How all sustainable aviation fuel (SAF) feedstocks and production technologies can play a role in decarbonising aviation
Sustainable aviation fuel (SAF), hydrogen and electric all have a role to play in the decarbonisation of aviation. SAF is vitally important as it can address decarbonisation of fuel over its lifecycle, and this is now available to be used in all turbine engines including in mid and long-range aircraft. Therefore, understanding the feedstock landscape as well as the technology pathways is integral to maximising the production and supply of SAF.
The production of SAF starts with one of five main families of raw materials: oils and fats, sugar and cereal, municipal solid waste, wood and agricultural residue, or renewable energy and carbon (see image 1) used to replace a proportion of the crude oil feedstock.
Each of these feedstocks uses a particular production technology, with each specific technology pathway needing approval from the fuel standard body ASTM before being commercially deployed.
There are two ways of producing SAF, with standalone units or through co-processing. Standalone units use sustainable feedstocks to produce the synthetic kerosene (SK), that is then blended with conventional jet fuel to produce SAF. Whilst producing SAF through co-processing up to 5% sustainable feedstocks being are processed alongside fossil feedstocks through hydro-processing in the refinery.
With standalone units the feedstock is converted in a biorefinery into SK and then certified to the relevant annex in ASTM D7566 standard. The approved technology pathways and associated feedstocks are shown in table 2. This SK is then blended up to 50% with conventional jet fuel and certified to ASTM D1655 or Defence Standard 91-091 and is supplied as a conventional Jet A/ Jet A-1 fuel.
Two of the technology pathways and associated feedstocks are approved to produce SAF through co-processing. These feedstocks are shown in table 3.
Hydrotreated esters and fatty acids (HEFA)
Driven by the lower cost of capital and the availability of feedstocks which are close in energy density to fossil fuels, most of the SAF supplied today is derived using the hydrotreated esters and fatty acids (HEFA) pathway. The primary feedstocks for this conversion pathway include waste fats, oils, and greases and following pre-treatment these can be processed in standard hydrocracker units.
While HEFA synthetic paraffinic kerosene (SPK) is currently the only commercial pathway being used at scale to produce SAF, current feedstocks are limited. There is need for rapid commercial large-scale sustainable feedstock mobilisation. Alternative high energy crops that are being trialled or have already been approved as HEFA feedstocks, include algae, camelina, pennycress, tallow tree and carinata. We promote the use of cover crops (crops grown for the protection and enrichment of the soil), such as carinata, as a feedstock when they do not require additional land demand and contribute to sustainable farming practices – supporting soil carbon accumulation, soil quality and biodiversity. bp has already entered into a long-term strategic offtake and market development agreement with Nuseed to purchase carinata oil.
While an increasing number of flights have been fuelled by SAF produced from the HEFA pathway, limited feedstocks mean we expect to see SAF produced from alcohol to jet (AtJ), Municipal Solid Waste (MSW) and second generation (2G) biomass increasing significantly beyond 2030.
First generation alcohol to jet (AtJ)
Alcohol to jet (AtJ) is another technology that has an approved pathway. It is a method whereby sugary, starchy biomass such as sugarcane and corn grain are converted via fermentation into ethanol or other alcohols which can then be shipped or piped before being converted to fuel. These feedstocks are easy to grow and transport by train, however sugarcane must be processed into ethanol within 48 hours of being cut. To achieve low logistical costs, reduce carbon emissions from transport and make better use of infrastructure, ethanol plants benefit from being placed close to feedstock production mills as well as to refineries.
In some regions, particularly in the Americas, feedstock such as corn and sugarcane are currently commercially used for fuel production. Demand from sectors such as ground fuel and petrochemicals means however that there is limited feedstock available to aviation. As a result, there are no commercial SK plants using the AtJ production pathway.
Timing is therefore the important factor to consider with the AtJ pathway. As ground fuels move more towards electrification this will free up feedstock supply for the aviation sector which in turn will lead to commercial SAF production from this pathway.
Another consideration with the AtJ pathway is that the reduction in carbon intensity is not as strong when compared to the alternative technologies. Implementation of solutions such as carbon capture and storage technology will be key to lowering greenhouse gas emissions (GHG) using this technology. Other options to evaluate include the use of biogas in place of natural gas in mills and converting farm machinery to run on biofuels rather than fossil fuels.
