Experimental investigation on charging characteristics

Journal of Electrostatics 67 (2009) 799–806

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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat

Experimental investigation on charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion enhanced by positive corona pulsed ESP Fei Xu a, b, Zhongyang Luo a, *, Wei Bo a, Lei Zhao a, Xiang Gao a, Mengxiang Fang a, Kefa Cen a a b

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Zhejiang Electric Power Design Institute, Hangzhou 310012, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 December 2008 Received in revised form 7 February 2009 Accepted 17 June 2009 Available online 3 July 2009

The electrostatic precipitator (ESP) has been extensively used for collecting aerosol particles emitted from coal combustion, but its collection efficiency of PM2.5 (Particulate matter whose aerodynamic diameter is less than 2.5 mm) is relatively low due to insufficient particle charging. The positive pulsed ESP is considered to enhance particle charging and improve collection efficiency. A laboratory-scale pulsed ESP with wire-plate electrode configuration was established to investigate the particle charging and penetration efficiency under controlled operating conditions of different applied impulse peak voltages, impulse frequencies, dust loadings and residence times. The results show that most particles larger than 0.2 mm are negatively charged, while most particles smaller than 0.2 mm are positively charged. For a given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 mm, and there is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. Under the same operating conditions, the particle penetration efficiency decreases with increasing impulse peak voltage and impulse frequency, but increases as the dust loading increases. The results imply that residence time of 4 s is optimum for particle charging and collection. PM2.5 number reduction exceeding 90% was achieved in our pulsed ESP. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: PM2.5 Positive corona Pulsed ESP Pulsed corona discharge Charging characteristics Penetration efficiency

1. Introduction ESPs have been used as one of the most common particulate control devices for decades at power plants [1–4]. The ESP removes more than 99% of the particulates from flue gas in terms of mass concentration. However, the particle collection efficiency of the conventional ESP in terms of number concentration is relatively low, especially for PM2.5, which results in a large number of aerosol particles emitted in the atmosphere [5–7]. New stricter environmental regulations for fine particle emissions with PM2.5 are taken in serious consideration throughout the world. With the adoption of increasingly stringent particulate emission regulations, more attention is being paid to PM2.5, which is leading to the development of ESP to improve collection efficiency [8–10]. Since most of PM2.5 escape from the ESP due to insufficient particle charging, short positive pulse corona energization has been used to enhance the particle charging and overcome back corona problems, which

* Corresponding author. E-mail address: [email protected] (Z. Luo). 0304-3886/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2009.06.002

appear at high resistivity dusts [11–14]. The short positive pulse corona energization of an ESP generates much higher electron density than DC energization and, hence, enhances the particle charging due to much larger mobility of electrons. As a result, it improves the particle collection efficiency. In order to maximize the particle charging, we need to know charges, which particles of a certain size can hold for given operating conditions of pulsed corona discharge. Mitchner and Self [15] carried out a series of experiments to measure the charge-to-mass ratio of the particles enhanced by corona charging and concluded that the charging process became less effective at higher concentration with constant voltage. J. H. Ji and J. Hwang [16] measured the particle charging when the polarities of two corona chargers were either positive or negative. Their results also implied that with constant voltage, the particle charging depended on the particle mass loading, i.e. higher mass loading will cause lower particle charging. Researchers in Zhejiang University achieved high capture efficiency of PM2.5 from coal-fired power plant by pulsed corona discharge combined with DC agglomeration [17]. However, the particle charging characteristics enhanced by pulsed corona discharge have not been discussed in more detail. In this paper,

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F. Xu et al. / Journal of Electrostatics 67 (2009) 799–806

8

7

6 3 4

13 2 5 9 1

10 11

Fig. 3. Electric diagram of the pulsed power supply.

