Biomass is an organic material which stores sunlight in the form of chemical energy. It is available on a renewable basis. Here, we specifically mention the lignocellulosic biomass from plants and residues from various agricultural activities. Biomass is an organic mate­rial that is composed of polymers that have extensive chains of carbon atoms linked to macromolecules. The polymer back bone consists of chemical bonds linking carbon with carbon, or carbon with oxygen, or sometimes other elements such as nitrogen or sulfur. Instead of describing polymers in terms of the atomic structure of the chain, most can be viewed as assemblies of some larger molecular unit. In the case of cellulose, that unit is the glucan moiety, essentially a molecule of glucose with one molecule of water missing (C6H10O5)n. For hemicellulose, the unit is often a 5-carbon sugar, called xylose. However, hemicellulose polymers are not linear chains as in the cellulose polymer. Some are branched and other monomer units have side chains, with acetyl groups being very common. The lignin polymers are composed of phenyl propane subunits linked at various points on the monomer through C—C and C—O bonds. In addition, there are often side chain moieties such as methoxy groups. Wood-based biomass is available in large quantities and is cheap. It consists of three major components, that is, lignin, cellulose, and hemicellulose.

(i) Cellulose: It contains linear polysaccharides in the cell walls of wood fibers, consisting of D-glucose molecules bound together by p-1,4-glycoside linkages. Biomass comprises 40-50% cellulose.

(ii) Hemicellulose: It is an amorphous and heterogeneous group of branched polysaccharides (copolymer of any of the monomers of glucose, galactose, mannose, xylose, arabinose, and glucuronic acid). Hemicellulose surrounds the cellulose fibers and is a linkage between cellulose and lignin (15-30%). Hemicelluloses are heterogeneous polymers

of pentoses (e. g., xylose, and arabinose), hexoses (e. g., mannose, glucose and galactose), and sugar acids. Unlike cellulose, hemicelluloses are not chemically homogeneous. Hemicelluloses are relatively easily hydrolyzed by acids to their monomer components consisting of glucose, mannose, galactose, xylose, arabinose, and small amounts of rhamnose, glucuronic acid, methylglucuronic acid, and galacturonic acid. Hardwood hemicelluloses contain mostly xylans, whereas softwood hemicelluloses contain mostly glucomannans. Xylans are the most abundant hemicelluloses. Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of 1,4-linked р-D-xylopyranose units. Xylans from different sources, such as grasses, cereals, softwood, and hardwood, differ in composition. Besides xylose, xylans may contain arabinose, glucuronic acid, and acetic, ferulic and p-coumaric acids. The degree of polymerization of hardwood xylans (150-200) is higher than that of softwoods.

(iii) Lignin: It is a highly complex three-dimensional polymer of different phenylpropane units bound together by ether (C—O) and carbon-carbon (C—C) bonds. Lignin is concentrated between the outer layers of the fibers, leading to structural rigidity and holding the fibers of polysaccharides together (15-30%). Generally, softwoods contain more lignin than hardwoods. Lignins are divided into two classes, namely, guaiacyl lignins and guaiacyl-syringyl lignins. Although the principal structural elements in lignin have been largely clarified, many aspects of their chemistry remain unclear.

In addition, small amounts of extraneous organic compounds, that is, extractives, proteins, and inorganic constituents are found in lignocellulosic materials (about 4%; Stocker, 2008). Biomass residues like wheat straw, corn stover, or sugar cane bagasse contain much ash and N, S, Cl, and these quantities also depend on the geographical source.