Abstract:
CO
2 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 CO
2 storage capacity mainly based on CO
2 adsorption. In this study, the anthracite from the Qinshui Basin was collected to conduct high-pressure CO
2 adsorption isothermal experiments under dry, moisture-equilibrated and moisture-saturated conditions at 45℃. The CO
2 excess adsorption curve was corrected by the reduction of CO
2 dissolution. The adsorption capacity and adsorption heat were calculated using the modified D-R adsorption model. On the bases of the comparison of three CO
2 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 CO
2 adsorption capacity of dry, moisture-equilibrated and moisture-saturated coal samples are 56.72 cm
3/g, 45.19 cm
3/g and 48.36 cm
3/g, and their adsorption heat are 29.42 kJ/mol, 26.23 kJ/mol and 27.24 kJ/mol, respectively. (2) With the CO
2 density less than 0.16 g/cm
3 (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 CO
2 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 CO
2 adsorption capacity of moisture-equilibrated coal samples. The interaction between the coal-H
2O system and CO
2 is stronger than that between coal and CO
2 under the competitive adsorption of CO
2 and H
2O, resulting in the higher adsorption capacity of moisture-saturated coal samples being higher than that of dry coal samples in the supercritical stage of CO
2. (4) Adsorption storage is the main form of CO
2 geological storage in coal. In deep coal reservoirs, moisture saturation plays a more obvious role in increasing the adsorption capacity of supercritical CO
2. High-pressure water injection into deep coal seams is an effective way to improve CO
2 storage capacity and the permeability of coal seams.