JPS6231614B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to an electrostatic precipitator (hereinafter sometimes referred to as EP), and in particular to an electrostatic precipitator that can maintain a constant dust collection performance regardless of fluctuations in boiler load. This article relates to a method of operating a dust collector.

The conventional method of controlling the operation of electrostatic precipitators is to detect the number of spark discharges and control the number of occurrences to a constant level, as described in the August 1980 issue of Kagaku Equipment. ing. In this method, when a spark discharge occurs, the charging voltage drops to nearly 0, but after that, charging is stopped for 1/2 to 1 cycle to prevent the spark discharge from re-igniting or transitioning to arc discharge, etc., and even when recharging. The charging voltage is increased over several tens of msec. Therefore, it takes about 0.1 sec from the generation of spark discharge to the time when a charging voltage is generated, corona discharge is started, and dust collection is performed. This time is a large loss for electrostatic precipitators that require high dust collection performance. For example, the charging time used as an index to indicate the scale of electrostatic precipitators installed at coal-fired power plants is set at more than 10 seconds, but spark discharges occur five times during this charging time. Then, about
There is a non-charging time of about 0.5 seconds, during which time no dust collection is performed. As a result, 99%
For electrostatic precipitators that require high dust collection performance, the size of the
This would require 2.5% larger equipment.

In addition, according to a recent patent publication, the dust concentration in the inlet flue and outlet flue of the electrostatic precipitator is detected to control the operation of the electrostatic precipitator, that is, to control the applied voltage and discharge current. Tend. The drawback of this method is that currently ON-
This depends on the existence of a device that detects the concentration of soot and dust in the flue. In other words, the current soot and dust concentration detection devices that can be used with ON-Line include devices that detect the concentration of soot and dust based on the transmittance of light, and devices that shine light on the soot and dust and detect the particle size and concentration of the soot and dust from the scattered light. However, in the former case, it is difficult to detect minute changes in the dust concentration, especially when the dust concentration at the outlet of an electrostatic precipitator is several tens of mg/m ³ N. It is difficult to detect changes in soot and dust concentration. The latter is the upper limit of detection in cases where the dust concentration is as high as 10 mg/m ³ N, such as in the flue at the entrance of a dust collector ^.
A dilution device of about 1000 times is required to reach the desired temperature, and a dilution device of about 10 times is required for the exit flue, but there are problems with the accuracy of this dilution device and low reliability due to dirt on the sensor. . Other measuring instruments measure dust concentration intermittently, so they are not suitable for ON-Line control of electrostatic precipitators.

An object of the present invention is to provide an operating method for controlling an electrostatic precipitator in response to changes in soot and dust properties accompanying fluctuations in boiler load.

The present invention detects the oxygen concentration or carbon monoxide concentration in the exhaust gas when electrostatically precipitating the soot and dust in the exhaust gas discharged from a coal boiler, and uses the detection result to determine the voltage applied to the electrostatic precipitator. and/or controlling the discharge current. The technical background that led to the invention of the present invention is outlined below.

As a countermeasure against nitrogen monoxide, there is a tendency to keep the amount of oxygen in coal as a fuel low, and as a result, it was thought that the properties of the generated soot and dust would change. Therefore, we measured the relationship between the oxygen concentration and carbon monoxide concentration in the exhaust gas, the soot and dust concentration, and the soot and dust properties, especially particle size and unburned matter. As a result, the relationship shown in Figure 1 was obtained. (In the figure, the horizontal axis shows the oxygen concentration and carbon monoxide concentration in the exhaust gas, and the vertical axis shows the soot and dust concentration.) In other words, the soot and dust concentration increases as the oxygen concentration decreases, and as the carbon monoxide increases. It is clear that this will increase as a result. Figure 2 shows the results of analyzing the properties of the soot and dust at this time. (In the figure, the horizontal axis shows the particle size, and the vertical axis shows the unburned particles.) From Figure 2, as the oxygen concentration decreases, the particle size of the generated soot and dust becomes smaller, and the smaller the particle size, the more unburned particles. It can be seen that there are more and more The following three effects can be considered on the electrostatic precipitator when such fine particles increase. One is that the dust collection performance decreases due to an increase in the number of fine particles, and the other is that the discharge current per unit throughput decreases due to the increase in the number of fine particles, resulting in an increase in the amount of charge that charges the dust. The last reason is that the electrical resistivity of soot and dust decreases with fine particles containing a large amount of unburned matter, and the electrical resistivity of soot and dust collected on the dust collection electrode decreases. Because the dust is low, it immediately releases the electric charge it had and is re-splattered, being entrained in the exhaust gas from the dust collection electrode again, reducing the dust collection rate.

