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Dynamic Simulation of Melting Process in Arc Furnace

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DYNAMIC SIMULATION OF MELTING PROCESS IN ARC FURNACES

1 INTRODUCTION

The melting in an arc furnace is a very complex process because of the physics of the arcs, scrap geometry, combustion of combustible material, oxidation of metal elements in the scrap and reduction of oxides in the slag, etc.

It is very difficult and complicated from physical and mathematical point of view to make exact models describing the process in detail.

However, it is possible to simplify some of the processes by models based on experience which together with a total energy balance give results of energy input, energy output, losses, consumables and process time with very good coincidence with practice.

The computer program EAFMELT for dynamic simulation of the melting process in AC and DC arc furnaces is a tool to simulate the melting process in arc furnaces.

The computer program can be used for simulate amongst other things the following:

- AC single shell and twin shell furnaces

- DC single shell and twin shell furnaces

- Impact of preheating of scrap with burners in waiting shell in a twin shell furnace concept

- Impact of process variables

- Impact of scrap composition and charging program of scrap

- Impact of charged molten crude iron or molten pig iron

- Impact of changed electrical melting program

- Impact of changed burner program

- Impact of changed oxygen and carbon injection program

- Impact of preheated scrap outside furnace shell

- Impact of post combustion with oxygen excess

- Impact of time controlled furnace pressure

- Impact of tight furnace

- Impact of refractory insulation on water-cooled surfaces

- Impact of series reactor

- Impact of time controlled parallel load (simultaneous melting in parallel connected furnaces) on production and specific consumption, resultant electrical load and flicker

- Impact of voltage stabilisation and flicker by means of SVC system

- Impact of fault levels on voltage stabilisation, flicker and production

 

2 SHORT DESCRIPTION OF PHYSICAL MODELS

Furnace

The furnace size is described by its dimensions of the shell, the roof and the electrodes. The thermal and physical coefficients and the initial temperature of the linings can be specified in the input data as an alternative to default data.

Water-cooled wall panels and water-cooled roof panels is described with its dimensions, slag thickness and initial slag temperatures.

Scrap

The scrap and melt in the furnace is modelled as a mass, which bulk density is increased by a linear function of the average temperature of the scrap and which heat transfer area is reduced by a non-linear function of the average temperature.

The scrap is described by its charging time, weight, bulk density, average size of scrap pieces, content of combustible like oil immersed turnings, content of solved carbon, solved silicon, solved manganese, solved nickel and chromium, yield of metals and average temperature or heat content from preheating outside the furnace.

The program calculates the real enthalpy versus temperature and melting temperature based on the solved metal elements specified for the scrap.

Sponge Iron (DRI)

The sponge iron is described by the time for feeding, feeding rate, metallisation grade, carbon content and initial enthalpy from preheating.

Lime, Slag Former

Lime and slag former is modelled by means of balls that need some time to melt. An energy balance model calculated the time for melting the charged lime based on the energy need for slag forming, density, heat transfer, fraction size of the charged lime and initial temperature.

Charged Carbon or Coke

Same model as for lime for solving of charged carbon and coke.

 

Charged Alloys

Same model as for lime for melting and solving charged alloys.

Wall Burner(s)

The wall burners are described by operating time, total feeding rate of fuel (oil or gas, CH4) and oxygen. The burner can be operated under or over stoichiometric and post combustion by means of oxygen excess can be modelled.

Door Burner(s)

The same as for wall burners.

Oxygen Lance

The oxygen lance is described by operating time, feeding rate of oxygen, carbon and lime.

Slag Door Operation

The operation of the slag door is used in the program to calculate the in leakage air into the furnace chamber. The total in leakage air with regards to other openings in the furnace shell described in the furnace dimensions and the average specified under pressure in the furnace chamber is calculated. The total amount of oxygen in the in leakage air and excess oxygen input in the burners are then used for calculation of the resultant degree of post combustion of carbon monoxide (CO).

