(325c) Conversion of Switchgrass Into Fermentable Sugars by Electrolyzed Water Pretreatment and a Two-Stage Pretreatment

Biofuel is a carbon-based and renewable energy source that is originally from solar energy captured and stored in the form of carbohydrate in plants. Biofuel can be produced from monomeric sugars released from plants by fermentation into ethanol, butanol, and other transportation fuels. Switchgrass (Panicum virgatum) is a warm season perennial grass that is native to North America. Besides a high cellulosic content, switchgrass also has some significant advantages, such as being CO2-neutral, pest and disease resistance, natural wildlife habitat, drought and flood tolerance, among others, making it one of the best energy corps. Electrolyzed water is a relatively new pretreatment catalyst which was first proposed and tested by scientists at University of Illinois at Urbana-Champaign (UIUC) in 2006. A two-stage pretreatment method for deconstructing biomass with a purpose of partially removing lignin and hemicellulose to achieve a mild pretreatment was also recently developed at UIUC. In this study, the two methods were evaluated for conversion of switchgrass into fermentable sugars. The fermentablility of hydrolyzates obtained from the two-stage pretreatment was also examined. Acidic electrolyzed water (AEW) and alkaline electrolyzed water (ALEW) were used as pretreatment catalysts to pretreat switchgrass at three temperatures (170°C, 185°C, and 200°C) and 3 treatment times (5min, 15min, and 25min). A solid loading of 20% w/w was used (8 g switchgrass (dry basis) to make a 40 g slurry) in pretreatment. The enzymatic hydrolysis was performed in 100mL VITLAB flasks in a shaker water bath at 50°C and 200rpm for 120h with a solid loading of 8%. The enzyme loading was: 15FPU cellulase/ g glucan and 40U beta-glucosidase/ g glucan. In two-stage pretreatment, 1.0%, 2.0% and 3.0% H2O2 solutions were used to partially remove hemicellulose and lignin during the 1st stage in a shaker water bath at 50°C and 150rpm for 16h, 24h, and 32h with a solid loading of 5%. The supernatants of the 1st stage were used for hemicellulose precipitation. The remaining solids were washed with DI water into VITLAB flasks and subjected to a 2nd treatment at 121°C for 30min in an auto-clave with a solid loading of 12.5%. After the 2nd treatment, the obtained slurry was used for enzymatic hydrolysis at a solid loading of 10%. The other conditions of the enzymatic hydrolysis were the same as those in the electrolyzed water pretreatment experiments except that the hydrolysis time was 96h. The highest glucose yield in the electrolyzed water pretreated samples was 29.75g glucose from 100g dry switchgrass when using ALEW at 200°C and 25min. Over 50% hemicellulose was removed in the 1st stage treatment of the two-stage process, and most of hemicellulose in the liquid portion could be recovered after a further treatment. Over 30g glucose was released from 100g dry switchgrass in the two-stage process. The hydrolyzate obtained from the treatment of 3% H2O2 solution for 16h was fermented by Baker yeast in a shaker water bath at 30°C for 96h without any concentration and detoxification. The final ethanol concentration is 18.89g/L and the productive yield was 119% of the theoretical yield. A mass balance for the two-stage process showed that 18g ethanol was produced from 100g dry switchgrass.