Discussion on the Corrosive Effect of Low-Temperature Corrosion on Air Preheaters and Desulfurization Equipment

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This post was last edited by Wang Genrong on 2022-3-26 at 16:20. Discussion on the Corrosive Effect of Low-Temperature Corrosion on Air Preheaters and Desulfurization Equipment. Planning Department, Huaneng Central China Branch (430077), Xiang Yuehua. Based on on-site operation experience with air preheaters and wet desulfurization systems in a certain power plant, this paper explores the corrosion issues associated with flue gas, calculates the acid dew point of the plant, proposes solutions, and analyzes their actual application effects. 　Low-temperature corrosion dew point temperature, air preheater, desulfurization: 0. Introduction: The power plant has a capacity of 4×350 MW, with coal-fired steam turbine generators; each boiler is equipped with two rotary air preheaters with three regeneration stages each. The coal used is low-quality anthracite, containing 3.5%–5% sulfur. The combustion of this high-sulfur coal results in flue gases with high concentrations of sulfur dioxide. To protect the local atmospheric environment, limestone/gypsum wet desulfurization systems are used in conjunction with the generator sets. The project is constructed in two phases. After the completion of the first phase, the cold and medium-temperature sections of the air preheaters in Units #1 and #2 suffered from severe corrosion and ash accumulation due to dew formation. The corrosion issue in the GGH equipment within the desulfurization system was also significant. The large temperature gradient in the GGH heat exchangers, along with the flue gas temperature at the outlet being below the dew point temperature of the flue gas, caused the equipment to remain in a highly acidic environment for extended periods, resulting in significant corrosion of the heat exchanger tubes. Since the proper operation of air preheaters and desulfurization units is crucial for the safe and environmentally friendly operation of power plants, it is necessary to explore the causes and hazards of low-temperature corrosion. The rotary air preheater and desulfurization facilities are installed at the rear of the boiler. After passing through the air preheater and the heat exchangers at the inlet of the desulfurization system, the flue gas loses heat, resulting in a lower temperature. When high-sulfur coal is used, the liquid sulfuric acid that condenses in the flue gas causes corrosion of the heat exchangers and flue ducts, leading to severe damage to the heat exchange elements. Additionally, this acidic liquid binds together the ash particles in the flue gas, causing accumulation of ash in the air preheater, electrostatic precipitator, and heat exchangers, which can lead to blockages. In severe cases, this affects the boiler and desulfurization facilities’ ability to operate at full capacity. According to the terms of the first-phase contract, the air preheater should be able to function properly at an inlet air temperature of 25°C and an outlet flue gas temperature of 145°C. However, after about half a month of operation, the air preheater became severely blocked, manifested by fluctuations in the pressure of the primary and secondary air streams. These fluctuations gradually increased in amplitude and became periodic, with the cycle matching exactly one full rotation of the air preheater. In severe cases, this caused the fans to experience surge conditions; the inability to maintain the proper pressure of the primary air led to the activation of the boiler’s MFT mechanism. As a result, the flue gas temperature had to be raised to 160–165°C in order to keep the boiler running until it could be shut down for cleaning. High flue gas temperatures also affect the proper operation of the desulfurization system, reducing the service life of the resin lining in the absorption tower and increasing maintenance costs, thereby severely impacting the safe and economic operation of the unit. 1. The corrosion issue associated with desulfurization is a major factor affecting the integrity of equipment; corrosion of GGH units in power plant desulfurization systems is particularly prominent. The flue gas heat exchange units in desulfurization installations use closed-loop water circulation heat exchange systems (MGGH). After entering the desulfurization unit, the flue gas first passes through a heat exchanger to be cooled, so as to reach the inlet flue gas temperature specified in the design of the absorption tower and facilitate smooth absorption. The temperature reduction achieved by the heat exchanger is set at 142°C to 100°C for the first phase of desulfurization design, and at 142°C to 120°C for the second phase. Due to the large temperature gradient in the heat exchanger and the fact that the flue gas temperature at the outlet is below the dew point temperature, large amounts of SO2 and SO3 condense on the surface of the tube bundles, exposing the equipment to a highly acidic environment over time. In particular, the concentration of SO3 increases significantly as the flue gas temperature rises, resulting in severe corrosion of the heat exchanger tube bundles. Significant costs are incurred during each major maintenance overhaul to carry out repairs or replacements in order to ensure the proper operation of the equipment. Furthermore, rubber-lined pipes are commonly used in the piping design of desulfurization units, and rubber-lined pumps are employed for most of the pumps as well. Based on operational performance, this approach proves to be effective; however, wear and corrosion still occur to a significant extent in certain equipment and parts of them, which increases the maintenance costs. 