Functional group-driven modulation of combustion-gas kinetics in underground coal enhanced combustion

Enhanced combustion coupled with in-situ thermal energy extraction in underground coal seams represents a novel paradigm-shifting strategy for coal resource extraction through in-situ chemical fluidized mining. Systematic investigation into the combustion dynamics and reactive gas evolution under injection-enhanced conditions provides validation for optimizing this in-situ thermo-chemical conversion process. This study elucidates the metamorphic dependence of coal combustion through combined FTIR and TG-FTIR analyses, delineating functional group evolution and gaseous emission dynamics across coals of different metamorphic grades. The results indicate that the intensification of the coal metamorphism, characterized by increased multi-substituted aromatic groups (2 adjacent H deformation) and stable OH-π structures, alongside reduced active oxygen-containing groups (conjugated CO and -COOH) and asymmetric aliphatic chains, collectively diminishes the coal ignitability. Combustion performance indices reveal that low-metamorphic-degree coal exhibits enhanced ignition and combustion efficiency at lower temperatures, whereas high-metamorphic-degree coals demonstrate inferior ignitability but achieve higher combustion rates. Highly metamorphosed coal typically exhibits lower CO₂ emission intensity and higher CO emission intensity due to incomplete oxidation. This divergence in combustion products directly correlates with enhanced oxygen demand requirements for complete oxidation at higher metamorphic stages. CH₄ emission predominantly occurs within the 200–700℃ thermal regime, with 400°C serving as a critical thermal demarcation point for the peak CH₄ release intensity from coals of varying metamorphic degrees. NO is the main component of the harmful gas NO_x, and highly metamorphosed coal releases less NO. Ignition propensity demonstrates linear dependence with -COOH, and the burnout efficiency exhibits proportional enhancement of the 3 adjacent H deformation. OH-OH, asym. R₂CH₂, and asym. RCH₃ collectively control the volumetric release of gaseous species during coal combustion. These findings offer a scientific basis for optimizing underground coal combustion systems by strategically utilizing natural variations in coal maturity.