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Energy & Fuels 1996, 10, 1215-1219
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Phase Diagram Approach to the Fluxing Effect of Additions of CaCO3 on Australian Coal Ashes H. J. Hurst,* F. Novak, and J. H. Patterson CSIRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde, New South Wales 2113, Australia Received December 26, 1995. Revised Manuscript Received August 12, 1996X
This paper presents a phase diagram approach to predict the melting temperatures of coal ash/flux mixtures and the viscosity versus temperature characteristics of the molten slags. This is illustrated by calcium oxide fluxing studies of three Australian bituminous coal ashes covering a range of silica to alumina ratios. Reasonable agreement was obtained between the observed melting temperatures of the calcium oxide/coal ash mixtures and predictions based on equilibrium phase diagrams for the ternary system silica, alumina, and calcium oxide or for the quaternary system silica, alumina, calcium oxide, and ferrous oxide. The approach is supported by energy dispersive scanning electron microscopy (SEM), SEM microprobe, and X-ray diffraction (XRD) studies of the slags. Slag viscosity measurements were made with a rotational viscometer over the range 1200-1600 °C. The measured viscosities were compared with predicted values using a model based on experimental results for the ternary system silica, alumina, and calcium oxide. The agreement between experimental results and predictions from this approach suggests that sensible estimates can be made of the amount of fluxing agent necessary for satisfactory slag tapping from the ash content and ash composition of the coal.
Introduction Concerns over greenhouse gas emissions and the need for greater efficiency in power generation have led to increased interest in integrated gasification combinedcycle (IGCC) technology. A number of demonstration plants (up to 300 MW) are now in operation. Several types of gasifiers are being tested, but it is likely that an entrained flow gasifier operating in a slagging mode would be best suited for gasification of Australian bituminous coals. Several such gasifiers have been developed, operating at high temperatures (up to 1800 °C) and pressures (15-20 bar). The mineral matter in the coal is melted, runs down the gasifier walls, and is then tapped as a molten slag. Thus, it is essential that the slag has a viscosity low enough for optimum slag tapping. The viscosity should be 1500 °C. This disadvantage may be overcome by either adding a flux (generally limestone) or blending with another coal with more suitable ash characteristics. Although overseas IGCC developers have tested a number of Australian coals, the results are proprietary. Initial work on slag viscometry measurements and the relationship of viscosity to successful slag tapping was carried out for U.S. coals2,3 and was confirmed and extended by British workers.4-6 Some data for AustraAbstract published in Advance ACS Abstracts, September 15, 1996. (1) Ely, F. G.; Barnhart, D. H. Chemistry of Coal Utilization; Lowry, H. H., Ed.; Wiley: New York, 1963; Suppl. Vol., Chapter 19, p 831. (2) Corey, R. C. U.S. Bur. Mines, Bull. 1964, No. 618, 64 pp. (3) Nicholls, P.; Reid, W. T. Trans. ASME 1940, 62, 141-153. (4) Shaw, J. T. Br. Coal Util. Res. Assoc., Mon. Bull. 1959, 23, 170190. X
lian bituminous coals were obtained nearly 30 years ago7 and some from more recent work.8 A recent review covers the characterization of Australian coals for use in IGCC power generation.9 The interest in slag properties has led to the development of models for the prediction of slag viscosity10-13 and flux requirements.14 This paper presents experimental measurements of melting points and viscosities of slags prepared by fluxing selected Australian coal ashes with calcium oxide and compares the results with model predictions, using a major component phase diagram approach suggested by Kalmanovitch.15,16 The work forms part of broader studies to cover the range of compositions observed in Australian bituminous coal ashes. (5) British Coal Utilisation Research Association Report to CEGB; Surrey, England, Dec 1963; Vol. I-III. (6) Hoy, H. R.; Roberts, A. G.; Wilkins, D. M. Inst. Gas Eng., Commun. 1965, 672, 444-469. (7) Boow, J. J. Inst. Fuel. 1965, 38, 3-12. (8) Hurst, H. J.; Novak, F.; Patterson, J. H. Proceedings of the 28th Newcastle Symposium on “Advances in the Study of The Sydney Basin”, Newcastle, NSW, Australia; University of Newcastle: Newcastle, Australia, 1994; pp 352-359. (9) Harris, D. J.; Patterson, J. H. Aust. Inst. Energy J. 1995, 13, 22-32. (10) Watt, J. D.; Fereday, F. J. Inst. Fuel 1969, 42, 99-103. (11) Riboud, P. V.; Roux, Y.; Lucas, D.; Gaye, H. Fachber. Huttenprax. Metallweiterverarb. 1981, 19, 859. (12) Urbain, G.; Cambier, F.; Deletter, M.; Anseau, M. R. Trans. Br. Ceram. Soc. 1981, 80, 35-43. (13) Streeter, R. C.; Diehl, E. K.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1983, 28, 174-191. (14) Ashizawa, M.; Inamaru, J.; Takahashi, T.; Hara, S.; Kobayashi, Y.; Hamamatsu, T.; Ishikawa, H.; Takekawa, T.; Murakami, N.; Koyama, Y. Central Research Institute of Electric Power Institute, Report EW90003, Feb 1990. (15) Kalmanovitch, D. P. Ph.D. Thesis, Imperial College, London, 1983. (16) Kalmanovitch, D. P.; Sanyal, A.; Williamson, J. Engineering Foundation Conference on Slagging and Fouling due to Impurities in Combustion Gases, Copper Mountain, CO, 1984; p 537-554.
