AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 24 ( end )

24 Electrostatic Precipitators 24.1 EARLY DEVELOPMENT The phenomena of electrostatic attraction amuses children who like to stick balloons to their heads. That opposite charges attract and like charges repel is a basic law of physics. It was noted as early as 600 B.C. that small fibers would be attracted by a piece of amber after it had been rubbed. Modern knowledge of electrostatics was developed throughout the last four hundred years, including the work of Benjamin Franklin on the effect of point conductors in drawing electric currents. The first demonstrations of electrostatic precipitation to remove aerosols from a gas were conducted in the early 1800s with fog and tobacco smoke. The first commercial electrostatic precipitator (ESP) was developed by Sir Oliver Lodge and his colleagues, Walker and Hutchings, for a lead smelter in North Wales in 1885. Unfortunately, this application was unsuccessful because of problems with the high-voltage power supply and the high resistivity of the lead oxide fume. As will be discussed in this chapter, resistivity is an extremely important factor affecting ESP performance. In the U.S., Dr. Frederick Cottrell, professor of chemistry at the University of California at Berkeley, and his colleagues developed and improved the technology for industrial application. Cottrell established the nonprofit Cottrell Research Corporation, which supported the experimental studies that formed the fundamental basis of precipitator technology. The technology was applied success-fully to control sulfuric acid mist in precious metal recovery kettles. Cottrell installed the next commercial system at a lead smelter. Although the high resistivity of the dust again made it a difficult application, the high-voltage power supply issues were resolved sufficiently well so that the ESPs could operate at about 80 to 90% removal efficiency. Within a few years, ESPs were being installed in Portland cement plants, pulp and paper mills, and blast furnaces. The first installation on a coal-fired boiler was at Detroit Edison Company’s Trenton Channel Station in 1924. Eventually, ESPs were specified for most coal-fired boilers until there were more than 1300 installa-tions servicing about 95% of the coal-fired boiler applications. 24.2 BASIC THEORY An ESP controls particulate emissions by: (1) charging the particles, (2) applying an electric field to move the particles out of the gas stream, then (3) removing the collected dust. Particles are charged by gas ions that are formed by corona discharge from the electrodes. The ions become attached to the particles, thus providing the charge. FIGURE 24.1 Tubular collection electrode. (Courtesy of Geoenergy International, Inc.) In a typical ESP, vertical wires are used as the negative discharge electrode between vertical, flat, grounded plates. The dirty gas stream passes horizontally between the plates and a dust layer of particulate collects on the plates. The typical spacing between the discharge electrode and the collector plate is 4 to 6 in. The dust layer is removed from the plates by “rapping,” or in the case of a wet ESP, by washing with water. An alternative to the plate and wire design is the tube and wire design, in which the discharge electrode wire is fixed in the center of a vertical tubular collection electrode. In this configuration, the gas flow is parallel to the discharge electrode. This configuration, shown in Figure 24.1, is common for wet ESP. 24.2.1 CORONA FORMATION An electrical potential of about 4000 volts/cm is applied between the wires (discharge electrodes) and collecting plates of the ESP. In most cases, the wires are charged at 20 to 100 kV below ground potential, with 40 to 50 kV being typical. For cleaning indoor air, the wires can be charged positively to avoid excessive ozone formation. However, the negative corona is more stable than the positive corona, which tends to be sporadic and cause sparkover at lower voltages, so negative corona is used in the large majority of industrial ESP. In the intense electric field near the wire, the gas breaks down electrically, producing a glow discharge or “corona” without spark-over, as depicted in Figure 24.2. In a negative corona, ionized molecules are formed from the corona glow caused by the high electrical gradient around the discharge wire. The space outside the corona is filled with a dense cloud of negative ions. The dust particles will collide with some of the ions giving them a negative charge. These charged particles will be driven by the electric field toward the plates where they are collected. FIGURE 24.2 Corona formation — plan view plate and wire configuration. FIGURE 24.3 Particle charging. 24.2.2 PARTICLE CHARGING As particles move through the electric field they acquire an electrostatic charge by two mechanisms, bombardment charging and diffusion charging, as illustrated in Figure 24.3. Both types of particle charging act simultaneously, but bombardment charging is of greater importance for larger particles and diffusion charging is more important for submicron particles. The magnitude of the charging for both mecha-nisms is lowest for particles in the size range of 0.1 to 1 microns, therefore, the minimum collection efficiency will occur for this size range. However, a well designed ESP will be capable of collecting greater than 90% of even these difficult to collect particles. Bombardment charging is of primary importance for particles greater than 1 micron. Ions and electrons move along the lines of force between the electrodes normal to the direction of flow of particles in the gas stream. Some of the ions and electrons are intercepted by uncharged particles, and the particles become charged. Because the particles are now charged, ions of like charge are now repulsed by the particle, thus reducing the rate of charging. After a time, the charge on the particles will reach a maximum that is proportional to the square of the particle diameter. Because extremely small particles (less than 0.1 micron) have an erratic path in the gas stream due to Brownian motion, they can acquire a significant charge by diffusion charging. Thus, an ESP can be an efficient collection device for submicron particles. However, these particles represent only a small fraction of the mass of dust entering an ESP, so they are often neglected in studies of ESP performance, even though they can be of great importance to particulate emissions. 24.2.3 PARTICLE MIGRATION Most charged particles migrate under the influence of the electric field towards the plate, although a few particles in the vicinity of corona discharge will migrate towards the wire. The presence of charged particles in the gas space affects the overall electric field. Near the plate, the concentration of charged particles will be high, and inter-particle interferences can occur. Finally, particles will collect as a dust layer on the plates, and a portion of their charge may be transferred to the collecting electrode. Ideally, charged particles will migrate to the plate before exiting the ESP, as illus-trated in Figure 24.4, and will stick to the dust layer on the collecting electrode until it is cleaned. When the plate is rapped, the dust layer should fall as a sheet into dust collection hoppers without re-entraining into the gas stream. FIGURE 24.4 Migration velocity vs. treatment time. The velocity at which charged particles migrate towards the plate can be calcu-lated by balancing the electrical forces with the drag force on the particle moving through the flue gas. The electric field produces a force on the charged particle proportional to the magnitude of the field and the charge: F = qE (24.1) where Fe = force due to electric field q = charge on particle E = strength of the electric field (volts/cm) However, several simplifying assumptions are needed for calculation of balancing electrical force with drag force: · Repulsion effects between particles of like charge are neglected · The effect of the movement of gas ions (electric wind) is neglected · Gas flow within the ESP is turbulent · Stokes’ Law can be applied for drag resistance in the viscous flow regime · Particles have been fully charged by bombardment charging · There are no hindered settling effects in the concentrated dust near the plate. After applying these simplifying assumptions, the migration velocity for parti-cles larger than 1 micron charged by bombardment charging is calculated using Equation 24.2: ω =  D+2 eo Ec Ep dp C¢ (24.2) g where D = dielectric constant for the particle eo = permittivity, 8.854 ´ 10–12 coulombs/volt-meter Ec = strength of the charging electric field Ep = strength of precipitating (collecting) electric field dp = particle diameter µg = gas viscosity C¢ = Cunningham slip correction factor Note that the migration velocity is proportional to the square of the electrical field strength, directly proportional to the particle diameter, and inversely proportional to the gas viscosity. ... - tailieumienphi.vn