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Experimental study on fly ash resistivity at temperatures above 673 K
Fuel 116 (2014) 650–654
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Experimental study on fly ash resistivity at temperatures above 673 K Jinjin Xu a,⇑, Zhongzhu Gu b,⇑, Jun Zhang a a b
School of Energy and Environment, Southeast University, Nanjing 210096, PR China School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, PR China
h i g h l i g h t s The fly ash resistivity at temperature of 673–1273 K was measured and discussed. Paper gave experimental data of fly ash resistivity at 673–1273 K. Variation of fly ash resistivity with temperature at 673–1273 K was explored. Influence factors on fly ash resistivity at 673–1273 K were analyzed. A new method to collect low resistivity fly ash was proposed.
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 26 August 2013 Accepted 27 August 2013 Available online 5 September 2013 Keywords: Fly ash Resistivity High temperature Electrostatic precipitation
a b s t r a c t Research on fly ash resistivity at low temperature has been studied a lot, but there are few experimental researches on fly ash resistivity at the temperature above 673 K. Eight samples were selected to be investigated. By DR2-1000 dust resistivity tester, experimental resistivity data of the fly ashes at high temperature were achieved. The experimental results at temperature of 673–1273 K were discussed. With the increase of temperature, the decrease of resistivity agreed with Arrhenius equation well. The existence of alkali metal and iron ions reduced the resistivity obviously. Size distribution showed influence on the electrical resistivity. But with the increase of temperature, chemical constitution played greater role in electrical characteristics. Most of the experimental fly ashes had resistivity value above 105 X cm at the temperature of 673–1273 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction High temperature flue gas cleaning is showing their importance in many fields including chemical industry, oil refineries, incinerations, biomass gasification and metal refining [1]. For energy, environment and technology reasons, it is necessary to develop filtration process at as high a temperature as possible. Electrostatic Precipitation (ESP) is a technique to remove suspended particles in gas using electrostatic force. It is widely used in pulverized coal combustion boilers, cement kilns, sinter plants and other industrial sources. Despite the effective and reliable performance of electrostatic precipitator at temperature below 673 K, there are some issues hindering its operation at high temperature. The latest high temperature scientific data of the traditional ESP date from the mid-1980s [2–4] and the results showed that the ESP working zone decreases with the increase of temperature [5,6]. Unstable corona discharge, electrical insulation, constructional problems and other issues are the main barriers of ESP ⇑ Corresponding authors. Tel.: +86 25 85891105; fax: +86 25 85891282. E-mail addresses: [email protected] (J. Xu), [email protected] (Z. Gu). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.075
development at high temperature. However, the thermionic emission ESP is an innovative improvement regarding the application of ESP at the temperature above 1073 K. It takes some low work function composite materials as an emission cathode. Heated by high temperature flue gas, the cathode emits thermal electrons to charge the particles. Thermionic emission ESP could overcome the drawbacks of traditional ESP very well in high temperature flue gas cleaning. Electrostatic precipitation [7], especially the thermionic emission ESP [8], offers numerous advantages over other filtration technologies. And it is prospective and preferable in the long term of their performance in particles removal at high temperature. Electrical resistivity of particle is an essential variable which significantly affects ESP collection efficiency [9]. By now, the research on the resistivity is restricted to the range of low temperature (<723 K). Resistivity value at high temperature is only achieved by extrapolation. None resistivity value has been measured at high temperature [7]. Not only the application of classical ESP in extreme temperature (723–1273 K), but also the research of novel thermionic emission ESP at high temperature (1073–1373 K) [8,10] demands research of particle resistivity at high temperature. In this paper, an experimental investigation was carried to explore
J. Xu et al. / Fuel 116 (2014) 650–654
the electrical resistivity characteristics of fly ash at high temperature.
651
JSM-5610LV). The size distribution was measured with a laser particle size analyzer (China Bettersize, type: BT-2003). 3.2. Electrical resistivity
2. Previous studies Electrical resistivity is one of the critical parameters influencing the behavior of electrostatic precipitation. It is influenced by a number of important variables, beginning at the coal mine and extending through the pulverizers, boiler furnace, and up to flue gas conditions at the precipitator inlet [11]. As for itself, the main factors include chemical composition, temperature and size distribution. As far as chemical composition is concerned, Bickelhaupt [12] has done a series of studies. He has verified that volume conduction is an ionic mechanism involving the alkali metal ions, principally sodium, as charge carriers. The presence of iron influences the number of alkali metal ions capable of migration. The effect of alkali metal was also fully analyzed by Qi and Yuan [13]. And there is an inverse proportionality between resistivity and iron concentration for a given level of lithium and sodium. Temperature is a parameter that has significant effect on the electrical resistivity. The dust resistivity consists of two separate mechanisms, volume and surface resistivity [14]. Volume resistivity dominates at high temperature and is inversely proportional to working temperature. Surface resistivity mainly occurs at low temperature and is proportional to the temperature. Usually at the temperature of 373–473 K, the resistivity arrives at a peak value as both the volume and surface resistivities are near the maximum. Size distribution has double impact on resistivity from reviews by Yuan [15]. The fine fly ashes have higher porosity and larger specific surface area than coarse particles. On one hand, since coarse particles have lower porosity, the volume ratio of air and solid particles is smaller. Therefore, volume conductivity of coarse particles is more advantageous than that of fine fly ashes. On the other hand, fine particles have larger specific surface. Large specific surface would increase the contact area and be beneficial to surface conductivity. As a result, it is difficult to estimate variation of resistivity with size distribution. Generally, fly ashes in wet flue gas have liquid film on the surface. The liquid film increases the surface conductivity and hence fine particles exhibit lower resistivity. However, with the increase of temperature, the water evaporates gradually and specific surface area shows less influence than porosity on the resistivity. For the dry fly ashes, coarse particles usually have lower resistivity value. The researches on fly ash electrical characteristics have been studied a lot, but most of them aim at the temperature below 723 K [7,11,16,17]. As none experimental value is measured at high temperature, more effort is needed in achieving experimental results of fly ash resistivity at high temperature.
