Optimization and Process Control of Air Quality Control Systems for Improved WFGD Oxidation Chemistry and Effluent Composition for Wastewater Treatment

Wastewater treatment (WWT) of wet flue gas desulfurization (WFGD) effluent, mercury and selenium control are becoming critical issues for many utilities operating with coal-fired power generation. Research has shown that absorber slurry oxidation reduction potential (ORP) is a primary factor for determination of control of WFGD chemistry. Previously unexplained swings in the ORP of operating WFGD units have been observed in excess of 300 mV, both positive and negative. Such swings, and the associated changes in the oxidizer content of the absorber stream, can result in mercury re-emission and have negative impacts upon WWT. Research indicates that control of WFGD ORP may be possible. Should such control be implemented, the fraction of selenate passing to WWT and the risk of mercury re-emission are reduced. Further, cost of additives may be lowered for some plants should ORP be controlled. The control of reagent feed is an important parameter for optimization, as high oxidizer content can result in deleterious side reactions and lower pH. Parametric testing was performed at a Midwestern utility to examine the impacts of upstream unit operation performance and traditional WFGD control parameters upon WFGD ORP. Herein is discussed results of this parametric testing and potential tuning of absorber and upstream operations for improved WFGD oxidation chemistry and effluent composition.