Third-body effects of N2 and Ar in ammonia pyrolysis: Shock tube experiments and modeling insights

Abstract

A physically-grounded description of the ammonia (NH₃) pyrolysis is essential to set up predictive kinetic models of NH₃ combustion. However, most reference experiments employed for model development and validation are conducted in argon-diluted conditions. When these experiments are applied directly to conventional combustion environments, they may introduce systematic biases due to bath-gas dependencies. To address these effects, ammonia pyrolysis was investigated in a shock tube at near-atmospheric pressure, using Ar, N₂ and their equimolar mixture as bath gases. At the same time, a recently developed kinetic framework, including an accurate description of collider-specific effects in pressure-dependent reactions, was leveraged to analyze the experimental measurements. It was found that the progressive replacement of Ar with N₂ significantly accelerated the ammonia decomposition rate by up to a factor of $\sim 2$. The kinetic model showed a satisfactory agreement with experimental data, in terms of both NH₃ time histories and half-life times under all the investigated conditions. Moreover, kinetic analysis showed that such acceleration in the decomposition rate was almost exclusively due to the change in the NH₃ decomposition reaction ${\mathrm{NH}}_3\mathrm{(+M)}\to {\mathrm{NH}}_2+H\mathrm{(+M)}$, because of the enhanced N₂ collider efficiency relative to Ar. These results provide direct experimental evidence of the recent theoretical findings on third-body effects in NH₃ chemistry, and highlight the importance of accounting for bath-gas effects when developing kinetic models.

Novelty and significance statement: By designing ad-hoc shock-tube experiments, this work advances the fundamental understanding of the collider-dependent decomposition rate of ammonia. To the authors’ knowledge, this is the first study providing time-resolved measurements of NH₃ mole fractions during pyrolysis including pure N₂ as a bath gas, assessing its role as a third-body collider. Most reference data for tuning and validating NH₃ decomposition rates have indeed been obtained in Ar-diluted systems, and only recent studies have considered N₂-containing mixtures at high temperatures. These results provide a novel and unique benchmark for assessing kinetic models, and consolidate recent theoretical findings on third-body collision efficiencies, bridging fundamental theory with experiments. Moreover, this work establishes an experimental protocol that can be systematically extended to investigate a wider range of colliders, supporting future improvement of NH₃ pyrolysis and combustion mechanisms.