Environ. Sci. Technol. 2004, 38, 1545-1553
Particle Collection Efficiency and Particle Re-entrainment of an Electrostatic Precipitator in a Sewage Sludge Incineration Plant T H O M A S F E R G E , †,‡ J U ¨ RGEN MAGUHN,† HANNES FELBER,§ AND R A L F Z I M M E R M A N N * ,†,‡,| Institut fu ¨r O ¨ kologische Chemie, GSF Forschungszentrum, Neuherberg, Germany, Stadtentwa¨sserungswerke Mu ¨ nchen, Mu ¨ nchen, Germany, Abteilung Umweltchemie und Prozessanalytik, BIfA Bayerisches Institut fu ¨ r Angewandte Umweltforschung und -technik, Augsburg, Germany, and Analytische Chemie, Lehrstuhl fu ¨ r Festko¨rperchemie, Institut fu ¨ r Physik, Universita¨t Augsburg, Augsburg, Germany
In several recent studies it was shown that high atmospheric loads of submicrometer particles in the size range below 500 nm have strong impact on human health. Therefore, extensive research concerning the reduction of fine particle emissions is needed to further improve air quality. Regarding health effects, especially the emission characteristics of fine and ultrafine particles emerging from anthropogenic sources such as combustion processes are of special interest. This study shows that the emission characteristic of an electrostatic precipitator (ESP) due to re-entrainment of fine particles and their subsequent release into the atmosphere can be significantly lowered by application of different operating conditions. For this purpose the particle collection efficiency of an ESP was studied in a municipal sewage sludge incineration plant. Particles were sampled under different operating conditions upstream and downstream from the ESP, and the particle number concentrations were measured simultaneously with aerodynamic particle sizers. In addition, the size distribution of the particles downstream from the ESP was measured with high time resolution by an electrical low-pressure impactor to investigate the particle re-entrainment into the flue gas. To determine the influence of operating conditions, different rapping cycles were investigated regarding their impact on the collection efficiency and the subsequent particle re-entrainment.
Introduction The control of particulate matter emission is an important aspect of industrial air pollution engineering. Historically, particulate emission control mainly dealt with the reduction of mass of the particulate material released into the atmosphere. However, epidemiological studies pointed out the impact of particles with diameters below 2.5 µm on human * Corresponding author (at GSF Forschungszentrum) phone: +49(0)89-3187-4544; fax: +49-(0)89-3187-3510; e-mail:
[email protected]. † GSF Forschungszentrum. ‡ Universita ¨ t Augsburg. § Stadtentwa ¨ sserungswerke Mu ¨ nchen. | BIfA Bayerisches Institut fu ¨ r Angewandte Umweltforschung und -technik. 10.1021/es034709s CCC: $27.50 Published on Web 01/23/2004
2004 American Chemical Society
health (1, 2). Although it is not yet clear whether the mass, the number concentration, or the surface area constitutes the most important parameter concerning health effects, according to recent studies, fine and ultrafine particles are suspect to have a considerably stronger impact on human health than coarse particles (3-5). This implicates that the particle number concentration, which is a reasonable measure for description of fine particles, may be a determinant for negative impacts on the environment and health. Especially anthropogenic particulate emission sources such as power plants, traffic, and industrial processes contribute considerably to the fine and ultrafine aerosol loading of the atmosphere. Anthropogenic combustion processes result in the formation of particles in the size range below 2.5 µm (6, 7), which are also known as carriers of toxic constituents (8). Therefore, the investigation of air pollution control technologies regarding the emission characteristics of fine and ultrafine particles will provide useful information for further improvement of particle abatement technologies. Electrostatic precipitators (ESPs) are air pollution control devices widely used in industry for control of particulate emission by capturing particles using electrostatic fields. In detail, electrostatic precipitation consists of three steps: (1) charging the particles by a high-voltage electric discharge, (2) collection of the particles on the surface of an oppositely charged electrode, and (3) cleaning the surface of the collection electrode. Thus, the collection efficiency of an ESP mainly depends on the rate of charging the particles. The charging mechanisms, ESP characteristics (e.g., geometry, applied voltage), and resulting