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 thermochemical 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 Hg(O) but only for inlet O2 levels that are much lower than those in full-scale FGDs.

Mercury Species and SO2 Adsorption on CaO(100)

“…The likely binding mechanism of Hg 0 to CaO is physisorption. This result is consistent with experiments reporting that elemental mercury may not be adsorbed by calcium-based sorbents 8,9 until it is first oxidized. The oxidized forms of Hg, i.e., HgCl, HgCl 2 , and HgO were also investigated in the following sections, in addition to SO 2 binding.…”

A novel semi‐dry method for the simultaneous removal of Hg and SO2 using spray drying absorption method

“…has ever expanded real‐time monitoring on a wet desulfurization system of a desulfurization plant, after which it was concluded that the pH effect was critical for the removal of Hg. While Niksa and Fujiwara believed that pH had no effect on Hg absorption in desulfurized slurry like lime. In order to verify the effect of pH in the SDA system on Hg removal, slurries with different pH were formed and the effect of Hg removal efficiency was studied.…”

Experimental Study on Demercurization Performance of Wet Flue Gas Desulfurization System

“…There are several methods to enhance Hg 0 oxidation rate, such as improving combustion condition, altering the flue gas composition (enhancing the content of HCl, Cl 2 , O 2 in flue gas), injecting active carbon and catalyst into flue gas, adding additive in coal, adding oxidant in absorption liquid, and so on. 17,18 Several types of additives, such as Fenton reagent, KMnO 4 , Na 2 S, and K 2 S 2 O 8 /CuSO 4 , were selected in order to investigate their effect on Hg speciation and removal efficiency.…”