S.Haoetal./ColloidsandSurfacesA:Physicochem.Eng.Aspects444 (2014) 104–113

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contentbeing88–98%[6]).CBMisretainedincoalbedsmainlyinadsorbedstate[7,8].CBMisnotonlythemajorrootofcoalminedisasterandatmosphericpollutionsource,butalsoavaluablenon-renewableenergy[9].TheinterestbothinrecoveryofCH4fromcoalseamandinoutbursthazardsrelatedtocoalmininghasledtoextensivestudyofgassorptionincoal[7,9–12].

Inrecentyears,increaseinatmosphericCO2concentrationhasraisedconcernaboutclimatechangeandledtoworldwideeffortsonreductionofCO2emissions[7,13–18].AlthoughseveraloptionsforCO2sequestrationarebeingconsidered,apotentiallyattractiveapproachisthestorageofCO2indeep,unmineablecoalbeds[7,19].SuchcoalbedsfrequentlycontainlargeamountsofrecoverableCH4,andtherecoveryofthisgascanbeenhancedbyinjectingCO2intothecoalbeds[7,20].TheinjectionofCO2canservedualpurposestosequesterlargeamountsofCO2andtosimultaneouslyprovideanincreasedsupplyofCH4.Consequently,someofthesequestrationcostscanberecoveredinthevalueoftheCH4produced[7,19].

ThedesignofoptimalrecoveryofCH4andCO2sequestrationincoalbedsreliesgreatlyontheavailabilityofhigh-pressure,supercriticaladsorptiondataforgassorptionaswellasreliableadsorptionmodelsthatarecapableofaccuratepredictionsofadsorptionphenomena[20].Tosimulatereservoirconditions,lab-oratorysorptionexperimentsaregenerallyconductedatelevatedtemperature,usuallybetween293and323K[7,11].However,thecoalbedtemperatureincreaseslinearlywithminingdepthincreas-ing[21].

MethanesorptionisothermsincoalarecommonlyIUPACtypeI[7].TheLangmuirmodeliswidelyusedintheCBMindustrybecauseofitssimplicityandprovidingareasonable ttomostexperimentaldata[7].However,theassumptionofanenergeticallyhomoge-neoussurfaceasproposedbyLangmuirtheoryisnottrueforcoal[11].Furthermore,theLangmuirmodelcannotpredictmethaneadsorptionamountatdifferenttemperaturesfromoneadsorptionisotherm.Thismeansthattoobtainmethanecapacityatdifferenttemperaturestherewouldbealotofexperimentstodo.

TheDubinin–Astakhov(D–A)modelhasbeencommonlyappliedtothedescriptionoftypeIisotherms[22–24].TheD–Aequationwasmainlydevelopedfortheadsorptionofvaporsbelowthecriticalpoint[22].However,experimentsofgasadsorptiononporoussolidshaveshownthatthereisnoabruptchangeintheadsorptionduringthetransitionfromsub-criticaltosuper-criticalconditions[22].ThissuggeststhattheD–Aequationcanbeempir-icallyappliedtosuper-criticalgasesaswell[22].ThenicefeatureofusingD–Aequationfordescriptionofgasadsorptionisthatasinglecharacteristiccurvewouldbeobtainedifthecharacteris-ticenergyisindependentoftemperature[22].Ifthecharacteristiccurveforanadsorptionsystemisknown,thenadsorptionuptakeoftheadsorbateatdifferenttemperaturescouldbepredicted.

TheproblemsofapplyingtheD–Aequationtosupercriti-cal uidsare[11,22,25]:(1)theestimationofthesaturationvaporpressure,ps.Abovethecriticaltemperature,Tc,thecon-ceptofsaturationvaporpressuredoesnotexist,hencetheuseofpseudo-saturationvaporpressureswasproposed;(2)theden-sityoftheadsorbedphaseatagiventemperature.Inordertoobtainthevolumeofadsorbedphase,itisnecessarytohaveavalueofadsorbedphasedensity.Theadsorbedphasedensityisnotdirectlymeasurableandasaconsequence,itsvalueisapproxi-mated;and(3)theformofthetemperature-invariantcharacteristiccurvetobeutilized.Thein uenceofthemethodsforestimatingpseudo-saturationvaporpressuresoncharacteristiccurvesofacti-vatedcarbon-methaneadsorptionsystemshasbeenreported[26].However,toourknowledge,nostudyhasbeenreportedforcoal-methanesystems,andnostudyhasbeenreportedonthein uenceofthecharacteristiccurveformontheD–Aequationpredictiontoo.Toavoidtheseproblemsabove,Kimetal.[27]usedgasdensityinsteadofgaspressure(andadsorbedphasedensity

Table1

Proximateandultimateanalysisofthecoals.

