PAPER NO: TP13-09
CATEGORY: REPAIR & CONSTRUCTION
COOLING TECHNOLOGY INSTITUTE
HOW STRIPPING BIOFILM FROM THE COOLING WATER LOOP IMPACTS POWER PLANT PRODUCTION OUTPUT
The studies and conclusions reported in this paper are the results of the author’s own work. CTI has not investigated, and CTI expressly disclaims any duty to investigate, any product, service process, procedure, design, or the like that may be described herein. The appearance of any technical data, editorial material, or advertisement in this publication does not constitute endorsement, warranty, or guarantee by CTI of any product, service process, procedure, design, or the like. CTI does not warranty that the information in this publication is free of errors, and CTI does not necessarily agree with any statement or opinion in this publication. The user assumes the entire risk of the use of any information in this publication. Copyright 2013. All rights reserved.
Presented at the 2013 Cooling Technology Institute Annual Conference
Corpus Christi, Texas - February 4-7, 2013
HOW STRIPPING BIOFILM FROM THE COOLING WATER LOOP IMPACTS POWER PLANT PRODUCTION OUTPUT
Tom Muilenberg and Cem Candir
Biofilm removal is a key step in maintaining the cooling water loop across power plants. However, it becomes crucial if the biofilm, visible or not, is present in condenser tubes because it eventually affect power plant production, especially if the plant is reaching peak capacity. This paper will present the challenges in cooling water loop maintenance, a new methodology to an efficient heat exchange and a case where biofilm removal helped increase a power plant’s production load.
Microbiological control and the prevention of aerosolization of bacteria within cooling towers has always been the primary driver of using hypochlorite, biocides and algaecides. One of the areas that is often overlooked is how well the biocide program is working and how it affects the power plant production capacity.
Cleanliness of condenser tubes is critical for effective cooling and therefore returning more water back into the boiler. In a typical power plant, there are two major water loops. 1) the cooling water loop is circulated between the cooling tower and condenser; 2) the boiler water loop is circulated through boiler, turbines and back to the boiler. The heat exchange happens when the cooling water loop "meets" the warmer boiler water loop at the condenser (see Chart 1).
Chart 1 – Typical cooling water and boiler water loop in a power plant. Courtesy of MIOX Corporation.
- General Discussion of Biofouling in Condenser Cooling Water Loop
Condenser efficiency in a power plant cooling loop is of critical importance in any power plant. The ability of the condenser to cool the steam coming off of the generating turbines, can often be the limiting factor in generator production. For example, if the condenser is operating at maximum efficiency, it can process more steam, allowing the generator to run at a higher rate. If the condenser is operating at lower efficiency, it can limit the amount of steam that can be processed or recovered. As a result, the steam coming off of the generator may need to be reduced, which in turn reduces the capacity at which the generator can be operated.
Many things can influence the efficiency of the condenser on both the tube and shell side of the condenser. Since this paper focuses on the water side, some tube side issues will be discussed. The cleanliness of the tube side walls has a significant effect on the efficiency of the condenser. The most common cause of loss of efficiency is the presence of surface fouling on tube internals. Mineral scaling, silica fouling and biofouling are the most common culprits of surface fouling. Of these, biofouling is arguably the most significant, possibly only second to silica fouling for its effect on heat transfer.
The reason biofouling is considered of paramount importance is that it can create multiple challenges for plant operation. Among these is corrosion, pitting corrosion, harboring of pathogenic organisms, and the significant thermal resistance it presents. In fact, when compared to mineral fouling, biofouling presents significantly more thermal resistance than the most common sources of mineral fouling. Chart 2, compares the reduction in heat transfer for various fouling layer thicknesses. In this chart, one can see, for example, that 1 mm of biofilm reduces heat transfer reduces efficiency by 50%, whereas calcium carbonate scale at that same thickness only about 10%.
Chart 2 – Effect of Heat Transfer vs. Thickness of Scale/Biofouling. Courtesy of MIOX Corporation.
While all forms of condenser fouling should be controlled or mitigated, if an operator could only choose one to control, the highest effect could come from control of the biofouling.
- Identifying Cooling Loop Biofilm
The presence of biofilm may present itself in a variety of ways. The most common and obvious is macro-fouling of the cooling tower hot deck, media and support structure and the basin itself. If routine inspection of the condenser tube is performed, it may be visible as a gelatinous mass partially or, in extreme cases completely, plugging the tubes. In addition, this biofilm may collect or capture additional foulants, such as algae, particulate or precipitated solids. In this case, it will appear as a hybrid fouling layer with chunks of material entrapped in the mass.
