SE507673C2 - Ways of regulating power supply to an electrostatic dust separator - Google Patents

Abstract

In an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes between which a varying high voltage is maintained, a pulsating direct current supplied thereto is controlled. The frequency, the pulse height and/or the pulse length of the pulsating direct current are varied, so as to obtain a plurality of frequency-height-length-combinations. The charges of the individual pulses are being measured or calculated. The frequency-height-length-combinations resulting in flashover between the discharge electrodes and the collecting electrodes are registered. The size of the pulse charge at flashover is used for selection of the frequency-height-length-combination of the pulsating direct current.

Description

507 10 15 20 25 30 35 673 is the most common disturbance the cleaning of the filter that must be applied to the precipitation electrodes stored dust must be able to be removed from the filter. At this cleaning, emissions are temporarily increased very sharply if 2 no special measures are taken.

An often very difficult problem to overcome when it occurs dust with high resistivity must be separated. At such operating cases, you are often forced to work with extremely unfavorable same operating parameters due to the risk of partial discharges in the gradually growing dust on the precipitating electrodes. the layer. When you get partial discharges in the dust layer becomes the effect i.a. emission of charges and dust from precipitates the electrodes, s.k. back corona.

To optimize operations and reduce energy consumption while the separation is improved has several methods for pulse supply of the current to the filter has been suggested. Example are found in US-4,052,177 and US-4,410,849. In the first mentioned it is suggested to input pulses which are of the order of magnitude microseconds, which means that the rectifiers become a lot expensive. In the latter, pulses of the order of magnitude are suggested milliseconds, which can be obtained quite easily by selective control of ordinary thyristor rectifiers such as supplied with AC voltage of mains frequency.

The continued technical development in the field of electronics has also made it possible to use a frequency conversion to an intermediate frequency of 10 to 50 kHz reduce the size of transformers and rectifiers.

By pulse width modulation of this frequency is obtained with relatively simple mean pulse lengths down to 0.02-0.10 ms and rapid regulation with reliable control of power supply the supply to the electrostatic precipitator. Such methods are described in i.a. DE 35 22 568 and WO 88/07413.

With the new technologies, the number of control parameters has increased and thus the complexity of the control systems. Unfortunately leads this also means that the regulation itself increases the disturbance in the function of the separator. In the same way as emissions increase during the cleaning of the filter, the emissions will increase 10 15 20 25 30 35 3 sn? 673 during the period of adjustment or control of the set control parameters are made.

If adjustment is made manually using the switch on an opacity meter (smoke density meter) takes so long to the adjustment that you can get very well with varying operation such large emissions during the adjustment itself that these become as much of the total emissions as those that depend on the cleaning. In addition, there is a risk that operational tions affect the adjustment so that the optimization fails significant changes in dust concentration or gas the temperature occurs during the time required for adjustment eringen.

Furthermore, as mentioned, the actual cleaning of precipitates electrodes to a temporarily sharply increased dust concentration in the outgoing gas. Each measurement of opacity ethylene for adjusting the power supply should therefore be done only during times when no cleaning is performed. Then this happens very often in the separator closest to the fireplace, or other source of dust, there is a high risk of cleaning nevertheless has a decisive negative effect on balancing.

It is therefore of the utmost importance that methods read for a fast and safe adjustment of the power supply to electrostatic precipitators based solely on electrical measurements in the separator itself or associated rectifier. It has been shown that even if the cleaning very strongly affects the dust concentration in it from the separator outgoing gas changes the relationship between current and voltage in a separator only marginally due to this.

Some experiments with optimization based on measurement only of electrical quantities have already been made and as an example reference is made to US-4,311, 491, EP-465 547 and EP-184 922. examples, however, have remaining shortcomings in terms of DQSSB. , in case of process changes, and reliability, when to find the setting that provides minimal energy consumption use under varying conditions when separating high-resistance dust. An advanced and in most cases the correct method is further described in WO 93/10902. This is, however 507 673 10 15 20 25 30 35 4 has the weakness that it is complicated to implement era in a control unit and is becoming increasingly difficult to use, of course shorter pulses are.

OBJECT OF THE INVENTION It has been shown that the methods tried so far have not always, when separating high-resistance dust, leads to the optimal parameter combination. This applies especially for the methods based on the measurement of dust integration, but this also applies to those proposed so far methods based on the measurement of electrical quantities.

In general, it also applies to all balancing activities means that normal operation is disturbed. The operating parameters must be changed slightly from the setting considered to be it best to be able to determine whether the parameter combination tion is still the best. The disturbance often lasts during a considerable time interval.

