International Journal of Coal Geology 85 (2011) 289–299

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International Journal of Coal Geology

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Methane and carbon dioxide sorption on a set of coals from India

Pratik Dutta ⁎, Santanu Bhowmik, Saumitra DasBengal Engineering and Science University, Shibpur, P.O. — Botanic Garden, Howrah — 711103, West Bengal, India

⁎ Corresponding author. Department of Mining EnginScience University, Shibpur, P.O. — Botanic Garden, HoIndia. Tel.: +91 33 2668 4561/2/3x477; fax: +91 33 26

E-mail address: dutta.pratik@gmail.com (P. Dutta).

0166-5162/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.coal.2010.12.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 October 2010Received in revised form 13 December 2010Accepted 13 December 2010Available online 20 December 2010

Keywords:Coalbed methaneAbsolute sorptionCarbon sequestrationCoal maceral composition

Complete sorption isotherm characteristics of methane and CO2 were studied on fourteen sub-bituminous tohigh-volatile bituminous Indian Gondwana coals. Themean vitrinite reflectance values of the coal samples arewithin the range of 0.64% to 1.30% with varying maceral composition. All isotherms were conducted at 30 °Con dry, powdered coal samples up to a maximum experimental pressure of ~7.8 MPa and 5.8 MPa formethane and CO2, respectively.The nature of the isotherms varied widely within the experimental pressure range with some of the samplesremained under-saturatedwhile the others attained saturation. The CO2 tomethane adsorption ratios decreasedwith the increase in experimental pressure and the overall variation was between 4:1 and 1.5:1 for most of thecoals. For both methane and CO2, the lower-ranked coal samples generally exhibited higher sorption affinitycompared to the higher-ranked coals. However, sorption capacity indicates a U-shaped trend with rank.Significant hysteresis was observed between the ad/desorption isotherms for CO2. However, with methane,hysteresis was either absent or insignificant. It was also observed that the coal maceral compositions had asignificant impact on the sorption capacities for both methane and CO2. Coals with higher vitrinite contentsshowed higher capacities while internite content indicated a negative impact on the sorption capacity.

eering, Bengal Engineering andwrah — 711103, West Bengal,68 2916.

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In a Coalbed Methane (CBM) reservoir almost the entire gas isstored into the micropores within the coal matrix in adsorbed form.The economic success of a reservoir depends, to a large extent, on thegas saturation level, which can be determined by knowing the gascontent vis-à-vis the gas storage capacity of the reservoir (GasResearch Institute (GRI), 1996). On the other hand, storage of CO2 indeep unminable coal beds is being considered as one of the promisingtechnologies of geological sequestration, which can also lead toEnhanced Coalbed Methane (ECBM) recovery. CO2 can access thefinest pores and adsorb firmly to the coal at a near-liquid density withminimal chances for its later release (Krooss et al., 2002). Further-more, injected CO2 can replace adsorbed methane due to the higheraffinity of coal to CO2, accelerating methane recovery in the ECBMprocess. Therefore, CO2–ECBM is considered as a value-added optionof CO2 sequestration. Although many other factors need to beconsidered for site suitability analysis of CO2–ECBM, relative sorptionaffinity of coal to methane and CO2 is the parameter of primaryconcern. An underlying requirement for the estimation of methanerecovery and CO2 storage capacity of a coal bed is an understanding oftheir sorption behavior on coal and this has to be measured

experimentally due to the complex and heterogeneous physical andchemical structures of coal (Bae et al., 2009).

