EP0734773B1 - Method for continuous optimization of the operating condition of an electrofilter - Google Patents

Abstract

Continually optimising the operating condition of an electrostatic filter comprises: (a) operating with a direct voltage with superimposed pules, derived by full-wave rectification of the mains voltage, the time spacing between successive pulses being variable by screening a selectable number of full mains waves; (b) operating with a constant number of pulses in the normal phase and subsequent test phase of each of successive operating periods; (c) measuring the residual voltage during the normal and test phases in each of successive cycles (1,2,3...,k,...) at a given limitation of the current or voltage and iteratively varying the limitation, depending on the residual voltage change from cycle, so that the residual voltage tends towards a max.; and (d) operating the test phase of each operating period with a number of pulses differing by 1 from that in the normal phase, measuring and comparing the mean voltage values in the normal and test phases and, depending on the difference, iteratively varying the number of pulses from operating period to operating period such that the mean voltage value tends towards a max..

Description

An electrostatic precipitator has two sets of electrodes: precipitation electrodes and spray electrodes. The precipitation electrodes usually exist from profiled sheet metal strips that lead to several parallel walls are composed. Two adjacent walls form an alley for the gas stream to be cleaned. They are in the middle of the alley Spray electrodes arranged. They often consist of wires or Ribbons with laces. Mostly they are Precipitation electrodes are grounded, and the spray electrodes are with a High voltage source connected.

The dust particles to be separated are separated by electrons the spray electrodes are released, ionized and in the between Spray electrodes and precipitation electrodes existing electrostatic field deflected from the gas flow and to the Precipitation electrodes deposited. The carried electrical They transfer charge to the precipitation electrode.

However, if the dust particles to be separated have a very high specific resistance (> 10 ¹¹ Ωcm), under certain operating conditions the electrical charge from the dust layer deposited on the precipitation electrodes cannot flow away as quickly as it is charged by the influx of further charged particles . The result is the so-called back spraying, ie a discharge which is opposite to the discharge occurring at the spray electrodes. The back spraying throws dust back into the gas stream. The degree of separation deteriorates.

The electrostatic separation of high-resistance dusts is one certain strength of the current flowing between the electrodes of a Electrofilter flows, optimally. The deposition deteriorates Increase in the current, this is an indication that the Back spraying has started. To optimize the deposition, it is therefore necessary to limit the current so that the Back spraying is just avoided.

The optimal operating point depends on the parameters of the cleaning gas flow. If the parameters change, then in a change in the operating point is also generally required. This be explained using a few simple examples:

An electrostatic filter can remove the fly ash from the flue gas separate the coal-fired boiler. With the steam generated in the boiler A generator can be used to generate electrical energy via a turbine operate. The need for electrical energy increases within of the daily load cycle, less steam is required. For Reduced steam generation, less coal is fired. Accordingly, the amount of fly ash that the e-filter decreases must cut off. By changing the mode of operation of the boiler from Full load to partial load thus changes an essential parameter, namely the volume flow of the fly ash.

To clean the boiler walls, they are used during operation Water vapor emitted. This process is called Soot bubbles. Soot blowing can be done 3 to 4 times a day and take half an hour to an hour each. The Steam leaves the boiler with the flue gas through the Dust collector. Part of the moisture is deposited on the fly ash particles and changes the electrical properties of fly ash and Flue gas. Soot blowing also changes the operation of the Electro filter essential parameters.

Load changes and soot bladders are only examples of perfectly ordinary ones Operations in the operation of an electrostatic filter, in which the Change the parameters of the gas flow to be cleaned.

EP 0 097 161 B1 discloses an electrostatic Operate separator with a current that is just the point of insertion of spraying back. Current and voltage are monitored in which one increases or decreases the arousal to the point determine at which the back spraying begins. The source from which the Excitation is powered by a 50 Hz network. The stream consists of one Pulse train, and the voltage is a DC voltage with superimposed AC component. With conventional two-way rectification the mains voltage results for the current and the AC component of the voltage a frequency of 100 Hz.

