The corrosion of the water wall tube on the fire side is high temperature corrosion. The high temperature corrosion of water wall tubes poses a serious threat to the safe and economical operation of boiler units. According to statistics, the power loss caused by the leakage of the water-cooled wall pipe every year accounts for a considerable part of the total power loss, and the main reason for the leakage of the water-cooled wall pipe is high temperature corrosion.
The frequent occurrence of boiler water wall tube burst accidents caused by high temperature corrosion not only causes huge economic losses, but also affects the safe and stable operation of the entire power grid.
According to the causes of high temperature corrosion and the analysis of the composition of corrosion products, the high temperature corrosion of the water wall of pulverized coal boilers can generally be divided into the following types: chloride type high temperature corrosion, sulfate type high temperature corrosion, sulfide type high temperature corrosion and high temperature corrosion caused by reducing atmosphere.
The corrosion of the water wall to the fire side cannot occur in the oxidizing atmosphere of the combustion zone, but in the reducing atmosphere of carbon monoxide formed by incomplete combustion. At this time, when the unburned carbon particles fly to the surface of the furnace tube, volatile sulfur and sulfide are released at the same time, which has a sulfide effect on the metal and accelerates corrosion. Therefore, the corrosion of the boiler water wall to the fire side is sometimes called “reductive atmosphere corrosion”. When continuous poor combustion or pulsating flame impinges on the furnace wall, a corrosive environment with high temperature and excess fuel is formed, which is conducive to the production of sodium and phosphorus pyrosulfate with a melting point of only 427℃. The higher the temperature of the water wall, the easier it is to form various corrosion products.
Some data show that in the range of 300~500℃, the corrosion degree of the flue gas side increases by 1 times for every 50℃ increase in the surface temperature of the pipe wall.
High temperature corrosion has a lot to do with coal type, and sulfur and sulfide in coal are the main factors for the formation of corrosion. High-temperature corrosion mainly occurs on boilers burning lean coal. The fastest corrosion rate is the front point of the boiler water wall to the fire side, and the combustion characteristics of coal are one of the main factors affecting the corrosion rate. When burning lean coal that is difficult to ignite and burn out, if the combustion conditions in the furnace are not good or in an atmosphere with low oxygen concentration, the coal and ash particles will rise and the oxygen concentration will drop, the concentration of carbon monoxide near the water wall increases, resulting in an increase in the fly ash of the boiler combustibles and aggravated high temperature corrosion of the water wall. Sulfide type high temperature corrosion is mainly caused by pyrite sulfur in coal, and its corrosion history is as follows.
①Pyrite powder arrives near the water wall along with unburned pulverized coal, and is decomposed by heat to release free atomic sulfur and ferrous sulfide:
FeS₂→FeS+S
When there is a certain concentration of H2S and SO2 near the tube, free atomic sulfur may also be generated:
2H2S+SO2→2H2O+3S
②In a reducing atmosphere, when the temperature of the tube wall reaches 623K, the single atomic sulfur will undergo vulcanization, that is:
Fe+S→FeS
③H2S can also act through the FeO complexed in the loose Fe2O3 and the denser magnetic ferric oxide layer Fe3O4 (Fe2O3-FeO):
FeO+H2S→FeS+H2O
④ Ferrous sulfide FeS generates black magnetic ferric oxide by slow oxidation:
3FeS+5O2→Fe3O4+3SO2
It can be seen from the above process that there are three fundamental reasons for the high temperature corrosion of the water wall: high sulfur content in coal, unburned pulverized coal scouring the water wall and forming a local reducing atmosphere, and high temperature of the water wall tube wall.
Both organic sulfur and pyritic sulfur in the fuel can participate in combustion to generate sulfur dioxide and sulfur trioxide:
S+O2→SO2
2SO2+O2→2SO3
Sulfur trioxide is formed by the combination of part of the combustion product sulfur dioxide and excess oxygen in the flue gas. Although its content is very small, due to the large combustion value of sulfur trioxide, a small amount of sulfur trioxide can significantly increase the dew point temperature of the flue gas. In the production process of sulfur trioxide, flue gas temperature is a very important corrosion factor. When the flue gas temperature is between 425℃ and 625℃, sulfur trioxide is most likely to be generated. In the process of pressing and firing the boiler, this temperature can be repeatedly formed in the furnace and last for a period of time. At the same time, after pressing the fire, the combustion produces sulfur dioxide, which is then converted into sulfur trioxide. The increase of sulfur trioxide increases the dew point temperature of the flue gas, which can reach above 160℃.
Sulfur dioxide and sulfur trioxide combine with water vapor in the flue gas to generate sulfurous acid and dilute sulfuric acid:
SO2+H2O→H2SO3
SO3+H2O→H2SO4
Water in the fuel forms water vapour when it burns. When the furnace is pressed, these water vapors are mixed in the flue gas of the furnace, making the flue gas moist, and the furnace wall contains a lot of moisture, and a large amount of water vapor is generated after heating. These vapors combine with sulfur dioxide and sulfur trioxide in the flue gas to form sulfurous acid vapor and sulfuric acid vapor. In the process of starting and stopping the boiler, the furnace temperature is not very high, and the internal pressure of the boiler is 0.1~0.2MPa. At this time, the saturation temperature of the medium is only 120~130 ℃, especially the back of the water wall tube is in direct contact with the humid furnace wall, and the temperature is lower, and it falls below the dew point temperature of the flue gas. The sulfurous acid vapor and sulfuric acid vapor in contact with it condense into sulfurous acid and dilute sulfuric acid on the surface of the water wall tube. And here is the dead corner of the flue gas. Once the dew is attached, it will not be easily taken away by the flowing flue gas, so that it will be in an acidic corrosion environment for a long time, so that the pipe wall will be chemically corroded:
H2SO3+Fe→H2+FeSO3
2FeSO3+O2→2FeSO4
H2SO4+Fe→H2+FeSO4
From the corrosion morphology, the length direction of the corrosion pit is along the axis of the pipe, which is the direction of the downward flow of the condensed dew. From the above analysis, we can see that the main reason for the corrosion and leakage of water-cooled wall pipes is that the acid flue gas generated by fuel combustion condenses at or below the dew point temperature to generate corrosive media, so that the water-cooled wall pipes are partially corroded and perforated. The high sulfur content in the fuel is the main cause of the sour flue gas.
