This project will provide information about the fate of mercury in synthetic gypsum produced by wet FGD systems on coal-fired power plants, when used as feedstock for wallboard production. Wet FGD systems play a key role in current and future efforts to limit the air emissions of mercury control from coal-fired plants. Potential emissions of mercury from FGD byproduct gypsum during wallboard production could limit overall mercury control levels achieved by the coal power industry. Furthermore, any adverse effects of elevated mercury levels in wallboard products could undermine the use of FGD gypsum as a feedstock for wallboard plants. Under a Cooperative Agreement with DOE-NETL, USG Corp., a major producer of wallboard, will provide high-quality data on the extent and location of mercury release during the wallboard production process, and provide additional information on the potential for mercury leaching at the end of the wallboard life cycle, when it is disposed in municipal landfills.

Data will be collected in commercial wallboard plants owned and operated by USG. A variety of synthetic gypsum sources will be investigated; all are from FGD systems on coal-fired power plants but reflect different coal and FGD conditions. Solid samples from various locations in the wallboard process, including the wallboard product, will be collected and analyzed for mercury content. Simultaneous flue gas measurements will be made using the Ontario Hydro Method to quantify any mercury releases to the atmosphere during wallboard production.

Any potential mercury releases from the synthetic gypsum solids are thought to result from thermal desorption. Most of the testing will be concentrated around the “mill” portion of the plant, where the synthetic gypsum is dried and calcined. It is in the mill portion of the process where the feedstock sees the highest process temperatures and where the evolution of waters of hydration may promote mercury desorption. A limited amount of testing will also be conducted in the downstream board plant where the calcined gypsum is slurried, mixed with proprietary additives and formed into wallboard.

The solid and flue gas mercury concentration and flow rate data will be used to calculate a mercury balance around the operating wallboard plant. Samples of each synthetic gypsum will be evaluated in laboratory simulated calcining tests to provide comparison data and evaluate a lab technique for screening synthetic gypsum samples. Also, wallboard produced from synthetic gypsum will be leached according to the Toxicity Characteristic Leaching Procedure (TCLP) to provide an indication whether wallboard disposed of in municipal landfills will have a tendency to release mercury into groundwater.

Seven different power plant/FGD feedstock variations will be investigated in this project.
Each of these variables is thought to impact the amount of mercury in the synthetic gypsum feedstock and/or the stability of that mercury in the wallboard production process:

1. A feedstock produced by a power plant that fires high-sulfur bituminous coal and that has a limestone, forced oxidation (LSFO) FGD system that produces a wallboard grade gypsum byproduct; in addition, the power plant will have an operating SCR unit for NOX control, which promotes mercury oxidation and thus, potentially greater mercury capture in the FGD byproduct;
2. A plant/feedstock configuration similar to the one above, except that the power plant will not be operating the SCR unit for NOX control;
3. A plant/feedstock configuration similar to the two above (with the SCR in service for NOx control), except that the LSFO system employs a gypsum fines blow down, which could potentially change the distribution of mercury in the synthetic gypsum;
4. A plant that fires a low rank coal (Texas lignite) rather than bituminous coal, with no SCR and no FGD blow down;
5. A feedstock from a power plant that fires high-sulfur bituminous coal and operating with the SCR bypassed and with a relatively high rate of FGD blow down;
6. A plant/feedstock configuration similar to Item 5 above, with the SCR in service for NOx control and testing a wet FGD additive for enhanced mercury capture and separation of mercury from FGD gypsum; and
7. A power plant firing Powder River Basin Coal with an SCR in service for NOx control and utilizing a LSFO FGD system with blow down.  

To investigate all plant/feedstock configurations above, testing will be conducted at five different USG wallboard plants because no wallboard plant uses all seven feedstock configurations. The proposed test matrix encompasses the range of variables that would be expected in synthetic gypsum produced in U.S. coal-fired plants.