collection efficiency have been extensively studied (see, for example, refs 9-15). There are two main charging mechanisms: diffusion charging and field charging. The chargeability of particles decreases with decreasing size; in contrary the mechanical mobility increases with decreasing size. Therefore, a minimum of collection efficiency can be found between 0.1 and 1 µm. This U-shape collection efficiency curve has been verified in many experimental (11, 16-18) and theoretical (12) studies. Furthermore, the collection efficiency of an ESP is dependent also on the elemental composition of particles (19, 20). Although the mass collection efficiency can exceed 99%, penetration of submicrometer particles easily reaches tens of percent when calculated in terms of number concentrations (13, 14). Therefore, extensive research has been done concerning several aspects, such as improving the collection efficiency for small particles (21, 22). On the other hand, besides the effects of particle charging, the collection efficiency of an ESP deteriorates with time due to accumulation of particles on the collection electrodes. ESPs perform best when particle deposits on the collection electrodes have a resistivity below 1010 Ω/cm (23). At higher resistivities the field strength in the space between the ionizing electrode and the top of the dust layer is reduced. This can cause a breakdown in the electrical field, and back corona can take place, lowering the efficiency of the ESP. To remove the dustcake from the surface of the electrodes and thus prevent a significant decline in efficiency, mechanical rappers are employed. Particles tend to fall into the hopper underneath the ESP; however, a fraction of dislodged particles remains suspended in the air and is re-entrained into the flue gas stream (23). Back corona naturally impairs the performance of an electrostatic precipitator. For laboratory-scale precipitators the efficiency can decrease during an operation period of 300 min from 95% down to 80% (10). The authors of the study investigated a laboratory-scale single-stage ESP and, VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (a) Schematic drawing of the incineration plant with the two sampling points. (b) Schematic drawing of the sampling and analysis system. in addition to collection efficiency, also paid attention to the performance subject to different rapping mechanisms. For instance, they examined the influence of time intervals between single raps on the rapping efficiency, a measure for removal of the dust layer, and on the particle re-entrainment. Their findings suggest that due to agglomeration the particle re-entrainment can be lowered by increasing the time interval between single raps. However, the collection efficiency of the ESP decreases constantly over time due to growing dust layers. Therefore, there exists an optimum time interval which represents a compromise between both effects. This is also in good agreement with investigations by Wadenpohl and Lo¨ffler (24), who report on agglomeration of soot particles. They found that agglomeration takes place in three steps: deposition of particles, agglomeration, and subsequent possible re-entrainment as larger agglomerates. Also industrial-scale ESPs have been studied regarding their particle collection efficiency (18) and the effects of electrode cleaning on re-entrainment of particles (25). In the latter study also a dependency of the rate of re-entrainment on time intervals between single raps was observed. However, these measurements only considered total suspended matter (25) or did not pay particular attention to the re-entrainment of particles due to rapping of the electrodes (18). In this work the effects of different rapping cycles for cleaning the electrodes of an ESP of a municipal sewage sludge incineration plant are investigated. On one hand, the collection efficiency of particles with diameters above 0.5 µm was determined by parallel measurements with two aerodynamic particle sizers (APSs). On the other hand, the re-entrainment of fine and ultrafine particles was investigated. Here, measurements were performed highly timeresolved with an electrical low-pressure impactor (ELPI), 1546
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which allows the measurement of ultrafine particles down to 30 nm. The objectives of this study are to elucidate whether results obtained from laboratory measurements can be transferred to industrial processes and to find out whether the performance of air pollution control devices, as, for example, an ESP, can be improved by applying appropriate operating conditions.
