Exergy and energy analysis of an AC steel electric arc...

380 Int. J. Exergy, Vol. 12, No. 3, 2013

Copyright © 2013 Inderscience Enterprises Ltd.

Exergy and energy analysis of an AC steel electric arc furnace under actual conditions

Ebrahim Hajidavalloo*, Hamze Dashti and Morteza Behbahani-Nejad Mechanical Engineering Department, Shahid Chamran University, Ahvaz, Iran Email: [email protected] Email: [email protected] Email: [email protected] *Corresponding author

Abstract: The energy and exergy analyses of an existing steel Electric Arc Furnace (EAF) are performed to estimate the potential for increasing the furnace efficiency. To obtain realistic results, the effect of air infiltration into the furnace was taken into account. The results of the analyses revealed that the energy and exergy efficiencies of the furnace are low and should be increased. The main sources of energy waste are stack gases followed by heat transfer to the cooling water, while the main sources of exergy destruction are combustion and heat transfer. Hot stack gases contain 18.3% and 12.2% of the total input energy and exergy, respectively. Increasing the air infiltration reduces the energy and exergy efficiencies of the EAF. By using the energy of flue gas to preheat the sponge iron, the electrical energy consumption of the furnace can be reduced by 89 GJ, dictating a 21.4% reduction in electrical energy consumption and a 13.6% increase in steel production.

Keywords: exergy analysis; steel electric arc furnace; preheating.

Reference to this paper should be made as follows: Hajidavalloo, E., Dashti, H. and Behbahani-Nejad, M. (2013) ‘Exergy and energy analysis of an AC steel electric arc furnace under actual conditions’, Int. J. Exergy, Vol. 12, No. 3, pp.380–404.

Biographical notes: Ebrahim Hajidavalloo is an Associate Professor in Mechanical Engineering Department of Shahid Chamran University of Ahvaz. He has completed his PhD from Dalhousie University in Canada and since then is working in current position.

Hamze Dashti is MSc graduate from Mechanical Engineering Department of Shahid Chamran University of Ahvaz.

Morteza Behbahani-Nejad is an Associate Professor in Mechanical Engineering Department of Shahid Chamran University of Ahvaz. He has completed his PhD from Tehran University in Iran and since then is working in current position.

This paper is a revised and expanded version of a paper entitled ‘Exergy analysis of steel electric arc furnace’ presented at the ‘ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis’, 12–14 July 2010, Istanbul, Turkey.

Exergy and energy analysis of an AC steel electric arc furnace 381

1 Introduction

The iron and steel industries are the largest industrial energy consumers. According to Camdali et al. (2005), approximately 12% of world energy production is used in the iron and steel sectors. Bisio et al. (2000) reported that after employee costs, energy costs represent the highest cost element in integrated steel works (about 30% of the total cost). The share of Electric Arc Furnace (EAF) technology in the iron and steel industry is increasing rapidly and was associated with at least 50% of the total steel production in 2010, based on the prediction of Raja et al. (2000).

There have been many investigations regarding the energy analysis of EAFs. Most recently, Kirschen et al. (2009) presented energy balances for 70 modern EAFs and found that the total energy requirements of these EAFs ranged from 510 to 880 kWh/ton, with energy efficiencies between 40% and 75%. They reported that a decrease in energy losses to off-gassing systems and cooling systems will increase energy efficiency and thereby decrease CO2 emissions. Fruehan et al. (2000) showed that the energy used to produce liquid steel by EAF facilities is significantly higher than the theoretical minimum energy requirements. They also indicated the potential for practical reductions in the EAF energy requirements by about 24–33%. Hajidavalloo and Alagheband (2008) investigated the effect of sponge iron preheating on the efficiency of electric arc furnaces. They proposed that, by using a neutral gas such as nitrogen as the working fluid, some portion of the exit flue gas can be saved and returned to the furnace.