Particle resistivity is normally determined by measuring the leakage current through a dust layer in high voltage conductivity cell [9]. According to this method, DR2-1000 type high temperature dust resistivity test facility was used to test the electrical resistivity of fly ashes at high temperature. Manufactured by North China Electric Power University, the instrument consists of control desk, electrode furnace and DC high voltage power supply system. Two parallel circular electrodes are settled in the furnace. The lower electrode is a circular ash tray to be filled with measured ash. The upper electrode is placed on the ash layer. Control desk is used to regulate the DC high voltage (0–20 kV), detect the micro-current signal (3 104–20 mA), and set the furnace temperature (<1473 K). The thickness of ash layer is 5 mm and the cross-sectional area of the upper electrode is 50.5 mm2. When it is tested, the ash is naturally and loosely loaded in the lower electrode. The upper electrode is placed on the central ash layer to keep fine parallelism with the lower electrode. After heating the furnace at target temperature, a high voltage field is applied to the particle layer. Then leakage current through the dust layer is detected and recorded. The resistivity is obtained from the given electrical field strength and the experimental data of current density, as follows [15]:
s d
q¼
U U ¼k I I
ð1Þ
where q is the resistivity of the particle (X cm), s is the cross-sectional area of the particle layer (cm2), d is the thickness of particle layer (cm), U is the voltage (V), and I is the current (A). k is the coefficient of electrode which is the ratio of cross-sectional area and thickness of particle layer. k is determined by geometric dimensions of the measuring device, and it is 10.1 in this paper. 4. Results and discussion 4.1. Particle characterization
3. Experimental apparatus and methods
The chemical composition, the particle size distribution, and the surface structure of the aerosols were investigated. Table 1 is the chemical characterization of the different ashes expressed in weight percent, exclusive of loss on ignition. Fig. 1 shows SEM photographs of the different ashes. The heterogeneity of fly ash was obvious. From the photos, it could be found that samples 2, 4 and 6 were mostly spherical in shape. Samples 1, 5, 7 and 8 were mostly flake and irregular in form. Sample 3 was mostly irregular in form and had some spherical particles. Table 2 is the mass median diameters (MMD) of the different fly ashes. Fig. 2 shows the size distributions of the particles.
3.1. Fly ash characterization
4.2. Electrical resistivity of industrial particles
Eight particle samples were taken as examples to be investigated. Sample 1 was obtained from electrostatic precipitator in a metallurgical company in Panzhihua. The rest samples were collected from electrostatic precipitator in different coal-fired power stations. Chemical composition of the fly ashes was measured with Energy Dispersive Spectroscopy (EDS) (American NORAN, NORANVANTAGE) and Rotating anode X-ray diffractometer (XRD) (Japan Rigaku, D/max 2500VL/PC). The shape and surface structure of the particles were investigated using a Scanning Electron Microscopy (SEM) manufactured by Japanese electronics company (type:
Fly ash samples were divided into three groups to analyze electrical resistivity according to their chemical characteristics. Figs. 3, 4, and 5 are their resistivity variation as a function of temperature from 373 to 1273 K. It could be seen that an increase in temperature resulted in a decrease in resistivity of the fly ashes at the temperature above 373 K, except that fly ash samples 1–5 had a peak value in the range of 373–573 K. As mentioned earlier, there are two mechanisms of resistivity of fly ashes: surface resistivity and volume resistivity. The total effective resistivity may be evaluated by considering these two kinds of
652
J. Xu et al. / Fuel 116 (2014) 650–654
Table 1 Chemical characterization of different ashes. Chemical composition
Ash no.
Chemical composition
Ash no.
1 Ca2SiO4 (%) SiO2 (%) Al2SiO5 (%) (Mn2O3)3MnSiO3 (%) K2Ca(SO4)2 (%) Na14Mn2O9 (%)
15.51 30.6 18.69 28.85 4.42 1.93
CaSO4 (%) SiO2 (%) Al2SiO5 (%) Ca(OH)2 (%) CaCO3 (%) Fe2O3 (%) Fe3O4 (%)
2
3
4
5
6
7
8
30.2 45.7 – – – 10.3 13.8
8.0 17.8 47.4 1.8 25.0 – –
26.1 42.9 – – – 14.2 16.8
3.3 2.5 – – 49.4 34.1 10.6
34.7 43.9 2.0 – – 6.6 12.9
24.5 69.2 – – – 6.3 –
– 75.0 15.5 – – 9.5 –
Fig. 1. Scanning electron micrographs of fly ash.
Table 2 Mass median diameters of fly ashes.
10 14
MMD (lm)
Ash no.
MMD (lm)
1 2 3 4
4.29 5.25 5.49 9.57
5 6 7 8
13.48 13.68 19.97 23.27
Cumulative mass distribution (%)
100 Sample 1 Sample 3 Sample 5 Sample 7
80
Sample 2 Sample 4 Sample 6 Sample 8
Sample 1
10 13
Sample 3
10 12
Sample 4
10 11
Resistivity (Ω⋅cm)
Ash no.
