Study on the influence of unloading disturbance of initial load stress on the microstructure and thermodynamic behavior of granular coal

Abstract

With the advancement of coal mining, the pre‐mining stress on the coal seam increases. After mining, the coal seam fractures and unloads, leaving granular coal in the goaf with a high risk of spontaneous combustion. To investigate the oxidation behavior and underlying mechanisms of granular coal in goafs at various depths, fresh coal was subjected to static stresses ranging from 4 to 16 MPa and then underwent unloading treatment to generate granular coal with varying initial stresses. Subsequently, simulations of granular coal in goafs at various depths were conducted. Structural characteristics (pores and functional groups) and oxidation heat production performance of the granular coal after unloading were analyzed using a low‐temperature nitrogen adsorption instrument, a Fourier infrared spectrometer, and a simultaneous thermal analysis system. The findings suggest that as the initial loading stress increases, the number of micropores and mesopores within the unloaded bulk coal decreases, while the number of macropores increases. Furthermore, important oxidation‐active structures, including ‐OH, ‐CH3, ‐CH2‐, C=O, and ‐COOH, gradually increase, with a slight decrease observed after exceeding 8 MPa. The pressure‐unloading process leads to a gradual decrease in the characteristic temperature of the bulk coal, with the characteristic temperature increasing after exceeding 8 MPa, although it still remains lower than that of the raw coal. As the burial depth of the goaf increases, the oxidation behavior of the unloaded granular coal becomes more pronounced, leading to an increased tendency and risk of spontaneous combustion. If the initial loading stress on deep coal seams is excessive, the oxidation heat production capacity of the resulting unloaded granular coal may be slightly diminished, yet it still poses a significant disaster risk. The research results can provide valuable insights for mitigating and managing spontaneous combustion risks in coal seam mining operations conducted at different depths.