Adsorption behavior ofcarbon dioxide and methane in bituminous coal:A molecular simulation study☆

Jing You ,LiTian ,Chao Zhang ,Hongxing Yao ,Wu Dou ,Bin Fan ,Songqing Hu ,*

1 Production Engineering Research Institute in HuabeiOil field Company ofCNPC,Renqiu 062552,China

2 College ofScience,China University ofPetroleum(East China),Qingdao 266580,China

3 Huabei Oil field ShanxiCoalbed Methane Exploration and Development Branch ofCNPC,Changzhi048000,China

4 Huabei Oil field ChangzhiCoalbed Methane Exploration and Development Branch of CNPC,Changzhi046000,China

1.Introduction

Carbon dioxide(CO2)is the main greenhouse gas resulting the globalwarming,which has been recognized as one of the key environmental issues.Due to the environmentalconcerns,a lotofresearch efforts have been made in the field of CO2capture and storage(CCS).Currently,the main storage media for CO2is geologicalmedia,such as oiland gas reservoirs,deep saline formations,or unminable coalseams[1].Among all storage media mentioned above,injecting CO2into unminable coal seams is considered to be a promising method for sequestering CO2.During the sequestering process,injected CO2can also simultaneously displace the adsorbed methane in the coal seams.Therefore,this method is called enhanced coalbed methane(CO2-ECBM)[2,3].

Although CO2-ECBMhas been applied in the United States,Canada,Australia and China,the fundamentals of this technology stillneed to be studied in order to enhance the understanding of the recovery process.Recently,lots oflaboratory experiments have been conducted to study the adsorption behavior of CO2and CH4on coals[4–10].The relationship between the properties ofcoaland the CO2adsorption capacity was investigated by Day et al.[5]on 30 different types ofcoals.Results showed that the adsorption capacity of CO2depends on the rank of coal.It can be minimum when the vitrinite re flectance of the coalis approximately 1.2%.Ottiger etal.[4]measured the adsorption isotherms of pure CO2and CH4on dry coals at 45 and 60°C.They found that there was more CO2adsorbed on coal than CH4in all cases.In addition,Zhang et al.[6]and Mastalerz et al.[7]investigated the adsorption of CO2/CH4mixtures on different ranks of coal.It was found that the preferentialadsorption ratio of CO2decreases with increasing pressure for all coals.With the only exception of anthracite,the preferential adsorption ratio of CO2also decreases with the increase of the coal rank.Although many experiments have been performed in the laboratory to study the adsorption of CO2and CH4on the coal,there is very scarce literature on the effect of geological conditions to assess the CO2storage and CH4production behavior.The laboratory experiments also could notdirectly demonstrate the microscopic adsorption mechanism and displacement during the CO2-ECBMprocess.

Molecularsimulation is a usefultoolwhich can be applied to explore the microscopic adsorption mechanism and displacement interactions for porous materials.Many molecular simulations have been performed to study the adsorption behavior of CO2and CH4on carbon-based porous materials under various conditions[1,11–18].Huang etal.[12]performed grand canonicalMonte Carlo(GCMC)simulations to investigate the effectoftemperature,pressure and pore size on the adsorption behavior of CO2/CH4mixtures in carbon nanotubes(CNTs).They found that CO2in the binary CO2/CH4mixtures was preferentially adsorbed in CNTs.They also found thatthe effectoftemperature and pressure on the adsorption behavior of the binary mixtures was dependent on the diameters of the CNTs.Tenney etal.[15]and Liu et al.[16]investigated the in fluence of surface heterogeneity on the adsorption of CO2in the functionalized graphitic slit-shape pores.The results of their work showed that the amount of adsorbed CO2increased with increasing concentration ofoxygen atoms on the surface.The adsorption of CO2was also enhanced on the heterogeneous surface compared with that on the homogeneous graphite surface.To investigate the effect of surface heterogeneity on the adsorption of CO2/CH4,CO2/N2,and CO2/H2O mixtures,Liu et al.[18]performed the GCMC simulations to calculate the adsorption selectivity of CO2in carbon-based systems with oxygen functionalized surfaces.They found that oxygencontaining functionalgroups could enhance the adsorption selectivity of CO2relative to CH4and N2,whereas water vapor was always adsorbed preferentially over CO2.For all molecular simulations mentioned above,the carbon-based porous materials were employed and intend to represent coals,which have a large discrepancy with actual coals and could notaccurately demonstrate the adsorption behavior of CO2and CH4in naturalcoals.Therefore,it is necessary to construct a more reasonable coal model to obtain some basic insights into the adsorption behavior of CO2and CH4in coals.

