The optimized SIS Injector for EAF Application

AISTech 2014 – The Iron & Steel Technology Conference and Exposition, 5-8 May 2014, Indianapolis, IN, USA

The optimized SIS Injector for EAF Application

Hans-Jürgen Odenthal1), Jan Bader1), Ralf Nörthemann1), Markus Reifferscheid1), Igor Klioutchnikov2), Herbert Olivier2) 1)

SMS Siemag AG, Eduard-Schloemann-Straße 4, 40237 Düsseldorf, Germany Phone: +49 (0)211-881-4143 Fax: +49 (0)211-881-4997 E-mail: [email protected] Web page: http://www.sms-siemag.com 2)

Shock Wave Laboratory, RWTH Aachen University, Schurzelter Straße 35, 52074 Aachen, Germany Web page: http://www.swl.rwth-aachen.de

Key words: Burner, CFD, coaxial jet, correlation model, DNS, EAF, energy input, injector, supersonic flow.

ABSTRACT The SIS (Siemag Injection System) injector is a combined burner/injector system to melt down the scrap and to superheat the melt in an electric arc furnace (EAF). The new SIS 4.0 injector generation is characterized by a simple and light design and a fluid dynamically optimized nozzle geometry. This paper describes structure, functioning and special features of the SIS 4.0 injector. In order to achieve a long supersonic oxygen jet, e. g. for enabling the injector to be used in furnaces of high construction, a new correlation model for the coaxial jet length in hot environment was developed. The geometry of the central oxygen nozzle and the coaxial shrouding gas nozzle as well as the gas consumption can be calculated by the new correlation model. Important gas-dynamic principles, which contribute to a better understanding of the SIS principle and which have been used for programming the advanced design tools (CARD, COAX) are presented. Oxygen injectors designed on the basis of the correlation model produce up to 40 % longer supersonic jets than conventional injectors. INTRODUCTION In order to increase the productivity of the EAF and to reduce the amount of electrical energy, combined gas burners/oxygen injectors are commonly used to melt down the scrap and to superheat the liquid metal. While the energy input and the melt-down power is highest in the furnace center, cold spots are formed at the wall panels. The installation of several burners/injectors helps to add thermal energy and to homogenize the heat distribution inside the furnace domain. The SIS injector is a combination of gas burner and oxygen injector. In the injector mode, oxygen is blown onto the melt at supersonic speed. Hereinafter, if not explicitly indicated, the term injector is always used. The deeper the oxygen jet penetrates the melt, the more intense is the reaction between melt, slag and gas and the faster is the decarburization. The SIS injector supports the melt-down of scrap and accelerates the decarburization of the melt. The SIS injector is designed such that the use of raw gases (O2, CH4, LPG, compressed air) is minimized. Operating costs are saved and the return on investment period of the injector equipment is short. In an EAF, the injector is located with corresponding distance to the melt surface, since the injector could be damaged by splashing or thermal loads. However, at higher positions the injector is further away from the melt surface. Sometimes, the injector nozzle is more than two meters away from the melt, especially in DRI furnaces with varying melt level. If the nozzle is not properly designed, gas

dynamic phenomena, such as compression (shocks) and expansion waves may lead to nozzle wear and a substantial decrease in the axial jet velocity. The jet momentum is lost and oxygen does not enter deeply enough into the melt. The momentum can even be so small that the foaming slag is no longer displaced and the oxygen does not reach the melt. This paper describes structure, function and theoretical aspects of the SIS 4.0 injector. These principles are unique, especially the oxygen injection mode. The latter is based on a central, cold oxygen jet leaving the nozzle at the speed of sound (Mach number M  2) and a surrounding, hot shrouding gas jet. Hereinafter, the central oxygen nozzle is referred to as primary (p) nozzle and the shrouding  s , temperature Ts, gap height Hs) of the secondary nozzle must be nozzle as secondary (s) nozzle. The process parameters (mass flow m perfectly adapted to primary nozzle. Therefore, a new gas dynamic concept has been developed and numerically validated by the Shock Wave Laboratory of the RWTH Aachen University. Based on numerical analyses of the mixing process and the determination of the optimal flow parameters of the secondary jet, to be considered in conjunction with the primary jet, an increased stability and length of the oxygen jet was found. For practical application, the oxygen jet has to be focused along a large distance so that it enters the melt at highest momentum. This causes the melt to be mixed properly and leads to an intensive foaming and decarburization. PRINCIPLE OF THE SIS INJECTOR Figure 1 illustrates a typical arrangement of SIS injectors in a 120 t EAF and the prospective energy distribution. The arrangement of the electrodes induces the well-known cold and hot spots on the furnace wall. The hot spot temperature depends, inter alia, on the transformer capacity, energy input and furnace diameter. The aim is to melt down the scrap as fast as possible and to transfer it into the flat-bath phase with stable, long electric arcs. In the flat-bath phase, coal or carbon dust is injected into the slag layer and blown onto the melt. The carbon reacts with FeO in the slag form CO, which foams up the slag. The slag covers the electric arcs and reduces the radiation and hot spot temperature. With foaming slag, the furnace can be operated with long arcs at high power, i. e. low electric currents. This leads to low energy consumption, short superheating periods and low use of refractory and electrode material. It is important to adjust the equilibrium between carbon and oxygen. Too much injected carbon causes the carbon to enter the off-gas as CO2 without being used, and too little injector carbon causes the foaming slag layer to collapse. SMS Siemag controls the carbon injection based on sound measurements at the EAF. The SIS injectors (SIS 1-3) in Figure 1 are installed in the injector boxes together with the carbon injectors. The installation angle towards the horizontal line depends on the EAF type, it is typical between 40° and 45°. Carbon is blown into those regions where the oxygen jets cause the melt to be stirred into vigorous movement. The fourth SIS injector (SIS 4) is installed to heat-up the tapping (EBT) area. Generally, the process of melting and superheating is optimized by the SIS injectors in a specific way. In the drilling phase, cooler furnace areas are heated-up and skulls are avoided. In the flat-bath phase, a stable foaming slag layer is formed. SIS injectors in their current design [10] have been successfully used in eleven EAFs so far. Some references, e. g., are Forpost Management (120 t, 120 MVA), Peiner Träger (125 t, 170 MVA), Jindal Steel & Power Ltd. (250 t, 200 MVA), PT Gunung Garuda (120 t, 100 MVA), JSW Steel Ltd. (160 t, 137 MVA).

