Process Development of Bioethanol Production from Wheat and Barley Residues Steam Pretreatment and SSF
Sammanfattning: Bioethanol has received increasing attention as an alternative to petrol due to the current dependency on supplies from other countries and due to the increased concentration of the greenhouse gas carbon dioxide in the atmosphere. In this thesis, the development of 2nd generation production processes for bioethanol production from the lignocellulosic part of biomass is discussed. The conversion of biomass to bioethanol is divided into three main steps: pretreatment, enzymatic hydrolysis and fermentation (SSF), which is the enzymatic hydrolysis step combined with the fermentation step. The raw materials were common agricultural residues: wheat straw, barley straw and dry-mill residues. Residues rich in lignocellulose are produced in the starch-to-ethanol dry-mill process. Microwave pretreatment of the residues showed that the ethanol yield in dry-mill processes could be improved by 14% when the dry-mill residues were also converted to ethanol. Investigation of steam pretreatment conditions for wheat straw and barley straw showed that they have different optima. Steam pretreatment of wheat straw resulted in high yields of glucose and xylose at the same pretreatment condition, which is exceptional. This was not the case for barley straw, which required different pretreatment conditions to obtain high yields of glucose and xylose. Sulphuric acid was used as a catalyst in pretreatment and the acid-impregnation techniques of soaking and spraying the straw were compared. The pretreatment was found to be more severe on the straw with soaking due to the buffering capacity of the straw. Various methods of increasing the ethanol yield and concentration in SSF were also investigated. Results showed that a concentration of water-insoluble solids of 10% in SSF resulted in an ethanol yield of only 50% using baker's yeast (Saccharomyces cerevisiae) cultivated on sugar solution. However, the ethanol yield and the concentration increased to 89% and 30 g/l, respectively, when the yeast was cultivated in hydrolysate. Prehydrolysis prior to SSF was also investigated. A decrease in viscosity of the steam-pretreated material was observed, which improved the mass transport of substrates, products, enzymes and yeast. However, it was found that prehydrolysis decreased the ethanol yield. Approximately 20% of the straw is xylose. Co-fermentation of glucose and xylose in SSF is a necessity to obtain high ethanol yields, and in this work xylose-fermenting recombinant strains of S. cerevisiae (TMB3400, FT30-7 and FT30-13) were studied. It was found that a slow release rate of glucose during fermentation increased the xylose consumption. Controlled enzymatic hydrolysis in SSF is thus necessary to increase the xylose consumption and to increase the overall ethanol yield.
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