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From stover to cellulosic ethanol
A hydraulic fork inserts a bale of stover into a commercial grinder, the same kind used elsewhere on tires and wood, while an employee starts to cut netting. POET has improved the conveyor that carries ground stover into its Scotland, S.D. pilot plant.
Pretreatment starts in this corner of the plant with heat and dilute sulfuric acid. The acid converts hemicellulose in stover to xylose, an inedible five-carbon sugar. Part of the process is proprietary and hidden from view. Enzymes convert cellulose into glucose.
Dave Carlson inserts yeast into the fermentation process to start turning the sugars from cellulose -- xylose and glucose -- into a beer that contains alcohol. Different yeasts are needed for each type of sugar.
Xylose to beer
These fermenters are turning xylose into beer. Similar fermenters hold glucose, the same sugar that makes corn ethanol.
A stripper removes the alcohol from the beer in a process that’s similar to conventional ethanol production. This is the first step of distillation.
Pumped into rail tank cars
Ethanol from both sugars, now identical to grain-based ethanol, is denatured with gasoline and pumped into rail tank cars. It goes toward the EPA mandate of 6.6 million gallons of cellulosic ethanol use this year, a fraction of 250 million gallons planned the 2007 energy law.
Here are the key stages of cellulosic ethanol: milled biomass, dark solids with sugars from pretreatment, xylose liquor from the beer tanks, and pure ethanol stripped from the beer.
POET's pilot plant
Manager Dave Bushong at the entrance to POET’s pilot plant and research facility in South Dakota. The 20,000-gallon plant is small enough to fit into this metal building but is a scaled working model for the 25-million gallon commercial plant planned by POET.
Genera Energy plant
Manager Keith Brazzell stands outside the larger 250,000-gallon Genera Energy test plant in Vonore, Tennessee. It’s run by Dupont Danisco Cellulosic Ethanol, using a similar chemical treatment fermentation process, but DDCE uses a patented bacterium to ferment xylose and glucose. The plant also uses its own optimized enzymes which can be generated on a licensed site.
Here's an inside look at how ethanol is made.