Competitive adsorption of CO2/CH4 mixture on dry and wet coal from subcritical to supercritical conditions

• Sorption of a CO2/CH4 mixture on an anthracite coal was measured up to 13 MPa.
• The high selectivity of CO2 on coal resulted in the negative excess sorption of CH4.
• The moisture effect on sorption in wet coal was decreased in supercritical region.
• The sorption behavior of mixture was similar to that of pure CO2 at the same density.
• The mixture density in free volume was an important variable of isotherm shapes.

Enhanced coal bed methane (ECBM) recovery enables captured CO2 to be stored while increasing the recovery of CH4. Competitive adsorption of a CO2/CH4 mixture on dry and wet anthracite coal was studied at 318 and 338 K and pressures up to 13 MPa and was compared with the adsorption behavior of pure CO2 and CH4. The negative excess adsorbed amounts of CH4 in the CO2/CH4 mixture were observed in both dry and wet coal due to the high selectivity of CO2 on coal. Therefore, the total excess adsorbed amount and isotherm shape of mixture on dry and wet coal were similar to the results of pure CO2. The moisture of the coal reduced the sorption amount compared to dry coal in the mixture system, showing a similar moisture effect on the isotherm of pure CO2 in the same range of fluid density. Above the critical point of CO2, the sorption difference between dry and wet coal decreased with an increase in pressure. The fluid density of a mixture in free volume was an important variable to estimate the ECBM recovery efficiently because the sorption behavior of the CO2/CH4 mixture was similar to that of pure CO2 based on the same density condition.