In 2022 aviation accounted for ca. 2% of global energy-related CO2 emissions, with around 800 Mt CO2 being produced by this sector per year. CO2 emitted from aircraft is released into the upper atmosphere causing a greater environmental impact than CO2 emitted at ground level. Flying also leads to the release of nitrogen oxides, water vapour and soot, which high up in the atmosphere can cause radiation leading to further negative effects on the climate.
Clearly, tackling aviation related emissions is a key part of addressing climate change. However, flying remains particularly hard to decarbonise due to the unique requirements for aircraft technology, including weight and size constraints, long innovation cycles, prioritization of safe operation and the relative cost associated with adopting new technologies in this sector at scale.
Traditionally, aviation fuel (kerosene) is produced from crude oil distillation and contains a mixture of longer chain hydrocarbons. Synthetic fuels are also well-known and combined with traditional aviation fuels to produce a hybrid-fuel. Both these well-used fuel types heavily rely on fossil fuels and when burnt lead to CO2 which has been captured for millions of years being released into the atmosphere.
Given the impact of aviation on global emissions, this area is ripe for innovation. A number of companies are currently working on ways to transition to a more sustainable aviation industry. Overall, there are three key technologies which are emerging in this space:
- Circular CO
- Hydrogen and
- Battery power.
Circular CO2
An exciting area in the pursuit of greener aviation is the application of the idea of a circular economy to aviation fuels.
As explained in our article Circular economy: new technologies rise to the challenge, the circular economy refers to an industrial model based on the principles of designing out waste and pollution where possible; keeping products and materials in use where their use is necessary; and regenerating natural systems. For aviation, this involves finding fuel alternatives which involve minimal by-products and emissions in the first place and, perhaps even more importantly, reusing these waste products by converting them back into more fuel.
There are a number of companies actively working to use the carbon dioxide emitted by combustion to produce greener synthetic fuels by combining carbon capture technology with novel conversion processes. The so called ‘e-Fuels’ have already been explored in our blog e-Fuel – the solution to green air travel. Here we seek to highlight some of the key innovators in this field and some recent developments in this technology.
Circular synthesis of new fuels using the Fischer-Tropsch reaction
A key reaction used in the production of liquid fuels from carbon dioxide is the Fischer-Tropsch reaction. This reaction was first developed by Franz Fischer and Hans Tropsch in 1926 and involves the conversion of carbon monoxide and hydrogen into alkanes. Scientists are able to convert CO2 released from combustion to carbon monoxide (using a Water-Gas-Shift reaction) and then use the Fischer-Tropsch reaction to produce synthetic e-Fuels. By recapturing the CO2 produced by air travel and using this to produce new fuel, aviation fuel can be made in a circular fashion.
One company operating in this space is Oxford University spin-out, OxCCU. OxCCU have developed a process that uses renewable electricity to combine CO2 from the air with hydrogen from water to make truly circular e-Fuels. OxCCU have developed a new Fe-Mn-K catalyst which enables a high CO2 conversion with high C5+ selectivity. Therefore, unlike with traditional Fischer-Tropsch processes which first require the formation of CO, OxCCU’s process allows efficient direct conversion of CO2 to jet fuel in a single step. OxCCU recently filed a patent application for this technology.
Impressively, this process is also compatible with CO2 from a variety of sources, including direct air capture. The H2 required for this process can be sourced from the electrolysis of water using power from renewable energy (see my recent blog on green hydrogen production for an insight into developments being made in the field of hydrogen electrolysers). With further developments in CO2 capture and increases in renewable energy this technology presents an exciting prospect for a net-zero jet fuel that avoids the need for modifications to current jet engines and fuel infrastructure.
