Causes and preventive measures of low-temperature corrosion of steam generators

What is boiler low temperature corrosion?

Sulfuric acid corrosion that occurs on the rear heating surface of the boiler (economizer, air preheater) is called low-temperature corrosion because the flue gas and tube wall temperatures in the rear heating surface section are low. After low-temperature corrosion occurs in the economizer tube, leakage may occur within a short period of time, posing safety risks. Shutting down the furnace for repairs will also cause greater economic losses.

The main cause of low-temperature corrosion of boilers

The sulfur in the fuel is burned to form sulfur dioxide (S+02=SO2). The sulfur dioxide is further oxidized under the action of the catalyst to form sulfur trioxide (2SO2+02=2S03). SO3 and the water vapor in the flue gas generate sulfuric acid vapor (SO3+H2O =H2SO4). The presence of sulfuric acid vapor significantly increases the dew point of flue gas. Since the air temperature in the air preheater is low, the flue gas temperature in the preheater section is not high, and the wall temperature is often lower than the flue gas dew point. In this way, sulfuric acid vapor will condense on the heating surface of the air preheater, causing sulfuric acid corrosion. Low-temperature corrosion often occurs in air preheaters, but when the sulfur content in the fuel is high, the excess air coefficient is large, the SO3 content in the flue gas is high, the acid dew point rises, and the feed water temperature is low (the turbine is deactivated at high temperature ), the economizer tube may also suffer from low-temperature corrosion.

Boiler low temperature corrosion case

A company’s circulating fluidized bed boiler was put into operation intermittently for less than a year, and multiple pipes in the lower economizer pipe suffered from perforations and leaks. The boiler fuel is a mixture of bituminous coal and sludge, the economizer tube material is 20 steel (GB/T 3087-2008), and the economizer inlet temperature is generally lower than 100°C.

The reasons for the perforation and leakage of the economizer tube were analyzed through material composition analysis, mechanical property test, metallographic analysis, scanning electron microscope morphology and energy spectrum analysis, X-ray diffraction phase analysis, etc. The analysis found that the economizer tube operates at a low temperature, and the corrosion products contain a large amount of S and Cl elements. The outer wall of the economizer tube suffers from low-temperature corrosion under low-temperature operation and acid corrosion during shutdown, which ultimately leads to coal saving. The pipe is corroded, perforated and leaking.

Low temperature corrosion prevention measures
1. Increase the wall temperature of the air preheater tube so that the wall temperature is higher than the flue gas dew point.
2. Add additives to the flue gas to neutralize SO3 and prevent the generation of sulfuric acid vapor. 3. Use low-temperature corrosion-resistant materials to make air preheaters and economizers.
4. Use low-oxygen combustion to reduce excess oxygen in the flue gas and prevent and reduce the conversion of SO2 into SO3.
5. By detecting the acid dew point temperature, the acid dew point under certain working conditions can be accurately known, thereby adjusting the exhaust gas temperature to achieve the best conditions for energy saving and extending the life of the boiler.