Sequential supplementary firing in natural gas combined cycle with carbon capture: A technology option for Mexico for low-carbon electricity generation and CO<inf>2</inf> enhanced oil recovery

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ContentslistsavailableatScienceDirect

International

Journal

of

Greenhouse

Gas

Control

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / i j g g c

Sequential

supplementary

ﬁring

in

natural

gas

combined

cycle

with

carbon

capture:

A

technology

option

for

Mexico

for

low-carbon

electricity

generation

and

CO

2

enhanced

oil

recovery

Abigail

González

Díaz

∗

,

Eva

Sánchez

Fernández,

Jon

Gibbins,

Mathieu

Lucquiaud

TheUniversityofEdinburgh,SchoolofEngineering,TheKing’sBuildings,EdinburghEH93JL,UnitedKingdom

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22March2016

Receivedinrevisedform1June2016 Accepted7June2016

Availableonline20June2016

Keywords:

Sequentialsupplementaryﬁring Supercriticalsteamcycle Postcombustioncarboncapture Enhancedoilrecovery

a

b

s

t

r

a

c

t

Combinedcyclegasturbinepowerplantswithsequentialsupplementaryﬁringintheheatrecovery

steamgeneratorcouldbeanattractivealternativeformarketswithaccesstocompetitivenaturalgas

prices,withanemphasisoncapitalcostreduction,andwheresupplyofcarbondioxideforEnhanced

OilRecovery(EOR)isimportant.Sequentialcombustionmakesuseoftheexcessoxygeningas

tur-bineexhaustgastogenerateadditionalCO2,but,unlikeinconventionalsupplementaryﬁring,allows

keepinggastemperaturesintheheatrecoverysteamgeneratorbelow820◦C,avoidingastepchangein

capitalcosts.Itmarginallydecreasesrelativeenergyrequirementsforsolventregenerationandamine

degradation.PowerplantmodelsintegratedwithcaptureandcompressionprocessmodelsofSequential

SupplementaryFiringCombinedCycle(SSFCC)gas-ﬁredunitsshowthattheefﬁciencypenaltyis8.2%

pointsLHVcomparedtoaconventionalnaturalgascombinedcyclepowerplantwiththesamecapture

technology.Themarginalthermalefﬁciencyofnaturalgasﬁringintheheatrecoverysteamgenerator

canincreasewithsupercriticalsteamgenerationtoreducetheefﬁciencypenaltyto5.7%pointsLHV.

Althoughtheefﬁciencyislowerthantheconventionalconﬁguration,theincrementinthepoweroutput

ofthecombinedsteamcycleleadsareductionofthenumberofgasturbines,atasimilarpoweroutputto

thatofaconventionalnaturalgascombinedcycle.Thishasapositiveimpactonthenumberofabsorbers

andthecapitalcostsofthepostcombustioncaptureplantbyreducingthetotalvolumeofﬂuegasby

halfonanormalisedbasis.Therelativereductionofoverallcapitalcostsis,respectively,15.3%and9.1%

forthesubcriticalandthesupercriticalcombinedcycleconﬁgurationswithcapturecomparedtoa

con-ventionalconﬁguration.Foragaspriceof$2/MMBTU,theTotalRevenueRequirement(TRR)–ametric

combininglevelisedcostofelectricityandrevenuefromEOR–ofsubcriticalandsupercriticalsequential

supplementaryﬁringisconsistentlylowerthanthatofaconventionalNGCCby,respectively,2.2and5.7

$/MWhat0$/tCO2andby4.9and6.7$/MWhat$50/tCO2.Atagaspriceof$4/MMBTUand$6/MMBTU,

theTRRofasubcriticalconﬁgurationisconsistentlylowerforanycarbonsellingpricehigherthan2.5$/t

CO2and37$/tCO2respectively.

license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

AnnualelectricitydemandinMexicoispredictedtogrowby72% from259to446TWhebetween2011and2026(MexicanMinistry

ofEnergy,2012).Itisexpectedthatthisrisingdemandfor electric-itywouldbemetbyanincreaseintheuseofbothcoalandgas, withnaturalgasbeingthedominantenergysourcein2027.Inthe past10years,thefractionofnaturalgasinelectricitygeneration inMexicoincreasedsigniﬁcantlyfrom17.1%(32.9TWhe)in2000

∗Correspondingauthor.

E-mailaddress:[email protected](A.GonzálezDíaz).

to50.4%(130.6TWhe)in2011(MexicanMinistryofEnergy,2012). Inthis contextofrapidelectriﬁcationdominatedbynaturalgas powerplants,Mexicointendsinparalleltoreduce“itsgreenhouse gas(GHG)emissionsby50%below2000levelsby2050”(CTF/TFC, 2009).In2012,theMexicanCongressapprovedthe“General Cli-mateChangeLaw”toreduceGHGemissions,andrecentpolicies recognisethepotentialforEnhancedOilRecovery(EOR)andshale gasopportunities. Oneof the strategies proposedtoreach this objectiveistheapplicationofCarbonCaptureandStorage(CCS)on fossilfuelpowerplantsforthepurposeofEORintheoilindustry, whichreliesontheavailabilityofthelargeamountsofCO2(Lacy

etal.,2013;MexicanMinistryofEnergy,2012)between2020and 2050.

http://dx.doi.org/10.1016/j.ijggc.2016.06.007

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The triple challenge of rapid electrification through natural gas,reducingCO2emissionsinpowergenerationandrollingout EnhancedOilRecoveryatnationallevelrequiresanimportantR&D efforttodevelopnationallyrelevantCCStechnologyoptions.The outcomecouldthen beimplementedin thecurrent technology roadmapforthedesignofnewbuildCCS-EORreadyNGCCpower plants,tofacilitateincorporatingCO2capturetechnologiesandEOR intothefutureenergymix.Thispaperpresentstheresultsfroma techno-economicstudyofpowerplantconfigurationsdedicatedto addressthistriplechallenge.Itinvolvesthesequential supplemen-taryfiringofnaturalgasintheheatrecoverysteamgeneratorofa naturalgascombinedcyclepowerplant,followedbytheremovalof carbondioxideinapost-combustionscrubbingamine-based cap-tureunittosupplyCO2forEOR.Thiscapturetechnologyhas,atthe timeofwriting,beendeployedatcommercialscaleattheBoundary DampowerplantinCanada.Itisparticularlyrelevantinthecontext ofatechnologyroadmapforCCSreleasedbytheMexicanMinistry ofEnergy,recommendingactionsatnationalleveluntil2024with aparticularfocusondevelopingsolventabsorptiontechnologies linkedtonaturalgascombinedcycleplants(MexicanMinistryof Energy,2014).

2. SequentialsupplementaryﬁringwithCO2capture

2.1. Introductiontotheconcept

Non-sequential,singlestage,supplementaryﬁringistypically used in Natural Gas Combined Cycle (NGCC) power plants to increasepoweroutputbyaround30%duringtimesofpeakdemand of electricity and highelectricity sellingprices (Kiameh,2003).

Liet al.(2012)proposedtoimplement supplementaryﬁringin

gas-firedpowerplantswithcarboncapture.Theyreporteda con-centration of O2 of 5.6% v/v in the exhaust gas, compared to 12.4%v/vwithoutsupplementaryfiring.Thetemperature differ-enceatthehighpressuresuperheaterheaderoftheheatrecovery steamgenerator (HRSG) increasesfrom 50◦C to800◦C leading to a gas temperature of 1280◦C and large heat transfer irre-versibilities, compared to a gas temperature around 530◦C. In both cases, high pressure steam temperature is 480◦C. Single stagesupplementaryfiringrequiresadvancedalloystocopewith themaximumtemperatureachievable,which thenrestrictsthe amountofsupplementaryfuelthatcanbeused.Modificationsto theHRSGdesigntowithstandhighertemperaturesare,however, compensatedby higher CO2 concentrations atthe captureunit inlet.

