Combined characterization of pore structurein deep medium-rank coal using mercury intrusion and liquid nitrogen adsorption methods and its pore fractal characteristics

To study the pore structure and fractal characteristics of deep medium-rank coal, combined characterization using mercury intrusion and liquid nitrogen adsorption methods was conducted on coal samples from the main coal seams in typical deep mining areas, including Shenyang Hongyang Third Mine, Kailuan Linxi Mine, Huainan Xinji Second Mine, and Pingdingshan Pingmei Sixth Mine. Parameters such as pore size, pore volume, and specific surface area were obtained, and the pore fractal characteristics were studied based on the Menger sponge model and the FHH model. The results showed that: (1) Among the pore structure parameters tested with mercury intrusion method, the average pore size ranged from 31.10 to 34.70 nm, pore volume from 0.048 3 to 0.059 4 mL/g, and specific surface area from 5.590 9 to 7.652 8 m2/g. The pore development in the main coal seams of typical deep mining areas was relatively similar. The pore volume distribution was dominated by macropores, with micropores and transition pores contributing roughly equally, and mesopores having a relatively small distribution. This indicated that macropores had better connectivity and mesopores were more closed. Micropores accounted for more than 70% of the total specific surface area, while the proportions of mesopores and macropores were minimal, indicating that micropores had the strongest adsorption capacity, which was negatively affected gas management in deep coal seams. The fractal dimensions based on the Menger sponge model ranged from 2.6 to 3.0, indicating irregular pore shapes, complex pore structures, and generally rough pore surfaces. (2) The effective pore size tested using liquid nitrogen adsorption method ranged from 3 to 177 nm with significant differences in total pore volume and specific surface area among the mining areas. Pore volume distribution was dominated by transition pores and mesopores, with a lower distribution of micropores and no macropores. This indicated that liquid nitrogen adsorption was effective for characterizing mesopores and transition pores but struggled to characterize macropore structures. Moreover, the connectivity of micropores was relatively poor. The specific surface area was mainly composed of transition pores, micropores, and mesopores, with no macropores. Among them, transition pores were mostly dominant and had relatively strong adsorption capacity. The fractal dimensions based on the FHH model ranged from 2.0 to 2.7, indicating a relatively simple and regular structure. (3) The differences in pore structures of deep medium-rank coal were discussed. The pore structure parameters (specific surface area and pore volume) determined by mercury intrusion and liquid nitrogen adsorption methods showed a non-linear concave curve variation with increasing burial depth. The fractal dimensions derived from the Menger sponge model and the FHH model showed a convex curve trend with increasing burial depth.