Now Boarding: Commercial Planes Take Flight with Biobased Jet Fuel

by Melissae Fellet (C&EN) Aviation industry hopes to cut emissions with renewable fuels -- This year, renewable jet fuel took off, graduating from demonstration and test flights to some regular commercial use. In January, some flights left the airport in Oslo, Norway, running on jet biofuel produced from an oilseed crop. In March, United Airlines became the first U.S. airline to use biofuel for regularly scheduled commercial flights leaving Los Angeles International Airport. Three months later, Alaska Airlines flew commercial flights using biofuel produced from renewable isobutyl alcohol.

By 2050, the global aviation industry aims to combat climate change by reducing net carbon emissions by 50% compared with 2005 levels. That’s a commitment to cut one-tenth the emissions projected for 2050. Improved engine efficiency and aircraft aerodynamics will provide some reductions. But transitioning to fully renewable jet fuel is key to meeting the targets suggested by the International Air Transport Association (IATA).

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Companies develop jet biofuels to chemically mimic petroleum-derived jet fuel so they can be used immediately with existing engine designs and fueling infrastructure. But the industry is far from supplying the more than 300 billion L of jet fuel used worldwide just in 2012.

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For now, fuel producers lack the funds, policy support, and renewable fuel incentives to build more factories and increase production volumes, though there are signs that the industry is ready to grow.

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Currently, feedstocks can be converted to jet biofuel through one of five different processes approved by ASTM International, a nonprofit group that develops international technical standards for materials and products.

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Neste, in Finland, and AltAir Fuels, in California, the two firms capable of making jet biofuels at commercial scale, use animal fat, plant oil, and used cooking oil to produce primarily linear and branched paraffins. To convert fat and oil to hydrocarbons, the companies first deoxygenate and hydrogenate them to make long, linear hydrocarbons, which are then cracked and isomerized to shorter linear and branched C₈ to C₁₆ hydrocarbons. This so-called hydroprocessed esters and fatty acids (HEFA) process is also used to produce renewable diesel that is chemically indistinguishable from petroleum-derived diesel.

Because current aviation biofuels contain only linear and branched paraffins, they have to be blended with petroleum-derived fuels to create a jet fuel with the physical properties specified by ASTM. The renewable fuel at Oslo Airport contains 50% biofuel produced by Neste, and United uses renewable fuel containing 30% biofuel from AltAir Fuels.

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In Minnesota, Gevo produces renewable jet fuel on a demonstration scale using isobutyl alcohol produced from fermenting cornstarch. The company provided fuel to Alaska Airlines for its test flights in June.

The first step in this process, called alcohol-to-jet, involves dehydrating isobutyl alcohol to form isobutylene, then oligomerizing isobutylene and hydrogenating the resulting compounds to yield mainly C₁₂ and C₁₆ branched hydrocarbons. According to guidelines set by ASTM, a renewable fuel can contain up to 30% of this fuel.

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A different alcohol-to-jet pathway starting with ethanol rather than isobutyl alcohol could create fuel that does not need to be blended, says Richard Hallen, a staff scientist at Pacific Northwest National Laboratory.

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To reduce production costs, some companies repurpose existing infrastructure, as AltAir Fuels did when it retrofitted an existing oil refinery near the Los Angeles airport. They also look for feedstocks available near a refinery, to save collection and transportation costs.

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Current policy support for jet biofuels includes targets for renewable aviation fuel from the U.S. and the European Union, along with the U.S. military, though the biggest push for sustainability comes from the aviation industry itself. At the end of this month, the International Civil Aviation Organization, a United Nations group that adopts standards and recommendations for international aviation, will meet to set the next level of emissions targets and global sustainability measures for the aviation industry, including clarifying the role of jet biofuels in meeting those targets. One country, Indonesia, has set a jet biofuel blending mandate by 2018. READ MORE