PERFORMANCE ANALYSIS OF INTERNALLY REFORMED MCFC/IT-SOFC COMBINED CYCLE

Transcription

1 HEFAT008 6 th Intenational Confeence on Heat Tansfe, Fluid Mechanics and Themodynamics 30 June to July 008 etoia, South Afica ae numbe: MA3 ERFORMANCE ANALYSIS OF INTERNALLY REFORMED MC/IT-SO COMBINED CYCLE Musa A.*, Huisseune H. and De aee M. *Autho fo coesondence Deatment of Flow, Heat and Combustion Mechanics, Univesity of Ghent, Sint-ietesnieuwstaat, 4,9000 Gent Belgium, Musa.Abdullatif@ugent.be, Michel.Deaea@ugent.be. ABSTRACT High temeatue fuel s such as the solid oxide fuel (SO) and molten cabonate fuel (MC) ae consideed extemely suitable fo electical owe lant alication. The intemediate temeatue (IT) SO and MC efomances ae calculated using numeical models which ae built in Asen custome modele fo the intenally efomed (IR) fuel s. These models ae integated in Asen lus TM. In this aticle, a new combined cycle is oosed: this combined cycle consisting of two-staged of MC and IT-SO. The MC&IT-SO combined cycle and single-staged cycle with MC only ae simulated in ode to evaluate and comae thei efomances. The simulations esults indicate that the net efficiency of MC&IT-SO combined cycle is 64.6 % unde standad oeation conditions. On the othe hand, the net efficiency of single-staged MCC cycle is 5.6%. In othe wods, the cycle with two-staged MC and IT-SO gives much bette net efficiency than the cycle with single-staged MC. INTRODUCTION The develoment of enegy system with eadily available fuels, high efficiency and minimal imact ae equied in ode to meet inceasing enegy demands and to esond to envionmental concens. Inceasingly, fuel systems ae being consideed as omising solutions. Fuel s convet chemical enegy into electical enegy and have the advantage of continuous suly of eactant gases. The fuel s used fo stationay enegy oduction ae tyically high temeatue fuel s (HTs) such as solid oxide fuel (SO) and molten cabonate fuel (MC). In combination with gas tubine (GT) systems the MC o SO ae now unde study as at of new highly efficient owe lants. The most tyical hybid configuation suggested in the liteatue is a ecueated gas tubine ocess with a MC as coe unit of the system. This tye of the system has been investigated in [3-7]. MCs can achieve an electical efficiency of %. A new aoach is using seially connect fuel s. The fuel s have a highe efficiency than gas tubine and by aising the at of the cycle owe they suly, cycle efficiency can be aised. The United State Deatment of Enegy (DOE) has investigated ways to achieve high owe geneation efficiency of 80% and has oosed a multi-staged fuel system with five seial stages of fuel s in ode to educe the egeneative heat fo fuel and ai, and to extend the oeating temeatue ange of fuel s. In [] Aaki and co-authos analysed a owe geneation system consisting of two- stages extenally efomed SOs with seial connection of low and high temeatue SOs. They showed that the owe geneation efficiency of the two-staged SOs is.3% and the total efficiency of owe geneation with gas tubine is 56.% unde standad oeating conditions. In this ae newly a combined cycle is oosed and investigated. Themodynamic models fo intenally efomed IT-SO and MC ae develoed. These fuel s ae combined in diffeent ways in ode constuct single-staged and two-staged fuel system combining diffeent tyes. The aim of the ae is to find the best configuation fo a singlestaged o a two-staged combined system. Theefoe, the efomance of two tyes of cycles is analysed: combined cycle consisting of two-staged of MC and IT-SO and singlestaged cycle with MC. NOMENCLATURE F [kc/kmol ] Faaday s constant, i [A/cm ] Cuent density Q & [kw] Heat [a] atial essue K [-] Equilibium constant h [kj/kmol] Enthaly

