FGD and DeNOx
NEWSLETTER       

                                                                                                         May 2005
                                                                                                             No. 325

Over 90 Percent Hg Removal by Wet FGD and SCR at Mt. Storm

The results of extensive field-testing at Dominion Energy’s 563-MW coal-fired Mt. Storm 2 power plant indicate that wet FGD systems can provide a cost-effective, near-term mercury emissions control option. This unit fires Eastern bituminous coal and is equipped with ESP and SCR systems. Babcock & Wilcox began the testing to evaluate its wet FGD mercury control enhancement process. B&W injects a liquid sodium hydrosulfide reagent into the wet FGD slurry to eliminate mercury re-emission.

During the first phase of testing, the focus was on analyzing the potential of Hg⁰ re-emission across the wet FGD system. The SCR system was bypassed and the result was the presence of rather significant quantities of Hg⁰ at the inlet of the FGD system. After NaHS was injected, the concentration of elemental mercury at the FGD outlet fell. During the second phase of testing, the SCR was put into operation and its effect on Hg speciation at the inlet of the FGD and on total Hg control across the wet scrubber was measured. In the third phase with the SCR online, the effect of B&W’s patented additive technology on Hg⁰ re-emission and control across the wet FGD system was studied.

Figure 2 gives the results from the second phase testing. The bituminous coal fired at Mt. Storm has sufficient chlorine content to produce significant quantities of HCl in the flue gas. The concentration of HCl was high enough to induce the oxidation of Hg⁰ by the SCR catalyst. Also of significant importance was the fact that nearly all of the oxidized Hg generated by the SCR catalyst was captured by the wet FGD system (>95 percent control of Hg²⁺). No re-emission of Hg⁰ was observed even in the absence of B&W additive. Scott Renninger, George Farthing, and S. Ghorishi of B&W and Chuck Teets and Jeanne Neureuter of Dominion Energy report that in the combustion of this coal, the combination of SCR and wet FGD constituted a very good Hg control technology (>90 percent Hg removal). Figure 3 gives the results of the third phase of testing where B&W’s additive was added. It is evident that with the SCR on-line, no re-emission of Hg⁰ occurred in the wet FGD. 

Figure 2. Second Phase Testing Results

Figure 3. Third Phase Testing Results

In summary, without the SCR system in service and no NaHS injection, the total mercury removal efficiency across the FGD was approximately 70 percent. Without the SCR system in service and with NaHS injection, the total mercury removal efficiency increased to 78 percent. With the SCR system operating, 90 percent mercury removal was achieved through the SCR, ESP and wet FGD. No further mercury removal was achieved with the addition of NaHS. The authors of the paper stress that further studies are needed on plants that have operated their SCR longer and/or burn Powder River basin fuels, before this data can be generalized.

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