by Anatol Jaworek and Andrzej Krupa

An effective method which allows to remove aerosol particles from exhaust gases or indoor air such as dust, smoke, bacteria or spores in micrometer and submicrometer size range is the electrostatic scrubbing. In an electrostatic scrubber the aerosol particles and the scrubbing droplets are charged to opposite polarities. The charged droplets act as small spherical collecting electrodes filling the precipitator chamber. Because the distances of the particles to these collectors are very short, the attractive Coulomb force are relatively high and they causes the particles to move toward the charged droplets up to the mechanical contact with it.

This type of scrubbers removes all shortcomings inherent to conventional electrostatic precipitators and inertial scrubbers, which do not allow to control effectively the dust particles in the submicrometer size range. The electrostatic scrubbing increases manifolds overall collection efficiency as compared to conventional inertial scrubbers. The scrubbers utilizing electrostatic deposition require lower water rate, and lower pressure drop through the equipment, operating at the same over-all collection efficiencies as inertial scrubbers. The equipment utilizing electrostatic forces operates at lower relative velocities than that in which inertial collection is dominant. The main trouble with practical use of the electrostatic scrubbers is the corrosion of metal elements, including electrodes.

A schematic diagram of an electrostatic scrubber is shown in the following figure.

The advantages of the electrostatic scrubbers are as follows:

1. Low investment costs,

2. Low power consumption (similar to electrostatic precipitators),

3. Low water consumption (lower than in inertial scrubbers),

4. Low pressure drop (a few cm H₂O),

5. Low sensitivity to physical and chemical particle characteristics,

6. High collection efficiency in submicrometer range.

The following mechanisms of particle deposition on a collector (a droplet) can be distinguished:

1. Coulomb and image electrostatic forces and repulsive action of the space charge of equally charged dust particles cloud,

2. Polarisation forces between the particles placed in an external electric field,

3. Vortices downstream of the collector, occurring for high Reynolds numbers,

4. Brownian motion,

5. Thermophoresis and dyfusiophoresis,

6. Inertial impact,

7. Interception,

8. Gravitational sedimentation.

The collection efficiency of the method was determined experimentally in a small scale electrostatic scrubber of the cross section of 66 x 67 mm, with the gas velocity ranged from 0.24 to 0.65 m/s. The droplets of the size in the range of 0.6 to 3 mm in diameter were charged by an electrohydrodynamic spraying method, that allowed to impart them a charge close to a half of the Rayleigh limit. The surface charge density on the droplets was up to 10 C/m². In the electrostatic scrubber the particles are charged by ion impact in an alternating electric field charger which allow to impart maximum theoretical value of charge, i.e., much more higher than that achieved by any other method. The collection efficiency was measured by means of the reduction of the charge on a series of scrubbing droplets after they passed through the channel.

The collection efficiency obtained experimentally for a single droplet was as high as 20 for aerosol particles of the mean radius of 3 mm. The collection efficiency increased with decreasing droplet radius. The collection efficiency of electrostatic deposition is enhanced with low relative velocities between the particles and the scrubbing droplets.

The theoretical model was developed for determination of the collection efficiency for a moving (i.e., not fixed) collector (e.g., a drop) in 3D space. The flow field around the collector was determined from the solution of Navier-Stokes equations. Both, the particle and the collector motion due to the gas viscosity forces, gravity and electrostatic force were taken into consideration. The equations of motion were solved simultaneously using the Runge-Kutta method. The deposition of aerosol particles on a moving spherical collector due to the electrostatic forces are dominant only for low relative particle-collector velocities. Also the collecting droplet should be as small as possible, and charged close to the Rayleigh limit in order to achieve high collection efficiency in electrostatic scrubbers. ¨