DEVICE FOR DISCHARGING AND DISTRIBUTING AN ABSORBENT MATERIAL IN A FLUE GAS DUCT

Abstract: In a vertical flue gas duct (10), in which flue gases containing gaseous pollutants are to be conducted upwards, a discharging and distributing device is arranged. The device is adapted to discharge and distribute a particulate absorbent material which is reactive with the gaseous pollutants in the flue gases and which, during cleaning of the flue gases, is to be introduced into these gases in moistened state in order to convert the gaseous pollutants to separable dust. The device comprises at least one distributing plate (35) which extends obliquely downwards in the flue gas duct (10) and which is adapted to receive on its upper side moistened absorbent material and which has two well-defined side edges converging obliquely downwards seen in the direction of inclination.

Description: DEVICE FOR DISCHARGING AND DISTRIBUTING AN ABSORBENT MATERIAL IN A FLUE GAS DUCT The present invention relates to a device arranged in a substantially vertical flue gas duct, in which flue gases containing gaseous pollutants, such as sulphur dioxide, are to be conducted upwards, for discharging and distributing in the flue gas duct a particulate absorbent material which is reactive with the gaseous pollutants in the flue gases and which, during cleaning of the flue gases, is to be introduced into these gases in moistened state in order to convert the gaseous pollu- tants to separable dust. When gaseous pollutants, such as sulphur dioxide, are to be separated from flue gases, the gases are con¬ ducted through a flue gas duct, in which a particulate absorbent material reactive with the gaseous pollutants is, in moistened state, introduced into the flue gases in order to convert the gaseous pollutants to separable dust. The flue gases are then conducted through a dust separator, in which dust is separated from the flue gases and from which the thus-cleaned flue gases are drawn off. Part of the dust separated in the dust separator is con¬ ducted to a mixer, where it is mixed and moistened with water, whereupon it is recycled as absorbent material by being introduced into the flue gases along with an addi¬ tion of fresh absorbent. As fresh absorbent, use is gene- rally made of slaked lime (calcium hydroxide). In order to clean the flue gases in an effective fashion it is, of course, essential that the absorbent material is supplied to the flue gases as a homogeneous mixture in which the moisture is evenly distributed. A device which is especially advantageous for making a homogeneous mixture of absorbent material and water in the above-described flue gas cleaning method is disclosed in WO 96/16727. This mixing device comprises a container, in which the particulate absorbent material is fluidised during the mixing operation. The container has an outlet end extending into the flue gas duct for discharging, through an overflow, a homogeneous mixture of absorbent material and water in the flue gas duct. In order that the flue gases should be efficiently cleaned, it is also essential that the moistened absor¬ bent material is uniformly distributed in the flue gases over the entire cross-section of the flue gas duct. Such a homogeneous distribution is not achieved by means of the overflow in the above-described mixing device. The object of the present invention therefore is to provide a device which can be used both in the above- described mixing device and in other mixing devices for discharging in the flue gas duct moistened absorbent material received from the mixing device and distributing this in a homogeneous manner over the entire cross-sec¬ tion of the flue gas duct. According to the present invention, this object is achieved by a device which is of the type mentioned by way of introduction and which is characterised in that it comprises at least one distributing plate extending obliquely downwards in said flue gas duct and adapted to receive on its upper side moistened absorbent mate¬ rial, said distributing plate having two well-defined side edges converging obliquely downwards seen in the direction of inclination. Preferably, the device has at least two distributing plates opposing one another. In a preferred embodiment, a discharging unit, from which the distributing plates project obliquely down¬ wards, is positioned centrally in the flue gas duct to discharge moistened absorbent material on the distribut¬ ing plates. The distributing plates are preferably of triangular shape. The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 is a schematic view of a plant for cleaning flue gases from a coal-fired central heating plant; Fig. 2 is a side view of a mixer provided with an inventive device, certain parts being broken away; Fig. 3 is a top plan view of the mixer in Fig. 2; and Fig. 4 is a cross-section taken along line IV-IV in Fig. 3. Fig. 1 illustrates schematically a plant