Source: The Economist 2022/08/20
https://www.economist.com/science-and-technology/2022/08/17/ways-to-make-aviation-fuel-green
Extrait
Travelling by air is by no means the biggest source of anthropogenic greenhouse-gases. At the moment, it contributes about 2.5% of them. But, after a covid-induced dip, air travel is once again growing (see chart), and its emissions are high-profile and hard to deal with. For short-range, small-capacity planes batteries show some immediate promise. But for bigger aircraft the technofantasy of using compressed hydrogen (made from green sources, natch) either as jet fuel in its own right, or to run fuel cells which then drive electric motors, is likely to remain just that—a fantasy—for decades.
Ideally, this CO2 will have come recently from the atmosphere, so that when it returns to the air no net greenhouse effect is created. At minimum, though, it will be extracted from the exhaust of an industrial process, enabling it at least to substitute for fossil-fuel emissions elsewhere before it is released.
Do this at a price competitive with stuff from a refinery and the world is your oyster. Airlines have already undertaken some 450,000 flights using saf as part of the fuel mix. The industry aims to be carbon neutral by 2050, and so far saf seems the only practical way to get there. But making saf is an uphill task. Stripped to its chemical essentials it means taking the equivalent of engine exhaust and turning it into something resembling what went into that engine in the first place. This requires “fixing” the carbon of carbon dioxide into big energy-rich molecules. Not surprisingly, the result is around three times as costly as ordinary jet fuel.
Such saf as has so far made it to market relies on photosynthesis to do the carbon fixing. It is derived from discarded cooking oil and animal fats, the triglyceride molecules of which trace their existence to the action of sunlight on chlorophyll.
To make saf, triglycerides are hydrotreated, an established way of producing biodiesel for ground transport. A triglyceride molecule consist of three hydrocarbon tails attached to an oxygen-containing head. Hydrotreating combines the oxygen with hydrogen, to yield water. This liberates the tails and turns the head into a molecule of propane. The liberated tails can then be processed into drop-ins.
https://www.neste.com/products/all-products/saf/key-benefits?