Further studies on the electro-aerodynamic precipitator

Powder Techdogy

- Ekevicr

SA,

Sequoia

Lausanne

- F’rintcd in the Netherlands

Fnrther Studies on the Electra-Aerodynamic S.

L. SO0

AND

L. W.

Depmrmen~ of ~fechanical (Received

March

RODGERS Engineering.

Unirersixy of Iihoir,

IS. 1971; in revised form May

Urbana. III. 61801 (UXA.)

18. 1971)

SUMMARY In an electro-aerodymmic precipitator a set of high coltage wire electrodes is installed at afinite distance from a set of collector plates of the opposite polarityThe direction of the electric field fomzed between the electrodes and the plates is aligned with the direction of the dust-laden gas jlowing into the precipitator. These alternately charged plates mounted on a drum are then rotated below cIeaning shoes where the dust is remowd and drained into a sump. In such a configuration, electric wind and field hare the same directions as the dusty gas flow_ This, together with small passages which maintain laminarjlow, eliminates reentrainment by electric wind and turbulence. The corona

current also increases

maintaining

high

operating

with dust loading, thus efficiency

at

heavy

loading. A small-scale preliminary experimental study has demonstrated that such a precipitator pror%ies consistently high emciency (abow 99 “4, compactness, and light weight. When collectingfly ash, it does not depend

on

the sulfia

operating eflciency. fm has been 99_98%

content

of the

ash for

Highest eficiency obtained with cigarette smoke_

high so

INTRODUCl-ION

A survey of requirements of dust collection in various industries and applications is given in Table 11m6. Particularly interesting are certain items which are not mentioned in Table 45 of the Air Pollution

Engineering

Manual’,

namely:

Item (2): The powder plant with magnetohydrodynamic (MHD) topping has been promoted as a means of reducing air poliution-a fallacy at best. In reality it reduces fly ash output by $ because it increases power plant efficiency from 40% to 60% by MHD topping but promoters have avoided l

Papap-

tedat theInternational Poxvdcr Tczchnologr and

Bulk Solids Conf-cc.

Precipitator*

Harro.pts

Yorks..

May 12-14. 1971.

mentioning seeding material for maintaining the electricaI conductivity of the gas. The seed material takes the form of K=O and Na,O (a minimum of 300 tons of seed materiai and 200 tons of fly ash per day for a 1000 megawatt power plant) in the combustion product. Extremely high collection efficiency beyond availabIe practice is needed because of the cost and caustic nature of the seed material ; fly ash aIone is inert and contains 14 y0 of alkaIi material. Item (13) : Electrostatic precipitators are available onIy for municipal incinerators but not in the capacity range for apartment incinerators. Item (14): There is no electrostatic precipitator for the capacity range of foundries. Item (15): There are continuous self-cleaning air filters for engines of earth-moving equipment but none based on the electrostatic principle. Item (16): There is no convenient form of selfcleaning electrostatic precipitator for household or central air conditioning systems_ When compared to other devices such as venturi scrubbers, fabric filters, and vortex colIectors an electrostatic precipitator has the distinct advantage of: (a) low power consumption for high efIiciency : 50 w/loo0 cfm as compared to 7 kw/lOOO cfm in a venturi scrubber_ (b) low pressure drop: & to 1 in. water as compared to 5 in_ water in vortex collectors, 30 in water in wet scrubbers. and 3 in water in fabric filters (c) low water consumption such that air poilution wiI1 not be shifted to water pollution. (d) negligible cooling of the gas, thus preserving the chimney draft; this is not the case with any wet type of collectors_ These items do not apply to the wet or inigatcd type of electrostatic precipitators. Tests show power consumption up to 6 kW per 1000 cfm in wet electrostatic precipitators We shall first consider the limitations of conventional electrostatic precipitators to bring out the need for new developments_ An overall survey gave typical sizes of electroPomfer rechnoI_, 5 (1971/72)

S L SOO,

44 TAE%LE 1 I

TYFWAL

DUST co~~cnos

REQUIRESES~S~

(minimumof 10% dust below 10 r.rm)

