Decarbonizing Aviation: Enabling Technologies for a Net-Zero Future

Executive Summary

Combustion of jet fuel is one of the most important mechanisms by which aviation contributes to climate change, accounting for an estimated 2%–3% of global carbon dioxide (CO₂) emissions before considering additional warming effects from contrails. Those emissions could triple by 2050, potentially accounting for 25% of CO₂ released into the atmosphere as emissions across other sectors fall. As demand for air transport of people and goods continues to grow, decarbonizing the aviation industry is in many ways the most difficult challenge facing the transportation sector – in large part because the weight and space constraints of air travel are most cost-effectively met using energy-dense fossil fuels. Despite the challenge, countries have made commitments to reducing the aviation industry’s climate footprint, with most efforts to date focused on increasing the use of sustainable aviation fuel (SAF). In practical terms, SAF typically refers to biofuels that are derived from waste oils and fats treated with hydrogen such that the final product is chemically identical to kerosene. These biogenically sourced SAFs are the only alternative to jet fuel that are fully commercially available today. Their climate impact depends on many factors, but, theoretically, some bio-SAF can deliver substantial CO₂ reductions. For example, typical production methods for hydro-processed esters and fatty acids (HEFA), a common form of SAF, are estimated to reduce greenhouse gas emissions by 50%–65% compared to conventional jet fuel on a lifecycle basis. 

However, exclusive reliance on bio-SAF to achieve stated decarbonization objectives is unlikely to succeed over the longer term given feedstock constraints and sustainability concerns related to the indirect effect of biofuels production on agriculture and land use. Comparing projections for sustainable biofuels supply against expected growth in aviation energy demand over the next several decades suggests that, by midcentury, aviation energy demand – at an estimated 21.5 quadrillion BTU (quads) – could be double the energy available from projected global biofuels supply. This shortfall points to the need for other low- or zero-carbon aviation technologies. Some of the main candidates are synthetic or e-fuels that, like SAF, could be used as drop-in substitutes for fossil kerosene; direct use of hydrogen and ammonia as aircraft fuels; and fully electric aircraft. This report draws from the literature and from interviews with key market players for a detailed examination of each of these non-fossil aviation decarbonization options. 

Synthetic fuels refer to hydrocarbon fuels that are typically synthesized from syngas, a mixture of carbon monoxide and hydrogen. Synthetic fuels can deliver substantial CO₂ reductions depending on the source of the CO₂ and the carbon intensity of the hydrogen used to produce them. These are sometimes referred to as “e-fuels”, if the synthetic fuels use hydrogen sourced from electrolytic processes. Several climate beneficial options for sourcing CO₂ are being investigated, including CO₂ capture at emissions point sources such as power plants, waste biomass gasification, direct air capture of CO₂ from the atmosphere, and oceanwater carbon capture. From a technical perspective, the most mature of these carbon sourcing options involve point-source carbon capture while the least mature utilize oceanwater carbon capture. Direct air capture (DAC) is likely the most readily scalable technology, but it is currently also the most expensive, at $600–$1,000 per metric ton of CO₂ captured¹. This is significantly higher than the cost of point-source CO₂ capture at $15–$130 per metric ton. Going forward, DAC costs are expected to drop, especially if the advantages of off-grid co-location with renewables are exploited. This could make DAC, alongside waste biomass gasification, one of the most economical and climate beneficial sources of CO₂. Capturing carbon from oceanwater is a relatively new idea but it may have promise, especially considering that the concentration of CO₂ in the ocean is 120–150 times its concentration in the atmosphere. To pursue this option, however, it will be important to address concerns about potential effects on ocean ecology and determine the rate at which the ocean will re-absorb CO₂ from the air with a high degree of certainty.   

Clean hydrogen with low or zero upstream carbon emissions is the other component needed to synthesize aviation fuel. From a climate perspective, hydrogen produced via water electrolysis using clean electricity resources is probably the best option, but hydrogen made from fossil fuel reforming with carbon capture will likely also play a role. Regardless, low-carbon hydrogen must be produced at a competitive price. Current global expectations range from $1 to $6 per kilogram (kg), with significant variation in production costs between specific projects and geographies owing to differences in production method, capital costs of renewables, capital costs of equipment, as well as operations and maintenance costs.  

Once both components have been sourced, captured CO₂ and low-carbon hydrogen can either be converted to syngas for subsequent Fischer-Tropsch conversion to liquid hydrocarbon fuels, or they can be synthesized into an alcohol, such as methanol, which can then be converted, via alcohol-to-jet technology, to synthetic kerosene. Current cost estimates for synthetic fuel range from $1.50 to $4.50 per gallon, which is two to five times the cost of traditional jet fuel. Prices are projected to fall as CO₂ and hydrogen costs decline and the market grows. That said, the high cost of synthetic fuels today does not diminish the importance of this technology. Like biofuels, synthetic kerosene, also referred to as synthetic SAF, offer a drop-in solution – they can be introduced slowly, and blended directly with current jet fuel, such that the market has time to develop even as emissions begin to decrease.  

The potential advantages of synthetic SAF notwithstanding, more ambitious zero-carbon aviation technologies are in development. Hydrogen can be used to power air travel, reducing a significant portion of aviation emissions to near zero depending on how the hydrogen is sourced. This option, however, requires significant development of hydrogen infrastructure and entails changes to aircraft design that would require a lengthy re-certification process; in addition, longer-range planes would lose passenger seating to hydrogen tankage. To date, efforts to develop hydrogen-fueled aircraft technology have focused on smaller aircraft for regional routes.  

Another zero-carbon option is ammonia-powered flight. Ammonia, a hydrogen carrier, has 49% more energy per volume than liquid hydrogen, which means that ammonia-fueled aircraft could use conventional configurations while flying distances up to twice that of hydrogen-fueled aircraft. However, fuel toxicity and, similar to hydrogen, the need for specialized infrastructure are downsides, with toxicity being of particular concern for passenger aircraft. Ammonia holds promise but the technology is approximately 10 years behind hydrogen, meaning that its contribution to decarbonizing aviation in the 2025–2050 timeframe is likely to be small.   

Fully electric aircraft, likewise, could end up having a niche role, but that technology is not expected to meaningfully reduce emissions relative to traditional air travel. The primary reason is that battery energy densities of approximately 750 watt-hours per kilogram (Wh/kg) would be needed to support commercial flights over regional distances. This is three to four times the energy density of battery chemistries available today. Even if battery technology advances, hydrogen or ammonia will likely remain the better option. Electricity as an energy source for air travel makes the most sense for intra-city operations using electric vertical take-off and landing (eVTOL) aircraft, a technology that could help to ease on-road vehicle traffic if its market grows.  

This study uses information from a literature study and from interviews with industry participants to develop scenarios for achieving net-zero aviation emissions by 2050 using a mix of four of the aviation fuels discussed above: bio-SAF, synthetic kerosene, hydrogen, and ammonia.² To ground our analysis, we compared our results to supply projections for each fuel from the International Energy Agency’s (IEA’s) net-zero-emissions-by-2050 (NZE) scenario. None of the fuel quantities required in our scenarios was allowed to exceed the IEA NZE supply projections, with the exception of ammonia, where the IEA projection reflects only existing uses and marine fuel usage. For hydrogen, we use the IEA NZE projection of hydrogen supply for transportation uses only, which makes up approximately half of the IEA’s projection for total hydrogen supply. The synthetic fuel supply was governed by the IEA projection for available CO₂ from DAC only.   

With these constraints and gathered know-how from interviews, our base case assumes that 90% of projected available biofuel feedstocks goes to SAF production. Additionally, airplanes operating on hydrogen fuel are assumed to cover 20% of regional flights (less than 750 miles, or 1207 km) while airplanes operating on ammonia fuel are set to cover 10% of flights less than 1650 miles (2655 km).  The remaining demand is assumed to be covered by synthetic fuels. With these assumptions, the 2050 fuel mix in the aviation market consists of 86% bio-and synthetic SAF while zero-carbon fuels make up the remaining 14% in our base case. The bottom of figure E.S.1. shows the fuel energy needed for aviation as a percentage of projected total supply from the IEA. 

Figure E.S.1: Results for the base case  

As noted on the figure, the IEA’s ammonia projections do not include aviation demand, meaning that global ammonia production would have to expand by an additional 19% over the IEA projection. According to the IEA’s estimates, the entire transportation sector will consume 12.5 quads of synthetic fuel and 23.3 quads of hydrogen in 2050. In our base case, aviation accounts for 68.5% and 81.7%, respectively, of the transportation sector’s demand for those two fuels, where our estimate for hydrogen demand in the base case includes fuel for direct hydrogen powered flight, as well as the hydrogen needed as a feedstock to produce the other three fuels. A scenario in which the aviation sector utilizes 81.7% of all the hydrogen projected to be used for transportation in IEA’s NZE case for 2050 (or 40.3% of the total hydrogen supply projection) would have significant implications and would likely require specific policy and planning interventions to avoid supply shortfalls that could elevate costs across the industry. 

Analysis of the other fuel mix scenarios highlights another challenge: increasing reliance on comparatively straightforward, drop-in synthetic fuels to meet future aviation demand requires the most hydrogen of any scenario analyzed – 3.18 quads more hydrogen than the base case discussed above. By contrast, the scenario that instead focuses on aggressive deployment of hydrogen-powered aircraft requires the least amount of hydrogen. This perhaps surprising result illustrates both (1) the extent to which future demand for hydrogen will be driven by its use as a needed input to synthetic SAF production and (2) the advantage of directly using hydrogen given its high specific energy. In sum, there are tradeoffs between the convenience of drop-in synthetic SAF, the ability to quickly scale low-carbon hydrogen production, and uncertainty about how quickly direct hydrogen-powered flight might mature.  

To help overcome these challenges and promote the tools and technologies needed to achieve aviation decarbonization, a wide range of policies are needed. One broadly applicable policy tool is a clean fuel standard, which would require gradual reductions in the carbon intensity of the transportation fuel mix. A strong clean fuel standard would include: a trajectory to net-zero by midcentury, robust and comprehensive lifecycle GHG accounting, safeguards against overreliance on unsustainable feedstocks, and multi-sector coverage with appropriate restrictions on cross-sector credit trading to avoid scenarios in which airlines buy inexpensive credits generated in other sectors rather than investing in the development of low-carbon aviation fuels. Other useful policies include synthetic SAF mandates or production subsidies as well as policies designed to support synthetic SAF production by boosting the supply of climate-friendly hydrogen and carbon feedstocks. Finally, governments should explore policies that help accelerate full commercialization of the critical technologies needed to decarbonize aviation by actively investing in research and development initiatives. 

In summary, the projected supply of climate-friendly biofuels is likely not sufficient to meet the aviation sector’s full energy needs. Other types of energy carriers will need to be investigated, developed, and potentially deployed. Our analysis shows that while many significant obstacles remain, the potential shortfall in low-carbon aviation energy supply due to biofuels’ sustainability issues can be managed using other enabling technologies. Achieving such a multi-fuel future, however, will require a massive amount of low-carbon hydrogen, significant cost reductions, and substantial innovation, driven by major strategic changes in the aviation industry and well-thought-out policies to create the correct incentives. That said, if successful, a multi-fuel approach could open the door to realizing ambitious decarbonization commitments in the aviation sector.

1. Introduction

The ability to transport people and goods over long distances by air is a defining feature of the modern age, and as incomes have risen around the world, demand for air travel has grown steadily – with direct consequences for aviation energy use and climate change impacts. Airplanes, which require energy-dense fuels because of their inherent weight and space constraints, today operate almost exclusively on kerosene, a carbon-intensive fuel that is produced from petroleum.³ Carbon dioxide (CO₂) emissions from jet fuel combustion are thus one important mechanism by which aviation contributes to global warming. In 2018 direct emissions from aircraft operations were estimated to account for about 2.5% of global CO₂ emissions.⁴ As emissions across other sectors begin to fall, at the current pace aviation could account for 25% of CO₂ released into the atmosphere by 2050.

Aircraft operations also affect Earth’s atmosphere in other climate-relevant ways. Typically summarized by the term “non-CO₂ effects”, additional warming occurs when engine emissions such as black carbon (soot), sulfate aerosols, and nitrogen oxides attach to atmospheric water vapor, forming cirrus clouds and contrails in the stratosphere that interact with atmospheric methane and tropospheric ozone. There is greater uncertainty about these direct and indirect impacts, but recent studies conclude that their net warming effect is roughly equal to or even greater than the contribution from aircraft CO₂ emissions.  

Several features of the aviation sector – most notably its multi-faceted climate impacts, strong projected growth trajectory, demand for energy-dense fuels, and significant capital stock of long-lived equipment and infrastructure that have been optimized for a single fuel type – present a significant challenge for decarbonization. Nonetheless, the industry has made a series of commitments in recent years to reduce its climate footprint. The International Civil Aviation Organization (ICAO), a technical body of the United Nations that serves as a global policy and standards forum for the aviation industry, in 2010 adopted Resolution A37-19 which sought to achieve 2% annual fuel efficiency improvements through 2050 and carbon-neutral growth from 2020 onwards, effectively maintaining existing aviation emissions through midcentury.⁵ In 2021, the International Air Transport Association (IATA), which represents 300 airlines, adopted a resolution calling on its members to achieve net-zero carbon emissions by 2050.⁶ ICAO followed suit in 2022 with its member states agreeing to an updated aspirational goal of net-zero CO₂ emissions by 2050.⁷ Numerous individual airlines, including many major carriers, have made similarly ambitious pledges. 

To date, efforts to begin making good on these commitments have largely focused on the expanded use of sustainable aviation fuels (SAF), a designation that could theoretically encompass a range of climate-friendly alternatives to conventional kerosene jet fuel. However, no clear definition of what constitutes “sustainable” in the aviation context has been established, and the term SAF has never been consistently applied. In practice, aviation biofuels – specifically, fats, waste oils and vegetable oils that have been treated with hydrogen to make them suitable for blending with kerosene – are the only alternative to petroleum-based jet fuel that is fully commercially available today. And while any fuel of biogenic origin is often assumed to qualify as sustainable by definition, the reality is that different biofuels vary widely in terms of their lifecycle carbon emissions, as well as their other ecological and social benefits. These impacts depend on a host of factors, including the land, water, and energy requirements associated with different types of feedstocks, cultivation practices, and fuel production processes. That said, from a climate standpoint alone the lifecycle carbon reductions obtained by using waste-based biofuels instead of conventional fossil fuels can be substantial. Other potential low- or zero-carbon options for meeting the aviation sector’s energy requirements, such as synthetic or e-fuels, direct hydrogen- or ammonia-powered flight, or electrification, show promise but are not yet commercially available.  

In September 2022, Clean Air Task Force (CATF) released a report on decarbonization challenges and opportunities for the aviation sector that reflects the current focus on biomass-based fuel alternatives. The report first defined the case for decarbonizing aviation, including by estimating aviation’s climate impact in 2050, then looked specifically at constraints on sustainable biofuels production in the context of those projections for future aviation energy demand.⁸ It concluded that biomass-based options for decarbonizing aviation will likely not be sufficient. Biofuels will certainly play a role, and their advantages in terms of compatibility with existing aircraft and fueling infrastructure are important, especially in the near term. But supplies of the SAF that are widely commercially available today are fundamentally limited by feedstock and land-use constraints, competition with food production, and ecological concerns that will ultimately limit the share of aviation emissions that can be addressed through biofuels use alone.  

