Knocking its SOx off
The FGD systems (Figure 3) are designed to remove 95% (30-day rolling average) of the SO2 produced when burning the design fuel. Wheelabrator guarantees that the stoichiometry of the scrubber's limestone reagent will not exceed 1.03 under any conditions. All guarantees are being met with one absorber recycle pump out of service.
3. Scrubbing it down. The inlet to the absorber section of the new scrubber for Unit 1, shown during installation. The system is designed to remove 95% of the unit's SO2 emissions when it's burning coal with a sulfur content of 0.5%. Courtesy: Washington Group International
All FGD process systems have been placed in a common building (Figure 4). The reagent preparation equipment is located in the center, between the two absorbers. Adjacent to each absorber is a dedicated primary gypsum dewatering hydroclone. Shared vacuum filter systems and a common storage area have been located at the end of the building.
4. Compact "Scrubber Island." FGD process equipment, limestone preparation, and gypsum dewatering and storage equipment for both units are all housed in a compact design. Courtesy: Washington Group International
To expedite construction, the FGD building was designed and built in two phases. The bottom structural mat, designed during the project's early stages, addressed general equipment arrangements, the placement of the FGD slurry pump and limestone ball mill equipment, and maximum floor loading limits. The design of the building's framing, flooring, and major structural bracing systems was frozen for up to one year while the locations of vertical columns were determined to facilitate the bottom mat design. At that point, the final design was completed and the top mat—including the sumps, trenches, and equipment pads—was issued for construction.
Makeup and cooling water for the FGD process is supplied by P4's existing cooling tower makeup system. The increase in demand required the upgrading of lakeside makeup pumps. Wastewater from the FGD process is discharged into the upgraded existing cooling tower blowdown system that drains into Lake Michigan, about five miles away.
The limestone used by the FGD process is delivered by truck, stored in an open pile on the plant site, and then sent via enclosed conveyor to replenish local day silos. The wallboard-quality gypsum that the scrubber produces is stored in an enclosed facility and later trucked offsite. Some 42,000 lb/hr can be produced when both units are burning the design fuel at full load.
P4 goes 3-D
A very interesting aspect of the AQCS upgrade project was its "availability by design" philosophy. To meet such an ambitious goal, Washington Group International and We Energies took a number of specific actions during the design process (see table).
Availability by design. A specific program for maximizing system reliability and availability was developed for the Pleasant Prairie Power Plant project. Source: Washington Group International
Among them, but not shown in the table, was the development of a 3-D model of the plant to facilitate collaborative design reviews. The 3-D model included all equipment, piping/supports, cable tray, and structural members. Using MicroStation Smart Plant from Intergraph Corp. (www.intergraph.com), Washington Group, We Energies, and subcontractors' site teams took a weekly walk though the design (Figure 5). The software and model enabled interactive reviews of valve and instrument locations, accessibility, egress, maintenance space, hoisting/removal plans, and the like. Without question, building the model was a terrific decision. Time and again it allowed design changes to be proposed, discussed, and finalized with minimal effect on project cost or schedule.
5. Virtual walk-through. Washington Group used 3-D engineering design tools to create a virtual representation of Pleasant Prairie Power Plant. The images were used in performance O&M studies and to identify design deficiencies prior to construction. Courtesy: Washington Group International
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