LU92944B1 - Installation for distribution of granular or powder material via pneumatic transport comprising a device for pressurizing and depressurizing a dispensing hopper for storage of said material - Google Patents

Abstract

An installation (20) for distribution of granular or powder material via pneumatic transport comprising at least two dispensing hoppers (22, 22', 22") for temporary storage of the material, the dispensing hoppers (22, 22', 22") being suited for being, alternately, pressurized for emptying and depressurized to permit filling thereof, and a device for pressurizing and depressurizing the dispensing hoppers (22, 22', 22"). The device comprises a pressurizing duct (24, 24', 24") associated with each of the dispensing hoppers (22, 22', 22") for feeding pressurizing gas into the dispensing hoppers (22, 22', 22"), the pressurizing duct (24, 24', 24") being fluidly connected to a feed line (26) for receiving pressurizing gas from a gas source; and a depressurizing duct (28, 28', 28") associated with each of the dispensing hoppers (22, 22', 22") for evacuating pressurizing gas therefrom, wherein a pressurized bag filter (30, 30', 30") suited to operating under pressure is arranged in the depressurizing duct (28, 28', 28"). The device further comprises a return line (34) fluidly connected between the depressurizing duct (28, 28', 28") and the feed line (26), wherein the return line (34) is connected to the depressurizing duct (28, 28', 28") at a location downstream of the pressurized bag filter (30, 30', 30") and to the feed line (26) at a location upstream of the pressurizing duct (24, 24', 24"). (Fig. 2) 92944

Description

Installation for distribution of granular or powder material via pneumatic transport comprising a device for pressurizing and depressurizing a dispensing hopper for storage of said material

Technical Field [0001] The present invention relates to an installation for distribution of granular or powder material via pneumatic transport, such as for example an installation for injecting coal into a blast furnace, comprising at least two dispensing hoppers for temporary storage of the granular or powder material. The dispensing hoppers are pressurized for emptying and they must be depressurized to permit filling thereof.

Background Art [0002] It is well-known, for example from EP 0 079 444 or EP 0 212 296 to inject granular or powder materials, in particular powdered coal, into a blast furnace. These materials are conventionally transported pneumatically. Typically, in a blast furnace, powdered coal may be injected at the level of each tuyere or at least into a plurality of tuyeres. Such a transport and injection installation is shown in Figure 1 where, in order to simplify the drawing, a single dispensing line towards the tuyeres has been shown, it being understood that there may be a plurality of these lines connected downstream of a single main storage hopper 1. The coal is supplied from the main storage hopper 1, which is maintained at atmospheric pressure, and distributed towards each injection point via a pneumatic dispensing line 2 which, in the vicinity of the blast furnace, divides into a plurality of injection lines, each connected to an injection point. The coal may also be transported via the dispensing line towards another vessel maintained under constant pressure, then known as injection hopper, located in the vicinity of the blast furnace. The injection lines are then connected to the injection hopper. Transport and injection installations conventionally comprise pressurized temporary storage vessels, or dispensing hoppers, which are suited to containing the material and are pressurized by a transport gas, for example nitrogen. This gas makes it possible to fluidize the powder material and to transport it to its point of use, conventionally located in the tuyere.

[0003] Transport and injection installations are intended, on the one hand, to enable control of the flow rate of injected material and, on the other hand, a continuous supply of powder material. To this end, as shown in Figure 1, a set of at least two dispensing hoppers 3 located in parallel in the transport line and provided with closing valves 4, 5 enabling control of filling and emptying of each hopper is conventionally used for each transport line. The dispensing hoppers 3 are used alternately, with one being emptied for supplying coal to the blast furnace tuyeres, while the other one is being filled from the main storage hopper. Furthermore, each dispensing hopper is provided with a weighing means 6 which makes it possible to determine the quantity of powdered coal introduced on each filling, so enabling control of the quantity of coal injected.

[0004] When filling a dispensing hopper from the main hopper, in order to avoid excess pressure which could prevent the coal from flowing well, the gas present in the dispensing hopper is discharged towards the upper part of the main hopper by a pressure-equalizing duct equipped with an isolation valve 16 which is open during the filling. Since the duct is closed by the valve 16 once filling of the dispensing hopper is complete, it therefore remains filled with moist gas containing powdered coal. In order to avoid the risks of condensation and clogging of the coal, or even blockage of the line, which may arise, these ducts are insulated and heated.

