High-pressure adsorption of methane, carbon dioxide and their mixtures on coals with a special focus on the preferential sorption behaviour

During recent years, extensive studies have been undertaken at RWTH Aachen to assess the gas adsorption capacities of coals of different rank with respect to CH₄, CO₂ and their mixtures [e.g. Int. J. Coal Geol. 51 (2002) 69; Proceedings JCOAL Workshop: Present Status and Perspective of CO₂ Sequestration in Coal Seams, Tokyo, Japan, (5 September 2002) 23-38]. Excess sorption isotherms of carbon dioxide recorded at 40, 60 and 80 °C on dry and moisture-equilibrated Carboniferous coals from the Netherlands exhibited distinct minima and even negative values in the 8-12 MPa interval. These anomalies are indicative of a strong volumetric effect. Evaluation of the experimental results in terms of absolute sorption assuming a range of different densities for the adsorbed phase could not eliminate the observed anomalies. In consequence, substantial swelling (up to 20%) of the (powdered) coal samples must be invoked to account for the observed phenomena. This interpretation is supported by the results of field tests in Alberta, Canada [Proceedings JCOAL Workshop: Present Status and Perspective of CO₂ Sequestration in Coal Seams, Tokyo, Japan, (5 September 2002) 59-66], which resulted in a significant reduction in coal-seam permeability upon CO₂ injection. The latest research focuses on the preferential sorption behaviour of CO₂ and CTLt of coals from the Silesian coal basin. Experiments are conducted at pressures up to 250 bar (25 MPa) at a temperature of 45 °C using the volumetric method. These measurements provide fundamental information for enhanced coalbed methane recovery (ECBM) and storage of CO₂ in deep unminable coal seams proposed as a potential means of the reduction of anthropogenic CO₂ emissions (RECOPOL-project: http://www.nitg.tno.nl/ recopol/). Preferential adsorption experiments on dry and moisture-equilibrated coals of different rank under identical conditions showed that adsorption is a function of coal type, moisture content and pressure. While at pressures above 50 bar, CO2 was always adsorbed preferentially to methane, preferential sorption of methane was observed in some instances at lower pressures. The unexpected phenomenon of preferential CH₄ adsorption on natural coals is presently an issue for further investigation. In the context of a round robin project initiated by the US Department of Energy, CO₂ excess sorption isotherms have been determined on five US premium coals at 22 °C in the dry state. Diversities of the excess sorption behaviour of these coals under different rank can be observed. Generally, excess sorption isotherms of lignite and subbituminous coals (0.25-0.46% VR_r) xhibited a monotonous increase over the entire experimental pressure range (up to ∼ 50 bar), while higher mature coals tended to approach a saturation level corresponding to a Langmuir isotherm.