An efficient technique for turning waste plant material waste into jet fuel has been announced, coincidentally a few days after a paper measured how far we have already come on this journey.
Air travel is less than 5 percent of global greenhouse emissions today, but its share is expected to rise like a plane at liftoff. Not only are more people flying more often, but larger sources of emissions are shrinking, or expected to start soon. Shorter flights can shift to high-speed rail, but for intercontinental travel we need a way to make carbon-neutral jet fuel.
Existing options, such as last year's demonstration flight across the Pacific, don't look likely to scale well, but Professor Ning Li of the Dalian Institute of Chemical Physics has found a way to turn cellulose into jet fuel that might do better. Moreover, Li claims in the journal Joule that the product beats petroleum-based fuels on energy density.
Li first converted cellulose, the polymer that gives plant cell walls their strength, to 2,5-hexanedione through hydrogenolysis. After separation from other molecules, the 2,5-hexanedione was reacted with hydrogen to produce a jet fuel that has 10 percent more energy by mass than those used today. The heavier the fuel, particularly on long-haul flights, the more that needs to be burned early on in order to move the portion that will be used later.
"Tthe aircraft using this fuel can fly farther and carry more than those using conventional jet fuel, which can... decrease the CO2 emissions during the taking off (or launching) and landing," Li said in a statement.
Cellulose is exceptionally abundant. Li used wheatgrass as a source, but it's also the main component of sawdust and straw. It's been converted to jet fuel previously, but the multi-stage process made it expensive. Li hopes the more direct path will be cheaper.
Airplane companies can offer customers offsets for the emissions their flights produce, but if the take-up rate was high enough, the world would quickly run out of suitable offset mechanisms. Some potential crops, such as Carinata, produce oils that can already be made into jet fuel. The carbon dioxide emitted when burning them is balanced by what the plants draw from the atmosphere. However, with oil being only a small portion of the plant's mass, this path takes a lot of land per flight.
Another approach has been to make bio-jet fuel by breaking plant carbohydrates into sugars and letting bacteria convert them to ethanol. A review of progress on this in Energy and Environmental Science notes the price has declined from a ludicrous $300,000 a gallon in 2007 to $16 a gallon. Petroleum-based jet fuel costs $2.50 a gallon, and the study shows it is realistic to match this through further modest improvements and efficiencies at each stage of the process.