Will Sustainable Aviation Fuels Take Flight?
How much would you pay to see the world and save the planet?
Key Takeaways:
- For those of us who fly, air travel has a massive effect on our carbon footprint. A single roundtrip flight from SFO to JFK generates over a tonne of CO2 per passenger. (The average American lifestyle produces about 15 tonnes of CO2 per year; world-wide, the average is closer to 4 tonnes annually.)
- The number of passengers flying each day is expected to continue climbing. At the current pace with today’s technology, global CO2 emissions from air transport could exceed the US’s total emissions in the next two decades.
- There isn’t a good replacement for jet fuel, especially for long-haul flights (5+ hours nonstop). Batteries are too heavy (even with big energy density improvements), and hydrogen takes up a lot of space. Even if a cost-effective solution to hydrogen-powered flight is developed, it will take years and trillions of dollars to replace the existing fleet of commercial airplanes that run on jet fuel.
- If the energy density of batteries improves significantly, batteries could theoretically power short passenger and cargo flights in the next decade. While electric Vertical Take-Off and Landing (eVTOL) craft are exciting for other purposes, they will not help me visit family in Florida.
- Though not a silver bullet, sustainable aviation fuels (SAF) are the best option to reduce the CO2 emissions from flying in the next two decades.
- Today there are economic incentives to make renewable diesel and transportation fuels (especially in California), but not sustainable aviation fuel. The amount of SAF produced today is therefore tiny. However, the tide of industry opinion appears to be turning, and many SAF projects are under construction despite the lack of firm incentives. If the EU or Biden Administration implements policies to decarbonize aviation, we could see even faster capacity buildout.
- Not all sustainable aviation fuels and technologies to make them are equal. The feedstock, yield (amount of feedstock converted to fuel), relative carbon intensity, and the track records / ability of the company to get financing are all critical.
This Thursday, I, along with 1,854,533 other people, passed through a US TSA checkpoint to fly somewhere for Memorial Day weekend. The US is back to ~75% of the number of daily travelers in 2019, and more than 5 times the number of people who traveled on this day last year during COVID.
As many of us take to the skies again, the CO2 footprint (plane-print?) of air travel is taking off again as well. Why are CO2 emissions from air travel such a big deal? As our family, friends and business connections increasingly span the globe, what options do we have to travel the world without harming it?
A Good Reason to Be Afraid of Flying — Climate Change
If you are like me and are lucky enough to fly at least once a year, it’s probably your biggest single contribution to climate change. A roundtrip flight from San Francisco to NYC produces over one tonne (t) of CO2 per passenger [1], more than people in some countries emit over an entire year:
That single roundtrip flight is about as bad for the climate as 162 hamburgers. [2] If everyone in the world took one long-haul flight a year, say SF to NYC, these flights would emit over 9 gigatonnes of CO2 per year, more than the annual CO2 emissions of the USA. [3]
COVID aside, passenger travel will continue going up as flying continues to become accessible to more people. Over 4.5 billion flights were taken in 2019, three times the number of flights taken twenty years ago. Emissions from air travel increased by 32% from 2013–2018, despite fuel efficiency increases of roughly 1% per year. While aircraft today are responsible for 2–3% of global CO2 emissions (significant), extrapolating the current growth in emissions into the future looks pretty scary.
How We Power Planes
Most passenger planes run on jet fuel, also called kerosene. Jet fuel, gasoline and diesel are all made from refining oil, and are very similar chemicals. [4] All three have very high energy densities — they let you store a lot of energy in a small fuel tank. Jet fuel freezes at a lower temperature than gasoline, making it better for cold temperatures at cruising altitudes. Jet fuel also has a higher “flash point”, which means it’s less likely to cause a fire in an accident, plus a few other helpful properties. [5]
The first big barrier to replacing jet fuel with more sustainable energy sources (hydrogen, batteries, or even natural gas) is the energy density. Powering planes with hydrogen or batteries would be much better from a CO2 emissions standpoint, but would take up a lot more weight and/or space on the airplane. Jet fuel stores 43 times more energy than a Lithium-ion battery of the same weight. Even if new types of batteries like lithium-metal or lithium-sulfur increase the energy density of batteries by 4x, jet fuel still weighs 10 times less for the same energy content.
Powering planes with hydrogen is hard for different reasons. Hydrogen contains 2.8 times more energy by weight than jet fuel. Since cutting weight makes airplanes more efficient, you’d think this would be a good thing. However, hydrogen takes up much more space and is harder to store. Even when chilled to liquid state (below -259 degrees C or -434 degrees F) it would take 5 gallons of liquid hydrogen to store the same amount of energy as one gallon of jet fuel. (Plus now a more expensive and likely heavier tank is needed).
The Elephant In the Air. In addition to an uphill battle with physics, the biggest barrier to switching fuels is that we’d have to replace the existing aircraft fleet and fueling infrastructure to switch to batteries or hydrogen. In the near term, we really need a more sustainable form of jet fuel.
So until the airlines come up with trillions of dollars to replace their current fleets (which will most likely end up parked in a desert somewhere at the end of their lives), we’ll be flying in jet fuel-powered airplanes. How do we make flying less bad than eating 162 burgers?