12

1.Air compressor; 2.Flow-meter; 3.Vibration feeder; 4.Pulsed ESP; 5.Electrode; 6.Negative DC power supply; 7.Pulsed power supply; 8.Oscilloscope; 9.Diluter; 10.ELPI; 11.Computer; 12.Vacuumpump; 13.Exhaust fan Fig. 1. Sketch of the experimental system.

a wire-plate ESP with positive pulse corona energization was designed and established. We carried out a series of experiments to investigate the particle charging characteristics under controlled operating conditions of different impulse peak voltages (Up), impulse frequencies (F), dust loadings (N) and residence times (t). Finally, the relationships between particle charging and penetration efficiency were also discussed. 2. Experimental setup and methodology 2.1. Experimental setup As shown in Fig. 1, the experimental system mainly consists of a fly ash feeder, a pulsed ESP, and a particle measurement system using electrical low pressure impactor. The test aerosol particles were first dried and then generated by a vibration feeder with constant dosage characteristics. After that, they were mixed with particle-free air at room temperature (25 C) to obtain desired flow rate and then introduced into the pulsed ESP, whose sketch is shown in Fig. 2. The pulsed ESP is of wire-plate structure, which consists of four electric fields with two polished stainless-steel plates as the collection plates and eight wire discharge electrodes in middle. The negative DC power supplies the first and last electric fields, of which the first electric field offers particle precollection and the last electric field offers particle agglomeration and collection after they were highly charged. While the second and third electric fields are supplied by positive pulse power, which enhances particle charging in our ESP. The diameter of the discharge electrode used is 1.0 mm with an effective length of 200 mm. The distance between wire and plate is 60 mm, and the wires are placed at intervals of 100 mm. The plates are grounded.

A short-duration and high-voltage positive pulsed power supply was used to apply varying impulse peak voltages and impulse frequencies to the central discharge electrode. The electric diagram of the pulsed power supply is shown in Fig. 3. A digital oscilloscope (TDS3034B, Tektronix, USA) with a high-voltage probe (P6015A, Tektronix, USA) was connected to the discharge electrodes to measure the applied positive pulse peak voltage and negative DC voltage. 2.2. Measurement of particle number distribution and electrostatic charges An isokinetic sampler and two-stage dilution system were coupled in line with an electrical low pressure impactor (ELPI, Dekati, Finland) to measure the particle number concentrations upstream and downstream of the ESP in real time. The ELPI setup for particle concentration and charging measurement is shown in Fig. 4. The particles are sampled through a unipolar corona charger, where particle surfaces are saturated with positive charges. Subsequently, the charged particles enter a low pressure cascade impactor where particles are separated according to their inertia and consequently their aerodynamic diameter. An ELPI with a filter stage was also used to characterize the charges on the aerosol particles ranging from 0.02 mm to 2.5 mm, corresponding to a total of 11 stages. Upon particle deposition on the impactor collection plates of the individual stages, charges carried by the particles are

Inlet Corona charger

Electrometer

Impactor negative DC positive pulse grounded outlet

inlet

Outlet discharge electrode Fig. 2. Sketch diagram of the pulsed ESP.

Vacuum pump Fig. 4. ELPI setup for particle concentration and charging measurement.


801

Table 1 List of experimental operating conditions. Case 1 2 3 4 5 6 7 8 9 10 11 12

Dust loading (particles/cm3) 5

3.4 10 3.4 105 3.4 105 3.4 105 3.4 105 3.4 105 1.1 106 3.5 106 3.4 105 3.4 105 3.4 105 3.4 105

Applied impulse peak voltage (kV)

Applied impulse frequency (Hz)

Flow rate (m3/h)

Residence time (s)

Temperature ( C)

