Experimental Studies of Novel Constructions of Discharge Electrode in ESP for the Reduction of Fine Particles and Mercury Emission

Transcription

1 Świerczok et al. 29 Experimental Studies of Novel Constructions of Discharge Electrode in ESP for the Reduction of Fine Particles and Mercury Emission A. Świerczok and M. Jędrusik Wrocław University of Technology, Poland Abstract Flue gases from coal-fired power plants in Poland are mainly cleaned by electrostatic precipitators (ESPs). New restrictive legislation (Directive of IED) regarding the emission of particulate matter to the atmosphere will require novel solutions and/or modification of the existing electrostatic precipitators in order to increase the collection efficiency, particularly in PM2.5 range. Although the mass percentage of PM2.5 particles in total emission is small, the impact of these fractions on human health is relatively large, particularly due to high content of mercury. For these reasons, new constructions of emission electrode applied in ESPs have been investigated in this paper. The current-voltage characteristics and current density distribution over the collection electrode for different types of emission electrodes have been presented. It was shown that for particles from coal-combustion boilers, the collection efficiency of fine particles can be increased for certain constructions of the emission electrode. The size of particles precipitated on the collection electrode varies depending on the place of deposition, relative to the axis of emission point. It was also confirmed that the percentage of mercury is higher in small particles fractions than in the larger ones. The results indicate that proper selection of emission electrode construction allows increasing total collection efficiency and decreasing of the emission of fine particles of high mercury content. It seems, therefore, that the selection of emission electrode is one of the prospective solutions, which will allow comply with new restrictive emission regulations. Keywords Electrostatic precipitator, discharge electrode, corona current, fly ash I. INTRODUCTION Electrostatic precipitators (ESP) are the most numerous exhaust gases cleaning devices in Polish coalfired power plants. A forecast for the nearest years predicts brown and a hard coal as the main power energy source. Characterized by low pressure drop and high cleaning efficiency, electrostatic precipitators (ESPs) becomes the investment and operational competitive devices, especially if their PM2.5 fractional collection efficiency could be increased [1]. Furthermore, there exists a possibility to reduce the mercury emission from coal combustion processes by mercury oxidation and its removal with ESP - together with dust particles [2, 3]. Although there are many factors influencing ESP performances, the construction of discharge electrode is still one of the most important factors because this electrode is not only the ion source for dust particles charging but also builds the electric field causing the motion of charged dust particles towards the collection electrode [4, 5]. Properly designed discharge electrode should ensure high corona current level for a given high voltage [6, 7] with possibly uniform current density distribution on collection plate. It is expected that more uniform corona current density distribution will improve total collection efficiency of ESP [8-11]. Hitherto many previous studies carried out by leading research centers point out that the mechanism of particle collection in regions of high corona currents differs from those with low or even zero corona current [12-14]. These reasons led the authors to examine how the Corresponding author: Arkadiusz Świerczok address: arkadiusz.swierczok@pwr.edu.pl discharge electrode construction affects the precipitation efficiency of industrial dust (from the combustion of coal). Particular attention has focused on the collection efficiency of fine particles and mercury adsorbed on their surface. II. LABORATORY STUDIES At the beginning of these studies it was decided to examine the influence of discharge electrode construction and uniformity of corona current distribution on collection efficiency of industrial dusts. Series of measurements of total and fractional collection efficiencies were carried out for selected constructions of discharge electrode (DE) mounted on a laboratory model of ESP. For those measurements, fly ashes collected from brown coal fired PC boiler (Pulverized Coal fired boiler) have been used. Its size fractional analyses were done with optical meter Mastersizer S of Malvern Instruments Ltd. Mercury content in the fly ash has been estimated with atomic spectroscopy absorption method (ASA), selective for enriched Hg sample at trap inside analyzer by amalgamation with gold, with apparatus AMA-254 by Altec. For these tests, two constructions of discharge electrode, shown in Fig. 1, installed in a horizontal ESP model, were used. The test bench comprised of one-stage electrostatic precipitator (ESP) model chamber with a set of discharge and collecting electrodes, air ducts, dust particle feeder, high voltage supply unit and exhaust fan. The ESP model chamber was made of plexiglass to enable visual Presented at the 14 International Symposium on Electrohydrodynamics (ISEHD 14), in June 14