Regarding the first item, it is clear from research and experience to date that electrostatic precipitators have poor collection performance for submicron particles of 1 μm or less. The following items are generally clear from the following formula.

I: Current, E: Electric field strength I = K ₁ E ² /1 + K ₂ S S: Specific surface area of soot and dust K ₁ , K ₂ : Coefficient In other words, even if the soot and dust concentration is the same, the discharge current decreases as the number of fine particles increases. However, the dust collection performance decreases because the amount of charge on the dust decreases.
This relationship is shown in FIGS. 3 and 4. Figure 3 shows the relationship between the rate of decrease in discharge current (vertical axis) and the dust concentration (horizontal axis).
This shows that the discharge current decreases. Figure 4 shows the relationship discovered by the present inventors, and shows the relationship between the apparent moving speed ratio of particles and the dust collection factor CF, which represents the dust collection performance of an electrostatic precipitator when generated under the same combustion conditions. Show relationships. The apparent moving speed ratio of particles on the vertical axis is the apparent moving speed ω of particles under a certain condition of the actual EP when the apparent moving speed ω of the particles is 100, and the apparent moving speed ω of the particles under other conditions of the actual EP or model EP.
The larger the speed ratio, the better the dust collection performance. Further, the dust collection factor CF on the horizontal axis is an index calculated from various factors that affect dust collection performance, and in the formula for calculating CF shown in the figure, each symbol means the following.

: Average particle diameter of soot and dust (μm) E: Average electric field strength of EP (KV/cm) i: Discharge current per unit dust collection area of EP (mA/cm)
m ² ) D: Spacing between the dust collection poles of the actual EP (cm) D ₀ : Spacing between the dust collection poles of the model EP (cm) a, b: Experimentally determined constants (-) From this figure 4, the dust collection factor CF The dust collection performance improves by increasing the dust collection factor CF.
It can be seen that in order to increase the average electric field strength E and the discharge current i, the average electric field strength E and the discharge current i can be increased.

Furthermore, it can be understood that even under the same applied voltage, the dust collection performance decreases as the discharge current decreases or the particle size of the dust decreases.

The last item is fine particles that contain a particularly large amount of unburned matter, that is, a large amount of soot deposited in the gas phase.Since the main component of this soot is carbon, it has a low electrical resistivity and is used in electrostatic precipitators. Because the electrical resistivity is at a level that causes re-splatter, re-splatter occurs;
This will cause a decrease in dust collection performance.

As mentioned above, the present invention is based on the knowledge that the concentration and properties of soot and dust in the exhaust gas of a coal boiler are closely related to the oxygen concentration and carbon monoxide concentration in the exhaust gas. Based on the detection results, the applied voltage and/or discharge current of the electrostatic precipitator is controlled to keep the dust collection performance constant.

An embodiment of the present invention will be described below with reference to FIGS. 5 and 6. The exhaust gas emitted from the boiler 1 has its oxygen concentration and carbon monoxide concentration detected by an exhaust gas analyzer 2, and is introduced into an electrostatic precipitator 4 through an air heater 3, where soot and dust are removed. It is discharged from the rear chimney. A high voltage is applied to this electrostatic precipitator 4 by a high voltage power supply 5. A comparator controller 6 is connected to the high voltage power supply 5 so that the voltage and current of the high voltage power supply can be controlled based on the output from the exhaust gas analyzer 2. This comparison controller 6 has figures 1 and 6 (horizontal axis: particle diameter, vertical axis, cumulative weight percentage) is input. FIG. 6 illustrates the tendency of the particle size distribution of soot and dust when the oxygen concentration in the exhaust gas is 2% and 5%. That is, the cumulative weight percentage on the vertical axis means the value obtained by dividing the cumulative weight of particles with a particle size smaller than the horizontal axis by the total weight. Therefore, in the case of this figure, it can be seen that there are more fine particles when the oxygen concentration is 2% than when the oxygen concentration is 5%. The voltage and current applied to the electrostatic precipitator are controlled in response to changes in these quantities. The method of this control is to keep the index (dust collection factor CF) shown in FIG. 4 above constant. By doing so, it becomes possible to control the electrostatic precipitator in a manner that can respond not only to changes in the amount of soot and dust but also to changes in the quality of soot and dust.

