Performance Analysis of Hybrid Solid Oxide Fuel Cell - Gas Turbine Power Generating System

Transcription

1 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland Performance Analyss of Hybrd Sold Oxde Fuel Cell - Gas Turbne Power Generatng System Senthl V. Vannvedu Umapath 1,*, Kran Rao Bhmma 1 and Parchur M. V. Subbarao 1 Department of Mechancal Engneerng., Indan Insttute of Technology, New Delh, Inda Department of Mechancal Engneerng, Indan Insttute of Technology, New Delh, Inda Abstract: Electrochemcal oxdaton of H, CO, CH 4 and hgher hydrocarbons n fuel cells that produces power also produces heat. By utlzng the thermal energy from the SOFC n a heat engne, a hybrd cycle wth hgh performance can be acheved. In the operaton of any fuel cell stack, some unburned fuel must reman n the products leavng the cell n order to mantan sgnfcantly generated voltage throughout the cell. The unutlzed fuel rejected from the cell can be fully oxdzed n the combuston chamber whch mproves the cycle performance. Objectve of ths work s to smulate a combned sold oxde fuel cell and gas turbne system thermodynamcally and explore varous optons n achevng hghly effcent and envronment frendly power generatng system. Stack effluent and ts temperature are calculated by solvng smultaneous equatons correspondng to the equlbrum constants of ndvdual reactons. It s consdered that the temperature of the product leavng the stack s equal to the stack temperature. Fractonal effluent from the fuel cell stack s evaluated by reverse substtuton method. A smple regeneratve Brayton cycle along wth combuston chamber at the downstream of the fuel cell stack s consdered for ths study. Here the fuel s consdered as methane whch s reformed nternally nsde the cell stack. Parametrc study has been made to dentfy the key development needs and mprove the operatng condtons. Hybrdzaton of SOFC-GT recovers approxmately 5% of the toppng cycle s capacty. Fuel-to-electrcty converson effcency goal of a plant s greater than 60% based on lower heatng value (LHV) of the fuel. The output s presented n the form of detaled study of varous parameters and the characterstcs of ndvdual components of the cycle. Keywords: SOFC-GT, Thermal Smulaton, Hybrd Cycle Performance, Combned Cycle Modelng 1. INTRODUCTION Fuel cell s a devce that converts chemcal energy drectly nto electrcty. Sold oxde fuel cell (SOFC) can drectly use hydrocarbon or any hydro- carbon derved fuels. SOFC operates at temperatures hgh enough to enable the drect reformaton of natural gas. SOFC converts hydrogen, reformed from natural gas to electrcal power and hgh-grade waste heat for combned heat and power (CHP) system. Heat produced by oxdaton n fuel cells arses from two sources: when the entropy decreases, S resultng from the overall oxdaton reacton, accompanyng the usual decrease n the number of moles of gases from reactants to products and due to the rreversble process occurrng n the operaton of the cell. The heat must be rejected from the fuel cell stack n order to mantan ts temperature at a desred level. Whenever sgnfcant quanttes of hgh temperature rejected heat are avalable, a bottomng cycle can add apprecably to mprove the overall power generaton effcency. In order to obtan the hghest possble effcency n electrcal generaton, both the thermal energy from the heat and the unburned fuel rejected from the cell must be recovered and converted nto addtonal electrcal energy. Combust A D Powe condton syste Exhau SOF Generat Fue A Ar A Flte Compresso G G Absolute Temperature compresson 1 Combustor 4 Fuel Cell Expanson 6 Recuperaton Natural Pum Recuperat Entropy Fg. 1 Reference SOFC - GT system consdered Power generaton communty s facng a major challenge to protect the envronment whle producng a plentful supply of clean low-cost energy. Hybrd sold oxde fuel cell and gas turbne (SOFC-GT) s expected to play a major role n electrcty power Correspondng author: vusenthl77@yahoo.co.uk 1

