First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 189
Biomass? 6.2 What Difference Should Be Considered Between Wood and Agricultural
and Agricultural Biomass?
Plant biomass is composed primarily of cellulose, hemicelluloses, lignin, and smaller
amounts of water, pectin, protein, extractive materials, and ash, which do not contribute
considerably in forming the structure of the material (Ravenet al., 1992). Cellulose,
hemicelluloses, and lignin are present in various amounts in the different parts of
the plant and they are intimately associated with each other to form the structural
framework of the plant cell wall.
Cellulose preserves the crystalline structure, and it appears to be the core of the
complex matrix. Hemicellulose is located both between the micro- and the macrofibrils
of cellulose. Lignin provides a structural role of the matrix in which cellulose and
hemicellulose are entrenched (Faulonet al., 1994).
Since cellulose is the major material of the plant cell walls, most of the lignin is
found in the interfibrous area, while a smaller part can also be found on the cell surface
(Kirk-Otmer, 2001).
The composition of lignocellulosic materials depends on plant species, age and growth
conditions. Distribution of cellulose, hemicellulose, and lignin as well as the content of
the different sugars of the hemicellulose, varies significantly between different plants
(Table 6.1).
There are four principal bonds recognized in lignocellulosic materials, including ether
bonds, ester bonds, carbon—carbon (C—C) bonds, and hydrogen bonds.
These are primary bonds that form the intrapolymer linkages of lignocellulosic
material components and connect the different units of lignocellulosic materials
(interpolymer linkages). The situation and function of bonds are shown in Table 6.2
(Faulonet al., 1994).
Table 6.1Composition of different lignocellulosic materials (Jorgensenet al., 2007).
Materials Glucosea) Xyloseb) Arabinoseb) Mannoseb) Lignin References
Hardwood
Birch 38.2 18.5 0 c) 1.2 22.8 Jørgensenet al. (2007)
Willow 43 24.9 1.2 3.2 24.2 Jørgensenet al. (2007)
Softwood
Spruce 43.4 4.9 1.1 12 28.1 Jørgensenet al. (2007)
Pine 46.4 8.8 2.4 11.7 29.4 Jørgensenet al. (2007)
Grasses
Wheat straw 38.2 21.2 2.5 0.3 23.4 Jørgensenet al. (2007)
Rice straw 34.2 24.5 n.dd) n.dd) 11.9 Jørgensenet al. (2007)
Corn stover 35.6 18.9 2.9 0.3 12.3 Jørgensenet al. (2007)
a) Glucose is mainly obtained from cellulose.
b) Xylose, arabinose, and mannose make up hemicellulose.
c) Below detection limit.
d) Not determined.
