Switchgrass Ethanol Yields Large Net Energy Gain

The five year study also found greenhouse gas emissions from cellulosic ethanol made from switchgrass were 94 percent lower than estimated greenhouse gas emissions from gasoline production.

"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," said principal researcher Ken Vogel, a U.S. Department of Agriculture-Agricultural Research Service geneticist in the university's agronomy and horticulture department.

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.

Plant geneticist Ken Vogel examines a field of young switchgrass. (Photo courtesy UNL)

Technology to convert biomass into cellulosic ethanol is now at the development stage. Six small commercial scale biorefineries are being built with scale-up support from the U.S. Department of Energy.

Vogel's 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.

The study took place on 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 the university 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.

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.

By contrast, corn grain ethanol biorefineries must 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.

But higher yields compared to corn can be expected in the future, said Vogel, who points out that corn grain conversion technology is mature, while cellulosic conversion efficiency technology is still developing.

Vogel said he does not expect switchgrass to replace corn or other crops on Class 1 farm land. He and his colleagues are developing the grass for use on marginal, highly erodible lands similar to that currently in the federal Conservation Reserve Program. All the fields in this study met the qualifying criteria for that program.

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 baseline 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.

Switchgrass in this study employed UNL's best management practices for switchgrass, including no-till seeding, herbicides, weed control and adaptive cultivars.

Six cellulosic biorefineries that are being co-funded by the U.S. Department of Energy also are in the works across the United States. These plants are expected to produce more than 130 million gallons of cellulosic ethanol per year.

Researchers reported their findings in the current issue of "Proceedings of the National Academy of Sciences," online at: http://www.pnas.org/cgi/content/abstract/0704767105v1.

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