Ignition, combustion modes and NO/N2O emissions in ammonia/n-heptane combustion under RCCI engine conditions

Abstract

Ammonia has been considered a promising carbon-free fuel for marine engines. However, its low flame speed and high nitrogen oxides (NO${}_x$) and nitrous oxide (N${}_2$O) emissions present significant challenges. To address these issues, novel combustion concepts, such as ammonia/diesel dual-fuel Reactivity-Controlled Compression Ignition (RCCI) engines, have been proposed. This paper presents a detailed investigation of ammonia/n-heptane combustion under RCCI engine conditions using direct numerical simulation (DNS) to gain insights into ignition, combustion modes, and emission formation mechanisms. A temporally evolving jet configuration is considered in the DNS, with the computational domain comprising two regions: a fuel-lean premixed ammonia/air mixture and a fuel-rich n-heptane jet/ammonia/air mixing region. The pressure and temperature in these regions are representative of typical marine engine operating conditions. The DNS results reveal multiple reaction layers, including the fuel-lean premixed flame (LPF), fuel-rich premixed flame (RPF), diffusion flame (DF), and rich ammonia oxidation layer (RAOL). The LPF propagates into the ambient ammonia/air mixture, significantly influencing combustion efficiency and NO formation, while the RPF propagates into the fuel-rich n-heptane/ammonia/air mixture due to low-temperature ignition. The DF oxidizes combustion intermediates and NO, while the RAOL facilitates ammonia oxidation, forming intermediate species such as hydrogen (H${}_2$), amino radicals (NH${}_2$), and nitrene radicals (NH), which eventually participate in the reactions in the DF and RPF. The back-supported propagation of the LPF is influenced by n-heptane mixing, heat, and radical transfer from the DF, and jet-induced vortices and turbulence. Increasing n-heptane jet speed enhances this effect, improving ammonia combustion efficiency. NO primarily forms in the LPF and is consumed in the DF, while N${}_2$O is generated in the LPF (continuously) and RPF (during the ignition stage), while being consumed in the RAOL. Higher n-heptane jet velocity accelerates NO consumption but increases N${}_2$O formation due to enhanced mixing and ammonia entrainment. Understanding these mechanisms provides valuable insights into optimizing RCCI combustion for reduced emissions and improved efficiency in ammonia-fueled marine engines.

Novelty and significance statement

• This research investigates ammonia-fueled RCCI engines using high-fidelity direct numerical simulations, examining the effects of turbulent jets and ambient ammonia concentration. The simulations resolve all fine structures and provide detailed insights, while the findings are applicable to practical marine engine scenarios.

• Multiple reaction layers in ammonia RCCI combustion are identified, including back-supported lean premixed flame propagation, cool flame, diffusion flame, and rich ammonia oxidation layer.

• The study elucidates the processes behind $\mathrm{NO}$ and N₂O emissions in ammonia RCCI engines, including their generation, consumption, and potential control.