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Harvesting opportunity with cellulosic biomass

Kevin J. Shinners Published on 16 September 2009

Although there are strong opinions about environmental impact and supply of fossil fuels, we know that sometime in the future we’ll have to rely on alternative sources of energy to sustain our nation’s economy and way of life.

There are many competing renewable energy technologies – including solar, wind and cellulosic biomass. Cellulosic biomass is attractive because it’s the one technology that can be readily converted to transportation fuel, although the conversion technology is immature.

 The term “cellulosic biomass” comes from the three constituents of the plant’s cell wall – cellulose, hemicellulose and lignin. All of us involved with hay and forages are very familiar with the plant’s cell wall. We measure the cell wall content by ADF, NDF and lignin when we conduct nutritional analysis of our crops.

With most hay and forage crops, we try to balance these three chemical components with other important animal nutritional components like protein and starch.

That’s not the case when we talk about cellulosic biomass – we want to maximize the cell wall content. You might think of this as trying to make the worst-quality hay that you could think of (Table 1*)!

Cellulosic biomass comes from three main sources – agricultural residues, dedicated energy crops and forest products. Examples of residues include corn stover and small grain straw.

Dedicated energy crops would include switchgrass, miscanthus and giant reed. Forest products would include waste wood, forest thinnings and hybrid poplars.

Each of these crops has pros and cons as a biomass feedstock. For instance, residue yields are often only 1 to 3 dry tons per acre compared to the 5 to 10 dry tons per acre with dedicated energy crops. Residues also often have high ash content. Meanwhile, dedicated energy crops are challenged by low income per acre compared with residues, which have the combined grain and biomass income.

We call the supply chain between biomass production and conversion at the biorefinery “feedstock logistics.” Logistics involves harvest, processing, storage and transport.

The economic competitiveness of cellulosic ethanol production is highly dependent on feedstock cost, which constitutes 35 to 50 percent of the total ethanol production cost.

Currently, the logistic system adopted from hay and forage production are being proposed for cellulosic feedstocks. However, the economic viability of these systems is challenged because they involve equipment with insufficient capacity, too many operations and often produce undesirable physical and chemical properties in the delivered feedstocks.

Researchers and product developers are looking at new ways for traditional hay and forage machines and storage systems to be adapted for cellulosic biomass.

One approach to reduce feedstock costs is to eliminate as many “non-value-added” operations as possible. We also want to make every operation a “value-added” step. The goal is to reduce cost and improve efficiency by adopting and improving current hay and forage harvest and storage technology. Let’s consider some examples.

Typically corn stover harvest involves shredding, raking/merging, baling, bale gathering, transport to the storage site and then placing into storage.

Some producers improve efficiency by raking only or combining shredding and merging. Harvesting in this manner not only has too many operations, but stover yield is low and soil contamination high. Additionally, round bales of stover do not shed water well, so uncovered storage can lead to high losses.

Machine developers have proposed modifications to the combine to streamline the stover harvest process into one or two passes. The single-pass system makes modification to the combine to capture the material-other-than-grain (MOG) exiting the combine. The amount and physical form of MOG can be altered by the type of head and rear MOG processor used.

Some grain producers are concerned about how the simultaneous harvest of grain and stover will affect grain harvest productivity. An approach to overcome this concern is a two-pass system where windrows of corn stalks and cobs would be formed at grain harvest. The second pass involves either chopping or baling.

Switchgrass is an often-mentioned cellulosic biomass feedstock. Switchgrass is a high-yielding perennial grass that is typically harvested as dry hay packaged in large round or square bales.

Just like traditional methods of harvesting corn stover, this approach has too many non-value-added steps and bales stored outdoors are subject to high storage losses. Bales also must be de-twined and tub-ground before conversion at the biorefinery.

Switchgrass for biomass will be harvested very mature, typically in the late summer or fall. At this late maturity, the moisture content of the standing crop allows it to be direct-cut and stored by ensiling, thus eliminating cutting, raking, field drying, bale handling, etc. An additional benefit is the crop leaves the field size-reduced and ready to be used at the biorefinery without additional processing.

Across the country, producers harvest, store and feed hay and forage crops in a wide variety of ways. The same will be true for cellulosic biomass feedstocks.

We’ll see everything from large square bales of switchgrass to direct-cut harvested giant reed to cubed or pelletized miscanthus. No matter the type of feedstock, hay and forage producers will be able to marshal their considerable machinery resources, harvesting experience and entrepreneurial spirit to take advantage of these new crops.  FG

*References and tables omitted but are available upon request at

Kevin J. Shinners
Biological Systems Engineering
University of Wisconsin

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