Lime Softening Sludge: A Potentially Important Source Of Sorbents For Wet Fgd Systems

The main objective of this project was to determine the suitability of lime sludge, a lime softening by-product from water treatment plants, for flue gas desulfurization in Illinois coal-fired power plants. Specifically, the project involved a detailed characterization of chemical, mineralogical, and physical properties and evaluation of reactivity of lime sludge collected from selected Illinois water treatment plants. The project had three main tasks. In Task 1, we collected lime sludge samples form 35 water treatment plants and analyzed each sample for its chemical, mineralogical, and physical properties. In Tasks 2 and 3, we measured the reactivity of selected sludge with respect to sulfur oxide capture using Titration and thermogravimetric analysis (TGA) methods. The results showed that lime sludge consists primarily of calcium carbonate (mostly \textgreater 80 weight percent CaCO3). Calcium carbonate polymorph aragonite is the dominant mineral composition in a few lime sludge samples studied. Other major component includes magnesium carbonate (MgCO3), which ranges between 2 to 15 percent and primarily occurs as solid solution within calcite. Total carbonate content is more than 90 percent in most samples collected and reached 95\% or higher in more than half of the samples studied. As with calcium carbonate, both magnesium carbonate and aragonite are highly soluble in dilute acids, an important property for desulfurization process. Lime sludge is also characterized by small particle size (mostly around 15 µm or less) and high surface area (4.5-11 m2 /g). Both titration tests and TGA tests data indicate that lime sludge are highly reactive compared to dolomite and dolomitic limestone. In contrast to dolomite and dolomitic limestone, MgCO3 in lime sludge does not have a negative effect on reactivity because MgCO3 contribution is not from the less reactive dolomite source. The results of this study, coupled with other studies from Kansas and Iowa, suggest that lime sludge can be very effective in capturing SO2. The high reactivity is attributed to its naturally fine particle size, high porosity, and large surface area. Lime sludge provides an excellent source of sorbent for wet flue gas desulfurization systems. Utilization of lime sludge for desulfurization could eliminate or reduce the need for limestone, especially in central and northeastern Illinois where high-calcium limestone deposits are generally absent or rare.