Moleular simulation of competitive adsorption of CO2/N2/O2 gas in bituminous coal

The main cause of coal spontaneous combustion is the oxidation and temperature rise of coal. Injecting composite inert gas for fire prevention and extinguishing is one of the important methods to prevent coal spontaneous combustion. A large number of studies have macroscopically investigated the fire prevention and extinguishing effects of CO2/N2 and their synergistic actions. However, there is currently a lack of research at the molecular level on the competitive adsorption mechanism between mixed inert gases (CO2, N2) and O2 molecules in coal. Therefore, this study conducted adsorption experiments using Qian Yingzi(QYZ) bituminous coal and compared them with the simulation data of the Wiser coal model to verify the reliability of the Wiser model. Subsequently, employing grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods, the competitive adsorption behavior of different proportions of binary mixed component gases (CO2/O2, CO2/N2, O2/N2) at a temperature of 303.15K was investigated from five perspectives: adsorption capacity, adsorption selectivity coefficient, potential energy distribution, competitive adsorption heat, and interaction energy. The results indicate that among the competitive component gases, CO2 exhibits the strongest adsorption effect on the entire system, which is closely correlated with the distribution range of adsorption energy sites. The adsorption heat and adsorption amount of CO2 consistently exhibit a negative correlation, whereas the relationship between the adsorption heat of N2 and O2 and the adsorption amount shows a positive correlation. Their adsorption capacities depend on the pressure and the proportion of the two gases. Based on the isothermal adsorption curve of CO2/N2, it is recommended that the gas pressure during actual inert gas injection be less than 2.5MPa. Under this pressure, the competitive adsorption effect is no longer significant, providing a certain theoretical basis for high-pressure injection. Moreover, the preliminary determination of the optimal injection ratio range is from 4:6 to 3:7. Furthermore, a systematic logical framework for selecting underground inert gas composition is proposed, and an underground mobile inert gas infusion process is adopted. The actual gas composition for infusion is determined based on cost factors in practical engineering. The research results can provide guidance for the selection of gas injection parameters for mixed inert gas fire prevention and extinguishing in goaf.