Energy and exergy analysis of an ethanol fueled solid oxide fuel cell power plant.

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1 Enrgy and xrgy analyss of an thanol fuld sold oxd ful cll powr plant. Yannay Casas a, Lus E. Artaga a, Mayra Morals a, Elna Rosa b, Lus M. Pralta a and Jo Dwulf c. a Chmcal Engnrng Dpartmnt. Cntral Unvrsty of Las Vllas. Road to Camauaní Km 5.5. Santa Clara, c/p 54830, Vlla Clara, Cuba. b Appld Chmstry Cntr, Cntral Unvrsty of Las Vllas. Road to Camauaní Km 5.5. Santa Clara, c/p 54830, Vlla Clara, Cuba. c Rsarch Group ENVOC, Ghnt Unvrsty, Coupur Lnks 653, 9000 Ghnt, Blgum Tlf: (53) (422) Fax: (53) (422) Abstract: A sold oxd ful cll (SOFC) systm ntgratd wth an thanol stam rformng stag s valuatd consdrng th frst and scond laws of thrmodynamcs. Th rrvrsblty losss dstrbuton and th plant nrgy and xrgy ffcncs ar studd undr dffrnt procss condtons (823<T<973K) and watr to thanol molar ratos (5<R AE <6.5). Th post combuston of th cll off gass for th hat rcovry s also takn nto account to mantan th systm opraton wthn th auto-sustanablty boundars. An ncras of ffcncy and rrvrsblty at th stack s rportd whn th ractants rato s ncrasd. Th hghr losss ar placd at th stam rformr (280kW 350kW) and th cll (400kW 590kW) du to th combnaton of th chmcal composton and stram condtons on chmcal and physcal componnts of th xrgy. Ky Words: Exrgy Effcncy, Ful cll, Irrvrsblty, Stam rformng. 1. Introducton Ful clls ar consdrd to b th propulson systm of th nar futur, snc thy can produc lctrcty wthout pollutng th nvronmnt, and possss th ncssary spcfc powr, powr dnsty and durablty to rplac convntonal ntrnal combuston ngns from thr currnt applcatons [1]. In rcnt yars, th sold oxd ful cll (SOFC) runnng on pur hydrogn or crud gass has drawn grat attnton, du to ts hgh ffcncy and dgr of ntgraton vn wth turbn cycls [2-4]. SOFCs support ntrnal convrson of lght hydrocarbons, alcohols and carbon monoxd wthout usng nobl mtals as lctrods [1,2,5]. A tradtonal mthod to study a powr gnraton systm s th nrgtc analyss applyng th frst law of thrmodynamcs; t has bn wdly usd to asss th sold oxd ful clls. In that papr th ffct of th stam rformng kntc pattrn was takn nto account and th pnch mthodology was appld to mnmz th us of utlts. Howvr, t s clar that nstad, an xrgtc analyss wth xrgy as th masur of th qualty (usful part, transformabl to work) of nrgy can b usd to spcfy dsgn optma whch ar dffrnt from thos rsultng from th nrgy consrvaton law [7]. In ths sns Douvartzds t al. [9] dvlopd an nrgy xrgy analyss n ordr to optmz th opratonal condtons of a SOFC powr plant, consdrng only th hydrogn oxdaton wthn th ful cll, and rctng th ffct of th cll losss, nstu mthan rformng and carbon monoxd convrson. Morovr, Douvartzds t al. [8-9] dd not tak nto account th ffct of th kntc pattrn of th thanol stam rformng (ESR) on syngas composton and nstad thy us th xtnt of th racton (ε). Th optmal condton was rachd for a SOFC ful utlzaton factor of 79.85%, an thanol convrson of 100%, watr to thanol rato 3:1 and no nrgy ntgraton was dvlopd. Th xrgy analyss of an ntgratd ntrnal mthan rformng - sold oxd ful cll - gas turbn (IRSOFC GT) powr gnraton systm was prformd by Pgah [10]. Corrspondng author: Yannay Casas Ldón mal:yannay@uclv.du.cu

