US20030108834A1

US20030108834A1 - Gas lance system for molten metal furnace

Info

Publication number: US20030108834A1
Application number: US10/004,850
Authority: US
Inventors: John Pelton
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2001-12-07
Filing date: 2001-12-07
Publication date: 2003-06-12

Abstract

A lance system for lancing gas into a molten metal furnace wherein primary gas is passed into a furnace as a coherent jet enveloped in secondary gas provided into the furnace through a plurality of secondary openings communicating with respective secondary passages having restrictions within the lance set back from the lance face.

Description

TECHNICAL FIELD

This invention relates generally to coherent jet technology and, more particularly, to the application of coherent jet technology in a molten metal furnace.

BACKGROUND ART

A recent significant advancement in the field of gas dynamics is the development of coherent jet technology which produces a laser-like jet of gas which can travel a long distance while still retaining substantially all of its initial velocity and with very little increase to its jet diameter. One very important commercial use of coherent jet technology is for the introduction of gas into liquid, such as molten metal, whereby the gas lance may be spaced a large distance from the surface of the liquid, enabling safer operation as well as more efficient operation because much more of the gas penetrates into the liquid than is possible with conventional practice where much of the gas deflects off the surface of the liquid and does not enter the liquid.

In a coherent gas jet system, one or more gas jets are surrounded by a flame envelope to maintain coherency over a long distance from the injection lance. When a coherent gas jet system is employed in a harsh environment such as a molten metal furnace, material within the harsh environment may plug some of the apertures on the lance from which gas is provided for forming the flame envelope. This requires periodic stoppage of operations and cleaning of the lance. This stoppage reduces the efficiency of the industrial operation, e.g. electric arc furnace practice or basic oxygen furnace practice, in which the coherent gas jet system is employed.

Accordingly it is an object of this invention to provide a system for establishing a coherent gas jet wherein plugging or fouling of the apertures or ports for the provision of the flame envelope gases is reduced or eliminated.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:

A lance comprising a lance face having at least one primary opening for passing at least one gas jet out from the lance, and a plurality of secondary openings arranged around the primary opening(s) on the lance face, each secondary opening communicating with a passage for passing secondary gas within the lance to the respective secondary opening, and each said passage having a restriction therein so that the area for gas flow at the restriction is smaller than the area of the secondary opening with which that passage communicates.

Another aspect of the invention is:

A method for providing at least one gas jet from a lance into a molten metal furnace comprising:

(A) passing primary gas in at least one primary gas jet out from a lance to form at least one primary gas jet within the molten metal furnace;

(B) passing secondary gas through a plurality of passages within the lance, each passage communicating with a secondary opening and having a restriction therein, and said secondary gas flowing within a passage having a higher pressure at the entrance to the restriction than at the secondary opening; and

(C) passing secondary gas out from the secondary openings and forming a gas envelope around the primary gas jet(s) within the molten metal furnace.

As used herein the term “coherent jet” means a gas jet which is formed by ejecting gas from a nozzle and which has a velocity and momentum profile along its length which is similar to its velocity and momentum profile upon ejection from the nozzle. Another way of describing a coherent jet is a gas jet which has little or no change in diameter along its length.

As used herein the term “length” when referring to a coherent gas jet means the distance from the nozzle from which the gas is ejected to the intended impact point of the coherent gas jet or to where the gas jet ceases to be coherent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a head on view of one preferred embodiment of a lance and [0014]
FIG. 2 is a cross sectional view of that embodiment of the lance which may be used in the practice of this invention. [0015]
FIG. 3 illustrates the embodiment of the invention illustrated in FIGS. 1 and 2 in operation. [0016]
FIG. 4 is a detailed view of one preferred embodiment of the restriction in the secondary gas passage in the practice of this invention. [0017]
The numerals in the Drawings are the same for the common elements. [0018]