Sequential combustion effectively makes use of the excess oxygennecessaryfor gasturbine combustiontogenerate addi-tionalCO2 and allows tokeep temperaturearound800–900◦C, anachievablerangewithinaheatrecoverysteamgeneratorwith supplementaryfiring(Kehlhoferetal.,2009).Thelaststageof sup-plementaryfiringbringsoxygenclosetostoichiometriclimits(1% v/v).Thiscorrespondstoanexcessairaround5%v/v.Gasandoil firedboilersusedinutilityandindustrialsteamgeneration appli-cationstypicallyoperatewithanexcessairintherangeof5–10% v/v,resultinginoxygenlevelsinthecombustiongasoftheorder of1–2%v/v(Steamitsgenerationanduse,2005,pp.11.4).Inthe contextofsequentialcombustioninHRSGatlowexcessoxygen, thissuggestthatcompletecombustionwithoxygenlevelsaslow as1%v/vmaybepracticallyachievablewithgoodair/fuelmixing withappropriateburnerdesign.

Theresultingfluegasofsequentialcombustionisthenmore comparabletothefluegasofacoal plant,which facilitatesthe incorporationofpost-combustionCO2capturebyaddressingthree specificchallengesassociatedwithnaturalgasfluegas:

A.)CO2 concentrationintheexhaustgas:alowconcentrationof CO2intheexhaustgasesaffectstheelectricityoutputpenalty ofcapturebecauseofalowerdrivingforceforCO2absorption andanassociatedincreaseinbothabsorbersizeandsolvent energyofregeneration(Lietal.,2012).CO2concentrationsin theexhaustgasesaretypically10–15%v/vinacoalpowerplant and3–4%v/vinagasturbine.Theyincreaseto9.4%v/vwith theconfigurationwithfivestagesofsequentialsupplementary firinginthisarticle.

B.)Largeexhaustgasvolumesleadingtohighercapitalcosts:With fivestagesofsupplementaryfiring,theoverallfluegasflowrate enteringthecaptureplantisaround50%oftheflowrateofa standardNGCCplantwithpost-combustioncapturewiththe samepoweroutput.

C.)O2 concentration:largeamounts ofexcess air necessaryfor gasturbineoperation,typically200%,resultinhighO2 concen-trationingasturbineexhaustcomposition,around12.3%v/v (IEAGHG,2012),increasingsolventoxidativedegradationand operationalcosts(GoffandRochelle,2004).Withﬁvestagesof sequentialsupplementaryﬁring,theO2concentrationisaround 1.3%v/vattheinletoftheabsorber.

Burningsupplementaryfuelinconsecutivestagesincreasesthe heatavailableintheHRSGandleadstoalargercombinedcycle poweroutputandareductionofthenumberoftheGTtrains,at con-stantpoweroutput.Thisalsohasapositiveimpactonthenumber ofabsorbersandthecapitalcostsofthepostcombustioncapture plantbyreducing thetotalvolumeoffluegasbyhalfona nor-malisedbasis.Itdecreasesmarginallytheenergyrequirementsfor solventregenerationandmarginallyreducesaminedegradation.In practice,theoverallthermalefficiencyofaSSFCCplantislowerthan thatofastandardNGCC.Oneusefulmetricisthemarginalthermal efficiencyoftheadditionalnaturalgascombustion,asproposedin Eq.(1).Thisisdefinedastheratiooftheincrementinpoweroutput totheaddedfuelinputintheHRSG:

marg=

_W

SF−W0 MSFLHV

(1)

wheremargisthemarginalefﬁciency,W0isthepoweroutputof steamturbineofconventionalNGCCplant(MW),WSFisthepower

outputofthesteamturbineofaplantwithsequential supplemen-taryﬁring(MW),MSFisthemassﬂowofsupplementaryfuelinthe

HRSGandLHVisthefuellowheatvalue(MJ/kg).Inprinciple,Eq.

(1)canbeusedtocomparepowerplantswithoutandwithcapture.

2.2. Steamcycleandheatrecoverydesignwithsequential

supplementaryﬁring

Aconfigurationwithtwostagesofsupplementaryfiringwith subcriticalsteamcycleispresentedinKehlhoferetal.(2009). Nat-uralgasfiredisburntattwolocationsintheprimaryheatexchange section.InformationrelatedtothevaluesoffinalCO2andO2 con-centrationinthefluegasisnotprovided.Thefluegastemperature isincreasedafterthegasturbineviaafirststageoffiringtoa max-imumtemperaturearound750◦Candentersasuperheaterheat exchanger.Naturalgasisthenfiredagaininasecondstagefollowed byanevaporator.

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exchangesectioninordertomitigatehighpeaktemperaturesin theHRSGwhengeneratingsupplementarypower.Thepeak tem-peraturereachedis760◦C,however,thevaluesofﬁnalCO2andO2 concentrationintheﬂuegasarenotprovided.Ontheotherhand,

Ganapathy(1996)suggeststhat highermaximumtemperatures

arepossiblebyintroducingothermodificationsintheHRSG.For instance,atemperatureof927◦Cisachievablewiththeuseof insu-latedcasingsandupto1316◦Cwhenequippedwithwater-cooled furnaces.Inordertoavoidincludingadvancedalloys,boilerdesign consistingofwater-cooledfurnacesandexcessivecapital expendi-ture,exhaustgastemperaturescanbekeptatamaximumof820◦C, atypicaltemperatureinaconventionalNGCCwithsupplementary firing(Thermoflow,2013).Boththesubcriticaland supercritical configurationsproposedherearebasedonthisconcept.

Two steam cycle conﬁgurations are possible with sequen-tialsupplementaryﬁring:Supercriticalsteamconditions:630◦C, 295bar (McCauley et al., 2012; Salazar-Pereyra et al., 2011; Satyanarayanaet al., 2011; Czieslaet al., 2009)and subcritical steamconditions:601.7◦C,172.5bar(IEAGHG,2012).Inbothcases, themaximumdesigntemperatureisacriticalparameterforthe designoftheHRSG.

3. Sequentialsupplementaryﬁringwithasubcritical

combinedcycle

Atechno-economicstudyofasubcriticalcombinedcycle con-figurationis firstcompared toa referenceplant consistingof a new-buildNGCCplantwithpost-combustioncaptureinthis sec-tion.The nextsectionof thearticle examinesthebenefits of a supercriticalcombinedcycleoverasubcriticalconfiguration.Both configurationsexaminedhereareequippedwithaconventional HRSG,wherethemaximumtemperatureachievableis820◦C.A modelofthepowercycleintegratedwiththecaptureplantisused tooptimise performance and providethebasis for the techno-economic study. Appendix A lists the parameters used in the modellingofthepowerplantsforallcasestudies.

3.1. ModellingandoptimisationofsubcriticalSSFCCcycle

alternative

Theparametersinvolvedintheoptimisationoftheoverall ther-malefﬁciencyandthemarginalthermalefﬁciencyoftheadditional naturalgascombustionintheHRSGare:

-thenumberofadditionalﬁringstages -theamountoffuelburnt

-thepinchpointtemperature

-numberofpressurelevelsintheHRSG,andsteampressure -thestacktemperature

PowerplantsconfigurationsaresimulatedusingAspenHYSYS®. SettingthemaximumHRSGtemperatureachievableallowsfora givennumberofstagesofsupplementaryfiringwithaminimum levelofexcessO2contentinthefluegasforcompletecombustion. Afterthefinalfiringstagetheoxygencontentinthefluegasis1% v/v(Steamitsgenerationanduse,2005,pp11.4),whichis suffi-cienttoachievecompletecombustion.TheoptimisationinAspen HYSISconsistsofmaximisingmarginalefficiencyandreducingheat transferirreversibilitiesasmuchaspossiblebyanalysing differ-entpressurelevelsofsteamproducedintheHRSG(triple,double orsinglepressure).Theintegrationbetweenthecombinedcycle andthecaptureplantconsistsofsolventregenerationsteambeing extractedfromthecrossoverpipebetweentheintermediate pres-sure(IP)andthelowpressure(LP)turbinesofthesteamcycleata

pressure3barinordertoallowoptimumsolventregenerationofa 30%wtMEAsolvent.

3.2. ModellingandoptimisationoftheCO2captureplantand

compressorunit

AllcasestudieshavebeenintegratedwithastandardCO2 cap-tureplantusing30%wtMEA,asshowninFig.1.TheCO2capture plantissimulatedinAspenplus®usingarate-basedapproach.The captureplantwasvalidatedbyseveralauthorsbasedonvarious datasets fromdifferent pilotplants(Razi etal., 2013;Sanchez Fernandezetal.,2014).Theperformanceoftheabsorberis esti-matedtoﬁndtheoptimumparameterssuchasleanloading,rich loading,absorberandstripperpackingheight,heattransferarea, andenergyremovedfromthecondenser,andtheelectricityoutput penalty(EOP)toachieve90%CO2capturerate.