This paper follows up on the key findings from CATF’s earlier report to present a fuller analysis of other non-biofuel options for the aviation sector, including synthetic liquid fuels that can be made using captured carbon and renewable hydrogen, as well as other fuel alternatives such as hydrogen and ammonia. For each of these alternatives, we take stock of the current outlook, drawing on findings in the literature and interviews with industry leaders to highlight recent cost and technology developments in the areas of synthetic fuel production, carbon capture, and low-carbon hydrogen production. Later sections touch briefly on non-CO₂ climate impacts as well as discuss scenarios for aviation sector decarbonization that rely on different mixes of low- and zero-carbon fuels. We also present policy recommendations for supporting the development of a wider range of climate-friendly aviation fuels.  Lastly, while it is an important topic, a rigorous assessment of whether, when, and to what extent captured carbon should be permanently sequestered or used to make synthetic fuels is a topic set aside for future work. 

2. The Role for Biofuels

As noted in the introduction, biofuels account for the vast bulk of what is today described as sustainable aviation fuel (SAF). Biogenically sourced aviation fuel first began to be used in significant quantities in 2016, with global production ramping up rapidly from less than 10 million liters per year initially to well over 100 million liters by 2019.⁹ More recently, global SAF utilization has expanded again, reaching 300 million liters in 2022, according to data published by the International Energy Agency (IEA).¹⁰ As a fraction of conventional jet fuel use, however, the SAF contribution remains small – on the order of 0.1% based on estimated global jet fuel demand in 2022 of 360 billion liters.¹¹  

Several factors explain the focus on biofuels as a preferred near-term pathway for reducing aviation carbon emissions. First, such fuels are already commercially available through mature supply chains, most of which make use of waste biogenic feedstocks such as used cooking oils, animal fats, or vegetable oils. Though a conversion process known as hydro-processed esters and fatty acids (HEFA), these fats and oils are processed with hydrogen to create hydrocarbon fuels that match the energy density of fossil kerosene and can be readily blended with conventional jet fuel. Under existing international (ASTM) regulations, biofuel blends of up to 50% are approved for use by commercial aircraft.¹² Operating on 100% biofuel is technically feasible and has been demonstrated on different engines for several flights over the past five years by GE Aerospace and others,¹³ but it still needs to be fully validated for all technical specifications that apply to commercial jet fuel, including aromatics content and other properties, before approval and widespread use.  

As a drop-in substitute for kerosene that requires no major modifications to aircraft engine design or fuel delivery infrastructure, HEFA-derived jet fuel has important advantages over other potential energy carriers such as hydrogen or ammonia, which have far lower energy density and cannot be easily integrated into existing fuel systems. And while biofuel for aviation end uses currently costs more than conventional jet fuel – $1.80 per liter of biofuel compared to $0.49 per liter for conventional jet fuel – it is the least costly lower-carbon fuel alternative available to the industry today.¹⁴ To that end, large airports like San Francisco Airport (SFO) already receive and are actively working to scale SAF intake in its pipelines – SFO is targeting 20% SAF by 2030 – and are also looking into ways airports can reduce the cost of SAF delivered. One option being considered is to use government grants or other incentives to offset the added cost of SAF purchases over the next few years and quickly ramp up the use of cleaner fuel.  

Exclusive reliance on biofuels to achieve longer-term decarbonization objectives is unlikely to be tenable, however, for reasons highlighted in CATF’s 2022 report (see fn. 8). One issue is that the carbon reduction benefits of biofuels are uncertain and can vary widely depending on feedstock choice.  The supply of feedstocks that are widely commercially available today and can be used to economically produce fuels that deliver clear environmental benefits — e.g., feedstocks like waste oils and fats — is inherently limited. In the face of these limitations, the biofuels market has relied on farm-grown feedstocks like corn and soy, but the additional demand for land-intensive starch and vegetable oil crops directly and indirectly contributes to carbon-releasing land use changes. Other factors will further constrain the supply of bio-SAF, including notably the methods and energy sources used to produce hydrogen, a necessary input for processing biogenic materials into aircraft-usable fuel.  

Assuming a global volume-weighted average of 88.7 grams CO₂-equivalent per megajoule (gCO₂e/MJ) for conventional jet fuel,¹⁵ lifecycle emissions models indicate that SAF generally achieve anywhere from a 25% to 85% reduction in lifecycle CO₂e emissions. By comparison, modeled lifecycle emission reductions for HEFA fuels are in the range of 50%–65%.¹⁶ This is a substantial range with considerable residual emissions that would have to be addressed to fully achieve the net-zero objective.¹⁷ To help constrain the definition, the U.S. government’s SAF Grand Challenge and its SAF Production Tax Credit set the bar for what qualifies as a sustainable aviation fuel at a minimum 50% reduction in lifecycle CO₂e emissions compared to conventional fuel.¹⁸  

There are other methods to produce SAF from other biogenic feedstocks, some of which are currently being used to produce vehicle fuels such as bio-diesel. For example, by employing advanced fermentation methods with sugar crops, such as corn, jet fuel can be created with alcohol as an intermediary, or via a novel process such as catalytic upgrading of lignin oils.¹⁹ Another option is Fischer-Tropsch synthesis of municipal solid waste and lignocellulosic biomass feedstocks, such as forestry and agricultural residues or switchgrass and miscanthus. However, these pathways entail higher costs and are not yet as mature as HEFA. Additionally, competition with other land uses, ecological concerns, as well as the scale and pace of feedstock and technology deployment and innovation may ultimately limit their potential. 

CATF’s 2022 report on aviation decarbonization compared current levels of biofuels production to projections for aviation energy demand over the next several decades. In 2019, aviation sector energy demand reached a historic high of approximately 14 quadrillion Btu (quads). That same year, global production of all types of transportation biofuels totaled approximately 3.65 quads. Air travel and associated energy demand declined steeply in the pandemic years of 2020 and 2021 but have since rebounded: according to the IEA, global aviation sector energy demand in 2022 was 10.8 quads. Global use of transportation biofuels, meanwhile, has continued to grow, reaching 4.08 quads in 2022 according to the IEA. Only a very small share of current biofuels production, however, is formulated for aviation uses – the vast majority is comprised of ethanol and biodiesel, which are primarily intended for light- and heavy-duty vehicle applications respectively.   

To substantially displace conventional jet fuel, in sum, global biofuels production would have to expand at least two- or three-fold, and a much greater share of production would have to be shifted to the aviation sector. Continued growth in overall aviation energy demand in coming years would add to the scaleup challenge, necessitating further expansion of the biofuels supply to keep up and potentially exacerbating related sustainability concerns, depending on the rate of development of advanced biofuels and feedstock technology. Feedstock constraints will also limit the potential for future cost reductions and biofuels could become more expensive over time if the cultivation of crops for aviation fuel competes with other land uses.²⁰ Current estimates suggest that global aviation energy demand by midcentury could be as much as 50% higher than the 2019 total of approximately 14 quads: specifically, estimates of future demand considered in CATF’s 2022 analysis are 15.9 quads in 2030, 18.8 quads in 2040, and 21.5 quads in 2050. Those estimates are used as the basis for the multi-fuel scenario analysis later in this report.  

The aviation industry itself recognizes that current SAF will not be sufficient. In its Fly Net Zero plan, for example, the IATA anticipates that SAF will achieve 65% of the CO₂ reductions needed to get the aviation sector to net-zero, with the remainder coming from a combination of new technologies and improvements in infrastructure and operations.²¹  

This is not to say that the continued deployment of commercially viable biogenically sourced SAF and research and development into non-commercially demonstrated biogenically sourced SAF will have no role to play in aviation decarbonization. From recent conversations with an airframe maker, it is clear that the industry views bio-SAF as the most critical technology to meet its decarbonization commitments. The sentiment is that what is needed right now is SAF at scale. This view was echoed in a discussion with World Energy, a major SAF supplier. World Energy believes, like many in the industry, that HEFA is only a partial solution, but argues that scaling current HEFA technology is necessary to the development of SAF infrastructure across the ecosystem. According to World Energy, continuing to improve the well-known HEFA process and increasing its efficiency will help the industry prepare for the eventual transition to new technologies like synthetic fuels closer to midcentury. Along these lines, World Energy is planning to increase SAF production to 25,000 barrels per day, investing $5 billion into the market via plants in California and Texas. The company’s view is that continuing public support, technology development, long-term off-takers, increased confidence in feedstock tracking/verification in parallel with growing supply, greater willingness to pay a premium for SAF, and private financing are some of the things that are needed to scale SAF and increase SAF market share relative to conventional jet fuel, such that aviation can begin to decarbonize. World Energy’s expectation is that increased production will be absorbed by demand that is expected to materialize in the near future, with Airbus, for example, aiming for 20% SAF blends by the end of 2024 and 30% blends by 2030. This push is concurrent with airframe makers looking at fleet renewals that target an increase in aircraft fuel efficiency of approximately 25%. 

Together, these perspectives illustrate the industry’s view of the status of low-carbon options while also bringing the scale of aviation’s decarbonization challenge into clearer focus. They lead to the inescapable conclusion that a broader array of strategies and tools will be needed to meet current climate commitments.  

3. Carbon Sourcing and Low-Carbon Hydrogen Production

The term synthetic fuel has typically been used to refer to hydrocarbon fuels produced via a sequence of chemical reactions from syngas – a mixture of carbon monoxide and hydrogen that can be derived from a variety of carbon-containing feedstocks. For purposes of this paper, we use the term synthetic fuel, or the term synthetic SAF, to refer to drop-in liquid aviation fuels that are made using CO₂ and hydrogen or syngas as feedstocks. Sometimes called electrofuels or e-fuels, or labeled power-to-liquids in some studies, synthetic fuels can deliver substantial carbon reduction benefits relative to conventional aviation fuels depending on the source of the carbon and the carbon intensity of the hydrogen needed as feedstocks.   

Technology considerations for supplying inputs like captured CO₂, biomass-derived syngas, and low-carbon hydrogen are key factors in assessing the potential viability of synthetic jet fuel as a decarbonization option for the aviation sector. These technologies exist at a wide range of technology readiness levels (TRL) and their costs are highly dependent on scale, availability of feedstocks, and market demand. The most established technology for sourcing CO₂ is point-source carbon capture, which is just starting to reach commercial scale, whereas marine-based carbon capture has only just been demonstrated. Direct air capture (DAC) and biomass gasification reside in the middle of the technology readiness spectrum, but, in all likelihood, all of these techniques could be needed to spark development in the CO₂ feedstock market. The carbon intensity and cost of hydrogen is another key factor in terms of the affordability and likely adoption rate of synthetic fuels for aviation.  

Clearly, industry players looking to participate in the burgeoning market for carbon as a feedstock for fuels will have many things to consider, including the question of whether it would be better to sequester captured carbon. Weighing the myriad considerations that would go into a rigorous assessment of the benefits and costs of sequestration versus utilization, including utilization as a feedstock for fuel production, is beyond the scope of this work. This is an important topic, however, and one that must be analyzed further to inform future policy discussions.  

3.1 Point-Source Carbon Capture

One option for supplying CO₂ for synthetic fuels production is to capture it from large point sources such as power plants or industrial facilities. The most commercially mature technologies available for this purpose use absorptive chemicals, sorbents, or a membrane to separate CO₂ from other components in the exhaust gas stream post combustion. Chemical systems use liquid alkanolamines or ammonia to bind to CO₂. In these systems, the flue gas is first “scrubbed” with an amine solution, then the CO₂-laden amine solution goes to a regenerator where heat (in the form of steam) is applied to liberate the CO₂. This process typically captures 85%–90% of the CO₂ present in the flue gas.  Systems that use solid sorbents or a membrane to separate CO₂ from other flue gases offer potential advantages in terms of reduced handling of hazardous chemicals and potentially higher tolerance for impurities, but membrane systems, in particular, are still at a low level of technology readiness. 

Regardless of the separation method used, CO₂ capture systems require energy to operate. Absorptive systems need energy to push flue gas through an absorber column or membrane, while amine-based systems need energy to strip the CO₂ out again once it has been captured so that the chemical solvent can be used again.  The higher the energy requirements of the capture system, the more expensive it will be to operate. As synthetic jet fuel production will demand high volumes of high-purity CO₂, this requirement, among other considerations, will play a role in determining which choice of capture system is optimal for the emissions source being utilized.  

Emissions associated with point-source capture are another consideration, as obtaining CO₂ with this method and using it as a feedstock for synthetic fuel production is akin to carbon recycling. The process will never be net-zero, but using the carbon more than once before it is released to the atmosphere results in a net climate benefit. Industry players like ETFuels agree with this view; in an interview with CATF, the company noted that using carbon extracted from a non-biogenic point source reduces net emissions at least 50% and thus should be allowed under low-carbon fuel policies. ETFuels also emphasized that while point-source capture is less climate beneficial than avoiding carbon-based fuels altogether, it is ultimately still helpful, so long as carbon sources that encourage additional fossil fuel extraction such as oil, natural gas, and coal refineries are avoided in favor of unavoidable process sources like cement and steel plants to prevent the development of perverse incentives in favor of fossil fuels. Finally, the company noted that lifecycle analysis (LCA) will be tricky but is important to get right.  Another e-fuel company, HIF Global, echoed this thinking, stating that the narrow focus among many policymakers on using biogenically sourced CO₂ misses the point. HIF Global’s opinion is that CO₂ is just the glue that makes clean hydrogen more useful. As such, reasonable existing CO₂ point sources are valid feedstocks so long as the LCA is rigorous and correctly accounts for these “less green” carbon sources. CATF also believes that obtaining CO₂ as cheaply as possible in the early stages of market development is critical. Once a market is established and prices begin to drop due to scale, policies can be designed to be more stringent about the carbon intensity of the feedstock.²²  

The comparatively low cost of point-source carbon capture is its biggest advantage over other, newer capture methods. Estimates in the literature range from as low as $15 per metric ton of CO₂ to $130/metric ton, depending on a host of factors, including the emissions source, the concentration of CO₂ in the flue gas, the capture technology used, and other variables.²³ According to the IEA, capture costs for sources with a lower concentration of CO₂ in the flue gas – this would include power plants and steel or cement plants – range from $40 to $120 per metric ton; capture costs are lower for facilities that emit a more concentrated stream of CO₂, such as bioethanol, ammonia, and natural gas processing plants.²⁴  

Research is ongoing in multiple areas to improve the performance and reduce the cost of CO₂ capture systems. Areas under active investigation include advanced sorbent and solvent systems that have higher absorption capacity, lower regeneration energy requirements, higher tolerance for impurities, and other advantages. R&D is also underway to develop improved membranes with lower cost, greater durability, better permeability and selectivity for CO₂, and higher tolerance to contaminants. Other potentially promising areas for technology advancement include hybrid systems that combine more than one capture technology and novel concepts such as cryogenic separation and electrochemical membranes.²⁵    

3.2 Biomass Gasification

Biomass, as an alternative to captured CO₂, can also provide the carbon needed to produce climate-friendly synthetic liquid fuels. In this pathway, a biomass feedstock is gasified to produce syngas which can then be converted via Fischer-Tropsch to drop-in-ready synthetic jet fuel. The main steps in the gasification process are captured in Figure 1. 

Briefly, biomass gasification involves first harvesting, pre-treating (e.g., chipping or grinding), and drying the biomass feedstock; a pyrolysis step, in which the biomass is subjected to high temperature in the absence of oxygen to produce hydrocarbon (“tar”) gases and carbonized biomass (charcoal); and finally, a gasification step, in which a sequence of oxidation and reduction reactions (typically accomplished by first burning the products from pyrolysis in air at high heat and then passing CO₂ and steam over a very hot bed of charcoal) is used to convert a significant fraction of the tar gases and charcoal to H₂ and CO. Because the gasification process generates byproducts and contaminants, including particulates, sulfur oxides, nitrogen oxides, and hydrocarbons such as tar, the syngas generally needs to undergo a final cleaning step before it can be used to synthesize a liquid hydrocarbon fuel or for other purposes. This process is often customized by companies to suit their specific process needs including directly using the CO₂ produced earlier in the gasification process for alcohol synthesis.  

