CO₂ adsorption of anthracite with different moisture contents and its implications for geological storage

CO₂ geological storage in deep coal seams is regarded as an important pathway to achieve “2030 carbon peak and 2060 carbon neutrality goals”. The moisture content in coal seams plays a significant role in the CO₂ storage capacity mainly based on CO₂ adsorption. In this study, the anthracite from the Qinshui Basin was collected to conduct high-pressure CO₂ adsorption isothermal experiments under dry, moisture-equilibrated and moisture-saturated conditions at 45℃. The CO₂ excess adsorption curve was corrected by the reduction of CO₂ dissolution. The adsorption capacity and adsorption heat were calculated using the modified D-R adsorption model. On the bases of the comparison of three CO₂ absolute adsorption curves at different moisture conditions, the microscopic mechanism of moisture saturation enhancing the adsorption capacity was explained. The results show that (1) the CO₂ adsorption capacity of dry, moisture-equilibrated and moisture-saturated coal samples are 56.72 cm³/g, 45.19 cm³/g and 48.36 cm³/g, and their adsorption heat are 29.42 kJ/mol, 26.23 kJ/mol and 27.24 kJ/mol, respectively. (2) With the CO₂ density less than 0.16 g/cm³ (6.48 MPa), the absolute adsorption capacity of anthracite in descending order is dry coal, moisture-saturated coal, and moisture-equilibrated coal, but when the CO₂ is in a supercritical state, the order changes to dry coal, moisture-equilibrated coal, and moisture-saturated coal. (3) The preferential occupation of high-energy adsorption sites by water molecules is the main reason for the reduction of the CO₂ adsorption capacity of moisture-equilibrated coal samples. The interaction between the coal-H₂O system and CO₂ is stronger than that between coal and CO₂ under the competitive adsorption of CO₂ and H₂O, resulting in the higher adsorption capacity of moisture-saturated coal samples being higher than that of dry coal samples in the supercritical stage of CO₂. (4) Adsorption storage is the main form of CO₂ geological storage in coal. In deep coal reservoirs, moisture saturation plays a more obvious role in increasing the adsorption capacity of supercritical CO₂. High-pressure water injection into deep coal seams is an effective way to improve CO₂ storage capacity and the permeability of coal seams.