1 May 2024 (Lloyd's List) - MAN Energy Solutions said last week it expected its new ammonia engine to emit “extremely low” amounts of nitrous oxide.
Just how low though is “extremely low”? This remains the elusive question.
Nitrous oxide (N2O), known as laughing gas, is a byproduct of burning ammonia — something shipping hopes to do a lot more of if it is to decarbonise. N2O is also a greenhouse gas nearly 300 times stronger than CO2.
A paper by the Maersk Mc-Kinney Møller Center for Zero-Carbon Shipping published last year said with one gram of N2O equivalent to 265g of CO2, “small quantities of N2O may invalidate the case for ammonia as a low-emission fuel”.
And there is a trade-off between N20 and Nitrogen oxide (NOx) pollution: burning ammonia at a lower temperature releases more N2O, while burning it hotter emits more NOx.
MAN head of two-stroke promotion Rasmus Bidstrup said the company would publish exact N2O figures upon full-scale testing. Its target is a 90% GHG reduction compared with fuel oil when using pilot fuel (diesel, needed because ammonia is hard to ignite).
“This means that effectively 10% of emissions will be left. 5% will be from pilot oil (if it’s fossil fuel — if it’s biofuel even less) and then some percentages from N2O,” Bidstrup said in an email.
“Our target is very much within reach, when we include all GHG emissions already now.”
The first ammonia dual-fuel engines are set for delivery in 2025.
WinGD, another engine maker, said development was still in progress. It would share detailed emissions data when its estimates were corroborated with proper testing of its first production engines.
Serkan Unalan, associate researcher at the International Council on Clean Transportation in Berlin, said for ammonia to be viable as marine fuel, N2O emissions would need to be about 270 times less than CO2 emitted by diesel engines today, if going by the Maersk Center’s calculations.
Engine makers expect about 0.06g of N2O emissions per kilowatt hour of CO2 equivalent.
In Unalan’s rough estimate, N2O would be equal to 16.2g of CO2 equivalent, compared to about 500g CO2e for VLSFO. “So if manufacturers achieve as low as they claim N2O emissions would be, than the switch would definitely be viable,” Unalan said.
But he added: “We need more data on real-world emissions and better test cycles to accurately estimate these emissions.”
Australian miner Fortescue, which is working on an ammonia-fuelled supply ship, said N2O can be significantly reduced through engine calibration.
A spokesperson said the effect of N2O was “significantly outweighed by the overall reduction in greenhouse gas emissions that is possible when using ammonia as a fuel”.
There are no readily available after treatment systems for N2O, but Fortescue and other engine makers are looking into this.
University College London associate professor Tristan Smith said N2O was in the International Maritime Organization’s GHG calculations so will be regulated and managed.
“Technologically it’s surmountable — the question is the trade-off with other parameters and ultimately what that then leaves the cost of ammonia at,” Smith said.
