Open AccessArticle

Molecular Energy of Metamorphic Coal and Methane Adsorption Based on Gaussian Simulation

Tao Yang

^1,2

Jingyan Hu

¹,

Tao Li

^1,*,

Heng Min

and

Shuchao Zhang

School of Mine Safety, North China Institute of Science and Technology, Sanhe 065201, China

Key Laboratory of Mine Filling and Safe Mining of National Mine Safety Administration, North China Institute of Science and Technology, Sanhe 065201, China

Author to whom correspondence should be addressed.

Processes 2024, 12(12), 2621; https://doi.org/10.3390/pr12122621

Submission received: 29 October 2024 / Revised: 18 November 2024 / Accepted: 19 November 2024 / Published: 21 November 2024

(This article belongs to the Topic Energy Extraction and Processing Science)

Download

Browse Figures

Versions Notes

Abstract

Effectively controlling the adsorption and desorption of coal and mine gas is crucial to preventing harm to the environment. Therefore, this paper investigated the adsorption of coal and methane molecules from the perspective of microscopic energy through Gaussian simulation. Gaussian 09W and GaussView 5.0 software were used to construct and optimize the molecular model of four different metamorphic coals, namely lignite, sub-bituminous coal, bituminous coal, and anthracite, and their adsorption structure with methane as well as the energy, bond length, vibration frequency, infrared spectrum, and other data on the optimal structure were obtained. The binding energy of coal molecules and methane from large to small was as follows: sub-bituminous coal (7.3696 KJ/mol), lignite (6.6149 KJ/mol), bituminous coal (5.2170 KJ/mol), and anthracite (4.9510 KJ/mol). The equilibrium distance was negatively correlated with the binding energy, and the molecular structure and position of coal largely determined the binding energy. Additionally, adsorption was more likely to occur between methane molecules and hydroxyl groups. Many new vibration modes were observed during the adsorption of coal and methane molecules. This paper is of practical significance, as studying the adsorption of coal and mine gas can prevent and control mine gas outbursts and ensure safe production.

Keywords:

Gaussian simulation; metamorphic coal; methane adsorption; molecular energy

1. Introduction

China’s energy is characterized by “rich coal, poor oil, and less gas”. Coal resources occupy an important position in China’s energy security. Coal has a complex porous structure with a large surface area, rich pore structure, and strong adsorption capacity for gas. Its large mining volume, high demand, and rich energy storage make it very useful. However, numerous pollutants are produced during coal mining, processing, and utilization, including harmful CO₂, SO₂, and CO. If these pollutants are not controlled, they will cause great harm to the environment. According to research, the release of these substances is related to the fracture of chemical bonds [1]. Therefore, studying the microstructure of coal molecules is vital. Mine gas is the accompanying product of complex chemical changes during coal formation, and its main component is methane. Mine gas diffuses easily and has strong permeability. It is usually free between cracks and pores in coal seams or adsorbed on the surface of coal molecules. Studying the adsorption law of methane gas on the surface of coal molecules is conducive to controlling mine gas adsorption and desorption as well as rationally recycling mine gas. This reduces the risk of underground operations and ensures the safety of people’s lives and property and smooth production activities [2].

At present, great strides have been made in the study of the adsorption of mine gas or methane gas by coal at the microlevel. Chilleri John et al. [3] performed a Gaussian molecular simulation to propose a numerical optimization framework, which can optimize the allocation of computing resources and analyze the parameters of coal adsorption mine gas. Chen et al. [4] analyzed the difference in water and methane adsorption by coal through molecular simulation. When the isothermal adsorption pressure was less than 15 MPa, the adsorption capacity of CH₄ and H₂O gradually increased with the increase in adsorption pressure, and the average adsorption capacity of H₂O was much higher than that of CH₄. When the pressure was higher than 15 MPa, the average adsorption capacity of CH₄ and H₂O reached saturation. Yue et al. [5] performed an isothermal adsorption test and molecular dynamics simulation to study the adsorption capacity and isosteric adsorption heat of coal to CH₄ and deuterated methane (CD₄) and revealed that the isosteric adsorption heat of coal to CH₄ and CD₄ decreased with the increase in adsorption equilibrium pressure. Moreover, the isosteric adsorption heat and mine gas pressure were found to satisfy the exponential function relationship. Yan et al. [6] used molecular dynamics and Monte Carlo methods, based on the coal macromolecular model, to simulate and analyze the adsorption mechanism of methane and CO₂ in coal under different pressure, temperature, and water content conditions. The microscopic variation in adsorption capacity, isosteric heat of adsorption, interaction energy, and coalbed methane of the coal macromolecular structure model has also been studied. Fan [7] performed molecular simulation to establish the macromolecular structure unit of long flame, coking, and lean coal and studied the adsorption of coal and methane at the macro- and microlevels. Li et al. [8] studied the relationship between coal seam gas flow law and coal pore structure using the high-pressure capacity method and high-pressure pump test. The higher the pressure, the smaller the pore size and the greater the influence of adsorption on permeability. Deng [9] used molecular simulation to analyze the chemical structure model of W. Fuchs coal. The results revealed that the different diffusion coefficients of CO₂, H₂O, and N₂ lead to different degrees of methane desorption in the CO₂–CH₄, H₂O–CH₄, and N₂–CH₄ systems in the slit hole. The mine gas desorption amount of coal reservoir is the macroscopic embodiment of methane desorption. Yan et al. [10] used Materials Studio (MS) to characterize the molecular structure parameters and constructed a molecular numerical model to calculate and analyze the macromolecular carbon skeleton structure of different coal samples. Yang et al. [11] used LAMMPS software (stable version 29Sep2021.3) to perform molecular dynamics research and identified the adsorption of methane by coal as typical monolayer adsorption. Moreover, they revealed that expansion deformation enhances the methane adsorption performance of coal. Tang et al. [12] studied the interaction energy, potential difference, and settling time between surfactant and bituminous coal molecules through molecular simulation, and the simulation results were consistent with the experimental results.

