The technology also uses biomass in another way. The company processes seed crops, such as soybeans or camelina, which contain large amounts of oil. After extracting that oil (which leaves behind cellulosic materials that are gasified), the oil is processed to remove oxygen atoms, forming long chains of straight hydrocarbon molecules. These are then treated to make the straight molecules into branch-like molecules that remain liquid at lower temperatures, making them useful in jet fuel.
The use of biomass reduces net carbon dioxide emissions, but so does the fact that direct liquefaction is more efficient than conventional gasification, says Daniel Cicero, the technology manager for hydrogen and syngas at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL), in Morgantown, WV. In gasification, only about 45 percent of the energy in the coal is transferred to the fuel produced. Accelergy claims efficiencies as high as 65 percent using direct liquefaction. Yields of fuel are also higher. Gasification methods produce about two to 2.5 barrels of fuel per ton of coal. Direct liquefaction produces over three barrels per ton of coal, and adding the biomass brings the total to four barrels per ton of coal.
All told, Fiato says, gasifying coal to produce liquid fuel produces 0.8 tons of carbon dioxide per barrel of fuel, while Accelergy’s process produces only 0.125 tons of CO2 per barrel. That makes it competitive with petroleum refining, especially the refining of heavier forms of petroleum. (The fuels produce about the same amount of carbon dioxide when they’re burned.)
In addition to reducing carbon emissions compared to conventional coal to liquids technology, a key advantage of the process is the ability to make high-quality jet fuels. The direct liquefaction of coal produces cycloalkanes, looped molecules that have high energy density, giving airplanes greater range. They are also stable at high temperatures, allowing them to be used in advanced aircraft.
One drawback to the process is that it costs more than refining petroleum. Indeed, Cicero says that an NETL study of coal and biomass to liquid fuels technology suggests it would not be competitive until petroleum prices stay above $86 to $93 a barrel. (The study was based on conventional gasification processes.) He says that supplying fuel to the Air Force could sustain one or two small Accelergy plants, but to move beyond this would require a price on carbon-dioxide emissions of about $35 a ton.