Feedstock to Flight: Carbon Accounting During SAF Production

Written By Emily Tobler

New to the concept of lifecycle analysis in SAF? Check out part one of this series:
Feedstock to Flight: The Importance of Lifecycle Analysis in SAF — 4AIR

In this second part of our exploration into SAF lifecycle analysis, we delve deeper into the methodologies used to calculate emissions reductions earlier in the chain of custody. Keep in mind these key takeaways: 

  • Lifecycle analysis (LCA) is a technique used to estimate the environmental impacts of a product, material, or service throughout its entire lifetime, and it is essential for understanding the environmental benefits of SAF. 

  • Measuring SAF through a lifecycle analysis reveals that while all SAF contributes positively to emission reductions, not all SAF is the same. 

  • This comprehensive view of fuel emissions, known as Well-to-Wake (WTW) emissions, includes both Well-to-Tank and Tank-to-Wake emissions, typically measured in grams of CO2 equivalent per energy (megajoule) of fuel (gCO2e/MJ). 


Key Production Concepts & Calculation Elements 

As covered in Part 1, while the final product of SAF is certified to be chemically identical to conventional jet fuel, the emissions impact of a given batch of SAF is highly variable depending on the feedstock and production processes used. Understanding the carbon intensity (CI) of SAF is complex and depends on the specific scenarios and models used for lifecycle analysis (LCA). To better grasp the concept, let's break down some key terms: 

Biogenic vs. Non-Biogenic Emissions 

  • Biogenic Emissions are generated from the combustion, decomposition, or processing of organic materials not derived from fossil fuels. In simpler terms, biogenic CO2 emissions originate from the natural carbon cycle, where plants absorb CO2 and, after their lifecycle, release it back into the atmosphere, where it can be reabsorbed by other plants. This process keeps the carbon within the current atmospheric system, as opposed to adding new carbon to it.  

    One critical aspect of biogenic CO2 is that it is generally considered neutral (zero) in lifecycle assessments. This is because the CO2 released during combustion is assumed to be offset by the CO2 absorbed by plants during their growth phase through photosynthesis, making it part of a circular carbon process. 

  • Non-Biogenic Emissions, however, are not considered part of the circular carbon cycle. These emissions originate from non-organic sources. An example of this is SAF created from captured CO2 off of an industrial plant processing petroleum and converted using advanced technologies like power-to-liquid (PtL). In Europe, this type of SAF is called Renewable Fuels of Non-Biological Origin (RFNBOs). Unlike biogenic SAF, non-biogenic CO2 cannot be considered neutral when calculating LCA because the carbon is sourced from outside the biological carbon cycle. 

Currently, most commercially available SAF is produced entirely from biogenic feedstocks. However, as SAF produced from non-biogenic sources becomes available, or as biogenic and non-biogenic sources are combined to create SAF, emissions calculation guidelines will need to adapt to account for these new pathways. 

Understanding the distinction between biogenic and non-biogenic emissions is essential for accurately assessing the carbon intensity of SAF. This diagram helps visualize SAF of biogenic and non-biogenic origin; any feedstock outside the biomass circle would be considered of non-biogenic origin:

Core LCA Components 

Core LCA components (sometimes known as “the core”) represent the essential elements in calculating the carbon intensity of SAF. These are direct emissions from the supply chain of SAF production and use. The boundaries of what is considered in the core components can vary based on the type of SAF being analyzed, or the methodology used:

Image description: This chart shows the core LCA boundary (visualized within the dotted line) used to calculate carbon intensity in feedstocks like residues, by-products and wastes (such as corn, tallow, and used cooking oil). 

Source: Lifecycle Analysis for Sustainable Aviation Fuels - Allroggen, Florian - ASCENT 2023 SAF Meeting 

Land Use Change  

This concept broadly encompasses the ways in which feedstock production affects the use of land, and it can be categorized into two types: Direct and Indirect (or Induced). 

  • Direct Land Use Change (DLUC) occurs when existing agricultural land is repurposed for the cultivation of biofuel feedstocks. For example, if a parcel of land previously used to grow food crops is converted to grow feedstocks for biofuel production, this transition is classified as DLUC. Or if land not previously used for agriculture was converted into agriculture or woodland, this would have DLUC emissions. 

  • Indirect Land Use Change (ILUC) refers to the broader, often unintended consequences of biofuel production on land use patterns, quantified in terms of greenhouse gas (GHG) emissions. According to the World Economic Forum, ILUC arises when increased biofuel production drives market forces that lead to land indirectly being converted from other uses, such as forested areas, to crop production. This conversion can result in a net increase in GHG emissions due to factors like deforestation, reduced carbon sequestration, and changes in soil organic carbon. Estimating ILUC emissions involves complex economic models that simulate potential future scenarios based on assumptions about crop prices, yields, and demand responses that vary based on feedstock. These models generate estimates of land conversion and assign ILUC emissions factors to different feedstocks, which can significantly affect the final CI score of SAF. It is important to note that there is considerable uncertainty and debate in estimating ILUC emissions, and not all methodologies for calculating SAF’s carbon footprint consider it. 

