Energy & Fuels 2007, 21, 3437–3445
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Characterization of Ash Cenospheres in Fly Ash from Australian Power Stations Ling-ngee Ngu,†,‡ Hongwei Wu,*,†,‡ and Dong-ke Zhang† Centre for Fuels and Energy and Department of Chemical Engineering, Curtin UniVersity of Technology, GPO Box U1987, Perth WA 6845, Australia ReceiVed June 18, 2007. ReVised Manuscript ReceiVed July 31, 2007
Ash cenospheres in fly ashes from five Australian power stations have been characterized. The experimental data show that ash cenosphere yield varies across the power stations. Ash partitioning occurred in the process of ash cenosphere formation during combustion. Contradictory to conclusions from the literature, iron does not seem to be essential to ash cenosphere formation in the cases examined in the present work. Further investigation was also undertaken on a series of size-fractioned ash cenosphere samples from Tarong power station. It is found that ∼70 wt% of ash cenospheres in the bulk sample have sizes between 45 and 150 µm. There are two different ash cenosphere structures, that is, single-ring structure and network structure. The percentage of ash cenospheres of a network structure increases with increasing ash cenosphere size. Small ash cenospheres (in the size fractions 250 µm) have a low SiO2/Al2O3 ratio, and a high proportion of the ash cenospheres are nonspherical and of a network structure. A novel quantitative technique has been developed to measure the diameter and wall thickness of ash cenospheres on a particle-to-particle basis. A monolayer of size-fractioned ash cenospheres was dispersed on a pellet, which was then polished carefully before being examined using a scanning electron microscope and image analysis. The ash cenosphere wall thickness broadly increases with increasing ash cenosphere size. The ratios between wall thickness and diameter of ash cenospheres are limited between an upper bound of ∼10.5% and a lower bound of ∼2.5%, irrespective of the ash cenosphere size.
1. Introduction Australia produces a considerable amount of fly ash from the coal-fired power generation industry and is one of the countries with the lowest ash utilization: only 10% of fly ash produced is used, mainly in the building and construction industry.1 The large quantities of fly ash impose significant pressure on environment and waste management. The impact, however, could be minimized by effective ash utilization through a range of potential industry applications, as summarized in a recent review.2 Ash cenospheres are hollow lightweight microspheres as part of fly ash produced during pulverized coal combustion for power generation.3–6 They have superior properties including a light weight, good packing factor, enhanced insulation, improved flow characteristics, reduced shrinkage, less water absorption, excellent mechanical strength, chemical inertness, good thermal resistance, and good electrical properties. Ash cenosphere utilization may offer significant manufacturing * To whom correspondence should be addressed. E-mail: h.wu@ curtin.edu.au. Telephone: +61-8-92667592. Facsimile: +61-8-92662681. † Centre for Fuels and Energy. ‡ Department of Chemical Engineering. (1) Heidrich, C. Historical data on total australia ash utilisation; Ash Development Association of Australia (ADAA): Wollongong, Australia, 2002. (2) Wang, S.; Wu, H. J. Hazard. Mater. 2006, B136, 482. (3) Raask, E. J. Inst. Fuel 1968, 339–344. (4) Raask, E., Mineral impurities in coal combution-behaVior, problems, and remedial measures; Hemisphere Publishing Corporation: Leatherhead, 1985. (5) Vassilev, S. V.; Menendez, R.; Diaz-Somoano, M.; MartinezTarazona, M. R. Fuel 2004, 83 (4–5), 585–603. (6) Vassilev, S. V.; Menendez, R.; Alvarez, D.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Fuel 2003, 82 (14), 1793–1811.
