Steel-plastic composite pipe air preheater

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I. Problems associated with air preheaters made of common materials 1. Corrosion of heat exchange tubes – Acid dew point corrosion: The sulfuric acid dew point, also known as the flue gas dew point, refers to the temperature at which sulfuric acid vapor condenses into liquid sulfuric acid. The main cause of low-temperature corrosion is sulfuric acid corrosion. In particular, with oils and coal fuels, the sulfur content in these fuels produces large amounts of sulfur dioxide upon combustion. Part of this sulfur dioxide reacts with the unburned oxygen in the flue gases to form sulfur trioxide, which then combines with water vapor in the flue gases to form sulfuric acid vapor. When this vapor passes through the air preheater in the low-temperature section, and the temperature of the flue gases is below the dew point of sulfuric acid, the sulfuric acid vapor condenses into liquid droplets that adhere to the surface of the metal tubes used for heat exchange. This leads to electrochemical reactions between the metal and these droplets, resulting in corrosion of the air preheater at low temperatures. Acid dew point corrosion is usually related to the sulfur trioxide concentration in the flue gas, the ash content in the flue gas, as well as the water dew point temperature. The water dew point falls within the range of moisture content in the flue gases of conventional boilers, typically between 30–60°C, which is much lower than the exhaust temperature of the boilers. At the initial stage of condensation, the dew point of sulfuric acid vapor is much higher than that of water vapor; as a result, the sulfuric acid liquid condenses first. The concentration of the resulting liquid sulfuric acid is very high. However, sulfuric acid with a concentration above 58% does not have a strong corrosive effect on metal surfaces. As the temperature of the flue gas further drops, some water vapor begins to condense, and the sulfuric acid droplets absorb a large amount of liquid water, causing the concentration of the sulfuric acid solution to decrease. When the sulfuric acid concentration drops to 56–58%, the corrosion rate of carbon steel materials reaches its peak. When the flue gas temperature is close to the dew point, the amount of sulfuric acid deposited is low and the corrosion rate is not high. However, when the flue gas temperature drops below the dew point, SO2 in the flue gas dissolves in the water film to form sulfurous acid (H2SO3), and HCl and HF in the flue gas also dissolve in the water film; the corrosivity increases as the flue gas temperature continues to drop. 2. Dust accumulation and blockage issue: Dust accumulation and blockage in air preheaters are mainly caused by two factors: Condensed acid liquid sticking to dust – After sulfuric acid droplets adhere to the metal surface, their adhesion strength increases, which accelerates the deposition of dust particles from the flue gas onto the metal surface, ultimately leading to blockage at the cold end of the preheater ； Ammonium salt crystal blockage – excessive ammonia injection during denitration results in the formation of NH4HSO4. At lower temperatures, NH4HSO4 aggregates with the large number of ash particles in the flue gas, causing caking and blockage of the air preheater plates. II. Performance comparison of air preheaters made from different materials III. Characteristics of fluoroplastic steel pipes 1. Definition of fluoroplastic steel pipes: Fluoroplastic steel pipes are composite pipes in which meltable polytetrafluoroethylene (PFA) or other extrudable fluoroplastics are directly molded onto the surface of a metal pipe through heat fusion, thereby forming a seamless bond with the metal pipe. 2. Characteristics of fluoroplastic steel pipes: ① Corrosion resistance on the outside of the pipe – stable chemical properties, with inertness to almost all chemicals and solvents. ②Anti-scaling and anti-dust accumulation – The solid material has a very low wetting tension, resulting in a smooth surface that is less prone to scaling and dust accumulation, making it easy to clean. ③Wear resistance – low surface friction coefficient, high lubricity, self-lubricating properties, and resistance to wear. ④Long lifespan – Fluoroplastics have strong resistance to aging, allowing the material to last for 15 to 20 years. ⑤High versatility – Fluoroplastic steel pipes possess good rigidity, thus offering wide applicability; they can be used in both horizontal and vertical flue ducts. Penguin: 346889887