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Case Study CH-1045

3D TRASAR® Technology Saves Over 281 Million Gallons of Water per Year

Economic REsultsEnviRonmEntal REsults

$47,503 savings in chemical and water pumping costs

281,000,000 gallons/year

eROI

SituationThe population of Nevada, the most arid state in the nation, grew by 66% between 1990 and 2000. Over two thirds of the population – 1.7 million people – live in the Las Vegas metropolitan area and depend on the Colorado River system for their drinking water. Droughts and population growth have strained that resource, so much so that Nevada has embarked on a project to develop distant groundwater supplies in northern Nevada. By pumping water through 327 miles of underground piping, officials hope to postpone serious water shortages.1

Nevada’s growing population needs more than water. It needs electric power too. Operating a power plant in this environment presents challenges. Water supplies are tight. Water chemistry is problem-atic. The need to prevent opera-tional problems competes with the need to conserve scarce natural resources.

Such was the situation at NV Energy’s Reid Gardner Gen-erating Station. The plant needs water to operate. Minimizing that water use is the environmentally responsible thing to do, but traditional water-use reduction techniques – such as raising cooling tower concentration ratios – were not technically viable.

River and well sources supplied make-up water to the cooling system. Both contained prob-lematic concentrations of silica, which had, historically, deposited on condenser tubes and cooling tower fill. The deposit, as shown in Table 1, was pure Silica (SiO2).

Deposit Analytical Report

Silicon (as SiO2) 82%

Aluminum (as Al2O3) 2%

Calcium (as CaO) 2%

Iron (as Fe2O3) 2%

Magnesium (as MgO) 1%

Sodium (as Na2O) 1%

Sulfur (as SO3) 1%

1Johnson, Jeff, “Water Authority Looks Beneath for Drought Protection”, Southwest Hydrology, September/October 2008, p.30-32

This deposit generally occurs only when the recirculating water silica concentrations exceed 150 ppm and the temperature is less than 70°F (21°C). In this system, silica concentrations were about 230 ppm on average. The incoming water temperature was about 80°F (27°C) with a 20° temperature drop across the tower: perfect conditions for SiO2 formation.

Silica deposits are tenacious, insulating and difficult to remove. They reduce condenser efficiency and require time-consuming, mechanical cleaning processes to remove. Prevention of these deposits is the best option.

3D TRASAR Technology: A New ApproachA number of parameters determine silica-scaling potential: silica concentration, pH, temperature, concentration of other multivalent ions – Ca, Mg, Al, Fe, Zn – and biological activity. Prevention of silica-based deposits requires simultaneous management of all these stresses.

3D TRASAR Technology prevents operational problems and minimizes operating costs by controlling a cooling system based on the actual stresses placed upon it. 3D TRASAR Technology measures the key parameters related to system stress. When changes or upsets are detected, 3D TRASAR Technology responds by taking appropriate, automatic, corrective action. It then communicates with system users through a variety of mechanisms (3D TRASAR Web,

automated reports, text messages, digital pages, etc.).

3D TRASAR Technology employs fluorescence-based control to pre-vent deposition. An inert fluorescent material provides baseline informa-tion correlating with the amount of dispersant polymer present in the recirculating water. A second fluorescent “tag” is incorporated onto the dispersant polymer itself. The tag is active. It reacts to system stresses just as the polymer does. By comparing the tagged polymer fluorescent signal to that generated by the inert TRASAR material – and the rates at which the two signals change – 3D TRASAR Technology continuously measures the scale-forming stress on the dispersant and makes necessary changes, auto-matically, to prevent deposition.

In the summer of 2007, this facil-ity started using 3D TRASAR Technology to control its cooling water chemistry. In addition to the

tagged dispersant polymer and inert TRASAR material necessary for proper program control, a new, Nalco-proprietary, silica dispersant was incorporated into the program.

ResultsSuccess in this system required tight control of cycle chemistry and better silica dispersion than had been had been previously available. 3D TRASAR Technology control, coupled with the new silica disper-sant, delivered the results the facility needed.

Throughout the trial period, Real-Time Monitoring (RTM) technology, a quartz crystal microbalance-based instrument licensed for use from Sandia National Laboratories, re-ported the amount of fouling in the system. As can be seen in Figure 2, no deposition was noted. These results were confirmed by visual inspections, as shown in Figure 3.

Figure 1 – Prior to 3D TRASAR control, deposition was evident in the cooling tower fill. Today, no deposition is seen.

By every measure – real-time monitoring, 3D TRASAR data and visual inspections – 3D TRASAR Technology helped this power plant operate better under very challenging circumstances. Water use was minimized, operational problems were prevented and efficiencies were maintained which translated directly into operating cost reduction.

Environmental ROIAccording to the United States Geological Survey, Americans use 80 – 100 gallons of water per day. The savings achieved at this plant will satisfy the annual water needs of about 9000 people per year.2

Evaporation removes heat from an open recirculating cooling water system. When recirculating cool-ing water passes through a power plant’s condensers, it picks up heat from the condensing steam and transports it over the cooling tower, where some of it evaporates. Evaporation from a cooling tower is almost pure water. All of the dissolved ions are left behind to concentrate in the remaining

Performance Under Upset ConditionsMost treatment programs deliver adequate results under routine operating conditions. Superior performance under upset or severe operating conditions requires a much more robust approach. 3D TRASAR Technology delivered at thisfacility under those conditions.

The 3D TRASAR program had been in operation for a number of months when a blowdown pump malfunc-tioned in one of the units. Cycles in this system increased from about 6 to over 17, severely stressing the dispersant polymer and increasing the silica concentration to over 600 ppm. Heavy deposition was expected, but, upon inspection, almost none was detected, as shown in Figure 3.

Figure 3 – Even under upset conditions, very little deposition occurred in the auxiliary heat exchanger tubes.

Figure 2 – RTM data showed no increase in deposit loading during the trial period. Quartz crystal Microbalance (QCM)a frequency confirms the finding.

2Source: U.S. Census bureau.

Reid Gardner Generating Station, Moapa, NV

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Nalco reports Environmental Return on Investment (eROI) values to customers to account for contributions in delivering both environmental performance and financial payback.

up water demand decreases, as shown in Figure 4. At this plant, the cycles of concentration increased from 4.5 to 7.0, an improvement that resulted in a make-up water use reduction of 281 million gallons per year.

Many power plants operate on a Zero Liquid Discharge (ZLD) basis, which means no liquid waste leaves the boundary of the facility. At this plant, liquid waste is pumped to evaporation ponds. There is a limit to the amount of water these ponds

Figure 4 – The annual water savings at this plant would meet the annual water use needs of 9,000 people. Increasing cycles of concentration in the cooling tower decreased make-up water demand by 281 million gallons and eliminated the need to dispose of an equal amount of blowdown water in the plant’s evaporation ponds.

recirculating water. If the concen-tration of dissolved ions exceeds its solubility, deposition occurs.

The concentration of ions in the recirculating water is controlled by a combination of bleeding off a certain portion of the concentrated recirculating water, adding in fresh make-up water and use of scale-inhibiting chemicals. As the con-centration ratio – the concentration of an ion in the recirculating water divided by the concentration in the make-up water – increases, make-

can accept, not only because they are limited in size, but also because the evaporation rate from the ponds varies with weather conditions. Increasing cycles of concentration reduced the load on the evaporation ponds by 281 mil-lion gallons per year, meaning the plant can operate in accordance with its ZLD requirement with-out risking operational problems caused by evaporation pond limits.