Air Pollution Control
Engineering
Prof. Jiakuan Yang
Huazhong University of Science and
Technology
Questions for the Last Lecture
1. Please give examples of primary
particulates and secondary particulates.
2. For particles too large or too small,
how to modify the Stokes’ law.
Answer
Primary particles are found in the
atmosphere in the form in which they
were emitted.
Fly ash
Coal dust
ZnO Fume
Cement dust
Answer
(3) secondary particles
Secondary particles are formed in the
atmosphere from gaseous pollutants which
are called as the precursors.
Secondary particles
precursors
Tobacco smoke
CxHy
sulfuric Mist
SO2
Black smoke of car
CxHy
Particles too large for Stokes’ Law
 4
D part g

Vt   3
 Cd  fluid


when Rp <0.3,
When 0.3≤ Rp ≤1000






1
2
24
Cd 
Rp

24
0.7
Cd 
1  0.14R p
Rp

Particles too Small for Stokes’ Law
gD  part
2
V
18
gD  partC
2
V
1  A D 
18
Lecture 9 Control of Primary Particulates
Most of fine particles in the atmosphere
are secondary particles, <2.5 μm.
They are difficult to be collected by a
control device. The best way to control
secondary particles is process change.
Lecture 9 Control of Primary Particulates
The control of primary particles is a
major part of air pollution control
engineering.
For the particulates in size range 1 µm
~ 70 µm. The control device can be
designed on the basis of Stokes’ law.
Lecture 9-1 The nature of Particulate Pollutants
Ⅰ Types of Particles collection devices
Ⅱ Gravity Settlers
Ⅲ Centrifugal Separators
Ⅳ Electrostatic Precipitators (ESP)
Ⅴ Summary
Ⅰ Types of Particles collection devices
gravity
settlers
(重力沉降式室)
wall collection device
(壁式收集设备）
cyclone separator
(旋风除尘器）
electrostatic precipitators
(静电除尘器）
Ⅰ Types of Particles collection devices
surface filter
(表面式过滤器）
dividing collection device
（分离式收集设备）
depth filter
(纵深式过滤器）
scrubber
(气体洗涤器）
The general idea of wall collection device
Driving the particles
to wall
①
Particles adhere to
each other to
②
agglomerate
③
Removed from the wall
2.Gravity settlers
(1) Schematic of typical gravity settler
chamber
Inlet duct
outlet duct
W
H
L
Cross-sectional area (WH) is much larger than
that of inlet duct, or that of outlet duct.
Some baffles: inlet baffles and out baffles
reducing the velocity of gas in gravity setter.
(2) Block flow model
Assumptions
① Average horizontal gas velocity in chamber
Vavg
Q

WH
② The horizontal velocity of the gas in the
chamber everywhere = Vavg
(2) Block flow model
③ The horizontal component of the velocity of the
particles =Vavg
④ The vertical component of the velocity is equal
to Vt
⑤ If a particle settles to the floor, it stays
there and is not re-entrained.
Separation of the particles from the dirty air
(2) Block flow model
collection efficiency
① The time the gas will take to pass through the
chamber in the flow direction.
L
t
Vavg
② During that time, the particle fall down a
distance by gravity.
L
Vertical Setting Distance(VSH) = t Vt = Vt 
Vavg
(2) Block flow model
collection efficiency
L
Vertical Setting Distance(VSH) = t Vt = Vt 
Vavg
If VSD<H, the particles will pass through the
chamber and will not be captured by the bottom
wall of chamber.
If VSD>H, the particles will be captured on the
bottom wall of chamber.
(2) Block flow model
collection efficiency
③ Efficiency
η= Fractional captured =
LVt
HVavg
Substituting Stokes’ law equation in the above equation.

LgD  part
2
HVavg 18
η
D2
(3) Mixed flow model
Assumptions, Page 252
Totally mixing in the z direction but not in the x
direction.
mixed
  LgD 2  part 

 1  exp  


HV
18

 
avg

mixed  1  exp( blowflow)
Collection efficiency, %
Mixed flow model and
Block flow model
block
mixed
Diameter of the particles, μ
Shortcoming of Gravity settlers
This type of device would be useful for collecting
particles with diameters of perhaps 100μm, but
not for smaller particles of air pollution interest.
<100μm
LgD 2  part

HVavg 18
L
Long and expensive
H
Subdividing the chamber with horizontal
plates, difficult to clean up
Vavg Cross sectional area, cost
g
Substituting some other force for gravity.
Ⅲ Centrifugal Separators
(1) centrifugal force and terminal setting velocity
(终极沉降速度).
Fd’
Vc
Fd
Centrifugal force
Vt
Vt
gravity
Ⅲ Centrifugal Separators
gravity
mg 

