Ethanol Production Potential of Acid Pretreated Switchgrass Varieties

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Title: Ethanol Production Potential of Acid Pretreated Switchgrass Varieties
Author: Yang, Ying
Advisors: Jay J. Cheng, Committee Member
Joseph C. Burns, Committee Member
Ratna R. Sharma-Shivappa, Committee Chair
George C. Allen II, Committee Member
Abstract: Three new experimental switchgrass germplasms (St6-1, St6-3E and St6-3F) containing 22.71 to 30.95% glucan, 13.27 to 19.37% xylan and 17.39 to 20.60% lignin (on dry matter basis) were studied. Oven- or freeze-dried switchgrass whole-plant samples were pretreated with dilute sulfuric acid at 10% solid loading in an autoclave at 121 °C/15 psi. The effect of three acid concentrations (0.5, 1.0 and 1.5% w/v) and residence times (30, 45 and 60 min) on switchgrass composition was investigated. Influence of plant part was also studied for freeze-dried samples that had been separated into stems and leaves prior to drying. The greatest hemicellulose removal observed was 83.58% for oven-dried and 85.87% for freeze-dried samples resulting from intense pretreatment involving greater acid concentration (1.5% w/v H2SO4) or longer residence time (60 min). Hemicellulose in leaf was easily solubilized during acid pretreatment and removed from the solids by effective washing. However, delignification was not significant during acid pretreatment and was limited to 10% for all samples investigated. For each oven- or freeze-dried germplasm, pretreatment conditions resulting in the least lignin content or greatest hemicellulose solubilization were selected for hydrolysis and fermentation. The pretreatment conditions selected based on lignin and hemicellulose content were: 60 min/0.5% H2SO4 and 60 min/1.0% H2SO4 for oven-dried St6-1, 30 min/1.0% H2SO4 and 60 min/1.5% H2SO4 for oven-dried St6-3E, 30 min/1.0% H2SO4 and 45 min/1.5% H2SO4 for oven-dried St6-3F, 45 min/1.0% H2SO4 (whole plant) and 45 min/1.0% H2SO4 (leaf) for freeze-dried St6-1, 60 min/1.0% H2SO4 (stem) and 60 min/1.5% H2SO4 (leaf) for freeze-dried St6-3E, 45 min/1.5% H2SO4 (whole plant) and 60 min/1.0% H2SO4 (leaf) for freeze-dried St6-3F. A cellulase (Novozymes NS 50013 cellulase complex) and cellobiase (Novozymes NS 50010 β-Glucosidase) mixture at an activity ratio of 1:4 FPU/CBU was added during hydrolysis at cellulase activities of 0, 15 and 30 FPU/g dry biomass. The effect of xylanase supplementation at 0.25% w/w dry biomass was also tested. Enzymatic hydrolysis was enhanced by acid pretreatments especially those resulting in greater hemicellulose solubilization. The greatest glucan to glucose conversion obtained was 104.70-106.65% for freeze-dried St6-3F leaf samples after being pretreated with 1.0% acid for 60 min at 121°C/15 psi and hydrolyzed by cellulase at 15 FPU/g dry biomass supplemented with xylanase or by cellulase alone at 30 FPU/g dry biomass. Addition of cellulase significantly impacted (P < 0.05) hydrolysis efficiency for all switchgrass samples tested while adding xylanase did not appreciably enhance glucose yield. Fermentation of switchgrass hydrolyzates by Saccharomyces cerevisiae (ATCC 24859) resulted in almost complete utilization of glucose for ethanol production, indicating that dilute acid pretreatment had no inhibitory effect on fermentation if pretreated solids were completely washed. The greatest ethanol yield from the most effective acid pretreatment was 0.082 g/g initial biomass obtained with oven-dried St6-3E and freeze-dried St6-3F whole-plant samples. A 60% theoretical ethanol yield at 0.092 g/g initial biomass was achieved through a 7-day simultaneous saccharification and fermentation (SSF) of oven-dried St6-3E switchgrass pretreated with 1.5% sulfuric acid for 60 min at 121°C/15 psi. Fermentation of the hydrolyzate from pretreated oven-dried switchgrass with immobilized yeast cells did not enhance ethanol yield compared with conventional fermentation. Based on switchgrass yields of 13450 kg/ha and its capability of producing 0.082 g ethanol/g initial biomass (consider cellulose conversion only), the estimated overall ethanol yield is 1398 L/ha switchgrass (149 gallon/acre), showing its potential as an energy crop for bioethanol production. However, a comprehensive economic analysis of lignocellulose-to-ethanol conversion and further investigation of treatment parameters are needed for facilitating the scale-up of this challenging process.
Date: 2008-11-10
Degree: MS
Discipline: Biological and Agricultural Engineering

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