A study of ammonia combustion induced by high reactivity fuel based on optical diagnostics and chemical kinetic analyses

Abstract

Ammonia is considered an optimal alternative fuel due to its non-emission of CO₂. However, the use of pure ammonia presents significant challenges. A dual fuel approach utilizing ammonia and high reactivity fuel (HRF) is expected to address these challenges. Nevertheless, the interaction mechanism between ammonia and HRF remains unclear. In the current study, various direct injection (DI) fuels such as n-heptane, n-dodecane, and n-dodecane mixed with 3%_vol 2-ethylhexyl nitrate (EHN) were selected. Optical diagnostic methods and kinetic analyses were employed to investigate the effects of DI fuel reactivity and DI energy ratio on the dual fuel method adopting HRF and ammonia. Experimental results reveal that DI fuel reactivity and DI energy ratio determine the ability to ignite ammonia and influence flame development mode, respectively. Notably, the n-dodecane/EHN blend can operate at a 4% DI energy ratio, with a flame speed of less than 5 m/s, while at a 40% DI energy ratio, the flame speed increases to 10–15 m/s. Emissions at the 40% DI energy ratio include 4373 ppm of NO, 41.4 ppm of N₂O, 71.2 ppm of NO₂, and 6391 ppm of unburned NH₃. Reducing the DI energy ratio from 40% to 20% decreases NO and NO₂ emissions by 14.6% and 7.3%, respectively, while N₂O and unburned NH₃ emissions increase by 129.7% and 105%, respectively. Chemical kinetic analyses suggest that the active atmosphere produced by HRF has a certain impact on reducing ammonia ignition delay in the initial phase of combustion. As combustion progresses, the impacts of the HRF-induced thermal atmosphere on reducing the ammonia ignition delay become more pronounced, with ambient temperature playing a critical role. Furthermore, as the combustion process develops, the influence of ambient pressure on reducing ammonia ignition delay becomes increasingly significant.