Many studies have been reported on CH4 and CO2 sorption on coalsin the past. It has generally been established that coal can adsorbhigher amount of CO2 than methane at the same temperature andpressure. Although the 2:1 adsorption ratio of CO2 to methane waswidely reported in the past, more recent works reported much higherratio in some coals (Arri et al., 1992; Busch et al., 2003; Fitzgeraldet al., 2006; Gentzis, 2000; Harpalani et al., 2006; Krooss et al., 2002;Mastalerz et al., 2004; Shagafi, 2010; Smith, 1999; Tang et al., 2005).Physical gas adsorption process is believed to be reversible in nature,but small to significant hysteresis in sorption isotherm curves hasbeen reported for many coals (Busch et al., 2003; Harpalani et al.,2006; Ozdemir et al., 2003, 2004). Understanding the factorscontrolling CO2 and CH4 adsorption in coals, although is importantfor modeling of both CO2 sequestration and CBMproduction, is not yetclearly understood. Experiments conducted on dry coals to studydependency of sorption capacity to coal composition and rank revealthat ash strongly reduces the sorption capacity by acting as a diluent(Gurdal and Yalcin, 2000, 2001; Laxminarayana and Crosdale, 1999).Presence of moisture also shows a strong effect on the sorptioncapacity (Krooss et al., 2002; Laxminarayana and Crosdale, 1999; Levyet al., 1997). Many studies were undertaken to establish correlationsbetween CH4 and CO2 sorption capacities with coal rank and maceralcomposition (Bustin and Clarkson, 1998; Carroll and Pashin, 2003;Chalmers andBustin, 2007; Crosdale et al., 1998; Faiz et al., 1992;Gurdaland Yalcin, 2000, 2001; Laxminarayana and Crosdale, 1999, 2002; Li

290 P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

et al., 2010; Mastalerz et al., 2004; Prinz et al., 2004; Unsworth, 1989).However, such correlations remain largely inconclusive till date.

Coalbed Methane (CBM) has emerged as an important source ofunconventional natural gas in countries like USA, Canada, China, andAustralia. India with a vast reserve of coal has been a center of intenseCBM exploration activities in the recent years, where the totalresource of CBM is estimated at 4.6 trillion cubic meters (DirectorateGeneral of Hydrocarbons, 2010). The paper discusses the results ofstudy on pure gas sorption characteristics of CH4 and CO2 on someIndian coals of various types and origin. The sorption characteristicsare then correlated with various petrographic compositions of thecoals tested. Themain objective of the study is to discuss the influenceof rank and macerals on the sorption of CO2 and CH4.

2. Samples

Fourteen coal samples were collected from the eastern part of thecountry, which is endowed with huge coal reserves (Fig. 1). Majority

Fig. 1. Coalfield

of India's coal mining and CBM exploration activities are concentratedwithin this region. Eight of the samples belong to the Raniganjcoalfield, four to the Jharia coalfield, and the remaining two to theSouth Karanpura coalfield. Information about the source of the coalsamples is given in Table 1. All samples except those from the SouthKaranpura coalfield were collected from freshly exposed faces ofunderground coal mines. The two South Karanpura coals, SKAC1 andSKAC3, were obtained from a CBM drilling well. Upon collection, coalblocks were immediately wrapped tightly with plastic, indexed, andtransported to the laboratory. The blocks were then crushed, in stages,to different size fractions. After crushing, the samples were kept inairtight packets and refrigerated to prevent oxidation. On an average30–50 g samples between −100 and +150 mesh size (0.100–0.149 mm) were used for sorption experiments. Samples were driedby keeping them in a vacuum-oven chamber maintained at 105 °C for48 h. The step also ensured that the samples were degasified (Bae andBhatia, 2006). Before the start of the experiment, a small amount ofcoal (~1 g) was used for proximate analysis following the standard

s in India.

Table 1General information about the coal samples.

Name Location Depth (m)

Bogra Raniganj Formation of Raniganj Coalfield 160Kenda 107Narayankuri 46Satgram 190Kalimati Barakar Formation of Raniganj Coalfield 172Local II 118Mehaladih 201Mugma Special 109SKAC 1 South Karanpura Coalfield 914SKAC 3 99615th Seam Jharia Coalfield 52016th Top Seam 65016th Bottom Seam 45018th Seam 380

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ASTM procedures (ASTM Designation: D 3172 — 07a, 2010). Anotherpart of the sample was used for coal petrographic analysis at theCentral Institute for Mining and Fuel Research, Dhanbad following theIndian Standard is 9127.

