Numerical study on the effect of oxygen concentration on flame evolution of a single coal particle in a flue gas environment

Understanding the mechanisms of ignition and combustion is crucial for the optimization and efficient management of coal-fired systems, especially because variations in ignition and combustion behaviors directly influence the evolution of the gas-phase flame. A transient model, developed by the authors in a previous study (Yuan et al. 2024), which employed the chemical percolation devolatilization model, a reduced gas-phase mechanism, and char reaction model, was used to investigate the impact of oxygen concentration (X_O2) on the evolution of the gas-phase flame within a flue gas environment at 1800 K. Online experimental measurements of CO and CO₂ volume fractions provided further validation for the numerical model, in addition to previous optical measurements. The results indicate that gas-phase reactions commence with volatile combustion, transition into a phase where volatile and CO combustion occur simultaneously, and ultimately culminate in CO combustion alone. Char oxidation is identified as the primary source of CO production. In the homogeneous and heterogeneous ignition modes, CO actively participates in gas-phase reactions during the phase of intensified volatile combustion, causing an initial increase and subsequent decrease in flame intensity. In contrast, under the homo-heterogeneous ignition mode, CO's involvement in gas-phase reactions become more pronounced as volatile combustion subsides, resulting in a secondary enhancement of flame intensity. As X_O2 increases, both volatile and char combustion are promoted, causing the flame front to shift closer to the particle surface. This shortens the duration of volatile combustion and reduces the time for gas-solid interactions, which are crucial for maintaining flame stability.