Page 381 - Refining Biomass Residues for Sustainable Energy and Bioproducts

344 Refining Biomass Residues for Sustainable Energy and Bioproducts

considered. Accurate component analysis of biomass is essential as it determines
the conversion efficiency based on the conversion method. Different components of
biomass are described later.
Cellulose is a natural polymer and repeating units of six carbon ring D-glucose.
The three hydroxyl groups in each unit are linked with one another forming intra-
molecular and intermolecular hydrogen bonds that provide cellulose, a crystalline
structure, and the stability (mechanical and chemical) (Saini et al., 2015). The ace-
tal bonds link the C-1 of one pyranose ring to C-4 of the succeeding ring forming a

long chain. The degradation temperature of cellulose is around 240 C 360 C and
it mainly produces liquid products after conversion.
Hemicellulose is the short and heterogeneous branched chain of polymers (pen-
tose and hexose), which surrounds the cellulose and also links the cellulose with
lignin. Unlike cellulose, it has a lower degree of polymerization and is amorphous
in nature.
Lignin is the aromatic, most complex, and high molecular weight polymer with
cross linking of phenolic groups (Lebo et al., 2000). It is the amorphous cross-link
resin that serves as a binder for the fibrous cellulose and hemicellulose components.
Lignin is located mainly on the exterior of the microfibrils and also covalently
bonded to hemicellulose and thus imparts rigidity to cell wall. Because it is cova-
lently bonded, lignin is not readily depolymerized.
Apart from these three components, biomass also contains some extractives and
inorganic elements. Inorganic elements (Na, K, Mn, Mg, Cl, Al, Zn, Si, etc.) are
basically present in the ash whereas extractives include proteins, fats, resins, gums,
pectins, etc. that can be extracted using polar and nonpolar solvents (reference).
Table 15.2 presents the compositional analysis of some of the agricultural wastes.

15.4 Pretreatment of agricultural waste biomass

Agricultural waste biomass generally requires certain modification in its structure
and properties before its utilization in any conversion process. Thus pretreatment of

Table 15.2 Lignocellulosic composition of different agricultural wastes from literature.

Agricultural waste Hemicellulose (wt.%) Cellulose (wt.%) Lignin (wt.%)
Rice husks 12.0 29.3 28.7 35.6 15.4 20.0
Rice straw 23.0 25.9 29.2 34.7 17.0 19.0
Wheat straw 23.0 30.0 35.0 39.0 12.0 16.0
Corn cob 31.9 36.0 33.7 41.2 6.1 15.9
Corn stalk 16.8 35.0 35.0 39.6 7.0 18.4
Sugarcane bagasse 28.0 32.0 25.0 45.0 15.0 25.0
Tea waste 19.9 30.2 40

Source: Cai, J., He, Y., Yu, X., Banks, S.W., Yang, Y., Zhang, X., et al., 2017. Review of physicochemical
properties and analytical characterization of lignocellulosic biomass. Renew. Sustain. Energy Rev. 76, 309 322;
Demirbas, A., Demirbas, A., 1997. Calculation of higher heating values of biomass fuels. Fuel 76, 431 434.