Modeling of NO formation in low pressure premixed flames

In this paper a detailed sub-mechanism NOMecha2.0 for NOx chemistry at high temperature is validated on a large experimental database obtained in flames, a jet-stirred reactor (JSR) and a plug-flow reactor under sub-atmospheric and atmospheric conditions. This sub-mechanism mainly based on the one implemented in the GDFkin®3.0_NCN model for prompt-NO formation has been thoroughly updated. Prompt-NO formation results from the reaction between CH and N2 yielding NCN and H. Then, NCN radicals react with H and O-atoms yielding HCN + N and CN + NO, respectively. Care was taken in the choice of the enthalpy of formation of the N-species, especially for NCN radicals for which the enthalpy values reported in the literature differ by more than 10 kJ mol−1. Critical rate constant values for the two main reactions (CH + N2 → NCN + H and NCN + H → HCN + N) and the branching ratio of the reactions NCN + H → products are discussed. Validation focuses mainly on prompt-NO formation in low pressure flames stabilized with different fuels (CH4, C2H6, C3H8 and C2H2) using data from the literature. It draws in particularly on the extended experimental database, including species profiles (CH, NCN, CN, HCN, NCO and NO) previously obtained in Lille in CH4 and C2H2 flames, which was extended in this work. Validation also considers experiments dedicated to HCN oxidation in a JSR or in flames, and to HNCO oxidation performed in a plug-flow reactor. In order to improve the prediction of HNCO, HCN/HNC isomerization has been implemented in NOMecha2.0. Finally the NOMecha2.0 sub-mechanism was satisfactorily coupled to a detailed chemical mechanism from the literature dedicated to the oxidation of acetylene at high temperature to predict NO formation.