Thermomechanical Analysis of Coal Ash: The Influence of the Material

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Thermomechanical Analysis of Coal Ash: The Influence of the Material for the Sample Assembly G. W. Bryant,* G. J. Browning, S. K. Gupta, J. A. Lucas, R. P. Gupta, 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 September 22, 1999

Concerns over the characterization of the thermal properties of coal ash using thermomechanical analysis (TMA) in carbon assemblies (crucible and penetrating ram) has led to a study which critically examines the effect of sample assembly materials on thermomechanical analysis measurements. This study quantifies the influence of the sample assembly material on the TMA traces. For temperatures up to 1600 °C and in inert conditions, ash chemistries were identified where the slag reacts with a range of sample assembly materials [molybdenum (Mo), platinum (Pt), carbon (C), zirconia (ZrO2), alumina (Al2O3), and boron nitride (BN)]. Only ZrO2 and Pt sample assemblies were found to be unreactive to ash samples during the tests. Iron-containing ashes were found to react with C assemblies, so that previous TMA data published for such assemblies and inert gases is not characteristic of ash fusibility alone. The implications of the reactions to the interpretation of this TMA data are outlined. For ashes that contain iron, TMA results in graphite sample assemblies may be considered to have been obtained under stronger reducing conditions than those used in the standard AFT test.

Introduction The ash fusibility test has been the most accepted method for assessing coal ash fouling and slagging properties upon utilization. Studies have indicated that characteristic temperatures obtained from the ash fusibility temperatures (AFT) test reflect trends in liquidus temperatures,1,2 contain significant proportions of liquid phase, and do not provide useful information for processes with high coal ash throughput and low residence times. Shortcomings of the AFT test in the prediction of coal performance are well documented.3,4 A new technique based on thermomechanical analysis (TMA) has been developed to characterize stages of coal ash melting during heating.5-7 These studies in carbon sample assemblies have shown that the melting events observed, as coal ash is heated, can be reproducibly measured using TMA with much greater accuracy (typically (10 °C) for a given assembly material. The accuracy of the AFT test from Australian Standards is quoted at (80 °C for IDT and FT and (50 °C for HT and ST.8 TMA is not proposed to predict standard ash * Author to whom correspondence should be addressed. (1) Huggins, F. E.; Kosmak, D. A.; Huffman, G. P. Fuel 1981, 60, 577-584. (2) Huggins, F. E.; Huffman, G. P.; Dunmyre, G. R. Fuel 1981, 60, 585-597. (3) Wall, T. F.; Creelman, R. A.; Gupta, R. P.; Gupta, S. K.; Sanders, R. H.; Lowe, A. Demonstration of the true ash fusibility characteristics of coals; ACARP Project C3039, 1995, Final Report. (4) Juniper, L. Combustion News, 1995, Australian Combustion Technology Centre, Feb, p 1. (5) Gupta, S. K.; Gupta, R. P.; Bryant, G. W.; Juniper, L.; Wall, T. F. Thermomechanical Analysis and Alternative Ash Fusibility Temperatures. Proceedings EF Conference, Impact Of Mineral Impurities During Solid Fuel Combustion, 1997; Wall, T. F., Baxter, L. L., Eds.; Kona, Hawaii, 2-7 Nov.

fusibility temperatures, rather to provide an alternative technique for ash fusibility characterization. Alternative ash fusibility temperatures based on the TMA technique have been proposed, which correspond to particular penetration levels (denoted as T (P%) for penetration levels of 25%, 50%, 75%, and 90%).5 These temperatures were suggested indicators for the onset of melting (T25), intermediate melting (T50), complete melting (T75), and slag flow (T90). These temperatures were found to be repeatable to (10 °C. Another characteristic temperature determined from TMA analysis was the onset of melting (Tm), which is believed to be associated with the formation of liquid phases and increased strength development in boiler deposits. Imaging of heat-treated samples indicated the characteristic TMA temperatures T25, T50, T75, and T90 were shown to represent 25 ( 15%, 60 ( 15%, 80 ( 5%, and 90 ( 5% melting, respectively. Although the proposed temperatures were low compared to conventional AFTs, especially for ironrich samples, the use of TMA appeared to be an excellent tool for routine characterization of ash fusibility of various coals. However, there was some concern regarding the effect of the graphite sample assembly material on the TMA measurements. The aim of this study was to critically examine sample assembly materials on TMA measure(6) Gupta, S. K. Ph.D. Thesis, University Of Newcastle, Australia, 1998. (7) Gupta, S. K.; Gupta, R. P.; Bryant, G. W.; Wall, T. F. The effect of potassium on the fusibility of coal ashes with high silica and alumina levels. Fuel 1998, 77 (11), 1195-1201 (8) Osborne, D.; Happ, J. Existing specifications for thermal coal. Proceedings, CRC Workshop on Coal Characterisation for Existing and Emerging Technologies, 1996; Brisbane, Australia, 14-15 Feb.

