WO1999003567A1

WO1999003567A1 - Absorbent material, apparatus and method for exhaust gas cleaning

Info

Publication number: WO1999003567A1
Application number: PCT/GB1998/001908
Authority: WO
Inventors: William Frederick Boylett
Original assignee: William Frederick Boylett
Priority date: 1997-07-16
Filing date: 1998-07-16
Publication date: 1999-01-28
Also published as: AU8226398A; GB9714927D0

Abstract

The present invention relates to exhaust gas cleaning. In one aspect, an absorbent material (400) for the absorption of elements of exhaust gases comprising a bead (400) having a core (404) of a first material and a coat of a second reactive material around the core. Also disclosed is a method of manufacturing such an absorbent material, which method comprises the steps of providing a plurality of cores of first material and rolling the plurality of cores in the second material. Also disclosed is an exhaust gas cleaning apparatus (2) comprising a frame (12) for supporting a plurality of beads (400) for cleaning the waste gas and means (4, 6, 8) for passing exhaust gas through the frame which beads are substantially static relative to the frame during the cleaning process.

Description

ABSORBENT MATERIAL, APPARATUS AND METHOD FOR EXHAUST GAS CLEANING

Field of the Invention

The present invention relates to absorbent materials, an exhaust gas cleaning method and apparatus for carrying out the process, and a method of manufacturing the absorbent materials. The invention is particularly, but not exclusively, concerned with the removal of acid from exhaust gases such as those of ceramic producing kilns and incinerators .

Background to the Invention

The use of limestone chippings for the removal of acid gases, such as hydrogen fluoride, sulphur dioxide and hydrochloric acid, present in the exhaust gases of ceramic producing kilns and incinerators is a well known phenomenon. The use of hydrated limestone for the same process is also well documented. Both processes have advantages and disadvantages. The limestone chipping method uses mechanical and/or gravitational means to move the chippings around the system which increases the efficiency of the process, but is both noisy and dusty. In addition, the movement requires management and maintenance. Furthermore, limestone chippings have a relatively low efficiency of absorption. Absorption is limited to approximately 1mm of penetration below the surface of the chipping which produces absorption efficiencies of between 10 and 50% depending upon external factors including retention time. Due to the variability of absorption, it is necessary to utilise additional chippings in the cleaning process which act as a buffer in the event of low efficiency absorption. A system capable of cleaning 100, OOOmVhr requires approximately 250 tonnes of limestone chippings. The requirement for a relatively high tonnage of chippings increases the expense of the process and, for instance, the cost of HF removal from such a system is approximately £2.20 per kg. In some circumstances, due to the use of blown air to carry cold chippings around the system, corrosion can occur. Due to the complexity of this system it is necessary to manage the process thus requiring valuable management time. With this system it is also necessary to set the rate of chipping removal to cope with the worst conditions of HF generation. Most ceramic companies use many different clays, some high in pollutants and some low. By setting the limit for the high HF products, great inefficiency occurs when low HF clays are used.

To improve efficiency, a hydrated lime module system has been developed. The system uses hydrated lime which has been compressed into perforated blocks and this gives more efficient absorption than the limestone chipping method. A disadvantage of such a system is the relatively high pressure drop across the perforated blocks. Furthermore, the blocks are prone to dust blockage, and for this reason, are generally utilised for small exhaust volumes in the range 5, 000-25 , 000m³ of exhaust gases. In such a system, a system cleaning 10,000m³/hr would require approximately 10 tonnes of modules. Advantageously, the system requires no moving parts and is designed to run unattended for long periods before the modules require replacement. Disadvantageously, this system could not be used in brick producing tunnel kilns due to the high levels of dust.

However, the modules are expensive to replace and cost approximately £1, 300/tonne . The system is more convenient but increase cost with the cost for removal of HF being approximately £9.00/kg compared with £2.20/kg with the limestone system.

