|If slag is allowed to build up inside a boiler, it can lead not only to efficiency problems but can also cause damage because of the weight. Photo courtesy of Norm Harty.|
By Justin Martino, Associate Editor
Nearly 50 years ago, Norm Harty, president of N.B. Harty General Contractors Inc., was a driller and dynamite blaster for road and railway construction when Kenneth Bridegroom, a supervisor at a power plant where Harty was subcontracting, asked if dynamite could be used to clean the inside of a boiler.
|Norm Harty has been using explosives to clean boilers for nearly 50 years, which helps increase boiler efficiency and heat transfer. Photo courtesy of Norm Harty.|
“Back then I was hungry,” Harty said. “I was really, really searching for anything.”
Although Harty had never seen the inside of a power plant before, he told Bridegroom he could do the job. After a labor strike and a buildup of slag prompted the plant to contact Harty, he used dynamite to remove a slag deposit, inventing the process as he went along.
“I didn’t really realize exactly what I had done, but Bridegroom did,” he said. “The rest is history. The next thing I knew I was going all over the east doing this, and it was all by word of mouth.”
Harty used his technique for 17 years before he had any competitors attempting to use the same process. Using dynamite is still an option for power plants, and Harty’s company cleans nearly 100 boilers a year.
Other operators are looking at different methods that have been developed to clean boiler tubes without taking the plant offline, including improving the soot blowers located in the boiler. Whichever methods is chosen, however, the importance of keeping a clean boiler is not in question.
The importance of boiler tube cleaning
Power plants burning anything other than natural gas will, over time, build up ash that forms slag deposits on the exterior of the tubes running through the boiler. The slag can be a safety hazard – the tubes expand and contract as they heat up and cool down and the deposits may break off and fall, a problem that arises most notably when a plant is taken offline for maintenance, Harty said.
Dirty boiler tubes can also affect the operation of a plant. Slag buildup on tubes will act as an insulation protecting the tube from the heat of the boiler, requiring more fuel to reach the same temperature and produce the same output as a clean boiler. Harty said cleaning the slag deposits inside a boiler can increase boiler efficiency from 1 percent to 4 percent.
“Even 2 to 3 percent in a boiler that generates 600 MW, that’s hundreds of thousands of dollars each day or week,” he said. “They’re still burning the coal, but the coal fire doesn’t get to the tube.”
Clean boilers can also help reduce the emissions produced by the plant because less fuel is needed to produce the same amount of power, according to Tim Martin, Director of Product Management for Clyde Bergemann Power Group Americas Inc., Boiler Efficiency Division.
Fuel switches leading to slag deposits
Slag deposits can be caused by multiple sources. Harty said older boilers, especially in the eastern U.S., were much smaller because the plants were burning high-sulfur eastern coal with a high Btu output. Less fuel was needed to produce the required heat, and the boilers did not produce as much ash.
|Although soot cleaning systems may be used to clean a boiler without taking it offline, a plant with older online systems may require a shutdown. Photo courtesy of Norm Harty.|
The high-sulfur coal released more pollutants into the air, so plants began using western coal, such as Powder River Basin coal, in their fuel mix. Western coals create more ash and produces less Btus, requiring more fuel for the same temperature. Plants that burn lignite, often located along the Missouri River, require even more fuel. Those boilers might be twice as large as a Midwest plant built to burn western coal and three or four times as large as plants built to burn eastern coal, many of which are being decommissioned right now, Harty said.
Fuel switches can create a problem in a plant. If more ash is produced than the plant’s current system can handle, the slag buildup can become uncontrollable over time. This can happen when units are burning Powder River Basin coals or other coals that have low melting points for the ash or significant sodium levels, which makes the ash more tenacious in areas such as the convection pass, said Joel Booher, a business manager for Diamond Power.
“Unless the furnace box is very large, what you’ll see is the boiler eventually can’t take it anymore, and you’ll have uncontrollable slag formation,” Booher said. “That was what we were fighting against.”
