1. Particle Control Technologies
Lecture notes adapted from
Prof. Dr. Benoit Cushman-Roisin
Thayer School of Engineering at Dartmouth

2. 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 T,P,Q,
Chem. Composition Chem. Composition
Resistance Pressure Drop PM
Control
Device

3. 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.

4. Collection Efficiency
Efficiencies calculated on mass basis:
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)

5. 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:
where
hj: collection efficiency for the jth size
mj: mass percent of particles in the jth size

6. Example 3.1 from the book

7. PM Control Devices Gravity Settler Cyclones ESP Filters and Baghouses
Wet Scrubbers

8. Settling Chamber
Efficient for particles with diameter of mm (depending on its density)
Velocity through chamber < m/s (to prevent reentrainment) V H L

9. Settling Chamber Settling time < transit time through chamberv H L
Settling time < transit time through chamber
t = H/vt = L/v 
Settling chambers are cheap to build and operate but not preferred due to their large space requirement

10. Settling Chamber Dp (um) Vt (m/s) Required Area 0.1 8.6 (10)-7 3 km2
Assuming unit density sphere at STP, vt and chamber Lw are tabulated below: Assumed flow rate Q = 150 m3/min
Dp (um)
Vt (m/s)
Required Area
0.1
8.6 (10)-7
3 km2
0.5
1.0 (10)-5
0.25 km2
1.0
3.5 (10)-5
71000 m2 5
7.8 (10)-4
3200 m2 10
3.1 (10)-3
810 m2

11. 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

12. Cyclones :

13. Cyclones :

14. Cyclone Geometry

15. Cyclone Geometry

16. Cyclone Theory

17. Cyclone Theory 2 3

18. Cyclone Theory

19. Cyclone Theory

20. Collection Efficiency

21. Collection Efficiency
(i) increase Vt (expensive, since DP a Vt2, as we will see in the next slides

22. Collection Efficiency

23. Collection Efficiency

24. Pressure Drop =Hv(rgasu2)/2Q
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)

25. Cyclone Analysis

26. Example

27. 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

28. Example
Calculate efficiency for each size range witch dpc = 5.79 um:

29. Example 4.5

30. Example 4.5

31. Example 4.5

32. ESP

33. ESP

35. ESP Geometry

36. ESP Theory

37. ESP Theory

38. + - - + Time = 0 Time = t1, x=x1 e M P e M M+ P e-+MM+ + 2e- e e e e
Negative electrode
Positive Plate e e e M e e e
Electrons are accelerated to the plate, on the way collide with gas molecules (much 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+ e

39. ESP Theory

40. - + + - Inter Electrode Area Particle Charging e M M+ P M- P- e M M- P
Gas molecules away from the negative electrode, they start slowing down. Instead colliding with them they bump up to them and are captured. e P e e M- P
Negative electrode e e e e P e e e e P M- e e
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.
Now molecules are negatively charged and they too want to move to the positive plate.

41. ESP Theory

42. 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.

43. 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)

44. Particle Collection
Particles reached to the plate can be removed either by water sprays or rapping of the plates
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.
Charges are slowly leaked to the grounded collection plate.
Particles are held onto plates by intermolecular cohesive and adhesive forces

45. Effect of Resistivity

46. 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

47. Resistivity

48. Resistivity

49. Resistivity

50. Sparking

51. ESP THEORY

52. ESP Theory

53. 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 increases with E and dp2

54. ESP Theory

55. ESP Theory

56. ESP Theory

57. Efficiency

58. Efficiency

59. Collection Efficiency vrs Particle Diameter

60. 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

61. 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

62. Typical Values for the Fly-Ash ESP
Parameter
Range of Values
Drift velocity
1-10 m/min
Channel (Duct) Width, D
15-40 cm
Specific Collection Area Plate area/Gas Flow
m2/(m3/min)
Gas velocity u
m/s
Aspect Ratio (R)
Duct Length/Plate Height
Corona Power Ratio Pc/Q
W/(m3/min)
Corona Current Ratio (Ic/A)
mA/m2
Plate area per electrical set As m2
Number of electrical sections
2-6
Table 5.1

63. 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 ( 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)

64. 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.

65. ESP Types
Collection Plate
Tubular Precipitator

66. ESP Types The method of charging Single-stage
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

67. ESP Types The temperature of operation
Cold-side (T<204 C) Good for high sulfur coal.
Hot-side
Good for low sulfur coal boilers since high resistivity of these particles can be lowered by higher temperatures

68. ESP Types The method of particle removal from collection surfaces Wet
Dry

69. An Example

70. Example

71. Example

72. 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

73. POWER REQUIREMENT
k: an adjustable constant in the range of for we in ft/sec and Pc/A in W/ft2

74. Corona Power vrs Efficiency, Figure 5.9

76. 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.

77. SOLUTION

78. SOLUTION

79. SOLUTION
H can be 1.5 to 3 times of plate height including hoppers, controls, and so forth.

80. SOLUTION

82. http://www.youtube.com/watch?v=y5w0IGuLR3A **
Video Demonstration on Electrostatic Precipitation ** **