Taking charge

• Apr 01, 2000

Industrial electrostatic precipitators (ESPs) were first commercially applied by Cottrell Inc. in 1907 at a gun powder factory in Pinole, Calif. and later at a lead smelter in nearby Selby, Calif. Both of these installations were designed to collect liquid sulfuric acid mist. Since that time, the electrostatic precipitation process has become widely adapted for the control of particulate emissions on numerous industrial processes worldwide.

ESPs are air pollution control devices that are used for separating dust particles and mist from a polluted air stream through the use of an electrostatic field. The polluted air passes over a high-voltage negative electrode. An electrostatic field then imparts a charge on the particles, which are then attracted to a collecting electrode of an opposite charge (opposite charges attract each other; the same charges repel each other). The collected particles are then removed by rapping or washing the collecting surface.

While the first precipitators were wet systems, most of the development efforts since the early days have been focused on dry precipitators. Wet ESPs are basically the same as dry ESPs with the exception of a continuous water flow over the collecting plate. In the past 20 years, however, wet precipitators have found wide acceptance beyond their original sulfuric acid mist application. This resulted from a recognition that wet precipitators can collect fine (<2 microns)="" particles="" and="" handle="" liquid="" or="" sticky="">

With this recognition, much has been done to address some of the difficulties in adapting electrostatic precipitation to the wet mode of operation. Effective water treatment systems have been developed to minimize wastewater discharges. Corrosion-resistant materials have also been developed. However, since the first application of wet precipitators, nothing of significance has been introduced to improve the basic particle-separation performance of the wet precipitation process — until now.

The precipitation process

Electrostatic precipitation, which is the process of charging, collecting and removing particles, is made possible by corona discharge that places an electrical charge on a particle. The particle is then "pushed" electrostatically to an adjacent surface of opposite charge. Gases in the vicinity of a high-voltage negative-discharge electrode form a plasma (glow) region when the imposed voltage reaches a critical level, the corona onset voltage, about 17 kilovolts. Free electrons in this region are then repulsed toward the positive (ground) surface until they finally collide with gas molecules to form negative ions. These ions, 30,000 to 60,000 times heavier than the electrons, are much less mobile. As a result, they form a slow moving space-charge cloud of the same polarity around the emitting surface. By restricting the further emission of high-speed electrons, this space-charge tends to stabilize the corona. With the corona established, particles in the area become charged by the ions and are driven to th e positive (ground) electrode by the electric field.

Once captured on the ground electrode, the particles are removed by either mechanical rapping as in a dry precipitator, or by irrigation as in a wet precipitator.

This process is described analytically by the Deutsch equation:

E = 1 - e^{(-Aw /Q)}

Here, E = efficiency; A = area of collecting surface; w = velocity of particle migration to the collecting surface and Q = gas flow rate.

Consideration of this relationship shows that w , the migration velocity, is the critical parameter that determines the size or efficiency of the precipitator. Any development that might increase the value of w would be a very important advancement in the design of electrostatic precipitators.

To understand the possibilities we should consider the factors that affect the value of w . Of primary importance is voltage. It can be shown that

w µ E_ave q

where E_ave is the average voltage and q is charge on the particle.

Further, it can be shown that for particles greater than about 0.5 microns in diameter¹

q µ E_max

where E_max is the peak voltage to which the particle is exposed.

For particles less than about 0.3 microns in diameter, it can also be shown that

q µ I

where I is the ion density in the gas stream.

Finally, it can be shown that I is proportional to the average voltage of the precipitator.

Thus, we can conclude that,

w µ E_ave E_max (for particles >0.5 microns)

and

w µ E_ave E_ave (for particles <0.3>

Or, as a general rule,

w µ E² .

In conclusion, we see that any increase that can be brought to the voltage of the precipitator has an exponential effect on the performance of the system.

Current power supply technology

Since the first installation, electrostatic precipitators have been energized with high-voltage transformer/rectifier systems that supply DC power at 50 or 60 Hertz (Hz). These power supplies have a characteristic waveform that is highly rippled and shows distinct peak, average and minimum voltages within each period of the sine wave.

The difference between the average and peak voltages in precipitators with 60 Hz power supplies is significant. Peak voltage can exceed the average voltage by as much as 40 percent. While the peak (sparking) voltage of a system is strictly a function of the gas stream conditions and the mechanical configuration of the precipitator, the spread between the peak and average voltage is a function of the type of power supply that is employed. A power supply which operates with equal peak and average voltage would be a significant improvement.

New power supply technology

New advances in solid state power electronics have made it possible to develop a high frequency power supply for use in electrostatic precipitators. This new device operates with an output frequency between 30,000 Hz and 50,000 Hz. Operation at these frequencies coupled with modern high-speed microprocessor controls makes it possible to operate any precipitator so that the average voltage is essentially equal to the peak voltage.

To illustrate how significant this improvement is, let us analyze these values in the context of the Deutsch equation. Assuming operation on particles greater than 0.5 microns in diameter, the migration velocities for the two types of power supplies could be calculated as follows:

For 60 Hz: w 1 µ E_max E_ave

w 1 µ (60)(45)

w 1 µ 2,700

For 30,000 Hz: w 2 µ E_max E_ave

w 2 µ (60)(60)

w 2 µ 3,600

Therefore, w 2 /w 1 = 1.33

Thus, we see a 33 percent increase in the migration velocity due to the implementation of this improved power supply.

Implications for wet precipitators

Wet ESPs are frequently utilized for applications involving high concentrations of fine particles. Examples of such applications are sulfuric or phosphoric acid mist control, wood dryers or hazardous waste incinerators. These applications frequently have very high concentrations of particles in the range of 0.1 to 0.5 microns and can exhibit a troublesome characteristic known as space charge corona suppression.

This phenomenon occurs when the fine particle concentration is so high that an intense space-charge cloud forms between the discharge and collecting electrode. This space-charge can have the effect of impeding the flow of electrons from the discharge electrode and reducing the performance of the precipitator.

As discussed earlier, in situations involving particles in the range of 0.3 microns and less, the migration velocity is directly proportional to the ion density. The ion density is very strongly affected by the voltage. Operation above the "knee" in the curve would be a significant improvement. The capability of the advanced high frequency power supply systems to operate at distinctly higher voltages may be even more significant in the following situation.

For example, given a very severe corona suppression, we can see that simply raising the average voltage from 38 kilovolts to 43 kilovolts would increase the operating current by a factor of two. Since the ion density is directly proportional to the operating current and the migration velocity is directly proportional to the ion density, we can conclude that the migration velocity will also increase by a factor of two. The precipitator size could be reduced by 50 percent for the same efficiency.

Present work of high frequency power supplies

Presently, at least two suppliers of high voltage power supplies (ABB Environmental Systems and CelPower LLC) are testing and offering high frequency systems for commercial sale. Results to date appear to bear out the theory discussed above on both wet and dry applications.

¹ A complete discussion of the mechanisms for particle charging is beyond the scope of this article. Further detail of the mechanisms for particle charging may be found in Industrial Electrostatic Precipitation by Harry J. White, published by Addison-Wesley, 1963, and Applied Electrostatic Precipitation edited by K.R. Parker, published by Blackie Academic and Professional, 1997.

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This article originally appeared in the 04/01/2000 issue of Environmental Protection.