Contrary to the energy analysis, there has not been much work to address the exergy analysis of EAFs. In the exergy analysis, the first and second laws of thermodynamics are used to evaluate the potential improvement in the performance of a thermal system. Summaries of the evolution of exergy analysis throughout the late 1980s are provided by Kotas (1985), Moran and Sciubba (1994), Bejan et al. (1996), Rosen (1999), and Dincer (2002). In recent years, many researchers have used exergy analysis for industrial processes. Camdali and Tunc (2003) studied the exergy analysis of an EAF and concluded that its exergy efficiency is about 55%. They also computed the chemical exergy of the different input and output materials of the EAF. Bisio et al. (2000) studied the effects of design parameters on the performance of the EAF. Based on their research, the exergy efficiency of the furnace is about 55% if a heat recovery scheme is used but, otherwise the exergy efficiency is around 52%. Ostrovski and Zhang (2005) studied the energy and exergy efficiencies of the blast iron making process and found that the overall efficiency strongly depends on the utilisation of off-gas. The efficiency of a natural gas-fired aluminum melting furnace in a die-casting plant was examined by Rosen and Lee (2009) using energy and exergy methods. They found that the overall-system efficiency was 10% for energy and 6% for exergy. Coskun et al. (2009) proposed a new approach for simplifying the calculation of flue gas specific heat and specific exergy value in one formulation depending on fuel chemical composition.

As seen, there are not many reports on the exergy analysis of EAFs in the literature. Moreover, those who have discussed the subject made simplifying assumptions that are not realistic to the best knowledge of the authors. For example, the effect of air infiltration in the EAF was neglected in previous researches (Camdali et al., 2003; Camdali and Tunc 2003), even though it has important effects on the energy and exergy balance of the furnace. In this study, energy and exergy analyses of the EAF were performed while considering the effect of air infiltration into the furnace.

382 E. Hajidavalloo, H. Dashti and M. Behbahani-Nejad

2 System description

In electric arc furnaces, high voltage is applied to electrodes to create an electric arc between a metal charge and the electrodes. Heat is generated by the electrical resistance of the metal charge against passing the electric current. Production of liquid steel in an electric arc furnace can be defined with the following order:

Charging with scrap and sponge iron

Melting the charge material

Superheating the melt

Discharging

The production of high-quality steel is the result of using proper materials with good compositions that produced the following chemical reaction in the EAF (Camdali et al., 2003):

‘2[Fe]+3[O] → [Fe2O3]’, ‘[Fe]+[O] → [FeO]’, ‘[C]+[O] → CO(g)’, ‘[Si]+2[O] → [SiO2]’, ‘[Mn]+[O] → [MnO]’, ‘2[P]+5[O] → [P2O5]’, ‘(CaO)+[S] → (CaS)+[O]’, ‘(CaCO3) → (CaO)+ CO2(g)’, ‘2[Al]+3[O] → [Al2O3]’, and ‘[Ca]+[O] → (CaO)’, ‘[Zn]+[O] → [ZnO]’.

where brackets and parentheses are used for materials in solid and liquid phase, respectively.

3 Analysis

Exergy analysis is a useful tool in furthering a more efficient use of energy-resources, because it enables the locations, types, and magnitudes of wastes and losses to be accurately identified and meaningful efficiencies to be determined (Dincer and Rosen, 2007). In the exergy analysis, a complete equilibrium of the system with its environment is considered, including the chemical and thermal equilibriums. The exergy balance can be expressed in different forms, depending on the inlet and outlet conditions. For an open system in a steady state which is in contact with n heat sources, as well as multiple inlets and outlets, and has a net input work equal to W, the exergy balance over a specific time period can be expressed as follows:

01

1n

W Ii

i in outi

TEx Q m ex m ex ExT

(1)

where ExW is the work exergy (MJ), Qi is the heat loss (MJ), ExI is the internal exergy destruction (MJ), and ex is the inlet or outlet exergy (MJ/kg). The specific time period is usually considered to be the batch time of the furnace. The temperature and pressure in a reference state, T0 and P0, are taken as 25°C and 100 kPa, respectively. Figure 1 shows a general system with all the exergy components.


