Influence of ethene and propene on fuel reactivity and cyanides formation in ammonia combustion kinetics

To improve reactivity, the zero-carbon fuel ammonia (NH3) is often blended with large hydrocarbon fuels with higher reactivity, such as diesel or n-heptane, which will rapidly convert into several critical small molecular intermediates, dominated by light olefins including ethene (C2H4) and propene (C3H6), under the high temperature engine combustion conditions. The reaction kinetics between the light olefins (C2H4 and/or C3H6) and NH3, fundamentally determines the combustion kinetics of the large hydrocarbon-NH3 binary fuel systems, which may also shed light on carbon-nitrogen cross-reactions, forming unconventional emissions pollutants, such as cyanides. However, proper understanding on the fundamental combustion characteristics of such binary fuels, as well as the resulting unconventional emissions is lacking. The primary objective of the present study is to elucidate the influence of two typical light olefins on the fundamental pyrolysis and oxidation reaction kinetics of NH3, as well as the generation of unconventional emission species containing nitrogen element. Firstly, a chemical reaction kinetic model for the pyrolysis and oxidation of NH3-C2H4/C3H6 is established, which is validated comprehensively by the experimental data of neat NH3 and the blended fuels, NH3-C2H4/NH3-C3H6, including intermediate species distribution, ignition delay time (IDT) and laminar flame speed (LFS). Then, the impact of C2H4 and C3H6 on the fuel reactivity of NH3, and formation of critical products are investigated. Results show that, C3H6 substantially improves the reactivity of NH3 during both the pyrolysis and oxidation reactions, as well as the generation of the hydrogen cyanide (HCN); while C2H4 tends to improve the NH3 reactivity during the oxidation reactions, but it has negligible effect during the pyrolysis reactions. Kinetic analysis reveals that the main cause for the enhanced reactivity of NH3 by C3H6 is the production of the reactive methyl radicals (CH3), which helps to destroy NH3 through the hydrogen-abstraction reaction CH3 + NH3 <-> CH4 + NH2. Meanwhile, the barrierless combination reaction between radicals CH3 and NH2 eventually leads to the generation of HCN, through the reaction path: CH3 + NH2 -> CH3NH2 -> CH2NH2 -> CH2NH -> HCNH/H2CN -> HCN.