Gas j&n\-

Output rt-mp.

range (IO3 cfm)

range en

(1) Steam power

xi-750

27cL6w

(1) Power plant. MHD’

10’

300-1500

(3) (4) (5) (6) (7)

50-103 X-100 Z-103 SO-x0

125-750 loo-ioo 150-I IO0 175-350

Yi-20 S-20 Xl-150 i-100 7-50 X-150 %20

loo-a00 70-x0 X0-550 100-700

.s_wem

plant (Upto 500 hi\\-l

Cement Steel Smelting MP and paper Chemical Acid mist Gas cleanin_e (8) Petroleum (catalyst) (9) Rock mill (10) Gas (lar) (I 1) Carbon black (1’7) Gypsum (13) Incinerators (14) Foundry’ (15) Intake air filter (automori\e)’ ( 16) Air conditioning6

0.5XXI

s-10 1-4 I-100

L W-RODGERS

Dust chroughpur. shorr jlow per day’

7400

2OO-TOOfly ash 300 seed material 25-1500 0.05-30 0.02-50@0 u-40 0.005-1 l-10 O.i+OO 02-500 O.oo~-1

50-150

0.06-7.5 0.75-10 0.01-10 01-6 0.00-0.06

300-700 X0-350 500-1800

700-1700 SO

z x IO-~-o.005

70

* Comerted from originaldata to short tons per da>-per cfm flow by the following relations: (gfm’) x 4.5 x IO-‘. (mg/ft’) x 1.59 x 1O-s, (grain,ft3) x 1.03 x IO=*_

static precipitators as shown in Fig. 1 for various applications3-‘-s.

LIMITATIOSS

OF CO&-OSAL

ELEClRO!3-A-A-K

PRECIPITATORS

present configuration of conventional electrostatic precipitators was introduced in 1906. The basic de+? of a conventional precipitator leads to the followmg limitations I (a) It consists of an assembly of collector plates with a row of high voltage charging electrodes mounted between each pair of plates and the entire assembly mounted in the dust-laden gaseous stream. Since the plates are cleaned by rapping and dropping of collected dust by _~vity, they are required to be separated at a sullicrent distance for the free fall of the dust. This leads to distances of 4-5 in. from the corona wires to each plate, and a high voltage of Xl-75 kV is necessary to produce the desired corona extent, tubular (corona dischargeg. To a Ii&x wire-in-tube) configuration has also been used, but the basic features are similar_ SuceessN rapping also depends on a certain range of conductivity of fly ash. If a design effickucy ia: 99 % for coal with 5 % sulfur when low sulfur (0.5 “4 coal is used, the operating efficiency might become as low as 70 oA1o_ Diminution of the usurdly speciEed efficiency also

The

” =

s

P

Powder TkhML.

5 (1971J72)

FURTHER

STUDIES

ON

ELECI-RO-AERODYNAMIC

45

PRECIPITATOR

occurs in the course of operation due to dust buildup. This is because rapping does not provide complete cleaning of electrodes. Starting with a clean unit, an increase in precipitator loss can be noticed af?er 15 min; the ultimate condition of continuous rapping may have an increase in loss from 2 to 4% yet the corona power input might be doubled from 30 to 70 W per 1000 cfmg. (b) With the plates separated at 8 in. apart, the accepted optimumgasflow velocityisapproximately 5-10 ft per second- This gives a high Reynolds number based on the gap between plates, causing reentrainment of collected dust by turbulence_ Rapping also causes reentrainment of dust (c) The number of wires between each pair of collector plates has to be large to insure a high probability of having the dust charged because of reentrainment by flow turbulence mentioned in the above and reentrainment by electric wind normal to the flow direction”_ (d) Non-uniformity of the corona discharge arises because a wire which is slightly off the center line between the plates is subjected to an electric force to push it further away from the center Iine. As shown in Appendix 1, even with an installation accuracy within 0.01 mm at the wire support, a deviation of 8 mm from the center may occur over a 5-m-long wire. Thrrc is thus a aviation in the distance between wire and plates along each wire. This reduces the effective passage height or tube length of a tubular precipitator_ Therefore a large number of wires is used between each pair of plates so that at least some are at an equilibrium position near to one plate even if most of the wires are nearer to the opposite plate. A further assurance of corona is obtained by operaing at an arc rate of up to 100 arcs per minute over the whole unit’. Such an arc rate causes ozone generation, another factor in air pollution_ (e) The need for large frontal area, long flow passages, and krge numbers of wires makes conventional precipitators large and bulky (Fig. 1). (f) Beta-zx the electric field is normal to the direction of gas flow, the corona current decreases with increase in dust loading, resulting in low operating effkiency with increased dust loading’_ (g) The basic configuration, even in tubular form’*, is unsuitable for high temperature, high pressure design that might be called for in a precipitator installed ahead of a preheater or for a coal burning gas turbine plant with ash removed ahead of the turbine.

