One- or two-step processes: Which have a lower GHG emissions intensity for production of synthetic aviation fuel via indirect CO2 electrolysis?

Abstract

The development of sustainable aviation fuel (SAF) could pave the way towards addressing the dual challenges faced by the aviation sector: meeting rising demand for air transport and achieving net-zero targets. In this study, the well-to-pump (WtP) and well-to-wake (WtW) greenhouse gas (GHG) emissions intensity (EI) of aviation fuel production via four CO₂-indirect pathways (intermediate products are required) is estimated and the WtW GHG EI compared to conventional fossil-based and bio-ethanol pathways. We aim to determine whether a one- or two-step electrochemical conversion is more likely to result in lower GHG intensity aviation fuel, under what conditions pathways incorporating these electrochemical processes have a lower GHG EI than conventional crude oil-based and biomass-based jet fuels, and whether these CO₂-derived sustainable aviation fuel (CO₂-SAF) pathways can approach “carbon neutrality.” The key findings from this work are: (1) processes using ethylene as an intermediate tend to have a lower GHG EI, although there is not a meaningful difference between one- and two-step pathways; (2) all pathways could achieve a lower GHG EI than fossil and biomass based routes if the location is carefully selected to minimize the GHG EI of electricity supply and if the CO₂ source is strategically chosen; and (3) while these pathways have the potential to approach zero GHG emissions, emissions from fuel manufacturing will be challenging to eliminate entirely. Notably, the GHG EI of CO₂-based SAF is far more sensitive to background system parameters, such as the carbon intensity of electricity and CO₂ supply, than to technical parameters. Therefore, we suggest that background factors may play a greater role in determining GHG EI than technical innovation.