Comparison of detailed reaction mechanisms for homogeneous ammonia combustion

Ammonia is a potential fuel for the storage of thermal energy. Experimental data were collected for homogeneous ammonia combustion: ignition delay times measured in shock tubes (247 data points in 28 datasets from four publications) and species concentration measurements from flow reactors (194/22/4). The measurements cover wide ranges of temperature T, pressure p, equivalence ratio φ and dilution. The experimental data were encoded in ReSpecTh Kinetics Data Format version 2.2 XML files. The standard deviations of the experimental datasets used were determined based on the experimental errors reported in the publications and also on error estimations obtained using program MinimalSplineFit. Simulations were carried out with eight recently published mechanisms at the conditions of these experiments using the Optima++ framework code, and the FlameMaster and OpenSmoke++ solver packages. The performance of the mechanisms was compared using a sum-of-square error function to quantify the agreement between the simulations and the experimental data. Ignition delay times were well reproduced by five mechanisms, the best ones were Glarborg-2018 and Shrestha-2018. None of the mechanisms were able to reproduce well the profiles of NO, N2O and NH3 concentrations measured in flow reactors

Testing of NH3/H2 and NH3/syngas combustion mechanisms using a large amount of experimental data

A possible solution to improve the combustion properties of ammonia is to blend it with other fuels. Two of the most usually used co-fuels are hydrogen and syngas (H2/CO). To investigate the chemistry of the co-combustion with these fuels, a large amount of indirect experimental data for the combustion of neat NH3, and NH3/H2 and NH3/syngas fuel mixtures were collected from the literature including ignition delay times measured in shock tubes, concentration measurements in jet stirred and flow reactors, and laminar burning velocity measurements. Altogether, 4898 data points (in 472 data series) were recorded which cover wide ranges of equivalence ratio, temperature, and pressure. These experimental data are available in data files in the ReSpecTh site (http://respecth.hu). The performances of 18 recently published detailed reaction mechanisms were quantitatively assessed using the collected experiments. There are significant differences between the performances of the models, and the performance of a mechanism may also vary significantly with the different types of experiments. The best-performing mechanisms are POLIMI-2020, Han-2020, and KAUST-2021 for NH3/H2 fuel mixtures, and Shrestha-2021, Mei-2021, and Mei-2020 for NH3/syngas systems. The results indicate that further mechanism development is needed to reproduce the measurements more accurately. Local sensitivity analysis was carried out on the kinetic and thermodynamic parameters of the best-performing mechanisms. Even though the investigated models have different parameter sets, the most important reactions and thermodynamic properties are similar. The most important reactions are not the same for the different types of experiments but most of them include the NH3, NH2, and/or NNH species. Among the thermodynamic parameters, model outputs are most sensitive to the data of NH3 and NH2