Supercritical gas sorption on moist coals

Available online at www.sciencedirect.com

eology 74 (2008) 203–214www.elsevier.com/locate/ijcoalgeo

International Journal of Coal G

Supercritical gas sorption on moist coals

Stuart Day ⁎, Richard Sakurovs, Steve Weir

CSIRO Energy Technology, PO Box 330, Newcastle NSW 2300, Australia

Received 23 December 2007; received in revised form 29 January 2008; accepted 29 January 2008Available online 14 February 2008

Abstract

The effect of moisture on the CO2 and CH4 sorption capacity of three bituminous coals from Australia and China was investigated at 55 °C andat pressures up to 20 MPa. A gravimetric apparatus was used to measure the gas adsorption isotherms of coal with moisture contents ranging from0 to about 8%. A modified Dubinin–Radushkevich (DR) adsorption model was found to fit the experimental data under all conditions. Moistureadsorption isotherms of these coals were measured at 21 °C. The Guggenheim–Anderson–de Boer (GAB) model was capable of accuratelyrepresenting the moisture isotherms over the full range of relative pressures.

Moist coal had a significantly lower maximum sorption capacity for both CO2 and CH4 than dry coal. However, the extent to which thecapacity was reduced was dependent upon the rank of the coal. Higher rank coals were less affected by the presence of moisture than low rankcoals. All coals exhibited a certain moisture content beyond which further moisture did not affect the sorption capacity. This limiting moisturecontent was dependent on the rank of the coal and the sorbate gas and, for these coals, corresponded approximately to the equilibrium moisturecontent that would be attained by exposing the coal to about 40–80% relative humidity. The experimental results indicate that the loss of sorptioncapacity by the coal in the presence of water can be simply explained by volumetric displacement of the CO2 and CH4 by the water. Below thelimiting moisture content, the CO2 sorption capacity reduced by about 7.3 kg t

−1 for each 1% increase in moisture. For CH4, sorption capacity wasreduced by about 1.8 kg t−1 for each 1% increase in moisture.

The heat of sorption calculated from the DR model decreased slightly on addition of moisture. One explanation is that water is preferentiallyattracted to high energy adsorption sites (that have high energy by virtue of their electrostatic nature), expelling CO2 and CH4 molecules.Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

Keywords: Coal; CO2 isotherms; Methane isotherms; Dubinin–Radushkevich; Moisture isotherms; Sequestration

1. Introduction

Geological storage of CO2 in deep, unmineable coals seamsis being actively investigated around the world as a means toreduce atmospheric emissions of greenhouse gasses. As well asproviding long-term disposal of CO2, the technology also offersthe possibility of recovering the methane associated with coalseams, which is a less greenhouse intensive fuel than coal (i.e.enhanced coal bed methane production, ECBM). One of thefundamental properties required to assess the CO2 sequestrationpotential of coal seams is their sorption capacity but it is

⁎ Corresponding author. Tel.: +61 2 4960 6052; fax: +61 2 4960 6054.E-mail address: [email protected] (S. Day).

0166-5162/$ - see front matter. Crown Copyright © 2008 Published by Elsevier Bdoi:10.1016/j.coal.2008.01.003

important to measure this under experimental conditions thatreplicate the high pressures and temperatures likely to be foundin deep seams suitable for sequestration.

Although CO2 and CH4 adsorption onto coal has beenstudied for many years, it is only very recently that data relatingto sorption of CO2 onto coals under supercritical conditions (i.e.TN31 °C, PN7.3 MPa) have begun to become widely available(e.g. Krooss et al., 2002; Busch et al., 2006; Siemons andBusch, 2007; Sakurovs et al., 2007). Gas sorption measure-ments are often made on dry coal but since coal seams arenaturally saturated, accurate sorption capacity estimates requirethat isotherms be measured in the presence of moisture.

There is general agreement that moisture tends to reduce thegas sorption capacity of coal. In some early work, Joubert et al.(1973, 1974) examined the effect of moisture on the methanecapacity of a number of American coals. They found that the

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http://dx.doi.org/10.1016/j.coal.2008.01.003

204 S. Day et al. / International Journal of Coal Geology 74 (2008) 203–214

methane capacity decreased linearly with increasing moisture toa certain moisture content, but above this level further moisturehad no effect on the methane sorption. Levy et al. (1997)examined an Australian bituminous coal and found that atmoisture levels below about 4%, methane capacity decreasedlinearly with increasing moisture.

There have been a number of studies where the adsorptionisotherms on dry and moist coals have been compared. Often,the moist coals are prepared by equilibrating the sample withmoisture in a sealed chamber containing a saturated solution ofpotassium sulphate (97% relative humidity). In one such study,Krooss et al. (2002) measured high pressure methane and CO2

adsorption isotherms on several dry and moisture-equilibratedDutch coals. For methane, they found the sorption capacity ofthe moist coals to be as much as 25% lower than the dry coal.They also found that CO2 capacity of moist coal was lower thanthat of the dry coal, but their moist coal isotherms showedunusual maxima and minima which they interpreted as a man-ifestation of CO2-induced swelling enhanced by the presenceof water. In another study, Fitzgerald et al. (2005) measuredmethane, nitrogen and CO2 isotherms on two American coalsunder in-seam conditions of 13.8 MPa and 55 °C. This groupused wet samples to simulate in-seam conditions but dry coalswere not measured so the extent to which the isotherms hadbeen affected could not be determined.

Siemons and Busch (2007) measured CO2 sorption iso-therms on both dry and moist coals of various ranks from coalbasins from around the world. These measurements were madeat temperatures of 45 °C and at pressures up to 20 MPa. Theirresults, in common with other workers, showed that the excesssorption was higher in the dry coals. In the dry samples therewas no obvious correlation between the sorption capacity andthe rank of the coal. However, in wet coals, the CO2 sorptioncapacity increased with increasing rank. Prinz and Littke (2005)also found that the sorption capacity of moist coals was rankdependent.