3. Sorption isotherm tests

To find out sorption capacity of the coals, a high-pressuremanometric gas-sorption apparatus was constructed similar to theone explained by Dutta et al. (2008), the schematic diagram of whichis shown in Fig. 2. The two stainless steel pressure vessels, termedreference cell and sample cell, were connected to the pressuretransducers (accuracy 0.05% of full scale) to monitor pressurevariations. The entire set up was immersed in a constant temperaturewater bathmaintained at 30 °C (±0.1 °C). The internal volumes of thereference and sample cells were calibrated with helium and employ-ing real gas law. Gas compressibility factor was determined usingSpan–Wagner Equation of state (Span andWagner, 1996). The samplecell was filled up with coal sample and the void volumewithin the cellwas similarly calibrated by helium expansion before the start of theexperiment. Thereafter, an adsorbate gas (methane or carbondioxide) was injected into the reference cell and allowed to stabilize.The valve between the cells was then opened and the gas was allowedto pass into the sample cell and adsorb onto the coal. Adsorption wasassumed to be complete when pressure in the sample cell attainedequilibrium and remained constant for at least 2 h. The amount of gasadsorbed was then calculated by employing mass balance equationusing real gas law and equation of state. The process was repeated for8–10 increasing pressure steps to obtain the adsorbed volume at eachpressure step. After reaching the highest pressure step of ~8 MPa,

Fig. 2. Schematic of the experimental setup (after Dutta et al., 2008).

desorption steps were carried out by venting out gas from the samplecell into the reference cell, in stages, until sample cell pressurereached near atmospheric level, and calculating similarly the volumeof gas desorbed in every step.

Although the coal samples originated from different depths andpressure–temperature conditions, all tests were carried out at 30 °Cand up to the same moderate pressure range. This was done toprovide a common basis to compare their sorption properties andassess the correlation between a few coal characteristics and thesorption properties.

The sorbed volume measured by the process is the excess sorptionor Gibbs sorption, which does not account for the volume of gassorbed into the pore spaces of the adsorbent. At low pressures, thissorbed gas volume is not significant and the excess and absolutevalues are almost the same. But at higher pressures, this sorbedvolume is high and the absolute values may be substantially differentthan reported excess values (Arri et al., 1992; Hall et al., 1994;Harpalani et al., 2006). Absolute sorbed values were calculated fromthe experimental excess values using the equation:

nabsolute = nexcess = 1− ρgas = ρadsorbed� �� �

ð1Þ

where, nabsolute is the absolute sorption, nexcess is the excess sorption,ρgas is the gas density at a given pressure–temperature, and ρadsorbed isthe sorbed phase density of the adsorbate gas, which was taken as0.421 g/cm3 and 1.18 g/cm3 for CH4 and CO2, respectively (Harpalaniet al., 2006).

The absolute adsorption values were statistically fitted to theLangmuir equation, shown below, by non-linear regression throughSPSS statistical package:

V = VL P= P + PLð Þð Þ ð2Þ

where, V is the adsorbed gas volume at equilibrium pressure P, VL isthe maximum storage capacity of the gas and termed as Langmuirvolume constant, and PL is the Langmuir pressure that corresponds tothe pressure at 50% of Langmuir volume and termed as the Langmuirpressure constant.

The experimental uncertainties in isotherm measurements werecalculated using the theory of error propagation (Ozdemir, 2004). Theobjective function in gas adsorption step is expressed by the equation:

Δnex = f ðPRi;PRf ;VR ;ZRi; ZRf ;PSi; PSf ;Vo;ZSi;ZSf ;w;TÞ ð3Þ

where, Δnex is the excess adsorption in a pressure step; PRi, PRf are theinitial and final pressures of the reference cell, respectively; VR is thevolume of the reference cell; ZRi, ZRf are the initial and final gascompressibility factors, respectively, at the corresponding pressure–temperature conditions within the reference cell; PSi, PSf are the initialand final pressures of sample cell, respectively; Vo is the void volumeof samples cell; ZSi, ZSf are the initial and final gas compressibilityfactors, respectively, at the corresponding pressure–temperatureconditions within the sample cell; w is the weight of the sample;and T is the temperature of isotherm measurement. The maximumuncertainty in the pressure steps was calculated by taking the squareroot of the sum of squares of each individual factors of isothermexperiment and given by the equation (Mavor et al., 2004):

dðΔnexÞ = df ðPRi; PRf ;VR; ZRi;ZRf ; PSi;PSf ;Vo; ZSi; ZSf ;w; TÞ

= √ ∂Δn∂PRi

dPRi

� �2

+ ………… +∂Δn∂T dT

� �2( )

:ð4Þ

The limit of error for each pressure reading was found from thetransducer calibration record. Uncertainty in computation of thereference cell and sample cell void volumes was calculated using thedispersion from the mean value during calibration. The limit of error

Table 2Proximate and petrographic analyses of coal samples.

