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| Fig. 1: Simplified design for HEFA production system. [8] (Image Source: A. Klingberg) |
The decarbonization of transportation remains a significant challenge in addressing global greenhouse gas emissions. Although the aviation sector accounts for only a fraction of these emissions, they are particularly difficult to mitigate. While advancements in battery technology and hydrogen fuel cells have shown potential, they face significant limitations in energy density, storage, and infrastructure development. [1] Given these technological hurdles, the aviation sector is unlikely to transition away from liquid fuels in the near future. Thus, an emphasis has been placed on drop-in biofuels, which can provide comparable performance to petroleum-based jet fuels without requiring modifications to existing aircraft engines. Despite their current high costs, these biofuels are uniquely positioned to address aviations emissions challenges while leveraging existing systems. [1]
Hydroprocessed Esters and Fatty Acids (HEFA) represents the most advanced and widely implemented Renewable Jet Fuel (RJF) technology to date. For HEFA production, there are presently multiple commercial facilities that are either operational or under development. [2] HEFA fuel received ASTM certification in 2011, permitting its use in blends containing up to 50% fossil jet fuel.
HEFA technology involves two main steps: hydrotreatment and hydroisomerization. During hydrotreatment, oxygen is removed from triglycerides through either hydrodeoxygenation (HDO) or hydrodecarboxylation/hydrodecarbonylation (HDCN/HDCX) at elevated temperatures and pressures. [3] In the decarboxylation pathway, the carboxyl group is removed from the fatty acid chains through direct carbon-carbon (CC) bond cleavage in the presence of hydrogen, resulting in the production of linear alkanes with one less carbon atom than the original fatty acid chain. [4] As a result, this process releases carbon dioxide. Similarly, in the decarbonylation pathway, the carboxyl group is removed, releasing carbon monoxide and water. [4] The resulting carbon dioxide and carbon monoxide produced can be separated from the gaseous stream using separators or scrubbing, although this may require additional energy input due to the need for gas purification equipment. This process fully deoxygenates the triglycerides and converts them into straight-chain paraffins, which help achieve a high cetane number. However, these straight-chain paraffins make the fuel less effective in cold temperatures, which is a significant issue for aviation fuels. [3] To solve this problem, the second step, hydroisomerization, transforms the straight-chain paraffins into branched isoparaffins. This conversion improves the fuels ability to flow in cold conditions while maintaining its desirable properties. [3] A simplified HEFA production system is shown in Fig 1.
The primary feedstocks for HEFA jet fuel production include vegetable oils, used cooking oils (UCO), and animal fats. Additionally, non-edible oils from crops like jatropha and camelina, as well as algae-derived oils, have been employed to diversify feedstock sources and mitigate competition with food production. [2,3]
Life cycle assessments have shown that HEFA pathways can significantly reduce fuel related greenhouse gas (GHG) emissions. The reduction in emissions can range from 55% for soybean based HEFA to 84% when using waste residuals like UCO. [5] However, these benefits depend heavily on feedstock origin and life cycle assumptions. For example, utilizing valuable byproducts such as beef tallow instead of waste can increase life cycle GHG emissions, potentially exceeding those of conventional fossil fuels due to the environmental impacts of feedstock production. [5] Additionally, the source of hydrogen used during production is a critical factor in determining HEFAs GHG performance. Currently, most HEFA production relies on hydrogen from steam methane reforming (SMR) of natural gas, which significantly contributes to lifecycle emissions. Transitioning to hydrogen produced via renewable electricity-based electrolysis could reduce GHG emissions by approximately 9% compared to SMR, highlighting its potential to improve the environmental impact of HEFA pathways. [6] In general, optimizing both feedstock selection and hydrogen sourcing is essential for maximizing HEFAs potential in reducing aviation-related GHG emissions.
Despite potential emission reduction benefits, the economic viability of HEFA poses significant barriers to its widespread adoption. For example, an economic assessment in Brazil found that all sustainable aviation fuel production pathways are currently more expensive than fossil fuel kerosene, with Minimum Selling Prices (MSP) ranging from 26.7 to 44.6 USD/GJ compared to 15.8 USD/GJ for fossil jet fuel. [7]
Despite its advanced status, HEFA faces challenges related to feedstock availability and cost. The reliance on waste oils, such as UCO, is limited by regional collection capacities and competition with biodiesel production. [2] Moreover, the high price of sustainable feedstocks and limited large-scale production capacity limit HEFA's market penetration. [2] Nevertheless, ongoing process optimizations and the potential for blending HEFA with conventional jet fuels may enhance economic feasibility and scalability, positioning HEFA as a critical component in the aviation sectors decarbonization efforts. [3]
In summary, HEFA is a robust and feasible pathway for producing sustainable jet fuel, leveraging existing refinery infrastructure and a variety of feedstocks. While technical and economic challenges remain, particularly concerning feedstock supply and fuel properties, HEFAs adaptability and ongoing advancements hold promise for significant contributions to reducing aviation-related greenhouse gas emissions. [2,3]
© Andrew Klingberg. 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] L.M. Fulton et al., "The Need For Biofuels as Part of a Low Carbon Energy Future," Biofuels Bioprod. Biorefin. 9, 476 (2015).
[2] R. Mawhood et al., "Production Pathways For Renewable Jet Fuel: A Review of Commercialization Status and Future Prospects," Biofuels Bioprod. Bioref. 10, 462 (2016).
[3] P. Vozka, P. Šimáček, and G. Kilaz, "Impact of HEFA Feedstocks on Fuel Composition and Properties in Blends with Jet A," Energy Fuels 32, 11595 (20018).
[4] R. C. Monteiro et al., "Production of Jet Biofuels by Catalytic Hydroprocessing of Esters and Fatty Acids: A Review," Catalysts 12, 237 (2022).
[5] R. S. Capaz et al., "Environmental Trade-Offs of Renewable Jet Fuels in Brazil: Beyond the Carbon Footprint," Sci. Total Environ. 714, 136696 (2020).
[6] G. Seber et al., "Uncertainty in Life Cycle Greenhouse Gas Emissions of Sustainable Aviation Fuels From Vegetable Oils," Renew. Sustain. Energy Rev. 170, 112945 (2022).
[7] R. S. Capaz et al., "Mitigating Carbon Emissions Through Sustainable Aviation Fuels: Costs and Potential," Biofuels Bioprod. Bioref. 15, 502 (2021).
[8] M. Pearlson, C. Wollersheim, and J. Hileman, "A Techno-Economic Review of Hydroprocessed Renewable Esters and Fatty Acids for Jet Fuel Production," Biofuels Bioprod. Bioref. 7, 89 (2013).