How ammonia could decarbonise shipping

With ammonia being toxic and poorly suited to internal combustion engines, why is the shipping industry considering it so seriously as a fuel? One reason for strong interest in cleaner marine engines is because shipping accounted for 2.6% of global carbon dioxide emissions in 2015. The International Maritime Organization has therefore set a target to decarbonise 50% of shipping fuel by 2050. A second reason is that it’s difficult for container ships to move away from fossil fuels, because they need as much space as possible for cargo. To power cargo ships with batteries, for example, you would need around 10–100 times as much space as their fuel tanks currently take up. You might be able to use a hydrogen fuel tank that’s four times as large as with oil – if you can keep it at –253°C and at least 350 times atmospheric pressure.

To provide the same propulsion, ammonia will take up around three times as much space as oil, cooled to either –33°C or compressed to 10 times atmospheric pressure. That’s a better proposition than hydrogen or batteries. Burning ammonia will mainly produce nitrogen and water but is only truly green if the ammonia has also been produced without greenhouse gas emissions.