Throughout history, humans have used plant and animal matter for heating and cooking. Today, technological advances and society's increasing demand for energy have led to an expanded role for these biomass fuels. The term bioenergy, or biomass, means any plant-derived organic matter available on a renewable basis, including crops and trees, agricultural food and feed crops, agricultural crop wastes and residues, wood and wood wastes and residues, aquatic plants, animal wastes, municipal wastes, and other waste materials. These sources can provide energy in the form of solid, liquid, or gaseous fuels and provide about three percent of all the energy used in the United States.
Biomass fuels get their energy from the sun. Photosynthesis converts solar energy striking the leaves of plants into chemical energy, which is stored in the plants themselves. Animals that eat plants store some of this energy in their bodies; some of it is also discharged in manure and other wastes. Biomass fuels are a renewable resource because they can be replaced fairly quickly (times ranging from one growing season to perhaps one or two decades) without permanently depleting Earth's natural resources. By comparison, fossil fuels such as coal, petroleum, and natural gas require millions of years to be produced. Thus, drilling and mining for fossil fuels are not regarded as renewable processes, since they deplete the Earth's resources at a rate that is millions of times greater than the rate at which they could be replaced.
Wood
Wood was once the main energy resource used during the early history of the United States, but now it plays only a small role in meeting the nation's energy needs (3%). Still, in certain parts of the country, including Wisconsin, wood provides people with a cheap and plentiful source of energy for heating. About 20 percent of U.S. homes get some heat from burning wood, while about four percent use it as their primary fuel. The forest products industry (pulp and paper manufacturers, mills, etc.) consumes almost two-thirds of all fuel wood.
Wood gets its energy from the sun. Photosynthesis converts sunlight that strikes the tree leaves into chemical energy in the form of cellulose, hemicellulose, and lignin, which are the main chemical compounds found in wood. For this reason, wood is sometimes referred to as a "lignocellulosic" biomass fuel or feedstock. Wood is a renewable resource, which means that additional resources can be grown to replace any wood that is cut down.
Wood for heating is sold in units called cords. A cord is a stack of wood 8 feet long, 4 feet high, and 4 feet wide (128 cubic feet). A face cord is a stack 8 feet long, 4 feet high, and 12 to 16 inches wide (32 to 40 cubic feet). A cord of hardwood such as maple, oak, or hickory may contain twice as much energy as a cord of softwood such as pine or balsam fir. This variation in energy is because a cord of hardwood weighs up to twice as much as a cord of softwood. For example, about twelve cords of white pine are needed to heat an average home is Wisconsin for the year, while only about seven cords of white oak are needed to provide the same amount of heat. These figures assume that the average Wisconsin home needs 80 million Btu for heating each year and uses a woodstove with an efficiency of 50 percent.
Forests cover about one-third of the total land area of the United States (766 million acres). About two-thirds of this forest is productive enough to grow commercially valuable trees.
Of Wisconsin's 35 million acres of land, about 17.1 million acres are forested. Forest area in Wisconsin has been steadily increasing since 1968, mostly due to the conversion of marginal agricultural land back into forests. Currently, forest covers about 48 percent of the total land area of the state. Since 1983, WI forestland increased almost nine percent, or 1.3 million acres. Most of this increase occurred in the northern part of the state, with forests between 61 to 100 years of age experiencing the largest increase in acreage. (see: Wisconsin Forests – A Quick Overview, WI DNR 2011 report)
The potential amount of wood energy from Wisconsin sources is about 200 trillion BTU per year, an amount that could heat about 2.5 million average-sized Wisconsin homes each year. Methods for harvesting wood range from simply cutting down a tree with an ax or saw to removing all the trees from a large area (clear-cutting) using chainsaws and other heavy equipment. Other than drying, wood does not require much processing before being used as a fuel. Some homeowners may burn wood pellets that are manufactured from finely ground wood fiber, which requires more processing. Wood pellets for burning in power plants are made by harvesting and shredding whole trees. Pellet fuel can also be made from sawdust and shavings leftover from processing trees for lumber and other wood products. The main advantage to pellet fuel is that it provides a more consistent fuel supply with more carefully controlled energy content, moisture content, particle size, and surface area.
