FR2782377A1 - After-burning equipment, e.g. for steel-making light-arc furnaces producing effluents in large amounts with substances to be incinerated, comprises combustion container with burners with separate combustion air supply and fuel supply - Google Patents

Abstract

After-burning equipment comprises a combustion container (1) with burners. The fuel supply means are formed by a number of elementary units (21) each comprising a gas injector (9) with multiple jets associated with a nozzle with high impulse thermal cone (10) and all directed transversely to the effluent flow in the container. After-burning equipment comprises a combustion container (1) with burners where the effluents to be treated reach incineration temperature for the substances harmful to the environment they possibly contain. The burners are made up by the combination of means supplying combustion air (8) with separate means bringing the fuel. The combustion air supplying means are arranged upstream relative to the effluent flow circulating direction. The fuel supply means are formed by a number of elementary units (21) each comprising a gas injector (9) with multiple jets associated with a nozzle with high impulse thermal cone (10) and all directed transversely to the effluent flow in the container.

Description

POST-COMBUSTION EQUIPMENT for INDUSTRIAL REACTORS.

  The invention relates to the treatment of effluents which can be produced, sometimes in large quantities, in liquid or particulate form, but most often in gaseous form, by industrial reactors, in particular metallurgical reactors, such as the melting furnaces of the steel industry, electric arc furnaces in particular. More specifically, the invention relates to the combustion, outside the reactor, of the unburnt residues contained in these effluents and the resulting incineration

  any species that are harmful to the environment.

  It is known that a steel arc furnace, which will be taken as an example of application of the invention to illustrate the rest of the presentation, is a large smoke producing apparatus, conventionally generating from 50 to 250,000 Nm3 / h of gas from the melting of the iron charge contained in the furnace and the controlled or uncontrolled air inlets. These gases, loaded with dust (oxides of

  Iron, Zn, Pb, Cu, Cd ...), are captured and then released into the atmosphere after treatment.

  The treatment consists, not only in dusting them by filtration, but also in thermally destroying the gaseous species contained which can cause explosions (CO, H2). Other species in the solid, liquid or gaseous state, liable to pollute the environment, such as residual CO, VOCs, PAHs, dioxins or furans are destroyed by calcination, but only partially. The operation is carried out in equipment placed outside the oven, and consisting essentially of a combustion pot, called "post-combustion" since it involves burning unburnt materials, therefore combustible residues which have no managed to burn in the oven. This "post-combustion" pot is provided, at its inlet end by which the fumes arrive, with one or more oxy-fuel oil or oxy-gas burners directed in the general direction of the flow of the gases so as to create a longitudinal flame which "captures" well all the unburnt during their passage in the pot, and which raises the temperature of the fumes (from 300 C at the inlet) to about 1000 C. Due to the high speeds imposed by the high gas flows to be evacuated, we use a very long pot that can reach twenty meters, or even more. It is estimated that several seconds (between 2 and 4 seconds), the necessary residence time of this gas around 1000 C once burned to pyrolyze the toxic organic substances to be eliminated that it may contain. In fact, even with these very long pots, it turns out that the flame-smoke contact time to be heated is such that a large quantity of smoke does not reside

  long enough at the high temperature required for pyrolysis.

  It should be noted that this type of equipment has become fairly widespread in the industry.

  However, its large size constitutes serious handicaps, both technical because of its size and limited efficiency, and economical by the high cost of manufacture,

  maintenance and operation, handicaps which the present invention proposes to resolve.

  To this end, the invention relates to post-combustion equipment for unburnt materials contained in the effluents of industrial reactors, such as electric arc furnaces in the steel industry, comprising a combustion pot whose inlet end is provided with burners and in which said effluents reach their incineration temperature of any substances harmful to the environment which they contain, equipment characterized in that said burners are constituted by means of introducing combustion air associated with separate fuel introduction means arranged downstream with respect to the direction of flow of the effluent stream, and in that said fuel introduction means are formed by a plurality of elementary entities oriented transversely to the stream d '' effiuents and each comprising a "multi-jet" gas injector associated with a dart nozzle

  thermal with high impulse, advantageously placed downstream of the outlet of the fuel jets.

  In accordance with a preferred embodiment, the "multi-jet" injector entities are

  parietal and distributed along the periphery of the periphery of the pot.

