Using hydrogen to spark ammonia engines into life

"Ammonia was first used as a fuel during World War II, when gasoline was hard to get hold of," explains David Emberson, associate professor at NTNU's Department of Marine Technology.

"It was actually quite a crazy idea because ammonia is a very poisonous, toxic gas."

Necessity, however, is the mother of invention.

Where fuel shortages drove the development of ammonia engines in the 1940s, the need for carbon-free propulsion is inspiring a new era today.

The spark of a good idea

Ammonia is abundant and common, and it can be produced using renewable energy. It is a zero-carbon fuel that produces no carbon dioxide emissions during its use. But ammonia has poor ignition characteristics and will not burn well on its own.

To power an engine, ammonia is usually used alongside another fuel to help ignition. These are known as dual-fuel engines.

Coal gas, biofuels and diesel are examples of ignition fuels—they are used to "spark" ammonia into action.

Diesel shortages during World War II prompted the first real-world use of ammonia as a fuel. A fleet of Belgian buses was retrofitted to run on ammonia, but the engines required a fuel mixture containing up to 40% coal gas to achieve stable combustion.

Ignition fuels reintroduce carbon emissions into the equation, which weakens the case for ammonia engines in today's market. Engineers are researching carbon-free alternatives and investigating how to use them as efficiently as possible.

Net-zero ammonia engine

Emberson has previously investigated how to minimize the amount of ignition fuel required in a dual-fuel system. Using similar techniques, he and his team have now investigated using hydrogen as the ignition fuel in an ammonia engine.

Hydrogen is also a zero-carbon fuel and can be created from the ammonia itself. Its use would require additional hydrogen storage or onboard equipment to separate the hydrogen from the ammonia as needed. Maximizing efficiency is therefore a key priority.

The research by Emberson and his colleagues suggests that as little as 7%–8% hydrogen is needed to help ignite the ammonia if administered in the right way.

The paper is published in the journal Energy & Fuels.

Powering the next generation of marine technology

The results could have a profound impact on the technology's viability in the marine sector.

"The reality is, the marine sector is where we are most likely to carry on using engines and burning fuels for the longest period of time," says Emberson. "Our focus is on how we can shift the sector to low- or zero-carbon fuels."

Shipping accounts for between 11% and 12% of all transportation emissions worldwide. To reduce emissions by 50% (from 2008 levels) by 2050, as agreed by the International Maritime Organisation, there will need to be widespread adoption of alternatives to diesel engines.

Ammonia is gaining favor in the shipping industry as a climate-friendly alternative to petroleum. But typically, ammonia engines use an ignition fuel that is not carbon-free.

Using hydrogen as the ignition fuel

"Hydrogen as a fuel is extremely difficult to compress, store and transport. It's one thing to have a 700-bar container for a hydrogen-powered car, like the Toyota Mirai, but to use it for a ship of any size is quite infeasible," explains Emberson.

Hydrogen could, however, be used as a carbon-free ignition fuel for an ammonia engine.

The researchers investigated techniques to ignite the ammonia with the least possible hydrogen. Hydrogen combustion produces no carbon but can produce nitrogen oxides, which are extremely damaging to the environment. Hydrogen must also be stored at very high pressure and is extremely explosive. There are significant storage challenges surrounding its use at sea.

Ammonia, although toxic, can be stored at a much more moderate pressure, is not explosive, produces more energy per liter and is more suited for long-distance shipping.

Using a small amount of hydrogen to ignite the ammonia represents a valid combination of the two fuels' key characteristics.

Cracking ammonia

Ammonia (NH₃) is a simple molecule composed of one nitrogen atom carrying around three hydrogen atoms. It can therefore be considered a chemical storage mechanism for hydrogen.

The hydrogen can be retrieved by "cracking" the ammonia—separating it into hydrogen and nitrogen. This process can occur onboard, removing the need to transport large quantities of the volatile fuel.

Emberson believes this technology could even be retrofitted to some engines: "Cracking the ammonia is an option, but will also require energy, so reducing that cracking demand is an important part of the development," he explains.

Emberson's research investigates direct injection. Fuel is sprayed into the combustion chamber rather than mixing with the ammonia at the intake. This differentiates the research from work at other labs and from the dual-fuel engines already in operation.

Pre-chamber flamethrowers

Duc Duy Nguyen is a Ph.D. at the Department of Marine Technology and co-authored the paper. He explains that the core of the research is to get the mechanics of the fuel injection correct while developing methods to record the process.

"If you have a really big bonfire and you need to light it, you need to create some small sparks, with really intense, high energy," says Nguyen. "It's the same within the small space of a pre-chamber. Here we supply hydrogen. As little of it as possible."

Small amounts of hydrogen are injected into a small "pre-chamber" in the engine, where the hydrogen is ignited by a spark plug. Flames then shoot out of this pre-chamber and ignite the ammonia in the rest of the engine, like miniature flamethrowers.

The team's work revolves around adjusting the precise alignment of the nozzle and the dimensions of the holes. They adjust the miniature flamethrowers for maximum efficiency and have developed sophisticated methods to document the process.

36,000 frames per second

Using advanced high-speed image capture, the researchers have refined the amount of fuel required.

"Typically, when an engine is running, we cannot see what's happening inside the engine," says Nguyen. "But using the optical techniques here, we can see inside and understand the fundamentals of ammonia combustion."

Nguyen uses two different cameras. The first is a high-speed luminosity camera, which is capable of capturing 36,000 frames per second. It observes the visible light from the chemical reaction and records the combustion's behavior in minute detail.

The second is a chemiluminescence camera, which captures behavior invisible to the naked eye.

"There are a lot of variables to be controlled," says Nguyen. "It is not just about how much ammonia to inject, and how much hydrogen to inject. It's when. We change the injection timing, and also the spark timing, based on when the piston is at the top or at the bottom."

Uncharted territory

Anders Valland is research manager at SINTEF Ocean and has been following the research.

"Emberson and crew are working on this in our Maritime Energy Systems Laboratory, and we collaborate with him along many interesting lines," says Valland.

"There is still quite a way to go since the results rely on inline ammonia cracking, which is a technology under development, and potentially will be affected by rules and regulations for use of hydrogen as fuel.

"Those R&Rs are not yet in place," explains Valland.

NTNU and SINTEF are both partners of HYDROGENi, a Norwegian research center supporting the adoption of hydrogen energy sources. Ammonia engines are seen as an important enabling technology for decarbonizing marine technology.

"Ammonia engines may not be the perfect solution, but we will likely see their use in certain areas," explains Emberson. "Making the fuel, distribution of the fuel and refueling the vessels are areas that have not been solved. Onboard safety and potential nitrogen emissions are also concerns, as is the impact on maintenance and engine life."

End of the line for ammonia buses

The original Belgian buses that ran on ammonia during World War II were decommissioned after the war's end, when diesel prices dropped and petroleum availability rose.

"The old infrastructure won out in the end," says Emberson. "Today's challenge is to ensure we make alternatives which are more attractive, and which can perform as well or better."