AECOM Process Technologies (legacy URS) provided engineering, procurement, and construction management (EPC) services to SECI to convert the flue gas desulfurization (FGD) system at its Seminole Generating Station from an inhibited-oxidation process to a forced oxidation process to produce gypsum. The forced oxidation (FOX) system included air blowers, blower building, air piping, and air spargers for the five modules on each of two units. Modifications were also made to the dewatering thickeners and drum filters to accommodate gypsum production. A conveyor was built in a separate project to deliver the gypsum to a new wallboard facility built adjacent to the power plant. About 500,000 tons per year of gypsum is now made into wallboard instead of entering the landfill at the plant.

To be able to produce wallboard grade gypsum, a purge stream was required to control the concentration of chloride in the FGD liquor. AECOM Process Technologies developed a patented purge water treatment process that allows water to be discharged from the FGD process while meeting plant discharge limits for selenium. The process requires pH adjustment (via lime) and addition of ferrous chloride. The purge water system was designed to process 400 gpm of supernate water. New equipment installed included ferrous chloride storage tanks and feed pumps, a 100-ton lime silo and feed tank, two 20,000-gallon reactor vessels with mixers and pumps, and two media filters. An existing thickener was integrated into the process flow diagram to reduce capital costs. Waste sludge from the process is either mixed with fly ash for landfill disposal or dewatered in a trailer mounted plate and frame filter press.

In 2007, SECI retained AECOM to upgrade the performance of the existing scrubbers, as well as install a new dewatering system for the existing Unit 1 and 2 FGD systems. AECOM was the EPC contractor for the project.

Scope of supply included:

• High performance trays for 10 absorber modules
• Additional oxidation air blow with building extension to serve as spare capacity with higher sulfur fuel
• New gypsum hydrocyclone system to serve as a spare primary dewatering capacity
• Two new horizontal belt filters for secondary dewatering
• New dewatering building
• New filter feed tank
• Additional conveyors to tie into existing conveying system to wallboard manufacturer