Experimental Study on Pore-fracture Evolution Law in the Thermal Damage Process of Coal

High temperature causes thermal damage to coal, alters its pore-fracture structure, and then influences its permeability and spontaneous combustion characteristics. In this paper, to study the evolution law of coal pore-fracture in the thermal damage process and the influence mechanism of thermal damage on pore-fracture development, the microfracture evolution characteristics and pore morphology of coal samples treated under different temperatures were qualitatively studied by susing a scanning electron microscope (SEM). Besides, the evolution of coal pore structure as a function of temperatures in the heating process was quantitatively studied based on Nuclear magnetic resonance (NMR) technique. In addition, the pore structure of coal samples was quantitatively characterized in combination with the fractal theory, and the variation of fractal dimension of coal sample pore structure with heat treatment temperature was obtained. The results show that after heat treatment, the development of fractures of coal was promoted, and the connectivity between the fractures was effectively enhanced. Moreover, the higher the temperature is, the more favorable it is for the development, expansion, and connectivity of fractures and for the formation of microfracture networks in coal. The high-temperature thermal damage changes the pore structure of coal, increases the pore volume and porosity of coal and enhances the pore connectivity. Moreover, after high-temperature heat treatment, the pore structure of DL coal with low metamorphism changed more significantly than that of XG coal. The porosity of DL coal and XG coal increased with the rising temperature, while the pore fractal dimension DmNMR, DMNMR, and DNMR of the two kinds coal samples show the opposite trend. Furthermore, the variation amplitude of DmNMR is significantly larger than that of DMNMR, and the variation of three fractal dimensions of DL coal is larger than that of XG coal. The research results can support a reference for coal fire prevention.