THEORETICAL AND EXPERIMENTAL STUDY OF SUPERSONIC OXYGEN JETS - INDUSTRIAL APPLICATION IN EAF B.Allemand (Air Liquide), P.Bruchet (Air Liquide), C.Champinot (Air Liquide), S.Melen (Air Liquide), F.Porzucek (SAM Neuves-Maisons) Introduction Oxygen use in EAF has increased a lot in the past few years and the trend will continue. Recent survey made by M. Birat 1 from IRSID shows that average oxygen consumption was around 30 Nm3/t in EAF in 1998 and that this consumption will reach 40 Nm3/t in 010. An other trend in EAF is to lower nitrogen level in EAF (to deliver a better quality steel); to achieve this, two points must be improved: Reduction of air entries in the furnace by trying to close partly the slag door Better foamy slag practice to avoid contact between air and liquid steel. Those two trends explain the emergence of new technologies for delivering oxygen into the steel bath. Recently, a number of technologies have been proposed with stationary wall mounted lances which can deliver oxygen at supersonic speeds to the molten steel bath. The novel feature of these lances is that the oxygen stream resists divergence as it is traveling in the furnace to the bath. The motivations to the steelmakers for using these lances in an EAF are as follows: Wall mounted lances allow decarburization to occur with the slag door closed. This should reduce the air ingress into the furnace, and save electrical energy. Injection is possible in several different locations. This can speed up the decarburization process and increase the homogeneity of steel bath temperature and chemistry. Lower maintenance costs are recognized because there are fewer or no moving parts associated with the wall-mounted injector. Wall mounted injectors greatly eliminate the need for replacing water-cooled lance tips or consumable pipe. A conventional lance manipulator contains many moving parts, to swing into position and extend and extract the lance, which needs heavy maintenance. An intensive research program on specific Laval nozzle design and jet behavior in EAF conditions has been developed by Air Liquide Research and Development division. In particular, the coherency concept is properly defined. At the conclusion of this program, Air Liquide proposes to the steelmakers, its new technology ALARC-JET, consisting of a water-cooled and wall-mounted injector equipped with an optimized supersonic nozzle that is able to deliver a high impinging jet of oxygen into the liquid bath from a long distance. This paper will describe with details : - theoretical fundamentals of supersonic jets ; - laboratory experiments ; - specific behavior of supersonic jets in EAF conditions ; - characteristics of ALARC-JET injector technology ; - industrial benefits for steelmakers, including results in industrial furnace.
1. Theoretical study of Supersonic jets 1-1 Principle In steel industry applications, oxygen is usually injected under supersonic condition. Supersonic jets are preferred over subsonic jets to increase gas penetration into the molten bath. The main objective is to maintain supersonic conditions at the point where the oxygen jet impacts the molten bath. Turbulent mixing layer Coherent region (potential flow core) Transition region Region of fully developed flow Fig 1 : Schematic diagrams of jet flow showing potential flow core and velocity profiles at various distances from orifice A supersonic jet exiting a nozzle interacts with the surrounding still air to produce a region of turbulent mixing and dragging. This process results in an increase in the jet diameter and a decrease in the jet velocity with increasing distance from the nozzle 3. Consequently, the original force of the jet decreases (U jets nozzle = Force) with increasing distance. The mixing process in supersonic jets is dominated by large-scale coherent structures (Brow et Roshko 4, Papamoschou 5 et Viswanathn et Morris 6 ). The compressible effect reduces the mixing process and then increases the potential flow core length 7. Observations in both shear layers and jets have shown that when the Mach number increases, the rate of growth of the mixing layer decreases. Papamoschou and Roschko have proposed that convective Mach number is the proper parameter to correlate the effects of compressibility on turbulent mixing in free shear flows. This convective Mach number (M c ) is a function of Mach number (M i ) and density ratios. Mukunda et al 8 proposed to define the convective Mach number as : c M jet jet M jet M Equation 1 The best method of decreasing the rate of growth of the mixing layer is to increase the convective Mach number without increasing the velocity ratio (Viswanathan and Morris 9 ) between jet (U jet ) and ambient air (U ). The region where the initial force is constant is defined as the coherent region. The coherent length is the length of the potential flow core. In this region, the axial velocity and the Mach number are constant. The coherent zone length is defined when the mixing layer between still air and the jet reaches the axis of flow (see figure 1). There are different ways to estimate this length. Theoretically the best value is the turbulent kinetic energy but it is very difficult to measure by experiment. Lau, Morris and Fischer 10 proposed to use empirical formulation to estimate the coherent region length. The generalization of this empirical formulation is: L d p j 4. 1.1( M j T 1 T j a ) Equation where subscript j represents the isentropic expended state from exit pressure to ambient pressure. This expression is applicable only for air in air, at the same density. The main objective is getting the longer coherent region with supersonic speed. With this in mind, we started to study the Laval nozzle for internal and external characteristics of the jet flow.
