WO2002029324A1

WO2002029324A1 - A device for feeding and dosing combustible particle material to a furnace area and the use of same

Info

Publication number: WO2002029324A1
Application number: PCT/NO2001/000405
Authority: WO
Inventors: Anne Margrete Nerland; Reidar Hestetun; Jørn Hembre SKAAR; Victoria Kielland; Anders Ruud; Jens Bugge Hatlevoll
Original assignee: Norsk Hydro Asa
Priority date: 2000-10-06
Filing date: 2001-10-03
Publication date: 2002-04-11
Also published as: NO20005038D0; NO315060B1; AU2002211103A1; NO20005038L

Abstract

A device for feeding and dosing combustible particle material to a furnace area comprising a store (2) which communicates with an ejector (5) for the supply of particle material and a pressurised gas from the ejector to the furnace area. An adjustable lock/valve arrangement (32) is fitted between the store (2) and the ejector (5) for dosing of material between the store (2) and the ejector (5). The present invention is particularly well suited for feeding and dosing carbon dust to a furnace for calcining carbon electrodes.

Description

A device for feeding and dosing combustible particle material to a furnace area and the use of same

The present invention concerns a device for feeding and dosing combustible particle material to a furnace area, in which the material is dosed from a store to the furnace area by means of a pressurised gas. The present invention is specially adapted for the combustion of carbon dust, which causes a major disposal problem within the metal industry, such as electrolysis plants for the production of aluminium using the prebake method. At a medium-sized aluminium plant, several hundred tonnes of recipient dust may be produced annually from the activity which produces prebaked anodes. The present invention makes it possible to use such carbon dust as a combustion medium in anode furnaces for the baking of anodes. Such utilisation of the dust produces financial benefits as it reduces the costs for oil or gas as a combustion medium and solves the problem of disposal of carbon containing dust.

US 4,313,386 (Figs. 2 and 3) shows a device for feeding solid fuel in particle form to a distribution unit for further introduction into several combustion areas. The feeding device comprises a funnel-shaped store for the fuel. An agitator, which is designed to rotate about a horizontal axis, is mounted in the lower part of the store. A feed screw is mounted under the agitator. It is driven by a motor to feed fuel out to a fall chamber. A venturi ejector arrangement is mounted at the base of the fall chamber. The quantity fed into the fall chamber can be adjusted by varying the speed of the screw's motor. The venturi ejector arrangement comprises a nozzle for the supply of pressurised air to a venturi. A chamber for the introduction of solid fuel is mounted between the nozzle and the venturi.

The above solution may result in problems if particulate material such as carbon containing dust is to be fed through the device described. Firstly, the material can easily cake or stick in the store, particularly in the part which is not in contact with the agitator. Moreover, problems can arise with the screw feeder. The finest fractions of the material can escape between the feed screw's outer edge and its surrounding housing. This can, in turn, lead to irregular feeding and high wear on the screw on account of particles being dragged along between the screw's outer edge and its surrounding housing. Another problem which can arise in connection with the transport of fine-grained dust which can stick or cake in this screw feeder is that the dust is subjected to a certain compression in the screw feeder. This can result in the material being fed in the form of large cohesive lumps down into the fall chamber, which can result in very irregular feeding of combustible material or clogging of the downstream screw feeder.

The above disadvantages can be avoided with the present invention. The present invention is specially adapted for precise feeding and dosing of fine-grained, carbon containing dust so that problems with caking and clogging can be avoided. The device in accordance with the present invention is, moreover, specially well suited for use in connection with dosing and feeding combustible material to furnaces for baking carbon electrodes within the electrolysis industry.

The present invention will be described in further detail in the following, where:

Fig. 1 shows a device in accordance with the present invention, seen in perspective, Fig. 2 shows a diagram of a section through a device in accordance with the present invention, Fig. 3 shows details of an ejector arrangement, Fig. 4 shows details of an agitator/valve device; a partial section is shown in perspective, Fig. 5 shows further details of the valve device shown in Figure 4.

