Page 244 - Synthetic Fuels Handbook

230 CHAPTER EIGHT

O
OH
O HO OH
HO OH
OH
OCH
CH O 3
3
CH O OCH 3 OCH 3 OH
3
O HO OH
OH
O
OH
HO
OH O
O OCH 3
HO O HO
O OCH 3
HO OCH 3 O
HO
OCH 3
HO
O OH
O O
OCH
CH O OH 3
3
O
OCH 3
OH
HO OH
OCH 3
FIGURE 8.2 Hypothetical structure of lignin to illustrate the complexity of the molecule.

Lignin is the final major constituent of plant material important to biomass processing
and it is a complex chemical compound that is most commonly derived from wood and is
an integral part of the cell walls of plants, especially in tracheids, xylem fibers, and scler-
eids. The chemical structure of lignin is unknown and, at best, can only be represented by
a hypothetical formula, the veracity of which is questionable.
Lignin is not a carbohydrate, but a polymer of single benzene rings linked with aliphatic
chains; the phenolic compound p-hydroxyphenylpropane is an important monomer group
in lignin. Like hemicellulose, lignin is amorphous and more soluble than cellulose. It may
be removed from wood by steaming or by dissolving in hot aqueous or aqueous bisulfite
solution. Lignin resists hydrolysis and is resistant to microbial degradation.
The term lignin was introduced in 1819 and is derived from the Latin word lignum
(meaning wood). It is one of most abundant organic compounds on earth after cellulose
and chitin. By way of clarification, chitin (C H O N) is a long-chain polymeric polysac-
5
n
13
8
charide of beta-glucose that forms a hard, semitransparent material found throughout the
natural world. Chitin is the main component of the cell walls of fungi and is also a major
component of the exoskeletons of arthropods, such as the crustaceans (e.g., crab, lobster,
and shrimp) and the insects (e.g., ants, beetles, and butterflies), and of the beaks of cepha-
lopods (e.g., squids and octopuses).