A further problem with the aforementioned systems is the requirement to shut down the process producing the exhaust gases whenever it is necessary to replace the absorbent materials.

One aim of preferred embodiments of the present invention is to obviate or overcome at least one of the above problems .

Summary of the Invention

According to a first aspect of the present invention, there is provided an absorbent material for the absorption of elements of exhaust gases comprising a bead having a core of a first material and a coat of a second reactive material around the core .

According to the present invention in a second aspect there is provided a method of manufacturing an absorbent material according to the first aspect of the invention, which method comprises the steps of providing a plurality of cores of first material and rolling the plurality of cores in the second material .

According to the present invention in a third aspect, there is provided an exhaust gas cleaning apparatus comprising a frame for supporting a plurality of beads for cleaning the waste gas and means for passing exhaust gas through the frame which beads are substantially static relative to the frame during the cleaning process. According to the present invention in a fourth aspect, there is provided a method of cleaning exhaust gas, the method comprising the steps of providing an exhaust gas cleaning apparatus according to the second aspect of the invention and passing exhaust gas through the frame while the beads are substantially static relative to the frame during the cleaning process.

The beads in the third or fourth aspects of the invention are preferably according to the first aspect of the invention.

Preferably, the beads comprise a coat of hydrated lime around a core made of a different material.

Preferably, the bead indentations on the surface which increases surface area. Preferably, the indentations are formed by rotating the beads during preparation.

Preferably, when coated, the core material may be any suitable material such as wood, polymer beads such as polystyrene beads or limestone chippings. Preferably, limestone chippings are used.

Preferably, the beads are between l-15mm in diameter, more preferably, 3 -12mm, most preferably 4.5-7.5mm. When coated, the coating is preferably between 1-10,000 microns, more preferably between 500-4,000, most preferably 750-2,500 microns.

Preferably, the bead includes additives of liquid and/or dry binders which can accelerate the process of binding the hydrated lime coating to the core material. Preferably, the beads are substantially in the form of spheres.

Acids to be cleaned may comprise any suitable acid including HF, S0₂ and HC1 gases. The beads may be of any suitable shape and need not be substantially spherical.

In such a case, the size of diameter above refers to the widest diameter of the bead.

Preferably, the beads are arranged in trays which are permeable to the exhaust gases. Preferably, the trays are permeable on the top and bottom and have substantially impermeable sides. Preferably, the trays are arranged in series so that the exhaust gases may, optionally, pass through 1, 2, 3, 4, or more trays. Preferably, the trays can easily be accessed, removed and replaced.

Preferably, the trays are arranged in a horizontal bed to provide a porous absorption barrier for the exhaust gases. Preferably, the beds of trays form a multi-layered absorption barrier with at least two beds of trays substantially parallel and vertically spaced from each other. Three, four or more beds may be provided.

The or each tray or within at least one tray beads of different reactive properties relative to the exhaust gas may be provided. If in one tray, the beads may be provided in layers of different reactive properties relative to the exhaust gas.

Preferably, a bypass is provided to redirect the exhaust gases during tray replacement .

The method of manufacture of the beads may be in accordance with any known technique to the skilled person and may, in particular, consist of rotating dry powders into a drum of suitable diameter so that the powder tumbles and rolls in such a way that the particles bond together to form a sphere. The additives of liquid and/or dry binders may be added to the drum to accelerate the process .

The diameter of the spheres can be controlled by the length of time that they spend in the rotating drum and also by the amount of powder that is incorporated into them. Typically, the drum may be rotated at a speed of between 10-50 rpm, more typically between 15-40 rpm and, most typically between 20-30 rpm.

Preferably, after manufacture, the beads are cured so that they are compact and easy to handle.

