Answering questions about SAF benefits

Sustainable Aviation Fuels (SAF) are claimed to reduce jet CO2 emissions by up to 80%.  Yet it is also argued that SAFs are not a “clean” aviation solution.  How can these contradictory claims be reconciled?

Fuels described as Sustainable Aviation Fuels are biofuels derived from crops.  When burned, they are not clean, as they create as much CO2 as petroleum-based fuels.  Their benefit derives from the CO2 absorbed by crops, such as corn, when grown.  In principle, growing SAF precursor crops absorbs as much CO2 as is later emitted when burned as fuel.  Various crops, and even wood, can be processed into aviation fuel.  Growing crops requires fertilizer, irrigation, and fuel for farm equipment, and subsequent processing requires chemicals and energy.  In the optimal case, where all these requirements are met with carbon-free processes (which is certainly not true today), the resulting SAF could be as much as 80% lower in CO2 than petroleum in lifecycle emissions.  

While an 80% reduction in CO2 appears beneficial, there are a number of practical issues that work together to eliminate this benefit, and even cause SAF to have a large greenhouse gas penalty when compared with fossil-based fuel. 

Processing SAF reduces its benefit

One factor affecting the benefit of SAF is biofuel processing.  SAF based on used cooking oil can result in nearly 80% less CO2 when compared with fossil jet fuel. Cooking oil is a SAF precursor similar to jet fuel, requiring less processing than other sources, such as wood, grass, or corn.    However, the worldwide supply of used cooking oil is infinitesimal compared with the volume of aviation requirements.  Processing of practical feedstocks for SAF, such as corn or soy, requires significant additional heat energy, and the overall benefit of such SAF is significantly less due to CO2 from the energy supply.  While 80% CO2 reduction is theoretically possible using used cooking oil, it is only available to replace a tiny fraction of aviation use.

Another factor affecting the benefits of SAF is the nature of the energy sources used in their creation.  Crops require fertilizer, which creates CO2 when manufactured.  Harvesting and transporting crops require machines that produce CO2.  The processing of crops into SAF requires a large amount of heat and electrical energy.   If this energy is provided using today’s sources, the amount of CO2 produced in processing will cancel most or all of the CO2 benefit of SAF based on crops.

The proposed answer to the processing energy problem is to ensure that all SAF processing energy is derived from low-carbon sources.    In theory, every part of the production process could be powered by carbon-free electricity.  This is certainly feasible if, at some point in the future, we have a fully decarbonized electrical power system, and have converted all industrial heat processes to electricity.  However, until that time, any low-carbon energy resource diverted to new SAF production is a resource that is not being used to decarbonize the existing uses, and those uses must continue to depend on fossil energy.  Therefore, SAF processing cannot be considered a net benefit to greenhouse gas generation until the entire grid is decarbonized, which now seems unlikely before 2100. 

Finally, there is considerable confusion about exactly what commercial SAF is today.  Any SAF purchased today is not pure SAF but is rather a blend of SAF and fossil fuel.   Fuel sold as “SAF” today may be only 10-20% actual SAF.  Therefore, SAF as sold today is mainly fossil petroleum, and any potential CO2 benefit ascribed to pure SAF must be divided by a factor of 5-10.

Growing SAF requires major land use changes

The above factors make any near-term reductions in CO2 by using SAF implausible.  However, the biggest problem, which makes SAF much worse than fossil jet fuel, is the land use area requirement and its consequences.  A single Boeing 737 using SAF would consume approximately 10,000 Tons of SAF per year, requiring 40,000 acres of cropland per year.  All commercial flights together are projected to consume nearly 100 billion gallons of fuel per year, and if they all used SAF, it would require nearly 2 billion acres of arable land, an area representing nearly ½ of the world’s available cropland.  Such a use of dwindling farmland would displace and disrupt food supplies.  The only feasible solution to this problem is to create extensive new cropland for SAF by cutting down forests.

SAF from cropland created by displacing forest is much worse for the environment than petroleum jet fuel.   Forests are natural absorbers of CO2, removing approximately 2 Tons of CO2 per acre per year.  Cutting down the amount of forest to supply a single Boeing 737 for a year causes a permanent increase of 80,000 tons of CO2 per year.  The same Boeing 737 using fossil jet fuel only generates about 30,000 tons of CO2 per year.  Therefore, any aircraft supplied from SAF that displaces forests contributes more than twice as much CO2 as an aircraft powered by fossil jet fuel!

Synthetic SAF

Another proposed approach to SAF is to synthesize jet fuel directly from the constituent elements without crops.  In theory, carbon-free energy could be used in an industrial process to assemble jet fuel molecules directly from carbon dioxide taken from the air.  Such fuel would be low-carbon and not affect land use.  This remains hypothetical and, as currently envisioned, would require very large amounts of energy.  No SAF based on this approach is currently produced, and even if it were demonstrated, it would not provide benefit until the entire electrical and industrial heat system were decarbonized, which is unlikely before 2100. 

Summary

SAF, as produced in tiny quantities today, does not reduce CO2 effects due to processing-related emissions.  If SAF were produced at a sufficient quantity to supply the aviation industry, it would require a huge amount of cropland, which would either directly replace forests or push some other food crop to replace forests.  The loss of carbon sequestration caused by the destruction of forests completely overwhelms any theoretical benefit of SAF.  The result is that any benefit claimed for SAF is more than cancelled out by the elimination of carbon-absorbing forests.