Optimization of enzymatic hydrolysis and fermentation of steam-pretreated sugarcane bagasse for ethanol production

In most tropical sugar producing countries, sugarcane bagasse represents a key source of lignocellulose that is already available at the sugarcane processing plant. Besides being used as a fuel for steam and electricity production, surplus bagasse may be used for ethanol production sharing part of the available conventional bioethanol production infrastructure. However, to this option become a reality there are still some challenges that must be faced in relation to its conversion process to ethanol.

 In the biological conversion of lignocellulosic biomass to ethanol, it is necessary to attain final ethanol concentrations that make distillation feasible from an economical point of view, estimated to be at least 40-50 g/l.  To reach this level, a sugar concentration of at least 8% (w/w) is required, which largely depends on the efficiency of conversion of carbohydrates to fermentable sugars through pretreatment and enzymatic hydrolysis. This in turn generally requires a minimum initial dry matter content of 15% in the enzymatic hydrolysis, with the restrictions in mixing due to the high viscosity and other associated problems.

In this work, process conditions leading to high ethanol concentrations by enzymatic hydrolysis and fermentation of steam-pretreated sugarcane bagasse were studied and optimized in experiments at high solids loadings of 20-25% (w/w). Final ethanol production above 5% (w/w) was achieved, corresponding to process yields from 60 to 70% of theoretical in CSTR reactors. Using other reactor designs, such as rotating drum reactor, higher ethanol production and yields could be obtained working at up to 30% total solids.