Research On Pretreatment And Saccharification Of Cotton Stalk For Bioethanol Production

Lignocellulosic biomass, as the resources for the production of second generationbioethanol, has caught great attention recently. But it is unrealistic to use a common method toconvert all types of biomass to bioethanol due to the complex structure of variouslignocellulosic biomass. The pretreatment of the biomass to alter the structure andcomposition is the key step during the conversion process, to enhance the rate of hydrolysisduring saccharification.Cotton stalk could be used to produce bioethanol by bioconversion because of its highcellulose content. But its high lignin content limits the accessibility of cellulose and the rate ofhydrolysis.This thesis will focus on the pretreatment of cotton stalk to remove the lignin in ethyleneglycol. Cotton stalk was pretreated with ethylene glycol at concentrations of70%~90%(v/v),liquid-solid rates of10:1~30:1, treatment times of0.5~2.5h, treatment temperatures of140~180℃, and with5‰HCl as catalyst. The modeling of organosolv pretreatment of cottonstalk was tried. The levels of ethylene glycol concentration, treatment time and temperaturewere optimized by using a Box–Behnken design to enhance cellulose content in the treatedsolid residue. And the pretreated cotton stalk was hydrolyzed at cellulase concentration of10~90FPU/g, substrate concentration of20~100g/L, treatment times of1~96h, treatmenttemperatures of40~60℃and pHs of4.4~5.2. The modeling of enzyme hydrolysis ofpretreated cotton stalk was tried. The levels of cellulase concentration, substrate concentrationand treatment time were optimized by using a Box–Behnken design to to optimize enzymaticsaccharification conditions. The key results showed as follows:The influences that ethylene glycol concentration, liquid-solid ratio, treatment time andtemperature had on cellulose and lignin content were significant and the models obtained hadstatistically significance. The influences on the cellulose and lignin content followed the orderof ethylene glycol concentration>treatment time>treatment temperature. The cellulose contentwas75.65%, higher than the original value34.83%, and the lignin content was7.12%, lowerthan the original value18.89%under the optimized conditions of ethylene glycolconcentration90%, liquid-solid ratio20:1, pretreatment time2.5h, and pretreatmenttemperature180℃. Images of Scanning Electronic Microscopy (SEM) showed that the original sample had a regular and compact surface, indicating a highly ordered surfacestructure. However, part of lignin was dissolved out during the pretreatment, thus the compactstructure of stalk was broken efficiently and some internal areas were exposed compared withthe image of untreated sample. Overall, the application of ethylene glycol had remarkableeffect on the pretreatment of cotton stalk.The influence of cellulase concentration, and treatment time had on reducing sugarsyield was highly significant, and the interaction between the two factors had a significantinfluence on the reducing sugars yield. And the model obtained had a statistically significance.The influences on reducing sugars yield followed the order of cellulaseconcentration>treatment time>substrate concentration. The reducing sugars yield was486.9mg/g, higher than the original value178.2mg/g under the optimized conditions ofenzyme concentration90FPU/g, substrate concentration47.2g/L, treatment time72h,treatment temperature50℃and pH4.8.The preliminary research of fermentation showed that fermentation underwent a fasterrate at the initial stage and the increment of ethanol yield was relatively larger. While after18h, the rate of fermentation slowed down and the increment of ethanol yield got smaller. Themaximum ethanol yield was47.3%at48h.