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PRINCIPLES AND STRATEGY OF EAF POST-COMBUSTION
Michael G. Grant Air Liquide America Corporation
5230 South East Avenue Countryside, IL, 60525
USA 708-579-7859
[email protected]
Key Words: Electric Arc Furnace, Post Combustion, Chemical
Energy, Oxygen in EAF
INTRODUCTION
The electric arc furnace (EAF) has been continuously improving
for the last 30 years. It has emerged as the steelmaking method of
choice when new steelmaking capacity is being considered for a
particular plant or when greenfield steel plants are being
constructed. In recent years, EAF steelmaking has been producing
high quality steel, at high production rates in facilities that
require much lower capital investment than integrated mills. Of all
the factors contributing to the success of the modern EAF
operation, including ultra-high powered furnaces and foamy slag
practices, one of the greatest contributions has been the increased
use of oxygen. The use of oxygen in the EAF has been steadily
increasing over the last 20 years. Energy input into the EAF
operation from chemical reactions involving oxygen has contributed
to the production rate of the steelmaking operation. Such tools as
oxy-fuel burners, O2 lancing and post-combustion have effectively
increased the power contribution of chemical reactions into the
EAF. Of these tools, post-combustion remains the tool that is least
widely used by steelmakers for boosting the productivity of their
EAF.
Post-combustion is a method where CO and H2 evolved from the
steelmaking charge during melting are
combusted to produce heat that is utilized in the melting
process. A typical EAF operation results in the evolution of
significant quantities of these gases. Typically, these gases are
burned in the gas collections system en route to the baghouse. The
intent of post-combustion is to capture the heat evolved from the
oxidation of these gases for utilization in the melting process.
However, the effectiveness of any post-combustion system depends on
the operating conditions of a specific EAF. Therefore, thorough
examination of any operation should be made prior to installing any
post-combustion system to estimate the benefits that can be
expected. This paper describes post-combustion theory and practice
and it shows how different operations can result in a variety of
post-combustion performance results.
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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DESCRIPTION OF POST-COMBUSTION Reactions
Post-combustion is a process for utilizing the chemical energy
in the off-gas of the EAF process. In the EAF, it consists of
burning the CO and H2 evolving off the steel bath. Potential
sources of CO and H2 in the steelmaking process are listed below:
CO originates from: Hydrocarbons present in the scrap during melt
down. Combustion of charge and foamy slag carbon. Partial oxidation
of carbon during lancing. Reduction of FeO during slag foaming
via:
)()( gs COFeCFeO (1) H2 originates from: The cracking of
hydrocarbons present in the scrap. The reduction of water from the
atmosphere, panel leaks and spray rings via:
(2) )(2)(2)()(2 gggg COHCOOH
)()(2)()(2 ggsg COHCOH (3)
During the oxidation of carbon, the reaction CO to CO2 produces
twice as much energy as the reaction of C to CO when represented
based on the volume of oxygen used. These reactions are listed in
Table I.
Table I. Heats of Reaction at 3000F for the Oxidation of Carbon
and Hydrogen
Reaction H (kWh/scf O2)
C + O2 CO -0.079 CO + O2 CO2 -0.180 C + O2 CO2 -0.130 H2 + O2
H2O -0.167
The majority of the combustible gases produced during melting
(CO and H2) are combusted after they exit
the furnace by air drawn into the duct through the break flange.
Therefore, most of the potential chemical energy produced from the
complete oxidation of carbon is contained in the off-gases of an
EAF. This energy is wasted in the gas collection system after these
gases leave the furnace because that is where CO is oxidized to
CO2. A post-combustion system is designed to capture a significant
amount of this potential chemical energy and transfer it to the
process. Post-Combustion in the EAF
Post-combustion occurs in almost every EAF operation even if
there is no post-combustion system installed on the furnace. Air
drawn into the furnace by the baghouse fans provides the oxidant
for burning a portion of the CO and H2 generated during
steelmaking. While this generates a significant amount of heat
for
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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melting, the heating of nitrogen (79% of the air) absorbs as
much as 50 % of this natural post-combustion heat. Nitrogen gas is
inert in the EAF process and does not participate in any reactions
that generate heat.
Many EAF operations worldwide have installed post-combustion
systems that utilize pure oxygen to
combust the CO and H2 generated by the steelmaking process. By
using pure oxygen to post-combust CO and H2, the inefficiency of
heating nitrogen is reduced because there is less nitrogen in the
EAF atmosphere due to the addition of post-combustion oxygen above
the bath, which displaces the cold air ingress to the furnace. As a
result, the higher concentration of oxygen enables more combustion
of the furnace gases to take place in the furnace where the energy
is needed. Methods of Post-Combustion
There are many different systems of post-combustion being used
on EAFs worldwide. Many of these are derivatives of the same basic
principle: to burn the CO and H2 above the bath when solid scrap is
still high in the furnace. This is when heat transfer between the
hot combustion gases and the scrap is most efficient because the
scrap is cold and the large surface area of the scrap promotes
convective heat transfer (the most prominent mode of heat transfer
from burners and post-combustion). As the charge melts down, it has
a lower surface area and the temperature difference between the hot
combustion gases and the scrap is lower. Therefore, the
driving-force for heat-transfer decreases. This contrasts
post-combustion in bath smelting processes where heat is
transferred to the molten metal. However, successful use of
post-combustion in bath smelting processes depends on delicate
control of slag layer thickness, bottom stirring intensity and the
location and velocity of the post-combustion oxygen1.
