The aviation industry is responsible for 2 percent of global carbon dioxide emissions, twelve percent of emissions from transport, and is one of the most carbon-intensive activities any individual can participate in. For all of these reasons, the race to make air travel carbon-neutral has become a central focus of governments and researchers who are determined to reduce the impact of greenhouse gasses on our environment. From electric planes to biofuels and hydrogen energy, there are plenty of new and exciting technologies in this space, but achieving parity with jet fuels in either cost or energy density is a formidable challenge. 

Take electric planes, for example, the size and weight of the batteries required to get a plane off the ground mean that the most promising projects vary from carrying 2 to 9 passengers. 

Then there’s biofuel, currently the most popular and commonly used of the ‘carbon neutral’ aviation fuels. Already, Bergen, Brisbane, Los Angeles, Oslo, and Stockholm airports provide a 50/50 mix of biofuel and jet fuel. But the biofuel sector is now coming under attack for being a false solution to the aviation industry’s carbon problem. Biofuel is considered carbon-neutral because the carbon that it releases when burned was absorbed by the organic material when it was growing. So the lifespan of biofuel is technically carbon neutral. But when you take into account the land used in producing the crops frequently turned into biofuel, or the transport of that fuel and the release of this carbon high in the atmosphere rather than into the soil as it would naturally be released, the biofuel argument begins to crumble. 

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Finally, there is hydrogen, the most abundant element in the universe, and one of the most popular clean energy sectors on the planet in 2020. The superior energy density of hydrogen fuel cells, when compared to batteries, means that this solution would extend the range of a flight – which is one of the major pitfalls of an electric plane. The downside to hydrogen, however, is that it has a poor specific power output and so planes would have to be entirely redesigned to carry the amount of hydrogen required for flight.

While all of these carbon-neutral aviation solutions battle to find a solution for their various shortcomings, a new competitor has entered the field. Ammonia jet fuel could well be the answer that the aviation industry has been looking for. While the use of ammonia as fuel isn’t new (it was famously used to fly the X-15 aircraft in the 1950s and 60s) it was never economically viable.

A recent concept study carried out by Reaction Engines and Britain’s Science and Technology Facilities Council (STFC) using a new propulsion system may well have solved that problem. This system stores chilled, pressurized liquid ammonia in the plane and then uses the heat of the engine to warm the ammonia as it is fed into a chemical reactor where a catalyst breaks some of it down into hydrogen. Then the ammonia-hydrogen mixture is fed into the engine and burns like conventional fuel and produces hydrogen, nitrogen, and water vapor. The most exciting part of this development is that the energy density of the fuel and the storage conditions would mean that currently available engines and aircraft could be adapted to run on this fuel. While the first flight with ammonia is still a few years away, it certainly is a solution to be taken seriously.

By Josh Owens for

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