Ethanol
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concentration of approximately 15%. The beer is at least 85% water, and is
distilled to separate out the ethanol. However, as the ethanol boils off and
is condensed into liquid, some water accompanies the ethanol. Distillation
cannot remove the last 5% of the water. Usually, a molecular sieve is used to
remove this water to produce anhydrous ethanol. The corn-to-ethanol process also produces a high protein material that, after drying, is called distillers dried grains (DDG) and is used as animal feed.
For sugary crops such as sugar cane or sugar beets, the conversion of starch
to glucose is not required. The sugar is extracted directly from the crop and
the remainder of the process is the same as for converting corn starch to
ethanol. Sugar cane is the primary feedstock for production of fuel ethanol
in Brazil.
If the ethanol production feedstocks are limited to corn, sugar cane, or
sugar beets, not enough ethanol can be produced to meet the total demand
for engine fuel. For example, corn-based ethanol can meet less than 10% of the
total demand for transportation fuel. Thus, there is great interest in expanding the feedstock pool to include cellulosic materials. The pool of potential
cellulosic feedstocks is large enough that, when converted into ethanol, could
potentially supply most if not all of the transportation fuel needs.
The technology for cellulose conversion is still in its infancy. A difficult
key problem is to deconstruct the waxy lignin that provides the plant structural support while containing and protecting the cellulose and hemicellulose within. The lignin itself contains no sugars but it must be deconstructed
to allow access to the cellulose and hemicelluloses. The growing interest
in cellulosic-based fuel ethanol has sparked a great amount of research on
lignin deconstruction. Three different approaches (physical, chemical and
biological, and combinations of those) are being studied. Pulverization is an
example of the physical approach. Treatment with concentrated acid is one
chemical approach. Genetic modification of plants to produce more easily
deconstructed lignin is a biological approach. A successful approach must
be able to free up a high percentage of the sugars, be reasonably fast, and
not consume excessive energy. Some of the sugars in cellulose and hemicellulose are not directly fermentable to ethanol. Thus, after the lignin is deconstructed and removed, enzymes are used to convert these unfermentable
sugars to a fermentable form. The remainder of the process is then similar to
the production of ethanol from corn.
Beyond the conversion of cellulose to ethanol, other problems must be
solved before cellulose can become a useful feedstock for ethanol production. These include a selection of the best plant species and development of
procedures for harvesting, densification, transport, and storage of the cellulosic crops. Among the species being considered are grasses such as switch
grass or miscanthus. Currently unutilized crop residues such as corn cobs,
corn stalks, or sugar cane bagasse are also possibilities. Products such as
waste paper or wood wastes could also be sources of cellulose. For each such
cellulose source, it would be necessary to develop economical procedures for
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concentration of approximately 15%. The beer is at least 85% water, and is
distilled to separate out the ethanol. However, as the ethanol boils off and
is condensed into liquid, some water accompanies the ethanol. Distillation
cannot remove the last 5% of the water. Usually, a molecular sieve is used to
remove this water to produce anhydrous ethanol. The corn-to-ethanol process also produces a high protein material that, after drying, is called distillers dried grains (DDG) and is used as animal feed.
For sugary crops such as sugar cane or sugar beets, the conversion of starch
to glucose is not required. The sugar is extracted directly from the crop and
the remainder of the process is the same as for converting corn starch to
ethanol. Sugar cane is the primary feedstock for production of fuel ethanol
in Brazil.
If the ethanol production feedstocks are limited to corn, sugar cane, or
sugar beets, not enough ethanol can be produced to meet the total demand
for engine fuel. For example, corn-based ethanol can meet less than 10% of the
total demand for transportation fuel. Thus, there is great interest in expanding the feedstock pool to include cellulosic materials. The pool of potential
cellulosic feedstocks is large enough that, when converted into ethanol, could
potentially supply most if not all of the transportation fuel needs.
The technology for cellulose conversion is still in its infancy. A difficult
key problem is to deconstruct the waxy lignin that provides the plant structural support while containing and protecting the cellulose and hemicellulose within. The lignin itself contains no sugars but it must be deconstructed
to allow access to the cellulose and hemicelluloses. The growing interest
in cellulosic-based fuel ethanol has sparked a great amount of research on
lignin deconstruction. Three different approaches (physical, chemical and
biological, and combinations of those) are being studied. Pulverization is an
example of the physical approach. Treatment with concentrated acid is one
chemical approach. Genetic modification of plants to produce more easily
deconstructed lignin is a biological approach. A successful approach must
be able to free up a high percentage of the sugars, be reasonably fast, and
not consume excessive energy. Some of the sugars in cellulose and hemicellulose are not directly fermentable to ethanol. Thus, after the lignin is deconstructed and removed, enzymes are used to convert these unfermentable
sugars to a fermentable form. The remainder of the process is then similar to
the production of ethanol from corn.
Beyond the conversion of cellulose to ethanol, other problems must be
solved before cellulose can become a useful feedstock for ethanol production. These include a selection of the best plant species and development of
procedures for harvesting, densification, transport, and storage of the cellulosic crops. Among the species being considered are grasses such as switch
grass or miscanthus. Currently unutilized crop residues such as corn cobs,
corn stalks, or sugar cane bagasse are also possibilities. Products such as
waste paper or wood wastes could also be sources of cellulose. For each such
cellulose source, it would be necessary to develop economical procedures for