Combined Effect of Alkali Soaking and Extrusion Conditions on Fermentable Sugar Yields from Switchgrass and Prairie Cord Grass

Abstract

Abstract: Serious efforts are on the move to meet the US DOE's target of replacing 30 percent of transportation fossil fuels with bio-fuels by 2025. Pretreatment methods using dilute acid, alkali, hydrothermal, steam explosion, AFEX, and controlled pH water are under investigation. A viable continuous pretreatment method might be found through extrusion due to its obvious merits. Extrusion pretreatments showed a significant improvement on sugar recovery from different biomasses. However, glucose and combined sugar recovery from extrusion pretreated switchgrass (SG) and prairie cord grass (PCG) was about 50 and 63.5%, respectively; there is still room to improve the sugar recovery. Hence, extrusion combined with other pretreatment method could improve the sugar recovery from SG and PCG. It is a well known fact that alkali acts as delignification agent at low concentration without degrading the carbohydrates. Therefore, the current study was proposed to use alkali soaking and extrusion in sequence as an effective pretreatment. In order to evaluate the combined effect of alkali soaking and extrusion, SG and PCG (2, 4, 6, 8 and 10 mm) was soaked at different alkali concentrations (0.5, 1.0, 1.5, 2.0, and 2.5 % w/v NaOH) for 30 min at room temperature and then extruded using a lab scale single screw extruder at various barrel temperatures (45-225°C) and screw speeds (20-200 rpm). An experimental design was developed using a central composite rotatable design in Design Expert software, which resulted in 36 treatments. The extruded samples were subjected to enzymatic hydrolysis and sugar measurement using HPLC as outlined in NREL LAPs. Statistical analysis confirmed that all the independent variables considered in this study had a significant effect on sugar recoveries. The following optimum pretreatment condition 180°C barrel temperature, 118 rpm screw speed, 2% alkali concentration, and 6 mm SG particle size resulted a maximum glucose, xylose, and combined sugar recovery of 90.5, 81.5, and 88%, respectively. The maximum glucose, xylose, and combined sugar recovery of 87, 85, and 82%, respectively were recorded for PCG at the optimum pretreatment condition of 114°C barrel temperature, 122 rpm screw speed, 1.70% alkali concentration, and 8 mm particle size.