FIELD OF THE INVENTION

The present invention relates to a method for controlling the cleaning of an electrostatic precipitator unit by rapping. The electrostatic precipitator unit comprises discharge electrodes and collecting electrodes between which a high voltage is maintained. Under the action of the electric field between the electrodes, the particles charged by the current between the electrodes are moved towards the collecting electrodes and deposited thereon. Dust deposited on the collecting electrodes is removed by mechanical rapping of the collecting electrodes by one or more impulses being periodically supplied to the electrodes individually or in groups in a predetermined manner. All the collecting electrodes of the unit are cleaned during recurrent, relatively short, rapping periods separated by rapping intervals of essentially longer duration.

BACKGROUND OF THE INVENTION

Electrostatic precipitators are suitable in many contexts, especially in flue gas cleaning. Their design is robust and they are highly reliable. Moreover, they are most effici- ent, 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. A procedure that is central to the function of an electrostatic precipitator is the rapping of the collecting electrodes. By rapping, the separated dust is released from the electrodes and falls down in collecting hoppers intend¬ ed therefor. The rapping frequency, i.e. how often the

rapping is effected per unit of time, is controlled mainly by two opposite requirements. Since the dust cake on the collecting electrode by its growth gradually deteriorates the function of the filter, rapping is desirable before the dust cake becomes too thick. On the other hand, in each rapping, a considerable amount of dust is released and reentrained to the flue gas, resulting in a momentarily reduced degree of separation. The selected rapping fre¬ quency will be a compromise whicli should, for instance, maximise the average degree of separation.

Other rapping parameters that may be varied are the number of raps during each rapping and the force thereof. Also the electric voltage between discharge electrode and collecting electrode may be reduced, disconnected or even reversed during the rapping in order to facilitate the release of the dust during rapping

An electrostatic precipitator consists of a number of precipitator units, which are connected in series. Since the amount of dust separated, in a certain unit, per unit of time decreases strongly with the increasing number of precipitator units passed by the flue gas, the rapping must be controlled separately for each precipitator unit. To make it possible to separate dust released in one precipi¬ tator unit during rapping once more in a succeeding preci- pitator unit, the rapping should, however, be co-ordinated so as not to be carried out at the same time in several precipitator units. Also the rapping sequence in one precipitator unit containing a plurality of collecting electrodes to be rapped is selected carefully, such that all electrodes are rapped once during a so-called rapping cycle, where the rapping sequence for the individual elec¬ trodes has been selected for the purpose of minimising the reentrainment of dust to the flue gas.

The collecting electrodes in a modern electrostatic precipitator unit usually consists of parallel steel cur¬ tains. Each curtain comprises several steel plates. The rapping is in most cases done by rapping one steel curtain

at a time. Rapping is commonly made only at one point per curtain, more seldom at two or more points, like top and bottom or at both bottom corners.

When rapping the collecting electrodes, like above mentioned one rap for a complete steel curtain, it is impossible to achieve the same acceleration in every part of the curtain. Acceleration values are commonly used as indicative figures for the rapping efficiency. Also in the individual collecting electrodes the acceleration varies between very different values. The maximum acceleration is usually to be found close to the position where the rapping hammer hits the electrode or curtain and decreases with the increasing distance therefrom. Tlie picture, however, is complicated by oscillation nodes for the various natural oscillations of the plates.

The safe way of achieving dust removal on the entire collecting electrode seems to be to rap it so forcefully that the minimum acceleration, which is required for releasing the dust, for instance 150 g, where g is the acceleration due to gravity g « 10 m/s ² , is exceeded practically all over the electrode.

However, this means that the acceleration reaches, on parts of the electrode, very high values, for instance 1000 g. By such high values, the dust no longer comes loose in large flakes but rather as being finely divided in the flue gas and partly leaving the precipitator unit together with the flue gas. As a rule, this causes clearly visible puffs of smoke and a temporary exceeding of the permissible emission values. One attempt to solve this problem is disclosed in US 4,928,456, in which several raps are applied to the collecting electrode in each rapping period. The force of the rapping is increased step-by-step, so as to first remove the dust on that part of the electrode, which is closest to the hammer and then, by increasing the rapping force, also knock off the dust in areas further and further away from the rapping hammer until the entire collecting

electrode is cleaned. A drawback of this method, however, is that it is very difficult to apply to applications with tumbling hammers. It is mainly for use with magnetic rappers. Further, also in this c;ιse the control range is comparatively narrow.

OBJECT OF THE INVENTION

It has been found that the methods used up to now, for rapping the collecting electrodes of an electrostatic precipitator do not always result in the equal removal of dust from the entire collecting electrode. Either parts of the collecting electrodes are insufficiently cleaned or large amounts of the dust cake is partly finely divided again, resulting in reentraining of the dust to the flue gas.

The main object of the invention is to combine effect¬ ive cleaning of the collecting electrodes, by rapping, with as small emissions of dust as possible during rapping.

