Abstract:
The invention relates to a method and device for supersonically injecting oxygen into a furnace, in particular a cupola furnace, in which the total oxygen required for the furnace operation is injected with the aid of two distinct circuits, i.e., the first circuit comprising at least one supersonic oxygen injecting nozzle and a second circuit which comprises additionally oxygen injecting means and is connected to the first circuit by pressure-sensitive means, such as a discharging device (or upstream pressure adjuster), in such a way that a stable pressure is obtained in the first circuit upon the attainment of the maximum flowrate thereof, wherein the first circuit can consists of several supersonic nozzle groups.

Description:
This application is a §371 of International PCT Application PCT/FR2006/051080, filed Oct. 23, 2006. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a method for super-sonically injecting oxygen into a melting furnace, especially a shaft furnace, in which the raw materials such as coke and scrap iron are loaded through the top and in which the combustion of the combustible materials is carried out by injecting air, generally preheated air, which reacts with the coke, the combustion having been initiated using preheated burners. These furnaces are especially cupola furnaces which comprise a toric annulus placed at the base of the cupola into which the blast air, preheated by heat exchange with the combustion gases, is injected through a multitude of nozzles connected to this toric annulus. 
     2. Related Art 
     To improve the operation of cupola furnaces, or to increase their production, it is known to inject oxygen by means of supersonic lances positioned in the center of each nozzle. One of the advantages of this technology is the penetration of oxygen into the center of the cupola due to the high oxygen injection velocity. 
     However, in the case of a low oxygen flow rate, the pressure of the oxygen in the lances decreases, this results in a decrease in the velocity of the oxygen injected into the cupola, (a velocity which becomes subsonic), the penetration of oxygen into the center of the cupola then being lower than at a high oxygen flow rate (with an upstream pressure of around 8 to 10×10 5  Pa in the case of the cupola). 
     In order to obtain a high oxygen velocity, the lances are generally sized for a working pressure of around 9×10 5  Pa (upstream of the convergent/divergent device that forms the supersonic injection nozzle positioned at the end of the lance). However, this pressure is only obtained at the nominal flow rate of the installation: it is only 4.5×10 5  Pa when operating at 60% of the nominal value. 
     To overcome this problem, it has already been proposed to make all the lances operate alternately either by alternating the “start” and “stop” regimes, or by alternating a “low flow rate” with a “high flow rate”. In both cases, the maximum flow rate is obtained at the working pressure of the lances. Thus, the lances are stopped from operating at low pressure which results in a low oxygen injection velocity. 
     These known techniques have, however, the following drawbacks:
         complexity of implementation (installation cost);   reliability of solenoid valves subjected to a very large number of opening/closing cycles;   knowledge of the average flow rate consumed is difficult to establish, which does not facilitate comparison of these techniques relative to a stable flow rate; and   control of the overall flow rate is not continuous, but is in increments of flow rate.       

     One alternative consists in operating an increasing number of lances, as a function of the flow rate in order to maintain the most stable pressure possible in the lances. Thus the low operating pressures are avoided when the oxygen flow rate is low. 
     However, there is generally an oxygen injection dissymmetry that is prejudicial to the correct operation of the cupola. 
     In all the case, the solutions described above require, in addition, the installation of an additional motor control. 
     SUMMARY OF THE INVENTION 
     The method and the device according to the invention make it possible to avoid these drawbacks. The method of the invention is characterized in that the total oxygen required for the furnace operation is injected using two separate circuits:
         a first circuit comprising at least one supersonic oxygen injection nozzle; and   a second circuit comprising additional oxygen injection means, the second circuit being connected to the first circuit by pressure-sensitive means, such as an overflow (or more generally upstream pressure regulating means), so as to obtain a stable oxygen pressure in the first circuit as soon as the maximum flow rate of this first circuit is attained.       

     In the first circuit, positioned inside each nozzle is a supersonic lance, the dimensions of which are provided for operating at the optimum pressure that gives the maximum oxygen velocity (i.e. 9 bar relative for a velocity of around Mach 2.1), this pressure being attained for a fraction of the total maximum flow rate. 
     In the second circuit, the additional oxygen for attaining the total flow rate is injected. This second circuit will inject the oxygen into the cupola through a second injection point, different from the injection point of the supersonic lances. The injection velocity over this second circuit will be lower, but the usage time of this second circuit will be low compared to the usage time of the first circuit. 
     Preferably, this second circuit will be directly fed by a “branch connection” in the first circuit by means of an overflow (or a pressure regulator placed upstream of the supersonic nozzle). 
