Study On Key Problems In Bioethanol Conversion From Lignocellulose

Lignocellulose converted to bioethanol is currently a hot subject in bioenergy conversion research. In this thesis, the key problems included pretreatment; enzymatic hydrolysis; fermentation and ethanol distillation in bioethanol conversion from straw were studied.Kinetic studies of hemicellulose hydrolysis of corn stover at low temperature were investigated in this thesis. Seaman model was used to analyze the experiments data and the kinetic parameters for xylose, glucose and furfural were obtained at different temperatures. The optimal conditions at different temperatures were obtained from the analysis of response surfaces, which were 5% H2SO4 at 80℃for 240 min with 13.21 g/L xylose; 5.5% H2SO4 at 100℃for 60 min with 18.73 g/L xylose and 2% H2SO4 at 121℃for 60min with 17.95 g/L xylose. Arrhenius equation was used to calculate the activity energy which was 56.3 kJ/mol.The central composite design was used to optimize the effect of acid concentration, treatment time and solid content on dilted sulfuric acid pretreatment of rapeseed strw at two temperatures. The results showed that the solid content was not significant on pretreatment at high temperature. After pretreated at 180℃, the xylose yield reached 68.4% and the concentration of xylose was 27.3 g/L at 1% sulfuric acid for 10 min, which was one time hiher than that of 10% solid content. However, the pretreatment was great affected by solid content at low temperature. After pretreated at 121℃, the xylose yield got to 73.6% at 10% solid content and the xylose concentration was 14.7 g/L at 2% sulfuric acid for 60 min. Scanning electron microscope(SEM) results showed that cellulose structure was more easily destroyed at high temperature than at low temperature.The effects of solid content on enzymatic hydrolysis and fermentation were studied in this thesis. The results showed that an acceptably higher solid content coul not prevent the enzymatic hydrolysis and fermentation process and on the contrary it could improve the final concentration of glucose and ethanol. The enzymatic hydrolysis and fermentation of rapeseed straw could be carried out at 25% solid content after pretreated at high temperature. And the final concentration of ethanol could get to 4.6% which was 2.07 times higher than that of 10% solid content. Moreover, the solid content could also reached 20% during enzymatic hydrolysis and fermentation after pretreated at low temperature and the final ethanol concentration was 3.5% which was 1.76 times higher than that of 10% solid content.Simultaneous ethanol fermentation and separation was carried out in a membrane distillation reactor. The ethanol production efficency, production rate and final ethanol concentration were compared between different fermentation mode, with membrane distillation and without membrane distillation. The results showed that fermentation with membrane distillation could separate ethanol from the broth which eliminated the inhibition of high ethanol concentration on fermentation. And the production efficency, production rate and final ethanol concentration were greatly improved in the fermentation with membrane distillation.