Electrostatic Precipitator (ESP)
Reading: Chap. 5
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Ying
Electrical migration
Positive
Electrical mobility
Cation
Man
Corona discharge
Love
ESP theory
Attraction
Charging mechanisms Hell
Ash resistivity
Republican
Flue gas conditioning
War
Power consumption
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Yang
Negative
Anion
Woman
Hate
Repel
Heaven
Democrat
Peace
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Electrical Migration
• Coulomb’s law
Electric Field
q1q2
FE  K E 2
r
E
FE
(q=ne)
q
– Statcoulomb (stC): the charge that causes a repulsive force of 1
dyne when 2 equal charges are separated by 1 cm (3.3310-10C)
– Unit charge: 4.8 10-10stC (1.610-19C)
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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
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Electrical Mobility
• Terminal velocity in an electrical field
(electrical migration velocity/drift velocity)
May the force be with the particles!
FE  FD
qE 
(force balance)
3d pVTE
Cc
qECc
 VTE  w 
 qEB
3d p
qCc
VTE
Z 

 qB
E
3d p
(for Re < 1)
Q: Difference between cyclone
and ESP in terms of forces
acting on the system? What’s
the effect?
Q: What is the physical meaning of electrical mobility?
Q: When does a particle have a higher mobility?
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Q: How can we generate charges?
Electron
Molecule
Corona Discharge
Particle
Positive Corona
+
-
Negative Corona
-
+
Step 1
-
+
Step 2
+
-
+
+
-
+
+
Step 3
-
-
+
Step 4
Electrode
+
-
+
Collection Plate
+
Electrode
Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html
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Collection Plate
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Electrostatic Precipitator
Turbulent Flow with Lateral Mixing Model
1
1
(20)
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2
3
2
3
(12)
(8)
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• 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 crosssection
• Deutsch-Anderson Equation
dN
2RVTE dt
2VTE dt



N
R 2
R
N (t )
2V t

 exp( TE )
N0
R
 VTE Ac 

   1  P  1  exp 
Ac/Q: Specific Collection Area (SCA)
Q 

Q: How to increase the efficiency?
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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?
6m
Internal Configuration: self-review
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3m
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Charging Mechanism: Diffusion Charging
• Random collisions between
ions and particles
The total number of charges on a particle
n
d p kT
2e 2
  d p ci e 2 N i t 
ln 1 

2kT


(ci ~ 2.4104 cm/s)
Use esu, not SI units.
The total charges on a particle
q  ne
Q: Does q depend on time?
Does q depend on dp?
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Charging Mechanism: Field Charging
• Bombardment of ions in the presence of a strong field
Total number of charges by field charging
2


Ed
 3  
p   eZi N i t 


n


   2   4e   1   eZi N i t 
(Zi ~ 450 cm2/stV•s)
Saturation charge
2

Ed
 3   p 
ns  

   2   4e 
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.
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Comparison of Diffusion & Field Charging
dp (um)
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
50
ndiff
0.10
0.30
1.1
2.8
7
21
48
108
311
683
1490
4134
nfield
0.02
0.06
0.40
1.6
6.5
40
161
646
4035
16140
64562
403510
ntotal
0.12
0.36
1.50
4.38
13.2
61.2
209
754
4346
16824
66052
407644
Zdiff
0.66
0.49
0.31
0.23
0.18
0.15
0.16
0.16
0.18
0.20
0.21
0.23
ZField
0.10
0.11
0.12
0.13
0.17
0.30
0.52
0.98
2.34
4.61
9.16
22.78
Z (stC•s/g)
0.76
0.60
0.43
0.36
0.35
0.45
0.68
1.14
2.52
4.80
9.37
23.0
Nit = 107 s/cm3
= 5.1
E = 5 KV/cm
T = 298 K
Number of Charges vs dp
106
Diffusion charging
Field Charging
105
104
Q: Does collection efficiency
increase as particle size increase
(because of a higher number of
charges)?
n
103
102
101
100
10-1
10-2
0.01
0.1
1
10
dp (um)
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ELectrical Mobility vs dp
Z (stC.s/g)
10
Diffusion charging
Field Charging
Combined Charging
1
Typical fly ash
size distribution
0.1
0.01
0.1
1
10
dp (um)
Q: If the ESP is used to collect the
fly ash, how will the particle size
distribution at ESP outlet look like?
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Resistivity/Conductivity
• Impact of particles’ resistivity on ESP’s performance:
109 - 1010 ohm-cm is desired
• Factors: temperature, composition
• Flue gas conditioning
Q: How does resistivity affect an ESP’s performance?
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Effects of sulfur content and temperature on resistivity
Q: Is S in coal good or bad?
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Flue Gas Conditioning
Water spray for cement kiln dust
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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 m3/min. The ash resistivity is 1.6×1010 ohm-cm.
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Good for moderate
collection efficiency
(90% ~ 95%)
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High Efficiency ESP (>95%)
Q: In designing a high
efficiency ESP, a smaller
drift velocity is to be used.
Why?
Matts-Ohnfeldt Equation
k
 A


  1  exp  C we  
  Q
 
Use k = 1 for fly ash
k = 0.5 or 0.6 for
industrial category
Rule of Thumb
• Below 95%, use Deutsch-Anderson Equation
• Above 99%, use Matts-Ohnfeldt Equation
• Between them, use an average
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Power Consumption
• Corona power
PC  ICVavg
• Drift velocity
kPC
we 
AC
Power density ~ 1-2 W/ft2
• Efficiency vs. Corona Power
  kPC 

  1  exp
 Q 
k = 0.55 for Pc/Q in W/cfs up to 98.5%
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Quick Reflection
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