A FUME-FILTRATION SYSTEM HAVING A MOVING BED OF STONE CHIPPINGS

Abstract: A fume-filtration system comprises a plurality of filter modules (1, 2, 3, 4, 5) connected in parallel and aligned in a battery, in which a flow of fumes introduced into the modules passes through a filter column of stone chippings housed in the modules and forming a first free surface (23, 24) for the inlet of the fumes and a second free surface (25, 26) for the outlet of fumes, the column being renewed in counter-current to the fume flow either with continuous or intermittent renewal, the intermittent renewal being performed on a module temporarily excluded from the set of modules in parallel.

Description: A FUME-FILTRATION SYSTEM HAVING A MOVING BED OF STONE CHIPPINGS FIELD OF THE INVENTION The present invention relates to a fume-filtration system having a moving bed of stone chippings serving both as a dust remover and as a neutraliser for the oxides of sulphur in the fumes. STATE OF THE ART Filtration systems have been known for some time which make use of moving beds of inert granular material instead of cyclones or textile filters whose effectiveness and useful life are limited and also instead of expensive and bulky scrubbers in thermal power plants. US Patent US-A-3 976 747, for example, describes a system in which relatively hot fumes, at temperatures of the order of 90 * C traverse, in cross flow, a space filled with limestone chippings (CaCO j ) which are continually renewed by downward movement from above. The temperature of the fumes is correlated with the vapour content in such a way that some of the water vapour condenses and activates the reaction of the /12659 PC17EP96/03940 2 sulphur dioxide and sulphur trioxide in the fumes with the carbonate, with the formation of calcium sulphate (CaS0 4 ) . As an evolution of this concept, systems have also been proposed which operate at higher fume temperatures with the addition of water which vaporises or with prior ionisation of the fumes by means of electrostatic cells. The document EP-A-0 321 914 describes a similar cross flow filter system which allows very hot fumes, up to 1200°C, to be cleansed of solid particles carried in them by means of a moving bed of basalt, in such a way that the fumes, already purified, traverse heat recovery devices. Filtration systems have also been proposed in which the inert filter material is a component of a product of a process which requires combustion and in which fumes are produced. For example, Italian Patent No. 1 188 668 describes a system for the preparation of a bituminous conglomerate in which the fumes and dust given off by the process are filtered by a moving bed of stone chippings which constitute a component of the conglomerate. As well as removing the dust, the filter process also /12659 PC17EP96/03940 3 enables a large proportion of the heat content of the fumes to be recovered and used in pre-heating the chippings. In practice all these arrangements have had limited industrial application because of a number of adverse factors such as: - large pressure drop in the filtration plant and consequent requirement for energy to drive the gas flow, - a filtering capacity which is not adequate to satisfy requirements imposed by regulations with the consequent need to use other types of filters in combination, - rigid plant structure, dimensioned for specific applications and the impossibility of achieving the benefits of mass production, - non-optimal exploitation of the filtering capacity of the moving bed due to the cross flow, with saturation and exhaustion of the filtering capability of the bed in its direction of movement and the consequent release of solid particles and pollutants in the fumes; - difficulty in the production of structures or shutters capable of retaining the moving bed while at the same time being permeable to the transverse flow of the fumes and such as to ensure a uniform or predetermined distribution without pressure losses. In practice much of the pressure loss along the path of the gas flow is due to these retaining structures. It is also known that the relative movement of the granules of inert material and their movement relative to the retaining structure at the output interface of the flow from the filter bed give rise, by friction, to the formation and release of dust which is entrained by the fume flow whereby its purification is inevitably imperfect. SUMMARY OF THE INVENTION These disadvantages are eliminated by the filtration system with a moving bed of stone chippings which is the subject of the present invention, in which the flow of fumes traverses the moving aggregate bed in counter¬ current so that the fumes leave the filter bed where this is most "clean", with an increase in the filtering action. Moreover, the fumes enter and leave the moving bed through a natural sloping interface where there is no relative sliding between the inert material and the retaining grill but only the formation of a natural slope with reduced mechanical forces between the granules of the inert material so that the formation of dust is minimised. According to a further aspect of the present invention According to a further aspect of the present invention the formation of dust due to the movement of the bed of aggregate is completely eliminated in a modular structure with several filter elements, with