The Fusibility of Blended Coal Ash - Energy & Fuels (ACS Publications)

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Energy & Fuels 2000, 14, 316-325

The Fusibility of Blended Coal Ash G. W. Bryant,* G. J. Browning, H. Emanuel, S. K. Gupta, R. P. Gupta, J. A. Lucas, and T. F. Wall Cooperative Research Centre For Black Coal Utilisation, Department Of Chemical Engineering, University Of Newcastle, Callaghan, NSW, 2308, Australia Received May 20, 1999. Revised Manuscript Received November 3, 1999

The fusibility of blended coal ash was studied by comparing the standard ash fusibility temperature test, temperatures at particular penetration levels measured by thermomechanical analysis (TMA), and predictions of liquid proportion with temperature thermodynamic equilibrium. Ash fusibility temperatures (AFT) of coal ash are found at temperatures below the predicted liquidus temperature and, for ashes from blended coals, are generally nonlinear with respect to the blend proportion. The conclusion that trends in AFTs with blend proportions are mirrored by changes in the liquidus temperature from ternary systems, as was found by previous investigators, is not supported. This study provides support for the use of TMA to characterize ash fusibility. That is, TMA temperatures change with blend proportions when AFTs do not, and also mirror temperature changes at defined liquid contents predicted at thermodynamic equilibrium.

Introduction Coal blending is used to provide a consistent feedstock of fuel for the power generation industry. Blending may also be used to reduce costs and compensate for undesirable characteristics of a coal, such as sulfur content. Several studies have indicated greater fouling and slagging upon coal blending or switching than would be expected upon combustion of either of the parent coals.1 Huggins et al.2 have concluded that the liquidus surfaces of the SiO2-Al2O3-XO (where X ) Fe, Ca, K2) ternary systems correlate with trends obtained for ash fusibility temperature (AFT) measurements, in particular the flow/fluid temperature, for coal ash-additive mixtures (using Fe2O3, CaO, K2CO3). This work leads to the conclusion that the change in liquidus temperature with coal ash blend ratio may correlate with changes in AFTs. Further, this study also indicated that the principal factors associated with coal ash melting and subsequent slag flow properties were the SiO2:Al2O3 ratio and basic oxide levels. The operational temperatures of pulverized fuel plants are in the range of 900 to 1500 °C,3,4 which is below the liquidus temperature of most ashes, and as such, the liquidus temperatures alone may not be capable of providing the correlation suggested. The AFT test and thermomechanical analysis (TMA) is used to examine this correlation. * Corresponding author. (1) Bogomolov, V. V.; Artemjeva, N. V.; Aleknovich, A. N.; Gladkov, V. E. The Slagging Behaviour Of Coal Blends In The Pilot Scale Combustion Test Facility. Proceedings EF Conference, Impact Of Mineral Impurities During Solid Fuel Combustion, 1997; Wall, T. F., Baxter, L. L., Eds.; Kona, Hawaii, 2-7 Nov. (2) Huggins, F. E.; Kosmak, D. A.; Huffman, G. P. Fuel 1981, 60, 577-584. (3) Benson, S. A.; Jones, M. L.; Harb, J. N. Ash Formation and Deposition, Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; 1993; Chapter 4, pp 299-373. (4) Singer, S. Pulverized coal combustion: recent developments, Energy Technol. Rev. 1984, 90, xxx.

Table 1. Ash Analysis of Coal Ash Samples as Oxide Weight Percent ash oxide analysis (wt %) SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O TiO2 Mn3O4 SO3 P2O5

