Coal matrix response to CO₂ adsorption and emission: implications for sustainable mining and carbon sequestration

This study investigates the coal matrix response to CO₂ adsorption and emission, exploring its potential for sustainable carbon sequestration in abandoned coal mines, mineable and un-nimeable coal layers. Unlike previous studies focusing on short-term desorption, this research uniquely examines long-term gas emission without vacuum intervention, simulating real-world conditions in abandoned mines. The results highlight coal's strong CO₂ retention, with higher pressures amplifying deformation but not necessarily adsorption, due to confining pressure effects. Optimal saturation times of 1500 min for pressures below 30 bar and 2700 min for higher pressures are recommended. Using a novel triaxial testing apparatus, coal samples from the Chamestan region, Iran, were subjected to CO₂ saturation at injection pressures of 15, 30, and 35 bar for 3000 min, followed by emission over 3150 min. Key findings reveal that CO₂ adsorption increases with pressure, inducing axial strains of 0.517%, 0.559%, and 0.678% at 15, 30, and 35 bar, respectively, driven by gas diffusion and fracture formation. Post-emission, axial strains reduced to 0.087%, 0.071%, and 0.054%, indicating irreversible structural changes. Cyclic tests showed a significant decline in adsorption capacity, with axial strain dropping from 0.560% to 0.331% in a second cycle without vacuum treatment, recovering to 0.445% after vacuum treatment. These findings advance the understanding of coal's long-term behavior for CO₂ sequestration, offering practical insights for enhancing carbon capture and storage technologies and supporting sustainable mining practices to mitigate climate change.