This paper aims to investigate the adsorption behavior of CO2and CH4in the bituminous coalat various conditions during the CO2-ECBM process.First,an amorphous bituminous coalmodelwas built by using molecular dynamic(MD)simulations and characterized by the simulation of nitrogen adsorption at 77 K.The bituminous coal model was then used to calculate the adsorption isotherms and isosteric heat of adsorption curves for pure CO2and CH4,and the adsorption selectivity of CO2in the binary CO2and CH4mixture by using GCMC simulations.

2.Computational Details

2.1.Coalmolecular model

Coalis a complex,strained,heterogeneous,cross-linked macromolecular solid[19],exhibiting an amorphous molecular structure[11].Many molecular models for bituminous coalhave been developed on the basis of the results of physical and chemical analysis.The Wiser coalmolecular model[20,21]is one of the typicalmolecular models,which is considered as a reasonable molecular modelfor bituminous coal(Fig.1).In our simulations,the methylis used to saturate the dangling bonds in the Wiser coalmolecular model.The composition of the bituminous coalin the builtmodelis 78.3 wt%carbon,11.2 wt%oxygen,5.8 wt%hydrogen,3.2 wt%sulfur and 1.4 wt%nitrogen,which is very close to the naturalbituminous coal.

An appropriate force field is crucialto molecularsimulation,which is needed be able to describe the interaction between atoms and quantitatively validate the experimental results.In the reported studies on coals,Dreiding[21–24],consistent valence force field(CVFF)[25],and condensed-phase optimized molecular potentialfor atomic simulation studies(COMPASS)[14]were employed to calculate the density of the coalmodel.The calculated bituminous coaldensities through different force fields and the experimental true density of coal are shown in Table 1.From the comparison of the differences between these simulated values and the experimental one,it can be clearly seen that the density computed through COMPASS force field is extremely close to the true value.In addition,Yang et al.[26]employed COMPASS force field to predict various thermophysical properties of smallinorganic molecules under a very broad range of conditions,and an excellent agreementhas been obtained between the calculated and experimental results.Therefore,in the presentwork,the Wiser coalmolecular model and the COMPASS[27]force field were used to presentthe bituminous coaland to describe the atomic interactions between gas fluids and coal,respectively.

Table 1 Bituminous coaldensity calculated by different force fields

In this work,fully atomistic representation was used for CH4,CO2,N2and coalmodel.In the atomistic simulations,the COMPASS force field was used to describe the intermolecular and intramolecular interactions.In order to validate the potentialmodel,a series of molecular simulations were performed to calculate the bulk density of the pure fluid(CO2and CH4)with the pressure varying from 0 to 20 MPa at a temperature of 318.15 K.Fig.2 shows the bulk densities of CO2and CH4obtained by molecular simulations,as wellas the corresponding experimentalvalues[29].It can be seen that the simulated data are in good agreement with the experimental values.All L-J parameters were taken from the COMPASS force field for the atoms of the fluids and coalmodel.

Fig.1.Wiser coalmolecular model[20,21].

Fig.2.Bulk densities of CO2 and CH4 obtained by molecular simulation and experimental state equation[28]at a temperature of318.15 K.

2.2.Grand Canonical Monte Carlo(GCMC)simulations

Grand Canonical Monte Carlo(GCMC)simulations of N2adsorption at 77 K(for the purpose ofcharacterization)as wellas of CO2/CH4adsorption at 318.15 K in the bituminous coal model were performed.GCMC simulations of equimolar CO2/CH4mixtures adsorption were also performed at the temperature ranging from 300 K to 340 K with the bituminous coalmodel.In addition,the CO2/CH4mixture adsorption with differentbulk CO2mole fractionsatdifferentgeologicaldepths was also considered.GCMC technique is a stochastic method thatsimulates a system having a constant chemicalpotentialμA(A=CO2,CH4and N2),pore volume V and temperature T.The absolute adsorption isotherm is given by the ensemble average of the number of each adsorbate molecule as a function of the fugacity fAof the reservoir[17].In each GCMC simulation,2×107con figurations were generated.The former 107con figurations were discarded to guarantee equilibration,and the latter 107con figurations were used to calculate the ensemble averages.AllGCMC simulations were performed using a cubic box with the size of 4 nm×4 nm×4 nm,and a cutoffradius of1.85 nm was used for the L–J potentialbetween allatoms.CO2,CH4and N2molecules and coalmodel were kept rigid in the molecular simulations.