A second object of the invention is to improve the performance of the electrostatic precipitator unit by giv¬ ing a thinner and more equal residual dust layer on the collecting electrodes after a rapping period. A third object of the invention is to supply a tool to make possible an adaptation of the rapping parameters to the prevailing situation in each electrostatic precipitator unit.

A fourth object of the invention is to optimise the rapping procedure in an electrostatic precipitator unit by purely electrical measurements, in the same unit.

SUMMARY OF THE INVENTION

The invention relates to a method for controlling the cleaning of an electrostatic precipitator unit by rapping.

The electrostatic precipitator unit comprises discharge electrodes and collecting electrodes between which a high voltage is maintained. Under the action of the electric field between the electrodes, the particles, charged by the current between the electrodes, are moved towards the collecting electrodes and deposited thereon. Dust deposited on the collecting electrodes is removed by mechanical rapp¬ ing of the collecting electrodes by one or more impulses being periodically supplied to tlie electrodes individually or in groups in a predetermined manner. All the collecting electrodes of the unit are cleaned during recurrent, rela¬ tively short, rapping periods separated by rapping inter¬ vals of considerably longer duration.

In the method according to the invention, the rapping period is divided into two or more rapping cycles. During each rapping cycle essentially all the collecting elect¬ rodes are supplied at least one mechanical impulse. The voltage between the electrodes o.: the precipitator unit or the current being supplied to the electrodes of the precip- itator unit is reduced step-by-step between the rapping cycles and is kept essentially constant during each indivi¬ dual rapping cycle.

GENERAL DESCRIPTION OF THE INVENTION

The dust cake on the collecting electrodes of an electro¬ static precipitator unit is held in place by i.a. electric forces. If the dust is highly resistive the compressive electrostatic forces are essential. To facilitate cleaning of the electrodes, in such cases, the voltage between discharge electrodes and collecting electrodes are often reduced during rapping.

However, a mechanical rapping by e.g. tumbling hammers give a very uneven cleaning effect. In parts of the elect¬ rodes the forces induced by the rapping may exceed the

holding forces, in other parts the holding forces may be too large to make a dust cake release possible.

To improve the situation th«,ι present invention sug¬ gests that a rapping period is divided into two or more rapping cycles. Essentially all collecting electrodes are rapped in each rapping cycle. Between the rapping cycles the voltage or current between the discharge and collecting electrodes is reduced in a step and a lower voltage/current is kept during the next rapping cycle. This is repeated, with an even lower voltage/current if needed.

The voltage may be zero or even be reversed during one or more last rapping cycles. The voltage/current may be reduced even before the first rapping period below the value that is prevalent before the rapping period. Optimal voltage/current levels in the step-by-step reductions and/or the number of rapping cycles are depend¬ ent on the operating situation of the precipitator unit and can, according to relations based on practical experience, be controlled in dependence on one or more of the operating parameters of the precipitator unit.

Such a parameter is, when using pulsating direct current, the optimal pulse frequency, which according to US-5,477,464 is determined by varying the frequency, pulse charge and/or pulse duration of the pulsating direct cur- rent, thereby obtaining a plurality of combinations of frequency, charge and duration, and an optimal combination of frequency, charge and duration for the operation of the precipitator unit is determined. The number of rapping cycles during a rapping period and/or the level of the current or voltage during the individual rapping cycles are adjusted in dependence of the pulse frequency for the determined optimal combination of frequency, charge and duration.

Another operating parameter which, when using puls- ating direct current, is suited for controlling the number of rapping cycles and/or the level of voltage or current during the individual rapping cycles, is the minimum level

of the varying high voltage immediately before the rapping period.

A further suitable operating parameter is the average level of the pulsating direct current immediately before the rapping period.

Within the inventive idea, it is also possible to put a short intermediate period between the individual rapping cycles in a rapping period. During this intermediate period, earlier operating data for current and voltage are essentially re-established. In a preferred mode of oper¬ ation an operating parameter obtained during this inter¬ mediate period is used in order to decide whether a further rapping cycle is to be effected and perhaps also which the new voltage/current level should be. A suitable operating parameter for deciding if a further rapping cycle is requested, is the minimum level of the varying high voltage obtained during this intermediate period.

A suggested operating parameter for deciding which the new voltage/current level should be in the next rapping cycle, is the average value of the pulsating direct cur¬ rent, obtained during this intermediate period.

The common situation in an electrostatic precipitator giving less and finer dust in the later units passed by the gas, gives as a basic rule that short intervals between the rapping periods usually means a low number of rapping cycles, while long intervals between the rapping periods usually result in a higher number of rapping cycles per rapping period. For a given cleanness of the collecting electrode after a rapping period, measured, for instance, as the weight of the dust remaining on the collecting electrode, the method gives less emission of dust compared with rapping without a step-by-step reduction of voltage/ current.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates schematically a precipitator for carry¬ ing out the inventive method.

Fig. 2 shows how the acceleration is distributed on a collecting electrode during rapping.

Fig. 3 shows schematically how the current and voltage are varied during a rapping period with three rapping cycles.

Fig. 4 shows schematically how the voltage, in short inter¬ mediate periods between the rapping cycles, briefly returns to essentially earlier operating data.

