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February 1, 2022
In the midst of managing an operational and economic recovery from COVID-19, the commercial aviation industry has set a long-term goal for carbon dioxide (CO2) emission reduction. In October 2021, the International Air Transport Association (IATA), which represents the global commercial aviation industry, and Airlines for America, a U.S. industry trade group for all the major U.S. passenger and cargo carriers, announced their members' commitment to net-zero carbon emissions by 2050. Achieving this goal, however, will be a difficult challenge.
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Aviation is a difficult sector to decarbonize. The enormous energy required to fly a heavy aircraft long distances takes an energy dense power source that liquid hydrocarbon fuel provides. Battery technology for electric flight is improving. However, batteries currently do not have an energy density anywhere close to liquid fuel and would be too heavy for large aircraft over long distances. Hydrogen (H2) is a potential liquid fuel that generates only water vapor in combustion and very small amounts of nitrous oxides. Shifting to H2 as a fuel source presents a significant set of challenges in production, storage and infrastructure, aircraft configuration and engine modification requirements. All electric- or hydrogen-powered flight for large aircraft and long distances could be decades away.
The strategy with the most promise for substantially reducing the sector’s carbon footprint in the near and medium term is expanding the use of sustainable aviation fuel (SAF). SAF is an alternative jet fuel made from renewable biomass or waste-based feedstock that has a lower life-cycle carbon intensity than conventional petroleum-based fuel. SAF can be made from a variety of feedstock such as oil seed plants and energy grasses; agricultural and forestry residue; organic municipal solid waste; fats, oils, and greases from cooking waste and meat production; algae; and industrial carbon monoxide waste gas.
Another type of sustainable jet fuel is e-SAF or Power to Liquid (PtL). PtL is produced by combining hydrogen, which can be split off from water (it’s the “H” in H2O) using renewable electricity in a process called electrolysis, with carbon extracted from the atmosphere or from industrial waste gas. Barriers to wider use of this technology are the cost of PtL, which is significantly higher than the cost of other types of SAF, and the availability of renewable electricity for the electrolysis. This high cost is expected to decrease substantially over the next three decades and the global industry trade group Air Transport Action Group (ATAG), in its 2021 report Waypoint 2050, projects PtL production of 42 to 57 percent of total SAF by 2050.
Benefits of Sustainable Aviation Fuels
SAF is considered a “drop-in” fuel and is certified for use blended with conventional fuel up to a maximum 50 percent mixture, requiring no change in aircraft engines or fueling infrastructure. There are currently seven SAF production processes (pathways) approved as safe for use in blended form by civilian jet aircraft.
SAF made from plant feedstock limits the introduction of carbon into the atmosphere because plants absorb CO2 through photosynthesis during growth. This creates a loop: plants absorb carbon from the atmosphere, then are turned into SAF, which releases the carbon back into the atmosphere when it is burned. In this manner, SAF is “defossilized” by using carbon already in the biosphere, not the fossil carbon used in petroleum-based jet fuel.
Using organic municipal solid waste as a feedstock for SAF also recycles carbon contained in the waste and reduces methane emissions, a potent greenhouse gas that can leak from landfills.
Life-cycle CO2 emissions from “neat” (unblended) SAF can be up to 80 percent lower than those from conventional petroleum-based fuel. When blended with 50 percent conventional fuel, the reduction in CO2 emissions is still a substantial 40 percent.
SAF also generates fewer conventional pollutants that contribute to local air quality concerns around airports. It emits fewer sulfur oxides and particulates, less carbon monoxide and fewer unburned hydrocarbons. With fewer particulates in aircraft exhaust, contrail formation is diminished. Contrails and the cirrus clouds they generate can produce an atmosphere-warming influence contributing significantly to aviation’s climate change impact.
To ensure that SAF is indeed more sustainable than petroleum fuel, the UN’s International Civil Aviation Organization (ICAO) set SAF standards that include requirements to “achieve net greenhouse gas (GHG) emissions reduction on a life-cycle basis (compared to conventional fuel), contribute to local social and economic development” and avoid “competition with food and water.”
According to the ATAG report, growing non-food plant feedstocks for SAF has the potential to use otherwise unproductive land, provide employment opportunities in rural areas, add revenue for farmers, and can enrich soil when feedstocks are grown as cover crops in between growing seasons for conventional crops.
Challenges of Sustainable Aviation Fuels
First approved for commercial aviation use in 2011 (when blended with petroleum jet fuel), SAF is much more expensive than fossil-based fuel, costing generally at least four or five times more than conventional jet fuel.
Increased production will improve the economies of scale for these sustainable fuels, which made up less than 0.1 percent of the 96 billion gallons of total global commercial aviation fuel consumption in 2019. In the United States, there were just over 4.5 million gallons of SAF produced in 2020, a substantial increase over 2019, but only a small fraction of the 18.3 billion gallons of jet fuel consumed in 2019 by scheduled U.S. carriers. Production capacity is limited but growing.
Fats, oils, and greases (FOG) are the feedstocks most in use today for SAF but are in limited supply. A diverse set of feedstocks is required for a shift from conventional jet fuel. More than one billion dry tons of biomass is potentially available annually in the coterminous United States as feedstock for bioenergy and chemical products, enough to exceed projected U.S. domestic aviation fuel demand.
SAF competes with renewable diesel (RD)—used in road transport—for feedstock resources and production capacity. RD benefits from favorable tax policy, making its production more profitable; it dwarfs SAF output nationally.
Momentum is picking up to encourage expanded production and use of SAF. Initiatives by the Biden-Harris Administration, legislation introduced in Congress, corporate partnerships with airlines, and a growing number of purchase agreements and commitments by airlines to buy and use SAF are helping support growth of the sustainable aviation fuels industry.
This first installment in a four-part series of articles reviewed the basics of SAF. The second article looks at the initiatives announced in 2021 by the federal government and legislation before Congress to help develop and incentivize production of SAF. The third article will examine strategies in the private sector to encourage greater production of SAF at lower cost, as well as contract agreements for purchase and production of SAF in the United States and internationally. The final article in the series will describe the important SAF developments during the 2021 international climate talks in Glasgow (COP26), including plans announced by the United States to advance the SAF industry.
Author: Jeff Overton
Read all of the articles in the Sustainable Aviation Fuel series
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