25 38 48 53 53 53 53 53 53 53 53 53

300 300 300 300 200 100 300 300 300 300 300 300

17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3 34.6 23.0 13.8 11.5

4 4 4 4 4 4 4 4 2 3 5 6

25 25 25 25 25 25 25 25 25 25 25 25

software. The instrument had been turned on for one hour before the measurements were preformed according to the manufacturer’s specifications. 2.3. Experimental operating conditions A list of the experimental operating conditions performed is shown in Table 1. The air flow rate was controlled to provide different residence times in the pulsed ESP. The aerosol particle charging and penetration efficiency characteristics were established under the pulsed ESP as a function of different applied impulse peak voltages (case 1,2,3,4), impulse frequencies (case 4,5,6), dust loadings (case 4,7,8) and residence times (case 4,9,10,11,12). 2.4. Data analysis The particle diameter dependent penetration efficiency, h, was evaluated with the number concentration of the aerosol particles in a certain size fraction measured at the inlet and outlet of the ESP using the equation below: Fig. 5. Oscillogram of negative DC and positive impulse with different peak voltages.

h ¼ 1 measured with sensitive (fA level) multichannel electrometers. The measured current values are inverted to yield particle number concentrations using transfer functions provided by the manufacturer. Both the negative DC power of the ESP and the corona charger voltage of the ELPI were turned off while measuring the particle charging. Particle bounce and re-entrainment were minimized by the application of a 1/20 diluted Apiezon-L grease carbon tetrachloride solution on the impactor collection plates. The solvent was allowed to evaporate before assembling the impactor stages. With the filter stage, an adaptor was employed. The air flow rate of the ELPI was 10 l/min, generated by a Sogevac model SV25 vacuum pump (Leybold, France). The particle size distributions and electrostatic charges were measured and recorded using Dekati ELPI 4.0

ðNinlet Noutlet Þ Ninlet

(1)

where Ninlet is the aerosol particle number concentration at the ESP inlet (particles/cm3), Noutlet is the corresponding aerosol particle number concentration at the ESP outlet (particles/cm3). The charges on each impactor stage were derived from the electric current data by calculating the area under the curve in the current versus time plot for each particular stage. The average number of charges per particle, a, for a given size fraction was calculated by dividing the charges by that particular particle number using the following equation:

a ¼

I

(2)

hQNe

Table 2 Survey of fly ash resistivity. Temperature ( C)

Moisture content

1st data (U$cm)

2nd data (U$cm)

3rd data (U$cm)

Average (U$cm)

25 100 110 120 130 140 150 160 170 180

10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

1.03Eþ11 1.08Eþ11 1.17Eþ11 1.36Eþ11 1.83Eþ11 2.14Eþ11 2.23Eþ11 3.68Eþ11 1.26Eþ12 1.31Eþ13





600000

Particle number concentration (particles/cm3)

Particle number concentration (particles/cm3)

802

500000 low dust loading middle dust loading high dust loading

400000 300000 200000 100000

3500000 3000000 low dust loading middle dust loading high dust loading

2500000 2000000 1500000 1000000 500000 0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

Paricle diameter (µm)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Paricle diameter (µm)

Fig. 6. Particle number distributions and accumulative distributions at different dust loadings.

where I is the current on each stage (fA), h is the penetration efficiency on each stage, Q is the flow rate of ELPI(10 l/min), N is the particle number concentration (particles/cm3), e is the elementary charge(1.602 1019 C).

48 kV and 53 kV respectively, with the impulse frequency constant at 300 Hz. The pulse rise time demonstrated in Fig. 5 was about 300 ns with a shortest pulse width of 550 ns. 3.2. Particle resistivity and number concentration distributions

3. Results and discussion 3.1. Applied power supply outputs The pulsed ESP was supplied by both negative DC power and positive pulse power. The negative DC power offered particle collection in the first and last electric fields. The negative DC voltage was about 15 kV. The positive pulse power supply was designed to generate different applied impulse peak voltages and impulse frequencies, upgrading the amount of particle charge. The minimum rise time of the pulse generated was about 300 ns and the maximum peak voltage was about 60 kV. The impulse frequency ranged from 7 Hz to 300 Hz. Fig. 5 shows the waveforms of the negative DC and positive impulses applied in the experiments. As shown in Fig. 5, the impulse peak voltages given were approximately 25 kV, 38 kV,

