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We combine several proven technologies to achieve a well functioning, integrated FGD treatment facility with appropriate performance guarantees, including:

• Lime neutralization / desaturation
• Heavy metal removal, including hard-to-remove priority species
• Clarification
• Filtration
• Sludge thickening and dewatering
• BOD / COD reduction
• Nitrates and ammonia reduction
• Selenium reduction
• Mercury removal

Scrubber Technology

Scrubbers are used in U.S. power plants to meet air quality emission standards set by the U.S. Environmental Protection Agency (EPA) and by individual state requirements.  Recent updates to the regulatory programs call for further reduction of sulfur dioxide (SO₂) emissions from coal-burning power plants.

FGD systems use either dry or wet scrubbers. Dry scrubbers generally use lime or limestone as a reagent to remove SO₂ and although they use water in the process, the water usage is lower than with wet scrubbers. When the power plant's coal contains less than 2% sulfur, as it does in western coals such as the Power River Basin (PRB) coal, the plant will typically consider using dry scrubbers. Wet scrubbers are preferred when the coal contains higher amounts of sulfur. Since a power plant being retrofitted with a new FGD system will be in service for 25+ years, a number of power plants are installing wet scrubbers, even if they are currently using low sulfur coal, to have the future flexibility to use coals from many different world sources that may have higher sulfur content.

Wet FGD systems most commonly are of the limestone-forced-oxidation (LSFO) type to convert SO₂ in the flue gas to gypsum. While this allows greenhouse gases to be released to the atmosphere, it removes the SO₂ that causes acid rain.  Future versions of FGD systems will capture CO₂ and dispose of it in an environmentally-friendly way.

Wet FGD scrubbers use an absorber tower in which flue gas is contacted by limestone slurry, resulting in conversion of SO₂, in the flue gas into calcium sulfate (gypsum), with carbon dioxide (CO₂) going up the stack.  Gypsum can be sold to wallboard manufacturers, thus turning a waste stream into a usable resource. To maintain required conditions in the scrubber (to control chlorides levels consistent with the scrubber’s materials of construction and to maintain ash fines levels low enough to maintain gypsum quality), a constant purge stream is discharged from the scrubber system.

The proper design of sludge dewatering systems and the selection of materials of construction are critical in FGD wastewater installations. Both of these key design areas can have a major impact on the treatment plant’s operation, reliability and cost.  Siemens Water Technologies has extensive expertise in FGD wastewater treatment and is the industry leader.  By having early discussions with the utility about their specific power plant, we are able to provide an optimized system at the best cost / benefit ratio.

Mercury Removal

Siemens is continually working on new mercury reduction technologies to reduce mercury levels below 12 parts per trillion (ppt).  These technologies are currently under evaluation and in pilot testing at various facilities nationwide.  For additional information on low level mercury removal please contact Adam Szczesniak, e-mail: adam.szczesniak@siemens.com.

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