A New Route to Cellulosic Biofuels

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.

ZeaChem is just one of a number of companies pursing cellulosic biofuel production. Warrenville, IL-based Coskata is also developing a hybrid thermochemical-biological process. Its feedstocks are first gasified under high temperatures, yielding synthesis gas, a mix of carbon monoxide and hydrogen. The synthesis gas is then digested by anaerobic bacteria that convert the gas directly to ethanol.

Wesley Bolsen, chief marketing officer for Coskata, says the company is getting yields of 100 gallons of ethanol for every ton of wood chips or carbon-equivalent feedstock at a pilot plant it recently opened in Madison, PA. “We can get one of the highest yields in the industry, and it’s demonstrated yield, not theoretical yield,” Bolsen says.

Mascoma, a cellulosic biofuels company based in Lebanon, NH, is pursuing an approach that uses genetically engineered microbes to simultaneously break cellulose into sugars and ferment the sugars into ethanol without the need for expensive enzymes. Michael Ladisch, chief technology officer for Mascoma, says its theoretical production limit is 100 gallons of fuel per ton of feedstock, but by having the microbes perform both tasks, the company has reduced cellulosic ethanol’s production costs by 20 to 30 percent compared to conventional processes.

Mascoma is testing different engineered microbes at its pilot plant in Rome, NY, and Ladisch says the company plans to break ground on a commercial-scale facility in Kinross, MI, in the next year or two.

According to McMillan, all three approaches could play a role in future fuel production. “There is room for more than one winner here,” he says. “If they can compete with the price of gasoline, then they can play.”