Microscopic and macroscopic characteristics for coal spontaneous combustion under pre-oxidation and stress

In order to investigate the risk of Coal Spontaneous Combustion (CSC) remaining in goaf, the micro- and macro-oxidation characteristics of pre-oxidized coal (pre-oxidation temperature: 70 °C, 120 °C, 180 °C) and raw coal under stress (0 MPa, 4 MPa, 8 MPa) were studied. The mechanism of oxidative properties and changes in functional groups were investigated by Fourier infrared spectroscopy (FTIR) and simultaneous thermal analysis (STA). The following are the acquired results: Firstly, pre-oxidation temperature dictates the trend of the active functional groups, and the magnitude of the elevation or decrease is determined by the stress. Hydroxyl generation corresponds with aliphatic hydrocarbon consumption. Secondly, the characteristic temperatures (T2, T3, and T5) are impacted differently by stress and pre-oxidation temperature coupling. Generally, coal oxidation activity increases with pre-oxidation temperature, while stress alters the pre-oxidation degree through changing permeability property. Furthermore, stress promotes the production of free radicals. Overall, the highest risk is the coal sample with pre-oxidation temperature of 70 °C and stress of 4 MPa. The two mathematical models for the risk pre-oxidation temperature and risk stress have been obtained from T2. The intersection of these models is (3.22, 84.63), signifying that the highest risk of coal spontaneous combustion occurs at a stress of 3.22 MPa and a pre-oxidation temperature of 84 °C.