The study aims to investigate changes in the spontaneous combustion characteristics of coal and its microscopic reaction mechanisms in a CO2 atmosphere. Programmed heating experiments and thermogravimetric analysis were conducted to examine index gas and combustion parameters in both air and CO2 atmospheres. Molecular functional group changes were analyzed via in situ infrared experiments, and their correlation with index gases was assessed using the grey correlation method. The results revealed that CO2 injection during the initial heating stage increased the production of CO, CH4, and C2H4. However, beyond 160 °C, CO2 effectively reduced the oxidation activity of functional groups and significantly inhibited the production of indicator gas. The characteristic temperature range of coal samples exhibited a 'lag' phenomenon under the influence of CO2, resulting in a prolonged combustion stage with reduced mass loss and heat discharge. Following CO2 injection, the -OH concentration significantly decreased, while C=O gradually increased. Additionally, the concentrations of -COOH, aliphatic hydrocarbons, and aromatic hydrocarbons initially increased with rising temperature and then declined. The contribution of these groups to gas generation followed the sequence of oxygen-containing functional groups > aliphatic hydrocarbons > -OH > aromatic hydrocarbons.
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