Many urgent programs are underway worldwide to find replacements for fossil fuels. Ammonia is one possibility for jet propulsion—but should it be?
Episode Summary:
The recent UN Climate Change Conference of the Parties (COP26) in Glasgow, Scotland, reaffirmed CO2 reduction targets by the major signatories to the Paris Climate Accords. Decarbonizing transportation is a cornerstone of these efforts, but aviation is difficult to wean from fossil fuels for technical reasons.
Kerosene is cheap, energy dense and easy to carry on aircraft, but research is ongoing into alternate propulsion technologies, bio-fuels and alternate fuels such as ammonia. Ammonia has sparked recent interest as a jet fuel, but Jim Anderton comments on how the disadvantages may outweigh the advantages.
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Transcript of this week’s show:
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At the recent United Nations Climate Change Conference (COP26) held in Glasgow, Scotland, the participating nations reiterated their goals for net zero carbon emissions by 2050, and added support for some industry-specific initiatives, including aviation.
For the aviation industry, the International Civil Aviation Organization (ICAO) is now the agreed managing body for international carbon reduction schemes, as well as the development of sustainable aviation fuels.
Up to now, drop-in replacements for aviation-grade kerosene such as Jet A have been bio-derived kerosene analogues, so it is not surprising that they perform as well as petroleum-derived fuels in turbine engines. But the global aviation industry consumes well over 5 million barrels per day of jet fuel, and replacing that kind of quantity is not going to happen from biological sources.
Right now, a major source of interest is in ammonia as an alternate fuel. There is plenty of research activity on ammonia as a fuel, and with good reason. Ammonia is almost 18 percent hydrogen by mass, and it can be made from renewable hydrogen with nitrogen sourced from the air. There is a huge industry already dedicated to manufacturing ammonia as a fertilizer, chemical feedstock and refrigerant, and there is an efficient transportation infrastructure to move it around.
That’s all good, but there several problems as well. Low flammability is one, which is a serious problem if you are burning it in a jet engine, and a lot of the research is focused on premixed and counterflow flames strategies and sophisticated swirl burner technologies that can support stable burning in jet engine combustors.
And there is another problem: all that useful hydrogen is bound to nitrogen, and at the kinds of combustion temperatures that make efficient jet engines, it binds with oxygen to produce nitrogen oxides, a major contributor to photochemical smog. Unlike automotive piston engines, urea after-treatment of the exhaust stream is not possible in aviation.
Handling the stuff is another factor. Ammonia is very toxic, and in all but the coldest sub-zero conditions, it exists as a gas and is a serious inhalation risk. For transportation pipelines or in railroad or truck tank cars, ammonia is refrigerated; or more commonly, liquefied by pressure. A railroad tank car is typically pressurized to 125 psi for this purpose.
Now, a Boeing 777 might carry 180,000 litres or 47 thousand gallons of fuel, distributed between a centre tank and left and right wing tanks. These, of course, are not pressurized, and if it is necessary to apply 125 psi to keep ammonia fuel liquid, that tankage will have to be either cylindrical or spherical, or use some advanced internal support structure, possibly 3D printed. Of course, the tanks must be vented, which is effectively a toxic gas leak source. Then there is question of what happens during an emergency fuel dump. Does emergency aircraft simply dump tens of thousands of gallons of liquid ammonia to flash to gas in the atmosphere?
I’m not saying that these technical challenges cannot be overcome, but the entire point of the exercise is to prevent CO2 emissions. Kerosene is an incredibly efficient and energy-dense fuel source, and as a liquid at normal atmospheric pressures and temperatures, it is very unlikely that any gaseous fuel source will make sense in large aircraft applications.
The goal is zero CO2, not necessarily zero CO2 from every source. It seems to me that a more sensible solution would be to simply remediate the CO2 emissions from aircraft by pulling it out of the atmosphere and sequestering it, something that has been demonstrated on a commercial scale for some time. There is lots of money sloshing around the world for research on advanced alternate fuel technologies, but if I were to choose a fuel to power jet engines, I’m sceptical that a fuel that is toxic, difficult to store and difficult to ignite is going to be a practical solution.
If you asked me, and no one has, we should just take the CO2 out elsewhere.
