Location: Bioenergy ResearchTitle: Conversion of switchgrass to ethanol using dilute ammonium hydroxide pretreatment: influence of ecotype and harvest maturity Author
|O Bryan, Patricia|
|Hector, Ronald - Ron|
|Mitchell, Robert - Rob|
Submitted to: Environmental Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2013
Publication Date: 6/1/2013
Citation: Dien, B.S., O'Bryan, P.J., Hector, R.E., Iten, L.B., Mitchell, R.B., Qureshi, N., Sarath, G., Vogel, K.P., Cotta, M.A. 2013. Conversion of switchgrass to ethanol using dilute ammonium hydroxide pretreatment: influence of ecotype and harvest maturity. Environmental Technology. 34(13-14):1837-1848. Interpretive Summary: Switchgrass is a perennial warm season grass that is being developed as a dedicated bioenergy crop by the USDA. Its advantages include high production, ability to grow on marginal farm lands, and it can be produced and harvested using conventional farm equipment. Switchgrass produced as bioenergy has been predicted to return 540% energy and decrease green house gas emissions by 97% relative to gasoline. Two important management decisions that a farmer needs to make are which variety to plant and when to harvest the biomass. This study addresses both questions by evaluating Cave in Rock and Kanlow varieties harvested at two and three maturities, respectively. Switchgrass was pretreated with dilute ammonia and converted to either sugars or ethanol using biochemical processes. It was determined that carbohydrate contents, available for ethanol fermentation, increased and conversion efficiencies decreased as the crop matured. The end result was that ethanol and sugar yields per kg of switchgrass were uniform. Ethanol yields were 184.4 – 202.0 l/Mg l/Mgf SG (dry basis). Our results recommend production of the Kanlow and harvesting at either anthesis or post-frost.
Technical Abstract: Switchgrass (Panicum virgatum L.) is a perennial C4 grass that is being developed as a bioenergy crop because it has high production yields and suitable agronomic traits. Five SG samples were included in this study that varied in ecotype and maturity. SG samples contained 317 – 385.0 g glucans / kg SG dry basis (db) and 579.3 – 660.2 g total structural carbohydrates / kg SG, db. Carbohydrate contents were greater for the upland ecotype versus lowland type and increased with harvest maturity. Pretreatment of SG with dilute ammonium hydroxide (8%w/w ammonium loading) at 170°C for 20 min was determined to be effective for preparing SG for enzymatic conversion to monosaccharides; glucose and xylose recoveries were 66.9 – 90.5% and 60.1- 84.2% of maximum, respectively and decreased with increased harvest maturity. Subsequently, pretreated SG samples were converted to ethanol by simultaneous saccharification and fermentation using engineered xylose-fermenting Saccharomyces cerevisiae strain YRH400. Ethanol yields were 145.5 – 159.4 l/ton of SG (dry basis) and followed a similar trend as observed for enzymatic sugar yields.