1 Preface
 
Flue gas desulfurization (FGD) is the process of removing sulfur oxides from the gaseous byproducts generated during the combustion of fossil fuels. Traditional combustion technologies cause most of the sulfur in fuel to be oxidized and released into the flue gas, contributing to the formation of "acid rain." Acid rain refers generally to the deposition of various acids in the atmosphere, which can harm both the environment and human health. Acid rain has become a major concern for governments worldwide.
Energy conservation and the substitution of low-sulfur fuels for high-sulfur fuels have played a role in reducing overall pollution; however, it remains necessary to adopt technologies for removing sulfur oxides. Currently, the primary method involves using alkaline slurries—such as lime/limestone slurry—in wet scrubbing processes to remove sulfur oxides generated after combustion. The resulting gypsum is either recycled or not recycled.
Nickel-containing materials demonstrate cost-effective performance in flue gas desulfurization systems, and the Nickel Development Institute (NiDl) has extensive experience in this area. This information can serve as a valuable reference for manufacturers of flue gas desulfurization equipment, end-users, material suppliers, and providers of process technologies worldwide.
 
2. Emission Standards
 
In the 1970s, Japan was the first country to introduce emission standards nationwide, followed by the United States. In the 1980s, as public awareness of environmental protection grew, the scope of application of these standards continued to expand and became increasingly stringent. This trend persisted throughout the 1990s, as people became increasingly aware of the dangers of air pollution and gained a deeper understanding of both the technical feasibility and economic viability of emission-control technologies.
These standards generally apply to large combustion facilities with capacities exceeding 50 MW, including boilers in most power plants that burn fossil fuels and other major industrial pollution sources. Some countries, such as the United States, also impose emission controls on smaller facilities, and increasingly stringent regulations have proven to be effective. Given the varying local conditions and needs across different countries, the standards stipulated by national regulations also differ. Moreover, differences in facility size, fuel types, and time constraints for meeting current emission standards further contribute to variations in regulatory requirements. In general, all newly constructed facilities are required to comply with the most stringent emission standards.
2.1 Europe
Currently, with the support of the United Nations Economic Commission for Europe, efforts are underway to garner international support for the more stringent regulation known as the “Convention on Long-range Transboundary Air Pollution.” A major measure adopted by the European Community is the enactment and implementation of the “Large Combustion Plant Directive (LCPD),” which sets emission limit values for solid, liquid, and gaseous fuels used in facilities with a capacity of 50 MW or greater.
2.2 North America
At the end of 1990, the United States passed the Clean Air Act Amendments (CAAA), which mandated nationwide reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides to levels below those recorded in 1980. The act required that SO2 emissions be reduced by 10 million tons in two phases before the year 2000. Thereafter, annual national SO2 emissions were to remain no higher than 8.9 million tons. By January 1, 1995, SO2 emissions had already been cut by half (the first phase). By January 1, 2000, all individual units with a capacity of 25 MW or more that burned fossil fuels were required to limit their emissions to no more than 1.2 pounds (0.5 kg/10^9 J) of SO2 per million BTU of heat input—a requirement that also stipulated the retention of allowances for each ton of SO2 emitted (the second phase). Originally, the target for nitrogen oxide reductions was 2,000 tons per year; however, this reduction target is now under renegotiation. It is estimated that between 1990 and 2010, U.S. power plants would spend $12 billion annually to achieve the goal of reducing SO2 emissions by 10 million tons per year.
2.3 Japan
Due to Japan’s small land area and large population, it was among the first countries to pay close attention to air pollution control. In 1968, the Japanese government enacted the “Air Pollution Control Law.” New process technologies have been continuously developed, and some of these technologies have already been transferred to companies in North America and Europe. By the mid-1980s, more than 1,500 air pollution control facilities had been installed across Japan’s various industrial sectors. Each factory is required to install emission-control equipment in accordance with established standards; currently, the total capacity of flue-gas desulfurization units installed in Japan has exceeded 15,000 MW.
2.4 Asia-Pacific Region
Currently, the region is experiencing rapid development, leading to a growing demand for electricity. To meet this demand, large quantities of fossil fuels are being consumed. Coal consumption is on the rise in India, China (including Taiwan), Indonesia, Thailand, and South Korea. These countries have already introduced air quality regulations, and their emission standards are expected to be comparable to current international levels.
 
