OCR Text |
Show chemical composition from a multilayered, milk-colored, sintered ash to a hard, black, crystalline substance.5-7 Chemical analyses have indicated some deposits are composites of complex sodium, iron, and nickel vanadates.5 The mechanism for deposition has been suggested as selective condensation of complex vanadium compounds and alkali sulfates, forming insoluble vanadates on the tube surfaces. 5»7»9 Figure 1 illustrates the crystalline deposit and chemical composition. Figure 2 gives the vapor pressure of the vanadium and sodium compounds as a function of temperature in the vicinity of the heat-transfer surface.10 The complex vanadates have low melting-point eutectics (1240°F), which are believed to be responsible for the slag growth and tube bond.5 The melting point of sodium sulfate is about 1635°F. This, too, can be reduced to as low as 1100 to 1200°F in the presence of other impurities such as CaSO,, or MgSO H. A list of the melting points of various compounds appears in Table 3. In a boiler where residence time is very short, molten ash or supercooled vapors can exist at temperatures much below their dew point or freezing point. Under these circumstances they are in nonequilibrium or in a metastable state. The condensation lines or freezing points may be as much as 100 to 200°F below the corresponding state of equilibrium-a partial explanation of why severe fouling can occur at tubewall temperatures as low as 950°F when firing an oil rich in alkalis but nearly void of vanadium. Fouling can also be caused by incomplete combustion, resulting in deposition of soot during operation or poor atomizing during shutdown. Vanadium has been retained in oil until the last of the petroleum is vaporized, hence the high concentration of vanadium and sulfur in petroleum cokes. Unspent oil or carbon 1-8 |