Electrical Power Input

The electrical power input in the furnace is calculated from given electrical power program describing operating time, secondary voltage, electrode current, impedance and flicker (PST/MW). A model for calculation of the impact of the temperature on arc stabilisation is used to calculate the change in powers, power factors, currents and flicker. Regards to the fault level on the electrical power supply bus and phase compensation for voltage stabilisation and flicker like SVC system are taken into consideration in the model. When specifying the electrical power program the electrical parameters can be calculated as function of a specified average temperature in scrap, which means that electrical operation parameters do not need to be pre-calculated with another kind of computer program.

 

Electrode Spraying with Water

The spraying of electrodes with water reduces the side oxidation surfaces on the electrodes. However, the spray water streaming on the electrodes into the furnace is vaporised, which requires energy and increases the off-gas flow. These factors increase the off-gas losses and the energy consumption. In the program these factors are considered and modelled.

Roof Opening Program

The operation of the roof is described by a program for closing and opening and is described by the operating time and closed and opened mode. The program calculates the energy losses

Slag Tapping Program

A model for tapping of slag because of slag foaming and tilting of the furnace is used for calculation of the total mass inside the furnace chamber.

Steel Tapping Program

A model for tapping of steel at the end of the melting process is used for calculation of the temperature in the remaining hot heel in the furnace.

3 ENERGY BALANCE

The process temperatures are calculated by means of a total energy balance combined with partial energy balances for calculation of the off-gas losses and energy transfer through the slag. Since the problem is non-linear in its nature it is solved by means of solving a non-linear equation system with a first iteration loop followed by a post iteration loop for improving and testing of the accuracy.

The off-gas temperature is calculated by means of an energy balance between the resultant combustion energy from oxidation of electrodes, oxidation of combustibles in scrap, energy from oxy-fuel burners and post combustion of carbon monoxide and heat transfer from the off-gas to scrap/melt, to refractory lining, to wall panels, to roof panels, to refractory centre part and to electrodes. The heat transfer between the off-gas and the heat absorbing areas is modelled by gas radiation. The gas emission coefficient is calculated with regards to temperatures and partial pressures of carbon dioxide and water vapour.

With lack of oxygen in the furnace chamber for combustion the combustion degree factor, λ = CO2/(CO2+CO), is calculated and the resultant combustion energy and combustion temperature are reduced with regards to the combustion factor less than 1 unit.

The slag temperature is calculated by means of an energy balance between the energy input and losses above the slag and energy consumption, losses and energy input from oxidation of carbon and metal elements in the scrap and melt.

Energy Input Distribution

The energy inputs used in the total energy balance are listed below.

- Electrical arc power

- Electrical losses in electrodes inside furnace chamber

- Energy from oxidation of electrodes

- Energy from combustion of combustibles in scrap like oil immersed turnings

- Energy from oxy-fuel burners

- Energy from oxidation of carbon

- Energy from post combustion of carbon monoxide

- Energy from oxidation of silicon

- Energy from oxidation of manganese

- Energy from oxidation of chromium

- Energy from oxidation of iron

- Energy input from charged preheated scrap or molten steel or iron

Energy Consumption and Losses

The energy consumption and losses used in the total energy balance are listed below.

- Calorimetric energy need for temperature changes in scrap

- Calorimetric energy need for reduction of sponge iron

- Calorimetric energy need for slag forming

- Calorimetric energy need for solving of charged carbon

- Calorimetric energy need for melting alloys

- Calorimetric energy need for reduction of iron

- Calorimetric energy need for reduction of chromium

- Calorimetric energy need for heating of electrodes

- Heat energy losses in bottom and EBT lining

- Heat energy losses in wall/slag line lining

- Heat energy losses in water-cooled wall panels

- Heat energy losses in water-cooled roof panels

- Heat energy losses in roof centre part lining

- Heat energy losses for electrode spray water

- Heat energy losses in off-gases

- Heat energy losses at roof opening

- Heat energy losses in slag

 

4 RESULTS FROM SIMULATION

The outputs from the simulation are a part of a result summary process temperatures, off-gas flow, off-gas composition, off-gas analysis, power input distribution, power and heat losses, temperature raise in panel water, distribution, energy input and energy losses distribution, consumable rates, specific consumptions, material balance, steel analysis, slag analysis and electrical parameters which can be presented in the output data in form of tables or graphic diagrams.

