Aviation biofuel. An aviation biofuel (also known as bio-jet fuel, sustainable aviation fuel (SAF) or bio-aviation fuel (BAF)) is a biofuel used to power aircraft. The International Air Transport Association (IATA) considers it a key element in reducing the environmental impact of aviation. Aviation biofuel is used to decarbonize medium and long-haul air travel. These types of travel generate the most emissions, and could extend the life of older aircraft types by lowering their carbon footprint. Synthetic paraffinic kerosene (SPK) refers to any non-petroleum-based fuel designed to replace kerosene jet fuel, which is often, but not always, made from biomass.

Biofuels are biomass-derived fuels from plants, animals, or waste; depending on which type of biomass is used, they could lower  emissions by 20–98% compared to conventional jet fuel.
The first test flight using blended biofuel was in 2008, and in 2011, blended fuels with 50% biofuels were allowed on commercial flights. In 2023 SAF production was 600 million liters, representing 0.2% of global jet fuel use.

Aviation biofuel can be produced from plant or animal sources such as Jatropha, algae, tallows, waste oils, palm oil, Babassu, and Camelina (bio-SPK); from solid biomass using pyrolysis processed with a Fischer–Tropsch process (FT-SPK); with an alcohol-to-jet (ATJ) process from waste fermentation; or from synthetic biology through a solar reactor. Small piston engines can be modified to burn ethanol.

Sustainable biofuels are an alternative to electrofuels. Sustainable aviation fuel is certified as being sustainable by a third-party organisation.

SAF technology faces significant challenges due to feedstock constraints. The oils and fats known as hydrotreated esters and fatty acids (Hefa), crucial for SAF production, are in limited supply as demand increases. Although advanced e-fuels technology, which combines waste  with clean hydrogen, presents a promising solution, it is still under development and comes with high costs. To overcome these issues, SAF developers are exploring more readily available feedstocks such as woody biomass and agricultural and municipal waste, aiming to produce lower-carbon jet fuel more sustainably and efficiently.

Environmental impact

Plants absorb carbon dioxide as they grow, therefore plant-based biofuels emit only the same amount of greenhouse gases as they had previously absorbed. Biofuel production, processing, and transport, however, emit greenhouse gases, reducing the emissions savings. Biofuels with the most emission savings are those derived from photosynthetic algae (98% savings) although the technology is not developed, and those from non-food crops and forest residues (91–95% savings).

Jatropha oil, a non-food oil used as a biofuel, lowers  emissions by 50–80% compared to Jet-A1, a kerosene-based fuel. Jatropha, used for biodiesel, can thrive on marginal land where most plants produce low yields. A life cycle assessment on jatropha estimated that biofuels could reduce greenhouse gas emissions by up to 85% if former agro-pastoral land is used, or increase emissions by up to 60% if natural woodland is converted.

Palm oil cultivation is constrained by scarce land resources and its expansion to forestland causes biodiversity loss, along with direct and indirect emissions due to land-use change. Neste Corporation's renewable products include a refining residue of food-grade palm oil, the oily waste skimmed from the palm oil mill's wastewater. Other Neste sources are used cooking oil from deep fryers and animal fats. Neste's sustainable aviation fuel is used by Lufthansa; Air France and KLM announced 2030 SAF targets in 2022 including multi-year purchase contracts totaling over 2.4 million tonnes of SAF from Neste, TotalEnergies, and DG Fuels.

Aviation fuel from wet waste-derived feedstock ("VFA-SAF") provides an additional environmental benefit. Wet waste consists of waste from landfills, sludge from wastewater treatment plants, agricultural waste, greases, and fats. Wet waste can be converted to volatile fatty acids (VFA's), which then can be catalytically upgraded to SAF. Wet waste is a low-cost and plentiful feedstock, with the potential to replace 20% of US fossil jet fuel. This lessens the need to grow crops specifically for fuel, which in itself is energy intensive and increases  emissions throughout its life cycle. Wet waste feedstocks for SAF divert waste from landfills. Diversion has the potential to eliminate 17% of US methane emissions across all sectors. VFA-SAF's carbon footprint is 165% lower than fossil aviation fuel. This technology is in its infancy; although start-ups are working to make this a viable solution. Alder Renewables, BioVeritas, and ChainCraft are a few organizations committed to this.

