Experimental Study of Corona Discharge Generated in a Modified Wire - Plate Electrode Configuration for Electrostatic Processes Applications A. Bendaoud, A. Tilmatine, K. Medles, M. Younes, IRECOM, University Djillali Liabès Sidi Bel Abbès 22000, Algeria
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Abstract— Parallel wire-plate electrode arrangements are widely used to generate corona discharges in various electrostatic processes. The present paper analyzes the characteristic features of a slightly-modified such electrode configuration. In all the experiments, a stainless steel wire, diameter 0.18 mm or 0.57 mm was employed as corona electrode, energized from a reversible DC high-voltage supply. The grounded electrode consisted in three plate segments, parallel to the wire and located respectively at 20 mm, 15 (or 10) mm, and 20 mm from it. In a first set of experiments, the current-voltage characteristics of this electrode arrangement were obtained for both polarities of the high-voltage supply, and compared to those of a standard wire-plate electrode system. In a second set of experiments, each plate segment was made of several Aluminum strips insulated from each other and successively connected to a micro-ammeter, in order to characterize the distribution of the corona current at the surface of the grounded electrode. The third set of experiments described in the paper was performed with the corona electrode energized from a rotary spark gap connected at the output of the DC high-voltage supply. Finally, the possibility to apply this electrode configuration for the precipitation of dust contained in flue gases is briefly discussed. Keywords – Corona discharge, electrostatic precipitator, rotary spark gap.
I.
INTRODUCTION
Precipitation of dusts [3], spraying of powders [4], separation of granular mixtures [5], and flue gas treatment [6] are only a few of the industrial applications of the corona discharges. The particles of dust, the powders or the granular materials processed respectively in electrostatic precipitators, sprayers or separators are charged by the ions generated in such discharges. The electric forces that act on the corona charged particulate matter achieve the dust collection, the powder deposition, or the granule separations [7-9]. In the plasma chemical reactors, the corona discharge transfers to the treated gas the energy necessary to decompose the pollutants [10]. A lot of work has already been done to characterize the various types of corona electrodes and to compute the electric field strength, as well as the ionic charge density they generate [12-16]. Indeed, the efficiency of any such electrostatic
O. Blejan, L. Dascalescu IUT - Angouleme University of Poitiers Angouleme, France
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process depends on the maximum magnitude but also on the spatial distribution of the electric field strength and of the ionic charge density in the corona discharge [11]. Various constructions have been described in the technical literature [12, 13], accompanied by current-voltage curves that enabled their comparison under certain well-defined conditions (distance to a grounded electrode, polarity of the high-voltage electrode). Corona discharge in wire-plate electrode arrangements has been thoroughly investigated, mostly in relation with the design of high-voltage DC or AC lines [13]. The experimental studies have clarified important practical issues, such as the onset voltage level of the corona discharge, the power losses, the influence of the ambient conditions (humidity, wind, ..) . The researchers elaborated sophisticated numerical models for calculating the electric field and the space charge generated by a ionizing wire facing a plate electrode [13]. Accurate numerical models have also been validated for the study of two other electrode configurations, which are typical for electrostatic precipitators: wires between parallel plates connected to the ground and co-axial wire-cylinder electrodes. In a recent paper, two of the authors pointed out the spectacular modification of the corona current density distribution generated by a local reduction in the crosssectional area of a standard coaxial wire-cylinder precipitator [13]. They report a significant improvement of the collection efficiency of the modified precipitator, as compared to the standard installations of similar dimensions. Would it be benefic to modify in a similar way the standard wire-plate electrode configurations? The aim of the present work is to give an answer to this question, by characterizing the corona discharge generated between a wire electrode, connected to a DC high-voltage supply, and several plate segments parallel to that wire and located at different distances from it. It is expected that the density of the corona current and the intensity of the electric field will increase in certain sections of the device, improving the corona charging of the particles, while the average speed of the gas will remain practically the same.
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1
(a)
(b) Fig. 1. “Dual-type” corona electrode facing a one-level grounded electrode (model #1) (a) and modified two-level grounded electrode (model #2.1 or 2.2), energized from a reversible DC high-voltage supply; d2 =15 mm for model # 2.1; d2 =10 mm for model # 2.2. Fig. 3. Electrode arrangement energized from a rotary spark gap connected to a DC high-voltage supply.
Fig. 2. Ground electrode arrangement model #2.1 (2.2) with :1 … 12: Aluminum strips.
The experiments were performed with several models of grounded electrodes. Model #1 consisted in a metallic (aluminium) plate, spaced at 20 mm from the ionizing wire, as shown in Fig. 1, a. Models #2.1 and #2.2 were confectioned of twelve Aluminium strips glued at the surface of a two-level insulting support, which was machined as shown in Fig. 1, b. The distance between two strips was 2.2 mm on the lower level, and 1 mm on the upper level of the grounded electrode. The spacing between the ionizing wire and the lower level of ground electrode was d = 20 mm. The upper level of the grounded electrode was located at d2 = 15 mm and d2 = 10 mm from the ionizing wire, for models #2.1 and #2.2, respectively. In some experiments, the various electrode arrangements were energized from a rotary spark gap connected at the output of the reversible DC high-voltage supply (Fig. 3). III.
