Patent Application
20100044930
The present invention generally relates to a method of injecting a supersonic coherent jet of an inert gas (either a pure inert gas or a high concentration of inert gas) into a molten metal bath located within a metallurgical furnace.
In steelmaking, it is desirable to form coherent jets to promote mixing of the molten steel and to dilute the carbon monoxide (CO) in the molten steel and encourage the carbon and oxygen to come out of the steel. However, the use of oxygen to form such coherent jets can result in oxidation of the steel and undesirable by-products. Thus, it would be useful to form coherent jets from inert gases that do not react with the steel. The most desirable inert gas is argon because it is truly inert. Argon does not react at all with steel. Other inert gases are also desirable, but may have some reaction with steel. For example, nitrogen may cause “nitrogen pickup” and add nitrogen into the steel, affecting the quality of the steel. Another inert gas such as carbon dioxide may oxidize the molten steel bath due to the dissociation of CO2.
In general, the prior art teaches utilizing the “external shroud” technique, whereby the main jets, including an inert gas, are surrounded by an externally produced flame shroud. U.S. patent application Ser. No. 11/476,039, filed on Jun. 28, 2006 and entitled “Oxygen Injection Method” (Mahoney et al.), disclosed the “internal shroud” method to form oxygen coherent jets for application to improving the top-blown refining process of molten metal baths (e.g., basic oxygen furnace (BOF) steelmaking).
The internal shroud technique described by Mahoney et al. incorporates the following elements:
A problem arises when this method is applied to pure inert gas or high concentration inert gas, balance oxygen. The internal shroud method is ineffective for producing coherent jets of inert gases due to the elimination or suppression of fuel combustion in the jet shear layer (i.e., combusting the injected fuel and oxygen in the shear layer to produce a coherent jet is not possible).
Therefore, a problem to solve is the production of coherent jets containing pure or a high concentration of inert gas, particularly argon, using the internal shroud technique. Another problem to solve is the improvement of the refining of molten metal, particularly the basic oxygen process, by the application of internal shroud coherent jets containing argon.
Japanese Patent Application No. JP2002-288115 (JFE/Nippon) is concerned with the process of flame stabilization within a duct. This is accomplished by injecting fuel, which mixes with a portion of the main oxygen stream. Upon ignition, the flame is stabilized within an annular groove located in the gas passage wall, which acts as a flame holder. As a result, this technique cannot be applied to produce argon coherent jets. Japanese Patent Application No. JP2003-0324856 discloses a single burner lance capable of supplying flame and an oxygen jet to a wide area in melting/refining of iron, but does not discuss injection of an inert gas or an internal shroud.
Because of the difficulties in applying an internal shroud method to an inert gas, it has not been achieved thus far. The present invention allows the relative advantages of the internal shroud method versus the external shroud method to now be applied to inert gases such as argon.
The present invention relates to a method of injecting a supersonic coherent jet of an inert gas into a melt located within a metallurgical furnace having a heated furnace atmosphere.
In accordance with the method, an inert gas stream is introduced into a nozzle having a passageway of a converging-diverging configuration. It is to be noted that the entire passageway does not have to have a converging-diverging configuration and in fact, a passageway in accordance with the present invention can have a converging-diverging configuration portion followed by a straight cylindrical portion extending to the face of the nozzle. Further more, the term “inert gas stream”, as used herein and in the claims, encompasses uniformly blended streams having an inert gas concentration of at least 40% by volume, and preferably at least 70% by volume. An oxygen stream is injected into the inert gas stream at inner circumferential locations of the passageway that are situated entirely within the passageway so that a combined inert gas and oxygen containing stream is formed within the passageway. In this regard, “oxygen stream” means a stream having an oxygen concentration of at least 75% by volume and preferably commercially pure oxygen at least 90% by volume. Then a fuel containing a hydrogen species is injected into the inert gas stream at inner circumferential locations of the passageway that are situated entirely within the passageway. In this regard, the term “hydrogen species” means molecular hydrogen or a molecule containing hydrogen or any substance containing hydrogen atoms or combinations thereof. As a result, a combined fuel, inert gas and oxygen stream is formed within the passageway having a structure composed of an outer circumferential region, comprising a mixture of the fuel, inert gas and oxygen, and an inner central (core) region that is surrounded by the outer circumferential region and containing the combined inert gas and oxygen and essentially no fuel.
