Thermodynamic characterization and spontaneous combustion mechanism of coal in continuous hot–humid air flow

In the coal storage areas of coastal ports and waterlogged areas at the bottom of mines, continuous exposure of the coal body to hot-humid airflow affects its low-temperature oxidation behavior. To determine the effect of hot-humid airflow on the mechanism of coupled coal–oxygen–water reaction, coal samples under different air relative humidity (ARH) conditions were analyzed by scanning electron microscopy, Fourier-transform infrared spectrometry, and thermogravimetry. The results show that under ARH, the number of pores in the coal body increase, and condensed moisture clogs the pore structure. Calculations using the Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Friedman kinetic models indicate that ARH 30 and 90 promote coal spontaneous combustion (CSC) before the transition temperature point, while inhibiting CSC after the transition temperature. The mean activation energies correlate well with oxygen–containing functional groups and aliphatic substances, and the good agreement of the correlation coefficient values between –C=O, –CH2, and activation energies indicate an increasing phase change with the progress of CSC. Furthermore, the presence of humidity promotes –COOH and –OH generation, and this effect is more pronounced at an ARH 30. The results of this study provide a new theoretical basis for heat and humidity control in mine environments and coal storage in coastal ports.