advantages, product improvement, and cost reduction in a wide range of industrial processes. Ash cenospheres can be used to produce various lightweight construction products, including lightweight cements,7 aggregates in lightweight concrete,8–10 and porous glass crystalline molded blocks for the removal and solidification of liquid and other hazardous wastes.11 They have also found extensive applications in manufacturing lightweight composites, including polyurethane composites,12 polyester composites,13 functionally gradient materials,14 poly(vinyl chloride) composites,15 PCV-U composite,16 nylon composites,17 syntactic polymer foam,18–21 (7) Lilkov, V.; Djabarov, N.; Bechev, G.; Petrov, O. Cem. Concr. Res. 1999, 29, 1641–1646. (8) Blanco, F.; Garcia, P.; Mateos, P.; Ayala, J. Cem. Concr. Res. 2000, 30 (11), 1715–1722. (9) McBride, S. P.; Shukla, A.; Bose, A. J. Mater. Sci. 2002, 37, 4217– 4225. (10) Barbare, N.; Shukla, A.; Bose, A. Cem. Concr. Res. 2003, 33 (10), 1681–1686. (11) Anshits, A. G.; Vereshchagin, N.; A., T.; Voskresenskaya, E. N.; Kostin, E. M.; Pavlov, V. F.; V., N.; Knecht, D. A.; Tranter, T. J.; Macheret, Y. Method for solidification of radioactive and other hazardous waste. US 647259, 20001127, 2002. (12) Chalivendra, V. B.; Shukla, A.; Bose, A.; Paramewaran, V. J. Mater. Sci. 2003, 38, 1631–1643. (13) Cardoso, R. J.; Shukla, A.; Bose, A. J. Mater. Sci. 2002, 37, 603– 613. (14) Parameswaran, V.; Shukla, A. J. Mater. Sci. 2000, 35, 21–29. (15) Wan, C.; Zhang, Y.; Xu, H.; Gu, H.; Huang, W. Gongcheng Suliao Yingyong 2003, 31 (9), 15–18. (16) Shen, Z.; Wang, M.; Ma, S.; Xing, Y.-S.; Liu, C.-H.; Hu, Z.; Chen, X.-R. Suliao 2002, 16 (5), 55–57. (17) Ding, X.; Lu, D.; Yu, D.; Zhang, L.; Xu, R.; Bai, J. Suliao 2003, 32 (2), 19–22.
10.1021/ef700340k CCC: $37.00 2007 American Chemical Society Published on Web 10/03/2007
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metal-matrix composites,22–24 glass fiber/epoxy resin composites,25 and conducting polymers.26–28 The properties and performance of the lightweight products utilizing ash cenospheres depend largely on ash cenosphere characteristics, particularly ash cenosphere physical structure such as diameter, particle size distribution, wall thickness, and shape.8,13–19,21,29 For example, when used as fillers, ash cenospheres of thicker walls are favored to increase the mechanical strength of the finished products.8,21 On the other hand, the shape of ash cenospheres may also be an important factor.16 In comparison to other shapes, spherical ash cenospheres have the lowest surface area to volume ratio and, thus, require less resin, binder, and water to wet out the surface. Besides, spherical particles offer the advantages of reduced shrinkage as well as improved workability and ease of use.19,29 Apart from these properties, the particle size distribution of ash cenospheres affectsthemechanicalpropertiesofthemanufacturedproducts.13,14,18 Therefore, a good understanding of ash cenosphere characteristics and its formation mechanisms is essential to not only the optimization of ash cenosphere market positioning but also the development of effective strategies to enhance and optimize ash cenosphere production in power plants, without affecting the main business of electricity generation. Previous studies3,5,30–32 investigated the yield, morphology, thermal properties, chemistry, and mineralogy of ash cenospheres collected from various power stations. However, these studies only considered bulk ash cenosphere samples, using random crosssection analysis without stereological corrections,33 which is complicated, and/or relying on indirect calculations based on bulk density to obtain the wall thickness.3 Therefore, the results derived were generally in the forms of average ash cenosphere wall thickness and average ash cenosphere diameter of bulk ash cenosphere samples.4 The difference in the structure of individual ash cenosphere particles is ignored, and obtaining correlations between the wall thickness and the size of the same single ash cenosphere particle is also impossible. In practice, “floaters”, that is, ash particles that have a bulk density less than that of water and therefore float on the top of (18) Shutov, F. Syntactic polymeric foams; Hanser Publishers: New York, 1991; pp 355–374. (19) Klempner, D.; Frisch, K. C. Handbook of polymeric foams and foam technology; Oxford University Press: New York, 1991; pp 356– 374. (20) Gupta, N.; Woldesenbet, E. Fem contact analysis and experimental investigation of the effect of radius ratio of cenosphere on syntactic foams. In Proceedings of the 10th U.S.