I was interested to read that a South Korean shipyard has received approval in principle for a large container ship powered by ammonia. It's the most recent use of ammonia as a fuel, but far from the only one. A Chinese shipyard has also received initial approval for such a design. However, no operational, production-line ammonia engine has yet been built; the concept has only been tested on a smaller scale.
I'd never thought of ammonia as an engine fuel, so I did some research. Ammonia was first used as a fuel in the 1870's and 1880's. It seems ammonia doesn't combust evenly or predictably on its own, but if another fuel (e.g. hydrogen, natural gas, etc.) is added to it, the combination burns very well. If hydrogen is used, there are no carbon emissions whatsoever - a completely "green" fuel. This is already attracting attention for use in jet engines for aircraft.
Another advantage is that ammonia is already produced in vast quantities for fertilizer and other applications. It's a very well-known industrial process, that could easily be expanded to meet increased demand. There's currently plenty of liquid natural gas to blend with it, and hydrogen production is ramping up for fuel cell use, which would be a preferable blend from a zero-carbon-emissions point of view. Finally, ammonia can also be used in fuel cells, making it a useful product from more than one aspect of power generation.
Ammonia has the disadvantage that it's toxic; but so is gasoline or diesel, in a different way. Hydrogen has its own hazards (as the Hindenburg disaster proved). However, if the hazards of other fuels can be overcome, I daresay ammonia can be safely managed as well.
I must admit, I'm intrigued. My late father (who worked in the oil industry) said for decades that hydrogen would be "the fuel of the future", because there are literally unlimited supplies available in seawater. Simply "cracking" it into its component elements of hydrogen and oxygen would produce fuel. However, I don't think he ever thought of hydrogen as a catalyst to burn another fuel like ammonia.
Will we ever see our own automobiles powered by ammonia? Not in the immediate future, I'm sure - all the research I've seen so far has been with large engines, for ship or jet aircraft. However, existing vehicles have been tested using ammonia fuel, with success. If distribution networks can be set up, so that it's as freely available as gasoline or diesel, I don't see why ammonia can't be more widely used. After all, if you can adapt an existing engine to use ammonia, why not go all the way and build ammonia automobile engines?
Of course, it might make the drug problem worse. Ammonia is used in the production of methamphetamines, as farmers already know to their cost - their agricultural ammonia, for use as fertilizer, is often targeted by drug-makers. If it were available at every service station, I'm sure it would be misused even more. I guess we'll have to see whether there are ways around that.