Structural Evolution of Coal During Coalification and Its Impact on the Adsorption Mechanism of Composite Inert Gases Using MD and GCMC Simulations

To analyze the impact of coalification on the fire-suppression efficacy of composite inert gases, molecular models for lignite, bituminous coal, and anthracite were built based on experimental characterization. Subsequently, molecular simulations were employed to analyze the ultra-micropore structures of coal, single-gas adsorption, and the competitive adsorption between O2 and composite inert gases (CO2-N2). The results indicate that the bridgehead-to-peripheral aromatic carbon ratio XBP exhibits a distinct upward trend with increasing coalification. The total potential energy of coal models reveals a decrease in the relative contribution of electrostatic energy and a corresponding increase in bonded energy. The evolution from lignite to anthracite is accompanied by an increase in both accessible surface area and pore volume of ultra-micropores, with the pore size distribution shifting from a unimodal pattern to a bimodal character. The adsorption amount for all gases increases with coal rank, but the isosteric heat of adsorption gradually decreases. Under multi-component competitive conditions, the adsorption amount of O2 drops rapidly initially and then declines slowly as the CO2 fraction increases, the optimal CO2:N2 ratio for fire suppression is identified as 6:4 for bituminous coal and anthracite, whereas a ratio of 8:2 is required for lignite. Under identical atmospheric conditions, the increase in coal rank results in a decreased proportion of electrostatic energy and an increased proportion of van der Waals energy within the total adsorption energy. These findings could contribute to the fire prevention strategies in mines.