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Particle Control Technologies
Lecture notes adapted from
Prof. Dr. Benoit Cushman-Roisin
Thayer School of Engineering at Dartmouth
Design Criteria
• Design of a system which remove solid or liquid
particulate matter from a gaseous medium
• Chacacteristics of gaseous medium and
particulate matter to consider in the design:
• Size
• Chem. Composition
• Resistance
T,P,Q,
Chem. Composition
Pressure Drop
PM
Control
Device
Collection Efficiency
• Considering the wide range of size of
particulates, efficiency will be different for
each size.
• The overall efficiency (h) can be calculated
on a basis of total number (or mass) of
particles
• Generally regulations are written based on
mass, and efficiencies are calculated on
mass basis.
Collection Efficiency
• Efficiencies
calculated on mass
basis:
M i  M e Li  Le
h

Mi
Li
h: overall collection efficiency (fraction)
Mi: total mass input rate (g/s or equivalent)
Me: total mass emission rate (g/s or equivalent)
Li: particulate loading in the inlet gas to the device (g/m3)
Le:particulate loading in the exit gas stream, (g/m3)
Collection Efficiency
• When the particulate size distribution is
known, and the efficiency of the device
is known as a function of particle size,
the overall collection efficiency can be
calculate:
h  h j m j
where
hj: collection efficiency for the jth size
mj: mass percent of particles in the jth size
Example 3.1 from the book
PM Control Devices
1.
2.
3.
4.
5.
Gravity Settler
Cyclones
ESP
Filters and Baghouses
Wet Scrubbers
Settling Chamber
• Efficient for particles with diameter of 1050 mm (depending on its density)
• Velocity through chamber < 0.3-3 m/s (to
prevent reentrainment)
V
H
L
Settling Chamber
v
H
L
• Settling time < transit time through chamber
vH
• t = H/vt = L/v 
v (d ) 
t
p
L
Q
vt (d p ) 
Lw
Settling chambers are cheap to build and operate but not preferred due to their
large space requirement
Settling Chamber
• Assuming unit density sphere at STP, vt and chamber Lw
are tabulated below: Assumed flow rate Q = 150 m3/min
Q
vt (d p ) 
Lw
Dp (um)
0.1
Vt (m/s)
8.6 (10)-7
Required Area
3 km2
0.5
1.0
5
1.0 (10)-5
3.5 (10)-5
7.8 (10)-4
0.25 km2
71000 m2
3200 m2
10
3.1 (10)-3
810 m2
Settling Chamber
• Baffled Settling
Chamber
– Large particles can not
make sudden direction
change and settle into
dead space of
chamber
– Baffle chambers are
used as precleaners
Cyclones
• :
Cyclones
• :
Cyclone Geometry
Cyclone Geometry
Cyclone Theory
Cyclone Theory
2
3
Cyclone Theory
mVi 2
dragforce  3md pVt 
D/2
Cyclone Theory
Collection Efficiency
Collection Efficiency
(i) increase Vt (expensive, since DP a Vt2, as we will see in the next slides
Collection Efficiency
Collection Efficiency
Pressure Drop
K: a constant depends on cyclone configuration and operating conditions. Theoretically K can
vary considerably but for air pollution work with standard tangential-entry cyclones values of K
are in the range of 12 to 18
Power needed = Energy per unit volume of gas x volume of gas per unit time
PowerDPxQ
=Hv(rgasu2)/2Q
Cyclone pressure drops range from about 0.5 to 10
velocity heads (250 to 4000 Pa)
Cyclone Analysis
Example
Example
Conventional Type (No:3)
N=(1/H) (Lb+Lc/2) = (1/0.5)(2+2/2)=6
•Vi=Q/WH =150 /(0.25*0.5) =1200 m/min 20
Vi=20 m/s
Example
Calculate efficiency for each size range witch dpc = 5.79 um:
hj 
1
1  (d pc / d pj ) 2
Example 4.5
Example 4.5
Example 4.5
ESP
ESP
ESP Geometry
ESP Theory
ESP Theory
Time = 0
Time = t1, x=x1
+
-
P
M
P
M
M
e e
e
M
P
M
e
e
M
Negative
electrode
e
M
P
P
M
Positive
Plate
M e
e
M
+
e
e
+
M
e
e
M
M
e
e-+MM+ + 2e-
-
ee
M
+
M
M
e
P
M
M
+
e
P
e
e M
e
+
e
M
e
e
M
M
M
P
e
e M
+ e
e
P
M
e
M
M e
e
M
+
P
+
e
Electrons are accelerated to the plate, on the way collide with gas molecules
(much
e
less frequently to particles)
If electron carries sufficient energy ionize the molecule, produce 2nd free electron.By
this way, each seed e- can create thousands of e- and M+
M
ESP Theory
Particle Charging
Inter Electrode Area
-
+
P
e
M
M
M
e
e
M
P
e
Gas
M
e
molecules
MM
away from the
M+
e
negative
eM e
P
electrode,
e
M
Negative
they start e
electrode
+ e
Mslowing down.
