Particle-charge magnitudes in electrostaticprecipitationE. T. Hignett, Ph.D.

SynopsisThe charge magnitudes acquired by fly-ash particles in the process of electrostatic precipitation have beenmeasured over ranges of treatment times, gas velocities and particle diameters. The measurements weregenerally in agreement with existing theories with two notable exceptions: the rate of charging wasconsiderably lower than predicted and the charge acquired by particles larger than about 50fim wasfound to increase more rapidly with diameter than the square law established for particles below this size.The latter effect is believed to be due to the greater surface area of the more irregular larger particles.

List of symbolsa = particle radiuse = electronic chargeE = electric field strength/ = corona current per unit length of discharge electrode

N = ion concentrationp = charging constantq = particle-charge magnitudeQ = limiting particle-charge magnitudet = timee = relative permittivity of the dust

/A = ion mobilityT = time constant

used which could be inserted into the laboratory precipitatorat a number of points along the duct to enable the rate ofcharging to be determined. To measure the charge on indi-vidual particles, it would have been necessary to detectelectric pulses from as many as 106 particles per secondentering the sampling probe with particle-charge magnitudesin the range 10~l6C to 10~"C, and the measurements wouldnecessarily have been made in the presence of electricaldisturbance from the coronal discharge. It was thereforedecided to measure the bulk charge on the dust collected inthe probe using samples of dust of narrow size range.Except where stated, carbon-free fly ash, obtained by heatinga typical fly ash to 800° C, was used.

1 IntroductionThe theory of particle charging in electrostatic preci-

pitation proposed by Pauthenier and Moreau-Hanot1 isbased upon the assumptions that the particles are smoothspheres and that they move in a constant electric field. Inpractice, the particles may have shapes other than spherical(only the finer particles of fly ash appear to be smoothspheres when examined under a microscope), and the electricfield strength decreases from a high magnitude close to thedischarge electrode to remain approximately constant acrossthe outer two thirds of the gap towards the collectingelectrode.2 However, measurements of the particle-chargemagnitude made by Edmondson,3 Hewitt,4 Pauthenier andMoreau-Hanot,1 Penney and Lynch,5 and White,6-7 show anorder-of-magnitude agreement with the theory. All of thesemeasurements were made downstream from the corona dis-charge zone in which the particles were charged, the theoreticalcharging rate quoted by Rose and Wood8 being assumed.With the exception of some measurements made by Whiteat the outlet of a full-scale electrostatic precipitator, all thecharge measurements were made using laboratory apparatus.

fn most of these experiments the particles were smoothspheres of diameters less than 1/xm, liquid droplets and solidparticles being used. Only those experiments carried outby Pauthenier and Moreau-Hanot covered the range ofparticle size, typically 1 /i,m-100/zm, which is of interest in theelectrostatic precipitation of pulverised fuel ash. Tt was con-sidered that a knowledge of the charge magnitudes acquiredby particles of fly ash in an electrostatic precipitator would beimportant in directing future research effort towards improve-ment in industrial electrostatic precipitation. For this purposeit was essential that fly ash should be used in the experiments,as it Was expected that the charge magnitudes might varyfrom the theoretical values due to the surface roughness andnonsphericity of the particles.

This paper describes measurements of the particle-chargemagnitude acquired by fly-ash particles in a laboratoryelectrostatic precipitator which were made as part of a studyof fundamental aspects of the electrostatic precipitation ofpulverised fuel ash (fly ash), A charge-measuring probe was

Paper 5339 J, first received 22nd September 1966 and in revised form13th March .1967Dr. Hignett is with the Central Electricity Generating Board, NorthEastern Region, Leeds 2, Yorks., England

PROC. 1EE, Vol. 114, No. 9, SEPTEMBER 1967

2 Apparatus and experimental methodA horizontal tubular electrostatic precipitator, 12 ft

long and 10in in diameter, was modified for use in theseexperiments (Fig. 1). From the entry to the tubular sectionthe discharge electrode was screened for 3 ft to obtain a zeroelectric-field zone by sheathing it with polythene tube of£in wall thickness, which itself was sheathed with an earthedlayer of aluminium foil. Tn this zone a honeycomb flowstraightener was set across the duct. In the precipitation zonewhich followed, £ in-high by ^in-diameter lugs were set intothe ^in-diameter discharge electrode at 4in intervals to helpdistribute the corona current evenly. The length of this zonecould be varied from 4in to 4ft in 4in steps. Immediatelyfollowing the precipitation zone was a second zero electric-field zone, 2ft long, in which the charge-measuring probewas inserted. The remainder of the tubular precipitator wasa continuation of the precipitation zone with the same formof discharge electrode.

|»~ precipitation zone-»|

air flow

Fig. 1

Electrode arrangement in the tubular electrostatic precipitator1 Zero electric-field section at entry2 Discharge electrode3 Collector electrode4 Polythene sleeve5 Aluminium foil6 Earth wire7 Probe

The flow velocity through the duct could be varied up tolOft/s, and dust was introduced, either at the air intake sothat the dust was uniformly distributed across the duct atthe beginning of the precipitation zone, or through a £in-diameter tube in the flow-straightening zone to produce afilament of dust at the precipitation zone. The method bywhich the dust was introduced into the air flow was foundto have no effect upon the particle-charge magnitude.

