Oxygen-Enhanced Ladle Preheating Systems: Improved Tap-to-Tap ...

Oxygen-Enhanced Ladle Preheating
Systems: Improved Tap-to-Tap Cycle
Time and Operating Cost Reductions
Energy costs and stricter emissions requirements are
a perennial concern of steelmakers. Although ladle
preheating consumes a relatively small portion of the
overall energy budget of a steelmaking plant, it is an inefficient
energy consumer and can benefit from improvements
in combustion technology. Furthermore, poor
ladle preheating can have a large effect on the energy
efficiency of the largest energy consumers within the
steelmaking shop, the EAF or BOF itself, and the LMF
station or other secondary metallurgical facility with
reheating capability. Modern steelmaking ladles need to
be preheated to in excess of 2,100°F which means that
more than 60% of the fuel energy is exhausted to the
stack when air fuel burners are used. Praxair’s Dilute
Oxygen Combustion (DOC) system provides a means of
cutting energy consumption by as much as a half, while
also lowering carbon emissions and reducing NOx.
Adequate control of ladle brick temperature is critical
to the operation of the modern steelmaking facility. Not
only is it necessary for temperature management at tap
and during secondary treatment, it is also necessary for
adequate control of temperature during casting. Cold
ladles can lead to excessive temperature drop during
casting and, in extreme cases, inability to complete the
cast. Figure 1 shows a schematic of the effect of time
from end cast on a ladle to tap on the subsequent rate
of temperature drop between the end of secondary
ladle treatment and the caster. As the time from end
cast to tap extends, the rate of temperature loss and
the variability in the rate of temperature loss increases.
The increase in variability is due to inconsistent use of
covers and ladle preheaters. Ladles with more than 120
minutes between end of cast and tap were consistently
put on preheaters and show better temperature control
than ladles in the 90- to 120-minute group. When the
expected time from end cast to tap was in the 60- to
90-minute range, operators noted that ladles which were
placed on the air-fired preheaters tended to be colder
than ladles that sat under a cover. Changing the practice
so that all ladles with more than 90 minutes from
end cast to tap were preheated improved temperature
uniformity. However, ladles with end-cast-to-tap times of
more than 60 minutes continued to present significant
temperature control issues. The air-fired preheaters
were unable to rapidly bring ladles up to temperature,
and preheated ladles tended to be cold on the bottom.
Authors
To compensate for the variability in ladle heat loss,
primary steelmaking has to tap at a higher temperature,
resulting in some heats that are too hot at secondary
treatment. Heats that are too hot or too cold at
Enhanced ladle preheating stations are
possible through the use of burners capable of
operating on either air or oxygen. Results for
reduced operating costs, as well as reduction
in tap temperature and tap-to-tap cycle time,
are presented.
secondary treatment incur a penalty in process time,
potentially hurting shop productivity. Higher tap temperatures
incur an energy and process time penalty at
primary steelmaking, particularly in EAF shops. Thus,
ladle preheating can have a large impact on the shop’s
overall energy efficiency and productivity. Praxair’s
oxy-fuel-based ladle preheating systems offer rapid and
uniform ladle heating, which can catalyze significant
improvements in shop productivity and in overall energy
consumption beyond the energy savings at the ladle
preheaters themselves.
This paper discusses laboratory work to characterize
the performance of Praxair’s oxy-fuel technology in
ladle preheating and outlines the results from customer
installations.
Technology Overview
Dilute Oxygen Combustion (DOC) — DOC technology,
incorporated in the J/L (Jet/Lance) burners developed
by Praxair, improves temperature uniformity, reduces
NOx and carbon emissions, and reduces the consumption
of natural gas compared to air-fuel burners.
Patents covering the J/L burner or DOC process or
equipment include:
• 5,076,779
• 5,266,025
• 5,387,100
• 5,449,286
James Kelly (left), senior process specialist, Praxair R&D, Indianapolis, Ind.
