Molecular Simulation of Thermodynamic Properties of CH4/CO2 Adsorption by Coal Molecules at Different Temperatures and Moisture Contents under Variable Pressure Conditions

In order to further elucidate the thermodynamic mechanism of CH4/CO2 adsorption by coal molecules, the adsorption behavior of a molecular model of coal (C206H128O36N2) at Wucaiwan, Zhundong was investigated by applying Materials Studio 2020 and Monte Carlo (GCMC) simulation methods, and the adsorption behavior of CH4 and CO2 was studied from the thermodynamic point of view under the conditions of different temperatures, pressures, and moisture contents. The results showed that at different temperatures or moisture contents, CH4 molecules had a low-density scattering distribution and CO2 molecules had a high-density polymerization distribution. Temperature and moisture content and adsorption constants a and b were negatively correlated. Under the same conditions, the relationship between single- and binary-component adsorption amounts was CO2 > CH4 and the relationship between heat of adsorption was CO2 > CH4. When adsorption potential energy or entropy of adsorption was the same, the adsorption capacity was CO2 > CH4. Temperatures and moisture contents were negatively correlated with the total adsorption capacity of CH4/CO2; pressure was positively correlated with the total adsorption capacity of CH4/CO2. The effect of temperature on the equivalent heat of adsorption was greater than that of pressure at different temperatures, and the entropy of adsorption was positively correlated between temperature and CH4/CO2, while the amount of adsorption was negatively correlated with the entropy of adsorption. The effect of moisture content on the equivalent heat of adsorption was greater than that of pressure at different moisture contents, and the entropy of adsorption was negatively correlated between moisture content and amount of adsorption. The adsorption entropy of CH4/CO2 was negatively correlated, and the adsorption amount was positively correlated to the adsorption entropy. At a temperature above 318 K or moisture content above 10%, the total CH4/CO2 adsorption decreased significantly and the CO2 adsorption decreased significantly. From a thermodynamic point of view, the presence of a large amount of H2O had a much greater effect on CO2 than on CH4, and an increase in temperature or moisture content was unfavorable for CO2 sequestration, CO2 stripping of CH4, and control of CH4 diffusion and desorption, whereas at low temperature, high pressure, and moisture content <1%, the effect of stripping, sequestration, and control was good.