Is SAF
a fair COP? Jan/Feb 2024

ICAO was out in force at the COP 28 meeting in Dubai last December. The organisation addressed the Presidency Roundtable on Energy Transition presenting the outcome of the third ICAO Conference on Aviation and Alternative Fuels (held, conveniently, in Dubai at the end of November), particularly the adoption of an ICAO Global Framework for Aviation Cleaner Energies. The message: aviation’s clean energy transition is on.

ICAO states that the framework is to "facilitate the scale-up of the development and deployment of Sustainable Aviation Fuel (SAF), Low Carbon Aviation Fuel (LCAF) and other aviation cleaner energies on a global basis, and mainly by providing greater clarity, consistency and predictability to all stakeholders, including those beyond the aviation sector”.

The short-term “aspirational vision” doesn’t appear ambitious: to reduce aviation’s CO₂ emissions by 5% by 2030 (from what it would otherwise have been without the use of “cleaner” energies). But it is a vital tool in the long-term target of net-zero by 2050.

In 2024 aviation is likely to be responsible for the emission of 1 billion tonnes (1 gigatonne) of CO₂ — back to pre-pandemic levels — still just 2% of global emissions. But over the next 26 years, passenger numbers are forecast to grow from 4.5bn to 10.5bn, and aviation’s emissions could double to 2Gt CO₂. The pathway to net zero is based on “carbon-neutral” growth to 2035 (mostly through carbon-offset) followed by aggressive carbon reduction: this, according to IATA, requires the mitigation of 21.2Gt CO₂ over the period.

The use of SAF will account for 65%-70% of the CO₂ abatement required to achieve the target.

How does SAF work?

The basic premise behind SAF is the creation of a fuel either from organic feedstocks (that have drawn in CO₂ from the atmosphere or soil as they have grown), recycling CO₂ emissions from waste sources (both biomass and effluent from industrial processes), or using CO₂ captured directly from the air: this is combined with hydrogen to generate the required hydrocarbon chains. The resultant fuel has a formula fundamentally the same as jet kerosene produced from fossil fuels and consequently produces the same amount and type of emissions as conventional fuel. But the production of the feedstock takes CO₂ out of the atmosphere thus creating a “loop” that has the potential to reduce lifecycle emissions substantially.

Lifecycle emissions reductions from SAF are not generally 100% of CO₂, as they depend on energy sources to process the raw feedstock into fuel and get it into the aircraft. If the production facilities are run on renewable energy, and transport is through pipelines or using alternative energy vehicles, the emissions reduction factor can be improved significantly: even beyond 100% with the addition of carbon capture utilisation and storage (CCUS) of CO₂ or methane (CH₄) from the production processes.

The emissions reduction (ERF) from SAF is a product of the proportion of SAF mixed with conventional fuel (currently limited to 50% but likely to rise to 100% maybe by 2030). The lifecycle reduction in comparison with fossil-based jet fuel — currently estimated to lie around 70%.

Therefore, the use of SAF at the maximum current blend ratio of 50% could “reduce” carbon emissions on a flight by 35%.

SAF Production Pathways

There are a number of approved pathways and processes to generate SAF, depending on the feedstocks used. Each of the pathways has to be tested rigorously and certified for use by ASTM International (the global standards-setting agency for fuels).

Fischer-Tropsch (FT)

The FT process takes any carbon containing material and breaks it into individual building blocks of H₂ and carbon monoxide (CO₁) gasified to form syngas. FT synthesis then combines these building blocks into synthetic pariffinic kerosene.

Two different FT processes have been certified by ASTM to date, one that produces a straight paraffinic jet fuel (SPK) and one that also produces additional aromatic compounds (SPK+A). Typical feedstocks include renewable biomass such as municipal solid waste (which would otherwise go to landfill), agricultural wastes and forestry residues, wood and energy crops.

Hydrotreated Esters and Fatty Acids (HEFA)

HEFA refines biological oils or fats into SAF through a process that uses hydrogen (hydrogenation). In the first step of the HEFA process, the oxygen is removed by hydrodeoxygenation. Next, the straight paraffinic molecules are cracked and isomerised to jet fuel chain length. The HEFA process for SAF is similar to that used for (and therefore competes with) renewable diesel production, only with more severe cracking of the longer chain carbon molecules. Typical feedstocks are used cooking oil, tallow (eg from pig, sheep and cattle carcases), distiller’s corn oil, tall oil, algae and vegetable oils from jatropha and camelina.

Hydrotreated Fermented Sugars to Synthetic Iso-Paraffins (SIP)

The SIP process is biological. Microbes are used to convert C₆ sugars into farnesene, which after treatment with hydrogen can be used as SAF. The maximum blend ratio with conventional jet fuel is 10%.

Alcohol to Jet (AtJ)

AtJ converts alcohols produced by fermenting bio-waste into synthetic pariffinic kerosene by removing the oxygen and linking the molecules together to get the desired carbon chain length (oligomerisation). Currently, there are two feedstocks approved for use in the AtJ technology: ethanol and iso-butanol, and the sources can be agricultural waste products such as stover, crop straws, grasses and forestry slash.

Catalytic Hydrothermolysis Synthetic Jet Fuel (CHJ)

CHJ converts fatty acid esters and free fatty acids into SAF via catalytic hydrothermolysis followed by any combination of hydrotreatment, hydrocracking, or hydroisomerization and fractionation. The feedstocks are triglyceride-based: plant oils, waste oils, algal oils, soyabean oil, jatropha oil, camelina oil etc.