The effect of moisture on mixed gas sorption has also beenexamined in recent years. Clarkson and Bustin (2000) studiedseveral Canadian coals and found that both methane and CO2

sorption capacity (in pure gases) was reduced by around 25–30% in the moist coals relative to the dry material. There wasalso evidence that the selectivity of CO2 to adsorb onto the coalwas reduced in the presence of moisture. Similar results werefound by Busch et al. (2006) for a range of European and U.S.coals. The latter results were obtained at very high pressures ofup to 23 MPa, whereas the maximum pressure in the Clarksonand Bustin study was only about 5 MPa for CO2.

Because of the effect that moisture has on the isotherm, thereare implications when comparing sorption data. Unless themoisture content of the coal is precisely defined, it is notpossible to make direct comparisons between data sets. Thesignificance of this has been recognised by the U.S. Departmentof Energy's National Energy Technology Laboratory (NETL)which recently conducted an inter-laboratory comparison ofsorption results (Goodman et al., 2007). In that study, sixlaboratories prepared moisture-equilibrated samples of Ar-gonne premium coals according to a prescribed procedure and

measured the isotherms of the moist coals. They found thatagreement was generally good up to 8 MPa but at higherpressures, even though the samples were prepared nominallyunder the same conditions, the reported sorption values di-verged significantly.

The interaction of moisture with coal, even in the absence ofother gases, is complex. Water can exist in coals in a number ofphysical states and can vary from less than a few percent in highrank coals to more than 70% in brown coals and lignites. Inlow rank coals, the moisture may itself represent a significantstorage reservoir for CO2 (Busch et al., 2007).

The nature of adsorption of water with coal is considered tobe by hydrogen bonding at oxygenated “primary” sites. Waterpreferentially adsorbs at these sites and can also form clustersaround these primary sites (McCutcheon et al., 2003). Theamount of water adsorbed is closely correlated to the oxygencontent of the coal but for CO2 and CH4, the situation is lessclear and the nature of the binding sites is not well defined. Inthis case adsorption occurs through dispersion forces which areweaker than the hydrogen bonds through which water binds topolar sites. It has been found that CO2 capacity is also related tothe carboxyl group content of coal (Nishino, 2001) suggestingthat there is a range of different sites with different CO2

affinities. However, Goodman et al. (2005) investigated CO2

bonding on coal with FTIR and found that there was nodifference in affinity. Day et al. (2007) also found very littlevariation in the affinity of CO2 with rank in dry coals.

Despite considerable research over a long period, the exactnature of gas sorption in the presence of moisture is still not wellunderstood. The aim of the work presented here, therefore, wasto examine the effect of moisture on the CO2 and CH4 sorptioncapacities of several coals under supercritical conditions likelyto be encountered in sequestration and ECBM applications.

2. Experimental

2.1. Samples

It is widely recognised that thermal drying of low rank coalscan cause irreversible changes to the coal structure. Theseeffects, however, diminish with increasing rank and it has beensuggested that drying does not significantly alter the structure ofbituminous coals (Miknis et al., 1996). To avoid the effects ofstructural changes induced by drying, which may affect thesorption results, the samples selected for this study were allbituminous coals. Coals in this range are also more likely to besuitable candidates for ECBM applications since low rank coalsgenerally occur at shallow depths and have little or no asso-ciated methane.

Three bituminous coals were used in these experiments; oneeach from the Hunter Valley and Illawarra coal regions inAustralia and another from the Tashan mine in China. Details ofthe three samples are shown in Table 1.

Samples for isotherm measurements were prepared by crush-ing and screening fresh air-dried lumps of coal to a particle sizerange of 0.5–1.0 mm. The prepared material was then placedin a freezer until required for use. Carbon dioxide, methane and

http://dx.doi.org/doi:10.1016/S1750-07)00120-

Table 1Properties of the three coals examined

Hunter Valley Illawarra Tashan (China)

Moisture (%, ad) 2.7 1.1 1.7Ash (%, db) 17.6 16.9 24.8Volatiles (%, daf) 28.4 21.7 38.8Carbon (%, daf) 83.9 88.9 81.8Hydrogen (%, daf) 4.6 4.5 5.3Nitrogen (%, daf) 1.9 1.6 1.4Sulphur (%, daf) 0.5 0.3 0.5Oxygen (%, daf) 9.2 4.6 11.1Rmax (%) 0.81 1.40 0.81

205S. Day et al. / International Journal of Coal Geology 74 (2008) 203–214

moisture isotherms, surface areas and chemical analyses wereall obtained using this material.

2.2. CO2 and CH4 sorption measurements

Carbon dioxide and methane adsorption isotherms weremeasured using a gravimetric system which has been describedin detail previously (Day et al., in press). Briefly, the apparatuscomprises a sample cell and a separate reference cell, both ofwhich have accurately known volumes. Coal is placed in thesample cell and the void volume determined from the coalvolume, which in turn is calculated from its mass and densitymeasured by helium pycnometry. Carbon dioxide (BOC GasesAustralia Ltd, SFC grade, 99.99% purity) or methane (AirgasInc. UHP methane, Radnor, PA) is admitted to the system in aseries of pressure steps up to a maximum pressure of 20 MPaand the mass of both the sample and reference cells is monitoredcontinuously with electronic balances capable of measuring upto ±1 mg. Since the volume of the reference cell is known, themass gain of the reference cell provides a direct measure of thegas density at each pressure, which is used to determine themass of gas within the void volume in the sample cell. Thisavoids the need to use an equation of state to estimate gasdensity, which is normally required with volumetric or somegravimetric systems.