Building on the literature and theoretical analysis, this paper uses Gaussian 09 W and GaussView 5.0, among other software, to perform Gaussian simulation. The adsorption capacity of different metamorphic coals to adsorb a single methane molecule is analyzed, and the molecular structure of four metamorphic coals is simplified and refined. The software is used to construct the coal molecular model and the adsorption methane molecular model at different positions. The adsorption energy value, bond length, vibration frequency, and infrared spectrum, among other parameters, are obtained through molecular simulation. Finally, analysis is conducted to refine and summarize the adsorption law of different metamorphic coals in adsorbing a single methane molecule. By studying the energy law of methane molecules adsorbed by coal with different degrees of metamorphism, the adsorption process of coal and methane can be predicted, and the adsorption capacity of methane by coal with different degrees of metamorphism and different positions can be compared horizontally. This can effectively control disasters such as gas outbursts and prevent safety production accidents caused by excessive energy released during the adsorption process.

2. Gaussian Simulation Method

Molecular simulation combines theoretical research with computer technology to simulate the structure and motion of molecules at the microlevel. Gaussian simulation, as a very extensive random simulation of continuous variables, can be used to perform the microscopic analysis of molecules using software, such as Gaussian 09 W (Revision D.01) and GaussView 5.0. By constructing, optimizing, and calculating the molecular model, the energy, bond length, charge, vibration frequency, infrared spectrum, and other information in the molecular motion can be obtained. Gaussian simulation is very powerful and commonly used in calculating the electronic structure and spectral properties of molecules. This article only studies these aspects and does not involve molecular mechanics, so Gaussian simulation was chosen for this research.

2.1. Base Set Selection

Gaussian is software used for quantum chemistry research. By calculating the molecular orbitals of a given molecule, the structure, energy, electron density, frequency, and sofelectrostatic potential, among other properties, can be obtained. Molecular orbitals are generally represented by a linear combination of atomic orbitals. The wave function describing the atomic orbital is called the basis function, and the set describing all the basis functions of an atom is called the basis set. Commonly used basis functions include Slater-Type Orbitals (STOs) and Gaussian-Type Orbitals (GTOs). The STO is a hydrogen wave function, which is the most primitive basis set. The function form has a clear physical meaning, but its mathematical properties are not good. The GTO has good computational properties. STOs can describe the real track, but it involves heavy computations. Although GTOs can greatly simplify the calculation, they have lower accuracy than STOs. Therefore, a linear combination of multiple GTOs is typically used to fit STOs, which can ensure accuracy and simplify the calculation amount to improve the calculation efficiency.

The minimum basis set is STO-nG, and the rough calculation results are generally not used. The people basis set includes the double zeta and three zeta basis sets and has good universality and accuracy. The notation is generally l-mnkG. Taking 6-31G as an example, 6 indicates that the inner orbit is represented by six GTOs fitting an STO, 31 indicates that the valence orbit is represented by two STOs, where 3 indicates that the STO near the nucleus is fitted with three GTOs, and 1 indicates that the STO far from the nucleus is fitted with one GTO. Increasing the basis set improves the accuracy of the simulation, but the amount of calculation and the time required are also greatly increased. In this study, only the adsorption energy of methane molecules adsorbed by different metamorphic coals was analyzed, and a very large basis set was not needed. Thus, time and computing resources can be saved. Here, 6-31G [13] was selected, which can not only meet the accuracy required for simulation but also reduce the amount of calculation and save computing resources.