1- Laval nozzle principle The theory of one-dimensional steady gas dynamics can be applied with some assumptions to the flow: short duct of small divergence and curvature the gas is a perfect gas, viscous stress is ignored energy loss and heat exchange are ignored. The supersonic nozzle generally consists of a low angle divergence with no curvature in addition to having a short duct. The gas velocity in the duct is too high to allow enough time for heat exchange to occur and therefore the motion of gas in the duct is an adiabatic process. On the other hand, the development of boundary layers will have little influence on the flow because friction forces have reduced the velocities in the boundary layer. Under these conditions, the motion of the gas in the nozzle can be considered as a isentropic process, and the equations of one-dimensional steady gas dynamics can be applied in the design of the supersonic nozzle. The change of speed, pressure, of cross-sectional area and density are easily established by Hugoniot equations : da ( M A d M VA constante da (1 M A M dv 1) V dv V ) dp P Equations 3, 4, 5, 6 where M is Mach Number (M=V/C, C is the speed of sound), is the ratio of specific heats, A is the crosssectional area, P is the pressure and V is the velocity. From these equations, it is possible to explain the main Laval nozzle principle 11 : If M < 1 ; In the subsonic motion of a gas, the speed of flow decreases with increasing crosssectional area and pressure. And conversely, the speed of flow increases with decreasing crosssectional area and pressure, which is the condition of convergent section in Laval nozzle. If M > 1 ; In the supersonic motion of a gas, the pressure increases with decreasing cross-sectional area and speed of flow. And conversely, the pressure decreases with increasing cross-sectional area and speed of flow, which is the condition of divergent section in Laval nozzle. If M = 1, then da = 0 ; It is a critical section or throat section in Laval nozzle where the velocity will achieve sonic speed. V = C. By applying these characteristics, a supersonic Nozzle can be designed to produce a gas jet with supersonic speed. The supersonic nozzle must be a kind of converging-diverging nozzle which consists of three sections : convergent (subsonic zone), throat (critical zone) and divergent (supersonic zone). 1-3 Design of ALARC-JET Just as pointed out above, the supersonic nozzle should consist of three sections: convergent, throat and divergent. However, in a real supersonic nozzle, there is a stable section before the convergent section in order to make the flow uniform with low turbulence 1.
R1 R Di b Dr a R3 Do Figure L1 Lconv Lr Ldiv L L1 : stable section Lconv : convergent section Lr : throat section Ldiv : divergent section L : exit section In order to produce an exit jet with high momentum, a good uniform and tidy boundary, as well as with low turbulence and energy loss, the dimensions of each section in supersonic nozzle need to be designed carefully and calculated precisely on the basis of gas dynamics. The quasi-one-dimensional theory gives no information about the proper contour of the duct. The method of characteristics provides a technique to properly design the contour of a supersonic nozzle for shock free, isentropic flow, taking into account the multidimensional flow inside the duct. In order to increase the coherent length, it is necessary to reduce the turbulence inside the jet. This turbulence may be created by variation of the section inside the duct or by the edge stresses (see figure 3). This property is characterized by the turbulent kinetic energy. Typically the tuyeres used in the industry are not particularly optimized, the internal turbulence is frequently very high. This reduces the injector efficiency. To improve the contour of the nozzle, computational fluid dynamics (software FLUENT 5) was used. The boundary conditions applied to the computational domain were : total pressure in the inlet side (at the convergent side) and ambient pressure far from the nozzle outlet (100*diameters) where the velocity become subsonic. The final objective was to get the tuyere configuration creating a jet as coherent as possible. Therefore the following values have been compared in order to evaluate the best tuyere : - evolution of the boundary layer inside the duct - level of turbulent viscosity of the exit jet - pressure waves inside the duct. Fig 3 commonly used tuyere creating a high turbulent jet Fig 4 :ALARC-JET design
The important design parameters of the nozzle are : - diameter of convergent, throat, divergent (Di, Dr, Do) - length of stable, convergent, throat, divergent and coherent section (L1, Lconv, Lr, Ldiv, L) - curvature radius at the different edges (R1, R, R3), - angles and. These parameters will be referred to as "tuyere geometry". They have been calculated in order to get a parallel uniform flow at the exit nozzle. From the usual simulated tuyere (see figure 3) to the optimized one (see figure 4), a rate of more 50 % in coherent length was gained. This study helped us to define the optimized design of the tuyere. Velocity (m/s) Standard jet ALARC-JET Fig 5.b : commonly used tuyere : Velocity field Velocity (m/s) Fig 5.a : x/d : exit jet distance/exit nozzle diameter Fig 5.c : ALARC-JET tuyere : Velocity field Both the conventional tuyere without any optimization (figure 5.b) and ALARC-JET with optimization (figure 5.c) were simulated with the same physical conditions and same diameter. Comparing the axial velocities of these two tuyeres (figure 5.a) shows the strong effect of the design on the coherence region length. 