Revolutionizing Aviation: Understanding The Transformative Power of Power-to-Liquid Fuels
Introduction
The journey to sustainable aviation is not a singular path—it's a sprawling landscape of possibilities, where cultivating diverse solutions will be necessary to meet industry net-zero goals. Within these solutions, SAF is expected to play a colossal role. According to the International Air Transport Association (IATA), SAF is poised to contribute a substantial 65% to the indispensable emissions reductions needed for the complete decarbonization of aviation. It stands as the linchpin in our commitment to achieving carbon neutrality, but the present reality paints a modest picture—SAF currently constitutes a mere 1% of the global fuel supply. This technology is still nascent and the need for a profusion of renewable sources is paramount if we are to scale up and make a transformative impact on aviation's carbon footprint. Power-to-liquid (PtL) fuel, or “e-fuels”, stand out as one particularly exciting possibility, poised to shape the aviation landscape for years to come. Industry experts predict that PtL will emerge as the principal contributor, generating the largest volume of SAF in the long run. So, what are e-fuels, and what’s their potential to redefine the future of flight?
Background
Broadly, SAF emerges from three broader categories of alternative fuel sources:
Biomass-to-Liquid (biofuels)
Waste-to-Liquid
Power-to-Liquid (e-fuels)
With many of the feedstocks potentially being a mix of categories, and as the aviation industry embarks on a trajectory toward sustainability, SAF is expected to undergo several generations of development over the coming decades. Different conversion pathways need to be ASTM-approved to refine and process renewable feedstocks into SAF, and can be divided into these larger categories:
Currently, eight pathways or “annexes” have been approved by ASTM, and nearly all these pathways convert biomass into SAF. This includes fats, oils, and greases (FOGs) and materials like tallow (animal fat) and used cooking oil (UCO). However, biomass feedstocks have inherent limitations, primarily due to its restricted global supply and lack of scalability. Identifying and locating the relevant feedstock is crucial to develop regional supply chains. Right now, the more expensive the feedstock, the more mature the technology:
Image Description: Feedstock costs and the technical effort to produce SAF are inversely proportional.
Source: T. Edward Yu’s presentation assessing the Impact of Preprocessing and Conversion Technologies on SAF Supply from Forest Residues in the Southeast US, University of Tennessee Knoxville
Looking ahead, the future of SAF is poised to witness a paradigm shift towards power-to-liquid fuels. This process involves converting carbon dioxide, and blending it with hydrogen, thus literally creating liquid fuel or hydrocarbons from hydrogen and carbon. Using renewable or even direct solar energy like solar-to-fuel concepts, can achieve near zero carbon score. Solar-to-fuel works by harnessing solar heat to drive a thermochemical reactor, where carbon dioxide and water undergo a transformative process, resulting in the creation of syngas.
How are e-fuels created?
Creating e-fuels involves three main components:
1. Clean Hydrogen Source:
A fundamental requirement of e-fuels is a source of clean hydrogen, constituting approximately 15% of the resultant SAF. Hydrogen can be sourced and created in several different ways, determining how sustainable it is. The primary recommendation within power-to-liquid fuels is to use "green hydrogen", which is created through water electrolysis, a process powered by renewable electricity.
2. Carbon Source:
Comprising approximately 85% of the final SAF, the carbon component in e-fuels must be renewable to prevent further contributions to atmospheric carbon. Methods for capturing carbon include Direct Air Capture (DAC), which extracts carbon dioxide directly from the atmosphere, and Point Source Capture (PSC), which focuses on concentrated industrial waste gases. While PSC tends to be the transitional method, it works by capturing CO2 that would’ve been emitted, rather than capturing CO2 already in the atmosphere. This captured carbon dioxide undergoes a conversion to carbon monoxide.
3. Fuel Synthesis:
To create the final product, a liquid SAF, carbon and hydrogen need to be fused into high-density liquid hydrocarbons. Various conversion pathways can accomplish this, including the ASTM-approved Fischer-Tropsch (FT) synthesis. Methanol synthesis is another possibly pathway for e-fuels, and it is currently under review.
Image Description: Visual representation of the power-to-liquid process. Electrolyzers, powered by renewable energy, split water into hydrogen and oxygen. Hydrogen and carbon dioxide are mixed to form a syngas, creating liquid SAF.
Source: Synhelion
This revolutionary process demonstrates the synergy of clean hydrogen, recycled carbon, and innovative synthesis methods, likely redefining the future of aviation fuel.
Sustainability of E-fuels
The sustainability of a given SAF hinges on feedstock choices and the carbon intensity of the conversion process, determining the emissions throughout the production lifecycle. In this way, e-fuels hold a huge amount of potential. For example, capturing and sequestering CO₂ play a pivotal role in power-to-liquid fuel production, creating a closed loop where the initially emitted CO₂ is repurposed to generate fuel. As a result, e-fuels can achieve a remarkable reduction of near 100% in its lifecycle emissions.
However, while the components of power to liquid fuels—hydrogen and carbon—may seem futuristic and easy to come by, there is more to the story. E-fuels demand a significant amount of energy for production, primarily to produce renewable hydrogen through electrolysis. The energy usage is so much so that there’s not enough global renewable energy supply to meet the energy demands jet fuel from e-fuel would require, making e-fuels, for the time being, a more niche production opportunity.
Image Description: Graph showing the greenhouse gas reduction and renewable energy required for distinct use of renewable electricity. Moving from left to right shows the best “bang for your buck” in terms of carbon reduced per unit of renewable energy deployed.
