Auto‐Oxidation Effect on Flue Gas Sludge Systems

Upsets to Flue Gas Desulfurization (FGD) sludge treatment systems may occur under auto‐oxidation conditions. Auto‐oxidation is the appreciable natural conversion of ${\mathrm{CaSO}}_3\cdot 1/2H_{2}O$ to ${\mathrm{CaSO}}_4\cdot 2H_{2}O$ due to lower ${\mathrm{SO}}_2$ loading (less than 1.2 lb inlet ${\mathrm{SO}}_2/\mathrm{MMBTU}$) and operating pHs of lime/limestone FGD Systems. A 75–79% auto‐oxidized $(75-79\%{\mathrm{CaSO}}_4)$ FGD sludge exhibits a mean particle diameter of 45 microns—considerably larger than unoxidized $\left({\mathrm{CaSO}}_3\right)$ particles. The oxidized sludge reaches terminal settling velocity within 8–10 min, five times faster than unoxidized material. Auto‐oxidized waste settles in gravity thickeners to higher densities (0.85 g dry solids/ml) and to thicker underflows (40% solids). Rakearm overload and blanket consolidation can be avoided by proper drive mechanism design and underflow solids/density control, providing an 8 hr underflow surge/storage tank is recommended instead of thickener sludge blanket storage. Vacuum filtration of an $80\%{\mathrm{CaSO}}_4$ sludge yields 25% greater solids in the cake as compared to a 30% oxidized material. To avoid cake‐cracking, reduce the degree of vacuum during incidents of autooxidation and provide dual filtrate receivers. Using predictive curves presented here, a Transportability Analysis was performed on a coal of 0.59–0.91% sulfur and of 10.00–21.50% ash. At an ${\mathrm{SO}}_2$ loading of 1.08 lb/MMBTU 38% solids (thickener underflow) or 63% solids (filter cake) could be achieved. Either filter by‐pass or adding less fly ash to the cake permits achieving a 75–85% solids transportability blend. Auto‐oxidized sludge/ash blends exhibit unconfined compressive strengths from 57–63 psi and permeabilities from $2.2-3\times {10}^{-5}\mathrm{cm}/s\text{.}$