Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 40

Figure 2.4. Basic building blocks of lignin.
keeping them separate and also preventing them from sliding past one another and causing the
fibril networks to collapse.
It has been proposed that hemicellulose absorbs energy when the plant is subjected to different
dynamic forces (e.g. wind) and static forces (e.g. snow, ice) since it serves as a link between
cellulose and lignin and can thus be used to redistribute stresses. Because some hemicellulose
remains associated with cellulose even after prolonged cooking of chemical pulps, it has been
suggested that at least one end of the branched hemicellulose chain may be adsorbed onto the
cellulose micro fibrils. This is observed for approximately one third of the short-sided hemicel-
luloses. In softwoods and hardwoods, these tend to be glucomannans and xylans, respectively,
for which both ends of the branched chain can bind to micro fibrils, forming snakelike patterns
of relatively inflexible chains (Walker, 2006).
2.3.3 Lignin
Lignin is an unusual aromatic biopolymer because it has a random, complex and cross-linked
network structure, i.e. its structure is not well defined. It is also difficult to study in its unmodified
form as it readily undergoes oxidation and condensation reactions. Cell wall lignins are always
associated with hemicelluloses and they form covalent bonds with each other.
Lignin is the second most abundant natural polymer in plant biomass: it accounts for 17-24,
18-25 and 27-33% of the total mass of grasses, hardwoods and softwoods, respectively. It is a
natural amorphous polymer that effectively functions as “glue” maintaining the plant's structural
integrity. The basic building blocks of in lignin are three monolignols: p -coumaryl alcohol,
coniferyl alcohol, and sinapyl alcohol (Fig. 2.4). They form, respectively, p -hydroxyphenyl,
guaiacyl and syringyl phenylpropanoid units when incorporated into the lignin polymer (Boerjan
et al ., 2003). Lignins exist as a complex branched and cross-linked network of phenylpropenyl
subunits.
2.3.4 Extractives
The extractives are a wide range of compounds that can be extracted using polar (e.g. water) or
non-polar (e.g. ether) solvents. Plant extractives include waxes, oils, fats, lipids, resins etc. and
typically have high-energy values; fats and oils can yield as much as 38 kJ/g. For example, palm
fruits contain approximately 30% oil by mass and resin acids account for up to 45% of the dry
weight of the knot wood from certain softwood trees (Boutelje, 1966).
2.3.5 Sugars
When discussing biomass, “sugars” is typically taken to mean sucrose, which consists of glucose
and fructose. Sugar cane is themost well-known example of a plant containing high concentrations
of sugar. A mature stalk of sugar cane typically contains 12-16% (wt%, dw) soluble sugars.