Storage Strategies for Cellulosic Feedstocks

EPM explores storage options for corn stover and switchgrass, both of which have the potential to serve as the next big feedstocks for cellulosic ethanol production.
By Anna Austin | January 12, 2009
Corn stover, which consists of the leaves and stocks of corn plants usually left in the field after harvest, makes up about half of the yield of a corn crop. Corn stover, if it can be efficiently harvested, stored and transported, has multiple uses including as feed for animals, fertilizer, a biomass source for steam generation or electricity, and as a feedstock for the production of biofuels.

Corn stover has been seriously considered as a potential feedstock for the production of cellulosic ethanol for several years, and has been the subject of much research and experimentation. Innovation is needed, however, in corn stover collection, transportation and storage methods to turn the idea into reality.

Eric Woodford knows corn stover. He and his wife, Mary, run Woodford Custom Inc., a family farm business in Redwood Falls, Minn., where their primary focus is baling and selling corn stalks.

Currently, the most widely accepted storage method is to turn the corn stover into large, round bales, according to Woodford. "The infrastructure to do this type of harvest is already in place on many farms across the Midwest," he says. Round bales generally require less expertise than square baling, shed water easier, and are typically less expensive (about $15 per dry ton for round bales, compared with $20 per dry ton for square).

The initial investment would be the purchase of a round baler at a cost of approximately $23,000, whereas a square baler would cost about $87,000.
Woodford has found that corn stover storage practices vary among the different regions of the U.S., mainly due to climate and end-user needs. "My experience is in the Midwest, where the climatic challenges are rain, snow, ice and humidity," he tells EPM.

The corn stover bales can be wrapped to protect them from inclement weather. Woodford has found that wrapping the bales with net/mesh is more effective than just wrapping the bales with twine. "Net wrap provides a certain amount of protection from the elements," Woodford says. "When water hits the net, it beads up and runs off the edge of the bale."

Although it's more expensive than other methods, mesh wrap sheds water better and prevents a great deal of loss while the bales are being moved, which may offset the higher cost.

According to Tom Schechinger, chairman of the board of managers at BioMass Agri Products in Harlan, Iowa, plastic from the twine or mesh wrap is also considered a serious contaminate. "De-twining is often a laborious task," he says. "Installing equipment that will de-twine or de-wrap adds cost and is not fail proof."

Storage Logistics
In a study conducted by the Department of Biological Systems Engineering at the University of Wisconsin and released in 2007, round and square stover bales, both dry and wet were stored indoors and outdoors, some wrapped in a plastic film tube, sisal twine, plastic twine and net wrap.

"Corn stover bales are usually made at a range of 7 percent to 18.5 percent moisture," Woodford says. "Bales that are wetter than this will heat, which will reduce the value of the feedstock and may cause a fire."

Consistent with Woodford's recommendation of using net wrap, the researchers found that bales in the net wrap and plastic twine had a much less dry-matter loss than the ones wrapped in sisal twine.

Bales wrapped in the tubes were removed after approximately 8 months of storage, and the researchers found there were no statistical differences in fermentation between bale types, but higher moisture bales produced significantly greater levels of fermentation products than low-moisture bales.

The study's overall findings were that storing wet corn stover by ensiling resulted in less loss and more uniform product moisture, compared with dry stover bales stored outdoors. Ensiling may also prevent or reduce the risk of fire.

Woodford recommends that the bales stored outside on the ground be lined up end to end, rather than side to side. "The area should have a good slope so rain water will run off and there should be no depressions where water can pond," he says.

In Midwest locations where harsh weather conditions can occur, Woodford suggests stover bales be placed where they can be accessed by roads, yet will not block roads with drifting snow. "If bales need to be stacked high in large piles I would recommend some sort of roof or tarp to cover the top of the stack, because when you stack bales, rain and snow has to soak in as it can't run off as easily," he says. "Extreme care must be taken to make sure that all bales placed in the stack are dry and will not heat."

Woodford prefers to have multiple satellite storage locations as opposed to one central location. "Multiple satellite locations help break up truck traffic and reduce potential loss in the event of a fire," he says.

Although corn stover looks to be a promising feedstock for cellulosic ethanol, Woodford thinks it should be considered as a stepping stone to more sustainable crops, such as native grasses and dedicated energy crops.

