Changes in the major functional groups of coal under long-term freezing and the effects of their spontaneous combustion characteristics

In order to reveal the potential effects of long-term freezing on the low-temperature oxidative properties and the risk of spontaneous combustion of coal, this study investigated the low-temperature oxidative properties of coal samples treated with different freezing durations (2, 4, 6, and 8 months) by SEM, gas chromatography, and Fourier transform infrared (FTIR) spectroscopy experiments. Experimental data reveal that the oxidation behavior of coal undergoes stage-dependent changes under long-term freezing, where the 4-month mark serves as a pivotal threshold: during the 4-month freezing period, the active sites in the coal continue to accumulate and the oxidative activity is gradually enhanced, while after 4 months, the generation of active centers is suppressed, and the effect of the promotion of oxidative behavior is weakened. Specifically, the F 4 sample (frozen for 4 months) exhibited the lowest apparent activation energy, with a maximum decrease of 5.35 kJ/mol from the original coal. The grey correlation and TOPSIS comprehensive analysis showed that the coal sample frozen for four months had the highest proximity index (gas generation: 0.65; heat release: 0.72), with its gas release, fire coefficient, and heat release intensity all reaching their peak values. The raw coal had the lowest proximity index (gas: 0.61; heat release: 0.30), with significantly lower gas production and heat release. This is because freezing treatment increases the number of active groups such as -CH 3 and -CH 2 in coal, which react with oxygen to form oxygen-containing intermediates such as C=O and -COOH. These intermediates then decompose to produce CO/CO 2 and release significant heat, indicating that freezing treatment can effectively enhance the self-ignition tendency of coal.