2.2. Simulation Method

First, the molecular model was drawn using GaussView, and the molecular structure model was constructed according to the properties of the molecular structure and bond length and bond angle. After optimizing the model, a Gaussian simulation of the adsorption process was performed using Gaussian software to obtain the energy, bond length, vibration frequency, infrared spectrum, and other information regarding molecular motion, as shown in Figure 1. The analysis steps of the Gaussian simulation were as follows:

(1): The molecular model was initially constructed using GaussView software based on the actual structure of the molecule. The initial model did not reach the optimal configuration, and the model needed to be optimized. The energy and boundary standard values generally revealed an inverse proportional function trend with the increase in the number of optimizations. When the boundary standard value approached 0 after Gauss optimization, the molecular stability optimization was completed, and the energy value stabilized near a specific value, which was the optimal energy value [14]. The optimized molecular structure model was the optimal configuration, and the configuration was saved and recorded in gif format.
(2): The optimized molecular optimal configuration file was opened in Gaussian software. The analysis method and basis set of the molecular structure need to be filled in the Route Section. The analysis method involved density functional theory, and the analysis method of the B3LYP optimized structure was selected. The basis set was also selected, and “# b3lyp/6-31g opt freq” was filled to run and save the file as an. out file.
(3): Reading the. out file obtained by the above operation can retrieve information such as energy, key length, and charge.
(4): Open the above file in GaussView and check “Read Intermediate Geometries (Gaussian Optimizations Only)”. Clicking on the Optimization Plot under the Result module will show the change in energy and boundary standard value with the increase in optimization times.
(5): Open the above file in GaussView and do not check “Read Intermediate Geometries (Gaussian Optimizations Only)”. Clicking on Vibrations under the Result module will show the molecular vibration frequency, infrared spectrum, and other information.

3. Molecular Modeling of Metamorphic Coal

3.1. The Metamorphic Degree of Coal

Under the combined action of various complex factors, coal undergoes slow metamorphism, and its physical and chemical properties change [15,16]. Coal metamorphism is a complex chemical change process in which dehydration, decarboxylation, demethanization, deoxidation, and polycondensation reactions occur. The influencing factors include temperature [17], pressure [18], and time but mainly temperature. Temperature is greatly affected by the depth of the bottom layer. As the depth of the formation increases, the temperature rises, the aromatic structure in the vitrinite undergoes chemical changes, the functional groups and bonds are reduced, and the chains are shortened and condensed, thereby increasing the metamorphic degree of coal [19]. Coal metamorphism includes deep, normal, and contact metamorphism. During normal metamorphism, the degree of metamorphism of lignite, sub-bituminous coal, bituminous coal, and anthracite [20] increases from low to high. During metamorphism, many coal indexes change. The vitrinite emissivity was not affected by the composition of coal and rock, ash content, and the representativeness of the coal samples but was slightly affected by the degree of reduction, which can better reflect the degree of coal metamorphism. Lignite, sub-bituminous coal, bituminous coal, and anthracite can also be divided according to the vitrinite emissivity. The greater the vitrinite emissivity, the higher the degree of coal metamorphism. The vitrinite reflectance of the four types of metamorphic coal is as follows: lignite (Rr < 0.4%), bituminous coal (0.4% ≤ Rr < 0.5%), bituminous coal (0.5% ≤ Rr < 2.0%), and anthracite (2% ≤ Rr < 6.0%).

The molecular formula and molecular structure of the simplified coal can be obtained by properly simplifying the four metamorphic coals as well as analyzing the coal chemistry and coal molecular structure. The order of increasing structure complexity is lignite (C₁₉H₂₁NO₆), sub-bituminous coal (C₁₆H₂₂O₂), bituminous coal (C₂₂H₁₆O), and anthracite (C₄₂H₂₀O₂); as the carbon content increases, the degree of metamorphism also increases. In the same order, the hardness of the four metamorphic coals increases; the shade increasingly deepens; and the moisture, volatile matter, and smoke and ash produced after combustion decrease. According to their physical and chemical properties, different metamorphic coals can be used for different purposes. By studying the properties of different metamorphic coals, we can explore the changes in coal under different geological conditions and coal-forming years to fully utilize these resources, protect the environment, and ensure safe production [21].

3.2. Simulation of Coal Molecular Energy

The four metamorphic coals are mainly composed of carbon, hydrogen, and oxygen, as well as other elements, including multiple aromatic rings and side chains. Due to differences in structures, the energy, vibration frequency, and infrared spectrum of the coal molecules are different. The optimized molecular plane structure of the four metamorphic coals is shown in Figure 2. The individual element contents of several main elements calculated based on the chemical formula and atomic mass of four types of coal molecules are shown in Table 1.

