- Sustainable air fuel (SAF) offers a reduction in carbon emissions over the aviation lifecycle
- Downsides remain in the form of production emissions, land use opportunity costs and the compounded effects of emitting at altitude
- However, modern fuels are drastically reducing these, bringing low-carbon flights closer to reality
Sustainable aviation fuel (SAF) is the umbrella term used by the aviation industry to describe alternative fuel types for powering jet flights. It includes sustainable biofuel (from animal and plant material) as well as sustainable fuels from non-biological sources. Some biofuels are not defined as SAF due to their negative environmental impacts (more on these below). The International Civil Aviation Organization (ICAO) defines “Alternative Fuels” as “any fuel that has the potential to generate lower carbon emissions than conventional kerosene on a life cycle basis.”
Aircraft flights cause between 2% and 5% of global warming. Traditional SAFs reduce the life cycle carbon impact of flights by using fuels whose carbon has already been absorbed from the atmosphere by plants and then converted, either directly or via animal consumption, into fuel. The chemistry of SAF is similar to conventional jet fuel; it’s the sourcing that makes the difference, with many derived from cooking oil or other non-palm waste oils, as well as solid waste from homes and businesses or food scraps that would otherwise go to landfill or be incinerated.
The sky, the limits
Biofuels are not necessarily good news; all have some environmental drawbacks, many of these serious. Fuels derived from palm oil, for example, the cultivation of which is responsible for extensive global rainforest deforestation, can’t be described as “sustainable”. Even the most sustainable alternative fuels, derived from waste or by-products, have negative impacts on climate change thanks to the equipment involved in growing and transporting feed crops, as well as opportunity costs of the land use that produces them. The ICAO’s definition of an alternative fuel includes any fuel that lowers carbon emissions relative to kerosene over its life cycle, and the extent of these reductions varies greatly depending on the fuel in question and how it’s been produced.
Additionally, combusting hydrocarbons at altitude has been shown to have a disproportionate effect on warming compared to combustion at ground level, so until commercial electric flights (powered by clean electricity) become a reality, no flight should be considered 100% clean, especially since most SAFs require mixing with conventional kerosene fuels.
Suppose, however, you could produce a fuel that directly replaced conventional fuels without requiring mixing, and that rather than biofuel crops requiring extensive land use, the carbon in this fuel was pulled straight from the air? Suppose, again, that the electricity to power this operation came entirely from renewable, low-carbon sources? Prometheus Fuel claim to have achieved exactly this, describing their fuel as zero-carbon, because its carbon content is extracted directly from the atmosphere and 100% of the energy used to produce it is derived from renewables like solar and wind.
Prometheus has announced a partnership to power test flights for Boom Supersonic, an aviation company hoping to reinvent supersonic air travel using rigorous sustainability principles. Ground tests using an 80% SAF blend from Prometheus were carried out in January 2019, and air tests for Boom’s XB-1 demonstrator craft are chalked for 2021. Boom claims these will be carbon neutral thanks to a combination of aircraft design, SAF, flight routing and carbon offsetting*. Boom hopes to commence commercial operations in its Overture vessel from 2026.
A caveat; however accurate Boom and Prometheus’s claims to carbon-neutrality, the technology still involves hydrocarbon combustion at altitude. Since studies have shown that nitrous oxide emissions and contrail vapour from aircraft engines generate more warming than all their carbon emissions combined, this doesn’t yet spell clean airline travel. However, with Prometheus promising upcoming technologies to permanently sequester atmospheric CO2, the way is open to carbon-negative technologies in future, and a reduction in emissions while electric aircraft technology is scaled.
*Carbon offsetting schemes have their own drawbacks from sustainability perspectives, which will be discussed by Cleu in future. There is as yet little information on exactly how much carbon Boom plans to offset in order to meet its carbon neutral claims.