Maine researchers use TDO to convert biomass into hydrocarbon oil
Located in Orono, Maine, deep in the heart of the wood products industry, the University of Maine has long been a hub where some of the brightest minds have worked toward finding efficient, cost-effective methods to break down cellulose derived from forest products into valuable hydrocarbon fuels and chemicals. According to Clayton Wheeler, associate professor of chemical and biological engineering at the University of Maine, he and his colleagues have developed a novel mixed-carboxylate platform that can convert a range of cellulosic materials into a high-energy, versatile hydrocarbon bio-oil.
The revolutionary two-step process is called thermal deoxygenation (TDO), a spin on chemistry that was used to make acetone in the 1800s, according to Wheeler. In the first step, biomass is hydrolyzed with dilute acid to isolate the sugars present in cellulose to form a mixed organic acid—or hydrolyzate—that resembles syrup. The hydrolyzate becomes neutralized when combined with calcium hydroxide to form a calcium salt. The salt is then heated in a reactor to 450 degrees Celsius (840 degrees Fahrenheit), a step that removes oxygen. What’s left is hydrocarbon oil that phase separates from water right out of the reactor.
Thus far, Wheeler said he and his team have produced between 4 and 10 liters of hydrocarbon oil per month on a bench-scale with an energy density of about 41 megajoules per kilogram. The acids and calcium hydroxide can also be recycled at the end of the process, he added.
Since the process removes most of the oxygen from the oil, a key step that distinguishes TDO from other conventional pyrolytic methods, Wheeler noted that it’s this deoxygenation step that can inhibit oil yield because removing oxygen affects the overall mass of the oil. Oxygen is removed as both carbon dioxide and water, and because the process requires no catalysts or outside hydrogen source, most of the energy in the original cellulose source is present in the bio-oil.
“Wood has the potential for about 3.2 barrels of oil equivalent in terms of energy, if you can maintain all the original energy in it as heating value and convert it at 100 percent efficiency,” Wheeler told Biorefining Magazine. “About one-third of that is lignin and two-thirds of that is from carbohydrates. We estimate that of the two-thirds that are in carbohydrates, based on the current state of the technology and extrapolating bench-scale experiments, we might at this point be able to achieve 1 barrel of oil equivalent, or 50 percent energy yield of the carbohydrate fraction [in cellulose].”
Wheeler illustrates his theoretical starting point for achieving bio-oil yield from the organic acid mixture. “If we start with cellulose at 17.5 megajoules per kilogram and we assume that our hydrocarbon oil has an energy density of 44 megajoules per kilogram, then what you really have is the potential for about 87 percent energy efficiency,” he said. “That equates to 67 percent carbon efficiency and 35 percent mass efficiency. That’s our theoretical basis for a starting point. What we have achieved is 78 percent of that in mass yield just for our step in the process based on staring with the organic mixture. Our energy density is slightly less than 44 megajoules per kilogram.”
The raw hydrocarbon oil produced out of the reactor contains more than 200 compounds that have a similar boiling point distribution of distillate cuts like diesel and jet fuel with smaller cuts of gasoline mixed in it, Wheeler said, adding that the oil might be compatible with No. 2 heating oil straight out of the reactor. Due to its low octane level—82—further upgrading would be required for it to be suitable as a transportation fuel in gasoline engines to reach the 87-octane mark.
“In some of our more recent results, we probably have higher aromatic content, such as toluene and xylene, in the gasoline cut,” Wheeler said. “I’m hopeful that our octane will actually be higher in our newest results.”
Another key attribute of the TDO process is that it can not only convert wood waste or agricultural residues into hydrocarbon oil, but the process can also handle inputs that are rife with contaminates such as municipal solid waste.
After accidentally stumbling upon this novel process almost a year ago, Wheeler said he’s now trying to figure out how to further optimize his process. A paper on the research has been published in the journal Green Chemistry. The research was funded through grants from DOE EPSCoR (Experimental Program to Stimulate Competitive Research) and the DOE’s Office of Biomass. Further work on TDO process evaluation is currently sponsored by the Logistics Research and Development Program at the Headquarters of the Defense Logistics Agency, Fort Belvoir, Va., as part of the wood to jet fuel project at the University of Maine’s Forest Bioproducts Research Institute.
“If anything there was some serendipity involved,” Wheeler said. “We were trying to do one thing and we came up with something completely different.”
While the initial work is promising, Wheeler said he’s careful not to oversell his results. But, he said, he’s open to potential partnerships or research collaborations with other parties that could help scale-up the technology in a pilot-scale environment, possibly at a pulp and paper mill.
“I think some fundamental work would be helpful moving this forward,” he said.
A video of Wheeler’s process can be seen by clicking here.