Figure 2 shows that the lignite molecule contains two benzene rings and three side chains, and the functional groups include hydroxyl, carbonyl, carboxyl, and other structures. The sub-bituminous coal molecule contains two heterocycles, one benzene ring, and two side chains, and the functional groups include hydroxyl groups. The bituminous coal molecule contains four benzene rings, a heterocyclic ring, and a side chain, and the functional groups include hydroxyl groups and other structures. The anthracite molecule contains six benzene rings, multiple heterocycles, and two side chains, and the functional groups include hydroxyl groups. The molecular structure was constructed by Gaussian simulation, and the molecules were optimized to obtain the optimal configuration, as shown in Figure 3. The energy value of the coal molecules is stable at the optimal energy value, and the energy values of the four coal molecules are shown in Figure 4. Noticeably, the energy values of the four coal molecules are all negative. For convenience, the absolute value was taken here. Hartree is the unit of energy of HF (Hartree–Fock), where 1 Hartree = 2625.5 KJ/mol. Figure 4 shows that the optimal energy of lignite, sub-bituminous coal, bituminous coal, and anthracite before single-molecule adsorption is −1242.4106 Hartree, −773.2023 Hartree, −923.0249 Hartree, and −1762.6828 Hartree, respectively. The order of the absolute value is anthracite > lignite > bituminous coal > sub-bituminous coal. Based on this size relationship, advance arrangements can be made for the mining of different types of coal to prevent energy accumulation from causing danger.

Figure 5 depicts the infrared spectra of the four types of metamorphic coal molecules simulated using GaussView. The horizontal axis is the vibration frequency, and the range of spectral frequency in the diagram is 0–4000cm⁻¹. The type of vibration frequency, the intensity of the absorption peak, and the corresponding vibration frequency are listed in Table 2. When the frequency is 1600–3000 cm⁻¹, there is almost no spectral line distribution in the spectrum. Different frequencies represent different vibration modes of different functional groups. The curve of lignite fluctuates markedly, indicating that many different functional groups (such as hydroxyl, carbonyl, carboxyl) are present in lignite. As the degree of metamorphism increases, the curve gradually smoothens, the types of functional groups gradually decrease, the chain shortens, polycondensation occurs, and the aromatic structure increases. A frequency greater than 3000 cm⁻¹ corresponds to the main absorption peak of oxygen-containing functional groups. With the increase in metamorphic degree, the peak and area of this part decrease first and then increase slightly, which conforms to the change rule of the oxygen atom content. The types and quantities of the oxygen-containing functional groups of the four coal molecules also conform to this rule. The frequency in the range from 1500 to 1600 cm⁻¹ is mainly the absorption peak of the aromatic structure. The larger the absorption peak value and area, the more benzene rings there are. The figure shows that anthracite has the highest number of benzene rings. The vibration frequency corresponding to the absorption intensity peak in the infrared spectrum of brown coal corresponds to the aromatic structure, and its vibration mode is also significantly related to the benzene ring and side chain directly connected to multiple hydroxyl groups. The vibration modes corresponding to the absorption intensity peaks in the infrared spectra of sub-bituminous coal, bituminous coal, and anthracite are all significant vibrations of hydroxyl-containing side chains. These three types of coal all have only one oxygen-containing functional group, with a relatively single type of functional group, and the peak does not appear within the vibration frequency range corresponding to the aromatic structure.

3.3. Adsorption of Single Methane and Coal Molecules

Methane (CH₄), an important part of mine gas, can be adsorbed on the surface of coal. By constructing the adsorption model of a single methane molecule and four different metamorphic coal molecules, the energy relationship, vibration frequency, and infrared spectrum during adsorption can be studied. The molecular structure of methane is simple, and the energy value obtained after optimization is −40.5106 Hartree. Methane is usually adsorbed at the benzene ring or side chain of the coal surface molecule. Methane is adsorbed with several benzene rings and side chains of lignite, sub-bituminous coal, bituminous coal, and anthracite, respectively. Through repeated optimizations, the optimal structure, energy value, bond length, vibration frequency, infrared spectrum, and other types of information of a single adsorbed methane molecule can be obtained. When constructing the molecular model of methane adsorbed on the benzene ring, the methane molecule is placed at a parallel distance of approximately two bond lengths on the benzene ring. In contrast, when constructing the molecular model of methane adsorbed on the side chain, the hydrogen atom in the methane molecule is aligned to the oxygen atom of the hydroxyl group (-OH) in the side chain, and the distance is approximately two bond lengths [22]. The adsorption of methane and coal molecules involves a dynamic equilibrium between the molecules. Methane changes from the free state to part of the coal molecules. During this process, energy needs to be released. The formula for calculating the released energy is [23]

E_{a d s} = E_{M} + E_{g a s} - E_{g a s / M}

where

E_{a d s}

is the binding energy of coal and methane molecules required to achieve equilibrium adsorption,

E_{M}

is the energy of coal molecules before adsorption,

E_{g a s}

is the energy of methane molecules before adsorption, and

E_{g a s / M}

is the adsorption energy of the coal and methane molecular system after adsorption.

