OCR Text |
Show depositing on tube surfaces is rich in vanadium and sulfur. Completion of the oxidation of this carbon at the tube surface temporarily raises the melting temperature of the ash, allowing it to melt and react with other ash on the tube surface. On cooling, it solidifies and remains tenaciously attached to the metal surface. The preceding discussions were based on the assumption that sufficient excess air is used to ensure complete oxidation of vanadium to V205. If insufficient air were present during combustion (i.e., less than 3 percent), the vanadium would be oxidized to V203 or V20^, which have melting points higher than 3500°F. In this case the vanadium is innocuous and leaves the steam generator as dry ash. The melting points of the various oxide states of vanadium explain why the fusibility of oil ash can be as low as 1200°F under oxidizing conditions and as high as 2000°F under reducing conditions. As will be shown shortly, the behavior is just the reverse of coal ash rich in iron. COAL Although the major elements found in the impurities in coal are similar to those in oil, the concentration levels, form, and relative distribution of individual elements are quite different (Table 4). With the exception of the alkalis in western coal, the impurities exist as insoluble, inorganic inclusions. In western coals the alkalis exist as metallic, organic compounds in the form of 1-12 |