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). Alternative energy sources considered for cars and trucks—things such as batteries, fuel cells, or liquefied hydrogen—would be technically impossible for planes or require too much infrastructure change in the global aviation industry to be practical.
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. For example, United has a contract to purchase 57 million L of renewable fuel over the next three years. Only two companies produce renewable jet fuel on a commercial scale, and airlines have already claimed all the jet biofuel to be produced from three future plants. 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.
“In the future, we imagine all jet fuel having some level of renewable content,” says Steve Csonka, executive director at the Commercial Aviation Alternative Fuels Initiative, a group of airlines, airports, aircraft and engine manufacturers, researchers, and U.S. government agencies.
The development of aviation biofuels lags about a decade behind the first biofuels for cars and trucks, a delay that has provided the jet fuel industry a chance to learn from previous mistakes. For instance, revised emissions estimates for first-generation biofuels—corn ethanol blended with gasoline and fatty acid methyl esters added to diesel—reveal that they may not be as green as they first seemed: Converting land to grow crops for biofuel releases carbon dioxide trapped in the soil, making thegreenhouse gas impact for corn ethanol and biodiesel potentially greater than their petroleum-derived counterparts.
In general, jet biofuel producers look to make renewable fuels that deliver about 50 to 80% net greenhouse gas reductions, Csonka says. To do this, they extract oils from nonfood crops such as camelina and jatropha grown on nonarable land or in rotation with grains. Companies also use waste products—such as animal fat, used cooking oil, forestry and agricultural waste, and household trash—as feedstocks. Having a variety of feedstocks makes it easier to produce renewable jet fuel around the world because refineries can use the feedstock most available in their region, says Robert Boyd, manager of the biofuel deployment program at IATA.
Currently, feedstocks can be converted to jet biofuel through one of five different processesapproved by ASTM International, a nonprofit group that develops international technical standards for materials and products. Each process, however, makes only some of the hydrocarbons found in petroleum-based jet fuel, which contains aromatic compounds along with linear, branched, and cyclic paraffins—saturated hydrocarbons containing eight to 16 carbon atoms. This mixture creates a fuel with the lubricity, freezing point, and energy density, among other specified properties, that keep it from boiling, freezing, or absorbing water in the variety of conditions a plane experiences on land or in the air. Gasoline and diesel have physical properties unsuitable for airplanes: The shorter hydrocarbons in gasoline can make the fuel too volatile, whereas the longer hydrocarbons in diesel can increase the fuel’s freezing point.
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.
Over the next three years, AltAir Fuels will deliver about 37% of its 150-million-liter yearly production capacity for renewable fuel to United; most of the rest of the company’s capacity will go toward producing renewable diesel. Neste, in Finland, produces 2 million L of renewable diesel, gasoline, and jet fuel each year.
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. Researchers at Washington State University are working on incorporating feedstocks other than cornstarch into this process, for example, sugars from wood waste fermented into alcohols.
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. That’s because starting with a two-carbon alcohol rather than four-carbon isobutyl alcohol enables scientists to build a wider range of hydrocarbons instead of being restricted to a handful of longer ones, as is the case for current jet biofuels. To simplify the ASTM certification process for this production pathway, companies are first working to transform ethanol to branched saturated hydrocarbons using the same strategies used in the approved isobutyl alcohol route.
At a demonstration-scale jet biofuel plant in Georgia, LanzaTech collects a mixture of carbon monoxide and hydrogen called syngas from industrial exhaust streams. Microbial fermentation of the syngas yields ethanol that is dehydrated to ethylene, then oligomerized and hydrogenated to form branched C10 to C16 paraffins.
ASTM is assessing this ethanol-to-jet pathway for official approval, a process that can take at least five years, Hallen says. Other companies are developing aviation biofuel that can be used without blending, as well as other fuels containing cyclic paraffins and aromatics that could be mixed with current jet biofuels to provide a fully renewable fuel. But the lengthy certification process slows the growth and commercialization of these processes.
In the next two years, three more commercial-scale jet biofuel plants are expected to open in the U.S., all supported by federal funds. Each plant will construct linear and branched hydrocarbons from syngas using a series of reactions called the Fischer-Tropsch process, although the factories will use different feedstocks. Red Rock Biofuels will gasify wood waste, Emerald Biofuels uses oil from nonfood crops, and Fulcrum BioEnergy will gasify municipal solid waste. Though these refineries are not open yet, airlines already anticipate the fuel: Cathay Pacific, an airline based in Hong Kong, has agreed to purchase 1.4 billion L of fuel produced by Fulcrum over 10 years. United has also invested $30 million into the company. Southwest Airlines and FedEx have agreed to purchase fuel produced by Red Rock.
Despite the demand and airline support for jet biofuel, producers struggle to make renewable fuel cost-competitive with petroleum-derived fuel. Fuel is already the second-largest operating expense for aviation, and renewable fuels can cost 40–75% more than petroleum-derived fuel, depending on the production process and current price of traditional jet fuel. Lacking blending mandates for renewable jet fuel, companies have fewer reasons to produce more costly jet biofuel when they could use the same feedstock or process to generate renewable ethanol or diesel, both of which are supported through 2016 by U.S. Environmental Protection Agency renewable fuel mandates. “When trying to develop a new market, there needs to be some incentives to get people and companies committed to developing the technologies to commercial scale,” Hallen says.
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. And by only considering processes that provide at least 50% net greenhouse gas reductions, companies position their fuels to be eligible for future renewable fuel subsidies, Csonka says.
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.
More than 2,500 commercial flights have flown using renewable fuel already, and the number is growing. Research, demonstration, and commercial projects are ongoing worldwide. And, Csonka says, “over the next several years, more refineries, feedstocks, and production pathways will appear, showing that the area is moving toward broader commercialization.”

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2016 American Chemical Society