beds which move intermitantly in alternate phases in tandem in the sense that, whilst one or more modules are in active operation, their filter beds being steady in order to filter the fumes, another module, excluded from the path of the fumes, is subjected to complete or partial renewal of its moving bed. According to a further aspect of the present invention the system is constituted by a plurality of filter modules so as to provide a plant whose potential can vary within wide limits by the combination of different numbers of modules while, at the same time, ensuring a uniform flow distribution between the modules as well as a uniformly distributed, or at least controlled, rate of flow of the aggregate materials between the various modules. The modular structure, as well as allowing economy of scale, also allows the production of small components which can be transported by road and minimises the operations needed to set up the plant at the work site. According to a further aspect, the present invention provides a filtration system with modules operating in 12659 PCI7EP96/03940 6 reverse flow. According to a further aspect of the present invention, in a fume-filtration system having a continuous cycle of stone chippings, a pneumatic system for recycling the chippings is provided which employs a proportion of the fumes to be filtered as the carrier fluid. BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics and advantages of the invention will become clearer from the following description of a preferred embodiment of the invention and of its variants, made with reference to the appended drawings, in which: - Figure 1 is a partially exploded perspective view of a first preferred embodiment of the moving-bed fume-filtration system according to the present invention; - Figure 2 is a sectional view, taken on the line I- I of Figure 1, showing a filtration module of the system of Figure l; - Figure 3 is a sectional view of a first variant of the filtration module; - Figure 4 shows in section a second variant of the filtration module; Figure 5 is a perspective view of a second preferred embodiment of the filtration system of the present invention; and - Figure 6 is a perspective view of an assembly of a third preferred embodiment of a fume-filtration system according to the present invention with pneumatic recycling of the stone chippings. DESCRIPTION OF A PREFERRED EMBODIMENT AND VARIANTS With reference to Figure 1, a filtration system formed in accordance with the present invention includes a plurality of filter modules 1, 2, 3, 4, 5. The modules, in the form of vertical containers of square cross-section, each terminating at its lower end in a discharge hopper which is dihedral in Figure 1 but may be pyramidal or conical, are juxtaposed in a direction of alignment perpendicular to the front and rear walls of the mutually juxtaposed modules. The upper regions of the modules are traversed, in the direction of alignment, by a feed-screw conveyor 6, shown in broken outline, housed in a suitable housing open towards the interiors of the modules. The screw conveyor is rotated by drive members 7 and its inlet 8 is connected to the outlet 9 of a rotary drum screen 16 housed in a container 10 preferably, but not necessarily, located over the modules 1, 2, 3, 4, 5. In this way much of the heat evolved by the modules is recovered in pre-heating the screened chippings. The screen is also rotated by drive members, not illustrated, which may also include the motor 7. The screw conveyor is supplied by the screen (not necessarily a rotary drum) with screened chippings, free from fine dust particles which are, instead, collected in the container 10 and discharged under gravity to a collection vessel, not visible. The chippings may be obtained from any type of rock available as long as it is not friable and may be chosen according to specific requirements of the plant. Preferably the rock is a carbonate, such as limestone or dolomite, because of the property that such rocks exhibit when hot and/or in the presence of vapour of reacting with sulphur dioxide and trioxide which are the most common pollutants in fumes, fixing them in the form of calcium sulphate. The chippings, as is more clearly illustrated in Figure 2, are supplied to the modules 1 to 5 and fill them to a predetermined level set by the position of the screw conveyor itself, the transport action of which, in known way, stops with the occlusion of the outlet opening. At the bottom of each of the discharge hoppers of the various modules 1, 2, 3, 4, 5 is a rotary discharge valve arranged with its axis in the direction of alignment of the modules and rotated by drive members 11 so as to extract a quantity of chippings from the various modules at a controlled rate and to discharge them into a rotary screw conveyor 12 driven by drive members not illustrated. The screw 12 conveys the chippings discharged from the modules to the base of a conventional elevator, of screw or bucket type, which raises the chippings to its top 14 and empties them under gravity into an inlet duct 15 of the screen 16. The continuous cycle for the chippings thus described may include a reservoir, for example immediately upstream of the screen or the screw conveyor 6, with a flow-metering valve, as well as a line for introducing additional chippings which, in time, are