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Table 1. Coal Ash Analyses A ash content, % ad AFT, °C, reducing atmos deformation hemispherical spherical flow ash composition, wt % SiO2 Al2O3 Fe2O3 TiO2 Mn3O4 CaO MgO Na2O K2O P2O5 SO3 total SiO2/Al2O3
B
C
9.3
18.0
8.8
>1600 >1600 >1600 >1600
1560 1580 1600 >1600
1180 1260 1280 1400
51.5 34.5 2.04 2.0 0.01 1.68 0.81 1.15 0.30 0.08 1.63
85.4 11.7 0.71 0.43 0.01 0.06 0.06 0.01 0.36 0.05 0.05
50.8 22.9 9.84 1.55 0.10 4.54 0.88 0.12 0.54 0.82 4.68
95.7 1.49
99.5 7.3
96.8 2.22
crucible combination had previously been found from measurement of a standard reference material18 over the range 10001300 °C. SEM Measurements. After the viscosity measurements, the Mo crucibles containing the slag were left in the furnace to cool overnight. The crucible was then sectioned, with half being used for SEM measurements and the other for chemical analyses by HF digestion and ICP analysis. The SEM measurements were made with a Cambridge S240 instrument and a Noran energy dispersive spectrometry unit. Quantitative measurements were made using a Cameca CAMBEBAX electron microprobe and wavelength dispersive spectrometry. XRD Measurements. The slags were crushed and ring milled to obtain a fine powder for analysis and XRD measurements, which were made using a Phillips Bragg-Brentano diffractometer. Typical slag spectra showed a line pattern superimposed on a broad background from the amorphous slag. The analysis was performed by the computer program SIROQUANT19 using Rietveld refinement of the full XRD profile.
Discussion Experimental Section Ash Properties. Three coals, A, B, and C, were selected to cover the range of silica to alumina ratio for Australian bituminous coals. The coals were crushed and ashed to completion at 810 °C. The relevant analyses are given in Table 1. Slag Melting Temperatures. A prediction of the amount of flux required was made from either the ternary SiO2Al2O3-CaO equilibrium phase diagram17 or the quaternary SiO2-Al2O3-CaO-FeO phase diagram.16 Liquidus temperatures were determined from these diagrams for the ash and for increasing additions of CaCO3 flux. A liquidus temperature of 90% of the total, it is reasonable to assume that the properties of the coal ashes may be predicted from these four major components. This approach is more successful for coal ashes fluxed with calcium oxide, because the addition of the flux ensures that the slag is more closely represented by the ternary silica, alumina, and calcium oxide or by the quaternary silica, alumina, ferrous oxide, and calcium oxide systems. The effects of the minor components in the coal ash are then masked. Table 2 shows the calculated compositions of three ash/flux mixtures from coal ashes A, B, and C and the slag analyses from the nine ash/flux mixtures. As might be expected, changes in composition occur with the losses in heating from 810 to 1500 °C. However, if the compositions of the mixtures and slags are compared using normalized major components (Table 3), little difference is observed between the predicted and experimental compositions, because the major components are unaffected during the melting of the coal ash. The BCURA study of British coal ash slags4 also used molybdenum components to minimize slag reactivity with the crucible, and found that normalized slag analyses were similar to those predicted from coal ash (18) Broadbent, C. P.; Franken, M.; Gould, D.; Mills, K. C. Proceedings of the 4th International Conference on Molten Slags and Fluxes, Sendai, Japan; Iron and Steel Institute of Japan: Tokyo, 1992; pp 439443. (19) Taylor, J. C. Powder Diffr. 1991, 6, 2-9.