The probe (Fig. 2) consisted of a paper filter thimble

1325

inside a Faraday cage, which was itself screened by a p.t.f.e.sheath whose outside surface was coated with an earthedlayer of conducting silver paint. A charged particle enteringthe probe induced a charge of the same sign and magnitudeon the Faraday cage; this effect was instantaneous and wasindependent of the resistivity of the particle or of the filter

i, 10 cm

26 cm

1 12Fig. 2Charge-measuring probe

1 Dust-laden air entry2 Faraday cage (brass)3 Paper filter4 P.T.F.E. cup, outer surface coated with silver paint5 Spring6 Wire gauze7 P.T.F.E. ring8 Brass tube9 P.V.C. tube

10 Insulated lead11 Earth lead12 Suction

material. A particle landing on the silver-painted surface hadno effect upon the Faraday cage. The Faraday cage and thesilver paint formed a cylindrical capacitor system, and themagnitude of the charge collected by the probe was deter-mined by measuring, with a valve electrometer, the potentialacross the capacitor system consisting of the probe, the leads,the meter and a reservoir capacitor. In practice, the combinedcapacitance of the probe, leads and meter was negligible incomparison with the 0 1 /xF reservoir capacitor. Suction wasapplied to the probe by a centrifugal fan, the airflow beingregulated to achieve approximate isokinetic sampling. Theprobe was positioned 1 in from the surface of the collectorelectrode for all the experiments. Although isokinetic samplingcould not be guaranteed owing to the width of the probehead, the collection efficiency was shown not to vary appre-ciably over the close ranges of particle size used in theexperiments by comparing the injected dust with that collectedby the probe. (These comparisons also showed that theefficiency of the filter was satisfactory.)

The dust samples used in the experiments were sieved intonarrow size ranges down to 44jum. Below this size a centri-fugal airborne classifier was used to separate samples ofdifferent mean sizes. Typical size gradings of the samples,obtained using a Coulter counter, are shown in Fig. 3. Themean size obtained from these gradings was converted to amean based upon surface area by the technique described byHerdan,9 which assumes that the particle size is distributedlog normally. This, however, does not take into account thesurface roughness of the particles, as the Coulter-countersize grading gives the diameter of a sphere of the same volumeas the particle.

Most measurements were made at a flow velocity of 5 ft/s,this being the order of magnitude of the flow velocity throughmodern industrial electrostatic precipitators. Prior tests hadshown that, at velocities above 6ft/s, particle re-entrainmentbecomes appreciable. Each measurement was made over aperiod of up to 5min, injecting the dust from a vibrating

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feeder into an initially clean apparatus. The maximumquantity of dust injected during this period was 200g, thequantity being kept to a minimum to avoid changes in theprecipitation conditions due to dust build up. Under theseconditions the quantity of dust collected by the probe wasabout lOmg. The collection of free ions was accounted for

100

o 10

1 95 99 99-95 20 40 60 80percentage below size

Fig. 3Size gradings of two of the samplesa Typical size grading of a sieved sampleb Typical size grading of a sample obtained from the airborne classifier

by measuring the rate of charge collection prior to dustinjection, and frictional charging during injection was shownto be negligible for fly ash by making measurements with thedischarge electrode not energised. (Samples of nylon andperspex were found to acquire a positive charge upon injec-tion and rapidly coated the discharge electrode when theprecipitator was energised, but their measured charge mag-nitudes compared with those for fly ash; so it is probablethat very few positively charged particles were able to travelas far as the probe.)

When the discharge electrode with £in lugs was used, thepotential was kept constant at 40kV negative, the associateddischarge current being 150/u.A/ft. For part of the experimentto determine the dependence of the particle charge magnitudeupon the treatment time, a redesigned discharge electrodewas used to give an improved distribution of the coronacurrent. This electrode consisted of a 3/16in-diameter rodinto which steel gramophone needles were set at 1 in intervalswith their points extending 3/16 in from the surface of therod. When using this electrode the discharge current was keptconstant at 75ju.A/ft, the applied potential being 23 kV. Allmeasurements were carried out in air at room temperatureand are expected to be true for flue gas, as the corona dis-charge characteristics of flue gas have been reported to beinsensitive to gas composition.110

