The influence of chlorine on the design of wet flue gas desulfurization processes
Thread Content
The impact of chlorine on the design of wet flue gas desulfurization processes I. Corrosion of the desulfurization system by chlorides In wet desulfurization systems, substances such as SO2, H2SO3, H2SO4, HCl, CaCO3, CaCl2, and CaSO4 are present; as a result, corrosion is a significant issue in these systems. However, SO2, H2SO3, H2SO4, and HCl react rapidly to form CaSO4 and CaCl2. Due to the very low solubility of CaSO4, the corrosive effect of Cl– becomes particularly severe. CaCl2 is highly soluble in water; at 30°C, its solubility can reach 102 g per 100 g of water. Due to the recirculation of water in the desulfurization system, chlorine gradually accumulates in the absorption slurry, with concentrations reaching up to 1%. When the CI content reaches 2%, most stainless steels can no longer be used; instead, chloroprene rubber, glass flake lining, or other corrosion-resistant materials should be employed. CI– is the main cause of pitting, crevice corrosion, stress corrosion, and selective corrosion in metals. II. Other effects of chlorides on the desulfurization system 1. Chloride ions have strong coordination capabilities; they can coordinate with metal ions to form complexes such as (AlCl2)+, (FeCl4)–, and (ZnCl4)2–. These complexes enclose Ca2+ or CaCO3 particles, reducing the amount of Ca2+ or CaCO3 available to participate in reactions. As a result, more inert substances are formed, which lowers the efficiency of desulfurization. 2. Chloride ions have a greater ability to penetrate compared to HSO3‑ or SO32‑; in other words, their diffusion coefficient is higher. CI‑ exerts a repulsive effect on HSO3‑ or SO32‑, thereby affecting the dissolution and reaction of SO2 and reducing the desulfurization efficiency. 3. The consumption of the absorbent increases as the chloride concentration rises; simultaneously, chlorides inhibit the dissolution of the absorbent. 4. The high chloride concentration in the absorption slurry leads to an increased amount of desulfurization residue in the gypsum (it is generally required that the excess desulfurization amount in gypsum be no more than 5%). 5. Chlorides cause difficulties in the subsequent dehydration of gypsum, resulting in an increased water content in the final gypsum product (the required water content for gypsum is generally <10%). 6. It affects the comprehensive utilization of gypsum. When gypsum is used as a cement retarder, strict requirements are placed on the chlorine content in gypsum, which generally needs to be less than 0.1%. Therefore, when the chloride content is high, additional dechlorination measures are required, which complicates the subsequent treatment processes and increases costs. 7. When the chloride content is high, the amount of inert substances in the absorption slurry that do not participate in the reaction increases, the relative density of the absorption slurry rises, and the electrical energy consumption of the slurry circulation system increases. The above analysis shows that although the chlorine content in coal is extremely low, it is a key factor in wet flue gas desulfurization. III. Sources and Emissions of Chlorine 1. Sources of chlorine: Chlorine is introduced from the flue gas imported from outside (with a chlorine content of about 0.1% in coal), process water (with a chlorine content of about 120 mg/L), and limestone (with a chlorine content of about 0.01%). Most of the chlorine in the desulfurization system comes from coal. Chlorine in coal exists mainly in inorganic forms, such as calcium chloride, potassium chloride, sodium chloride, magnesium chloride, etc. The chlorine content in coal used in our country is generally 0.1%, while in some types of coal it ranges from 0.2% to 0.3%. 2. Chlorine emission: Chlorine is emitted through the flue gas at the outlet (with a chlorine content of about 1 mg/Nm3), gypsum, and wastewater. IV. Simplified design of the desulfurization process under different assumptions 1. If comprehensive utilization of gypsum is not considered and the power plant adopts a hydraulic ash removal method: the gypsum slurry can be concentrated to a moisture content of 40%–50% in one stage before being sent to the ash dump. All chlorine present in the desulfurization system will be removed along with the gypsum slurry, so corrosion protection does not need to focus on chlorides. No vacuum dehydrator and wastewater treatment system will be designed. 