Investigation of the Impact of Moisture in Various States on the Adsorption of CH4, N2, and CO2 by Coal

To investigate the impact of moisture in various states on the adsorption of CH4, N2, and CO2 by coal, this study conducted isothermal adsorption experiments on dry coal for moisture and on coal with varying moisture content for CH4, N2, and CO2. The results revealed that coal moisture adsorption progresses through four stages: monolayer adsorption, multilayer adsorption, cluster formation, and condensation into free moisture filling pores. The presence of moisture in coal varies across these stages, with its impact on CH4, N2, and CO2 adsorption decreasing as moisture content increases. As the moisture content rises, the adsorption constants (a and b values) progressively decline and eventually level off. Based on differences in coal moisture states, delineated by a relative humidity of 0.50 (corresponding to a moisture content of 2.39% in experimental samples), adsorption potential theory identified low moisture content stages where the monolayer and multilayer adsorption potentials for moisture (E1 = 4.99 kJ/mol, E2 = 39.71 kJ/mol) exceeded those for CH4, N2, and CO2, suggesting competitive adsorption effects. In the high-water content stage, the mesoporous structural fractal dimension D2 of the coal samples was found to be 2.77, nearly reaching 3, as determined through liquid nitrogen adsorption experiments and fractal dimension theory. This suggests that the intricate pore structure leads to water condensation, free water formation, and pore blockage, making it the primary factor influencing the adsorption of CH4, N2, and CO2 in coals, which is inherently linked to the characteristics of coal.