Zero Petroleum are another company working on producing “drop in” e-Fuels for the aviation industry. Their process also involves using hydrogen derived from the electrolysis of water and CO2 from the air. The hydrogen and carbon are then combined in a process called Direct-FT synthesis. This is a proprietary version of the Fischer-Tropsch process used to directly manufacture target fuels at a high yield with no need for refinery upgrading. Recently, Zero made headlines as the British RAF were able to use its fuel to achieve the first flight powered by synthetic e-Fuel. Zero are now working with the RAF to mass produce sustainable aviation fuel for their fleet.
Fulcrum Bioenergy are following a slightly different approach to e-Fuel synthesis and using household waste as the source of the syngas. In this process, household waste is sorted to remove metals and other inorganic materials. The waste is then processed and dried to create a “confetti-like” feedstock. This is then heated in a low-oxygen environment in the presence of steam to form syngas in a process referred to as gasification. The syngas is then fed through a wet scrubbing process to remove unwanted impurities and adjust the carbon monoxide and hydrogen ratios, so that it is ready for use in a Fischer-Tropsch reaction. The final stage in this reaction involves cracking the crude product of the Fischer-Tropsch reaction into smaller chain hydrocarbons, the jet fuel fraction is then separated from the mixture and longer chain hydrocarbons are recycled back into the cracker. Fulcrum Bioenergy have filed a patent application to cover their technology, with claims directed to a fuel with a high biogenic content.
Direct air capture using alkaline hydroxides
Other companies operating in this area are more focused on the step of capturing CO2 from the air itself. One such company is 1PointFive, who have developed an AIR TO FUELSTM process, allowing them to convert CO2 into drop-in compatible fuels. This process involves using large vessels of alkaline hydroxide solution which react with CO2 in the air to form a carbonate solution. Calcium hydroxide is then added, which converts the captured carbon dioxide into solid CaCO3 and regenerates the alkaline hydroxide solution. This captured carbon dioxide can then be used to produce drop in synthetic fuels using the Fischer-Tropsch process described above.
Biofuels
The Fischer-Tropsch process isn’t the only process which can be used to produce sustainable “drop in aviation” fuels - bio-based fuels provide another possible alternative.
There are several ways of producing these fuels, such as:
- hydrogenation of organic materials (HEFA processes);
- converting sugars into paraffins using micro-organisms (synthesised Iso-Paraffins);
- converting ethanol or iso-butanol into aviation fuel by removing oxygen and oligomerisation of the carbon chains; and
- catalytic thermolysis of fatty acids esters and fatty acids followed by hydrocracking.
Often Biofuels are considered impractical as the requirement for agricultural land to produce the biological feedstocks can mean that less land is available for food or feed. To avoid this issue, a number of companies are looking at producing Biofuels from waste.
One company working in this area is Finnish firm Neste, who have developed a process which allows waste materials such as fat or vegetable oils to be converted into sustainable aviation fuel. According to their recent patent application, their process is a HEFA process, which involves refining the waste oils through hydrogenation, followed by cracking and isomerising the paraffinic molecules to jet fuel chain length. Neste were awarded the European inventor award in 2023 for this technology in the industry category.
In an alternative approach, LanzaJet have developed a process using ethanol obtained by the fermentation of waste, to produce jet fuel. Their process converts ethanol to synthetic paraffinic kerosene by dehydrating the ethanol to produce ethylene, followed by oligomerisation, hydrogenation and then fractionation.
Excitingly Virgin Atlantic recently flew the first transatlantic flight using a commercial passenger plane powered by 100% sustainable aviation fuel (SAFs). The sustainable aviation fuel was supplied by Air BP and Virent and was a blend of 88% waste fats and 12% waste from US corn production. This flight is considered to be a significant milestone in sustainable UK aviation, with the lifetime emissions for these kinds of fuels being up to 70% lower than conventional aviation fuels. Currently, SAFs are added to traditional aviation fuels in small amounts; however, they are more costly than conventional aviation fuel and hence are used in an amount of less than 0.1% overall. Although, there are not yet any commercial plants for producing SAFs in the UK, the government aims to have five being constructed by 2025.