Figure 1: Biomass gasification explained²⁶  

Biomass gasification is currently drawing interest as a potential alternative or complement to e-fuel production using captured CO₂. Costs for biomass gasification processes used in refineries today vary widely with costs for liquid fuel production (including ethanol, methanol, and F-T fuels) ranging from $2,023 to $36,200 per metric ton.²⁷ The examples on the lower end are for ethanol and methanol synthesis, and while the details of these processes are undoubtedly different, these production pathways are similar to ones being planned for HIF Global’s commercial scale projects in Chile, Uruguay, and Tasmania. HIF Global says that each of these plants will use waste biomass from nearby forests as a source of sustainable carbon to produce synthetic fuels. Together they represent a combined capacity of over 1.5 gigawatts (GW) of electrolyzers and will produce over 1 million metric tons per year of synthetic methanol. HIF Global also aims to begin construction of a larger plant in Texas in 2024. Plans for this facility include 1.8 GW of electrolyzer capacity and an expected output of 1.4 million metric tons of synthetic methanol. Synthetic methanol can be converted to aviation fuel via alcohol-to-jet technology. 

These types of projects reflect optimism that biomass gasification could be an attractive, near-term option for producing climate-friendly aviation fuel, especially in locations where low-cost biomass or waste feedstocks are available in proximity to low-cost, low-carbon energy inputs. Areas of ongoing investigation include process improvements and gasifier technologies that increase syngas yield while reducing system energy demand and syngas cleaning requirements. Additionally, the type of biomass being gasified affects the carbon intensity of the resulting fuel, so an assessment of the biomass feedstock’s alternative fate – i.e., what would have happened to the feedstock had it not been collected for fuel production – needs to be undertaken as part of the lifecycle GHG assessment. For example, the carbon intensity of SAF made with carbon derived from an agricultural residue like corn stover is likely to be relatively low, because if the corn stover had not been collected for fuel production at least some of it would have decomposed and released carbon back into the atmosphere. If the carbon was instead obtained by harvesting and gasifying a still-growing tree, the carbon intensity of the resulting SAF could be relatively high because the tree might otherwise have continued to absorb and store carbon for years. Finally, as with point-source carbon capture, emissions accounting will be tricky and certain parts of the gasification process are not easily electrified. It will be important to get the details right to ensure that climate-beneficial solutions flourish.

3.3 Direct Air Carbon Capture

In contrast to point-source carbon capture, direct air capture (DAC) technologies remove CO₂ from the ambient air. This approach has the advantage that it can be implemented anywhere: because DAC installations are stand-alone, their operations don’t need to be integrated with those of another industrial facility. This makes the technology highly scalable, a point that was noted in discussions with University College London (UCL).  Carbon dioxide is multiple orders of magnitude less concentrated in the atmosphere, however, than it is in the exhaust gas stream from a combustion source. Thus, the techno-economic challenge for DAC technology centers on developing methods to remove meaningful quantities of CO₂ cost-effectively and energy-efficiently from a dilute source.  

Figure 2: Schematic of a DAC liquid solvent system²⁸  

A schematic illustrating the basic approach is provided in Figure 2. As with point-source carbon capture, most current designs rely on a liquid solvent or solid sorbent that selectively absorbs CO₂ when exposed to ambient air. Because DAC systems have to move much larger volumes of air to capture a meaningful amount of CO₂, they typically include large fan arrays. In some designs, the CO₂ reacts with a liquid solution to produce a solid that will release pure CO₂ gas when heated; in other designs the filter or sorbent is directly heated to produce a concentrated stream of CO₂. As with point-source capture and biomass gasification, energy requirements to operate the DAC system are a significant consideration.  

A variety of DAC systems have been proposed and more than a dozen pilot and demonstration facilities have been constructed in the United States, Canada, and Europe over the last decade. The three largest companies currently active in DAC are U.S.-based Global Thermostat with pilot plants in California and Alabama; Canadian-based Carbon Engineering, with a pilot plant in British Columbia; and Swiss-based Climeworks, with plants in Iceland, Switzerland, and Italy.²⁹ At present, two commercial DAC facilities are operating: the first, developed by Climeworks, is located in Iceland and has been operating since 2021; the second, located in California and developed by Heirloom Carbon Technologies, was launched in 2023. Both facilities use renewable energy (geothermal in the case of the Climeworks plant), employ a solid sorbent (calcium oxide, in the case of Heirloom’s technology), and are designed to capture CO₂ on a scale of thousands of metric tons per year. Specifically, the Climeworks plant, outside of Reykjavik in Iceland, captures approximately 4,000 metric tons of CO₂ per year; the Heirloom plant, in Tracy, California, is designed to capture approximately 1,000 metric tons per year.³⁰  

Carbon Engineering’s first plant is expected to come online in 2025. In an interview with CATF, company representatives highlighted plans for the first plant to capture 0.5 million metric tons per year and a second plant, to be built in Texas, to capture up to 1 million metric tons per year. Carbon Engineering plans to use already developed commercial processes and then knit them together as a way to quickly improve energy efficiency while bringing the first plants online. With these facilities, as with others planned for the future, the focus is on finding renewable, additional, and low-carbon energy sources – a difficult challenge that is partially mitigated by the siting flexibility of DAC technology. Essentially, Carbon Engineering is looking for locations where wind and solar are abundant and where there is little concern about competing land uses that could constrain DAC development. HIF Global is another company that expresses strong support for DAC. It has a collaboration agreement with Baker Hughes for the development and testing of DAC units in the United States. HIF Global is also planning a demonstration DAC unit to be installed at its Haru Oni facility using a technology in development with Porsche and MAN. 

A focus on co-location, tested processes, energy efficiency, and strong partnerships as highlighted in some of our conversations, is critical as the DAC market grows – otherwise, high costs could stop further development. Moving from the specific cases discussed above, cost estimates for currently available DAC systems in the literature range from $600 to more than $1,000 per metric ton of CO₂ – significantly higher than the cost of point-source capture and other climate mitigation options.³¹ While IEA’s current cost range for DAC systems excluding storage, based on a 2019 assessment, is more optimistic ($134–$342 per metric ton)  it is likely that costs will have to come down further: $100/ metric ton is often cited as a rough cost target to justify DAC deployment on a larger scale.³²  

How quickly the market for captured CO₂ is likely to take off, however, is debated. Some companies that are currently developing DAC technology, like Carbon Engineering, say they expect significant cost reductions from replication and scale-up, while advocates point to recent corporate and government investments in DAC as grounds for optimism about the technology’s future trajectory.³³ The expanding e-fuels market should also increase demand for CO₂, spurring investment and eventually reducing prices. Other analysts, however, warn that current levels of support remain inadequate to deliver the technological breakthroughs needed to dramatically lower costs. A recent assessment by BCG, for example, warns that DAC is in danger of falling short of its potential because of “comparatively low support from governments and other players” in moving down the cost curve; according to this assessment, it will be critical for DAC costs to fall to $150/metric ton or less if it is to be widely deployed by 2050.³⁴  

3.4 Ocean Carbon Capture

Another, newer concept for supplying CO₂ for synthetic fuel production involves capturing CO₂ from oceanwater. Oceans, which are estimated to have absorbed about 25% of anthropogenic fossil CO₂ emissions since the start of the industrial era,³⁵ are the world’s largest carbon reservoir, and the concentration of CO₂ in oceanwater is about 120–150 times the concentration of CO₂ in the atmosphere.  

Electrochemical pathways for removing CO₂ from oceanwater have been proposed and are currently under investigation. In simple terms, these pathways use electricity to rearrange water and salt molecules in seawater into acidic and basic solutions. Gaseous CO₂ can then be stripped from the acidic stream using electro-deionization or electrodialysis. Such a system could be designed to return slightly more basic, CO₂-depleted water to the ocean, which could help counter ocean acidification and boost the ocean’s capacity to absorb CO₂ from the atmosphere – delivering further climate benefits. Figure 3 illustrates one vision for a comprehensive ocean capture system.  

Figure 3: Electrochemical ocean carbon dioxide removal³⁶ 

These theoretical benefits notwithstanding, ocean CO₂ removal pathways face cost and technology challenges. At the scale of CO₂ capture needed to supply mass production of synthetic fuels, moreover, impacts on ocean ecology and associated infrastructure demands could give rise to significant concerns. As with DAC, the electrochemical processes involved are energy intensive; in addition, the need to pump, treat, and cool large quantities of oceanwater incurs further large energy and cost penalties. Exact costs are unknown, but early estimates suggest that they may be in excess of $100 per metric ton of CO₂ for some prototype systems.  Additionally, scientific verification of the ocean’s capacity to re-absorb an equivalent amount of CO₂ is still ongoing. The current expectation is that most of the CO₂ removed from the ocean would be re-absorbed from the air after 1-2 years and the remainder would take another 5-10 years, but that has yet to be definitively confirmed. 

These unanswered questions and apparent disadvantages are balanced against the fact that the technology should be scalable (given the vast surface area available to interface with capture systems), can be combined with offshore wind to mitigate energy concerns, and, unlike DAC, won’t compete for land with other uses. All these points, pros and cons, were highlighted in CATF’s conversation with World Energy. Work is ongoing to address ocean capture challenges, with efforts to develop the needed technologies proceeding via relatively small demonstration projects. One example is Captura, a company looking to remove CO₂ from seawater without releasing additives or by-products back into the ocean.³⁷ Captura’s first pilot project in 2022 captured 1 metric ton of CO₂ per year. A 100-metric ton-per-year system was then installed at the port of Los Angeles in 2023 and there are plans to scale to 1000 metric tons per year shortly. Concurrently, significant effort is being invested in monitoring, reporting, and verifying the impact of the capture system on the ocean.³⁸ In summary, capturing carbon from oceanwater is a relatively new concept but depending on the results of future R&D, it may play a role in sourcing CO₂ for synthetic aviation fuels among other decarbonization efforts. 

3.5 Low-Carbon Hydrogen Production

Hydrogen is a critical input to all production pathways for sustainable aviation fuel – indeed, hydrogen is even required to produce the kinds of bio-SAF that are already commercially available. For synthetic aviation fuels to be considered low- or zero-carbon, the hydrogen used to produce them must also have low or zero upstream greenhouse gas emissions. This discussion focuses primarily on hydrogen produced via water electrolysis using electricity from renewable resources – i.e., what is typically known as “green” hydrogen. However, we also include a brief discussion of cost estimates for “blue” hydrogen – i.e. hydrogen made using conventional methods from fossil fuel feedstocks (usually natural gas), with carbon capture. Low-carbon blue hydrogen is likely to play a role as a transition option, especially in the early stages of decarbonization when competing demands for renewable electricity could limit producers’ ability to supply electrolytic green hydrogen. This point was echoed in some of CATF’s interviews, where the importance of having production options while the market is still nascent was highlighted, especially given the current cost and technology risk associated with electrolysis.  

More generally, hydrogen as a process feedstock is not a new concept; in fact, hydrogen is already produced and used on a large scale in a variety of industrial applications, including in petroleum refining, to make ammonia for fertilizer, and in the production of methanol.³⁹ In 2020, global hydrogen production reached 90 million metric tons, according to the IEA.⁴⁰ However, only a small share of current hydrogen production qualifies as low-carbon. According to the IEA, production via electrolysis accounts for approximately 2% of global hydrogen supply at present, while electrolysis using renewable electricity accounts for an even smaller share: on the order of one-tenth of one percent of the global hydrogen supply.⁴¹  

Instead, virtually all dedicated hydrogen production today uses fossil-fuel feedstocks and is relatively carbon intensive. Globally, about 59 million metric tons of hydrogen are produced annually from natural gas, using steam methane reforming. Another 20 million metric tons per year are produced from coal, using gasification (mostly located in China), with the balance of global production from oil and electricity.  About 16% of global hydrogen production in 2022 consisted of by-product hydrogen, which is most often consumed on site. In fact, current methods of hydrogen production are estimated to account for 6% of global natural gas consumption, while generating close to 900 million metric tons of CO₂ emissions annually.⁴²  

By contrast, producing hydrogen via electrolysis – that is, by passing an electrical current through water to split the hydrogen atoms from the oxygen atoms – can have very low associated carbon emissions, provided the electricity used is generated by non-emitting sources such as renewables or nuclear. Figure 4 provides a schematic illustration of the steps involved in electrolytic hydrogen production. Unlike carbon capture, where many of the underlying technologies and processes are still in the early stages of commercial development, water electrolysis is relatively well developed and understood. Scale-up challenges exist, but the bigger hurdles to greatly expanding green hydrogen production to serve the synthetic fuels market center on economics and access to low-cost renewable electricity sources.  

Figure 4: System-level schematic of an electrolyzer⁴³ 

In 2021, for example, the levelized cost of producing hydrogen from natural gas in different parts of the world ranged from a low of $0.50 per kilogram (kg) to as much as $1.70 per kg, according to the IEA. By contrast, current costs for hydrogen production via renewable-powered electrolysis range from $3 to $6 per kg.⁴⁴ Some optimistic outlooks predict that production costs for green hydrogen will fall below $1 per kg. 

A recent CATF analysis is less bullish.⁴⁵ It finds that average production costs for clean hydrogen are highly unlikely to fall below $3/kg in the foreseeable future, in large part because prices for clean firm electricity are unlikely to fall below $40 per megawatt-hour (MWh). That said, there will be significant variation in production costs between different projects and geographies owing to many factors including capital costs of renewables, cost of capital, and operations and maintenance costs.  

A green hydrogen plant being developed by World Energy in Newfoundland provides an example of how advantageous geography can be leveraged to reduce production costs. In conversations with CATF, World Energy noted that its Newfoundland plant will use 3 GW of wind energy to power electrolyzers for hydrogen and eventually ammonia production. The company’s goal is to lower the carbon intensity of the hydrogen it needs to upgrade bio-SAF, but a demonstration of the scale-up potential of green hydrogen will have wide applicability regardless. World Energy’s view is that, ultimately, there will be ways to scale green hydrogen production and that cost-effective CO₂ sourcing is likely the bigger challenge for synthetic fuels if they are to replace bio-SAF in the future. Discussions with HIF Global, meanwhile, pointed to another issue: overall hydrogen demand. HIF Global would be happy to sell the hydrogen it produces, but noted the difficulty of transporting hydrogen and identifying markets that are ripe for direct hydrogen uses. In the company’s view, the direct-use market remains immature, and while the U.S. Inflation Reduction Act’s hydrogen provisions will be helpful in this respect, access to cheap, clean electricity is the most important near-term priority for growing the green hydrogen market. 

