Ethanol, also known as ethyl alcohol, drinking alcohol or grain
alcohol, is a flammable, colorless, slightly toxic chemical compound, and is best
known as the alcohol found in alcoholic beverages. In common usage, it is often
referred to simply as alcohol. Combustion of ethanol forms carbon dioxide and water:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O.
Currently the main feedstock in the United States for the production of ethanol
is corn. Approximately 2.8 gallons of ethanol (10 liters) are produced from one
bushel of corn (35 liters). While much of the corn turns into ethanol, some of the
corn also yields by-products such as DDGS (distillers dried grains with solubles)
that can be used to fulfill a portion of the diet of livestock. A bushel of corn
produces about 18 pounds of DDGS. Critics of ethanol as fuel decry the use of corn
to produce ethanol because corn is an energy-intensive crop that requires petroleum-derived
fertilizers; however, using corn to produce alcohol could save farmers additional
petroleum if the farmers are feeding the byproduct to livestock and if the excrement
from the animals is then used as fertilizer for the corn [Lynn Ellen Doxon; The
Alcohol Fuel Handbook].
Although most of the fermentation plants have been built in corn-producing regions,
sorghum is also an important feedstock for ethanol production in the Plains states.
Pearl millet is showing promise as an ethanol feedstock for the southeastern U.S.
Trials of new crops, such as agricultural residues, wood wastes, and various grasses,
show much lower yields using conventional, commercialized processes. These crops
are cellulosic rather than starchy, and have fewer accessible sugars for fermentation.
Newer, more complex processes are necessary to release plant sugars, primarily by
disrupting lignin networks. However, the appeal of such crops is their lower requirement
for fertilizer and other inputs, and in some cases lower cost or higher availability
as "waste" products. The dominant ethanol feedstock in warmer regions
is sugarcane. The directly-accessible sugars simplify the fermentation process.
In temperate regions, this accessibility has been somewhat replicated by selective
breeding of the sugar beet. In some parts of Europe, particularly France and Italy,
wine is used as a feedstock due to massive oversupply. Japan is hoping to use rice
wine (sake) as an ethanol source.
At petroleum prices like those that prevailed through much of the 1990s, ethylene
hydration was a decidedly more economical process than fermentation for producing
purified ethanol. Later increases in petroleum prices, coupled with perennial uncertainty
in agricultural prices, make forecasting the relative production costs of fermented
versus petrochemical ethanol difficult.
The Canadian firm Iogen brought the first cellulose-based ethanol plant on-stream
in 2004. The primary consumer thus far has been the Canadian government, which,
along with the United States government , has invested millions of dollars into
assisting the commercialization of cellulosic ethanol. Realization of this technology
would turn a number of cellulose-containing agricultural byproducts, such as corncobs,
straw, and sawdust, into renewable energy resources. Other enzyme companies are
developing genetically engineered fungi which would produce large volumes of cellulase,
xylanase and hemicellulase enzymes which can be utilized to convert agricultural
residues such as corn stover, distiller grains, wheat straw and sugar cane bagasse
and energy crops such as Switchgrass into fermentable sugars which may be used to
produce cellulosic ethanol.
Cellulosic materials typically contain, in addition to cellulose, other polysaccharides,
including hemicellulose. When hydrolysed, hemicellulose breaks down into mostly
five-carbon sugars such as xylose. S. cerevisiae, the yeast most commonly used for
ethanol production, cannot metabolize xylose. Other yeasts and bacteria are under
investigation to metabolize xylose and so improve the ethanol yield from cellulosic
material.
Prospective technologies:
The anaerobic bacterium Clostridium ljungdahlii, recently discovered in commercial
chicken wastes, can produce ethanol from single-carbon sources including synthesis
gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial
combustion of either fossil fuels or biomass. Use of these bacteria to produce ethanol
from synthesis gas has progressed to the pilot plant stage at the BRI Energy facility
in Fayetteville, Arkansas.
Another prospective technology is the closed-loop ethanol plant. Ethanol produced
from corn has a number of critics who suggest that it is primarily just recycled
fossil fuels because of the energy required to grow the grain and convert it into
ethanol. However, the closed-loop ethanol plant attempts to address this criticism.
In a closed-loop plant, the energy for the distillation comes from fermented manure,
produced from cattle that have been fed the by-products from the distillation. The
leftover manure is then used to fertilize the soil used to grow the grain. Such
a process is expected to have a much lower fossil fuel requirement. However, general
thermodynamic considerations indicate that the total efficiency of such plants,
in combination with the production of cellulose/sugar, will remain relatively low
Source:
Wikipedia