Theelectricityoutputpenaltycanbecalculatedfromthenet poweroutputwithoutcapture;thenetpoweroutputwithCO2 cap-ture,whichincludeslosesforsteamextractionandelectricalenergy forCO2compressorsandotherarchilleries;andtheCO2captured asshowninEq.(2).

EOP=MWwithout/capture−MWwith/capture CO2 captured

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where EOP is the electricity Output Penalty (kWh/t CO2), MWwithout/capture isthenetpoweroutputwithoutcapture(kW), MWwith/captureisthenetpoweroutputwithCO2captureand com-pressorunit(kW),and_CO2capturedistheamountofCO2capture (t/h).

TheleansolventloadingoftheMEAisvariedtofindthe min-imumEOPforagivenCO2concentrationinthefluegases.While studyingtheeffectofdifferentleanloadingonthecapture pro-cess,thestripperreboilerpressureisvariedtochangethevalues oftheleanloadingandthetemperatureiskeptconstant.The rec-ommendedtemperatureofthereboilerforMEAis120◦C(Kohland Nielsen,1997;IEAGHG,2010;Rochelle,2009).Itwasverifiedtobe optimalintheexperimentalresultsofKnudsen(2011)inapilot plantwithcapacitytocapture1t/hofCO2fromthefluegas gen-eratedatthecoalfiredpowerplantoperatedbyDonginEsbjerg, Denmark(SanchezFernandezetal.,2013afterKnudsen,2011).For eachleanloadingspecified,theheightoftheabsorberisthen var-ied.Atagivenabsorberheight,theabsorptionsolventcirculation rateisvariedtoachievethesameCO2removalcapacity(90%).

Theconﬁgurationofthecompressorisselectedwithtwotrains ofagear-typecentrifugalcompressorwith7stagesand intercool-ingaftereach stage.It isdesigned fora nominalpressureratio 80andaCO2temperatureof40◦Caftertheintercoolersbasedon

LiebenthalandKather(2011)andSiemens(2009).

3.3. Conventionalnaturalgascombinedcycleconﬁguration

Theconventionalcaseisa NGCCplantintegratedwith MEA-basedCO2 capture.Theconfigurationand operatingparameters fortheconventionalcaseisbeentakenfromParsonsBrinckerhoff (IEAGHG,2012).TheconfigurationoftheNGCCconsistsoftwogas turbinesandthreesteamturbines.EachtraincomprisesofoneGE 937IFBgasturbinewithfluegasexitingintoaHRSG.Thetotal steamgeneratedinbothHRSG’ssupplysteamtoasubcriticaltriple pressuresteamcyclecomprisingofthreesteamturbines,asshown inFig.2.

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Fig.1.ProcessﬂowdiagramoftheCO2captureprocess.DCC–directcontactcooler.

Fig.2. SchematicprocessﬂowdiagramoftheconventionalnaturalgascombinedcycleconﬁgurationwithtwoGE937IFBgasturbine,twotriplepressureHRSGsandone subcriticalsteamturbine.

3.4. SubcriticalSSFCCpowerplantconﬁguration

Fig.4showsthepinchdiagramforaconfigurationwherethe totalamountofsupplementaryfuelisburntusingasingleduct burnertoreach1%v/vO2inthefluegas,representingbyablack dashedline.Thetemperaturerisesupto1700◦C.Withsequential supplementaryfiring,thetotalamountofnaturalgasisdividedinto fivestagesthroughtheHRSG.Asaresult,thepeaktemperatureis droppedtoaround820◦CasshowninFig.4representingbyablack continueline.Thetotalamountofnaturalgasburnedinfivestages intheHRSGis22.3kg/s.Thiscorrespondsto57%ofthetotalfuel inputofthegasturbineandtheHRSG.

TheschematicprocessﬂowdiagramshowninFig.5isthe opti-mumsubcriticalSSFCCconﬁgurationandconsistsofasingleGE937 IFBgasturbinefollowedbyasingleHRSGunit.TheHRSGoperates withasinglepressureandprovidessteamtoasinglereheatsteam cycle.SimilarmaterialstothatofaconventionalHRSG(stainless steel304)canbeused.

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0 100 200 300 400 500 600 700

0 50 100 150 200 250 300 350 400 450

Te

mp

er

at

ur

e (

°

C)

Heat duty (MW)

Exhaust gas HP steam IP steam LP steam

Fig.3.Temperature/heatdiagramfortheHeatRecoverySteamGeneratoroftheNaturalGasCombinedCycleplant(Fig.2)withsubcriticalsteamconditions(601.7◦_C,

601.5◦C,172.5bar).

0 200 400 600 800 1000 1200 1400 1600 1800

0 250 500 750 1000 1250 1500

Te

m

p

era

tu

re

(°C

)

Heat duty (MW)

Exhaust gas with single stage firing Exhaust gas with sequential firing HP steam

IP steam

∆T3 ∆T2

∆T1

Fig.4.Temperature/heatdiagramoftheHeatRecoverySteamGeneratorofafivestagesequentialsupplementaryfiringconfigurationwithasinglepressureHRSG,with asinglereheatcombinedcycleandsubcriticalsteamconditions(601.7◦C,601.5◦C,172.5bar).ThethreepinchtemperaturesT1,T2,T3arerespectively76◦C,70◦C, 79◦C.

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Table1

Temperature,O2concentration,CO2concentrationattheinletofeachductburnerforasubcriticalsequentialsupplementaryﬁringpowerplant.

Temperature InletO2concentration InletCO2concentration ExitequivalentexcessAir

Inlet(◦_C) _Outlet₍◦_C) _%_v/v _%_v/v _%_v/v

Ductburner1 643 820 11.9 4.2 100

Ductburner2 712 809 10.2 5.01 69

Ductburner3 608 802 8.0 5.45 39

Ductburner4 453 778 5.8 6.33 26

Ductburner5 480 774 4.0 7.85 6

totheﬁrstthreeburnersarepresent.Hightemperaturecan com-pensateforthelowlevelsofoxygenandthecombustioncanbe stabilisedwithsimpleburnerramps(Lietal.,2012).Itisalsoworth reiteratingthatgasandoilﬁredboilersusedinutilityandindustrial steamgenerationapplicationstypicallyoperatewithanexcessair intherangeof5–10%v/v(Steamitsgenerationanduse,2005,pp 11.4),comparabletothe6%v/vofequivalentexcessairattheinlet ofthelastburner.

Althoughthespecificdesignofductburnerstooperatewithin thisrangeisoutsidethescopeofthisstudy,itisworthnotingthat thepresenceofhigherlevelsofCO2comparedtoconventionalgas andoilboilersrequiresfurtherinvestigationofcombustionstability andefficiency.Ifsatisfactorycombustionprovedtobechallenging inthefinal ductburner,thiscouldleadtoremovingtheburner andoptimisetheconfigurationtooperatingwithonefewerburner, withpossiblehigheroutlettemperaturetomaximisenaturalgas usage.

3.5. Effectofsequentialsupplementaryﬁringonpowerplant

performance

KeyparametersfortheconventionalNGCCconﬁgurationand subcriticalSSFCCwithCO2capturearedescribedinTable2.When supplementaryfuel is burnt sequentially in a single one HRSG attachedtoa295MWgasturbine,thecapacityofthesteamcycle increasesfrom245MWto545MW.Thecorrespondingtotalnet poweroftheSSFCCis781MW,similartotheconventionalNGCC conﬁgurationwith794MW. AsinSSFCConlyonegasturbineis used,thetotalvolumeoftheexhaustgasesisreducedbyhalf.It hasapositiveimpactonthenumberofdirectcontactcoolers(DCC) andabsorbersofthecaptureplants.

AlthoughtheefficiencyofthesubcriticalSSFCCconfigurationis oftheorderof43.1%LHV,comparedto51.3%LHVforastandard NGCCplantwithcapture,therearesignificantcapitalcost impli-cationsforthegasturbine,theheatrecoverysteamgenerator,the steamcycle,theabsorbertrainsandthestripper/compressionpart ofthecaptureplantandthepotentialforadditionalrevenuefrom EOR:

•TheSSFCCconﬁgurationmakesuseofasinglegasturbine/HRSG traincomparedtotwogasturbine/HRSGtrainsfora standard conﬁguration.