II.
EXPERIMENTAL SETUP
The corona discharged was generated using a so-called “dual-type electrode”, composed of a thin tungsten wire, length 198 mm, diameter 0.18 mm or 0.57 mm, attached to a copper cylinder, diameter 20 mm, as shown in Fig. 1. The ionizing wire and the supporting cylinder were energized from the same reversible DC high-voltage supply (model SL300, SPELLMAN, Hauppauge, NY). The two elements were parallel to and the plane defined by their axis was perpendicular to the grounded electrode.
EXPERIMENTAL PROCEDURE
In a first set of experiments, the current-voltage characteristics of models #2.1 and #2.2 were compared to those obtained for the standard wire-plate electrode arrangement (model #1), at both polarities of the high-voltage supply. The current was measured with an analog micro-ammeter. The high-voltage was read on the front-panel of the power supply. An experiment was carried out using also a separate highvoltage probe, to confirm the accuracy of the readings. For the second set of experiments, a micro-ammeter was connected successively between the ground and each of the 12 aluminum tapes of the collector electrode.
2
100
I(µA)
two level one level I (µA)
80 60 40 20 0 6
7
8
9 10 11 12 13 14 15
U (kV)
Fig. 4. Current-voltage characteristics obtained at negative polarity of the corona wire, for models #1 (one-level) and #2.1 (two-level).
It was thus possible to determine the distribution of the current density along the two surfaces (inferior level and superior level) of the electrode arrangement. Each experiment was replicated three times. In a third set of experiments, the current-voltage characteristics and the distribution of the corona current at the surface of the grounded electrode were obtained for the wire electrode energized from the rotary spark gap connected at the output of the reversible high-voltage supply. IV.
120 100 80 60 40 20 0
negative polarity positive polarity
5
7
9
11
13
15
17
19
U (kV) Fig. 5. Current-voltage characteristics obtained at negative and positive polarity of the corona wire, for model #2.1 (d2 = 15 mm and wire diameter 0.18 mm)
TABLE I. CORONA ONSET AND SPARK-OVER VOLTAGE FOR MODEL #2.1
Wire diameter [mm]
φw = 0.18 mm
φw = 0.57 mm
Voltage [kV] Current [ȝA]
Uc =7.5; Us =17 Ic = 0.15; Is = 100
Uc = 10; Us =18.5 Ic = 0.3; Is = 72
TABLE II. CORONA ONSET AND SPARK-OVER VOLTAGE FOR MODEL #2.2
Wire diameter [mm]
φw = 0.18 mm
φw = 0.57 mm
Voltage [kV] Current [ȝA]
Uc =6.6; Us = 13.5 Ic = 0.9; Is = 95
Uc = 9.5; Us = 16 Ic = 0.35; Is = 70
RESULTS AND DISCUSSION
A. Current-Voltage Characteristics The current-voltage characteristics obtained for models #1 and #2.1 at negative polarity can be examined in Fig. 4. At a given voltage, the corona current generated by the modified electrode arrangement is higher than that obtained with the standard wire-plate system. The explanation is that the electric field is more intense and hence the corona current density is higher at the upper level of model #2.1. At negative polarity, the corona current is higher, in absolute value, than at a similar high voltage of positive polarity, as can be seen by examining the current-voltage characteristics in Fig.5. The corona onset voltage Uc and the spark-over voltage Us for d2 = 15 mm (Model #2.1) and d2 = 10 mm (Model #2.2) , as well as the respective currents Ic and Is can be read in Tables I and II. This is why the negative polarity is preferred in most electrostatic precipitation applications. However, most of the experiments that will be presented in the following subsections of the paper were done at positive polarity, for which the corona discharge was more stable. B. Corona Current Density Distribution The distribution of current density for model #2.1 (d2 = 15 mm) with wire diameter 0.18 mm , is given in Fig. 6, for U = 17 kV et –17 kV. The density is higher for the negative polarity, in each point at the surface of the collector electrode. Similar results were obtained for model #2.2. The aspect of the current density distribution curves is similar for the 0.57 mm diameter wire. As expected, smaller current densities values were obtained at a given voltage level.
negative polarity
J (A/m2)
positive polarity
0,05 0,04 0,03 0,02 0,01 0,00 0
1
2
3
4
5
6
7
8
9 10 11 12 13 Band
Strip Number Fig. 6. Corona current densities obtained at a high voltage U = +/- 17 kV for model #2.1 (d2 = 15 mm and wire diameter: 0.18 mm).
