Quantitative study on transformation effect of supercritical CO₂ on multi-scale pore structure of coal

In order to more accurately characterize the effect of supercritical CO₂ on the pore structure of the coal and explore the mechanism of its transformation on the pore structure of the coal, low-temperature nitrogen adsorption and desorption tests and mercury intrusion porosimetry (MIP) tests were carried out based on the literature research and the coupled-hole method. It was concluded that the pore distribution results obtained from mercury intrusion porosimetry test and low-temperature nitrogen gas absorption and desorption test showed differences, and joint characterization should be carried out to reduce the test error; supercritical CO₂ can make the microscopic and small pores in the coal rupture gradually into medium and large pores, and the pore structure becomes more open and connected. The pore volume percentage of macropores, mesopores and small pores after supercritical CO₂ treatment was found to be increasing, with increments of 3.66%, 1.84% and 2.92% respectively, while the pore volume percentage of micropores was decreasing, with a reduction of 8.42%; the specific surface area of macropores, mesopores and small pores of the coal were also on the trend of increasing, with increments of 0.47%, 3.67% and 57.65% respectively, while that of the micropores was decreasing, with a reduction of 61.79%. The main reason for this phenomenon is that supercritical CO₂ has certain extractive pore expansion effect and adsorption and swelling effect on the pore structure of the coal, which can react with the mineral components and organic functional groups in the coal, leading to chemical dissolution process inside the coal, making the degree of adhesion between the coal particles deteriorate, and increasing the dispersion of the particles of the coal, and the coal has a strong adsorption on the supercritical CO₂, resulting in the expansion of the pore volume and pore specific surface area of the coal. Moreover, the strong adsorption of coal to supercritical CO₂ leads to the adsorption and swelling process inside the coal, resulting in the expansion of the coal matrix and the widening of the pore space, which leads to the increase of the pore volume and pore specific surface area of the coal.