The inert gas stream is introduced into an inlet section of the passageway at or above a critical pressure. As a result, a choked flow condition is established within a central throat section of the passageway, the combined fuel, inert gas and oxygen containing stream is accelerated to a supersonic velocity within a diverging section of the passageway, and the combined fuel, inert gas and oxygen containing stream is discharged as a structured jet from the nozzle into the furnace atmosphere. The structured jet has the structure of the combined fuel, inert gas and oxygen containing stream and the supersonic velocity upon discharge from the nozzle.
Ignition and combustion of the fuel while within the passageway is prevented by not introducing an ignition source and providing the passageway with an inner surface uninterrupted by any discontinuity within which the outer circumferential region could otherwise decelerate and provide a site for stable combustion of the fuel.
A flame envelope is produced that surrounds a jet of inert gas formed from the inner central region of the structured jet and that initially has the supersonic velocity. The flame envelope inhibits velocity decay and concentration decay of the jet of inert gas. Velocity would otherwise decay without the flame envelope due to interaction of the jet of inert gas with the furnace atmosphere. Such interaction also causes a dilution of the jet of inert gas to produce a concentration decay. As used herein and in the claims, the term “flame envelope” means a flame that surrounds the jet of inert gas and propagates along the length thereof by active combustion of the fuel and any reactants that may be present within the heated furnace atmosphere, wherein such combustion is supported in whole or in part by oxygen supplied by the structured jet of inert gas. In the present invention, the flame envelope is produced entirely outside of the nozzle through contact of the outer circumferential region of the structured jet with the heated furnace atmosphere. This contact creates a shear-mixing zone containing a flammable mixture composed of the fuel, the argon, the oxygen and the heated furnace atmosphere and auto-ignition of the flammable mixture through heat supplied by the heated furnace atmosphere.
The jet of inert gas is directed into the melt, while surrounded by the flame envelope. In this regard, the term “melt” as used herein and in the claims with respect to a steelmaking furnace, electric arc furnace (EAF) or BOF, means both the slag layer and the underlying molten pool of metal. As a result, in such furnace, the jet of inert gas would first enter the slag layer. In case of a metallurgic furnace in which a slag layer is not produced, the “melt” at which the jet of inert gas enters would constitute the molten metal. An example of this would be a non-ferrous refining vessel.
Although not known in the prior art, a discharge of a structured jet, such as described above, when contacted by the heated furnace atmosphere will produce a region within an outer shear-mixing zone that will ignite to form a flame envelope that will surround and inhibit velocity decay and concentration decay of a supersonic jet of inert gas formed by the inner central region of the structured jet. This allows a nozzle of the present invention to be positioned at some distance away from the melt and allows the beneficial stirring action of the melt to be enhanced.
As indicated above and as known in the prior art, the production and injection of a jet of inert gas while at a supersonic velocity has the advantage of minimizing any oxidization of the metal contained within the melt for refining purposes while at the same time producing a vigorous stirring action of the melt. Additionally, there are no external fuel passages that can plug requiring removal of the lance from service and extraction of deposits, known as skull, from the face of the nozzle. Furthermore, as can be appreciated from the above discussion, the disadvantages of mixing, igniting, stabilizing and combusting an oxygen and fuel containing stream at high velocity within a combined space (nozzle) are avoided by the present invention because ignition, stabilization and combustion of the mixture of fuel and oxygen is prevented while within the nozzle.
The combined fuel, inert gas and oxygen containing stream can be fully expanded upon discharge thereof as the structured jet from the nozzle. The fuel can be introduced to inert gas and oxygen containing stream while within the diverging section of the nozzle. As a safety measure, the combined fuel, inert gas and oxygen containing stream can be over expanded upon the discharge thereof as the structured jet from the nozzle such that the stream has a sub-ambient pressure while within the diverging section of the nozzle. The fuel can be introduced into the inert gas and oxygen containing stream at a location within the diverging section at which the inert gas and oxygen containing stream is at the sub-ambient pressure. As a result, upon failure of the fuel supply system, inert gas and oxygen will not back-flow through fuel passages creating a potentially dangerous condition. Another beneficial result is the fuel delivery system is not required to overcome positive back-pressure in the nozzle, thereby minimizing the supply pressure required for fuel delivery into the nozzle.