Many EAF operators have attempted to perform post-combustion in
their furnaces by using super-
stoichiometric ratios of oxygen to fuel in their burners.
Success using this technique is limited because it is difficult to
perform post-combustion during periods when it is advantageous to
use burners at their full power and at a stoichiometric oxygen to
fuel ratio. There are periods during most heats, when it is most
efficient to use both the burner and post-combustion systems at
their full power at the same time. Performing post-combustion using
the burners does not allow this to occur. Also, the velocity of the
oxygen coming out of the burners is often very high which can
interfere with adequate mixing between the oxygen and the furnace
atmosphere. In addition to these, high velocity oxygen from burners
has been known to attack the electrodes and the furnace delta.
For these reasons, Air Liquide decided that it was more
effective to use a post-combustion system that is
separate from the burners that delivers low velocity oxygen from
multiple locations counter current to the flow of furnace gases to
the fourth hole. By using multiple injectors, adequate volumes of
oxygen can be delivered at low velocity to destroy the large
volumes of CO and H2 evolving from the scrap. Furthermore, a
furnace atmosphere analyzer is usually used to control the Air
Liquide post-combustion system to deliver appropriate volumes of
oxygen to achieve maximum O2 efficiency. A schematic of the Air
Liquide post-combustion system is presented in Figure 1. In this
example, six (6) injectors are used to deliver the post-combustion
oxygen to the steelmaking process. The injectors are angled so the
direction of flow can be directed against the flow of the furnace
gases toward the fourth hole.
STRATEGY FOR IMPLEMENTING EAF POST-COMBUSTION
The effectiveness of a post-combustion system will vary
depending on the state of many operating conditions. Examples of
factors that will determine the performance of post-combustion are
as follows: Cleanliness of the scrap. Scrap that contains
considerable amounts of oil and grease will generate a lot of
CO and H2 during the early stages of melting due to the cracking
of hydrocarbons. This will affect the concentration of CO and H2 in
the furnace atmosphere.
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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Amount of charge and foamy slag carbon. Most of the carbon
charged to the heat, whether it is charge carbon dissolved in the
bath or foamy slag carbon, will eventually be oxidized to form CO
inside the furnace.
INJECTOR
INJECTOR
INJECTOR
INJECTOR
INJECTORINJECTOR
ALARC-ASTM
ALARC-PCTM
To bag houseTo bag house
injectors
Off-gas sample conditioning
Analysissystem
Oxygen valve stand
Furnace computer
ALARCTMcontrol system
Sampling probe
Figure 1 Schematic of the Air Liquide Post-Combustion System
with Fourth Hole Analysis2
Rate of melting. The speed of steelmaking (rate of O2 injection)
will influence the rate of CO evolution and can therefore affect
the performance of the post-combustion system. This necessitates a
thorough study of the furnace practice and conditions of any given
operation before an effective post-combustion system can be
designed.
Analysis of EAF Atmosphere
Before designing a post-combustion system for a particular
operation, a firm understanding of the furnace conditions is
required. To gain this understanding, it is necessary to have
knowledge of the composition of the EAF atmosphere. Furthermore,
this information along with data describing the material and energy
inputs and outputs must be used in a complete mass and energy
balance of the operation so that the appropriate volumes of
post-combustion oxygen can be calculated. Mass and energy balance
calculations also predict the benefits a post-combustion system can
provide the EAF operation in question.
Figure 2 illustrates the configuration of the sampling probe for
capturing samples of the furnace
atmosphere. A specially designed water cooled probe must be
placed as close to the gap as possible and it must extend into the
duct to a point where samples representative of the EAF atmosphere
can be extracted. The furnace gases must then be filtered for dust
and cooled to condense the water vapor that is present in the
fourth hole gases. After this treatment, the gases travel to the
analyzer. An example of the variation in gas composition over an
entire heat is illustrated in the graph in Figure 2.
Mass and Energy Balance
Before a post-combustion system can be designed for a particular
operation, the measured EAF gas composition must be incorporated
into a mass and energy balance model along with other data
describing the EAF operation. Results from the model estimate the
benefits that can be expected from using post-combustion. In
addition, required oxygen flow rates for destroying CO and H2
formed during steelmaking are calculated. The following data is
entered into the model to perform the mass and energy balance:
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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Scrap, DRI, pig iron, etc. weight and composition. Tap weight,
temperature and composition. Weight and compositions of charge and
foamy slag carbon. Electrical energy consumption. Electrode
consumption. Natural gas usage (scf/heat). Oxygen usage (scf/heat).
Flue (fourth hole) gas analysis for the heat. Tap-to-tap time and
power-on-time.
Based on these inputs, the mass and energy balance will provide
the following information:
Energy Balance. Carbon and oxygen balance. Total flue gas volume
in scf/ton and scfm. Volume of air ingress into the furnace is
scf/ton and scfm. Water (steam) volume produced and leaving the
fourth hole. Make predictions of future operational changes
(scenario development).