3 Wet scrubbing process
 
Although flue gas desulfurization is a relatively simple process carried out under relatively mild operating conditions compared to other chemical processes, numerous material-related issues have persistently arisen. Figure 1 shows the specific areas in flue gas desulfurization units that are prone to problems, as well as the conditions under which various materials are used.
Influenced by early experiences in the United States and Japan, carbon steel with plastic and rubber linings is often used. Failure of these linings can lead to power plant shutdowns—for example, in January 1987, a fire broke out in the flue gas desulfurization equipment at the Niederrhein power plant operated by RWE in Germany; in May 1990, a fire occurred at the Kainan power plant operated by Kansai Electric Power Company in Japan; in August 1993, a fire broke out at the Sulven power plant operated by FFB in Germany; and in 1994, fires occurred at the Big Bend power plant operated by Tampa Electric Company in North America.
In this regard, the development of a “wallpapering” technique for applying thin sheets of nickel-containing materials—such as stainless steel—onto carbon steel liners, with a thickness of 1.6 mm, is noteworthy. Initially, it was believed that using carbon steel coated with non-metallic materials would be the most cost-effective approach for flue gas desulfurization systems. However, this turned out not to be the case. This is because such coatings are prone to mechanical damage and demand highly precise application conditions; if these conditions are not met, the coating will fail to adhere properly. In terms of life-cycle costs and lost benefits, all-alloy structures generally prove superior to coated-steel systems.
However, metal systems can also suffer damage due to improper alloy selection, poor welding and machining quality, insufficient understanding of their operating conditions, or inadequate control over these conditions. Today, based on practical experience and extensive corrosion test results obtained both in the field and in the laboratory, we can confidently select alloys.
The acidic/chloride environment inside flue gas desulfurization scrubbers is highly corrosive. It is essential to consider the corrosive effects of sulfuric acid under various pH conditions, especially in the presence of chlorides and fluorides—both originating from the fuel and enriched in the scrubbing medium. When fouling and residues are present, these environmental conditions can lead to crevice corrosion.
Considerable progress has been made in the development and operation of various flue gas desulfurization systems. Further understanding of the chemical processes involved, limitations on the materials used in construction, and equipment design has led to improved operational performance of these systems.
The proper selection and use of nickel alloys and stainless steels can indeed provide cost-effective solutions to most material-related issues encountered during the operation of flue gas desulfurization equipment. As a result, throughout the equipment’s intended service life—typically defined by conventional power-generation practices—low maintenance costs and high equipment availability can be ensured.
Lime-limestone wet flue gas desulfurization equipment, which utilizes nickel-containing materials to overcome corrosion issues, has already accumulated extensive experience. Among the wet limestone/gypsum absorption towers constructed worldwide, more than 40% have adopted nickel-alloy materials. Moreover, it is projected that by 2005, approximately 70% of absorption towers will be equipped with stainless steel and nickel alloys. Factors affecting corrosion
① Sulfur content
Sulfur oxides produced by coal combustion react with water to form corrosive acids. The ideal operating conditions for flue gas desulfurization systems are a maximum sulfur content of 4% in the coal and removal of at least 95% of the sulfur oxides.
② Temperature
Although the normal operating temperature range within the flue gas desulfurization scrubber is not high, its corrosive effect on the environment is significant. The incoming flue gas typically has a temperature of 160℃, but as it passes through the scrubber, its temperature is reduced to between 50 and 65℃. The outlet temperature of the scrubber is very close to the condensation temperature of sulfuric acid or sulfurous acid. Consequently, acid condensation occurs in the outlet duct, dampers, and chimney, necessitating the use of acid-resistant materials. It is crucial to recognize that when the bypass is fully open, untreated flue gas will bypass the scrubber entirely; therefore, the system must also be designed to accommodate this scenario.
Under normal operating conditions, the sulfuric acid concentration in the condensate is roughly between 26% and 55%, and can exceed 80% when the bypass is fully open. Heating the system entirely to 170°C will prevent condensation, but it will reduce system efficiency.
③ pH control
To minimize scaling in the scrubber system, it is essential to closely monitor the process conditions. By maintaining the slurry’s pH within an appropriate range, the crystallization of calcium sulfite can be effectively controlled. If the pH is too low, the scrubber’s efficiency will decline. Typically, limestone is used to keep the dilute scrubbing slurry slightly acidic (with a pH between 4.5 and 5.5). In addition, to further reduce scaling, the use of sulfur or thiosulfate additives is becoming increasingly common. To cool the flue gas to the optimal reaction temperature, water treated with a neutralizing agent must be employed to prevent the pH from dropping excessively (to 1.5 or below). The optimal temperature varies depending on the fuel being used—for instance, higher temperatures are required when lignite is utilized. These operational constraints must be carefully considered when selecting materials.
④ Chloride content
Another factor affecting the corrosivity of the medium within washing equipment is chloride, which originates from the water used in the scrubbing tower and from hydrogen chloride generated by the combustion of coal containing chlorides. Chloride concentrations can become highly concentrated due to water evaporation and recirculation; reportedly, in single-loop recirculating systems, concentrations can exceed 100,000 ppm, forming highly corrosive acidic chloride solutions. Therefore, it is crucial to strictly control chloride emissions in wastewater. It is necessary to employ methods such as multi-stage flash distillation to remove chlorides from the process water discharged by flue gas desulfurization plants.
In a dual-loop system, environments with high chloride concentrations may be confined to the quench zone of the scrubber tower. In other parts of the scrubber tower, chloride levels are in the range of several thousand ppm, and the medium exhibits relatively low corrosivity, so it may not be necessary to use nickel alloys with the highest corrosion resistance.
⑤ Fluoride content
The fluoride content in coal can be as high as 3,000 ppm. Fluorides tend to accumulate beneath deposits of dirt on metal surfaces, exacerbating crevice corrosion of stainless steel caused by acidic chlorides. When the combined concentrations of chlorides and fluorides reach 100,000 ppm or higher, nickel-based alloys with elevated chromium and molybdenum contents must be used to prevent corrosion induced by chlorides and fluorides.
 
