Understanding the direct LCA emissions of SAF production pathways is crucial for making informed decisions and advancing sustainability goals in aviation. This analysis explores the direct LCA emissions associated with three prominent SAF production methods: HEFA, FT synthesis, and SIP production. Additionally, we introduce the Greenhouse gases, Regulated Emissions, and Energy use in Technologies (GREET) model, a comprehensive LCA tool that provides deeper insights into the environmental impact of SAFs. Evaluating these pathways and their associated emissions is essential in shaping the future of sustainable aviation.

Overall, this section provides valuable insights into the emission rating of different SAF production pathways and highlights the importance of feedstock selection and technological advancements in production.

HEFA Direct LCA Production Emissions

The HEFA process, pivotal in SAF production, involves transforming feedstocks like vegetable oils and animal fats into cleaner aviation fuels. Crucially, the emission impact of this method hinges on the chosen feedstock, as each carries its unique emissions profile.

Analyzing the direct LCA emissions reveals a significant disparity among feedstocks in HEFA production. For instance, used cooking oil (UCO) exhibits the lowest emissions at 13.9 gCO2e/MJ. This efficiency is attributed to UCO being a recycled waste product, thereby bypassing the need for additional land use or agricultural processes. In contrast, palm oil without methane capture records the highest emissions, peaking at 60 gCO2e/MJ. This figure reflects the substantial environmental impact of palm oil cultivation, including deforestation and significant methane emissions from palm oil mill effluent (POME), particularly when methane capture is not employed [57] .

The International Energy Agency (IEA) brings into focus a significant sourcing challenge for SAF feedstocks. UCO and animal fats, despite their lower greenhouse gas emission intensity and compatibility with EU feedstock requirements, are in soaring demand. Consequently, their use is projected to nearly exhaust all estimated supplies within the period up to 2027 [58] . This limitation highlights the need for diversifying feedstock sources to sustainably meet the growing SAF demand.

As a well-established technology, HEFA has been instrumental in pioneering SAF production. However, its dependency on limited feedstocks, each with their significant emissions benefits, poses a challenge. Although HEFA is a viable starting point, the advancement of alternative technologies and feedstocks is vital for adequately addressing the escalating demand for SAFs.

Table: Direct LCA Emissions for Hefa Production (GCO₂E/MJ) 

Fischer-Tropsch (FT) Production Direct LCA EMISSIONS

The Fischer-Tropsch (FT) synthesis is an advanced production process, involving gasification and catalytic chemical reactions. It starts with gasification, a high-temperature process converting solid materials like coal, biomass, or MSW into syngas—a mixture of hydrogen and carbon monoxide. This gas then undergoes purification to remove impurities, ensuring its suitability for the FT process.

Table: Direct LCA Emissions for Fischer-Tropsch Production (GCO₂E/MJ) 

In FT production, direct LCA emissions vary widely depending on the feedstock. For instance, agricultural residues demonstrate minimal emissions at 7.7 gCO2e/MJ, benefitting from being a readily available byproduct, thereby preventing methane release from decomposition. This contrasts starkly with non-biological MSW, which, when constituting over 45% of waste content, shows significantly higher emissions (86.2 gCO2e/MJ). The high emissions from non-biological MSW stem from the extra energy needed to process these non-biodegradable materials into suitable syngas.

FT synthesis offers a versatile approach to SAF production, capable of utilizing a variety of feedstocks, including biological materials such as agricultural waste and MSW. This flexibility presents a dual advantage: addressing waste management issues and generating cleaner liquid fuels. The method’s capacity to handle diverse feedstock types not only reduces dependence on limited resources but also broadens the spectrum of viable materials for SAF production.

As the aviation industry marches towards its sustainability objectives, FT synthesis emerges as a crucial technology. Its ability to produce more environmentally friendly and cost-effective SAFs, coupled with its diverse feedstock compatibility, marks it as a pivotal innovation in the journey toward greener aviation.

Table: Direct LCA Emissions for SIP (GCO2E/MJ)

SIP Production Direct LCA EMISSIONS

SIP (Sugar-to-Hydrocarbon) production offers a transformative approach in the realm of SAF production. This innovative process utilizes plant-derived sugars, typically from sugarcane or sugar beets, and converts them into jet fuel. Unlike traditional fermentation methods that produce alcohol, SIP involves cultivating sugar-rich plants, extracting their sugars, and then utilizing specific microbes to produce farnesene (C15H24), a longer chain hydrocarbon. These hydrocarbons are subsequently purified and refined to meet aviation fuel standards.

The SIP method for producing SAF is particularly noteworthy for its innovative use of sugar-rich crops, transforming them into a renewable jet fuel alternative. The direct LCA emissions from SIP, ranging between 32.4 and 32.8 gCO2e/MJ, illustrate its effectiveness in reducing greenhouse gas emissions. Furthermore, the widespread cultivation of sugarcane and sugar beets around the globe offers a flexible and regionally adaptable feedstock source.

