Turning farm waste into the jet fuel of tomorrow

“Basically, the recipe for how they do this is written in their DNA, so we take that recipe and copy it into a friendly microorganism, making it able to replicate the process.”

The organism of choice for the Senior Researcher and his colleagues is carefully selected fungi that they feed with renewable, carbon-rich substrates—such as farm waste or acetate produced from renewable energy—which the fungi turn into hydrocarbon rings.

The researchers have been able to produce rings that consist of just three carbon atoms and in doing so, they have made a burnable fuel with an energy density that is greater than some of the most common commercially available rocket fuels.

Next hurdle: Scaling up

“We have evidence that it’s possible to create this energy-dense fuel. Now we are looking at the viability of scaling up production and producing it at a reasonable cost,” Pablo Cruz-Morales says.

This process is likely to take a decade. And even then, the invention will not be usable in its pure form in standard aircraft engines, which are designed to take a different type of fuel. However, it could likely be blended with traditional jet fuel to increase energy density and make plane trips more environmentally friendly in line with new EU requirements (see box).

The fuel also looks like a suitable alternative for flying rockets into space, which could prove important in relation to cost as the space industry has deep pockets. At least that is what industry representatives have led Pablo Cruz-Morales to believe. They aren’t fazed by a higher price for fuel that weighs less. 

“It seems that a couple of million dollars in extra costs is not that important when you have projects that cost hundreds of millions of dollars – especially if this would allow you to bring two more astronauts or satellites per launch,” he explains.

Brilliant bacteria and clever chemistry

In laboratories elsewhere at DTU, a different group of researchers is also tackling the challenge of reducing the environmental impact of aviation. Here another Pablo—a PhD student who answers to the full name Pablo Doménech—is working with researchers in the fields of both biotechnology and chemistry with the aim of developing a two-phased approach for turning CO2 captured from the production of biogas into jet fuel.

“This way, when the fuel burns up in the air, it will not lead to an excess emission of CO2 because the CO2 has already been trapped and made into fuel. It’s closing the carbon cycle,” he explains.

The first phase is a fermentation process where carefully selected, temperature-resistant bacteria are fed CO2 and renewable hydrogen derived from, for example, wind power by electrolysis. The bacteria turn this into acetate.

But acetate is not the desired compound itself, and this is therefore fed to a different type of bacteria that can turn it into a variety of other derived compounds that all have oxygen in their structure, so-called oxygenates. These oxygenates are commonly used solvents or alcohols such as acetone and butanol. 

The second phase involves chemical catalysis – which is a process that facilitates a chemical reaction – to combine the oxygenates with each other. This approach creates longer hydrocarbons with structures of the types that are used for aviation fuels because of their very specific fuel properties.

“But these molecules still contain oxygen, which is undesirable in fuels because it will decrease the energetic capacity. So as part of the chemical catalysis we remove the oxygen by adding more renewable hydrogen which basically attacks the oxygen in these molecules and takes it out in the form of water. That way we end up with alkanes which are long chains of hydrocarbons that are the ones that are suitable for use as jet fuel,” he says.

The work is showing great promise as the researchers have been able to produce all four types of hydrocarbons (linear and branched alkanes, cyclic hydrocarbons, and aromatic compounds) that are the main ingredients in the type of jet fuel currently in use in commercial aviation.

All alternatives are welcome

Over the next few years, work will continue to test the energetic potential of these compounds as actual jet fuels, bring down the use of energy in the production process, and assess if the process can be carried out on a larger scale at an acceptable cost.

In the meantime, Pablo Doménech is excited that researchers worldwide are committed to fighting climate change from a range of perspectives because we need all hands on deck to explore different pathways.

“It’s not one or the other, but all alternatives are welcome in order to reduce our dependence on fossil fuels,” he emphasizes.