Figure 1 Exergy balance of an open system at steady condition

Specific flow exergy is generally divided into thermo-mechanical and chemical exergies as follows:

tm chex ex ex (2)

Thermo-mechanical exergy includes kinetic, potential and physical exergies that can be represented as follows:

tm kin pot phyex ex ex ex (3)

The physical exergy of the flow is calculated from the following relation:

1 0 0 1 0phy i i i iex h h T S S (4)

The kinetic and potential energies of a material stream are ordered forms of energy, so these are fully convertible to work and can be defined as follows:

2

2kin iVex (5)

potE iex g Z (6)

Since the changes in the potential and the kinetic exergies are negligible, so they are not taking into account in the calculations. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environmental state to dead state by processes involving heat transfer and exchange of substance only with environment (Kotas, 1985) The standard chemical molar exergy of the fuel constituents (

chex ) can be found in thermodynamic tables (Moran and Shapiro,

2000). The molar chemical exergy of a gas mixture is obtained from the following relation (Moran and Shapiro, 2000):

01 1

lnj jch ch

i i i ii i

Ex n y ex RT y y

(7)

where yi is the molar ratio of the i-th gas constituent.


3.1 Exergy balance of electric arc furnace

An actual AC EAF, working at the Khouzestan Steel Company in Iran, was considered for this study in order to obtain realistic data. Figure 2 shows a schematic diagram of the EAF based on the exergy balance terms. The following assumptions are made during the exergy analysis:

The heating process in the furnace occurs in a steady state.

This steady process can be integrated over a specific time period (e.g. the batch time period of the EAF, which is about 110 min.)

Stack gases can be treated as ideal gases.

Hence, the exergy balance equation for the EAF can be expressed as follows:

, ,phy ch in phy ch out TIin out

W m ex Ex m ex Ex Ex (8)

where ExTI is the total irreversibility including the internal and external parts. In addition, the inlet and outlet exergies of the materials are expressed as follows:

phy phy phy phy physcr scr sp sp elk elk ck ckin

phy phy phy phylim lim dlm dlm oxy oxy cw cw

m ex m ex m ex m ex m ex

m ex m ex m ex m ex

(9)

phy phy phy phy phyls ls st sl st sl dst dst sg sgout

phycw cw

m ex m ex m ex m ex m ex

m ex

(10)

The effect of differences between the ambient and the inlet and the outlet pressure is usually negligible, so the enthalpy and entropy changes of materials can be calculated as follows:

01 0

T

i i PTh h C dT (11)

0

1 0

T

i i PTS S C T dT (12)

2PC a bT cT

(13)

The coefficients (a, b and c) used in equation (13) can be found in thermo-chemistry tables (Kubaschewski et al., 1989).

Owing to the presence of induced draft fan (ID fan) at the end of the exhaust duct, ambient air is generally drawn into the furnace through the opening in the external surface of the furnace. This opening is used to discharge slag from the EAF. In most investigations (Camdali et al., 2003; Camdali and Tunc 2003) the effect of air infiltration has not been taken into account by the energy and exergy analyses of the EAF. Since the rate of air infiltration in the furnace is not negligible, its effects on the energy and exergy analyses are significant and must be considered.


Figure 2 Total exergy balance for EAF

The first-law efficiency of EAF can be defined as the ratio between the energy in the liquid steel output to the energy input, which is mostly electrical energy. This can be expressed as follow:

ls ls

in in

m Im I

(14)

Exergy efficiency is defined as the ratio of the recovered exergy to the supplied exergy (Cengel and Boles, 1994). Accordingly, the exergy efficiency of the EAF is the ratio between the recovered exergy of the liquid steel output and the mostly electrical exergy input. This can be expressed as follows:

ls ls

in in

m exExergy recoveredEvergy supplied m ex

(15)

4 Results and discussion

The energy and exergy balances of the EAF can now be calculated on the basis of the data available in the plant. Most investigations have not considered the effect of air infiltration in the energy and exergy analyses. This neglect is not accurate, because air is drawn into the furnace through the external opening and thus affects the thermal balance of the system (Fruehan et al. 2000). To quantify the effect, the analysis was performed with and without air infiltration.

