In this article, we will learn about Boiler Cold End Corrosion Mechanism and prevention methods
In boilers they are two types of corrosion will take place. One is the hot corrosion and another one is the cold end corrosion. As the name indicate the cold end corrosion will take place in the cold region of the boilers. Usually, the cold region refers to the pre-heater installed just before the flue gas exit to the stack or induced draft fan.
Mechanism of Cold end corrosion
The sulfur content in the fuel is the major cause for the cold end corrosion. When the sulfur burns in the boiler, it forms sulfur dioxide. Which is relatively harmless. However, the SO2 is further oxidized to form SO3 as a result of the catalytic effects of vanadium pentoxide(V2O5) or iron oxide(Fe2O3) in the fuel itself and.
Presence of even a very small quantity of SO3 reacts with the steam or water in the flue gas.
Due to this reaction, the SO3 present in the flue gas forms sulfuric acid. When the flue gas temperature falls below the dew point (temperature below which condensation of moisture takes place). Which leads to a severe corrosion problem at the cold end of the boilers. ie., air pre-heater installed just before the flue gas exit to the stack or induced draft fans.
Prevention Methods
The following methods are used to prevent cold corrosion in boilers.
Reducing the excess airflow
Minimizing the excess air supply to the combustion chamber will drastically reduce the formation of sulfur trioxide. The excess air supply shall be maintained less than the 5% of total airflow, the formation of sulfur trioxide is drastically reduced.
Reducing the fuel sulfur content
The boiler fuel contains very high sulfur, then sulfur burns in the burner and form sulfur trioxide. Hence it is always preferred to use less sulfur fuel. (Less sulfur fuel cost is more when compared with high sulfur fuel).
If high sulfur content fuel usage in a boiler is not eliminated then other prevention methods (Fuel additives, we will discuss below) are implemented to reduce the occurrence of cold end corrosion.
Sulfur content plays a major rule when oil is used as boiler fuel when compared with the coal. In the case of coal fire boilers, the formation of Sulfur trioxide in the exhaust region (Cold end of the boiler) will suppress by the formation of ash in coal fire boiler.
Reducing Moisture Content in the Fuel gas
As we have seen above the moisture play a vital role in the formation of sulfuric acid. In the coal fire boiler, the fuel moisture will be reduced by various methods by proper storage and preheating of coal. But in the case of fuel oil it very difficult to remove the moisture.
Reducing the steam leakage rate
The leakage of boiler tubes or excess steam used in steam-assisted soot blower may lead to adding the moisture content. Also, it will alter the dew point of the exhaust flue gas. Increasing the moisture content in the flue gas cause increasing the dew point of the fuel gas. ie., the dew point is raised to 150 oC from 120 oC. This will leads to severe cold end corrosion.
Fuel Additives
By adding magnesium base additives in the fuel will lead to form magnesium oxide. Thereby it will reduce the formation of sulfur trioxide. Also, these additives will control the catalytic reaction of vanadium pentoxide and iron oxides.
Flue gas Additives
By adding magnesium oxide, magnesium carbide and ammonia are the major flue gas additives using in the boiler. in the hot zone of the boiler (before the economizer) to suppress the sulfur trioxide formation at the cold end.
At high temperature, the magnesium reacts with the sulfur to form magnesium sulfide. Thereby the sulfur trioxide formation will be reduced. The major disadvantage is that the magnesium sulfide is deposited on the pre-heater tubes and increasing the fouling resistance.
The quantity of additives to be added with fuel gas is determined by the presence of sulfurtrioxide.
Flue gas exhaust temperature
The usual method employed to minimize the cold end corrosion by maintaining the flue gas exhaust temperature above the dew point. The temperature difference shall be of 20oC between the flue gas and dew point.
The boiler efficiency will be reduced with the increasing of temperature difference.
Removing the catalyst
Vanadium pentoxide (V2O5) and ferric oxide (Fe2O3) act as a catalyst for the SO3 formation reaction. These catalysts which are commonly found on the surfaces of oil-fired boilers. Catalytic effects are influenced by the amount of surface area of catalyst exposed to the flue gas. Therefore, boiler cleanliness, a reflection of the amount of catalyst present, affects the amount of sulfur trioxide formed.