However, even in cases where macro-fouling is not evident, a loss of cooling efficiency that cannot be attributed to other operational problems may present the need for biological control.
- Impact on Operating Costs & Electricity Production
For systems operating near or at peak production, the inability to cool the steam effectively can cause the power plant to reduce electricity production. This results in lower efficiencies, which will increase operating cost per kilowatt produced. Furthermore it may limit the total electricity that the plant can produce, resulting in the loss of revenue.
Identifying and eliminating biofilm in the cooling loop can result in increased profitability and/or increased bulk sale of electricity. This was exactly what was happening at Purto Rico's Palo Seco power plant in 2010.
- Background of Palo Seco Power Plant
Puerto Rico has few natural energy resources that can be used as a source of fuel for power generation. As a result, shipped-in petroleum products are the dominant energy source for generating electricity on the island. Due to the high cost of shipping in fuels, as well as the relative volatility of petroleum prices, the cost to generate electricity, and hence the price to customers is relatively high when compared to other countries, including the rest of the United States. In 2011, 68 percent of Puerto Rico’s electricity came from fuel oil, 16 percent from natural gas, 15 percent from coal, and 1 percent from hydroelectric power. Price for electricity varies widely based on the fuel source, and the final price to customers is an amalgam of the electricity prices from the various sources. Since the cost of power generated from fuel oil (approximately $0.34/kWhr) is significantly higher than natural gas and or coal, there is a high motivation to maximum generation efficiency of the island’s fuel oil plants, such as the Palo Seco facility.
The 590 MW Palo Seco power plant is part of the public company of Puerto Rico Electric Power Authority (PREPA), and is one of the 5 main suppliers of electricity on the Puerto Rico island, population of 4 million people. Palo Seco plant is a fuel oil based plant which was originally built in 1970. With the growing electricity demand, the plant has been getting close to its capacity until the summer of 2010 when the cooling became a major bottleneck in the system.
Palo Seco switched from hypochlorite to a more effective disinfectant to control the biofilm, in this case the Mixed Oxidant Solution chemistry. The power plant increased its efficiency by about 40 MW within just a month of installing the new MIOX generators. Today, two years after the new solution was introduced, cooling loop water and the two towers are clean; the generators are operating at the specified temperatures and the heat exchangers at optimal conditions; the compressors operate smoothly; and only one pump per tower is required to keep water running through the system.
The Palo Seco facility had historically used a variety of chemicals in attempt to keep biofilm formation under control, but had struggled with finding a regime that would consistently keep the condenser clean. The facility suffered from a particularly challenging makeup water source, which is influenced by its proximity to the San Juan port and large industrial parks in the vicinity. A combination of poor surface water quality along with plentiful airborne contamination means that an ideal combination of initial organisms along with ample food source is created. In other words, once the organisms of concern are introduced to the cooling environment, they can continue to be fed via the introduction of a consumable food source from airborne particulates as well as through the makeup water. This results in a tower and cooling loop that is particularly difficult to maintain from the perspective of limiting algae and biofilm growth.
These issues reached a peak at Palo Seco plant in 2010. The heat exchangers were plugged and temperatures rose well above manufacturer specification due to serious biofouling in the plant’s cooling tower system. The biofilm at Palo Seco was described by one technician as “like mud growing in all the walls, all the pipes and all the valves. The water in the basin was slurred. We couldn’t see the bottom.” PREPA decided to seek an effective, less costly, greener and safer solution for cooling water and loop disinfection. This led them to change their cooling water disinfection from bulk hypochlorite to using the Mixed Oxidant Solution chemistry.
- The Technology
MIOX systems use a proprietary electrolytic process used for over 100 years, electrolyzing a brine solution and producing primarily two different chemistries: 1) sodium hypochlorite 2) Mixed Oxidant Solution. The Mixed Oxidant Solution is proven to be more effective compared to straight sodium hypochlorite especially in controlling biofilm due to the trace amounts of Hydrogen Peroxide resides in the solution along with hypochlorite up to 24 to 48 hours. Although it is not the only biofilm removing chemistry, the technology offered a cost effective, safe environmentally friendly solution for Palo Seco. The operational cost of the MIOX generator including power and salt was approximately $0.40 per 1 gallon equivalent of bulk hypochlorite at 12.5% concentration. The prior treatment of the purchased bulk hypochlorite was on average $1.25 per 1 gallon at 12.5% concentration.
