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Sulphite treatment to enhance the ethanol production of softwood whole slurries Zhong, Na

Abstract

To try to compete with conventional bioethanol production from sugar/starch feedstocks, “whole-slurry”, high solids loading hydrolysis and fermentation of steam pretreated softwood biomass was employed to obtain high sugar, high ethanol concentrations. However, two major challenges were encountered; substrate recalcitrance limiting effective enzymatic hydrolysis and high concentrations of inhibitors inhibiting effective sugar fermentation. The major focus of the work described in this thesis was to assess the benefits of integrating sulphite treatments prior, during and after steam pretreatment to enhance the effectiveness of the enzymatic hydrolysis of the whole cellulose and hemicellulose derived slurry and the fermentation of the biomass derived sugars. Initial work focused on improving fermentation at high sugar concentrations (up to 25% w/v) by improved strain selection, nutrient supplementation and the use of high cell density growth Subsequently, sulphite post treatment was assessed to see if it could improve both the hydrolysis of the whole slurry, containing both cellulose and hemicellulose, and the fermentation of the softwood derived sugars. To try to maximize the beneficial influence of sulphite treatment the softwood chips were treated prior to steam pretreatment, to result in sulphonation ahead of lignin condensation, hopefully improving the extent of sulphonation with less sulphite loading. This approach resulted in some improvements. However, it proved difficult to balance some of the factors that influence the effectiveness of enzyme mediated hydrolysis, such as the extent of suphonation, substrates size reduction and lignin condensation, although an improvement in whole slurry fermentation was observed. To try to further improve this strategy a two stage, alkali- followed by acid-sulphite approach was assessed, using sulphite in the first stage to sulphonate the lignin and SO₂ in the second stage to further sulphonated the lignin while decreasing particle size. This resulted in high degree of sulphonation, enhanced delignification and substantial substrate size reduction. Minimum lignin condensation and fermentation inhibitors were detected. More than 160 g/L fermentable sugars and 80 g/L ethanol could be achieved when using the two stage (alkali and acid) sulphite pretreatment of lodgepole pine approach to generate a hemicellulose and cellulose whole slurry that could be readily hydrolysed and fermented.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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