Figure 1: Arrangement of SIS injectors and typical energy distribution in a 120 t EAF

The SIS injector concept has been further developed, so that now the SIS 4.0 generation is available. Figure 2a shows the layout of the SIS 4.0 injector. The major difference to the previous generation is the lighter weight of the injector, facilitating installation and replacement. Unlike the previous generation consisting of a separate, laterally installed flame generator and burner/injector unit [10] the combustion chamber is now integrated into the SIS 4.0 unit and designed as pipe-in-pipe solution. a) b) c)

Figure 2: a) Layout of the SIS 4.0 injector; b) Implementation into the injector box; c) Burner mode in operation The inner injector part, Figure 2a, consists of copper and contains the central, primary nozzle (supersonic). The nozzle is welded onto the steel pipe containing the connections for oxygen, fuel gas (CH 4, LPG) and compressed air. In the injector mode, the lower fuel gas connection is pressurized with compressed air. All hoses for the gas supply are equipped with quick-action type couplings. The inner injector part weighs around 18 kg and is flange-mounted on the outer injector part. A refractory seal is placed between inner and outer part preventing back-flames. The outer injector part is designed such that an aerodynamically optimized annular gap is formed. This coaxial nozzle has a shape such that the subsonic, hot off-gas exits the gap at very specific values for Ms and Ts. The outer contour is integrated into the injector box, Figure 2b. Since the supersonic nozzle is slightly retracted, it is protected from splashes of slag and melt and thus from clogging. Primary and secondary nozzles are water-cooled via inner ducts and form a long-life system for reliable operation and high availability. The refractory lining is protected against excessive wear by adapting the injector position to each furnace individually. Due to the split-shell furnace design, every SIS 4.0 injector can be replaced easily. The flow rates (O2, CH4, LPG, air) are controlled by the valve station. The maintenance interval of the SIS 4.0 unit is longer than that of the furnace shell; maintenance is carried out when the shell is changed or the furnace is relined. The SIS 4.0 unit operates in three modes, i. e. the pilot, burner, and injector mode, Figure 3. a)

Pilot mode

b) Burner mode

c)

Injector mode

Figure 3: Layout and basic function of the SIS 4.0 injector

The pilot mode, Figure 3a, is a standby mode that is provided in case of an inactive injector. In the pilot mode, a mixture of natural gas (CH4, LPG) and compressed air is internally ignited by a spark plug. The pilot flame prevents slag and melt splashes from sticking to the SIS tip; the injector featuring the retracted nozzle and a concentrically arranged annular gap prevents clogging of the nozzle. In the burner mode, Figure 3b, thermal energy is supplied into the EAF. Oxygen and natural gas are conveyed unburnt through the injector. Inside the furnace a combustible mixture is generated that is ignited by the hot furnace atmosphere. The powerful flame supports the melt-down of scrap in the cold spots. During the flat-bath period, operation is changed over to the injector mode, Figure 3c. The main factor for fast decarburization is an intensive transition of oxygen into the melt. This is accomplished by the supersonic jet (Mp  2). Similar to the pilot mode, the coaxial nozzle supplies a shrouding jet of hot off-gas which covers the cold oxygen jet and increases the supersonic jet length. With the use of compressed air and a small amount of natural gas, the costs for the shrouding gas generation are approx. 70 % lower than with conventional systems. Table 1 shows typical gas consumptions of a 6 MW SIS 4.0 injector. Table 1: Characteristic operating conditions of a 6 MW SIS 4.0 injector

