Construction of Buertai Coal Macromolecular Model and GCMC Simulation of Methane Adsorption in Micropores.

With the increase in high gas mines in the low coal rank mining area in the northwestern part of China, high gas mines in the low-rank coal mining area have caused many gas emission accidents. Coal is a porous material, containing a large number of micropores (<2 nm), which can absorb large amounts of methane, so it is necessary to explore methane adsorption in micropores of low-rank coal. In this work, FTIR, HRTEM, and 13C-NMR were used to test the macromolecular structural parameters of Buertai coal, which was a kind of low-rank Jurassic coal in northwestern China. The results showed that the aromatic structural units in the Buertai coal structure mainly consist of naphthalene, anthracene, and phenanthrene. The fat structure mainly occurs in the form of aliphatic side chains, cycloalkanes, and other compounds. The oxygen atoms are present in the form of carbonyl groups, ether bonds, and phenol groups with a ratio of about 6:4:9. The nitrogen atoms are present in the form of pyrrole and pyridine compounds. Finally, the macromolecular structure model of Buertai coal was built, and the calculated NMR spectrum from the model was very consistent with the experimental NMR spectrum of Buertai coal. The relationship between the macromolecular density and energy of Buertai coal was explored using the Amorphous Cell module in the simulation software, Materials Studios 8.0 (MS 8.0), and the density value at the lowest energy was determined to be about 1.23 g/cm3. The pore structure parameters of Buertai coal were also calculated. It was found that both pore volume and void fraction decreased evenly as the diameter of the probe molecule increased, but the surface area decreased rapidly when the diameter of the probe molecule was 3.46 Å. All pore sizes were found to be smaller than 10 Å from the pore size distribution (PSD) curve of Buertai coal, which provided a lot of adsorption sites for methane (CH4). The results of the CH4 adsorption simulation from Grand Canonical Monte Carlo (GCMC) showed that CH4 is adsorbed inside the micropores of coal, and the adsorption capacity of CH4 depends on the diameters of micropores when the micropores are less than 8.5 Å. There are many micropores where CH4 did not appear because these micropores are closed and did not provide a channel for CH4 to enter. The results of experimental methane adsorption indicate that the excess adsorption capacity from the GCMC simulation was very close to the experimental results of Buertai coal. This work provides a new perspective to study the methane adsorption behavior in micropores of coal.

Introduction

China is a major coal resource country.[1,2] In recent years, the fully mechanized top coal caving mining process has significantly increased coal production and also caused an increase in high gas mines in the low-rank coal mining area in the northwestern part of China, specifically in Inner Mongolia, Shaanxi, Gansu, and Ningxia provinces. High gas mines in the low-rank coal mining area caused a lot of gas emission accidents and have been the main serious natural disaster during underground coal mining.[3] As an energy source, coal-bed methane (CBM) has many advantages, exploring the adsorption mechanism of methane is significant because it plays a crucial role in the CBM utilization and the prevention and prediction of gas accidents.[4]

The structure of coal can be inferred from various parameters representing coal’s chemical structure, which is a combination of aromatic layer, heteroatom, branched functional groups, and connection modes and modes of action occurring between different carbon atoms.[5] The macromolecular structural parameters of coal have been characterized by many analytical techniques. High-resolution transmission electron microscopy (HRTEM), solid-state 13C nuclear magnetic resonance spectroscopy (13C-NMR), Fourier transform infrared spectroscopy (FTIR), and several advanced analytical techniques provide more detailed information on the macromolecular coal structure.[6] Over the past 70 years, more than 134 structures of coal have been studied according to Mathews et al.[7]

Molecular simulation has been widely used in studying the macromolecular structure of coal[8,9] and methane adsorption in micropores.[10] It provides valuable insights into the coal structure and methane adsorption at atomistic scales.[6] At the end of the 20th century, Carlson[11] first applied the computer molecular design method to establish the macromolecular coal structure model, and the structure of coal was rapidly developed using quantitative methods. Shi et al.[12] used infrared spectroscopy and 13C-NMR spectroscopy to calculate the macromolecular structure parameters of nitric acid oxidized coal from Fushun City and also conducted functional group and elemental analysis on such coal and were able to construct a coal structure consistent with the experimental results. In terms of molecular calculation,[13] based on the Beer–Lambert law, a linear relationship between structural parameters and the distribution of some functional groups was obtained using the methods of quantum chemistry for the quantification of coal infrared spectra. Yu et al.[14] built the vitrinite macromolecular model through 13C-NMR, FTIR, and HRTEM, and the crystal parameter was 15.8 Å.

