In this article, we see the Boiler Hot End Corrosion – Mechanism and prevention methods. In boiler they are two types of corrosion will take place. They are
- Hot end corrosion
- Cold end corrosion
The Hot End Corrosion take place in the hot region of the boiler. It’s usually in the furnace region and superheater section.
Mechanism Of Hot End Corrosion
Due to the presence of sodium, vanadium and sulfur in the fuel oil cause Hot end corrosion. During the combustion at the boiler furnace, the sodium, vanadium and sulfur present in the fuel oil react and form oxides, sulfates, and vanadates. Even a small amount of the above components cause slag deposits on the boiler tubes and induce corrosion. This type of corrosion called Hot End Corrosion.
Based on the presence of vanadium in fuel oil, it’s classified as two types. They are
Low Vanadium Fuel: The vanadium content is less than 100 ppm
High Vanadium Fuel: Vanadium content above 200 ppm
The vanadium reacts with oxygen and forms vanadium oxides in the boiler furnace. Due to the high temperature in the furnace region, the various form of vanadium oxides sticks to the metal surface. The deposits of vanadium oxides promote the transport of oxygen to the metal surface and induce Corrosion in the boiler tubes. The subsequent deposition of vanadium oxides accelerates corrosion in boiler tubes.
If the temperature is low, they pass through the boiler as fly ash. Normally this problem occurs when the steam temperature is more than 538oC or higher. It’s very difficult to reduce the presence of vanadium in heavy fuel.
Prevention Methods
The following are the prevention methods to control hot end corrosion in boilers.
Reducing Excess Air
Reducing the amount of excess air into the boiler combustion chamber can reduce the formation of vanadium oxides and sulfur trioxide. The excess of air supply into the combustion chamber shall be maintained less than the 5% of total airflow.
Material of Construction
Using of High-chromium alloys will reduce the hot end corrosion. Some of the high chromium alloys are C-276, Alloy 625, 50% Ni 50% Cr or 40% Ni 60% Cr.
Fuel Additives
Fuel additives such as magnesium and aluminium oxides used to control the high-temperature corrosion problem in boilers. The function of additives had to elevate the melting point of the deposits present in the fuel oil. Thereby deposition of materials on boiler tubes reduced.
Magnesium oxide added in fuel oil has to the metallic constituents to remain in the solid phase as they contact the metal surface.
Lower Fuel Ash Content
Based on Ash content in Fuel oil, it’s classified into two types, they are
High Ash Fuel: The Ash content in fuel oil is more than 0.05%
Low Ash Fuel: The Ash content is less than 0.02% Fuel oil
If more ash content present in the fuel, during the combustion process more ash generated and deposition on the metal surface (boiler tubes) and induce corrosion. Hence it’s always recommended to use low ash fuel to prevent corrosion and shoot formation in boilers.
Conclusion
Proper sampling methods and techniques, the quality of fuel shall be ensured before feeding into the boiler.
Monitoring the fuel oil quality, this type of corrosion shall be prevented and increase the service life of boilers.