US3809524A

US3809524A - Injection of liquid fuels into shaft furnaces

Info

Publication number: US3809524A
Application number: US00269556A
Authority: US
Inventors: P Bruhlet; G Quillet
Original assignee: Individual
Current assignee: Compagnie Francaise de Raffinage SA; Wendel Sidelor SA
Priority date: 1971-07-08
Filing date: 1972-07-07
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: NL7208930A; AU4427572A; LU65671A1; BE785612A; ZA724643B; GB1356218A; SU524530A3; DE2232637B2; DD98998A5; AT320997B; AU470953B2; NL159141B; SE376018B; IT956988B; PL75754B1; JPS5223964B1; DE2232637A1; CA975556A; CS188879B2; FR2145089A5

Abstract

Method and apparatus capable of atomizing and mixing large, even stoichiometric, amounts of liquid fuel (with or without oxygen) in the blast of tuyeres supplying a shaft furnace without ignition of the mixture prior to reaching the furnace, accomplished by atomizing the fuel by injection into a blast having a speed of between mach 0.3 and mach 1 and thereafter reducing the speed of the blast to velocities acceptable for supplying the furnace requirements, before ignition of the fuel can occur, by constantly increasing the cross-section of the tuyere conduit in a smooth curve to give a flow path without recirculation streams. In a further aspect of the invention, any generatrix of the conduit is wholly convex with the tangent at the discharge end forming an angle of at most 15* to the axis of generatrix.

Description

Bruhlet et al.

[451 May 7, 1 974 INJECTION OF LIQUID FUELS INTO SHAFT FURNACES [75] Inventors: Paul Bruhlet, Nilvance; Gerard Quillet, Thionville, both of France [73] Assignee: Compagnie Francaise de Raffinage,

Paris, France 22 Filed: July 7, 1972 211 Appl. No.: 269,556

[30] Foreign Application Priority Data- FOREIGN PATENTS OR APPLICATIONS 537,892 5/1922 France 239/433 154,674 5/l95l Australia ..239/433 Primary Examiner-Carroll B. Dority, Jr. Attorney, gent, or F irm-Curtis, Morris & Safford; A. Thomas S. Saffqrd 57 ABSTRACT Method and apparatus capable of atomizing and mixing large, even stoichiometric, amounts of liquid fuel July 8, 1971 France 71.25093 (with or without oxygen) in the blast of tuyeres supplying a shaft furnace without ignition of the mixture [52] US. Cl 431/181, 75/42, 266/30, prior to reaching the furnace, accomplished by atom- 266/ 9/433 izing the fuel by injection into a blast having a speed [51] Int. Cl. F23c 5/08 of between machO 3 and mach l and thereafter rel Field-0f Search ducing the speed of the blast to velocities acceptable 239/4 4.5; 431/ 181, 75/42 for supplying the furnace requirements, before ignition of the fuel can occur, by constantly increasing the References Cited cross-section of the tuyere conduit in a smooth curve UNITED STATES PATENTS to give a flow path without recirculation streams. In a 290,343 12/1883 Morgan et al. 75/42 furthe? aspect 0f the i any generatrix W 966,704 8/1910 'Pickles 239 433 condult 1S Wholly Convex with the Hinge"t a! the 3,558,! 19 'l /1971 Demalander 75/42 a ge end form ng an angle of at most 15 to the axis 3,596,894' 8/1971 Duthion et al 75/42 of generatrix. 3,608,881 9/197! Yordanov et al. 266/41 18 Claims, 3 Drawing Figures 6 w 77% D I, iiii z? 1 of D i A 4 X'- A X l INJECTION OF LIQUID FUELS INTO SI-IAFI FURNACES The present invention relates to the injecting of liquid fuels into a shaft furnace, and more particularly to the injection of fuel into a blast furnace.

Methods and devices are known for injecting fuel into the nozzle or tuyere of a shaft furnace in which the fuel is injected into the blast via injectors or blowpipes under a relatively high pressure. The average size of the injected droplets of fuel is generally too large and the combustion becomes defective when the rate of the injection of the fuel is increased.

