Experimental study on inhibition mechanism of composite inhibitors for coal spontaneous combustion at critical temperature

To elucidate the inhibition mechanism of composite inhibitors during coal spontaneous combustion under a selected critical-temperature condition, three coals with different ranks were investigated. Temperature-programmed oxidation coupled with gas chromatography, in-situ Fourier transform infrared spectroscopy (in-situ FTIR), and electron paramagnetic resonance (EPR) were employed to evaluate the effects of individual inhibitors (MgCl 2 , TEMPO, and PX4B) and PX4B–TEMPO composite inhibitors on coal oxidation behavior and reactive structures at 70 °C. The results showed that the inhibitory effects of the inhibitors became more pronounced after treatment at 70 °C, with PX4B exhibiting the strongest overall inhibition. The inhibitory effectiveness of TEMPO increased with increasing coal rank. PX4B and TEMPO exhibited a distinct non-additive synergistic effect, as evidenced by CO emissions lower than the theoretically weighted values, with a maximum inhibition efficiency of 61 %. The optimal PX4B:TEMPO mass ratio was 7:3 for low- and medium-rank coals and 5:5 for high-rank coking coal, indicating a coal-rank-dependent matching relationship. FTIR analysis showed that the composite inhibitor altered the evolution of hydroxyl hydrogen bonds and reactive oxygen-containing functional groups, whereas EPR measurements revealed a decrease in residual free-radical concentration after inhibitor treatment. Based on these results and the known inhibition characteristics of the individual components, a complementary synergistic mechanism is proposed: PX4B mainly regulates reactive functional-group structures, whereas TEMPO suppresses the propagation of coal–oxygen chain reactions through free-radical scavenging. Their complementary actions at different stages of coal oxidation collectively suppress coal spontaneous combustion. These findings provide a theoretical basis for the targeted formulation and application of composite inhibitors for coal spontaneous combustion prevention.