The effect of heating rate on secondary oxidation of coal spontaneous combustion under hypoxic conditions

Coal spontaneous combustion (CSC) presents significant safety risks in coal mining. This study investigates the causes through experiments on lignite from southwestern China. The coal samples underwent varying heating rates (HRs) in a programmed temperature furnace under 5% O₂ to produce experimental samples. Custom-engineered heating apparatuses, gas chromatography (GC), and Fourier-transform infrared spectroscopy (FTIR) were enlisted to analyze the effects of HR on coal oxygen consumption, CO emissions, and functional group conversions under hypoxic circumstances.The principal discoveries indicate that HR significantly influences secondary oxidation and the transformation of functional groups, thereby affecting the CSC process. The optimal HR for secondary oxidation depends on the initial oxidation temperature. At an initial oxidation temperature of 80°C, coal samples subjected to a heating rate of 0.4 °C/min demonstrated an elevated rate of oxygen consumption, along with markedly higher levels of aromatic hydrocarbons and oxygen-containing functional groups in comparison to other samples. At HRs of 0.8 °C/min and 1.2 °C/min, the oxygen consumption rate increased with the initial oxidation temperature, ultimately exceeding the aliphatic hydrocarbon group content in samples treated at other temperatures. The hydroxyl content decreased with rising temperature but was maintained at lower HRs at lower temperatures.