An experimental and kinetic modeling study of ammonia/n-heptane blends

Ammonia is carbon free and hence is a promising renewable fuel to achieve a reduction in CO₂ emissions. However, due to its relatively low reactivity, ammonia is often blended with other high reactivity fuels in practical combustors. This study aims to understand the chemical kinetics of ammonia blended with n-heptane, which is a primary reference fuel and an important component in diesel and gasoline surrogate models. A high-pressure shock tube is used to measure the ignition delay times of ammonia/n-heptane blends with different blending ratios, for stoichiometric mixtures at 10 atm pressure in the temperature range 1000–1400 K. The experimental results show that fuel reactivity decreases with increasing ammonia concentration. The oxygen concentration also shows a large effect on the reactivity of ammonia/n-heptane blends. A new detailed kinetic model is developed to simulate these new ignition delay times in addition to experimental data available in the literature. Overall, the current kinetic model can predict well the auto-ignition behavior and laminar burning velocities of ammonia/n-heptane blends over a wide range of experimental conditions. Flux and sensitivity analyzes show that the interaction reaction pathways between ammonia and n-heptane via H-atom abstraction from n-heptane by ṄH₂ radicals are important in predicting the fuel reactivity of ammonia/n-heptane blends.