The effect of CO2 purity on the development of pipeline networks for carbon capture and storage schemes

ContentslistsavailableatScienceDirect

International

Journal

of

Greenhouse

Gas

Control

jo u r n al hom e p 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

The

Effect

of

CO

2

Purity

on

the

Development

of

Pipeline

Networks

for

Carbon

Capture

and

Storage

Schemes

B.

Wetenhall

_,

_J.M.

_Race

_,

_M.J.

_Downie

a_{SchoolofMarineScienceandTechnology,NewcastleUniversity,Newcastle,NE17RU,UK}

b_{DepartmentofNavalArchitecture,OceanandMarineEngineering,UniversityofStrathclyde,Glasgow,G40LZ,UK}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received14December2013

Receivedinrevisedform8September2014 Accepted17September2014

Availableonline8October2014

Keywords:

CO2pipelines CO2specification Pipelinenetworks Hydraulicmodelling Costanalysis

a

b

s

t

r

a

c

t

Oneofthekeyaspectsrelatingtothetransportationofanthropogeniccarbondioxide(CO2)forclimate

changemitigationaspartofCarbonCaptureandStorage(CCS)schemesisthecompositionoftheCO2

streamtobetransported.Thespecificationofthisstreamhasbothtechnicalandeconomicimplications and,asCCSschemesstarttobecomerealised,therequirementtospecifytheCO2 streamqualityis

becomingmoreimportant.

TheaimofthisworkhasbeentoanalysetheeffectsofthecompositionoftheCO2streamfrom

post-combustion,pre-combustionandoxyfuelcaptureprocessesonthehydraulicnetworkdesignandthe relativecostsofthenetwork.Severalkeyconclusionshavebeendrawntoinformtheprocessofspecifying theCO2purityandtoguidepipelineoperatorsonthespecificationofaCO2stream,fordensephase

pipelineoperation,onthebasisofhydraulicdesign.

Theanalysishasshownthatimpurityadditionsupto2mol%didnotaffecttherelativecost/kmforthe networkswhencomparedtoapureCO2equivalentintermsofthepipelineinternaldiameterandlength.

However,theinletpressuretothenetworkisincreasedforallofthecompositionsstudiedandinthis respect,levelsofhydrogeninparticularshouldbelimitedtolessthan1mol%toreduceinletpressure andtherebycompressioncosts.

Ithasbeendemonstratedthatdirectconnectionpipelinesfromsourcetosinkarethemostexpensive networkoptionshowever,whendesigningapipelinenetwork,thesizeoftheemitters,thephasingof entryintothenetworkandthestabilityofthenetworkintheeventofinterruptionsinflowneedstobe considered.

©2014TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).

1. Introduction

CarbonCaptureandStorage(CCS)isrecognisedasoneofasuite ofsolutions required toreducecarbon dioxide(CO2)emissions

intotheatmosphereandcontributetowardsglobalclimatechange mitigation.InCCSschemes,CO2 is capturedfrompowerplants

orotherlargestationarysourcesandtransportedtoappropriate geological sites either for Enhanced Oil Recovery (EOR) or for storage.Unlessthecaptureandstoragesitesareco-located,allCCS schemeswillinvolvethetransportationofCO2fromthecapture

planttothestoragesiteeitherviaapipelinenetworkorbyship basedtransportation.

OneofthebarrierstotherapidimplementationofCCSisthe highcapitalcostofdemonstrationschemes(BBC,2011).Thereis

∗Correspondingauthor.Tel.:+00441415485709.

E-mailaddress:[email protected](J.M.Race).

thereforeanurgentrequirementtoreducethecostsofCCSinorder

that theimplementationof largescaleCCSschemesbecomes a

moreviableandattractiveoptionpostdemonstration(Sweeney, 2012).StudiesofthecostsofthefullCCSchainindicatethatthe largestcosts(whetherthatbeintermsoftheincreasedcostof elec-tricityorthecostpertonneofcapturedCO2)areassociatedwith

thecaptureprocess(MiddletonandBielicki,2009;Yanetal.,2008; ZEP,2011).However,withinthatchain,thecostoftransportation hastobeconsideredandcostreductionssoughtwherepossible

(GCCSI,2011).

Thecostofthepipelinesystemhasbeenshowntobeprimarily influencedbycapitalexpenditure(CAPEX)andtobeapproximately proportional tothe lengthof the network (ZEP, 2011; Knoope, 2013).Consequently,many studieshave investigatedthe

devel-opment of modelsto provide an optimaldesign for a pipeline

networkthatminimisesthepresentvalueforthecapitaland oper-atingcostsofthesystem(Brunsvoldetal.,2012;Kazmierczaketal.,

2009;Kubyetal.,2010;MiddletonandBielicki,2009;Vandeginste

http://dx.doi.org/10.1016/j.ijggc.2014.09.016

Table1

CompositionrangesforCO2streams(Anhedenetal.,2005;IEAGHG,2011;IPCC,2005;KatherandKownatzki,2011;OosterkampandRamsen,2008).

Component Pre-combustion Post-combustion Oxyfuel

Min Max Min Max Min Max

CO2 vol% 95.6 99.7 99.8 99.97 85 99.94

SOx vol% 0.001 0.01 0.007 2.5

NOx vol% 0.002 0.01 0.01 0.25

H2S vol% 0.01 3.4

CO vol% 0.03 0.4 0.001 0.002

Ar vol% 0.03 1.3 0.003 0.045 0.01 5.7

O2 vol% 0.03 1.3 0.003 0.03 0.01 4.7

N2 vol% 0.03 1.3 0.021 0.17 0.01 7

H2 vol% 0.002 1.7

CH4 vol% 0.035 2 0.01

Hydrocarbons vol% 0.003 0.01

HCN vol% 0.0005

NH3 vol% 0.003 0.005

CH3OH vol% 0.02

andPiessens,2008).MiddletonandBielicki(2009)haveindicated

thattherearesevenparametersthatshouldbeoptimised simulta-neouslyinacomprehensivenetworkmodel;theamountofCO2to

becaptured;thelocationofthesources;therouteofthepipeline; thepipelinedimensions;thelocationofthesinks;theinjection volumeateachsinkandthedistributionoftheCO2inthepipeline

network.Howeveranotherkeyparameterthatneedstobe consid-eredisthepurityoftheCO2.

