Posting this about ammonia as ship fuel as ship propulsion seems to be discussed a lot here.
Takeaways;
Takeaway after reading takeaways;
Every time I hear another “ammonia” story, I cringe. After more than a century of internal combustion engines, if it were a viable option, there would be such a system in use. To me, it falls into “Professor Crackpot’s” “All ya gotta do is…” global warming derangement syndrome. Most of these improbable scenarios seem to result from that, powered by “Single-factor-analysis”. 
Ammonia[edit]
Ammonia has been used as a lifting gas in balloons,[3] but while inexpensive, it is relatively heavy (density 0.769 g/L at STP, average molecular mass 17.03 g/mol), poisonous, an irritant, and can damage some metals and plastics.
https://www.osha.gov/laws-regs/standardinterpretations/1994-06-02 :
The National Fire Protection Association (NFPA) rates anhydrous ammonia as a 3 (on a scale of 4) - a most severe toxic health hazard and as a 1 (on a scale of 4) - a flammable gas. There is an extensive history of catastrophic releases of NH(3) in the nations workplaces resulting in severe injuries and death. Although NH(3) would not be flammable as defined in 1910.1200(c), it does have lower (16 percent by volume) and upper (25 percent by volume) flammable or explosive limits. As such, NH(3) is flammable or explosive when subjected to a flame or in a fire. In consideration of the hazards noted above, OSHA believes that the release of anhydrous ammonia from a workplace process containing a TQ or greater amount presents a potential catastrophic situation including potential exposure of employees and therefore is covered by the PSM standard.
This may be an option for uninhabited AWES including an ammonia-filled balloon over the high seas or deserts, in addition to manufacturing ammonia for other uses including ship fuel.
Dont they use ammonia in exhaust treatment systems on some ships to get rid of NOx gasses? But here they say it can cause NOx emissions…
And this seems like a step backward, not anything innovative. After all they had engines running on 100% ammonia decades ago. If there is problems igniting the ammonia in the engine you can use a iron catalyst using waste heat from the engine to crack some of the ammonia into hydrogen so it is easier to ignite and you get a higher temperature flame.
Any way it is more efficient to use a solid oxide fuel cell to get the energy out of the ammonia than putting it into a combustion engine.
One more project. Sorry its in Norwegian but you all know how to get it translated I guess (chatgpt)
One major socioeconomic factor is ammonia is the main basis for Nitrogen fertilizers. The development of the ability to synthesize ammonia is credited with “The Green Revolution”, allowing a great multiple of the number of people that can be fed worldwide. Similarly to using corn (food) to create ethanol fuel, a shift of ammonia from its main use of feeding people, to fuel for internal combustion engines would meet social resistance, as in “you are starving people by burning their food as your fuel”. Just sayin’. Meanwhile, it seems OK as an industrial refrigerant, but seems to have too many negative aspects to burn as fuel. An endless “talking point”. “All ya gotta do is…”. Are there any working examples?
You obviously have to increase the production massively if you are going to use it as fuel. But it is not like you are taking farmlands away from food production like a lot of bio fuels are doing.
And when it comes to safety the systems are already in place because we already transport and synthesize ammonia on an industrial scale for fertilizer production.
But when making “green” ammonia the efficiency is really bad. You get the already bad efficiency of making hydrogen with electrolysis, pluss the bad efficiency of the Haber-Bosch process on top of that. And if you then chose a combustion engine to get the energy out of the ammonia after all of that you end up with a truly horrible efficiency from electricity to ammonia and then to the propeller shaft of the ship.
I can’t validate this, but close to a 100 per cent efficiency electrolysis might be possible. And what is the efficiency of the Haber process, 50 per cent? That maybe sounds viable, but perhaps not for use in a combustion engine.
But then you have this:
https://cen.acs.org/environment/green-chemistry/Chemists-make-N-NH-efficient/100/i27
Ammonia is a crucial fertilizer for the world’s food production, and now scientists have figured out how to make it with nearly 100% efficiency, using nitrogen gas and electricity
The nitrogen oxides, NO, NO2, NOx, are often confused with nitrous oxide, N2O, which is a powerful oxidizer and is also known as laughing gas, an anaesthetic, and “nitrous” for drag racing engines.