(509d) Improvements in Dilute-Acid Steam Pretreatment and SSF of Softwood, Using Yeast Cultivated On Hydrolyzate, for Bioethanol Production

Bioethanol as one of the most promising alternative fuels contributes to the reduction of environmental impacts generated by fossil fuel consumption through low net-emission of carbon dioxide. Reducing the production costs of ethanol from lignocellulosic material is essential in enabling its commercialization. A process based on enzymatic hydrolysis and fermentation is today regarded as the most promising alternative in converting the biomass to ethanol with high yields and low production cost. However, to reach high yields and high ethanol concentration, cellulosic biomass must be pretreated. Pretreatment is a crucial step to increase the enzymatic digestibility of biomass and has a key role in the ethanol production cost.

Previous studies have shown that dilute-acid steam pretreatment, using SO2, is considered to be one of the most feasible methods of pretreatment for softwood. Simultaneous saccharification and fermentation (SSF) of the pretreated slurry, using yeast cultivated on the hydrolyzate, is also shown to be a process option that results in higher ethanol yield due to reduced end-product inhibition of the enzymatic hydrolysis. The integration of enzymatic hydrolysis and fermentation results also in decreased investment costs. However, in order to reduce the energy demand in down stream processing, and thereby the production cost, the ethanol concentration after SSF has to be high, which requires a high concentration of solids.

This study deals with certain drawbacks involved with the pretreatment, SSF and yeast cultivation processes in large-scale, using spruce as substrate. The aim is to improve sugar and ethanol yields through enhancing the digestibility of the cellulose chain, lowering the dilution of sugar solution, and minimizing sugar degradation products. We also looked into the influence of cultivation of yeast on hydrolysates containing suspended material, due to difficulties in separation of liquid and solids in large scale, on the ethanol yield from the SSF at high dry matter content and low yeast concentration, using both commercial and recombinant yeasts.