  According to another embodiment, the separate means for introducing combustion air are constituted by boxes attached to the pot coming to cap calibrated lights formed in the wall of the pot in the vicinity of the injector entities and which puts them in

  i5 communication with the inside of the pot.

  In a particularly advantageous embodiment, the means for introducing combustion air into the pot are controlled by the instantaneous flow of effluents at the inlet of the pot, or at the target temperature after combustion. The same applies to the number of injectors

  to be put into instant operation.

  As will no doubt have been understood, the invention is based on the discovery that a few tenths of a second at 600-1000 C is in fact sufficient, and not the 2-3 seconds considered hitherto necessary, to allow the incineration of organic components to be removed from the fumes. Therefore, a transverse flame combustion system, instead of the longitudinal flame burner, becomes possible for the realization of the post-combustion pot, which is thus considerably reduced in length (by a factor of 2 practically or even

beyond).

  This presupposes, however, that the fumes can be present in the combustion zone already premixed with combustion air and that, moreover, it is possible to have a short and powerful flame burner technology enabling the temperature to be reached. incineration as quickly as possible. To this end, an original design of mini-burners is retained for the invention, consisting of a plurality of elementary entities provided with injectors propelling multiple jets of combustible gas in a stream of smoke previously mixed with combustion air . As soon as it leaves the injectors, this combustible gas is suddenly heated up to its ignition temperature (around 800 C for natural gas) by a hard jet of very hot burnt gases (which we call "thermal dart") a pilot flame nozzle present on each entity in the immediate vicinity of the injectors. The "tearing off" effect which almost inevitably causes the flow of effluents on the radial jets of combustible gases coming from the injectors leads to opt for a location of the pilot flame nozzle downstream of the injectors, so that obstacles the flow of effluents from these darts

  gaseous from the nozzles create behind them, in the direction of flow, thermo-

  aerodynamics conducive to the early establishment of a homogeneous and stationary flammable mixture which, brutally overheated by these same gaseous darts at high temperature, then burns almost instantaneously, thus propagating a short and powerful flame. Combustion air, added separately and upstream of the gas injectors, is supplied to the flue gas in sufficient quantity to satisfy not only the lively combustion of the combustible gas introduced by the injectors - and whose released thermal energy makes it possible to quickly reach the ignition temperature of the unburnt fumes to be treated, namely, CO, VOC, PAH, BTX, dioxins and furans usually present, but also the combustion of these unburnt products. As these are delivered in an inevitably fluctuating quantity by the reactor which produces them, an adjustment of the combustion air flow blown into the post-combustion pot will advantageously be carried out in order to constantly adjust it to the oxygen requirements required for combustion. total of the fuels brought into the pot by the stream of effluents to be treated on the one hand, and by the gas injectors, on the other hand. Likewise, since the arrival temperature of the fumes to be incinerated is also inevitably variable, the number of gas injectors in service at any time will advantageously be adapted accordingly in order to maintain the thermal conditions required for the incineration of organic species. to destroy. Of course, these two regulations can be connected in cascade, the final temperature control of the fumes after post-combustion managing the number of injectors to activate, which manages the flow of combustion air to be blown, which can be adjusted in response to variations in the incoming smoke flow. Of course, provision will be made in this case for a regulation time constant different from the first cited to ensure the convergence of the automation processes of these

related regulations.

  However, it will have been observed that the invention in fact consists, in its essential characteristics, in treating the fumes according to a three-stage scheme: preparation of the fumes by premixing with oxidizing air, then their combustion by strong injection of combustible gas assisted by powerful thermal darts, followed by thermal exhaustion of

  smoke over a very short time, less than a second in any case.

  The invention will in any case be well understood and other aspects and advantages

  will emerge more clearly in view of the following description of a given embodiment

  with reference to the accompanying drawing plates in which: - Figure 1 shows schematically in longitudinal section a post-combustion pot of the prior art which has been mentioned previously; - Figure 2 shows schematically in longitudinal section a post-combustion pot according to the invention provided with combustion air intake boxes; - Figure 3 is a sectional view at said boxes along the plane of Figure 2; - Figure 4 is an elevational view showing the detail of embodiment of a combustion air supply box; - Figures 5a and 5b show the constitution of an injector entity "multi jets" associated thermal dart used by the invention, first seen in vertical section in Figure 5a according to the section plane AA of Figure 5b , and plan view from above in FIG. 5b along the section plane BB of FIG. 5a; - Figure 6a is a partial view in longitudinal section of the periphery of the pot at the location of the combustion air introduction ports, according to an alternative embodiment of the invention;

  - Figure 6b is a sectional view of a light, along the plane D-D of Figure 6a.

