Investigation of desorption and diffusion of gas within low-rank bituminous coal under moisture-stress constraints

Gas extraction is one of the primary methods for guaranteeing secure coal mining and the effective use of high-quality energy. It is crucial to study the CH₄ desorption and diffusion pattern of low-rank bituminous coals under moisture and stress conditions. Because in low-rank bituminous coals, moisture in the coal will occupy the adsorption position and diffusion channel of methane (CH₄), affecting the efficiency of coalbed methane extraction. Herein, low-rank bituminous coals with varying moisture contents were prepared using the salt solution method, and their physical characteristics were described. CH₄ desorption characteristics were investigated across particle and lump coal to analyze the impact of moisture, adsorption equilibrium pressure, and confining pressure. The applicability of the unipore and time-varying diffusion models in elucidating CH₄ diffusion characteristics was evaluated. The small-molecule models of low-rank bituminous coals with different moisture contents were established using the molecular simulation method, which was also used to conduct diffusion kinetic simulations to characterize the diffusion behaviors. Finally, the mechanism of moisture, confining pressure, and adsorption equilibrium pressure on CH₄ desorption and diffusion properties were investigated. The findings can serve as a theoretical foundation for estimating the risk of coal and gas outbursts and guiding methane extraction management in low-rank bituminous coal mines.