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Second generation biofuels failed commercially when they were first introduced, but continue to show promise for sustainable aviation fuel production

Summary​

Second generation biofuels were developed to address the food vs fuel controversies of first generation biofuels by using non-food lignocellulosic feedstocks. Despite being technologically mature and capable of commercial operation, they have faced significant challenges including policy failures like the RFS2 mandate, technical difficulties, and lack of price competitiveness, resulting in most second generation biofuel plants closing or operating at a low capacity. However, they remain promising for sustainable aviation fuel production through pathways like Alcohol-to-Jet and Fischer-Tropsch processes, offering potential for decarbonizing the aviation industry in the short to mid term, despite ongoing concerns about sustainability and land use impacts.

Second generation biofuels were developed to address the controversies that arose from the feedstock needs of first generation biofuels, specifically land usage and food scarcity issues. Second generation biofuels are considered the most mature and primed option of the three generations of advanced biofuels to make an impact in the short to mid term due to their technological readiness and their proven ability to operate at a commercial scale 1 2. However, they have not been without significant challenges, controversies, and oftentimes perceived failures.

In the years following the 2007 – 2008 food crisis, which many attribute to the rapid growth of corn-based ethanol in the United States domestic market, many researchers, policymakers, and companies began to explore alternative feedstocks for biofuels. This resulted in the passing of the Energy Independence and Security Act of 2007 (EISA), which introduced the revised Renewable Fuel Standard (RFS2) that obligated petroleum refiners to blend in 8.5 million gallons of cellulosic second generation biofuel into the US gasoline supply by 2019 and 16 million gallons by 2022 3. The long-term goal of the RFS2 mandate was to replace corn-based ethanol as the leading blended biofuel in the US market 3. As is now apparent, the RFS2 mandate proved to be a significant failure due to a number of reasons including the over-ambition and subsequent watering down of the mandate that shook investor confidence, and the emergence of the ethanol 'blend wall' that limited the amount of ethanol that could be blended into the US gasoline supply 3 4 5. Although proven technologically with a significant number of pilot and commercial plants having been operated, due to technical challenges and lack of price competitiveness with first generation biofuels, the vast majority of second generation biofuel plants have been closed, canceled or been operated at a low capacity 3. In addition to the technological challenges, second generation biofuels that rely on lignocellulosic feedstock have received a significant level of criticism challenging their credentials as a 'sustainable' or 'renewable' biofuel due to their effect on indirect land use change (ILUC) and soil organic carbon (SOC).

The technological readiness of second generation biofuels and the large availability of feedstock options make them suitable in assisting with decarbonization of the aviation industry. The majority of Sustainable Aviation Fuel (SAF) in use today are Hydroprocessed Esters and Fatty Acids (HEFA), or sometimes known as Hydroprocessed Renewable Jet Fuels (HRJs), that use vegetable oils, animal fats, and waste oils as their feedstock. However, the supply of naturally occurring oils such as vegetable oil from oil palm is constrained, meaning that they cannot be scaled up to meet the demands of the SAF market 6. Sustainable aviation fuels that use second generation biofuels as their feedstock such as Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) and Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) could be scaled up to meet the demands in the short to mid term.

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Footnotes​

  1. Mohr, A., & Raman, S. (2013). Lessons from first generation biofuels and implications for the sustainability appraisal of second generation biofuels. Energy Policy, 63, 114–122. https://doi.org/10.1016/j.enpol.2013.08.033 ↩

  2. Cavelius, P., Engelhart-Straub, S., Mehlmer, N., Lercher, J., Awad, D., & Brück, T. (2023). The potential of biofuels from first to fourth generation. PLoS Biology, 21(3), e3002063. https://doi.org/10.1371/journal.pbio.3002063 ↩

  3. Brown, T. R. (2019). Why the cellulosic biofuels mandate fell short: A markets and policy perspective. Biofuels, Bioproducts and Biorefining, 13(5), 889–898. https://doi.org/10.1002/bbb.1987 ↩ ↩2 ↩3 ↩4

  4. Breetz, H. L. (2020). Do big goals lead to bad policy? How policy feedback explains the failure and success of cellulosic biofuel in the United States. Energy Research & Social Science, 69, 101755. https://doi.org/10.1016/j.erss.2020.101755 ↩

  5. Peplow, M. (2014). Cellulosic ethanol fights for life. Nature, 507(7490), 152–153. https://doi.org/10.1038/507152a ↩

  6. Detsios, N., Theodoraki, S., Maragoudaki, L., Atsonios, K., Grammelis, P., & Orfanoudakis, N. G. (2023). Recent advances on alternative aviation fuels/pathways: A critical review. Energies, 16(4), 1904. https://doi.org/10.3390/en16041904 ↩