EP 0 140 855 B1 describes a method for changing one of the Electrodes of an electrostatic dust collector Known voltage, in which the voltage by one of the Grid frequency derived pulse train is generated and the change by causing the length of the interval between two successive individual pulses by hiding a straight one Number of pulses is varied. The number of pulses per second is depending on the number of hidden pulses, this means that the number is 33, 20, 14, 11, etc. degraded. The mains voltage is one via thyristors High voltage transformer supplied, the secondary side with a Two-way rectifier is connected. The output voltage of the The rectifier is connected to the electrodes of the electrostatic filter. The thyristors are controlled by a control loop that is switched so that it between two pulses that are fed to the electrostatic filter, one Deletes even number of pulses from the mains voltage.

It is known from EP 0 465 547 B1 for controlling the power supply the discharge electrodes of an electrostatic filter in order to achieve a max.

Dedusting the discharge electrodes current pulses with a given Supply current and the number of pulses per second according to vary the aforementioned document. In doing so, each other corresponding instantaneous values of the voltage between discharge and Precipitation electrodes for a number of different pulse frequencies is measured, and the current pulse supply is then adjusted to the pulse frequency for which the largest instantaneous value has been measured. The pulse current is taken into account the capacity of the Power supply unit and possible flashovers between the Discharge and precipitation electrodes to a maximum set.

The invention has for its object a method for operating an electrostatic precipitator at which the set operating point is continuously monitored and tracked so that the filter is constantly in works close to the optimal operating point. This task will solved according to the invention by the features specified in claim 1.

Further advantageous features are specified in the subclaims.

Figures 1-3 serve to illustrate the invention with reference to Diagrams.

Figure 1 illustrates the course of current and voltage for different cases, characterized by different length of the interval distinguish between two successive pulses.

Figure 2 illustrates the succession of operating periods.

Figure 3 illustrates the sequence of cycles within a single one Phase.

In Figure 1, five diagrams arranged one above the other show the course of current and voltage for conventional operation with of current and voltage for conventional operation with Mains frequency and two-way rectification (above) as well as in the so-called "Semi-pulse operation" with different pulse numbers 1 - 4. At the The current path is the same as the two-way rectification Voltage curve through a chain of pulses at intervals of 10 ms characterized. The voltage pulses are a DC voltage overlaid. At pulse number 1, the high voltage part of the Power supply between two pulses for a full network oscillation not controlled. The remaining pulses therefore have one increased interval of 30 ms. Every voltage pulse has at the beginning a steeply rising flank. The other flank falls until the onset of the next following pulse approximately exponentially to one Residual voltage. The current pulses rise steeply up to one Maximum current and then drop just as steeply to 0. Below "current" always means the maximum current. If the number of pulses is 2 Two full network oscillations do not occur between two pulses switched through. The remaining pulses are therefore at a distance of 50 ms. The corresponding characteristics for the pulse numbers 3 and 4 are readily apparent from Figure 1.

FIG. 2 shows several successive operating periods m, m + 1, m + 2, ... shown schematically. Each operating period includes one Normal phase and a subsequent test phase. The normal phase takes much longer than the test phase. The duration of the normal phase is preferably about 4: 1 to 20: 1 for the duration of the test phase. The normal phase lasts e.g. B. 1 h, the test phase 5 - 10 min.

A constant pulse number is used in each normal phase, also in every test phase. However, the pulse number deviates from the test phase the pulse number of the immediately preceding normal phase by ± 1 as explained below.

The normal phase and the test phase each comprise a sequence of cycles, which are consecutively numbered 1, 2, 3, ..., k, k + 1, ... The cycles follow one another at intervals of 20-40 s, preferably approximately 30 s. In each cycle, one turns on the control device of the electrostatic filter upper limit for the current, d. H. a current limit, entered. The associated residual voltage is measured and due to of the measured value obtained becomes the current limit for the following Cycle set, as explained below with reference to Figure 3.

In Figure 3 there are several along the horizontal time axis successive cycles k, k + 1, ... are represented symbolically. The In this case, there is a time interval between the individual cycles 30 s. A normal phase is considered first.

