Although recirculating seawater cooling tower is seen as a sustainable and cost-effective alternative to single-pass cooling, deposit formation in condensers, heat exchangers or cooling tower fill is a niggling problem. George A Peabody and Geoff Townsend posit the view that an advanced deposit control programme could be the answer.

INTRODUCTION

Engineering and operating companies are increasingly turning to the use of recirculating seawater cooling tower systems in regions with limited fresh water supplies to provide condenser cooling for electric power plants, as well as process cooling for the refining, chemical, and primary metals industries. The use of recirculating seawater cooling is replacing single-pass cooling as a means of reducing water withdrawals, and limiting the thermal impact of blow-down water on the marine environment.

In addition to the positive sustainability impact, seawater cooling tower systems provide cost benefits through reduced make-up and blow-down water flows, and the corresponding reduction in pipeline construction and water pumping costs. Figure 1 illustrates the potential reduction in water flow as a function of the concentration factor (CF) for a condenser cooling system serving a modern 800 MW combined cycle power plant. The most common and practical concentration range for seawater cooling tower operation is 1.2 to 1.8 for most seawater.

Deposit formation in condensers, heat exchangers or cooling tower fill can quickly reduce production efficiency and output. Successful deposit control in seawater cooling tower systems depends on both the design engineering and water treatment practices. This article examines advancements in calcite scale control – one of the primary issues related to seawater cooling tower treatment.

CALCITE SCALE CONTROL

Seawater is naturally very stable, but severe scaling may occur as temperature and concentration increase. Seawater cooling tower systems typically operate with a concentration factor of 1.2 to 1.8, resulting in a calcite saturation that is 10 to 20 times above equilibrium. Operation at higher CF risks the formation of gypsum and magnesium scales, which are more difficult to inhibit chemically.

Acid feed is one option to prevent the formation of calcite scale in the cooling system. Due to the very large size of many seawater cooling systems, the amount and cost of acid required can be substantial. The trend today is to eliminate the acid use for scale control in seawater cooling tower systems, and rely on calcite scale inhibitors. The desire to operate at higher concentration factors to reduce cost and environmental impact means that more is expected from the scale inhibitors, and increases the need for advanced deposit control. However, advanced deposit control requires the right inhibitor, the ability to monitor and control dosage on-line, and the ability to accurately monitor deposition in a timely manner.

SELECTION OF A SCALE INHIBITOR

The ideal scale inhibitor should:

• Have an excellent environmental profile
• Provide robust and predictable calcite inhibition
• Be able to be measured and controlled on-line
• Be cost effective.

Specific phosphonates and polycarboxylates are selected as the best low or non-phosphorous compounds for the application.

ON-LINE CONTROL OF SCALE INHIBITOR DOSING

The ability to measure and control the concentration of the scale inhibitor on-line may be achieved through measurement of an inert fluorescent moiety present with the inhibitor. Seawater is naturally very low in background fluorescence, making fluorometry an ideal means to detect and control low concentrations of scale inhibitor. The control system should also include measurement and data management for pH, conductivity, redox, turbidity, and temperature to provide a complete picture of the cooling water chemistry.

ON-LINE DEPOSIT MONITORING

Although increasing the cycles of concentration provides significant benefits, it is also accompanied by an increase in the scaling potential. Having deposit monitoring in place allows the cycles of concentration to be increased with confidence. It, therefore, forms a key element of the programme strategy. On-line deposit monitoring using the concept of a quartz crystal microbalance with a heated surface is currently under development, and has been applied to seawater cooling systems.

ADVANCED DEPOSIT CONTROL DEMONSTRATION

A demonstration was conducted at a modern combined cycle power plant using a seawater cooling tower system to provide condenser cooling. The system was operating at a concentration factor of 1.3, using a scale inhibitor, but no acid. The focus of the demonstration was on improved control of calcite deposition through the application of a traced scale inhibitor, combined with on-line deposit monitoring.

The necessity of a scale inhibitor is demonstrated in Figure 2, which depicts the increase in deposit mass when the scale inhibitor dosing was temporarily switched off. Note the concentration of the inhibitor falling abruptly, and the calcite mass increasing steadily thereafter.

The introduction of feedback control of the inhibitor is illustrated in Figure 3. With manual control, the concentration of inhibitor varies over a wide range. Feedback control maintains the inhibitor at the desired dose to protect system performance, without periods of over or under treatment.

Once reliability in the dosage control and the sensitivity of the deposition detection systems had been established, the next phase was to reduce the concentration of the scale inhibitor while closely monitoring for signs of deposition. (Figure 4)
Note that the deposition trend is flat, while the inhibitor dose is reduced by over 20%.

During the course of the demonstration, the cycles of concentration were increased from an average of 1.2 to 1.8 without the need for pH control, resulting in a reduction in seawater usage of 1,360 m3/hr, with an associated pumping cost in excess of €135000 per annum. In addition, despite the higher scaling potential, the concentration of scale inhibitor applied was reduced to 1.6 mg/L from the 2.8 mg/L dose of the prior scale inhibitor. There was no evidence of scale deposition.

SUMMARY

An advanced deposit control programme for seawater cooling tower systems can provide substantial energy and chemical savings while reducing the environmental impact. It is a practical and cost-effective way to improve cooling system cleanliness and reliability. The programme helps plant operators to achieve better cooling system performance, supporting both production and sustainability goals.

George A Peabody is Technical Director, Power, Nalco Co and Geoff Townsend is Industry Fellow, Research and Development, Nalco Co. They can be contacted at: gpeabody@nalco.com and gtownsend@nalco. com respectively.