Methods The measurements were performed at a sewage sludge incineration plant with an operational capacity of 3 t/h dry matter. A schematic drawing of the plant with the sampling points is depicted in Figure 1a. The investigated ESP (model ETH-1×12×4.5-0.3, Rico/AEG, To¨nisvorst, Germany) is the first step of applied flue gas cleaning technologies downstream from the boiler chamber and consists of one filter zone with 12 electrodes, which can be operated from 30 to 65 kV. The clearance between the single electrodes measures 300 mm. When operated under full load of the incineration plant, the electrodes are typically set to a voltage of 45 kV, resulting in an electric field strength of 1.5 × 105 V/m. The electrodes are arranged in parallel and are rapped periodically by a rotating system of hammers. Particle measurements were performed during full load of the incineration plant and two different rapping cycles for cleaning the electrodes. The so-called short cycle is operated as follows. One of the electrodes is cleaned by rapping for 10 s followed by an intermission of 90 s with no rapping applied. After this period the next electrode is rapped for 10 s followed again by an intermission of 90 s. This cycle is performed continuously. The second operating condition which was investigated is the so-called extended rapping cycle. Here the rapping events
are prolonged to 30 s, and intermissions between these events are extended to 10 min. The combustion aerosol in the waste gas of the incineration plant was analyzed regarding its size distribution using on-line techniques. Particle size distributions were measured up- and downstream from the ESP with two APSs (model 3320, TSI Inc., Minnesota; measuring range 0.5-20 µm). The fine and ultrafine particle fraction was measured on-line with an ELPI (Dekati Ltd., Tampere, Finland) only downstream from the ESP. The ELPI consists of 12 stages, which cover a size range from 30 nm to 10.2 µm. Isokinetic sampling was performed with a home-designed sampling system. An adjusted sampling pump (Stro¨hlein GmbH, Korschenbroich, Germany) was employed for drawing the flue gas sample stream. The sampling probe (quartz glass), all sampling lines, and the first part of the dilution system were heated to temperatures above 150 °C to avoid condensation effects in the probe. Large particles were intercepted by a heated cyclone (home-designed). The cut-point diameter of the cyclone was estimated to be 40 µm according to ref 26. The sampled aerosol was rapidly diluted by a factor of 10000 upstream and a factor of 1000 downstream from the ESP with clean dry air by heated ejector dilution units (VKL 10-E, Palas GmbH, Karlsruhe, Germany) prior to size analysis. The high dilution ratios were obtained by installing several dilution units in line, whereas isokinetic dilution was provided by adjusting the air flows at the ejector nozzles via the appropriate pressure of the dilution air, which was known due to previous calibration (Palas GmbH). All sampling lines were kept as short as possible to avoid large particle losses. Specific losses in the sampling line were not addressed because the sampling systems up- and downstream from the ESP were identical; hence, the calculation of number concentration based collection efficiency was not influenced by particle losses. Furthermore, in the interesting size range accessible with the ELPI (between 30 nm and 1 µm) losses due to gravitational settling as well as diffusion losses can be neglected (9). The sampling and analysis schemes are depicted in Figure 1b; a detailed description of the sampling system can be found in ref 27. Due to a lack of detector response at the low end of the size distribution, the APS underestimates the number concentration of particles with diameters below 1.0 µm (28). With the two APS systems applied, size distributions from 1.0 to 20 µm could be measured simultaneously up- and downstream from the ESP. The size distributions were averaged over 90 s periods. For further analysis regarding the collection efficiency the single-channel data of the APS systems were summed up to result in accumulated particle number concentrations, which give the number of all particles in a specified size range. Due to the above-mentioned underestimation of the APS below 1.0 µm and the fact that above 10 µm the particle number concentrations can be neglected (see Figure 2), only particles with diameters from 1 to 10 µm were considered for the calculation of the accumulated particle number concentrations. The accurate formula
∑
a-b
)
dN dD ∫ dD b
a
with a and b meaning the lower and upper limits of the respective size range taken into account, was approximated according to
∑
1-10
ch42
)
∆N
∑ ∆D∆D ch8
and
∑
1-2.5
ch23
)
∆N
∑ ∆D∆N ch8
where ∑1-10 means the accumulated number concentration from 1 to 10 µm and ∑1-2.5 the accumulated number
FIGURE 2. Comparison of typical particle size distributions. (A) Size distribution of particles above 1 µm measured with the APS and ELPI. (B) Size distribution of particles between 30 nm and 10 µm measured with the ELPI as well as mass distribution of the aerosol in the same size range. concentration from 1 to 2.5 µm. The limiting channels of the APS data were chosen to fit into the desired size range (in this case channel 8 as lower limit and channels 23 and 42 as upper limits, respectively). Data of the size distribution and number concentrations of fine and ultrafine particles with diameters below 1 µm were accessible via the ELPI measurements. These are notably important for evaluation of the re-entrainment of fine and ultrafine particles into the flue gas stream due to rapping. Measurements with the ELPI were performed only downstream from the ESP because only one instrument was available at the time; therefore, no simultaneous measurements were possible. Thus, no ESP efficiency data for fine and ultrafine particles are available in the current study. However, the size distributions were recorded with a very high time resolution (1 s intervals). Therefore, detailed information about the re-entrainment of particles was obtainable also for the short rapping cycle. Additionally, the collected particle samples of the ELPI were weighed to determine the mass distribution of the aerosol. For this, the collection plates of the ELPI were coated with cleaned and VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dried aluminum foil, which was weighed before and after several hours of measurement. The long sampling time was necessary to obtain weighable amounts of aerosol on each of the collection substrates. To maintain the conductivity needed for simultaneous on-line number concentration measurements, the foils were not greased, certainly resulting in a slight underestimation of the particle mass due to bouncing effects. The collection efficiency of the ESP was calculated from the APS data according to
(
ηESP ) 1 -
∑ ∑
)
us 1-10
ds 1-10
× 100
where ∑us represents the accumulated number concentration upstream from the ESP and ∑ds the accumulated number concentration downstream from the ESP in the respective size range. Note that the collection efficiency calculated here only includes particles above 1 µm and therefore does not include any effect of re-entrainment of fine and ultrafine particles. Also, the collection efficiency used in this work is based on number concentrations and therefore does not correspond to conventional efficiency values, which usually are mass-based (PM2.5 and PM10).
