Cleaner Coal through Computation
Stanford researcher uses Ranger to design better pollution controls
In the 1970s, scientists in the U.S. noticed that noxious chemicals discharged from coal-fired power plants — specifically nitrogen oxide (NOx) and sulfur dioxide (SO2) — were reacting in the atmosphere to produce acid rain, which was killing forests and spoiling waterways.
The government reacted swiftly, enacting regulation to restrict the amount of NOx and SO2 that could be released. Power plant operators installed pollution control devices, or scrubbers, that pulled these chemicals from the smoke. In a matter of years, acid rain had been significantly reduced.
But does that mean that coal-fired power plants are now safe? Not even close, says Jennifer Wilcox, an assistant professor of Energy Resources Engineering in the School of Earth Sciences at Stanford University. She specifically cites 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," explained Wilcox, who received an NSF CAREER award in 2005 to investigate trace metals.
About 5,000 tons of mercury is released worldwide from coal-fired power plants each year. Recently, scientists have determined that mercury and other trace metals emitted from industrial sources travel long distances and wind up in remote regions, like the Arctic, where they enter the food chain and accumulate 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, show elevated levels of toxic metals that have been correlated to birth defects and other ailments. While mercury's chemistry has been studied for decades, its story is still not completely elucidated. Even less is known about reactions involving selenium and arsenic.
Understanding trace metal adsorption aids in determining the pathway by which the trace metals are emitted into the atmosphere. In this example, elemental selenium is adsorbed onto an iron oxide nanoparticle through a surface oxidation mechanism. [selenium: blue-colored atom; oxygen: red-colored atoms such that hatched and solid correspond to surface and bulk, respectively; iron: rust-colored atoms]
It's not possible to see inside the flue where the gases are interacting, so Wilcox simulates the interactions of these particles using the Ranger supercomputer at the Texas Advanced Computing Center (TACC). Her studies of the dynamics of trace metals inside the flue of a power plant are helping her design and improve the technologies capable of removing heavy metals from the combustion process.
Specifically, Wilcox's simulations of capture devices show how the size, pore structure, and composition impact the success of a material as a collector or oxidizer of heavy metals. 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.
Some trace metal particles are captured in particulate matter devices, like electrostatic precipitators, but the majority are released into the air. The Environmental Protection Agency (EPA) requires filters to capture particulate matter at all industrial plants, and approximately 25-30 percent of power plants have scrubbers that capture nitrogen oxide and sulfide dioxide as well. However, these traditional scrubbers are limited in their ability to capture mercury, arsenic and selenium, and the methods to develop new technologies are crude.
"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," said Wilcox. "This strategy makes it difficult to optimize an existing scrubbing technology for multi-pollutant capture since the mechanism is unknown."
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 catalytic or absorbing 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 said.
Her simulations help scientists like Nick Hutson, a senior research engineer at the EPA, better understand the combustion process.
"Experimental results can be difficult to explain, other than, ‘This is what we saw,'" Hutson said. "Understanding the materials at a fundamental level allows you to progress to the point where you can start looking for real engineering solutions."
Ranger allows Wilcox to predict the behavior and tendencies of these particles with more detail and granularity than is possible in an experiment. The system runs endless variations to optimize the characteristics of the materials within a filtering device so that the reaction at the surface can be as effective as possible. One optimization calculation can take up to 20 days — the equivalent of more than 20 years on a desktop system.
Jennifer Wilcox, assistant professor of Energy Resources Engineering in the School of Earth Sciences at Stanford University.
In terms of Wilcox's research, optimizations can lead to changes in the spacing of pores in a charcoal filter, or the creation of never-before-seen monolayer alloys of palladium and gold that, Wilcox predicts, will better oxidize heavy metals in gasification.
Today, Shell, Chevron, the Electric Power Institute, the Air Force, and many others support Wilcox's work, believing that computational approaches to pollution control offer unmatched potential for insight in material improvements, and may revolutionize the design of scrubbers, including those for carbon capture. But not long ago, her methods were raising eyebrows.
"I gave a talk at the electric utilities environmental conference and many of the audience walked out," Wilcox recalled. "This field is very new to computational modeling, but I think it's going to make an impact in areas like fuel chemistry and catalysis."
In 2011, according to Hutson, the EPA will release regulations limiting the release of mercury and other heavy metals into the atmosphere. At that time, power companies will be forced to adopt more efficient and effective technologies to protect the environment and avoid fines, and 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," said Wilcox. "But as long as we're using it, we need to learn how to minimize its environmental impact."
May 19, 2010
The Texas Advanced Computing Center (TACC) at The University of Texas at Austin is one of the leading centers of computational excellence in the United States. The center's mission is to enable discoveries that advance science and society through the application of advanced computing technologies. To fulfill this mission, TACC identifies, evaluates, deploys, and supports powerful computing, visualization, and storage systems and software. TACC's staff experts help researchers and educators use these technologies effectively, and conduct research and development to make these technologies more powerful, more reliable, and easier to use. TACC staff also help encourage, educate, and train the next generation of researchers, empowering them to make discoveries that change the world.
- High levels of mercury, arsenic and selenium discharged from power plants are public dangers and have been shown to cause birth defects and other ailments, especially in the Arctic.
- Using the Rangersupercomputer, researchers simulated the behavior of trace metals in the flue of a power plant to learn how these metals interact and how they can be removed from the air.
- Their findings are helping to improve pollution control technology in advance of the predicted 2011 EPA regulations limiting the release of mercury and other metals into the atmosphere.
To find out more about this project and its sponsors, visit the Clean Energy Conversions Laboratory website.
- B. Padak, J. Wilcox, Understanding Mercury Binding on Activated Carbon, Carbon, 47(12), 2855-2864, 2009.
- E. Sasmaz, S. Aboud, J. Wilcox, Hg Binding on Pd Binary Alloys and Overlays, J. Phys. Chem. C, 113(18), 7813-7820, 2009.
- J. Wilcox, A Kinetic Investigation of High-Temperature Mercury Oxidation by Chlorine, J. Phys. Chem. A, 113, 6633-6639, 2009.
Science and Technology Writer