2 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland generaton n the comng decades. It promses to acheve hgh electrcal effcences and decentralzed power generaton concept. The desgn and optmzaton of a fuel cell hybrd power system s very complex because of the number of requred systems, components and functons. Recently, several fuel cell gas turbne hybrd system confguratons have been suggested by varous research groups around the world [1] [] [3] & [4]. An exact thermodynamc analyss s appled to analyze the system performance of the SOFC-GT cycle. Parametrc study has been done for regeneratve Brayton cycle to dentfy the key development needs and operatng mprovements for ths cycle. Pure Methane s consdered as fuel. Ths partcular presentaton emphaszes on the pont of nterest as shown n Fg.. Modelng crteron for the regons before and after that juncton s taken from the prevous work of the same author [5] & [6].. SOFC EFFLUENT COMPOSITION In case of methane as fuel, the fractonal effluent from the stack cannot be calculated drectly n a sngle step because of ts nature of mult-reacton mechansm. Varous reactons correspondng to the methane are lsted below: Reformng: CH 4 + H O CO + 3H Shft gas reacton: CO + H O CO + H Electrochemcal: H + ½ O H O.1 Ideal cell potental of the stack The deal performance of a fuel cell s defned by Nernst potental, where E, s the deal cell voltage. The overall reactons for varous types of fuel cells are presented. General form of Nernst equaton, In case of pure hydrogen as a fuel, E 0 RT = E + ln nf [ reactant actvty] [ product actvty] H + 1/ O H O H = 41 kjmol 98 1 E P P 1/ o RT H O = EH + ln F P HO For externally or nternally reformed methane, the possble electro chemcal reactons are H + 1/ O H O H = 41 kjmol 98 1 CO + 1/ O CO H = 83 kj mol / 1/ o RT PH P O o RT PCOPO E= EH + ln + E + ln CO F PHO F PCO Neglectng carbon monoxde electrochemcal reacton, E P P 1/ o RT H O = EH + ln F P HO E actual = E (losses or polarzaton). All the losses are calculated from the same authors past work [5] whch was exclusvely done on polarzaton dstnctveness. For analyzng the hybrd cycle, temperature of the fuel cell stack and ts effluent are the mperatve crteron as mentoned n Fg.. Effluent fractons of the fuel are calculated by reverse substtuton method.

3 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland Fuel nlet SOFC Stack Pont nterest of Bottomng Cycle Fg. Conventonal fuel recyclng.1.1 Effluents calculaton As an ntal guess, neglectng shft gas reacton[9], CH 4,anode + H O, anode 4H,anode + CO,anode (Steam reformng reacton wthout shft gas reacton) 4H,anode + O,cathode 4H O,anode CH 4, anode + O, cathode H O, anode + CO, anode (Combned reformng and FC reacton) For 1 MW, kg/s of methane are requred. (Consderng FC utlzaton factor as 85% and 100% nternally reformed case) Table 1 Fuel fracton wthout shft reacton FRACTION speces flow rate (gmol/s) FRACTION GAS INLET FC INLET COMBINED ONLY REFORMATION FC OUTLET % CH CO CO H H O TOTAL Steam reformaton reacton wthout shft gas reacton s combned wth electrochemcal reacton to gve overall reacton of the stack. Fuel fractons neglectng shft gas reacton are tabulated n table 1 as an ntal guess. At the SOFC operatng temperature shft reacton s very fast and assumed always to be n equlbrum. Fractonal consumpton of ths reacton s obtaned by solvng quadratc equaton whch s obtaned from equaton (1). Incluson of ths fracton wll gve the real value from the stack as shown n table. CO + H O CO + H R CO = ( 476 / T(K) e ) (Shft gas reacton) and R CO = [ CO + p][ H + p] [ CO p][ H O p] (1) Table Fuel fracton wth shft reacton Fracton FC output Gas W/O shft Effect of shft CO 0 -(-6.8) 6.8 CO H H O (-6.8) For 1173 K, R CO = , solvng the quadratc equaton we get p = and Among these, practcally possble root s Fndng p from the above expresson, modfed actual effluent from the stack s calculated. One of the mportant aspects for the bottomng cycle analyss s temperature of the effluent. Ths s calculated by applyng the energy balancng to the fluds. Flow rates of ndvdual speces are obtaned by solvng ther correspondng chemcal equlbrum constant expressons (3), (4) & (5) smultaneously. The equlbrum constants n the reformng and shft gas reactons are temperature dependent and can be expressed by a polynomal equaton. Other constants are taken from Chan et al. [1]. 3

