8

Aviation and Maritime Fuels

AVIATION FUELS

The life-cycle climate impacts of aviation fuels have been evaluated in the academic literature and as part of regulatory assessments for several fuel policies. There has been analysis of both conventional, petroleum-derived jet fuel and of a variety of alternative fuels produced through a wide array of conversion processes (i.e., alternative aviation fuels). In this section, the term “alternative aviation fuels” (AAFs) is used to refer to alternatives to conventional fossil aviation fuel. The term “sustainable aviation fuels” has been commonly used to describe alternative (non-petroleum) aviation fuels and is used in some policy contexts to refer to aviation pathways that satisfy certain sustainability criteria. However, this term in a general context may not necessarily indicate environmental benefits. Therefore, the term “sustainable aviation fuels” is not used in this report because it suggests an endorsement of the environmental benefits for all non-petroleum aviation fuels. For the purposes of evaluating the climate impacts of AAFs within fuels policies, this section discusses several key areas that may require special consideration beyond the approaches used for alternative fuels used in other sectors: (1) the non-carbon dioxide (CO₂) effects of aviation fuels when combusted at high altitudes, (2) the impacts of alternative fuels on airplane efficiency, and (3) the impact of a flexible product slate on the life-cycle emissions calculations for aviation fuels. Some of these effects are discussed further in the subsequent sections.

The standards organization American Society for Testing and Materials (ASTM) International has certified seven types of AAFs under its ASTM D7566 standard; this certification ensures the physical and chemical characteristics of fuels and their operational performance up to a specific blend level for each fuel (Prussi et al., 2021). Key criteria include composition, volatility, and stability; these fuels are suitable for commercial use as “drop-in” fuels when blended with conventional jet fuel up to their maximum blend level. ASTM International certification does not determine the technology-readiness level or sustainability of certified fuels. An overview of the ASTM International–approved and pending approval upcoming AAF pathways and their likely feedstocks is provided in Table 8-1. In addition to liquid fuels, energy supplied by electricity and hydrogen to alternative airframe designs can also be considered an AAF, though these technologies do not go through the ASTM International liquid fuel certification process (Viswanathan et al., 2022). Given that each pathway may utilize different feedstocks with varying environmental implications, a life-cycle analysis (LCA) is used to evaluate the impacts of each pathway relative to petroleum jet fuel, as well as to assess the impacts of different feedstocks of the production systems within each pathway.

Prior LCAs of AAFs have used a variety of analytical approaches and scopes, with a wide variation in results based on the authors’ assumptions and methodology. Across the literature, a major source of variation in the emissions estimates for AAFs is associated with the types of feedstocks; within a given conversion pathway, there are often some variations in emissions outcomes as across different conversion pathways. As shown in Table 8-2, most analyses have taken a primarily attributional approach, estimating on the energy and emissions directly attributable to feedstock production through to final use, without attributing market-mediated effects to AAFs. We note that the existing literature largely consists of attributional analyses and relatively few consequential assessments of the impact of AAFs; this is a limitation within the literature and is not meant to imply that market-mediated emissions or the consequential LCA (CLCA) approach in general are not an important consideration in the assessment of AAFs. The ratio of attributional LCA (ALCA) to CLCA approaches in Table 8-2 may not represent the importance or significance of the CLCA approach for AAFs. Across the literature, allocation approaches vary considerably by study; in some cases, studies utilize process-level allocation, wherein the allocation approach may vary depending on the life-cycle stage. For example, the impacts of producing biomass co-products such as soy

oil and soy meal may be allocated on a mass basis, whereas the products from a bio-refinery may be allocated on an energy basis. Some studies, such as Han et al. (2017), utilized a system expansion approach to attribute impacts from co-products with a different performance metric than energy, such as for the dried distillers’ grains with solubles co-products of corn alcohol to jet production. Many studies described in Table 8-2 include a sensitivity analysis to illustrate the impact of allocation assumptions on their emissions estimates. The Allocation section (Chapter 6) provides additional background on the methodology of allocation and its role in LCA.

Carbon Offsetting and Reduction Approaches

Several fuels policies also incorporate the LCAs of aviation fuel climate impacts, which are examined in the following sections.

