High-pressure sorption isotherms and sorption kinetics of CH 4 and CO 2 on coals

Abstract

Using a manometric experimental setup, high-pressure sorption measurements with CH 4 and CO 2 were performed on three Chinese coal samples of different rank (VR r = 0.53%, 1.20%, and 3.86%). The experiments were conducted at 35, 45, and 55 °C with pressures up to 25 MPa on the 0.354–1 mm particle fraction in the dry state. The objective of this study was to explore the accuracy and reproducibility of the manometric method in the pressure and temperature range relevant for potential coalbed methane (CBM) and CO 2-enhanced CBM (CO 2-ECBM) activities ( P > 8 MPa, T > 35 °C). Maximum experimental errors were estimated using the Gauss error propagation theorem, and reproducibility tests of the high-pressure sorption measurements for CH 4 and CO 2 were performed. Further, the experimental data presented here was used to explicitly study the CO 2 sorption behaviour of Chinese coal samples in the elevated pressure range (up to 25 MPa) and the effects of temperature on supercritical CO 2 sorption isotherms. The experiments provided characteristic excess sorption isotherms which, in the case of CO 2 exhibit a maximum around the critical pressure and then decline and level out towards a constant value. The results of these manometric tests are consistent with those of previous gravimetric sorption studies and corroborate a crossover of the 35, 45, and 55 °C CO 2 excess sorption isotherms in the high-pressure range. The measurement range could be extended, however, to significantly higher pressures. The excess sorption isotherms tend to converge, indicating that the temperature dependence of CO 2 excess sorption on coals at high-pressures (>20 MPa) becomes marginal. Further, all CO 2 high-pressure isotherms measured in this study were approximated by a three-parameter excess sorption function with special consideration of the density ratio of the “free” phase and the sorbed phase. This function provided a good representation of the experimental data. The maximum excess sorption capacity of the three coal samples for methane ranged from 0.8 to 1.6 mmol/g (dry, ash-free) and increased from medium volatile bituminous to subbituminous to anthracite. The medium volatile bituminous coal also exhibited the lowest overall excess sorption capacity for CO 2. However, the subbituminous coal was found to have the highest CO 2 sorption capacity of the three samples. The mass fraction of adsorbed substance as a function of time recorded during the first pressure step was used to analyze the kinetics of CH 4 and CO 2 sorption on the coal samples. CO 2 sorption proceeds more rapidly than CH 4 sorption on the anthracite and the medium volatile bituminous coal. For the subbituminous coal, methane sorption is initially faster, but during the final stage of the measurement CO 2 sorption approaches the equilibrium value more rapidly than methane.