|Cleaning built-up slag deposits from a fossil fuel boiler can increase efficiency between 1 percent and 4 percent, helping maximize revenues for a utility. Photo courtesy of Clyde Bergemann.|
By Justin Martino, J.D., Associate Editor
For power plants burning anything other than natural gas, boiler cleaning is an important part of keeping the plant working efficiently.
Over time, boilers will build up ash that will form slag deposits on the boiler. Slag acts as an insulation that protects the tube from the heat of the boiler, creating a need to use more fuel to reach the same temperature and produce the same output as a clean boiler. Cleaning slag deposits inside a boiler can increase boiler efficiency between 1 percent and 4 percent. Clean boilers can also reduce emissions produced by a power plant because less fuel is required to produce the same amount of power.
While the importance of boiler cleaning is clear, power plant operators have many options when it comes to techniques used to clear slag and ash buildup. Cleaning practices vary from offline cleaning, which requires a planned outage, to online cleaning that can occur while the plant is still operating. Methods can use everything from sound waves to dynamite.
One option for boiler cleaning is an acoustic cleaning system, which can knock ash off boiler tubes or selective catalytic reduction systems with acoustic energy without risking damage or fatigue to the units. While this can be done by using acoustic horns that can be retrofitted onto boilers, Advanced Acoustic Technologies LLC uses a technique that is engineered specifically for a plant.
“Unlike the horn suppliers, our acoustic devices are part of an engineered, integrated system where the frequency produced by the acoustic device is determined by 3D element modeling,” AAT co-founder Robert van Dam said. The process is similar to flow modeling, except the company looks at the acoustic aspects of how sound waves behave in an enclosure.
The company’s WaveMaster Acoustic Cleaning systems provide continuous, online, volume cleaning with soot blowers used in a remedial role, van Dam said. The company uses acoustic modeling to determine the natural frequency of the flue gas and decide the proper frequency for the custom system as well as where it needs to be placed in the boiler.
“Our acoustic cleaners are designed to be operated continuously if that is required for the application,” van Dam said. “There’s no erosion. It’s online, so it’s running when the boiler is running, and by being specific with the frequency and the location we can place the acoustic effective area where we want it. We’re not just cleaning what is directly in front of our device. We’re cleaning a volume because the acoustic cleaner is in resonance with flue gas.”
Acoustic cleaning works especially well on dry and dusty deposits, van Dam said. The company works with areas where ash is below the fusion point, such as horizontal tube banks in a utility boiler. The technology is not as effective in superheaters or other other areas where the ash is likely partially molten.
According to van Dam, the company has seen excellent results on a wide variety of fuels, including Powder River Basin, pet-coke, bituminous and sub-bituminous coals. The fully-optimized sound field can penetrate throughout several tube banks with the application of only one or two acoustic cleaners. Van Dam said the system is used to clean the entire convection pass at Tennessee Valley Authority’s 1,300-MW Cumberland plant, which is 67 rows of tubes, with two acoustic devices.
|Slag deposits on boiler tubs will act as insulation, preventing the heat produced by the boiler from transferring to the water inside the tubes. Photo courtesy of Clyde Bergemann|
Another method for cleaning slag from a boiler is using high-pressure water jets. Thompson Industrial Services uses high-volume, specialized hydroblasting equipment, with pumps that can send up to1,200 gallons per minute through the hoses. The company also uses remote-controlled robotic cleaning systems and other automated tools to clean boilers.
Thompson Senior Business Development Manager Carl Wise said the company is able to use water lances for both online and offline cleaning, although whether online cleaning is possible may depend on the specific boiler.
“You have to do online cleaning from strategic positions because you have to be very careful spraying the water directly into the boiler while it’s operating,” he said. “It’s an extremely critical process.”
Whether cleaning can be done while the boiler is online depends on the particular boiler. Although the company routinely cleans boilers without having to shut them down, Wise said it may not be possible in every situation.
Online cleaning has multiple advantages when it is possible, however.
“If you take a boiler completely down, it takes a lot of time to bring it back up,” Thompson Senior Sales Representative Jim Walker said. “It saves the utility time and money to have that boiler up and running when we do our deslagging.”
Because of the high-volume pumps, Wise said the company is able to clean farther distances with their lances and can be more effective because the lance can go out more than 40 feet on either side of the lance.
Thompson also performs many other types of cleaning for power plants, including hot ash removal and using a process involving dry foam to remove combustible dust, which can create a safety hazard on the external part of the boiler.
|Areas that collect dry coal ash, such as convection passes, are suited for acoustic cleaning. Photo courtesy of Advanced Accoustic Technologies|
Soot blowers use compressed air, steam or water to keep slag buildup from occurring without the necessity of taking the plant offline. Soot blowers have been in use for some time now, but, like many other aspects of power plants, are becoming more sophisticated as time goes on.
“We have a basic technology, and we have our advanced technology, which we call SmartClean,” said Tim Martin, director of product management for the boiler efficiency product division with Clyde Bergemann Power Group Americas Inc.. “Our basic technology does the bare minimum — it’ll keep the boiler clean and keep it online and running. Our advanced technology is where we get into targeting the areas of boilers that really need to be cleaned with the proper intensity, so we really get into monitoring the boiler performance and adjusting the cleaning parameters in real time, and that’s where the plant can gain efficiencies from using the advanced technology.”
Intelligent soot blowing systems have several advantages over traditional systems, Martin said. The company’s advanced technology can increase the efficiency by 1 percent over basic systems. Intelligent soot blowing systems also only clean when necessary, which avoids boiler tube erosion.
“Basic systems are blind to what’s going on inside the boiler,” Martin said. “There may be areas in your boilers that are clean, but you’re running a soot blower because it’s time in the sequence, so it’s blowing high pressure steam on a bare tube, which can lead to erosion. In addition, you may have areas of your boiler that foul much more quickly and because of the way the sequence is set up, it may not be able to hit that area for several hours. In those hours, that area could really foul up significantly and plug the boiler and cause a clinker or severely restrict the heat transfer, forcing them to have a shutdown.”
Many coal-fired boilers built in the past 10 years were installed with intelligent soot blower technology, Martin said. Clyde Bergemann is also working on retrofits for power plants that weren’t originally equipped with intelligent soot blower technology.
Using explosives to clean slag from boilers isn’t a new process, but it’s one still in use that many plant operators prefer.
The method was first used by Norm Harty of N.B. Harty General Contractor Inc. Over the years, Harty said he and his staff have built the procedure into a state-of-the art technique that can quickly clean the slag from a boiler and have it back online.
To clean a boiler using explosives, Harty said his company will use primer cord around tubes that are close to avoid damage. The cord has connectors to delay the chargers, which he said is important to avoid destroying the wall or insulation of the boiler.
Harty said using explosives has several advantages, including speed and convenience. “With dynamite, you can put all of it in a pickup truck and clean any boiler,” he said.
Explosives also have an advantage in areas where plants are concerned about water use, he added.
“Water is really scarce out west, and this is another reason explosives are being used predominately in the western plants,” he said. “They can’t afford to waste a drop of water, and by using dynamite they’re able to save their water and clean their boiler at the same time.”
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