Water utility lime sludge reuse for flue gas desulfurization

There are hundreds of water utilities across the U.S. that generate significant amounts of lime sludge as a result of the water lime softening process. Beneficial use of the lime softening sludge is a sustainable solution to the current landfill disposal practice. We have recently evaluated the feasibility of utilizing lime sludge in the flue gas desulfurization process of coal-fired power plants.1,2 A supply-demand evaluation showed that the water treatment plants in the U.S. generate approximately 3.2 million tons of lime sludge per year at an estimated disposal cost of approximately US$90 million. Furthermore, power utilities are using approximately 6.3 million tons of limestone per year. A life cycle assessment study showed that the environmental impact of lime sludge utilization in power plants was two orders of magnitude lower than that of the landfill disposal option. We further performed laboratory-scale studies using a simulated flue gas at a flow rate of ~0.5 LPM to evaluate performance of several lime sludge samples for SO2 capture, while investigating themercury re-emission during the scrubbing process. Results indicated that all tested lime sludge samples, at doses less than the dose of a baseline limestone sample, reduced the SO2 concentration from 2,000 to less than 0.5 ppm. Furthermore, tested lime sludge samples exhibited lower or similar mercury re-emission compared to the limestone sample.We have further validated our recent laboratory-scale work by performing additional experiments using a small prototype scrubber at a flue gas rate of 5-10 LPM. Several experiments were performed to test two lime sludge samples and a baseline limestone sample using a simulated flue gas containing 2,000 ppm SO2. Similar to the previous smaller scale experiments, the SO2 removal and mercury re-emission profile were monitored during the scrubbing experiments. Overall, the data trends observed for the small prototype tank scrubber are in good agreement with the trends observed for the laboratory-scale experiments. Work is in progress to perform field-testing of selected lime sludge samples for desulfurization of 5-10 LPM flue gas, obtained as a slipstream from the exhaust duct of a coal boiler at a local power plant.