3 Results and discussion3.1 The rate of charging of particles

The rate at which a particle acquires charge in anelectrostatic precipitator can be expressed by

i = 1 +q Q Qt

where r =

Initially, measurements were made using a dust of 13-5/xmmean diameter and varying the teatment time by varying theflow velocity with a precipitation zone of length 1 ft or 2ft.To eliminate the possibility of velocity effects the measure-ments were repeated at constant velocity, varying theprecipitation-zone length from 4 in to 4ft. The lattermeasurements were made at a discharge current of 75/uA/ftusing the redesigned discharge electrode and a finer dust,having a mean diameter of 6-5jnm. The results are shownin Figs. 4 and 5, the values of the limiting particle-chargemagnitude derived from the intercepts being 2-4 x 10~14Cfor the 13-5ju,m-diameter fly ash and 1-8 x 10~l5C for the6 • 5/xm-diameter fly ash. The measurements, expressed asfractions of the respective limiting charge magnitude, areplotted in Fig. 6, showing that a particle acquires 50% ofits limiting charge in 0-2s and 80% in 1 s. This charging rateis much lower than the theoretical rate of 80% in 0 1 s

PROC. IEE, Vol. 114, No. 9, SEPTEMBER 1967

quoted by other authors.2-7>8 The ion concentrations calcu-lated from the gradients in Figs. 4 and 5 are, respectively,7 x 106 ion/cm3 and 4 x 106 ion/cm3. These values are lowin comparison with a typical value of 5 x 108 quoted byother authors2-7>8 for laboratory experiments. Although theconcentration was not measured there is no reason to doubt

111-

- 7

-la 5

Fig. 4Variation of particle-charge magnitude with treatment time forcarbon-free fly-ash particles of 13-5 ftm mean diameter

these values, particularly as the laboratory electrostaticprecipitator was operated at potentials well below the flash-over potential, the intention being to simulate the low currentdensities at which industrial precipitators operate. Fig. 6 isof use in converting measurements obtained with this appa-ratus to limiting charge magnitudes which should be inde-

14

12

10

10 14 16

Fig. 5Variation of particle-charge magnitude with treatment time forcarbon-free fly-ash particles of6-5f.im mean diameter0 Flow velocity = 5ft/sO Flow velocity = 3ft/s

pendent of the ion concentration. It is possible that, with lowcurrent densities as in this apparatus, the concentration, andconsequently the rate of charging, in industrial precipitators,are low. Lowe et al.u quote a typical figure for currentdensities in industrial precipitators as 40/xA/ft at 50kV,which is lower than the minimum current density at which

0-8

1= 0-4o8 0-2

oi 0 0-1 02 0-3 0-4 0-5 0-6 0-7

treatment time, s0-8 09 10

Fig. 6Rate at which a particle acquires charge in an electrostatic precipitator% 6-5|xm-diameter dustI- 13-5 |/.m-diameter dust

PROC. 1EE, Vol. 114, No. 9, SEPTEMBER 1967

the laboratory electrostatic precipitator could be controlled.However, a reduction in the rate of charging to the extentfound in these experiments need not affect design calculationsfor power-station plant, as the residence time of 5 s is adequatefor particles to acquire charges approaching their limitingvalues.

The deviation of the experimental points from the meanline at high values of 1// in Fig. 5 is attributed to the use of avery short precipitation zone for these measurements (4inand 8 in). It was thought at first that, as the length of theprecipitation zone was varied up to 4 ft, the size grading ofthe dust collected in the probe might have become pro-gressively finer and that this might have caused the apparentreduction in the charging rate. However, no measurablechange was observed in the size grading compared with thefeed size.

3.2 Variation of the particle charge magnitude withthe particle diameterThe limiting charge magnitude acquired by a smooth

spherical particle of radius a, when it is bombarded withions in a constant electric field E, is expressed by theequation''7i8

Q = pEa2 (e.s.u.)

where p = 3 for a conducting particle

and p = 3e/(e + 2) for a particle of relative permittivity e

In practice, the electric field strength is not constant, beingvery high close to the discharge electrode but decreasingrapidly to remain approximately constant over most of thedistance to the collector electrode. The electric field strengthover most of the precipitator duct is given approximatelyfor low dust concentrations by

E = (2///X)"2

/ being the current per unit length of the discharge electrode.Thus, the particle-charge magnitude may be expected to beproportional to the square of the particle diameter, and it isthis relation which has been investigated.

