Corrosion due to flue gas acid dew point in waste heat recovery equipment

Cause of corrosion: In boilers and industrial heating furnaces that use heavy oil or sulfur-containing gas as fuel, sulfur particles are oxidized during combustion; in the low-temperature areas of such equipment such as flue ducts and air preheaters, these particles combine with water to form H2SO4, resulting in corrosion known as flue gas acid dew point corrosion or sulfuric acid dew point corrosion. Typically, heavy oil used as fuel contains 2% to 3% sulfur compounds, which produce SO2 upon combustion. Approximately 1% to 2% of this SO2 is converted into SO3 under the catalysis of metal oxides present in the ash. These substances then combine with the moisture contained in the combustion gases (about 5% to 10%) to form sulfurous acid or sulfuric acid, which further combine to form wet sulfates. As a result, condensation can occur at temperatures much higher than the normal dew point, leading to severe corrosion. Studies show that the dew point of combustion products containing as little as 0.025% (by volume) of SO3 is 171°C. Within the temperature range of 20–45°C below the acid dew point, the general corrosion rate is at its highest. Meanwhile, sulfuric acid that condenses on the surface of the equipment reacts with dust in the flue gas to form deposits that are difficult to remove; these deposits affect heat transfer, resulting in lower surface temperatures of the wall tubes, which further accelerates condensation and promotes corrosion. This type of corrosion caused by the \"dew point\" of sulfurous or sulfuric acids occurs mostly during shutdown periods, as the flue gas contains a certain amount of water vapor; when the temperature drops to the dew point during shutdown, water accumulates in areas where it is prone to stay, leading to corrosion. Protection measures: Measures to prevent corrosion caused by flue gas dew point can be considered from two aspects. From a process perspective, using low-sulfur heavy oil (with a sulfur content of less than 0.5%) can prevent flue gas dew point corrosion ； Or by adding additives to the heavy oil that can react with SO3 to form corrosive substances (such as adding 0.06% magnesium hydroxide slurry to the heavy oil), the concentration of SO3 can be significantly reduced, thereby mitigating dew point corrosion ； During the design and operation process, care should be taken not to lower the flue gas temperature too much in an attempt to save energy ； Reduce dust in the flue gas, and take advantage of downtime to promptly clean the dirt on the equipment surface. From the perspective of equipment surface treatment and material selection, coatings such as polytetrafluoroethylene or non-metallic linings like ceramics can be used, but there are many limitations in practical applications. The company uses perfluoroplastics (polytetrafluoroethylene) to design and manufacture flue gas heat exchangers, thereby improving and addressing acid dew point corrosion issues, effectively extending the equipment’s service life and yielding significant economic benefits.