Electrodynamics

Electrical Precipitation

Electrical precipitation is used widely in power plants for removing particles from discharging smokestacks. In the most common type, the dusty gas flows between parallel plate electrodes. The particles are charged by ions generated in a corona discharge surrounding rods or wires suspended between the plates.  Figure 1 shows schematics of electrostatic precipitators.

Figure 1.  Schematics of electrostatics precipitators.

In the precipitators the cloud of electrons and negative ions moves towards the collecting electrodes.  Particles are charged by field or diffusion charging, depending on their size. The Reynolds number is of the order of  or greater and hence the flow is turbulent. The flux normal to the collecting plate is given by

,                                                                       (18)

where  is the particle migration velocity towards the plate given by

,                                                                                       (19)

Here,  is the charge on the particle and  is the intensity of the electric field.

For  being a constant, assuming  remains constant, it follows that

,                                                                     (20)

and

                                         (21)

where

,                                                                                  (22)

is the deposition velocity due to combined Brownian and turbulent diffusion.

For , it follows that  and deposition is controlled by diffusion. When  as is the case for electrical precipitators, . For a length length,

,                                                              (23)

Hence,

,                                                                     (24)

For a length ,                                                                          

,                                                                        (25)

Figure 2 shows typical collection efficiencies of electrostatic precipitators.

Figure 2. Variation of collection efficiency of an electrostatic precipitator.