It is a principal object of the present invention that indicate an improved method for selecting operating parameters for electric dust separators when separating so-called difficult dust. Exv. dust with high resistivity.

Another object of the present invention is to assign a method that, based on the measurement of electrical only quantities, generally provides a faster and safer integration electrostatic precipitators.

It is especially an object of the present invention to indicate a method that can be used for all heart rate feeding all the way down to pulse lengths of the order of 1 microsecond and below and as without complicated calculation programs with high safety provides the best operating point for the static dust collector.

SUMMARY OF THE INVENTION The present invention relates to a method of static dust collector unit, including emission electrons electrodes and precipitation electrodes, between which a varying 10 15 20 25 30 35 5,507,673 high voltage is maintained, control a pulse supplied to them erande DC. In the method according to the invention it is varied pulsating direct current frequency, pulse height and / or pulse length, so that a plurality of frequency-height-length combinations obtained.

For each of these combinations is measured or the charge of the individual pulses is calculated. The frequency-height length combinations leading to estimates between emissions sion electrodes and precipitation electrodes, are registered.

The size of the pulse charge at overflow was used for selecting the frequency-elevation of the pulsating direct current length combination.

GENERAL DESCRIPTION OF THE INVENTION One of the basic observations in the operation of the static dust separator is that the separation becomes more effective when the voltage between emission electrodes and precipitation electrodes are increased. essentially one finds that the physical velocity of the particles depends on the square of the electric field strength and thus also on the square of the voltage.

Effective separation therefore requires a high voltage.

The higher the better. The useful voltage range limited upwards by having an overshoot between the electrodes.

With trouble-free operation, you therefore seek to work so closely the projection limit as possible.

High voltage also generally means the current density gets high. If the dust that is separated conducts current well means this is not a problem, but if the dust has a high resistivity the density and current density of the gas is high, it will be separated the dust layer on the precipitation electrodes to be charged so that the electric field strength in the layer becomes sufficient to drive the same high current density through the dust layer.

This often leads to electrical degradation of the dust layer, s.k. back corona if not the current density limited. 507 673 6 10 15 20 25 30 35 It has long been known, more than 50 years, that pulse feeding of the current to electrostatic precipitators provides improved performance of the separator when the dust is difficult to separate, i.e. highly resistive. As mentioned above, this has led to one with sometimes very complicated equipment sought to introduce necessary energy in the separator even with very short pulses.

Eventually, the knowledge grew that it worked excellent even with pulses of the same order of magnitude as the waves in the usual AC voltage used in the distribution networks. This was explained by charge changes in the dust layer, which during charge growth causes the so-called the radiation, has a time constant of about 1 second.

However, it must not be interpreted as meaning that it takes 1 second to upload the layer, although many make this mistake, without as if it takes about a second for the layer to discharge when charging has stopped. Charging is controlled only by applied charge, i.e. of the size of the current.

The charging can thus take place in less than a millisecond the current is large enough.

Pulse supply of the current also affects the override voltage. to some extent, and the current at overflow at very high degree. The charge in each individual pulse is further, at over- stroke, in principle a function of the pulse rate.

Of course, you can generate estimates with any size pulses. Here and in the future is often implied, with charge at flashover, the smallest charge as at current frequency, or other given conditions, gives estimates.

If the pulses are assumed to be very short and carried out at such intervals that the dust layer has time to lose the bulk of its charge between the pulses it becomes charge that must be applied in a single pulse to cause projections substantially constant, regardless of the pulse the frequency. If you increase the heart rate, one gradually comes increasing part of the charge from the previous pulse to remain in the dust collector when the next pulse comes and thereby reducing the charge supply required for the transfer kind. If the pulses are very close, each individual pulse can be very small when projected. 10 15 20 25 30 35 507 673 If we assume that the resistivity of the dust layer is so 7 low that there is no risk of re-radiation is the limit the value of the pulse size at high frequencies is zero. Conversely it can be said that the required pulse charge during flashover grows with the interval between the pulses, from zero when the interval is very short, to a constant value when the interval is long, usually seconds. Figure 1 illustrates this principle. By intervals between the pulses is meant here and in continuation T = 1 / f where f is the pulse rate.

If the shape of the pulses changes, it is found that a longer pulse will mean that a larger charge is required for than with a shorter pulse if the pulse frequency is kept constant aunt. This is because the filter loses some of its charge. already during the pulse, i.e. the efficient charge can be considered to have decreased. A curve mass according to Fig. 2 is obtained.

The relationship between the curves obviously depends to a large extent on the resistivity of the fabric layer, if any.

