high cycles cooling tower operation using a revolutionary alkaline treatment program Continuum* AEC (Alkyl Epoxy Carboxylate)

Transcription

1 Water Technologies & Solutions technical paper high cycles cooling tower operation using a revolutionary alkaline treatment program Continuum* AEC (Alkyl Epoxy Carboxylate) Authors: Joseph Janeczko (SUEZ), Mark Tylec (Southern Energy), Kal Tari (Praxair, Inc) abstract Operating cost minimization is an everyday goal for the Power Generation Industry. The cost-effective treatment of open recirculating cooling systems for corrosion, mineral scale, fouling, and microbiological growth is critical to ensure optimal generation of power. Capitalizing on an advancement in alkaline cooling water technology enabled a northeastern cogeneration plant to reduce water consumption, discharge costs, and treatment costs. This paper discusses the conversion from a conventional phosphonate technology to Continuum AEC, a revolutionary cooling water treatment program. It details the increased cycles of concentration, improved treatment performance, and reduced overall operating costs provided by the new treatment program. introduction Corrosion and deposition in power generation systems can be costly. It is estimated that one-half of the annual maintenance expense for power plants is traceable to corrosion. 1 Failure to control corrosion and deposition can result in reduced power generation rates, increased maintenance, unplanned outages, and losses in unit availability. A conventional, phosphonate-based cooling water treatment program was used at a northeastern cogeneration plant for several years. Although the program was acceptable, the plant was interested in reducing water consumption and sewer discharge, while maintaining or improving the current performance levels and reducing treatment costs. This required an increase in the cycles of concentration for the cooling tower which was not possible because of the limitations of the phosphonate program. In order to achieve performance and costs goals, Continuum AEC was evaluated in the cooling system over a 12-month period. The program combines alkyl epoxy carboxylate (AEC) with a patented polymeric dispersant, a phosphate-based steel corrosion inhibitor package, and an azole for yellow metal corrosion protection. AEC represents the first significant advancement in calcium carbonate scale control for industrial cooling water systems in the past 25 years. AEC has proven to be an extremely effective scale inhibitor, especially in high ph, high alkalinity, and at high calcium levels and temperature conditions. The two major advantages of the AEC component over traditional phosphonates are its exceptional stability in the presence of oxidizers and its superior solubility. Compared to conventional alkaline treatments, a Continuum AEC program also provides an improved level of program confidence and enhanced environmental acceptability. plant description The UDG Niagara Falls cogeneration facility, operated by Southern Energy Inc., has a gross power production capacity of approximately 56 MW. Steam is produced by a Pyropower circulating fluidized bed boiler, burning a combination of coal and limestone. The condensing/extraction steam Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. tp438.docx Jun-03

2 turbine generator has design throttle conditions of 1,500 psig/950 F (103.5 bar/510 C) and an exhaust pressure of 3.3 pounds (1.5 kg) absolute. to the tower basin based on level demand. The tower blowdown is discharged to the municipal sewer system. A closed-loop cooling system provides cooling water to various high heat transfer components within the boiler system (Figure 1). The water in the closed-loop system is cooled by a plateand-frame exchanger using cooling tower water. The tower makeup water analysis is shown in Table 1. The condenser metallurgy consists of a carbon steel tubesheet, 90/10 copper/nickel tubes, and carbon steel water boxes. Tower design criteria are shown in Table 2. Operating History Figure 1: UDG Niagara cooling system schematic Table 1: Makeup water analysis Components Calcium as CaCO 3 Magnesium as CaCO 3 Manganese as Mn Sodium as Na Bicarbonate as CaCO 3 Sulfate as SO 4 Chloride as Cl Silica as SiO 2 ph 7.3 Conductivity Levels 83 ppm 35 ppm 0.02 ppm 24 ppm 84 ppm 35 ppm 22 ppm 0.7 ppm 229 mmhos Table 2: Cooling tower and condenser design criteria Parameters Conditions Hot Water Temperature 106 F Cold Water Temperature 85 F Flow rate Drift Evaporation Condenser Tube Velocity Condenser Heat Duty 27,000 gpm 2.0 gpm 557 gpm 7 ft/sec 257 (10 6 ) BTU/hour Condenser Heat Flux 710 BTU/h/ft 2 -F The cooling tower is a mechanical draft, PVC-fill, counterflow design, made of wood. The makeup water is purchased from the City of Niagara Falls and added Since operations began in 1991, the cooling tower treatment has used a conventional, phosphonatebased technology for operation in a less corrosive, alkaline environment. This treatment included a phosphonate for scale control and mild steel corrosion protection, an azole (tolyltriazole) for copper corrosion control, and a polymeric dispersant. This treatment program operated at a ph range of 8.3 to 8.5, controlled with sulfuric acid. The tower was controlled at a level of 900 to 1,000 ppm calcium (as CaCO 3 ) to yield 10 to 12 cycles of concentration. These control ranges corresponded to a Langelier s Saturation Index (LSI) range of +1.8 to Tower cycles were restricted to 12 because of the limited capabilities of the phosphonate treatment program. Relatively high calcium levels in the makeup could lead to scaling on the condenser at cycles above 12. Program performance was also affected by high levels of airborne particulates from surrounding coal piles. The alkaline program performed well when operating within specified guidelines. Results from corrosion coupons indicated an average corrosion rate of 2.8 mpy for mild steel and 0 mpy for both 90/10 copper-nickel and stainless steel. Annual inspections of plant equipment confirmed acceptable corrosion and deposition results. Microbiological control is maintained through the use of a granular bromochlorohydantoin (BCDMH) product. The BCDMH is fed during the evening and early morning for a total duration of 10 hours, targeting for 0.2 to 0.4 ppm free halogen residual. Microbiological counts are monitored weekly with the use of dip slides. In addition to Page 2 tp438tp438.docx