for clean- ing flue gases from a coal-fired central heating plant 1, said flue gases containing dust, such as fly ash. A pre- heater 2 is arranged to transfer heat from the hot flue gases to combustion air, which via a duct 2a is supplied to the central heating plant 1 by means of a fan 3. The hot flue gases are conducted through a duct 4 to a dust separator 5 which in the embodiment illustrated is a fabric filter, which in a known manner contains a plu¬ rality of rows of filter bags and through which the flue gases are conducted to be cleaned. The thus-cleaned flue gases are conducted through a duct 6 to a flue gas fan 7, which through a duct 8 feeds them further to a chimney 9 to be discharged into the atmosphere. The dust separator may also be, for instance, an electrostatic precipitator. The duct 4 has a vertical portion 10. A mixer 11 communicates with this portion 10 in the lower part thereof. The mixer 11 introduces in a manner described in more detail below moistened dust particles into the flue gases in the lower part of the duct portion 10. The dust particles separated in the dust separator 5 are collected in the hoppers 12 of the dust separator 5. Part of the collected dust particles is recycled in the system by being conducted to the mixer 11 (arrow PI). The remainder of the collected dust particles is remov¬ ed in a manner that is not described in more detail, for instance by means of a screw conveyor. The mixer 11 is a mixer of the type as disclosed in WO 96/16727 and is shown in more detail in Figs 2-4. The mixer 11 illustrated in Figs 2-4 comprises a container 13, which essentially is in the shape of an elongate, parallelepipedal box. The container 13 has two vertical side walls 14 and 15, a vertical rear end wall 16, a vertical front end wall 17, a horizontal upper bot¬ tom 18, a horizontal lower bottom 19 and a horizontal top or lid 20. At the rear end, the container 13 has an inlet 21, through which dust particles from the hoppers 12 are introduced from above into the container 13 (arrow PI in Figs 1 and 2), and at the front end, an outlet 22, through which a homogeneous mixture of dust particles and water is discharged (arrows P2 in Figs 3 and 4) . In the drawings, the front end of the container 13 is inserted in the vertical duct portion 10, through which the flue gases are conducted upwards (arrows P3 in Figs 1, 2 and 4). In this application, the outlet 22 is an overflow formed as a result of the side walls 14 and 15 being lower in the main part of the container part inserted in the duct portion 10, than in the con¬ tainer part located outside the duct portion 10. As appears from Figs 2 and 3, the top 20 extends from the inlet 21 to the outlet 22, i.e. up to the duct portion 10. Between them, the two bottoms 18 and 19 define a chamber 23 which, in the lateral direction, is delimited by the two side walls 14 and 15 and, in the longitudinal direction, by the two end walls 16 and 17. The ceiling of the chamber 23, i.e. the upper bottom 18, consists of an air-permeable fluidisation cloth of polyester mounted in stretched state in the container 13. An air-supply means, which here consists of two air inlets 24 and 25, is arranged to supply air to the chamber 23 (arrows P4 in Figs 2 and 3) , so as to fluidise the dust particles in the container 13. A water-supply line 26, which is disposed above the container 13, is connected to a plurality of nozzles 27 arranged in the upper part of the container 13 to spray water in finely-divided form over the dust particles in the container. The nozzles 27, of which but a few are shown in the drawings, are arranged in two parallel rows extending along the container 13. Two juxtaposed, horizontal shafts 28, 28' extend along the entire container 13 and are rotatably mounted in the two end walls 16 and 17 with the aid of bearings 29, 29' and 30, 30', respectively. A motor 31 is arranged to rotate the shafts 28, 28' via a transmission unit 32. Each shaft 28, 28' supports a plurality of elliptic discs 33, 33' which, about their minor axes, are mounted in inclined state on the shaft 28, 28' at an axial dis¬ tance from one another. The shafts 28, 28' extend through the centres of the respective discs 33, 33'. In the exam¬ ple illustrated, each disc 33, 33' is so inclined in relation to the shaft 28, 28' that the angle α between the major axis of the disc and the shaft 28, 28' is about 60° (see Fig. 2). This angle α may vary between 45° and 80°. The discs 33, 33' are so inclined in relation to the respective shafts 28, 28' and have such an elliptic shape as to have a circular axial projection, as illustrated in Fig. 4. The discs 33, 33' are so positioned on the respective shafts 28, 28' that the discs of the one shaft project into the spaces between the discs of the other shaft. Each of the discs 33, 33' arranged and designed in the manner indicated above performs, during the rotation of the shafts 28, 