A visit to switchgrass? trial plots run by Iowa State University researchers; near Ames, IA. Photo by CL Williams, 2010.
Biomass? materials, after pre-processing into suitable forms for various conversion technologies, provide feedstock? for a variety of bioenergy? end products and end uses. The majority of biomass for bioenergy feedstocks comes from three sources: forests, agriculture, and waste. However, non-forest conservation lands, such as grasslands? and savannahs, and algaculture (cultivation of algae) are also potential sources of bioenergy feedstocks. Regardless of source, biomass materials can be divided into two broad categories: woody and non-woody (figure 1). While forests provide only woody materials, agriculture sources provide both woody and non-woody biomass for bioenergy production (figure 2).
Figure 1. Two broad categories of biomass materials, and four composition types. (CL Williams, 2011.)
Forests, as one might expect, are a source of woody biomass. However, woody materials can also be sourced from agriculture (figure 2). For example, fast-growing tree species such as hybrid willow (Salix) and poplar have been developed for production in agricultural settings (i.e., grown like row crops on farms). Agriculture is also the source of non-woody materials used to make bioenergy (figure 2). Agricultural systems produce four types of non-woody biomass materials: cellulosic materials such as plant leaves, stems and stalks; sugar; starch (i.e., grains); and oil-producing plant materials (e.g., soybeans)(figure 2).
Figure 2. Sources and types of biomass materials for conversion into bioenergy. (CL Williams, 2011.)
Although forest-based wood makes up the majority of biomass used in production of bioenergy in the U.S., grassland- and agriculture-based biomass materials are likely to play a bigger role in the national energy portfolio over time. Agricultural systems – including lands in row-crop production and grasslands, are the source of four types of biomass materials: sugars, starches, non-woody lignocellulosic materials and woody lignocellulosic materials.
Starches and sugars make up only a small portion of plant matter. The majority of most plants consists of cellulose?, hemicellulose and lignin. Cellulose and hemi-cellulose are made up of chains of sugars. Advanced bioethanol technology can break down these chains into fermentable sugars. As a result, ethanol can be produced from nearly any plant material. Cellulose and hemicellulose can be readily used in thermal and thermochemical conversion processes. Therefore, grasslands and agricultural systems are a vast and important potential sources of feedstocks for every bioenergy product and end-use. Agriculture-based biomass materials come from annual commodity crops like corn and soybeans, residues collected after harvest of annual crops grown for food or feed, and perennial crops such as grass and tree crops (figure 3).
Figure 3. Sources of biomass from agricultural systems. (CL Williams, 2011.)
Agriculture-based biomass materials can be grown in production systems that are dedicated for bioenergy purposes, or as a secondary benefit from systems dedicated to plant production for other purposes (figure 3). That is, dedicated bioenergy cropping systems can include annual crops grown for their sugars or starches, such as sugarcane and grains, and perennial herbaceous and woody crops grown for their cellulose.
Annual bioenergy crops
Today, corn grain and soybeans are the primary feedstocks used in the U.S. to produce liquid transportation fuel (ethanol and biodiesel, respectively). Nearly all of the bioethanol produced in the U.S. is made from corn grain. Since most of the corn grown in the U.S. is in the Midwest and Upper Midwest, the greatest concentration of ethanol plants is also in the Midwest and Upper Midwest (figure 4). Biodiesel is an alternative fuel made from vegetable oils. Soybean oil is the dominant oil used in biodiesel production in the U.S. Biodiesel can be blended with petroleum-based diesel. Biodiesel is made through a refining process called transesterification.
Annual crops, including those used for biofuel? production, germinate, flower and die in a single year or season. They must be replanted each year or, depending on the crop, they may germinate voluntarily from seeds that fell to the ground before harvest or during harvest operations. Annual crops, like corn, are a dedicated bioenergy crop only when they are grown specifically for bioenergy, such as when a farmer grows the crop under contract with a bioenergy company like an ethanol producer. However, corn and soybeans can be grown and sold to commodity firms who then broker their sale to other companies which may include food producers, bioenergy producers or industrial chemical producers, for example. This is an example of a production system not dedicated to bioenergy production.
Figure 4. Midwestern states are the location of a high concentration of bioethanol plants, and the largest supply of corn grain. (USDA.)
In annual cropping systems, above ground, non-grain portions are leftover after harvest, including stalks, leaves, chaff, husks etc. These residues are composed of lignocellulose and may also be used as bioenergy feedstock. The residues of food crops may be burned to produce biopower? (i.e., electricity) or converted to liquid fuels, energy gases and chemicals through a variety of conversion technologies. However, the conversion of residues to biofuels is not yet widely available due to scale?-up challenges in commercial production systems. Food crop residues are considered a secondary benefit, not the primary purpose of a crop.
Figure 5. Corn stover? gathered into large square bales. (NREL.)
In Wisconsin and the Midwest, corn stover (stalks and leaves) is recognized as the leading candidate for production of cellulosic bioethanol because it is abundant and relatively inexpensive. Today, most stover is collected after grain is harvested (figure 5). That is, the farmer must make additional passes with equipment over the field to collect and bale stover. Increased trips over fields increases costs (i.e., fuel use) and increases soil compaction, To improve fuel efficiency and soil quality, researchers are developing single-pass systems in which grain and stover are harvested at the same time and each fraction separated, processed and transported from field machinery in separate streams.
Perennial bioenergy crops
Perennial plants in the Midwest grow over the spring and summer, die back every autumn and winter, and then return from root-stock the following spring. In the case of trees, seasonal growth occurs from the previous season’s standing wood. Perennial bioenergy crops include herbaceous and woody crops grown for their cellulose. Leaves, stems and stalks - the cellulosic parts of plants, may be burned directly through combustion to generate electricity, or these materials can be converted to liquid fuels, energy gases and chemicals through a variety of conversion technologies. Perennial herbaceous crops include grasses? like switchgrass? and Miscanthus?. Perennial non-forest woody crops include hybrid poplar? (figure 6) and other fast-growing trees. Herbaceous plants may be grown in a monoculture?, or in mixtures of varying diversity. Woody crops are typically grown in monocultures. Perennial crops have received considerable attention as bioenergy feedstock sources because they provide both long-term yield potential and environmental benefits like wildlife habitat, soil erosion prevention, and water quality improvement.
Figure 6. Hybrid poplar trees are a fast-growing perennial woody bioenergy crop suitable to many locations in the U.S. (NREL.)
When agronomically managed, perennial grasslands can be maintained in long-term rotations (10+ yr) offering many advantages over traditional annual crops. In Midwest experiments, high diversity grassland mixtures (figure 7) produced higher biomass yields than monocultures of switchgrass at the same location. Cool-season grasses, such as orchard grass, timothy, Kentucky bluegrass, and Canada wild rye, are prominent livestock forages, but not generally recommended for bioenergy harvest because of their poor feedstock quality. Perennial, warm-season tall-grasses currently demonstrate the most promise for yield and feedstock quality of all grassland plants. Warm-season grasses however, are slow to establish; depending on the species, peak yields are not usually achieved until 3 to 5 years after planting. Even with slow establishment, however, overall operating costs may be lower than conventional row crops since fields do not need to be planted and fertilized every year, and therefore reducing fuel and labor expenses.
Figure 7. Natural grasslands are being considered as potential sources of biomass feedstocks when periodic ecosystem management activities may result in harvest of merchantable materials. (Cordelia McGehee, 2003.)
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