The Impact of Wet Flue Gas Desulfurization Scrubbing on Mercury Emissions from Coal-Fired Power Stations

Abstract

This article introduces a predictive capability for Hg retention in any Ca-based wet flue gas desulfurization (FGD) scrubber, given mercury (Hg) speciation at the FGD inlet, the flue gas composition, and the sulphur dioxide (SO₂) capture efficiency. A preliminary statistical analysis of data from 17 full-scale wet FGDs connects flue gas compositions, the extents of Hg oxidation at FGD inlets, and Hg retention efficiencies. These connections clearly signal that solution chemistry within the FGD determines Hg retention. A more thorough analysis based on thermo-chemical equilibrium yields highly accurate predictions for total Hg retention with no parameter adjustments. For the most reliable data, the predictions were within measurement uncertainties for both limestone and Mg/lime systems operating in both forced and natural oxidation mode. With the U.S. Environmental Protection Agency’s (EPA) Information Collection Request (ICR) database, the quantitative performance was almost as good for the most modern FGDs, which probably conform to the very high SO₂ absorption efficiencies assumed in the calculations.

The large discrepancies for older FGDs are tentatively attributed to the unspecified SO₂ capture efficiencies and operating temperatures and to the possible elimination of HCl in prescrubbers. The equilibrium calculations suggest that Hg retention is most sensitive to inlet HCl and O₂ levels and the FGD temperature; weakly dependent on SO₂ capture efficiency; and insensitive to HgCl₂, NO, CA:S ratio, slurry dilution level in limestone FGDs, and MgSO₃ levels in Mg/lime systems. Consequently, systems with prescrubbers to eliminate HCl probably retain less Hg than fully integrated FGDs. The analysis also predicts re-emission of Hg⁰ but only for inlet O₂ levels that are much lower than those in full-scale FGDs.