WO1997041959A1

WO1997041959A1 - Method for controlling an electrostatic precipitator

Info

Publication number: WO1997041959A1
Application number: PCT/SE1997/000730
Authority: WO
Inventors: Hans Jacobsson
Original assignee: ABB Fläkt Aktiebolag
Priority date: 1996-05-09
Filing date: 1997-04-30
Publication date: 1997-11-13
Also published as: SE9601763L; AU2797497A; SE506423C2; SE9601763D0

Abstract

A method for controlling an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes, between which a high voltage is maintained by a direct current supplied to the electrodes. Dust deposited on the collecting electrodes is removed by mechanical rapping of the collecting electrodes by mechanical impulses being supplied to the electrodes individually or in groups. The collecting electrodes of the precipitator unit are cleaned during recurrent, relatively short, rapping periods separated by rapping intervals, of considerably longer duration. A strategy of controlling the electrical parameters is selected and this strategy is maintained during the optimisation. The length of the interval between the rapping periods and possibly also other rapping parameters are varied. The current and voltage during the interval between the rapping periods are measured. The average ofthe current, voltage, or a quantity derived from the currrent and/or voltage, during the interval between the rapping periods, is calculated. This calculated average is used for controlling the length of the rapping intervals and/or other rapping parameters.

Description

Method for Controlling an Electrostatic Precipitator

FIELD OF THE INVENTION

The present invention relates to a method for controlling an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes, between which a high voltage is maintained, by a constant or pulsating direct current supplied to the electrodes. Dust deposited on the collecting electrodes is removed by mechanical rapping of the collecting electrodes by one or more mechanical impuls¬ es being supplied to the electrodes individually or in groups, in a predetermined manner. All the collecting electrodes of the precipitator unit are cleaned during recurrent, relatively short, rapping periods separated by rapping intervals of considerably longer duration. The duration of the rapping intervals and/or other rapping parameters are controlled.

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. Besides, a too high rapping frequency results in the formation of a hard coating that adheres to the collecting electrode and is very difficult to remove by rapping. The selected rapping frequency will be a compromise which should, for instance, maximise the average degree of separation. Other rapping parameters that may be varied are the number of raps during each rapping period and the force thereof. The electric voltage between discharge electrodes and collecting electrodes may be reduced, disconnected or even reversed during the rapping in order to facilitate the release of the dust during rapping. An electrostatic precipitator usually consists of a number of precipitator units which are connected in series. Since the amount of dust separated, in a given 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 a precip¬ itator unit during rapping, once more in a succeeding precipitator 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 a precipitator unit containing a plurality of collecting electrodes, to be rapped, is selected carefully, such that all electrodes usually are rapped once during a so-called rapping cycle, in a sequence selected for the purpose of minimising the reentrainment of dust to the flue gas.

If adjustment of e.g. rapping frequency is effected manually by means of the reading of an opacimeter (tester for the optical density of smoke) , this takes such a long time that unfavourable values of the rapping frequency during the adjustment, which result in increased emissions during the time of adjustment, has an impact also on the long time average of the emission.

Furthermore, there is a risk that operational vari¬ ations in the equipment producing the gas to be cleaned, e.g. a coal fired boiler, affect the adjustment negatively if considerable changes in the concentration of dust, the composition of dust or the gas temperature occur during the time needed for the adjustment. This already applies to the adjustment of the electric parameters of the precipitator and is a still more difficult problem in the adjustment of, for instance, the rapping frequency, since the rapping frequency varies between minutes for the first precipitator unit to several hours or even days for the last one.

US 4,432,062 discloses an automatic optimisation of the rapping frequency in terms of the average value of the remaining dust content in the flue gas after the precipi- tator. The drawbacks of this method are a dependence on the measuring of the remaining dust content in the flue gas and the fact that the rapping frequency varies over several orders of magnitude between the precipitator units. When selecting the rapping frequencies of the precipitator units as independent parameters, this leads to simultaneous optimisation of many parameters, which easily results in sub-optimisation or the absence of convergence of the optimising algorithm. If, on the other hand, predetermined functional relations between the rapping frequencies are selected, e.g. constant relative proportions, the number of degrees of freedom is restricted too much, involving a risk of sub-optimisation. The corresponding conditions apply to other rapping parameters, such as the voltage or current during rapping. OBJECT OF THE INVENTION

It has been found that the methods tested up to now, for controlling the rapping parameters do not always result in the optimal combination of parameters and, above all, are far too slow.

