Positive Results
When the unit came back into operation after the spring 2007 outage, there was an immediate reduction in the number of tons of landfill material discharged from the centrifuge process. This reduction was directly related to the relocation of the #1 clarifier lateral pump suction (underflow take-off), the change in centrifuge technology, and the optimization of dewatering polymers. The amount of material sent to the landfill decreased from an average of 733.5 tons/day to 489.9 tons/day, which added up to an annual savings of $256,500 in landfill costs (Figure 4).
4. A slowdown in sludge generation. This chart illustrates the number of tons of sludge sent by the Zimmer Station to the landfill on a monthly basis during the evaluation period from May 1, 2006 through July 31, 2008. The numbers reveal a large overall reduction in the volume of sludge disposed during the last 12 months of the evaluation period. Courtesy: Duke Energy
The test centrifuges that were installed during the spring 2007 outage did not perform as well as expected and resulted in higher polymer consumption and decreased cake solids. Consequently, the team made a decision to abandon the test centrifuges and rent centrifuges with back drives. An immediate improvement in both cake solids and polymer consumption was realized. Ultimately, Utter Construction purchased new centrifuges with back drives. Cake solids improved approximately 3% for the time frame associated with this evaluation. Currently, cake solids operate in the high 40% range on average for both centrifuges (Figures 5 through 8).
5. A piece of cake. Switching to centrifuges with back drives led to a marked improvement in cake solids and polymer consumption. The data shown are from Centrifuge No. 1. Courtesy: Duke Energy
6. A solid success. Cake solids improved approximately 3% during the evaluation period from May 1, 2006 through July 31, 2007. The data shown are from Centrifuge No. 2. Courtesy: Duke Energy
7. Less is more. The new automated make-down system provides a more efficient and reliable dewatering operation. Courtesy: Duke Energy
8. Paring down polymer use. The reduction in polymer consumption translates into greater savings in operating costs. Courtesy: Duke Energy
Lime kiln dust (LKD) is added to the sludge as it is discharged from the centrifuges via a pug mill. LKD is used simply to increase the dryness of the sludge prior to shipping it to the landfill. Pebble lime performs the same function but is used on weekends in lieu of LKD to avoid the overtime associated with the pug mill operation. The pebble lime is mixed with the sludge by a front-end loader prior to loading the trucks for the landfill.
The decreased landfill tonnage combined with increased cake solids and the resulting reduced LKD and pebble lime usage have resulted in additional annual savings of $310,356 (Table 1), for total annual savings of $566,856.
Table 1. A money-saving move. By significantly reducing the number of tons of centrifuge solids sent to the landfill each year, the Zimmer Station has been able to realize substantial savings. The dewatering process also avoids burning 2,800 gallons of diesel fuel. Courtesy: Duke Energy
—Craig Moyer (craig.moyer@duke-energy.com) is the FGD coordinator at Duke Energy's Zimmer Station. Juan Fernandez (juanm.fernandez@ge.com) and Bob Carraro (francis.carraro@ge.com), area manager, both work at GE Water & Process Technologies.
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