Switchgrass production for biomass (Research Brief #51)
Posted January 2001
Switchgrass offers a wide range of environmental benefits as an alternative energy crop. Switchgrass can reduce soil erosion and provide warm season pasture for most ruminants. Harvesting this crop in August can improve habitat for grassland birds. Burning switchgrass together with coal can generate electricity with lower emissions than coal alone.
Currently, no biomass crop is economically competitive with coal. But with changes in energy policy and technology, switchgrass offers promise on highly erodible lands formerly enrolled in the Conservation Reserve Program (CRP).
A 1996-1997 Wisconsin study evaluated the co-firing of switchgrass and coal in a power plant to generate electricity. With support from the Center for Integrated Agricultural Systems (CIAS), a team from the UW-Madison departments of agronomy and mechanical engineering worked with researchers from the Wisconsin Department of Natural Resources to study this biomass crop from harvest to smokestack emissions. (Researchers are listed on page two.)
Increased concerns about global warming and air pollution have raised interest in alternative energy. Biomass crops are renewable, potentially less polluting than conventional energy sources, and typically do not add to environmental levels of carbon dioxide. Several herbaceous plants and trees are candidates for biomass crops. Switchgrass is a perennial warm season grass native to the North American prairie from the Gulf of Mexico to Canada.
“For biomass purposes, several warm season grasses are interchangeable because of similar yield and growing characteristics,” says Laura Paine, formerly with the UW-Madison Agronomy Department, “but switchgrass is commonly used in biomass studies because it is the easiest to establish.” It grows three to six feet tall in clumps, providing excellent wildlife habitat. The leaf litter provides shelter for animals while protecting the soil from erosion. This hardy species tolerates drought and annual cutting.
Switchgrass is harvested using conventional haying equipment. An annual cutting in fall or winter suffices for biomass production since its value is based on total tonnage, not nutritional quality.
The researchers harvested switchgrass from five CRP fields in southern Wisconsin in August, 1996. A total of 150 acres was harvested, from fields ranging in size from 2 to 19 acres. The large, round switchgrass bales averaged 740 pounds each, with an average yield of 2.3 tons per acre. Yields of six to eight tons per acre are possible with fertilization.
Researchers monitored a harvested and unharvested area in each field for vegetative structure and bird activity during the months of May, June, and early July, 1997 to coincide wtih the grassland bird breeding season.
Energy production characteristics
Switchgrass is not only a renewable source of fuel, it is also environmentally friendly. “Unlike fossil fuels, it recycles atmospheric carbon and creates smaller amounts of atmospheric pollutants, such as sulfur and nitrogen oxides, than fossil fuels,” says Danny Aerts, researcher in the UW-Madison Mechanical Engineering Department.
When switchgrass is burned, potassium compounds in the grass are deposited in the combustion chamber. This process is called slagging. To prevent slagging, switchgrass was mixed with coal at a ratio of 90 percent coal and ten percent switchgrass. Since the researchers observed no slagging at this level, the percentage of switchgrass could be increased to further reduce emissions. “The amount of switchgrass included is limited by the current burner design, but technical modifications would allow a greater percentage of switchgrass to be burned,” says Aerts.
Even with switchgrass at only ten percent, the co-firings had lower emissions than coal alone for two out of three pollutants tested. Opacity-the amount of particulate matter in the smoke plume leaving the power plant-was reduced by 50 to 60 percent when switchgrass and coal were fired together compared to combustion of coal alone. Dark plumes of smoke are caused by particles from incomplete combustion, and switchgrass helps make a more complete burn.
Nitrogen oxide emissions were reduced by 17 percent at 40 megawatts with the ten percent switchgrass co-firing compared to coal only combustion. At 49 megawatts, nitrogen oxide emissions were reduced by 31 percent. Power plants are not currently required to meet standards for nitrogen oxides, but will in the future.
Sulfur dioxide emissions did not differ between co-firing tests and coal-only combustion. Power plants are required to meet standards for sulfur dioxide emissions, and those that exceed limits can buy credits from those with emissions below the standard.
On a heating value basis based on 1997 prices, switchgrass costs about the same as natural gas, but 3.7 times as much as coal. The cost of growing and harvesting the switchgrass was overstated in the study because of the small scale and transportation and storage complications.
If the switchgrass had been harvested, transported, and processed in the most efficient way, total costs would have been about $23 per bale. Harvesting costs included $6 per acre for cutting, $6 per acre for raking, and $8 per bale for baling. Transportation costs were included at $8 per bale. A profit margin for the farmer was not included, as the switchgrass for this study was harvested from CRP land. The costs do not include fertilization expenses. Increased yields from fertilizer applications should more than cover costs.
Grassland bird habitat
Both harvested and unharvested switchgrass areas in the study provided habitat for declining grassland birds. Grassland bird populations have undergone a steady decline, documented since the 1960s. The study looked at several species of grassland birds identified by the Wisconsin DNR as having low or restricted populations.
Researchers measured the vegetative structure of the areas in the study. Different bird species prefer a different range of conditions, from tall, dense vegetation to lower, sparse vegetation with patches of bare ground.
Harvested areas experienced very little regrowth following the August cutting. But these areas began to grow earlier the following spring than unharvested areas. This difference disappeared by July. Harvested areas had higher percentages of bare ground and lower levels of standing residual than unharvested areas. The litter layer was thinner and the standing residual was shorter in the harvested area. These areas also had a higher percentage of legumes and non-grass herbaceous plants than unharvested areas.
The research showed that several species of birds, such as the American goldfinch, ring-necked pheasant, and song sparrow, tolerate both harvested and unharvested switchgrass areas. Unharvested areas were inhabited by birds like the dickcissel, Henslow’s sparrow, and sedge wren that prefer tall, dense vegetation. Birds preferring harvested areas include the bobolink, and Eastern and Western meadowlark.
The researchers found that harvesting switchgrass in August created habitat the following year favoring six declining grassland bird species. These birds require short, structurally diverse vegetation. Overall, there were more pairs of grassland birds in harvested than in unharvested parts of the fields. Providing a balance between harvested and unharvested areas creates habitat for the greatest number of declining grassland bird species.
Switchgrass could offer an environmentally sound crop on highly erodible land if energy economics change. Changes in energy policies favoring renewable fuel sources, green pricing, and improvements in technology are all possibilities that could make switchgrass a competitive fuel. Its value as a fuel and for bird habitat make it a possible crop for former CRP ground.
Researchers involved in switchgrass project
The researchers included Dan Undersander from the UW-Madison College of Agricultural and Life Sciences department of agronomy, Laura Paine formerly with the department of agronomy and now crops and soils extension faculty in Columbia County, and Ken Ragland and Danny Aerts from the UW-Madison College of Engineering, department of mechanical engineering.
They were assisted by Jerry Bartelt and Dave Sample from the Wisconsin Department of Natural Resources Bureau of Integrated Sciences Services. The project was also supported by Madison Gas and Electric Company, the Great Lakes Regional Biomass Energy Commission, and the Electric Power Research Institute.
Published as Research Brief #51