January 25, 2009 | General

Biomass Harvesting Strategies

BioCycle January 2009, Vol. 50, No. 1, p. 39
Developing methods to economically supply biomass feedstocks to biorefineries is critical to the commercial-scale production of biofuels.
Diane Greer

A 2005 STUDY by the Oak Ridge National Laboratory found that U.S. forest and agricultural lands could produce over 1.3 billion dry tons per year of biomass – enough to meet more than one-third of the country’s demand for transportation fuel. Its assessment, contained in what came to be known as the “Billion Ton Study,” included 428 million dry tons/year of crop residues and 377 million tons/year of perennial crops.
Producing biomass at these volumes calls for new equipment and harvesting and storage methods on our nation’s farms. Farmers must be able to supply feedstocks profitably and at prices which allow production of biofuels that are cost competitive with gasoline.
Kevin Shinners, a professor of Agricultural Engineering at the University of Wisconsin, understands the challenges of collecting and handling biomass crops. His current research focuses on improving the efficiency and economics of harvesting and storing corn stover and perennial grasses. “One of our goals is to try to bring as much value to the farm level as possible with biomass,” Shinners says.
Current techniques for collecting stover, which involve shredding, drying, raking and baling the material, are inefficient and labor intensive. “If you add up all the steps involved in the process, it can be quite a costly endeavor to harvest a ton of biomass,” Shinners notes.
Equipment used to harvest the grain and stover must make multiple passes over a field, first to collect the grain and then to process and collect the stover. Stover collected off the ground during a second pass is often contaminated with soil, Shinners explains. “Any type of soil or rock in that material is not very desirable, no matter what energy process we are using with biomass.”
Drying the stover prior to baling can be difficult in certain parts of the country, such as the Midwest, where farmers must race to collect the residues before the onset of winter. Baling wet stover or storing the bales outdoors results in losses due to material degradation.
About five years ago Shinners started exploring the best means for eliminating the nonvalue added steps involved in collecting stover, along with producing a cleaner crop. His solution was to design and engineer a single pass harvester. “The single pass harvest system takes some fraction of the corn plant and harvests it at the same time we harvest the grain,” he explains. “By the way we configure the machine we can harvest different amounts of the available stover.” Since the stover never hits the ground, the process produces a uniform clean product.
The equipment can be set up to collect just the cob and the husk or to take the entire stalk. Adjustments permit the stalk to be cut at different heights. “We have options to reconfigure the harvester to choose fractions that are most desirable for us and to optimize the amount of material we are pulling off the field to maintain sustainable soil fertility and organic material,” he adds. Rather than developing a piece of equipment from scratch, Shinners made changes to a standard grain combine. “We envisioned a system which would not require farmers to go out and buy a whole new machine.”
On the front end of the combine, Shinners modified the head, a piece of equipment that cuts and gathers the crop. Normally farmers use an ear snapper head that snaps the ear off the stalk and drops everything else on the ground. “We have modified the head so that we can target the cob and the husk or the whole plant can be brought into the combine,” he explains.
A chopper along with a blower and a spout were added to the backend of the combine. The mechanisms chop the material to reduce its size and blow it through a spout into a wagon that is pulled behind the combine or a truck that runs along side.
As a general rule of thumb, every pound of grain harvested by the modified combine will yield about a pound of stover at the maximum. Harvesting the maximum amount of stover possible using the new system will yield about 400 tons of dry material per acre.
Collecting stover at the same time as the grain does affect harvest productivity. The exact impact depends on how much stover the farmer decides to collect. “If they are using the ear snapper head and are just collecting the cob and the husk it will not slow them down,” Shinners says, although he notes there are issues related to handling the stover in addition to the grain. “In terms of how fast they go down the field, it will not really change their harvesting rate.”