C. Pulsed Energization The aspect of the pulse generated by the rotary spark gap device can be examined in Fig. 7. At the frequency f = 125 Hz, the corona current is higher, in absolute value, than the value measured obtained at a similar high voltage of f = 62.5 Hz, as shown in Fig. 8. The corona onset voltage is the same for the two frequencies, but the spark-over voltage at a frequency f = 125 Hz is higher than that obtained at f = 62.5 Hz. The overall current is smaller than in the case of DC energization. This observation is confirmed by the current density curves obtained for various pulse frequencies generated by the roraty spark gap (Fig. 9). The superposition of pulsed and DC energization should be examined as a solution for electrostatic precipitator applications.
3
J (A/m 2) f=62,5 Hz
0,03 0,025 0,02 0,015 0,01 0,005 0 0
1
2
3
4
5
f =125 Hz
6
7
8
9 10 11 12 13 BandNumber Strip
Fig. 9. Current dstribution obtained for pulsed energization of the corona wire for model #2.1 (d2 = 15 mm and wire diameter: 0.18 mm)
VI. Fig. 7. High-voltage pulses generated by a rotary spark gap, at the frequency f = 62.5 Hz
[1] [2] [3]
I(µA)
45 40 35 30 25 20 15 10 5 0
f=62,5 Hz
f=125 Hz [4]
[5]
[6]
[7]
10 11 12 13 14 15 16 17 18 19 20 21 22 U(kV) Fig. 8. Current-voltage characteristics obtained for pulsed energization of the corona wire for model #2.1 (d2 = 15 mm and wire diameter: 0.18 mm)
[8]
[9] [10]
V.
CONCLUSIONS
The modified electrode arrangement has current-voltage characteristics that are more advantageous for electrostatic precipitation applications than those of standard devices. At a given size of the electrode system, the total current/total length ratio of the modified design is higher, ensuring better particle charging. Current density distribution mapping might be useful for the interpretation of particle collection efficiency results. These experimental results will serve to validate a numerical model of the corona field generated by the modified electrode arrangement.
[11]
[12]
[13]
REFERENCES
H.J. White, Industrial Electrostatic Precipitation, Reading, MA: Addison-Wesley, 1963. L.B. Loeb, Electrical Corona. Their Basic Physical Mechanisms. Berkeley, CA: Univ. of California Press, 1965. L.M. Dumitran, P. Atten, D. Blanchard, and P. Notingher, “ Drift velocity of fine particles estimated from fractional efficiency measurements in a laboratory-scaled electrostatic precipitator,” IEEE Trans. Ind . Appl., vol. 38, no. 3, May-June 2002, pp. 852-857. G.A. Kallio and D.E. Stock, Flow visualisation inside a wire-plate electrostatic precipitator,” IEEE Trans. Ind. Appl., vol. 26, no. 3, May/June 1990, pp. 503-514. L.M. Dumitran, O. Blejan, P. Notingher, A. Samuila, and L. Dascalescu, “Particle charging in combined corona-electrostatic fields,” Conf. Rec. IEEE/IAS 40 Ann. Meet., Hong Kong, 2005, vol. 2, pp. 1429-1434. A. Bendaoud, A. Tilmatine, K. Medles, M. Rahli, M. Huzau, L. Dascalescu, Characterisation of dual corona electrodes forelectrostatic processes applications, in: Conference Recordings of IEEE/IAS Annals Meeting, Seattle, pp. 1552–1558. K. Adamiak, Particle charging by unipolar ionic bombardment in an AC electric field, IEEE Trans. Ind. Appl. 33 (1997) 421–426. L. Dascalescu, A. Mizuno, R. Tobaze´ on, A. Iuga, R. Morar, M. Mihailescu, A. Samuila, Charges and forces on conductive particles in roll-type corona-electrostatic separators, IEEE Trans. Ind. Appl. 31 (1995) 947–956. L. Dascalescu, An Introduction to Ionized Gases: Theory and Applications, TUT Press, Toyohashi, 1993. L. Dascalescu, R. Morar, A. Iuga, A. Samuila, V. Neamtu, I. Suarasan, Charging of particulates in the corona field of roll-type electroseparators, J. Phys. D: Appl. Phys. 27 (1994) 1242–1251. L. Dascalescu, A. Iuga, R. Morar, V. Neamtu, I. Suarasan, A. Samuila, D. Rafiroiu, Corona and electostatic electrode for hightension separators, J. Electrostat. 29 (1993) 211–215. L. Dascalescu, A. Samuila, D. Rafiroiu, A. Iuga, R. Morar, Multipleneedle corona electrodes for electrostatic processes application, IEEE Trans. Ind. Appl. 35 (1999) 543–548. Blejan, O.; Notingher, P.; Dumitran, L.M.; Younes, M.; Samuila, A.; Dascalescu, L.,Experimental Study of the Corona Discharge in a Modified Coaxial Wire-Cylinder Electrostatic Precipitator, Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE, Volume , Issue , (2007) 1111 – 1114.
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