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Power plants’ flue gas desulphurization (FGD) systems require chemically resistant materials to protect them from corrosion. Whether it’s the scrubber module, ductwork, stacks or associated equipment and tankage, maintenance costs can be high. It therefore makes sense to keep required maintenance and its associated costs under control by selecting the correct FGD materials.
Fortunately, several choices exist, encompassing a wide array of materials and chemistries. These are proven technologies with successful installations worldwide.
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A technician applies a polymer lining to protect an FGD unit from corrosion. Photo courtesy of Sauereisen Inc.
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Materials engineering and science expertise, coupled with knowledge of chemistry, enables personnel to identify and examine problems and select construction materials resistant to the high temperature corrosion common to FGD systems. The chemistry involved is well-documented and will not be discussed here. This article will instead focus on the chemical and physical properties that some of the materials used in FGD systems must have.
Manufacturers supply the physical property data for restorative materials and the test methods used to obtain them. The specifier/engineer then uses this data to develop the specification. Once chosen, the corrosion resistant construction materials must be applied correctly. Part of the material selection process includes evaluating whether the desired materials can be properly applied under project specific conditions within the resource budget (money, time and manpower).
Another part of the materials selection process involves specifying the materials and the properties they must exhibit. Many times this part of the process is subcontracted to the materials manufacturers, or worse, the applicator. Rarely, if ever, does the engineer give any thought to whether the specified materials’ properties are sufficient, whether they are obtainable in the field and whether the test method is appropriate, accurate and informative. The attitude is “if the ASTM (or other organization) published a method, it has to be the way to test.” The reality is that ASTM, like other organizations, is a “voluntary consensus organization.” This means a coalition of users, producers, general interest, engineers and others agreed on every document. This does not necessarily result in the best method or standard.
The specifier/engineer therefore must at least read the specified methodology to ascertain if it makes sense, if it’s accurate and if it’s applicable to his or her project and to the materials being specified. A simple way to do this is to reference the test methods appropriate to the class of materials and not, for example, use a plastics standard or method to test paints or coatings. Likewise, steel standards should not be used to test concrete. This is a simple concept, yet such inappropriate methods are commonly used.
The specifier/engineer may always ask for additional data and request a modification to any test method if it improves the specification for the intended application. Other times, pieces of disparate standards or methods may have to be combined. It is the specifier’s/engineer’s responsibility to make sure the specified test methodologies provide the information required to determine whether a proposed construction material is suitable for the project.
The specifier/engineer also should specify the property attainment levels appropriate to the application. In other words, if all calculations show that a tensile strength of 800 psi (including safety margins) is sufficient, there is no valid reason to require 2,000 psi because one manufacturer claims it is better. This could result in perfectly acceptable products being eliminated; products that may save money. Keep in mind that corrosion resistance is the primary requirement. Large physical properties do not automatically translate to good chemical resistance and values obtained during and after prolonged exposure (greater than one year) are most important, not the value on the manufacture’s literature.
The environmental attributes of the materials under consideration are seldom considered. For some materials such as steel and glass this is not a concern because there is little difference between one manufacturer’s material and another’s. For others, such as concrete and refractories, it is important for specifications and revisions to be current.
With industrial paints and coatings, environmental factors are a legitimate concern. The applicator must be aware of environmental and worker health and safety concerns. The utility/plant owner may have to contend with emission reports and requirements. Solvent borne coatings and paints often contain volatile organic compounds (VOC) and emit hazardous air pollutants . The federal government, through the Environmental Protection Agency and Occupational Safety and Health Administration, along with state agencies regulate these environmental hazards. The safest products are those with low VOCs; less than 100 grams per liter (gm/l). Some manufacturers are aware of these standards and have consistently formulated their products’ VOCs to 50 gm/l or less. As an added benefit, the corrosion resistance, thermal resistance, permeability and other physical properties are improved for low VOC products, so the added value comes at no additional cost to the user.
In addition to a lining material’s ability to resist attacks from the chemical resistant material of construction (CRMOC) itself, its permeability is of paramount importance. The lower the water vapor transmission rate, the lower the permanence and permeability. As permeability decreases, corrosion resistance increases. This is one property that, despite its importance to the selection process, is often overlooked by specifiers/engineers.
