Lignocellulosic biomass is typically nonedible plant material, including dedicated crops of wood and grass, and agro-forest residues. Lignocellulosics are mainly composed of cellulose, hemicellulose, and lignin.

Cellulose is a homopolysaccharide composed of P-D-pyranose units, linked by P-1, 4- glycosidic bonds. Cellobiose is the smallest repetitive unit and it is formed by two glucose monomers. The long-chain cellulose polymers are packed together into microfibrils by hydrogen and van der Waals bonds. Hemicellulose and lignin cover the microfibils (Fig.1). Hemicellulose is a mixture of polysaccharides, including pentoses, hexoses and uronic acids. Lignin is the most complex natural polymer consisting of a predominant building block of phenylpropane units. More specifically, p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol are the most commonly encountered alcohols (Harmesen et al., 2010). Lignocellulosic materials also contain small amounts of pectin, proteins, extractives (i. e. no — structural sugars, nitrogenous material, chlorophyll and waxes) and ash (Kumar et al., 2009).

The composition of the biomass constituents can vary greatly among various sources (Table 1). Accurate measurements of the biomass constituents, mainly lignin and carbohydrates, are of prime importance because they assist tailored process designs for the maximum recovery of energy and products from the raw materials.

Since 1900, researchers have developed several methods to measure the lignin and carbohydrates content of lignocellulosic biomass. Globally recognized Organizations, such as American Society for Testing and Materials (ASTM), Technical Association of the Pulp and Paper Industry (TAPPI) and National Renewable energy and Laboratory (NREL) have developed methods to determine the chemical composition of biomass, based on modifications of the two main procedures developed by Ritter (Ritter et al., 1932) and by Seaman (Saeman et al., 1954), (Table 2).

LIGNIN

Fig. 1. Lignocellulosic materials: composition of major compounds (Kumar, 2009)