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Coherent Jets in Steelmaking: Principles and Learnings
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3. INTRODUCTION
The iron and steel industry is one of the largest users of industrial gases such as oxygen,
nitrogen, and argon. A majority of this usage occurs during molten metal processing,
wherein the gases can be injected into the molten metal bath by the following methods:
• From above the surface, with lances, as in the BOF converter and the electric
arc furnace (EAF)
• Sub-surface, with tuyeres (as in AOD and Q-BOP processes) or submerged
lances (in some EAFs)
Figure 1 - (a) Top lance in a BOF, and (b) bottom blowing tuyeres in a Q-BOP
Sub-surface injection with tuyeres is generally most efficient, but suffers many practical
problems that have hindered its widespread application. Similarly, submerged lances are
continually consumed by the molten metal and need to be constantly replaced.
Therefore, lances above the molten metal surface are the predominant method of gas
injection into molten steel.
Some of the critical factors in injecting gases from above the metal bath are:
• Penetration of the jet below the bath surface
• Degree of splashing
• Intensity of bath mixing
• Lance height above the metal bath surface
• Flow rate per nozzle
• Nozzle design and angle
Generally, the gas jet from the lance needs to be injected at supersonic velocities to allow
greater penetration and lance height.
2
4. Supersonic Jets
In steelmaking applications, supersonic gas jets are preferred over subsonic jets to
increase gas penetration into the molten bath. In order to obtain a supersonic jet, a
converging-diverging or Laval nozzle must be used at the lance tip, Figure 2 (1). The
exit velocity of oxygen jets in steelmaking is typically in the range of Mach 2.0-2.3 or
about 550-575 m/s.
Exit
Throat
Figure 2 - Schematic of a Laval nozzle
The behavior of supersonic gas jets issuing from Laval nozzles has been extensively
studied (1-5). The profile of a jet after it exits the nozzle can be broadly classified into
three regions as a function of distance from the nozzle exit, Figure 3. In the initial part of
this region, termed as the potential core region, the axial jet velocity is equal to the exit
velocity at the nozzle. Downstream of the potential core region, the jet expands as a
result of turbulent mixing and entrainment of the ambient atmosphere, but its velocity is
still supersonic. Subsequently its axial velocity and force decays, and the jet becomes
subsonic. Downstream of the potential core, supersonic jets expand at an angle of about
10° from the longitudinal jet axis (1).
Mach 2
Mach 2
Mach 1
Potential
Core
20-35D
Steel
Bath
Sub-sonic
Figure 3 – Profile of a jet issuing from a conventional Laval nozzle
Several studies have investigated the length of the potential core and the supersonic
length (3-10). The potential core length is affected directly by the gas pressure upstream
of the nozzle (or the Mach number of the jet) and inversely by the square root of the ratio
of the ambient gas density to the jet gas density. Typically for steelmaking applications,
3
5. the length of the potential core region of an oxygen jet ranges from approximately 20D in
room temperature ambient atmosphere, to approximately 35D in hot furnace conditions;
where “D” is the throat diameter of the correctly designed Laval nozzle for the selected
flow rate.
The distance to the steel bath should be within the potential core region for a “hard blow”
impact, penetration and less splash. Penetration of supersonic jets in metallic baths has
been investigated (9,11,12). However, there is considerable disagreement between the
various studies in regard to the estimated penetration depth for a supersonic jet impinging
on a steel bath. Nevertheless, it is well understood that the penetration and mixing
provided by a jet is a strong function of the jet conditions at impact.
Conventional supersonic gas jets delivered through top lances are limited in the distance
over which the supersonic velocity is maintained, the gas flow rate that can be injected
without excessive splashing, and the extent of stirring provided to the molten bath.
Coherent Jets
The potential core length of a supersonic jet can be significantly increased by shrouding it
in a flame envelope (13-15). This shrouded jet is also called a Coherent Jet. CoJet®
technology for coherent jets was developed by Praxair over a number of years. Basic
research on jet behavior led to the concept of jet coherency, and ways to extend jet length
using optimum flame shrouds. The flame shroud is created using a fuel and oxidant in the
optimum configuration. A coherent jet injector or lance comprises a Laval nozzle and a
surrounding nozzle arrangement for the flame envelope. The operating conditions for the
injector can be altered so that it is capable of producing a jet in which the supersonic core
length can be varied. The injector is also capable of producing a conventional supersonic
gas jet. With optimum flame conditions, the potential core length of an oxygen coherent
jet can be extended beyond 70 nozzle diameters, Figure 4.