  In each figure, the same elements are designated with identical references.

  Turning first to Figure 1, we see that the known pot 1 ', like post-combustion pots currently used, consists of a steel cylinder internally lined with refractory material and provided at its end inlet 2 'of a front oxy-gas burner 3. This develops a longitudinal flame 4 along the axis 5' of the pot, so as to propagate parallel to the inlet flow 6 'of the gaseous effluents to be treated and thus to accompany them in

  their progression in the pot to the exit end 7 '. Ordinarily, such a post pot

  combustion is located in the vicinity of an industrial oven of which it constitutes an element of the smoke suction line placed close to the capture orifice to benefit from the enthalpy again

  high smoke in this place.

  The pot 1 'conventionally has between 3 and 4 meters in diameter, and its length can exceed 20 meters. As already explained, this provision, although very restrictive, was considered necessary to produce a flame 4 propagating over a great length by burning the combustible unburnt therein and thus providing the gases, having regard to their high speed, a residence time at high temperature long enough (ie 2 to 3 sec) essential,

  it was thought, the incineration of harmful substances possibly contained.

  Taking the opposite foot, the applicants were able to verify, thanks to a compact pot as shown in fig. 2, that the duration of the residence time was in fact a less demanding parameter than what was admitted hitherto, as soon as the pyrolysis temperature of the harmful species to be eliminated was reached. Consequently, and provided that the combustion air blown into the fumes and the injected combustible gas can quickly form with the fumes to be treated a homogeneous mixture, the faster this critical temperature is reached by the gaseous effluents passing through the pot, the more this can be short, while delivering a gas at its outlet

  completely oxidized and chemically inert.

  This is why the pot I according to the invention shown in FIG. 2 comprises parietal inlets 8 of combustion air followed, in the direction of gas flow, by parietal injectors with multiple jets 9 of combustible gas associated with high impulse thermal darts 10, the whole being distributed along the periphery of the pot in its effluent inlet zone 2 and directed transversely to this stream 3 of effluents. The combustion air thus begins to mix with the lukewarm effluents 3 before the arrival of the combustible gas. The whole is then quickly homogenized by stirring during the forced crossing of the double curtain formed by the ring of jets 9 of combustible gas coming from the multi-jet injectors and by the ring of hard and very hot gaseous darts 10 which emerge from the pilot flame nozzles , which have the additional function of heating the constituents of the mixture to allow them to quickly reach their ignition temperature. A short, intense and homogeneous flame 11, which consumes the unburnt ones, is thus established in the pot on one to two meters long downstream of the thermal darts 10, which

  shows that a very homogeneous and stationary mixture forms just behind this curtain.

  The pyrolysis temperature of the organic species to be incinerated is obtained there (700 to 1000 C), so that an additional 6 to 8 meters in pot length is sufficient to ensure the effluents a residence time of a few tenths of a second at this temperature. necessary for the desired incineration. Thus, the post-combustion pot 1 according to the invention has, for a diameter retained from the prior art (3 to 4 m), a length which can be reduced to 8 or 10 m,

  instead of the 20 m and more known pots currently used for the same purposes.

  As can be seen, by referring jointly to FIGS. 2, 3 and 4, the arrival of the combustion air through the openings 8 made through the metal wall 20 at the entrance to the pot 1 is produced by means of sheet metal boxes 12, 12 '. These boxes are attached by welding on two opposite faces 13, 13 ′ of the cylindrical post-combustion pot, having, in the example considered, a cross section of rectangular shape. Each of the two boxes 12, 12 ′ has a general shape as a "policeman's whistle", with a half-moon body 14 which can accommodate a fan 17 (fig. 2) and extended by a straight whistle leg 15 intended to come cap the row of lights 8 distributed along the width of each face 13,13 'of the pot. It will be noted that this whistle shape, which makes it possible to progressively reduce the useful section of the box when one traverses the row of lights 8 going from that located closest to the blowing body 14 towards the most distant, ensures an almost homogeneous distribution. air outlet speeds from the lights 8 of the same row. To perfect the homogeneity of the combustion air intake in the effluent stream to be treated, it will be advantageous, as shown in fig. 3, to place the boxes 12 and 12 'at head to tail, one relative to the other on

  their respective receiving face of the pot.

