Microscopic competitive adsorption characteristics of CH4–CO2 on coal surface

To clarify microscopic competitive adsorption characteristics of methane (CH 4 ) and carbon dioxide (CO 2 ) on coal surfaces, a constitutive model for CO 2 –CH 4 competitive adsorption on coal surfaces is derived based on the Lennard-Jones intermolecular potential theory. The model accuracy has verified against both molecular dynamics simulation results and competitive adsorption experimental data. Model analysis results reveal that: the densities of CO 2 and CH 4 both increase initially and then decrease with decreasing distance from coal surface. The location of maximum CO 2 density is closer to the surface than that of CH 4 , which is a characteristic parameter characterizing the CO 2 competitive adsorption superiority. The average molar fraction of CO 2 in adsorption region is significantly higher than that in the free phase. The CO 2 competitive adsorption selectivity coefficient decreases by a power law and gradually approaches 1 with increasing CO 2 mole fraction in free region. The CO 2 competitive adsorption superiority weakens significantly when the CO 2 mole fraction in free region exceeds 40%. During pressurization below 10 megapascal (MPa), the free CO 2 gradually transforms into the adsorbed state, and CO 2 competitive adsorption superiority is enhanced. The CO 2 competitive adsorption selectivity coefficients show linearly upward trend with increasing pressure and power-law downward trend with increasing temperature.