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Energy & Fuels, Vol. 10, No. 6, 1996 1217
Table 2. Analyses for Ashes, Fluxed Mixturesa (Calculated), and Slags (Weight Percent) SiO2 Al2O3 Fe2O3 TiO2 Mn3O4 CaO MgO Na2O K2O P2O5 SO3
AF1 m
AF1 s
AF2 s
AF3 s
AF4 s
BF1 m
BF1 s
BF2 s
Cs
CF1 s
CF2 m
CF2 s
45.2 30.3 1.79 1.75 0.01 13.8 0.71 1.01 0.26 0.07 1.43
45.8 30.0 1.92 1.60 0.01 13.4 0.85 0.87 0.25 0.08 0.22
41.1 27.9 1.92 1.53 0.02 23.2 0.84 0.86 0.25 0.08 0.25
36.3 24.8 1.61 1.37 0.02 29.8 0.81 0.77 0.21 0.07 0.34
33.0 22.8 1.44 1.29 0.02 36.4 0.82 0.73 0.18 0.07 0.43
61.3 8.43 0.51 0.31 0.01 28.2 0.04 0.01 0.26 0.04 0.04
59.0 8.72 0.53 0.38 0.01 27.4 0.24 0.05 0.26 0.05 0.46
52.2 9.11 0.55 0.33 0.01 31.6 0.24 0.05 0.26 0.05 0.52
54.7 23.7 9.98 1.92 0.09 4.66 0.95 0.12 0.54 0.88 0.11
48.0 22.8 9.01 1.60 0.08 10.2 1.07 0.11 0.75 0.79 0.19
45.7 21.0 7.95 1.24 0.06 14.7 0.96 0.07 0.67 0.79 3.89
45.1 21.9 8.33 1.51 0.07 14.7 0.99 0.12 0.69 0.77 0.25
95.0
97.9
96.0
97.2
97.1
95.0
97.7
94.6
total aAF1
94.4
m, coal ash A, flux addition 1, predicted from ash. AF1 s, coal ash A, flux addition 1, slag analysis.
Figure 1. Representation of (a, left) 1600 °C. The addition of calcium oxide changes the composition along the indicated straight line from the mullite region to the anorthite and gehlenite regions with an (20) Yamanaka, T.; Mori, H. Acta Crystallogr. 1981, B37, 10101017. (21) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60, 577-584. (22) Schobert, H. H.; Witthoeft, C. Fuel Process. Technol. 1981, 5, 157-164. (23) Mills, K. C.; Rhine, J. M. Fuel 1989, 68, 193-200.
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1218 Energy & Fuels, Vol. 10, No. 6, 1996
Figure 2. Observed and melting termperature for weight percent addition of CaO to coal ash. A, anorthite; C, cristobalite; CW, pseudowollastonite; G, gehlenite; M, mullite; T, tridymite.
initial reduction in liquidus temperatures. Figure 2 shows reasonable agreement between the experimentally determined melting temperatures and the liquidus temperatures predicted from the phase diagram. An upper limit to flux addition is provided by the increase in melting point for mixtures in the gehlenite region with >90% CaCO3 addition. Prediction of the fluxing behavior of coal B is also shown in Figure 1a. The composition of the ash lies in the mullite region, and the addition of calcium carbonate moves the composition of the mixtures into the cristobalite, tridymite, and pseudowollastonite regions. Figure 2 again shows reasonable agreement between the experimental melting points of the fluxed mixtures and the predicted liquidus temperatures. An increase in
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Hurst et al.
