electrical power of fuel cell is: el 3 ( ) OP () W = i A E () GASIFICATION GAS REFORMING AND PURIFICATION The gasificatio gas of biomass has a hydroge molar fractio that ca be maximized by the reformig of the molar fractios of methae (CH 4 ), hydrocarbos (CxHy), ad carbo mooxide (CO). Brazilia Joural of Chemical Egieerig Vol. 26, No., pp , Jauary - March, 29

4 62 A. Sordi, E. P. Silva, L. F. Milaez, D. D. Lobkov ad S. N. M. Souza For maximum coversio of these gaseous species ito hydroge two reactors are eeded: oe for hydrocarbo reformig ad aother for CO processig. The process of reformig is defied as a thermochemical ad catalytic coversio of a liquid, solid or gaseous fuel ito a hydroge-rich mixture. Accordig to Silva (99), most processes use light hydrocarbos for extractig hydroge. Light hydrocarbos are those with molecular mass betwee methae ad aphtha, ad a boilig poit below 25 C. These compouds ca react with water at temperatures of 65-9 C. I the case of methae, a ickel/alumia catalyst is used. The best kow reformig methods are: steam reformig, partial oxidatio, ad auto-thermal reformig. I this work, steam reformig was cosidered. The global gasificatio gas reformig reactios are described by Rcs. (2) to (4). CH4 + ah2o bco + cco2 + dh2 + eh2o Rc.(2) CH fho 2 gco+ hco2 + ih2 + jho 2 Rc.(3) CO + lh2o mco2 + H2 Rc.(4) The applicatio of H 2 for power geeratio i PEMFC requires that the aode ilet gas have a CO cocetratio lower tha µmol.mol -, sice CO is a poiso to the fuel cell eletrocatalyst (Zalc ad Löffler, 22). If hydroge is produced from hydrocarbos reformig (i.e. biomass gasificatio gas), purificatio is required i order to reduce the CO levels to cell requiremets. This task is partially accomplished by a water gas shift (WGS) reactor (reactio 4) (Giuta et al., 26). The fial CO cleaup occurs i a preferetial oxidatio (PrOx) or i the pressure swig adsorptio (PSA) uit. The WGS reactio is moderately exothermic with a heat reactio, Δ H = -4 kj.mol -. The high temperatures favor itrisic kietics while lower temperatures favor high equilibrium CO coversio. Adiabatic operatio with a ilet-outlet temperatures of C yields poor performace because the temperature of the process stream icreases to a poit at which equilibrium coversio is low (<8%). Isothermal operatio with ilet-outlet temperature of 25 C iitially yields coversios lower tha those obtaied adiabatically, but the coversio curve cotiues to icrease to a equilibrium coversio value of 9% (Zalc ad Löffler, 22). Sigificatly better performace ca be achieved by operatig at a relatively high temperature ad exploitig reactio kietics whe the gas compositio is far from equilibrium ad the lowerig the temperature as thermodyamics begis to limit the CO coversio (Zalc ad Löffler, 22). This task is accomplished i two adiabatic stages usig two differet catalysts with itermediate coolig. The first reactor operates betwee 3 ad 5 C (high temperature stage HTS) ad uses a iro-based catalyst (Fe/Cr). The secod reactor operates at lower temperatures (8-3 C) (low temperature stage LTS) ad uses a copper-zic catalyst supported o alumia Cu/Z/Al (Fracescoi et al., 27). Hydroge fial cleaup ca be achieved by a PSA system, which is widely used for gas purificatio. PSA is geerally employed for oxyge or itroge from air, ad for hydroge geerated by processes such as hydrocarbo reformig. This techology has bee i commercial use for hydroge purificatio sice 966, ad is curretly widely used (Myers et al., 22). Basically, a PSA works by the actio of a adsorbet bed selective for certai gaseous species. A gaseous mixture is itroduced ito the bed uder high pressure, ad the adsorbig solid selectively adsorbs certai compoets, allowig the o-adsorbed compoet to pass through the bed as a purified gas. The PSA systems operate i cycles, where three steps are basic to ay process: pressurizatio, adsorptio ad depressurizatio. I the depressurizatio occurs the adsorbet regeeratio ad desorptio of compoets retaied, ad the the process returs to its iitial coditio. Therefore, the