A Clearer Flight Plan for Tackling Aviation’s Climate Impact
Air travel represents around 2.5% of global CO2 emissions – a proportion that is likely to grow if air travel continues on a business-as-usual trajectory, and as other sectors start to decarbonize. While many forms of land-based transport can rely on electricity via batteries or fuel cells, aviation, particularly long-haul aviation, relies on high energy-density fuel, which cannot be easily substituted by the battery technology of today, so scaling up sustainable aviation fuel (SAF), which can offer up to 100% net emission reductions and work with existing fleets, has emerged as the best option for the sector.
In response, we’ve seen intense innovation in the SAF industry, with major breakthroughs in production technology and new feedstock utilization pathways – as well as widespread policy support, as countries around the world have established SAF mandates and incentives, requiring flights to start using small, but increasing, percentages of SAF.
But the required scaling curve is steep – in an assessment published in Bloomberg, SAF supply needs to grow 16-fold by 2030 to stay on track to meet the industry’s net-zero goals.
The Feedstock Challenge
One of the biggest challenges with SAF is the search for the right sources of sustainable feedstocks, with many SAF projects struggling with feedstocks that are too expensive, too scarce, or both.
Today, SAF is overwhelmingly produced using the hydro-processed esters and fatty acids (HEFA) process, which converts fats, oils, and greases from sources like used cooking oil (UCO) and animal fats. Some SAF is also produced via the HEFA process using crop-based feeds like canola oil and soybean oil, but because such feeds compete with food production, these are usually not considered part of the long-term SAF solution. HEFA-based
SAF has paved the way for the industry, but there are already feedstock constraints for this process and SAF adoption is still in the low single-digit percent range.
China, the world’s largest producer of UCO is on track to run out in the near future. Pressure on the supply chain not only means HEFA feedstocks are more expensive, but it also creates a strong incentive towards fraudulent behaviours, like bad actors classifying virgin oils as UCO. HEFA fuels have created the foundation for the industry – but those looking to scale up SAF production ethically are struggling to meet demand using waste streams like UCO alone.
Developing new (and more scalable) carbon feedstock streams, and the process technologies to convert them cost-efficiently into drop-in liquid fuel, is therefore a core requirement of the next phase of SAF growth.
Broadening Horizons
In response to the supply challenges with UCO-derived fuels, the industry has begun exploring using green hydrogen, combined with captured CO2, to produce “e-fuels” as the ultimate solution for SAF, because the feedstocks are functionally unlimited. E-fuel boosters argue that such theoretical feedstock abundance means that no other pathways are needed, and in the long run there will only be e-fuels. E-fuel skeptics on the other hand argue that high cost of green hydrogen makes e-fuels prohibitively expensive, and argue that it will be decades, if ever, that e-fuels make economic sense (when compared to other sustainable fuel routes.)
Underlying the sceptics’ concerns, however, is the vast amount of renewable power that e-fuels require, and the increasing market competition for such renewable power, from, e.g., electric vehicles and data centers – much larger and more powerful industries. As is often the case, the future reality will likely be more complex – and more than one feedstock will be needed.
This argues for the deployment of versatile technologies capable of processing a variety of feedstocks, including green H2 and CO2 (as in the e-fuel process), but also sustainably sourced CO and CH4 and bio-derived H2 which in many places are more readily available at reasonable costs today. Such streams can be obtained from forestry and agricultural residues, municipal solid waste and biogas. Taking a diversified approach will enable the SAF industry to scale production faster and more cost effectively in the medium term than if it remains focused solely on HEFA or e-fuel, which is critical to insuring the nascent recent growth doesn’t stall.
Further Optimizing the Fischer-Tropsch Process
One key way to scale SAF economically is to further optimize, intensify and simplify the well-established Fischer Tropsch (FT) process through Aether Fuel’s Aurora technology. There are three basic steps to producing SAF through the FT process: syngas generation (where the feed gases are converted into H2 and CO), the FT step (where the syngas is converted into raw hydrocarbons), and upgrading (where the raw hydrocarbons are converted into finished fuels).
Over the last decade, the FT step has seen great innovation at the scales relevant to sustainable fuel production, such that it no longer dominates the plant capital expenditure for sustainable fuel projects. With our Aurora technology, we slash the CAPEX of the syngas generation and upgrading steps, so that combined with state of the art FT, we can realize dramatically lower overall costs at the right scale to make use of a diverse range of feedstocks.
Aether’s Aurora technology was also engineered from the beginning for high feedstock flexibility, and can utilize six different types of input streams:
Biogas from anaerobically digested biomass waste
Forestry waste, such as forestry residues (e.g. slashes or thinnings) and wood processing waste (e.g. bark, shavings, and sawdust)
Agricultural waste such as waste from sugar cane production (e.g. bagasse), vegetable oil production (e.g. empty fruit bunches and fronds from oil palm processing), grain production (e.g. rice husks, wheat straw, corn stover/stalks), or natural fabric production (e.g. cotton or hemp stalks)
CO2 and Hydrogen (H2)
Industrial off-gases, produced by various industrial processes (like steel production, chemical production, and oil refining)
Municipal solid waste
All of these feedstocks require certain kinds of preprocessing using existing technology to get them into a gas phase form suitable for feeding into a plant. In some cases, the input streams are already gas phase and may require just clean-up to remove certain trace impurities. In other cases, the input streams are solids and require gasification (a known process for making a mixture of CO2, CO, CH4 and H2 gases from solids) followed by clean-up. With this technology, we then further process the gas in our electric Tri-Converter, followed by third-party FT conversion into raw hydrocarbons and our proprietary upgrading, with by-products recycled to the Tri-converter to maximize liquid hydrocarbon fuel yield.
As Aether Aurora can efficiently convert CO2 and H2 feeds, it is ideally suited to produce e-fuel and scale e-fuel production commercially once DAC CO2 and green H2 costs come down. At the same time, it can also be applied (and crucially, scaled commercially) today using other feed stocks that are already available and cost effective right now.
This means the industry can get access to more economical sustainable fuels in the near term, and we can start accumulating the critical learning (and resulting cost efficiencies) that will come from repeat builds of production facilities.
SAF has a steep slope to climb to scale up; but the route the industry needs to follow is becoming much clearer.