10.1021/ef9900942 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000

Thermomechanical Analysis of Coal Ash

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Table 1. Ash Analysis of Ash Samples Prepared for the Experimental Program A B C D refractory high Fe high Fe/Ca high Ca ash oxide analysis SiO2 Al2O3 CaO Fe2O3 MgO K2O SO3 ash fusibility temperatures (°C reducing) DT ST HT FT ash fusibility temperatures (°C oxidizing) DT ST HT FT

85.50 12.20 0.14 0.90 0.00 0.37 0.10

42.20 32.00 1.80 18.30 1.00 0.10 1.70

39.70 18.20 17.54 17.10 0.86 0.33 4.50

12.90 14.60 31.19 0.30 3.09 0.05 37.57

1400 1590 >1600 >1600

1210 1460 1480 1520

1160 1200 1220 1300

1340 1340 1350 1390

1430 1590 >1600 >1600

1500 1520 1520 1570

1260 1280 1290 1360

1350 1390 1430 1470

ments, and to recommend suitable sample assembly materials for various applications. Experimental Section Ash Samples. Ash analysis and ash fusibility temperatures for the four coal ash samples selected for this study are presented in Table 1. Ash sample composition and ash fusibility temperatures were determined in accordance with the relevant Australian Standard.9,10 Ash A is a refractory ash with SiO2 + Al2O3 (>95 wt %), which is characterized by high ash fusibility temperatures and was selected on the basis of forming an alumino-silicate slag during heating. Ash B contains significant quantities of Fe2O3, which produces intermediate ash fusibility temperatures and was selected on the basis of forming an iron alumino-silicate slag during heating. Ash C contains significant proportions of both Fe2O3 and CaO that leads to low ash fusibility temperatures observed for this sample, and was selected on the basis of forming an iron/calcium alumino-silicate slag during heating. Ash D contains significant proportions of CaO such that it is essentially the dominant fluxing component producing intermediate ash fusion temperatures; it was selected for the formation of a calcium alumino-silicate slag during heating. Thermomechanical Analysis Measurements. TMA experiments were carried out in a Setram TMA92 Thermo Mechanical Analyzer. A schematic diagram of the apparatus is shown in Figure 1. Approximately 50 mg of sample was placed into a crucible, compacted with 260 kPa pressure, and then a penetrating ram was inserted into the crucible. The entire sample assembly was placed into the TMA instrument and purged with high purity argon for 10 min. The sample assembly was heated from ambient conditions at a rate of 50 °C per min to 700 °C and then at 5 °C per min to 1600 °C as per a previously developed standard procedure.11 A load of 100 g is applied to the penetrating ram resulting in a pressure of 140 kPa at the interface between the penetrating ram and sample surface. As the sample is heated, the penetrating ram is forced into the ash, and may eventually contact the base of the crucible if the slag completely flows into the annulus between the crucible and penetrating ram. Output from the (9) Australian Standard AS1038: 14, 1972. Methods for the Analysis and Testing of Coal and Coke, Part 14: Analysis of Coal Ash, Coke Ash and Mineral Matter. (10) Australian Standards AS1038: 15, 1995. Methods for the Analysis and Testing of Coal and Coke, Part 15: Fusibility of Coal Ash and Coke Ash. (11) Saxby, J. D.; Chatfield, S. P. Proceedings 7th Australian Coal Science Conference, 1996; Monash University, Australia, pp 391-398.

