Environmental Engineering Reference
In-Depth Information
straight to the cattle as feed. Ideally, the corn will pass through the cow,
and the manure may return to the plant and go into an anaerobic digester
to produce methane, natural gas.
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Corn is not the only feedstock chemically suitable for ethanol. Research
is underway to derive ethanol from wheat, oats, and barley, and others
are investigating the use of microbes genetically engineered to produce
enzymes that will convert the cellulose in crop waste, wood chips, and
other plants to ethanol. Cellulose, essentially any kind of plant fi ber, is
available all over the United States, not only in the Midwest where corn
is grown and most ethanol plants are located. Ethanol plants using cel-
lulose could be located near coastal areas, alleviating the problem of
transporting ethanol from the Midwest. But ethanol is corrosive and
absorbs water, making it diffi cult to transport in pipelines.
Cellulosic ethanol is much more energy effi cient than ethanol from
corn and has a higher productivity. Using cellulose from native rather
than from cultivated plants (“grassoline”) does not require fertilizer or
the petroleum-using machinery to spread it. Using waste materials such
as corn stover, wood pulp, or trash can be even more effi cient and has
the additional positive effect of putting waste materials to work. The
energy bill that Congress passed in December 2007 requires that 3
percent of the nation's federally mandated ethanol be derived from cel-
lulosic sources by 2012 and 44 percent by 2022. As of mid-2009, there
was no economically competitive commercial production of cellulosic
ethanol, but there is intensive research by many corporations, and tech-
nological barriers may soon be overcome.
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The amount of available cellulosic biomass in the United States could
produce more than 100 billion gallons of grassoline a year, about half
its current annual consumption of gasoline and diesel. Similar projections
estimate that the global supply of cellulosic biomass has an energy
content equivalent to between 34 billion and 160 billion barrels of oil a
year, numbers that exceed the world's current annual consumption of
30 billion barrels of oil. Cellulosic biomass can also be converted to any
type of fuel: ethanol, ordinary gasoline, diesel, even jet fuel.
Ethanol distilled from sugar cane is much cheaper to produce and
generates eight times more energy per unit of input than corn does.
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The
United States does not use sugar cane to produce ethanol for several
reasons that are related to politics rather than to science. The favors
granted to the American sugar industry, such as a guaranteed share of
85 percent of the market, keep the price of domestic sugar so high that
using it for ethanol is not cost-effective. And the tariffs and quotas for
