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Index for Iron-Based Slagging for Pulverized Coal Firing in Oxidizing and Reducing Conditions A. R. McLennan,† G. W. Bryant,*,† C. W. Bailey,† B. R. Stanmore,‡ and T. F. Wall† Cooperative Research Centre For Black Coal Utilization, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW, Australia, 2308, and Department of Chemical Engineering, University of Queensland, St. Lucia, Old, Australia, 4072 Received June 16, 1999. Revised Manuscript Received December 1, 1999
A model for the prediction of iron-based slagging precursors from the combustion of ironcontaining coals is detailed. The model accounts for the form of iron (pyrite or siderite), the distribution of iron within the pulverized coal, temperature, and oxidizing or reducing conditions. The input required for the model is a CCSEM analysis of the pulverized coal. For oxidizing conditions, the index predicts similar behavior for pyrite, and siderite-containing coals, with iron alumino-silicate ash particles becoming sticky at temperatures greater than 1400 °C. This suggests that for oxidizing conditions, the extent of included iron minerals is the most important factor. For reducing conditions, the index predicts sticky ash particles are formed at lower temperatures, as low as 1000 °C for pyrite-containing coals as a result of the decomposition and partial oxidation of pyrite-forming sticky particles, and 1100 °C for siderite-containing coals. For reducing conditions, the level of excluded pyrite mineral for pyrite-containing coals and the level of included iron-containing minerals associated with clays for siderite- and pyrite-containing coals are the most important factors determining slagging.
Introduction Slagging refers to deposition in the region of the boiler where radiant heat transfer is dominant, including the burners, the main boiler waterwalls, the bottom hopper, and, usually, the bottom of the first bank of superheater tubes.1 Molten particles deposited on heat exchange surfaces are retained, whereas much of the dry ash material rebounds and is re-entrained in the flue gas.2 These ash deposits first undergo a sintering stage, but as thickness increases, the outer layer of the deposit is transformed to a tacky material of low viscosity 2 which can significantly increase the growth rate of deposits. Subsequently, large masses of semi-molten slag can build up.2 Boiler slagging experience with pyrite coals suggests that deposition of molten or semi-fused particles of pyrite residues may have a significant role in the initial stages of slagging,3 with severe slagging of pulverized fuel (pf) boilers usually being associated with coal ashes rich in iron.2 The more common iron containing minerals found in coals are listed in Table 1.4 North American and United Kingdom coals are known to contain iron predominantly in the form of pyrite.1,5-7 Australian and South African * Corresponding author. † University of Newcastle. ‡ University of Queensland. (1) Couch, G. Understanding slagging and fouling in pf combustion; IEA Coal Research: London, 1994. (2) Raask, E., Mineral impurities in coal combustion, behaviour problems and remedial measures; Hemisphere Publishing: Bristol, PA, 1985. (3) Bryers, R. W. The physical and chemical characteristics of pyrites and their influence on fireside problems in steam generators. J. Eng. Power 1976, 517-527.
coals have usually no more than a few percent of the mineral matter as pyrite, with siderite generally the dominant iron-bearing mineral.8,9,10 Indian coals are known to have low sulfur contents (50%) oxidizing to Fe3+ form for oxidizing conditions.23 Residence time was assumed to be sufficient for complete oxidation of iron minerals under oxidizing conditionssi.e., pyrite/siderite oxidized to magnetite/ hematite, and significant proportions of iron present in glass oxidized to Fe3+ form. To establish the temperatures at which such ash particles become sticky, reference was made to equilibrium phase diagrams. The FeO-Fe2O3 phase diagram,26 shown in Figure 1, indicated that melting commenced for wustite, magnetite, and hematite phases at 1370, 1600, and 1585 °C, respectively. For the FeO-FeS and Fe-glass phases, where melting commences at a eutectic temperature and continues over a temperature range, the criteria for the temperature above which ash particles become sticky was 25 wt % molten phases. Referring to the FeS-FeO phase diagram,27,28 also shown in Figure 1, it is apparent that at a temperature of 1000 °C for compositions from 6 to 87 wt % FeO greater than 25 wt % molten phase exists. Previous work has established the vast majority (>80 number %) of FeO-FeS ash particles formed under reducing conditions fall within this composition range,23 so FeOFeS phase ash particles are assumed to be sticky for temperatures >1000 °C. As Fe-glass phase ash formed under reducing conditions contain iron in the Fe2+ state, the melting behavior can be determined from the FeO-SiO2-Al2O3 phase diagram, shown in Figure 2.29 The triangular region for typical ash particle compositions 24 is shown, with the hatched sections indicating the solidus temperatures. (26) Muan, A.; Osborn, E. F. Phase equilibria among oxides in steelmaking; : Reading/MA, 1965. (27) Oelsen, W.; Mitt. K. Wilh. Inst. Eisenhu¨ ttenwes. 1961, 32, 741751. (28) Schu¨rmann, E.; von Hertwig, I. O. Gieβerei, techn.-wiss. Beih. 1960, 14, 31-36. (29) Osborn, E. F.; Muan, A. Phase Equilibrium Diagrams for Oxide Systems, Am. Ceram. Soc.: Columbus, OH, 1960.