Offsets: The Easy Button
The “easy” solution for now is to reduce or sequester CO2 emissions somewhere else, and use accounting to say that a particular flight, passenger’s seat, etc. is effectively carbon neutral. This practice is called carbon offsetting, and this topic really deserves its own discussion! Focusing on aviation, European airlines have their own offset programs that passengers can sign up for, and a growing number of private companies offer carbon offsets. For example, voluntary programs like The Good Traveler, Terrapass and MyClimate let travelers estimate the emissions from their flights and to contribute to projects that reduce greenhouse gas (GHG) emissions elsewhere to “offset” or make up for it. Offsetting one tonne of CO2 (a roundtrip SFO → JFK) through Good Traveler site is currently $25, and goes to projects like protecting forests and reducing methane emissions from wastewater treatment plants. [6]
While this is a nice start, there are several challenges here. One is that while some of these programs are transparent about the projects supported and provide verification of the offsets, many do not. I also see this as a short term solution, because the current types of projects don’t scale as emissions from air transport climb to billions of tonnes of CO2 per year.
The logical endpoint of the offset argument is direct air capture (DAC) processes, which capture CO2 from the air for sequestration or use. This would also be a very expensive mitigation strategy: it currently costs over $600 per tonne of CO2, and few facilities have been built (one of our portfolio companies, Carbon Capture, is working on this!) Even at a future “expected cost” of $200 per tonne of CO2 captured with DAC, this would add $200+ to every roundtrip SF-NYC flight (which remember emits ~1 tonne of CO2).
Sustainable Aviation Fuels: The Pegasus-Unicorn That Will Save Us?
Many airlines including British Airways, United, Delta, Southwest Airlines, and others have promised to reduce their CO2 emissions by using sustainable aviation fuels (SAF), or jet fuel made from renewable feedstocks (waste oil, garbage, even CO2 and hydrogen). [7] Today’s SAF technologies could produce up to 80% less net CO2 than conventional jet fuel. Since SAF made through approved pathways can be blended up to 50% with conventional jet fuel, this means that the net CO2 emissions from flying could be reduced by up to 40%. Not a silver bullet, but still a huge reduction in a growing source of CO2 emissions.
The amount of sustainable aviation fuels produced today is currently tiny. Only 4 million gallons of SAF were produced world-wide in 2018, mostly at one refinery outside of LA (run by World Energy). Global consumption of jet fuel was over 95 BILLION gallons in 2019... SAF is still less than 0.1% of jet fuel produced around the world.
The reason that little sustainable jet fuel is made currently is that there hasn’t been any financial incentive to make it. Most biofuel producers has focused on making renewable diesel, because a gallon of renewable diesel can be sold at a high premium with the federal Renewable Fuel Standard (RFS) and California’s Low Carbon Fuel Standard (LCFS) credits. According to the EPA, the US consumed about 900 million gallons of renewable diesel last year, nearly all of it in California to benefit from LCFS credits.
Unlike diesel and gasoline, there are currently no requirements for airlines to pay a premium for SAF, promises aside. The public pledges by British Airways and United — purchasing one million gallons a year by 2030, and 3.4 million gallons this year with partners — are fairly unimpressive. These non-binding commitments are barely a drop in the bucket (or fuel tank) compared to the 100+ billion gallons of jet fuel we’re set to consume next year.
Despite this, the outlook for sustainable aviation fuels is promising. While only a handful of facilities are producing SAF today [8], a fleet of new production facilities are under construction worldwide. Oil majors (Total, Shell, Sasol, BP, Phillips) and biofuels technology companies (Neste, REG, LanzaTech, Gevo, Velocys, Red Rock Biofuels, Fulcrum BioEnergy) have placed big bets that sustainable aviation fuels will command a premium.
The flight path to more SAF will likely see some turbulence. Most of the facilities producing SAF and renewable diesel today start from waste or plant-based oils. (Remember that diesel and jet fuel are chemically very similar, and so are the routes to make them from renewable feedstocks.) These oil-to-jet processes are sometimes called HVO or HEFA. Feedstock cost and availability for these processes is a challenge. There isn’t that much waste cooking oil to go around. Palm oil is a sought-after feedstock, but it’s edible (competes with food) and increasing the size of palm plantations in SE Asia creates biodiversity, deforestation and other environmental concerns.
Meanwhile, facilities to make sustainable aviation fuels from less limited starting materials are under construction. Red Rock Biofuels, Fulcrum Bioenergy and Aemetis are leading the charge in converting organic stuff like garbage, corn husks, and woodchips first into simple molecules: a mixture of hydrogen (H2) and carbon monoxide (CO) called syngas. This syngas is then converted to jet fuel via the well-known Fischer-Tropsch (FT) process (which allowed Germany to make jet fuel from gasified coal during WWII). The gasification step has proved challenging, and completion of both Red Rock Biofuels and Fulcrum’s first projects have been delayed by several years. Ethanol-to-jet and alcohol-to-jet facilities from LanzaJet (spinout from LanzaTech) and Gevo are also under construction.
