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Although there are various methods of particle charging, for example tribo- electric, ultraviolet and radiation effects, for industrial precipitator applications corona charging is universally used as being the most efficient and cost-effective approach. The physics of corona or ion production therefore occupies an essential position in the practice of electrostatic precipitation. Investigations into the physics of ionisation date back to the middle of the nineteenth century and are still ongoing, particularly since the development and application of computational fluid dynamics (CFD) using fast computers capable of deriving satisfactory solutions to Poisson and Laplace equations.

The early investigations were concerned with developing voltage current rela- tionships of corona discharge and the effects of different means of electrical energisation. In 1862, Gaugain [2], working with concentric cylinders, found that for a given outer diameter electrode, the electrical breakdown voltage was primarily dependent on the radius of the central emitter electrode.

Qualitatively, the following equation represents the breakdown potential of such a system, where the radius of the inner electrode, r, is very much smaller than R, the radius of the outer passive electrode, i.e. r << R:

E= A + C/r1/3_{, (2.1)}

where E is the electrical breakdown field, r is the radius of the inner electrode, and A and C are experimental constants.

Röentgen [3] in 1878, working with point/plane electrodes, found that a cer- tain voltage had to be applied to initiate a corona current flow. This corona onset voltage was dependent on the sharpness of the point, gas pressure and polarity of the point electrode. Further investigations produced a parabolic relationship for negative corona current flow as:

I= AV (V − M), (2.2)

where I is the corona current flow, V is the applied voltage and A and M are experimental constants.

The next significant finding, as regards conventional early concentric wire and tube precipitator arrangements, can be attributed to work carried out by Townsend [4]. It was noted that with negative ionisation of the central discharge element, with the outer being connected to the positive terminal of the supply voltage, the appearance of the corona discharge was very different to when the inner electrode was positively energised. In the case of negative energisation the corona appears as bright flares which tend to move across the surface of the electrode, whereas with positive energisation the corona appears as a diffuse glow surrounding the electrode. Other differences found were that with negative energisation the corona had a distinctive hissing sound, the corona initiation voltage was lower and the breakdown potential higher.

Figure 2.1 indicates the difference in electrical characteristics of the two forms of energisation for the same electrode arrangement.

In general, these findings probably explain why negative energisation is nor- mally used for industrial gas cleaning operations, although for air cleaner type duties, because of a lower ozone production rate, positive energisation is gener- ally used.

The conduction of electricity through gases is fundamentally different to that in solids and liquids, which contain ‘charge carriers’ that move under the influence of the electric field to produce a current flow. For metallic materials, the current arises from the movement of electrons that migrate through the crystal lattice; for semiconductors, the charge carriers can be either electrons or ‘holes’ that migrate through the material; for insulating materials, e.g. silica and alumina, ionic conduction only occurs at high temperature, when the mean free electron path is significantly increased, and in the case of conductive liquids, current flow results from electrolytic ion conduction through the liquid. Whereas with gases, ions need to be provided, or produced from some outside force or agency, to induce a current flow that will cross the inter-electrode space.

For electrostatic precipitators applied to gaseous applications, the major outside agency for ion production is by high voltage electric input. Figure 2.2 indicates a typical electric field distribution across the inter-electrode area, between a small diameter emitter and a much larger passive electrode, i.e.

r < < R, for a tubular type of precipitator.

It will be noted that the electric field adjacent to the emitter is extremely high and it is this electrical stress which excites any free electrons in the immediate vicinity. These fast moving electrons acquire sufficient energy from the applied Figure 2.1 Comparison of discharge characteristics for negative and positive energisation

of a wire–tube electrode arrangement

electricfield to collide with other gas molecules to produce further free electrons and positive ions. Townsend, working in this area, proposed the concept of a chain reaction or electron avalanche, in which each new electron produced generates new electrons by ionisation in ever increasing numbers.

The number of ions at a distance x from the active zone can be represented by

n= n0eαx, (2.3)

where n0 represents the number of ions at a distance x= 0 and α is the Townsend

ionisation coefficient, which varies with the gas temperature, pressure and resultant field strength.

