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Conversion yields of biomass to ethanol and ethanol to jet fuel are critical in determining the feedstock and land requirements for sustainable aviation fuel production

Summary​

Biomass conversion yields are critical for determining land and resource requirements for sustainable aviation fuel production. This analysis focuses on two key conversion processes: biomass to ethanol and ethanol to jet fuel. Ethanol yields vary significantly based on biomass type, pretreatment methods, and fermentation processes, with practical limitations preventing achievement of theoretical maximum yields due to pentose removal during pretreatment. For ethanol to jet fuel conversion, yields are influenced by market conditions and co-product utilization, with higher incentives leading to higher yields. The analysis establishes conservative benchmarks of 280 L ethanol per ton of dry biomass and 2.0 L ethanol per L jet fuel for subsequent calculations, recognizing that these yields directly impact land requirements for biomass production.

In order to calculate the amount of land and resources required to produce a given amount of sustainable aviation fuel (SAF), it is important to define the yields of the two primary conversion processes: biomass to ethanol, and ethanol to jet fuel. This is important because variations in these yields can result in vastly different estimates of the amount of land required to produce biomass. Two different yield values are utilized in this analysis: the first is the ethanol yield, which is measured in liters of ethanol per ton of dry biomass (L/ton), and the second is the jet fuel yield, which is measured in liters of jet fuel per liter of ethanol (L/L). Put simply, if the ethanol yield is lower, more biomass and therefore more land is required to produce the same amount of ethanol. If the jet fuel yield is lower, meaning that more ethanol is required to produce the same amount of jet fuel, more ethanol and therefore more biomass and land is required to produce the same amount of jet fuel.

Biomass to ethanol conversion​

Ethanol conversion yields depend on a number of factors. For lignocellulosic biomass, cellulose is the compound that is most easily converted into a sugar via enzymatic hydrolysis after which it can be fermented into ethanol. However, hemicellulose (chain of pentose sugars) and lignin, which are the other main compounds present, are not as easily converted into sugars and therefore play an important role in dictating the overall yield of the conversion process. Other factors that can affect yields are:

  • The type of lignocellulosic biomass used. Some biomass may have high portions of lignin or hemicellulose that reduce the overall efficiency of the conversion process.
  • Pretreatment utilized.
  • Fermentation methods (sequential hydrolysis and fermentation, simultaneous saccharification and fermentation, sequential hydrolysis and co-fermentation, and simultaneous saccharification and co-fermentation).
  • The type of enzymes and yeast utilized.
  • General plant efficiencies.

Listed below in table one is a number of different ethanol yields used in the literature for different types of lignocellulosic biomass.

Table 1: Ethanol yields for different types of lignocellulosic biomass

MaterialYield (L/ton dry biomass)Cellulose yieldPentose yieldEthanol yieldTheoretical maximum yield (L/ton)Source
Sugar cane bagasse19280%70%80%*1
Rice straw353.6--85%*4162
Wheat straw367.2--85%*4322
Sugar cane bagasse363.8--85%*4282
Corn stover358.7--85%*4222
General cellulosic3753
Corn stover339.280%*4244
Corn stover26165%65%65%4025
Switchgrass271 – 3766

* Best performing microbial conversions

Although it is theoretically possible to achieve yields close to the theoretical maximum, in practice, yields tend to be lower as pentose (from hemicellulose) is generally removed from the biomass during pretreatment and not converted into ethanol. This is because pentose requires genetically modified yeast strains to ferment pentose, which exist but are not used on a commercial scale 7.

Due to this limitation, an achievable and conservative ethanol yield of 280 L/ton of dry biomass will be utilized in the remainder of this analysis.

