Combined Monte Carlo and molecular dynamics simulation of methane adsorption on dry and moist coal

• Combined MD and MC methods was first used to address coal swelling.
• Temperature has no effect on the maximum adsorption expressed on a volume basis.
• Quantify CH4 absorption for both dry and moist coal.
• Effect of water on CH4 adsorption and coal swelling mechanism were investigated.
• Water is preferably adsorbed on oxygen and nitrogen atoms.

A quantitative understanding of methane (CH4) adsorption on dry and moist coal and the mechanism of coal swelling is vital for successful coal bed methane (CBM) projects. CH4 adsorption isotherms of coal with moisture contents ranging from 0 to 3 wt% water, the temperature effect on maximum adsorption capacity, coal swelling, and adsorbed phase density have been modeled by performing combined Monte Carlo (MC) and molecular dynamics (MD) simulations at temperatures of 308 and 370 K (35 and 97 °C) and at pressures up to 10 MPa. Simulation results demonstrate that absolute adsorption (expressed as a mass basis) divided by bulk density is independent of temperature for CH4 on dry coal when pressure is over 8 MPa. Both the adsorption capacity and adsorption rate of CH4 decrease, while coal swells as moisture content increases. These results show that the presence of water in the coal matrix reduces the interaction between the coal and methane. Our results indicate that coal–water interaction dominates and is the main contributing factor to the coal swelling. The interaction of CH4–H2O and CH4–CH4 is negligible and the absolute adsorption of CH4 on both dry and moist coal follows the Langmuir isotherm for the pressure range simulated. This study provides a quantitative understanding of the effects of moisture and temperature on CH4 adsorption, coal swelling, and the adsorbed CH4 density from a microscopic perspective. Molecular modeling proves to be a valuable and cost-effective tool for studying gas adsorption behavior in complex and complicated systems.