GT COUPLING OF BIOMASS STEAM GASIFICATION AND AN SOFC - GAS TURBINE HYBRID SYSTEM FOR HIGHLY EFFICIENT ELECTRICITY GENERATION

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1 Proceedgs of ASME Turbo Expo 4: Power for Land, Se and Ar June 14 17, 4, Venn Austra GT4-539 COUPLING OF BIOMASS STEAM GASIFICATION AND AN SOFC - GAS TURBINE HYBRID SYSTEM FOR HIGHLY EFFICIENT ELECTRICITY GENERATION Tobas Proell Insttute of Chemcal Engeerg Venna Unversty of Technology Getredemarkt 9/166, 16 Venn Austra Phone: , Fax: tproell@mal.zserv.tuwen.ac.at Rehard Rauch Insttute of Chemcal Engeerg Venna Unversty of Technology Getredemarkt 9/166, 16 Venn Austra Phone: , Fax: rrauch@mal.zserv.tuwen.ac.at Chrstan Acherng Repotec Umwelttechnk GmbH Europastrasse 1, 754 Guessg, Austra Phone: , Fax: c.acherng@repotec.at Hermann Hofbauer Insttute of Chemcal Engeerg Venna Unversty of Technology Getredemarkt 9/166, 16 Venn Austra Phone: , Fax: hhofba@mal.zserv.tuwen.ac.at ABSTRACT An energetc model of an ternal reformg sold oxde fuel cell (IRSOFC s developed. It s tegrated a process couplg fludzed bed steam gasfcaton of bomass and an IRSOFC-gas turbe hybrd cycle. Process smulaton s performed usg the software package IPSEpro. The model of the gasfcaton and gas condtong secton s based on data from the 8 MW (fuel power plant Guessg/Austr whle the fuel cell s modeled based on recent lterature data. Heat utlzaton for power generaton s consdered coverg both hybrd cycle exhaust and heat from the gasfcaton process. Electrc effcences up to 43 % are expected for combed heat and power applcaton even at small plant capactes the range of 8 MW fuel power. Keywords: Bomass, Gasfcaton, Sold oxde fuel cell, Hybrd system, Combed heat and power INTRODUCTION Along wth the present dscusson about gradually substtutg fossl fuels by sustaable sources, bomass gasfcaton systems have been developed based on dfferent approaches. In Austr a dual fludzed bed gasfcaton system has been developed usg steam as the gasfcaton agent and provdg the necessary heat the gasfcaton reactor by crculatg hot bed materal [1]. It s heated up a second fludzed bed reactor by combuston of resdual char. An 8 MW (fuel power demonstraton plant [] s operated Guessg/Austra sce December 1 and reached 65 hours of operaton November 3. For power generaton, a MW el gas enge s used requrg a two step cold gas cleang system consstg of a bag flter and an organc solvent scrubber for tar removal. The plant s operated combed heat and power (CHP-mode and reaches an electrc effcency of 5 % (gross at a total fuel utlzaton of about 7 %. The steam gasfcaton producer gas s contrast to the ar gasfcaton systems almost free of ert ntrogen and shows lower heatg values (LHV between 1 and 14 MJ/m 3 N (dry gas. The gas composton shown Table 1 largely resembles the composton of partly reformed natural gas and the spectrum of possble gas applcaton s consderably hgher than for ar gasfcaton systems. The most prospectve technologes to utlze the producer gas apart from conventonal combuston turbes or enges are hgh temperature fuel cells for electrcty generaton and syntheses ether of hgh qualty lqud fuels or of synthetc natural gas. Gas turbes (GT have been dscussed combaton wth gasfcaton of bomass for pressurzed gasfers and hgh temperature producer gas cleang. However, contuous sold feed to the pressurzed systems s stll a key-problem. Another crtcal pot s the avalablty of hot gas condtong technologes order to meet the gas turbe specfcatons for long-term operaton. On the other hand, atmospherc gasfcaton systems can produce fuel for ternal combustg GT. Because of the need for fuel gas compresson, the producer 1 Copyrght 4 by ASME

2 gas must be cooled, what s usually done along wth gas condtong (cold gas cleang. In ths case, the GT s drectly competg to gas enges, whch are stll advantageous wth respect to gas cleanless requrements and effcency the power range of 1-5 MW el [3], whch s the typcal sze of bomass CHP-applcaton. Table 1 Typcal dry composton of producer gas from the Guessg steam gasfcaton process CH 4 v-% (dry 1 11 C H 4 v-% (dry.5 C 3 -Fract. v-% (dry.5.7 CO v-% (dry 4 6 CO v-% (dry H v-% (dry 38 4 N v-% (dry 1.. LHV MJ/m 3 N (dry Owg to the hgh hydrogen content and the potental of steam reformg for the hydrocarbon fracton, the steam gasfcaton producer gas represents a fuel well adapted to the requrements of hgh temperature fuel cells. Recently, sold oxde fuel cell (SOFC-GT hybrd concepts have been publshed for natural gas applcatons [4-1] and also combaton wth bomass gasfcaton [11]. SOFC stacks work the temperature range from 8 to 1 C and the fuel cell performance deally creases wth creasg pressure [4]. Therefore, the SOFC represents a sutable toppg cycle for external combustg GT-systems. The hgh operatg temperature allows ternal steam reformg of methane and CO-shftg at the anode surface, whch guarantees hgh fuel converson rates. Chan et al. [1] treat the energetc modelg