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A technician applies Gunite containing a chemical-resistant refractory within a brick chimney. Photo courtesy of Sauereisen, Inc.
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According to a recent survey sponsored by Sauereisen, a chemically-resistant lining’s permeability to corrosive substances is the most important consideration during product selection. Although nearly 70 percent of those surveyed said permeability to corrosive substances is the most critical factor for chemically resistant linings, more than 64 percent said permeability is the most commonly overlooked aspect of corrosion-resistant lining selection. The study also revealed chemical resistance to be the most important quality of protective linings, with porosity ranked as the number one factor in determining chemical resistance.
Obviously, the specifier’s/engineer’s task can be made easier by relying on material manufacturers to supply the required information. A reputable and experienced manufacturer can provide this data and discuss its significance.
One caveat bears repeating: A physical property with exceptionally high values does not translate to chemical resistance. One product, for example, could have a compressive strength of 50,000 psi and another only 5,000 psi. The weaker material may well have better chemical resistance. It’s the percent change in the property after exposure that indicates the relative degree of chemical resistance. For example, if a 50,000 psi product loses 40 percent of its strength after exposure and drops to 30,000 psi while the 5,000 psi system loses only 20 percent and drops to 4,000 psi, the weaker material is more chemically resistant.
In the mid-1980s, a major New York power company had a problem with the independent brick liner of one of its stacks. The unit, which is wet-scrubbed, operates at or below the sulfuric acid dew point. The brick liner developed several vertical cracks and acidic gasses leaked into the annulus. The cracks, which were already two-thirds of the liner’s total height, were judged to be active and growing. A replacement liner was far too expensive and time consuming to replace or rebuild. Calculations showed that a reduction in the temperature of the brickwork would greatly reduce the crack propagation. A repair system comprised of a lightweight, corrosion resistant refractory applied over a chemical-resistant elastomeric membrane was applied over the cracked brickwork. This same system has been applied in several other brick liners with existing cracks. Performance records show that in this case, as well as others, this repair method successfully extended the life of the liner by more than 10 years on average.
The materials used were specifically designed for service in FGD environments. The refractory was based on potassium silicate chemistry while the elastomeric membrane was organic in nature. Potassium silicates are particularly resistant to sulfuric acid. Other refractory materials, such as calcium aluminates, phosphoric acid-bonded, sodium silicates, and so on do not work well in the moisture-laden, highly acidic environments. The sulfuric acid chemically attacks and destroys them. The membrane chosen for this application is also resistant to sulfuric acid and is many times more impermeable than both refractories and even the dense brick. Because it is more flexible than the brick and the refractory, the membrane is not as prone to cracking. The combined benefits of the two materials result in a durable repair system. This same system has been used successfully on steel stacks since the early 1980s. Here the service records show a typical service life of 12 to 15 years before major repair or replacement.
In addition to silicates, calcium aluminate refractories are often used as repair materials. They make superior refractories and are economical. They will withstand temperatures higher than 2000 F and offer excellent thermal shock resistance. Their limitations lie in their chemical resistance. They are readily attacked by acidic species such as hydrochloric acid and wet strong sulfuric acid. Therefore, when used in FGD systems, they should not be exposed to wet environments or operate below the acid dew point. This eliminates them from use in systems with scrubbers. With the U.S. Supreme Court’s recent ruling regarding installation of pollution abatement controls on older plants wishing to upgrade, the use of calcium aluminate refractories in FGD systems is limited.
Steel is a common substrate for scrubbers, ductwork, breechings and stacks, as well as most of the associated tankage of today’s scrubbed flue gas system. Unfortunately, steel corrodes quickly and catastrophically in an FGD environment; it is often too expensive and time consuming to consider. Today’s engineers can take advantage of some remarkable developments in the corrosion-resistant protective coatings arena. There is a clear distinction in the types of coatings discussed here. These coatings are not what the typical consumer would call “paint.” While formulated to be applied by what is essentially paint technology, these coatings and linings are especially designed for highly corrosive environments.
These coatings are typically based upon one of three resin types: Novolak Epoxy, Novolak Epoxy Vinyl Esters and specific polyesters, usually isophthalic, chlorendic or fumarate. These resins may also be used as the binder in fiber reinforced plastics or layers. The coatings are specifically designed to be highly chemically resistant, to withstand fairly high temperatures, be abrasion resistant and perhaps most importantly, very impermeable.