1-4 Supersonic free jet analyze and modeling Adapted and Non-adapted jets condition The performance of the supersonic free jet is very close to the aerodynamic behavior of the jet. When the design of the nozzle is fixed, the exit jet depends only on the total inlet condition (total temperature and total pressure). As a function of total inlet condition (pressure and temperature), the exit pressure Pe can be more or less close to ambient pressure (pressure in the furnace). There are three fundamental steady-flow regimes when a compressible gas is discharged from a passage into a region having a given pressure, as follows : 1. Adapted jets, where the passage exit pressure and ambient pressure are the same. Non-adapted under-expanded jets, where the passage exit pressure is higher than the ambient pressure 3. Non-adapted over-expended jets, where the passage exit pressure is lower than the ambient pressure. In the non-adapted jets, steady flow only occurs when the Mach number at the passage exit is sonic or supersonic. Pressure equalization is achieved in multidimensional external expansion or compression waves, and consequent discontinuities in density. The non-adapted jets can cause energy loss, which reduce the momentum
and produce turbulent flow in the jet exit, which shorten the coherent length. When the jet is adapted, an exit flow with high momentum, good uniformity and a tidy boundary is obtained. In conclusion, for a supersonic nozzle design to achieve a flow with long coherent length, when the shape of the nozzle is determined, the exit Mach number as well as the ratio between inlet pressure and the exit pressure are fixed. The exit pressure must be very closed to ambient pressure. Any greater deviation from the computed value of inlet pressure can result in a deterioration of the jet. Ambient density effects on jet growth rate To predict the effect of ambient density on the jet growth rate, a numerical simulation was realized. The result is presented below. For an adapted supersonic jet at Mach.1 with exit diameter of 7.7 mm : Case 1 : In cold furnace where the ambient temperature is Ta= 300K, and the exit jet temperature is Tj = 156 K - the ambient temperature Ta = 300K and the ambient density is = 1.14 kg/m 3, - the exit jet temperature Tj = 156 K and the exit jet density is 0 =.3 kg/m 3 The coherent region length is 1 times the equivalent diameter (figure 6.b) Case : In hot furnace where the ambient temperature is Ta = 000K, and the exit jet temperature is Tj = 156 K - the ambient temperature Ta = 000K and the ambient density is = 0.176 kg/m 3, - the exit jet temperature Tj = 156 K and the exit jet density is 0 =.3 kg/m 3 The coherent region length is 35. times the exit nozzle diameter (figure 6.c) Velocity (m/s) Fig 6.b : cold furnace; velocity field Velocity (m/s) Fig 6.a : axial velocity comparison between hot furnace simulation and cold furnace simulation Fig 6.c : hot furnace; velocity field As shown above, the effect of density changes by hot still air in a furnace has a very important benefits on the coherent region length. Coherency of the jet is significantly increased when the ambient temperature is high. New formula to predict supersonic coherent jet length As we have seen, the coherent region length can be approached for supersonic jets with the equation (see above) :
L d p j 4. 1.1( M j T j 1 ) T Equation a If we use this empirical formulation for the same supersonic jet in cold ambient temperature and in hot ambient temperature, we obtain a very poor comparison with computational fluid dynamic (CFD) : x/d Equation CFD Result Ta = 300 K 9.6 1 Ta = 000 K 10 37 A new modification for the empirical formulation is proposed here. Thring and Newby 13 first proposed an equivalent diameter to account for density differences between the jet and the ambient fluids in axisymetric subsonic jets without heat release. We propose to include this equivalent diameter in the equation as : L d p * T j 4. 1.1( M j 1 ) Equation 7 T with far field equivalent diameter : 0 d * a 0 1 d Equation 8 where d 0 is the exit nozzle diameter, 0 is the jet fluid density and is the ambient density. If we use this new formulation in the last example, the result is : x/d Equation Equation 7 CFD Result Ta = 300 K 9.6 13,4 1 Ta = 000 K 10 35,5 37 The confrontation between CFD result and the new analytical law shows very good agreement. It is now possible to estimate in hot furnace the coherent region length with this scaling law. - Parametric study Results from modelling as well as the experiment highlighted some parameters that may have an important influence on the jet of ALARC-JET expansion. These parameters are : the upstream pressure of the jet the ambient temperature the O temperature the flow rate. Changing the upstream pressure means keeping the flow rate constant, the ambient temperature constant and changing the Mach number and therefore the tuyere diameters. Mach number and upstream pressure are linked as well as tuyere diameters. Changing the O temperature will change the speed of sound, therefore in order to get the Mach number, it will be necessary to change the jet velocity and therefore the tuyere diameters. Changing the flow rate means keeping all other parameters constant except, of course the tuyere diameters in order to keep the same Mach number. The jet expansion has been characterised by the coherent length of the jet (see definition in 1-1). The table below describes the parameter variation :
