Experimental and modeling study on the ignition delay times of ammonia/methane mixtures at high dilution and high temperatures

Ammonia is a promising alternative clean fuel due to its carbon-free character and high hydrogen density. However, the low reactivity of ammonia and the potential high NO x emissions hinder its applications. Blending methane into ammonia can effectively improve the reactivity of pure NH 3 . In addition, lean combustion, as a high-efficiency and low-pollution combustion technology, is an effective measure to control the potential increase in NO x emissions. In the present work, the ignition delay times (IDTs) of NH 3 /CH 4 mixtures highly diluted in Ar (98%) with CH 4 mole fractions of 0%, 10%, and 50% were measured in a shock tube at an equivalence ratio of 0.5, pressures of 1.75 and 10 bar and a temperature range of 1421 K - 2149 K. A newly comprehensive kinetic model (named as HUST-NH 3 model) for the NH 3 /CH 4 mixtures oxidation was developed based on our previous work. Four kinetic models, the HUST-NH 3 model, Glarborg model [19] , Okafor model [7] , and CEU model [10] , were evaluated against the ignition delay times, laminar flame speeds, and species profiles of pure ammonia and ammonia/methane mixtures from the present work and literature. The simulation results indicated that the HUST-NH 3 model shows the best performance among the above four models. Kinetic analysis results indicated that the absence of NH 3 + M = NH 2 + H + M (R819) and N 2 H 2 + M = H + NNH + M (R902) in the CEU model and Okafor model cause the deviations between the experimental and simulation results. The overestimation of the rate constants of NH 2 + NO = NNH + OH (R838) in the Glarborg model is the main reason for the overprediction of the NH 3 laminar flame speeds.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Ammonia is a promising clean fuel due to its carbon-free nature and high hydrogen density, but its low reactivity and potential high NOx emissions limit its applications. Blending methane into ammonia enhances its reactivity, while lean combustion is an effective approach to control NOx emissions. This study developed a comprehensive kinetic model for ammonia/methane mixtures and found that it performs better than existing models.