[Weekly Topic] Discuss what measures can be taken to prevent low-temperature corrosion of flue gas during boiler operation (November 15---November 21)

Reply to 1# dongjiang1001: Corrosion prevention on the flue gas side of boilers. Methods to prevent low-temperature corrosion: (1) Low-oxygen combustion； (2) Additives can be used: dolomite powder can be added to fuel, and ammonia can be added when burning high-sulfur coal in coal-fired boilers for extended periods; the temperature at which ammonia is added should be between 200 and 600 degrees, and sufficient amounts of ammonia must be used ； (3) Raise the wall temperature of the heating surface so that it remains above the dew point of the flue gas. Hot air recirculation and warm air heaters are generally used to increase the air temperature at the inlet of the air preheater. (4) Corrosion-resistant metal or non-metallic materials, as well as non-metallic cladding materials, are used to improve the corrosion resistance of the heated surfaces. Such as borosilicate heat-resistant glass replacing steel pipes, etc. Prevention of sulfur corrosion: 1. To prevent corrosion of the water wall, method A is to improve combustion conditions – the excess coefficient should not be too low – and to avoid contact between tube walls. B: Control the tube wall temperature to prevent scaling inside the tubes and excessive local heat load on the water wall. C: Introduce air to create an oxidative gas film along the furnace walls, thereby diluting the SO2 concentration in the flue gas and ensuring that substances decomposed from the deposit layer diffuse outward rather than inward. To prevent high-temperature corrosion on the smoke side of the superheater and reheater, there is A: limiting the parameters of the superheated steam. B: When arranging the superheater, care should be taken to ensure that the steam outlet section is not located in areas where the flue gas temperature is too high. Corrosion protection on the water side of boilers – Methods for preventing oxygen-induced corrosion. In heat recovery systems and pipelines before deoxygenation, corrosion-resistant metal materials or protective coatings should be used. Appropriate deaerators should be selected; Table 1 lists three commonly used types of deaerator equipment (namely, vacuum deaerators, open-type deaerators, and wet-type deaerators), among which wet-type deaerators are widely used in high-pressure boilers. Chemical deoxygenation method is employed. Depending on the operating pressure and temperature, an appropriate deoxidizer should be selected. For high-pressure boilers, it is advisable to use organic deoxidizers that do not increase the amount of dissolved solids; hydrazine (N2H4) is commonly used in such cases. For medium- and low-pressure boilers, sodium sulfite can be employed. Due to the high toxicity of hydrazine, at present organic deoxidizers are gradually being used to replace hydrazine with less toxic alternatives, such as p-cyclohexylphenol, cyclohexylamine, ethylenediamine, hydroquinone, and carbohydrazide. Ammonia (NH3) treatment of feed water. Through ammonia addition, the pH value of the feedwater is increased; it is advisable to keep the pH value within the range of 8.5 to 9.2. Methods for preventing and controlling corrosion under deposits: Before a newly installed boiler is put into service, it must undergo chemical cleaning. After the boiler is put into operation, it must be cleaned regularly to promptly remove deposits and corrosion products from the metal pipe walls. Reduce the ion content of copper and iron in the feed water. ? Carry out proper protection measures for the boiler when it is not in use to prevent corrosion during that period; this helps avoid the accumulation of corrosive substances on the metal surface of the boiler tubes, as well as prevents an increase in iron content in the boiler water during operation due to corrosion products formed while the boiler is idle. Improve the quality of the feedwater to minimize its corrosive components as much as possible. Select an appropriate boiler water treatment method to adjust the quality of the boiler water. Methods for preventing acidic corrosion: Install continuous water quality monitoring equipment to keep track of changes in water quality at all times, so that necessary actions can be taken promptly. Add neutralizing amines, etc., to suppress the occurrence of acidic corrosion. Control the ratio of Na to PO4 to prevent acidic corrosion. Methods for preventing alkaline corrosion: strictly control the PH/PO4 ratio ； Use an appropriate dispersant to prevent the formation of deposits ； Control the free alkali in the boiler water to prevent the formation of local high concentrations of free alkali. Methods to prevent alkaline embrittlement? Use welding instead of riveting or expansion joining for boilers ； Maintain the relative alkalinity of boiler water at less than 0.2 (Relative alkalinity = amount of free NaOH / total salt content) ； Select a suitable method for treating water inside the boiler; regulate the levels of pH and PO4. As for preventing corrosion caused by copper-ammonia compounds, ensure that there is an appropriate residual amount of deoxidizer present, so as to guarantee the absence of dissolved oxygen and prevent the aforementioned reactions from occurring. Select deoxidizers and neutralizing amines with good thermal stability to keep free ammonia below 0.3 ppm in saturated steam. Disabling corrosion prevention methods? Preventing air from entering the boiler’s steam system. Methods such as nitrogen filling and protected steam pressure methods can be employed. Reduce the humidity inside the moisture system. Methods such as drying and desiccant methods can be used. Add a corrosion inhibitor to form a protective film on the metal surface, or add an oxygen scavenger to remove dissolved oxygen from the water. Methods for preventing erosion-corrosion: In the design, minimize the number of bends and turbulent flow areas as much as possible, and optimize the locations of the deaerator and feed water pump.

