Abstract
Jet fuel is relatively small in terms of energy consumption and carbon dioxide emissions (10% of U.S. transportation sector in 2021, expected to increase to 14% by 2050). Still airlines have ambitious goals to reduce their greenhouse footprints from carbon-neutral growth beginning this year to reducing greenhouse gas emission for international flights by 50% by 2050 compared to 2005 levels. The challenge is heightened by the longevity of the current fleet (30–50 years) and by the difficulty in electrifying the future fleet because only 5% of the commercial aviation greenhouse gas footprint is from regional flights that might, conceivably be electrified using foreseeable technology. Therefore, large amounts of sustainable aviation fuel will be needed to reach the aggressive targets set by airlines. Only 3 million gallons (11.4 ML) of sustainable aviation fuel (SAF) (with a heat of combustion totaling about 400 TJ = 0.0004 EJ) was produced in the U.S. in 2019 for a 26 billion gallon per year market (3.6 EJ/year). Fischer-Tropsch and ethanol oligomerization (alcohol-to-jet) are considered for producing SAF, including the use of renewable electricity and carbon dioxide. In sequencing the energy transition, cleaning the U.S. grid is an important first step to have the largest greenhouse gas emissions reduction. While carbon dioxide and clean electricity can potentially provide the SAF in the future, an ethanol oligomerization option will require less energy.
Highlights
Commercial aircraft rely on the combustion of hydrocarbon fuels because they offer high specific energy and high energy density
Neither of those flight-critical characteristics can yet be matched by rechargeable power trains consisting of modern batteries or fuel cells and electrical motors in multi-aisle long-haul aircraft
U.S airlines have committed to net-zero carbon emissions by 2050 and carbon-neutral growth relative to a 2019 baseline for domestic and international flights (Airlines for America, 2021)
Summary
Commercial aircraft rely on the combustion of hydrocarbon fuels because they offer high specific energy (energy per unit mass) and high energy density (energy per unit volume) Neither of those flight-critical characteristics can yet be matched by rechargeable power trains consisting of modern batteries or fuel cells and electrical motors in multi-aisle long-haul aircraft. Addressing the bulk of the emissions requires a long -haul solution, which, through 2050, will mean the introduction and use of SAFs. This paper considers the problem from an energy perspective and does not consider all routes that might contribute to the practical solution of GHG reduction in the transportation sector. We will consider routes to renewable fuels, starting with renewable or waste sources of carbon and noncarbogenic sources of energy. The roughly 3.6 EJ/year employed by the aviation sector, Paviation, averaged across a year, is equivalent to the continuous consumption of more than 100 GW of power: Paviation
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