2 Th n-stu partal oxdaton of mthan and th lctrochmcal oxdaton of hydrogn at th cll anod wr takn as man ractons. Ful cll off gas was usd to fd a turbn fulfllng n ths way th powr rqurmnts for ful comprsson. Th thrmodynamc losss n ach unt wr calculatd and no nrgy ntgraton stratgy was takn nto consdraton. Th us of rnwabl fuls coupld to SOFCs hav bn also rportd by Panopoulus t al. [11-12] and Fryda t al. [13] whch studd th xrgy ffcncy of a bomass stam gasfcaton ractor ntgratd wth a hgh tmpratur SOFC n a combnd hat and powr schm. In th prsnt papr, th frst and scond laws of thrmodynamcs ar combnd to obtan a systm confguraton and optmal opratonal condtons for an xtrnal catalytc thanol stam rformr coupld to a sold oxd ful cll systm. Th systm combns th rnwabl charactr of thanol wth th tchncal advantags of ful clls to dsgn a nar zro msson systm wth a hgh dgr of ffcncy. On th othr hand, a dtald thrmodynamc modl for th valuaton of th SOFC s provdd and a kntc modl s usd to asss th convrson of mthan wthn cll anod. Th dstrbuton of rrvrsblty on ach dvc and th whol procss ar rportd ond to th xrgy and nrgy ffcncs. Th study starts wth an ntgratd procss flow dagram as bas cas [14] and all th analyss ar dvlopd takn nto account th ffct of th hat xchangr ntwork (HEN) dsgn throw pnch mthodology. Th mathmatcal procssng of th modls s carrd out usng th Aspn Hysys gnral purpos modlng-nvronmnt. 2. Dscrpton of th systm Th thanol stam rformr sold oxd ful cll systms ar dpctd n Fg. 1. Th ntal fluxs of watr and thanol ar pumpd nto th Mxr whr an sothrmal mxng taks plac at atmosphrc condtons (298 K and 1atm). Aftr that, th lqud mxtur s vaporzd and prhatd on dvcs HEX-1 and HEX-2 pror to th Ractor nlt. Th ndothrmc stam rformng of th thanol H ( = kj/mol) s studd consdrng a packd bd ractor chargd wth a N/Al hydrotalct catalyst [15-16] and among th varous racton pattrns rportd prvously [14,16-17] th Lagmur-Hshlwood kntc modl rportd n Artaga t al. [6,14] s usd to dscrb a sx stp racton schm ncludng th cok dposton on catalyst surfac Th mxtur lavng th Ractor s thn fd nto a sold oxd ful cll modul whr an ar flux (21%O 2, 79%N 2 ) s usd as oxdant whch s prvously comprssd and hatd n a Comprssor, HEX-3 and HEX-4. Th SOFC modl s usd to study th procss and to dsgn varous scnaros consdrng varatons of th rformr opratonal paramtrs. Snc th λ SOFC rato of th syngas fd nto th anod s hghr than two, no carbon dposton problms ar supposd to occur on th anod of th SOFC and th n-stu mthan rformng; ths s studd usng a kntc ractor modl whch consdrs th powr law pattrn wth a ngatv racton ordr for watr [23]. Morovr, th shft convrson of th carbon monoxd s smulatd usng a Gbbs ractor modl (qulbrum), n ths way th offgas composton s calculatd rgorously and th nrgy rcupraton n th post-combustor agrs wth th ral pctur of th problm. Th ful utlzaton factor s dfnd by (Eq. 1). Th hat balanc n th cll consdrs th hat consumd n th mthan rformng and th hat producd by th lctrochmcal racton and shft convrson. U f ( ) f n H2 + ( 4 X CH4 f n CH4 + X CO f n CO f out H2 n H2 X n CH4 X n CO ) f + 4 f + f = (1) CH4 Th SOFC xhaust contanng H 2, CH 4, O 2, N 2, CO, H 2 O, and CO 2 s coold (to avod NO x formaton) and burnd downstram n th post-combuston systm. Th post-combuston unt s modld as an adabatc convrson ractor (ConvRact) and th dpltd hat s usd to balanc th nrgy rqurmnts n th procss. CO Corrspondng author: Yannay Casas Ldón mal:yannay@uclv.du.cu

3 Fg 1. Procss flow dagram (PFD). 3. Scond Law Analyss physcal and chmcal; mor prcsly, th physcal xrgy xprsss th usful work that a substanc can produc whn brought rvrsbly from ts stat to th rstrctd dad stat and t can b wrttn as: ph = ( H H ) T ( S S ) (2) o o o Chmcal xrgy s obtand whn th componnts of th nrgy carrr ar frst convrtd to rfrnc compounds and thn dffus nto th nvronmnt, whch s n rfrnc (dad) stat. For a gasous stram flow, th molar chmcal xrgy ( ch ) of all spcs s gvn by th (2) Fryda t al. [13]: ch = x o RT o x ln(x ) + (3) No dvatons btwn ral nvronmntal and rfrnc condtons ar