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At present, there are few studies on the development, evolution, and quantitative analysis of internal pores and cracks after heat treatment of coal. On the basis of this, low-temperature nitrogen adsorption, scanning electron microscopy, and X-ray diffraction (XRD) experiments were carried out on five kinds of temperature-treated bituminous coal and anthracite. The pore cracks of coal samples were studied qualitatively and quantitatively using linear fitting and the fractal theory. Through the analysis of the original diffractograms in the XRD experiment, the quantitative analysis of mineral composition in coal was realized. The results show that, with the increase of the temperature, the thermal damage of coal changes obviously and the micromorphology of the coal surface changes from the initial compact structure to the appearance of pores and small cracks and then to the mutual connection and evolution into large cracks. The development of pores and cracks of bituminous coal is more obvious than that of anthracite. Because anthracite has better heat resistance, there is no expansion and crack in its internal structure during the experiment. Through the fractal calculation of the coal nitrogen adsorption capacity, it is obtained that the overall pore volume and specific surface area of bituminous coal and anthracite are positively correlated with the fractal dimension (D 1) of the pore structure and V daf is negatively correlated with the fractal dimension (D 2) of the pore surface. In the temperature range from a normal temperature to 100 °C, the mineral crystal structure inside the coal body changes significantly, a large number of pores appear on the coal surface, and the total pore volume increases accordingly to reach the measured maximum value. The research of this paper is of great significance to reveal the effect of the temperature on coal damage and deformation and crack evolution.

Acoustic emission (AE) technology has the advantage of online localization to study the change laws of AE in the process of coal spontaneous combustion and to reveal the generation mechanisms of AE signal during the process of heating and rupture of coal body from a microscopic perspective. This paper first constructs a large-scale coal spontaneous combustion AE test system and conducts experimental tests on the AE signal in the process of coal spontaneous combustion. The results show that with the increase of temperature in the process of coal spontaneous combustion, the AE signal shows a trend of increasing fluctuations. Low-temperature nitrogen adsorption experiments studied the pore structure of coal spontaneous combustion, and the results showed a correspondence between the development of pores and the temperature of coal spontaneous combustion. Further, through the analysis of the evolution of the pore structure of coal through Fourier transform and fractal theory, it is found that the high-frequency main frequency AE signal and average frequency are continuously enhanced with the increase of temperature. The fractal dimension of the pore structure and the fractal dimension of the AE count of the coal body first rise and then decline. The mechanism of coal spontaneous combustion AE of coal is revealed, and the pore development caused by thermal stress when coal heats up is the main source of AE signal generation. The research in this paper is of great significance for applying AE technology to detect the position of coal spontaneous combustion.