Switchgrass grown for biofuel production produced 540% more energy than needed to grow, harvest and process it into cellulosic ethanol, according to an on-farm study by University of Nebraska researchers.
Results from the five-year study involving fields in three states—Nebraska, South Dakota and North Dakota--highlights the potential of this prairie grass as a biomass fuel source, says Ken Vogel, a USDA Agricultural Research Service geneticist at UNL. 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 joint USDA-ARS and UNL study also found greenhouse gas emissions from cellulosic ethanol made from switchgrass were 94% lower than estimated greenhouse gas emissions from gasoline production.
Researchers reported their findings last month in Proceedings of the National Academy of Science. The research paper that is available online at http://www.pnas.org/cgi/content/abstract/0704767105v1.
Vogel does not expect switchgrass to replace corn or other crops on the most productive farmland. Researchers are developing it for use on marginal, highly erodible land similar to that currently in the conservation reserve programs.
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% of current U.S. petroleum consumption, according to Vogel.
Technology to convert biomass into cellulosic ethanol is 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.
Switchgrass grown in this study yielded 93% more biomass per acre and an estimated 93% more net energy yield than previously estimated in a study done elsewhere of planted prairies in Minnesota that received low agricultural inputs, Vogel says.
Researchers point out in their 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 the five-year, researchers found that 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," says Vogel.
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.