Are hydrocarbons the key to sustainable aviation? Photo by Jose Lebron on Unsplash
Air travel is the foundation of our interconnected society. With the development of airplane technology came the ability for humans to fly, to travel across oceans in mere hours, and to create a society where places once a weeks’ journey away now take a fraction of the time to reach. Our society has developed a dependence on this technology by creating a market to travel and experience the world, trade goods, share cultures, and visit friends and family. But the very thing fuelling this travel—hydrocarbon fuel—is contributing to the decline of our Earth. Aircraft that run on conventional jet fuel—the fossil fuel known as kerosene—emit carbon dioxide, which contributes to the greenhouse effect. The best way to tackle this? Run our planes on something else. Sustainable aviation fuel (SAF) is a buzz-word in the aviation industry today, and Oxford Carbon Capture Utilisation (OXCCU), a spin-out company from the University of Oxford, is at the forefront of its development with their novel process combining carbon dioxide and hydrogen (both components found in the air) to generate the hydrocarbons needed to power an airplane. Andrew Symes, an Oxford MChem graduate, as well as CEO and Cofounder of OXCCU, spoke to the Oxford Scientist about the future of sustainable aviation and the role they may play in it.
Our society has developed a dependence on this technology…
Back in 2017, Symes came across a paper in Nature detailing how carbon dioxide could be converted directly into gasoline and jet fuel range hydrocarbons in a single step. The researchers took the already established Fischer-Tropsch process (used to convert carbon monoxide to liquid hydrocarbons) to the next level by eliminating the energy intensive intermediary step of converting carbon dioxide to carbon monoxide before it is then converted to fuel. This concept became the backbone of OXCCU, who have used this method to generate jet fuel. Earlier this year, they opened their demonstration plant at Oxford Airport showcasing how the process is possible outside of a laboratory environment and could have the potential for larger-scale production, providing a green solution to power jet engines.
Finding a balance between maintaining global connectivity and decreasing climate impacts is key.
When asked about the best way to increase the sustainability of aviation, Symes was confident in the need for the widespread transition to SAF. At the moment, aviation accounts for around 2.5% of global carbon emissions, but this is set to rapidly increase. Adopting an “eco-Marxist” approach and banning all flying would be one way to stop emisions, but realistically, whilst some excessive use can be restricted and replaced by trains and other ground transportation, aviation demand is not just going to disappear overnight. Finding a balance between maintaining global connectivity and decreasing climate impacts is key. The solution, therefore, is to clean up the jet fuel. In regard to SAF, people often talk about using hydrogen as a fuel. This, however, presents numerous problems. Due to the nature of hydrogen gas, with its low energy density by both mass and volume, containing it for transport and storage is difficult, even if energy is used to cool it to the incredibly low temperatures required for it to be a liquid. Current aviation technology is designed to work with liquid hydrocarbons, which has far superior energy density, and simple refuelling, meaning a whole set of technology would need to be developed for hydrogen fuelled aircraft to become a reality—our climate does not have that sort of time. Symes remarked that the promise of the hydrogen economy is tantalising because it has zero emissions; it comes from water and makes water—avoiding carbon entirely. This being said, he went on to detail the huge problems associated with using hydrogen as a fuel regarding the storage, weight, and technology. The solution, he claimed, is ‘using hydrogen in the fuel not as the fuel’. Power-to-Liquid fuels (PtLs), such as the fuel produced by OXCCU, are hydrocarbons, just like kerosene, but created artificially by combining hydrogen and carbon dioxide rather than forming underground over millions of years. Current aircraft are already equipped to carry and run on this fuel, so no technological challenges stand in the way. Furthermore, hydrocarbons themselves are incredibly energy-dense resources: as Symes put it, ‘nature discovered the best way to store hydrogen as a fuel’ in the form of hydrocarbons. In this regard, OXCCU aims to create an efficient, low-cost method to produce PtLs; this way, hydrocarbons are still used as jet fuel but its effect on the climate has decreased.
As a society we think of hydrocarbons as being inherently bad…
The idea of hydrocarbons being green goes against everything we are taught in school. As a society we think of hydrocarbons as being inherently bad, but the reality is that the negative effects come from taking carbon out of long-term reservoirs, where nature stores it in the form of fossil fuels such as oil and gas, and emitting it back into the atmosphere as carbon dioxide. If the carbon required to make the hydrocarbons comes from sources other than fossil fuels, the process can be made a lot greener, as no new carbon is being taken from long term storage and released into the air. The ideal solution to this would be direct air capture, where carbon dioxide is taken directly from the atmosphere. Unfortunately, our current technology is not yet advanced enough for this to be a reality. The more serious option in the short term is taking carbon dioxide from biogenic sources, where it is produced as a byproduct and currently just released to the atmosphere when making fuels such as bioethanol as well as biogas and in sugar production. The final option is capturing and releasing carbon dioxide released from the burning of fossil fuels. This final option is not considered a fully green solution as it still involves fossil fuel burning, but it does mean any carbon dioxide emitted is being used multiple times before reaching the atmosphere—decreasing the amount released in the long run. OXCCU is currently developing a larger scale plant, which plans to source carbon dioxide from bioethanol production, combining the carbon dioxide with hydrogen gas generated by water electrolysis. During this process, electricity is used to split water into hydrogen and oxygen. The oxygen is then harmlessly released into the atmosphere, and the hydrogen is captured. A local green electrolyser, sourcing its energy renewably, will be used for OXCCU’s plant. Once the carbon dioxide and hydrogen are combined, a hydrocarbon fuel, which can be used in jet engines, is left.
At present, OXCCU’s current plant at Oxford Airport is only used to demonstrate the possibility of the process, and although plans are underway for a bigger plant to produce the fuel on a larger scale, no OXCCU-produced fuel has made it to a jet engine so far. The transition to sustainable fuel is aviation is starting off slow. Only a handful of planes have run using PtL fuel, and even then, it is combined with kerosene, only making up a small part of the fuel the engine is running on. However, the potential for rapid growth is there. The ReFuel EU mandate, which applies to all flights originating in the EU regardless of destination, aims for 2% of jet fuel to be SAFs by 2025, with this target growing to 70% by 2050, demonstrating the plans for rapid acceleration in the fuel’s adoption. Other countries such as the USA and Japan also have SAF mandates in place, and the UK is set to adopt its own at the start of next year. The aims are high and achievable, but a few barriers still stand in the way. The cost of electrolysis needs to be reduced to make PtLs economically viable, and green electricity sources need to be readily available for use during the electrolysis process. Symes also pointed out that using SAFs does not mean the aviation industry will be 100% clean. Residual emissions remain from soot produced by engines: however, technological developments in engineering could be used to combat this.
