EP0500752A1

EP0500752A1 - Electrocyclone for gas cleaning

Info

Publication number: EP0500752A1
Application number: EP19900917632
Authority: EP
Inventors: Jean-Yves Deysson; Jean-Noel Dumont; Alain Esteve; Martine Ramel; Jacques Vendel; Francis Daniel
Original assignee: Bertin et Cie SA; Charbonnages de France CDF; Commissariat a lEnergie Atomique CEA
Current assignee: Charbonnages de France CDF; Bertin Technologies SAS; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1989-11-21
Filing date: 1990-11-20
Publication date: 1992-09-02
Also published as: FR2654648A1; FR2654648B1; WO1991007230A1

Abstract

Electrocyclone pour le dépoussiérage de gaz, comprenant des conduits (16) de passage du gaz à dépoussiérer, et dans lesquels sont disposées des électrodes axiales (22) formées chacune par l'empilement axial de pièces annulaires (34) à aiguilles radiales, les pointes de ces aiguilles se trouvant sur des cercles ayant un diamètre compris entre 20 et 40 % environ du diamètre interne des conduits (16).Electrocyclone for the dedusting of gas, comprising conduits (16) for passing the gas to be dedusted, and in which are arranged axial electrodes (22) each formed by the axial stack of annular pieces (34) with radial needles, the tips of these needles located on circles having a diameter of between 20 and 40% approximately of the internal diameter of the conduits (16).

Description

ELECTROCYCLONE FOR GAS DUST COLLECTION

The invention relates to an electrocyclone for the dedusting of gases, in particular of the type comprising multiple dedusting units which are arranged in parallel in the same enclosure.

Electrocyclones combine the principle of dedusting a gas by the effect of centrifugal force (the gas being put in vortex movement around an axis so that the solid particles conveyed by the gas move away from this axis under the effect of centrifugal force and are collected on a surrounding wall) and the principle of electrostatic dusting, which consists in electrically charging the solid particles by corona effect and in creating a radial electric field to precipitate the particles on a surrounding electroconductive wall.

The efficiency of separation of the particles by centrifugal effect decreases when the size of the solid particles decreases and is low for particles having a size less than 1 μm, while conversely the efficiency of the electrostatic precipitation remains substantially constant since the hundredth of μm. These two actions are therefore complementary and have combined performances which are superior to those of cyclones for dedusting by centrifugal force in the submicronic domain, with a space / flow ratio much lower than that of electrostatic dedusting installations.

The efficiency of the electrostatic precipitation is also a function of the intensity of the ionization current, emitted during the corona discharge. The efficiency is maximum when the value of the ionization current is maximum for a given value of the high voltage applied to the electrode. The object of the present invention is in particular to improve the efficiency of the electrostatic separation in an electrocyclone by increasing the value of the ionization current corresponding to a given voltage of electrodes.

It also aims to simplify the construction of this electrocyclone, when it is intended to treat gases having a temperature below 200 ° C.

It also aims to maintain the effectiveness of the electrocyclone for an extended period of time.

The invention therefore proposes, for this purpose, an electrocyclone for the dedusting of gases, in particular of the type with multiple dedusting units arranged in parallel in a single enclosure, each unit comprising a vertical cylindrical duct carrying at its upper end a blading annular inlet of the gas to be cleaned, an axial chimney for the outlet of the purified gas extending upwards in the upper part of the aforementioned conduit, an electrode mounted in the axis of this conduit, and means for connecting the electrode and of the conduit at the poles of a continuous high voltage source, charac¬ terized in that said electrode carries radial needles whose free ends formed in a point are located on circles having a diameter smaller than that of the chimney and between approximately 20 and 40% of the internal diameter of the flue.

The shapes and dimensions of the electrodes of the dedusting units according to the invention result from a compromise between the following imperatives: depositing the maximum of electrical charge on the solid particles conveyed by the gas circulating in the conduit and per¬ turbering as little as possible. flow of gas descending at the periphery of the conduit, then ascending in the axis of this conduit. It has been found that an electrode according to the invention constitutes the best possible compromise and allows in particular, compared to a simple axial electrode, to multiply by a coefficient of at least 1.5 to 2.5 the intensity of the ionization current, for a given value of the voltage applied to the electrode. This results in a considerable improvement in the efficiency of the electrostatic separation.

