Production of Ethanol from Spruce at High Solids Concentrations - An Experimental Study on Process Development of Simultaneous Saccharification and Fermentation
Abstract: Replacing fossil fuels by biofuels such as ethanol is considered a promising alternative to reduce greenhouse gas (GHG) emissions and mitigate climate change. Biofuels produced from lignocellulosic biomass, so-called second generation biofuels, result in decreased GHG emissions and limit competition with food and animal feed production. Interest in producing ethanol from lignocellulosic biomass has therefore increased rapidly during recent years. Several pilot and demonstration plants for the biochemical conversion of lignocellulose to ethanol have been built, and the first commercial plants are planned to start large-scale production within the coming years. However, a great deal remains to be done in process development to increase the production efficiency and decrease production cost. The work presented in this thesis focuses on the biochemical conversion of spruce to ethanol, using enzymatic hydrolysis and fermentation in simultaneous saccharification and fermentation (SSF). The main aim of this work was to achieve a high ethanol concentration after fermentation, in order to reduce the energy required in distillation, thus reducing the production cost. A final ethanol concentration of 65 g/L was achieved, which is well above the 4 wt% considered to be the limit for economically feasible distillation. Furthermore, these experimental studies on the production of ethanol from spruce have contributed to a better understanding of some of the fundamental steps in the production process. Enzymatic hydrolysis and fermentation must be performed at higher solid substrate concentrations in order to increase the ethanol concentration after fermentation. In the first part of this work, it was shown that the decrease in ethanol yield in SSF with high solids concentration is a result of both increased mixing difficulty and increased inhibition of the yeast, and possibly the enzymes, due to increased levels of inhibitory substances. In the second part of the work, it was shown that the ethanol yield in high-solids SSF could be significantly increased by adding a prehydrolysis step prior to SSF. It was also shown that this positive effect on ethanol production from spruce is a result of fibre degradation rather than decreased viscosity, as often suggested when using other lignocellulosic materials such as straw and grass. The addition of a prehydrolysis step prior to SSF shifts the process from being fermentation-limited to being hydrolysis-limited. Prehydrolysis thus enhances fermentation, rather than the overall performance of hydrolysis. The initial dry matter content in SSF was increased from 5-10% water-insoluble solids (WIS) to 20% WIS. The process configuration in enzymatic hydrolysis and fermentation has been shown to significantly influence the overall ethanol yield. The highest ethanol concentration (65 g/L) with an overall ethanol yield of 72% was obtained in fed-batch SSF, where prehydrolysed steam-pretreated spruce was fed to the reactor over a period of time. Approximately a quarter of the cellulose was, however, not converted to glucose, and was thus not fermented to ethanol. There is thus further potential for improvements in the process, which could increase the ethanol concentration and yield.
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