0
5
10
15
20
25
30
35
0:00
:02
0:02
:58
0:05
:54
0:08
:51
0:11
:47
0:14
:43
0:17
:40
0:20
:36
0:23
:33
0:26
:29
0:29
:25
0:32
:22
0:35
:18
0:38
:14
0:41
:11
0:44
:07
0:47
:03
0:50
:00
0:52
:56
0:55
:52
0:58
:49
1:01
:45
1:04
:41
Tim e
0
5
10
15
20
25
30
35 % O2% H2% CO2% CO
% C
O,
% H
2, %
CO
2, %
CO
CO
H2
O2
CO2
Probe
Elbow
Roof
Electrode
Air inlet
Projectionsaccumulation
Air
Combustion area
Unoxidized areaSampling point
Figure 2. Probe Arrangement for Sampling EAF furnace gases and
Example of Furnace Gas Composition2.
Figure 3 contains example results of calculations using the EAF
mass and energy balance program at Air Liquide. Calculation of
Post-Combustion O2 Flow Rates
The mass and energy balance model has also been adapted to
calculate post-combustion oxygen flow profiles. For the operation
producing the graph in Figure 2, the oxygen flow rate profile
required for destroying the CO and H2 is graphically presented in
Figure 4. The calculation results displayed in Figure 4 demonstrate
how post-combustion oxygen volumes are determined when designing a
post-combustion system for a particular EAF shop.
58th Electric Furnace Conference Orlando (USA) November 12-15th,
2000 5
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Mass Balance Energy Balance
OK
Cancel
0.8062
1.7
0.252
2
119
100
0
0
0.535
6
167756.525000
.1
Total Scrap Charged
Foamy Slag Carbon
Tons Tapped
OxygenNatural Gas
107
7109.
15.82%19.95%12.65%
1784
49.58%2.00%
Air Ingress Steam
COCO2H2N2O2
scf/ton
scf/ton3341. scf/ton
scf/ton
Dry Flue Gas(average composition)
lbs/ton
ton9.999 lbs/ton
scfscf/tonscf
233.64 1567.82
%%%
% Cton
COH
%%%%
tonAssumptionsC
OHH2O
4765. scfm
10138 scfm
Units
1.07Charge Carbon
ton
20.0 lbs/ton
PC OxygenON/OFF off 0 scf/ton
Steel
OK
Cancel
kWh/ton
Assumptions
UnitsSlag
Dry Off GasAnalysis:
15.82%19.95%12.65%
2.00%135.9
kWh/ton
Chemical Reactions (incl. Lancing)
Si + O2(g) = SiO2Fe + 1/2 O2(g) = FeO
Mn + 1/2 O2(g) = MnOC + 1/2 O2(g) = CO
-7.0Electrode oxidization
Electrical Energy
Foamy Slag Carbon -11.6kWh/ton
Burners-61.1kWh/ton
Air/O2 Post-Combustion-105.0kWh/ton
-82.2 kWh/ton-22.2 kWh/ton-11.9 kWh/ton-22.0 kWh/ton
Total -138.3 kWh/ton
Charge Carbon -23.7 kWh/ton
Slag Formation -2.8 kWh/ton
Heat Losses - Electrical + Water Cooling
217.7 kWh/ton
Air Ingress 48.9 kWh/tonN2 = 46.9 kWh/ton O2 =2.0 kWh/ton
354.4kWh/ton
COCO 2H2O 2
61.8 kWh/ton
-420.2 kWh/ton
Mass Balance
Figure 3. Example Mass and Energy Balance Results
0
500
1000
1500
2000
2500
3000
0:00:00 0:07:12 0:14:24 0:21:36 0:28:48 0:36:00 0:43:12 0:50:24
0:57:36 1:04:48 1:12:00
Time
O2
FLO
W R
ATE
(scf
m)
PC O2 injection
Figure 4. Post-Combustion Oxygen Injection Profile Calculated
for Gas composition profiles in Figure 2.
The EAF atmosphere profile shown in Figure 2 represents data
from 15 heats of normal operation, which includes the complete
spectrum of steel grades produced by that shop using all the types
of scrap normally consumed. It is important to study the furnace
under all (normal) conditions that it operates to be able to
understand the diversity of furnace atmospheres that can be
generated at a particular shop. The information is then used to
generate O2 flow profiles such as the one in Figure 4 so that a
post-combustion system can be suitably designed. Estimating the
Impact of Potential Melting Practice Changes
Thus far in this report, the method for determining the
post-combustion oxygen flow rate profiles for an existing EAF
practice has been presented. However, in many cases, a steel
company may be considering post-combustion as one of several EAF
productivity or quality improvements that are being planned for the
future. Therefore, any mass and energy balance model must be
capable of including these potential changes to the practice when
formulating O2 flow profiles. The model must also be able to make
predictions of the benefits that post-combustion will bring for
each practice change.
The Air Liquide model is capable of using current operational
data to make projections of the effects of
different practice changes on the furnace performance. Examples
of how different furnace practices affect the
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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productivity of the furnace and how each of these cases will be
benefited by installing post-combustion are described below. These
scenarios are based on the operation of the furnace where
atmosphere profiles similar to Figure 2 are generated. The
following scenarios are examined in this paper: Base Case: No
charge or foamy slag carbon is charged and there are no burners
being used. 618 scf lance
O2/ton. Scrap charge includes 12.5 % pig iron. Case 2: 20
lbs/ton Charge Carbon added to Base Case and a total of 939 scf
lance O2/ton. Case 3: 10 lbs/ton foamy slag carbon added to Case 2
with a total of 1100 scf lance O2/ton. Case 4: 25 % DRI added to
Case 3 with a total of 810 scf lance O2/ton. Case 5: Burners with
an average firing rate of 5 MW (over the whole heat) added to Case
4.