2 Causes of low-temperature sulfur corrosion and analysis of fouling in heat exchangers When coal burns, flue gas is generated; the moisture content in this gas depends on the type of fuel used, the amount of excess air, and the moisture present in the air. Steam cleaning also increases the moisture content in the flue gas. If water vapor does not combine with other substances, and there is little moisture in the fuel, its partial pressure is low, resulting in a low dew point for the water vapor—usually between 30 and 60°C. Under such conditions, dew formation does not occur on the low-temperature heating surfaces ； In fact, during the combustion of coal, especially when high-sulfur coal is used, apart from some sulfates remaining in the ash, most of the sulfur is burned to form SO2. Approximately 0.5–5% of this SO2 is converted into SO3 under the catalytic action of excess oxygen in the flue gas and Fe2O3 present in the ash. SO3 reacts with water vapor in the flue gas to form sulfuric acid vapor. The dew point of this sulfuric acid vapor – also known as the acid dew point or flue gas dew point – is relatively high. Even a small amount of SO3 in the flue gas can significantly raise its dew point, causing large amounts of sulfuric acid vapor to condense on the heat surfaces at temperatures lower than the dew point, thereby leading to corrosion. The designed sulfur content of the coal used in the power plant is 4.02%, and the actual operating conditions are consistent with this value. Based on the elemental analysis of the coal, and using the thermal calculation standards for Soviet boiler units from 1973 that are widely employed in China, by calculating the partial pressure of water vapor in the flue gas (PH2O, in %) and then referring to tables of saturated moist air, it can be determined that the dew point temperature is approximately 35.5°C. If the excess air coefficient for flue gas emission is set at 1.35, and the share of fly ash is taken as 0.92 based on the actual conditions of the power plant, the acid dew point temperature can be calculated to be around 123°C. When the preheater is not in use, the overall temperature at the cold end of the air preheater is only 90°C, which inevitably leads to low-temperature corrosion. 　　The actual corrosion condition on the heated surface at the tail is related both to the concentration of condensed dew and to the wall temperature. As the boiler load changes, the extent of corrosion and its most severe areas also change; as a result, the cold end of the air preheater and the heat exchangers at the sulfur removal inlet and outlet suffer the most severe corrosion. 　　When the wall temperature at the cold end of the air preheater is low, and if the warm air heater stops operating for various reasons, the wall temperature drops well below the dew point of the water vapor in the flue gas; this leads to the condensation of large amounts of water vapor and dilute sulfuric acid. Moreover, since there is a significant amount of ash in the flue gas, this ash deposits on the wall surfaces and, after reacting chemically with water and acid, forms hard deposits. The persistent low-temperature weather has further exacerbated ash accumulation on the heated surfaces, blocking most of the air preheaters and forcing the unit to shut down. At the same time, the heat transfer elements in the air preheater are arranged closely, allowing fly ash in the flue gas to easily accumulate on the heated surfaces, thereby increasing the gas flow resistance and affecting the normal operation of the air preheater. Furthermore, ash accumulation on the heated surface at low temperatures causes the temperature of the metal wall to drop further; sulfuric acid vapor can penetrate through the ash layer and reach the metal wall, where it forms sulfuric acid, causing the ash to harden and making it more difficult to remove. 　　The factors that contribute to the deterioration of the corrosive environment in desulfurization equipment are complex; changes in the composition of flue gas are one of the main causes. These include variations in the SO3 content in the flue gas, increases in dust concentration, and rises in moisture levels. Frequent start-up and shutdown of the units also exacerbate equipment corrosion. Generally speaking, in wet flue gas desulfurization systems, the dew point temperature of the flue gas also tends to decrease. However, the corrosivity level of the flue gas does not decrease; on the contrary, it increases significantly. The reason for this is that although the SO3 content in the flue gas after desulfurization decreases, the total amount of corrosive substances in the flue gas increases. These include acidic fumes and acidic moisture generated during wet flue gas desulfurization, as well as highly corrosive substances such as chlorides and fluorides present in the water used for coal combustion and in the desulfurization slurry preparation. The presence of chlorides and fluorides leads to a significant increase in the corrosivity level of the flue gas. If the temperature of the flue gas after desulfurization is below the acid dew point temperature, the corrosivity level of the flue gas will increase further. 