At equilibrium, excess adsorption on the coal at eachpressure, Wads, was calculated by subtracting the mass of freegas within the void space from the mass gain of the sample cell:

Wads ¼ Wmeas � qg Vcell � Vsample

� � ð1Þ

where Wmeas is the mass gain of the sample cell, ρg is thedensity of the gas as determined from the reference cell and Vcell

and Vsample are the volumes of the empty sample cell and thecoal sample, respectively.

The excess sorption data were fitted to a modified Dubinin–Radushkevichmodel (Eq. (2)) which has been found to accuratelyrepresent sorption data over a wide range of temperatures andpressures (Sakurovs et al., 2007, 2008; Day et al., in press).

Wads ¼ W0 1� qg=qa� �

e�D ln qa=qgð Þ½ �2 þ kqg ð2Þ

W0 is the maximum sorption capacity of the coal, ρg is thedensity of the gas at the temperature and pressure, ρa is the

density of the adsorbed phase, D is a constant which is afunction of both the heat of adsorption and the affinity of the gasfor sorbent and k is a constant. The k term is strongly influencedby errors in cell volume and coal density and thus the value of kpotentially has a high measurement uncertainty. The density ofthe adsorbed phase, ρa, was taken to be 1000 kg m−3 for CO2

and 420 kg m−3 for CH4.The heat of adsorption, which is a measure of the bond

energy of the adsorbate molecules to the adsorbent, can beextracted from the D term in the Dubinin–Radushkevich modelaccording to the expression:

D ¼ RT=bE½ �2 ð3Þwhere R is the universal gas constant, T is the temperature, β isan affinity constant for the gas onto the coal and E is the heatof adsorption (Sakurovs et al., 2008). The β term relates theaffinity of the adsorbate to that of a reference adsorbate, whichby convention is benzene and is given a value of β (C6H6)=1(Prinz and Littke, 2005). For CO2, a value of 0.35 for β has beenused in relation to sorption onto coal (Ozdemir et al., 2004), butthere are various values reported (Wood, 2001) and hence thereis considerable uncertainty associated with this value.

The heat of adsorption, E, in Eq. (3) is related to the isostericheat of adsorption (i.e. the heat of adsorption at a particularsorbate loading),Qst, according to Eq. (4) (Ozdemir et al., 2004):

Qst ¼ DHvap þ bE ð4ÞwhereΔHvap is the enthalpy of vaporisation of CO2. Thus, βE isthe net heat of adsorption of CO2 onto the surface of the coal.

Isotherms were measured on each coal at 55 °C and atvarious moisture contents ranging from 0 to about 8% moisture.Dry samples were prepared by vacuum drying the sized coal at60 °C for 2 days, which was found to reduce the moisturecontent to less than 0.1%. These relatively mild drying con-ditions also reduced the likelihood of producing irreversiblechanges in the samples.

Moist samples were prepared from the sized “as-received”coal by either air drying in the laboratory at 21 °C, equilibratingover saturated salt solutions at 21 °C or adding additional liquidwater to the coal and thoroughly mixing to ensure that there wasan even distribution of moisture throughout the sample. Themoisture content of each sample was measured using a Mettler–Toledo HG63 moisture analyser and the average moisture con-tent determined from at least five analyses of each coal.

Since water itself occupies space, its presence must be ac-counted for. Direct helium density measurements on wet coalgave inconsistent density values, which we attribute to theevaporation of moisture during the evacuation and measure-ment stages. Hence, the volume occupied by the wet coal in thesample cell was calculated from the helium density of the drycoal (measured using a Quantachrome Ultrapycnometer 1000helium pycnometer) and assuming that any adsorbed water hada density of 1000 kg m−3.

Dissolution of gas into coal moisture can provide some stor-age capacity which should be subtracted from the measuredcapacity to yield the true sorption capacity of moist coals. An

http://dx.doi.org/doi:10.1016/S1750-07)00120-


estimate of the amount of CO2 dissolved in water under theexperimental conditions was made using published solubilitydata (Duan and Sun, 2003) and was found to be relatively lowcompared to the total sorption capacity. In coal containing 5%moisture at 55 °C and 20MPa, dissolved CO2 would account forapproximately 2.8 kg t−1 of the total capacity of the coal.However, since the volume of the water will increase as CO2 isdissolved, the void volume of the sample cell is reduced. If anappropriate correction is not applied, the calculated sorptioncapacity tends to be overestimated, which at high pressures maybe similar in magnitude to the error introduced by gasdissolution. The net result is that the two effects tend to canceleach other. Moreover the solubility of carbon dioxide in boundwater may well be different to its solubility in free water. In viewof these uncertainties and the relatively small errors involved inany case, the dissolution of CO2 or CH4 into the sorbed waterwas neglected.

Experiments were performed by placing approximately 150–200 g of prepared coal into the sample cell and exposing tovacuum for 30 s to remove air (but not affect the moisturecontent). Gas sorption was measured at numerous pressuresranging from 0.1 to 20 MPa. Samples were held at each pressurestep for at least 4 h before changing the pressure in the cell. Thishas previously been shown to be sufficient time to allowequilibrium to be reached in this apparatus (Sakurovs et al., 2007).

Sorption isotherms were compared on a dry-ash-free (daf)basis.

2.3. Moisture sorption measurements

Moisture adsorption isotherms were determined for eachcoal at 21 °C by measuring the equilibrium uptake of watervapour by dried coal under constant humidity.

Approximately 1–2 g of the 0.5–1.0 mm coal fraction wasweighed into glass weighing jars and dried overnight at 105 °Cunder nitrogen. Each sample was then placed into a chambercontaining a saturated salt solution to maintain the relativehumidity inside the vessel at a specific level. Seven salt solutionswere used to provide relative humidities ranging from 10 to 97%.

