Molecular Simulation of Methane Adsorption Behavior on Coal: Effects of Maturity and Moisture Content

The adsorption behaviors of multiphase fluids in coal are vital for estimating their transfer and flow mechanisms. In this study, taking the classical coal structures as instances, the impacts of maturities and moisture contents of coal on the methane adsorption capacities and thermodynamic properties were analyzed and determined by molecular simulations. Specifically, the simulated pressures were varied from 0 to 16 MPa, and three temperatures (298 K, 323 K, and 348 K) were considered. In addition, the competitive adsorption capacities of water/methane molecules were evaluated and the critical value of moisture content affecting the isosteric adsorption heat of methane was determined. Furthermore, a quantitative relationship between the gas adsorption amount and adsorption equilibrium pressure under a three-phase “solid–gas–liquid” action was established. The simulated results showed that taking the intermediate-rank coals as examples, the absolute adsorption capacities of methane molecules on different moist coals reduced by 28.3%, 35.1%, 46.9%, and 62.3%, respectively (at uniform pressure–temperature status) in comparison to their dry status. In addition, when the moisture content was lower than 1.4 wt%, the average isosteric adsorption heat remained almost stable. However, when the moisture content exceeded 1.4 wt%, the average isosteric adsorption heat increased sharply with the increase in the water content. Finally, linear function formulas yielded a good fitting accuracy range for the low- and intermediate-rank coals; in comparison, for the high-rank coal, an exponential function formula obtained a better fitting accuracy than a linear one.