Making sense of our options to decarbonize aviation
The importance of air travel to the global economy is readily apparent: over $6 trillion of goods are moved annually via air, and over 8 billion passengers flew for work or leisure last year alone. Furthermore, global air travel, responsible for around 800 million metric tons of CO2 emissions in 2022 (~2-3% of total emissions), is only expected to grow, with some projections estimating it will make up as much as 25% of global CO2 emissions by midcentury. That means it is a critical sector to decarbonize if we want to achieve our global climate goals. However, this is an immensely challenging issue due to:
- Airplane design favoring energy-dense fossil kerosene because of inherent weight and space constraints; and
- A significant stock of long-lived equipment and infrastructure has been optimized for that specific fuel type.
Nonetheless, numerous major airlines and airports, among others in the industry, have made a series of commitments to reduce aviation’s climate footprint.
One popular way forward is increasing the use of sustainable aviation fuel (SAF), an alternative typically derived from biomass treated with hydrogen such that the final product is chemically identical to kerosene. Biomass-derived SAF (bio-SAF), however, is unlikely to be able to support a fully decarbonized aviation sector given feedstock constraints and sustainability concerns related to the indirect effect of biofuels production on agriculture and land use. Cross-sector competition, especially with the food industry, must be avoided, forcing the aviation industry to consider a mix of multiple fuel alternatives to make up for the coming biofuel shortfall. But what options are available, and which of those are feasible? A newly released report from CATF examines these options and finds that aviation decarbonization will likely require a mix of fuels. Here’s what we found:
Options for a future clean aviation fuel mix vary widely
From fuels that are synthesized from carbon dioxide pulled from the air to planes that use larger versions of batteries that power electric vehicles on the road today, the options that are being researched, developed, and tested by the aviation industry vary widely. These include:
Low-Carbon Synthetic Fuel: a hydrocarbon fuel synthesized from syngas, a mixture of carbon monoxide and hydrogen, to produce jet fuel. It requires climate-friendly carbon and low-carbon hydrogen as feedstocks and may offer many of the advantages of bio-SAF without the sustainability concerns. Cost, currently estimated at 3-5 times more than fossil jet fuel, is a concern along with the life cycle emissions associated with sources of carbon and hydrogen feedstocks. There are several options for climate-friendly carbon and a variety of hydrogen production methods.
Point-source carbon capture
CO2 that is captured from large point sources such as power plants or industrial facilities. Low cost is the biggest advantage, but emissions associated with this method will never reach net zero as it is more akin to carbon recycling, where the carbon is used more than once before being transferred to the atmosphere.
Biomass gasification
Biomass feedstock is gasified to produce syngas which can be converted to synthetic jet fuel. This option could be attractive in specific locations where low-cost biomass or waste feedstocks are available near low-cost, renewable energy sources.
Direct air carbon capture (DAC)
Carbon is removed directly from the ambient air using a liquid solvent or solid sorbent that selectively absorbs CO2. It can be implemented anywhere and is scalable, but the low CO2 concentration in air means that a large amount of renewable energy is needed to power huge fan arrays to capture meaningful amounts of CO2.
Ocean carbon capture
Oceans are the world’s largest carbon reservoir with CO2 concentrations of about 120–150 times ambient air. This technology is also scalable and can be combined with offshore wind to mitigate energy concerns. However, unknown technology and cost risks, along with questions about negative effects on ocean ecology, exist with this method.
Hydrogen production
Low-carbon hydrogen can be produced via water electrolysis using renewable or nuclear energy, natural gas reforming with carbon capture, or less common techniques like methane pyrolysis. Regardless of the production method, cost is a major hurdle and that must be solved before hydrogen can function as a feedstock for synthetic fuels.
Hydrogen, Ammonia, and Electric Propulsion: Certain plane types on some routes may be able to utilize fuels like hydrogen and ammonia, or batteries – in lieu of bio-based SAF or synthetic fuels – to reduce aviation CO2 emissions more significantly. These fuels avoid the carbon atom altogether, whereas captured carbon, that would otherwise be part of natural processes or intentionally sequestered, is ultimately released when burning either biogenic or synthetic SAF. However, there are cost, technology, and infrastructure challenges with these alternative fuels that must be overcome.
Direct hydrogen-powered flight
Efforts to develop hydrogen-fueled aircraft technology have focused on smaller aircraft for regional routes. Companies are aiming for the introduction of these aircraft post-2035 as it requires significant development of hydrogen infrastructure and changes to aircraft design that would require a lengthy recertification process.
Direct ammonia-powered flight
Ammonia (NH3) has 49% more energy per volume than liquid hydrogen, meaning that ammonia-fueled aircraft could replace SAF on longer distance flights where it would be difficult to use hydrogen. However, the need for new infrastructure and fuel toxicity are downsides and the technology is approximately 10 years behind hydrogen.
Fully electric aircraft
This makes sense for drones or electric vertical take-off and landing aircrafts (eVTOLs), but battery energy density limitations lead to a lack of range and small aircraft that are not suitable for most marketable routes. Fully electric aircraft are expected to have little or no impact on aviation CO2 emissions.
