Molecular Dynamics Mechanism of CH₄ Diffusion Inhibition by Low Temperature in Anthracite Microcrystallites

Understanding the adsorption/diffusion characteristics of CH₄ at low temperatures (<273.15 K) is of great significance not only for coal bed methane estimation but also for gas disaster prevention and methane storage in deep coal beds. In this work, the adsorption configurations of anthracite macromolecules were constructed with Materials Studio, and then, the adsorption and diffusion behaviors of CH₄ at 233.15-363.15 K were simulated, respectively, using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) algorithms. The results show that the absolute adsorption capacities of CH₄ at low temperatures are substantially larger than those at high temperatures, and the adsorption amount further increases with the continued cooling at a given sorption pressure. The isosteric heat of CH₄ adsorption ranges from 8.715 to 11.746 kJ/mol, belonging to a spontaneous physical adsorption. The self-diffusivity D _s of CH₄ at low temperatures is substantially smaller than that at high temperatures and further decreases with cooling. The most probable velocity of CH₄ molecules (v _p) greatly decreases, and the number of gas molecules with a higher energy is significantly reduced by a low temperature, resulting in the diffusion inhibition of CH₄.