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Sustainable Aviation Fuels (SAF): A Path to Cleaner Skies

29th August 2023

Sustainable aviation fuels could modernise aviation and its emissions

The aviation industry is responsible for 3.5 per cent of annual global warming, calculated using radiative forcing. Greenhouse gas (GHG) emissions, non-renewable feedstocks and volatile operational costs are all issues associated with traditional aviation fuels. Here, Paul Vanden Branden, director at SciMed, the UK’s largest distributor of lab equipment, discusses the key test methods used to demonstrate sustainable aviation fuels’ (SAFs) suitability to mitigate the environmental impact of air travel.

Traditional aircraft turbine fuel (ATF) is derived from crude oil and is mostly comprised of kerosenes, C9-16 hydrocarbon chains. It also has up to 25 per cent aromatic compounds, such as alkylbenzenes and naphthalenes. Although essential for ATF compatibility with engine combustion systems and maintaining fuel energy density, increasing aromatic content causes a proportional increase in particulate matter (PM) emissions.

Due to the environmental problems caused by ATF, SAFs are held up by political and industrial leaders as a route towards sustainable global aviation. SAFs are still carbon-based fuels and produce CO2 upon combustion, but they’re sourced from renewable carbon sources, such as biomass, used cooking oil and even algae.

Each feedstock is at a different stage of development, but, regardless of starting material, investment in scaling up SAF manufacturing is crucial. Although the volume produced internationally in 2022 tripled from the previous year, 300 million tons up from 100 million, serious action is required to meet EU quotas for SAF to make up 70 per cent of overall aviation fuels at EU airports by 2050. That number is currently 0.05 per cent.

Increasing production is only half the battle. SAFs are “drop-in” fuels, meaning they’re interchangeable with petroleum fuels in transport, blending and combustion. To ensure long-term uptake of sustainable alternatives to kerosene-based fuel, SAF manufacturers have to demonstrate that the quality and physicochemical properties are at least comparable. To do this, there are multiple ASTM methods to satisfy.

Principal quality tests

Lubricity is a key property for fuel. The ball on cylinder lubrication evaluation (BOCLE) adheres to ASTM D5001 and determines the lubrication function. Under controlled conditions, a steel ball is pressed against the outer surface of a rotating ring and immersed in a bath of the test fuel. After rotating the test ring for 30 minutes, the test stops and the wear scar on the test ball is measured.

Insufficient lubricity can lead to reduced fuel system efficiency, fuel pump damage and even safety risks during critical flight stages, when the engine demands optimal performance.

Assessing fuel corrosivity is also important to inform fuel storage and transportation decisions. The copper corrosion test, common in the petroleum industry, adheres to ASTM D130. By placing a glass tube filled with the test fuel and a copper strip into a steel pressure vessel then warming to a stable temperature in a water bath, fuel manufacturers can simulate long-term conditions to evaluate the fuel’s ability to corrode its environment.

The analyst compares the copper strip to an ASTM colour chart to assess the relative degree of corrosion. The steel pressure vessel simulates the different conditions that the fuel would experience at various altitudes during flight.

One final metric used to investigate sustainable aviation fuels’ suitability is ignition temperature testing, conducted in adherence with ASTM E659. Unlike flash point testing, the ignition temperature is one reached without an ignition source. It’s defined as the lowest temperature at which fuel will be vapourised into a gas that ignites without an external flame or ignition source.

The years leading to the 2050 production tipping point will be key for SAF manufacturers. Ensuring that their products meet the high regulatory standards set in the aviation sector is a crucial part of bringing one of the world’s highest-polluting industries into a new, more sustainable era.

What next?

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