A

B

C

D

E

F

42.2 32.0 1.8 18.3 1.0 0.2 0.1 2.1 0.4 1.7 0.1

49.4 31.6 1.8 11.8 1.0 0.1 0.2 1.8 0.2 1.9 0.1

60.8 22.3 3.2 5.6 1.2 1.0 2.1 0.8 0.09 2.2 0.13

85.8 12.2 0.1 0.9 1600

FT

100% C 80% C/20% D 60% C/40% D 40% C/60% D 20% C/80% D 100% D

996 1039 1070 1061 1189 1470

1095 1117 1195 1242 1465 1533

1159 1211 1396 1450 1507 1566

1229 1275 1472 1486 1537 1594

1300 1230 1210 1290 1290 1430

1390 1390 1430 1490 1550 1590

1410 1430 1470 1510 1550 >1600

1450 1470 1490 1550 1590 >1600

1240 1230 1210 1230 1270 1400

1330 1370 1410 1470 1530 1590

1350 1390 1430 1490 1550 >1600

1410 1450 1490 1530 1590 >1600

100% E 80% E/20% F 60% E/40% F 40% E/60% F 20% E/80% F 100% F

923 983 955 946 954 1010

1034 1062 1075 1085 1071 1097

1176 1175 1099 1175 1170 1303

1335 1258 1113 1223 1201 1557

1240 1210 1170 1150 1230 1430

1240 1210 1210 1270 1310 1590

1240 1210 1210 1270 1330 >1600

1260 1230 1210 1290 1370 >1600

1190 1170 1130 1090 1130 1200

1200 1210 1130 1210 1270 >1600

1200 1210 1130 1230 1290 >1600

1240 1210 1170 1250 1350 >1600

Table 3. Species and Phases Considered in FACT Version 2.1 Calculations gas phase liquid polynomial solution

inert (Ar), O2 slag

solid pure components

SiO2, Al2O3, FeO, Fe2O3, Fe3O4, Fe, CaO, Al6Si2O13, CaAl2O4, Ca2Al2SiO7, Ca3Al2O6, CaSiO3, Ca2SiO4, Ca3SiO5, Ca3Si2O7, CaAl2Si2O8, Fe2Al4Si5O18, FeAl2O4, 2(FeO).SiO2, CaFe2O4, Ca2Fe2O5, CaFe4O7, CaFeSi2O6, Ca3Fe2Si3O12

polynomial solid solutions

wollastonite (WOLL), dicalcium silicate (CASI), corundum (CORU and AL2O), iron spinel (FESP), melilite (2)

mullite are the dominant crystalline phases present.13,14 As mentioned earlier, TMA output provides an indication of the relative proportion of liquid and solid phases (particularly at penetration levels >30%), FACT predicts this and as such a direct comparison is justified. • The calculations do not consider the specific mineral forms of the ash used in the TMA and AFT test. • The elements considered in the calculations were Si, Al, Ca, Fe, and O. Na, K, and Mg were not considered, as the database is not adequate for these elements. Most samples used in this study have ash oxide analyses where SiO2 + Al2O3 + FeOn + CaO is greater than 90%, but some differences between FACT predictions and experiments appear due to this simplification. Development of the FACT database to incorporate Na, K, and Mg into the current five-component system is in progress. In all calculations relating to inert atmospheres, as has been observed for laboratory ashes, it was assumed the initial iron species in the system was distributed as 80% Fe(II) and 20% Fe(III).15-17 For calculations in oxidizing atmospheres the oxygen partial pressure was fixed at 0.21.

Results and Discussion Blending on the Basis of Varying Fe2O3 Content. Coal ash samples A and B were blended to give coal ash samples with FeOn content varying between 18.3% and 11.8%. Lines AB on the SiO2-Al2O3-FeO and SiO2-Al2O3-CaO ternary phase diagrams (Figures 1 and 2) present the range of the blend compositions. All (13) Unuma, H.; Takeda, S.; Tsurue, T.; Ito, S.; Sayama, S. 1986, Fuel 65, 1505-1510. (14) O’Gorman, J. V.; Walker, P. L., Jr. Fuel 1973, 52, 71-79. (15) Nowok, J. W. Energy Fuels 1995, 9 (3), 534-539. (16) McLennan, A. R. Ph.D. Thesis. University of Newcastle, Australia, 1998. (17) Bailey, C. W. Ph.D. Thesis. University of Newcastle, Australia, 1999.

