The use of ammonia as fuel generally has the potential to reduce emissions of CO₂ and particulate matter, including black carbon. In marine applications, substituting marine fuels with ammonia will also reduce sulfur dioxide emissions.

Combustion of pure ammonia results in the formation of water (H₂O), nitrogen (N₂) and oxides of nitrogen (NO, NO₂ and low ppm concentrations of N₂O). In fuel rich combustion, emission of unburned ammonia and hydrogen will increase, while oxides of nitrogen decrease due to competition for oxygen.

Emissions of N₂O are potentially concerning, since the GWP of N₂O is 273 [20] on both a 20-year and a 100-year timescale. Emissions are however expected to be less than 50 ppm on average and may furthermore be reduced by avoiding low load ammonia combustion, which promotes formation of N₂O.

Emissions of hydrogen is also a growing concern. Although hydrogen does not absorb light in the infrared spectrum, it does affect the composition of the troposphere by inhibiting breakdown of methane by hydroxyl radicals, while also contributing to production of ozone. The CO₂ equivalent GWP of hydrogen has recently been estimated to be 11.6 +/- 2.8 [21]. It is therefore important to be observant of hydrogen emissions when operating engines near the stoichiometric value, where hydrogen can be formed due to oxygen depletion. This is most relevant for small engines operating around the stoichiometric condition, whereas large marine engines operating with excess air will most likely not have significant formation of hydrogen.        

Upstream emissions of GHG related to the production and transportation of the ammonia must be considered as well. Ammonia is an energy carrier, and the CO₂ emitted when producing, transporting, and storing it emits more than 3 tons of CO₂ per ton of ammonia, which is comparable to the CO₂ emitted directly when burning fossil fuels [22]. Today, ammonia is produced from methane which is steam reformed and combined with atmospheric nitrogen. For ammonia to come closer to being a zero-carbon fuel, it must be produced using energy from renewable sources.

Dual fuel engines are however designed to operate with fuel oil as ignition source. The fuel oil substitution rate effectively determines the direct emissions of CO₂, as well as all other pollutants formed with fuel oil combustion.    

Emission aftertreatment options

Ammonia combustion can create high emissions of NOx, which is harmful to both humans and the environment. NOx can however be reduced to very low concentrations with SCR catalysts, which use ammonia as reductant. This allows for optimizing the engine emissions with respect to both NOx formation and ammonia slip, such that these can react together in the SCR and result in low emissions of both.  

Leading catalyst developers in the chemical industry have made catalytic materials for SCR solutions that are also capable of reducing N₂O, which is not reduced with conventional catalyst materials.

Large slips of ammonia from the fuel system or through defective fuel injectors into the exhaust is a safety hazard to ship crew and passengers. The most effective way of neutralizing this hazard is by absorbing the ammonia in spray towers. SOx scrubbers will be effective in the exhaust stream and may be used for cleaning air purged from contaminated areas as well.