Overall, circular CO2 provides a promising route for providing a drop-in replacement for aviation fuel.
Pure H2
Hydrogen has been extensively discussed as an alternative aviation fuel (see our blog Aiming for the sky: aviation carbon reduction goals for a more in-depth look at this topic).
Approaches for hydrogen powered planes generally involve using modified engines with compressed gaseous hydrogen fuel or hydrogen fuel cells powering electric motors.
Hydrogen is particularly enticing as a replacement fuel because it does not produce any carbon emissions when burnt, avoiding release of CO2 and other nasties into the upper atmosphere.
However, like many proposed solutions there remain a number of issues to be resolved before a full conversion to hydrogen fuel would be possible. Primarily these issues include the problems of storage, re-fuelling, and sourcing the hydrogen.
Hydrogen has a high specific energy compared to liquid fuels, but a much lower energy density. Therefore, although gaseous hydrogen might be suitable for short-haul flights, it is likely that liquid hydrogen would be needed for long-haul aircraft, as there simply isn’t enough space to store such large volumes of hydrogen on conventional planes. These issues mean that a switch to hydrogen would require an overhaul of aviation infrastructure as well as the design of planes themselves in order to implement.
Green hydrogen production
Hydrogen gas itself is usually obtained from fossil fuels and hence is currently not renewable at large scale. Therefore, for hydrogen to be a truly sustainable fuel the hydrogen involved must be produced using renewable energy sources, such as from the electrolysis of water.
One business that is tackling this challenge is Bristol based, oort energy, who are seeking to make hydrogen electrolysis economical and sustainable. Their technology involves the use of Polymer Electrolyte Membrane electrolysis to produce hydrogen. Oort have gone from proof-of-concept stage in 2019 to testing a 10 kW electrolyser stack in 2021 and they are now working on a 250 kW demonstrator system.
Being able to produce green hydrogen more cheaply and efficiently is a key piece in the jigsaw if hydrogen is to be a viable green alternative to aviation fuel. There is significant interest in innovation in this space.
Hydrogen powered planes
ZeroAvia Aviation are one company working on hydrogen powered planes. They envisage using modular electrolysers to produce hydrogen on-site at airports. They are also developing liquid hydrogen refuelling to allow longer flights. ZeroAvia Aviation have already flown nine test flights with an electric engine powered by a hydrogen fuel cell and are working with the Ecojet (part of Ecotricity) in 2025 starting with an Edinburgh to Southampton route with 19 seater turboprop aircraft.
Big aerospace firms such as Airbus, Rolls-Royce and Boeing are also involved in research and development of hydrogen-powered aircraft, albeit on slightly longer timescales than ZeroAvia. Airbus’s ZEROe program hopes to develop the world’s first hydrogen-powered commercial aircraft by 2035. The ZEROe concept involves using hybrid-hydrogen aircraft, powered by hydrogen combustion through a modified gas turbine engine. It is envisaged that liquid hydrogen will be used as the fuel for the modified turbines. The modified gas turbine engines are complemented by hydrogen fuel cells, which create power that complements the gas turbine, this results in a hybrid-electric propulsion system. In 2022, Airbus launched their ZEROe demonstrator to test hydrogen combustion technology on a multimodal platform.
This technology feels a little further away than circular CO2; in particular because it would require significant changes to aviation infrastructure. Nevertheless, in the longer term a move to hydrogen may have benefits as it reduces the levels of CO2 emitted high up in the atmosphere.
Battery power
Another way to power planes with no emissions is to use batteries.
Charging batteries is the most efficient way to power vehicles using green energy, with around 73% of energy in an electric car being converted to useful work. However, even the best batteries of today store only a fraction of the energy per unit weight of conventional jet fuels. As the batteries required for long haul flights would be simply too heavy, based on current technologies, electric planes are envisaged for air travel within urban settings and for short haul flights.