As mentioned above, the operating voltage of an electrostatic precipitator is generally set to the highest voltage that can be applied. That is, since the voltage immediately before spark discharge is applied, it is difficult to change the electrical input, that is, the amount of charge of the dust, by changing the voltage. Therefore, there is a method described below as a possible method of controlling the method according to the present invention.
Generally, the voltage waveform applied to an electrostatic precipitator is a waveform as shown in Figure 7a (horizontal axis: time, vertical axis: applied voltage), whereas the input voltage waveform is as shown in Figure 7b ( The horizontal axis: time, the vertical axis: primary voltage). That is, in FIG. 7a, the waveform changes due to the capacitance of the electrostatic precipitator. Therefore, the seventh waveform for control is
This will be explained based on Figure b. The applied voltage is kept constant, that is, the effective voltage is kept constant, and the current is controlled by changing the conduction angle of the control element on the input side, and the electrical input is kept constant in response to changes in dust. It is something. For example, during normal operation, the current is suppressed with the conduction angle set to A, but when the load on the boiler changes, the state of the dust changes, and there is not enough charge to charge the dust. In other words, when the current decreases, the conduction angle A is gradually expanded to the position B, and by controlling the index shown in Figure 4 to remain constant, the dust collection performance of the electrostatic precipitator is maintained constant. be able to. This control method is applicable not only to changes in the physical properties of soot and dust due to load fluctuations, but also to changes over time that occur in electrostatic precipitators for coal-fired power plants, in other words, the phenomenon in which the dust collection performance at the initial stage of operation deteriorates over time. is also applicable.
In this case, the reason for this change over time is that the dust adhering to the dust collection electrode remains without being peeled off by hammering, and the discharge current decreases even when the applied voltage is constant. , the index shown in FIG. 4 decreases, and the dust collection performance deteriorates, so the control method of the present invention is applicable.

Furthermore, the waveform shown in Figure 7b is a full-wave rectification waveform, but depending on the coal used in the coal-fired boiler, the electrical resistivity of the dust generated is high.
High resistance disturbances and reverse ionization phenomena may occur, so in order to respond to changes in coal type,
It is preferable to allow half-wave rectification of the waveform shown in FIG. 7b. In other words, in the case of a high resistance fault or reverse ionization phenomenon, an abnormal current generally flows, and an invalid current other than the current actually involved in dust collection flows, so in order to suppress these phenomena, It is effective to use half-wave rectification and control the conduction angle A to provide a time during which no discharge phenomenon occurs.

As described above, according to the present invention, there is an effect that the dust collection performance of the electrostatic precipitator can be kept constant in response to changes in the quantity and quality of soot and dust and changes in discharge current.

According to the present invention, there is an effect that the dust collection performance of the electrostatic precipitator can be maintained constant in response to changes in the boiler load and exhaust gas properties.

[Brief explanation of the drawing]

Figure 1 shows the relationship between oxygen concentration, carbon monoxide concentration, and soot and dust concentration, Figure 2 shows the relationship between particle size and unburned content,
Figure 3 shows the relationship between dust concentration and discharge current reduction rate.
Figure 4 shows the relationship between the dust collection factor CF and the apparent moving speed ratio of particles, Figure 5 shows the configuration of the present invention, Figure 6 shows the particle size distribution, and Figure 7 shows the control principle of the present invention. . 1... Boiler, 2... Exhaust gas analyzer, 3... Air heater, 4... Electrostatic precipitator, 5... High voltage power supply, 6...
Comparison control.

Claims (1)

[Claims] 1. Detecting the oxygen concentration or carbon monoxide concentration in the exhaust gas when collecting soot and dust in the exhaust gas discharged from a coal boiler using an electrostatic precipitator,
As the oxygen concentration decreases or the carbon monoxide concentration increases, the applied voltage and/or discharge current of the electrostatic precipitator is controlled to increase, and the oxygen concentration increases or the carbon monoxide concentration increases. A method for operating an electrostatic precipitator, characterized in that the applied voltage and/or discharge current of the electrostatic precipitator is controlled to decrease as the voltage decreases.