Electric Arc Furnace

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, 2000 1

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


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.


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:


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).

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35

0:00

:02

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:58

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:54

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:51

0:11

:47

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:43

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:40

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:36

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:33

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:29

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:25

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:22

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:18

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:14

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:11

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:07

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:03

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:56

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:49

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:45

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:41

Tim e

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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.


Mass Balance Energy Balance

OK

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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

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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

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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


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%


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



Table III. CASE 2: 20 lbs/ton of Charge Carbon added to Case 1, 940 scf/ton Total Lance Oxygen.


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



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


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



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.


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



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.


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

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WITH PC

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Time in Heat (min)

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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.


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.


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.


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

Electric Arc Furnace

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