Paper 12-040.pdf, Page 1 of 8
AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
Increasing Melting Energy Efficiency in Steel Foundries
S. Biswas, K.D. Peaslee, S. Lekakh
Missouri University of Science & Technology, Rolla, MO, USA
Copyright 2012 American Foundry Society
PRESENT STATE OF U.S. FOUNDRIES—
STATISTICAL AND TECHNOLOGICAL ANALYSIS
ABSTRACT
Steel foundries are one of the most energy intensive
industries. The increasing concerns over volatile energy
cost and carbon dioxide emission have pushed the
foundries to improve melting efficiency and hence
decrease electrical energy consumption. This paper is a
review of the research efforts during the last five years at
Missouri S and T under a grant from the U. S. Department
of Energy’s, Energy Saving Melting and Revert
Reduction Technology (“Energy SMARRT”) Program.
Statistical analysis of industrial measurements
(thermocouple, infrared camera) and operating data were
combined with thermodynamic and computational fluid
dynamics (CFD) modeling to investigate best industrial
practices and opportunities to improve energy efficiency.
Improvements in melting efficiency and productivity were
investigated through industrial trials using supplemental
chemical energy through additions of SiC and oxyfuel
burners in electric arc furnaces. New ladle designs and
practices were investigated to reduce energy losses in the
ladle. A dynamic model of heat losses in the ladle from
furnace tap to mold pouring is being developed to aid
foundries in energy optimization.
Within the steel foundry industry, there is great variety in
furnace capacity, power supply, age of equipment, rate of
production, melting schedule and operating practice, all of
which have major influences on energy consumption. A
study of melting efficiency in steel foundries using
statistical analysis provides examples of material and
energy savings from improvements in technology and
melting practices. That study was based on information
gathered at 19 North American industrial foundries
including a combination of historical data and industrial
measurements by the research team. Information and data
were collected on the type of melting equipment, melting
practices, energy use and ladle practices. The data was
statistically analyzed using commercially available
statistics software. A multiple regression analysis allowed
evaluation of the influence of the melting furnace (type,
size, age and transformer power) and operating parameters
such as tap temperature, tap to tap time and furnace
productivity on the energy consumption for melting steel.
TYPES OF MELTING FURNACE USED IN STEEL
FOUNDRIES
The types and age of melting furnaces used in steel
foundries are summarized in Table 1. The average steel
foundry furnace is 28 years old. Electric arc furnaces
(EAF) are generally significantly older installations than
induction furnaces (IF). EAFs used in steel foundries
average 45 years in age with the oldest installation built in
1938 and the newest installation in 1977. Older EAFs are
typically less energy efficient than newer furnaces,
especially in the area of electricity distribution and
control. Coreless induction furnaces used in steel
foundries are typically newer installations averaging just
over 10 years in age with several furnaces installed within
the last five years. Many IF-based foundries have
installed new furnaces with the newest generation of
power supplies which are more energy efficient than
previous generations of equipment.
Keywords: steel foundries, energy, ladle, oxyfuel.
INTRODUCTION
The higher temperatures required for the melting of steel
results in significantly higher energy losses in comparison
with melting other industrial cast alloys. The energy costs
associated with heat losses during melting are
significantly higher for steel foundries than foundries
melting other cast alloys. Today’s steel foundries use both
induction furnaces (IF) and electric arc furnaces (EAF)
for melting steel. To benchmark current energy use and
investigate opportunities for energy improvement, a
survey was conducted among US steel foundries.1 The
results of this work provided ideas as to the best practices
in the foundry industry and a series of industrial trials
focused on improving energy efficiency.2 This paper
summarizes the energy benchmarking from this research
and some of the improvements in energy efficiency that
can be achieved through the addition of chemical energy
and ladle practice development.3, 4, 5
Table 1. Installation Year by Type of Melting Furnaces
Furnace
Type
449
Number
Average
Year
Installed
Oldest
Year
Installed
Newest
Year
Installed
All
58
1977
1938
2003
EAF
24
1960
1938
1977
IF
34
1992
1976
2003
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AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
The capacities of melting furnaces also vary from 400 lb
to 11,0000 lb as shown in Table 2. EAFs are generally
much larger capacity than the IFs.