•Thenumberofabsorbertrainsisreducedfromfourtotwo,as previouslydiscussed.

•Thecapacityofthestripperandthecompressiontrainisincreased byaround17.7%.

3.6. EffectofincreasedCO2concentrationonsolventenergyof

regenerationandabsorbercolumndesign

ThecombustionofadditionalnaturalgasintheHRSGincreases theCO2 concentrationinthefluegasfrom4.2%v/vto9.36%v/v, whilstreducingtheexcessoxygento1.3%v/v.Theoptimumlean loadingfor aNGCCconfigurationwithcapturereaches0.27mol CO2/molMEAand0.26molCO2/molMEAforaSSFCCconfiguration.

ThehigherrichloadingachievedwithhigherCO2 concentration leadstoanincreaseinsolventcapacityandthespeciﬁcreboilerduty decreasesfrom3.56to3.42GJ/tCO2foraconﬁgurationwith21m ofstructuredpackingheightintheabsorbercolumnsasshownin

Fig.7.ThisisillustratedinFig.6wheretheoptimisationofthe over-allelectricityoutputpenaltywithsolventleanloadingisreported. Columnswithverylargediametersarenotrecommended.There isamaximumvolumeflowrateof300,000m3_/h_(292.5_t/h approx-imately) which couldbetreated in anabsorber column due to economiclimitsofthesizeoftheabsorber(DesideriandPaolucci, 1999;Yagietal.,1992).Forsystemsthatrequiretheprocessing ofa largerflow, amodulardesignwithseveraltrainsoperating inparallelisadopted.Rezazadehetal.(2015)afterReddyetal. (2003)reportedthatthemaximumdiameterforanabsorber col-umnunderoperationis18.2m.InsubcriticalSSFCC,thetotalflue gasflowrateis696kg/scontaining93kg/sofCO2comparedwith aconventional NGCCwhere totalfluegasflowrateis1347kg/s whichcontain79kg/sthatcanbeseeninTable2.Thenbasedon theargumentdescribedpreviouslyrelatedtothecapacityofthe absorber,thefluegasflowrateofonetrainofSSFCCis348kg/s whichcontain46.5kg/sofCO2 andthefluegasflowrateofone trainoftheNGCCis336.8kg/swhichcontain19.75kg/s.Thefinal configurationofSSFCCis:twoDCCandtwoabsorbers;andtwo strippercolumnsand tworich/leanheatexchangers.ForNGCC: fourDCCandfourabsorbers;andtwostrippercolumnsandtwo rich/leanheatexchangers.

Thereductionbyapproximately50%oftheoverallgasﬂowrates hasapositiveimpactonthecapitalcostsoftheDCCandabsorber columnswhicharereducedfromfourtotwocolumns.Theheight oftheabsorberforSSFCCandtheNGCCareoptimised,basedonthe reductionofthereboilerduty,fortheCO2contentinﬂuegasand 90%captureandbotharriveatthesameheightof21mofpacking ineachabsorbercolumn.

3.7. Comparisonofcostofelectricity

Themainobjectiveofthiseconomicstudyistocomparethe expected cost of electricity, taking into account revenues from EOR,ofa SSFCCconfigurationwithaconventional NGCC.There are importantdifferences betweenboth configurations,suchas thermalefficiency,sizeofcriticalpiecesofequipment,operational costs.Inthisstudy,thedirectcomparisonoftheexpectedcostsof sequentialsupplementaryfiringwithaconventionalconfiguration, usingconsistentsourcesofinformationensuresthaterrorand inac-curaciesincapitalcostsaremitigated.Asensitivityanalysisisalso providedtoexaminetherobustnessofthefindingsoverarange of capitalcost estimates toaccountfor theassociated estimate uncertainties.

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Table2

SummaryofkeyparametersofaSSFCCwithsinglepressuresubcriticalsteamcyclewithCO2capture(Saturatedvapourat3barisusedinthereboiler).

Concept Unit NGCC SSFCCsubcritical

LHVnetelectricefﬁciencya _% _51.3 _43.1

GasturbinepoweroutputGT MW 590 296

Steamcyclepoweroutput MW 245 545

TotalLHVgrosspoweroutput MW 835 840

TotalLHVnetpoweroutput(includingCO2compressionandotherancillaries) MW 794 781

Massﬂowrateofnaturalgastogasturbine kg/s 33.2 16.6

Massﬂowrateofnaturalgasforsupplementaryﬁring kg/s NA 22.2

MarginalefﬁciencyofnaturalgasﬁredinHRSG(LHV) % NA 36

MarginalefﬁciencyofnaturalgasﬁredinHRSG(LHV)without post-combustioncapture(forcomparativepurposepurposesonly)

% NA 44.7

Electricityoutputpenalty(EOP) kWhe/tCO2 408 362

Carbonintensityofelectricitygeneration kgCO2/MWh 39.8 47.5

Fluegasmassﬂowrate kg/s 1347 696

Fluegascompositionafterdirectcontactcooler

Water(H2O) %vol 4.29 4.29

Carbondioxide(CO2)b %vol 4.37 10.87

Oxygen(O2) %vol 12.5 1.3

Nitrogen(N2) %vol 78.8 83.5

CO2massﬂowtopipeline kg/s 79 93

Capturelevel % 90 90

Solventenergyofregeneration GJ/tCO2 3.56 3.42

Steammassﬂowtosolventreboiler kg/s 145 146

Numberofabsorbertrains 4 2

Diameter m 15.5 15.5

Absorberheight m 21 21

Fluegasmassﬂowrateateachabsorberinlet kg/s 337 348

VolumeofpackingusedforCO2capture(notincludingwaterwashsections) m3 16,260 8130

a_LHV_net_electric_efﬁciency_includes_CO

2compressionandparasiticlossesandtransformedlosses. b _The_{concentration}_of_the_CO

2presentedinTable2istheconcentrationoftheexhaustgasafterthedirectcontactcooler.ItishigherthanintheHRSGaftercondensation

ofafractionofthewatercontainedintheﬂuegas.

1.75 1.80 1.85 1.90 1.95 2.00

352 362 372 382 392 402 412 422 432

0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31

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/ to

n

e C

O

2

)

Lean loading (kmol CO

₂

/ kmol MEA)

Pressure

NGCC

SSFCC

Fig.6. Optimisationofelectricityoutputpenaltyforanaturalgascombinedcycle(NGCC)andforsequentialsupplementaryﬁringcombinedcycle(SSFCC)asafunctionof solventleanloading,withaCO2removalrateof90%andstrippertemperatureof120◦C.TheCO2concentrationintheﬂuegasis,respectively,4.2mol%and9.4%foraNGCC

configurationandaSSFCCconfiguration.Thebluedottedlinesindicatetheoptimumsolventleanloading.(Forinterpretationofthereferencestocolorinthisfigurelegend, thereaderisreferredtothewebversionofthisarticle.)

costofcaptureplantforaSSFCCconﬁguration.Thetotalvolume ofpackingoftheabsorberandthestripperandtheareaofheat exchangersareusedtoanalysetheimplicationsontherequired capitalexpenditure(CAPEX).

ThespeciﬁcinvestmentsoftheconventionalNGCCand subcrit-icalSSFCCcaseshavebeenevaluatedandarereportedinTable3. ThenetspeciﬁcinvestmentestimatedfortheNGCCcaseis773 $/kW,whichincreasesto1698$/kWwhentheCO2captureunitis incorporated.TheresultsoftheNGCCareingoodagreementwith

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3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

0.40 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49

5 7 9 11 13 15 17 19 21 23

R

ic

h

l

o

ad

in

g (m

o

l

C

O2

/m

o

l M

EA

En

erg

y

re

bo

iler

(G

J/

to

nn

e C

O2

)

Absorber height (m)

Rich loading at 4.27% CO2 Rich loading at 9.36% CO2 Energy reboiler at 4.27% CO2 Energy reboiler at 9.36% CO2

Fig.7.EffectofCO2concentrationintheﬂuegasonsolventenergyofregenerationforarangeofabsorbercolumnheights.Thecapturerateis90%.Theleanloadingand

pressureinthereboilerare,respectively,0.27and1.9barfortheNGCCconﬁgurationand0.26and1.9barfortheSSFCCconﬁguration.