Mach 2 Mach 2 Mach 1
Potential Core Length = ~70D
Supersonic Length
Steel
Bath
Sub-sonic
Figure 4 – Profile of a jet issuing from a Coherent Jet nozzle
4
6. A comparison of the potential core lengths for different oxygen jets is shown below:
Jet lengths
0 10 20 30 40 50 60 70 80
Supersonic with
O2 shroud only
Coherent Jet
(Fuel + O2
shroud)
Supersonic Jet
(No shroud)
Nozzle diameters
Room Temp. Hot atmosphere
Figure 5 – Potential core lengths of oxygen jets with different shroud conditions
The following observations are drawn from Figure 5:
a) A supersonic jet is longer in a hot furnace atmosphere than in a room
temperature atmosphere – this is clearly due to the lower density of the
ambient gases at high temperatures.
b) A coherent jet is longer than a supersonic jet in both room temperature as well
as hot furnace conditions. In addition, the length of an optimum coherent jet
is not significantly affected by its ambient temperature (i.e. the shroud is its
own ambient condition). Hence, coherent jet injectors can be positioned well
above the bath and/or in the sidewall of the furnace and carry out effective
bath lancing.
c) A supersonic jet shrouded with oxygen alone (no fuel) could conceivably use
the fuel from the furnace atmosphere (e.g. CO) to produce coherency.
However the chilling effect of the shroud oxygen and the slow kinetics of the
furnace gases actually serves to reduce, rather than extend, the jet length.
A fundamental attribute of coherent gas jets relevant to steelmaking is the ability to
deliver to a molten bath precise amounts of gas at high velocity and impact pressure from
a longer stand-off distance with lesser splashing and greater penetration. Also, when the
coherent jet of oxygen produced by the nozzle impinges the molten steel bath, the
concentrated momentum of the oxygen jet dissipates in the steel as fine bubbles,
providing deeper penetration and more effective slag-metal mixing. This results in high
5
7. efficiency lancing and decarburization. Coherent jets can be applied to steelmaking to
inject gas for refining reactions (oxygen), for stirring and slag-metal mixing (nitrogen or
argon), to provide chemical heat (fuel and oxygen), and to convey solid materials such as
coal or other powders.
CoJet®
TECHNOLOGY IN ELECTRIC ARC FURNACES
The introduction of Praxair’s CoJet gas injection technology several years ago, was a
significant step in effectively injecting chemical energy in electric arc furnace steel
making. This breakthrough technology was the first to introduce the concept of fixed
wall mounted injectors in an electric arc furnace, with each injector designed to perform
multiple functions such as:
- Burner,
- Lancing,
- Post combustion, and
- Carbon/powder injection
This arrangement replaced the traditional lance manipulator and burners on the furnace,
as shown in Figure 6.
CoJet®
Injector
CoJet®
Injector
CoJet®
Injector
Burner
Lancing
PC
Carbon
Burner
Lancing
PC
Carbon
Burner
Lancing
PC
Carbon
Door
Closed
Lancing
Carbon
Manipulator
Conventional Lance Multi-point Injectors
Figure 6 – Comparison of furnaces using conventional system vs CoJet® system for
chemical energy in an EAF
Historically, conventional EAF operation has been carried out by manual lancing and
carbon injection, usually through an open slag door. Under these operating conditions,
6
8. there can be considerable variation in the amount of chemical energy utilized from one
heat to another or from one shift to another. Furthermore, the oxygen injected for
lancing ends up being added entirely in a limited local area of the molten steel bath,
usually just behind the slag door. This can result in localized over-oxidation, high
refractory wear at the banks, and excessive electrode consumption of the phase directly
in line with the door lance.
All such drawbacks can be immediately eliminated with the installation of a CoJet®
system on the furnace. By using coherent jet technology, the furnace is converted from a
manual operation to a fully automated sequenced operation with the slag door closed, as
depicted in Figure 6. In addition, the oxygen and carbon are now uniformly introduced
through multiple injectors around the furnace in a homogeneous manner as shown in
Figure 6. This allows for the use of higher overall chemical energy with added benefits
achieved in return.