  As can also be seen in the figures, and more clearly still in FIG. 4, each box is advantageously provided with an additional leg 16 which extends parallel to the whistle leg 15 for distributing combustion air, but has a constant cross section all along. This leg 16 constitutes a chassis intended to come into line with the orifices 18, better visible in FIG. 6a, formed through the large faces 13 and 13 ′ of the wall of the pot downstream of the air inlet lights 8 to house and fix therein the entities with “multi-jet” injector and associated thermal dart of which it is refers to the beginning of this brief. For this purpose, it comprises a row of openings 19, to be aligned with the orifices 18 during the mounting of the boxes 12 and 12 ', for the installation of these entities which will be described in more detail

  in what follows, with reference to FIGS. 5.

  As can be seen, each multi-jet entity 21 is constituted by a cylindrical envelope 22 provided with a flange 23 for fixing on the edge of the reception opening 19 on the leg 16 of the box 12. In this envelope 22, are arranged in an arc several (here three) tubes 24 of a few millimeters in diameter, held at the end on one side by a front plate 25 receiving the narrowed outlet end 26 of the tubes 24, and on the other side through the bottom 28 of a gas supply chamber 27 of each of the three tubes 24, this chamber 27 itself being supplied with pressurized combustible gas by an inlet nozzle 29. The tightness of the chamber 27 is provided by an upper cover 30. These three tubes 24 together form a "multi-jet" injector 9 which is mentioned earlier in the description. The chamber 27 is traversed by a passage 35 defined by a ferrule 36 and putting the interior volume of the envelope 22 in communication with the external environment through an opening 31 provided for this purpose in the cover 30. In fact, this conduit 35 is used to install a pilot flame 32, confined and powerful, produced by a generator 33 (for example a mini plasma torch) shown here mounted prominently on the cover 30 by means of a plate 34. This pilot flame 32 extends inside the envelope 22 to project a thermal dart 10 of burnt gases at high impulse into the combustion pot through the orifice 37 formed for this purpose in the front plate 25. Thus , the assembly formed by the casing 22, the passage 35 at one end and by the outlet orifice 37 at its other end, constitutes an embodiment of the "pilot flame nozzle" mentioned in the first

part of this brief.

  It will also be understood that it is this powerful thermal dart 10 from the nozzle, which, reproduced in as many copies as there are multi-jet entities 21 distributed around the periphery of the pot 1, builds the dam radial, the flow of which through the effluent stream to be treated ensures, by heat exchange, the heating thereof to the combustion temperature of the unburnt products, and, by hydrodynamic stirring, obtaining a homogeneous mixture between these effluents, the combustion air provided by the prior ventilation through the openings 8 and the combustible gas

injected through the tubing 24.

  As already explained, the conditions are thus met in favor of a lively and rapid combustion of this mixture, which is then manifested by an intense and compact flame which already arises in the vicinity of each entity 21 in a radial direction and which, by "flushing" effect due to the flow of effluents then tilts towards the longitudinal direction as one approaches the central axis 5 of the pot. It is precisely because of this "hunting" effect that it is advantageous to have the thermal dart 10 downstream of the multijets 9 of combustible gas. It goes without saying that the invention cannot be limited to the embodiments exemplified above, but extends to multiple variants and equivalents insofar as its definition is respected.

Claims (5)

