Inhibitory effects and mechanisms of citric acid with different concentrations on spontaneous combustion of long-flame coal

Coal spontaneous combustion poses a serious threat to mine safety, and the application of inhibitors is an important approach for preventing and controlling such hazards. Because metal ions in coal can exert significant catalytic effects during low-temperature oxidation, their activity can be reduced through complexation with chelating agents, thereby suppressing coal oxidation. Environmentally friendly citric acid is employed as a chelating agent, and its mass-fraction-dependent inhibitory effects and synergistic mechanisms on the spontaneous combustion of long-flame coal are investigated. Temperature-programmed oxidation, simultaneous thermal analysis, Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), low-temperature nitrogen adsorption, and scanning electron microscopy (SEM) are used to characterize the macroscopic and microscopic changes in coal treated with different mass fractions of citric acid. The optimal inhibitory performance is obtained at a citric acid mass fraction of 5%, at which the crossing-point temperature increases by 40.0 °C and the peak CO release decreases by 79%. At citric acid mass fractions above 5%, CO₂ generation increases because of the thermal decomposition of citric acid; nevertheless, CO release remains more than 24.6% lower, indicating that the oxidation chain reactions of coal are continuously suppressed. The inhibition mechanism is attributed to synergistic chemical and physical effects. Chemically, ICP-MS and XPS analyses show that citric acid promotes the removal of Fe, Ca, and other metal ions through chelation and decreases the relative content of Fe³⁺. ESR analysis further demonstrates a pronounced decrease in free radical concentration, thereby suppressing oxidation chain reactions. FTIR analysis shows that reactive structures, including aliphatic hydrocarbon and carbonyl groups, decrease after citric acid treatment, whereas the relative contents of oxygen-containing groups such as —OH and —COOH increase. Physically, BET and SEM analyses demonstrate that citric acid and its reaction products modify the pore structure of coal, decrease the specific surface area by approximately 20%, and produce a denser surface morphology, thereby restricting oxygen adsorption and transport. Overall, the spontaneous combustion tendency of long-flame coal is substantially reduced through the combined mechanisms of metal-ion chelation, free-radical suppression, and pore blockage, with the strongest inhibitory effect obtained at a citric acid mass fraction of 5%. These findings provide a theoretical basis for the targeted application of environmentally friendly inhibitors for coal spontaneous.