In this section, ethanol is obtained from raw cellulosic materials or from materials with high content of cellulose and hemicelluloses. Even though it is not possible to register the industrial scale production of cellulosic origin at the worldwide level, the possibility of using lignocellulosic residuals in this manner has initiated many research and development activities, particularly in the United States.

The main components of the lignocellulosic biomass are cellulose and hemicel — luloses, and being made of fermentable sugars, they can be used to obtain ethanol. One of the critical points that characterize this spinneret section is the physical separation of cellulose and hemicelluloses from lignin. This can be achieved by chemical-physical (the most well known of which is called steam explosion, which applies saturated vapours at high pressure), chemical (with acids) and mechanical (with press systems) treatments. Subsequently, the cellulose and hemi­celluloses are subjected to hydrolysis, which can be carried out in two ways: chemical or enzymatic.

The acid chemical hydrolysis can take place in only one step or in two different steps. In the first case, hydrolysis is carried out using concentrated (at a 77% con­centration) sulphuric acid (H2SO4) which is added to the cellulose material in a ratio of 1.25:1 and at a temperature of 50°C. In the second case, dilute sulphuric acid is used: first, the hemicelluloses are attacked by H2SO4 at a concentration of

0. 4% and at a temperature of 215°C. In enzymatic hydrolysis, the cleavage of the cellulose and hemicelluloses chains is achieved using enzymes that are called cellulases, which have been discovered in the micro-organism Trichoderma reesei and have also been subsequently identified in many other microbic groups.

Enzymatic hydrolysis is preferred to chemical hydrolysis. The hydrolysis of cellulose yields glucose molecules, which is an easily fermentable six-carbon atom sugar; the hydrolysis of hemicelluloses gives five-carbon atom sugars that are ethanol fer­mented with more difficulty. The total yield of bio-ethanol in the cellulosic section is still a matter of high concern, especially with regard to hemicelluloses.