EP3156132A1 - Method and device for filtering coal dust from exhaust steam from the coal drying process - Google Patents

Abstract

The inventive method for filtering carbon dust from the exhaust gas (7) of the drying of coal is characterized in that the exhaust steam (7) are subjected to a filtering in an electrostatic filter (6), wherein between at least one spray electrode (10) and at least a voltage is applied to a precipitating electrode (11), the voltage being generated by rectifying a three-phase alternating voltage. The corresponding device (1) according to the invention comprises an electrostatic filter (6) with at least one spray electrode (10) and at least one deposition electrode (11) and a voltage supply (9) with a three-phase AC voltage source (18) connected to a rectifier (19) for delivering a rectified three-phase alternating voltage, which can be applied between the at least one spray electrode (10) and the at least one deposition electrode (11) is connected.

Description

The present invention is a method and an apparatus for filtering carbon dust from the exhaust steam coal drying. In particular, in the drying of coal, such as lignite, arises due to the desorption of water from the coal exhaust steam, which contains a not inconsiderable proportion of coal dust. These must be at least partially removed from the exhaust steam before emission of the exhaust steam into the environment.

On the one hand, gases such as exhaust gases or exhaust steam, which contain particles such as dust, have to be filtered out before being released into the environment to meet the environmental requirements that exist in many countries. On the other hand, because of the potential for spontaneous combustion of coal dust and in combination with the large surface area of dusts, exhaust steam with coal dust poses a hazard due to the potential risk of explosion, which can be reduced by reducing the coal dust load of the exhaust steam. Conventional filter methods used in pulverized coal filtration can be improved in terms of their efficiency

On this basis, the present invention has the object, at least partially overcome the disadvantages known from the prior art and in particular to provide a method and apparatus for filtering carbon dust from the exhaust steam drying coal, with a good filtration efficiency at the same time a reduced Probability of ignition of coal is given.

These objects are achieved by the features of the independent claims. The respective dependent claims are directed to advantageous developments.

The inventive method for filtering carbon dust from the exhaust steam of the drying of coal is characterized in that the exhaust steam is subjected to a filtering in an electrostatic filter, in which between at least one spray electrode and at least one deposition electrode, an electrical voltage is applied, wherein the electrical voltage is generated by rectification of a three-phase AC voltage.

In the context of this document, the exhaust steam is understood to mean a gas which comprises vapors, that is to say a gas mixture containing steam, and pulverized coal.

The filtering of dusts from the flue gas in particular of power plants by means of electrostatic precipitators is to be assumed to be known. In this case, corresponding electrostatic filter with discharge electrodes and separation electrodes of different electrical potential are used, in which a charge transfer to the dust particles in the flue gas is carried out by an ionization of gas molecules in the flue gas, so that they are electrically attracted to the deposition electrode. However, the dust particles in the flue gas of power plants are usually not selbstentzündlich, so that the skilled artisan first assumes that a direct transfer of the technology to the filtering of coal dust especially because of the risk of breakdown always present in electrostatic precipitators, which may be to ignite the coal particles with a corresponding deflagration can lead, is excluded. For this reason, it is also not possible to use the corresponding filter units, which are used in the power plant for filtering dust from the flue gas of the power plant, easily to the filtering of To transfer carbon dust from the exhaust gas in particular from coal dryers. The existing risk of explosion would prevent a person skilled in the art from transmission because the corresponding electrostatic precipitators are immediately and inevitably damaged in the event of an explosion. Therefore, in principle, another structure of the electrostatic precipitator in the case of filtering coal dust from the exhaust steam of coal dryers is advantageous.

The efficiency of an electrostatic precipitator is particularly influenced by the applied electrical voltage. The higher the voltage, the higher the amplitude of the electric field. Thus, the level of electrical voltage affects the acceleration of the particles in the direction of the resistance electrode and thus the filter efficiency.

