The aviation sector is on the brink of a crisis. Its future is in limbo as the world moves towards
decarbonisation. Planes are currently only responsible for 2-3% of the world’s
carbon dioxide emissions, but that’s expected to rise to 25% by 2050. [1]
Most major polluters have clear technology pathways to a cleaner future. The automotive
industry has batteries and electric motors. The shipping industry has a range of potential
alternative clean fuels to choose from. Our electrical grids are rapidly investing in solar
and wind, and future nuclear energy projects are being researched intensively. There is
still plenty of work to do, but the path ahead for these sectors has been surveyed and marked.
However, the aviation industry has no clear way forward for replacing kerosene,
and if the aviation sector can’t find answers to this problem, it’s projected that with the
continued growth of passenger numbers and the expected decarbonisation across other industries,
that it could represent as much as 25% of total world wide emissions by 2050. [1]
To understand this problem, and the potential technologies we could see in the future,
we first need to understand the current state of aviation fuel.
Today, nearly all jet engines use kerosene,
but internal combustion turbine engines are not actually that picky about the fuel they consume.
Gas powered turbines power grids all over the world [2] , and many of them are being
converted to run on bioethanol [3].Early jet engines were powered by mostly gasoline. If it
burns hot and can be pumped into a combustion chamber, chances are it can drive a turbine.
But, it’s not quite so simple for a jet engine that flies and carries humans.
There are two main types of jet fuel used for commercial aviation.
Jet A and Jet A-1. Jet A is primarily used in the United States and Jet A-1
is used in the rest of the world. [4] So is this just another case of the United
States insisting on being different because they are too stubborn to admit the rest of the world
may just have a better system? In this case, no.
The primary difference between the two is their freezing point,
with Jet A-1 having a lower freezing point of -47 degrees versus Jet A at -40.
For domestic flights within the US, Jet A’s freezing point is just fine, but
for colder climates, or colder international routes like those that fly over the arctic,
a lower freezing point is needed to prevent the fuel from turning to wax. So,
a lower freezing point is desirable, but it comes at a price.
The United States uses Jet A because it is cheaper. To understand why,
we need to understand how crude oil is refined.
Crude oil is essentially just a blend of many different hydrocarbons,
all with different carbon chain lengths. [5] We have short chain gas molecules like
methane and butane, with 1-4 carbon atoms in each chain. Then we have longer gasoline molecules,
with chain lengths between 5 and 10. While, kerosene molecules range from around 10 to 16.
We can separate each fuel type from crude oil thanks to these chain lengths impacting the
boiling point of each component, which allows us to separate them with fractional distillation.
We simply heat the crude oil up and pump it into a distillation tower.
The longer chain hydrocarbons liquify lower in the distillation tower,
thanks to their lower boiling point, and when they do so, they are tapped off.
The shorter chain molecules will remain gaseous and continue rising through the tower,
but the tower gets gradually colder as it rises. Soon Kerosene will turn to liquid and be removed,
then gasoline, and finally the lightest methane and butane gases rise right to the very top.
So how does this explain Jet A-1’s lower freezing point? Freezing points and boiling points are
generally linked, so Jet A-1 can lower its freezing point by excluding hydrocarbons
with longer chains, and therefore excludes lower boiling point molecules from the mix.
Jet A, in comparison, is less picky about the freezing point
and can take a larger cut of this distillate. Meaning, there is a broader percentage of
the crude oil that can be included in Jet A, making it cheaper than Jet A-1.
So, it makes perfect sense for a country like the United States,
that doesn’t need to worry too much about low temperatures,
to manufacture a cheaper wider cut fuel for their domestic airline industry.
So, these are our first two properties we need to consider when choosing a future aviation fuel:
freezing point and cost. The freezing point issue rules out longer chain molecules like diesel.
Diesel powered vehicles in Canada and Alaska actually have to cut their
fuel with kerosene to prevent the fuel from freezing in the winter months. [6]
This is the same reason a different jet fuel, Jet B, is used in parts of Canada and Alaska. It’s
also known as wide-cut fuel, which gets its name because it takes a much larger cut of the crude
oil distillate, with a mix of 30% kerosene and 70% gasoline, giving it an even lower freezing point
of -60. So if this wide-cut fuel can be used in engines, why isn’t it used in all engines?