4. Corrosion Resistance of Nickel-Containing Materials

     The corrosion resistance of stainless steels and nickel alloys stems from their ability, in atmospheric environments, to spontaneously form a thin, chromium-rich surface oxide passivation film. All alloys with a chromium content exceeding 12% typically possess this capacity to develop such an oxide film. Obviously, the composition of the oxide film varies depending on the alloy’s chemical makeup; however, even if the film is damaged, it still retains its corrosion resistance in many different media. For nearly every application, there is a steel grade that can deliver satisfactory performance at a cost-effective price.
 
5 Corrosion in Flue Gas Desulfurization Equipment
5.1 Stress Corrosion Cracking
Within the typical operating temperature range of 50–65℃ for scrubbing towers, there is no risk of stress corrosion cracking in austenitic stainless steels. However, when the temperature rises to 150–175℃ or higher, stress corrosion cracking can become a significant issue. It is crucial to recognize that even if the pH and chloride content of the bulk solution are well controlled, chloride concentrations can still become locally enriched beneath deposits or on heat-transfer surfaces, thereby creating conditions conducive to stress corrosion cracking.
5.2 Pitting Corrosion
① The effects of chloride concentration and pH value
Extensive research has been conducted on the effects of chloride ion concentration, pH value, and temperature on the performance of nickel-containing materials used in processing industries and for seawater transport. Most of this research has focused on the conditions encountered in flue gas desulfurization equipment. Localized corrosion exhibits a clear correlation with both pH value and chloride content. The accumulation of chloride ions and the presence of oxygen-deficient crevices can lead to localized corrosion when solid deposits form. When selecting materials, it is essential to take into account the possibility of such deposit formation during operation. Experiments conducted under controlled conditions can help determine the upper limits of pH and chloride content for various nickel alloy materials. These findings can be used to identify under which conditions stainless steel is appropriate and under which conditions nickel alloys with superior corrosion resistance should be employed.
Comparing materials using the Pitting Resistance Equivalent Number (PREN) facilitates the selection of appropriate alloys that can meet the operating conditions of flue gas desulfurization equipment. The Pitting Resistance Equivalent Number can be calculated according to the following formula: PREN = %Cr + 3.3%Mo + 16%N When tungsten is present, the formula becomes: PREN = %Cr + 3.3%Mo + 16%N + 1.65%W The higher the PREN value, the better the alloy's resistance to pitting corrosion. The PREN value is an extension of the Pitting Index—a metric that has been successfully applied to stainless steels in seawater environments.
Figure 3 (omitted) shows the performance curves for 316L, 317L, and 317LM stainless steels, providing a conservative guideline for alloy selection in flue gas desulfurization scrubbing systems under conditions where the pH ranges from 3.5 to 7.0 and the chloride content is up to 5,000 ppm. It also illustrates the application of the PREN value.
② The influence of temperature
Generally speaking, rising temperatures can increase the corrosion rate or enhance the susceptibility of alloys to corrosion. To accommodate the higher operating temperatures within flue gas desulfurization equipment or to account for potential temperature fluctuations during operation, higher-grade nickel alloy materials that can withstand more demanding conditions are available.
③ The effect of molybdenum
As everyone knows, molybdenum can enhance the resistance of austenitic stainless steels and nickel alloys to general corrosion, particularly their resistance to pitting corrosion in chloride solutions and sulfur dioxide vapor. For example, recognizing the value of increasing molybdenum content, a 317LM stainless steel with a molybdenum content of 4.0–4.5%—exceeding the typical composition range of 317 stainless steel—has been developed for certain highly corrosive service environments. Furthermore, a 6% molybdenum ultra-austenitic stainless steel has been developed with even higher molybdenum content, offering superior resistance to corrosion in acidic chloride media.
 
