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 (D1) and full-scale matrix pores (DT) 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 (D₂) 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 D₂ and negatively correlated with D1 and DT. 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.