Ammonia in Dual-Fueled Internal Combustion Engines: Impact on NOx, N2O, and Soot Formation

Abstract

The combustion of ammonia in internal combustion engines (ICE) releases nitrogen-related exhaust emissions. Numerous studies have shown that the increased formation of nitrous oxide (N2O) may offset ammonia’s carbon-free advantages, leading to a higher greenhouse gas potential than fossil fuels. Moreover, nitrogen contained in ammonia further promotes an increase in NOx formation. This study aims to expand the understanding of emission formation in dual-fuel ICEs when using ammonia as a fuel. By constant-pressure reactor simulations coupled with detailed reaction kinetics, the concept of equivalence ratio–temperature diagrams was employed to characterize conditions featuring high NOx, N2O, and soot concentrations. The diagrams were obtained for pure ammonia, pure n-heptane, and three blends with ammonia energy shares (AES) of 20, 50, and 80%. Our findings strengthen the perception that high concentrations of N2O in ICEs are related to incomplete combustion. A higher AES leads to increased N2O concentration during the ignition, going from single-digit ppm levels for pure n-heptane to conditions featuring levels 3 orders of magnitude higher for pure ammonia. In fully burned mixtures, N2O emissions feature a low fuel dependency and single-digit concentration levels only at low equivalence ratios and high temperatures. Further, varying contributions from the fuel NO, prompt NO, and thermal De-NOx mechanisms were observed with fuel composition; however, the thermal NO contribution led to a fuel-independent behavior for NOx emissions at temperatures above 2600 K. The soot concentration decreases as the carbon content in the fuel decreases. In our configuration, the lowest equivalence ratio at which the 0.1% soot yield limit was observed was 2.20 for pure n-heptane, 2.65 for AES of 20%, 5.05 for 50% AES, and not attained for higher AES. Ultimately, it was found that in fuel-rich regimes and at fully burned conditions, low concentrations of NOx and N2O emissions are observed.