2. Without considering the comprehensive utilization of gypsum, power plants adopt a dry ash removal method: the gypsum slurry can be concentrated in two stages to form gypsum cakes with a 20% moisture content, which are then transported to dry ash storage areas; the gypsum helps to remove the excess chlorine from the slurry. A vacuum dehydrator is designed, but a wastewater treatment system is not. 3. Comprehensive utilization of gypsum: the chlorine limit in gypsum is 0.1%; it is necessary to wash the gypsum in order to reduce its chlorine content, and vacuum dehydrators as well as wastewater treatment systems must be designed for this purpose. V. Based on the above analysis, the following conclusions can be drawn: 1. Chlorine exists mainly in the form of CaCl2 in flue gas wet desulfurization systems; it is difficult to remove from such systems, leading to its accumulation, which causes corrosion, affects the desulfurization efficiency, and complicates subsequent treatment processes. Therefore, chlorine must be taken into consideration during the design of desulfurization systems. 2. The selection of the gypsum treatment system and wastewater treatment system in the desulfurization system depends to a large extent on the chloride content in the absorption slurry and the fate of the gypsum. Factors such as the limit on chloride content in the absorption slurry, the degree of gypsum concentration, whether gypsum is washed with clean water, whether wastewater treatment is necessary, and the volume of wastewater to be treated must all be considered; the process parameters should be selected based on the specific conditions of each project, rather than simply adopting foreign models. 3. When designing the materials for the desulfurization system, a chloride concentration of 6% in the absorption slurry is taken into account, which meets both safety and economic requirements. Internationally, this is also generally used as the design limit. 4. The control of the chloride concentration in the absorption slurry during the operation of the desulfurization system can be determined based on the chlorine content in the coal; for low-sulfur coal, the chloride content in the absorption slurry can be controlled at 2% ； For high-sulfur coal, a concentration of 3% to 3.5% can be selected; the specific choice should be determined based on technical and economic considerations. 5. The chlorine content in domestic limestone is generally between 0.001% and 0.024%, and the chlorine content in the process water used in desulfurization systems is also around 0.01%. When performing simplified calculations of the chlorine balance in desulfurization systems and making decisions regarding the process design, the chlorine content in both the process water and limestone can be ignored to simplify the calculation process. 6. The investment in the gypsum dewatering treatment and wastewater treatment systems accounts for 1/5 to 1/4 of the total investment in the desulfurization system. Moreover, these systems require a large amount of space, have complex processes, and present difficulties in operation and management. Therefore, if the chlorine problem can be effectively resolved, this system can be simplified or eliminated, **reducing construction costs and operational management expenses.
Applications of Hastelloy C-276 and Hastelloy C-22 in FGD systems 1.0 Overview: China has experienced rapid development in recent years, but power shortages, particularly in developed regions, have become a bottleneck hindering economic growth. As a large number of coal-fired power plants are built in China, this issue will improve to some extent. The pollution problems caused by coal combustion have also become a focal point of public concern. **To this end, specific policies have been introduced, requiring thermal power plants in the two control areas to install desulfurization and denitrification devices. Since desulfurization units (FGD systems) are considered environmental protection investments that do not generate profits, non-metallic corrosion-resistant materials (paints, resin flakes, etc.) were widely used in the desulfurization units installed in the early stages to reduce initial costs. However, as environmental awareness grows among people, and due to the increasingly high maintenance costs and unplanned power outages associated with non-metallic anti-corrosion systems, more and more power plants are starting to use high-nickel corrosion-resistant alloys, particularly Hastelloy. This article will discuss the material selection, processing, and application examples of Hastelloy corrosion-resistant alloys. 