Gasoline, thanks to it’s shorter carbon chain lengths,
is too volatile for general use in aviation. It’s flash point is much lower than kerosene.
Flash point is the lowest temperature vapors can form from a liquid to create an ignitable mixture
in air. So low flash points make unintended explosions and fires much more likely,
not something airports and planes are particularly fond of. The lower temperature of vaporization can
also cause problems with vapor locks in plumbing. Where gas bubbles can form and cause blockages.
This becomes an even larger issue for jet engines, as boiling points lower as pressures
decrease at altitude. So gasoline is not a desirable jet fuel for general applications.
The US Navy and US Airforce even use two different Kerosene grades for a similar
reason. The U.S. Air Force uses JP-8 [1], which is similar to Jet A-1, but with the addition of
corrosion inhibitors and anti-icing additives that are not required for the Jet A-1 standard.
While the US Navy uses JP-5. The primary difference between the NAVY and Air Force
fuels is that the navy fuel has a higher flash point. 60 degrees versus 38 degrees.
This makes it much safer to handle during refueling operations on aircraft carriers,
and makes explosions much less likely in the event of an attack. This was a constant worry during WW2
with the predominantly gasoline powered piston engines. Fuel fires were not a
rare occurrence during the war. [7] This is the third property we need to consider: flash points.
But we aren’t done yet. We haven’t even mentioned the most obvious. Energy content.
The primary function of aviation turbine fuel is to power the aircraft. This is achieved by
igniting the fuel, which releases heat, which raises the pressure,
which causes air flow. To fulfill this role most effectively we want high energy content.
We can measure the energy content of a fuel pretty easily. It’s simply the
heat released when a known quantity of the fuel is burned under specific conditions.
There are two “quantity” measurements however. Energy per unit mass,
measured in megajoules per kilogram, and energy per unit volume, measured in megajoules per liter.
In general a dense fuel with a high volumetric energy content is desired,
especially for military aircraft that always take off with their fuel tanks filled to the brim,
so volumetric energy density is a more important metric. Commercial aircraft only fill their tanks
with enough fuel to reach their destination, with a little extra in case of emergency,
but volumetric energy density is still generally a better measurement.
Let’s add this to our shopping list, and start looking at potential alternative fuels.
First, let’s look at the numbers for our 4 main identified properties with a typical
kerosene jet fuel. Cost, freezing point, flash point and volumetric energy density.
These will be our measuring sticks for our alternative fuels.
The first stop on our proverbial shopping trip is the biofuel aisle.
We have a tonne of options to choose from here.
In terms of production volumes, bioethanol and biodiesel
are currently the most available biofuels.
Ethanol is a short chain alcohol. Similar to the short chain hydrocarbons, it’s freezing
and flash point is quite low, minus 115 degrees celsius and 13 degrees respectively. [8] The
low freezing point is useful, but the low flash point is a problem. This makes ethanol volatile,
which makes it undesirable as a jet fuel. It’s volumetric energy density is about 61%
of kerosene, meaning range would be reduced if fuel tanks remained the same size. [9]
Biodiesel suffers from the opposite problem to bioethanol because it’s carbon chain lengths
are much longer. As a result it’s flash point is very high, between 98 and 150 degrees depending on
the feedstock used, and as expected comes with a very high freezing point of about 1 degrees.
This fuel would turn to wax in the fuel tanks. It’s unusable.
However, we can further process these biofuels to create fuels that are so similar to kerosene that
they can even be used in current generation planes with very little modification. [10]
Airbus began testing a fuel composed entirely of biofuel this year in an A350 powered by Rolls
Royce XWB engines. [11] Testing the plane's performance and emissions using the fuel,
which was manufactured be Neste. A company that manufactures biofuels from palm oil
and waste oils, like cooking oil. Results of this test have not yet been published,
but NASA has already published data from their own tests with a 50-50 fuel blend or traditional
jet fuel and a similar plant oil derived biofuel. [12] Their tests showed, with only a 50-50 blend,
that particulate emissions in the contrail were reduced by up to 70%. That’s important,
because those particulates have a much larger impact on earth’s atmosphere
than the carbon emissions. This is positive news, but these biofuels
are a long way from being cost effective or even environmentally friendly to manufacture.