4.1 Energy and exergy analyses of EAF without air infiltration

Tables 1 and 2 show the chemical components of input and output materials without air infiltration. Also, the physical exergies of all input and output materials are listed in Tables 3 and 4, respectively. The chemical exergies of the input and output materials are listed in Tables 5 and 4, respectively. Because the masses and temperatures of the input and output materials are fixed, the difference between chemical exergy input and output is constant and is defined for simplicity as follows:

, , , 1419324.3 1290107.2 129217.1 MJch net ch in ch outEx Ex Ex


Figures 3 and 4 compare the chemical exergies of the input and output materials, respectively. It should be noted that sponge iron has the highest chemical exergy of the input materials and liquid steel has the highest exergy of the output materials.

Figure 3 Chemical exergy of input materials

Figure 4 Chemical exergy of output materials

Table 1 Chemical components of the input materials

Ch. Component (%) m (kg) M (kg/kmole) n (kmole) Scrap Iron

Fe 97 19,400 55.847 347.3777 C 1 200 12.011 16.6514 Si 0.37 74 28.086 2.6348

Mn 0.66 132 54.938 2.4027 P 0.06 12 30.974 0.3874 S 0.06 12 32.06 0.3743 Cr 0.42 84 51.996 1.6155 Ni 0.12 24 58.71 0.4088


Table 1 Chemical components of the input materials (continued)

Ch. Component (%) m (kg) M (kg/kmole) n (kmole) Mo 0.11 22 95.94 0.2293 Cu 0.2 40 63.546 0.6295

TOTAL 100 20,000 484.108 372.7113 Sponge Iron

Fe 89.22 151,674 55.847 2,715.8845 C 1.87 3,179 12.011 264.6740 P 0.41 697 30.974 22.5027 S 0.35 595 32.06 18.5590

MnO 0.43 731 70.937 10.3049 SiO2 3.82 6494 60.084 108.0820

Al2O3 1.1 1870 101.961 18.3403 CaO 1.6 2720 56.079 48.5030 MgO 1.2 2040 40.304 50.6153

TOTAL 100 170,000 460.257 3,257.4658 Cock

C 89 1,780 12.011 148.1975 SiO2 4 80 60.084 1.3315

Al2O3 3 60 101.961 0.5885 H2O 2 40 18.015 2.2204

Fe2O3 0.75 15 159.691 0.0939 CaO 0.75 15 56.079 0.2675

S 0.5 10 32.06 0.3119 TOTAL 100 2,000 439.901 153.0111

Electrode C 99.3 425.997 12.011 35.4672 Si 0.15 0.6435 28.086 0.0229 Fe 0.15 0.6435 55.847 0.0115 Ca 0.15 0.6435 40.08 0.0161 Al 0.05 0.2145 26.982 0.0079

SiO2 0.05 0.2145 60.084 0.0036 Al2O3 0.05 0.2145 101.961 0.0021 Fe2O3 0.05 0.2145 159.691 0.0013 MgO 0.05 0.2145 40.304 0.0053

TOTAL 100.00 429 525.05 35.54 Lime

CaO 95 7,600 56.079 135.52 MgO 5 400 40.304 9.92

TOTAL 100 8,000 96.383 145.45


Table 1 Chemical components of the input materials (continued)

Ch. Component (%) m (kg) M (kg/kmole) n (kmole) Dolomite

CaO 65 2,600 56.079 46.36 MgO 35 1,400 40.304 34.74

TOTAL 100 4,000 96.383 81.10 Oxygen

O2 100 11,377 31.998 355.57 TOTAL 100 11,377 31.998 355.57

Cooling Water H2O 100 1,146,107 18.02 63619.57

TOTAL 100 1,146,107 18.02 63619.57

OVERALL 215,806.4 (without CW) 4,400.8

Table 2 Chemical components of the output materials

Ch. Component (%) m (kg) M (kg/kmol) n (kmol) Liquid Steel

Fe 99.10 163,123.92 55.85 2,920.91 C 0.08 137.65 12.01 11.46 Si 0.16 262.34 28.09 9.34