2.1 Selection of Hastelloy corrosion-resistant alloys Throughout the entire FGD system, from the boiler exhaust inlet to the chimney outlet, where processes such as cleaning, demisting, and heat exchange take place, the degree of corrosion varies in different locations. Moreover, different process designs also affect the choice of materials for specific locations. Among them, the inlet area is recognized as the most corrosive location, and Hastelloy®C series alloys are typically used there. Hastelloy® S21 is a registered trademark of Haynes International Inc. in the United States, and is commonly referred to as Hastelloy in China. This is because the acid concentration at the wet-dry interface of the inlet is quite high, the pH value is very low, resulting in strong corrosivity. At the same time, since raw coal usually contains certain amounts of chlorides and fluorides, the flue gases generated during combustion contain chloride and fluoride ions, which can cause severe corrosion in iron-based alloys (super austenitic stainless steels, duplex steels, etc.). Therefore, Hastelloy is used in the inlet flues of desulfurization systems in the vast majority of power plants. Hastelloy C series alloys belong to the Ni-Cr-Mo alloy family. Looking at the development history of Ni-Cr-Mo alloys, Hastelloy C was introduced as early as the 1940s. Hastelloy C-276 was invented in the 1960s, but its corrosion resistance at the weld seams is significantly lower than that of the base material. In environments with high corrosivity, especially in cases of localized corrosion such as pitting and crevice corrosion, the weaknesses of the welded area become apparent. These conditions are more evident in the desulfurization systems of power plants with high sulfur and chlorine content in their raw coal. Therefore, after years of research and development, Haynes International Inc. in the United States invented the C-22 alloy in the 1980s. By adjusting its chemical composition, C-22 achieves a more stable crystal structure, enabling it to maintain excellent corrosion resistance even in a welded state. Of course, the base material of C-22®s19 also possesses superior corrosion resistance compared to C-276 (Figure 1). Especially in the environment of FGD systems, C-22 exhibits superior resistance to uniform corrosion and localized corrosion. Among the alloys invented in the 1990s, C-2000 has a high degree of alloying and excellent properties; it is the most modern alloy in the Hastelloy C series. The main components of various alloys are shown in Table 1. Economic considerations are taken into account in all engineering projects, and the same is true for FGD projects. Therefore, cost-performance becomes a key factor in material selection. Among the aforementioned Ni-Cr-Mo alloys, C-22 and C-276 have the same price, while the other newer Ni-Cr-Mo alloys are more expensive. At the same time, the specific gravity of C-22 (8.69 g/cm3) is slightly lower compared to that of C-276 (8.89 g/cm3). When using the same amount of material, the total cost of C-22 is lower. More importantly, the C-22 is widely used in FGD systems of foreign power plants and has demonstrated excellent performance. In some power plants in the United States, its service life is even twice that of C-276, and it has been in use for over a decade to date. Some experienced engineering companies in the country have also begun to recognize the advantages of C-22, and are using it to replace C-276 in domestic power plants. Therefore, C-22 alloy is an extremely cost-effective material for corrosion protection in FGD systems, and can fully replace C-276 alloy. Table 1: Major chemical components of the alloys – Ni, Cr, Mo, W, Fe, Si, C, and others.
Hastelloy®C-22®: Balance of elements: 22%, 13%, 3%, 3%; 0.08*, 0.010*; V: 0.35*.
Hastelloy®C-276: Balance of elements: 16%, 16%, 4%, 5%; 0.08*, 0.010*; V: 0.35*.
Hastelloy®C-2000®: Balance of elements: 23%, 16%; other elements: –; Cu: 1.6%, 2.2%.
