By Bill Looney and Brian Baleno, Solvay Advanced Polymers, L.L.C.; Greg L. Boles, Koch-Otto York Separations Technologies; and Jacob Tetlow, Arizona Public Service Co./Cholla Plant
Among the many types of pollution-abatement systems in use today, wet fluegas desulfurization (FGD) systems – also known as wet scrubbers – are notoriously capital- and maintenance-intensive.
Mist eliminators are an integral part of most wet FGD systems. The ongoing maintenance and frequent replacement of these components represent a large part of a wet scrubber system’s overall operating costs. Today, mist eliminators used in wet scrubber towers are available in a variety of materials: polypropylene, fiberglass-reinforced polymer (FRP), polysulfone and stainless steel. Each has its own advantages and disadvantages. Material selection has a direct effect on operations and maintenance requirements, as well as on capital and operating costs associated with mist eliminators used in wet FGD systems.
While the high-performance engineering of thermoplastic polysulfone is more costly than other non-metallic materials, extensive use of this material for wet scrubber mist eliminators has been shown to significantly reduce the number and duration of routine cleaning cycles, to trim operation and maintenance costs, to extend overall service life of the mist eliminator components and to improve the long-term reliability of these capital- and maintenance-intensive pollution-abatement systems.
The reliable operation of wet scrubbers has a direct effect on a regulated facility’s ability to ensure compliance with mandated SO2 limits. Problems with mist eliminators can lead to unplanned or excessive wet scrubber downtime. Complicating things, when backup scrubbing capacity is not available, the resulting time required for mist eliminator cleaning can lead to megawatt losses, the need for supplemental or replacement power and even the possibility of power plant service interruptions.
Wet Scrubber Basics
Inside a wet scrubber, the incoming sulfur-laden exhaust stream comes in contact with a circulating stream of an alkaline solution, most often a slurry of caustic reagents. Contact between the two streams converts the acid gases in the fluegas into neutral salts and other solid byproducts. These eventually are removed and raise the flue gas pH to 7 to 8 prior to discharge.
Lime and limestone are the most widely used reagents. With such an approach, a slurry containing calcium hydroxide or calcium carbonate is used to convert SO2 to calcium sulfite and calcium sulfate, which then precipitate out of the solution as a sludge byproduct. Although several different scrubber configurations are available, spray towers using banks of high-pressure nozzles to atomize the scrubbing liquid into a fine cloud of tiny, reactive droplets are most common. The surface-area-to-volume ratio of the droplets produced by wet scrubber spray towers intensifies contact between the acid-laden fluegas stream and the scrubbing liquid. This maximizes mass transfer of the pollutant from the gas phase to the liquid and promotes the neutralizing reactions.
Mist Eliminators Remove Entrained Droplets
Mist eliminators installed near the top of the spray tower typically rely on one or more chevron-shaped trays or baffles that remove fine droplets of the alkaline reactant slurry that become entrained in the fluegas stream inside the tower during operation. These droplets typically contain water, salt (either unreacted alkali or reacted sulfate/sulfide compounds) and small amounts of the acidic gases containing SO2, SO3 and other acidic species that must be removed.
Chevron mist eliminators can be installed and operated either in a horizontal or a vertical orientation. Two layers are typical. The first removes the bulk of the entrained liquids and the second removes residual liquids.
By design, chevron mist eliminators provide a tortuous path for the fluegas stream. This path forces any entrained droplets to impinge upon the baffle plates and coalesce into larger droplets, which fall back into the vessel. These falling droplets mix with the spray from the slurry nozzles and create a mist that promotes intense mixing between the acid gases and the alkaline reactants.
Additional reactions occur in the slurry and in the byproduct sludge that collect in the tower base. This byproduct sludge is ultimately dewatered using settling ponds, belt filters, plate-frame filters or other means. This allows the solids to be disposed of and the water to be reused. The returned scrubbing liquids can be recirculated and reused, while the cleaned flue gas stream is discharged, essentially free of mist.
Because the environment inside a wet scrubber can wreak havoc on all wet scrubber internal components, including the mist eliminator, it is critical to understand the range of cost and performance attributes of the leading materials used to build them: polypropylene, FRP, polysulfone and stainless steel. Proper material selection has enormous direct implications for the overall operation, reliability and lifecycle costs associated with both the mist eliminator and the wet scrubber.
Several coal-fired power plants were surveyed. Table 1 summarizes the operating experience of each.
Under normal operating conditions, the temperature inside a wet scrubber is about 130 F. But temperature excursions above 250-350 F are not uncommon, the result of upset conditions that can be caused by unexpected power outages, problems with the gas bypass system, clogging and failure of the slurry piping or spray header system or loss of slurry flow.