Natural gas CH4, LPG, shrouding/secondary jet Oxygen O2, central/primary jet Compressed air, shrouding/secondary jet Power

V [m3 (s.t.p.) / h] P [MW]

Pilot mode 11 300

Burner mode Injector mode 600 11 1200 3000 300 6

Advantages of the SIS 4.0 injector are:  Injector weight is only approx.18 kg, thus easy installation and removal is possible.  Coaxial nozzle designed on new models developed by RWTH Aachen University and SMS Siemag, thus highest oxygen injection efficiency.  Coaxial jet is designed for hot furnace environment and oxygen jet parameter leading to a reduction of natural gas (CH 4, LPG) as compared to standard SIS, especially during pilot mode.  Pipe-in-pipe solution, the hot shrouding gas is generated in the annular injector tube.  Conventional spark plug instead of spark electrode; option that must be tested: Glowing plug with the further advantage that an ignition transformer would not be necessary.  Optimized panel layout with injector box designed as cast part.  Re-machined sealing surface avoiding back-flames.  No excessive wear of refractory lining below injector box.  Injector is used for new installations and for upgrades for already existing furnaces, low maintenance costs. SIS 4.0 INJECTOR - LAYOUT The shape of a conventional supersonic nozzle with parallel outlet jet can, in terms of total pressure p 0, total temperature T0, flow rate  0 , p, the V 0 , and ambient pressure p, be designed for just one point, the design (d) point, Figure 4a. With given values for p0, T0, V nozzle contour can be calculated by the isentropic theory [2]. If, for a certain contour, only one of these values differs from the design point, local pressure, temperature, and density variations arise inside and outside the nozzle. These flow disturbances induce straight or inclined compression and/or expansion waves that may cause premature nozzle wear and unsteady, non-reproducible EAF conditions. In general, the supersonic jet can be subdivided into potential core and supersonic region. In the potential core, the flow variables (M, u, p, T, ) are nearly constant. The axial length L* of the supersonic jet is characterized by the point M = 1. As a rule of thumb, the normalized length L*/D1 of a supersonic jet entering a cold environment is L*/D1  10 to 20. If the same gas jet enters a hot environment at T = 1600°C, it is L*/D1  20 to 30. More details about supersonic nozzles operated at off-design conditions are described in [2,10]. Two software tools have been developed in C++ to determine the geometry and process gas data for the SIS 4.0 injector: The first program is CARD (Charakteristikenverfahren zur Auslegung Rotationssymmetrischer Düsen – method of characteristics for dimensioning rotationally symmetric nozzles) and is based on the methods of characteristics. CARD calculates the optimum bellshaped contour of the primary nozzle. Supersonic nozzles designed by CARD produce an undisturbed jet with highest exit momentum. The second program is COAX (Computerprogramm zur Optimierung und Auslegung von Koaxialdüsen – computer program for optimizing and dimensioning coaxial nozzles) and determines the secondary nozzle, but always to be seen in combination with the previously CARD-designed primary nozzle. COAX ensures that the cold oxygen jet and the hot shrouding gas jet are adapted to each

other such that the maximum length of the oxygen jet is attained. However, several classes of certain coaxial nozzles which are designed by CARD and COAX have been applied for patent. The basic principles of both programs are explained in the following.

Figure 4: Notation: a) Conventional nozzle; b) Coaxial nozzle with primary (p) supersonic nozzle (M > 1) and secondary (s) subsonic nozzle (M < 1); cold oxygen is supplied to the central nozzle, hot off-gas (CO) to the shrouding nozzle; Mcsp: Convective Mach number between secondary (s) and primary (p) jet; Mc∞s: Convective Mach number between ambience (∞) and secondary (s) jet CARD – software to design the central supersonic nozzle CARD uses the method of characteristics and solves the partial differential equations for gas-dynamics for steady-state, isentropic, non-rotational, and axisymmetric flows

(u 2  a 2 ) u, v: x, r: a:

u  v   u v  a 2 v  (v 2  a 2 )    u v     , x r  r   r x 

(1)

Flow velocity in axial and radial direction, Axial and radial coordinate, Speed of sound.

