Sustainable aviation fuel: prepare for take-off!

Despite the unprecedented drop in global air traffic due to the Covid-19 crisis in the past years, passenger numbers and cargo volumes are anticipated to increase in the coming decades. Greenhouse gas (GHG) emissions from aviation contribute to 2% of total GHG emissions, with just over 600 Mt of CO2 production in 2020.

Yvon Bernard

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Article Summary

While a range of solutions, including technical, operational, and behavioural, are required to reduce aviation emissions, it is recognised that liquid fuels will continue to be the dominant fuel for air travel through to 2050, and there are many technology options to transition from fossil-derived jet fuel to sustainable aviation fuel (SAF).

SAF is an alternative to fossil jet fuel and a promising solution to decarbonise the aviation sector. It is produced from either:
- Renewable or waste-derived bio-based resources that meet sustainability criteria depending on the source, such as renewables oils and fats, lignocellulosic biomass, wastes, low-carbon intensity (CI) inedible starches, and sugars
- Captured carbon dioxide combined with green hydrogen (produced via the electrolysis of water using electricity from renewable sources (so-called e-kerosene)

SAF is a drop-in fuel, meaning it can be blended with traditional jet kerosene (currently up to 50 vol%), and the blend does not require any equipment change, special infrastructure, or modification of the supply chain, limiting supply-chain investment requirements.

According to the IEA, SAF currently accounts for 0.1% of global jet fuel consumption: producing SAF today is currently more expensive than producing fossil-based jet fuel. A combination of consumer pricing, regulatory, and incentive programmes is therefore required to expand SAF production, regardless of the pathway.

All roads leading to SAF
Among the seven pathways currently certified under the ASTM D-7566 specification for synthetic kerosene to be blended into Jet A1 pool, Axens provides de-risked technology for the three main pathways.

Each of these pathways can significantly reduce fuel lifecycle GHG when compared to fossil baselines, with reductions well over 90% possible when utilising advanced cellulosic feedstocks and/or green power:

Vegan®: Hydroprocessed Esters and Fatty Acids (HEFA) pathway: Vegan is a flexible solution to produce renewable diesel and SAF through the hydrotreatment of a wide range of lipids (renewable vegetable oils and animal fats). This technology allows producers to effectively address environmental regulations and sustainability targets, and secure energy diversification with drop-in premium quality products. Vegan technology includes a hydrotreatment (HDT) section and a hydroisomerisation (HDI) section. The HEFA pathway offers the possibility to revamp existing hydrotreatment units and turn them into HEFA units to produce SAF.

Gasel®: Fischer Tropsch (FT) pathway: Gasel technology converts synthesis gas (Hâ‚‚+CO) from various origins — biomass, captured carbon oxides — into a flexible slate of ultra-clean liquid fuels (XTL), including SAF. This Fischer-Tropsch route is commonly accepted as one of the most promising mid-term solutions for the production of alternative fuels and petrochemicals, including biomass-to-liquids (BTL) and e-fuels. It includes syngas purification and Fischer-Tropsch synthesis and product upgrading.

BioTfueL®: Gasification pathway: BioTfueL technology unlocks SAF and advanced biofuels production from energy crops and agricultural and forestry residues (including wood industry residues) via a thermochemical pathway. This technology enables access to a wide range of biomass and positions itself as a sustainable alternative to vegetable oil and food-based renewable fuels. Axens is the only licensor offering a BTL solution from biomass to liquid. It relies on a four-step process: pretreatment, gasification, syngas conditioning, and Fischer-Tropsch synthesis and upgrading.

Ethanol to Jet pathway: Ethanol to Jet (ETJ) is the process by which 1G or advanced bioethanol (2G) is converted to SAF via different steps: dehydration, oligomerisation, hydrogenation, fractionation (see Figure 1). This differentiating technology bundle approach utilises already commercially proven processes, with over 100 homogeneous and heterogeneous reference units, 70 hydrogenation references, and four dehydration references.

Futurol®: Enzymatic conversion pathway: Futurol converts lignocellulosic biomass from from energy crops and agricultural and forestry residues (including wood industry residues), into cellulosic ethanol (advanced bioethanol). This cellulosic ethanol can then be converted to SAF with the ATJ process described above. Futurol relies on a four-step process: pretreatment, biocatalysts production, hydrolysis and fermentation, and products recovery. Futurol was selected by INA, in Croatia, to produce 55,000 tonnes, equivalent to 70 million litres, of advanced bioethanol in synergy with green energy production.

Combination of Enzymatic conversion + Alcohol to Jet pathways: Combining Futurol and the Alcohol to Jet processes provides the possibility of producing ultra-low CI — or even carbon-neutral, advanced SAF from lignocellulosic biomass. This chain has many advantages, including:
- A two-stage investment plan to either produce advanced bioethanol first (with Futurol), then subsequently build an Alcohol to Jet block
To produce SAF from low-CI, 1G ethanol (with ATJ), decarbonise the SAF over time with the introduction of advanced bioethanol
- Utilising a ‘hub and spoke’ approach by building multiple small Futurol plants near feedstock sources that all feed a centralised ATJ plant that captures economies of scale. These advantages make the ATJ pathway unique in today’s continuously evolving environment.

Many different types of technologies and pathways are now available to produce SAF, and Axens is uniquely positioned to bring de-risked technologies to three of the primary pathways (HEFA, ATJ, and Biomass to Liquids via Gasification and Fischer-Tropsch). Each pathway has distinct regional feedstock-specific, economies of scale and techno-economic advantages, with each one having a role to play depending on the individual project context. Meeting the demands of the aviation sector will likely require the implementation of multiple project pathways, including those listed here. Axens can perform feasibility studies to select the right pathway(s) with the right business model in order to come up with the most optimised solution to fit any need.
The common threads running through these technologies are flexibility, reliability, and the realisation of decades of technology development, demonstrating that Axens is ready to meet the challenges of scaling up SAF capacity in the coming years.

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