Sustainable aviation fuels, or SAFs, are a critical part of aviation’s decarbonisation journey. But not all SAFs are as green as the others, with some marked differences in sustainability across their production pathways. For more on how the science is shaping up, we talked with experts from energy nonprofit RMI and researchers from Rhodium Group.
At their sustainability core, says RMI principal Andrew Chen, “not all SAFs and feedstocks are created equal. They will display a different range of lifecycle carbon benefits or total emissions. In part, when you ask the question about sustainability, there are measures included in lifecycle analyses that can give you an inference of that.”
Aviation is starting to get to grips with lifecycle analysis within its supply chain, but in many ways calculations for fuel production purposes are already more advanced.
Eric O’Rear, senior analyst with Rhodium Group, tells us that, “outside of Hydroprocessed Esters and Fatty Acids (HEFA), which is the only existing commercial SAF production pathway, we view the Fischer-Tropsch (FT) and Alcohol-to-Jet (AtJ) production pathways as the most promising in the near- to medium-term. This is largely based on current investment trends, recent announcements for new SAF facilities, and the fact that they are the closest to being at commercial scale compared to the other ASTM-approved pathways.”
HEFA questions revolve around the sources of the fats, oils and greases — and the hydrogen
One philosophical question is how to account for feedstocks that would otherwise be waste, such as used cooking products that are used for much of today’s HEFA production.
When it comes to sustainability, “the HEFA pathway really runs the gamut,” Hartej Singh, a manager in RMI’s climate-aligned industries practice, tells us. Here, it “really depends on whether the fat, oil or grease that is fed into it as an input is truly waste in nature — or is it virgin?”
The production of the hydrogen that is also a part of the feedstock for HEFA is also critical to its sustainability: whether it is black, brown, grey, blue or green, in other words.
Furthermore, newer HEFA feedstocks may make a real difference to how sustainable the resulting SAFs are, both environmentally and economically.
RMI senior associate Corey Stewart explains that “when we get into the second generation feedstocks for HEFA is where it starts to really get interesting, because the economics start to really favour HEFA over others.”
This includes waste fats, oils and greases from multiple sources, including agricultural products and animal fats, as well as the yellow grease along the lines of what is used today as waste cooking oils, fats and greases.
Third generation feedstocks, Stewart says, “go to a lot of different pathways, but they’re also really interesting. That’s where a lot of the waste comes in, like forestry waste, agricultural waste — municipal solid waste is a really interesting one as well, that isn’t just for HEFA.”
The big issue for AtJ is about food versus fuel
Alcohol-to-jet and Fischer-Tropsch processes are very promising when it comes to sustainability, but neither is without its hurdles.
“Compared to conventional jet fuel, both pathways also have the potential to reduce total life-cycle emissions by more than 100 percent when coupled with carbon capture,” Rhodium’s O’Rear says. “It must be noted, however, the degree to which these pathways will ultimately scale up will greatly depend on how much public and private sector support there continues to be, as well as how well the industry is able to address feedstock supply constraints.”
The big question for alcohol-to-jet as an early pathway is one of specific feedstocks for the alcohol used within the process. Primarily, the issue is one of “food versus fuel”, namely the replacement of arable land currently or potentially used to feed people rather than to fly airplanes.
This is an especially sensitive issue when it comes to the use of corn ethanol, a very controversial and politically charged feedstock, particularly in the US context.
However, RMI’s Singh says, when it comes to future technology, the food versus fuel problem might be solvable, but new issues arise. “There are next generation sources that produce ethanol, like gas fermentation, things of that sort. Those need to be commercially scaled. So that’s another point of challenge for AtJ,” Singh says.
The FT pathway’s blockers are ones of scale and logistics
While the Fischer-Tropsch process is very promising, there the big question is one of how to manage the logistics of the large-scale manufacturing process.
[link to or embed Helge Sachs YOA — https://public.yocova.com/news-insights/yocovaonair/yocova-on-air-with-dr-helge-sachs-senior-vice-president-sasol-ecoft/]
“Fischer-Tropsch is only economical when you have a massive plant — an enormous 100 barrel per day type facility. The challenge is, if you have that type of facility and can build that type of plant, how are you going to get enough feedstock to keep that plant running at full capacity?” RMI’s Singh asks. “You’re playing the balancing act: I need to build a big plant for the plant to be economical, but how am I going to keep that plant fed with bio feedstock? There’s only so much woody biomass available, and the transporting of something like a woody biomass or wood pellets is lot more difficult than something like oil.”
Future fuels and new technologies offer promising benefits
As these pathways and other technologies develop from research to proof-of-concept and mature into industrialised processes, new technologies including AI may well lead to breakthroughs in the way the production pathways are optimised.
“Where we see AI coming into play is catalyst selection,” Singh says. “Most of the SAF production pathways and the reactors use a catalyst. There’s an infinite amount of permutations and combinations of catalyst versus reactor size versus operating pressure versus operating temperature. It’s just unfathomable to think about the different permutations in one’s head, because each catalyst is a different material as well. That’s where AI could really come into play: finding an optimal catalyst for an optimal operating condition. The National Energy Technology Laboratory here in the United States has actually done a lot of work on that already.”
Looking to the future, beyond the initial three kinds of pathway lie synthetic fuels and power-to-liquid, also known as e-fuels.
Here, RMI’s Chen concludes, “the challenge is obviously the nature of the technology, the uncertainties and costs of the potential underlying renewable electricity and other feedstocks. It’s just further off in terms of commercial deployment.”
;
