Impact of nitrogen substitution on polycyclic aromatic hydrocarbon growth in ammonia-hydrocarbon blended fuel combustion

Combustion of ammonia-hydrocarbon blended fuels suppresses polycyclic aromatic hydrocarbon (PAH) and soot formation while generating nitrogen-substituted PAHs (NPAHs). This work theoretically investigates the impact of N-substitution on the hydrogen abstraction C₂H₂ addition (HACA) mechanism by introducing N at distinct sites: the phenyl (forming pyridyl C₅H₄N) and acetylene (forming HCN). Reaction pathways were characterized using high-level CCSD(T)/CBS//M062X/def2TZVPP calculations, and rate coefficients were calculated with Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Results show that N-substitution in phenyl stabilizes radicals through through-space/through-bond interactions, modestly elevating initial addition barriers and reducing reaction rates. However, at elevated temperatures, NPAHs exhibit dehydrogenation branching ratios and kinetics similar to PAHs, demonstrating sustained growth potential. N-substitution in C₂H₂ restructures transition states, increasing entropic loss and reducing addition reaction rates. HCN addition lowers dehydrogenation barriers but elevates cyclization barriers, favoring stable product formation over ring closure. This dual-pathway inhibition suppresses PAH growth. Theoretical analysis reveals that N-substitution suppresses PAH growth but that NPAHs can grow at similar rates to PAHs at high temperatures, indicating that initial NPAH growth must be considered.