Ammonia For A Sustainable Propulsion System And Low Emissions For The Aircraft Of The Future

Reaction Engines and the Science and Technology Facilities Council of Great Britain (STFC) have completed a concept study on the utility of using ammonia as aviation fuel.

By pairing Reaction Engines’ heat exchange technology with STFC’s advanced catalysts, they hope to produce a sustainable, low-emission powertrain for the aircraft of the future.

Modern jet engines use a variety of kerosene-based fuels that have a very high energy density that can propel aircraft well beyond the speed of sound and carry passengers and cargo around the world.

Unfortunately, these fuels are also derived from fossil fuels and produce significant carbon dioxide emissions, which the aviation industry and many governments have pledged to radically reduce by 2050.

One way to achieve these reductions is to look for alternatives to conventional jet fuels. The problem is that most of those alternatives have a much lower energy density than standard aviation fuels, in addition to other drawbacks.

For example, current battery technology would require future aircraft to be very small, short-range, and with little payload capacity.

Whereas liquid hydrogen could be a viable alternative, but such a large quantity would need to be transported that aircraft would have to be completely redesigned and new infrastructure would have to be built.

The idea of ​​using ammonia as aviation fuel is not new. Although it is only a third the energy density of diesel, it is relatively easy to liquefy and store, and was already used in the famous X-15 rocket plane, propelling it into space on a series of suborbital missions in the 1950s and 1960s. Plus, it’s carbon-free.

The hard part is finding an economically viable way to use it in aviation. To solve this problem, Reaction Engines produced a new propulsion system based on heat exchange technology that it developed for its SABER hypersonic engine, which was evaluated by STFC’s Rutherford Appleton Laboratory near Didcot in Oxfordshire.

In this new system, ammonia is stored as a refrigerated and pressurized liquid in the wings of the aircraft, just as kerosene-based fuel does today. The heat collected from the engine by the heat exchanger would heat the ammonia as it is pumped and fed into a chemical reactor where a catalyst breaks down some of the ammonia into hydrogen. The ammonia and hydrogen mixture is fed into the jet engine, where it is burned like conventional fuel, although the emissions consist mainly of nitrogen and water vapor.

According to Reaction Engines, the energy density of ammonia is high enough that the aircraft does not need significant modifications and the engine can be overhauled in a relatively short time. A ground test is in the works with a first flight in a few years.

The combination of Reaction Engines heat exchange technology and STFC’s innovative catalysts will enable the development of a rule-changing class of green ammonia-based aviation propulsion systems. Our study showed that an ammonia-fueled jet engine could be retrofitted from currently available engines, and ammonia as a fuel does not require a complete rethink of civil aircraft design as we know it today. This means that a rapid transition to a sustainable low-cost aviation future is possible; Ammonia-powered aircraft could be available for short-haul routes long before 2050.

Dr. James Barth, Head of Engineering for Reaction Engines.

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