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Title: Lime Pretreatment of Switchgrass at Mild Temperatures for Ethanol Production

item XU, JIELE - North Carolina State University
item CHENG, JAY - North Carolina State University
item SHARMA-SHIVAPPA, RATNA - North Carolina State University
item Burns, Joseph

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2010
Publication Date: 4/10/2010
Citation: Xu, J., Cheng, J.J., Sharma-Shivappa, R.R., Burns, J.C. 2010. Lime Pretreatment of Switchgrass at Mild Temperatures for Ethanol Production. Bioresource Technology. 101:2900-2903

Interpretive Summary: Ethanol can be produced on a renewable basis from a variety of high-carbohydrate containing biomass as found in sugarcane, corn, sweet potato and from lignocellulosic biomass, as found in switchgrass. This study was conducted, using switchgrass as a source of lignocellulose, to explore a pretreatment source that was readily available, relatively non-toxic, yet would release the phenolic bonding within the cell wall matrix. This would provide access to the cellulose, and possibly hemicellulose, for potential monomer formation with subsequent conversion into ethanol. This was accomplished using three temperature variables including ambient temperature, 50oC and 121oC. Lime pretreatment was effective in improving the production of ethanol from switchgrass. Increasing temperature from ambient to 121oC during lime treatment, increased cross -linked lignin molecules which actually interfered with the reduction of lignin. However, as long as the bonds were disrupted through calcium ion bonding, the biomass porosity was improved and breakdown (digestibility) was greatly increased. Furthermore, calcium bonds led to improved carbohydrate preservation, in spite of greater lignin concentrations, and subsequently total simple sugar production. The data reported provides the basis for future conversion-plant scale-up studies using methodology that is relatively inexpensive, safe to handle, and environmentally friendly, all important for industrial application.

Technical Abstract: Lignocellulosic biomass is a promising alternative feedstock for ethanol production. However, its recalcitrant structure necessitates a pretreatment step to break up the lignocellulosic matrix, thus improving the accessibility of carbohydrates to hydrolytic enzymes for sugar production. In this study, lime pretreatment of switchgrass was explored at 121 oC, 50 oC, and room temperature, and the effects of residence time, lime loading, and biomass washing on the sugar production efficiency were investigated. Pretreatments were evaluated based on the yields of biomass-derived sugars in the following enzymatic hydrolysis. Under the best pretreatment conditions (50 oC, 24 h, 0.1 g Ca(OH)2/g raw biomass, and wash intensity of 100 ml water/g raw biomass), the yields of glucose, xylose, and total reducing sugars reached 239.6, 127.2, and 433.4 mg/g raw biomass, which were respectively 3.15, 5.78, and 3.33 times of those of untreated biomass. The study on calcium-lignin bonding showed that calcium ions crosslinked lignin molecules under alkaline conditions, which substantially decreased lignin solubilization in pretreatment, but the resulting high lignin contents of the pretreated biomass did not compromise the improvement in enzymatic digestibility.