–Japan Conference on Composite Materials; Chang, F.-K., Ed.; DEStech Publications, Inc.: Lancaster, PA, 2002. (21) Gupta, N.; Woldesenbet, E.; Mensah, P. Composites, Part A 2004, 35A (1), 103–111. (22) Rohatgi, P. K.; Guo, R. Q.; Iksan, H.; Borchelt, E. J.; Asthana, R. Mater. Sci. Eng. 1998, 244A, 22–30. (23) Wang, D.; Shi, Z.; Hong, G.; Lopez, H. F. J. Mater. Synth. Process. 2001, 9 (5), 247–251. (24) Souvignier, C. W.; Sercombe, T. B.; Huo, S. H.; Calvert, P.; Schaffer, G. B. J. Mater. Res. 2001, 16 (9), 2613–2618. (25) Wang, M.; Zhao, X.; Shen, Z.; Ma, S. Beijing Hangkong Hangtian Daxue Xuebao 2003, 29 (12), 1064–1067. (26) Shukla, S.; Seal, S.; Akesson, J.; Oder, R.; Carter, R.; Rahman, Z. Appl. Surf. Sci. 2001, 181 (1–2), 35–50. (27) Shukla, S.; Seal, S.; Rahaman, Z.; Scammon, K. Mater. Lett. 2002, 57 (1), 151–156. (28) Akesson, J.; Seal, S.; Shukla, S.; Rahma, Z. AdV. Mater. Proc. 2002, 160 (2), 33–34. (29) Ashida, K. Syntactic foams; Noyes Publications: Park Ridge, NJ, 1995; pp 147–163. (30) Fisher, G. L.; Chang, D. P. Y.; Brummer, M. Science 1976, 192, 553–555. (31) Ghosal, S.; Self, S. A. Fuel 1995, 74 (4), 522–529. (32) Spears, D. A. Appl. Clay Sci. 2000, 16 (1–2), 87–85. (33) Sokol, E. V.; Maksimova, N. V.; Volkova, N. I.; Nigmatulina, E. N.; Frenkel, A. E. Fuel Process. Technol. 2000, 67 (1), 35–52.
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ash ponds, are collected as ash cenospheres.3,34,35 In other words, regardless of whether the particles are of spherical shapes or nonspherical, of a network structure, inorganic particles which have apparent density less than that of water are all considered to be ash cenosphere products. Therefore, it becomes essential to carry out a systematic study on the structure of the collected “ash cenospheres” as the quality of the products utilizing these collected “ash cenospheres” are largely determined by the structure of these “ash cenospheres”. In this paper, a novel technique is developed, which combines size fractionation, monolayer pellet setting, controlled fine polishing, scanning electron microscopy (SEM), and image analysis, to characterize ash cenospheres on a particle-to-particle basis. Besides structure characterization, this technique achieves not only the determination of the true size and wall thickness of ash cenospheres but also the development of correlations between wall thickness and size of a single ash cenosphere particle. In combination with other structure and chemistry analyses, the data provide further insights into the ash cenosphere properties and formation mechanisms. 2. Experimental Section 2.1. Sample Collections and Preparations. Fly ash samples were collected from five Australian power stations, including Muja and Collie in Western Australia, Tarong in Queensland, and Wallerawang and Mount Piper in New South Wales. In practical applications ash cenospheres are harvested from the surface of ash lagoons and have a density less than that of water. Therefore, in this study, a sink–float method using water as the medium was employed to separate ash cenospheres from the bulk fly ash samples. The float samples were then heated to 830 °C and held at that temperature for 1 h in a furnace, with air continuously flowing through the furnace to remove unburnt carbon. The yield of the float samples after density separation and heat treatment is taken as the ash cenosphere yield. Further sample preparation was also carried out to sieve ash cenospheres from the Tarong power station into a series of nine narrow size fractions (25–45, 45–63, 63–75, 75–90, 90–106, 106–125, 125–150, 150–250, and >250 µm) for subsequent characterization. 2.2. Sample Characterization. The chemical compositions of ash cenospheres and fly ash samples were determined using X-Ray fluorescence (XRF) spectroscopy. Characterization of ash cenosphere structure was carried out using a new technique. The technique combines size fractionation, monolayer pellet setting, controlled fine polishing, SEM, and image analysis. Measurements of the true diameter and wall thickness of ash cenospheres were achieved on a particle-to-particle basis. A detailed description of the technique is discussed in the next section.
