OCR Text |
Show individual particles of pure mineral compounds and blended mixtures of minerals or portions of the 10 basic elements found in coal: aluminum, silicon, titanium, iron, calcium, magnesium, sodium, potassium, phosphorus, and sulfur-each with its own particle size distribution and physicochemical properties upon heating. Furthermore, the juxtaposition of the mineral species during combustion will determine the temperatures they will reach, their ultimate composition as fly ash particulate, and the degree of melting or volatilizing they will undergo. Figure 2 is an oversimplification of the composition of coal for a given particle size, illustrating several basic species that may be introduced into a furnace during combustion. The first species is pure coal, which rarely exists. Theoretically, it contains no ash and thus presents no problems. In reality, this species contains as much as 2 percent inherent ash, which may or may not have the same chemistry and melting temperature as the bulk source. The specific gravity of this species is usually less than 1.30. A second species exists, which contains a blend of FeS2 and non-pyritic ash included in the coal particle. The specific gravity of this particle may range from approximately 1.30 to 2.80, depending upon the percentage of each constituent present. During ashing, pyrites form one of the oxide states of iron, which act as a fluxing agent for the clays present (i.e., aluminosilicate compounds). Therefore, the melting temperature of the resultant ash may range from 1900 to 2800°F, depending on the percentage of pyrites and aluminosilicates present. Two other coal-bearing species may exist, containing either pure pyrites or pure clays. The former will have a high specific gravity because of the iron present. Its melting temperature upon complete ashing should exceed 2800°F. The latter will have a somewhat lower specific gravity, since it is a blend of alumina, silica, and small amounts of calcium, magnesium, sodium, and potassium. Depending upon the fluxing action of the minor and trace constituents, the melting temperature of the ash may vary from about 2200°F to 2800°F. All the coal-bearing species are expected to reach the maximum furnace temperatures because of their intimate contact with the combustibles in the coal. Three other species may exist-one containing pure aluminosilicate compounds; one consisting of pure pyrites; and the third, a blend of the two. All these species should reach a maximum temperature somewhat lower than the flame temperature because of the flow of heat from the flame to 8 |