Comprehensive characterization and full pore size fractal characteristics of coal pore structure

To explore the pore structure and full pore size fractal characteristics of coal, with the help of mercury intrusion and low temperature liquid nitrogen adsorption, combined with Micro-CT scanning system, the pore structures of coal samples taken from 5-2 coal seam in Haiwan mine (#1) and central 1 coal seam in Gubei mine (#2) were characterized respectively. The complexity of full pore size distribution of coal samples was compared and analyzed based on fractal theory. The results have shown that:1) The pore structure of # 1 coal developed, micro-pore to macro-pore are open, and the fracture extend from the upper face to the lower end face, both of which form a reticular topological structure in space. However, there were a large number of solitary pores in #2 coal, most of micro-pores belonged to closed pores or semi-open pores, and the number of fracture structures was relatively small, which was difficult for them to form pore fracture topological structure and not conducive to gas seepage. 2) The pore specific surface area of #1 and #2 coal mass was mostly contributed by micro-pores and transition pores, but there was a significant difference in pore volume between them, that is, the volume ratio the macro-pore and meso-pore of #1 and #2 coal mass was relatively high, while the pore volume of #2 coal mass was mainly contributed by micropore and transition pore; 3) After a comprehensive comparison of the defects of three characterization methods, a new method was proposed to characterize the full pore size fractal dimension. It was concluded that the pore size distribution of #1 coal was more heterogeneous than that of #2 coal, that is, the pore structure was more complex in the pore size dominant interval characterized by low temperature liquid nitrogen adsorption method. While the pore structure of #2 coal was more complex than that of #1 coal in the pore size dominant interval characterized by high pressure mercury pressure method and there was the most significant difference in pore development between the two coal samples in the range of medium pores. In the pore diameter dominant range characterized by the Micro-CT scanning system, the heterogeneity of pore structure was roughly the same, and the difference was the number of pores; 4) Multi-mean joint characterization of pore structure and full pore size distribution of coal can correct "compression effect" errors of coal matrix, compression failure of pores and fractures and "shielding effect" in the determination of micro-pores and transition pores in the high pressure stage, as well as errors caused by the neglect of macro-pores and visible pores due to the sample size, which is the basis for the research and evaluation of solid-gas coupling and gas occurrence, diffusion and seepage. © 2022, Editorial Board of Journal of Mining & Safety Engineering. All right reserved.