Multi-step reaction kinetics and heat release characteristics of coal smoldering in a sequential pyrolysis and oxygen-depleted combustion process

Coal fires pose persistent threats to mining safety and ecological environments, and their effective control requires understanding coal smoldering intrinsic mechanisms and thermal behavior. This study investigates the multi-step reaction kinetics and heat release characteristics of gas coal smoldering by reproducing the sequential process of inert-atmosphere pyrolysis followed by oxygen-depleted combustion. Thermogravimetric experiments were conducted using a TG-DSC analyzer, and Genetic Algorithm–based inverse modeling approach combined with Gaussian multi-peak fitting was applied to identify optimal reaction schemes and quantify the effects of pyrolysis temperature and oxygen concentration on multi-step reaction pathways, kinetic parameters, and reaction heat. The results indicate that coal pyrolysis at different isothermal temperatures over 30-500°C can be consistently described by a 2-step mechanism involving drying and depolymerization. The oxidation of coal pyrolyzed at 30-300°C follows a 3-step mechanism, including oxygen adsorption, combustion of oxygen-enriched intermediates, and semi-coke combustion, whereas oxidation of coal pyrolyzed at 400-500°C is dominated solely by semi-coke combustion. The total heat release Q , average heat release rate q ave , and semi-coke combustion heat ΔH sco decrease linearly with decreasing oxygen concentration and exhibit critical oxygen thresholds, below which their attenuation rates increase sharply. Coal pyrolyzed at 300°C exhibits the highest Q and ΔH sco , together with the lowest critical oxygen concentration (∼3%), indicating the strongest smoldering sustainability. Reducing the oxygen concentration in coal fire areas below this critical threshold significantly suppresses heat generation, thereby more efficiently inhibiting smoldering propagation. These findings provide new insights into coal smoldering mechanisms and offer theoretical basis for efficient control strategies.