Prevention of sulfur corrosion: 1. To prevent corrosion of the water wall, method A is to improve combustion conditions – the excess coefficient should not be too low – and to avoid contact between tube walls. B: Control the tube wall temperature to prevent scaling inside the tubes and excessive local heat load on the water wall. C: Introduce air to create an oxidative gas film along the furnace walls, thereby diluting the SO2 concentration in the flue gas and ensuring that substances decomposed from the deposit layer diffuse outward rather than inward. To prevent high-temperature corrosion on the smoke side of the superheater and reheater, there is A: limiting the parameters of the superheated steam. B: When arranging the superheater, care should be taken to ensure that the steam outlet section is not located in areas where the flue gas temperature is too high. Methods to prevent low-temperature corrosion: (1) low-oxygen combustion ； (2) Additives can be used: dolomite powder can be added to fuel, and ammonia can be added when burning high-sulfur coal in coal-fired boilers for extended periods; the temperature at which ammonia is added should be between 200 and 600 degrees, and sufficient amounts of ammonia must be used ； (3) Raise the wall temperature of the heating surface so that it remains above the dew point of the flue gas. Hot air recirculation and warm air heaters are generally used to increase the air temperature at the inlet of the air preheater. (4) Corrosion-resistant metal or non-metallic materials, as well as non-metallic cladding materials, are used to improve the corrosion resistance of the heated surfaces. For example, borosilicate heat-resistant glass can be used to prevent corrosion on the flue gas side of boilers: under the same potential difference, the corrosion rate of molten salts is higher. In molten salt systems, it is often the migration rate of the oxidant that becomes the controlling step for the overall corrosion.

1. Start with the material; ND steel can be used； 2. Structurally, a radial heat pipe economizer can be used, with a flue gas temperature of around 85 degrees Celsius.

Reply to 1# dongjiang1001: Firstly, I believe that low-temperature corrosion is an inevitable problem in boilers. Unless desulfurization is carried out prior to combustion, low-temperature corrosion will inevitably occur during actual operation. This is a piece of corroded pipe I took from a waste heat boiler in an aluminum plant previously. There’s also its scanning electron microscope image. The third image shows a relatively high magnification level; it can be seen that corrosion has penetrated into the pipe wall, forming a complex transition zone with the pipe itself. The corroded layer on the surface of the pipe wall tends to flake off easily. During actual operation, this corroded layer continuously peels off, causing further corrosion of the pipe. Eventually, the pipe becomes thinner and thinner, ultimately leading to bursting or leakage. The main component of the corrosion layer is ferrous sulfate with crystallization water. Firstly, low-temperature corrosion is closely related to the sulfur content in the fuel; it is therefore essential to control this sulfur content. As a solution, blending fuels can be employed during boiler combustion – using coal with low sulfur content helps to control SO2 concentrations and reduces the formation of sulfuric acid vapor in the economizer and air preheater sections. The anti-corrosion material layer allows anti-corrosion materials to be applied to surfaces exposed to low temperatures; this approach seems to work well. Personally, I think this added solution isn’t very feasible. Firstly, the addition of additives affects the boiler’s ignition and stable combustion, which may impact its output. Secondly, the wear on the heating surfaces caused by these additives must be taken into account, plus there is also the issue of cost. Furthermore, it is essential to increase the intensity of soot blowing, so as to minimize the adhesion of fly ash particles to the pipe walls. This helps maintain a relatively high temperature on the pipe walls, preventing the condensation of sulfuric acid vapor; this approach can also yield fairly good results. These are just my personal opinions; please feel free to correct me.