Name of the sample Moisture(%)

Volatile matter(%)

Ash(%)

Fixed Carbon(%)

Vitrinite(%)

Semi-vitrinite(%)

Liptinite(%)

Inertinite(%)

Mineral matter(%)

MeanRo%

Bogra 7 38 10 45 73.9 0.4 4.2 15.8 5.7 0.64Kenda 4 40 12 44 65.3 1 9.7 16.6 7.4 0.62Narayankuri 7 41 15 37 73.8 0.8 6 11.4 7.9 0.64Satgram 4 40 17 39 49.7 0.8 11.3 27.2 11 0.61Kalimati 1.2 36 28 34.8 12 1.3 7.3 61.3 18.1 0.96Local II 2.3 41.5 22 34.2 12.9 1.8 5.8 62.1 17.4 1.01Mehaladih 1.1 35 30 33.9 16.7 2.3 8.3 64.6 8.1 0.96Mugma Special 2.2 38 27 32.8 7.7 0.7 12.8 61.9 16.9 1.06SKAC 1 1.4 34 31 33.6 9.3 0.5 0.3 53.4 36.5 1.94SKAC 3 1.8 32 48 18.2 0.2 0 0 18.6 81.2 1.3015thSeam 1.2 45 20 33.8 86.1 0.2 0.2 5.5 8 1.2916thTop Seam 1.6 36.5 24 37.9 62 1.4 0.3 29.5 6.8 1.2216thBottom Seam 6 35 34 25 58.4 2.3 0.3 33.7 5.3 1.1118thSeam 0.5 38 22 39.5 84.2 0.5 1 9.8 4.5 0.97

Note: Proximate analysis is conducted as per ASTMDesignation: D 3172 - 07a, 2010 andpetrographic analysis is done as per IS 9127. Coalmacerals are reported as % of volume, as received basis.

292 P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

for the compressibility factor was calculated by using the minimumand maximum experimental pressure–temperature combination. Themaximum uncertainty for the weight and temperature was takenfrom the instrumental accuracies of balance and water bath,respectively.

4. Results and discussion

4.1. Proximate and petrographic analyses

The results of proximate and petrographic analyses of the coalsamples are given in Table 2. The proximate analysis was carried outafter drying of the samples as explained earlier and, therefore, themoisture content is representative of the residual moisture of thesamples. Indian coals generally have very high ash content. The ashcontent of the samples ranges from 10% to 48%. Raniganj formationcoals have higher moisture but lower ash content than the othersamples. All samples have high volatile matter content ranging from32% to 45%. Mean vitrinite reflectance, Ro, representative of thematurity of the coal samples, varies from 0.61% to 1.94% indicatingthat the coals are bituminous. South Karanpura coals are the most

Fig. 3. Methane adsor

mature ones followed by the Jharia, Barakar formation, and Raniganjformation coals. All Raniganj and Barakar formation coals are brightwhereas the others are dull as indicated by their high vitrinite andintertinite contents, respectively.

4.2. Methane and carbon dioxide adsorption on coal samples

Methane adsorption isotherms on dry-ash-free (DAF) basis for allcoal samples are shown in Fig. 3 and an overview of the experimentalresults is presented briefly in Table 3. It can be seen from Table 3 thatthe Langmuir equation matches the experimental adsorption valuesquite well. The uncertainty associated in the isotherm experiment,calculated by the process explained earlier, varied within the range of5.6%–8.6%.

As can be seen in Fig. 3, all plots follow Type 1 isotherm behavior.Some of the samples show steady increasing nature while the othershave attained saturation or nearing saturation at the final point of thepressure range. Data presented in Table 3 show that the Langmuirvolume constant varies widely from a low of 12 mL/g for the Local IIcoal to a high of 36 mL/g for the Narayankuri seam. The difference innature of the isotherms can be explained by the variation in their

ption isotherms.

Table 3Overview of methane adsorption experiments.