consdrd. Th standard chmcal xrgs of all componnts of ach stram rprsntd n th PFD (Fg. 1) ar showd n th Tabl 1. Exrgy analyss s a thrmodynamc mthod of usng th consrvaton of mass and nrgy prncpls togthr wth th Scond Law of Thrmodynamcs for th dsgn and analyss of thrmal systms. Th purpos of an xrgy analyss s gnrally to dntfy th locaton, th sourc, and magntud of tru thrmodynamc nffcncs n a gvn procss. Exrgy s th maxmum work that can b producd whn a hat or matral stram s brought to qulbrum n rlaton to a rfrnc nvronmnt. In ths study a tmpratur of T o = K, prssur P = bar and nvronmnt composton of % N 2, %O 2, 0.03 %CO 2, 3.03 % H 2 O and 0.92 % Ar ar assumd as rfrnc [18]. In th prsnt artcl th xrgy of ach matral stram s xprssd as th sum of two componnts, Tabl1.Standards Chmcal Exrgy (Stds.Chm.Exrgy) of ndvdual spcs. [26] Spcs Formula Stds. Chm. Exrgy Ntrogn N Oxygn O Watr H 2 O g Watr H 2 O l 3120 Carbon CO Doxd Carbon CO Monoxd Argon Ar Ethanol C 2 H 5 OH g Ethanol C 2 H 5 OH l Mthan CH Hydrogn H Carbon C Not: All th xrgy valus ar prsntd n kj. Kmol -1. An xrgy balanc for a control volum at stady stat s formulatd to calculat th xrgy dstructon (E Dst ) of th systm at dffrnt opratonal condtons. In th prsnt work all th nlt and outlt strams ar consdrd to dtrmn th rrvrsblty dstrbuton, whch nclud th sum of mattr, nrgy and powr. Th mxr, hat xchangrs (HEX-1 to HEX-6),

4 thanol stam rformng ractor, th SOFC modul, th post-combuston unt and th auxlary qupmnts (pumps and comprssor) ar ncludd nto th lmts of th systm (Dashd lns ara n Fg. 1). Th procss global xrgy balanc, by gnorng th changs n kntc and potntal xrgs, s xprssd as: E rrv = ( f ) ( f ) nlt outlt T 1 o (4) + Q W T Whr E rrv =To*S gn (th Gouy -Stodola thorm) rprsnts th rat of xrgy dstructon nto th dvc du to rrvrsblts, s th total xrgy of ach chmcal spcs, whch s th sum of th physcal and chmcal xrgs Dfnton of th rrvrsblts Irrvrsblty at ach stag of th procss s calculatd basd on th approachs dscrbd prvously. Th quatons for th valuaton of ths paramtr ar prsnt blow Stam rformng ractor Exrgy dstructon of th ESR can b xprssd by th followng quaton. f + f E = S05 S05 S14 S14 (5) ESR f f S06 S06 S15 S Hat Exchangr qupmnt Th xrgy balancs n all hat xchangr dvcs can b xprssd as follows: T o ph ph E = 1 Q f _ HEX + T k nlt k outlt hot ph ph + f k nlt k outlt cold (6) Whr s th hat xchangrs and k rprsnt ach stram of procss (nlt and outlt). Th prvous quaton s not affctd by chmcal xrgy du to th chmcal compostons of th hot and cold nlt strams ar constants n th hat xchang qupmnt Sold Oxd Ful Cll Th chmcal transformatons nto th ful cll cathod and anod, th dsspatd hat to th nvronmnt and th powr dlvrd by th lctrochmcal racton ar consdrd nto th rrvrsblts. Thn th xrgy dstructon n th SOFC s calculatd as blow: E SOFC + + = 1 [( f f )] S09 S09 S11 S11 [( f f )] S06 S06 T o T SOFC S10 Q SOFC P SOFC S10 cathod anod (7) Comprssor Th xprssons to dtrmn th comprssor rrvrsblts ar rportd by Pgah [10]. It taks nto account th comprsson ratos, polytropc ffcncs and flud condtons. 1 η p,c E C = f ar R T o ln( CPR) (8) η p,c Post-Combuston unt In th aftr burnr th rrvrsblty s dfnd consdrng th chmcal and physcal componnts, th losss by hat transfr ar zro bcaus th combustor oprats adabatcally: E = ch ch PC f f S13 S13 S14 S14 (9) ( f H f H ) S13 S13 S14 S14 + T ( f S f S ) 0 S13 S13 S14 S14 4. Systm nrgy and xrgy ffcncs Enrgy ffcncy of th whol systm (Eq.10) s dfnd by th rato btwn th dlvrd powr and th amount of nrgy contand n th thanol molcular structur t s rfrrd to th LHV. W η nrgy = (10) ( f n LHV) thanol Whr W s th global powr gnratd by th systm (work producd work consumpton).