Result Summary

Production

- Time power on

- Time power off

- Time tap to tap

- Tapping temperature

- Yield (tapped steel/charged scrap, sponge iron)

- production rate

Specific Consumptions

- Electrical energy

- Electrodes

- Carbon, coke

- Fuel (oil or natural gas)

- Oxygen in burners

- Oxygen in lances

- Lime , slag former

Energy and Material Balances

- Energy input distribution

- Energy loss distribution

- Material balance

- Steel analysis

- Slag analysis

Average Electrical Power Input

- Average electrical power input in arc furnace

- Average electrical power input in parallel load

- Average electrical power input in harmonic filters

- Average electrical power input in SVC (TCR reactor)

- Average electrical power input on common furnace busbar

- Average electrical power input on electrical supply busbar

 

Process Temperatures

- Scrap- and melt temperature

- Slag temperature

- Resultant combustion temperature

- Off-gas temperature

- Bottom lining temperature

- Wall and slag line lining temperature

- Wall panel slag temperature

- Wall panel "tube" temperature

- Roof panel slag temperature

- Roof panel "tube" temperature

- Roof centre part lining temperature

- Electrode temperature

The following diagram shows some of the process temperatures.

 

Other Furnace Process Parameters

- Scrap area

- Scrap height

- Free furnace volume

Off-Gas Parameters

- Furnace Pressure

- In leakage air

- Total input of oxygen

- Off-gas flow

- Off-gas composition as CO2, CO, H2O, SO2, O2 and N2

- Off-gas analysis as CO2, CO, H2O, SO2, O2 and N2

- Gas radiation emission factors

The following diagram shows off-gas temperature and off-gas flows.

 

Power Input

- Electrical power

- Oxidation of electrodes

- Oxidation of combustibles and oil

- Oxy-fuel burners

- Oxidation of carbon

- Post combustion of carbon monoxide

- Oxidation of iron Fe

- Oxidation of metals like Si, Mn, Cr

The following diagram shows the power input distribution.

 

Power Losses and Heat Losses

- Electrical losses

- Bottom lining losses

- Wall- slag line lining losses

- Wall panel losses

- Roof panel losses

- Roof centre part losses

- Electrode spray water losses

- Off-gas losses

The following diagram shows the power and heat losses distribution.

 

Temperature Raise in Cooling Water in Wall and Roof Panels

- Wall panel losses

- Input temperature in cooling water

- Temperature rise in cooling water

- Roof panel losses

- Input temperature in cooling water

- Temperature rise in cooling water

Energy Input

- Electrical power

- Oxidation of electrodes

- Oxidation of combustibles and oil

- Oxy-fuel burners

- Oxidation of carbon

- Post combustion of carbon monoxide

- Oxidation of iron Fe

- Oxidation of metals like Si, Mn, Cr

- Sensible heat in charged scrap or molten steel or iron

Energy Losses

- Electrical losses

- Bottom lining losses

- Wall- slag line lining losses

- Wall panel losses

- Roof panel losses

- Roof centre part losses

- Electrode spray losses

- Off-gas losses

- Roof opening losses

- Slag losses

Consumable Rates

- Electrical power

- Oxidation of electrodes

- Fuel rate in burners

- Oxygen rate in burners

- Carbon injection rate in lance(s)

- Oxygen rate in lance(s)

- Lime injection in lance(s)

The following diagram shows the consumable rates plotted on screen and printed on line printer.