Photo Courtesy of NREL
About 1.2 million cords of wood are cut and burned for energy in Wisconsin each year. The total amount of wood energy used in Wisconsin in 2012 was more than 46 trillion BTU, about three percent of all the energy used in the state. In 2019, about 2.3 percent of total U.S. annual energy consumption was from wood and wood waste. Worldwide, one-half of all wood that is cut down is used for fuel, while in developing countries 90 percent is used for fuel. Sweden is a world leader in using wood as an energy source; most of the wood they use is for fueling district heating plants.
Certain electric power plants in the United States and the rest of the world burn wood to generate electricity. Like coal and fuel oil, wood is burned in a boiler that heats water into steam. The steam then spins a turbine connected to an electric generator. Power plants usually burn wood along with other fuels; they rarely burn wood exclusively. Approximately 85 power plants in the United States burn wood to produce and sell electricity, including the Bay Front Plant in Ashland, Wisconsin.
Wood is a major fuel source for industries that produce wood products. About 1,000 wood-fired power plants currently operate in the United States, two-thirds of which are owned by industries such as the paper and pulp industry. Many of these industries use wood energy to provide steam, heat, and electricity (this multiple use is called cogeneration). In Wisconsin, roughly 8 trillion BTU of wood energy is used to operate lumber and papermills, representing about 1/6 of the state's total wood energy production.
In parts of the U.S. where wood is plentiful, many rural homeowners burn wood for space heating. Roughly 500,000 homeowners in Wisconsin burn wood to meet some or all of their space heating needs. Wisconsin residents use about one-half of all wood fuel, while the other half is used for commercial and industrial purposes.
Wood is also used to make building materials, pulp, and paper. Other uses include consumer products such as toys, sporting equipment, furniture, and musical instruments. Wood and its derivatives are used in as many as 10,000 products. Generally speaking, different species of trees are harvested to make wood products than those grown specifically for producing energy. Trees such as ash, maple, walnut, and cherry are frequently used for consumer wood products, whereas pine and other softwoods tend to dominate the construction lumber and paper industries. Faster growing (short rotation) species such as willow, poplar, alder and hazel are typically chosen for dedicated wood energy sources.
Energy Crops
Many crops that have been traditionally raised for food can also serve as a source of biomass energy. The most dominant examples in use today include corn, sugar cane, soy, and canola. In many cases, a portion of the crop can be used for energy (for example soy oil) while the other portion can be used for food consumption (for example soy meal). Many types of energy crops can be raised on marginal farmlands that are not capable of supporting high yield food crops on an economically competitive basis.
Energy crops can furthermore be classified based on the type of feedstock material that they provide. Carbohydrate based energy crops provide either sugar or starch-based feedstocks, and include crops such as corn, sugar cane, sugar beets, and sweet sorghum. Oil based energy crops include crops such as soy, canola, sunflower, oil palm, and jatropha. Cellulosic based energy crops include many species that have traditionally been raised for industrial fibers (such as cotton, hemp, jute, flax, etc), as well as other naturally occurring species that have been identified and cultivated specifically for their fast growth and ability to produce large amounts of cellulosic biomass (e.g. switch grass, elephant grass, and reed canary grass).
Algae
Aquatic algae are capable of photosynthesis, and many species demonstrate incredible growth rates compared to land-based plants. Because they are aquatic algal growth is not limited by the availability of water. Furthermore, because these organisms are suspended in the water column, they do not need to expend energy in the creation of cellulosic structural materials to support them and counteract the force of gravity. As a result, algae have the luxury of being able to store a lot more energy in the form of stored sugars, starches, or oils. In fact, the composition of some algal species can be over 50% stored oil as a percentage of their body mass.
Algal are generally classified as either microalgae or macroalgae. Macroalgae are large multicellular organisms that are plantlike in their appearance (with stem, stalk, and leaf- like structures). Examples of macro algae would include kelp, red algae, green algae, and other types of seaweeds (including the Nori seaweed used to make Japanese sushi rolls). Microalgae are unicellular organisms that can exist either individually or in small colonies that are small enough that they are completely suspended in the water column by buoyant forces. Examples of microalgae include spirogyra, chlorella, spirulina, volvox, and diatoms.
