A SUMMARY OF THE CAUSES OF CORROSION AND A PATH TO ITS ELIMINATION
Greg Odenthal, Director of Engineering & Technical Operations, ITC - International Technical Ceramics, LLC and Steve Williams, COO of JC Industrial Services, summarize baghouse corrosion and provide a solution for its elimination.
Corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, expensive over design and also can jeopardize safety. In general, corrosion in cement plants occurs when process gases containing moisture, SOx, HOI, and NOx, operate at dew point temperatures. The economic Impact of corrosion damage has become a significant problem in cement plants worldwide. The best time to stop corrosion is before it begins and ITC - International Technical Ceramics, LLC has developed several ceramic thermal barrier coatings that have all but eliminated baghouse corrosion due to condensing flue gases.
Carbon steel and even stainless steel corrodes in flue gas service. Equipment like electrostatic precipitators, baghouses,
cooling ducts, conditioning towers and stacks frequently fall due to corrosion. Corrosion is worse when there is presence
of acidic compounds in the flue gas. Figure 1 shows severe corrosion on the Inside of a baghouse lined with conventional epoxy based coatings.
The source can be from sulfur content in the feed or fuel, chloride content in the feed or air and C02 and NOx from combustion. The moisture content in the gas stream produces hot acid condensation on the steel shell walls, and in most cases, intermittent for short periods of time however the cumulative impact can be up to 1.0 mm/yr. of metal loss equating to less than five (5) years life. This loss of metal thickness can be seen as thick rust flakes and also pinholes caused by localized attack. Atmospheric air entering the system through these pinholes gives rise to an increase in energy consumption during operation of the baghouse. Air pollution control devices, the fans and stack are also candidates for corrosion. Water spray towers used to control temperatures, amplify the problem. Some processes have acid gas scrubbers, which are also problematic if they are not protected. In these systems, the stack is also a problem area. In general, equipment operating in the cooler end of the process is where most of the corrosion develops. These areas are sensitive to cold air in leakage, low external temperatures and startups and shutdowns.
The corrosion of pollution control devices in cement plants is most severe during the acidic condensation of the process gases containing moisture, S03, S02, C02, HCI and NOx. The condensation can be more frequent and more aggressive depending on several variables such as moisture concentration, inlet gas temperature, defects in the thermal Insulation, leaks of cold air into the baghouse and low ambient temperature. When there Is a large fluctuation In the gas temperature entering the baghouse, an additional complication occurs; at lower temperatures there Is severe corrosion due to condensing flue gases, at higher temperatures there can be thermal degradation of the corrosion protection (conventional) coating.
Many coatings have been developed in the past. Epoxy and silicone coating materials can resist the effects of acid condensation to some degree. Acrylics, alkyds, or polyesters will not withstand high operating temperatures. The failure mode for these types of coatings are oxidative degradation and delamination, see figure 2. Oxidation damage occurs when the process equipment operates above 150° C (302° F). Undercut corrosion, dis-bonding and delamination occur when there is any surface damage or imperfection in the surface preparation. There are high temperature silicone coatings that can operate up to 500° C (932° F) in dry environments, but in hot combustion gas systems with even a small percentage of moisture content, they tend to fail in a few months.
CERAMIC COATING TECHNOLOGY
Today high temperature, energy efficient ceramic coatings are being used to eliminate baghouse corrosion along with the high cost of maintenance associated with this problem. ITC - International Technical Ceramics, LLC has engineered and developed several different ceramic coatings that are successfully being used to prevent baghouse corrosion. These cement coatings have no VOCs and are water based, spray applied products with excellent adhesion and the ability to handle high temperature, moist and acidic environments without de-bonding see figure 3.
Instead of applying a barrier that slows the heat conduction through insulation and substrate, ITC coatings use reflective, low transmittance technology to improve insulating values and prevent the formation of a dew point thus eliminating condensing flue gases and acid attack. Substrate surface preparation is critical to the success of ITC coatings and must be abrasively blasted to remove chemical debris and contaminants from the shell, which are the catalysts for the onset of corrosion. Application thicknesses for ITC coatings are only mills thick compared to 100+ mills of conventional epoxy-based coatings. ITC and its alliance partner JC Industrial Services to date, have several successful installations, each with two (2) to three (3) years of service life without any metal loss or coating maintenance required. The major benefits attained through the use of ITC protective coatings are:
- Baghouse longevity due to the elimination of metal loss
- Reduction in downtime
- Increase in efficiency due to elimination of condensing flue gases leads to reductions in bag and cartridge corrosion allowing for longer campaigns.
Despite the developments in corrosion resistant alloys over the past few decades, carbon and stainless steels continue to rust and corrode in harsh acidic environments. The need for corrosion prevention is greater today than ever before due to the new more stringent pollution and emission regulations imposed by the EPA. Without the use of coatings, surface substrates will continue to corrode leading to premature equipment failure costing hundreds of thousands of dollars per year in steel replacement, maintenance, unscheduled downtime and possible fines. ITC’s line of ceramic protective coating provides a cost-effective path toward corrosion elimination through the use of thermal barrier cement coating that extends the service life of cement plant equipment.
1. "The Benefits of Internal Coatings" Debra Ashley - Specialized Coating Technology, LLC
2. "Solutions For Severe Corrosion" Linas Mazelka - 3L&T Inc. USA