While insufficient data are available to determine the effect of the different scrubbing reagents or type of coal on the fouling of mist eliminator components, the experience of the utilities surveyed (along with anecdotal evidence from many other customers of the authors’ various companies) shows that proper material selection has a direct effect on both the frequency of the solids buildup on the mist eliminator chevrons and the ease and frequency associated with their cleaning, Proper material selection can also help mitigate thermal aging problems that typically plague most wet scrubber mist eliminators.
Table 2 summarizes material properties, performance attributes and relative cost differences associated with each of the four mist eliminator materials discussed here. When specifying for an industrial duty, steady state operating conditions should be taken into account. To the furthest extent possible, any excursions (such as short-term temperature spikes) that could occur due to potential upset conditions should also be anticipated and factored.
Polypropylene: Among the four prevailing materials, polypropylene is the lowest-cost option (on a material-cost basis) and is easily formed, molded and extruded. Polypropylene is also considered to be a broadly chemically resistant polymer, especially in the face of both acidic and alkaline constituents found inside a typical wet scrubber. As a result, polypropylene is currently the most widely used among the three polymer-based options for wet scrubber mist eliminators.
Fiberglass-reinforced polymer (FRP): FRP is comprised of a vinyl ester or polyester resin matrix, to which continuous glass fibers have been added to provide structural reinforcement. FRP is widely used in corrosion-resistant piping and vessels due to its low cost relative to the corrosion-resistant metals, broad resistance to chemical attack and corrosion, light weight, strength, rigidity and load-bearing capabilities. The use of FRP for mist eliminator construction has followed from its success in other industrial applications.
Polysulfone: This high-temperature engineering polymer is more costly than both polypropylene and FRP from a materials standpoint. However, for use in mist eliminator applications, a variety of factors help to justify its premium cost over the mist eliminator unit’s lifetime. These include its resistance to both oxidation and hydrolysis in the wet scrubber’s hot, moist, acidic environment and its improved retention of flexural modulus, toughness, rigidity and other thermal and mechanical properties at elevated temperatures, compared to the other polymeric options. Polysulfone’s particular performance advantages in mist eliminator applications are discussed in greater detail below.
Stainless steel: Stainless steel is the most costly of the four prevailing mist eliminator materials. It is valued for its superior mechanical strength and rigidity under prolonged exposure to elevated temperatures. It is also perceived to be easier to clean, because its superior mechanical strength allows the operator to use water blasting at significantly higher pressure to remove solids buildup. However, as discussed below, stainless steel’s particular vulnerability to surface pitting by oxidative attack makes this perception more of a misperception under real-life wet scrubber conditions.
Problems in the Field
Problems that mist eliminators routinely experience fall into two basic categories: Fouling and corrosion, and heat-related damage to the mist eliminator components.
Fouling and corrosion have the largest effect on system operating efficiency and on operation and maintenance requirements. Fouling occurs when solids and salts produced during the wet scrubbing process accumulate on the chevron mist eliminators. Corrosion occurs when oxidation or chemicals attack the surfaces and crevices of mist eliminator components.
When solids are allowed to accumulate, the buildup can close off the available open area within the chevron baffles. This leads to increased pressure drop, raises local velocities in the constricted passageways and reduces the collection efficiency of the mist eliminator. That can cause mist carry-over out of the tower. Such solids buildup increases the frequency and duration of cleaning, meaning extensive system downtime and maintenance.
Material buildup on the chevrons causes excessive mechanical loading that can be especially damaging to mist eliminators, related connectors and support bracing made of polymeric materials, which tend to lose their mechanical strength inside a wet scrubber. For instance, under prolonged exposure to elevated operating temperatures, components made from polypropylene can soften and melt. Coupled with excessive loading from solids buildup, this loss of thermal and mechanical integrity can lead to warping, buckling and premature failure. In some cases, polypropylene mist eliminators require extra bracing to accommodate the excessive load on the chevron baffles caused by solids buildup.
Wear, Tear and Cleaning
Wear and tear associated with frequent, high-pressure water blasting – the most widely used cleaning method – coupled with thermal aging combine to shorten the chevron’s overall service life and lead to more frequent component replacement.
Once acid gases in the coal-combustion flue gas are converted into calcium sulfite and calcium sulfate during wet scrubbing, they precipitate and drop out of suspension. These byproducts then accumulate on surfaces and passageways of the chevron mist eliminator. In general, calcium sulfite precipitates as a soft, white or yellow sludge that is easily washed from any surface onto which it settles. By comparison, calcium sulfate tends to form a harder precipitate. When allowed to settle on surfaces and in crevices such buildup is hard to remove during routine cleaning operations and also makes surfaces rougher. This further accelerates precipitation requiring still more frequent cleaning and downtime.