5 On th othr hand, xrgy ffcncy (Eq.11) s rfrrd to th xrgtc potntal of th prmary ful (standard xrgy of thanol). W η xrgy = (11) f n o ( ) thanol 5. Rsults and dscusson 5.1. Exrgy analyss consdrng opraton paramtrs An xrgy study has bn dvlopd n ordr to smulat th powr plant usng th Sold Oxd ful cll tchnology rprsntd n th Fg. 1. Ths smulaton program s abl to calculat th flow rat, tmpratur, prssur, nrgy and th xrgy contnt n vry stram of nlt and outlt as wll as th xrgy dstructon by rrvrsblts of ach stag nvolvd n th plant. Th rsults of th xrgy analyss appld to th bas cas ar shown n th Tabl 2. Th condtons slctd wr T ESR = T SOFC = 923 K, R AE = 6.0, U f = 80 %, V cll = 0.64 V, comprssor polytrophc ffcncy η p,c = 90 % and λ SOFC > 2. Consdrng th standard chmcal potntal of thanol as th nlt flow avalablty, th rrvrsblts rprsnt approxmatly 66 % of th flow avalablty, so th xrgy ffcncy taks a valu of 34 %. wll as th xrgy ffcncy. Accordng to th xplanaton abov; th xrgy ffcncs ar favord by hghr rformr tmpraturs and watr to thanol fd ratos. Exrgtc ffcncs rach valus rangng from 32 % to 35 % approxmatly n all analyss; th hghr ffcncs ar obtand at 973 K and R AE of 6.5. From an rrvrsblty pont of vw, th systm studd s not favord by th ncras of th rformr opraton varabls. Th lowr rrvrsblty valus ar obtand at lowr tmpratur and R AE, nvrthlss t corrsponds to lowr xrgy ffcncy of th procss (lss than 30%), and so thr should always b a balanc btwn systm ffcncy and systm rrvrsblty. Furthr on, t s mportant to xprss that th flow dagram s dsgnd on ntgraton prncpls allowng an optmal us of th hot strams and kpng th dsgn of th plant to oprat autonomously, wthout any ntracton btwn xtrnal sourcs of hat and powr (auto-sustanablty). Tabl 2. Bas cas smulaton rsults. Cll voltag Inlt flow avalablt Stack powr Nt powr Enrgy ffcncy Exrgy ffcncy 0.6 V kw kw kw % % Th ffct of th rformng tmpratur and ractants molar rato on xrgtcs ffcncy and losss ar prsntd n th Fg. 2 and Fg. 3 rspctvly. Th ful cll powr and th thanol flow hav a notabl nflunc on th xrgy ffcncy accordng to th dfnton wrttn prvously. On th othr hand, th cll powr s drctly proportonal to th hydrogn obtand n th rformr. Bcaus of ths; th hghr hydrogn yld allows th ncras of th SOFC powr as Fgur 2. Effct of th rformng tmpratur and R AE on xrgtc ffcncy.