given by the attached Claims. In particular, it will be even better to hang the birth of the combustion flame 11 in the vicinity of each injection entity 21 by virtue of a blowing of combustion air also directed towards the gas injectors 26. In the embodiment of this variant, illustrated in FIGS. 6, this oriented blowing of combustion air is obtained by fitting the pre-ventilation air inlet ports 8 so as to cause them to produce a double flow of combustion air: a main flow directed transversely and intended to oxygenate the effluent stream to be treated as already explained, and a secondary flow 39 developing downstream of the main flow 38 along the inner surface of the wall 20 of the post-combustion pot and in the same direction as the effluents to be treated, therefore directed towards the close openings 18 made through this wall to accommodate the outlet end of the blowing entities 21. Such an arrangement can simply consist in lining the lights 8 with the help of a lining 40 provided with a separating partition 41 which longitudinally divides the interior passage into two adjacent compartments 42, 43, and which is extended inside the pot by a bent flange 41a curved towards the passages 18 in order to channel in this direction the flow of secondary combustion air 39 coming from the smaller compartment 43. Likewise, as already indicated, it may prove useful to regulate the flow of combustion air blown into the post-combustion pot depending on the flow rate or the unburnt content of the effluents to be treated. Metallurgical reactors, furnaces with steel arcs in particular, have an operating regime essentially of the transient type. It follows gas productions in quantity often irregular in time which are characterized mainly by phenomena of "puffs". The post-combustion equipment must be able to follow these variations in effluent flow rates by adjusting the flow of combustion air supplied as a consequence so as to remain permanently in stoichiometric combustion conditions, or in any case of complete combustion at regard to unburnt. Thus, the equipment according to the invention may incidentally be provided with an analyzer of the content of unburnt materials (in particular CO) in the effluents before they enter the pot 1 and of a device for measuring their flow rate (pressure-reducing element , such as a Pitot tube for example). These measuring instruments then deliver their signals to a controller having in memory the abacuses specific to pot I of values for adjusting the flow rate of combustion air necessary for complete combustion as a function of the gas flow rate and of its unburnt content. An actuator receives from the controller a signal representative of the value resulting from the reading of the charts and consequently controls the motors of the fans 17, 17 'in the ventilation boxes 12, 12'. It will be recalled that if the invention was conceived for ovens producing gas at high flow rates, such as furnaces with steel arcs, it nevertheless remains applicable to any industrial furnace emitting effluents containing unburnt materials, such as melting, for example cupola furnaces, reverberatory furnaces for melting metal waste, or even pyrolysis heating systems for waste containing hydrocarbon co-materials. Furthermore, the embodiment described with reference to Figures 2, 3 and 4 relates to an afterburner having at least its rectangular entry to be able to fit into, or around a pipe of the same shape carrying the fumes treat. This is only an exemplary embodiment, the inlet of the pot according to the invention can of course take a completely different shape, in particular round to adapt if necessary to a pipe of circular shape. CLAIMS   1) Post-combustion equipment for unburnt materials contained in the effluents of industrial reactors, comprising a combustion pot whose inlet end is provided with burners and in which said effluents reach the incineration temperature of any substances harmful to the environment. environment which they contain, equipment characterized in that said burners are constituted by means for introducing combustion air (8) associated with separate means for introducing fuel arranged downstream with respect to the direction of flow of the effluent stream, in that said fuel introduction means are formed by a plurality of elementary entities (21) oriented transversely to the effluent stream and each comprising a "multi-jet" gas injector (9) associated with a thermal dart nozzle with strong impulse (10).   2) Post-combustion equipment according to claim 1, characterized in that, on each injector entity (21), the nozzle (35) with thermal dart (10) is arranged downstream of the outputs of the multi-jets (9) of combustible gas, in the direction of the flow of the effluent stream. 3) Post-combustion equipment according to claim 1, characterized in that the injector entities (21) are parietal and distributed around the periphery of the periphery of the inlet of the pot (1).   4) Post-combustion equipment according to claim 1, characterized in that the separate means for introducing combustion air consist of calibrated ports (8) formed in the wall (20) of the pot (1) in the vicinity of entities with injector (21) and upstream of   the latter in the direction of flow of the effluent stream.   ) Post-combustion equipment according to claim 1, characterized in that it comprises means for adjusting the flow of combustion air to control the means (8) for introducing combustion air into the pot (1) to the flow instant effluent to be treated which present at the entrance of the pot.   6) Post-combustion equipment according to claim I or 5, characterized in that it comprises means for adjusting the number of gas injectors (24) to be put into operation   depending on the temperature of the effluents to be treated which are present at the inlet of the pot (1). o10   7) Post-combustion equipment according to claims 1 or 4, characterized in that   the means (8) for introducing combustion air include provisions (41) for producing a transverse flow of combustion air (38) and, downstream of the latter in the direction of flow of the effluent stream, a longitudinal air flow (39) flowing in the direction of flow of the effluent stream. 8) Post-combustion equipment according to claim 7, characterized in that said arrangements are constituted by a separating partition (41) with bent end (41a) extending inside the pot (1) and sharing said means ( 8) introducing air into   two adjacent compartments (42, 43).