Energy & Fuels 1998, 12, 257-261
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Use of Thermomechanical Analysis To Quantify the Flux Additions Necessary for Slag Flow in Slagging Gasifiers Fired with Coal G. W. Bryant,* J. A. Lucas, S. K. Gupta, and T. F. Wall Cooperative Research Centre for Black Coal Utilisation, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW, 2308, Australia Received June 3, 1997X
Coals with high ash fusibility temperatures are utilized in slagging gasifiers with the addition of a flux in order to lower ash melting temperatures and slag viscosity. Recently, thermomechanical analysis (TMA) has emerged as a technique that can characterize coal ash and slag behavior. The progressive shrinkage of heated coal ash samples mixed with calcium flux (CaO) has been measured using TMA. The resulting shrinkage has been correlated with the viscosity measured in a rotating bob viscometer for the same samples in order to quantify flux levels necessary for the viscosity required for coal fired slagging gasifiers (15-25 Pa‚s). For this viscosity range, shrinkage levels in the range of 10-95% were obtained, indicating that the TMA technique has the potential to quantify the necessary flux additions.
Introduction Integrated gasification combined cycle (IGCC) plants commonly utilize entrained flow slagging gasifiers, in which the mineral matter of coal is converted to a slag on the internal refractory wall of the gasifier. The slag is required to have a viscosity which allows it to flow down the internal wall of the gasifier to the base where it is tapped. To ensure the slag can be tapped, the slag viscosity is optimally 15 Pa‚s and must be less than 25 Pa‚s at the tapping temperature which is usually in the temperature range 1400-1500 °C.1 Coals with low ash fusibility temperatures and low viscosity at elevated temperatures are therefore preferred for use in IGCC applications. These coals typically contain elevated levels of calcium, iron, potassium, and sodium. Conversely, coals which have high ash fusibility temperatures have more widespread use in conventional pulverized fuel technology as these coals generally do not form slags or fouling deposits on heat transfer surfaces. For coals with high ash fusibility temperatures and high coal ash viscosity, fluxing agents (which are additives that lower slag viscosity) must be added to allow these coals to be utilized in IGCC technology. A common flux is limestone (CaCO3), which when heated decomposes to lime (CaO) prior to its retention in the slag. A benchtop experiment is required to facilitate rapid determination of flux requirements to alter coal ash viscosity appropriately for utilization in slagging gasifiers. The TMA technique measures the penetration of a ram into an ash pellet as the sample is heated. Previous work2 has identified the potential for shrinkage measurements obtained from the TMA technique to provide a basis for a new standard ash fusibility test. Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Harris, D.; Patterson, J. H. Aust. Inst. Energy News J. 1995, 13, 22-32.
Figure 1. Schematic diagram of TMA crucible containing ash prior to heating.
In the present study, a potential correlation between the shrinkage of an ash pellet during heating and the viscosity of the sample has been investigated in order to allow rapid determination of coal ash viscosity as a function of flux addition. Experimental Section Coal Ash Samples. Table 1 presents the ash analysis and ash fusibility temperatures for the three samples selected for this study. Coal sample A contains high levels of silica (85.4% db) and low levels of basic oxides in ash, which is reflected in its high ash fusibility temperatures. Coal sample B contains
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(2) Gupta, S.; Wall, T. F.; Creelman, R.; Gupta, R. P. Proc. ACS Conf. New Orleans, 1996, 41, 647-651.
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Table 1. Ash Analysis of Coal Ash Samples as Oxide Weight Percent of Ash and Ash Fusibility Temperatures ash analysis oxide (wt %) SiO2 Al2O3 Fe2O3 TiO2 Mn3O4 CaO MgO Na2O K2O P2O5 SO3
A
B
C
85.40 11.70 0.71 0.43 0.01 0.06 0.06 0.01 0.36 0.05 0.05
51.50 34.50 2.04 2.00 0.01 1.68 0.81 1.15 0.30 0.08 1.63
50.80 22.90 9.84 1.55 0.09 4.54 0.88 0.12 0.54 0.82 4.68
ash fusibility temp (°C) reducing deformation spherical hemispherical flow
A
B
C
1500 >1600 >1600 >1600
>1600 >1600 >1600 >1600
1180 1260 1280 1400
ash fusibility temp (°C) oxidizing deformation spherical hemispherical flow
A
B
C
1530 >1600 >1600 >1600
>1600 >1600 >1600 >1600
1320 1360 1390 1420
Figure 2. Measured viscosity curves for coal ash sample B as a function of temperature at the flux levels indicated (as g of CaCO3/g of ash). The low-temperature limits correspond to experimentally determined temperatures of critical viscosity.
