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. 2019 Nov 26;4(24):20762–20772. doi: 10.1021/acsomega.9b03165

Crystallite Structure Characteristics and Its Influence on Methane Adsorption for Different Rank Coals

Junqing Meng †,‡,§,*, Shichao Li †,‡, Jiaxing Niu †,‡
PMCID: PMC6906950  PMID: 31858063

Abstract

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The ability of coal to adsorb methane depends on the coal microstructure; however, the research on its exploration is still underway. In this paper, a new method was adopted to investigate the evolution characteristics of the crystallite structure of eight different rank coals and its influence on the methane adsorption capacity. The crystallite lattice parameters, including d002, Lc, La, Nave, and fa, were determined by curve fitting analysis of X-ray diffraction (XRD) spectra. The methane adsorption experiments were carried out through a static capacity method, and the methane adsorption parameters (VL, PL) were measured. Correlations were established for the crystallite lattice parameters and the methane adsorption parameters. From the results obtained, there is a good negative linear relationship between VL and d002 and a good exponential relationship between PL and d002, indicating that the increasing d002 can weaken the methane adsorption capacity. VL displays an exponential increase with increasing Lc and Nave, whilePL presents a linear decrease, but reverse variations are emerged in the process of change for both, and the methane adsorption capacity is weaken temporarily. VL presents a lognormal distribution with increasing La, and the minimum value appears at La = 1.85–1.9 nm. VL and PL both obey lognormal distribution with increasing La/Lc, but their trends are completely opposite, and the methane adsorption capacity is the strongest at La/Lc = 0.85–0.9. As fa increases, VL and PL present an overall exponential increase and an overall exponential decrease, respectively, but reverse changes also emerge. The methane adsorption is related to the crystallite structure characteristics of coal. Finally, the influence mechanism of the crystallite structure evolution on the methane adsorption capacity was analyzed, which has great significance for prevention of gas disasters in underground coal mines.

1. Introduction

Coal is one of the main energy resources in the world,1 and it is essentially a polymer organic compound that is evolved by biochemistry and physical chemistry during long geological history. Due to the coalification and the original characteristics of coal-forming plants, different rank coals have different microstructure characteristics. Gas exists in coal in the form of adsorbed and free states, and gas in an adsorbed state accounts for about 80–90%.2 Gas disasters, such as coal and gas outburst, occur in the process of massive mining of coal seams, and the possibility of gas disasters increases with the increasing mining depth and stoping rate.3−5 It is helpful for engineers to master the mechanism of gas disasters by investigating the interaction between different rank coals and gas and then taking effective prevention and control measures. Because of heterogeneity and complex physical and chemical structures, the microstructure characteristics of coal have a significant influence on the interaction between coal and gas.6−8 Therefore, it is especially important to investigate coal macromolecular structure and its effect on the methane adsorption.

Many analytical techniques have been widely applied for the quantitative evaluation of coal macromolecular structures because of the advancement in technical innovation and software improvement in recent years, such as X-ray diffraction (XRD),9−12 high-resolution transmission electron microscopy (HRTEM),13,14 Raman spectroscopy,15,16 solid-state nuclear magnetic resonance (NMR),17,18 etc. Based on considerable research on coal macromolecular structure, scholars have found that coal macromolecular structure is the crystallite structure that evolves from amorphous to crystal, which is similar to graphite crystal, and the view is universally recognized.19−21 Compared with other instrumental techniques for exploring internal structure of coal, XRD exhibits prominent advantages. It not only obtains the maximum structural information about carbonaceous materials from a wide range of scattering angles but also the average value of sample properties instead of local features,10 which is very important for coal, a material with heterogeneity. Therefore, XRD has been widely applied in the research of the crystallite structure. Warren applied XRD to quantitatively evaluate the structure of carbonaceous materials at first and proposed a structural model that is suitable for a nonideal layer material, such as carbon black, i.e., random lattice, and the conclusion provided a theoretical basis for a follow-up study on coal crystallite structures.22 Houska et al. and Iwashita et al. used the crystallite lattice parameters (interlayer spacing of the aromatic layer (d002), crystallite height (Lc), and crystallite diameter (La)) to evaluate the stacking structure of carbon materials, and subsequently, these parameters were widely applied to analyze the coal crystallite structure.23,24