Hastelloy sheet linings: Although Hastelloy offers superior corrosion resistance compared to other materials, its high cost often dissuades many users from using it. In many parts of the FGD system, stainless steel suffers severe corrosion, whereas Hastelloy may show less than 1 mm of corrosion even after ten years. It would be a great waste to use Hastelloy merely for the strength and stiffness of the equipment. At this point, some people choose to use explosive composite panels. However, since FGD systems usually operate under positive pressure or atmospheric pressure conditions, we can also apply thin-sheet lining technology. In this technique, thin sheets are welded to different substrates to form a leak-proof corrosion protection layer. This processing method is very simple; it requires no special tools or equipment, nor highly trained workers. The most important feature of this type of installation is that during seal welding, the welded metal generally does not suffer from a reduction in the degree of alloying of the weld due to the base material, as all seal welding takes place between alloys. Furthermore, this manufacturing technique **reduces the production time, as stacking thin sheets one by one is quite simple. We will use 1.6 mm thick Hastelloy®C-22 alloy sheets as an example to explain the key aspects of manufacturing thin-sheet lining technology: designing the installation scheme. As shown in Figure 3, the sheets used for lining are not joined together by splicing but rather by overlapping. Generally, we recommend a lap length of 25mm. Therefore, it must be taken into account when designing the installation plan and material layout. Additionally, at each corner, thin plates with matching shapes need to be designed ； At the end connected to the substrate, an expansion connector may need to be installed. Generally, the installation of the side walls and ceiling is completed first, followed by the floor. The welding process is developed in advance; it is the most critical aspect of this technology. GMAW (Metal Arc Welding) is the most commonly used welding process in this technology; GTAW (Tungsten Arc Welding) and SMAW (Shielded Metal Arc Welding) can also be used, but their welding efficiency is lower than that of GMAW, making them less suitable for welding large areas. The selection of shielding gas is an important factor in the development of welding processes. In automatic welding, the gas mixture of 90% helium + 7.5% argon + 2.5% carbon dioxide (A-1025) is commonly used. This shielding gas has a certain oxidizing property, but it provides high arc stability. Another shielding gas composition of 75% argon + 25% helium is non-oxidizing, results in smooth welds, but has low arc stability. The NiCoBrite™ shielding gas produced by Haynes International achieves a good balance in these two aspects, making it an excellent choice for synergistic systems. (For specific welding parameters, please contact Haynes International in the United States.) Cutting, drilling, and rough machining are carried out at the factory on the thin sheets; holes need to be drilled in these sheets for plug welding. The thin sheets at the edges and corners must be folded. If these tasks can be done at the factory, it will save a lot of time during on-site installation. Since the thin plates are stacked, there is no need to align the joints. Sealing welding can be achieved by stacking them crosswise, without the need for precise cutting of the alloy sheets. With a pre-formed sheet having one or two 90-degree corners, it can be conveniently installed on the ceiling or floor, as shown in Figure 4, thereby eliminating the problem of installing it using edges. Intermittent fillet welds and attachments in the center of the plate allow the thin plates to be fixed to the ceiling, while the seal welds on the side walls begin at the completed edges. Among them is a thin plate with a 90-degree bend, on which holes are drilled to install expansion joint sealers. Under normal conditions, the ductility of C-22®s21 sheets is very good (with an elongation at break of up to 50%). In this way, cold forming of the components is also no problem. No cracking or tearing occurs even when the bending radius reaches twice the wall thickness. Generally, cold-formed thin sheets should be installed under cold-working conditions. Any hot working of alloy sheets must be followed by solution treatment to restore the material’s corrosion resistance. Substrate surface treatment: For used equipment, the substrate must be thoroughly cleaned before attaching the alloy sheet. Generally, the corroded surface of the substrate is first abraded with sandblasting, and then cleaned with water. Sandblasting and cleaning can open up corrosion pits and wash away corrosion products and dirt. In a new device, sandblasting or gentle grinding of the substrate is generally used to remove heavy rust or oxide layers. These are applied only to the areas where the alloy and the base material are welded. The thin plate is attached to the substrate; during the installation process, we can observe two distinct welding states. The first is a thin plate structure attached to the substrate. Next, the alloy thin sheet is given a sealed structure through peripheral seal welding. The thin plate and the substrate are attached through a intermittent welding structure. These