Although algae have enormous potential as a biomass source due to their rapid growth rates, there are numerous challenges that must be addressed to facilitate their widespread adoption. These include issues of cultivation, harvesting, de-watering, drying, and extraction of oil or carbohydrate feedstocks.
Agricultural Wastes
Agricultural wastes are plant parts left over after farmers have harvested their crops. These wastes include stalks, husks, prunings, straw and corn cobs. Agricultural wastes can be collected, dried, and burned to produce energy. Burning these wastes in small power plants can provide a convenient source of energy for rural areas and developing countries. The ash that remains can sometimes be used as a fertilizer. Agricultural wastes are used to produce energy in many parts of the world. In Hawaii and Brazil, bagasse, a residue left over after sugarcane is harvested and processed, is burned in power plants to produce electricity. In Denmark, straw is burned to produce heat for farms. Wisconsin produced about 1.7 trillion Btu of energy from crop residues in 1994, an amount equal to the energy used to heat about 20,000 average-sized Wisconsin homes.
Agricultural wastes are primarily composed of cellulose, hemicellulose, and lignin and have been the focus of much attention in recent years for the production of cellulosic ethanol fuel. The main challenge in processing cellulosic fuels is in first separating the cellulose from the lignin, followed by the breakdown of cellulose into simple sugars (e.g. glucose). Efforts in this area are the focus of intense research and development being conducted by the Great lakes BioEnergy Research Center at the University of Wisconsin (see www.glbrc.org)
Animal Wastes
Animal waste products such as manure have long provided biomass fuel for rural societies. People in developing countries often burn dried manure for heating and cooking. Manure can also be placed in tanks called anaerobic digesters, where it is broken down by bacteria and various chemical processes to produce biogas (approximately 60 percent methane and 40 percent carbon dioxide). Small-scale production of biogas can provide fuel for space heating and cooking, while larger-scale production of biogas can generate electricity or provide heat for manufacturing.
A number of biogas-fueled power plants are in operation throughout the world. Plants in the U.S. typically use manure from swine or dairy farms and cattle feedlots. For example, one plant in California that processes cattle manure supplies electricity to as many as 20,000 homes. As of December, 2013, over 40 Wisconsin dairy farms generate biogas from cow manure. Furthermore there are over 750 additional farms with at least 200 cows having the technical potential to generate biogas, and 250 farms that have over 500 cows and are considered good economic candidates for biogas development (see: Biogas Opportunity in Wisconsin – 2011 Strategic Plan)
Photo Courtesy of NREL
Human Waste
Wastewater treatment facilities from municipal sewer systems and from food processing operations can also be utilized to generate energy, through anaerobic digestion. Much like animal waste digesters, these systems produce energy in the form of methane gas. The methane gas can then be used to as a source of heat and/or to generate electricity. The benefits of this technique include odor control, waste reduction and valuable byproducts (soil fertilizer). McCain Foods in Plover, Wisconsin built a wastewater treatment facility on site that converts waste into energy. In 2002 this technology saved the company $50,000 in fuel bills. A plant in Madison, Wisconsin reports $370,000 in electrical savings and $75,000 in gas heating.
Municipal Solid Waste (MSW)
Waste disposed of by residents and businesses, called municipal solid waste (MSW), can provide a source of fuel. A large percentage of this waste is made up of organic materials such as wood, paper products, food waste, and yard waste. Therefore, some MSW is a form of biomass fuel. Specially equipped waste-to-energy power plants can use MSW to produce electricity or heat. The waste is separated and non-combustible materials are removed before the remaining waste is taken to the power plant to be burned. There are roughly 100 waste-to-energy facilities in the United States that are capable of burning about 100,000 tons of waste per day.
Another source of fuel from MSW is landfill gas. This gas in produced by the breakdown of organic material. Landfill gas, which is similar to natural gas, can be burned to provide heat and/or to generate electricity. One-third of all landfill gas facilities are located in California. Several also exist in Wisconsin.
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