OCR Text |
Show There is little reason for application of viscosity to the low-rank coals. Because of its relationship to the softening temperature, Tcv may be used as a fusibility index in the same manner as the softening temperature. SODIUM CONCENTRATION Full-scale operation of steam generators fired on lignite and Illinois high-alkali fuels, as well as experimentation at the pilot plant level, has taught investigators about the importance of sodium concentration in coal. Sodium volatilizes in the furnace and condenses in the convection passes. The quantity that condenses depends on its reaction with other elements, fly ash and tube surface area available, and partial pressure of sodium in the flue gas. This last is influenced by the level of sodium in the fuel. From experience, investigators have learned that the severity of fouling is dependent on the level of sodium in the coal. Calcium in large concentrations suppresses sodium fouling. Quantitative ratios have not been published. European experience has influenced some to include an equivalent amount of potassium with the sodium; thus Na2o + 0.6589K2O as postassium and sodium behave in a similar manner. Since we are dealing with condensation and partial pressure levels in the flue gas, others have elected to express the total alkalis in the coal rather than the ash. SLAGGING AND FOULING INDICES The concern for the influence of coal-ash fusibility on fireside slagging and sodium concentration on fouling has led many investigators to develop 1-32 |