10 10 10 9 10 8 10 7 10 6 10 5 10 4
60
10 3 40
600
800
1000
1200
Temperature (K) Fig. 3. Resistivity variation of samples 1, 3, and 4 with temperature.
20
0
400
0.1
1
10
100
Particle size (μm) Fig. 2. Particle size distribution of fly ash samples.
1000
resistivity together. At low temperature, the resistivity is mainly governed by surface condition. It is therefore related to the absorption or adsorption of water vapor in the air. At high temperature volume resistivity is of primary importance. Although the experimental data covered the entire resistivity–temperature range
J. Xu et al. / Fuel 116 (2014) 650–654
10 14
Sample 5
10 13
Sample 6
10 12
Sample 7
Resistivity (Ω⋅cm)
10 11 10 10 10 9 10 8 10 7 10 6 10 5 10 4 10 3
400
600
800
1000
1200
Temperature (K) Fig. 4. Resistivity variation of samples 5, 6, and 7 with temperature.
1014
Sample 2
1013
Sample 8
Resistivity (Ω⋅cm)
1012
653
contained 34.1% Fe2O3 and 10.6% Fe3O4. As the action of iron ions, sample 5 exhibited a very low resistivity of 1.06 104 X cm at 1073 K. And the resistivity of sample 5 decreased obviously with the increase of temperature. With 6.6% Fe2O3 and 12.9% Fe3O4, sample 6 had more iron ions than sample 7 which only contained 6.3% Fe2O3. Thus, sample 6 would have lower resistivity value than sample 7. But from the figure, it only occurred at the temperature above about 873 K. In the temperature range of 673–873 K, sample 7 exhibited lower resistivity than sample 6. Size distribution was conjectured to be the cause of that. To further confirm the influence of size distribution, Fig. 5 presented resistivity variation of samples 2 and 8 as a function of temperature. From the chemical characteristics in Table 1, resistivity value of sample 2 was thought to be lower than that of sample 8 at high temperature for the sample 2 had more iron ions. However, in the range of 673–873 K the result turned out to be opposite. As explained earlier, coarse particles have better volume conduction and exhibit lower resistivity than fine particles at high temperature. As a result, sample 8 with coarser size distribution exhibited lower resistivity at the temperature below 873 K. But when the temperature was above 873 K, chemical constitution played greater role and hence sample 2 which had more iron ions presented lower resistivity, as shown in Fig. 5. From the experimental results above, the resistivity of the fly ashes decreased with the increase of temperature. According to the research [12], the variation of fly ash resistivity with the increase of temperature could be described by Arrhenius equation. The logarithmic form of Arrhenius equation is
1011
log q ¼ log q0 þ ½ðh=kÞ log eð1=TÞ
1010
where q is the resistivity of the fly ash, q0 is a complex material parameter, h is experimental activation energy, k is Boltzmann’s constant, and T is absolute temperature. The resistivity was plotted on a log-scale ordinate. And the horizontal ordinate is the reciprocal of absolute temperature multiplied by 1000. Results of the entire samples fit linear laws well, and three typical resistivity data of them are shown graphically in Fig. 6. The linear curves in Fig. 6 demonstrated that the experimental resistivity data still had a good agreement with the Arrhenius equation at the temperature above 673 K.
10 9 10 8 10 7 10 6 10 5 10 4
400
600
800
1000
1200
ð2Þ
4.3. Discussion and prospects
Temperature (K) Fig. 5. Resistivity variation of samples 2 and 8 with temperature.
(373–1273 K), the paper only discussed the region of above 673 K, in which the resistivity is mainly influenced by the temperature and the amount and chemical constitution of the various microconstitutions making up the fly ash. Resistivity data of samples 1, 3 and 4 were show in Fig. 3. The figure reveals that sample 1 had the lowest resistivity among samples 1, 3 and 4. The data was an expected result as the sample 1 was found to have sodium and potassium ions from Table 1. Alkali metal ions represented by sodium and potassium ions served as the principal charge carriers in ionic conduction mechanism of volume resistivity. And sample 1 exhibited a very low resistivity of 1.0 104 X cm at 1123 K. As far as the chemical constitution was concerned, sample 4 had 14.2% Fe2O3 and 16.8% Fe3O4. The iron oxide of sample 3 was too few to be detected. Iron ions played important role in resistivity. And samples 3 and 4 had different content of iron ions in the fly ash micro-constitution. That was the reason for their difference of the resistivity value with the increase of temperature. The results for resistivity of samples 5–7 at various temperatures were plotted and shown in Fig. 4. From Table 1, sample 5
Electrical resistivity of fly ash is an essential variable which significantly affects ESP and thermionic emission ESP collection
Fig. 6. Typical experimental and theoretical resistivity data.
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J. Xu et al. / Fuel 116 (2014) 650–654
efficiency. Particle resistivity could be classified into three regimes: low resistivity (<104 X cm), normal resistivity (104–1010 X cm), and high resistivity (>1010 X cm) [18]. If the electrical resistivity of particle exceeds a critical value of about 1010 X cm, corona currents will be limited by electric breakdown of the collected dust layers. In the contrary case, if resistivity is below 104 X cm, particles reaching the collecting electrode rapidly lose their electrical charge and may be re-entrained in the gas stream [7]. From the experimental data, samples 2, 3, 4, 6, 7, and 8 had the resistivity value all above 105 X cm at high temperature (673– 1273 K). It could be concluded that they were suitable for ESP at high temperature. And it could also be demonstrated from the collection efficiency of thermionic emission ESP [8]. As far as samples 1 and 5 were concerned, the resistivity was below 104 X cm at the temperature above 1123 K. It was well below the normal range for good electrostatic precipitator performance. Re-entrainment caused by low resistivity was the main reason for poor collection efficiency by ESP. For the low resistivity particles collection, Yamamoto et al. developed an electrohydrodynamically (EHD) assisted electrostatic precipitator. According to the research [19], the collection plate is designed to be attached with pocket zone. As the action of voltage, ionized gas near the electrode drifts toward the grounding plate. This secondary flow is generally called electric wind, corona wind, or ionic wind [20]. Low-resistivity dust could be effectively collected inside the pocket zone with the assistance of ionic wind. The thermionic emission ESP has 1015–1016 or more ions per cubic meter in charging section [10]. That is two or over two orders of magnitude higher than traditional ESP and hence thermionic emission ESP have better ionic wind effect. It could be inferred that lowresistivity fly ashes could be more effectively collected with thermionic emission ESP enhanced by EHD, but it still needs to be explored in further research.