The Mach lines, i. e., the lines of weak pressure disturbances which propagate at the speed of sound and which are arranged at a defined angle relative to the local velocity vector, describe the right-running and left-running characteristics. The Mach lines are characteristic curves of the gas-dynamic equation. In the present case, the method of characteristics is combined with a boundary layer correction, i. e. the momentum-reducing influence of the boundary layer is taken into account. Within the boundary layer, the gas is decelerated from maximum velocity to zero velocity at the wall. Exemplarily, the shape of a supersonic nozzle is shown in Figure 5a. The nozzle consists of a converging subsonic section and a diverging supersonic section. The nozzle areas for M < 1, M = 1, and M > 1 are marked. CARD iteratively calculates the entire contour  0 , exit pressure p1, ambient pressure p, for a set of the following input parameters: Total pressure p0, total temperature T0, flow rate V heat capacity ratio , and specific gas constant R0. For certain groups of curves, the solution of Eq. (1) is possible. Curve groups c+ and c- are shown in Figure 5b and are called the Mach lines. The characteristics with the flow angle ( - ) are referred to as right-running characteristic lines (in flow direction to the right of the flow line), those with the flow angle ( + ) as left-running characteristic lines (in flow direction to the left of the flow line). The ascending gradient of the characteristic lines is

 dr    c   tan(  )  dx  c-, c+: : :

and

 dr    c   tan(  ) ,  dx 

(2)

Right/left-running characteristic line, Fluid flow angle, i. e. angle between the local velocity vector and the system of coordinates, Mach angle with  = sin-1·(1/M).

The compatibility conditions along the characteristic lines are given as

d (   ) c   M: :

1

dr M 2  1  cot  r

and

Mach number, Prandtl-Meyer angle [2].

d (   ) c   

1

dr , M 2  1  cot  r

(3)

a)

b)

c)

d)

Figure 5: a) Notation used for the method of characteristics (CARD); b) Left- and right-running characteristics; c) Subsonic and throat region; d) Example for a Mach number distribution calculated by CARD: Design conditions are V 0,O 2 = 3068 m3(s.t.p.)/h, p0,O2 = 8.4 bar, T0,O2 = 20°C, p = 1.23 bar, O2 = 1.43, R0,O2 = 259.83 J/kgK, rk = 2.1, R2 = 4.0, R = R1 = 4.0,  = 30°; diagram shows the nozzle contour with and without boundary layer correction To start the calculation, the sonic line (red line in the throat, Figure 5b) and the initial line for the iteration are determined on the basis of the equation for the non-dimensional perturbation velocity potential  for axisymmetric compressible flows (   1) x ' xx rr 

r  0. r

(4)

This method according to Sauer [12] provides a solution of the perturbation equation using a power series expansion. The perturbation velocities are calculated using the critical speed of sound a*, i. e. u  x and   r u( x, r )  bx 

b:

(   1) b 2 r 2 4

and

v( x, r ) 

(   1) b 2 xr (   1) 2 b3r 3 ,  2 16

(5)

Constant.

CARD calculates the initial values of the initial line up to the initial characteristic. CARD uses an iterative method to determine the coordinates of the grid points and the corresponding flow parameters and takes into account the curvature of the characteristics. The expansion section of the nozzle with positive curvature is calculated from the initial to the last expansion characteristic. The flow parameters are determined on the basis of special contour functions such as

r  a  bx  cx 2 a, b, c:

and

dr  tan   b  2cx , dx

(6)

Constants.

The expansion section of the nozzle with negative curvature is calculated between the recent expansion characteristic and the Mach line originating from the point on the axis, where M = M1. There, the contour is determined by means of backward characteristics and the wall streamline. The pressure value on the contour is controlled for the optional case p 1 > p and the nozzle contour is determined with a corresponding exit angle 1. CARD determines the subsonic section of the nozzle using predetermined shape functions and generates a flow-optimal configuration of the subsonic wall, Figure 4c. For given data of rk, R1, R2 and , contour functions in the form of circular arcs defined r = f (xk,rk,R2) for x  x2, r = f (x1,x2,r1,r2) for x2  x  x1, r = f (xt,rt,R1,R2) for x1  x  xt.

(7a) (7b) (7c)