3. Technique for Ash cenosphere Characterization Figure 1 illustrates the detailed procedure of the developed technique to characterize ash cenospheres. First, a bulk ash cenosphere sample was sieved into a series of narrow size fractions. To prepare the pellet specimen for SEM examination, a thin layer of epoxy resin was brushed on a finely polished epoxy pellet. The ash cenosphere sample of one of the narrow size fractions was then dispersed on the resin, typically forming a monolayer of ash cenosphere particles on the pellet, as shown in Figure 2. After the resin solidified, ash cenosphere particles within the monolayer were fixed and the contact points were aligned with the finely polished surface of the pellet. The three largest spherical ash cenosphere particles were then identified and measured under a microscope. It should be noted that as (34) Vassilev, S. V.; Vassileva, C. G. Fuel Process. Technol. 1996, 47 (3), 261–280. (35) Kruger, R. A. Fuel 1997, 76 (8), 777–779.
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Figure 1. Schematic diagram (not to scale) of the procedure of the cenosphere characterization technique.
Figure 2. Typical SEM image of monolayer dispersion of cenosphere particles on a pellet.
the samples were of narrow size fractions, it was common that the several largest particles, randomly distributed on the pellet, had similar sizes. Additional epoxy resin application was followed to set all the ash cenosphere particles in the resin, allowing necessary time for epoxy resin solidification before controlled fine polishing being applied to the epoxy pellet. During the process of controlled fine polishing, the pellet was repeatedly examined by microscope to monitor the size of the identified particles in the cross section for two reasons. One is that during the progress of polishing, the fine polishing needs to be monitored and controlled to make sure that the size of
the three identified particles be similar at any cross ection. This is important to ensure that the cross section was always parallel to the surface of the pellet before the second layer of resin was applied. The other reason is to ensure that the polishing is completed at the right level. Indeed, the polishing should be terminated when those identified particles in the cross section of the pellet reached their measured (i.e., maximum) sizes. This would indicate that the polishing had intercepted the center of the largest particle. SEM images of the ash cenosphere particles in the fine-polished cross sections were then taken, followed by image analysis to measure the inner and outer diameters of all concentric annuli in the cross sections. The combined technique of SEM and image analysis is commonly used for char, ash, and deposit characterization in the literature.36–44 (36) Wu, H.; Wall, T.; Liu, G.; Bryant, G. Energy Fuels 1999, 13, 1197. (37) Wu, H.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels 2000, 14, 282. (38) Wu, H.; Bryant, G.; Wall, T. Energy Fuels 2000, 14, 745. (39) Robinson, A. L.; Buckley, S. G.; Baxter, L. L. Energy Fuels 2001, 15, 66. (40) Yu, J. L.; Strezov, V.; Lucas, J.; Liu, G.-S.; Wall, T. Proc. Combust. Inst. 2002, 29, 467. (41) Kweon, S. C.; Ramer, E.; Robinson, A. L. Energy Fuels 2003, 17, 1311. (42) Yu, J.; Lucas, J.; Strezov, V.; Wall, T. Energy Fuels 2003, 17, 1160. (43) Yu, J.; Harris, D.; Lucas, J.; Roberts, D.; Wu, H.; Wall, T. Energy Fuels 2004, 18, 1346. (44) Wee, H. L.; Wu, H.; Zhang, D.-K. Energy Fuels 2007, 21, 441.