This post was last edited by dongjiang1001 on 2011-11-16 at 14:11: 1. Raise the temperature of the metal walls of the air preheater； 2. Use a heat-pipe air preheater ； 3. Use corrosion-resistant materials ； 4. Use low-oxygen combustion ； 5. Use additives to lower the dew point or inhibit corrosion.

The last edit to this post was made by zgj2405 on 2011-11-20 at 13:53. To make full use of the waste heat from the flue gases, reduce the flue gas temperature, and improve the thermal efficiency of boilers, air preheaters are installed at the rear of industrial boilers. However, as an air preheater at the rear of a boiler, it is typically located in an area with low-temperature flue gas containing water vapor and sulfuric acid vapor; its operating conditions are quite harsh, making it prone to low-temperature corrosion and ash blockage. In the low-temperature area of the boiler, air preheaters are prone to low-temperature corrosion and ash deposition; these issues can lead to blockages in the flue gas passages, an increase in draft resistance, and positive-pressure combustion in the boiler. This not only reduces the boiler’s output but can even lead to forced shutdowns. The result of corrosion is leakage and damage to the air preheater tubes, leading to severe air leakage and a deterioration in the combustion conditions. In severe cases, the heating surfaces have to be replaced frequently, which not only increases the amount of maintenance work and material consumption but also affects the normal operation of the boiler. The entry of cold air into the flue gas side reduces the flue gas temperature, accelerating low-temperature corrosion and ash deposition, thereby affecting the safe operation of the boiler. This article explains the mechanism of low-temperature corrosion in boiler air preheaters and the preventive measures. Keywords: boiler, fuel, air preheater, corrosion mechanism. There are two reasons for the low-temperature corrosion of the heating surfaces at the rear part of the boiler: one is the presence of sulfur trioxide in the flue gas ； Second, the metal wall temperature of the heated surface is lower than the acid dew point temperature in the flue gas. Boiler fuel contains more or less sulfur. When burning fuel with a high sulfur content, most of the sulfur in the fuel is converted into sulfur dioxide after combustion; under certain conditions, a small portion of it is further oxidized into sulfur trioxide gas. Sulfur trioxide gas can combine with water vapor to form sulfuric acid vapor. Its condensation dew point temperature exceeds 120°C. The higher the dew point temperature, the greater the acid content in the flue gas, and the more severe the corrosion and ash fouling become. When the wall temperature of the air preheater is lower than the dew point of the generated sulfuric acid, the sulfuric acid condenses on the pipe walls, causing corrosion known as low-temperature corrosion. The degree of corrosion of the metal wall depends on the amount of sulfuric acid condensation, the concentration, and the temperature of the metal wall. Sulfuric acid acts like a film; it adheres to the tube wall and causes corrosion there, while at the same time it continues to accumulate soot, forming various sulfates. Over time, this layer becomes thicker – and this is what constitutes low-temperature slag formation. 2 Minimum allowable temperature of the air preheater tube wall: The sulfur content in coal affects the determination of the boiler’s flue gas temperature. At the same time, taking into account factors such as heat loss from boiler exhaust gases and the prevention of low-temperature corrosion in the rear heating surfaces, the designed exhaust temperature for boilers equipped with air preheaters is currently generally between 160 and 190°C. In fact, due to large fluctuations in load when certain units use steam, or operation at low load for extended periods, improper operations occur, resulting in an excessive amount of excess air ； Due to equipment malfunction and failure to clean the dust in a timely manner, the flue gas temperature remains below 140°C for an extended period, that is, below the dew point of the flue gas. From the perspective of the overall flue gas flow in the furnace, the flue gas passage of the air preheater has a small cross-sectional area and high resistance, which increases the likelihood of ash accumulation and slag formation. When loose ash adheres inside the tubes for an extended period of time, it may change from a loose form to a compact one, as some of the ash can absorb sulfur dioxide, sulfur trioxide, and water vapor from the flue gas, resulting in the formation of sulfates and sulfites. It is the formation of these salts that causes the loose ash to transform into compact ash. These dust deposits react with the inner wall of the air preheater to form ferric sulfate and ferrous sulfate, further increasing the stability of the dust deposits and sludge. The aforementioned changes in the dust accumulation properties first occur on the inner wall of the tubes at the cold end of the counterflow air preheater (on the inlet side). This is because at this location, where low-temperature air exchanges heat with low-temperature flue gas, the tube wall temperature is low; as a result, corrosion and dust accumulation tend to spread gradually from the cold end of the tubes toward the hot end, and they often accumulate in the dead corners where the flue gas flow velocity is lower. When a boiler is started and stopped frequently, and the ash and scale accumulation is not removed in a timely manner, the rate of corrosion and ash accumulation inevitably increases. 