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. 2020 Dec 30;6(1):523–532. doi: 10.1021/acsomega.0c05008

A Study on the Effect of Coal Metamorphism on the Adsorption Characteristics of a Binary Component System: CO2 and N2

Hongqing Zhu 1, Xin He 1,*, Yuyi Xie 1, Song Guo 1, Yujia Huo 1, Wei Wang 1
PMCID: PMC7807769  PMID: 33458504

Abstract

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At present, there is little research on the multicomponent gas adsorption characteristics of coal with different metamorphisms. In this study, DH (low metamorphism), FGZ (medium metamorphism), and DSC (high metamorphism) coal samples were selected as the microscopic research objects, and the molecular models of them were constructed by means of elemental analysis, 13C NMR, and X-ray. The adsorption characteristics of coal with different metamorphisms under the binary component system (CO2 and N2) were explored by experiments and simulations at 298 K and 1000 kPa. The results showed that in the binary component system gas environment, the adsorption strength of CO2 is stronger than that of N2. DH has the highest isosteric heat of adsorption, and the adsorption strengths of CO2 and N2 is stronger than that for FGZ and DSC. The adsorption amounts of CO2 and N2 by three coal molecules are ranked as DH > FGZ > DSC. The sequence of adsorption selectivity of CO2/N2 is DH > FGZ > DSC > 1, which demonstrates the stronger competitiveness of CO2 than N2. The adsorption selectivity of CO2/N2 for DH is stronger than that for DSC. However, with the increase of the CO2 component, the adsorption selectivity of CO2/N2 for DH has a great influence, while DSC is relatively stable. The simulation results display a good agreement with the experimental results. The research can improve the accuracy and efficiency of inert injection measures and has guiding significance for the prevention and control of coal spontaneous combustion accidents by inert injection.

1. Introduction

Coal is widely used as a kind of primary fossil fuel and a chemical raw material all over the world.1,2 However, the coal spontaneous combustion easily leads to fire disasters during coal production, transportation, and utilization, which causes serious resource waste, environmental pollution, and is even threatening to workers’ lives and health.3−5 For nearly half a century, coal chemistry scholars at home and abroad have been devoted to exploring the molecular structure properties of coal from a microscopic perspective, which not only provides a methodological basis for the understanding of the nature of coal but also has important significance for the study of the spontaneous combustion characteristics of coal.

Different from other macromolecular organic matters, coal has no united physical and chemical structural forms, so the compositions and structures and morphological structures of coal molecules with different metamorphism degrees vary significantly. According to the coal metamorphism degree from low to high, coal can be divided into three categories: lignite, bituminous coal, and anthracite.6 Researchers7−10 analyze the molecular structure changes of coal by a variety of chemical experiments, such as pyrolysis, polycondensation, etc., and explore the effect of small molecular structure products produced by splitting decomposition on the changes of coal molecular structure characteristics, to obtain the molecular structure of coal by inverse deduction. Through continuous improvement and development, the coal molecular structure model has successively gone through the stages of the Fuchs model,11 Given model,12 Wiser model,13 and Shinn model.14 In the present period, modern techniques, such as X-ray, Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), 13C NMR, and so on, are often used for research and analysis. Lian et al.15 based on X-ray photoelectron spectroscopy, 13C NMR, and ultimate analysis and constructed the molecular model of the DMC scaffold (DMC-S). Wu et al.16 used XRD, FTIR, 13C NMR, SEM, and AFM techniques.

