Sustainable aviation fuel – what’s in the tank?

written by Air bp | November 20, 2023

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 is now available for use in all turbine engine aircraft.

Australia has enormous potential to develop a homegrown SAF industry. bp’s Kwinana Renewable Fuels project in Western Australia plans to produce SAF by 2026. The KRF project is expected to produce 10 thousand barrels per day of renewable fuels (which will include SAF and other products). The KRF project will leverage existing infrastructure from bp’s former refinery on the site, which is already integrated with tankage, pipelines, and terminal operations, including into Perth Airport.

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.

Click the diagram to enlarge

Each of these feedstocks uses a particular production technology, all of which require 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. When producing SAF through co-processing, up to 5 per cent sustainable feedstocks are being processed alongside fossil feedstocks through hydro-processing in the refinery.

Hydroprocessed esters and fatty acids (HEFA)

Driven by the lower capital costs and the availability of feedstocks, which are close in energy density to fossil fuels, most of the SAF supplied today is derived using the HEFA pathway. The primary feedstocks for this conversion pathway include waste fats, oils, and greases. Following pre-treatment, these can be processed in standard hydrocracker units.

One of the challenges of SAF produced from HEFA is that current feedstocks are limited. Alternative high energy crops that are being trialled or have already been approved as HEFA feedstocks, include algae, camelina, pennycress, tallow tree and carinata.

First-generation alcohol to jet (AtJ)

The AtJ pathway uses 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.

Demand from sectors such as ground fuels and petrochemicals currently mean there is limited feedstock available to aviation. As ground fuels move more towards electrification, this may free up feedstock supply for the aviation sector.

Fischer-Tropsch (FT) for municipal solid waste (MSW)

For SAF produced from MSW using FT technology, an advantage is the fact that it uses waste which may otherwise be left to decompose in landfill sites.

Although a capital-intensive process to get the infrastructure in place, there is ongoing work to research and develop technologies that will lead to more efficient production using this pathway.

Technologies to convert 2nd generation biomass

For these feedstocks, there is no pathway that is currently commercially deployed. However, work is progressing with ASTM for pyrolysis of biomass through both standalone production and co-processing in refineries.

eSAF: Fischer-Tropsch (FT) for power-to-liquid (PtL)

Possibly one of the most promising pathways for SAF in the longer term is PtL technology (producing what is called eSAF), which is still very much in its infancy. Renewable electricity (from sources such as solar, hydro or wind) is used in an electrolysis process to extract hydrogen from water. This green hydrogen is first used to convert carbon dioxide (from the air, biogenic or industrial sources) to carbon monoxide. Then, using 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. The availability and cost of renewable energy and carbon dioxide, as well as the expansion and improvement of green hydrogen plants, must be addressed to meet market demand.

In summary, 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. Utilising a wide range of feedstocks is key to the production of SAF, as is the ongoing evolution of production pathway options. Air bp will continue to work alongside stakeholders, governments and NGOs, and others to help meet future SAF demand.

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