Figure 1. Schematic diagram of TMA apparatus and ash sample assembly prior to heating. TMA consists of ram penetration into the sample, expressed as a percentage of the original height of the sample, at a specific temperature. From the TMA output, the T25, T50, T75, and T90 temperatures are obtained. TMA measurements were conducted in sample assemblies constructed from graphite (C), zirconia (ZrO2), silica (SiO2), alumina (Al2O3), magnesia (MgO), molybdenum (Mo), platinum (Pt), and boron nitride (BN). Thermodynamic Equilibrium Calculations. The Facility for the Analysis of Chemical Thermodynamics (F*A*C*T)12 was used to calculate liquidus and solidus temperatures as well as equilibrium product distributions for simplified coal ash systems. Calculations were simplified by using a five component SiO2-Al2O3-FeO-Fe2O3-CaO system. In previous studies the liquidus temperature of ternary systems1 and the proportion of liquid phase5 has been related to the fusibility of coal ash. FACT is not proposed here as a predictive tool, it is used to provide an insight into TMA data. The limitations in the use of FACT in studying ash fusibility are the following: • TMA and AFT experiments are conducted on samples which are being progressively heated while FACT calculations are approximating the crystallization of systems which are being cooled. Several studies have shown that at temperatures greater than 1300 °C SiO2 and mullite are the dominant (12) Bale, C. W.; Pelton, A. D.; Thompson, W. T. F*A*C*T User Manual; Ecole Polytechnique de Montreal/Royal Military College, Canada, 1996. (http://www.crct.polymtl.ca).

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Table 2. Species and Phases Considered in FACT Version 2.1 Calculations liquid polynomial solution

slag (SLAG,1)

solid pure components polynomial solid solutions

SiO2, Al6Si2O13, CaAl2O4, CaAl12O19, Ca3Al2O6, CaSiO3, Ca2SiO4, Ca3SiO5, Ca3Si2O7, CaAl2Si2O8, Fe, Fe2Al4Si5O18, CaFe2O4, Ca2Fe2O5, CaFe4O7 monoxide rocksalt structure (MONO), wollastinite (WOLL), dicalcium silicate (CASI), olivine (OLIV), corundum (CORU and AL2O), iron spinel (FESP), melilite (2)

Figure 2. TMA data for ash A. (A) Measured penetration (%) data for heating ash, (B) rate of penetration (µm/5 °C), and (C) alternate ash fusibility temperatures. 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. (13) Unuma, H.; Takeda, S.; Tsurue, T.; Ito, S.; Sayama, S. Fuel 1986, 65, 1505-1510. (14) O’Gorman, J. V.; Walker, P. L., Jr. Fuel 1973, 52, 71-79.

• Na, K, and Mg are not considered in the calculations, as the errors associated with extrapolation of the existing database into the multiple component system are not well characterized. Most samples used in this study have ash oxide analyses where SiO2 + Al2O3 + Fe2O3equiv + CaO is >90% and as such simplification of calculations to the 5-component system was deemed satisfactory. It should be noted that development of the FACT database to incorporate Na, K and Mg in the current 5 component system is in progress. In all calculations relating to inert atmospheres, it was



Figure 3. TMA data for ash B. (A) Measured penetration (%) data for heating ash, (B) rate of penetration (µm/5 °C), and (C) alternate ash fusibility temperatures. assumed the initial iron species in the system was distributed as 80% Fe(II) and 20% Fe(III). A list of species and phases considered in the calculations is provided in Table 2.

Results and Discussion Two materials were discounted during preliminary work due to poor thermal performance upon heating. Fused silica and magnesia deformed at temperatures of 1200 °C, indicating significant sintering of the material. Alumino-Silicate (refractory) Ashes. TMA traces, rate of penetration as a function of temperature, and TMA alternate ash fusion temperatures are presented in Figure 2 for ash A. ZrO2 follows the accepted Pt trend ((5%), however all other sample assemblies used produced significant differences. Al2O3 appears to have a

significantly different thermal performance when compared to all other sample assembly materials. The onset of penetration and its magnitude is similar for Mo, BN, and C as shown in Figure 2b. The TMA temperatures are similar for all sample assembly materials with the exception of T25 and are comparable with the conventional ash fusibility temperature measurements in magnitude and trend, as clearly shown in Table 1. A related study has shown that decreasing the SiO2: Al2O3 ratio increases TMA temperatures significantly.6 There were two contributing factors identified, the first being a general increase in liquidus temperatures as the SiO2:Al2O3 ratio decreases, second, a higher solids content at a given temperature for these samples may also prevent penetration of the ram resulting in higher TMA temperatures.