It has been attempted to increase the fuel injection rate up to stoichiometric conditions and even beyond, while retaining good combustion, by atomizing the fuel in the form of fine droplets and partially or totally burning the atomized fuel, mixed with the blast, in the nozzle or the tuyere.

A process is known in which the combustion also takes place in the nozzle or tuyere, the holding of the I flame being assured in this process by a physical obstacle or by a sudden widening which creates a recirculation zone which captures and stabilizes the flame (French Patent 1,558,425 or its two patents of addition corresponding to US. Pat. No. 3,596,894).

Another process is known in which the fuel is burned in the nozzle or tuyere, which forms a combustion chamber, where the holding of the flame is assured by a prior rotating of the blast (French Patent 1,559,679).

In addition, a process is known in which the fuel is injected into a tuyere, preferably in the formof a Laval tuyere. The fuel, previously atomized by an'auxiliary gas, is injected therein into the blast, in a convergence. The convergence isfollowed by a divergence of linear profile, the'half angle of which is between and 7.5, and preferably between 0 and 35. This process has the purpose of imparting to the blast-fuel mixture at speed of departure from the tuyere which is equal to or greater than mach 1 so as to permit the penetration of said mixture to the center of the blast furnace (Belgian Patent 39,431).

Furthermore,-other processes are known in which the fuel is burned in a reactor outside the shaft furnace, the burned gases in thiscase being injected through the tuyeres into the hearth.

Some of these last-mentioned processes make it possible to employ fuel injection rates which reach or exceed stoichiometric conditions with good conditions of combustion. On the other hand, they have the drawback of causing considerable variations in the impulse" of the gases entering-the hearth, that is to say, the amount of movement, as a function of the variations in the amount of fuel injected. These variations in impulse cause changes in operation in the shaft furnaces, which changes, if they cannot be corrected, may result in substantial disturbances when one is compelled to vary the fuel injection rates substantially with time.

An object of the present invention is to avoid these substantial variations in the impulse of the gases entering the hearth as a function of the amount of fuel injected, which may vary between zero and stoichiometry and even beyond, while assuring the best conditions of combustion. .that is to say, reducing the formation of carbon soot to a minimum.

This result is obtained by burning at the nose (i.e., the discharge end) of the tuyeres a "blast-fuel mixture prepared under special conditions and by avoiding any combustion of this highly inflamable mixture before it arrives at the nose of the tuyeres.

For this purpose, an embodiment of the present invention is a process for introducing through the tuyeres into the hearth of a shaft furnace, a blast mixture, whether with added oxygen or not, and large quantities of atomized liquid fuel, in which process:

liquid fuel isatomized by the blast at a speed of be tween mach 0.3 and mach l;

mixture is transported to the nose of the tuyeres within a time which is less than the ignition time of the said mixture;

the speed of the mixture is constantly decreased towards the nose of the.tuyeres;-

the said decrease in speed is assured by a conduit a constantly increasing cross-section;

recirculation flows of the said mixture are avoided so as to prevent any ignition, said result being obtained by creating a smooth flow path through a conduit of constantly increasing crosssection with a curved profile, this curved profile being possibly extended by a linear profile.

Another embodiment of the invention consists of a shaft furnace tuyere for the carrying out of said process, in which:

the feed conduit for the atomized liquid fuel/blast mixture has the shape of a curved diffuser any generatrix of which is a curve the equation of which has a second derivative which is always positive;

a line tangent to the end of any curved generatrix forms with a line tangent to the point of origin of said generatrix an angle at most equal to 15, and preferably between 10 and 12.

A further embodiment of the invention is a shaft furnace tuyere for the carrying out of the said process, in

which:

the feed conduit for the atomized liquid fuel-blast mixture has the shape of a curved diffuser any generatrix of which is a curve whose equation has a second derivative which is always positive, the said curved diffuser being extended by a linear diffuser;-

the generatrix of the linear portion forms with a line tangent to the origin of the curved generatrix with which it connects, an angle at most equal to 15, and preferably between 10 and 12;

a line tangent to the end of any curved generatrix forms with the line tangent to the origin of said generatrix an angle at most equal to 15 and preferably between 10 and 12.