welds are generally 25 mm long, with a spacing of 150 mm. (See Figure 3) It should be noted that the structural strength of all lining systems derives from the intermittent fillet welds between the thin plates and the substrate. The strength of structural welding is shown in Tables 2 and 3. Therefore, high-quality welding is necessary. The best is a welding technique that allows for seamless welding to the substrate. If this cannot be achieved, it is recommended that the fillet weld be level with the bottom surface of the alloy sheet before bonding the substrates. This helps to maintain a tight fit between the thin plates. Because additional thin plates need to be installed, the overlap between the plates is nominally 25 mm. To fix a position in structural fillet welding, fixed welds between alloys are required. Before welding, the base material and alloy sheets generally need to be cleaned to remove grease, oil, corrosion products, scale/sediments, water, and other contaminants. At least operations similar to welding stainless steel are required. When processing Hastelloy sheets, it is not necessary to use stainless steel tools and cutters. Stainless steel wire brushes are generally used in welding and routine cleaning operations. The attachment at the center of the plate provides the lining system with additional structural strength and stiffness through the structural welding between the thin plate and the substrate. In many installations, a row of plug holes is welded along the 1219 mm wide sheet, at a position 600 mm from the center. Under positive pressure, the welding joint of these additional structures is not mandatory and is determined by the designer. Generally, there are the following 3 methods for socket welding. These methods are: first, direct plug welding ； Second, apply a layer of welding metal over the seaming weld ； Third, use seal welding to weld a cover plate onto the plug hole. The corrosion resistance obtained using these methods is shown in Table 4. Full-seal welding: After the alloy-lined structure is attached to the substrate, seal welding must be performed on the overlapping plates. Seal welding not only provides structural strength but also creates a leak-free system. As mentioned in the previous sections, the overlapping thin plates must be secured with reliable fixing welds so that the structural fillet welds can be completed. Before seal welding, additional fixing welding is required to ensure that the two overlapping thin plates are closely fitted together. These fixed welds are usually very small, about 6 mm long, with a spacing of 75 mm. An excessive gap or the space between the two overlapping thin plates increases the likelihood of damage to the seal weld, which may ultimately lead to system leakage. It is essential to grind and thin these fixing welds before performing continuous seal welding. For fixed welding, it is sufficient to ensure a tight fit between the thin sheets during seal welding. The larger these welds are, the more difficult it is to achieve a secure seal. The beginning and end of the seal weld also need to be ground and shaved thin. It should be emphasized that corrosion in sealed welds usually occurs at the beginning, end, or where the weld is secured. Careful development of welding procedures can significantly reduce repair costs. Inspection: Non-Destructive Testing (NDE) is the most important part of this manufacturing process. It is important to note that seal welding can only be performed once; therefore, any insufficient penetration or other welding defects may lead to leaks in the lining system. It is recommended to install holes in the substrate through which water can drip, so that leaks in the lining system can be detected and repaired promptly. Visual inspection can detect 80-90% of welding defects. It is recommended to conduct a thorough visual inspection before using NDE testing; repairing the suspected weld seam is quick, and there is no need to worry about contamination from NDE testing fluids (dye penetrants or vacuum box soaps). Currently, the vacuum leakage testing method seems to be the most accurate and cost-effective (due to its widespread use) testing method. This is because the test has high sensitivity (the ability to detect leaks) and operates at a fast speed. Liquid dye penetrant inspection is another alternative method that can be used in small areas that cannot be accessed with a vacuum box. Repairing GTAW, GMAW, or SMAW welding defects can be accomplished using repair welding processes. The GTAW welding process is considered the most suitable. Repair using the GTAW welding process can involve the use of filler material or not, depending on the length of the defect. Small defects with low penetration depth can be remelted and washed away without any problem. Large areas that need grinding may require additional welding material. Figures 3 and 4: Shear strength of structural fillet welds. Welding material: HASTELLOY C-22; Base material: carbon steel. Welding position: fillet weld. Weld length: ¾” to 1”. Welding method: flat welding. Shear force (pounds): 6000, 6900, 7200. Average of the two tests: 2.3. Application example: Desulfurization is still in its infancy in China; although many power plants have installed FGD systems, they have only been in operation for a few years. In contrast, foreign countries began constructing and operating desulfurization projects in the 1970s and 1980s, thus possessing extensive practical experience. Our country has adopted a great deal of technology from foreign engineering companies. Many engineering design firms have improved further while learning from foreign technologies, making adjustments tailored to the actual conditions of domestic power plants, and have achieved considerable results as a result. In this regard, it is necessary to draw on some experiences from the construction and operation of power plants abroad. The following are some material replacement records from power plants abroad for reference by designers. 