To minimize fouling, chemical additives such a magnesium, sulfur and dibasic acid (DBA) are sometimes added to inhibit precipitate formation and deposition. Meanwhile, the use of spray headers with optimized spray patterns helps minimize the amount of fouling that occurs during normal operations. However, neither of these approaches can completely eliminate scaling and fouling.
As a result, wet scrubbers with mist eliminators must be shut down routinely for manual cleaning. Periodic cleaning methods range from fire hose (with the mist eliminator remaining installed in the tower) to removing mist eliminator components from the scrubber tower and cleaning them with high-pressure water jets.
Making matters worse, many wet scrubber operators report that the environment inside a wet scrubber can roughen mist eliminator component surfaces, particularly those made from polypropylene, FRP and stainless steel. While different mechanisms cause problems affecting each of the different materials, the end result is the same: increased surface roughness that makes removing accumulated solids more difficult and that accelerates solids buildup over time, shortening the time between required cleanings.
APS Cholla Plant Experiment
The APS Cholla power plant recently performed an experiment in which mist eliminators constructed from each of the four prevailing mist eliminator materials were installed in one of the plant’s four wet scrubber towers (which is 80 feet tall by 30 feet in diameter, and housed in the portion of the facility known as Unit 2). (Figure 1).
Figure 1 Mist eliminators made from (l to r) : polysulfone, FRP, stainless steel and polypropylene
Figures 2 through 5 provide a visual comparison of the solids buildup that occurred on mist eliminator chevrons of each of the different materials after two months in service. Scale is shown on the polypropylene, FRP and stainless steel ribs (Figures 2, 3 and 4), while the chevrons made from polysulfone show minimal buildup (Figure 5. Note that solids buildup did occur on the polypropylene brackets used to support the polysulfone chevron blades).
Figure 2 Polypropylene mist eliminator showed solids buildup
Original polypropylene ME after 2 months of operation in 202B Tower
Figure 3 Chevrons made of FRP also saw solids buildup
FRP ME after 2 months of operation in 202B Tower
Figure 4 Buildup on stainless steel was found mainly at the edges
317 SS ME after 2 months of operation in 202B Tower
Figure 5 Polysulfone remained largely free of solids buildup. (supports made of polypropylene did see buildup.)
Polysulfone ME after 2 months of operation in 202B Tower
Based on its experiments comparing the four leading mist eliminator materials and its favorable operating experience with polysulfone, the APS Cholla facility is retrofitting its remaining wet scrubber towers in Unit 2 with mist eliminators constructed from polysulfone as each of the existing polypropylene mist eliminators reaches the end of its useful life. Plans are also being made to retrofit the existing wet scrubbers in Unit 1 and Unit 4 of the same plant with polysulfone mist eliminators.
How Each Material Fared
Polypropylene: Polypropylene is subject to oxidation when exposed to the high temperatures inside a scrubber, and often experiences softening at elevated operating temperatures. A surface morphology examination of used polypropylene and polysulfone mist eliminator components from the APS Cholla facility, carried out using scanning electron microscopy (SEM), revealed an interesting finding.
When particles become embedded in the polypropylene chevrons softened under elevated temperatures, a significantly roughened surface on a microscopic level is created. This helps to explain the experience of many operators – including those at APS Cholla – who report that once polypropylene mist eliminators have been put into service, no amount of cleaning can restore the mist eliminator chevrons to a “good as new” quality.
FRP: When subjected to temperatures of 140 F to 160 F and relative humidity in the neighborhood of 85 percent, FRP tends to experience hydrolytic attack that weakens the bonds between the resin matrix and the glass fibers. Different rates of thermal expansion and contraction between the glass and resin matrix cause the formation of tiny cracks and voids, which increases the potential for delamination. The cracks and exposed glass fibers create a rough surface similar to what happens to polypropylene mist eliminators. This makes it difficult to restore the components to a truly pristine condition during periodic cleaning.
Polysulfone: Polysulfone shows the greatest chemical/oxidative stability in mist eliminator applications in terms of resistance to fouling, solids buildup and corrosion from acid attack. Polysulfone withstands damage from exposure to mineral acids, alkali and salt solutions. In many utility applications, it shows a considerably longer service life, compared to polypropylene and FRP under the range of operating conditions found inside a typical wet scrubber. Meanwhile, because it demonstrates greater resistance to the surface roughness, pitting and surface buildup problems commonly experienced by polypropylene, FRP and stainless steel, polysulfone is also the easiest to clean of all of the competing mist eliminator materials.
Cholla plant operators report they are able to run their polysulfone mist eliminators 50 percent longer than polypropylene before the unit must be taken down to remove solids buildup. This is a cleaning frequency improvement of from once every three to six months for polypropylene to an average of once every 10 to 12 months for polysulfone.
