Adsorption–Desorption–Seepage Characteristics and Deformation Mechanism of Confined Coal for Flue Gas under Different Water Saturations

The excessive discharge of flue gas degrades air quality, while its injection into coal seams mitigates pollution and enables carbon dioxide storage. This study investigates the adsorption, desorption, and seepage mechanisms of flue gas in coal seams through gas–solid coupling tests under triaxial stress for coal samples with varying water saturation. The results indicate that under constant coaxial and confining pressures, the adsorption capacity, desorption capacity, and seepage rate of flue gas exhibit a significant decline with increasing water saturation. Additionally, the equilibrium times for desorption and seepage are reduced, while the equilibrium time for adsorption is extended. During the processes of adsorption, desorption, and seepage, both axial and radial strains in the coal decrease with increasing water saturation or axial/confining pressure, with the axial strain consistently lower than the radial strain. Gas component analysis reveals nitrogen (N₂) dominance during initial desorption, with carbon dioxide (CO₂) and methane (CH₄) increasing as desorption progresses. Further analysis demonstrates a significant linear relationship between the axial and radial strains of coal and the gas seepage rate. As the effective stress or water saturation increases, both the diffusion coefficient and dimensionless permeability exhibit a declining trend under the same water saturation conditions. Under dry conditions, the effective stress sensitivity coefficient decreases with increasing effective stress. In contrast, under high water saturation conditions, the effective stress sensitivity coefficient displays a more complex pattern, becoming more pronounced as water saturation rises. These studies help to understand the influence of water saturation and stress on the behavior of flue gas in coal and provide a theoretical basis for optimizing coal bed methane extraction and carbon sequestration.