Methane Adsorption Characteristics and Adsorption Model

Article Cite This: Energy Fuels 2018, 32, 7485−7496

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Methane Adsorption Characteristics and Adsorption Model Applicability of Tectonically Deformed Coals in the Huaibei Coalfield Guanwen Lu,†,‡ Chongtao Wei,*,†,‡ Jilin Wang,†,‡ Gaoyuan Yan,†,‡ Junjian Zhang,†,‡ and Yu Song†,‡ †

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Key Laboratory of Coal bed Methane Resource & Reservoir Formation Process, China University of Mining & Technology, Ministry of Education, Xuzhou 221008, China ‡ China University of Mining & Technology, Xuzhou 221116, China ABSTRACT: Based on methane isothermal adsorption experiments, the supercritical methane adsorption characteristics of middle-rank tectonically deformed coals (TDCs) screened from the Huaibei coalfield were analyzed. The applicability of different adsorption models to different kinds of TDCs was discussed using the standard deviation method, and the mechanism of methane adsorption in TDCs was explored. The following results were obtained. First, the experimental maximum adsorption capacity of TDCs increases gradually with enhanced tectonic deformation, and the experimental maximum adsorption capacity of ductile TDCs is significantly higher than those of primary coal and brittle TDCs. The adsorption potential of TDCs gradually decreases with increasing adsorption space, and the adsorption potential of ductile TDCs is generally higher than those of primary coal and brittle TDCs. Second, for the applicability of adsorption models to TDCs, the highly applicable models of primary, cataclastic, and scaly coals are the Toth, Langmuir−Freundlich, and Dubinin−Astakhov models; the highly applicable models of schistose coals are the Langmuir, Toth, Langmuir−Freundlich, three-parameter Brunauer−Emmett−Teller, Dubinin−Radushkevich, and Dubinin−Astakhov models; the highly applicable models of wrinkle coals are the Toth, Langmuir−Freundlich, Dubinin−Radushkevich, and Dubinin−Astakhov models; and the highly applicable models of mylonitic coals are the Langmuir, Toth, Langmuir−Freundlich, expand-Langmuir, three-parameter Brunauer− Emmett−Teller, and Dubinin−Astakhov models. Third, the three-parameter Brunauer−Emmett−Teller model is suitable to study the adsorption state of TDCs. As the deformation degree increases, the adsorption state of TDCs transforms from monolayer unsaturated adsorption to multilayer adsorption. TDCs have larger adsorption potential and adsorption space with the enhancement of tectonic deformation, which increases the number of adsorption layers on the coal surface. ability is significantly stronger than that in high-rank coal.17 Ju et al. discovered that the mechanism of methane adsorption and desorption in TDC is different from that of primary coal because of differences in pore and chemical structures.18 Normally, we refer to a fluid whose temperature exceeds its critical temperature as a supercritical fluid, regardless of whether its pressure or density exceeds the critical value. One characteristic of supercritical fluids is that they will not be liquefied, irrespective of the pressure. The density of fluid will gradually increase with increasing pressure, and it will gradually approach its liquid density.8,19 Methane is generally in a supercritical state in MIAE as the critical temperature of methane is 190.6 K and the experimental temperature is 303 K. The viscosity of methane under a supercritical state is close to that of the gas. This will make the experimental adsorption capacity less than the actual adsorption capacity of coal, affecting the development of CBM and the accuracy of gas disaster prediction.20 In adsorption characterization methods, researchers use molecular mechanics, quantum chemistry, and other methods to study the adsorption mechanism of methane in coal macromolecules. Qiu et al. used multiple density functional theory methods to calculate the dominant adsorption position

1. INTRODUCTION Due to the transformation of multiperiod tectonic movements, tectonically deformed coals (TDCs) are widely developed in major coal basins in China.1,2 The special coal structure and pore and permeability characteristics of TDC cause its gas adsorption characteristics to be different from those of undeformed coal (hereafter referred to as “primary coal”), which affects the effect of coal and gas outburst prevention and coalbed methane (CBM) development.3,4 For example, the brittle deformation sequence of TDCs may be the target layer for CBM development, and the development of mylonitic coals may lead to the occurrence of coal and gas outbursts.5,6 Methane adsorption characteristics of TDC are influenced by the degree of coalification, macro- and microcompositions, deformation, pore structure, chemical structure, and other factors.2,7,8 Many studies on the adsorption characteristics of TDC have been done by various researchers, and methane adsorption mechanisms and influencing factors have mainly been revealed by methane isothermal adsorption experiments (MIAE).9−13 Using MIAE, Guo et al. demonstrated that moisture in coal suppresses the adsorption of methane.14 Wang et al. considered that, under the effect of entrapment and thermal evolution of the igneous sill, the metamorphic degree of coal increases, the coal pore structure develops, the gas adsorption capacity enhances, and gas outburst risk gradually increases.15,16 Jian et al. found that microcomponents in lowrank coal affect pore volume and that its methane adsorption © 2018 American Chemical Society

Received: April 19, 2018 Revised: June 13, 2018 Published: June 19, 2018 7485

DOI: 10.1021/acs.energyfuels.8b01397 Energy Fuels 2018, 32, 7485−7496

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Energy & Fuels

Figure 1. (a) Schematic map showing the Xuzhou-Suzhou arcuate duplex-imbricate fan thrust system and sampling location in the Huaibei area.8,31 (b) Strata column of coal measures in the study area.

of methane molecules on the carbon surface model.21 Ricca and Bauschlicher calculated the binding energy of methane molecules to the graphite surface and showed that methane molecules were first adsorbed in the groove and then on the outer layer of carbon nanotubes.22 Liu and Meng explored the law of energy changes in the adsorption process of coal using the adsorption energy calculation model. Their results indicate that the adsorption potential decreases with increasing adsorption space; adsorption is first carried out in the micropores (pore diameter < 2 nm) as the adsorption potential of micropores is much larger than that of mesopores (2 nm < pore diameter < 50 nm) and macropores (pore diameter > 50 nm).23,24 Parameters such as the critical desorption pressure of adsorbed gas, maximum gas content, and recovery rate of CBM can be obtained effectively through MIAE and adsorption model, and then CBM resources can be estimated. Therefore, the adsorption model is a key tool to obtain the above parameters.25 Previous studies have established a variety of adsorption models from different perspectives and evaluated the applicability of the corresponding models.26,27 Nounou et al. believed that the Langmuir model (L model) is suitable for the characterization of uniform surface adsorption, but this characterization is inadequate for nonuniform surfaces. The Freundlich model (F model) and Langmuir−Freundlich model (L−F model) are suitable for characterizing low-pressure adsorption, but their estimates will significantly deviate under high pressure.28 Yang et al. discovered that the more parameters the adsorption model has, the higher fitted accuracy the model possesses. The fitted results of the Toth model (T model) and L−F model are better than those of the

