Biofuel: Major Net Energy Gain From Switchgrass-based Ethanol

Jan 14, 2008 - ScienceDaily

Switchgrass grown in this study yielded 93 percent more biomass per acre and an estimated 93 percent more net energy yield than previously estimated in a study done elsewhere. (Credit: Image courtesy of University of Nebraska-Lincoln)

Switchgrass grown for biofuel production produced 540 percent more energy than needed to grow, harvest and process it into cellulosic ethanol, according to estimates from a large on-farm study by researchers at the University of Nebraska-Lincoln.

Results from the five-year study involving fields on farms in three states highlight the prairie grass' potential as a biomass fuel source that yields significantly more energy than is consumed in production and conversion into cellulosic ethanol, said Ken Vogel, a U.S. Department of Agriculture-Agricultural Research Service geneticist in UNL's agronomy and horticulture department.

The study involved switchgrass fields on farms in Nebraska, North Dakota and South Dakota. It is the largest study to date examining the net energy output, greenhouse gas emissions, biomass yields, agricultural inputs and estimated cellulosic ethanol production from switchgrass grown and managed for biomass fuel.

"This clearly demonstrates that switchgrass is not only energy efficient, but can be used in a renewable biofuel economy to reduce reliance of fossil fuels, reduce greenhouse gas emissions and enhance rural economies," Vogel said.

The joint USDA-ARS and Institute of Agriculture and Natural Resources study also found greenhouse gas emissions from cellulosic ethanol made from switchgrass were 94 percent lower than estimated greenhouse gas emissions from gasoline production.

In a biorefinery, switchgrass biomass can be broken down into sugars including glucose and xylose that can be fermented into ethanol similar to corn. Grain from corn and other annual cereal grains, such as sorghum, are now primary sources for ethanol production in the U.S.

In the future, perennial crops, such as switchgrass, as well as crop residues and forestry biomass could be developed as major cellulosic ethanol sources that could potentially displace 30 percent of current U.S. petroleum consumption, Vogel said. Technology to convert biomass into cellulosic ethanol is being developed and is now at the development stage where small commercial scale biorefineries are beginning to be built with scale-up support from the U.S. Department of Energy.

This study involved 10 fields of 15 to 20 acres each with four in Nebraska near Atkinson, Crofton, Lawrence and Douglas; four in South Dakota near Highmore, Bristol, Huron and Ethan; and two in North Dakota near Streeter and Munich. Trials began in 2000 and 2001 and continued for five years. Farmers were paid for their work under contract with UNL and documented all production operations, agricultural inputs and biomass yields. The researchers used this information to determine the net energy estimates. Switchgrass grown in this study yielded 93 percent more biomass per acre and an estimated 93 percent more net energy yield than previously estimated in a study done elsewhere of planted prairies in Minnesota that received low agricultural inputs, Vogel said. The study demonstrates that biomass energy from perennial bioenergy crops such as switchgrass can produce significantly more energy per acre than low input systems. Less land will be needed for energy crops if higher yields can be obtained.

Researchers point out in the study that plant biomass remaining after ethanol production could be used to provide the energy needed for the distilling process and other power requirements of the biorefinery. This results in a high net energy value for ethanol produced from switchgrass biomass. In contrast, corn grain ethanol biorefineries need to use natural gas or other sources of energy for the conversion process.

In this study, switchgrass managed as a bioenergy crop produced estimated ethanol yields per acre similar to those from corn grown in the same states and years based on statewide average grain yields.

"However, caution should be used in making direct ethanol yield comparisons with cellulosic sources and corn grains because corn grain conversion technology is mature, whereas cellulosic conversion efficiency technology is based on an estimated value," Vogel said.

Vogel said he does not expect switchgrass to replace corn or other crops on Class 1 farm land. He and his colleagues are developing it for use on marginal, highly erodible lands similar to that currently in the Conservation Reserve Programs. All the fields in this study met the criteria that would have qualified for this program. Using a conservation cellulosic conversion value, researchers found that switchgrass grown on the marginal fields produced an average of 300 gallons of ethanol per acre compared to average ethanol yields of 350 gallons per acre for corn for the same three states.

The researchers point out that this was a base-line study. The switchgrass cultivars used in this study were developed for use in pastures. New higher yielding cultivars are under development for specific use in bioenergy production systems.

Switchgrass yields continue to improve, Vogel said. Recent yield trials of new experimental strains in the three states produced 50 percent higher yields than achieved in this study.

"Now, we really need to use an Extension effort to let farmers know about this new crop," Vogel said.

Richard Perrin, UNL agricultural economist, was the primary economic analyst for this study. Other authors were Marty Schmer, USDA-ARS agricultural science research technician and UNL doctoral student, and Robert Mitchell, USDA-ARS agronomist at UNL.

Decades of switchgrass research at UNL put scientists in the position to start studying the crop as a biomass energy source in 1990.

"UNL and the USDA-ARS have been pioneers in switchgrass research since the 1930s, domesticating it as a pasture grass,"Vogel said.

Vogel has led research to develop switchgrass cultivars for biomass production. The UNL-USDA team also has developed recommendations for how best to manage switchgrass to maximize biomass yields.

Future research will include further studies of improving management practices including work on improving establishment and harvesting methods, improving biomass yield, and improving conversion efficiency and net and total energy yields, Vogel said.

Switchgrass in this study employed UNL's best management practices for switchgrass, including no-till seeding, herbicides, weed control and adaptive cultivars. This study was also based on farm fields up to 20 acres instead of smaller research-scale plots typically less than about 100 square feet.

Six cellulosic biorefineries that are being co-funded by the U.S. Department of Energy also are in the works across the U.S. that should be completed over the next few years. These plants are expected to produce more than 130 million gallons of cellulosic ethanol per year, according to the U.S. Department of Energy.

Researchers reported their findings in the Proceedings of the National Academy of Sciences January 7.

Adapted from materials provided by University of Nebraska-Lincoln.