ELECiR3AERODYNAWC

PRECIPITATOR

The above limitations of the conventional electrostatic precipitator are alleviated in the electroaerodynamic prccipitator’3 which is being developed at present, in the following manner: (a) With thorough cleaning of collector plates by slowly (l-3 r-p-m.) rotating the dust-laden (alternately charged) plates mounted on a drum below the cleaning shoe .znd cleaning by a liquid or a gas (or steam) or both, close spacing of collector plates can be used. Corona discharge is produced by a ring of corona wires spaced at a controlled distance from the plates forming a corona wire cage (Figs 2 and 3)_ (b) Even at an interstitial flow velocity of gas of 10 ft per second, the flow area is wrapped around, so large frontal area is not needed. With gaps of 4 in., laminar flow between the plates does not cause reentrainment ; the Reynolds number based on gap was below 2000. The advantage of small spacing between plates and low diffusivity in Iaminar flow is shown in Appendix 2_ (c) The electric wind and electric field are in the flow direction, causing no reentrainment(d) The distance between the wire and plates is closely controlled by wire tension, preventing nonuniformity of corona. Efficient operation v.%hout arcing is proven to be feasible. Scrub Air Ver.t

S I_ SOO,

46

F-i-f 3. Basic subassemblies .Zlp1tat0r.

of an elecctro-aerodynamic

pre-

(e) The resulting compactness is such that for a lo6 cfm flow capacity, the volume of the clcctroaerodynamic precipitator is & of that of a corresponding conventional precipitator (Fig_ 1)r3. Scaling-up is readily done_ (f) The corona current increases with dust loading because the dust flow is in the direction of the field and removes a number ofions in proportion to its own concentration_ The operating effkiency is unaffected by increased dust loading. (g) With plates cleaned and regenerated by a steam blas& mud is collected by subsequent condensation. When cleaning is by water, efhcient contact with dust calls for a very small amount of water; collected dust is drained in the form of a paste into a sump (Fig 3). The gas is cooled only to a negligible extent. (h) Because of thorough plate cleaning operating effkiency is independent of the conductivity of the dust or fly ash. For instance, it does not require high sulfur content in coal for proper functioning.

ExF%FtIMENTAL.

STUDY

All these items have been experimentally demonstrated_ Our recent development of an electro-aerodynamic precipitator of 30 cfm flow capacity has demonstrated that such a precipitator design has

I_ W. RODGERS

consistently high efficiency, compactness, and light weight. Experience from an earlier study’ led to the present model for 30 cfm air flow featuring: (1) a “corona cage” carrying the wires (0.005 in. diam. tungsten) inside the rotor with the wire-to-plate spacing of OS or 1 in, (2) a vertical rotor with aluminum collector plates, and a set of cleaning shoes Figures 2 and 3 show the nature of the corona cage and the vertical unit. The rotor of 7 in. 0-d by 2 in deep with 3.5 in plate length has a slanted passage to allow water drainage at the bottom Effective passage height is about 2.25 in. The “corona cage” fits inside the rotor, and the housing has an inlet pipe of 1.5 in. id. and outlet of l-75 in_ o-d_ A drive using ;I combination of speed reducers was installed to operate in the range 0.5-3 r-p-m A vacuum cleaner was used to exhaust the air, and corona power was furnished by a “Del Electronics” power supplyA duct unit as shown in Fig 4 was also built for studying phenomena such as dust build-up. sparks, clcctric-wind-induced motion of dust. and to serve Tabs

