Micropore distribution and methane adsorption process and mechanism in bituminous coals: A molecular dynamics simulation study

The research of micropore distribution and the mechanism of methane adsorption in coal are significant for methane emission. In this study, the macromolecular structure models of bituminous coals were constructed to analyze microporous gas adsorption capacity, isosteric heat of adsorption and adsorption sites. The results show that the distribution of micropores is extremely heterogeneous, mainly composed of simple near-spherical pores and complex near-ink bottle pores. The simulated measurable micro-PV of coal samples was very close to the results of LP-CO2GA. The simulated limit gas adsorption volume by the Grand Canonical Monte Carlo method was 10.35 m3/t and 10.81 m3/t, respectively, accounting for 70.5% and 60.3% of the experimental results. The gas adsorption volume in the form of micropore filling accounted for 99% of the calculated limit gas adsorption volume. It was confirmed by theory, experiment and simulation that micropores were the main adsorption space of gas in coal. The simulated isosteric heat of adsorption at different pressures varied in the range of 23.3–24.29 kJ/mol, validating that coal adsorption on gas was a physical process. During adsorption, methane molecules were more easily adsorbed in pores consisting of aromatic rings and aliphatic structures and were more easily adsorbed on the side of the pore near the aromatic structure.