ITMI952009A1 - FUME FILTRATION SYSTEM WITH MOVABLE BED OF PIETRISCO - Google Patents

Abstract

Fume filtration system comprising a plurality of filter modules (1, 2, 3, 4, 5) connected in parallel and aligned in a battery in which a flux of fumes introduced into the modules passes through a crushed stone filter column contained in the modules and forming a first free surface (23, 24) for the intake of fumes and a second free surface (25, 26) for the emission of fumes, the column being renewed in counter-current with respect to the flow of fumes with continuous renewal or in the intermittent case being performed on a module temporarily excluded from the parallel.

Description

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a fume filtration system with a mobile bed of crushed stone acting both as a deduster and as a neutralizer of the sulfur anhydrides contained in the fumes.

STATE OF THE ART

Filtration systems have been known for some time which make use of mobile beds of inert granular material, replacing cyclones or fabric filters whose efficacy and useful life are respectively limited and also, for thermal power plants, replacing expensive and bulky smoke washing towers.

For example, the patent US-A-3,976,747 describes a system in which the relatively hot fumes at temperatures of the order of 90 ° C cross-flow a perforated cavity filled with limestone crushed stone (CaCO3) which is continuously renewed with movement from above. downward.

The temperature of the fumes is correlated to the vapor content so as to have partial condensation of water which activates reaction processes of the sulfur dioxide and sulfuric dioxide of the fumes with the carbonate, with the formation of calcium sulphate CaSO4.

As an evolution of this concept, systems have also been proposed that operate at higher temperatures than the fumes with the addition of water that vaporises or with preliminary ionization of the fumes by means of electrostatic cells.

Document EP-A-0321914 describes a similar cross-flow filtering system that allows to purify very hot fumes, up to 1200 ° C from the transported solid particles, by means of a mobile basalt bed, so that the fumes pass through recovery devices. heat already purified.

Filtration systems have also been proposed in which the filtering inert is a component of a process product requiring combustion and development of fumes.

For example, Italian Patent No. 1,188,668 describes a bituminous conglomerate preparation system in which the effluent and dusty fumes of the process are filtered by a mobile bed of crushed stone which constitutes a component of the conglomerate.

The filtering process ensures, in addition to the abatement of dust, also, and to a large extent, the recovery of the thermal content of the fumes and the preheating of the crushed stone.

In practice, all these construction solutions have led to limited industrial applications due to a plurality of negative factors such as:

- high pressure drops of the filtration system and consequent energy expenditure to ensure the flow of fumes.

inadequate filtering capacity to meet regulatory requirements and consequent need for use in combination with other types of filters,

- rigid plant structures, with specific sizing on a case-by-case basis and impossibility of achieving the benefits of mass production.

non-optimal exploitation of the filtering capacity of the mobile bed, due to the cross flow, with saturation and exhaustion of the filtering power of the bed in its direction of movement and consequent release of solid particulate and pollutants in the fumes;

- difficulty in making structures or shutters for containing the movable bed that are at the same time permeable to the transverse flow of fumes and such as to ensure uniform or predetermined distribution without pressure losses.

In practice, most of the head losses along the flue gas path are caused by these containment structures.

It has also been noted that the relative movement between inert granules and relative to the containment structures at the outlet interface of the flue gas flow from the filtering bed, gives rise by friction to the development and release of powders which are entrained by the flue flow whose purification it is inevitably imperfect.

SUMMARY OF THE INVENTION

These drawbacks are eliminated by the filtration system with mobile bed of crushed stone object of the present invention, in which the flow of the fumes crosses the mobile and inert bed in countercurrent, so that the fumes leave the filter bed where it is more "clean" with a increased filtering action.

Furthermore, the fumes enter and leave the mobile bed through a natural slope interface where, since there is no relative sliding between inert and containment grids, but only the formation of a natural slope with reduced mechanical stresses between the aggregates of the aggregate, the development of dust is minimal.

