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Erosion wear mechanisms of coal–water–slurry (CWS) ceramic nozzles in industry boilers
Materials Science and Engineering A 417 (2006) 1–7
Short survey
Erosion wear mechanisms of coal–water–slurry (CWS) ceramic nozzles in industry boilers Deng Jianxin a,∗ , Ding Zeliang b , Yuan Dongling a a
b
Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Department of Mechanical Engineering, Zhuzhou Institute of Technology, Zhuzhou 412008, Hunan Province, PR China Accepted 26 October 2005
Abstract Al2 O3 /(W, Ti)C ceramic composites were prepared for the use of coal–water–slurry (CWS) nozzles in industry boilers. The erosion rates of the CWS ceramic nozzles were measured. Eroded bore surfaces of the nozzles was examined by scanning electron microscopy. Finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution inside the ceramic nozzle. Results showed that the primary wear mechanisms of the CWS ceramic nozzle exhibited polishing action in the inner center hole and thermal shock damage with chipping at exit. The temperature, temperature gradient and thermal stress at exit surfaces of the CWS ceramic nozzle were higher than those of other parts of the nozzle. Greater temperature gradient and higher thermal stress were the main reason that caused the failure of the exit surface of the CWS ceramic nozzle. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramic nozzles; Erosion wear; Wear mechanisms; Coal–water–slurry
1. Introduction Coal–water–slurry (CWS) holds promise to offer a longterm alternative to fuel oil, and also it is being conceived as an attractive fuel for power generation industry. CWS has been developed for more than 30 years as a new coal fuel and a substitute for oil mainly in industry boilers. They are also a considerable potential for gasification applications. Recently, CWS has been successfully applied around the world in various boilers for power generation, petroleum, chemical and metallurgical industries, etc. The attraction of the CWS is its complete independence of an oil supply. CWS can be stored without the danger of a coal dust explosion and burned in a similar way to heavy fuel oil in existing oil fired equipments with a few modifications, and can be transported in pipeline, leading to reduction in transportation costs and pollution compared to coal [1–3]. CWS preparation processes have been developed and commercialised with large-scale production plants in China, Russia, Japan and Italy. CWS technology has been successfully demonstrated in Canada, France, UK, Germany and other countries. Research
∗
Corresponding author. Tel.: +86 531 88392047. E-mail address: [email protected] (D. Jianxin).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.10.058
and development activities are in progress on various aspects of the CWS technology [4,5]. CWS typically contains 70–75 wt.% coal (ranged from 40 to 120 m), 25–30% water and approximately 1% chemical additives (stabilizer and dispersant). The effects of a number of variables, such as the properties of the coal, the particle sizes and their distribution, the type and the amount of chemical additives, the method of preparation of the slurry and the effect of its rheological properties on the behavior of CWS are very important. CWS also contains high hardness minerals such as iron pyrites and quartz. The hardness of these materials can be up to HV 2500. So CWS can be regarded as an admixture of soft abrasives and hard abrasive [1]. In CWS burning process, the nozzle is eroded continuously by the abrasive action of CWS, and the working environmental temperature of nozzle can reach up to 1000 ◦ C [1]. Also, there is a very high temperature gradient inside nozzle. So the nozzle is subjected to low angle of sliding impingement and thermal shock of CWS [1]. The nozzle materials in CWS burning must have good erosion, oxidation and thermal shock resistance. Ceramics have intrinsic characteristics, such as: high melting point, high hardness, good chemical inertness and high wear resistance, that make them promising candidates for high temperature structural materials and wear resistance materials, in
2
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
situations where metallic components do not achieve satisfactory service lives, owing to inadequate heat and wear resistance. The industrial applications of advanced ceramics include nozzles, dies, cutting tools, drawing or extrusion, seal rings, valve seats, bearing parts, a variety of engine parts, etc. The nozzle is the most critical part in a CWS atomizing and burning system. There are many factors that influence the nozzle wear such as: material properties, microstructure, nozzle geometry, the mass flow rate and temperature, etc. Various studies have already been published on the erosion behavior and mechanisms of ceramic materials such as in sandblasting surface treatment, water-jet cutting, abrasive air-jet machining [6–10], etc., in recent years. Although there is considerable interest in the erosion behavior of ceramic materials, little work has been published on the erosion wear of CWS ceramic nozzles. In this study, Al2 O3 /(W, Ti)C ceramic composite was produced to be used as the CWS nozzle material. The erosion rates of this CWS ceramic nozzle were measured. Effect of the factors (such as: the temperature, the temperature gradient, and the thermal stress, etc.) that influence the CWS ceramic nozzle wear was investigated. The erosion wear mechanisms of the CWS ceramic nozzles were examined with microstructure analysis on the eroded surface. The purpose was to characterize the erosion wear of the CWS ceramic nozzle in industry boilers.