2 n [mol s - ] Mole flow ate s [kj/kmolk] Entoy SO [kw] SO stack electical owe R t [Ω m²] Total esistance W & cv [kw] Wok in a contol volume σ cv [kw/k] Entoy oduction in a contol volume Com [kw] Comesso owe tub [kw] Tubine owe A [m ] Active aea T [ C] Cell temeatue u f [-] Total fuel utilization V [V] Cell voltage S/C [-] Steam-cabon atio Subscits and Suescits 0 [-] At standad temeatue and essue i [-] Initial [-] oduct R [-] Reaction o eactants Geek symbols [-] Efficiency ohm ol, an ol, ca [Ω m²] Ohmic esistance [Ω m²] Anodic eaction esistance [Ω m²] Cathodic eaction esistation V [V] Oveotential loss CYCLES DESCRITION The SO and MC s cuently in oeation ae fuelled with natual gas. The high temeatue inside the s stack allows fo efoming the methane diectly inside the if the steam is ovided at the inlet. The heat necessay fo this efoming eaction is deliveed by the electochemical eaction in the. Fuel is ovided at atmosheic conditions. The fuel is ue methane (CH 4 ). In the cycles at of the anode gasses is ecycled, as the anode gasses contain steam needed in the efoming eaction. This is a way of avoiding a steam geneato in the cycles. MC/IT-SO Combined Cycle Configuation Fig. shows a cycle diagam of the combined cycle consisting of an MCC and IT-SOF. In this cycle the anode flow of the MC and IT-SO is in aallel connected. Methane is admitted into the heat exchange H/E to eheat the methane. The eheated methane is slit into two ats. at of the eheated methane is mixed with the ecycling anode gases; the mixtue is sulied to the anode side of MC stack. The emaining at of the eheated methane is mixed with the at of ecycling anode gases and then entes into the anode side in the IT-SO stack. In both stacks the emaining at of anode gases is ecycled to the combusto. The combusto exit gas which contains a majo at of ai, and CO is slit into two ats. The fist at is the cathode inlet gas of the MC stack. The emaining at of the combusto exit gas and cathode outlet gas of MC ae mixed, the mixtue is sent to a gas tubine and heat exchange (H/E) esectively. The cathode gases of the IT-SO stack is ecycled to the combusto. The comessed ai fom the comesso (AC) is sulied to the heat exchange (H/E) and then entes into the cathode side of the IT-SO stack. Single-staged MC Cycle Configuation The single-staged MC cycle is simila to the combined cycle (Fig. ), excet that thee is no IT-SO stack. The combusto exit gas is slit into two ats. The fist at of the combusto exit gas and the comessed ai fom the comesso (AC) ae mixed. This mixtue is sent to the heat exchange (H/E) and then entes into the cathode side of the MC stack. The emaining at of the combusto exit gas is mixed with at of the cathode outlet gas, the mixtue is sent to a gas tubine and heat exchange (H/E) fo ecoveing enegy. The emaining at of the cathode outlet gas is ecycled to the combusto. In this cycle at of the eheated methane is byassed to the combusto. C Exhaust 353 C 6 C H/E Anode Anode Ai 5 C CH4 5 C 368 C MC Cathode IT-SO Cathode AC GT IT-SO 634 C MC 800 C 30 kg/h H/E 7 C 0 C Fuel Combusto Figue Schematic diagam of MC&IT-SO combined cycle (AC: ai comesso; : fuel comesso; GT: gas tubine; H/E: heat exchange)

3 CYCLES ANALYSIS METHODE Assumtions and calculation conditions of the models The assumtions and conditions of the models used in the simulation ogam ae as follows: Steady state conditions with negligible fictional losses Negligible changes of otential and kinetic enegies in any ocess Changes in the comosition of the anode and cathode gases ae only significant in the flow diection. Oxidant and fuel ae consideed to be ideal gases. Nenst otential is indeendent of hydostatic essue gadient. The oeating temeatue of the is equal the outlet cathode and anode temeatue. Themochemical asects: wate gas shift and methane efoming eactions In the models the chemical eactions ae assumed to be in equilibium. This means that the eactions occu instantaneously and each the equilibium condition sontaneously at each osition. Fo SO models the electochemical eaction is imlemented: O + e O cathode () H + O H O + e anode () H + O H O oveall eaction (3) Fo MC model the electochemical eaction is imlemented: + O + e CO3 CO3 HO + CO H O H O CO cathode (4) H + + e anode (5) + oveall eaction (6) The IT-SO and MC oeate at a temeatue high enough to efom the steam efoming inside the anode. The chemical eactions in the steam efoming and shifting of