Numerical study of performance and combustion characteristics of ammonia–hydrogen and ammonia–methanol fuel blends in a marine engine

Ammonia is a promising carbon-free fuel for marine engines due to its high energy density and global availability. However, its low flame speed, high ignition temperature, and poor combustion performance, along with elevated NOx emissions, limit its effectiveness in conventional engines. These issues can be mitigated by blending ammonia with hydrogen or hydrocarbons, which improves ignition, combustion stability, and emissions. This study analyses the combustion behaviour of ammonia blended with hydrogen and methanol. Validated mechanisms based on laminar flame speeds were used. A dual-fuel, medium-speed marine engine was simulated using a sector model under compression ignition conditions. Mechanisms with 4063 reactions (220 species) for ammonia–hydrogen and 4553 reactions (221 species) for ammonia–methanol blends were employed. Results showed enhanced combustion efficiency with increased diesel (n-dodecane) in the blend. A 20% hydrogen blend reduced CO emissions by approximately 14 times compared to pure ammonia and diesel-rich blends, but increased NOx levels. The engine achieved optimal thermal efficiency of 48% using a blend of 80% ammonia and 20% methanol under lean conditions (EQR 0.43–0.44), at an initial cylinder temperature of 326 K and pressure of 1.605 bar, with a constant piston wall temperature of 450 K and n-dodecane direct injection of 0.05 g. However, increasing methanol content also raised carbon emissions.