Research on the Functional Group Changes of Coal under the Erosion of High Temperature Airflow

When residual coal in high-temperature mine goafs collapses, it is subjected to prolonged exposure to hot airflow before reaching spontaneous combustion conditions. This process significantly affects the coal's oxidation activity and spontaneous combustion risk. To determine the influence patterns of high-temperature environment and high-temperature airflow on the functional groups in coal, tests, and analysis of the functional groups of coal under different temperatures without airflow and different airflow rates were conducted. The research findings indicate that the influence patterns of environmental temperature and airflow rate on different functional groups in coal molecules vary. The high-temperature environments of 35 and 65 °C without airflow result in an increase in the relative contents of most oxygen-containing functional groups, aliphatic units, and hydroxyl functional groups. Under the long-term erosion of different temperatures and airflow rates, most functional groups increase or decrease. Notably, oxygen-containing functional groups exhibited a significant increase under low-flow 65 °C conditions. In comparison with raw coal, under the circumstances of 35 and 65 °C, the aromatic hydrogen ratio (AHR) of the coal samples exhibited varying degrees of increase resulting from erosion in both windless and low wind flow conditions. With the exception of the coal sample eroded by a 200 mL/min 35 °C hot airflow over an extended period, where the aromatic ring condensation degree (AOC) decreased, the AOC of other coal samples rose to different extents. Whether subjected to high-temperature alone or high-temperature airflow, the length of the aliphatic chain of the coal samples all manifested a decreasing tendency to varying degrees. Nevertheless, the aliphatic chain (CL) value of the coal samples eroded by hot airflow for a prolonged time decreased relatively significantly, generating more oxygen-containing functional groups such as hydroxyl and carboxyl. Save for the coal samples eroded by 200 and 100 mL/min 35 °C hot airflow over an extended period, where the CO to CC ratio (C) of the raw coal decreased, the C value of the coal molecules of other coal samples increased markedly. The experimental results of functional groups and related indicators suggest that long-term erosion in a high-temperature windless environment or by low-flow high-temperature airflow might cause strong oxidation reactivity of coal, facilitating oxidation and increasing the propensity for spontaneous combustion.