(james_kelly@praxair.com); F. Dentella, ESA Pyronics International, Bergamo,
Italy (dentella@esacombustion.it); A. Recanati, SIAD SpA, Bergamo, Italy
(anselmo_recanati@Praxair.com); Jorge Visus (center), product manager, combustion
and heat treating, Praxair Espana, Madrid, Spain (jorge_visus@praxair.com);
and Emmanuel Miclo (right), Praxair SAS, Rungis, France
May 2011 ✦ 307

Figure 1
Effect of end-cast-to-tap time on the rate of temperature loss between
the end of secondary processing and casting.
Figure 2
Comparison of the firing rate and temperature versus time for an airfired
and a DOC-fired preheater heating a cold ladle to 2,200°F.
Table 1
Summary Statistics From Ladle Preheater Comparison
308 ✦ Iron & Steel Technology
DOC
Units Air-fired burner Improvement
Setpoint °F 2,200 2,200 —
Average Fce Temp °F 2,224 2,311 —
Cycle average Fire rate mmBTU/hr 2.85 1.44 50%
Time to T min 170 70 59%
NOx lbs/mmBTU 0.073 0.044 40%
NOx lbs/hr 0.21 0.063 70%
• 5,601,425
• 5,755,818
• 5,924,858
• 6,007,326
• 6,394,790
In Dilute Oxygen Combustion, the fuel gas
and oxygen are injected separately via highvelocity
jets, creating rapid mixing of the fuel
and oxidant with the hot furnace gases. This
mixing and dilution produces a thermally uniform
heat release with low peak flame temperatures.
The uniform heat release results in more
even temperature distribution throughout the
furnace. The low flame temperatures are key to
inhibiting the production of thermal NOx.
DOC Concept —
• Separate fuel and oxygen injection at high
velocity.
• Rapid dilution of fuel and oxygen with
furnace gases.
• Diffuse flame results in very uniform heating.
• Low peak flame temperature minimizes
NOx production.
The J-burner portion of the J/L burner consists
of a fuel injection assembly retained within
a concentric oxygen tube. The design of the
J-burner makes use of Praxair’s Coherent Jet
(CoJet ® ) technology to delay mixing of the fuel
gas and oxygen. This permits the J-burner to be
recessed within the burner refractory block for
protection from heat, without risk of damage
due to combustion within the recess cavity. The
burner’s fuel assembly is tipped with a fieldreplaceable
nozzle, thereby permitting adjustment
of flame characteristics. The J-burner
includes a spark igniter and UV flame sensor
for self-ignition and flame safety requirements.
The lance portion of the burner is used to
inject oxygen whenever the furnace is above the
fuel auto-ignition temperature. It is separated
from the jet by several inches in order to entrain
furnace gases and prevent mixing of
the oxygen and fuel close to the burner,
thereby delaying combustion. This
allows the oxygen to become “diluted”
with the furnace gases and results in a
much lower flame temperature. Since
combustion takes place throughout the
furnace chamber instead of directly at
the burner, a more diffuse flame is produced
instead of a point heat source,
resulting in more even distribution of
energy throughout the furnace.
Key Burner Characteristics —
• Field-replaceable nozzles —
allows different velocities to be

selected. This allows luminosity, burner
momentum, flame intensity, flame length
and heat flux profile to be controlled for
each application.
• Includes automatic spark ignition system
and provisions for UV flame detector for use
with flame safety.
• Burner capacities of 6–9 mmBtu/hr.
• Low maintenance.
The characteristics of DOC technology are presented
in greater detail elsewhere. 1
Laboratory Measurements
To characterize the performance of DOC technology
in a ladle preheating-type application,
the ladle preheating process was simulated in
the laboratory. An existing high-temperature
furnace was modified to simulate a ladle with
a cross-sectional area of 1,600 inches and depth of 91
inches. The furnace was lined with 8.75 inches of highalumina
refractory. The air firing condition was simulated
with a 3.4 mmBTU/hr Bloom burner. A Praxair 2.4
mmBTU/hr JL burner was used to simulate the DOC
oxy-fuel firing condition. Figure 2 shows the evolution
of the burner firing rate over time. After 1 hour, the
DOC burner had heated the ladle to the temperature
setpoint of 2,200°F, at which point the burner firing rate
started to modulate and within six hours had cut back to
less than 1 mmBTU per hour. The air-fired burner took
almost three hours before the setpoint temperature was
reached and had cut back to only 1.8 mmBTU per hour
after six hours.
