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

Abstract

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 NOx emissions hinder its applications. Blending methane into ammonia can effectively improve the reactivity of pure NH3. In addition, lean combustion, as a high-efficiency and low-pollution combustion technology, is an effective measure to control the potential increase in NOx emissions. In the present work, the ignition delay times (IDTs) of NH3/CH4 mixtures highly diluted in Ar (98%) with CH4 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-NH3 model) for the NH3/CH4 mixtures oxidation was developed based on our previous work. Four kinetic models, the HUST-NH3 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-NH3 model shows the best performance among the above four models. Kinetic analysis results indicated that the absence of NH3 + M = NH2 + H + M (R819) and N2H2 + 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 NH2 + NO = NNH + OH (R838) in the Glarborg model is the main reason for the overprediction of the NH3 laminar flame speeds.