400,000-kilowatt units: coal with 2.3% sulfur content; Bawe technology. 625,000-kilowatt units: coal with sulfur content of 1.0–3.6%; Peabody technology. FGD systems were put into operation in 1981 and again in 1984. The original design for the bypass reheat area used calcium aluminate refractory bricks; the original design for flues used carbon steel with glass flake coating. The original design for unit control dampers used stainless steel, while the seals for these dampers were made of 625 alloy. The original design for outlets used carbon steel with resin flake coating, and the expansion connectors were also made of 625 alloy. In 1982, calcium aluminate refractory bricks were replaced with potassium silicate concrete. In 1985, the glass flake coating developed bubbles, so it was repainted. In the same year, stainless steel dampers were replaced with C-276 alloy. In 1985, the 625 alloy seals became corroded and damaged, so they were replaced with C-276 alloy. In 1986, potassium silicate concrete was replaced with C-276 and C-22 alloys. In 1988, some of the damaged flake coatings were replaced using C-22 and C-276 alloys. In 1987, resin flakes were replaced with C-22 alloy linings. In 1989, C-276 thin plates were welded together using C-22 welding material to replace the damaged flake coatings. In total, 23 tons of alloy materials, including C-276 and C-22, were used. A total of 6 tons of alloy materials, including C-276 and C-22, were also used. 3.0 Summary: 1. Hastelloy C series alloys are widely used in power plant desulfurization systems (FGD systems). 2. Hastelloy C-22 alloy performs excellently as a corrosion-resistant material in such systems, offering good cost-effectiveness, and can serve as a substitute for Hastelloy C-276 alloy. 3. Thin plate lining technology, being a relatively economical anti-corrosion solution, has been adopted by many power plants around the world
Hastelloy®C-22®: Balance of elements: 22%, 13%, 3%, 3%; 0.08*, 0.010*; V: 0.35*.
Hastelloy®C-276: Balance of elements: 16%, 16%, 4%, 5%; 0.08*, 0.010*; V: 0.35*.
Hastelloy®C-2000®: Balance of elements: 23%, 16%; other elements: –; Cu: 1.6%, 2.2%.
Hastelloy sheet linings: Although Hastelloy offers superior corrosion resistance compared to other materials, its high cost often dissuades many users from using it. In many parts of the FGD system, stainless steel suffers severe corrosion, whereas Hastelloy may show less than 1 mm of corrosion even after ten years. It would be a great waste to use Hastelloy merely for the strength and stiffness of the equipment. At this point, some people choose to use explosive composite panels. However, since FGD systems usually operate under positive pressure or atmospheric pressure conditions, we can also apply thin-sheet lining technology. In this technique, thin sheets are welded to different substrates to form a leak-proof corrosion protection layer. This processing method is very simple; it requires no special tools or equipment, nor highly trained workers. The most important feature of this type of installation is that during seal welding, the welded metal generally does not suffer from a reduction in the degree of alloying of the weld due to the base material, as all seal welding takes place between alloys. Furthermore, this manufacturing technique **reduces the production time, as stacking thin sheets one by one is quite simple. We will use 1.6 mm thick Hastelloy®C-22 alloy sheets as an example to explain the key aspects of manufacturing thin-sheet lining technology: designing the installation scheme. As shown in Figure 3, the sheets used for lining are not joined together by splicing but rather by overlapping. Generally, we recommend a lap length of 25mm. Therefore, it must be taken into account when designing the installation plan and material layout. Additionally, at each corner, thin plates with matching shapes need to be designed ； At the end connected to the substrate, an expansion connector may need to be installed. Generally, the installation of the side walls and ceiling is completed first, followed by the floor. The welding process is developed in advance; it is the most critical aspect of this technology. GMAW (Metal Arc Welding) is the most commonly used welding process in this technology; GTAW (Tungsten Arc Welding) and SMAW (Shielded Metal Arc Welding) can also be used, but their welding efficiency is lower than that of GMAW, making them less suitable for welding large areas. The selection of shielding gas is an important factor in the development of welding processes. In automatic welding, the gas mixture of 90% helium + 7.5% argon + 2.5% carbon dioxide (A-1025) is commonly used. This shielding gas has a certain oxidizing