Advantageously, the use of beads allows flexibility in varying the pressure drop and absorption capacity of the process. The use of larger beads will decrease the pressure drop across the absorbent but will also decrease the surface area available for absorption and, consequently, decrease absorption of material. However, by varying the diameter of the beads the appropriate absorption capacity can be provided which gives the minimum pressure drop across the absorbent. Pressure drop variability may also be effected by varying the depth of bed in which the absorbent beads are found.

A further advantage lies in the arrangement of the trays into spaced beds which decrease the overall pressure drop of the reactor and, therefore, allow a relatively long retention time without the commensurate high pressure drop across the bed. Preferably, dampers are provided which, typically, extend vertically between the beds and thereby provide multi-pass or single pass processes depending upon the operating conditions required.

Preferably the beads are recycled and reused.

Preferably, the process uses beads coated with hydrated lime which, advantageously, when fully reacted, can be recycled, the spent hydrated lime being separated as calcium fluoride, calcium chloride, calcium sulphate etc and used for industrial use and the old limestone chippings or other core material being recycled with hydrated lime.

The invention is particularly useful for cleaning exhaust gases from tunnel kilns, especially brick producing tunnel kilns.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures; in which:

Figure 1 shows a double bed process and apparatus in accordance with the present invention;

Figure 2 shows a single bed process and apparatus in accordance with the present invention;

Figure 3 shows a quadruple bed process and apparatus in accordance with the present invention; Figure 4 is a schematic cross-sectional illustration of a bead according to one aspect of the present invention; and

Figure 5 shows another single bed process and apparatus according to an aspect of the present invention.

Description of the Preferred Embodiments

Referring to Figure 1, an exhaust gas cleaning apparatus 2 has an inlet 4 for exhaust gases, an outlet 6 for cleaned exhaust gas and a cleaning chamber 8 between the inlet 4 and outlet 6 for carrying out the waste gas cleaning .

The inlet 4 terminates in an opening 10 in the main cleaning chamber 8 which comprises four vertically spaced absorption beds 12A, 12B, 12C, 12D which are centrally disposed in the middle of the chamber . 8 in parallel relationship. The beds are rectilinear with respective vertically aligned sides which do not extend outwardly as far as the outer walls of the chamber 8, leaving a vertical channel on each side of the beds extending from the top to the bottom of the chamber on each side of the chamber respectively. An inlet channel 12 communicates with the inlet 4 and an outlet channel 14 communicates with the outlet 6 of the chamber 8. Both the bottom 12D and top 12A trays are respectively spaced from the bottom and top of the chamber so that a bottom 16 and top 18 channel are also formed around the beds.

Between the top and the bottom of the chamber on the outlet side, a damper 20 extends between the top of the chamber 8 and the distal side of the bed 12B and another damper 22 extends between the bottom of the chamber 8 and the distal side of the bed 12C, thus preventing the exhaust gases from passing directly through the cleaning chamber, via the top 18 or bottom 16 channel, without passing through the absorbent beds 12A-12D. A further damper 24 extends from the upper bed 12A to the lower bed 12D and abuts the vertical sides on the proximal side of the beds in respect of the inlet 4 thus providing only two routes of access 26A, 26B to the absorbent beds 12A-12D for the exhaust gas. The first route of access 26A for the route for the exhaust gas is via the upper channel 18 and then down through the two uppermost beds 12A, 12B whereafter the gas may pass out between the second and third beds 12B, 12C respectively to the outlet side channel 14 and thence through the outlet 6. The second route of access 26B for the exhaust gas is via the bottom channel 16 and up through the two lowermost beds 12C, 12D respectively, similarly allowing the gas to exit between the second and third beds 12B, 12C respectively and via the outlet side channel 14 and the outlet 6.