The time 0 in FIG. 3 can be any time during the Operation, z. B. the switch-on time or the start of a Normal phase. At this point the current limit for the Cycle k set to 450 mA. The associated residual voltage is according to the upper diagram of Figure 3 at about 25 kV. Subsequently the current limit is increased to 500 mA to try out whether a higher residual voltage now arises. The increased Current limitation results in a residual voltage of in cycle k + 1 25.8 kV. As the increase in the current limit leads to an increase in the Residual voltage has led to k + in the following cycle Current limit increased again, this time to 550 mA. It follows again an increased residual voltage, namely 26.2 kV. Because the change the residual voltage also has a positive sign this time the current limit in the same direction changed again, d. H. increased to 600 mA. There is an increased cycle k + 3 Residual voltage of 26.5 kV. Another try through one Increasing the current limit to 650 mA is an even higher one Reaching residual voltage, however, fails in cycle k + 4. The Residual voltage drops to 26.2 kV. This is an indication of the beginning of the Spraying back. Therefore, the current limitation is restored in cycle k + 5 set lower, namely to 600 mA, with the effect that the Residual voltage rises to 26.5 kV. After in cycle k + 5 the Lowering the current limit to an increased residual voltage has led, the current limitation for the cycle k + 6 is also degraded. This time, however, there is a lower residual voltage of 26.2 kV. Therefore, the current limit for the next cycle changed again in the opposite direction, namely increased to 600 mA. This leads to an increased residual tension in cycle k + 7. Apparently the condition has stabilized from cycle k + 3. The consequence is that from now on the current limitation between 550 and 650 mA now and then commutes here. The residual voltage is quasi stationary Value. It fluctuates slightly around 26.4 kV. That state is at the set pulse number and the current parameters of the optimal gas flow to be cleaned. Should the parameters change however change during the normal phase under consideration, so gropes Control device analogous to the cycles k to k + 3 at the optimal Operating point that corresponds to the changed parameters. Experience has shown that the changes in the parameters are relative slow, measured by the period of the cycles. The changes the current limitation from cycle to cycle that at the specified Example each ± 50 mA, are in any case small compared to that Current to which is limited, preferably about 5 to 15%. Therefore, the voltage changes are also in the stationary state relatively small, so that the filter operation through them is not noticeable is affected.

It goes without saying that considering the Current-voltage characteristic, which is to be determined experimentally, instead of Current can also limit the voltage.

During the entire duration of the normal phase or at least during a period of time that spans several cycles at the end of the Normal phase extends by evaluating a large number of An average value of the voltage is calculated and saved. As an average z. B. selected the effective voltage.

A test phase follows the normal phase. In the test phase should be tried out whether with a changed pulse number Deposition can be improved.

In the test phase, a pulse number is used that differs from the Pulse number of the immediately preceding normal phase by ± 1 differs. Also in the test phase are analogous to the normal phase go through numerous cycles. After becoming a quasi stationary State has been set, an average of the Voltage calculated and saved. This mean is compared with the Average of the associated normal phase compared.

In the example illustrated in FIG. 2, for Operating period m that the mean value in the test phase is lower than in the normal phase. The change in the pulse number - in this case from 5 to 6 - has no improvement in terms of increasing the Average voltage result. Therefore, in the normal phase following period m + 1 worked again with the pulse number 5. Because in the Operating period m an increase in the number of pulses was unsuccessful, is in the Test phase of the operating period m + 1 reduced the number of pulses to 4. The This time, too, the effect is that the mean value of the voltage sinks. Therefore, in the normal phase of the operating period m + 2 Pulse number reset to 5. In the subsequent test phase tried again the pulse number 6, again with the success that the Average voltage drops. Consequently, also in the Operating period m + 3 in the normal phase the pulse number 5 again set. In the test phase, the pulse number 4 is tried again, this time with success, apparently because a characteristic of cleaning gas flow has changed. It turns out an increased Average voltage. Since the decrease in the number of pulses in the Operating period m + 3 was successful, will be in operating period m + 4 keep the pulse number 4 in the normal phase. In the subsequent Test phase, the pulse number is reduced again, to 3. The effect but is negative. Therefore, in the normal phase of the operating period m + 5 reset the pulse number to 4.