Rather than requiring transportation of the disinfectant to the site, the MIOX equipment generates the disinfectant on site using only salt, water, and power. The concentration of the solution is <1% as Free Available Chlorine (FAC); a level well below any safety thresholds. Most biocides have U.S. National Fire Protection Association safety ratings of 2 or 3. Salt, the starting material for on-site generated MOS, has a safety rating of 0. Many of these biocides also have reportable quantities for spills. No handling of chlorine is required, personnel do not have to wear protective gear, and there are no storage compatibility issues. An environmentally “Green” feed-stock of sodium chloride is all that is needed. The potential to form chlorine gas by accidentally combining high pH bulk hypochlorite and acid is eliminated. Corrosion damage to the facility is also reduced providing savings on building repair.
Another major difference between Mixed Oxidant Solution and hypochlorite is that Mixed Oxidant Solution performs at the elevated pH values typically found in cooling towers. Often, pH adjustment is not needed. This is primarily due to the trace Hydrogen Peroxide produced in addition to hypochlorite during the proprietary electrolysis process. Mixed Oxidant Solution is also a direct replacement to bulk bleach, and performs well, even at higher pH and TDS.
- The Implementation at the Palo Seco Plant
During the summer of 2010, an improvement program was put in place to attempt to identify alternative technology to improve biofilm control. On-site generation of the Mixed Oxidant Solution chemistry was identified as a potential solution.
Once the course of action was determined, the technology was tested using a portable pilot unit right next to one of the cooling towers prior to installation of the full scale system (Chart 3). Initial results appeared promising, thus later in the year, a full scale system was installed. The full-scale equipment consisted of two generators and ancillaries installed as a unitized and containerized design. Each generator was designed to provide 15 lbs/day (ppd) as free available chlorine (FAC). Ancillaries included feed water softening, brine tank, oxidant day tank and chemical dosing pumps and controls. Chlorine concentration is controlled based on ORP with feedback control to the dosing pumps.
The system replaced the existing bulk bleach system entirely.
Chart 3 – Palo Seco Installation of on-site disinfection unit of MIOX (left picture demonstrates the pilot trailer by the cooling tower, right picture demonstrates the actual unit). Courtesy of PREPA.
Once the systems were treating the cooling towers of Palo Seco, results were seen almost immediately. The solution began to remove biofilm and reduce the temperatures in the condensers thereby increasing the energy load of the plant. The efficiencies gained at the Palo Seco plant alone resulted in overall savings of roughly $34 million a year according to PREPA. As mentioned above, the cost efficiency is doubly important, as 68 percent of PREPA’s power is generated from fuel oil, whose price is also very volatile. Palo Seco has four generating units – two rated for 85 MW and two rated for 210 MW. Due to biofouling, the generating units were operating close to their high temperature limit. As a result, the outputs of the generators were reduced to 80 MW and 195 MW respectively.
Shortly after the new treatment began, the temperature drop across the condenser was increased approximately 6 – 7○C. As a result, the system began operating within its design parameters, and the associated generating capacity lost due to biofouling was regained. The graph below depicts a typical plot of temperature and associated load for one of the generators at the Palo Seco plant.
Chart 4 – Palo Seco power production load output during the summer of 2010. The biofilm control (Mixed Oxidant Solution ) was implemented on May 21, 2012. Courtesy of PREPA.
Based on the above graph, it can be seen that the improved performance resulted in approximately 20 MW of additional capacity out of unit #3. Overall, the plant achieved 40 MW of additional capacity in just one month, representing an increase of 7 percent.
Visual inpection was performed after several months of operation and virtually all macrofouling of the tubes as well as the cooling tower surfaces was removed as can be seen on Chart 5. Additional observations were made with regard to bacteria counts in the system. Prior to the change in disinfectant, bacteria counts measured well in excess of one million colony forming units per milliliter (CFU/ml). Results after implementation were less than 100 CFU/ml.
Chart 5 – On the left hand side, prior to treatment visual inspection to tube sheets demonstrates the significant Biofouling. The picture on the left hand side was taken after 3 months of controlling the biofilm. Courtesy of PREPA.