for

Low

voltageCornechon Corona at High

wxres Vo!:oge

Low Fid

Adpsroble Corona FrfXl-le -J Fi_e -I.

A CHo-dimensional

duct dectro-zxrodynamic

precipitator_

quick-change test rig for experimentation. Experiments were conducted with dusts of magnesia and alumina, fly ash and house dust When collecting fly ash, the precipitator does not depend on high sulfur content in ash for high operating effkiency. Magnesia and alumina, because of their high resistivity’*~ would have caused difficnhy in a conventional precipitator. Particle feeding was done initially with a S. S. White dental unit The dust was dried in an oven and then stored in the sealed fluidized bed-chamber of ;1s a

Powder Technol_ 5 [1971/z)

FLaTHER

STUDIES

ON ELECTRO-AERODYNAMIC

47

PRECIPITATOR

the feeder- The fly ash was hand sieved to remove clinkers. The feeder was found to be somewhat erratic with magnesia and alumina particles; a motor-driven belt with an adjustable hopper gate was used with a copper chute and tygon tubing to feed into the inlet pipe Modified vacuum cleaner bags were used to gather a complete dust sample at the inlet and exit The effect of moisture was corrected by a two hour “soak” in the weighing room before and after tests; this time was found to be enough to eliminate the effect of change in moisture. and control bags were used for checkmg Feed rate was determined by using a Whatman extraction thimble, and steady 95-PP”A efficiencies collecting fly ash were obtained at conditions which allowed steady operation without arcing for a 9 kV, 0.7 mA input and f in. wire-to-plate spacing Provision for 1.0 in. wire-to-plate spacing which allows operation of higher power inputs (20 kV). is built into the present corona cage of the rotary unit When testing with room dust, the extremely low airborne dust concentrations encountered are measured indirectly by using properties of suspensions such as forward light scattering_ The SinclairPhoenix photometer avaiIabIe in the Iaboratory utilizes this property to measure dust concentration (in number of particles). Particles from 0.05 to 40 p can be sensed. By far the most remarkable results were obtained with the duct unit in Fig 4, in which basic dimen_sions are readily altered for studying dust build-up, arcing, and the electric wind effect. With the recessed plates insulated from earth, a plate voltage of 1 kV was obtained_ Operating efftciency at 22 cfm flow with magnesia dust was 99_6°/0 based on bag collection at the exit; photometer readings on tests with cigarette smoke gave 99.98°/0 effteieney by mass.

DISCIJSSION

A conventional precipitator depends on fly ash from cod with more than 2% sulfur content to perfcm satisfactorily. It also performs differently at flue gas temperatures above and below the dew pomt (270-28S’F) of sulfuric acid formed in the combustion of coal with 2 o/0sulfur_ A conventional precipitator giving 92 o/0efficiency at a gas temperature of 310°F may achieve only 55% efficiency at 27dF when 1 p_p_m_ condensed acid existsloT because of the field configuration and the dielectric properties

of sulfuric acid mist Just as it is insensitive to the properties of dust material, the electro-aerodynamic system is not expected to be subject to the above limitation. Present development includes a 30 cfm unit and a 1000 cfm unit (just begun) and is, of course, far from what is called for in a 1000 megawatt power plant and field experience is needed. The electro-aerodynamic system is readily scaled up_

ACKKOWLEDGEXtEXT The authors wish gestions.