According to another characteristic of the invention, the radial needles are distributed over an electrode length in planes perpendicular to the axis of this electrode.

This length of electrode carrying the pins extends advantageously from the lower end of the chimney to the level where the aforementioned vertical cylindrical conduit, of circular section, is connected to a frustoconical conduit whose section decreases progressively ¬ down and which leads into a hopper.

In a preferred embodiment of the invention, the number of needles in each plane perpendicular to the axis of the electrode is between three and six.

This number of needles is chosen for reasons of symmetry in the duct and of gas ionization efficiency: below three, it does not allow an arrangement with any symmetry with respect to the duct and the efficiency is scaled down; above six, the needles are unnecessarily close to each other.

Advantageously, each plane perpendicular to the axis of the electrode comprises four radial needles at 90 ° from each other. This arrangement corresponds to the most favorable setting of the field lines in the conduit.

By the way ", in this preferred embodiment of the invention, the electrode comprises an axial stack of annular parts each carrying radial coplanar needles. The pitch between annular pieces stacked axially is at least a few millimeters.

The radial needles of an electrode can have constant angular orientations relative to the axis of the electrode from one annular piece to another, so that they then lie in common vertical planes which intersect on the axis of the electrode, or the annular parts carrying the needles can be mounted free to rotate about the axis of the electrode.

According to another characteristic of the invention, when the gas to be dusted is at a temperature below about 200 ° C., each electrode has mounting fittings made of a plastic material such as polytetrafluoroethylene, resistant to acid fumes and not containing inclusions which could be the cause of electrical ignition.

This avoids using ceramic parts for mounting and insulating the electrodes. Advantageously, at least the lower part of the chimney of each unit can then be made of a similar plastic material.

Preferably, this lower end of the chimney comprises parallel circular grooves or ribs, which make it possible to increase the length of path of the bypass currents, and therefore to reduce the risks of electrical ignition between the chimney and the electrode. .

To avoid the accumulation of dust on the sides of these ridges or ribs, the latter make with the horizontal an angle greater than a determined value.

Finally, the invention provides means for de-clogging the dedusting units, these means comprising for example hammers connected to drive means for periodically striking the aforementioned conduits of said units.

To reduce the size, the units can then be staggered in the aforementioned enclosure. The invention will be better understood and other characteristics, details and advantages thereof will appear more clearly in the description which follows, given by way of example with reference to the appended drawings in which: FIG. 1 is a view schematic in vertical section of part of an electrocyclone according to the invention; Figure 2 is a plan view of an annular electrode part according to the invention; Figure 3 is an enlarged view of the end of a radial needle of this annular piece of electrode; Figure 4 is a schematic view in axial section of the lower end of a chimney forming part of a dust removal unit; Figure 5 is a schematic top view of the electrocyclone, showing the arrangement in quin¬ design of dust collection units; FIG. 6 is a diagrammatic view of part of the electrocyclone, showing the means of cleaning.

Reference is first made to FIG. 1, in which a part of an electrocyclone according to the invention is shown diagrammatically, seen in axial section. This electrocyclone comprises an enclosure 10 in which are arranged a plurality of dust removal units 12 above a dust collection hopper 14, which forms the lower part of the enclosure 10. Each dust removal unit 12 comprises es¬ essentially a duct 16 of dry cylindrical shape circular tion, connected to a lower duct 18 of frustoconical shape narrowing downwards and debou¬ song in the hopper 14, a vertical chimney 20 which is arranged in the axis of the duct 16 and which extends partly upwards upper part of this duct, and an axial electrode 22 which extends in the duct 16 between the lower end of the chimney 20 and the upper end of the lower duct 18.