6
 part D g  Fd  3DVt
3
Vt 
Centrifugal force
gD  part
2
18
V

3V
m
  part D
 Fd  3DVt
r
6
r
2
c
2
c
Vc D  part
2
Vt 
2
18r
Ⅲ Centrifugal Separators
Page 255, Example 9.2
Centrifugal force is two orders of magnetic larger
than the gravity force.
Centrifugal force is more powerful than the
gravity force, so centrifugal particle separators
are higher efficiency than gravity settlers. In the
cyclone, gravity can be ignored.
Questions for the Last Lecture
What is the general idea of wall
collection device?
The general idea of wall collection device
1. Driving the particles to wall.
2. Particles adhere to each other to agglomerate.
3. Particles are removed from the wall.
Ⅲ Centrifugal Separators
(2) Schematic of a cyclone separator
D0
S
De
Wi
H
① H1 , D 0
vertical cylindrical body
②H2, Dd
conical bottom
H1
③Wi, H:
rectangular inlet
④S, De:
H2
Dd
cylindrical outlet
Ⅲ Centrifugal Separators
(3) gas flow and efficiency
Because rectangular inlet is arranged
tangentially to the circular body of cyclone, so
that the entering gas flows around the
circumference of the cylindrical body, not
radically inward.
Ⅲ Centrifugal Separators
(3) gas flow and efficiency
① Outer helix around the outer part of the
cylindrical body with downward component.
Contributing to collection
② Inner helix is at the center of cylindrical
body, flowing upward to the gas outlet.
Particles are not collected.
Outer helix is equivalent to the gravity settler.
旋风除尘器
Ⅲ Centrifugal Separators
(3) gas flow and efficiency
L=NπD0
L
Vt
Centr. force
Wi
W
H
Vc
Fd
Fd
H
Vt
gravity
Vavg
Ⅲ Centrifugal Separators
(3) gas flow and efficiency
L = NπD0
L — the length of the flow path
N — the number of turns that the gas makes the
traversing the outer helix of the cyclone
D0 — the outer diameter of the cyclone
Ⅲ Centrifugal Separators
(3) gas flow and efficiency
cyclone separator
LVt

HVavg
LVt

WiVc

ND0  V  D   part
2
c
2
WiVc 18r
NVc  D   part
2

Gravity settlers
9Wi 

LgD  part
2
HVavg 18
Ⅲ Centrifugal Separators
(4) Cut diameter
Cut diameter is the diameter of a particle
for which the efficiency curve has the value
of 0.50.
1/ 2
Dcut
 9Wi  


 2NV  
c part 

Ⅲ Centrifugal Separators
(4) Cut diameter
Efficiency can be derived from
experimental data , expressed as the
following Empirical Data-fitting Equation.

D / Dcut 

2
1  D / Dcut 
2
A Multiclone
Page 261, Fig. 9.5
(5) Pressure drop of cyclone

Pressure drop  Pin  Pout  K ρg V /2
2
i

ρg — the gas density.
Vi — the velocity at the inlet to the cyclone.
K — constant, K=8 for most cyclone
separators.
(6) Location of cyclone
Before the blower, air will be sucked in
cyclone, degrading the overall collecting
efficiency.
After the blower, bearing and collecting
on the blower’s blades.
Ⅳ Electrostatic Precipitators (ESP)
(1）The basic idea
The basic idea of all ESPs is to give the
Particles an electrostatic charge and then
put them in an electrostatic field that drives
them to a collecting wall.
ESP includes two steps:
 charging the particles.
 Collecting the particles.
(2) The charge on the particles by field charging
  
2
q  3 
 0 D E0
 2
ε — dielectric constant of the particle,1 for
vacuum, 4-8 for typical solid particles.
ε0 — the permittivity of free space, 8.85×10-12
C/(V·m)
D — particle diameter.
E0 — the local field strength.
(3) Electrostatic force
F  qE p
Ep — the local electric field strength
causing the force.
Using an average E
E = E0=Ep
(3) Electrostatic force
  
2
F  3 
 0 D E0 E p
 2
  
2 2
 3 
 0 D E
 2
(4) drift velocity (静电拖曳速度)
Fd  3DVt


  
2 2
electrostatic force  3 
 0 D E 
 2

  
D 0 E 

 2

 Vt 

2

电除尘器外观图
宽间距卧式电除尘器
HHD型宽间距卧式电除尘器
电除尘器
Dust-collection
plate
High –voltage
wire for
corona
discharge
集尘板
L
2H
h
Dirty gas
烟气
Corona
discharge along
the length of
wire
电晕线长度
Clean gas
Dust removed from
plates to hoppers
清洁气体
Collected dust
on plate
收集在集尘板上的尘
电除尘器
电除尘器
烟道气
风板的
距离
烟道气
清洁气体
详图：严密的放电极
收集的粉尘
（5) Diagrammatic sketch of a simplified ESP
Page 267 Fig. 9.7
With two plates, four wires, and one flow channel.
H
H
From A direction
Vavg
A
ES force
Vt
Highvoltage
wires for
corona
discharge
Dustcollection
plates
Fd
h
L
（5) Diagrammatic sketch of a simplified ESP
ESP is two gravity settlers back to back.
LVt
L
Lh



Vavg H Vavg H Vavg Hh
Area of plate
A  Lh
Q  HhVavg Volumetric flow

A
Q
（5) Diagrammatic sketch of a simplified ESP

A
For block flow
Q
  1  exp( 
A
Q
)
For mixed flow
Page 269 Fig. 9.8
Modern
ESP
Summary
Driving forces
Terminal settling velocity
Efficiency
Cut diameter
Centr. force
figure
Vt
ES force
ω
Vt
gravity
Gravity
Driving force

6
Terminal
settling
velocity
efficiency
Vt 

Cut diameter
 part D g
3
gD 2  part
18
LgD 2  part
HVavg 18
50 μ
For small particles ineffective
Suitable for
particles’ size
>100 μ
Centrifugal force
ES force
  
2 2
qE  3 
 0 D E
 2

2 
2
2
D

E


0
Vc D  part


2


Vt 


18r
Vc2
m
r

NVc  D 2   part
9Wi 
5μ
effective
15~100 μ

A
Q
0.5 μ
More effective
5~15 μ
This Lecture and the Next Lecture
 This Lecture:
 Chapter 9
Page 249~280
 The Next Lecture:
 Chapter 9
page 280~314
Exerciser
Page 315 9.7
9.10
DISCUSSION
Topics about Air pollution:
 Introducing yourself
 Interesting news or information about air pollution your
having read or heard
 Your opinions on this Air Pollution course
 Your suggestions for Chinese Air Pollution
 Other familiar issues about Air Pollution