the samples resulting from blends of A and B lie in the mullite primary phase field, as indicated in Figure 5b. Figure 4a presents the TMA traces obtained for the coal ash sample blends of A and B, as a function of temperature, for the blend compositions indicated. It is apparent that there is little change in penetration at given temperatures, as the FeOn content is varied between 11.8% and 18.3%. This indicates that the amount of slag formed at a given temperature varies little as the FeOn content is varied between 11.8% and 18.3%. At approximately 1150 °C, rapid penetration occurs for all samples, indicating rapid slag formation at this temperature. Equilibrium calculations performed using FACT (Figure 4b), show that the predicted amount of slag formed at given temperatures (above 1100 °C), increases as the iron content increases from 11.8% to 18.3%. Rapid slag formation is predicted to occur at temperatures between 1100 and 1150 °C for blends of A and B. The rapid slag formation predicted is reflected in the TMA traces (Figure 4a), where there is rapid penetration of all samples of blends of A and B at 1150 °C. The equilibrium calculations performed using FACT suggest that the penetration observed in the TMA traces may be correlated with the formation of slag phases during the TMA experiments for blend samples of A and B. The low temperature penetration occurring between 850 and 1150 °C in the TMA experiments (Figure 4a), possibly results from the fluxing effects of other basic oxides (Na, K, and Mg) in the samples, which were not considered in the equilibrium calculations. Repacking of the sample due to vibrations, gas evolution from incomplete mineral decomposition, softening of clay particles, and sintering of the sample will cause some

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Figure 3. Schematic diagram of TMA apparatus and ash sample assembly prior to heating.

of the observed penetration (generally less than 10%) at the lower temperatures. Figure 5a presents the TMA temperatures as a function of the blend composition. Observations indicate that the TMA temperatures generally increase slightly as the iron content decreases from 18.3% to 11.8%. Figure 5b presents the predicted FACT temperatures, with varying amounts of slag formed as a function of the blend composition of samples A and B. There is good agreement between the trends in the TMA temperatures (Figure 5a), and the predicted FACT temperatures (Figure 5b). The trends observed for 10, 30, 50, and 70% slag curves correlate with the trends in the T25, T50, T75, and T90 curves, respectively. The liquidus temperature increases slightly as the iron content decreases, which is as expected from the phase diagrams, where all blends of samples A and B occur in the mullite primary phase field. There is a ∼300 °C difference between the liquidus and 70% slag curve, indicating that after rapid initial slag formation up to ∼1320 °C, the slag slowly continues to form until the samples are 100% slag at between ∼1625 °C and 1675 °C. This melting behavior can be seen in the TMA traces for all the blend samples of A and B, which level off above ∼1320 °C, with penetration slowly approaching 100% as the temperature increases to 1600 °C. Figure 6, parts a and b, presents the AFTs for various blend compositions of A and B in reducing and oxidizing

Figure 4. Comparison of data for blends of A and B (varying iron). (A) TMA traces, and (B) slag wt % calculated using FACT.

conditions, respectively. In reducing conditions, the AFTs, with the exception of IDTs, increase slightly as the FeOn content decreases from 18.3% to 11.8%. The changes in liquidus, solidus, 50% slag, and 90% slag with Fe2O3 content are also shown in Figure 6a. The liquidus parallels the ST, HT, and FT curves, with a ∼150 °C difference between the liquidus and AFT curves (except the IDT curve). The AFT’s are ∼200 °C higher than the experimental TMA temperatures. From the results obtained, predictions from FACT can be related to TMA temperatures and AFTs as FeOn content varies between 11.8% and 18.3%, for the ash compositions studied. High AFT temperatures obtained, compared to the experimental TMA temperatures, may be explained by the dissolution of Al2O3 into the ash sample from the mounting tile used in the test. This increase in Al2O3 level in the ash would result in higher AFTs than would expected.18 Blending on the Basis of Varying Silica-toAlumina Ratio. Coal ash samples C and D were

The Fusibility of Blended Coal Ash

Figure 5. Comparison of data for blends of A and B (varying iron). (A) TMA temperatures as a function of blend composition, and (B) calculated FACT temperatures for varying amounts of slag formed, as a function of blend composition.

