Minnkota Power Cooperative in Grand Forks, ND provides all of northeastern North Dakota and northwestern Minnesota with electricity through two coal-fired units at the Milton R. Young Station near Center, ND. The Young Station began to have problems with wear and corrosion in the flyash piping for the scrubber of Unit 2. The plant tried stainless steel pipes which were able to withstand the abrasiveness of the flyash, but it was very expensive for the station to use and the flyash would build up in the pipes. Fiberglass pipes were tried, but required continuous maintenance and repair. Minnkota then turned to Abresist Corporation, a Kalenborn Company in Urbana, IN, for wear-resistant, lined piping for an effective and cost-efficient answer to their wear problems.
The Milton R. Young Station
Milton R. Young Station, operated by Minnkota, has two units, which produce a combined total of about 705,000 kilowatts of electricity. The Young 1 unit began producing electricity in 1970 and is also owned by Minnkota. Generating 250,000 kilowatts of electricity, Young 1 serves over 110,000 customers. Young 2 has been producing electricity since 1977 with a generating capacity of 455,000 kilowatts. Combined, the units consume around 4.3 million tons of coal annually.
Consistently ranked as one of the lowest cost, coal-fired power plants in the U.S., Young Station receives lignite coal from a nearby mine and is burned in cyclone-fired boilers.
Flyash causes wear and corrosion in piping
When lignite coal is burned it produces two types of ash, flyash and bottom ash. The flyash is light and travels through the boiler and is captured by an electrostatic precipitator. The flyash is then used in the scrubber with a mixture of lime to scrub the sulfur dioxides from the exhaust gases. The bottom ash, however, is heavy and drops out of the bottom of the boiler.
Wear and corrosion from the flyash in the Young 2 scrubber became a problem and the search began for wear-resistant piping. “We have a very abrasive situation,” says Dennis Ziniel, Mechanical Maintenance Supervisor for Minnkota Power. “The flyash is very abrasive and very corrosive.”
Young Station operates two scrubber towers to remove the sulfur dioxide from the exhaust gases. Nozzle heads within the towers spray a mixture of limestone and flyash through the gas stream. The calcium in the limestone and flyash spray is able to scrub the sulfur dioxide out of the gas before it is released into the atmosphere. The resulting slurry of limestone, flyash and sulfur dioxide is then sent to settling ponds and eventually water will be pulled from the top of the ponds and be reused in the scrubbers.
Before trying Abresist piping for transporting flyash, Young Station also tried fiberglass and stainless steel pipes, but those pipes provided new problems. “Lined fiberglass stands up fairly well for a limited time in some applications,” Ziniel explains. “We’ve got some fiberglass here that’s been in service about ten years, but we’re starting to see some real issues with it.”
“We also have other applications where we’ve gone with 3/16 stainless steel because it is so abrasive and so corrosive,” Ziniel continues. “But stainless steel is very, very expensive. So this piping from Abresist Corporation is kind of a blessing.”
Young 2 needed wear-resistant piping that was cost-efficient and low maintenance. Minnkota Power began searching for solutions and looked to Abresist Corporation. Approximately 400 feet of 2”, 3” and 4” Abresist lined piping, including about 35 elbows and tees, was purchased by Young Station in 1988 for transporting flyash to the Young 2 scrubbers. Abresist cast basalt lining is made from hard volcanic rock, offering high resistance against sliding abrasion.
Abresist lined pipes versus stainless steel and fiberglass
The lined pipe from Abresist Corporation requires less maintenance and offers
cost savings compared to the fiberglass and stainless steel pipes. Fiberglass pipes can cost up to half as much as an Abresist lined pipe while stainless steel pipes cost approximately twice as much as the lined pipe being used in their facility.
Young Station also faced issues with build-up in their stainless steel pipes. When it got colder outside, there would be a reaction in the slurry and the flyash would stick to the inside of the pipes. To remove the build-up, a cleaning contractor is brought in with high pressure washers. “That is a very lengthy process. And very costly, too,” Ziniel admits. “Depending on how badly the pipes are built up depends on how long the contractors are here. We’ve had cleaning contractors out here for up to two weeks at $600 an hour. But, with the Abresist basalt lined pipes we haven’t had any build-up, so Abresist lined pipes kind of give you the best of both worlds.”
Though fiberglass pipes offer the benefit of cost savings, Abresist basalt-lined pipes provide a longer service life since fiberglass pipes require more maintenance and repair, especially within the joints. The joints can erode quickly if not matched up correctly. “We have several spots like that where we are continually repairing it because they were not perfectly matched up when they were put together,” Ziniel notes. Since installing the Abresist lined pipes, the pipes have not needed to be repaired or relined.
“As a maintenance supervisor I have a lot of problem areas I have to worry about,” Ziniel states. “But one of the areas I don’t have to worry about is the Abresist lined pipe because I know it’s there and it’s working and I know I’m not going to have an issue with it.”
The future of flyash at Young Station
Young Station will soon be changing some of their processes in accordance with an agreement made with the Environmental Protection Agency (EPA). The plant will be modifying its scrubbers to scrub 98-99% of the exhaust gas. Currently, 92-94% of the gas is being scrubbed. The plant will also be sending limestone to the scrubber as a scrubbing medium rather than the flyash. Limestone does a better job of scrubbing than the flyash. The plant plans to use as much of the existing system as they can for the scrubber modifications. The Abresist lined piping system may also be used for transporting limestone.