Low-temperature oxidation characteristics and their association with functional-group evolution of long-flame coal subjected to water-immersion pretreatment at different temperatures

Residual coal in deep goafs may show altered low-temperature oxidation behavior after water immersion, drainage, and drying. To distinguish the effect of pretreatment temperature from the additional differences associated with water immersion, long-flame coal was subjected to dry and water-immersion pretreatments at 25, 35, 50, and 65 °C, with three independent sample batches prepared for each condition. Temperature-programmed oxidation, in situ FTIR spectroscopy, apparent kinetic analysis, Pearson correlation analysis, and grey relational analysis (GRA) were used to characterize oxidation behavior and functional-group evolution. Increasing dry pretreatment temperature enhanced subsequent oxidation, and the oxygen consumption rate of D65 at 200 °C was 25.2% higher than that of D25. Compared with D65, S65 showed a 33.3% higher oxygen consumption rate at 200 °C and 51.0% and 29.8% higher CO and CO 2 concentrations, respectively, at 130 °C. Two-way ANOVA showed significant interactions between treatment mode and pretreatment temperature for the three macroscopic oxidation indicators (P < 0.05). In situ FTIR results showed that pretreatment altered the initial relative intensities and temperature-dependent changes of –OH, aliphatic structures, C=O, phenolic C–O, and aromatic ether C–O–C. Pearson analysis showed significant correlations between functional-group evolution and CO and CO 2 evolution (P < 0.01), with –CH 2 – generally showing relatively strong correlations. GRA showed stage-dependent associations between functional-group evolution and gas evolution, while the top-ranked functional group varied in some independent replicate analyses. These results provide an experimental basis for distinguishing pretreatment-temperature effects from the additional differences associated with water immersion, filtration, and redrying under the present experimental conditions.