Table3

Estimatedspeciﬁcinvestmentforthenaturalgascombinedcyclewithandwithoutcaptureandsubcriticalsequentialsupplementaryﬁringcombinedcyclewithcapture.

Plantcomponent Unit NGCC NGCCw/capture SubcriticalSSFCCw/capture

Grosspoweroutput MW 928 835 839.7

Netpoweroutput MW 909 794 781

Powerplantmainitems

Gasturbine,generatorandauxiliaries M$ 137 137 68

HRSG,ductingandstack M$ 65 65 33

Ductburner M$ 0 0 2

Steamturbinegeneratorandauxiliaries M$ 66 55 88

Coolingsystemandmiscellaneous,BalanceofPlant(BOP)system M$ 69 36 68

Subtotal M$ 336 293 260

TotalInstallationcosta _M$ ₁₆₃ ₁₄₂ ₁₂₆

BareErectedCost(BEC) M$ 499 434 386

Indirectcostb _M$ ₇₀ ₆₁ ₅₄

EngineeringProcurementandConstruction(EPC) M$ 569 495 440

Contingencies,owner’scostsc _M$ ₁₃₄ ₁₁₆ ₁₀₃

TotalCapitalRequirement(TCR)powerplant M$ 703 611 544

Captureplantmainitems

Fluegascooling M$ NA 21 11

CO2absorber&ﬂuegasre-heater M$ NA 110 55

Rich/leanaminecirculation M$ NA 6 6

Strippingsection M$ NA 139 139

Ancillaries M$ NA 5 5

Suportingfacilities&labor(directandindict)d _M$ _NA ₆₁ ₄₇

Subtotal M$ NA 342 262

Installationcoste _M$ _NA ₁₂₈ ₉₈

BEC M$ NA 470 361

EPC,Contingenciesandowner’scostsf _M$ _NA ₂₁₆ ₁₆₆

TCRcaptureplant M$ NA 687 527

TCRCO2compressiong M$ NA 49 53

Speciﬁcinvestment–Gross $/kW 757 1614 1337

Speciﬁcinvestment–Net $/kW 773 1698 1438

a_49.8%_of_subtotal_cost₍_IEAGHG,₂₀₁₂_). b_14%_of_BEC_cost₍_Franco_et_al.,₂₀₁₂_). c _23.5%_of_EPC₍_IEAGHG,₂₀₁₂_).

d _2.7%_of_the_total_equipment_cost₍_IEAGHG,₂₀₁₂_). e_37.5%_of_subtotal_cost₍_IEAGHG,₂₀₁₂_). f _46%_of_BEC₍_IEAGHG,₂₀₁₂_).

g_Hendriks_et_al.₍₂₀₀₃₎_includes_{installation,}_indirect_costs,_{contingencies}_and_owner’s_costs.

thecomplexityandthenumberofheatexchangersissmallerasthe HRSGisasinglepressuresysteminsteadofatriplepressuresystem. Thecontributionofthegasturbinetotheoverallpoweroutputis muchlowerthanintheNGCC.Effectively,thenumberofgas tur-binetrainsisreducedfrom2to1.Theadditionalinvestmentsinthe steamcyclearecompensatedbythereductioninthegasturbine train,leadingto11%lowerpowerplantspeciﬁcinvestmentthan NGCCwithcapture.Theinvestmentinthesteampartofthepower

cycle(steamturbines,coolingsystemandBOP)increasesto gener-atemorepowerfromtheheatrecoveredinSSFCC.Inallthecases, asthepowerplantsaredesignedtooperatewithCO2capture,the LPsteamturbinesizeissmallerthanifitwouldoperatewithout capture.

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Table4

Operatingandmaintenancecost(O&M)ofthepowerplantandCO2captureplantforthenaturalgascombinedcycleandsubcriticalsequentialsupplementaryﬁringcombined

cycle.

Unit NGCC NGCCwithcapture SubcriticalSSFCC

Powerplant M$ M$ M$

FixedO&Mcostsa _M$ _13.3 _11.6 _11.4

Variablecosta _M$ _17.6 _15.4 _15.2

CO2captureandcompression

FixedO&Mcostsb _M$ _NA _14.7 _11.6

Variablecostc _M$ _NA _10.9 _7.4

Total M$ 30.9 52.6 45.5

TotalO&M–net $/kWh 4.85 9.46 8.32

a_COPAR₍₂₀₁₃₎_. b _2%_TCR_CO

2captureplantincludingcompression(IEAGHG,2011). c _Solvent_make_up_is_estimated_as_2.4_kg_MEA/t_CO

2fortheNGCCcasewith13%v/vO2intheﬂuegas(Gorsetetal.,2014)and1.5kgMEA/tCO2fortheSSFCCcasesforO2

concentrationssimilartocoalﬂuegas(below4%v/v)(RubinandRao,2002;DOE,2007).

Table5

TotalcostofCO2transportforthenaturalgascombinedcycleandsubcritical

sequen-tialsupplementaryﬁringcombinedcycle.

Unit NGCC NGCCwith

capture

Subcritical SSFCC

CO2transporta M$ NA 7.0 8.2

$/kW NA 8.9 10.5

a_3.65_$/tCO

2in2011dollar(DOE/NETL,2013a,b)isupdatedto3.51$/tCO2in

2013dollarusingtheChemicalEngineeringindex(2013).

to52.6M$(milliondollar),whencaptureisadded.Thevariable operatingcostsofthecaptureunitdecreasefortheSSFCC config-urationcomparedtotheNGCCwithcapture.Mainlyitreflectsthe benefitsofhavinglowersolventdegradationwithloweroxygen concentrationsinthefluegas.

ThetotalcostofCO2transportisprovidedinTable5.TheCost ofgeologicalstorageisnotincludedastheCO2producedis con-sideredforEOR.TheCO2conditionsconsideredinthisstudyareat apressureof150barand95%CO2purityforthepurposeofEOR projects(DOE/NETL,2012)with100kmfromthepowerplantto theoilﬁeld,asindicatedintheDOEstudy.

3.8. Totalrevenuerequirementanddecisiondiagram

ThevaluesprovidedinTables2–4areusedtoestimatethe lev-elisedcostofelectricity(LCOE)usingEq.(B2)inAppendixBandthen calculatethetotalrevenuerequirement(TRR).TheTRRisdeﬁned asthetotalrevenuenecessaryfortheprojecttobreakeven.Itis quantiﬁedatdifferentCO2sellingpricesusingEqs.(3)and(4).

TRR= LCOE−EORrevenue (3)

whereTRRisthetotalrevenuerequirementin$/MWh,LCOEisthe levelisedcostofelectricity$/MWh.EORrevenueistherevenuefor sellingCO2in$/MWhandiscalculatedusingtheEq.(4).

EORrevenue=CO2sellingprice×levelisedﬂowofCO2captured

netpoweroutput (4)

whereCO2sellingpriceisin$/tCO2,levelisedﬂowofCO2capture int/h,andnetpoweroutputinMW.

Thefollowingunderlyingassumptionsareusedinthisanalysis:

-Thereisnocarbonpriceassociatedwiththeresidualcarbon emis-sions

-ItisﬁnanciallyworthbuildinganewNGCCplantwithcapture intheelectricitymarketwhereallthepossibleconﬁgurationsof plantswouldoperate

-Thereforetheelectricitysellingpriceaveragedoverthelifeof aplantisatleasthigherthantheLCOEoftheNGCCplantwith capture

ThecostimpactsofCO2-EORsalesareinvestigatedwitha sensi-tivityanalysisofthecapitaloftheSSFCCconﬁgurationinFig.8.The subcriticalSSFCCconﬁgurationisnaturallymoresensitiveto varia-tionoftheCO2sellingpricebecauseoftheadditionalrevenuefrom sellingCO2forEOR,normalisedperunitofenergy.Withrespectto thepriceofcrudeoil,thecommercialCO2pricedecreasedwithin therangefrom25to$65/twhenthecrudeoilpriceis $100/bar-relto45$/tCO2 atoilpriceof$70/barrel(ZhaiandRubin,2013;

NationalEnergyTechnologyLaboratory,2012,2010).Inour anal-ysis,theCO2salepricecoversarangefrom0to$50/tCO2andthe gaspricefrom2to6$/MMBTU,indicatingthat:

-For a gas price of 6 $/MMBTU (5.69 $/GJ), the total revenue requirementlinesofthesubcriticalSSFCCconﬁgurationintersect withthetotalrevenuerequirementlineoftheNGCC conﬁgura-tionatabreakevenCO2sellingpriceof37$/tCO2,asshownin

Fig.8.Witharelativereductionofcapitalcostof10%oftheSSFCC conﬁguration,thelinesintersectat7.5$/tCO2.ForaCO2selling priceabovethebreakevenvalueintheintersection,the subcrit-icalSSFCCplantwithCO2-EORhasasmallerTRRthantheNGCC plantandwouldgenerateadditionalrevenuesifboth conﬁgura-tionsreceivethesameelectricitysellingprice.