3.Results and Discussion

3.1.Construction and characterization of the bituminous coalmodel

3.1.1.Construction of the bituminous coalmodel

Molecular mechanics(MM)and molecular dynamics(MD)simulations were employed to construct the bituminous coalmodel(Fig.3).Two Wiser coalmolecular models were first packed in a periodic cube to construct an amorphous cell.This amorphous cellwas then minimized and relaxed by annealing dynamics in order to search for the global energy-minimized con figuration.At last,MD simulation was run in an NPT ensemble(i.e.,constant atom number N,pressure P=101.325 kPa and temperature T=298 K)so that the density and the energy could fluctuate around some stable values,indicating that the system reached the state of thermodynamic equilibrium[14].The COMPASS force field was employed to carry out MD simulations.The result shows that the equilibrium density of the bituminous coal model was 1.20 g·cm-3,which is in fair agreement with the experimentaltrue density[21].

Fig.3.Molecular con figuration of the bituminous coalmodel.The atoms are colored as follows:C,gray;O,red;N,blue;S,yellow;and H,black.

3.1.2.Characterization of the pore structure ofbituminous coalmodel

In order to characterize the pore structure ofbituminous coalmodel,the simulated N2adsorption isotherm at 77 K on the bituminous coal model was performed,as shown in Fig.4.p0is the bulk saturating vapor pressure for N2at 77 K.The amount of adsorbed N2increases rapidly at relatively low pressure due to the very smallpore sizes in the bituminous coalmodel.The speci fic surface area(S)and the pore volume(Vp)of the adsorbed N2(Table 2)were calculated by the BET method[29]and hard sphere probe molecule method,respectively.In the probe molecule method,a hard sphere probe molecule with the diameter of a nitrogen molecule(0.36 nm)was used,and the hard sphere probe molecule is rolled across the surface of the matrix atoms to calculate the speci fic area and the pore volume.From Table 2,it can be seen thatthe values obtained by above two methods wellmatch each other.Meanwhile,the probe molecularmethod presents a largerspecific surface area and a relative smaller pore volume.

Fig.4.Simulated adsorption isotherm of N2 at 77 K in the bituminous coalmodel.

Table 2 Characterization results for the bituminous coalmodel

Fig.5 shows the pore size distribution obtained in the bituminous coal model using the method proposed by Gelb and Gubbins[28].In this method,the hard sphere probe molecules with different diameters were used to calculate the cumulative pore volume curve,and the derivative of the cumulative pore volume curve with respect to the pore diameter gave the pore size distribution.From Fig.5,allpore sizes were found to be smaller than 1 nm.In combination with the results shown in Table 2,the pores in the bituminous coalmodelconsistofmicropores.Itiswellknown thatthe adsorption ofCO2and CH4in naturalcoalmostly takes place in micropores which have diameters ranging from 0.4 to 2 nm and low permeability[30]due to very high adsorption energies ofmicropores.Zhang etal.[31]measured the pore diameter distribution of Nantong bituminous coal.They found that the diameters of the coal range from 0.6 to 2 nm.Therefore,in terms ofadsorption sites,the bituminous coalmodelcan be used to describe the adsorption behavior of fluids in bituminous coal.

Fig.5.Simulated pore size distribution of the bituminous coalmodel.

3.1.3.Characterization of the adsorption behaviorofbituminouscoalmodel

Fig.6.Excess adsorption isotherms of CH4 and CO2 measured by experiments and molecular simulations in bituminous coalat 318.15 K.

In order to further verify the rationality of the bituminous coal model,we compared the simulated excess adsorption isotherms of CH4and CO2to experimental ones that measured by Dongyong Li et al.[32]at 318.15 K,as shown in Fig.6.Our simulated results cannot be directly compared with the experimentaldata because the adsorbed amounts of both CO2and CH4measured by Dongyong Li are excess amounts.The output from GCMC simulation is the absolute amount adsorbed which is the totalnumber ofmolecules presentinside the adsorbent[33].The excess amount adsorbed is the absolute amount adsorbed minus the amount that would occupy the microporosity if the fluid was at its bulk density in the micropores.The excess amount adsorbed could be calculated by the equation as follows:[33]

whereρbulkis the fluid density in the bulk phase at the same temperature and pressure for adsorption(which can be obtained from the library of thermodynamic data or obtained from the simulated data in Section 2.2)and Vporeis the pore volume of the bituminous coal model which can be obtained using the computational method described in Section 3.1.