“palm oil without methane capture records the highest emissions, peaking at 60 gCO₂e/MJ.“  

The integration of SIP technology into the broader SAF production landscape is significant. It introduces a novel approach that doesn’t solely depend on conventional feedstocks. By tapping into the global sugar crops sector, SIP production contributes to the diversification of SAF feedstock sources. This diversification is essential, offering an alternative and potentially more sustainable route to SAF production, distinct from the traditional pathways. The SIP method’s ability to efficiently convert sugars to jet fuel expands the possibilities for SAF, making it a valuable addition to the spectrum of technologies striving to meet the aviation industry’s sustainability goals.

AtJ Isobutanol and Ethanol Production Direct LCA Emissions

Alcohol-to-Jet (AtJ) technology innovatively converts biomass-derived alcohols like isobutanol and ethanol into jet fuel. This process involves fermenting biomass to produce these alcohols, which are then chemically transformed and refined to meet the strict standards of aviation fuel.

Table: Direct LCA Emissions For AtJ Isobutanol (GCO₂E/MJ) 

Table: Direct LCA Emissions For AtJ Ethanol (GCO₂E/MJ) 

The direct LCA emissions from the AtJ process provide important insights into its environmental impact. For example, switchgrass processed via the Isobutanol AtJ pathway results in emissions of 28.6 gCO2e/MJ. This figure is notably higher than the emissions from sugarcane processed through FT synthesis, which is 10.4 gCO2e/MJ. This discrepancy is largely due to the AtJ process’s higher energy requirements, indicating a substantial opportunity for emission reduction through the use of more sustainable energy sources.

“The AtJ pathway is still developing and quickly catching up to the maturity levels of established technologies like HEFA and FT.“

The AtJ pathway is still developing and quickly catching up to the maturity levels of established technologies like HEFA and FT. To enhance its viability as a sustainable fuel production method, ongoing research and development are essential. This research should focus on improving process efficiency and reducing environmental impacts, thereby strengthening AtJ’s role in the future of SAFs. As the industry evolves, the optimization of processes like AtJ will be crucial for meeting the stringent environmental goals of the aviation sector.

The GREET model – Greenhouse Gases, Regulated Emissions, and Energy Technologies

The GREET (Greenhouse gases, Regulated Emissions, and Energy use in Technologies) model, developed by the Argonne National Laboratory, stands as a comprehensive LCA tool. It evaluates energy use, greenhouse gas emissions, and air pollutant emissions across various transportation fuels and technologies, including SAFs [59].

Distinguished from the ICAO CORSIA framework, which centers on greenhouse gas emissions for carbon offsetting, GREET delves deeper. It offers an extensive analysis of environmental impacts, considering a broader range of factors and providing detailed emissions and energy consumption data. This holistic approach renders GREET invaluable for researchers and policymakers needing in-depth environmental insights into different SAF production pathways.

The GREET model’s application in assessing various SAFs—derived from waste cooking oil, algae, agricultural residues—has revealed their potential for significantly lower greenhouse gas emissions compared to conventional jet fuel, with some SAFs cutting emissions by up to 80%.

In policy circles, the GREET model’s influence is growing. On July 28, 2023, a bipartisan group of 21 federal lawmakers urged the U.S. Treasury Department to adopt GREET as the secondary methodology for calculating the SAF tax credit. They highlighted GREET’s accuracy and relevance to domestic practices, emphasizing its role in enabling farmers to engage in the clean energy market and its importance in carbon reduction efforts [60].

For policymakers and investors, familiarity with the GREET model is crucial. Policymakers can leverage GREET to compare environmental performances of SAF pathways, assess emission reduction potential of SAFs, and strategize to foster SAF adoption. Investors can use GREET to evaluate the environmental impact of potential SAF investments, aligning their choices with sustainability objectives.

The versatility of the GREET model in evaluating different SAF pathways and its capacity to provide detailed data on emissions and energy usage make it an essential tool. It not only informs decision-making but also guides strategic investments in SAF technologies, playing a pivotal role in the journey toward sustainable aviation.

Considering LCA to Advance SAF Production

In the pursuit of sustainable aviation, the LCA implications of SAF production emerge as a critical consideration. This involves selecting sustainable feedstocks and ensuring optimal returns on capital for both investors and policymakers who aim to support and implement the most effective strategies. The in-depth analysis of various SAF production pathways highlights the profound influence of feedstock choice on direct LCA emissions, land-use change emissions, and the overall environmental impact.

The availability and land requirements of feedstocks vary considerably, and their selection is pivotal in determining the environmental sustainability of SAF production. For instance, HEFA production primarily depends on limited feedstocks like used cooking oil UCO, which may not suffice to meet the escalating demand for SAF. In contrast, FT synthesis and SIP production provide more versatility in feedstock options, allowing the use of diverse sources such as agricultural residues, MSW, and sugar-rich crops.

Moreover, the emissions from land-use changes, especially ILUC are essential factors in assessing the sustainability of different SAF production methods. LCA offers a comprehensive framework to evaluate the environmental impacts of SAFs, taking into account both direct and indirect emissions from the production of feedstock to the combustion of fuel.

Policymakers and investors need to conduct thorough assessments of the available feedstock in their regions before committing to investments in production facilities. Ultimately, achieving successful SAF production necessitates a multifaceted approach that includes feedstock diversification, technological innovations, and thorough LCA-based evaluations. 

“Policymakers and investors need to conduct thorough assessments of the available feedstock in their regions before committing to investments in production facilities. “