An experimental and modeling study of ammonia pyrolysis

Ammonia (NH3) is a promising carbon-free fuel and a hydrogen carrier. In recent years, there has been a large number of experimental and numerical studies to understand the chemical kinetics of NH3 in moderate to extremely complex systems. This study focused on understanding the chemical kinetics of NH3 in a simple system (pyrolysis). Shock-tube experiments were performed to monitor the NH3 time history profiles during pyrolysis, with and without the presence of H2, using a new laser absorption diagnostic near 10.4 µm. The pyrolysis experiments were conducted for mixtures of ∼ 0.5% NH3/Ar and ∼ 0.42% NH3/2% H2/Ar behind reflected shock waves, near atmospheric pressure, and over a temperature range of 2100–3000 K. Using the data from the present study as a guide, along with NH3 pyrolysis data from the literature, a detailed chemical kinetics mechanism for NH3 pyrolysis is proposed herein. This mechanism was assembled using available reaction rate constants from the literature. The mechanism showed excellent agreement with the experimental results, as well as with the literature data. Additionally, an assessment of 15 detailed NH3 chemical kinetics mechanisms on their capabilities of predicting the new pyrolysis experiments was performed. The assessment showed that these literature mechanisms yield significantly different predictions, with only one model producing acceptable results for the majority of the NH3 pyrolysis experiments. The effect of pyrolysis reactions on the prediction of oxidation data was investigated by updating the pyrolysis sub-mechanism of selected literature models with the reactions of the present pyrolysis model. The prediction of the updated literature models significantly improved for ignition delay time and flame speed literature data, indicating the importance of pyrolysis reactions for high-temperature oxidation chemistry.