Aviation is near collapse. Decarbonisation threatens its future. By 2050, planes will account for 25% of global carbon dioxide emissions, up from 2-3% now.
Most large polluters have clean-tech paths. Batteries and electric motors power cars. Shipping offers many clean fuel options. Our electrical networks are spending heavily in solar and wind, and nuclear energy projects are being studied. Some industries have a roadmap, but there is still more to accomplish. Nevertheless, the aviation industry has no apparent method to replace kerosene, and with the continuous expansion of passenger numbers and the planned decarbonisation of other industries, it might account for 25% of global emissions by 2050. We must first study aviation fuel to grasp this dilemma and future technology.
Kerosene’s hard to replace.
Most jet engines utilize kerosene, however internal combustion turbine engines may use any fuel. Gas turbines power networks worldwide, and many are being converted to bioethanol. Early jet engines ran on gasoline. It can operate a turbine if it burns hot and is injected into a combustion chamber. A human-carrying jet engine is more complicated.
Commercial aviation uses two jet fuels. Jet A/A-1. Jet A is used in the US and Jet A-1 elsewhere. Is this another example of the US stubbornly refusing to accept that other countries may have a superior system? Here, no. Jet A-1 freezes at -47 degrees, whereas Jet A freezes at -40. Jet A’s freezing point is adequate for US domestic flights, but for colder areas or foreign routes like those over the arctic, a lower freezing point is needed to avoid waxing. Hence, a lower freezing point is desirable yet costly.
Jet A is cheaper for the US. Understanding crude oil refining helps explain why. Crude oil is a mixture of hydrocarbons with varying carbon chain lengths. Methane and butane have 1-4 carbon atoms per chain. Gasoline molecules with chain lengths of 5–10 are next. Kerosene molecules are 10–16. Due to these chain lengths affecting boiling points, fractional distillation may separate each fuel type from crude oil.
We pump heated crude oil into a distillation tower. Due to their lower boiling point, longer-chain hydrocarbons liquify lower in the distillation tower and are tapped off. The tower grows colder as it rises, but the shorter chain molecules stay gaseous. Kerosene will convert to liquid and be evacuated, followed by gasoline and the lightest methane and butane gases.
Why is Jet A-1’s freezing point lower? Jet A-1 lowers its freezing point by removing hydrocarbons with longer chains, which also removes lower boiling point molecules. Jet A is less finicky about freezing point and may accept a bigger cut of this distillate. Jet A can include more crude oil than Jet A-1, making it cheaper. Hence, a country like the US, which doesn’t need to worry about cold temperatures, may produce cheaper wider cut fuel for its domestic airplane business. The freezing point and expense of a future aircraft fuel are our first two considerations. Diesel cannot freeze because of its length.
Kerosene is added to diesel fuel in Canada and Alaska to avoid freezing. Canada and Alaska employ Jet B for the same purpose. Wide-cut fuel has a freezing point of -60 and is made from a bigger cut of crude oil distillate with 30% kerosene and 70% gasoline. Why isn’t all engine gasoline wide-cut? Gasoline is too volatile for aircraft due to its shorter carbon chain lengths. It flashes significantly lower than kerosene. Flash point is the lowest temperature liquid vapors may form an ignitable combination in air. Low flash points increase the likelihood of unexpected explosions and fires, which airports and airlines dislike.
Vapor locks in pipes can also occur at lower vaporization temperatures. where gas bubbles obstruct. When pressures drop at altitude, boiling points drop, making jet engines increasingly more vulnerable. Gasoline is not a good aircraft fuel. For identical reasons, the US Navy and Airforce employ two separate Kerosene grades. JP-8, which is comparable to Jet A-1, is used by the U.S. Air Force. US Navy employs JP-5. Navy gasoline has a higher flash point. 60 vs. 38. This makes aircraft carrier refueling safer and reduces attack-related explosions.