The main object of the invention is to maximise the dust separation, in an electrostatic precipitator unit, in order to minimise the dust emission of a plant comprising the electrostatic precipitator.

A second object of the invention is to maximise an average degree of dust separation, in an electrostatic precipitator unit, during the rapping intervals.

A third object of the invention is to maximise an average degree of dust separation, in one electrostatic precipitator unit, during the rapping intervals without any essential risk of influence from the performance in down¬ stream or upstream units.

A fourth object of the invention is to maximise an average degree of dust separation, in one electrostatic precipitator unit, during the rapping intervals by purely electrical measurements, in the same unit.

SUMMARY OF THE INVENTION

The present invention relates to a method for optimisation of the performance of an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes, between which a high voltage is maintained by a constant or pulsating direct current supplied to the electrodes. Under the action of the electric field between the electrodes, the particles, charged by the current therebetween, are moved towards the collecting electrodes and are deposited thereon. Dust deposited on the collecting electrodes is removed by mechanical rapping of the collecting electrodes by one or more mechanical impulses being supplied to the electrodes individually or in groups, in a predetermined manner. All the collecting electrodes of the precipitator unit are cleaned during recurrent, relatively short, rapp¬ ing periods separated by rapping intervals of considerably longer duration. The duration of the rapping intervals and/or other rapping parameters are controlled.

In the method according to the invention, a strategy of controlling the electrical parameters is selected and this strategy is maintained during the optimisation. The length of the interval between the rapping periods and possibly also other rapping parameters such as number of raps, rapping force and current or voltage between the electrodes during the rapping period are varied. The cur¬ rent and voltage during the interval between the rapping periods are measured. The average of the current, voltage, or a quantity derived from the current and/or voltage, during the interval between the rapping periods is calcul¬ ated. This calculated average is used for controlling the length of the rapping intervals and/or other rapping para- meters.

GENERAL DESCRIPTION OF THE INVENTION

When a dust layer is built up on the collecting electrodes, this results in a voltage drop across the dust layer, which is essentially proportional to the thickness of the dust layer. The thicker the layer the greater part of the volt¬ age between the discharge electrodes and the collecting electrodes lies over the dust layer. This results in reduced current if the voltage between the electrodes is maintained. In order to maintain the same current, the voltage must increase.

If this would be the only effect no serious problem should arise. However, the general experience is that with an increasing thickness of the dust layer, the degree of separation decreases. The reasons for this are not fully understood or generally accepted.

Presuming a linear relation between current and volt¬ age in the dust cake, the derivative of the voltage should, in this case, be essentially proportional to the amount of separated dust per unit of time. The decreasing efficiency is shown by the fact that, when operating with a constant current, the derivative of the voltage decreases in course of time, as shown in Fig. 3, see the description below. The decreasing degree of separation during the inter¬ val between the rapping periods could apparently be met by making the rapping intervals as short as possible. This would, at first sight, make it possible to avoid the grad¬ ual deterioration of the efficiency. However, it has been found that when rapping, there will always be a residual layer after rapping and that this residual layer of dust has a thickness which increases with a decreasing duration of the rapping interval. As a consequence, one begins, at short rapping intervals, higher up on the voltage curve immediately after rapping and cannot include that part of the voltage curve, which has the greatest derivative when selecting longer rapping intervals.

Therefore, when comparing short and long intervals between the rapping periods, it will be found that with short intervals, a relatively seen lower degree of separ¬ ation is obtained at the beginning, which does not decrease very much during the interval, while with long intervals, a higher degree of separation is obtained at the beginning, but a more drastic decrease to a relatively seen low value is obtained at the end.

