Aviation is hard to decarbonise with electric planes a long way off. In the interim, jet fuel from bio-based and recycled sources is urgently needed to reduce greenhouse gas emissions. But which is the best technology to invest in? There are signs that the Fischer Tropsch process, invented in the 1920s to convert coal to liquid hydrocarbons, is undergoing a renaissance. The number of companies involved continues to grow.
Airlines, oil companies and governments are announcing roadmaps and strategies for sustainable aviation, nearly every day. The UK government is the latest to launch a new strategy, committing to have at least five UK SAF plants under construction by 2030 and new funding for a net-zero 100% SAF-driven transatlantic flight. Winners of a precursor competition in the UK relied heavily on Fischer Tropsch type technology.
The most technologically straightforward option for SAF is to hydrotreat virgin or used vegetable oils (HVO) to produce a drop-in fuel, which can be blended directly with conventional aviation kerosene. But these raw materials are not available in large enough quantities to make a significant contribution. So, companies have been looking for technology that can treat a wide range of feedstocks from both bio-based and recycled sources.
Biomass and fossil wastes are the most abundant and sustainable raw materials, and these are mainly solids. So, the first step is usually gasification to produce syn-gas, a mixture of carbon monoxide and hydrogen. Adapting existing syn-gas technology to convert such a wide range of materials is not straightforward. Enerkem was the first to commercialise the conversion of mixed municipal waste to methanol via syn-gas in Canada. Last year they announced that their project in Rotterdam will be repurposed to produce sustainable aviation fuel using Shell’s Fischer Tropsch technology.
Sasol has been developing and operating Fischer Tropsch technology since the 1950s using South Africa’s coal resources to produce diesel and gasoline. It recently announced a tie-up with German aircraft manufacturer Deutsche Aircraft to develop SAF, based on Sasol’s experience. The hydrogen required is produced with renewable energy.
BP has been working on better Fischer Tropsch catalysts for many years. Together with Johnson Matthey, it offers a process for license; Fulcrum Bioenergy is the first licensee and their commercial plant in the US started commissioning a few months ago.
But Fischer Tropsch technology produces a range of hydrocarbons, some of which are too heavy for SAF and have to be sold as diesel substitutes or be reprocessed. ExxonMobil has claimed that its technology, which converts methanol to SAF, has higher yields than ‘other options’. A mix of alcohols can also be used as feedstock, providing extra flexibility.
Airlines are increasingly focussing on waste and residues as raw materials. Although waste biomass provides low carbon SAF, there is no consensus that end-of-life plastics and other fossil residues provide substantial GHG savings. The European Commission has yet to pronounce on the GHG savings of recycled carbon fuels. There is also the problem of availability. There are not enough of the most easily processed wastes on the market. I have previously written about the displacement of used cooking oil (UCO) from road transport to aviation fuel by misguided policy measures.
In the future, the world must move to using captured CO2 and renewable energy to provide enough low carbon SAF to decarbonise all aviation. How adaptable will today’s processes be?
Exxon could use one of the electrochemical routes under development to produce methanol directly from CO2 but further scale-up will be required. Johnson Matthey is bringing a new reverse water gas shift technology to its partnership with BP. This uses hydrogen to reduce CO2 to carbon monoxide, whilst minimising the competing route to methane. This would produce the necessary syn-gas.
There may be other technologies at the laboratory or pilot scale, which offer better performance. But, Fischer Tropsch has stood the test of time and it is commercially available. It has been developed by different companies over the years. It may not be perfect but, as the saying* goes; “Don't let perfect be the enemy of good”. The pace of climate change is such that we may have to go with the best we have right now.
Published: 19 August 22
