Biomass Characteristics - Biomass Gasification, Pyrolysis and Torrefaction - page 56

The secondary wall (inside the primary layer) is made up of three layers: S1,
S2, and S3 ( Figure 3.5 ). The thickest layer, S2, is made of macrofibrils,
which consist of long cellulose molecules with embedded hemicellulose.
The construction of cell walls in wood is similar to that of steel-reinforced
concrete, with the cellulose fibers acting as the reinforcing steel rods and
hemicellulose surrounding the cellulose microfibrils acting as the cement-
concrete. The S2 layer has the highest concentration of cellulose. The highest
concentration of hemicellulose is in layer S3. The distribution of these com-
ponents in the cell wall is shown in Figure 3.6 .

3.3.2 Constituents of Biomass Cells
The polymeric composition of the cell walls and other constituents of a
biomass vary widely but they are essentially made of three major polymers:
cellulose, hemicellulose, and lignin.

3.3.2.1 Cellulose
Cellulose, the most common organic compound on Earth, is the primary
structural component of cell walls in biomass. Its amount varies from 90%
(by weight) in cotton to 33% for most other plants. Represented by the
generic formula (C 6 H 10 O 5 ) n , cellulose is a long-chain polymer with a high
degree of polymerization ( , 10,000)

and a large molecular weight

( 500,000). It has a crystalline structure of thousands of units, which are

,

Primary cell wall

S1

Middle
lamella

S2

Center fluid
passage

S3

FIGURE 3.5 Layers of a wood cell. The actual shape of the cross-section of a cell is not
necessarily as shown.