Experimental investigation into the mechanism of secondary oxidation of coal under hot air flow erosion

The residual coal in the goaf of high-temperature mines is highly susceptible to oxidation and spontaneous combustion (SC) due to the erosive effects of hot airflow. Moreover, the risk of secondary oxidation may be significantly elevated. To elucidate this issue, a temperature-programmed experiment device on secondary oxidation was conducted to investigate the characteristics of coal's secondary oxidation under the influence of hot air flow with varying flow rates and temperatures. The analysis focused on O₂ consumption rate, heat release intensity (<i>q</i>), the generation rate of carbon-oxygen gas, and the apparent activation energy (<i>E</i> _a) during the secondary oxidation process. The results indicate that during the low-temperature oxidation (LTO) process, an increase in air flow rate leads to a higher O₂ consumption rate and <i>q</i> for both raw coal (<i>R</i> _C) and coal samples treated with 35 °C hot air flow (<i>T</i> ₃₅). Additionally, the <i>E</i> _a in the surface oxidation stage is lower, which implies a greater risk of coal spontaneous combustion (CSC). The temperature of the hot air flow showed a negative correlation with the rate of CO gas generation, and treatment with a hot air flow at 35 °C will promote the generation of CO₂ gas during the later stages of oxidation. Compared with the <i>T</i> ₃₅ coal sample, the coal sample treated at 65 °C (<i>T</i> ₆₅) exhibits greater sensitivity to variations in air flow rate. Under an airflow volume of 200 mL min^-1, the erosion of hot airflow at various temperatures can effectively reduce the risk of CSC. However, under lower airflow conditions (50-150 mL min^-1), the <i>T</i> ₃₅ coal sample exhibit the highest risk of SC during the later stages of LTO. Under all airflow conditions, treatment with a hot airflow temperature of 65 °C inhibits the CSC in secondary oxidation. This investigation provides a theoretical foundation for further investigation into the mechanisms of CSC.