Source: T. Elizabeth Lindstad, Tor Øyvind Ask, Pierre Cariou, Gunnar S. Eskeland, Agathe Rialland, Wise use of renewable energy in transport, Transportation Research Part D: Transport and Environment, Volume 119, 2023
When considering carbon reduced per unit of renewable energy deployed, it makes sense to focus on the biggest decarbonization opportunities, the “biggest bang for our buck”. In a time of limited renewable energy production capacity, aviation is one of the least efficient places to deploy that renewable energy. Per $ invested, using renewable energy to decarbonize other industries like road transport, achieves a cheaper mt of CO2 reduction because of the efficiencies gained of converting an internal combustion car to an electric one. Only if aviation can help install new renewable energy capacity and accelerate overall renewable energy production, do e-fuels help on the energy side.
In addition to the high electricity usage, power-to-liquid fuels are also resource-intensive. They require a substantial amount of hydrogen, which is currently most often sourced from fossil fuels, termed “gray hydrogen” as opposed to “green hydrogen”, which would reduce the process's overall sustainability. In addition, hydrogen has more impactful opportunities in other hard-to-abate sectors like steel production. Additionally, the production of e-fuels demands large quantities of water and other resources, so care must be taken to not restrain local ecosystems or cause additional environmental damage.
While e-fuels exhibit significant promise in contributing to a sustainable aviation future, the challenges posed by their high energy usage and resource demands underscore the need for continuous innovation and efficiency improvements to fully realize their potential as a key player in the sustainable aviation landscape.
Other Barriers to Production
As discussed, large-scale e-fuel production is not currently feasible due to the global demand for jet fuel exceeding the capacity of all the world’s renewable electricity.
In addition to e-fuel production’s inefficient use of renewable electricity, several other barriers impede their widespread production. Chief among these challenges is the high cost of renewable energy, which constitutes a majority of power-to-liquid production expenses. According to the International Civil Aviation Organization (ICAO), “under current conditions, costs for PTL jet fuel are expected to be 3 to 5 times higher than costs for conventional fuel.” The timeframe for extensive adoption remains hampered by this economic infeasibility, emphasizing the need for ongoing innovations and cost-reduction measures.
Should We Still Be Excited About E-fuels?
Despite some large barriers, power-to-liquid fuels offer immense promise in steering long-haul aviation towards sustainability. E-fuels are expected to be a crucial component of future SAF supplies, influenced significantly by European SAF sub-mandates for advanced pathways and power-to-liquid fuel. Because they are produced using renewable energy, they have the transformative potential to significantly slash aviation's CO2 emissions through a clean synthesis process. Moreover, they require minimal land and biogenic materials resulting in little-to-no land use impact, unlike other biofuel or crop-based fuels might.
The excitement about power-to-liquid fuels lies in the potential. Combining a biogenic CO2 source with new green hydrogen and renewable energy creates a very strong sustainability story, completely recycling extant CO2 and using minimal land resources. For places without significant land availability for biofuels (like Europe), or in locations where biofuel yields may be too low or difficult to collect/transport efficiently (like the Middle East or Australia), power-to-liquid can fill a very important gap in SAF production needs.
Not only for aviation, but power-to-liquid can also assist in decarbonizing other industries. For example, Fischer-Tropsch synthesis conversion process produces an array of hydrocarbons as intermediate products which could be used for other sectors, like shipping or the chemical industry. Cross-industrial partnerships like this could lead to lower costs, bringing e-fuels to market more quickly. Another exciting potential lies in the Middle East, where solar and wind energy resources abound. With some of the highest levels of solar irradiation globally, countries like Saudi Arabia, the UAE, and Qatar are already spearheading renewable energy programs. This positions the Middle East as a key player in fostering the growth and implementation of PtL technologies.
In addition, several fuel producers and start-ups are going all-in on power-to-liquid fuels. Air Company is focused on scaling carbon capture and sequestration technologies with the goal of providing decarbonization solutions to several industries, including aviation. They entered a purchase agreement with Jet Blue for 25 million gallons and signed partnerships with Boom Supersonic, Virgin Atlantic and the US Air Force. Twelve’s power-to-liquid technology “works like industrial photosynthesis”, taking carbon dioxide, water and renewable electricity to produce carbon-based products like SAF, dubbed “e-jet”. They are scaling up a production facility in Washington state and expect the first regular passenger service on e-jet in early 2025, as well as penning firm offtake agreements with the International Airlines Group (IAG) and more.
Conclusion
In the future, the landscape of sustainable aviation will certainly see a significant growth of power-to-liquid fuels. The synergy of clean hydrogen, renewable carbon sources, and innovative synthesis methods position e-fuels as a transformative force. While there are barriers, such as high production costs and energy usage, the journey toward large-scale adoption is underlined by its significant potential in reducing greenhouse gas emissions without feedstock limitations. Cooperation across sectors and utilizing geographical advantages, particularly in the Middle East with its abundant solar and wind energy, amplify the exciting prospects for power-to-liquid’s's integration into aviation. As we navigate challenges and foster collaboration, power-to-liquid fuels emerge not just as an alternative but as a necessity for sustainable aviation, propelling us towards a future where flights traverse the skies with reduced environmental impact.
Sources
1. IATA - Sustainable Aviation Fuel (SAF)
2. Sustainable Aviation Fuels (SAF) Powerlist 2023 by SimpliFlying - Issuu
3. Conversion processes (icao.int)
4. WEF_UAE_Power_to_Liquid_Roadmap_2022.pdf (weforum.org)
5. Synthetic fuels explained | Synhelion
6. Wise use of renewable energy in transport - ScienceDirect
7. ICAO Working Paper - Power-to-liquids (ptl): Sustainable Fuels for Aviation
8. Greening the Energy Mix: Saudi Arabia, the United Arab Emirates, and Qatar - Al Tamimi & Company
9. AIR COMPANY | Carbon Technology Leader for a Decarbonized Future
10. Twelve | carbon transformation