Ammonia and ammonia/hydrogen combustion: Comprehensive quantitative assessment of kinetic models and examination of critical parameters

Ammonia (NH3) stands as a pivotal player in the global shift towards carbon-free energy systems. Reliable chemical kinetic models are crucial for advancements in NH3-based combustion technologies. Despite the existence of quite a large number of individual models, their validations occur under different, and most often, under limited sets of conditions and are predominantly based on graphical comparisons with experimental data. This study performs a comprehensive quantitative assessment of 16 recent models based on an extensive experimental database for pure NH3 and NH3/H2 mixtures. The foundation for this quantitative assessment lies in a similarity score computed between smoothly interpolated experimental and corresponding prediction curves. The assessment leverages the extensive range of experimental data sets available in the literature and was categorized according to distinct target quantities, including species concentrations, ignition delay times, and laminar burning velocities. The species concentration assessment was further sub-categorized according to pyrolysis, high-, intermediate- and low-temperature oxidation, and the thermal DeNOx process. The comprehensive evaluation revealed significant differences between the models' performances, with some models exhibiting better agreement than others. None of the models delivered satisfactory agreement across all conditions, emphasizing the need for further improvements. The model performances were scrutinized under the different categories to examine critical kinetic parameters and offer insights for potential improvement. In the broader context, consolidating a comprehensive NH3/H2 model necessitates amalgamating insights from diverse kinetic modeling, experimental, and theoretical computation studies. This work serves as a foundational step in this direction, contributing to the ongoing efforts to refine the understanding of NH3 combustion. Novelty and significance statement This study comprehensively evaluates 16 recent NH3 combustion kinetic models using a holistic similarity score and extensive experimental data on NH3 and NH3/H2. Through analysis of representative experiments across different kinetic regimes, we pinpoint key kinetic parameters and their impact on prediction agreement. This work serves as a foundational step towards establishing a consolidated model that integrates insights from various kinetic modeling as well as theoretical and experimental kinetic data, enhancing our understanding of NH3 combustion through a unified approach.