A tale of two towers

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Source: MIOX Corporation

By Paul E. Schrock, senior chemist, NIPSCO Valparaiso Service Center and Tom Muilenberg, global business director for power and heavy industry, Miox Corp.

A new onsite mixed oxidant system at the R.M. Schahfer Generating Station of Northern Indiana Public Service Co. (NIPSCO), in Wheatfield, Ind., has yielded substantial treatment chemical cost savings and improved Summer operation compared to the control unit cooling tower.

The NIPSCO project holds the double honor of being Miox’s first power plant cooling tower installation as well as its largest cooling tower installation.

The Schahfer Generating Station lies southeast of Chicago near Valparaiso, Ind., along the Kankakee River. The station’s National Pollutant Discharge Elimination System (NPDES) permit requires compliance with a chlorine residual level that cannot be exceeded when discharging into the Kankakee River’s final receiving waters.

Until spring 2010, NIPSCO’s Chemical and Environmental Compliance Department had used a “tri-cocktail” regimen of sodium hypochlorite, sodium bromide, and a photosynthesis blocker (or algaecide) to treat water supplied to four cooling towers at the 1,780 MW coal-fired power plant.

The cooling water is allowed to reach 2 to 3 cycles of concentration, with the cooling tower blowdown being discharged into a 200-acre settling pond, which then may be discharged into the Kankakee River. Due to the NPDES permit restrictions, disinfection dosing is limited to two hours a day for each tower. During the summer months, it was not uncommon to see spikes in Legionella above 200 colony forming units per milliliter (CFU/ml), when an action level of 100 CFU/ml triggers a recommendation to workers servicing the towers to wear respiratory equipment for health safety.

At the time the utility first began considering a new disinfection regimen, sodium hypochlorite prices had nearly doubled to about $1.60 to 1.65 a gallon, so saving money was central to the idea of a new regimen. The savings potential of also reducing the number of chemicals used in bio-control from three to one only made the proposal more attractive to officials at NIPSCO, who have a forward-thinking reputation of working closely with environmental regulators to stay ahead of the regulatory curve.

NIPSCO has invested significantly in both the boiler water and cooling water areas concerning automated chemical feed control, process chemistry data acquisition, and compliance with the Electric Power Research Institute recommended cycle chemistry guidelines. The company has also stayed ahead of the air emissions regulatory curve for NOx and SO2 removal by complying early with the Clean Air Act rules.

NIPSCO installed a Miox RIO on-site mixed oxidant disinfection system in late 2009 early 2010 on one of the four cooling towers at R.M. Schahfer Generating Station, Unit 15, which has a flow rate of 168,000 gallons per minute (gpm). Unit 14, the tower most like Unit 15 (but with brass versus stainless steel main condenser tubes), was kept on the biocide/algaecide tri-cocktail treatment as a control comparison to help evaluate the new system’s effectiveness. The hypochlorite-to-bromide biocide ratio on Unit 14 ranges from 4:1 to 8:1, depending on the tower’s biological conditions. Unlike the other two towers, towers 14 and 15 have open-air hotdecks, which makes algae control more problematic.

The ongoing evaluation criteria for comparing Unit 15 to Unit 14 included:
Mild steel corrosion rate dataDipslide total bacteria countsAquaFlour (chlorophyll A) countsProgram costs - financial efficiencyCondenser visual cleanlinessCondenser differential pressureLegionella bacteria counts.
After two full quarters of use, the Unit 15 cooling water system has experienced a longer summer run without a cleaning than historically has been experienced, said Bert Valenkamph, NIPSCO’s director of Chemical and Environmental Compliance.

Except for a time at the end of September when record high temperatures promoted increased biological growth, the Unit 15 tower outperformed Unit 14. In the future, NIPSCO plans to shift the dosing regimen from an hour in the morning and an hour in the evening to four times a day in half-hour dosing periods. The company continues to investigate alternative dosing regimens that will yield optimal results, and has also discussed the possibility of applying for permit revisions in order to gain a longer allowable daily dosing period. Algal growth was still minimized compared to the previous years, even while tweaking the dosing strategy and volume.

As for corrosion, Steve Barnes, a NIPSCO senior chemist involved in the project, said the utility is currently in the process of establishing better corrosion data for the cooling water systems.

Unit 15 is normally the dirtiest cooling tower and has to be cleaned regularly to clear condenser tubes. After a record long run, though, the condenser was brought down for a scheduled maintenance check in June.

Financially, the system has delivered savings. The Unit 14 tower’s treatment employs sodium hypochlorite, sodium bromide and an algaecide for a total cost of $225,000 annually. Meanwhile, the Miox-treated Unit 15 tower (with yearly forecast estimated on the high side) has expenses of about $66,500 a year. The annual savings with the Miox system amounts to about $158,500 a year. There has also been no increased costs for operating and maintaining the Miox system compared to the previous system.

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