The diverging section of the nozzle can extend from the central throat section to a nozzle face of the nozzle exposed to the heated furnace atmosphere. Other possibilities will become apparent from the detailed discussion below.
Preferably, the supersonic velocity of the structured jet of combined fuel, inert gas and oxygen is at least about Mach 1.7.
The metallurgical furnace can be an electric arc furnace (EAF).
Alternatively, the metallurgical furnace can be a basic oxygen furnace (BOF). In such cases, the fuel is preferably introduced into the oxygen stream at a specific equivalence ratio. The equivalence ratio between the shroud fuel (F) and oxygen (O) is defined as the ratio of the fuel/oxygen ratio to the stoichiometric fuel/oxygen ratio:
(F/O)/(F/O)stoich. (Equation 1)
For example, a shroud composed of CH4 and O2, the F/Ostoich=0.5. For pure argon, experiments indicate that the shroud requirement would be about F/O=0.2 to 0.13 (very oxidizing). Thus, the equivalence ratio would be between 0.26 to 0.4. However, the invention would still be operable outside of these ranges so these are preferred, but not required. For pure argon, it would be preferable to inject about 5-15% oxygen of the argon as shroud oxygen and less would be required for argon/oxygen ignition jets.
In either type of furnace, the heated furnace atmosphere will contain carbon monoxide and the flammable mixture used in forming the flame envelope will in turn contain the carbon monoxide. Where the metallurgical furnace is a basic oxygen furnace, the nozzle can be mounted in a water-cooled lance at a lance tip of the water-cooled lance. It is understood, however, that the application of the present invention is not limited to such furnaces and in fact can be used in a furnace having a heated furnace atmosphere that contains no carbon monoxide or any other substance that can serve as part of the flammable mixture used in forming the flame envelope. All that is necessary with respect to the “heated furnace atmosphere” is that it be of sufficient temperature to cause auto-ignition of the flammable mixture.
In any embodiment of the present invention, the fuel can be introduced into the inert gas and oxygen containing stream at the inner circumferential locations of the passageway by injecting the fuel into a porous metal annular element having an inner annular surface. The inner annular surface forms part of the throat section or the diverging section of the converging-diverging passageway. (The shroud fuel and shroud oxygen can be injected together into the inert gas or can be injected separately)
In a further aspect of a method of the present invention applied to injecting inert gas into melt located within a metallurgical furnace having a heated furnace atmosphere containing carbon monoxide, inert gas streams can be introduced into nozzles having passageways of converging-diverging configuration wherein the nozzles are situated at a tip of a water-cooled lance and angled outwardly from a central axis of the water-cooled lance. Such a metallurgical furnace can be a basic oxygen furnace. The fuel containing a hydrogen species and an oxygen stream are injected into the inert gas streams in the manner outlined above to form structured jets, flame envelopes and individual jets of inert gas, which initially have a supersonic velocity. The water-cooled lance can be situated within the basic oxygen furnace and the jets of inert gas are directed into the melt.
In basic oxygen furnace lances, there are typically between 3 and 6 nozzles and the nozzles are outwardly angled at between about 6 degrees and about 20 degrees from the central axis. As indicated above, in case of a basic oxygen furnace, the fuel can be introduced into the oxygen streams at an equivalence ratio of between 0.26 and 0.4 (although not required) and the supersonic velocity of each of the structured jets of combined fuel, inert gas and oxygen can be at least about Mach 1.7. In a specific embodiment, the fuel can be introduced into a fuel chamber and the nozzles are positioned to pass through the fuel chamber. The fuel is introduced into the passageways through fuel passages located within the lance tip and communicating between the inner circumferential locations of the passageways and the fuel chamber. In this regard, there can be between about 4 and about 12 fuel passages for each of the passageways. It is to be noted that more or less fuel passages can be used. The same can be said here for the internal shroud oxygen, i.e., both fuel and oxygen can be injected into the same chamber or separate chambers.
