The use of corn as a feedstock for producing the biofuel-ethanol has presented problems due to its linkage to the food supply. Efforts to find an alternative biomass feedstock have focused on cellulosic biomass that is not used in food manufacturing.

One viable feedstock is switchgrass. Dr. John L. Field, research scientist, Natural Resource Ecology Laboratory at Colorado State University in Fort Collins, Colo., says, “Switchgrass is a native perennial grass species that is found naturally throughout the central U.S. Switchgrass has been a focus in efforts to find a feedstock that is not linked to the food chain because it is well adapted to the climate and the landscape.”

In a previous TLT article, switchgrass and other biofeedstocks were converted to bio-oil in a process known as fast pyrolysis (1). Rapid heating at a rate of 1,000 C per second and a limited residence time in the reactor led to the pyrolysis occurring at 500 C. The resulting bio-oil is a complex mixture that can be used as a fuel in boiler applications.

Besides identifying a proper feedstock, there is a need to ensure that the environmental impact of producing it is minimal and will satisfy regulations such as the U.S. Renewable Fuel Standard. One aspect that needs to be considered is biogenic emissions, exchanges of greenhouse gases between an agricultural ecosystem and the atmosphere.

Field says, “A big issue is the organic carbon content present in the soil. Losing it as carbon dioxide leads to an increase in biogenic emissions and gaining it means that the Greenhouse Gas (GHG) footprint is reduced. Other factors that need to be considered include the amount of water and nitrogen fertilizer required. The latter is the source of nitrous oxide an even more potent GHG.”

The challenge in working with a crop such as switchgrass is to maximize crop yields while limiting biogenic emissions. Field says, “The current U.S. Renewable Fuel Standard requires 56 grams of carbon dioxide equivalent (gCO₂) per megajoule (MJ^-1) lifecycle emissions reduction compared to the 93 gCO₂ MJ^-1 emissions attributed to gasoline.”

Field continues, “Switchgrass has an important advantage as a biomass feedstock because it requires only small amounts of fertilizer and water to grow.”

To demonstrate the potential for using switchgrass as a feedstock, Field and his colleagues conducted a modeling study from a 30,000-foot view to simulate how targeted landscape design can produce high crop yields in combination with a reduction in biogenic emissions.

Southwestern Kansas
The researchers evaluated the potential for growing switchgrass in the area near a currently idle celluloslic biorefinery in the southwestern region of the U.S. state of Kansas. The study included not only the county where the biorefinery is located but also all adjacent counties that cover a total area of approximately 4 million acres. Figure 3 shows an aerial view of one part of the study area.


Figure 3. One part of the area in the Southwest section of Kansas is shown where researchers conducted a modeling study to show how switchgrass can be grown in an optimal manner while minimizing GHG emissions. (Figure courtesy of Colorado State University.)

Field says, “We took into consideration such engineering issues as supply chain emissions, the amount of diesel fuel used in growing the switchgrass and transporting it to the biorefinery. Ecological issues included where on the landscape switchgrass was grown, and the optimal amount of fertilizer required also were used in the study.”

The researchers used the DayCent ecosystem model that had been developed at Colorado State University several decades ago and widely used. This model estimated not only crop yield but also biogenic GHG emissions. 

Switchgrass production was simulated on a variety of land types with different soil, clay, sand and silt content, including marginal lands such as those classified under the Conservation Reserve Program, which are considered to be environmentally sensitive. But growing switchgrass on these lands led to higher emissions footprints due to a reduced ability to store additional organic carbon in the soil. 

The researchers found that optimizing to reduce GHG emissions led to the greater use of silty soils that have a greater capacity to store organic carbon and less use of fertilizer. Doing so could reduce the footprint of biofuel production by 22 gCO₂ MJ^-1. 

Field says, “We intend to more closely evaluate marginal agricultural land, which is on the edge of what is economically sustainable. This land type is often vulnerable to erosion and has lower crop yields. We believe that the use of a switchgrass, a perennial, will be advantageous because it will leave the soil less vulnerable to erosion.”

Additional information on the modeling analysis can be found in a recent article (2) or by contacting Field at John.L.Field@colostate.edu. 

REFERENCES
1. Canter, N. (2013), “Production of bio-oil,” TLT, 69 (4), pp. 10-11.
2. Field, J., Evans, S., Marx, E., Easter, M., Adler, P., Dinh, T., Willson, B. and Paustian, K. (2018), “High-resolution techno-ecological modelling of a bioenergy landscape to identify climate mitigation opportunities in cellulosic ethanol production,” Nature Energy, 3 (3), pp. 211-219.