Competitive adsorption of CO2/CH4 on coal: Insights from thermodynamics

The main reason for CO2 enhanced coalbed methane recovery is the competitive adsorption of CO2 and CH4 on coal. Therefore, it is necessary to study the equilibrium adsorption of CO2, CH4 and their mixed gases on coal over a wide range of pressures. This work investigates the mechanism adsorption of CO2/CH4 on coal by thermodynamics. The findings indicate that CO2 demonstrates a greater increase in Gibbs free energy and a larger decrease in entropy loss related to CH4, indicating that the adsorption process of CO2 is more to occur spontaneously. In CO2/CH4 mixed gas adsorption system, the amount of gas adsorbed for different gas ratios (yCO2:yCH4 = 1:3; yCO2:yCH4 = 2:2; yCO2:yCH4 = 3:1) falls between the amounts adsorbed for pure CO2 and CH4 components. As CO2 concentration rises, the amount of mixed gases adsorbed similarly increases. Additionally, the amount of CH4 adsorbed in any combination is lower than that of CO2, suggesting that CO2 has a predominant influence regarding the adsorption process. Furthermore, isosteric heat of adsorption for CO2 in the mixed gases exceeds that of CH4, suggesting a stronger connection between CO2 and the surface of the coal matrix during the competitive adsorption stage.