In the study of gas adsorption, CO2, N2, CH4 and other mixed gases are mostly studied. Wu et al.17 took bituminous coal as the research object, and under the conditions of 298.15–318.15 K and up to 10,000 kPa, he carried out the grand regular Monte Carlo and molecular dynamics simulations for the single, binary, and ternary component systems of CO2, N2, and O2. Wu et al.18 studied the ignition characteristics of bituminous coal under the conditions of O2/N2 and O2/CO2 atmospheres. Gao et al.19 studied the adsorption characteristics of CH4, CO2, N2, and H2O for lignite. Zhao et al.20 and Brochard et al.21 studied adsorption characteristics of CO2–CH4 binary component systems for a single type of coal. Li et al.22 studied the molecular simulation of adsorption characteristics of CH4, CO2, and N2 multicomponent gases in coal and the influence of gas adsorption on coal spontaneous combustion. Zheng et al.23 studied the multicomponent gases’ competitive adsorption properties and its effects on the coal oxidations, which were investigated by a gas adsorption analyzer and in situ FTIR spectroscopy. Gao et al.24 studied the synergistic mechanism of CO2 and active functional groups during the low temperature oxidation of lignite. Zhang et al.25 used thermal analysis and a programmed temperature rise to study the influence of N2 on secondary coal oxidation from the aspects of thermal behavior and fire-extinguishing ability of secondary coal oxidation. In the study of coal with different metamorphic degrees, the scholars mainly studied the adsorption of methane gas. Wang et al.26 studied the changes of rock physical properties and adsorption capacity of different ranks of coal. The results show that the content of organic matter in vitrinite is the most, but the content of organic matter is controlled by sedimentation and has nothing to do with the coal rank. The lower the ratio of vitrinite to inert rock, the greater the inherent water content, the more serious the adsorption of methane. Li et al.27 calculated and simulated the energy of methane adsorption by different rank coals, and the relationship of adsorption capacity of different coal samples was as follows: high bituminous coal and anthracite > low bituminous coal > medium bituminous coal; Gao et al.28 found that the unit structure of coal was very important to the accuracy of methane content through the simulation of methane adsorption on different rank coals. The structure of the supermonomer indicates the degree of coal metamorphism and has a great influence on the adsorption of methane in coal. Meng et al.29 used the method of combining GCMC and DFT to study the adsorption mechanism of coal methane with different degrees of deterioration at a 298 K temperature and 0–100 bar pressure. The CO adsorption experiments of different metamorphic coals at a constant temperature and pressure were studied by Liu et al.30 The results show that with the increase of coal rank, the adsorption capacity of CO increases at first and then decreases.

The above studies mainly focus on the multicomponent gas adsorption of a single type of coal, while the multicomponent gas adsorption characteristics of coal with different metamorphism are rarely mentioned, and there is a lack of targeted measures for the prevention and control of spontaneous combustion of different types of coal. In order to explore the microscopic mechanism of inert gas prevention and control of coal spontaneous combustion accidents and improve the accuracy and efficiency of inert gas measures, lignite from Danhou mine, bituminous coal from Fangezhuang mine, and anthracite from Dashucun mine were selected as the research objects in this manuscript. Elemental analysis and 13C NMR and X-ray detection methods were adopted to analyze and study coal samples and build a molecular structure model. By using molecular mechanics (MM) and molecular dynamics (MD) simulations and then the accuracy of simulation is verified by experiments, the adsorption characteristics of coal with different metamorphisms under CO2 and N2 binary component system environments were explored from a microscopic perspective.

2. Results and Discussion

2.1. Molecular Structure Modeling and Optimization

2.1.1. Test Results of the Total Elemental Analysis

After the experiment, the elemental analysis results are shown in Table 1. In order to obtain more accurate elemental analysis results, the samples were tested for elements C, H, N, S, and O, respectively. The relative content values of the listed elements were determined by experiments instead of the common subtraction method.

Table 1. Element Composition of Coal Samples.
name weight (mg) C (%) H (%) O (%) N (%) S (%)
DH 1.6530 58.88 4.36 25.38 0.65 0.21
FGZ 1.6770 80.17 4.76 4.728 1.51 1.29
DSC 1.8250 82.48 3.62 3.399 1.26 0.18
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2.1.2. Result Analysis of the Nuclear Magnetic Resonance Carbon Spectra (13C NMR) Test

From the results of nuclear magnetic resonance carbon spectra (13C NMR) tests of DH, FGZ, and DSC, it can be seen that the main chemical shifts of DH and FGZ samples appear at 0–70 × 10–6, 100–170 × 10–6, and 210–250 × 10–6, and the main chemical shifts of DSC appears at 100–150 × 10–6, as shown in Figure 1.

Figure 1.

Figure 1

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13C NMR test carbon spectra and split peak fitting diagram of DH (a), FGZ (b), and DSC (c).