2. Materials and experimental procedures 2.1. Materials and processing The starting powders used to fabricate the ceramic composites are listed in Table 1 with their particle sizes, purities and manufacturers. The average particle size of Al2 O3 starting powders was less than 1.0 m, and (W, Ti)C particles (aver-
Table 1 Particle size, purity and manufacturer of the starting powders Starting powder
Average particle size (m)
Purity (%)
Manufacture
Al2 O3 (W, Ti)C
<1 1–2
>99.9 >99.0
Zibo aluminum works Zhuzhou cemented carbide works
age particle size 1.0–2.0 m) were added to Al2 O3 . The Al2 O3 and (W, Ti)C combined powders were prepared by wet ball milling in alcohol with cemented carbide balls for 80 h. Following drying, the final densification of the combined powders was accomplished by hot pressing with a pressure of 32 MPa in argon atmosphere for 15 min to produce a ceramic disk. The sintering temperature employed for hot pressing was 1750 ◦ C. Densities of the hot-pressed ceramics were measured by the Archimedes’s method. Test pieces of 3 mm × 4 mm × 36 mm were prepared from the disk by cutting and grinding using a diamond wheel and were measured to determine the flexural strength, Vickers hardness and fracture toughness. A three-point bending mode was used to measure the flexural strength over a 30 mm span at a crosshead speed of 0.5 mm/min. Fracture toughness measurements were performed using indentation method in a hardness tester (ZWICK3212) using the formula proposed by Cook and Lawn [11]. On the same apparatus the Vickers hardness was measured on polished surface with a load of 98 N. Data for hardness, flexural strength, and fracture toughness of Al2 O3 /(W, Ti)C ceramic nozzle materials were gathered on five specimens and averaged. 2.2. Coal–water–slurry burning tests Coal–water–slurry burning tests were conducted with a DNS2-1.0-SM industry boiler (made in China). The schematic
Fig. 1. Schematic diagram of the CWS boiler.
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
3
Fig. 4. Photo of the CWS ceramic nozzles. Fig. 2. Photo of the CWS industry boiler.
made from Al2 O3 /(W, Ti)C were manufactured by hot pressing (see Fig. 4). The mass loss of the worn nozzles was measured with an accurate electric balance (minimum 0.01 mg). The erosion rates (W) of the nozzles are defined as the nozzle mass loss (m1 ) divided by the nozzle density (d) times the burn mass of CWS (m2 ): W=
Fig. 3. SEM micrograph of the coal powders in the coal–water–slurry.
diagram and the practical photo of this equipment are shown in Figs. 1 and 2, respectively. The CWS drawn by pump passed through pipeline, accelerated and mixed in the spray-gun (nozzle) by gas stream commonly compressed air. The properties of the CWS used in this study are listed in Table 2. The SEM micrograph of the coal powders in the coal–water–slurry is shown in Fig. 3. The atomizing air pressure was set at 0.4 MPa, and CWS pressure was set at 0.2 MPa. Ceramics nozzles with internal diameter 4.5 mm, external diameter 10 mm, and length 15 mm
m1 (dm2 )
(1)
where the W has the units of volume loss per unit mass of CWS (mm3 /kg). Finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution of the CWS ceramic nozzle when used in a CWS industry boiler. The microstructures of sintered materials were studied on polished surface and fracture surfaces by scanning electron microscopy (SEM). For observation of the micro damage and determination of erosion mechanisms, the worn nozzles were sectioned axially. The eroded bore surfaces of the ceramic nozzles were examined by scanning electron microscopy. 3. Results and discussion 3.1. Microstructure and mechanical properties Data for density, flexural strength, hardness and fracture toughness were gathered on five specimens and are listed in Table 3. The typical microstructure from the polished surface
Table 2 Properties of the coal–water–slurry Consistency (%)
Quantity of heat (MJ/kg)
Ash (SiO2 , A12 O3 , Fe2 O3 , CaO, MgO, FeS2 , K2 O, Na2 O) (%)
Sulphur (%)
Volatility (%)
Adhesiveness (MPa s)
Grit number (m)
65 ± 2
18.81–20.48
<12
<0.8
>15
1000–2500
40–80
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
4
Table 3 Composition and mechanical properties of the CWS ceramic nozzle Compositions (vol.%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2 )
Hardness (GPa)
Density (g/cm3 )
Al2 O3 /45% (W, Ti)C
850 ± 43
4.9 ± 0.3
21.5 ± 0.6
6.64
Fig. 5. Typical microstructure of the polished surface of Al2 O3 /(W, Ti)C ceramic composite.
of hot-pressed Al2 O3 /(W, Ti)C ceramic composite is shown in Fig. 5. Specimens were etched using a hot solution of phosphoric acid. The black areas were identified by EDX analysis as (W, Ti)C, and the grey phases with clear contrast were Al2 O3 . It can be seen that the (W, Ti)C phases are quite uniformly distributed throughout the microstructure. Fig. 6 shows the SEM micrograph of the fracture surfaces of Al2 O3 /(W, Ti)C ceramic composite. From this SEM micrograph, different morphologies of the composite can be seen clearly. The Al2 O3 /(W, Ti)C composites exhibited a rough fracture surface,
Fig. 7. TEM micrograph of Al2O3/(W, Ti)C ceramic composite.