NASA has determined that 50% aviation biofuel mixture can cut particulate emissions caused by air traffic by 50–70%. Biofuels do not contain sulfur compounds and thus do not emit sulfur dioxide.

History

The first flight using blended biofuel took place in 2008. Virgin Atlantic used it fly a commercial airliner, using feedstocks such as algae. Airlines representing more than 15% of the industry formed the Sustainable Aviation Fuel Users Group, with support from NGOs such as Natural Resources Defense Council and The Roundtable For Sustainable Biofuels by 2008. They pledged to develop sustainable biofuels for aviation. That year, Boeing was co-chair of the Algal Biomass Organization, joined by air carriers and biofuel technology developer UOP LLC (Honeywell).

In 2009, the IATA committed to achieving carbon-neutral growth by 2020, and to halve carbon emissions by 2050.

In 2010, Boeing announced a target 1% of global aviation fuels by 2015.

By June 2011, the revised Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons (ASTM D7566) allowed commercial airlines to blend up to 50% biofuels with conventional jet fuel. The safety and performance of jet fuel used in passenger flights is certified by ASTM International. Biofuels were approved for commercial use after a multi-year technical review from aircraft makers, engine manufacturers and oil companies. Thereafter some airlines experimented with biofuels on commercial flights. As of July 2020, seven annexes to D7566 were published, including various biofuel types:
 Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK, 2009)
 Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK, 2011)
 usHydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP, 2014)
 Fischer-Tropsch Synthetic Paraffinic Kerosene with Aromatics (FT-SPK/A, 2015)
 Alcohol to Jet Synthetic Paraffinic Kerosene (ATJ-SPK, 2016)
 Catalytic Hydrothermolysis Synthesized Kerosene (CH-SK, or CHJ; 2020).

In December 2011, the FAA awarded US$7.7 million to eight companies to develop drop-in sustainable fuels, especially from alcohols, sugars, biomass, and organic matter such as pyrolysis oils, within its  and  programs.

Biofuel provider Solena filed for bankruptcy in 2015.

By 2015, cultivation of fatty acid methyl esters and alkenones from the algae, Isochrysis, was under research.

By 2016, Thomas Brueck of Munich TU was forecasting that algaculture could provide 3–5% of jet fuel needs by 2050.

In fall 2016, the International Civil Aviation Organization announced plans for multiple measures including the development and deployment of sustainable aviation fuels.

Dozens of companies received hundreds of millions in venture capital from 2005 to 2012 to extract fuel oil from algae, some promising competitively-priced fuel by 2012 and production of  by 2012-2014. By 2017 most companies had disappeared or changed their business plans to focus on other markets.

In 2019, 0.1% of fuel was SAF: The International Air Transport Association (IATA) supported the adoption of Sustainable Aviation fuel, aiming in 2019 for 2% share by 2025: .

By that year, Virgin Australia had fueled more than 700 flights and flown more than one million kilometers, domestic and international, using Gevo's alcohol-to-jet fuel. Virgin Atlantic was working to regularly use fuel derived from the waste gases of steel mills, with LanzaTech. British Airways wanted to convert household waste into jet fuel with Velocys. United Airlines committed to  of sustainable aviation fuel for 10 years from Fulcrum BioEnergy (of its  fuel consumption in 2018), after a $30 million investment in 2015.

From 2020, Qantas planned to use a 50/50 blend of SG Preston's biofuel on its Los Angeles-Australia flights. SG Preston also planned to provide fuel to JetBlue Airways over 10 years. At its sites in Singapore, Rotterdam and Porvoo, Finland's Neste expected to improve its renewable fuel production capacity from  a year by 2020, and to increase its Singapore capacity by  to reach  in 2022 by investing €1.4 billion ($1.6 billion).

By 2020, International Airlines Group had invested $400 million to convert waste into sustainable aviation fuel with Velocys.

In early 2021, Boeing's CEO Dave Calhoun said drop-in sustainable aviation fuels are "the only answer between now and 2050" to reduce carbon emissions. In May 2021, the International Air Transport Association (IATA) set a goal for the aviation industry to achieve net-zero carbon emissions by 2050 with SAF as the key component.

The 2022 Inflation Reduction Act introduced the Fueling Aviation's Sustainable Transition (FAST) Grant Program. The program provides $244.5 million in grants for SAF-related "production, transportation, blending, and storage." In November, 2022, sustainable aviation fuels were a topic at COP26.