One company leading the way in innovation of electrical planes is Rolls-Royce. Rolls-Royce are focusing on three main areas, small propeller aircraft, commuter aircraft and urban air mobility. Small propeller aircraft have two to four seats and are often used for training pilots, aerial surveying, light cargo operations and banner towing. Recently, Rolls-Royce’s Spirit of Innovation, an all-electric small propeller aircraft, was named the worlds fastest electric vehicle following a test flight reaching speeds of 345 mph in November 2021. The battery in the Spirit of Innovation contains 6,480 cells, having a weight of 700 kg, Rolls-Royce have described it as “the most power-dense propulsion battery pack ever assembled in aerospace”. The Spirit of Innovation also has a motor power of 400 kilowatts equivalent to a 535 BHP supercar, allowing it to achieve such fast speeds.
In the field of commuter travel Rolls-Royce have developed a hybrid-electric power generation system comprised of an embedded gas turbine driving a 2.5 MW generator and a 3000V power electronics and an electric propulsion unit. This was developed as part of the E-Fan X project in collaboration with Airbus. This involved adapting a British Aerospace 146 aircraft to carry two tons of batteries and converting one of the jet engines to run on electricity. Although this project has now been wound up, Rolls-Royce are still looking to develop competitive electrical products for use in the commercial aviation market, with a particular focus on retrofitting existing small passenger planes.
Rolls-Royce are also looking at developing fully electrical aircraft for urban air mobility. These aircraft of the future are adapted to allow Electric Vertical Take-Off and Landing (eVTOL) and have electric propulsion systems, which offer distributed, electrically powered propellers which can be placed in different positions on the aircraft and/or be tilted to allow for both lift and cruise operation. They also allow the aircraft to hover by individually controlling their torque. These aircraft are initially envisaged for fixed point-to-point connections in large urban centres on routes already served by helicopters today. Rolls-Royce are planning to build on experience gained from their CityAirbus demonstrator scheme, where they developed an all-electric octocopter (an eight propellor driven aircraft) designed to carry up to four passengers at speeds of up to 75 miles per hour and with relatively low propellor rotation speeds of less than 1000 rpm. These futuristic drone-like aircraft could provide relief for commuters in congested urban areas.
Another business working in this area are American company Wisk. They have developed a four-seater driverless eVTOL, which they claim has a range of up to 144 km and a cruising speed of about 140 mph. With six propellors mounted on each wing once aloft it flies like a small plane, but the ability to tilt the propellors allows efficient vertical take-off. Wisk have recent signed a memorandum of understanding with the Queensland Council of Mayors with the aim to bring their air taxis to Brisbane by 2032 - just in time for the summer Olympics being hosted there.
Not wanting to be left behind UK start-ups are also innovating in this space, one key example of this is Vertical Aerospace. Working with Rolls-Royce, they have developed the VX4 an eVTOL. They claim that these eVTOLs have a range of greater than 100 miles, a top speed of 200 mph and a power train of greater than 1 MV. The VX4 is also said to be 100 times quieter than a helicopter. Their vehicles make use of a proprietary battery system designed to limit weight and drive unit economics. Vertical Aerospace have filed patent applications for their rotor assembly system, their propulsion system thermal management and even the arrangement of access doors and seats in the aircraft.
Conclusion
There is a significant need to reduce the impact of plane travel on the environment. Despite the number of flights taken decreasing dramatically during the global Covid 19 pandemic, flight numbers are now starting to increase to pre-covid levels. It seems that a range of different solutions are required to tackle this problem, with electric and hydrogen planes being currently more suited to short haul and urban travel and circular CO2 providing the best option for long haul routes. We look forward to seeing new developments in this exciting and rapidly advancing field.
Authors
Helen Eastmond
Helen is a trainee patent attorney in our chemistry team. Helen has an MSci degree in Natural Sciences, specialising in Chemistry, from the University of Cambridge. Her final year research focused on new materials for carbon dioxide capture.
Email: helen.eastmond@mewburn.com
