Molecular Simulation Study on Adsorption and Diffusion Behaviors of CO₂/N₂ in Lignite

Understanding the adsorption and diffusion of CO₂ and N₂ in lignite at high temperature is of great significance for fire prevention and control. Considering the influence of temperature on coal structure, molecular structure models of lignite at 298.15, 323.15, 423.15, and 523.15 K were constructed by molecular mechanics and dynamics, and grand canonical Monte Carlo molecular simulation was conducted for single-component and two-component systems under different temperatures, pressures, and gas ratios. The adsorption capacity was positively correlated with the pressure and molar ratio of CO₂ but negatively correlated with the temperature. The adsorption amount of CO₂ (1.060 mmol/g) was generally larger than that of N₂ (0.069 mmol/g), showing a greater selectivity. However, CO₂ was more sensitive to temperature, and the adsorption amount decreased faster with the increase in temperature. At high temperature, the adsorption amount of CO₂ and N₂ is basically equal, both of which are at a low level. The CO₂ isosteric heat of adsorption (7.46-8.84 kcal/mol) varies significantly with temperature. The interaction energy is consistent with the change trend of adsorption quantity, and van der Waals energy plays a dominant role in adsorption. Injecting CO₂ and N₂ at the high temperature stage has a poor extinguishing effect, which can only dilute oxygen content and exchange heat, and the advantage of CO₂ will be lost. It may be more effective using liquid nitrogen which has the properties of low temperature and high specific heat capacity. The results are of great significance to improve the efficiency of fire prevention and suppression in underground coal mines.