Each of these scenarios is simulated with and without
post-combustion to demonstrate how operating practice affects the
performance of a post-combustion system. Calculations were made
assuming constant yield of 90 percent. Therefore, as charge and
foamy slag carbon are added, lance oxygen must be increased to
balance additional carbon. In the Case 4 where DRI is added, oxygen
was removed because the oxygen present in the 8 % FeO contained in
the DRI is capable of performing significant amounts of
decarburization.
Case 1 - Table II describes the Base Case scenario where there
is no charge or foamy slag carbon used in the operation.
Additionally, the base case uses no burners. Approximately 39 % of
the energy going into the process is chemical energy. This chemical
energy originates from the oxidation reactions occurring in the
bath (138.3 kWh/ton) and from natural post-combustion (146.8
kWh/ton). Natural post-combustion occurs in almost every electric
arc furnace by the oxidation of CO and H2 with air drawn into the
furnace by the baghouse fans. A small amount of energy is produced
by the oxidation of electrodes and by slag forming reactions. Note
also that the unused air ingress (nitrogen from air ingress)
consumes about 75 kWh/ton. The other off-gases (CO, H2, CO2, H2O)
leaving the furnace account for 59 kWh/ton.
Table II. CASE 1: Base Case No Carbon Added, 617 scf O2/ton
INPUTS BASE CASE BASE CASE WITH POST-COMBUSTIONElectrical Energy
-454.6 kWh/ton -458.8 kWh/tonBath Reactions -138.3 kWh/ton -138.3
kWh/tonCharge Carbon 0.0 lbs/ton 0.0 kWh/ton 0.0 lbs/ton 0.0
kWh/tonFoamy Slag Carbon 0.0 lbs/ton 0.0 kWh/ton 0.0 lbs/ton 0.0
kWh/tonElectrode oxidization 6.0 lbs/ton -7.0 kWh/ton 6.0 lbs/ton
-7.0 kWh/tonPost combustion heat 0.0 scf/ton -146.8 kWh/ton 272
scf/ton -146.8 kWh/tonHeat from burners 0.0 MW 0.0 kWh/ton 0.0 MW
0.0 kWh/tonHeat of slag formation -2.8 kWh/ton -2.8 kWh/tonLance O2
618 scf/ton 618 scf/tonAir Ingress into EAF 5369 scf/ton 5319
scf/tonTotal -749.5 kWh/ton -753.7 kWh/ton
OUTPUTSHeat in Steel 354.4 kWh/ton 354.4 kWh/tonHeat of N2 in
Off-gas 59.0% 75.4 kWh/ton 56.7% 74.7 kWh/tonHeat of O2 in Off-Gas
3.6% 4.8 kWh/ton 7.0% 9.7 kWh/tonHeat of CO in Off-Gas 0.0% 0.0
kWh/ton 0.0% 0.0 kWh/tonHeat of CO2 in Off-Gas 9.8% 20.4 kWh/ton
9.5% 20.4 kWh/tonHeat of H2 in Off-Gas 0.0% 0.0 kWh/ton 0.0% 0.0
kWh/tonHeat of H2O in Off-Gas 27.7% 33.4 kWh/ton 26.8% 33.4
kWh/tonSlag at Tap Temperature 61.8 kWh/ton 61.8 kWh/tonLosses
199.3 kWh/ton 199.3 kWh/tonTap to Tap Time (min) 80 80Power on Time
(min) 61 61
Total 749.5 kWh/ton 753.7 kWh/tonPercent Chemical Energy 39.3%
39.1%
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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Table II also shows that in this case, where there is no charge
carbon, there is enough air ingress to oxidize all the CO and H2
formed during steelmaking. The lower left section of Table II shows
that even without post-combustion, there is no CO and H2, formed
during steelmaking, exiting the fourth hole. Therefore, there is no
need to add post-combustion oxygen because there is nothing to
post-combust. Never the less, the model was used to calculate the
effect of adding a post-combustion system when it was not needed.
The results show that there is an energy penalty in this case.
Electrical energy increases from 455 kWh/ton to 459 kWh/ton when
post-combustion is turned on. Post-combustion oxygen is added to
the furnace where it simply heats up on its way to the fourth hole.
The heat lost by excess oxygen flowing out the fourth hole doubles.
It is also interesting to note that when post-combustion oxygen is
injected into an EAF, the amount of air ingress into the furnace
drops. Also, it can be expected that electrode oxidation will
increase when adding post-combustion to Case 1, however, the model
is incapable for accounting for this probable effect.
Case 1 is an example of an operation where post-combustion will
not be viable because of the lack of CO
and H2 produced by the process. Examples like Case 1 demonstrate
that the EAF operation must be characterized and understood so that
reliable estimates of return on investment of a post-combustion
system can be made.
Case 2 Charge carbon is commonly added to the electric arc
furnace to provide additional heat and to
ensure that there will be sufficient carbon in the bath at
melt-in. A portion of carbon that is charged with the scrap will
dissolve in the molten steel and the furnace atmosphere will
oxidize the rest. An important consideration when increasing the
amount of carbon charged to an EAF is the amount of extra oxygen
that must be used to balance the carbon. This is the reason why the
lance oxygen increased from 618 scf/ton with no carbon charged in
Case 1 to 940 scf/ton with 20 lbs/ton of charge carbon in Case
2.
