Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 39

Figure 2.3. Structure of cellulose.
contain N, which generates NO x when biomass is combusted. In addition, the S in biomass can
cause problems during gasification and may contaminate catalysts.
Extractives have the highest energy values of all the organic components. The high oxygen
content of cellulose reduces its energy content per unit mass.
2.3.1 Cellulose
Cellulose is a polymer that accounts for a large proportion of plant biomass. It is the most
abundant natural polymer; having crystalline and amorphous regions. Pure cellulose consists
of linear chains of 7000 to 15,000 linked glucose (C 6 H 10 O 5 ) monomers. Its molar H/C and O/C
ratios are 1.67 and 0.83, respectively. The bond between the glucose molecules is formed between
the first carbon atom of one ring and the oxygen atom bound to the fourth carbon atom of the
next, as shown in Figure 2.3.
These bonds are stabilized by hydrogen bonding between a hydroxyl group on one ring and
the oxygen atom within the adjacent ring. This hydrogen bonding causes cellulose to form micro
fibrils with a flat ribbon-like conformation. These micro fibrils are relatively stable and have
high axial stiffness. Networks of stacked cellulose fibrils in the S2 cell wall structure provide the
bulk of the plants' structural reinforcement. To withstand bending forces, cellulose fibrils have
regions where the cellulose chains are disordered (amorphous) while others are highly ordered
and crystalline.
Oxygen accounts for nearly half the total mass of cellulose, which therefore has a relatively
low energy content; its calculated gross calorific value is 17.68 kJ/g (Gaur and Reed, 1998) and
according to Walawender et al . (1985) ( α )-cellulose has a gross calorific value of 17.492 kJ/g on
a dry ash-free basis.
2.3.2 Hemicellulose
In contrast to cellulose, which consists exclusively of glucose and forms a long un-branched
polymer, hemicellulose contains many different sugar monomers. That is to say, it is a mixed
polymer including rings with five carbon atoms (C5; pentose) and also those with six (C6; hex-
oses). Hemicellulose has short side chains and some varieties are heavily branched, such as
arabinogalactan, which is frequently found in larch wood. Individual hemicellulose molecules
typically consist of 80-200 hexose and pentose monomers. While individual cellulose molecules
stack to form crystalline micro fibrils and networks, hemicelluloses are characterized by their
non-bonding steric repulsion, which disfavors their close packing. As a result, hemicellulose is
amorphous in its native form and is therefore more reactive than cellulose. In addition, the more
extensive the branching in the hemicellulose chain, the more soluble it is. The solubility of hemi-
cellulose is further enhanced by the availability of its hydroxyl (O-H) groups for hydrogen bonding
with water.
The term hemicellulose encompasses a very wide range of different molecules. The individual
hemicelluloses in a biomass sample along with their structures, composition, and relative abun-
dance vary depending on the genus, species, structural origin (tissue type, cell type) and location
in the cell wall. In the secondary cell wall, hemicellulose acts as a bridge between micro fibrils,