Technology▼Solutions Regenerating mercury-loaded sorbents In anticipation of upcoming U.S. EPA regulations, researchers are developing new technologies to control mercury emissions from coal-fired power plants. Although such devices capture mercury from flue-gas streams, they transfer it to a solid phase, eliminating one problem but creating another. Scientists are concerned that the
from gas streams don’t have to end up as waste. As long as a sorbent is electrically conductive, the mercury can be recovered electrochemically and the sorbent can be reused. Unlike most air pollutants, mercury is a volatile metal, which can be easily oxidized and reduced electrochemically. The trick is to find a sor-
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Hg2+ Mercury sorbent
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Elemental mercury in gas streams can be captured on an activated carbon cloth sorbent as shown in the backscatter electron micrograph (inset) and recovered at the cathode of an electrochemical cell.
amount of mercury in combustion residues will increase over the next decade, if EPA requires enhanced mercury controls on power plants. Some of the most promising controls, they say, could create a hazardouswaste disposal problem and render fly ash unusable in commercial products like cement. Research reported in the October 15 issue of ES&T (pp. 4430–4435) by Paul Bolger and David Szlag, both formerly with EPA’s National Risk Management Research Laboratory in Cincinnati, offers a potential solution to the disposal problem. Using electrochemistry, they show that carbonbased sorbents used to trap mercury 408 A
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bent that is electrically conductive and captures mercury by a physical adsorption process, Bolger says. “If you have a sorbent material with a lot of sulfur in it, you might get bonding between mercury and sulfur. Such chemical bonding is irreversible in that you can’t recover the mercury electrochemically,” he explains. In laboratory experiments at ambient temperature, the researchers tested various carbon-based sorbents, including powdered activated carbon, carbon felt, ultrafoam, carbon nanofiber composites, and activated carbon cloth, using a nitrogen gas stream with a specified amount of elemental mercury (Hg0) in it. The cloth turned
ENVIRONMENTAL SCIENCE & TECHNOLOGY / NOVEMBER 1, 2002
ENVIRON. SCI. TECHNOL. 2002, 36, 4430–4435
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out to adsorb nearly 20 times more mercury than powdered activated carbon, and unlike the powdered form, was electrically conductive. The other sorbents did not adsorb appreciable amounts of mercury. To remove trapped mercury from the carbon cloth, the researchers developed an electrochemical system that regenerates the cloth sorbent at the anode and captures pure Hg0 at the cathode. In the first step, Hg0 is oxidized at the anode in an undivided cell, forming Hg2+. The Hg2+ is then deposited onto a high surface area cathode, reduced to Hg0, and collected. Although both forms of mercury—Hg0 and Hg2+—can be trapped on the cloth sorbent, the researchers only tested Hg0 in the gas stream because Hg2+ is readily removed in scrubbers, which are typically placed upstream of mercury-trapping sorbents to control power plant SO2 emissions, Bolger says. The work provides a proof of principle that mercury in gas streams can be trapped on an activated carbon cloth and recovered at the cathode of an electrochemical cell. The experiments, however, were performed at ambient temperature. At higher temperatures, which are more typical of a gas stream emitted by a power plant, less mercury is likely to adsorb to the cloth, Bolger acknowledges. In addition, flue gases typically contain a mixture of compounds that might interfere with the adsorption capacity, he says. Experts say that whether such an approach is feasible really boils down to the cost of regenerating cloth sorbents compared with the cost of existing sorbent systems, such as activated carbon injection. “It is a matter of economics whether we recover the mercury and regenerate the carbon or simply dispose of them,” says Ramsay © 2002 American Chemical Society
Chang, manager of air pollution control at the Electric Power Research Institute, who sees the new electrochemical approach as just another way of heating up carbon to drive off the mercury. “Commercial processes of heating the carbon to recover the mercury already exist,” he adds. Scientists at EPA say they are not convinced there is a need to regenerate sorbents loaded with mercury. “My sense is that when mercury is attached to the carbon, it is probably not going to volatilize or leach out. There is no evidence yet that it is going to pose a hazard if it is disposed of in a normal fashion,” says Jim Kilgroe, a mechanical engineer with EPA’s Research Triangle Park facility in North Carolina. Nonetheless, EPA is investigating the leaching of mercury from coal ash and plans to release its findings in a couple of months, says Susan Thorneloe, a senior environmental engineer with the agency. “One of the concerns that we have is that a lot of the protocols and procedures that are being used now really aren’t applicable for these types of waste,” she says, referring to the wide range of pH values and solid/liquid ratios that combustion byproducts can be exposed to in the environment. “If we find that mercury is not leaching, then we will have much more comfort in knowing that it won’t be a problem in terms of the environment. If we find that it is leaching, then we will be able to more clearly define under what conditions that may be occurring.” In the United States, electric utilities generate more than 100 million tons of coal ash each year, 30% of which is reused in commercial products, according to the University of North Dakota’s Energy & Environmental Research Center. If the release of mercury from coal ash does turn out to be significant, EPA may have to reconsider the use of powdered activated carbon injection, one of the key technologies it is reviewing for power plant mercury control. In such systems, mercury-loaded sorbent cannot easily be separated from the ash because the sorbent is injected as a fine powder. A carbon cloth, on the other hand, could be dropped into place and easily removed when saturated, so the mercury does not end up in the ash. The agency has until December 15, 2003, to propose its new regulations. —BRITT E. ERICKSON NOVEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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