Coal sample Max. exp.pressurekPa

Langmuir volumeconstant, VL, mL/g

Langmuir pressureconstant, PL, kPa

Correlationcoefficient,R2

Bogra 5085 33 2701 1.000Kenda 6341 24 3082 0.998Narayankuri 6858 36 7351 0.996Satgram 4194 23 2553 1.000Kalimati 5838 16 1894 0.999Local II 4602 12 1316 0.999Mehaladih 7782 23 3607 0.995Mugma Special 4625 13 1852 0.998SKAC 1 5705 22 1103 0.999SKAC 3 5204 22 1058 1.00015th Seam 6856 24 2026 0.99916th Top Seam 6249 27 1823 0.99916thBottomSeam 7038 28 2410 0.99818th Seam 6286 22 1749 0.999

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Langmuir volume and Langmuir pressure constants. For example,both Bogra and Narayankuri coals have similar Langmuir volumeconstant, but the Langmuir pressure constant of the Bogra coal ismuch lower than that of the Narayankuri coal. This explains thedifference in nature between these two isotherms. Due to its higherLangmuir pressure constant value, Bogra coal shows very highsorption capacity in low-pressure range whereas, the increase inadsorption capacity of the Narayankuri coal is more gradual.Furthermore, the adsorption plots indicate that, except the Bogracoal, which shows unexpectedly high sorption behavior, all othersamples are markedly separable by their location and adsorbed gascapacity. The Raniganj formation coals show consistently highersorption capacities within the entire pressure range compared to themore mature Barakar formation coals. But, the higher ranked Jhariaand South Karanpura coals have much higher methane sorptioncapacities compared to the lower ranked coals of Raniganj coalfieldwithin the same pressure range. A notable observation, however, isthat the adsorption isotherms of the Raniganj formation coals aresteadily increasing and remain under-saturated within the experi-mental pressure range. As a consequence, these coals have compa-

Fig. 4. Carbon dioxide ad

rable or even higher Langmuir volume constant compared to theBarakar and South Karanpura coals.

Carbon dioxide adsorption isotherms for all coal samples areshown in Fig. 4 and an overview of the results of the experiments ispresented briefly in Table 4. Like methane, carbon dioxide isothermplots also exhibit Type 1 behavior and they are also markedlydistinguishable based on their location and adsorbed gas capacities.All Raniganj formation coals show steady increasing nature but theothers have attained saturation within the experimental pressurerange. Results presented in Table 4 show that the Langmuir volumeconstant varies widely from a low of 18 mL/g for the Local II coal to ahigh of 89 mL/g for the Narayankuri coal. For methane as well, theLocal II and Narayankuri coals exhibit the minimum and maximumLangmuir volume constant, respectively. However, unlike methane,the lower-ranked Raniganj formation coals have much higher CO2

adsorption capacities compared to the other higher ranked coals. Notonly the lowly ranked Raniganj samples have higher Langmuirvolume constants, their isotherm plots consistently lie much abovethose of the other coals for the entire experimental pressure range.But similar to methane, Barakar formation coals also exhibit thelowest sorption capacities.

The results, presented above, do not demonstrate any convincingrelationship between rank and adsorption capacity. Earlier studiesalso failed to reach any definitive conclusion about this relationship.Chalmers and Bustin (2007) showed that rank has a dominatingcontrol on the methane adsorption capacity with anthracite coalsexhibiting higher adsorption capacity than the bituminous coals.Crosdale et al. (1998) and Unsworth et al. (1989) made similarobservations. However, Laxminarayana and Crosdale (1999) andPrinz et al. (2004) found that methane adsorption capacity generallydisplays a U-shaped curve with coal rank. A similar U-shaped trendwith coal rank for CO2 was observed by Ozdemir et al. (2004) forlignite to low volatile bituminous coals. On the other hand, Stantonet al. (2002) could not find any specific correlation between CO2

sorption capacity and coal rank. Li et al. (2010) observed a similarU-shaped correlation for CO2-sorption capacity with coal rank. How-ever, in contrast to methane and exactly similar to the findings of thisstudy, they found that the sub-bituminous coal sample exhibited thehighest CO2 adsorption capacity. Gurdal and Yalcin (2000, 2001)observed that, for CO2 up to Ro value of about 1.1, the Langmuir volume

sorption isotherms.

Table 4Overview of carbon dioxide adsorption experiments.