The main challenges facing biofuels are scaling the feedstocks in an environmentally friendly
way and cost. Waste oil products as feedstocks are fantastic and every country should be working on
ways to collect waste products to feed this growing industry, but sourcing oil from the
palm oil industry is obviously problematic, as the palm oil industry is driving the destruction
of the Borneo rainforest. Sourcing enough feedstocks to completely replace fossil fuels
in the aviation industry is going to be a massive problem to solve, and right now we have no answer.
Cost is also a huge issue. Norway announced a 0.5%
biofuel mandate for the aviation sector in 2019. [13]
This is a tiny fraction of the total fuel used, but Scandavian Airlines has said that this 0.5%
mandate will add an additional 3.3 million dollars in fuel costs a year. Making it 100%, assuming
prices wouldn’t rise with the extra demand, would cost 660 million dollars extra a year. That would
Completely wipe out Scandinavian Airlines' 2019 profit of 84 million dollars. [14] So,
these biofuels currently fail the cost metric, despite being suitable alternatives to kerosene.
Even if we ignore the questionable environmental benefit of the feedstocks,
the real issue here is the difficulty in scaling up feedstocks to meet demand.
So, are there any other alternatives?
Hydrogen is also being explored as a potential future fuel.
Airbus has published several concept aircraft that could utilize hydrogen,
because, unlike biofuels, hydrogen cannot be used in existing planes. This would require a
complete overhaul of airlines plane inventories and would cost trillions over several years.
Hydrogen’s main advantage is that’s feedstock is just water, and we are surrounded by water.
However, hydrogen currently needs very pure fresh water to prevent corrosion
to the electrodes that split the water apart during electrolysis. Researchers are
working on ways to extend the life of these electrodes while preventing the salt ions,
like chloride, that are found in seawater, from breaking down the electrodes. [15]
The alternative is simply pairing the system with desalination process, but this would draw
even more electricity for what is already a very expensive process.
Hydrogen, right now, does not satisfy our cost requirement.
But let’s move forward with the expectation that we will have massive amounts of excess
renewable energy looking for a home in the future and assume these costs will come down.
Hydrogen has insanely good gravimetric energy density,
at 120 MJ/kg. [16] Completely blowing kerosene out of the water at around 44 MJ/kg. However,
hydrogen’s volumetric energy density, the quantity we actually care about, is complete dog trash.
The only way to get it to a reasonable number is by pressurizing it or making it cold,
but even then it’s volumetric energy density is terrible. At 700 bar, that’s 700 times
atmospheric pressure, hydrogen still only has a volumetric energy density
of 5.6 MJ/L, compared jet fuels 38.3 MJ/L. [17] Pressurizing a fuel tank to 700 bar comes with its
dangers, as repeated pressure cycles can lead to rapid failure due to fatigue. This is made worse
by hydrogen’s habit of attacking and embrittling materials, a phenomenon that is also accelerated
by higher pressures. [18] So, most designs for hydrogen fuel tanks instead call for cryogenic
storage. Where the hydrogen is cooled to achieve a higher volumetric energy density
with much lower pressures. [16] This also results in higher energy densities of 8 MJ/L,
but still much lower than the 38 MJ/L of traditional fuels.
This low volumetric energy density, and need to pressurize, makes hydrogen fuel tanks a
nightmare to integrate to an aircrafts airframe. Planes these days place a large amount of fuel
inside the wings. [19] This is ideal for several reasons. It takes up no useful space inside the
cabin of the plane. Aircraft wings need to be hollow to increase the strength of the wings.
The weight of the fuel being located so close to the center of lift means the plane does not need
to adjust it’s control surfaces during flight to compensate for changes in center of gravity
as the fuel gets used up, which reduces drag. Finally, when flying, the wings deflect upwards
due to the upwards lift force they create. This creates stress in the supporting structures
of the plane. So, by putting the fuel in the wings it actually helps the wings deflect less
as the weight of the fuel pushes them down, and as the fuel is used up,
the lift the wings need to generate reduces, and the upwards lift forcing the wings up reduces.
