Influence of Maximum Pressure on the Path of CO₂ Desorption Isotherm on Coal

Coal seams with a high CO 2 content may have outburst risk, and degasification of CO 2 has to be conducted before these coal seams can be safely extracted. For geosequestration of CO 2 in unminable coal seams, injected CO 2 may desorb with the reduction of CO 2 pressure. Desorption of CO 2 dominates these processes, while adsorption isotherms are widely used assuming that the adsorption–desorption process is fully reversible. To understand the difference between CO 2 adsorption and desorption isotherms, i.e., sorption hysteresis, as well as the dependence of CO 2 sorption hysteresis on maximum pressure, four cycles of CO 2 adsorption–desorption experiments are conducted continuously with increasing maximum pressure (1, 2, 3, and 4 MPa). The difference of CO 2 emission volume between adsorption and desorption isotherms is compared, and a significant deviation (0.059 and 0.032 mol/g) has been observed. The adsorption isotherms show a good repeatability, indicating that the gas holding capacity does not change during a long-term contact with CO 2 . However, a distinct difference between the desorption isotherms is observed. The path of desorption isotherm depends upon the maximum pressure, and higher maximum pressures can reduce the proneness of CO 2 desorption in the pressure range of this study (0–4 MPa). We suggest that desorption isotherms should be used to predict the CO 2 emission volume and long-term storage stability, and the maximum pressure of the laboratory sorption test should be decided according to the in situ coal seam pressure.