Development of Simultaneous Saccharification and Fermentation for Production of Ethanol from Softwood

Sammanfattning: This thesis deals with the development of simultaneous saccharification and fermentation (SSF) in order to improve the economy of the softwood-to-ethanol process. The effect of process stream recirculation on ethanol production from steam-pretreated softwood based on SSF was investigated for two process cases. In the first case, part of the stillage stream after distillation was recycled and in the second case the liquid after SSF was recycled. The aim was to minimize the energy demand in the distillation of the fermentation broth and in the evaporation of the stillage, as well as the use of fresh water. It was possible to replace of 60% of the fresh water by recirculation of stillage, while maintaining the ethanol yield. This would result in a reduction in the ethanol production cost of 17%. In the second case, where the stream after SSF was recirculated, up to 40% of the fresh water could be replaced without affecting the final ethanol yield, which would result in a 12% reduction in the estimated production cost. Recirculation of the condensates from the evaporation step to SSF could be employed to replace all the fresh water without affecting the ethanol yield. This will make it possible to further reduce the use of fresh water and thereby reduce the volume of wastewater. Increased water-insoluble material (WIM) content in SSF often gives rise to severe inhibition of yeast metabolism due to the increased concentration of toxic compounds formed in the pretreatment step. One way to overcome this problem is to adapt the baker’s yeast to the increased amount of inhibitors in SSF by cultivating it on steam pretreatment hydrolysate. The WIM load was increased from 5 to 8% while maintaining the ethanol yield when the adapted yeast was used in SSF. This could result in a lowering of the production cost by 19%. In order to increase the WIM further, SSF was run in fed-batch mode, using the adapted yeast. Fed-batch SSF with up to10% WIM had no effect on the ethanol yield. A combination of high WIM in fed-batch SSF and recirculation of the process stream will definitely reduce the ethanol production cost. The effect of adding detergent to SSF was investigated using Tween-20, a non-ionic detergent. Tween-20 at 2.5 g/L increased the ethanol yield in SSF by 8%, and could be used to reduce the amount of cellulases required by 50% while maintaining the ethanol yield.