Evolution and oxidation properties of the functional groups of coals after water immersion and air drying

The evolution of functional groups, the gas generation laws in oxidation and temperature increase, and the oxidation kinetics parameters of the coal samples immersed in water for 30 days, 90 days, and 150 days (hereinafter referred to as 30 d coal, 90 d coal, and 150 d coal, respectively) were analyzed through Fourier transform infrared spectrometer, programmed temperature increase test system, and thermal gravimetric analyzer to disclose spontaneous combustion and the influencing mechanism of long-flame coals immersed in water for a long period. Results show that the aliphatic hydrocarbon and the hydroxyl contents of the coal samples increase after water immersion and air drying. In particular, the aliphatic hydrocarbon contents in 30 d coal, 90 d coal, and 150 d coal increase from 12.61 % for raw coal to 15.84 %, 14.08 %, and 13.90 %, respectively, whereas the hydroxyl content increases from 48.75 % for raw coal to 50.77 %, 50.13 %, and 49.23 %, respectively. In the low-temperature oxidation process, the CO output increases, and the apparent activation energy declines. When the temperature is 200 °C, the CO outputs of 30 d coal, 90 d coal, and 150 d coal are 28088, 25421, and 22112 ppm, which are 37.4 %, 24.4 %, and 8.2 % higher than the CO output of raw coal (20437 ppm), respectively. The apparent activation energy decreases from 65.7 kJ mol−1 for raw coal to 61.12 kJ mol−1, 63.08 kJ mol−1, and 64.5 kJ mol−1. According to the comparison of characteristic temperature points, the temperature of the mass extreme (T3) and the temperature of ignition point (T4) of the immersed coal samples both decrease. The T3 values of 30 d coal, 90 d coal, and 150 d coal decrease from 282.71 °C for raw coal to 274.87, 278.33, and 280.50 °C, whereas their T4 values decrease from 424.24 °C for raw coal to 419.23, 420.14, and 423.42 °C, respectively. This study proves that water immersion increases the oxidative activity of coals, thereby simplifying spontaneous combustion.