A Comparison of Chemical Pretreatment Methods for Converting Cotton Stalks to Ethanol

Abstract

The objective of this study was to investigate the effectiveness of sulfuric acid, sodium hydroxide, hydrogen peroxide, and ozone pretreatments for conversion of cotton stalks to ethanol. Sulfuric acid, sodium hydroxide, and hydrogen peroxide at concentrations of 0.5, 1, and 2% (w/v) were used to pretreat ground cotton stalk samples at a solid loading of 10% (w/v). Treatment temperatures of 90 degrees Celsius and 121 degrees Celsius at 15 psi were investigated for residence times of 30, 60, and 90 minutes. Ozone pretreatment was performed in the liquid phase at 4 degrees Celsius with constant sparging. Lignin, carbohydrate, and moisture content analyses were performed on the pretreated solids. The pretreated solids from sulfuric acid, sodium hydroxide, and hydrogen peroxide pretreatment (at 2%, 60 min, 121C/15psi) showed significant lignin degradation and/or high sugar availability and hence were hydrolyzed by cellulases from Trichoderma reesei and β-glucosidase at 50 degrees Celsius. The results showed that time, temperature and concentration were all significant (p is less than or equal to 0.05) factors in delignification for NaOH and xylan removal for H2SO4. Sulfuric acid pretreatment resulted in the highest xylan reduction (95.23% for 2% acid, 90 min, 121C/15psi) during pretreatment and the lowest cellulose to glucose conversion during hydrolysis (23.85%). Sodium hydroxide pretreatment resulted in the highest level of delignification (65.63% for 2% NaOH, 90 min, 121C/15psi) and the highest cellulose conversion (60.8%). Hydrogen peroxide pretreatment resulted in significantly lower (p is less than or equal to 0.05) delignification (maximum of 29.51% with 2%, 30 min, 121C/15psi) and cellulose conversion (49.8%) than sodium hydroxide pretreatment, but had a higher (p is less than or equal to 0.05) conversion than sulfuric acid pretreatment. Ozone pretreatment showed no significant changes in lignin, xylan, or glucan contents with increasing time. Quadratic models using time, temperature, and concentration as numeric variables were developed to predict xylan reduction for H2SO4 pretreatment and lignin reduction for NaOH pretreatment. In addition, linear models relating a modified severity parameter (log Mo) combining the pretreatment parameters with xylan or lignin reduction were developed and resulted in R-squared values of 0.89 and 0.78, respectively.