According to a further aspect of the present invention, the development of powders due to the movement of the inert bed is completely eliminated with a modular structure of several filtering elements, with intermittent and alternating phase moving bed, operating in tandem, in the sense that while one or more modules are operationally active, with the filter bed stopped, to filter the fumes, another module, excluded from the path of the fumes, is subjected to a complete or partial renewal operation of the mobile bed.

According to a further aspect of the present invention, the system consists of a plurality of filtering modules to obtain variable plant potentials within wide limits, with the combination of a variable number of modules and at the same time to ensure a uniform distribution of flow between modules and a uniformly distributed and in any case controlled flow rate of the aggregates between the different modules.

The modular structure allows, in addition to economies of scale, the creation of components with reduced dimensions, susceptible to road transport and the reduction to a minimum of the set-up operations of the plant at the place of operation.

According to a further aspect of the present invention, a filtration system with modules operating in tandem with initial self-cleaning of the reverse flow modules is provided.

According to a further aspect of the present invention, in the case of a continuous-cycle filtration system for crushed stone, a pneumatic system for recycling the crushed stone is provided which uses, as a carrier fluid, a fraction of the fumes to be filtered.

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 its variants, made with reference to the attached drawings in which:

Figure 1 is a partially exploded perspective view of a first preferred embodiment of the mobile bed fume filtration system in accordance with the present invention;

Figure 2 is a sectional view, according to the view I-I of Figure 1, of a filtration module of the system of Figure 1;

Figure 3 is a sectional view of a first variation of the filtration module embodiment;

Figure 4 is a sectional view of a second variant embodiment of the filtration module;

Figure 5 is a perspective view of a second preferred embodiment of a filtration system in accordance with the present invention.

Figure 6 is an overall perspective view of a third preferred embodiment of a fume filtration system in accordance with the present invention, with pneumatic recirculation of the crushed stone.

DESCRIPTION OF A PREFERRED REALIZATION AND OF VARIATIONS

With reference to Figure 1, a filtration system made in accordance with the present invention comprises a plurality of filter modules 1,2,3,4,5.

The modules, in the form of vertical containers with quadrangular section, terminated at the bottom in a dihedral discharge hopper in figure 1 but also pyramidal or conical, are juxtaposed to each other according to an alignment direction perpendicular to which the front walls extend and posterior of the mutually juxtaposed modules.

The modules are crossed at the top in the direction of alignment by a feed screw 6 indicated in broken lines and housed in a suitable seat open towards the inside of the module.

The auger is rotated by motor members 7 and connected at its inlet 8 with the discharge 9 of a rotating drum screen 16 housed in a container 10 preferably but not necessarily arranged above the modules 1,2,3,4, 5

In this way the heat dispersed by the modules is recovered to a good extent with a preheating action of the sieved crushed stone.

The screen is also rotated by motor members not shown which may also consist of motor 7.

The auger is fed by the sieve (not necessarily with a rotating drum) with sieved crushed stone without fine particulate matter which is instead collected in the container 10 and discharged by gravity into an invisible collection tank.

The crushed stone can be obtained from any available rock as long as it is not friable, which can be chosen according to specific plant needs.

Preferably the rock is of the carbonate type, such as limestone and dolomite, due to the marked properties that these rocks have when hot and / or in the presence of steam to react with sulfur dioxide and sulfur dioxide which are the most common pollutants of the fumes, fixing them in shape of calcium sulphate.

The crushed stone, as more clearly illustrated in figure 2, is unloaded in modules 1 to 5 and fills them up to a predetermined level imposed by the position of the screw itself, whose transport action is exhausted, in a known way, with the occlusion of the exhaust port.

At the lower end of the unloading hopper of the different modules 1,2,3,4,5, an unloading rotovalve is arranged axially oriented in the direction of alignment of the modules and rotated by motor members 11, so as to extract from the different modules a controlled quantity and flow rate of crushed stone which is discharged into a rotating auger 12, driven by motor members not shown.

The auger 12 conveys the crushed stone discharged from the modules at the base of a conventional bucket or screw type elevator device 13, which discharges the crushed stone raised up to its top 14 by gravity into an inlet duct 15 of the screen 16.

