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Water for Energy and Fuel Production
by the catalytic methods and the resulting phenol groups are reacted with methanol
to produce methyl aryl ethers. These substances are high-value octane enhancers
and can be blended with gasoline. We now examine each of the steps outlined in
Figure 9.5 in detail.
9.4.1 PreTreATmenT
Unlike in the production of grain ethanol, in the production of cellulosic ethanol,
pretreatment is essential to achieve the reasonable rates of yields in the enzymatic
hydrolysis of biomass [2]. Pretreatment has generally been practiced to reduce the
crystallinity of cellulose, to lessen the average degree of polymerization of the cellulose and the lignin–hemicellulose sheath that surrounds the cellulose, and to alleviate the lack of the available surface area for enzymes to attack. The importance of
pretreatment can be better understood by examining the hydrolysis process in which
the interaction between the enzymes and the substrates must occur. The hydrolysis of
cellulose into sugars and other oligomers is a solid-phase reaction in which enzymes
must bind to surface to catalyze the reaction. Cellulase enzymes (which are commonly used) are large proteins with molecular weight ranging from 30,000 to 60,000
and are thought to be ellipsoid with major and minor dimensions of 30°A–200°A.
The internal surface area of wood is very large; however, only about 20% of the
prevolume is accessible to cellulose-sized molecules. By breaking down the tight
hemicellulose–lignin matrix, hemicellulose or lignin can be separated and the accessible volume of cellulose can be greatly increased. This removal of materials greatly
enhances the enzymatic digestibility.
A typical pretreatment consists of size reduction, pressure sealing, heating, reaction, pressure release, surface area increase, and hydrolyzate/solids separation [23].
Mechanical pretreatments such as intensive ball milling and roll milling to expose
more surface area have been found to be very expensive. The hemicellulose–lignin
sheath can be disrupted by either acidic or basic catalysts. While basic catalysts
simultaneously remove lignin and hemicellulose, its consumption is very large due to
its use in neutralization by ash and acidic groups in the hemicellulose. In the recent
years, more acidic catalysts such as mineral acids and organic acids generated in situ
by autohydrolysis of hemicellulose have been tested.
The five important pretreatment processes that are currently being examined and
implemented are as follows [2]:
1. Rapid steam hydrolysis (RASH) or autohydrolysis steam explosion
2. Dilute acid prehydrolysis
3. Organosolv pretreatment
4. Combined RASH and organosolv pretreatment
5. Ionic liquid pretreatment
Most pretreatment approaches are not intended to actually hydrolyze cellulose to
soluble sugars, but rather to generate the pretreated cellulosic residue that is more
hydrolyzable by cellular enzymes than native biomass. Here we examine each pretreatment process in detail.
Water for Energy and Fuel Production
by the catalytic methods and the resulting phenol groups are reacted with methanol
to produce methyl aryl ethers. These substances are high-value octane enhancers
and can be blended with gasoline. We now examine each of the steps outlined in
Figure 9.5 in detail.
9.4.1 PreTreATmenT
Unlike in the production of grain ethanol, in the production of cellulosic ethanol,
pretreatment is essential to achieve the reasonable rates of yields in the enzymatic
hydrolysis of biomass [2]. Pretreatment has generally been practiced to reduce the
crystallinity of cellulose, to lessen the average degree of polymerization of the cellulose and the lignin–hemicellulose sheath that surrounds the cellulose, and to alleviate the lack of the available surface area for enzymes to attack. The importance of
pretreatment can be better understood by examining the hydrolysis process in which
the interaction between the enzymes and the substrates must occur. The hydrolysis of
cellulose into sugars and other oligomers is a solid-phase reaction in which enzymes
must bind to surface to catalyze the reaction. Cellulase enzymes (which are commonly used) are large proteins with molecular weight ranging from 30,000 to 60,000
and are thought to be ellipsoid with major and minor dimensions of 30°A–200°A.
The internal surface area of wood is very large; however, only about 20% of the
prevolume is accessible to cellulose-sized molecules. By breaking down the tight
hemicellulose–lignin matrix, hemicellulose or lignin can be separated and the accessible volume of cellulose can be greatly increased. This removal of materials greatly
enhances the enzymatic digestibility.
A typical pretreatment consists of size reduction, pressure sealing, heating, reaction, pressure release, surface area increase, and hydrolyzate/solids separation [23].
Mechanical pretreatments such as intensive ball milling and roll milling to expose
more surface area have been found to be very expensive. The hemicellulose–lignin
sheath can be disrupted by either acidic or basic catalysts. While basic catalysts
simultaneously remove lignin and hemicellulose, its consumption is very large due to
its use in neutralization by ash and acidic groups in the hemicellulose. In the recent
years, more acidic catalysts such as mineral acids and organic acids generated in situ
by autohydrolysis of hemicellulose have been tested.
The five important pretreatment processes that are currently being examined and
implemented are as follows [2]:
1. Rapid steam hydrolysis (RASH) or autohydrolysis steam explosion
2. Dilute acid prehydrolysis
3. Organosolv pretreatment
4. Combined RASH and organosolv pretreatment
5. Ionic liquid pretreatment
Most pretreatment approaches are not intended to actually hydrolyze cellulose to
soluble sugars, but rather to generate the pretreated cellulosic residue that is more
hydrolyzable by cellular enzymes than native biomass. Here we examine each pretreatment process in detail.