A high-resistance dust layer will change the appearance on the curves. It is found that it is sometimes possible to feed a very high current without even coming close to expected surge voltage. Sometimes you still get an estimate a very low voltage, sometimes you can input available rectifier maximum current without getting an estimate.

In cases where a pulse pulse is sought for an estimated pulse charge when you have a high-resistance dust layer on the precipitation the electrode you will find that at low frequencies gets about the same result as when the dust is low resistance, while at high frequencies a growing pulse charge is needed instead of a decrease. Principle connection between pulse charge at flashover and frequency too short pulses are shown in Fig. 3.

If a new curve is registered for another pulse shape find one that it, like what applied to low-resistance dust is higher when the heart rate is longer. One finds further that certain pulse lengths do not lead to estimates over a certain frequency. In general, one can find a tendency towards a mini- mum for a certain frequency. This can be more or less sov 675 8 10 15 20 25 30 35 pronounced and depends on the resistivity of the substance in question. One curve mass is shown in Fig. 4.

This minimum can be said to be due to the electrostatic limitation of the dust collector at short intervals between the pulses are set by the dust layer, while at long intervals The choice between the pulses is determined by what happens in the gas that flows through the dust collector. The present invention is based on the unpredictable discovery that a relationship exists between the efficiency of the dust collector and that minimum pulse charge required for flash. One should choose to work in the area where you are close to the minimum pulse charge for estimates with the given limitations such as pulse devices and other equipment has.

According to the present invention, therefore, it is proposed that varies the length, height and frequency of the pulses so that for example, for each frequency determines the minimum charge leading to estimates and then repeating this procedure for other frequencies and uses these values on the pulse charge the estimate for selecting parameters for the set the operation. Appropriately, a survey of a sufficiently large frequency range so that one finds one certain frequency which gives the very lowest value of the pulse at the estimate, over which frequency you get one again increased pulse charge, and continues operation at a frequency close to this value. In step with changes in the gas that should purified, the size of the pulse is then varied at this frequency so that you are close to the estimate limit. Preferably you choose exactly the frequency that gave the minimum for charging.

The preferred variation of the appearance of the pulses is such as to select the maximum height of the pulse and for to get a varying charge varies the width of the pulse.

An alternative is to keep the relationship between length and height substantially constant. Within the scope of the invention of course also that the frequency is kept constant and pulse length and pulse height. The choice of control algorithm depends on the structure of the pulse device. A constant height is preferable at use of pulse width modulated high frequency converters. At use of mains frequency and thyristors one is forced to 10 15 20 25 30 35 507 675 9 accept that width and height covariate in a way that determined by the mains voltage and that the pulse frequency can only varied as submultiples of the mains frequency. When used of high-frequency converters, the pulse frequency can be varied e.g. between 1 Hz and 10 kHz, preferably between 1 Hz and 1 kHz.

One of the great advantages of the invention is the measurement technology the simplicity. By keeping track of the controls easily frequency, the charge of the pulses can be calculated from the current size by dividing the current by the number of pulses per unit of time. When using pulse width modulated high frequency frequency converter, the pulse charge can be calculated from the pulse width.

BRIEF DESCRIPTION OF THE FIGURES The invention will now be described in more detail in connection with attached drawings there Fig. 1 shows the principal relationship between pulse charging at estimate and the interval T between the pulses at separation fabric of low resistivity; Fig. 2 shows the corresponding for varying pulse lengths; Fig. 3 shows the principal relationship between pulse charging at estimate and the interval T between the pulses at separation fabric of high resistivity; Fig. 4 shows the corresponding for varying pulse lengths; Fig. 5 shows a simplified wiring diagram for a device suitable for carrying out the proposed method.

DESCRIPTION OF THE PROPOSED EMBODIMENT Figure 5 shows in a principle circuit diagram a voltage converting device that supplies high voltage direct current to a dust separator 1. The device consists of a three-phase rectifier bridge 2, a pulse generator 3, a transformer 4, a 507 673 10 15 20 25 30 35 10 full-wave rectifier bridge 5 for single-phase, a choke 6 and a control equipment 7 with associated measuring resistors 8, 9 and 10.

The three-phase rectifier bridge 2 comprises six diodes 21 to 26 and are connected via three conductors 27, 28, 29 to an ordinary three-phase AC network.

The pulse generator 3 consists of four transistors 31 to 34 and four diodes 35 to 38. The transistors are controlled by their bases are connected to the control equipment 7.

The full-wave rectifier bridge 5 consists of four diodes 51 more 54.

The control equipment 7 is in addition to the connection to the resistors 31-34 connected to a measuring resistor in series with dust collector 1, for measuring the current to the dust collector the electrodes of the separator, and to a voltage divider consisting of two resistors 9 and 10 connected between the dust collector electrodes for measuring the current voltage between them electrodes.

