Study On Key Technologies In Ethanol Production From Corn Stover

Lignocellulosic biomass is a cheap, renewable, abundantly available resource, and its bioconversion to fuel ethanol is very meaningful in the sustaining development of our society and economy. In this dissertation, key technologies in ethanol production from lignocellulosic biomass including pretreatment, enzymatic hydrolysis of cellulose and hemicellulose, ethanol fermentation from hexose and pentose, and comprehensive utilization of lignocellulosic feedstock were investigated.Corn stover was pretreated by dilute acid, aqueous ammonia /dilute acid, lime, and sodium hydroxide, respectively, for enhancing the enzymatic susceptibility of substrate. Pretreatment with 2% NaOH at 85 °C for 75 min removed 73.9% lignin and almost all the acetate from the raw material, and the deligified corn stover residues were readily to degrade by cellulase complex. The alkaline liquor separated from the reaction mixture could be recycled for 3-4 times in repeated pretreatemnt batches.Enzymatic hydrolysis of corn stover by cellulase enzymes was thoroughly investigated to understand the mechanism of cellulase. It was found that the lignin removal plays a crucial role in enhancing enzymatic digestibility of substrate, and that hemicellulose removal is not necessary for high hydrolysis yield. The enzymatic hydrolysis of corn stover was highly affected by the cellulase complex composition. Supplementing cellobiase greatly reduced the inhibitory effect caused by cellobiose due to low cellobiase activity in T. reesei cellulase. Xylose and arabinose were detected out during the enzymatic hydrolysis of corn stover, indicating the hydrolysis of hemicellulose by xylanase in cellulase complex. It is difficult to get high reducing sugar concentration in batch hydrolysis due to the porous property of cellulosic substrate. Therefore fed-batch hydrolysis was performed to increase the reducing sugar concentration, thus helping to obtain higher ethanol concentration in subsequent ethanol fermentation.Effective conversion of xylose to ethanol is a key step for ethanol production from lignocellulosic materials. Sacchromyces cerevisiae has traditionally been used for ethanol fermentation, but it can't utilize xylose. Several yeasts, such as Pichia stipitis and Candida shehatae, are able to ferment xylose to ethanol. However, these strains are constrained by their low ethanol tolerance and the requirement of some aeration for optimal ethanol productivity. A recombinant yeast of Sacchromycescerevisiae ZU-10 was examined for its capability of fermenting xylose to ethanol. It was found that S. cerevisiae ZU-10 had the capability of high ethanol tolerance, stable genetics character and effective cofermentation of glucose and xylose to ethanol under anaerobic conditions. 50 g/L glucose and 30 g/L xylose were fermented to 32.8 g/L ethanol, giving a ethanol yield of 0.41. However, the growth rate of S. cerevisiae ZU-10 was slow and the strain was sensitive to inhibitors such as higher than 0.25 g/L acetic acid and higher than 0.08 g/L furfural.It was found that the recombinant yeast S. cerevisiae ZU-10 could effectively ferment glucose and xylose in corn stover enzymatic hydrolysate to ethanol under anaerobic conditions, and that 66.9 g/1 glucose and 32.1 g/1 xylose were fermented to 41.4 g/L ethanol, giving a ethanol yield of 0.418.. The ethanol yield was increased by 24.2% due to the conversion of xylose to ethanol by recombinant yeast S. cerevisiae ZU-10.In order to eliminate end-product feedback inhibition of glucose on cellulase, simultaneous saccharification and fermentation (SSF) of corn stover for ethanol production was performed with S. cerevisiae ZU-10. The results showed that glucose and xylose liberated from cellulose and hemicellulose in corn stover were rapidly fermented to ethanol by S. cerevisiae ZU-10. The bioconversion efficiency of corn stover was highly enhanced because of the elimination of end-product feedback inhibition.In view of low grow rate of recombinant S. cerevisiae ZU-10, the cells were immobilized by entrapping them into calcium alginate gels. Comparing with free cells, fermentation of corn stover hydrolysates by immobilized S. cerevisiae ZU-10 cells had the advantages of shorter period, enhanced tolerance to acetic acid and furfural, and convenient recycling of cells. During 12 repeated batches of ethanol fermentation of corn stover enzymatic hydrolysate by immobilized cells, glucose was completely utilized and more than 90% xylose was consumed during each batch, giving an average ethanol concentration of 40.4 g/L and ethanol yield of 0.41.During fuel ethanol production from corn stover, bioconversion of cellulose and hemicellulose in NaOH-pretreated corn stover to ethanol, lignin recovery and alkali-soluable hemicellulose sugar recovery could be integrated into a biorefinery scheme, thus realizing the comprehensive utilization of overall biomass. Based on integration of key technologies in ethanol production from corn stover, a model product line with the productivity of 300 tons ethanol per year has been set up inHenan Tianguan Fuel Ethanol Co. Lmt.The characteristics and novelty in this dissertation was obvious in the following aspects: cofermentation of glucose and xylose to ethanol by recombinant yeast S. cerevisiae ZU-10, ethanol production from hydrolysates of cellulose and hemicellulose in corn stover by recombinant yeast S. cerevisiae ZU-10, and integration of key technologies in ethanol production from lignocellulosic biomass. These results were meaningful not only in academic research, also in utilization of renewable lignocellulosic biomass, decreasing the production cost of fuel ethanol , and alleviating the world grain crisis.