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"Biofuels represent a grand challenge in
technology," Prather told the Senate Committee on Energy and Natural
Resources. "There is no single silver bullet that will make a robust
transportation fuels industry a reality."
Most of the 5 billion gallons of ethanol produced
annually in the United States comes from corn, but there's not enough
corn available to make it a viable long-term source, according to
Stephanopoulos.
Right now, about 16 percent of the U.S. corn crop
is going into ethanol production, but the fuel makes up less than one
percent of U.S. demand for liquid fuels, once you take into account
the amount of energy needed to produce the ethanol, Stephanopoulos
said. Even if all U.S. corn went into ethanol production, there would
only be enough for 4 to 5 percent of U.S. annual liquid fuel
consumption.
To replace corn, scientists are turning to
cellulose found in grasses and agricultural wastes. In his Science
article, Stephanopoulos outlined several challenges to producing
ethanol from cellulose and avenues of research scientists are pursuing
to overcome them.
"The technology to produce cellulosic ethanol is
not there yet," he said. However, he estimates that large-scale,
economically feasible production of ethanol from cellulose could
happen within 10 to 15 years.
One of the major advantages of cellulosic material
is its abundance, according to Stephanopoulos. He cited a recent
Department of Energy report that estimated the United States could
sustainably produce about 1.4 billion tons of such material per year.
"If we can convert that into liquid fuel, that
could become a pretty significant percentage of liquid fuels in the
U.S.," he said.
In addition, the energy balance in making ethanol
from cellulosic material is much more favorable than producing ethanol
from corn.
There are two major steps to producing ethanol from
plant energy crops and agricultural wastes. First, the plant material
must be broken down into its main components (cellulose and
hemicellulose), which is done by treatment with heat, acid, ammonia or
steam. In the second step, the cellulose is broken down into sugars,
such as glucose, and fermented into ethanol. That is usually done by
yeast or other microorganisms.
To maximize efficiency, scientists need to improve
both the amount of plant material that can be produced per acre and
the amount of ethanol that can be produced from the biomass,
Stephanopoulos says.
Current lines of research described in his Science
article include:
Consolidating the ability to break down cellulose
and ferment sugar into ethanol in a single microorganism-"That would
be a wonderful possibility, to have a single reactor," Stephanopoulos
said.
Last year, Stephanopoulos and MIT colleagues
reported that they had developed a new way to engineer the genome of
yeast to produce desirable traits-in that case, the ability to
tolerate high levels of ethanol, which is normally toxic to yeast. The
technique holds promise for the development of other traits that would
make yeast more efficient ethanol producers.
In her prepared statement during last week's Senate
hearing, Prather pointed to that study as an example of the kind of
research that needs to be done if the United States wants to commit to
converting to alternative fuels like ethanol.
"If we're going to be serious about it, the
government has to be serious about funding long-term research about
how we're going to transform our infrastructure-our economy, really,"
Prather said this week.
Prather emphasized to the committee that the
development of biofuels is a "systems problem, meaning that there are
lot of components and each one has an impact on all the others."
While some researchers pursue genetic manipulation
of microorganisms, they need to also coordinate with others involved
in fuel production and consumption-for example, engineers designing
new engines and agricultural scientists working with new crops.
"Those are conversations that have to be had all
along the pipeline," Prather said.
Her other primary message to the committee was that
while ethanol is promising, other potential biofuels, such as
biodiesel, butanol and hydrogen, should not be excluded from study.
Ethanol has only 70 percent of the energy density
of gasoline, making it less efficient, and its tendency to absorb
water makes corrosion a concern for the current U.S. petroleum storage
and distribution network, she said.
"A broader vision should include the possibility of
alternative biomass-derived fuels with better physical properties and
better integration into the infrastructure-that is, fuels that will
work in existing cars and which can be transported through existing
networks," she told the committee. |