Due to the complexity of the coal structure and the limitation of nuclear magnetic resonance technology, it is necessary to split the peaks of the spectra to obtain more detailed structure information. The 13C NMR carbon spectrum of coal samples is used for the peak splitting operation, and the peak position was added as completely as possible to ensure a high fitting degree and accuracy so that the result of the peak splitting was more consistent with the experimental results. After the split-peak fitting operation, data analysis is performed on the results, and the structural attribution is determined according to the chemical shift value; then, the relative area value of each coal sample structure is used to calculate the 12 structure parameters of the coal sample, as shown in Table 2.

Table 2. Structure Parameters of Coal Samplesa.
namea fa faC fa′ faN faH faP faS faB fal fal* falH falO
DH 60.12 0.96 59.16 36.86 22.30 7.45 9.94 19.46 39.88 13.63 19.46 6.79
FGZ 82.69 0.64 82.05 26.28 55.77 2.56 5.77 17.95 17.31 6.41 10.26 0.64
DSC 98.22 1.93 96.29 19.25 77.04 0.53 9.34 9.38 1.78 0.24 0.26 1.29
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a

fa: total aromaticity carbon; faC: carbonyl group carbon; fa′: carbon in aromatic nucleus; faN: nonprotonated aromaticity carbon; faH: protonated aromaticity carbon; faP: oxygen-linked aromaticity carbon; faS: side branch aromaticity carbon; faB: bridging aromaticity carbon; fal: total aliphatic carbon; fal*: methyl carbon or quaternary carbon; falH: methylene carbon; and falO: oxygen-linked aliphatic carbon.

According to the value of structural parameters, the important parameter XBP of the macromolecular structure can be calculated, namely, the ratio of aromaticity bridge carbon to perimeter carbon. This parameter reflects the average value of the degree of condensation of aromatic rings in the coal structure.Inline graphic. According to the formula, the XBP of DH is 0.49, the XBP of FGZ is 0.28, and the XBP of DSC is 0.11.

2.1.3. Result Analysis of the X-ray Photoelectron Spectroscopy (XPS) Test

The results of X-ray photoelectron spectroscopy (XPS) of DH, FGZ, and DSC show that different peak positions correspond to different existing states of an element. As shown in Tables 3–5, the corresponding structural attribution and proportion of the elements were obtained from the split-peak fitting operation for each element.

Table 3. Structure Attributions of C, O, and N Atoms in DH.
atom peak position (BE) area (P) relative area (%) structure attribution
C 283.81 17955.22 66.85 C–C
284.99 5584.85 20.8 C–H
286.25 824.32 3.07 C–O
287.81 2491.69 9.28 C=O
O 531.13 12761.57 29.31 C–O
531.27 12816.36 29.44 C–O
531.78 4710.54 10.82 C–O
532.55 13244.37 30.43 C=O
N 399.22 943.18 70.88 pyridinic nitrogen N-6
400.04 301.11 22.63 pyridinic nitrogen N-5
401.29 50.74 3.82 quaternary nitrogen
402.08 35.59 2.68 oxidized nitrogen
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Table 5. Structure Attributions of C, O, N, and S Atoms in DSC.
atom peak position (BE) area (P) relative area (%) structure attribution
C 283.61 31471.28 67.64 C–C
284.42 11660.33 25.07 C–H
286.94 1171.96 2.52 C–O
289.18 2216.69 4.77 C=O
O 531.12 1639.02 12.63 C–O
531.73 11111.67 85.64 C=O
535.97 224.74 1.73 COOH
N 397.82 1054.98 51.25 N-6
399.41 428.13 20.81 N-5
401.13 105.38 5.12 N-Q
401.89 76.20 3.71 N-Q′
402.94 392.88 19.11 N–O
S 163.14 252.17 100 thiophene-S
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Table 4. Structure Attributions of C, O, N, and S Atoms in FGZ.
atom peak position (BE) area (P) relative area (%) structure attribution
C 283.54 31776.32 70.74 C–C
284.45 10376.05 23.10 C–H
287.64 1908.42 4.25 C–O
289.73 856.57 1.91 C=O
O 531.60 7920.54 73.25 C–O
531.78 2538.82 23.48 C–O
532.29 215.63 1.99 C=O
532.57 138.31 1.28 C=O
N 396.99 256.79 18.60 pyridinic nitrogen N-6
397.76 194.67 14.10 pyridinic nitrogen N-6
398.37 354.90 25.71 pyridinic nitrogen N-6
399.04 573.99 41.59 pyridinic nitrogen N-6
S 163.04 680.93 100 thiophene-S
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2.1.4. The Establishment of a Coal Macromolecular Structure Model