exhausted through use. In applications in which the chippings are a component of the productive process, as, for example in the case of bituminous conglomerate, obviously there is no closed cycle but once the chippings have traversed the filter modules and performed their role as the fluid filter bed, they are forwarded to the subsequent processing steps. Figure 2 is a section taken on the line I-I of Figure 1 showing one of the modules 2 and clearly shows the feed screw 6 housed in a suitable seat 17 which is open downwardly towards the interior of the module which is bounded by two parallel vertical sides 18, 19, a lid 20 and two inclined lower walls 21, 22 forming a conveying hopper which opens downwardly to the rotary valve 23. Between the walls 18, 19, and 21, 22 respectively are two inlet openings Pl, P2 for the fumes to be cleansed. Between the walls 18, 19 and the lid 20 are two outlet openings P3 , P4 respectively for the cleansed fumes. The chippings introduced into the filter module by the screw 6 fill the entire module, distributing themselves under gravity and as a result of friction between them so as to form inclined free surfaces 23, 24 extending close to the openings Pl, P2 between the walls 18, 21 and 19, 22 respectively. At the top the chippings define an interface constituted by two free surfaces 25, 26 extending between the seat 17 and the walls 18, 19 respectively. As a result of the rotation of the valve 23 and the introduction of chippings by the screw 6, the entire mass of chippings in the module moves uniformly downwardly under gravity and is continually renewed so that the levels of the free surfaces remain unchanged. The uniformity of the movement, if necessary, can be ensured by suitable deflectors, such as 27, which provide local resistance to the fall of the chippings, whose rate of descent is very low and can be continuous or intermittent. The uncleansed fumes are introduced through the apertures Pl, P2 , pass through the entire column of chippings between the walls 18, 19 in counter-current with the flow of chippings, and leave the module through the apertures P3, P4. Gradually as the fumes traverse the bed of chippings, they deposit solid particles on the surfaces of the chips which, if reactive towards the oxides in the fumes, also fix these gradually as sulphates. Thus, the concentration of dust and oxides gradually decreases from the bottom upwardly, both in the fumes and in the aggregate, and the bed of aggregate can exert its maximum filtering action. It is in fact evident that, to a good approximation, in one unit volume of the fluid bed, the quantity of particles (and oxides) retained can be considered proportional to the concentration of particles in the fumes and to the filtering activity of the bed, which decreases as a function of the quantity of particles already retained. Thus, the upper layers of the filter bed, which are practically clean as they are renewed continually, can filter the fumes effectively even though the particle concentration in the fumes has been reduced to very small values. It can also be seen that the formation of dust due to the movement of the granules relative to each other and to the walls, which is obviously correlated with the contact pressures exerted (which by analogy with hydraulic systems can be considered proportional to the static load) is higher at the foot of the filter column and negligible at the free surfaces 25, 26 where, essentially, it is due solely to the rolling of the granules on the inclined surfaces, caused by the renewal of the filter mass, with contact pressures due solely to the weight of the rolling granules. The quantity of dust transported by fumes leaving the module is therefore very small and can be further reduced by dampening the chippings immediately before their introduction into the modules or even by dampening the free surfaces 25, 26 by sprinkling. In particular operating conditions this dampening can also be advantageous to increase the capture of noxious, gaseous pollutants such as oxides and acids. The dampening can also result from the condensation of vapour in the fumes as a result of their cooling by release of heat to the filter bed if the working temperature of the filters is relatively low. A further important consideration is that, to penetrate the bed of chippings, the fume flow does not have to traverse retaining structures and deflector shuttering and is subject only to the pressure drop imposed by the reduction in section and the tortuousness of the labyrinth formed by the granular bed. By way of example, the module of Figure 2 can have a depth perpendicular to the plane of the drawing, of the order of 4 m, a width of the column of granules of the order of 1.5 m, a height of the column of granules traversed by the fumes of the order of 1.5 and free inlet and outlet passage sections of the order of 3.2 2 (divided into two pairs of inlet and outlet openings) . For fume flow rates of the order of 3 m /s and hence velocities of the order of 0.5 m/s, the pressure drop is of the order of 15 mbar (150 mm of water column) and is exclusively due to the traversal of the bed. The filtering efficiency is extremely high, of the order of 99.9% and, in absolute terms, the solid particulate residue in the fumes is less than 20 mg per m 3 of fumes. The particulate residue is due