Coal is a porous material,[15] containing a large number of micropores (<2 nm), which can absorb large amounts of methane.[16] The methane adsorption capacity is mainly determined by micropores in coal.[17] In recent years, Grand Canonical Monte Carlo (GCMC) simulation has been an effective method for solving the problem of methane adsorption in coal’s micropores in microcosm.[18] Li[19] and Mosher[20] used the GCMC simulation to study the adsorption characteristics of methane molecules on coal pores from another aspect. The diffusion and adsorption of methane could also be systematically simulated via GCMC simulation.[21−23]

The adsorption capacity of different coal is not the same.[24,25] Recently, although many models of the macromolecular structure of coal in China have been published,[26] most of them are high-rank and middle-rank macromolecular coal structure models. With the increase in high gas mines in the low-rank coal mining area in the northwestern part of China, high gas mines in the low coal rank mining area have caused a lot of gas emission accidents. The CH4 adsorption and micropore characteristics of low-rank coals are not the same as those of high-rank coals,[27] so it is necessary to explore the methane adsorption mechanism and the interaction between methane molecules and low-rank coal molecule. Coal comprised of macromolecules and small molecules. These small molecules have different effects on methane adsorption, we do not consider the influence of small molecules in this study.[24]

In this paper, FTIR, HRTEM, and 13C-NMR were used to test the macromolecular structural parameters of Buertai coal. A macromolecular structure model of Buertai coal and the microporous structures were built. Combining high-pressure CH4 adsorption experiments and GCMC simulations, the effects of pressure and temperature on the adsorption of methane by coal macromolecule were analyzed. Different probes were used to analyze the changes in the pore parameters. Finally, the way of methane adsorption in coal’s micropores was proposed.

Results and Discussion

Coal Structural Characteristics

Figure shows the infrared spectrum of Buertai coal. The spectrum was roughly divided into four parts where the 700–900 cm–1 band represents the aromatic hydrocarbons absorption peak, the 1000–1800 cm–1 band represents the absorption peaks of the oxygen-containing functional groups and the heteroatom functional group, the 2800–3000 cm–1 band indicates the absorption peak of aliphatic hydrocarbons, and the 3000–3600 cm–1 band indicates the absorption peak of the hydroxyl functional group. The fitting results and details of the four bands of the infrared spectrum are shown in S2 (Tables S1–S4 and Figures S1–S4).

Figure 1

Fourier transform infrared spectrum of Buertai coal.

The 13C-NMR spectrum and the fitted spectra of the sample are shown in Figure , while the peak positions are summarized in Table S5.[30−32] As seen in Figure , the Buertai coal 13C-NMR spectrum was clearly divided into two large peaks and one small peak. The two large peaks at 0–50 and 100–165 ppm represent the aliphatic zones and aromatic carbon zones, respectively. Also, the small peak of about 220 ppm represents the carbonyl carbon zone. From the comparison of peaks in Figure , it was apparent that the peak area of the fat zone was much smaller than the peak area of the aromatic carbon zone. This indicates that the majority of carbon atoms in Buertai coal were aromatic carbon atoms, and the role of fatty carbon was to connect these aromatic carbon atoms. Peak assignments in the 13C-NMR spectrum are summarized in Table S6. Twelve structural parameters of Buertai coal are summarized in Table .

Figure 2

Buertai coal 13C-NMR spectrum.

Table 1

Structural Parameters of Buertai Coala

sample	fa	fac	fa′	faN	faH	fap	fas	faB	fal	fal*	falH	falO
Buertai coal	84.4	4.00	81.45	32.75	48.51	5.60	11.50	15.65	14.34	2.94	8.66	2.74

fa: total sp2-hybridized carbons; fac: carbonyl or carboxyl group carbons; fa′: aromatic carbons; faH: protonated aromatic carbons; faN: nonprotonated aromatic carbons; faP: aromatic carbons bonded to hydroxyl or ether oxygen; faS: alkylated aromatic carbons; faB: aromatic bridgehead carbons; fal: total sp3 carbons; fal*: methyl carbons; faH: CH or CH2; falO: aliphatic carbons bonded to oxygen.

