Effect of thermal damage on the pore–fracture system during coal spontaneous combustion

Abstract

Understanding the variation law of pore–fracture system during coal spontaneous combustion (CSC) plays a crucial role in understanding the mechanism of CSC and its prevention as well as control. In this study, three different ranks of coal samples were heat-treated (25–500 °C). Through nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and uniaxial compression (UC) tests, the changes in the pore-fracture system of different types of coal during CSC were comprehensively summarized. Based on the experimental results, the thermal rupture behavior of coal was simulated using PFC2D software. The results show that During CSC, the pore development of low-rank coal was obvious, the pore development of middle-rank coal was mainly concentrated in the high-temperature stage (T >300 °C), and the pore structure of high-rank coal was more stable. Meanwhile, the heterogeneity of the pore–fracture system of middle- and low-rank coals decreased, and the adsorption and seepage capacities increased. The heterogeneity of the pore–fracture system of high-rank coal was slightly influenced by temperature. The compressive strength of coal decreases with increasing temperature during CSC. The numerical simulation results were similar to the experimental results, i.e., the microcracks in the coal samples increased with temperature during heating; the effect of cracking caused by the high temperature will change the way the sample breaks, and the samples changed from ductile to brittle. The results of this study can serve as a reference for coal-fire control.