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20 July 2022

Biologists increase energy density with new carbon negative jet fuel

It could be biologists – not engineers – that have solved the aviation industry’s multi-billion-dollar problem of how to reach net zero emissions. Recent advances in synthetic biology claim to have produced a genetically modified soil bacteria that can produce a fuel even more energy dense that fuels available on the market today.

Published in the journal Joule, academics from the Lawrence Berkeley National Laboratory, California, have developed a way to produce cyclopropane (CP) rings using bacteria, producing one of the most energy-dense hydrocarbon structures known.

Unlike normal hydrocarbons, where carbon atoms bond with up to four other atoms in a tetrahedral – often hydrogen – CP rings are only composed of three carbon atoms. The acute angles between the atoms are 60 degrees rather than 109.5 degrees, placing greater strain on the bonds between them.

Under such strain, these bonds contain a larger amount of energy; more than enough to fuel an aircraft if deployed at scale. Many CP rings have an energy density of over 40 megajoules per liter, surpassing even the most advanced fuels used in aviation and space exploration, which typically average around 35 megajoules. CP rings have the largest net heat of combustion per carbon among all cycloalkanes.

Perhaps more importantly, it provides an energy density that is greater than gaseous hydrogen (2 megajoules per liter), liquid ammonia (11.5 megajoules per liter), and liquid hydrogen (8.5 megajoules per liter), all of which are being explored thoroughly for their potential to decarbonize air travel. It would absolutely smash the energy density of batteries, which with an equivalent energy density of around 2 megajoules per liter (0.6 megajoules per kilogram) and would also beat the sustainable fuel made from biomass and waste products which is already being used by some airlines.

Engineers in the industry have been aware of cp rings for decades. In the 1960s, Soviet scientists developed used them in Syntin, a rocket fuel used to launch Soyuz and Proton rockets. The fuel provided a 3% increase in thrust per unit of the fuels weight against kerosene – not insignificant when looking at the fine margins of space flight. But making such fuels has remained both difficult and expensive, and often uses a fossil fuel feedstock. The fuel also involved toxic and hazardous compounds, so production stopped in the 1990s.

Lawrence Berkeley’s innovation takes inspiration from how the Streptomyces roseoverticillatus soil bacteria produced an anti-fungal substance called Jawsamycin, which is full of CP rings.

Researchers altered this process by re-engineering the biosynthesis that produces Jawsamycin. The new process interrupts the production process at a step where a nitrogen-containing ring would otherwise be added, and then produced a fatty acid using an enzyme that is found in a similar type of bacteria.

The final compound contains either six or seven CP rings along a carbon backbone – called fuelimycins. One final step is required to create methyl esters and turn the fatty acid product into a usable fuel.

With the bacteria growing using carbon dioxide, the production of the fuel from start-to-finish can – in theory – be dubbed as carbon negative.

Obviously, we’re far from reaching scale from this lab-based innovation. Researchers are currently only producing around 10 milligrams of fuelimycin per liter of bacteria solution. There will need to be at least a 100-fold increase in the production rates of the process for it to be commercialized, and it will need to become repeatable at scale.

The idea of repurposing existing aircraft to run on CP rings would be a foolish strategy for airline operators. While the technology will allow planes to fly further with less weight, and could theoretically be blended into existing hydrocarbon fuels, such planes may not still be in operation within the likely timeframes of commercialization.

The other thing to consider is cost. The laboratory’s success in the past has been to genetically engineer bacteria to produce malaria-fighting drugs, where customers are happy to pay a premium. For jet fuel in a competitive aviation space, customers will be more reluctant to pass costs on to consumers.

The aviation industry – responsible for 2.5% of greenhouse gas emissions – is split in its approach towards decarbonization. At Rethink Energy, we anticipate a split between hydrogen fuel cell aircraft in smaller planes, with sustainable fuel – produced through waste rather than biomass – for long-haul flights. For the latter, CP rings could play a role in improving performance, without increasing carbon emissions, but alone they will not provide the imminent decarbonization needed. Companies that push such approaches in their transition strategies will, in truth, have no real plans for transition.

One route that’s been suggested for such innovators is partnership with the country’s air force which faces public pressure to decarbonize. Such a route to scaling would provide a consistent market, without commercial restrictions.

The study’s abstract states, “Freeing the global economy from its dependance on petroleum is key to slow down the pace of climate change. Energy-demanding applications like rocketry, aviation, and shipping are fueled with petroleum-derived hydrocarbons that are difficult to replace. These fuels are rich in cyclic molecules with strained bond angles allowing them to store more energy than non-cyclic molecules. The highest amount of energy can be stored in cyclopropanes, but these molecules are hard to produce via organic synthesis.”

“We produced polycyclopropanated fatty acid methyl ester (POP-FAME) fuels in bacteria. The POP-FAMEs can have energy densities of more than 50 MJ/L, which is larger than the energy of the most widely used rocket and aviation fuels. Although the next step is to scale up their production until the process is commercially viable, the availability of a biobased production method opens the possibility to replace fossil fuels in a very constrained sector.”