5. Conclusions Eight samples were taken as example to investigate the electrical resistivity characteristics. This paper focused on the experimental resistivity in the temperature region of 673–1273 K and came to the conclusions as follows. (1) The resistivity of the fly ashes decreased with the increase of temperature. The variation agreed with Arrhenius equation well. (2) At high temperature, volume resistivity was mainly influenced by chemical characterization. Alkali metal and iron ions played important role in ionic conduction mechanism of volume resistivity. Their existence reduced the resistivity obviously. (3) Size distribution showed influence on the electrical resistivity. Coarse particles had better volume conduction and lower resistivity. But with the increase of temperature, chemical constitution played greater and major role in electrical characteristics.
(4) At high temperature, most of the experimental fly ashes had resistivity value above 105 X cm and were suitable for collection by ESP. For low resistivity particles collection, it may be a good way to try thermionic emission ESP enhanced by EHD.
Acknowledgements The research was supported by National Natural Science Foundation of China (No. 50778090) and Industrialization Projects of Jiangsu Provincial Education Department (No. JBH2012-15). They are greatly acknowledged for the funding of this work. The authors would like to thank Dr. Li Gang in Nanjing Normal University Center for Analysis and Testing, for performing analysis of microstructure and data processing of chemical composition. References [1] Heidenreich S. Hot gas filtration – a review. Fuel 2013;104:83–94. [2] Rinard G, Rugg DE, Yamamoto T. High-temperature high-pressure electrostatic precipitator electrical characterization and collection efficiency. IEEE Trans Ind Appl 1987;23:114–9. [3] Tassicker OJ. High temperature-pressure electrostatic precipitator for electric power generation technologies: an overview of the status. Inst Chem Eng 1986;99:331–49. [4] Brown RF, Walker AB. Feasibility demonstration of electrostatic precipitation at 1700 F. J Air Pollut Control Assoc 1971;21:617–20. [5] Reijnen K, Van Brakel J. Gas cleaning at high temperatures and high pressures: a review. Powder Technol 1984;40:81–111. [6] Weber E, Hübner K, Pape HG, Schulz R. Gas cleaning under extreme conditions of temperature and pressure. Environ Int 1981;6:349–60. [7] Villot A, Gonthier Y, Gonze E, Bernis A, Ravel S, Grateau M, et al. Separation of particles from syngas at high-temperatures with an electrostatic precipitator. Sep Purif Technol 2012;92:181–90. [8] Gu ZZ, Xi XL, Yang JC, Xu JJ. Properties of RE-W cathode and its application in electrostatic precipitation for high temperature gas clean-up. Fuel 2012;95:648–54. [9] Barranco R, Gong M, Thompson A, Cloke M, Hanson S, Gibb W, et al. The impact of fly ash resistivity and carbon content on electrostatic precipitator performance. Fuel 2007;86:2521–7. [10] Xu JJ, Gu ZZ, Xi XL, Yang JC. Cathode discharge and dust removal characteristics of a lanthanum-tungsten cathode high-temperature electrostatic precipitator. J Eng Therm Energy Power 2011;26:576–81. 634 [in Chinese]. [11] Estcourt VF, Frisch NW. Measuring and reporting fly ash resistivity. IEEE Trans Power Appl Syst 1980;PAS-99:573–81. [12] Bickelhaupt RE. Volume resistivity-fly ash composition relation. Environ Sci Technol 1975;9:336–42. [13] Qi LQ, Yuan YT. Mechanism of the effect of alkali metal on the electrostatic precipitability of fly ash. Fuel 2013;107:848–51. [14] Tang MK, Feng GJ. The analysis on affecting factors of dust ratio resistance and their responding measures. J Jiangxi Univ Sci Technol 2007;28:44–6 [in Chinese]. [15] Yuan YT. Electrostatic precipitation in power plant. 1st ed. Beijing: Chemical Industry Press; 2004 [in Chinese]. [16] Bickelhaupt RE. Electrical volume conduction in fly ash. J Air Pollut Control Assoc 1974;24:251–5. [17] Bickelhaupt RE. Surface resistivity and the chemical composition of fly ash. J Air Pollut Control Assoc 1975;25:148–52. [18] Ahn YC, Lee JK. Physical, chemical, and electrical analysis of aerosol particles generated from industrial plants. J Aerosol Sci 2006;37:187–202. [19] Yamamoto T, Abe T, Mimura T, Otsuka N, Ito Y, Ehara Y, et al. Electrohydrodynamically assisted electrostatic precipitator for the collection of low-resistivity dust. IEEE Trans Ind Appl 2009;45:2178–84. [20] Liang WJ, Lin TH. The characteristics of ionic wind and its effect on electrostatic precipitators. Aerosol Sci Technol 1994;20:330–44.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Experimental study on fly ash resistivity at temperatures above 673 K Jinjin Xu a,⇑, Zhongzhu Gu b,⇑, Jun Zhang a a b
School of Energy and Environment, Southeast University, Nanjing 210096, PR China School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, PR China
h i g h l i g h t s The fly ash resistivity at temperature of 673–1273 K was measured and discussed. Paper gave experimental data of fly ash resistivity at 673–1273 K. Variation of fly ash resistivity with temperature at 673–1273 K was explored. Influence factors on fly ash resistivity at 673–1273 K were analyzed. A new method to collect low resistivity fly ash was proposed.