FIGURE 3. Typical particle size distribution above 1 µm measured with the APS upstream and downstream from the ESP.
Results and Discussion Figure 2 shows a typical size distribution downstream from the ESP measured with an APS and an ELPI. In addition, the mass distribution of the aerosol derived from the gravimetric analysis of the impaction plates of the ELPI is shown. The measurements of the size distribution with both the APS and ELPI are in good agreement as can be seen in Figure 2A. On the other hand, Figure 2B shows the difference between the number concentration and mass distribution of the aerosol. While the main fraction of particles referring to number concentration is below 1 µm, the mass distribution shows that the major part of the particle mass is due to particles above 1 µm. This size range is described very well by the APS measurements. Although conventional PM standards are mass-based and collection efficiencies in industrial applications usually refer to these mass-based standards, we calculate the collection efficiency from the APS number concentrations. In PM2.5 and PM10 standards, the efficiency is technically based on reduction of total suspended matter (TSP). Because the mass distribution is made up to a major degree by particles above 1 µm, the efficiency values derived only from number concentrations of particles in this size range can be compared to the usual collection efficiency. Taking into account that particles below 1 µm only contribute to a lesser extent to the overall measured particle mass, the error in the estimated collection efficiency is relatively low. Furthermore, size-resolved measurements of short-time effects during and after rapping require a high time resolution, which is not achievable with size-resolved impactor-based mass distribution measurements. In contrast, the number concentration especially of the fine and ultrafine particle fraction was one of the main objectives in this work, which could be observed very well with the on-line ELPI measurements. This size fraction is especially important regarding health effects (5) of aerosols. Typical particle size distributions measured with the APS up- and downstream from the ESP are depicted in Figure 3. At both sampling points the shapes of the measured size distributions resemble each other. This shows that the ESP has relatively consistent collection efficiency in the size range above 1 µm. Furthermore, the particle number concentration above 10 µm is relatively low, and the main fraction of particles is in the size range below 2.5 µm. This is shown in more detail in Figure 4 too. Here the accumulated particle 1548
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FIGURE 4. Comparison of the accumulated particle number concentrations ∑1-2.5 and ∑1-10. concentrations ∑1-2.5 and ∑1-10 are compared for two measurements upstream and downstream from the ESP. Only a slight difference between the two size ranges can be seen for both sampling points. This indicates that the main fraction adding up to the accumulated number concentration consists of particles below 2.5 µm as can be expected from known properties of number size distributions of combustion aerosols (6, 7, 27, 29, 30). Downstream from the ESP ∑1-2.5 covers 97.5% of ∑1-10; however, upstream this fraction covers only 91.5%. This indicates a slightly decreased collection efficiency of the investigated ESP for particles smaller than 2.5 µm possibly due to the mentioned minimum in collection
FIGURE 5. Particle collection efficiency for the short (top) and the extended (bottom) rapping cycles measured with the APS. Only the ∑1-10 values were taken into account. efficiency, which extends even to particle sizes above 1 µm (U-shape collection efficiency curve). However, the calculation of the collection efficiency of the ESP is based on the ∑1-10 values throughout this work. For this calculation an averaged value for the accumulated number concentration upstream from the ESP was used to reduce the effects of possible sampling errors on the calculated values. Due to the high concentration of dust particles, it was only possible to sample consistently for short periods (i.e., to avoid clogging of the sampling system). The comparability of the measurements was ensured because the incineration plant was operated throughout under constant burning conditions. Measurements downstream from the ESP were performed continuously to receive long-term data especially for evaluation of the particle re-entrainment. The collection efficiencies for both the short and the extended cycles are shown in Figure 5. Note that these efficiency values are based on number concentrations and not on particle mass. The single rapping events are clearly visible in the case of the extended cycle. Here, the efficiency temporarily drops to values of 83-87%. This is due to a significant amount of re-entrained particles in the considered size range (1-10 µm) downstream from the ESP. Neglecting these short-time effects, the ESP shows a collection efficiency of approximately 96% for both cycles. The extended cycle has a slightly higher collection efficiency, when the rapping itself is not taken into account (97% compared to 95.5% for the short cycle). Immediately after the rapping events these efficiency values increase slightly in the case of the extended cycle. It can be further observed that the collection efficiency