Instead e
colliding with M e
Mthem they
e
bump upeto M
MP
them and are +
captured.
Now molecules are negatively charged
and they too want to move to the
positive plate.
+
MM
P- -
-
e
e
e
M
e
e
M
P
e
M
e
e
e
e
e
M
P-M
-
M
M
PM
-
e
e
M M
- PM
P
P
M
-
P
P
e
PM
-
e e
e
e
M
e
e
P
Next , negative gas ions stick to the particles.
Small particles (Dp<1 um) can absorb tens of
ions, larger ones with tens of thousands of ions
acquire.
ESP Theory
Charging of Particles
Ions are transported by the electiric field and/or
thermal diffusion.
Field Charging: Ions are transported to particles
along with the field lines as shown in left figure.
Important mechanism for larger particles (dp>2
um)
The ions will continue to bombard a particle until
the charge on that particle is sufficient to divert
the electric lines away from it.
A particle is calle «saturated « when it no longer
receives an ion charge.
Charging
2. Diffusion charging: random Brownian motion of the negative gas ions
charges the particles. The random motion is related to the velocity of the gas
ions due to thermal effects, the higher the temperature, the more movement.
collision with gas ions due to thermal Brownial motion. Important for smaller
particles (dp<0.2 um)
Particle Collection
Particles reached to the plate can be
removed either by water sprays or
rapping of the plates
Charges are
slowly leaked
to the
grounded
collection
plate.
Particles are held onto plates
by intermolecular cohesive and
adhesive forces
In rapping deposited dry particles are
dislodged from the plates by sending
mechanical impulses or vibrations to the
plates.
The plates are rapped while the ESP is
on-line.
Rapping is applied when the dust layer
is relatively thick (0.08 cm-1.27 cm) so
that they will fall off as large aggregate
sheets (no dust reentrainment)
Dust is collected in the hoppers. Dust
should be removed from here as soon as
possible to avoid dust packing which
makes removal very difficult.
Effect of Resistivity
Resistivity
Resistivity (P) is resistance to electrical conduction and can vary widely
P of a material is determined experimentally by establishing a current flow
through a slab (of known geometry) of the material
P = (RA/L)=(V/i)(A/L)
[ohm-cm]
R:resistance, ohm
A: area normal to the current flow, cm2
L:path length in the direction of current flow, cm,
V: voltage, i: current, A
Resistivity
Resistivity
Resistivity
Sparking
ESP THEORY
ESP Theory
ESP Theory
Amount of charge of the particle controls its velocity. What is the maximum charge a
particle can carry?
As the more charge a particle collects, it produces its own electiric field and charging
process slows down due to growing repulsion. The maximum surface charge, qsat, is
reached when the electric field at particle surface is zero.
