With the aviation industry world wide scrambling to replace crude oil as the feedstock for turbine fuel production, fuel companies are working furiously in the background to identify and exploit the most sustainable and viable replacement.
But that is not as simple as it sounds; development of sustainable aviation fuel (SAF) means finding raw materials that do the job and are not limited in supply themselves.
One company at the leading edge of SAF development is Air BP, which this week released a paper outlining the five major raw materials that are firming up as the most likely new feedstocks for the aviation fuels of the future. These are the materials that aircraft will be burning in lieu of the avgas and Jet-A1 that is in the bowsers today.
As the paper states, the most likely form of SAF in the short term is a blend of sustainable kerosine (SK) and fossil fuel.
"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 used to replace a proportion of the crude oil feedstock," the Air BP paper states.
"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 stand-alone units or through co-processing. Stand-alone 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.
"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."
HEFA feedstocks include algae, camelina, pennycress, tallow tree and carinata; however, these products are in limited supply, which BP says will likely see more SAF produced from alcohol to jet (AtJ), municipal solid waste (MSW) and second generation (2G) biomass increasing significantly beyond 2030.
Air BP explains that AtJ 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."
According to the World Bank, the world produces no less than 2 billion tonnes of what is termed municipal solid waste (MSW). This is a fancy name for the garbage you put out in your wheelie bin. But according to Air BP, that garbage may hold the key to SK production.
"Access to MSW as a feedstock is widely available across the globe and it is typically a lower cost feedstock than other raw materials," the Air BP paper states. "In some regions aviation is in competition with other sectors, including the energy industry, for access to MSW.
"In the EU, BP is–among other feedstock and technology pathways–advocating for recycled carbon fuels made from the non-organic portion of MSW to be recognised as SAF under the EU’s planned SAF blending mandate.
"We are not however advocating for the use of recycled plastics as a standalone feedstock source for SAF."
Second generation biomass (2G) is material such as woody, non-food crops, animal waste and agricultural residues; material such as stems, stalks and husks that remain after the food has been extracted. Although it appears to be an ideal source for SAF production, Air BP is less than optimistic.
"For these feedstocks there is no pathway that is commercially deployed," the paper states. "However, work is progressing with ASTM for pyrolysis of biomass through both stand-alone production and co-processing in refineries.
"Once a pathway is approved, demo plants would then need to be established to prove the technology at scale before it is commercially deployed.
"While second generation biomass such as agricultural and forestry residue is in vast supply as a feedstock, once aggregated it must be transported by road or rail – as a solid it can’t be moved via pipeline.
"Ultimately this means we’ll end up with small production plants near where forestry residues are processed. While a handful of these plants already exist, none are being used currently to produce SAF."
Air BP is more optimistic about the use of power-to-liquid (PtL) technology, which produces what is called eSAF. This process used renewable electricity such as solar, hydro or wind to extract hydrogen from water using electrolysis.
"This green hydrogen is first used to convert carbon dioxide (from the air, biogenic or industrial sources) to carbon monoxide," the paper says. "Then using Fischer-Tropsch (FT) synthesis technology, this carbon monoxide along with more green hydrogen is converted into a wax that can be upgraded to SK.
"The challenge currently with eSAF technology is cost. To be commercially viable and competitive with conventional jet fuel this fuel (which is expected, in the short-term, to be three to eight times the cost of conventional jet fuel and up to around four times the cost of SAF made from HEFA) needs to be produced at low cost.
"The availability and cost of the renewable energy and carbon dioxide, as well as the expansion and improvement of green hydrogen plants must be addressed to meet market demand."
Carbon dioxide could be sourced as an industrial by-products from manufacturing companies as well as via technology that removes carbon directly from air. Manufacturers are looking for ways to reduce the carbon they emit, and eSAF production may prove the ideal destination. It's a win-win for both.
"In terms of increasing green hydrogen production, as it is required by many industries to decarbonise, aviation will benefit from the focus being placed on increasing production," the paper continues. "In addition, German mandates have specific requirements for eSAF from 2026 and European mandates currently being finalised are expected to follow suit in 2030.
"Longer-term, eSAF will also benefit from on-going work in developing new pathways in addition to those already approved. In particular, methanol to jet could provide a competitive alternative production method for these feedstocks."
With the aviation industry racing down the SAF path, it is clear that companies like Air BP will need to find the right feedstock to be able to first increase SAF supplies to commercial levels, then maintain the demand to provide a reliable source of fuel.
"Aviation is one of the hardest-to-abate sectors when it comes to reducing fuel lifecycle carbon emissions, with SAF currently the only way to decarbonise the industry at pace and at scale," Air BP concludes.
"Utilising a wide range of feedstocks is key to the production of SAF, as is the ongoing evolution of production pathway options.
"BP will continue to work with stakeholders across the energy supply chain as well as governments, NGOs, authorities and other businesses to help meet future SAF demand."