Unlikely Mix ­ Used Tires, Pistachio Shells ­ Can Clean Up Pollution

"Mercury is a toxic pollutant that can enter rivers, lakes and the human food chain," said Massoud Rostam-Abadi, a chemical engineer and head of Energy and Environmental Engineering at the Survey. "Coal-fired power plants are one of the largest anthropogenic sources of mercury emissions. With funding provided by EPRI (the Electric Power Research Institute) and the Illinois Office of Solid Waste Research, we looked for materials that could effectively remove mercury from the combustion flue gases."

In a collaborative research program, Rostam-Abadi and Scott Chen, another chemical engineer at the Survey, and their UI colleagues -- civil and environmental engineering professor Mark Rood and graduate students Hsing-Cheng Hsi and Chris Lehmann -- prepared adsorbents from a variety of feedstocks, including Illinois high-sulfur coal, waste tires and pistachio shells. The adsorbents were then evaluated for their effectiveness at removing two forms of mercury emissions -- elemental mercury and mercuric chloride -- from several different simulated combustion gas streams.

"We found that mercury removal was affected by both the properties of the adsorbent and the flue gas compositions," Rostam-Abadi said. "In one flue gas, the adsorbents were equally effective in removing both forms of mercury. In another flue gas, the tire and pistachio carbons had nearly five times larger capacity for the adsorption of mercuric chloride than their coal-derived counterpart."

Because utilities produce different amounts of the two mercury emissions, some tailoring of the feedstocks will be required to achieve effective mercury removal for individual power plants.

"We are currently examining the mixing of adsorbents to more effectively remove both forms of mercury from individual power plants," Rostam-Abadi said. "Adsorbents produced from different precursors will have different properties due to variations in their chemical and physical compositions. Also, depending on plant location, certain adsorbents may be more economical than others."

In their work, the researchers also showed that activated carbons containing sulfur additives were significantly more effective at removing mercury emissions from the flue gases. "But adding the sulfur usually requires additional processing steps and production costs," Chen said.

"Tire rubber already contains sulfur -- which makes the rubber more durable -- so activated carbon from tires might prove more cost-effective than existing products," Rood said. "And there is certainly a plentiful supply: each year, more than 200 million tires are disposed of in the United States alone."

The researchers will present their latest findings at the American Chemical Society meeting, to be held Aug. 20-24 in Washington, D.C.