Advanced Biofuels in Aviation

Fig. 1: Comparison of biojet GHG emissions and CO2 savings across routes. [1] (Image Source: S. Banerjee)

In the United States, the impact of advanced biofuels in aviation is vast, with estimates showing an even greater influence in the future. The FAA's goal for renewable jet fuel is based on the use of advanced fermentation (AF) derived from grass, and estimates show that currently, renewable jet fuel accounts for approximately 1.4 percent of total fuel emissions in commercial aviation, as seen in Fig, 1. [1] The graph shows that Gasification and Fischer-Tropsch with forestry residues achieve the highest CO₂ savings (95%) and lowest emissions (6 gCO₂/MJ), while routes like HEFA with oilseed rape vary widely (20-54% savings), underscoring the challenges in standardizing biofuel impacts in aviation. However, how would one account for the total number of renewable emissions currently being emitted? Through what means would this 1.4 percent figure be computed? And what are the mechanisms that ensure that this is an accurate measure?

First, it is key to understand the specific biofuels being counted. Biofuels from advanced fermentation (AF) are classified as such because, unlike traditional biofuels from crops or oily grains, fuels obtained via AF can more efficiently convert biomass from grasses than traditional biofuels. [2] This is done by using lignocellulosic (or woody) biomasses and converting using hydrogen to renewable jet fuel. [1] In this context, AF would not include feedstocks like starch and sugar or vegetable oils and fats. The vast majority of carbon dioxide emissions from fossil fuels are emitted when burned in the engine, accounting for about 84 percent of these emissions. The remaining 16 percent of emissions result from the production phase.

AF can potentially reduce total emissions anywhere from 20 percent to 95 percent, but this can differ based on how the reductions are calculated. [3] For instance, the emissions tied to feedstock production can have a significant impact. A case in point would be feedstocks that are derived from agricultural and forestry residues and municipal wastes, as they produce lower emissions overall. [4] Emissions reductions beyond 90 percent can even be achieved through particular conversion techniques such as gasification and Fischer-Tropsch synthesis despite the high energy intensities involved because of the energy being derived from the biomass itself. With multiple approaches all fitting within the AF umbrella, it becomes more complicated to reliably arrive at the 1.4 percent figure.

Moreover, the use of advanced biofuels in aviation is dependent on a range of factors including international standards, certification, and technological readiness. Their emissions can vary based on viscosity, material compatibility, and other elements particular to AF-derived fuels. Given that there are still no widely employed national standards established in this arena, it would seem a quite complex endeavor to be able to compute an accurate measure of what percent of total fuel emissions renewables account for.

As such, if one is to gauge the percent of total emissions that renewables comprise, the emissions reduction potential must be determined. Currently, though, it is known that advanced biofuels' reliance on different kinds of feedstocks makes the differential in emissions reduction so wide, ranging from 50 percent to 95 percent as seen in Table 1, that knowing what percent of emissions it can be considered to contribute to is a difficult proposition. Until this is known to a more dependable extent, it would appear that the arrived upon 1.4 percent of total commercial fuel emissions that renewables represent is an estimate that cannot be fully trusted.

Crucially, the financial element looms large, given that aviation biofuels are more expensive than conventional jet fuel. [1] Because fuel costs comprise such a significant proportion of the operating costs of airlines, their environmental benefits might not outweigh their financial costs. For instance, conventional jet fuel has trended downward in price, with a 45% drop from an average monthly cost of $0.77 per liter in 2014 to $0.42 per liter in 2017. This can blunt the industry momentum to invest in and transition to alternatives like biofuels, emphasizing the need for financial incentives in this growing but still limited arena.

© Samir Banerjee. 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.

References

[1] J. Bosch et al., "Aviation Biofuels: Strategically Important, Technically Achievable, Tough to Deliver," Grantham Institute, Briefing Paper No. 23, November 2017.

[2] D. Tilman et al., "Beneficial Biofuels - The Food, Energy, and Environment Trilemma," Science 325, 270 (2009).

[3] "Directive (EU) 2015/1513 of the European Parliament and of the Council of 9 September 2015 Amending Directive 98/70/EC Relating to the Quality of Petrol and Diesel Fuels and Amending Directive 2009/28/EC on the Promotion of the Use of Energy from Renewable Sources," Official Journal of the European Union, 2015 O. J. (L 329) 1.

[4] N. Winchester et al., "Economic and Emissions Impacts of Renewable Fuel Goals for Aviation in the US," Transp. Res. A: Policy Pract. 58, 116 (2013).