According to the variation of the crystallite lattice parameters of coal (d002, La, and Lc), Li et al. used XRD and thermogravimetry coupled with mass spectrometry (TG/MS) to investigate the evolution characteristics and mechanism of agglomerate structures in the process of coal pyrolysis.25 Takagi et al. estimated the stacking structure (Ps), the average number of the effective aromatic layer (Nave), and the influence of heat-treatment conditions on the carbon stacking structure in several kinds of coals by means of XRD and concluded that the heating rate required for the development of the stacking structure of lower-rank coals was slower than that required for higher-rank coals.26 Boral et al. analyzed the relationship between the crystallite lattice parameters (d002, La, etc.) and the chemicals as well as petrographic parameters of coals.27 Su et al. found that the essence of coalification is the evolution of coal crystallite structure by XRD.28 Zhu et al. compared the difference in graphitization degree, the crystal structure types, and the crystal size for three needle cokes by XRD.29 Zhang et al. showed the crystallite lattice parameters of four different rank coals, its relationship with oxidation and combustion characteristics of coal (crossing point temperature) was discussed, it was concluded that a positive correlation was observed between the coal rank and the combustion characteristics, and negative correlation was observed between the interlayer spacing and combustion characteristics.30

Methane adsorption on coal results from the interaction between CH4 molecules and coal microstructure.31−33 Therefore, coal microstructure plays a decisive role in methane adsorption. Based on the view, coal chemists have done some research. You et al. constructed a coal molecule and simulated the electrostatic interaction between the heteroatom group on the surface of the coal molecule and methane molecules by molecular dynamics (MD).34 Zhu et al. studied the optimal adsorption sites of methane molecules on shale using density functional theory.35 Based on molecular mechanics (MM) and MD, Song et al. observed that methane molecules were adsorbed preferentially at the edge of crystallite structure and tended to aggregate around the branched chain of the crystallite structure.36 Meng et al. analyzed the effects of its oxygen-containing functional group on methane adsorption by constructing the Zhaozhuang coal molecule model, and it was discovered that carbonyl has a greatest effect on methane adsorption.37 In addition, the influence of microscopic pore structure on the methane adsorption sites and the methane adsorption capacity was confirmed by Mosher et al., Li et al., and Liu et al.38−40

As mentioned, much research has focused on the interaction between methane molecules and coal microstructure by molecular simulation. However, the influence of crystallite structure on methane adsorption is less researched through the experimental method. The combination of the high-pressure methane adsorption experiments and the XRD analytical method was adopted to study the relationship between the crystallite lattice parameters and the methane adsorption capacity for eight different rank coals, and the influence of coal crystallite structure on the methane adsorption capacity was analyzed in this paper, which will provide theoretical foundation from another view for understanding the methane adsorption mechanism and the prevention of gas disasters in underground coal mines.

2. Experimental Procedure

2.1. Sample Preparation

Eight fresh coal samples ranging from lignite to anthracite were collected from multiple coalfields in China. The samples were collected according to Chinese standard GB/T 482-2008 and placed in sampling bags, and then, the bags were tightened and vacuum-sealed to prevent the contamination and oxidation of the samples. After collection, the samples were immediately delivered to the laboratory for sample preparation. All of the samples were pulverized, screened to pass through size 60 mesh to 80 mesh sieves, and dried continuously for 12 h at 80 °C. A proximate analysis of samples was performed following the international standards ISO 11722:2013 and ISO 1171:2010, an ultimate analysis of samples was conducted according to international standards ISO 17247:2013 and ISO 19579:2006, and the vitrinite reflectance of samples (R0) was determined in accordance with international standard ISO 74045:2009. The results of conventional analysis are presented in Table 1.

