Conference Report: Learning From Experience
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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INCREASING THE SERVICE LIFE of pollution abatement equipment depends on the level of
care taken to select alloys and to ensure quality fabrication. |
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THIS PARELLEL WET SCRUBBER and wet electrostatic precipitator is at an industrial facility in the
U.S. where S31603 was used extensively. |
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MIRANT'S DICKERSON STATION in Maryland is a 853-MW coal-fired electric station. |
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For more information from the Nickel Institute on the use of nickel-containing
materials in the chemical process industry, |
Building pollution abatement equipment can now benefit from 30 years of operating experience with nickel alloys. By Patrick Whiteway
Nickel Magazine, November 2004 -- Coal is by far the most commonly used fuel to generate the world’s electric power. But coal is not the cleanest fuel in the world. It is a major source of airborne pollutants such as particulate matter, sulphur dioxide and oxides of nitrogen. The cost of reducing these pollutants is high. Which is why users are looking hard for more cost-effective ways to clean up the gases generated by coal-burning plants.
The operators who build and run coal-fired electric generating stations are looking for ways to build pollution abatement equipment that has a longer service life. Alloy selection and quality of fabrication are seen as ways to increase reliability and to reduce maintenance costs.
The percentage of chromium, nickel and molybdenum in the alloys used to make this equipment determines the material’s resistance to corrosion. And many operators now have up to 30 years of experience working with different nickel alloys and stainless steels.
That’s why, in August, NACE International arranged for William Mathay, a Nickel Institute consultant and Ronald Richards of Enerfab to chair a meeting of engineers in Washington D.C. During the one-day symposium some valuable operating lessons were heard.
For example, in 1986, duplex stainless steel S32550 was used to line the absorber tower at Gibson Unit #5 in Owenville, Indiana. During inspections in 2004, the material was found to exhibit minimal signs of chemical attack or erosion damage, reported Steven Alston and Ronald Richards.
"Even at the inlet wet/dry interface, the duplex alloy is holding up with only generalized shallow surface pitting," the authors said.
Gibson is one of the largest electric-generating stations in the U.S. It is operated by PSI Energy Inc., a wholly-owned subsidiary of Cincinnati-based Cinergy Corp.
In addition, nickel alloy N06022 was used in 1986 as a wall-paper lining in the outlet ductwork in Unit #5. "To date, this lining has eighteen years of service and looks essentially the way it did the day it was installed," the authors say.
In another example, experience with corrosion in a pilot wet electrostatic precipitator was presented by CR Clean Air Technologies and Enerfab Inc.
The question that perplexed these authors is why corrosion appeared in one part of the unit and not in another part when both areas were constructed of the same material: S31603.
What they discovered after a corrosion investigation is that strong oxidizing agents, such as ozone generated within the precipitator, acted to passivate the collecting tubes which exhibited no corrosion. This suggests that a slightly lower grade material can be used in this section of the plant; a potentially valuable observation for the designers of future such units.
One company that should benefit from its more than 30 years of experience in pollution abatement equipment is Oregon-based PacifiCorp. In August 2004 it announced it will spend US$120 million to, among other things, install a new wet-lime scrubber at its Huntington 2 plant near Salt Lake City, Utah. The investment will reduce sulphur dioxide emissions from the plant by about 15,000 tonnes per year.
The company has operated flue gas desulphurization (FGD) systems in more than 20 coal-fired plants in the
U.S. for more than 30 years. So it has lots of experience with various FGD materials of construction that can
be applied at Huntington. A summary of that experience was presented in Washington by G. W. Betenson.
Nickel alloy N10276 was recently installed at the company’s Hunter 2 unit using pulsed arc welding, for example. The same material was used in the unit in 1986, but suffered a "massive failure" because it had been welded using short circuit gas metal arc welding procedures.
"Corrosion of the liner attachment welds to the carbon steel substrate coupled with vibration of the duct mechanically fatigued the remaining attachment welds," the authors say, "resulting in a massive failure of the liner system. Improvements were made to the design of the dissimilar metals welds near the duct expansion joints and the outlet duct."
Other operating experiences related to relatively simple things such as good housekeeping. High concentrations of fluoride in FGD systems can cause corrosion challenges even for nickel alloy N10276, for example. However, corrosion can be greatly reduced by installing a water wash system that allows duct floors to be washed regularly, report Michael Hoydick, project manager for Wheelabrator Air Pollution Control and Kevin Frizzell, technical services superintendent of Owensboro Municipal Utilities (OMU).
This lesson was learned after 10 years of operating experience at OMU’s Elmer Smith 441-MW coal-fired electric generating station in Owensboro, Kentucky, U.S.A.
High fluoride concentrations were blamed for what is called "under-deposit" corrosion of the nickel alloy. The alloy was used in the inlet ductwork of the limestone-based wet FGD system. That system was installed in 1994.
If proper heed is given to these experiences, new pollution abatement equipment that will be required due to tightening regulations should be long-lasting and reliable.
Patrick Whiteway is the Editor of Nickel Magazine.
PHOTOS: PHOTOS.COM
 The complete proceedings of this symposium, which includes papers on high alloyed stainless steels, nickel alloys, clad, non-metallic materials and electrochemical protection, are available on CD from NACE International. |
