Delft University of Technology Solid oxide fuel cell (SOFC)

178 Pages·2017·5.64 MB·English

by  Aditya Thallam Thattai

Delft University of Technology

Solid oxide fuel cell (SOFC) integrated power plants
System and kinetic studies
Thallam Thattai, Aditya
DOI
10.4233/uuid:2c54c1c6-3f5f-43cd-897c-78d59db28e04
Publication date
2017
Document Version
Final published version
Citation (APA)
Thallam Thattai, A. (2017). Solid oxide fuel cell (SOFC) integrated power plants: System and kinetic studies.
https://doi.org/10.4233/uuid:2c54c1c6-3f5f-43cd-897c-78d59db28e04
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SOLID OXIDE FUEL CELL (SOFC) INTEGRATED
POWER PLANTS
SYSTEM AND KINETIC STUDIES
Aditya THALLAM THATTAI

SOLID OXIDE FUEL CELL (SOFC) INTEGRATED
POWER PLANTS
SYSTEM AND KINETIC STUDIES
Proefschrift
terverkrijgingvandegraadvandoctor
aandeTechnischeUniversiteitDelft,
opgezagvandeRectorMagnificusprof.ir.K.C.A.M.Luyben,
voorzittervanhetCollegevoorPromoties,
inhetopenbaarteverdedigenopmaandag26juni2017om12:30uur
door
Aditya THALLAM THATTAI
MasterofScienceinMechanicalEngineering
DelftUniversityofTechnology,theNetherlands
geborenteNagpurVidarbhaMS,India.
Ditproefschriftisgoedgekeurddoorde
Promotoren: Prof.dr.ir.B.J.BoersmaenProf.dr.J.J.C.Geerlings
Copromotor: Dr.P.V.Aravind
Samenstellingpromotiecommissie:
RectorMagnificus voorzitter
Prof.dr.ir.B.J.Boersma TechnischeUniversiteitDelft,promotor
Prof.dr.J.J.C.Geerlings TechnischeUniversiteitDelft,promotor
Dr.P.V.Aravind TechnischeUniversiteitDelft,copromotor
Onafhankelijkeleden:
Prof.dr.ir.C.A.RamirezRamirez TechnischeUniversiteitDelft
Prof.dr.ir.G.Brem UniversiteitTwente
Prof.dr.E.Worrell UniversiteitUtrecht
Dr.A.Moreno ENEA,Italy
Prof.dr.D.J.E.M.Roekaerts TechnischeUniversiteitDelft,reservelid
Copyright©2017byA.ThallamThattai
ISBN978-94-6186-823-7
Anelectronicversionofthisdissertationisavailableat
http://repository.tudelft.nl/.
ThesiscoverdesignedbyReshuGupta
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Salutations to Lord Hayagreeva, the Supreme Efful-
gence, whose gifts of knowledge are as innumerable as they
areimperishable.

