Effect of oxygen-supply on the reburning reactivity of pyrolyzed residual from sub-bituminous coal: A reactive force field molecular dynamics simulation

The lean-oxygen combustion of incompletely pyrolyzed char dominated at the combustion front of coalfield fires. Functional group distributions and carbon micro-crystal dimensions were characterized for developing the molecular model (C184H124N2O16S) of pyrolyzed coal char at 400 °C (TLG P400). The overall combustion process of TLG P400 under various oxygen/fuel ratios (Ω) were simulated in an intuitive approach with ReaxFF. The findings indicated that the primary carbon-containing intermediates transformed from large pieces of aromatic fragments C40+ with self-similarity to heavy components of C15–C40 in initial combustion stage, then to light components of C5–C14 in late stage. With Ω increasing from 1.0 to 5.0, the final carbon conversion was found to rise from 78.50% to 96.32%, while the CO/CO2 ratio in the products sharply decreased from 2.575 to 0.213. The apparent activation energy was calculated as 194.49 kJ/mol for TLG P400 combustion at Ω = 1.0. Higher oxygen concentration enhanced the adsorption proportion of edge carbon, resulting in the generation of unstable intermediates featuring meta- or para-semiquinone. These transformations promoted the detachment of edge carbon and weakened the bottle-neck effect of the single-benzene ring on rapid oxidation. This study provides valuable insights into combustion mechanism and monitoring approaches for controlling coalfield fires.