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Figure 4. TMA data for ash C. (A) Measured penetration (%) data for heating ash, (B) rate of penetration (µm/5 °C), and (C) alternate ash fusibility temperatures.

Iron Alumino-Silicate Ashes. TMA traces, rate of penetration as a function of temperature, and TMA alternate ash fusion temperatures are presented in Figure 3 for ash B. ZrO2 follows the accepted Pt trend ((5%); however, all other sample assemblies used produced significant differences. All sample assemblies showed similar temperatures for the onset of melting with the exception of BN. For this sample there appears to be a increase in the Fe(II)/Fe(III) ratio due to contact with the C sample assembly when compared to the raw coal ash. Further, a chemical reduction of iron oxide to metallic iron is occurring during the TMA test for C and BN sample assemblies, as observed by SEM examination of polished cross sections of the sample assemblies at the completion of each test. Direct reduction of iron oxide liquid by C has been reported to occur due to the presence of

saturated C in hot iron metal,15 as shown in eq 1. Dissolved C has been reported to be present in molten slags at levels of 5 wt %.16 This increased C in slag results in higher viscosity and an increase in the slag fusibility temperature.17 This reduction is thermodynamically possible at temperatures greater than 1100 °C. There is also potential for residual oxygen present in the purge gas to react with the C sample assemblies to produce CO gas and further reduce FeO to iron, although this is minor in comparison to other mechanisms of FeO reduction. (15) Bosworth, C.; Bell, H. Physical Chemistry Of Iron And Steel Manufacture, 2nd ed.; Longman Group Limited: London, 1972; p 204. (16) Slag Atlas, 2nd ed.; Eisenhuttenleute, V. D., Ed.; Verlag Stahleisen Gmbh, D-Du¨sseldorf, 1995. (17) Bachinin, A. A.; Nesterenko, S. V.; Khomenko, V. M.; Bykov, L. V.; Zotov, A. V. Russian Metallurgy (Metally) 1997, 4, 32-38.



Figure 5. TMA data for ash D. (A) Measured penetration (%) data for heating ash, (B) rate of penetration (µm/5 °C), and (C) alternate ash fusibility temperatures.

FeO(l) + C(s) f Fe(l) + CO(g)

(1)

The formation of iron carbide on the internal surface is also possible as shown in eq 2:18

3FeO(l) + 4C(s) f Fe3C(s) + 3CO(g)

(2)

The removal of iron oxide from the system moves the composition of the slag away from the initial bulk composition. The new bulk composition would have higher liquidus temperatures and subsequently a greater solid content at that temperature where iron is being reduced. Crystallization of solids from the liquid phase would increase the solid content of the slag, which would (18) Raask, E. J. Inst. Energy, March 231-239, 1984.

prevent the high-temperature penetration expected when considering the low-temperature behavior. This effect is evident in Figure 3 where the high-temperature penetration is observed to be low although the penetration at low temperatures is high for C sample assemblies compared to most other materials. Direct reduction of iron oxide by BN is thermodynamically possible at temperatures of 140 °C as shown in eq 3 and behaves similarly to that outlined for C sample assemblies.