Switch to Switchgrass
Switchgrass is another potential cellulosic ethanol feedstock that has garnered a lot of attention. University of Tennessee researchers are well on their way to discovering the most economical and beneficial methods of storing switchgrass.

Switchgrass is a warm-season perennial commonly found in prairies, pastures and along roadsides. It is considered a high-yielding, versatile, adaptable plant, which is able to thrive in many different weather conditions, and requires lower fertilizer and herbicide amounts compared with typical crops.

Several companies, including Missouri-based CleanTech Biofuels Inc., have expressed interest in the crop. The research team at UT co-led by agricultural economics professor Burton English, agricultural economist James Larson and soil scientist Don Tyler began experimenting with switchgrass in January 2008 through the Tennessee Biofuels Initiative. "Logistics is a big issue in regard to biomass," English says. "We realize that one can transport more rectangular bales in a truck than round bales, but storage is another matter."

The group established 720 acres of switchgrass in eastern Tennessee to perform a storage study, comparing round or rectangular bales stored on three different surfaces, covered and uncovered.

Five-by-4-feet round bales, and 4-by-8-feet rectangular switchgrass bales were stored on well-drained ground, gravel and pallets. Some bales on each of those surfaces were covered with a plastic tarp, wrapped in plastic or left uncovered.

Every 100 days starting in January 2008, the group conducted bale destruction tests to see what was happening inside of them, Burton explains. "We do this by cutting them open with a chain saw, selecting four different weathered surfaces to take samples from and sending the samples to the National Renewable Energy Laboratory in Golden, Colo., for testing," he says. The researchers have collected samples from January, May and September, and are now preparing to collect the December samples. "We are doing this every 100 days for 500 days, even though a season is 365 days," Burton says. "We believe there will have to be some extra storage time, just in case not enough switchgrass is produced in every given year. At this time, we think the material will be stored at farms, so this study will look at the cost of each specific method."

Cost and Findings
According to English, the least expensive switchgrass storage method is as round bales, without a tarp and on pallets. "It costs about $4 per ton that way," he says. "If it doesn't have a tarp it's going to weather about 6 inches, but we don't know if that's bad or good for conversion processes, because we don't have conversion process data yet."

The samples the group has sent to the NREL will be chemically analyzed to answer that question. Among several things that were measured, English says the bale's ethanol content will be significant.

A round bale covered with a tarp and stored on a gravel pad will cost about $12 a ton, English says, noting that a round bale typically weighs 1,300 pounds; a square bale weighs about 1,700 pounds. "If you don't cover that square bale and leave it open to rain, it can increase in weight by up to 3,100 pounds," English says, adding that the square bales soak up water rather quickly, which may damage their quality. All bales that were left uncovered were waterlogged when the first samples were taken in May.

Taking cost and overall effectiveness into consideration, the UT researchers found that tarp-covered bales stored on pallets resulted in less degradation compared with those stored on a gravel pad. "A gravel pad is more expensive, and it doesn't add quality," he says.

In the experiment, round, tarp-covered bales showed little signs of weathering, and decreased in weight by about 37 pounds per bale. Weathering on covered rectangular bales varied and significant decomposition was observable along the bottom edge and exposed sides of most bales. The average decrease in weight was about 1,313 pounds per bale.

A 25-by-100-feet tarp costs $500 and will cover about 144 rectangular bales, or 120 round bales, according to English. Pallets cost about $6.50 each, and only one bale can be placed on a pallet. Gravel pads cost approximately 60 cents per square foot.

English says UT should begin receiving experiment results from the NREL at any time and that the study should be completed by next summer.

Feedstock Future
Determining the cost and effectiveness of feedstock storage is important to the commercial development of cellulosic ethanol production, which is why so many companies and universities are working to put the pieces of this puzzle together. For example, in August 2008, Illinois-based Archer Daniels Midland Co., Deere & Co., and Missouri-based
Monsanto Co. announced a collaboration to identify environmentally and economically sustainable methods to harvest, transport and store corn stover. Other universities, such as Iowa State University and the University of Kentucky have developed programs to study biomass-to-ethanol crops, with an emphasis on switchgrass.

It is clear researchers are beginning to develop a solid storage infrastructure for the use of these crops as a source of clean, renewable fuel.

Anna Austin is an Ethanol Producer Magazine staff writer. Reach her at or (701) 738-4968.