Parameters : upstream pressure Ambient T O T Flow rate Tuyere dimension Mach number Variable fixed Fixed fixed variable variable Fixed variable Fixed fixed fixed fixed Fixed fixed Variable fixed variable fixed Fixed fixed Fixed variable fixed fixed One-dimension calculations have been performed to evaluate the effect of these parameters in an adapted tuyere thanks to the equation 1, presented in (1-4) and validated by the modeling : L d p * T j 4. 1.1( M j 1 ) Equation 7 T a d * 1 with 0 0 d Equation 8-1 Upstream pressure We wish to keep the flow rate and the ambient temperature constant. Changing the upstream pressure will automatically change the Mach number and therefore the tuyere diameters (convergent, throat and divergent). Coherent length Fixed flow rate (3500Nm3/h), fixed temperature (1873K).4 jet coherent length (m). 1.8 1.6 1.4 1. 1 1 3 4 Total pressure (10 5 Pa) Fig 7 : effect of upstream pressure on coherent length This curve shows that increasing the upstream pressure, increases the length of the coherent jet. However, gaining 0.m in coherent length means increasing the upstream pressure by 10 bars, which may be too impractical to achieve. - Ambient temperature The objective here is to keep everything constant, except the ambient atmosphere temperature in order to simulate the high temperature of the gases inside furnace. A supersonic jet is divided into main zones : - coherent region : constant velocity, no expansion - transition region and region of fully developed flow where there is aspiration of ambient gas into the jet, expansion of the jet, and the jet momentum is dispersed by a larger ambient gas quantity that will decrease the velocity of the jet.
With a hot atmosphere, the density of the ambient atmosphere is lower. Therefore the mass added to the jet will be lower and the velocity will decrease more slowly. This will lead to a more confined jet. This result has been observed with modeling as well as with experiment. The following figure shows how the jet coherent length increases with the temperature level in the furnace atmosphere due to a decrease in the density of gases around the jet: 1.8 Coherent length Fixed flow rate (3500Nm3/h), Fixed pressure(10bar) Coherent length (m) 1.6 1.4 1. 1 0.8 0.6 typical furnace temperature 0.4 30 530 1030 1530 030 Ambiant temperature (K) Fig 8 : jet coherent length according to ambient temperature. Some measurements have been made inside a furnace using a thermocouple close to the jet. Temperature between 1800 and 1900K have been measured. For an ambient temperature around 1900K the jet has coherent length around 1.6m for a flow rate of 3500Nm3/h (red square in Fig 8.). -3 O upstream temperature Calculations were made at the same flow rate and Mach number and the delivered oxygen temperature was increased. Since the speed of sound increases when temperature increases, it will be necessary to increase the jet velocity to keep the same Mach number, and therefore, increase the tuyere diameters. By increasing the jet temperature, the ratio between the ambient temperature and the jet temperature will decrease and therefore according to equation 1, the coherent length decreases. 1.8 Coherent length Fixed flow rate (3500Nm3/h), Fixed pressure(10bar) Coherent length (m) 1.6 1.4 1. 1 0.8 0.6 0.4 50 90 330 370 410 450 490 O temperature (K) Fig 9 : jet coherent length variation with O temperature
This curve shows that heating O will have a negative effect on the jet coherency. The red square on the curve represents the situation of oxygen at room temperature. -4 Flow rate Changing the flow rate means keeping all other parameters constant except, of course, the tuyere diameters in order to keep the same Mach number. Coherent length (m).1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 Coherent length Fixed ambiant temperature ( 1873K) Fixed pressure (10bars) 500 1000 1500 000 500 3000 3500 4000 4500 flow rate ( Nm3/h) Fig 10 : jet coherent length variation with flow rate Since the coherent length of the jet is directly proportional to the tuyere diameter, increasing the flow rate increases its coherent length. 3- Experimental Studies 3-1 Measurement equipment After a theoretical study which allowed us to - design an optimized tuyere, - define best parametric conditions for a specific jet requirement, - develop formula to predict jet length in various conditions and confirm the modeling study, experiments have been performed in Air Liquide research center pilot furnace to characterize supersonic jets. The characterization methods were : - laser visualization thanks to Mie scattering method and - velocity measurement with Pitot tube measurement. Mie scattering visualization The pilot furnace is 6 m long and m large and allows several possibilities of measurement tools and access. Moreover it is possible to change the ambient temperature inside the furnace. In order to visualize the expansion of the jet, the Mie scattering method has been used. This method consists of seeding a 300 Nm3/h ALARC-JET with ZrO particles. Then with a laser sheet and a video camera, it is possible to get images from the shape of the jet. The horizontal laser sheet enters furnace through side doors and the visualization is made using a video camera located on furnace roof.