The enclosure 10 further comprises a manifold 24 for supplying the units 12 with gas charged with solid particles, and a manifold 26 for the outlet of purified gas, which is located above the manifold 24. The upper ends of the conduits 16 therefore open into the supply manifold 24 and are provided with a fixed casing 28 making it possible to impart a vortex movement to the gas inside the conduits 16, while the chimneys 20 pass through the manifold d supply 24 and open at the top into the outlet manifold 26. The electrodes 22 extend axially upwards inside the chimneys 20 and are connected at their upper end, by a common conductor 30, to a terminal 32 from a continuous high voltage source. The conduits 16 of the dedusting units are generally connected to earth or to ground. The different electrodes 22 are for example supported at their upper end by their common supply conductor 30 and are provided with conventional insulating means for centering in the chimneys 20. They also include, inside these chimneys, sheaths or sheaths of coating made of electrically insulating material, to reduce the risks of electric ignition between the electrodes and the walls of the chimneys 20.

When the gas to be dusted is at a temperature below about 200 ° C, the various means of support, centering and protection of the electrodes can wind be plastic, preferably po¬ lytétrafluoroethylene (marketed under the name "Teflon"), which resists corrosion of acid fumes and does not contain inclusions increasing the risk of electric ignition.

The lower parts of the electrodes 22, which are inside the conduits 16 below the chimneys 20, are formed by axial stacking of annular parts 34, one of which is shown on a larger scale in FIG. 2. Each annular part 34 is made of stainless steel and comprises radial needles 36 distributed uniformly around the axis of the electrode. Preferably, these radial needles are four in number and are 90 ° from each other. Their free ends 38 formed in sharp point are on circles of diameter smaller than that of the chimney and comprised between 20 and 40% approximately of the internal diameter of the conduit 16.

In the example shown, each part 34 comprises an annular central part 40 with a radial slot 42 facilitating the mounting of the annular part on an electrically conductive rod. The ends of the radial needles 36 are as pointed as possible, as can be seen in the enlarged view of FIG. 3. In the example shown, the annular pieces 34 have a thickness of 0.1 mm, the external and internal diameters of the annular central part 40 are 20 and 8.5 mm respectively, and the ends 38 of the radial needles are on a circle having a diameter of 75 mm. The stacking pitch of the annular parts 34 is approximately 3 mm, the total length of the electrode 22 formed by the stacking of the annular parts is approximately 300 mm, the duct 16 having a diameter of the order of 250 mm , while the chimney 20 has a diameter of approximately 150 mm.

These dimensions and this form of electrode constituted by the stack of parts 34 make it possible to obtain an ionization current of 8 milliamps per meter of electrode approximately when the voltage applied to the electrode is 50 kV. This ionization current is 1.5 to 2.5 times greater than that which could be obtained with conventional axial electrodes, for example made of expanded metal.

In addition, the flow of gas through the conduit 16 is not disturbed, so that the efficiency of separation by centrifugal force is not diminished.

In general, the diameters of the duct 16 and of the chimney 20 are in a ratio close to 2, for which the areas of the gas passage sections are substantially equal, on the one hand between the duct 16 and the chimney 20, on the other hand in the chimney 20.

The stack of annular parts 34 extends generally, substantially between the lower end of the chimney 20 and the upper end of the frustoconical duct 18. To reduce the risk of priming between the electrode and the chimney 20, provision can be made to form circular grooves 44 on the lower part of the chimney, as shown diagrammatically in FIG. 4. The grooves 44 increase the length of path of the bypass currents.

In addition, the flanks of the ridges make an angle with the horizontal which is greater than a determined value, corresponding to the slope angle of the solid particles. This avoids the accumulation of dust on the sides of the ridges 44.

The chimneys 20, at least at their lower part, can be made of a plastic material such as polytetrafluoroethylene, when the gas to be dusted has a temperature below about 200 ° C. This results in a simplification of the assembly of these chimneys. Improving the separation by electrostatic effect generally requires providing means for unclogging the dust removal units, the solid particles tending to stick to the internal walls of the conduits 16 of these units.

The invention therefore provides, as shown diagrammatically in FIGS. 5 and 6, to have the units 12 staggered in the enclosure 10, the units 12 therefore forming parallel rows and being laterally offset with respect to one another by a row to the next row.

