Investigation on the potential of using carbon-free ammonia in large two-stroke marine engines by dual-fuel combustion strategy

Ammonia (NH3) as a carbon-free fuel is gaining much attention in the shipping industry for its enormous potential of global decarbonization, which is expected to achieve large-scale applications in marine engines by dual-fuel (DF) combustion technology. As such, this study was conducted to explore the emission reduction potential of NH3/diesel DF combustion strategy via low-pressure gas injection in large low-speed two-stroke marine engines using computational fluid dynamics (CFD) modeling coupled with chemical kinetics. An established in-house CFD platform, KIVA-CHEMKIN-CDAC, was employed to perform engine simulations, while an DF combustion mechanism was constructed in this work to mimic the oxidation behaviors of NH3 and diesel as well as the emissions formation. It was found that NH3 admission exhibits a significant inhibiting effect on the autoignition of pilot fuel. Increasing ammonia substitution ratio (ASR) will prolong the ignition delay, resulting in high intensity of premixed combustion. Generally, NH3/diesel DF combustion mode shows two-stage heat release shape. The first stage is characterized by the premixed combustion of diesel–NH3–air mixtures, whereas the dominant combustion regime of the second stage highly depends on the NH3 concentration in the premixed charge. In the very lean NH3-air mixtures, the second stage corresponds to the mixing-controlled diffusion combustion phase; but for the richer NH3-air mixtures, it could be controlled by the turbulent flame propagation. NOx emission decreases when the ASR does not exceed 40%, otherwise increases significantly. The former is probably due to the Thermal DeNOx process dominated by NH2 + NO = N2 + H2O, while the latter is due to the fuel-bound nitrogen. As expected, CO2 emission is reduced monotonically for the same total fuel energy with the increase of ASR, confirming that the utilization of zero-carbon fuel is the most direct means to reduce CO2 emissions. Moreover, there is a trade-off relationship between NOx and N2O emissions. This is because N2O formation usually occurs at lower temperatures (i.e., T < 1400 K). Furthermore, advancing the pilot-fuel injection timing could reduce the unburned NH3 and N2O emissions. Therefore, optimization of injection timing can achieve lower emissions while maintain higher efficiency.