Lignocellulosic Ethanol Production: Studies on Sugarcane Bagasse, Paja Brava, Wheat Straw, Quinoa Stalks and Curupaú
Abstract: Lignocellulosic biomass refers to plant material that is composed mainly of cellulose, hemicellulose and lignin. These materials are today of large interest to researchers in the field of biofuel and bioenergy. Residues for forestry and agriculture are particularly interesting since they result from crops with established cultivation procedures and technology for harvest and transportation are already in place. In the current work, the utilisation of a number of biomass materials for bioethanol production was studied. The materials used were sugarcane bagasse, wheat straw, paja brava (the brave straw), quinoa stalks and hardwood (Anadenanthera colubrina). These materials were initially analysed to determine the contents of carbohydrates, lignin, extractives and ash. The second part of the work dealt with the production of pentose-rich hydrolysates from steam pretreatment of the materials either without a catalyst or using SO2 or H2SO4 to enhance the hydrolysis. Most emphasis was placed on SO2-catalysed steam pretreatment, which was found to give high pentose sugar recovery from feedstocks such as sugarcane bagasse, paja brava, wheat straw, and quinoa stalks. For the pretreatment of Anadenanthera colubrina, known in Bolivia as the Curupaú tree, the use of H2SO4 was also investigated. By-products originating from the degradation of carbohydrates and lignin, were found in only low amounts in the hydrolysates obtained at suitably chosen conditions. The solid, cellulose rich, fraction of four materials was subjected to enzymatic hydrolysis (using commercial cellulases). In most cases, a relatively high degree of hydrolysis (typically > 80%) was obtained, at least at dilute conditions. Specific analytical efforts were made to analyse aromatic compounds in the liquid phase after pretreatment. Glycosylated aromatics, primarily arabinosylated p-coumaric and ferulic acids, were found in the liquid fraction after pretreatment. The concentrations showed a clear dependence on the severity conditions and the type of catalyst agent. The third part of the work focused on the fermentabilities of sugarcane and paja brava hydrolysates. A genetically engineered xylose-utilizing strain of Saccharomyces cerevisiae (TMB3400) was found to give a high xylose conversion during fermentation of the xylose rich pretreatment hydrolysates from sugarcane bagasse and paja brava, whereas the yeast Pichia stipitis was found less robust in these hydrolysates. For both yeasts, the process option simultaneous saccharification and fermentation (SSF) was found to be preferable to separate hydrolysis and fermentation (SHF) in terms of overall yields.
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