Each absorption bed 12 is substantially similar. For ease of reference, upper bed 12 will be described in more detail. Bed 12A comprises a parallelepiped structure having a wire mesh base to allow the exhaust gas to pass therethrough. The side walls of bed 12A are solid (ie impermeable to the gas) . The upper face of the bed 12A is open. Extending from the each short top edge of bed 12A is a metal flange 28 which sits on a projecting ledge 30 of damper 20 on one side and damper 24 on the other. Also extending from dampers 20, 24 are further projecting ledges 32 which extend underneath bed 12A. Compressible seals 34 are disposed between the bottom of bed 12A and each of ledges 32. The arrangement is dimensioned so in use the bed 12A sits on ledges 30 creating a substantially gas impermeable seal, and compresses seals 34 sufficiently that these also create gas impermeable seals. Additional soft seals are provided at each juncture of tray and frame, including between flange 28 and ledge 30.

A by-pass passageway 36 is centrally mounted on the upper side of the cleaning chamber 8 to provide an alternative passageway for the exhaust gases. In use, the inlet 4 is closed by damper 38, sealing the cleaning chamber 8 from the incoming exhaust gases and a further damper 40 opens the other end of by-pass passageway 36, to allow exhaust gases to pass directly from the inlet 4 to the outlet 6 without entering the cleaning chamber 8. During this time, the beds 12A-12D in the chamber 8 can be replaced or replenished.

Figure 2 shows a similar arrangement to that shown and described with respect to Figure 1 except that the dampers are arranged so that only a single pass through the beds is carried out. A damper 42 extends from the top of the chamber on the distal side as far as the distal side of the upper bed 200A but does not extend as far as the second bed 200B so that the gas passing down through the upper bed 200A via the upper channel 18 can pass directly out between the first and second bed 200A, 200B to the outlet channel 14 and outlet 6. Similarly, a damper 44 extends from the bottom of the chamber on the distal side as far as the distal side of the bottom bed 200D but does not extend between the lower bed 200D and the second bed from the bottom 200C so that gas may escape after passing through the lower bed 200D between the third and fourth bed 200C, 200D respectively and via the outlet channel 14 and the outlet 6. Further dampers on the proximal side between first and second beds (damper 46) and the third and fourth bed (damper 48) and on the distal side between the second and third bed (damper 50) are provided to provide passages for gas through the second and third beds 200B, 200C respectively and to prevent exhaust gases passing through more than one bed.

Referring to Figure 3, a multiple four pass bed is provided by providing a damper 52 from the top of the chamber to the proximal side of the upper bed 300A and connecting the proximal sides of all the beds, leaving a gap between the bottom of the chamber and the bottom bed, into which the gas enters. Thus the gas is forced to pass down the inlet side of the beds and up through the beds via the bottom channel 302. Dampers 304 also extend from the distal side of the bottom of the chamber 28 to the top of the distal side of the upper bed 300A so that the gas is forced to pass through all four beds 300D-300A before exiting via the upper passage and the outlet .

Thus exhaust gas passes through a plurality of bead filled trays for cleaning.

Referring to Figures 1-3 and especially to Figure 4, the hydrated lime beads 400 are made of coated dual material in the form of pure hydrated lime 402 bonded to limestone chippings 404. Each bead 400 has a central core 404 in this case of limestone chippings, with a reactive outer shell 402, in this case hydrated lime.

As an alternative the trays can be layered with a mixture of beads as shown in Figure 5. Four beds 500A- 500D provide cleaning between an inlet 502 and an outlet 504. For instance, a tray may include hydrated lime for HF, sodium bicarbonate for S0₂, magnesium lime for HC1 , activated charcoal for dioxins etc. The configuration shown in Figure 5 is especially suitable for multi- contaminated exhaust gas. Equally different reactants can be disposed in separate trays.

A method of manufacture of the beads consists of rotating the limestone chippings in a drum of suitable diameter so that the hydrated lime powder, present in the drum, tumbles and rolls in such a way that the particles bond together to form a sphere around the chippings . Additives of liquid and/or dry binders are also added to accelerate the binding process and the diameter of the spheres is controlled by the length of time that they are present in the rotating drum.