2 3 International Journal of Plasma Environmental Science & Technology, Vol.9, No.1, APRIL 15 Fig. 1. Typical discharge electrodes: a) (diameter of the needle 2.2 mm, curvature radius of the tip.3 mm) and b), dimension in mm m out ηt 1 (1) m where: m in - fly ash mass at the inlet of ESP, m out - fly ash mass at the outlet of ESP In order to estimate the influence of discharge current density on fine particle collection it was necessary to determine the fractional collection efficiency. For this purpose, fly ash samples were collected at the inlet and outlet of an ESP, and particulate size distributions were carried out with a Malvern analyzer. From these distributions, the fractional collection efficiency was determined from the following equation: q3 inlet ( 1 ηc ) q3outlet ( η f q (2) in 3inlet where: q 3inlet percentage of particles in a given size interval at the ESP inlet, ( ) q 3outlet percentage of particles in a given size interval at the ESP outlet, ( ). B. Current-voltage Characteristics Fig. 2. Experimental setup for measuring of ESP efficiency with marked cross sections of taking of dust samples. 1 fly ash feeder, 2 high voltage supply unit, 3 final filter, 4 exhaust fan with controlled rotation, 5 gravimetric dustmeter. observations of the phenomena occurring in the interelectrode space. The collecting electrodes were made of flat steel plates, also spikes of discharge electrodes were steel. Additionally, the discharge electrodes could be easily replaced. Basic dimensions of the ESP were as follows: length of the active electric field L = 2 mm; active height of electrodes H = 45 mm; spacing between discharge electrodes s = 17 mm. A sketch of the ESP is shown in Fig. 2. A. Measurements of Collection Efficiency Measurements of collection efficiency were carried out with air flow at temperature ca. 293K and relative humidity of =45%. The ESP has been energized with DC high voltage of negative polarity connected to the discharge electrode. The concentration of fly ash particles at ESP model outlet was measured by means of gravimetric dust meter, isokinetically taking gas samples with a probe. The dust was collected on a highly-efficient absorbent paper of a measuring filter. The fly ash was injected by a feeder to inlet channel with concentration of.3 g/m 3. The total collection efficiency was estimated by measuring the fly ash mass at the inlet and at the outlet and calculated according to a formula: The current-voltage (I-U) characteristics of selected discharge electrodes were measured in clean air and dustfree collection electrodes, by measurement of corona current with micro-ammeter connected between the collection electrode (CE) and ground. C. Corona Current Distribution For selected designs of discharge electrode, corona current density distribution was measured on the collection electrode surfaces as on testing bench shown in Fig. 3. Implementation of a moving measuring probe with a surface area of 1 cm 2 - in a CE plane, enables determination of corona current distribution on CE surface in measurement areas of 3 mm (Vert x Hor). In each of the 1 measurement points (27 ) the probe current is measured with digital pico-ammeter no 6485 of Keithley Fig. 3. Photograph of experimental ESP model used for measurements of current density distribution: 1 HV supply, 2 discharge electrode, 3 measurement area, 4 collection electrode, 5 pico-ammeter. 4 5

3 Świerczok et al. 31 X - direction, length of ESP X - direction, lenght of ESP Cumulative percent, vol % Y - direction, height of ESP Y -direction, height od ESP Fig.4. Fractional size distribution of fly ash, cumulative size distribution. Corona current, Fig.5. Current-voltage characteristics of ESP. III. RESULTS OF LABORATORY BENCH MEASUREMENTS A. Fly Ash Size Analysis Supply voltage, kv Results of fly ash size analysis are shown in Fig. 4. The fly ash median diameter was d 5 = 63 µm and modal diameter d max = 89 µm, that is typical for fly ashes from coal fired PC boilers. B. Discharge Electrode Current-voltage Characteristics Discharge electrode current-voltage characteristics are presented in Fig. 5. It can be noticed that the I-U characteristic of electrode is more steep than for, and the corona onset voltage for is at a level of U = 1 kv, whereas for the corona onset is at U = kv. Fig. 6. Discharge current (in amps) distribution on the collection electrode surface for: electrode, electrode for supply voltage of U = 5 kv. Collection efficieny, % Supply voltage, kv Fig. 7. Dependence of average collection efficiency on DC supply voltage for different tested discharge electrode constructions; average gas flow velocity in the ESP model chamber w g =.8 m/s. C. Corona Current Distribution The results of corona current distribution measured with moving measurement probe are shown in Fig. 6. For the construction of discharge electrode the corona discharge current covers much larger surface at the collection electrode than the. For this electrode the standard deviation of corona current distribution is lower and equals RSD = 61% and the average value of corona current is higher, i d = 91.7 na. For the electrode these values were as follows: RSD = 162%, i d = 3.3 na that presents rather poor covering of collection electrode with corona current of low average values.