Changes in the way coal plants are used
As power plants operators use coal plants differently, the methods of cleaning the boilers have changed. Although coal plants previous ran at full load as much as possible, the drop in price of natural gas has caused many owners to cycle the plants more so that they’re not always running at full load.
“It’s been a huge change in the way the coal plants are operated,” Booher said. “Not only is natural gas at record lows, which causes coal plants to be dispatched lower, but there’s also extensive renewables online now. So in a place like Texas, where many large coal plants are designed to run full load, these are now up and down on load.”
Harty said that bringing the boiler down and back up can create some efficiency for the plant by partially removing the slag through cycling. Boiler tubes will expand up to 18 inches from top to bottom when hot, and the expansion and contraction of the tubecan knock off slag.
Jeff Kite, principal engineer for boiler performance at Diamond Power, said boilers in plants that are being cycled more often don’t have to be cleaned as often, but the boilers were not designed to be cycled so it makes it difficult on operations in general.
“Supercritical units do not easily accommodate being asked to go from full load to half load on a regular basis,” Booher added. “They’re designed to be run at stable high loads, and it’s hard on the metal throughout the boiler to be cycled up and down.”
Current cleaning systems
Most power plants have some sort of soot blower system that works to clean the plant when the boiler is online and producing power, Martin said. Soot blower systems have been around since at least the 1930s, he said, adding thaThe has seen photographs of soot blowers that were operated by a chain raised and lowered by a hand-operated crank.
Soot blowers and other online cleaning systems use compressed air, steam or water to keep slag buildup from occurring without the necessity of taking the plant offline. As companies look for solutions that allow the plant to keep operating, newer soot blowing systems may allow a plant to keep its boilers clean without requiring a planned maintenance where the boiler is taken offline, although older systems may require a shutdown for cleaning.
“Some of the online cleaning products might be 20 years old,” Martin said. “Once slag becomes a problem, their cleaning devices aren’t strong enough or in the proper location to remove it.”
If buildup reaches a point the online cleaning products installed by the plant are unable to remove the slag, the plant must be taken offline for cleaning. Offline cleaning may involve water jets, compressed air or dynamite.
Using dynamite to clean boiler tubes
Harty said that older methods being used for offline cleaning when he first started cleaning boilers were “very crude” and could involve using shotguns to fire slugs at tubes to knock off slag or using a large iron ball suspended on a chain or chisels and hammers to strike the tubes to remove any build up.
“It was one massive thing to do, and it was very dangerous,” he said. “They had a lot of people get hurt.”
Modern methods of offline boiler tube cleaning have made many improvements over older methods, and Harty said using dynamite as a method of offline cleaning has multiple advantages over other methods, such as high-pressure water, with one of those being that no moisture is introduced into the boiler.
“The water and moisture will go down into the ash tanks and set like concrete because that slag is in such a fine state,” he said. “They buy that right now to put in concrete and strengthen the concrete. If you dry clean it with explosives, you don’t have any moisture and the slag will fall into the hopper and down into the grinder and out the sluice area.”
Harty also said he can reduce downtime for plants that have to be taken offline for cleaning. Whereas using high pressure water requires setting up pipes and hoses, dynamite can be used quickly and with less equipment. He added that his company recently cleaned a boiler in Nevada in less than 36 hours.
“Downtime is money,” he said. “Downtime is loss of revenue. That’s why they want you in and want you out.”
Using dynamite to clean slag is popular in western coal plants because of the need to conserve water, he said.
“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.”
In order to clean a tube using explosives, Harty said his company will use primer cord around tubs that are close together to avoid damage. The cord has connectors that will delay the charges. Without using the connectors, he said the process could destroy the wall or insulation of the boiler.
Sticks of dynamite can be used in more open places where there is a heavy amount of slag. Harty said he has seen slag reach 60 to 70 feet deep in some boilers. At one point, his company cleaned 150 truckloads of slag from a boiler, though he called that “an extreme case.”
