Mar. 2, 2000 COLUMBUS, Ohio -- Ohio State University engineers have found a way to use methane to remove toxic nitric oxide emissions from the stack gases of coal-burning power plants.
This new method of catalytically reducing nitric oxide with methane removes up to 100 percent of nitric oxide from stack gases in a safer and less expensive way than any currently available.
Nitric oxide is a common byproduct of combustion, and cars and fossil-fuel burning power plants are two chief sources of the molecule. Once released, nitric oxide reacts with oxygen to produce nitric acid -- a main component of acid rain -- and depletes ozone in the upper layers of the stratosphere.
"Although the technology exists to reduce nitric oxide emissions, it is not a problem-free technology by any stretch of the imagination," said Umit Ozkan, professor of chemical engineering at Ohio State.
In a recent issue of the journal Catalysis Today, Ozkan and her graduate students, Junko Mitome and Enrique Aceves, described a method that may provide a better way.
Currently, plants that produce nitric oxide inject ammonia into their exhaust gases to convert the pollutant into nitrogen and water. A vanadium-oxide-based catalyst helps the chemical conversion take place.
But ammonia is expensive and difficult to handle, Ozkan said. It is highly corrosive, and toxic if released. Also, if the temperature inside a smokestack climbs too high, the ammonia may react with oxygen over this catalyst to form more nitric oxide.
Methane is environmentally more compatible than ammonia, Ozkan said, as well as cheap and plentiful. It is the main component of natural gas, and is already present in some smokestack emissions. Methane is also highly stable -- not reactive like ammonia.
This stability presents the only major difficulty for reducing nitric oxide emissions with methane, Ozkan said.
The researchers reported in Catalysis Today that having the right catalyst formulation to activate methane can overcome this obstacle. Under ideal conditions, Ozkan and her graduate students removed nearly 100 percent of nitric oxide from gas in a laboratory reactor.
With the help of the palladium-based catalyst, methane reacted with the nitric oxide to form nitrogen, carbon dioxide and water.
Ozkan acknowledged that palladium is expensive, but said the process requires very little. The catalyst thinly coats the interior surface of a ceramic support that contains thousands of tiny honeycomb-shaped passages.
"We are making the precious metal go a long way," she said. "Gas flows through the small channels inside the ceramic support and reacts with the catalyst that is coating the interior surfaces." In form and function, the catalyst bed resembles the inside of an automobile catalytic converter.
After testing the use of methane over palladium-based catalysts on emissions of nitric oxide alone, the engineers tested how well methane would remove the pollutant from a gas containing other compounds. They used a combination similar to real-world smokestack emissions -- nitric oxide, methane, oxygen, water, and sulfur dioxide -- for this part of the experiment.
Oxygen and water did not interfere with the reaction, but the sulfur dioxide did. Over 6 hours, it rendered the catalyst partially inactive, and lowered the amount of nitric oxide removed from the gas to 85 percent.
The engineers are working to improve the formulation of the catalyst, but Ozkan thinks part of the problem could be solved if smokestacks had two catalytic reactors and switched between them. That way one reactor would have time to recover from exposure to sulfur dioxide while the other one was actively removing nitric oxide.
Coal-burning power plants in the Northeastern and Midwestern United States need an emission control system that works despite the presence of sulfur dioxide, because coal from this area is sulfur-rich.
"For a state like Ohio that has large coal reserves, it is very good for the economy to make use of those reserves," said Ozkan. "But even a small amount of sulfur may deactivate these nitric-oxide reduction catalysts."
According to Ozkan, the switch from an ammonia-based emission control system to methane shouldn't cost companies much. "The same catalyst bed design common in power plants today can be used with this new formulation," she said, and any initial cost resulting from the switch can be made up in the savings that result from not using ammonia.
Ozkan also explained that methane may be considered a pollutant under certain circumstances, such as when it emanates from decomposing landfill materials. But removing nitric oxide with methane doesn't just replace one pollutant with another, she said.
"Of course, we don't want anything to be emitted, but nitric oxide is a much more problematic pollutant than methane. Also, once you activate the methane as we suggest, there is more than enough oxygen in the stack gas to burn it all off. It converts completely to carbon dioxide and water," she said.
This work was funded by the National Science Foundation and by the Ohio Coal Development Office.
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