工学电池及其电化学.pptx

Towards a Sustainable Energy Future2011SpringFuelCellandElectrochemistryLecture6:MCFCTowards a Sustainable Energy FutureMoltenCarbonateFuelCellTowards a Sustainable Energy FutureMoltenCarbonatefuelcellAdvantagesnoble metal catalysts are not required;being developed for natural gas and coal-based power plants;High efficieny;Fuel flexibility;for industrial,electrical utility,and military applicationsTowards a Sustainable Energy FutureNeedforCO2atcathodetransfertheCO2fromtheanodeexitgastothecathodeinletgas(CO2transferdevice);produceCO2bycombustingtheanodeexhaustgas,whichismixeddirectlywiththecathodeinletgas;supplyCO2fromanalternatesourceTowards a Sustainable Energy FutureHightemperatureEffectAdvantagesachieve higher overall system efficiencies;greater fuel flexibilityDisadvantagessevere demands on the corrosion stability and life of cell components,particularly in the aggressive environment of the molten carbonate electrolyte;Need special structure to hold molten electrolyte-capillary pressures to establish the electrolyte interfacial boundariesInPAFC,PTFEservesasabinderandwet-proofingagenttomaintaintheintegrityoftheelectrodestructureandtoestablishastableelectrolyte/gasinterfaceintheporouselectrode.ThephosphoricacidisretainedinamatrixofPTFEandSiCbetweentheanodeandcathode.Towards a Sustainable Energy FutureLargestfloodedporesintheporouscomponentsTowards a Sustainable Energy FutureOver the past 28 years,the performance of single cells has improved from about 10 mW/cm2 to 150 mW/cm2Towards a Sustainable Energy FutureTowards a Sustainable Energy FutureElectrolytefabricationElectrolytemanagementcontrol over the optimum distribution of molten carbonate electrolyte in the different cell components1980:hotpressing(about5,000psi)mixturesofLiAlO2andalkalicarbonates;1)void spaces(700C,whereasthephaseisstableat600to650C.Theindustrytrendistoswitchfrom-LiAlO2to-LiAlO2forbetterlong-termphaseandparticle-sizestabilities.FCEisdevelopingalow-costLiAlO2,aqueous-basemanufacturingsystem,butmustresolveslowdryingrateofLiAlO2anditsinstabilityinwaterTowards a Sustainable Energy FutureDevelopmentComponents3)ElectrolytePresentelectrolyteshavethefollowingchemistry:lithiumpotassiumcarbonate,Li2CO3/K2CO3(62:38molpercent)foratmosphericpressureoperationandlithiumsodiumcarbonate,LiCO3/NaCO3(52:48o60:40molpercent)thatisbetterforimprovedcathodestabilityunderpressurizedoperationandlifeextension;Evaporationoftheelectrolyteisalife-limitingissueforthemoltencarbonatefuelcell.Li/NaelectrolyteisbetterforhigherpressureoperationthanLi/Kbecauseitgiveshigherperformance.Towards a Sustainable Energy FutureDevelopmentComponents4)Coal Gas Trace SpeciesTowards a Sustainable Energy FuturePerformancesFactorsaffectingtheselectionofoperatingconditionsstack size,heat transfer rate,voltage level,load requirement,and cost.TypicalMCFCswillgenerallyoperateintherangeof100to200mA/cm2at750to900mV/cell.Towards a Sustainable Energy FuturePerformances1)EffectofPressureTowards a Sustainable Energy FuturePerformancesAthigherpressureTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureTowards a Sustainable Energy FuturePerformances2)EffectofTemperatureThewatergasshiftreactionachievesrapidequilibriumattheanodeinMCFCs,andconsequentlyCOservesasanindirectsourceofH2WaterGasShift:CO+H2OCO2+H2Towards a Sustainable Energy FuturePerformances2)EffectofTemperatureinthetemperaturerangeof575to650C,1/3ofthetotalchangeincellvoltagewithdecreasingtemperatureisduetoanincreaseinohmicpolarization;Beyond650C,however,therearediminishinggainswithincreasedtemperatureTowards a Sustainable Energy FuturePerformances3)EffectofReactantGasCompositionThehalf-reactionatcathode:O2+CO2+2eCO32Towards a Sustainable Energy FuturePerformances4)EffectofUtilizationTowards a Sustainable Energy FutureTowards a Sustainable Energy FuturePerformances5)EffectofImpuritiesGasifiedcoalisexpectedtobethemajorsourceoffuelgasforMCFCs,butbecausecoalcontainsmanycontaminantsinawiderangeofconcentrations,fuelderivedfromthissourcealsocontainsaconsiderablenumberofcontaminants.Towards a Sustainable Energy FuturePerformances5)EffectofImpurities-SulfurTowards a Sustainable Energy FuturePerformancesTheadverseeffectsofH2Soccurbecauseof:Chemisorption on Ni surfaces to block active electrochemical sites,Poisoning of catalytic reaction sites for the water gas shift reaction,Oxidation to SO2 in a combustion reaction,and subsequent reaction with carbonate ions in the electrolyte.Atanode,Towards a Sustainable Energy FuturePerformances6)EffectofinternalreformingTwointernalreforming:indirectinternalreforming(IIR)anddirectinternalreforming(DIR).IIRThe section is separate,but adjacent to the fuel cell anode.Advantages:1)where the exothermic heat of the cell reaction can be used for the endothermic reforming reaction;2)the reformer and the cell environments do not have a direct physical effect on each other.Disadvantage:the conversion of methane to hydrogen is not promoted as well as in the direct approach.DIRHydrogen consumption reduces its partial pressure,thus driving the methane reforming reaction,Equation(6-34),to the right.Towards a Sustainable Energy FutureEndothermicExothermicTowards a Sustainable Energy FutureMethaneisacommonfuelininternalreformingMCFCs,wherethesteamreformingreactionTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureTowards a Sustainable Energy Future