Hydrogen can also be produced in a low-carbon manner from natural gas if CO₂ emissions from conventional production methods, such as steam methane reforming, are captured at the plant site and sequestered underground or otherwise kept out of the atmosphere. In addition, to be low-carbon, hydrogen produced from fossil fuels must minimize all upstream methane and CO₂ emissions from the fossil fuel supply chain, including methane leakage from natural gas production and transport. The operation of carbon capture systems, however, increases overall plant energy demand and thus cost – for this reason, blue hydrogen production is not yet occurring on a commercial scale anywhere in the world. The IEA estimates that adding carbon capture increases production costs for blue hydrogen to around $1–$2 per kg.⁴⁶   

A more recent, CATF-commissioned analysis of options for importing hydrogen to Europe looked at costs for producing blue hydrogen in different parts of the world.⁴⁷ It estimated production costs for blue hydrogen as low as $0.68 per kg to as much as $2.45 per kg, depending on the export location and the hydrogen production volume being modeled. These cost estimates include plant capital and operating costs, assuming hydrogen production using auto-thermal reforming (ATR) with natural gas as the feedstock, which allows for high rates of carbon capture (>97%) at lower cost than conventional steam methane reforming systems; all-renewable electricity inputs; 97% pre-combustion carbon capture at the plant site; and a flat $20-per-metric ton cost for CO₂ sequestration. The low end of this cost range ($0.68/kg) reflects high-volume production with low-cost natural gas, whereas the high end ($2.45/kg) reflects low-volume production using much more expensive natural gas.⁴⁸ Roughly speaking, these cost estimates – spanning a range of input assumptions from most to least favorable for blue hydrogen production – are consistent with the $1–$2 per kg range from the IEA. 

A final point is that the cost of hydrogen delivered to a synthetic fuel production facility is likely to include costs for transportation and storage. Because hydrogen is inherently challenging to handle in large quantities, these costs could be significant, in the range of $0.05–$0.15/kg for hydrogen storage in salt caverns and $0.20–$0.50/kg for hydrogen transport by pipeline,⁴⁹ where the higher costs are expected if hydrogen inputs are sourced from a distant supplier. Hydrogen’s low volumetric density means that it must be compacted, through compression and/or refrigeration, to be efficiently transported or stored in large quantities. Once compressed, gaseous hydrogen can be moved economically via pipeline; alternatively, it can be liquefied or converted to a chemical “carrier” that can be shipped over long distances where pipeline transport is not feasible. World Energy has identified one way to temporarily tackle this issue. The company is advocating for a book and claim system in which one entity pays the production premium while another entity, nearer to production, uses the clean hydrogen in lieu of fossil-derived grey hydrogen. World Energy sees an intervention like this as critical in the early days to show investors that there is interest in low-carbon hydrogen and thereby help raise capital for future production plants. To be effective, however, a hydrogen book and claim system would need to include rigorous greenhouse gas emissions accounting for hydrogen production. Others, like HIF Global, are exploring the potential to co-locate hydrogen production and synthetic fuels production at the same site, which would make it possible to economically ship barrels of liquid fuel product in a traditional fashion, rather than needing to transport the hydrogen.  

4. The Role for Synthetic Fuels

As a drop-in fuel that can directly substitute for kerosene, the role for synthetic jet fuels is of particular interest because it offers many of the advantages of bio-SAF without the feedstock constraints and sustainability concerns that apply to conventional large-scale biofuels production. Given uncertainty over the long-term land-use impacts of biofuels, synthetic fuel made with captured carbon and low-carbon hydrogen may also deliver larger and more reliable climate benefits. The actual emissions benefits depend on the details of the production process, and any resulting fuel would have to be subject to validation via a rigorous lifecycle analysis. But unlike bio-SAF, this option would, in the best case (i.e., where fuel is synthesized from clean hydrogen and CO₂ extracted from the air, with renewable energy used throughout the process), enable sustainable, net-zero aviation. However, climate-friendly synthetic fuel production comes with its own challenges. First, the process entails large upstream energy requirements, including significant inputs of electricity and heat to make hydrogen and run carbon capture systems. Technology readiness is another issue: while underlying methods such as water electrolysis and Fischer-Tropsch (F-T) synthesis are well understood, some of the systems and technologies needed for zero-carbon synthetic fuel production have yet to be demonstrated and commercialized at scale.  

4.1 Pathways for Synthetic Fuels Production

Two main pathways for climate-friendly synthetic jet fuel production are being actively explored at present. The first pathway involves converting CO₂ to carbon monoxide (CO) and water (H₂O) via a reverse water-gas shift reaction and then using F-T to catalytically synthesize jet fuel from the CO and hydrogen (H₂). Climate-friendly sources of CO₂ for this pathway, discussed in detail in the previous chapter, might include CO₂ captured from a large industrial point source, directly from the atmosphere, or from oceanwater. Another potentially climate-friendly option, which does not rely on captured CO₂, is to gasify waste or biomass feedstocks to produce syngas, which can then be converted via F-T to synthetic jet fuel. Direct application of the F-T process, which is well-understood and has a decades-long track record of producing liquid hydrocarbon fuels for a range of applications, is a potential advantage of this pathway. F-T was also the first process to be certified for the production of drop-in aviation fuels.  

A second pathway that involves the conversion of alcohol to jet fuel, and does not require F-T, is also of note. In this pathway, an alcohol (such as ethanol or methanol) is produced using a technique known as CO₂ hydrogenation. That alcohol is then dehydrated and formed into more complex, long-chain hydrocarbons using catalysts via a process called oligomerization. The long-chain hydrocarbons obtained through oligomerization can then be further processed into synthetic jet fuel. A variant of this pathway that features methanol (CH₃OH) production as the intermediate step is currently being investigated by several companies.  

At present, companies in the nascent synthetic fuels market still face many commercial and technical challenges, and any production process solution will have to make sense from both perspectives. In the case of ETFuels, the effort to identify a process that is viable economically led ETFuels to focus on a single value chain using an off-grid, renewable energy model. Given that, the company is currently focused on producing e-methanol via CO₂ hydrogenation instead of direct SAF synthesis with Fischer-Tropsch. ETFuels’s view is that methanol makes a good business case due to the off-grid model acting to lower costs and the versatility of low-carbon synthetic methanol providing access to multiple offtake markets. Additionally, from a technical perspective, the synthesis process is more modular and flexible. This helps keep capital costs down and enables scalability. Finally, methanol production involves less technical risk than F-T purely from a fuels synthesis and purification point of view, because it requires less process heat and does not result in many non-SAF products. ETFuels believes F-T might make sense in the future, but for now the company favors the methanol pathway.   

HIF Global is focused on the same pathway for many of the same reasons. The company points out that F-T lacks flexibility as it requires a near 100% feedstock capacity factor. With CO₂ hydrogenation to produce methanol, by contrast, input energy can be ramped up and down, which fits well with the fluctuations inherent to renewable energy. Reliance on F-T would likely require the fuels producer to purchase grid electricity or buy lots of batteries for onsite energy storage, both of which are very expensive options that would diminish the cost advantages of siting a refinery in an off-grid, purpose-selected location. Furthermore, the reverse water-gas shift reaction has not been demonstrated at a large scale to date, but is nonetheless required for F-T, adding technical risk. HIF Global also notes the issue of generating multiple products in the F-T process that then need treating, cracking,⁵⁰ and upgrading. Carbon Engineering is electing to focus on scaling DAC over F-T derived e-fuels given the overall status of the key technology components (reverse water gas shift, low carbon hydrogen production) needed for e-fuels production today. The company recognizes that, in the future, combining CO₂ captured from the atmosphere with low-cost clean hydrogen may be part of the path forward for decarbonizing hard-to-abate sectors like aviation.  

Whether the business case for the F-T process begins to make more sense or companies continue using methanol as an intermediary, resulting e-fuels will come with infrastructure requirements that must be managed. These requirements depend on location, but if the synthetic fuel refinery cannot be co-located with a CO₂ source, CO₂ must be brought to the refinery via pipeline or, as ETFuels mentioned, by truck or rail. Infrastructure will be needed on the airport side as well. SFO, for example, has a zero-carbon target that aims to reduce the airport’s energy use and remove all onsite carbon-intensive energy sources by 2030. As part of this initiative, SFO plans to increase its use of SAF to meet the California goal of a 20% blend by the end of the decade. SFO completed an infrastructure, supply chain, and feasibility study in 2018, which enabled SFO to continuously receive SAF via pipeline beginning in 2019. Although SAF is drop-in fuel that can be received, distributed, and loaded onto the aircraft without any changes to the current airport infrastructure, SFO will need to better understand the logistics of connecting SAF from its production point to airport facilities. As new and diverse SAF suppliers look to enter the market, new transportation nodes to connect SAF to existing pipelines or on-airport infrastructure may need to be developed. 

While bio-SAF is the most advanced aviation fuel alternative available today and while supply agreements for bio-SAF are already in place with many in the industry, UCL notes that costs may increase in the future as demand for SAF increases beyond the amount that cheaper feedstocks can support at projected capacity, forcing the market to turn to synthetic fuels. Based on insights gleaned in CATF’s discussions with industry participants, the budding market for synthetic fuels and related technologies needs to take hold in the coming decades such that a low-carbon, blend-ready, drop-in fuel option is available that can scale to meet aviation demand.  

4.2 Cost Estimates for Low- or Zero-Carbon Synthetic Jet Fuel

Producing synthetic fuels is expensive at present – much more so than producing traditional jet fuel or bio-SAF. A recent analysis by Bergero et al.⁵¹ estimates the cost of synthetic jet fuel production at about $2.60 per liter, assuming the cost for electrolytic hydrogen is $4.50 per kg and the cost of captured CO₂ is $250 per metric ton. The study notes that a production cost of $2.60 per liter is more than three times higher than the global average cost of fossil jet fuel in 2022. According to the authors, their estimates are broadly consistent with other recent studies that report synthetic fuel costs ranging from $1.30 to $4.70 per liter. Bergero et al. also point out that economies of scale and learning-by-doing could substantially reduce electrolyzer and carbon capture costs in the future, making synthetic fuels more competitive. 

Another recent study, by Gössling and Humpe, arrives at similar estimates for the current cost of e-fuels production:  $2.53–$2.63 per liter, depending on whether electricity is supplied by solar vs. onshore wind, respectively, with only modest cost declines – to a range of $2.20–$2.38 per liter – projected for 2050.  The cost declines result primarily from a reduction in the electricity required to produce e-fuels, from 32 kWh/liter in 2025 to 20 kWh/liter in 2050.⁵²   

Estimates from a 2024 IEA analysis of synthetic fuel options for decarbonizing the transport sector are likewise broadly consistent with the results from Bergero et al. and Gössling and Humpe.⁵³ Specifically, the IEA finds that the cost of producing synthetic kerosene today can be expected to range from $2.50 to $4.20 per liter,⁵⁴ where the low end represents an optimized case of a large-scale plant located at a site with access to high-quality, high-capacity-factor wind and solar photovoltaic (PV) resources and low-cost biogenic feedstocks, while the high end reflects unoptimized conditions.  

Additional cost estimates can be found in the literature. For example, an analysis by the UK’s Royal Society estimates the cost of synthetic jet fuel at $3.47–$4.52 per liter, depending on whether needed CO₂ inputs are provided by point-source carbon capture versus more expensive DAC.⁵⁵ Another study that explores the feasibility of adding synthetic fuel production at an existing biomass-fueled combined heat and power (CHP) plant in Östersund, Sweden, by contrast, arrives at substantially lower cost estimates: approximately $1.5/liter, with very slightly (3%) higher costs for the alcohol-to-jet-fuel process compared to the F-T process.⁵⁶ The Swedish results reflect the importance of intentional co-location of feedstocks and renewable energy to minimize costs while the market is still nascent, a point that came up in several interviews. At the Swedish location the CO₂ is supplied by implementing amine-based capture at an existing CHP plant that operates primarily on woody biomass; in addition, the CHP plant offers opportunities for energy integration that improve the overall thermal efficiency of the process.⁵⁷ Most importantly, the site offers access to abundant supplies of low-cost renewable electricity. This advantage is critical because the Swedish analysis, consistent with other cost studies, finds that overall e-fuel production costs are dominated by the cost of renewable electricity, which is needed in significant quantities for hydrogen production via electrolysis and to power other steps in the fuel synthesis process.  

The Swedish example and examples from CATF’s interviews with companies active in this space illustrate an important broader point: that the best near-term opportunities for synthetic fuel production, in terms of minimizing cost and maximizing environmental benefit, are likely to exist at locations that offer a combination of advantages, including an abundance of renewable energy resources, access to low-cost feedstocks, and the ability to maximize efficiency by integrating e-fuels production with systems for meeting other energy or industrial needs.   

Finally, most studies find that production costs for synthetic fuels can be expected to decline in the future with improvements in process efficiencies and cost reductions for carbon capture and zero-carbon electricity generation. The aforementioned 2024 IEA study, for example, estimates that a further 60% reduction in electrolyzer cost and 25% reduction in renewable electricity cost would allow synthetic kerosene production costs to fall to $1.75 per liter, still more than triple the cost of conventional jet fuel but roughly competitive with currently available bio-SAF. Similarly, an analysis by Martin et al. estimates that the cost of aviation e-fuels could fall as much as 68% by 2050.⁵⁸ Translating increased fuel costs to a measure of levelized cost for transport, this analysis finds that the use of e-fuels could increase the cost of aviation transport by a factor of 2.0–2.6 currently, decreasing to a factor of 1.5–1.7 in 2050.  

Table 1. Estimated production costs for synthetic SAF: findings from selected studies. The current average cost for conventional, fossil-based jet fuel is $0.49 per liter.

Study	Current Cost  ($/liter)	Projected Cost  ($/liter)	Notes, key assumptions
IEA (2024)	$2.50 – $4.20	$1.75 (by 2030)	Current costs represent optimized to unoptimized conditions. Future cost assumes 60% reduction in electrolyzer cost and 20% reduction in renewable electricity cost.
Bergero, et al. (2023)	$2.60	n/a	Assumes cost of $4.50/kg for electrolytic H2 and $0.25/kg for captured CO2.
Gössling & Humpe (2023)	$2.53 – $2.63	$2.20 – $2.38 (by 2050)	Range for current cost reflects difference in electricity cost for offshore wind versus onshore PV. Projected future cost is based on reducing the renewable electricity requirement from 32 kWh/liter to 20 kWh/liter.
UK Royal Society (2023)	$3.47 – $4.52	n/a	Range reflects cost difference between supplying CO2 using point-source carbon capture versus (more expensive) DAC.
CHP Plant  Östersund, Sweden  (2021)	$1.50 – $1.55	n/a	Plant in an optimal location with abundant renewable energy and a co-located source of CO2.

This summary of current market and technology conditions for synthetic fuels is not meant to imply that such fuels will have no future in aviation due to high cost. Synthetic fuel production is theoretically extremely scalable, allows for flexible siting as plants can be located where renewable resources are plentiful, and may cost less than biofuels production in 2050 – all while still offering the advantages of a drop-in solution. That last point is key as airframe makers are making a big push into bio-SAF today knowing that nearer to midcentury there will likely be upwards of 40,000 aircraft in fleets that need to be capable of operating on clean fuel. The ability to make a seamless move to synthetic SAFs, once the technology is developed, markets are established, and prices are more reasonable, is one possible way aviation can still thrive in a net-zero future.    

5. Beyond Fuels with Carbon: Hydrogen, Ammonia, Batteries, and Non-CO₂ Effects

Take-off weight and energy density requirements have understandably resulted in a focus on non-fossil, synthetic or biogenic hydrocarbon fuels as potential solutions for aviation decarbonization. In addition to the fact that the complex global system needed to fuel aircraft with kerosene-derived Jet A fuel is already in place, the largest planes on international routes have little choice but to rely on such fuels for the foreseeable future. Fuels with carbon can be produced in a more climate beneficial fashion, as explained in earlier chapters, but that should not preclude the pursuit of options for deeper decarbonization where feasible. Specifically, there are plane types and routes where alternative, zero-carbon fuels like hydrogen and ammonia, as well as battery-powered aircraft, can reduce aviation CO₂ emissions in a more significant way than bio-based SAF or synthetic fuels. This is because carbon that would otherwise be part of natural processes or intentionally sequestered is released when burning either biogenic or synthetic SAF. A fuel’s climate impact will always depend both on its exhaust emissions as well as the specifics of the process used to produce the fuel, as determined by a rigorous lifecycle analysis that takes into account all pollutants; generally, however, a larger emissions benefit is expected with non-carbon-based fuels. Playing this advantage in appropriate segments of the aviation space while also understanding the non-CO₂ effects of air travel on climate could bring strategies for reaching the goal of net-zero impact from aviation by midcentury into much sharper focus.   