Basically, it comes from a certain voltage to ionize the molecules in the exhaust steam, which then attach to the coal dust. This is because only above a certain lower limit voltage can trigger electrons from the spray electrode. To the top, the so-called flashover voltage limits the voltage to be applied. When this flashover voltage is exceeded, so-called flashovers or breakdowns occur, that is to say as arcs which form between the spray electrode and the precipitation electrode and thus lead to a short circuit. From the prior art, the control of an electrostatic precipitator, in which this is subjected to a rectified two-phase AC voltage. This is a possible procedure in the field of filtering the flue gases of power plants. In this case, there is no pure DC voltage at the electrostatic precipitator, but rather a residual ripple which is generated by the rectification of the two-phase alternating voltage and which is intrinsic to the principle and can only be eliminated by complex measures. This will also be explained below with reference to the accompanying figures. Arrows find here usually in the range of the peak value of the control voltage instead. As is known, the arithmetic mean of the voltage at a rectified two-phase AC voltage is well below the amplitude voltage, usually up to 63% of the peak value.

Thus, there are two contradictory boundary conditions in the electro-filtering of gases, such as exhaust steam with carbon dusts. In order to achieve the highest possible filter efficiency, the highest possible voltage is therefore advantageous, while at the same time high voltages increase the risk of flashovers with a simultaneous explosion risk.

Now, if a three-phase AC voltage - commonly referred to as three-phase current - used to drive the electrostatic precipitator, as proposed in the context of the present invention, the result is a different picture. The corresponding ripple of the rectified three-phase AC voltage is different from that of a rectified two-phase AC voltage, also results in a completely different arithmetic mean, which is up to 95% of this amplitude voltage. In comparison with a rectified two-phase alternating voltage, the arithmetic mean of the voltage is significantly higher in the case of a three-phase alternating voltage. Consequently, the use of a rectified three-phase alternating voltage at the same amplitude voltage for operation of the electrostatic filter compared to operation with a rectified two-phase alternating voltage leads to a significantly increased deposition rate of the electrostatic precipitator. Thus, a compromise between the largest possible separation efficiency while reducing the risk of explosion with significantly greater separation efficiency than when operating with rectified two-phase voltage is possible.

Therefore, the present invention, by employing an electrostatic precipitator operated with a rectified three-phase AC voltage, enables a marked increase in the deposition efficiency so that a relatively high rate of coal dust removal from the exhaust of a coal drying process can be achieved while minimizing the risk of explosion ,

According to an advantageous embodiment of the electrostatic filter is freely flowed through for the exhaust steam.

The use of an electrostatic filter makes it possible in an advantageous manner to be able to provide freely flowable cross sections for the exhaust gas. In contrast to filter techniques in which, for example, a depth filter is used, in which therefore the gas to be cleaned flows through a filter medium and the particles are retained in this, an open structure can be realized, as even if the exhaust only at the spray and deposition electrode passes a deposition of the pulverized coal particles takes place. This has the advantage that in the event of an ignition of the pulverized coal and thus a possible explosion in the exhaust gas, the corresponding explosion gases can freely leave the filter, so that damage to the filter - if any - do not lead to a major damage to the system.

According to a further embodiment, if an electrical breakdown between at least one spray electrode and at least one deposition electrode is registered, the amplitude of the electrical voltage is reduced by a predeterminable value.

The detection of the presence of a breakdown is possible via the absorption of the flowing electrical current via the spray electrode and the deposition electrode, because a breakdown or flashover leads to a strong increase in the flowing current. If this is the case, the amplitude of the applied electrical voltage is reduced by a predefinable value. As a result, on the one hand, the deposition efficiency is reduced by a certain amount, but on the other hand, in the long-term operation, carbon copies are effectively avoided. Since a breakdown is disadvantageous due to the combustibility of the carbon particles, so a safe operation of the electrostatic precipitator for filtering carbon dust from the exhaust steam of coal dryers can be ensured.

According to a further advantageous embodiment, the electrical voltage is increased in predetermined levels up to a predetermined nominal value.

This allows a safe start-up of the electrostatic precipitator, in particular in combination with the monitoring of an electrical breakdown and a corresponding reduction of the applied voltage, since in this way a slow increase of the voltage with simultaneous prevention of breakdowns can be achieved.

