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| Fig. 1: Utility of Biofuels. (Source: Wikimedia Commons). |
In the United States, commercial air travel accounts for 3% of total greenhouse gas emissions yearly and the United Nations expects airplane emissions of carbon dioxide to triple by 2050. [1] One potential mitigation approach is the use of biofuel alternatives. Biofuels present a possible alternative to non-renewable fuel for transportation as well as heating and electricity generation. (Fig. 1) The first commercial aviation flight using a blend of biofuel and traditional jet fuel occurred in 2008 by Virgin Atlantic and United Airlines performed a commercial flight in 2021 with 1 of the 2 engines containing 100% biofuel. [2] A major appeal of biofuels is it's ability to be "drop-in ready" meaning it can mix with conventional fuel or be used on it's own, and is already compatible with current aircraft engines. A major obstacle of biofuels though is the increased cost compared to traditional jet fuel.
A study in October 2021, by the American Society for Testing and Materials (ASTM), identified 9 pathways for generating sustainable aviation fuel (SAF). [3] For each conversion process, there are various eligible feedstock (fats, biomass, algae, etc.) as well as an approved blending ratio by volume. Techno-economic analysis (TEA) is a popular approach for estimating the cost of various feedstock/conversion process combinations. The approach utilizes empirical data to estimate capital/operating costs and mass/energy balances to help assess the viability of potential biofuels before commercialization.
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| Table 1: Minimum jet fuel selling price from the techno-economic assessment of Beal et al. [4] |
An earlier study in November 2020 completed a techno-economic analysis for 9 different jet fuel pathways with feedstocks including land crops (soy, corn, etc.), marine crops (microalgae), and by-products (municipal waste, oil, etc.). [4] A key term looked at was the minimum jet fuel selling price (MSFP) which is the minimum price the fuel can be sold at to break even and is reported in $/Liter.
No pathway was found to have a lower MSFP than conventional jet fuel ($0.78/Liter) with waste oil and grease (WOG) having the lowest of the biofuels ($0.81/Liter). Capital costs were found to play a large role in the MSFP of each pathway.
Another study in June 2022 completed a techno-economic analysis for a combination of 16 different feedstocks and 6 different conversion processes for a total of 30 SAF production routes. [5] Key factors that decreased MSFP were the jet fuel yield, process efficiency, and scalability of the feedstock. Hydrotreated Esters and Fatty Acids (HEFA) were found to currently be the most economic option based on MSFP due to low capital and operating costs. This conclusion is supported by the fact that HEFA is the most commercially developed SAF. [6] Feedstock supply chain logistics and optimization of production processes were found to be the key areas needing improvement.
Although biofuels show promise in lowering GHG emissions within the commericial aviation industry, the cost of production must be reduced in order to be competitive with conventional jet fuel.
© Jacob Beardslee. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Growth in Greenhouse Gas Emissions from Commercial Aviation", Environmental and Energy Study Institute, June 2022.
[2] P. Wilson, "Airliners Powered by Sustainable Fuel Remain a Distant Goal", New York Times, 29 Jun 22.
[3] "SAF Grand Challenge Roadmap: Flight Plan for Sustainable Aviation Fuel", U.S.Department of Energy, September 2022.
[4] C. M. Beal, A. D. Cuellar, and T. J. Wagner, "Sustainability Assessment of Alternative Jet Fuel For the U.S. Department of Defense," Biomass Bioenergy 144, 105881 (2021).
[5] M. F. Shahriar and A. Khanal, "The Current Techno-Economic, Environmental, Policy Status and Perspectives of Sustainable Aviation Fuel (SAF)", Fuel 325, 124905 (2022).
[6] K. S. Ng, D. Farooq, and A. Yang, "Global Biorenewable Development Strategies For Sustainable Aviation Fuel Production," Reneew. Sustain. Energy Rev. 150, 111502 (2021).