INPUTS CASE 2 CASE 2 WITH POST-COMBUSTIONElectrical Energy
-439.2 kWh/ton -413.0 kWh/tonBath Reactions -138.3 kWh/ton -138.3
kWh/tonCharge Carbon 20.0 lbs/ton -23.7 kWh/ton 20.0 lbs/ton -23.7
kWh/tonFoamy Slag Carbon 0.0 lbs/ton 0.0 kWh/ton 0.0 lbs/ton 0.0
kWh/tonElectrode oxidization 6.0 lbs/ton -7.0 kWh/ton 6.0 lbs/ton
-7.0 kWh/tonPost combustion heat 0.0 scf/ton -137.8 kWh/ton 483.1
scf/ton -206.1 kWh/tonHeat from burners 0.0 MW 0.0 kWh/ton 0.0 MW
0.0 kWh/tonHeat of slag formation -2.8 kWh/ton -2.8 kWh/tonLance O2
939.6 scf/ton 939.6 scf/tonAir Ingress into EAF 4586.1 scf/ton
4497.0 scf/tonTotal -748.8 kWh/ton -790.9 kWh/ton
OUTPUTSHeat in Steel 354.4 kWh/ton 354.4 kWh/tonHeat of N2 in
Off-gas 51.0% 64.4 kWh/ton 49.8% 63.2 kWh/tonHeat of O2 in Off-Gas
2.1% 2.7 kWh/ton 3.3% 4.4 kWh/tonHeat of CO in Off-Gas 9.6% 12.1
kWh/ton 0.0% 0.0 kWh/tonHeat of CO2 in Off-Gas 9.4% 19.3 kWh/ton
18.9% 39.0 kWh/tonHeat of H2 in Off-Gas 1.0% 1.2 kWh/ton 0.0% 0.0
kWh/tonHeat of H2O in Off-Gas 27.0% 32.2 kWh/ton 27.9% 33.4
kWh/tonSlag at Tap Temperature 61.8 kWh/ton 61.8 kWh/tonLosses
200.6 kWh/ton 234.8 kWh/tonTap to Tap Time (min) 78 74Power on Time
(min) 59 55
Total 748.8 kWh/ton 790.9 kWh/tonPercent Chemical Energy 41.3%
47.8%
Table III. CASE 2: 20 lbs/ton of Charge Carbon added to Case 1,
940 scf/ton Total Lance Oxygen.
58th Electric Furnace Conference Orlando (USA) November 12-15th,
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Table III shows that 20 lbs/ton of charge carbon (with oxygen to
balance) produces enough energy to bring the electrical energy
consumption down from 455 kWh/ton to 439 kWh/ton. With this drop in
electrical energy consumption, there is an associated increase in
furnace productivity. The tap-to-tap time drops from 80 minutes in
Case 1 to 78 minutes in Case 2. In addition, CO and CO2 formed from
the oxidation of the charge carbon displace enough air to reduce
the cold air ingress to the furnace by almost 15 %. As a result,
there is not enough oxygen from air ingress to completely oxidize
the CO and H2 in the furnace. There is now a significant proportion
of CO and H2 in the furnace atmosphere to consider using
post-combustion.
Table III demonstrates that with 20 lbs/ton of charge carbon
added to Case 1, installing post-combustion
results in electrical energy savings of approximately 26 kWh/ton
(439 kWh/ton to 413 kWh/ton). Furthermore, post-combustion is able
to reduce the tap-to-tap time from 78 minutes to 74 minutes by
increasing productivity by more than 5 %. Almost half (48 %) of the
energy needed for making steel is supplied by chemical energy.
Case 3 In Case 3, 10 lbs/ton of foamy slag carbon was added to
the operation described in Case 2. This
extra carbon was balanced with an additional 160 scf/ton of
oxygen to bring the total lance oxygen to 1100 scf/ton. The results
are displayed in Table IV. The model predicts a decrease in
electrical energy consumption of only 2- kWh/ton. In reality, the
production of a good foamy slag will induce much higher electrical
energy savings that those shown in Table IV. The model is unable to
estimate the energy savings incurred by shielding the arc with a
highly foaming slag. The addition of foamy slag carbon further
decreases the amount of cold air ingress to the furnace by
8.5%.
Table IV. CASE 3:Case 2 with the Addition of 10 lbs/ton Foamy
Slag Carbon, Total Oxygen is 1100 scf/ton.