3 Prevention and control measures To prevent low-temperature corrosion and ash deposition in air preheaters, measures can be taken from four aspects: 3.1 Regarding fuel and combustion products: Sulfur can be removed from the fuel and flue gas to prevent the formation of sulfur trioxide, thereby reducing the dew point temperature of the flue gas. ①The fundamental solution is to remove sulfur from fuel and flue gas; however, at present the technology is not yet mature, making practical application very difficult. The sulfur content in coal used for industrial boilers is usually between 1% and 1.5%, with some types reaching 3% to 5%; therefore, boilers should avoid using coal with a sulfur content higher than 2%. ②During boiler operation, minimizing the excess air volume and reducing the excess oxygen in the flue gases can significantly decrease the formation of sulfur trioxide. Consequently, the dew point temperature of the flue gases also drops, which in turn reduces the likelihood of corrosion on the low-temperature heating surfaces. Under normal circumstances, the critical value of the excess air coefficient in the combustion chamber is approximately 1.05; values lower than this significantly help reduce low-temperature corrosion. 3.2 Regarding boilers: Methods such as increasing the wall temperature of the low-temperature heating surfaces or positioning the wall temperatures away from areas where the flue gases cause severe corrosion are employed. ①Appropriately increase the flue gas temperature; raising the boiler’s flue gas temperature can correspondingly raise the wall temperature of the air preheater. For most fuels, a wall temperature of 105°C is required to prevent or reduce corrosion. This can be achieved by increasing the inlet air temperature of the air preheater or raising the water temperature at the inlet of the economizer. ②To reduce or avoid operation of the boiler at low or high loads, operating the boiler at low load inevitably causes the flue gas temperature to drop below the dew point of the flue gases, leading to corrosion of the air preheater tube walls. When the boiler operates under overload conditions, both the coal feed rate and the exhaust gas volume increase accordingly. The preheater finds it difficult to cope with this sudden increase in the amount of dust and smoke; as a result, the resistance of the flue gases rises, leading to ash accumulation and blockages inside the pipes. Due to the use of coal with high sulfur content and prolonged operation at overload conditions, severe corrosion in the lower sections of the air preheater as well as perforations leading to air leakage can easily occur. 3.3 Changing the layout of the heated surfaces: ① Using a horizontal tubular air preheater. Horizontal tubular air preheater, with flue gas flowing outside the tubes and air flowing inside them. Compared to vertical types, horizontal types can generally increase the wall temperature by 10–30°C at the same flue gas and air inlet temperatures. ②Change the heat transfer method. In common air preheaters, a counter-current arrangement is generally used in order to achieve a higher temperature of the preheated air with a smaller heating surface area. To prevent low-temperature corrosion in the air preheater, counter-current heat transfer can be changed to forward-flow heat transfer, or a sequence of forward-flow and then counter-current heat transfer can be used. Both can correspondingly increase the metal wall temperature in the low-temperature section of the air preheater. 3.4 Strengthen the cleaning of air preheaters; understand the patterns of ash accumulation and carry out regular cleaning. It can increase the smoke flow area, reduce smoke resistance, and accordingly decrease corrosion of the heated surfaces. When cleaning dust from pipes, they can be soaked in 5% alkaline solution and then rinsed with clean water. To reduce pipe blockages, increasing the pipe diameter can also yield satisfactory results. 4 Conclusion There are many methods to prevent corrosion and ash accumulation in air preheaters; those mentioned above are merely the common approaches currently used to avoid low-temperature corrosion in the rear heating sections of boilers. The specific method to be adopted should depend on the conditions of each individual facility.