of ternal reformg SOFC (IRSOFC to detal. The same authors present a fuel cell-gt hybrd system based on performance data of a tubular IRSOFC [8,9], on whch the present work s largely based. The Guessg steam gasfcaton process cludg gas cleang has been already modeled and mplemented to the equaton-orented process smulaton tool IPSEpro [3]. Wth the present work, the smulaton has been extended by the model of the IRSOFC stack and a couplg between the gasfcaton process and an IRSOFC-GT hybrd concept s realzed the smulaton program. A steam cycle and a compact organc Ranke cycle (ORC are dscussed as possble concepts for heat recovery across the plant. NOMENCLATURE A eff effectve sectonal area of the cell stack m E electrc potental of the fuel cell V E theoretc fue cell potental for lhv of H V E molar standard exergy of speces J. mol -1 E A actvaton energy J. mol -1 e specfc exergy of a stream J. kg -1 F Faraday constant (F A. s. mol -1 G R Gbbs free reacton enthalpy at.1 MPa J. mol -1 f H 98 standard enthalpy of formaton J. mol -1 h specfc enthalpy of a stream J. kg -1 mean current densty of the cell stack A. m - exchange current densty A. m - L lmtg current densty A. m - L total length of fuel cell channel m LHV molar lower heatg value J. mol -1 lhv specfc lower heatg value J. kg -1 M mean molar mass of a stream kg. mol -1 m& mass flow kg. s -1 n number of electrons per mol H (n --- n& mole flow mol. s -1 P el electrc power W p pressure Pa Q & exchanged heat W trans R general gas constant (R J. mol -1. K -1 s specfc entropy of a stream J. kg -1. K -1 T temperature K V polarzaton voltage drop (overvoltage V w mass fracton of water fuel kg. kg -1 X fuel converson rate --- x length coordate fuel cell channel m y molar fracton of speces mol. mol -1 Greek symbols: β exchange coeffcent --- δ k thckness of layer k m energetc effcency 1 ν stochometrc coeffcent of speces --- ρ specfc electrc resstance Ω. m φ SF steam/fuel rato (total water to dry fuel kg. kg -1 φ bypass part of total ar or fuel resp. bypass --- Subscrpts: AC alternate current a anode act actvaton polarzaton c cathode chem chemcal conc concentraton polarzaton cond condenser of steam or ORC cycle conv fuel converson the fuel cell stack DC drect current el electrc exp effectvely exported from the plant FC fuel cell FU fuel utlzaton (electrcty and utlzed heat referrg to bomass fuel G gasfcaton HC hybrd cycle HRC heat recovery cycle speces gas mxture or reacton feed stream to the system v verter k component of the fuel cell net electrc consumpton subtracted ohm ohmc polarzaton out stream leavg the system Plant referrg to the whole CHP plant Q, q heat r reversble operated fuel cell react actually reactg SG steam generaton for fludzaton vol voltage effcency due to polarzaton Copyrght 4 by ASME

3 MODELING General aspects Wth the followg, the term modelg largely refers to the energetc descrpton of the process. The energetc performance s conventonally descrbed terms of energy based on LHV and sensble heat. The advantage of ths method s the compatblty to common effcency deftons. On the other hand, exergy may be used as the characterstc stream qualty for process evaluaton. Accordg to Baehr [13], the exergy of a stream conssts of the exergy of heat and chemcal exergy. For deal gas mxtures, the specfc exergy s defed by: e e q + e chem (1 e q e [ s( T, p s( T, ] h( T h( T T p ( chem 1 M ( y E + R T ( y ln y (3 The thermal envronment defed for the present study s K,.1 MPa. Enthalpy and entropy Eq. ( are the propertes of the gas mxture. Whle the pressure dependency of exergy s represented by the entropy term Eq. (, the entropy of mxg effect on exergy s not covered by Eq. (. The reason s that the mxg entropy enters both entropy expressons the square brake term of Eq. ( the same way. Therefore, the mxg rreversblty s part of the chemcal exergy and represented by the rght hand sde addend the square brake of Eq. (3. The molar exergy of pure substances at thermal envronment condtons depends on the defton of a chemcal envronment. Baehr [13] reports the exergy at standard condtons (98.15 K,.1 MPa for numerous chemcal elements based on an equlbrum envronment calculated by Dederchsen [14]. The standard exergy of chemcal compounds can be calculated from element exergy and standard free enthalpy. For pure water and steam the exergy s defed by equatons smlar to Eqs. (1 and ( usg IAPWS-IF97 [15] data for temperature and pressure dependent enthalpy and entropy. Equaton (3 reduces the case of pure water to a constant. The exergy of sold mxtures s expressed analogy to deal gases wth the smplfcaton that the pressure dependency of entropy can be neglected. For organc mxtures defed just by elementary analyss, the entropy of formaton s not avalable whle the enthalpy of formaton can be calculated from the heatg value. The chemcal exergy s set equal to the hgher heatg value for these substances, what should be a good approxmaton [16]. Gasfcaton and gas