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 (NH3), a carbon-free fuel, is a good candidate; however, its combustion produces NOx, unburnt NH3 and N2O-a strong greenhouse gas (GHG). This work conducts a preliminary assessment of the emission control challenges of NH3 application as fuel in the maritime sector. Commercial catalytic technologies are applied in simulated NH3 engine exhaust to mitigate NH3 and NOx while monitoring N2O 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 NOx and NH3 in the exhaust gas: (a) shortage and (b) excess of NH3. The simulation results indicate that NOx conversion can be optimized to meet the IMO limits with minimal NH3 slip in both cases. Excess of NH3 promotes N2O formation, particularly at higher NH3 concentrations. Engine-out N2O emissions are expected to increase the total N2O emissions; hence, both sources need to be considered for their successful control.