Switchgrass as an energy crop has untapped potential
A decade ago, switchgrass promised farmers an optional bioenergy crop. Researchers at the USDA-ARS Northern Great Plains Research Laboratory at Mandan, North Dakota, found the deep-rooted native perennial could store significant amounts of carbon in the soil – even on marginal land. Also, it outperformed corn in energy efficiency.
“For every unit of nonrenewable energy required to grow switchgrass, we got 6.4 units of energy back,” said agronomist Marty Schmer in a 2009 interview with Successful Farming magazine. “With corn grain, for every unit of nonrenewable energy put into producing the crop, there are 1.2 to 1.8 units of energy that come out.”
Schmer was part of a team of researchers conducting a five-year study evaluating switchgrass’s potential as an ethanol crop. A hardy native grass adapted to a wide range of growing conditions, the perennial has long proven its ability to reduce or eliminate wind and water erosion, provide feed for livestock, and offer habitat for wildlife.
Beyond showing its efficiency in producing ethanol, the research documented switchgrass’s ability to sequester significant and rapidly accruing levels of carbon in the soil. Data was gathered from 10 on-farm switchgrass fields in Nebraska, South Dakota, and North Dakota. Fields were on marginal land and were fertilized with nitrogen for maximum yield.
Though there were variations from site to site, soil organic carbon (SOC) within the top 12 inches of soil increased across all sites at an average rate of 980 pounds per acre per year. Four sites in Nebraska sampled to a depth of 48 inches had SOC increases averaging 2,590 pounds per acre per year.
“Given the levels of soil carbon sequestration we found, coupled with the high net energy balance and low emissions of greenhouse gases, switchgrass is a great energy crop for marginal cropland,” Schmer says.
Potential Remains Untapped
Today, the perennial’s potential as a bioenergy crop remains untapped. “Producing ethanol from switchgrass has never materialized at the scale we hoped it would,” says Schmer, who now works as a USDAARS research agronomist and adjunct assistant professor at the University of Nebraska-Lincoln.
High costs for processing the crop into fuel blocked its inroads into the market. Converting switchgrass into ethanol costs even more than the conversion of corn into ethanol because of the additional processing required.
Other forces have hamstrung the energy prospects for the crop, as well. “Oil production in the U.S. has increased dramatically in the last decade, lowering the cost of fuel,” Schmer says. “Along with that, there’s been an increase in car fuel efficiency and a steady adoption of electric-powered vehicles. As a result, demand for ethanol has not increased like we had expected.”
Beyond its promise for ethanol, switchgrass has also signaled its worth as a cofiring feedstock with coal for generating electricity. From 2007 to 2014, researchers at the University of Kentucky looked at the crop’s feasibility in this area.
“Our project showed switchgrass can be burned with coal, and farmers were interested in growing the crop,” says Ray Smith, Extension forage specialist at the University of Kentucky (UK). “Burning only 10% switchgrass with coal generates electricity with lower emissions than does burning coal alone.”
The goal of the UK research was to expand and diversify farm-income options for farmers while potentially contributing to the state’s renewable energy goals. With an eye on developing a market for switchgrass co-fired with coal for generating electricity, project manager Tom Keene engaged 20 farmers in northeastern Kentucky to plant 5 acres each of the perennial.
Taking Delivery of Switchgrass
The Spurlock Power Station at Maysville, Kentucky, owned by East Kentucky Power Cooperative, ramped up to take delivery of the switchgrass and co-fire it with coal in a unit modified to burn alternative fuels.
“At the time, it was looking like there could be state or federal mandates coming that would require plants to burn a certain amount of renewable fuels,” Smith says. Other encouraging forces included the possible emergence of a carbon-credit system rewarding use of renewable fuels.
Initially, farmers hauled switchgrass to the power station in big bales to be ground on-site. Then, high transportation costs and problems with the ground material feeding into the power system led researchers to produce briquettes made from blending the ground switchgrass with waste wood shavings. Unable to continue accessing wood shavings at low cost, the team then adopted an on-farm pelletizing of the switchgrass using mobile pelletizers.
Some growers adopted an efficient, whole-farm system of producing switchgrass. “They would take the first crop at the leafy stage for hay and harvest the switchgrass regrowth after frost for the power plant,” Smith says. “When the crop is harvested after frost, the nutrients, like nitrogen and potassium, have all gone into the roots.”
This not only saved producers fertilizer dollars but also benefited the power plant.
“High amounts of nitrogen cause the production of nitrous oxide after burning, and this pollutant must be scrubbed out of the power unit,” he says. “Potassium leaves salt residues that lead to corrosion of the boiler. The two-harvest system worked well, which benefited producers by having a low-maintenance perennial they could produce on marginal land.”
While the farmers’ switchgrass production replaced a small fraction of the power plant’s coal, the project showed proof of concept for its workability.
The political winds switched again, blowing away the near-term possibilities of either mandates or a significant carboncredit system rewarding broader adoption of renewable fuels such as switchgrass.
The relatively low cost of coal caused the sidelining of farmers’ switchgrass as an energy source. “The switchgrass cost the plant $20 to $60 more per ton than the per-ton price for coal delivered,” Smith says. “Delivering the switchgrass is a significant expense.”
Yet, he says, there’s still a huge justification for growing it for energy, as examples emerged of switchgrass working on a small scale. “In the broader scheme of things, we need a system that pays farmers fairly for their crop.”
Schmer agrees, pointing to ongoing research by diverse entities focused on producing other switchgrass bioproducts that could generate economic value for producers. “Much of our research is now also focusing on switchgrass production related to natural resources,” he says. “For instance, how do we best transition from a perennial system back into an annual system?”
New switchgrass cultivars (Liberty and Independence) developed by USDA-ARS and the University of Illinois at Urbana-Champaign have the potential to nearly double yields obtained from switchgrass crops grown from previous cultivars.
“Adding perennials to a rotation is especially beneficial to marginal cropland in order to build soil carbon and to reduce erosion,” Schmer says. “Perennials also reduce nutrient losses and benefit wildlife. Adding them back into the ag landscape benefits farmers as well as society as a whole.”