As of 2023, 90% of biofuel was made from oilseed and sugarcane which are grown for this purpose only.

Production
Jet fuel is a mixture of various hydrocarbons. The mixture is restricted by product requirements, for example, freezing point and smoke point. Jet fuels are sometimes classified as kerosene or naphtha-type. Kerosene-type fuels include Jet A, Jet A-1, JP-5 and JP-8. Naphtha-type jet fuels, sometimes referred to as "wide-cut" jet fuel, include Jet B and JP-4.

"Drop-in" biofuels are biofuels that are interchangeable with conventional fuels. Deriving "drop-in" jet fuel from bio-based sources is ASTM approved via two routes. ASTM has found it safe to blend in 50% SPK into regular jet fuels. Tests have been done with blending synthetic paraffinic kerosene (SPK) in considerably higher concentrations.

 HEFA-SPK 
 Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosine (HEFA-SPK) is a specific type of hydrotreated vegetable oil fuel.  this was the only mature technology (but by 2024 FT-SPK was commercialized as well). HEFA-SPK was approved by Altair Engineering for use in 2011. HEFA-SPK is produced by the deoxygenation and hydroprocessing of the feedstock fatty acids of algae, jatropha, and camelina.
 Bio-SPK 
 This fuel uses oil extracted from plant or animal sources such as jatropha, algae, tallows, waste oils, babassu, and Camelina to produce synthetic paraffinic kerosene (bio-SPK) by cracking and hydroprocessing. Using algae to make jet fuel remains an emerging technology. Companies working on algae jet fuel include Solazyme, Honeywell UOP, Solena, Sapphire Energy, Imperium Renewables, and Aquaflow Bionomic Corporation. Universities working on algae jet fuel are Arizona State University and Cranfield University. Major investors for algae-based SPK research are Boeing, Honeywell/UOP, Air New Zealand, Continental Airlines, Japan Airlines, and General Electric.
 FT-SPK 
 Processing solid biomass using pyrolysis can produce oil or gasification to produce a syngas that is processed into FT SPK (Fischer–Tropsch Synthetic Paraffinic Kerosene).
 ATJ-SPK 
 The alcohol-to-jet (ATJ) pathway takes alcohols such as ethanol or butanol and de-oxygenates and processes them into jet fuels. Companies such as LanzaTech have created ATJ-SPK from  in flue gases. The ethanol is produced from CO in the flue gases using microbes such as Clostridium autoethanogenum. In 2016 LanzaTech demonstrated its technology at Pilot scale in NZ – using Industrial waste gases from the steel industry as a feedstock. Gevo developed technology to retrofit existing ethanol plants to produce isobutanol. Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is a proven pathway to deliver bio-based, low-carbon fuel.

Future production routes 
Systems that use synthetic biology to create hydro-carbons are under development:
 The SUN-to-LIQUID project is examining Fischer-Tropsch hydro-carbon fuels (solar kerosine) through the use of a solar reactor. 
 Alder Fuels is proposing to convert lignocellulosic biomass (a common type of waste from forestry and agriculture) into a hydrocarbon-rich "greencrude" via pyrolysis (see: pyrolysis oil). Greencrude can be turned into fuel in refineries like crude oil.
 Universal Fuel Technologies is marketing its Flexiforming technology that can use different feedstocks and even the byproducts from existing renewable fuel manufacturing processes to produce SAF.
 Arcadia eFuels is constructing a plant that will use renewable electricity to perform electrolysis of water to produce green hydrogen coupled with CO2 capture to produce syngas and use Power-to-X gas to liquid processes to produce SAF. The plant at the port of Vordingborg, Denmark is expected to begin production in 2028.

Piston engines 
Small piston engines can be modified to burn ethanol. Swift Fuel, a biofuel alternative to avgas, was approved as a test fuel by ASTM International in December 2009.

Technical challenges 
Nitrile-based rubber materials expand in the presence of aromatic compounds found in conventional petroleum fuel. Pure biofuels that aren't mixed with petroleum and don't contain paraffin-based additives may cause rubber seals and hoses to shrink. Synthetic rubber substitutes that are not adversely affected by biofuels, such as Viton, for seals and hoses are available.