With the installation of the CoJet injector in September 1996 (14), Macsteel-Arkansas
became the first electric furnace in the world to operate with fixed multi-functional
sidewall injectors. Today, steel makers have widely accepted this concept, as it has
enhanced their efforts to lower costs, improve productivity, and in general, optimize
their melting process. Indeed, the industry has shifted to this new standard for chemical
energy input in EAFs.
That the CoJet program has been a success, is clearly evident. The testimony to its wide
acceptance in the EAF melting process, can be summarized as follows:
• Seventy (70) furnaces authorized to operate this technology worldwide over
the last six years
• A wide geographical distribution with customers in the United States,
Canada, Mexico, South America, Europe, and Asia.
• Furnaces ranging in capacity from 30 to 200 metric tonnes tap weight.
• The number of injectors installed per furnace ranging from one (1) to four
(4).
• Various furnace types that include AC, DC, Shaft, and Consteel®
process.
• Raw material input to the furnace with wide ranging variation - 100% scrap,
100% DRI, a mixture of scrap and DRI, alternate irons, hot metal,
continuous scrap feeding.
• Furnaces operating under constant flat bath conditions with continuously
varying bath heights.
• Inherent burner capacity of 3MW to 6MW per injector.
• Designed lancing capability from 775 Nm3/hr to 4200 Nm3/hr per injector
• A wide spectrum of oxygen practice from 12 Nm3/ton to 55 Nm3/ton
A furnace operated with the CoJet ® system can reap benefits from this technology -
typical cost benefits derived are from a combination of following parameters:
7
9. ♦ Reduced power consumption
♦ Increased productivity
♦ Elimination of supersonic lances and manipulators
♦ Significantly reduced maintenance
♦ Improved yield
♦ Reduced refractory wear at banks
♦ Reduced gunning
♦ Reduced electrode consumption
♦ Reduced injected carbon
♦ Improved delta life
Some of the other factors that lend added value to using a coherent jet technology are:
♦ Automation – less operator dependent
♦ Consistency – from heat to heat
♦ Improved slag foaming – higher rate of power input
♦ Non water cooled injectors – easy to check
♦ Total flexibility – option to selectively lance with any injector(s)
Specific details on results from various furnaces have been reported elsewhere (16-22).
KEY LEARNINGS
With 70 furnaces around the world authorized to use CoJet® technology, considerable
knowledge has been garnered at Praxair from their experience with a wide range of
furnace types and operating practices. This could be broken down into several
categories.
Jet Characteristics
It is now clear that coherent jets can be designed for remote location on the sidewall
even at a height as high as 2 meters above the bath. Concurrently, coherent jet length
can be achieved where jet coherency of supersonic speeds can be offered up to 3 meters
in length from the nozzle face.
This factor becomes even more critical when furnaces are operated with 100% DRI
feed, hot metal or continuous feeding of scrap. Under these conditions, the furnaces are
in a flat bath condition all the time. Given these conditions, where the bath height
changes significantly over the extent of the heat, it becomes critical to have accurate
quantitative data on jet length. This helps ensure that a truly coherent jet is designed
with the appropriate requirements, and with its characteristics maintained over the
requisite distances. Such well designed coherent jets can function effectively in such
8
10. furnaces with continuous feeding, or even in furnaces where there are bath height
changes due to bottom wear.
It is critical to have quantitative data on coherent jet length and “perfect” coherent jets
to handle variations in bath height without splashing or creating negative effects on the
furnace. A good example is the use of CoJet technology on the 3 x 200 metric ton
furnaces at Sidor in Venezuela (19). The furnaces operate with 100% DRI that is
continuously fed from the top. The bath height changes by 800 mm (31") from the
initial 50T heel to the 200T full level, see Figure 7. Lancing is done exclusively with 3
fixed CoJet injectors, installed 850mm (33.5”) above final bath level. The jet length
required to impact the bath was 2260mm (89") at start to 1340mm (53") at the end. A
perfect coherent jet with the CoJet system could handle the bath height change and start
lancing and slag foaming early in the heat.
