The UK's Reaction Engines has announced a joint venture to create compact, lightweight ammonia reactors it says can be used to decarbonize difficult sectors like shipping and off-grid energy generation – and surprisingly, also aviation.
We've written before about ammonia's potential in the clean transport sector; check out our ammonia clean fuel primer piece from September. Compared against hydrogen, ammonia's much easier and cheaper to store and transport, and although it only carries about 20 percent as much energy as hydrogen by weight, it carries about 70 percent more energy than liquid H2 by volume.
The weight issue generally rules ammonia out of aviation discussions; at less than half the specific energy of jet fuel it looks less attractive than hydrogen. But hydrogen's volume issues must also be taken into account. Today's airliners are built for jet fuel so retro-fitting large-volume long-range hydrogen tanks can mean you lose seats. And anyone who's flown economy can attest, airlines really like fitting in as many seats as they can.
With that in mind, maybe ammonia becomes another option to put back on the table. And that's one of the things Reaction Engines has proposed in its new joint venture with IP Group and the UK-Government-funded Science and Technology Facilities Council (STFC).
The partnership would use heat exchanger technology Reaction developed for its Synergetic Air-Breathing Rocket Engine (SABRE), designed for hypersonic and space travel applications – and combine it with STFC's work in ammonia catalysts.
As we explained when Reaction first brought the idea to light a few months ago, here's the rough idea: the heat exchanger would capture heat from a jet engine's exhaust and use it to power a cracking reactor. The reactor would catalytically convert pure ammonia into an ammonia-hydrogen blend that'll work as an easily combustible fuel that's more or less a drop-in replacement for jet fuel.
So an airline operator can now choose between weight and volume, with the added benefit that this system wouldn't require an entirely new electric powertrain. In a shipping setting, this ammonia-hydrogen fuel would be used as a pilot fuel, and it would also be suitable for use in generators for off-grid power production.
How green will it be? Well, the combustion products will be nitrogen and water. But both ammonia and hydrogen tend to produce nitrous oxides, or NOx emissions, when burned. Carbon emissions may contribute to climate change, but NOx emissions are harmful to humans, animals and entire ecosystems, creating smoggy air and contributing to the formation of acid rain. So we certainly hope this consortium takes care of that side of things.
It's an interesting idea, and we're fascinated to learn how it pans out economically. Ammonia and recyclable aluminum-air batteries offer interesting alternatives to hydrogen in the nascent clean commercial aviation space, and with cost being such a powerful driver we'd love to know how they stack up. Not to mention, how they'll stack up with regards to efficiency, considering that all three of these fuel alternatives will be created from green energy that the world can't afford to throw away.
That's another issue with ammonia, too; green ammonia is rare and expensive at this point, and green hydrogen is one of the required inputs. Clean combustion engines won't be so clean if they're fueled by ammonia produced using methane gas, like the vast majority of what's available today.
Still, we're fascinated to see how this all pans out. Stay tuned.
Source: Reaction Engines