Finally, in addition to the increase power available to sell, reductions in operation and maintenance costs were achieved due to savings from bulk-purchased hypochlorite as well as reduced manual cleaning of the condenser tubes. Prior to the switch in the bio-control regime, PREPA’s operation and maintenance costs were approximately $2.5 million and higher a year. After the system was running on all units, costs fell down to $1.1 million; nearly a 57 percent reduction.
Prevention of biofouling is shown to be critical in power generation output that could result in significant savings as demonstrated with the Palo Seco plant installation. MIOX's Mixed Oxidant Solution provided a good cost effective, safe and environmentally friendly alternate by effectively removing biofilm at a fraction of operational cost compared to bulk hypochlorite.
- Additional Information Sources
- T. Muilenberg, How Using On-site Generated Mixed-Oxidant Solution to Replace Delivered Chemicals for Cooling Tower Treatment Can Save Money and Improve Safety and Performance; Energy, Utility & Environment Conference; Phoenix, AZ; January 31, 2012
- T. Muilenberg, New Bio-Control Approach Boosts Power Plant Production by 7%; T. Muilenberg; Energy, Utility & Environment Conference; Phoenix, AZ; January 29, 2012.
- P.E. Schrock and T. Muilenberg; Mixed Oxidant Replaces “Cocktail” of Chemicals; Industrial Water World; May/June 2011
- P.E. Schrock and T. Muilenberg; A Tale of Two Towers; Power Engineering; February, 2011
- Venczel, L.V., M. Arrowood, M. Hurd, and M.D. Sobsey, 1997, “Inactivation of Cryptosporidium parvum Oocysts and Clostridium perfringens Spores by a Mixed-Oxidant Disinfectant and by Free Chlorine”, Appl. Environ. Microbiol., 63(4):1598-1601
- Donlan, R.M. 2002 “Biofilms: Microbial Life of Surfaces” Emerg. Infect. Dis. Volume 8, Number 9, Sept. Available from: URL: http://www.cdc.gov/ncidod/EID/vol8no9/02-0063.htm
- Wiatr, C. L., “Bacterial Resistance to Biocides in Recalculating Cooling Water Systems.” 2006 CTI Journal: 27, 1, 46-58
- Barton, L., 1996, “Disinfection of Simulated Cooling Tower Water,” University of New Mexico, Albuquerque, NM, March, 1996
- Crayton, C., et al. Montana Water Resources Center, Montana State University. Final Report on the Validation of Mixed Oxidants for the Disinfection and Removal of Biofilms from Distribution Systems. October 1997
- Petersen, P., and Bradford, W.L., “Mixed Oxidant Application in Cooling Tower Maintenance” January, 2000
- Michael Fehr, Ph.D., “An Alternative Cooling Tower Disinfectant: On-site Generated Mixed Oxidants” Industrial WaterWorld, September, 2006
- Michael Fehr, Ph.D., Technical Manager of Fehr Solutions, LLC. Personal communication, 2005
PAPERS AUTHORED AND PRESENTED
- Microfiltration - A New Barrier to Turbidity and Pathogens; T. Muilenberg; American Water Works Association (AWWA) Engineering and Construction Conference; Denver, CO; March 1996
- Microfiltration Basics: Theory and Practice; T. Muilenberg; AWWA Membrane Technology Conference; New Orleans, LA; February 1997
- Microfiltration for Backwash Waste Recovery; T. Muilenberg; AWWA National Convention; Atlanta, GA; June 1997
- Membrane Technology and Microfiltration; T. Muilenberg; Finger Lake Water Works Conference; Montour Falls, NY; June 1998
- Membrane Filter Techniques; T. Muilenberg; Minnesota Section AWWA Annual Conference; Duluth, MN; September, 1999
- Conference Digest - Membrane Technology Advancing; T. Muilenberg inverview; AWWA MainStream periodical; May 1996
- Microfiltration - How Does it Compare?; T. Muilenberg; Article in Water Quality Dealer, Vol. 1, Number 4; August, 1996
- Microfiltration - How Does it Compare?; T. Muilenberg; Water & Wastes Digest; December, 2000
- Water Management with Membrane Technologies: The Benefits of Microfiltration; T. Muilenberg; Pollution Engineering, Vol. 35 Issue 5; May 2003