resulting from the mixed transgranular and intergranular fracture modes. Typical TEM micrograph of the ceramic composite is presented in Fig. 7. It can be seen that there is a large proportion of fine (W, Ti)C grains heavily embedded in matrix grains with inhomogeneous size distribution around 10–30 nm. This structure is very beneficial for the strength, as the existence of such fine grains may inhibit the grain growth of Al2 O3 [12]. 3.2. Cumulative mass loss and erosion rates of the CWS ceramic nozzles Fig. 8 shows the variation of cumulative volume loss with the operation time for Al2 O3 /(W, Ti)C ceramic nozzles in CWS
Fig. 6. SEM micrograph of the fracture surface of Al2 O3 /(W, Ti)C ceramic nozzle.
Fig. 8. Cumulative volume loss of CWS ceramic nozzle with the operation time (atomizing air pressure: 0.4 MPa; CWS pressure: 0.2 MPa).
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
Fig. 9. Variation of CWS ceramic nozzle erosion rates with the operation time (atomizing air pressure: 0.4 MPa; CWS pressure: 0.2 MPa).
burning process. It can be seen that the cumulative volume loss continuously increased within the operation time. Fig. 9 shows the variation of erosion rates with the operation time for the CWS ceramic nozzles. The erosion rates showed a significant decrease up to 48 h operation, and within further 80 h operation it almost remained constant.
Fig. 11. Temperature distribution in the CWS ceramic nozzle.
Al2 O3 /(W, Ti)C ceramic by hot pressing. Fig. 10 shows the FEM gridding model of the CWS ceramic nozzle. Fig. 11 shows the temperature distribution in the CWS ceramic nozzle. It can be seen that the temperature at the exit surfaces of the CWS ceramic nozzle is higher than that of other
3.3. Temperature, temperature gradient and thermal stress analysis Three-dimensional finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution of the CWS ceramic nozzle when used in a CWS boiler. Owing to the symmetry, an axisymmetric calculation was preferred. Presume that it was a steady state boundary conditions. The materials constants are as follows [13]: the elastic modulus of Al2 O3 /(W, Ti)C is 450 GPa, the Poisson’s ratio is 0.23, the density is 6.64 g/cm3 , the thermal conductivity is 28.0 W/m K, the thermal expansion coefficient is 7.25 × 10−6 /K. CWS nozzles with internal diameter 4.5 mm, external diameter 10 mm, and length 15 mm were made from Fig. 12. Temperature gradient in the CWS ceramic nozzle.
Fig. 10. Finite element method gridding model of the CWS ceramic nozzle.
5
Fig. 13. Thermal stresses in the CWS ceramic nozzle.
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D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
Fig. 14. Wear profiles of the exit bore surface of the CWS ceramic nozzle (a) before tests; (b) after 120 h operations.
part of the ceramic nozzle, and the highest temperature is 269 ◦ C. The temperature at the entry surfaces of the CWS ceramic nozzle is relatively small, and the minimum temperature at this area is only 149 ◦ C. The temperature gradient in the CWS ceramic nozzle is shown in Fig. 12. It was found that there is greater temperature gradient inside the exit surface of the CWS ceramic nozzle, and the highest temperature gradient is 20,126 ◦ C/m. While the temperature gradient at the entry surfaces of the CWS ceramic
nozzle is very small, and the temperature gradient at this area is only 4236 ◦ C/m. The damage of ceramics subjected to high temperature gradient environments is a major limiting factor in relation to service requirements and lifetime performance. High temperature gradient inside ceramics often yields an instantaneous thermal stress, which is, in some case, sufficient to cause considerable cracking damage or even catastrophic failure [14–16]. Fig. 13 shows the thermal stress distribution in the CWS ceramic nozzle. As
Fig. 15. SEM micrographs of the worn inner bore surfaces of the CWS ceramic nozzle at different areas after 120 h operations, (a) Schematic diagram, (b) center bore surface of the worn nozzle and (c) exit bore surface of the worn nozzle.
