Effects of oxidation temperature on microstructure and spontaneous combustion characteristics of coal: A case study of Shendong long-flame coal

To identify effects of the pre-oxidation process on the spontaneous combustion characteristics of Shendong low-rank long-flame coal, the microstructure and oxidation reactivity of coal samples pre-oxidized at different temperature including 70 °C, 100 °C, 200 °C, 300 °C, and 400 °C were studied and compared with those of raw coal. The surface morphology, pore features, functional groups, and microcrystalline structure of coal were studied using SEM, N2 adsorption, XPS, and XRD analyses. Moreover, TG-DSC experiments were conducted to investigate the changes in the spontaneous combustion characteristics of coal. The results indicated that low-temperature pre-oxidation at 70 °C and 100 °C does not alter the physical and chemical structure significantly, so the pore parameters, functional group contents, and microcrystalline structure of these two samples were very similar to those of raw coal. When the pre-oxidation temperature reached and exceeded 200 °C, the specific surface area and total pore volume of coal samples exhibited a significant decrease due to extensive thermal decomposition and consumption of coal components via oxidation. Moreover, high-temperature pre-oxidation treatment above 200 °C sharply reduced the aromatic and aliphatic carbon C–C/C–H content, and this is ascribed to the substantial consumption of primordial organic matter, this made coal macromolecules more tightly packed with an increase in carbon crystal size and carbon matrix order. In addition, with increasing temperature from 200 °C to 400 °C, the C–C/C–H contents decreased and aromaticity of coal increased, which revealed the weak oxidative activity. The heat release △H1 from 30 °C to 300 °C of coal samples after pre-oxidation at 200–400 °C was significantly lower than that of raw coal, while the △H1 values of coal samples treated at 70 °C and 100 °C were close to that of raw coal. Notably, that oxidation treatment at 70 °C resulted in an increase in the amount of heat released, because of the largest specific surface area that increases the risk of spontaneous combustion. Thus, the pre-oxidation treatment increases the risk of spontaneous combustion of coal when the pre-oxidation temperature is lower than a critical temperature.