Fractal characteristics of nano-pore structure in tectonically deformed coals in Hancheng mining area

The nano-pore structure can be changed under tectonic deformation, and the different deformation mechanisms influence also the pore structure in various degrees. Pore structure of coals is very inhomogeneous, and is exactly described difficultly by the conventional experimental methods, but the fractal theory provides a quantitative method for describing the complexity of the pore structure. The influence of tectonic deformation on nano-pore structure of different deformed coals collected from Hancheng mining area in Weibei coalfield was quantitatively analyzed by nitrogen gas adsorption experiment based upon the fractal Frenkel-Halsey-Hill (FHH) theory. The results show that ductile deformed coals with higher pore fractal dimensions have higher heterogeneity and more complicated pore structure compared to brittle deformed coals, resulting in higher capillary condensation and prominent adsorption hysteresis. Fractal dimensions of tectonic coal increase with reduction of the average pore diameter and the increase of meso-pore content, indicating the stronger the tectonic deformation, the smaller the average pore size, the more complicated the pore structure. With the stress strengthening, fractal dimension shows an increasing trend. Thus, the fractal dimension can be used as an indicator of the deformation degree of the nano-pore structure.