To produce a bead of approximately 5-7mm from a 4mm limestone chipping it is necessary to rotate the drum at 25 rpm for approximately 20 minutes. Further rotation causes the beads to form a surface similar to that of a golf ball, having numerous indentations on the surface, it is calculated that this phenomenon produces approximately 10% more surface area and thus produces greater absorbency of the beads in the system.

The beads are then packed into trays which are of dimensions 1.2m x 1.2m x 0.6m to provide a bed of packed beads approximately 0.6m deep. Sixteen such trays are arranged 4 x 4 in each of four vertically spaced beds, providing a cleaning chamber of 64 trays which holds approximately 55 tonnes of beads and has an absorption area of approximately 52,000m². Beads of 5mm diameter give a weight to each tray of approximately 680kg. There are approximately 7 million - 8 million beads/tray at 5mm diameter and 3.7 million beads at 7mm diameter. The surface areas depend upon the size of beads and would vary from 815 m²/tray for the 5mm diameter bead to 600 m²/tray for beads of 7mm diameter. Beads of this size or other sizes can be mixed to give maximum absorption and minimum pressure drop depending upon the application and absorption required. Another possible configuration is 36 trays 6 x 3 x 2. Tray sizes and numbers can vary depending upon the application.

In the beds described, exhaust gases could pass at a rate of 542 m³/hr/m² of a reactor bed area, whilst producing a dwell or retention time of 2 seconds and a pressure drop of 12mm - 16mm. By arranging the beds in spaced vertical relationship a longer retention time is possible and the efficiency of absorption increases to approximately 40% - 60% and the reactor is capable of absorbing approximately 21 tonnes of, for example, hydrogen fluoride. In this arrangement, the reactor could be run at approximate 552 m³/hr/m² of reactor area so that 50,000 m³/hr would pass through the reactor at a pressure drop of approximately 16-20mm.

Such a reactor has the ability, when situated on a brick tunnel kiln, to remove approximately 50mg of HF/m³ of products of combustion for a period of approximately 8,400 hrs before needing replenishment/replacement.

By operating the reactor in this manner a considerable saving in costs is achieved compared to the limestone chippings and hydrated lime module methods. Although specific figures for flow and retention time have been given, the flows and retention times can be varied to cope with a variety of operating conditions. Similarly, the number of beds of absorbent material can be varied to suit particular requirements. The whole of the bed may be set on a removable trolley so that the trays of beads can be rapidly replaced. The provision of humidity control is also envisaged so that the reactor may operate at high levels of reactivity.

A major benefit of this system is the ability to be able to recycle the used beads. Since the inner core is generally inert, the beads can be re-rolled in hydrated lime to be replenished and recycled. Even if some hydrated lime remains, it is just covered by the re- treatment. This is possible and necessary, as the acid gas penetration into the bead is restricted to approximately 1mm, at which point the conversion of HF into CF stops, producing a 1mm coating over the whole of the surface area. If the beads are re-coated with any of the available coatings, ie Calcium Hydroxide, Sodium Carbonate, Sodium Bicarbonate, Magnesium Lime, they can be re-used in the cleaning system. By treating the used beads in this manner, major environmental and financial benefits are achieved.

A further benefit of this design is the ability of the system to remove approximately 90% of all air borne dust particles.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification ( including any accompanying claims , abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. An absorbent material for the absorption of elements of exhaust gases comprising a bead having a core of a first material and a coat of a second reactive material around the core .

2. An absorbent material according to claim 1, in which the absorbent material comprises a coat of hydrated lime around a core made of a different material.

3. An absorbent material according to claim 1 or claim 2, in which the bead has indentations on the surface.

4. An absorbent material according to any preceding claim, in which the core material is selected from wood, polymer beads such as polystyrene beads or limestone chippings .

