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 “Why Electric Planes are Inevitably Coming”.
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