Molecular simulation study of CO₂ and O₂ adsorption characteristics and diffusion pattern in coal body at atmospheric pressure

In order to investigate the adsorption characteristics and diffusion law of CO₂ and O₂ in coal, the adsorption and diffusion processes of CO₂ and O₂ in coal at different molar ratios, temperatures and pressures were simulated using the grand canonical system Monte Carlo and molecular dynamics methods. The findings indicated that: ① CO₂ was distributed in clustered aggregation and O₂ was distributed in point-like dispersion in coal. Under identical conditions, the adsorption quantity, equivalent adsorption heat, average mass density, and adsorption selectivity were CO₂>O₂. ② With the increase of the molar ratio of CO₂, the adsorption amount, the equivalent adsorption heat, and the diffusion coefficient of CO₂ exhibited a tendency to increase. The mass density of CO₂ increased with the increase of the mole fraction, but an excessive increase in the mole fraction of CO₂ results in an increase in the O₂ diffusion coefficient. With increasing pressure, the adsorption selectivity coefficient of CO₂ decreases from 7.67 to 3.69. ③ The adsorption of O₂ is weaker than that of CO₂, and the diffusion of O₂ shows a decreasing trend, while the diffusion of CO₂ shows an increasing trend. An increase in temperature has a detrimental effect on the adsorption capacity of coal for CO₂ and O₂. However, the diffusion of O₂ and CO₂ exerts a significant promotional influence, with a higher temperature facilitating the diffusion of the two gases and accelerating the desorption from coal. Consequently, the diffusion coefficient demonstrates an increasing trend. The findings of this study may serve as a valuable reference for the prevention and control of spontaneous coal combustion and other related disasters.