blended in varying proportions to give coal ash samples with SiO2:Al2O3 ratios between 7 and 2.7. Lines CD on the SiO2-Al2O3-FeO and SiO2-Al2O3-CaO ternary phase diagrams (Figures 1 and 2) represent the range of the blend compositions. All samples resulting from blends of C and D lie in the mullite primary phase field, as indicated in Figure 8b. Figure 7a presents the TMA traces obtained for the blends of coal ash samples C and D, as a function of temperature, for the blend compositions indicated. It is apparent that decreasing the SiO2:Al2O3 ratio results in greater penetration at a given temperature. This indicates that there is a decrease in the amount of slag formed at a given temperature as the SiO2:Al2O3 ratio is increased. Equilibrium calculations were performed using FACT on a simplified five-component system of SiO2-Al2O3(18) Australian Standard AS1038, Part 15, 1995, Methods for the Analysis of Coal and Coke, Part 15: Fusibility of Coal Ash and Coke Ash.

Energy & Fuels, Vol. 14, No. 2, 2000 321

Figure 6. Ash fusibility temperatures (AFTs) for blends of A and B (varying iron). (A) AFTs in reducing atmospheres. Also plotted are the liquidus, solidus, 50% slag, and 90% slag curves calculated using FACT. (B) AFTs in oxidizing atmospheres. Also plotted are the liquidus and solidus curves calculated using FACT.

FeO-Fe2O3-CaO. The calculations suggest that the penetration observed in the TMA traces may be correlated with the formation of slag during heating. Figure 7b presents the calculated weight percent of sample as slag, as a function of temperature, for the blend compositions indicated. There is a good agreement between the FACT predictions and the TMA traces. There is however, a shift of approximately +200 °C in the FACT prediction, when compared to the TMA traces. The TMA traces for blends of 0-60% D, show an initial rapid penetration at about 930 °C, which is 210 °C lower than the temperature of initial slag formation predicted by FACT (Figure 7b). The variation in the temperature of initial melting can be explained by the presence of 3.3 wt % Na2O + K2O in coal ash sample C. It is known that sodium and potassium reduce the initial melting temperature of ash, and a correction term can be applied to determine the

322


Figure 7. Comparison of data for blends of C and D (varying SiO2:Al2O3). (A) TMA traces, and (B) slag wt % calculated using FACT.

approximate reduction in the solidus temperature.19 The correction term has a value of -100 °C × (Na2O + K2O) wt % for 0-2 wt % Na2O + K2O and for subsequent amounts an additional correction of -30 °C × (Na2O + K2O) wt % for 2-5 wt % Na2O + K2O. Blends of C and D have (Na2O + K2O) contents of between 0.5 and 3.3 wt % Na2O + K2O, which results in a lowering of the solidus temperature of 50-240 °C. This effect can be observed in the TMA traces of blends 0-60% D (Figure 7a), which contain significant amounts of Na2O + K2O, where there is rapid initial penetration at about 930 °C. FACT predicts that initial slag formation for these blends occurs at about 1140 °C (Figure 7b). Figure 8a presents the TMA temperatures T25, T50, T75, and T90, as a function of the blend composition. Observations from Figure 8a, indicate an increase in the TMA temperatures as the SiO2:Al2O3 ratio increases from 2.7 to 7. The T25 and T50 temperatures increase slightly from 0 to 60% D and then rise sharply from 60 to 100% D, whereas the T75 and T90 temperatures show (19) Mills, K. C.; Rhine, J. M. Fuel 1989, 68, 193-200.

Bryant et al.

Figure 8. Comparison of data for blends of C and D (varying SiO2:Al2O3). (A) TMA temperatures as a function of blend composition, and (B) calculated FACT temperatures for varying amounts of slag formed as a function of blend composition.

the opposite trend. Figure 8b presents the predicted FACT temperatures, with varying amounts of slag formed as a function of the blend composition. TMA temperatures compare well with the predicted FACT temperatures, with the trends observed in the TMA temperatures (Figure 8a) clearly reflected in the FACT prediction (Figure 8b). The difference in the liquidus and 70% slag lines for 0-40% D are reflected in the T90 temperatures. Significant amounts of Na2O and K2O in coal ash sample C account for the ∼100 °C difference between T25 (Figure 8a) and the 10% slag curve (Figure 8b) for blends of 0-60% D. The presence of Na2O and K2O also explains the significant lowering of TMA temperatures (Figure 8a) for blends of 0-20% D, when compared to the predicted equilibrium slag formation (Figure 8b). Figure 9, parts a and b, presents the AFTs for various blend compositions of C and D in reducing and oxidizing conditions, respectively. In general, the AFTs, with the exception of IDTs, increase as the SiO2:Al2O3 ratio increases from 2.7 to 7. The IDTs in reducing conditions


