June 18, 2008 | General

Turning Field Residue Into Ethanol Feedstock

Corncobs will be used to produce cellulosic ethanol at new biorefinery plant in Emmetsburg, Iowa.
Diane Greer

DARRIN IHNEN is looking at corncobs in a new light. Once left on the fields to rot, Ihnen is now harvesting the cobs along with the corn from 4,000-acres on his farm in Hurley, South Dakota – hoping to turn the field waste into income producing feedstock for making ethanol. Typically corn is harvested by a combine that strips the ears from the corn stalks, separates the grain from the ears and then deposits the stover, consisting of both the cobs and the stalks, back on the field.
Ihnen is part of a group of farmers and equipment manufacturers assembled by Sioux Falls, South Dakota-based Poet Energy to develop and test the on-farm logistics of harvesting and storing corncobs. The cobs will be used to produce cellulosic ethanol at Poet’s new $200 million biorefinery, which will be built on the site of the company’s existing 50-million gallon per year (mgpy) dry mill ethanol plant in Emmetsburg, Iowa.
Slated for completion in 2011, the biorefinery will require harvesting, storing and transporting 275,000 acres of cobs, weighing between 450 and 500 tons. Once at the plant, the cobs will be combined with 300 to 350 tons of corn fiber, generated by Poet’s corn fractionation process, to produce 25 mgpy of cellulosic ethanol.
Poet is targeting cobs as a biomass feedstock to produce cellulosic ethanol for several reasons. “From a scientific point of view, you want to find a biomass source that is in abundant supply, has a large potential for fermentable sugars and can be easily collected,” explains Dr. Mark Stowers, Poet Vice President for Research and Development. “Cobs score a 10 out of 10 on all three of those categories.”
From an environmental sustainability perspective, cobs comprise only six percent of the stover left on a field and are low in nutrients, such as nitrogen, phosphorous and potassium. So removing cobs from a field has minimal or no impact on soil fertility. Several landlords leasing land to Ihnen to grow corn do not permit the removal of stover due to concerns about soil fertility. “But they do not care if I remove the cobs,” says Ihnen.
Developing the best methods for harvesting and storing the cobs is presenting the research team at Poet with several challenges. Currently there is limited farm machinery available to harvest the cobs and little experience with cob storage. “We have a lot to learn about the process,” Stowers says.
The top priority is to develop methodologies to harvest the cobs that will not impede a farmer’s ability to collect the grain, he adds. The goal is to collect the cobs at the same time as the grain, so that farmers do not have to go back to the field and collect cobs off the ground.
Simultaneous collection is important in South Dakota and other parts of the Corn Belt where the fall harvest window is short. Snow can come as early as the first part of November. “You do not want to go back later to pick up the cobs off a snow covered field,” Stowers says. Preventing the cobs from hitting the ground keeps dirt and other impurities out of the feedstock. “This is a critical factor when processing the cobs in the plant, because we do not want to process all that dirt,” he adds.
To address the cob harvest and storage challenge, Poet realized it had to engage farmers, original equipment manufacturers and system suppliers early in the process to meet its massive harvest goals. Equipment manufacturers collaborating with Poet on the project include Case IH, John Deere, Kinze, Claas, Vermeer, AGCO, Demco and Feterl.
Ihnen volunteered to take part in the initial field tests after hearing discussions about the Emmetsburg project during a board meeting of the farmer-owned Great Plains Ethanol plant in Chancellor, South Dakota. Poet designed, built and manages the Chancellor plant where Ihnen is the chairman of the board. “We started kicking ideas back and forth,” he says. Ihnen’s farm was the right size for a test and located close to Poet’s headquarters in Sioux Falls. Ihnen’s background is also well suited for the project. He is a board member of the National Corn Growers Association, is passionate about ethanol and “is willing to try just about anything,” Stowers says. Adds Ihnen: “We consider our farm progressive and always like to be the first to try new technologies.”
During the harvest last fall, Ihnen tested two different single pass harvesting techniques for Poet. The first method connected Ihnen’s existing combine to a second piece of equipment called a cob caddy. The cob caddy took all the field waste coming out of the back of the combine and separated the cobs from the rest of the stover. “The machine had a fan that blew out the lighter materials and ended up catching the cobs,” he says. Since cobs are more than twice as dense as corn stalks, the equipment uses the bulk density differential between the materials to separate the cobs from the stalks, Stowers explains.
The second method employed a modified combine designed by John Deere that collected the corn and the cobs together. Separation of the grain from the cobs occurred in a second operation at the side of the field. Both methods enabled collection of the corn and cobs over Ihnen’s 4,000 acres without slowing down the harvest. “We hit all the harvest timelines,” Stowers says.
Probably the biggest lesson learned last year was that there are multiple feasible equipment options for collecting the cobs. “It is a matter of personal preference,” Ihnen says. Some bugs were discovered with the equipment and the manufacturers are making modifications. “This fall we are testing the equipment again,” he explains. “It is kind of exciting to think about this harvest and what we might be seeing out here in terms of equipment.”
Cobs collected during the harvest are stored at the farm in piles up to 30 feet high. The idea is to store them at the farm until they are needed at the biorefinery. On-farm storage in outdoor piles is viable, in part, because the high bulk density of the cobs causes them to degrade more slowly than other portions of the stover. Ihnen notes that it is not unusual to find cobs in a field two to three years after the field was harvested.
Part of Poet’s testing of storage options is focused on how the cob piles survive the winter and the spring, and how the length of time the cobs are stored affects the ethanol yield, Stowers explains. “The results to date are encouraging, and we are pleasantly surprised to see the quality of the cobs that are still coming out of the piles, even today.”
Ihnen is sharing his experience harvesting cobs with other area farmers and expects to see more of his neighbors closely watching this year’s harvest. “A lot of our neighbors are investors and owners in Poet plants and are interested in how cobs can add to a plant’s profitability.”
Harvesting cobs promises to add revenue to a farm’s bottom line. Depending on yields, an acre of corn produces between 1,500 and 2,500 pounds of cobs, Ihnen says. Poet is still modeling what it will pay for the cobs but initial estimates peg the rate at $30 and $60 per ton, which would generate between $90,000 and $300,000 of additional income from 4,000 acres of corn.
For this year’s harvest Poet is expanding its testing to include farms in Iowa adjacent to the Emmetsburg plant and additional farms in South Dakota. Research and testing are also continuing on farm to plant logistics and processing steps required prior to applying pretreatment technologies to extract the sugars from the cellulose.
Back in Hurley, Ihnen is excited about the opportunity presented by harvesting cobs. “As corn farmers we know how to grow corn so the cob is already there,” he says. “You do not have to change crops and grow switchgrass or sorghum. If we can get cellulose from corn cobs, that is a real benefit to the Corn Belt.”
Diane Greer is a Contributing Editor to BioCycle. She can be reached at

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