PreliminarystudieshaveindicatedthatthepurityoftheCO2

has a significant impact on pipeline transport costs

particu-larlyat lowerflowrates andlongerdistances(Yanet al.,2008) and, on this basis, it could be considered to be insufficientto

consider the stream as pure CO2 when conducting hydraulic

calculations. This paper investigates the effect of the purity of

theCO2 onpipelinecapacityandnetworkdevelopmentinmore

detail.

2. EffectOfImpuritiesOnPipelineHydraulics

2.1. CO2PuritySpecifications

AnthropogenicCO2capturedfromapowerplantoranyother

industrialsourcewillcontainnon-CO2componentsoftenreferred

toasimpurities.Theamountandtypeofimpuritiesthatcouldbe presentintheCO2streamfrompowerplantcapturewill

primar-ilybedependentonthecaptureprocess,thecapturetechnology usedandthefuelsource.Inaddition,legislativeandeconomic con-straintswillalsoplayapartindeterminingallowableorachievable levelsofcertainimpurities.Anumberofstudieshavebeen

con-ductedinto thecompositionof theCO2 streams capturedfrom

powerplant(Anheden et al.,2005; IEAGHG, 2011;IPCC, 2005;

Kather and Kownatzki, 2011; Oosterkamp and Ramsen, 2008).

Theresultsof thesestudiesfor differentcaptureprocesses and technologiesaresummarisedinTable1,whichillustratesthata largevariationexistsinthepublishedliteratureregarding poten-tiallevelsofimpuritiesintheCO2streamscaptured.Analternative

approachtodefiningaCO2specificationwastakenbydeVisseretal.

(2008)intheDynamistransportspecification,whichrecommends

CO2puritylevelsbasedontherequirementsofthepipeline.Itis

recognisedthattheDynamisspecificationisnotaCO2composition

andtheuseoftheDynamisstudyfordefiningCO2compositions

mustbeconsideredwithcare(Raceetal.,2012).

IntheDynamisstudy,safetyandtoxicitylimits,infrastructure durabilityandtransportefficiencyareconsideredinthe develop-mentofthespecificationshowninTable2.Thestudypresented inthispaperisrestrictedtotheeffectsofimpuritiesonhydraulic behaviour,althoughitishighlightedthatindeterminingapipeline CO2 stream specification, theeffects on all aspects of pipeline

Table2

PipelinespecificationsproposedbytheDynamis(deVisseretal.,2008). Component DynamisSpecification

CO2 vol% >95.5

H2O ppm <500

SOx ppm <100

NOx ppm <100

H2S ppm <200

CO ppm <2000

N2 vol%

Totalnon-condensablegases<4vol%

Ar vol%

O2 vol%

H2 vol%

CH4 vol% Aquifer<4vol%EOR<2vol%

designand operationmustbeconsidered(Raceetal.,2012).In respectofthehydraulicbehaviouritisinterestingtonotethat,the Dynamisspecificationsetsalimitof4vol%onthenon-condensable componentsofN2,O2,H2,CH4 andArinordertominimisethe

impactonpipelinecapacity,capital costandcompressioncosts.

Yanetal.(2008)havealsostudiedthetechno-economicimpactof

non-condensablesatdifferentlevels(13%,4%and1%byvolume) onthetransportationofCO2fromoxyfuelcapture.Theyconclude

thatthelimitonnon-condensablecomponentsof<4vol%isa rea-sonablepurificationlimitintermsofthecostbalanceoftheCCS chain.However,theyindicatethat,forshortdistancesand,where thestorageconditionspermit,thelevelofnon-condensablescould beraisedto10vol%,althoughlevelsashighasthismayrequire specialattentiontomeetregulatoryrequirements(i.e.healthand safetyconsiderations),basedontheconcentrationoftheindividual impuritiespresent.

Theindividualcomponentsofthecapturestreamsconsideredin thispaperaretakenfromIEAGHG(2011)andarepresentedindetail

inTable3.Forthenetworkstudy,onepre-combustionstreamand

onepostcombustionstreamwerechosen.Thesestreamsbothhave ahighpercentageofCO2andareconsideredtoberepresentativeof

achievablelevelsofsecondarycomponents.Twooxyfuel composi-tionswerealsochosen,onebeingarelativelypurestream(oxyfuel 2)andtheothercontainingalargerpercentageofimpurities (oxy-fuel1).Thelasttenyearsofdevelopmentinoxyfuelcombustion powerplantswithCO2 capturehaveestablishedthat the

Cryo-genicProcessingUnit(CPU)ofthepowerplantcouldbedesigned todeliverCO2 withcomposition rangingfromaslowas80%to

Table3

DetailedcompositionofCO2streamsfromIEAGHG(2011).

Component Pre-combustion Post-combustion Oxyfuel(1) Oxyfuel(2)

CO2 vol% 97.95 99.81 85.0 98.0

O2 vol% - 0.03 4.70 0.67

N2 vol% 0.9 0.09a 5.80 0.71

Ar vol% 0.03 - 4.47 0.59

H2O ppmc _{600 600 100 100}

NOx ppm - 20 100 100

SO2 ppm - 20b _{50 50}

SO3 ppm - - 20 20

CO ppm 400 20 50 50

H2S+COS ppm 100 - -

-H2 vol% 1 - -

-CH4 ppm 100 - -

-Cricondenbar bara 77.54 73.93 93.26 75.95

a_{TotalconcentrationofN2+Ar;} b_{TotalconcentrationofSO2+SO3;}

c _{Althoughthelevelsofwaterarequotedhere,waterisnotconsideredinthehydraulicanalysiscalculations.}

0 10 20 30 40 50 60 70 80 90

0 5 10 15 20 25 30 35 40

Pure Carbon Dioxide 2% Hydrogen

2% Nitrogen Dioxide 2% Hydrogen Sulphide

Pr

essur

e /bar

Temperature / o_C

Fig.1.PhasediagramforbinarycombinationsofCO2and2mol%H2,H2SandNO2(calculatedusingthePengRobinsonequationofstate).

datafromoperationalpowerplantcaptureplants,theyare con-sideredtoberepresentativecompositionsforthepurposesofthis study.

