EP0461687A1 - Process for cleaning electrostatic dust separators - Google Patents

Abstract

A process and a device for cleaning the precipitation surfaces of electrostatic dust separators are proposed, in which large particulate cleaning dust is placed in the dust separator and electrostatically precipitated alone or together with the dust in the crude dust in the dust separator and the precipitated dust is cleaned off at intervals from the precipitation surfaces and discharged from the dust separator. In this case it is proposed that the cleaning dust is placed above the areas of the electrostatic dust separator in the dead space and is distributed in accordance with the cleaning requirement. <IMAGE>

Description

The invention relates to a method for cleaning the precipitation surfaces of electrostatic dust separators, wherein coarse-grained cleaning dust is introduced into the dust separator and electrostatically separated on its own or together with the dust in the raw gas in the dust separator, and to a device for carrying out the method.

Such a method is known from DE-PS 861 382. It had been found that, in certain applications, the precipitation electrode surfaces were coated with a layer of firmly adhering fine dust which cannot be removed with the customary cleaning methods and which necessitates interruptions in operation for mechanical cleaning if the separation performance is not to drop to unacceptably low values. The problem could be solved by adding coarse-grained cleaning dust, which is deposited on the precipitation electrodes and, when detached, also removes the otherwise inseparable fine dust by means of a type of sanding effect, so that the effectiveness of the precipitation electrodes is retained.

However, it has been shown that the known method for modern large filters is still in need of improvement. According to the invention it is therefore proposed that the cleaning dust is introduced above the fields of the electrostatic dust separator in the flow-free space and distributed according to the cleaning requirement.

In the conventional method, it was not possible to introduce the cleaning dust in such a way that all areas of the precipitation electrode surfaces were supplied with cleaning dust, ie the fine dust adhered to growing parts of the precipitation electrodes over time, with the result that that the separation performance was reduced accordingly. Since the preferred dust separators for horizontal gas passage above the fields, ie above the gas flow cross-section, have a not inconsiderable free space that is required for the removal and suspension devices for the spray electrodes and precipitation electrodes, but is only partially filled, this space can be inventively can be used for the targeted and metered introduction of the cleaning dust without any other structural changes being necessary.

Further details emerge from method claims 2 to 6 and device claims 7 to 9.

Fig. 1

zeigt einen Längsschnitt durch den oberen Teil eines Staubabscheiders.

Fig. 2

zeigt einen Horizontalschnitt durch den Staubabscheider oberhalb der Prallteller.

Fig. 3 und 4

zeigen in Seiten- und Vorderansicht Teilschnitte mit den Flugbahnen des Reinigungsstaubes.

Fig. 5

zeigt in perspektivischer Darstellung einen Teil der Förder- und Verteileinrichtung.

An embodiment of the inventive concept is shown in FIGS. 1 to 5.

Fig. 1

shows a longitudinal section through the upper part of a dust collector.

Fig. 2

shows a horizontal section through the dust collector above the baffle plate.

3 and 4

show in side and front view partial sections with the trajectories of the cleaning dust.

Fig. 5

shows a perspective view of part of the conveyor and distribution device.

1 shows parts of three separating fields (14) to (16) lying one behind the other in the gas flow direction (17), which are arranged in a housing with an inlet connection piece (21), ceiling (13) and box-shaped roof supports (transverse to the gas flow direction (17)) 12) are arranged. They essentially consist of parallel plate-shaped precipitation electrodes (6) which run on the gas stream and which hang from supports (5), and mostly wire-shaped spray electrodes which are stretched in frames (not shown). The frames for the spray electrodes are removed via insulators (22) in the roof racks (12). Outside the housing are arranged transversely to the gas flow distribution devices (11) which supply through the filter ceiling (13) sloping downwards distribution pipes (1) with cleaning dust that exits through lower outlet openings (2) from the distribution pipes (1) and first on the Baffle plate (3) and then falls further into the separation fields (14) to (16).

The horizontal section according to FIG. 2 shows how the baffle plates (3) are arranged above the precipitation electrodes (6) in the fields (14) to (16). With (12) again the roof rack, with (17) the gas flow direction, with (21) the inlet connection and (23) the side wall of the filter housing.