The fly ash particle resistivity was measured by a DR type resistivity analyzer. We characterized the particle resistivity under different test temperatures with 10% moisture for three times. Measurements were made on ascending and descending mode of temperatures between 25 C and180 C. The experimental data is shown in Table 2. It is found that the experimental fly ash is of high resistivity. The results in Table 2 also show good data reappearance. An electric low pressure impactor (ELPI) with charger on was employed to measure the aerosol particle number concentration distributions at different dust loadings. The number distributions and their accumulative distributions are shown in Fig. 6. The measured size fraction ranged from 0.02 mm to 2.5 mm, which was divided into 11 fractions. The total number concentrations were about 3.4 105 particles/cm3, 1.1 106 particles/cm3 and 3.5 106

0

4.5

-400

-600

-800

-1000

Average numberof charges per particle

Average number of charges per particle

-200

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

25kV 38kV 48kV 53kV

-1200 0.01

0.1

Particle diameter (um)

-1400 0.01

0.1

1

Particle diameter (µm) Fig. 7. Number of elementary charges versus particle size with peak voltage as a parameter. Minus means negative charge and plus means positive charge. F ¼ 300 Hz, N ¼ 3.4 105 particles/cm3, t ¼ 4 s.


0.26 0.24

Penetration efficiency

0.22

25 kV 38 kV 48 kV 53 kV

0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.01

0.1

1

Particle diameter (µm) Fig. 8. Grade efficiency versus particle size with peak voltage as a parameter. F ¼ 300 Hz, N ¼ 3.4 105 particles/cm3, t ¼ 4 s.

particles/cm3, which were defined as low, middle and high dust loadings, respectively. As shown in Fig. 6, for all dust loadings, PM2.5 occupied a large particle number proportion in the total number, and a bimodal distribution was found with the peak values at 0.07 mm and 0.48 mm. 3.3. Effects of impulse peak voltage on particle charging and penetration efficiency Experiments were carried out to investigate the effects of impulse peak voltage on the particle charging and penetration efficiency. The air flow rate was controlled at about 17.3 m3/h to provide residence time of approximate 4 s in the pulsed ESP. Experiments were performed by changing the impulse peak

803

voltage with the dust loading and impulse frequency constant at 3.4 105 particles/cm3 and 300 Hz, respectively. By equation (1) and equation (2), the average number of charges per particle and the penetration efficiency were calculated. The results are shown in Figs. 7 and 8, respectively. In Fig. 7, in order to make it clear, two combined curves were given, in which minus means negative charge and plus means positive charge. Fig. 9, Fig. 11 and Fig. 13 below are also in the same case. Because of the different values of electron and positive ion mobilities, the pulse corona discharge is a bipolar charging process, which consists of two periods, the electron charging in pulse corona discharge duration and the ion charging after the pulse corona discharge duration [18,19]. During the pulse corona discharge, most of the particles are negatively charged by electrons due to a field charging mechanism. The amount of negative charges captured is dependent on its diameter. The larger the particle is, the more negative charges it captures [20–22]. After the pulse corona discharge, a large number of electrons get lost, which results in positive ions remaining in space. At the same time, a diffusion charging mechanism takes place between the positive ions and particles because of the random thermal motion of the positive ions in the gas, especially for ultra-fine particles [23]. The smaller the particle is, the stronger the thermal motion is. Hence, two competing simultaneous processes take place: particles are negatively charged by the field charging and neutralized by the diffusion charging. As a result, the final particle charging state depends on the combined effects of the field charging and diffusion charging mechanisms. As shown in Fig. 7, most particles larger than 0.2 mm are negatively charged, and the larger the particle is, the more negative charges it captures. However, most particles smaller than 0.2 mm are positively charged, and the smaller the particle is, the more positive charges it captures. This indicates that for particles larger than 0.2 mm, the field charging mechanism is predominant, while for particles smaller than 0.2 mm, the diffusion charging mechanism is predominant [24]. And for particles with a diameter near 0.2 mm, the field charging and diffusion charging mechanisms are equivalent, which results in charge neutralization. As a bipolar

100

-200 -300 -400 -500 -600 -700 -800 -900 -1000 -1100 -1200



0 -100

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

100 Hz

-0.5

-1300

0.01

200 Hz

0.1

300 Hz


-1400 -1500 0.01

0.1

1

Particle diameter (µm) Fig. 9. Number of elementary charges versus particle size with pulse frequency as a parameter. Minus means negative charge and plus means positive charge. Up ¼ 53 kV, N ¼ 3.4 105 particles/cm3, t ¼ 4 s.

