Charging of Particles among Field Lines | Environmental Engineering

When high voltage is established between a pair of electrodes, an electric field is established. Charged particles like electrons and ions, under certain conditions, move along the field lines. Particles with a negative charge migrate in the direction of the positive electrode and particles with a positive charge in the opposite direction towards the negative electrode. Fig. 9.1 gives a schematic diagram of the ESP and its accessories.

Ionization of a gas by moving electrons or ions is known as collision or impact ionization. With the increase in voltage across the electrodes, current intensity rises almost directly with the voltage. However, after a particular point, the growth of current intensity slows down and the gas ionization is stabilized.

The highest current possible for a given ionization intensity is termed ‘saturation current’. At a sufficiently high voltage, the moving charge carriers within the gas i.e., electrons and ions are accelerated and ionization, takes place by collision. This is known as collision ionization of the gases. The collision ionization in the neighbourhood of the central conductor is called a ‘corona discharge’. Fig. 9.2 shows the current intensity voltage characteristics.

The Corona discharge occurs in an inhomogeneous electric field with definite electrodes shape and arrangement. In the electrostatic precipitators, two types of electrode arrangements are made-a conductor enclosed in a cylindrical pipe type precipitators or a row of conductors located midway between the two plates as in plate type precipitators. Fig. 9.3 shows the inhomogeneous electric field generation in pipe and plate precipitators and charging of dust particles on plate type collection electrode.

Normally, the discharge or central electrode has negative voltage. For industrial ventilation and air conditioning two stage positive corona tubular precipitators are preferred, since negative corona is accompanied with ozone formation. For a negative charged central electrode negative corona discharge takes place. The positive ions that are formed are neutralized on the discharge electrode.

The negative ions and the electrons are propelled to move toward the collecting electrode. If dust laden gas moves between the electrodes, the negative ions transfer their charge to the dust particles. Thus, ions, electrons and dusts particles having some negative charge move away from the discharge electrode towards collection electrode.

The migration velocities of electrons, ions and dust particles are of widely different magnitude with electron velocity being thousand times more than that of ions and dust particles velocity being much smaller than that of the ions.

Due to friction with the charged particles, neutral gas molecules are forced to move toward the collection electrode generating secondary gas motion, the so called electric wind having a magnitude of several metres per second. The electric wind supports the dust collection away from the discharge electrode but near the collection electrode, sweeps back the dust particle away from it.

If the free electrons are unable to attach themselves to gas molecules, most of the free electrons will move to the positive electrode and a spark occurs. This is the beginning of the breakdown of corona, much as a capacitor breaks down if charged to too high a voltage.

Nitrogen is unsuitable for negative corona operation, unless some electron absorbing gases like sulphur di-oxide, oxygen, water vapour and Carbon dioxide are also present. Since these gases are present in large quantities in the exhaust gases the negative corona is usually suitable.

The primary gas motion is characterized by turbulent eddies between the two electrodes and randomizes the particle motion in zig-zag fashion. However, close to the collection electrode, the laminar boundary layer prevails which smoothens the random motion and enforces a velocity component parallel to the surfaces of the collection electrode.

Turbulence and electric wind exert a decisive influence in electric precipitation and thereby complicate and general mathematical quantization of the migration velocity and efficiency of collection.

Fig. 9.4 shows the charging of dust particles in an electric field with an electric wind. Fig. 9.5 shows the movement of charged particle in turbulent flow field and shows how it is collected once it touches the laminar boundary layer.

Critical Initial Onset Voltage:

Once a voltage is applied across the discharge and collecting electrode, an inhomogeneous electric field is generated whose strength and rate of decay is greatest in the very vicinity of the active points near the discharge electrode and lowest in the proximity of the collecting electrodes.

This electric field has electrostatic nature until the applied voltage remains below a level known as critical initial voltage or corona onset voltage, which we denote as V_C corresponding to electric field strength, E_C Fig. 9.6 shows the distribution of the electric field strength along the distance between discharge and collecting electrodes.

It may be seen that the electric field strength in passive zone near the collection is made up of electro static field strength, E_e; and the field strength induced by the space charge of the particles. Passive zone is larger with field strength too low to have any collision ionization and has unipolar ions, as that of the discharge electrodes.

The active or discharge zone is a narrow zone around the discharge electrode generating both types of charge carriers, but particles having the polarity of discharge electrodes are expelled into passive zone. Because of recombination of ions of opposing polarities, some amount of energy is released, a part of which is transformed into light and causes the visible glow of the discharge.

For tube precipitators, the corona onset voltage is given by,

Whitehead in 1987 suggests the following equation for V_C based on experimental observations:

where V_c is in kV, r_d ,and p_c are discharge wire and collecting tube radii, in cm, k₁ and k₂, are constants depend on the gas characteristics, configuration of electrode system, polarity of the electrodes and the type of voltage employed (i.e. d.c.. or rectified) is the relative gas,

The current flow is produced by the oriented motion of carriers of unipolar charges during corona discharge. These carriers of charges are either ions or charged admixture of particles. The current produced per unit area of the cross section are given as,

where E_n is the field strength component normal to the surface area d_s; M_i and M_p are the mobility of ions and particles, respectively, in (cw²/V_s) and n_i and n_p are the number densities of the ions and particles present in the charged space. Since n_p < n_i, and motion of ions are very large as compared to particles, equation (4) is written as,

neglecting the particle contribution.

Thus, the current flowing is a function of number density of ions, individual charge of the ions and the velocity at which the ions follow the lines of force from the discharge towards the collecting electrodes.

In denser gases, where the motion of ions is obstructed by large number of neutral molecules, the ions move in an irregular fashion, accelerating gently over short paths between successive collisions with gas molecules.

Mobility of the ions is the factor that governs their velocity in an electric Held of unit strength. It is a weak function of the gas characteristics and its state and electric field strength but is very strongly affected by the gas pressure which limits the free paths available to the ions.

The mobility varies with the temperature and pressure of the gas in the following way:

where T_R and P_R are the reference states and M_iR is the mobility at reference state.