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
can be seen, the stresses along the axis direction of the nozzle increased from entry to exit. The tensile thermal stress at the exit surface of the CWS ceramic nozzle is higher than that of other part of the ceramic nozzle. The highest tensile stresses are located on the exit area of the nozzle, and the maximum value is 469.0 MPa. 3.4. Wear surface studies Fig. 14 shows the SEM micrograph of the exit profiles of the CWS ceramic nozzles before tests and after 120 h operations. It can be seen that the exit zone of the worn CWS ceramic nozzle appeared to be entirely brittle in nature with the evidence of large scale-chipping, and exhibited a brittle fracture-induced removal process. For observation of the micro damage and determination of erosion mechanisms, the worn ceramic nozzles were sectioned axially. Fig. 15 shows the SEM micrographs of the inner bore surface of the CWS ceramic nozzles at different areas after 120 h operations. From these SEM micrographs, the center bore surface of the CWS ceramic nozzle is very smooth, the microstructure can be seen clearly (Fig. 15(b)). The “white” phase with clear contrast is (W, Ti)C, and the black phase is Al2 O3 . Plowing features characteristic at center zone of the nozzles could not be seen. Since the CWS particles hit the target at low angles at the nozzle center wall section. Most of the particles traveled parallel to the nozzle wall and the wear mode was mainly abrasion. Also as the erodent particles in CWS were much softer than the Al2 O3 /(W, Ti)C ceramic, sliding abrasive particles in the CWS acted as a polishing process at the inner center bore surface of the nozzle. It is suggested that the primary wear mechanisms of the inner center bore surface of the CWS ceramic nozzles is polishing by the abrasive particles. While the exit zone of the CWS ceramic nozzle appeared to be brittle fracture in nature with the evidence of large scalechipping. Fig. 15(c) shows the SEM micrograph of the exit surface of the worn nozzle. Form this figure a lot of cracks can be observed at the exit bore surface. Since the nozzle exit section suffers form severe temperature gradient, and generates large thermal residual stress (see Figs. 12 and 13). The wear mechanisms in this area of the CWS nozzle appeared to be brittle fracture owing to the large thermal residual stress that causes the cracks on the nozzle surface and facilitates removal of the materials. 4. Conclusions Al2 O3 /(W, Ti)C ceramic composite was produced by hot pressing for the use of coal–water–slurry (CWS) nozzles. Finite element method (FEM) was used as a means of numerically eval-
7
uating temperature, temperature gradient, thermal stress and its distribution inside of the CWS ceramic nozzle. Detailed observations and analyses of the nozzle wear surface have revealed that the primary wear mechanisms of this CWS ceramic nozzle exhibited polishing action in the center surface and thermal shock damage with chipping at exit. The temperature, temperature gradient and thermal stress at exit surfaces of this CWS ceramic nozzle were higher than those of other parts of the ceramic nozzle. Greater temperature gradient and higher thermal stress were the main reason that caused the failure of the exit surface of this CWS ceramic nozzle. Acknowledgements This work was supported by “the National Natural Science Foundation of China (50475133)”, “Specialized Research Fund for Doctoral Program of Higher Education (20030422105),” “Natural Science Foundation of Shandong Province (Y2004F08, Z2003F01),” and “the Program for New Century Excellent Talents in University (NCET-04-0622).” References [1] Cen Kefa, Yao Qiang, Cao Xinyu, et al., Theory and Apllication of Combustion, Flow, Heat Transfer, Gasification of Coal Slurry, Zhejiang University Press, Hang zhou, 1997 (in Chinese). [2] K.S. Narasimban, Proc. Int. Tech. Conf. Coal Util. Fuel Syst. 22 (1997) 843–852. [3] B.C. Meikap, N.K. Purohit, V. Mahadevan, J. Colloid Interf. Sci. 281 (1) (2005) 225–235. [4] F. Boylu, H. Dinc¸er, G. Atesok, Fuel Process. Technol. 85 (4) (2004) 241–250. [5] F. Boylu, G. Atesok, Coal-water mixtures and their technologies, in: Proceedings of the 5th Coal Utilisation and Technology Symposium, Ankara, Turkey/Lebib Yalkin, Istanbul, 2000, pp. 195–212. [6] Deng Jianxin, Feng Yihua, Ding Zeliang, Shi Peiwei, J. Eur. Ceram. Soc. 23 (2003) 323–329. [7] Madhusarathi Nanduri, D.G. Taggart, T.J. Kim, Int. J. Mach. Tools Manuf. 42 (5) (2002) 615–623. [8] M. Hashish, J. Tribol. 116 (1994) 439–444. [9] S. Lathabai, D.C. Pender, Wear 189 (1–2) (1995) 122–135. [10] Y. Zhang, Y.B. Cheng, S. Lathabai, J. Eur. Ceram. Soc. 21 (13) (2001) 2435–2445. [11] R.F. Cook, B.R. Lawn, J. Am. Ceram. Soc. 66 (11) (1983) 200–201. [12] Byung Koog Jang, Manabu Enoki, in: R.C. Bradt (Ed.), Fracture Behaviour and Toughening of Alumina-Based Composites Fabricated by Microstructural Control, Plenum Press, New York, 1996. [13] Ding Zeliang. Development of assembled coal–water–slurry ceramic nozzles and study on its failure mechanisms, Dissertation, Shandong University, Jinan, 2004 (in Chinese). [14] M. Collin, D. Rowcliffe, Acta Mater. 48 (8) (2000) 1655–1665. [15] H.L. Ekkehard, V.S. Michael, J. Am. Ceram. Soc. 74 (1) (1991) 19– 24. [16] R.B. John, L. Edgar Forsythe, L.R. Richard, J. Am. Ceram. Soc. 69 (8) (1986) 634–637.
