Kinetic analysis on the hydrogen cyanide formation in the premixed methane/ammonia flame

Hydrogen cyanide (HCN) is a highly toxic trace emission that may be generated during ammonia (NH3) blended hydrocarbon combustion. In this study, a new kinetic model was developed and validated to predict HCN formation in NH3-blended combustion. Key rate coefficients for nitrogen-substituted hydrocarbon species were critically reevaluated. Using a one-dimensional freely propagating flame model, the formation of HCN in laminar premixed CH4/NH3/air flames was systematically analyzed under various conditions. Results indicate that HCN emission increases with increasing equivalence ratio (phi) and non-monotonically changes with NH3 blending ratio (alpha). Under lean conditions, HCN is primarily formed from the decomposition of imine radicals and oxidized by O/OH inside the flame front. While under rich conditions, the HCN oxidation takes place in the post-flame region, making the emissions at the outlet highly dependent on the reaction residential time. Consequently, attention should be paid to HCN emissions under high NH3 blending fraction (alpha > 50 %), and ultra-rich conditions (phi > 1.2) at normal temperature and pressure. The peak emission of HCN occurred at alpha = 0.7 and phi = 1.45, reaching a lethal concentration (310 ppm) in 1D premixed flame. Complex CHi and NHi interaction reactions are exhibited in the formation of HCN. The overall formation route can be divided into methylamine oxidation routes and radical combinations routes, and the former is dominant under all conditions. Independently increasing temperature or pressure will reduce the HCN emission due to the increased post-oxidation of HCN and diminished reaction characteristic time, respectively. More complicated changes in HCN were shown when temperature and pressure were increased at the same time. However, emissions will fall to a negligible level when temperature and pressure exceed 600 K and 2 atm, suggesting minimal concern for HCN emissions in CH4/NH3 combustion under elevated pressure and temperature conditions.