The samples were removed periodically from the chambers,covered with lids and immediately weighed. This procedurewas continued until all samples had reached equilibrium.

Isotherms were constructed for each coal by plotting theequilibrium moisture content against relative pressure, P/P0, i.e.the vapour pressure, P, divided by the saturated vapour pressureat the temperature of the experiment, P0.

The moisture adsorption data were fitted to the Guggen-heim–Anderson–de Boer (GAB) adsorption model. The GABmodel is very similar to the well known BET expression exceptthat a third adjustable parameter, K, is introduced as shown inEq. (5):

W ¼ WmCGP=P0

1� KP=P0

� �1þ CG � 1ð ÞKP=P0

� � ð5Þ

where W is the amount of moisture adsorbed, Wm is the mono-layer capacity, P/P0 is the relative pressure, CG is a constant

related to the heat of adsorption and K is a constant whichaccounts for enthalpy differences between the layers above themonolayer and the bulk condensed liquid (Timmermann, 2003).When the value of K is equal to 1, Eq. (5) reduces to the BETequation.

According to the BET theory, the net heat of adsorption isgiven by Eq. (6) (Gregg and Sing, 1982):

Qst � DHvap ¼ RT lnC ð6Þ

Since C=KCG (Timmermann, 2003), Eq. (6) was used todetermine the heat of adsorption of water for the three coals.

The experimental data were fitted to the GAB model usingthe Solver routine supplied with Microsoft Excel. The values ofWm, CG and K were adjusted until the root mean squared (rms)residuals were minimised.

2.4. Surface area measurements

Surface areas of the coals were measured by low pressure gasadsorption using a Micromeritics Tristar 3000 apparatus. Mea-surements were made at 0 °C with CO2 as the sorbate andsurface areas were calculated from the experimental data usingthe Dubinin–Radushkevich isotherm model.

3. Results

3.1. CO2 isotherms

The CO2 excess adsorption isotherms for each coal at fourmoisture contents are shown as a function of gas density inFig. 1. Note that all three coals have been plotted with the sameexcess sorption scale for ease of comparison.

The modified Dubinin–Radushkevich model, depicted bythe line plots in Fig. 1, provided an excellent fit to theexperimental data under all conditions. A summary of the modelparameters, W0, D, k and the rms residuals is shown in Table 2.

For the dry coals, the maximum CO2 sorption capacityvaried considerably between coals, but was independent ofrank. Although the Tashan and Hunter Valley coals were ofsimilar rank (81.8% and 83.9% C, respectively), the HunterValley material had a sorption capacity about 60% higher thanthe Tashan coal. The Tashan coal also had the lowest CO2

surface area of the three coals (Table 5). The CO2 capacity ofhighest rank Illawarra coal (88.9% C) was slightly higher thanthat of the Tashan sample.

In all cases, the CO2 sorption capacity of moist coal was lowerthan the corresponding dry coal. However, the extent of the effectwas variable across the range of coals. In the highest rank coal, thesorption capacity was least affected by moisture, whereas in theother two samples, and particularly the Hunter Valley coal, theCO2 capacity was substantially reduced. The effect of moisturecontent on sorption capacity is better illustrated in Fig. 2where themaximum sorption capacity, W0 (on a daf basis), is plotted as afunction of moisture content for the three coals.

The reduction in sorption capacity of the Illawarra coal wasrelatively low, decreasing by about 8% from 65 kg t−1 for the

Table 2Modified Dubinin–Radushkevich model parameters and residuals for CO2

adsorption for each coal

Coal Moisture(%, ar)

W0

(kg t−1, daf)D βE

(kJ mol−1)k rms deviation

(kg t−1, daf)

HunterValley

0.0 98.0 0.044 13.0 0.024 0.48

1.4 88.8 0.055 11.7 0.022 0.841.8 86.4 0.054 11.7 0.024 0.602.0 81.5 0.050 12.2 0.028 0.393.0 69.8 0.067 10.5 0.029 0.573.9 70.1 0.065 10.7 0.026 0.766.5 69.8 0.072 10.1 0.027 0.43

Illawarra 0.0 64.5 0.042 13.3 0.012 0.750.9 56.7 0.050 12.2 0.013 0.972.0 58.6 0.054 11.8 0.009 0.805.1 59.3 0.055 11.7 0.008 1.36

Tashan 0.0 60.2 0.045 12.8 0.016 0.561.5 45.2 0.063 10.9 0.021 0.512.0 45.7 0.066 10.6 0.012 0.397.9 48.7 0.068 10.5 0.013 0.55

Fig. 2. Maximum sorption capacity,W0, as a function of moisture content for thethree coals.

Fig. 1. CO2 adsorption isotherms of the three coals showing the effect ofmoisture. The line plots are the fits calculated using the modified DR model.(Note: some of the Hunter Valley coal isotherms have been omitted for the sakeof clarity).


dry coal to around 60 kg t−1 for the moist coal. The value ofW0

for the Hunter Valley coal, on the other hand, decreased from98 kg t−1 for the dry material to about 70 kg t−1 for coalcontaining more than about 3% moisture, i.e. a reduction ofalmost 30%.

For all three coals, there was a certain moisture contentabove which the CO2 sorption capacity did not reduce further

with additional moisture. For the Hunter Valley sample, at least3% moisture was required before the CO2 sorption capacityreached its lowest level. This is approximately equivalent to themoisture content that would be attained if the sample had beenexposed to a relative humidity of about 50%. In the case of theIllawarra coal, no further reduction in sorption capacityoccurred beyond about 0.9% moisture (i.e. equilibrium moisturecontent at 40% relative humidity). The Tashan sample laybetween these two extremes.