The peak fitting results of 13C-NMR spectrum of the Buertai coal can also be used to calculate the ratio of aromatic bridge carbon to the surrounding carbon (XBP), XBP = faB/(faH + faP + faS) = 0.23. This parameter was especially important because it can calculate the size of the aromatic clusters in the coal.[33,34]

HRTEM can be used to observe the samples, even fine atomic structures. The original image produced by the electron microscope was black and white (Figure a). This image was striped lattice extraction, and image analysis was performed,[35] enabling us to obtain lattice stripes of aromatic layers (Figure b,c). In this paper, the distribution of the aromatic layer in Buertai coal was calculated by the Daniel Van Niekerk[36] classification method (Figure S5). The calculation results are shown in Table S7. The calculation results of the statistical distribution of aromatic sheets are shown in Figure . It can be seen from Figure that the number of aromatic condensation rings in Buertai coal mainly aggregates below 3 × 3, which indicates that the degree of condensation of aromatic rings in Buertai coal is low.

Figure 3

(a) Raw HRTEM image of Buertai coal; (b) stripe extraction image; and (c) false-color image.

Figure 4

Statistical distribution of aromatic sheets in Buertai coal.

Macromolecule Characteristics of Buertai Coal

Aromatic Structure

According to the 13C-NMR results, the aromatic carbon content of the coal was 0.81, the ratio XBP of the bridge carbon atom to the peripheral carbon atom in the coal structure was 0.23, while the XBP of naphthalene is 0.25 and the XBP of the anthracene is 0.40. The HRTEM analysis shows that in the Buertai coal structure, anthracene, benzene, naphthalene, and phenanthrene are the main aromatic groups, and only 1–2 aromatic groups were selected. Therefore, in developing the structure model of Buertai coal in this experiment, naphthalene was assumed to be the main component supplemented with other aromatic condensation rings, and the number of aromatic condensation rings was continuously adjusted to maintain the XBP value of the model close to 0.23. Based on the results of instrumental analyses using FTIR, 13C-NMR, and HRTEM, the type and number of aromatic ring structures composing the Buertai coal structure model are summarized in Table , and the calculated total number of aromatic carbons is 145.

Table 2

Aromatic Carbon Structure of Buertai Coal

Aliphatic Structure

Based on the FTIR analysis results, the aliphatic carbon atoms in Buertai coal mainly occur in the form of aliphatic side chains, naphthenes, and other similar compounds. The infrared spectrum of the 2800 to 3000 cm–1 band showed that the ratio of methylene to methyl in the macromolecular structure of Buertai coal is about 3:1. According to the 13C-NMR analysis results, the ratio of faH:fal* is also 3:1, and thus, it can be found that the number of aliphatic rings in Buertai coal is higher than that of aliphatic chains. From Table , it can be seen that there are 179 C atoms in the structure model of Buertai coal, of which 34 are aliphatic C atoms.

Heteroatom Structure

Using the results of the ultimate analysis and the number of C atoms in the structure model, the number of O and N atoms in the model was determined to be 19 and 2, respectively. Using the results of the ultimate analysis and the number of carbon atoms in the structure model, according to the 13C-NMR results, the oxygen-containing functional groups in Buertai coal exist in the form of six carbonyl carbons, four ether oxygen bonds, and nine phenol groups. Nitrogen in the coal structure is usually present in the form of pyrrole-type N and pyridine-type N. Therefore, combining with the proximate and ultimate analyses, N in the Buertai coal structure model mainly occurs as pyrrole-type N and pyridine-type N. Since the sulfur content of the Buertai coal is very low, only 0.17%, the sulfur element was not considered.

Macromolecular Structure Construction

From the information summarized in Table , we can obtain the classification form and the ratio of the aromatic layer in the Buertai coal structure model. By continuously changing the connections between carbon atoms, different models of the coal macromolecular structure were obtained, and the 13C chemical shift of the structural model was calculated separately until it corresponded with the experimental peak shape,[27] as shown in Figures and 6. This iterative process allowed the structural model to be refined so that it more closely resembled the true macrostructure of Buertai coal. Finally, the plane macromolecular structure of Buertai coal was built, as shown in Figure . The flowchart outlining the process used to develop the model is shown in Figure S6, and the ultimate analysis of the macrostructure model of Buertai coal is summarized in Table .

