Insight into ScCO2 adsorption mechanism and differences in medium- and high-rank coal

Insight into the adsorption characteristics and mechanisms of supercritical CO₂ (ScCO₂) in deep coal seams is crucial for predicting CO₂ geological storage capacity and enhancing coalbed methane recovery. This study conducts high-pressure CO₂ isothermal adsorption experiments, combined with pore structure analyses using low-temperature CO₂ adsorption, low-temperature N₂ adsorption, and mercury intrusion, focusing on gas coal from Qidong Mine and anthracite from Sihe Mine. Results show that CO₂ adsorption in coal involves both micropore filling and monomolecular layer adsorption. An SDR-Langmuir mixed model was developed to describe the ScCO₂ adsorption behavior, with R² values ranging from 0.982 to 0.996. Modeling results show that in medium- and high-rank coals, micropore filling contributes 54 %–99 % of the adsorption capacity, which decreases with increasing pressure (>7.5 MPa) while monomolecular layer adsorption increases. Micropore filling is governed by van der Waals forces, which weaken with temperature rise, leading to reduced CO₂ adsorption capacity. In medium-rank coals, monomolecular layer adsorption increases with temperature due to more meso- and macropores and oxygen-containing groups, while in high-rank coals, monomolecular layer adsorption decreases as temperature increases. This study provides a theoretical basis for modeling and evaluating ScCO₂ storage in coal seams.