Replotting the data shown in Fig. 2 as the difference in themaximum CO2 sorption capacity between the moist and drycoal (i.e. ΔW0) yielded the curves shown in Fig. 3. To removethe effects of mineral matter, note that the moisture content hasbeen recalculated to a dry-ash-free basis.

The initial sections of the curves (i.e. the region wheremoisture affected the sorption capacity) in Fig. 3 wereessentially linear with approximately the same slope for allthree coals. The average slope of this region is represented by

Fig. 3. Reduction in W0 compared to dry coal (for CO2). Dashed line representsbest fit for results below the critical moisture content.

Fig. 4. Net heat of adsorption of CO2 as a function of moisture content for thethree coals.


the dashed line in Fig. 3 and indicates that about 0.3 moleculesof CO2 are displaced per water molecule held by the coal.

The net heats of adsorption of CO2, βE, were evaluated forthe three coals at each moisture content using Eq. (3). Thesevalues are listed in Table 2 and are also plotted in Fig. 4 as afunction of moisture content for each coal.

In all three coals, βE decreased with increasing moisturewith a trend similar to that observed for the sorption capacity asa function of moisture content (Fig. 2) and, like that relation-ship, the effect was most pronounced for the Hunter Valley coal.

For all three dry coals, βE was about the same, at around13 kJ mol−1, despite the difference in rank. This is similar toresults which have been reported previously. Sakurovs et al.(2008), for example, found the βE values for three dry Argonnepremium coal samples varied only slightly between about 11.7and 13.0 kJ mol−1 for CO2 at 55 °C. Similarly, Ozdemir et al.(2004) found that the heat of adsorption of CO2 across the fullrange of dry Argonne premium coals measured at 22 °C (i.e.sub-critical conditions) was around 26. 6 kJ mol−1, regardlessof rank. Subtracting the heat of vaporisation of CO2 (17.15 kJmol−1; Ozdemir et al., 2004) yields a net heat of adsorption ofabout 9.5 kJ mol−1, which is comparable to the values foundhere that were measured at 55 °C. In another study, Prinz andLittke (2005) determined the heats of adsorption of CO2 for awide range of German coals to be around 10 kJ mol−1, againwith little effect of rank. Goodman et al. (2005) also found thatthe heat of adsorption was not affected by the rank of the coal.

Most estimates of the heat of adsorption have been made bymodelling the isotherm, however, Glass and Larsen (1994)directly measured the isosteric heat of adsorption, Qst, of var-ious sorbates onto Illinois No 6 coal using an inverse chro-matographic method. For CO2, the isosteric heat of adsorptionwas 27.6 kJ mol−1, which is equivalent to a net heat ofadsorption of 10.5 kJ mol−1. This value compares well withthose determined from adsorption models.

There have been relatively few studies which have comparedthe heats of adsorption of dry and moist coal but Prinz andLittke (2005) noted that the net heat of adsorption of CO2 was

slightly lower in the moist samples, which is consistent with theresults presented here.

3.2. CH4 isotherms

Fig. 5 shows the CH4 isotherms for the three coals at severalmoisture contents plotted as a function of gas density. The opensymbols in Fig. 5 represent the measured adsorption CH4

capacity at each pressure while the corresponding DR modelisotherms are shown as line plots.

As with the CO2 isotherms, the modified DR model fitted themeasured data very well. The CH4 DR model parameters areshown in Table 3.

The order of maximum CH4 sorption capacity,W0, of the drycoals was the same as for the CO2 capacity; the Hunter Valleycoal had both the highest CO2 and CH4 capacities while theTashan sample had the lowest CO2 and CH4 capacities. On amass basis, the methane capacity was about 4 to 5 times lessthan the CO2 capacity.

As expected, the presence of moisture reduced the methanecapacity of the coal, again, displaying some critical moisturecontent above which the sorption capacity was unaffected(Fig. 6). Like the CO2 results, this critical moisture contentincreased with decreasing rank, however it was higher for CH4

than for CO2. For CH4, the limiting moisture content wasequivalent to the moisture content that would be attained if thecoal was equilibrated at about 60 to 80% relative humidity.

Although the general trend of reducing CH4 capacity wassimilar to that for CO2, in relative terms, the effect was greaterfor methane. Moisture reduced the CH4 capacity more than itdid for CO2 over the range of coals examined. For example, theCO2 capacity of Hunter Valley coal was reduced by about 30%when moisture was present at or above the critical moisturecontent whereas for methane, the reduction was around 50%.The effect was less pronounced in higher rank Illawarra coal butthe CH4 capacity was still more affected than CO2.

The plot ofΔW0 for the CH4 results is similar to those for theCO2 isotherms in that the reduction in CH4 capacity was linear

Table 3Modified Dubinin–Radushkevich model parameters and residuals for CH4

adsorption for each coal

Coal Moisture(%, ar)

W0

(kg t−1, daf)D βE

(kJ mol−1)k rms deviation

(kg t−1, daf)

HunterValley

0.0 21.8 0.054 11.8 0.006 0.12

2.1 19.3 0.061 11.1 0.001 0.125.3 10.9 0.091 9.1 0.004 0.166.5 11.4 0.074 10.0 0.008 0.10

Illawarra 0.0 16.7 0.057 11.4 −0.006 0.120.8 15.6 0.066 10.6 −0.002 0.101.1 14.7 0.074 10.0 −0.007 0.165.5 15.2 0.085 9.4 −0.011 0.09

Tashan 0.0 13.2 0.072 10.2 −0.006 0.071.2 9.6 0.078 9.8 −0.001 0.052.1 8.3 0.092 9.0 −0.003 0.085.0 8.9 0.093 8.9 −0.006 0.06

Fig. 6. Reduction in W0 compared to dry coal (for CH4). Dashed line representsbest fit for results below the critical moisture content.