Figure 3. Measured shrinkage data for heating coal ash samples at several flux (as g of CaCO3/g of ash) levels indicated for (A) coal ash sample A, (B) coal ash sample B and (C) coal ash sample C.
moderate levels of silica (51.5% db) and alumina (34.5% db) and low levels of the basic oxides, resulting in high ash fusibility temperatures (>1600 °C). Coal sample C contains moderate levels of silica (50.8% db) and alumina (22.9% db) and higher levels of the basic oxides Fe2O3 and CaO and therefore has lower ash fusibility temperatures than encountered for in samples A and B. Lime flux was prepared by calcining pulverized analytical reagent grade CaCO3 at 1000 °C for 6 h. These samples were then sealed under vacuum in polyethylene bags to prevent loss of lime activity associated with water absorption. 3 Fluxed coal ash samples were generated by weighing and mixing the appropriate masses of ash and flux in a rotating sample splitter prior to being placed in the TMA crucibles. Thermomechanical Analysis Measurements. TMA experiments were carried out in a Setram TMA92 thermo mechanical analyzer. Approximately 50 mg of sample was
placed into a graphite crucible, the ash was then compacted with a force of approximately 3 N, and a penetrating ram was inserted into the crucible containing ash. The entire assembly was then placed into the TMA instrument and heated to 700 °C at a rate of 50 °C/min and then at 5 °C/min until a temperature of 1600 °C.4 A schematic diagram of the TMA crucible, ash pellet, and tapered rod assembly is shown in Figure 1. As the sample is heated, the penetrating ram sinks into the ash and will eventually contact the base of the crucible when the slag fully flows into the annulus between the crucible and penetrating ram. Output from the TMA consists of the shrinkage of the ash expressed as a percentage of the original height of the ash as a function of temperature. The first derivative of the shrinkage gives the rate of change of shrinkage of the pellet ash temperature. Viscosity Measurements. The viscosities of the samples were determined from measurements in a rotary Haake hightemperature viscometer. Details of the technique are contained elsewhere.5
(3) Russell, R. O. J. Metals 1967, 104-106. (4) Saxby, J. D.; Chatfield, S. P. Proc. 7th Aust. Coal Sci. Conf. Monash University, Aust. 1996, 391-398. (5) Hurst, H. J.; Novak, F.; Patterson, J. H. Proc. 5th Int. Conf. Molten Fluxes Salts, Sydney, 1997.
Results and Discussion Viscosity Measurements. Figure 2 illustrates the effect of flux addition on measured viscosity for coal ash
Quantifying Flux Additions Necessary for Slag Flow
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Figure 6. Liquidus (top) and solidus (bottom) temperatures as a function of CaCO3 addition as determined by FACT for coal ash sample B. The flux levels for the samples considered are indicated by the vertical lines.
Figure 4. Schematic diagram of the effect of flux additions on TMA traces: (A) raw coal ash and (B) fluxed coal ash.
sample B as a function of temperature for the flux level indicated. The shaded region indicated in Figure 2 represents the temperature and viscosity regimes required for operation of a slagging gasifier.1 The general observation derived from Figure 2 is that increasing flux addition results in a significant reduction in viscosity at a given temperature, until at increased flux additions the viscosity is not significantly affected by flux levels. At even higher levels of flux addition the viscosity would start to increase: this is due to flux additions being so great that the composition of CaO is greater than the CaO eutectic composition, thereby increasing liquidus
temperatures, and the proportion of solids in the melt at a given temperature. The temperature at which the viscosity of a liquid becomes non-Newtonian is termed the temperature of critical viscosity. For the samples shown in Figure 2 this is the lower temperature limit and is the point at which solids formed during cooling start to influence the observed viscosity. Shrinkage Measurements. Figure 3 presents the TMA traces obtained for coal ash samples as a function of temperature for the fluxing levels indicated. The general observation to be drawn from Figure 3 is that increasing flux levels generally results in a greater shrinkage at a given temperature. The temperature of maximum shrinkage occurs at progressively lower temperatures as fluxing levels increase. At the highest fluxing levels there may be an increase in the temperature of the maximum rate of shrinkage for the same reasons outlined above. Not all flux added to the sample in the experiment melts into the slag initially formed. A gradual dissolution of CaO into the slag is expected to occur, limited by availability of components, the diffusivity of components, temperature and time. Other experiments2 where pellets of ash have been heated to the temperature where significant shrinkage occurs have indicated that substantial melt phase has formed. This melt phase is usually greater than 50% of the mass at the deformation temperature of the ash. The shrinkage measured is therefore related to the flow of the melt phase around the ram as it penetrates the sample. A schematic diagram of the possible effect of flux on TMA experiments is shown in Figure 4. For a raw coal ash the formation of liquid occurs by dissolution of ash
Figure 5. Correlation of measured shrinkage from TMA tests and viscosity for coal ash sample and flux level as indicated for viscosities of 15, 25, and 50 Pa‚s (the codes on the bars give the sample number and flux level as g of CaCO3/g of ash).