Figure 7 - CoJet® system installation for a 100% DRI operation
Imperfect coherent jets when installed in such furnaces, can render considerable damage
to the furnace and negatively impact the operation. Typical problems that can be
encountered with imperfect coherent jets are:
• Excessive splashing
• Excessive overheating of panels
• High delta wear
• High refractory wear
• High levels of FeO
9
11. • Poor yield
Another key aspect of coherent jets are the shroud ports, which are integral to the nozzle
design. Maintaining the appropriate gas flows through the flame shroud (fuel gas and
shroud oxygen) is critical for proper operation in each of the modes. If the flows to the
shroud ports are not independently controlled, the integrity of the coherent jet can be
compromised during a campaign. Such occurrences lead to negative effects on the
furnace, some of which are listed above.
High Chemical Energy Utilization
Another aspect of a well-designed coherent jet system is the significant impact of
chemical energy addition. Experience gained shows that CoJet® systems can be
effectively used even with high oxygen practice. Some customers are operating furnaces
at > 45 Nm3/ton of oxygen and still reaping the benefits of improved yield, reduced
power consumption, and higher productivity. Indeed, the observed general benefit of
injecting oxygen in to the furnace using a CoJet system, yields a power reduction of
about 4.0-4.5 kWh/Nm3
of oxygen used. This clearly demonstrates that the coherent jet
technology can be used to avail of high chemical energy utilization with positive results.
This is mainly due to the following features:
• Multiple injectors offer multi-point injections
• Homogeneous distribution of energy in the furnace
• Uniform early foamy slag generation
• Efficient coherent jets provide better stirring and mixing resulting in lower
FeO in the slag
• Post-combustion, especially at high oxygen levels
The most striking feature is that high levels of chemical energy have been consistently
delivered without compromising on yield or slag FeO levels.
Lancing Efficiency and Carbon
It is now quite apparent that when lancing with coherent jets, the decarburization
efficiency improves considerably. This aspect becomes a useful tool to reduce refining
times, improve productivity, and make concomitant gains in power savings. Since
overall oxygen usage can be increased with multi point injection, without any detriment
but rather added benefit to the practice, the balancing factor then has to be carbon
availability. If this is overlooked, inevitably melt samples will register very low carbon.
While injected carbon is a good tool to generate early slag foaming, it is really the
charge carbon that is needed to provide acceptable final carbon in the melt. Injected
carbon has not proved to be an effective vehicle to recarburize with any degree of
consistency.
10
12. Indeed, experience has now shown that levels of injected carbon should be closely
monitored to ensure that unnecessary wasteful addition is avoided. Generally, carbon
injection with the CoJet® system has demonstrated that best results are achieved with
additions that are less than 8-10 kg/ton. Increasing injected carbon additions excessively
beyond this level do not serve to shorten the heat or improve foamy slag. The charge
carbon should be adequate enough, not only to get reasonable carbon in the melt
samples, but also to contribute to some foaming in the furnace.
The improvements in jet penetration, uniformity of lancing around the furnace and
better slag foaming have also translated into lower N levels in the steel. Reductions up
to 10ppm have been achieved, both in scrap and DRI charged furnaces.
Simplicity and Consistency
A salient feature of the coherent jet technology has always been the simplicity of the
concept and its effectiveness in delivering consistency, heat after heat. While the general
piping and site work can be initiated early without any interruption to regular
production, the final conversion of a furnace to the CoJet system can be executed within
a day. Advance training provided in the basic concepts of the CoJet technology
operation, coupled with simulation practice runs, makes the actual start-up easier to
commission. Once the system is running in operation, the logic and simplicity of its
operation becomes evident, as the injectors function in different programmed modes
based on the appropriate signals received.
The operators then can quickly grasp the versatility of the system and the flexibility it
offers for changes in operating practice. These changes include factors such as raw
material/scrap mix, final product mix, and the number of buckets charged. Since the
injectors run almost entirely in a programmed sequence, the oxygen and carbon
injection always remains consistent This translates to a fairly consistent input of
chemical energy for every heat, resulting in overall improved consistency of operation.
Maintenance
A frequent concern of any melt shop is the kind of attention and the level of
maintenance necessary for any new system and hardware under consideration for use.
This aspect is crucial since any down time needed to carry out maintenance, comes at a
price. At times it can be significant, especially, if production has to be halted to correct
the problem.