From the comparison,it can be seen that the simulated excess adsorption isotherms of CH4and CO2in the bituminous coalmodel are qualitatively consistent with the experimental results.Although the simulated results were calculated in micropores and that the experimentalvalues were measured in both micropores and mesopores,the predicted and experimentalcurves presentsimilartrends.This suggests that the most of the CO2and CH4adsorption in naturalcoaltakes place in micropores.Therefore,the bituminous coalmodelis a reasonable coal modelwhich could be used to describe the adsorption behaviors ofCO2and CH4in bituminous coal.

3.2.Adsorption ofpure carbon dioxide and methane

3.2.1.Adsorption isotherm

The adsorption isotherm is crucialto estimate the adsorption capacity of CO2and CH4in the coalreservoir under different temperatures and pressures.Hence,the simulations ofadsorption isotherms ofpure CO2and CH4were performed at318.15 K as this temperature is related to the field of Enhanced Coal Bed Methane(ECBM)recovery[1].In the GCMC simulations,the absolute amounts adsorbed as a function of fugacity of CO2and CH4were calculated,and then the fugacity was converted into pressure by using the Peng–Robinson(PR)equation.Fig.7 depicts the simulated adsorption isotherms ofpure CO2and CH4in the bituminous coalmodelat 318.15 K.Both adsorption isotherms are of the type I according to the IUPAC classi fication[34].At lower pressure,the adsorbed amount of CO2increases more rapidly with increasing pressure compared with that of CH4.Such a difference is owing to the fact that the interactions of micropores with CO2are stronger than that with CH4.At higher pressure,the adsorbed amounts ofboth CO2and CH4tend to reach a plateau as the micropores get filled.When the pressure increases up to 15 MPa,the maximum adsorbed amount for CO2and CH4is approximately 1.41 and 1.07 mmol·g-1,respectively.Itsuggests thatthe adsorption amount of CO2in micropores is much larger than that of CH4at the same conditions.

Fig.7.Simulated adsorption isotherms for CO2 and CH4 in bituminous coal model at 318.15 K.

3.2.2.Isosteric heatofadsorption

The isosteric heat of adsorption(Qst)as a function of the absolute amount adsorbed for CO2and CH4is shown in Fig.8.Overall,Qstdecreases with increasing adsorption amount for both CO2and CH4in the bituminous coal model.For the lowest absorbed amount,Qstfor CO2and CH4is 33.76 and 24.72 kJ·mol-1,while it decreases to 30.80 and 23.62 kJ·mol-1forthe largestabsorbed amount,respectively.This decrease may be due to the presence ofheteroatoms functional groups leading to the heterogeneous surface of the bituminous coal model.Thus,at low adsorbed amount,CO2and CH4molecules preferentially interact with the active adsorption sites,which present a higher Qst.

Fig.8.Isosteric heat of adsorption Q st as a function of the absolute amount adsorbed for CO2 and CH4.

From Fig.8,the isosteric heat of CO2adsorption is also found to be larger than that of CH4adsorption at the same adsorbed density,which indicates that CO2has stronger interaction with bituminous coalthan CH4.This stronger interaction of CO2with bituminous coal leads to the larger CO2adsorption amount.In addition,the isosteric heat of adsorption for CO2exhibits a more signi ficant decrease than that for CH4,which suggests that the heterogeneity surface of the bituminous coal model has a more obvious in fluence on the CO2adsorption.

3.2.3.Radialdistribution functions(RDF)

To explore the structuralinformation for the adsorbed CH4and CO2in the bituminous coal model,the same number of CH4and CO2molecules(50 molecules each within the bracket)were added to the structure at 318.15 K.Fig.9 displays the typicalmolecular con figurations for the absorbed CH4and CO2in the bituminous coalmodel.The interaction energy value of the typical molecular con figuration for adsorbed CO2is lower than that for CH4,which indicates that CO2would be adsorbed more easily than CH4in the bituminous coalmodel.