Summary
Increased climate change over past decades has resulted in an increase in the av-
erage temperature (also called global warming) of Earth’s climate system. At the
recent Paris climate conference (COP21) in 2015, 195 countries in the world have
agreed upon a stringent plan to limit global warming below 2oC. This demands a
significantreductionintheindustrialemissionofgreenhousegases,predominantly
carbon dioxide (CO ). Existing fossil fuel (coal, natural gas) fired power plants
2
account for the majority share in global carbon dioxide (CO ) and other harmful
2
(SO , NO )emissions. Thereforeclean, efficientandflexiblepowerplantconcepts
x x
need to be developed towards upgrading existing power plants and to meet the
strictCO emissiontargets. Combinedcyclepowerplantsliketheintegratedgasifi-
2
cationcombinedcycle,IGCC(coalbased)andintegratedreformingcombinedcycle,
IRCC (natural gas based) can be utilized to produce electricity using fossil fuels at
relativelyhighefficienciescomparedtoconventionalsinglecycleplants.
Possible approaches to make IGCC/IRCC power plants cleaner, efficient and
moreflexibleincludebiomassutilization(renewableenergysource),applicationof
CO capturetechnologies,retrofittingwithhighlyefficientfuelconversiontechnolo-
2
gies like solid oxide fuel cells (SOFCs) and energy/fuel storage. This dissertation
primarily aims to provide design concepts and thermodynamic system analysis for
large scale IGCC and IRCC power plants with a focus on achieving high electrical
efficiencies, low CO emissions and high operational flexibility. SOFCs have been
2
explored as an efficiency augmenting technology and metal hydride based hydro-
gen storage as a flexibility option. Furthermore, future development of safe and
optimally operating hydrocarbon (like natural gas (methane)) fuelled SOFC units
on the basis of system and numerical models, requires reliable experimental data
and understanding in the underlying reaction kinetics. Thereupon, an extended
experimental study has been carried out in this work on methane steam reforming
(MSR)kineticsinsingleoperatingSOFCs.
Thedissertationcomprisesof4mainparts: a)anexperimentalmodelvalidation
studyonhighpercentage(upto70%)biomassco-gasificationinIGCCpowerplants
(called bio-IGCC) based on an existing coal based power plant in the Netherlands.
b) a thermodynamic system study towards retrofitting SOFCs and CO capture in
2
existing IGCC power plants with a focus on near future implementation. c) a ther-
modynamic system design study on flexible IRCC power plants with metal hydride
based hydrogen storage and a preliminary study on integrating SOFCs in natural
vii
SUMMARY
gas (methane) based power plant systems. d) an experimental study on methane
steamreforming(MSR)reactionkineticsinsingleoperatingSOFCs.
Co-gasificationofbiomasslikewoodpelletscombinedwithcarbondioxide(CO )
2
captureinexistingcoalbasedIGCCpowerplantshasalargepotentialtoreduceCO
2
emissionsinthenearfuture. Woodybiomassislargelyconsideredacarbonneutral
fuelbasedonahypothesesthatitremovesasmuchCO fromtheenvironmentdur-
2
ing its growth as is emitted after its conversion in industrial plants. In order to
assess biomass co-gasification as a clean energy technology, high percentage (upto
70% energy based) biomass co-gasification tests were carried out in the past by
NUON/Vattenfall at the currently defunct 253 MW coal based Willem-Alexander
e
Centrale(WAC),BuggenuminTheNetherlandsutilizingsteamexplodedwoodpel-
lets. Chapter 2 of this dissertation presents the obtained experimental data with a
detailed and validated steady state thermodynamic off-design model developed as
an aid to assess future plant operations. The validation study shows a reasonably
accurate model prediction for a net power output of 173 MW and a net plant ef-
e
ficiency of about 37.2%. Furthermore, the need to carry out co-gasification with
high lower heating value (LHV) torrefied wood pellets has also been pointed out.
Confirming previous reports in literature, an exergy analysis of the complete sys-
tem indicates largest exergy destruction in the gasifier and gas turbine combustor,
suggesting an additional scope for process improvements. Despite unavoidable in-
consistenciesintheobtainedplantdata, ithasbeenshownthatoff-designthermo-
dynamicmodelscanbeeffectivelyutilizedtopredictpowerplantperformancewith
arelativelyhighaccuracy(within3%relativedeviation).
Solid oxide fuel cells (SOFCs), operating at high temperatures (700-1000oC)
are fuel flexible and highly efficient electrochemical devices for electricity produc-
tion. Existingcoal/biomassfiredIGCCpowerplantscouldberetrofittedwithSOFCs
andnovelCO capturetechnologiestoreduceCO emissionsandincreasenetelec-
2 2
trical efficiencies in near future. Utilization of SOFCs also promotes lower SO
