Experimental investigation on microstructure evolution and spontaneous combustion properties of secondary oxidation of lignite

Effective prevention and control of spontaneous fire of low-rank coal at underground goafs are critical to ensure coal mine safety. The characteristics of secondary oxidation of two lignites types were investigated. The results show that the spontaneous combustion tendency of the preoxidized lignite is higher than that of the raw coal, especially in the early stages of low-temperature oxidation. After preoxidized by air under different conditions, coal samples were used in an adiabatic oxidation experiment (by H2O2) to simulate the self-heating process. The experimental results show that after being pre-oxidized 100-150 degrees C, the oxidation rate of coal sample re-oxidation increased significantly. and this effect is closely correlated to the treatment time and temperature. The TG/DSC tests show that preoxidation gradually increases the self-heating risk of coal; however, when the treated temperature exceeds 175 degrees C, the calorific value of coal decreases sharply. The preoxidation step weakens some functional groups in coal to a certain degree, but the proportion of C-O structure fluctuates, decreasing the activation energy of the secondary oxidation of coal between 40 and 120 degrees C. The proportion of coal particles with porosity >10 nm increases in coal samples studied after the treatment, which favors the oxidation diffusion of coal. In general, secondary oxidation causes microstructural changes in lignite and increases the risk of spontaneous combustion, but preoxidation at an excessively high temperature may over consume the organic constituents, decreasing the liability of self-heating. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.