Hydroprocessed Hydrocarbons, Esters and Fatty Acids (HC-HEFA)

Biologically-derived hydrocarbons and free fatty acids and fatty acid esters are upgraded in the HC-HEFA process similar to the HEFA process: they are hydroprocessed to saturate the hydrocarbon molecules and remove all oxygen. A recognised bio source is the Botryococcus braunii species of algae. The maximum blend ratio is 10%.

Co-processing

Co-processing is an additional method to accelerate SAF production. This involves the use of sustainable feedstocks (vegetable oils, waste oils and fats, or Fischer-Tropsch wax) as a small input into the refining of conventional fossil-based jet fuel in existing crude oil refineries. It is not a SAF-focused production pathway, but an opportunity to accelerate SAF production using existing infrastructure while dedicated SAF facilities scale up. ASTM limits the blend ratio to 5%.

Power-to-Liquids (PtL)

PtL is a non-biological pathway to produce sustainable fuel. Electricity from renewable sources is used to split water molecules into “green” hydrogen and oxygen through electrolysis. CO₂ is captured from industrial sites, or direct from the air, converted into carbon monoxide and combined with the H₂ to form syngas and through the FT process converted to generate the required hydrocarbon chains. To be sustainable, the power source has to be entirely from renewable sources such as solar or wind (and the resulting H₂ “green”) and not diverted from other uses. The feedstocks are (green) electricity and air and water.

Green Hydrogen

IATA estimates that the industry will have a need for 120m tonnes of green hydrogen by 2050: 20% of which may be for hydrogen powered flight, but the majority for the production of SAF. (It is needed for HEFA processes as well as PtL.)

Currently, according to the IEA, a mere 1% of the global 100m tonne production of hydrogen can be classed as low-emission, and virtually all of that is generated by facilities using fossil-fuels mitigated through CCUS (known as “blue” hydrogen).

The agency estimates that by 2030 there may be global capacity for 15m tonnes of green hydrogen through electrolysis. In the meantime, SAF produced from PtL is economically prohibitive with production costs, net of the value of carbon reduction, estimated at nearly $4,500/tonne, five times the current cost of conventional jet.

Is there enough feedstock?

Commercial aviation this year is forecast to use nearly 300m tonnes of jet fuel — slightly higher than pre-pandemic levels. By 2050 the industry total requirement could be for energy double this amount, ie equivalent to 600m tonnes.

It is expected that battery, hybrid, and hydrogen powered aircraft will increasingly become available for short haul flying during the 2030s, but by 2050 may account for only 15% of the power required. As a result the industry could need c500m tonnes of hydrocarbon fuel.

The Air Travel Action Group (ATAG) in its Waypoint 2050 report of 2021 presented analysis suggesting that there is a current feedstock availability to produce 205m tonnes of SAF from biomass, waste and industrial gases.

The analysis considered three key constraints:

• Sustainability: environmental and social limitations on feedstock availability, such as the constraints on land that can sustainably be used for energy crops, or the fraction of agricultural residues that must be left on the land to maintain the soil quality.
• Fairness: the balance of feedstocks that can be used for SAF production, compared to use by other sectors. Many of these feedstocks are used today for heating, animal bedding or feed, soaps, lubricants, and as raw material for oleochemicals industry.
• Economics: volumes of feedstock that can used to produce fuels and enable flights at economically sustainable socially acceptable prices.

Over time, some of these sources will change: industrial waste gases are likely to fall as other industries decarbonise, but competition for liquid fuels from road transport is likely to diminish with an accelerating uptake of electrically powered vehicles. New plant based material may become economically viable. By 2050 it estimates the total potential little changed at 198m tonnes.

Under the group’s central scenario, biological feedstocks become fully utilised by 2040, requiring a massive shift of production to PtL using renewable energy. By 2050 PtL could account for 60% of total SAF production, but at a cost: ATAG estimates that PtL production costs even in 2050 would be $1,470/tonne before accounting for the value of carbon reduction.

What if SAF doesn’t work?

There is no plan “B”. There is currently no foreseeable economic source of energy to power long haul flights (which account for 80% of the industry’s emissions) that is not based on hydrocarbons. Consequently, to continue to allow flights of more than 3,500km, the airline industry will continue to emit 3.16tCO₂ for each tonne of kerosene burnt. Current forecasts for growth from ATAG and IATA — of say 3.1% in passenger numbers (and 3.5% in passenger kilometres) over the next 26 years — already account for a slowing in growth from past trends, partly on a normal S-curve path as the industry matures, but also taking account of the increase in costs inherent in decarbonisation efforts. If the quantity of SAF produced cannot be scaled to fit the anticipated fuel needs under current forecasts, the forecasts will have to adjust to meet the commitment to net zero. And that growth rate may have to be negative.

As an exercise we reverse-engineered those forecasts to add in a variable for the effectiveness of SAF implementation. The chart shows a (rather depressing) representation of the level of growth that the industry would be allowed to achieve to ensure CO₂ neutrality by 2050 at various levels of effectiveness (100% effectiveness means SAF fully achieves its projected role in emissions reduction and air travel grows at IATA’s forecasted base rate; 25% means that only a quarter is achievable through SAF and emissions reduction has to come mostly from a reduction in air travel).

However, IATA’s chief economist Marie Owens Thomsen, quoted in ATW, maintains: “There are solutions for replacing fossil fuels. There are no solutions for replacing aviation.”