Figure 9. Ash fusibility temperatures (AFTs) for blends of C and D (varying SiO2:Al2O3). (A) AFTs in reducing atmospheres. Also plotted are the liquidus, solidus, 10% slag, and 50% slag curves calculated using FACT. (B) AFTs in oxidizing atmospheres. Also plotted are the liquidus and solidus curves calculated using FACT.

are similar, being between 1210 and 1240 °C, for compositions of 0-60% D. The IDTs then increased to ∼1400 °C for 100% D. The changes in liquidus, solidus, 50% slag, and 10% slag with SiO2:Al2O3 ratio are also shown in Figure 9a. The liquidus and solidus do not parallel the experimental AFT curves in either oxidizing or reducing conditions; however, the 50% slag curve correlates well with the ST and HT. The 10% slag curve shows a similar trend as the IDT curve with increasing SiO2:Al2O3 ratio, which is supported by a previous study where it was shown that at the IDT there is extensive liquid formation.2 For the range of oxide compositions considered, the liquidus does not correlate with the variation in TMA temperatures or AFTs as the SiO2:Al2O3 ratio varies between 2.7 and 7. The predicted FACT temperatures for varying amounts of slag formed (Figure 8b) as the SiO2:Al2O3 ratio varies, correlates well with the experimental TMA temperatures obtained (Figure 8a). The


Figure 10. Comparison of data for blends of E and F (varying calcium and iron). (A) TMA traces, and (B) slag wt % calculated using FACT.

experimental TMA temperatures for blends of 0-20% D, are lower than that expected from the FACT prediction, and can be explained by the significant amount (4.3 wt %) of other basic oxides (Na, K, and Mg) in sample C, which were not considered in the FACT equilibrium calculations. The predicted 50% slag and 10% slag curves correlate well with the ST, HT, and IDT, respectively, in reducing conditions, as the SiO2: Al2O3 ratio increases. Blending on the Basis of Varying CaO Content. Coal ash samples E and F were blended coal ash samples with CaO content varying between 0.4% and 29.6%. Lines EF on the ternary phase diagrams of SiO2-Al2O3-FeO (Figure 1) and SiO2-Al2O3-CaO (Figure 2) systems represent the range of blend compositions. As the CaO content increases, the bulk composition moves from the mullite, to the anorthite and then the gehlenite primary phase fields, as indicated in Figure 11b. As the blend compositions move across three primary phase fields, the variation in liquidus temperature with coal ash blend proportions will have two minima.

324


Figure 11. Comparison of data for blends of E and F (varying calcium and iron). (A) TMA temperatures as a function of blend composition, and (B) calculated FACT temperatures for varying amounts of slag formed, as a function of blend composition.

Figure 10a presents the TMA curves obtained for the coal ash samples of blends of E and F, as a function of temperature, for the blend compositions indicated. For temperatures up to ∼1100 °C, there is little difference between the TMA curves for each blend, indicating that the amount of slag formed at a given temperature up to 1100 °C is similar as the CaO content is varied between 0.4% and 29.6%. Between 950 °C and 1050 °C, rapid penetration occurs for all samples, indicating initial formation of a sodium- and potassium-rich slag at this temperature range. For temperatures above ∼1120 °C, there is a significant difference between the TMA curves (Figure 10a), suggesting that the amount of slag formed varies as the CaO content is varied. The TMA curve for the blend of 60% E shows there is significantly more slag formation above ∼1120°, compared to the two parent coal ashes E and F. Blends of 20%, 40%, and 80% E have similar melting characteristics, and their behavior is intermediate of ash samples 60% E and 100% E. Ash samples containing 60% E exhibit the most rapid slag formation

Bryant et al.

