Study on Secondary Oxidation Risk of Coal with Variable Soaking Time under the Influence of Oxygen Concentration

Primary and secondary low-temperature oxidation experiments were conducted on lignite immersed in water for short (50 days) and long (200 days) periods under three different oxygen concentrations (8, 18, and 21%), which provided a theoretical basis for the identification and risk judgment of a spontaneous combustion zone in goaf. The results indicate that in both the short-term water-immersed coal (STWIC) and long-term water-immersed coal (LTWIC), the apparent activation energy (Ea) for the three stages of secondary oxidation is lower than that for primary oxidation under 18% oxygen concentration, suggesting a greater risk of spontaneous combustion. The STWIC and LTWIC that have experienced a primary oxidation in the goaf are more prone to secondary oxidation spontaneous combustion when the oxygen concentration is 8 and 18%, while the risk of secondary oxidation spontaneous combustion is lower than that of primary oxidation at 21%. During the experimentation, a temperature inflection point was observed, which decreased with an increase in the oxygen concentration conditions. Beyond this temperature inflection point, the oxygen consumption rate and heat release intensity during the primary oxidation of STWIC surpassed those during secondary oxidation. Moreover, the generation rates of CO and CO₂ during the secondary low-temperature oxidation process of LTWIC were lower than those of STWIC.