Table 2. Capacities of Steel Melting Furnaces (lb)
Number
Average
Capacity
(lb)
Minimum
Capacity
(lb)
Maximum
Capacity
(lb)
All
58
12368
400
110000
EAF
24
26433
6000
110000
IF
34
2440
400
9500
Furnace
Type
Fig. 2. Graph illustrates the availability of measuring
devices in foundries.
Figure 1 compares the types of refractory linings used in
the various melting furnaces. IFs use alumina-based
refractories exclusively. EAFs are split with nearly 2/3
using basic refractory linings (magnesia) and 1/3 using
acid refractory linings (silica). The steel foundry industry
has made significant progress in the last 10 years in
moving towards more basic refractory installations in
EAFs to take advantage of the steel quality improvements
associated with basic slag practices.
Statistics of energy consumption in steel foundries is
given in Table 3. Reported energy consumption varies
between 350 kWh/ton to 700 kWh /ton with an average of
527 kWh /ton.
Table 3. Statistics of Energy Consumption (kWh /Ton)
for Steel Melting
Average
Standard
Deviation
Minimum
Maximum
527
65
350
700
Multiple regression analysis was done for the purpose of
determining how operating practice variables and
equipment type (independent variables) influence the
energy consumption in kWh/ton for melting steel
(dependent variable). Figure 3 is a graphical analysis of
the component effect which shows the relative magnitude
of the influence of individual independent variables (tap
temperature, tap-to-tap time, year of installation and
furnace capacity) on the value of the dependent variable
(kWh/ton energy consumption).
Fig. 1. Graph illustrates the refractory practices in
furnaces.
KWH/ton =1364 - 169*(EAF=1; IF=0) - 1.3*Year +
0.91*Tap to tap time, min + 0.57*Ttap,°F
REPORTED ENERGY CONSUMPTION FOR
MELTING STEEL IN CURRENT PRACTICES
Successful energy management in steel foundries is
difficult without monitoring energy consumption.
Unfortunately, this is an area that the steel foundry
industry is poorly equipped. Only 38% of the electric arc
furnaces and 15% of the induction furnaces in operation
are equipped with electric meters for monitoring electric
consumption (Fig. 2). Over one third of the foundries
surveyed have no equipment for monitoring their energy
consumption during steel melting.
The R2 for this equation was 0.54, indicating fairly good
correlation of the data with this equation. The multiple
regression analysis showed that the following independent
variables had an influence on the energy consumption for
melting steel (from strong to weak influence):
• increasing “tap temperature” increased energy
consumption (strong influence),
• increasing “tap to tap time” increased energy
consumption (strong influence),
• •EAF has lower energy consumption than IF (strong
influence),
• newer equipment (“Year of installation”) decreased
energy consumption (strong influence) and
• increasing “furnace capacity” decreased energy
consumption (weak influence).
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AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
(a)
(b)
(c)
(d)
Fig. 3. Multiple linear regression model shows the effect of a) tap temperature, b) tap to tap time, c) installation year
and d) furnace capacity on electrical energy consumption in kWh/ton.
In addition to the statistical data collected at foundries,
foundry operators were asked to report on what they
considered to be the major factors with the greatest
influence on energy losses during melting at their facilities.
The three major factors most frequently cited in the survey
(Fig. 4) were: refractory (75% of surveys), scheduling
(70% of surveys) and casting yield (25% of surveys).
improvements including new melting equipment
(furnaces, power supply PLC, and water cooled panels)
and improved ladle practices (improved linings, ladle
preheaters, alloy wire feeders and argon stirring).
Table 4. The Major Improvements Implemented In 19
Steel Foundries during Last 15 Years
New
Furnaces
9
Lining
7
Melting Furnaces
Power
PLC
Supply
5
4
Ladles
Preheat Alloy Wire
2
2
Water
Cooling
3
Ar Stirring
1
EXAMPLE OF INDUSTRIAL MEASUREMENTS
AND MELTING HEAT BALANCES
A team from MS and T visited five foundries, observed
the melting of several heats and calculated heat balances.
Figures 5 and 6 show examples of the heat balances from
an induction furnace and an electric arc furnace,
respectively. From these figures there are several areas
that could be used to reduce energy consumption in both
types of furnaces.
Fig. 4. Graph shows the survey results for the cause
of decrease in energy efficiency.
According to the survey (summarized in Table 4), many
North American steel foundries have introduced melting
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AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
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preheating the charge,
oxy-fuel burners for heating cold regions of the solid
charge during melt down,
post-combustion of CO produced in the furnace to
CO2 and
exothermic heat from oxidation reactions within the
melt.