L model, F model, and Dubinin−Radushkevich model (D−R model). The T and L−F models have three parameters, while the L, F, and D−R models have two parameters.29 In summary, the study of the methane adsorption characteristics of coal is mainly developed for primary coal, but research is less developed for TDCs. The adsorption of methane in coal is supercritical adsorption, and its adsorption mechanism is essentially different from that of nonsupercritical gas. Previous research has resulted in controversies associated with the selection of adsorption models, and the criterion for the fitted applicability of models is not standardized. Studying the adsorption characteristics of different types of TDCs and the corresponding adsorption models is one of the keys to solving the above problems. At the same time, it has important theoretical and practical significance for the analysis of the mechanism adsorption characteristics in TDCs and the evaluation of CBM resources.25−27 Therefore, middle-rank (maximum vitrinite reflectance, Ro,max = 0.71−1.14%; photometer model MSP UV−vis 2000 in accordance with ASTM Standard D2797/D2797M-11)30 TDC samples were screened from the Xutuan and Qinan coal mines in the Huaibei coalfield in Anhui province, North China. Based on MIAE of the coal samples, the supercritical methane adsorption characteristics of TDCs were analyzed. The applicability of different adsorption models to different kinds of TDCs was discussed using standard deviation, and the microscopic mechanism of methane adsorption in TDCs was explored.

2. GEOLOGICAL SETTING The Huaibei coalfield is located at the southeast margin of the North China plate. The Xuzhou-Suxian arcuate double thrust7486


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Figure 2. Structure outlines of Xutuan (a) and Qinan (b) coal mines.8,31

Table 1. Basic Properties of Tectonically Deformed Coal Samplesa sample no.

coal mine

Pr01 Pr02 Ca03 Ca04 Sc05 Sc06 Sca07 Sca08 Wr09 Wr10 My11 My12

Xutuan Qinan Xutuan Qinan Xutuan Qinan Xutuan Qinan Xutuan Qinan Xutuan Qinan

TDC type primary coal cataclastic coal schistose coal scaly coal wrinkle coal mylonitic coal

Mad (%)

Aad (%)

Vdaf (%)

Ro,max (%)

TRD (g/cm3)

porosity (%)

Vmax (cm3/g)

1.86 1.28 1.27 1.14 1.47 1.54 1.26 1.46 1.02 0.91 1.18 1.23

36.51 26.8 12.3 17.75 9.49 7.43 12.93 12.07 8.56 8.38 12.31 9.32

39.27 38.43 35.58 32.3 33.67 33.25 34.22 30.1 36.05 31.94 34.38 35.05

0.85 0.9 0.85 0.79 0.75 0.71 1.07 1.04 1.01 1.07 0.9 1.14

1.6 1.52 1.37 1.43 1.36 1.34 1.38 1.38 1.33 1.33 1.38 1.37

4.38 4.61 1.46 4.2 3.68 2.24 2.17 3.62 3.76 2.26 2.9 2.92

4.59 5.53 5.67 5.88 6.83 6.94 7.24 7.91 8.09 8.83 10.32 11.32

a

Mad, air-dry basis moisture content, %; Aad, ash content of air-dry basis, %; Vdaf, volatile dry ash-free basis production rate, %; Ro,max, maximum reflectance of vitrinite; TRD, true relative density, g/cm3; Vmax, experimental maximum adsorption capacity for air-dried basis.

imbricate fan thrust fault system, which consists of linear compactly closed folds and thrust-imbricate fan faults, formed in the Yanshanian Orogenic Period and is the main structural feature of the Huaibei coalfield (Figure 1a).10,13,31 The strata in the Huaibei coalfield consist of the upper Proterozoic, Sinian, Cambrian, Ordovician, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary systems. Among them, the lower Permian Shanxi Formation and the middle Permian Lower Shihezi Formation are coal-bearing strata. The main mineable coal seams in this area are the No. 8 coal seam of the Lower Shihezi Formation and the No. 10 coal seam of the Shanxi Formation (Figure 1b).15,16,31 The Qinan and Xutuan coal mines are located in the south part of the Huaibei coalfield, and they are bounded to the north by the Suzhou faults (Figure 1a). The coal structure in the Suxian mining area manifests as strong structural deformation in the overlying system in the northeast wall of the Xisipo fault and relatively weak deformation in the

underlying system in the southwest wall of the Xisipo fault. The coal seams in the Huaibei coalfield have been strongly deformed as a result of multistage tectonic events. Thus, TDCs are widely developed in the Xutuan and Qinan coal mines (Figure 2).31

3. EXPERIMENTS AND MODELS 3.1. Samples and Experiments. Of the 12 middle-rank TDC samples studied, six samples are from the Xutuan coal mine and the others are from the Qinan coal mine (Figure 2). According to the structure-genetic classification system of TDCs which is put forward by Ju et al.,32 the samples can be divided into primary coal, cataclastic coal, schistose coal, scaly coal, wrinkle coal, and mylonitic coal (Table 1). The samples are composed of primary coal, cataclastic coal, schistose coal, scaly coal, wrinkle coal, and mylonitic coal (Table 1). As maceral type has a significant influence on methane adsorption, vitrain bands were sorted by hand to constrain the influence of organic type on the results of MIAE.18 The macro- to microscale deformation characteristics of various TDCs are shown in Figure 3. The TDCs resulting from brittle 7487


