Molecular Simulation Study on Adsorption and Diffusion Characteristics of CH₄ in Cylindrical/Slit Pores of Lignite

Low-rank coal reservoirs have diverse pore structures, making it necessary to investigate their impact on the adsorption and diffusion characteristics of small-molecule gases at the microscopic scale. To investigate the interaction mechanism between the pore structure characteristics of brown coal and CH₄, brown coal from Linglu Mine was selected as the research subject. Low-temperature nitrogen adsorption experiments were conducted in conjunction with a nonlocalized density functional theory model analysis to characterize the pore structure of the coal. The Materials Studio software was employed to construct molecular models of lignite for two typical pore types─cylindrical and slit-pore─at scales of 1, 3, and 5 nm. Monte Carlo simulations were conducted using the grand canonical Monte Carlo and molecular dynamics methods to investigate the isothermal adsorption capacity, density distribution, isothermal adsorption heat, radial distribution function, diffusion mechanism, and fluid type of CH₄ across different pore size models. The results showed that pores with diameters between 0 and 5 nm were most abundantly developed in the coal, exhibiting both slit pore and cylindrical characteristics. The slit pore structure was more favorable for the adsorption of CH₄ molecules. Furthermore, the isothermal adsorption heat of the cylindrical pores was 1.3-1.4 times that of the slit pores. The interaction strength between the oxygen-containing functional groups in lignite and CH₄ followed the order C═O > C-O-C > -OH. The RDF peak for slit pores is higher than that for cylindrical pores. The diffusion coefficient of CH₄ in the slit pores was 1.3-3.5 times that in the cylindrical pores. Knudsen number calculations indicated that CH₄ exhibited transitional diffusion and Knudsen flow characteristics within the brown coal pores.