Ever tried to save a broken item, so you don't have to replace it or change your habits? For some, this involves sewing up a pair of holey jeans over and over again. Others continue to listen to music on an old iPod, even though smartphones are super sophisticated and there are zillions of music streaming platforms. The charger doesn’t really work though, so it has to be charged at a funny angle now. For some, it means driving around with an old banger, repairing it with parts from a bygone era, to avoid switching to an electric car. Evidence of the aviation industry clinging to old technology is the SAF: Sustainable Aviation Fuels - which is, as its name suggests, sustainable fuels for aviation, which are an alternative to kerosene.
Sustainable aviation fuels are an attempt to keep 23,000 planes in the sky, but with a lower ecological cost, since they’re meant to be more sustainable. “There are three generations of bio-sourced fuels: biofuels which use the starchy or oleaginous reserves of plants (grain), biofuels resulting from the conversion of support tissues formed of cellulose and lignin (straw), and finally photosynthetic microorganisms such as algae, which are capable of synthesising large quantities of carbon molecules, particularly oils,” says Charlotte de Lorgeril, Energy Partner at Sia Partners. “The first generation is industrially mature and certified for lipid raw materials: hydrogenated oils and fats, also called HEFA (Hydroprocessed Esters and Fatty Acids).”
Two other processes are still in the development phase, explains de Lorgeril. “The first is biological processes for converting sugars, xyloses and alcohols, also called ATJ (Alcohol to Jet),” she says. “The other is gasification-synthesis processes also called FT (Fischer-Tropsch), which consist of converting lignocellulosic biomass (wood residues, cereal straw, forest waste, etc.) into synthetic fuels.” Beyond biofuels, there are also e-fuels, which are completely synthetic fuels. “Synthetic e-fuels are produced based on hydrogen,” says de Lorgeril. “First, we produce “green” hydrogen from renewable energies, which we then mix with CO2 to obtain a synthetic fuel.” This is based on a process called PtL (Power to Liquid) which is still in the research and development phase, but consists of producing the biofuel by electrolysis of carbon dioxide captured in the atmosphere or in industrial gas residues.
There you go - it’s a bit technical, but you get the idea. There are generally two main categories of SAF - the first includes fuels made from biomass and waste, the second are synthetic. It’s important to understand that SAF is versatile, but its production processes are complex and the sectors are still largely disparate. Obviously, when humans are producing so many fuels, it's a struggle to know what’s possible and what we can do with all the different kinds of waste. It’s hard to offer a solid sustainable solution.
Despite their differences, sustainable aviation fuels all have one thing in common - they’re intended to be sustainable. As Gro Harlem Brundtland, former Prime Minister of Norway, said: it’s about responding “to the needs of the present, without compromising the ability of future generations to meet their own needs.” Except that of course, just saying we’re sustainable isn’t good enough. “Standards and certifications, such as the ISCC-EU (International Sustainability and Carbon Certification), can be used to assess and ensure the sustainability of SAF,” says de Lorgeril. “It’s important to note that stricter standards and technological advances continue to improve the sustainability of SAF, however producing fuels usable in aviation is complex.” Fuels must meet numerous criteria (energy density, molecular density, melting points, etc.) in order to meet aviation performance and safety requirements. And to produce SAF, de Lorgeril explains that “these specifications must be respected, which makes production very complex, and most often requires mixing SAF with kerosene or conventional aviation fuel.”
Most SAF, despite the promise of partial decarbonation of aviation, must be mixed with good old kerosene, the extraction of which is still terrible for the planet. Therefore the future of sustainable aviation is dubious - at least until we’ve really interrogated their sustainable credentials.
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