Our analysis finds that low-carbon synthetic fuel, hydrogen-fueled, and ammonia-fueled planes are all good options to pursue if coupled with bio-SAF, which is the most developed and cost-reasonable option available today. Next, we analyzed how these fuels might fit together in a market looking to minimize costs while decarbonizing by midcentury.
Methodology
This analysis draws from literature reviews and interviews with key market players for a detailed examination of each of the above non-fossil aviation decarbonization options. It uses this information to develop scenarios for achieving net-zero aviation emissions by 2050 using a mix of four aviation fuels: bio-SAF, synthetic fuel, hydrogen, and ammonia. These four fuels cover all the clean aviation fuel technologies currently in development, with the exception of fully-electric propulsion which, as mentioned above, was removed from consideration due to lack of range. This exercise helped determine which fuel mix might result in a near-net-zero future for aviation and provided insight into potential implementation challenges.
The full report looks at four 2050 scenarios, two of which are highlighted below. To ground our analysis, we compared our results to supply projections for synthetic fuel, hydrogen, and ammonia from the International Energy Agency’s (IEA’s) net-zero-emissions-by-2050 (NZE) scenario. Only ammonia was allowed to exceed its NZE projection (the IEA assumes that ammonia will be consumed only for existing uses and as a marine fuel; it does not assume any ammonia will be used as aviation fuel). The hydrogen supply numbers from the IEA NZE scenario only include hydrogen supply for transportation uses, which makes up approximately half of the total projection; and 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 CATF 2022 to maintain consistency. Our 2050 scenario analysis assumes no further growth in biofuel supply during 2030-2050, mainly due to land constraints. Of the biofuel supply that is available, across all cases we expect a large percentage to go to the aviation industry given the comparatively few good low-carbon options. Most of the rest of the demand will likely be met with synthetic fuels, followed by direct hydrogen- or ammonia-powered flight.
Different fuel mixes present different advantages and challenges for implementation
Case 1: Advanced synthetic fuels
This case represents a scenario where synthetic fuel prices drop faster than many projections anticipate, allowing it to capture a large piece of the market. As part of determining the amount of synthetic fuel necessary the case assumes very small contributions from hydrogen aircraft and none from ammonia variants.
Figure 1: Results for the advanced synthetic fuel case
The result is synthetic fuels make up 56% of the mix with the bio-SAF contribution at 41%. A key benefit is that few infrastructure changes and improvements will be needed beyond what is required to support pipelines to the airport from new synthetic fuel production facilities. Regional flights powered directly with hydrogen have a very small impact at 3% of total energy demand. However, the bar chart on the right shows that both synthetic fuel and total hydrogen demand are very close to the IEA’s 2050 NZE supply projection, at 95.7% and 95.4%, respectively.
Even though this case assumes little impact from hydrogen aircraft, owing to the huge amount of hydrogen needed to produce synthetic fuels, this level of demand for low-carbon hydrogen from the aviation sector would put stress on a supply chain that today has yet to scale. Furthermore, synthetic fuel being close to the projection means that nearly all of the CO2 sourced from DAC will need to go toward synthetic aviation fuels rather than being sequestered. Both of those results have challenging implications for the low-carbon hydrogen production market and climate impact of DAC. If this comes about, it will likely be because of a strong preference for preserving fuel and infrastructure uniformity in the aviation sector, along with rapid technology and business growth in the synthetic fuel market.
Case 2: Advanced hydrogen
This case puts emphasis on hydrogen-powered regional flights, such that planes with zero CO2 engine emissions are aggressively added to fleets, airport infrastructure for hydrogen is prioritized over infrastructure for other clean technologies, and the CO2 used to produce synthetic fuel remains expensive through 2050.
Figure 2: Results for the advanced hydrogen case
This scenario shows that increased direct hydrogen use in aircraft 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-CO2 engine emissions on planes operating over longer-ranges.
Interestingly, this scenario requires the least amount of low-carbon hydrogen production – just 69.6% of the IEA NZE supply projection for transportation uses. This result illustrates:
- The extent to which future demand for hydrogen will be driven by its use as a needed input to synthetic SAF production; and
- The advantage of directly using hydrogen whenever possible, given its high specific energy.
Because this scenario requires only a small (9.5%) expansion of ammonia supply, relatively few quads of hydrogen, and reduces 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 handle hydrogen safely, and developing needed hydrogen transmission and distribution infrastructure around the country. This scenario offers some advantages, but it is less likely due to infrastructure and technology risk. 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.
While challenging, aviation decarbonization is likely best served by a mix of fuels
It is possible to decarbonize aviation with a mix of technologies being developed today. As illustrated above, different fuel mixes present different advantages and challenges to implementation. Any transition from a single fuel to a three-or-four-fuel mix will be very complex and expensive due to hydrogen supply requirements, infrastructure needs, the cost of sourcing CO2, and hydrogen aircraft adoption. One alternative could be DAC or ocean-based carbon capture offsets (combined with continued fossil kerosene use). However, the climate benefits and costs related to this must be thoroughly examined, which CATF intends to do in future work.
While a transition to multiple fuels would require sufficient scale-up of several nascent industries and other changes that can be expected to stress airport infrastructure and operations, in comparison to exclusive reliance on biofuels it represents a more achievable, more climate-beneficial approach to decarbonization. If the entire sector rises to this challenge, then aviation decarbonization becomes an achievable goal by midcentury.