5.1 Hydrogen-Powered Aircraft

The idea of hydrogen-powered aviation is not new. Examples involving military applications can be found dating back to the 1950s. In fact, the U.S. government commissioned the National Aeronautics and Space Administration (NASA), Lockheed Martin, and others to conduct in-depth research on a theoretical 400-passenger hydrogen-fueled airplane with a 5500-mile (8851 km) range during the oil crisis of the 1970s.⁵⁹ The size and range of the hydrogen-powered aircraft targeted in this early research is just as important as the dates when the research began. Not only has hydrogen aircraft technology been studied for decades, at least some of the focus has been on planes capable of international flights. In theory, low-carbon hydrogen can power aviation applications for a very large segment of the market, reducing a significant portion of emissions to near zero. Furthermore, a hydrogen aircraft actually offers some design advantages, including the potential to improve weight distribution efficiency by optimizing the hydrogen tank layout. This is in comparison to kerosene fuel, which must always be carried in the wings.⁶⁰ However, any redesign would force a lengthy aircraft re-certification process, and the new planes would lose passenger seating to hydrogen tankage, an issue that becomes especially problematic for longer flights that are already associated with higher ticket prices. As a result, more recent interest in hydrogen as a potential aviation fuel has focused on smaller aircraft and drones, with an eye toward the feasibility of powering single-aisle planes for regional routes in the future.  

Industry opinions on the business case for hydrogen aircraft are somewhat fractured. ETFuels does not see direct hydrogen-powered technologies working for aviation applications and cites hydrogen distribution, storage, refueling infrastructure, as well as generally low technology readiness as major issues. HIF Global doesn’t entirely rule out hydrogen-fueled aircraft, but anticipates that certification will be very difficult especially considering the testing, safety, and logistics challenges that would have to be overcome. 

The solution for the near term seems to be to start small. Recently, Honeywell Aerospace partnered with the National Renewable Energy Laboratory (NREL) to look at converting some electric unmanned aerial drones to hydrogen,⁶¹ with the aim of increasing range while eliminating the excessive noise, vibration, and emissions of current long-range drones that use combustion engines. ZeroAvia and Universal Hydrogen are thinking bigger. ZeroAvia completed a 10-minute test flight of a 19-seat hydrogen-powered plane early in 2023, and Universal Hydrogen is looking at designing a hydrogen turboprop aircraft for 50-seat applications. Both companies are using fuel cells, which chemically convert hydrogen to electricity that is used to power the aircraft engines, and are currently targeting flights of a few hundred miles. Their plans include scaling the fuel cells to a size needed to power 100–150 seat planes (similar to a Boeing 737) and investigating hydrogen combustion.⁶² 

Airbus’s ZEROe program is even more ambitious; its goal is to bring the world’s first hydrogen-powered commercial aircraft to market by 2035. Airbus is targeting ranges of greater than 2000 nautical miles with a variety of designs from a blended-wing body (BWB) that would use hydrogen combustion for longer hauls, to a fully electric hydrogen fuel cell turboprop design for shorter distances. The company is using its A380 aircraft as a test bed for hydrogen combustion while separately testing high-power fuel cells in the lab at 1.2 megawatts, a power level similar to engine requirements at takeoff. Airbus expects that use of electrolytic hydrogen produced from renewable electricity, either directly or as an input to synthetic fuels, has the potential to reduce total aviation sector CO₂ emissions by up to 50%.⁶³  

While the experience of hydrogen-powered flight will seem very similar from a passenger perspective, hydrogen-powered aircraft will differ from today’s planes in several key technical aspects. The largest difference can be seen in the fuel properties summarized in Table 2. Compared to Jet A fuel, hydrogen is far more energy dense by weight, but far less energy dense by volume. In fact, hydrogen’s specific energy, or energy content per kilogram of fuel, is three times that of kerosene fuel. But hydrogen contains, at best, four times less energy per liter. Hydrogen used for aviation fuel must be stored as a liquid, which entails keeping it very cold (i.e., below its boiling point of -253.15℃) in large tanks, instead of in the form of comparatively easier to handle compressed gas, which has even lower volumetric energy density. For this reason, gaseous hydrogen will only be useful for very small planes or drone applications, as well as, potentially, other segments of the transportation sector. In addition, liquid hydrogen fuel tanks must be cylindrical to carry pressure loads, which prevents storage in the wings. This will necessitate changes in passenger seating, including the removal of some seats to make room for tanks in the fuselage. Finally, the wings in hydrogen plane designs will need to be made of stronger materials to compensate for the loss of fuel, which, as part of the wing structure in conventional designs, acts as a stiffener to prevent bending during takeoff. This adds weight.    

Table 2: Properties of Jet A fuel, liquid hydrogen, and gaseous hydrogen 

	Jet A  (Kerosene)	Liquid Hydrogen	Compressed Gaseous Hydrogen
Specific Energy (MJ/kg)	43.2	120	120
Energy Density (MJ/liter)	34.9	8.5	4.8
Storage Temperature (℃)	Ambient	-253.15	Ambient
Storage Pressure (bar)	Ambient	2	700

Hydrogen tank design itself is also extremely important.  As illustrated in figure 5, hydrogen tanks would have to be large: so, their number, location, the amount of insulation needed to keep the fuel at cryogenic temperatures, and the system to manage ullage, or the boil-off gas that accumulates during flight, are all critical. Each of these design choices affects weight and thus the energy required at takeoff, a key design parameter.  

Adler and Martins have investigated the effect of gravimetric fuel tank weight, or the relative weight of the fuel in comparison to tankage weight, as a function of flight length. A value of 100% corresponds to a theoretical case where the tankage weight is zero. Running their model, they find that a tank efficiency of 55% is the cutoff: below this threshold (i.e., when the tanks are heavy), flying long distances on hydrogen requires much more energy than flying on kerosene.⁶⁴ Above this cutoff, or for very light tankage, less energy, compared to kerosene, was required to fly long distances due to hydrogen’s superior specific energy. 

Figure 5: Airbus rendering of a hydrogen-powered aircraft with visible tanks⁶⁵

Also noteworthy is the fact that the curve for regional flight distances is mostly flat, effectively decoupling tankage weight from performance for this market segment. Considering that, to date, very high gravimetric tankage efficiency has only been achieved in aerospace grade applications, like on the Space Shuttle (tank efficiency ~80%),⁶⁶ it makes sense that short of a technical breakthrough in tankage material that is suitable for more mainstream applications, hydrogen-powered aircraft will be largely limited to regional routes. One other option is a BWB, or flying-wing-type design, but in addition to forcing major infrastructure changes at airports, the benefit of this approach for hydrogen applications compared to traditional tube and wing designs could be relatively small – on the order of only a few percent change in energy efficiency.⁶⁷  

The technical challenges associated with burning 100% hydrogen fuel in an airplane engine should also be mentioned. These challenges do require a partial engine redesign, but such a redesign has been determined to be feasible and is being researched by large companies like GE as part of a larger portfolio of climate initiatives. Most of the engine design would remain the same, with major changes being confined to the combustor. Hydrogen has a wider flammability limit such that it is much easier to burn than Jet A fuel, however, this property confers a greater risk of flashback, a dangerous condition where the flames travel upstream from the combustor into the fuel mixing zone. These, and other technical challenges can be overcome with the correct combustor design without much loss in efficiency or increased emissions of nitrogen oxides (NO_x) relative to kerosene. Additionally, fuel cells will have an overall higher efficiency compared to gas turbines, meaning that hydrogen fuel cells might be preferred for smaller aircraft.  

Overall, despite its theoretical advantages, hydrogen’s impact on aviation as a direct aircraft fuel in the 2050 timeframe is likely to be limited for a few reasons. As a UCL researcher pointed out in discussions with CATF, the feasibility of ongoing projects is uncertain given the high cost of hydrogen relative to conventional jet fuel, the aforementioned range issues, and the need to re-certify aircraft. UCL researchers do not expect significant use of hydrogen aviation earlier than 2035, and since that timeframe is likely to slip if anything, 50% penetration into regional markets – at the very most – may be reasonably anticipated by 2050. Long timescales for fleet turnover will make even that best case very difficult to achieve, however. It was also noted in the conversation that while long-range designs are possible, reduced-range aircraft are more likely due to tank weight complications, echoing the discussion above.  

Hydrogen infrastructure at the airport is another key issue as the buildout of hydrogen storage and refueling systems will be expensive and time consuming. At SFO, complete infrastructure improvements for alternative fuels can take 5–20 years to roll out using a cost effective, phased development approach. To that end, SFO is getting ahead by both readying its upstream utilities infrastructure to accommodate new electrical demands (at the substation, medium voltage levels, etc.) and bringing hydrogen and electric equipment into the airport, on a trial basis, for demonstration, commissioning, and analysis. The airport has 10 fuel cell light-duty vehicles in operation, but expects more across its ground support equipment in the year ahead, which should help staff become comfortable with operating, maintaining and refueling these hydrogen vehicles. Going forward, issues with handling hydrogen, like leakage from airport equipment, cannot be allowed to foul operations. That said, though SFO seeks to incorporate future electric and hydrogen aircraft, with guidance from airline investments in both technologies, the airport expects SAF to be the primary non-conventional fuel it is handling for at least the next 20 years, with only limited hydrogen penetration in the medium term.  

In summary, there is a long way to go on fuel, aircraft, and infrastructure before hydrogen technology can be scaled in such a way as to substantially reduce aviation emissions. Widespread adoption will likely be limited to after 2050. But hydrogen-powered flight is real and successful campaigns to test hydrogen flight, like the one recently kicked off by Universal Hydrogen,⁶⁸ help show the potential of the technology. With continued development, hydrogen planes could begin delivering large reductions in aviation emissions from regional routes in the coming decades.  

5.2 Ammonia-Powered Aircraft

Zero-carbon aviation technology could play a significant role in reaching the industry’s stated climate commitments but restricting this technology to direct hydrogen propulsion limits applicability. Hydrogen aircraft are theoretically capable of completing flights of almost any distance; however, as discussed above, longer flights require increasingly large, expensive, and difficult-to-manufacture lightweight tankage that must be stored in the fuselage. This reduces passenger revenue and ultimately acts as a practical limit on climate benefits. One solution is to use a zero-carbon hydrogen carrier instead. Ammonia (NH₃) has a volumetric energy density of 12.7 megajoules per liter (MJ/L), or 49% more energy per liter than liquid hydrogen, meaning that ammonia carries more hydrogen per unit volume than pure liquid hydrogen. Ammonia aircraft could use conventional fuel and tank configurations, which opens the possibility of retrofitting existing engines to create zero-carbon airplanes from the existing fleet. Disadvantages of ammonia include its toxicity and the need for additional ammonia infrastructure, with toxicity being of particular concern for passenger-facing operations, as a UCL researcher noted in conversations with CATF. If those hurdles can be overcome, however, ammonia could offer double the range of a hydrogen jet in 2050. That said, while ammonia is a widely used and globally transported chemical for usage in other applications like fertilizers and industrial refrigeration, for aviation applications it is a novel fuel that is currently still being actively researched at institutions like the University of Central Florida (UCF). The technology is approximately 10 years behind hydrogen-powered flight.  

Greater volumetric energy density in combination with a considerably higher boiling point – 33℃ for ammonia versus -253.15℃ for hydrogen – means that ammonia can be stored in the wing, paralleling on-board storage for Jet A fuel. This effectively eliminates all the tankage-related design and manufacturing constraints that are present with hydrogen aircraft. An additional advantage, noted by Otto et al., is that ammonia, because it does not contain carbon, does not coke. This allows for aircraft designs to achieve system-wide performance improvements, such as improved intercooling and pre-cooling of the air to be used for turbine thermal management, while also reducing NO_x in airplane exhaust as much as two orders of magnitude by using some of the ammonia fuel for selective catalytic reduction (SCR).⁶⁹  

Figure 6: Ammonia gas turbine layout⁷⁰ 

A schematic illustration of the theoretical ammonia gas turbine system being explored in this research, which is being led by UCF and funded by NASA, is shown in Figure 6. Major companies and organizations – namely, GE, Boeing, Southwest Research Institute, Greater Orlando Aviation Authority, ANSYS, Purdue, and Georgia Tech – are also part of the research team.   

The system includes low- and high-pressure compressors and turbines (abbreviated LPC, HPC, HPT, LPT in the figure), all of which is turbomachinery present in any gas turbine regardless of fuel type. Ammonia is piped in from the wings; partially cracked into hydrogen, ammonia, and nitrogen; then, to an optimal extent, fed into the combustor. The nitrogen doesn’t meaningfully affect the primary combustion reaction. Essentially the engine is burning an ammonia/hydrogen blend in lieu of combusting 100% ammonia. This is preferred as ammonia is reticent to burn without a pilot fuel. The waste heat recovery unit and the heat exchanger (PHX) are used to minimize heat loss while also reaping some of the cooling benefits mentioned earlier – both are ways to boost overall efficiency.  

A conversation with the UCF researchers involved in this work revealed some further details about the project. Their investigation is based around a reference Boeing 737-8 operating out of Orlando, a real-world scenario with applicability outside of regional routes. While ammonia’s energy density does impose some flight distance limitations relative to kerosene, calculations show that approximately 75% of domestic flights can be fueled by ammonia with no reduction of passengers, a percentage that covers most routes flown by single-aisle planes. Twin-aisle planes, which are mostly reserved for international routes, would still need to use SAF. A blend ratio of roughly 75% ammonia to 25% hydrogen is being targeted to promote the ideal combustion characteristics that result in a flame speed similar to kerosene without increased risk of flashback, where the optimal ratio will be obtained through system optimization as a part of the ongoing efforts. Researchers are also still determining the best cracking technique for converting ammonia to hydrogen, but in general, highly efficient ammonia cracking techniques are starting to come to market in a variety of forms.⁷¹ The surrounding turbomachinery should be mostly unchanged, though work with GE to confirm this assumption is ongoing.  

The UCF researchers are also working with the Orlando airport to design the fuel handling systems that would be needed for ammonia in order to ensure the safety of the flying public and population around airports as well as to ensure smooth ground operations. This includes cost and infrastructure analysis as well as toxicity safety studies. Fueling on the ground would involve liquid ammonia at temperatures close to ambient, which would ease airport operations. Overall, ammonia should be much easier to handle than hydrogen, especially considering that it is already safely used for many applications around the world.  

Despite its apparent benefits, ammonia is expected to play an even smaller role than hydrogen by 2050, with any potential to outpace hydrogen and erode SAF use likely reserved for after midcentury. This is mostly an issue of timing: because switching to ammonia in aviation applications requires a complex system overhaul, this option is less developed than hydrogen and less likely to be commercially ready within the next 30 years. Participants in the UCF project, for example, expect that it will take approximately 5 years to complete the current design work, 5 years to complete demonstration and deliver results to Boeing and GE, and 10–15 years more for those companies to design, test, and certify new planes. This timeline puts commercialization in the 2045–2050 timeframe at the earliest, assuming everything proceeds smoothly. Furthermore, there are ammonia supply and infrastructure concerns. Supply challenges stem mostly from cross-sector competition for ammonia. Competition with agricultural uses, notably fertilizer production, could cause prices to rise, especially if the marine shipping industry is also buying ammonia fuel to decarbonize. The infrastructure challenges for ammonia generally mirror those for hydrogen with less concern about undetected leakage and more concern about general fuel toxicity. These issues should be solvable; however, representatives from SFO voiced concern about splintering airport infrastructure investments to support a push for such a broad range of fuels. The additional cost and complexity of adding ammonia infrastructure to what is needed for SAF and hydrogen must be taken into consideration.   