2.2. EffectsofImpuritiesonCO2FluidProperties

TheadditionofimpuritiesintotheCO2streamaffectsseveral

importantparametersintheanalysisofpipelinehydraulics includ-ingthephasebehaviour,density,viscosityandcompressibilityof thefluid.Priortothepresentationofthehydraulicstudy,itis there-foreimportanttounderstandtheinfluenceofdifferentimpurities ontheseCO2propertiesandultimatelyonthedesignofthepipeline

network.

2.2.1. EffectofImpuritiesonPhaseBehaviour

Toillustratetheeffectofimpuritiesonphasebehaviour, con-siderthephasediagramsofFig.1forbinarycombinationsofCO2

with2mol%ofhydrogen(H2),nitrogendioxide(NO2)andhydrogen

sulphide (H2S).Thesecomponentshave beenselectedfor

illus-tration asthey representcomponents that couldbe present in

theCO2 stream(Table1)and theyalsodemonstrateimportant

behaviourinthiscontext.Allcomponentswhosecritical tempera-tureandpressureisabovethatofpureCO2willopenupatwo-phase

regionbelow that of pureCO2 (e.g.H2S and NO2).Conversely,

componentswithcriticaltemperaturesandpressuresbelowthose ofpureCO2 willopenupatwo-phaseregionabovethatofpure

CO2.Theeffectofeachofthecomponentsconsideredonthephase

envelopeisillustratedinTable4.Althoughalloftheimpurities raisedthecriticalpressure,atthelevelsstudied,componentswith acriticaltemperaturebelowthatofpureCO2 loweredthe

criti-caltemperatureofthemixturerelativetopureCO2,whilstthose

withahighercriticaltemperaturethanpureCO2raisedthecritical

temperatureofthemixture.

Theseeffectsareimportantasthechangeinphasebehaviour limitstheallowableoperatingregionofthepipeline.CO2is

trans-portedmostefficientlybypipelineasadensephaseorsupercritical liquid1_{.However it is essential for operating efficiency,and to}

preventdamagetocomponentssuchasvalves,pumpsand

com-pressorsthatthefluid remainsina singlephase.Consequently, itisdesirabletomaintainthepressureinthepipelineabovethe cricondenbarofthefluid.Additionsofimpuritieswithcritical tem-peraturesandpressuresbelowCO2willthereforerequirehigher

Table4

Relativecriticalpressuresofkeyimpuritiesandtheireffectonthephaseenvelope.

MolecularWeight Criticaltemperature(◦C) Criticalpressure(bar) Effectonphaseenvelope

Hydrogen 2 -240.0 13.0

PhaseenvelopeaboveCO2

Nitrogen 28 -147.0 33.9

Carbonmonoxide 28 -140.35 35.0

Argon 40 -122.4 48.7

Oxygen 32 -118.6 50.4

Methane 16 -82.8 46.0

Carbondioxide 44 31.0 74.1

Hydrogensulphide 34 100.1 89.4

PhaseenvelopebelowCO2

Sulphurdioxide 64 157.7 78.8

Nitrogendioxide 46 157.9 101.0

operatingpressurestobespecifiedresultinginincreasedcostsfor compressionandpumping.

2.2.2. EffectofImpuritiesonDensity

Fig.2illustratesthenon-linearrelationshipbetween temper-ature,pressureanddensityforpureCO2.Ingeneral,thedensity

of CO2 decreases with increasing temperature and decreasing

pressure,however,thebehaviourisnon-linear andasharp dis-continuityindensityoccursclosetotheVapour-LiquidEquilibrium (VLE)lineduetothephasechangefromtheliquidtogaseousphase. Inthisregion,smallchangesintemperatureandpressurecanhave largeinfluencesondensity.Theadditionofimpuritiesmovesthe locationofthediscontinuitytohigherpressuresforcomponents withlowercriticaltemperaturesandpressuresthanCO2,andto

lowerpressuresforcomponentswithhighercriticaltemperatures andpressuresthanCO2,asshowninFig.3.Thisbehaviour,and

theeffectonCO2 pipelinetransportation,hasbeendiscussedin

Seevametal.(2007).However,akeyconclusionthatisemphasised

hereisthatloweringtheinlettemperaturewillincreasepipeline capacityasitincreasesthedensityofthefluid.Inaddition,limiting

theamountofcomponentswithlowercriticaltemperaturesand

pressuresthanCO2willalsoimprovepipelinecapacity.

2.2.3. EffectofImpuritiesonViscosity

Ingeneral,theviscosityofthefluidincreaseswithincreasing pressureanddecreasingtemperature.Asharpdiscontinuityin vis-cosityisobservedattheVLEand,intheliquid phase,theeffect oftemperatureonviscosityismoredominantthaninthegaseous

phase(Fig.4).TheimpactofimpuritiesontheviscosityofCO2isalso

illustratedinFig.4.Inthegaseousphase,theviscosityofthefluid isnotsignificantlyaffectedbytheadditionofimpurities.However, inthesupercriticalphase,theviscosityisdramaticallyaffectedby theadditionofimpurities,withanincreaseinviscosityoverpure CO2 observed forcomponentswithhighercritical temperatures

andpressuresthanCO2(e.g.NO2)andadecreaseinviscosityover

pureCO2 observedforcomponentswithlowercritical

tempera-turesandpressuresthanCO2(e.g.H2).Decreasingtheviscositywill

reducetheresistancetoflowofthefluidinthepipeline.

3. ModellingMethodology

Inthisstudy,theeffectsofimpuritieshavebeenstudied,firstly onasinglesourcetosinkpipelinetomodeltheeffectofindividual impuritiesonpipelinediameterandthenonthreenetwork scenar-iostostudytheimpactoftheproductstreamcompositiononthe systemsizeandconfiguration.Thehydraulicmodelling methodol-ogyforbothofthesestudiesisdescribedinthefollowingsections.