cleang As mentoned the troducton, the modelg of the gasfcaton process has been the subject of prevous work and wll not be descrbed to detal here. The present work ams at an authentc representaton of the gasfcaton process takg measured plant data to account. The plant layout of the gasfcaton and gas cleang secton s shown Fg. 1. The gasfer s fludzed wth superheated steam and the bomass fuel s troduced to the statonary bed usg a screw feedg system. An mportant parameter s the total water to dry fuel rato, the followg shortly called steam/fuel rato: m& w + m& Steam ϕ SF (4 m& ( 1 w The raw producer gas water content (7 48 v-% strongly depends on the steam/fuel rato, whch takes values between.5 and 1. practcal operaton. Gasfer Producer gas cooler Combuston secton Bag flter Flue gas cooler Scrubber Producer gas recycle Bag flter Clean gas Stack Bomass Ar for combuston Steam Bed ash Fly ash Fg. 1: Layout of the Guessg steam gasfcaton process. The temperature spread between combuston and gasfcaton reactor s determed by the necessary energy for gasfcaton and the bed materal crculaton rate. Further parameters wth energetc sgnfcance are the amount of resdual char that leaves the gasfcaton secton wth the bed materal and the gasfcaton temperature. The pressure both gasfer and combuston reactor s close to atmospherc condtons. The gas condtong secton conssts of producer gas cooler, bag flter, and a tar scrubber usg rape ol methyl ester (RME as solvent. The water content clean producer gas s lmted to the value for 1 % humdty at scrubber ext. Varaton of the scrubber ext temperature (45 7 C results clean gas water contents between 1 and 3 v-%. The condensate/rme soluton s separated an equalzaton tank and the water fracton s troduced the hot flue gas le after part evaporaton. A small part of the clean producer gas s recycled to the combuston reactor order to control the gasfcaton temperature. The bed materal leavg the gasfer carres about 1 % of the dry fuel as resdual char whch s the ma fuel for the combuston reactor. After leavg the fast fludzed bed combustor, the hot bed materal s separated from flue gas a cyclone and enters the gasfer through a steamfludzed loop seal. After fal heat recovery the flue gas cooler, dust s separated a bag flter whle the gas goes to the stack. 3 Copyrght 4 by ASME

4 For IRSOFC operaton, all hydrocarbons except CH 4 must be elmated from the producer gas because of the rsk of carbon deposton on the fuel electrode [17]. The reformg can be realzed sde the gasfer usg catalytc actve bed materal or a separate reformg unt drectly after the gasfer. The present work assumes reformg of all hgher hydrocarbons sde the gasfer. Measured data from the Guessg stallaton show, that the CO-shft reacton CO + H + (5 O CO H s very close to equlbrum at the ext of the gasfer. Therefore, the model assumes React. (5 to be equlbrum at the gasfer ext. The CH 4 concentraton s estmated based on the operatg experence on dfferent FICFB gasfcaton systems. Two effcency values based on thermal fuel power descrbe the overall behavor of the gasfcaton process. The chemcal effcency refers to the amount of energy combed the exported producer gas: m& PG,exp PG, clean (6 chem, G ( m& The thermal effcency of the gasfcaton process relates the exportable heat, whch s the heat transferred producer gas cooler and flue gas cooler reduced by the heat needed for steam generaton, to the prmary fuel power: + Q& trans, FG cooler ( m& Q& trans PG cooler Q&, trans, SG (7 Q, G The am of the optmzaton of the gasfcaton process s a hgh chemcal effcency. The fluence of addtonal bomass fuel to the combuston chamber on the chemcal effcency wll be dscussed the results secton. Internal reformg sold oxde fuel cell (IRSOFC Sold oxde fuel cells typcally work at temperatures between 8 and 1 C. The electrolyte s normally an onconductg ZrO -Y O 3 sold soluton wth an Y O 3 content of 8-1 mol-%. The porous electrodes are electron conductg. The anode (fuel electrode materal s a cermet of N and ZrO, a largely appled cathode (ar electrode materal s Sr-doped LaMnO 3. Dfferent SOFC developers present ether tubular or planar cell desgn. The electrochemcal reacton takg place at the three-phase boundary fuel/anode/electrolyte s: H + O HO + e (8 On the cathode sde, oxygen s electrochemcally reduced to oxygen ons whch are actually transported the electrolyte: 1 O + e O (9 The overall oxdaton reacton s therefore: H 1 + O H O (1 At the operatg temperatures of the SOFC, a drect electrochemcal oxdaton of CO at the anode/electrolyte boundary would be theoretcally possble. However, the CO-shft reacton (React. 