Static and dynamic back-corona characteristics


Voltage-current characteristics are a basic tool to analyse the operational conditions of an electrostatic precipitator, and to detect the onset of Back-corona, that strongly affects the collection performances. In this investigation, a laboratory set-up has been arranged, with a classical ESP wireplate configuration; both static and dynamic voltage-current characteristics have been measured and analysed with different geometries, dust characteristics, and layer thickness. Similar results have been also obtained in an industrial ESP, and compared with the laboratory results.

In the laboratory ESP, the static voltage-current characteristics and the collection efficiency have been measured as functions of time; with high resistivity dust, back-corona takes place as layer thickness increase, with a decrease of the efficiency and a sharp raise of the current. The effect of electrode type and powder resistivity has been analysed.

With varying applied voltages, the dynamic voltage-current characteristics show an hysteresis cycle, the recorded values depending on the slope of the applied waveform. The shape of the dynamic characteristics changes significantly with the onset of back-corona. A self-consistent numerical model has been applied to simulate static and dynamic voltagecurrent
characteristics in absence of back-corona. These simulations allowed to analyse the retarding effects of the different physical process (capacitive charging, ion drift, space charge movements), and to estimate the corresponding time constants. Finally, the dynamic effects of back-corona have been analysed.

A second set of experiments have been realized in a 600MW power plant: ESP static and dynamic characteristics have been measured with fly ash of low and high resistivity: the results are very similar to those obtained in the laboratory ESP, and confirm that it is possible to define a specific “signature”, on both static and dynamic V-I characteristics, of the different physical processes associated to the particle captation, including back-corona.

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