removal of species adsorbed is doe by total pressure reductio, which gives the PSA systems a faster pace i cycles ad greater productio per uit volume of adsorbet bed tha other types of adsorptio processes (Neves ad Schvartzma, 25). The fastess ad the operatio with two or more syc beds allows iput ad output of products cotiuously, these features are essetial for fuel cell systems. The eergy required for this separatio of gaseous species is obtaied from the mechaical work of compressig the gaseous mixture. Eergy expeded i this mechaical work is a sigificat compoet of the operatioal cost of a PSA system. SYSTEM PERFORMANCE ANALYSIS The methodology used to aalyze the performace of the system was the eergy ad exergy balace. Iitially the simulatio of the gasificatio gas reformig was carried out, i which the fial mixture compositio, correspodig to the chemical equilibrium i a give thermodyamic coditio, was calculated usig Lagrage multipliers Brazilia Joural of Chemical Egieerig

5 Hydroge from Biomass Gas Steam Reformig for Low Temperature Fuel Cell: Eergy ad Exergy Aalysis 63 to fid the miimizatio poit of the total Gibbs free eergy of the system. I this work, the EES (Egieerig Equatio Solver) was used i the simulatio of the gasificatio gas reformig. The hydroge molar fractio i the mixture is a fuctio of the temperature, pressure ad the steam/carbo ratio (γ). I the preset work, the pressure was.3 kpa, ad the parameter γ was equal to 2 for CH 4 reformig, 3 for C 2 H 4, ad for CO reformig. With regard to iput coditios, the gasificatio gas may have compositios depedig o the gasificatio process ad biomass type. Table illustrates the molar fractios of some gaseous mixtures produced by differet gasificatio processes reported by Bai (24). Iitials i Table stad for: CFB: circulatig fluidized bed FB: fluidized bed IFB: idirect fluidized bed ICFB: idirect circulatig fluidized bed DRF: dowdraft UPF: updraft Oce the hydroge molar fractio i the gaseous mixture i the shift reactor (shift gas) is calculated, the ext step is the hydroge purificatio by PSA. I the preset work, a PSA system was cosidered with a operatio pressure of 65 kpa, ad a hydroge recovery factor of.85 of the volumetric flow of the molar fractio of this species i the shift gas. These characteristics were chose based o research ad developmet data from the Hydroge Laboratory of the Uiversity of Campias Physics Istitute. Thermodyamic aalysis of the system illustrated i Fig. assumed the ideal gas model. Equatios (2) ad (3) represet the ethalpy ad etropy calculatios for a ideal gas mixture. T hi = yi hi + cp i.dt i= T (2) T s = y s +.dt R.l y cpi ( ) (3) i i i i T i= T The mass ad eergy balaces i reactors ad heat exchagers are represeted by Eqs. (4) to (6). m = m (4) i out i i P i i i= i= ( ) ( ) h (T) h (T) = (5) i i out out i= i= h h = Q (6) The exergy balace for a cotrol volume is: R ( ) ( ) W I= ex ex (7) CV out out i i The defiitio of specific physical ad chemical exergy, accordig to Szargut et al. (988), is represeted by Eqs.(8) ad (9): ( ) ( ) ex = h h T s s (8) PH i i i i i= CH = i CHi + i i i= i= (9) ex y.ex R.T. y.l.y Where the total specific exergy is: ex = ex + ex (2) CH PH The referece eviromet proposed by Szargut et al. (988), where stadard temperature ad pressure (298.5 K ad.325 kpa) ad the stadard atmosphere compositio ca be foud was used to defie physical ad chemical exergies. Table GGAS compositio for differet gasificatio processes. Process CFB ICFB FB IFB IFB FB DRF UPF Fluid reaget air steam air steam steam air air air Biomass Bagasse Wood Wood Black liquor Wood Wood Wood Wood mol.mol - GGAS GGAS2 GGAS3 GGAS4 GGAS5 GGAS6 GGAS7 GGAS8 Y H Y CO Y CO Y N Y CH Y C2H Brazilia Joural of Chemical Egieerig Vol. 26, No., pp , Jauary - March, 29

