Sustainable Aviation Fuel - The Future of Flight?
It’s no secret that aviation and air travel is a major contributor to air pollution, the increase of greenhouse gasses, and climate change. According to NOAA, aviation accounts for 2.5% of global CO₂ emissions, and has contributed around 4% to global warming to date. Of that, approximately 86% of these emissions are from commercial aviation, and the remaining are generated by military, GA, and cargo traffic. And the problem is only compounding. According to Climate Central, a for - profit climate change research organization, emissions from aviation have steadily increased, quadrupling from 1966 to 2018.
The vast, vast, majority - about 90% - of emissions from air travel are directly caused by the engines burning fuel.
In the simplest sense, airplane engines work by sucking in air, mixing it with fuel, and lighting it on fire. The resulting gas propels the airplane forwards. The process of producing jet fuel is extremely labor intensive, pollutive, and complicated.
Step 1: Crude Oil Extraction and Refining
This is the start of most fuels, whether it be for boats, planes, or cars. Jet fuel is derived from crude oil, a complex mixture of hydrocarbons. After extraction from the ground or offshore sources, crude oil is transported to refineries. At the refinery, crude oil is heated, and its components are separated based on boiling points. Jet fuel is collected from the mid-range distillation, typically between 150°C to 300°C. The fuel undergoes hydrotreating to remove sulfur and other impurities. Hydrotreating also enhances thermal stability, ensuring the fuel performs well at high temperatures and doesn’t form deposits inside engines, which can lead to all sorts of issues.
Step 2: Additives and Blending
A number of agents are added and mixed into the pure fuel, including anti-icing agents and anti-coagulents, so the mixture doesn’t gum up fuel lines.
Step 3: Shipping and Loading
The last step in jet fuel’s journey to the aircraft might take place in the back of a truck, on a barrel in a ship, or even in a pipeline directly to the airport from the fuel refinery. It’s then loaded onto planes by the thousands of gallons.
HEFA (Hydroprocessed Esters and Fatty Acids)
HEFA fuels are made from oils and fats, such as used cooking oil, animal fats, and plant oils like camelina or jatropha. During production, triglycerides are hydroprocessed to remove impurities and converted into hydrocarbons, which are refined into aviation-grade fuel which works exactly like conventional fuel.Advantages:
- Certified for commercial aviation and already in use.
- Easily integrated with existing fuel infrastructure.
Disadvantages:
- Limited availability of feedstocks due to competition with other industries.
Fischer-Tropsch Synthetic Fuel
FT fuels use municipal solid waste, agricultural residues, forestry waste, or biomass as feedstock. Waste materials are converted into syngas (a mixture of carbon monoxide and hydrogen), which is processed into hydrocarbons via the Fischer-Tropsch method to produce jet fuel.
Advantages:
- Uses non-edible biomass and reduces landfill waste.
- Scalable with appropriate infrastructure investment.
Challenges:
- Requires complex technology and is extremely expensive
Alcohol-to-Jet Fuel
ATJ fuel is derived from ethanol (from crops like corn or sugarcane) or isobutanol (from biomass or waste). During production, the alcohol undergoes dehydration and chemical conversion to form hydrocarbons suitable for aviation.
Advantages:
- Can leverage existing ethanol production facilities.
- Reduces reliance on fossil fuels.
Challenges:
- Production competes with food crops used for people
Power-to-Liquid (PtL) / E-Fuels
PtL fuels are made using carbon dioxide captured from the atmosphere or industrial sources, combined with hydrogen produced through electrolysis powered by renewable energy. These components undergo chemical reactions to produce synthetic hydrocarbons suitable for aviation.
Advantages:
- Offers a zero-emission solution when powered by renewable energy.
- Addresses both carbon capture and fuel demand.
Challenges:
- Requires significant energy input and advanced CO₂ capture technology, which is extremely expensive
Hydrogen-Derived SAF
While hydrogen could directly power future aircraft via fuel cells or combustion engines, it can also be used to create SAF. In this process, hydrogen reacts with captured CO₂ to form hydrocarbons for aviation use.
Advantages:
- Holds potential for carbon-neutral flight.
Challenges:
- Requires advances in hydrogen storage and aircraft design to become feasible.
Currently, the University of Minnesota operates the Minnesota SAF Hub, which their website describes as a "the nation’s first large-scale hub committed to scaling production of sustainable aviation fuel to replace conventional jet fuel. The Minnesota SAF Hub is a coalition anchored by Bank of America, Delta Air Lines, Ecolab, and Xcel Energy. These anchor companies are joined by other leading organizations, such as the State of Minnesota, the Metropolitan Airports Commission (which owns and operates MSP Airport), the University of Minnesota, and many others.
The MN SAF Hub had a major breakthrough in their research on September 25th, 2024, when a Delta Air Lines Airbus A321, operating a routing flight from Minneapolis to New York City’s LaGuardia Airport, took off powered solely by sustainable aviation fuel made with camelina, a common plant found throughout North America and Europe closely related to flax. I did a school project on this, and got in touch with Dr. Nick Jordan, one of the SAF program directors.
Dr. Jordan explained in great detail why the school picked camelina for their SAF project. For one, it’s abundant and requires little care and upkeep. It can survive harsh Minnesota winters and sprouts relatively early in the Spring. Camelina has no uses for humans or livestock (and is traditionally treated as a nuisance weed), so there’s no competition there. Camelina is a very oily plant, meaning the yield is high when it’s processed using the HEFA process I described above.
Minnesota is going full steam ahead with SAF. A massive SAF processing facility is being built in Rosemount (a suburb of Saint Paul), which is supposed to be able to process 30 billion gallons of SAF annually. Delta Air Lines has pledged to use SAF for 10% of its fuel at MSP Airport by 2027, and half by 2035. Minnesota is perfect for such an endeavor. For one, there’s lots of farmland to grow camellia on. The state is also home to major agriculture/innovation companies like EcoLab, Cargill, and Xcel Energy, who already have the infrastructure to take on such a project. Lt. Governor Peggy Flanagan told CBS News that “We’re going to blend it here [Minnesota], we’re going to buy it here, we’re going to make it here, and we’re going to make it even better here.”
Sustainable Aviation Fuel has a lot of promise. It’s not perfect (it still has some environmental impact, especially with current crop cultivation methods), but I think as science improves and we become more efficient at growing SAF producing crops, it will get better.
https://www.energy.gov/eere/bioenergy/sustainable-aviation-fuels