Table 1. Conventional Analysis of Coal Samplesa.

  proximate analysis (wt %)
   ultimate analysis (wt %, daf)
  
sample ID Mad Aad Vdaf FCad R0 (%) C H O N S coal rank
MTG 3.55 10.65 29.45 56.35 0.42 80.86 3.37 13.01 1.21 1.55 lignite
EEDS 4.74 9.63 23.11 62.52 0.5 81.77 2.98 12.56 1.33 1.36 lignite
PX 2.51 8.82 24.62 64.05 1.46 84.36 4.15 8.3 1.35 1.84 bituminous
DSC 0.56 13.27 18.24 67.93 2.13 87.63 3.41 6.95 0.74 1.27 bituminous
HL 3.77 10.56 16.69 68.98 2.19 89.24 2.95 5.17 1.22 1.42 bituminous
TL 2.25 9.88 7.95 79.92 3.18 91.38 2.01 5.27 0.69 0.65 anthracite
ZZ 1.82 12.46 7.58 78.14 3.21 89.94 3.26 4.81 1.15 0.84 anthracite
YQ 0.88 10.53 7.35 81.24 3.32 89.55 3.05 5.77 1.11 0.52 anthracite
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a

Note: ad, air dried basis; daf, dried ash free basis; Mad, moisture; Aad, ash; Vdaf, volatile matter; FCad, fixed carbon.

2.2. High-Pressure Methane Adsorption

The adsorption process of methane is usually described by adsorption isotherms,41−43 following the international standard ISO 18871:2015 for the measure of the methane adsorption capacity of coal samples. Methane adsorption isotherms were tested by the static capacity method in a 3H-2000PH1 apparatus (Beishide Instrument Technology (Beijing) Co., Ltd.). Prior to the methane adsorption experiment, the pulverized coal samples required for the experiment (each coal sample weighed approximately 6 g) passed through the size 60–80 mesh were dried in a vacuum drying oven at 80 °C for 12 h, and then placed in the apparatus. Methane adsorption experiments were conducted at an equilibrium pressure range of 0–5.5 MPa at 25 °C.

2.3. X-ray Diffraction

To reduce the influence of inorganic minerals on XRD analysis, pulverized coal with a particle size of <75 μm (200 mesh) was demineralized using a typical three-step HCL–HF–HCL procedure.44,45 The influence of demineralization on crystallite lattice parameters measured by XRD was negligible.26 Coal samples (10 g), ethanol (2 mL), and an HCL solution (50 mL, 5 mol/L) were mixed in a beaker, and then, the beaker was placed in a constant-temperature water bath and heated for 1 h. The mixture was removed for filtration to obtain coal sample. Next, the coal sample was mixed fully with an HF solution (50 mL) and the above heating process was repeated. After heating, the mixture was removed and filtered, thus obtaining an HF-treated coal sample. Finally, the coal sample was mixed with a concentrated HCL solution (50 mL, 1.19 g/mL). Repeating the above heating and filtration process, the coal sample was washed with excess distilled water until the pH of the filtrate was neutral.

X-ray diffraction (XRD) experiments were conducted on selected samples by an XRD diffractometer with a CBO crossover optical system (SmartLab, Rigaku Corporation, Japan). Pulverized coal was fixed on the support in the sample chamber, Cu Kα (40 kV, 40 mA) radiation was used as an X-ray source, and the X-ray intensities scattered from the examined coal samples were measured in the range of 5° < 2θ < 80° with a step size of 0.02°. The results were analyzed using Origin Pro 9.1.0 software (OriginLab Corporation). The average carbon crystallite lattice parameters (interlayer spacing of aromatic layers (d002), crystallite height (Lc), crystallite diameter (La), and average number of effective aromatic layer per carbon crystallite (Nave)) were calculated using the Bragg’s equations and empirical equations derived from Scherrer (eqs 1–4).27,29,46 The characterizations of these parameters in coal crystallite structure are shown in Figure 1

2.3. 1
2.3. 2
2.3. 3
2.3. 4

where λ is the X-ray wavelength (0.15406 nm); θ002 and θ100 are the peak positions of the (002) and (100) bands, respectively (°); β002 and β100 are the full width at half-maximum (FWHM) of the (002) and (100) peaks, respectively (°); and kc and kα are constants depending on the X-ray refection plane (0.89 for the (002) band and 1.84 for the (100) band).

Figure 1.

Figure 1

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Characterization of the average carbon crystallite lattice parameters in coal crystallite structure.

If it is assumed that the areas under the (002) band and the γ band (A002 and Aγ) are equal to the number of aromatic carbons and aliphatic carbon atoms, respectively, the aromaticity of the coal can be calculated using eq 5(47−49)

2.3. 5

where A002 and Aγ are the areas under the (002) and γ bands, respectively.