Prevention of low-temperature corrosion: 1. Increase the temperature of the air preheater tube walls so that it is higher than the dew point of the flue gas. For example, increase the flue gas temperature, enable hot air recirculation, and use a preheater to raise the inlet temperature of the air preheater. The advantage of this method is its simplicity, while the disadvantage is the reduced efficiency of the boiler. 2. Additives are introduced into the flue gas to neutralize SO3 and prevent the formation of sulfuric acid vapor. The advantage of this method is that it does not reduce boiler efficiency, while the disadvantages are increased operating costs and the need to remove the products formed through neutralization. 3. Air preheaters should be made from corrosion-resistant materials, such as glass tubes, enamel-coated tubes, or ceramic materials; these materials provide good corrosion protection without reducing the boiler’s efficiency. However, they are costly and have a high air leakage coefficient. 4. Low-oxygen combustion is employed to reduce the excess oxygen in the flue gases, thereby preventing and minimizing the conversion of SO2 to SO3. Low-oxygen combustion can reduce the power consumption of fans for introducing and exhausting air; it is a technically effective measure with high economic value and great potential for development. However, low-oxygen combustion requires boilers to be equipped with advanced combustion equipment and monitoring instruments, as well as operators with high technical skills.

Reply to 7# 39392069: The analysis is excellent; you’re a real expert!

1. Increase the temperature of the metal wall surface of the air preheater. 2. A heat-tube air preheater is used. 3. Use corrosion-resistant materials. 4. Use low-oxygen combustion. 5. Use additives to lower the dew point or inhibit corrosion. 6. Ensure that the overall temperature at the cold end is higher than the dew point of the flue gas (using warm air heaters, etc.). 7. Soot blowing of the air preheater helps to reduce soot accumulation, which in turn lowers corrosion.

1 Strengthen the control of sulfur content in coal entering the plant; when drafting contracts for coal procurement, it is necessary to include provisions for controlling the sulfur content in the coal. By using economic incentives, it is possible to reduce the amount of coal with high sulfur content that reaches the furnace at the source. However, in recent years, due to shortages in electricity supply and the fact that coal has become a commodity sold on the market, this approach is less feasible. Nevertheless, efforts can be made to improve the mixing of different types of coal, in order to prevent coal with high sulfur and ash content from being used in large quantities in the furnaces. 2 Increase the wall temperature of the low-temperature heating surfaces; this is the most effective method during boiler operation. By keeping the wall temperature of the low-temperature heating surface above the dew point temperature, sulfuric acid vapor cannot condense on the metal surface, thereby reducing corrosion. To increase the wall temperature, it is necessary to raise both the flue gas temperature and the air temperature. Raising the flue gas temperature increases the heat losses from the boiler, thereby reducing its efficiency. However, for the safe operation of the unit, this measure is necessary even before any modifications are carried out. In the first phase of Luohuang Power Plant, two boilers had some of their heat exchange surfaces removed, which allowed the flue gas temperature to be increased from 145°C to around 168°C; this approach helps to reduce corrosion and prevent blockages in the air preheater ； Additionally, it is possible to increase the temperature of the cold air at the inlet of the air preheater in order to raise the wall temperature of the cold-end heating surfaces. This can be achieved by installing warm air heaters between the exhaust of the blower and the primary fan and the air preheater; during operation, these warm air heaters are turned on or off as needed based on the temperature at the fan inlets, so as to maintain the cold air temperature at the inlet of the air preheater within the range of 20–50°C, thereby ensuring that the overall temperature of the cold end of the air preheater remains within the specified limits. During the second phase of construction, Luohuang Power Plant specifically added a warm air heater between the primary fan outlet and the air preheater, which was not present in the first phase, thereby ensuring its long-term safe operation and achieving good results. 