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Sustainable Aviation Fuels

Decarbonisation essential

Time seems to be running out on the global need to remove dependency on fossil fuels, and the world needs to decarbonise. To do so is going to be expensive. The UN estimates that the investment needed to achieve net-zero carbon emissions by 2025 will reach $125tr. A recent report from McKinsey suggests that capital spending on physical assets for energy and land-use systems would top $275tr between 2022 and 2050, or over $9tr a year, a 66% increase in annual investment costs from today’s levels.

Aviation is one of the most difficult sectors to decarbonise. As IATA succinctly puts it, the industry is constrained by three main factors which limit its options:

  • Aircraft have long useful lives. They can remain in service for 30 years, and the renewal process can take decades.
  • Aircraft have lengthy development times. Because of necessary strict certification requirements and safety standards the development of new aircraft incorporating the latest technology can take over 10 years. (Airbus is developing a hydrogen-powered zero-carbon fuel cell engine for a 100-seat aircraft that might be launched later this decade but not enter service until at least 2035).
  • Aircraft use a lot of energy. Very few energy storage solutions can currently provide the power-to-weight ratio provided by fossil fuels to enable a tin can to carry hundreds of passengers with their luggage at 1,000km/h more than 10km above the ground.

In 2021, IATA adopted a resolution at its annual meeting committing its member airlines to achieving net-zero carbon emissions from their operations by 2050 (see Aviation Strategy, December 2021). The following year ICAO adopted a similar long term aspirational goal. At this year’s annual IATA meeting the organisation reiterated its commitment to the resolution and produced a series of documents highlighting the pathways required to meet the Fly Net-Zero ambition.

IATA also restates how expensive the process will be, suggesting that the investment needed to bring about aviation’s transition to net -zero by 2050 could touch $5tr, over $180bn a year (equivalent to a quarter of current airline annual revenues).

Aviation accounts for a modest 2-2.5% of global CO₂ emissions, but is expected to continue growing: without taking additional efforts annual CO₂ emissions from the industry could double to 2 gigatonnes by the middle of the century.

Sustainable Aviation Fuels

By far the largest contribution to the path to net-zero is expected to come from the use of Sustainable Aviation Fuels (SAF) — accounting for 62% of the planned mitigation of carbon emissions in 2050 and 80-90% of aviation fuel use.

A huge ramp-up of production of this contentious source of energy is required. SAF output in 2022 trebled to 300m litres — roughly equivalent to 0% of the industry’s total 350bn litre fuel consumption.

Path to Net Zero

IATA estimates that there will be a need for 5,000-7,000 biorefineries to supply the industry’s needs, at an investment cost of $1.08-1.45trn (roughly equal to $12/gallon capacity) which it estimates equates to 6% of historical annual fossil fuel investments. At the moment SAF production costs a multiple of the jet fuel price: the premium to fossil fuels may come down over time, but the blended price will be passed on to the passenger through ticket prices.

SAF is a contentious issue because it is neither well understood nor well explained. At the moment a key method for its production is from hydrotreated vegetable oils (HVO) and hydroprocessed esters and fatty acids (HEFA): transforming “waste” fats into a kerosene equivalent. Other methods include alcohol-to-jet (ATJ) from waste fermentation, and pyrolitic conversion of CO₂ and hydrogen (syngas) through the Fischer-Tropsch process into liquid hydrocarbons. The key idea is that it is using carbon from a renewable source to burn and release into the atmosphere which can then be reabsorbed through photosynthesis into the food chain: a perfect circle and carbon neutrality.

Unfortunately, current SAF technology cannot produce the aromatics present in fossil fuel production that provide lubricating, sealant and anti-corrosive properties in pipes. The regulations require a particular proportion of these aromatics for a fuel source to qualify as jet kerosene, and SAF needs to be blended with conventional aviation fuel to achieve these levels. Although some aircraft have successfully tested 100% SAF flights, the safe proportion is deemed a 50% blend. Depending on the source of power used to create the biofuel, blended SAF flights could achieve no more than a 42% reduction in net life-cycle carbon emissions.

Technology in the aircraft will no doubt improve, and by the 2030s IATA in its road maps suggest that all new aircraft will be able to operate safely on tanks entirely consisting of biofuels. But, the ground infrastructure may cause problems: delivery from refinery to airport and, within airports, from fuel farm to wing may require significant investment for pure SAF deployment.

Pigs Can Fly

And then there are the feedstocks: the source of non fossil fuel carbon to transform into juice. Aviation here is competing with ground transport, a sector of the economy equally needing to decarbonise with a fuel requirement ten times that of air transport (albeit that ground transport is in the process of transitioning to battery powered propulsion). And bio-diesel to power trucks doesn’t have quite the same level of structural safety requirements to which airlines are subject, and its production may appear more profitable.

A recent report from Brussels-based Transport & Environment (T&E, a non-governmental organisation that lobbies and campaigns for zero-emission mobility) highlighted the dubious sustainability credentials of using used cooking oil and “waste” fats from animal carcases to generate biofuels in general, and SAF in particular. In Europe the use of animal fats (some of which according to a bizarre European rule can be double-counted for carbon reduction accreditation) to generate biodiesel has grown from 30,000 tonnes a year in 2006 to 1.4m tonnes in 2021. It is forecast almost to triple to 3.9m tonnes by 2030. There are only so many pigs you can slaughter to power the trucks to take the carcases to the refinery.

Regarding SAF, the group suggests that the HEFA process will only be able to provide a sustainable source of animal fats for 1.4% of the aviation industry’s requirement by 2050 — without taking into account that there is a need to reduce livestock farming to fit in with greenhouse gas emission reduction commitments.

The report highlights the group’s analysis that a 100% SAF flight between London and New York would require the equivalent of 8,800 pig carcases each way.