5 s ketcally faster and the equlbrum s drven to the rght by the hydrogen consume of React. (1. The anode s as well a catalyst for steam reformg of CH 4 accordg to CH + H O CO + 4 H. (11 4 The equlbrum of React. (11 s far on the rght at the operatg condtons of the SOFC, ketc hbton s the only reason for possble complete converson. In analogy wth the lterature [4], the present work assumes that CH 4 s completely reformed when passg the anode. In order to prevent sold carbon deposton on the anode surface, the molar steam to combustble carbon rato (S/C rato the anode feed must be hgh enough. Typcal values for the S/C rato order to effectvely avod carbon deposton are [18]. Incomplete fuel converson s expressed terms of CO and H passg by. Reacton (5 s assumed to be equlbrum the anode exhaust. In order to vestgate the combaton of the exstg gasfcaton process and fuel cell technology for power generaton, a model has been developed takg the energetc behavor of the fuel cell stack to account. To allow drect comparson to other power generaton unts lke gas turbe or enge, the bass for the effcency formulaton s the thermal fuel power of the anode feed based on LHV. The overall electrc effcency of the cell stack cludg the DC/AC verter s: P el, AC el, FC (1 m & The overall effcency s a product of reversble cell effcency, voltage effcency, fuel converson effcency, and verter effcency: el, FC r vol conv v (13 The reversble cell effcency used wth the present work largely corresponds to the commonly used thermodynamc effcency, whch s an upper bound for the fuel cell effcency comparable to the Carnot cycle effcency for heat enges. The reversble cell effcency relates the open crcut voltage at operatg condtons to the theoretcal cell potental f the enthalpy of formaton of gaseous H O at K would account for the voltage: E r r E (14 Ths defton s compatble to effcency deftons for enges based on the LHV of the fuel gas. The open crcut voltage depends on temperature and on the partal pressures of the gas speces React. (1. The present work presumes a constant cell stack temperature. The gas composton, however, 4 Copyrght 4 by ASME

5 changes along the fuel cell channel. The reversble cell potental for changg partal pressures s generally gven by: dependency of onc conducton, the followg correlaton s reported the lterature [19]: E r L ( ν GR T 1 R T p n F L ln x ( x dx (15 1 ρ( T A T E exp R T ohm A, ohm (1 For means of smplfcaton, the concentraton changes are assumed to be lear between gas feed and dra on both anode and cathode sde. The analytcal soluton of Eq. (15 s then [3]: GR ( T Er + n F + R T ν p p, out, p, p, ln p out, + ln ( p, out ( The theoretcal potental that corresponds to the enthalpy of formaton of gaseous H O at K s a constant: E H H O( g f 98( n F 1.53 V (17 If current s drawn from the fuel cell, the voltage decreases wth creasg current densty due to rreversblty the dfferent parts of the cell. Ths voltage drop s termed polarzaton or overvoltage. Most authors dvde the polarzaton effect actvaton overvoltage, ohmc loss, and concentraton overvoltage. The actvaton polarzaton can be descrbed by the Butler-Volmer equaton accordg to: β F V exp T R T ( (1 β F V exp R T act act (18 The exchange coeffcent s assumed to be.5 for the fuel cell applcaton [1]. The exchange current densty s proportonal to the forward and reverse reacton rate at stack temperature. The temperature dependency of s descrbed by an Arrhenus approach: E A, T ( exp (19 R T The constants Eq. (19 could be obtaed from data reported by Chan et al. [8] for a tubular IRSOFC (Semens Westghouse Desgn as 7196 A. m -1 and E A, kj. mol -1. If data s avalable, the actvaton can be expressed separately for each electrode. The ohmc polarzaton s determed by the resstance of the on conductg electrolyte and the electron conductg electrodes and terconnectons. The voltage drop for constant effectve area (e.g. planar cell s: V ohm ( k ρ δ ( k k In the case of tubular cells, the current densty changes the radal drecton must be consdered. For the temperature Data from the lterature [9] has been used to determe the constants Eq. (1 descrbg the overall ohmc resstance of the tubular fuel cell mentoned above for an estmated thckness δ of m: A ohm and E A,ohm 5.4 kj. mol -1. Concentraton overvoltage occurs due to the decreasg partal pressure of the reactants at the three phase boundares because of lmted dffuson the electrodes. Complex polarzaton models have been developed by Chan et al. [1]. Wth the present work, a smplfed descrpton accordg to Chan et al. [8] s appled: V R T ln 1 n F ( T conc ( L Concentraton polarzaton s not sgnfcant f the cell stack s operated well below the lmtg current densty, whch aga s a functon of operatg temperature. The temperature dependency s expressed by the smple correlaton ( T a + b T (3 L L L The coeffcents have been ft from lterature data [9] for the tubular IRSOFC: a L 175 A. m -, b L 5.65 A. m -. K -1. The actual cell voltage at a certa current densty s: E Er V (4 The voltage effcency of the charged cell s therefore: E vol (5 E r In a real SOFC, the fuel wll never be completely converted. Therefore, a converson effcency of the cell stack can be generally defed as n& H react LHV, H conv (6 m& The amount of actually reactg hydrogen can be related to the total current of the cell stack: [ H ( H O( g ] A n& eff H LHV, react H f 98 (7 n F It s mportant to notce that the converson effcency defed by Eq. (6 mples the energetc effects of possble fuel conversons lke reformg and shftg. In the case of hgh rates of endothermc reformg, the converson effcency can take values above unty even though H and CO may be left the exhaust. In ths case, a part of the heat produced by 5 Copyrght 4 by ASME

