ORP as a Predictor of WFGD Chemistry and Wastewater Treatment

Recent studies have shown that system oxidation-reduction potential (ORP) is not only an important factor for predicting wet flue gas desulfurization (WFGD) absorber chemistry but also may be a predictor of process equipment corrosion and wastewater treatment requirements.

Purge streams of wet flue gas desulfurization (WFGD) units, which are one byproduct of controlling SO2 emissions from coal combustion, are being increasingly subjected to stricter wastewater regulations. Consequently, coal-fired power generators need a method for controlling the operational chemistry of these WFGD units. Upon implementation of a suitable control method, WFGD bleed stream chemistry and flow rate may be optimized, thereby resulting in improved performance of one or more downstream unit operations. A further benefit is reduced reagent and additive costs in various devicies, including the WFGD unit.

One control parameter of interest is the oxidation-reduction potential (ORP) of the bleed stream. Much like pH, the measurement of ORP can be taken in real time and integrated with other plant-monitoring data. By incorporating ORP measurements into a process control scheme for limestone forced-oxidized WFGD absorbers—along with various other control variables such as SO2 removal, absorber pH, reagent flow rate and/or one or more reaction stoichiometries, and/or gypsum purity—generators are able to manage the oxidation states of various dissolved metals in the slurry and the potential reemission of mercury. (Also see “How to Measure Corrosion Processes Faster and More Accurately,” May 2009 in the POWER archives and “Mercury Control: Capturing Mercury in Wet Scrubbers, Parts I and II,” July and September 2007, respectively, in the COAL POWER archives—both available at powermag.com.)

A further benefit is control of the corrosion rate of the absorber recirculation tank (ART) and other alloy parts within the system. Many utilities have had ORP excursion events in WFGD wastewater discharge where the ORP readings changed from 150 millivolts (mV) to 300 mV to a reading above 500 mV. Previously, these fluctuations have gone largely unexplained. We have determined that this magnitude of change in ORP, in an ART, due to coal composition and upstream air quality control system (AQCS) effects on WFGD absorber chemistry, can accelerate corrosion.

One potential solution to fluctuating ORP readings is to use integrated process controls designed to tune the upstream operation of the AQCS train to produce consistent inlet flow parameters to the WFGD tower, rather than operating each as an independent process. The control of the ORP level in a WFGD system may produce improved plant operations by reducing the amount of wastewater treatment necessary and helping mitigate mercury reemission.

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