Short survey
Erosion wear mechanisms of coal–water–slurry (CWS) ceramic nozzles in industry boilers Deng Jianxin a,∗ , Ding Zeliang b , Yuan Dongling a a
b
Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Department of Mechanical Engineering, Zhuzhou Institute of Technology, Zhuzhou 412008, Hunan Province, PR China Accepted 26 October 2005
Abstract Al2 O3 /(W, Ti)C ceramic composites were prepared for the use of coal–water–slurry (CWS) nozzles in industry boilers. The erosion rates of the CWS ceramic nozzles were measured. Eroded bore surfaces of the nozzles was examined by scanning electron microscopy. Finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution inside the ceramic nozzle. Results showed that the primary wear mechanisms of the CWS ceramic nozzle exhibited polishing action in the inner center hole and thermal shock damage with chipping at exit. The temperature, temperature gradient and thermal stress at exit surfaces of the CWS ceramic nozzle were higher than those of other parts of the nozzle. Greater temperature gradient and higher thermal stress were the main reason that caused the failure of the exit surface of the CWS ceramic nozzle. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramic nozzles; Erosion wear; Wear mechanisms; Coal–water–slurry
1. Introduction Coal–water–slurry (CWS) holds promise to offer a longterm alternative to fuel oil, and also it is being conceived as an attractive fuel for power generation industry. CWS has been developed for more than 30 years as a new coal fuel and a substitute for oil mainly in industry boilers. They are also a considerable potential for gasification applications. Recently, CWS has been successfully applied around the world in various boilers for power generation, petroleum, chemical and metallurgical industries, etc. The attraction of the CWS is its complete independence of an oil supply. CWS can be stored without the danger of a coal dust explosion and burned in a similar way to heavy fuel oil in existing oil fired equipments with a few modifications, and can be transported in pipeline, leading to reduction in transportation costs and pollution compared to coal [1–3]. CWS preparation processes have been developed and commercialised with large-scale production plants in China, Russia, Japan and Italy. CWS technology has been successfully demonstrated in Canada, France, UK, Germany and other countries. Research
∗
Corresponding author. Tel.: +86 531 88392047. E-mail address: [email protected] (D. Jianxin).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.10.058
and development activities are in progress on various aspects of the CWS technology [4,5]. CWS typically contains 70–75 wt.% coal (ranged from 40 to 120 m), 25–30% water and approximately 1% chemical additives (stabilizer and dispersant). The effects of a number of variables, such as the properties of the coal, the particle sizes and their distribution, the type and the amount of chemical additives, the method of preparation of the slurry and the effect of its rheological properties on the behavior of CWS are very important. CWS also contains high hardness minerals such as iron pyrites and quartz. The hardness of these materials can be up to HV 2500. So CWS can be regarded as an admixture of soft abrasives and hard abrasive [1]. In CWS burning process, the nozzle is eroded continuously by the abrasive action of CWS, and the working environmental temperature of nozzle can reach up to 1000 ◦ C [1]. Also, there is a very high temperature gradient inside nozzle. So the nozzle is subjected to low angle of sliding impingement and thermal shock of CWS [1]. The nozzle materials in CWS burning must have good erosion, oxidation and thermal shock resistance. Ceramics have intrinsic characteristics, such as: high melting point, high hardness, good chemical inertness and high wear resistance, that make them promising candidates for high temperature structural materials and wear resistance materials, in
2
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
situations where metallic components do not achieve satisfactory service lives, owing to inadequate heat and wear resistance. The industrial applications of advanced ceramics include nozzles, dies, cutting tools, drawing or extrusion, seal rings, valve seats, bearing parts, a variety of engine parts, etc. The nozzle is the most critical part in a CWS atomizing and burning system. There are many factors that influence the nozzle wear such as: material properties, microstructure, nozzle geometry, the mass flow rate and temperature, etc. Various studies have already been published on the erosion behavior and mechanisms of ceramic materials such as in sandblasting surface treatment, water-jet cutting, abrasive air-jet machining [6–10], etc., in recent years. Although there is considerable interest in the erosion behavior of ceramic materials, little work has been published on the erosion wear of CWS ceramic nozzles. In this study, Al2 O3 /(W, Ti)C ceramic composite was produced to be used as the CWS nozzle material. The erosion rates of this CWS ceramic nozzle were measured. Effect of the factors (such as: the temperature, the temperature gradient, and the thermal stress, etc.) that influence the CWS ceramic nozzle wear was investigated. The erosion wear mechanisms of the CWS ceramic nozzles were examined with microstructure analysis on the eroded surface. The purpose was to characterize the erosion wear of the CWS ceramic nozzle in industry boilers.
