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2022-11-01
Ammonia Fuels Hope for Carbon-free Aviation
Air traffic is predicted to reach over 10 billion passengers a year by 2050 and contributes a disproportionate percentage of global carbon emissions. Decarbonising the aerospace industry has thus become one of the planet’s biggest challenges.
Most modern aircraft are powered by jet-fuel which is flammable and produces carbon dioxide, sulphur oxides and soot. The emissions generated from such propulsion contribute a huge amount to the total global greenhouse gas production.
The answer to cleaner sustainable flight might, interestingly, be right under your kitchen sink.
Ammonia — the same stuff that has been keeping glass clean for eons — is the focus of a Raytheon Technologies Research Center project to investigate and test fuels that produce no carbon emissions.
Ammonia has been considered by various industries for use in power generation as it is carbon free, offers high energy density and has an established transportation network in contrast to hydrogen, which has issues with both storage and distribution.
Compared with hydrogen, ammonia’s much easier and cheaper to store and transport, and although it only carries about 20 per cent as much energy as hydrogen by weight, it carries about 70 per cent more energy than liquid hydrogen by volume.
What rules ammonia out of aviation discussions is generally the weight issue; at less than half the specific energy of jet fuel it looks less attractive than hydrogen. But on this hydrogen’s volume issues also needs to 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.
US$2.6 m. Research Grant
Under a US$2.6 million research grant from the Department of Energy Advanced Research Projects Agency-Energy, or ARPA-E, Raytheon Technologies is developing a turboelectric aviation propulsion system that would use ammonia as both a fuel and a coolant.
Ammonia is good at conducting and absorbing heat, meaning a lot of energy comes out of every drop. It’s far less flammable than jet fuel, a bonus in terms of fire safety. It doesn’t form “coke,” or the residue that hydrocarbon fuels leave behind under extreme temperatures. That also makes ammonia an attractive coolant — it can absorb heat as it flows through the engine without dirtying things up like a hydrocarbon fuel would.
The biggest draw of all is that, unlike other alternative fuels, it produces absolutely no carbon emissions.
Lance Smith, the Raytheon project’s principal investigator, explains: “People are talking a lot about biofuels. In all those cases, you’re still depositing carbon in the atmosphere. There are only two fuels that don’t do that. One is hydrogen. The other is ammonia.”
Both can be used to power a plane, but ammonia has one key advantage: it’s much easier to store as a liquid. Liquid ammonia needs to stay around -33 degrees Celsius. In the air, that’s easy: -33 degrees Celsius is just about the temperature of the air at cruising altitude. On the ground, you’d either have to refrigerate or sacrifice a bit as boil-off.
Hydrogen, by contrast, would require a lot more refrigeration, both on the ground and in flight; it needs to stay below 20 Kelvin, or approximately -253 degrees Celsius. Here’s the interesting part: when ammonia decomposes, it breaks down into molecules of nitrogen and — hey, look at that — hydrogen. So basically, using ammonia as a fuel is “kind of a workaround to the challenges of carrying hydrogen on an airplane,” Smith insists.
Things to Explain
Before any pilots start dumping glass cleaner into their fuel tanks — not recommended, for the record — there are a few things to explain.
The kind of ammonia they use in cleaning products is called aqueous ammonia, meaning it’s diluted in water. The kind of ammonia that might power a plane is anhydrous ammonia — basically, the pure stuff. And here’s how that would work.
It starts as a liquid in the fuel tank, likely in the wing of the aircraft. From there it goes into a pump for pressurisation, then to a heat exchanger, which warms it to a gaseous state. Then it’s off to something called a catalytic cracking unit, where it receives even more heat and starts to decompose. By this point, the fuel has significantly more energy than when it left the fuel tank, and now it’s ready to go into the combustor and propel the plane forward.With, of course, no carbon emissions.
Need for Procedures
If ammonia is such good fuel, why didn’t we start using it on a huge scale a long time ago?
For starters, it’s heavy, and whenever you add weight to an airframe, you increase the amount of energy it takes to propel it.
“It’s not an impossible hurdle to get over,” Smith said. “You can carry the extra weight. But we’re trying to minimise the penalty.”
It’s also toxic. That means procedures need to be in place to transport it en masse. Luckily, industries such as agriculture have been doing that for a long time.
“People know how to handle it. There are procedures in place, but those procedures haven’t been implemented in the aerospace world,” pointed out Smith.
Potential Drawbacks
Current strategies being explored have some potential drawbacks for the aerospace sector. Battery technology does not have the power density required to give a standard narrow-body jet (such as the A320 or 737) sufficient range. Hydrogen would need to be used in its deeply cryogenic liquid state, requiring new infrastructures and major changes to aircraft configurations. Synthetic fuels and biofuels require novel processes or arable land for production and leave the issue of soot emissions unsolved. Further solutions are certainly needed.
Australian company Aviation H2 hopes to clean up commercial flight by converting existing aircraft to burn green ammonia instead of standard Jet-A jet fuel. It’s planning to have a nine-seat passenger jet in the air and flying on ammonia by the middle of next year.
