This week the UK government welcomed the first transatlantic flight by a commercial airliner using 100% Sustainable Aviation Fuel (SAF). Prime Minister Sunak said ‘SAF is primarily made of waste oils and fats. … SAF will be key to decarbonizing aviation. .. It could create a UK industry with an annual turnover of almost £2.5 billion, which could support over 5000 UK jobs’.[1]

Unfortunately, this isn’t correct. Aviation fuel made from waste oil and fats is not zero carbon. Perhaps more importantly, the quantities available for use in the UK and elsewhere are not sufficient to ‘decarbonize aviation’. And published official reports show that the government knows this. The actual share of aviation needs that can be met by these two sources is almost certainly less than 2%, even if these raw materials are entirely used for this purpose, rather than existing uses. Mr Sunak estimates the potential industry value at £2.5 billion but even if all the UK’s waste oil was used for aviation, the size would be about a tenth of this number.

How much waste cooking oil is potentially available in the UK?

In 2013, consultants Ecofys produced a report, then published by the Department for Transport, that estimated that the total volume of used cooking oil (usually called UCO) produced in the UK was about 250 million litres.[2] This figure was taken from estimates produced by the UK Sustainable Biofuels Association and submitted to the House of Commons.[3]

Most of this UCO, then and now, is used to make biodiesel for road use. And not all is collected for reuse. But let’s assume that all the 250 million litres would be available for aviation. Put through the most efficient process, this would turn into about 160,000 tonnes of aviation fuel.

The total current demand at UK airports runs at about 12.2 million tonnes. So UK-sourced UCO could produce about 1.3% of the country’s needs. But, to stress the point, this is assuming that every litre produced was efficiently turned into aviation fuel with no losses. Every single takeaway in the country, every restaurant and catering establishment would have to devote all its UCO to one particular use. Biodiesel and other uses have no access to the UCO even though at the moment, for example, McDonalds uses its own waste oil for biodiesel for its distribution fleet.

What do other government reports say about the maximum availability of UCO?

In 2017, the government’s business and energy department, then called BEIS, asked Ricardo to estimate the real availability of all forms of waste biomass in 2030.[4] (UCO is included as waste biomass because cooking oil is made from oil seeds such as rapeseed).

The consultants reported that the energy value of all UCO available for use was 7 Petajoules (PJ). This was described as ‘the accessible resource in 2030, if no barriers to supply are overcome’. If all these barriers were surmounted, the number rises to 9 PJ.

In an efficient process, 90% of the energy value of UCO can be converted to aviation fuel. That means that the maximum energy available would be 8.1 PJ, equivalent to 2.25 terawatt hours (TWh). The energy value of all the aviation fuel used in the UK is about 145 TWh, implying that UCO could provide about 1.45% of the total requirement if all is devoted to aviation fuel. That’s slightly more than the Ecofys figure of about 1.3%.

What might be the actual amount that the aviation industry could use?

In a consultation document published earlier this year, the Department for Transport made its own estimates of the volume of UCO that could be available for aviation purposes in 2030.[5] It used further work by Ricardo and a body entitled the Aviation Impact Accelerator, a team based at Cambridge University. Most of the external team members of this second body are part of the aviation industry, including Boeing, Rolls Royce and Heathrow Airport. It won’t be a surprise to learn that the Accelerator produces some estimates for availability which are an order of magnitude greater than the figures from the specialised consultants.

The 2023 forecasts developed by Ricardo assume that the UK can devote 3% of all available domestically produced UCO for aviation fuels and also purchase 1% of all internationally produced used oil. These assumptions therefore result in much lower assumed availability. Rather than estimating a total energy value of 2.25 Terawatt hours, it suggests a figure of less than a tenth of this level.[6] This figure is then assumed to fall as the availability of internationally sourced UCO declines. Other countries will need that oil for their own fuels.

These Ricardo figures, published by the government as the lower bound of its forecasts, would allow only about 0.1% of all aviation needs to be fulfilled by UCO (from the UK and elsewhere) in 2030.