to

thank

the

referee for his sus-

REFERENCES Air Pohrion Engineering Bfaonuol.National Air Poilution Conrrot Administrarion, U.S. Dept. of Health. Education. acd Welfare_ 1967. p_ 140. z S. L. Soo. Direcr ficrg_a- Conrcrsion. Prenricc-Hall. En+ uood cliffs_ NJ.. 196% p_ ??I_ 3 R. L BLXP. The USCof electrostatic precipitators on municipal incinerators. and Anonymous. Ekctroslaric precipitator for municipal incineration_ PolIurion E@umrr;r .\~ests. Feb. 1970_ 1 P. S. CowEs. Various approaches to air poliurion control foi cupola meking units. Gra~andDircrile Iron .Vrss_ June 1967. A. GULOFF. personal communication. K. T. WHITX%YAX= B. Y. H. LIU. Dust. Dlqclopcdia 0.r

1 J_ A. D~wnsox.

7

8

9 10

Chemical TechnoIogF_ Vol. 7. Wile>-_ Neu York. 1965. pp. 429--153. M. J. ARCHBOLD. Observations and experiments resulting from a precipirator improrerncnt pro_nam_ Proc. _4m. Porrer Conj, 23 (1961) 371-390. S. L_ ‘Zoo AX;D L- W_ RODGERZ.. An electro-aerodgamic precipitator. Paper Xo. GS-lM.6lsf Ann. Mfg.. Air Polizuionn Conrrol Associaiion. Sr. Par& .Winn., 1968. H. J. Wm Indusrrial Eiecirosraric Prccipirarinn. AddisonWesley. Reading. Mass.. 1963. pp. 33, 169. 203. J. T. REESEALD J. GRECO. Esperience with elatrostatic nj-

ash collection equipment serving steam-electric gencrating plants, Paper n’o. 67-WAiAPC-3. AS3fE. 1967. 11 0. E. RA&UDAS *XT S. L. Soo. Electroh>drod>namic secondary flow. PhFs_ Fhzi&. 12 (1969) 19ZL5. I7 R. F. BROWS ANJ A. B. WALKER, Fcasibiiity demonstrarion of clatrostatic praipirarion ar liCiO=F. Paper X0. 70-7. 63rd Ann. Mzg.. Air Pol.‘urion Conrrol Associarion. SY. Louis. MO.. :9x. precipitator. I_;S. Pa~cn~ So. 13 S. L !5oo. Ekctro-aerodynamic 3JOO.611 (March 17.1970); Crom~ Parem 1301_129 (Dccnnher 2, 1970). 14 C. D. HODG~IAS. R. C. W~sr A\;D S. M. SELB~. Hundbook of Chemisfr3_ and Physics_ Chemical Rubber Pub. Co_ Cleveland. Ohio. 1959. p_ 55’6.

Powder Technol_ 5 (1971.7’)

S L SOO, L W. RODGERS

48 APPEN?)I?i

6/X)_ The wire hangs down a vertical distance y.

1

Equilibrium position of a corona n-ire For a wire of radi.us R and voltage V at distance _X from a grounded conducting plane, electrostatics gives the force per unit length of wire as: Fr =

(XE, V/x

,)(ln

k)- ‘, n/m

= z\-, R

Fl =

(51EoV’/b)

(1)

where Ed is the permittivity (8.854 x lo-” joule m) and k is given by k;F;-’

holding a weight of mass M. An element of the wire at distance p from the top will have a deviation x due to a net force F, produced by the offset and the above electrostatic force I

coul’/ (2)

and I;--2sJR for sr 9 R. A conventional precipitator consists of wires hanging, normally by a weight. between pairs of grounded conductor plates. As shown in Fig 5,2b is the distance bet\\een the parallel plates and the top of the wire is installed \vith an ofkt ‘1 (accuracy

This force produces the following relation for the deflection, under a gravitational acceleration g: d-r/dy = - K/W)

bo

- Y)

(4)

which can be integrated to determine the deflection x along the height of the wire, with maximum deflection s, at the bottom, or F;