But targeting a high yield of stover by either collecting the whole plant or a large fraction of the whole plant can reduce productivity by 20 to 50 percent. Farmers must go slower because some of the combine’s power is now used to run the equipment collecting the stover. Logistically, simultaneously handling grain and high stover yields will also slow down operations since wagons accumulating the chopped stover need to be emptied periodically.
Shinners points out that his research is focused on engineering the process, not optimizing equipment performance, productivity or fuel use. “At some point you need to hand this off to an industrial concern and let them make the modifications they feel are warranted from a commercial standpoint.”
How the economics of harvesting stover will play out is still an open question. “You may have a 25 percent reduction in productivity but are getting more material off the field that you can sell for $50, $75 or a $100/ton,” he says. “It all has to workout economically. At the end of the day are you getting a bigger or smaller check from this process? We feel that economically it is going to be more viable than a dry bale system.”
He estimates single pass harvesting reduces collection costs by one-third compared to traditional multipass methods. But farmers need to consider other costs. For example, if removing stover requires additional fertilizer to maintain soil fertility, then the value of the stover needs to offset the increased cost of fertilizer. Stover harvested using a single pass method is also wetter, containing between 35 to 55 percent more moisture than stover left on a field to dry. Wet stover is heavier than dry stover, resulting in higher transportation costs.
Shinners is also investigating alternatives for storing the material that will increase its value. “Instead of just selling a commodity we want to sell a value added product.” Biorefineries employing biochemical processes to produce cellulosic biofuels include pretreatment systems that breakdown the material to extract fermentable sugars. One potential means of increasing the value of these biomass feedstocks is to start the pretreatment process on the farm.
“Pretreatment costs are roughly one-third of the cost of producing ethanol,” Shinners explains, noting that pretreatment processes at biorefineries often involve high temperatures and pressures to overcome the recalcitrance of cellulose and hemicelluloses structures forming the cell walls of plants. This improves throughput rates. But time is not a factor when pretreating the material on the farm using less expensive processes. “We have this material in storage and can begin the pretreatment process that makes the cell wall contents more available once it gets to the biorefinery,” he says.
Laboratory testing of on-farm pretreatment systems employed dilute acid, alkali, ozone or novel enzymes on two types of perennial grass, switchgrass and reed canarygrass. Experiments compared the ability of these on-farm pretreatments to improve enzymatic breakdown of cellulose and hemicelluloses biomass at the biorefinery. Similar testing is under way with corn stover but results are still being analyzed.
Results for the perennial grasses show that all pretreatment methods improved cellulosic degradation, but sulfuric acid, lime and ozone pretreatment technologies yielded the highest conversion rates. Acid and lime were also judged to be the easiest to integrate into existing on-farm storage systems, while incorporating gaseous ozone would be more challenging.
Beyond screening the technologies, Shinners is trying to determine if on-farm pretreatment can be profitable to farmers. Pretreated materials will be shipped wet to the biorefinery, adding to transportation costs. The question comes down to how much more will biorefineries be willing to pay for partially pretreated material and will that price increment offset the on-farm costs? If Shinners determines on-farm pretreatment is potentially profitable, then the next step in his research is to repeat the tests of each pretreatment option in farm-scale systems to determine the challenges of scaling the technologies.
Shinner’s third research project, which is still in the initial stages, is developing harvesting equipment for perennial grasses and legumes. The idea is to engineer a piece of equipment that separates the leaf from the stem fraction when harvesting the plants. The leaf fraction would be left on the farm for animal feed while the stem fraction would be sent to the biorefinery, he explains.
Legumes are interesting because of their ability to fix nitrogen in the soil. “Alfalfa might make a very attractive biomass feedstock if you can use it both as an animal feed and a biomass feedstock and get all the nitrogen fixing benefits of the plant,” Shinner says. Results of this work will probably be available sometime in 2010.
Diane Greer is a Contributing Editor to BioCycle. Field trials of these projects were on the tour at BioCycle’s 8th Annual Renewable Energy Conference in October 2008.

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