Table 1 shows summary statistics from the ladle preheating
tests. The DOC burner was able to heat the ladle
Figure 4
Figure 3
Initial temperature rise versus simple heating model.
(a) (b)
to a higher temperature in less time with 50% lower
fuel consumption and 70% less total NOx output. The
ladle average temperature was higher at the end of the
heating cycle for DOC, even though for both burners
the setpoint was 2,200°F. In the DOC test, the flame was
purposely directed toward the bottom of the ladle, giving
a temperature increase with distance from the ladle
mouth. With the control thermocouple placed at the
ladle mouth, there was approximately 200°F temperature
increase toward the ladle bottom.
Figure 3 compares the heat-up profile of the two
burners for the first hour with a simple spreadsheet
model. The model is based on the solution presented
by Carslaw and Yeager 2 for heat transfer at constant heat
flux into an infinite slab starting at room temperature.
The model suggests that, over the first 70 minutes of the
heating cycle, 85% of the heat energy from the fuel is
Typical DOC-type ladle preheater installation showing the piping connections on the cold face (a)
and the hot face of the burner block (b).
May 2011 ✦ 309

available to heat the refractory for the oxy-fuel-fired system.
Less than 50% of the energy from the air-fired system
is available. The model does not agree very well in
the first 30 minutes of the heat-up cycle. This is for two
Figure 5
Table 2
Operating Results Achieved at Various Customer Sites
310 ✦ Iron & Steel Technology
Benefit
Customer Fuel consumption Tap temperature Ladle life Cold ladles Aborted casts
A 50% No change Improved Improved Reduced
B Not documented — — Improved Reduced
C 50% 40°F decrease No change No change No change
D 50% 11°F decrease No change No change No change
E 51% No change No change No change No change
F 50% No change No change No change No change
Change in average furnace tap temperature with increases in the ladle
preheater setpoint.
Table 3
Breakdown of Oxy-Fuel Preheating Economics at Customer C
Based on hypothetical fuel ($6/mmBTU), oxygen ($0.25/100 scf) and electricity
prices ($0.04/KWhr)
Air fuel preheating Oxy-fuel preheating
Natural gas 0.055 mmBTU/T 0.027 mmBTU/T
0.055 x 6.0 = $ 0.33/T 0.027 x 6.0 = $0.16/T
Oxygen $0.00/T 54 scf/T
0.54 x 0.25 = $0.14/T
Gas subtotal $0.33/T $0.30/T
Electricity 0.16 x 40 = 6.4 KW/T $0.00/T
6.4 x 0.04 = $0.26/T
Total $0.59 T $0.30/T
reasons. First, when the ladle is cold, a greater percentage
of the energy in the combustion gases is available
to heat the refractory. Second, the thermocouples for
measuring the brick surface temperature were located
slightly proud of the refractory surface, so at
low temperatures they were influenced quite
strongly by the temperature of the combustion
gases. In the first 30 minutes of the heat-up,
the thermocouple reading reflects more on the
surrounding gas temperature than the surface
temperature of the refractory itself.
Installation
Installation of the DOC burner technology on
existing preheaters is a simple matter of installing
a flow control skid for the fuel and oxygen
flow and a burner block in the ladle preheater
cover. The volume of flue gases is reduced
eightfold, so any flues in the existing covers can
be closed and the waste gases can be vented
through the gap between the preheater cover
and the ladle. The reduction in volume of flue
gases greatly reduces the potential for damage
to the cover, the burner block and the cover
support system over the life of the cover system.
As with any ladle preheat application, controlling
the gap between the cover and the ladle is
important for maximizing efficiency.
Figure 4 shows a view of the hot face
and the cold face of the burner installation.