The molecular adsorption configuration with the highest binding energy can be obtained by comparing the binding energy of coal molecules adsorbing methane molecules at different adsorption positions. Under this configuration, the adsorption between coal molecules and methane molecules is the highest. Through Gaussian simulation, the energy value, vibration frequency, infrared spectrum, bond length, and other types of information under this configuration can be obtained and analyzed. Moreover, the relationship between the metamorphic degree, adsorption capacity, and equilibrium distance of coal can be obtained.

4. Gaussian Simulation Results

4.1. Relationship Between the Adsorption Position and the Binding Energy

The two benzene rings of lignite were labeled as benzene ring 1 and benzene ring 2 from left to right, the two side chains connected to benzene ring 1 were labeled as side chain 1 and side chain 2 from top to bottom, and the side chain connected to benzene ring 2 was labeled as side chain 3. Models were constructed for several adsorption positions. The binding energy obtained from the binding energy formula is shown in Figure 6. The maximum binding energy is 6.6149 KJ/mol, and the adsorption position is side chain 3, which contains hydroxyl. The two side chains of sub-bituminous coal are labeled as side chain 1 and side chain 2 from top to bottom. As shown in Figure 7, the maximum binding energy is 7.3696 KJ/mol, and adsorption occurs in side chain 2, which contains the hydroxyl group.

The four benzene rings of bituminous coal are labeled as benzene rings 1–4 from top to bottom, as shown in Figure 8. The maximum binding energy is 5.2170 KJ/mol, and adsorption occurs in the side chain containing hydroxyl groups. The molecular structure of anthracite is symmetrical, so only half of the adsorption sites can be discussed. The six benzene rings of anthracite are labeled as benzene rings 1–6 from top to bottom, as shown in Figure 9. The maximum binding energy is 4.9510 KJ/mol, and adsorption occurs in the side chain containing hydroxyl groups.

Figure 6, Figure 7, Figure 8 and Figure 9 reveal regularity in the binding energy of coal molecules adsorbing methane release at different positions. When methane molecules are adsorbed on different benzene rings of the same coal molecule, the energy released is similar. However, when they are adsorbed on different side chains of the same coal molecule, the energy released will be different due to the structure and position of the side chain, but the difference is small. In contrast, the difference in the energy released by adsorption on the benzene ring and side chain is large. This shows that the structure and position of methane molecules adsorbed on coal molecules are similar, and the energy released is similar. The energy released by adsorption on the same coal molecule depends largely on the structure and position of adsorption. The binding energy released by methane molecules adsorbed on the hydroxyl-containing side chain is significantly greater than the binding energy released by methane molecules adsorbed on the benzene ring. This is because the oxygen atom is negatively charged, and the hydrogen atom is positively charged. According to the principle of opposite attraction, when methane is adsorbed on the coal molecule, one hydrogen atom of methane will adsorb the oxygen atom on the coal molecule; that is, the oxygen atom in the -OH of the coal molecular structure and the methane molecule and the hydroxyl group of the side chain in the coal molecule have the largest charge change. Therefore, adsorption mainly occurs between the methane molecule and the hydroxyl group. The optimal configuration of the four different metamorphic coal molecules adsorbing the best position of the methane molecule is shown in Figure 10.

The simpler the structure connected by hydroxyl side chains the stronger the adsorption ability of hydroxyl groups and methane molecules, which is consistent with the binding energy law of different adsorption positions of the four types of metamorphic coal. Methane molecules in sub-bituminous coal are adsorbed on hydroxyl-containing side chains connected to heterocycles. In this equilibrium state, the adsorption position is far away from the benzene ring structure, and the adsorption capacity and binding energy are the strongest. The strongest position for lignite to adsorb methane is also the side chain containing hydroxyl groups, but the hydroxyl groups are not directly connected to the benzene ring, so the binding energy is secondary. The hydroxyl groups of bituminous coal and anthracite are directly connected to the benzene ring, and their aromatic structures gradually increase, resulting in a gradual decrease in adsorption capacity, while anthracite has the smallest binding energy. In actual production, it is necessary to design appropriate ventilation and heat dissipation systems and mine gas extraction methods for coal with different adsorption capacities. For example, the adsorption capacity of sub-bituminous coal is strong. For coal mines with more sub-bituminous coal, it is necessary to do a good job with the design to prevent the release of a large amount of energy in the process of adsorbing mine gas. Anthracite has the weakest adsorption capacity for methane. A large amount of mine gas in coal mines containing more anthracite is difficult to be adsorbed, so it exists in a free state. Therefore, it is necessary to take measures to prevent mine gas outburst.