Fig. 5. CH4 adsorption isotherms of the three coals showing the effect ofmoisture. The line plots are the fits calculated using the modified DR model.


with respect to increasing moisture content, up to the criticalmoisture content. The slope of the line in this region (dashedline in Fig. 6) was the same for all three coals and wasequivalent to around 0.2 molecules of CH4 displaced for everyH2O molecule.

These results compare well to the work performed by Joubertet al. (1973), who measured the effect of moisture on themethane sorption by several U.S. bituminous coals. Note thatJoubert et al. (1973) calculated the Langmuir volume (Vm)rather than W0, however, for CH4 the difference between the

two values is generally not great. For the dry Hunter Valley coal,for instance, W0 was 18.0 kg t−1 whereas Vm was 18.5 kg t−1.The Joubert et al. (1973) results for coals below the criticalmoisture content are plotted in Fig. 6 as solid markers and lieclose to the line of best fit for the three coals examined in thepresent study. This suggests that the mechanism responsible forreducing the sorption capacity is the same in all cases.

The net heat of adsorption of CH4 was slightly less than forCO2. For the dry coals, βE varied between about 10 to 12 kJmol−1, compared to around 13 kJ mol−1 for CO2. There wasalso a slight decrease in βE with increasing moisture content(Fig. 7), although the drop was less than observed for CO2. Thedifferences in the heats of adsorption of CO2 and CH4 in thepresence of water are consistent with differences in theadsorption selectivity which have been observed previouslyon moist coals (Clarkson and Bustin, 2000; Busch et al., 2006).

3.3. Moisture isotherms

The moisture adsorption isotherms for the three coals areshown in Fig. 8. In each case, the isotherms are plotted as afunction of relative pressure.

Fig. 7. Net heat of adsorption of CH4 as a function of moisture content for thethree coals.

Table 4GAB model parameters for moisture adsorption onto the three coals

Coal Wm

(kg t−1, daf)CG KG rms deviation

(kg t−1, daf)Net heat ofadsorption(kJ mol−1)

Hunter Valley 27.0 24.5 0.70 0.91 6.9Illawarra 7.8 45.7 0.72 0.16 8.5Tashan 16.2 26.0 0.58 0.49 6.7


Moisture uptake by coal is well known to increase withdecreasing rank and the results shown in Fig. 8 are consistentwith this trend. However, although the Hunter Valley andTashan samples are of similar rank, the Hunter Valley coal had asignificantly higher isotherm. The Hunter Valley sample alsohad the highest CO2 sorption capacity of the three samples butthe Illawarra coal, with the lowest moisture isotherm, had ahigher CO2 capacity than the Tashan.

Plots of the results calculated with the GABmodel are shownin Fig. 8 as line plots. In all cases, the predictions of the GABmodel agreed well with the measured data.

A summary of the model parameters and net heat of ad-sorption for each coal is provided in Table 4.

The net heats of adsorption (i.e. Qst−ΔHvap) of these coalsare shown in Table 4 and were within a fairly narrow range ofabout 7 to 8 kJ mol−1.

The monolayer capacity, Wm, in each case was about 30 to40% of the maximum capacity at P/P0=1. Although there was a

Fig. 8. Moisture isotherms for the three coals plotted as a function of relativepressure. The open markers represent the measured points and the line plots arethe corresponding GAB model results.

significant difference in Wm between the coals, the relativepressure corresponding to this value was between about 0.2 to0.3 for each coal.

The net heat of adsorption for water estimated from the GABmodel was significantly lower than the heats of adsorptionestimated for CO2 and CH4. This is somewhat surprising giventhat hydrogen bonding associated with water adsorption wouldbe expected to result in stronger bonds than the dispersionforces involved in CO2 adsorption. However, determining heatsof adsorption from BET and GAB isotherms has been criticisedbecause these models assume that the heat of adsorption isconstant during monolayer adsorption but drops to zero once themonolayer has been completed. The K term in the GAB modelis intended to compensate for this effect but the energy term inthe GAB model is usually around 35 to 40% lower thanestimated from the BET model (Timmermann, 2003). Conse-quently, heats of adsorption estimated by the GAB model arelikely to be lower than experimentally determined values.

Heats of adsorption can also be determined experimentallyby measuring adsorption isotherms at several temperatures. Inthis method, isosteres are constructed by plotting the pressure(or gas density) against the reciprocal temperature at a constantadsorbate loading. From the Clausius–Clapeyron equation, theheat of adsorption is given by:

Qst ¼ �RAlnP

A 1=Tð Þ� �

ð7Þ

A plot of lnP versus 1/T yields an adsorption isostere, theslope of which can be used to calculate Qst. If P/P0 is usedinstead of P, the net heat of adsorption is obtained.

Adsorption data at temperatures other than 21 °C were notavailable for these coals, however, moisture isotherms havebeen previously measured at various temperatures between10 °C and 60 °C on another Hunter Valley coal, very similar tothat used in the current study (Day, 2001). The results from thatstudy were therefore reprocessed to estimate the heat ofadsorption over a range of moisture contents to compare withthe values estimated from the GAB model.

The net heat of adsorption of the earlier sample is plotted as afunction of moisture content in Fig. 9.

At low moisture contents, the heat of adsorption was about12 kJ mol−1 but above about 2% moisture, decreased rapidly toapproach zero by around 3.5% moisture. The onset of the rapiddecline in the heat of adsorption corresponded closely to themonolayer capacity, Vm, predicted by the GAB model for thiscoal. For comparison, the heat of adsorption for this coal

Fig. 9. Net isosteric heat of adsorption of moisture on a coal similar to the HunterValley sample as a function of moisture content.


determined from the GAB model was very similar to the otherHunter Valley coal at about 8 kJ mol−1.

