OCR Text |
Show the particles. The actual temperature will depend upon particle size and physical location within the combustor. The pure aluminosilicate compounds have a specific gravity between 2.0 and 2.8, depending upon composition, and melting temperatures between 2200 and 2800°F, depending upon the concentration of minor elements. The blended species of pyritic and nonpyritic ash has a specific gravity ranging between 2.0 and slightly less than 5.0, depending upon the percentage of pyrites present. Melting temperatures may range between 1900 and 2800°F, again depending upon the percentage of pyrites present. The heavier particulate will have the lower melting temperatures until the pyrites exceed 50 percent by weight concentration. A further increase in pyrite concentration will increase this weight and the melting temperature of the oxidized ash. The species consisting of pure pyrites has a specific gravity of about 5.0. Its melting temperature depends upon the degree of sulfur reduction and iron oxidation. Melts as low as 1112°F have been detected during the early stages of desulfurization. 11 Figure 3 is a thermogram illustrating the formation of a melt under reducing conditions. The phase diagram in Figure 4 shows a eutectic is formed between FeS and FeO at 1740°F, indicating a substantial melt is formed after 50 percent of the sulfur has been released. The final melting temperature upon completion of the oxidation depends on the percentage of contaminant present; it can exceed 2300°F. Depending upon the distribution and orientation of the minerals within the coal, the actual melting temperatures of individual species of ash entering the furnace may vary anywhere from 2000 to 2800°F, while the composite ash may have a melting temperature of 2400°F. It is quite evident that a small portion of the total ash may be responsible for a fireside problem because of low melting temperatures, and these low temperatures are not de-tectable by traditional methods of analysis. Any blending or alteration of the impurities in the coal must address the disposition of the minor constituents whose melting temperatures are undetected by composite sample analysis because of dilution bv innocuous constituents. The size distribution of the mineral matter in coal will influence which species are released during crushing and pulverizing and hence influence which species are readily removable and which have the greatest potential for contacting heat-transfer surfaces during and after combustion. 10 |