INPUTS CASE 3 CASE 3 WITH POST-COMBUSTIONElectrical Energy
-436.7 kWh/ton -401.3 kWh/tonBath Reactions -138.3 kWh/ton -138.3
kWh/tonCharge Carbon 20.0 lbs/ton -23.7 kWh/ton 20.0 lbs/ton -23.7
kWh/tonFoamy Slag Carbon 10.0 lbs/ton -11.6 kWh/ton 10.0 lbs/ton
-11.6 kWh/tonElectrode oxidization 6.0 lbs/ton -7.0 kWh/ton 6.0
lbs/ton -7.0 kWh/tonPost combustion heat 0.0 scf/ton -128.9 kWh/ton
593.1 scf/ton -213.2 kWh/tonHeat from burners 0.0 MW 0.0 kWh/ton
0.0 MW 0.0 kWh/tonHeat of slag formation -2.8 kWh/ton -2.8
kWh/tonLance O2 1100.3 scf/ton 1100.3 scf/tonAir Ingress into EAF
4194.5 scf/ton 4085.1 scf/tonTotal -749.0 kWh/ton -797.9
kWh/ton
OUTPUTSHeat in Steel 354.4 kWh/ton 354.4 kWh/tonHeat of N2 in
Off-gas 46.6% 58.9 kWh/ton 45.4% 57.4 kWh/tonHeat of O2 in Off-Gas
1.9% 2.5 kWh/ton 3.1% 4.1 kWh/tonHeat of CO in Off-Gas 11.9% 15.0
kWh/ton 3.4% 4.4 kWh/tonHeat of CO2 in Off-Gas 11.7% 24.0 kWh/ton
20.1% 41.2 kWh/tonHeat of H2 in Off-Gas 5.2% 6.2 kWh/ton 0.0% 0.0
kWh/tonHeat of H2O in Off-Gas 22.8% 27.2 kWh/ton 28.0% 33.4
kWh/tonSlag at Tap Temperature 61.8 kWh/ton 61.8 kWh/tonLosses
199.1 kWh/ton 241.2 kWh/tonTap to Tap Time (min) 77 73Power on Time
(min) 58 54
Total 749.0 kWh/ton 797.9 kWh/tonPercent Chemical Energy 41.7%
49.7%
Table IV demonstrates the benefits of post-combustion when added
to Case 3. In this situation, post-
combustion is expected to save the operation approximately 35
kWh/ton while reducing the tap-to-tap time by approximately 4
minutes. These savings result from less air drawn into the furnace
and more CO available for post-combustion due to the additional 10
lbs/ton of foamy slag carbon. The addition of post-combustion to
Case 3 increases the chemical energy contribution to the process to
approximately 50%. However, there is a
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significant increase in the heat losses going to the cooling
water (42 kWh/ton) during the installation of post-combustion as
shown in Table IV. This could create some problems for operations
where the furnace cooling system is already close to maximum
capacity.
Case 4 In Case 4, 25 % DRI was included in the scrap mix while
the total lance oxygen was reduced to
811 scf/ton. The lance O2 was reduced because there is 8 % FeO
in the DRI. The FeO contains oxygen that participates in the
decarburization reactions. When 25% DRI is included in the scrap
charge, the electrical consumption increases by 12% to 489 kWh/ton
(see Table V). This is the extra energy required for the reduction
of FeO in the DRI and the melting of the additional gangue
material. This extra energy consumption occurs despite the fact
that there is a 9% drop in cold air ingress with the addition of
DRI. In Case 4, the DRI was assumed to contain 2 percent carbon,
which was oxidized to CO to further displace the cold air ingress
from the Case 3 example.
Table V shows that the addition of post-combustion in Case 4
requires considerably more oxygen than the
previous example in Case 3. It takes 703.9 scf O2/ton to destroy
the CO and H2 evolved from the Case 4 operation due to the large
amount of carbon that is present in the DRI. Electrical energy
savings are 43 kWh/ton with post-combustion and tap-to-tap time is
reduced by 6 minutes. It is interesting to note that
post-combustion has the potential to restore almost all of the
productivity that would be lost by using DRI. However this depends
on how the DRI is added to the furnace. If DRI is continuously
charged into a flat bath, it is expected that heat transfer
efficiency may not be as high as indicated in Table V because there
will be less surface area to heat the steel during flat bath
continuous charging.
Table V. CASE 4:Case 3 with 25 % DRI in the Charge, 811 scf/ton
total lance oxygen.
INPUTS CASE 4 CASE 4 WITH POST-COMBUSTIONElectrical Energy
-488.5 kWh/ton -445.4 kWh/tonBath Reactions -74.3 kWh/ton -74.3
kWh/tonCharge Carbon 20.0 lbs/ton -23.7 kWh/ton 20.0 lbs/ton -23.7
kWh/tonFoamy Slag Carbon 10.0 lbs/ton -11.6 kWh/ton 10.0 lbs/ton
-11.6 kWh/tonElectrode oxidization 6.0 lbs/ton -7.0 kWh/ton 6.0
lbs/ton -7.0 kWh/tonPost combustion heat 0.0 scf/ton -120.1 kWh/ton
703.9 scf/ton -217.0 kWh/tonHeat from burners 0.0 MW 0.0 kWh/ton
0.0 MW 0.0 kWh/tonHeat of slag formation -7.1 kWh/ton -7.1
kWh/tonLance O2 810.6 scf/ton 810.6 scf/tonAir Ingress into EAF
3802.6 scf/ton 3672.8 scf/tonTotal -732.2 kWh/ton -786.1
kWh/ton
OUTPUTSHeat in Steel 354.9 kWh/ton 354.9 kWh/tonHeat of N2 in
Off-gas 42.3% 53.4 kWh/ton 40.8% 51.6 kWh/tonHeat of O2 in Off-Gas
1.7% 2.3 kWh/ton 3.1% 4.2 kWh/tonHeat of CO in Off-Gas 14.1% 17.9
kWh/ton 7.4% 9.4 kWh/tonHeat of CO2 in Off-Gas 13.9% 28.6 kWh/ton
20.6% 42.4 kWh/tonHeat of H2 in Off-Gas 9.4% 11.2 kWh/ton 0.0% 0.0
kWh/tonHeat of H2O in Off-Gas 18.6% 22.2 kWh/ton 28.0% 33.4
kWh/tonSlag at Tap Temperature 47.1 kWh/ton 47.1 kWh/tonLosses
194.6 kWh/ton 243.1 kWh/tonTap to Tap Time (min) 84 78Power on Time
(min) 65 59
Total 732.2 kWh/ton 786.1 kWh/tonPercent Chemical Energy 33.3%
43.3%
Case 5 - Table VI contains model calculations predicting the
effect of adding burners to Case 4. In normal
EAF operations, the burners are operated at high power early in
the heat when the scrap is high in the furnace and then as the heat
progresses toward flat bath conditions, the burners are fired at
low power because heat transfer efficiency is low3. For calculating
the contribution of the burners to the operation, an average firing
rate of 5 MW was used. This is roughly equivalent to 234 scf/ton of
natural gas use.
