Molecular Simulation of H2O, CO2, and CH4 Adsorption in Coal Micropores

Based on the chemical model of coal, slit micropores with different pore sizes are established and structures are optimized in the software of materials studio. As the temperature rises, absolute adsorption capacities of H2O are slightly affected, while absolute adsorption capacities of CO2 and CH4 gradually decrease. As the fugacity rises, excess adsorption curves of CO2 experience increase-decrease-gentle three stages, while the curves of CH4 gradually decrease. With the increase of pore size, adsorption capacities of H2O increase, while adsorption capacities of CO2 and CH4 gradually decrease. H2O firstly adsorbs on the oxygen-containing functional group, so the walls of pore are the preferential area for H2O, while CO2 and CH4 choose to adsorb on-C-C-, therefore the walls are the primary area for CO2 and CH4. Strong potential in micropores and hydrogen bond among water molecules will promote the water adsorption, while the adsorptions of CO2 and CH4 are only induced by the Van der Waals interaction, but the difference between adsorption density and bulk density of CO2 and CH4 decides the change of excess adsorption capacity.