Figure 5

Calculated spectrum of Buertai coal.

Figure 6

Experimental spectrum of Buertai coal.

Figure 7

Macromolecular structure model of Buertai coal.

Table 3

Structural Parameters of Buertai Coal

molecular formula	molecular weight	element content (%)
C182H142O19N2	2658	C	H	O	N
		82.15	5.38	11.42	1.05

Based on the structural model that was developed for Buertai coal, the molecular mechanics (MM) and molecular dynamics (MD) were optimized, and the energy minimum configuration for Buertai coal macromolecules was found. Details of the spatial configuration optimization of the macromolecular structure model could be found in S10. The relationship between the density and energy of Buertai coal samples was studied by the Amorphous Cell module (AC) in the Materials Studio software. The density corresponding to the lowest energy was the density of the optimal configuration.[37] The energy minimum was reached when the density increased to 1.23 g/cm3, as shown in Figure . To get closer to the real macromolecular structure, an amorphous cell containing 10 coal molecules was first built and used as the initial configuration for subsequent simulations, as shown in Figure , the unit cell size was 33.36 Ǻ × 33.36 Ǻ × 33.36 Ǻ.

Figure 8

Energy and density relationship of Buertai coal.

Figure 9

Energy optimal spatial configuration of Buertai coal.

Microporous Structures of Coal

Visualization of Microporous Structures

The pore structure affects many properties of coal.[17] The pores in the coal macromolecular structure are mainly micropores (<2 nm); these pores have a great influence on methane adsorption. The Atom Volumes and Surfaces tools in the MS software were used to analyze the micropores in coal. The models of porous structures were built using different probe diameters, as shown in Figure . When using H2O as the probe, it showed the largest number of pores because the dynamic diameter of H2O was smaller. On the contrary, the dynamic diameter of CH4 was larger, so fewer pores could be measured. To more intuitively compare the size of the pores, two slices were cut at the same position of the three microporous structure models. As we can see from Figure , there were more micropores in slice 1 and slice 2 and the micropore size was also bigger than others. This can also be explained from another aspect why the adsorption ability of H2O in coal is greater than that of CO2 and CH4.[22][22]

Figure 10

Micropores seen in H2O (a), CO2 (b), and CH4(c) in macromolecule.

Parameters of Microporous Structures

Eight different gas molecules were used as probes to obtain the surface area, micropore volume, and void fraction of the Buertai coal macromolecule. The results are shown in Figure . When He molecule was used as the probe, the value of the surface area, pore volume, and void fraction were the maximum, which can reach 90 m2/g, 0.1 cm3/g, and 12%, respectively. As the diameter of the probe molecule increased, both pore volume and void fraction decreased evenly but the surface area decreased rapidly when the probe diameter was 3.46 Å. When the diameter of the probe molecule CH4 reached 3.8 Å, the value of the surface area was the minimum. The same result was also confirmed in Figure , where the pore volume curve was calculated using the different probe molecules, and the derivative of the curve with respect to the pore diameter gave the pore size distribution (PSD). The value of the PSD curve reached its maximum when the probe molecular diameter was 2.0 Å, and all pore sizes were found to be smaller than 10 Å, this was consistent with the simulated results reported by You et al.[41] These tiny pores provided a lot of adsorption sites for methane.

Figure 11

Pore structure parameters of different probe molecules.

Figure 12

Pore volume and PSD line at different cavity diameters.

GCMC Simulation

Methane Isothermal Adsorption

First, the methane adsorption isotherms were calculated. As shown in Figure , the absolute adsorption amount of methane increased with the increase in pressure, and the maximum value was about 0.6 mmol/g; however, when the pressure exceeded 5000 kPa, the maximum value did not change anymore. This was because the methane adsorption of coal conforms to the pore-filling mechanism,[39] the gas fills the pores under the action of pressure and will not change after being filled; therefore, the adsorption amount of methane would not increase. Generally, the experimental adsorption amount is the excess adsorption amount, and the excess adsorption amount of methane from the GCMC simulation is almost equal to that of experimental adsorption. The dual-site Langmuir model (DSL) was used to fit the adsorption isotherms.

