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Show 12 depending upon input conditions. The surfaces within a utility boiler which would correspond to this type of deposit collection would be the topmost sections of the water-wall tubes in the combustion chamber or even the radiant superheater tubes. Deposits for future studies will be collected on a temperature controlled tube made of 304 stainless steel, and preferably collected at lower surface temperatures corresponding to water-wall tubes. Ash Deposit Sample Analysis The main tool used in the analysis of the mineral matter deposits is a Scanning Electron Microscope (SEM) equipped for Energy Dispersive X-ray Analysis (EDAX). The SEM can produce a micrograph of up to 40,000 magnification while showing exquisite surface detail. The sample to be examined requires that a conductive coating be applied to the surface to obtain an SEM image. For the samples analyzed in the study, gold coating was used for its superior clarity in producing micrographs. Elemental analysis can be obtained simultaneously with the SEM image through EDAX. This analysis is based upon the ejection of inner shell electrons of the sample atoms by the high energy beam of the SEM. As the electrons drop back to their inner shell positions, a characteristic X-ray frequency (or energy) is emitted for each element due to its unique electronic structure. By analyzing the x-ray spectrum a qualitative elemental composition of the surface under SEM observation can be obtained. A limitation of the gold coating of the surface necessary for good SEM resolution is that it masks the signal for the element sulfur in the EDAX scan. If the sulfur in the sample is to be identified by EDAX analysis then an electrically conducting coating other than gold, e.g. copper, has to be used. Carbon, hydrogen, and nitrogen contents of the deposits are determined using a Perkin-Elmer Model 240B Elemental Analyzer. This instrument reacts the sample with pure oxygen at elevated temperatures and then measures the thermal conductivity of the resulting off-gases to evaluate the CO , HpO, and N? produced. These data are then used to calculate the amount of carbon, hydrogen, and nitrogen present in the deposit sample. Results The results of analysis of the deposits collected in CVS flames at the MIT CRF are very similar to those obtained in parallel studies of coal-oil mixture flames. Two features were found to be common: evidence that the deposited mass has passed through a molten stage, and the presence of tiny iron-rich crystals on the surface of the slag mass. Both features can be clearly seen in Figure 26. The appearance of once molten material in this case is surprising since the reported initial deformation and ash fusion temperatures of the ash from the coal used in the CVS are greater than 1540°c, which is almost 100 C higher than the highest peak flame temperature recorded in any of the CVS flames. This behavior is best explained by the influence of the char burn-out on deposit formation in CVS combustion. In the previous section, Figures 20-25 show large agglomerates of char formed from single CVS droplets. Such large particles require a rather long residence time to be completely consumed and, further hindering burn- |