FUEL TREATMENT FUNDAMENTALS (Amtek Research Inc. Canada)
Deposit,Slagging and Corrosion on the Fire side
During combustion of the fuel a layer of slag gets deposited on the fire side of the water tube. This reduces heat transfer to water and failure of the tube. Slagging depends upon the presence of slag forming components such as Na, K, V, S, Pb, Ni, Al, and Si. Low ash softening temperature is the major cause of slagging. Slag in molten condition attracts fly ash resulting in more deposits. The temperature in the superheater area of the boiler is controlled by the temperature of dissociation of water (steam) into hydrogen and oxygen. The maximum superheated steam temperature is about 600°C. The most common types of corrosion attacks are caused by Na2SO4, V2O5 and sodium vanadates, 2Na2O.V2O5 and Na2O.V2O4.5V2O5. The melting point of V2O5 is 650-680°C. Melting points of eutectics formed between vanadium pentoxide, sodium and potassium oxides can range in the 550-600°C. Since their melting temperatures are very low, the alkali sulfates and vanadates can form a liquid slag at the hot metal surfaces, which then provide a condition conducive to accelerated oxidation and corrosion. |