4 32 International Journal of Plasma Environmental Science & Technology, Vol.9, No.1, APRIL Active regions Fractional collection efficieny, Non active regions Fractional collection efficieny, Fig. 8. Fractional collection efficiency of ESP for two tested discharge electrodes: supplied with DC voltage of 3 kv, supplied with DC voltage of kv. D. Collection Efficiency For fly ash having size distribution shown in Fig. 4 the collection efficiency characteristics are shown in Fig. 7. The results are given as average values of three measurements done for the same conditions. The ESP fractional collection efficiency was estimated on the base of fly ash size distribution analyses for ESP inlet and outlet accordingly to equation (2). Some examples of the results for selected voltage levels are shown on Fig. 8. From the measuring tests it can be concluded that the total as well as the fractional collection efficiency for electrode is higher than for. The measurement results had also shown that electrode is better than electrode with respect to collecting fine particles. The biggest differences occur for particles with diameter from 2 to 1 µm, for supply voltage of kv and from 2 to 7 µm for a voltage of 3 kv. It can also be noted that the construction of appears particularly advantageous with respect to total collection efficiency for lower supply voltages (greater collection Fig. 9. Photographs of fly ash samples deposited on collection electrodes for electrodes and. Cumulative percent, vol % Cumulative percent, vol % virgin fly ash virgin fly ash Fig. 1. Characteristics of fractional size analysis (cumulative size distribution) of fly ash collected at the outlet measurement cross section x = 19 mm active region, of high corona current; non active region, with very low corona current, supplied with DC voltage of kv.

5 Świerczok et al. 33 Fig. 11. Changes of particle median diameter along the ESP length for two tested discharge electrodes: ;. TABLE I MERCURY CONTENT IN FLY ASH SAMPLES FROM DIFFERENT CROSS SECTIONS OF THE ESP DE type Median diameter, m Median diameter, m active regions non active regions 1 1 x - lenght of ESP, mm active regions non active regions 1 1 x - lenght of ESP, mm Mercury content Cross section mg/kg Active region Non active region x = 3 mm x = 99 mm x = 19 mm x = 3 mm x = 99 mm x = 19 mm efficiency differential - similar like in case of total collection efficiency). Taking that into account, it was decided to compare fractional size analyses of fly ash samples collected from different ESP regions (Fig. 9): different cross sections along the ESP (inlet, center and outlet), from regions opposite to the spike of tested DE (active regions with high corona current), and also from non active regions (i.e. regions practically without current). The ESP for these tests was energized with HV supply voltage of kv DC. The measurement cross sections were fixed at the ESP model inlet (x = 3 mm), at its center (x = 99 mm) and at the outlet (x = 19 mm). Besides the fly ash samples fractional size analysis the mercury content was also determined. Examples of fractional size analysis are presented on Fig. 1. In each case the composition of the fly ash at the inlet to the model (virgin fly ash) was given. The obtained results have shown that fine particles are collected at the outlet part of ESP; the tendency to shifting the fly ash size distribution characteristics towards fine sizes is clearly visible for both and electrodes as well as for active and non active regions. Especially important is the fact that fine particles are better collected in non active regions than in active regions that was also confirmed by fly ash size analysis from other measurement cross sections of the ESP model. This effect is presented in Fig. 11, where are shown changes of particle median diameter along the ESP length. The characteristics presented in Fig. 11 show that mean size of dust particles collected in active regions is more shifted towards larger particles than in non active regions. The particle median diameter significantly decreases along the ESP in active regions but only slightly in non active regions. E. Mercury Contents During these tests also the mercury content in collected fly ash samples was determined, and the results are listed in Table I. The results show that in active regions of the ESP, mercury content depends mainly on a place of sampling; the mercury content in the fly ash increases along the ESP length. That agrees with other observations [2] and confirm that mercury is mainly transported by fine particles which are prevailing at the ESP outlet (see Fig. 11). In the non active regions of ESP, the dependence of mercury content in fly ash sampling place (along the ESP length) is not so clear and most probably results from inaccuracy of size analyses carried out for too small samples available (from ESP outlet). It may be seen that for electrode the mercury content in fly ash samples taken from all measuring cross sections of ESP is larger than for electrode (for active regions). IV. CONCLUSION The obtained test results show that construction of discharge electrode (DE) has a significant influence on ESP collection efficiency. The type of electrode gives much better collection efficiency of ESP cleaning exhaust gases after a coal fired PC boiler than electrode. Electrode with more aggressive currentvoltage characteristics and more uniform corona (discharge) current distribution is characterized by better

