**1. Introduction**

Coal is an important basic energy source and industrial raw material in China, and it accounts for more than 90% of the fossil resource reserves [1,2]. The gradual depletion of shallow coal resources has increased the depth of coal mining. Fire in mines caused by coal spontaneous combustion often occurs and becomes one of the main disasters in coal mine production [3], which seriously affects economic development and threatens the lives of coal miners [4,5]. The prevention of coal spontaneous combustion fires has been major topics of research in the field of coal mine safety [6–8]. At present, many studies generally agree that coal spontaneous combustion is caused by coal−oxygen recombination [9]. However, coal−oxygen recombination is a complex process. The adsorption mechanism of oxygen in coal is difficult to reveal by experimental means. A mutual reflection relationship exists between the properties and structure of coal [10]. As a result, the construction and optimization of the coal macromolecule model is the theoretical basis for revealing the mechanism of coal spontaneous combustion.

The construction of the coal molecular model provides foundation for molecular simulation. The research methods are mainly divided into physical and chemical methods [11–15]. 13C nuclear magnetic resonance spectroscopy (13C-NMR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other physical methods were used to obtain relevant information on the construction of the molecular structure model [16–18]. Early coal chemical structure models, such as the Wiser, Given, and Shinn models [19–21], were widely recognized by scholars. The rapid development of computer technology in recent years has encouraged scholars to use modern analysis methods combined with computer-aided software in building coal molecular models. The mechanism and reaction

**Citation:** Qu, L.; Liu, L.; Chen, J.; Wang, Z. Molecular Model Construction and Optimization Study of Gas Coal in the Huainan Mining Area. *Processes* **2023**, *11*, 73. https://doi.org/10.3390/pr11010073

Academic Editors: Feng Du, Aitao Zhou and Bo Li

Received: 18 November 2022 Revised: 18 December 2022 Accepted: 25 December 2022 Published: 28 December 2022

**Copyright:** © 2022 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/).

characteristics of coal spontaneous combustion were investigated by molecular simulation [22–25]. Carlson [26] simulated the three-dimensional structures of four bituminous coal models using computer software and concluded that molecular modeling is an effective means to study coal structure. Wu et al. [27] systematically investigated the competitive adsorption characteristics of smoke from coal-fired power plants using molecular dynamics (MD) simulation software and on the basis of the construction of the molecular structure of coal. The results showed that the absolute adsorption decreases with the increase in temperature and water content in coal. However, the absolute adsorption increases with the rise in pressure and the corresponding volumetric mole fraction. Hong et al. [28] structured the coal char model and studied the combustion and gasification properties of coal char using the reaction MD (ReaxFF MD) simulation method. Hao et al. [29] investigated the adsorption characteristics of different gasses (CH4/CO2/N2) on the coal surface by establishing the molecular model of anthracite coal and revealed the thermodynamic parameters of coal surface adsorption using the molecular simulation. The results showed that the adsorption amounts of the three gases on the coal surface are shown in descending order: CO2 > CH4 > N2, according to adsorption affinity and thermodynamic parameters. Meng et al. [30] revealed the relationships between different grades of coal and methane adsorption using the method of the grand canonical Monte Carlo and the density functional theory based on studying four different grades of coal models. Wei et al. [31] constructed a macromolecular structure model of Zaoquan coal using proximate analysis, ultimate analysis, XPS, 13C solid-state NMR, and other physical methods together with computer software. They determined the effects of different final temperatures and heating rates on pyrolysis behavior. Chai and Zeng [32] probed the changes in macroscopic and microscopic characteristics of Wucaiwan coal at different temperatures by constructing a molecular model. Ping et al. [33] analyzed the mirror and inert groups of Shanghai Miao bituminous coal by using 13C solid-state NMR, Fourier-transform infrared spectroscopy, and XPS. The differences between the structure and functional groups in the molecules were explored to construct molecular structure models.

The studies of the abovementioned scholars indicate that the characteristics of coal oxygen adsorption were deeply explored by molecular simulation. The accuracy of the coal molecular model is the focus of our subsequent research, which has an important effect on the research results. The coal macromolecular model was a collection of analytical results of experimental data based on the characterized coal microstructure parameters, which must be further analyzed or verified. However, the verification methods used by the abovementioned scholars need to be further improved. The authors are mainly engaged in the research on coal spontaneous combustion fire prevention and control and coal mine safety. Thus, the gas coal in the Huainan mining area was further studied in this work based on comprehensive research methods. Proximate analysis, ultimate analysis, 13C-NMR, XPS, and XRD were used to realize the information characterization of gas coal. The rationality of the macromolecular structure model based on the characterization information was verified by the carbon spectrum and density simulation of coal molecules. A more stable and accurate molecular structure model was obtained using the Materia Studio (MS) software optimization, which provided a method for predicting the macromolecular model of coal and theoretical support for preventing the occurrence of coal spontaneous combustion fire at the molecular level.
