Investigation into the thermal behavior and FTIR micro-characteristics of re-oxidation coal

To investigate pore structure, free radical concentrations, thermal behavior and FTIR micro-characteristics of the re-oxidation coal, we used Scanning Electron Microscope (SEM), Electron Spin Resonance (ESR), Fourier Transform Infrared Spectroscopy (FTIR), and a Synchronous Thermal Analyzer (STA). We conclude that the extension of pores and micropores on the coal surface due to initial oxidation, lead to an increase in the combination of coal and O2 in the re-oxidation of coal (ROC). Moreover, as the initial degree of oxidation increases, the number of aliphatic hydrocarbons and hydroxyl groups show a decreasing trend, while the content of oxygen-containing functional groups (–COO–, CO, COOH) and free radical concentrations increase significantly. Differences between the aliphatic hydrocarbon content in raw coal, 80 °C-Y coal (i.e., coal pre-oxidized at 80 °C for 30 min) and 180 °C-Y coal are small. However, the content of aliphatic hydrocarbons in 280 °C-Y coal decreased sharply from 35.26% to 25.83% compared to that of 180 °C-Y coal. The free radical concentrations of the 380 °C-Y coal increased by 9.84% compared to the raw coal. The initial oxidation can directly increase the free radical concentrations of coal, and promote the oxidation reaction of the re-oxidation coal. The consumption of combustibles during initial oxidation results in the rate of ROC being slower than the oxidation of raw coal. With the increase in the degree of initial oxidation, these changes become more obvious.