According to the element, 13C NMR, and XPS detection result analyses, the proportion and quantity of each structure in the coal macromolecular structure can be roughly calculated. The macromolecular structure diagrams are drawn and revised to get the final plan view of the DH, FGZ, and DSC macromolecular structure models. Materials studio 8.0 can be used for structure optimization and annealing simulation, a three-dimensional view of the coal macromolecular structure model after optimization is obtained, as shown in Figure 2.

Figure 2.

Figure 2

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Macromolecular structure diagram of DH (a), FGZ (b), and DSC (c).

2.1.5. Optimization of the Unit Cell Density Structure

The result shows that the density corresponding to the lowest energy of coal molecules reached for the first time cannot reflect the true density of coal, and the density corresponding21 to the lowest energy value is more accurate after exceeding the value. After the simulation, the DH macromolecular model density is 1.15 g/cm3 and the energy is 1008.244 kcal/mol; the FGZ macromolecular model density is 1.1 g/cm3 and the energy is 1652.33 kcal/mol; the DSC macromolecular model density is 1.15 g/cm3 and the energy is 895.073 kcal/mol, as shown in Figure 3.

Figure 3.

Figure 3

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Macromolecular structures of DH (a), FGZ (b), and DSC (c) with their unit cell structures.

2.2. Data Analysis of Adsorption Characteristics

The molecular models of CO2 and N2 were drawn in Materials Studio 8.0 software, and both were optimized by using the geometric optimization and energy tasks in the force module. Figure 4 shows the optimized molecular model of CO2 and N2, with a C=O bond distance of 1.160 Å and a N≡N bond distance of 1.098 Å, for the adsorption simulation study.

Figure 4.

Figure 4

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Molecular models of CO2 (a) and N2 (b).

2.2.1. Isosteric Heat of Adsorption

Adsorption heat refers to the direct interaction between the adsorbent and the adsorbate, and the heat released in the process of gas adsorption reflects the adsorption strength to some extent.17,31Figure 5 shows that the isosteric heat of adsorption of CO2 for DH is about 8.8–9.0 kcal/mol, that for FGZ is about 8.6–8.9 kcal/mol, and that for DSC is about 8.2–8.4 kcal/mol; the isosteric heat of adsorption of N2 for DH is about 4.5–4.8 kcal/mol, that for FGZ is about 4.2–4.5 kcal/mol, and that for DSC is about 4.4–4.7 kcal/mol. Therefore, the isosteric heat of adsorption is CO2 > N2, and the isosteric heat of CO2 is about twice as much as that of N2, that is, the adsorption strength of CO2 is obviously stronger than that of N2. The isosteric heat of adsorption of CO2 in the mixed gas environment by three coal molecules are ranked as DH > FGZ > DSC; but that of N2 are ranked as DH > HD > DSC. It can be seen that the DH lignite with low metamorphism has the strongest adsorption strength for CO2 and N2 compared with other two kinds of coal. Because the O contents of DH is much higher than that of FGZ and DSC, as shown in Table 1, the O contents can significantly enhance the isosteric adsorption heat of CO2 and N2.32−34

Figure 5.

Figure 5

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Isosteric heat of adsorption for DH, FGZ, and DSC at 298 K and 1000 kPa.

It can be seen from the isosteric heat of adsorption of CO2 and N2 in the CO2/N2 mixed gas environment for DH, FGZ, and DSC that the minimum value of CO2/N2 appears at the CO2 component that is less than 10%. At this point, the adsorption strength is weak, and the possibility of desorption is greater. When the CO2 component is greater than 10%, the isosteric heat of adsorption of CO2/N2 for the three coal samples generally shows an upward trend. With the increase of the component of CO2, the adsorption strength of CO2 and N2 also increases, and the adsorption becomes more firm, and the possibility of desorption reaction decreases.

