STANFORD / U. TEXAS-AUSTIN (US)—Pollution control devices known as scrubbers, installed to restrict the amount of nitrogen oxide and sulfur dioxide release from coal-fired power plants, may have helped to reduce acid rain, but they haven’t made those plants safe.
Not even close, says Jennifer Wilcox, assistant professor of energy resources engineering at Stanford University. She specifically cites the high levels of mercury, arsenic, and selenium that are still being discharged from power plants as public dangers.
“During combustion, coal is not burned all the way, and metals are released into the atmosphere,” explains Wilcox.
About 5,000 tons of mercury are released worldwide from coal-fired power plants each year, according to the Environmental Protection Agency (EPA). Recently, scientists have determined that this mercury travels long distances and ends up in remote regions, like the Arctic, where it enters the food chain and accumulates within organisms.
Fish and polar bears have been found with dangerous amounts of mercury in their systems, and the people of the Arctic, who eat these animals, display elevated levels of toxic metals, which are correlated to birth defects and other ailments.
It’s not possible to see inside the flue of the power plant where the gases interact, so Wilcox simulates the interactions of these particles using the Ranger supercomputer at the University of Texas at Austin’s Texas Advanced Computing Center.
Her studies are helping to improve on current technologies and design new ones that can remove heavy metals from the coal combustion process.
Specifically, Wilcox’s simulations show how the size, pore structure, and composition of a material affects its success as an oxidizer of heavy metals. According to Wilcox, the goal is to create a structure that will bind trace metal molecules and convert them into a water-soluble form that can be easily removed.
“The strategy of a company that makes scrubbers is to find something that mostly works and to go with it, but they have no interest in finding out why something works and even less interest in optimizing for enhancement,” Wilcox says.
That’s where Wilcox and her colleagues come in. Their simulations map the electronic structure and electron placement in the trace metals as they react with the materials in scrubbers. This helps develop a clear idea of why certain materials react better than others.
“Once we understand what the pathway is, then we can tune the catalyst or sorbent,” she says.
Such simulations help scientists better understand the combustion process.
“Experimental results can be difficult to explain,” says Nick Hutson, senior research engineer at the EPA. “Understanding the materials at a fundamental level allows you to progress to the point where you can start looking for real engineering solutions.”
These engineering solutions can mean changes in the spacing of pores in a charcoal filter, or the creation of a never-before-seen alloy of palladium and gold that, Wilcox predicts, will better oxidize heavy metals in gasification.
In 2011, the EPA is expected to release regulations limiting the release of mercury and other heavy metals into the atmosphere. Power companies will be forced to adopt more efficient and effective technologies to protect the environment and avoid fines. Wilcox will be leading the way, having developed the theory and methods needed to produce optimal capture devices.
“The idea of ‘clean coal’ is not realistic. Coal will never be clean,” Wilcox says. “But as long as we’re using it, we need to learn how to minimize its environmental impact.”
The work is supported by the National Science Foundation.
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