3.1. DeterminationofPipelineDiameter

Oneofthefirststagesinthedesignofapipelineistocalculate therequiredinternaldiameterfortheanticipatedflowrate. Sev-eralsimplemodelsfordeterminingtherequiredpipelinediameter exist,manyofwhichformthebasisoftechno-economicmodelsfor CO2 pipelinetransportation(Heddleetal.,2003;Hendriksetal.,

2003;IEA,2002,2005a,b;McCoyandRubin,2007;Ogdenetal.,

0 100 200 300 400 500 600 700 800 900 1000

0 20 40 60 80 100 120 140 160

5degC

30degC

D

en

sity /

kg

/m

3

Pressure /bara

0 100 200 300 400 500 600 700 800 900 1000

0 20 40 60 80 100 120 140 160

Pure Carbon Dioxide

4mol% Hydrogen

4mol% Nitrogen Dioxide

D

en

sity /

kg

/m

3

Pressure /bara

Fig.3.EffectofimpuritiesonthedensityofCO2forbinarycombinationsofCO2-4mol%H2andCO2-4mol%NO2at30◦_{C(calculatedusingthePengRobinsonequationof}

state).

2004).AreviewofthesesimplemodelsbyGhaziandRace(2012)

recommendedthatdiametercalculationsbasedonfluidmechanics principles(ratherthanmassbalanceequationsorrules-of-thumb) shouldbeusedintechno-economicmodelsastheyrequirefewer initialassumptionstobemade.Althoughthesesimplemodelsare adequatefortherequirementsofinitialpipelinesizingandcosting, detailednetworksizingstudiesrequiretheuseofmore sophisti-catedsteady-statehydraulicmodelswhichaccountfortheeffects ofpressureandtemperaturedropalongthepipelineandthe con-sequentchangeinfluidpropertiesthatresult.

Ingeneral,thecalculationofsteadystatefluidflowinpipelines requiresthesimultaneoussolutionoftheequationsfor

conserva-tionofmass,momentumandenergy.Fromthesolutionofthese

equations,foranyknownfluidcompositionandgiventwoofthe parametersofinitialpressure,finalpressureorflowrate,itis possi-bletocalculatethepressureandtemperaturedropalongapipeline length.Alternatively,aswasconductedinthisstudy,foragiven

outletpressure,required pressuredropandflow rate,itis pos-sibletocalculatetheoptimumpipelinediameter.Thehydraulic

modelling in this studyhasbeen conductedusing the PIPESIM

steady-statemultiphaseflowsimulatorsoftware(Schlumberger, 2010).ThenumericalprocedureemployedinPIPESIMisbasedon themethodoffinitedifferences.Thepipelineisdividedinto seg-mentsandthepressureandtemperaturegradientcalculationsare performedinthedirectionofflowoneachsegmentbasedonthe averagefluidconditionsinthesegment.Avalueoftheunknown parameteris setand iterativelyadjusteduntil theoutputvalue matchesthecalculatedvalue.Onceconvergencehasbeenachieved, thecalculationmovestothenextpipelinesegment.Fig.5presents aflowdiagramforthecalculationsconductedbythePIPESIM soft-ware,indicatingthemodelsthathavebeenselectedforthestudy. TheCO2 physicalandphasepropertieswerecalculatedusing

thesoftwarepackageMultiFlash(Infochem,2011)withthe

Peng-Robinson equation of state (Peng and Robinson, 1976). This

0 20 40 60 80 100 120

0 20 40 60 80 100 120 140 160

Pure Carbon Dioxide (5degC)

4mol% Hydrogen (5degC) 4mol% Nitrogen Dioxide (5degC)

Pure Carbon Dioxide (30degC)

4mol% Hydrogen (30degC)

4mol% Nitrogen Dioxide (30degC)

Vi

sc

os

ity

/

μ

Pa

s

Pressure /bara

• Equation of State –Density, compressibility enthalpy, heat capacity

• Viscosity model • Thermal conductivity

Mass and momentum balance - Flow equation

Pressure drop Energy balance and heat

transfer calculations

Temperature drop

Simultaneous solution

Fig.5. Flowdiagramindicatingthecalculationsconductedinthehydraulicanalysis.

equationofstatewasselectedasitishasbeenshowntobe suf-ficientlyaccurateforthemixturecompositionsandthepressure andtemperaturerangesexploredinthispaper(LiandYan,2009). Althoughthereislittlepublishedinformationavailableonthe calculationofCO2viscosity,numerouscorrelationsexistfor

calcu-latingtheviscosityinoilandreservoirfluids.Inordertodetermine whetheroneofthesemodelscouldbeextendedtocalculatethe viscosityofdensephaseCO2mixtures,thecalculationsfromtwo

viscositymodelsavailableinPIPESIM,Pedersen(Pedersenetal.,

1984)andLBC(Lohrenzetal.,1964)werecomparedwith

experi-mentaldataforpureCO2publishedbyVanDerGulik(1997).The

resultsarepresentedin Fig.6.On thebasis oftheseresultsthe Pedersenmodelwasselectedasitwasseentoalwaysover-predict theexperimentaldataandthereforewouldbeaworst-case predic-tionforthehydrauliccalculations.

TheflowequationselectedforthisanalysiswastheBeggsand Brillcorrelation(BeggsandBrill,1973)withtheMoodyfriction factor(Moody,1944)asdefinedinBrillandMukherjee(1999).The

BeggsandBrill-Moodymethodhasbeendemonstratedtobe

par-ticularlyapplicableforsingleandmultiphasefluidsandhasbeen usedforthemodellingofotherCO2pipelines(Hein,1985).This

methodalsohastheadvantagethatitcanaccuratelypredictsmall amountsofliquidformation.

ThemethodologyadoptedinPIPESIMtocalculatetheheat trans-fercoefficientbetweenahorizontalburiedpipelineandtheground surfacefollowstheapproachofKreithandBohn(2001)todefine

Table5

OutputfromemittersconsideredinthestudyAMEC(2008).

CO2outputperannum(Mt) IEAtier

Station1 0.60 1

Station2 1.50 0

Station3 1.65 0

Station4 2.03 0

Station5 2.88 0

Station6 2.89 0

Station7 3.12 0

Station8 6.20 0

Station9 7.68 0

Station10 22.37 0

aconductionshapefactor,S,fromwhichthegroundheattransfer coefficient,hg,iscalculatedusingtheequation:

hg=

ks g

R (1)

Where kg=ground thermal conductivity and R=reference length(takentobethepiperadius).

