Role of N2O and equivalence ratio on NOx formation of methane/nitrous oxide premixed flames

Nitrous oxide (N2O) is often used as an oxidizer in rocket technology with plenty of potential advantages. However, the N2O/hydrocarbon combustion results in the staggering increase of NOx emission. To scrutinize the role of N2O and equivalence ratio on NO formation characteristics of N2O/CH4 premixed flame, mixtures of N2O/CH4 and N-2/O-2/CH4 were investigated by employing one-dimensional opposed-jet model with detailed reaction mechanism. The flame structures, NO reaction pathways, and NO formation routes were discussed. In the heat release rate analysis, the heat release is dominated by N2O route, resulting in high flame temperature in N2O/CH4 premixed flame. Regarding the mechanism of NO formation, NO is mainly produced from N-2 through the thermal routes and from HNO through the HNO-intermediate routes for the N-2/O-2/CH4 flame but mainly comes from N2O through the N2O and HNO-intermediate routes in the N2O/CH4 flame. The thermal routes alter to the primary chemical mechanism of NO consumption. The NO production in fuel-lean N2O/CH4 flames is enhanced through the N2O route due to the increase of the mole fraction of O atom. The thermal route becomes forward reactions to produce NO for the extreme fuel-rich N2O/CH4 flame. Regarding the reaction pathways of the prompt route, it can be approximately divided into three categories: (1) N-2 -> CN -> NCN, NO (in terminal) -> N-2 ; (2) CN -> NCN, NO (in terminal) -> NO, CN; (3) N-2 -> CN -> NCN, NO (in terminal) -> N -> N-2 . In NO production rate analysis, the main NO formation comes from the N2O and HNO-intermediate routes, while NO consumption associates with the thermal and NNH-intermediate routes. In addition, the prompt-NO formation is dominated by species of N2O instead of CH radical in the extreme fuel-rich N2O/CH4 flames. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.