In addition, when rapid turnaround is required, polysulfone mist eliminators can be cleaned in place. However, to ensure complete cleaning and maximize the time between cleaning cycles, both the polypropylene and polysulfone components are typically removed from the tower. Once removed, the polysulfone components can withstand aggressive cleaning techniques because they have greater mechanical strength than polypropylene.
Stainless steel: While valued for its improved mechanical strength and rigidity at elevated temperatures, stainless steel is prone to corrosion under conditions inside a typical scrubber tower, mainly pitting and crevice corrosion. These types of corrosion typically result in areas where there is a high concentration of chlorides or other halogens. Pitting not only increases surface roughness, which promotes the buildup of solids and makes cleaning more difficult, but in extreme situations, can also lead to perforations. This was the case at AEP’s Gavin Plant.
There, the plant’s two units produce 2,600 MW and include six wet scrubber towers, each 41 feet in diameter. After unfavorable experience with Type 317L stainless steel mist eliminators, AEP switched all six wet scrubber mist eliminators to polysulfone. The upper level of mist eliminators is particularly vulnerable to pitting corrosion because rinse water often does not reach the top of the upper deck. This creates a wet/dry interface where acidic components can concentrate.
The Cholla facility carried out an economic analysis to support its decision to switch from polypropylene to polysulfone mist eliminators exclusively in Unit 2. As part of that analysis, plant operators noted that each offline cleaning cycle for polypropylene costs more than $100,000 in manpower alone. Based on its ability to cut in half the frequency of its off-line cleaning – from once every three to six months to once every 10 to 12 months – the plant is saving $50,000 per tower per year by switching to polysulfone.
APS Cholla’s economic analysis also found that polysulfone components do not have to be replaced as frequently as polypropylene components. Despite being two to three times more expensive, a polysulfone mist eliminator can operate longer than one made from polypropylene before it must be replaced. At APS Cholla, the polypropylene mist eliminator would be replaced in less than a year because the increasing difficulty in cleaning them made breakage more common.
When APS Cholla upgraded the first of four wet scrubbers in Unit 2 from polypropylene to polysulfone mist eliminators, it spent an additional $52,000 on the premium components. But that material replacement has saved the facility more than $146,640 over the first three years of operation, an ROI of 13 months.
Switching from stainless steel to polysulfone has also proven to be a good strategy. PPL Montana operates 22 wet scrubbers, six of which use polysulfone mist eliminators; the rest use stainless steel. When PPL Montana carried out an analysis to determine the economic advantages of polysulfone versus stainless steel for wet scrubber mist eliminators, they found that polysulfone’s ability to withstand solids buildup and corrosion allowed the operators to take the scrubbers down for cleaning twice a year, while the stainless steel mist eliminators must be taken offline for cleaning eight times a year.
And because PPL Montana has no backup scrubbing capacity, the downtime required for each scheduled mist eliminator cleaning also led to costs associated with an 80 MW load reduction. According to PPL Montana (and assuming an energy cost of $32 a MW/hour for the 13-hour downtime sessions), the annual losses alone incurred by the towers using polysulfone mist eliminators (taken offline twice/year) are $76,560. The annual megawatt losses incurred by the scrubbers equipped with stainless steel mist eliminators (taken offline eight times/year) cost the utility $306,240.
As a result, the ability to stem the megawatt losses associated with excessive downtime by using polysulfone mist eliminators instead of those made from stainless steel is helping PPL Montana save $229,680 per year. And plant operators note that if power demand is high, the utility often has to buy replacement power during those downtimes, at a cost of $50 to $100/MW.
Bill Looney is Global Market Manager for Solvay Advanced Polymers with 25 years experience in high performance polymers. He earned his BS in Chemical Engineering from Michigan Technological University. For the last 15 years Bill has worked extensively in helping Solvay’s customer understand where high performance plastics can be used to replace corrosion resistant metals.
Greg Boles has spent 20 years with the Koch Chemical Technology Group, in various capacities of sales and marketing. Currently, Greg is the FGD Marketing Manager for mist eliminators associated with scrubbers for wet flue gas desulphurization applications.
Jacob Tetlow is planning manager for the Arizona Public Service Corp., Cholla Power Plant, responsible for planning, scheduling and executing overhauls on four coal fired units. With more than 12 years of experience in engineering, operations and maintenance, Tetlow has worked on many power plant projects and overhauls.
Brian Baleno is a Business Development representative for Solvay Advanced Polymers. Brian has spent nearly a decade managing material solutions for customers in automotive, aerospace, chemical processing, healthcare and, of course, the power plant industry.
Acknowledgements: The authors wish to thank Paul Shook of PPL’s Colstrip Station and Frank Fetty of AEP’s Gavin Station, as well as other power plant personnel, for sharing their experience in applying different materials for mist eliminator components in FGD equipment. The authors also wish to thank Suzanne Shelley for her efforts during the writing and editing of this article.