In this chapter, the methods of overcoming the difficulties of a highly resistive fly ash are based on changing the properties of the gas or particulate at the inlet to the precipitator, whether it is upstream or downstream of the air heater. There are, however, alternative methods of mitigating the effect of reverse ionisation on performance, which are of an electrical nature. These methods include intermittent energisation, where instead of a continuous energising voltage, the supply is intermittent, thereby allowing any charge arriving at the collector sufficient time during the non-energising period to neutralise before the next arrival of corona following re-energisation. Another approach is to superimpose a high voltage -60 kV, short duration 100 μs, pulse onto a reduced energising voltage. Again the mechanism is that during the pulse period sufficient corona is produced to charge the particles and that there is sufficient time between successive pulses for the particle charge to neutralise. Thus with either approach, voltage build up on the fly ash surface is avoided, which would have given rise to a positive ion emission, i.e. reverse ionisation operation.

Inspec keywords: electrical resistivity; electrostatic precipitators; fly ash; ion emission; corona

Other keywords: electrostatic precipitator; highly resistive fly ash surface; precipitator performance; time 100 mus; successive pulse; positive ion emission; corona; electrical resistivity; particle charge; air heater; operating condition

Subjects: Electrostatic devices