The function of the device is as follows. Via lines 27-29 the rectifier bridge 2 is supplied with three-phase alternating current. This similar is rectified and transmits a direct voltage via lines 11 and 12 to the pulse generator 3. With the control equipment 7, the istorernes 31-34 lead times so that a pulse width modulated voltage, substantially shaped like a square wave, via 13 and 14 are applied to the primary side of the transformer 4.

The secondary winding induced in the transformer 4 the voltage is rectified by the rectifier bridge 5 and via smoothing The electrodes of the pin separator 1 are fed with it obtained direct current.

The control equipment 7 controls the transistors 31-34 as mentioned and also monitors the current and voltage of the dust collector via resistors 8 and 10. By the lead times for transistors the pulses can be controlled by the pulse width of the generated, substantially square-shaped current, is varied and thus you control both current as voltage in the dust separator.

In an exemplary use of the invention, it is contemplated the device as above operate at an intermediate frequency of 50 10 15 57673 ll kHz and a current pulse frequency of 10 Hz. The heart rate is it for the device maximum and the current is regulated so that the limit is continuously sensed by varying the pulse length slightly around an average of about 1 ms.

During the adjustment, the pulse frequency is reduced to 5 Hz and the pulse length is increased gradually until an estimate is made. The pulse charge registered. With gradually increasing heart rate, up to values above 10 Hz, the process is repeated so that for each the minimum override charge is determined. The frequency for which the minimum pulse charge noted during regulation the estimate obtained was used in the continued operation.

At this frequency, the pulse rate is then varied in the usual manner. the length so that substantially maximum current can be supplied.

The interval between adjustments must be determined unitary, but since the adjustment does not mean a total interruption or a decisive disturbance can it without inconvenience performed at short intervals if the operating parameters vary.

Claims (10)

507 673 12 10 15 20 25 30 35 PATENT CLAIMS In an electrostatic precipitator unit, comprising emission electrodes and precipitation electrodes, between which a varying high voltage is maintained, control a pulsating direct current supplied thereto, the charge of the individual pulses being measured or calculated characterized in that the DC and pulsed current or pulse length is varied so that a plurality of frequency-height-length combinations are obtained; that the frequency-height-length combinations leading to flares between emission electrodes and precipitating electrodes are recorded, and that the magnitude of the pulse charge at flips is used to select the frequency-height-length combination of the pulsating direct current. A method according to claim 1, wherein the pulses are generated by a pulse device, characterized in that the height of the pulses is selected as the one which the pulse device can generate at a maximum and that the adjustment takes place by varying the pulse length and frequency. 3. A method according to claim 1, characterized in that the relationship between the height and length of the pulses is kept constant during the adjustment. 4. A method according to claim 1, characterized in that the pulse frequency is kept constant and that the adjustment takes place by varying the pulse length and pulse height. A method according to claim 1, wherein the pulse is generated by phase-controlled rectifiers transmitting at least a part of a half-wave from a substantially sinusoidal mains voltage which after up-transformation and rectification is supplied to the electrostatic precipitator unit, characterized in that the frequency is varied by holding the phase-controlled rectifiers during a part of or a whole half-wave and then substantially non-conductive during one or more half-waves. A method according to any one of claims 1 to 4, wherein the pulses are generated by a pulse device, in the form of a high frequency converter, by pulse width modulation of an intermediate frequency and the intermediate frequency of the pulse device is between 1 and 100 kHz, preferably between 10 and 50 kHz, characterized in that the pulse frequency is varied between 1 Hz and 10 kHz, preferably between 1 Hz and 1 kHz. A method according to any one of the preceding claims, characterized in that the frequency-height-length combination for which the required pulse charge at flashover is least determined is determined and that a pulse frequency close to that obtained with this frequency-height-length combination selected for continued operation. 507 673 1 ,, 10 15 20 25 30 Method according to one of Claims 1, 2, 3, 5 or 6, characterized in that the adjustment starts with a frequency which is lower than that which applied to the operation just before the adjustment and for the current frequency, the minimum required. pulse charge at flashover until at gradually increasing frequency determines, one has passed a minimum for pulse charge at flashover and then select pulse parameters close to those that gave the current minimum for continued operation. 9. A method according to any preceding claim, characterized in that the current to the electrostatic precipitator unit is measured and that the charge of the individual pulse is calculated as the average value of the current divided by the pulse frequency. 10. A method according to any one of claims 2, 6, 7 or 8, characterized in that the charge of the individual pulse is calculated by means of the pulse width.