The Uncertain Future of Jet Fuel

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.

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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.

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pexels photo 9119884

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.

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[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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