Figure 3. Fe3O4-SiO2-Al2O3 equilibrium phase diagram.18
For an Fe-glass phase ash formed under oxidizing conditions with iron in both Fe3+ and Fe2+ forms, the melting behavior can be determined from the Fe3O4SiO2-Al2O3 phase diagram, shown in Figure 3. For the entire range of typical ash particle compositions, the solidus temperature corresponds to the eutectic point at 1382 °C. Thermodynamic equilibrium calculations for a range of Fe-glass compositions for oxidizing and reducing conditions determined that at a temperature of 20 °C in excess of the solidus temperature indicated by phase diagrams, greater than 25 wt % molten slag phase exists. Having established the temperatures at which the various iron-containing ash phases for oxidizing and reducing conditions become sticky, each phase was assigned a smoothed step function to indicate the mass fraction of sticky ash, as shown in Figure 4. The onset of formation of sticky particles corresponds to the solidus temperature. A stickiness factor of 1.0 (or 100% of particles being sticky) is assigned to that temperature where thermodynamic equilibrium calculations indicate 25 wt % slag formation for that particular composition. For the Fe-glass phase under reducing conditions (i.e., the FeOn-Al2O3-SiO2 system, Figure 2), it was assumed that 30 wt % of ash particles fall within the
Index for Iron-Based Slagging for Puliverized Coal Firing
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Figure 4. Determination of ash phases sticky as a function of temperature.
1205-1210 °C, 1083 °C, and 1088 °C regions, and 10 wt % within the 1148 °C region, each contributing to the function indicated in Figure 4. The slagging index is then calculated as a function of temperature using the CCSEM weight percentages of included and excluded minerals, the functions defined in Figure 4, the degree of association of included minerals (R, assumed 0.5), and the ash content of the coal as follows:
X(T)OX ) [(FeCO3 + FeS2 + FeS)excl*E(T)OX + (1 - R)*(FeCO3 + FeS2 + FeS)incl*E(T)OX + R*(FeCO3 + FeS2 + FeS + SiO2 + *kg ash/tonne coal SiO2 - Al2O3)incl*D(T)OX] X(T)RED ) [(FeCO3)excl*B(T)RED + (FeS2 + FeS)excl*A(T)RED + (1 - R)*(FeCO3)incl*B(T)RED + (1 - R)*(FeS2 + FeS)incl*A(T)RED + R*(FeCO3 + FeS2 + FeS + SiO2 + *kg ash/tonne coal SiO2 - Al2O3)incl*C(T)RED] Results and Discussion The values of the index for Coals A, B, C, and D are plotted for the temperature range 1000-1800 °C for oxidizing and reducing conditions in Figure 5. Comparison of the plots for oxidizing conditions reveals similar behavior for all coals. At approximately 1400 °C the contribution from Fe-glass phase becomes effective, with a further increase at ∼1600 °C due to magnetite and hematite. The Fe-glass phase is formed from the coalescence of included pyrite or siderite and alumino-silicate minerals, which is dependent on the proportion of included minerals and their degree of association. The magnetite and hematite is formed from excluded pyrite or siderite, and included pyrite or siderite not closely associated with other minerals. Hence for oxidizing conditions, ash deposition and slagging potential is most influenced by the level of included iron minerals and their degree of association with alumino-silicates, not by the total iron content. For siderite-containing Coal A the reducing atmosphere plots show the contribution of the Fe-glass phase becoming significant at 1100 °C, 300 °C lower than for oxidizing conditions. A further contribution at ∼1370
Figure 5. Slagging index for oxidizing and reducing conditions as a function of temperature for (a) Coal A, (b) Coal B, (c) Coal C, and (d) Coal D.
°C is due to wustite derived from excluded siderite and included siderite not closely associated with other minerals, ∼200 °C lower than for oxidizing conditions. Hence for siderite-containing coals under reducing conditions, the ash deposition and slagging potential is again most influenced by the level of included iron
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minerals and their degree of association with aluminosilicates, not by the total iron content. For pyrite-containing Coals B, C, and D the plots show the contribution of the FeO-FeS phase is significant at temperatures as low as 1000 °C, 400 °C lower than for oxidizing conditions. Further contributions to sticky ash proportions are noted at 1100 °C due to Fe-glass derived from included pyrite or siderite and aluminosilicate minerals, and at ∼1370 °C (Coal B) due to wustite derived from excluded siderite and included siderite not closely associated with other minerals. Hence for pyrite-containing coals under reducing conditions, sticky ash particles are formed at low temperatures from both included and excluded pyrite mineral, thus ash deposition and slagging potential is influenced by the total iron content. Under oxidizing conditions, the residence time may not be sufficient for the complete oxidation of pyrite to magnetite or haematite,24 as assumed by the model, which may cause the index to underestimate the mass of sticky ash. Under reducing conditions, some oxidation of pyrite and siderite to form magnetite in the early stages of combustion before oxygen depletion may occur,13 rather than forming FeO-FeS phase and wustite as assumed by the model, and may cause the index to overestimate the mass of sticky ash. Considering these issues, the values given by the index may be viewed as indicating the extremes of behavior for oxidizing and reducing conditions. However, incorporation of a kinetic model for the oxidation of pyrite and siderite to introduce residence time as a variable would allow more accurate predictions to be performed using the index. Mineral particle size can also have significant effects on slagging mechanisms. Generally, smaller ash particles (5µm) are associated with slagging. Thus the larger excluded minerals are likely to produce ash particles that contribute to slagging. These larger excluded minerals are also slower to oxidize, further enhancing their slagging potential. Coalescence of included minerals also produces ash particles large enough to contribute to slagging. However, recent work30 has shown the degree of coalescence of included minerals to be primarily dependent on the extent of char fragmentation, which in turn is influenced by char structure. As CCSEM analysis of a coal sample also yields the particle size of the minerals, consideration of these effects could be incorporated into the model to further improve it. Overall, the index reveals a significant increase in the ash deposition and slagging potential of high-iron coals for reducing conditions compared to oxidizing conditions. At lower temperatures (