Thesemodelsareusedtocalculatetheheattransferfromoiland gaspipelinesanditisconsideredthatthesamemethodologycan beappliedtoCO2pipelinesasthematerialsandcoatingsusedwill bethesame.

Oncetheoptimuminternaldiametershavebeencalculatedfor each pipeline using the proceduredefined above, the required externaldiameterandwallthicknessiscalculatedusingthethin wallformulaforallowablehoopstressinPD8010-1(2004):

h=p.Do

20.t ≤e.a.SMYS (2)

Where, h=hoop stress (MPa), p=internal pressure (barg),

Do=externaldiameter(mm),t=wallthickness(mm),e=weld

fac-tor(assumedtobe1),a=designfactorand␴SMYS=theSpecified

MinimumYieldStress(SMYS)inMPa.Forthenetworkscenarios

considered,itwasassumedthatthepipelinewouldbelocatedin aClass1locationasdefinedinPD8010-1(2004)andthereforea designfactorof0.72wasused.

Ingeneral,pipelinesaresuppliedinstandard,discreteranges ofexternaldiameterandwallthickness.Althoughitisrecognised thatacustomercanspecifyanyexternaldiameterandwall thick-ness,BSEN10208-2(2009)indicatesthat,whereappropriate,the

40 50 60 70 80 90 100 110 120

40 50 60 70 80 90 100 110 120

Pedersen (1984)

LBC (1964)

Predicted

Viscosity/

μ

Pa

s

Experimental Viscosity Measurement - Van der Gulik (1984)/ μPa s

1 2

3

4

5

6 7

8

9

10

Terminal P10

P9

P8 _P7

P6 P5 P4

P3

P2 P1

Fig.7. SchematicrepresentationoftheCase1pipelinenetwork.

external diameter and wall thickness should be within these

standardranges.Consequently,oncetherequiredexternal

diam-eter and wall thickness have been determined based on the

hydraulicandstressanalysisconstraints,thevalueswereincreased toselectstandarddiametersandwallthicknessesasspecifiedinBS

EN10208-2(2009).

Afinal checkwasthenmadewithrespecttothevelocityof thefluidinthepipelineforthecalculatedflowrateanddiameter. Thiscalculationensuresthatthecalculatedvelocityisnotsoslow thatitwouldaffecttheoperationandmaintenanceofthepipeline andneitherisitsofastthatitcouldcauseerosionofthepipeline.

TochecktheerosionalvelocitytheprocedureoutlinedinAPIRP 14E(1991)forthecalculationoferosionalvelocitywasadopted.In ordertoensurethaterosionisnotathreattothepipeline,theactual velocity mustbeless than theerosionalvelocity.The erosional velocityiscalculatedusingthefollowingequation:

v

mix (3)

Wheree=erosionalvelocity(m/s);mix=densityofthefluid

mixture (kg/m3_{) and C=is anempirical constant as defined in}

API14E(1991).ThevalueofCinEquation3hasbeendetermined

1

3

2+4

5

6 7

8

9

10

Terminal P10

P9 P8

P7

P6 P5

P3

P2+4 P1

Table6

LengthsandflowratesofpipelinesfornetworkCases1-3.

Pipeline Case1(km) Pipeline Case2(km) Case3(km)

Distance(km) Flowrate(MT/yr) Distance(km) Flowrate(MT/yr) Distance(km) Flowrate(MT/yr)

P1 53 0.54 P1 4 0.54 1 0.54

P2 43 1.35 P2&P4 18 3.18 17 3.18

P3 70 1.49 P3 7 1.49 2 1.49

P4 43 1.83 – – –

P5 46 2.59 P5 14 2.59 16 2.59

P6 36 2.60 P6 12 2.60 14 2.60

P7 52 2.81 P7 23 2.81 26 2.81

P8 108 5.58 P8 19 5.58 10 5.58

P9 98 6.91 P9 9 6.91 98 25.70

P10 90 20.13 P10 90 45.83 90 20.13

Total 639 45.83 Total 194 45.83 274 45.83

empiricallyfromexperimentsconductedintheoilandgas indus-tryandarangeofvaluesforCareprovidedinAPIRP14E(1991) dependingontheserviceanderosivenatureofthefluid.For solids-free,continuousserviceavalueof122kg/m2_{sisrecommendedfor} C(APIRP14E,1991)andthisvaluehasbeenadoptedforthiswork. AlthoughthisvalueforCisconsideredtobeconservativeintheoil industry(Salama,2000),itisrecognisedthatthereisno compara-bleexperimentaldatabasefromwhichtodetermineanappropriate valueofCforCO2inthegaseousordensephase.

3.2. ModellingAssumptionsandInputData

3.2.1. FluidConditions

AsmentionedpreviouslyinSection2.2.1,itisessentialtoavoid

two-phaseflowin thepipelinenetworkby keepingthesystem

pressureabovethecricondenbarofthefluidfordensephase opera-tion.Consequently,forthisstudy,theminimumoperatingpressure

inthesystemhasbeentakentobe10%abovethecricondenbar

calculatedforthegivenfluid compositiontoprovidean operat-ingmarginonthepipelinepressureandavoidtherequirementfor intermediatecompression.Forthestudyontheeffectofdiameter, binarycombinationsofCO2with2mol%,4mol%and15mol%

impu-ritieshavebeenconsidered.Itisrecognisedthattheseimpurity levelsarenotrealisticorrepresentativeofpotentialCO2streams,

buttheyhavebeenchosenatanexaggeratedlevelinorderthat thequalitativeeffectofeachimpuritycanbeobserved.Forthe net-workstudy,thecompositionsforeachofthecapturetechnologies presentedinTable3havebeenusedandithasbeenassumedthat everyemitterinthenetworkisusingthesamecapturetechnology withthesamecompositionofCO2forthattechnology.