While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings.
a) and 1(b) are schematics of an injector used to inject a jet of inert gas at a supersonic velocity into a melt for use in accordance with the method of the present invention, viewed from the injector face and cross-sectionally, respectively.
a) and 13(b) are schematic, cross-sectional views showing an injector for an argon jet without an internal shroud of the present invention and an injector for an argon jet with an internal shroud of the present invention, respectively.
The problem of producing internal shroud inert gas coherent jets, in particular, argon coherent jets, is solved by the method of the present invention by introducing a mixture of fuel and oxygen into the outer periphery of the inert gas jet. The resultant supersonic “structured jet” is composed of a central region of argon gas and is surrounded by an outer circumferential region composed of argon, fuel and oxygen gas. The technique effectively transforms the surface of the argon jet into an oxygen-like jet, thereby rendering the internal fuel injection effective for producing a coherent jet of argon.
The furnace atmosphere contacts the jet through the formation of a shear (mixing) layer and activates combustion between the fuel and oxygen and results in the production of an argon coherent jet.
Relative to the external shroud, the primary advantages of positioning the fuel and oxygen injectors within the nozzle (i.e., internal shroud) include one or more of the following:
The internal shroud method of the present invention is an enabling technology for applying the coherent jet principle to the BOF converter, which will provide process benefits coupled with a more practical lance design.
An improved inert gas coherent jet, particularly an argon coherent jet, should enable more steelmaking benefits per volume of inert gas supplied and therefore, possibly render the top lance argon blowing process economical for BOF.
The internal shroud inert gas coherent jet apparatus incorporates the following elements:
1. An injector or lance with lance body and lance tip;
2. A means for introducing argon, oxygen and hydrogen containing fuel into the lance body;
3. A tip containing one or more converging-diverging nozzles for the production of a supersonic argon stream(s);
4. A means for injecting oxygen into the outer perimeter of the argon streams, either into the diverging section or any other section of the nozzle;
5. A means for injecting hydrogen-containing fuel into the outer perimeter of the argon stream, preferably into the diverging section of the nozzle.
Experiments were conducted in an apparatus used to simulate the hot furnace gas. The apparatus used in Examples 1 and 2 is shown in
Experiments were conducted to try to produce a pure argon coherent jet injecting only internal shroud fuel. The internal shroud inert gas coherent jet injector used is illustrated in
The argon was injected at 100 psig and 3795 scfh and the fuel was natural gas (NG). The nozzle exit (D) and throat (T) diameters were 0.38-in. and 0.26-in., respectively. In a simulated furnace gas, the internal injection of fuel resulted in no change in jet length, as shown in Table 1.
The coherent jet length is defined as the axial centerline distance from the nozzle exit to where a Pitot tube registers 50 psig, which corresponds to a position within the supersonic core of about Mach 1.7.
The experimentally measured temperatures above, when corrected for radiation losses, result in actual simulated furnace gas temperatures near to commercial furnaces, in the range of about 3000° F.
In this set of experiments, the same injector design as in Example 1 was used and both oxygen and fuel were pre-mixed and injected via the passageways (13, 14) into the internal shroud ports to try to produce a coherent argon jet. However, injecting only internal oxygen (up to 2% relative to the argon flow) and injecting both fuel (0.66%) and oxygen (0.97%) resulted in no changes in jet length (i.e., L/Lo=˜1 for all experiments), as shown in Table 2.
Further experiments were run using the injector shown in
Another such embodiment that uses two separate conduits to supply the shroud fuel and oxygen is shown in
The data plot in
The central core of the jet consists of very high velocity pure argon. At the outer circumferential region, the gas contains oxygen, methane and argon; the gas is not burning within the nozzle as determined by the lack of detection of combustion products in the range of −1 to 1 (−1<R/Rn<1). At about −1.5≦R/Rn≧1.5, the methane and oxygen peaks precipitously drop due to reaction with the furnace atmosphere to produce carbon dioxide and carbon monoxide. This position marks the location of the inner edge of the flame front. R is the radial coordinate and Rn is the nozzle exit radius (Rn=D/2)
Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that these are other embodiments within the spirit and the scope of the claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/87607 | 12/14/2007 | WO | 00 | 9/25/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60875112 | Dec 2006 | US |