Since no pressure disturbances can occur in the subsonic range, this approach is sufficient. The variables r k, R1, R2, and  are empirically based, they are optimized from nozzle to nozzle. This is necessary because the subsonic section too has a clear influence on the supersonic jet length L*. A boundary layer correction of the nozzle contour is performed by means of a displacement thickness function based on the reference temperature method [6]. The contour is smoothed using a Lagrange polynomial of high order. Besides the optimized nozzle contour, the CARD iterations give the exit Mach number M1 [-], exit pressure p1 [Pa], exit velocity u1 [m/s], exit temperature T 1 [K], mass flow  [kg/s], exit momentum I1 [Ns], specific exit momentum I1s [N/m2], and supersonic jet length L* [m]. rate m Figure 5d shows an example for a nozzle designed by CARD. The main difference between a nozzle design based on the isentropic theory and on the method of characteristics is that the first method is a one-dimensional method whereas the latter is a two-dimensional, rotational one. Supersonic nozzles designed by CARD generate an undisturbed, lossless flow with maximum momentum and supersonic jet length L*. SMS Siemag uses CARD for dimensioning all types of nozzles. COAX – software to design the coaxial nozzle To further increase the supersonic length L*, an annular-shaped, hot shrouding gas is blown parallel to and around the central, cold oxygen jet by combusting CH4 or LPG with air. The principle of the coaxial jet, Figure 4b, is based on the mechanism of lowering the density of the secondary in comparison to that of the primary jet. Since the secondary jet is moving related to the furnace ambience, the primary jet is protected from the ambience and dissipation inside the shear layers between primary jet and atmosphere is reduced. Decreasing the density simply based on its temperature dependency, however, does not accomplish the objective. If the overall design is not correct, the shear layers between primary jet, secondary jet and ambience will become unstable, roll up and the length of the primary jet decreases. The Shock Wave Laboratory of the RWTH Aachen University has been carried out extensive Direct Numerical Simulations (DNS) in order to understand and visualize the interaction of the involved gas jets (oxygen, CO) and the hot EAF environment. Based on these results, all process variables could be adjusted in such a way that the primary jet length L*p is maximized. From the literature [9, 11] is known that the interaction of the gases is characterized by the parameters at the exit of the secondary (coaxial) nozzle:   

Exit Mach number Ms and exit velocity us, Exit temperature Ts, Ratio Hs/Dp of the secondary nozzle height Hs to the exit diameter Dp of the primary nozzle.

By variation of these parameters, the DNS simulations that it is not the exit Mach number and exit velocity of the primary and secondary nozzle that need to be the same, but the convective Mach numbers. The convective Mach number according to Bogdanoff [5] is based on the relative convection speed of large-scale flow structures and the corresponding speed of sound. In the shear layer of a single jet, the convective Mach number Mc is given by

Mc  M c: M: u: a: : : 1,2:

M1 (1   u ) (1  

 0.5

) 

0.25

with  u 

u2 , u1

 

2 , 1

 

2 , 1

(8)

Convective Mach number, Mach number, Mean jet velocity, Speed of sound, Gas density, Specific heat ratio, High speed side (1) and low speed side (2) of the shear layer with equal static pressure p 1 = p2.

If the convective Mach number Mc is calculated on the geometric average of Mc1 and Mc2 and moreover the specific heat ratios are the same (1 = 2), then Eq. (8) can be simplified

Mc 

u1  u 2 a1  a 2

(valid for single jet).

(9)

More complex phenomena than for the single jet are observed for the coaxial jet with two shear layers. The SIS injector, Figure 3c, is a good example for a coaxial jet entering a hot ambience with a supersonic central jet and a subsonic shrouding gas jet. A detailed study of coaxial jets is given by Murakami and Papamoschou [9]. The authors investigated the impact of a coaxial jet on the potential core length of the primary jet with emphasis on supersonic jet noise production and proposed a semi-empirical model. However, the investigations are limited to cold ambient conditions; the compressible shear layer in a hot environment was not well understood so far. In Figure 4b, the regions where the flow remains homogeneous at constant gas properties are the primary (p) and secondary (s) potential cores. Again, the supersonic jet length L*p is defined by the location where the local Mach number is Mp = 1.

However, in comparison to a single jet, now two convective Mach numbers between the secondary (s) and primary (p) jet on the one hand (Mcsp), and between the ambience (∞) and the secondary (s) jet on the other hand (Mc∞s) are of importance. In the case that p ≠ s ≠ , the equations for Mcsp and Mc∞s can be found in [8]. For p = s = , a simplified form of the convective Mach numbers applies, analogous to Eq. (9)

M csp 

u p  us , a p  as

Mcs 

us as  a

(valid for coaxial jet).

(10)

Based on the DNS simulations, the following, new optimization criterion for the interaction between primary jet, secondary jet and furnace environment could be found

Mcs  Mcsp (central optimization criteria for coaxial flows).

(11)

Eq. (11) is the new approach for dimensioning coaxial jets entering a hot environment. The new criterion allows the prediction of flow conditions resulting in a maximum jet length. Previous approaches merely assume that the velocities of the different jets are identical. Eq. (11) represents the prediction core of the COAX software. Figuratively speaking, COAX adapts the predominating gas flow conditions such that L*p becomes maximum. However, the mathematical realization for determining the nozzle contour and operational parameters by means of COAX is extremely complex. Thus, the implementation is only roughly described. Coming back to the more complex form of Eq. (10), see [8], and considering the new optimization criterion, Eq. (11), the convective Mach numbers for the coaxial jet are

M p s / p (1  M csp 

us ) up

(1  s / p )( s /  p )

, u,  : , p, s :

 M cs 

0.25

M s   / s (1   / s )(  / s )0.25

,

(12)

Density, flow velocity, specific heat ratio, Ambience, primary, secondary.