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The addition of burners reduces the electrical energy
consumption by almost 17 kWh/ton, which reduces
the tap-to-tap time by 2 minutes. The hot gases produced by the
burners displace the cold air ingress into the furnace and reduce
it by 22 %. Since there is less air drawn into the furnace, there
is less natural post-combustion (post-combustion by air). The
contribution of natural post-combustion to the operation drops from
120 kWh/ton to 96 kWh/ton (compare Table V and Table VI).
Therefore, the combined proportion of CO and H2 in the furnace
atmosphere increases. The burners also increase the heat load to
the walls as indicated by the losses.
When post-combustion is added to Case 5, it reduces the
electrical energy consumption by almost 50 kWh/ton and decreases
the tap-to-tap time by 7 minutes. Post-combustion produces better
results than other cases because there is less post-combustion by
air ingress taking place in the furnace while there is the same
amount of CO and H2 produced by the steelmaking process as in Case
4. The burners themselves do not produce extra CO for
post-combustion. They reduce the air ingress that concurrently
participates in the post-combustion reactions.
Table VI. CASE 5: Case 4 with oxy-fuel burners averaging 5MW
over the entire heat.
INPUTS CASE 5 CASE 5 WITH POST-COMBUSTIONElectrical Energy
-472.0 kWh/ton -423.0 kWh/tonBath Reactions -74.3 kWh/ton -74.3
kWh/tonCharge Carbon 20.0 lbs/ton -23.7 kWh/ton 20.0 lbs/ton -23.7
kWh/tonFoamy Slag Carbon 10.0 lbs/ton -11.6 kWh/ton 10.0 lbs/ton
-11.6 kWh/tonElectrode oxidization 6.0 lbs/ton -7.0 kWh/ton 6.0
lbs/ton -7.0 kWh/tonPost combustion heat 0.0 scf/ton -96.1 kWh/ton
784.5 scf/ton -201.1 kWh/tonHeat from burners 5.1 MW -61.1 kWh/ton
5.1 MW -61.1 kWh/tonHeat of slag formation -7.1 kWh/ton -7.1
kWh/tonLance O2 810.6 scf/ton 810.6 scf/tonAir Ingress into EAF
2949.7 scf/ton 2805.0 scf/tonTotal -752.9 kWh/ton -808.9
kWh/ton
OUTPUTSHeat in Steel 354.9 kWh/ton 354.9 kWh/tonHeat of N2 in
Off-gas 32.8% 41.4 kWh/ton 31.2% 39.4 kWh/tonHeat of O2 in Off-Gas
1.3% 1.8 kWh/ton 2.9% 3.9 kWh/tonHeat of CO in Off-Gas 14.1% 17.9
kWh/ton 9.8% 12.5 kWh/tonHeat of CO2 in Off-Gas 17.2% 35.3 kWh/ton
21.5% 44.2 kWh/tonHeat of H2 in Off-Gas 13.7% 16.3 kWh/ton 0.0% 0.0
kWh/tonHeat of H2O in Off-Gas 20.9% 24.9 kWh/ton 34.6% 41.2
kWh/tonSlag at Tap Temperature 47.1 kWh/ton 47.1 kWh/tonLosses
213.2 kWh/ton 265.7 kWh/tonTap to Tap Time (min) 82 75Power on Time
(min) 63 56
Total 752.9 kWh/ton 808.9 kWh/tonPercent Chemical Energy 37.3%
47.7%
Design of a Post-Combustion System
The five cases presented above demonstrate how the operating
conditions of an EAF will affect the results of using
post-combustion. For this reason, it is important to understand the
operation of a furnace prior to designing such a system. The EAF
operation affects the amount of CO and H2 generated which in turn,
affects the amount of oxygen required for the destruction of those
gases. These factors must be known before any return on investment
calculations can be made for justification of a post-combustion
system.
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ACTUAL RESULTS FROM POST-COMBUSTION Benefits of
Post-Combustion.
The benefits realized by steelmakers that have used the Air
Liquide Post-combustion system are as follows: Utilization of the
fuel (CO and H2) evolving from the process to produce heat for
melting steel. This
reduces electrical energy requirements and increases the
productivity of the electric arc furnace. Reduction of baghouse CO
emissions. Heat from post-combustion is absorbed in the charge so
that less combustion occurs in the off-gas system
which, Reduces the temperature of the off-gas system, Minimizes
high temperature spikes associated with rapid CO evolution
(cave-ins).