Flue gas exit laser High T Furnace four Alice four Hades ALARC JET O Fig 11 : experimental set-up for laser visualization : furnace top view The light sheet is generated with pulsed Nd:YAG laser with a wavelength =53 nm, 90 mj/pulse in 10-8 s. Video recording was performed with monochrome CCD camera with a gating time = 1 ms. The size of the ZrO particle was around 1 micron. Pitot tube measurement The Pitot tube is commonly used in subsonic flow to measure velocity. However, it is rarely used in supersonic flows. A Pitot tube immersed in a supersonic flows will create a shock wave in front of its nozzle. Therefore the pressure measured by the Pitot is the stagnation pressure in back of the shock wave. The standard formula of supersonic flow may be applied to the pressure of the back of the upward shock wave 14. Shock wave Supersonic flow M1>1 P1 Subsonic flow M<1 P Fig 1: shock wave formation in front of the Pitot tube 3- Results Different experimental configurations have been tested : Influence of oxygen pressure variation on the shape of the jet in cold atmosphere The supersonic nozzle ALARC-JET was tested. It has been designed to deliver an adapted jet at Mach.1 for an oxygen pressure of about 9.1 bars. However, if the inlet pressure of oxygen is lowered, the jet can no longer be considered adapted. Results show that the further the pressure is below 9.1 bar, the greater the cross-sectional area of the jet becomes. Fig 13 : : jet visualized in cold ambient temperature for non-adapted tuyere (Po=3bar)
Fig 14 : jet visualized in cold ambient temperature for adapted tuyere (Po=9.1 bar) Influence of hot atmosphere : Results show that increasing the temperature of the ambient atmosphere around the jet ( from room temperature to more than 1300K), increases the coherency and coherent length of the jet : Fig 15 : jet visualized with a hot ambient atmosphere Using the Pitot tube, pressure measurements have been performed for configurations : jet of adapted tuyere in a cold atmosphere ( 300K) jet of adapted tuyere in a warmer atmosphere ( 1050 K) The results are described in the following curve : Pressure along the jet 7 6 5 jet in cold atmosphere jet in atmosphere at 1050K 4 3 1 0 50 100 150 00 50 300 350 400 450 500 550 600 Distance along the jet (x/d) Fig 16 : pressure measured with Pitot tube along the jet From Fig. 16, one can see that the higher the pressure is maintained, the greater is the jet impulsion and the longer is the coherent length of the jet. The effect on the gas jet of lower gas density produced by hot quiescent air in the furnace is highly dependant on the pressure evolution inside the jet. Moreover, the experiment have been performed for warm furnace but not at the level of EAF temperature (more than 1800K), therefore it can be expected that the coherency of the jet will be significantly higher than in the experimental furnace atmosphere at 1050K.
3-3 Conclusion of experiments Laser visualization as well as Pitot tube measurement demonstrates the significant effect of the temperature of the ambient atmosphere on the coherency of the jet. The differences considered here result from density changes produced by exothermic reaction. These are shown to be similar to those produced by fluid density differences in non-reacting flows in accordance with Kathleen et al (1998). Therefore these theoretical and experimental studies have confirmed the optimized design of the tuyere and the parametric study results obtained in the part. Based on complete theoretical studies around the jet formation, Air Liquide has developed and now owns validated tools to design and dimension supersonic tuyeres that are adaptable to a variety of electric arc furnace conditions for any steel plant. Based on furnace operating practise, desired flow rate, available oxygen pressure or injector location, Air Liquide can quickly design the best tuyere configuration to suit any EAF. 4. Industrial implementation 4.1 objectives The supersonic oxygen injector, ALARC-JET, mounted in the water-cooled panels, enables the injection of oxygen at supersonic velocity into the metal liquid bath. The objectives of this injector are to accelerate the metal bath decarburization and to produce a better foaming slag practice by improving oxygen distribution in the furnace. Wall-mounted ALARC-JET Lance manipulator Fig 17: effect area of ALARC-JET Depending on the number of injectors implanted on a furnace, this injector could: Improve decarburization when added to existing lances Replace door lances allowing the slag door remain closed for longer periods of time (with the use of several injectors). 4. action of the oxygen O injected by lance or tuyere into liquid steel Iron and carbon in the steel bath compete with each other for reaction with oxygen (reactions 9 and 11). According to (5), carbon oxidation (decarburization) should occur preferentially. This occurs when carbon content in the liquid steel is higher than 0.%. Below 0.%, iron oxidation occurs due to the carbon mass transfer kinetic limitation.