According to a further advantageous embodiment, the nominal value is predetermined by the comparison with historical data.

Historical data is understood to mean, in particular, the voltage applied in the past. Thus, the nominal value can be set faster in continuous operation than with other methods and in particular so punches can be effectively avoided.

According to an alternative to this advantageous embodiment, the nominal value is determined by an increase in the electrical voltage in predeterminable stages until an electrical breakdown takes place.

Such a procedure ensures that the highest possible electrical voltage is present without punctures. This leads to a high efficiency of the separation of pulverized coal particles from the exhaust steam of coal dryers.

When terminating the filtering, it is accordingly also advantageous to reduce the electrical voltage in predeterminable stages. Usually, the drying of the coal in the coal dryer is stopped first. This leads to a change in the composition of the exhaust steam, since usually both the water vapor content and the proportion of coal dust decreases. It is advantageous if a matched to the composition of the exhaust steam control of the amplitude of the electrical voltage during shutdown, on the one hand to ensure the most effective filtering of the pulverized coal while reducing the risk of breakdowns and thus to deflagrations in the filter, since due to falling water vapor content in the exhaust gas the risk of explosion generally increases.

According to a further aspect of the present invention, an apparatus for filtering carbon dust from the exhaust steam of a coal dryer is proposed, comprising at least one electrostatic filter having at least one spray electrode and at least one deposition electrode and a voltage supply for applying an electrical voltage between the at least one spray electrode and the at least one A deposition electrode, wherein the voltage supply comprises a three-phase AC voltage source connected to a rectifier for outputting a rectified three-phase AC voltage.

Thus, a three-phase generator can be used as a three-phase AC voltage source whose output voltage is rectified by a rectifier and then applied as part of the power supply to the spray electrode and the deposition electrode.

More preferably, the device according to the present invention is suitable for carrying out the method according to the present invention.

According to an advantageous embodiment, the electrostatic filter free for the exhaust steam durchströmbare cross sections.

This allows safe operation, as in the case of a deflagration in the electrostatic precipitator, as it can be triggered for example by a breakdown, the explosion gases can flow freely through the electrostatic precipitator and thus make at most a little damage the same.

According to a further advantageous embodiment, the device comprises a control device for carrying out the method according to the invention.

All details and advantages disclosed in the context of this document for the method according to the invention can be transferred and applied to the device according to the invention and vice versa. The features listed individually in the claims can be combined with one another in any technologically meaningful manner and can be supplemented by explanatory facts from the description and details from the figures, wherein further embodiments of the invention are shown.

Fig. 1:

ein erläuterndes Bild zum Betrieb eines Elektrofilters mit einer gleichgerichteten zweiphasigen Wechselspannung;

Fig. 2:

ein erläuterndes Bild zum Betrieb eines Elektrofilters mittels gleichgerichteter dreiphasiger Wechselspannung;

Fig. 3:

ein Beispiel einer erfindungsgemäßen Vorrichtung;

Fig. 4:

ein Beispiel einer Vorrichtung zum Trocknen von Kohle mit einer erfindungsgemäßen Vorrichtung; und

Fig. 5

ein Beispiel einer Spannungsversorgung in einer erfindungsgemäßen Vorrichtung.

The invention and the technical environment will be explained in more detail with reference to FIGS. The figures show particularly preferred embodiments, however, to which the invention is not limited. In particular, it should be noted that the figures and in particular the illustrated proportions are purely schematic. Show it:

Fig. 1:

an explanatory image for operating an electrostatic filter with a rectified two-phase AC voltage;

Fig. 2:

an explanatory image for operating an electrostatic precipitator by means of rectified three-phase AC voltage;

3:

an example of a device according to the invention;

4:

an example of a device for drying coal with a device according to the invention; and

Fig. 5

an example of a power supply in a device according to the invention.