Coal sample Max. exp.pressurekPa

Langmuir volumeconstant, VL, mL/g

Langmuir pressureconstant, PL, kPa

Correlationcoefficient,R2

Bogra 5750 74 2732 0.997Kenda 5240 68 2406 0.997Narayankuri 5855 89 2586 0.996Satgram 5784 58 1976 0.995Kalimati 4956 25 1074 0.998Local II 4644 18 800 1.000Mehaladih 5380 31 1271 0.999Mugma Special 4755 26 1167 0.998SKAC 1 5488 33 841 0.999SKAC 3 4689 41 984 1.00015th Seam 5428 35 1082 0.99916th Top Seam 5624 42 1418 0.99816thBottomSeam 5604 42 1231 1.00018th Seam 5745 39 1169 0.996

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decreases with increasing maturity and then increase with furtherincrease in coal rank as also observed by Day et al. (2008). Therelationship of VL and PLwith Ro for the coal samples tested in this study

Fig. 5. Relationship of VL and PL with

Fig. 6. CO2/CH4 adsorption

is presented in Fig. 5, where it can be seen that both VL and PL exhibit U-shaped trend. This observation is consistent with Day et al. (2008) andGurdal and Yalcin (2000, 2001).

To compare the relative adsorption behavior of CH4 and CO2 for allcoal samples, their adsorption ratios at different pressures werecalculated and are presented in Fig. 6. It can be observed from thisfigure that the CO2:CH4 ratio reduces with increasing pressure andattains almost a constant value. The Narayankuri coal exhibits veryhigh CO2:CH4 ratio of ~8:1 at low pressure, reduces with increasingpressure, and finally reaches a value of ~4:1. For rest of the coalsamples, this ratio change mildly within the experimental pressurerange. Overall, the CO2:CH4 ratio varies approximately between 1.5and 4. However, in terms of the Langmuir volume constants, the ratiovaries from 1.3 for the Mehaladih coal to 2.9 for the Kenda coal withthe lower-ranked Raniganj formation coals exhibiting consistentlyhigher ratio compared to the other higher-ranked coals. The resultsare in tune with other similar studies (Busch et al., 2003; Harpalaniet al., 2006; Krooss et al., 2002; Mastalerz et al., 2004; Shagafi, 2010).It is well-established that coals have higher sorption affinity for CO2

compared to methane and this may be attributed to a number ofreasons. Harpalani et al. (2006) and Mastalerz et al. (2004)enumerated the reasons citing the works of Cui et al. (2004), Clarksonand Bustin (1999), Milewska-Duda et al. (2000) and Reucroft and

mean vitrinite reflectance (Ro).

ratio with pressure.

Fig. 7. Hysteresis in sorption of CH4 and CO2.

295P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

Fig. 7 (continued).

296 P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

Sethuraman (1987). CO2 has smaller kinetic diameter and higheradsorption energy than methane. As a result, CO2 can even diffusethrough the ultramicropores in coal, which otherwise remaininaccessible to methane (Cui et al., 2004). While adsorption ofmethane molecules occur within the micropores of coal, CO2

molecules can get absorbed into the solid/elastic organic structurein addition to being adsorbed into the micropores (Milewska-Duda etal., 2000). Reucroft and Sethuraman (1987) suggested that up to 50%of the total uptake of CO2 could be due to CO2 dissolved (absorbed)into the coal structure for some coals. Clarkson and Bustin (1999)discussed the possibility of CO2 being adsorbed, in addition to themicropore surface, into the mesopores in multi-layers. Mastalerz et al.(2004) also observed that mesopores influence CO2 sorption. If themesopores play significant role in CO2 sorption, and as the proportionof mesopores decreases with rank in favour of micropores, CO2/CH4

ratio should decrease with rank, which is generally consistent withthe observations of this study as seen in Fig. 6.

4.3. Adsorption–desorption hysteresis

Complete adsorption and desorption plots of all coal samples, bothfor methane and carbon dioxide, are shown in Fig. 7. From the plots, itcan be observed that methane adsorption and desorption isothermsfor all samples almost fall on each other except those of the Bogra and

Fig. 8. Relationship of VL and PL wi

Mehaladih coals, which exhibit very small hysteresis. However,carbon dioxide adsorption and desorption isotherms show significanthysteresis for Bogra, Kenda and 18th Seam coals, while small tomedium hysteresis for the rest of the samples.