In the continuous cycle of the crushed stone thus described, a buffer tank can be provided, for example immediately upstream of the screen or of the auger 6 with a metering and flow valve as well as a line of integration of the crushed stone which in the long term is subject to exhaustion due to wear.

In applications where the crushed stone constitutes a component of the production process, as for example in the case of bituminous conglomerate, obviously a closed cycle is not created, but the crushed stone, once the filtration modules have been passed through and played the role of a filtering fluid bed, it is initiated towards subsequent stages of the process.

Figure 2 is a section according to the view I-I of Figure 1 of one of the modules (2) and clearly shows the feed screw 6 housed in a suitable seat 17 open at the bottom towards the inside of the module, delimited by two parallel vertical sides 18, 19, by a lid 20 and by two lower oblique walls 21,22 forming a conveying hopper open at the bottom on the rotary valve 23.

Between the walls 18,19 and 21,22 respectively there are two inlet ports P1, P2 for the fumes to be filtered.

Between the walls 18,19 and the cover 20 respectively two outlet ports P3, P4 are open for the purified fumes.

The crushed stone introduced into the module, filtering from the auger 6 fills the whole module, distributing itself by gravity and friction between the granules with free inclined surfaces 23.24 extended near the ports P1, P2 between the walls 18.21 and 19.22 respectively. Above the particulate matter defines an interface formed by two free surfaces 25,26 extended between seat 17 and walls 18,19 respectively.

The whole mass of crushed stone contained in the module, due to the rotation of the valve 23 and the addition of crushed stone to the screw conveyor 6, moves uniformly from top to bottom by gravity and is continuously renewed, keeping the level of the free surfaces unchanged.

The uniformity of movement, if necessary, can be ensured by suitable deflector tiles such as 27 which locally introduce a resistance to the falling of the crushed stone, whose descent speed is very low and can be continuous or even intermittent.

The non-purified fumes are introduced through the openings P1, P2, pass through the entire column of crushed stone contained between the walls 18,19 in countercurrent relative to the flow of the crushed stone and leave the module through the openings P3.P4.

As the fumes pass through the bed of crushed stone they deposit the solid particulate on the surface of the granulate which, if of an active type with regard to the anhydrides contained in the fumes, also provides for a gradual fixation of these as sulphates.

Thus the concentration of dust and anhydrides gradually decreases from the bottom upwards both in the fumes and in the aggregates and the inert bed can perform its maximum filtering action.

It is in fact evident that with a good approximation, in a unit of volume of the fluidized bed, the quantity of particulate (and anhydrides) retained can be considered proportional to the concentration of particulate matter in the fumes and to the filtering activity of the bed, which decreases according to the particulate already retained.

Thus the upper layers of the filter bed, practically clean because they are continuously renewed, can effectively exert their filtering activity on the fumes even if the concentration of the particulate in the fumes has dropped to minimum values.

It can also be observed that the development of dust due to the relative movement of the granules between them and with the walls, obviously correlated to the contact pressures exerted (which by analogy with hydraulic systems can be considered proportional to the static load) is higher at the base. of the filtering column and negligible at the free surfaces 25,26 where it is essentially due to the sole rolling mechanism of the granules on the slope, caused by the renewal of the filtering mass with contact pressure due only to the weight of the rolling granules.

The quantity of dust carried by the fumes exiting the module is therefore minimal and can be further reduced by providing for humidification of the crushed stone immediately before entering the modules or even rain humidification of the free surfaces 25,26.

In particular operating conditions, this humidification can also be advantageous for enhancing the processes of capturing harmful gaseous pollutants, such as anhydrides and acids.

Humidification can also be the result of a condensation of the vapor contained in the fumes, consequent to their cooling by transferring heat to the filter bed, if the working temperature of the filters is relatively low.

A further important consideration is that the flow of fumes, in order to penetrate the bed of crushed stone, must not pass through containment structures and deflecting shutters and is subject only to the pressure drops imposed by the reduction in section and by the tortuosity of the labyrinth formed by the bed of granulate. .

