OCR Text |
Show of iron in the heaviest gravity fraction, as seen by comparing Figure 12 with Table 7. In addition, when the degree of slagging for each coal, as depicted in Figure 12, is compared with the ash-softening temperatures of individual size and gravity fractions within each coal, as presented in Figure 5, it is seen that the greatest degree of slagging occurred with coals where the variability in melting temperatures between individual components was greatest. As previously mentioned, the Upper Freeport and Lower Freeport coals displayed the greatest scatter in melting temperatures of individual size and gravity fractions, and indeed, these coals experienced the greatest amount of slag formation during firing. The enrichment of iron in the slag removed from the wall of the furnace and the slagging probes was not as dramatic as was expected, based on the quantitative analyses of deposits removed, as shown in Tables 8 and 9 for Upper and Lower Freeport coals, respectively. This was probably due to contamination of the heaviest gravity fractions by nonpyritic minerals rich in Al203, Si02, and CaO whose gravities are greater than 2.85. The minerals responsible for this contamination could include calcite, quartz, illite, and ankerite. All these were found to be major or, at least, accessible minerals in the coals tested. The bond of slag to the slagging probes was extremely light in all cases. Scanning Electron Microscope Analysis (SEM) and Energy Dispersive X-Ray Analysis (EDAX) of the layers of deposit adjacent to the tube surface indicate ash deposition was initiated by particles 8y or less in size, rich in alumina, silica, and iron. Compared to the coal ash, fly ash, and outer layers of slag, there is also an enrichment of potassium. Figure 13 presents typical SEM and EDAX scans showing the variation of potassium level with location in the slag for the Lower Freeport center slagging probe. By extending the scope of the analyses to results obtained during other nonrelated, yet relevant, firing tests which included fuels not characterized by size and gravity techniques, and a number of subbituminous as well as bituminous fuels, it was deduced that the degree of slagging was dependent upon the formation of an initial dust layer related to the potassium level of the coal ash. A Western fuel fired with an ash 39 |