Bioethanol Production from Lignocellulosic Materials: Some studies on Sugarcane Bagasse and Paja Brava
Abstract: The commercial feasibility of ethanol production from biomass is dependent on the availability of lignocellulose in large amounts at low cost. Various kinds of agricultural residues are of interest, due to the fact that these feedstocks have well-established cultivation procedures already in place, as well as technology for harvest and transportation. In the current work, the utilization of sugarcane bagasse and the straw material Paja Brava for bioethanol production was studied. Sugarcane bagasse is a widely available feedstock, in particular in South America and the Caribbean. It has a high carbohydrate content (about 43% glucan and 24% xylan) and is available at current sites of ethanol production. Paja Brava (the brave straw) is an abundant lignocellulosic material from the Bolivian Altiplano. This material is to some extent similar to sugar cane bagasse, and it has a glucan content of about 34 % and a xylan content of about 25%. In the current work it was shown that the steam pretreatment of these lignocellulosics using SO2 (2 or 2.5% based on water content) as catalyst gave a high yield of monomeric xylose for both materials (around 60%) at suitably chosen temperatures and holding times. For the sugar cane bagasse, the highest xylose yield was obtained at a temperature of 190 ºC with a holding time of 5 min, whereas the corresponding values for Paja Brava was 200 ºC and 5 min. The pretreatment giving a high xylose yield was also the best in terms of giving a high enzymatic digestibility. By-products derived from the degradation of carbohydrates and lignin, were found only in low amounts in the steam-pretreated hydrolyzates. The fermentabilities of both hemicellulose hydrolyzates (from the pretreatment) and the cellulose hydrolyzates (from the enzymatic hydrolysis) were also assessed. The engineered xylose-utilizing strain Saccharomyces cerevisiae TMB3400 was found to give a significant xylose conversion during fermentation of the xylose rich pretreatment hydrolyzates from both feedstocks. The yeast Pichia stipitis was, however, found less robust in these hydrolyzates. Simultaneous saccharification and fermentation (SSF) was found to be a promising option for conversion of pretreated Paja Brava. At a solid loading (water insoluble solids, WIS) of 5% almost quantitative xylose conversion could be obtained. However, this decreased to about 50% when a WIS content of 10% was used. Still an ethanol yield of 0.36 g g-1 was obtained at this high WIS content.
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