6 64 A. Sordi, E. P. Silva, L. F. Milaez, D. D. Lobkov ad S. N. M. Souza Exergetic efficiecy calculatios adopted the iput/output criterio defied by Kotas (995) as ratio efficiecy. The ratio efficiecies of the followig cotrol volumes were calculated: reformig ad shift reactors, heat exchagers, PSA, PEMFC, ad the system as a whole. Eqs.(2) to (26) represet these efficiecies, respectively. ( ex CH (REFGAS)) ( ) ψ,rr = ex (H O) + ex (H O) + CH 2 PH 2 ( + ) ex (GGAS) ex (GGAS) CH PH ( ex CH (SHIFTGAS)) ( + ) ψ,sr = ex (REFGAS) ex (REFGAS) CH ( ) ex (H ) PH 2 ψ,psa = W PSA PH ( PH COLD,out PH COLD,i ) ( PH HEAT,i PH HEAT,out ) ex, ex, ψ,he = ex, ex, W el ψ,pemfc = ex (H ) + W ( ) CH 2 AI W η II,FC = ex (GGAS) + W + W ( ) el CH AI PSA (2) (22) (23) (24) (25) (26) RESULTS Figures 2 ad 3 show the molar fractios of the equilibrium compositio of GGAS ad GGAS2 reformig simulatios respectively. The potetial for hydroge productio is higher from GGAS2 tha from GGAS because of its higher CH 4 ad CO molar fractios. The temperature at which hydroge productio reaches its peak depeds o the amout of CH 4, CO ad iert gases such as N 2 ad CO 2. The CH 4 reformig reactio is ehaced by higher temperatures (65-85 C), while the CO reformig reactio is ehaced by lower temperatures (25 C) i a isothermal reactor as Zalc ad Löffler (22). Thus, gas coolig is ecessary at the reformig reactor exit to allow the coversio of the remaiig CO molar fractio i the shift reactor. I order to allow the use of the gasificatio gas physical exergy at the reformig reactor, the gas precleaig must be performed by the hot system. If the system is cold, the gaseous mixture must be heated up to the reformig operatio temperature agai, icreasig the eergy cosumptio of the hydroge productio system. Tables 2 ad 3 illustrate the results of thermodyamic simulatio for PEMFC / GGAS system i Fig.. Figure 4 shows that the largest cotributio i the exergy destructio (irreversibility) occurs i the PEMFC. The process of electrical work trasfer i the fuel cell is associated to the chemical potetial gradiet, ad the geeratio of irreversibility is proportioal to this gradiet. The exhaust gas temperature of the PEMFC is very close to the ambiet temperature, ad therefore there is ot much heat produced by the PEMFC available..6,6,6.6.5,5.5,5 H 2 N 2.4,4,4.4 y i [mol.mol - ].3,3.2,2 H 2 CO 2 y i [mol.mol - ],3.3,2.2 CO 2 H 2 O., H 2 O CO CH T [K] Figure 2: Molar fractios from GGAS reformig simulatio., CO C 2 H 4 CH 4 N T [K] Figure 3: Molar fractios from GGAS2 reformig simulatio. Brazilia Joural of Chemical Egieerig

7 Hydroge from Biomass Gas Steam Reformig for Low Temperature Fuel Cell: Eergy ad Exergy Aalysis 65 Table 2: Simulatio of PEMFC / GGAS system. Q GGAS [kw] Ex CH [kw] W el [kw] W AI [kw] W PSA [kw] η I,FC [%] η II,FC [%] Positio m Table 3: Simulatio of PEMFC / GGAS system. T [ºC] ex PH ex CH Ex [kw] [kg/s] [kj/kg] [kj/kg] (gas) ,497., (water) ,2., ,2., ,68.,426, ,., ,., ,., ,92. 7,3., ,3., , , PEMFC 5 I [%] RR HE PSA SR COMP HE2 EV Figure 4: Percet Irreversibilities of PEMFC / GGAS system. The gaseous mixture compositio has a importat effect o the system performace. For the gaseous mixtures i Table, Figure 5 shows the variatio o reformer exergetic efficiecy as a fuctio of the itroge molar fractio i the gasificatio gas of biomass. The higher the amout of this iert gas is, the lower the reformer efficiecy. I practice, the presece of iert gases has a egative ifluece o the reactio kietics. Although the effectiveess of the reformig process is very importat for the performace of the system, the PSA has the most strikig ifluece o the eergy cosumptio. Hydroge purificatio is the mai determiat of eergy cosumptio, due to the eed to compress the gaseous mixture, ad due to the use of part of the purified hydroge for adsorptio bed regeeratio. Figure 6 shows that purificatio performace is proportioally better for higher molar fractios of hydroge i the shift gas. This favorable coditio is obtaied i the reformig of GGAS2, GGAS4 ad GGAS5 (Figure 7). These gaseous mixtures are produced by the gasificatio process with steam ijectio ad idirect heatig, which yields gases of a higher quality. I a pilot plat, steam productio should be provided for the gasificatio process; part of the required heat ca be obtaied by burig the PSA exhaust gas. Brazilia Joural of Chemical Egieerig Vol. 26, No., pp , Jauary - March, 29