2.2.2. Adsorption Amount and Adsorption Selectivity

2.2.2.1. Adsorption Amount

The adsorption amount simulated by the sorption module in Materials Studio 8.0 software is in the form of the number of adsorbed gas molecules, and the unit is the number of average molecules/cell. Therefore, it is necessary to convert the simulated value into the actual value of a unified unit and introduce the conversion formula,19Inline graphic, where a means the number of adsorption molecules and M indicates the relative molecular weight of coal molecules, in g/mol; A is the result of unit conversion, in mmol/g.

The molecular formula of DH is C215H191O30N, MDH = 3265 g/mol, the molecular formula of FGZ is C156H121O10N3S, MFGZ = 2227 g/mol, and the molecular formula of DSC is C166H108O7N2, MHD = 2240 g/mol. The calculation results are shown in Figure 6a.

Figure 6.

Figure 6

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Adsorption amounts of CO2 and N2 for DH, FGZ, and DSC from simulation (a) and experiment (b) at 298 K and 1000 kPa.

In the pure CO2 adsorption environment (the component of CO2 is 100%), the adsorption amount for DH is 7.03 mmol/g, that for FGZ is 5.67 mmol/g, and that for DSC is 4.95 mmol/g; in the pure N2 adsorption environment (the component of CO2 is 0%), the adsorption amount for DH is 2.59 mmol/g, that for FGZ is 2.38 mmol/g, and that for DSC is 1.64 mmol/g. The adsorption amounts of CO2 and N2 in the mixed gas environment by three coal molecules are ranked as DH > FGZ > DSC, which closely agrees with the simulation result from Gao et al.28 that the high metamorphism of coal is negative to the adsorption amounts of CO2 and N2. This is contrary to the fact that researchers generally believe that the adsorption amount of coal is positively correlated with the degree of metamorphism34,35 However, many researchers have found that the coal metamorphism is not linearly related to the adsorption amount.27,29,36,37 According to our research, DH is a lignite with low metamorphism, and its oxygen-containing functional groups (OCFGs) are much higher than that of the other two coal samples. There are more OCFGs, including 2 —C=O, 16 −O–, 1 −COOH, and 5 —C(=O)—O; FGZ is a medium metamorphism bituminous coal, and its molecular structure consists of six —C=O and four −OH; DSC is anthracite with high metamorphism, including two —C=O, two −OH, one −O–, and one —C(=O)—O. The surface OCFGs can significantly enhance the CO2 and N2 adsorption amount.31

As can be seen from the adsorption amount curve, when the component of CO2 is less than 10%, CO2 adsorption is in a state of rapid growth, N2 adsorption is in a state of rapid decline, and the adsorption amount of N2 is less than 1 mmol/g. When the component of CO2 was divided into 10–60%, the CO2 adsorption showed a slow upward trend, and the N2 adsorption showed a slow downward trend, tending to 0 mmol/g. When the component of CO2 is greater than 60%, the CO2 adsorption tends to be saturated, and the N2 adsorption is close to 0 mmol/g.

In order to ensure the validity of experimental data, error analysis was carried out for each experiment of three kinds of coal samples. The concepts of uncertainty and confidence interval are introduced. The uncertainty of the measured adsorption amount of CO2/N2 can be estimated as Xi = Xi (measured) ± δXi, Inline graphic, where the value Xi (measured) is the mean value of the n set of repeated experiments, σ is the standard deviates of repeated experiment data.38,39Figure 6b plotted the mean value and error bar of adsorption amount of CO2/N2 after repeated experiments and determined the uncertainty of CO2/N2 measured. The relative uncertainty of the adsorption amount of CO2/N2 measurements is less than 6% (±2σ/–T) with 95% confidence.

