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Wet limestone scrubbing has been the dominant choice for sulfur dioxide (SO2) removal from power plant flue gas streams. The technique is proven and systems are now available that can remove 99 percent of SO2 from gas streams. Perhaps the single greatest drawback with wet flue gas desulfurization (WFGD) is that toxic impurities in the coal will come out of the flue gas in the scrubber." These impurities include compounds of mercury, selenium, arsenic and boron. While the concentration of any of these materials in the FGD purge stream is typically quite low, it does not take much of any to affect aquatic life if these streams enter natural bodies of water.
Wet Limestone Scrubber Operation
The type of scrubber can have a large impact on the species of impurities that come out in the byproduct or bleed streams. Because most wet-limestone scrubbers are of the forced-oxidation type, this article will focus upon that variety. Consider the basic process of a wetlimestone system, in this case the spray tower design illustrated in Figure 1.
Sulfur dioxide is first absorbed into the liquid phase as it contacts the slurry sprays, where it forms sulfurous acid (H^sub 2^SO^sub 3^).
Some theoretical chemists argue that true H^sub 2^SO^sub 3^ does not exist and that SO2 retains its molecular character and is simply surrounded by water molecules. However, when SO2 is added to water the pH drops, which suggests that Equation 1 is correct and that the following dissociation reaction is accurate.
The principal component of limestone, calcium carbonate (CaCO3), reacts with the aqueous acid.
Equations 1, 2 and 3 combined illustrate the simplified but fundamental scrubbing process.
In the absence of any other reactants, calcium and sulfite ions will precipitate as a hemihydrate, where water is actually included in the crystal lattice of the scrubber byproduct.
This material is difficult to handle, so most scrubbers are equipped with air injection to oxidize sulfite ions to sulfate (SO42). Sulfate then precipitates with calcium as gypsum (CaSO^sub 4^-2H^sub 2^O).
The critical process stream for this discussion is the bleed to the byproduct system based on the chemistry of Equation 6. This, of course, is the stream that feeds belt or drum vacuum filters for gypsum dewatering and subsequent sale...