8 66 A. Sordi, E. P. Silva, L. F. Milaez, D. D. Lobkov ad S. N. M. Souza ψrr [%] ,.2,2.3,3.4,4.5,5.6,6 y N2 [mol.mol - ] Figure 5: Exergetic efficiecy of the reformig reactor as a fuctio of itroge molar fuctio ψpsa [%] ,25.3,3.35,35.4,4.45,45,5.5, ,6.65,65 y H2 [mol.mol - ] Figure 6: Exergetic efficiecy of PSA as a fuctio of hydroge molar fractio i the SHIFTGAS I [kw] GGAS GGAS2 GGAS3 GGAS4 GGAS5 GGAS6 GGAS7 GGAS8 Figure 7: Total irreversibilities of system for differet GGAS compositio Figure 8 shows the variatio of the first ad secod law efficiecies of the fuel cell system as a fuctio of the hydroge molar fractio i the shift gas. The first law efficiecy is a adequate parameter to measure system performace but is ot adequate to measure the gas quality, sice the lower heatig value does ot reflect the quality of the gaseous mixture for hydroge productio. The secod law efficiecy, o the other had, is idicative of the gasificatio gas quality i the sese that the smaller the gas flow required to produce a give amout of hydroge, the higher the system performace will be. For the costructio of a pilot plat, the higher exergetic efficiecy illustrated i Figure 8 results i compoets of a smaller volume. Figure 9 also idicates that the system will be smaller due to the lower irreversibility. A pilot plat with smaller volume compoets will have a correspodigly lower cost. Brazilia Joural of Chemical Egieerig

9 Hydroge from Biomass Gas Steam Reformig for Low Temperature Fuel Cell: Eergy ad Exergy Aalysis η I,FC η [%] η II,FC ,25.3,3.35,35.4,4.45,45,5.5.55,55.6,6.65,65 y H2 [mol.mol - ] Figure 8: First ad secod law efficiecies as a fuctio of the hydroge molar fractio I [kw] , ,3.35,35,4.4, ,5.55,55.6,6,65.65 y H2 [mol.mol - ] Figure 9: Total system irreversibility as a fuctio of the H 2 molar fractio i the SHIFTGAS CONCLUSIONS The simulatio demostrated that the hydroge productio varies with the operatio temperature of the reformig reactor, ad with the gas mixture compositio. These two variables are iheretly depedet because of the particular chemical equilibrium state for each thermo-chemical reactio of system species. The temperature of 85 C i output gasificatio reactor is sufficiet for maximum methae coversio ad hydroge productio. But hot gas cleaig should be used i gasificatio process so that gasificatio gas physical exergy ca be utilized i steam reformig. I cold gas cleaig the coolig of the gasificatio gas to the eviromet temperature i the wet scrubber results i a thermal eergy loss, which is a disadvatage of the cold cleaig. Gas reheatig for the steam reformig reactio also causes the destructio of the exergy iheret to the heat trasfer process, ad makes the system more complex. The exergetic efficiecy of the reformig reactor also depeds o the gas compositio; for the lower quality gas (GGAS) the efficiecy was 85%. For GGAS2, which is the best quality gaseous mixture, the reactor efficiecy preseted a value of 92.5%. Both the exergetic efficiecy ad the total irreversibility values of the power geeratio system with PEMFC were, respectively, 26.2% ad 775 kw for GGAS ad 3.5% ad 575 kw for GGAS2. Therefore, to achieve the maximum performace of the hydroge productio ad electricity geeratio i the fuel cell systems, the gasifier should be desiged to obtai gas mixture similar to GGAS2. The mai resposible for the differece betwee the first ad secod law efficiecies of the PEMFC system is the hydroge purificatio process (PSA). Thus, for a fuel cell / gasificatio gas system to be competitive whe compared with the traditioal thermal machies, further research ad developmet i gas purificatio techology is ecessary. O the other had fuel cells operatig at high temperatures like the SOFC (Solid Oxide Fuel Cell) ad MCFC (Molte Carboate Fuel Cell) are adequate to operate with gases from gasificatio because there is o eed for hydroge purificatio. Brazilia Joural of Chemical Egieerig Vol. 26, No., pp , Jauary - March, 29

