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Role of Sulfides in the Sequestration of Mercury by Wet Scrubbers
Behrooz Ghorishi, Bill Downs, and Scott Renninger
Wet scrubber systems, depending on the form of gas-phase mercury, can provide cost-effective mercury emissions control for coal-fired utility boilers. However, field- and bench-scale tests suggest that significant increases in elemental mercury (Hg0) concentration may occur across wet scrubbers. The Babcock & Wilcox Company (B&W) observed this phenomenon around 1997 and subsequently focused its research on finding solutions. During pilot-scale tests aimed at enhancing the mercury removal performance of wet scrubber systems, B&W discovered the important role of sulfides on the sequestration of soluble ionic mercury (Hg2+) and was subsequently awarded two patents utilizing sulfide chemistry. A detailed mechanistic interpretation of the role of sulfide on the sequestration of Hg2+ is presented here with the aid of a commercial electrolytic equilibrium model (OLI software, www.olisystems.com). B&W’s pilot-scale results and OLI mechanistic modeling revealed the important role of transition metal ions (specifically iron) and soluble sulfites in exacerbating re-emission of Hg0 and the significance of the presence of sulfides in suppression of this re-emission. The significant effect of scrubber pH on the consumption of sulfides by metal ions was another important finding of this study that has considerable field implications. One of B&W’s patented technologies that utilizes sodium hydrosulfide has been applied on several operating wet scrubbers to prevent re-emission of Hg0. The mechanistic information compiled by B&W has been used to further improve the performance of this technology in the field. |
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Behrooz Ghorishi, The Babcock & Wilcox Company Dr. Behrooz Ghorishi received his Ph.D. in chemical engineering from Rensselaer Polytechnic Institute in 1995. He currently is an Advisory Engineer with the Babcock & Wilcox Company Research Center, where he directs his research toward commercialization of mercury control technologies for pulverized-coal-fired units. He is also responsible for developing state-of-art technologies for the control of carbon dioxide emissions and commercialization of catalyst-based air pollution control technologies.
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