| Computer controlled scanning electron microscopy (CCSEM) is an invaluable tool that can be utilized to gather information on size as well as composition of a sample on particle-by-particle basis. Graphical representation of such information often reveals features that are not obvious from a tabular form of data presentation. The CCSEM technique and several graphical representations developed during the course of this research have been used to investigate the reactions of calcium, sodium and potassium in pulverized coal combustion. By a comparative study, it is established that the form of occurrence is of primary importance in understanding the behavior of basic elements in the coal during combustion and that the scale of the combustion furnace is of secondary importance. Calcium, in either discrete or dispersed form in the coal, reacts with clays to form Ca-aluminosilicate slag. Relatively larger Ca-aluminosilicate slag particles are formed when calcium is in a discrete form in the coal whereas smaller Ca-aluminosilicate slag particles are formed when calcium is in dispersed form in the coal. Calcium is also transformed to CaO fumes which either reacts with SO{dollar}sb2{dollar} to form CaSO{dollar}sb4{dollar} or reacts with Na vapors and SO{dollar}sb2{dollar} to form Ca-S-Na containing particles. Sodium volatizes during combustion and may condense on heat transfer surfaces as Na{dollar}sb2{dollar}SO{dollar}sb4{dollar} or may form a sticky layer on aluminosilicate particles to form Na-aluminosilicate which on cooling crystallizes into what is believed to be nepheline particles. Potassium, which is present in the mineral illite, in the coal, does not appear to volatize significantly but simply melts to form K-aluminosilicate slag. Comparison of the combustion reactions of dispersed sodium and potassium show that both elements behave similarly. Ion-exchanged potassium introduced in to a lignite mimics the reactions of sodium that is believed to be atomically dispersed in lignites. During combustion, the ion-exchanged potassium in a lignite forms kalsilite, a compositional analogue of nepheline. Formation of Ca-S-K containing particles with composition similar to that of Ca-S-Na is also observed. |
