Thermal role and reaction kinetics of functional groups during coal smoldering at high temperatures: A case study of long-flame coal

Understanding the microscopic reaction mechanisms of high-temperature smoldering combustion (SC) is essential for efficient fire management. This study adopted high-temperature in-situ FTIR and DSC techniques to investigate the real-time evolution laws of 11 typical functional groups and their correlation with heat release during SC of long-flame coal. The reaction kinetics mechanism of typical functional groups under time-scale effects (TSE) was revealed. The results demonstrated that reduced oxygen level (100%–21%, 16%–1%) mainly affects coal combustion performance by restricting or delaying the rapid consumption of typical functional groups. Heat release restriction follows a two-stage linear model, with sensitivity to this limitation being about 21 times higher from 21% to 3% to 3%–1% oxygen level. Aliphatic hydrocarbons at low temperatures and carboxyl/carbonyl groups at high temperatures exhibit the highest correlation degree with heat release. Aliphatic hydrocarbons determine the early-stage ignition capability of coal, while aromatic hydrocarbons (benzene rings) govern the burnout capability, and oxygen-containing functional groups dictate the burnout characteristics and maximum heat release intensity. The sensitivity to TSE follows the sequence: benzene rings ≈ oxygen-containing functional groups > aliphatic hydrocarbons > hydroxyl groups, and oxygen-limited conditions > normal oxygen conditions. Kinetic studies confirm that the activation energies under oxygen‑limited conditions (3%, 50–100 kJ/mol) are lower than those under normal‑oxygen conditions (140–200 kJ/mol). An oxygen level of 3% can be adopted as a critical safety threshold for the on‑site sealing management of fire zones.