energy use are part of
ongoing UW research
Jan Shepel
Associate Editor
ARLINGTON
Several challenges face the move to biomass energy systems. One is “aggregating” the product - whether that product is switchgrass, corn stover or other materials high in cellulose. Another is handling it to get it ready for eventual energy use.
Getting enough of the product together in one place and processing it will be necessary if the biomass feedstocks, as they are called, are ever going to become a big part of the nation’s energy use. Those factors will also be important if such energy systems are going to be profitable for farmers.
Kevin Shinners, a University of Wisconsin engineer in biological systems, is working on a number of different machines that may lead to more economical ways to harvest energy crops. He talked about some of that research with farmers and ag consultants at a recent field day at the Arlington Agricultural Research Station.
Shinners believes Wisconsin has great potential for biomass development. “Wisconsin is capitalized like no other state for producing, harvesting, storing and transporting biomass,” he said. “We have it right here and ready to go.”
Custom harvesting operators are always looking for more work, especially if that work falls at different times than the more dominant crops, he noted. Another plus is the livestock industry in the state; as biomass harvest removes the material from the fields, manure nutrients will be available to make up for it. Removing stover as a biomass crop can also make working the field the following spring easier because the residue from the previous crop won’t be in the way, he said.
The biomass being looked at for energy production is the woody fiber part of the plant - the cell wall that includes cellulose and hemicellulose.
Shinners said that while switchgrass is often mentioned as a crop for biomass, there are other, probably more viable crops. Irrigated sorghum fields in Texas produce as much as 15 to 20 tons of dry matter per acre.
But the top biomass producer is likely to be corn stover. Corn is produced on 90 million acres of cropland in the U.S., he said, and can produce 3 to 6 tons of stover dry matter per acre. “It is the most important residue we have,” he said.
Shinners estimated that there are 10 million tons of stover to be harvested in Wisconsin alone in an average year. Every 40 bushels of corn that is produced equals about a ton of stover dry matter, he said.
Switchgrass, in the trials he has worked on, yields from 4 to 10 tons of dry matter in small plots. But he said that in real-world growing conditions, he would expect an average of 5 tons. Alfalfa can produce from 3 to 8 tons per acre, he said.
Much of Shinner’s work has been aimed at reducing the number of operations needed to make a biomass crop useable. Shredding, raking, baling, gathering bales, storing them and taking them to the refinery are followed by a number of processes at the energy facility - de-twining among them.
The first problem with this system is that it involves too many operations, he said. Because of the raking and shredding, it also contains a high volume of “ash,” dirt from the field. Another problem is that there is a low-cob capture in raking and baling, and the cobs are valuable in the biomass mix.
As the cobs drop out of the combine it’s hard to get them in a position where they will end up in a bale destined for a biomass refinery, he said. Another problem is that there is often a great deal of storage loss with round bales.
Biomass refiners are going to want “number 2 yellow stover” he said, referring to the physical and chemical uniformity that can be found in No. 2 yellow corn.
Shinners believes many of the problems can be managed by ensiling the stover. Some equipment companies are working on a machine that can harvest the corn grain in one bin while also chopping the stover.
Since that system will significantly slow down harvest, another option is a two-pass system that decouples the grain and stover harvest; it captures the grain like a combine while laying down a windrow of stover, cob and husk.
Shinners is working on a system that might make alfalfa or forage grasses and even switchgrass a better biomass crop. The crop is “fractionated” at harvest with a modified head that strips the high-protein leaves off the crop and allows the woody stems to remain in the windrow for harvest as biomass.
“That protein is not wanted at the biorefinery,” he said, “and it is a good livestock feed.”
A stripping rotor from a snap bean head followed by a cutter bar on a chopper captures the leaves in a wagon, while the alfalfa stems are laid in a windrow. “This is a way to get alfalfa to be a biomass feedstock for us,” he said. The head has been used in grasses, too, like reed canary grass.
Another process he’s experimenting with is cubing of grass or alfalfa. Shinners noted that power plants wanting to use biomass are interested in products that can be utilized with existing equipment. Cubes of biomass, which are nearly the size and shape of lumps of coal, allow the utilities to cofire the renewable product with coal.
At about 7,500 British thermal units in a pound of biomass cubes, they will never be able to match the energy power of coal, which has 12,000 Btu per pound, he said. But he is optimistic they can be a part of a renewable energy system because they allow power plants to utilize their existing equipment and require fewer steps than baled biomass feedstocks.
There are 16 steps from field to boiler with baled material like switchgrass or corn stover. Using the field cuber allows the system to reduce the number of operations to nine from field to boiler, he said.
Farmers at the field day were especially interested in looking over the old John Deere 425 hay cuber Shinners has been using. Shinners said John Deere made only about 400 of the machines in the 1960s and he believes there are only about 100 of the machines left. He has two of them to work with.
There are a few operating commercially in the western U.S., where growers use them to cube alfalfa for export to Asia. The machine cubes alfalfa right in the field as compared to other cubing systems that are stationary with the hay brought to them.
Growers using the cubers in the field are able to get a premium price for the field-cubed product because their customers believe they are getting a top-quality product. “When you use a stationary process, these buyers know that other less premium materials might get into the cube,” he said.
The John Deere machine can only move about 1.5 mph, he said, and uses a 250-horsepower engine. When the company made the machine, they had to purchase those engines because at that time they didn’t manufacture any that large, he said.
Shinners is using the machine as a “test mule” to explore the idea of field cubing. He said cubes are 180 degrees when they come out of the machine.