Then the publication of the report raised concerns in other industrial sectors.

Fats from animal carcases are not necessarily unwanted waste, and the anticipated high demand for their use in biofuel production may have unintended consequences. Some categories of fats are burnt by other industries to provide heat and power: their alternative might be fossil fuels.

Others, those deemed fit for human consumption, are used in the oleochemical industry for the production of soaps, cosmetics and candles; or by the pet food industry to make their products “more palatable”.

The cosmetics industry might be under pressure to source fats from less sustainable sources (such as palm oil — the cultivation of which generally requires land-use change and is seen to contribute to deforestation). Pet food suppliers, with fewer alternative replacements, will see significant price pressure.

Quoted by the BBC, Nicole Paley, deputy chief executive of UK Pet Food, the manufacturers' trade association said: “These are really valuable ingredients for us and they are hard to replace, and they’re put to good use already in a very sustainable way. So actually diverting these ingredients to biofuels... put us in competition with the aviation industry. And when it comes to the purse strings of the aviation sector, the pet food industry would find it really difficult to compete.”

Demand for used cooking oils for biofuel production is causing additional problems. The Times of London reported that French police as saying that theft of used cooking oil from outside restaurants has reached industrial levels. This followed the arrest of a gang suspected of stealing 385 tonnes of the stuff over a three month period, worth nearly €0.5m, shipping it abroad for conversion to biodiesel. The price of used cooking oil, it said, has risen ten-fold from €150/tonne in 2008.

Meanwhile, the Vegan Society of Canada has warned that many flights would no longer be “vegan certifiable”, encouraging airlines to specify that their sources of SAF do not contain animal products.

Other sources of feedstock are needed, and plant-based sources such as Jatropha oil and Camilina Sativa, grown on marginal land, and even algae have been used. But the cause has not been particularly helped by a recent briefing document published by the UK’s Royal Society, suggesting that demand for crop-based aviation biofuels would require up to 68% of the available agricultural land in the UK.

Willie Walsh, IATA’s Director General, was scathing of the report in his address to the airline association’s AGM this year. In particular he noted that the authors had based their analysis on the fuel burn performance data for flights between London and New York on a 737-300 (an aircraft that went out of production in 1999). “Now, I’ve flown the 737-300”, he said, “so I know a bit about it and what I know for certain is you cannot get the minimum of 21 tonnes of fuel that they estimated you would require into the fuel tanks that can only take a maximum of 16 tonnes. So, if we know that that section of the report is rubbish what confidence can we have in the rest of the document?”

Another contentious element of SAF is that currently there are so few production facilities that physical delivery is impossible: transporting SAF to a specific airport or flight could lead to higher greenhouse gas emissions. To counter this IATA is calling for a policy decision to establish global standards for a SAF book-and-claim system, and especially one that avoids double-counting.

On the principle that a gallon of fuel burnt in the USA is the same as 3.78 litres burnt in the EU or China it does not matter where SAF is bought or consumed. The sustainable fuel would go into the fuel system at an airport near the refinery, the body covering the fuel premium (“buying” the SAF) would be credited with the CO₂ mitigation no matter where they were. This is the accepted practice in renewable electricity generation. It even gives corporate entities the option to “purchase” any volume of SAF, including 100% of fuel needs, to satisfy their requirements for ESG credentials.

But it is also a beautifully confusing concept for the man in the street: he may be willing to pay a premium for a ticket on an airline that advertised its use of sustainable fuel, but he would not even know if such fuel were used on the flight he takes. The Canadian vegan would never be able to find out if the kerosene on their flight were porcine in origin.

The real problem though is that SAFs are hydrocarbons and, when burnt, release the same amount of CO₂ as do conventional fuels.

Green Hydrogen

Further down the path to fly net-zero, the use of hydrogen will play a part. IATA estimates that by 2050 maybe 10% of aviation’s fuel requirement will be satisfied by the use of green hydrogen. But this presents another series of problems.

Generating hydrogen by electrolysis takes a lot of energy — it may take 50kWh to generate 1kg of hydrogen which could produce 33kWh (an energy efficiency of 67%). But it takes up a lot of space: 1kg of H₂ needs a container with a volume of 11m3. To cool it to 20°K and compress to manageable liquid quantities requires the use of another third of that output energy, reducing the energy efficiency to 50%. To qualify as “green” hydrogen and a zero-carbon fuel, the power used to generate it has to come from renewable sources.

IATA suggests that by 2050 the industry will have a need for 100m tonnes of hydrogen — equivalent to the total hydrogen production worldwide in 2023. The majority of that need will be used to create liquid hydrocarbon fuel: only around 10% is expected to be used to power zero-carbon aircraft.

According to the organisation’s aircraft technology road map, it anticipates that regional turbo-props and commuter aircraft retrofitted with hydrogen fuel cell power plants may first enter service in the late 2020s, but that it will be at least 2035 before we see the entry into service of a clean sheet short- to medium-haul H₂ powered “jet”. Widebody H₂ combustion aircraft may be possible in the 2040s.

As well as the technical difficulties of storing and delivering hydrogen on board an aircraft. airports will need to find the space and build the facilities for in-plane delivery, and there is a strong need for the development of the supply chain to get the hydrogen to the airports. T&E estimates the total cost of the transition in Europe to H₂ aircraft will amount to €300bn by 2050 — allocating, somewhat speciously, three quarters of this cost to hydrogen generation and liquefaction. By then it argues that hydrogen aircraft could be cheaper to operate — but, unhelpfully, only if demand (particularly business travel) is restricted and jet kerosene is taxed.

Fuel Consumption: a scenario

This is part of an article which appeared in Aviation Strategy Issue #275 May/Jun 2023.