Decarbonisation Technology - May 2024 Issue

fermentation, can be converted into jet fuel via alcohol-to-jet (ATJ) technology. Another option is to produce methanol from CO₂ and hydrogen (via syngas manufacturing steps or directly) and then convert the methanol into SAF via oligomerisation and hydroprocessing steps. Technology for methanol-to-jet has yet to be fully de-risked and scaled up. However, it is at the advanced stage, and the resulting jet fuel is undergoing certification to qualify as a blend-in component of conventional jet fuel. As previously mentioned, aviation fuel made via Fischer–Tropsch pathways and ethanol-to-jet has been approved for aviation use in blends up to 50% with conventional refinery kerosene, viz. standard Jet A1. Although a proven technology, the production of bio-SAF relies on the availability of sustainable biomass residues, a resource not universally accessible across all regions. This geographical variability, together with potential competition with food production, has prompted EU policymakers to restrict the production and use of bio-SAF to 50% of the total SAF in 2050 (Table 1), with eSAF making up the balance. eSAF made using renewable power, water, and CO₂ eSAF is produced through a PTL process, eliminating the need for organic feedstocks. As shown on the left of Figure 1, the process uses water electrolysis to generate hydrogen using renewable sources such as solar, wind, and hydropower and the reverse water gas shift (RWGS) reaction to produce carbon monoxide (CO) from CO₂. The configuration shown in Figure 1 is the most technically mature option at present; in the future, water electrolysis and RWGS could be replaced by co-electrolysis of CO₂ and water in solid oxide electrolyser cells, or co-SOEC, which would make syngas, or a mixture of hydrogen and CO, in one step. The CO₂ could be obtained from point sources or direct air capture (DAC). Subsequently, hydrogen and CO undergo Fischer–Tropsch synthesis, followed by product upgrading. This approach provides flexibility in the products generated, enabling the production of various paraffinic fuels and/or products. When SAF is the desired product, the plant can be designed such that 80% of all production

is aviation fuel, the rest being naphtha (used as a chemical feedstock or for blending in the gasoline pool). However, producing renewable hydrogen and sourcing the right type of CO₂ present challenges. At present, renewable hydrogen production is constrained by the need for significant investment and infrastructure development to scale up electrolysis technology and manage the intermittency of renewable energy sources. Both must be addressed to achieve cost-competitive and scalable renewable hydrogen production. For eSAF to meet the required GHG savings of at least a 70% reduction vs a fossil benchmark of 94 gCO₂/MJ(fuel) under EU regulations, it must be made from acceptable CO₂ sources. The European Commission published a delegated act specifying which CO₂ sources will be allowed over a defined timeline. Until 2040, the use of specified industrial (fossil origin) point sources of CO₂ will be permitted, whereas CO₂ originating from power generation can only be used until 2035. Beyond 2040, the CO₂ will be limited to that originating from the production or combustion of sustainable biofuels, bioliquids or biomass fuels, and DAC. Regulations in non-EU regions are yet to be shaped. There are many ongoing developments in DAC technology worldwide, driven by both direct air carbon capture and storage (DACCS) as a form of offset and growing demand for synthetic fuels. Scaling up DAC, which is recognised as a key technology for the net zero pathway, depends on the development of policy and regulation, as well as unpredictable growth dynamics. Nonetheless, in the IEA Net Zero Emissions by 2050 Scenario, DAC technologies are expected to capture more than 85 Mt of CO₂ in 2030 and about 980 MtCO₂ in 2050, requiring a large and accelerated scale- up from almost 0.01 MtCO₂ today. This outlook positions DAC as a relevant source of CO₂ in the context of SAF production by mid-century. Combined production of eSAF and bio-SAF: A sweet spot When the options and challenges in SAF production are considered, the joint production of bio-SAF and eSAF emerges as a strategic choice due to several advantages: • Full utilisation of biomass carbon: The relatively low hydrogen-to-carbon ratio of