Improvements in online boiler cleaning
Many companies would prefer to not take their plants offline for cleaning, and both Diamond Power and Clyde Bergemann have developed online cleaning solutions that are improvements over the previous soot blowers.
Previous soot blowers had “simplistic controls,” Kite said. Although the operator of the system was able to make a sequence and run the devices in order, operators were unable to get any sort of feedback without a visual inspection of the boiler.
“Realistically, it wasn’t until the ‘90s that we started implementing heat transfer sensors, which are devices made out of sections of boiler tubing placed within the cleaning radius of the water lances that provide feedback of heat transfer and can be used to determine how clean or dirty that section is,” Kite said.
Older soot blower systems might also utilize an “across-the-boiler” system, where a nozzle will need to spray water across a 60-foot space in order to reach the place that needed to be cleaned. That system might use 200 gallons of water per minute, which, after hitting the hot boiler tube, could create more steam than the boiler could handle and even cause a change in the power produced by the boiler.
“Many units have induced draft fans that really run on the ragged edge, and they can’t handle that extra gas volume going through there,” Booher said. “Oftentimes these high-flow cleaning devices can cause disturbances of the unit.”
Modern systems can use retractable soot blowers, such as Diamond’s Power HydroJet™ water cleaning systems, allowing less water to be used since less force can be used over the shorter distance. Systems using retractable soot blowers can create a 60 or 70 percent water use reduction and provide as much or better cleaning effectiveness, Kite said.
The systems can also interpret where the boiler is developing a buildup of slag, which can help operators avoid attempting to clean sections of tube that are already clean. If water or steam hits a clean section of a boiler tube, it can cause tube erosion over time. Cold water hitting a hot tube can also cause the tube to contract, leading to tube damage and a possible blow out.
“Water cleaning can damage tubes if done improperly,” Kite said. “If an operator has this wonderful tool that can clean his boiler, he has a habit of using it too much if he doesn’t have input coming back to him letting him know what’s going on.”
Modern systems use a variety of solutions to provide “intelligent soot blowing,” which can help prevent outages and damage to the boiler.
Goal-based intelligent soot blowing
Intelligent soot blowing, or ISB, uses automatic control systems to analyze the cleaning needs of the boiler. Introduced in the ‘90s, it is becoming the standard for boilers, Booher said, with many new boilers using an ISB system. Improvements can still be made on previous generation of intelligent soot blowing systems, however.
“What is coming to light is that these systems can be complicated in their calculations, and so they can be difficult for the operations staff to understand in order to make the right decisions,” he said. “Goal-based ISB uses intelligent soot blowing to achieve the goals the operators already want to achieve – keeping steam temperature in the right range or keeping their gas temperatures in the right range. It’s simpler and quicker to commission.”
Although gas temperature may be a common goal for many boiler operators, Booher said different operators may have a different goal that is important to them. Diamond Power will work with the plant engineering staff to configure the ISB system, and if a goal can be quantified, it can be incorporated into the system.
Once the system is in place, it will analyze data from individual cleaning devices installed in the system and that device’s ability to impact the goals set by the plant operator. Ultimately, the system will allow an operator to clean tubes only when and where they need cleaning.
“The idea would be to use it as little as possible because it can cause damage to the unit,” Booher said. “If the system can automatically determine where to clean and which blowers make the biggest effect, then in the end you can end up blowing less soot blowers to maintain the same general boiler cleanliness.”
Variable pressure blowers
Clyde Bergemann has also introduced a variation in intelligent soot blowing that uses variable pressure when cleaning boiler tubes. Martin said the goal of the system is to provide just enough pressure to clean the tube to avoid creating any damage.
“We could go in with a very, very high pressure, and we know that whatever is on the tube is going to be cleaned, but there’s a downside to that, and that’s tube erosion,” he said. “What the SMART Clean™ does is look at cleaning with the proper intensity. We clean with just enough intensity to clean the ash off the tube, but not too much intensity to cause tube erosion.”