The second-most produced chemical in the world today, ammonia is primarily used as a fertiliser, but as the clean energy revolution kicks in, it’ll start to be used effectively as an easier way to move and store green hydrogen.
Often, clean energy potential sits an inconvenient distance from where the demand is. If that clean energy is used to electrolyse water and produce hydrogen, it can be stored and transported. But that hydrogen can also be mixed with atmospheric nitrogen to produce ammonia, which travels better than either gaseous or cryogenic liquid H2.
Feasibility Study
Following a three-month feasibility study this year, Aviation H2 has selected the use of liquid ammonia to turbofan combustion as the best route to carbon-free flight and will soon start modifying turbofan engines to test and prove the concept.
After launching a capital raise, the company says the results from their studies are very positive.
Their research shows that converting a Falcon 50 to Liquid Ammonia Turbofan Combustion is the most efficient and commercially viable avenue to building a hydrogen-powered plane.
The company’s team of engineers say they now have a clear pathway to having Australia’s first hydrogen-fuelled aircraft in the skies by the middle of 2023.
“By implementing this power path, Aviation H2 can fly aircraft with hydrogen fuel using significantly less weight than alternative power paths while generating the same amount of power,” says Aviation H2 Director, Dr Helmut Mayer.
Safety Factor
Safety will be under extreme scrutiny throughout the clean aviation revolution, and to that end, Aviation H2 and other companies will eventually have to have all their powertrains certified by relevant aviation authorities. Emissions will also be under the microscope, and here, ammonia combustion runs into a problem. As the hydrogen in ammonia is broken off and joined with atmospheric oxygen to form water, a percentage of the nitrogen also gets oxidised in the flame, causing environmentally harmful nitrous oxides.
Mayer says the company is working on solutions.
The company’s initial target is to get a small regional nine-seat jet built and flight tested. After three months of feasibility studies, it’s signed an agreement with charter operator FalconAir, giving Aviation H2 access to FalconAir’s hangars, facilities and operating licenses.
FalconAir will help acquire turbofan engines for ground-based testing, as well as the aircraft itself, most likely a Dassault Falcon 50 business jet, since it’s got three engines, but can run on two.
The initial plan is to build an aircraft capable of hour-long flights, with the same engine thrust and performance characteristics as it would get on Jet-A. It’ll be a useful range to begin with, says Mayer, better than other zero-carbon options.
The plan is to have a plane in the air, with at least one engine running on ammonia, by mid-2023, to prove the concept. The company will then go public on a stock exchange to raise the funds needed to get an ammonia powertrain patented, certified and commercialised as a product. Eventually, Aviation H2 hopes to begin retrofitting existing planes as a transitional step for carriers looking to decarbonise.
UK’s Reaction Engines, the company behind the SABRE rocket engine, is also working on a similar project.
UCF Project
The University of Central Florida (UCF) has embarked on a five-year ammonia aviation project in collaboration with Boeing, General Electric and other partners, funded by NASA to the tune of US$10 million.
UCF is developing new technology that is expected to make airplane engines emission free.
“We don’t want to create something that will be too cumbersome and expensive to implement,” says lead investigator and UCF Engineering Professor Jay Kapat. “If we want people to adopt this green tech, it needs to be scalable. To adopt hydrogen, for example, we can’t expect every airport to set up large cryogenic liquid hydrogen systems like Kennedy Space Center.”
Kapat put together a team of experts from UCF, Georgia Tech and Purdue and with industry experts from Boeing, General Electric, ANSYS, Southwest Research Institute and the Greater Orlando Aviation Authority. The team landed a US$10 million five-year NASA University Leadership Initiative grant to get the ball rolling.
Kapat says: “By having our partners in industry we know we’ll fine tune and be ready for technology transition, so we can provide a greener future for our children.”
Kapat and several of his UCF colleagues in engineering and the Florida Space Institute propose using liquid ammonia (NH3) as the fuel for aircraft which, upon combustion, will produce harmless emissions that are green while still providing enough power to keep the aircraft aloft.
Reaction Engines JV
At COP26 last year, a group led by UK’s Reaction Engines, a company pioneering space access and sustainable technologies for over 30 years, launched a design for a lightweight, modular cracking unit which uses engine exhaust heat to partially crack ammonia into a mixture similar to jet fuel — perfect for aviation applications.
In 2020, Reaction Engines completed a design concept for ammonia-fuelled jet engines, inspired in part by NASA’s ammonia-fuelled, X-15 aircraft research programme.
Reaction Engines has announced a joint venture (JV) to create compact, lightweight ammonia reactors it says can be used to decarbonise difficult sectors like shipping and off-grid energy generation — and also aviation.
It has proposed its 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.
Ammonia and recyclable aluminium-air batteries offer interesting alternatives to hydrogen in the nascent clean commercial aviation space, and with cost being such a powerful driver it would be useful 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, and green hydrogen is one of the required inputs. Clean combustion engines won’t be so clean if they’re fuelled by ammonia produced using methane gas, like the vast majority of what’s available today.
Reference Text/Photo:www.ucf.edu, https://reactionengines.co.uk . aviationh2.com.au
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