By John Guffre, P.E., Paragon Airheater Technologies
In a typical coal fired power plant, the rotary regenerative air heater is responsible for between 5 percent and 10 percent of the boiler’s total efficiency, thus making it a critical component for attaining low heat rates and reducing fuel consumption.
The three biggest threats to air heater performance deterioration are corrosion of the heat exchange surfaces, plugging and air heater leakage through the seals. This article will focus primarily on the dramatically increased level of corrosion and plugging issues associated with installing SCR (selective catalytic reduction) and SNCR (selective non-catalytic reduction) systems for the control of nitrous oxide (NOX) emissions.
Until the advent of post-combustion NOX control systems, air heater plugging/corrosion issues on coal boilers were primarily associated with sulfuric acid condensation in the cold end layer. As sulfur in the coal is burned, some SO2 (sulfur dioxide) is converted to SO3 (sulfur trioxide), which then combines with flue gas moisture to form sulfuric acid vapor. This vapor/moisture combination has a dew point between 280 F and 320 F; the higher the SO3 concentration, the higher the dew point. In addition to causing corrosion in the cold end layer, this condensate combines with fly ash to cause cold end plugging.
Historically, plugging issues were addressed with simple on-line soot blowers and occasional deluge or high pressure water washes during outages. The cold end acid corrosion problem was addressed by the use of a shallow (12″) layer of heavy gauge (18ga) Cor-Ten steel element designed for ease of cleaning (Figure 1). This layer is considered a “sacrificial layer” intended to include the entire acid dew point temperature range and is designed for easy, independent replacement at relatively low cost. This concept has been a standard for power plant regenerative air heaters for at least 50 years.
Unfortunately, the traditional design concept does not effectively address the flue gas conditions associated with the SCR process; the injection of ammonia in the presence of a catalyst to convert NOX into molecular nitrogen and water. This process does not capture all of the injected ammonia participates in NOX reduction – some reaches the air heater as ammonia “slip” where it reacts with SO3 to form ammonium sulfate and bisulfate (ABS). ABS precipitates on air heater element surfaces located in the temperature zone between 300 F and 450 F. ABS, combined with flyash, becomes a sticky deposit that is both very difficult to remove and, being located farther in the air heater than traditional acid deposits, is difficult to reach with an adequate amount of soot blowing energy (Figure 2). A further complication can occur with some high alkali Western fuels, where calcium-sulfur compounds form in the fly ash causing cement-like deposits to form inside the air heater.
For many plants, the ammonia deposition-based air heater deposits has resulted in:
- Rapid corrosion of cold and intermediate air heater layers
- Plugging-related load reductions due to the inability of boiler fans to overcome the added pressure drop
- Increased air heater leakage due to increased differential pressure between the hot and cold ducts in the air heater (which can also cause fan related load reductions)
- Frequent outages for high pressure water washes
- Reduction in air heater thermal efficiency resulting from deposition-related disruption of required design flow and turbulence patterns in the air heater element.
While every plant has its own personality with regard to these problems, it is possile for combinations of these issues to result in added costs or revenue losses in excess of $10 million annually. Reports suggest that some plants have made capital expenditures of as much as $6 million on air heater improvements to try to mitigate these problems.
Suppliers, academics and research groups have been hard at work developing and testing potential solutions to combat the ABS problem. These include:
- New coatings that add lubricity and corrosion resistance to the element surface to minimize ABS buildup and make it easier to remove with soot blowing
- Use of enhanced corrosion-resistant steels to extend the life of heat exchange element in the ABS and acid deposition zones
- Reconfiguration of air heaters from multiple temperature zone layers to a two layer design, with all ABS formation confined to a single layer
- Improvements in air heater soot blowers
- New technologies for removing ammonia slip from the exit gas stream before it enters the air heater
- Newer generation catalysts.
While each of these improvements has demonstrated a degree of success, none have proven to be a panacea for all ABS issues.
Applying coatings to the heat transfer element can reduce the ABS accumulation rate by maintaining a smooth surface finish. Figure 3 illustrates the reduced accumulation rate of an element coated with porcelain enamel (a coating similar to that used on the interior of a kitchen oven). Although newer enamel formulations are being tried, established traditional enamel coatings have at least two deficiencies.
First, being composed primarily of glass melted onto steel, porcelain enamel is rather brittle and tends to develop cracks which form crevices that provide attachment points for the ABS to affix and build up. These cracks also provide paths for acid to reach the underlying steel. Brittleness is a particular problem in horizontal shaft air heaters where elements flex with each rotation. Cracking is a particularly difficult problem to prevent while handling the elements during installation.
Second, enameling must typically be applied over steel that has very little resistance to corrosion, due to the enamel application process, which involves curing at temperatures around 1,500 F. Special enameling steels must be used to prevent carbon in the steel from oxidizing and forming CO2 gas ,which can bubble through the uncured coating, leaving penetrations for ABS attachment and acid attack. Steels with low corrosion resistance mean that corrosion can also rapidly progress along the steel surface and undermine the otherwise intact coating, causing peeling.
