Aircraft fuel from plant waste

New synthetic route effectively converts cellulose into high-performance fuel

So far, aircraft mostly fill up with kerosene. But a new bio-fuel from plant remains could soon become economically competitive. © Chalabala / iStock)
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Airborne biofuel: Researchers have developed a synthetic route that produces high-performance fuel for aircraft from plant waste - from straw, corn stalks or sawdust. From the cellulose of these residues arises a mixture of hydrocarbons, which is suitable as aircraft fuel or kerosene. The trick: This biofuel has a higher energy density than kerosene and could therefore be economically rewarding.

Almost all commercial aircraft today use kerosene as fuel - a fossil fuel considered to be low in terms of climate and the environment. In addition to switching to electric drives in aviation, biofuels could also change that. Several airlines are already testing the addition of biokerosene to normal aviation fuel. Although these so-called drop-in fuels generate greenhouse gas when burned. But if they are made from plant material, their net balance may be CO2 neutral.

High performance fuel instead of kerosene

The problem, however: So far, biokerosene is two to three times more expensive than normal aviation fuel and therefore not very economical. Researchers are therefore trying to find cheaper ways to synthesize biokerosin. But there is another way - the production of biofuel with a higher energy density than normal kerosene. Such high-performance fuels have been used mainly by military jets because they are too expensive for most airlines.

But if you could produce these high-density fuels cheaply from plant material, you would fly cheaper and at the same time environmentally friendly. "The use of high-density aircraft fuel can significantly increase the range and payload of an aircraft without the need for larger tanks, " said Ning Li of the Dalian Institute of Chemical Physics in China. "So far, however, a synthetic route for these polycycloalkanes from cellulose is lacking."

Cellulose becomes fuel

Now Li and his team have developed such a synthetic pathway. It makes it possible to produce the mixture of ring-shaped hydrocarbons typical of these fuels from plant wastes such as straw, corn stalks or sawdust. In the first step, the cellulose is removed from the plant residues and converted into the compound 2, 5-hexanedione using dichloromethane. This reaction provides more than 70 percent recovery of this hydrocarbon at normal pressure and temperature, the researchers explain. display

In the next step, the 2, 5-hexanedione is added by repeated reaction with hydrogen to form longer chains of the ring-shaped hydrocarbons. Finally, polycycloalkanes with twelve and 18 carbon atoms are formed by means of different catalysts. "To our knowledge, this is the first report on the synthesis of polycycloalkanes from 2, 5-hexanedione and hydrogen, " Li and his team say.

Also economically rewarding

The resulting hydrocarbon mixture is best suited as a high-density aircraft fuel, the researchers said. At the same time, it could also be used as an additive to conventional kerosene to increase its energy density and improve the environmental footprint. "Our biofuel can reduce aviation's CO2 emissions because it's made from biomass and has a higher energy density than conventional kerosene, " says Li.

According to the scientists, the production of bio aviation kerosene could also be economically viable according to this pattern. Because of the favorable starting materials, only a few reaction steps and low energy costs, the production is even more favorable than with conventional high-performance fuel. That could make this biofuel competitive.

Looking for a replacement for toxic solvents

However, there is a major shortcoming: it still requires the toxic and environmentally harmful solvent dichloromethane for the effective conversion of cellulose. "We now want to look for a sustainable and environmentally friendly solvent that can replace dichloromethane in the hydrogenolysis of cellulose, " say the researchers. If this succeeds, there may be new, more competitive biofuels for aviation in the future. (Joule, 2019; doi: 10.1016 / j.joule.2019.02.005)

Source: Cell Press

- Nadja Podbregar