3. Results and Discussion

3.1. Methane Adsorption Capacity

Figure 2 presents the methane adsorption isotherms of coal samples fitted by the Langmuir model at 25 °C, and all of the curves in Figure 2 correspond to type-I isotherm based on the IUPAC classification. The experimental data clearly demonstrate that the methane adsorption capacity is greatly affected by the coal rank. The good fits (R2 > 0.99) in Figure 2 illustrate that the use of the Langmuir model for describing methane adsorption on coal surfaces is reasonable and convincing; therefore, the Langmuir volume (VL) and Langmuir pressure (PL) can be used to evaluate the methane adsorption capacity.50−52VL is a direct indicator of the CBM gas storage capacity and is proportional to the total number of available sites for adsorption. PL is closely related to the affinity of a gas on a solid surface and the energy stored in coal formation.52

Figure 2.

Figure 2

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Adsorption isotherms of methane on coal samples.

Table 2 and Figure 3 present the Langmuir fitting parameters (VL and PL), and the fitting degree is high. In Figure 2, the methane adsorption amount increases gradually and flattens with the increasing adsorption equilibrium pressure and there is a positive correlation between the methane adsorption isotherm and the coal rank (except for PX (R0 = 1.46%)). The results demonstrate that there is a remarkable discrepancy in the methane adsorption capacity for different coal samples, varying widely from 10.05 to 31.17 mL/g for VL and from 0.45 to 2.70 MPa for PL. The sample YQ presents the greatest VL and the smallest PL, and the sample MTG appears to show the smallest VL and the greatest PL. Therefore, the affinity of methane molecules is the strongest toward the surface of sample YQ and the weakest for sample MTG. VL shows an up–down–up trend with the increase of R0, and there is a reverse trend in the change of PL. The two inflection points in the trends occur at R0 = 0.5% and 1.46%, respectively. This phenomenon may be related to the coalification of different stages.11

Table 2. Langmuir Fitting Parametersa.

sample ID VL (mL/g) PL (MPa) R2
MTG 10.05 2.70 0.9969
EEDS 17.42 1.20 0.9985
PX 11.77 1.86 0.9996
DSC 17.79 0.71 0.9992
HL 21.50 0.88 0.9997
TL 26.96 0.64 0.9999
ZZ 28.03 0.50 0.9998
YQ 31.17 0.45 0.9998
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a

Note: VL, Langmuir volume; PL, Langmuir pressure.

Figure 3.

Figure 3

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Variation of VL and PL with the coal rank.

3.2. XRD Spectral Characteristics and Parameter Evolution

3.2.1. XRD Spectral Characteristics of Coal Samples

Figure 4 shows the original XRD spectral curves of selected coal samples, and it can be seen that these demineralized coal samples have the same graphite characteristics as reported in previous articles.53,54 The (002) and (100) diffraction peaks are all found in the XRD spectra of selected coal samples, and the peak value appears at around 2θ = 25 and 44°, respectively. As the coal rank increases, the (002) diffraction peak sharpens and the (100) diffraction peak becomes more obvious. The (002) diffraction peak is reflected comprehensively by the carbon crystal peak ((002) band), which is arranged regularly, and the carbon peak of the amorphous structure (γ band);10,29,55 therefore, the (002) diffraction peak has a certain asymmetry. It can be seen that the peak position of the γ band appears at 2θ = 21° by curve-fitting the (002) diffraction peak, and the curve fitting of (002) diffraction peaks for four representative coal samples (MTG, PX, DSC, and ZZ) is shown in Figure 5. The (002) band reflects the stacking height of the aromatic layer, and it corresponds to the crystallite formed by polycondensation of the aromatic nucleus, namely, aromatic crystallite. In theory, the 2θ value of the (002) band gradually increases with the increase of coal rank until it reaches 26.6°. The γ band is related to nonaromatic structures, such as aliphatic structures; in other words, it is caused by aliphatic side chains, functional groups, and aliphatic hydrocarbons attached to aromatic nucleus. The (100) diffraction peak reflects the condensation degrees of the aromatic rings, namely, the size of the aromatic layer.