4.2. Relationship Between Binding Energy of Adsorption and Adsorption Equilibrium Distance

In Gaussian simulation, the bond length between atoms is measured in Angstroms (Å), which is a common unit for expressing the length of a light wave and the molecular diameter, and 1 Å is equal to 0.1 nm. The bond length of the two atoms with the shortest distance between methane molecules and coal molecules is taken as the equilibrium distance, and this length can be obtained using Gaussian software. The equilibrium distance of the optimal adsorption configuration of the four metamorphic coals is shown in Figure 11. Lignite, sub-bituminous coal, bituminous coal, and anthracite have equilibrium distances of 2.3952 Å, 2.2221 Å, 2.5282 Å, and 2.5726 Å, respectively, and the size relationship is anthracite > bituminous coal > lignite > sub-bituminous coal.

The binding energy released by the optimal adsorption configuration of the four metamorphic coals is sub-bituminous coal > lignite > bituminous coal > anthracite, as shown in Figure 12. The comparison reveals that the adsorption energy is inversely proportional to the adsorption equilibrium distance.

4.3. Vibration Frequency and Spectral Analysis of Coal with Different Metamorphic Degree

Figure 13 depicts the infrared spectrum of the four types of metamorphic coal molecules after methane adsorption obtained using GaussView simulation. The types of vibration frequency, the absorption peak intensity, and the corresponding vibration frequency are listed in Table 3. Comparing Figure 13 with Figure 5 reveals slight differences in the frequency, intensity, and spectral simulation curves corresponding to the infrared spectral absorption peaks before and after the adsorption of methane molecules by coal molecules. However, the overall gap is not large, indicating that the functional groups are largely unchanged. The types of vibration modes have increased, indicating the presence of many new vibration modes during the adsorption of coal and methane molecules, which corresponds to the high activity of oxygen-containing functional groups. As the degree of metamorphism increases, the oxygen-containing functional groups decrease, and the properties gradually stabilize. The absorption peak of sub-bituminous coal shifts greatly on the horizontal axis, which is related to its strongest adsorption capacity. The absorption peaks of the other three coals change little.

5. Conclusions

(1): Lignite, sub-bituminous coal, bituminous coal, and anthracite were selected as the research objects in this study (which are arranged according to the metamorphic degree from low to high). The energy value relationship before single-molecule adsorption was anthracite > lignite > bituminous coal > sub-bituminous coal, the binding energy relationship of release was sub-bituminous coal > lignite > bituminous coal > anthracite, and the equilibrium distance relationship was anthracite > bituminous coal > lignite > sub-bituminous coal. Moreover, the adsorption energy was inversely proportional to the adsorption equilibrium distance.
(2): The structure and position of methane molecules adsorbed on coal molecules were similar, and the energy released was also similar. The energy released by adsorption in the same coal molecule depends largely on the structure and position of the side chain and benzene ring. Moreover, the binding energy released by methane molecules adsorbed on the hydroxyl-containing side chain was significantly greater than that released by adsorption on the benzene ring. Therefore, adsorption mainly occurred between methane molecules and hydroxyl groups.
(3): Different frequencies represented different vibration modes of different functional groups. As the degree of metamorphism increased, the curve gradually smoothened, the variety in the functional groups gradually decreased, the chain shortened, polycondensation occurred, and the aromatic structure became more prominent. There were slight differences in the frequency, intensity, and spectral simulation curves corresponding to the absorption peaks of infrared spectra before and after the adsorption of methane molecules by coal molecules, but the overall gap was not large. The functional groups were largely unchanged, and the types of vibration frequencies increased, indicating that many new vibration modes are present during the adsorption of coal and methane molecules.
(4): In actual production and life, coal mining enterprises can roughly predict the energy released during the adsorption process of coal and gas underground according to the degree of deterioration of coal, which is conducive to preventing the danger caused by the excessive accumulation of energy underground. The design of underground ventilation and a heat dissipation system can be adjusted according to the proportion of different types of metamorphic coal in the mine. The adsorption capacity of methane varies with different types of metamorphic coal, and suitable gas extraction methods and equipment can be selected according to the proportion of different types of metamorphic coal to improve work efficiency and save costs while ensuring safe production.

Author Contributions

Methodology, J.H.; software, J.H.; investigation, H.M.; data curation, T.L.; writing—original draft, J.H.; writing—review and editing, J.H.; visualization, S.Z.; supervision, T.L.; project administration, T.Y.; funding acquisition, T.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52274200), the Natural Science Foundation of Hebei Province for Outstanding Youth (No. E2023508019), and the Fundamental Research Funds for the Central Universities (No. 3142023007; 3142021008).