6 polarzaton s recombed to the fuel by the reformg reacton. The total fuel converson rate often used the lterature s defed for the IRSOFC terms of H equvalents. Accordg to React. (5 and (11, one mole of CO equals one mole of H, whle one mole of CH 4 equals 4 moles of H. The overall fuel converson rate can be wrtten as: X n&, 1 n& a out ( y H, equv., out ( y H, equv., For pure hydrogen as fuel, conv s equal to X fuel. (8 Fally, the effcency of the DC/AC verter, defed as P el, AC v (9 Pel, DC must be consdered. v s set to 96.5 % for the present work. SOFC-GT hybrd process Massardo et al. [4] present four basc optons for the layout of a SOFC-GT cycle wth heat recovery steam generaton. One of these concepts has been further vestgated by Chan et al. [8,9] who also focus on part load behavor [1]. The process set up dscussed the present work s based on these sources. However, the fal heat recovery from exhaust gas s left to a further process downstream of the hybrd cycle secton. The scheme of the fuel cell-gt secton s shown Fg.. fuel gas GAS ternal ar preheater fuel preheater water WAT steam generator GAS fuel bypass ar preheater exhaust IRSOFC Anode FUEL CELL Cathode ar compressor combustor A F ar bypass GT Fg. : IRSOFC-GT hybrd process. generator ambent ar Addtonal steam to the fuel gas may be necessary order to reach the S/C rato requred for preventg carbon deposton on the electrode. The fuel cell stack s modeled sotherm and adabatc. Heat produced due to dsspaton s therefore affectg the temperature sde the IRSOFC stack. In contrast to natural gas fuelled cells, the heat consumpton by endothermc reformg s low for producer gas, whch may requre adopton of the cell desgn order to use the comg gas streams to cool the cell stack. The two exhaust streams of the fuel cell are mxed a combuston chamber, where AMB complete combuston of the fuel s assumed. The fuel bypass around the fuel cell s used durg start-up and for control purposes. The ar bypass allows the lmtaton of the turbe let temperature (TIT. The GT s descrbed by ts sentropc effcency. The ar compressor s drectly coupled to the GT, whle the fuel compressor s electrc powered. Wth respect to practcal operatg stablty, Chan et. al. [8,1] suggest a set up wth two GT, one to drve the ar compressor and a power turbe coupled to the generator. The present work focuses on energetc behavor. Therefore, the smpler concept wth just one GT has been taken to consderaton. The turbe exhaust s used to preheat both ar and fuel stream and for anode steam generaton before leavg the secton towards heat recovery. Heat recovery cycle (HRC The exhaust temperature of the hybrd cycle s typcally about 4 C. Producer gas and flue gas coolg the gasfcaton secton also provdes a consderable amount of hgh level heat. Steam cycle and ORC have been vestgated as possble heat recovery concepts. Electrcal effcences are summarzed Table. The steam parameters are 45 C/8. MPa/45 C/. MPa for the -stage cycle, 45 C/1.8 MPa for the 1-stage cycle. Steam turbe sentropc effcency s 8 %. For the ORC, the hot sde temperature level s 8 C and the rato el / Carnot.4. Table Heat-to-electrcty effcency for steam cycle and ORC at dfferent condensaton temperatures. el [%] T cond [ C] p cond [MPa] 1-stage steam cycle stage steam cycle ORC RESULTS AND DISCUSSION Gasfcaton secton The put parameters for the smulaton are derved from data measured at the Guessg plant. The water content of the bomass fuel s wt.-% and the LHV s MJ. kg -1. The gasfer s operated at 85 C and a steam/fuel rato of.75. The concentraton of CH 4 after the tegrated reformg step s set conservatvely to 1. v-%(dry. The resultg producer gas composton s specfed Table 3. The tar scrubber s operated at 65 C, what results a clean gas water content of 5.7 v-%. Table 3 Producer gas composton from FICFB steam gasfcaton at 85 C cludg pre-reformg CH 4 v-% (dry 1. CO v-% (dry 1.6 CO v-% (dry 1. H v-% (dry 45.8 N v-% (dry 1.4 LHV MJ/m 3 N (dry 11.3 H O v-% Copyrght 4 by ASME

