Molecular simulation of thermodynamic properties of CH4 and CO2 adsorption under different moisture content and pore size conditions

In order to further clarify the thermodynamic properties of CH4 and CO2 adsorption in coal, a Monte Carlo method with a giant regular system was used to simulate the adsorption behavior of CH4 and CO2 in coal at different moisture contents and pore sizes. The results show that the adsorption of CH4 and CO2 in coal molecules is negatively correlated with the moisture content. As the pore size increases, the wall superimposed effect decreases, the stability of adsorption becomes less stable, and the selectivity of CO2 adsorption decreases, but the space to accommodate gas molecules becomes larger, which makes the adsorption amount rise. The average heat of adsorption of CH4 and CO2 showed a decreasing and then increasing relationship with the moisture content, and a negative correlation with the pore size, and the effect of H2O on CO2 is stronger than that of CH4. With the increase of moisture content, the absolute value of adsorption energy increases overall, and the high potential of water clusters tends to attract CO2 molecules more. With the increase of pore size, the wall superimposed effect weakens and the adsorption stability becomes worse, which makes the absolute value of adsorption energy smaller. The overall adsorption entropy of CH4 and CO2 has a negative correlation with moisture content and a positive correlation with pore size. The comparison of the slope of the fitted curves of adsorption entropy at different moisture contents adds that CO2 mainly displaces CH4 molecules in the middle of the pores at 3% moisture content.