In a previous post various Sustainable Aviation Fuels, or SAFs for short, are discussed. Except for some minor differences in their exact composition, SAFs are basically just conventional jet fuel. The main difference is that most SAFs lack aromatics. Aromatics cause swelling used in the seals of aircraft fuel systems which prevents fuel leaks. This lack of aromatics does require some changes, either to aircraft fuel system design or the chemical composition of SAFs themselves such as the FT-SPK/A variant which specifically includes aromatics, before they can be used at 100%.
But, compared to the massive design changes needed to aircraft wanting to operate using things like liquid hydrogen, these seem like minor challenges. And many airlines and other organizations in the aviation industry herald SAF as a key path to reducing harmful emissions that contribute to climate change. But what can really be achieved and what’s just clever marketing?
Consider the following claim:
Boom Supersonic is transforming air travel with Overture, the fastest and most sustainable supersonic airliner. Carrying 65–80 passengers, Overture will fly at twice the speed of today’s airliners and will run on 100% sustainable aviation fuel (SAF).
Boom Supersonic on their supersonic airliner in development, the Overture (source)
At a glance this sounds nice, but let’s consider this statement in a little more detail. First of all, if and when the Overture goes into service it would almost certainly be the only supersonic airliner in service. So, saying it is the most sustainable one is pretty meaningless when it has no competition. Further, claiming it will run on 100% SAF seems like a stretch. Ultimately, it is up to airlines to decide how they fuel their aircraft, and it seems unlikely that they would fuel it with 100% SAF at all times. There may not even be sufficient availability of SAF at the introduction of the Overture at all airports it will operate to in order to achieve this. Stating it could run on 100% SAF would be a more realistic statement. But Boom seems confident that they will be able to run on 100% SAF producing entirely on net-zero carbon emissions, stating in an announcement for their Net Good Summit that their aircraft is “…expected to be the first large commercial aircraft to be net-zero carbon from day one…”. We’ll see.
In a whitepaper published by Boom, they optimistically assume exponential growth of SAF production over the years to come, but this seems based mostly on wishful thinking of how other technologies have advanced. Even if this exponential growth does happen, can we ramp up this whole new way of producing in a way that is truly sustainable? A study published in early 2022 on the current use of corn to produce bioethanol suggests an overall net negative impact on the environment, with “at least 24% higher” more carbon released into the environment and other negative effects associated with a change in land use to produce the necessary corn. To be fair, many studies on this have been done with varied outcomes, a key reason being that there are a lot of variables one can consider to determine the overall lifecycle impact on global warming, and not all studies pick the same ones. Further, this is specifically looking at the use of corn, and other processes may work out much better. However, it isn’t a promising precedence for SAF. Extremely thorough analysis and monitoring of the entire lifecycle of the fuel’s production, refinement, transportation and eventual use is needed to ensure it is truly sustainable. And can this be done at the scale needed to support the entire aviation industry and potentially other forms of transport as well?
This clever marketing works however as even reputable media do not seem capable of interpreting what such statements actually mean. In 2021, Business Insider reported on United Airlines’ decision to purchase the Overture and described the aircraft as being “a zero-emission supersonic jet” (article). CBS News also repeated this claim of the Overture being a “zero-emission aircraft” (article). Obviously, this is not true. By using SAFs as a fuel for the Overture, it may be able to offset some of its carbon emissions as the feed stock used to produce the SAF may have captured carbon dioxide from the atmosphere during its growth. That does not mean it has no emissions, and depending on the method used, may only offset a portion of the total carbon emissions. And considering that supersonic flight burns considerably more fuel per passenger-kilometer flown, it is quite likely that the Overture running on 100% SAF would still have greater net carbon emissions than a conventional jet aircraft flying on conventional jet fuel. For example, an August 2022 report from International Council on Clean Transportation, which is supportive of SAF generally, estimates that carbon dioxide emissions from the Overture would be about five times worse than that of conventional airliners per passenger-kilometer flown if it were to use soy as its feedstock. The report also indicates that for United Airlines to operate its planned 15 Overture aircraft, it would require more than the entire 2020 soybean production of South Dakota just to produce enough fuel for these 15 aircraft.
There is a second issue, which is that terminology is applied in inconsistent and misleading ways. Consider the following statement from this IATA blog post found on the IATA website:
We believe that SAF is both a game-changer and the future of our industry because it can cut life-cycle emissions by about 80%. It can be made from a wide variety of sources – used cooking oil, urban or agricultural waste, non-food crops, even algae – and it can be dropped straight into modern jet engines or without any need of modification.
Robert Boyd, former Assistant Director Aviation Environment and IATA lead on Sustainable Aviation Fuel (source)
There are several problematic issues in these two sentences alone, which come from a highly authoritative source. First, as with many articles, it isn’t very clear about which emissions are being cut. It just talks about “life-cycle emissions”. What is meant is CO2 emissions. However, it isn’t just carbon emissions that contribute to climate change. It is estimated that only about a third of aviation’s impact on climate change comes from CO2 emissions. NOx and contrails are the other major contributors (source). By claiming SAF “cut life-cycle emissions by about 80%” and not indicating this refers to CO2, the statement becomes misleading. Many articles do call this out properly by specifically referring to carbon emissions, but this mistake or willful omission is all too common. Further, the post then lists a variety of potential sources of SAF. Many of these paths would not yield an 80% net saving in CO2 emissions and could in fact be worse. Given the concerns around using corn for bioethanol today and this potentially being worse for the environment than traditional fuel, getting to an 80% life-cycle reduction seems optimistic.
While it may seem as if the industry has fully embraced SAFs as a key strategy to pursue for reducing the climate impact of aviation, other voices from industry leaders do exist:
…in my mind, SAF and carbon offsets are more greenwashing than real at this point…
Jozsef Varadi, CEO of Wizz Air (source)
Concerns are voiced by Barclays who investigated Sustainable Aviation Fuels from a financial investment perspective. Their report published in April of 2022 calls out many major hurdles when it comes to SAF being produced and adopted at the scale envisioned by the aviation industry to meet their goals. Some key challenges presented are the massive investment required, the resulting increase in fuel prices for airlines, competition with other forms of transportation also wanting to use biofuels, land-use challenges and more. Overall, the plans are described as “…ambitious, if not aspirational…”
So, are Sustainable Aviation Fuels really the path forward? Or are they just clever marketing to make the flying public believe that aviation is doing its part and there’s no need to feel bad about flying? The reality is probably somewhere in between. Overall, if done well, Sustainable Aviation Fuels can reduce net carbon emissions. But as of right now, evidence does not point towards a path where they would lead towards a net-zero carbon emission future for aviation in the foreseeable future, plus they ignore the other emissions that contribute to climate change. Worse, if not properly implemented or monitored, the production of SAF could end up being a net-negative for our environment. However, marketing teams at airlines and elsewhere are quick to publish the most optimistic view of things possible. But we should consider their statements with a considerable dose of skepticism and not consider us to be on a path to aviation being truly sustainable just by going down the SAF road. Doing something is still better than doing nothing at all, but many claims made are clearly not realistic.