Iron powder could end up as a fuel for marine applications, it is not as if the world will run out of iron.

As for ammonia..the big problem is that on combustion or oxidation in a fuel cell you get NOx, in amounts that make diesel exhaust look very clean.....!!

Yes, emissions are a problem for ammonia, but it is looking more likely to be adopted in marine applications. Large installations have the capacity to include treatment for the exhaust products.

https://www.ammoniaenergy.org/topics/maritime-engine/