CORROSIVE WET FGD SYSTEMS CAUSED BY COMBINATION OF FACTORS, EPRI SAYS
A combination of materials, operating conditions and environmental factors is likely to have caused the premature corrosion of dozens of recently-commissioned wet flue gas desulfurization (FGD) systems across the country, according to a technical root cause analysis recently released by the Electric Power Research Institute. The primary cause of the corrosion is likely the use of newly-developed stainless steel and metallic alloys that line the inside of absorber vessels that have inadequate corrosion resistance, the analysis says, combined with the use of forced-oxidation systems, currently the industry standard for FGDs. Absorber vessels made with materials such as Duplex Alloy 2205, 317LMN and nickel-based stainless steels have led those wet FGD systems to be particularly susceptible to premature corrosion attacks, said head author John Shingledecker, a senior project manager for EPRI’s fossil materials and repair program. “From our root cause analysis, we see that the current alloys that are being used didn’t have the adequate corrosion resistance for the forced oxidation chemistry,” Shingledecker said in an interview with GHG Monitor.
Wet FGD scrubbers are used widely by utilities as a way to remove sulfur dioxide from flue gas, required under various state and federal environmental regulations. The technology combines limestone slurry with flue gas and uses scrubbers to absorb the SO2. Recent incarnations of the technology from multiple manufacturers over the last decade have switched from traditional linings in absorber vessels such as organics and plastics to metallic alloys. The technology was supposed to be cheaper and higher efficiency, capturing more than 98 percent of a unit’s SO2 emissions while also producing a gypsum byproduct that could be sold to make wallboard. “It was supposed to be kind of the new big thing for scrubber installations,” American Electric Power Spokeswoman Melissa McHenry previously told GHG Monitor. “It was supposed to be the perfect alloy and have a lot of strength, low corrosion and be resistant to a lot of things.” However, utilities found that the technology, which costs hundreds of millions of dollars per unit and is supposed to last 25 to 30 years, has begun “aggressively” corroding in some instances after several months of use.
EPRI Began Studies Following Industry Complaints
EPRI began investigating the issue in late 2010 after several utilities complained about early instances of corrosion. Shingledecker and his team began by publishing a list of best practices for utilities to identify cases of corrosion. The group then looked at data provided by member utilities, examined available literature on the topic and conducted field tests using slipstreams off of currently operational absorber vessels to test the effects of corrosion on sample alloy coupons. Shingledecker said that part of the initial problem was that the combination of the metallic alloys and forced-oxidation systems were not adequately tested together before they were sold beginning around 2004. “It was a combination of a change in process and the introduction of new materials for this application,” Shingledecker said. “There was not a large amount of data on forced oxidation chemistry with an alloy vessel prior to 2004 to begin with.”
The analysis finds that more work needs to be done to find the best practices and materials that can be used to mitigate corrosion. “The combination of the oxidizing power of the slurry, plate surface preparation, time of service, scaling on vessel walls and, possibly, scale chemistry needs to be better characterized to help prevent this type of widespread deterioration of assets,” EPRI’s report says. EPRI is continuing three slipstream field tests to gain samples of the effects of corrosion on coupons. Shingledecker said those tests will continue through the end of the year. In the meantime, utilities said they are doing their own temporary spot fixes while EPRI continues its analysis.
At the end of the day, Shingledecker said many of the final mitigation decisions will depend on individual units. “Every utility’s situation is unique, the size of units differ and they operate in different fashions, so the mitigation strategies and the severity of corrosion is variable. All of those factors play into the utility’s decision-making on what they should do moving forward,” he said. EPRI estimates that there are more than 360 wet scrubbers currently in use in North America, more than 160 of which have been commissioned since 2004. Of those 160 units, Shingledecker estimated at almost half of which use absorber vessels lined with metallic alloys. However, he estimated that production of those systems have largely come to a halt since the corrosion concerns surfaced. “I’m pretty confident that a number of these alloys will not be used going forward in the future,” he said.