Fig. 5 shows schematically the pulsating direct current during the short intermediate periods between the rapping cycles.

DESCRIPTION OF EMBODIMENTS

Fig. 1 illustrates schematically a precipitator for carry¬ ing out the inventive method. The precipitator has an inlet duct 41 and an outlet duct 42, and comprises three precipi¬ tator units 1, 2, 3 each having a dust hopper 11, 12, 13. The precipitator units are supplied from three rectifiers 21, 22, 23. The rectifiers 21-23 are controlled and monit¬ ored by a control unit 30. The control unit 30 also commun- icates with devices 51, 52 and 53 for rapping of the col¬ lecting electrodes in the precipitator units 1, 2 and 3. Fig. 2 shows one example of how the acceleration is distributed, on one collecting electrode 4, during rapping. It is given as multiples of the acceleration due to grav- ity, and varies in this example from 990 g, close to the rapping device 5, down to 145 g in the corner opposite to the rapping device 5. In a first zone 6 the acceleration

exceeds 400 g, in a second zone 7 it is between 400 g and 300 g, and in a third zone 8 the acceleration is below 300 g.

Fig. 3 shows schematically current I and voltage U, without showing their pulsating nature, during a rapping period S, which comprises three rapping cycles SI, S2 and S3, between which the voltage and current are reduced in three steps. In this example, the current supplied to the electrostatic precipitator in the intervals between the rapping periods has been controlled so as to be constant, which means a gradually increasing voltage between the electrodes in the precipitator during the intervals between the rapping periods.

Fig. 4 shows schematically the current I, without showing its pulsating nature, during a rapping period S, which comprises three rapping cycles SI, S2 and S3, where the voltage and current are reduced step-by-step. During intermediate periods of short duration PI, P2, between the rapping cycles, current and voltage is raised to essenti- ally earlier operating data.

Fig. 5 shows schematically the pulsating voltage dur¬ ing the short intermediate periods, between the rapping cycles S1-S3 in Fig. 4, with essentially the same average current as before the rapping period in question. The maxi- mum voltage will be lower than before, due to the thinner dust cake. However presuming a highly resistive dust cake the minimum voltage levels U _m i _n ι, U _m i _n2 will be higher and the minimum voltage levels U _m i _n ι, U _m j_ _n2 ,... increases.

In a preferred embodiment the invention functions as follows:

The precipitator unit 1 is supplied with pulsating direct current from the rectifier 21. Pulse frequency, pulse charge and pulse duration are varied, thereby obtaining a plurality of combinations of frequency, charge and dur¬ ation. The combination of frequency, charge and duration is

established and used, which is optimal for the operation. One way to find the optimal operating parameters is reveal¬ ed in US-5,477,464.

The average value of the direct current and the maxi- mum value and minimum value of the voltage are measured immediately before the point for the start of the rapping period. These values are, together with the prevailing pulse data, transmitted to the control unit 30 and stored therein. If the pulse frequency is below a determined limit, the current to the precipitator unit is reduced in a predetermined manner at the beginning of the rapping period. During one rapping cycle, with the reduced current supplied, all the collecting electrodes are being rapped. Then, full current is again supplied to the precipitator unit, with earlier established pulse parameters. After a few seconds the average current and the minimum voltage are measured once more.

If the minimum voltage has increased above a predeter- mined or calculated value, the operation continues with a new optimisation of the pulse parameters. If the minimum voltage is below said value, the rapping period is prolong¬ ed with one additional rapping cycle during which cycle the current is reduced further. After this rapping cycle the need for additional rapping cycles is tested in the same way. The same procedure is repeated until the minimum volt¬ age level has reached above the determined limit.

The other precipitator units 2, 3, are treated in the same way to get optimised rapping parameters.

If the first rapping cycle during a rapping period is carried out at a current too low, the dust is coming loose all over at the same time and is finely divided where the acceleration is as highest. Consequently, a large amount of dust is reentrained to the gas. On the other hand, by way of example, the dust comes loose, during a rapping period comprising three raps with step-by-step reduced current,

within the first zone 6 (see Fig. 2) , where the acceler¬ ation exceeds 400 g during the first rapping, the dust within the second zone 7, where the acceleration is between 400 g and 300 g during the second rapping and the dust within the third zone 8, where the acceleration is below 300 g at the last rapping, when the current is as lowest. A considerably less amount of dust is finely divided and reentrained to the gas, resulting in less emissions and less visible puffs of smoke.

ALTERNATIVE EMBODIMENTS

The method according to the invention is of course not limited to the embodiment described above, but may be modified in a number of ways within the scope of the appended claims.

The method according to the invention can be used in all strategies of controlling voltage and current, for example constant voltage between the rapping periods instead of constant current as shown in Fig. 3, see the description of the drawings above.

In Figs 3 and 4, the voltage and the current are greater than zero also during the third and last rapping cycle S3. The voltage/current can during the last rapping period be zero or reversed in relation to the ordinary direction of polarity of the precipitator unit, thereby facilitating the dust coming loose from those areas of the collecting electrode, which have the lowest acceleration.