This is just a snapshot of some of companies and technologies in the emerging sustainable aviation fuels landscape. Pyrolysis and hydrothermal processing methods to make SAF from biomass are less technically mature, but could be game-changing in the mid- to long-term. A fleet of startups is working on these as well as smaller scale or more creative gasification technologies.
Commercial-scale SAF production facilities typically take 3–6 years and several hundred million dollars to construct, giving veteran technology companies a clear advantage in project development and financing. Still, given that the cost of a single commercial jet-fuel powered airplane is $100–300mm, perhaps we should consider this a bargain?
To summarize the SAF situation: of the ways to reduce CO2 emissions from air travel, sustainable aviation fuels are by far the one I am most bullish about in the near term.
…and for those really serious about reducing their carbon footprint in the air, there are ways to fly without an engine!
Notes
- This is just the CO2 produced from burning jet fuel. NOx and other greenhouse gas emissions associated with flying also contribute to climate change. The water contrails left behind at high altitude are also believed to have a warming effect.
Also, your mileage may vary. Depending on the number of stops, type and age of the airplane, and how full the plane is, the CO2 emissions per passenger will be different. Since a large amount of emissions are from take-off and landing, long-haul flights emit less CO2 per mile than shorter flights with connections. Here are a few examples of carbon footprint calculators from Atmosfair and the International Civil Aviation Organization, ICAO. These calculators make slightly different assumptions, and only consider CO2 emissions from burning jet fuel, not the GHG effects of water vapor (contrails) and non-CO2 emissions. - Assumptions for 162 hamburgers = 1 roundtrip flight from SFO-NYC: 60 kg CO2 equivalents are emitted per kg of beef. One burger = 1/4 lb = 0.11 kg per burger. This gets us to 6.8 kg of CO2 emitted per burger.
- The world’s population is currently 7.7 billion humans, and one round-trip long-haul flight (say San Francisco to NYC) generates over 1 tonne of CO2. At one long flight per global citizen, 7.7 billion humans x 1 tonne CO2 = 7.7 billion tonnes of CO2. This is even greater than the 5.1 billion tonnes of CO2 per year the US emits annually.
- Gasoline, jet fuel, and diesel are all made from refining oil. After oil is refined, it is separated into “fractions” depending on how big the hydrocarbon molecules are. (Here’s a helpful graphic.) Hydrocarbons are molecules that are mostly carbon atoms, “C”s and hydrogen atoms, “H”s. The smallest hydrocarbon, methane, has only one carbon atom and four hydrogen atoms (C1). The fraction that becomes gasoline is a mix of molecules with 5 to 10 carbon atoms (C5–C10). The jet fuel or kerosene fraction contains slightly larger molecules with 10 to 16 carbon atoms (C10–C16). The hydrocarbons that make up diesel are even bigger on average — 14 to 20 carbon atoms (C14-C20). The biggest hydrocarbon molecules (>C70) are tar-like and used as asphalt.
- Jet fuel / kerosene also has lower viscosity than gasoline, meaning that it flows more like water and is less likely to clog parts of the engine at low temperatures. Two versions of jet fuel are widely used, Jet A and Jet A-1. The difference between the two fuels is that Jet A-1 has a lower freezing temperature (Jet A: –40°C vs. Jet A-1: –47°C), and contains a chemical to reduce static.
I should also note that many club and small private planes with piston-driven rather than turbine engines run on “aviation gasoline” or AVGAS, which is closer to gasoline than kerosene (thanks David!). But I digress… - To me, the challenge is that many of these projects feel like either a) something we should be doing anyway (e.g. reducing methane coming off of landfills), or b) preventing future emissions to justify flying today (e.g. protecting forests… which also, shouldn’t we be doing anyway?) The Good Traveler is my current favorite despite these shortcomings because the projects are concrete and verified. If you have a different favorite, please share!
Carbon offset projects need to meet three criteria. First, additionality — the project wouldn’t have happened if it hadn’t been funded by the money from the carbon offsets. Second, has to be no leakage — if CO2 emissions are reduced in one place, this can’t result in an increase somewhere else. Last, it also has to be permanent, which can be a challenge for forestry projects — how can you guarantee trees will never be cut down? This is one reason why I’m a fan of capturing CO2 and putting it back underground… that accounting is easy! - The US Department of Energy recently completed a study suggesting that the US could generate more than 20 billion gallons of SAF a year from agricultural waste and other sources, although other estimates put the amount of available biomass in the US at 340 million tonnes.
- Burning bio-based jet fuel still produces roughly the same amount of CO2 and H2O — burning a molecule with 10 carbon atoms emits 10 molecules of CO2, and there’s no way around that. However, if the source of these carbons was CO2 in the air (extracted by plants or otherwise) versus from oil in the ground, less net CO2 is emitted. To classify as a “sustainable aviation fuel”, companies have to complete a Life Cycle Analysis to calculate how the CO2 emitted in making and transporting their product compares to using conventional jet fuel.
- World Energy, Neste, and Total are currently producing SAF; all three are expanding their production facilities and/or constructing new ones.
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