5. An absorbent material according to any preceding claim, in which the bead is between l-15mm in diameter.

6. An absorbent material according to claim 5, in which the bead is between 3 -12mm in diameter.

7. An absorbent material according to claim 5 or claim 6, in which, the bead is between 4.5-7.5mm in diameter..

8. An absorbent material according to any preceding claim, in which the coating is between 1-10,000 microns.

9. An absorbent material according to claim 8, in which the coating is between 500-4,000 microns.

10. An absorbent material according to claim 8 or claim 9, in which the coating is between 750-2,500 microns.

11. An absorbent material according to any preceding claim, in which the bead includes additives of liquid and/or dry binders which can accelerate the process of binding the hydrated lime coating to the core material.

12. An absorbent material according to any preceding claim, in which the bead is substantially in the form of a sphere .

13. A method of manufacturing an absorbent material according to the first aspect of the invention, which method comprises the steps of providing a plurality of cores of first material and rolling the plurality of cores in the second material .

14. A method of manufacturing an absorbent material according to claim 13, which consists of rotating dry powders into a drum of suitable diameter so that the powder tumbles and rolls in such a way that the particles bond together to form a sphere .

15. A method of manufacturing an absorbent material according to claim 14, in which additives of liquid and/or dry binders may be added to the drum to accelerate the process .

16. A method of manufacturing an absorbent material according to claim 13 or claim 14, in which drum is rotated at a speed of between 10-50 rpm.

17. A method of manufacturing an absorbent material according to claim 16, in which the drum is rotated more typically between 15-40 rpm.

18. A method of manufacturing an absorbent material according to claim 16 or claim 17, in which the drum is rotated between 20-30 rpm.

19. A method of manufacturing an absorbent material according to any one of claims 13 to 18, in which the beads are cured.

20. A method of manufacturing an absorbent material according to any one of claims 13 to 19, in which the beads are recycled and reused.

21. An exhaust gas cleaning apparatus comprising a frame for supporting a plurality of beads for cleaning the waste gas and means for passing exhaust gas through the frame which beads are substantially static relative to the frame during the cleaning process.

22. An exhaust gas cleaning apparatus according to claim 21, in which the beads are according to any one of claims 1 to 12 and/or manufactured according to any one of claims 13 to 20.

23. An exhaust gas cleaning apparatus according to claim 21 or claim 22, in which the beads are arranged in trays which are permeable to the exhaust gases.

24. An exhaust gas cleaning apparatus according to claim 23, in which the trays are permeable on the top and bottom and have substantially impermeable sides.

25. An exhaust gas cleaning apparatus according to claim 23 or claim 24, in which the trays are arranged in series so that the exhaust gases may, optionally, pass through 1, 2, 3, 4, or more trays.

26. An exhaust gas cleaning apparatus according to any of claims 21 to 25, in which the trays are arranged in a horizontal bed to provide a porous absorption barrier for the exhaust gases .

27. An exhaust gas cleaning apparatus according to claim 26, in which the beds of trays form a multi-layered absorption barrier with at least two beds of trays substantially parallel and vertically spaced from each other.

28. An exhaust gas cleaning apparatus according to any one of claims 23 to 27, in which the or each tray or within at least one tray beads of different reactive properties relative to the exhaust gas may be provided.

29. An exhaust gas cleaning apparatus according to any one of claims 21 to 28, in which a bypass is provided to redirect the exhaust gases during tray replacement .

30. An exhaust gas cleaning apparatus according to any one of claims 21 to 29, in which dampers are provided which extend vertically between the beds and thereby provide multi-pass or single pass processes depending upon the operating conditions required.

31. A method of cleaning exhaust gas, the method comprising the steps of providing an exhaust gas cleaning apparatus according to any one of claims 21 to 30 passing exhaust gas through the frame while the beads are substantially static relative to the frame during the cleaning process.

32. A method of cleaning exhaust gas, comprising the steps of using a bead according to any one of claims 1 to 12 and/or manufactured according to any one of claims 13 to 20.