Study on the Spontaneous Combustion Characteristics and CO-CH4 Formation Law of Low-Rank Bituminous Coal Based on In Situ Infrared

Abstract

Low-rank bituminous coal exhibits strong chemical reactivity and a relatively high spontaneous combustion tendency due to its high volatile matter content. The relationship between chemical structure evolution and gas generation during the oxidation of low-rank bituminous coal was studied by temperature-programmed and in situ infrared spectroscopy. The results show that the reaction between aliphatic side-chain −CH3/–CH2– groups in the coal and O2 molecules is the main reason for CO generation. The aliphatic side-chain group decreased by 80.57% in the stage of coal oxidation at 180–380 °C. During the high-temperature oxidation stage, the oxygen-containing functional groups C═O/C–O–C in the coal are thermally decomposed to generate CO. The CH4 formation in the process of coal oxidation is not affected by O2 concentration, and it is more sensitive to temperature. The adsorbed gas in the residual coal is gradually released by heating during the low temperature oxidation stage. When the coal is oxidized to 260 °C, it thermally decomposes to produce CH4. As the temperature continues to rise to 330 °C, the CH4 formation rate increases exponentially. The thermal decomposition of the aromatic ring branched-chain structure Ar–C–H– is the main reason for the CH4 generation during the spontaneous combustion of residual coal. The apparent activation energy Ea of coal with different O2 concentrations during the slow oxidation stage at 60–100 °C shows that an 8% O2 concentration can be used as a criterion for determining the critical O2 volume fraction of low-rank bituminous coal spontaneous combustion in the Shendong Mining Area . Monitoring of spontaneous combustion of residual coal in goaf should pay attention to O2, CO, and CH4 at the same time, especially when the CH4 concentration rises abnormally, which indicates that the spontaneous combustion of residual coal in goaf has entered the stage of high-temperature oxidation. Continuous air leakage into the goaf can lead to residual coal combustion, thereby endangering mine safety.