Hysteresis in adsorption–desorption curves were observed byearlier researchers. Busch et al. (2003) observed significant hysteresis,both for methane and CO2, for a set of dry Argonne premium coals andno specific trend could be observed with respect to maturity of thecoals. Similar observation was made by Tang et al. (2005) on dryPowder River Basin coal. Ozdemir et al. (2004) observed hysteresis inCO2 adsorption–desorption curves for some dry Argonne coals and thehysteresis was more for lower ranked coals. Harpalani et al. (2006)reported such hysteresis, both for methane and CO2, on a set ofmoisture-equilibrated American coals. They attributed this behaviorto changes in adsorbent properties/structure, to capillary condensa-tion in the adsorbentmicropores, or to loss of moisturewhile bleedinggas from the sample cell during desorption. Tang et al. (2005)postulated that the surface geometry heterogeneity may account forthe adsorption–desorption hysteresis. They mentioned the work ofSeri-Levy and Avnir (1993), who used Monte-Carlo simulations ofgas–solid systems to examine gas adsorption on rough surfaces ofvarious geometries and computed significant hysteresis in equilibriumisotherms as a result of path dependent configurations of adsorbedmolecules.

th ash for CH4/CO2 adsorption.

Fig. 9. Relationship of VL and PL with mineral matter for CH4/CO2 adsorption.

297P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

For methane, the present study, conducted on dry coals, showsalmost no hysteresis for most of the coal samples except for the Bograand Mehaladih samples. Therefore, the significant hysteresis in CH4

ad/desorption curves, as observed by Busch et al. (2003) and Tanget al. (2005), could not be observed in the present study. The verysmall hysteresis in some of the samples may well be due toexperimental uncertainty or due to less than perfect drying. For CO2

however, all coals show medium to very high hysteresis and no trendwith maturity is discernible. Other than the explanations cited abovefor adsorption–desorption hysteresis, the nature of sorption of CO2

molecules on coal may well be a reason for the hysteresis. As alreadymentioned, CO2 may also be absorbed/dissolved into the coalstructure and during desorption, only the adsorbed molecules comeout of the pore-spaces leaving behind the dissolved molecules in thecoal structure.

For CO2 only, except the Raniganj Formation coals, all samples showa slight increase in sorption during the first desorption step. Busch et al.(2003) observed similar phenomenon for some samples, both formethane and CO2. They attributed it to the small inaccuracies in theexperimental values/equation of state or to the changes in the coalvolume due to swelling. In the present study, since no such increase insorption was observed with methane, changes in coal volume withswelling by CO2 may well be the reason for the phenomenon.

4.4. Relationship between sorption behavior and petrographic composition

As can be seen from Figs. 3 and 4, the nature of the adsorptionisotherms in experimental pressure range for all coal samples is notthe same. Isotherms of some coals have attained saturation, some aremildly increasing, and the others are steeply increasing with pressure.The calculated Langmuir constants, VL and PL, were correlated withthe ash and maceral compositions of all coal samples.

VL and PL values for CH4 and CO2 against ash content are plotted inFig. 8. It can be seen from the graphs that, except for the SKAC3

Fig. 10. Relationship of VL and PL with fixed

sample, ash has a negative impact on the maximum gas adsorptioncapacity and Langmuir pressure for all coals. Except for the SKAC3sample, an increase in ash content by 22% reduces the sorptioncapacity by 20 mL/g for CH4 and 46 mL/g for CO2. On an average, it canbe stated that 1% increase in ash reduces the sorption capacities by0.9 mL/g formethane and 2.0 mL/g for CO2. Levy et al. (1997) reportedthat, for Bowen basin coal, the average decrease in methaneadsorption capacity for 1% increase in ash was 0.38 cm3/g. Studiesby Gurdal and Yalcin (2000) and Laxminarayana and Crosdale (1999)match well with this negative relationship.

The same VL, PL values were plotted with mineral matter contentand the results are shown in Fig. 9. It can be seen that the graphs showthe same characteristics as for ash, i.e., decreasing adsorptioncapacities with increasing mineral matter in coal for all samplesexcept SKAC3. For 1% increase in mineral matter, sorption capacitydecreases ~1 mL/g formethane and ~2 mL/g for CO2. However, similarwork by Mastalerz et al. (2004) showed a poor correlation betweenmineral matter and the sorption capacities of Indiana coals. All theseresults can be attributed to the fact that gas is adsorbed by the organicmatter only and not by the inorganic/mineral content, which act asdiluents in coal.

