OCR Text |
Show impact directly on convective heat exchange surfaces, selective deposition of the heavier gravity fractions occurred. Table 8 compares the chemistry of the deposits with the coal ash and the sink 2.85 gravity fraction of the coal ash for Upper Freeport coal. The very high concentration of iron on the first fouling bank tube deposit suggests that only a portion of the pyrites in the sink 2.85 gravity fraction is contaminated. Further separation at some gravity between 2.85 and 5.0 might reveal the true level of pure pyrites. The quantity of deposits formed on these convective tubes depends upon the total pyrites liberated from the coal and nonpyritic mineral matter. There is evidence in the SEM scan in Figure 17 that oxidation of the pyrites on the fouling probe tubes is not completed until after deposition, as the deposit appears to be completely fused. The melting temperature of this deposit under oxidizing conditions, as determined by ASTM methods, is in excess of 2375°F, which is higher than the local temperatures encountered. The local flue gas temperatures vary from about 1000°F at the tube surfaces to 1800°F in the bulk flue gas stream. Eutectics in the FeS-FeO system were reported to melt at 1740°F, and therefore, it is possible that fusion had taken place prior to complete oxidation of the iron. It is also seen in Figure 17 that deposits removed from downstream tubes are not fused in such a manner since the flue gas temperatures at this location were lower than 1600°F. Ash deposited during advanced stages of deposition in Zone B closely resembled the coal ash for all coals fired except Upper Freeport, Indiana County coal. There was a slight enrichment in the iron concentration, producing a slag which appears to be a blend of the -1.80/+2.85 gravity fraction and -2.85 gravity fraction. Deposition rate increases in the advanced stage of slagging when a molten surface forms, and fly ash retention approaches 100 percent. The iron enrichment is not as great on the furnace walls as it is on the furnace roof or convection tubes, as the heavy iron-rich gravity fractions are probably concentrated along the center of the flow stream. Thus, they do not readily come into contact with the existing molten ash along the furnace walls, whereas particles of lower gravity rich in silica, alumina, and calcium do. CONCLUSIONS Selective deposition of mineral matter in the furnace is sensitive to 47 |