Molecular Simulation of Methane Adsorption Properties of Coal Samples with Different Coal Rank Superposition States

Obvious differences exist in the structures of different coal rank coals, resulting in many differences and similarities in the amount of methane (CH4) gas adsorbed by coal and the control mechanism. In this study, we conducted adsorption simulations of three different coal rank coals in the superposition state using the Materials Studio software with simulated temperatures of 293.15, 313.15, and 333.15 K and adsorption pressures ranging from 0 to 10 MPa. We used the grand canonical ensemble Monte Carlo calculation method to calculate and study the adsorption amount, adsorption process, isosteric heat, and diffusion coefficient of CH4 in detail. We found that the adsorption of CH4 by coal samples of three coal rank coals (i.e., anthracite, bituminous coal, and lignite), which were mixed and stacked separately, was concentrated in the effective pores of coal molecules. Their effect on the amount of CH4 adsorption was dominated by the coal samples with more effective pores in the coal molecules. The isosteric heat of the coal samples showed an obvious exponential relationship with the adsorption pressure, and the isosteric heat of coal samples as a nonhomogeneous adsorbent gradually decreased with an increase in adsorption pressure. During the transformation of free-state CH4 into an adsorbed state, we obtained the best adsorption effect for bituminous-lignite superposition-state coal samples, and the diffusion coefficient of CH4 was the lowest.