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Fuels used for steam generation contain a large variety of impurities in the form of inorganic material apart from the organic material that provides the heat energy. During combustion these impurities undergo changes in their chemical form by combining with other constituents in the combustion regime. The effect of such combined materials being formed will be different at different sections of boiler starting from the furnace to the air pre-heaters. The various effects can be listed in general as a few given below
- Furnace wall corrosion
- Furnace wall deposition
- Furnace slagging
- High temperature deposition in superheaters
- High temperature corrosion in superheaters
- Fouling in the low temperature superheater and economizer area
- Air pre-heater plugging
- Low temperature corrosion
- Metal wastage due to erosion
High temperature corrosion in a boiler firing coal is mainly caused due to the presence of impurities like sulphur, alkali metals and chlorine. In fuel oil fired boilers, the main impurities are vanadium, sulphur, sodium, and chlorine. During combustion, these elements combine to form various types of vapors and condensed phases. The chemistry of these reactions taking place during combustion is complex and is widely varying. However, all the reactions undergo certain changes that are simple to understand. The sulphur in the fuel combines with oxygen to form sulphur dioxide and trioxide, depending upon the availability and temperature.
Vanadium combines with oxygen and vanadium oxides. These oxides act as catalysts to help the sulphur dioxide conversion to sulphur trioxide. The sulphur trioxide acts as catalyst for the conversion of vanadium oxides to vanadium pentoxide. The vanadium pentoxides combine with sodium in the fuel to form sodium meta venadate, which is a low melting point inorganic. Vanadium sulphates are also formed during combustion depending on the environment and the amount available in that environment.
These alkali vanadates and vanadium sulphates will condense along the flue gas path in superheaters and reheaters where metal temperatures are in a range for condensation of these types. These vanadium deposits are very hard and difficult to remove by soot blowing. Depending on the level of metal temperatures of the tubes the vanadates remain as hard deposits that are difficult to remove by even mechanical means- or corrode the tube. Different corrosion mechanisms are believed to operate on tube surfaces, as dictated by the local chemistry of combustion gases and deposits, the tube material compositions, the flue gas temperature, and the tube metal temperatures. The melting points of these deposits have been measured to be approximately 550°C to 620°C. So, it is expected that regions of tubing, both ferritic and austenitic, operating above 540°C are prone to this type of attack.
Corrective actions will depend on the severity of the high temperature ash corrosion problem. Measuring the melting temperatures of the fireside scale/ash constituents would provide a measure of ash corrosivity. Long-term solutions to fireside corrosion problems are mainly:
- Fuel blending
- Use of additives
- Lowering the tube crown temperatures by
- Burner tuning
- Steam flow redistribution
- Replacing tubes using more corrosion resistant materials
- Thicker tube walls
- Maintaining sufficient and proper air distribution in secondary, and tertiary
- Avoid excessive oxygen in furnace
While firing fuel oil, lowering flue gas oxygen contents to around 0.25% will result in a drop in liquid ash corrosion rates, especially when the fuel contains high vanadium and sodium levels. The main reason is for fuel oils with high vanadium to sodium ratios, low excess air operation produce lower oxides of vanadium than V2O5 in the ash deposits. Hence the formation of low melting point compounds of V2O5 and sodium oxide is reduced to a much lower level. Magnesium or calcium oxide based additives are used to control the corrosiveness of oil fuels. An EPRI study has concluded that the use of additives as a corrective action has had a successful effect and has proven to be economically feasible. The addition of MgO results in the formation of Magnesium Vanadate complex (3MgO V2O5), which has a higher melting temperature. It has been seen that designing and operating the units at lower excess air levels can also reduce the corrosion.