TABLE 8-2 Sample of Published LCAs of Aviation Fuels (inclusion in this list does not imply endorsement by this committee)

The Carbon Offsetting and Reduction Scheme for International Aviation

The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) was developed and adopted by the International Civil Aviation Organization (ICAO) to reduce and offset a portion of the growth in greenhouse gas (GHG) emissions for international aviation. This offsetting scheme includes an assessment of the life-cycle emissions of a selection of various qualifying AAFs (ICAO, 2019; Prussi et al., 2021). These default values are intended to provide an accounting method for tracking some GHG reductions from petroleum displacement through the use of AAF, and are not presented as reflecting the view of this committee. In CORSIA, GHG emissions reductions may be generated by subtracting the life-cycle emissions for AAFs from the fossil fuel baseline, calculated as 89 gCO2e for jet fuel and 95 gCO₂e/MJ for aviation gasoline. More information on the variation in upstream carbon intensity (CI) of fossil fuels is in Chapter 7.

The methodological approach for LCA in CORSIA is summarized by Prussi et al. (2021) and presented in detail in ICAO (2019). The assessment takes a primarily attributional approach using energy allocation, in addition to a consequential induced land use change (ILUC) (as described by CORSIA) assessment for each crop-based fuel pathway. Emissions estimates from each pathway are broken into emissions values for “core LCA values” and “ILUC LCA value.” The core-LCA reflects the attributional emissions for each fuel, from its feedstock production through to final combustion. The ILUC assessment is based on a set of pathway-specific demand shocks input into the Global Trade Analysis Project BIO (GTAP-BIO) and the Global Biosphere Management (GLOBIOM) economic models, which reflect policy demand for a mix of alternative jet and road transportation biofuels co-products. Added together, the default core-LCA and ILUC emission factors for pathways can be compared to the fossil fuel baseline to determine a fuel’s eligibility relative to the 10 percent GHG reduction threshold for CORSIA eligibility.

CORSIA has defined a set of sustainability criteria. According to these criteria, an eligible AAF cannot be produced from feedstock “made from biomass obtained from land converted after 1 January 2008

that was primary forest, wetlands, or peat lands and/or contributes to degradation of the carbon stock in primary forests, wetlands, or peat lands as these lands all have high carbon stocks” (CORSIA, 2021). CORSIA also obligates AAF producers to also determine direct land use change (LUC) emissions for eligible land conversions that occurred after January 2008 and replace ILUC emissions with direct LUC emissions in cases where the estimated direct LUC is larger than ILUC. This approach, though implemented as a safeguard, may nevertheless ignore some ILUCs.

On the other hand, CORSIA suggests that certain land types, land management practices, and innovative agricultural practices can be considered to contribute to low risk for land area change. As a result, aviation feedstocks produced from these lands, upon check and verification, can receive a value of zero for ILUC in the LCA of a batch of fuels.

CORSIA specifies two approaches for low LUC risk aviation fuel feedstock production: the Yield Increase Approach, and the Unused Land Approach. For the Unused Land Approach, CORSIA (ICAO 2019, 10-11) states: “Eligible lands for the unused land approach could include, among others, marginal lands, underused lands, unused lands, degraded pasture lands, and lands in need of remediation.” In order to qualify as sustainable aviation fuel feedstock under the low land use risk category, certification is required by one of the CORSIA approved certification schemes. The certification schemes, in turn, work with CORSIA on the technical implementation of the policy. As a result, technical documents have been submitted to CORSIA to evaluate which types of land could in practice qualify under the low LUC risk land category. For example, a report by the University of Illinois at Chicago and Southern Illinois University Edwardsville (Mueller et al., 2021) explored qualifying reclaimed coal mining land for the carbon credits under this category. The low LUC risk categories as implemented within CORSIA are policy instruments intended to promote low LUC GHG values of a fuel produced within this framework, though their precise impact on emission remains uncertain.

CORSIA is unique as it is a global rather than national or regional low-carbon transportation policy effort. Therefore, when it came to ILUC modeling its guiding technical committee, the Committee on Aviation Environmental Protection (CAEP), had to oversee and negotiate the use of a larger range of possible models and input parameters than was the case for past national efforts. On the one hand, multiple models can reduce methodological uncertainties. The CORSIA “Life Cycle Assessment Methodology Document” states: “Because no model can pretend completeness of the representation, the comparison of different model results can help address model design uncertainty.” On the other hand, differing results and parameters had to be reconciled as part of the CORSIA LCA process. CORSIA’s ILUC modeling is based on two economic models, the GTAP-BIO developed at Purdue University and the GLOBIOM model developed by the International Institute for Applied Systems Analysis in Austria. These models represent two very different modeling approaches. The results from both models were reviewed by ICAO’s Fuels Task Group experts and reconciled into default ILUC values. The alignment between the two models varied across the 17 pathways reported by Prussi et al. (2021). In most cases, the differences were relatively small and close across the two models. Therefore, the average of the model’s results for each of these pathways has been adopted as the default ILUC value. However, for other pathways results were significantly different across the two models. For these pathways, after a review process, the Fuels Task Group assigned the lower of the two ILUC values from the modeling and added an adjustment factor of 4.45 gCO₂e/MJ.