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Figure 3. (a) Deformation sequence and macro- and microdeformation characteristics of tectonically deformed coals.2,33 (b1) Pr01, primary coal, hand specimen, original bedding can be clearly seen. (b2) Pr01, primary coal, electronic microscope, ×100, simple fractures are sporadically developed. (b3) Ca04, cataclastic coal, hand specimen, primary structures are well-preserved. (b4) Ca04, cataclastic coal, electronic microscope, ×100, small fractures gradually develop. (b5) Sc05, schistose coal, hand specimen, original bedding is visible. (b6) Sc05, schistose coal, electronic microscope, ×100, one to two groups of dominant fissures develop normally, and fracture width is stable and uniform. (b7) Sca07, scaly coal, hand specimen, primary structure is illegible, and the coal body is almost completely covered by smooth and bright friction surfaces. (b8) Sca07, scaly coal, electronic microscope, ×100, shearing fissures develop extensively. (b9) Wr10, wrinkle coal, hand specimen, original bedding is illegible. (b10) Wr10, wrinkle coal, electronic microscope, ×100, friction surfaces and fissures develop densely with irregular curves or are twisted. (b11) My12, mylonitic coal, hand specimen, primary structure is illegible as the coal body is seriously broken. (b12) My12, mylonitic coal, electronic microscope, ×100, friction surfaces and fissures develop densely and chaotically, crushing the coal into small fragments and powders. models and their expanded models are divided into three categories, and the parameters of these models all have physical significance. The first category of adsorption model is based on adsorption kinetics, including the L model and its extended models, such as the F model, T model, L−F model, and expand-Langmuir model (E-L model).20,28 The basic assumptions of these models are that the adsorption heat is constant, the surface of the adsorbent is uniform, there is no interaction between the adsorbed molecules, and the molecular adsorption is mainly monolayer adsorption.25,35

deformation processes are cataclastic coal, mortar coal, and granulitic coal (in order of increasing brittle deformation).31,32 The TDCs resulting from shear deformation processes are (in order of increasing shear deformation) schistose coal (which forms from granulitic coal) and scaly coal with squamous structures.31,32 Under the effect of ductile deformation, wrinkle coal develops.33 It is noteworthy that the mylonitic coal associated with ductile deformation is developed through two different processes. The first type of mylonitic coal is formed from wrinkle coal as the result of strong rheomorphismin. The other is formed from scaly coal through plastic deformation (Figure 3). The MIAE analyses were carried out using an IS-100 high-pressure gas isothermal adsorption/desorption apparatus manufactured by Terra Tex in the United States. According to the Chinese national standard for MIAE (GB/T 19560-2008),34 samples were crushed to a size of less than −60 mesh. The samples prepared for adsorption analysis were first degassed at 303 K under high vacuum for >24 h to remove air, free water, and other gases. This temperature and time had been found to be sufficient to degas samples prior to MIAE analyses.2,8 The final pressure in high vacuum was stable at 0.1 MPa, indicating that the impurities were removed completely. The experimental temperature was 303 K, and the error was within 0.2 °C. The maximum pressure was approximately 8 MPa, and the pressure accuracy was 3.51 KPa. The adsorption equilibrium time was generally more than 12 h at each pressure point. The adsorption medium was methane gas with a purity of 99.99%. The experimental maximum adsorption capacity values of samples are shown in Table 1. 3.2. Models. 3.2.1. Adsorption Models. The study of adsorption models is not only to find a convenient mathematical expression for the relationship between adsorption capacity and adsorption conditions but also to understand the microscopic mechanism of adsorption. Although polynomial fitting has a higher correlation coefficient (R2) for the methane adsorption of coal, its parameters have no practical significance.19 In this paper, typical adsorption

L model:

V = VL

p pL + p

(1)

F model:

V = Kbpn

(2)

T model:

V=

VLKbp [1 + (Kbp)n ](1/ n)

(3)

L−F model:

V = VL

Kbpn 1 + Kbpn

(4)

E-L model:

V=

VLKbp 1 + Kbp + n Kbp

(5)

where V is the adsorption volume when the experimental pressure is p, m3/t; p is the experimental pressure, MPa; VL is the Langmuir volume, which represents the gas volume of coal adsorbed when the 7488


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Energy & Fuels unit mass of coal is in a saturated adsorption state at a given temperature, m3/t; pL is the Langmuir pressure, which indicates the pressure corresponding to the gas adsorption volume of a unit mass coal when it reaches half of the Langmuir volume, MPa; n is the ratio of the number of surface active sites in the pores of coal to the number of adsorbed methane molecules; and Kb is the binding constant. The second category of adsorption model is the Brunauer− Emmett−Teller (BET) multilayer adsorption model, which includes the biparameter Brunauer−Emmett−Teller model (B-BET model) and the three-parameter Brunauer−Emmett−Teller model (T-BET model).19 These models are actually extensions of the Langmuir monolayer adsorption model. The model assumptions are that the adsorption of molecules is multilayered, and when the molecules are adsorbed as a second layer, the first layer does not necessarily need to be saturated; the molecular adsorption heat of the first layer is constant, and the adsorption heat of the second layer and above is the liquefaction heat; and the absorption and desorption of adsorbates only occur on the surfaces of adsorbents exposed to gas.36

temperature, 303 K; and Tc is the critical temperature of methane, 190.6 K.39 When the adsorption volume is calculated according to the definition of the Gibbs equilibrium adsorption capacity, the excess adsorption capacity (actual adsorption capacity) needs to be converted into the absolute adsorption capacity (Gibbs equilibrium adsorption capacity).40 The conversion formula is

B-BET model:

where ρb is the boiling point density of methane, 0.424 cm3/g; Tb is the boiling temperature of methane at atmospheric pressure, 111.7 K; and T and ρad have the same meanings as in eqs 10 and 11, respectively. The best-fit values of the adsorption model parameters were calculated based on the Levenberg−Marquardt method using the firstOpt15pro software. This algorithm adopts the standard Levenberg−Marquardt and general global optimization mode, setting the convergence criterion to 1.00 × 10−10 and the maximum number of iteration steps to 1000, and the number of real-time control output is 20. The calculation of each parameter needs to be performed more than three times until the difference between the values of the two adjacent fitted parameters is within the specified range to ensure that the parameters are all greater than 0 and that the correlation coefficient R2 is greater than 0.95. 3.2.2. Adsorption Potential and Adsorption Space. Methane molecules are mainly adsorbed on the surfaces of nanopores in the coal matrix. Adsorption also occurs on the surfaces of pores with larger diameters, but nanopores have overall more surface area than larger pores for the same pore volume.41 During the process of isothermal adsorption, the thermodynamic energy of methane molecules changes in the adsorption field on the surface of the coal as the pressure increases. This is a macroscopic representation of the interaction between coal macromolecules and methane molecules. Adsorption is based on energy changes. In the process of methane adsorption, adsorption potential (ε) and surface free energy (γ) can be used not only to measure the molecular strength of coal matrix but also to reflect the law of energy changes.23 Assuming that the equilibrium pressure of the ideal gas is Pi, and the pressure of the adsorption layer is the saturated vapor pressure P0, the work (adsorption potential) ε per unit mass of the adsorbate converted from the nonadsorption phase to the adsorption phase is23

