Analysis of the causes of low-temperature flue gas corrosion and scaling in petrochemical air preheaters

Thread Content

This post was last edited by Wang Wei2 on 2021-2-15 at 12:36. Analysis of the causes of low-temperature flue gas corrosion and scaling in petrochemical air preheaters. 1. Mechanism of low-temperature dew point corrosion: SO3 is formed; generally, fuel oils or fuel gases contain small amounts of sulfur, which turns entirely into SO2 upon combustion. Due to the excess oxygen present in the combustion chamber, a portion of this SO2 further reacts with oxygen to form SO3. Under normal excess air conditions, about 1–3% of all SO2 is converted into SO3. In high-temperature flue gas, SO3 does not corrode metals. However, when the flue gas temperature drops below 400°C, SO3 reacts with water vapor to form sulfuric acid vapor; the reaction equation is as follows: SO3 + H2O = H2SO4. When this sulfuric acid vapor condenses on the heated surfaces at the rear of the furnace, low-temperature sulfuric acid dew point corrosion occurs. At the same time, this sulfuric acid liquid that condenses on the low-temperature heating surfaces also adheres to the dust in the flue gas, forming stubborn deposits that are difficult to remove, thereby obstructing or even blocking the flue gas passages. 2. Relationship between corrosion rate and wall temperature: Sulfuric acid vapor and water vapor in the flue gas begin to condense when they come into contact with a cold surface, and the concentration of sulfuric acid in the condensed liquid is very high. Due to the condensation of some steam, the concentrations of sulfuric acid and water vapor in the flue gas decrease (with a greater decrease in the former and a smaller decrease in the latter), and as a result, the dew point of the flue gas also drops. As the flue gas continues to flow forward and encounters colder surfaces, the vapor in it keeps condensing, but the concentration of sulfuric acid in the condensed liquid gradually decreases. Therefore, the concentration of sulfuric acid in the flue gas gradually decreases. The concentration of sulfuric acid in the flue gas condensate has the greatest impact on the corrosion rate of the heat exchange surface. Concentrated sulfuric acid has a very slow corrosion rate on steel, while dilute sulfuric acid has the fastest corrosion rate. The above is only the effect of sulfuric acid concentration on the corrosion rate. However, during operation, the actual corrosion rate is also related to the temperature of the steel. At high temperatures, chemical reaction rates are faster, and the corrosion rate (for sulfuric acid of the same concentration) is also faster. The actual corrosion condition on the heated surface at the tail is, of course, related both to the dew point concentration and to the wall temperature. When the wall temperature is high and no dew formation occurs, the corrosion rate is very low ； At the onset of dew formation, the high concentration of sulfuric acid in the dew results in a corrosion rate that is not very high, even though the wall temperature is high ； On heat exchange surfaces at lower temperatures, although the wall temperature decreases, the concentration of sulfuric acid in the condensation becomes weaker, which accelerates the corrosion rate; this rate reaches a maximum at a certain point (generally considered to be 10–40°C below the dew point temperature) ； Thereafter, due to the lower sulfuric acid concentration and lower temperature, the corrosion rate decreased. Finally, due to the very low wall temperature, a large amount of water vapor condenses, resulting in a relatively high corrosion rate. 3. Analysis of the causes of scaling: The main reasons for this issue are as follows: (1) The structural design of the preheater is unreasonable; there are grooves in its lower part, which allow liquid and impurities to accumulate, leading ultimately to chemical reactions between these substances and the formation of scale. (2) Without soot blowing equipment, the ash accumulated on the fins cannot be removed in a timely manner; this ash mixes with water vapor to form a solid substance, leading to local blockages in the finned tubes. (3) The temperature of the air entering the preheater is too low; the designed temperature is 20°C, while the ambient temperature in winter is -26°C. As a result of intense heat exchange between the air and the flue gases, the temperature at the end of the preheater becomes excessively low. Low temperatures facilitate the formation of water vapor, which, together with particles from the flue liner, accumulates between the fins, ultimately leading to blockages in the preheater and a reduced negative pressure in the furnace. (4) The 100 mm thick castable lining, covering an area of about 150 square meters over a length of over 40 meters from the furnace top to the preheater flue, has a high thermal conductivity, resulting in significant heat loss. At temperatures as low as -26°C in winter, condensation forms on the inner surface of this lining, causing it to deteriorate; the resulting particles are carried by the smoke flow into the preheater, thereby exacerbating dust accumulation there. (5) The preheater does not provide adequate insulation; the temperature near the wall of the preheater can easily drop below 120°C, leading to low-temperature corrosion and condensation. For the above reasons, a large amount of scale accumulates on the preheater tube bundle in the end. The structure of the flue gas is relatively complex, but it mainly consists of sulfate components. Its thermal conductivity is much lower than that of metals, generally ranging from 0.058 to 5.8 W/(m•℃). The thermal conductivity of metal steel pipes is 6 to 10 times greater than that of scale; in other words, for the same heat transfer area, the thermal resistance of scale is 6 to 10 times higher than that of metal. This increases the thermal resistance of the heat exchange tubes, eventually rendering them unusable.