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 26 August 2013 Accepted 27 August 2013 Available online 5 September 2013 Keywords: Fly ash Resistivity High temperature Electrostatic precipitation
a b s t r a c t Research on fly ash resistivity at low temperature has been studied a lot, but there are few experimental researches on fly ash resistivity at the temperature above 673 K. Eight samples were selected to be investigated. By DR2-1000 dust resistivity tester, experimental resistivity data of the fly ashes at high temperature were achieved. The experimental results at temperature of 673–1273 K were discussed. With the increase of temperature, the decrease of resistivity agreed with Arrhenius equation well. The existence of alkali metal and iron ions reduced the resistivity obviously. Size distribution showed influence on the electrical resistivity. But with the increase of temperature, chemical constitution played greater role in electrical characteristics. Most of the experimental fly ashes had resistivity value above 105 X cm at the temperature of 673–1273 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction High temperature flue gas cleaning is showing their importance in many fields including chemical industry, oil refineries, incinerations, biomass gasification and metal refining [1]. For energy, environment and technology reasons, it is necessary to develop filtration process at as high a temperature as possible. Electrostatic Precipitation (ESP) is a technique to remove suspended particles in gas using electrostatic force. It is widely used in pulverized coal combustion boilers, cement kilns, sinter plants and other industrial sources. Despite the effective and reliable performance of electrostatic precipitator at temperature below 673 K, there are some issues hindering its operation at high temperature. The latest high temperature scientific data of the traditional ESP date from the mid-1980s [2–4] and the results showed that the ESP working zone decreases with the increase of temperature [5,6]. Unstable corona discharge, electrical insulation, constructional problems and other issues are the main barriers of ESP ⇑ Corresponding authors. Tel.: +86 25 85891105; fax: +86 25 85891282. E-mail addresses: [email protected] (J. Xu), [email protected] (Z. Gu). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.075
development at high temperature. However, the thermionic emission ESP is an innovative improvement regarding the application of ESP at the temperature above 1073 K. It takes some low work function composite materials as an emission cathode. Heated by high temperature flue gas, the cathode emits thermal electrons to charge the particles. Thermionic emission ESP could overcome the drawbacks of traditional ESP very well in high temperature flue gas cleaning. Electrostatic precipitation [7], especially the thermionic emission ESP [8], offers numerous advantages over other filtration technologies. And it is prospective and preferable in the long term of their performance in particles removal at high temperature. Electrical resistivity of particle is an essential variable which significantly affects ESP collection efficiency [9]. By now, the research on the resistivity is restricted to the range of low temperature (<723 K). Resistivity value at high temperature is only achieved by extrapolation. None resistivity value has been measured at high temperature [7]. Not only the application of classical ESP in extreme temperature (723–1273 K), but also the research of novel thermionic emission ESP at high temperature (1073–1373 K) [8,10] demands research of particle resistivity at high temperature. In this paper, an experimental investigation was carried to explore
J. Xu et al. / Fuel 116 (2014) 650–654
the electrical resistivity characteristics of fly ash at high temperature.
651
JSM-5610LV). The size distribution was measured with a laser particle size analyzer (China Bettersize, type: BT-2003). 3.2. Electrical resistivity
2. Previous studies Electrical resistivity is one of the critical parameters influencing the behavior of electrostatic precipitation. It is influenced by a number of important variables, beginning at the coal mine and extending through the pulverizers, boiler furnace, and up to flue gas conditions at the precipitator inlet [11]. As for itself, the main factors include chemical composition, temperature and size distribution. As far as chemical composition is concerned, Bickelhaupt [12] has done a series of studies. He has verified that volume conduction is an ionic mechanism involving the alkali metal ions, principally sodium, as charge carriers. The presence of iron influences the number of alkali metal ions capable of migration. The effect of alkali metal was also fully analyzed by Qi and Yuan [13]. And there is an inverse proportionality between resistivity and iron concentration for a given level of lithium and sodium. Temperature is a parameter that has significant effect on the electrical resistivity. The dust resistivity consists of two separate mechanisms, volume and surface resistivity [14]. Volume resistivity dominates at high temperature and is inversely proportional to working temperature. Surface resistivity mainly occurs at low temperature and is proportional to the temperature. Usually at the temperature of 373–473 K, the resistivity arrives at a peak value as both the volume and surface resistivities are near the maximum. Size distribution has double impact on resistivity from reviews by Yuan [15]. The fine fly ashes have higher porosity and larger specific surface area than coarse particles. On one hand, since coarse particles have lower porosity, the volume ratio of air and solid particles is smaller. Therefore, volume conductivity of coarse particles is more advantageous than that of fine fly ashes. On the other hand, fine particles have larger specific surface. Large specific surface would increase the contact area and be beneficial to surface conductivity. As a result, it is difficult to estimate variation of resistivity with size distribution. Generally, fly ashes in wet flue gas have liquid film on the surface. The liquid film increases the surface conductivity and hence fine particles exhibit lower resistivity. However, with the increase of temperature, the water evaporates gradually and specific surface area shows less influence than porosity on the resistivity. For the dry fly ashes, coarse particles usually have lower resistivity value. The researches on fly ash electrical characteristics have been studied a lot, but most of them aim at the temperature below 723 K [7,11,16,17]. As none experimental value is measured at high temperature, more effort is needed in achieving experimental results of fly ash resistivity at high temperature.