property, but it provides high arc stability. Another shielding gas composition of 75% argon + 25% helium is non-oxidizing, results in smooth welds, but has low arc stability. The NiCoBrite™ shielding gas produced by Haynes International achieves a good balance in these two aspects, making it an excellent choice for synergistic systems. (For specific welding parameters, please contact Haynes International in the United States.) Cutting, drilling, and rough machining are carried out at the factory on the thin sheets; holes need to be drilled in these sheets for plug welding. The thin sheets at the edges and corners must be folded. If these tasks can be done at the factory, it will save a lot of time during on-site installation. Since the thin plates are stacked, there is no need to align the joints. Sealing welding can be achieved by stacking them crosswise, without the need for precise cutting of the alloy sheets. With a pre-formed sheet having one or two 90-degree corners, it can be conveniently installed on the ceiling or floor, as shown in Figure 4, thereby eliminating the problem of installing it using edges. Intermittent fillet welds and attachments in the center of the plate allow the thin plates to be fixed to the ceiling, while the seal welds on the side walls begin at the completed edges. Among them is a thin plate with a 90-degree bend, on which holes are drilled to install expansion joint sealers. Under normal conditions, the ductility of C-22®s21 sheets is very good (with an elongation at break of up to 50%). In this way, cold forming of the components is also no problem. No cracking or tearing occurs even when the bending radius reaches twice the wall thickness. Generally, cold-formed thin sheets should be installed under cold-working conditions. Any hot working of alloy sheets must be followed by solution treatment to restore the material’s corrosion resistance. Substrate surface treatment: For used equipment, the substrate must be thoroughly cleaned before attaching the alloy sheet. Generally, the corroded surface of the substrate is first abraded with sandblasting, and then cleaned with water. Sandblasting and cleaning can open up corrosion pits and wash away corrosion products and dirt. In a new device, sandblasting or gentle grinding of the substrate is generally used to remove heavy rust or oxide layers. These are applied only to the areas where the alloy and the base material are welded. The thin plate is attached to the substrate; during the installation process, we can observe two distinct welding states. The first is a thin plate structure attached to the substrate. Next, the alloy thin sheet is given a sealed structure through peripheral seal welding. The thin plate and the substrate are attached through a intermittent welding structure. These welds are generally 25 mm long, with a spacing of 150 mm. (See Figure 3) It should be noted that the structural strength of all lining systems derives from the intermittent fillet welds between the thin plates and the substrate. The strength of structural welding is shown in Tables 2 and 3. Therefore, high-quality welding is necessary. The best is a welding technique that allows for seamless welding to the substrate. If this cannot be achieved, it is recommended that the fillet weld be level with the bottom surface of the alloy sheet before bonding the substrates. This helps to maintain a tight fit between the thin plates. Because additional thin plates need to be installed, the overlap between the plates is nominally 25 mm. To fix a position in structural fillet welding, fixed welds between alloys are required. Before welding, the base material and alloy sheets generally need to be cleaned to remove grease, oil, corrosion products, scale/sediments, water, and other contaminants. At least operations similar to welding stainless steel are required. When processing Hastelloy sheets, it is not necessary to use stainless steel tools and cutters. Stainless steel wire brushes are generally used in welding and routine cleaning operations. The attachment at the center of the plate provides the lining system with additional structural strength and stiffness through the structural welding between the thin plate and the substrate. In many installations, a row of plug holes is welded along the 1219 mm wide sheet, at a position 600 mm from the center. Under positive pressure, the welding joint of these additional structures is not mandatory and is determined by the designer. Generally, there are the following 3 methods for socket welding. These methods are: first, direct plug welding ； Second, apply a layer of welding metal over the seaming weld ； Third, use seal welding to weld a cover plate onto the plug hole. The corrosion resistance obtained using these methods is shown in Table 4. Full-seal welding: After the alloy-lined structure is attached to the substrate, seal welding must be performed on the overlapping plates. Seal welding not only provides structural strength but also creates a leak-free system. As mentioned in the previous sections, the overlapping thin plates must be secured with reliable fixing welds so that the structural fillet welds can be completed. Before seal welding, additional fixing welding is required to ensure that the two overlapping thin plates are closely fitted together. These fixed welds are usually very