Experimental Investigation of the Matrix Pore Size Distribution and Inner Surface Fractal Dimension of Different-Structure High Rank Coals

To investigate the nanopore characteristics of different-structure high-rank coal matrices, four samples were collected from the Zhaozhuang Mine, a typical mine highly prone to coal and gas outbursts in Shanxi Province, China, and the nanopore size distribution was measured using a lowtemperature nitrogen adsorption method for these four samples. Based on the nitrogen adsorption isotherms, the inner surface fractal dimensions of micropores, meso- and macropores, and full-scale matrix pores were estimated using a Frenkel–Halsey–Hill (FFH) fractal model, and the relationships between the fractal dimensions and pore parameters were discussed. The inner surface of the high-rank coal matrix is heterogeneous. The inner surface fractal dimensions of micropores (D ₁) and full-scale matrix pores (D _T) can be arranged in a descending sequence for the differentstructure coals: mylonitized coal > granulated coal > cataclastic coal > intact coal, while the inner surface fractal dimensions of meso- and macropores (D ₂) are in an ascending sequence. With the increasing deformation degree of coals, some macropores and mesopores are transformed into smaller pores, such as micropores; the total pore volume (PV) and total specific surface area (SSA) increase; the SSA percentage and PV percentage of meso- and micropores increase; and those of macropores decrease. The average pore width (APW) is positively correlated with D ₂ and negatively correlated with D ₁ and D _T. The tectonic deformation enhances the irregularity of micropores and the heterogeneity of the whole coal matrix pores and decreases the inner surface roughness of meso- and macropores.