Low-Temperature Oxidation Kinetics and Multi-Scale Inhibition Mechanisms of Coal Treated with Fly Ash Hydrogels

Fly ash was incorporated into a polymer hydrogel system to prepare a fly ash composite hydrogel for inhibiting coal spontaneous combustion. The inhibition behavior of hydrogel-treated coal was examined by Fourier transform infrared spectroscopy (FTIR), thermogravimetry – differential scanning calorimetry (TG–DSC), apparent kinetic analysis, and density functional theory (DFT)-based frontier orbital calculations. FTIR results showed that hydrogel treatment reduced the relative activity of hydroxyl-related structures. Based on a coordination equilibrium model and an apparent kinetic approximation, the estimated instantaneous rate ratio of hydroxyl-related oxidation decreased to 0.02% under the selected assumptions. The C=O absorption band shifted from 1718 to 1703 cm−1 after hydrogel treatment, indicating variation in the local electronic environment of carbonyl-containing structures. A band-shift-based empirical correlation provided estimates of the DSC-derived apparent activation energy increments with deviations below 3% for the selected coal samples. Frontier orbital calculations were further used to interpret the possible influence of functional-group coexistence on the apparent oxidation activity of coal. The hydrogel-treated samples showed a 15–33% increase in the DSC-derived apparent activation energy and a 15.8–46.8% reduction in heat release. These results suggest that the inhibition effect of the fly ash composite hydrogel is associated with Fe3+-related coordination, carbonyl electronic-environment variation, and frontier orbital energy-gap modulation.