Slag phenomena

The heat transfer efficiency is significantly affected by the slag layer properties and behavior therefore, those factors other than slag phenomena that affect HTE are presented in Table 5. As for the PCR, it is desirable to keep the HTE as high as possible. An increase in HTE at the same PCR decreases fuel consumption (Fruehan et al., 1989 Keogh et al.,... 

Table 5. Factors Other Than Slag Phenomena That Affect the HTE...

A common thread runs throughout the behavior of low-rank coal inorganics in fouling, agglomerating, and slagging phenomena the important role of sodium and the formation of melilite phases. As yet no one has developed a "grand synthesis" of a unified theoretical explanation of these phenomena. It seems very likely that such a synthesis is possible, and we may hope that it will be forthcoming in the not-too-distant future. 

Although the data obtained in these studies have been valuable in interpreting the slagging phenomena observed in gasifier tests, a more fundamental objective of the current work is to develop correlations that can be used to predict a priori the viscosity behavior of low-rank-coal slags from a knowledge of the ash or slag composition. 

As it has been shown by Roy [80] the alkahs content in the pore solution of cement paste was significantly lower at 50% slag addition (Fig. 6.35). It should be tmderlined that this phenomenon is observed irrespectively of the alkali content in mineral additions, which can be higher than in cement. Similarly, the sodium and potassirrm content decrease in pore solution occirrs in case of fly ash cement paste [115]. The effect of fly ash is, however, not clear, because in some cases Diamond [129], as well as Glasser and Marr [130], observed the increase of these ions concentration in the liquid phase of cement paste. However, these authors are of the opinion that silica firme arrd, after longer period of time, fly ash, reduce the sodium and potassium soluble corttpotmd content [129, 130]. 

Figures 3a and 3b show the calculated flow fields for the 260 mm immersion case. The predominant flow direction in the slag was upwards from the interface to the top and back down in a loop. This flow was driven by both natural convection and the buoyancy generated by the carbon monoxide gas produced by the electrode-slag reactions. Hence, it was strongest around the electrodes, oftheordra-of 0.1 m/s, and decreased to die ordra-ofO.OI m/s away from the electrodes. In addition to the principal recirculatory flow loop around the electrodes, a second weaker recirculatory loop formed below the electrodes due to the temperature difference between the electrode and the interface. By contrast, the bullion was relatively quiescent, except for a weak flow loop near the side wall, apparently in a counterintuitive direction (Figure 3b). More work is required to establish whether this loop is a real physical phenomenon or an artefact of the mesh geometry.

Changes in the cement manufacturing process by using slag as a substitute for clinker, proportionally decreases the Cr(VI) concentration of cement. This phenomenon is considered to be one of the reasons for the decline in the prevalence of cement dermatitis among construction workers in recent years in Singapore (Goh and Gan 1996). 

The temperature phenomenon described above was consistent with measurements of slagging and fouling. During many of the tests sootblowing was not permitted, and the evaluation focused upon the rate of temperature increase at the economizer exit as a function of time. In virtually all cases the rate of temperature rise was faster when no biomass was in the fuel mix than when biomass was incorporated in the fuel mix. The rate of temperature rise measures deposition indirectly, since it measures reduction in heat transferred from the combustion products to the steam as a function of fouling. 

It is significant to point out that the present method is based on the relationship between interionic associations and their consequential effects on slag viscosities, which was shown by Seetharaman et al [7], In principle, any property that reflects the interionic interactions and the structural evolution, cluster behavior at different temperature intervals should show the same characteristic. If agglomeration or atomic rearrangement is manifested in the viscosities of slags, other physical properties like electrical conductivity should reflect this phenomenon as well. 

Another form of deterioration in concrete involves the chemical reaction between the products of cement hydration and carbon dioxide. This reaction decreases the pH of the solution, and this may lead to the corrosion of the embedded steel. Carbonation shrinkage is another phenomenon that has been documented. Maslehuddin, et al., examined by DTA/TG, the products formed in mortar specimens exposed to CO2 at 55-75°C for 54 weeks. Some were contaminated with chloride and sulfate ions. The amount of Ca(OH)2 and CaC03 formed in several cement mixes containing fly ash, silica fume, and slag was determined (Table 9). In general, the amount of Ca(OH)2 is lower in samples exposed to CO2, and the amount of CaC03 in samples exposed to CO2 is higher. There is also an accelerated carbonation in contaminated specimens. 

A major problem with the gas-stirring method of fluid mixing and impurity removal is the re-entrainment of the slag into the molten steel. This phenomenon is generally associated with the mixing and flow characteristics at the slag/metal interface and has attracted the interest of many researchers in recent years [3-8]. 

The refractories exhibit open porosity which allows impregnation by slags. This impregnation depends on the nature of the materials. It occurs for carbonless refractories, however, impregnation is veiy limited and even non-existent for catbon refractories (since carbon has an anti-wetting effect). For a given porosity and porosimetiy, the amplitude of the phenomenon depends on the capillary motor... 

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