DETAILED DESCRIPTION

The invention will be described in detail with reference to the Drawings. [0019]
Referring now to FIGS. 1, 2, [0020] 3 and 4, primary gas 2 is passed within lance 1 through primary gas passage or channel 26 and then through a nozzle 20, preferably a converging/diverging nozzle, and then out from lance 1 through primary gas opening 11 to form primary gas jet or coherent gas jet stream 30. Typically the velocity of the primary gas jet is within the range of from 500 to 3000 feet per second (fps). Preferably the velocity of the primary gas jet is supersonic when it is formed upon ejection from the lance face and remains supersonic for a distance of at least 20 d where d is the exit diameter of nozzle 20. Although the Drawings illustrate an embodiment employing only one primary gas jet or coherent gas jet injected into a molten metal furnace from the lance, more than one primary gas jet may be injected from the lance in the practice of this invention. When more than one primary or coherent gas jet from the lance are employed, generally the number of such primary or coherent gas jets is within the range of from 2 to 6.
Any effective gas may be used as the primary gas in the practice of this invention. Among such gases one can name oxygen, nitrogen, argon, carbon dioxide, hydrogen, helium, steam and hydrocarbon gases. Also mixtures comprising two or more gases, e.g. air, may be used as the gas in the practice of this invention. [0021]
Secondary gas is passed through the lance and out from the lance through a plurality of secondary openings arranged on the [0022] lance face 5 around the primary opening(s) so that the secondary gas forms a gas envelope or gas shroud around and along the length of the primary gas jet(s) provided from the lance. The gas envelope combusts to form a flame envelope around and along the length of the primary gas jet(s). Preferably, as illustrated in the Drawings, two secondary gases, fuel and oxidant, are provided in this manner from the lance, and these two secondary gases mix and combust to form the flame envelope around and along the length of the primary gas jet(s).
Fuel and oxidant are provided from lance [0023] 1 from one or more sets of secondary openings. The fuel is preferable gaseous and may be any fuel such as methane or natural gas. The oxidant may be air, oxygen-enriched air having an oxygen concentration exceeding that of air, or commercial oxygen having an oxygen concentration of at least 90 mole percent. Preferably the oxidant is a fluid having an oxygen concentration of at least 25 mole percent. The fuel and oxidant may be passed out from one set of secondary openings as a mixture or may be passed individually as fuel and oxidant from respective secondary openings on the lance face. The embodiment of the invention illustrated in the Drawings is a preferred embodiment wherein, when the primary gas is oxygen, the fuel and oxidant are passed out from lance 1 through an inner ring or circle of secondary openings 9 for the provision of fuel and through an outer ring or circle of secondary openings 10 for the provision of oxidant. The diameter (ds) of each of the secondary openings, and thus the area for gas flow for each secondary opening, is smaller than the diameter (d) of each of the primary opening(s).
[0024] Fuel 3 is passed within lance 1 through inner annular passage 25 which is annular to primary gas passage 26, and oxidant 4 is passed within lance 1 through outer annular passage 27, which is annular to primary gas passage 26 and inner annular passage 25. Proximate the face 5 of lance 1, typically at a distance of at least 3 ds from the face 5 of lance 1, inner annular passage 25 communicates with a plurality of secondary passages 7 which each communicate with a secondary opening 9 on lance face 5, and outer annular passage 27 communicates with a plurality of secondary passages 8 which each communicate with a secondary opening 10 on lance face 5. In this way fuel passes through lance 1 through inner annular passage 25 and then through secondary passages 7 and out from the lance through secondary openings 9 while oxidant passes through lance 1 through outer annular passage 27 and then through secondary passages 8 and out from the lance through secondary openings 10. The fuel and oxidant passed out from the lance form a gas envelope around the primary gas jet(s) which combusts to form a flame envelope or flame shroud 33 around the primary gas jet(s) within the molten metal furnace. Flame envelope 33 around primary gas stream 30 serves to keep ambient gas from being drawn into the gas stream 30, thereby keeping the velocity of gas stream 30 from significantly decreasing and keeping the diameter of the gas stream 30 from significantly increasing, for at least a distance of 20 d from the nozzle exit. That is, the flame envelope or flame shroud serves to establish and maintain gas stream 30 as a coherent jet for a distance of at least 20 d from the nozzle exit.
Due to the relatively small size of the secondary openings, which typically have a diameter within the range of from 0.15 to 0.75 inch, the secondary opening may be partially or even totally blocked by material, such as a molten metal or slag, from within the molten metal furnace. This may occur by globs of molten material being pasted over some holes. It may also occur due to pressure surges in the furnace which can create a momentary reverse flow in the auxiliary gas holes. This carries small molten particles into the holes, and these deposit on the walls of the holes, gradually closing them off. Such partial or total blockage vitiates the integrity of the flame envelope thus reducing its efficacy which has a deleterious effect on the coherent primary gas jet(s) passed out from the lance. [0025]
The pressure to push away a plug at the exit of an auxiliary gas hole, or to prevent a reverse flow due to a pressure surge in the furnace, can only go as high as the pressure in the [0026] annular feed channels 25 and 27. This may be only a few pounds per square inch. In order to provide a higher pressure for this purpose each secondary passage has a restriction which reduces the gas flow area at the restriction so that it is less than the gas flow area of the secondary opening with which that secondary passage communicates. The effect of this is to require a higher pressure in the annular gas channels and at the entrance to the restrictions in order to maintain the design flow. The available pressure in the auxiliary gas holes is correspondingly increased to provide more pressure to push away plugs and to resist pressure surges in the furnace.
Generally the gas flow area at the restriction within the secondary passage is from 20 to 80 percent of the gas flow area of the secondary opening with which that secondary passage communicates. The secondary gas passing through the restrictions generally requires 50 to 1000 percent higher inlet pressure (compared to the pressure required to achieve this flow without the restrictions) to achieve the designed flow through the restrictions and subsequent secondary openings. Even higher pressures can be advantageously used, but may not generally be available. This serves to reduce the occurrence of secondary opening plugging since, in the event that one or more of the secondary openings becomes blocked by metal or slag splash or other foreign matter, the 50 to 1000 percent higher pressure in the [0027] annular passages 25 and 27 is better able to dislodge and clear the blockage or prevent it from occurring in the first place.
The downstream termination of the restriction within each secondary passage is preferably located at a distance of at least 3 ds upstream from the exit openings of [0028] secondary openings 9 and 10 to allow the secondary gas to expand and reestablish fully developed flow conditions with pressure and velocity that are characteristic of the secondary passage without a restriction. One convenient location for restrictions 35 and 36 is shown in FIGS. 2 and 3 as at the entrance of the secondary passages. If this location will result in a hole length substantially greater than 5 ds, it is desirable to locate the restrictions within the hole about 5 ds from the exit, as shown in FIG. 4. The smaller volume downstream of the restriction will result in a faster pressure buildup to rapidly resist plugging and reverse flow.
Although the invention has been described in detail with reference to a certain preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims. [0029]

