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Understanding Variation in Switchgrass Biomass Composition

M. R. Schmer, K. P. Vogel, R. B. Mitchell, B. S. Dien, H. G. Jung, M. D. Casler
CSA News Magazine 2012 v.57 no.3 pp. 14
foods, enzymes, field experimentation, business planning, feedstocks, scientists, Panicum virgatum, petroleum, cell walls, glucose, industry, xylose, bioenergy, near-infrared spectroscopy, weather, forage, cell wall components, cultivars, equations, ethanol production, teams, biomass, ethanol, prediction, Agricultural Research Service, microorganisms, growing season, pentoses, researchers, United States
Switchgrass is being developed as a cellulosic ethanol feedstock for the temperate regions of North America. Cellulosic ethanol sources, such as switchgrass, have the potential to displace a significant portion of current United States petroleum consumption. Cellulosic refineries using biochemical processing will convert biomass cell wall carbohydrates into simple sugars and microorganisms will then ferment the simple sugars into ethanol. A reliable switchgrass supply will be important to cellulosic refineries to ensure stable operational costs. Switchgrass ethanol production (L ha-1) is influenced by both biomass yield and biomass composition. Switchgrass biomass yield potential data is becoming available but the potential effects of production practices, cultivars, and environment on switchgrass biomass composition and subsequently on ethanol yields has not been previously determined. To better understand the extent of switchgrass biomass composition variation, USDA-ARS researchers utilized newly developed near-infrared reflectance spectroscopy (NIRS) calibrations for switchgrass biomass to estimate biomass composition. With this information, the ethanol yield per ton of switchgrass biomass can be calculated. Near-infrared reflectance spectroscopy is a non-destructive technology that uses the spectral characteristics of a sample to obtain composition information and has been widely used in the food, forage, and pharmaceutical industries. The switchgrass biomass NIRS composition calibrations which were developed by a team of USDA-ARS scientists are able to accurately estimate 20 components in switchgrass biomass including all cell wall and other carbohydrates which determine the amount of ethanol that could be produced in a biorefinery. In the January-February 2011 issue of Agronomy Journal, switchgrass NIRS prediction equations were used to estimate cell wall composition and theoretical ethanol yields on both a per ton and per hectare basis from 10 switchgrass fields in a three state region. Ethanol conversion rates differed across locations and over time but ethanol conversion rates (L Mg-1) within a switchgrass field from a single growing season were relatively stable. Results indicate that the amount of hexose (six-carbon) sugars and pentose (five-carbon) sugars varied over years which impacted ethanol conversion rates. Cell wall glucose was the primary hexose sugar and xylose was the primary pentose sugar. Annual biomass yield and cell wall glucose concentrations were positively correlated across all experimental field locations. According to the authors, it should be feasible to increase ethanol production by improving both biomass yield and conversion efficiency. Proper management and improved switchgrass cultivars can reduce biomass composition variation but weather conditions will have a strong impact on ethanol yield potential. Results of this study demonstrate the biorefineries can expect significant year-to-year variation in biomass cell wall composition from a given switchgrass field and across production regions. Cellulosic refineries will need to consider this variation in biofuel yields when implementing their biochemical conversion technology and business plans. Switchgrass bales will need to be sampled prior to conversion to quantify cell wall composition. Results from sampled switchgrass bales could possibly be used to modify pretreatment and enzyme requirements to optimize ethanol conversion rates. The USDA-ARS switchgrass NIRS bioenergy calibrations provide a rapid, cost-effective method to accurately estimate switchgrass cell wall composition. A non-funded cooperative agreement between the USDA-ARS and the Near Infrared Spectrometry Consortium (NIRSC) (http://nirsconsortium.org/default.aspx) has been established for the purpose of transferring the NIRS calibrations for switchgrass composition to other public and private laboratories and to industries that are using or will use switchgrass for biomass energy.