For coal-fired and industrial power plants, air-related emissions of mercury can be controlled by a number of technologies. For coal-fired electric steam generators, the technology chosen will many times depend on the existing equipment in-place at the plant. While Activated Carbon Injection ACI has found widespread use, for those with Wet Flue Gas Desulphurization WFGD systems, many times it is less expensive to use the SO2 absorber for mercury mass transfer.
For those units with Wet Flue Gas Desulfurization WFGD units, adding a sulfide-based precipitant to the WFGD liquid phase will react with any aqueous mercury to form a solid precipitate, and generally phase-partition to the solid. Within the absorber, the sulfur dioxide must transfer from the gas-phase to the liquid phase to then form the aqueous sulfur species such as bisulfite/sulfite. Then, dissolved alkali such as calcium, can react with the dissolved sulfur to form calcium sulfate (gypsum) in most instances, although many still make a calcium sulfite by-product. As long as sufficient up-front gas-phase oxidation of mercury is achieved, either via catalyst or the addition of halogen on the coal, the precipitant added in sufficient quantity will allow the mercury emissions limit to be achieved. Those few US sodium-based WFGD are a bit of a different animal.
While the industry has generally described the WFGD control of mercury in terms of “mercury re-emission”, relying on this description of how changes occur in the WFGD mercury emissions, leaves much to be desired. The first thing we need to realize is that elemental mercury is not insoluble. To learn more about mercury vapor pressure, please refer to this NIST document here.
Once sufficient up-front gas-phase oxidation of mercury is achieved either via the mechanical oxidation resulting from the NOx SCR catalyst, or the addition of halogens onto the coal, the sulfide precipitant ensures the material tends to report to the WFGD slurry solids.
Please see the typical equipment set-up shown below for testing precipitant additive into the WFGD. While there are many areas where the precipitant can be added, the fastest acting is to add the material to the Absorber Recirculation Pump piping. This will lead to sub-saturation of the liquid with respect to aqueous mercury. Within seconds, this recirculated slurry liquid, which is now sub-saturated with respect to mercury, enters the mass transfer zone via the spray header/nozzles. And just like the absorption of the sulfur dioxide which depends on the continued sub-saturation of the liquid-phase with respect to sulfite, the mercury similarly can be transferred from gas to liquid via the liquid’s sub-saturation. Of course there are differences between sulfur dioxide solubility and elemental mercury solubility, and that has led to a lot of the misunderstanding. Once mercury is in the aqueous phase of the absorber slurry, we can now precipitate the aqueous mercury as mercuric sulfide. While the upstream side of the pump is preferred for sulfide-additive injection location, the down stream side can also be utilized. The precipitant can also be added in the return water line if that is preferred, or even some other location. It takes a bit longer to provide the mercury sub-saturation of the liquid phase that enters the mass transfer zone of the WFGD absorber, but with sufficient time it can be achieved in this way.
Click here for a look at the workshop I gave in July 2018 at Reinhold in Lexington, KY. titled “Cycling Load Impacts on WFGD Mass Transfer”.
Here is a mercury workshop I gave at the 2014 Reinhold Conference titled “Achieving Cost-Effective MATS Compliance for WFGD’s through the lens of Hg Absorption”. While this presentation was made while I was a Babcock & Wilcox employee, it is in the public domain.