Environmental Engineering Reference
In-Depth Information
the GHG impact depends on farming practices, particularly the use of fertilizers.
This is specifically true for ethanol made from corn. When ethanol is made from
cellulosic sources there is considerable reduction in GHGs [48]. This is because
producers of cellulosic ethanol burn lignin to heat the plant sugars whereas most
producers of corn ethanol burn fossil fuels to provide the energy for fermentation.
Cellulosic ethanol is a renewable, biodegradable, clean burning, alternative fuel.
Cellulosic biomass typically contains 40-50% cellulose, 20-30% hemicellulose,
and the remainder, 15-30%, is lignin and other components [49]. Cellulose consists
of glucose monomers linked by a
β
-1,4 bond which forms a linear polymer [50].
Hemicellulose is a highly-branched complex polymer that is composed mainly of
xylose and other five-carbon sugars [50]. Lignin is a phenyl propane polymer that
acts as a binder, which cannot be converted into useful products. The hemicellulose
is randomly acetylated and acts as an interface between the cellulose and lignin.
The cellulose and hemicellulose can be broken down into simple sugars that are
used to produce ethanol, while the lignin can be burned to produce heat, which
helps to increase overall efficiency. What makes cellulosic ethanol promising is the
diverse, abundant, low cost feedstock that is readily available. There are two main
methods for the production of ethanol from biomass; enzymatic saccharification and
fermentation, and fermentation by cellulolytic microorganisms.
However, cellulosic ethanol is not without its challenges and drawbacks.
Commercial production of cellulosic ethanol currently requires high initial capital
costs and involves risk. In 2002, a DOE study determined that for cellulosic ethanol
to be competitive, the production cost would need to be $1.07 per gallon or less [51].
One of the most expensive steps in the production of cellulosic ethanol involves the
pretreatment of biomass.
4.2 Comparison of Pretreatment and Manufacturing Processes
Pretreatment is required to alter the physical and chemical properties of the biomass
to make it easier to process. The methods of pretreatment are similar for either enzy-
matic or microbial cellulosic ethanol processing. Removing or altering the lignin
allows access to carbohydrates in the biomass. Higher lignin sources require chemi-
cal treatment to reduce the level to below 12% to enhance digestibility [50]. To gain
access to the cellulose fiber, de-crystallization of the hemicellulose that is cova-
lently bound with the lignin via hydrolysis is required [52]. The conversion of all
the sugars derived from hemicellulose is highly desired to increase efficiency and
minimize by-products. Pretreatment of the biomass is also required to increase the
surface area and pore size, thus making it easier to digest. The increase in surface
area is from the combination of hemicellulose solubilization, lignin solubilization,
and lignin redistribution caused by various methods of pretreatment [53].
There are several methods by which pretreatment is performed: physical, chem-
ical, and biological. Physical methods include ball and compression milling that
shear or shed the biomass to de-crystallize the cellulose and increase the surface area
and digestibility. However, these processes do very little to degrade hemicellulose
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