Micro-characteristics of low-temperature coal oxidation in CO2/O2 and N2/O2 atmospheres

•Effect of CO2 on limiting low-temperature coal oxidation was better than N2.•Micro-properties of coal oxidised in CO2/O2 and N2/O2 atmospheres were compared.•Grey theory was adopted to illustrate key attractive functional groups. Frequent spontaneous combustion of coal is a major threat to coal mine safety. CO2 and N2 are often used to prevent coal spontaneous combustion. To determine the differences between their reactions with coal, Fourier transform infrared spectroscopy (FT-IR) was adopted to explore the functional groups distinction of coal samples oxidized in CO2/O2 and N2/O2 atmospheres. The curve-fitting method and grey relational theory were selected to analyse the type, relative content and variation of major functional groups. The main functional groups in the samples were elucidated as oxygen-containing functional groups, aliphatic hydrocarbons and aromatic hydrocarbons. Oxygen-containing functional groups and aliphatic hydrocarbons are prone to react in low-temperature oxidation. As the oxygen concentration increased, hydroxy and CO were continuously reduced. The aliphatic hydrocarbons content exhibited a significant decrease from the critical to crack temperatures in all atmospheres, indicating that they were extremely active in different oxidation stages. The effect of CO2 on inhibiting spontaneous combustion of coal was superior to that of N2 when comparing the hydroxy contents in CO2/O2 and N2/O2. Meanwhile, the results of the grey correlation analysis also verified this conclusion. The grey correlation grade cleared that hydroxy, CO, aliphatic hydrocarbons and COO were key functional groups relating to oxygen concentration. Their differences in the CO2/O2 and N2/O2 atmospheres demonstrated that low-temperature oxidation of coal became slower under the influence of CO2 than N2. The results from this study can clarify the difference between CO2 and N2 in preventing coal spontaneous combustion from a micro view and provide a guide to selecting appropriate inhibitors in the goaf and working face.