The estimates from the Aviation Impact Accelerator are far more optimistic, largely because it assumes that the volumes of used cooking oil available in the UK will grow. (There is no justification presented for this opinion). This industry body uses the Ricardo UK figures from 2017 for total availability (2.25 TWh for the energy value of UCO produced in the UK) and then almost doubles this figure by 2040. But even under these unrealistic assumptions, the total percentage of all aviation fuel produced in 2030 is no more than around 2% of current needs.[7]

To summarise, if ALL the UK’s UCO was used to make aviation fuel, meaning that other major uses, such as biodiesel were stripped of their share, government data suggests that no more than 1.45% of energy needs could be met. Even adding in substantial growth (which is highly implausible) and some imports only increases that figure to around 2% in the Aviation Impact Accelerator figures.

Would the use of other waste oils change this position?

The UK Prime Minister also mentioned waste oils in his statement. The principal source for aviation fuel today is animal fats derived from slaughterhouses. This is sometimes called ‘tallow’. The volumes are far smaller than for UCO. As importantly, tallow loses more energy in the conversion process to aviation kerosene than does UCO.

The Ricardo 2017 analysis suggests that the total availability of all tallow in the UK is equivalent to about 4 PJ, or 1.1 TWh. After conversion to jet fuel, the energy value might be around 0.4 TWh, or around a quarter percent of the UK’s needs. For reasons which are not explained, the industry-led Aviation Impact Accelerator sees the amount of tallow rising over 50% between 2025 and 2030, even though the amount of meat being eaten in the UK is stable or even falling. The more pessimistic assumptions by Ricardo in its 2023 projections suggest that considerably less than 0.1% of aviation demand can be met by tallow.

What are the implications of this analysis?

For aviation to be fully decarbonised the world will need a mixture of battery aircraft for short trips, some use of hydrogen in medium-sized aircraft and a very large scale replacement for aviation kerosene for long distance travel. Although using UCO is appealing because of the ease of conversion to fuel, the volumes are tiny in the context of the global need. We will need alternatives that offer orders of magnitude more output for full decarbonisation of aviation, even though they are far more complex and costly than UCO. SAF from waste oils is a dead-end.

What are the realistic options for the future? Government reports in the UK focus on forestry residues and municipal solid waste. In the case of wood products, the concern is the risk of deforestation, the lack of available supply and the technological complexity of turning lignocellulosic materials into kerosene. No-one is doing it at scale yet. Municipal waste suffers in addition from a small supply that is likely to decline as recycling plastics becomes more common.

So the answer has to be synthetic fuels, made from hydrogen and direct air captured CO2. This is an early stage industry but is the only conceivable way of meeting aviation’s needs with a low carbon impact. Follow Infinium and Norsk e-Fuel as good examples.

Appendix

The International Energy Agency’s views

It isn’t just the UK which doesn’t have enough waste oils. Here’s what the IEA wrote in December of last year.

Used cooking oil and animal fats are unlikely to provide relief (to biofuel producers), as they are in even higher demand because they offer lower GHG emissions intensity and meet EU feedstock requirements. In fact, the use of used cooking oil and animal fats nearly exhausts 100% of estimated supplies over the forecast period. Even when a broader range of wastes (such as palm oil mill effluent, tall oil and other agribusiness waste oils) is considered, demand still swells to nearly 65% of global supply.

(From ‘Is the biofuel industry approaching a feedstock crunch’, IEA December 2022.)

And we almost certainly need to fly a lot less. Not least because even the best SAF still releases water vapour when burnt, adding to the global heating caused by contrails.

[1] https://www.livemint.com/news/world/sustainability-landmark-virgin-airlines-takes-off-first-saf-based-flight-rishi-sunak-calls-it-very-exciting-11701242225845.html

[2] https://assets.publishing.service.gov.uk/media/5a74ebade5274a3cb286840c/ecofys-trends-in-the-uco-market-v1.2.pdf

[3] https://publications.parliament.uk/pa/cm201012/cmselect/cmenvaud/1025/1025vw08.htm

[4]https://assets.publishing.service.gov.uk/media/5a7f9007e5274a2e87db69a8/Biomass_feedstock_availability_final_report_for_publication.pdf

[5] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1147351/uk-sustainable-aviation-fuel-mandate-consultation-stage-cost-benefit-analysis.pdf

[6] The exact figures are not given in the report and I have estimated this number from Figure 5.

[7] Once again, I should stress that the government report does not provide the precise figures and I am estimating the percentage from Figure 5.