Storing the heavy fuel in the wings is an incredibly elegant solution, and it’s not possible
with hydrogen. There simply is not enough space in the narrow hollow structure of wings to fit
the equipment needed. This space is also getting even smaller as newer generation composite planes
enter the market [19], with their sleek elegant wings being much thinner than older metal versions
Because hydrogen needs to be pressurized and cooled, it requires specialized fuel tanks that
are too bulky to fit into these small spaces. The matter is only made worse because of hydrogen’s
dismal volumetric energy density. Some designs for hydrogen planes simply call for the massive fuel
tanks to be placed inside the fuselage, replacing valuable space that could be used for passengers
or cargo. This just compounds the issue of cost even more, as airlines will now be making less,
while also having to pay more for fuel. While some have proposed a more drastic change in
flight architecture, the blended wing. The blended wing offers fantastic drag characteristics and
leaves plenty of space within the wing to store the large fuel tanks. There is a lot more to be
said about this design, but we will explore this kind of plane in more detail in a future video.
Now we need to deal with the safety concerns. Hydrogen is a gas in normal conditions,
so flash point is not a relevant quantity. It’s gases are going to ignite at all ambient
temperatures if exposed to an ignition source. It is a difficult fuel to handle for this reason.
Hydrogen also has no odor and it’s flame is nearly invisible, so detection of leaks is difficult.
It’s also difficult to mix odorising agents, like the sulfur odorants we add to natural gas, because
the freezing temperatures of liquid hydrogen would simply turn them solid in the tanks and they
wouldn’t exit with the gas when there was a leak. These odorants would also contaminate any fuels
cells using hydrogen to generate electricity. [18] This is a problem because many future hydrogen
powered jet engines, including all of Airbus’ concepts, call for hybrid engines,
mixing electric motors powered by hydrogen fuel cells with combustion turbines burning
hydrogen. [20] Gas alarms will be essential early warning systems and they will need
to be located anywhere large quantities of hydrogen are stored. In the case of a leak,
modular tanks, with shut off valves between each section will be essential to minimize risk.
These storage and handling difficulties are likely the largest barrier for hydrogen moving forward,
and this is why some have proposed an extra step, that will use hydrogen to
generate a new type of hydrocarbon fuel. E-Fuels. This would be done by combining carbon dioxide,
which will be drawn directly from the atmosphere using direct air capture,
with hydrogen to produce methanol. This methanol would be liquid at ambient temperatures and
could be further processed, like our ethanol from earlier, to produce kerosene efuels. E-Fuels are
fuels that are created entirely using sustainable feedstocks and renewable electricity. This would
solve the scalability issues of biofuels, but more than likely cost a lot more due to the
sheer amount of energy needed to both create hydrogen and draw carbon dioxide from the air.
It’s hard to make predictions on the future of the air travel industry.
If I was placing bets, I think biofuel mandates, despite their questionable environmental benefit,
will continue to be introduced, and then, as excess renewable electricity floods the market,
energy intensive processes like efuels may take over. Primarily because these fuels are
compatible with current jet engines. Hydrogen has a chance of succeeding, but it will require
massive investments to completely overhaul airport and plane architecture,
which alone will cost trillions of dollars. This cost barrier is going to be something
the aviation industry is going to have to accept in the near term. It’s more than likely that air
travel will get vastly more expensive during this transitional period. That cost inflation
can be minimized by a gradual introduction of biofuels and efuels that are compatible with
current generation infrastructure. However, as we saw in Norway, even just a 0.5% biofuel
mandate increased fuel costs significantly. And this may just be a hard truth we as a society
need to accept if we truly want to become a carbon neutral civilisation and save our planet,
that the aviation industry's historic decline in ticket prices may be beginning to reverse.
There is one facet to the future of aviation fuel that I have not mentioned in this video. The
electric future. There are several small planes already in flying, powered by batteries. Their
ranges are severely limited, but a niche market could be developing for them in the near future.
This is a topic my friend, Sam from Wendover Productions, covers in detail in his video
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