E-Book Content
Lesson 1 Electrostatic Precipitator Operation Goal To familiarize you with the operation of electrostatic precipitators (ESPs). Objectives At the end of this lesson, you will be able to do the following: 1. Describe the theory of precipitation 2. Describe how an ESP operates to collect particulate matter 3. Describe the two ESP designs for particle charging and collection: high voltage single-stage and low voltage two-stage 4. Distinguish between cold-side and hot-side ESPs 5. Briefly describe wet ESP operation 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. They can collect a wide range of particle sizes, and they can collect particles in dry and wet states. For many industries, the collection efficiency can go as high as 99%. ESPs aren't always the appropriate collection device, but they work because of electrostatic attraction (like charges repel; unlike charges attract). Let's see how this law of physics works in an ESP. 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. The particles would quickly collect on the plate, creating a dust layer. The dust layer would accumulate until we removed 2.0-2/98 1-1 Lesson 1 it, which we could do by rapping the plate or by spraying it with a liquid. Charging, collecting, and removing—that's the basic idea of an ESP, but it gets more complicated. Let's look at a typical scenario using a common ESP construction. 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. You can mentally divide this field into three regions (Figure 1-2). The field is strongest right next to the discharge electrode, weaker in the areas between the discharge and collection electrodes called the inter-electrode region, and weakest near the collection electrode. The region around the discharge electrode is where the particle charging process begins. Figure 1-1. Typical dry electrostatic precipitator Electric field strength Weakest Strongest Weakest Interelectrode region Figure 1-2. ESP electric field 1-2 2.0-2/98 Electrostatic Precipitator Operation Corona Discharge: Free Electron Generation Several things happen very rapidly (in a matter of a millisecond) in the small area around the discharge electrode. The applied voltage is increased until it produces a corona discharge, which can be seen as a luminous blue glow around the discharge electrode. The free electrons created by the corona are rapidly fleeing the negative electric field, which repulses them. They move faster and faster away from the discharge electrode. This acceleration causes them to literally crash int