B.B. Beamish and G. O'Donnell. Coalseam Gas Research Institute, James Cook Universit. Sorption testing of coals from the Bowen Ba- sin has been performed ...
Microbalance applicationsto sorption testing of coal by B.B. Beamish and G. O'Donnell CoalseamGas ResearchInstitute,JamesCook Universit Sorptiontestingof coalsfrom the BowenBasin has beenperformedusinga high pressure microbalance.The gravimetric metbod is applied, enabling adsorption isothermsto be determinedat different coal moisturestates. The apparatusis ideal for testingof side-wall material, drill cuttings or maceral concentrates,as well as core material. No singlecoal parametercan be used to accuratelypredict the sorption capacityof the Bowen Bash coalstested.A generalpositi\€ correlationwith coal rank and a negatirecorrelation with ash content is observed.Bright coals have a higher sorption capacitythan dull coals of equiralent rank (up to 30 %), rel.atedto their greater degree of microporosity. At 4 MPa gaspressureand 23.5"Cthe sorption capacityof the dried coalstestedranged tuom 2U29 scc/g (daf) of methane.Methane sorption capacity decreaseswitb increasing moisture content and the relationshipwith the Langnuir Volume of VwA/a= 1/(1.02+ Am) applies,whereA = 0.35.
INTRODUCTION Major researchon methanegas production from coal seamsto date has been conducted in the United Slates,with limited inwstigations in Australia being related to shallower coalsfor mine safetypurposes. Methaneproduction from coal searnsfacesseraral technology gaps" wbich require research and fresh ideas,particularly for de.aeloping new basins,deeper coals, and geologicallycomplex settings(Scbrauftagelet al., 1990).The Bowen Basin of Queenslandposesa major challengein tbis contet. Sbarer(1992) statesthat besidesgascontent and permeability effects, two other characteristicsare critical in understandingthe nature of fluid flow throughcoal.Theseare the Townnille19-21November,1992
adsorptior/desorption isothermwhichdehnes the relationshipbetweenpressureand sorption capacity of tbe coal, and the diffusion parameters(ditrrsivity) of the coal, which control the gas flow rate through tbe main coal matrix.Sorptiontestingof coal is rherefore a major componentof coalbedmethane assessment. Maror er al (1990)providea review of sorption testingproceduresapplied in the US. The uansport of methane through coal is generallyconsidereda two-stageprocess: .
.
diffusion of metlane through the pore structureof the coal to naturallyoccurring cractq followed by the simulrqneous flow of water and gas throughthe crack structure.
Smith and Williems (1984),conductednumerous tests on American coals,looking at the rate effectsand assessing the current acceptedpbpical models.They pointedour inrcstigatorsstudying the relatira magnitudes of the aborre two steps" including Kissell QnD and Kuuskraz et aL (1979),ignored the coupling of the diffusional and laminar flow processes.This same finding has been emphasisedby the work of Beamish er c/. (191), particularly in the case of dul and bright coal behaviour, which will be addressedby a further paper by Gamsonand Beamishin theseproceedings.The work on coal microstructureat the CGRI adds a further dimensionto the complexitiesof gas flow behaviour throug.hcoal, whereby rhe macropores'6temis seenas more thanjust a cleat network. Previousstudieson g:ur sorption capacities harc revealed that the methane is primarily stored as an adsorbedlayer on tbe internal surfacsof tle coal pores (Curl, 1978).The anount of gas stored is pressure,temperature and moisturecontentdependent(Patching, 1970). Kitrr (1977) reported that the CoahedM ethaneSynposium
7')
B.B.Beamishand G. O,Donnell
sorption capaclty of coal generally increases Option l: Custor.nbuild a s)stem al.ongtie with ranlr and depth' Limited dala reportec tin'esor paines (196g) or Lama and Bartoto date sa {gsu'qlian coals(Bartosiewiczand siewicztfgS2l. Hargrarcg l98j) ha.venot been able to subsrqntiatea raak dependenceof sorption caOption 2: Purchasea readymade unit from a pacity' Coal type effectson sorptiorcapacity reputablemanufacturet*ii.u is capableof ha'rcshownconflictingresults"i inAi.utra accurateand efficient adsorption/desorptiou Uy -J Ettinger et aL (196e. testing. Recently, a new approach,to gas sorption studieshas been "l*fJgd by Levine tigqrl and Beemish et aL (r99r). Tne rpp"rutu, lt"d-'- -io work performed independentlyin the US 4nfl {u3u'elia, is a high pru.urr'gliThe gravimetricmettrod(Daines, i"-!dg*' 1968;Lema and Bartosiewicz,t982j is apcoal samplesdown to mittigramsize. *:d l9 This allows detailedcomparisonor-coattype 'no sorption behaviour. Core sanpl., ur,
liltniffi
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rial'
Constructingequipmentin-houseas per option was consideredtime consumingand .1. requiringa major debuggingperiod. The accuracy of data -"asure-"ni is reasonable (with multiple sampling),but manually time intensive.Recent dirclopments to compurerise-resultsbave been *"or uy Siahaan er az. (1989)usinga high precisionbuLr,.r, bur the manual.component;f disconnectingand reconnectingvesselsto the slstemsdll remains.
