Competitive Adsorption Behavior of CO2 and CH4 in Coal Under Varying Pressures and Temperatures

The CO2 injection technology for replacing CH4 to enhance coalbed methane (CBM) recovery (CO2-ECBM) offers dual benefits, i.e., reducing CO2 emissions through sequestration and increasing CBM recovery, thereby leading to economic gains. However, there is no clear consensus on how temperature and pressure affect the competitive adsorption characteristics of CO2 and CH4 mixed gases in coal. Therefore, the competitive adsorption behavior of CO2 and CH4 mixed gases at various pressures and temperatures were investigated using the breakthrough curve method. Anthracite was selected for the adsorption experiment conducted under three gas injection pressure levels (0.1 MPa, 0.5 MPa, and 1 MPa) and at three temperature levels (20 °C, 40 °C, and 60 °C). This study showed that, when the temperature remained constant and the pressure ranged from 0.1 to 1 MPa, the adsorption rates of CO2 and CH4 increased as pressure rose. Additionally, the selectivity coefficient for CO2/CH4 decreased with an increase in pressure, suggesting that higher pressures within this range are not conducive to the replacement efficiency of CH4 by CO2. As the temperature increased from 20 to 60 °C under constant pressure conditions, both the selectivity coefficients for CO2/CH4 and the adsorption rates of CO2 and CH4 exhibited a downward trend. These findings imply that, within this temperature range, a reduced temperature improves the ability of CO2 to efficiently displace CH4. Moreover, CO2 exhibits a higher isosteric heat of adsorption compared to CH4.