7 The calculated energetc data of the gasfcaton and gas cleang secton are summarzed Table 4 for a total fuel power of 8. MW. Further mprovement of the gasfcaton process mght be reached by lowerg the gasfcaton temperature or by tegrated fuel dryg. However, these aspects are beyond the focus of ths study. Table 4 Energetc performance of the 8 MW steam gasfcaton process. kw 8 Heat producer gas cooler kw 9 Heat flue gas cooler kw 66 Total heat steam generaton kw 59 Power of exported PG kw 5788 Electrc power put kw 5 chem % 7.4 Q,G % 1.7 IRSOFC performance The energetc performance of the IRSOFC stack s vestgated for producer gas wth a dry composton accordg to Table 3 and a water content of 53 %. The S/C rato of the fuel mxture s 3.5. In the desgn of cell stacks, effectve coolg by an adequate ar dstrbuton s a key ssue. Wth the present work the cell stack s modeled sothermal and the ar utlzaton s set to 5 % (15.3 v-% O the cathode exhaust. The heat balance for operaton at 1 C/ A. m - s fulflled wth anode feed preheated to 65 C and cathode feed to 696 C. From all factors determg the total electrc effcency of the fuel cell accordg to Eq. (13, only polarzaton s dependg on current densty. Therefore, the effcency s presented together wth the effectve voltage Fg. 3 for a certa fuel utlzaton and stack pressure. Fgure 4 shows the power densty of the cell on current densty. The workg pot must be fxed accordg to economc aspects regardg stack sze and stack effcency. To assure operaton stablty, the actual current densty should be to the left of the power densty peak. The pressure mpact on cell voltage and electrc effcency s shown Fg. 5. Note that the x-axs shows absolute pressure and atmospherc pressure s therefore at.1 MPa. Polarzaton effcency creases together wth the reversble cell voltage because the voltage drop due to polarzaton remas constant. SOFC-GT hybrd cycle The hybrd cycle shown Fg. s fuelled wth clean producer gas. The water content anode feed s creased from 6 v-% to 53 v-% by jecton of steam after producer gas compresson. The settgs for the smulaton of the hybrd cycle are summarzed Table 5. The fuel utlzaton s 85 % the fuel cell and 1 % the combustor. The fuel bypass dcated Fg. s zero for standard operaton whle the ar bypass follows the requred turbe let temperature (TIT. The power for fuel compresson must be subtracted from the brut power produced. Effectve cell voltage [V] X fuel 85 % X ar 5 % p.5 MPa IRSOFC temperature T 11 C T 1 C T 9 C T 8 C T 7 C Current densty [A. m - ] Electrc effcency fuel cell el,fc [%] Fg. 3: Effectve voltage and electrc effcency of the fuel cell vs. current densty. Power densty [W. m - ] IRSOFC temperature T 11 C T 1 C T 9 C T 8 C T 7 C X fuel 85 % X ar 5 % p.5 MPa Current densty [A. m - ] Fg. 4: Power densty of the fuel cell vs. current densty. The energetc potental of the exhaust s descrbed by Q,HC as the transferred heat f the gas were cooled to a stack temperature of 15 C. The performance of the hybrd cycle 7 Copyrght 4 by ASME

8 Effcency [%] X fuel 85 % X ar 5 % T 1 C A. m - v conv eta_v eta_conv eta_r eta_vol eta_el_fc Absolute stack pressure [MPa] r vol el,fc Fg. 5: cell performance vs. stack pressure. versus operatg pressure Fg. 6 shows a flat maxmum electrc effcency between.6 and.9 MP whch may be shfted to rght f the ar compresson would clude terstage coolers. The behavor s domated by the GT cycle characterstcs, where the TIT s set to 9 C. At.9 MPa the ar bypass approaches zero and fuel must be bypassed order to reach the TIT. In order to reach hgh electrc effcences stable operaton and at bearable vestment costs, the practcal workg pot wll be left to the peak Fg. 6. The heat effcency shows, how much of the thermal power put to the hybrd cycle can be used by a HRC coolg the exhaust stream to 15 C. Plant performance cludg heat tegraton The hybrd cycle s coupled to the gasfcaton process by usg the exported producer gas as fuel. In order to crease the plant electrc effcency, a heat recovery concept that covers both hybrd cycle exhaust enthalpy and the net coolg energy from the gasfcaton process s consdered. The performance of dfferent technologes has already been presented the modelg secton. In the practcal mplementaton of a steam cycle concept, the whole le of preheatg, evaporaton, and superheatg must be mplemented separately for GT exhaust and flue gas unless the flue gas flter and blower operate at heat recovery let temperature (55 C. The heat for the generaton of fludzaton steam must be decoupled from the steam cycle. If an ORC s used for heat recovery, heat carrer ol s used to brg the energy from the coolers to the actual Ranke cycle. In ths case, fludzaton steam generaton can also be powered by the heat carrer ol. Table 5 Parameters the smulaton of the SOFC-GT hybrd cycle shown Fg.. Pressures [MPa]: ambent ar.11 clean producer gas.97 anode and cathode feed turbe exhaust.15 Temperatures [ C]: ambent ar 15 clean producer gas 65 fuel cell let anode sde 39 * ar feed to ternal ar preheater 47 * fuel cell let cathode sde 771 * fuel cell exhaust anode sde 1 fuel cell exhaust cathode to ternal preheater 1 cathode exhaust after ternal preheater 691 * turbe let temperature (TIT 9 Pressure drops [%]: fuel preheater (fuel/exhaust 1 / 1 ar preheater (ar/exhaust 1 / 1 fuel cell stack anode sde cathode sde cl. ternal ar preheater 3 combustor (anode exhaust/cathode exhaust / Effcences [%]: fuel compressor (sentropc/shaft/motor 8 / 99 / 96 ar compressor (sentropc/shaft 8 / 99 gas turbe (sentropc/shaft/generator 8 / 99 / 97 * Temperatures changg wth pressure/load due to constant heat exchanger are values for.5 MPa cell stack pressure. Hybrd cycle effcences [%], fuel cell bypass of ar and fuel resp. [%] TIT 9 C el,hc,net eta_el_net eta_el_fc el,fc eta_q_15 Q,HC eta_el_gt_net el,gt,net ph_ar_bypass ϕ fuel,bypass ph_ar_bypass ϕ ar,bypass Cell stack pressure [MPa] Fg. 6: Hybrd cycle performance vs. fuel cell operatg pressure wth a TIT lmt set to 9 C. 8 Copyrght 4 by ASME