In general terms, where the field varies with distance across the field x, as in a precipitator, then the equation takes the integral form

n= n₀e冮 x

0 α dx

(2.4) (note the term 1/α is the mean free path of the electron between collisions). Figure 2.2 Relative field strength between the electrodes of a tubular precipitator

arrangement

Figure 2.3 indicates the relationship between the Townsend ionisation coef- ficient (ion pair production) for air at atmospheric pressure and at different temperatures and field strengths.

Although few industrial gases comprise pure air (79 per cant (N2+ CO2)+ 21

per cent O2) as the carrier gas, the curve indicates the effect of increasing field

strengths on the ionisation characteristics. At 20°C, doubling of the field strength results in the number of ion pairs, or Townsend coefficient, increasing by a factor of 20. The impact of a rise in gas temperature producing signifi- cantly increased ionisation because of the larger mean free electron paths is also indicated.

In practice, with the system negatively energised, although there are a large number of ion pairs immediately adjacent to the discharge element, as the electrons rapidly move across the field area they collide with and attach them- selves to gaseous molecules to produce negative ions, while the positive ions are attracted towards the discharge element and, although initially during transit producing further ion pairs, on reaching the element take no further part in the process.

Adjacent to the discharge element there is an abundance of electrons, but as the distance from the element increases, because of attachment, the number of electrons decrease and there is a corresponding increase in the number of nega- tive ions. The net number of electrons at a distance x from the electrode is represented by the following equation:

n= n₀e冮 x

0

(α − η) dx _(2.5)

where η = coefficient of attachment or the Townsend second coefficient. Figure 2.3 Effect of electric field strength and temperature on ion pair production

Note. Those gases in the right hand region of the Periodic Table, such as Cl2,

HF, O2, SO2 and SF6, all being deficient in electrons in their outer shell, have

a great affinity for electron attachment and are termed electronegative gases. Their affinity for electron attachment, even if the gases are present in minor quantities, tends to reduce ionisation and suppresses the electrical discharge in gases. Electropositive gases such as N2, H2, Ar, etc., in high concen-

trations have little affinity for electron attachment and consequently do not produce negative ions and result in little current flow up to their breakdown voltage.

An example of a uniform stable self-maintaining gas discharge between an emitter and passive or receiving electrode is illustrated in the case of a concentric wire and tube precipitator arrangement, where the radius of the emitter is small compared with that of the passive electrode, as shown in Figure 2.4.

In the case of positive energisation, normally found in air cleaning applica- tions, the primary electrons produced at the boundary of the visible glow are attracted towards the emitter. In moving through the field they collide and pro- duce new ion pairs by impact ionisation, the positive ions migrating towards the passive earthed electrode. Except for the collection of electrons, the emitter itself plays little part in the ionisation phenomena, which is essentially a gas process with the primary electrons being released from the gas molecules through photoelectric effects in the plasma region.

Negative corona is a feature of gases or admixtures that exhibit appreciable electron attachment and as such are sensitive to gas composition and tempera- ture. The corona can range from virtually zero to being a highly stable discharge

Figure 2.4 Schematic tubular precipitator arrangement indicating active and space charge areas

dependent on operating characteristics. Unlike positive energisation, the emitter plays an important role in the ionisation phenomena; any positive ions gener- ated in the plasma region are attracted towards the emitter because of the high field strength, to produce further electrons on impact.

The ultraviolet radiation in the plasma region releases even further electrons; hence we have an abundance of fast moving electrons in the plasma region. These electrons move rapidly away from the electrode to produce ion pairs on impact with the gas molecules. The emitter immediately captures the positive ions, while the negative ions and any free electrons migrate towards the passive electrode.

As indicated earlier, the visual appearance of negative corona takes the form of tufts or bright glow points or Trichel pulses [5], which on a plain smooth emitter rapidly move about. The number of tufts and their luminescence increases as the energising voltage is raised producing a higher corona current discharge.