 

Specific Consumption

- Electrical energy

- Electrode consumption

- Fuel consumption in burners

- Oxygen consumption in burners

- Carbon, Coke consumption

- Oxygen consumption in lance(s)

- Lime, slag former consumption

Material Balance

- Charged scrap

- Total charge input

- Sponge iron input

- Charged alloy

- Oxidised iron

- Steel

- Slag

 - Yield (tapped steel/charged scrap)

Steel Analysis

- Percentage Fe, C, Si, Mn, Cr and Ni

Slag Analysis

- Percentage CaO, SiO2, MnO, Cr2O3 and FeO

Electrical Parameters for Arc Furnace

- Primary current

- Primary voltage

- Apparent power

- Active power

- Reactive power

- Power factor

- Electrode current

- Arc voltage

- Arc power

- Refractory index

- Dynamic operation impedance factor

The following diagram shows some of the electrical parameters for the arc furnace.

 

Electrical Parameters for Parallel Load

- Primary current

- Primary voltage

- Active power

- Reactive power

Electrical Parameters for Phase Compensation (Fixed Filters and SVC)

- Primary current

- Primary voltage

- Active power

- Reactive power

Resultant Electrical Parameters for Common Furnace Bus

- Primary current

- Primary voltage

- Apparent power

- Active power

- Reactive power

- Power factor

 - Flicker

Resultant Electrical Parameters for Electrical Power Supply Bus

- Primary current

- Primary voltage

- Apparent power

- Active power

- Reactive power

- Power factor

- Flicker

The following diagram shows the 1 minutes and 10 minutes PST values on the electrical power supply line bus.

 

 

5 RESULT PRESENTATION

The results can be presented in form of tables with process parameters versus time on computer screen or be printed out in a printer. Most of the process parameters can also be presented in graphic diagrams on screen or be plotted in a printer. All output data can be saved on data files for further treatment in spreadsheet programs like EXCEL or LOTUS.

6 ACCURACY OF SIMULATIONS

The results of simulations compared with actual operation results of a number of arc furnaces in Europe and North America shows very good coincidences.

With about the same power on and tap to tap time, tapping temperature, input of fuel, carbon and metal elements in scrap and input of oxygen the simulated electrical energy consumption show an accuracy better than 2-3 % for all compared furnaces.

The following tables give some brief ideas about the accuracy of the simulations.

 

Furnace No 1 (Italy)

Operation results

Simulated results

Transformer power

MVA

55

55

Shell diameter

m

5.8

5.8

Charged scrap and pig iron

ton

103.9

103.9

Bucket 1 Scrap

ton

31.0

31.0

Pig iron

ton

19.0

19.0

Bucket 2 Scrap

ton

27.7

27.7

Pig iron

ton

9.3

9.3

Bucket 3 Scrap

ton

16.9

16.9

Pig iron

ton

0.0

0.0

Charged carbon

kg

0

0

Charged slag former

kg

2255

2255

Burners

Fuel (natural gas)

Nm3

565

565

"

Nm3/min

43.5

43.5

Oxygen

Nm3

1130

1130

"

Nm3/min

87.0

87.0

Oxygen post combustion

Nm3

956

956

"

Nm3/min

86.9

86.9

Oxygen lances

Carbon

kg

547

550

"

kg/min

25.0

25.0

Oxygen

Nm3

3195

3182

"

Nm3/min

86.7

86.0

Production

Tap to tap time

min

51.0

51.0

Power on time

min

39.0

39.0

Power off time

min

12.0

12.0

Tapping temperature

C

1614 1)

1627 (power off)

Yield (tapped steel/charged scrap)

p.u.

0.88

0.88

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

25756

25910

"

kWh/ton

248

249

Electrodes

kg/ton

1.40

1.05 2)

Natural gas

Nm3/ton

5.5

5.4

Carbon

kg/ton

5.3

5.2

Oxygen

Nm3/ton

50.8

49.9

Slag former

kg/ton

21.7

21.5

1) The temperature is taken 0.5 to 1 minutes before tapping.