VL and PL values, both for the CH4 and CO2, against fixed carboncontent of the samples are plotted in Fig. 10. As seen from the graphsin Fig. 10, with an exception of SKAC3, the adsorption capacitygenerally increases as the fixed carbon content in coal increases.SKAC3 coal, although has the lowest carbon content, shows a veryhigh adsorption. For the rest of the coals, an average 15% increase infixed carbon content increases sorption capacity by 15 mL/g formethane and by 40 mL/g for CO2. However, Gurdal and Yalcin (2000)found a highly scattered relationship between fixed carbon and gasadsorption capacity. On the other hand, Levy et al. (1997) have shownthat there is a continuous increase of methane adsorption capacitywith increasing fixed carbon values of the moisture-equilibratedBowen Basin coals in Australia.

carbon content for CH4/CO2 adsorption.

Fig. 11. Relationship of VL and PL with vitrinite for CH4/CO2 adsorption.

298 P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

VL and PL values of all samples against their vitrinite content areplotted in Fig. 11. The figure shows a general increasing polynomialrelationship of VL and PL with vitrinite content, both for CH4 and CO2.It can be seen from the graphs that roughly a 60% increase in vitrinitecontent increases the sorption capacities by ~20 mL/g and 50 mL/g formethane and CO2, respectively.

The effect of vitrinite content on gas sorption capacity is not clearas the experimental results throughout the world show significantvariations. Earlier work of Crosdale et al. (1998) reported thatvitrinite-rich bright coals have significantly higher gas adsorptioncapacity than that of dull coal samples of same rank, though somevariation to this observation exists and this variation in adsorptioncan be related to both coal type and rank dependent pore/microporestructure development. Chalmers and Bustin (2007) observed a veryweek correlation between vitrinite content and methane adsorptioncapacity for sub-bituminous coals. However, they also observed thatbright bituminous coals have higher gas capacities than their dullcounterparts. Mastalerz et al. (2004) found no strong relationshipbetween gas adsorption capacity and vitrinite content. The study alsopointed out the micropore dependency and its un-correlation withadsorption capacity. Lamberson and Bustin (1993) and Levine (1993)mentioned in their work that methane sorption capacity increaseswith increasing vitrinite content. Crosdale et al. (1998) observed a U-shaped curve for adsorption capacity with vitrinite content andcommented that the decrease in adsorption capacity in lower range ofvitrinite content for high to low volatile bituminous coal can beattributed to plugging of the micropore system. However, studiesconducted previously on Indian coals reported that the vitrintecontent and adsorption capacity are uncorrelated (Laxminarayanaand Crosdale, 2002).

To investigate the relationship between sorption behavior of thecoals and their inertinite content, VL and PL values are plotted againstinertinite content in Fig. 12. It can be seen from the figure that theinertinite content shows a mild, negative relationship with both theLangmuir constants VL and PL for all coals. Like vitrinite content, the

Fig. 12. Relationship of VL and PL with

effect of inertinite content on gas sorption capacity is not clear andsignificant variation exists. Earlier, Ettinger et al. (1966) reported thathigher methane adsorption capacity can be associated with theinertinite-rich coal where as Faiz et al. (1992) and Gurdal and Yalcin(2000, 2001) observed no correlation between them.

5. Conclusions

The study presented here provided experimental results of CH4

and CO2 pure gas sorption on fourteen dry Indian bituminous coalsamples. The following conclusions can be drawn from the study:

1. Under similar temperature and pressure conditions, coals exhibithigher affinity to CO2 compared to methane and the ratio of CO2 tomethane sorption varied from 2:1 to 4:1 and the ratio decreaseswith pressure.

2. The ratio of CO2:CH4 adsorption decreases with increasing rank.3. Sorption capacities of the coals, both for CO2 andmethane, indicate

a U-shaped trend with rank.4. Significant hysteresis is observed between the adsorption and

desorption isotherm curves for CO2. However, such hysteresis isinsignificant or absent when methane is the sorbent gas.

5. Ash and mineral matter in coal act as a diluents to significantlyreduce the adsorption capacity.

6. The effect of fixed carbon on sorption capacity is positive for mostof the coal samples chosen for the study.

7. Increase in vitrinite content increases the sorption capacity of thecoals, both for methane and CO2.

8. Inertinite content indicates a negative impact on gas sorptionalthough the correlation is not strong enough to draw a conclusion.

Acknowledgements

We are thankful to BP International and Department of Science andTechnology, Government of India for funding of the research work.

inertinite for CH4/CO2 adsorption.

299P. Dutta et al. / International Journal of Coal Geology 85 (2011) 289–299

The comments/suggestions of the reviewers and that of Dr. ÖzgenKaracan are highly appreciated.

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