Page 315 - Synthetic Fuels Handbook

FUELS FROM WOOD 301

10.2 COMPOSITION AND PROPERTIES

The cellulose content of wood varies between species in the range of 40 to 50 percent. Some
lignocellulosic materials can have more cellulose than wood. Cellulose is an organic poly-
mer, consisting solely of units of glucose held together in a straight chain macromolecule.
These glucose units are bound together by β-(1,4)-glycosidic linkages which establish as
the repeat unit for cellulose chains; cellulose must be hydrolyzed to glucose before fermen-
tation to ethanol.
Lignocellulose is a complex matrix combining cellulose, hemicellulose, and lignin,
along with a variable level of extractives. Cellulose is comprised of glucose, a six-carbon
sugar, while hemicellulose contains both five- and six-carbon sugars, including glucose,
galactose, mannose, arabinose, and xylose. The presence of cellulose and hemicellulose
therefore makes lignocellulose a potential candidate for bioconversion. The ability of the
bioconversion platform to isolate these components was initially limited, as the wood
matrix is naturally resistant to decomposition. Recent advances, however, have made this
process more commercially viable. Costs remain higher than for starch-based bioconver-
sion, but there is added potential for value-added products that can utilize the lignin com-
ponent of the wood.
By forming intramolecular and intermolecular hydrogen bonds between hydroxyl
groups within the same cellulose chain and the surrounding cellulose chains, the chains
tend to be arranged parallel and form a crystalline macromolecular structure. Bundles of
linear cellulose chains (in the longitudinal direction) form a microfibril which is oriented
in the cell wall structure (Hashem et al., 2007). Cellulose is insoluble in most solvents and
has a low accessibility to acid and enzymatic hydrolysis.
Unlike cellulose, hemicelluloses consist of different monosaccharide units. In addition,
the polymer chains of hemicelluloses have short branches and are amorphous. Because of
the amorphous morphology, hemicelluloses are partially soluble in water. Hemicelluloses
are related to plant gums in composition, and occur in much shorter molecule chains than
cellulose. The hemicelluloses, which are present in deciduous woods chiefly as pentosans
and in coniferous woods almost entirely as hexosans, undergo thermal decomposition very
readily. Hemicelluloses are derived mainly from chains of pentose sugars, and act as the
cement material holding together the cellulose micelles and fiber (Theander, 1985).
The backbone of the chains of hemicelluloses can be a homopolymer (generally consist-
ing of single sugar repeat unit) or a heteropolymer (mixture of different sugars). Among
the most important sugar of the hemicelluloses component is xylose. In hardwood xylan,
the backbone chain consists of xylose units which are linked by β-(1,4)-glycosidic bonds
and branched by α-(1,2)-glycosidic bonds with 4-O-methylglucuronic acid groups (Hashem
et al., 2007). In addition, O-acetyl groups sometime replace the OH groups in position C 2
and C . For softwood xylan, the acetyl groups are fewer in the backbone chain but softwood
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xylan has additional branches consisting of arabinofuranose units linked by α-(1,3)-glycosidic
bonds to the backbone. Hemicelluloses are largely soluble in alkali and, as such, are more
easily hydrolyzed.
Lignins are polymers of aromatic compounds the function of which is to provide struc-
tural strength, provide sealing of water-conducting system that links roots with leaves, and
protect plants against degradation. Lignin is a macromolecule, which consists of alkylphe-
nols and has a complex three-dimensional structure. Lignin is covalently linked with xylans
in the case of hardwoods and with galactose-glucose-mannose units in softwoods. Lignin
is a natural polymer which together with hemicelluloses acts as a cementing agent matrix
of cellulose fibers in the woody structures of plants.
Most lignin applications are based on technical lignins which are separated during pulp-
ing processes, and hydrolysis of lignin, which is obtained during wood acidic hydrolysis.