Scrubbers' Aggressive Corrosion Can Be Fixed--At a Cost

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Written by John Egan for Industrial Info Resources (Sugar Land, Texas)--Aggressive corrosion has appeared in dozens of power plants' wet flue-gas desulfurization (FGD) units, in some cases as shortly as three months after construction of the scrubber was complete. A variety of utilities, including units of American Electric Power Company (NYSE:AEP) (Columbus, Ohio), Duke Energy Corporation (NYSE:DUK) (Charlotte, North Carolina) and FirstEnergy Corporation (NYSE:FE) (Akron, Ohio), are working in collaboration with the Electric Power Research Institute (EPRI) (Palo Alto, California) to identify the root cause of the problem and determine a remedy. There has been some litigation between affected power generators and engineering and construction (E&C) companies.

"There has been no leakage or catastrophic failure of the scrubber structure," AEP spokeswoman Melissa McHenry told Industrial Info in an interview. "During our regular inspection and maintenance of scrubbers in 2009, we discovered corrosion and pitting, and the steps we've taken seem to be fixing the problem. Mitigation has involved a trowel- and spray-applied coating system on the tank walls and floor with an acid-resistant Stebbins tile overlay on the floor and up the walls of the tank. We did not have to take the scrubbers down for unscheduled repair. The remediation costs--$30 million--are manageable, and we are confident the fix addresses the problem."

With the remediation, McHenry said AEP is confident that the scrubbers "will continue to have their full design life of 20 to 25 years."

Although EPRI and some utilities are being closed-mouthed about the issue now, the problem apparently has been traced to particular types of steel alloy, duplexes 2205 and 255, which were widely used in scrubber construction over the last decade. "This material was considered state-of-the-art when we built our scrubbers," McHenry said. "But I would be surprised if 2205 was still being used today, given what we know."

"This is a complex industry issue, and the EPRI task force is looking at materials issues, chemistry issues, and a variety of other possible factors," Rita Sipe, a spokeswoman for Duke, said in an interview. "It looks like a variety of factors are in play. People are focused on fixing the problem. We have spent over $5 million over the last few years across our fleet to fix the problems."

At least one case of litigation between a large utility and a large E&C firm has been settled confidentially, sources told Industrial Info.

Sipe said scrubbers at Duke's Cayuga Power Station in Indiana and Allen Power Station in North Carolina have experienced aggressive corrosion. McHenry said that a number of plants that AEP owns or operates for others have been affected, including:

• Mountaineer Power Station, West Virginia
• John Amos Power Station, West Virginia
• Mitchell Power Station, West Virginia
• Conesville Power Station, Ohio
• Cardinal Power Station, Ohio
• Kyger Creek Power Station, Ohio
• Clifty Creek Power Station, Indiana

In addition, news reports have identified several other power plants whose scrubbers have been hit with aggressive corrosion, including:

• AEP and Duke's Miami Fort Power Station in Ohio
• FirstEnergy's W.H. Sammis Power Station in Ohio, and
• GenOn Energy's (NYSE:GEN) Cheswick Power Station in Pennsylvania

EPRI declined to comment for this article. But in an article in the Spring 2011 issue of EPRI Journal, the organization observed: "Corrosion has been serious in some plants and in extreme cases has penetrated vessel walls. The corrosion has been found in many designs produced by several manufacturers. To date, the cause is unknown, but in preliminary investigations, the common factor appears to be absorber vessels and installations constructed since about 2004 using duplex stainless steel alloy 2205 and possibly other duplex stainless steels. Initial EPRI surveys show that at least 20% of the approximately 360-370 FGD systems in the United Sates have this material in major components."

"A structural compromise in significant systems such as FGD units is a serious concern for plant operators," said Chuck Dene, an EPRI project manager quoted in the EPRI Journal article. "Corrosion that violates the minimum wall thickness can jeopardize the structural integrity of the tank. If a tank were to rupture, it could have catastrophic effects on surrounding equipment and shut down a plant. In addition, emissions regulations require that a unit not run without the FGD unit in place."

One of Dene's colleagues, John Shingledecker, an EPRI senior project manager, told the journal, "Once we saw the seriousness of the attacks and their prevalence throughout the industry, we knew we had to act quickly." The article said aggressive corrosion has not been detected at dry FGD systems.

Until about a decade ago, scrubbers used a different type of steel, Type 317L stainless steel or a variation, such as Type 317 LMN. But around 2000, a "fundamental shift occurred in the way scrubbers were designed and operated." Utilities, seeking higher rates of sulfur dioxide (SO2) removal, began using different chloride concentrations during scrubber operations. The price of nickel-based alloys shot upward, quadrupling or worse. So manufacturers sought other alloys, such as duplex stainless steels, the ERPI Journal article noted.

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