## 2 Composition of Lignocellulosic Biomass  
The chemical composition of biomass depends strongly on its source [6; 15]. The main components of LB are polysaccharides, which are cellulose (40-50%), hemicellulose (25-35%), and lignin (15-20%) [16; 17]. The ratios between these fractions vary with the plant age, stage of growth and other conditions [3; 16; 18; 19]. These polymers are organized in complex non-uniform three-dimensional structure with different degrees and varying relative composition [20]. Table 1 resumes the average content of these three components in some LB types.  
Cellulose is the major mass constituent of most natural biomass and it is found in the walls of the cells of plants. It is a structural polysaccharide in plants and it is a part of supporting tissues.  
The wall of a young plant cell contains approximately 40-45% cellulose; wood 50%, while the purest example of cellulose is cotton with a percentage greater than 90% [21; 22]. It is made up of interlinked glucose units which are \(\beta\)-1,4-O-glucoside bonds. These bonds result in significant hydrogen bonding, both intramolecular and intermolecular cellulose molecules, which makes cellulosic material relatively hard to hydrolyze [23]. Thus, cellulose has crystalline and amorphous parts [19].  
Hemicelluloses are the second most abundant polymers on the world after cellulose in lignocellulosic materials [4; 24]. Hemicelluloses are heteropolysaccharides (i.e., containing more than one type of sugar unit) with branches attached to the main backbone. Nevertheless, hemicellulose is formed by a single type of monosaccharides linked by \(\beta\)-bonds (1-4), mostly containing pentoses (d-xylose and l-arabinose), hexoses (d-glucose, d-mannose and p-galactose) and smaller amounts of l-rhamose, in addition to uronic acids such as glucuronic acid, 4-O-methyl-d-glucuron acid, and galacturonic acid. The proportions of these substituents may vary according to biomass kinds [3]. Hemicelluloses connect cellulose and lignin fibers to give consistency and flexibility to the structure of the cell wall [3; 19].  
Figure 1: Lignocellulosic biomass production areas and wastes included in each category.  
Lignin is a structurally important polymer in biomass and is one of the most abundant organic substances on earth, next to cellulose [25]. Lignins are major structural components of higher plants and the primary cell wall provides structural support, impermeability confers to biomass its resistance to hydrolysis and microbial degradation [26; 27]. Structurally, lignin is a complex three-dimensional polymer of phenylpropane units (approx. 25). The phenylpropane units are derivatives of carbohydrates, coming from the dehydration and cyclisation of sugars. They are mostly either 4-hydroxycinnamyl alcohol (para-coumaryl alcohol, H) or its 3- and/or 3,5-methoxylated derivatives--coniferyl (guaiacyl, G) alcohol, and sinapyl alcohol (or syringal, S), respectively. The ratio of these units varies according to the plant and biomass type. According to previous studies, lignin content may vary both between species and among different tissues of an individual plant [3]. It has been found that a high lignin content is correlated with the recalcitrance of polysaccharides to enzymatic hydrolysis, which makes pretreatments necessary previously to any biological process aiming to produce bioenergy [13; 15]. The lignin content and its partial degradation is related with some problems in the AD process.  
The elemental analysis of biomass provides the mass concentrations of the major elements (carbon, oxygen, hydrogen, nitrogen). These concentrations depend on the chemical composition of the LB and allow the estimation of the theoretical biomethane yield and the subsequent bioenergy yield using the Buswell equation [29]. The average composition of the main wastes used in the AD process is shown in Table 2.