The U.S. Renewable Fuel Standard

The U.S. Environmental Protection Agency (EPA) has also assessed the LCA emissions of various AAFs pathways to determine their eligibility under the Renewable Fuel Standard (RFS). Approved pathways include hydroprocessed esters and fatty acid fuels produced from soy, algae, waste fats, oils and greases, camelina, corn oil, and sorghum oil (EPA, n.d.). Within the RFS, these fuels’ life-cycle emissions have been assessed by EPA and meet, according to EPA’s methods, a 50 percent GHG reduction threshold in 2022 relative to the diesel comparator of 94 gCO₂e/MJ, including the impacts of ILUC; they are eligible for D4 and D5 Renewable Identification Numbers (see Chapter 3) as either biomass-based diesel or advanced biofuel, respectively.

The California Low-Carbon Fuel Standard

The California low-carbon fuel standard (CA-LCFS) was amended in 2019 to include aviation fuels. Unlike petroleum fuels used in the road sector, jet fuel is not subject to the CA-LCFS regulation and does not generate deficits. Rather, the production and blending of AAFs can generate credits as an “opt-in” fuel; credits are generated by subtracting the AAF’s CI from benchmarks calculated by the California Air Resources Board (CARB). Starting with a baseline CI of 87 gCO₂e/MJ that reflects the average CI of jet fuel consumed in California, the jet fuel benchmark remains fixed at the 2010 baseline CI for conventional jet fuel, with a zero percent reduction in each year, until the benchmark for diesel substitutes declines below the CI baseline for jet fuel in 2023 (CARB, 2020). From 2023 onward, the CI decline for jet fuel will move in parallel with that for diesel fossil fuel.

Non-CO₂ Effects of Aviation Fuels

Beyond the climate forcing contributions of conventional GHGs such as CO₂, methane (CH₄) and nitrous oxide (N₂O) throughout the life cycle of aviation fuels, jet fuel combustion at altitude may also contribute to climate change through other forms of radiative forcing. Incomplete combustion of jet fuel may result in nitrogen oxides (NO_x) formation, aerosolized sulfates and soot, contrail formation, and contribution to cirrus cloud formation (IPCC, 1999). These emissions, in conjunction with NO_x, sulfates, and water vapor, may contribute to contrails and increased cirrus cloud formation, together called aviation-induced cloudiness. Notably, the magnitude and sign of these effects differs substantially—for example, sulphate aerosols are a negative radiative forcer whereas contrails provide a net warming effect. Figure 8-1 illustrates the possible routes that incomplete aviation fuel combustion may contribute to radiative forcing. These impacts may warrant consideration in LCA approaches for aviation fuels, as they are partly attributable to the types of fuels combusted; therefore, a portion of these impacts may be attributable to fuels. This raises issues of both how to set a baseline for aviation fuels and how to estimate the difference in overall climate impact between conventional aviation fuels and AAFs.

A recent analysis of the cumulative contribution of aviation to the climate through 2018 estimates with greater precision the contribution of aviation-induced cloudiness. Lee et al. (2021) estimates that non-CO₂ effects comprise approximately 2/3 of aviation’s current radiative forcing. Figure 8-2 separates out the effective radiative-forcing impacts of AAFs into several discrete categories in milliWatts per square meter (mW/m²). Each component, which may have either a cooling or a warming effect, is shown with a best estimate and a confidence interval. NO_x emissions at high altitudes can have a variety of different effects that may both warm and cool the atmosphere. Summed together, these impacts add up to 17.5 mW/m². However, the largest contributor to the total impact is contrails and increased cirrus cloud formation, with an impact of over 50 mW/m². When taking into account non-CO₂ effects, the CO₂ combustion impact of aviation alone declines to approximately 34 percent of the total. In particular, the impacts of NO_x, soot, and contrail formation are estimated to add significantly to the overall climate impact of aviation.

The aromatic content of jet fuel is strongly correlated to the soot emissions. The aromatic content of jet fuel is an important factor in determining its operational performance, as it affects density, boiling point, smoke point, and freeze point for the fuel. Aromatic content is limited to a maximum of 25 percent and minimum of 8 percent by volume as part of the ASTM International certification for conventional jet fuel. Within that range, there may be substantial variation in aromatic content for fossil jet fuel (Hemighaus et al., 2006). Reduced aromatic content can generate a decrease in ice crystal numbers in contrails, but also an increase in water vapor. The net impact of the change in the fuels’ aromatic content remains highly uncertain and requires additional testing and measurement, particularly to quantify the impact of sustainable aviation fuel blending. Early research suggests that reduced aromatic content in fuel could lead to a reduction in optical depth of the contrails, shorter contrail lifetimes, and overall decreased radiative forcing. (Bräuer et al., 2021). Klower et al. (2021) parameterized the impact of low aromatic AAFs on contrail cirrus formation as a function of the square root of the fossil fuel share, implying that a 50 percent reduction of jet fuel aromatic content via AAF blending reduces contrail cirrus radiative forcing by about 30 percent.

