One-Pass Stover Harvest

January 10, 2009 11:53 AM


Front and rear attachments allow a conventional combine to harvest corn stover or cobs, as well as grain.

Ethanol producer Poet is already generating ethanol from corncobs on a pilot scale and expects to go commercial with cellulosic ethanol in 2011.

Now it's up to the ag machinery industry to develop efficient equipment that gets cobs from the field to the biorefinery without adding major complications to the grain harvesting process, says Stuart Birrell, an Iowa State University ag engineer. "This will be critical to the success of the bioeconomy," he says.

With that in mind, Birrell and fellow Iowa State researchers have developed a dual-stream harvesting system that harvests corn kernels, as well as the stalks, leaves and/or cobs, in one pass. The system uses a stover attachment that can be put on a standard combine for an additional cost of $35,000 to $50,000, Birrell estimates.

"The advantage of this machine is it allows producers to use conventional harvest machinery to harvest two products, spreading capital costs over two different crops and production enterprises," says Scott Weishaar, director of business development for Poet.

The system includes a modified header and corn reel attached to the front of the combine and a chopper and blower on the back. The header and reel feed leaves and stalks into the combine so the biomass can be harvested before it touches the ground and is contaminated with soil. The chopper cuts cobs, stalks and leaves into approximately 2" pieces. A blower throws the chopped stover into a wagon that is either pulled behind the combine or following alongside.

If the stover wagon becomes full or any other part of the cob/stover transportation system is unavailable, the combine operator can shut off the blower and flip a switch that lets the stalks, cobs and leaves drop onto the ground and continue to harvest grain. There is no need to disconnect, unhook or remove any component to go from harvesting residue and grain to just grain.

Speed stop. Although tests with the prototype machine have been successful, Birrell says there is more development work to do on harvest capacity.

He is working to get the speed to at least 80% of a normal grain harvest, no matter how much stover is collected. Then farmers can decide how much stover they want to harvest without significantly affecting the time it takes to harvest grain.

Another concern is stover density for transport. The chopper is designed to increase density between 3 lb. to 4 lb. per cubic foot. However, to fill a truck to its allowed weight limit that density would need to be closer to 12 lb. to 14 lb. per cubic ft. "That's a limiting factor," Birrell says, explaining that hay bales are typically about 9 lb. to 11 lb. per cubic foot.

To get up to a hay-bale density for corn, the stover would require a more complex and expensive combine that would bale the product as it comes out of the back of the harvester. However, these systems have yet to be developed.

"Our preference is to have a baler run beside the combine and not attached, in order to keep the combine flexible for conventional grain harvest," Birrell says. A baler pulled alongside would also better use horsepower from a tractor instead of the combine, he adds.

Even with these obstacles yet to overcome, Birrell says the one-pass system is about 90% of where he would like it to be. "I believe it will take at least one more year of full-scale field tests before commercialization decisions can be made," he says.

You can e-mail Jeanne Bernick at

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