This means that it is possible to find an optimal rapping interval with on average a maximum degree of separ¬ ation, while disregarding rapping losses, by selecting a suitable strategy of controlling. In addition, the losses during the rapping periods will of course increase by having short rapping intervals, but this is beyond the scope of the present invention. The manner in which one tries to find the optimal rapping interval largely depends on the electric control¬ ling of the electrostatic precipitator unit.

According to the present invention, a strategy of controlling the electrical parameters, for each precipi¬ tator unit, is selected and this strategy is maintained during the optimisation. The length of the interval between the rapping periods and possibly also other rapping para¬ meters are varied. The average of the current, voltage, or a quantity derived from the current and/or voltage, during the interval between the rapping periods, is calculated and used for controlling the length of the rapping intervals and/or other rapping parameters.

One suggested mode of operation comprises measuring of the average of the current during the interval between the rapping periods. The length of the rapping intervals and/or other rapping parameters are controlled so as to obtain as high an average as possible. Another suggested mode of operation comprises measuring of the average of the voltage during the interval between the rapping periods. The length of the rapping intervals and/or other rapping parameters are controlled so as to obtain as low an average as possi¬ ble.

Further suggested modes of operation comprises measur- ing of the average of the product of current and voltage or measuring the average of the time derivative of the voltage during the interval between the rapping periods, and by controlling the length of the rapping intervals and/or other rapping parameters so as to obtain as high an average as possible.

Within the inventive idea a further strategy of con¬ trolling maximises or minimises the average value of some other quantity derived from the current and/or voltage than the average power or the time derivative of the voltage during the rapping interval.

If the current is kept constant, it may be convenient to try to get the minimum average voltage or the maximum average increase (voltage derivative) . If the voltage is kept constant, it may be convenient to try to get the maxi¬ mum average current. If the current must be limited owing to so-called back corona from the dust layer, resulting in a decrease of the voltage when the current increases above a given value, it may be convenient to try to maximise the average power. If the precipitator unit is controlled so as to try to operate close to the limit of electric flashover, this corresponds basically to operation with constant cur- rent. It is the field strength in the gas, rather than the voltage between the electrodes, that decides the flashover level, and the principle of controlling can be selected to be about the same as if the current is kept constant.

The main parameter of rapping to be controlled is the length of the interval between the rapping periods.

However, within the scope of the inventive idea, it is possible to control several other rapping parameters such as, but not limited to, number of raps per rapping period, rapping force and current or voltage during the rapping period.

Within the scope of the inventive idea, it is also possible to choose to carry out a simultaneous optimisation of two or more rapping parameters. During a rapping period, it is also possible to apply to each collecting electrode a number of consecutive raps and, between the consecutive raps, successively decrease the voltage between the elec¬ trodes or the current to the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 2 shows schematically the current as a function of the time during some rapping intervals when keeping the voltage constant for two different durations of the rapping inter¬ vals, Figs 2a and 2b.

Fig. 3 shows schematically the voltage as a function of the time during some rapping intervals when keeping the current constant for two different durations of the rapping inter¬ vals, Figs 3a and 3b.

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 with direct current from three rectifiers 21, 22, 23. The rectifiers 21-23 are controlled and monitored by a control unit 30. The control unit 30 also communicates with devices 51, 52 and 53 for rapping of the collecting electrodes in the precipitator units 1, 2 and 3.

Fig. 2 shows schematically how tiie current I in a precipi¬ tator unit varies in time during a rapping interval T if the voltage U between the electrodes is kept constant.

Owing to the increase of the thickness of the dust layer, the current decreases in time. The maximum current immedi¬ ately after a rapping period S is higher after a long rapping interval (I_amax' Fig- ^2a) than after a short one (ϊbmax' ^Fi9- ^2b) ^•

Fig. 3 shows schematically how the voltage U between the electrodes in a precipitator unit varies in time during a rapping interval T if the current I in the precipitator unit is kept constant. Owing to the increase of the thick¬ ness of the dust layer, the voltage increases in time. The minimum voltage immediately after a rapping period S is lower after a long rapping interval (U_ami_n, Fig. 3a) than after a short one (Uj-jmi_j-,, Fig. 3b) .