9 The plant electrc effcency s:, produced Pel, ( m& fuel fuel Pel consumed (3 el, Plant The heat effcency the case of CHP operaton s gven by: ( chem, G Q, HC + Q, G Q HRC (31 Q, Plant, If the coolg temperature of the HRC s such that no heat can be exported, Q,HRC becomes zero. In the case of the one or two stage steam cycle there s stll the heat from producer gas coolg (wth the evaporaton energy for scrubber condensate and fludzaton steam subtracted avalable for heat export. The plant performance for the three dfferent heat recovery concepts s summarzed Table 6. The gas generaton s operated accordg to Table 4, the hybrd cycle parameters are those of Table 5 wth.5 MPa fuel cell operatg pressure. The heat recovery cycles behave accordg to Table. Producer gas, flue gas, and GT exhaust are cooled to 15 C. Table 6 Plant effcency data calculated for dfferent heat recovery technologes. Concept T Q,exp [ C] el,hrc [%] Q,HRC [%] el,plant [%] Q,Plant [%] FU,Plant [%] 1-stage steam cycle stage steam cycle ORC For the rather complex concept wth a two stage steam cycle and a condensaton turbe, the plant reaches an electrc effcency of 45 %. Economcally, such a confguraton may be of terest for stallatons above 1 MW fuel power. The dfference effcency to the one stage steam cycle of only two percentage pots may be outweghed by economc aspects. Sce compact ORC unts are avalable on the market, ths technology allows a smple plant confguraton. For CHP the capacty range below MW fuel power, the ORC concept s probably the most promsg from the economc pot of vew. The energy consumpton by steam generaton for both gasfer fludzaton and anode feed humdfcaton results rather low global effcency values of only about 6 % CHP operaton. Optmzaton potental and exergy Any optmzaton approach has to take the rato between possble mprovement effcency and related costs to account. The theoretcal potental for mprovement of a certa process unt s quantfed by ts exergy loss. To get a more practcal approach towards the optmzaton potental, the rreversbltes must be dvded to avodable and unavodable losses [3]. In order to emphasze the dfferent qualty of dfferent forms of energy lke electrcty, chemcal combed energy, and sensble heat, the exergetc behavor s shown Fg. 7 for a typcal CHP operaton usg an ORC for heat recovery. The exergy of the streams s defed by Eqs. (1-(3 and electrcty s consdered pure exergy. The ma exergy loss can be found the gasfcaton secton. These losses are practcally not avodable due to the rreversble nature of hgh temperature reactons far from chemcal equlbrum (.e. gasfcaton, combuston. Improvement s possble by operaton at lower gasfcaton temperatures and at hgher bed materal crculaton rates (lower temperature dfference between gasfcaton and combuston reactor. The loss gas cleang s relatvely hgh due to the coolg of the scrubber solvent to the envronment. The loss due to producer gas coolg s practcally not avodable because low gas temperatures are requred for the fuel compressor. El. cons Loss PG cooler 3 PG cooler Gasfcaton 939 Loss gasfc Fludzaton Loss preheatg, FC, combuston Loss gas cleang 1517 El power fuel cell Gas cleang FC & comb. 598 Loss GT GT Ar and PG compresson 1335 Evap. add. steam Splt Loss evap Heat carrer ol 8 C FG cooler 7 Evaporaton El power output Loss exhaust gas cooler Loss ORC Exh. cooler Exp. heat 985 ORC 7/9 C Loss FG cooler Loss evaporaton C Heat carrer ol 13 C All values kw! Fgure 7. Exergy streams of a CHP plant wth an ORC for heat recovery (ambent ar and lqud water exergy omtted. The exergy loss of the fuel cell cludg feed compresson, feed preheatg, and post combuston of the exhaust s partly avodable as far as the polarzaton losses the fuel cell are concerned. However, the polarzaton exergy loss s only about 44 kw, 84 kw are lost the DC/AC verter for the effcency chosen. Polarzaton decreases wth creasg stack area. An optmum has to be chosen accordg to economcs. Another crtcal pot wth respect to plant effcency s the hgh amount of steam added to the anode feed. If the IRSOFC can be operated at a lower S/C rato, the reversble cell potental creases and the energy consumpton for steam generaton decreases. The ma part of the electrcty s produced the hybrd cycle whle the HRC contrbutes only wth about 1 % to the total electrcty producton. It s obvous, that the HRC effcency fluences the plant performance only margally and rather the cheapest than the most effcent HRC concept should be selected. Summarzg, the greatest potental for short-term optmzaton of the process can be located the gasfcaton secton and fuel cell operaton at a reduced S/C rato. On a long term bass, rgorous changes the operaton mode lke pressurzed gasfcaton combaton wth hot producer gas cleang can lead to sgnfcant mprovement of the energetc performance. 87 Stack Copyrght 4 by ASME

