I recently heard a radio programme about shipping. They claimed that presently world wide shipping was responsible for 3% of CO2 pollution. Most heavy ships use heavy oil ( almost like tar) as their fuel. Is the dregs that remain from the refining processes of other lighter fuels.
There is a concern that as world transport moves away from the traditional Petrol and diesel fuels there will be less refining and possibly the availability of heavy oil will decrease, which could affect world wide shipping.
Some shipping lines have been researching alternatives. Whist Batteries and electric motors may well suit short hop routes like ferries, other present day fuels are too refined and expensive and would put costs up astronomically.
As has been suggested else where "What about Hydrogen?" The storage problems relating to the volume of storage necessary to provide motive power for large cargo ship was estimated to use up to a third of the cargo capacity, which presents both engineering and economic challenges.
But someone has spotted that a lot of hydrogen we produce these days is converted into ammonia (NH3), which is hydrogen rich by density. Its considerably easier to contain and compress or cool down to a liquid phase(-33C) than pure hydrogen.
Engineers are working on an engine that can burn ammonia, and there is a growing expectation that it could be the beginnings of an alternative low carbon propulsion system.
Its not without its downsides though. There are several significant hazardous issues, any leaks of Ammonia in habitation areas or work spaces pose sever risks to humans or other animals.
Spills of ammonia will over time dilute in sea water, but whilst there is even only low concentrations it is fatal to marine life.
In most practical internal combustion engines (ICEs), burning Ammonia will usually form nitrogen oxides (NOx), nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O).
We also have to consider how the majority of Ammonia is presently produced through a process called the "Haber–Bosch process in which nitrogen (N2) and hydrogen (H2) react at high pressure (80 to 300 atm) and high temperature (300 to 500 °C) in the presence of a catalyst (usually magnetic iron oxide, Fe3O4) to form ammonia: N2 + 3H2→ 2NH3 (2)." (Wickipedia)
This is highly energy dependant process which usually used Hydrogen derived from Steam Reforming which has also a high energy dependant process.
This is still a research project, and from these brief checks it still seems to have a lot of awkward dilemmas to resolve before it as any chance of becoming a commercially viable scheme, But the teams are still working on it so who knows what new low cost manufacturing schemes they may come up with.