Investigation of high-pressure selective adsorption/desorption behaviour of CO₂ and CH₄ on coals: An experimental study

While an increasing amount of data is becoming available on the sorption capacity of natural coals for pure CH₄ and CO₂ at different temperatures and pressure ranges, only few measurements have been reported for mixtures of two or more gas components under the conditions of competitive sorption. High-pressure adsorption/desorption experiments with CO₂/CH₄ gas mixtures have been carried out on 10 coals of different rank and composition to investigate their selectivity with respect to the sorption of the two gases. The sample set comprised two coals from the Netherlands, ranging in vitrinite reflectance from 1.19% to 1.56% VR_r, three coals from the Silesian Basin in Poland (VR_r 0.68% to 0.78%) and five coals from the Argonne Premium Coal Sample Programme (VR_r 0.25% to 1.68%). The experiments were performed in the dry and moisture-equilibrated state. In addition, the effect of grain size on the competitive sorption was examined. The results are reported as a guideline for predicting adequate coal properties with respect to CO₂ injection and CO₂ enhanced coal bed methane (ECBM) projects in the future. The volumetric sorption experiments were performed with a sequential arrangement of calibrated reference and measuring cells and a sample loop for transfer of gas samples to an on-line gas-chromatograph. The experiments were carried out at 45 °C and pressures up to 23 MPa. Only few measurements showed preferential adsorption of CO₂ and preferential desorption of CH₄, as commonly expected. Distinct variations were observed in the competitive adsorption and desorption behaviour of the different dry and moist samples, ranging from preferential adsorption of CH₄ in the low pressure range to preferential desorption of CO₂ over the entire pressure range. The observed preferential sorption phenomena give rise to the assumption that CO₂ sequestration in coal seams with subsequent CO₂-enhanced CBM production might only be an option in specific coal basins.