Reducing aromatic content through lower-aromatic conventional fossil jet fuel or added AAF content could therefore reduce contrail cirrus radiative forcing.

The uncertainty of non-CO₂ contributions to the climate impact of aviation may be significantly higher than that of CO₂ from jet fuel combustion; Lee et al. (2021) estimate that non-CO₂ forcing terms contribute about eight times more than CO₂ to the uncertainty in aviation’s net effective radiative forcing. The non-CO₂ effects of aviation fuels raise several important questions for LCA of aviation fuels and in turn, for aviation fuel policy.

Though existing LCAs of aviation fuels have generated a set of estimates for the emissions of conventional GHGs released from the well-to-wake production and use of aviation fuels, compared to the quantity of existing literature on the life-cycle GHG emissions for AAF production, the data are more sparse on the non-CO₂ effects of AAFs. Preliminary research suggests that lower aromatic content in aviation fuels may result in reduced contrail formation (Voight et al., 2021). The magnitude and direction of some effects may differ based on the use case, route, and altitude of the flight in question; for example, non-CO₂ emissions at stratospheric altitudes from supersonic airplanes would differ from the impacts of subsonic flights consuming the same fuel at lower altitudes.

It is challenging to present the non-CO₂ impacts of aviation fuel combustion on a consistent basis alongside the climate impacts of the fuel cycle, largely due to the different time scales. The fuel cycle is dominated by CO₂ emissions and thus is not sensitive to assumptions of time horizon; in contrast, the impact of non-CO₂ emissions is much more sensitive to the assumptions of the time horizon and operating conditions (altitude and atmospheric conditions). Non-CO₂ effects have the highest warming impact at short time-scales, with the CO₂ impact overshadowing other warming effects on longer time scales (Fahey et al., 2016). Alternative metrics such as global temperature potential (GTP) and average temperature response (ATR) suffer from the same problem, as they remain sensitive to assumptions of time horizon (EASA, 2020).

Combusting aviation fuels in flight releases CO₂ in addition to a mix of other pollutants such as soot, water vapor, sulfates, and NO_x capable of generating climate impacts by interacting with the atmosphere at high altitudes.

Impacts of Alternative Aviation Fuels on Aircraft Efficiency

Using alternative fuels or electrification technologies (e.g., batteries or fuel cells) for aviation will alter aircraft efficiency if they impact the total weight of the aircraft, which is particularly important during takeoff. For example, Bills et al. (2020) illustrated the impact of batteries on aircraft weight in their 2020 article (see Figure 8-3). During the climb phase of the flight, energy consumption per unit time approximately doubles relative to the cruise phase of the flight. Because of restrictions on takeoff weight, increasing or decreasing the weight of the aircraft through the use of alternative fuels or electrification technologies will impact the payload an aircraft is capable of carrying and the maximum range for long flights. The length of a given flight will also affect the overall energy use and life-cycle emissions per passenger-kilometer or tonne-kilometer transported; shorter flights will have larger impacts per unit cargo-kilometer because of the outsized impact that the climb phase will have on total trip-level energy use.

So far, there is no standard set of aircraft design and flight duration assumptions used by researchers or LCA practitioners for comparative analysis of AAFs. For liquid fuels that can be combusted in conventional jet engines, the Breguet Range Equation can be used to estimate the impact of more or less energy-dense fuels on total aircraft efficiency, given a set of specific aircraft specifications and trip length, as illustrated in Baral et al. (2019). In most cases, the fuel savings associated with a more energy-dense liquid fuel will be small because these fuels will be blended with conventional petroleum jet fuels. Given the small expected differences in aircraft efficiency, comparing emissions savings on a per-MJ higher heating value content basis for most AAFs is appropriate. Table 8-3 provides an overview of different fuels and energy carriers used in aviation and provides their energy density and density.

