Plant waste might soon help make flying a little greener. A fuel made from cellulose has such a high energy density that it could make an even better jet fuel than those made from fossil fuels.
Flying produces 2 per cent of global carbon emissions, a figure projected to soar to in excess of 10 per cent by 2050. So numerous groups are working on ways of limiting the emissions generated by flying.
Ning Li of the Dalian Institute of Chemical Physics in China and his team have found a relatively simple and efficient way to turn cellulose, which is abundant in plants, directly into compounds called polycycloalkanes, which should make an ideal jet fuel.
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Although the team has yet to test the fuel in an aircraft, its properties suggest it will be good for the job. “Aircraft using this fuel can fly further or carry more than those using conventional jet fuel,” says Li.
Fuels for planes need to pack as much energy into the limited space of the fuel tanks as possible, yet still have a very low freezing point so fuel pipes don’t freeze at high altitude. The standard Jet A-1 fuel has a freezing point of -47°C and an energy density of 35 megajoules per litre.
Li says his mix of polycycloalkanes freezes at -48°C and should have an energy density of more than 37 megajoules per litre. However, this is an estimate based on the properties of similar molecules rather than direct measurements.
Fuel from waste
Many passenger jets have already flown using fuels made partly from biofuels. But airlines won’t routinely use biofuels until large quantities are available at competitive prices, and it is hard to produce lots of cheap biofuel without wrecking the environment.
For instance, Indonesia wants to , but cutting down rainforests to plant oil palms increases greenhouse gas emissions and destroys precious habitat.
Li and his team think their biofuel’s desirable properties will help make it commercially viable. However, their process currently uses a particularly hazardous chemical called dichloromethane, which means an alternative will have to be found before their fuel could be made in large quantities.
The environmental advantage of cellulose is that it can be obtained from plant waste such as the nonedible parts of crops, but the amount of waste is limited. And according to the Committee on Climate Change, the official advisor to the UK government, we must use the world’s finite supply of biomass in ways that maximise emissions reductions overall.
It says biofuels should supply no more than . So completely replacing fossil fuels in flying will require other technologies too, such as turning electricity from renewable energy directly into liquid fuels.
Joule
