A New Route to Cellulosic Biofuels
ZeaChem’s pilot plant will make ethanol using termite microbes.
Biofuel startup ZeaChem has begun building a biofuel pilot plant that will turn cellulosic feedstocks into ethanol via a novel approach that uses microbes found in the guts of termites. The company says the ethanol yields from the sugars of its cellulosic feedstocks are significantly higher than the yields from other biofuel production processes. ZeaChem says its process also has the potential to produce a plastic feedstock.
The company employs a hybrid approach that uses a combination of thermochemical and biological processes. It first uses acid to break the cellulose into sugars. Then, instead of fermenting the sugars into ethanol with yeast, as is typically done, the company feeds the sugars to an acetogen bacteria found in the guts of termites and other insects. The bacteria converts the sugar into acetic acid, which is then combined with hydrogen to form ethanol.
“It’s a little more complicated than a conventional process. It’s not the obvious, direct route, but there is a high yield potential,” says Jim McMillan of the U.S. Department of Energy’s National Renewable Energy Laboratory in Golden, CO.
In more conventional biofuel processes, much of the carbon content locked up in the sugars is lost to the formation of carbon dioxide when the sugars are fermented into ethanol. Converting the sugars into acetic acid and then ethanol, however, yields no carbon dioxide. As a result, this method has the potential to raise biofuel yields by as much as 50 percent, according to ZeaChem.
What the company gains in sugar-to-ethanol conversion, however, comes at a cost elsewhere, says McMillan. In cellulosic fuel production, the feedstock typically goes through a pretreatment stage that separates out lignin, an energy-dense plant material. The lignin is then typically burned to produce the heat that drives sugar fermentation and other processes. With ZeaChem’s approach, the lignins are gasified to yield the hydrogen that is later combined with acetic acid to form ethanol. Because of this, ZeaChem will likely have to make up the lost heat source elsewhere. “You may actually have to bring in more feedstock just to power your process,” McMillan says.
ZeaChem CEO Jim Imbler says the company has achieved at a laboratory scale yields of 135 gallons per ton of feedstock, 35 percent higher than those of its competitors. Expanding from the lab to a 250,000-gallon pilot plant will go a long way toward proving the effectiveness of the process, and will also allow the company to test the production of another potential product: by swapping out its acetogen microbe for one that converts sugars into propionic acid, ZeaChem says it can combine the new acid with hydrogen to form propanol, a feedstock for plastic.
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