Making bio-based aviation fuel from lignocellulosic biomass

US scientists have developed a new technique to accelerate the production of renewable jet fuel from corn cobs and wood chips.

Globally, air travel contributed 815 million US tons of CO_2­ emissions in 2016, 2% of the global manmade total, according to the International Air Transport Association (IATA). It is predicted that by 2035, 7.2 billion passengers will travel by air in 2035.

Alternative jet fuels are increasingly seen as a means to reduce the environmental impact of jet fuel, and scientists from the US Catalysis Centre for Energy Innovation (CCEI) and University of Delaware are working to improve the production of alternative jet fuel from waste biomass. Their research has been documented in the latest edition of the journal ChemSusChem.

 

Overcoming the obstacles to renewable jet fuel

According to Basudeb Saha, associate director of CCEI, an Energy Frontier Research Centre supported by the US Department of Energy, one of the major hurdles to making renewable jet fuel is increasing the speed of two critical chemical processes – coupling and deoxygenation.

The plant material the scientists work with has a low carbon content once its broken down from a solid into a liquid, meaning the carbon molecules need to be chemically stitched together or “coupled” to create high-carbon molecules in the jet fuel range. Once this has been completed, the oxygen must be removed to form branched hydrocarbons. This branching is essential to improving the flow of jet fuel at the freezing temperatures of commercial flight.

"International planes may fly at an altitude of 35,000 feet, where the outside temperature could be as low as -14° Centigrade," says Saha, who is leading a renewable jet fuel project at the center. "That's the temperature at which a plane has to run, and the fuel can't be frozen."

Saha and colleagues have developed new catalysts, dubbed ‘chemical goats’, that kickstart the chemical reactions needed to transform plant materials into fuel. One of these catalysts is made from graphene and increases the speed of the coupling reaction while operating at a low temperature of 60°C. Another catalyst removes oxygen in an energy efficient way and produces high yields of branched molecules, up to 99%, which is suitable for jet fuel.

According to a statement, both catalysts are recyclable and the processes are scalable.

As a point of comparison, several US companies are currently producing renewable jet fuel from materials such as triglycerides extracted from used oil and grease, or from a combination of carbon monoxide and hydrogen called syngas. However, processing these materials requires high temperatures (350°C), and high pressure, making their production expensive and energy intensive.

“The low temperature and high selectivity of our process can enable cost-competitive and sustainable bio-based aviation fuels from lignocellulosic biomass,” Saha concludes.