242
Water for Energy and Fuel Production
Cellulose
bundles
Lignin
Hemicellulose
FiGUre 9.4 A universal description of plant cell wall. (Adapted from Lee, S. and Shah, Y.,
Biofuels and Bioenergy—Processes and Technologies, CRC Press, Boca Raton, FL, 2012.)
Cellulose is a large polymeric molecule composed of many hundreds or thousands
of monomeric sugar (glucose) molecules, and in this regard, it can be considered
as polysaccharide. The molecular linkages in cellulose form linear chains that are
rigid, highly stable, and resistant to chemical attack. It is also crystalline and may be
somewhat soluble in a suitable solvent [18]. However, cellulose molecules (which are
the predominant source of glucose for ethanol) in their crystalline form are packed
so tightly that even small molecules of water cannot easily permeate the structure. It
is even more difficult for large enzyme molecules to permeate and diffuse into the
cellulose structure. To break the crystalline structure of cellulose and make them
more exposed to enzymatic hydrolysis, all processes of cellulosic ethanol require
pretreatments. As discussed earlier, this step was not required in the production of
grain ethanol.
Starch and sugar can also come from hemicellulose that consists of short and
highly branched chains of sugar molecules. It contains both five-carbon sugars (such
as d-xylose and l-arabinose) and six-carbon sugars (such as d-galactose, d-glucose,
and d-mannose as well as uronic acid. For example, galactan found in hemicellulose is a polymer of sugar galactose. Since hemicellulose is amorphous due to highly
branched structures, it is relatively easy to hydrolyze to its constituents—five- and sixcarbon sugars [18]. While both five- and six-carbon sugars are in principle fermentable to ethanol, the fermentation chemistry, yeast requirement, and process chemistry
for six- and five-carbon sugars (pentose and xylose) are considerably different. In
general, five-carbon sugars are more difficult to ferment than six-carbon sugars.
Lignin molecule is a complex and highly cross-linked aromatic polymer that is
covalently linked to hemicellulose (Figure 9.4). Lignin contributes to the stabilization of mature cell walls. Due to its high calorific value, it provides more energy
than cellulose or hemicellulose, but it cannot be fermented to ethanol. Lignin is a
Water for Energy and Fuel Production
Cellulose
bundles
Lignin
Hemicellulose
FiGUre 9.4 A universal description of plant cell wall. (Adapted from Lee, S. and Shah, Y.,
Biofuels and Bioenergy—Processes and Technologies, CRC Press, Boca Raton, FL, 2012.)
Cellulose is a large polymeric molecule composed of many hundreds or thousands
of monomeric sugar (glucose) molecules, and in this regard, it can be considered
as polysaccharide. The molecular linkages in cellulose form linear chains that are
rigid, highly stable, and resistant to chemical attack. It is also crystalline and may be
somewhat soluble in a suitable solvent [18]. However, cellulose molecules (which are
the predominant source of glucose for ethanol) in their crystalline form are packed
so tightly that even small molecules of water cannot easily permeate the structure. It
is even more difficult for large enzyme molecules to permeate and diffuse into the
cellulose structure. To break the crystalline structure of cellulose and make them
more exposed to enzymatic hydrolysis, all processes of cellulosic ethanol require
pretreatments. As discussed earlier, this step was not required in the production of
grain ethanol.
Starch and sugar can also come from hemicellulose that consists of short and
highly branched chains of sugar molecules. It contains both five-carbon sugars (such
as d-xylose and l-arabinose) and six-carbon sugars (such as d-galactose, d-glucose,
and d-mannose as well as uronic acid. For example, galactan found in hemicellulose is a polymer of sugar galactose. Since hemicellulose is amorphous due to highly
branched structures, it is relatively easy to hydrolyze to its constituents—five- and sixcarbon sugars [18]. While both five- and six-carbon sugars are in principle fermentable to ethanol, the fermentation chemistry, yeast requirement, and process chemistry
for six- and five-carbon sugars (pentose and xylose) are considerably different. In
general, five-carbon sugars are more difficult to ferment than six-carbon sugars.
Lignin molecule is a complex and highly cross-linked aromatic polymer that is
covalently linked to hemicellulose (Figure 9.4). Lignin contributes to the stabilization of mature cell walls. Due to its high calorific value, it provides more energy
than cellulose or hemicellulose, but it cannot be fermented to ethanol. Lignin is a