Towards jet zero

As the aviation industry searches for ways to reduce its carbon footprint, an unlikely solution is taking flight – grease, recycled cooking oil, and animal fats. This recycled kitchen byproduct is proving to be an innovative option to allow air travel to take a greener flight path.

Currently, aviation is responsible for around 2 per cent of global CO2 emissions.¹ With demand for flying expected to increase by the middle of the 21st century, this could grow significantly without action. In response, the industry has adopted the goal of net zero emissions by 2050.

It’s not an easy goal. One of the solutions could be sustainable aviation fuel (SAF) – a class of biofuel made from plant or animal materials rather than fossil sources. A biofuel is considered sustainable if its ingredients do not compete with food crops or resource usage, and more broadly, do not engender environmental challenges such as deforestation, soil productivity loss, or biodiversity loss. 

“We have to make sure we’re not investing in something that’s actually worse than fossil fuels,” says Jane O’Malley, a researcher at the International Council on Clean Transportation.

Materials that avoid these pitfalls include waste fats, oils and greases, municipal solid waste, and agricultural and forestry residues. Some additional emissions are released during SAF production due to the energy needed to transport raw materials and refine the fuel; however, overall, it has the potential to cut CO2 emissions from aviation by 80 per cent, making significant headway towards meeting the sector’s net zero goals.²

Other fossil fuel alternatives, like electric aircraft and hydrogen power, will also likely play a role in a cleaner aviation sector, but they are less feasible in the short term and in the context of existing infrastructure. It may take until 2030 or later for commercial electric aircraft to be technically and financially feasible. Even then, such flights are likely to be only short range as the relatively modest energy capacity of batteries significantly limits flight range. Distance is also an issue for hydrogen, as storing it in liquid form is difficult for long-haul flights.

By contrast, SAF is a ‘drop-in’ replacement for aviation fuel, meaning that it is compatible with existing aircraft engines and fuelling infrastructure.

“We need technologies that leverage the existing infrastructure and feedstocks, specifically those that don’t have worse impacts on the environment,” according to Derek Vardon, CTO of Alder Renewables, a US-based SAF company.

Tim Obitts, the company’s CEO, agrees: “We need to have a 100 per cent drop-in solution. We can’t have two different infrastructures at airports.”

Alder Renewables aims to solve this with a sustainable fuel that can be processed in traditional oil refineries. “Real, economically viable SAF production is going to take off once the refineries enter the market,” explains Obitts.

Production pathways

Various processes can convert feedstocks into SAF, but the only one currently used at scale is HEFA (Hydrotreated Esters and Fatty Acids), which refines fats, oils, and grease into fuel and has the highest conversion rate of any pathway at 90 per cent. Finnish company Neste, for example, produces fuel from cooking oil and animal waste fats, which is available at airports worldwide. Air BP is exploring the same process.

“Biofuels made from oils can essentially be produced using existing refinery infrastructure because they’re quite similar to the hydrocarbons you get from fossil petroleum,” says O’Malley. “Some companies are already working to reconvert their facilities.”

As there is a limited supply of fat, oil, and grease, a broader range of technologies will be needed. One is the alcohol-to-jet (AtJ) pathway, taking sugary or starchy biomass such as sugarcane, sugar beet, plant dry matter, and corn grain, converts this into ethanol or other alcohols, and then converts these to fuel. Another is the Fischer Tropsch (FT) pathway, which involves converting carbon materials, such as municipal solid waste, coal, ash, and sawdust, into fuel. Alder Renewables is exploring using agricultural residues, wood waste products, and non-food crops such as miscanthus – a tall ornamental flowering grass – for SAF products.

Scale or fail

Once produced, SAF still needs to be blended with traditional aviation fuel. The maximum blend rate allowed on commercial flights is 50 per cent, but there are hopes that airlines will be able to use 100 per cent SAF by 2030. In November 2023, Virgin Atlantic flew the first-ever flight with 100 per cent SAF from London Heathrow to New York JFK, under special dispensation and with no paying passengers. It was powered by 88 per cent HEFA made from waste fats and 12 per cent synthetic kerosene made from plant sugars.

In 2023, SAF production tripled to 600 million litres.³ However, this still accounts for just 0.2 per cent of aviation fuel use – compared to a goal of 19 per cent by 2040, under International Energy Agency’s sustainable development scenario.⁴

The main obstacles are the high costs associated with innovation and production methods; none is yet cost-competitive with traditional jet fuel. There are initiatives underway to make the switch less painful. In a recent revision of the EU Emissions Trading Scheme (ETS), funds paid into the scheme by airlines will now be reinvested into the market as SAF allowances for airlines seeking to replace conventional fuel with SAF. These allowances will cover as much as 95 per cent of the cost differential between SAF and traditional fuels. The US Congress, meanwhile, has proposed the Sustainable Skies Act to incentivise SAF use through a tax credit and a grant to expand SAF production. There is also significant interest in Asia; China has exported over 1 million megatonnes of used cooking oil for SAF production, including substantial amounts to Malaysia and Singapore. Indonesia is looking to develop domestic processing of used cooking oil.

Another challenge is that SAF has a lower energy density than traditional jet fuel, so planes storing the same amount of fuel will not be able to fly as far – albeit still further than electric planes, based on current technology.

Then there is the process required to approve new aviation technologies, which are covered by the American Society for Testing and Materials (ASTM), a voluntary global standards organisation led by technical experts and business professionals in 140 countries. “The process of getting fuel approved through ASTM takes years, and boatloads of money, so it’s hard for small companies to move quickly,” says Obitts. “Airlines don’t want to pay any more right now unless they’re forced to. It’s a bit of a David and Goliath situation when new technologies try to emerge and go forward, because refineries are also very conservative in their approach.” 

Ultimately, SAF will only be one part of the solution for decarbonising air transport, but it could be a crucial part. “The path to net zero will be made up of a diversified mix of feedstocks,” says O’Malley.