electrostatic precipators (esp)
TRANSCRIPT
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
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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
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2013/10/23Aerosol & Particulate Research Lab
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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2013/10/24Aerosol & Particulate Research Lab
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?
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2013/10/25Aerosol & Particulate Research Lab
Positive Corona Negative Corona+ -
+ -
+
+ -
+
+
+
-
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+-
+
- +
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-
-
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+
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
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1 2 3
1 2 3
(20) (12) (8)
Turbulent Flow with Lateral Mixing Model
Electrostatic Precipitator
7/27/2019 Electrostatic Precipators (ESP)
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• 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?
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2013/10/28Aerosol & Particulate Research Lab
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
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2013/10/29Aerosol & Particulate Research Lab
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.
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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)
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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
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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?
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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?
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2013/10/214Aerosol & Particulate Research Lab
Effects of sulfur content and temperature on resistivity
Q: Is S in coal good or bad?
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2013/10/215Aerosol & Particulate Research Lab
Water spray for cement kiln dust
Flue Gas Conditioning
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2013/10/216Aerosol & Particulate Research Lab
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.
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2013/10/217Aerosol & Particulate Research Lab
Good for moderate
collection efficiency
(90% ~ 95%)
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2013/10/218Aerosol & Particulate Research Lab
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?
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2013/10/219Aerosol & Particulate Research Lab
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%
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Quick Reflection