Both preheating of the scrap charge and use of oxyfuel
burners have the potential of increasing the melting
efficiency of the solid scrap charge. Two supplementary
chemical energy methods, post-combustion of CO in the
furnace to CO2 and exothermic heat from oxidation
reactions to the melt, could both increase energy
efficiency during the flat bath period. Opportunities to
increase the energy efficiency are greatest during the
superheating and correction period because the electrical
energy efficiency drops significantly when heating
liquid steel with an open arc in air. A significant portion
of the arc energy is reflected from the arc and bath
surface to the sidewalls and roof where the energy is lost
in heating (and often melting) refractory rather than
steel. In addition to using chemical energy, there is a
future potential of increasing arc efficiency by utilizing
more energy efficient long arcs (higher voltage and
lower current) with a foamy slag to decrease the heat
losses by blanketing the arc.
Fig. 5. Sankey-diagram (energy flow) of heats in hot
lining with 200 lb heel in 2 ton IF is illustrated.
Scrap preheating systems, oxyfuel burners and postcombustion of CO require additional capital investment.
By comparison, the addition of a material such as SiC
which produces exothermic reactions during the oxygen
blow does not require any capital investment. Several
practical examples of increasing electrical energy
efficiency by using different sources of chemical energy
are given below.
OXYFUEL BURNERS
Chemical energy from oxygen combustion of natural gas
was introduced in a 4 ton EAF through installation of an
oxyfuel burner through the door. Effective combustion of
natural gas provides energy to the solid charge during the
melting period. Table 5 shows the heat balance for a trial
heat. In this case, chemical energy through the addition of
natural gas is providing 10.7% of the total energy used.
The oxyfuel burner use decreased melting time which
increased productivity and decreased overall energy
losses. Therefore, the electrical energy consumption was
decreased from 480-500 kWh/t without oxyfuel burners to
400-420 kWh/ton with burners. Figure 7 shows the
energy usage during steel melting cycle.
Fig. 6. Sankey-diagrams (energy flows) of melting
steel in 15 ton EAF is illustrated.
IMPROVEMENTS IN MELTING ENERGY
EFFICIENCY BY ADDITION OF CHEMICAL
ENERGY
Supplemental chemical energy is a promising way for
decreasing electrical energy consumption and increasing
the efficiency and productivity of melting steel in steel
foundry EAFs. There are many technologies that are
possible for introducing supplemental chemical energy
into the EAF steel melting process including:
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AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
Table 5. Energy Balance of Heat Using Oxyfuel Burner
Input energy:
Electrical
Chemical (Natural Gas)
Chemical (from C oxidation)
Output energy
To melt
To slag
Losses
KWH/t
418.1
56.4
51
%
79.5
10.7
9.8
353
53.81
118.71
67.17
10.24
22.59
Because the heat of oxidation reaction is generated within
the liquid steel, heat transfer efficiency from the
exothermic reactions should be nearly 100%. This
expected efficiency is much higher than the typical 40%
efficiency for post-combustion of CO above the bath. In
this research work, the amount of exothermic heat
generated during oxygen boiling was increased by adding
SiC with the solid charge.
Fig. 8. Mechanism of energy produced by the addition
of SiC in steel is illustrated.
The energy and operational effects of adding enough SiC
with the scrap charge to represent 0.4- 0.6% of the charge
weight was investigated in a 20 ton EAF. Figure 9 shows
the energy balance diagram after the use of SiC as a
source of chemical energy in the foundry. The trial results
are summarized in Table 7. The addition of SiC reduced
the electrical energy consumption by 7.1% and increased
the productivity by nearly 5%.
Fig. 7. Graph shows the sequence of energy input
during melting steel in EAF with oxyfuel burner.
OXYGEN LANCE
Direct injection of oxygen by a lance to the solid charge
and melted steel can reduce the electrical energy
consumption by decreasing scrap melting time and direct
generation of chemical energy from oxidation reactions in
the melt. Table 6 shows the data collected from a
participating foundry for over a period of time after the
start of use of coherent jet system for direct oxygen
injection. The introduction of coherent jet has decreased
the electrical energy consumption by 10% and also
reduces the melt down time by 13%.