By means of the present invention, the conditions of flow of the fuel injection blast, and in particular the amount of impulse of the mixture at the nose of the tuyere and the losses in pressure in the blast feed conduit, are not substantially changed as compared with a blowing of the shaft furnace without fuel injection, whatever the amount of fuel injected.

The invention furthermore makes it possible to vary the rate of injection without modifying the diameter at the nose of the tuyeres. ln particular, it makes it possible to inject. under good conditions, very large quantities of fuel into the shaft furnace and to do this with all the progressiveness desired.

Other characteristics and advantages of the description will become evident from the following detailed description in whichreference is had to the accompanying diagrammatic drawing. In said drawings, which are not of a limitative nature:

FIG. 1 is a cross-section through the inner profile of the blast feed conduit at the outlet into the shaft furnace in accordance with the invention;

FIG. 2 is a section through the same profile, the downstream part of which has'been modified in accordance with adifferent embodiment of the invention;

FIG. 3 is a longitudinal diagrammatic section through a blast furnace tuyere in accordance with the invention.

The invention consists in feeding to the nose of the tuyere a mixture of fuel and combustion-supporting agent, ready to be ignited and burned'intensely upon its arrival at the nose of the tuyeres, without prior ignition in the blast conduit and with a minimum cracking of the fuel.

In order to obtain good combustion, it is known that the fuel must be atomized and distributed as uniformly as possible in the blast.

In accordance with the invention, the atomization is effected economically in known manner by injection of the liquid fuel into the hot blast brought to high speed by passage through a portion of conduit of suitable section a.

Moreover, the distribution-of the liquid fuel in the hot blast improves increasingly rapidly in the direction of flow, the finer the atomization is. For this purpose, the speed of the blast in the injection zone must be at least equal to mach 0.3, which establishes the upper limit of the cross-section a." i r However, it is also necessary to avoid combustion of the atomized fuel mixed with the blast in front of the nose of' the tuyere so as to prevent the impulse of the jet entering the furnace from varying with the amount of fuel injected. For this purpose one could contemplate effecting this atomization very close to the nose. However, the velocity of the blast, upon its entrance into the hearth, is a function of the operating conditions of the shaft furnace of which-it constitutes one of the adjustment parameters. Now, in general, the speeds of introduction of the blast used are definitely less than the speeds which assure good atomization. The outlet cross-section A"-at the nose of the tuyeres, which is imposed by the operating conditions of the shaft furnace, is therefore generally larger. than the cross section a previously determined.

Inorder to pass from speeds of more than about mach 0.3, which assures good atomization, to the lower speeds imposed by the operation of the apparatus, the invention employs a conduit of constantly increasing cross-section which connects the injection zone to the nose of the tuyere.

The mixture of fuel and combustion supporting agent must be transported to the nose of the tuyere within a time which is at most equal to the time which would be necessary for the droplets of fuel to ignite by progressive heating in contact with the hot blast. This maximum time of stay is equal to the ignition time. In accordance with the characteristics ofthe blast and the fuel (temperature, rate of flow, etc.), under the different operating conditions of the shaft furnace, this time of stay leads to limiting the length fL between the injection zone and the nose of the tuyere. We shall designate by L.- the critical value of this length; LJ is therefore the maximum value of L under given conditions. I

' In order to avoid the ignition of the atomized fuel mixed with the blast before it reaches the nose of the tuyere, it is also necessary to avoid within the conduit of increasing cross-section anything which might cause said ignition by'creating recirculation streams, or any phonomenon presenting the same drawbacks, and in particular the appearance of a shock wave. In order to avoid this last-mentioned phenomenon, the speed of the blast must at all times remain subsonic. The crosssection a of the blast feed conduit in the injection zone is therefore determined in such a manner thatthe speed of the blast there is between about mach 0.3 and about mach 1, with due consideration of the maximum characteristicsof the blast which are capable of being used (rate of flow, temperature, pressure, etc.).

The conduit of increasing cross-section whose dimensions have just been determined must furthermore have an overall shape such as not to create at any point of the flow of the fluid at the wall points of particular loss of pressure where recirculation streams could appear. It is necessary to reduce the risk of ignition in the boundary layer of the stream of blast flowing in said conduit.