Figure 4.

Figure 4

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XRD spectra of selected coal samples.

Figure 5.

Figure 5

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Curve fitting of XRD spectra of representative coal samples for the (002) peak: (a) MTG, (b) PX, (c) DSC, and (d) ZZ.

The diffraction angle (2θ), the full width at half-maximum(β), and the peak area (A) of the (002) band and γ band were obtained by curve-fitting of the (002) diffraction peak, and similarly, the diffraction angle (2θ) and full width at half-maximum(β) of the (100) peak were obtained by curve-fitting the (100) diffraction peak. The crystallite lattice parameters (d002, Lc, La, Nave, aromaticity (fa), and La/Lc) were calculated according to eqs 1–5, and La/Lc reflected the morphological characteristics of aromatic structures. The statistics for the crystallite lattice parameters of eight coal samples are shown in Table 3.

Table 3. Statistics of Crystallite Lattice Parameters.
sample ID 2θ002 (deg) 2θ100 (deg) β002 (deg) β100 (deg) d002 (nm) Lc (nm) La (nm) Nave (−) La/Lc (−) fa (−)
MTG 25.38 43.07 5.07 9.71 0.3507 1.5881 1.7969 5.53 1.1315 0.6853
EEDS 25.50 43.14 4.14 9.59 0.3495 1.9331 1.8221 6.53 0.9426 0.7236
PX 25.43 42.40 3.97 9.09 0.3499 2.0281 1.9171 6.80 0.9453 0.7588
DSC 25.59 43.70 3.43 8.84 0.3479 2.3473 1.9793 7.75 0.8432 0.7758
HL 25.63 44.56 3.37 8.67 0.3474 2.3873 2.0234 7.87 0.8476 0.7923
TL 25.75 43.85 3.31 7.88 0.3458 2.4333 2.2462 8.04 0.9247 0.8386
ZZ 25.73 44.80 3.29 8.19 0.3461 2.4459 2.1456 8.07 0.8772 0.8228
YQ 25.76 44.56 3.28 8.36 0.3456 2.4506 2.1005 8.09 0.8571 0.8357
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3.2.2. Crystallite Lattice Parameter Evolution of Coal Samples

Figure 6 depicts the relationship between crystallite lattice parameters (d002, Lc, La, Nave, La/Lc, and fa) and R0. As seen in Table 3 and Figure 6, the crystallite lattice parameters show obvious stage evolution characteristics with the change of coalification. The d002 ranges from 0.3456 to 0.3507 nm, which indicates that the selected coal samples have a lower-level ordered crystallite unit relative to graphite (0.336–0.337 nm).56d002 shows three different trends with the increase of R0; d002 decreases rapidly at R0 = 0.4–0.6%, d002 increases slightly and slowly at R0 = 0.6–1.25%, and d002 decreases again, and the speed is slower than that of R0 = 0.4–0.6% at R0 = 1.25–3.4% (Figure 6a). These trends are related to the jump point in the coalification process reported by Bustin and Guo.57Lc ranges from 1.5881 to 2.4506 nm, La ranges from 1.7969 to 2.2462 nm, and the variations indicate that the crystallite size of coal increases with the enhancement of coalification. Lc increases rapidly and then gradually flattens; the demarcation point is approximately R0 = 1.5% (Figure 6b). However, there is a notable discrepancy between the fitting curve and experimental data at R0 = 1.46%, and Song’s report indicated that the increasing speed of Lc will slow down at R0 = 1.2–1.5%.58 Therefore, there is a certain correlation between the trends of d002 and Lc if the error is ignored. La increases slowly with the increase of R0, and the demarcation point is approximately R0 = 0.6% (Figure 6c). Nave is related to Lc and d002, varying from 5.53 to 8.09, and its trend is similar to that of Lc (Figure 6d). La/Lc exhibits a U-shape curve correlation with R0. La/Lc decreases slowly at R0 = 0.4–2.25%; it presents transitory stagnation and varies little at R0 = 2.25–2.75% and increases slowly at R0 = 2.75–3.4% (Figure 6e). fa ranges from 0.6853 to 0.8386, and it increases linearly with the increase of R0 (Figure 6f).

Figure 6.

