Lignocellulose biomass, including wood waste, agricultural waste, household waste, etc. represents a renewable resource which has stored solar energy in its chemical bonds [120]. It has great potential for bioethanol production, when compared to ethanol produced from grain, tubers, and sugar plants, because it is a widely available cheap feedstock which does not compete with human food products.

9.7.12.1 Pretreatment

It is known that the main difficulty in converting lignocellulose biomass into second-generation ethanol consists in breaking down structural and chemical biomass complex. In the course of the breakdown process, cellulose feedstock is affected by enzymes which allow further recovery of ethanol. Biomass consists of polysaccharides-cellulose and hemicellulose, which are hydrolyzed into single sugar components, followed by further recovery of ethanol by well-known and elaborated fermentation technologies. Enzymatic activity in lignocellulose hydrolysis gives a good yield and minimum amount of by-products; it has lower energy consumption, milder operating conditions, and represents an environ­mentally friendly processing method [157, 194]. Considering that the sugars required for fermentation are bound to the lignocellulose structure, pretreatment of biomass is required in order to remove and/or modify lignin and hemicellulose matrix before enzymatic hydrolysis of polysaccharides. Unlike starch which is a crucial source of energy in plants, cellulose has mostly a structural role as it provides plant cells with mechanical durability with hemicellulose and lignin. Natural cellulose materials do not have high reactivity; therefore, fermentable saccharification requires a large cellulose surface and broken cellulose microfilm structure. Reactivity of natural substrates is also reduced by lignin. The most commonly applied methods can be classified into two groups: chemical hydrolysis (dilute and concentrated acid hydrolysis) and enzymatic hydrolysis. In addition, there are some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by thermal treatment, wet — oxidation, gamma-rays or electron-beam irradiation, or microwave irradiation. However, these processes are commercially unimportant.