In practice, the limitations imposed by the ratio a/A of the upstream and downstream cross-sections and the maximum possible length L for this conduit of increasing cross-section do not make it possible to impart to it the shape of a traditional divergent cone with sufficiently small angle to avoid any separation. For this reason, the invention proposes imparting to said conduit the shape of a curved diffuser permitting laminar flow at the wall.

Referring to FIG. 1, the hot blast is fed through the conduit 1 of cross-section a at a speed of between mach 0.3 and mach The liquid fuel is injected into the zone 2 via one or more injectors which feed the fuel in sheets substantially perpendicular to the flow of the blast. The mixture of blast and atomized fuel is then guided by the curveddiffuser 3 up to the nose of the tuyere 4 which debouch es into the furnace 5.

The upstream cross-section a'of the diffuser 3 is the cross-section of the blast conduit in-the injection zone 2; the downstream cross-section A of the diffuser 3 is that of the nose of the tuyere 4 upon discharge into the shaft furnace. The length L is at most equal to the length L as determined previously, that is to say, the maximum distance permitted between the injection zone and the nose of the tuyere. If the length of the curved diffuser is designated by A, then L is equal to X when the injection is effected at the entrance to the diffuser. L is greater than A when the injection is effected upstream of the entrance to the diffuser. The generating curves of this diffuser 3 are curvilinear and characterized by equations whose second derivative is always positive. Each generatrix has its origin a point B located on the connecting line between the point of origin of the diffuser and the blast feed conduit and passes through a point D of the downstream section of the diffuser which is imposed by the outlet cross-section of the tuyere.

If the envelope surface of the fuel which has diffused from the injection zone 2 is inscribed within the diffuser 3, there will be no fuel on the inner wall of the diffuser, which wall will in practice be protected by a thin layer of blast, avoiding the dangers of ignition in the boundary layer.

If this envelope surface encounters the wall of the diffuser 3 before the end, there will be observed a slight trickling of liquid fuel on the inner wall of the diffuser. However, in order that the danger of ignition remain small in this boundary layer, despite the reduced speed of flow (the velocity gradient varies very rapidly in the vicinity of the wall), the negative temperature gradient in this boundary layer is increased by intense cooling of the wall of the diffuser, and any possibility of catching of the flame on the wall is eliminated by a very smooth surface of this wall. I

In either of these hypotheses, the boundary layer must remain very thin. As a matter of fact, it generally creates a heterogeneity of distribution of the fuel in the blast, which heterogeneity must be reduced, particularly when the amount of fuel injected is high and close to stoichiometric.

Furthermore, there is a limit angle of opening of the diffuser 3 beyond which there takes place within the boundary layer a turbulent flow which favors the igniting of the mixture before it arrives at the nose of the tuyere. Atevery point on a curved generatrix, the angle a defined by the tangent at this point and the tangent at the origin must remain less than half of the limit angle. In particular, the angle a as shown in FIG. 1, defined by the lines tangent to the points B and D must be at most equal to I5", and preferably be between and 12.

If, for the dimensions of the diffuser and the shape of the generatrices contemplated, the half angle of opening .at the point D must be greater than this limit, the curvilinear generatrix BD of the diffuser is then replaced by a curvilinear generatrix BD extended by a straight line DD, tangent at D to the generatrix BD and forming with the tangent at B to BD an angle equal to said limit angle, as indicated at 6 in FIG. 2. The point D can be determined since it is common to the curvilinear generatrix BD and to the straight line DD and the tangent at this point to the generatrix BD is identical with the straight line DD.

The liquid fuel, mixed with the blast, which is guided by the diffuser 3 in which it cannot ignite, arrives at the nose of the tuyere where it ignites and burns very rapidly under the normal operating conditions of the furnace. The variation in impulse of the injected gases as compared with a conventional injection does not result in any disturbances in the operation of the furnace.

In one particular case, the equation of each curvilinear generatrix may advantageously be of the form:

(1 the abscissa axis X'X being the axis of flow and the ordinate axis being an axis perpendicular to the preceding one in the plane determined by the generatrix and the axis of the flow.

, A: length of the curved part of the diffuser;-

Y ordinate of point D;

Y,;: ordinate of point B.

In this case, the pressure gradient remains constant in the direction of flow and the losses in pressure are minimal.