Figure 6

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Variation of crystallite lattice parameters with the coal rank: (a) d002, (b) Lc, (c) La, (d) Nave, (e) La/Lc, and (f) fa.

According to the evolution characteristics of the crystallite lattice parameters for selected coal samples, the condensation degree of aromatic rings increases gradually and the arrangement of aromatic layers becomes gradually regular. Although the variation process does not conform to a linear relationship, it presents phased evolution characteristics. Coal crystallite structures have a complex evolution mechanism.

When R0 = 0.4–0.6%, nonaromatic structures in the coal, such as aliphatic side chains (−CH3, −CH2, etc.) and oxygen-containing functional groups, begin to exfoliate; the branching degree of aliphatic structure begins to decrease, and the aliphatic hydrogen content is reduced.59 It is worth noting that the abscission of oxygen-containing functional groups is most prominent at R0 = 0.43–0.5%.60 After the abscission of nonaromatic structures, nonhydrocarbons such as soluble organic matter are formed, the proportion of aromatic carbon atoms in coal is increased, and the new active sites in the structural units of coal are gradually formed.61,62 The abscission of nonaromatic structures results in d002 decreases rapidly, it also increases the possibility and speed at which the crystallite structure is reconstructed in the longitudinal direction,63 and thus Lc and Nave increase rapidly. However, compared with the abscission of nonaromatic structures, the condensation of aromatic rings in the transverse direction occurs later,64 so the trend of La shows temporary gentleness. The changing speed of the aromatic structural unit in the longitudinal direction is higher than that in the transverse direction, so the crystallite structure changes from a “flat type” to a “lanky type” (flat type and lanky type represent the morphological characteristics of the crystallite structural unit), and La/Lc decreases. At this stage, a large number of nonaromatic structures remain in the coal structure and molecular structures are arranged irregularly and relatively loosely.65

When R0 = 0.6–1.25%, aliphatic structures begin to dramatically exfoliate and methylene is detached faster than other aliphatic compounds.66,67 Meanwhile, oxygen-containing functional groups continue to be exfoliated, the partial bridge bond in the structural unit begins to rupture,28 and the number of new active sites increases. Therefore, aromatic rings are condensed gradually at active sites in the transverse direction and La increases gradually. In addition, dehydrogenation is initiated in the aromatic system and partial residual functional groups connected to the stable aromatic layers form at all angles, such as hydroxyl and some hydrogenated aromatic rings. There is a certain steric hindrance between the aromatic layers,11 which causes d002 to increase slightly, and the increasing speeds of Lc and Nave are slower. The gap between the increasing speed of La and Lc decreases, the decreasing speed of La/Lc slows down gradually, and crystallite structural units present as a lanky type. This stage is mainly based on the evolution of nonaromatic structures; the numbers of aliphatic structures and oxygen-containing functional groups dramatically decrease, the aromatic structural system becomes slightly larger, the regularity of the molecular arrangement is enhanced slightly, and the coal structure is denser.

When R0 = 1.25–3.4%, the content of nonaromatic structures in coal is reduced, they decrease slowly with the enhancement of coalification, and this stage is mainly based on the evolution of aromatic structures.64 The condensation between aromatic rings is enhanced gradually, the increasing speed of La is faster than that at R0 = 0.6–1.25%, and dehydrogenation in the aromatic system and the adjustment of steric hindrance occur synchronously; this step is completed very quickly.68 Therefore, d002 decreases again and Lc and Nave increase quickly at first and then increase slowly. The increasing speed of La is gradually greater than that of Lc, which causes La/Lc to present a U-shape curve at this stage. The increasing speed of the crystallite structural unit in the longitudinal direction is gradually smaller than that in the transverse direction. At this stage, the regularity of the molecular arrangement is enhanced, the aromatic rings are condensated more intensely, the aromatic layers are arranged more closely, and a strong shrinkage stress is produced, which results in the production of micropores.64

