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Show MFC when fIred with an eastern coal. There is a distinction between predicted and anticipated properties. Predicted properties are direct outputs from the model and are limited to deposit elemental composition. Anticipated properties are inferred from predicted rates of, for example, inertial impaction versus condensation are not quantitatively predicted. Anticipated properties include deposit morphology, strength, emissivity, and removability. Qualitative Predictions and Observations Rate of Deposition: Experimental results indicate that char particle fragmentation is more substantial for the cenosphere-forming bituminous coals than for subbituminous or lignite coals al). Therefore, the rate of impaction of particles on the tubes for the Wyoming coal is predicted to be higher than for the bituminous coal normally used in the boiler due to the size distribution of the fly ash. However, the subbituminous coal fly ash contains a higher proportion of constituents with low capture efficiencies than the bituminous coal fly ash. On balance, the rate of accumulation was qualitatively predicted to be about the same for the Wyoming coal as for the coals previously used in the boiler. This result, as with most of the results to be discussed in this paper, is specific to this coal under these operating conditions and is not a general property of western subbituminous fuels (see, for example, ill. The observed rate of deposition during the test bum is consistent with the predictions discussed above; The rate of deposit accumulation was about the same, perhaps slightly slower, than that experienced with previous bituminous coals in this boiler under similar operating conditions. Morpholo~y and Stren~th: The Wyoming coal contains a relatively high proportion of free silica compared to other coals of similar rank, ash chemistry, and geographical origin. The free silica particles are slow to fuse or sinter in the deposit and contribute to both the granular nature and the lack of deposit strength. This high percentage of free silica is also largely responsible for the high ash fusion temperatures of this coal compared to other similar coals. A second major contribution comes from the calcite. Calcium that originates as calcite (as compared to organically bound or siliceous calcium) has an effect similar to free silica on the deposit with respect to granularity and deposit strength, although less pronounced. This anticipated deposit strength is specific to this coal and is, in fact, the opposite of typical utility experience with coals similar in geographic origin and organic composition (18). The observed morphology and strength of the deposits is consistent with the predicted results. Specifically, the deposits were granular, friable, and showed no indication of significant sintering. Removability: Sintering and fluid formation in these coals is often associated with (a) the incorporation of alkali material in silica to form low-melting-point silicates and (b) sulfation of sodium or calcium on the relatively cool heat transfer surface to form their respective sulfates. We anticipated a lack of sintering or fluid formation based on our experience with other coals in MFC tests and the mineralogy of the Wyoming coal. Deposits generated from Wyoming-type coals that sinter in the rvfFC typically are associated with coals that have relatively high sodium and/or high sulfur contents relative to the ash content. The Hanna Basin coal (on which we have yet to perform combustion tests in the MFC) has a modest sodium content and a low ratio of sulfur to total ash compared to the other western coals. Therefore, we anticipated little sintering or formation of fluid phases. Western coals often fonn sintered deposits in utility boilers <lID. During the three-week test burn, deposits were easily removed using standard maintenance procedures, consistent with the anticipated behavior. There wa n indication of fluid or condensed phases next to heat transfer surfaces. 10 |
