BIOETHANOL PRODUCTION FROM LIGNOCELLULOSIC BIOMASS - REVIEW.

In the last decade, environmental protection is one of the major challenges. It is necessary to ensure the protection of the environment and the conservation of natural resources, in accordance with the requirements of a sustainable economic and social development. The most important impact of modern human activities is the release of large amounts of different compounds after burning of fossil fuels, which are responsible for increasing of greenhouse gases (GHG) concentrations in the atmosphere. The depletion of fossil fuels and necessity to increase energy reserves, especially for the propulsion of transport, contributed to search and use of alternative fuels. Partially or completely substitution of gasoline with bioethanol is an alternative method to reduce GHG emissions. Currently, biofuels (first generation) are produced from sources which are used to feed the population. The competition between food vs. biofuel could be solved if biofuels were obtained from renewable resources, such as lignocellulosic biomass (LCB). Second-generation biofuels are obtained from raw materials such as agricultural (straws, sugarcane bagasse, corn stalks and cobs) and forestry residues (sawdust, bark, branches, etc.) which do not interfere with global food production. In 2019, the main producers of bioethanol were USA, Brazil and EU which produced about 54%, 30% and 5% respectively of the worldwide bioethanol. This paper reviews the most important steps in bioethanol production are pretreatment, hydrolysis and fermentation of the mash. There are a lot of pretreatments, such as physical, chemical, physicochemical, biological and combined pretreatments. The combined pretreatments have been shown to be more effective compared to single pretreatments, and there is a wide range of combinations which can be applied in the future. Hydrolysis step contributes to the release of fermentable sugars, which are further converted by microorganisms into bioethanol. There are a lot of hidrolysis methods, such as Separated Hydrolysis and Fermentation (SHF), Simultaneous Saccharification and Fermentation (SSF), Simultaneous Saccharification and co-Fermentation (SSCF) and Consolidated Bioprocessing (CBP). The liquid fraction obtained after pretreatment and hydrolysis contain various predominant carbohydrates such as glucose and xylose. Therefore, special microorganisms are needed to perform co-fermentation. [ABSTRACT FROM AUTHOR]