Influence of sequestered supercritical CO2 treatment on the pore size distribution of coal across the rank range

The influence of supercritical CO2 (SC-CO2) fluids on pore structure characteristics of coal plays a vital role in geo-sequestration of CO2 into deep coal seams. Changes in coal pore structure and mineral content after exposed to SC-CO2 fluids are quantitatively evaluated by N2/CO2 adsorption and X-ray diffraction (XRD) for a lignite, bituminous and anthracite coals. The SC-CO2 exposure has a notable effect on coal pore distribution. Affected by SC-CO2, the micropore volume within coal is reduced by 12.6 to 34.8%, while both the average pore diameter and macropore volume are remarkably increased. XRD analysis shows that no newly-formed mineral compositions were detected in treated coals, but the mineral content is altered. As the main carbonate minerals of coal, both calcite and dolomite are consumed during SC-CO2 treatment. Due to the strong extraction capacity for small organic molecules, the influence of SC-CO2 is related to coal rank, and the largest increment in macropore volume is observed in lignite. The greatest reduction of micropore volume is identified in anthracite treated sample, indicating that SC-CO2 has the most impact on micropores in high-rank coal. Mineral dissolution likely accounts for this reduction in micropore contribution by increasing mesopores and macropores.