Figure 13

Three kinds of adsorption amount of methane at 303.15 K.

The effect of temperature on the adsorption amount of methane was also significant, shown in Figure . The CH4 adsorption capacity on the macromolecule model of coal increased with increasing pressure and decreased with increasing temperature. At low pressure (<500 kPa), the isolines were more concentrated, changes in pressure had a greater impact on the adsorption capacity, and the temperature had little effect on adsorption capacity. At high pressure (>500 kPa), the isolines were more dispersed, the effect of pressure on adsorption was weakened, and changes in temperature had a greater impact on adsorption capacity. Under high-temperature conditions, it was not conducive to the adsorption of CH4 by the coal structure model because the adsorption of coal molecules to methane molecules was physical adsorption;[40] from the equation of state of an ideal gas, we can conclude that when the pore volume was constant, the increase in the temperature would lead to a decrease in the number of methane molecules. This is also exemplified in the work undertaken by Yu et al.[14]

Figure 14

Absolute adsorption amount of methane at different temperatures.

Adsorption of Methane in Micropores

To explore the methane adsorption mechanism in the micropores, the structure of micropores with the density map after methane adsorption were combined; in this way, the adsorption state of methane in the macromolecular structure could be observed more clearly (Figure ). Three slices were cut at equal distances on the three axes of XYZ, and from these slices, it was found that methane adsorption in micropores showed pore volume filling mechanism;[42,43] all of the methane was adsorbed inside the pores. When the diameter of the pore was 8.2 Å, the methane in the pore began to scatter. When the diameter of the hole was 8.5 Å, the methane in the pore was dispersed and aggregated into two parts, as shown in Y-Slice 1 and Z-Slice 2. So, the amount of methane adsorbed depended on the pore size when the diameter of the hole was less than 8.5 Å; the larger the pores, the more methane was adsorbed.[44] Also, we also observed that there were many micropores where methane did not appear because these micropores were closed micropores and did not provide a channel for methane to enter.

Figure 15

Overlay of methane density and pore structures.

Conclusions

Ultimate analysis, FTIR, HRTEM, and 13C-NMR analytical techniques were performed on Buertai coal to quantify its structural properties. Based on these structural parameters, the macromolecular structure model of Buertai coal was built. The ratio XBP of the bridge carbon atom to the peripheral carbon atom in the coal structure is 0.23; the aromatic layer of Buertai coal is mainly 3 × 3 or less. The macromolecular structure model of Buertai coal was refined to obtain a coal structure model in great agreement with the experimental NMR spectra. Also, the density of the low-rank coal macromolecular model was 1.23 g/cm3, calculated by Materials Studio software.

The pore structure parameters were calculated then; it was found that as the diameter of the probe molecule increased, both pore volume and void fraction decreased evenly. However, the surface area decreased rapidly when the probe diameter was 3.46 Å. All pore sizes were found to be smaller than 10 Å from the PSD curve of Buertai coal, these micropores provided a lot of adsorption sites for methane. CH4 was adsorbed inside the micropores of coal. The adsorption capacity of CH4 depends on the diameters of micropores when the micropores are less than 8.5 Å. There are many micropores where CH4 did not appear because these micropores are closed and do not provide a channel for CH4 to enter. The methane adsorption simulation results showed that temperature and pressure had different effects on methane adsorption, and the excess adsorption capacity from the GCMC simulation was very close to the experimental result of Buertai coal.

Materials

The coal samples used in this study were from the Buertai Mine of the Shenhua Group, in Fugu County, Shaanxi Province, China. The Buertai coal is located in Jurassic coalfields with low-rank coal in Northwest China. The coal samples collected in the mining area were packed in the multilayer plastic bags and shipped back to the laboratory and then crushed, passed through a 200 mesh sieve, dried at 353.15 K for 24 h, and stored in the dark. The details of the ash removal of coal samples can be found in S1. The proximate analyses and ultimate analyses were conducted on the standards introduced in 2006 and 2008 (GB/T212-2008, GB/T213-2008, GB/T476-2008, and GB/T19227-2006). The density of Buertai coal was obtained according to the Chinese National Standard GB/T 217-2008. The results of the proximate analysis, ultimate analysis, and density of Buertai coal are shown in Table .