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Figure 7. Comparison between FACT equilibrium calculations and TMA shrinkage data for coal ash sample B at flux levels of (A) 0.00, (B) 0.45, and (C) 1.00 g of CaCO3/g of ash.
into the melt, this is a gradual process reflected in the TMA trace. High solids loading, combined with higher melt viscosity results in gradual shrinkage increases with increasing temperature. Addition of flux is expected to result in the formation of a melt phase enriched in calcium at a particular temperature. Increasing CaO content of the melt dramatically reduces the viscosity giving rise to a sharp increase in shrinkage with increasing temperature as is observed in Figure 3B.
Correlation of Shrinkage with Viscosity. A direct correlation between melt viscosity and shrinkage is complicated by a difference in the state of the solid liquid melt at a given temperature. The viscosity measurements used a slag which had been heated to a temperature significantly above the liquidus and then cooled to the required temperature for measurements. Conversely, shrinkage measurements are obtained on samples of ash and flux which progressively react on heating.
Quantifying Flux Additions Necessary for Slag Flow
The measured shrinkage corresponding to estimated viscosities (at the temperature of the measured shrinkage) of 50, 25, and 15 Pa‚s are shown in Figure 5. The data in Figure 5 was obtained as follows. For a given sample the temperature for a specific viscosity (say 15 Pa‚s) was obtained from the viscosity curves (Figure 2). If the temperature was less than the temperature of critical viscosity, the data point was ignored. Otherwise, the shrinkage at the established temperature was estimated from the TMA traces (Figure 3). Data obtained for a temperature less than the temperature of critical viscosity has resulted in the omission of six data points. There is a reasonable degree of scatter in the data; however, for a viscosity of 50 Pa‚s an average shrinkage of 18% is noted with a standard deviation of 9.5%. For a viscosity of 25 Pa‚s an average shrinkage of 65.5% is noted with a standard deviation of 32.7%. For a viscosity of 15 Pa‚s the average shrinkage was 83% with a standard deviation of 19.5%. The scatter observed is possibly related to the differing ash compositions in the samples and further experiments are necessary in order to establish corrections for particular ash chemistries. FACT Predictions. The Facility for the Analysis of Chemical Thermodynamics (F*A*C*T)6 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 system based on SiO2-Al2O3-FeO-CaONa2O. Figure 6 shows the predicted liquidus and solidus temperatures for coal ash sample B, as determined by thermodynamic equilibrium calculations as a function of flux additions. From Figure 6 there is a minimum in the liquidus temperature at a flux addition of approximately 0.92 g of CaCO3/g of ash. The reduction in the measured shrinkage with the change of flux level from 0.75 to 1.00 in Figure 3 at 1250 °C is consistent with this minimum. The temperatures for rapid shrinkage appear to correlate with the solidus (6) 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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temperatures, where slag is expected to form rapidly as ash dissolves. From Figure 3B these temperatures are approximately 1400 and 1250 °C for samples with flux additions of 0.25 and 0.75 g of CaCO3/g of ash respectively, corresponding to the solidus temperatures shown in Figure 6 of 1360 and 1230 °C. Equilibrium calculations conducted using FACT suggest that the shrinkage observed in the TMA trace may be correlated with the formation of liquid phases during the TMA experiment. Figure 7 gives the comparison between calculated weight percent of sample as slag and TMA shrinkage measurements for coal ash sample B and coal ash sample B fluxed with 0.00, 0.45, and 1.00 g of CaCO3/g of ash. A reasonable comparison is observed indicating that the temperature of rapid shrinkage does appear to correspond to the formation of slag which the code predicts. Conclusions Measurements of the progressive shrinkage of heated samples of three coal ashes and calcium oxide flux have been obtained in order to evaluate the technique as a means of quantifying the flux needs for coals in slagging gasifiers. For the viscosity range which needs to be established for slag to flow (15-50 Pa‚s), the average shrinkage levels range from 83% to 18% with a general increase as viscosity decreases. Although further work is required to clarify the reasons for differences in results, the technique appears to have the potential for characterizing flux requirements. 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. Dr. J. Saxby of the CSIRO Division of Coal and Energy Technology undertook the TMA measurements, and Dr. J. Patterson and Dr. H. Hurst provided the viscosity data. EF9700846