Particle resistivity is normally determined by measuring the leakage current through a dust layer in high voltage conductivity cell [9]. According to this method, DR2-1000 type high temperature dust resistivity test facility was used to test the electrical resistivity of fly ashes at high temperature. Manufactured by North China Electric Power University, the instrument consists of control desk, electrode furnace and DC high voltage power supply system. Two parallel circular electrodes are settled in the furnace. The lower electrode is a circular ash tray to be filled with measured ash. The upper electrode is placed on the ash layer. Control desk is used to regulate the DC high voltage (0–20 kV), detect the micro-current signal (3 104–20 mA), and set the furnace temperature (<1473 K). The thickness of ash layer is 5 mm and the cross-sectional area of the upper electrode is 50.5 mm2. When it is tested, the ash is naturally and loosely loaded in the lower electrode. The upper electrode is placed on the central ash layer to keep fine parallelism with the lower electrode. After heating the furnace at target temperature, a high voltage field is applied to the particle layer. Then leakage current through the dust layer is detected and recorded. The resistivity is obtained from the given electrical field strength and the experimental data of current density, as follows [15]:
s d
q¼
U U ¼k I I
ð1Þ
where q is the resistivity of the particle (X cm), s is the cross-sectional area of the particle layer (cm2), d is the thickness of particle layer (cm), U is the voltage (V), and I is the current (A). k is the coefficient of electrode which is the ratio of cross-sectional area and thickness of particle layer. k is determined by geometric dimensions of the measuring device, and it is 10.1 in this paper. 4. Results and discussion 4.1. Particle characterization
3. Experimental apparatus and methods
The chemical composition, the particle size distribution, and the surface structure of the aerosols were investigated. Table 1 is the chemical characterization of the different ashes expressed in weight percent, exclusive of loss on ignition. Fig. 1 shows SEM photographs of the different ashes. The heterogeneity of fly ash was obvious. From the photos, it could be found that samples 2, 4 and 6 were mostly spherical in shape. Samples 1, 5, 7 and 8 were mostly flake and irregular in form. Sample 3 was mostly irregular in form and had some spherical particles. Table 2 is the mass median diameters (MMD) of the different fly ashes. Fig. 2 shows the size distributions of the particles.
3.1. Fly ash characterization
4.2. Electrical resistivity of industrial particles
Eight particle samples were taken as examples to be investigated. Sample 1 was obtained from electrostatic precipitator in a metallurgical company in Panzhihua. The rest samples were collected from electrostatic precipitator in different coal-fired power stations. Chemical composition of the fly ashes was measured with Energy Dispersive Spectroscopy (EDS) (American NORAN, NORANVANTAGE) and Rotating anode X-ray diffractometer (XRD) (Japan Rigaku, D/max 2500VL/PC). The shape and surface structure of the particles were investigated using a Scanning Electron Microscopy (SEM) manufactured by Japanese electronics company (type:
Fly ash samples were divided into three groups to analyze electrical resistivity according to their chemical characteristics. Figs. 3, 4, and 5 are their resistivity variation as a function of temperature from 373 to 1273 K. It could be seen that an increase in temperature resulted in a decrease in resistivity of the fly ashes at the temperature above 373 K, except that fly ash samples 1–5 had a peak value in the range of 373–573 K. As mentioned earlier, there are two mechanisms of resistivity of fly ashes: surface resistivity and volume resistivity. The total effective resistivity may be evaluated by considering these two kinds of
652
J. Xu et al. / Fuel 116 (2014) 650–654
Table 1 Chemical characterization of different ashes. Chemical composition
Ash no.
Chemical composition
Ash no.
1 Ca2SiO4 (%) SiO2 (%) Al2SiO5 (%) (Mn2O3)3MnSiO3 (%) K2Ca(SO4)2 (%) Na14Mn2O9 (%)
15.51 30.6 18.69 28.85 4.42 1.93
CaSO4 (%) SiO2 (%) Al2SiO5 (%) Ca(OH)2 (%) CaCO3 (%) Fe2O3 (%) Fe3O4 (%)
2
3
4
5
6
7
8
30.2 45.7 – – – 10.3 13.8
8.0 17.8 47.4 1.8 25.0 – –
26.1 42.9 – – – 14.2 16.8
3.3 2.5 – – 49.4 34.1 10.6
34.7 43.9 2.0 – – 6.6 12.9
24.5 69.2 – – – 6.3 –
– 75.0 15.5 – – 9.5 –
Fig. 1. Scanning electron micrographs of fly ash.
Table 2 Mass median diameters of fly ashes.
10 14
MMD (lm)
Ash no.
MMD (lm)
1 2 3 4
4.29 5.25 5.49 9.57
5 6 7 8
13.48 13.68 19.97 23.27
Cumulative mass distribution (%)
100 Sample 1 Sample 3 Sample 5 Sample 7
80
Sample 2 Sample 4 Sample 6 Sample 8
Sample 1
10 13
Sample 3
10 12
Sample 4
10 11
Resistivity (Ω⋅cm)
Ash no.
10 10 10 9 10 8 10 7 10 6 10 5 10 4
60
10 3 40
600
800
1000
1200
Temperature (K) Fig. 3. Resistivity variation of samples 1, 3, and 4 with temperature.