essentially to the transport of inert dusts generated by the granules at the output interfaces of the filter bed whilst the unburnt pollutants in the fumes are retained upstream. Several modules operating in parallel, as illustrated in Figure 1, make it possible, according to the number of modules installed, to provide higher potential, for example up to 15-20 m 3 per second or even more, by providing a greater number modules or more batteries of modules operating in parallel. Figure 1 shows, by way of example, five identical modules aligned in a battery. The flow of fumes is distributed between the modules by a first distributor duct 27 of variable cross-section with an inlet opening 28 and a plurality of outlets connected to the inlet openings 29, 30, 31, 32, 53 on one side of the various modules. A similar distributor 49, obviously connected to the first, is provided on the opposite side of the battery of modules. A fume manifold 50 of variable cross-section is connected to the openings of each of the modules on one side thereof and conveys the filtered fumes towards a suction system represented as a centrifugal fan 51. A similar manifold 52 connected to the first is provided on the opposite side. Figure 3 shows in section, again on the line I-I of Figure 1, a variant of the filter module which achieves even more thorough filtration of the fumes by completely eliminating the transport in the fumes of dust formed at the output interfaces of the filter bed due to the movement of the granules constituting the bed. This variant can be used to advantage in modules operating at high temperatures in which the bed cannot be moistened. The module of Figure 3 differs from the module of Figure 2 in that it includes two pivoted shutters 33, 34 for closing the outlet openings of the module, controlled independently for each module, the shutter 33 being represented in its open position and the shutter 34 in its closed position. These shutters, which in any event can be provided to regulate and distribute the flows in the various modules precisely, to advantage have the further function of allowing the fume flow to be excluded from one of the modules. The rotary discharge valve of each of the modules (which, as illustrated in Figure 3, can be replaced by a transverse shutter 35) is also controlled by an actuator 36 independently of the others but coordinated with the closure of the fume shutters. In this case, rather than being renewed continuously, the filter beds can be renewed intermittently and cyclically as each of the modules is temporarily excluded from the flow of fumes. Thus, whilst at least one module performs its filtering action with the bed closed, and hence without the formation of dust, the bed of another module, from which the flow is excluded, can be entirely or partly, and therefore gradually renewed, giving time for the dust to settle before the fume flow is reintroduced into the module. Subsequently the same operation can be performed on one of the other modules and so on, cyclically. Clearly, to achieve this manner of operation it is necessary to have at least two modules operating in tandem . Even more advantageously, by exploiting the low pressure developed by suction systems located downstream of the filter modules, the module excluded from the fume flow can be subjected to a reverse flow in order to transport the dust generated by the movement of the aggregate towards the lower part of the bed. To this end the pivoted shutters 33, 34 can be shaped in such a way as to perform a dual action of closing the fume outlet openings and opening a passage 37 communicating with the external environment, the said opening 37 normally being closed. As there must necessarily be a furnace or a boiler upstream of the filter system, where the fumes are generated and in which there is necessarily a pressure drop, the fume pressure in the distributors, such as 27, 29 of Figure 1, will be below ambient. Therefore, when a module is excluded from the fume flow, a flow of air develops from the opening 37 to the distribution ducts 27, 29 which traverses the bed of chippings in the reverse direction and removes the dust formed at the interfaces 25, 26. Any dust formed at the interfaces 23, 24 is filtered by the other modules. Clearly the same result can also be achieved in filter modules which operate at high rather than low pressure by the provision of means for blowing air into the filter chambers. With this arrangement the filter system is particularly suitable for high-temperature operation with an appropriate choice of material for the bed such as a mixture of, for example, basalt and a small proportion of additives, such as rock carbonates for de-sulphurising, and can be located upstream of the heat-exchangers to ensure their perfect efficiency by preventing the formation of dust deposits. Figures from 1 to 3 show a preferred embodiment and a variant thereof, but it is clear that other variations may be introduced. For example, Figure 4 shows, in section, a module 40 similar to those described but having an asymmetric section with a single inlet opening 38 on one side and a single outlet opening 39. The module 40, particularly suitable for low potential uses, can be provided (like the other modules already described) with a plurality of accessories, such as a modulation/exclusion shutter 41 and, for