6 Nickel-containing materials—applications in flue gas desulfurization equipment
 
Due to a lack of understanding of the operating conditions encountered and the absence of reliable data on the performance of stainless steels and nickel alloys, many early flue gas desulfurization systems were designed with carbon steel components featuring non-metallic coatings and rubber linings. Although coating technologies have advanced significantly, problems still persist—and these issues can be effectively addressed by selecting appropriate nickel alloy materials.
Increasing the contents of chromium, nickel, molybdenum, and nitrogen can enhance corrosion resistance. Chromium forms a protective surface passivation film, while molybdenum and nitrogen improve resistance to pitting corrosion and crevice corrosion. Nickel helps restore damaged protective films and also improves machinability and weldability.
The challenge faced by designers is to select a material that contains these alloying elements in an appropriate combination, capable of resisting corrosion in the flue gas desulfurization scrubber environment, while keeping material costs as low as possible.
6.1 Use of Stainless Steel
Wherever stainless steel can be used, standard grades of stainless steel are typically sufficient. However, given the increasingly corrosive environments encountered today, there is a growing trend toward using stainless steel grades with higher contents of molybdenum, chromium, and nickel. Low-carbon stainless steels or stainless steels stabilized with titanium or niobium are resistant to intergranular corrosion. For example, alloys with a nickel content of over 42% or duplex stainless steels featuring an austenite-ferrite microstructure and containing 22% to 25% chromium can effectively address the issue of chloride stress corrosion cracking (CSCC).
6.2 Use of Nickel-Chromium-Molybdenum Alloy
Improved grades of conventional stainless steels, such as 317LM or nitrogen-containing 317LMN, exhibit insufficient corrosion resistance in scrubber environments characterized by high chloride concentrations and low pH values. Guided by the Pitting Resistance Equivalent Number (PREN), and drawing on extensive laboratory testing combined with practical field experience, we have developed a ranking of the performance of nickel-based stainless steels and nickel-chromium-molybdenum alloys. The purpose of this ranking is to assist in selecting the most effective materials for resisting specific corrosion conditions, including those involving low pH values and high chloride concentrations. The guidelines proposed take into account various operating conditions with differing degrees of corrosion severity at typical service temperatures (50–65°C). It is crucial to recognize that deviations from these standard temperatures, the presence of fluorides, severe scaling, poor design, and suboptimal fabrication practices can all impact alloy performance and should not be overlooked by relying solely on these simplified guidelines. Furthermore, it should be kept in mind that even if scrubber designs appear similar, the environmental conditions within each individual scrubber are unique. Subtle differences in the fuels used, water quality, equipment operation, and equipment design can all influence material performance.
Figure 4 shows a material selection guide for chloride concentrations ranging up to 200,000 ppm and pH values from 1 to 6.5. To facilitate detailed consideration, the corrosion conditions—ranging from “mild” to “very severe”—are indicated along with their corresponding pH values and chloride concentrations.
Similar data on the effects of fluorides and chlorides have already been published in “New Cost-Effective Metallic Materials for Flue Gas Desulfurization,” [J. Charles, European Stainless Steel, November 1993, Vol. 5(9)].
Table 1 provides the standard composition of the nickel material described in Figure 4.
 