The present invention has been reduced to practice on a tuyere having the following characteristics:

Diameter at the nose which tuyere was used on a blast furnace having the following characteristics:

Diameter of the hearth 8.80 m Number of tuyeres 20 Average temperature of the blast Average rate of flow of the blast without additional oxygen 6,000 m NTP/hr In FIG. 3, which shows the blast feed device in the hearth of the blast furnace, the tuyere 7 which has the characteristics enumerated above is cooled in known manner. It has a water chamber 8 with one or more water inlets 9 and one or more outlets 10.

The blast conduit of the tuyere 7 having the axis X'X consists from upstream to downstream of a succession of portions of conduits of revolution around the axis X'X. The upstream frustoconical portion is of convergent form 13. The area of the inlet cross-section of the convergent portion 13 is equal to that of the cross section of the conduit of the nozzle 11 and the area of its downstream cross-section is equal to that of the cross-section of the cylindrical portion of a diameter of 1 10 mm forming the neck 12 which follows it. With due consideration of the requirements of manufacture, positioning, placing of the injection pipe, cooling, etc., there is sought a convergent shape 13 and a length of the neck 12 which result in minimum losses in pressure. The neck 12 has a length of2 I0 mm. The convergence I3 and the neck 12 improve the stabilization of the flow of the blast.

The area of the cross-section of the neck 12 is smaller than that of the conduit of the nozzle II so as to increase the speed of the blast in order to assure a satisfactory pneumatic atomization of the fuel by the blast. The speed of the blast is then from 350 to 400 m/sec.

h9 ne s. stens qfilown tbya di fuse which is initially curved at 3 over a length k 252 mm and then linear at 6 over a length of 38 mm. Since the injection in this case is effected at the entrance to the diffuser, the total length of the diffuser is equal to the length L. The upstream cross-section of the diffuser is circular and its diameter is equal to that of the neck 12. Its downstream cross-section is also circular, with a diameter equal to that of the nose of the tuyere 7.

As the directrices of the curved portion of the diffuser are in this case circular, all the generatrices are represented by a single equation. This equation, by application of formula I, then becomes:

+[(R/ l[ x/A] 2) in which R represents the radius of the downstream section of the curved portion of the diffuser and r the radius of the upstream section.

As this diffuser has a linear end part, the radius R is therefore not the radius of the nose of the tuyere at the point D, but that of the blast conduit at the point D,

and )1 represents the abscissa of this point D.

R 75.5 mm

r 55 mm I The straight line DD forms an angle of 1 1 with the tangent to B' (which in this case is parallel to the axis X'X).

Under conditions of constant production, or a circular blast with a temperature of 950C and a blast containing 25 percent additional oxygen, the production of liquid fuel added being close to stoichiometry, namely about 535 1/hr. per tuyere of heavy'fuel oil No. 2, the fuel mixed with the blast does not ignite within the tuyere but rather directly at the nose of the tuyere in the furnace. The variation of the impulse is very light and does not disturb the operation of the furnace. The com-.

bustion is much more intense and complete than with the customary injection devices. We claim: 1. In a method for blowing shaft furnaces through at least one tuyere with a blast capable of containing large quantities of atomized liquid fuel, the improvement for maintaining a substantiallyconstant impulse at the outlet of the tuyere, whatever the amount of fuel injected, with rapid combustion of the mixture atthe nose of the tuyere, said improvement comprising the steps of:

atomizing the liquid fuel in a stabilized blast in an injection zone where the speed of said blast is between mach O.3 and mach l; conducting the resulting fuel-blast mixture to the nose of the tuyerewithina time which is less than v the ignition time of the said mixture;

constantly and smoothly decreasing the, speed of the mixture towards the nose of the tuyere to the desired furnace-blowing velocity by constantly increasing the cross-section of the flow path along a curved diffuser profile the interior surface of which -is always convex and which profile at one end connects to the injection zone and at its other end extends towards the nose of the tuyere and the tangent of which profile at said other end connects to said nose, which profile avoids recirculation steams. I

2. Method according to claim 1 wherein the tangent to said curved profile has finite dimensions such that the said flowpath is extended at the discharge end by a linear profile.

