Biological Ways Of Producing Ethanol
Biological Ways Of Producing Ethanol
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.
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."
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.
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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