After some transformation and with u = 0, the optimum exit Mach number of the secondary jet is

us u p  a p 1  Ms   as as  as 

 1  p / s   1  s /  

1

  p    0.25    .   s2   

(13)

With p = s = , the following simplification applies

M csp 

u p  us us  M cs  , a p  as as  a

(14)

and finally Ms 

us u p (a s  a  ) .  as a s (a p  2a s  a  )

(15)

Considering Eq. (14) and Eq. (15), fifteen DNS simulations have been performed to find the maximum supersonic jet length L*p . Based on these simulations, a new correlation model has been proposed which allows the calculation of L*p in hot environmental conditions, i. e. the empirical model is valid for cold primary and hot secondary coaxial jets in a hot environment. The correlation model has been validated by the DNS simulations as described in [8]. Three quadratic functions based on the convective Mach number Mc1 for the single jet, and the two convective Mach numbers Mcsp and Mcs for the coaxial jet were defined. The empirical functions predict the jet length L*p of the coaxial jet configuration as compared to the single jet L*, i. e. L*p  L*p (L* ) . For generating the hot shrouding gas, fuel (CH4, LPG) is combusted with air inside the SIS 4.0 injector. The hot off-gas exits the shrouding gap with certain values for Mach number Ms and temperature Ts and covers the oxygen jet. Depending on the fuel type, COAX determines the required volumes of fuel and air such that the optimum off-gas volume, i. e. the optimum exit Mach number Ms and off-gas temperature Ts are attained.  air / m  air,st . In the injector mode, COAX controls the off-gas temperature of the secondary jet via the air-fuel equivalence ratio   m  air for combustion and the minimum required stoiThe combustion-air ratio is the quotient from the actually available air volume m  air,st required for the complete combustion. The following applies for CH4: chiometric air flow rate m ~ O  m ~ N  m ~ CO  m ~ H Om ~ ( 1)O  m ~ N , CH  m (16) 4

O2

2

N2

2

CO 2

2

H 2O

2

O2

2

N2

2

~: m

Molar mass ratio of the component, Air-fuel equivalence ratio.

:

In the burner mode, the burner capacity P is usually specified  CH 4 / LPG h CH 4 , PV u

h

CH 4 u

:

(17)

Upper heating value.

 O2 .  CH 4 / LPG and m From the energy balance, COAX calculates the required mass flow rates m With the calculated flow rates of the secondary jet, the outer coaxial nozzle is determined by specific contour functions (circular arcs, straight lines, etc.) inducing a largely undisturbed subsonic flow in the shrouding duct. An essential feature of the shrouding gas is that it exits horizontally from the shrouding duct, i. e. parallel to the primary gas. RESULTS So far, CFD methods have been used to develop and confirm the SIS 4.0 concept, plant tests are scheduled for summer 2014 on a 120 t AC EAF. On the one hand, Reynolds Averaged Navier-Stokes (RANS) simulations with ANSYS FLUENT 15.0 were carried out to investigate the flow inside the primary and secondary nozzle as well as the near nozzle flow. Since it is known from literature [1, 4] that RANS methods only hardly predict the potential core and supersonic length of a jet entering a hot atmosphere, progressing DNS simulations with the WENO5-DNS (Weighted Essentially Non-Oscillatory) code developed by the Shock Wave Laboratory were performed. Compared to RANS computations, DNS does not require any turbulence modelling since the structure of turbulent scales is fully resolved; the DNS model is valid for compressible jets in hot ambient conditions. Figure 6 shows results of steady-state, two-dimensional (2D), axisymmetric RANS simulations with the RNG k-turbulence model.  0,O 2 = 3000 m3(s.t.p)/h, p0,O2 = 10 bar, T0,O2 = 20°C, p1 = p = 1.013 bar and Here, the SIS 4.0 injector (type A) was designed for V T = 1650°C; the corresponding thermic power in burner mode is P = 6.0 MW. The two-dimensional grid consists of 1.31 million hexahedral cells, the grid could be used for both conventional and coaxial calculations. In the latter case, ANSYS FLUENTs species transport model is used to consider the gaseous mixing between oxygen and CO [3]. Generally, pressure boundary conditions are applied to the computational domain at the inlets and ambient outlets. Second-order upwind schemes are used to discretize the governing equations, i. e. flow, energy, turbulent kinetic energy k and turbulent dissipation rate .