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

Zhang, Z.; Wang, L.; Li, H. Study on model construction and optimization of molecular structure. Coal Sci. Technol. 2021, 49, 245–253. [Google Scholar] [CrossRef]
Shi, H.; Qi, Y.; Zhang, G.; Jiang, X.; He, X.; Sun, Y. Risk assessment of coal and gas outburst based on combination weighting and grey clustering. China Saf. Sci. J. 2023, 33, 52–57. [Google Scholar]
Chilleri, J.; He, Y.; Bedrov, D.; Kirby, R.M. Optimal allocation of computational resources based on Gaussian process: Application to molecular dynamics simulations. Comput. Mater. Sci. 2021, 188, 110178. [Google Scholar] [CrossRef]
Chen, X.; Nie, S.; Kang, N.; Zhao, S.; Qi, L. Molecular Simulation on Adsorption Difference of Water and Methane on Long-Flame Coal. Adsorpt. Sci. Technol. 2023, 2023, 2615946. [Google Scholar] [CrossRef]
Yue, J.; Li, H.; Sun, Y.; Shao, H.; Lyu, Z.; Li, H. Study on adsorption properties of coal for methane and deuterated methane. Coal Sci. Technol. 2022, 50, 93–99. [Google Scholar]
Yan, J.; Jia, B.; Liu, B.; Zhang, J. Simulation Study on Molecular Adsorption of Coal in Chicheng Coal Mine. Molecules 2023, 28, 3302. [Google Scholar] [CrossRef] [PubMed]
Fan, Z. Study on Macromolecular Modeling and Adsorption Mechanism of Coal with Different Coal Rank. Master’s Thesis, China University of Geosciences, Beijing, China, 2020. [Google Scholar] [CrossRef]
Li, X.; Xue, H.; Chen, L.; Shen, Z.; Xu, M.; Xu, S. Micropore structure of outburst coal seam in Guizhou Area and its effect on gas flow. Coal Sci. Technol. 2020, 48, 67–74. [Google Scholar] [CrossRef]
Deng, S. Study on Dynamic Simulation of Methane Desorption Under the Transformation of Tectonic Coal Reservoir. Master’s Thesis, Guizhou University, Guiyang, China, 2023. [Google Scholar] [CrossRef]
Yan, H.; Nie, B.; Peng, C.; Liu, P.; Wang, X.; Yin, F.; Gong, J.; Wei, Y.; Lin, S. Molecular Model Construction of Low-Quality Coal and Molecular Simulation of Chemical Bond Energy Combined with Materials Studio. Energy Fuels 2021, 35, 17602–17616. [Google Scholar] [CrossRef]
Yang, J.; Zhao, X.; Zheng, B. Study on the Porous Structure and the Methane Adsorption of Tectonic Coal at Molecular Level. Coal Geol. China 2024, 36, 8–12. [Google Scholar]
Tang, M.; Jin, J.; Jiang, B.; Zheng, C.; Zhou, L.; Gao, S.; Wang, L. Molecular simulations and experiments on the effect of different types of surfactants on the wetting properties of bituminous coal. J. China Coal Soc. 2024, 49, 2986–2996. [Google Scholar]
Li, Y. Coal structure evolution and its fuel, raw material and functional material properties development. J. China Coal Soc. 2022, 47, 3936–3951. [Google Scholar]
Yu, H. Molecular Simulation on the Infuncene Mechanism of Coal Oxidation on CO₂/CH₄ Adsorption. Master’s Thesis, Harbin Institute of Technology, Harbin, China, 2020. [Google Scholar] [CrossRef]
Li, T.; Zhang, J.; Jin, Z. Numerical research into solid-gas-thermal coupling of coal and containing gas. J. Heilongjiang Univ. Sci. Technol. 2017, 27, 17–21. [Google Scholar]
Li, T.; Zhang, H.W.; Han, J.; Lv, Y.C.; Lan, T.W. Controlling effect of tectonic stress field on coal and gas outburst. J. Xi’an Univ. Sci. Technol. 2011, 31, 715–718. [Google Scholar] [CrossRef]
Qiu, Y.; Long, H.; Bai, Y.; Lin, H.; Yan, M.; Xiao, T. Thermodynamic and kinetic characteristics of gas adsorption by coal under temperature effect. China Saf. Sci. J. 2023, 33, 147–155. [Google Scholar]
Jiang, Y.; Bai, G.; Zhou, X.; Wang, Y.; Fu, T.; Hu, K. Test and analysis of coal adsorption volume of CH₄. Coal Sci. Technol. 2022, 50, 144–152. [Google Scholar]
Shu, Z.; Xu, X.; Wei, Y.; Wang, A.; Wang, L.; Liu, Z.; Chen, G.; Cao, D. Study on macromolecular structure of different types of contact metamorphic coals. Coal Sci. Technol. 2023, 51, 147–157. [Google Scholar]
GB/T5751-2009; The National Standards Compilation Group of the People’s Republic of China. China Coal Classification. China Standards Press: Beijing, China, 2009; pp. 3–6.
Liu, G.; Li, B.; Zhang, Z.; Liu, H.; Guan, W.; Si, N. Gas expansion energy of coals with different metamorphic degrees: Evolutionary characteristics and their implications for the outburst prediction. Coal Geol. Explor. 2023, 51, 1–8. [Google Scholar] [CrossRef]
Zhou, C.; Yang, X. Theory Simulation of Function between Different Adsorbates and Coal Surface Molecules. Coal Technol. 2017, 36, 203–205. [Google Scholar] [CrossRef]
Dutta, D.; Wood, B.C.; Bhide, S.Y.; Ayappa, K.G.; Narasimhan, S. Enhanced gas adsorption on graphitic substrates via defects and local curvature:A density functional theory study. J. Phys. Chem. C 2014, 118, 7741–7750. [Google Scholar] [CrossRef]