Ammonia is interesting because it is the zero-carbon aviation fuel that offers the most range while requiring the fewest number of changes to the structural design of the aircraft. Engines would need to be redesigned, but the characteristics of combusting an ammonia/hydrogen blend make aspects of that redesign more straightforward than designing for 100% hydrogen. As already noted, the technology is new and won’t be ready for commercialization until well into the 2040s at best, but longer term, ammonia fuel has the potential to enable large emission reductions in the single-aisle aircraft market segment.   

5.3 Fully Electric Aircraft

Electrification is generally seen as the preferred method to decarbonize much of the transportation sector due to its clear energy efficiency advantages over using the combustion of low-carbon fuel or electricity output from fuel cells to provide motive power. Energy efficiency is an important parameter for all potentially electrifiable end-uses, especially considering the relative scarcity of renewable energy during the transition period to a net-zero economy, but aviation, which is typically considered a “hard-to-abate” sector, presents particular challenges. This is because the unique power and energy requirements of air travel, coupled with aircraft weight restrictions, make electrification extremely difficult. The realm of the possible as it pertains to aviation electrification has been researched for years, with early efforts focused on hybrid electric aircraft based on the successful experiences seen with on-road vehicles. Siemens, Airbus, and others debuted the first manned electric-hybrid aircraft in 2011; that prototype achieved a 25% reduction in emissions but provided no technical basis for scaling up to commercial transport.⁷² The fact that emission reductions at the 25% level can be achieved with SAF, which does not require certifying a new aircraft design that combines a gas turbine and battery powerpack, greatly limits the applicability of hybrid technology.  

Fully electric aircraft, which use only batteries and a motor, are technologically more straightforward and produce no direct emissions. However, NASA studies⁷³ show that a battery energy density of 400 watt-hours per kilogram (Wh/kg) is the threshold energy density needed for general aviation and 750 Wh/kg would be needed for commercial regional air service. Both these energy densities are well above the 225–270 Wh/kg seen in advanced, lithium-ion car batteries on the road today.⁷⁴ To enable longer-haul flights beyond the regional market segment, new battery technologies with energy density well above 1,000 Wh/kg will be needed, illustrating why aviation is considered a hard-to-abate sector.  

Nonetheless, aircraft electrification is likely to still play some role going forward. Putting aside electric drones, which is an existing market, small aircraft (less than 10 seats, flying fewer than 400 miles (644km)) and eVTOLS providing urban air mobility (UAM) could go fully electric. EVTOLs could have the largest impact, but conventional options may take to the skies first. One example is the Beta Technologies CX300, a 5-seat plane that can fly up to 386 miles (621km) on a single charge.⁷⁵ Another is Alice, which can carry 9 passengers up to 250 miles (402km).⁷⁶ These planes are well suited to make short, zero-emission hops, but because routes flown by planes with 19 seats or fewer make up just 4% of departures,⁷⁷ their potential to deliver meaningful emission reductions is quite limited.  

The most impactful market for electric aircraft may be replacing helicopters with eVTOLs in UAM applications. The novelty of these vehicles implies a long regulatory process, however, where companies that are relatively new to the aviation space would need to meet Federal Aviation Administration (FAA) design and safety standards and obtain a vehicle production certification as well as an air carrier certification. This would likely push their timeline to market behind that of traditionally designed electric planes. If the regulatory hurdles can be cleared, some industry experts project that the eVTOL market could grow to $28.5 billion by 2030.⁷⁸   

Figure 7: Hyundai four-passenger eVTOL set for 2028⁷⁹  

An upcoming eVTOL prototype from Hyundai (shown in Figure 7) is expected to be capable of a 25–40-mile (40– 64-km) trip at an altitude of 1500 feet (457 meters), making it suitable for intra-urban travel as an air taxi or emergency vehicle. Ideally these craft could serve as a more efficient replacement for helicopter⁸⁰ thereby reducing some on-road congestion while using battery technology that is already available today. 

Based on that potential, industry participants are getting ready to support the electric infrastructure that will be needed for these aircraft. SFO convened a working group on eVTOLs and advanced electric air mobility to help assess related infrastructure needs. Currently, the airport’s peak load is 55 MW and each eVTOL charging point would require an additional 1 MW, a challenge considering that SFO’s electrical infrastructure already approaches the 55-MW cap on high-load days. One option under discussion is to move to micro-grids and smart grids as a way to combine improved energy management with renewable energy generation and onsite energy storage. However, airports will quickly be confronted with how to equitably distribute electricity to serve the maximum amount of commercial air service. Given this extra complexity, the airport could decide to zone and segment alternative aircraft and related infrastructure. It is possible to imagine a zero-emission terminal of some sort in the future, where electric, hydrogen, and ammonia aircraft would interface with new infrastructure. According to SFO officials, however, building something on the scale of a new terminal could require a capital investment of at least $2 billion and could take at least 5 years to complete. Due to these challenges, to date, SFO has not committed to providing charging for eVTOLs,  

In comparison to other alternative, zero-emission options, electrification is likely to play a niche role, but it is not expected to meaningfully reduce emissions from traditional air travel by 2050. If eVTOLs grow in popularity, they may have an indirect impact on on-road emissions, but limited aircraft size and range for airport-to-airport operations make it hard to imagine that this technology could make a dent in aviation sector emissions. For this reason, the aviation fuel mix case study in Chapter 6 does not consider electrification as an option (only bio-SAF, synthetic SAF, hydrogen, and ammonia are included as plausible solutions).  

5.4 Non-CO₂ Effects

Efforts to decarbonize the aviation sector have understandably focused on direct CO₂ emissions from aircraft; however, other, non-CO₂ effects are also being investigated in the literature. Some of this research suggests that non-CO₂ effects may account for as much as two-thirds of aviation’s overall climate impact.⁸¹ Particular attention is being focused on contrails and the cirrus cloudiness they create, in an attempt to quantify the amount of net positive radiative forcing – or total atmospheric warming – attributable to this phenomenon. The complex, developing science behind contrails is briefly introduced in the text box; an upcoming CATF report will address the subject in more detail. 

6. Case Study: Potential Clean Fuel Mixes in a Decarbonized Future

Decarbonizing the aviation industry by midcentury is an immense challenge that will be especially difficult to surmount with a single type of fuel. Bio-SAF is the most developed technology and the cheapest option available today, but biogenic feedstock sustainability and related land-use considerations make it difficult to scale this option at reasonable cost in the context of growing overall aviation energy demand. Synthetic kerosene is a drop-in solution that can be blended with fossil kerosene or bio-SAF, but the costs associated with low-carbon hydrogen production and sourcing CO₂ are a big barrier. Meanwhile, hydrogen and ammonia fueled planes require new infrastructure and re-certified aircraft to access their zero-emission potential. This suggests that, in a market looking to minimize costs while decarbonizing on a reasonable timeline, a multiple fuel solution may be pursued. This case study uses information gained from a literature study and from the discussions with industry participants that informed the previous chapters to look at several plausible fuel mix scenarios that would result in a near-net-zero future for aviation.  

To ground the case study, we use supply projections from the IEA’s net zero emissions by 2050 (NZE) scenario. To develop these estimates, the IEA assumes that the goal of net-zero emissions is met, and then projects energy and fuel demand for the entire market. In other words, the projections represent an estimate of how much of a given fuel or feedstock will be needed to meet decarbonization goals worldwide. As such, our analysis required that all cases not exceed the IEA’s total projected supply for any fuel except ammonia, where only existing uses and marine fuel were originally included in the IEA projection. Regarding the other fuels, the hydrogen supply numbers from the IEA NZE scenario used in this case study only include hydrogen supply for transportation uses, which makes up approximately half of the total IEA hydrogen projection. The synthetic fuel supply was governed by the IEA projection for available CO₂ sourced only from DAC. The biofuel supply projection (originally from an IEA analysis of potential 2030 supply) is the same as in earlier CATF work (see fn. 8, figure 3) to maintain consistency. Our 2050 scenario analysis assumes no further growth in biofuel supply during 2030-2050, mainly due to land constraints. 

The IEA’s NZE supply projections as well as expected aviation energy demand, both summarized in table 3, were used to set the baseline requirements for the case study. Starting with these demand numbers, the analysis uses actual 2014 aviation energy usage, at 11.1 quads,⁸³ as well as projected demand for 2050, at 21.5 quads (the 2050 projection was chosen to maintain consistency with earlier CATF work (see fn. 8)). On the right of the table, projected hydrogen supply (for transportation end uses only), as well as the projection for metric tons of captured CO₂ (DAC only) – i.e., 205 megatonnes (MT) and 1.1 gigatonnes (GT), respectively – are from the IEA World Outlook 2023.⁸⁴ The ammonia supply projection, which includes existing uses such as agriculture and new applications like marine fuel, is from the same NZE scenario but is referenced in the IEA’s Ammonia Technology Roadmap.⁸⁵ As there is no projection for ammonia use as an aviation fuel, any ammonia needed for aviation end uses would be in addition to the IEA projections.  

Table 3: Data used for case study; 2030–2050 aviation demand from fn. 8  

The supply of synthetic SAF is projected to reach 102.5 billion gallons (Bgal) in 2050, based on the IEA projection for CO₂ available via DAC and a report from Atsonios et al., which looks at designing advanced pathways for synthetic jet fuel production. That paper sets out to maximize the volume of product that can be classified as aviation fuel, since a common challenge of fuel synthesis is that it leaves shorter-chain hydrocarbons that cannot be used as SAF. Atsonios et al. describe a novel Fischer-Tropsch based process that can produce 90.7% synthetic jet fuel relative to other products and a methanol synthesis process that can produce 85.8%, with plant efficiencies of 72.6% and 49.5% respectively.⁸⁶  Such fuel synthesis technologies, as introduced in earlier chapters, operating at these relatively high efficiencies, would be considered theoretical today; with further technology advances, however, these parameters could be representative of real plant operations several decades from now, when every potential efficiency has been exploited. Additionally, while it is reasonable to expect that a large part of the CO₂ used as a feedstock for synthetic fuel production would be captured from the atmosphere in 2050, realistically, DAC-sourced CO₂ is not going to make up the entire universe of carbon used for synthetic fuels. Considering these factors and making use of the data in this report to compute the amount of synthetic fuel derivable from the projected 1.1 GT supply of CO₂ results in a supply estimate of 12.5 quads. This data point is used as the upper limit for synthetic fuel demand in our case study analysis. 

The last dataset fed into the analysis, from a 2018 NASA report, provides information on the worldwide distribution of flight distances in July 2014.⁸⁷ This data was used to calculate annual gallons of kerosene used in each flight distance segment using typical aircraft fuel economy and passenger counts; dividing the results by total aviation energy use in 2014 (11.1 quads) yields an estimate of the share of overall aviation energy use attributable to each flight distance segment. The result of these calculations is shown in Figure 8. The non-dimensionalized shares shown in the figure were then scaled up using projected overall aviation energy demand for 2050 (21.5 quads) to estimate fuel demand as a function of flight distance for future airline operations.  

Figure 8: Percentage of global fuel use as a function of flight distance

6.1 Base Case

The base case is meant to represent a net-zero aviation future that is reasonably attainable given the state of the technologies discussed in previous chapters. A foundational assumption for this case, one that is replicated across all cases given the relative certainty of the technology, is that biofuel will be used to meet a large part of total aviation energy demand in 2050. As for the specific breakdown of biofuels going to various transportation end uses, while we view it as unlikely that 100% of biofuel feedstocks will be used for SAF, there is reason to expect that a very large percentage will go to the aviation industry as that is most difficult to decarbonize and most willing to pay a premium. Accordingly, this case assumes 90% of projected sustainable biofuel feedstocks in 2050 will go towards SAF production.  

Most of the rest of demand in 2050 will likely be met with synthetic fuels; however, it is difficult to make a direct assumption about the contribution from this category of fuels due to competing factors: on one hand, synthetic fuels are very expensive; on the other hand, they are also, as many noted in interviews, highly scalable. As a remedy, we first compute the role of other, newer technologies – namely direct hydrogen- or ammonia-powered flight – whose market penetration by 2050, while likely to be much smaller, is also easier to project. We then assume that remaining demand is met by synthetic fuels. Fully electric aircraft are not considered because they are not projected to have a sizable effect on sector emissions, as discussed previously.  

First, for hydrogen aircraft, the base case assumes that range will be limited, especially before 2050. Extended range variants with extremely light hydrogen tankage that minimize energy use relative to kerosene may be developed, but the consensus from discussions with companies and findings in the literature is that hydrogen will be used first for smaller planes and/or regional flights. The maximum flight distance for a regional flight is assumed to be 750 miles (1207 km). As shown in figure 8, a 750-mile band would include a moderate number of flights as the distance distribution is heavily skewed towards the 500-to-1100-mile (805–1770 km) range. Specifically, flights less than 750 miles make up about one-third of daily traffic. That said, it is unreasonable to assume that hydrogen-powered flights will account for all of this volume by 2050 given the state of the technology today; the difficulty of building hydrogen infrastructure, both in terms of transport to the airport and onsite fueling systems; challenges in handling liquid hydrogen; and required changes to safety/operating procedures. Additionally, many larger planes make regional trips for multiple reasons having to do with airline system operations and route popularity, and those planes – given that they must also regularly fly over 750 miles – are not likely targets for early hydrogen adoption. Accordingly, this case study assumes that only 20% of flights less than 750 miles will be powered by hydrogen in 2050. 

Ammonia-powered flight is the newest of the technologies we considered and is still being researched at universities. It offers potential advantages compared to hydrogen, however, which explains why some major industry players are backing related R&D work. As laid out previously, ammonia’s greater volumetric energy density means that approximately 75% of domestic flights could be covered using ammonia-powered aircraft. Given the size of the United States landmass, this approximation can be used directly with figure 8, resulting in the potential of ammonia-powered flights up to approximately 1650 miles (2655 km), a notable improvement over hydrogen. Ammonia does present downsides related to fuel toxicity, especially for passenger applications, but in general, airports would be able to handle ammonia more easily than hydrogen.  

Ammonia technology is well behind hydrogen, however, making commercialization before the 2045–2050 timeframe unlikely. Given that, our base case assumes that just 10% of flights less than 1650 miles (2655 km) will be covered by ammonia aircraft. This leaves synthetic fuels to cover the remaining demand. This method of assigning a percentage of demand to biofuels, hydrogen, and ammonia first, and then assigning the remainder to synthetic fuels, is used for all cases. The assumptions for each case are summarized in Table 4. 

Table 4: Summary of assumptions used in the case study 

Aviation Fuel Demand Breakdown by Energy Carrier in 2050: Scenario Assumptions 

Results for the base case are shown in figure 9. Projected aviation energy demand in 2050 is broken down such that 86% is met using bio-and synthetic SAF, both drop-in fuels. Emissions do remain when using these fuels, but HEFA will reduce the CO₂ footprint by 50%–65% and synthetic fuel derived from DAC will be nearly net-zero. A scenario that relies heavily on these two fuel types is particularly likely as it is the least demanding in terms of changes to infrastructure.  

