Slag behavior

Recent work has shown that pulverized coal, if separated by gravity fractionation, can yield important information relative to slagging potential due to the iron content. (6,7). Results of this work have shown that the percentage of iron in the heavy fractions correlates very well to the slagging behavior in commercial boilers. (See Figure 3). This technique appears to identify the proportion of relatively pure pyrites particles that are generated in the pulverized coal feed and that are capable of melting at relatively low temperatures and that would account for enrichment of iron in lower furnace waterwall deposits. 

Ash yield between 6% and 25% for bituminous coal, or 4% and 25% for subbituminous coal—the ash must have particular slag viscosity characteristics since ash slag behavior is critical to satisfactory operation, and the maximum temperature at which the slag has a viscosity of 250 cP is 1340°C (2445°F) for bituminous coal and 1260°C (2300°F) for sub-bituminous coal. 

A frequently used estimation of the slag behavior in coal gasification systems is based on the CaO/Si2 ratio [29,96,97]. Figure 3.6 shows the semiempirical diagram that distinguishes slags according to their acidity. The minimum of the temperature, where 25 Pa-s can be achieved, is around the point of balance... 

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.,... 

A compositional analysis of the ash in coal is often useful in the total description of the quality of the coal. Knowledge of ash composition is also useful in predicting the behavior of ashes and slags in combustion chambers. Utilization of the ash by-products of coal combustion sometimes depends on the chemical composition of the ash. In addition, concern over release of certain trace elements to the environment as a result of coal utilization has made determination of these elements an increasingly important aspect of coal analysis. 

The vaporization behavior of the "Eastern" slag, in the composition range 24 to 21 wt.% K2O, is very different to that... 

Note that at T > 1400 K, the potassium pressures fall below those expected from KAIO2, but that the SO2, CO2, and H2O pressures are still relatively high. Apparently, at this stage, the K produced by sulfate and carbonate decomposition is retained in the bulk slag. After further heating, the sample was virtually depleted of Na, SO2, and CO2 H2O also continued to fall-off in pressure to a negligible level. Following this initial clean-up period, the sample showed a more no,rmal vaporization behavior and representative data are summarized in Table II. 

Synthetic Low Melting Slag (K >). A lower liquidus temperature ( V 1480 K), less viscous (as compared with the K sample) synthetic slag was prepared for studies analogous to those performed for the K4 system. It was hoped that this slag would not show the same anomalous activity behavior, at lower temperatures, as for the K4-MHD coal slag sample. 

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. 

Efforts are still being made to refine these empirical correlations and, in particular, to understand the reasons why certain slags fail to fit the correlations. The ultimate objective is to interpret slag rheological behavior in terms of specific phases present in the slag, to the extent that such phases can be identified and quantified. 

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