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Analysis of the High-Temperature Cohesion Behavior of Ash Particles Using Pure Silica Powders Coated with Alkali Metals Hidehiro Kamiya,* Akira Kimura, Mayumi Tsukada, and Makio Naito† Graduate School of Bio-Applications and Systems Engineering, BASE, Tokyo University of Agriculture & Technology, Koganei, Tokyo 184-8588, Japan Received August 8, 2001. Revised Manuscript Received December 8, 2001
This paper analyzes the increasing mechanism of cohesive behavior of ash powders at elevated and high-temperature conditions. Model ash powders were prepared from fine pure amorphous silica powder coated and heat-treated on the surface with 0.5 wt % sodium and/or potassium. The cohesive behavior and deformation properties of model ash powder beds were determined by using a new split-type tensile strength tester of powder beds. The adhesion behavior of fly ash collected in a pulverized coal combustion (PC) and a pressurized fluidized bed combustion (PFBC) system was compared with the results of model ash powder samples. In the hightemperature range above 800 °C, a rapid increase of tensile strength and plastic rupture deformation occurred in both the natural ash samples and the model ash powder samples prepared from alkali metal and pure silica powder. It is suggested that the coating alkali metal reacted with the amorphous silica phase and formed a small amount of low-melting-point eutectic materials, such as K2O‚4SiO2. Since these eutectic materials formed a small amount of liquid phase at interparticles at about 800 °C, the tensile strength and plastic rupture deformation rapidly increased at high temperatures above 800 °C.
1. Introduction The increase of the stickiness and adhesion force of ash powder at high temperatures hinders the stable operation and scale-up design of various high-efficiency coal-combustion power-generation systems, for example, the integrated coal gasification combined cycle (IGCC)1 and pressurized fluidized bed combustor (PFBC) systems. Dry ash particle deposition and the growth of a deposited ash layer on the water-cooled wall blocked the gasifier in IGCC systems and pulverized-coal combustors.1,2 In PFBC systems, the operation of a filter cake detachment on rigid ceramic filters by reverse-pulsing is difficult with the increase of the adhesive force of ash.3-5 The cohesive properties of ash particles at these high temperatures depend on many physical and chemical factors. However, these phenomena have been measured and analyzed in a few papers.6-11 Since the testing cell and probe were prepared from metal, the * Corresponding author. † Japan Fine Ceramics Center, Atsuta-ku, Nagoya, Japan. (1) Koyama, S.; Morimoto, T.; Ueda, A.; Matsuoka, H. Fuel 1996, 75, 459-465. (2) Smith, D. H.; Haddad, G. J.; Ferer, M. Energy Fuels 1997, 11, 1006-1011. (3) Abbott, M. F.; Austin, G. Fuel 1985, 64, 832-833. (4) Walsh, P. F.; Sayre, A. N.; Loehden, D. O.; Monroe, L.; Beer, J. M.; Sarofim, A. F. Prog. Energy Combust. Sci. 1990, 16, 327-346. (5) Kamiya, H.; Deguchi, K.; Gotou, J.; Horio, M. Powder Technology 2001, 118, 160-165. (6) Jimbo, G. Annual IFPRI Report, ARR-11-8 (1988, May). (7) Jimbo, G.; Yamazaki, R. Prepr. Eur. Symp. Particle Technol. 1980, B, p 1064 (Amsterdam, June 3-5, 1980). (8) Oshima, T.; Hirota, M.; Morishita, H.; Arimoto, K. J. Soc. Powder Technol. Jpn. 1982, 19, 703-708.
