Responsible Flying

On Wednesday, 1^st December, a United Airlines flight 737 MAX8 with more than 100 passengers took off from Chicago’s O’Hare International Airport to Washington DC. What was remarkable about this flight was that it used 500 gallons of Sustainable Aviation Fuel (SAF) in one of its two engines. The other engine used the same amount of conventional jet fuel to establish that there are no operational differences between the two fuels. Currently airlines are allowed to carry only a maximum of 50% SAF. United Airlines will go down in history as the first commercial carrier to operate a full passenger flight using 100% sustainable fuel. 

SAF is made from renewable biomass and waste resources. It has properties that are similar to conventional jet fuel, but a significantly smaller carbon footprint. Depending on the feedstock used, the reduction in GHG emissions from use of SAF can be as much as 80% compared to traditional jet fuel. Typical feedstocks for SAF are cooking oil, animal waste fat, agricultural residues, municipal solid wastes and animal manure. Other potential sources are forestry waste such as waste wood and fast growing crops and algae.

In order to maintain the same level of engine performance, SAF should have an identical molecular composition as that of traditional jet fuel, which is made up of n-alkanes, iso-alkanes, cycloalkanes and aromatics. The emission profile of an engine operating on SAF would thus be same as that with conventional fuel. However, SAF is almost carbon-neutral over its lifecycle because it is derived from renewable feedstocks.

SAF can be produced in many ways, which are broadly classified under two heads – thermochemical process and biochemical process. A typical thermochemical process is the well-known Fischer-Tropsch synthesis, in which carbon-rich biomass is gasified to produce syngas which is then catalytically converted to liquid fuel. The requirement of high pressure and temperature together with the need for a catalyst makes this an expensive process. Another thermochemical process is pyrolysis, in which the biomass is heated up to 600 degrees C in the absence of oxygen to yield among other things pyrolysis oil. Pyrolysis oil is converted to jet fuel by hydrotreating to eliminate the oxygenated molecules which are detrimental for the fuel. In a typical biochemical route for SAF, alcohols are produced first by fermentation of biomass which are subsequently converted into long-chain hydrocarbons that have similar properties as jet fuel. In another biochemical pathway, sugars are directly transformed to hydrocarbons without the alcohol intermediary.

Presently, ASTM has certified eight processes for producing SAF of acceptable quality. However, only the HEFA (Hydrogenated Esters and Fatty Acids) process has been successfully commercialised as of now and accounts for almost 95% of SAF production. In the HEFA process, waste oils and fats are hydrogenated and then isomerised to yield long-chain hydrocarbons, which are then selectively cracked to produce aviation fuel. The production of SAF is currently not economically favourable. However, SAF is becoming increasingly imperative if the aviation industry has to meet its 2050 target of cutting down emissions by 50%. In USA, the department of energy is working with the department of agriculture and other agencies to develop a strategy for scaling up technologies to produce SAF on a commercial scale.