6 Fgur 3. Irrvrsblts, varyng th rformng tmpratur and R AE. Th rrvrsblty of th stags nvolvd n th plant at rformng tmpratur of 923 K s llustratd n Fg. 4. Th total xrgy dstructon n th systm s causd manly by th SOFC, rformr and post-combustor, whch rprsnts mor than th 50 % of th nlt flow avalablty, smlar rsults ar rportd by Pgah [10]. Changs n th chmcal xrgs ar mor rlvant than th changs of th physcal contrbutons; ths phnomnon s rlatd fundamntally wth th chmcal ractons xtnts. Th xrgy dstructon of th rformr s ncrasd from kw to kw for fd molar ratos of 5 and 6.5 rspctvly. That chang n prformanc s du to th ncras on th dmands of hat n th rformr wth R AE, whch s causd by th ncras of ractons convrson takng plac n ths stag and th total flow. Fgur 4. Exrgy dstructon by rrvrsblty n cycl stags varyng R AE. In ordr to nvstgat th prformanc of th xrgtc losss n th ful cll, w assumd that th hat rctd by th lctrochmcal racton s a wast, bcaus of ths; th xrgy dstructons by hat losss ar sgnfcant, rachng valus of 70% of th xrgy dstructon of th SOFC for watr to thanol molar rato of 6.5. Th ffcnt us of wastd hat by th cll n a turbn, hat ngns or n a combnd cycl can rduc th rrvrsblts n ths stag and th global systm, as wll as to mprov th xrgy and nrgy ffcncs [19]. Th ncras of R AE produc a hghr hydrogn flow to th ful cll, convrtng mor chmcal nrgy nto lctrcty, whch mans mor currnt and powr producd. Nvrthlss at th sam condton, th rrvrsblty s ncrasd n th ful cll stack du to th ncras of th wast hat and dsspatv phnomna (ovrpotntal losss). Th xrgy dstructon n th post-combustor prsnts a lttl dcras for dffrnt R AE. Thos losss ar assocatd to th rrvrsblty of th combuston procss of H 2, CO and CH 4, th amounts of ractants, th tmpratur of th combustor and th stochomtrc rato of th combuston. In ths cas th amounts of fuls tak an mportant rol; spcally th nrgtc contnt of th nlt stram s rducd wth

7 th ncras of th watr to thanol molar rato, allowng lowr losss by th hat transfr. 6. Conclusons Th valuaton prsntd n ths papr allows obtanng a complt da of th ral work dlvrd by an ntgratd sold oxd ful cll and an thanol stam rformng unt and t rlatonshp wth th most rlvant opraton varabls. Th ffct of watr to thanol rato and th rformng tmpratur on th nrgy and xrgy ffcncy of th systm was also dscussd. Th hghr xrgy ffcncs (35% at 973 K and R AE =6.5) also concd wth hghr total rrvrsblty losss (1050kW) du to th ffct of th chang n both th chmcal and physcal componnts. It was also dmonstratd that a balanc should b stablshd by th procss ngnr consdrng qulbrum btwn powr producton and procss ffcncs. Nomnclatur CPR comprsson ratos E xrgy dstructon, kw. total xrgy of ach chmcal spcs, kj/kmol. ch molar chmcal xrgy of spcs, kj/kmol. ph molar physcal xrgy of spcs, kj/kmol. F n thanol thanol flow fd to th ractor, kmol/s. H molar absolut nthalpy, kj/mol. H o molar nthalpy at rfrnc stat, kj/mol. P SOFC cll powr output, kw. Q hat duty of componnt, kw. R unvrsal gas constant, J/(K mol). R AE watr/thanol molar rato. S molar absolut ntropy, kj/(mol K). S o molar ntropy at rfrnc stat, kj/(mol K). T tmpratur of componnt, K. T 0 tmpratur of rfrnc stat, K. x mol fracton of spcs. Grks Lttrs η xrgy xrgy ffcncy, %. η nrgy nrgy ffcncy, %. Subscrpts C comprssor. ESR rformr. HEX hat xchangs. Hot stram of HEX. Cold stram of HEX. PC post-combuston. SOFC sold oxd ful cll. ach chmcal spcs of th stram. ach componnt of systm (mxr, rformr ful cll). k strams of procss ( nlt and outlt) S06, S07 S15 strams of th flow dagram 7. Rfrncs [1]. Hrschnhofr, J.H., t al., 2006, Ful Clls Handbook, sxth Ed., EG & G Tchncal Srvcs, Natonal Enrgy Tchnology Laboratory. P.O. Box 880. Morgantown, Wst Vrgna [2]. Thorud, B., Dynamc Modllng and Charactrzaton of a Sold Oxd Ful Cll Intgratd n a Gas Turbn Cycl. Doctoral Thss. Dpartmnt of Enrgy and Procss Engnrng. Norwgan Unvrsty of Scnc and Tchnology. [3]. Cals, F., Palomb, A., Vanol, L., 2006, Dsgn and partal load xrgy analyss of hybrd SOFC GT powr plant, Journal of Powr Sourcs, (158), pp [4]. Cals, F., t al., 2006, Smulaton and xrgy analyss of a hybrd Sold Oxd Ful Cll (SOFC) Gas Turbn Systm, Enrgy, (31), pp [5]. Ballard., 2007, Cas Study - Rsdntal Cognraton. Groundbrakng Ful Cll Soluton. Ballard Powr Systms Inc Glnlyon Parkway Burnaby, Brtsh Columba. [6]. Artaga-Prz, L.E., t al., 2009, An autosustanabl sold oxd ful cll systm fuld by bo-thanol Procss smulaton and hat xchangr ntwork synthss, Chmcal Engnrng Journal, 150, pp [7]. Hotz, N., Sn, S.M., Poulkakos, D., 2006, Exrgy analyss of a sold oxd ful cll

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