Zero-carbon fuels make up the remaining 14% of projected energy demand (8% from ammonia, and 6% from hydrogen). Hydrogen aircraft technology has a higher TRL but less potential applicability given the challenges with tankage weight and range. The expectation is that more flights over short distances with hydrogen do occur in the base case, but ammonia use on longer (less common) routes leads to ammonia being responsible for a slightly larger contribution to meeting the total energy demand. Regardless, aircraft certification, fleet turnover, and infrastructure buildout are expected barriers to greater adoption of either of these fuels. 

Figure 9: Results for the base case  

On the right of the figure, the percentage of total projected 2050 supply is shown for each fuel. As noted on the figure, the IEA’s ammonia projections do not include aviation, meaning that an additional 19% needs to be supplied relative to out-of-sector projections to meet demand. This is not necessarily infeasible, but it does have indirect implications for the agriculture industry. Projected demand for synthetic fuel and hydrogen is well below the relevant IEA supply projections (at 68.5% and 81.7% respectively). That amount of hydrogen includes fuel for direct hydrogen powered flight, as well as the hydrogen needed as a feedstock for the production of ammonia, synthetic, and HEFA fuels. Note that, computed using assumptions from Atsonios et al.,⁸⁸ the amount of hydrogen needed as a feedstock to synthesize e-fuels is very significant, totaling 15.3 quads or 80.5% of the total 19.1 quads of hydrogen projected to be needed by the aviation sector in this analysis. Also of note is the aviation sector utilizing 81.7% of the IEA NZE hydrogen projection for all transportation uses in 2050 (40.3% of the total hydrogen supply projection). This result is significant and implies that specific policy and planning interventions would be required to avoid supply restrictions that elevate costs across the industry. 

The base case helps to show that the sustainability issues for biofuels surfaced in the first CATF aviation report (see fn. 8) are manageable. Insofar as that report concluded that supply shortfalls could emerge if the aviation industry attempts to rely exclusively on bio-SAF to support its decarbonization efforts, these results show that a combination of other technologies currently under development can make up the shortfall while honoring IEA NZE 2050 supply projections. Since it is unlikely that the future aviation fuel mix will actually align with all of these assumptions, however, our remaining analysis looks at some additional scenarios to provide more context. These include an advanced synthetic fuels case, an advanced hydrogen case, and an advanced ammonia case.  

6.2 Advanced Synthetic Fuels Case

The advanced synthetic fuels case represents a scenario where synthetic fuel prices drop faster than many current projections anticipate, allowing this fuel to capture a larger piece of the market than expected in the base case. In this scenario, an increased contribution from synthetic aviation fuel reduces demand for hydrogen and bio-SAF and entirely eliminates the need for ammonia aircraft. Compared to bio-SAF, synthetic fuels are chemically identical but currently more expensive, especially if the CO₂ used to make these fuels is sourced from DAC. Even if more economical CO₂ sources are available, producing synthetic SAF requires around 100 times more hydrogen than producing bio-SAF, cementing its cost disadvantage. However, truly climate beneficial biofuel feedstocks are projected to become scarcer, raising the possibility that synthetic fuels will be more competitive in 2050 despite the fact that they require more hydrogen. Thus, the advanced synthetic fuels case assumes a slight drop to 80% of the total projected sustainable biofuels feedstock supply going toward SAF production, due to the cost of biofuels beginning to eclipse the cost of synthetic fuels near midcentury.  

Turning to ammonia, in a scenario where synthetic fuels are cheaper than biofuels, the biggest advantage of using ammonia over synthetic fuels – i.e., fuel cost – is muted. The Haber-Bosch process used to produce ammonia consumes less hydrogen than is needed for e-fuel synthesis, and nitrogen is cheaper than CO₂ to source. On the other hand, synthetic fuels are drop-in compatible and have no range restrictions, while ammonia needs a large amount of new infrastructure as well as redesigned aircraft engines. If synthetic fuels are adopted at an accelerated pace and prices fall nearer to prices for ammonia, there is little reason for the market to pursue the more risky, newer technology option in lieu of the drop-in solution. Thus, the advanced synthetic fuel case assumes 0% of flights less than 1650 miles (2655 km) will be covered by ammonia-powered aircraft.  

For hydrogen, the tradeoffs aren’t as clear; however, looking at the market today there is a demonstrated appetite for truly zero-carbon fuel solutions in the aviation market. Additionally, any captured CO₂ that is permanently sequestered instead of being used as a feedstock for synthetic fuel production affords a greater climate benefit. Considering that hydrogen aircraft are already being tested and that there are incentives to find the most reasonable, beneficial solutions, hydrogen aircraft are not likely to be eliminated from consideration; rather, their penetration will probably be limited to small planes over very short distances. For these reasons, our advanced synthetic fuel case assumes a drop of one half compared to the base case, or just 10% of flights less than 750 miles (1207 km) being covered by hydrogen in 2050. 

Compared with the base case, figure 10 shows synthetic fuels making up the largest part of the 2050 aviation energy mix at 56%, with the biofuel contribution reduced to 41%. In this case, carbon-containing fuels still account for 97% of the mix; however, there may be a small climate benefit compared to the base case since synthetic SAF with CO₂ sourced from DAC likely has lower lifecycle emissions than bio-SAF. There is also little need for infrastructure changes and improvements in this case, beyond what is required to support pipelines to the airport from new synthetic fuel production facilities. Additionally, a 0% role for ammonia means that there is no concern about adversely affecting the agriculture and marine shipping markets. Direct hydrogen flights are relegated to a niche market at 3% of total energy demand. 

Figure 10: Results for the advanced synthetic fuel case

That said, the bar chart on the right shows that both synthetic fuel and total hydrogen demand are much closer to the IEA’s 2050 NZE supply projection, at 95.7% and 95.4%, respectively, where the percentage for hydrogen is relative to the supply projection only for transportation uses. Owing to the huge amount of hydrogen needed to produce synthetic fuels, additional demand for low-carbon hydrogen from the aviation sector would put stress on a supply chain that today has yet to scale. If aviation demands nearly all the projected hydrogen supply for transportation uses, then other transportation areas looking to decarbonize, like heavy trucking, will experience increased competition for a product that has wide applicability for decarbonizing sectors such as: steel and cement production, existing uses of hydrogen as a feedstock in refineries, as well as ammonia production for the agriculture sector. Thus, this case presents cost and supply risks that are less present in the base case. If it comes about, it will likely be because of a strong preference for preserving fuel and infrastructure uniformity in the aviation sector, in combination with explosive technology and business growth in the e-fuels market.  

6.3 Advanced Hydrogen Case

The advanced hydrogen case looks at a scenario where emphasis is put on hydrogen-powered flight, such that planes with zero CO₂ emissions are aggressively added to fleets, airport infrastructure for hydrogen is prioritized over infrastructure for other clean technologies, and the CO₂ used to synthesize e-fuel remains expensive through 2050. In this situation, increased hydrogen use is expected to lower demand for synthetic fuels and ammonia, but not affect demand for bio-SAF.  

First, with respect to bio-SAF, any carbon-based fuel is going to be preferred, especially in the near term, over alternatives like hydrogen due to infrastructure costs and aircraft technology risk. In a scenario where CO₂ prices remain elevated, either due to limitations of DAC or challenges associated with scaling fuel synthesis technology, the aviation industry is likely to look first to maximize its use of bio-SAF. For that reason, this case assumes no change from the base case in the biofuel contribution – in other words, it still assumes that 90% of projected sustainable biofuel feedstocks will be used for SAF production. 

With ammonia, we expect that rapid adoption of hydrogen-powered flight reduces the need for this more novel technology.  Ammonia’s advantages over hydrogen are range and ease of handling. However, if hydrogen effectively covers a large percentage of regional flights, ammonia will be forced to compete with SAFs, of both bio- and synthetic origin, in the 750–1650-mile (1207–2655-km) range where, in contrast to the comparison with hydrogen, ammonia is more difficult to handle and has a shorter range. Furthermore, any handling advantages over hydrogen are relevant only if ammonia infrastructure is in place, and in this scenario, hydrogen infrastructure development is assumed to be favored. Once technology timelines are factored in, this scenario sees ammonia becoming a niche solution in certain markets that are partial to zero-carbon fuels but where range is a hard requirement, or in markets that host other large ammonia users (e.g., ports). As such, the advanced hydrogen case assumes 5% of flights less than 1650 miles (2655 km) – i.e., half as many as in the base case – will be covered by ammonia-powered aircraft.  

Lastly for hydrogen itself, the percentage of flights 750 miles (1207 km) or less that operate on hydrogen increases in this scenario, but not to 100%. The primary reason is that larger, single-aisle planes that fly both regional and longer domestic routes will necessarily have reduced hydrogen uptake. That said, Airbus and others expect to have hydrogen aircraft by 2035, and even if that target date is missed, the technology is far enough along that it should be ready for widespread adoption for regional flights if market conditions are right and the infrastructure is aggressively rolled out as assumed in this scenario. For these reasons, the advanced hydrogen case assumes that 80% of flights less than 750 miles (1207 km) will be hydrogen-powered in 2050. 

The biofuels share on the top of figure 11 is unchanged as explained above. Instead, increased hydrogen use results in a sharp reduction in demand for synthetic fuels, dropping their share to 25%. That equals the share for direct hydrogen-powered flight. This leaves ammonia to cover the remaining 4% of demand from more niche, zero-carbon but longer-range use cases. 

Figure 11: Results for the advanced hydrogen case  

Interestingly, the advanced hydrogen case is the case where the least amount of low-carbon hydrogen needs to be produced – just 69.6% of the IEA NZE supply projection for transportation uses. This finding illustrates the extent to which future demand for hydrogen will be driven by synthetic fuel uptake as well as the advantage of directly combusting a fuel like hydrogen that has a superior specific energy, or energy content per kilogram, in comparison to carbon-based fuels. Because this scenario requires only a small (9.5%) expansion of ammonia supply and relatively few quads of hydrogen, while also reducing reliance on expensive synthetic fuels (down to 42.9% of projected supply), it is likely the case with the lowest fuel costs in our analysis. Other costs, however, would be born elsewhere, primarily from outfitting airline fleets with hydrogen aircraft, installing hydrogen infrastructure at airports, re-training airport staff to safely handle hydrogen, and developing needed hydrogen transmission and distribution infrastructure around the country. This scenario offers some advantages, but it is less likely than the previous two cases. If it does materialize, it will likely be due to a relatively uniform industry and policy push to develop hydrogen aircraft and associated infrastructure, together with heavy airline investment in fleet turnover to add zero-carbon planes.  

6.4 Advanced Ammonia Case

This case examines a situation where the new technology needed to enable ammonia-powered aviation, along with required infrastructure, advances rapidly to allow for a sizable increase, relative to the base case, in coverage of the less-than-1650-mile (2655-km) flight segment. That range, which corresponds to approximately 75% of flights, implies that ammonia can be used for most single-aisle aircraft worldwide. Twin-aisle aircraft for international flights, however, would still need a low-carbon, kerosene-like jet fuel which is why this case assumes the same percentage of biogenically sourced SAF as the base case. Consequently, SAF production in this case still requires 90% of projected sustainable biofuel feedstocks. 

Demand for hydrogen-powered flight, on the other hand, is completely obviated by ammonia in this scenario. If ammonia technology advances rapidly, ammonia infrastructure is built out, and airports and airlines get comfortable with the safety measures needed to manage fuel toxicity risks, then there will be little reason for the market to select hydrogen over ammonia. Both are zero-carbon fuels, but ammonia makes for easier ground operations and allows for longer flights. If hydrogen’s primary advantage of technology readiness is assumed to be minimal, as it is in this scenario, the market should select ammonia over hydrogen so long as ammonia prices are near expectations. Thus, this case assumes 0% of flights less than 750 miles (1207 km) are covered by hydrogen in 2050. 

Finally, looking at ammonia itself, while its theoretical advantages are many, there are key limitations in just how much the less-than-1650-mile (2655-km) flight segment can be realistically covered by ammonia in 2050. Even assuming that the expected commercialization timeframe for ammonia technology shifts forward from 2045–2050 by five to ten years, once fleet turnover and the relative novelty of the technology are factored in, the best case for ammonia is likely moderate adoption by midcentury. This leaves a window for synthetic fuels to make up the remaining demand where the ease afforded by their drop-in compatibility would compete against a lower price point for ammonia. Considering the timeline and these competing factors, our advanced ammonia case assumes a modest increase from the base case to 33% of flights less than 1650 miles (2655 km) being covered by ammonia aircraft in 2050. 

Figure 12: Results for the advanced ammonia case 

Figure 12 shows results for this case, with the biofuels portion unchanged and the synthetic fuel and ammonia fuel mix nearly evenly split at 29% and 25% of overall aviation energy demand, respectively. The bar chart shows a slightly reduced need for hydrogen compared to the base case, at 79.5% of IEA-projected hydrogen supply for transportation uses, but more demand than the advanced hydrogen case. This is mostly due to the additional hydrogen needed to increase ammonia production and, to a lesser extent, to meet slightly heavier reliance on synthetic fuel, at 49.5% of IEA-projected supply, for this case.  

The required increase in ammonia production itself is the key result, as this case implies an increase in ammonia supply of 62.7% over projections based on ammonia use in other sectors. In contrast to the other cases, this increase is likely large enough to drive some cross-sector effects, potentially creating a situation where ammonia supply and demand dynamics lead to widespread price increases. These supply concerns, as well as the state of technology for ammonia-powered flight, make this the least likely of all cases considered in our analysis. Ammonia has advantages as an aviation fuel, but there would need to be an industry-wide shift in thinking for it to play a greatly expanded role in the midcentury timeframe. A scenario like the base case, in which airlines experiment with adopting this newer, longer range zero-carbon technology on certain routes, is more likely. 

Lastly, figure 13 compares demand for hydrogen in each of the advanced cases to hydrogen in the base case. As highlighted in the foregoing discussion, our results show significant changes in demand for hydrogen – on the order of approximately ±15% or 6 quads – depending on the case.  

Figure 13: Comparison of hydrogen production demand versus the base case

The biggest driver is synthetic fuels production, which requires hydrogen as a feedstock and leads to a set of tradeoffs: the convenience of drop-in synthetic fuels, versus the ability to quickly scale clean hydrogen production, versus uncertainty about how quickly the technologies and infrastructure needed to enable direct hydrogen-powered flight might mature.  This three-way conundrum will be heavily influenced by costs, technology considerations, industry preferences, policy commitments, and public will. 

7. Policy Recommendations

Efforts to commercialize synthetic SAF and other aviation decarbonization options face numerous challenges, many of which are discussed in this report. Nonetheless, the development and deployment of massive volumes of non-biogenic, climate-friendly aviation fuel is essential to eliminate GHG emissions from the aviation sector – which, in turn, is a critical step in achieving full decarbonization of the transportation sector. Policymakers in the United States, Europe, and elsewhere are starting to address these challenges directly and indirectly, but more needs to be done to promote the production, distribution, and use of synthetic SAF and zero-carbon aviation fuels. The suite of useful policies ranges from broadly applicable measures that reduce the carbon intensity of transportation fuels to highly targeted provisions that support the build-out of supply chains for appropriately-sourced carbon and hydrogen feedstocks. 

7.1 Clean Fuel Standards

Clean fuel standards (CFS) require gradual reductions in the carbon intensity (CI) of the transportation fuel mix. Typically, these policies subsidize low-carbon alternative fuels and energy carriers (including electricity) by awarding tradable credits to producers of fuels that beat the CI target. At the same time, they increase the cost of conventional fuels by requiring producers of such fuels to buy enough credits to cover any excess carbon from failing to meet the CI target.  

