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

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 338K and pressures up to 13MPa 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.