Researchers at the National Research Council of Canada (NRC) have developed strategies for using ammonia to reduce greenhouse gas emissions from heavy-duty diesel engines, achieving overall reductions of up to 50% to 60% in lab tests.
As Canada moves towards decarbonizing its transport sector, electrification and hydrogen-fuelled engines are showing great promise as zero-emission technologies. But for certain transport applications, the drawbacks of these systems—the relatively low energy density of batteries, for example, or the complex transport and storage infrastructure needed to manage hydrogen—are a significant barrier to replacing tried-and-true diesel engines.
"There's no way you could recharge a battery on a ship on its way from Canada to Europe," explains NRC Research Officer Hongsheng Guo. He gives the examples of marine transport, heavy-duty mining vehicles and long-haul locomotives as cases where it is especially difficult to find a substitute for the efficiency and versatility of diesel power.
With research that is at the forefront of its field, Guo, NRC Research Officer Shouvik Dev and their team in the NRC's Advanced Clean Energy program have broken new ground in exploring ammonia's potential to fill that gap as a carbon-free fuel for use in diesel engines in these hard-to-decarbonize sectors.
Ammonia: readily available but with a few drawbacks of its own
Widely used in agriculture, ammonia is among the top 10 most commonly produced industrial chemicals in the world. A gas at ambient temperature and pressure, ammonia can be compressed and liquefied at much lower pressures than hydrogen, making it significantly easier to store and transport. It also has an energy density almost double that of hydrogen.
Past interest in ammonia as a fuel has ranged from exotic applications to more down-to-earth research into ammonia's potential as a carbon-free alternative fuel for internal combustion engines. For example, NASA and the US Air Force broke records in the 1960s with the experimental X 15 aircraft powered by an ammonia-fuelled rocket engine.
But until now, engineers have had limited success in achieving diesel-like efficiency from ammonia while managing the safety and pollution concerns that come with using the substance as a fuel. Although it is carbon-free, ammonia is corrosive and toxic at low concentrations. When ammonia is burned, by-products include nitrogen oxides (NOx), a cause of acid rain, and nitrous oxide (N2O), a greenhouse gas 300 times as potent as carbon dioxide (CO2). What's more, ammonia's relatively low flame propagation speed and narrow flammability limits mean a significant amount of the substance can pass through an engine without burning at all—a phenomenon known as ammonia slip.
Finding a way forward for ammonia–diesel dual-fuel technology
In 2 papers published in Fuel in April and May 2022 that now rank among the most cited literature in the field, the NRC team investigated the performance of a heavy-duty diesel engine running on an ammonia–diesel fuel mix.
The team used a computational fluid dynamic model validated by data from experiments conducted at the NRC's clean-combustion engine facilities in Ottawa to examine the effects of varying the ratio of ammonia to diesel in the fuel mix and adjusting the timing of the diesel injection. The researchers also looked at the impact of splitting the diesel injection into 2 distinct pulses during the engine's power cycle. By optimizing these parameters, they were able to achieve close-to-diesel efficiency while delivering lower overall greenhouse gas emissions.
"We identified the fundamental mechanisms and the causes of potential issues with ammonia combustion," Guo says. "Then we developed strategies to address those issues, such as improving the mixing process during combustion, enhancing flame propagation to reduce ammonia slip and reducing NOx and N2O emissions.
"We found we were able to replace about 70% of the diesel with ammonia, with the potential to reduce overall greenhouse gas emissions by 50% to 60%."
Reducing emissions while maintaining efficiency
In partnership with other government agencies, industry and academia, the NRC team continues its work on developing ammonia–diesel dual fuel technology. They aim to gain further ground in addressing the challenges of ammonia slip, and NOx and N2O emissions, all while maintaining the efficiency for which diesel engines are renowned.
"When it comes to managing these emissions challenges, we have to remember that the key reason the diesel engine is so popular is because of its inherent efficiency," says Dev. "Whatever we do to address these challenges, we still have to maintain that efficiency, if not improve it."
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