Figure 1. Gaussian simulation operation interface. (a) Energy change trend. (b) Vibration frequency.

Figure 2. Molecular structure diagram of different metamorphic coals: (a) lignite (b) sub-bituminous coal, (c) bituminous coal, (d) anthracite.

Figure 3. Optimal structure model of different metamorphic coal molecules: (a) lignite (b) sub-bituminous coal, (c) bituminous coal, (d) anthracite.

Figure 4. Molecular energy values of different metamorphic coals.

Figure 5. Infrared spectra of different metamorphic coal molecules. (a) Lignite. (b) Sub-bituminous coal. (c) Bituminous coal. (d) Anthracite.

Figure 6. Binding energy of different adsorption positions of lignite.

Figure 7. Binding energy of different adsorption positions of sub-bituminous coal.

Figure 8. Binding energy of different adsorption positions of bituminous coal.

Figure 9. Binding energy of different adsorption positions of anthracite.

Figure 10. Optimal structure model of methane molecules adsorbed by different metamorphic coal molecules. (a) lignite, (b) sub-bituminous coal, (c) bituminous coal, (d) anthracite.

Figure 11. Adsorption equilibrium distance of different metamorphic coal molecules.

Figure 12. Binding energy of different metamorphic coal molecules after adsorption.

Figure 13. Infrared spectra of the optimal configuration of methane adsorbed by different metamorphic coal molecules. (a) Lignite. (b) Sub-bituminous coal. (c) Bituminous coal. (d) Anthracite.

Table 1. Coal molecular element content.

Coal Molecular Type	Molecular Weight	Carbon Content/%	Hydrogen Content/%	Oxygen Content/%	Nitrogen Content/%
Lignite	359	63.5	5.8	26.7	4.0
Sub-bituminous coal	250	76.8	8.8	14.4	0.0
Bituminous coal	296	89.2	5.4	5.4	0.0
Anthracite	560	90.0	3.6	6.4	0.0

Table 2. Infrared spectral absorption peaks of different metamorphic coal molecules.

	Lignite	Sub-Bituminous Coal	Bituminous Coal	Anthracite
Types of vibration frequency	135	114	111	186
Frequency (cm⁻¹)	1683.20	299.265	392.262	1187.20
Infrared (max)	325.2524	173.4121	142.3788	765.4109

Table 3. Infrared spectrum absorption peak of the methane adsorbed by different metamorphic coal molecules.

	Lignite	Sub-Bituminous Coal	Bituminous Coal	Anthracite
Types of vibration frequency	150	129	126	201
Frequency (cm⁻¹)	1683.18	324.524	397.786	1187.95
Infrared (max)	326.5804	180.7527	171.9651	752.3717

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

© 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Share and Cite

MDPI and ACS Style

Yang, T.; Hu, J.; Li, T.; Min, H.; Zhang, S. Molecular Energy of Metamorphic Coal and Methane Adsorption Based on Gaussian Simulation. Processes 2024, 12, 2621. https://doi.org/10.3390/pr12122621

AMA Style

Yang T, Hu J, Li T, Min H, Zhang S. Molecular Energy of Metamorphic Coal and Methane Adsorption Based on Gaussian Simulation. Processes. 2024; 12(12):2621. https://doi.org/10.3390/pr12122621

Chicago/Turabian Style

Yang, Tao, Jingyan Hu, Tao Li, Heng Min, and Shuchao Zhang. 2024. "Molecular Energy of Metamorphic Coal and Methane Adsorption Based on Gaussian Simulation" Processes 12, no. 12: 2621. https://doi.org/10.3390/pr12122621

APA Style

Yang, T., Hu, J., Li, T., Min, H., & Zhang, S. (2024). Molecular Energy of Metamorphic Coal and Methane Adsorption Based on Gaussian Simulation. Processes, 12(12), 2621. https://doi.org/10.3390/pr12122621

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Menu