Climate Smart Agriculture 

Climate-Smart Agriculture (CSA) practices optimize the carbon efficiency of biomass feedstocks used in SAF production. These practices can include no-tillage farming, crop rotation, using animal manure, and planting cover crops. By employing CSA practices, farmers can enhance soil health and resilience, leading to lower DLUC emissions and a lower carbon intensity in the resulting fuel. Despite the potential benefits, many LCA methodologies and current clean transportation fuel programs do not assign lower CI estimates to crops grown using climate-smart practices compared to those grown with conventional farming methods.

 

SAF Calculation Methodologies 

Producers are primarily concerned with calculating the LCA of SAF to determine whether their fuel meets sustainability standards to qualify as a sustainable alternative and secure additional incentives. The LCA of the same gallon of SAF can differ depending on the methodology used, which affects the net emissions benefit and often the eligible incentives. Two primary methodology categories are used to assess the CI of SAF: ICAO and GREET. 

ICAO 

The International Civil Aviation Organization (ICAO) developed a methodology consisting of two main components: 

  1. Core LCA Components: The upstream boundaries of ICAO’s core LCA includes feedstock cultivation; feedstock harvesting, collection and recovery; feedstock processing and extraction; feedstock transportation to processing and fuel production facilities; feedstock-to-fuel conversion processes; and fuel transportation and distribution. To calculate a value for this, their methodology accounts for emissions from ongoing operational activities, such as running a fuel production facility and cultivating feedstocks, as well as emissions from associated resources and utilities, like processing chemicals, electricity, and natural gas. However, emissions from one-time construction or manufacturing activities, such as building fuel production facilities or manufacturing equipment, are not included. 

  2. Indirect Land Use Change (ILUC): ICAO includes ILUC emissions to address potential unintended consequences of biofuel production, such as deforestation or changes in land use patterns.  

ICAO uses these two components to publish a set of default LCA values (measured in gCO2e/MJ): 

Image Description: This table contains a part of ICAO’s list of default SAF CI values, which vary by region, feedstock and conversion process.

Source: ICAO document 06 - Default Life Cycle Emissions - March 2024.pdf 

In March 2024, ICAO updated the list of default CI values for SAF LCAs, adding new pathways and feedstocks, and introduced pathway specifications for more detailed ILUC calculations.

GREET 

The Greenhouse gases, Regulated Emissions, and Energy use in Technologies (GREET) model, developed by the Department of Energy’s Argonne National Laboratory, evaluates the lifecycle emissions of various fuels, including SAF. It is adaptable to incentives and reporting schemes, such as the 40BSAF-GREET for the US Federal SAF Blenders Tax Credit and CA-GREET for California’s Low Carbon Fuel Standard.  

Like ICAO, GREET considers both core LCA and ILUC, but uses different data to inform its models, leading to differing ILUC estimates for feedstocks like soy and corn. 

When to Use Which Methodology 

Utilizing ICAO or GREET methodologies for calculating the LCA of SAF depends on regional regulations and the specific sustainability standards required to qualify for incentives:

As new incentives emerge, this has sparked significant debate over which methodology should be used. In 2023, the Department of Treasury had to determine which methodologies could be used to qualify for the Inflation Reduction Act’s Federal SAF Blenders Tax Credit (BTC). The decision became a focal point of contention, particularly over the inclusion of ethanol-based SAF. Ethanol could qualify under a version of GREET but not under ICAO. Ultimately, the BTC allowed either ICAO or a newly adapted version of GREET, which includes a pilot program for climate-smart agriculture practices—potentially paving the way for new carbon intensity reduction strategies within the biofuel supply chain. 

This debate highlighted key differences between the two methodologies. While ICAO/CORSIA is widely recognized as the universal standard, it relies on older data. In contrast, GREET, which is updated annually and tailored to U.S. regulations, offers more current information but has faced criticism. Environmentalists argue that GREET "waters down" sustainability criteria for SAF. They point to the strong support GREET receives from the airline and ethanol industries as a potential red flag, and desire to instead incentivize next-generation SAFs—such as those derived from forestry residues and municipal waste—rather than continuing to incentivize ethanol-based fuels. The models also diverge on their estimates of ILUC, with GREET providing lower ILUC estimates for feedstocks like soy and corn compared to ICAO.  

As the U.S. gears up for the next federal SAF credit (available from 2025-2027), stakeholders are keenly observing which methodologies will be approved, given the tight timeline for production readiness.

Part 3 of this series will unpack downstream SAF reporting and the impact on operators.

 
Emily Tobler
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