[0005] In order to empty each dispensing hopper via pneumatic transport of the powdered coal, these hoppers are supplied with pressurized gas via a duct equipped with a valve 7, which makes it possible to maintain the pressure required for transport during the entire duration of emptying. These dispensing hoppers, which are maintained under pressure when in use, must therefore periodically be depressurized each time before being refilled from the main storage hopper. The gas escaping from the hopper during depressurization inevitably entrains some powder product still present in the hopper. Bag filters are conventionally used in order to avoid discharging significant quantities of powdered coal into the atmosphere together with the pressurized gas during depressurization. Typically, each dispensing hopper is connected via a depressurizing duct equipped with a valve 8 to a bag filter 9 located on the main storage hopper 1, which makes it possible to recover the powdered coal retained by the bag filter directly into the main hopper, the gas being discharged, in accordance with the arrow F, into the atmosphere or recovered elsewhere. Since the main hopper is at atmospheric pressure, the filter too is therefore also under atmospheric pressure, the drop in pressure arising from the loss of load between the dispensing hopper 3 and the bag filter 9 and being controllable by the depressurizing valve 8 located on the duct connecting the hopper and filter.

[0006] During depressurization, the pressure in the dispensing hopper declines gradually. Furthermore, the bag filters permit a maximum volumetric flow rate which is determined by the area of the filter's filtration surface. In order to optimize depressurization, it is therefore desired to maintain a maximally constant volumetric flow rate through the filter throughout depressurization, which makes it possible to minimize the necessary filtration surface and hence the overall size of the filters and the cost thereof. The flow rate control provided by the valve 8 located on the duct between the dispensing hopper 3 and the bag filter 9 makes it possible to ensure the desired constancy of flow rate.

[0007] One drawback of these systems resides in the fact that the control valve 8 therefore necessarily acts on gas streams laden with powder material. Another drawback arises from the fact that, in a typical blast furnace installation, two, or even three, dispensing hoppers are connected to a single low pressure bag filter and, consequently, variations in pressure in a duct connecting a dispensing hopper to the filter during depressurization may have a disruptive effect on the weighing equipment of the other dispensing hoppers, and in particular on the dispensing hopper currently carrying out injection.

[0008] Another drawback arises from the fact that, during depressurization, a certain quantity of powdered coal is returned with the depressurization gas stream towards the bag filter and then onwards towards the main storage hopper. It is not possible to determine this quantity with precision. As a result, not only is the true quantity of coal injected into the tuyeres less than the quantity detected by the dispensing hopper weighing operations, but the quantity is also not known with precision.

[0009] Still another drawback originates from the fact that the duct between the depressurizing valves and the bag filter are relatively long and of large diameter, so resulting in elevated equipment costs.

[0010] It is suggested in WO 2014/006073 to provide a pressurized bag filter for each dispensing hopper and to place the pressurized bag filter in direct fluid communication with the associated dispensing hopper. Any valves or control means regulating the flow of pressurizing gas evacuated from the dispensing hopper is placed downstream of the pressurized bag filter.

Technical Problem [0011] It is an object of the present invention to provide an improved installation for distribution of granular or powder material, wherein pressurizing and depressurizing of the dispensing hopper is enhanced. This object is achieved by an installation as claimed in claim 1.

General Description of the Invention [0012] In the light of these aims, the invention provides an installation for distribution of granular or powder material via pneumatic transport comprising at least two dispensing hoppers for temporary storage of the granular or powder material, the dispensing hoppers being suited for being, alternately, pressurized for emptying the dispensing hopper and depressurized to permit filling thereof.

[0013] The installation further comprises a device for pressurizing and depressurizing the dispensing hoppers with a pressurizing duct associated with each of the dispensing hoppers for feeding pressurizing gas into the respective dispensing hopper, the pressurizing duct being fluidly connected to a feed line for receiving pressurizing gas from a gas source. The device further comprises a depressurizing duct associated with each of the dispensing hoppers for evacuating pressurizing gas from the respective dispensing hopper, wherein a pressurized bag filter is arranged in the depressurizing duct, the pressurized bag filter being suited to operating under pressure.

[0014] According to an aspect of the invention, the device for pressurizing and depressurizing the dispensing hoppers further comprises a return line fluidly connected between the depressurizing duct and the feed line, wherein the return line is connected to the depressurizing duct at a location downstream of the pressurized bag filter and to the feed line at a location upstream of the pressurizing ducts.