decreases between the single raps from about 97% to approximately 95.5%. This can be interpreted in terms of formation of dust layers. This decrease in efficiency is comparable to the values determined by Kim and Lee in the laboratory-scale ESP (10). However, these effectssthe immediate drop in collection efficiency during rapping as well as the slow decrease between rappingsscannot be observed for the short cycle, which all along exhibits a rather constant level of collection efficiency. However, this is also due to the 90 s average data obtained by the APS measurements compared to the time span of only 100 s for a whole rapping cycle. Due to the fact that with APS measurements no information on the fine and ultrafine particles can be obtained, an ELPI was used for investigation of the size distribution of this fraction downstream from the ESP. The size resolution is lower compared to that of a scanning mobility particle sizer (SMPS), because only 12 size channels are available. However, due to the nonscanning operation of the ELPI, a high time resolution can be achieved. A complete size distribution can be measured every second, so that the short rapping cycle could be investigated too, regarding its effect on particle re-entrainment. Figure 6 shows a three-dimensional plot of the particle size distribution vs time for both the short and the extended rapping cycles. Because of the high time resolution of the ELPI, even the concentration peaks during the rapping events of the short cycle are well resolved. Note that for better representation of the second maximum during the extended cycle here dN values are shown. The main difference between the two cycles regarding the particles below 1 µm is clearly visible. During rapping the number concentration is very high for both cycles due to the mechanical removal of deposited particles from the electrodes. However, between the rappings, the number concentrations differ to a great extent. This is depicted in more detail in Figure 7, where the size distributions are shown for both the short and the extended cycles. The exact values are listed in Table 1. Size distributions during rapping are displayed as solid lines; between these events the size distributions are represented by dotted lines. Additionally, in Figure 7b, the size distribution 60 s after the rapping is shown as a dashed line. During rapping the number concentration of particles with diameters above 1 µm is increased by a factor of only 2-3 for the short cycle and a factor of 6-9 for the extended cycle (see Table 1). This is in agreement with the course of the collection efficiency derived from the APS measurements (see Figure 5). In contrast, the number concentration of particles below 1 µm shows a different behavior. In this size range, during rapping the re-entrained particles exhibit roughly the same number concentration, regardless of which cycle is applied. The major difference appears in the intermittent periods, when no rapping occurs. During the short cycle, the number concentration of particles between single rapping events stays at a quite high level, whereas for the extended cycle the number concentration is decreased by a factor of 6. This higher level of re-entrained particles is obviously an effect of the applied rapping cycle as is shown in Figure 7b. Here, additionally the size distribution measured shortly after the rapping (60 s time delay) is depicted. Regarding the re-entrainment characteristics in the early postrap period, this size distribution is comparable to the distribution measured between the rappings of the short cycle, where a 60 s delay corresponds to the intermittent period. The early postrap size distribution of the extended cycle shows no major difference from the size distribution recorded over the whole intermittent period. On the other hand, it is different compared to the corresponding size distribution of the short cycle (in which the postrap period VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Three-dimensional plots of the particle size distribution time course downstream from the ESP measured with the ELPI. The enlarged plots showing 15 min measurements show clearly the elevated level of fine and ultrafine particles during the short cycle. represents the intermittent period as well). Thus, the number concentration of fine and ultrafine particles decreases much faster during the extended cycle in the early postrap period. These results indicate that particles undergo agglomeration and aggregation during deposition, according to the literature (10, 25). Additionally, the rate of aggregation in the dust layer is dependent on the time between the raps. For 1550
