Corona Generation

When the potential difference between the wire and plate electrodes increases, a voltage is reached where an electrical breakdown of the gas occurs near the wire. When gas molecules get excited, one or more of the electrons can shift to a higher energy level. This state is transient; once the excitation has ceased, the molecule reverts to its ground state, thereby releasing energy. Part of this energy converts

FIG. 5.17.1 Single-stage, parallel plate ESP with accessories.

to light. The bluish glow around the wire is the corona discharge. A different situation occurs when an electron or ion imparts additional energy on an excited molecule. This process causes a cascade or avalanche effect described next.

The space between the wire and the plate can be divided into an active and a passive zone (see Figure 5.17.3). In the active zone, defined by the corona glow discharge, electrons leave the wire electrode and impact gas molecules, thereby ionizing the molecules. The additional free electrons also accelerate and ionize more gas molecules. This avalanche process continues until the electric field decreases to the point that the released electrons do not acquire sufficient energy for ionization. The behavior of these charged particles depends on the polarity of the electrodes; a negative corona is formed if the discharge electrode is negative, and a positive corona is formed if the discharge electrode is positive.

In a negative corona, positive ions are attracted toward the negative wire electrode, and electrons are attracted toward the positive plate or cylinder electrode. Beyond the corona glow region, the electric field diminishes rapidly, and if electronegative gases are present, the gas molecules become ionized by electron impact. The negative ions move toward the plate electrode. In the passive zone, these ions attach themselves to aerosol particles and serve as the principal means for charging the aerosol. The ion concentration is typically 107 to 109 ions/cm3.

The corona current, and therefore the charge density in the space between the electrodes, depends on factors such as the ionic mobility, whether the gas is electropositive or electronegative, and whether the corona is positive or negative. If the gas is electropositive with low electron affinity like N2, H2, or an inert gas, its molecules absorb few electrons. Thus, the current is predominantly electronic. Due to the higher mobility of electrons, the corona current is high. Conversely, an electronegative gas like O2 has high electron affinity and absorbs electrons easily. Here, the current is due to negative ions, and thus the corona current is low due to the lower mobility of the gas ions.

When a corona is negative, the free electrons leaving the active zone are transformed into negative ions with a substantially lower mobility on their way to the plates. The negative charge carriers thus cover the first part of their path as fast, free electrons and the second part as slower ions; their average mobility is lower than that of free elec

Electrode

FIG. 5.17.2 Parallel plate, two-stage ESP.

Electrode

FIG. 5.17.2 Parallel plate, two-stage ESP.

trons but higher than that of the large ions. On the other hand, when a corona is positive, the positive charge carriers are large, slow ions by origin and retain this form throughout their motion. Consequently, a negative corona always has a higher corona current than a positive corona for an applied voltage.

Negative coronas are more commonly used in industrial applications, while positive coronas are used for cleaning air in inhabited spaces. A negative corona is accompanied by ozone generation and, therefore, is usually not

Wire Electrode

Electrical Breakdown

Wire Electrode

Electrical Breakdown

Plate Electrode

FIG. 5.17.3 Variation of field strength between wire and plate electrodes.

Distance

Plate Electrode

FIG. 5.17.3 Variation of field strength between wire and plate electrodes.

used for cleaning air in inhabited spaces. However, most industrial gas-cleaning precipitators use a negative corona because of its superior electrical characteristics which increase efficiency at the temperatures at which they are used.

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