V=

VmCp (p0 − p)[1 + (C − 1)(p /p0 )]

Vad = Vex /(1 − ρg /ρad )

where Vad is the Gibbs equilibrium adsorption capacity, cm3/g; Vex is the measured equilibrium adsorption capacity, cm3/g; ρg is the density of the gas phase, g/cm3; and ρad is the density of the adsorption phase and is calculated by the empirical formula of the density of adsorption phase proposed by Ozawa et al.,25 which is 0.263 g/cm3. The empirical formula of the density of the adsorption phase is25 ρad = ρb exp[−0.0025(T − Tb)]

(6)

T-BET model:

V=

VmCp[1 − (n + 1)(p /p0 )n + n(p /p0 )n + 1] (p0 − p)[1 + (C − 1)(p /p0 ) − C(p /p0 )n + 1]

(7)

where Vm is the maximum adsorption capacity of monolayer in BET model, m3/t; C is a constant related to the heat of adsorption; p0 is the saturated vapor pressure, MPa; V, p, and n have the same meanings as in eqs 1−5. The third category of adsorption model is based on the adsorption potential theory, including the D−R model and the Dubinin− Astakhov model (D−A model). The adsorption potential theory reflects the change in the Gibbs free energy of the adsorbent after adsorbing a unit molar mass of the adsorbate.37 In contrast to the monolayer adsorption theory described by the L model, the Polanyi adsorption potential theory does not need to establish a physical model of the adsorption layer. The adsorption of methane molecules in micropores is different from the adsorption of monolayers or multilayers on mesopore or nonpore surfaces, and the pore volume is filled in order according to the adsorption potential. The adsorption potential theory is the basis of the Dubinin micropore filling theory, and it is the most mature and practical theoretical system for describing the gas phase adsorption behavior of micropore adsorbents, especially carbonaceous micropore adsorbents.38 D−R model:

V = V0 exp[−D ln 2(p0 /p)]

(8)

D−A model:

V = V0 exp[−D ln n(p0 /p)]

ε= (9)

ps = pc (T /Tc)

∫p

p0

i

where V0 is the volume of pores, m3/t; D is the constant associated with the net heat of adsorption; V, p, p0, and n have the same meanings as in eqs 6 and 7. Methane cannot be liquefied at supercritical temperatures. Therefore, the virtual saturated vapor pressure ps is used instead of saturated vapor pressure p0 to extend the B-BET, T-BET, D−R, and D−A models to supercritical conditions. In this paper, the Dubinin method (eq 10) is used to calculate the supercritical saturated vapor pressure ps, and the result is 11.69 MPa.39 2

(11)

ij p yz RT dp = RT lnjjjj 0 zzzz jpz p k i{

(12)

(13)

where ε is the adsorption potential, J/mol; pi is the equilibrium pressure obtained from the isothermal adsorption curve, MPa; R is the gas constant, 8.314J/(mol·K); and p0 and T have the same meanings as in eqs 6 and 10, respectively. The adsorption space ω is the parameter that characterizes the pore structure, and it represents the space occupied by gas in the adsorption state in coal.23 ω = Vad

(10)

M ρad

(14)

where ω is the adsorption space, cm3/g; M is the molecular weight of gas, g/mol; and Vad and ρad have the same meanings as in eq 11.

where ps is the supercritical saturated vapor pressure, MPa; pc is the critical pressure of methane, 4.62 MPa; T is the experimental 7489


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Figure 4. Typical isothermal adsorption curve types and experimental maximum adsorption capacity (Vmax) of tectonically deformed coals.

Table 2. Experimental and Fitted Maximum Adsorption Capacities of Different Adsorption Models VL (cm3·g−1) −1

Vm (cm3·g−1)

V0 (cm3·g−1)

sample no.

Vmax (cm ·g )

L

T

L−F

E-L

B-BET

T-BET

D−R

D−A

Pr01 Pr02 Ca03 Ca04 Sc05 Sc06 Sca07 Sca08 Wr09 Wr10 My11 My12

4.59 5.53 5.67 5.88 6.83 6.94 7.24 7.91 8.09 8.83 10.32 11.32

5.82 7.05 7.01 6.85 8.22 8.41 8.75 9.54 9.85 11.13 15.69 17.28

5.82 5.74 5.87 6.56 7.49 8.07 7.52 8.26 8.61 11.13 15.69 17.28

5.82 5.87 5.94 6.62 7.59 8.2 7.63 8.38 8.77 11.13 15.69 17.28

7.48 8.94 8.67 8.25 9.88 10.58 10.79 11.84 12.36 14.61 23.95 25.73

2.22 2.77 2.88 2.93 3.5 3.42 3.61 3.92 3.98 4.27 4.7 5.42

11.29 8.24 8.64 10.95 12.03 12.37 10.95 11.71 11.95 20.64 8.28 35.44

4.93 6.1 6.25 6.27 7.51 7.36 7.83 8.49 8.67 9.26 10.48 11.9

5.02 5.72 5.84 6.16 7.26 7.31 7.42 8.08 8.33 9.49 11.59 13.12

3

4. RESULTS AND DISCUSSION 4.1. Analysis of MIAE Results. MIAE were performed on the 12 samples. The experimental temperature was 303 K, and the maximum pressure was approximately 8 MPa. From Figure 4, it can be found that the isothermal adsorption curves of all samples belong to the type I adsorption curve classified by IUPAC,5,13 but there are obvious differences in the adsorption characteristics of different types of TDCs. The isothermal adsorption curves of different types of TDCs reveal three features. The isothermal adsorption curves of primary and cataclastic coals have maximum values when the pressure is between 5 and 6 MPa. When the pressure is higher than 5−6 MPa, the adsorption capacity begins to decrease (Figure 4a). The supercritical adsorption capability of these TDCs is the lowest. The adsorption capacities of schistose and scaly coals rise rapidly in the low-pressure stage (1−4 MPa), while it does not change much in the high-pressure stage (5−8 MPa) (Figure 4b). These kinds of TDCs have moderate adsorption capability. The adsorption capacities of wrinkle and mylonitic coals continue to increase with increasing pressure (Figure 4c). These TDCs have the highest adsorption capability.