2. Materials and experimental procedures 2.1. Materials and processing The starting powders used to fabricate the ceramic composites are listed in Table 1 with their particle sizes, purities and manufacturers. The average particle size of Al2 O3 starting powders was less than 1.0 m, and (W, Ti)C particles (aver-
Table 1 Particle size, purity and manufacturer of the starting powders Starting powder
Average particle size (m)
Purity (%)
Manufacture
Al2 O3 (W, Ti)C
<1 1–2
>99.9 >99.0
Zibo aluminum works Zhuzhou cemented carbide works
age particle size 1.0–2.0 m) were added to Al2 O3 . The Al2 O3 and (W, Ti)C combined powders were prepared by wet ball milling in alcohol with cemented carbide balls for 80 h. Following drying, the final densification of the combined powders was accomplished by hot pressing with a pressure of 32 MPa in argon atmosphere for 15 min to produce a ceramic disk. The sintering temperature employed for hot pressing was 1750 ◦ C. Densities of the hot-pressed ceramics were measured by the Archimedes’s method. Test pieces of 3 mm × 4 mm × 36 mm were prepared from the disk by cutting and grinding using a diamond wheel and were measured to determine the flexural strength, Vickers hardness and fracture toughness. A three-point bending mode was used to measure the flexural strength over a 30 mm span at a crosshead speed of 0.5 mm/min. Fracture toughness measurements were performed using indentation method in a hardness tester (ZWICK3212) using the formula proposed by Cook and Lawn [11]. On the same apparatus the Vickers hardness was measured on polished surface with a load of 98 N. Data for hardness, flexural strength, and fracture toughness of Al2 O3 /(W, Ti)C ceramic nozzle materials were gathered on five specimens and averaged. 2.2. Coal–water–slurry burning tests Coal–water–slurry burning tests were conducted with a DNS2-1.0-SM industry boiler (made in China). The schematic
Fig. 1. Schematic diagram of the CWS boiler.
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
3
Fig. 4. Photo of the CWS ceramic nozzles. Fig. 2. Photo of the CWS industry boiler.
made from Al2 O3 /(W, Ti)C were manufactured by hot pressing (see Fig. 4). The mass loss of the worn nozzles was measured with an accurate electric balance (minimum 0.01 mg). The erosion rates (W) of the nozzles are defined as the nozzle mass loss (m1 ) divided by the nozzle density (d) times the burn mass of CWS (m2 ): W=
Fig. 3. SEM micrograph of the coal powders in the coal–water–slurry.
diagram and the practical photo of this equipment are shown in Figs. 1 and 2, respectively. The CWS drawn by pump passed through pipeline, accelerated and mixed in the spray-gun (nozzle) by gas stream commonly compressed air. The properties of the CWS used in this study are listed in Table 2. The SEM micrograph of the coal powders in the coal–water–slurry is shown in Fig. 3. The atomizing air pressure was set at 0.4 MPa, and CWS pressure was set at 0.2 MPa. Ceramics nozzles with internal diameter 4.5 mm, external diameter 10 mm, and length 15 mm
m1 (dm2 )
(1)
where the W has the units of volume loss per unit mass of CWS (mm3 /kg). Finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution of the CWS ceramic nozzle when used in a CWS industry boiler. The microstructures of sintered materials were studied on polished surface and fracture surfaces by scanning electron microscopy (SEM). For observation of the micro damage and determination of erosion mechanisms, the worn nozzles were sectioned axially. The eroded bore surfaces of the ceramic nozzles were examined by scanning electron microscopy. 3. Results and discussion 3.1. Microstructure and mechanical properties Data for density, flexural strength, hardness and fracture toughness were gathered on five specimens and are listed in Table 3. The typical microstructure from the polished surface
Table 2 Properties of the coal–water–slurry Consistency (%)
Quantity of heat (MJ/kg)
Ash (SiO2 , A12 O3 , Fe2 O3 , CaO, MgO, FeS2 , K2 O, Na2 O) (%)
Sulphur (%)
Volatility (%)
Adhesiveness (MPa s)
Grit number (m)
65 ± 2
18.81–20.48
<12
<0.8
>15
1000–2500
40–80
D. Jianxin et al. / Materials Science and Engineering A 417 (2006) 1–7
4
Table 3 Composition and mechanical properties of the CWS ceramic nozzle Compositions (vol.%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2 )
Hardness (GPa)
Density (g/cm3 )
Al2 O3 /45% (W, Ti)C
850 ± 43
4.9 ± 0.3
21.5 ± 0.6
6.64
Fig. 5. Typical microstructure of the polished surface of Al2 O3 /(W, Ti)C ceramic composite.
of hot-pressed Al2 O3 /(W, Ti)C ceramic composite is shown in Fig. 5. Specimens were etched using a hot solution of phosphoric acid. The black areas were identified by EDX analysis as (W, Ti)C, and the grey phases with clear contrast were Al2 O3 . It can be seen that the (W, Ti)C phases are quite uniformly distributed throughout the microstructure. Fig. 6 shows the SEM micrograph of the fracture surfaces of Al2 O3 /(W, Ti)C ceramic composite. From this SEM micrograph, different morphologies of the composite can be seen clearly. The Al2 O3 /(W, Ti)C composites exhibited a rough fracture surface,
Fig. 7. TEM micrograph of Al2O3/(W, Ti)C ceramic composite.