[0015] As the pressurized gas available downstream the pressurized bag filter is clean, it can be injected into the dispensing hoppers via the return line and the “existing” pressurizing gas supply and distribution piping, instead of requiring a dedicated pressure equalizing piping as suggested in the prior art. The use of existing piping instead of dedicated piping for injecting recycled pressurizing gas reduces related equipment costs.

[0016] Advantageously, the return line comprises flow control means for regulating the flow rate through the bag filter. Preferably, the flow control means of the return line comprises static control means without moving elements. Such static control means may e.g. be embodied in the form of a de Laval nozzle, of a predetermined cross-section and shape, suited to permitting the passage of a maximum volumetric flow rate which is less than or equal to the maximum admissible flow rate for the bag filter. Alternatively, a plate with an orifice of predetermined cross-section may be used. Providing that expansion of the gas in the orifice or de Laval nozzle remains critical, the latter produce a constant effective volumetric flow rate through the bag filter located upstream, insofar as depressurization proceeds at a substantially constant temperature. When the pressure at the inlet to the de Laval nozzle, or upstream of the orifice, declines continuously as a result of depressurization of the hopper, the mass flow rate of the mixture of gas and powder product likewise declines proportionally. On the other hand, the volumetric flow rate remains constant independently of the pressure upstream of the de Laval nozzle or the plate orifice, provided that the upstream gas temperature remains constant.

[0017] The feed line may comprise a buffer branch comprising a buffer vessel for storing pressurizing gas. Such a buffer vessel is fed with pressurizing gas from the feed line at limited flow rate. When a dispensing hopper is to be pressurized, compressed gas buffered in the buffer vessel may be supplied at large flow rate from the buffer vessel into the dispensing hopper. Pressurizing of the dispensing hopper is completed by means of pressurizing gas supplied at limited flow rate directly from the feed line, through a bypass branch, bypassing the buffer vessel.

The buffer branch and bypass branch are equipped with respective valves for opening and closing the respective branches as required.

[0018] According to one embodiment of the present invention, the return line feeds pressurizing gas evacuated from one dispensing hopper back into the feed line at a location between the buffer branch and the pressurizing ducts. The pressurizing gas recovered from one dispensing hopper can thus be used to pressurize another dispensing hopper.

[0019] According to another embodiment of the present invention, the return line feeds pressurizing gas evacuated from one dispensing hopper back into the buffer branch of the feed line. Thus, recovered pressurizing gas may be used to fill the buffer vessel.

[0020] According to an improved embodiment of the present invention, the return line comprises an auxiliary buffer vessel for storing pressurizing gas evacuated from the dispensing hoppers. Indeed, the recovered pressurizing gas is fed into such an auxiliary buffer vessel before feeding the pressurizing gas, at larger flow rate, from the auxiliary buffer vessel into the dispensing hopper to be pressurized.

[0021] According to a further embodiment of the present invention, the return line comprises an injector. Such an injector may be used to increase the pressurizing gas recovery efficiency from a gas releasing dispensing hopper toward a gas receiving dispensing hopper or buffer vessel, against a negative pressure gradient. The injector is fed with propellant gas, which may advantageously be pressurizing gas fed from the gas source which also feeds pressurizing gas to the feed line. Downstream of the injector, the return line may feed the pressurizing gas into the feed line at various locations; either downstream of the buffer branch, into the buffer branch or upstream of the auxiliary buffer vessel.

[0022] According to a further embodiment of the present invention, the return line comprises a compressor arranged in a compressor branch of the return line. Once the maximum amount of pressurizing gas has been extracted from the gas releasing dispensing hopper by pressure equalizing, the compressor can be activated to extract more pressurizing gas from the gas releasing dispensing hopper. The compressor is stopped once the gas releasing dispensing hopper is almost at atmospheric pressure. Downstream of the compressor, the return line may feed the pressurizing gas into the feed line at various locations; either downstream of the buffer branch, into the buffer branch or upstream of the auxiliary buffer vessel.

[0023] Finally, the depressurizing duct may comprise an evacuation line for evacuating pressurizing gas from said dispensing hopper to the atmosphere or for use elsewhere. Such an evacuation line preferably comprises flow control means, preferably static control means, most preferably a de Laval nozzle of predetermined cross-section and shape.