4 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland log R p = AT 4 + BT 3 + CT + DT + E () Where A, B, C, D and E are constants. x, y and z are the respectve molar flow rates of CH 4, CO and H partcpatng n the reactons and when the ntal temperature s known, the equlbrum constants can be calculated from the equaton () at gven fuel utlzaton rate and nlet condtons of the flow. R 3 [ CO ] + y [ H ] + 3x + y z n + x n + x = CO + x y H O x y+ z n + x n + x shft [ ] [ ] (3) R ref [ CO] + x y [ H ] + 3x + y z = n + x n + x [ CH 4] x [ H O] x y + z p n + x n + x cell (4) z = U (3 x+ y ). (5) f Where superscrpt s for nlet and n s the al molar flow rate and [CO ], [CO], [H ], [H O] are molar flow rates of the speces. Both reformng and shft gas reactons are endothermc. The respectve heats assocated wth these reactons can be calculated from the followng equatons: Q r = x[ h CO + 3 h - h O - h ] (6) H H CH 4 Q s = y[ h + CO h - h O - h CO ] (7) H H Assgnng the heat generated from the electrochemcal process of the fuel cell stack as Q rxn, the al heat transfer from the stack becomes, Q=Q rxn Q r - Q s. (8) Net heat generaton rate obtaned from the above expresson (8) s used to calculate the stack temperature that s consdered as same as the effluent comng from the stack. Iteratons are made between changes n the speces fractons and the net heat generaton rate to calculate the product temperature. Hot unutlzed fuel from the fuel cell stack s oxdzed n the combuston chamber. Excess ar s used to dlute the product gas from the combuston chamber for mantanng the desgn temperature of the gas turbne. Compressor and turbne are modeled by mass and energy balancng. Thermal smulaton for the bottomng cycle s adapted from the past work of the same author [6]. 3. RESULTS AND DISCUSSIONS 3.1 SOFC stack model Power output of the cell can be determned from the cell voltage and current densty. Fg. 3 shows that peak power s obtaned at average power densty and hgh voltage at low current densty. Desgnng a stack to operate at maxmum power densty that peaks at a hgher current densty may seem logcal, but t s obtaned at lower cell voltage whch results n lower cell effcency. It s usual practce to operate on the left sde of the peak power densty, at a pont that yelds a compromse between low operatng cost ( hgh cell effcency that occurs at hgh voltage and low current densty ) and low captal cost ( less cell area that occurs at low cell voltage and hgh current densty ). 3. Parametrc study of SOFC/GT system A reference SOFC/GT system s mathematcally developed accordng to Fg.1. In ths cycle, effluent from the fuel cell stack whch conssts of hgh temperature unutlzed fuel s made to burn n the combuston chamber. Excess ar s suppled from the compressor to the combuston chamber for dluton. Fuel cell stack of one mega watt capacty s consdered for ths analyss. Bottomng cycle s consdered wthout the effect of recuperaton after the turbne exhaust for smplcty. Parametrc study s performed by varyng only one parameter at a tme whle keepng others at ther base case values. Further, effect of varaton of all the parameters at a tme can be studed Cell voltage and turbne nlet temperature The nfluence of turbne nlet temperature (TIT) on system performance s shown n Fg.4. Increasng the TIT does not lead to much mprovement n the effcency of the system. Indeed, more fuel consumpton at hgh nlet temperature leads to less utlzaton of fuel n the fuel cell stack. 4

5 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland 1.0E E+03 Cell voltage =0.85 volts and pressure= 4 bar Cell potental (volts) 8.0E E E-01.0E E+03.5E+03.0E E E E+0 Power densty (W m - ) Effcency (%) E Current densty (A m - ) 0.0E Turbne nlet temperature (K) Fg. 3 Cell voltage and power densty as a functon of current densty Fg. 4 Influence of turbne nlet temperature to the system effcency 3.. Gas turbne power supplement effect on pressure Turbne power output for one mega watt fuel cell system decreases at a pressure greater than 4.5 bar. Ths s because of the decrement n TIT due to the shft of heat recovery. In order to mantan the same effcency extra fuel can be njected Turbne Net Output (MW) Combned cycle effcency (%) Pressure (bar) Pressure (bar) Fg. 5 Influence of pressurzaton to the net turbne output Fg. 6 Influence of pressurzaton to the cycle effcency 3..3 Pressurzaton At low pressure ratos, prmary fuel n the gas turbne must be reduced to meet constant exhaust temperature and more fuel can be sent to the SOFC stack. Ths means less output from the gas turbne and more from the SOFC, thus ncreasng the effcency and decreasng the pressure. At a pressure rato of 4.5 and above the overall cycle effcency gets reduced due to hgh power consumpton because of pressurzaton. Maxmum effcency of nearly 60% s found at the pressure rato of 4.5. At hgh pressures, the TIT ncreases to mantan the exhaust gas temperature and thus ncreasng the gas turbne net output Cell voltage and al effcency The voltage range between 0.65 and 0.95 s taken for consderaton. Cell voltage s drectly proportonal to the fuel cell effcency. But, when t comes to the hybrdzaton, t has nomnal mpact on the overall system effcency Influence of utlzaton factor Reactant utlzaton and gas composton have major mpacts on fuel cell effcency. It s apparent from the Nernst equatons that fuel and oxdant gases contanng hgher partal pressures of electrochemcal reactants produce hgher cell voltages. Utlzaton (U f ) refers to the fracton of the al fuel or oxdant ntroduced nto a fuel cell that reacts electrochemcally. Due to the ncrease n losses n the fuel cell stack and reduced fuel n the combuston chamber, ncreased utlzaton factor leads to decrease n cycle effcency. 5

