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Air Pollution Control Engineering. Prof. Jiakuan Yang Huazhong University of Science and Technology. Questions for the Last Lecture. Please give examples of primary particulates and secondary particulates. 2. For particles too large or too small, how to modify the Stokes’ law. Answer.
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Air Pollution Control Engineering Prof. Jiakuan Yang Huazhong University of Science and Technology
Questions for the Last Lecture • 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 CxHy Black smoke of car
Particles too large for Stokes’ Law when Rp <0.3, When 0.3≤ Rp≤1000
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 9-1The 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 cyclone separator (壁式收集设备） (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 depth filter （分离式收集设备） (纵深式过滤器） scrubber (气体洗涤器）
① ② ③ The general idea of wall collection device Driving the particles to wall Particles adhere to each other to agglomerate Removed from the wall
Gas flow out Gas flow in 2.Gravity settlers (1) Schematic of typical gravity settler outlet duct chamber Inlet duct W L H 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 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.
collection efficiency (2) Block flow model ① The time the gas will take to pass through the chamber in the flow direction. ② During that time, the particle fall down a distance by gravity. Vertical Setting Distance(VSH) = t Vt =
(2) Block flow model collection efficiency Vertical Setting Distance(VSH) = t Vt = 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.
D2 η (2) Block flow model collection efficiency ③ Efficiency η= Fractional captured = Substituting Stokes’ law equation in the above equation.
(3) Mixed flow model Assumptions, Page 252 Totally mixing in the z direction but not in the x direction.
Mixed flow model and Block flow model block Collection efficiency, % mixed Diameter of the particles, μ
L Long and expensive H Subdividing the chamber with horizontal plates, difficult to clean up Vavg cost Cross sectional area, g Substituting some other force for gravity. 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
Fd’ Vc Fd Centrifugal force Vt Vt gravity Ⅲ Centrifugal Separators (1) centrifugal force and terminal setting velocity (终极沉降速度).
Ⅲ Centrifugal Separators gravity Centrifugal force
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.
②H2, Dd conical bottom ③Wi, H: rectangular inlet ④S, De: cylindrical outlet Ⅲ Centrifugal Separators (2) Schematic of a cyclone separator De ① H1, D0 S Wi D0 vertical cylindrical body H H1 H2 Dd
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 Contributing to collection Particles are not collected. Outer helix is equivalent to the gravity settler. ① Outer helix around the outer part of the cylindrical body with downward component. (3) gas flow and efficiency ② Inner helix is at the center of cylindrical body, flowing upward to the gas outlet.
Ⅲ Centrifugal Separators L=NπD0 L Vt Fd W Centr. force Vc H Vavg Vt Fd gravity (3) gas flow and efficiency Wi H
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 Gravity settlers
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.
Ⅲ Centrifugal Separators (4) Cut diameter Efficiency can be derived from experimental data , expressed as the following Empirical Data-fitting Equation.
A Multiclone Page 261, Fig. 9.5
(5) Pressure drop of cyclone 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 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 Ep — the local electric field strength causing the force. Using an average E E = E0=Ep
电除尘器外观图 卧式 立式
High –voltage wire for corona discharge Dust-collection plate 集尘板 L 2H h Dirty gas 清洁气体 Clean gas 烟气 Corona discharge along the length of wire Collected dust on plate Dust removed from plates to hoppers 电晕线长度 收集在集尘板上的尘