resulting from the mixed transgranular and intergranular fracture modes. Typical TEM micrograph of the ceramic composite is presented in Fig. 7. It can be seen that there is a large proportion of fine (W, Ti)C grains heavily embedded in matrix grains with inhomogeneous size distribution around 10–30 nm. This structure is very beneficial for the strength, as the existence of such fine grains may inhibit the grain growth of Al2 O3 [12]. 3.2. Cumulative mass loss and erosion rates of the CWS ceramic nozzles Fig. 8 shows the variation of cumulative volume loss with the operation time for Al2 O3 /(W, Ti)C ceramic nozzles in CWS
Fig. 6. SEM micrograph of the fracture surface of Al2 O3 /(W, Ti)C ceramic nozzle.
Fig. 8. Cumulative volume loss of CWS ceramic nozzle with the operation time (atomizing air pressure: 0.4 MPa; CWS pressure: 0.2 MPa).
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Fig. 9. Variation of CWS ceramic nozzle erosion rates with the operation time (atomizing air pressure: 0.4 MPa; CWS pressure: 0.2 MPa).
burning process. It can be seen that the cumulative volume loss continuously increased within the operation time. Fig. 9 shows the variation of erosion rates with the operation time for the CWS ceramic nozzles. The erosion rates showed a significant decrease up to 48 h operation, and within further 80 h operation it almost remained constant.
Fig. 11. Temperature distribution in the CWS ceramic nozzle.
Al2 O3 /(W, Ti)C ceramic by hot pressing. Fig. 10 shows the FEM gridding model of the CWS ceramic nozzle. Fig. 11 shows the temperature distribution in the CWS ceramic nozzle. It can be seen that the temperature at the exit surfaces of the CWS ceramic nozzle is higher than that of other
3.3. Temperature, temperature gradient and thermal stress analysis Three-dimensional finite element method (FEM) was used as a means of numerically evaluating temperature, temperature gradient, thermal stress and its distribution of the CWS ceramic nozzle when used in a CWS boiler. Owing to the symmetry, an axisymmetric calculation was preferred. Presume that it was a steady state boundary conditions. The materials constants are as follows [13]: the elastic modulus of Al2 O3 /(W, Ti)C is 450 GPa, the Poisson’s ratio is 0.23, the density is 6.64 g/cm3 , the thermal conductivity is 28.0 W/m K, the thermal expansion coefficient is 7.25 × 10−6 /K. CWS nozzles with internal diameter 4.5 mm, external diameter 10 mm, and length 15 mm were made from Fig. 12. Temperature gradient in the CWS ceramic nozzle.
Fig. 10. Finite element method gridding model of the CWS ceramic nozzle.
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Fig. 13. Thermal stresses in the CWS ceramic nozzle.
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Fig. 14. Wear profiles of the exit bore surface of the CWS ceramic nozzle (a) before tests; (b) after 120 h operations.
part of the ceramic nozzle, and the highest temperature is 269 ◦ C. The temperature at the entry surfaces of the CWS ceramic nozzle is relatively small, and the minimum temperature at this area is only 149 ◦ C. The temperature gradient in the CWS ceramic nozzle is shown in Fig. 12. It was found that there is greater temperature gradient inside the exit surface of the CWS ceramic nozzle, and the highest temperature gradient is 20,126 ◦ C/m. While the temperature gradient at the entry surfaces of the CWS ceramic
nozzle is very small, and the temperature gradient at this area is only 4236 ◦ C/m. The damage of ceramics subjected to high temperature gradient environments is a major limiting factor in relation to service requirements and lifetime performance. High temperature gradient inside ceramics often yields an instantaneous thermal stress, which is, in some case, sufficient to cause considerable cracking damage or even catastrophic failure [14–16]. Fig. 13 shows the thermal stress distribution in the CWS ceramic nozzle. As
Fig. 15. SEM micrographs of the worn inner bore surfaces of the CWS ceramic nozzle at different areas after 120 h operations, (a) Schematic diagram, (b) center bore surface of the worn nozzle and (c) exit bore surface of the worn nozzle.