-Foragaspriceof4$/MMBTU(3.79$/GJ),thetwototalrevenue requirementlinesintersectatabreakevenCO2sellingpriceof2.5 and33.5$/tCO2forvariationsofthecapitalcostsoftheSSFCC conﬁgurationsof0and10%.

-Foragaspriceof2$/MMBTU(1.896$/GJ),thecurrentpriceat thetimeofwritinginMexico(RegulatoryCommissionofEnergy, 2016),thesubcritical SSFCCconﬁgurationpresentsthelowest totalrevenuerequirementforallCO2sellingpriceandforcapital costsvaryingfrom−20%to10%.Atanincrementof20%relative, thelinesintersectatabreakevenCO2sellingpriceof30$/tCO2.

ThisanalysisissummarisedinadecisiondiagraminFig.9fora differencerangeofcapitalcostestimatesforthesubcriticalSSFCC conﬁgurationandarangeofgasprices.

4. Sequentialsupplementaryﬁringwithasupercritical

combinedcycle

4.1. Performanceassessment

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20 30 40 50 60 70 80 90

0 5 10 15 20 25 30 35 40 45 50

T

o

ta

l R

ev

en

u

e R

eq

u

im

e

re

n

t (

$

/M

W

h

)

CO2 price for EOR ($/tonneCO2)

NGCC

Subcritical SSFCC

∆CAPEX -10%

∆CAPEX + 10% ∆CAPEX -20%

∆CAPEX + 20%

Fig.8.Totalrevenuerequirementforasubcriticalsequentialsupplementaryfiringconfigurationandaconventionalnaturalgascombinedcycleconfigurationforarange ofrepresentativeCO2priceforEORandfuelprices.Therelativevariationincapitalcostsofthesubcriticalsequentialsupplementaryfiringconfiguration(CAPEX)ranges

from−20%to20%.TheverticallinesindicatethebreakevenCO2priceofequaltotalrevenuerequirements.

-15 -10 -5 0 5 10 15 20 25 30

0 5 10 15 20 25 30 35 40 45 50

)

%(

n

oit

ar

u

gi

f

n

oc

C

F

S

e

ht

f

o

st

s

oc

la

ti

pa

c

ni

n

oit

ai

ra

v

e

vit

al

e

R

breakeven CO2price of equal total revenue requirement ($/tonne)

6 $/MMBTU 4 $/MMBTU 2 $/MMBTU

35 4400 445 5

NGCC configuration has lower Total Revenue Requirement

SSFCC configuration has lower Total Revenue Requirement

Fig.9. Decisiondiagramforarangeofcapitalcostestimatesandgasprices.

(Innovative Steam Technologies Company, 2012). Nevertheless, advanced alloys,suchas IncoloyAlloy800&825, a nickel and iron-chromiumenrichedalloywithadditionsofmolybdenumand

copper,arerequiredcomparedtoaconventionalHRSG,with Stain-lessSteel304(InnovativeSteamTechnologiesCompany,2012).

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Fig.10.SchematicprocessflowdiagramofasupercriticalsequentialsupplementaryfiringconfigurationwithoneGE937IFB/singlepressureHRSGtraincombinedcycle withadoublereheatsteamcycle.

thecombined cycleis higher than thesubcritical configuration sincethereisanincrementinthemarginalthermalefficiencyof theadditionalgasusagewithsupercriticalsteamconditions.The configurationofthesteamturbinesisadaptedfromaconfiguration describedbyKjaer(1993)forapulverisedcoalpowerplant.Asin thesubcriticalSSFCCconfiguration,supplementarygasisburned in5stagesthroughouttheHRSGtoreducetheexcessO2downto aconcentrationof1%v/v.

With sequential supplementary firing, supercritical steam conditions of 630◦C, 601.5◦C, 290bar (McCauley et al., 2012; Salazar-Pereyraetal., 2011;Satyanarayana etal., 2011;Cziesla etal.,2009)increasetheaveragetemperatureofheatadditionto thesteamcycle.Theabsenceofphasechangebetweenthe evap-oratortothesuperheaterallowsforareductioninheattransfer irreversibilitieswithlower temperaturedifferencebetweenthe fluegasandtheturbineworkingfluid.ThepinchpointoftheHRSG isreducedwithsupercriticalconditionsfrom70◦Cto27◦Cseenin

Fig.11andthemarginalthermalefﬁciencyofnaturalgasusageis increasedfrom36to40.2%showninTable6.

For completeness, Fig.12 shows the expansion lines of the supercriticaldoublereheatcombinedcycleandthesubcritical sin-glereheatcombinedcycleonanenthalpy-entropydiagram.Inthe supercriticalRankinecyclewithdoublereheat,steamisexpanded from295barto80barin theVeryHigh Pressure(VHP)turbine andsentbacktotheHRSGwhereitisreheatedinReheaterRH2 ofFig.10.SteamthenexpandsintheHPsteamturbinedownto around42barandissentbacktotheHRSGwhereitisreheatedin ReheaterHR1.Thesteamtemperaturerisesto601◦Cbeforeitis expandedintheIPsteamturbine.

Table6 presentsthe performance assessment of the

super-criticalSSFCC configurations and comparesit totheequivalent subcriticalconfiguration.Withadditionalfuelbeingburntinone HRSG with supercritical steam conditions, the capacity of the steamturbineincreasesfrom245MWto589MWcomparedtothe conventionalNGCCconfiguration.Thetotalnetpowerofthe super-criticalSSFCCconfigurationis824MW,comparedto794MWwith aNGCCand781MWwithsubcriticalSSFCC.Thethermalefficiency ofthesupercriticalSSFCCconfigurationwithpost-combustion cap-tureis45.6%LHV,comparedto43.1%forasubcriticalSSFCCplant, asshowninTable6.However,therearecostimplications:TheHP partofthecombinedcycle,includingtheHPsteamturbine,valves, pipework,andtheHPsuperheaterrequiresbeingofsupercritical design.

Table8

Operatingandmaintenancecost(O&M)ofthepowerplantandCO2captureplant

forthesupercriticalsequentialsupplementaryﬁringcombinedcycle.

Unit SubcriticalSSFCC

Powerplant M$ M$

FixedO&Mcostsa _M$ _12.0

Variablecosta _M$ _16.0

CO2captureandcompression

FixedO&Mcostsb _M$ _11.6

Variablecostc _M$ _7.4

Total M$ 47.0

TotalO&M–net $/kWh 8.14

a_COPAR₍₂₀₁₃₎_. b_2%_TCR_CO

2captureplantincludingcompression(IEAGHG,2011). c _Solvent_make_up_is_estimated_as_2.4_kg_MEA/t_CO

2fortheNGCCcasewith13%

v/vO2intheﬂuegas(Gorsetetal.,2014)and1.5kgMEA/tCO2fortheSSFCCcases

forO2concentrationssimilartocoalﬂuegas(below4%v/v)(RubinandRao,2002; DOE,2007).

4.2. CostestimationofsupercriticalSSFCC

Themethodologyusedtoestimatethecostofthesupercritical SSFCCconﬁgurationisidenticaltothesubcriticalonedescribedin Section3.6.Forsupercriticalsteamconditions,thecostestimateof theHRSGinsectionswithhightemperatureisbasedonEq.(B1)in

AppendixB,whereafactorisusedtoaccountfortheuseofmore expensivealloystosupportsupercriticalconditions(Worldsteel prices,2013).

Thespeciﬁc investmentof supercriticalSSFCC is reportedin

Table7.WhencomparedwiththeconventionalNGCC

configura-tion,thereisareductioninthetotalspecificinvestmentof9.1%, equivalentto75M$,lowerthanforthesubcriticalconfiguration with15.3%and264M$respectively.Theoperatingand mainte-nancecosts(O&M)areprovidedinTable8.Sincethefuelthermal inputisthesameforbothconfigurationswithsequentialfiring,the amountofCO2generatedisthesame.TotalcostofCO2transportis providedinTable5.

