Utilization of Water Utility Lime Sludge for Flue Gas Desulfurization in Coal-Fired Power Plants: Part II. Lime Sludge Characterization and Mercury Reemission.

The feasibility of utilizing lime sludge in the flue gas desulfurization process of coal-fired power plants was evaluated through laboratory-scale studies. Eight lime sludge samples, collected from various water treatment plants, and a high-purity limestone sample were extensively characterized and tested for their ability to capture SO2 from a simulated flue gas, while investigating the mercury reemission profiles during the scrubbing process. The reactivity of lime sludge samples for acid neutralization was significantly higher than the reactivity of the tested limestone sample. At doses less than that of the limestone sample, the lime sludge materials reduced the SO2 concentration from 2,000 to <0.5 ppm. The residual lime, higher surface area, and more accessible pores in lime sludge samples were the major factors contributing to their higher reactivity. Concentrations of several elements including B, Mg, Mn, Fe, Cu, Zn, As, Sr, and Ba in some of the tested lime sludge samples were considerably higher than those elements in the limestone. However, no significant leaching of these elements into the scrubber solutions was observed. To investigate mercury reemission during the scrubbing process, ionic mercury was introduced into the simulated slurry and mercury reemission was monitored continuously. Results showed that compared with the limestone sample, the lime sludge samples tested had lower or similar cumulative mercury reemissions. However, different lime sludge samples showed different emission profiles. No conclusive correlation between the composition or trace element content of lime sludge samples and their mercury reemission could be identified. This result was likely due to the oxidative condition of the scrubbing process, which prohibited the reducing species from transforming the ionic mercury into elemental mercury.