05-04-2012, 12:18 PM
Electrostatic Precipitator Operation
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Introduction
As you may know, particulate matter (particles) is one of the industrial air pollution problems
that must be controlled. It's not a problem isolated to a few industries, but pervasive across a
wide variety of industries. That's why the U.S. Environmental Protection Agency (EPA) has
regulated particulate emissions and why industry has responded with various control devices.
Of the major particulate collection devices used today, electrostatic precipitators (ESPs) are
one of the more frequently used. They can handle large gas volumes with a wide range of inlet
temperatures, pressures, dust volumes, and acid gas conditions.
Theory of Precipitation
Every particle either has or can be given a charge—positive or negative. Let's suppose we
impart a negative charge to all the particles in a gas stream. Then suppose we set up a
grounded plate having a positive charge.What would happen? The negatively charged particle
would migrate to the grounded collection plate and be captured.
Particle Charging
Our typical ESP as shown in Figure 1-1 has thin wires called discharge electrodes, which
are evenly spaced between large plates called collection electrodes, which are grounded.
Think of an electrode as something that can conduct or transmit electricity. A negative,
high-voltage, pulsating, direct current is applied to the discharge electrode creating a negative
electric field.
Ionization of Gas Molecules
As the electrons leave the strong electrical field area around the discharge electrode,
they start slowing down. Now they're in the inter-electrode area where they are still
repulsed by the discharge electrode but to a lesser extent. There are also gas molecules
in the inter-electrode region, but instead of violently colliding with them, the electrons
kind of bump up to them and are captured (Figure 1-5). This imparts a negative charge
to the gas molecules, creating negative gas ions. This time, because the ions are negative,
they too want to move in the direction opposite the strong negative field.
Particle Charging Mechanisms
Particles are charged by negative gas ions moving toward the collection plate by one
of these two mechanisms: field charging or diffusion charging. In field charging (the
mechanism described above), particles capture negatively charged gas ions as the ions
move toward the grounded collection plate.
Electric Field Strength
In the inter-electrode region, negative gas ions migrate toward the grounded collection
electrode. A space charge, which is a stable concentration of negative gas ions, forms
in the inter-electrode region because of the high electric field applied to the ESP.
Increasing the applied voltage to the discharge electrode will increase the field
strength and ion formation until sparkover occurs. Sparkover refers to internal sparking
between the discharge and collection electrodes.