The particle-charge magnitude was measured with a numberof sieved samples of dust in the size range 4/xm-l 30/xm whenoperating the laboratory electrostatic precipitator at a flowvelocity of 5ft/s with a discharge potential of 40kV negative,giving a current of 150/xA/ft. The measurements were madeat treatment times of either 0 2s or 0-4s, and the chargemagnitudes were converted to the limiting values using factorsderived, from Fig. 6. Samples of fly ash, both carbon-freeand with the original carbon content, were used for themajority of the experiments, but two measurements were alsomade with samples of ground coke. The carbon content ofthe untreated fly ash was 39% by weight for the 66/xm-diameter sample and varied from 10% to 20% for the samplesof smaller diameter. The carbon content of the coke wasabout 85%.

The results are shown in Fig. 7, the broken line representingthe theoretical charge magnitude. The particle-charge magni-tude varied as the square of the particle diameter for diametersup to 50/xm, but the charge was three or four times greaterthan the theoretical value. This could have resulted from theparticles being carried by the turbulence into the high-electric-field region close to the discharge electrode, or it could havebeen because the calculated electric field strength was low. Ithas been shown by Lowe and Lucas2 that the electric fieldstrength given by the above equation is lower, possibly bya factor of 3 or 4, than the field strength which occurs overmost of the precipitator duct when dust is passing through.

Above 50/xm diameter the particle-charge magnitudeincreased at a higher rate than that predicted by the theory.For larger particles it might be expected that the surfaceroughness would be greater and the particle shape wouldbecome less spherical. The resultant increase in the surfacearea of a particle might cause it to acquire a charge greaterthan that predicted by the theory for a smooth particle. Anattempt was made to examine the effect of particle shape bymeasuring the particle-charge magnitudes with sieved samples

1327

of perspex spheres and of ground nylon. Although theperspexspheres were found to acquire a lower charge than the nylonparticles, it was not certain that this was caused by the

difference in shape; back ionisation7 was evidently occurringwith the highly resistive perspex and may have caused adecrease in the measured particle-charge magnitudes.

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1 10 100 200

mean particle diameter, p

Fig. 7Variation of limiting particle-charge magnitude with mean particlediameter% Carbon-free fly ashO High-carbon-content fly ash+ Coke

4 ConclusionsThe rate at which particles became charged in the

laboratory electrostatic precipitator was found to be muchlower than was quoted by other authors.2'7>8 This wasthought to be due to the low ion concentration, resultingfrom the operation of the precipitator at low current densitiescomparable with those of industrial precipitators. A similarlylow charging rate in industrial plant must not be ruled out,but it is unlikely to affect the design calculations of electro-static precipitators with residence times of about 5 s as atpresent. It could, however, become important in the designof an electrostatic precipitator based upon a short prechargingzone followed by a noncorona, collecting zone (such as aparallel-plate system) which is at present being investigated.

The particle-charge magnitude was found to vary in propor-tion to the square of the particle diameter up to 50/xm, but ata greater rate for diameters above 50/xm. At present 70%-80 % of a typical fly ash may be particles of diameter below20jiim. In this size range the particles become charged toapproximately three times the theoretical value; so it isreasonable, for future development in precipitation, to assumeorder-of-magnitude agreement with the theory.

5 AcknowledgmentsThe work was carried out at the Central Electricity

Research Laboratories, Leatherhead, and the paper ispublished by permission of the Central Electricity GeneratingBoard.

References1 PAUTHENIER, M. M., and MOREAU-HANOT, M.: / . Phys. Radium, 1932,

3, p. 5902 LOWE, H. J., and LUCAS, D. H.: 'The physics of electrostatic pre-

cipitation', Brit. J. Appl. Phys., 1953, 4, Suppl. 2, p. 403 EDMONDSON, H. : Ph.D. Thesis, University of Leeds, 19614 HEWITT, G. w.: 'The charging of small particles for electrostatic

precipitation', IEEE Trans. Commun. Electronics, 1957, 76, p. 3005 PENNEY, G. w., and LYNCH, R. D. : 'Measurements of charge imparted

to fine particles by a corona discharge', ibid., 1957, 76, p. 2946 WHITE, H. j . : 'Particles charging in electrostatic precipitation',

Trans. Amer. Inst. Elect. Engrs., 1951, 70, p. 11867 WHITE, H. J.: 'Industrial electrostatic precipitation' (Pergamon

Press, 1963)8 ROSE, H. E., and WOOD, A. J.: 'An introduction to electrostatic

precipitation in theory and in practice' (Contable, 1956)9 HERDAN, c : 'Small particle statistics' (Butterworths, 1960) •0 BOYLETT, F. D. A., and LOOMS, j . s. T. : 'Effect of discharge products

upon corona discharge and spark breakdown voltage', Proc. IEE,1963, 110,(12), pp. 2292-2296

1 LOWE, H. J., DALMON, j . , and HIGNETT, E. T. : 'The precipitationof difficult dusts', IEE Colloquium on electrostatic precipitators,1965, session 1, paper (ii)

1328 PROC. IEE, Vol. 114, No. 9, SEPTEMBER 1967