28', a throwing movement conducive to thorough mixing of the dust particles. By a partition 34 in the front part of the container 13, the chamber 23 is divided into a front part chamber 23a, which is situated in the duct portion 10, and a rear part chamber 23b. As appears from Fig. 2, the air inlet 24 opens into the rear part chamber 23b, while the air inlet 25 opens into the front part chamber 23a. With this division of the chamber 23, it is possible to achieve different fluidisation conditions in the two part cham¬ bers 23a and 23b, especially with a view to so adapting the air supply to the front part chamber 23a that one there obtains a suitable fluidisation state for the mate- rial discharge. That part of the dust particles collected in the hoppers 12 of the dust separator 5 which is to be recycl¬ ed in the system is introduced into the mixer 11 through the inlet 21. The dust particles are moistened with water which is supplied through the nozzles 27. By the con¬ struction of the mechanical mixing mechanism and by the fluidisation of the dust particles supplied to the mixer, the mixer 11 produces a homogeneously moistened, homoge¬ neous mixture of dust particles which is continuously introduced into the duct portion 10 through the overflow 22 of the mixer 11. In order to distribute the homogeneously moistened, homogeneous mixture of dust particles uniformly in the flue gases over the entire cross-section of the duct portion 10, the above-described mixer 11 has been supple¬ mented with a device according to the present invention. This distributing device consists, in the example shown, of four flat distributing plates 35, each having the shape of an isosceles triangle. The proportion of the base to the height of the isosceles triangle is 0.2-0.4, preferably 0.25-0.35. Two of the plates 35 are attached, in a juxtaposed manner, to one side wall 14 of the container 13 in the container part inserted in the duct portion 10. The remaining two plates 35 are attached to the other side wall 15 of the container 13 each in front of one of the plates attached to the side wall 14. The base of each distributing plate 35 is arranged on the same level as the overflow 22 and extends in parallel with the respec- tive side walls 14, 15. The distributing plates 35 are inclined and extend obliquely downwards in the duct por¬ tion 10. The plates 35 make an angle β with the horizon- tal plane, which in the shown embodiment is about 45°. The angle β may vary between 10° and 80° and is prefer¬ ably 35-50°. The proportion of the horizontal distance a of each distributing plate 35 from the neighbouring wall of the duct portion 10 to the length b thereof in the direction of protrusion, projected in the horizontal plane (see Fig. 4), is 0-0.5, preferably 0.1-0.3. The projection of the distributing plates 35 in the horizontal plane occupies 5-50%, preferably 20-30% (in the shown example, about 30%), of the total cross-sec¬ tional area of the duct portion 10, reduced by the cross- sectional area occupied by the mixer part projecting into the duct portion. The mixer 11 discharges homogeneously moistened dust particles on the distributing plates 35 through the over¬ flow 22. These dust particles slide by the action of gra¬ vity down along the plates 35 and successively reach the well-defined side edges thereof, where the flue gas flow in the duct portion 10, owing to the design and position¬ ing of the plates 35, obtains a high speed gradient, which causes lumps of dust particles, if any, to be torn apart and the dust particles to be dispersed in the flue gases and uniformly distributed in the flue gases over the entire cross-section of the duct portion. Besides, the plates 35 produce, owing to their design, a vorticity in the flue gases, which yields an enhanced global mixing of the dust particles in the flue gases. As will be appreciated, the distributing device described above can be modified in various ways within the scope of the invention. Thus, the distributing plates 35 may have a different triangular shape than the shown isosceles triangular shape. The plates 35 need not even be triangular but may have a completely different shape, if they just have two well-defined side edges converging obliquely downwards seen in the direction of inclination. The distributing plates 35 need not be flat either, but may, in some applications, be convex or concave. The num¬ ber of distributing plates 35 is, of course, adapted to the application involved. In some cases, it may be suffi¬ cient to use one plate 35 only. It will of course also be appreciated that the in¬ ventive distributing device can also be used in other mixing devices or discharging units than the mixer 11 described in detail above. That part of the mixer 11 which extends into the duct portion 10 can be replaced by, for instance, a horizontal plate to which moistened dust particles are supplied from a mixer of any kind.

