A fat load of good? Sourcing feedstocks for aviation biofuels looks not-so-simple

Biodiesel production in the laboratory.

Are fats and oils the key to the aviation fuels of the future? Envirotec writes

Many are betting on sustainable aviation fuels (SAF) as the pathway to meeting ambitious net zero targets in the sector, with 2040 and 2050 being oft-cited deadlines. Biofuels have been an important ingredient in the mix of technologies that might solve this puzzle. But there is contention over what constitutes “sustainable”, and doubts about whether many of the plant-based and food-fat-based alternatives really cut the mustard.

What is SAF?
The term “sustainable aviation fuel” (SAF) covers a range of chemicals, in a field that can be divided into the two categories of “biofuels” and “renewable fuels of non-biological origin”. Amongst the latter are synthetic fuels, or efuels, an emerging class of low carbon, drop-in replcaements, whose manufacture combines captured CO2 and renewable energy, and whose commercial deployment still faces many hurdles. Other speculative chemicals and materials in the future-fuels mix include ammonia, methanol and hydrogen.

SAF is “a core part” of the UK government’s Jet Zero Strategy, for example, announced in July 2022, which commits UK aviation to achieving net zero emissions by 2040.

The Biden administration’s Inflation Reduction Act (IRA) – unveiled last year – includes tax credits for SAF production. And in 2021 they announced a Grand Challenge to ramp production from the current 16 M gallons per year to 3 billion per year by 2030.

Support from politicians has been important for SAFs, and biofuels more widely, because of the need for subsidies to help close the gap between production costs and market prices. SAF currently is at least twice the price of traditional jet fuel, and accounts for less than 1% of global aviation fuel consumption.

Destined for a back seat?
In late May, Boeing chief executive Dave Calhoun seemed to dampen hopes that SAF might achieve price parity with Jet A, a kerosene-based fuel that is the most commonly used in commercial aviation. As the Financial Times reported, he said “I don’t think that will ever happen”, in comments that seemed to suggest a role for SAFs, in his eyes, but a limited one.

There’s no clear long-term demand for SAF as yet, and without that, investors will be reluctant to finance what’s needed to scale production, so it remains a niche product.

The question of where the feedstock to produce these fuels should come from also seems contentious and complex. The Biden administration’s ambitious projections seem to depend on a mix of agricultural waste (from corn and soya bean production) and woody biomass.

While bioenergy remains the world’s largest source of renewable energy, there is still at this point a sense that there might not be enough to go around, to meet the demands of aviation.

Food fats have grown in significance as a feedstock for biofuels in recent years – in other words, the use of used cooking oils and animal fat (the portion deemed unfit for human food consumption). Prized for their lower carbon intensity than fossil fuels, and the fact that they don’t compete with arable farming land (unlike crop-based fuels), demand in recent years has outstripped production.

EU legislation has encouraged the use of biodiesel – a biofuel derived from animal fat – as a transport fuel for road vehicles, with maritime and air transport now coming into view. An analysis by Stratas Advisors has predicted that in Europe the demand for biofuels based on animal fat will grow from 1.4 to 3.9M tonnes from 2021 to 2030.

Animal fats and cooking oil can be used to produce biodiesel and renewable diesel, the two principal biofuels dependent on these feedstocks (see section, “Fuels from fats”, beneath this article).

A recent report from Transport & Environment (“The Fat of the Land”, released on 31 May) pinpoints a danger with the existing EU legislation, and seems to raise serious doubts about the sustainability of using animal fats as an aviation fuel.

A difficulty arises with categorising animal fats so as to distinguish the “waste” material (for which no other uses can be found, one might suppose) from the stuff that already finds an important use.

The EU legislation categorises this material to communicate its suitability as a waste feedstock, with category 1 used for animal fat that is unfit for human consumption (and may present a risk of disease) while category 2 and 3 cover fats of a medium and low risk, respectively (category 3 is fit for human consumption). The use of all categories of material is permitted as feedstocks for aviation biofuels, but categories 1 and 2 count for double (as does used cooking oil) – and it has successfully incentivized their use in the manufacture of biofuels.

It’s “waste” (honest, guv)
A potential for fraud is one problem, with there being an incentive to mix in category 3 fats with category 1 and 2, and simply mis-label the resulting mixture as category 1 or 2, so as to help meet renewable transport targets.

And it’s maybe stretching it a bit to classify some of these fats as “waste”, suggests the report, as they already find a home in certain classes of product including soaps, pet foods, and cosmetic products (and also, until recently, to generate heat and power, often within rendering facilities). Between 2006 and 2021, these kinds of usage fell precipitously as more and more of this material went into making biodiesel (mostly for vehicle transport).

As demand grows for aviation fuel, it’s likely the producers of cosmetics and pet food will
have to look elsewhere for a replacement for these fats, and it will more than likely be palm oil, the cheapest alternative, but also associated with deforestation. As the report outlines, this could result in a 70% increase in emissions, compared to simply carrying on using kerosene as an aviation fuel.

What the aviation industry ought to be doing, suggested NewScientist, in a recent article on the topic, is investing in more advanced biofuels able to use cellulosic or other plant material feedstocks. These are more abundant and closer to the designation “waste”, although this is a more expensive route than relying on animal fats so you can see why the aviation sector is dragging its feet.

If all the animal fat produced in the EU was siphoned into making aviation biofuels it would still only meet about 3% of the demand predicted for the jet fuel market in 2030, say the authors. And the only alternative then would be to ramp up industrial meat production, not considered a sustainable avenue.