6 The nd Jont Internatonal Conference on Sustanable Energy and Envronment (SEE 006) A-00 (O) 1-3 November 006, Bangkok, Thaland Total effcency (%) Effcency MW FC stack, 4 bar Cell voltage (volts) Fg. 7 Influence of operatng voltage on system performance Utlzaton factor Fg. 8 Influence of utlzaton factor to the net turbne output 4. CONCLUSION Methane fed hybrd SOFC-GT power generaton system of 1.5 MW capacty s successfully modeled. For smulatng the fuel cell stack, earler expressons of the same author are adapted. It has been demonstrated that SOFC-GT can acheve 60% net electrcal effcences and thus shown that t s feasble for SOFC to be ntegrated wth a gas turbne engne. If we consder the recuperaton at the downstream of the gas turbne, effcences can be further mproved. Hgh current densty results n low captal cost whch holds good for automoble applcatons. But t lowers system effcency because of low cell voltage and hgh operatng cost. Low current densty wth hgher voltage results n hgh effcency and low operatng cost whch are more sutable for statonary power generaton. At the same tme hgh pressurzaton and more surface area lead to hgher captal costs. The TIT and cell voltage dd not show much mpact on system performance. Influence of cell voltage s nomnal to overall system effcency. The maxmum effcency of the system s found to be nearly 60 % at a pressure rato of 4.5. The base case peak can be mproved by excess fuel supply n the bottomng cycle. Thus, ths supports the dea that SOFC-GT combnaton provdes an effcent opton for power generaton. 5. REFERENCES [1] S.H.Chan, H.K.Ho, Y.Tan(003) Mult-level modelng of SOFC-gas turbne hybrd system, Int. J Hydrogen Energy pp [] J. Palsson, A. Selmovc, L. Sjunnesson (000), Combned sold oxde fuel cell and gas turbne systems for effcent power and heat generaton, Journal of Power Sources, vol. 86, pp [3] Jens Palsson (00), Computatonal flud dynamcs modelng of Sold Oxde Fuel Cells, Doctoral thess, Dvson of Thermal power engneerng, Lund Unversty, Sweden. [4] Chrstoph Stller, Bjørn Thorud, Stenar Seljebø, Østen Mathsen, Havard Karolussen, Olav Bolland (004), Fnte-volume modelng and hybrd-cycle performance of planar and tubular sold oxde fuel cells, Journal of Power Sources. [5] V.U.Senthl Vadvel, Rayapat subbarao, P M V Subbarao (006), Mathematcal modelng of ncorporated physcoelectrochemcal processes and polarzaton dstnctveness n sold oxde fuel cells, Proceedngs of the Internatonal Conference on Modelng and Smulaton. [6] Rayapat Subbarao, V.U.Senthl Vadvel, P.M.V.Subbarao (006), Thermodynamc modelng of hybrd sold oxde fuel cell gas turbne power plant, Proceedngs of Internatonal Congress on Renewable Energy. [7] Westnghouse Report Hgh temperature sold electrolyte fuel cell power generatng system, annual report 6/1/81-5/31/8. [8] B. Fredrksson Möller, J. Arragada, M. Assad, I. Potts (004), Optmsaton of an SOFC/GT system wth CO -capture, Journal of Power Sources, vol. 13, pp [9] MarK C. Wllams (00) Fuel Cell Handbook (sxth edton), EG&G Techncal Servces, Inc., West Vrgna. 6