243
Hydrolysis and Fermentation Technologies for Alcohols
Lignocellulose Prehydrolysis
pretreatment
Acid or
enzymatic
hydrolysis
Glucose
fermentation
Distillation
Ethanol
Lignin
Xylose
Xylose
fermentation
Lignin
processing
Ethanol
Beer
Other value-added
chemicals
FiGUre 9.5 Conversion of lignocellulose to ethanol. (Adapted from Lee, S. and Shah, Y.,
Biofuels and Bioenergy—Processes and Technologies, CRC Press, Boca Raton, FL, 2012.)
macromolecule whose typical molecular weight exceeds 10,000. Because of its crosslinked structure, it is difficult to process, extract, and hydrolyze. The major purpose
of the pretreatment step of the cellulosic ethanol process is to degrade the cross-linked
structure so that both cellulose and hemicellulose are more exposed for subsequent
hydrolysis and fermentation steps. An efficient conversion of lignin results in a substantial increase in the overall fuel yield of the cellulosic ethanol process.
A general scheme for the conversion of lignocellulose to ethanol is shown in
Figure 9.5. The lignocellulose is pretreated to separate the xylose and sometimes the
lignin from the crystalline cellulose. This step is very important because the efficiency of the pretreatment affects the efficiency of the subsequent steps. The xylose
can then be fermented to ethanol, whereas the lignin can be further processed to
produce other liquid fuels and valuable chemicals. Crystalline cellulose, the largest
(about 50%) and most useful fraction, remains behind as solids after pretreatment
and is sent to an acid or enzymatic hydrolysis process to break down the cellulose to
glucose. Enzymatic hydrolysis (which is more popular now) is very specific and does
not break down further sugars. Enzymatic processes are capable of achieving 100%
yield. The glucose is then fermented to ethanol and combined with the ethanol from
xylose fermentation. This dilute ethanol–water solution is further concentrated by
distillation and other dehydration processes.
For an overall efficiency of the conversion process, it is important to convert
hemicellulose (which can be up to 25% of lignocellulose) to xylose and xylose to
ethanol. Hemicellulose is primarily composed of xylan that can be easily converted
to xylose. xylose constitutes about 17% of woody angiosperms and accounts for a
substantially higher percentage of herbaceous angiosperms. Though the fermentation of xylose to ethanol is more difficult than that of glucose, it is very essential
for the overall efficiency of the process. Significant new yeast developments for this
purpose are currently pursued. Methods have been identified using new strains of or
metabolically engineered yeasts [22], bacteria, and processes containing enzymes
and yeasts.
Lignin (around 25% of lignocellulose) is a large random phenolic polymer. In
lignin processing, the polymer is broken down into fragments containing one or
two phenolic rings. Extra oxygen and side chains are stripped from the molecules
Hydrolysis and Fermentation Technologies for Alcohols
Lignocellulose Prehydrolysis
pretreatment
Acid or
enzymatic
hydrolysis
Glucose
fermentation
Distillation
Ethanol
Lignin
Xylose
Xylose
fermentation
Lignin
processing
Ethanol
Beer
Other value-added
chemicals
FiGUre 9.5 Conversion of lignocellulose to ethanol. (Adapted from Lee, S. and Shah, Y.,
Biofuels and Bioenergy—Processes and Technologies, CRC Press, Boca Raton, FL, 2012.)
macromolecule whose typical molecular weight exceeds 10,000. Because of its crosslinked structure, it is difficult to process, extract, and hydrolyze. The major purpose
of the pretreatment step of the cellulosic ethanol process is to degrade the cross-linked
structure so that both cellulose and hemicellulose are more exposed for subsequent
hydrolysis and fermentation steps. An efficient conversion of lignin results in a substantial increase in the overall fuel yield of the cellulosic ethanol process.
A general scheme for the conversion of lignocellulose to ethanol is shown in
Figure 9.5. The lignocellulose is pretreated to separate the xylose and sometimes the
lignin from the crystalline cellulose. This step is very important because the efficiency of the pretreatment affects the efficiency of the subsequent steps. The xylose
can then be fermented to ethanol, whereas the lignin can be further processed to
produce other liquid fuels and valuable chemicals. Crystalline cellulose, the largest
(about 50%) and most useful fraction, remains behind as solids after pretreatment
and is sent to an acid or enzymatic hydrolysis process to break down the cellulose to
glucose. Enzymatic hydrolysis (which is more popular now) is very specific and does
not break down further sugars. Enzymatic processes are capable of achieving 100%
yield. The glucose is then fermented to ethanol and combined with the ethanol from
xylose fermentation. This dilute ethanol–water solution is further concentrated by
distillation and other dehydration processes.
For an overall efficiency of the conversion process, it is important to convert
hemicellulose (which can be up to 25% of lignocellulose) to xylose and xylose to
ethanol. Hemicellulose is primarily composed of xylan that can be easily converted
to xylose. xylose constitutes about 17% of woody angiosperms and accounts for a
substantially higher percentage of herbaceous angiosperms. Though the fermentation of xylose to ethanol is more difficult than that of glucose, it is very essential
for the overall efficiency of the process. Significant new yeast developments for this
purpose are currently pursued. Methods have been identified using new strains of or
metabolically engineered yeasts [22], bacteria, and processes containing enzymes
and yeasts.
Lignin (around 25% of lignocellulose) is a large random phenolic polymer. In
lignin processing, the polymer is broken down into fragments containing one or
two phenolic rings. Extra oxygen and side chains are stripped from the molecules