Electrostatic Precipators (ESP)

7/27/2019 Electrostatic Precipators (ESP)

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2013/10/21Aerosol & Particulate Research Lab

Electrostatic Precipitator (ESP)

• Electrical migration

• Electrical mobility

• Corona discharge

• ESP theory

• Charging mechanisms

• Ash resistivity

• Flue gas conditioning• Power consumption

Reading: Chap. 5

Positive Negative

Republican Democrat

Love Hate

Ying Yang

Man Woman

Hell Heaven

Cation Anion

War Peace

Attraction Repel




Electrical Migration

• Coulomb’s law

– Statcoulomb (stC): the charge that causes a repulsive force of 1

dyne when 2 equal charges are separated by 1 cm (3.3310-10C)

– Unit charge: 4.8 10-10stC (1.610-19C)

221

r

qq K F E E E

F

q

E

(q=ne)

Electric Field




Millikan Experiment

(Robert Millikan,

US, 1868-1953;

Nobel Prize

Laureate, 1923)

Hinds, Aerosol Technology, 1999

http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html








Electrical Mobility• Terminal velocity in an electrical field

(electrical migration velocity/drift velocity)

c

TE p

C

V d qE

3

qEBd

qEC wV

p

cTE

3

qBd

qC

E

V Z

p

cTE 3

(force balance) D E F F

(for Re < 1)

Q: What is the physical meaning of electrical mobility?

Q: When does a particle have a higher mobility?

May the force be with the particles!

Q: Difference between cyclone

and ESP in terms of forces

acting on the system? What’sthe effect?




Positive Corona Negative Corona+ -

+ -

+

+ -

+

+

+

-

-

+-

+

- +

-

-

-

+

+

Corona Discharge

Step 1

Step 2

Step 3

Step 4

Collection Plate Collection Plate

Electron

Molecule

Particle

ElectrodeElectrode

Q: How can we generate charges?

Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html

http://www.mtcnet.net/~jdhogg/ozone/ozonation.html







1 2 3

1 2 3

(20) (12) (8)

Turbulent Flow with Lateral Mixing Model

Electrostatic Precipitator




• Deutsch-Anderson Equation

R

dt V

R

dt RV

N

dN TE TE

22

2

)2exp()(0 R

t V N

t N TE

Q

AV P cTE exp11 Ac /Q: Specific Collection Area (SCA)

• Turbulent flow: uniformly mixing

• Perfect Collection

• The fraction of the particles

removed in unit time = the ratio of

the area traveled by drift velocity

in unit time to the total cross-

section

Q: How to increase the efficiency?




Q: An ESP that treats 10,000 m3/min of air is

expected to be 98% efficient. The effective

drift velocity of the particles is 6.0 m/min. (a)

What is the total collection area? (b) Assuming

the plates are 6 m high and 3 m long, what is

the number of plates required?

6 m

3 mInternal Configuration: self-review




Charging Mechanism: Diffusion Charging

• Random collisions between

ions and particles

kT t N ecd

ekT d n ii p p

2 1ln

2

2

2

Q: Does q depend on time?

Does q depend on d p ?

The total number of charges on a particle

(ci ~ 2.4104 cm/s)

neq

The total charges on a particle

Use esu, not SI units.




Charging Mechanism: Field Charging

• Bombardment of ions in the presence of a strong field

eZ1

eZ

4

2

3

i

i

2

t N

t N

e

Ed n

i

i p

Total number of charges by field charging

Q: Is the charging rate dependent on

particle size? On field strength? On time?

On material?

Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999.

e

Ed n

p

s4

2

32

Saturation charge (Zi ~ 450 cm2/stV•s)




Comparison of Diffusion & Field Charging

Q: Does collection efficiency

increase as particle size increase

(because of a higher number of

charges)?

dp (um) ndiff nfield ntotal Zdiff ZField Z (stC•s/g)

0.01 0.10 0.02 0.12 0.66 0.10 0.76

0.02 0.30 0.06 0.36 0.49 0.11 0.60

0.05 1.1 0.40 1.50 0.31 0.12 0.43

0.1 2.8 1.6 4.38 0.23 0.13 0.36

0.2 7 6.5 13.2 0.18 0.17 0.35

0.5 21 40 61.2 0.15 0.30 0.45

1 48 161 209 0.16 0.52 0.68

2 108 646 754 0.16 0.98 1.14

5 311 4035 4346 0.18 2.34 2.52

10 683 16140 16824 0.20 4.61 4.80

20 1490 64562 66052 0.21 9.16 9.3750 4134 403510 407644 0.23 22.78 23.0

Number of Charges vs dp

dp (um)

0.01 0.1 1 10

n

10-2

10-1

100

101

102

103

104

105

106

Diffusion charging

Field Charging

Nit = 107 s/cm3

= 5.1

E = 5 KV/cm

T = 298 K




ELectrical Mobility vs dp

dp (um)

0.01 0.1 1 10

Z

(stC.s/g)

0.1

1

10 Diffusion charging

Field Charging

Combined Charging

Typical fly ash

size distribution

Q: If the ESP is used to collect the

fly ash, how will the particle size

distribution at ESP outlet look like?




Resistivity/Conductivity

• Impact of particles’ resistivity on ESP’s performance:

• Factors: temperature, composition

• Flue gas conditioning

109

- 1010

ohm-cm is desired

Q: How does resistivity affect an ESP’s performance?




Effects of sulfur content and temperature on resistivity

Q: Is S in coal good or bad?




Water spray for cement kiln dust

Flue Gas Conditioning




Effective drift velocity as a function of resistivity by measurement

Use the same Deutsch-Anderson Equation with new we.

Q: Estimate the total collection area required for a 95% efficient fly-ash ESP

that treats 8000 m

3

/min. The ash resistivity is 1.6×10

10

ohm-cm.




Good for moderate

collection efficiency

(90% ~ 95%)




High Efficiency ESP (>95%)

Matts-Ohnfeldt Equation

k

eC w

Q Aexp1

Use k = 1 for fly ash

k = 0.5 or 0.6 for

industrial categoryRule of Thumb

• Below 95%, use Deutsch-Anderson Equation

• Above 99%, use Matts-Ohnfeldt Equation

• Between them, use an average

Q: In designing a high

efficiency ESP, a smaller

drift velocity is to be used.

Why?




Power Consumption

avg C C V I P

C

C

e

A

kP

w

Power density ~ 1-2 W/ft2

QkP C exp1

• Corona power

• Drift velocity

• Efficiency vs. Corona Power

k = 0.55 for Pc/Q in W/cfs up to 98.5%



2013/10/220A l & P ti l t R h L b

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