The United States Air Force found harmful bacteria and fungi in their biofueled aircraft, and use pasteurization to disinfect them.

Economics
In 2019 the International Energy Agency forecast SAF production should grow from 18 to 75 billion litres between 2025 and 2040, representing a 5% to 19% share of aviation fuel. By 2019, fossil jet fuel production cost was $0.3-0.6 per L given a $50–100 crude oil barrel, while aviation biofuel production cost was $0.7-1.6, needing a $110–260 crude oil barrel to break-even.

 aviation biofuel was more expensive than fossil jet kerosene, considering aviation taxation and subsidies at that time.

As of a 2021 analysis, VFA-SAF break-even cost was . This number was generated considering credits and incentives at the time, such as California's LCFS (Low Carbon Fuel Standard) credits and the US Environmental Protection Agency (EPA) Renewable Fuel Standard incentives.

Sustainable aviation fuels

Sustainable biofuels do not use food crops, prime agricultural land or fresh water. Sustainable aviation fuel (SAF) is certified by a third-party such as the Roundtable For Sustainable Biofuels.

As of 2022, some 450,000 flights had used sustainable fuels as part of the fuel mix, although such fuels were ~3x more expensive than the traditional fossil jet fuel or kerosene.

Certification
A SAF sustainability certification ensures that the product satisfies criteria focused on environmental, social, and economic "triple-bottom-line" considerations. Under many emission regulation schemes, such as the European Union Emissions Trading Scheme (EUTS), a certified SAF product may be exempted from carbon compliance liability costs. This marginally improves SAF's economic competitiveness versus fossil-based fuel.

The first reputable body to launch a sustainable biofuel certification system was the European-based Roundtable on Sustainable Biomaterials (RSB) NGO. Leading airlines and other signatories to the Sustainable Aviation Fuel Users Group (SAFUG) pledged to support RSB as their preferred certification provider.

Some SAF pathways procured RIN pathways under the United States's renewable fuel standard which can serve as an implicit certification if the RIN is a Q-RIN.

Criteria

 EU RED II Recast (2018) 
 Greenhouse gas emissions from sustainable fuels must be lower than those from the fuels they replace: at least 50% for production built before 5 October 2015, 60% after that date and 65% after 2021. Raw materials cannot be sourced from land with high biodiversity or high carbon stocks (i.e. primary and protected forests, biodiversity-rich grasslands, wetlands and peatlands). Other sustainability issues are set out in the Governance Regulation and may be covered voluntarily.

 ICAO 'CORSIA'
 GHG Reduction - Criterion 1: lifecycle reductions of at least 10% compared to fossil fuel. Carbon Stock - Criterion 1: not produced from biomass obtained from land whose uses changed after 1 January 2008 from primeval forests, wetlands or peatlands, as all these lands have high carbon stocks. Criterion 2: For land use changes after 1 January 2008, (using IPCC land categories), if emissions from direct land use change (DLUC) exceed the default value of the induced land use change (ILUC), the value of the DLUC replaces the default (ILUC) value.

Global impact
As emissions trading schemes and other carbon compliance regimes emerge, certain biofuels are likely to be exempted ("zero-rated") by governments from compliance due to their closed-loop nature, if they can demonstrate appropriate credentials. For example, in the EUTS, SAFUG's proposal was accepted that only fuels certified as sustainable by the RSB or similar body would be zero-rated. SAFUG was formed by a group of interested airlines in 2008 under the auspices of Boeing Commercial Airplanes. Member airlines represented more than 15% of the industry, and signed a pledge to work towards SAF.

In addition to SAF certification, the integrity of aviation biofuel producers and their products could be assessed by means such as Richard Branson's Carbon War Room, or the Renewable Jet Fuels initiative. The latter works with companies such as LanzaTech, SG Biofuels, AltAir, Solazyme, and Sapphire.

Along with her co-authors, Candelaria Bergero of the University of California's Earth System Science Department stated that "main challenges to scaling up such sustainable fuel production include technology costs and process efficiencies", and widespread production would undermine food security and land use.

Certified processes

See also 
 Biodiesel
 Fossil fuel phase-out
 List of emerging technologies
 Vegetable oil fuel

References

Further reading

External links
 
 
 
 
 
 
 

Algae fuel
Aviation and the environment
Aviation fuels
Biofuels