Lignocellulosic Ethanol Production Based on Steam Pretreatment and SSF: Process Development through Experiments and Simulations

Sammanfattning: Lignocellulosic materials, such as wood, waste products from the forestry industry, agricultural residues and herbaceous crops, serve as an abundant and comparatively cheap feedstock with the potential to be used for large-scale ethanol production. However, the complex structure of lignocellulosic materials, the presence of various hexose and pentose sugars in hemicellulose, and the presence of various compounds that inhibit the fermenting organism constitute physical barriers that add to the production cost and make full-scale introduction economically more risky. The work presented in this thesis was aimed at improving the economics of lignocellulosic ethanol production, by investigating more cost-efficient ways to convert the carbohydrates to ethanol. Steam pretreatment of Salix was extensively assessed. Glucose yields of above 90% and xylose yields higher than 80% of the theoretical were obtained after enzymatic hydrolysis. Simultaneous saccharification and fermentation (SSF) of steam-pretreated Salix using baker’s yeast (S. cerevisiae) resulted in ethanol yields of above 80% of the theoretical, based on the glucan and mannan content of the raw material. Cultivating the yeast on part of the pretreatment liquid resulted in a more tolerant yeast, which made it possible to perform SSF with higher substrate loadings. The highest ethanol concentration obtained was 32.6 g/L. There is, however, room for further improvements, as discussed in this thesis. Process simulations were performed utilizing a model including all major process steps, which was implemented in the commercial flow-sheeting program Aspen Plus. The model input was based on experimental data recently obtained on lab scale or in a process development unit. An economic evaluation, including estimations of the ethanol production cost for different process configurations, was carried out based on the simulation results. The results clearly demonstrate the importance of a high ethanol yield and the necessity of utilizing the pentose fraction for ethanol production to reduce the production cost, especially when using a pentose-rich feedstock. A less energy-demanding process reduces the capital cost and results in higher co-product revenue, which is beneficial for the overall process economics. Although ethanol was shown to be the main product, i.e. yielding the major part of the income, the co-product revenue has considerable impact on the process economics, and the importance of good utilization of the entire feedstock was clearly shown. Utilization of the excess solid residue for heat and power production was highly favourable from the economic aspect. A spruce-based ethanol plant with such a configuration was shown to be competitive with, and in many ways more advantageous than, a biomass-based heat and power plant in terms of overall economics.