The adsorption experiment results showed that the variation trend of the adsorption amount was consistent with the simulation results, as shown in Figure 6; the adsorption amount of CO2 was greater than N2, and the adsorption amount of CO2 and N2 in the mixed gas environment by three coal molecules are also ranked as DH > FGZ > DSC. There are errors between the adsorption amount of experiment and the average adsorption amount from simulation, as shown in Figure 7. The simulation results of the CO2 adsorption amount are generally higher than the experimental results, which may be affected by coal ash. The experimental results of the N2 adsorption amount were higher than the simulated results, which is due to the fact that the N2 adsorption amount was small and tended to be 0 mmol/g after the CO2 component was greater than 20%. Therefore, the experimental instrument accuracy is not enough and the measurement value is relatively large. Meanwhile, the errors may be caused by insufficient adsorption, inaccurate measurements, and improper storage of samples. The mean absolute error (MAE) between experimental and simulated results of CO2 is less than 1.2 and that of N2 is less than 0.15. Therefore, it is feasible to study gas adsorption behavior by constructing coal molecules.

Figure 7.

Figure 7

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Mean absolute error (MAE) between experimental and simulated for DH (a), FGZ (b), and DSC (c).

2.2.2.2. Adsorption Selectivity

Adsorption selectivity is a criterion used to assess the performance of a sorbent in preferentially adsorbing one species in a binary mixed atmosphere.29 The adsorption selectivity,17,20,29Inline graphic, is defined as SCO2/N2 where xCO2 (or xN2) and yCO2 (or yN2) represents the mole fraction of species CO2 (or N2) in the adsorbed phase and bulk phase, respectively. Therefore, the fact that the adsorption selectivity SCO2/N2 is larger than 1 indicates that the competitive capacity of adsorbate CO2 in the mixed components is stronger than adsorbate N2, and the greater the selectivity, the stronger the adsorption. If the SCO2/N2 is less than 1, the competitive adsorption capacity of adsorbate N2 is stronger than adsorbate CO2. The SCO2/N2 is equal to 1, which means that the competitive adsorption capacity of adsorbates CO2 and N2 is equal.

To ascertain the competitive ability of the coal with different metamorphisms for CO2 and N2, the adsorption selectivity is presented in Figure 8a, with various components of CO2 at 298 K and 1000 kPa. The SCO2/N2 values of three coal samples are greater than 1, which indicates that CO2 is much more competitive than N2. The SCO2/N2 ranges of DH is between 44 and 66, the SCO2/N2 ranges of FGZ is between 40 and 56, and the SCO2/N2 ranges of DSC is between 37 and 44; the sequence of SCO2/N2 was DH > FGZ > DSC. It can be seen that the adsorption selectivity of CO2/N2 for DH with low metamorphic is stronger than that for DSC with a high metamorphic degree, indicating that the adsorption selectivity of CO2/N2 on the metamorphic degree is negative. After linear fitting of the adsorption selectivity values, the adsorption selectivity of CO2/N2 to coal samples was negatively correlated with the component of CO2 in the mixed gas environment; the |k| of the fitted curve is DH > FGZ > DSC. With the increase of the CO2 component, the adsorption selectivity of CO2/N2 for DH with low metamorphism has a great influence, while DSC with high metamorphism is relatively stable.

Figure 8.

Figure 8

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Adsorption selectivity of CO2/N2 for DH, FGZ, and DSC from simulation (a) and experiment (b) at 298 K and 1000 kPa.

According to the experimental results, the adsorption selectivity of CO2/N2 is calculated as shown in Figure 8b. The trend and |k| of the fitted curve of DH, FGZ, and DSC from experiment are similar to simulation. However, the SCO2/N2 of DH ranges from 29 to 41, the SCO2/N2 of FGZ ranges from 25 to 34, and the SCO2/N2 of DSC ranges from 23 to 30. All the SCO2/N2 from experiment are smaller than those from simulation. According to the MAE analysis in Section 2.2.2.1, due to the small N2 adsorption amount and the insufficient precision of the experimental instrument, the measured value is relatively large, and the calculation result of adsorption selectivity is relatively small.

To a certain extent, the results of simulation and experiment reveal the adsorption characteristics of coal with different metamorphic degrees in a binary mixed gas environment. In order to study this phenomenon further, we will continue to analyze and summarize more coal samples to obtain more perfect research conclusions.