At the very outset, this aspect of the CoJet system has been a major benefit to steel
makers. The injector itself is non water-cooled and weighs barely 13 kg. A quick
inspection of an injector, as timed by some customers, takes about 10 minutes. This
includes removal, check, and re introduction in to the side panel. Generally, such an
inspection is advised at least once a week on a down day to ensure the integrity of the
nozzle face. However, a special screen is also available at the operator station to alert
11
13. the operator, in case any specific changes occur. This feature is of great benefit since it
can not only help troubleshoot with ease, but also identify the area of the equipment that
may need attention.
Another key benefit from a maintenance perspective is the life of the coherent jet
injector. Experience shows, if adequate cooling water as required is maintained, basic
flows and pressures of oxygen and fuel are held and sustained in the various modes -
especially during charging, the injector life is significant. Customers have been using
CoJet® injectors and panels for more than a year, and in some cases, even up to 18
months or more . This is a key benefit attained since the coherent injectors are truly
remote in location, and thus not subject to the severe and constant harsh conditions of
the proximity of the bath, as is the case with supersonic lances or some imperfect
coherent jets.
Environmental Factors
The CoJet system was basically developed as a novel approach to provide chemical
energy effectively and efficiently, to the electric arc furnace to facilitate melting. While
the equipment, at the outset, was not designed specifically to reduce CO and NOx, it
was also not expected to increase these constituents generated at a customer’s facility.
Since post combustion is offered as part of the CoJet® program, it was expected that
combusting a good portion of the CO in the furnace should result in possibly some
reduction of this product at the bag house.
Secondly, an added feature of the CoJet technology, is to operate the furnace with the
slag door closed for almost the entire heat. This reduces the air infiltration and heat
losses through the door, and at the same time, provides additional safety to the
operation. Additionally, a closed door operation also reduces the potential to form NOx
in the furnace. Figure 8 illustrates the typical NOx evolution profile observed, and
shows that most of the NOx emissions occur with the opening of the slag door towards
the end of the heat.
NOx History for CoJet Case
0
100
200
300
400
500
0 6 12 19 25 30 36 42 48 54 61 67
Time (mintes)
NO
x
(P
PM
)
Slag door open Tapping
Figure 8 – Example of NOx emissions
12
14. It is now apparent that the coherent jet technology can assist in helping companies
reduce their CO and NOx emissions. There may also be other steps necessary to modify
the operation within the plant to reduce generation of these constituents. Nevertheless,
whatever these steps may be, in conjunction with the CoJet technology, taken together,
can result in reduced emissions and help in meeting environmental compliance.
FUTURE ENHANCEMENTS
While the CoJet program has been widely accepted worldwide, based on steady
feedback received from customers and the market place, several programs are underway
in development and commercialization. Automation is an area of interest, and stemming
from it are models that have been developed to predict end point carbon and provide
closed loop carbon injection, as depicted in Figure 9. Both these models are geared
towards further optimization of the process, increased consistency, and added
automation.
Arc Stability
Measurement
CoJet Closed
Loop System
Carbon
injection Low,
Medium, Hi
Carbon
Injection
System
Figure 9: Closed loop carbon injection arrangement
Coherent jet injectors are also now in commercial testing to inject solids/powders in the
furnace. This would not only be helpful to recycle arc furnace dust, but also facilitate
controlled recarburization through carbon injection. Since the requirement to add carbon
in to the bath to recarburize arises often due to scrap quality variation, this aspect of the
program has considerable value to furnace operators.
As the CoJet® technology is adopted in different regions of the world, it has become
necessary to modify the design to adapt to different fuel feedstock. Besides natural gas
and LPG, injectors are now available for use with alternate fuels - including liquid fuels.
This also entails programming and functional changes that are needed to operate
effectively and prevent residue contamination at the nozzle surface.
The use of coherent jet technology is also available for application to other areas such as
stainless steels. With inert gas coherent jets developed for use, this can be of great
benefit to stainless steel melt shops to improve cost benefits from:
- Metallic and chromium yield improvement
- foamy slag operation
- deoxidizer savings
13
15. - productivity improvement
Application on a commercial scale is underway. Historically, stainless steel
manufacturers have been unable to utilize oxygen effectively in the EAF. This is
because chromium and other alloying elements are highly susceptible to oxidation,
particularly at the elevated temperatures. This property, when factored in with the
alloys’ high cost, has made it prohibitive to use high levels of oxygen. With inert gas
coherent jet technology, these limitations can be overcome and oxygen can be used
effectively utilized to achieve significant benefits.
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