Ammonia was first tried as a fuel during World War II, when gasoline became hard to obtain. David Emberson, an associate professor in the Department of Marine Technology at the Norwegian University of Science and Technology (NTNU), recalls this. "It was a fairly crazy idea, because ammonia is a very toxic gas," he says. However, fuel shortages pushed engineers to experiment: at the time, a fleet of Belgian buses was converted to run on ammonia, but the engines required a mixture with up to 40% coke oven gas added for stable combustion.
Today interest in ammonia has returned - now for climate reasons. Its use produces no carbon dioxide, and it can be produced using renewable energy. The problem is that ammonia by itself ignites poorly. In engines it is usually ignited with a second fuel - so-called dual-fuel systems. But such pilot fuel (diesel, biofuel) again adds carbon emissions.
Emberson's team investigated how to ignite ammonia with hydrogen - also a carbon-free fuel. Hydrogen can be obtained from the ammonia itself on board, but this requires additional devices and energy. The scientists looked for a way to get by with a minimal amount of hydrogen. The results, published in the journal Energy & Fuels, showed that if hydrogen is supplied in a special way, only 7-8% of the total fuel volume is enough.
Technically it works like this: a small portion of hydrogen is injected into a special pre-combustion chamber, where it is ignited by a spark plug. The resulting flame, like a miniature blowtorch, shoots into the main chamber and ignites the ammonia. The researchers adjusted the shape and arrangement of the nozzles and also used high-speed filming to look inside the running engine and understand exactly how combustion occurs. According to one of the authors of the work, doctoral student Duc Duy Nguyen, it is important to calculate precisely not only how much fuel to supply, but also at what moment to do so.
According to the International Maritime Organization, shipping accounts for 11-12% of all transport emissions in the world, and by 2050 they are planned to be halved compared with 2008. Ammonia is considered a promising fuel for long voyages: it is easier to store than hydrogen, which requires very high pressure and is explosive. However, ammonia has its own limitations: it is toxic, and the combustion of hydrogen can produce dangerous nitrogen oxides. In addition, the technology for producing hydrogen from ammonia on board is still only being developed, and rules for using hydrogen as a fuel have not yet been adopted.
"Ammonia engines may not be the ideal solution, but in some areas they will likely find use," Emberson concludes. He also notes that issues of production, delivery and refueling with this fuel, as well as safety and the effect on engine service life, have not yet been resolved. So this is a promising development, not a ready-made technology that will replace diesel at sea tomorrow.