Apressuregradientof0.2bara/kmhasbeenassumedforthe net-work.Thispressuregradientisconsideredappropriatebasedon operatingexperiencequotedforCO2pipelinesintheUSA(Seevam

etal.,2010)andisalsoinlinewiththepressuregradientsassumed

intheworkofVandeginsteandPiessens(2008).Theinlet

tempera-tureoftheflowintothepipelinewasassumedtobe30◦C.Although itisnotedthattheoutputfromthecompressorcanbeashighas

40-50◦C(Farris,1983),alowertemperaturehasbeenadoptedforthis

studyasithasbeenassumedthatcoolingwouldbeconductedafter thefinalstageofpressurisationinordertomaximisethedensityof thefluidinthepipeline(asdescribedinSection2.2.2).

CO2emissiondatahasbeentakenfromtenpowerstationsin

atypicalregionalcluster(AMEC,2008).Theemittershavebeen classifiedaccordingtotheIEATierclassification2_{(AMEC,2008)and}

theannualCO2outputconsideredfromeachemitterispresented

2 _{TheIEATierClassification classifiesCO2emittersbyemission size:Tier0} includesallCO2sourcesemittingover1Mt/year,Tier1ismadeupofallsources withanoutputofbetween50kt/yearand1Mt/yearandTier2includesallother sourcesemittingunder50kt/year.

Table7

Hydraulicmodelinputassumptions.

FluidConditions Unit

Inlettemperature 30 ◦C

Pressuregradient 0.2 bar/km

Arrivalpressureatterminal Cricondenbar+10%

Flowrate 90%ofCO2emissions MT/yr PipelineandEnvironmentalConditions

Pipelineroughness 0.0457 mm

Pipelineburialdepth 1.2 m

Pipelinematerialyieldstrength 450 MPa Pipelineinsulation None

Pipelinethermalconductivity 60.55 W/m.K Soilthermalconductivity 2.595 W/m.K

Groundtemperature 5 ◦_C

inTable5.Inthecalculationsofflowrateintothepipeline,a90%

captureratehasbeenassumedfromeachemitter.

3.2.2. PipelineandEnvironmentalConditions

All of the pipelines in this study are plain carbon steel of gradeEN10208L450(BSEN10208-2,2009).Aroughnessvalueof

0.0457mmhasbeenusedastherecommendedvaluefor

commer-cialsteelpipelines(Mohitpouretal.,2003).Ithasbeenassumed thatthemanufactureandconstructionstandardsandpracticesfor CO2pipelineswillbesimilartothoseusedfornaturalgaspipelines

andthereforenoinsulationhasbeenappliedtothepipelinesinthe hydraulicmodelandthepipeshavebeenburiedtoadepthof1.2m. Thisfigurewasassumedinthecalculationstoberepresentativeof themaximumdepthofcoverrequiredbyfortheconstructionof onshorepipelinesin theUK(PD8010-1,2004).Thesoilthermal conductivityhasbeentakentobe2.595W/m.K,whichistypicalof awet,sandysoil(McAllister,2005).Thesoilthermalconductivityis consideredtobeconstantalongthewholepipelinelength,although itisrecognisedthatsoiltypeswillchangeoverthedistances mod-elled.Thegroundtemperaturehasbeentakentobe5◦C,whichis therecommendeddesignconditionfornaturalgaspipelinesinthe

UK(IGEM/TD/1,2008).

3.3. NetworkConfigurations

Threepipelineconfigurationshavebeendevelopedtostudythe effectofimpuritiesonnetworkdevelopment;

Case1. Directconnectionbetweenthesourceandtheonshore

terminal.Thiscasehasthelargestoveralllengthforthenetworkat

639km.

1

3

2+4

5

6 7

8

9

10

Terminal P10

P9 P8

P7

P6 P5

P3

P2+4 P1

Fig.9. SchematicrepresentationoftheCase3pipelinenetwork.

Case3. AtrunklineconnectingStation9totheterminalwith theother sources(expectfor station10) feedinginto this line.

The CO2 from Station 10 runs in its own directly connected

pipeline fromthe source tothe terminal. In this configuration

Stations 2 and 4 share a common pipeline. Case 3 has been

designed to overcome potential operational problems in Case

2 that could arise from having one large source (Station 10)

connected in a pipeline network that is linking much smaller

sources.

ThenetworksarerepresenteddiagrammaticallyinFigs.7–9.For Cases2and3,atreetypenetworkhasbeenmodelledwithalarge trunklineasthisisconsistentwiththefindingsofpreviousstudies

(AMEC,2008;Loneetal.,2010;Odenbergeretal.,2008;Pershad

etal.,2010).EachpipelineisdefinedbythelabelPi,whereiisthe

numberofthepowerstationfromwherethepipelineoriginates. Thelengthsofthepipelinesandthetotallengthofthenetworkfor eachcaseareshowninTable6.Allofthepipelineconnectionsare straightconnectionsandnoattempthasbeenmadetoaccountfor

Fig.11. Effectofimpuritytypeonpipelineexternaldiameterat15mol%impuritylevel.

topographyintherouteing.Eachnetworkcasewasmodelledfor apureCO2andforeachofthefourcapturestreamspresentedin

Table3.Thesinglesource-to-sinkpipelinethatwasselectedwas

P10.

3.4. CostModelling

ThecostmodelthathasbeenadoptedtoestimatetheCAPEXof eachofthepipelinesandnetworksisthatduetoGhaziandRace

(2012).ThismodelisbasedontheIEAmodelaspresentedinIEA

(2005a,b)withtheinclusionofalocationfactorFL.aspresentedin

IEA(2002).

CAPEX(MD)=FL.FT.[C1.L+C2+(C3.L−C4).]D+(C5).L−C6).D2 (4)

where,foronshorepipelines,C1=0.057;C2=1.8663;C3=0.00129;

C4=0;C5=0.000486;C6=0.000007; D=pipeline internal

diame-ter (inches); FL=locationfactor (taken tobe1.2 for theUnited

0.90 0.95 1.00 1.05 1.10 1.15 1.20

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 Total

Relative

cost/k

m

length

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

Fig.13. Relativecost/kmforeachpipelineinCase1.

Kingdom);FT=terrainfactor(takentobe1.1forcultivatedland);

L=pipelinelength(km).

Asummaryoftheinputdataandassumptionsforthestudyare presentedinTable7.

