Influence of dynamic supercritical CO2 on nanopore structure of coals based onlow-temperature nitrogen adsorption

The dynamic interaction of SCCO2 with coking coal and lean coal was simulated on a dynamic SCCO2 extraction system under conditions of temperature of 55 ℃ and pressure of 12 MPa. The low-temperature nitrogen adsorption was adopted to address the alterations in nanopore structure of coal samples, and the influence of SCCO2 on nanopore structure of coal samples was discussed. The results show that SCCO2 cannot alter pore shape of nanopores. For coking coal after SCCO2 exposure, the specific surface area and pore volume increase, the microporosity rapidly increases, while the mesoporosity and average pore size decline. However, the reverse trend is found for lean coal after SCCO2 exposure. Fractal analysis shows that SCCO2 enhances the heterogeneity of nanopore surface of coking coal, while the nanopore surface of lean coal after SCCO2 exposure becomes smooth. The net effect of SCCO2 extraction and swelling effect due to SCCO2 adsorption contributes to the different variations in nanopore structure of coking coal and lean coal. For coking coal, SCCO2 extraction effect is superior to swelling effect, which is favorable to the development of nanopores, while the reverse trend occurs in lean coal.