Thermodynamic and Chemical Kinetic Parameters in Ammonia Oxidation: A Comparison of Recent Studies and Parameter Recommendations

Abstract

Ammonia is a promising energy storage vector for renewable hydrogen and could become an essential part of the future energy mix. To efficiently utilize ammonia as a fuel, overcome its relatively low reactivity and high tailpipe emissions, and optimize the ratio of different additives, it is crucial to develop accurate chemical kinetic predictive abilities for this system. In this study, we review and compare thermo-kinetic parameters from 19 chemical kinetic reaction mechanisms published in the five-year period of 2018–2023. This comparison reveals a concerning inconsistency in the thermodynamic parameters used for many species in the H/N and H/N/O subsets. One species was even double-counted (having two distinct labels with two similar sets of reactions) in six of these recent mechanisms. Twelve reactions were identified for which the reported rate coefficient deviates by 3 orders of magnitude or more at 1000 K among the reviewed sources. Not all parameters in the literature mechanisms are trackable and properly cited. Ammonia modeling suffers from the “many-model” problem, with significant inconsistency in the parameter values used by different studies. The present work highlights some of these concerns and attempts to advance the current state of NH3 oxidation modeling by suggesting a coherent set of parameters justified by other work in the literature or by quantum chemistry calculations reported here. It is recommended that future studies of ammonia modeling will properly justify the thermokinetic parameters they use─especially deviations from established values─make all parameter sources trackable, and provide a glossary of chemical structures to all species in the suggested reaction mechanism.