specific applications, with sprinkler systems 44 or systems with adsorbent substances for treating the filter bed to promote the removal of the oxides of sulphur and other pollutants, such as electrostatic generators 42 immediately upstream of the filter bed for ionising the fumes (and hence the particles contained therein) and increasing the filter power of the bed in known manner (by electrostatic adhesion) , the bed moving continuously or intermittently, and also devices 43 for blowing steam into the hot fumes upstream of the filter column. Figure 5 is a perspective view of a second preferred embodiment of the filtration system of the present invention which differs from that shown in Figure 1 in that the rotary screen 101, housed in a sealed box 102, is disposed immediately downstream of the discharge screw conveyor 12 of the filter modules. In this way the screening takes place when the chippings are at their highest temperature, before cooling during transport and storage. This avoids any condensation which, in the case of particularly humid fumes, would cause the dust to adhere to the chippings, rendering the screening ineffective. The dust is deposited on the bottom of the container 102 from which it is removed by known means, for example, a screw conveyor and/or rotary valve. The screened chippings are discharged to the bottom of an elevator 103 which raises them and discharges them into a storage reservoir 104 with a discharge hopper which opens to the feed screw 105 of the filter modules. An oscillating valve 106 (or even a rotary valve) is suitably arranged immediately upstream of the reservoir 104 (or even downstream of it) to prevent unwanted air being sucked by the elevator into the filter modules. The elevator can also function as a washing tower for the chippings. Figure 6 is a perspective view of a third embodiment of the invention which differs from the preceding embodiments in that the chippings are recycled to the moving-bed filter modules by pneumatic means, with the use of the fumes themselves which are to be filtered as the transport medium. A high-power centrifugal fan 60 draws a relatively small proportion, of the order of 10%, of the fumes to be filtered from a duct 61 which delivers the fumes to a battery 62 of filter modules. This proportion of the fumes is conveyed through a duct 63 to a convergent duct portion 64 downstream of which a high-speed, low-pressure flow is established in a duct 65 into which the conveyor which receives the chippings from the modules discharges the chippings to be cleansed of dust and recycled. The speed of the fumes in the duct 65 is sufficiently high, of the order of 40-80 m/s, to entrain the chippings effectively and raise them through a column 66 which opens through an elbow 67 into a syphon chamber 68, of greater diameter, where the speed of the fumes falls significantly so that the chippings, no longer supported by the flow of fumes, fall under gravity into a storage reservoir 69, through an oscillating or rotary valve 70, whilst the finer dust and solid particles are carried away by the fumes. For safety, a metal mesh 71 may be located close to the head of the syphon for capturing any chippings larger than a predetermined limit should they reach the mesh due to inertia. As a result of their entrainment by the flow of fumes and their many impacts with the walls of the ducts 56, 66, 67, the film of powder formed on the surfaces of the chippings becomes separated and the chippings collected are perfectly clean and are separated from the dust even better than by means of a screening operation. Downstream of the head of the syphon, the diameter of the duct is gradually reduced so that the speed of the flow of fumes, rich in dust, increases. The fumes, thus accelerated, enter a cyclone 72 of conventional type where a good proportion of the solid particles is captured and removed from the cyclone by known means, such as rotary valves and the like. The fumes, cleansed to the limits allowed by the efficiency of the cyclone, are drawn through a duct 73 by the fan 60 and from here returned to the duct 61 for delivery to the battery 62 of filter modules where, mixed with that proportion of the fumes not used for the pneumatic transport of the chippings, they are subject to much more thorough dust removal by the moving bed filter modules. With this arrangement, all mechanical devices for conveying and raising the chippings are eliminated, and the plant is therefore very reliable. Moreover, the pneumatic recycling system operates with a fluid and transported chippings which are substantially at the same temperature so that heat is not extracted from the chippings and they are not subject to thermal stress which is a possible cause of fragmentation. Since the proportion of fumes used for recycling the chippings is modest and the fume velocity is extremely high, small-section ducts are employed which limits the heat-dissipation and which, in any case, can easily be insulated. It should also be noted that the circuit for the recycling of the chippings operates at low pressure compared with the pressure in the duct 61 which, in turn, as already mentioned, is preferably below ambient pressure. Consequently, should there be any imperfect sealing of the joints between the various sections of the circuit, no fumes are emitted into the environment.

