OCR Text |
Show The concept of using ash fusibility as a prediction index of clinker formation was introduced by Prost in 1895. :1»12 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 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 temperature 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. 1-16 |