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Bioconversion of corn fibre to ethanol Bura, Renata

Abstract

Corn fibre, due to its chemical and structural properties, was evaluated as a potential technically viable feedstock that might be used to demonstrate the effectiveness of a biomass to ethanol bioconversion process. One of the major goals of the thesis was to see if we could define the optimum pretreatment conditions which would result in maximal sugar recovery and ethanol production, with a minimum number of subprocess steps. Corn fibre was pretreated using SO₂-catalysed steam explosion at thirteen different severity conditions, chosen based on response surface modeling, with the goal of finding optimal pretreatment conditions. These conditions were defined to allow us to recover most of the hexose hemicellulose and cellulose derived sugars in an hydrolysable and fermentable form. The chosen severity conditions had a pronounced effect on the total amount of sugars recovered from corn fibre. Due to the chemical and biological nature of corn fibre (low lignin and high carbohydrate content), we were able to establish optimum, mild steam pretreatment conditions - 190°C for 5 minutes after exposure to 3% SO₂ - which yielded 93% of the total sugars in a hemicellulose-rich water soluble fraction, and a readily hydrolysable cellulose fraction by enzymes. The optimum pretreatment conditions also resulted in the production of a limited amount of sugar decomposition products, which allowed us to effectively ferment hexose sugars to ethanol using Saccharomyces cerevisiae. Efforts to increase the low sugar concentrations in the water soluble fraction were evaluated in an attempt to increase the final ethanol concentrations recovered following fermentation. Increasing the sugar concentration by using a high consistency (12% w/v) cellulose-rich water insoluble fraction in acetate buffer resulted in the generation of a glucose stream of 55 g L⁻¹. However, mixing this higher concentration water insoluble, cellulose stream was problematic, as evident by incomplete hydrolysis. As an alternative strategy, the hemicellulose-rich water soluble fraction was added to the cellulose-rich stream, effectively increasing the sugar concentration while reducing the number of subprocess steps to pretreatment, hydrolysis and fermentation. Although combining the hemicellulose and cellulose streams increased the overall sugar concentration, cellulose hydrolysis was inhibited due to end-product inhibition. Increased hydrolysis time, and enzyme loadings were able to further increase cellulose hydrolysis yields. In an effort to increase overall cellulose conversion, minimize end-product inhibition, and simplify the overall process, the simultaneous saccharification and fermentation of the entire slurry after steam explosion was investigated. This permitted a simplified two-stage bioconversion process, i.e., pretreatment and SSF, while alleviating the problem of end-product inhibition, as shown by good (86%) hexose to ethanol conversion at 8% solids consistency with minimum enzyme loading (10 FPU g cellulose⁻¹). The overall efficiency of the process was also improved, as shown by the fact that ethanol productivity was approximately five times greater for an SSF when compared to the SHF approach. This study elucidated the technical merits of process integration during the bioconversion of corn fibre to ethanol when using steam explosion of corn fibre, followed by simultaneous saccharification and fermentation of the pretreated slurry. A preliminary techno-economic modeling study indicated that the contribution of each of the subprocess steps to the final total cost was highly dependant on the nature of the feedstock. For example, during the bioconversion of softwoods to ethanol, the delignification process was shown to be the main cost driver of the process, while for corn fibre, the model pointed out that the hydrolysis step was the main contributor to the total production cost for all the cases analysed in this thesis. It also indicated the greater economic viability of the proposed process options (SO₂ -catalysed steam explosion and SSF), compared with the bioconversion of softwoods to ethanol, or compared to the separate hydrolysis and fermentation option for corn fibre (SHF), as determined by reductions in relative costs by 52% and 21%, respectively.

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