The radialdistribution functions(RDFs)g(r)between CH4,CO2and heteroatoms(O,N,S)are shown in Fig.10.For CO2and O atoms,there are a distinct peak at r≈0.3 nm and a relatively weaker peak at r≈0.8 nm.The first distinct peak is generated by the stronger interaction between CO2and Oatoms,and the second peak is attributed to the interaction ofCO2with the Oatom ofa neighboring ligand.However,no obvious peak in g(r)for CH4and Oatoms is observed,which indicates a weaker interaction between CH4and O atoms.Furthermore,pronounced peaks in g(r)for CO2and N atoms are observed at r≈0.35 nm,and a broad peak for CH4and N atoms is observed at r≈0.5 nm.In addition,the pronounced peak at r≈0.35 nm for CO2and N atoms is sharper and higher than that for CH4and N atoms,which suggests a stronger interaction between CO2and N atoms.In the case of g(r)for CO2,CH4and S atoms,two pronounced peaks at r≈0.35 nm and r≈0.45 nm were also seen for CO2,CH4and S atoms.Similarly,the peak at r≈0.35 nm for CO2and S atoms is sharper and higher than that for CH4and S atoms as that seen in g(r)for CO2,CH4and N atoms,which indicates a stronger interaction between CO2and S atoms.

Based on the above RDF results,CO2have stronger interactions with heteroatom groups in the bituminous coalmodelthan CH4.This is consistent with the results of the adsorbed density and the isosteric heat discussed in Sections 3.2.1 and 3.2.2,respectively.From the above,the adsorption capacity of CO2is stronger than that of CH4in coal,which enables the occurrence of the CO2sequestration and CH4enhancing process(CO2-ECBM).

Fig.9.Typicalmolecular con figurations for the adsorption of CH4 and CO2 in bituminous coalmodelat318.15 K.(a)The molecules ofCH4 are in blue;(b)the molecules ofCO2 are in red.

Fig.10.Radialdistribution functions of CO2 and CH4 around heteroatoms in bituminous coalmodelat 318.15 K.(a)O;(b)N;and(c)S.

3.3.Adsorption ofcarbon dioxide and methane mixtures

In order to investigate the competitive adsorption,the adsorption of binary CO2/CH4mixtures in the bituminous coalmodelwas simulated using GCMC method.When a system consists of two adsorbates,the difference in the interaction energy of the adsorbates willlead to the enhanced adsorption of one adsorbate relative to the other.This is de fined as adsorption selectivity[12].The adsorption selectivity of CO2relative to CH4in a CO2/CH4mixtures is de fined as:[11–13]

where xCO2and xCH4are the mole fractions of CO2and CH4in the adsorbed phase,respectively.yCO2and yCH4are the mole fractions of the corresponding molecules in the bulk phase,respectively.According to the above equation,if the adsorption selectivity SCO2/CH4is greater than 1,it implies that CO2is preferentially adsorbed compared to CH4.In contrary,CH4is preferentially adsorbed.The effect oftemperature,pressure,bulk CO2mole fraction,and geologicaldepth on the adsorption selectivity of CO2is studied in this work.

3.3.1.Effect oftemperature and pressure

In this work,GCMC simulations of the adsorption of equimolar CO2/CH4mixtures at temperatures ranging from 300 K to 340 K in the bituminous coalmodelwere performed.The effects of temperature and pressure on the adsorption selectivity of CO2in the bituminous coalmodelare plotted in Fig.11.In the investigated ranges of pressure and temperature,the adsorption selectivity of CO2relative to CH4varies in the range of 2.75–6.77,which is always greater than 1,indicating that the adsorption of CO2is preferential to CH4in bituminous coal.Under alltemperatures,the adsorption selectivity of CO2initially decreases rapidly and then gradually with increasing pressure.Atlower pressure,the high selectivity means more CO2is adsorbed in the bituminous coalmodel.With increasing pressure,CO2tends to reach the maximum adsorbed density,while CH4continues to be adsorbed in the micropores,causing the selectivity to drop down.

In addition,atlowerpressures(i.e.,belowthe pressure of8 MPa),the selectivity shows an obvious decrease with increasing temperature.However,when the pressure is larger than 8 MPa,the selectivity is insensitive to temperature because the temperature and pressure could hardly exert their effects on the selectivity when the CO2and CH4molecules are all at supercritical conditions.This is consistent with the adsorption of CO2and CH4molecules in carbon nanotubes[12]and carbon slit-pores[13].

In order to validate the simulated results,the simulated adsorption selectivity of CO2was compared with the experimental ones that measured by Zhang etal.[6],as shown in Fig.12.From the comparison,it can be seen that the simulated adsorption selectivity of CO2in the bituminouscoalmodelisqualitatively consistentwith the experimental results,the selectivity is overestimated in the simulated results.Such a discrepancy might be due to that the simulated data were calculated in micropores and that the experimental values were measured in both micropores and mesopores.

Fig.11.Adsorption selectivity of CO2 in bituminous coal model for the various temperatures and pressures considered.