By way of example, the module of figure 2 can have a depth, perpendicular to the plane of the figure, of the order of 4 m, width of the granulate column of the order of 1.5 m, height of the granulate column crossed by the fumes of the 'order of 1.5 m and free sections of passage in entry and exit of the order of 3.2 m <2> (divided into two pairs of entry and exit ports).

For a flow rate of the fumes of the order of 3 m / s and therefore a speed of the order of 0.5 m / s, the pressure drops are of the order of 15 mbar (150 mm of water column) and are exclusively due to crossing the bed.

The filtering efficiency is extremely high in the order of 99.9% and in absolute terms the residual solid particulate of the fumes is less than 20 mg per m of smoke.

The residual particulate is essentially due to the transport of inert dust generated by the granulate to the outlet interfaces from the filter bed while the unburnt pollutants of the fumes are retained upstream.

More modules operating in parallel, as illustrated in figure 1, allow to obtain, depending on the number of installed modules, higher potentialities, for example up to 15-20 m <3> per second and even more, providing for a greater number of modules or more batteries of modules operating in parallel.

Figure 1 shows for example 5 identical modules, aligned in a battery.

The distribution of the flue gas flow between the modules is carried out by a first distributor duct 27 with variable section with inlet 28 and a plurality of outlets which are connected to the inlet ports 29,30,31,32,53 on one side of the different modules.

A similar distributor 49, obviously connected to the first, is provided on the opposite side of the battery of modules.

A variable section flue gas collector 50 is coupled to the light of each of the modules on one side of these and conveys the filtered flue gases to 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 represents in section, again according to view I-I of Figure 1, an embodiment variant of a filtering module that allows an even more thorough filtration of the fumes, completely eliminating the transport of the fumes of dust developed at the outlet interface of the filtering bed from the movement of the constituent granules of the bed.

The embodiment variant can be advantageously used in modules operating at high temperatures in which the humidification of the bed is not feasible.

The module of figure 3 differs from the module of figure 2 in that it includes two rotating shutters 33,34 for closing the outflow ports of the module, independently controlled for each module, the shutter 33 being shown in the open position and the shutter 34 in the closed position .

These dampers, which can in any case be provided for precisely regulating and distributing the flow of fumes in the various modules, advantageously have the further function of allowing the exclusion of one of the modules from the flow of fumes.

Also the discharge rotovalve of each of the modules (which can be replaced as illustrated in Figure 3 by a transversal shutter 35) is controlled by an actuator 36 independently from the others and coordinated with the closing of the smoke dampers.

In this case, the renewal of the filter bed can be performed intermittently and distributed cyclically on each of the modules that are temporarily excluded from the flue gas flow instead of continuously.

Thus, while at least one module carries out its filtering action, with a stationary bed, therefore without the development of powders, the bed of another module, excluded from the flow, can be renewed in whole or in part, therefore gradually, giving time to the powders. develop to settle before reinserting the module into the flue gas flow.

Subsequently the same operation can be performed in one of the other modules and so on, cyclically.

Clearly, to achieve this operating mode, the installation of at least two modules operating in tandem is required.

More advantageously, by exploiting the vacuum developed by the suction systems arranged downstream of the filter modules, the module excluded from the flue gas flow can be subjected to a reverse flow which has the function of transporting the dust developed by the movement of the aggregate, towards the lower part of the bed.

For this purpose, the rotating shutters 33.34 can be shaped in such a way as to perform a double action of closing the flue outlet port and opening a port 37 for communication with the external environment, said port 37 being normally closed.

Since upstream of the filtering system there must necessarily be a furnace or a boiler, where the generation of fumes takes place, in which there is necessarily a pressure drop, the fumes pressure in the distributors as 27.29 of figure 1 will be lower than that environment.

Therefore, when a module is excluded from the flue gas flow, a flow of air develops from the port 37 to the distribution ducts 27,29 which crosses the bed of crushed stone in the opposite direction and eliminates the dust developed at the interface 25,26.

Any dust developed at interface 23,24 is filtered by the other modules.

It is clear that the same result can also be obtained in filtering modules operating under pressure rather than under vacuum by providing means for blowing air into the filtering chambers.