20
0
400
0.1
1
10
100
Particle size (μm) Fig. 2. Particle size distribution of fly ash samples.
1000
resistivity together. At low temperature, the resistivity is mainly governed by surface condition. It is therefore related to the absorption or adsorption of water vapor in the air. At high temperature volume resistivity is of primary importance. Although the experimental data covered the entire resistivity–temperature range
J. Xu et al. / Fuel 116 (2014) 650–654
10 14
Sample 5
10 13
Sample 6
10 12
Sample 7
Resistivity (Ω⋅cm)
10 11 10 10 10 9 10 8 10 7 10 6 10 5 10 4 10 3
400
600
800
1000
1200
Temperature (K) Fig. 4. Resistivity variation of samples 5, 6, and 7 with temperature.
1014
Sample 2
1013
Sample 8
Resistivity (Ω⋅cm)
1012
653
contained 34.1% Fe2O3 and 10.6% Fe3O4. As the action of iron ions, sample 5 exhibited a very low resistivity of 1.06 104 X cm at 1073 K. And the resistivity of sample 5 decreased obviously with the increase of temperature. With 6.6% Fe2O3 and 12.9% Fe3O4, sample 6 had more iron ions than sample 7 which only contained 6.3% Fe2O3. Thus, sample 6 would have lower resistivity value than sample 7. But from the figure, it only occurred at the temperature above about 873 K. In the temperature range of 673–873 K, sample 7 exhibited lower resistivity than sample 6. Size distribution was conjectured to be the cause of that. To further confirm the influence of size distribution, Fig. 5 presented resistivity variation of samples 2 and 8 as a function of temperature. From the chemical characteristics in Table 1, resistivity value of sample 2 was thought to be lower than that of sample 8 at high temperature for the sample 2 had more iron ions. However, in the range of 673–873 K the result turned out to be opposite. As explained earlier, coarse particles have better volume conduction and exhibit lower resistivity than fine particles at high temperature. As a result, sample 8 with coarser size distribution exhibited lower resistivity at the temperature below 873 K. But when the temperature was above 873 K, chemical constitution played greater role and hence sample 2 which had more iron ions presented lower resistivity, as shown in Fig. 5. From the experimental results above, the resistivity of the fly ashes decreased with the increase of temperature. According to the research [12], the variation of fly ash resistivity with the increase of temperature could be described by Arrhenius equation. The logarithmic form of Arrhenius equation is
1011
log q ¼ log q0 þ ½ðh=kÞ log eð1=TÞ
1010
where q is the resistivity of the fly ash, q0 is a complex material parameter, h is experimental activation energy, k is Boltzmann’s constant, and T is absolute temperature. The resistivity was plotted on a log-scale ordinate. And the horizontal ordinate is the reciprocal of absolute temperature multiplied by 1000. Results of the entire samples fit linear laws well, and three typical resistivity data of them are shown graphically in Fig. 6. The linear curves in Fig. 6 demonstrated that the experimental resistivity data still had a good agreement with the Arrhenius equation at the temperature above 673 K.
10 9 10 8 10 7 10 6 10 5 10 4
400
600
800
1000
1200
ð2Þ
4.3. Discussion and prospects
Temperature (K) Fig. 5. Resistivity variation of samples 2 and 8 with temperature.
(373–1273 K), the paper only discussed the region of above 673 K, in which the resistivity is mainly influenced by the temperature and the amount and chemical constitution of the various microconstitutions making up the fly ash. Resistivity data of samples 1, 3 and 4 were show in Fig. 3. The figure reveals that sample 1 had the lowest resistivity among samples 1, 3 and 4. The data was an expected result as the sample 1 was found to have sodium and potassium ions from Table 1. Alkali metal ions represented by sodium and potassium ions served as the principal charge carriers in ionic conduction mechanism of volume resistivity. And sample 1 exhibited a very low resistivity of 1.0 104 X cm at 1123 K. As far as the chemical constitution was concerned, sample 4 had 14.2% Fe2O3 and 16.8% Fe3O4. The iron oxide of sample 3 was too few to be detected. Iron ions played important role in resistivity. And samples 3 and 4 had different content of iron ions in the fly ash micro-constitution. That was the reason for their difference of the resistivity value with the increase of temperature. The results for resistivity of samples 5–7 at various temperatures were plotted and shown in Fig. 4. From Table 1, sample 5
Electrical resistivity of fly ash is an essential variable which significantly affects ESP and thermionic emission ESP collection
Fig. 6. Typical experimental and theoretical resistivity data.
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J. Xu et al. / Fuel 116 (2014) 650–654
efficiency. Particle resistivity could be classified into three regimes: low resistivity (<104 X cm), normal resistivity (104–1010 X cm), and high resistivity (>1010 X cm) [18]. If the electrical resistivity of particle exceeds a critical value of about 1010 X cm, corona currents will be limited by electric breakdown of the collected dust layers. In the contrary case, if resistivity is below 104 X cm, particles reaching the collecting electrode rapidly lose their electrical charge and may be re-entrained in the gas stream [7]. From the experimental data, samples 2, 3, 4, 6, 7, and 8 had the resistivity value all above 105 X cm at high temperature (673– 1273 K). It could be concluded that they were suitable for ESP at high temperature. And it could also be demonstrated from the collection efficiency of thermionic emission ESP [8]. As far as samples 1 and 5 were concerned, the resistivity was below 104 X cm at the temperature above 1123 K. It was well below the normal range for good electrostatic precipitator performance. Re-entrainment caused by low resistivity was the main reason for poor collection efficiency by ESP. For the low resistivity particles collection, Yamamoto et al. developed an electrohydrodynamically (EHD) assisted electrostatic precipitator. According to the research [19], the collection plate is designed to be attached with pocket zone. As the action of voltage, ionized gas near the electrode drifts toward the grounding plate. This secondary flow is generally called electric wind, corona wind, or ionic wind [20]. Low-resistivity dust could be effectively collected inside the pocket zone with the assistance of ionic wind. The thermionic emission ESP has 1015–1016 or more ions per cubic meter in charging section [10]. That is two or over two orders of magnitude higher than traditional ESP and hence thermionic emission ESP have better ionic wind effect. It could be inferred that lowresistivity fly ashes could be more effectively collected with thermionic emission ESP enhanced by EHD, but it still needs to be explored in further research.