4. Results

4.1. EffectofImpuritiesonPipelineDiameterandInletPressure

Theresultsfortheeffectofimpurityonpipelineinternal diam-eter,relative to pureCO2,are represented diagrammatically in

Fig.10.Thisfigureindicatesthat,withtheadditionofupto4mol% ofN2,O2,Ar,CO,H2,H2SandCH4,theimpurityhasnoeffectonthe

calculationoftheoptimumdiametersizei.e.theinternaldiameter specifiedforthebinarycombinationofCO2withimpurityisexactly

thesameasthatwouldbespecifiedforpureCO2.Thisresult

con-curswiththeworkofdeVisseretal.(2008)andYanetal.(2008)

whosetalimitof4vol%fornon-condensablesintermsofhydraulic efficiency.Forsomeimpurities(NO2andSO2)theadditionofthe

impurityhasreducedthediameterpipelinethatwouldbe speci-fiedoverpureCO2andthelargertheleveloftheseimpurities,the

smallerthepipelinethatisrequired.Howeverforthebinary com-binationsof15mol%impurityforN2,O2,Ar,CO,H2andCH4,the

diameterofthepipelinemustbeincreasedbybetween4-6%over thediameterforapureCO2 pipelinetoaccommodatethehigher

levelofimpurities.Thiswillobviouslyhaveacostimplication.

A comparison of internal diameter only takes into account

theeffectofimpuritiesonpipelinecapacity.However,itis also instructivetoconsidertheeffectofimpuritiesonexternalpipeline diameterasillustratedinFig.11.Forupto4mol%impurities,there

0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14

P1 P3 P2&4 P5 P6 P7 P8 P9 P10 Total

Relative

cost/k

m

length

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

0.90 0.95 1.00 1.05 1.10 1.15

P1 P3 P2&4 P5 P6 P7 P8 P9 P10 Total

Relative

cost/k

m

length

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

Fig.15.Relativecost/kmforeachpipelineinCase3.

isnorequirementtoincreasetheexternaldiameterofthepipeline overthatwhichwouldbespecifiedforpureCO2.However,forH2

inparticular,theadditionof15mol%increasestheinternal diam-eterby4.6%overpureCO2,buttheexternaldiameterisincreased

by6.2% over pure CO2. The reason for this is that the relative

cricondenbarandthereforetheinletpressure(Pi)forthe15%H2

mixture is significantly higher than for the other binary

com-ponents (Fig. 12).Consequently, based onEquation 2, thewall

thickness (and therefore the external diameter) needs to be

increasedforthesamepipelinematerial.Thisresultindicatesthat externalratherthaninternaldiametershouldbeusedincost calcu-lationsformoreimpurestreamstotakethecostofthisadditional materialintoaccount.

Fig. 12 also illustrates the relative effect of the types and amountsofdifferentimpuritiesontheinletpressureandallows thedifferent impurities tobe rankedin terms of theirefficacy

in increasinginletpressure. Therefore it canbeconcluded that atthe2mol%level,allimpuritieshaveasimilareffectonraising

the inlet pressure by 3% on average over pure CO2.

How-ever, the addition of 15mol% H2 doubles the inlet pressure

required compared to the 2mol% mixture, whereas the

addi-tion of 15% H2S has very little effect compared to the 2mol%

mixture.

4.2. EffectofImpuritiesonNetworkSizeandConfiguration

4.2.1. EffectonCost/kmLength

Inordertocomparethecostsforeverypipelineineachcase study,thecost/kmlengthforeachpipelinecarryingthefour differ-entCO2streamshasbeencalculatedrelativetopureCO2usingthe

methodologyoutlinedinSection3.4.Theresultsarepresentedin

Figs.13–15.Fromthisanalysisitcanbeseenthat,forthemajority

0.90 0.95 1.00 1.05 1.10 1.15 1.20

P1 P3 P5 P6 P7 P8 P9 P10 Total

Relative

cost/k

m

length

Case 1 Case 2 Case 3

0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

Relative

inlet

pressure

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

Fig.17. RelativeinletpressureforeachpipelineinCase1.

ofpipelinesinCases1to3,thepost-combustion,pre-combustion andoxyfuel2compositionshavenoeffectontherelativecostper kmforthepipelinesinthenetwork.However,theoxyfuel1 com-position,themostimpurecomposition,canaffecttherelativecost

ofthepipelinebyupto16%andis themostexpensive

compo-sitiontobetransportedforthemajorityofpipelinesinthethree cases.

OneresultofnotefromthisanalysisisthatforP10,themain

trunk line in Case 2. In this case, shown in Fig. 14, the

pre-combustioncompositionhasa highercost/kmthan theoxyfuel

1composition.The reasonforthis isthat, althoughtheoutside diametersarethesame,theinternaldiameters differ, highlight-ingagainthatacostmodelbuiltonexternalratherthaninternal diameterwouldbemoreappropriate.

For theoxyfuel1composition,a comparisonhasbeenmade

ofthecost ofthethree differentnetworkcasesand theresults arepresentedinFig.16.Theseresultsconfirmtheworkofother researchersthatdirect connectionpipelines fromsourceto

ter-minal are more expensive than network options. Of the two

networkedcases,thenetworkwiththetwotrunklines(Case3) isamoreexpensiveoptionoverallintermsofcapitalcostofthe network,however,itisoperationallymorestable.Toillustratethis effect,theCase2networkwasmodelledundertheconditionthat Station10,thelargestemitterinthenetwork,hadbeenshutdown. Consequently,theflowrateinpipelineP10wasreducedby44%and

thenetworkbecameunstableastheflowvelocitywastoolowfor thediameterofthepipeline.Asaresult,duringtimeswhenthe flowfromStation10wasstoppedforeitherplannedorunplanned

0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25

P1 P3 P2&4 P5 P6 P7 P8 P9 P10

Relative

inlet

pressur

e

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25

P1 P3 P2&4 P5 P6 P7 P8 P9 P10

Relative

inlet

pressure

Post-combustion Pre-combustion Oxyfuel 1 Oxyfuel 2

Fig.19.RelativeinletpressureforeachpipelineinCase3.

outages,thewholepipelinenetworkwouldbeaffected.However, inthesamescenarioinCase3,theflowfromStation10isina sin-glesourcetoterminalpipelineandthereforedoesnotdominate theflowinthenetwork.

