ReaxFF simulations on the transformation pathway of nitrogen elements in the heavy tar under oxy-coal combustion

Abstract

Heavy tar is a crucial intermediate product during coal combustion. To explore the transformation pathway of N atoms in heavy tar under oxy-coal combustion, a comprehensive molecular model of heavy tar with typical N-containing functional groups was constructed. Different temperatures and chemical equivalence ratios were set for the oxy-coal combustion. The ReaxFF was employed to study various products' distribution and molecular numbers. The reaction network among different precursors and NOx was extracted, and the NO to N₂ conversion mechanism was summarized. The results indicated that, like char combustion, the proportion of heavy tar gradually declined, the proportion of light tar and organic gas first rose and then gradually declined, and the proportion of inorganic gas continuously rose during heavy tar combustion. As the temperature increased, the proportion of cyanide precursors decreased, while the proportion of amine precursors and NOx increased. The oxidation of N-containing intermediates became more intense as the O₂ content rose, but this oxidation effect was inhibited, and the NOx generation was reduced as the O₂ content further increased. NO could bond with NH_i, HNO, CN, and activate NO, decomposing to produce N₂O, and N₂O reacted with H radical to produce N₂.