The system is designed to create maximum efficiency for the plant. By keeping slag from building up on the tubes, the plant is able to operate with maximum heat transfer, and by avoiding erosion, the operator can prevent future tube leaks that can shut down the boiler in a forced outage.
Martin said the system can save money for a plant operator by completely eliminating planned outages to clean the tubes. He said the company has clients with boilers that were going offline every three months who eliminated the problem by installing the SMART Clean™ system. Those boilers only come down for typical maintenance issues now, he said.
He added the system is not a large investment for a boiler operator, and the investment will be returned within six months to a year, with the owner of the boiler receiving the benefits of not requiring outages for the next 20 years.
Each job requires a different approach, and not all of the SMART Clean™ products may not be necessary.
“We use a lot of different techniques,” Martin said. “Some are more advanced than others. Really, every boiler is unique. We look at it from a fresh perspective and look at their specific needs, and then we can propose the right technology to meet those needs.”
Interior tube cleaning
Although cleaning slag from the inside of the boiler and exterior of the boiler tubes is a key aspect of keeping a boiler efficient, cleaning any deposits that may form on the interior of the tubes is also important.
If deposits from impurities in the water form on the interior of a tube, it can create an insulation problem, according to George Bodman of George H. Bodman Inc.
“if you put a tenth of an inch of scale in there, your temperature will go from 600 degrees farenheit to probably 700 to 750 degrees farenheit,” he said. “If that tube gets to 809 degrees farenheit, the tube is going to blow out.”
Scaling can also reduce the efficiency of the boiler by requiring more heat, and corrosion can form under the scale that will create a hole in the tube itself.
The process of removing the scale is individualized for each tube, Bodman said, and the preplanning for the cleaning should start at least six months in advance by getting a tube sample of the boiler. The sample can be used to determine the deposit weight density as well as the scale matrix.
Bodman said he also talks to the water treatment representatives to find out what has previously been done and why the deposit has formed. The next step after that is to speak with plant personnel and the plant chemist or relibility engineer to set up a program on how to remove the deposit.
Around 90 percent of boilers are are currently chemically cleaned, Bodman said, although some can be cleaned with high-pressure water.
Plant operators can choose to use a variety of chemicals, including hydrochloric acid, ethalene diamene tetracedic acid (EDTA) and hydroxyaceticformic acid.
EDTA is more expensive, but also has less environmental impact, Bodman said. It requires 13.6 pounds of EDTA to remove one pound of iron oxide. The same amount of iron oxide can be removed by 2.5 pounds of citric acid or 1.58 pounds of hydrochloric acid.
“When you look at costs, I can clean a 30,000 gallon boiler for around $50,000 to $60,000,” he said. “Cleaning that same boiler with EDTA will cost you over $100,000.”
Bodman said his company will run solubility tests with hydrocholic acid, citric acid and EDTA and allow the company to choose the method used.
Once the cleaning starts, he said it takes approximately 2 to 3.5 days. Depending on the boiler and water treatment, some boilers require cleaning every two years, while other boilers, like ones used in paper mills, may be cleaned every five to seven years.
A variety of options
Although boiler tube cleaning is a very important part of the industry, choosing the right method for a boiler is up to the operator. Modern advances in the industry, however, can make the process simpler and safer than it used to be, however.
“Soot blowing has really changed over the years, going from a product that you just install and turn on into really getting more into the engineering aspect and studying how it affects the boiler performance,” Martin said.
Whether an operator chooses to install a new online system or use an offline system, each boiler may have different problems and require a unique solution to reach maximum efficiency. For Harty, who went from blasting passes for roadways to using dynamite to clean multi-million dollar pieces equipment almost 50 years ago and has seen cleaning systems continue to evolve during that time, finding solutions to unique problems is a familiar concept.
“In a power plant, you have to be flexible and innovative,” he said.
Power Engineerng Issue Archives
View Power Generation Articles on PennEnergy.com