CH₄ Adsorption Behavior on Coals with Different Ranks by Grand Canonical Monte Carlo Simulations

To investigate the adsorption characteristics of CH₄ on coals with different ranks under high-temperature conditions, the Grand Canonical Monte Carlo simulations were employed, three typical coal sampleslignite, bituminous coal, and anthracitewere selected, and two temperature ranges were set: a low-temperature range (293.15-323.15 K) and a high-temperature range (373.15-1173.15 K). The results indicated that high-rank coals possessed more developed pore structures and larger specific surface areas, providing abundant adsorption sites. In addition, their increased aromaticity, reduced polar functional groups, and more uniform surface energy distribution favored CH₄ adsorption. At low-temperature conditions, adsorption was controlled by thermodynamics and dominated by micropore filling, whereas at high-temperature conditions, it was mainly controlled by kinetics and dominated by two-dimensional surface coverage on mesopores and macropores. As a result, adsorption isotherms increased nearly linearly with pressure without approaching saturation, and the influence of coal rank on CH₄ adsorption capacity was reduced. Based on these findings, differentiated strategies should be adopted in coalbed methane (CBM) development: for high-rank coals, increasing reservoir temperature can promote production, whereas for low-rank coals, the method of decreasing reservoir pressure by producing water is more effective. The research results provide molecular-level insights into CBM occurrence mechanisms and offer theoretical foundations for optimizing CBM development.