’ dx = -(yo-y))‘/2Mg

(5)

b-

and the maximum deflection s,, is given by, for h px and h + R. In(1 +x/b)-x/b

1~here rr=4[1+2(ln 26/R)-‘]. For example, in a precipitator section of plate-to-plate spacing 2b of 20 cm, corona wire diameter 2R of 2 mm, length y, of 5 m and the wire is weighted by a mass M of 20 kg we get a=5.33, and the quantity in the exponential of eqn. (6) has a magnitude of 6.7. giving x,/6800. Thus, even with a precision giving S=O_Ol mm at the top, the deflection at the bottom x0 is 8 mm This magnitude of x,, has an important effect on the distribution of corona discharge Other excitations such as fluid turbulence may give still larger magnitude of x,,_ When the corona wire is fixed at both top and bottom under a tensile force, the above calculation is readily extended.Theconfrgurationofasingle-stagetubular precipitator with a central corona wire can be treated in a similar manner, and it is readily shown that eccentricity of the above magnitude may occur This explains why the tubular type precipitator in general performs worse than the conventional platewire type precipitator_ The latter offers changes of repeated charging and collection.

r Fi_e 5. Ddlaion

of a corona wire between two

poundedphtes POW&r Tedurol. 5 (1971/72)

FURTHER

!STUDIES ON ELECl-RO-AERODYNAMIC

APPENDM

2

Effect

49

PRECIPITATOR

and the eigen values k’s are given by

of dl$%sion on electrostatic

precipitation

tan ka

Consider the flow at velocity U of a suspension of charged particles of charge-to-mass ratio (q/m) between oppositely charged parallel plates at potential difference V, at distance b apart_ The flow and the plates are ahgned with the x-direction with coordinate y normal to it. Neglecting space charge effect, for mobility K of the particles, under the influence of diffusivity 0, the density of particles p is given by the diffusion equation taking the form:

2gk

=

(12j

k’+l--2q

and Lb = KE _u*=

(k’i

b/2D

1)i’

Dx/Li

(13) = (k’+

(14)

l)(K’E’j4DU)s

The fraction of input dust collected is given by u{~(zo--P)d~ Upob

(7) where E, the electric field, is uniform. For negligible deposition at the boundary y=O, where &

CT,-

-

KEp=O

@)

At the collector plate _r= b u$

cx

J

b

0

pdy=-VKEp

(9)

where 9 is a sticking coefficient or fraction of particles drifting toward the plate being collected I q= 1 for complete sticking and subsequent removal; q=O for zero adhesion of particles on this wa!l_ Equation (7) is readily solved for p subjected to the boundary conditions given by eqns. (8) and (9) For p=pO at -x=0, we get:

x

cos ki_b + k sin k2b [I

1

[l-e-xiJ

which gives the efficiency of collection over length x from the inlet Note that for relaxation time ‘E of momentum transfer to the particle, K = (&~)-r, i-b = 4 [(q/m) Vi b’/D’]

CD s/b’]

= 2 hi,

number, the ratio of displacement by electrical effect to that by diffusion. and Nr,r is the diffusion response number, the ratio of relaxation time to diffusion time. It is seen that ib should be as large as possible and small D is obtained in laminar flow. Further. for q< f and k=O

e

x* = (KE

GsinkLb)]

X

f

&

v

(17)

1 -XZ (10)

At = (, +I;‘) -

( 16)

D/bU is the ratio of transport time to diffusion time. and x* should also be as large as possible, and small passage height is desirable. Further, we can express xf as:

where the Fourier coefficient A, is given by: k’

[l-e-“b(coskLb

iVDF

NED is the electro-diffusion

x* = NV’ izo%(D/bW(x/b)

1 _ + k sm kiy

(15)

sin 2ki.b -F -

2.

2-b

sm’ iii b

J (11)

siUb)(KE

b/D)

(IS)

This is a more general relation than that of Deutsch*_ which includes (KE x/U b) only, neglecting the effect of diffusion. The above relations are also applicable to conventional plate and wire precipitator system with wires situated at y=O and negligible electric wind with closely spaced corona wires

Pow-der 7-echoL

5 (1971/7’)