Since the test phase is relatively short compared to the normal phase and the Pulse numbers in the normal phase and the subsequent test phase only Differences by ± 1 are the fluctuations caused by this relatively small and have little influence on the quality of the individual Deposition. However, if the Change the parameters of the gas flow to be cleaned sustainably by the method of operation illustrated in FIG. 2 causes the Electrostatic filters always close to the respective optimal operating point is working.

Claims (6)

1. Method for continuous optimisation of the operational state of an electrostatic filter, with the following features:

   a) operation is with a direct voltage, which is derived from the mains voltage by full-wave rectification, with superimposed pulses, wherein the spacing in time between successive pulses is variable by masking a selectable number of full mains waves;

   b) successive operating periods (1, 2, 3, ..., m, ...) each comprise a respective normal phase and consecutive test phase;

   c) operation in each operating period is with a constant pulse number in the normal and in the test phase;

   d) during the normal phase and the test phase the residual voltage is measured in each of successive cycles (1, 2, 3, ..., k, ...) with a given limitation of the current or the voltage and the limitation is iteratively varied in dependence on the change in the residual voltage from cycle to cycle so that the residual voltage tends to a maximum;

   e) in each operating period (1, 2, 3, ..., m, ...) operation in the test phase is with a pulse number which is changed by comparison with the normal phase by ± 1 and the mean values of the voltage in the normal phase and in the test phase are measured and compared with one another, and in dependence on the difference the pulse number is iteratively varied from operating period to operating period so that the mean value of the voltage tends to a maximum.
2. Method according to claim 1 for continuous optimisation of the operational state of an electrostatic filter, with the following features:

   a) operation is with a direct voltage, which is derived from the mains voltage by full-wave rectification, with superimposed pulses, wherein the spacing in time between successive pulses is variable by masking a selectable number of full mains waves;

   b) successive operating periods (1, 2, 3, ..., m, ...) each comprise a respective normal phase and consecutive test phase;

   c) operation in each operating period is with a constant pulse number in the normal and in the test phase;

   d) the pulse number of the test phase deviates from the pulse number of the immediately preceding normal phase by ± 1, wherein the sign is determined in accordance with the following features h and i;

   e) the normal phase and the test phase each comprise a sequence of cycles (1, 2, 3, ..., k, ...) according to the following pattern:

   ea) the current is limited to i_k;

   eb) the residual voltage u_k is measured;

   ec) the current is limited to a value ik+1 = ik + Δik deviating from i_k, wherein Δi_k is positive or negative and the absolute value | Δi_k | is small relative to i_k;

   ed) the residual voltage u_k+1 is measured;

   ee) the difference Δu_k = u_k+1 - u_k is ascertained;

   ef) the current is limited to a value ik+2 = ik+1 ± Δik deviating from i_k+1, wherein the sign of Δi_k corresponds with the sign according to ec) when and only when the sign of Δu_k according to ee) is positive;

   f) a mean value of the voltage is ascertained and stored at least in the end section in both the normal phase and the test phase;

   g) the mean value of the test phase is compared with the mean value of the associated normal phase;

   h) if in the period (m) the mean value in the test phase is not greater than in the normal phase, then in the following period (m + 1) operation is with the same pulse number in the normal phase as in the normal phase of the period (m) and on transition to the test phase the pulse number is changed in reverse direction to that in the period (m);

   i) but if in the period (m) the mean value in the test phase is greater than in the normal phase, then in the following period (m + 1) operation in the normal phase is with the same pulse number as in the test phase of the period (m) and on transition to the test phase the pulse number is changed to the same direction as in the period (m).
3. Method according to claim 1 or 2, characterised in that the duration of the normal phase relative to the duration of the test phase is in a ratio such as 4:1 to 20:1.
4. Method according to one of claims 1 to 3, characterised in that the duration of an operating period amounts to 1 to 2 h.
5. Method according to one of claims 1 to 4, characterised in that the duration of a cycle amounts to 10 to 30 s.
6. Method according to one of claims 2 to 5, characterised in that the absolute value |Δi_k| amounts to between 0.05 i_k and 0.15 i_k.

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