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Show - 8 - ASH DEPOSIT COMPOSITION One additional area of study is the relationship between composition of the parent coal and the composition of deposits formed on tube or water-wall surfaces. The partitioning of major ash elements plays an important role in determining the melting tendencies of ash, in the case of slagging, or in determining the bonding characteristics of the condensing materials in fouling deposits. Both thermal conductance and radiative absorptance have also been shown to display compositional dependencies (15-17). Moreover, the ability to predict deposit composition based on coal composition and operating parameters will aid in the accurate projection of impacts accompanying fuel switching or blending. A complete discussion of elemental partitioning in the Cons01 1.5 MM Btu/hr combustor has been reported elsewhere.(12) Fe 203 enrichment and Na 20 depletion contributed to the most significant shifts in deposit composition. The degree of enrichment for Fe 203 was dependent on location (water-wall panel or fouling probes), heat release rate, as-fired Fe 203 concentration, and firing mode. Na 20 depletion in the bulk deposits (a consequence of vaporization) depended on location and heat release rate. Figure 9 is a plot of location resolved base-to-acid ratios (as a fusibility indicator) with firing mode notation. It is interesting to note the similarities between fouling probe deposits and the as-fired ash. As most boilers are successfully designed around limiting radiant section exit gas temperatures below the as-fired fusion temperatures, this is seemingly consistent. Lastly, the differences in radiant section gas flow patterns and subsequent deposition behavior (a consequence of differing firing modes), appear to produce distinct effects on the bulk fouling probe deposit compositions. This unders ta ndab 1 e di fference wou 1 d appea r otherwi se to be in contras t to the intuitive independence of fouling behavior on firing mode effects. Moreover, the firing-mode dependence appears to be directly substantiated by the significance of this term in the presented linear discriminant analysis of fouling behavior. CONCLUSIO 'S A data base comprised of over 130 combustion tests, using 33 different fuels in three different firing modes (wall, opposed-wall, tangential) was used to resolve ash deposition related phenomena in a 1.5 MM Btu/hr pilot-scale combustor. In addition to coal quality considerations, significant firing-mode dependencies were observed for slagging-induced heat flux reductions to a simulated water-wall, locational deposit elemental oxide compositions, fouling deposit removal problems, and convective section plugging difficulties. Operating conditions also contributed substantially in these four behavioral areas. A consideration of the combined effects of operating conditions and coal characteristics would appear to be necessary for improving predictive capabilities of existing slagging/fouling indices. |