Study on the variation in key functional groups of coal and the mechanism of CO and CO2 generation under the thermal effect of lean-oxygen conditions

Most closed fire areas in coal mines encounter rapid reignition after unsealing. To prevent the reignition of residual coal, it is necessary to study the impact of the lean oxygen conditions (LOCs) in closed fire areas on the characteristics of coal spontaneous combustion (CSC), evolution of the pore and fissure structures, and generation mechanisms of gases such as CO and CO2. A systematic study was conducted using in situ infrared, synchronous thermal analyzer (STA-DSC), scanning electron microscope (SEM), N2 adsorption, and programmed heating experiments. The results show that during the low-temperature oxidation process (LTOP), the temperature and oxygen concentration had a significant impact on the content variations of functional groups such as –OH, CO, COOH, CH2/CH3, and CC. Moreover, the higher the oxygen concentration, the higher was the effect. The concentration and heat release rate of CO and CO2 generation were positively correlated with the oxygen concentration. As the temperature increased, cracks developed gradually on the coal surface, and micropores transformed into mesopores and macropores. The specific surface area decreased gradually. When the temperature was below 110 ℃, the microstructure of the coal sample did not vary significantly. When the temperature was above 110 ℃, the effect was significant. The higher the oxygen concentration, the more significant was the impact on the pore characteristics of the coal. Two main pathways exist for CO and CO2 generation. CO2 generation requires a higher temperature than that required for CO. The research results provide a theoretical basis for the unsealing of closed fire areas and prevention of reignition.