In regard to the experimental maximum adsorption capacity, the largest are mylonitic and wrinkle coals, followed by scaly and schistose coals, and the lowest are primary and cataclastic coals (Figure 4d). An overall feature is that the experimental maximum adsorption capacity values of TDCs gradually increase with enhanced tectonic deformation, and the experimental maximum adsorption capacity values of ductile TDCs (mylonitic and wrinkle coals) are significantly higher than those of primary coal and brittle TDCs (wrinkle and cataclastic coals). 4.2. Fitted Maximum Adsorption Capacity. There are differences in the meaning of the fitted maximum adsorption capacity in different adsorption models. The fitted maximum adsorption capacity refers to the Langmuir volume (VL) in the first category model, the maximum adsorption capacity Vm of the BET equation in the second category model, and the pore volume V0 calculated by the adsorption model in the third category model. The MIAE data of 12 TDC samples were fitted using various adsorption models (Table 2). The fitted maximum adsorption capacities of different TDCs, which are calculated by different adsorption models, have a similar variation tendency. Taking the L model as an 7490


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Figure 5. Adsorption characteristic curves of different tectonically deformed coals.

example, the fitted maximum adsorption capacities of mylonitic and wrinkle coals are greater than those of scaly and schistose coals, and those of primary and cataclastic coals are the smallest. The fitted maximum adsorption capacities of TDCs gradually increase as the degree of deformation increases. The fitted maximum adsorption capacities calculated by other adsorption models also have a similar trend (Table 2), indicating that the fitted value and the experimental value of the adsorption capacity of maximum adsorption capacities have a similar variation tendency. Different adsorption models are used to fit the maximum adsorption capacity of the same category of TDCs, and some differences between the fitted results are found. Taking schistose coal as an example, in the first category model, the fitted maximum adsorption capacities calculated by the L, T, L−F, and E-L models are larger than the experimental maximum adsorption capacities of schistose coal (Tables 1 and 2). The fitted maximum adsorption capacities calculated by the L, T, and L−F models are similar, but they are significantly smaller than those calculated by the E-L model. In the second category model, the fitted maximum adsorption capacities calculated by the B-BET and T-BET models are smaller and larger than the experimental maximum adsorption capacities of coal, respectively. In the third category model, the fitted maximum adsorption capacities calculated by the D−R and D−A models are greater than the experimental maximum adsorption capacities, but the fitted maximum adsorption capacities calculated by the third category model are closer to

the experimental maximum adsorption capacities of coal than those calculated by the first category model and the second category model. Other kinds of TDCs also have a similar variation tendency (Table 2), which shows that different kinds of adsorption models have different fitted adaptabilities to TDCs. 4.3. Adsorption Potential and Adsorption Space. Based on the results of MIAE, the Polanyi adsorption potential theory and the adsorption space calculation formulas eqs 13 and 14 were used to calculate the adsorption space (ω) and adsorption potential (ε) of different types of TDCs, and the corresponding adsorption characteristic curves were made (Figure 5). These adsorption characteristic curves can be quantitatively fitted with second-order polynomials: ε = aω 2 + bω + c

(15)

where a, b, and c are constants obtained by second-order polynomial quantitative fitting; ε and ω have the same meanings as in eqs 13 and 14, respectively.23 It can be seen from Table 3 that the maximum adsorption potential is between 6.334 and 6.770 kJ/mol and the maximum adsorption potential of TDCs increases with the gradual increase in the coal deformation degree. At normal temperatures, the adsorption of methane by coal is physical adsorption. The force that causes physical adsorption is the van der Waals force that prevails between atoms and molecules and can be divided into electrostatic force, induced force, and dispersive force. For most molecules, the dispersion force is 7491


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The nine adsorption models adopted in this paper are nonlinear models. Although all the fitted correlation coefficients, R2, are higher than 0.95, they cannot be used to evaluate model applicability. Even for adsorption models that can be linearized, such as the L, F, and B-BET models, the parameters obtained after transformation do not reflect the fitted goodness of the original models. Therefore, the standard deviation S is introduced to quantitatively evaluate the applicability of the adsorption models.36

Table 3. Maximum Adsorption Potential and Maximum Adsorption Space of Tectonically Deformed Coal Samples sample no. Pr01 Pr02 Ca03 Ca04 Sc05 Sc06 Sca07 Sca08 Wr09 Wr10 My11 My12

TDC type primary coal cataclastic coal schistose coal scaly coal wrinkle coal mylonitic coal

εmax (kJ/mol)

ωmax (cm3/g)

6.334 6.350 6.353 6.457 6.584 6.592 6.601 6.661 6.664 6.692 6.735 6.770

0.013 0.016 0.016 0.017 0.019 0.020 0.020 0.022 0.023 0.024 0.028 0.030

S=

1 N−f

N

∑ (Ve − Vf )2 (16)

i=1

where S is the standard deviation, cm3/g; N is the number of data points; f is the number of model parameters; and Ve and Vf are the experimental adsorption capacity and the fitted adsorption capacity at equilibrium pressure, respectively, cm3/ g. The standard deviations of different adsorption models are shown in Table 3, with values between 0.15 and 2.80 cm3/g. The smaller the standard deviation is, the better the applicability of the model will be. According to the principle of small probability events, in the same kind of TDCs, if the difference between the standard deviation of the model and its lowest value is less than 0.05 cm3/g, higher than 0.05 cm3/g, or higher than 1.0 cm3/g, the model will have high applicability, medium applicability or low applicability, respectively.47 The applicability of the adsorption models to primary, cataclastic, and scaly coals is relatively consistent, in which the T, L−F, and D−A models have universal high applicability, the L, E-L, T-BET, and D−R models have medium applicability, and the F and B-BET models have low applicability. There are six highly applicable models (the L, T, L−F, T-BET, D−R, and D−A models), one medium applicable model (the E-L model), and two low applicable models (the F and B-BET models) for schistose coal. For wrinkle coal, the T, L−F, D−R and D−A models have high applicability, the L, E-L and T-BET models have medium applicability, and the F and B-BET models have low applicability. There are also many models adapted to the analysis of mylonitic coal, of which the L, T, L−F, E-L, T-BET, and D−A models have high applicability, the D−R model has medium applicability, and the F and B-BET models have low applicability (Table 4). For adsorption models, the standard deviations of the T, L− F, and D−A models are significantly smaller than those of other models (Table 4), and these models are highly applicable to all kinds of TDCs. For the T and L−F models, the parameter n is the ratio of the number of surface active sites in the pores of the coal to the number of adsorbed molecules, which indicates the nonuniformity of the adsorbent surface, and the fitted adsorption capacity of the model has a maximum

