Effect of molecular structure change on the properties of persistent free radicals during coal oxidation

This study is aimed at exploring the effect of molecular structure changes on the properties of persistent free radicals in coal during the heating process of coal spontaneous combustion (CSC). Therefore, the persistent free radicals and main functional groups variation characteristics in the process of coal oxidation were analyzed by means of in situ electron spin resonance and in situ infrared spectroscopy, and the change relationship between the molecular structure of coal and the persistent free radicals in coal during the reaction was further discussed by correlation analysis. The results showed that the concentration of persistent free radicals showed a stage change of “first decreasing, then increasing and then decreasing”. The g-factor decreases gradually and the heteroatom free radicals are consumed continuously. Dehydrogenation oxidation reaction causes the weakening of the “electron-proton” interaction, which narrows the line-width (ΔH) of free radicals. The phenolic hydroxyl as the electron donating group is converted into -CO functional group through dehydrogenation and oxidation reaction, which promotes the formation and accumulation of persistent free radicals in coal. Under the influence of phenolic hydroxyl, the relationship between the stretching vibration strength of aromatic ring CH and the properties of persistent free radicals in coal shows a similar change law with phenolic hydroxyl. In addition, the chain structure represented by –CH2-, –CH3/–CH2- is continuously consumed in the oxidative dehydrogenation reaction, triggering a series of free radical reactions. The free radicals formed in this process are affected by the steric hindrance of polymer networks and the electron delocalization of aromatic group in coal, so the persistent free radicals can be formed and accumulated. The dehydrogenation and oxidation reaction of the active group led to the reduction of g-factor and ΔH. Persistent free radicals play an important role in the free radical chain reaction that triggers CSC, so preventing the dehydrogenation and oxidation reaction of hydroxyl (especially phenolic hydroxyl) and aliphatic hydrocarbons (especially –CH2- and –CH3/–CH2-) is the key to prevent CSC.