Chemiluminescence during the high-temperature pyrolysis and oxidation of ammonia

Abstract

Chemiluminescent emissions from NH₂*, NH*, NO*, and OH* during the pyrolysis and oxidation of ammonia (NH₃) are quantitatively characterized to gain insight into their reaction mechanisms. Time profiles of light emitted from high-temperature reactions of NH₃/Ar and NH₃/O₂/Ar mixtures have been measured behind reflected shock waves at temperatures of 2300–2600 K and pressures of 1.6–1.9 bar in a high-repetition-rate shock tube. The emission intensities have been calibrated based on the well-characterized OH* chemiluminescence in a hydrogen-oxygen system and converted to photon emission rates for quantitative comparison with kinetic simulations. A kinetic model describing the pyrolysis and oxidation of ammonia and reactions of excited species has been constructed by combining the reaction mechanisms proposed in recent modeling studies. With only modest updates of the thermodynamic functions and the rate constants for the formation and quenching of excited species, the observed chemiluminescence profiles could be reasonably reproduced, with a few exceptions. The rate of production analysis indicates that NH₂* is produced by the reaction of NH₃ with H as well as thermal excitation of NH₂, that the energy transfer reactions from ³N₂ to NH and NO are responsible for the formation of NH* and NO*, respectively, and that the formation of OH* is competitively contributed by the reactions of H with N₂O, ³N₂ with OH, and NH with NO. Remaining discrepancies between the experiment and modeling are noted, and potential directions for further model improvement are discussed.