Fig.12.Experimentalselectivity ofCO2 in bituminous coalmeasured by Zhang etal.[6]at 323 K,and simulated selectivity of CO2 in bituminous coalmodelat 320 K.

3.3.2.Effects ofbulk CO2mole fraction and geologicaldepth

It should be noted that in the actualcoalbed methane(CBM)reservoir,during the CO2injection process,the coalmatrix is exposed to a mixture of CO2and CH4whose bulk composition varies when CO2molecules gradually displace CH4molecules.Moreover,the depth ofinjection site has an obvious in fluence on the CO2sequestration and the CH4enhanced recovery process.Therefore,in this work,the adsorption selectivity of CO2at a wide range of bulk mole fractions and injection site depths were also investigated.According to the real CBM reservoir conditions,five depths of the injection sites were selected(300 m,600 m,900 m,1200 m and 1500 m).It was supposed that the surface temperature and pressure are 288 K and 101.325 kPa,respectively.The corresponding temperatures and pressures at different geological depths were obtained by using an average geothermal gradient of 0.025 K·m-1and a hydrostatic pressure gradient.The various geologicaldepths and corresponding temperatures and pressures are listed in Table 3.

Table 3 Geologicaldepths and corresponding temperatures and pressures

The effects ofbulk CO2mole fraction and geologicaldepth on the adsorption selectivity of CO2in the bituminous coalmodelare shown in Fig.13.Overall,it can be seen that the selectivity of CO2decreases with increasing bulk CO2mole fraction and injection site depth.The variations of the selectivity of CO2adsorption indicate that excessive CO2injected into the coalseamwillcause the decreased ratio ofCO2sequestration,and there willbe less CO2sequestered in the deep injection sites than in the shallow ones for the same amount of replaced CH4.This is qualitatively consistentwith the simulated results reported by Brochard et al.[11]for CO2and CH4molecules adsorbed in the CS1000 model.However,the adsorption selectivity ofCO2in this work is quantitatively higher than the results reported by Brochard[11]at the same conditions.This is due to the stronger interactions of CO2molecules with the bituminous coalmodelresulted from the presence of heteroatom groups compared to thatwith CS1000 model.Moreover,the bituminous coalmodelused in this work is also closer to the actualcoalthan the CS1000 model.This also can accountforthe difference ofresults obtained from the above two models.

Fig.13.Adsorption selectivity ofCO2 in the bituminous coalmodelforthe various bulk CO2 mole fraction and geologicaldepths considered.

4.Conclusions

In this study,MMand MD simulations were employed to construct an amorphous bituminous coalmodel.

The pore structure of bituminous coalmodelwas characterized by the simulation ofnitrogen adsorption at 77 K,and then the modelwas con firmed to be reasonable by comparing the simulated results with the experimentalones.GCMC simulations were then performed to investigate the single and binary mixtures adsorption of CO2and CH4in the bituminous coalmodel.These investigations lead to the following conclusions.

For the single componentadsorption,the adsorption amountof CO2is much larger than that of CH4in the bituminous coal model at the same conditions due to the much greater isosteric heat of CO2adsorption compared with thatofCH4adsorption.Additionally,the RDF results indicate that CO2have stronger interactions with the heteroatom groups of the bituminous coalmodelthan CH4.Therefore,the adsorption capacity of CO2is stronger than that of CH4in coal,which enables the occurrence of the CO2sequestration and CH4enhancing process(CO2-ECBM).

For the binary adsorption of CO2/CH4mixtures,the simulation results show that CO2is adsorbed preferentially to CH4under the studied conditions.The adsorption selectivity of CO2initially decreases rapidly,and then reduces slowly with increasing the pressure.Atlowerpressure(i.e.,below the pressure of8 MPa),the selectivity shows an obvious decrease with increasing temperature.As the pressure becomes larger than 8 MPa,the selectivity is insensitive to temperature.In addition,the adsorption selectivity of CO2decreases with increasing bulk CO2mole fraction and injection site depth.This means that when excessive CO2was injected into the coalseam,the ratio of CO2sequestration will decrease.In addition,there willbe less CO2sequestered in the deep injection sites compared with that in the shallow ones for the same amount of replaced CH4.This suggests that we should optimize the temperature,pressure,geologicaldepth and bulk CO2mole fraction in order to obtain a satisfying adsorption selectivity of CO2in the practical CO2sequestration and CH4enhancing process,considering their effects on the competitive adsorption of CO2relative to CH4.

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