4.2.2. EffectonRelativeInletPressure

Asmentionedpreviously,thecostcalculationsindicatedabove donottakeintoaccounttheinletpressureofthepipeline,which willaffectthestressinthepipelineandconsequentlythewall thick-nessrequirement.Increasinginletpressurewillalsoaffectthecost ofcompression.

The relative inlet pressure (i.e. the inlet pressure for each pipelinerelativetopureCO2)hasbeencalculatedforeach

composi-tionandeachnetworkcase.TheresultsarepresentedinFigs.17–19. Theresultsindicatethat,forallcases,theoxyfuel1compositionhas thehighestrelativeinletpressure.Thisstreamisthemostimpure stream,containingupto15mol%impurities,andbasedonthe anal-ysispresented inFig.12,thehigherthelevel ofimpurities,the highertheinletpressurehastobetomaintainthefluidinthedense phase.Similarlythepost-combustionstream,whichcontainsless than0.2mol%impurities,hasthelowestinletpressureforallcases. Itisof particularinteresttoobservetheresultsforthe oxy-fuel1andthepre-combustioncompositions,whichbothcontain 2mol%impurities.ForCases1and3,thepre-combustionstream resultsinhigherinletpressuresthantheoxyfuel2composition.In ordertoaccountforthis,comparisonneedstobemadebetweenthe breakdownofimpuritiesinthestreaminTable3andtheresultsof

Fig.12.Althoughthetwocompositionscontain2mol%impurities, thepre-combustioncasecontains∼1%H2and∼1%N2whichhave

thegreatesteffectinincreasingtheinletpressure.Theoxyfuel2

compositioncontainsnoH2andtheothermajorcomponentsof

O2andArdonothaveasgreataneffectoninletpressure.

How-ever,forCase2,wheretheoverallpipelinelengthisshorter,the effectofthesedifferencesincompositiononinletpressureisnot aspronounced.

5. Conclusions

As CCS projects start to move from preliminary design to

detaileddesignandtoeventualcommercialprojects,the compo-sitionoftheCO2streamwillbecomeofincreasingimportancein

realisingcostreductionsinboththecaptureandtransportpartsof thechain.Theaimofthisworkhasbeentoanalysetheeffectsof thecompositiononpipelinecostandnetworkdesignforadense phasepipelinenetworkandseveralkeyconclusionscanbedrawn toinformtheprocessofspecifyingtheCO2purityonthebasisof

hydraulicdesign.

Inbinary combinationin singlepipelines,additions ofupto 4mol%ofimpuritiesdonotaffectthediameterandwallthicknessof pipelinethatwouldbespecifiedduetotheuseofdiscretepipeline sizesinthisanalysis.Inanetworksituationwithmultiple impuri-tiespresent,thecompositionofthe“non-CO2”partofthestream

doesbecomeimportant.Theanalysispresentedinthispaper indi-catesthatimpurityadditionsupto2mol%didnotaffecttherelative cost/kmforthenetworksintermsofthepipelineinternaldiameter andlength.However,theinletpressureisincreasedforallofthe compositionsstudiedandthiswillaffectthecompression require-mentsandthereforeoperationalcost.Inthisrespect,ithasbeen shownthatthelevelsofH2andN2inparticularshouldbelimited.

Onthebasisoftheworkconducted,evenlevelsofH2upto1mol%

wereincreasingtherequiredinletpressurebyover6%.

Aninterestingconclusioncanbedrawnfromtherelativeinlet pressureanalysisforCase2.Inthiscase,therelativeinletpressures

forthe98% purestreams(oxyfuel 2and pre-combustion)were

almostidentical.Therefore,ithasbeenpossibletonegatetheeffect oftheH2inthepre-combustionstreamcompositionbydecreasing

thelengthsofthepipelines.Itisrecognisedthatthisoptionmight notalwaysbepossibleduetootherconstraintsofrouteingcaused byterrainandriskcriteriabutitisachoicethatcouldbeconsidered. Anotherkeyobservationfromthisworkrelatestotheoperation ofnetworkswithmultipleemitters.Ifanumberofemittersareto beincorporatedintoanetworkitisimportanttoconsiderthe rela-tivecontributionsofeachoftheemitterstotheflowrate.Ifthereis onelargeemitterinthenetworkandthisemitterisnotinputting

into thesystem,for whatever reason, then thewhole network

couldbecomeunstabledependingontheoverallcontributionthat thisemittermakestotheflowrate.Thisscenariocouldarisedue

toplannedor unplannedmaintenance atthe source,but could

alsooccurintheinitialstagesofstartingupanetwork.Ithasbeen suggestedthatonescenarioforinfrastructuredevelopmentcould betooversizetrunklinesintheanticipationoffutureadditions

in this paper indicatesthat, depending ontherelative sizesof theemitters,this scenario wouldonly befeasibleifthelargest emittersin thesystemwerethefirst toinputinto thepipeline system.

Althoughthisstudyhasconcentratedontheeffectofimpurities onthehydraulicanalysisofpipelinesandnetworks,itis reiter-atedthat,whendeterminingapipelineCO2streamspecification,

theeffectsonallaspectsofpipelinedesignandoperationmustbe considered.Inparticularitisimportanttorecognisethatfracture

control,corrosion and cracking mechanisms,hydrate formation

andhealthandsafetyissuescanallinfluencetheacceptable lev-elsofCO2impurities.Itisfurtherhighlightedthattheconclusions

fromthisworkarespecifictodensephaseCO2pipelines.Gaseous

phasepipelineswillhavedifferentdesignconstraints,particularly withrespecttopreventingtwo-phaseflow,wheretemperatureis amoreimportantconstraintthanpressure.Furtheranalysiswould berequiredtodeterminetheimpactofimpuritiesonthese con-straints.

Acknowledgements

This work has been conducted under the auspices of the

MATTRANproject(MaterialsforNextGenerationCO2 Transport

Systems) and the authors gratefully acknowledgethe financial

supportofEPSRCandE.ONforthis research(E.ON-EPSRC Grant

ReferenceEP/G061955/1).The authorswouldalsoliketothank

SchlumbergerforthedonationofthePIPESIMsoftwarethrough

theSchlumbergerUniversityDonationscheme.

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