methane ae shown below: CH 4 + H O CO + 3H (7) CO + HO CO + H (8) The electochemical and wate gas shift eactions ae exothemic and, on the othe hand, fuel efoming is a stongly endothemic eaction. SO AND MC MODELS DESCRITION The fuel is teated as a single contol volume on which the steady state flow enegy equation is alied with the assumtion of negligible change of kinetic enegy and otential enegy. To detemine actual efomance, the oveotential must be deducted fom the Nenst otential ( E ) which eesents the ideal efomance. V = E (9) V loss Evaluation of voltage dos in IT-SO The model of the IT-SO stack used in this ae is based on an existing IT-SO model []. In the model the will utilize an electolyte with the thickness of 5µm, a µm thick cathode and µm thick anode. The s ae assumed to be stacked between biola inteconnect lates. Diffeently fom the high temeatue (HT) SO, which utilizes ceamic inteconnect, the lowe oeating temeatue of the IT-SO endes ossible alication of metal alloys. It is difficult to beak down eoted chaacteistics in these individual contibutions, and in the esent context whee the aim is stack and system modelling all losses is simly lumed togethe in one equivalent esistance. The esistance is assumed to have the following temeatue deendence []. Ε R tot = Αex (0) Κ T Whee T eesents the oeating temeatue and Κ is the Boltzmann constant. The activation enegy 9 ( Ε =.0 0 J) and the e-exonential facto 4 ( Α =.98 0 ) ae chosen to fit the exeimental data eoted by [8]. The voltage of the IT-SO can be calculated by V RT RT H K = ln + ln F F O H O ( irtot ) () Evaluation of voltage dos in MC In this aticle, the intenal esistances of the MC stack can be calculated as in [3]. The ohmic esistance ( ), anodic eaction esistance ( esistance ( ol, ca ol, an ohm ) and cathodic eaction ) ae eflected in the functions of the eaction temeatues and atial essue of gas constituents. The ohmic esistance included the ionic and electic esistance; it is calculated using an Ahenius equation as function of the oeating temeatue (eq ). ohm = 0.5ex 306 () T ,.7 0 ex ol an = H CO HO (3) T , ex ol ca = O CO (4) T The voltage of the MC can be calculated by

4 RT F RT F H ( + + ) H O COan V = ln K ln i ohm ol, an ol, ca COca (5) Electical owe of the fuel s The electical owe oduced by the fuel is calculated by: & (6) W = V I Similaly, the heat lost to the envionment at an equilibium state can be detemined eadily by evaluating the entoy ate balance fo a contol volume. Q & cv = T ( S σ ) cv (7) Whee: ( ) S = o o ( ) o R + H O S H ln O S H S O ( H ) O The themodynamic entoy oduction, which is actually the ievesibility, is elated to the electochemical oveotential as [4]: F T σ cv = V loss (8) CYCLE ERFORMANCE VARIABLES The steam cabon atio is defined as the atio between the mole flow ate of steam and the CH4 mole flow ate to the anode. S / C H O = (9) CH 4 O The fuel utilisation facto is defined by: u f H in + H consumed = (0) COin The fuel efficiency ( + 4 CH in 4 ) is defined as the atio of owe oduced by the fuel (IT-SO o MC) to the lowe heating value ( LHV ) of the total amount of fuel ( Q = m LHV ) sulied to the system. tot tot * = () Qtot The net cycle efficiency ( net owe oduced by the fuel s ( ) is defined as the atio of the ) and the tubine, minus the total comesso owe, to the ( LHV ) of the total amount Q tot of fuel ( ) tot s s + tub com net = () Q Table : Setting aametes of the cycles A m² T of IT-SO 7 C 6 ba i of MC 0.5Acm - u f 0. Fuel eciculation ate 0. S/C atio fo comesso 0.8 T of MC 6 C fo tubine 0.85 RESULTS AND DISCUSSION The cycle s layout unde investigation is simulated unde the standad oeating conditions shown in table. The aim of this analysis is to evaluate and comaed the efomance of the cycles. efomance of the MC/IT-SO combined cycle In these simulations each one of the standad oeating conditions was changed to see its effect on the MC&IT- SO combined cycle efomance. The total fuel utilisation ate of the combined cycle is 85% (4.5% in the MC, 4.5% in the IT-SO). In the following analyses, the tiangles ( ) indicate