3 Measures to prevent corrosion at low temperatures 3.1 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 of 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 reaching the furnace in large quantities. 3.2 Raising the wall temperature of the low-temperature heating surfaces 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 the 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 prevents 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-side heat exchange surfaces. This can be achieved by installing warm air heaters between the outlets of the supply fans and primary fans and the air preheater. During operation, these warm air heaters should be turned on or off as needed, depending 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. This ensures that the overall temperature on the cold side 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 exhaust fan and the air preheater – a component that was not present in the first phase – to ensure its long-term safe operation, achieving good results. 3.3 Strengthen the monitoring of the differential pressure at the inlet and outlet of the air preheater, paying particular attention when temperatures drop sharply in winter. It is also necessary to 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 differential pressures 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 improvements are not seen even after taking these measures, the air preheater must be washed with water during a shutdown period; before putting it back into operation, forced ventilation using the boiler’s residual heat or by starting the exhaust and intake fans is necessary to ensure thorough drying and prevent soot from adhering to the heat exchange surfaces, thus avoiding further blockages. 3.4 To address the issues of ash accumulation and corrosion in the rear heating surfaces, the design for the second phase of the power plant includes improvements to the material used in the air preheaters as well as to their heat transfer area. Corrosion-resistant Kalden steel is used in the medium-temperature section, while the thickness of this steel is increased in the low-temperature section. The heat transfer area of the air preheaters has been increased to 28,500 m2, which is higher than the 24,400 m2 in the first phase; the gaps between the heat transfer elements have also been increased accordingly, along with the gaps in the air preheaters themselves. The flue gas temperature at the boiler outlet remains unchanged. Additional primary air warmers have been installed, and based on the operation of the units in the second phase, this approach effectively prevents 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 maintenance session, significant investment was made to maintain or replace the desulfurization heat exchangers and pipelines to ensure the proper functioning of the equipment. 3.5 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 essential to ensure that the demisters and flue gas heaters are properly installed and functioning correctly. In other words, the reheat temperature of the flue gas after desulfurization should be raised above the acid dew point temperature, with a sufficient margin to reduce moisture in the flue gas and prevent condensation of residual gaseous SO3 vapor. At the same time, it is necessary 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 condensation corrosion at the inlet. 3.6 Since the further oxidation of SO2 to SO3 in flue gas occurs under certain conditions, an excess amount of air and a high temperature at the center of the furnace flame lead to greater production of SO3. Therefore, it is necessary to use an appropriate excess air coefficient and to arrange the air supply properly in order to achieve the best combustion conditions and reduce the formation of SO3 ； Furthermore, air leakage in the tail flue reduces the smoke temperature, deteriorates heat exchange with the heating 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 deposition; therefore, it is necessary to strengthen equipment maintenance in order to reduce the air leakage coefficient. 4 　Conclusion 　The corrosion of the heating surfaces at the rear of boilers is related to the acid dew point temperature. The low-temperature corrosion of these heating surfaces caused by flue gas is often indicated by the level of the acid dew point; the higher the dew point, the greater the extent of corrosion. With the reform of the power industry and increasing pressure on the national power supply system, there are higher demands for the safe and efficient operation of boilers. Meanwhile, more thermal power plants are being designed with flue gas desulfurization technologies. Preventing corrosion in boilers and desulfurization facilities during maintenance periods, ensuring their long-term efficient and safe operation, and reducing maintenance costs will undoubtedly have a positive impact on the ability of power plants to generate and supply electricity.