Study on the oxidized gas production characteristics and spontaneous combustion tendency of pre-oxidized water-soaked coal in lean-oxygen environments

Abstract In order to study the oxidation characteristics and spontaneous combustion (SC) tendency of pre-oxidized long-term water-soaked coal under different oxygen concentrations, raw long-flame coal (RC), water-soaked 200-day coal (S200), pre-oxidized water-soaked coal at 200 ℃ (O200S200), and pre-oxidized water-soaked coal at 300 ℃ (O300S200) coals were used for the study. The coal spontaneous combustion(SC) program heating test system was adopted to conduct experimental analyses at 21%, 15%, 10%, 5%, 3% oxygen concentration(OC) for experimental analysis. The results show that pre-oxidation water-soaked treatment(PWT) promotes the coal-oxygen complex reaction and increases the rate of coal oxygen consumption(OCR) and the rate of carbon and oxygen compounds production. The rate of CO and CO2 production of the water-soaked(WS) coal increased by 0.329 mol·(cm3·s)−1 and 0.922 mol·(cm3·s)−1, respectively, compared with that of the original coal sample. PWT reduces the activation energy of coal in the low-temperature oxidation stage (the maximum difference can be up to 110.99 KJ/mol) and enhances the oxidizing and heat-releasing capacity. There is a large amount of water in the WS coal, and its cross-point temperature and SC propensity index increased. There was a synergistic effect between the pre-oxidation(PO) and WS treatment, and the lowest comprehensive determination index of the SC propensity of coal in O200S200 samples was 831.92 which was 4.72 lower than that of RC samples, presenting a more SC tendency. Low oxygen concentration has inhibitory effect on the oxidation characteristic parameters of coal, and the apparent activation energy of low-temperature oxidation stage of pre-oxidized water-soaked coal(PWC) increased to 206.418 KJ/mol at 3% oxygen concentration. Both WS and PWT can improve the oxidation activity of coal, and the S200 samples have the most carbon oxides production in the anoxic environment, and the SC risk of the O200S200 samples is the highest. The lower the oxygen concentration of the anoxic environment, the lower the risk of SC of the coal samples. The oxidation characteristics and SC tendency of the PWC under different oxygen concentrations revealed in the study can provide theoretical guidance for the identification and prevention of SC disasters in coal seams with shallow burial and close spacing.