Revealing the mechanisms of ammonia dual-fuel combustion for decarbonization in marine transportation

Ammonia dual-fuel combustion offers a promising pathway for decarbonization in marine transportation, but nitrogen oxide (NOx) emissions present a significant challenge. This study investigates ammonia dual-fuel combustion under engine and constant volume chamber conditions using high-fidelity large eddy simulations (LES). The implemented combustion model has been extensively validated under both fundamental and engine combustion conditions. A novel three-dimensional species budget analysis tool, developed and implemented for the first time, quantifies the roles of convection, diffusion, and chemical sources in species transport. Unlike previous studies relying on zero-dimensional simulations and chemical pathway analysis, this approach captures the coupled effects of turbulent mixing, and local transient flows, offering a more physically representative understanding. A multidimensional reaction pathway analysis reveals the influence of ammonia fraction on heat release and NOx formation. Distinct flame structures were observed: the constant volume chamber shows a standard jet flame, while the engine presents a distributed flame due to complex piston-flow interactions. Nitric oxide (NO) forms mainly in high-temperature regions (T > 2000 K), nitrous oxide (N2O) accumulates in low-temperature zones (T < 1200 K). Chemical reactions are found to dominate NO and N2O formation, though convection contributes over 30 % to N2O, with diffusion remaining minimal. Furthermore, the formation mechanisms of key combustion species were mapped using probability density functions, elucidating the role of thermal mixing during combustion and providing actionable insights for optimizing ammonia dual-fuel combustion strategies. These findings contribute to overcoming the NOx challenge and advancing sustainable, low-carbon technologies for marine transportation.