The partial sections according to FIGS. 3 and 4 show how the cleaning dust falls through the outlet openings (2) of the distributor pipes (1) over the drop height (10) onto the baffle plate (3), bounces from there and according to the (8) and ( 9) indicated trajectories falls, whereby initially only the gravitational acceleration acts (8), then also the attraction of the electrostatic field (9). The carriers (5) for the precipitation electrodes (6) also carry the baffle plates (3) and have roof-shaped deflectors (4). They are attached to the roof rack (12) at the end. The frames (7) of the spray electrodes are also indicated in FIG. 4 between the precipitation electrodes (6).

The conveying and distribution system can essentially be seen from the greatly simplified perspective illustration according to FIG. 5. The distribution devices (11) arranged above the roof rack (12) are connected via a (Mechanical or pneumatic) dust supply system (18) supplied with cleaning dust. The cleaning dust flows from the distribution devices (11) (e.g. trough chain conveyor or screw conveyor) into the distribution pipes and through the filter cover into the dust separator, from which the fields (14) and (15) are indicated, which lie one behind the other in the gas flow direction (17). Excess cleaning dust flows via a return system (19) into a separate dust collecting container (20), from which the cleaning dust is introduced into the dust separator via the conveyor (18).

Experiments have shown that, using the inventive concept, keeping the precipitation electrode surfaces clean by means of cleaning dust is easily possible even with large, horizontally flowed dust separators, and that the distribution and metering of the cleaning dust can be adapted to all operational requirements.

When using cleaning dust with a high specific electrical dust resistance (eg quartz sand), the cleaning dust is pressed against the precipitation electrodes by the electrical field forces. When large amounts of cleaning dust are added, it can be observed that the cleaning dust flows down like water. When the high voltage is switched off or lowered, the cleaning dust is released from the precipitation electrode and falls freely downwards. When switched on again or when the high voltage is increased, the cleaning dust is suddenly thrown back to the precipitation electrode by the field forces. The cleaning effect is enhanced with the kinetic impact effect of the dust particles. This process can also be achieved by using appropriately pulsed high voltage.

Depending on the application, sand, iron ore, slag, limestone, coal, coke or similar can be used as cleaning dust. in a grain size between median 80 µm and 300 µm. It has been shown that the amount required is almost independent of the specific weight due to the electrical adhesive forces. The amount required per hour is in the range of 0.1 dm³ to 10 dm³ per running meter of precipitation electrode length in the gas direction. This does not apply to the last electrical field seen in the gas direction. Only the length of the electrodes of the precipitation electrode area to which cleaning dust is applied is taken into account here. The required amount of cleaning dust per hour and running meter of precipitation electrode length does not have to be introduced continuously. It can also be inserted at intervals with break times of several minutes to hours.

Example: dedusting the exhaust gas of an iron ore sintering belt

Exhaust gas volume 500,000 Nm³ / h effective amount of exhaust gas 800,000 m³ / h Dust content in the raw gas 1,000 mg / Nm³ required dust content in the clean gas 50 mg / Nm³ separated dust mass 475 kg / h Bulk density of the dust 1,000 kg / m³

Data of the selected electrostatic filter:

Assuming ideally uniform gas / dust distribution over the electrostatic filter cross-section, the dust masses separated in the individual electrical fields or field sections and to be transported down can be calculated using the extended German formula, whereby k = 0.5 is set from experience and measurement results . This results in the following scheme for the separated amounts of dust:

This diagram shows that the degree of separation falls disproportionately with increasing electrostatic filter length. Even if one does not consider the selective separation effect, the amount of dust accumulating in the output part is too small to achieve a sanding effect when knocked off, as a result of which the precipitation electrodes could be kept shiny metallic.

The selective separation of the grain fractions still contains a relatively large proportion of coarse grain fractions in the dust entering field 1. The addition of cleaning dust in field 1 can therefore as in DE-PS 861 382 limited to 10% of the dust collected in field 1, i.e. 39 kg / h for the numerical example. Conversely, the dust entering field 4 only contains the finest grain fractions, which are correspondingly difficult to clean from the precipitation electrodes. It has been shown here that, based on the dust to be separated, a relatively much higher proportion of cleaning dust is to be introduced (50% to 200%). In the numerical example, an addition of 12 kg / h results for a cleaning dust quantity of 100%. In order to prevent cleaning dust from being discharged into the clean gas, field 4 only applies a maximum of 75% of the length of the cleaning dust. In the numerical example, fields 2 and 3 are treated with a cleaning dust quantity of 50% and 100% of the separated fine dust quantity.