Table 1: Composition of Representative Nickel-Containing Materials Used in Flue Gas Desulfurization Equipment (abridged)
 
 
7. Minimize the occurrence of problems to the greatest extent possible.
 
In the design and operation of metal flue gas desulfurization scrubbers, the following factors will help minimize or prevent corrosion issues, ensuring long-term, trouble-free operation of the equipment.
· Monitor chloride content and pH range
· Prevent chloride concentration
· Avoid gaps during design and construction.
· Keep the surface free of accumulated fluid or dirt.
· Select the appropriate alloy based on the operating conditions within the scrubbing tower.
Monitoring chloride levels alone cannot prevent corrosion, but it can serve as a valuable indicator of potential problems. Whenever there is recirculation, there is a risk that chloride concentrations could rise to corrosive levels. Controlling the pH value is essential for ensuring efficient scrubbing and preventing scaling. A significant drop in pH can accelerate corrosion. Preventing chloride concentration buildup is primarily a matter of process design. If chloride concentration does become concentrated, it is best to confine the concentration to a small, localized area within the system and to use materials in that area that can withstand the highest anticipated chloride-induced corrosion. For the design and construction of any chemical equipment, avoiding crevices is always a good practice—this is especially true in areas where condensate might accumulate and concentrate to levels far higher than the expected acid concentration. Since flue gas condensate in scrubbing towers is highly corrosive, it is crucial to prevent condensation from occurring. If a reheater is used, it is important to maintain heat within the tube bundle during shutdown to avoid condensation. It should be noted that most current flue gas desulfurization designs do not include reheaters. It is recommended to keep equipment free of deposits. Accumulated scale increases the risk of pitting corrosion and crevice corrosion; therefore, cleaning should be performed during shutdown periods.
 
8. Spray application
   
People are increasingly emphasizing the adoption of spray coating technology. This technology is already a mature process and has been used in the chemical and processing industries for over thirty years. Moreover, the United States and Europe have more than fourteen years of extensive experience in using this technology in flue gas desulfurization equipment.
People are concerned that structural steel might dilute the weld metal, leading to poor quality in spot or plug welds. By applying an alloy overlay to plug welds, this latter issue can be effectively eliminated. As for arc spot welds, continuous application of filler metal to cover and overlay the weld metal during the completion of the welding process can effectively prevent weld dilution. Researchers have long been studying the thermal stress issues arising from the differing coefficients of thermal expansion between structural steel and nickel-containing materials; however, both experimental studies and practical experience have shown that, at typical service temperatures, this is not a significant problem.
 
9. Life-cycle cost
 
A life-cycle cost comparison was conducted for the various materials used in the scrubbing tower. The report titled “Life-Cycle Cost Advantages of Constructing Flue Gas Desulfurization Systems with Stainless Steel and Nickel-Based Alloys,” published by the Nickel Institute, provides detailed explanations.
The cost comparison involves a combined flue gas desulfurization scrubbing system composed of the following components: a Venturi tube made entirely of C-276 alloy, an absorption tower constructed from C-296 alloy composite panels, and a flue duct fabricated from C-276 alloy composite panels.
The results show that even when using the most expensive nickel-alloy material, the lifecycle cost is still lower than that of a similar system made from carbon steel with neoprene lining.
According to the data, different regions of the same equipment can utilize different alloys—not necessarily all C-276 alloy. This approach allows for further reduction of actual costs, depending on the specific circumstances. Moreover, using thin-sheet linings can further lower costs as well.
Therefore, it can be clearly stated that nickel-containing materials offer a highly cost-effective solution to corrosion problems in flue gas desulfurization equipment, resulting in low maintenance costs throughout the equipment’s entire service life and high equipment utilization rates.
 
10. Conclusion
 
Advantages of nickel-containing materials
It should be recognized that the corrosion resistance of nickel-containing materials is an intrinsic property of these materials. In general, nickel-chromium alloys containing molybdenum and other alloying elements possess a stable passivation film, which can effectively prevent corrosion in the harsh corrosive environments commonly encountered in flue gas desulfurization systems.
If the equipment’s temperature exceeds the design temperature, the nickel-containing materials used in the scrubbing tower of the flue gas desulfurization system can maintain their high-temperature strength, ensuring the structural integrity of the equipment without the need for an auxiliary cooling system, as is required in non-metallic systems.
For slurries with moderate abrasiveness, especially in corrosive environments, nickel-containing materials exhibit sufficient resistance to both erosion and corrosion. In wet-process systems, erosion and corrosion typically occur simultaneously rather than as two separate phenomena.
In the construction of scrubbing towers, the excellent mechanical properties of nickel-containing materials also represent a significant advantage. The vessel design can utilize thin-gauge materials with external reinforcement.
Nickel-containing materials exhibit excellent machinability, and they can be cut, formed, and joined using methods similar to those employed for carbon steel. However, it is essential to carefully preserve their inherent corrosion resistance. Moreover, factory processing is less costly than on-site processing, which can further reduce installation costs.