     Thus, the pressure in the first circuit will be stable as soon as the maximum flow rate of the first circuit is attained. 
     Preferably, the first circuit is sized so as to obtain a supersonic oxygen injection velocity as soon as a fraction, for example 60 vol %, of the maximum total oxygen flow rate is attained. According to one embodiment variant, the method of the invention is characterized in that the oxygen from the second circuit is injected into the blast air of the cupola or concentrically around the supersonic oxygen jet or directly into at least one of the blast-air injection nozzles, preferably at a subsonic velocity. 
     The invention also relates to a device for implementing this method characterized in that it comprises means for injecting oxygen, having a maximum flow rate, a first circuit comprising at least one supersonic oxygen injection nozzle, a second circuit for additional oxygen injection, the first and second circuits being connected to the oxygen injection means, pressure-sensitive means, such as an overflow (or an upstream pressure regulator), being interposed between the oxygen injection means of the first circuit and of the second circuit. 
     Also preferably, the first circuit comprises a plurality of groups of at least one oxidant injection lance, each lance group being activated successively in order to maintain a supersonic injection of oxidant into the first circuit while the oxidant flow rate of the first circuit is increasing. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be better understood with the aid of the following exemplary embodiments, given non-limitingly, together with the figures that represent: 
         FIG. 1 , a diagram of a cupola and of its oxidant (hot blast air) supply system according to the prior art. 
         FIG. 2 , a block diagram for injection of oxidant according to the invention. 
         FIG. 3 , the flow rate curves of the oxidant in the various circuits. 
         FIG. 4 , an exemplary embodiment of  FIG. 2 . 
         FIG. 5 , a schematic cross-sectional view of an oxidant injection nozzle and its supersonic oxygen injection system. 
         FIG. 6 , the oxidant flow rate curves in a multi-lance system operating in increments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  represents a diagram of a cupola  1  according to the prior art. The metallic substances  5 , the coke  4 , etc. are introduced through the opening  2  (in successive layers) located at the top of this cupola. Near to the top  2  is a circuit  3  for recovering hot gases. 
     The air belt  6  is supplied through  7  with air preheated by contact with the flue gases from  3 , the blast air being distributed through ducts, such as  18  having a plurality of nozzles such as  8  and  9  in the bottom part of the blast furnace. The molten metal is recovered in  11 , then  12 , whereas the slag is recovered in  10 . 
       FIG. 2  represents a block diagram of the system according to the invention. The total oxygen flow rate  21  is controlled by flow rate regulating means  22 , so as to obtain an oxygen (vol.) enrichment of X % of the hot blast air from the cupola. The first circuit ( 26 ) corresponds to the supersonic oxygen injection circuit. The second circuit ( 27 ) corresponds to the low-velocity, additional oxygen flow rate circuit. 
     Downstream of point  28 , is the first circuit  26  for injecting oxygen  24 : circuit  1  is supplied with oxygen, the maximum pressure of 9×10 5  Pa is attained with a maximum flow rate Q 1  as a function of the diameter of the supersonic nozzles positioned at the end of the lances. (Q 1 =flow rate of each lance×number of lances). 
     The second circuit  27  is also here, connected to the common point  28  via an overflow  23  (controlled, for example, for an upstream pressure of 9 bar) and a duct  25 . 
     This second circuit makes it possible to supplement the oxygen flow rate required for the cupola operation above the flow rate Q 1 . 
     In the example from  FIG. 2 , the circuit  26  injects oxidant through supersonic lances. The dimensions are provided for operating at the optimum pressure that gives the maximum oxygen velocity (i.e. 9 bar relative for a velocity of around Mach 2.1). 
       FIG. 3  illustrates the distribution of the flow rates between the first (supersonic) circuit and the second (additional) circuit and also the change in pressure in the supersonic lances. The pressure of 9 bar is attained as soon as the flow rate of 360 Sm 3 /h is attained (flow rate determined by the choice of the supersonic injector size). 
     The cupola furnace with hot blast air operates optimally when the production and operating parameters are stable. Thus, the consumption of oxygen is generally stabilized. 
     The oxygen flow rate may be increased temporarily during restarting or during an occasional increase in production, generally for relatively short durations. 
     With the system of supersonic lances that operate continuously, the lances are sized for the maximum flow rate. In the general case of stabilized operation, the velocity of the oxygen is much lower than anticipated with the supersonic system. (Throughout the text, except in particular cases, the term “oxygen” denotes an oxidant in general, that is to say commonly a gas containing at least 21 vol % of oxygen up to 100 vol % of pure oxygen). 