[0024] Using a bag filter that is suited to operating under pressure, allows any flow control means to be located downstream of the bag filter. Thus, any flow control means in the depressurizing duct or the return line has a dedusted stream of pressurizing gas passing therethrough and is therefore less subject to the risks of abrasion, clogging and reduction in cross-section, and therefore in flow rate, which could consequently arise.

[0025] The above installation is preferably used for injecting powdered coal into a blast furnace.

Brief Description of the Drawings [0026] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a simplified schematic view of a prior art installation for injecting coal into a blast furnace, which has already been described and commented upon in the introductory part of the present specification;

Fig. 2 is a simplified schematic view of an installation according to a first embodiment of the invention;

Fig. 3 is a simplified schematic view of an installation according to a second embodiment of the invention;

Fig. 4 is a simplified schematic view of an installation according to a third embodiment of the invention;

Fig. 5 is a simplified schematic view of an installation according to a fourth embodiment of the invention; and

Fig. 6 is a simplified schematic view of an installation according to a fifth embodiment of the invention.

Description of Preferred Embodiments [0027] Figure 2 shows an installation 20 for distribution of granular or powder material via pneumatic transport comprising at least two dispensing hoppers 22, 22’, 22” for temporary storage of the granular or powder material, the dispensing hoppers being suited for being, alternately, pressurized for emptying the dispensing hoppers and depressurized to permit filling thereof, and a device for pressurizing and depressurizing the dispensing hoppers 22, 22’, 22”. The device for pressurizing and depressurizing comprises a pressurizing duct 24, 24’, 24” associated with each of the dispensing hoppers 22, 22’, 22” for feeding pressurizing gas into the dispensing hopper, the pressurizing duct being fluidly connected to a feed line 26 for receiving pressurizing gas from a gas source.

[0028] A depressurizing duct 28, 28’, 28” is associated with each of the dispensing hoppers 22, 22’, 22” for evacuating pressurizing gas from the dispensing hopper. Each depressurizing duct comprises a pressurized bag filter 30, 30’, 30” suited to operating under pressure.

[0029] Such a pressurized bag filter has a reinforced casing to allow for the higher pressure resulting from the fact that the filter is located directly downstream of the dispensing hopper, i.e. upstream of any flow control means, and is therefore under a relatively high pressure, which may be as high as 25 bar or more. Furthermore, the casing of the filter will be dimensioned so as also to allow for the fatigue stresses arising from cyclic operation, the casing being pressurized and depressurized in approx. 12 to 15 minute cycles, i.e. typically 4 to 5 times per hour, over many years of service. Furthermore, the filter surface of the bag filter will be determined to permit a volumetric flow rate through the bags of the order of 2 to 60 m3/minute.

[0030] Each depressurizing duct 28, 28’, 28” shown in Figure 2 is provided with an evacuation line 30, 30’, 30” for evacuating gas into the atmosphere. Each evacuation line is provided with a flow control means 32, 32’, 32” such as e.g. a de

Laval nozzle for controlling the flow through the pressurized bag filter and an isolation valve (not shown) for opening or closing the evacuation line.

[0031] The device for pressurizing and depressurizing the dispensing hoppers further comprises a return line 34 fluidly connected between the depressurizing ducts 28, 28’, 28” and the feed line 26, wherein the return line 34 is connected to the depressurizing ducts 28, 28’, 28” at a location downstream of the respective pressurized bag filters 30, 30’, 30” and to the feed line 26 at a location upstream of the respective pressurizing ducts 24, 24’, 24”. At a location near the depressurizing ducts 28, 28’, 28”, the return line 34 comprises flow control means 36, 36’, 36” such as e.g. a de Laval nozzles for controlling the flow through the pressurized bag filter and isolation valves (not shown) for opening or closing communication between the return line 34 and the respective depressurizing ducts 28, 28’, 28”.

[0032] In the installation shown in Figure 2, the feed line 26 further comprises a buffer branch 38 comprising a buffer vessel 40 and a bypass branch 42 bypassing the buffer vessel 40. Pressurizing gas is fed from the gas source into the buffer vessel 40 at limited flow rate. When one of the dispensing hoppers 22, 22’, 22” is to be pressurized, pressurizing gas buffered in the buffer vessel 40 is supplied at large flow rate from the buffer vessel 40 into the respective dispensing hopper. Pressurizing of the dispensing hopper is completed by means of pressurizing gas supplied at limited flow rate directly from gas source into the dispensing hopper through the bypass branch 42. Isolation valves (not shown) are arranged in the buffer branch and the bypass branch for opening or closing the respective branches.