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the short cycle it is obvious that only thin layers can be formed on the surface of the electrodes between the single rappings. These thin layers are loosely held together. Thus, these layers are disaggregated during rapping, and particles are easily re-entrained into the flue gas channel in relatively high concentrations. In contrast to the short cycle, during the intermittent periods of the extended cycle, the dust layers can agglomerate more effectively. Here the aggregating force
FIGURE 7. Comparison of size distributions of particles in the flue gas downstream from the ESP measured with the ELPI: (A) short cycle, (B) extended cycle. Straight lines represent particle size distributions during rapping; dotted lines denote the intermittent periods when no electrodes are rapped. The dashed line in (B) represents the postrap period of the extended cycle (for details see the text). of deposited particles is sufficiently strong, so that particles and larger agglomerates, when impacted with shear forces (provided by the hammer system), become separated from the electrodes and either enter the hopper or are re-entrained mainly as larger particles. For industrial ESPs, the collection efficiency can be as low as 50% for particles in the size range from 0.3 to 0.6 µm (18). Strand et al. argue that this could be due to additional agglomeration of even smaller particles, which are transported out of the ESP due to re-entrainment and therefore have an additional effect on the even low efficiency for this particle size. By extending the time between cleanings of the electrodes, this agglomeration can take place more effectively, thus reducing the re-entrainment of particles in this size range. These findings are consistent with the laboratory results found by Kim and Lee (10), who observed a dependence of the particle re-entrainment by rapping on time for a laboratory-scale ESP. The concentration of re-entrained particles increased with time and decreased again after reaching a maximum. From these results they predicted that, depending on thickness, the dust layers become disaggregated during rapping to different extents. After a distinct layer thickness is reached, the aggregating force is sufficiently strong, causing the particle agglomerates not to break up and either to be re-entrained as larger particles or to fall into the hopper. In agreement with this study it was observed at the plant that, due to the longer time intervals between rappings of the electrodes, the formed dust layers can aggregate more efficiently and fine particles are released into
the flue gas stream as larger aggregates with diameters above 1 µm. Accordingly, during the short cycle the time between the rapping events is not sufficient for agglomeration to take place. Therefore, no particles with diameters above 1 µm are re-entrained as can be seen by comparing the particle size distributions during rapping of both cycles. In contrast to the extended cycle, where a second maximum arises due to agglomeration, during the short cycle the size distribution stays unimodal. In this context also the difference in the postrap periods, which is shown in Figure 7, can be discussed. For the short cycle the particle concentrations of re-entrained particles stay at elevated levels when compared to those of the postrap period of the extended cycle. Consequently, the duration of the rapping itself might have an influence on the reentrainment of particles. During the short cycle, agglomeration of particles is not sufficiently strong to prevent fine particles from being re-entrained into the flue gas channel. Furthermore, relatively short rapping of the electrodes results in even looser dust layers releasing small particles also after the rapping, thus producing high number concentrations of re-entrained fine particles even in the intermittent phase of the cycle. In contrast, the extended cycle allows the agglomerates to grow for a longer time. Hence, these dust layers exhibit stronger cohesion and thickness. A rapping of 30 s removes these thicker layers for the most part. Accumulated material on the electrodes, which possibly remains on the electrode surfaces, is aggregated to such a degree that no re-entrainment of fine particles can be observed. On the other hand, higher numbers of larger aggregates are released from the electrodes immediately during rapping, showing the agglomeration on the electrodes. Figure 8 shows averaged size distributions of particles downstream from the ESP (exact values are listed in Table 2). The size distributions were averaged over a period of 1 h for both cycles to eliminate the short-term effect of reentrainment during rapping and to evaluate differences of size distributions downstream from the ESP due to both operating conditions. The size distributions of particles with diameters