3. Method according to claim 1 wherein said flow path has an outer boundary whose shape is wholly defined by a generatrix the equation for the curve of which has a second derivative which is always positive.

4. A method according to claim 2 wherein the curved diffuser profile portion of said flow path has an outer boundary hose shape is defined by a generatrix the equation for the curve of which has a second derivative which is always positive.

5. Method according to claim 3 wherein the tangent to the end of any curved generatrix forms an angle of at most 15 with the tangent to the origin of said generatrix.

6. Method according to claim 4 wherein the generatrix of the linear flow path boundary is tangent to the curved generatrix with which it connectsand forms an angle of at most 15 with the tangent to the originof said generatrix.

7. Method according to claim 5 wherein said angle is between 10 and 12.

8. Method according to claim 6 wherein said angle is between 10 and 12.

9. In a shaft furnace tuyere having a discharge conduit of fixed shape with a narrowed inlet at a fuel injection zone and an enlarged outlet nose and means for injecting and atomizing variable amounts of liquid fuel into the blast flowing through said zone and into said discharge conduit, an improvement in said discharge conduit comprising means for supplying through said injection zone a substantially stabilized blast at a speed of between mach 0.3 and 1, a curved diffuser with a constantly and smoothly increasing cross-section throughout its length from said inlet and extending tangentially at its other end towards said discharge outlet nose to flow said blast within said conduit without any recirculation andto deliver it at said discharge outlet nose at conventional furnace blowing velocities, said curved diffuser having a shape the generatrix of which is a curve whose equation has a second derivative which is always positive, and the axial distance from said inlet to said outlet nose being no greater than L 10. In a tuyere according to claim 9, the improvement further comprising a tangential linear extension to said diffuser.

11. In a tuyere according to claim 9, the improvement further comprising the tangent to the end of any curved generatrix forming an angle of at most l5 with the tangent to the origin of said generatrix.

12. In a tuyere according to claim 10,- the improvement further comprising the generatrix of the linear part being tangent to the curved generatrix with which it connects and forming an angle of at most 15 with the tangent to the origin of said curved generatrix.

13. In a tuyere according to claim 11 the improvement further comprising the angle being between 10 and 12.

14. In a tuyere according to claim 12, the improve ment further comprising the angle being between 10 and 12. g

15. A tuyere according to claim 11, wherein the generatrix of the curved diffuser is defined by the following equation:

= radius of the discharge end of. the curved difat least 0.3 mach in the injection zone to wfithin acceptable furnace blowing velocities at said nose,

r radius of the inlet of the diffuser at the injection zone,

)1 the axial length of the curved diffuser, and

A s L,., wherein L is the maximum length of the tuyere as measured from the injection zone to the nose of the tuyere which for said given furnace, tuyere, and blast means conducts the fuel atomized in said blast to the nose of said tuyere before ignition.

16. Method according to claim 3 wherein the generatrix of the curved diffuser profile is defined by the following equation:

Y=R {1 [(R/r) 4 -11 [l-x/M wherein R radius of rotation of the generatrix at the discharge end of the curved diffuser profile portion, of the flow path,

blast to the nose of said tuyere before ignition. 17. In a tuyere according to claim 9 wherein the said other end of said curved diffuser terminates at said outlet nose without any linear extension.

18. Method according to claim I wherein the curved profile terminates at said other end at said nose.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,809,524 t d May 7, 1974 Inventor) Paul Bruhlet et a1.

It is certified that error appears 1T1 the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 4 (Column 7), line 56 "hose" should be -whose--.

Signed and sealed this 24th day of September 1974.

(SEAL) Attest: McCOY M. GIBSON JR. c. MARSHALL D ANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 u.s GOVERNMENT PRINTING OFFICE: Ins 0-366-33.