2) Exclusive breakage

Furnace No 2 (Belgium)

Operation results

Simulated results

Transformer power

MVA

120

120

Shell diameter

m

7.0

7.0

Charged scrap and pig iron

ton

165.0

165.0

Bucket 1 Scrap

ton

85.0

85.0

Pig iron

ton

0.0

0.0

Bucket 2 Scrap

ton

80.0

80.0

Pig iron

ton

0.0

0.0

Charged carbon

kg

0

0

Charged slag former

kg

4400

4400

Burners

Fuel (natural gas)

Nm3

480

475

"

Nm3/min

19.2

19.0

Oxygen

Nm3

960

950

"

Nm3/min

38.4

38.0

Oxygen post combustion

Nm3

0

0

"

Nm3/min

0.0

0.0

Oxygen lances

Carbon

kg

845

825

"

kg/min

25.0

25.0

Oxygen

Nm3

1835

1850

"

Nm3/min

49.6

50.0

Production

Tap to tap time

min

84.0

84.0

Power on time

min

63.0

63.0

Power off time

min

21.0

21.0

Tapping temperature

C

1660

1662 (power off)

Yield (tapped steel/charged scrap)

p.u.

?

0.924

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

72996

71860

"

kWh/ton

442

438

Electrodes

kg/ton

1.66

1.57 1)

Natural gas

Nm3/ton

2.9

2.9

Carbon

kg/ton

5.1

5.0

Oxygen

Nm3/ton

17.0

17.0

Slag former

kg/ton

26.7

26.7

1) Exclusive breakage

 

Furnace No 3 (Canada)

Operation results

Simulated results

Transformer power

MVA

43

43

Shell diameter

m

4.7/5.3

4.7/5.3

Charged scrap and pig iron

ton

73.6

73.0

Bucket 1 Scrap

ton

37.5

37.5

Pig iron

ton

4.1

4.1

Bucket 2 Scrap

ton

32.0

32.0

Pig iron

ton

0.0

0.0

Charged carbon

kg

240

240

Charged slag former

kg

2340

2340

Burners

Fuel (natural gas)

Nm3

544

544

"

Nm3/min

27.2

27.2

Oxygen

Nm3

1088

1088

"

Nm3/min

38.4

38.0

Oxygen post combustion

Nm3

111

90

"

Nm3/min

34.0

34.0

Oxygen lances

Carbon

kg

125

125

"

kg/min

25.0

25.0

Oxygen

Nm3

843

782

"

Nm3/min

46.0

46.0

Production

Tap to tap time

min

72.0 1)

55.0

Power on time

min

42.0

42.0

Power off time

min

30.0

21.0

Tapping temperature

C

1652

1652 (power off)

Yield (tapped steel/charged scrap)

p.u.

0.92

0.92

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

27747

27820

"

kWh/ton

377

378

Electrodes

kg/ton

2.62

1.48 2)

Natural gas

Nm3/ton

8.9

8.6

Carbon

kg/ton

5.0

5.0

Oxygen

Nm3/ton

32.7

31.9

Slag former

kg/ton

31.8

31.5

1) Average tap to tap time

2) Exclusive breakage

 

Furnace No 4 (Sweden)

Operation results

Simulated results

Transformer power

MVA

85

86

Shell diameter

m

6.8

6.8

Charged scrap and pig iron

ton

136.0

136.0

Bucket 1 Scrap

ton

66.4

66.4

Pig iron

ton

13.6

13.6

Bucket 2 Scrap

ton

56.0

56.0

Pig iron

ton

0.0

0.0

Charged carbon

kg

0

0

Charged slag former

kg

5000

5000

Burners

Fuel (oil)

litre

299

299

"

litre/min

13.0

13.0

Oxygen

Nm3

644

645

"

Nm3/min

28.0

28.0

Oxygen post combustion

Nm3

0

0

"

Nm3/min

0.0

0.0

Oxygen lances

Carbon

kg

325

320

"

kg/min

20.0

20.0

Oxygen

Nm3

2400

2350

"

Nm3/min

50.0

50.0

Production

Tap to tap time

min

65.0

65.0

Power on time

min

47.0

47.0

Power off time

min

18.0

18.0

Tapping temperature

C

1625

1638 (power off)

Yield (tapped steel/charged scrap)

p.u.

