Life Cycle CO2e Intensity of Sustainable Aviation Fuels and their Specific Use Cases

Abstract

Sustainable aviation fuel (SAF) is widely accepted as the primary emission reduction mechanism for the aviation industry in the short-to-medium term. SAF is produced from renewable sources and undergoes a qualification process to ensure its technical quality is suitable for use as a `drop-in� fuel with existing aircraft and infrastructure. There are currently eight technical pathways approved, and once certified, SAF can be blended at up to 50% with conventional aviation fuel.

Recognising the pivotal role of SAF in reducing emissions from aviation, governments worldwide are enacting policies to stimulate its production and adoption. The European Commission�s ReFuelEU Aviation legislation mandates that airlines uplift increasing amounts of SAF in the EU, starting at 2% in 2025 and rising to 70% by 2050. Regulatory frameworks also require SAF to achieve a minimum emission reduction over the full life cycle, relative to conventional aviation fuel. Therefore, confidence in the life cycle emissions of SAF is crucial. It is found that there is a lack of consistency, rigour, and transparency in the literature for sustainability assessments of SAF.

This work utilises a life cycle assessment (LCA) methodology developed from first principles to account the life cycle CO2 equivalent (CO2e) emissions associated with scenarios of sustainable aviation fuel in a specific, granular, and transparent manner. The methodology is underpinned by physical relations and mathematical operations. Realistic supply chains and scenarios that represent real world operations are constructed. Additionally, uncertainty analyses of each SAF scenario are conducted using Monte Carlo simulations.

The methodology is first applied to the commercially mature hydroprocessed esters and fatty acids (HEFA) pathway with used cooking oil as a feedstock. The HEFA SAF production is modelled to take place in Finland, while three locations of feedstock sourcing (Finland, Germany, and China), two hydrogen sources (grey hydrogen produced via the steam methane reforming of natural gas, and green hydrogen produced via the electrolysis of water using renewable electricity), and three energy sources (grid electricity, onsite wind electricity, and a mixture of 95% natural gas and 5% grid electricity) are analysed. The CO2e intensity of the HEFA SAF is calculated to range from 2 to 22 gCO2e/MJ across all energy input and supply chain scenarios, representing reductions of 76 to 97% relative to the conventional aviation fuel baseline.

The methodology is then applied to the Power-to-Liquid (PtL) SAF pathway utilising captured CO2 and renewable hydrogen as feedstocks and the Fischer-Tropsch conversion technology. The PtL scenarios are based in Bilbao, Spain, and consider four CO2 capture technologies (post combustion capture from fossil and biogenic sources, and direct air capture using high and low temperature technology) and two sources of electricity (grid and onsite-generated renewable electricity). The results of the life cycle assessment (LCA) show a large range in CO2e intensity of the PtL SAF, from 11 to 101 gCO2e/MJ. The use of renewable electricity is fundamental to all scenarios that achieve the 70% reduction, as required under the ReFuelEU Aviation legislation. Furthermore, alternative fuel production options such as replacing natural gas with biomethane to provide high temperature heat, electrifying the reverse water gas shift (RWGS) unit process, and employing electrochemical CO2 reduction technology offer further life cycle CO2e reductions of up to 7, 22, and 16% for some scenarios, respectively.

Finally, specific use cases of the HEFA and PtL SAF are modelled on real world Ryanair flights using actual flight data which informs on the mass of fuel utilised by specific aircraft on specific routes. The CO2e emission intensity of these commercial flight scenarios are reported in the industry standard reporting metric of gCO2e per revenue passenger kilometre (gCO2e/RPK). Numerous combinations of SAF and route are considered for both Boeing 737-800 and 737 MAX 8 aircraft. It is found that the flight route has a significant effect on the gCO2e/RPK intensity, with short routes resulting in higher CO2e intensities due to a higher proportion of the flight spent in the energy-intense climb stage. While operational measures such as increased load factors, and fleet renewal are important to reduce the gCO2e/RPK intensity, the greatest potential for reduction comes from SAF. However, the extent of the reduction depends on the blend ratio and the life cycle CO2e intensity of the SAF, which in turn is influenced by factors such as the supply chain configuration, feedstock choice, conversion technology, and the calculation methodology applied. This underscores the necessity of specific, granular, and transparent analysis.