Environmental Engineering Reference
In-Depth Information
process called lignification. The three classes of plant lignins are gymnosperm (soft-
wood), angiosperm (hardwood), and grass lignins. Lignin is primarily aromatic in
nature, which is obvious from the chemical structure. Its basic molecular units are
linked mostly by ether bonds, but carbon-to-carbon linkages exist as well. Different
functional side groups can be distinguished, such as hydroxyl, methoxyl, and carbonyl
groups. Covalent bonds exist with both hemicelluloses and cellulose (Jacobsen and
Wyman, 2000), rendering improved mechanical strength to the plant.
Lignin has multiple functions in plant biomass. Apart from its contribution to the
mechanical strength, it gives protection to plants against attacks from, e.g., bacteria, as
it is normally not digested by animal enzymes. Moreover, it stimulates the circulation
of water in the vascular system of plants. Finally, it is the least degradable part of dead
plants, making it the most important component of humus.
Lignin is relatively stable in thermal conversion (pyrolysis), and it has a larger
higher heating value (HHV) than carbohydrates. Lignin is produced in large quantities
as by-product of the pulp and paper industry, which needs cellulose fibers with a low
lignin content, and it mostly serves as heat supply fuel. It can also be used as substrate
for the production of high-added-value products, such as vanillin, phenol, and derived
phenolic resins. Lignin also plays a role in rubber production and antioxidant
generation (Pye, 2006). Chapter 15 concerning biorefineries presents some of the
possibilities of lignin valorization.
The aforementioned three major biopolymers in plants combine to form so-called
tracheids, which are elongated cells in vascular plants that facilitate transport of water.
Figure 2.6 shows a schematic illustration of these tracheids
structure indicating the
entanglement in the secondary walls of lignin, hemicelluloses, and cellulose.
To summarize the relative presence of the three main biopolymers in plant species,
an example of the composition of biomass in terms of main organic constituents is
given in Table 2.4.
'
2.2.4 Chitin and Chitosan
Chitin is a naturallyoccurringbiopolymer,whichprotects insects and sea life species and
for this purpose is incorporated in the formof enclosing skeletons. After cellulose, chitin
is themost abundant polysaccharide innonplant life nature and is primarilypresent in the
exoskeletons of crustaceans (such as crabs, shrimp, lobsters, etc.) and also in various
insects, worms, fungi, and mushrooms in varying amount (Arcidiacono and Kaplan,
1992). Chitin innature is usually associatedwithother components to reinforce the struc-
ture, such as calciumcarbonate (CaCO
3
) andproteins. Chitosan is derived fromchitin by
deacetylation using a sodiumhydroxide solution; the structures are shown in Figure 2.7.
2.3 MINOR ORGANIC CONSTITUENTS
Together with the major support structures described in Section 2.2, biomass contains
other components, such as oils, fats (lipids), proteins, starch, and sugars, as well as a
spectrum of organic extractives. These compounds are widely used as (ingredients of )
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