Abstract :
A two-stage type electrostatic precipitator composed of prechargers and collection fields has been considered to provide an effective alternative for controlling difficult high-resistivity fly ash. However, there are many varieties in the construction of both prechargers and collection fields, as well as their combination modes. In order to clarify the features specific to this precipitator, tests were made in the laboratory. Boxerchargers, free of discharge, were used as the prechargers, and three different types of collection fields were tested: conventional dc-energized twin-electrode fields, pulsed twin-electrode fields, and dc-energized parallel-plane fields. Two fly-ash samples were used in these tests: one with a medium resistivity (1010¿1011 ¿.cm at 100°C) and another with an extremely high resistivity (1015¿1016 ¿.cm at 100°C). In the case of the medium resistivity fly ash, no back discharge occurred, and each type of collection field, combined with the prechargers, showed a satisfactory performance. However, dust reentrainment began to occur in the parallel-plane fields after the growth of dust deposit on the collection electrodes, and the collection performance indicated a time-dependent degradation. The reentrained particles, larger in size and brighter in scattering light than the original particles, could beclearly observed when irradiated by a laser beam. Moreover, the surface of the dust layer indicated many pockmarks to appear when the reentrainment took place. Very severe back discharge occurred for the extremely high-resistivity fly ash in the twin-electrode fields, both dc-energized and pulse-energized, and the electrical collection performance was completely lost.