dominant: only for molecules with large dipole moments is the electrostatic force dominant; the induced force is usually small.42 Kaplan argued that the structure of coal is similar to that of graphite; the binding energy between graphitic aromatic carbon layers is approximately 5.4 kJ/mol, and the electronrelated energy ratio is approximately 80%.43 The binding energy is close to the average adsorption potential in this paper, indicating that the van der Waals force absorbed between coal and methane is mainly a dispersion force. The adsorption potential of ductile TDCs is larger than that of brittle TDCs (Table 3), as they are subjected to different dynamic metamorphisms caused by different mechanisms. In the brittle deformation process, rapid mechanical friction on the coal fracture surface generates thermal energy and causes changes in the chemical structure and composition of coal. In contrast, during the process of ductile deformation, damage to the coal structure is caused by the accumulation of local area strain energy.44−46 Figure 5 demonstrates that the adsorption potential of TDCs gradually decreases with increasing adsorption space. The maximum adsorption space of TDCs increases with increasing deformation intensity, and its variation tendency has good consistency with the experimental maximum adsorption capacity (refer to Tables 1 and 3). The adsorption space and adsorption potential of mylonitic and wrinkle coals are larger than those of other kinds of TDCs and are the result of the comprehensive effect of the pore structure and macromolecular structure of coal.1,3,5 4.4. Adaptability of Adsorption Models. In section 4.2, experimental data were processed using different adsorption models, and the obtained sample adsorption parameters were different from each other, indicating that various adsorption models have differing characterization accuracy for adsorption phenomena. In this section, the issue will be further analyzed.

Table 4. Standard Deviations of Different Adsorption Models, S (cm3·g−1) TDC type

L

F

T

L−F

E-L

B-BET

T-BET

D−R

D−A

primary coal cataclastic coal schistose coal scaly coal wrinkle coal mylonitic coal

0.27b 0.28b 0.30c 0.32b 0.28b 0.34c

1.70a 2.21a 2.55a 2.80a 2.70a 2.02a

0.18c 0.21c 0.32c 0.16c 0.24c 0.39c

0.20c 0.21c 0.33c 0.16c 0.25c 0.39c

0.36b 0.39b 0.41b 0.49b 0.39b 0.39c

1.11a 1.34a 1.54a 1.76a 1.70a 1.36a

0.31b 0.39b 0.33c 0.35b 0.31b 0.38c

0.26b 0.28b 0.29c 0.32b 0.26c 0.47b

0.18c 0.20c 0.30c 0.15c 0.22c 0.38c

a

Low applicability. bMedium applicability. cHigh applicability. 7492


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Figure 6. Parameter 1/n of different tectonically deformed coals.

value at high pressure, which fits well with the actual adsorption process of coal;29 therefore the T and L−F models have high applicability. It was found that the pore size distribution in coal is not strictly Gaussian distribution but Weibull distribution. The D−A model is based on this optimization of the D−R model, making it closer to the actual situation, so it has a better fitted applicability.48 The L, E-L, T-BET, and D−R models have generally medium applicability to all kinds of TDCs and have high applicability to mylonitic and schistose coals (refer to Table 4). The L model includes the assumption that adsorption takes place on an isotropic homogeneous adsorption surface. However, coal is a complex mixture of organic and inorganic materials, and the adsorption level of each active site on the adsorbent surface varies greatly. When the pressure is low (less than 2 MPa), the adsorption capacity of the pore surface of coal is heterogeneous, leading to poor fitness. As the pressure increases, this kind of energy inhomogeneity gradually decreases, and the model fitted applicability gradually improves.49 The T-BET model not only considers the nonuniformity of the adsorbent surface but also assumes that the number of adsorbed layers is finite, which is consistent with the actual situation, thus improving the fitting.49 The D−R model is a micropore filling model developed on the basis of the Polanyi adsorption potential theory. This model assumes that methane molecules fill the pore volume in turn based on the magnitude of adsorption potential in the micropores, without establishing a physical model of the adsorption layer.

The D−R model analyzes the process of methane adsorption by coal from the perspective of energy conversion and therefore has medium applicability.37 The F and B-BET models have low applicability to all kinds of TDCs, and their standard deviations are significantly larger than those of other models (refer to Table 4). The F model does not comply with Henry’s law at low pressures, and the calculation results increase with pressure at high pressure. However, in the actual adsorption process, the saturated adsorption capacity maintains a constant value after the pressure increases to a certain degree, and it does not continue to increase as the pressure increases.29 The B-BET model assumes that adsorption takes place on an isotropic uniform adsorption surface and that the number of adsorption layers is infinite, which is inconsistent with actual conditions.49 Therefore, the fitted effects of the F and B-BET models are poor. In summary, for TDCs, the first category of adsorption models has the best applicability, including two highly applicable models (the T and L−F models), two medium applicable models (the L and E-L models), and one low applicable model (the F model). The applicability of the third category models takes second place, including one high and one medium applicable model (the D−A and D−R models). The models of the second category of adsorption models have the lowest adaptability, including one medium applicable model (the T-BET model) and one low applicable model (the B-BET model). 7493


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Energy & Fuels 4.5. Adsorption State of TDCs. The adsorption state of TDCs can be expressed by the number of adsorption layers and adsorption saturation. The number of adsorption layers refers to the number of methane molecules adsorbed on the surface of TDCs. The number of adsorption layers ≤1 indicates monolayer adsorption, and >1 means multilayer adsorption.42 The traditional methane adsorption saturation of coal refers to the ratio of the volume of methane adsorbed by coal to the pore volume of coal, but the adsorption saturation here refers to the ratio of the number of adsorbed methane molecules to the number of surface active sites in the pores of coal.8 The adsorption saturation 2) occurs (Figure 6e,f). In summary, the T-BET model is suitable to study the adsorption state of TDCs. As the degree of deformation increases, the adsorption state of TDCs transforms from monolayer unsaturated adsorption to multilayer adsorption. TDCs have larger adsorption potential and adsorption space with the enhancement of tectonic deformation, which increases the number of adsorption layers on the coal surface.