Almost no post-combustion of CO to CO () is possible in the liquid phase due to the effect of carbon or iron oxidation by CO (4 or 6). Therefore O lance or tuyere injection to liquid steel leads to CO emissions from the bath into the atmosphere Further, at solid-gas interfaces, carbon and iron are oxidized by O and CO to form CO and FeO (9, 11, 1 and 14), however, with important kinetic limitations. But, an optimized tuyere injects the oxygen onto the bath, where the carbon is available to react. Conversely, a non-optimized tuyere or a subsonic lance does not produce a high momentum impinging jet. In these cases, the oxygen reacts with the carbon and with the iron (the carbon isn t sufficient to consummate all the oxygen). Thermodynamic principles of oxygen use in EAF Here is a brief summary on oxygen effects in EAF. The main effects of oxygen can be explained by equilibrium between O, Fe, FeO, C, CO and CO. Main reactions between those elements between 100 and 300K are: O oxidizes C, CO and Fe: C + ½ O CO (Equation 9) CO + ½ O CO (Equation 10) Fe + ½ O FeO (Equation 11) C reduces CO and FeO: C + CO CO (Equation 1) C + FeO CO + Fe (Equation 13) Fe reduces CO Fe + CO FeO + CO (Equation 14) This reaction defines the stability domains of either Fe or FeO according to the temperature and to the simplified post-combustion ratio. The reactions above enable one to understand the main effects of O use in the EAF. Decarburization Principles Decarburization is the reaction that removes carbon from the liquid steel. It is obtained by making carbon and oxygen react together. Decarburization depends strongly on the carbon level in the liquid metal. For liquid steel with high level of carbon (C > 0.%), decarburization is limited by oxygen transfer into the liquid phase: dc dt (Equation 15) k a G P O For liquid steel with low level of carbon(c < 0.%), decarburization is limited by carbon diffusion in the metal bath: dc k L a C (Equation 16) dt Decarburization speed depends on the metallurgical reactor in which it takes place. Here are some figures about this speed: Basic Oxygen Furnace (BOF) : 0.5 to 0.3%C/min Electric Arc Furnace (EAF) : 0.03 to 0.05%C/min In EAF, for low carbon steel, in the end of the decarburization period, it is important to reduce slag quantity to reduce iron losses in the slag. Foamy slag The first goal of oxygen injection in liquid metal bath is to remove different elements from it, like phosphorus, silica, carbon. Now, oxygen is also used to improve the slag foaming by injecting carbon into the slag simultaneously with oxygen to generate CO bubbles. This produces slag foaming thus enabling a longer arc practice with protection
of water-cooled panels. Depending on the steel s carbon content, the slag oxidation level, O and carbon flowrates into the slag, the effect will be either oxidizing or reducing at the slag/steel interface Based on the results from the modeling studies and the experimental tests, it appears that: There is no need to use a flame to decrease the atmosphere density around the jet because temperature of the atmosphere in the EAF during the refining period is very hot (we measured 1500 to 1650 C) By using an optimized nozzle, it is possible to have a high momentum impinging jet. 4.3 Industrial implementation The steelplant is a mini-mill located in the East of France. 800 000 tons/year of wire rod, mainly for reinforced concrete, are produced from scrap. Melting is performed by a 145 tons, 80 MVA furnace equipped with watercooled panels and roof, eccentric bottom tapping, carbon and oxygen devices. More precisely, the furnace is equipped with 5 burners, each having maximum power of 5 MW, 3 postcombustion lances located on the roof (each lance delivers 1500 Nm3/h of oxygen), and one BSE door lance manipulator ( oxygen lances of 400 Nm3/h each, one carbon lance). The furnace has a diameter of 7.3 m. 4.4 Implementation 4.4.1. Objective of Trials The objective of the trials is to improve its productivity, with a goal of lowering its Power-On-Time by 1.5 minutes. These trials will be conducted with a 500 NM3/h injector. This implementation is the first step of a multi-injector program. Figure 18 : initial implementation Figure 19 : nozzle after several months 4.4. Characteristics and location of the injector The injector (Flow-rate: 500 Nm3/h; speed: Mach.1) is located in the back of the furnace in the area of the EBT, just under the panel #. The pictures below show the exact location for injector. Flow rate Measured Coherence Distance temperature Length nozzle bath 500 Nm3/h 1900 K 1.4 m 1.5 m