With a bulk weight of cleaning dust of 1000 kg / m³, the following values result:

Assuming an average gas velocity of approx. 1.0 m / s and based on a migration speed of 80 cm / s for the cleaning dust, the path of the coarse dust furthest away from the precipitation electrode is (near the spray electrode = distance 20 cm) to the precipitation electrode 25 cm (from 20: 80x100 = 25). The field length is 4.32 m, 39 kg / h are entered. In the worst case (all coarse particles near the precipitation electrode), 2.3 kg / h or 5.8% are transferred to the next field. This results in:

As can be seen from this table, only 0.7 kg / h of coarse dust of the 75% panel length are entered in the last quarter of the last field according to the example. However, an electrode length of 1.08 m is still available for the deposition, which ensures that practically no coarse dust can be discharged on the clean gas side and would thus contribute to an increase in the clean gas dust. (A discharge of 10% corresponding to 1.2 kg / h of the flushing dust introduced in field 4 would, for example, only result in an increase in the clean gas dust content of 2.4 mg / Nm³.)

The amount of cleaning dust introduced in the individual force fields is achieved by the corresponding running time or pause time of the metering devices. The intervals between the dosing of the cleaning dust can be synchronized with the tapping of the precipitation electrodes, in such a way that the knocking or cleaning takes place shortly after the dosing into the corresponding field.

Dust originating from the process and which can be returned to the process can be used as cleaning dust. On the other hand, external dust can also be used. There is also the possibility of removing the coarse-grained cleaning dust from the separated dust view and thus drive in a cycle. Are suitable:
Fine sand, coarse dust from cyclone separators, iron ore, clinker, slag, limestone, coke, coal or similar with good pouring properties (low slope angle of the fill).

Claims (10)

A process for cleaning the precipitation surfaces of electrostatic dust separators, wherein coarse-grained cleaning dust is introduced into the dust separator and electrostatically separated on its own or together with the dust in the raw gas in the dust separator, and the separated dust is periodically cleaned from the precipitation surfaces and removed from the dust separator that the cleaning dust is introduced above the fields of the electrostatic dust separator in the dead space and distributed according to the cleaning needs. Method according to claim 1, characterized in that the high voltage is periodically briefly lowered or switched off during the application of the cleaning dust. A method according to claim 1 or 2, characterized in that only 25 to 75% of cleaning dust is applied to the length of the electrical field as viewed from the last electrical field in the gas direction. Method according to one of claims 1 to 3, characterized in that the amount of cleaning dust applied is 0.1 to 10 dm³ / h per running meter of the applied precipitation electrodes, viewed in the gas direction. Method according to one of claims 1 to 4, characterized in that the cleaning dust is introduced depending on the cycle of the tapping devices for the precipitation electrodes. Method according to one of claims 1 to 5, characterized in that the cleaning dust has a grain size between median 80 µm to 300 µm and a weight of> 0.9 kg / dm³. Method according to one of claims 1 to 6, characterized in that the cleaning dust is obtained from the dust which has been completely separated and removed from the electrostatic dust separator by separating off the fine dust by means of screening or washing out and drying. Device for carrying out the method according to claims 1 to 6, characterized by a conveyor (18) and distribution device (11) for the cleaning dust (8,9), arranged obliquely falling above the fields (14, 15, 16) of the electrostatic dust separator Manifolds (1) with outlet openings (2) for the dust and baffle plates (3) provided below the outlet openings. Device according to claim 8, characterized in that the conveying and distributing device (18, 11) is supplemented by a return system (19) with a dust collecting container (20). Apparatus according to claim 8, characterized in that the distribution device (11) is arranged above the filter ceiling (13) and that the distribution pipes (1) pass through the filter ceiling (13) in a gas-tight manner and at an incline.

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