Ethanol production from lignocellulosic biomass

The reported study intends to describe the state of the art in the domain of ethanol production from lignocellulosic biomass. It was sustained ana managed by a specialized group of the French Agrice (Agriculture for Chemical and Energy Organization). Its first goal was to pinpoint the main technical and economical bottlenecks of the processes which are today under consideration, and to identify which research and development efforts could be implemented to overcome them (in the short or middle term). Lignocellulosic biomass is a complex substrate, and essentially made of cellulose, hemicellulose and lignin. The processes which have been considered, attempted to recover a maximum amount of sugars from the hydrolysis of cellulose and hemicellulose, and to ferment them into ethanol. The hydrolysis processes used in the past are essentially chemical processes, but the acid recovery costs and the formation of toxic products make them uncompetitive. They are now substituted by enzymatic processes, which are more specific and allow higher hydrolysis yields under less severe conditions. However, the cellulose that is the target of the enzymatic hydrolysis, is not directly accessible to the enzymes. It is the reason why a pretreatment step has to precede the enzymatic hydrolysis, in order to improve the enzymatic susceptibility of the cellulose, and to hydrolyse the hemicellulosic fraction. Different types of pretreatment have been studied, but three methods appear more efficient: dilute acid hydrolysis, steam explosion wit catalyst addition and thermohydrolysis. These pretreatments could result in high hydrolysis yields of the cellulose fraction (close to 100%), and in a maximum recovery of the sugars from the hemicellulosic fraction. Enzymatic hydrolysis has yet to be improved in order to reduce the cost of consumption of the enzymes. Research works will have to focus upon the enzyme specific activity, in order to achieve higher efficiencies such as those obtained wit amylases. The SSF (Saccharification and Simultaneous Fermentation) process improves the enzyme efficiency by reducing the feed-back inhibition from the hydrolysis products. The screening of efficient fermentative microorganisms under high temperature conditions (45 degrees C) has thus to be further implemented. The last technological barrier of the process concerns the ethanolic fermentation of the pentoses. Indeed, the pentoses, originating from the hemicellulosic fraction, can represent up to 40% of total sugars in some lignocellulosic substrates. Nobody has yet identified a microorganism which is able to ferment the pentoses into ethanol with performances similar to those of Saccharomyces cerevisiae on glucose. But recent genetic improvements focused on the transformation of Saccharomyces cerevisiae and Zymomonas mobilis could result in good fermentative performances on pentoses.