It's easy to get
carried away with the advantages of ethanol as a transportation fuel.
It is a clean-burning, renewable, domestically produced product made
from fermented agricultural products, such as corn. The use of ethanol
does not contribute the amount of noxious fumes and volatile organic
compounds that standard gasoline spews into the air. Ethanol contains
oxygen, which provides a cleaner and more efficient burn of the fuel.
In E-85 fuel, made of 85% ethanol and 15% gasoline, ethanol lowers emissions
of unhealthy carbon monoxide by 30% and carbon dioxide by 27%. Although
ethanol emits carbon dioxide when burned, much of this greenhouse gas
is absorbed, or recycled, by the types of crops from which the ethanol
was made. As a result, burning ethanol contributes very little net carbon
dioxide to the atmosphere.
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This
bus in Peoria, Illinois runs on fuel made mostly of ethanol produced
from corn. (Photo by Greater Peoria Mass Transit District, Courtesy
of DOE/NREL.)
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In terms of cost, however,
E-85 doesn't compete with gasoline yet. Dick Ziegler, manager of ORNL's
Transportation Technology Program, who often drives one of ORNL's green
ethanol cars, says, "On one of my trips to Detroit, when gasoline cost
$1.34 a gallon, E-85 fuel cost $2.26 a gallon. A 50 cent per gallon
tax credit would be required to make ethanol cost the same as the wholesale
price of gasoline."
Several ORNL researchers
are working on lowering the cost of ethanol production. In the first
step of this process, cellulose from waste wood and paper or harvested
corn, switchgrass, or hybrid poplars is pretreated with enzymes and
acids at the appropriate pressures and temperatures. Cellulose is a
polysaccharide containing chains of 6-carbon sugars. Jonathan Woodward
and his colleagues in ORNL's Chemical Technology Division (CTD) are
trying to find a more efficient, cost-effective enzyme that can split
cellulose into individual sugar molecules, such as glucose and xylose,
for direct conversion to ethanol.
The next step
is fermentation—converting sugar into ethanol using microorganisms.
Researchers Nhuan Nghiem, Brian Davison, and Tanya Kuritz, all of CTD,
are working on lowering the cost of this step.
Nghiem and Davison
are experimenting with a syrup of simple sugars, called lignocellulosic
hydrolyzate, supplied to them by Arkenol Inc. They pump the syrup up
through a fluidized-bed bioreactor (FBR) containing gel beads stuffed
with Zymomonas mobilis bacteria. The bacteria eat the sugars
and excrete ethanol, which comes out of the top of the bioreactor looking
like beer froth.
"By using immobilized
biocatalysts in an FBR, we produce ethanol 10 to 20 times faster than
do traditional batch processes using suspended bacteria in stirred tank
bioreactors," Nghiem says. "If our technique were widely used to produce
ethanol, the cost of the fuel could be lowered by 3 to 6 cents per gallon."
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Nhuan
Nghiem checks the bioreactor in which microorganisms stuffed into
gel beads convert sugar (from enzyme-degraded cellulose) to ethanol
for use as fuel. (Photo by Curtis Boles.)
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To reduce costs
further, the researchers envision a single system for using both cellulose-degrading
enzymes and microbes that ferment the resulting syrup to produce ethanol
(which would then be distilled to yield a transportation fuel). The
problem is that Zymomonas mobilis bacteria prefer a temperature
of 35°C and cannot tolerate the 55°C temperature at which the
enzymes work best.
"Tanya Kuritz
and I are trying to genetically engineer two candidate sugar-eating
microbes that thrive at 55°C," says Nghiem. "We will add the genes
that make the two key enzymes that produce ethanol as the desired final
product, which is a waste product of the bacteria. We will knock out
the genes responsible for the excretion of other products, to maximize
ethanol production."
DOE's Bioenergy Feedstock
Development Program, which is managed at ORNL by Janet Cushman and Lynn
Wright, is using genetic manipulation in a quest to maximize carbon
production in hybrid poplar trees and switchgrass, a native perennial
prairie grass. One goal is to increase the yield of ethanol from these
plants.
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Janet
Cushman, co-manager of DOE's Bioenergy Feedstock Development Program
at ORNL, examines species of fast-growing switchgrass. (Photo
by Curtis Boles.)
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Sandy McLaughlin
of ORNL's Environmental Sciences Division and Marie Walsh of the Energy
Division are studying the economics of using switchgrass to produce
ethanol. They are evaluating the soil and water quality and farm income
benefits that result from producing switchgrass in place of corn, wheat,
and other annual crops. They are also comparing the greenhouse gas emissions
that result from producing and using ethanol made from switchgrass with
those from the use of gasoline and other alternative transportation
fuels. Inclusion of environmental and social benefits into the market
price of transportation fuels has the potential to alter significantly
the relative economics of the different fuels.
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Bioenergy
Feedstock Development Program
ORNL's Transportation
Technology Program
ORNL's
Chemical Technology Division
ORNL's Environmental
Sciences Division
ORNL's
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