The promotion mechanism of persistent free radicals on low-temperature oxidation of coal

Abstract

This study aims to reveal new reaction pathways for low-temperature oxidation of coal from the perspective of persistent free radicals (PFRs), thereby enriching the theory of free radical reactions in coal spontaneous combustion (CSC). To this end, combining electron spin resonance technology, spin trap technology, and quantum chemical calculations, the occurrence characteristics and thermal reaction characteristics of PFRs during CSC were studied, and the reaction mechanism of PFRs promoting coal oxidation was studied using density functional theory (DFT). The results indicate that coal always contains abundant PFRs during the low-temperature stage of CSC (30 ℃–300 ℃), the concentration remains at the level of 1019 spin/mm3. These PFRs can induce the production of •OH and O2•− under the action of heat starting from 45 ℃–50 ℃. The higher the initial content of PFRs or the higher the heating temperature, the more favorable it is for the generation of •OH (the concentration of •OH remains at the level of 1011 spin/mm3). •OH and O2•− have stronger oxidizing properties than O2, and when they oxidize the active groups in coal, the required energy barrier is lower. In addition, both O2 and O2•− cannot trigger free radical reaction of coal oxidation at room temperature, while •OH has this ability (energy barrier < 40 kJ/mol), and will release a large amount of reaction heat (average enthalpy change of the oxidation reaction of active groups is −72.68 kJ/mol). From both thermodynamic and kinetic perspectives, the thermal reaction characteristics of PFRs in coal play a positive promoting role in the low-temperature stage of coal oxidation, and have great potential for triggering CSC. Therefore, the efficient quenching of PFRs or inhibiting the production of •OH and O2• − will be beneficial for preventing early oxidation of coal. The research results provide new perspective for the study of CSC and its prevention.