3. Conclusions

In order to study the adsorption characteristics of coal with different metamorphisms in the environment of binary component system gases of CO2 and N2, the macromolecular structure models of DH, FGZ, and DSC were constructed by using elemental analysis and 13C NMR and XPS detection methods. By means of simulation and experiment, the microscopic mechanism of CO2 and N2 mixed gases adsorbed by coal with different metamorphisms was explored.

Molecular mechanics (MM) and molecular dynamics (MD) simulations were used to optimize the molecular structure, and GCMC simulation was used to calculate the isosteric heat of adsorption and the adsorption amounts of three kinds of coal samples in the multicomponent gas environment of different proportions at 298 K and 1000 kPa. After comparison and analysis with the experimental values, the following relationships were obtained: (1) For the same coal sample, the adsorption strength of CO2 is stronger than that of N2; for different coal samples, the DH with low metamorphism has the highest isosteric heat of adsorption, namely, the strongest adsorption strength for CO2 and N2, which is stronger than FGZ and DSC. (2) The adsorption amounts of CO2 and N2 from three coal molecules are ranked as DH > FGZ > DSC, indicating that high metamorphism of coal is negative to the adsorption amounts of CO2 and N2. (3) The sequence of adsorption selectivity of CO2/N2 was DH > FGZ > DSC > 1, which demonstrates the stronger competitiveness of CO2 than N2. (4) The adsorption selectivity of CO2/N2 for DH with a low metamorphic degree is stronger than that for DSC with a high metamorphic degree. However, with the increase of the CO2 component, the CO2/N2 adsorption selectivity for DH with low metamorphism has a great influence, while DSC with high metamorphism is relatively stable. The research can improve the accuracy and efficiency of inert injection measures and has guiding significance for the prevention and control of coal spontaneous combustion accidents by inert injection.

4. Experimental and Simulation Methods

4.1. Experimental Methods

4.1.1. Sample Preparation

Select lignite from Danhou mine (DH), bituminous coal from Fangezhuang mine (FGZ), and anthracite from Dashucun mine (DSC) samples remove respectively the surface part of large fresh coal samples and crush them into 60–80 mesh pulverized coal; use hydrofluoric acid to demineralize the pulverized coal to reduce the influence of minerals in the pulverized coal on the experiment analysis results; then, clean the demineralized pulverized coal until the solution becomes neutral; finally, put the pulverized coal into the drying box, and vacuum packaging is carried out immediately after drying for 12 h.

4.1.2. Elemental Analysis

The elemental analysis of the sample is conducted on the basis of the national standard (GB/T 476-2008) to determine the dry mineral–matter free content of elements C, H, N, S, and O in the coal macerals.

4.1.3. Nuclear Magnetic Resonance Carbon Spectra (13C NMR) Test

The nuclear magnetic resonance carbon spectra (13C NMR) are tested on the JNM-ECZ600R spectrometer at the Zhong Ke Bai Ce laboratory; the resonance frequency is 150.91 MHz. Use a 3.2 mm probe to record the spectra at a 15 kHz rotation rate at room temperature. The experiment delay time is 3 s, and the contact time is 2 ms. The number of scans is 2000.

4.1.4. X-ray Photoelectron Spectroscopy (XPS) Test

X-ray photoelectron spectroscopy (XPS) test is performed on a Thermo escalab 250Xi X-ray photoelectron spectrometer (XPS) at the Zhong Ke Bai Ce laboratory. Use a 180° hemispherical energy analyzer, Al target, power of 150 W, 650 μm beam spot, voltage of 14.8 kV, and current of 1.6 A; and use the pollution carbon C1s = 284.8 eV to complete the charge calibration. The common energy for a narrow scan is 20 eV and 100 eV for a wide scan, and the vacuum degree is 1 × 10–10 mbar.

4.1.5. Adsorption Experiments

The principle of the adsorption experiment system is shown in Figure 9, including the vacuum pumping system, the adsorption reaction system, and the data acquisition system. The vacuum pumping system includes a vacuum pump, a vacuum pressure gauge, and an adsorption cylinder; the adsorption reaction system includes a high-pressure gas cylinder, pressure gauge of mixed gas, hose, and adsorption cylinder; the data acquisition system includes an adsorption cylinder, pressure gauge, and a gas chromatograph. Through direct observation, read the pressure difference between the adsorption cylinder and the pressure gauge before and after the adsorption reaction, and calculate the total adsorption amount of the system; the content of each component was determined by gas chromatography.