Figure 1 shows a device in accordance with the present invention for feeding and dosing combustible particle material 1 comprising a storage tank 2 for particle material with a removable lid 36, a frame structure 30 and a discharge nozzle 31 for feeding combustible particle material.

Figure 2 shows a diagram of a section through a device for feeding and dosing combustible particle material 1 , comprising a rotating agitator 3 in a storage tank 2, an ejector 5 and a lock/valve arrangement between the storage tank and the ejector. The storage tank 2 for particle material can expediently be cylindrical with an inclined or conical base 21. The figure shows a removable lid 36 on the top of the store for refilling the store with material. In the centre of the store, at its base, is an agitator 3 which is designed to rotate about a vertical axis. The agitator may consist of a drive shaft 6 with one or more rods 7', 7", 7'". The rods may be made of bar steel which is tapered towards its outer end. The agitator 3 may have a different design to that shown in the figure, but the design shown has proved to be very efficient in connection with handling carbon dust. The agitator's drive shaft 6 is connected to a drive shaft 8 driven by a motor 9 via an angular gear 10. The motor and angular gear are fixed to a bracket 11 in the lower part of the storage tank.

As shown in Figures 2, 3 and 4, there is an opening 22 in the base of the storage tank, which communicates with the ejector 5. This opening may be designed as a slit, be sector-shaped or have any other expedient geometric shape. The size of the opening can be varied using the lock/valve arrangement 32 (Figure 4), comprising a disc-shaped element 4 which can be provided with one or more openings 23; see also Figure 5. The element shown in the figure comprises a hub part 25, a peripheral part 26 and one or more ribs 24 which connect these parts to each other. The described design of the element 4 has been shown, when in use, to cause regular, adjustable distribution/feeding of particle material to the ejector and on to the furnace area. In a different embodiment, the disc-shaped element may have at least one closed sector which can cover the opening between the storage tank and the ejector so that the latter can be closed completely.

The disc-shaped element is shown, in Figures 2 and 4, mounted between guide elements 12' and 12". The disc-shaped element 4 can expediently be permanently connected to the drive shaft 8 so that it rotates and will alternately open/close the opening between the storage tank and the ejector completely or partially while the agitator is rotated. Alternatively, the element can have a separate rotation mechanism not connected to the drive shaft 8 so that it can be rotated to the desired position either manually or using an actuator or similar, regardless of whether the agitator is in motion or not (not shown).

When the disc-shaped element is rotating continuously at a fixed speed, the projected area of its opening(s) and its thickness will be significant for the quantity of material fed out. During the rotation of the disc-shaped element, its openings can be filled with material in the sector which is outside the opening between the storage tank and the ejector. When the said openings pass over the inlet to the ejector chamber, the material will be dropped down into the chamber in a relatively predictable quantity, which is an important feature in relation to the use of the device.

As Figures 2 and 3 show, the ejector 5 comprises a chamber 13 in which there is a downward-facing pipe 14 for the supply of a pressurised gas. The gas expediently consists of air which can be supplied from a compressor or fan 16 driven by a motor 17. In this example, the fan's outlet is connected to the pipe 14 (not shown) via a valve 33. It will be up to the man skilled in the art to adapt the gas quantities and pressure in relation to the quantity of material to be fed out and the pressure conditions in the furnace area in question. The pipe 14 can, at its outlet end, be narrower in order to increase the speed of the gas which flows out of the pipe and into the chamber 13. In the base of the chamber there is an outlet pipe 15 for transport of particle material and gas out of the chamber and possibly on to an injection device in a furnace (not shown) or an injection nozzle 31. The outlet pipe may be fitted with a valve 32. The opening 22, which connects the ejector to the storage tank, is located in the upper part of the chamber 13. Transport between the storage tank and the ejector chamber takes place mainly by means of gravitational forces. However, a certain negative pressure may also occur between the chamber 13 and the storage tank as a consequence of the effect of the ejector. The figure shows a diagonal side wall 18 belonging to the chamber 13, which is located partially below the opening between the storage tank and the ejector. This side wall may expediently be designed with an angle which is steeper than the angle of fall of the material in question.

