(14c) Speciation Control of Mercury for Coal-Fired Power Plants

Mercury species in flue gas from coal-fired power plants consist of three types, ionic (Hg²⁺), elemental (Hg⁰), and particulate (Hg^p).Previous research showed that elemental mercury is most difficult to be captured with available technologies. Ionic mercury, however, can be removed by AQCD (Air Quality Control Devices) such as cold side ESP, Fabric Filters or wet FGD scrubbers. Mercury speciation in flue gas correlates with the amount of chlorine content in coals. Bituminous coals contain high-chlorine (i.e. >200 ppm) and thereby yields primarily ionic mercury in the flue gas. In contrast, sub-bituminous and lignite coals contain low-chlorine (i.e. <50 ppm) and as such produce flue gas containing mostly elemental mercury.¹ To increase mercury capture in coal-fired power plants, a strategy is to control the speciation of mercury in the flue gas from coal combustion. Treatment program that can increase the relative ionic mercury content in the flue gas will maximize the mercury capture co-benefit of AQCDs. Such an approach is particularly appealing to those plants utilizing low-chlorine fuels.

Nalco Mobotec offers a patented mercury oxidant, MerControl^® 7895, for mercury speciation control. MerControl^® 7895 is an aqueous solution that is easily applied to the fuel, injected directly into the boiler using fireside injection, or in the ductwork prior to the economizer. This application can enhance mercury oxidation and thereby increasing mercury capture from coal pile to the stack. This paper will present the results of MerControl^® 7895 trials showing the efficiency of mercury speciation control and its impact on mercury capture. The paper will discuss impact on boiler operation and unit operations downstream of the boiler due to application of this technology.

Reference: 1 S.A.Benson, M.J. Holmes, D.P. McCollor, J.M. Mackenzie, C.R.Crocker, L. Kong, K.C. Galbreath, K. Dombrowski, C. Richardson ?Large-Scale Mercury Control Technology Testing for Lignite-Fired Utilities ? Oxidation Systems for Wet FGD? EERC and URS corporation report (September 26, 2003 ? March 31, 2007) for the U.S. Department of Energy (Cooperative Agreement No: DE-FC26-03NT41991).

MerControl^® is a registered trademark of Nalco Company.