4. Discussion

The results of this study, in common with a number of others,show that the sorption capacity of coal for both CO2 and CH4 isreduced up to some critical moisture content, beyond whichmoisture has no effect on the sorption capacity. This criticalmoisture content, for bituminous coals, seems to be dependenton rank and is roughly equivalent to the equilibrium moisturecontent that would be achieved if the coal was exposed to arelative humidity of between about 40 to 80% (i.e. relativepressures between 0.4 and 0.8). However, according to the GABisotherm model, monolayer completion occurs at a relativepressure between about 0.2 and 0.3 so it is clear gas sorption

Fig. 10. Schematic representation of CO2 and moisture adsorption in coal pores (CO2

pore structure represent polar sites. (a) CO2 only; (b) CO2 with water monolayer; (c

capacity continues to be reduced even as multiple layers ofwater are adsorbed onto the coal.

Adsorption of gases onto coal has traditionally been in-terpreted with the Langmuir adsorption model, which assumessurface monolayer coverage. Research over the last few years,however, has shown that alternatives such as the Dubinin–Astakhov and Dubinin–Radushkevich models provide betterfits to experimental data, especially at high pressures (Ozdemiret al., 2004; Ottinger et al., 2006; Sakurovs et al., 2007). Thisindicates that the mechanism of supercritical gas adsorption isactually one of pore filling and this concept was used bySakurovs et al. (2008) to explain the sorption behaviour ofgases onto coal. They suggested that there is a maximum porediameter above which pores cannot be fully filled by sorptionwhen the gas is supercritical. In this model, the density of theadsorbed gas is high at the surface (i.e. 1000 kg m−3 for CO2

and 420 kg m−3 for CH4) but progressively decreases atdistances further away from the surface to approach the densityof the free gas. Narrow pores will therefore be filled by theadsorbate but at larger diameters, the pores will only be partiallyfilled because under supercritical conditions, the gas is unable tocondense. This is shown schematically in Fig. 10a.

When considering the effect of water on sorption capacity itis useful to examine the results in terms of this pore fillingmechanism.

Water molecules attach to polar sites, such as hydroxylgroups, on the coal surface which are represented as blackmarks in Fig. 10. These sites are preferentially occupied bywater and in the process reduce the capacity for CO2 and CH4

by physical displacement. The point at which all of the polarsites are occupied by water corresponds to the monolayercoverage (i.e. Wm, in the GAB model) as shown in Fig. 10b.Since water only attaches to the polar sites, the hydrophobicsites on the coal remain available for adsorption of CO2 or CH4.Hence the moisture capacity of the coal is related strongly to the

—grey circles; water—small black circles). The black marks on the edge of the) CO2 with water clusters.


number of polar sites available but the CO2 and CH4 capacitiesare essentially independent of the type of surface. Instead, theCO2 and CH4 capacities depend on the size of the pores; a largenumber of smaller diameter pores corresponds to highercapacity (although sufficient macro-porosity would be requiredto allow access to these pores). This accounts for the lack of aclear correlation between sorption capacity for CO2 and CH4

and the rank of coal.This interpretation is supported by comparing the surface

areas of the coals determined from low pressure CO2 and wateradsorption isotherms. Table 5 shows the surface areas of thethree coals calculated from CO2 adsorption on dry coal (at 0 °C)and the corresponding values estimated from the water mo-nolayer capacity, Wm, calculated by the GAB model at 21 °C.

Even though water molecules are smaller than CO2 withcross sectional areas of 0.11 and 0.20 nm2, respectively(Nishino, 2001) and should therefore be able to access moreof the pore volume, the surface areas reported by the wateradsorption data are significantly lower than the correspondingCO2 values. This shows that only a small proportion of the totalsurface area is accessed by water because the water moleculesare restricted to polar sites. Since low rank coals contain agreater proportion of polar sites, their water surface areas arehigher than for high rank coals. The higher proportion of polarsites in low rank coals also explains why the reduction in CO2

and CH4 sorption capacity in the presence of water is greatest inlow rank coals.

At moisture contents above the monolayer capacity, clustersof water molecules form around the polar sites through hy-drogen bonding between adjacent water molecules (Fig. 10c).These clusters occupy space that would otherwise be availableto other sorbates and thus further reduce the sorption capacityfor these gases. For CO2, sorbate molecules can probably form alayer over the top of the adsorbed water (as shown in Fig. 10c)but CH4 is unlikely to attach to water so it will be restricted tothe hydrophobic sites not occupied by water. This is reflected inthe greater proportional effect of moisture on methane sorptioncompared to CO2. In the Hunter Valley sample, for example, thereduction in CO2 capacity was around 30%, but for CH4, thecapacity reduced by more than 50% when saturated with water.The trend was the same for the other coals, but in the high rankIllawarra coal the effect was substantially diminished; thereduction in CH4 capacity was around 10% compared to 8% forCO2. The higher limiting moisture content apparent for methaneadsorption compared to CO2 also suggests that CH4 is unable toattach to adsorbed water molecules.

When the moisture content of the coal is above the criticallevel, the CO2 and CH4 sorption capacity is unaffected. This

Table 5Comparison of surface areas determined by CO2 adsorption and wateradsorption

Coal CO2 surface area(m2 g−1, daf)

H2O surface area(m2 g−1, daf)

Hunter Valley 202.8 97.4Illawarra 148.6 28.1Tashan 120.7 58.7

water must therefore occupy volume in the coal particles that isnot accessible to other sorbates. It is probable that this volume iswithin large pores or interparticle voids.