     In the system according to the invention, the velocity of the oxygen injected is supersonic as soon as a significant fraction of the flow rate is attained (for example, 60% of the maximum flow rate). Above this flow rate, the additional oxygen is diverted toward the second injection circuit, this second circuit only being used transiently: the fact of having a lower velocity, and therefore a reduced effectiveness of this fraction of the oxygen flow rate, becomes secondary faced with the advantage of continuously injecting 60% (in the case of exceptional operation) or 90 to 100% (in the case of normal operation) of the oxygen flow rate used at very high velocity. 
     This solution has the advantage of a simple implementation and complete transparency for the operator who can still control the total flow rate of oxygen continuously. 
     Furthermore, no additional motor control is introduced. 
     The curve  30  represents the oxygen flow rate in the first circuit in the form of supersonic injection. This flow rate reaches a maximum toward 350 Sm 3 /h that corresponds to the maximum pressure attained in  21 , i.e. around 9×10 5  Pa (curve  31  is in bar with around 1 bar equal to 10 5  Pa). The increase in the flow rate (curve  32 ) is then achieved via circuit  2  ( 27 ). 
     Thus, defined in  FIG. 3  is a zone of “normal” operation  33  (supersonic injection of oxygen via  26 ) and a zone of exceptional operation  34  that corresponds to the startup of the installation, to a high transient production, etc. via the circuits  26  and  27 . 
       FIG. 4  describes an example of implementing the block diagram from  FIG. 2 . 
     The oxidant passes successively through a filter  40 , a flow meter  41 , a safety valve  42 , a metering valve  43 , the outlet of which is connected to the point  47  where the ducts  45  for the first circuit ( 26 ) and  46  for the second circuit ( 27 ) which supplies the overflow  44 , separate. 
       FIG. 5  is a cross-sectional view of the injection nozzle  8 , modified according to the invention. 
     The oxygen duct  16  passes through the jet of hot blast air  13  coming from  14  in order to terminate in the vicinity of the end of the nozzle  15  via a (convergent/divergent) supersonic injection nozzle  17 . 
       FIG. 6  illustrates the distribution of the flow rate between the first circuit  26  and the second circuit  27 , in the case where the first circuit  26  is composed of three groups of lances with successive opening of the groups in flow rate increments. 
     In order to increase the flexibility of the technique, use is made of n groups of lances (for example, three groups of lances) that open one after the other as explained below. Above the maximum flow rate of the first group of lances, the operation of the lances (circuit  1 ) in service will always be supersonic. 
     Circuit  2  injects oxidant in dilution into the blast air of the additional flow rate A (difference between the total flow rate A+B and the flow rate of the lances in service B). The oxidant injection velocity of this second circuit is lower, but the fraction of flow rate of this second circuit is low (15% on average). 
     Circuit  2  is directly supplied by a branch connection in circuit  1  by means of an overflow. Thus, the pressure in circuit  1  is stable as soon as the maximum flow rate of the first group of lances is attained. 
     In the example from  FIG. 6 , the various zones numbered 1 to 4 correspond to the following operation:
         Non-supersonic operation (flow rate of less than 500 Sm 3 /h)
           Zone 1: first group of lances and zero flow rate in circuit  2 .   
           Supersonic operation (flow rate between 500 and 1100 Sm 3 /h).
           Zone 2: first group of lances, curve  60  (hold) then flow rate in circuit  2  (ramp  61  in the figure) which in total gives the flow rate A+B (ramp  62  in the figure) from  FIG. 6 .   Zone 3: the first and second groups of lances of circuit  1  operate, to which is added a flow rate in the form of a ramp ( 61 ) in circuit  1 . When, in zone 3, the constant flow rate of circuit  1  ( 60 ) and increasing flow rate of circuit  2  ( 61 ) have attained 900 Sm 3 /h, then the third group of supersonic lances is activated, the flow rate of circuit  2  returns to zero and one is then in zone 4 again.   Zone 4: the three groups of lances of circuit  1  are activated, with an increasing flow rate in circuit  2 . (The curves  64  and  63  (or C and D) represent the air flow rate of the blast air enriched respectively with 3 vol % and 2 vol % of oxygen).   
               

     The air flow rate corresponding to an enrichment of 2% (curve D) and 3% (curve C) is given in  FIG. 6 . An enrichment of 3% makes it possible to decrease the amount of coke. Compared to the operation according to the prior art, the air flow rate is reduced by 10 to 15%, this drop being compensated for by the additional oxygen flow rate and the reduction in the coke flow rate. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.