Variation laws of CO2/CO and influence of key active groups on it during low-temperature oxidation of coal

The CO2/CO ratio produced by the reaction of active groups in coal with oxygen can truly reflect the state of coal spontaneous combustion. However, there are various paths for coal to produce CO and CO2, including oxidation, pyrolysis, and desorption. Gas production from coal desorption and pyrolysis influences CO and CO2 produced from oxidation, especially pyrolysis manifests as the production of gas and active sites. For this reason, a method that can exclude the interference of gas production from desorption and pyrolysis is designed, and based on this method, the variation of CO2/CO ratio of coal under different pathways is studied. The active sites generated from coal pyrolysis was also investigated using pyrolytic coal, and the intrinsic causes of CO2/CO ratio variation were studied based on microanalysis and model compounds. The experimental results show that the proposed method can effectively eliminate the effect of desorption and pyrolysis, and the CO2/CO ratio produced from oxidation is about 3. The active sites generated from coal pyrolysis strongly affect the concentrations of CO and CO2 at the beginning of oxidation, but they barely influence their ratio which is always about 3. The intermediates product by the oxidation of methylene linked to multiple benzene rings are the same precursor to produce CO and CO2, which are the main cause of a CO2/CO ratio of 3. The gradual involvement of different active groups in the reaction considerably affects the ratio of CO2/CO produced by oxidation path. Therefore, the low-temperature oxidation to produce CO and CO2 can be divided into three stages, which provides theoretical support for predicting the state of coal spontaneous combustion based on the change law of CO2/CO ratio.