An element 19 for the supply of a pressurised gas to the storage tank may expediently be fitted in the storage tank above the outlet opening which communicates with the ejector. The intention of this is that if any clogging/caking of material should occur in this area, for example as a consequence of excessive moisture in the dust, it can be remedied using the element 19. The element may be in the form of a nozzle or a fluidising element and may be connected to a source of pressurised gas via a line 20. Equivalent elements may be fitted at other locations in the device for feeding and dosing such as on the inside of the diagonal wall 18 of the ejector chamber.

The device in accordance with the present invention may be fitted with a programmable controller for automatic control of the supply of combustible particle material to a furnace area. The admission of material to be supplied to a furnace area may easily be increased by adjusting the rotational speed or opening of the rotating disc-shaped element 4 (not shown). For automatic control of the supply, the speed of the motor 9, which drives the element 4, may be adjusted by means of an output signal from the controller on the basis of a signal which is proportional to the temperature measured in the furnace area. The output signal is based on a comparison between the furnace area temperature measured and a predetermined setpoint value. For use in connection with an anode furnace, the furnace's firing curve may be entered in the controller so that the fuel supply is in accordance with the energy requirement in the furnace. The equipment is particularly well suited for use in connection with furnaces which are fitted with lids over each chamber (closed type), where the nozzle for the supply of combustible material is in a horizontal position, preferably level with the lower edge of the lid.

The device in accordance with the present invention is specially designed to handle dry dust. Carbon dust which is stored in a dry place contains approximately 0.5% moisture. However, the device has been shown to be able to handle dust with up to 10% moisture. This will, nevertheless, depend on how much the dust adheres, among other factors. The device has also been shown to be able to handle crushed tar from the furnaces' extraction system and dust with a high content of coarse coke.

Those of the device's parts which come into contact with the particle material may expediently be made of stainless steel. This material has a relatively smooth surface and is resistant to corrosion. Using this material, it has been shown that the angle of the base of the store may be as low as 45 degrees without the carbon dust bridging, which means that the store may be designed with a low overall height. Moreover, the lid of the store may be dimensioned so that the store can easily be filled with material stored in big bags.

Claims

1. A device for feeding and dosing combustible particle material to a furnace area comprising a store (2) which communicates with an ejector (5) for the supply of particle material and a pressurised gas from the ejector to the furnace area, characterised in that an adjustable lock/valve arrangement (32) is fitted between the store (2) and the ejector (5) for dosing of material between the store (2) and the ejector (5).

2. A device in accordance with claim 1 , characterised in that the lock/valve arrangement (32) comprises a rotating disc-shaped element (4) with at least one opening.

3. A device in accordance with claim 2, characterised in that the disc-shaped element (4) has a circular form.

4. A device in accordance with claim 2, characterised in that the disc-shaped element (4) is mounted on a drive shaft (8) which is driven by a motor (9) which rotates the element.

5. A device in accordance with claim 1 , characterised in that the store (2) comprises a vertical agitator (3) with a drive motor (9) which rotates the agitator.

6. A device in accordance with claims 4 and 5, characterised in that the agitator (3) is driven by the same motor (9) as the disc-shaped element (4).

7. A device in accordance with claims 2-4, characterised in that the rotational speed of the disc-shaped element (4) can be adjusted to feed a dosed quantity of combustible particle material to the furnace area.

8. A device in accordance with claim 7, characterised in that the rotational speed of the disc-shaped element (4) is adjusted using a controller which records the temperature in the furnace area and adapts the speed in accordance with a predetermined temperature, preferably in accordance with a predetermined firing curve (temperature/time interval curve).

9. Use of a device in accordance with claims 1-8 in connection with the firing of a furnace for baking or calcining carbon electrodes.

10. Use as stated in claim 9 in which the combustible particle material is carbon dust from an electrolysis plant.