With these measures, the filtering system is particularly suitable for operating at high temperatures, with a convenient choice of the bed material as a mixture of basalt and a small fraction of additives, such as carbonate rocks, with a desulphurizing function, and can be arranged upstream. of heat exchangers of which it ensures perfect efficiency, preventing the formation of dusty deposits.

Figures 1 to 3 represent a preferred embodiment and a variant thereof, but it is clear that other variants can be made.

For example, Figure 4 is a sectional view of a module 40 similar to those described but having an asymmetrical section, with a single inlet port 38 on one side and a single outlet port 39.

The module 40, particularly suitable for low capacities, can be provided (like the modules already described) with a plurality of accessories, such as a modulation / exclusion damper 41 and, for specific applications, rain humidification or water treatment systems 44. filter bed with adsorbing substances to favor the abatement processes of sulfur anhydrides and other pollutants. as well as electrostatic generators 42, immediately upstream of the filtering bed, to favor the ionization of the fumes (therefore of the particulate matter in these contents) and enhance in a known way (by electrostatic adhesion) the filtering power of the bed, which is mobile with continuous or intermittent motion, as well as devices for blowing steam 43 into the hot fumes upstream of the filtering column.

Figure 5 is a perspective view of a second preferred embodiment of a filtration system in accordance with the present invention which differs from that shown in Figure 1 in that the rotating screen 101, housed in a watertight box 102, is arranged immediately downstream of the discharge screw 12 of the filter modules.

In this way the sieving takes place with the crushed stone at the maximum temperature, before it cools down during transport and stowage operations. In this way, possible condensation phenomena are avoided which, in the case of particularly humid fumes, could cause the dust to adhere to the crushed stone, making the sieving operation ineffective.

The powders are deposited on the bottom of the container 102, from where they are extracted by known means, for example a transport screw and / or rotary valve.

The screened crushed stone is unloaded at the base of an elevator 103 which raises it, unloading it into an accumulation tank 104 with an open discharge hopper on the feeding screw 105 of the filtering modules.

A pendulum valve 106 (or also a rotary valve) is suitably arranged immediately upstream of the tank 104 (or even downstream of this) to prevent the aspiration of false air from the elevator into the filtration modules.

The elevator device can also act as a crushed stone washing tower.

Figure 6 is a perspective view of a third embodiment which differs from the previous ones in that the recycling of the crushed stone in the moving bed filtration modules is obtained pneumatically using, as a transport vector, the same fumes that must be filtered. .

A high-head centrifugal fan 60 draws a relatively modest fraction, of the order of 10%, of the fumes to be filtered from a flue gas delivery duct 61 to a battery 62 of filtering modules.

This fraction of fumes is conveyed, through a duct 63, to a convergent 64, downstream of which a high velocity and depression flow is established, conveyed in a duct 65 into which the crushed stone discharge screw from the modules pours the crushed stone to be dedusted and recycled.

The speed of the fumes in the duct 65 is sufficiently high, of the order of 40-80 m / s, to exert an effective transport and lifting action of the crushed stone in a column 66 which, through an elbow 67, opens into a siphon chamber reverse 68 of greater diameter, where the speed of the fumes is considerably reduced, so that the crushed stone, no longer supported by the. flow of the fumes, falls, by gravity, into an accumulation tank 69, through a pendular valve or rotary valve 70, while the dust and the finer solid particulate are transported by the fumes.

For safety, a metal mesh 71 can be arranged near the head of the siphon to capture the crushed stone, with dimensions greater than a predetermined limit, which should, by inertia, reach the mesh.

As a result of the transport action exerted by the flow of fumes and the multiple impacts with the wall of the ducts56,66,67, the powdery veil formed on the surface of the crushed stone detaches and the collected crushed stone is perfectly clean and separated from the dust even better than for by means of a sieving operation.

Downstream from the head of the siphon, the diameter of the duct is gradually reduced, so that the flow of fumes, rich in dust, increases its speed.

The fumes, thus accelerated, enter a conventional cyclone 72, where a large part of the solid particulate is captured and extracted from the cyclone by known means, such as rotary valves and the like.

