Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry

Ammonia oxidation and ignition delay time measurements for pressures above 10 atm are scarce. In addition, NH3 is known to adsorb on stainless steel, so measurement results could be in question if wall passivation is not employed for apparatuses utilizing steel. To overcome these measurement difficulties and overall lack of high-pressure data for ammonia, new and methodical ignition delay time measurements have been performed behind reflected shock waves over a wide range of temperatures (1560–2455 K), pressures (around 1.4, 11, and 30 atm) and equivalence ratios (0.5, 1.0, and 2.0) for mixtures of ammonia highly diluted in Ar (98–99%). The new set of data from the present study was compared to several models from the literature. It was found that a large majority of the models do not predict the ignition delay times with accuracy, and there is a surprisingly wide variation amongst the predictions. One satisfactory model, from Dagaut et al. (2008), was selected and extended to compounds other than NH3 using H2/O2/CO, N2O, NO2, and NNH sub-mechanisms from the literature. The resulting comprehensive mechanism predicts well the ammonia ignition delay time data from the present study along with other NH3, NO2, and N2O data from the authors as well as from the literature with high accuracy. In addition to the new ammonia oxidation data and related model comparisons, the present paper documents a state-of-the-art NOx sub-mechanism that can be used for a wide range of combustion calculations when added to, for example, baseline mechanisms involving hydrogen and hydrocarbon kinetics.