To overcome these deficiencies, a newer, non-brittle and highly flexible coating has recently been commercially released by Paragon Air Heater Technologies. This coating contains a proprietary silicone resulting in a high lubricity surface resistant to both acid and erosion. Unlike enamel, this coating does not require high temperatures for curing and can be applied over steels with enhanced corrosion resistance – such as Cor-Ten. This eliminates any potential for peeling if a portion of the coating is compromised. This silicone coating is also resistant to the demands placed upon the material during installation
Figure 3 also illustrates that steels with resistance to corrosion can also slow the rate of ABS deposition. When specifying steels for use in ABS air heaters, avoid “generic” descriptions such as “LACR” (low alloy corrosion resistant), which have no fixed properties or chemical makeup. Using an established ASTM standard and insisting on mill certifications is critical for steels exposed to ABS. The ASTM standard under which Cor-Ten can be manufactured is ASTM A606-04 Type 2 or Type 4 (note that “-04” stands for the 2004 update). The ASTM description for this steel is “high strength low alloy (HSLA) steel with enhanced corrosion resistance.”
Air Heater Reconfiguration
Some plants have elected to convert their air heaters to a “two layer” design, which eliminates the cold end layer (Figure 4). The idea is to have a single cold/intermediate layer, which will contain all of the ABS deposition area and provide better cleanability by eliminating the gap between layers. This design modification can also be combined with the aforementioned coated element to further enhance the improvement. This conversion has become popular among SCR owners. The conversion can be done at the time of SCR installation, or during a later extended outage.
While beneficial, the conversion can be costly as it requires replacing all of the baskets in the air heater as well as some rotor modifications. In addition, the conversion eliminates the easy to replace “sacrificial” cold end layer. Currently, manufacturing limitations also mandate the use of steels that are thinner than the traditional heavy 18-gage cold end steel. The reduction in steel thickness on the combined cold/intermediate layer can result in a shorter element life compared to the traditional multilayered design.
Soot Blower Improvements
Cleaning a rotating air heater is a considerable departure from cleaning a bank of stationary tubes. Soot blower manufactures have introduced new sootblowers, nozzles and controls specifically designed for air heater ABS issues. New blower controls can “index” in a controlled sequence to allow blowing at a set radius point while the air heater makes one or more full rotations. The distance between index points is typically set according to nozzle characteristics to assure full coverage.
ABS formation can be greatly reduced if the ammonia slip is eliminated or reduced before reaching the air heater’s ABS temperature zone. One established technology for eliminating excess ammonia is to coat all or part or the hot end layer of the air heater element with a coating containing NOX catalyst. Since this layer is at the same operating temperature as the SCR, this active catalyst layer can “consume” ammonia slip by combining it with any remaining NOX. The well established process was first implemented in the 1980’s to supplement NOX reduction after a traditional SNCR installation.
Another promising technology is ADS (adsorption desorption system). In this system, sections of the air heater are surfaced with zeolite or another adsorptive material. Ammonia is captured as the flue gas enters the air heater, before it reaches the ABS temperature zone. The adsorbed ammonia is desorbed and released when the hot zeolite rotates into the incoming air duct and the zeolite cools. The small amount of ammonia introduced into the combustion air by this process has no significant effect on boiler NOX production. This patented technology has been successfully demonstrated on pilot plants and versions of this technology have been used for many years on industrial air pollution control applications.
Current SCR catalysts have the undesirable side effect of also catalytically converting significant amounts of SO2 to SO3. In some cases, the excess SO3 can form a visible plume at the stack outlet.
More importantly, increasing the SO3 concentration can also increase the amount of ABS that is formed. If the concentration of SO3 entering the air heater is less than one half the concentration of ammonia, ammonium sulfate will be formed in lieu of ammonium bisulfate. Since ammonium sulfate is a relatively benign powder that is readily cleanable it can be advantageous to minimize the SO3 concentration entering the air heater.
To combat this phenomenon, catalyst manufacturers have been developing and introducing new SCR catalysts that significantly reduce the oxidation of SO2 to SO3. For coals with low sulfur content, the reduction in SO3 formation may have a noticeable affect on ABS formation.
The problems associated with ABS will continue to multiply since a significant number of new SCR installations are forecasted to be installed on coal fired boilers in the near future. In addition, minor ABS problems may transform into major problems as regulatory changes move SCR’s in continuous year round operation as opposed to the currently limited “ozone season” operation.
Both current and future SCR owners need to be aware of the extent of potential ABS problems as well as the fact that ABS formation rates may significantly increase as modes of boiler operation or changes in fuel occur.
In its present state of development, there are a variety of available air heater improvements to help with the problem ABS, but unfortunately there is no current single technology that can be labeled as a cure. For the foreseeable future, SCR owners should anticipate the need for a multifaceted approach to minimizing ABS buildup, and reducing its impact on plant operation and reliability.
Author: John Guffre has over 30 years experience in the power generation field with emphasis on improving power plant efficiency, and the design and operation of pollution control equipment for power plants. His experience includes hands on operation and maintenance of large coal fired power plants. He has also managed several companies that manufacture custom engineered products for the power generation industry. Mr. Guffre graduated from Rutgers University with a degree in Mechanical and Aerospace engineering and is a licensed professional engineer. Mr. Guffre oversees all new product research and development in the position of Research Scientist.