adhesive force in these papers was measured below a range of 700 °C. In our previous paper, we measured the tensile strength of fly ash powder beds by using a diametal compression test of ash powder pellets12 and a new splittype tensile strength tester of ash powder beds13 at high temperature at a range from room temperature to 950 °C. We reported that the tensile strength of ash powder beds rapidly increased over a range of 800 °C. This paper focuses the effect of alkali metal and amorphous silica phase on the mechanism of the increasing cohesive behavior of ash powders at elevated temperatures. Model ash powders prepared from fine pure silica powder coated on the surface with 0.5 wt % alkali metal. The cohesive behavior and deformation properties of model ash powder beds were determined by using a new split-type tensile strength tester of powder beds.13 On the basis of the comparison with these results of fly ash and model ash powders, the increasing mechanism of ash powder stickiness at high temperature is discussed. (9) Oshima, T.; Hirota, M.; Suzuki, M. J. Soc. Powder Technol. Jpn. 1983, 20, 357-364. (10) Berbner, S.; Loffler, F. Powder Technol. 1994, 78, 273-280. (11) Dockter, B. A.; Hurley, J. P. High-Temperature Gas Cleaning; Schmidt, E., et al., Eds.; Institut fur Mechnische Verfahrenstechnik und Mechanik, Universitat Karlsruhe (TH), 1996; pp 157-168. (12) Kamiya, H.; Kimura, A.; Horio, M.; Seville, J. P. K.; Kauppinen, E. J. Chem. Eng. Jpn. 2000, 33, 654-656. (13) Kamiya, H.; Kimura, A.; Yokoyama, T.; Naito, M.; Jimbo, G. Powder Technol., accepted.
10.1021/ef010208l CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
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Figure 2. Schematic illustration of a split-type tensile strength testing system for ash powder beds on high-temperature condition.
Figure 1. TG-DTA analysis of potassium oxalate. Table 1. Mean Diameter and Chemical Component of Pure Silica and Fly Ash Powders powder sample
mean diameter
SiO2
pure silica fly ash
5.3 µm 4.75 µm
99.99 45.0
chemical component [wt %] Al2O3 Fe2O3 K2O Na2O 38.5
13 ppm 4.6 0.3
0.5
2. Experimental Section 2.1. Model Ash Powder Preparation and Characterization. The original powder samples used commercial pure fused amorphous silica powder and fly ash. Fly ash powder was collected in a commercial pulverized coal combustion plant. The chemical components and particle-size distribution of each sample determined using ICP and the centrifugal sedimentation method were shown in Table 1. The total impurity (including Na, K, etc.) in the high-purity amorphous silica powder was about 13 ppm. The concentration of sodium and potassium in the fly ash was about 0.5%. The mean particle diameter of each sample was similar at about 5.0 µm. On the basis of the above results, model ash powders were prepared from a commercial pure fused silica powder. First, pure silica powder was added to the sodium or potassium oxalate solution. The concentration of alkali oxalate in aqueous solution was adjusted at 0.5 mol % for the silica particles, which was the mean alkali content in ash particles. The silica suspension was mixed for one minute by a planetary-type mixer and dried in an oven at 120 °C. After grinding with a pestle and mortar, dried silica powders with alkali oxalate were heat-treated at 620 °C for 1 h, causing thermal decomposition and removal of the oxalate. This heat-treatment temperature of dried silica particles after coating of sodium or potassium oxalate was determined by the TG-DTA analysis of potassium oxalate, as shown in Figure 1. The thermal decomposition and combustion reaction of potassium oxalate were completely finished at about 600 °C. A similar TG-DTA curve was obtained even in sodium oxalate. On the basis of this TG-DTA analysis, the heattreatment temperature was set over 600 °C. 2.2. Split-Type Tensile Strength Tester for Powder Beds. The details of a split-type tensile strength test at high temperatures are shown in Figure 2. The original system is a commercial split-type adhesion behavior testing system for powder beds (Hosokawa Micron Co. Ltd., Cohetester).14 The circular cell for the powder filling, which was 5 cm in diameter and 1 cm in depth, can be divided equally into a stationary part and a movable part. Both parts were prepared by highpurity silica glass with a low thermal-expansion coefficient to eliminate the effect of the thermal expansion of the cell on the powder bed strength in the wide temperature range from room temperature to 1373 K. The moving part was installed (14) Yokoyama, T.; Fujii, K.; Yokoyama, T. Powder Technol. 1982, 32, 55-61.
over a flat plate made of fused silica, and this flat plate was hung in a leaf spring of 3 sheets. After fixing the movable part using a hook, powder samples were packed into the cell and consolidated by uniaxial pressing (2.5 kPa for 600 s). Tensile testing was carried out at various temperatures ranging from room temperature to 1173 K using an electric furnace at a heating rate of 600 °C/h. After heat treatment at each temperature for 540 s before the tensile test, the hook used to fix the movable cell was removed, and the movable cell was pulled in each high-temperature condition. The relationship between load and displacement during loading was then measured. To discuss the mechanism of increasing the adhesive behavior at high-temperature conditions, the shrinking property of each powder bed sample was measured by a thermomechanical analysis (TMA) under a temperature condition of up to 1373 K.
