Chemistry of nitrogen-containing polycyclic aromatic formation under combustion conditions

In recent years, ammonia as a carbon-neutral fuel additive has attracted attention for blending with hydrocarbons in combustors. However, the chemical effects of blending ammonia with hydrocarbons are not well understood, e.g., the direct chemical effects on the formation of polycyclic aromatic hydrocarbons (PAHs), which are well-known soot precursors. In this study, we perform a systematic study of the reaction pathways describing the chemistry of nitrogen-containing polycyclic aromatic compounds (NPACs) in comparison with the hydrogen-abstraction-acetylene(C2H2)-addition (HACA) mechanism, employing the G3-type quantum chemistry composite method for accurate energy calculations and RRKM-one dimensional master equation approach for pressure- and temperature-dependent phenomenological rate constants. The calculated energies using G3//B3LYP/6-311G(d,p) procedure and rate constants were compared with available literature. While the formation of nitrogen-embedded PACs (e.g., 4-azapyrene) is less favoured compared with forming cyano group at high temperatures, a considerable amount of NPACs could still be formed. The proposed pathways provide possible molecular structures of several NPAC mass peaks inferred from the mass spectra of the soot particle samples from an ammonia doped ethylene laminar premixed flame. A deeper look into the reaction steps for NPAC and HACA pathways in a counterflow ethylene/ammonia flame simulation reveals that the addition of hydrogen cyanide to the aromatic structure is highly reversible at high temperatures, thus the direct chemical effect of doping ammonia on PAH formation is primarily by temporarily blocking the reactive sites and preventing further growth of PAHs via HACA pathways. These findings help explain the discrepancies between the small amount of NPACs detected in soot particle samples, while the observed direct chemical effect on PAH reduction is significant by doping ammonia in the flames.