Ethanol to jet fuel conversion​

The ethanol (alcohol) to jet (ATJ-SPK) conversion is a more straightforward process than the biological conversions of feedstock into ethanol. However, unlike the biomass to ethanol conversion, where every stage is optimized to try and reach the maximum yield, the yield efficiencies in the ethanol to jet conversion are more of a decision by the plant operator to balance yields with the costs of the process. This is because when converting ethanol to jet fuel, a range of different sized hydrocarbons are produced in the oligomerization and hydrogenation steps, of which only some are suitable for jet fuel usage. The products that are unusable can consist of other molecules such as diesel, LPG and naphtha. If there is a market for these products, they can be considered as co-products that can be sold to generate additional revenue. If there is no market for these products, they can be filtered and recycled back into the alcohol to jet conversion process, which can incur additional costs and other penalties such as more hydrogen usage. In other words, the ethanol to jet conversion is responsive to the market conditions regarding the usage of sustainable aviation fuel, with higher incentives (rebates, tax credits, subsidies, etc.) leading to higher yields. Below in Table 2 is a list of some ethanol to jet fuel conversion yields from the industry and the literature.

Table 2: Ethanol to jet fuel conversion yields

Author / InstitutionYield (L ethanol/ L jet fuel)Source
LanzaJet & DOE1.6*8
Han, Tao & Wang2.419
Geleynse et al.1.8410

* Approaching the theoretical maximum yield

Assuming that a market for co products exist along with robust incentives for the production of sustainable aviation fuel, a yield of 2.0 L ethanol/ L jet fuel will be utilized in the remainder of this analysis.

Sources​

Footnotes​

  1. Maga, D., Thonemann, N., Hiebel, M. et al. (2019). Comparative life cycle assessment of first- and second-generation ethanol from sugarcane in Brazil. International Journal of Life Cycle Assessment, 24, 266–280. https://doi.org/10.1007/s11367-018-1505-1 ↩

  2. Singh, R., Shukla, A., Tiwari, S., & Srivastava, M. (2014). A review on delignification of lignocellulosic biomass for enhancement of ethanol production potential. Renewable and Sustainable Energy Reviews, 32, 713-728. https://doi.org/10.1016/j.rser.2014.01.051 ↩ ↩2 ↩3 ↩4

  3. Wang, M., Han, J., Dunn, J. B., Cai, H., & Elgowainy, A. (2012). Well-to-wheels energy use and greenhouse gas emissions of ethanol from corn, sugarcane and cellulosic biomass for US use. Environmental Research Letters, 7(4), 045905. https://doi.org/10.1088/1748-9326/7/4/045905 ↩

  4. NREL (2002). Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover. https://doi.org/10.2172/15001119 ↩

  5. NREL (2000). Determining the Cost of Producing Ethanol from Corn Starch and Lignocellulosic Feedstocks: A Joint Study Sponsored by U.S. Department of Agriculture and U.S. Department of Energy. https://doi.org/10.2172/766198 ↩

  6. Haque, M., & Epplin, F. M. (2012). Cost to produce switchgrass and cost to produce ethanol from switchgrass for several levels of biorefinery investment cost and biomass to ethanol conversion rates. Biomass and Bioenergy, 46, 517-530. https://doi.org/10.1016/j.biombioe.2012.07.008 ↩

  7. Callegari, A., Bolognesi, S., Cecconet, D., & Capodaglio, A. G. (2019). Production technologies, current role, and future prospects of biofuels feedstocks: A state-of-the-art review. Critical Reviews in Environmental Science and Technology, 50(4), 384–436. https://doi.org/10.1080/10643389.2019.1629801 ↩

  8. U.S. Department of Energy, Bioenergy Technologies Office. (2021, March). Systems development and integration: Project peer review 2021 presentation by Harmon et al. [PDF]. U.S. Department of Energy. https://www.energy.gov/sites/default/files/2021-05/beto-35-peer-review-2021-sdi-harmon_r1.pdf ↩

  9. Han, J., Tao, L., & Wang, M. (2017). Well-to-wake analysis of ethanol-to-jet and sugar-to-jet pathways. Biotechnology for Biofuels, 10(1), 21. https://doi.org/10.1186/s13068-017-0698-z ↩

  10. Geleynse, S., Brandt, K., Garcia-Perez, M., Wolcott, M., & Zhang, X. (2018). The Alcohol-to-Jet Conversion Pathway for Drop-In Biofuels: Techno-Economic Evaluation. ChemSusChem, 11(21), 3728–3741. https://doi.org/10.1002/cssc.201801690 ↩