Saturated charge for a sphere with a diameter of dp :
qsat  3 /   2 0d p2 E
qsat increases with E and dp2
ESP Theory
ESP Theory
ESP Theory
Efficiency
Efficiency
Collection Efficiency vrs Particle
Diameter
Internal Configuration
• Internal configuration design is more art than
science
• The even distribution of gas flow through the
ducts is very important to the proper operation of
an ESP
• The number of ducts (Nd) is equal to one less
than the number of plates (n-1)
Nd = Q/uDH
(eq 5.15)
u: linear gas velocity (m/min)
D: Channel width (plate separation), m
H: plate height, m
Internal Configuration
• At the start of the design, use 5.15 to
estimate Nd by assuming a value for H and
choosing representative values of u and D
Typical Values for the Fly-Ash ESP
Parameter
Drift velocity
Channel (Duct) Width, D
Range of Values
1-10 m/min
15-40 cm
Specific Collection Area Plate
area/Gas Flow
Gas velocity u
0.25-2.1 m2/(m3/min)
Aspect Ratio (R)
Duct Length/Plate Height
Corona Power Ratio Pc/Q
0.5-1.5
Corona Current Ratio (Ic/A)
50-750 mA/m2
Plate area per electrical set As
Number of electrical sections
• Table 5.1
460-7400 m2
2-6
1.2-2.5 m/s
1.75-17.5 W/(m3/min)
Internal Configuration
• The overall length of the precipitator (Lo)
– Lo=NsLp + (Ns-1)Ls + Len +Lex
• Lp:: length of plate
• Ls: spacing between electrical sections (0.5-1.0 m)
• Len: entrance section in length (several meters)
• Lex:exit section in length (several meters)
• Ns: number of mechanical fields
Ns ranges between 2 and 6.
Ns=RH/Lp R is the aspect ratio (Duct Length/Plate Height)
Internal Configuration
• When the numbers of ducts and sections
have been specified, the actual collection
area (Aa) can be calculated as:
Aa=2HLpNsNd
• During the design process several plate
sizes and numbers of ducts are tried until
one combination is found such that Aa is
equal to the required collection area.
ESP Types
Tubular Precipitator
Collection Plate
ESP Types
The method of charging
1. Single-stage
2. Two-stage (Charging and collection is in different places.
Prefererd when dust loadings less than 7.35 mg/m3 and to
collect finely divided liquid particles
ESP Types
The temperature of operation
1. Cold-side (T<204 C)
Good for high sulfur coal.
2. Hot-side
Good for low sulfur coal boilers
since high resistivity of these
particles can be lowered by
higher temperatures
ESP Types
The method of particle removal from collection surfaces
1. Wet
2. Dry
An Example
Example
Example
POWER REQUIREMENT
• Power requirement: DP across ESP is very
small (< 1 in. H2O). Therefore Corona is the
main power requirement:
• Corona Power (W) = IcxV
– Ic:Corona current in Coul/s (amper)
– V:Average voltage (a function of resistivity)
Corona power input may range from 50 to 500 W
per 1000 cfm
POWER REQUIREMENT
k: an adjustable constant in the range of 0.5-0.7 for we in ft/sec and
Pc/A in W/ft2
Corona Power vrs Efficiency,
Figure 5.9
Problem 5.10
Provide a reasonable design for a 99.4% efficient ESP treating 30,000
m3/min of gas. The dust has a resistivity of 7.1 (10)10 ohm-cm. Specify the
total plate area, channel width, number and size of plates, number of
electrical sections (total and in the direction of flow), and total corona
power to be supplied, and estimate the overall dimensions.
SOLUTION
SOLUTION
SOLUTION
H can be 1.5 to 3 times of plate
height including hoppers, controls,
and so forth.
SOLUTION
Video Demonstration on Electrostatic Precipitation
http://www.youtube.com/watch?v=y5w0IGuLR3A **
http://www.youtube.com/watch?v=9kauc7OmmLQ
http://www.youtube.com/watch?v=x5YFK8mmeRQ
http://www.youtube.com/watch?v=BdRk3op2zpE
http://www.youtube.com/watch?v=iUXHzYLgrB0
**