5. CONCLUSIONS Based on MIAE, the supercritical methane adsorption characteristics of middle-rank TDCs screened from the Huaibei coalfield were analyzed. The applicability of different adsorption models to different kinds of TDCs was discussed using the standard deviation method, and the microscopic mechanism of methane adsorption in TDCs was explored. The main conclusions are as follows. 1. The experimental maximum adsorption capacity values of TDCs gradually increase with enhanced tectonic deformation, and the experimental maximum adsorption capacity values of ductile TDCs are significantly higher than those of primary coal and brittle TDCs. The adsorption potential of TDCs gradually decreases with increasing adsorption space, and the adsorption potential of ductile TDCs is generally higher than those of primary coal and brittle TDCs. 2. For the applicability of adsorption models to TDCs, the most applicable models for primary, cataclastic, and scaly coals are the T, L−F, and D−A models; the most applicable models for schistose coals are the L, T, L−F, T-BET, D−R, and D−A models; the most applicable models for wrinkle coals are the T, L−F, D−R, and D−A models; and the most applicable models for mylonitic coals are the L, T, L−F, E-L, T-BET, and D−A models. 3. The T-BET model is suitable to study the adsorption state of TDCs. As the degree of deformation increases, the adsorption state of TDCs transforms from monolayer unsaturated adsorption to multilayer adsorption. TDCs have larger adsorption potential and adsorption space with the enhancement of tectonic deformation, which increases the number of adsorption layers on the coal surface. 4. In the CBM reservoirs where TDCs are ubiquitous, the T, L−F, T-BET, and D−A models can be used to calculate the amount of methane adsorbed and then to evaluate the CBM resources. The T-BET model can be used to study the number of adsorption layers of methane molecules on the coal surface, and then to analyze the mechanism adsorption characteristics in TDCs. 7494


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(15) Wang, L.; Cheng, L. B.; Cheng, Y. P.; Liu, S. M.; Guo, P. K.; Jin, K.; Jiang, H. N. A new method for accurate and rapid measurement of underground coal seam gas content. J. Nat. Gas Sci. Eng. 2015, 26, 1388−1398. (16) Wang, L.; Cheng, L. B.; Cheng, Y. P.; Yin, G. Z.; Xu, C.; Jin, K.; Yang, Q. L. Characteristics and evolutions of gas dynamic disaster under igneous intrusions and its control technologies. J. Nat. Gas Sci. Eng. 2014, 18, 164−174. (17) Jian, K.; Fu, X.; Ding, Y. M.; Wang, H. D.; Li, T. Characteristics of pores and methane adsorption of low-rank coal in China. J. Nat. Gas Sci. Eng. 2015, 27, 207−218. (18) Ju, Y. W.; Jiang, B.; Hou, Q. L.; Tan, Y. J.; Wang, G. L.; Xiao, W. J. Behavior and mechanism of the adsorption/desorption of tectonically deformed coals. Chin. Sci. Bull. 2009, 54, 88−94. (19) Yu, H. G.; Fan, W. T.; Sun, M. Y.; Ye, J. P. Study on fitting models for methane isotherms adsorption of coals. J. China Coal Soc. 2004, 29, 463−467. (20) Men’shchikov, I. E.; Fomkin, A. A.; Arabei, A. B.; Shkolin, A. V.; Strizhenov, E. M. Description of methane adsorption on microporous carbon adsorbents on the range of supercritical temperatures on the basis of the Dubinin−Astakhov equation. Prot. Met. Phys. Chem. Surf. 2016, 52, 575−580. (21) Qiu, N. X.; Xue, Y.; Guo, Y.; Sun, W. J.; Chu, W. Adsorption of methane on carbon models of coal surface studied by the density functional theory including dispersion correction (DFT-D3). Comput. Theor. Chem. 2012, 992, 37−47. (22) Ricca, A.; Bauschlicher, C. W. B., Jr. The physisorption of CH4 on graphite and on a (9, 0) carbon nanotube. Chem. Phys. 2006, 324, 455−458. (23) Liu, S. S.; Meng, Z. P. Study on energy variation of different coal-body structure coals in the process of isothermal adsorption. J. China Coal Soc. 2015, 40, 1422−1427. (24) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Recommendations for the characterization of porous solids. Pure Appl. Chem. 1994, 66, 1739−1758. (25) Ozawa, S.; Kusumi, S.; Ogino, Y. Physical adsorption of gases at high pressure. IV. An improvement of the DubininAstakhov adsorption equation. J. Colloid Interface Sci. 1976, 56, 83−91. (26) Hu, H.; Li, X.; Fang, Z.; Wei, N.; Li, Q. S. Small-molecule gas sorption and diffusion in coal: molecular simulation. Energy 2010, 35, 2939−2944. (27) Zhang, Q. L. Adsorption mechanism of different coal ranks under variable temperature and pressure conditions. J. China Univ. Min. Technol. 2008, 18, 395−400. (28) Nounou, M. N.; Nounou, H. N. Multiscale estimation of the Freundlich adsorption isotherm. Int. J. Environ. Sci. Technol. 2010, 7, 509−518. (29) Yang, F.; Ning, Z. F.; Kong, D. T.; Peng, P.; Zhao, H. W. Comparison Analysis on Model of Methane Adsorption Isotherms in Shales. Coal Sci. Technol. 2013, 41, 86−89. (30) ASTM Standard D2797/D2797M-11; ASTM International: West Conshohocken, PA, 2011. (31) Jiang, B.; Qu, Z. H.; Wang, G. G. X.; Li, M. Effects of structural deformation on formation of coalbed methane reservoirs in Huaibei coalfield. Int. J. Coal Geol. 2010, 82, 175−83. (32) Ju, Y. W.; Jiang, B.; Hou, Q. L.; Wang, G. L. The new structuregenetic classification system in tectonically deformed coals and its geological significance. J. China Coal Soc. 2004, 29, 513−517. (33) Song, Y.; Jiang, B.; Shao, P.; Wu, J. H. Matrix compression and multifractal characterization for tectonically deformed coals by Hg porosimetry. Fuel 2018, 211, 661−675. (34) GB/T 19560-2008: Experimental Method of High-Pressure Isothermal Adsorption to Coal; China National Coal Association: 2008. (35) Zhou, L.; Zhang, J.; Zhou, Y. A Simple Isotherm Equation for Modeling the Adsorption Equilibria on Porous Solids over Wide Temperature Ranges. Langmuir 2001, 17, 5503−5507.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chongtao Wei: 0000-0002-3224-1867 Yu Song: 0000-0001-6602-3895 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was sponsored by the Major National Science and Technology Projects (No. 2016ZX05044002-003) and the National Natural Science Foundation of China (No. 41472134).

