Development of integrated cellulose- and starch-based ethanol production and process design for improved xylose conversion

Transportation fuels from renewable resources such as ethanol are one of the alternatives to ensure energy security and decrease the net emission of carbon dioxide. First-generation ethanol production from sugar- and starch-based raw materials (1G) is today well established in many countries, and the focus of research has thus shifted to the development and demonstration of the production of second-generation bioethanol from lignocellulose (2G). This thesis deals with the development of process configurations for bioethanol production from wheat straw integrated with wheat starch-based ethanol production. One part of the work focused on integration in simultaneous saccharification and fermentation (SSF) of steam-pretreated wheat straw (SPWS) with pre-saccharified, completely saccharified or fermented wheat meal using baker’s yeast, Saccharomyces cerevisiae. Mixing wheat straw and pre-saccharified or saccharified wheat meal was shown to be beneficial for both 1G and 2G ethanol production. Not only the ethanol concentrations, but also the ethanol yields, increased when pre-saccharified wheat meal was mixed with SPWS. The highest ethanol yield achieved was higher than that obtained with SSF of either raw material alone. Ethanol yields above 80% of the theoretical (from the hexose sugars) and ethanol concentrations of about 6% (w/v) were achieved in batch SSF. Ethanol concentrations at such levels reduce the energy demand in distillation, thus lowering the production cost. Since wheat straw contains a large amount of xylose, integrated process configurations were developed and investigated in an attempt to improve xylose utilization by a xylose-fermenting, genetically modified strain, S. cerevisiae TMB3400, in the second part of the work. The most promising configuration for co-fermentation of glucose and xylose was separate hydrolysis and co-fermentation of SPWS, as this allowed the glucose concentration to be controlled by the wheat-starch hydrolysate feed. An ethanol yield of 92% was achieved after fermentation based on glucose and xylose, and almost complete xylose consumption was achieved. In the last part of the work, differences in the performance of two mutated strains of S. cerevisiae TMB3400 were revealed. It was shown that KE6-13i was more tolerant to inhibitors, while KE6-12 performed better in less inhibitory environments