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can be seen, the stresses along the axis direction of the nozzle increased from entry to exit. The tensile thermal stress at the exit surface of the CWS ceramic nozzle is higher than that of other part of the ceramic nozzle. The highest tensile stresses are located on the exit area of the nozzle, and the maximum value is 469.0 MPa. 3.4. Wear surface studies Fig. 14 shows the SEM micrograph of the exit profiles of the CWS ceramic nozzles before tests and after 120 h operations. It can be seen that the exit zone of the worn CWS ceramic nozzle appeared to be entirely brittle in nature with the evidence of large scale-chipping, and exhibited a brittle fracture-induced removal process. For observation of the micro damage and determination of erosion mechanisms, the worn ceramic nozzles were sectioned axially. Fig. 15 shows the SEM micrographs of the inner bore surface of the CWS ceramic nozzles at different areas after 120 h operations. From these SEM micrographs, the center bore surface of the CWS ceramic nozzle is very smooth, the microstructure can be seen clearly (Fig. 15(b)). The “white” phase with clear contrast is (W, Ti)C, and the black phase is Al2 O3 . Plowing features characteristic at center zone of the nozzles could not be seen. Since the CWS particles hit the target at low angles at the nozzle center wall section. Most of the particles traveled parallel to the nozzle wall and the wear mode was mainly abrasion. Also as the erodent particles in CWS were much softer than the Al2 O3 /(W, Ti)C ceramic, sliding abrasive particles in the CWS acted as a polishing process at the inner center bore surface of the nozzle. It is suggested that the primary wear mechanisms of the inner center bore surface of the CWS ceramic nozzles is polishing by the abrasive particles. While the exit zone of the CWS ceramic nozzle appeared to be brittle fracture in nature with the evidence of large scalechipping. Fig. 15(c) shows the SEM micrograph of the exit surface of the worn nozzle. Form this figure a lot of cracks can be observed at the exit bore surface. Since the nozzle exit section suffers form severe temperature gradient, and generates large thermal residual stress (see Figs. 12 and 13). The wear mechanisms in this area of the CWS nozzle appeared to be brittle fracture owing to the large thermal residual stress that causes the cracks on the nozzle surface and facilitates removal of the materials. 4. Conclusions Al2 O3 /(W, Ti)C ceramic composite was produced by hot pressing for the use of coal–water–slurry (CWS) nozzles. Finite element method (FEM) was used as a means of numerically eval-
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uating temperature, temperature gradient, thermal stress and its distribution inside of the CWS ceramic nozzle. Detailed observations and analyses of the nozzle wear surface have revealed that the primary wear mechanisms of this CWS ceramic nozzle exhibited polishing action in the center surface and thermal shock damage with chipping at exit. The temperature, temperature gradient and thermal stress at exit surfaces of this CWS ceramic nozzle were higher than those of other parts of the ceramic nozzle. Greater temperature gradient and higher thermal stress were the main reason that caused the failure of the exit surface of this CWS ceramic nozzle. Acknowledgements This work was supported by “the National Natural Science Foundation of China (50475133)”, “Specialized Research Fund for Doctoral Program of Higher Education (20030422105),” “Natural Science Foundation of Shandong Province (Y2004F08, Z2003F01),” and “the Program for New Century Excellent Talents in University (NCET-04-0622).” References [1] Cen Kefa, Yao Qiang, Cao Xinyu, et al., Theory and Apllication of Combustion, Flow, Heat Transfer, Gasification of Coal Slurry, Zhejiang University Press, Hang zhou, 1997 (in Chinese). [2] K.S. Narasimban, Proc. Int. Tech. Conf. Coal Util. Fuel Syst. 22 (1997) 843–852. [3] B.C. Meikap, N.K. Purohit, V. Mahadevan, J. Colloid Interf. Sci. 281 (1) (2005) 225–235. [4] F. Boylu, H. Dinc¸er, G. Atesok, Fuel Process. Technol. 85 (4) (2004) 241–250. [5] F. Boylu, G. Atesok, Coal-water mixtures and their technologies, in: Proceedings of the 5th Coal Utilisation and Technology Symposium, Ankara, Turkey/Lebib Yalkin, Istanbul, 2000, pp. 195–212. [6] Deng Jianxin, Feng Yihua, Ding Zeliang, Shi Peiwei, J. Eur. Ceram. Soc. 23 (2003) 323–329. [7] Madhusarathi Nanduri, D.G. Taggart, T.J. Kim, Int. J. Mach. Tools Manuf. 42 (5) (2002) 615–623. [8] M. Hashish, J. Tribol. 116 (1994) 439–444. [9] S. Lathabai, D.C. Pender, Wear 189 (1–2) (1995) 122–135. [10] Y. Zhang, Y.B. Cheng, S. Lathabai, J. Eur. Ceram. Soc. 21 (13) (2001) 2435–2445. [11] R.F. Cook, B.R. Lawn, J. Am. Ceram. Soc. 66 (11) (1983) 200–201. [12] Byung Koog Jang, Manabu Enoki, in: R.C. Bradt (Ed.), Fracture Behaviour and Toughening of Alumina-Based Composites Fabricated by Microstructural Control, Plenum Press, New York, 1996. [13] Ding Zeliang. Development of assembled coal–water–slurry ceramic nozzles and study on its failure mechanisms, Dissertation, Shandong University, Jinan, 2004 (in Chinese). [14] M. Collin, D. Rowcliffe, Acta Mater. 48 (8) (2000) 1655–1665. [15] H.L. Ekkehard, V.S. Michael, J. Am. Ceram. Soc. 74 (1) (1991) 19– 24. [16] R.B. John, L. Edgar Forsythe, L.R. Richard, J. Am. Ceram. Soc. 69 (8) (1986) 634–637.