Reactivation of Fluidized Bed Combustor Ashes: Economic Evaluation and Implementation

When high-sulfur-content coal or coke is used as fuel in fluidized bed combustors, a large excess of limestone or dolomite must be added for good SO_x capture. All of the limestone is calcined but only 30–40% is actually sulfated. The resultant ashes are difficult to dispose of because of the free calcium oxide. These ashes can be reactivated for further SO_x capture. A proposed, economic process involves wet grinding of the ashes with sufficient excess water to allow both complete hydration and good grinding conditions. To prevent cementitious solidification of the wet product, it is then mixed with selected dry materials, for example fine coal, to absorb the excess water. Wet waste coal fines or sludges may also be used, then both to provide the water and prevent solidification. The product is then granulated with the cementitious reactions providing a binder for the granules. Good results with large additional SO_x capture have been observed both in a small pilot-sized CFBC and during a 54-hour utility boiler test in a 35 MW_t boiler. Calcium utilization was nearly doubled, with significant reduction of CO₂ emissions. Based on the test results, quick equity payback is expected with savings from reduced limestone purchase and ash disposal costs. In collaboration with The Babcock and Wilcox Company (B&W), a long test program at the Southern Illinois University in Carbondale, IL is planned.