(719d) Modeling Ammonia Combustion to Develop a Comprehensive Reaction Set for N/H/O Kinetics

Gas-phase combustion of ammonia (NH₃) offers valuable insights because it is de-coupled from hydrocarbon chemistry. Studies of combustion chemistry focus naturally on hydrocarbon fuels, but the role of nitrogen in combustion has been studied extensively for the formation of smog-forming NO_x compounds. With the increase in production of biogenic fuels, the role of nitrogen in combustion is commanding a closer analysis. Potential new fuels and fuel sources are being introduced whose amine and heterocylic chemistry may play a larger role in both major combustion pathways and pollutant formation.

Several reaction sets have been proposed to date for describing nitrogen combustion chemistry; however, these reaction sets tend to focus either on NO_x chemistry or amine chemistry.  In this work, we have created a comprehensive reaction set that can model both NO_x and amine chemistry as observed in several previously published experimental studies, including ammonia shock-tube pyrolysis experiments [1] and laminar, pre-mixed flames of various NH₃, N₂O, NO, H₂, and O₂ mixtures ranging from fuel-lean to fuel-rich with varying diluent (Ar) concentrations [2-5]. All experiments were selected to exclude carbon species so as to avoid complications due to isotopic resolution between carbon and nitrogen species. By biasing our study to exclude carbon chemistry, we also are able to focus specifically on nitrogen kinetics without the additional uncertainties of including carbon chemistry. Analyses of the new reaction set and sensitivity analyses reveal major reaction pathways for nitrogen-containing fuels and key reaction rate constants for kinetic modeling of nitrogen-species combustion chemistry.

Acknowledgments: This work is supported by the US Department of Defense under MURI contract W911NF-08-1-0171 and the NDSEG Fellowship Program.  Computational support was provided by the High Performance Computer Modernization Program of the Department of Defense.

References:

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