With gasoline-powered piston engines, WW2 was always a concern. Wartime fuel burns were common. Flashpoints are our third property. We’re not done. Not even the most obvious. Energy level. Turbine fuel powers airplanes. Igniting gasoline boosts pressure and airflow by releasing heat. High energy helps us do this. Fuel energy is easily measured. When a certain amount of fuel is burnt under specified conditions, heat is emitted. Two “quantity” measures exist.
Energy per kilogram and megajoules per liter. Military aircraft always take off with full fuel tanks, hence volumetric energy density is more significant. Commercial airplanes only carry enough fuel to reach their destination, with a little more for emergencies, but volumetric energy density is still a superior assessment. Add this to our shopping list and start researching other fuels.
Let’s examine our 4 key features with a common kerosene jet fuel. Cost, freezing, flash, and volumetric energy density. They will measure alternative fuels. We start shopping in the biofuel aisle. We have several choices. Bioethanol and biodiesel produce the most biofuels. Short-chain alcohol ethanol. It freezes at minus 115 degrees Celsius and flashes at 13 degrees, like short-chain hydrocarbons.
Low freezing and flash points are advantageous and problematic, respectively. Ethanol is flammable and unsuitable for aviation fuel. If fuel tanks were the same size, its volumetric energy density is 61% of kerosene, reducing range. Biodiesel has a different issue than bioethanol due to its longer carbon chains. Its flash point is between 98 and 150 degrees, depending on the feedstock, and its freezing point is roughly 1 degree. Fuel tanks would wax this gasoline. No use.
Yet, we can process these biofuels to make fuels so similar to kerosene that they can be utilized in modern planes without any modification. This year, Airbus tested a biofuel-only A350 with Rolls Royce XWB engines. Neste fuel testing the plane’s performance and emissions. A palm oil and cooking oil biofuels manufacturer. NASA has published data from their testing with a 50-50 fuel blend or regular fuel.
jet fuel and a plant oil-based biofuel.
Using simply a 50-50 mix, contrail particle emissions were decreased by up to 70%. That’s essential because particles affect Earth’s atmosphere more than carbon emissions. This is good news, but biofuels are expensive and ecologically unfriendly to produce. Biofuels face economic and ecologically friendly feedstock scaling issues.
Every government should be collecting waste oil products to fuel this booming business, but acquiring oil from the palm oil industry, which is destroying the Borneo rainforest, is difficult. We have no idea how to source enough feedstocks to replace fossil fuels in aircraft.
Cost is a major problem. Norway mandated 0.5% aviation biofuel in 2019. Scandavian Airlines estimates that this 0.5% requirement will cost 3.3 million dollars a year. Making it 100% would cost 660 million dollars a year, assuming prices wouldn’t grow with demand. It would erase Scandinavian Airlines’ 2019 84-million-dollar profit.
Despite being viable kerosene substitutes, these biofuels fail the cost metric. The problem is scaling up feedstocks to fulfill demand, regardless of their environmental impact. Other options? Hydrogen is another fuel under consideration. While hydrogen cannot be utilized in existing planes like biofuels, Airbus has published numerous hydrogen-powered concept aircraft. This would cost trillions and take years to rebuild airline jet inventories. Hydrogen is advantageous since its source is water, which is everywhere.
Hydrogen requires clear fresh water to prevent electrode corrosion during electrolysis. Researchers are trying to prevent saltwater salt ions like chloride from degrading these electrodes. Pairing the technique with desalination would take significantly more power for an already expensive operation.
Hydrogen currently fails our cost criterion. So let’s imagine we’ll have vast amounts of extra renewable energy searching for a home and that these costs will drop. At 120 MJ/kg, hydrogen possesses incredible gravimetric energy density. At 44 MJ/kg, completely outperforming kerosene. Hydrogen’s volumetric energy density, which we care about, is garbage. It can only be reduced by pressurizing or cooling it, although its volumetric energy density is poor.
