The global shipping industry drastically needs to clean up its game, but dragging these massive machines through the ocean requires extraordinary amounts of energy. Cheap, plentiful, filthy marine diesel will be extremely hard to replace. Last year we examined a promising high-density clean fuel candidate in green ammonia, but now we're learning that the industry is starting to potentially tilt toward methanol as the solution instead. So let's take a look at why, and examine methanol's potential as a clean fuel.

Why not green ammonia?

The answer appears to be fairly simple, according to a report from Recharge News. Ammonia is viewed as unsafe, it's difficult to handle and it simply doesn't carry enough energy.

In terms of safety, ammonia becomes harmful to humans at a very low concentration of 30 parts per million (ppm) over long-term exposure, or 300 ppm if exposure lasts an hour. So small leaks in any part of a ship's system could pose a persistent risk to the crew, and any larger release of fuel – say, if the ship runs aground, or hits another ship and the hull's breached, or there's a significant leak during refueling, or somebody drops a shipping container at port and damages part of the fuel system – could easily result in numerous fatalities.

Together In Safety's Future Fuels Report lists no less than six risk assessment scenarios in which ammonia fuels could pose an "intolerable risk."

Handling is difficult, says the same report, since ammonia only remains liquid under low to medium pressure, or when it's kept below -34 °C (-29 °F), and the shipping industry doesn't currently have regulations, rules or guidelines in place for its use as a fuel – mind you, it's most certainly already being carried in bulk as a cargo.

And then there's its volumetric energy density, which is key to viability in the shipping racket. At 3.5 kWh/l, ammonia is considerably more attractive than gaseous or cryogenic liquid hydrogen in this regard, but you get nearly 24 percent more energy per liter with methanol, and it's both easier and safer to deal with.