Flying high on biomass

Converting biomass to airplane fuel presents a few key challenges. Among them is developing an economically viable process that can be used at a commercial plant such as the one owned by Seattle’s biofuel producer Imperium Renewables.

The first step in the process entails the conversion of biomass to alcohol. While biocommodity technology company LanzaTech works on that step, Pacific Northwest National Laboratory (PNNL) is collaborating with Imperium to develop a catalytic process for the next one: converting biomass-based alcohol into drop-in fuels—that is, biofuels that are completely interchangeable with conventional ones.

Conventional biofuels are based on biomass-derived ethanol, which can be used in gasoline engines, and on biomass-derived methyl esters, which can be used as diesel fuel. Both fuels are oxygenated—that is, they contain hydrocarbons with oxygen atoms attached. However, oxygenates are not permitted in aviation fuel. The processing of biomass to aviation fuel must therefore require deoxygenating the hydrocarbons. Meeting that requirement forms the core research area for Richard Hallen of PNNL’s Bioproducts, Sciences, and Engineering Laboratory.

“You want to get the deoxygenation to occur without competing side reactions,” Hallen explains. One mechanism is hydrodeoxygenation, in which hydrogen is used to covert carbon–carbon double bonds into single bonds and produce water. But the mechanism can also cause the undesirable breaking of carbon–carbon bonds.

Making a jet fuel to the right specifications is not easy. Jet fuel requires hydrocarbon chains in the range of 10–14 carbons. That range is much narrower than the permissible range for gasoline. Besides removing oxygen, converting ethanol requires going from a 2-carbon chain up to 10–14 carbons. What’s more, jet fuel must have specific physical properties: It needs to flow at the low temperatures that prevail at cruising altitude and requires a freezing point of −45°C. By contrast, a freezing point of 0°C suffices for diesel.

“One problem is that petroleum doesn’t have the typical amount of oxygen that we see when making jet fuel out of bioproducts,” explains Hallen. “So catalysts need to be somewhat modified to handle higher oxygen and water co-products.” Commercial catalysts for biomass-derived jet-fuel production are based on catalysts developed for processing petroleum’s raw ingredients, or feedstocks. Hallen’s team is developing a longer-lived catalyst that will not deactivate over the longer times required for producing jet fuel from biomass-based alcohol.

Feedstock cost continues to be an issue for making biomass-derived aviation fuels commercially viable. “There are a lot of different feedstocks that are used for biofuels today, and there are a lot that can be used in the future,” says John Plaza, CEO of Imperium Renewables. His company’s jet-fuel initiative allows for the use of feedstocks from existing biomass sources in the Pacific Northwest. Those sources are sustainable and economically viable in the long term. Imperium’s current business operation, which is separate from its aviation-fuel production, converts canola oil into methyl ester for biodiesel.

Imperium’s work with PNNL looks to use a range of biomass—algae, wood residue, and agricultural wastes, in addition to the more standard vegetable oil crops—for conversion into alcohol and then jet fuel. Exploiting that diversity involves oils of different carbon chain lengths, which need to be processed appropriately. Dealing with impurities and getting a high yield of specific products is also part of the puzzle.

With healthy support from the Department of Energy, the collaboration of LanzaTech, Imperium Renewables, and PNNL—along with its partners, National Renewable Energy Laboratory, Orochem Technologies, and Boeing—seem to be flying toward their common goal of replacing “many portions of the barrel” with biomass-based jet fuel.