Assigning appropriate functional units when comparing across battery-electric aircraft, fuel cell electric aircraft, and aircraft with jet engines running on liquid fuels is more complex. Such cross-comparisons have been done in the scientific literature for passenger vehicles by using 1 km or mile traveled by a typical sedan as a functional unit (e.g., see Yuksel and Michalek, 2015). Chester and Horvath (2009) similarly explored vehicle-km and passenger-km as different functional units, noting that ridership impacts the latter unit on mass transportation modes where ridership may be well under maximum capacity in some cases. This approach works well for research applications, where introducing additional complexity is more acceptable when that complexity advances the goal of providing the most accurate comparison possible.

Effects of a Mixed Product Slate

Many AAF production pathways generate a variety of hydrocarbons as part of their product slate; in some cases, jet fuel is not even the largest share of the end product (either on an energy or market value basis) (Pearlson et al., 2013; Stratton et al. 2010). For some AAF processes, such as hydroprocessed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK) and Fischer–Tropsch synthetic paraffinic kerosene (FT-SPK), it may be impossible to generate a 100 percent jet product slate. Though these processes are currently optimized to maximize the share of middle distillates in their product slate, it may be possible in some cases to increase the jet fraction, while simultaneously increasing the share of less-valuable light ends such as naphtha and propane (Pearlson et al., 2013). Increasing the jet fraction requires additional hydrogen and may therefore increase the overall energy consumption and emissions for a biorefinery.

For the purposes of an LCA of jet fuel, the operating parameters of a biorefinery that produces jet fuel as a part of an overall product slate may have important implications on its climate impact. For example, a middle distillate-optimized biorefinery may have a different overall energy consumption and product slate than a jet-optimized biorefinery, changing overall emissions and the relative shares of co-products.

For AAF production pathways that generate both an aviation and a road fuel co-product, maximizing the share of AAF output requires additional energy and may reduce overall biofuel yields, increasing emissions attributable to AAF production.

MARITIME FUELS

International goods movement on ocean-going vessels carried 90 percent of international merchandise trade (11 billion tons) in 2016 (Li, 2020). In transporting this large amount of goods, marine vessels consume about half of fuel oil demand (IEA, 2020). Fuels for these vessels are undergoing notable changes mandated in the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI treaty. These rules require cuts in fuel sulfur levels from 3.5 percent to 0.5 percent m/m for vessels operating outside of designated emission control areas, which have a different set of more stringent emissions limits. They went into effect in 2020 and affect the consumption of predominant marine fuels including higher sulfur fuel oil. In 2018, the International Maritime Organization adopted a resolution on the initial strategy to reduce GHG emission from ships. It has set targets of 20 percent GHG emissions reductions from ships in 2020, 40 percent by 2030, and 50 percent by 2050. The method for achieving these

targets remains under development. In the meantime, MARPOL Annex VI undoubtedly will influence lifecycle GHG emissions of marine fuels and goods transport as the marine sector adopts technology to comply with it.

One main MARPOL Annex VI compliance strategy is using low sulfur fuels. Oil-derived options include very low sulfur fuel oil and marine gas oil. Refineries are adjusting to the increased demand for very low sulfur fuel oil, which is expected to dominate the marine fuel market (IEA, 2020). To produce lower sulfur fuels, refineries may install hydrodesulfurization technologies that consume energy and hydrogen beyond current levels or produce more distillates to blend with higher-sulfur fuels (Van et al., 2019). These changes would influence the life-cycle GHG emissions of oil-derived marine fuels and would vary from refinery to refinery. Installing on-board scrubbers that remove sulfur oxides (SOx) from ship exhaust streams is a second compliance option. These scrubbers take up space and, if releasing scrubber waste streams to the ocean in open loop systems, can have negative environmental consequences. A third option is to use liquefied natural gas (LNG), which has a higher calorific value (~20 percent) than liquid fuels, but requires vessels to be retrofitted for its use. Current trends indicate existing vessels are more likely to switch to lower-sulfur, oil-derived fuels or install scrubbers whereas new vessels may be built to use LNG as a fuel (Li, 2020). Using other marine fuels is also an option: see Table 8-4.

In sum, future climate regulations from the International Maritime Organization may further influence the mix of fuels supplied to the marine sector. Sulfur regulations such as MARPOL Annex VI are already driving changes in the production and use of fuels in the marine sector, including in refineries that are experiencing increasing demand for very low sulfur fuel oil. Importantly, marine fuels have similar supply chains to other transportation fuels (e.g., aviation, road transport). Unique aspects of their life cycle that are relevant to quantifying their emissions primarily come in the operations stage, such as CH₄ slip from LNG combustion in marine engines. CH₄ slip from LNG engines will vary based on the engine type and the load profile of the engine (Ushakov, 2019). In general, LCA methodological considerations for marine fuels are similar to those for other transportation fuels (Tan et al., 2021).

TABLE 8-4 Marine Fuels That Could Be Included in an LCFS

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