COLUMBUS, Ohio -- Researchers at Ohio State University have found a way to use hydrated lime to reduce the amount of selenium emitted by fossil-fuel combusters and incinerators.
Selenium is one of the most volatile and toxic byproducts produced by coal-fired power plants. An essential trace element for humans and other animals, large quantities of selenium can cause infertility and damage to the liver, kidneys and lungs.
About 2,500 tons of selenium are discharged into the atmosphere each year in the United States alone. Regulations being proposed by the Environmental Protection Agency will seek to control emissions of selenium and other metals found in fly ash and flue gas, said Liang-Shih Fan, professor and chair of chemical engineering at Ohio State.
Fan and Suhas Mahuli, a post-doctoral researcher in chemical engineering, have studied ways to capture selenium from flue gas before it escapes into the atmosphere. The researchers found that hydrated lime, which has been shown effective in controllingsulfur dioxide emissions in flue gas, can also be used to reduce selenium.
The fine, powdery lime is injected into the plant's boiler chambers through nozzles or injectors that can be retro-fitted to existing plants. The lime particles capture selenium and sulfur dioxide. As the flue gas moves upward, an electrostatic precipitator removes the particles by electrical charging.
The method, called dry sorbent injection, has been studied for several years as a way to remove sulfur dioxide from flue gas. But this is the first time studies have shown that hydrated lime chemically removes selenium by means of an irreversible chemical reaction.
"The hydrated lime -- which is calcium-based -- forms with the selenium to create calcium selenite, the first time this reaction has been documented," Fan said. "The compound is then captured by the dry sorbent and can be disposed of with the fly ash."
The researchers found that the hydrated lime was most effective at 400 to 600 degrees Celsius, and able to "capture" up to five percent of its weight in selenium. At higher temperatures, calcium selenite is thermally unstable, which reduces the efficiency of selenium capture.
The scientists compensated for this by injecting the dry sorbent further away from the firing point in a power plant, where temperatures can exceed 1,000 degrees Celsius.
"At 400 to 600 degrees Celsius, the rate of capture should be more than sufficient to effectively remove the selenium from the gas environment," Mahuli said.
More work is needed to discover the best way to stabilize and dispose of fly ash and the lime sorbent, which will contain the selenium, Fan said. There are currently no regulations regarding selenium disposal. The researchers are also trying to find out how much it would cost to retro-fit a plant for this technology.
"It's unclear just how strict the EPA regulations on selenium are going to be," Fan said. "Some plants may find it easier to install a very expensive wet scrubber to remove all of the harmful byproducts from flue gas and fly ash."
But dry sorbent injection technology could be more cost efficient for the removal of selenium, and possibly arsenic. Another advantage to the technology is the abundance of the raw material limestone, especially in the Midwest, Fan said.
The study was co-authored by Rajeev Agnihotri, a doctoral student in chemical engineering, and Abhijit Ghosh-Dastidar, a former post-doctoral researcher, both from OhioState. The work, which was published in a recent issue of the journal Environmental Science and Technology, was supported by the OhioCoal Development Office, a collaborative effort between OhioState and four other Ohio universities.