6 34 International Journal of Plasma Environmental Science & Technology, Vol.9, No.1, APRIL 15 total as well as fractional collection efficiency for most of the analyzed particle sizes. Obtained results confirm a thesis that corona current distribution uniformity has an effect on improvement of collection efficiency, mainly for fine particles. There has been observed an interesting effect in non active regions (with very low or close to zero discharge current values) are precipitated mostly fine particles (with smaller median diameter), however, bigger particles are deposited in regions with high corona current. Most probably it may be explained by an electric ion wind effect [1]. Taking into account the amount of dust precipitated in active regions (large quantity) and non active regions (small quantities) [13] it seems to be advisable to distribute the corona (discharge) current over the entire collecting electrode area. The mercury content in samples precipitated on collecting electrode (CE) depends mainly on fractional size of the sample; the finer is the fly ash size the higher is mercury content. The application of discharge electrode (DE) type with uniform corona current distribution may lead to reduction of mercury emission from coal combustion processes [15]. Considerations," presented at Power Gen 94 Conference, Orlando, Florida, [12] D. Blanchard, P. Atten, and L. M. Dumitran, "Correlation between current density and layer structure for fine particle deposition in a laboratory electrostatic precipitator," IEEE Transactions on Industry Applications, vol. 38, pp , 2. [13] J. Miller, H. -J. Schmid, E. Schmidt, and A. J. Schwab, "Local deposition of particles in laboratory-scale electrostatic precipitator with barbed discharge electrodes," in Proc. 6th International Conference on Electrostatic Precipitation, ICESP 1996, Budapest, Hungary, pp [14] M. Chambers, G. J. Grieco, and I. C. Caine, "Customized rigid discharge electrodes show superior performance in pulp & paper applications," in Proc. 8th International Conference on Electrostatic Precipitation, ICESP 1, 1, Birmingham, Alabama, USA. [15] S. J. Lee, Y. C. Seo, H. N. Jang, K. S. Park, J. I. Baek, H. S. An, and K. C. Song, "Speciation and mass distribution of mercury in a bituminous coal-fired power plant," Atmospheric Environment, vol., pp , 6. REFERENCES [1] A. Jaworek, A. Krupa, and T. Czech, "Modern electrostatic devices and methods for exhaust gas cleaning: A brief review," Journal of Electrostatics, vol. 65, pp , 7. [2] M. Jedrusik and A. Swierczok, "The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator," Journal of Electrostatics, vol. 71, pp , 13. [3] F. Scala and H. L. Clack, "Mercury emissions from coal combustion: Modeling and comparison of Hg capture in a fabric filter versus an electrostatic precipitator," Journal of Hazardous Materials, vol. 152, pp , 8. [4] K. R. Parker, Applied Electrostatic Precipitation, Blackie Academic & Prof., London [5] H. J. White, Industrial Electrostatic Precipitation (prep.), International Society for Electrostatic Precipitation, Library of Congress Catalog Card No , USA, 199. [6] A. Mizuno, "Electrostatic precipitation," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 7, pp ,. [7] J. D. McCain, "Estimated Operating V-I curves for rigid frame discharge electrodes for use in ESP modeling," in Proc. 8th International Conference on Electrostatic Precipitation ICESP 1, Vol. 1, Birmingham, Alabama, USA. [8] J. Miller, E. Schmidt, and A.J. Schwab, "Improved discharge electrode design yields favourable EHD-field with low dust layer erosion in electrostatic precipitator," in Proc. 6th International Conference on Electrostatic Precipitation, ICESP 1996, Budapest, Hungary, pp [9] P. J. McKinney, J. H. Davidson, and D. M. Leone, "Current distributions for barbed plate-to-plane coronas," IEEE Transactions on Industry Applications, vol. 28, pp , [1] J. Miller, B. Hoferer, and A. J. Schwab, "The impact of corona electrode configuration on electrostatic precipitator performance," in Proc. 3rd International Conference on Applied Electrostatics, 1997, Shanghai, China, pp [11] G. J. Grieco, "Electrostatic Precipitator Discharge Electrode Geometry and Collecting Plate Spacing - Selection