Sample

Proximateanalysiswt.%

Ultimateanalysiswt.%

Ash

VM

FC

Moisture

C

H

O

N

Xingq-1–5#8.4029.8371.170.8077.426.5814.601.40Qingh-2–3#1.6621.4778.530.6884.666.477.081.79Xujd-1#11.7111.6688.340.6788.703.564.353.39Leiy-1#

18.45

5.55

94.45

0.15

90.98

1.26

5.53

2.23

Ashwascalculatedonadrybasis;Volatilematter(VM)and xedcarbon(FC)werecalculatedonadried,ash-freebasis.

ratherthanpseudo-saturationvaporpressure)tomodifytheclas-sicDubinin–Radushkevich(D–R)equation,andthenappliedthemodi edD–Requationtothegasesadsorptiononcoalsundersupercriticalconditions.However,themodi edD–RequationisnotconvenienttouseinCBMindustryforthegasdensityisnotasintuitiveaspressure.

Inthepresentwork,methaneadsorptiondataonfourdiffer-entrankcoalswereobtainedatsupercriticaltemperatures.TheD–Aequationisappliedtomethane-coalisothermdataatele-vatedtemperature.Thepurposesofthecurrentstudyare:(1)todeterminewhichempiricalmethodpresentedintheliteraturetoevaluatethepseudo-saturationvaporpressureismostsuitableforanalyzingtheadsorptiondataabovetheTcofmethane.Thecri-terionusedinouranalysisisthatthedatashouldbereducedtotemperature-independentplots;and(2)todeterminetheformofthetemperature-invariantcharacteristiccurve.Thecriterionispre-dictionaccuracy.

2.Experimental

2.1.Materials

Inthiswork,Xingqingcoal(bituminouscoal,QinghaiChina,labeledasXingq-1–5#),Qinghuacoal(bituminouscoal,QinghaiChina,labeledasQingh-2–3#),Xujiadongcoal(bituminouscoal,HunanChina,labeledasXujd-1#),andLeiyangcoal(anthracite,HunanChina,labeledasLeiy-1#)wereused.Theproximateanal-ysisandultimateanalysisforthecoalsaregiveninTable1.Theoxygencontentwasdeterminedbydifference[28].Thesulphurinthefourcoalswasnotdetected.Thecoalsampleswerecrushedtoparticleswithsizeof60–80mesh.

2.2.Methods

2.2.1.2.2.1.Characterization

TexturalcharacterizationofthecoalsampleswasconductedusingaNOVA1000esurfaceareaandporesizeanalyzer(Quanta-chromeCompany,USA)withN2(at77K)asadsorbate.Priortoanalysis,samplesweredegassedat383Kfor24h[29].Thespe-ci csurfaceareawascalculatedbythemultipointBETmethod[30,31];themicroporevolumewasdeterminedbytheD-Requation[31,32];thetotalporevolumeVtwasevaluatedfromthenitrogenadsorptionatp/p0=0.98–0.99[33];andtheporesizedistributionwascalculatedbythebythenonlocaldensityfunctionaltheory(NLDFT)method[31].

Surfacemorphologywasinvestigatedbyscanningelectronmicroscopy(SEM)(JEOL/EOJSM-5900,Japan).

2.2.2.Adsorptionofmethane

Theadsorptionisothermswereobtainedthroughanequilib-riumvolumetricmethod.Thedetailedexperimentalmethodwaspresentedelsewhere[29].Toavoidthein uenceofwaterinthecoalsonthecalibrationofthereferencecellvolume,thesamplesweredriedinanovenovernightat383Kbeforemethaneadsorp-tion[29,32].