Study on thermal damage characteristics and micromechanical behavior of structures induced by spontaneous combustion of low-rank coal

Thermal damage induced by coal spontaneous combustion (CSC) has a significant impact on its development. In this study, low-rank coals were heat-treated (25-500 degrees C). Experiments such as nuclear magnetic resonance, scanning electron microscopy, and uniaxial compression were used to visualize and quantify the evolution of pore structure and structural strength during CSC. A multi-component discrete-element model of coal thermal fracture was developed and a sensitivity analysis of the micromechanical behavior of CSC was carried out. The results showed that during CSC, the micropores in coal gradually evolved into mesopores or macropores, and the pore diameter, porosity and permeability can be increased to 5.51 mu m, 21.05 % and 1.51 mD respectively with the temperature. Significant thermal damage occurred in low-rank coal at 100 degrees C, leading to a sharp decrease in its load-bearing capacity, which made it easier to reach the damage condition and produce more cracks. And then the thermal damage was fluctuating with the increase of temperature. From 100 degrees C to 500 degrees C, the micromechanical behavior of low-rank coal is similar, and the variation of the critical value with temperature is small. The study results can serve as a reference for controlling coal fires.