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Figure 3. Schematic diagram illustrating the determination of the true size and wall thickness of an ash cenosphere particle using the novel characterization technique developed in this study.
As shown in Figure 3, the inner and outer diameters of the largest ash cenosphere particle, that is, Min and Mout, are then measured and can be taken directly as true sizes of the largest ash cenosphere particle. Similarly, the inner and outer diameters, that is, Xin and Xout, of the concentric annulus of any other ash cenosphere particle in the cross section can also be directly measured. However, Xin and Xout are not true inner and outer diameters, that is, Din and Dout of the ash cenosphere particle. Simple calculations (see eqs 1–3) are necessary to derive Din, Dout, and therefore the average wall thickness t of the same ash cenosphere particle as Dout )
Mout2 + Xout2 2Mout
Din ) √Dout2 - Xout2 + Xin2
(1) (2)
Dout - Din (3) 2 It should be noted that to ensure that the rest of the ash cenosphere particles are intercepted when the controlled fine polishing reaches the center plane of the largest ash cenosphere particle, all ash cenosphere particles must be of a narrow size fraction. This is the reason that in this study, the bulk ash cenosphere samples were sieved into a series of nine narrow size fractions before the characterization of ash cenosphere particles in each narrow size fraction. Additionally, this paper has found that ash cenospheres may be of two different structures; a singlering structure and a network structure (see section 4.3). Therefore, it should also be noted that the technique developed t)
Figure 4. Ash cenosphere yield in fly ash samples from the five Australian power stations.
in this paper is limited for characterizing ash cenosphere particles of a single-ring structure. Ash cenosphere particles of a network structure are nonspherical and have irregular shapes; therefore, the technique is not applicable, and this paper only estimates the abundance of these particles in the samples. 4. Results and Discussion 4.1. Cenosphere Yields. Figure 4 presents the data on ash cenosphere yield in fly ash collected from the five Australian power stations. There is a significant variation in ash cenosphere yield across these power stations. The highest ash cenosphere yield (3.82 wt. %) was found in the fly ash from Tarong power
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Table 1. Chemical Composition of Fly Ash Samples from Different Power Stations (from This Work)a bulk fly ash (wt %) SiO2 Fe2O3 Al2O3 TiO2 P2O5 Mn3O4 CaO MgO Na2O K2 O SO3 V2O5 ZnO BaO SrO a
P1
P2
P3
P4
54.00 10.50 28.30 1.70 1.10 0.08 1.70 1.20 0.28 0.73 0.06 0.03 0.07 0.35 0.26
65.10 2.00 27.30 1.20 0.14 0.04 0.37 0.35 0.36 3.20
68.60 1.30 25.50 1.10 0.10 0.02 0.22 0.25 0.37 2.50
62.60 0.54 34.90 1.90 0.04 0.03 0.11 0.05 0.19
0.03
0.03
0.06 0.04
0.04 0.03
0.03
P1, Collie; P2, Wallerawang; P3, Mount Piper; P4, Tarong.
Table 2. Chemical Compositions of Ash Cenospheres from Different Power Stations (from This Work)a cenospheres (wt %) SiO2 Fe2O3 Al2O3 TiO2 P2O5 Mn3O4 CaO MgO Na2O K2O SO3 V2O5 ZnO BaO SrO
P1
P2
P3
P4a
P4b
P4c
51.60 5.10 36.20 1.50 0.84 0.05 0.88 0.87 0.23 1.70
57.90 1.20 34.80 1.10 0.10 0.02 0.29 0.35 0.31 3.50
58.10 0.73 36.70 0.85 0.04
67.40 0.94 26.20 1.08
