OCR Text |
Show with low-sulfur Western fuels. Ten different fuels have been fired at this time. BACKGROUND Fireside deposits and corrosion are attributed to the formation of a molten phase between the various constituents in the fly ash as a result of interaction of the mineral matter in the coal upon heating, chemical reaction with components in the flue gas, or condensation of the volatile constituents. Since the major portion of the inorganic mineral matter does not volatilize during combustion, bench scale tests and empirical indices have focused on identifying the melting temperature of the ash. Historically, the fusibility of the ash has been based on the composite analysis of the coal. The concept of using ash fusibility as a prediction index of clinker formation was introduced by Prost in 1895. ' He also proposed the first index for correlating fusion temperature to the chemical analysis of the ash. The ASTM ash fusion cones are used as a measure of fusibility in the United States. A Lietz apparatus is used in most experiments overseas. The ASTM method determines the temperatures at which a small tetrahedron, compressed to a standard size and shape from an ash/glucose mixture, melts when heated at a constant rate in a controlled environment. The ash is obtained from coal ashed in a muffle furnace at 1600°F. The cones are heated in an oxidizing as well as reducing environment. These tests are performed under a slow heating cycle, whereas fly ash is cooled rapidly. An unaccounted-for hysteresis temperature difference occurs between the superheating in the laboratory and supercooling in the flue gas stream. Ash fusion cones are highly subjective. Since the coal ash is heated to 1600°F, there is the possibility of losing some elements, such as sodium, that volatilize at low temperatures. The cones do not identify minor low-melting phases, and they cannot account for melts formed by sulfates between tube temperatures and 1600°F. To compensate in part for these deficiencies, some investigators are using thermoanalytical techniques with low-temperature ash. These procedures pick up minor melts at intermediate temperatures or transitory melts that may cause ash to stick. From the very beginning, investigators found a need to correlate ash fusion characteristics with ash chemistry to determine the elements responsible for lowering melting temperatures and to what degree they were V 3 |