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REFERENCES

(1) Jiang, B.; Li, M.; Qu, Z. H.; Liu, J. G.; Li, W. Current Research Status and Prospect of Tectonically Deformed Coal. Adv. Earth Sci. 2016, 31, 335−346. (2) Song, Y.; Jiang, B.; Li, F. L.; Liu, J. G. Structure and fractal characteristic of micro-and meso-pores in low, middle-rank tectonic deformed coals by CO2 and N2 adsorption. Microporous Mesoporous Mater. 2017, 253, 191−202. (3) Qu, Z. H.; Jiang, B.; Wang, J. L.; Li, M. Micropore properties and its origin of tectonically deformed coals. Journal of the China Coal Society. J. China Coal Soc. 2015, 40, 1093−1102. (4) Mark, C. Coal bursts that occur during development: A rock mechanics enigma. Int. J. Min. Sci. Technol. 2018, 28 (1), 35−42. (5) Jiang, B.; Qin, Y.; Ju, Y. W.; Wang, J. L.; Li, M. The coupling mechanism of the evolution of chemical structure with the characteristics of gas of tectonic coals. Earth Sci. Front. 2009, 16, 262−271. (6) Gurba, L. W.; Gurba, A.; Ward, C.; Wood, J.; Filipowski, A.; Titheridge, D. The impact of coal properties on gas drainage efficiency. In Geological Hazards - The Impact on Mining; Doyle, R., Moloney, J., Eds.; NSW Department of Industry - Resources and Energy: 2001, pp 215−220. (7) Zhang, W. J.; Ju, Y. W.; Wei, M. M.; Wang, W. C. Study on characteristics and mechanism of adsorption/desorption on different metamorphic-deformed coal reservoirs. Earth Sci. Front. 2015, 22, 232−242. (8) Song, Y.; Jiang, B.; Liu, J. G. Nanopore structural characteristics and their impact on methane adsorption and diffusion in low to medium tectonically deformed coals: case study in the Huaibei coal field. Energy Fuels 2017, 31, 6711−6723. (9) Yuan, J. H.; Zhang, H.; Guo, Y. L.; Cai, N. Thermodynamic properties of high-rank tectonically deformed coals during isothermal adsorption. Arabian J. Geosci. 2017, 10, 278. (10) Wang, L.; Cheng, L. B.; Cheng, Y. P.; Yin, G. Z.; Cai, C. C.; Xu, C.; Jin, K. Thermal effects of magmatic sills on coal seam metamorphism and gas occurrence. Bull. Volcanol. 2014, 76, 803. (11) Li, H. M.; Li, H. G.; Wang, K. L.; Liu, C. Effect of rock composition microstructure and pore characteristics on its rock mechanics properties. Int. J. Min. Sci. Technol. 2018, 28 (2), 303. (12) Liu, Y. W.; Wang, D. D.; Hao, F. C.; Liu, M. J.; Mitri, H. S. Constitutive model for methane desorption and diffusion based on pore structure differences between soft and hard coal. Int. J. Min. Sci. Technol. 2017, 27, 937−944. (13) Wang, L.; Cheng, Y. P.; An, F. H.; Zhou, H. X.; Kong, S. L.; Wang, W. Characteristics of gas disaster in the Huaibei coalfield and its control and development technologies. Nat. Hazards 2014, 71, 85−107. (14) Guo, H.; Cheng, Y.; Wang, L.; Lu, S. Q.; Jin, K. Experimental study on the effect of moisture on low-rank coal adsorption characteristics. J. Nat. Gas Sci. Eng. 2015, 24, 245−251. 7495


Article

Energy & Fuels (36) Zhao, L.; Qin, Y.; Yang, Z. B.; Shen, J.; Han, B. B.; Zhang, Z. Study on Supercritical Isothermal Adsorption Model of Methane in Coal. Nat. Gas Geosci. 2014, 25, 753−760. (37) Huan, X.; Zhang, X. B.; Wei, H. W. Research on parameters of adsorption potential via methane adsorption of different types of coal. J. China Coal Soc. 2015, 40, 1859−1864. (38) Amankwah, K. A. G.; Schwarz, J. A. A modified approach for estimating pseudo-vapor pressures in the application of the DubininAstakhov equation. Carbon 1995, 33, 1313−1319. (39) Dubinin, M. M. The Potential Theory of Adsorption of Gases and Vapors for Adsorbents with Energetically Nonuniform Surfaces. Chem. Rev. 1960, 60, 235−241. (40) Xiong, J.; Liu, X. J.; Liang, L. X.; Lei, M. Improved DubibinAstakhov model for shale-gas supercritical adsorption. Act. Pet. Sin. 2015, 36, 849−857. (41) Moore, T. A. Coalbed methane: A review. Int. J. Coal Geol. 2012, 101, 36−81. (42) Song, Y.; Zhu, Y. M.; Li, W. Macromolecule simulation and CH4 adsorption mechanism of coal vitrinite. Appl. Surf. Sci. 2017, 396, 291−302. (43) Kaplan, I. G. Theory of Molecular Interactions; Elsevier: New York, 1986; pp 178−251. (44) Ju, Y. W.; Hong, L.; Li, X. S.; Fan, J. J. Tectonic deformation and dynamic metamorphism of coal. Earth Sci. Front. 2009, 16, 158− 166. (45) Jiang, B.; Ju, Y. W. Tectonic Coal Structure and Its Petrophysical Features. Nat. Gas Ind. 2004, 24, 27−29. (46) Ju, Y. W.; Jiang, B.; Hou, Q. L.; Wang, G. L.; Fang, A. M. Structural evolution of nano-scale pores of tectonic coals in southern North China and its mechanism. Act. Geol. Sin. 2005, 79, 269−285. (47) Tsao, M. Bounds on coverage probabilities of the empirical likelihood ratio confidence regions. Ann. Stat. 2004, 32, 1215−1221. (48) Cui, Y. J.; Zhang, Q.; Zhang, H.; Zhang, Q. L. Adsorption of different rank coals to single component gases. Nat. Gas Ind. 2005, 25, 61−65. (49) Ou, C. H.; Li, S. L.; Guo, P.; Zhao, Y. F.; Chen, J. Study on isothermal model of paraffinic hudrocarbon adsorption in underground porous media. J. SW Pet. Inst. 2002, 24, 53−56.

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