3 dipslides, a MonitAll* heated surface monitoring device is used for visual confirmation of microbiological control. Velocity and skin temperature through the MonitAll equipment can be set to match or exaggerate cooling system conditions in tester and the glass tube where the tube can be continuously monitored. The hot tester contains a heated test section which can be removed for inspection. The voltage to the hot tester s heater can be varied to target a wide range of heat flux settings. Two thermocouples in the hot tester enable a test section temperature and bulk water temperature to be monitored and compared for heat transfer monitoring. The fouling factor can then be calculated from the change in temperature difference over time. Temperatures were recorded daily by the MonitAll device. In addition, turbine backpressure data from the plant s DCS system was monitored. These monitoring tools indicated that the phosphonate treatment program inhibited scaling under the tower control limits set for the program. Figure 2: Corrosion rates using Phosphonate-based treatment the condenser. Visual inspection of the tower and heat exchanger components indicated acceptable control of microbiological activity. With deregulation of the Power Industry in New York State looming, the plant was interested in any competitive edge gained through lowering operating costs, while maintaining the current level of performance in their water systems. The BetzDearborn Continuum AEC program was recommended to meet the plant objectives. Because of the advancements that Continuum AEC offered over traditional phosphonate-based technology, increased cycles of concentration were proposed to reduce operating costs while maintaining performance. performance monitoring The plant installed a MonitAll unit to the surface condenser outlet for monitoring both deposition and microbiological growth. The MonitAll assembly consists of a heat test assembly with a sample specimen tube, hot tester, piping accessories, and temperature monitoring meter. The test assembly is visually monitored through a section of glass tubing through which a system sample flows. This is ideal for viewing the fouling tendencies of the water in the actual equipment without opening the condenser. Corrosion coupons were used to measure corrosion rates for mild steel, copper-based metals, and stainless steel. Exposure periods for the corrosion coupons were 30, 60, and 90 days. Industry guidelines for acceptable uniform corrosion rates are as follows: Mild steel corrosion rates: < 3 mpy is very good 3 to 5 mpy is good 5 to 8 mpy is fair Copper-based alloys corrosion rates: < 0.2 mpy is very good 0.2 to 0.3 mpy is good > 0.3 is poor Stainless steel corrosion rates should be 0.0 mpy The phosphonate-based treatment maintained corrosion rates within these guidelines. Corrosion rates averaged 2.8 mpy, 0.0 mpy, and 0.0 mpy for mild steel, 90/10 copper/nickel, and stainless steel, respectively. The average corrosion rates are shown in Figure 2. The hot tester fits into the test assembly, enabling sample flow through the annular area between the hot tptp438.docx Page 3