The following are intended to be based on the Fig. 1 and 2 the advantages of driving an electrostatic precipitator by means of rectified three-phase AC voltage in comparison to a control with rectified two-phase AC voltage can be shown. Fig. 1 schematically shows a rectified two-phase AC voltage. The voltage curve 1 follows in each case the sine half waves, but always with identical sign. Thus, the voltage curve consists of a sequence of positive half sine waves. The voltage is given in relative units of 0 - 100%, the time in milliseconds, adjusted to a mains frequency of 50 Hz.

Fig. 2 shows the formation of the rectified three-phase AC voltage. In this case, the respective individual phases and their corresponding single-phase voltage curves 2 as well as the resulting superposed voltage curve 1 are shown. Furthermore, both in Fig. 1 as well as in Fig. 2 the arithmetic mean 3 of the superimposed flows 1 shown. At the in Fig. 1 and 2 Theoretical voltage curves shown are arithmetic means two straight lines, each at 63% in Fig. 1 in the case of a rectified two-phase AC voltage and at about 95% in Fig. 2 in the case of a rectified three-phase alternating voltage. Real power supplies have deviating voltage curves from these theoretical voltage curves, which have a different course, for example due to smoothing effects. For the sake of simplicity, however, the basic idea is to be explained here on the basis of the theoretical voltage curves.

It is clear from the voltage curves 1 shown that, in the case of a rectified three-phase alternating voltage, the arithmetic mean is significantly higher than in the case of the rectified two-phase alternating voltage. As a result, with the same amplitude 4 of the voltage, a significantly increased filter efficiency can be achieved through the use of a rectified three-phase alternating voltage.

In real DC operation, the separation efficiency of an electrostatic filter depends on the DC voltage applied between the spray electrode and the deposition electrode. In the case of a rectified AC voltage, this dependence on the voltage in the arithmetic mean 3. This is why the Figures 1 and 2 clearly that by operating the electrostatic precipitator with identical amplitude 4 of the voltage waveform 1 when applying a rectified three-phase AC voltage significantly higher separation efficiency can be achieved.

Fig. 3 shows an example of a corresponding device 5 for filtering carbon dust from the exhaust steam of a coal dryer. This comprises an electrostatic filter 6, which is flowed through by the exhaust steam 7 of a coal dryer from bottom to top. The exhaust steam 7 flows through a space in which a plurality of precipitation plates 8 are formed. The deposition plates 8 are electrically connected to a power supply 9 so that each adjacent plates have different electrical potential or different electrical polarity. That is, a number of the plates function as a discharge electrode 10, while the remaining precipitation plates 8 function as separation electrodes 11. The structure is chosen so that in each case spray electrodes 10 and deposition electrodes 11 are formed alternately. Adjacent to a deposition electrode 11 formed in the center of the electrostatic filter 6 are thus two spray electrodes 10, while adjacent to a spray electrode 10 formed in the middle of the electrostatic filter 6, two deposition electrodes 11 are formed.

By applying the rectified three-phase AC voltage in the high voltage range, in particular in the range of up to 100 kV, there is an ionization of gas particles that transmit their electrical charge, for example, by attachment to the pulverized coal particles on this. As a result, the particles provided with an electrical charge are guided to the deposition electrodes 11 and are deposited there. Already before the addition, agglomeration of pulverized coal particles can occur, through which various particles accumulate on one another and are then deflected together towards the precipitation electrode 11. The distraction follows the principle of Coulomb's law, according to which an electric field acts on a charge in the field.

The exhaust steam 7 is the exhaust steam 7 of a coal dryer, which contains a not inconsiderable part of pulverized coal particles. Furthermore, the exhaust steam 7 usually comprises vapors, so a water vapor-containing gas mixture. The water vapor is formed by the desorption of the water from the coal to be dried. Preference is given to a process procedure in which the vapor represents a gas flow saturated with water vapor. By passing the exhaust steam 7 through the electrostatic precipitator 6, a proportion of the pulverized coal particles is deposited on the precipitation electrodes 11. These are regularly excited by mechanical impactors, not shown in this figure, through which the particles accumulating thereon fall off and down in one Chute 12 is collected and discharged.

