What Is Woody Biomass?
Article #159, July 2010
By Bill Cook
So, we’re hearing more and more about woody biomass. What, exactly, does that mean? Potentially, lots of things.
Different kinds of woody biomass are used for different purposes. Some products require fairly narrow specifications. Other products can use just about anything. Add-in other kinds of biomass, such as switchgrass, and then the field of variability expands even more.
Biomass is simply tissue that was created by living things. Woody biomass is tissue that contains large amounts of cellulose, which is a complex sugar molecule. Hemicellulose and lignin are other important parts of wood, and becomes important with chemical processes. Plants other than trees have cellulose with different chemical structures. Sometimes, the chemistry makes a big difference.
Conventional boilers and furnaces can often accept a wide range of woody biomass in order to function but there may be preferences in what is bought. Sometimes the wood needs to be pre-processed into certain sizes or mixed with other combustibles, such as tires, in order to maximize combustion efficiency. These boilers can make heat, electricity, or both. Both, of course, are much better because there’s less energy waste.
Pellets and briquettes require particular recipes of small fiber material in order for the presses to make a product that sticks together and concentrates the amount of energy into smaller volumes. Glues are not used. That’s part of the reason why moisture and storage can be an issue with pellets. They need to be kept dry.
Something like cellulosic ethanol or other useful chemicals requires fairly tight specifications, especially if a biochemical process is used. The catalysts and living organisms sometimes have particular diets. If a thermo-mechanical process is used, then the specifications might be a bit more relaxed. The processes use biomass to produce a range of biofuels.
Biochemical processes require wood to be pretreated, meaning it must be broken down into the right sized small pieces. The complex sugar molecule is then broken down by “bugs” into simple sugars and then fermented. Sometimes this can be done in one step. Afterwards, the mix must be distilled and impurities removed. Ethanol goes to the market and the by-products are converted to other uses, such as generating heat, electricity, or making other chemicals.
Thermochemical processes use heat, usually in low oxygen environments. Fast pyrolysis (pyrolysis means heat without oxygen) creates a bio-oil that can be graded and fractioned into fuels or chemicals. Gasification creates gases that are cleaned and reformed to produce a syngas. The syngas can be fermented into ethanol or be fed into a “Fischer-Tropsch” process that makes things like biodiesel, alcohols, ammonia, and other products. Torrefaction is lower heat environment than pyrolysis and that slowly cooks wood rendering it water resistant and more energy rich. The dried material can be used for improved pellets, solid wood products, or other high-BTU fuel.
You can’t simply use any old wood for many of these newer energy applications.
Sources of wood can vary widely. Sometimes, the limbs and tops from timber harvests can be chipped and used for boiler fuel. Sometimes, whole trees might be used, especially species that are otherwise non-commercial. In places where few forest product markets exist, pulpwood or even sawlog sized material might be used for energy purposes.
However, where conventional markets exist, energy producers usually cannot afford to be price competitive.
Residues from sawmills and other primary forest industries have been used for energy production or other products for decades. Today, there is little “waste” in the wood manufacturing industry. So, most of the proposed energy facilities will need to use fresh wood.
For higher-end energy markets, companies may be quite particular about the wood they purchase. The proposed Frontier Renewable Resources ethanol plant in the eastern Upper Peninsula will begin by purchasing pulpwood grade hardwood, maple in particular.
Lastly, energy plantations may become feasible in the future. Plantations offer several advantages; wood quality can be better controlled, they can produce several times the volume of useable wood per acre than natural forests, and plantations can be grown close to a mill so that transportation costs can be greatly reduced. However, plantations are still quite expensive to plant, tend, and harvest. New technologies will help to lower these costs to the point where plantations can become economically viable.
Understanding some of the emerging wood energy technologies can be challenge. As the future grows closer, we shall see how well we learn to use our vast forest resources in an efficient and sustainable manner.
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Bill Cook is an MSU Extension forester providing educational programming for the entire Upper Peninsula. His office is located at the MSU Upper Peninsula Tree Improvement Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at email@example.com or 906-786-1575.
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: firstname.lastname@example.org
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