[0033] The embodiment of Figure 2 comprises pressurizing gas recovery by means of pressure equalizing between dispensing hoppers: as the pressurized gas available downstream the pressurized bag filter is clean, it can be injected into the dispensing hoppers via the “existing” pressurizing ducts 24, 24’, 24”. No dedicated pressure equalizing ducts are required for each dispensing hopper. Thus, it is expected that related equipment costs are reduced. In order to ensure continuous supply of coal to hot blast tuyeres of a blast furnace, the installation requires at least three dispensing hoppers.

[0034] Figures 3 to 6 show variants of the embodiment of Figure 2. Identical features are not specifically repeated in the present description for the sake of conciseness.

[0035] According to the embodiment shown in Figure 3, the return line 34 is connected to the buffer branch 38 of the feed line 26. The recovered pressurizing gas is returned from the dispensing hopper 22, 22’, 22” into the buffer vessel 40. The additional equipment required for pressurizing gas recovery is minimal. Due to the use of the buffer vessel 40, only two dispensing hoppers are required to permit continuous supply of coal to hot blast tuyeres of a blast furnace.

[0036] Taking the amount of gas recoverable by pressure equalizing between two dispensing hoppers as a reference, analytical calculations on a typical pulverized coal injection plant suggest that the relative recovery efficiency of the recovery into the buffer vessel is at maximum about 40%.

[0037] According to the embodiment shown in Figure 4, the return line 34 is connected to an auxiliary buffer vessel 44 for storing recovered pressurizing gas before the pressurizing gas is fed, at larger flow rate, from the auxiliary buffer vessel 44 into the dispensing hopper 22, 22’, 22” to be pressurized. Due to the use of the auxiliary buffer vessel 44, only two dispensing hoppers are required to permit continuous supply of coal to hot blast tuyeres of a blast furnace.

[0038] Taking the amount of gas recoverable by pressure equalizing between two dispensing hoppers as a reference, analytical calculations on a typical pulverized coal injection plant suggest that the relative recovery efficiency of the recovery into the auxiliary buffer vessel is about 65% in case the volume capacity of the auxiliary buffer vessel equals the coal volume capacity of the dispensing hopper and about 78% in case the auxiliary buffer vessel is twice that size.

[0039] The above embodiments have a common limitation: They stop once the pressure levels in the gas releasing dispensing hopper 22, 22’, 22” and in the gas accepting hopper or vessel are or close to equal. More pressurizing gas could obviously be recovered and the recovery efficiency increased by transporting additional gas from the gas releasing dispensing hopper into the gas accepting hopper or vessel, against a negative pressure gradient.

[0040] According to the embodiment shown in Figure 5, an injector 50 is arranged in the return line 34 as a conveyor for such additional gas transport. The pressurizing gas supplied to the feed line 26 from the gas source may be used as propellant via a propellant gas line 52. The propellant would, together with the recovered pressurizing gas, be supplied to the dispensing hopper 22, 22’, 22” to be pressurized and thus be used as pressurizing gas as well. As shown in Figure 5, the return line 34 is connected to the buffer branch 38 of the feed line 26. The recovered pressurizing gas is returned from the dispensing hopper 22, 22’, 22” into the buffer vessel 40. Due to the use of the buffer vessel 40, only two dispensing hoppers are required to permit continuous supply of coal to hot blast tuyeres of a blast furnace.

[0041] Although not shown in the Figures, such an injector may also be placed in a return line connected directly to the feed line as shown in Figure 2 or in a return line connected to the feed line via an auxiliary buffer vessel as shown in Figure 4.

[0042] It is clear that recovered pressurizing gas contaminated with solid material particles fed through an injector at gas velocities well above 100m/s would produce devastating wear or require most resisting wear protection in the injector. Such contamination is however avoided as the recovered pressurizing gas is cleaned by passing through the pressurized bag filter before reaching the injector 50.