4.3. Totalrevenuerequirementandsensitivitytogaspriceand

CO2sellingprice

TheTRRofthesupercriticalconﬁgurationisevaluatedandthen comparedwiththecorrespondingsubcriticalconﬁguration.

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Table6

SummaryofkeyparametersofaSSFCCwithsinglepressureHRSGandadoublereheatsupercriticalsteamcyclewithCO2capture(Saturatedvapourat3barisusedinthe

reboiler).

Concept Unit SupercriticalSSFCC SubcriticalSSFCC

LHVnetelectricefﬁciencya _% _45.6 _43.1

GasturbinepoweroutputGT MW 296 296

Steamcyclepoweroutput MW 589 545

TotalLHVgrosspoweroutput MW 884 834

TotalLHVnetpoweroutput(includingCO2compressionandotherarchillaries) MW 824 781

Massﬂowrateofnaturalgastogasturbine kg/s 16.6 16.6

Massﬂowrateofnaturalgasforsupplementaryﬁring kg/s 22.2 22.2

MarginalefﬁciencyofnaturalgasﬁredinHRSG(LHV) % 40.2 36

MarginalefﬁciencyofnaturalgasﬁredinHRSG(LHV)without post-combustioncapture(forcomparativepurposepurposesonly)

% 49.1 44.7

Electricityoutputpenalty kWhe/tCO2 350 362

Carbonintensityofelectricitygeneration kgCO2/MWh 45 47.5

Fluegasmassﬂowrate kg/s 696 696

Fluegascompositionafterdirectcontactcooler

H2O %vol 4.29 4.29

CO2 %vol 10.87 10.87

O2 %vol 1.312 1.3

N2 %vol 83.52 83.5

CO2massﬂowtopipeline kg/s 93 93

Capturelevel % 90 90

Solventenergyofregeneration GJ/tCO2 3.42 3.42

Steammassﬂowtosolventreboiler kg/s 157 146

Numberofabsorbertrains 2 2

Diameter m 15.5 15.5

Absorberheight m 21 21

Fluegasmassﬂowrateateachabsorberinlet kg/s 348 348

VolumeofpackingusedforCO2capture(notincludingwaterwashsections) m3 8130 8130

a_LHV_net_electric_efﬁciency_includes_CO

2compressionandparasiticlossesandtransformedlossesareincluded.

Table7

Estimatedspeciﬁcinvestmentforthesupercriticalsequentialsupplementaryﬁringcombinedcyclewithcapture.

Plantcomponent Unit SupercriticalSSFCCw/capture

Grosspoweroutput MW 884

Netpoweroutput MW 824

Powerplantmainitems

Gasturbine,generatorandauxiliaries M$ 68

HRSG,ductingandstack M$ 88

Ductburner M$ 2

Steamturbinegeneratorandauxiliaries M$ 108

Coolingsystemandmiscellaneous,BalanceofPlant(BOP)system M$ 64

Subtotal M$ 331

TotalInstallationcosta _M$ ₁₆₁

BareErectedCost(BEC) M$ 492

Indirectcostb _M$ ₆₉

EPC M$ 561

Contingencies,owner’scostsc _M$ ₁₃₂

TotalCapitalRequirement(TCR)powerplant M$ 693

Captureplantmainitems

Fluegascooling M$ 11

CO2absorber&ﬂuegasre-heater M$ 55

Rich/leanaminecirculation M$ 6

Strippingsection M$ 139

Ancillaries M$ 5

Suportingfacilities&labor(directandindict)d _M$ ₄₇

Subtotal M$ 262

Installationcoste _M$ ₉₈

BEC M$ 361

EPC,Contingenciesandowner’scostsf _M$ ₁₆₆

TCRcaptureplant M$ 527

TCRCO2compressiong M$ 53

Speciﬁcinvestment–Gross $/kW 1439

Speciﬁcinvestment–Net $/kW 1544

a_49.8%_of_subtotal_cost₍_IEAGHG,₂₀₁₂_). b_14%_of_BEC_cost₍_Franco_et_al.,₂₀₁₂_). c _23.5%_of_EPC₍_IEAGHG,₂₀₁₂_).

d _2.7%_of_the_total_equipment_cost₍_IEAGHG,₂₀₁₂_). e_37.5%_of_subtotal_cost₍_IEAGHG,₂₀₁₂_). f _46%_of_BEC₍_IEAGHG,₂₀₁₂_).

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0

100

200

300

400

500

600

700

800

900

0

250

500

750 1000

1250

1500

Te

m

p

era

tu

re

(°

C)

Heat

duty

(MW)

Exhaust gas HP steam Reheat 1 Reheat 2 ∆T3

∆T2

∆T1

Fig.11.Temperature/heatdiagramfortheHeatRecoverySteamGeneratorofafivestagesequentialsupplementaryfiringconfiguration,withadoublereheatcombined cycleandsupercriticalsteamconditions(630◦C,601.5◦C,295bar).ThetwopinchtemperaturesT1,T2,T3arerespectively27◦C,36◦C,88◦C.

Fig.12.Enthalpy-entropydiagramofthesupercriticalRankinecyclewithdoublereheatandthesubcriticalRankinecyclewithsinglereheat.

sellingpriceandgaspriceinarangefrom2to6$/MMBTU.The supercritical SSFCC configuration presents overall a lower TRR thanasubcriticalconfiguration.Thisisduetoanimprovementin efficiencyassociatedwithsupercriticalsteamconditionsandthe factthatrevenue fromCO2 salesareidenticaltothesubcritical

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-4.5 -4.0 -3.5 -3.0 -2.5 -2.0

-1.5 0 5 10 15 20 25 30 35 40 45 50

A

b

so

lu

te

c

h

an

ge i

n

T

o

ta

l Re

ven

u

e

Req

u

im

er

en

t (

$

/M

W

h

)

CO₂price for EOR ($/tonneCO₂)

2 $/MMB

TU

4 $/MMB

TU

6 $/MMB

TU

Fig.13.Reductionintotalrevenuerequirementforsequentialsupplementaryﬁringcombinedcycleplantwithsupercriticalsteamconditionscomparedtoasubcritical conﬁguration,forarangeofrepresentativeCO2priceforEORandfuelprices.

combinedcycleinthiscontext,asitpresentsconsistentlyalower TRRinarangeofgaspricefrom6to2$/MMBTUandwhentheCO2 capturedisutilizedforEORatcommercialpricesfromzeroto50 $/tCO2.

5. Conclusions

Theintegrationofsequential supplementaryfiringcombined cycle plants is examinedin the context of deployingCCS with EnhancedOilRecoveryinMexico.Anewdesignofheatrecovery steamgeneratorisproposedwhereadditionalfueliscombusted toincreasethevolumesofcarbondioxideavailableforEOR.The maximumamountofCO2isproducedbyreducingexcessoxygen levelsaslowaspracticallypossible(oftheorderof1%v/v).The totalpoweroutputofa sequentialsupplementaryfiring config-urationwithCO2 capture, consistingofa singlegasturbineand heatrecoverysteamgeneratortrain,is824MWwitha supercrit-icalcombinedcycle,781MWwithasubcriticalcombinedcycle, comparedto794MWforaconventionalNGCCconfigurationwith capturewithtwogasturbinesandtwoHRSGs.Thedifferencein thepoweroutputisduetothedesignoftheheatrecoverysteam generatorwhereadditionalfuelburntincreasesheatavailablefor steamgenerationinthecombinedcycle.Thisallowsareduction byhalfofthenumberofGT/HRSGtrainsandofthetotalvolume offluegas.Thishasapositiveimpactonthenumberofdirect con-tactcoolerandabsorbersrequiredinthepostcombustioncapture plant.Thereductionofoverallcapitalcostsis,respectively,9.1% relativeand15.3%relativeforthesupercriticalandthesubcritical configurationscomparedtotheconventional configurationwith capture.Bothsequentialsupplementaryfiringconfigurationsalso presentareductionintheelectricityoutputpenaltycomparedtoa conventionalNGCCplantwithcapture.

