Lignocellulose is a class of biomass that consists of three major compounds: cellu­lose, hemicellulose, and lignin. Lignocellulose is the most abundant renewable bio­mass; its annual production has been estimated in 1 x 1010 MT worldwide (Sanchez and Cardona 2008). Inside the lignocellulose complex, cellulose is a major struc­tural component of cell walls, and it provides mechanical strength and chemical stability to plants; cellulose is found in both the crystalline and the noncrystalline structure and resistant to hydrolysis.

Hemicellulose is a copolymer of different C5 and C6 sugars that also exist in the plant cell wall. Important aspects of the structure and composition of hemicellulose are the lack of crystalline structure, mainly due to the highly branched structure, and the presence of acetyl groups connected to the polymer chain (Kirk-Otmer 2001a, b). Hemicellulose is positioned both between the micro — and the macrofibrils of cellulose.

Lignin is a complex polymer and is the binding of the matrix in which cellulose and hemicellulose are embedded. Considering that cellulose is the main material of the plant cell walls, most of the lignin is found in the interfibrous area, whereas a smaller part can also be located on the cell surface (Kirk-Otmer 2001a, b).

Apart from the three basic chemical compounds that lignocellulose consists of water and inorganic component are also present in the complex. Among inorganic contents, ash is one of the largest constituent. Ash typically comprises of calcium, potassium, magnesium, manganese, and sodium oxides, and lesser amount of other oxides of iron, aluminum, etc.

The cellulosic ethanol feedstock can be broadly categorized into agriculture residue, waste product, woody biomass, and energy crops. The composition of lignocellu — lose highly depends on its source. The composition of lignocellulose encountered in the most common sources of biomass is summarized in Table 20.1.

Bioethanol production from this feedstock could be attractive for disposal of these residues. Importantly, lignocellulosic feedstock does not interfere with food security. Moreover, bioethanol is very important for both rural and urban areas in terms of energy security reasons, environmental concern, employment opportunities, agricultural development, foreign exchange saving, socioeconomic issues, etc.

Since different lignocellulosic biomass have different physicochemical charac­teristics, it is necessary to adopt suitable pretreatment technologies based on the lignocellulosic biomass properties of each raw material.