Influence mechanism of pre-oxidation on the secondary oxidation characteristics of coal: insights from TG-DSC-FTIR...

The secondary oxidation behavior of coal represents a critical process governing its spontaneous combustion propensity and pollutant emissions. To elucidate the influence mechanism of pre-oxidation on the secondary oxidation characteristics of coal, this study selected raw coal from Dananhu, Xinjiang (DNH) as the research subject. Pre-oxidized coal samples (DNH-80) were prepared via temperature-programmed heating to 80 °C with isothermal holding for 1 h. The thermal behavior, reaction kinetics, and Gas emissions characteristics of both coal samples were systematically investigated using thermogravimetry-differential scanning calorimetry-Fourier transform infrared spectroscopy (TG-DSC-FTIR) at three heating rates (5, 8, and 12 K/min). The results demonstrate that pre-oxidation treatment significantly altered the physicochemical properties of coal: proximate analysis revealed a substantial decrease in moisture content from 11.56% to 1.38%, accompanied by significant increases in fixed carbon and volatile matter contents. Fourier transform infrared (FTIR) spectroscopy further confirmed substantial microstructural alterations, characterized by a marked increase in oxygen-containing functional groups (e.g., carbonyl C=O) and a reorganization of aliphatic structures. Kinetic analysis revealed that the apparent activation energy of pre-oxidized coal during the main combustion stage (conversion degree of 0.2-0.6) was systematically lower than that of raw coal, indicating a marked enhancement in reactivity. Thermal analysis data further confirmed that pre-oxidized coal exhibited more intense combustion behavior, with its maximum mass loss rate (DTG_max) increasing by 3.0-10.1% and total heat release increasing by 6.6-7.2%. Gas emissions analysis demonstrated that pre-oxidized coal produced sharper CO₂ release peaks, with cumulative total release increasing by 7.27-10.13% relative to raw coal. This study elucidates the mechanism by which pre-oxidation intensifies the secondary oxidation process of coal through reducing activation energy and altering reaction process, providing essential theoretical foundation and experimental data for accurately assessing the spontaneous combustion risk and environmental impact of pre-oxidized coal.