Claims

1. A lance comprising a lance face having at least one primary opening for passing at least one gas jet out from the lance, and a plurality of secondary openings arranged around the primary opening(s) on the lance face, each secondary opening communicating with a passage for passing secondary gas within the lance to the respective secondary opening, and each said passage having a restriction therein so that the area for gas flow at the restriction is smaller than the area of the secondary opening with which that passage communicates.

2. The lance of claim 1 wherein the area for gas flow at the restriction is within the range of from 20 to 80 percent of the area of the secondary opening.

3. The lance of claim 1 wherein the plurality of secondary openings are arranged in at least one ring around the primary opening(s).

4. The lance of claim 1 wherein the plurality of secondary openings are arranged in two rings, an inner ring and an outer ring, around the primary opening(s).

5. The lance of claim 4 wherein the passages communicating with the secondary openings arranged on the inner ring communicate with an inner annular passage within the lance, and the passages communicating with the secondary openings arranged on the outer ring communicate with an outer annular passage within the lance.

6. The lance of claim 1 wherein the downstream termination of each restriction is a distance of at least 3 ds from the lance face where ds is the diameter of the secondary opening corresponding to that restriction.

7. A method for providing at least one gas jet from a lance into a molten metal furnace comprising:

8. The method of claim 7 wherein the gas envelope comprises a flame envelope.

9. The method of claim 7 employing one primary gas jet.

10. The method of claim 7 employing from 2 to 6 primary gas jets.

11. The method of claim 7 wherein the primary gas jet(s) comprises oxygen.

12. The method of claim 7 wherein the primary gas jet(s) has a supersonic velocity.

13. The method of claim 7 wherein the secondary gas pressure at the entrance to the restriction exceeds the secondary gas pressure at the secondary opening by at least 50 percent.