off-the-sher equipment exisred, namery sar-
pressure microbalances, which :":i!-t ltgh fulfiIled the requirementsof option 2. These As part of an ERDC fundedproject,samples fuuy self-containedunits capable of 3e. from tbe Boc€n Basinharreu.ro ,.rt.J highly accurate me:uiurementsoi weig.bt fo-rl the microbalancesorpriontechnique. cbangesin semplesassociatedwith adsorption/desorptionunder a wide range of pres_ sureand temperatureconditions. Tbe unit is amenable to automated measuremenrand I-ABORATORYSORTIIIONTESTING providespossibilitiesof future flexibility. This Equlpment for Adsorptlon/Desorption Tests equipmentbad previouslybeen used LnAuson CoaI tratii by There are two main methodsto consider (i) cslRo, Division of coal x1d Fne.,., ia the equipment for adsorp- Technologyfor Tlt:Fg adsorption testing .;T:i] tion/desoqptiontestingof coal samples.These lared carbon. are Volumetric (Ruppel et al., 1972) ana (ii) Gravimetric (Daincs, 196g;Lama -d;;;;: Gritrt"bs unirersity, Department of scisiewicz,1982).other factorsto be considerld t::t rechnology for adsorptiontesting .*d ot hydrogen i[o, the purpos€ of testing,sample size in hydrides. frac_ tion and quantity0obe used'Thi majorityof consuliationwith botb theseinstitutionsinditestingfor gasstoragecapacityusestine coa cared that . i"rtoriou, M 25D-p microbatsize ( < 250 tt-)' Bulk quantitiesare often ance would produce the resultsto sarisfy the used (80 - 150 d), which normally provides requiremensof rhe researchwork at .GRI. broad relationshipsbetween parameters. To obrqina closerunderstanding of tle coattype EquipmentOperatlon and microstructurc influence there is a neea The microbelqncearr.ngement is ilustrated to usc pieces of coal wbich can be used for in rigurel-.8,1-. was introduced to the sorptiontestingand sM work' Approximate .o.r l"rpirr'J, urpp.d pressures\arylng I g solid srmples are ideal ror tnis purpose. from 0.5 trap" to I Mpa up to a maximum of After reviewingthe techniquesbeing 7 MPa, and tbe sorption capacitymeasured applied o\€rseasand previouslyused in Australia-, at each step. Two particle sire r"nge, *rr" the gravimetric-techniquewas selected used, crushed(-272microns)and soltiJfs-to with two optionsto be consideredto meet CGRI's urml, with approximatelyI g of t*rpir u.ing researchobjectires. used in each case. A nxeJ tempeiatureof 23.5oC was used for all trrt, und the mois_ d.rill cuttingsor maceralconcenrrates.
CoalbedM ethaneSynposium
Townsvillel9-21 N ovember 1 992
ture coDtentof each test run was coutrolled by encuation. The crushed samples were dried and the solid samples were partiaily dried. Due to the volume di-fferenceson either side sf ths lelence point of the beam,one side of the unit wiil be more buoyant than the other and a correction is thereforerequired for this effect.This hasbeen determinedby calculating the volume difference between the two sideg and con"erting this volume difference to a weigbt factor which is pressuredependent. Pardcle SlzeEllects on StorageCapactty Reducing coal particle size to speedup rhe sorptionprocesshas often been confusedas a possiblesoruceof falselyincreasingstorage capacity.Experimentalresulrs by Ruppel er aL Qn$ and Bielicki et aL (7912) have sbownthat storagecapacitiesof coalstested at particle sizesranging ftom 3.36 mm - 44 differ witb any degree of sutistical signigcanc€. This comes as no suq)rise when the srrm sf thc e)certralsurface area of progressirrly smrller coal particles is compared to the interaal surface area of the pores availabls for sorption. Take for examplen gtamsof bituminouscoal made up of m sphericalparticlesof radius r (cm) aud density# (g/cc),then (1) A + - 3 = "=t mP 3
n-
(2)
r=
3t. p AiEr'
The ertcrnal surfacearea of tbeseparticles is giveu by (g t
sA=Arrzm substituting for m fron equarion (2) into equation(3) gres ({)
sA=I:#!
=E smz rp
Considerthe normal isothermtestingcrushed fractionof 60 mesh(250p:n;, r = 0.0125cm. From equation(5), 100 g of this coal with a density of 1.35 g/cc would have an external surfacearea of:
5.4=
3x700 0.0125xi.35
=t7 8s
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CoalbedM ethaneSyn posium
Townsyille 19-21Novembet1992
tion testitrgof coal
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Ftgun3, Variatiouin gassorptioncapacitywithfixedcatboncontentfor dull andbrigbtcoals fromthc BowenBasin. 19-21November,1992 T owns,tilk
eSWposium Coah ed Methan
40
B.B.Beamishand G. O,Donnell
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