Technically, alternative aviation fuels have made rapid progress, with new fuels approved, and several more in the pipeline. Commercially, progress has been slower; challenges are rife when it comes to producing feedstocks and fuels at high volumes and low prices.

Now, Boeing says, a biofuel supply sufficient to meet up to 1% of the aviation sector's fuel needs could be available “instantly” if green diesel is cleared for use in aircraft, and at a price competitive with petroleum jet fuel. The company is working with partners to gain approval by the end of 2014.

Certifying road-transport fuel for aircraft use would overcome the hurdle of scaling up biofuels to commercial volumes at competitive prices. Sufficient capacity already exists in the U.S., Europe and Singapore to produce 600 million gal. of jet fuel a year, according to Boeing.

Unlike conventional biodiesel, which is chemically unsuitable for use in aircraft, green diesel is produced from vegetable oils and animal fats via the same process as hydroprocessed esters and fatty acids (HEFA) jet fuel, which is already greenlighted for aviation use in blends up to 50% with petroleum jet fuel.

“Green diesel approval would be a major breakthrough in the availability of competitively priced, sustainable aviation fuel,” says James Kinder, a technical fellow in Boeing Commercial Airplanes' (BCA) propulsion division. Wholesale cost is about $3 per gal. with government incentives, he says.

“This is one small step in the total aviation fuels story, but a giant leap toward commercial availability,” says Julie Felgar, BCA's managing director for environmental strategy and integration. “The first one percent is the hardest and now, boom, it can be there overnight. The capacity exists today, at price parity.”

Boeing set meeting 1% of commercial aviation jet-fuel needs from biofuels by 2015 as a target en route to enabling aviation's goal of achieving carbon-neutral growth by 2020, but the scaling up of production has been hampered by lack of investment. “We selected 1% as an inflection point, significant enough to be able to say we are really going to get enough volume to get the economies of scale and to get an idea where the economics of biofuels are going,” Billy Glover, vice president of market strategy, told Aviation Week last year.

Green diesel is chemically similar to HEFA, but the aviation fuel requires additional hydroprocessing to achieve the lower freeze point needed for use in aircraft. The increased cost and waste from this additional processing is a disincentive for producers to make bio-jet fuel, Kinder says.

“When they refine the fuel, there is a diesel cut and a jet cut. They make green diesel then distil it to get light jet fuel,” Kinder explains. “They could keep hydroprocessing the fuel to get it all to a lower freeze point, but that increases waste. Diesel hydroprocessing means less cost and higher yield.”

Although green diesel freezes at higher temperatures, Kinder says the freeze point required for aviation use can be achieved by adjusting the blend ratio with conventional jet fuel. This approach would allow a biofuel already in commercial production for road transport to be used in aviation.

Boeing is working with engine manufacturers and fuel producers to generate a research report on green diesel that will be submitted to fuel-standards developer ASTM International for approval. This is hoped for by the end of this year, he says.

ASTM has already produced annexes to the jet-fuel standard for two alternative fuels: HEFA and synthetic paraffinic kerosene produced via the Fischer-Tropsch process. Six other fuel pathways are going through approval, including alcohol-to-jet and direct sugar-to-hydrocarbon.

There are two approaches to approving green diesel, Kinder says: Creating another new annex or modifying the HEFA approval to remove the freeze-point requirement and allow green diesel to be blended up to 50% with conventional jet fuel. “The final blend controls the freeze point,” he says.

Producers Diamond Green Diesel and Dynamic Fuels in the U.S. and Neste Oil in Europe are involved, as is process developer Honeywell UOP. “There is 800 million gallons of green diesel capacity available today, with the regional diversification we like,” says Felgar.

“Aviation use will not meaningfully impact trucking operations,” she says, adding, “They are already moving to liquid natural gas and hybrid engines.” Aviation, meanwhile, will be able to use one fuel in both aircraft and ground vehicles. “It will be a great breakthrough if we get this approved.”