Jetting further with bio-based fuels

Scientists have come up with various ways for converting plant material into jet fuel, including catalytically upgrading ethanol and syngas. Many airlines, meanwhile, have successfully flown aircraft powered by these bio-based versions of jet fuel, either on their own or in a mixture with petroleum-based jet fuel.

So far, scientists have mainly focused on producing bio-based versions of conventional jet fuel, but have struggled with their expense. Bio-based versions cost around $16 a gallon to produce, compared with just $2.50 for petroleum-based versions. Now, though, a team of Chinese scientists led by Ning Li at the Dalian Institute of Chemical Physics has developed a way to convert cellulose into an advanced, high-density jet fuel that might be better placed to compete economically.

Conventional jet fuel is a mixture of alkanes and cycloalkanes, whereas advanced jet fuels comprise a mixture of polycycloalkanes that give the fuel a higher energy density. These advanced jet fuels are much more expensive than conventional jet fuel, at around $25 a gallon, and so have mainly been used in military aircraft and missiles, which need to travel long distances on fairly small fuel tanks.

This high price clearly offers more leeway for producing a bio-based version that is competitive. If it can be produced cheaply enough, a bio-based version could even be used by commercial aircraft, as a fuel or fuel additive, allowing them to fly faster and further on the same amount of fuel.

As reported in a paper in Joule, Zhang and his team have now come up with a two-stage catalytic process for converting cellulose into polycycloalkanes. The first stage involves immersing the cellulose in a dichloromethane solution with hydrochloric acid and palladium on carbon as catalysts. This promotes a series of hydrodechlorination and hydrogenation reactions that convert the cellulose into an organic compound called 2,5-hexanedione.

The second stage involves passing the 2,5-hexanedione with hydrogen over a dual-bed catalyst system, in which the first bed contains a catalyst made from copper, nickel and magnesium oxide and the second bed contains a nickel-based catalyst. Between them, these catalysts promote a series of hydrogenation reactions that convert the 2,5-hexanedione into a range of polycycloalkanes with carbon chains containing between 12 and 18 carbon atoms.

By optimizing the reaction conditions, Zhang and his team were able to convert cellulose into 2,5-hexanedione with a yield of 71% and then convert 2,5-hexanedione into polycycloalkanes with a yield of 75%. Furthermore, testing confirmed that this mixture of polycycloalkanes has similar properties to petroleum-based advanced jet fuels, including an energy density around 10% higher than conventional jet fuel and a low freezing point.

"The aircraft using this fuel can fly farther and carry more than those using conventional jet fuel," says Li. In part because of this, Li and his colleagues are confident that this bio-based advanced jet fuel could be produced economically at commercial scales.

This assertion was supported by a recent paper in Energy & Environmental Science by researchers at Lawrence Berkeley National Laboratory and the Joint BioEnergy Institute (JBEI) in the US, who have also been working on bio-based versions of advanced jet fuel. They wanted to know whether their basic approach could produce an economically viable jet fuel, at least theoretically, before embarking on extensive experimental work.

To do this, they created computer models of five different pathways for producing advanced jet fuels, in order to determine whether modest improvements in these pathways could produce jet fuel at a cost of $2.50 a gallon. They found that this was possible for all five pathways, with the most effective improvement being to convert the leftover lignin into a valuable chemical. In addition, however, they found that airlines may be willing to pay up to $0.50 a gallon more for an advanced jet fuel that would allow planes to fly for longer.

"The development of plant-based compounds that have a performance advantage over their petroleum-based counterparts is an important factor in determining their marketplace viability," said Blake Simmons, chief science and technology officer at JBEI.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.