Raytheon Technologies Research Center is studying whether ammonia, a crucial ingredient in glass-cleaning products, could also be used in aviation fuel.
Ammonia is the key to a Raytheon project which is investigating and testing fuels that produce no carbon emissions, the company said in a news release. With the help of a $2.6 million research grant from the Department of Energy Advanced Research Projects Agency Energy, Raytheon is developing a turboelectric aviation pro- pulsion system that would rely on ammonia as a fuel and a coolant.
WHY AMMONIA MIGHT MAKE GOOD AIRPLANE FUEL
As a fuel, ammonia has some advantages, Raytheon said. It conducts and absorbs heat, meaning you can get a lot of energy out of every drop.
Ammonia is also less flammable than jet fuel, a bonus in terms of fire safety. And it doesn’t form “coke,” the residue that hydrocarbon fuels leave behind under extreme temperatures.
That makes ammonia an attractive coolant because it can absorb heat as it flows through the engine without making things dirty, like a hydrocarbon fuel would. The biggest advantage is that, unlike other alternative fuels, ammonia does not produce carbon emissions.
“People are talking a lot about biofuels. In all those cases, you’re still depositing carbon in the atmosphere,” said Lance Smith, the project’s principal investigator. “There are only two fuels that don’t do that. One is hydrogen; the other is ammonia.”
Both types of fuel can be used to power a plane, but ammonia has one major advantage: It’s 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 roughly the temperature of the air at cruising altitude. On the ground, you’d either have to refrigerate the fuel or sacrifice a bit as boil-off.
Hydrogen, by contrast, requires a lot more refrigeration on the ground and in-flight; it needs to stay below 20 Kelvin, or approximately -253 degrees Celsius.
And when ammonia decomposes, it breaks down into molecules of nitrogen and hydrogen. So using ammonia as a fuel is “kind of a workaround to the challenges of carrying hydrogen on an airplane,” Smith said.
HOW AMMONIA-FUELED PLANE ENGINES WOULD WORK
Before pilots start putting glass cleaner in their fuel tanks – which is not recommended – they should keep a few things in mind, Raytheon said.
First, the ammonia used in cleaning products is aqueous ammonia, which is diluted in water, Raytheon said. The kind of ammonia that might power a plane is anhydrous ammonia – basically, the pure stuff.
Here’s how that works: The ammonia starts as a liquid in the fuel tank, likely in the wing of the aircraft. From there it goes into a pump for pressurization, then to a heat exchanger, which warms it to a gaseous state.
Then the ammonia enters a catalytic cracking unit, where it receives more heat and starts decomposing.
By this point, the fuel has significantly more energy than when it left the fuel tank, and it’s ready to go into the combustor and propel the plane forward. With, of course, no carbon emissions.
CHALLENGES
But using ammonia as jet fuel presents some challenges. First, it’s heavy. And whenever pilots add weight to an airframe, they increase the amount of energy needed to propel it.
Ammonia is also toxic, which means pilots need procedures in place to transport the fuel in large quantities. Fortunately, industries such as agriculture know how to do that.
“People know how to handle it,” Smith said. “There are procedures in place, but those procedures haven’t been implemented in the aerospace world.”
Smith said those problems can be solved, and the effort required to solve them will be worth it if it means putting carbon-free planes in the air.