It was less than a decade ago that the power industry was enthusiastic about the new Duplex Stainless 2205 Alloy being used in the construction of wet flue gas desulfurization (FGD) vessels. The alloy appeared to be a cost-effective solution to the traditional nickel-based, stainless steel alloys, including 317L, C276, and C22.
Unfortunately, many U.S. power plants experienced severe pitting and corrosion deposits under the scale building up on vessel surfaces, sometimes within the first year of service. At times the corrosion was so aggressive it caused through-wall leaks within 18 months of service.
The accelerated corrosion test data performed on the 2205 alloy did not take into account the effects of high concentrations of chloride and fluoride rising under the scale, and creating aggressive crevice corrosion cells. Since the alloy could not withstand these higher concentrations of halides, the ferritic phase of the stainless steel was preferentially attacked. Moreover, the heat affect zones of the welds were particularly susceptible to attack. Wormholes through the alloy could easily be seen under a radiograph.
Many options, including alloy wallpaper and tile linings, have been used to help remedy this corrosive backfire, but vinyl ester linings have proven one of the most reliable and cost-effective methods for lining FGD vessels (Figure 1). Vinyl ester linings also have an excellent track record in providing both chemical and temperature resistance—a specification requirement for vessels in immersion environments. Of course, a number of variables within a vinyl ester system’s chemistry, primarily application conditions and environmental controls, can determine the lining’s long-term efficacy.
Once a vinyl ester’s correct polymer base is selected, other additives such as fillers, wetting agents, inhibitors, promoters, flexibilizers, and resins, among others, must be considered to achieve optimal, long-term performance. A detailed list of a vessel’s operating conditions and the owner’s goals for usage should always be given to the lining manufacturer to ensure that a lining can meet the project’s performance requirements.
While installing a proper vinyl ester lining system is essential to long-term corrosion protection, the cleaning of FGD absorbers during routine 18- to 24-month maintenance cycles is equally important to ensure long-term lining performance. But it can be costly.
A coal-fired generation facility in the eastern U.S. was spending about $100,000 per outage to clean each of the plant’s 70-foot-high by 70-foot-diameter FGD absorber units. The expensive and labor-intensive process of removing hard and thick gypsum buildup on wall surfaces—a common byproduct of ground-up limestone reacting under forced oxidation during the flue gas desulfurization process—often required the facility to use high-pressure, and sometimes ultra-high-pressure water washing equipment over several days to clean out the vessels before crews could enter and safely perform other routine maintenance. The aggressive hydroblasting often caused damage to the vinyl ester lining, necessitating further repairs or lining replacement.
The power utility installed proven vinyl ester lining systems in the plant’s three Alloy 2205 FGD absorbers in 2010 to protect the units from chemical corrosion. But after experiencing the expensive cleanup process during routine maintenance, plant management wondered whether there was a coating with low surface energy and good chemical resistance that could also reduce the cost for gypsum removal.
This led to a discussion with the facility’s coatings representative about a proprietary, new flake-filled vinyl ester resin topcoat technology in development for the auto industry. The technology was designed to provide strong chemical resistance and improve the flow of highly viscous solids for fast cleanup (Figure 2). The key ingredient in the topcoat is polytetrafluoroethylene (PTFE), which acts as a release agent and is commonly referred to as Teflon.
Fusion of a Good Idea and New Technology
That information inspired further discussion: Why not apply the advanced topcoat over the existing vinyl ester lining systems to determine whether it could help reduce downtime and labor costs associated with cleanup? Management made the decision to initiate a scope-of-work change to spray-apply one coat (15–25 mil dry film thickness) of the PTFE-based topcoat to two of the FGD absorber reaction tank shells and floors using standard spray equipment and scaffolding. The topcoat has fast cure properties, with a final cure in 24 hours at 70F.
During the next maintenance outage, a standard spray nozzle and firehose were used to flush the gypsum buildup off interior walls. The utility was impressed at how quickly the gypsum buildup was removed. With the topcoat in place, the buildup washed off with no damage to the existing lining system. With a regular firehose, the absorber units can now be cleaned with a significant reduction in time of several days per unit. Adding the PTFE-based topcoat has more than paid for its incremental installation cost, and has since been installed in the third FGD absorber unit. ■
—Les Prysock is regional linings manager for AkzoNobel ([email protected]).