1.5 m 50 The injector has been installed at an angle from the horizontal of 50. Distance between the tip and the metal bath is around 1.5m. 4.5 Process When the injector is not running, it is necessary that a small gas flow-rate (around 100 Nm3/h) be maintained through the injector to avoid the clogging of the injector. The gas could be either air, nitrogen or oxygen. Fig 1 illustrates the firing profiles of the ALARC-Jet along with those of the post-combustion, door lance manipulators and the burners as a function of electrical power consumption. 1st basket nd basket ALARC-JET Lances PC Lance manipulator Burners 4.6. Industrial results 0 18 3 39 55 Figure 1 Elec (MWh) The following results have been obtained with an oxygen consumption of 3 Nm3/t. Injector reliability The injector works very well, without problem (no clogging, no cooling problem ) after 9 months of operation. Influence of the injector on the furnace
The injector creates a better slag coating because of an improved foaming slag. This slag coating has been noted on the panels and on the roof, and this result has been obtained without metal splashing. The injector didn t have detrimental effects on the refractory of the furnace, and even improved the roof working life. The electrode consumption has not changed, with the injector utilization. Bath oxidation and iron losses After adjustment of the carbon injected, the utilization of the injector didn t affect the bath oxidation and the iron losses. Temperature distribution in the furnace The bath temperature is more homogenous. This is due to the fact that reaction between oxygen and the bath also takes place and generates heat in the back of the furnace. This enables one to reduce tapping temperature at EAF (as measured at the slag door) because the steel back of the furnace is no longer colder than the steel near the slag door.. By having a more uniform temperature distribution throughout the bath, the furnace operators are able to reduce power-on-time and electricity consumption. (for example, use of one injector at this steelplant has increased liquid bath temperature at the ladle furnace by 0K when tapping temperature at EAF remains constant). There is also a reduced standard deviation of the steel temperatures at the ladle furnace while using the injector. Tape hole clogging The tape hole clogging has decreased of 7% though use of the injector, because of the supplementary energy in the cold part of the furnace. Power On Time (POT) Power-on-time has decreased of about 45 seconds, but the temperature at the Ladle Furnace has increased by 15K. This increase of ladle furnace steel temperature of 15K results in 1mn POT and in a gain of 8 kwh at the EAF. Electricity consumption Electricity consumption has decreased by about 4 kwh/t at the EAF and kwh/t at the Ladle Furnace, even when only injecting 3 Nm3/t through the ALARC-JET. Foaming slag A better foaming slag practice is obtained due to a better distribution of oxygen and carbon in the furnace. Exemple of results Week 11 Without injector Week 1 With injector Difference Number of heats 70 91 O bath (ppm) 89 799-30 O injector (Nm3/t) 0 3.3 +3.3 Tapping temperature (K) 194 191-1 Ladle furnace température 1847 1857 + 10 POT (in mn) 46.8 46.4-0.4 Tap to Tap (in mn) 59.1 58.7-0.4 Productivity (t/h) 147.9 148.9 + 1 KWh/t EAF and LF 403 397-6 O total (Nm3/t) 30.8 34.7 + 3.9 C total (kg/t) 13.4 15 + 1.6 Conclusion of the ALARC-JET utilization The high efficiency of wall-mounted injector has been proven. The advantages of the injector are : reduction of the power-on-time reduction of electrical energy consumption. reduction of the tape hole clogging incidents no detrimental effects on iron losses (yield) or electrodes consumption
improved working life of the refractory. And these results are obtained with excellent reliability of the injector. Also, the ALARC-JET Is a very simple design that is nearly maintenance free Is very easy to implement on a furnace And is easy to integrate in the furnace process control and regulation. 4.7. Next steps The first injector is operating for more than 9 months. The next steps for this furnace are : - Power-on-time saving of 1 30 with only one 500 Nm3/h injector - To extend the trial to include additional ALARC-JET injectors. 4.8. Injectors Range Flow-rate of each injector will depend on the size of the furnace. Below are given the ranges of injectors that could be used on a furnace depending on its size. Furnace size (t) 50 75 100 15 150 Injector size (Nm3/h) 1500 1500-000 1500-500 000-3000 500-3500 Each ALARC-JET injector has specific operating conditions (O pressure and flow-rate) that must be followed for producing a good coherent jet. If ALARC-JET is used as a complementary tool to burners and door lances, one or two ALARC-JET injectors must be implanted in the back of the furnace, near the EBT if there is one. Specific oxygen consumption through the ALARC-JET injectors should reach 5 to 10 Nm3/t. If ALARC-JET is used to replace door lances, a minimum of three ALARC-JET injectors must be implanted around the furnace. The location the injectors should be close to the furnace cold spots but they should also be located to try to optimize oxygen distribution around the furnace. Specific oxygen consumption through ALARC-JET injectors should reach 15 to 5 Nm3/t. ALARC JET complete set Find below more specifications on ALARC-Jet injectors. Flow-rate (Nm3/h) 1500 000 500 3000 3500 Speed minimum (Mach).1.1.1.1.1 O pressure minimum (bars) 9 9 9 9 9 External diameter (mm) 80 100 10 135 150 Cooling flow-rate (m3/h) 5 6 6 8 10 5. Conclusion Supersonic injection has become a widely-used way to decarburize steel in the EAF using O. Air Liquide, through an extensive research and development program, has developed the ALARC-JET to help steelmaker optimize its use of oxygen in the EAF. Results of theoretical studies, confirmed by a sophisticated experimental program and long-term industrial test are:
.The geometric design of the nozzle is the most important parameter to create a regular jet with minimized turbulence, thus allowing to increase the jet s coherency length. To achieve this, design of the converging-diverging nozzle must include very specific parameters. Conventional supersonic nozzles being used in the steel industry do not incorporate in their design the specific parameters to create an adapted jet..quality of manufacturing, especially inner surface quality is also a key factor to prevent from creating any disturbance of the supersonic gas flow at the exit..the coherent zone of the jet is defined as the length of the potential core flow, that is to say the region in which the axial velocity is constant and supersonic..an innovative formula is proposed to calculate and predict the coherency length of the jet. The validity of this formula has been confirmed with an extensive 3D-modeling study. This formula is very useful to quickly select nozzle design and operation parameters for a specific application required by a steelmaker..according to this formula, the key parameters are: a / upstream pressure of the jet, b/ ambient temperature and c/ gas flow-rate. In industrial conditions, theoretical optimized coherency length can vary from 1,3 m up to 1,8 m for oxygen flow rate of 500 Nm3/h to 4500 Nm3/h. During decarburization process, the ambient conditions are very favorable to increase the coherency length. Temperatures of 1900 K have been measured. Coherency length under these conditions is 3 to 4 times the coherency length in room temperature conditions. ALARC-JET is made of a range of supersonic oxygen injectors with enhanced properties for Electric Arc Furnaces. ALARC-JET injectors can be located in EAF walls, in combination with or without burners, depending on location and whether or not there are existing devices at a given location. ALARC-JET technology is especially attractive for all oxygen injection technologies retrofits operation. ALARC-JET efficiency has been shown on a 165-ton furnace, on which only one injector installed in November 1999 has the tap-to-tap time by 0,5 to 1,5 minutes. The extreme simplicity of ALARC-JET design ensures it has maximum reliability for decarburazing. Additional implementations are in process, in several BOF plants (converters and secondary metallurgy) as well as in non-ferrous converters. References 1 J.P.Birat : «Une étude prospective de l évolution technologique du four électrique à arcs à l horizon 010», 0èmes journées sidérurgiques internationales, décembre 1999 W.J.A. Dahm and P.E. Dimotakis. «Measurements of entrainment and mixing in turbulent jets.», AIAA J., 5 : 116-13, 1987. 3 G.N. Abramovich «The theory of turbulent jets», M.I.T. Press, 1963 4 L. Brown and A. Roshko. «On density effects and large structure in turbulent mixing layers», J. Fluid Mech., 64(4) : 775-816, 1974 5 D. Papamoschou and Roscko. «The compressible turbulent shear layer : an experimental study», J. Fluid Mech., 197 : 453-477, 1988. 6 K. Viswanathan and P.J. Morris. «Predictions of turbulent mixing in axisymmetric compressible shear layers», AIAA Journal, Vol 30, N 6, 199 7 Schadow, Gutmark, Wilson. «Compressible spreading rates of supersonic coaxial jets», Experiments in fluids, 10, 1990 8 H.S. Mukunda, B. Sekar, M.H. Carpenter et al. «Direct simulation of hight-speed mixing layers», L-1699, NASA TP-3186, 199 9 K. Viswanathan and P.J. Morris. «Predictions of turbulent mixing in axisymmetric compressible shear layers», AIAA Journal, Vol 30, N 6, 199 10 Lau, Morris and Fisher. «Measurements in subsonic and supersonic free jets using laser velocimeter», Journal of Fluid Mechanics, 93, 1979 11 Anderson J.D. «Modern compressible flow», second edition, Mc Graw Hill
1 H.C. Man, J. Duan and T.M. Yue «design and characteristic analysis of supersonic nozzles for high gas pressure laser cutting», Journal of Materials processing technology, 63 : 17-, 1997. 13 Thring M.W. and Newby M.P. «Combustion length of enclosed turbulent jet flames», Proc. 4 th Int l. Symp. Comb. 789-796, Williams & Wilkins, Co., Baltimore. 14 Gas Dynamics, Multidimensional flow, Maurice j. Zucrow and Joe d. Hoffman