5. Conclusions Eight samples were taken as example to investigate the electrical resistivity characteristics. This paper focused on the experimental resistivity in the temperature region of 673–1273 K and came to the conclusions as follows. (1) The resistivity of the fly ashes decreased with the increase of temperature. The variation agreed with Arrhenius equation well. (2) At high temperature, volume resistivity was mainly influenced by chemical characterization. Alkali metal and iron ions played important role in ionic conduction mechanism of volume resistivity. Their existence reduced the resistivity obviously. (3) Size distribution showed influence on the electrical resistivity. Coarse particles had better volume conduction and lower resistivity. But with the increase of temperature, chemical constitution played greater and major role in electrical characteristics.
(4) At high temperature, most of the experimental fly ashes had resistivity value above 105 X cm and were suitable for collection by ESP. For low resistivity particles collection, it may be a good way to try thermionic emission ESP enhanced by EHD.
Acknowledgements The research was supported by National Natural Science Foundation of China (No. 50778090) and Industrialization Projects of Jiangsu Provincial Education Department (No. JBH2012-15). They are greatly acknowledged for the funding of this work. The authors would like to thank Dr. Li Gang in Nanjing Normal University Center for Analysis and Testing, for performing analysis of microstructure and data processing of chemical composition. References [1] Heidenreich S. Hot gas filtration – a review. Fuel 2013;104:83–94. [2] Rinard G, Rugg DE, Yamamoto T. High-temperature high-pressure electrostatic precipitator electrical characterization and collection efficiency. IEEE Trans Ind Appl 1987;23:114–9. [3] Tassicker OJ. High temperature-pressure electrostatic precipitator for electric power generation technologies: an overview of the status. Inst Chem Eng 1986;99:331–49. [4] Brown RF, Walker AB. Feasibility demonstration of electrostatic precipitation at 1700 F. J Air Pollut Control Assoc 1971;21:617–20. [5] Reijnen K, Van Brakel J. Gas cleaning at high temperatures and high pressures: a review. Powder Technol 1984;40:81–111. [6] Weber E, Hübner K, Pape HG, Schulz R. Gas cleaning under extreme conditions of temperature and pressure. Environ Int 1981;6:349–60. [7] Villot A, Gonthier Y, Gonze E, Bernis A, Ravel S, Grateau M, et al. Separation of particles from syngas at high-temperatures with an electrostatic precipitator. Sep Purif Technol 2012;92:181–90. [8] Gu ZZ, Xi XL, Yang JC, Xu JJ. Properties of RE-W cathode and its application in electrostatic precipitation for high temperature gas clean-up. Fuel 2012;95:648–54. [9] Barranco R, Gong M, Thompson A, Cloke M, Hanson S, Gibb W, et al. The impact of fly ash resistivity and carbon content on electrostatic precipitator performance. Fuel 2007;86:2521–7. [10] Xu JJ, Gu ZZ, Xi XL, Yang JC. Cathode discharge and dust removal characteristics of a lanthanum-tungsten cathode high-temperature electrostatic precipitator. J Eng Therm Energy Power 2011;26:576–81. 634 [in Chinese]. [11] Estcourt VF, Frisch NW. Measuring and reporting fly ash resistivity. IEEE Trans Power Appl Syst 1980;PAS-99:573–81. [12] Bickelhaupt RE. Volume resistivity-fly ash composition relation. Environ Sci Technol 1975;9:336–42. [13] Qi LQ, Yuan YT. Mechanism of the effect of alkali metal on the electrostatic precipitability of fly ash. Fuel 2013;107:848–51. [14] Tang MK, Feng GJ. The analysis on affecting factors of dust ratio resistance and their responding measures. J Jiangxi Univ Sci Technol 2007;28:44–6 [in Chinese]. [15] Yuan YT. Electrostatic precipitation in power plant. 1st ed. Beijing: Chemical Industry Press; 2004 [in Chinese]. [16] Bickelhaupt RE. Electrical volume conduction in fly ash. J Air Pollut Control Assoc 1974;24:251–5. [17] Bickelhaupt RE. Surface resistivity and the chemical composition of fly ash. J Air Pollut Control Assoc 1975;25:148–52. [18] Ahn YC, Lee JK. Physical, chemical, and electrical analysis of aerosol particles generated from industrial plants. J Aerosol Sci 2006;37:187–202. [19] Yamamoto T, Abe T, Mimura T, Otsuka N, Ito Y, Ehara Y, et al. Electrohydrodynamically assisted electrostatic precipitator for the collection of low-resistivity dust. IEEE Trans Ind Appl 2009;45:2178–84. [20] Liang WJ, Lin TH. The characteristics of ionic wind and its effect on electrostatic precipitators. Aerosol Sci Technol 1994;20:330–44.