804


penetration efficiency decreases as the particle diameter increases; while for particles smaller than 0.2 mm, the penetration efficiency increases as the particle diameter increases. The highest penetration efficiency is for particles with a diameter near 0.2 mm due to insufficient particle charging. The results show a negative correlation between the particle penetration efficiency and the average number of charges per particle.

0.24 0.22

100 Hz 200 Hz 300 Hz


0.20 0.18 0.16 0.14 0.12

3.4. Effects of impulse frequency on particle charging and penetration efficiency

0.10 0.08 0.06 0.04 0.02 0.01

0.1

1

Particle diameter (µm) Fig. 10. Grade efficiency versus particle size with pulse frequency as a parameter. Up ¼ 53 kV, N ¼ 3.4 105 particles/cm3, t ¼ 4 s.

charging process, the particle equilibrium charging state depends on the combined effects of the field charging and diffusion charging mechanisms. The average number of charges per particle increases with increasing impulse peak voltage. It has been known that with higher applied impulse peak voltage, the electric intensity increases, so there are more electrons and positive ions produced in the ESP. Hence, the particle charging is enhanced by increasing impulse peak voltage until the spark-over limit. In our experiments, peak voltage of 53 kV is just lower than the spark-over voltage, which provides the best particle charging. As shown in Fig. 8, the particle penetration efficiency under the pulsed ESP has been significantly degraded due to the enhancement of particle charging. For particles larger than 0.2 mm, the

Experiments were carried out to investigate the effects of impulse frequency on particle charging and penetration efficiency. Experiments were performed by setting the dust loading at about 3.4 105 particles/cm3 and the impulse peak voltage constant at 53 kV, which were considered to be the optimum operating conditions for particle charging. The residence time was controlled at about 4 s in the pulsed ESP. Results are represented in Fig. 9. As shown in Fig. 9, the impulse frequency was found to play an important role in particle charging. The average number of charges per particle increases rapidly with the increase of impulse frequency. The larger the impulse frequency is, the more energy is produced in the electric field in unit time, which results in higher particle charging. The maximum particle charging effect was obtained at impulse frequency of 300 Hz. Typical particle penetration efficiencies at different applied impulse frequencies are shown in Fig. 10. For a given applied dust loading and impulse peak voltage, the particle charging amount increases with the increase of impulse frequency. As a result, the particle penetration efficiency is affected greatly by the impulse frequency. Fig. 10 shows that with the applied dust loading at about 3.4 105 particles/cm3 and impulse peak voltage at 53 kV, the larger the impulse frequency is, the lower the particle penetration efficiency is, which can be explained by the increasing average number of charges per particle. The results also show a negative

200

-200 low dust loading middle dust loading

-400

-600

-800

-1000

-1200



0

-1400

0.01

4.5

high dust loading

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.01

0.1


0.1

1

Particle diameter (µm) Fig. 11. Number of elementary charges versus particle size with dust loading as a parameter. Minus means negative charge and plus means positive charge. Up ¼ 53 kV, F ¼ 300 Hz, t ¼ 4 s.


charges each particle captures. With relatively low dust loading condition, corona current almost did not change compared to that in the case of no dust loading. However, the situation would be different with higher dust loading for test (about 3.5 106 particles/cm3), in which condition the particle charging amount were reduced compared to the case of relatively lower dust loading. Fig. 12 gives the particle penetration efficiency as a function of dust loading. This indicates that the particle penetration efficiency also increases with increasing dust loading, which is mainly due to insufficient particle charging. The results imply that the particle penetration efficiency depends on the particle loading, higher particle loading will cause lower particle charging.