4 Continuum AEC evaluation The goal of the trial was to reduce operating costs and maintain condenser efficiency by using Continuum AEC technology for cooling water treatment. Increased cycles was the most effective way to reduce operating costs. Although higher cycles create a greater potential for scaling, AEC s superior calcium carbonate inhibition and calcium tolerance properties permit higher LSI and calcium levels to be achieved without fouling compared to traditional phosphonate technology. Table 3: Cooling Tower Operating Parameters with AEC. Parameter Calcium as CaC03 Total Hardness as CaCO3 Control Range 1,600 to 1,800 ppm 2,400 to 2,600 ppm Conductivity 4,300 to 4,500 μmhos ph 8.3 to 8.5 Orthophosphate as PO4 1.7 to 2.1 ppm Cycles of Concentration 16 to 18 Figure 3: MonitAll monitoring device results Figure 4: Corrosion rates using AEC treatment The phosphonate-based program was replaced with a Continuum AEC program in early August AEC treatment contains a complete corrosion and scale inhibition package designed specifically for the cooling system at UDG. For the evaluation, cycles were roughly doubled, producing approximately half the blowdown that was required previously. Increased cycles required a calcium control range of 1,600 to 1800 ppm ; the ph control range was set at 8.3 to 8.5 to minimize the potential for silicate deposition. The control ranges for the first eight months are outlined in Table 3. The orthophosphate control range was set at.7 to 2.1 ppm to work in conjunction with AEC to provide effective mild steel corrosion protection. The typical orthophosphate guidelines for an AEC treatment program are in the range of 4 to 7 ppm (as PO4) for cycled water with 250 to 800 ppm calcium levels. The calcium concentration control range of 1,600 to 1,800 ppm reduced the quantity of orthophosphate needed to provide the desired mild steel corrosion inhibition results. The key to successful corrosion protection for systems operating in the alkaline ph range, especially with 1,600 ppm calcium, is maintaining the solubility of the mild steel corrosion inhibitors. For the UDG tower, the orthophosphate and polymeric dispersant levels were selected to optimize both corrosion and deposit control. Continuous monitoring with corrosion coupons and the MonitAll equipment affirmed the program selection. Page 4 tp438tp438.docx

5 During an eight-month evaluation at higher cycles, the Continuum AEC program effectively controlled scaling and corrosion in the cooling system. As expected, the superior scale control capability of AEC was confirmed by regular deposition monitoring. Figure 3 represents clean heat transfer surfaces from the MonitAll device. In addition, turbine backpressure was monitored by the plant s DCS system and indicated cleanliness. Effective scaling control was also demonstrated by visual inspection of the condenser during a planned outage. During the evaluation, the MonitAll equipment at UDG Niagara Falls was adjusted to exaggerate system conditions; consequently, the MonitAll was more susceptible to scaling than the condenser. This monitoring technique was used to show that the AEC treatment program was able to handle even more severe conditions than normal operation. Voltage and flow through the MonitAll monitoring device were adjusted to yield a higher than design heat flux and a lower tube-side velocity, approximately 3 ft/sec (0.9m/sec). Corrosion coupon test results were excellent, an indication that the level of treatment was optimal for the ph and level of calcium. Not only did the AEC treatment program maintain corrosion rates within industry guidelines, but it also improved on the corrosion rates that were obtained with the previous phosphonate program. Corrosion rates averaged 1.7 mpy, 0.0 mpy, and 0.0 mpy for mild steel, 90/10 copper/nickel, and stainless steel respectively. The average corrosion rates are shown in Figure 4. The most recent mild steel corrosion coupon rate result was 0.3 mpy after an approximate 90-day exposure for mild steel metallurgy. During the evaluation, a high level of suspended solids contamination was a regular occurrence. The suspended solids were primarily airborne coal fines from an adjacent active coal storage pile. The cooling water appeared black and saturated with coal dust, particularly on very windy days. Initially, it was thought that the coal dust would interfere with the performance of the treatment program; however, extensive testing revealed that the superior solubility of AEC, in combination with the specified polymeric dispersant level, provided effective dispersion of the coal fines while maintaining phosphate solubility. economic evaluation The Continuum AEC evaluation produced the expected operational cost savings. Overall water consumption was reduced by the ability to nearly double cycles of concentration to the range of 19 to 22 cycles. The plant reduced tower blowdown from million pounds (75.5 million kg) per year to 91.5 million pounds per year (41.5 million kg), resulting in 45% cost savings in water (plant water is supplied by the City of Niagara Falls for US1.00/1,000 gallons ( L), with sewer costs of $0.92/1,000 gallons ( L), plus various surcharges). In addition, the cooling water treatment program cost was reduced by 36% because of the reduction of blowdown and optimization of product feedrates. The use-cost basis of the treatment program per million pounds of blowdown was not significantly increased, even at the exceptionally high calcium level of 1,800 ppm. The goal to reduce utility costs without sacrificing results was achieved. conclusions The results of the Continuum AEC program were: Operating Costs were reduced by 45% Cycles of concentration increased from 10 to 19 Corrosion results were improved Condenser cleanliness was maintained acknowledgments We would like to acknowledge the efforts of Robert Bethin of Southern Energy for his hard work throughout this project in the areas of monitoring, control, and troubleshooting. We would also like to acknowledge the efforts of Jim Scattolini of SUEZ Water for coordinating the writing of this technical paper and technical plant audit. references Whitaker, R., Roberts, A., and Syrett, B., "Corrosion", EPRI Journal, volume 6, pp.6-13, September tptp438.docx Page 5