10 CONCLUSIONS The energetc potental of a couplg between bomass steam gasfcaton and an SOFC-GT hybrd cycle s vestgated. Therefore, an energetc model of an IRSOFC stack has been developed based on data from the lterature. The characterstc behavor of the fuel cell presented agrees well wth other models [8,9]. For the gasfcaton and gas cleang secton, measured data from the 8 MW fuel power commercal plant Guessg/Austra are consdered. The chemcal effcency of the gasfer reaches 7.4 % based on LHV for a fuel water content of wt.-%. The smulaton results for three dfferent concepts of heat recovery are presented: one and two stage steam cycle and a compact ORC unt. The whole plant s mplemented the process smulaton tool IPSEpro. The plant electrc effcences wth heat recovery are 4-43 % for CHP operaton dependg on the HRC technology. For a -stage steam cycle condensaton operaton as HRC, 45 % electrc effcency may be reached. The largest exergy losses occur the gasfcaton secton. Cell polarzaton, feed preheatg and exhaust combuston cause the ma exergy loss of the hybrd cycle. Sgnfcant short-term mprovement of the process can therefore be reached by reducg the rreversbltes durg gas generaton. Hgher fuel and ar utlzaton rates and a lower S/C rato the SOFC also result a better plant performance. The HRC contrbutes only margally to the total electrcty output. The choce, whch HRC concept can be economcally realzed depends therefore strongly on the capacty of the plant. Further work wll am at the stallaton of an SOFC test unt a sde stream of the Guessg plant. ACKNOWLEDGMENT The authors gratefully acknowledge fancal support from the Austran funds program Knet. REFERENCES [1] Hofbauer, H., Rauch, R., Loeffler, G., Kaser, S., Fercher, E., and Tremmel, H.,, Sx Years Experence wth the FICFB-Gasfcaton Process, Proc., 1 th European Bomass Conference, W. Palz et al., eds., ETA, Florence, Italy, pp [] Hofbauer, H., Rauch, R., Bosch, K., Koch, R., Acherng, C., 3, Bomass CHP plant Guessg a success story, In: Pyrolyss and Gasfcaton of Bomass and Waste, Brdgwater, A.V., Ed., CPL Press, Newbury, UK, pp [3] Kaser, S., 1, Smulaton und Modellerung von KWK- Verfahren auf Bass Bomassvergasung, Ph.D. thess, Venna Unversty of Technology, Austra. [4] Massardo, A.F., Magstr, L.,, Internal Reformg Sold Oxde Cell-Gas Turbe Combed Cycles (IRSOFC-GT: Part A- Cell Model and Cycle Thermodynamc Analyss, ASME J. Eng. Gas Turbes Power, 1, pp [5] Campanar, S.,, Full Load and Part-Load Performance Predcton for Integrated SOFC and Mcroturbe Systems, ASME J. Eng. Gas Turbes Power, 1, pp [6] Magstr, L., Costamagn P., Massardo, A.F., Rodgers, C., McDonald, C. F.,, A Hybrd System Based on a Personal Turbe (5 kw and a Sold Oxde Cell Stack: A Flexble and Hgh Effcency Energy Concept for the Dstrbuted Power Market, ASME J. Eng. Gas Turbes Power, 14, pp [7] Selmovc, A., Palsson, J., Networked sold oxde fuel cell stacks combed wth a gas turbe cycle, J. Power Sources, 16, pp [8] Chan, S.H., Ho, H.K., Tan, Y.,, Modellg of smple hybrd sold oxde fuel cell and gas turbe power plant, J. Power Sources, 19, pp [9] Chan, S.H., Ho, H.K., Tan, Y., 3, Mult-level modelg of SOFC-gas turbe hybrd system, Int. J. Hydrogen Energy, 8, pp [1] Chan, S.H., Ho, H.K., Tan, Y., 3, Modellg for part-load operaton of sold oxde fuel cell-gas turbe hybrd power plant, J. Power Sources, 114, pp [11] Andres, J., Buhre, B.J.P.,, Small-scale, dstrbuted generaton of electrcty and heat usg tegrated bomass gasfcaton-gas turbe-fuel cell systems, DGMK Tagungsber, -1, pp [1] Chan, S.H., Khor, K.A., X Z.T., 1, A complete polarzaton model of a sold oxde fuel cell and ts senstvty to the change of cell component thckness, J. Power Sources, 93, pp [13] Baehr, H.D,, Thermodynamk, Sprger, Berl. [14] Dederchsen, Ch., 1991, Referenzumgebungen zur Berechnung der chemschen Exerge, Fortschrtt-Ber. VDI, Rehe 19, Nr. 5, VDI-Verlag, Duesseldorf, Germany. [15] The Internatonal Assocaton for the Propertes of Water and Steam, 1997, Release on the IAPWS Industral Formulaton 1997 for the Thermodynamc Propertes of Water and Steam, Erlangen, Germany. [16] Baehr, H.D., 1979, De Exerge der Brennstoffe, Brennst.-Waerme-Kraft, 31, pp [17] Eguch, K., Kojo, H., Takeguch, T., Kkuch, R., Sasak, K.,, flexblty power generaton by sold oxde fuel cells, Sold State Ioncs, , pp [18] Twgg, M.V., 1989, Catalyst Handbook, nd Ed., Wolfe Publshg Ltd., London, UK. [19] Pmenov, A., Ullrch, J., Lunkenhemer, P., Lodl, A., Ruescher, C.H., 1998, Ionc conductvty and relaxatons ZrO -Y O 3 sold solutons, Sold State Ioncs, 19, pp Copyrght 4 by ASME