Competitive adsorption equilibria of CO2 and CH4 on a dry coal

Gases like CO₂ and CH₄ are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO₂ and CH₄ on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45?°C and up to 190 bar for pure CO₂, CH₄ and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO₂ using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH₄. Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO₂, CH₄ and He at 45 and 60?°C and up to 140 bar. For CO₂ and CH₄ a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO₂ storage capacity and the dynamics of the replacement of CH₄ by CO₂.