A PAH growth mechanism for nitrogen-containing aromatics in ammonia-doped hydrocarbon flames

Ammonia (NH₃) is a hydrogen carrier that can facilitate the sustainable energy transition by enabling co-firing with hydrocarbon fuels. In this work, we investigated the direct chemical effect of NH₃ on the suppression of aromatic formation by employing a combination of ab initio/RRKM-ME study and kinetic modeling. New reaction pathways for the formation of nitrogen-containing polycyclic aromatic compounds (NPACs) in direct reaction with NH₃ were studied using the high-level G3//B3LYP/6-311G(d,p) composite method. The pressure- and temperature-dependent phenomenological rate constants for the amino group and pyrrolic ring formation and thermodynamic data for new NPACs were calculated. Our analysis of equilibrium constants in a C₂H₄/NH₃ counterflow diffusion flame reveals that NH₃ plays a role akin to HCN; both temporally block the reactive sites and compete with C₂H₂ addition step of the HACA mechanism. Furthermore, a PAH mechanism dedicated for NH₃-doped hydrocarbon combustion was developed and partially validated, highlighting the key reactions to reproduce experimentally measured reduction quantities, especially in diffusion flames. After merging two literature models, proposed NPAC pathways and recent literature on small nitrogen-containing species (e.g., NH₂, NH₃, and HCN) reacting with C₃ and PAHs were included. Simulations show that the four channels of C₃H₃ ＋ NH₂ noticeably consume C₃H₃ radicals and significantly suppress the formation of aromatics, which is directly responsible for around 10% reduction in pyrene peak mole fraction for both premixed and diffusion flames. Amino groups are more likely to form on the aromatic edges than cyano groups in a C₂H₄/NH₃ diffusion flame. The chemical effect of NH₂ radical and NH₃ appears to be larger than that of HCN in NH₃-doped diffusion flames. Overall, compared to the merged mechanism, our new kinetic model predicts a more significant chemical effect of NH₃ on the reduction of major PAHs, bringing the model predictions closer to experimental measurements without the need for empirical adjustments.