In the example shown, the clogging means comprise hammers 46 which are rotatably mounted on horizontal drive shafts 48 extending between the various rows of units 12. Each unit 12 can be associated with two hammers 46 the flap on two opposite sides, at two different levels, one on the part of the duct 16 which extends between the base of the chimney 20 and the frustoconical duct 18, the other on the upper part of the duct A. The shafts 48 on which the hammers are mounted: 6 can be rotated continuously, always in the same direction, and be connected by engre¬ nages to the hammers 46, one of the pinions of each gear being devoid of teeth on a part of its periphery, so that each hammer 46 is raised over part of a revolution of the corresponding shaft 48, and falls under the effect of its own weight , to collide with conduit 16 or 18, for r the rest of this turn of tree 48.

Of course, the hammers 46 are designed so as not to damage the conduits at the time of striking.

In general, the electrocyclone according to the invention is applicable to the dedusting of gases conveying solid particles of dimensions comprised by example between 1 and 200 μm, with good efficiency.

Furthermore, the invention also applies to electrocyclones with a single gas dedusting unit.

Claims

1. Electrocyclone for the dedusting of gas, in particular of the type with multiple units of dedusting arranged in parallel in the same enclosure, each unit (12) comprising a vertical duct (16) cylindrical carrying at its upper end a bladed annu ¬ gas inlet area to be dedusted, an axial chimney (20) for purified gas outlet, extending upwards in the upper part of the duct (16), an electrode (22) mounted in the axis of this duct , and means for connection of the electrode (22) and of the conduit to the poles of a direct high voltage source, characterized in that said electrode (22) carries radial needles (36) whose free ends (38) formed in a point are located on circles having a diameter smaller than that of the chimney (20) and comprised between 20 and 40% approximately of the internal diameter of the conduit (16).

2. Electrocyclone according to claim 1, characterized in that the radial needles (36) are distributed over an electrode length in planes perpendicular to the axis of the electrode, each plane per¬ pendicular to the axis of l 'electrode comprising a number of radial needles (36) between three and six.

3. Electrocyclone according to claim 2, characterized in that each aforementioned plane comprises four radial needles (36) at 90 ° from each other.

4. Electrocyclone according to one of claims 1 to 3, characterized in that the electrode (22) has an axial stack of annular parts (34) each carrying coplanar radial needles (36).

5. Elr ^ trocyclone according to claim 4, aractérisé in that the pitch between annular parts (34) stacked axially is at least a few millimeters.

6. Electrocyclone according to one of Claims 1 to 5, characterized in that the part of the electrode (22) carrying the radial needles extends substantially from the lower end of the chimney (20) to the lower end of the circular section duct (16), connected to a frustoconical duct (18).

7. Electrocyclone according to one of the preceding claims, characterized in that the radial needles (36) of each electrode (22) are contained in common vertical planes intersecting on the axis of the electrode.

8. Electrocyclone according to one of the preceding claims, for the dedusting of gases having a temperature lower than approximately 200 ° C., characterized in that each electrode comprises linings of protection and protection made of a plastic material such as polytetrafluoroethylene, resistant to corrosion by acid fumes and not containing inclusions.

9. Electrocyclone according to one of the preceding claims, for the dedusting of gases having a temperature below about 200 ° C., characterized in that at least the lower part of the chimney (20) is made of a plastic material such as polytétra¬ fluoroethylene, resistant to corrosion of acid fumes and containing no inclusions.

10. Electrocyclone according to one of the preceding claims, characterized in that the lower end of said chimney (20) comprises parallel circular grooves or ribs (44).

11. Electrocyclone according to claim 10, characterized in that the flanks of said ridges or ribs (44) make with the horizontal an angle greater than a determined value, avoiding the accumulation of dust on said flanks.

12. Electrocyclone according to one of the preceding claims, characterized in that it comprises means for unclogging said units (12), these means comprising hammers (46) mounted for rotation on horizontal shafts (48) and connected to drive means for periodically striking the aforementioned conduits of said units.

13. Electrocyclone according to claim 11, characterized in that the hammers (46) strike the conduits of the units at two different levels located below the base of the chimney, on the cylindrical (16) and frustoconical (18) parts respectively of said conduits.

14. Electrocyclone according to claim 12 or 13, characterized in that, to reduce the bulk, said units (12) are staggered in the enclosure (10).