Download TAKING CHARGE By Steve A. Jaasund, PE and J. Easel Roberts

TAKING CHARGE
By Steve A. Jaasund, PE and J. Easel Roberts, PE
High Frequency power supplies for wet electrostatic precipitators enhance air pollution control of
particulate emissions
Industrial electrostatic precipitators (ESPs) were first commercially applied by Cottrell Inc. in 1907 at a
gunpowder 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 emissions.
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
Electrostatic precipitators such as this one can help control
particulate emissions.
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. As shown in Figure 1, 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 the 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(-AO)/Q)
Here, E = efficiency; A = area of collecting surface; ω = velocity of particle migration to e collecting
surface and Q = gas flow rate.
Consideration of this relationship shows that ω, the migration velocity, is the critical parameter that
determines the size or efficiency of the precipitator. Any development at might increase the value of co
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 ω. Of primary
importance is voltage. It can be shown that:
ω α Eave q
where Eave 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 α Emax where Emax 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 α I where I is the ion density in the gas
stream.
Figure 1
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Finally, it can be shown that I is proportional to the average voltage of the precipitator. Thus, we can
conclude that:
ω α Eave Emax (for particles >0.5 microns)
and:
ω α Eave Eave (for particles <0.3 microns)
Or, as a general rule, ω α 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 (see Figure 2).
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 modem high-speed
microprocessor controls makes it possible to operate any precipitator so that the average voltage is
essentially equal to the peak voltage (See Figure 2).
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To illustrate how significant this improvement is, let us analyze the values shown in Figure 2 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:
ω1 α Emax Eave
ωl α (60)(45)
ωl α 2,700
For 30,000 Hz:
ω2 α Emax Eave
ω 2 α (60)(60)
ω 2 a 3,600
Therefore,
ω 2 /ωl = 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. An example of the effect is shown in Figure 3.
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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. As can be seen in Figure 3, 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 situation shown in Figure 3. For example,
given the very severe corona suppression situation shown in Figure 3, 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
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.
Steven A. Jaasund, PE, is the president and J. Easel Roberts, PE, is in charge of product development
at Geoenergy International Corp. in Seattle, Wash. The authors can be reached via their Web site at
wwwgeoenergy.com.
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