Page 293 - Materials Chemistry, Second Edition
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                       264                       Waste Management Practices: Municipal, Hazardous, and Industrial













                                   Raw gas
                                    inlet












                                                                                  descharge
                                                                       Collecting  electrodes
                                                                     electrode plates








                       FIGURE 9.9 Electrostatic precipitator (U.S. EPA, Air Pollution Engineering Manual, 2nd ed., 1973).


                          The stream of “dirty” gas is passed through a series of discharge electrodes (Figure 9.10). The
                       electrodes are negatively charged, usually in the range of 1000 to 6000 V. At this voltage a corona,or
                       cloud of charge, is generated. Most particles passing through this corona, regardless of initial compo-
                       sition, will develop a negative charge. A grounded (positive) surface, or collector electrode, is situated
                       near the discharge electrode. The negatively charged particulates will be attracted to and collect on the
                       grounded surface. Lastly, the particulate matter is removed from the collector surface by cutting off
                       the voltage to each electrode and then striking them with rappers at regular intervals or by wetting
                       down the plates. The collected particulates are then disposed. In some cases, particulates may physi-
                       cally or chemically resist changing charge. These will pass through the ESP without being captured.
                          The advantages of using the ESP for flue gas cleaning include:

                           ● Highly efficient removal of particulates
                           ● Relatively insensitive to high effluent gas temperatures
                           ● No wastewater treatment requirements
                          Disadvantages include:

                           ● High capital costs (a simple model will cost several million dollars)
                           ● Large space requirements
                           ● The ESP is often sensitive to the chemical composition of flue gas. Acid gases will cor-
                             rode metallic components
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