Claims: CLAIMS 1. A fume-filtration system constituted by at least one filter module comprising: - a vertical container terminating at its lower end in a discharge hopper (21, 22) and having a lower inlet opening (Pl, P2) for the fumes and an upper outlet opening (P3, P4) for the fumes, - first means (6) for filling the container with stone chippings, controlled to feed the container with chippings and to form a vertical column of chippings with a first free surface (23, 24) below the inlet opening (Pl, P2) and below the column and a second free surface (23, 26) of the chippings at the top of the column, - second means (12, 23) for the removal of chippings from the container at the bottom of the hopper, controlled in a manner coordinated with the first filling means (6) for renewing the chippings in the column so that they move downwardly from the top, whereby fumes introduced into the module through the inlet opening (Pl, P2) traverse the column of chippings in counter-current to the movement of the column of chippings and exit through the outlet opening (P3, P4) . 2. A system as in Claim 1, including sprinkler means for moistening the first free surface. 3. A system as in Claim 1, including an electrostatic ionisation cell for ionising the fumes upstream of the first free surface. 4. A system as in Claim 1, including means (43) for blowing water vapour into the module upstream of the first free surface (23, 24) . 5. A fume-filtration system comprising a plurality of filter modules (1, 2, 3, 4, 5) as in Claim 1, connected in parallel and aligned in a battery, in which the first filling means comprise a feed screw (6) common to the plurality of modules and extending in the direction of alignment of the modules. 6. A system as in Claim 5, in which the second removal means (12, 23) comprise a rotary valve (23) common to the plurality of modules and having an axis extending in the direction of alignment of the modules. 7. A system as in Claim 5, in which the second removal means (12, 13) comprise independent removal means (23, 35) for each module, these being controlled independently. 8. A system as in Claim 7, in which the filter modules each include closure means (33, 34) for closing the outlet opening (P3, P4) controlled independently in each module for the selective exclusion of a module from the set of modules in parallel. 9. A system as in Claim 7, including means (37) for introducing a reverse air flow into one of the modules excluded from the set of modules in parallel. 10. A system as in Claim 1 or Claim 5, including recycling means (13, 14, 15, 16, 9, 8) for recycling the chippings removed by the second means (12, 13) to the first means (6) , the recycling means including a screen (16) for screening the recycled chippings located above the module or plurality of modules (1, 2, 3, 4, 5). 11. A system as in Claim 1 or Claim 5, including recycling means (13, 14, 15, 16, 9, 8) for recycling the chippings removed by the second means (12, 13) to the first means (6) , the recycling means including a screen (101) for screening the recycled chippings located immediately downstream of the second removal means (12, 13) . 12. A system as in Claim 1 or Claim 5, including pneumatic recycling means (60, 64 65, 66, 67, 68, 69, 70) for recycling the chippings removed by the second means (12, 13) to the first means (6) , the pneumatic recycling means utilising, as a carrier fluid, a proportion of the fumes taken upstream of the at least one filter module and reintroduced into the fumes upstream of the at least one filter module after having been cleansed of dust by dust-removal means (72) .

Inventors: MAGRINI GIORGIO, MAGRINI, GIORGIO
Applicants: GIORGETTI & MAGRINI S R L, MAGRINI GIORGIO, GIORGETTI & MAGRINI, S.R.L., MAGRINI, GIORGIO
Application year: 1996
Main classification: B01D46
IPC codes: B01D46, B01D53