Figure 12. Ash fusibility temperatures (AFTs) for blends of E and F (varying calcium and iron). (A) AFTs in reducing atmospheres. Also plotted are the liquidus, solidus, and 50% slag curves calculated using FACT. (B) AFTs in oxidizing atmospheres. Also plotted are the liquidus and solidus curves calculated using FACT.

at lower temperatures, when compared to all other blends samples of E and F. This indicates that a ternary eutectic occurs at a composition close to that of this mixture of E and F. A eutectic for mixtures of E and F, simplified to the three-component system SiO2-Al2O3CaO, from the ternary phase diagram (Figure 2) occurs at ∼85% E, with a eutectic temperature of 1266 °C. A shift in the eutectic point from 85% E to 60% E, may occur due to the significant presence of other oxides in the samples which were not considered20 (sample E: 5% MgO, 5.2% Na2O; sample F: 3.5% K2O). Figure 10b presents the calculated FACT weight percent of sample as slag as a function of temperature, for the blend compositions indicated. Figure 10b indicates that initial slag formation for ash samples with compositions between 40 and 100% E, occurs between ∼990 and 1020 °C. This prediction is similar (∼40 °C (20) Slag Atlas, 2nd ed.; Eisenhuˆttenleute, V. D., Ed.; Verlag Stahleisen GmbH: D-Du¨sseldorf, 1995.


difference) to the temperature where there is rapid initial penetration for these samples (Figure 10a), which represents initial slag formation. The predicted initial slag formation for ash samples of 80% and 100% E occurs at ∼1140 °C, approximately 150 °C above the temperature where initial slag formation occurs for these samples (Figure 10a). The variation in the temperature of initial slag formation between the FACT prediction (Figure 10b) and the TMA experiments (Figure 10a) can be explained by the presence of ∼45% of (Na2O + K2O) in coal ash samples E and F. This can reduce the temperature of initial slag formation (solidus) for these samples to less than 1000 °C. Comparisons between the TMA temperatures (Figure 11a) and FACT equilibrium calculations (Figure 8b), indicate that the T90 curve may be predicted by the liquidus and 90% slag curve for blends of 60-100% F. For blends of 0-40% F, the T90 curve shows a shift of +30 °C when compared to the liquidus and T90 curves. The minima obtained for the T90 and T75 curves (Figure 11a) can be considered to occur near the eutectic composition for mixtures of E and F. The 25% difference in the minima for the TMA curves, which occurs at 40% F, and the FACT calculations, which occurs at 15% F, may be a shift in the eutectic point. This shift may occur due to the significant presence of other basic oxides in the samples, which were not considered in the simplified five-component SiO2-Al2O3-FeO-Fe2O3-CaO FACT calculations. The T75 curve (Figure 11a) closely parallels the predicted 50% slag curve (Figure 11b). The T25 and T50 curves (Figure 11a) parallel the predicted 10% slag curve (Figure 11b) for blends of 0-60% F. The predicted higher temperatures for the formation of 10% slag for


blends 80-100% F, are due to the presence of ∼4-5% (Na2O + K2O) in samples E and F. Figure 12, parts a and b, present the AFTs for varying blend compositions of E and F, in reducing and oxidizing conditions, respectively. In reducing conditions there are some similarities between AFT characteristic temperatures and FACT predictions. The AFTs in oxidizing conditions (Figure 12b) are slightly higher than in reducing conditions; however, the trends of the AFTs as the CaO content is varied are similar. The liquidus parallels the AFT curves for blends of 60-100% F. Conclusions The conclusion of previous investigators that trends in AFTs with blend proportions are mirrored by changes in the liquidus temperature from ternary systems is not supported. This study provides support for the use of TMA to characterize ash fusibility. That is, TMA temperatures change with blend proportions when AFTs do not, and they also mirror temperature changes at defined liquid contents predicted at thermodynamic equilibrium. Acknowledgment. The authors acknowledge the financial support provided by the Cooperative Research Centre for Black Coal Utilisation, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. The authors thank Dr. Evgueni Jak of the University of Queensland, and Mr. Jim Happ and Ms. Michelle Liddle of Rio Tinto Research and Technology Development for their assistance in the application of the FACT code. EF990093+

The Fusibility of Blended Coal Ash - Energy & Fuels (ACS Publications)

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