0.92

0.92

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

49547

49900

"

kWh/ton

364

367

Electrodes

kg/ton

1.66

1.57 1)

Oil

litre/ton

2.3

2.2

Carbon

kg/ton

4.6

4.6

Oxygen

Nm3/ton

22.4

22.0

Slag former

kg/ton

36.8

36.8

1) Exclusive breakage

 

Furnace No 5 (Stainless Steel ,Sweden)

Operation results

Simulated results

Transformer power

MVA

50

50

Shell diameter

m

5.8

5.8

Charged scrap and pig iron

ton

76.1

76.1

Bucket 1 Scrap

ton

55.1

55.1

Bucket 2 Scrap

ton

21.4

21.4

Charged carbon

kg

0

0

Charged slag former

kg

3000

3000

Burners

Fuel (oil)

litre

0

0

"

litre/min

0.0

0.0

Oxygen

Nm3

0

0

"

Nm3/min

0.0

0.0

Oxygen post combustion

Nm3

0

0

"

Nm3/min

0.0

0.0

Oxygen lances

Carbon

kg

531

529

"

kg/min

11.0

11.0

Oxygen

Nm3

504

502

"

Nm3/min

22.0

22.0

Production

Tap to tap time

min

90.0 1)

75.0

Power on time

min

58.0

58.0

Power off time

min

32.0

17.0

Tapping temperature

C

1658

1657 (power off)

Yield (tapped steel/charged scrap)

p.u.

?

0.94

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

36124

36390

"

kWh/ton

472

476

Electrodes

kg/ton

2.85

2.25 2)

Oil

litre/ton

0.0

0.0

Carbon

kg/ton

3.3

3.3

Oxygen

Nm3/ton

6.6

6.6

Slag former

kg/ton

39.2

29.2

1) Average tap to tap time

2) Exclusive breakage

 

Furnace No 6 (Germany)

Operation results

Simulated results

Transformer power

MVA

75

75

Shell diameter

m

6.3

6.3

Charged scrap and pig iron

ton

90.0

90.0

Bucket 1 Scrap

ton

51.0

51.0

Pig iron

ton

0.0

0.0

Bucket 2 Scrap

ton

39.0

39.0

Pig iron

ton

0.0

0.0

Charged carbon

kg

200

200

Charged slagformer

kg

4600

4600

Burners

Fuel (natural gas)

Nm3

448

449

"

Nm3/min

15.5

15.5

Oxygen

Nm3

900

898

"

Nm3/min

31.0

31.0

Oxygen post combustion

Nm3

1100

1100

"

Nm3/min

50.0

50.0

Oxygen lances

Carbon

kg

400

400

"

kg/min

40.0

40.0

Oxygen

Nm3

1800

1800

"

Nm3/min

60.0

60.0

Production

Tap to tap time

min

46.0

46.0

Power on time

min

34.0

34.0

Power off time

min

12.0

12.0

Tapping temperature

C

1625-1630

1637 (power off)

Yield (tapped steel/charged scrap)

p.u.

0.89

0.89

Specific Consumption (per ton charged scrap)

Electrical energy

kWh

30440

30710

"

kWh/ton

338

341

Electrodes

kg/ton

1.51

1.47 1)

Natural gas

Nm3/ton

5.0

5.0

Carbon

kg/ton

6.7

6.7

Oxygen

Nm3/ton

42.2

42.2

Slag former

kg/ton

50.7

51.1

1) Exclusive breakage

   

Dr. Alberto Fragiacomo (Engineering Coordination Manager)

North-East, ITALY;  Winter Local Time: GMT+1
Tel. :                                     
Mob.:  +39 347 2618940                          
Email: wemes@wemes.it 
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Update the: 03 febbraio 2018