Monte Carlo and molecular dynamics simulations of CH4 molecules adsorption behavior in bituminous coal

In this study, we established a macromolecular structure model of the Wiser bituminous coal (C192H166N4O19S9) to study the adsorption characteristics of CH4 molecules in bituminous coal. We then combined the grand canonical Monte Carlo and molecular dynamics simulation methods to study the adsorption behavior of CH4 molecules in coal at different temperatures and moisture contents with fugacity values of 0-12 MPa. The simulation results showed that the adsorption isotherms of CH4 gas are consistent with those of the Langmuir model, and the adsorption of CH4 molecules is negatively correlated with temperature and positively correlated with fugacity. The adsorption amount and rate of CH4 molecules in bituminous coal decreased as the moisture content in the coal molecules increased. The presence of H2O molecules in coal reduced the interaction between the coal and CH4 molecules. H2O molecules exhibited a stronger interaction with the heteroatomic groups in coal molecules than that of the CH4 molecules. Finally, the main factors causing a linear negative correlation between moisture content and the adsorption of CH4 molecules were the preferential occupation of H2O molecules in effective adsorption sites in coal molecules and the linear reduction of free volume in coal.