The exhaust steam 7 leaves after filtering in the electrostatic precipitator 6, the device 1 as purified exhaust steam 13. The flow paths within the electrostatic precipitator 6 are open, that is, with the exception of the precipitation plates 8, the exhaust steam 7 can flow freely through the electrostatic precipitator 6, so there are in particular no filter mats or the like, which are placed in the flow path of the exhaust steam 7. The deposition is therefore exclusively by the Coulomb's force.

Fig. 4 shows very schematically a coal dryer 14. This is designed for example as a classic tube dryer, in which the carbon migrates in tubes from top to bottom through the coal dryer 14, which is heated with hot supersaturated water vapor. At the lower end of the coal falls into a Kohleaustragselement 15, through which the coal 16 is discharged downward, while the exhaust steam 7 is passed through the trigger 17 in the electrostatic precipitator 6. This electrostatic filter 6, as in Fig. 3 is shown formed, leaves the exhaust steam as purified exhaust steam 13. Further shows Fig. 4 a control device 20, which is also part of the device 1 shown. This control device 20 is connected to the not shown power supply 9, the discharge electrodes 10 and the deposition electrodes 11 connected. On the one hand, it monitors the flowing current, in particular for the detection of breakdowns, and on the other hand it controls the discharge electrodes 10 and the deposition electrodes 11. The control device 10 is suitable and intended for carrying out the method according to the invention.

Fig. 5 shows very schematically a power supply 9. This power supply 9 has a three-phase AC voltage source 18 which is connected to a rectifier 19, so that at the outputs 20 of the voltage source 9 is applied a rectified three-phase AC voltage. With this power supply 9, the precipitation plates 8 are connected accordingly, so that discharge electrodes 10 and deposition electrodes 11 are formed.

The device 1 according to the invention and the method according to the invention allow efficient and reliable filtering of pulverized coal from the exhaust steam 7 of coal dryers 14 with a significant reduction of the risk of explosions resulting from breakdown of the electrostatic precipitator 6.

LIST OF REFERENCE NUMBERS

1

voltage curve

2

Single phase voltage curve

3

arithmetic mean

4

Amplitude of the voltage

5

Device for filtering carbon dust

6

electrostatic precipitator

7

exhaust steam

8th

Precipitation plate

9

power supply

10

spray electrode

11

deposition electrode

12

chute

13

purified exhaust steam

14

coal dryer

15

Kohlenaustragselement

16

coal

17

deduction

18

Three-phase AC voltage source

19

rectifier

20

control device

Claims (9)

Method for filtering carbon dust from the exhaust steam (7) of the drying of coal, wherein the exhaust steam (7) is subjected to a filtering in an electrostatic precipitator (6), wherein between at least one spray electrode (10) and at least one deposition electrode (11) voltage is applied, wherein the electrical voltage is generated by rectification of a three-phase AC voltage. The method of claim 1, wherein the electrostatic filter (6) freely for the exhaust steam (7) can be flowed through. Method according to one of the preceding claims, in which, if an electrical breakdown between at least one spray electrode (10) and at least one deposition electrode (11) is registered, the amplitude (4) of the electrical voltage is reduced by a predefinable value. Method according to one of the preceding claims, in which the electrical voltage is increased in predefinable stages up to a predefinable nominal value. Method according to Claim 5, in which the nominal value is predetermined by comparison with historical data. The method of claim 5, wherein the nominal value is determined by an increase in the electrical voltage in predetermined levels until an electrical breakdown occurs. Apparatus (1) for filtering carbon dust from the exhaust steam (7) of a coal dryer (14), comprising at least one electrostatic filter (6) with at least one spray electrode (10) and at least one deposition electrode (11) and a voltage supply (9) for applying a electrical voltage between the at least one spray electrode (10) and the at least one deposition electrode (11), the power supply (9) having a three-phase AC voltage source (18) connected to a rectifier (19) for delivering a rectified three-phase AC voltage. Device (1) according to claim 7, in which the electrostatic precipitator (6) has free cross-sections through which the exhaust steam (7) can flow. Apparatus (1) according to claim 7 or 8, further comprising a control device (20) for carrying out the method according to one of claims 1 to 6.

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