By means of Fig. 3, one embodiment of the method can now be elucidated. A starting value for the length of the rapping interval for the precipitator unit 1 is chosen. The voltage curve is assumed to be like in Fig. 3a. The average of the time derivative of the voltage is calculated and the value is stored in the control unit 30. Then the rapping interval is shortened and the voltage curve is assumed to be like in Fig. 3b. The new average of the time derivative of the vol¬ tage is calculated and the value is stored in the control unit 30. The two average values of the voltage time deriva¬ tive are compared and if the second value is higher a new test is made with an even shorter length of the rapping interval. If the second value is lower a new test is made with a longer rapping interval than the first one. By iteration a best value for the length of the interval can be fixed. Simultaneously, the other two precipitator units 2, 3, are optimised in the same way.

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.

Claims

1. A method for optimisation of the performance of an elec¬ trostatic precipitator unit (1, 2, 3) comprising discharge electrodes and collecting electrodes, between which a high voltage is maintained by a constant or pulsating direct current supplied to the electrodes, such that, under the action of the electric field between the electrodes, the particles, charged by the current therebetween, are moved towards the collecting electrodes and are deposited there¬ on, dust deposited on the collecting electrodes being removed by mechanical rapping of the collecting electrodes by one or more mechanical impulses being supplied to the electrodes individually or in groups, in a predetermined manner, such that all the collecting electrodes of the precipitator unit are cleaned during recurrent, relatively short, rapping periods separated by rapping intervals of considerably longer duration, by controlling the duration of the rapping intervals and/or other rapping parameters, c h a r a c t e r i s e d by

selecting a strategy of controlling the electrical para¬ meters and maintaining this strategy during the optimis¬ ation,

varying the length of the interval between the rapping periods and possibly also varying the other rapping para¬ meters such as number of raps, rapping force and current or voltage between the electrodes during the rapping period,

measuring current and voltage during the interval between the rapping periods,

calculating the average of the current, voltage, or a quantity derived from the current and/or voltage, during the interval between the rapping periods, and

using this calculated average for controlling the length of the rapping intervals and/or other rapping parameters.

2. The method as claimed in claim 1, c h a r a c t e r i s e d by measuring the average of the current during the interval between the rapping periods, and by controlling the length of the rapping intervals and/or other rapping parameters so as to obtain as high an average as possible.

3. The method as claimed in claim 1, c h a r a c t e r i s e d by measuring the average of the voltage during the interval between the rapping periods, and by controlling the length of the rapping intervals and/or other rapping parameters so as to obtain as low an average as possible.

4. The method as claimed in claim 1, c h a r a c t e r i s e d by measuring the average of the product of current and voltage during the interval between the rapping periods, and by controlling the length of the rapping intervals and/or other rapping parameters so as to obtain as high an average as possible.

5. The method as claimed in claim 1, c h a r a c t e r i s e d by measuring the average of the time derivative of the voltage during the interval between the rapping periods, and by controlling the length of the rapping intervals and/or other rapping parameters so as to obtain as high an average as possible.

6. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by varying the length of the rapping interval, but keeping the other rapping parameters constant.

7. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by varying the current supply to the electrodes or the voltage between the electrodes during the rapping period, but keeping the length of the rapping interval constant.

8. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by varying the number of raps or the size of the supplied impulse in each rap, but keeping the length of the rapping interval and the other rapping

> parameters constant.

9. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by keeping the voltage constant during the interval between the rapping periods.

10. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by keeping the current constant during the interval between the rapping periods.

11. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by controlling during the interval between the rapping periods, current and/or voltage in a manner known per se, such that the electrostatic precipi¬ tator unit operates close to its flashover limit under the prevailing conditions.

12. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by applying, during each rapping period, a number of consecutive raps to each collecting electrode and, between the consecutive raps, successively decreasing the voltage between the electrodes.

13. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d by applying, during each rapping period, a number of consecutive raps to each collecting electrode and, between the consecutive raps, successively decreasing the current between the electrodes.