Ammonia as a Marine Fuel towards Decarbonization: Emission Control Challenges

Decarbonization of the maritime sector to achieve ambitious IMO targets requires the combination of various technologies. Among alternative fuels, ammonia (NH 3 ), a carbon-free fuel, is a good candidate; however, its combustion produces NO x , unburnt NH 3 and N 2 O—a strong greenhouse gas (GHG). This work conducts a preliminary assessment of the emission control challenges of NH 3 application as fuel in the maritime sector. Commercial catalytic technologies are applied in simulated NH 3 engine exhaust to mitigate NH 3 and NO x while monitoring N 2 O production during the reduction processes. Small-scale experiments on a synthetic gas bench (SGB) with a selective-catalytic reduction (SCR) catalyst and an ammonia oxidation catalyst (AOC) provide reaction kinetics information, which are then integrated into physico-chemical models. The latter are used for the examination of two scenarios concerning the relative engine-out concentrations of NO x and NH 3 in the exhaust gas: (a) shortage and (b) excess of NH 3 . The simulation results indicate that NO x conversion can be optimized to meet the IMO limits with minimal NH 3 slip in both cases. Excess of NH 3 promotes N 2 O formation, particularly at higher NH 3 concentrations. Engine-out N 2 O emissions are expected to increase the total N 2 O emissions; hence, both sources need to be considered for their successful control.