3.3. Impact of the Crystallite Structure on Methane Adsorption

In Figures 3 and 6, although the methane adsorption capacity and crystallite lattice parameters do not evolve synchronously with the increase in R0, both have similar evolution trends, indicating that the crystallite structural evolution of coal has a certain influence on the methane adsorption capacity, but it is not the only influencing factor.69,70 The variations of VL and PL with crystallite lattice parameters are shown in Figure 7, and it can be seen that crystallite lattice parameters have a good fitting degree with VL and PL. There is a good linear relationship between VL and d002, and an obvious exponential relationship exists between PL and d002 (Figure 7a). The methane adsorption capacity decreases with the increasing interlayer spacing of the aromatic layer. VL presents an overall exponential increase with the increase of Lc and Nave, and PL presents an overall linear decrease with the increase of Lc and Nave, but it is worth noting that significant reversal changes existed for VL and PL at Lc = 1.9331–2.0281 nm and Nave = 6.53–6.80 (Figure 7b,d). Therefore, the methane adsorption capacity decreases temporarily during the evolution process of the aromatic structure in the longitudinal direction. VL shows a lognormal distribution with increasing La, and the minimum value appears at La = 1.85–1.9 nm. Although PL presents an overall exponential decrease as La increases, significant reverse variation occurs at La = 1.8221–1.9171 nm (Figure 7c). The methane adsorption capacity first decreased to the minimum and then increased slightly with the crystallite diameter increasing. Although the trends of VL and PL with La/Lc are opposite, both present lognormal distributions. VL reaches the maximum and PL reaches the minimum when La/Lc = 0.85–0.9, and there is no longer obvious variation for VL and PL when La/Lc > 1.0 (Figure 7e). Therefore, the methane adsorption capacity is enhanced to the greatest degree when the ratio of the crystallite diameter to the crystallite height increases gradually to 0.85–0.9; then, the methane adsorption capacity begins to decrease until the ratio is 1, at which point it is no longer obvious. The overall trends of VL and PL with fa display exponential increases and exponential decreases, respectively, but reverse variations are also produced at fa = 0.7236–0.7588 (Figure 7f).

Figure 7.

Figure 7

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Variation of VL and PL with crystallite lattice parameters: (a) d002, (b) Lc, (c) La, (d) Nave,(e) La/Lc, and (f) fa.

Although the influence of the partial crystallite structural characteristics in coal on the methane adsorption capacity can be reflected clearly by the variations in VL and PL with a single crystallite lattice parameter, different crystallite lattice parameters have different evolutionary laws with the increasing coal rank and thus every crystallite lattice parameter needs to be combined to analyze the influence of crystallite structural evolution on the methane adsorption capacity. Figure 2 shows that the adsorption capacity is the weakest for the sample MTG (R0 = 0.42%) and the adsorption capacity is the strongest for the sample YQ (R0 = 3.32%). Compared with the other samples, the sample MTG has the largest d002 (0.3507 nm) and the smallest La (1.7969 nm), Lc (1.5881 nm), Nave (5.53), and fa (0.6853); additionally, its crystallite morphological characteristics are shown as a flat type. The crystallite structure of MTG is just beginning to be formed, the interlayer spacing of the aromatic layer is large, the longitudinal stacking degree and the transverse extended degree are very small, the average number of effective aromatic layers is low, and the crystallite structure is very irregular. Research has shown that the number of effective aromatic layers has a great influence on the micropore structure and the micropore structure is easily formed between the effective aromatic layers or at the edges of aromatic structures.71 However, because the crystallite structure is in a rudimentary stage for sample MTG, the formation of a new micropore structure is restricted, the aromatic structure is formed as a flat type, and the average number of effective aromatic layers is less. In addition, aromatic carbon has a stronger adsorption capacity than aliphatic carbon and methane molecules are always adsorbed preferentially on aromatic carbon and then an aliphatic carbon.72,73 The presence of oxygen-containing functional groups is not conducive to the adsorption of methane molecules.74 Although there are large amounts of primary pores in the coalification stage in which the sample MTG can be located in theory,75 aliphatic structures and partial oxygen-containing functional groups play a controlling role in the formation of these primary pores,64 i.e., primary pores are dominated by aliphatic structures and oxygen-containing functional groups. Therefore, for the sample MTG, VL is the smallest, PL is the largest, the saturated adsorption amount of methane is the smallest, and methane is mainly present in these primary pores in a free state.76 The sample YQ belongs to high-rank coal; the non-aromatic structures have been largely ruptured, d002 (0.3456 nm) is the smallest, La (2.1005 nm) and fa (0.8357) are relatively large, Lc (2.4506 nm) and Nave (8.09) are the largest, and the crystallite morphological characteristics are shown as a lanky type (La/Lc = 0.8571). The crystallite structure of sample YQ is arranged closely and regularly; there is a large volume of crystallite structure, the average number of effective aromatic layers is more, and strong shrinkage stress is produced. Therefore, a large number of micropore structures dominated by aromatic carbon are produced and the aromatic carbon content increases. Meanwhile, due to the disappearance of nonaromatic structures, primary pores are decreased. Methane is adsorbed heavily around micropores and aromatic carbons.