An important consequence of volumetric displacement isthat estimates of wet coal capacity can readily be made from thedry coal isotherms, which are experimentally more convenientto measure. In the case of CO2, approximately 0.3 molecules ofCO2 are displaced by each water molecule, which is equivalentto a reduction in CO2 capacity of about 7.3 kg t−1 for each 1%increase in moisture content up to the limiting moisture content.For CH4, each molecule of water displaces 0.2 molecules ofCH4 so that the methane capacity is reduced by about 1.8 kg t−1

for each 1% increase in moisture content. This is somewhatlower than the results of Levy et al. (1997) who estimated thatCH4 adsorption capacity was reduced by 4.2 mL g−1, or about3.0 kg t−1, for each 1% increase in moisture for an Australianbituminous coal. However, their results were measured at 30 °C(compared to 55 °C in our study) and the capacities at only5 MPa were considered rather than the maximum sorptioncapacity, which makes direct comparison difficult.

It is interesting that the heat of adsorption of both CO2 andCH4 reduced slightly with increasing moisture content since itimplies that the adsorption sites have a range of affinities forthese sorbates. The higher energy sites, which are available toCO2 and CH4 on dry coal, are progressively occupied whenwater is present, so they are presumably the polar functionalgroups. This is contrary to some previous work that hasindicated that, for CO2 at least, there are no specific adsorptionsites (e.g. Amarasekera et al., 1995). In a recent study, Goodmanet al. (2005) directly examined the interaction of CO2 with coalusing a reflectance FTIR technique and they too, found noevidence of specific interactions between CO2 and oxygenatedsites on the coal surface. This group did, however, allude to theexistence of CO2-specific sites on coal when comparing theirresults to the slightly higher isosteric heats of adsorptionmeasured by Glass and Larsen (1994). Goodman et al. notedthat Glass and Larsen's results were obtained under conditionsapproaching infinite adsorbate dilution where the adsorbateloading would be very low with most of the adsorptionoccurring on the more energetic sites (since these would beexpected to be the first occupied). As a result, measurementsmade at low sorbate partial pressures would be expected to yieldhigher heats of adsorption than those calculated from isothermmodels, which provide a global or average value for the entiresurface.

Other evidence supporting the view that CO2 may bepreferentially adsorbed at carboxyl functional groups wasreported by Nishino (2001). These sites would be among thefirst to be occupied by water and would therefore lead to areduction in the heat of adsorption for both CO2 and CH4.

Another explanation for the observed reduction in heat ofadsorption is related to the suggestion that the heat of adsorptionincreases with decreasing pore size (Stoeckli and Ballerini,1991). In this situation, progressively filling the finest pores (i.e.highest energy pores) by water molecules would reduce the heatof adsorption of CO2 and CH4. However, if this were true, thenit would be expected that for the distinctive displacement ratios


of CO2 and CH4 by water to occur, only fine pores could absorbwater, since any moisture sorption on polar sites not in finepores would have no effect on the heat of sorption of CO2. Thisexplanation therefore seems unlikely.

5. Conclusions

Carbon dioxide and methane sorption isotherms were mea-sured on dry and moist coals at 55 °C and pressures up to20 MPa. In all cases the isotherms were well described by amodified Dubinin–Radushkevich model. Moisture isothermswere also measured for these coals at 21 °C and the Gug-genheim–Anderson–de Boer (GAB) adsorption model, anextension of the BET model, fitted the measured data wellover relative pressures between 0 and 1. The GAB model,however, appears to significantly underestimate the heat ofadsorption of water onto coal.

The CO2 and CH4 capacities of the three coals were highestin the dry material but decreased sharply in the presence ofmoisture. Sorption capacity continued to decrease withincreasing moisture until reaching a limiting value thatdepended on the rank of the coal. This limiting moisturecontent was highest in low rank coal and corresponded to theequilibrium moisture content that would be achieved had thecoal been exposed to a relative humidity of between about 40and 50% for CO2 and 60 and 80% for CH4. Further increases inmoisture content above this limit did not affect the CO2 or CH4

capacity of the coal. This suggests that laboratory sorptioncapacity measurements made on samples prepared by thecommonly used procedure of equilibrating at 97% relativehumidity are likely to represent the maximum effect on thesorption capacity and will therefore probably be indicative ofthe in situ capacity of the coal.

In low rank coals, moisture had a greater effect on thesorption capacity than in high rank coals, probably because ofthe greater proportion of polar sites which are preferentiallyoccupied by water at the expense of CO2 and CH4. The re-duction in gas sorption capacity with increasing moistureappears to be the result of simple volumetric displacement ofsorbate (i.e. CO2 and CH4) molecules by the water. Hence,below the limiting moisture content the reduction in sorptioncapacity with increasing moisture was approximately the samefor all coals. For CO2, the sorption capacity reduced by around7.3 kg t−1 for each 1% increase in moisture whereas the CH4

capacity decreased by 1.8 kg t−1 for each 1% increase inmoisture.

The fact that CO2 and CH4 continue to be adsorbed in thepresence of free water indicates that the free water resides inregions of the coal which are not accessible to gas sorbates. It islikely that this water is in the larger pores as well as interparticlevoids and the cleat network.

The heat of adsorption of both CO2 and CH4 on the coal wasreduced slightly in the presence of moisture. This suggests thatthere are some higher energy adsorption sites within the coalthat have greater affinity for these sorbates, but these sites aremore readily occupied by water molecules. Hence CO2 and CH4

are forced to lower energy sites when water is present. Water

appears to be adsorbed at specific, presumably polar, siteswhereas CO2 and CH4, although showing some degree of pre-ferential adsorption, are generally able to access all of theadsorption sites throughout the coal.

Acknowledgement

We are grateful to the CSIRO Energy Transformed Flagshipfor the financial support for this research.

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Supercritical gas sorption on moist coals