Figure 3. Viscosity-temperature relations for coal ash/CaCO3 mixtures.
melting point is seen in the cyclowollastonite region for mixtures with >70% CaCO3 addition. The prediction of the fluxing behavior of coal C was obtained using the quaternary phase diagram for the silica, alumina, calcium oxide, and 10% ferrous oxide system.16 The addition of calcium carbonate changes the composition of the ash from the high melting point mullite region to the lower melting point anorthite and gehlenite regions shown in Figure 1b. Only three experimental melting points were obtained (Figure 2), but comparison with the predicted temperatures shows that reasonable agreement was obtained and that this covered the mullite to anorthite boundary region (Figure 1b). Slag Viscosity Measurements. Suitable compositions of fluxed mixtures for viscosity measurements
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Energy & Fuels, Vol. 10, No. 6, 1996 1219
Table 4. Comparison of Experimental Viscosities (Pa‚s) with Urbain Models 1400 °C
1500 °C
Urbain12 modified24 exptl Urbain12 modified24 exptl AF1 s AF2 s AF3 s AF4 s BF1 s BF2 s Cs CF1 s CF2 s
70 15.6 6.3 3.3 17.7 8.0 329 80 33.5
3.6 1.9 10.1 3.9
32.2 7.3 5.8 3.2 9.6 4.9 212 22.2 10.5
24.5 6.4 2.8 1.6 7.1 3.5 98 27.5 12.6
1.6 0.8 4.2 1.7
8.4 2.5 1.2 1.2 3.5 2.1 66 7.0 3.6
were obtained from the melting point data and are marked in parts a and b of Figure 3. Figure 3 shows the experimental viscosity-temperature relationships for these fluxed mixtures obtained from coals A-C. The temperature of critical viscosity (Tcv), the temperature at which the melts start to crystallize, was also determined and is indicated in Figure 3. These results can then be used to determine the amount of flux required at 1400 °C for each of the coals examined. On an ash basis, this would be 40% calcium carbonate for coal ash A, 65% for coal ash B, and 15% for coal ash C. Using the ash contents given in Table 1, the amount of flux required would be 3.7% for coal A, 11.7% for coal B, and 1.3% for coal C. Slag Viscosity Models. Although many slag viscosity models have been proposed, the Urbain12 model is the most accepted because it is based on a fit of viscosity measurements for the SAC system and supplies reasonable predictions.13,23 Table 4 shows a comparison of the experimental values for the viscosities of the fluxed mixtures at 1400 and 1500 °C with the values predicted from the Urbain model, using the normalized compositions given in Table 3. The predicted results are generally larger than the experimental values, so that improved viscosity predictions are needed for accurate estimation of the amount of flux required. The Urbain model12 has the advantage of covering the whole range of composition of SAC mixtures. However, some improvements8,24 at the expense of compositional range may be made to the empirical fit by using more (24) Hurst, H. J.; Novak, F.; Patterson, J. H. Proceedings of the Fifth International Conference on Molten Slags, Fluxes and Salts, Sydney, Australia; Iron and Steel Society: Warrendale, PA, 1997 (to be published).
data points25 for compositions closer to those expected for Australian gasifier slags. This involves covering the anorthite, pseudowollastonite, and gehlenite regions of the phase diagram, since most Australian coal ashes have relatively high silica contents and higher silica to alumina ratios compared with Northern Hemisphere coals. Table 4 shows that some improvement is obtained using the modified model. Conclusions Although changes in chemical composition occur between the coal ashes and melts, the ratios of the major components remain unaltered. Thus, predictions of ash melting temperatures and slag viscosities can be made from the coal ash or slag compositions, as required. Due to the complexity of the system, no completely satisfactory models are available for the prediction of coal ash slag viscosities. Although treatments based on the major components provide an indication, the effect of minor components cannot be discounted. The situation is more favorable for fluxed mixtures, where these effects are masked, and sensible estimates may be made from viscosity models based on the composition. The present work demonstrates that reasonable predictions for slag viscosities and flux requirements may be made for the behavior of fluxed mixtures in entrained flow gasifiers, using a modified Urbain type model for viscosity predictions and equilibrium phase diagrams for predicting flux additions. Acknowledgment. We acknowledge the contributions of members of the CSIRO Divisions of Coal and Energy Technology and Exploration Geoscience, including G. Hansen and A. Gulliver for sample preparation, H. Orban and H. Kirkpatrick for ashing, P. Marvig for SEM sample preparation, J. Corcoran for SEM measurements, K. M. Kinealy for microprobe measurements, Dr. J. C. Taylor and C. E. Matulis for XRD measurements, and K. Riley for analysis of the coal ashes and melts. Ash fusion temperatures were measured by Carbon Consulting International Pty. Ltd., Newcastle. EF950264K (25) Machin, J. S.; Yee, T. B.; Hanna, D. C. J. Am. Ceram. Soc. 1952, 35, 322.
