Sustainable aviation fuel

Sustainable aviation fuel is a broad term for aviation fuels made through approved pathways from non-fossil or other eligible sources. Feedstocks can include used fats, oils and greases, biomass residues, municipal waste fractions, and, in some pathways, alcohols or renewable energy and carbon. SAF is designed to meet aviation fuel specifications; many approved synthetic components are blended with conventional jet fuel before being delivered through existing fuel systems. The permitted blend fraction depends on the pathway and applicable standards, so there is no single blend limit for all SAF.

Different processes convert feedstocks into hydrocarbons suitable for aircraft turbines. Examples include hydroprocessed esters and fatty acids (HEFA), gasification followed by Fischer–Tropsch synthesis, and alcohol-to-jet conversion. Other routes use sugars, fats or oils, or waste-derived intermediates. Pathway approval includes technical evaluation of fuel properties and compatibility; the existence of a promising feedstock or process does not by itself mean a fuel is approved for commercial use at any blend level.

A fuel's climate performance is evaluated over its lifecycle, not only at the aircraft engine. Under the International Civil Aviation Organization's CORSIA framework, the lifecycle value can include emissions from cultivating or collecting feedstocks, processing and transport, fuel conversion and distribution, and combustion. It may also account for direct or indirect land-use change; certain municipal-solid-waste fuels may qualify for specified optional credits. Results therefore depend on the feedstock, conversion process, production region and assessment method. Sustainability certification and eligibility criteria are intended to constrain harms and support credible emissions claims. SAF still emits carbon dioxide when burned, and lifecycle reductions are not identical for all products.

SAF can be used within established aircraft-fuel systems when it meets the relevant technical specifications and is blended as required. This compatibility can make it a near-term option for reducing aviation's lifecycle emissions, especially where electrification is difficult for long-distance flights. However, present supply is limited relative to total aviation fuel demand, and production can face high costs, competition for suitable feedstocks, investment needs and infrastructure constraints. Announced plant capacity is not the same as actual output: projects may be delayed, produce other fuels, or operate below design capacity. SAF is one component of aviation decarbonization, not a substitute for efficiency, operational changes or other technologies.

ICAO's Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) recognizes eligible sustainable aviation fuels and lower-carbon aviation fuels for specified emissions-accounting purposes, subject to sustainability certification and lifecycle rules. Operators must substantiate eligible-fuel claims under the scheme's monitoring, reporting and verification requirements. Certification and accounting frameworks help distinguish qualifying fuel from any alternative fuel marketed as sustainable, and allow claimed reductions to be assessed consistently.