x
andNO emissions. Chapter3presentsadetailedthermodynamicanalysistowards
x
retrofittingSOFCsandCO captureinbio-IGCCpowerplantsusingoff-designmod-
2
elsdevelopedbasedonvalidatedmodelspresentedinChapter2. Twosystemshave
been presented: i) a system based on WAC design with partial SOFC-CO capture
2
retrofit i,e only part of the syngas fuel is utilized in the SOFC and CO capture
2
unit ii) a newly designed integrated gasification fuel cell (IGFC) system with CO
2
capturewhereinsyngasfueliscompletelyconvertedintheSOFCunit. Thetwosys-
temstogetherhavebeenusedtopinpointthatexistingIGCCpowerplantscouldbe
operatedwithmorethan40%netelectricalefficiencywithoutmajorprocessmodi-
ficationswhenpartiallyretrofittedwithSOFCs(upto40MW )andoxy-combustion
e
CO capture. ThestudyfurtherrevealsthatfullscaleCO captureandSOFCintegra-
2 2
tion requires major redesign of the gas turbine and heat recovery steam generator
(HRSG). The reduction in thermodynamic losses in both systems compared to the
basesystem(presentedinChapter2)withoutSOFCandCO capturehasalsobeen
2
clearlypointedoutwithanexergy(2nd law)analysis.
Natural gas, a relatively cleaner fossil fuel compared to coal, is an additional
primary fuel utilized for electrical power production. Chapter 4 of this disserta-
viii
SUMMARY
tion with an aim to assess operational flexibility presents a thermodynamic system
study on IRCC power plants with metal hydride based hydrogen (H ) storage and
2
pre-combustion CO capture. The central idea explored in this study to introduce
2
operational flexibility is, the storage of H in a metal hydride (Magnesium hydride
2
(MgH )) during low power demand and utility of the stored H for power produc-
2 2
tion during high power demand. Metal hydrides (MH) as a H storage option in
2
power plants offer multiple advantages in terms of relatively high storage capaci-
ties (%wt) and extensive possibilities for heat integration within the system. The
comparativestudyusingsteadystateIRCCsystemmodelswithandwithoutH stor-
2
age shows that addition of MH based H storage in IRCC power plants causes an
2
insignificant penalty of the net system efficiency and that these system could be
operatedwithatimebasedaverageefficiencyabove45%withappropriateheatin-
tegration. TheH splitfractionandchoiceofthemetalhydride(reactionenthalpy)
2
are identified as two important design parameters. Additional aspects regarding
the temperature pinch in the HRSG and feed water preheaters (FWP) have also
been addressed. The reformer and gas turbine combustor have been identified as
sources for the largest thermodynamic irreversibilities. As a preliminary investiga-
tion, Appendix 4A of this dissertation presents a system study towards retrofitting
suchsystemswithSOFCstowardsreducingtheseirreversibilities.
FuelconversioninSOFCscannotbecompletelyunderstoodbasedonlyonther-
modynamic investigations. It is of vital importance to also investigate the underly-
ingreactionkineticstodeveloplarger,saferSOFCunitsforpowerplantintegration
andtopreciselypredictundesirabletemperaturegradientsinthecell. Furthermore,
it is also important to obtain reliable experimental data for developing accurate
systemandnumericalmodels. Chapter5ofthisdissertationpresentsanexperimen-
tal study on MSR kinetics in an operating single SOFC with Ni-GDC (gadolinium
doped ceria) anodes. The study has been carried out for relatively low inlet steam
concentrations (steam to carbon (S/C) ratio around 1) and moderate current den-
sities upto 3000 A/m2. Based on experimental methane conversion obtained at
various operating temperatures, gas compositions and current densities, a kinetic
model has been developed to calculate and compare relevant kinetic parameters
using two approaches - power law (PL) and general Langmuir-Hinshelwood (LH)
kinetics. Resultsusingbothapproachesindicatethatelectrochemicalhydrogenox-
idation (current) marginally promotes methane conversion and the MSR reaction
rate. However, the inlet methane partial pressure and the operating temperature
have been identified as the most important factors affecting the rate. Although
both approaches predict the same net MSR reaction rates, a significant difference
isobservedinthepredictedrateandspeciespartialpressuredistributionalongthe
channel length. Furthermore, experiments indicate that methane reforming on an-
odecurrentcollectorsinalwaysnotnegligible,particularlyathighertemperatures.
In a nutshell, the work presented in this dissertation is an important step for-
ward towards the conceptual design and development of clean, efficient and flexi-
ble SOFC integrated IGCC/IRCC power plants. The work additionally exposes the
importanceandthermodynamicadvantagesofemployingsolidoxidefuelcellsand
metalhydridebasedhydrogenstorageinlargescale(>150MW )powerplants. As
e
ix