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This article which was recently trending got me thinking about biofuels: Alaska Airlines makes first flight using jet fuel made from trees.

I am not familiar with the numbers for biodiesel, but I know that the energy in to energy out (EIEO) ratio for ethanol is about 1.2 to 1.4. Assuming biodiesel follows the same pattern, this leaves only a 20%-40% margin of energy "left" to process further to jet fuel, before the ratio drops below one.

So, how do bio jet fuels compare to ethanol? Is there a positive energy balance?

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    Am I correct that the EIEO ratio is the ratio of two values: firstly the total energy input required to make the fuel (eg energy required to make fertilisers and fuel etc needed to grow a crop, harvest it, process it), and the second value being the energy obtained from using the fuel. This would mean you're wasting your time if the ratio is less than 1.
    – Highly Irregular
    Nov 16, 2016 at 22:03
  • It looks like this was a POC project, so even if there any EROI numbers available it doesn't mean anything at this stage. We'll have to wait until there is an EROI of a large-scale continuous production process for bio jetfuel. BTW more info also here: nararenewables.org/sustainable-biojet
    – THelper ♦
    Nov 16, 2016 at 22:54
  • @HighlyIrregular, that's correct. However, there can be other considerations - if solar or wind power is used to process biofuels, it can be considered a form of energy storage.
    – LShaver ♦
    Nov 20, 2016 at 19:30

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I looked into this question a bit today and it seems that the industry is not yet mature enough to determine a narrow, consistent range for the energy in to energy out (EIEO) ratio of biofuels for aviation.

"Are Advanced Biofuels for Airplanes Ready for Takeoff?" (Scientific American, May 2017)

This excellent SA piece discusses the industry forces behind development of biofuels for aviation. Like many nascent industries, a lot of players entered the field when airlines and governments began expressing interest in using biofuels, however very few survived very long. It seems the economics of producing jet fuels from biofuel sources are still being worked out, despite significant interest.

"Greenhouse Gas Emissions and Land Use Change from Jatropha Curcas-Based Jet Fuel in Brazil" (Environmental Science and Technology, October 2010)

The article is behind a paywall so I wasn't able to read the full text (and it's a bit old) but one quote from the abstract stands out: "...aggregate emissions vary from ... an 85% decrease from the reference scenario, to ... a 60% increase over the reference scenario." The difference in total life cycle emissions was attributed largely to whether the crops were grown on existing agricultural lands, or if new forest was cleared for biofuel crops.

"Life-cycle analysis of greenhouse gas emissions from renewable jet fuel production" (Biotechnology for Biofuels, March 2017)

While again behind a paywall, this more recent article includes this conclusion in the abstract:

Fischer–Tropsch pathways yield the highest GHG emission reduction compared to fossil jet fuel (86–104%) of the pathways in scope, followed by Hydrothermal Liquefaction (77–80%) and sugarcane- (71–75%) and corn stover-based Alcohol-to-Jet (60–75%). Feedstock cultivation, hydrogen and conversion inputs were shown to be major contributors to the overall WtWa GHG emission performance. The choice of allocation method mainly affects pathways yielding high shares of co-products or producing co-products which effectively displace carbon intensive products (e.g., electricity).

Based on these articles and few others which I read, it seems like more research and development into aviation biofuel production processes is needed before a definitive conclusion regarding potential impact on GHG emissions from the aviation sector can be reached.

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