Mechanism of Dust Particle Charging | Environmental Engineering

In this article we will discuss about the mechanism of dust particle charging.

The dust particles charging takes place by:

(i) Diffusion charging, and

(ii) Field charging or ion bombardment charging.

Diffusion charging is the dominant mode of particle charging for particles with a diameter smaller about 0.2 µm; and field charging dominates for particles with a diameter greater than about 1 µm. Diffusion charging occurs due to random thermal motion of the ions. After impingement the ion will adhere to the dust particle and thus transfer its charge.

Field Charging:

Field charging occurs when the ions and electrons moving along electric field lines intercept the dust particles and give up their charge to dust particle. The electric charge of a dust particle is therefore a function of the electric field strength E and the dielectric properties of the dust particle.

A non-conductive dust particle with a dielectric constant (= 1), [The dielectric constant is the rates of the permittivity of the particle to the permittivity of the free space], causes no distortion of electric field lines. The conductive unchanged dust particles with a dielectric constant dd1 < ԑ < ∞, causes distortion of the field lines so that more electrons and ions are attracted by the field lines so that more electrons and ions are attracted by the dust particles.

The distortion of the electric field diminishes with increasing charge of the particle. The limiting or saturation charge is attained when no electric field lines intercept the dust particle trajectory.

Assuming the dielectric constants of the gas to be ϵ I, the following equation for the electric charge, Q_f of a spherical dust particle due to field charging is given:

Where E = electric field strength, volts/m, d_p = dust particle diameter, m; N = free charge (ion) density (number per m³), ԑ = 1.602 x 10^-19 (C) electronic charge, t = charging time (S)

and M_i= mobility of ions, (m³/vs)=V_i/E if N e M_it>>1, equation (9.3) reduces to

The saturation charge is a linear function of the electric field strength E and the surface area of the dust particle. The dielectric constant to ԑ of a particle has y a weak influence. For example, the variation in e from ԑ = 1 to ԑ = ∞, makes a change I C_C, value from 1 to 3 and thus raises Q_fs by only a factor of 3. The C_C value, mostly used for all kinds of particles, is between 2.5 to 3 since the Q_fs depends very strongly on the particle size, d_p. For a highly conductive particle with ԑ = ∞; the saturation charge is given as,

The particles attain their saturation charged state within a very short period (a fraction of a second). This is because of a large number N of free ions being of the order of 10¹² to 10¹⁴ as suggested by Lowe and Lucas (3).

The following table gives the field charging rate:

Diffusion Charging:

Particle Charging by diffusion in terms of practical situations are given by white (1963) as,

Where Q_D is the charge acquirement by a particle by diffusion; t is the charging time; v_ef and V_i are the effective root mean square velocity of gas molecules and the mean ion velocity, respectively; and I_i. is the ionic current density. Q and Q_s are the instantaneous and saturation equilibrium charge of the particle in almost infinite time.

The Complex Charging Process:

Both coarse and fine particles are generally charged by both ion bombardment and ion diffusion concurrently. The particle charge thus formed induces an electric field which then in turn affects the further course of the charging process. Bohm (1982) have dealt with the charging phenomena and has given a relation which gives the combined action of the both the mechanism on a particle charging.

He has also given a relation for charging of poly-disperse particles in gas stream. He has shown that the finer the particles, the greater will be the current attenuation and the smaller the proportion of ion emitted by the discharge electrode which actually reaches the collecting electrode. This is the reason why the currents ascertained across the upstream ends of precipitators are commonly lower than those measured at the downstream ends.