Figure 9.

Figure 9

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Adsorption experiment system.

The pulverized coal is placed in the adsorption tank, because the pressure is proportional to the amount of the material at room temperature. Therefore, the gas pressure ratio is the gas component ratio. The mixed gas was mixed in different proportions and injected into the adsorption cylinder for the adsorption experiment. The experiment lasted for 24 h. The difference value of the pressure gauge and the gas data of each component detected by the gas chromatograph were read to calculate the adsorption amount. It was set that the CO2 group was divided into a group of 0–100% and every interval of 20%. Coal is a known substance whose structure may vary greatly.40 In order to reduce the experimental error, a total of 18 experiments were carried out on the three kinds of coal samples, each of which was conducted for three times.

4.2. Simulation Methods

4.2.1. The Establishment of a Coal Molecular Structure Model

We perform the results of nuclear magnetic resonance carbon spectra (13C NMR) and X-ray photoelectron spectroscopy (XPS) to split-peak fitting operation. According to the fitting results, attribution and parameters of the carbon skeleton structure as well as the heteroatom structure attribution are determined, respectively. Then, build a plan view of a coal macromolecular structure model with reference to the test results of elemental analysis, and make the nuclear magnetic resonance carbon spectra (13C NMR) prediction about the plan view drawn. By comparing the predicted spectra with the nuclear magnetic resonance carbon spectra (13C NMR) test, the structural connection position and method are further adjusted to obtain a relatively accurate plan view of the coal macromolecular structure model.

By using the Geometry Optimization task in the Forcite module of Materials Studio 8.0 software to geometrically optimize the coal macromolecular structure model plan view, in which the Dreiding force field41,42 is selected, the maximum number of iteration steps is 50,000; annealing kinetics simulation is performed on the optimized model in the previous step to overcome the energy barrier.43 Select the Anneal task in the Forcite module, and set the temperature to 300–600 K, the heating step to 5, the cycle step to 10,000 steps, and the cycle to 10 times. Set the time step to 1 fs and the temperature to constant with the Nose temperature control. In this way, we can obtain the final optimized coal molecular structure model diagram.

4.2.2. Optimization of a Unit Cell Density Structure

Amorphous Cell module in Materials Studio 8.0 software is used to construct a unit cell structure. Set an initial density value of 0.7 g/cm3, a final density value of 1.4 g/cm3, an interval of 0.05 g/cm3, and a number of molecules of three; thus, periodic boundary conditions are added to coal molecules to explore the optimal geometric structure under periodic boundary conditions.

4.2.3. Grand Canonical Ensemble Monte Carlo (GCMC) Simulation

Use the Fixed pressure task in the Sorption module of Materials Studio 8.0 software to calculate the isothermal and isobaric adsorption simulation processes. The optimized molecular models of DH, FGZ, and DSC are used as adsorbents, and the gas mixture containing CO2 and N2 is used as adsorbent. The simulated temperature is 298 K, the pressure is 1 MPa, and the balance steps and production steps are 106 and 107, respectively; the simulated force field continues to use Dreiding, the charge balance method is QEq, the maximum calculation steps are 5000 steps, and the convergence standard is 5e–4 e; van der waals and electrostatic interactions are calculated based on Atom-based and Ewald and Group methods, respectively, and the cutoff distance is 18.5 Å.

By setting values of different partial pressure ratios and adjusting the composition of gaseous substances, multiple sets of data simulation and comparative analysis of the macromolecular structures of DH, FGZ, and DSC are carried out. The component of CO2 was set as a group from 0 to 100% every 10%, and 1, 5, 95, 99%, and pure N2 were set, respectively, which means that there were 45 groups of simulation tasks for three kinds of coal samples.

Acknowledgments

This study was funded by the National Key R&D Program of China (grant no. 2016YFC0801800), the National Natural Science Foundation of China (grant nos. 51704299 and 51804311), and the Open Research Fund of State Key Laboratory of Coal Mine Safety Technology (project no. sklcmst102).

The authors declare no competing financial interest.

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