Most FGD systems coatings contain the same base resins. Novolak epoxies are combined with a hardener that makes them resistant to deterioration by the environment. The proportions of epoxy resin-based coatings are determined by both the resin and the hardener used. This is one tool the coatings chemist uses to get the properties desired. Not all epoxies, therefore, will offer equal performance. The hardener choice is as important as the resin choice. The hardener molecules actually become part of the cured polymer as they serve as the cross linkers.
Vinyl esters and polyesters are all cured in the same basic manner. A promoter initiates the reaction by splitting the highly reactive organic peroxide “hardener.” The peroxide forms two free radicals. These free radicals react with the styrene, usually present, which reacts with the vinyl ester/polyester resin to “cure” the coating. The means of cross linking here is the same from coating to coating-styrene.
The coatings formulator has many ways to modify and improve specific properties of any given coating. One of the most important properties for a corrosion-resistant coating is its permeance. Permeance determines the rate and the extent to which a particular coating will be penetrated by the exposure medium. Even if two given coatings are equal in all other properties, the one with the lowest permeance will offer the best corrosion protection. Permeance may be determined by the ASTM E-96 or ASTM D-1653-03. These tests, which are inexpensive and easy to run, offer the specifier/engineer a way to specify the degree of protection desired. Most major coating manufacturers will readily supply this data. A metric permanence of 10-9 US perms (standard measure of permeance) has been shown to be a threshold in terms of corrosion protection. Those coatings with permeance lower than 10-9 US perms will give superior corrosion protection to those with higher permeance. Below this point, corrosion rates tend to flatten and are low.
Note that every material that is known or can be conceived of will be permeable to some extent, to some medium or media. Since everything is permeable to some extent, the key is to reduce permeability and permeance by modifying the coatings. Such techniques include adding certain fillers to the coating that are chosen because of their ability to reduce permeance. Some of the most successful reinforcement fillers include glass or specific mineral flakes or platelets, specific fibers and fiber blends, and nanoparticles. These coatings can provide long-term corrosion protection for eight years or more before recoating becomes necessary. Additionally, they are more economical and far less permeable than either brick or refractory liners. This often makes these types of liners the CRMOC of choice. Table 1 lists some common lining materials and their permeance and permeability in dry film thickness (DFT).
One example where permeability and permeance both were managed well involved a desulphurization system in a plant in the southern United States. At this plant, a Novolak epoxy vinyl ester (reinforced with the fiber described above) was in service for more than 13 years. Samples taken from the stack revealed under a scanning electron microscope that neither the chloride species nor the sulfur species present in the flue gas had penetrated more than one-third of the lining. The graph in Figure 1 shows the results.
Manufacturers of corrosion resistant coatings and refractories have a variety of materials and systems to protect steel and concrete. The refractory materials can withstand temperatures of 1200 F or higher. The organic coatings offer superior protection at temperatures of 400 F or lower. The primary refractory materials are based upon potassium silicates or calcium aluminates in dry environments. Organic coatings are usually based upon epoxy novolaks or epoxy novolak vinyl esters. The best of these coatings have been modified to permeance levels below 10-9 US perms for maximum corrosion protection.
Upcoming changes in flue gas cleaning technology will also generate chemistry changes. For example, certain mercury removal processes will increase chloride levels along with fluoride levels. In a closed-loop scrubber system, the chloride and fluoride levels will increase potentially by a thousandfold. This can have potentially catastrophic effects on some materials such as the lower grades of alloyed steel. Specifiers/engineers should discuss specific applications in detail with coating materials manufacturers. This can help with design details and system recommendations, and provide technical support.>
Author: Gary Hall is currently manager of organic technology at Sauereisen Inc., a manufacturer of adhesives, refractories, polymer linings and coatings. Mr. Hall is responsible for product development and improvement for Sauereisen and manages the research and development efforts for the organic product line. He has been with Sauereisen for 39 years and is a graduate chemist from the University of Pittsburgh. He is active in the National Association of Corrosion Engineers, the American Society for Testing and Materials and the American Institute of Chemical Engineers.
Power Engineering June, 2007
Author(s) :
Gary Hall
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