Figure 6: CFD simulation (2D-RANS): Comparison between a) conventional nozzle with single jet and b) coaxial nozzle with primary and secondary jet; velocity distribution in hot furnace atmosphere and detail of the near nozzle flow; SIS 4.0 injector (A) with design  0,O 2 = 3000 m3(s.t.p)/h, p0,O2 = 10 bar, T0,O2 = 20°C, p1 = p = 1.013 bar, T = 1650°C conditions V

Figure 6a shows the conventional nozzle with flat converging and diverging sections and a 20 mm long throat section. The contour with D* = 24.7 mm, D1 = 34.2 mm and a diffusor length of 77.9 mm was designed according to the isentropic theory. Although diameter ratio D*/D1 and pressure ratio p*/p0 have been matched to one another, clear flow disturbances arise inside and outside the nozzle which cause the jet quality to deteriorate and the supersonic jet length L* to decrease. At the location L* = 0.794 m (x/D1 = 24.4), the oxygen changes from supersonic to subsonic state. Figure 6b shows the coaxial jet simulation with CARD- and COAX-designed nozzles and gas consumptions. The primary cold jet is  CH 4 / m  air . The RANS solver calculates a covered by the hot shrouding jet, the hot gas temperature is adjusted by the mass flow ratio m supersonic jet length of L*p  0.943 m (x/Dp = 27.6) which implies an increase of around 19 % as compared to the conventional nozzle. The oxygen flow inside the primary nozzle is undisturbed due to the flow-adapted wall contour. Although the gas flow downstream of the nozzle indicates slight velocity disturbances, the effective cross-section of the supersonic oxygen jet is decreased by the covering subsonic CO jet. Table 2 compares representative results of the isentropic theory and CFD. The latter method matches well with the analytical solution. The relative error of the critical values (*) is smaller than that of the exit values (1,p). Table 2: Comparison between isentropic theory and CFD simulation; nozzle design point: V 0 = 3000 m3(s.t.p.)/h, p 0 = 10 bar, T0 = 20°C, 0 = 13.13 kg/m3 CFD – 2D/RANS

Isentropic theory (IT)

p/p0 T/T0 /0 M u [m/s]  0 [kg/s] m |M/M| [%]1)  0 /m  0 | [%]1) | m 1)

Conventional nozzle

Conventional nozzle

Coaxial nozzle (p)

throat (*) exit (1) 0.529 0.101 0.834 0.522 0.634 0.194 1 2.15 297.6 505.8 1.188

throat (*) exit (1) 0.530 0.105 0.839 0.540 0.632 0.196 0.987 2.07 294.6 495.7 1.172

throat (*) exit (p) 0.533 0.098 0.838 0.530 0.636 0.186 0.987 2.12 294.5 501.4 1.257

-

-

1.26

3.54 1.32

1.30

1.45 5.81

 means (IT - CFD)/IT

Turbulence modelling as necessary in RANS methods always includes empirical constants and requires modifications, especially with respect to EAF applications. The results shown in Figure 6 seem to underestimate L* and L*p , respectively. Currently, SMS Siemag is testing several Unsteady (U)-RANS models (SST-SAS – Shear Stress Transport - Scale Adaptive Simulation, LES – Large Eddy Simulation) applied to the SIS 4.0 technology in order to better describe the transient mixing behavior of the shear layers. However, this work is ongoing and a final conclusion is so far outstanding. For this reason, the Shock Wave Laboratory has carried out detailed DNS simulations for different classes of CARD- and COAX-designed SIS 4.0 injectors using their own CFD code. Again, the WENO5DNS code based on fifth-order finite difference WENO scheme was used for the compressible oxygen jet under hot furnace conditions. The fully three-dimensional, compressible, unsteady Navier-Stokes equations are solved on a three-dimensional grid with 7.4 million cells and a time resolution of t = 3.610-7 s. About 30,000 time steps, i. e. three times the convective time unit, were performed to achieve the criteria of convergence and used for time-averaging. For information about the DNS method, see [7, 8, 10]. Within the scope of a recent EAF modernization, the concept of the CARD- and COAX-designed nozzles was applied to the SIS injectors of the previous generation. All injectors were equipped with the new coaxial nozzles, but still with external hot gas generators,  0,O 2 = 1400 m3(s.t.p)/h, p0,O2 = 10 see [10]. Exemplarily, Figure 7 shows DNS results for this SIS 4.0 injector (type B) designed for V bar, T0,O2 = 20°C, p1 = p = 1.013 bar, T = 1600°C and P = 4.5 MW. Both time-averaged Mach number distributions, for the conventional nozzle with single jet, Figure 7a, and for the coaxial nozzle with primary and secondary jet, Figure 7b, predominantly shows a narrow, axisymmetric jet. The conventional nozzle shows a supersonic jet length of L* = 0.770 m (x/D1 = 30.7) whereas the coaxial nozzle leads to L*p  1.174 m (x/Dp = 43.8) which is an increase of 43 %. Figure 8 contains the DNS results of the time-averaged axial velocity and temperature for the SIS 4.0 injector type B. The length of the potential core can be taken from the diagram; it is x/D 1  20 for the conventional and x/Dp  25 for the coaxial nozzle. The latter one shows lower oxygen jet temperatures because the cold primary jet is protected from the hot ambience by the moderate-temperature secondary jet. DNS predict larger jet lengths than RANS computations, because, among other factors, the energy dissipation with DNS is lower and more realistic than with RANS. DNS resolves the turbulent flow structure in space and time more accurately than RANS, so that the jet lengths predicted by DNS are more reliable.

