|The dual-stream, single-pass harvester. Click to enlarge. Source: Iowa State University|
Researchers at Iowa State University are developing a single-pass system that will harvest corn kernels as well as the stover: stalks, cobs and leaves. The stover can serve as feedstock for cellulosic ethanol.
That dual-stream, single-pass harvesting system, based on a John Deere 9750 STS combine, was developed by Stuart Birrell, an Iowa State associate professor of agricultural and biosystems engineering, and graduate students Mark Dilts and Ben Schlesser.
Significant resources have been dedicated to the process of converting cellulose into ethanol. But very little has gone into answering how do you get a supply of stover from the field to the biorefinery. This will be critical to the success of the bioeconomy.—Stuart Birrell
The researchers are developing stover attachments that can be used on standard combines. The result would be an additional cost to farmers of about $10,000 to $15,000 instead of the six figures it would take for a separate combine to harvest stover. The attachments would also allow farmers to harvest grain and stover with one pass through a field.
The system the researchers have come up with includes a modified row crop header and corn reel attached to the front of the combine and a chopper and blower attached to 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 stover into 2-inch pieces. And the blower throws the chopped stover into a wagon.
Although tests with the prototype machine have been successful, Birrell said there is more development work to do:
Harvest capacity. The stover harvesting equipment is capable of speeds equal to a normal grain harvest when less than 50% of the stover is collected. When all of a field’s stover is collected, harvest speeds are about half of a normal grain harvest. Birrell said that would be unacceptable to farmers. Birrell is working to get the speed to at least 80% of a normal grain harvest no matter how much stover is collected. That would allow farmers to decide how much stover they want to harvest without significantly affecting the time it takes to harvest their fields.
Transportation. Researchers need to figure out how to pack the harvested stover so it can be economically transported. Stover comes off the combine at a density of about 3 to 4 pounds per cubic foot; it needs to be about 10 to 12 pounds per cubic foot for efficient trucking.
Storage. Researchers need to figure out how huge quantities of biomass can be stored. The US Department of Energy has estimated a biorefinery would need at least 2,000 tons of biomass per day. A year’s supply would cover 100 acres with 25 feet of biomass.
Fertility. Researchers need to determine how much stover can be removed from fields while still returning sufficient organic matter to the soil and protecting the soil from winter erosion.
Birrell’s stover harvesting research has been supported by a three-year, $180,000 grant from the US Department of Agriculture and the US Department of Energy and a two-year, $50,000 grant from Deere & Company of Moline, Ill.