How Is The Low Temperature Corrosion of Boiler Caused
The sulfuric acid corrosion of the heating surface (economizer and air preheater) is caused by flue gas and the lower tube wall temperature at the tail heating surface, so which is called low temperature corrosion.
The sulfur in the fuel burns to produce sulfur dioxide, and sulfur dioxide is further oxidized to produce three sulfur oxides under the action of the catalyst, and steam of sulfuric acid is generated by SO3 and water vapor in the flue gas. The dew point of flue gas is obviously increased by the presence of sulfuric acid vapor. For example, the pressure of water vapor in the flue gas of an oil fired boiler is about 0.008~0.014Mpa, and the corresponding thermodynamic dew point is 41~52℃. When the concentration of sulfuric acid vapor in the smoke is 10%, the dew point is increased to 190℃. The wall temperature of preheater pipe is related to the flow rate and temperature of flue gas and air. It is approximately equal to the average temperature of flue gas and air. Because the air temperature in the lower part of the air preheater is lower, the flue gas temperature in the lower part of the preheater is not high, and the wall temperature is often lower than the dew point of the flue gas. The sulfuric acid vapor will condense on the heating surface of the preheater, causing sulfuric acid corrosion.
Low temperature corrosion often occurs in the air preheater, but when the fuel sulfur content is higher, the boiler air coefficient is larger, so that SO3 in the flue gas content is higher. The dew point is higher, and the water supply temperature is low, the economizer tube may also occur low temperature corrosion.