Table 4

Proximate Analysis and Ultimate Analysis of Buertai Coal Samples

	proximate analysis (wt %)				ultimate analysis (wt %)
coal sample	Mad	Aad	Vdaf	FCada	Cdaf	Hdaf	Odafa	Ndaf	St,d	R0,max (%)	density (g/cm3)
Buertai	5.68	0.42	31.49	62.41	81.91	4.10	11.55	2.26	0.17	0.63	1.40

By difference; M: moisture; A: ash; V: volatile matter; FC: fixed carbon; ad: air dry; daf: dry-and-ash-free basis; t,d: total dry; R0,max: maximum reflectance; wt %, weight percentage.

Characterization of the Coal Macromolecule

FTIR

A Brüker Tensor 27 FTIR spectrometer was used to acquire the infrared spectra. Before the experiment, KBr was dried at 393.15 K for 5 h; after that, an appropriate amount of coal was mixed with the dried KBr and ground in an agate mortar until both components were fully ground. The powdered mixture was then pressed and scanned. The spectrum was recorded in the absorbance mode from 400 to 4000 cm–1, and 32 scans were co-added.

13C-NMR

The carbon spectrum of the Buertai coal sample was analyzed with a 600 MHz Brüker Advance III NMR spectrometer. The probe type was a solid double-resonance probe with a sampling time of 0.05 s, a resonance frequency of 75.43 MHz, a ZrO2 rotor with an outer diameter of 6 mm, and a rotor rotation speed of 6 kHz. The pulse width was 4.2 μs, the number of scans was 7000, and the spectra width was 3000 Hz.

HRTEM

The coal samples were analyzed on a JEM-2010 HRTEM with a dot resolution of 0.19 nm, a voltage of 200 kV, and a lattice resolution of 0.14 nm. In the preparation for the analysis, the Buertai coal sample was placed into a glass beaker with an ethanol solution and shaken for 30 min. After the coal sample and the ethanol solution were uniformly mixed, two or three drops of the mixture were deposited onto the microgrid. Once the ethanol was volatilized, the coal sample was loaded into the sample stage and then inserted into an electron microscope for analysis.

Construction of Coal Microporous Structures

The model of coal microporous structure was constructed by the MS Atom Volumes and Surfaces tool. The grid resolution was fine, and the grid interval was 0.025 nm. The calculated parameters were converted by the following equation.[28]

High-Pressure Methane Isotherms

The CH4 adsorption performances of the coal were evaluated using a 3H-2000PH2 high-temperature and pressure adsorption instrument produced by the Beishide Instruments Technology (Beijing) Co., Ltd. The temperature was 303 K and the pressure range of 0–6 MPa, respectively. First, the coal samples were dried at 378 K and held for 24 h to eliminate moisture and then moisture and gases were evacuated at 378 K and held for 10 h. Second, the coal samples were filled with methane at a pressure of up to 6 MPa. After the impurities were completely removed, the sample tube was placed in a water bath at 303 K for over 10 h to ensure an adsorption equilibrium was achieved at every point. Ten measurement points were made in the experiment; the adsorption equilibrium time was approximately 12 h. The static volumetric method was used to measure the adsorptive capacities of the materials under different pressures.

Based on Materials Studio (MS) software, the methane isotherms were calculated using the GCMC simulation method. The transformation between pressure and fugacity was calculated by the Peng–Robinson equation.[29] The adsorbent molecules were optimized with the minimum energy geometry after optimizing the annealing of Buertai coal. The isothermal adsorption temperature range was 273.15–333.15 K, the temperature interval was 10 K, and the pressure range was selected from 10 to 6000 kPa. In this paper, the Lennard–Jones (LJ 9–6) potential energy model was used to describe the interactions of van der Waals, and the force field was Compass II.

The adsorption amount obtained by MS software was an absolute adsorption amount that needs to be converted to excess adsorption amount by the following equation to compare with experiment adsorption amount.where Nexcess is the excess adsorption amount, Nabs is the absolute adsorption amount, NA is the Avogadro constant, P is the pressure (Pa), Vf is the free volume, R is the universal gas constant (8.314 J·mol–1·K–1), and T is the temperature (K).[38]