[0043] In the case of a typical pulverized coal injection plant already referred to above, the related net recovery efficiency improvements have been estimated as follows, the reference (100%) still being the pressurizing gas recovery by pressure equalizing between two dispensing hoppers (without injector), and net recovery meaning that the propellant gas is not included in the efficiency calculation, as it is actually not recovered (not used a second time), although it is mixed up with the recovered gas proper.

[0044] In case of recovery by pressure equalizing between two dispensing hoppers, as suggested by the embodiment shown in Figure 5, recovery efficiency is increased from 100% to about 120%.

[0045] An injector may also be provided for the return lines shown in the embodiments of Figures 3 and 4.

[0046] In case of recovery into the buffer vessel 40 as suggested in Figure 3, the addition of an injector would allow increasing recovery efficiency from about 40% to above 50%.

[0047] In case of recovery into the auxiliary buffer vessel 44 as suggested in Figure 4, the addition of an injector would allow increasing recovery efficiency from about 65% to above 85% for a vessel volume capacity equal to the hopper coal capacity.

[0048] A further embodiment is shown in Figure 6, wherein the return line 34 is connected to the buffer branch 38 of the feed line 26. Furthermore, the return line 34 comprises a compressor branch 60 having a compressor 62 arranged therein and a direct branch 64 bypassing the compressor 62.

[0049] The recovery is performed in two steps. In a first step, pressurizing gas is returned from the dispensing hopper 22, 22’, 22’” into the buffer vessel 40 by means of pressure equalizing between the hopper and the vessel. In the second step, pressurizing gas is conveyed from the dispensing hopper 22, 22’, 22’” into the buffer vessel 40 by means of the compressor 62. Isolation valves (not shown) are arranged in the compressor branch and the direct branch for opening or closing the respective branches. The compressor 62 is stopped when the pressure level in the gas releasing dispensing hopper has reached the filling pressure level of the dispensing hopper, i.e. normally (close) to atmospheric pressure. The constant filtering actual volume flow rate, controlled by the de Laval nozzle downstream the pressurized bag filter, perfectly fits with the operation of a volumetric compressor, e.g. a rotary screw compressor.

[0050] Using the same reference as for all the embodiments described above, i.e. the recovery by pressure equalizing between dispensing hoppers, the recovery efficiency of the embodiment shown in Figure 6 would be about 240%.

[0051] The invention is not restricted to the above embodiments and to the specific application relating to injecting coal into a blast furnace. It may also be applied to other installations comprising pressurized hoppers containing granular or powder materials and requiring periodic pressurization and depressurization of the hoppers through bag filters.

Legend of Reference Numbers: 1 main storage hopper 28 depressurizing duct 2 dispensing line 30 pressurized bag filter 3 dispensing hopper 32 flow control means 4 closing valve 34 return line 5 closing valve 36 flow control means 6 weighing means 38 buffer branch 7 valve 40 buffer vessel 8 valve 42 bypass branch 9 bag filter 44 auxiliary buffer vessel 16 isolation valve 50 injector F arrow 52 propellant gas line 20 installation for distribution 60 compressor branch 22 dispensing hoppers 62 compressor 24 pressurizing duct 64 direct branch 26 feed line

Claims (9)