3. Strengthen the monitoring of the pressure difference at the inlet and outlet of the air preheater, paying particular attention when temperatures drop sharply in winter. Ensure that the temperature at the cold end of the heat exchanger is higher than the dew point temperature of the flue gas. When abnormal pressure differences are detected for the primary air, secondary air, and flue gas at the inlet and outlet of the air preheater, adjustments should be made promptly, and soot blowing should be intensified. Before carrying out soot blowing, all steam condensate must be completely drained, and a high load should be maintained as much as possible. If there is no improvement despite these measures, the air preheater must be washed with water taking advantage of a shutdown period. Before putting it back into operation, forced ventilation using the residual heat from the boiler or by starting the exhaust and supply fans is necessary to ensure thorough drying, thereby preventing ash from sticking to the heat exchange surfaces and avoiding further blockages. 4. To address the problems of ash accumulation and corrosion in the rear heating surfaces, the design of the second phase of Luohuang Power Plant incorporated improvements to the material used for the air preheaters as well as their heat transfer area. Corrosion-resistant Kordovan steel was used in the medium-temperature section, while the thickness of this steel was increased in the low-temperature section. The heat transfer area of the air preheaters was increased to 28,500 m2, which is higher than the 24,400 m2 of the first phase. The gaps between the heat transfer elements were increased accordingly, as was the gap between the air preheaters themselves. The flue gas temperature at the boiler outlet remained unchanged. Additional primary air warmers were installed; based on the operation performance of the units in the second phase, this approach has proven effective in preventing corrosion and ash accumulation in the air preheaters. For the desulfurization system, four sets of heat exchange components were replaced, and comparisons were made in terms of price and manufacturing processes. Ultimately, \"ND steel resistant to low-temperature dew point corrosion\" was chosen; its corrosion resistance is 5 times that of ordinary carbon steel, and it performs well in operation. In addition, equipment maintenance was strengthened – during each major repair, significant investment was made to maintain or replace the desulfurization heat exchangers and pipelines to ensure the proper functioning of the equipment. 5　Due to the increased corrosivity of flue gas after wet flue gas desulfurization, in addition to enhancing the corrosion resistance of the desulfurization equipment, it is necessary to ensure that the demisters and flue gas heaters are properly installed and functioning correctly. That is, the reheating temperature of the flue gas after desulfurization should be raised above the acid dew point temperature, with a sufficient margin to reduce flue gas humidification and prevent the condensation of residual gaseous SO3 vapor. At the same time, it is important to keep the temperature of the GGH heat transfer fluid low enough, typically around 75°C, in order to maintain an appropriate surface temperature on the tubes of the heat exchanger and avoid dew point corrosion at the inlet. 6 Since the further oxidation of SO2 to SO3 in flue gas occurs under certain conditions, an excess amount of air leads to a higher temperature at the center of the furnace flame, resulting in more SO3 formation. Therefore, it is necessary to use an appropriate excess air coefficient and to arrange the air supply properly in order to maintain the optimal combustion conditions and reduce SO3 production ； Furthermore, air leakage in the tail flue reduces the smoke temperature, deteriorates heat exchange with the heated surfaces, increases the volume of flue gas, raises exhaust losses, and increases the power consumption of the exhaust fans. It also causes corrosion and ash buildup; therefore, it is necessary to strengthen equipment maintenance in order to reduce the air leakage coefficient.