Claims: CLAIMS 1. A device arranged in a substantially vertical flue gas duct ( 10), in which flue gases containing gaseous pollutants, such as sulphur dioxide, are to be conducted upwards, for discharging and distributing in the flue gas duct ( 10) a particulate absorbent material which is reactive with the gaseous pollutants in the flue gases and which, during cleaning of the flue gases, is to be introduced into these gases in moistened state in order to convert the gaseous pollutants to separable dust, c h a r a c t e r i s e d in that it comprises at least one distributing plate (35) extending obliquely downwards in said flue gas duct (10) and adapted to receive on its upper side moistened absorbent material, said distributing plate having two well-defined side edges converging obliquely downwards seen in the direc¬ tion of inclination. 2. The device as claimed in claim 1, c h a r a c ¬ t e r i s e d in that it has at least two distributing plates (35) opposing one another. 3. The device as claimed in claim 2, c h a r a c ¬ t e r i s e d by a discharging unit (11, 22) which is positioned centrally in the flue gas duct (10) and from which the distributing plates (35) project obliquely downwards and which is adapted to discharge moistened absorbent material on the distributing plates. 4. The device as claimed in claim 3, c h a r a c - t e r i s e d in that the proportion of the horizontal distance (a) of each distributing plate (35) from the neighbouring wall of the flue gas duct, to the length (b) thereof in the direction of protrusion, projected in the horizontal plane, is 0-0.5, preferably 0.1-0.3. 5. The device as claimed in claim 3 or 4, c h a r ¬ a c t e r i s e d in that the projection of the distri¬ buting plates (35) in the horizontal plane occupies 5-50%, preferably 20-30%, of the total cross-sectional area of the flow gas duct (10), reduced by the cross-sec¬ tional area occupied by the discharging unit (11, 22). 6. The device as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that each distributing plate (35) makes an angle (β) of 10-80°, preferably 35-50°, with the horizontal plane. 7. The device as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that each distributing plate (35) is of triangular shape. 8. The device as claimed in claim 7, c h a r a c ¬ t e r i s e d in that each distributing plate (35) is in the shape of an isosceles triangle. 9. The device as claimed in claim 8, c h a r a c - t e r i s e d in that the proportion of the base to the height of the isosceles triangle is 0.2-0.4, preferably 0.25-0.35.

Inventors: AAHMAN STEFAN, BRINGFORS NILS, JOHANSSON LARS-ERIK, AAHMAN, STEFAN, BRINGFORS, NILS, JOHANSSON, LARS-ERIK
Applicants: AAHMAN STEFAN, BRINGFORS NILS, FLAEKT AB, JOHANSSON LARS ERIK, AAHMAN, STEFAN, ABB FLAEKT AB, BRINGFORS, NILS, JOHANSSON, LARS-ERIK
Application year: 1997
Main classification: B01D53
IPC codes: B01D53, B01J8, B01J19