Table 6. Statistics for Electrical Energy Consumption
in a Basic EAF
Before
CoherentJet
After
CoherentJet
627
561
Mean (kWh/t)
513.9
464.8
Standard deviation (kWh /t)
37.7
47
Mean melt-down time (min)
104
90
Standard deviation (min)
29
30
Number of heats evaluated
SiC ADDITIONS IN EAF CHARGE
Figure 8 illustrates the possible advantage of using SiC
additions in solid EAF charge as an additional source of
chemical energy.
Fig. 9. Sankey diagram shows the decrease in
electrical energy consumption by the addition of
chemical energy (0.4% SiC in charge).
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AFS Transactions 2012 © American Foundry Society, Schaumburg, IL USA
Table 7. Effect of SiC Additions and Melting Practice
on Energy Consumption and Productivity of
20 T Capacity EAF
Heats
Without
SiC
With
SiC
Electrical energy
consumption
Decrease,
kWh/ton
%
467.3
434.1
of the thermal properties of different ceramic materials
typically used for steel ladle linings on heat losses during
use was analyzed. From this work, a new type of ladle
lining was developed at Missouri S and T. It was based on
porous ceramics with the potential of significantly
decreasing the heat losses during use in addition to saving
considerable ladle preheat energy.
EAF productivity
Increase,
Ton/hour
%
EFFECT OF LADLE SIZE AND HOLDING TIME
During industrial observations, detailed thermal data was
generated and collected from three to five typical heats at
each plant. The steel and ladle surface temperatures were
measured frequently throughout the holding period in the
ladle from tap to pouring using immersion thermocouples
and an infrared camera. In addition to the detailed data
collected during the foundry trials, information was
collected from the surveys of the 19 steel foundries. This
data included: type of lining materials, ladle sizes and
time/temperature profiles. The data collected through the
survey and trials was analyzed to determine the factors
that were most important to energy losses in the ladle.
One of the most important factors was found to be the
ladle capacity. Figure 10 summarizes the effect of ladle
capacity on the average tap temperature and temperature
losses during holding in the ladle. The tap temperature
was found to be significantly lower for higher capacity
ladles. A computational fluid dynamics (CFD) model was
used to study the effects of ladle size and validate the
industrial measurements. Figure 11 summarizes the
results of the study indicating the major difference in
cooling rates based on ladle size.
6.22
7.1
6.54
4.8
IMPROVEMENT ON MELT LADLE PRACTICES
Effective ladle design, preheat practices and use are
important for steel casting production. In foundry
operations, the tap temperature of the liquid steel is
typically superheated 250F to 500F (121C to 260C) above
the steel’s liquidus to compensate for heat losses during
tapping and holding in small ladles with large surface area
to volume ratios. This ratio changes from 0.5-0.6 1/m for
20 t ladles to 0.8-0.9 1/m for 5 t ladles to 3.5-3.8 l/m for
the 100 lb experimental ladle used in this study. Higher
superheat is also necessary to provide sufficient steel
fluidity to properly fill the mold cavity. In spite of the
relatively short time that the steel is in contact with the
ladle lining, the huge thermal gradients in the lining drive
high values of heat flux through the refractory surface.
Initial information about heat loses during steel ladling
was taken from a survey of steel foundries and from
industrial measurements at seven foundries. The influence
(a)
(b)
Fig. 10. Graphs show the statistical effect of industrial ladle capacity on a) tap temperature and b) rate of
temperature loss.
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easy to install lining in a way that results in
consistent properties
Three types of lining materials were studied in the
Missouri S and T foundry.4 In addition to standard
industrial high alumina castable and fiber magnesia
board/crucible, a specially designed low density porous
alumina castable was also introduced. The densities of the
lining materials used for the study are given in Table 8.
The thermal conductivity of these three linings in relation
to temperature and density are given in Fig. 12. It can be
seen that the porous alumina castable has very low
thermal conductivity and has the potential of improving
energy efficiency in the ladle.
Fig. 11. Results from CFD model show influence of
ladle size on steel temperature loss during holding.
Ladle inserts were prepared from different lining
materials (Fig. 13). High carbon iron was melted in an
induction furnace and tapped at 3000F (1649C) into the
ladles and steel temperature was monitored until the iron
reached 2420F (1326.7C). The holding time for each of
the materials is compared in Table 9. In addition, CFD
modeling was done for: (i) open top and (ii) isolated melt
top (efficient cover). Porous alumina castable can be
preheated, while fiber magnesia does not allow
preheating. In comparison to both high alumina castable
and fiber magnesia boards, low density highly porous
alumina lining significantly decreased the temperature
loss. However, lining erosion from the falling tap stream
resulted in bottom erosion. Later designs used standard
high alumina castable for the areas of high erosion.