the net efficiency of the MC&IT-SO combined cycle and the solid squaes ( ) indicate the efficiency added by both MC and IT-SO. Cuent density Fig. shows the changes in the net efficiency of the combined cycle with mean cuent density and gives an idea about the contibution of the s and the gas tubine to the net efficiency in the combined cycle. Inceasing the cuent density inceases the oveotential of stacks, esulting in decease of the efficiency of MC and IT-SO. When the cuent density inceases the MC outlet temeatue and the exhaust fuel, which is sent to the gas tubine incease togethe with the inceasing tubine outut. Since the decease in the efficiency of both MC and IT-SO is lage than the incease in the efficiency of gas tubine, the net efficiency of the combined cycle goes down MC&IT-SO cycle MC+IT-SO Cuent density (A/cm²) Figue Change of net efficiency of combined cycle with cuent density

5 Oeating temeatue of MC Fig. 3 shows the changes in the net efficiency of the combined cycle with the oeating temeatue of the MC. Inceasing the oeating temeatue of the MC deceases the electolyte esistance in MC stack, the efficiency of MC incease. In the bottoming gas tubine cycle, incease the oeating temeatue of the MC causes an incease in the outlet temeatue of the MC. This means that the tubine inlet temeatue inceases with inceasing tubine outut. Since the incease in the MC and gas tubine efficiencies ae lage than the decease in the IT-SO efficiency, the net efficiency of the combined cycle goes u MC&IT-SO cycle MC+IT-SO Oeating temeatue of MC ( C) Figue 3 Change of net efficiency of combined cycle with oeating temeatue of MC Oeating essue Fig. 4 shows the changes in the net efficiency of the combined cycle with essue. Raising the essue causes an incease in the Nenst otential. Also inceasing the oeating essue leads to some decease in the oveotential, esulting in an incease in the net efficiency in the combined cycle. An incease in the comessos outlet temeatue due to the incease of the oeating essue causes an incease in the tubine inlet temeatue, esulting in an incease of gas tubine outut. In aticula, the highe the oeating essue, the highe the voltage detemining a emakable imovement of the electical of the system. It is also obvious that when the value of the oeating essue is inceased, highe comessos and tubine costs must be consideed [5] Fuel eciculation Fig. 5 shows the changes in the net efficiency of the cycle with fuel eciculation ate. By inceasing the fuel eciculation ate the CH 4 concentation in the outlet of the stacks anode goes down. This means an incease in the mass flow ate of steam esults in moving the efoming and gas shift eaction equilibium to the H side. Inceases in the fuel eciculation ate leads to incease the electical outut of the stacks, esulting in a slight incease in net efficiency of the cycle MC&IT-SO cycle MC+IT-SO Cell essue (ba) Figue 4 Change of net efficiency of combined cycle with oeating essue MC&IT-SO cycle MC+IT-SO Fuel eciculation ate (-) Figue 5 Change of net efficiency of combined cycle with fuel eciculation ate Comaison of efomances between the single-staged MC cycle and MC/IT-SO combined cycle In this aagah, the efomance of the single-staged MC cycle is evaluated. In the combined cycle the efomance is analysed with vaying cuent density of MC and coesonding of diffeent cuent density of IT-SO stack setting fom 0. to 0.34 Acm -. Fig. 6 shows the changes in the total owe of the cycles with mean cuent density of MC. The total owe of the cycle is the owe oduced by the fuel s and tubine, minus the comessos owe. Inceasing the cuent density inceases the fuel stacks oveotential, esulting a decease of the voltage. Since the incease in the cuent density is lage than the decease in the voltage, the electical oututs of the fuel stacks (Fig. 7) goes u. The incease in the total fuel consumtion in the fuel stacks and the combusto in the cycles with mean cuent density (Fig. 8) causes an incease in the amount of inlet gas to the gas tubine; this means the electical outut of gas tubine also goes u.