Based on the evolutionary characteristics of crystallite lattice parameters, when R0 = 0.4–0.6%, nonaromatic structures begin to exfoliate and primary pore structures begin to decrease. New active sites begin to form, the proportion of aromatic carbon begins to increase, the interlayer spacing of the aromatic layer is decreased rapidly, the average number of effective aromatic layers is increased rapidly, and the crystallite structure changes from a flat type to a lanky type; therefore, micropore structures dominated by aromatic carbon are gradually formed. At this stage, the dominant adsorption sites of methane are increased, saturated adsorption amounts of methane are increased, and the amount of free methane is decreased. When R0 = 0.6–1.25%, the structural evolution is based mainly on the abscission of nonaromatic structures and the primary pores in coal are dramatically decreased. Meanwhile, the interlayer spacing of aromatic layers is increased sharply because of the existence of steric hindrance, the crystallite size and the average number of effective aromatic layers increase slowly, and aromatic structures are rather unstable; therefore, although the number of micropores increases, the increase is not obvious. In summary, the adsorption sites of methane in nonaromatic structures decrease dramatically, but the dominant adsorption sites of methane increase less, and thus, the amount of methane saturated adsorption is decreased and the amount of free methane is increased. When R0 = 1.25–3.4%, the structural evolution is based mainly on aromatic structures. The number of micropores dominated by aromatic carbons is increased dramatically as a result of the gradual increase of shrinkage stress, the content of nonaromatic structures decreases slowly, and primary pores vary slightly. For these reasons, methane is adsorbed robustly around the dominant adsorption sites, the methane saturated adsorption amount is increased, and the amount of free methane is decreased.

4. Conclusions

In this paper, correlations were established for crystallite lattice parameters and methane adsorption parameters using a method that combines X-ray diffraction (XRD) and high-pressure methane adsorption experiments. The influence of crystallite structural evolution on the methane adsorption capacity was analyzed and discussed, and the following conclusions are obtained:

(1) VL, PL, and crystallite lattice parameters all present staged variations with the increasing coal rank. Although the demarcation points of VL and PL are not exactly the same as those of crystallite lattice parameters, there is a certain correlation apparent in these trends.

(2) With the increase of d002, VL and PL present linear decreases and exponential increases, respectively. VL presents an overall exponential increase with the increase in Lc and Nave, and PL presents an overall linear decrease with the increase of Lc and Nave, but there are obvious and temporary reverse changes for VL and PL in their variation processes. As La increases, VL presents a lognormal distribution and PL presents an overall exponential decrease, but there is also obvious reverse variation for PL at a specific stage. VL and PL all obey lognormal distribution with the variation of La/Lc. As fa increases, VL and PL present an overall exponential growth and an overall exponential decline, respectively, but the reverse change still exists for VL and PL in a specific evolutionary stage of fa.

(3) The variations of crystallite lattice parameters reflect the evolution of the coal crystallite structure. In different stages of coalification, the aromatic structured and nonaromatic structures have different evolution speeds, which results in the number of total adsorption sites and dominant adsorption sites being different at every coalification stage; therefore, there are significant differences in the methane adsorption capacity.

Acknowledgments

This work is financially supported by the Fundamental Research Funds for State Key Research Development Program of China (No. 2016YFC0600708), the Fundamental Research Funds for the Central Universities (No. 2011QZ02), and the Yue Qi Distinguished Scholar Project, China University of Mining & Technology, Beijing; the authors are grateful for their support. This work described has not been submitted elsewhere for publication, in whole or in part.

The authors declare no competing financial interest.

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