Study of the Characteristics of Free Radical Changes in Coal During Heating in Dry Air and Nitrogen

This study explores the micro – mechanism of coal spontaneous combustion (CSC). The aim is to prevent self – ignition of residual coal in goaf areas when it contacts oxygen, thereby avoiding mine fires. A self – built CSC heating and oxidation system and an electron spin resonance spectrometer were utilized. Experiments on coal samples were carried out in both dry air and N2, focusing on the changes in free radicals within coal during heating. The free radical growth rate can be divided into three phases: slow growth (40–120°C), rapid growth (120–320°C), and decreased growth rate (320–400°C). Comparisons of free radical parameters between the two atmospheres showed that in dry air, the concentration and g – factor of free radicals were higher, and the linewidth was narrower than those in nitrogen. Further analysis of the difference – value (D – value) in free radical parameters between the two atmospheres indicated that the D – value in Ng increased linearly at different temperature stages. The D – value in the g – factor increased significantly at 100°C. The variation trend of linewidth was similar to that in dry air, and its change degree was affected by the free radical concentration. A gray relational analysis of the relationship between free radical parameters and gas products revealed a significant correlation between Ng and CO, with a correlation coefficient R ≥ 0.8. This study shows that the D – value in free radical parameters between dry air and N2 can reflect the change trend of coal sample free radicals after oxygen contact, which is crucial for understanding the coal – oxygen intrinsic oxidation reaction and the inherent mechanism of CSC.