3. Results and Discussion 3.1. Characterization of Ash, Pure Silica, and Model Ash Powders. Figure 3 shows the SEM observation of pure silica powder and fly ash powder collected at the pulverized coal combustion plant. Pure silica powders were prepared by a melt and cool process, producing spherical particles, as seen in Figure 3a. Because fly ash powders were produced by the same melt and cool process in pulverized coal combustion system, the shape of the ash powder was spherical, as shown in Figure 3b. Ultra-fine powders of less than 10 nm in diameter were observed on spherical ash and pure silica particles. It seemed that these ultra-fine powders were formed by the condensation of the vaporized phase in coal during the pulverized combustion and heatexchanging process. The SEM observation of model ash particles prepared from pure silica powder by mixing in a potassium oxalate aqueous solution and heat-treating at 620 °C for 1 h is shown in Figure 4. The surface of the particle appeared to become smooth after superfine powders left the surface of spherical silica particles during mixing in alkali oxalate aqueous solution. Figure 5 shows dilatometry data for powder beds of fly ash, pure silica, and model ash at a heating rate of 5 K/min. Powder samples were packed in an alumina pan and consolidated by a uniaxial compression at 5 kPa. The shrinkage of powder bed height, ∆L, during elevating temperature was measured by a differential transducer. The dilatometry data of each powder sample were given by the ratio of shrinkage to initial bed height, L0. All powder samples started to shrink at 200 °C, and densification continued to 750 °C. Pure silica powder gradually continued to shrink up to 1100 °C. Fly ash and model ash samples showed rapid shrinkage above
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Figure 3. SEM observation of sample powders. (a) Pure silica powder, and (b) fly ash collected in pulverized combustion plant.
Figure 4. SEM observation of model ash powder prepared from pure amorphous silica and alkali oxalate aqueous solution.
Figure 5. Dilatometry data for each sample powder bed during elevated temperature condition.
750 °C. The rapid shrinking temperature of model ash powder samples prepared from pure silica and alkali oxalate aqueous solution was equal to that of fly ash powders. In our previous paper,13 it was estimated that
this rapid shrinking phenomenon above 750 °C is generated by the liquid phase formation and viscous flow sintering at contact points between ash powders. Based on this result, the reaction with amorphous silica phase and alkali metal generated a low-melting-point eutectic material and formed a small amount of liquid phase between particles. It is estimated that the shrinking of the particle bed and the rapid increase in adhesive strength are promoted by the formation of a small amount of liquid phase. Figure 5 shows that, as temperature elevated above 900 °C, while the shrinkage of fly ash powder layer continued to increase with the rise in the temperature, the shrinkage of the model ash powder was almost completed at about 1000 °C. Since the K2O-SiO2-Al2O3 eutectic material, in which the melting point is 990 °C,15,16 was formed in the ash particles, the shrinkage of the ash increased when the temperature increased above 990 °C. On the contrary, since model ash powders prepared from pure silica and alkali metal did not include an alumina phase, the shrinkage by the eutectic material between amorphous silica and alkali metal finished at 1000 °C. 3.2. Tensile Strength and Deformation Behavior of Fly Ash and Pure Fused Silica. Examples of the tensile load and displacement relationship at room temperature and 850 °C are shown in Figure 6. The relatively brittle fracture of both powder beds was observed after elastic deformation at room temperature. In the case of the pure silica powder sample, the brittle fracture of the powder bed appeared at 850 °C. On the contrary, in the case of fly ash samples, viscous deformation appeared near the maximum load at 800 °C. Under conditions of relatively low temperatures below 800 °C, the adhesive force of both powders increased gradually in proportion to the temperature. The tensile strength of the pure silica powder bed gradually increased without the increase of deformation, even in elevated temperatures from room temperature to 800 °C. A rapid increase of tensile strength and deformation (15) Schairer, J. F.; Bowern, N. L. Bull. Soc. Geol. Finlande 1947, 20, 74-82. (16) Reifenstein, A. P.; Kahraman, H.; Coin, C. D. A.; Calos, N. J.; Miller, G.; Uwins, P. Fuel 1999, 78, 1449-1461.
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Figure 6. Examples of tensile stress and displacement relationship of ash and silica powder beds.