0.26 0.24


0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.01

805

low dust loading middle dust loading high dust loading 0.1

3.6. Effects of residence time on particle charging and penetration efficiency

1

Particle diameter (µm) Fig. 12. Grade efficiency versus particle size with dust loading as a parameter. Up ¼ 53 kV, F ¼ 300 Hz, t ¼ 4 s.

correlation between the particle penetration efficiency and the impulse frequency. 3.5. Effects of dust loading on particle charging and penetration efficiency Effects of dust loading on particle charging and penetration efficiency were then investigated. Experiments were performed by setting the applied impulse peak voltage and impulse frequency constant at 53 kV and 300 Hz, respectively. The residence time was controlled at about 4 s in the pulsed ESP. Typical results are represented in Figs. 11 and 12. As shown in Fig. 11, it was found that the average number of charges per particle would decrease if dust loading was high in the ESP. It has been known that for a certain given impulse energy, the larger the particle number is, the smaller

Figs. 13 and 14 present the average number of charges per particle and particle penetration efficiency as a function of gas residence time in the pulsed ESP. Experiments were performed by setting the applied impulse peak voltage and impulse frequency constant at 53 kV and 300 Hz, respectively. The dust loading was controlled at about 3.4 105 particles/cm3. The flow rate was adjusted from 11.5 m3/h to 34.6 m3/h to provide gas residence time of 6 s to 2 s in the pulsed ESP. As shown in Fig. 13, it was found that the average number of charges per particle increases as the residence time increases until 4 s. After that, the particle charging amount does not increase any more, which means the particles are almost satured charged at residence time of 4 s. Besides, gas ion wind and gas turbulence induced by gas velocity are another important parameters to particle charging. In Fig. 14, we find that for the comprehensive influences of gas velocity and residence time of 4 s, PM2.5 number reduction exceeding 90% is achieved in our ESP. The results indicate that the particle charging amount and penetration efficiency are strongly influenced by the gas residence time. For our pulsed ESP, residence time of 4 s is optimum for particle charging and collection.

200

-200

-400

-600

-800

-1000

-1200

-1400



0

2s 3s 4s 5s 6s

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0.01

0.1


-1600 0.01

0.1

1

Particle diameter (µm) Fig. 13. Number of elementary charges versus particle size with residence time as a parameter. Minus means negative charge and plus means positive charge. Up ¼ 53 kV, F ¼ 300 Hz, N ¼ 3.4 105 particles/cm3.

806


0.22 2s 3s 4s 5s 6s

0.20


0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.01

0.1

1

Particle diameter (µm) Fig. 14. Grade efficiency versus particle size with residence time as a parameter. Up ¼ 53 kV, F ¼ 300 Hz, N ¼ 3.4 105 particles/cm3.

4. Conclusions In this paper, a laboratory-scale wire-plate ESP with pulsed corona energization was proposed. Experimental studies were performed to investigate the charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion. ELPI was developed for measuring particle charges. Polarity and charge level at different particle size fractions were used to characterize the electrostatic charge profile of PM2.5. It provides a better understanding of electrostatic charging of PM2.5 enhanced by pulsed corona discharge. The experimental results show that in our pulsed ESP, for particles larger than 0.2 mm, most are negatively charged, while for particles smaller than 0.2 mm, most are positively charged. For a certain given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 mm. There is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. The average number of charges per particle and particle collection efficiency increase with increasing impulse peak voltage and impulse frequency, but decrease if the dust loading is high in the ESP. Residence time of 4 s with a fixed peak voltage of 53 kV is found to be optimum to achieve PM2.5 number reduction exceeding 90% in our pulsed ESP.

Acknowledgements This work is part of the Project Supported by State 863 High Technology R&D Project of China (No. 2007AA061804) and Science

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