A fat rush?
Animal fats and used cooking oil are now emerging as major global commodities, as analyst group S&P Global Commodity Insights notes in an early June report (“Biofuel Feedstock Trade Flows: First Come, First Served?”). And biofuel producers will be in a race to secure an adequate supply. “Animal fats and used cooking oil are to them what lithium, cobalt and copper are to battery makers,” said the group’s Juan Sacoto, Executive Director – Agribusiness Consulting.

Latin America and Southeast Asia look likely to emerge as strategic suppliers, say the authors, “where meat and vegetable oil consumption are expected to grow at a robust pace for decades.”
“The collection of animal fats and used cooking oil in these regions will be critical to serve North American and European countries where the production of these feedstocks has plateaued,” said a press release about the report.

It predicts a boom in renewable diesel production in 2030, and subsequent growth in SAF – “crucial to decarbonization of that sector”.

Changes in the pipeline
Another factor that might complicate the uptake of aviation biofuels is that not all of them are compatible with existing engines and fuelling infrastructure. For example, biodiesel (see “Fuel from fats”, opposite) can only be used as a relatively dilute blend in existing engines, whereas fuels based on renewable diesel, such as HEFA, are considered “drop-in replacements” with no modifications needed even when used in their pure form.

One issue is that hydrocarbon molecules (i.e., from fossil fuels) contain aromatic molecules that help to seal pipes (penetrating the elastomer materials that create a tight seal between pipes and gaskets, for example). Many biofuels omit this property, and so preparing this infrastructure to handle biofuels will require changes to sytems like pipes and tanks.

In existing SAF usage, aircraft will be fuelled with at most a 50% blend of SAF and normal jet fuel. Airbus has said all its planes can fly with a blend of up to 50% SAF. Qantas has pledged to use 10% SAF in its overall fuel mix by 2030. The UK’s Jet Zero Strategy (targeting net zero emissions in aviation by 2040) includes a mandate for at least 10% of jet fuel to be made from sustainable sources by 2030.
Boeing and Airbus have committed to using 100% SAF by 2030.

Measuring the emissions produced by HEFA biofuel during a world-first trial of 100% SAF on both engines of a commercial aircraft in April 2021. A Falcon chaser plane trails in its wake, equipped with instruments for monitoring emissions such as particulates.

Clearing the air
A world-first trial of 100% SAF use on both engines of a commercial jet reported promising results in April 2021. An A350 flew three flights over the Mediterranean Sea pursued by a Falcon chaser plane (pictured, below), which was equipped with instruments to compare in-flight emissions of both kerosene and Neste’s HEFA fuel (see “Fuels from fats”, below, for more on HEFA).

Another potential selling point of aviation biofuels is the potential for air quality improvements around airports. Neste said the study demonstrated lower particulate pollution levels for HEFA at all engine operating conditions.

Aircraft emissions appears to be an area where there are many unknowns, and the UK’s strategy document for net zero aviation includes – as one of six priority areas – “improving our understanding of the non-CO2 impacts of aviation such as contrails and nitrogen oxides”.

Governments seem to be doing their best to try and kick start a new SAFs industry, with incentives and tax credits looking likely to remain crucial. Apart from its 10% blend mandate, the UK government is also providing a £165 million Advanced Fuels Fund, with the aim that there will be at least five comercial SAF plants under construction in the UK by 2025. The winners of the AFF were announced in December, and include several projects that will convert black bin bag waste into SAF, using technologies like gasification and Fischer-Tropsch (an approach that involves converting hydrogen and CO2 into hydrocarbons under exacting conditions of temperature and pressure).

These kinds of synthetic fuels are also considered “SAFs” and have the advantage of allowing drop-in replacement, and might offer other advantages, including the possibility of using more sustainable feedstocks. Airbus, for example, signed a deal in March to develop the approach, with clean energy firm Masdar.

Crops are also a part of many biofuel plans. Quantas and Airbus announced a joint investment of $2m in June, to develop a production facility in Queensland that will convert sugar cane into SAF, with stated hopes of producing up to 100M litres per year when it opens in 2026.

The big aircraft companies are naturally hedging their bets, and SAFs likely place within the aviation of the future has yet to become clear. Understandably, they will want greater clarity before investing the billions it takes to develop a new aircraft, and the current uncertainty has been blamed for the apparent slowness of development of late (“How’s that 797 coming along?”).

Fuel from fats – SIDE PANEL

The two principal biofuels made from animal fats and cooking oils are biodiesel and renewable diesel.

Biodiesel uses the long-established technique of transesterification to produce “fatty acid methyl esters” (aka “biodiesel). It is the most common biofuel used in Europe, and is most often used as a diesel additive, to reduce levels of pollutants like particulates and carbon monoxide, from vehicles. There is a limit to how much can be added to unmodified fossil fuel engines, because of a fundamental difference in the composition of these molecules, compared to hydrocarbons.

Renewable diesel uses a newer production method, which involves reacting the fats and oils with hydrogen to yield “hydrotreated vegetable oils” (or HV01). Its molecules are sufficiently similar to hydrocarbons to permit the use of 100% composition HV01 in existing diesel engines.

A third, newer option is generating interest in aviation: hydrotreated jet fuel, also known as “renewable jet”. Produced using a similar process to renewable diesel, it is sometimes called “hydroprocessed esters and fatty acids”, or HEFA.