Hydrogen has 5.6 MJ/L volumetric energy density at 700 bar, 700 times atmospheric pressure. Jet fuels have 38.3 MJ/L. Pressurizing a gasoline tank to 700 bar can cause fatigue and quick failure. Hydrogen attacks and embrittles materials, which greater pressures increase. Most hydrogen fuel tank designs use cryogenic storage.
Hydrogen is chilled to increase volumetric energy density at lower pressures. This raises energy density to 8 MJ/L, although still much behind conventional fuels’ 38 MJ/L. Hydrogen fuel tanks are difficult to incorporate into airplane airframes due to their low volumetric energy density and pressurization.
Modern airplane wings carry a lot of fuel. It’s perfect. It uses no aircraft cabin space. Hollow wings strengthen aircraft. The weight of the fuel being so near to the center of lift means the plane does not need to modify its control surfaces during flight to compensate for variations in center of gravity as the fuel is used up, reducing drag.
Lastly, flying wings bend upward due to lift force. This stresses aircraft support structures. Once the fuel is used up, the lift the wings require to create decreases, and the upward force driving the wings aloft decreases. Hydrogen cannot store heavy fuel in the wings, which is an elegant solution. Wings are too thin to fit the necessary equipment.
When new composite planes hit the market, their sleek, graceful wings are substantially thinner than earlier metal ones, making this area even smaller. Hydrogen requires pressurized and cooled fuel tanks that are too large for these small locations.
Hydrogen’s low volumetric energy density makes matters worse. Some hydrogen plane designs put enormous fuel tanks inside the fuselage, taking up space that could be utilized for passengers or freight. Airlines will make less and pay more for fuel, which increases cost. Others suggest the blended wing, a more radical flying design alteration. The blended wing has great drag and room for huge fuel tanks. This design has plenty to say.
Safety is now our priority.
Normal hydrogen is a gas, hence flash point is irrelevant. If ignited, its gases will ignite at any temperatures. It’s hard to handle. Hydrogen has no odor and a very undetectable flame, making leak detection difficult. Adding sulfur odorants to natural gas is challenging because liquid hydrogen’s freezing temperatures would solidify them in the tanks and prevent them from leaking with the gas. These odorants would pollute hydrogen fuel cells. This is a concern since many prospective hydrogen-powered jet engines, including all Airbus ideas, combine hybrid engines with electric motors fueled by hydrogen fuel cells and combustion turbines burning hydrogen.
Gas alarms will be necessary early warning devices in hydrogen storage areas. Modular tanks with shutoff valves between sections will reduce danger in a leak. Others have suggested using hydrogen to create a new hydrocarbon fuel, as storage and handling issues are likely the biggest hurdle for hydrogen.
E-Fuels. Methanol will be made by mixing hydrogen with direct air capture carbon dioxide. Methanol, like ethanol, is liquid at ambient temperatures and may be converted into kerosene efuels. E-Fuels are made using renewable power and sustainable feedstocks. Due to the energy required to make hydrogen and pull carbon dioxide from the air, this would address biofuel scaling difficulties but cost more.
Air travel projections are difficult.
If I had to wager, biofuel regulations, despite their uncertain environmental impact, would persist, and as abundant renewable power floods the market, energy-intensive processes like efuels may take control. Because contemporary jet engines can use these fuels. Hydrogen may succeed, but it will need trillions of dollars to rebuild airport and plane infrastructure. Aviation must soon face this cost hurdle.
Air travel will undoubtedly skyrocket during this transformation. Biofuels and efuels compatible with present generating infrastructure can reduce cost inflation. In Norway, a 0.5% biofuel requirement raised petrol prices considerably.
If we want to create a carbon-neutral civilization and rescue the earth, we may have to accept that the aviation industry’s record ticket price drop may be reversing.
This video doesn’t cover one aspect of aviation fuel’s future. Electric future. Many battery-powered tiny aircraft fly. Their offerings are restricted, but they may find a niche market soon.