Green to Greener

POET has implemented anaerobic digestion technology at their pilot scale cellulosic facility in Scotland, S.D., making the process of producing cellulosic ethanol even greener.

Farmers, rural communities and ethanol producers have long been partners in driving innovation to achieve goals of energy independence, sound environmental stewardship and economic prosperity. A new technology by the world’s largest ethanol producer will go even further to solidify this partnership.

Anaerobic digestion technology will help move POET operations away from natural gas in the 2011 opening of a commercial scale cellulosic ethanol plant in Emmetsburg, Iowa. Liquid waste from cellulosic ethanol distillation will be used as a source to power the cellulosic plant and reduce even further the carbon footprint of POET’s adjacent grain ethanol plant.

“You’re essentially making a green energy source greener,” said Dave Bushong, General Manager for POET Research Center in Scotland, S.D., where the anaerobic digester is in the pilot stage.


The idea is fairly simple: bacteria devour waste and produce fuel in the form of a gas.

During cellulosic ethanol production a liquid stream is created that contains the unused parts of corn cobs (POET’s feedstock for cellulosic ethanol). Rather than paying to dispose of the liquid as waste, the anaerobic digester — essentially a large cauldron of anaerobic bacteria (bacteria that grows without oxygen) – is now fed that liquid. The bacteria breaks down the stream to produce biogas (an energy rich gas) and clean water. The water will be reused for ethanol production and the biogas is available as an energy source to make the ethanol plant more efficient and less dependent on fossil fuels.

“It’s an actual model of the biorefinery of the future,” Bushong said.

The model digester used at the Scotland, S.D. plant, installed on May 20, was designed and built by New Jersey-based Biothane, and the bacteria had to be conditioned to digest corn materials.

The biogas produced at the plant replaces natural gas, a fossil fuel. The digester, which produces the biogas and water around the clock, takes little electrical power to operate.

When the technology is implemented in the Emmetsburg, Iowa plant, called Project LIBERTY, it will be on a significantly grander scale. LIBERTY, a $200 million, 25 million gallon-per-year cellulosic ethanol plant, will use the biogas as a source to fuel steam systems used to make ethanol. It will be a renewable energy source created at the plant, for the plant.


Though anaerobic digesters are not a new technology, this is the first time the science has been applied to the waste generated by cellulosic ethanol production. Digesters have been used to produce methane from manure and other materials throughout the world and the Japanese use a form of anaerobic digestion in some of their alcohol distilleries.

The idea of using anaerobic digestion in ethanol production has been on the table for some time. POET became interested in using the waste stream from its ethanol processes about four years ago, said Mark Stowers, Vice President of Science and Technology for POET. It also had been an early goal to operate Project LIBERTY with a new energy source to make the plant as environmentally friendly as possible, he said. By doing so, cellulosic ethanol becomes more attractive and financially promising.

“You wouldn’t go forward on a commercial process if you were paying someone to take away this liquid stream as waste,” Stowers said. “It’s not economically feasible.”

An anaerobic digester fits squarely within POET’s long-term vision of ethanol production divorced from fossil fuels. In June, POET CEO Jeff Broin spoke of this vision and the digester’s place among other alternative energy strategies used by POET including a solid waste fuel boiler, landfill gas and cogeneration.

“This technology will cut fossil fuels out of our cellulosic ethanol production process and further improve the benefits of grain-based ethanol,” Broin said. “Over the long-term, POET would like to eliminate the use of fossil fuels at all of our plants through a variety of alternative energy sources.”


Bushong points out that the digester could produce yet another product for use in agriculture: fertilizer.

Eventually, the process does create a residual material that needs to be removed, Bushong said. But POET will look at whether this material could be used for crop fertilizer based on its nitrogen and phosphorus content, providing additional Earth-friendly benefits from cellulosic ethanol production. POET is also looking at using this material in animal feed applications.

The digester technology will benefit communities where POET operates by making the facilities more efficient and increasing their profitability. That would be a boon for economic development.

In 2008, a study sponsored by the Agricultural Utilization Research Institute claimed the use of anaerobic digestion in ethanol production could save producers money and boost profits by millions of dollars. It also could earn carbon credits. That study was conducted by Moorhead, Minn.-based Rein & Associates, a wastewater engineering company.

Finally, because the digester produces clean water as it breaks down the waste, POET can pump that water back into the front end of its production line, further reducing a facility’s usage of local water supplies.


Although Project LIBERTY will have an anaerobic digester, exactly what version is still being discussed, Stowers said. POET continues to study the process to find the most efficient and cost-effective model.

“We’re still looking for the best, most optimal anaerobic digester technology,” Stowers said.

The company plans to install another version of an anaerobic digester into the Research Center in Scotland to continue improving the process.

“This process hadn’t been done before with this feedstock, so we’re still evaluating a variety of research questions,” Stowers said. “We’re still learning, but I definitely think the experience of this will be invaluable as far as Project LIBERTY goes.”


The logistics surrounding cellulosic feedstock are complicated, and there is indeed much to learn.

In June, Broin announced the formation of a new division at the company — POET Biomass — which will devote itself to managing harvest and transportation logistics for corn cobs, waste wood and other feedstocks to be used for cellulosic ethanol and alternative energy projects.

POET officials say both corn and cellulosic ethanol can replace gasoline as the main transportation fuel of the nation. Cellulosic ethanol will become an increasingly integral part of the U.S. fuel supply, according to the government’s Renewable Fuels Standard.

Through new technologies, like cellulosic ethanol production, or those used in the current production of ethanol, innovations like the anaerobic digester will undoubtedly help change the landscape of ethanol production into a more sustainable endeavor.




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