1. Installation pour la distribution de matière granulaire ou pulvérulente par un transport pneumatique comprenant au moins deux trémies de distribution pour stockage temporaire de ladite matière granulaire ou pulvérulente, les trémies de distribution étant appropriées à être, de manière alternée, pressurisées pour vider les trémies de distribution et dépressurisées pour permettre le remplissage de celles-ci, et un dispositif pour pressuriser et dépressuriser lesdites trémies de distribution, ledit dispositif pour pressuriser et dépressuriser comprenant une conduite de pressurisation associée avec chacune desdites trémies de distribution pour alimenter un gaz de pressurisation dans ladite trémie de distribution, ladite conduite de pressurisation étant fluidiquement connectée à une ligne d’alimentation pour recevoir un gaz de pressurisation d’une source de gaz ; une conduite de dépressurisation associée avec chacune desdites trémies de distribution pour évacuer un gaz de pressurisation de ladite trémie de distribution ; un filtre à sacs pressurisé agencé dans ladite conduite de dépressurisation, ledit filtre à sacs étant approprié à fonctionner sous pression, caractérisée en ce que ledit dispositif pour pressuriser et dépressuriser comprend en outre : une ligne de retour fluidiquement connectée entre ladite conduite de dépressurisation et ladite ligne d’alimentation, dans laquelle ladite ligne de retour est connectée à ladite conduite de dépressurisation en un emplacement en aval dudit filtre à sacs pressurisé et à ladite ligne d’alimentation en un emplacement en amont de ladite conduite de pressurisation.1. Installation for the distribution of granular or powdery material by a pneumatic transport comprising at least two distribution hoppers for temporary storage of said granular or powdery material, the distribution hoppers being suitable to be, alternately, pressurized to empty the hoppers dispenser and depressurized to allow filling thereof, and a device for pressurizing and depressurizing said dispensing hoppers, said device for pressurizing and depressurizing comprising a pressurization line associated with each of said dispensing hoppers for supplying a pressurizing gas in said distribution hopper, said pressurizing line being fluidly connected to a supply line for receiving a pressurizing gas from a gas source; a depressurization line associated with each of said distribution hoppers for discharging a pressurizing gas from said dispensing hopper; a pressurized bag filter arranged in said depressurization line, said bag filter being adapted to operate under pressure, characterized in that said device for pressurizing and depressurizing further comprises: a return line fluidically connected between said depressurizing line and said feed line, wherein said return line is connected to said depressurizing line at a location downstream of said pressurized bag filter and said feed line at a location upstream of said pressurizing line. 2. Installation selon la revendication 1, dans laquelle ladite ligne de retour comprend un moyen de contrôle de flux, dans laquelle ledit moyen de contrôle de flux comprend préférablement un moyen de contrôle statique, le plus préférablement une tuyère de Laval de section transversale et de forme prédéterminées.The plant of claim 1, wherein said return line comprises flow control means, wherein said flow control means preferably comprises static control means, most preferably a Laval cross-section and flow nozzle. predetermined form. 3. Installation selon la revendication 1 ou 2, dans laquelle ladite ligne d’alimentation comprend une branche tampon comprenant un récipient tampon pour stocker un gaz de pressurisation.The plant of claim 1 or 2, wherein said feed line comprises a buffer branch comprising a buffer vessel for storing a pressurizing gas. 4. Installation selon la revendication 3, dans laquelle ladite ligne de retour est connectée à ladite branche tampon de ladite ligne d’alimentation.4. Installation according to claim 3, wherein said return line is connected to said buffer branch of said feed line. 5. Installation selon l’une quelconque des revendications 1 à 3, dans laquelle ladite ligne de retour comprend un récipient tampon auxiliaire pour stocker un gaz de pressurisation évacué desdites trémies de distribution.5. Installation according to any one of claims 1 to 3, wherein said return line comprises an auxiliary buffer container for storing a pressurization gas evacuated from said dispensing hoppers. 6. Installation selon l’une quelconque des revendications 1 à 5, dans laquelle ladite ligne de retour comprend un injecteur, ledit injecteur comprenant une ligne d’alimentation de gaz propulseur connectée à ladite ligne d’alimentation.6. Installation according to any one of claims 1 to 5, wherein said return line comprises an injector, said injector comprising a propellant supply line connected to said supply line. 7. Installation selon l’une quelconque des revendications 1 à 5, dans laquelle ladite ligne de retour comprend un compresseur.7. Installation according to any one of claims 1 to 5, wherein said return line comprises a compressor. 8. Installation selon l’une quelconque des revendications précédentes, dans laquelle ladite conduite de dépressurisation comprend une ligne d’évacuation pour évacuer un gaz de pressurisation de ladite trémie de distribution dans l’atmosphère ou pour une utilisation ailleurs, dans laquelle ladite ligne d’évacuation comprend un moyen de contrôle de flux, préférablement un moyen de contrôle statique, le plus préférablement une tuyère de Laval de section transversale et de forme prédéterminées.An apparatus according to any one of the preceding claims, wherein said depressurizing line comprises an evacuation line for evacuating a pressurizing gas from said distribution hopper into the atmosphere or for use elsewhere, wherein said pressure line comprises The evacuation comprises a flow control means, preferably a static control means, most preferably a Laval nozzle of predetermined cross section and shape. 9. Installation selon l’une quelconque des revendications précédentes, caractérisée en ce qu’elle est utilisée pour injecter du charbon pulvérulent dans un haut-fourneau.9. Installation according to any one of the preceding claims, characterized in that it is used to inject pulverulent coal into a blast furnace.