Fuel Ethanol Production from Corn Stover: Optimization of Steam Pretreatment and Improvement of Simultaneous Saccharification and Fermentation
Abstract: Fuel ethanol, produced from biomass, could well be tomorrow's replacement for gasoline. This thesis deals with the production of fuel ethanol from corn stover using an enzymatic process consisting of the following main steps: steam pretreatment of the raw material, enzymatic hydrolysis, fermentation of the sugars and ethanol refining. SO2-catalysed steam pretreatment was shown to give high glucose and xylose yields after enzymatic hydrolysis. The pretreatment could be conducted at high dry matter content in the raw material (40%) with negligible effects on the sugar yields, making it possible to achieve a high ethanol concentration in the fermented broth, which will decrease the energy demand in the distillation step. Several ways of increasing the overall sugar yield were investigated. Delignification of the pretreated material increased the sugar yield in the enzymatic hydrolysis step, but resulted in sugar degradation, especially xylose, leading to a decrease in overall xylose yield and only a slight increase in overall glucose yield. Addition of xylanases to the enzymatic hydrolysis, on the other hand, increased the overall glucose and xylose yields significantly. A comparison of separate enzymatic hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) showed that SSF resulted in a higher ethanol yield. Washing the solid material after pretreatment increased the overall ethanol yield in SHF but the yields obtained in SHF with washed material never reached the yields obtained with SSF of the whole pretreated slurry. Ways of increasing the overall ethanol yield and ethanol concentration after SSF were also investigated. By increasing the concentration of water-insoluble solids (WIS) in SSF from 5 to 10% the ethanol concentration was increased considerably, from 13 g/L to 25 g/L (using Saccharomyces cerevisiae), without a decrease in ethanol yield. Enzymatic prehydrolysis of the pretreated slurry prior to SSF decreased the viscosity of the slurry drastically, improving the possibility of increasing the total amount of sugars in SSF. However, this process configuration did not increase the overall ethanol yield.Corn stover contains around 23% xylan, which is not naturally fermented by Saccharomyces cerevisiae. A recombinant strain of S. cerevisiae (TMB3400) was used to co-ferment glucose and xylose during SSF. TMB3400 was shown to co-ferment xylose, giving high ethanol yields, at 5% WIS. At higher WIS (11%), complete co-fermentation was not achieved due to TMB3400's sensitivity to inhibitors. Both naturally occurring Saccharomyces cerevisiae and TMB3400 became more tolerant to inhibitor by cultivating the yeast in the liquid fraction obtained after pretreatment. Increased concentration of pretreatment liquid in the cultivation increased the inhibitor tolerance significantly, which resulted in higher ethanol productivity in SSF and thus a faster decrease in glucose concentration. This offers a promising prospect for complete xylose co-fermentation in the future.
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