Thesensitivityoftotal revenuerequirementsforlow-carbon electricitygeneration,ametric combininglevelised costof elec-tricityandrevenuefromEOR,toCO2pricesandfuelpricesisused

tocompareconfigurations.Sincecapitalcostestimatesarebound toincludelargebiasesanduncertainties,weperformasensitivity analysisshowingthatourconclusionsarerobustoverarangeofgas pricesandCO2pricesforEOR,andthatsequentialsupplementary firingisadvantageousinthecontextofNorthAmericangasprices. AcomparisonbetweenasubcriticalandasupercriticalSSFCC configurationsshowthatimprovementsinpowerplantefficiency withsupercriticalsteamconditionsconsistentlyresultinalower TRR.Atgaspricesrangingfrom2to6$/MMBTU,supercriticalSSFCC mayreceiveadditionalrevenuesrangingfrom1.5to4$/MWhfor CO2 pricesrangingfrom0to50$/tCO2 comparedtosubcritical configurations.

Furtherworkisneededtoincludesitespeciﬁcconsiderations anddetailedcapitalestimatesbeyondtheworkincludedinthis article,whichiseffectivelyaveryﬁrstattemptatassessingthe fea-sibilityandvalidityoftheconcept,withaccesstoaffordablenatural gaspricesandlikelyrevenuesfromEnhancedOilRecovery.

Acknowledgements

TheauthorswouldliketothanktheMexicanNationalCouncil forScienceandTechnology(CONACyT)forﬁnancialsupportto Abi-gailGonzalezDiaz,theGAS-FACTSprojectfundedbytheUKEPSRC (EP/J020788/1),andfundingforDrMathieuLucquiaudfromaUK RoyalAcademyofEngineeringResearchFellowship.Wearealso gratefultoM.AgustínAlcaráz(IIE)forvalidatingthesimulationof theNGCCwithThermoﬂow.

AppendixA.

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TableA1

Ambientconditionsandmodellingbasisforallcasestudies.

Concept Unit Value

Ambienttemperature ◦_C ₁₅

Ambientpressure bar 1.013

Relativehumidify % 60

Coolingwatertemperature ◦_C ₂₅

Coolingwatermaximumtemperaturerise ◦_C ₁₀

Fuelcaloriﬁcvalue(LHV) kJ/kg 46,510

Pressureratiocompressor 19.5

Pressureincondenser bar 4.38

Adiabatic/polytropicefﬁciencycompressor % 87.4/82

Adiabatic/polytropicefﬁciencygasturbine % 88/83.2

TableA2

Inputdataforallcasestudies.

Concept Unit NGCC SSFCCSupercritical SSFCCsubcritical

Pressuresupercriticalsteam bar NA 295.0 NA

Temperaturesupercriticalsteam ◦_C _NA _630.0 _NA

PressureHPsteam bar 172.5 80.0 172.5

TemperatureHPsteam ◦_C _601.7 _601.0 _601.0

PressureIPsteam bar 41.4 42.6 42.6

TemperatureIPsteam ◦_C _601.5 _601.0 _601.0

PressureLPsteam bar 3.0 3.0 3.0

TemperatureLPsteam ◦C 292.4 229.5 229.5

Isentropicefﬁciencysupercriticalsteamturbinea _% _NA _92.0 _NA

IsentropicefﬁciencyHPsteamturbine % 86.0 86.0 86.0

IsentropicefﬁciencyIPsteamturbine % 90.0 90.0 90.0

IsentropicefﬁciencyLPsteamturbine % 87.6 87.6 87.6

a_Franco_et_al.₍₂₀₁₂₎_.

TableA3

SummaryofkeyassumptionsfortheevaluationofplantrevenuesandCAPEX.

Capturelevelforpost-combustioncaptureplant % 90

Powerplantﬁxedcost(COPAR2012) $/MW-year 14,594

Powerplantvariablecost(COPAR2012) $/MWh 2.77

AnnualﬁxedcaptureplantrelatedtoCAPEX % 2.0

Interestrateordiscountrate % 10

Plantlife(COPAR2012) years 30

Loadfactorfornewplant,assumedtobeallatfulloutput(COPAR2012) % 80

Runninghoursperyearforretroﬁtloadfactor h/yr 7008

Variablecostsfornewplant,beforecapturebasis $/MWh 2

CO2emissionprice $/tCO2 0

TableB1

ReferencesofcapitalcostforpowerandCO2capture,CO2compressorplants.

Equipment Reference

Gasturbine,generatorandauxiliaries

9F5-seriesmodel

GasTurbineHandbook(2013)

HRSG,steamturbine,andbalanceof plantBOP

Francoetal.(2012)

Inductﬁring Thermoﬂow(2013)

Supercriticalsteamturbine DOE/NETL(2013a,b)

Captureplant IEAGHG(2012)

CO2compressor Hendriksetal.(2003)

CO2transport DOE/NETL(2013a,b)

listthebasicparametersusedinthemodellingofthepowerplants forallcasestudies).

AppendixB.

1. CAPEXestimate

SourcesofinformationforcapitalcostsareshowninTableB1. TheChemicalEngineeringPlantCostIndex2013isusedtoupdate thecostof equipmentto2013 and acurrency exchange of 0.8 EUR/USDin2014isused.

Thesumofallequipmentcosts,togetherwiththebalanceof plant(BOP),coolingwatersystem,andinstallationcostsis,asit isdescribed byRubinet al.(2013),thebareerectedcost(BEC). Followingthemethodology,theBECincludingindirectcosts, engi-neeringprocurementandconstruction(EPC)costs,contingencies, andowner’scostsgivesthetotalcapitalrequirement(TCR)forthe powerplantaswellasforcaptureplantandcompressionsystem.

ThecostoftheHRSGforallthreecasesiscalculatedusingthe Eq.(B1)proposalbyFrancoetal.(2012):

C=C0

_UA

U0A0

f

(B1)

whereC0isthereferenceerectedcostcomponent($),U0A0(MW/K) is the reference size component, UA is the scaling parameter

(MW/K)(UheattransfercoefﬁcientandAistheheattransferarea), f is thescalefactor(−).Forsupercritical SSFCC,Eq.(B1)isused toestimatethecostoftheHRSGinsectionswithlow tempera-ture,andforsectionswithhightemperatureEq.(B1)ismultipleby N=3.3.Nisthefactorforusingmoreexpensivematerialtosupport supercriticalconditions(Worldsteelprices,2013).

2. OperationandmaintenancescostO&M

Informationfortheoperationandmaintenanceﬁxedand vari-ablecosts(O&M)forcasestudiesforthepowerplantsectionare providedbyCostsandbenchmarksforthedevelopmentof invest-ment projects in the Mexicanelectricity sector(COPAR, 2013), whichgivesinformation forMexicoregardingnewpowerplant projectsinMexicanFederalcommissionofelectricity.The estima-tionincludestheexpensesforconsumablesandchemicalsolvent make-ups(variable)aswellascostsformaintenanceandlabor.

(16)

MEA/tCO2fortheSSFCCcasesforO2concentrationssimilartocoal ﬂuegas(below4%v/v)(RubinandRao(2002),DOE(2007)).

3. Levelisedcostofelectricity

Thelevelisedcostofelectricity(LCOE)iscalculatedby annual-izingthetotalcapitalcostandthetotaloperatingandmaintenance costsand variablecostsin$/MWh.Thenetelectricityproduced andsold,theoperating,maintenanceandfuelcostareconsidered constantoverthelifeoftheplantbasedonconstantdollar.Carbon pricesarenotincludedinthisanalysis.Then,thesimpliﬁed equa-tionfortheseconditionsisexpressedbyEq.(B2)reportedbyRubin

etal.(2013).

LCOE=_PowerTCR_output×FCF_×+_CFFOM_×₈₇₆₀+VOM+HR×FC+TCO2 (B2)

FCF=r×(1+r)T

(1+r)T−1

where

TCRisthetotalcapitalrequirement($),FCFﬁxedchargefactor,

FOMistheﬁxedO&Mcosts($),Poweroutputisthenetpower

gen-eratedbythepowerplant(MW),CFcapacityfactor(−),VOMisthe variableO&Mcosts($/MWh),HRnetpowerheatrate(MJ/MWh), FCfuelcostperunitofenergy($/MJ),andTCO2CO2transportcost ($/MWh).r(−)istheinterestrateandTistheeconomiclifeofthe plant(30yearsinthisstudy).

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