10 68 A. Sordi, E. P. Silva, L. F. Milaez, D. D. Lobkov ad S. N. M. Souza ACKNOWLEDGEMENTS We are thakful to CAPES for the fiacial support of this work, ad to the Hydroge Laboratory ad the Mechaical Egieerig Faculty at UNICAMP. NOMENCLATURE a,b,c.. molar umbers of a specie (-) i a chemical reactio A active area of cell cm 2 COMP compressor (-) c specific heat J.kmol -.K - p e elemetary charge.62 x -9 C E equilibrium potetial V E OC ope circuit voltage V E OP real voltage of cell V ex specific molar exergy J.kmol - Ex exergy trasfer rate W EV expasio valve (-) F Faraday s costat x 7 C. kmol - FB fluidized bed (-) FC fuel cell (-) g specific Gibbs free eergy J.kmol - G Gibbs free eergy J GGAS gasificatio gas of (-) biomass h specific molar ethalpy J.kmol - H ethalpy J HE heat exchager (-) HTS high temperature stage (-) i curret desity ma.cm -2 I irreversibility rate W IFB idirect fluidized bed (-) LHV lower heatig value J.kmol - LTS lower temperature stage (-) m mass flow kg.s - molar flow kmol.s - N Avogadro s umber 6.2 x 23 OFFGAS exhaust gas of PSA (-) P pressure kpa PSA pressure swig (-) adsorptio Q heat trasfer rate W R uiversal gas costat 8,34 J.kmol.K - REFGAS gas of reformig reactor (-) RR reformig reactor (-) s specific molar etropy J.kmol -.K - S etropy J.K - SHIFTGAS gas of shift reactor (-) SR shift reactor (-) T absolute temperature K) W work J) w specific work J.kmol - W work trasfer rate, power output W WGS water gas shift (-) y molar fractio mol.mol - Y molar fractio percet %mol.mol - Greeks Letters γ steam-carbo rate (-) η efficiecy % ψ ratioal efficiecy % Subscripts referece state.3 kpa ad K AI air iput (-) CH chemical (-) el electrical (-) elq electrochemical (-) GGAS gasificatio gas of biomass (-) g gas (-) HT heat exchager (-) H 2 hydroge (-) i ith specie (-) I first law of thermodyamics (-) II secod law of (-) thermodyamics i ilet (-) l liquid (-) out outlet (-) P Product (-) PH physical (-) PRT practical (-) R reaget (-) RR reformig reactor (-) SR shift reactor (-) TH Thermodyamic (-) CV cotrol volume (-) Superscripts referece state.3 kpa ad K Brazilia Joural of Chemical Egieerig

11 Hydroge from Biomass Gas Steam Reformig for Low Temperature Fuel Cell: Eergy ad Exergy Aalysis 69 REFERENCES Bai, R. L., Biomass Gasificatio Overview, NREL Natioal Reewable Eergy Laboratory. US DOE Uited States Departmet of Eergy, p. 48 (24). DOE U.S. Departmet of Eergy. Fuel Cell Hadbook, EG & G Services Parsos, Ic. Sciece Applicatios Iteratioal Corporatio, West Virgiia, USA, p. 352 (22). Fracescoi, J. A., Mussati, M. C., Aguirre, P. A., Aalysis of desig variables for water-gas-shift reactors by model-based optimizatio, Joural of Power Sources, v. 73, p (27). Giuta, P., Amadeo, N. ad Laborde, M., Simulatio of a Low Temperature Water Gas Shift Reactor Usig the Heterogeeous Model/Applicatio to a PEM Fuel Cell, Joural of Power Sources, v. 56 (7) p (26). Kotas, T. J., The Exergy Method of Thermal Plat Aalysis. Krieger Publishig Compay, Florida (995). Larmiie, J. ad Dicks, A. Fuel Cell Systems Explaied, Joh Wiley Sos (23). Matelli, J. A., Bazzo, E., A Methodology for Thermodyamic Simulatio of High Temperature Iteral Reformig Fuel Cell Systems, Joural of Power Sources, v.42, p (25). Myers, B. D., Ariff, G. D., James, B. D., Lettow, J. S., Thomas, C. E. ad Kuh, R. C., Cost ad Performace Compariso of Statioary Hydroge Fuelig Appliaces, The Hydroge Program Office; Office of Power Techologies; U.S Departmet of Eergy. 23 p. (22). Neves, C. F. C ad Schvatzma, M. M. A. M., Separação de CO 2 por meio da tecologia PSA, Química Nova, v.28,. 4, p (25). Silva, E. P., Itrodução a Tecologia e Ecoomia do Hidrogêio. Campias: Editora da Uicamp (99). Szargut, J., Morris, D. R. ad Steward, F. R., Exergy Aalysis of Thermal, Chemical ad Metallurgical Processes, Joh Bejamis Publishig Co. (988). Zalc, J. M. ad Löffler, D. G., Fuel processig for PEM fuel cells: trasport ad kietic issues of system desig, Joural of Power Sources, v., p (22). Brazilia Joural of Chemical Egieerig Vol. 26, No., pp , Jauary - March, 29