larger than 1 µm are quite similar. In contrast, the behavior of the fine particle fraction is rather different for both cycles. During the short cycle the number concentration of particles below 1 µm is increased by a factor of 3 compared to the values observed for the extended cycle. The higher concentration values of particles with diameters of about 0.077 µm during the extended cycle remain unclear. Possibly, this is due to agglomeration of even smaller particles. These ultrafine particles are readily precipitated in the ESP; for what reason they are not released due to rapping still remains unclear. Although there were no data available for the fine and ultrafine particle fraction upstream from the ESP, it can be concluded that the overall collection efficiency for particles in the size range below 1 µm is dependent not only on the charging mechanism or the mechanical mobility of particles in the ESP, but also on the applied rapping system. Thus, it is possible to increase the efficiency of ESPs by adjusting the operating conditions regarding the short-time effects of particle re-entrainment downstream from the ESP too, with special respect to the fine and ultrafine particle fraction. This is especially important for further flue gas cleaning, as there also exists a minimum of collection efficiency also on filters for particles in the size range from 50 to 500 nm (9). The particles in this size range are generally too large for diffusion to become an effective mechanism for particle reduction and too small for impaction or interception to be effective. Because these competing mechanisms are most effective in distinct size ranges, filters generally have the above-mentioned size, which gives minimum collection efficiency. Therefore, ESP operating VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. ELPI Data for the Particle Size Distribution Measured for Both Cycles during and between Rapping Events number concn [1/(cm3 µm)] particle diameter (µm)
rapping
short cycle no rapping
rapping
extended cycle no rapping
postrapping
0.041 0.077 0.13 0.2 0.31 0.5 0.8 1.272 2 3.14 5.14 8.23
0 0 1.14E+06 4.751E+05 2.486E+05 7.471E+04 2.944E+04 1.071E+04 7.397E+03 1.406E+03 1.333E+02 0
0 0 6.072E+05 3.072E+05 1.759E+05 5.770E+04 2.075E+04 5.703E+03 3.475E+03 5.474E+02 5.040E+01 0
0 2.932E+05 9.532E+05 2.404E+05 1.725E+05 5.246E+04 5.042E+04 3.199E+04 2.261E+04 5.016E+03 7.140E+02 0
0 1.352E+05 1.968E+05 7.326E+04 3.667E+04 9.262E+03 7.799E+03 4.960E+03 3.239E+03 5.795E+02 7.942E+01 0
0 1.364E+05 2.114E+05 7.833E+04 3.700E+04 9.633E+03 6.865E+03 4.617E+03 2.612E+03 5.414E+02 8.527E+01 0
to further ameliorate the emission characteristics of industrialscale incinerators.
Acknowledgments This study was carried out within the scope of the GSF-Focus “Health relevance of aerosols”, which coordinates aerosolrelated research within the GSF Research Center. We thank H. Nordsieck and T. Adam for contributions and support during the measurement campaign. T.F. thanks the Deutsche Bundesstiftung Umwelt (DBU) for a Ph.D. scholarship.
Literature Cited
FIGURE 8. Averaged size distributions measured with the ELPI downstream from the ESP. The averaging time was 60 min (6 rappings of the extended cycle and 36 rappings of the short cycle).
TABLE 2. Averaged Size Distributions Derived from ELPI Measurements for Both Rapping Cycles number concn [1/(cm3 µm)] (averaged size distribution)
number concn [1/(cm3 µm)] (averaged size distribution)
particle diameter (µm)
short cycle
extended cycle
particle diameter (µm)
short cycle
extended cycle
0.041 0.077 0.13 0.2 0.31 0.5
0.000E+00 8.455E+03 7.604E+05 3.694E+05 1.955E+05 6.181E+04
0.000E+00 1.329E+05 2.408E+05 8.731E+04 4.628E+04 1.190E+04
0.8 1.272 2 3.14 5.14 8.23
2.393E+04 7.030E+03 4.330E+03 7.229E+02 7.199E+01 1.152E-01
9.302E+03 5.798E+03 3.776E+03 7.080E+02 9.305E+01 0.000E+00
conditions where particles in this critical size window are reduced as far as possible are a useful tool for optimization of particle abatement strategies. Due to agglomeration effects on the surface of the collection electrodes, it is possible to reduce the particle re-entrainment very effectively by extending the rapping cycle. This makes up an important contribution to minimizing the emissions of particles in the size range below 1 µm, which are critical regarding their effects on human health. Further investigations should be performed focusing on advanced changes of operating conditions (as there should, for example, exist an optimum time interval for rapping cycles) as well as on different types of gas cleaning units (such as dry and wet ESPs, wet scrubbers, etc.). The intensive knowledge of the behavior of different gas cleaning aggregates will allow improving the operation of existing technologies 1552
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Received for review July 3, 2003. Revised manuscript received December 2, 2003. Accepted December 8, 2003. ES034709S
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