small, about 6 mm long, with a spacing of 75 mm. An excessive gap or the space between the two overlapping thin plates increases the likelihood of damage to the seal weld, which may ultimately lead to system leakage. It is essential to grind and thin these fixing welds before performing continuous seal welding. For fixed welding, it is sufficient to ensure a tight fit between the thin sheets during seal welding. The larger these welds are, the more difficult it is to achieve a secure seal. The beginning and end of the seal weld also need to be ground and shaved thin. It should be emphasized that corrosion in sealed welds usually occurs at the beginning, end, or where the weld is secured. Careful development of welding procedures can significantly reduce repair costs. Inspection: Non-Destructive Testing (NDE) is the most important part of this manufacturing process. It is important to note that seal welding can only be performed once; therefore, any insufficient penetration or other welding defects may lead to leaks in the lining system. It is recommended to install holes in the substrate through which water can drip, so that leaks in the lining system can be detected and repaired promptly. Visual inspection can detect 80-90% of welding defects. It is recommended to conduct a thorough visual inspection before using NDE testing; repairing the suspected weld seam is quick, and there is no need to worry about contamination from NDE testing fluids (dye penetrants or vacuum box soaps). Currently, the vacuum leakage testing method seems to be the most accurate and cost-effective (due to its widespread use) testing method. This is because the test has high sensitivity (the ability to detect leaks) and operates at a fast speed. Liquid dye penetrant inspection is another alternative method that can be used in small areas that cannot be accessed with a vacuum box. Repairing GTAW, GMAW, or SMAW welding defects can be accomplished using repair welding processes. The GTAW welding process is considered the most suitable. Repair using the GTAW welding process can involve the use of filler material or not, depending on the length of the defect. Small defects with low penetration depth can be remelted and washed away without any problem. Large areas that need grinding may require additional welding material. Figures 3 and 4: Shear strength of structural fillet welds. Welding material: HASTELLOY C-22; Base material: carbon steel. Welding position: fillet weld. Weld length: ¾” to 1”. Welding method: flat welding. Shear force (pounds): 6000, 6900, 7200. Average of the two tests: 2.3. Application example: Desulfurization is still in its infancy in China; although many power plants have installed FGD systems, they have only been in operation for a few years. In contrast, foreign countries began constructing and operating desulfurization projects in the 1970s and 1980s, thus possessing extensive practical experience. Our country has adopted a great deal of technology from foreign engineering companies. Many engineering design firms have improved further while learning from foreign technologies, making adjustments tailored to the actual conditions of domestic power plants, and have achieved considerable results as a result. In this regard, it is necessary to draw on some experiences from the construction and operation of power plants abroad. The following are some material replacement records from power plants abroad for reference by designers. 400,000-kilowatt units: coal with 2.3% sulfur content; Bawe technology. 625,000-kilowatt units: coal with sulfur content of 1.0–3.6%; Peabody technology. FGD systems were put into operation in 1981 and again in 1984. The original design for the bypass reheat area used calcium aluminate refractory bricks; the original design for flues used carbon steel with glass flake coating. The original design for unit control dampers used stainless steel, while the seals for these dampers were made of 625 alloy. The original design for outlets used carbon steel with resin flake coating, and the expansion connectors were also made of 625 alloy. In 1982, calcium aluminate refractory bricks were replaced with potassium silicate concrete. In 1985, the glass flake coating developed bubbles, so it was repainted. In the same year, stainless steel dampers were replaced with C-276 alloy. In 1985, the 625 alloy seals became corroded and damaged, so they were replaced with C-276 alloy. In 1986, potassium silicate concrete was replaced with C-276 and C-22 alloys. In 1988, some of the damaged flake coatings were replaced using C-22 and C-276 alloys. In 1987, resin flakes were replaced with C-22 alloy linings. In 1989, C-276 thin plates were welded together using C-22 welding material to replace the damaged flake coatings. In total, 23 tons of alloy materials, including C-276 and C-22, were used. A total of 6 tons of alloy materials, including C-276 and C-22, were also used. 3.0 Summary: 1. Hastelloy C series alloys are widely used in power plant desulfurization systems (FGD systems). 2. Hastelloy C-22 alloy performs excellently as a corrosion-resistant material in such systems, offering good cost-effectiveness, and can serve as a substitute for Hastelloy C-276 alloy. 3. Thin plate lining technology, being a relatively economical anti-corrosion solution, has been adopted by many power plants around the world