Figure 5 shows how CO emissions and baghouse temperatures are
reduced when post-combustion is
installed on the EAF. CO emissions are lower when using
post-combustion with pure oxygen because a greater proportion of
the CO is combusted in the furnace. Baghouse temperatures are
reduced when using post-combustion because most of the CO and H2
generated by the steelmaking process is combusted inside the
furnace where that heat is transferred to the steel leaving less
energy to heat the baghouse. In operations where post-combustion is
not being used, CO and H2 are oxidized in the gas collection system
where the only heat sink is the water-cooled duct. Therefore, much
more heat travels to the baghouse.
0 10 20 30 40 50 60 70 80 90
Time in Heat (m in)
PP
M C
O
WITHOUT PC
WITH PC
100
110
120
130
140
150
160
170
180
190
200
0 20 40 60 80 100
Time in Heat (min)
Tem
pera
ture
( F
)WITHOUT PC
WITH PC
Figure 5. CO emissions and baghouse temperature with and without
post-combustion4. Furnaces using ALARC-PC
Table VII contains data describing the benefits that were
measured before and after the Air Liquide post-combustion system
was installed on several selected furnaces2. These measurements
were made on numerous heats representative of the operations before
and after the installation of post-combustion on each of these
furnaces. The benefits vary between furnaces. In these examples,
Electrical energy savings varied between 22 kWh/t and 59 kWh/ton,
Furnace productivity increased by 0.9 to 2.3 heats/day, Tap-to-tap
time was reduced by 1.9 to 9.7 minutes.
The variation in results between the six operations chosen as
examples in Table VII is attributed to a large variety in operating
conditions. The wide variety of results explains the need for
understanding the operation and the benefits that a post-combustion
installation will bring prior to its installation.
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Table VII. Comparison of EAF Operations with and without
Post-Combustion2.
BSW Von Roll Valsabbia Cascade Steel
Alfa Acciai
Kisco
Tapping Weight (t) 78 70 70 95 80 65Melting Power (MW) 50 45
36/28 45 - 60 63 35REFERENCE PERIOD:Power On (min) 40.5 46 51.3
53.7 40.2 41.5Tap-to-Tap (min) 51.5 64 64.5 85.6 56.6
54.6Electrical Consumption (kWh/t) 372 447 398 515 414 393O2
Consumption (Nm
3/t) 35.6 14 26.3 28.3 36.3 57ALARC-PC SAVINGS ONPower On (min)
-3.7 -6 -3.9 -7.1 -3.9 -2.3Tap-to-Tap (min) 3.7 -6 -3.6 -9.7 -1.9
-4.7Electrical Consumption (kWh/t) -25 -44 -22 -59 -33 -42Heats per
day 2.1 2.3 1.4 2.2 0.9 3O2 Consumption (Nm3/t) - ALARC-PC 12 18
12.7 23.6 13 14.5O2 Consumption (Nm3/t) - Total 45.6 32 34.1 49.1
33.7 55
SUMMARY
Post-combustion is a process for utilizing the chemical energy
in the off-gas of the EAF process. The
chemical energy is evolved when CO and H2 are combusted by
oxygen inside the EAF. This post-combustion energy is used to
assist in melting the steel scrap.
However, prior to making the decision to install a
post-combustion system, an understanding of the EAF operation in
for which the system is being considered is important. This can be
achieved by studying a representative number of normal heats. The
study must comprise of a complete furnace gas analysis for all
heats. In addition to this, an accurate accounting of all the raw
material, fuel and oxygen inputs to the furnace must be made. All
product outputs and temperature must be recorded in order to be
able to complete the mass and energy balance of the furnace. Using
a model that may incorporate anticipated strategic practice
changes, the benefits of using post-combustion can be estimated.
Therefore, it can be decided whether or not a post combustion
system can be justified. Additionally, the aforementioned mass and
energy balance study is needed for calculating oxygen flow rates
required for designing a post-combustion system.
ACKNOWLEDGEMENTS The author wishes to thank the Air Liquide
America Corporation for its moral and financial support throughout
this work.
REFERENCES
1. Jones, J.A.T.; Oliver, J.F.; A Review of Post-Combustion in
the EAF A Theoretical and Technical Evaluation; 5th European
Electric Steel Congress, June 19 23, 1995; Paris, France.
2. Air Liquide America Corporation, Internal reports and
presentations.
3. R. Fruehan, Editor, The Making Shaping and Treating of Steel,
11th Edition, Steelmaking and Refining Volume, 1998, Chapter 10
Electric Furnace Steelmaking.
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4. Gregory, D.S.; Ferguson, D.K.; Slootman, F.; Viraize, F.;
Luckhoff, J.; Results of ALARC-PC Post-Combustion at Cascade
Rolling Mills; 53rd Electric Furnace Conference Proceedings, Volume
53, November 12-15, 1995, page 211.
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INTRODUCTIONDESCRIPTION OF
POST-COMBUSTIONReactionsReactionPost-Combustion in the EAFMethods
of Post-Combustion
STRATEGY FOR IMPLEMENTING EAF POST-COMBUSTIONAnalysis of EAF
AtmosphereMass and Energy BalanceCalculation of Post-Combustion O2
Flow RatesEstimating the Impact of Potential Melting Practice
ChangesCase 2 Charge carbon is commonly added to the e
Design of a Post-Combustion System
ACTUAL RESULTS FROM POST-COMBUSTIONBenefits of
Post-Combustion.Furnaces using ALARC-PC
SUMMARYACKNOWLEDGEMENTSREFERENCES