Figure 7: CFD simulation (3D-DNS): Comparison between a) conventional nozzle with single jet and b) coaxial nozzle with primary and secondary jet; time-averaged Mach number distribution in hot furnace atmosphere and detail of the flow downstream the nozzle  0,O 2 = 1400 m3(s.t.p)/h, p0,O2 = 10 bar, T0,O2 = 20°C, exit (x/Dp = 0); SIS 4.0 injector (B) with design conditions V p1 = p = 1.013 bar, T = 1600°C

Figure 8: CFD simulation (3D-DNS): Comparison between conventional and coaxial nozzle; time-averaged axial Mach number and  0,O 2 = 1400 m3(s.t.p)/h, temperature distribution in hot furnace atmosphere; SIS 4.0 injector (B) with design conditions V p0,O2 = 10 bar, T0,O2 = 20°C, p1 = p = 1.013 bar, T = 1600°C

CONCLUSIONS The new generation of the SMS Siemag injector for EAF installations is discussed. The SIS 4.0 injector is a combined injector/burner system patented by SMS Siemag. The SIS 4.0 injector convinces by its fluid- and thermodynamically optimized functionality, it is available for new furnaces and for upgrades. The new SIS 4.0 generation has been developed for maximum availability and efficiency. The unique characteristic of the SIS 4.0 injector is the principle of adapted coaxial flow operation, i. e. the best possible combined central (primary) oxygen and shrouding (secondary) gas jet. If the nozzle contours and operating conditions are carefully adapted to each other, the coaxial nozzle produces a supersonic oxygen jet of maximum length. Compared to injectors with a single conventional nozzle, the new SIS 4.0 injector with coaxial nozzle produces around 40 % longer oxygen jets. This large jet length ensures that oxygen is transferred into the metal even for large distances between injector and melt surface. From a theoretical point of view, much effort has been made to develop a mathematical model helping to design the SIS 4.0 injector. For this reason, the CARD code was developed in a first step. CARD is based on the method of characteristics for axisymmetric supersonic flows and predicts the optimum shape of the primary nozzle. This nozzle induces an undisturbed oxygen flow of maximum length and high transition of oxygen into the melt. In a second step, the oxygen jet length was further increased by a flow-optimized secondary gas jet. Based on highly accurate Direct Numerical Simulations (DNS), a new correlation model was proposed which predicts both potential core and supersonic length of the primary oxygen jet entering into a hot atmosphere. The new correlation model was programmed in the COAX code. CARD and COAX calculate all injector parameters such as nozzle contours and process parameters, e. g. oxygen, natural gas and air flow rates. REFERENCES [1] M. Alam, J. Nase, G. Brooks, Computational fluid dynamics simulation of supersonic oxygen jet behavior at steelmaking temperature, Metall. Mater. Trans. B 41 (2010) pp. 636. [2] J.D. Anderson, J.D., Modern compressible flow, 3rd ed. McGraw-Hill Series, 2004. [3] ANSYS FLUENT, User´s Guide, 2008. [4] R.A. Baurle, J.R. Edwards, Hybrid Reynolds-averaged/large-eddy simulations of a coaxial supersonic freejet experiment, AIAA J 48 (2010) 3, pp. 551. [5] D.W. Bogdanoff, Compressibility effects in turbulent shear layers, AIAA J, 1983; 21(6), pp. 926. [6] E.R.G. Eckeret, Engineering relations for friction and heat transfer to surfaces in high velocity flow, J. Aero. Sciences 22 (1955) 8, pp. 585. [7] V. Hermes, I. Klioutchnikov, H. Olivier, Linear stability of WENO schemes coupled with explicit Runge–Kutta schemes, Int. Journ. Numer. Meth. Fluids 69 (2012), pp. 1065. [8] I. Klioutchnikov, H. Olivier, H.J. Odenthal, Numerical investigation of coaxial jets entering into a hot environment, Computers & Fluids 86 (2013), pp. 490. [9] E. Murakami, D. Papamoschou, Mean flow development in dual-stream compressible jets, AIAA J 40 (2002) 6, pp. 1131. [10] H.J. Odenthal, P. Bui, M. Reifferscheid, E. Hovestaedt, J. Nies, I. Klioutchnikov, H. Olivier, Advanced design of burner/injector systems in electric arc furnaces (EAF), Proc. of the 4th Int. Conference on Modelling and Simulation of Metallurgical Processes in Steelmaking (SteelSim), 2011, pp. 1. [11] D. Papamoschou, A. Roshko, The compressible turbulent shear layer: an experimental study, J. Fluid Mech. 197 (1988), pp. 453. [12] R. Sauer, General characteristics of the flow through nozzles at near critical speeds, NASA TM, No. 1147, 1947.