An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature

Abstract

Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21-30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0-30 vol.%) at elevated pressure (1-10 bar) and temperature (298-473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N2H2 formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH2+NH=N2H2+H, NH2+NH2=N2H2+H2, N2H2+H=NNH+H2 and N2H2+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.

Comparative Effect of Ammonia Addition on the Laminar Burning Velocities of Methane, n-Heptane, and Iso-octane

“…It should be noted that, although several chemical kinetic mechanisms for NH 3 and NH 3 /hydrocarbon combustion are available in the literature (e.g. refs , , , and − ), most of them were built to predict the combustion behavior of hydrocarbons over the C 1 –C 3 range; therefore, they are not applicable for the present comparison, where n -heptane and iso-octane have been used. On the other hand, some comprehensive chemical mechanisms for the combustion of n -heptane, iso-octane, and mixtures of both exist in the literature, but they have not been expanded to include NH 3 modeling capability.…”

“…To advance this understanding, a considerable amount of experimental studies dedicated to the evaluation of S L for NH 3 -based mixtures in a wide range of the temperature, pressure, and stoichiometry has already been reported by several researchers using a variety of experimental methods. The mixtures examined were NH 3 /air, ,− NH 3 /O 2 , − NH 3 /O 2 + N 2 , , NH 3 + H 2 /air, ,,,,− NH 3 + H 2 + N 2 /air, NH 3 + CO/air, , NH 3 + H 2 + CO/air, , and NH 3 + H 2 + CH 4 /N 2 O + N 2 + air …”

A Review on Combustion Characteristics of Ammonia as a Carbon-Free Fuel

“…They also found that mixing ammonia with the addition of H 2 is the most effective way to improve the flame propagation by comparing with the burning velocity of NH 3 /CH 4 mixture, NH 3 /H 2 mixture, and NH 3 /CO mixture. The flame speeds of NH 3 /H 2 flames (the hydrogen concentration ranged from 0.0 to 0.3) and pure ammonia at oxygen rich conditions (the oxygen concentration ranged from 0.21 to 0.30) were measured by Shrestha et al at P 1-10 atm and T 298-473 K (Shrestha et al, 2021). They also developed a mechanism for the oxidation of ammonia/hydrogen mixtures and ammonia.…”

DNS Study of Spherically Expanding Premixed Turbulent Ammonia-Hydrogen Flame Kernels, Effect of Equivalence Ratio and Hydrogen Content

“…The UC San Diego [17] mechanism was used for this study due to its sufficient accuracy combined with its short number of reactions and species. Although several other mechanisms for ammonia combustion exist [27][28][29][30][31][32][33], these have large numbers of reactions and species, restricting their use in DNS studies and hence were not considered for use in this study.…”