Bioethanol: State and perspectives

Abstract

Processes of bioethanol production currently applied all over the world are reviewed in this paper. Attention is focused on potentially cheap biomass sources, as well as the most important operating factors controlling the progress and result of saccharification and fermentation reactions and affecting the yield of fermentable sugars and ethanol, respectively, such as: the type and concentration of acid, the type of enzyme, the type of working microorganism, operating temperature, duration time and pH. The hydrolysis conditions, namely duration time, temperature and sulfuric acid concentration, were combined in a single parameter, known as the "combined severity" (CS), in order to estimate the efficiency of bioethanol production from biomass. When the CS increases, the yield of fermentable sugars also increases. The decrease in the yield of monosaccharides coincides with the maximum concentrations of by-products, such as furfural and 5-hydroxymethylfurfural, which are well-known as yeast inhibitors. The highest ethanol yields has been obtained using the yeast Saccharomyces cerevisiae. With low oil prices and political reluctance to implement carbon taxes, fuel-ethanol production will remain uncompetitive unless some other form of cost reduction can be made, such as feedstock preparation costs.