Chemical Composition of the Lignocellulosic Biomass
Lignocellulose is the term used to describe the three-dimensional polymeric composites formed by plants as structural material. It consists of variable amounts of cellulose, hemicellulose and lignin  besides other minor compounds (Figure 2). The high heterogeneity of this feedstock mainly depends on its origin (Table 1) but also on other less manageable factors related to growing, harvesting and storage conditions.
Figure 1. World production (Ton) of some crops and their distribution (% of total production) in different areas (data source FAO, 2012).
Figure 2. Structure and main constituents of lignocellulosic biomass (adapted from ).
The carbohydrate constituents of the lignocellulosic materials are cellulose and hemicelluloses. Other forms of carbohydrates present in biomass are starch, pectin and chitin.
The cellulose constitutes the main support of the plant cell wall and provides strength to the lignocellulosic structure. It consists on long linear straight chains of glucose units, bound together by P-1,4-glycoside linkages, leading to highly polymerized crystalline fibres packed so tightly that not only enzymes, even water cannot enter the complex framework .
Hemicelluloses are an amorphous and heterogeneous group of branched polysaccharides constituted by pentoses (xylose and arabinose), hexoses (mannose, glucose and galactose) and sugar acids (glucuronic and galacturonic acids) which act as linkage between cellulose and lignin. Xylans are the most abundant hemicelluloses, consisting in homopolymeric backbone chains of 1,4-linked xylose units from that ramify secondary heterogeneous chains formed by different monomeric sugars and acids. The degree of polymerization of xylans (150-200) and its composition differs significantly between different lignocellulosic feedstocks.