California pioneered the CFS approach in 2009 when it adopted a statewide Low Carbon Fuel Standard (LCFS) for ground transportation fuels.⁸⁹ Other states on the U.S. West Coast followed suit over the next decade, as did British Columbia. Beginning in 2018, suppliers of low-carbon aviation fuels could “opt in” to the California LCFS and generate saleable credits;⁹⁰ in 2024, the California Air Resources Board initiated a regulatory revision process aimed at applying the LCFS carbon intensity standards to all aviation fuel used on flights that take off and land in California.⁹¹ 

Efforts are also underway to develop a nationally applicable clean fuel standard for the United States. Key elements of a strong federal CFS include: 

• Trajectory to zero by midcentury. A CFS must drive down the carbon intensity of transportation fuels to near-zero by approximately midcentury. 
• Robust and comprehensive lifecycle GHG analysis. The issuance of credits and deficits under a CFS hinges on determinations of fuels’ lifecycle GHG emissions. Clean fuel standards must utilize the best available models and tools for assessing full lifecycle emissions, including direct upstream emissions from fuel production, transport, and handling as well as significant indirect emissions (for example, from land-use changes in the case of transportation fuels made from biogenic feedstocks). 
• Safeguards against overreliance on unsustainable feedstocks. Safeguards, such as a limit on the number of credits that can be issued to fuels made from a particular feedstock, are needed to prevent regulated entities from over-relying on specific fuel types, especially land-intensive biofuels. 
• Multi-sector coverage. To promote planning and align incentives across highly related fuel markets, a CFS must apply to fuels used by on- and off-road vehicles, as well as to fuels used in the aviation and marine sectors. That said, a CFS should restrict cross-sector credit trading to ensure that the policy drives critical technology advances in the aviation sector and avoid scenarios in which, for example, airlines buy inexpensive credits generated by battery-powered passenger cars rather than investing in the development of low-carbon aviation fuels.  

7.2 Mandating Synthetic SAF

Governments can accelerate the deployment of synthetic SAF through consumption mandates. The European Union’s ReFuelEU Aviation policy requires fuel providers to gradually increase the availability of sustainable aviation fuels to civil aviation flights operating out of large airports, beginning at 2% of fuel consumption in 2025 and culminating at 70% in 2050. Nested within the ReFuelEU requirement is a specific sub-mandate for synthetic aviation fuels that grows over time: synthetic aviation fuel must account for 1.2% of SAF available at EU airports in 2030–2031, 10% by 2040, and 35% by 2050.⁹²     

By specifying that a particular fuel type must account for a predetermined share of the fuel market, consumption mandates are less effective than performance standards at promoting innovative and cost-effective compliance strategies. Mandates can, however, avoid some of the implementation problems that are associated with CFS-type performance standards’ heavy dependence on the complicated and imperfect tools needed to assess lifecycle GHG emissions.⁹³ 

7.3 Production Subsidies

Public subsidies that reward the production of low-carbon aviation fuels can also be effective, provided that eligibility for subsidies is limited to fuels that demonstrate clear and substantial improvements in carbon intensity as compared to conventional aviation fuel. The 2022 Inflation Reduction Act introduced a new subsidy for sustainable aviation fuel in the form of a tax credit to be issued by the Treasury Department. The value of the credit ranges from $1.25 per gallon for fuels that achieve a 50% reduction in carbon intensity to $1.75 per gallon for fuels that achieve a 100% reduction.⁹⁴ The tax credit is equally available to producers of synthetic SAF and bio-SAF, although few if any synthetic SAF producers will be positioned to claim the credit in the program’s early years. (Concerns around the methodology that the Treasury Department would use for quantifying the carbon intensity of bio-SAF led to extensive lobbying and a complicated intra-agency review process.)    

The most effective clean energy subsidies strike a balance. A well-designed subsidy for synthetic SAF would help insulate a fledgling industry from overwhelming price competition during its early years, while also ensuring that public funding is reserved for fuels that deliver clear and substantial climate benefits. As discussed in the context of clean fuel standards, the use of robust and comprehensive lifecycle GHG assessment tools is imperative. If, for example, a lifecycle assessment tool used to determine subsidy levels for SAF producers fails to adequately account for indirect land-use-change emissions associated with land-intensive methods of supplying feedstocks for bio-SAF, synthetic SAF producers will be unfairly disadvantaged while bio-SAF producers with significant indirect GHG emissions could receive financial benefits.    

7.4 SAF Contracts for Differences

A contract for differences (CfD) is a financial tool that obligates one party to compensate another party for the difference between the current market price of an asset and the value of that asset at the time the parties entered into their contract. Modified versions of this tool are gaining traction in the realm of energy policy as a way to reduce cost barriers to the deployment of less carbon intensive fuels by making a third party responsible for covering per-unit fuel costs that exceed an agreed upon threshold. For example, a government that recognizes the need to pull synthetic SAF into the marketplace could create a CfD with a synthetic SAF producer in which the government guarantees that the producer will earn a pre-specified amount for each gallon of synthetic aviation fuel it sells to airlines over a five-year period. The pre-specified amount could be established at a level that is, say, $2.00 above the current average per-gallon sale price of bio-SAF, allowing the synthetic SAF producer to compete in SAF markets while the government temporarily bears some or all of the additional cost of synthesizing SAF from non-biogenic feedstocks. 

The European Union is pursuing a similar objective by allocating up to 20 million free allowances under the Emissions Trading System (ETS) to aircraft operators that use SAF, with the intention of mitigating some or all of the price differential between SAF and conventional aviation fuel.⁹⁵ 

7.5 Boosting the Supply of Low-Carbon Hydrogen

Any massive scale-up of synthetic SAF production – and indeed, success in overall aviation decarbonization through almost all possible pathways – is contingent on the availability of low-cost hydrogen and carbon atoms obtained through climate-friendly processes. In the case of hydrogen, public policy is needed to defray the extra costs of producing hydrogen through processes involving low GHG emissions and/or to guarantee demand for such hydrogen.  

The United States is pursuing two main strategies to promote the production of low-carbon hydrogen. Both show significant promise, but both are also in need of refinement. The first focuses on direct investments to demonstrate hydrogen technology and is called the Regional Clean Hydrogen Hubs (H2Hubs) Program. Enacted as part of the Infrastructure Investment and Jobs Act of 2021, this program makes $8 billion available to regional coalitions of industrial and commercial participants who partner to bring low-carbon hydrogen production, storage and transport infrastructure together with end-users (including, in some instances, SAF producers).  

To ensure that some development of low-carbon hydrogen supply and infrastructure is directed to aviation applications, the U.S. Department of Energy (DOE), which is implementing the H2Hubs Program and is currently negotiating awards for seven proposed hubs, should consider hydrogen as an aviation fuel or feedstock and a key end-use for supporting broader decarbonization goals. In particular, DOE should consider directing some portion of the $1 billion in federal funding that was set aside for a demand-side support initiative as part of the H2Hubs Program to aviation end-uses to help demonstrate the needed technologies.⁹⁶ Additionally, any future funding for hydrogen hubs (in the United States or globally) should focus on the most essential end-uses sectors that need hydrogen to decarbonize – of which aviation is one. 

The second major national-level U.S. policy for advancing clean hydrogen is a production tax credit established by the Inflation Reduction Act of 2022. Under this policy, hydrogen producers can claim a tax credit of up to $3.00 per kilogram of low-carbon hydrogen. The U.S. Department of Treasury is currently finalizing guidance for the clean hydrogen tax credit (also known as “Section 45V” in reference to the relevant section of the U.S. Tax Code), but future amendments or extensions to the credit could focus on incentivizing necessary development of very low-carbon hydrogen production methods (for example by limiting future credits to the lowest levels of carbon intensity that are currently eligible for Section 45V). Future federal policies for low-carbon hydrogen could also tie incentives to climate-beneficial end-uses (namely, heavy industrial applications and heavy transportation applications, including aviation). This would help ensure that limited supplies of low-carbon hydrogen go first to high-priority sectors and end-uses that lack other viable decarbonization options, including uses of hydrogen as a clean fuel or fuel feedstock for aviation.  

The European Union is working to boost the supply of low-carbon hydrogen through a variety of financing programs, including the EU ETS Innovation Fund’s Hydrogen Bank, which will distribute €800 million in the form of fixed premiums per kilogram of renewable hydrogen produced;⁹⁷ and Horizon Europe’s Clean Hydrogen Partnership, a public-private initiative to support research and innovation in the production, transmission, distribution, and storage of renewable hydrogen, as well as in selected fuel cell technology areas.⁹⁸ By ensuring that a substantial portion of these and other pools of available funding are directed toward aviation-related end-uses, policymakers in Europe could accelerate the sector’s progress toward decarbonization.   

Another way governments could prioritize the use of climate-friendly hydrogen for aviation decarbonization would be to create a rebate available to synthetic SAF producers. Under this approach, a producer of synthetic SAF would be entitled to a fixed rebate (for example, $1/kg) for any low-carbon hydrogen it sources and incorporates into its product. 

7.6 Boosting Access to Climate-Friendly Carbon

Because the provenance of the carbon atoms used to make synthetic SAF significantly impacts the climate compatibility of the resulting fuel, policymakers and other stakeholders must (1) develop protocols for assessing the climate impact of various carbon-sourcing technologies and (2) implement mechanisms for testing and eventually deploying the most promising options.  

Policies can account for the climate impact of mechanically extracting carbon from the atmosphere and using it as a SAF feedstock with reasonable confidence, mainly because the key factors are measurable. The major impediment to direct air capture (DAC) appears to be the high cost of separating carbon from ambient air. Capturing carbon from seawater could be comparatively inexpensive but the technology’s net climate impact is less well understood, in part because it cannot be measured (and thus needs to be modeled).  

Carbon can also be obtained by capturing CO₂ emissions from “point source” facilities like factories and electric generating units. If the facility is powered by a fossil fuel, the climate benefits of using the captured CO₂ to then synthesize fuel are likely to be relatively modest, given that the process ultimately involves a transfer of geologic carbon to the atmosphere. If the facility is powered by bioenergy, synthetic SAF made from the captured CO₂ could have a very low CI, depending on the type of biomass that was used and what its alternative fate would have been had it not been combusted.   

Sorting out the climate impact of these pathways requires careful analysis that is transparent and standardized across markets and regulatory jurisdictions. Policymakers should work with academic institutions, public interest organizations, and industry to launch a variety of efforts designed to minimize the uncertainties surrounding carbon capture and, more pointedly, to clarify its role in synthesizing SAF. One such effort was initiated in 2024 by the U.S. National Science Foundation to “establish a comprehensive Federal marine [carbon dioxide removal] research program to accelerate the development of knowledge needed to determine,” among other things, “the climate-mitigation potential of marine [carbon dioxide removal] approaches, including their efficacy, permanence, scalability, energy and other resource demands, and costs.”⁹⁹  

As the cost and efficacy of carbon-sourcing technologies become better understood, policies should be implemented to support the initial deployment of capture technologies and processes that clearly reduce atmospheric CO₂ levels, are additional to existing carbon removal efforts, do not result in leakage, are subject to safeguards that prevent double-counting of claimed removals, and do not create any unintended harms.¹⁰⁰  Eventually, governments will also need to help develop protocols and supporting analytics for assessing the net benefits or disbenefits of using captured carbon for fuel synthesis, rather than permanently sequestering it. 

7.7 Reducing Non-CO₂ Effects

While the warming effect is not yet fully understood, recent analyses indicate that contrails might double or triple the aviation sector’s total impact on climate change. More research is urgently needed on numerous aspects of this phenomenon. CATF’s forthcoming report on non-CO₂ impacts of aviation outlines numerous steps that policymakers and other stakeholders can take to strengthen scientific understanding of contrail formation and how it can be reduced. 

7.8 Promoting Innovation in Zero- and Low-Carbon Aviation Fuels

Regional and national governments can help accelerate the commercialization of critical technologies for decarbonizing aviation by investing in research and development initiatives. Funding directed by agencies like the European Union Aviation Safety Agency, the Ministry of Economy, Trade and Industry in Japan, and DOE and the Federal Aviation Administration in the United States should support programs that, among other things: 

• Improve the efficiency and reliability of low-GHG systems for producing and delivering the hydrogen needed to scale up synthetic SAF and zero-carbon aviation fuels.  
• Optimize propulsion systems and on-board storage systems for zero-carbon fuels, including ammonia-powered gas turbines and low-weight tanks for hydrogen. 
• Prototype modified aircraft designs, including blended wing bodies designed to accommodate hydrogen.  

8. Conclusion

One of the most challenging aspects of decarbonizing the transportation sector is finding climate-friendly solutions that will work for the aviation industry. This industry, which has heavily relied on energy-dense, carbon-intensive fossil kerosene, must transition its aircraft engines, airport facilities, storage capabilities, supply chain infrastructure, operating practices, and safety regulations away from the use of conventional jet fuel to realize stated climate goals.  

Commonly referred to as sustainable aviation fuel or SAF, the most mature technology available today is biofuel derived from fats and oils processed with hydrogen to create hydrocarbon fuels that match the energy density of fossil kerosene. This drop-in solution is necessary; by itself, however, it is likely insufficient.  Previous CATF work based on supply projections from the IEA finds that scaling sustainable biogenic feedstocks in ways that do not adversely affect agricultural land uses to match growing aviation demand is unrealistic, even with stronger policies and private investments to improve supply going forward. Thus, a market that has relied on a single fuel will have to explore a transition to a combination of energy sources: biofuels, synthetic kerosene synthesized from low-carbon hydrogen and CO₂ (where the CO₂ might be obtained from point sources, biomass gasification, DAC, or other capture technologies), direct hydrogen or ammonia fuel, and, in some specific cases, fully electric propulsion.  

That transition would require sufficient scale-up of multiple nascent industries and other changes that can be expected to stress airport infrastructure and operations, but a multi-fuel future represents a more achievable, more climate beneficial approach to decarbonization compared to a strategy that relies exclusively on bio-SAF. Nearer to midcentury, as sustainable biogenic feedstocks become scarcer, costs for bio-SAF will likely approach parity with costs for synthetic kerosene; additionally, since hydrogen- and ammonia-powered flights are the cleanest options, these technologies should begin to play a role as they mature, albeit within a limited flight range. Ammonia and hydrogen should cost less than synthetic fuel given the costs associated with sourcing CO₂ for synthetic fuel production and the amount of hydrogen used in the process.  

Considering all those factors, our analysis shows that a combination of four fuels – bio-SAF, synthetic jet fuel, hydrogen, and ammonia, where the first two fuels are used to meet 86% of overall demand – represents a plausible fuel mix for the aviation industry in 2050. That said, there are significant technological and economic obstacles that will, at best, complicate the aviation industry’s transition to such a multi-fuel future. To have any chance of success the transition will need to be supported by substantial private investment as well as by public policies that encourage the deployment of synthetic and alternative zero-carbon fuels. Furthermore, those obstacles are also likely to sharpen interest in the use of direct air capture (and possibly direct ocean capture) of CO₂ to offset the emissions associated with simpler but higher-emitting strategies for fueling the aviation sector. CATF will more fully examine the role of offsets in subsequent analyses. 

Notwithstanding those challenges, the results of this analysis help illustrate a potentially viable way forward.  Remaking the aviation fuel market will be an ambitious undertaking, but if the entire sector rises to the challenge, aviation decarbonization is possible on a timeline that meets national and international climate goals.  

Footnotes