NEW REFRACTORY
The temperature of the liquid steel at tap typically varies
between 2950F (1621 C) and 3200F (1760C) at steel
foundries. These temperatures are close to the softening
temperature of the complex Al, Ca, Si, and Mg oxide
compounds which are often used for ceramic linings.
Also, the high rate of chemical reactions between the
lining and components of the liquid steel and slag takes
place at these temperatures. As a rule, foundry ladles are
not fully soaked even when used multiple times and are
therefore used under unsteady state heat transfer
conditions. Even in cases where the lining is preheated
prior to tap, a significant part of the heat energy from the
liquid steel accumulates inside the lining during the first
5-30 minutes after tap.
Table 8. Three Types of Lining Materials Studied
Foundry ladle operations require special ceramic lining
materials. These materials need to meet the following
requirements:
- chemically inert to molten steel and slag for
prevention of lining erosion and alloy contamination;
- thermal properties that minimize the heat losses from
the liquid steel;
- mechanical properties for prevention of failure from
impact of the tapping stream and thermal cracks and
(a)
Ceramics
Density, kg/m3
70% alumina castable
2300
Low density magnesia
1400
Porous high alumina castable
900-950
(b)
Fig. 12. Effect of (a) temperature and (b) density on thermal conductivity of different types of ladle lining is graphed.
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ACKNOWLEDGMENTS
(a)
(b)
The authors wish to thank the Steel Founders Society of
America and the member companies that have provided
support for this work. This work is supported by the U. S.
Department of Energy Assistance Award No. DE-FC3604GO14230, Energy Saving Melting and Revert
Reduction Technology (“Energy SMARRT”) Program,
Subtask No. 2.2. Such support does not constitute an
endorsement by DOE of the views expressed in the
article. The authors also wish to thank the personnel who
were involved in this project for the last five years and
contributed to this effort.
(c)
Fig. 13. Lining inserts are pictured: a) 70% alumina
castable, b) low density magnesia and c) porous
alumina castable.
Table 9. Ladle Holding Time for Melt Temperature to
Drop from 3000F (1649C) to 2420F (1326.7C)
REFERENCES
1.
Lining
Alumina
castable
Low density
magnesia
board
Alumina
porous
castable
Preheat,
F(C)
Measured
time, min
Modeled time,
min
Open Isolated
top
top
1290(699)
7
5
10
No
preheat
9
7
13
1290(699)
18
12
30
2.
3.
4.
CONCLUSIONS
The statistical analysis of a survey of multiple steel
foundries in the United States provides information about
the areas that need attention for energy efficiency
improvements in foundry melting. Identifying this
opportunity is still a challenge as many foundries are not
well equipped with modern devices for measurement of
electrical power, oxygen and natural gas consumption.
From the survey conducted by Missouri S and T, major
opportunities were identified as: (i) improvement in
scheduling and decreasing heat delay; (ii) addition of
chemical energy for melting steel and (iii) improvement
in ladle practice. CFD modeling, industrial and laboratory
trials were conducted to determine the effects of these
changes in reducing electrical power consumption. This
data will be used in the future for development of a
spreadsheet type model to allow foundries to calculate
energy usage and melt temperature losses.
5.
456
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Progress,” Proceedings of 59th SFSA T and O
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Peaslee, K.D., Lekakh, S., Martinez, E.,
“Improvements in Steel Melting Efficiency –
Industrial Trials,” Proceedings of 62nd SFSA T and O
Conference, Chicago, IL (2008).
Peaslee, K.D., Lekakh, S., Richards, V.L., Carpenter,
J., Wang, C., “Decreasing Electrical Energy
Consumption Through SiC Additions,” Proceedings
of 60th SFSA T and O Conference, Chicago, IL
(2006)..,
Peaslee, K., Lekakh, S., Smith, Vibhandik, M.,
“Increasing Energy Efficiency through
Improvements in Ladle Materials and Practices,”
Proceedings of 61st SFSA T and O Conference,
Chicago, IL (2008).
Peaslee, K.D., Lekakh, S., Sander, T., Smith, J.,
“Efficiency in Steel Melting: Ladle Development,”
Proceedings of 59th SFSA T and O Conference,
Chicago, IL (2005).