Investigation of hydrogen cyanide and hydrogen isocyanide on PAH growth in the pyrolysis of HCN/C₂H₂

The suppression of polycyclic aromatic hydrocarbon (PAH) growth, a critical precursor to soot formation, is essential for optimizing ammonia-hydrocarbon co-combustion systems to reduce emissions. While ammonia blending with hydrocarbons mitigates soot, existing kinetic models lack robust mechanisms to characterize interactions between nitrogen-containing species (HCN and HNC) and PAHs, limiting accurate predictions of PAH inhibition in practical pyrolysis or combustion environments. This study combines quantum chemistry and Rice-Ramsperger-Kassel-Marcus (RRKM) methods to calculate temperature-dependent rate constants for HNC and HCN reactions with phenyl (A1-) and naphthyl (A2-) radicals. The effects of HNC+PAH and HCN+PAH chemistry on the PAH growth in a practical C₂H₂/HCN/N₂ pyrolysis system were extensively investigated. The potential energy surface analyses revealed that the HNC addition to A1- or A2- via the C-atom was more favorable than the N-atom channel. Comparisons in the rate constants and branching ratios showed that HNC maintained a certain degree of competitiveness against C₂H₂ for A1- and A2- in the temperature range of 300-1500 K, especially at temperatures below 1000 K. While for the pyrolysis of C₂H₂/HCN/N₂ in a jet-stirred reactor, the competitiveness of HNC against C₂H₂ for A1- and A2- was negligible. The dominant addition channel for A1- was ranked as HCN > C₂H₂ > HNC, contrasting with predictions based solely on rate constants or branching ratios. This highlights the limitations of branching ratio-based evaluations in determining the role of HNC + PAH chemistry in PAH growth. The HCN + PAH chemistry demonstrated a suppression effect on the formation of four- and five-rings PAHs, with the inhibitory effect intensifying as the inlet mole fraction of HCN increased. The rate of production (ROP) analysis indicated that A2- + HCN = C₁₀H₇CN + H and A1- + HCN = C₆H₅CN + H were the two main factors for the decrease of mole fractions of pyrene (A4) and five-rings PAH (A5), and the latter exhibited a stronger suppression effect. This work uniquely establishes HNC-PAH chemistry and real-system behaviors, underscoring HCN's critical role in mitigating soot precursors under practical conditions. Novelty and significance statement: The novelty of this research lies in the first development of HNC + PAH chemistry and the detailed analyses of HCN/HNC + PAH chemistry on the formation of PAH in the pyrolysis of C₂H₂/HCN/N₂. The significance of HCN/HNC + PAH chemistry on the formation of PAH is determined using the rate constants in earlier research, but it has not been fully assessed in a real combustion or pyrolysis system. Through detailed analyses of the formation pathway of PAH predicted with and without HCN/HNC + PAH chemistry, this paper concluded that HNC + PAH may not play a key role in the formation of PAH, which contrasts with the results obtained using rate constants and branching ratio analyses. However, HCN could compete against C₂H₂ for A1- and A2-, inhibiting the growth of larger PAHs.