2BN(s) + 3FeO(s) f B2O3(s) + 3Fe(s) + N2(g)

(3)

Calcium/Iron Alumino-Silicate Ashes. TMA traces, rate of penetration as a function of temperature and TMA alternate ash fusion temperatures are presented

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Figure 6. Scanning electron micrographs of coal ash sample C sample assemblies containing slag at the completion of experiments (1600 °C) for (A) Mo sample assembly, (B) C sample assembly, (C) BN sample assembly, (D) Al2O3 sample assembly, and (E) ZrO2 sample assembly.

in Figure 4 for ash C. ZrO2 follows the accepted Pt trend ((5%), however all other sample assemblies used produced significant differences. Although the magnitude of the differences are much less than that observed for slags containing only iron as the fluxing component. Iron oxide reduction by C and BN sample assemblies was observed for coal ash C during heating. The presence of CaO reduces the effect of iron removal so that the sample remains “fluxed”. Calcium Alumino-Silicate Ashes. TMA traces, rate of penetration as a function of temperature, and TMA alternate ash fusion temperatures are presented in Figure 5 for coal ash D. It is apparent from observation of Figure 5a that there is little difference in the results for the temperature of onset of melting obtained from thermal analysis with different sample assembly

materials as shown in Figure 5b. ZrO2 follows the accepted Pt trend ((5%); however, all other sample assemblies used produced significant differences. The TMA temperatures are similar for Mo and C sample assembly materials (with the exception of T90) and are comparable with the conventional ash fusibility temperature measurements in magnitude and trend, as clearly shown in Table 2. Al2O3 increases TMA temperatures significantly as shown in Figure 5c. BN sample assemblies produce similar TMA temperatures to Mo although the T75 and T90 temperatures are significantly lower for BN. The remainder of the discussion will summarize experimental observations comparing the effects of the material of construction of the sample assembly on the thermal characteristics of the ash as it is heated. Figure



6 presents the SEM images of cross-sectioned sample assemblies at the completion of TMA experiments for coal ash C. Role of Platinum. The well-known high-temperature stability and resistance to reaction under inert conditions make this material a suitable reference point for this study. Role of Molybdenum. Mo is a refractory metal with a very high melting point (2610 °C) and is the main material used for crucibles in high-temperature experiments reported in the literature.19-21 Mo is observed to have an intermediate behavior to Pt and C sample assembly materials as shown in Figure 6a. Reduction of iron in the system to the Fe(II) state is possible. Mo, however, is not stable in atmospheres containing oxygen at temperatures greater than 770 °C, the rate of oxidation of Mo is equal to the rate of vaporization of MoO3.22 FACT predicts that Fe2O3 and Mo can react at elevated temperatures as shown in eq 4:

3Fe2O3(s) + Mo(s) f 6FeO(s) + MoO3(g)

(4)

Therefore the MoO3 generated, as a result of reaction with Fe2O3, would escape the system in the gas phase; the result is that all iron is in the form of Fe(II). There is currently no direct experimental information to support this hypothesis. This reduction reaction would not proceed to the formation of metallic iron resulting in the TMA trace for Mo being intermediate to that of the reactive C and unreactive Pt and ZrO2 sample assemblies. Role of Carbon. C significantly affects the behavior of samples containing iron oxide in ash by reduction of any Fe(III) to Fe(II) and then to metallic iron and its effective removal from the slag as shown in Figure 6b. The bulk slag contains little iron oxide when compared to the levels of iron oxide in the parent ash. The onset of penetration occurs at the same temperature as found with Mo sample assemblies as shown in Figures 2a, 3a, 4a, and 5a even for very low levels of iron. The rates of penetration encountered for C sample assemblies at low temperatures are similar to those found for Mo as shown in Figures 2b, 3b, 4b, and 5b. Although this penetration occurs at low temperatures 1100 °C) does not follow the same trends. The removal of iron can be modeled as shown in Figure 7. In C sample assemblies the atmosphere is highly reducing and the iron alumino-silicate slag prediction calculated using FACT presented in Figure 7 reflects this. The proportion of iron alumino-silicate slag was calculated using FACT, in equilibrium with metallic iron, which reflects highly reducing conditions. A FACT calculation for the same composition with no flux is also presented in Figure 7. The TMA trace for ash B is overlaid in Figure 7 which indicates that there is a transformation from iron-rich slag behavior to that (19) Vaisburd, S.; Brandon, D. G. Meas. Sci. Technol. 1997, 8, 822826. (20) Nowok, J. Energy Fuels 1995, 9, 534-539. (21) Hurst, H. J.; Novak, F.; Patterson, J. H. Proceedings 5th Int. Conf. Molten Fluxes and Salts, 1997: Sydney. (22) Harwood, J. J. The Metal Molybdenum. Chapter 19 The Protection of Molybdenum Against High-Temperature Oxidation; Harwood, J. J., Ed.; American Society for Metals: Cleveland, OH, 1956; Chapter 19, p 421.