The purified itimi, within the limits allowed by the efficiency of the cyclone, are sucked, through a duct 73, by the fan 60 and from this conveyed back into the duct 61 for the delivery of the fumes to the battery 62 of filter modules where, mixed with the fraction of the fumes not used for the pneumatic transport of the crushed stone, they undergo the much higher dedusting ensured by the mobile bed filter modules.

With this solution any mechanical device for transporting and lifting the crushed stone is eliminated and a high reliability of the plant is achieved.

Furthermore, the pneumatic recycling system operates with carrier fluid and transported crushed stone which are substantially at the same temperature, so that its thermal content is not removed from the crushed stone and thermal stresses are not imposed on it, causing possible fragmentation.

Since the fraction of the fumes used for the recycling of the crushed stone is modest and their speed is extremely high, small section ducts are used, which involve a modest heat dispersion and which in any case can be easily insulated.

It should also be noted that the crushed stone recirculation circuit operates in depression relative to the pressure existing in the duct 61 which in turn is, as already mentioned, preferably in depression with respect to the environment. Consequently, in the event of imperfect sealing of the joints between the different sections of the circuit, there is no emission of fumes into the environment.

Claims (12)

REVISED 1. Fume filtration system consisting of at least one filtering module comprising: a vertical container., terminated at the bottom in an unloading hopper (21,22) and equipped with a lower port (P1, P2) for the inlet fumes and an upper port (P3, P4) for the outflow of fumes, - first filling means (6) of said container with crushed stone controlled to feed said container with crushed stone and form a vertical column of crushed stone with a first free surface (23,24) below said inlet port (PI, P2) and said column and a second free surface (25,26) of crushed stone above said column, - second extraction means (12.23) of crushed stone from said container, at the end of said hopper, controlled in a coordinated manner with said first filling means (6) to renew the crushed stone of said column with a movement from top to bottom , whereby fumes introduced into said module through said lower port (P1, P2) exit from said outflow port (P3, P4) crossing said column of crushed stone in countercurrent with respect to the movement of said column of crushed stone. 2. System as in claim 1, comprising rain humidification means of said first free surface. 3. System as in claims 1 or 2 comprising an electrostatic smoke ionization cell upstream of said first free surface. 4. System as in claims 1 or 2 or 3 comprising means (43) for blowing water vapor into said module upstream of said first free surface (23,24). 5. 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 said first filling means comprise an auger (6) of power supply common to said plurality of modules, extended in the direction of alignment of said modules. 6. System as in claim 5, wherein said second extraction means (12,23) comprise a rotary valve (23) common to said plurality of modules, having an axis extended in the direction of alignment of said modules. 7. System as in claim 5, wherein said second extraction means (12,13) consist of independent extraction means (23,35) for each module, and independently controlled. 8. System as in claim 7, wherein said filter modules each comprise closing means (33,34) of said outflow port (P3, P4) independently controlled in each module, for the selective exclusion of a module from said parallel of modules. System as in claim 7, comprising means (37) for introducing a reverse flow of air into one of said modules excluded from said parallel of modules. 10. System as in claims 1 or 5, comprising means (13,14,15,16,9,8) for recycling said crushed stone extracted from said second means (12,13) to said first means (6), said means comprising a screen (16) of said recycled crushed stone, arranged above said module or plurality of modules (1,2,3,4,5). 11. System as in claims 1 or 5, comprising means (13,14,15,16,9,8) for recycling said crushed stone extracted from said second means (12,13) to said first means (6), said means of recycling comprising a sieve (101) of said recycled crushed stone, disposed immediately downstream of said second extraction means (12,13). 12. System as in claims 1 or 5 comprising pneumatic recycling means (60,64,65,66,67,68,69,70) of said crushed stone, extracted from said second means (12,13), towards said first means (6), said pneumatic recycling means using as carrier fluid a fraction of said fumes, taken upstream of said at least one filtering module and reintroduced into said fumes, upstream of said at least one filtering module, after dedusting with dedusting means ( 72