6 0 single-staged MC cycle Total owe (kw) single-staged MC cycle MC&IT-SO combined cycle MC&IT-SO combined cycle 400 owe (kw) 00 Cuent density of MC (A/cm²) Figue 6 Total owes of the cycles as a function of cuent density of MC MC IT-SO 00 Cuent density of MC (A/cm²) Figue 7 Fuel stacks owe of the cycles as a function of cuent density of MC 00 Cuent density of MC (A/cm²) Figue 9 Change of the net efficiency with mean cuent density of MC CONCLUSION In this aticle, a new combined cycle was oosed: combined cycle consisting of two-staged of MC and IT- SO. The MC&IT-SO combined cycle and singlestaged cycle with MC only wee simulated in ode to evaluate and comae thei efomances. By changing the oeating aametes, cuent density oeating temeatue of MC, essue, and fuel eciculation ate, the effects of these oeating aametes on combined cycle efficiency was investigated. The simulations esults indicate that the net efficiency of MC&IT-SO combined cycle and single-staged MC cycle ae 64.6% and 5.6% esectively unde standad oeating conditions of the cycles wee mean cuent density of 0. Acm - fo MC, oeating temeatue of 6 C fo the MC, essue of 6 ba, oeating temeatue of 7 C fo the IT-SO, and fuel eciculation ate of %. In othe wods, the cycle with two-staged MC and IT-SO gives much bette net efficiency than the cycle with single-staged MC. Fuel (kg/h) 5 00 single-staged MC cycle MC&IT-SO combined cycle 5 Cuent density of MC (A/cm²) Figue 8 Total fuel consumtion on the cycles as a function of cuent density of MC Fig. 9 shows the changes in the net efficiency of the cycles with mean cuent density. As the total owe of the cycles goes u (Fig. 6) and total fuel consumtion in the fuel stacks and the combusto in the cycles changes geatly, net efficiency of the cycles goes down. Though the fuel consumtion is lowe in the single-staged MC cycle, the total owe is also lowe, causing the eduction in efficiency. REFERENCES [] Aaki T, Ohba T, Takezawa S, Onda K, Sakaki Y. Cycle analysis of lana SO owe geneation with seial connection of low and high temeatue SOs. J. owe Souces 006; 58: [] lasson J, Selimovic A, Hendiksen. Intemediate temeatue SO in gas tubine cycles. J. ASME 00; GT: 009. [3] Kimijima S, Kasagi N. Cycle analysis of mico gas tubine-molten cabonate fuel hybid system. J. JSME 005; seies B: 48-. [4] Chan SH, Low CF, Ding OL. Enegy and exegy analysis of simle solid-oxide fuel owe systems J. owe Souces 00; 03: [5] Bocci E, Oecchini F, Di Calo A, MC and micotubine owe lant simulation. J. owe souce 006; : [6] Musa A, Steeman H, De aee M. efomance of intenal and extenal efoming molten cabonate fuel systems. J. ASME 007; 4: -7. [7] Baanak M, Atakül H, A basic model fo analysis of molten cabonate fuel behavio. J. owe Souces 007; 7: [8] Mogensen M, Solid oxide technology: Status, challenges and visions. oceeding, Hydogen Electochemisty and Enegetics, Noway, 999; [9] Calise F, Dentice d Accadia M, alombo A, Vanoli L. Simulation and exegy analysis of a hybid Solid oxide fuel (SO)-Gas Tubine system J. Enegy 006; 3: