Gas separation: processes – With control responsive to sensed condition – Electric or electrostatic field
Patent
1995-09-19
1997-06-17
Chiesa, Richard L.
Gas separation: processes
With control responsive to sensed condition
Electric or electrostatic field
95 7, 95 81, 96 21, 96 82, 323903, B03C 368
Patent
active
056392940
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for controlling the power supply in case of flashover between the electrodes of an electric precipitator. Power is supplied by a controllable high-voltage direct-current source.
The case in which the advantages of the method are particularly great, is the one in which the electrostatic precipitator operates with an exceedingly high flashover frequency. At the present level of technology, e.g. modulated high-frequency high-voltage rectifiers are suitable means for carrying out the method.
The invention is applied when the dust to be separated does not have such high resistivity that there is a risk of breakdown in the dust layer formed on the collecting electrodes. The invention is of no particular use when separating dust of such high resistivity that the voltage or current must be restricted owing to back-corona.
BACKGROUND OF THE INVENTION
In many contexts, especially in flue gas cleaning, electrostatic precipitators are the most suitable dust collectors. Their design is robust and they are highly reliable. Moreover, they are most efficient. Degrees of separation above 99.9% are not unusual. Since, when compared with fabric filters, their operating costs are low and the risk of damage and stoppage owing to functional disorders is considerably smaller, they are a natural choice in many cases. In an electrostatic precipitator, the polluted gas is conducted between electrodes connected to a high-voltage rectifier. Usually, this is a high-voltage transformer with thyristor control on the primary side and a rectifier bridge on the secondary side. This arrangement is connected to the ordinary AC mains and thus is supplied at a frequency which is 50 or 60 Hz.
The power control is effected by varying the firing angles of the thyristors. The smaller the firing angle, i.e. the longer conducting period, the more current supplied to the precipitator and the higher the voltage between the electrodes of the precipitator.
When separating dust of low or moderate resistivity, the degree of separation increases as the voltage between the electrode increases. The separation will thus be more effective at high voltage. The possible voltage is, however, not restricted by the construction of the high-voltage rectifier only, but also by the fact that at sufficiently high voltage, there will be flashover between the electrodes in the precipitator.
The optimal separation is therefore obtained when the voltage applied is just below the one causing flashover. Since the flashover limit may vary strongly according to varying operating conditions, a constant voltage is, unfortunately, not possible if one tries to obtain optimal separation, but instead one must frequently test the flashover limit by permitting flashover between the electrodes.
This is effected by slowly increasing the current until flashover occurs. Subsequently, the current is reduced in a predetermined manner and then again slowly increased until the next flashover. The procedure is repeated periodically. If the circumstances result in a highly varying flashover limit, more than 100 flashovers a minute may be acceptable. In more stable processes, 10 flashovers a minute may be involved. In certain processes, the best separation is however obtained at very high flashover frequencies although the operation is very stable. Up to now, this has not been explained in a satisfactory manner, but is verified by experience.
Examples of the technique of controlling are to be found in, inter alia, GB 1,402,149, FIG. 8 showing the fundamental reasoning. In case of flashover, the current is interrupted during a first time interval, and then the current is rapidly increased from zero, during a second time interval after which it is increased slowly when a given value, depending on the value before the flashover, has been achieved.
To ensure that the flashover does not lead to a permanent arc and, thus, sets the precipitator out of operation for a long time, the first time interval, during which the current is inte
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ABB Flakt AB
Chiesa Richard L.
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