Sustainable aviation fuels could cut airline CO2 emissions by up to 23% — Is that enough?

CO2 emissions are a big number composed of a lot of smaller numbers that add up to the totality of the problem. The goalwith participation from every sector those emissions come from is to, by the year 2050, reduce overall emissions and limit average temperature growth to less than 2˚C (3.6˚F) higher than the average pre-industrial temperature. That target is laid out by the Paris Agreement, the international treaty on climate change, adopted by almost 200 parties in 2015.

The aviation industry accounts for about 2.5 percent of global carbon emissions, and all hands are on deck to lower the number. European aviation, for example, set a goal to attain net-zero CO2 emissions by 2050. The International Civil Aviation Organization (ICAO) initiated its Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), creating incentives for the airlines to use sustainable aviation fuel (SAF) to keep sector emissions at their 2020 level.

SAF technology has been under development for several years, and it already has been demonstrated by United Airlines. United flew the first commercial flight with passengers on board using “drop-in” SAF — meaning it was interchangeable with conventional jet fuels, with no modifications to the engine — for one of the aircraft’s two engines. SAF, which blends biofuels and traditional petroleum-derived jet fuels, has a lower life-cycle carbon footprint than conventional jet fuels because the biofuels come from crops that absorb carbon dioxide when they are grown, decreasing their net carbon emissions.

The U.S. aviation trade organization, Airlines for America, along with the U.S. Departments of Agriculture, Energy and Defense; the Federal Aviation Administration (FAA); Boeing; and private partners like the Commercial Aviation Alternative Fuels Initiative (CAAFI), have launched a project called Farm to Fly 2.0, with the goal to supply some 1 billion gallons of SAF. The first commercial-scale biofuel production took place in 2016, and the U.S. aviation sector used more than 1 million gallons. A June 2018 study predicted about 19 biofuel production facilities would be operating in the U.S. by 2022, with an expected combined production capacity of 1 billion gallons per year. For context, before the COVID-19 pandemic, U.S. airlines consumed about 18 billion gallons of jet fuel in a year.

SAF costs more to produce, and ultimate production capacity is uncertain, so it’s not clear to what degree commercial airlines will use SAF to pursue their goal of meeting emission targets. The airlines may seek more SAF than can be produced; higher production costs could result in increased ticket prices, reducing demand (although that also would lower travel and emissions).

The upshot is that there is a vital need to assess, as accurately as possible, how effective introducing SAF into commercial air transportation would be toward lessening carbon emissions. Our team developed an approach that uses an operations-based model to predict emissions of future airline operations, depending on varying demand scenarios, improvement of aircraft technology, levels of biofuel utilization, and effects of biofuel pricing on what passengers pay for tickets.

The computational tool we created is called the Fleet-Level Environmental Evaluation Tool (FLEET). This modeling system allowed us to simulate different scenarios of SAF use and travel demand. FLEET’s optimization algorithm balances the maximization of airline profit with fulfilling passenger demand and addressing operational constraints. The model even considers several new technologies that airlines are expected to adopt to help cut fuel consumption.

Normalized fleet-level CO2 emissions predicted by FLEET for various SAF scenarios, considering passenger demand recovery to pre-COVID-19 levels by 2024 and future passenger growth rates at 75 percent of pre-COVID-19 levels until 2030. (Purdue University image/Samarth Jain, et al.)

We ran the FLEET simulation for the years 2005-2050, accounting for differing post-pandemic demand recovery scenarios that eventually bring passenger air travel back to pre-pandemic levels and add demand growth into the future. Depending on that demand recovery, all the SAF scenarios showed a reduction in CO2 by 2050, ranging from 4 percent to 23 percent lower than baseline emissions.

Our findings clearly demonstrated that even if airlines experience and meet higher passenger demand for travel, their carbon emissions still could be lower through the use of SAF than if they use conventional jet fuel. The up-to-23 percent decrease is not enough on its own to achieve aggressive CO2 reduction goals. However, it shows that SAF is one more tool in aviation’s toolbox — and a very valuable one — in addressing and mitigating global temperature growth.

William A. Crossley, PhD

J. William Uhrig and Anastasia Vournas Head and Professor of Aeronautics and Astronautics

School of Aeronautics and Astronautics

College of Engineering

Purdue University

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