Second generation bioethanol

Abstract

Second generation bioethanol, i.e., ethanol from lignocellulosic biomass is envisaged as a renewable source of transport fuel in the next years to complement or replace the first generation and the fossil fuel.At present, the most economic solution seems to be the enzymatic way that produces fermentable sugars from cellulose and hemicellulose.To make this process commercially viable, several improvements are needed to enhance the process.Currently, stocks of biomass are constituted by available agro-industrial residues such as corn stover and straws, wood and wood processing residues and dedicated crops such as miscanthus and other grasses.The biological process for converting the lignocellulose to fuel ethanol requires: a pretreatment to liberate cellulose and hemicellulose from their complex with lignin, a depolymerization of the carbohydrate polymers to produce free sugars, and a fermentation of mixed hexose and pentose sugars to produce ethanol.Pretreatment must be cost effective and must be adapted to the type of biomass.They must not produce high amount of inhibitors such as 5-hydroxymethyl furfural, even if different technique are currently available to detoxify fermentation broths.Saccharification by enzymes and fermentation should be conducted separately (SHF) or simultaneously (SSF).Integration of the process the development of cofermentation of pentoses produced through the pretreatment of the biomass (SSCF).Progress in genetic engineering allows us to consider the development of Consolidated BioProcesses (CBP) in which a single microbial strain is able to ferment polysaccharides to produce ethanol in one step.The economic viability of ethanol recovery from the fermentation mash depends of the overall process, as distillation heat balance is dependant of ethanol concentration.The future of ethanol from biomass is widely dependent on the oil price and on the political will of the different countries.