Figure 7. An example of the tensile stress and displacement relationship of model ash powder beds prepared from pure amorphous silica powder and alkali metals.
of fly ash powder beds was only observed in the hightemperature range (>800 °C). It is estimated that these increasing phenomena of strength and deformation were related with the rapid shrinkage of the fly ash layer. 3.3. Tensile Strength and Deformation of Model Ash Powders at Elevating Temperature. To analyze the rapidly increasing tensile strength of fly ash above 800 °C, the relationship between tensile strength and deformation behavior of alkali metal-coated pure silica powder beds with different coating conditions was measured and is shown in Figure 7. The rapid increase of tensile strength with viscous tensile deformation was observed by a coating of 0.5% alkali metal on the surface of pure silica powders in a high-temperature range of 800 °C. The effect of the testing temperature on the tensile strength of model ash powder beds with different alkali coating conditions is shown in Figure 8. The result13 of fine ash collected by a ceramics filter in pressurized fluidized bed combustion system, PFBC, is shown in this figure. Regardless of the alkali metal used, the tensile strength of the model powder beds exhibited an increase with a temperature rise above 800 °C equal to that shown in the fly ash samples. This indicates that the alkali metal coating on the silica powders reacted with the amorphous silica phase and formed a small amount of low-melting-point eutectic materials. Hence, above 800 °C, a small amount of liquid phase formed between
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Figure 8. The effect of temperature on tensile strength of model ash powders prepared from pure amorphous silica powders coated with sodium and potassium.
Figure 9. The effect of temperature on tensile fracture deformation of model ash powders prepared from pure amorphous silica powders coated with sodium and potassium.
silica particles, consisting of amorphous silica and alkali metal. It is suggested that a small amount of lowmelting-point eutectic materials, consisting of amorphous silica and alkali metal, for example, K2O‚4SiO2, whose melting point was about 770 °C,17,18 formed a liquid phase and that this was responsible for the majority of the increase in the adhesive force mechanism of the ash powders at high temperature. Furthermore, since Na2O‚4SiO2, has almost the same melting temperature,18 and the rapid increase of adhesion force of Na-coated silica powder occurred at the same temperature. Since the amount of this alkali and silica eutectic phase is too small to be characterized by an X-ray crystallography method, it is impossible to confirm these phase. This liquid phase was estimated from the increase in rupture displacement during the tensile test of powder beds at high temperature. Figure 9 shows the effect of temperature on the critical tensile rapture displacement. The critical rupture displacement, i.e., the separation displacement was observed during the tensile test shown in Figures 6 and 7 as the maximum tensile (17) Morey, G. W.; Kracek, F. C.; Bowen, N. L. J. Soc. Glass Technol. 1930, 14, 149-187. (18) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists; The American Ceramic Society, Inc: Westerville, OH; Vol. 7, pp 170-171.
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stress. The rupture displacement of pure amorphous silica powder was almost constant in the whole temperature range from room temperature to 1000 °C. The rupture displacement of fly ash and model ash powder beds prepared from alkali-coated pure silica was equal to that of pure amorphous silica in the relatively low temperature below 800 °C. However, the rapid increase of rupture deformation of fly ash and both model ash powders was observed in a high-temperature range above 800 °C. It is suggested that a small amount of liquid phase of low-melting-point eutectic materials between alkali metal and amorphous silica forms the liquid-bridge between particles. We conclude that the main source of rapid increase of tensile strength and rupture displacement of fly ash powder ranging from 750 to 850 °C is the liquid phase of low-temperature eutectic materials consisting of amorphous silica phase and alkali metal in ash particles. 4. Conclusion Model fly ash powder was prepared from 0.5 wt % sodium- or potassium-coated and heat-treated pure
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amorphous silica powder. Both model ash samples can reproduce the rapid-increase phenomena of tensile strength and the plastic deformation behavior of ash samples collected in pulverized combustion and pressurized fluidized bed coal combustion systems in high temperatures above 800 °C. The alkali metal and the amorphous silica phase in ash particles reacted and formed low-melting-point eutectic materials during the coal combustion process, for example, K2O‚4SiO2, whose melting point was about 770 °C, and formed a small amount of liquid phase between particles at about 800 °C. The liquid phase appeared to form a bridge between ash particles and promote the increase of ash adhesion and plastic deformation behaviors.
Acknowledgment. This work was supported by the Proposal-Based New Industry-Type Technology R&D Promotion Program from NEDO of Japan and a Grantin-Aid for Scientific Research (B), Japan. EF010208L