FORM PO-105O (10-69)

Claims

1. In a method for blowing shaft furnaces through at least one tuyere with a blast capable of containing large quantities of atomized liquid fuel, the improvement for maintaining a substantially constant impulse at the outlet of the tuyere, whatever the amount of fuel injected, with rapid combustion of the mixture at the nose of the tuyere, said improvement comprising the steps of: atomizing the liquid fuel in a stabilized blast in an injection zone where the speed of said blast is between mach 0.3 and mach 1; conducting the resulting fuel-blast mixture to the nose of the tuyere within a time which is less than the ignition time of the said mixture; constantly and smoothly decreasing the speed of the mixture towards the nose of the tuyere to the desired furnace-blowing velocity by coNstantly increasing the cross-section of the flow path along a curved diffuser profile the interior surface of which is always convex and which profile at one end connects to the injection zone and at its other end extends towards the nose of the tuyere and the tangent of which profile at said other end connects to said nose, which profile avoids recirculation steams.

2. Method according to claim 1 wherein the tangent to said curved profile has finite dimensions such that the said flow path is extended at the discharge end by a linear profile.

5. Method according to claim 3 wherein the tangent to the end of any curved generatrix forms an angle of at most 15* with the tangent to the origin of said generatrix.

6. Method according to claim 4 wherein the generatrix of the linear flow path boundary is tangent to the curved generatrix with which it connects and forms an angle of at most 15* with the tangent to the origin of said generatrix.

7. Method according to claim 5 wherein said angle is between 10* and 12*.

8. Method according to claim 6 wherein said angle is between 10* and 12*.

9. In a shaft furnace tuyere having a discharge conduit of fixed shape with a narrowed inlet at a fuel injection zone and an enlarged outlet nose and means for injecting and atomizing variable amounts of liquid fuel into the blast flowing through said zone and into said discharge conduit, an improvement in said discharge conduit comprising means for supplying through said injection zone a substantially stabilized blast at a speed of between mach 0.3 and 1, a curved diffuser with a constantly and smoothly increasing cross-section throughout its length from said inlet and extending tangentially at its other end towards said discharge outlet nose to flow said blast within said conduit without any recirculation and to deliver it at said discharge outlet nose at conventional furnace blowing velocities, said curved diffuser having a shape the generatrix of which is a curve whose equation has a second derivative which is always positive, and the axial distance from said inlet to said outlet nose being no greater than Lc.

10. In a tuyere according to claim 9, the improvement further comprising a tangential linear extension to said diffuser.

11. In a tuyere according to claim 9, the improvement further comprising the tangent to the end of any curved generatrix forming an angle of at most 15* with the tangent to the origin of said generatrix.

12. In a tuyere according to claim 10, the improvement further comprising the generatrix of the linear part being tangent to the curved generatrix with which it connects and forming an angle of at most 15* with the tangent to the origin of said curved generatrix.

13. In a tuyere according to claim 11 the improvement further comprising the angle being between 10* and 12*.

14. In a tuyere according to claim 12, the improvement further comprising the angle being between 10* and 12*.

15. A tuyere according to claim 11, wherein the generatrix of the curved diffuser is defined by the following equation: Y R ( 1 + ((R/r) 4 -1) (1 - x/ lambda ) ) 1/4 wherein R radius of the discharge end of the curved diffuser, which R is large enough to reduce a blast of at least 0.3 mach in the injection zone to within acceptable furnace blowing velocities at said nose, r radius of the inlet of the diffuser at the injection zone, lambda the axial length of the curved diffuser, and lambda < or = Lc, wherein Lc is the maximum length of the tuyere as measured from the injection zone to the nose of the tuyere which for said given furnace, tuyere, and blast means conducts the fuel atomized in said blast to the nose of said tuyere before ignition.

16. Method according to claim 3 wherein the generatrix of the curved diffuser profile is defined by the following equation: Y R ( 1 + ((R/r) 4 -1) (1- x/ lambda ) ) 1/4 wherein R radius of rotation of the generatrix at the discharge end of the curved diffuser profile portion, of the flow path, r radius of rotation of the generatrix at the injection zone end of the curved diffuser profile portion of the flow path, lambda axial length of the curved diffuser, and lambda < or = Lc, wherein Lc is the maximum length of the tuyere as measured from the injection zone to the nose of the tuyere which for said given furnace, tuyere, and blast conducts the fuel atomized in said blast to the nose of said tuyere before ignition.

17. In a tuyere according to claim 9 wherein the said other end of said curved diffuser terminates at said outlet nose without any linear extension.

18. Method according to claim 1 wherein the curved profile terminates at said other end at said nose.