Figure 7. Calculated slag wt % calculated using FACT in reducing conditions for an iron alumino-silicate and aluminosilicate slag. Also plotted is the TMA trace for ash B in a graphite sample assembly showing the relationship between the TMA trace and the predicted proportion of slag.

Figure 8. Comparison of characteristic TMA temperature differences with iron oxide in graphite and Mo assemblies for (A) T(25%), (B) T(50%), (C) T(75%), and (D) T(90%).

of an alumino-silicate slag depleted in iron oxide during the TMA test. The difference in temperature for T(P%) values as a function of iron oxide levels in ash obtained for C and Mo sample assemblies for 14 ash samples is provided in Figure 8. The compositions of the ash samples are provided in Table 3. The temperature difference for T(25%) values does not exceed 50 °C except for sample E, while the difference for T(50%) for all samples is within 25 °C. The T(50%) values show the least temperature difference obtained for the two sample assembly materials used. T(75%) and T(90%) vary significantly for the two assemblies and show a dependence on the iron oxide level in ash. Figure 8 also shows that the temperature difference of T(75%) and T(90%) decreases with increasing levels of iron oxide. However, if the iron oxide levels in ash exceed 15 wt %, the T(75%) values in C crucibles become lower compared to Mo crucible. At higher iron levels, some of the iron is still left in the liquid. In general, the effect of sample

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Table 3. Ash Analysis for Ash Samples Used to Determine the Extent of Iron and C Reactions during TMA Experiments sample

SiO2

Al2O3

CaO

Fe2O3

MgO

Na2O

K2O

TiO2

Mn3O4

SO3

P 2 O5

E F G H I J K L M N O P Q R

85.8 39.7 12.9 63.4 51.0 42.2 43.8 41.6 67.6 60.8 59.9 66.8 53.9 49.3

12.2 18.2 14.6 20.2 22.4 32.0 24.4 14.5 22.5 22.3 26.1 26.5 24.7 31.0

0.14 17.54 31.19 1.68 1.88 1.8 2.59 5.75 1.4 3.2 1.7 0.4 3.1 3.2

0.9 17.1 0.3 8.6 17.4 18.3 22.9 28.1 3.6 5.6 6.7 0.9 11.0 9.1

0.00 0.86 3.09 1.41 1.85 1.0 0.63 0.60 0.8 1.2 1.1 0.3 1.2 1.0

0.01 0.05 0.00 0.64 1.34 0.2 0.39 1.02 0.7 1.0 0.4 0.1 0.3 0.6

0.37 0.33 0.05 2.22 1.91 0.1 1.77 1.66 1.5 2.1 1.3 2.7 1.7 1.2

0.56 0.77 0.13 0.93 0.96 2.1 0.92 0.74 0.9 0.8 1.1 1.2 1.3 1.5

0.02 0.37 0.20 0.07 0.06 0.4 0.10 0.07 0.06 0.09 0.09 0.02 0.10 0.10

0.10 4.50 37.57 0.73 0.83 1.7 2.38 5.63 0.40 2.20 1.00 0.03 0.40 0.90

0.04 0.59 0.02 0.16 0.29 0.1 0.13 0.37 0.30 0.13 0.19 0.15 1.90 1.90

Table 4. Summary of Key Reactions and Temperatures for the Various Sample Assembly Materials material Mo Pt C ZrO2 Al2O3 BN a

chemical reaction

FeO(l) + C(s) f Fe(l) + CO(g) ZrO2 f ZrO2 (octa) f (mono) Al2O3(s) f Al2O3(l) 3FeO(s) + 2BN(s) f B2O3(s) + 3Fe(s) + N2(g)

temperature range for reaction (°C)

maximum temperature recommended (°C)

gaseous atmospheres for satisfactory operation

1000-1100 1300-1400 1000-1600 1000-1100

>1600 >1600 determined by Fe levels in ash

Thermomechanical Analysis of Coal Ash: The Influence of the Material

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