The dimensions of the piping are
considerably reduced from a typical
air-fired installation. This installation
includes a connection from the preexisting
combustion air fan. This fan
is operated only in ladle dryout mode
to provide excess air to help remove
moisture and maintain the slow temperature
ramp-up necessary for ladle
dryout. The ladle cover shows a flue
that was closed off after installation
of the oxy-fuel burner. Table 2 shows
results from various customers who
have used Praxair oxy-fuel fired ladle
preheating systems. In all the cases
the expected 50% reduction in fuel

consumption has been achieved. Customer C was able
to use oxy-fuel preheating to great advantage by reducing
the tap temperature and as a result saving considerably
on electrical power in the furnace. This customer
increased the preheater setpoint temperature by 500°F
to 2,350°F and still achieved a 50% reduction in fuel
consumption. The net worth of the savings was about
$0.3 per ton of steel produced. Other customers (D and
E) have achieved significant savings in electrical energy
for secondary steelmaking although the magnitude of
these savings have not been documented. Customer B
noted that the performance at the caster improved significantly
with much lower temperature drop during the
course of teaming. This reduced the number of short
casts and improved the liquid process yield.
Over a period of three months after the start-up of the
oxy-fuel ladle preheating system, customer C found that
it could increase the ladle preheat temperature setpoint
and achieve significant process benefits. In particular, it
could reduce the tap temperature and still obtain good
temperature control through secondary metallurgy and
the caster. Figure 5 illustrates how the increasing ladle
preheater setpoint temperature influenced the tap temperature
at the furnace. This reduction in tap temperature
had a positive effect on furnace tap-to-tap times,
which in turn allowed improvements in shop productivity.
At the highest preheat temperature setpoint, the slag
would start to melt on the ladle walls and bottom if the
preheat time was extended. Care was taken to ensure
ladle preheat times were not excessive.
Table 3 breaks down the economics achieved at customer
C. The actual fuel, oxygen and energy costs are
hypothetical customer values and not representative
of prices extant at the startup at this customer. In this
scenario the savings achieved are $0.29/tonne. These
savings do not include the benefit of improved productivity
and actual results may vary from those achieved by
customer C.
Conclusion
Dilute Oxygen Combustion technology has been proven
in the laboratory and in commercial furnace installations
to provide more uniform temperature distribution,
reduce NOx and carbon emissions, and consume
less fuel than conventional combustion technologies.
Since the development of this technology by Praxair in
the late 1990s, it has been successfully applied to glass
making furnaces, metal reheat furnaces and other applications.
In the ladle preheating application, it offers
more than 50% fuel savings, 70% reduction in NOx
emissions, and improved ability to heat ladles in a more
timely and effective fashion. The customer experience
has shown that the improved ladle heating performance
leads to significant shop operational benefits, including
the ability to lower tap temperatures, improve process
time at secondary metallurgy and improve performance
at the caster. The magnitude of the combined energy
savings can reach $0.29 per ton of steel produced.
Benefits from improved productivity due to faster processing
at the furnace, secondary metallurgy and the
caster can be much greater.
Acknowledgment
The authors greatly appreciate the help of Kelly Tian,
Robert Miller, Lee Rosen and Larry Cates in conducting
experiments, assembling data, and providing analyses of
the data presented in this paper
References
1. Cates, Larry, “Improved Temperature Uniformity in Batch
Reheat Furnaces With Praxair’s Dilute Oxygen Combustion (DOC)
Technology,” AISTech 2010 Conference Proceedings.
2. Carslaw, H.S., and Yeager, J.C., Conduction of Heat in Solids,
2nd ed., Clarendon Press, Oxford, 1959. ✦
This paper was presented at AISTech 2010 — The Iron & Steel Technology Conference and Exposition,
Pittsburgh, Pa., and published in the Conference Proceedings.
Did you find this article to be of significant relevance to the advancement of steel technology?
If so, please consider nominating it for the AIST Hunt-Kelly Outstanding Paper Award at AIST.org/huntkelly.
May 2011 ✦ 311