Lanyun Wang

Underground coal fires pose severe hazards to safe mining and environmental sustainability, with fracture development in coal serving as critical pathways for oxygen supply and heat transfer. However, the dynamic evolution of fractures under coupled high-temperature and triaxial stress conditions remains poorly understood. In this study, bituminous coal samples from a fire-affected mining area in Xinjiang, China, were subjected to high-temperature treatments (200 and 400 °C) and subsequently loaded under triaxial compression (confining pressure 5 MPa) while being monitored by an industrial CT scanning system. Full stress–strain curves were obtained, and VG Studio MAX software was used to quantitatively analyze fracture morphology, volume, density, porosity, and 3D fractal dimension across five successive loading stages. The results reveal that temperature governs the mechanical behavior of thermally treated coal. With increasing temperature, the compaction stage shortens, elastic modulus decreases, and peak stress first increases then decreases, while the durations of plastic deformation and postpeak failure stages first shorten then lengthen. The porosities of RC, C200, and C400 coal samples at the first scan were 0.07%, 0.28%, and 0.70%, respectively, indicating that high-temperature treatment increased the initial porosity of the coal samples. Based on fracture density, porosity, and fractal dimension, fracture evolution is divided into three stages: smooth development, slow growth, and rapid growth. The 3D fractal dimension further confirms this staged evolution. These findings provide a theoretical basis for predicting fracture network evolution in coal fire zones and offer practical guidance for fire prevention strategies.

In order to understand the influence of underground coal fires on coal fractures and pores, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) are combined to study the development of coal pore and fracture under high-temperature treatment and calculate the fractal dimension to analyze the relationship between the development of coal pore and fracture and the fractal dimension. The results show that the volume of pores and fractures of the coal sample (C200) treated at 200 °C (0.1715 mL/g) is greater than that of the coal sample (C400) treated at 400 °C (0.1209 mL/g), and both are greater than the original coal sample (RC) (0.1135 mL/g). The volume increase is mainly due to mesopores and macropores, and the proportions of mesopores and macropores in C200 were 70.15 and 59.97% in C400. The MIP fractal dimension shows a decreasing trend with the increase of temperature, and the connectivity of coal samples improved with the increase of temperature. The changes in volume and three-dimensional fractal dimension of C200 and C400 showed the opposite trend and are related to the different stress of coal matrix at different temperatures. The experimental SEM images confirm that the connectivity of coal fractures and pores improves with the increase of temperature. Based on the SEM experiment, the larger the fractal dimension, the more complex the surface is. The SEM surface fractal dimensions indicate that the surface fractal dimension of C200 is the smallest and that of C400 is the largest, which is consistent with the observations made by SEM. The combination of the two fractal dimensions is used to characterize the self-similarity of coal using the fractal dimension difference. When the temperature increased to 200 °C, the unordered expansion of the coal sample resulted in the largest fractal dimension difference and the lowest self-similarity. When heated to 400 °C, the fractal dimension difference of the coal sample is the smallest, and the microstructure of coal shows a regular groove-like development.

Coal is the one of foundations of energy and economic structure in China, while the unsealing of coal mine fires would cause a great risk of coal re-ignition. In order to explore the influence of pressure-bearing state on the re-ignition characteristics for residual coal, the uniaxial compression equipped with a temperature-programmed device was built. The scanning electron microscope, synchronous thermal analyzer and Fourier transform infrared absorption spectrometer was applied to investigate the microscopic structure and thermal effect of the coal samples. Moreover, the microscopic effect of uniaxial stress on coal re-ignition is revealed, and the re-ignition mechanism is also obtained. As the uniaxial stress increasing, the number, depth and length of the fractures of the pre-treated coal increases. The application of uniaxial stress causes the thermal conductivity to change periodically, enhances the inhibition of injecting nitrogen on heat transfer and prolonges the duration of oxidation exothermic. The content of oxygen-containing functional groups has a high correlation with apparent activation energy, and coal samples at 6 MPa is more probability to re-ignite while the fire zone is unsealed. Uniaxial stress could control the re-ignition mechanism by changing the structure of fractures and pores. The side chains and functional groups of coal structure are easier to be broken by thermal-stress coupling. The higher the ·OH content, the more difficult coal samples would be re-ignited. The research results would lay a solid theoretical foundation for the safe unsealing of closed fire-areas underground, tighten the common bond between the actual industry and the experimental theory in closed fire-areas underground, and provide the theoretical guidance for coal re-ignition preventing.