Measurement and interpretation of supercritical CO2 sorption on various coals

While the amount of CO2 sorption data on various natural coals has increased in recent years, only few measurements have been reported under the experimental condition of supercritical CO2 (scCO2) at very high pressure (> 5 MPa). The estimation of realistic CO2-sorption capacities for different coals is crucial for the understanding of the processes associated with CO2 storage and enhanced coalbed methane (ECBM) production.In this study CO2-sorption experiments up to 20 MPa at 45 °C have been performed on dry and water-containing coals from various coal basins. The coal samples cover a broad spectrum in rank, ranging from 0.52 to 2.41% vitrinite reflectance (VRr). As shown in various studies on activated carbon, scCO2 surface excess sorption isotherms do not increase continuously with pressure up to complete surface coverage of the sample. This is mainly caused by the static interpretation of excess sorption isotherms, not taking into consideration changes in the sample volume such as the volume of the sorbed phase, coal swelling, etc. This leads to difficulties in the interpretation of the experimental data in terms of adsorption isotherms, as they require models for all volumetric effects. In this context, this study provides an approach to account for the volumetric effects and, hence for the estimation of total sorption capacities. For the fitting procedure, one generalized correction factor for the volume increase was calculated for each coal sample and applied to the whole experimental pressure range. Generally, it was observed that coals containing water show a smaller volume increase than their corresponding dry samples and no specific trend with coal rank was observed. Contrary, a trend for the dry samples was observed: The sample volume increase follows a U-shaped trend, i.e., decreasing from 0.5 to 1.1% VRr and increasing again from 1.1 to 1.7% VRr.