Mn 0.17 277.31 54.94 5.05 P 0.16 271.48 30.97 8.76 S 0.26 421.83 32.06 13.16 Cr 0.02 28.81 52.00 0.55 Ni 0.01 23.80 58.71 0.41 Mo 0.01 21.82 95.94 0.23 Cu 0.02 39.67 63.55 0.62

TOTAL 100.00 164,608.63 484.11 2,970.49 Steel in Slag

Fe 99.10 1,365.26 55.85 24.45 C 0.08 1.15 12.01 0.10 Si 0.16 2.20 28.09 0.08

Mn 0.17 2.32 54.94 0.04 P 0.16 2.27 30.97 0.07 S 0.26 3.53 32.06 0.11 Cr 0.02 0.24 52.00 0.00 Ni 0.01 0.20 58.71 0.00 Mo 0.01 0.18 95.94 0.00 Cu 0.02 0.33 63.55 0.01

TOTAL 100.00 1,377.69 484.11 24.86


Table 2 Chemical components of the output materials (continued)

Ch. Component (%) m (kg) M (kg/kmol) n (kmol) Dust

Fe2O3 79.90 9,195.14 159.691 57.58 Al2O3 4.98 572.97 101.961 5.62 CaO 3.93 451.85 56.079 8.06 MnO 0.52 60.00 70.937 0.85 Cr2O3 0.09 10.41 151.989 0.07 SiO2 7.97 917.73 60.084 15.27

C 2.61 300.12 12.011 24.99 TOTAL 100.00 11,508.22 612.752 112.43

Slag CaO 49.01 12,148.84 56.08 216.64 FeO 0.82 204.21 71.85 2.84 SiO2 21.18 5,250.24 60.08 87.38 MnO 1.94 480.37 70.94 6.77 Fe2O3 0.03 8.49 159.69 0.05 Al2O3 5.48 1,357.65 101.96 13.32 Cr2O3 0.28 69.90 151.99 0.46 P2O5 4.02 997.29 141.94 7.03 CaS 1.74 431.22 72.14 5.98 MgO 15.49 3,840.21 40.30 95.28

TOTAL 100.00 24,788.43 926.97 435.75 Stack Gas

CO 69.55 9,405.10 28.01 335.78 CO2 30.16 4,078.35 44.01 92.67 H2O 0.30 40.00 18.02 2.22

TOTAL 100.00 13,523.46 90.03 430.67 Cooling Water

H2O 100.00 114,106 18.02 63,619.57 TOTAL 100.00 114,106 18.02 63,619.57

OVERALL 215,806.4 ( without water) 3,974.2

Table 3 Physical exergy of input materials

Input Materials m (kg) T (K) Δh (kJ/kg) Δs (kJ/kg K) phyxE (MJ)

Scrap Iron 20,000 303 2.32 0.008 0.37 Sponge Iron 170,000 303 3.57 0.012 3.39 Coke 2,000 303 4.33 0.014 0.07 Electrode 429 303 3.51 0.012 0.01


Table 3 Physical exergy of input materials (continued)

Input Materials m (kg) T (K) Δh (kJ/kg) Δs (kJ/kg K) phyxE (MJ)

Lime 8,000 303 4.16 0.014 0.26 Dolomite 4,000 303 4.16 0.014 0.14 Oxygen 11,377 303 4.59 0.015 13.87 Cooling Water 1,146,107 308 41.81 0.138 786.71 OVERALL 215,806.4 (without CW) 804.8

Table 4 Physical exergy of output materials

Output Materials M (kg) T (K) Δh (kJ/kg) Δs (kJ/kg K) phyxE (MJ)

Liquid Steel 164,609 1933 1,355.10 1.34 168,345.01 Steel in Slag 1,378 1933 1,355.10 1.34 1,408.96 Slag 24,788 1933 1,739.64 1.85 29,596.52 Dust 11,508 1933 1,574.11 1.72 12,100.57 Stack Gas 13,523 1933 2,321.05 2.51 17,652.07 Cooling Water 1,146,107 318 83.63 0.27 3,079.97 OVERALL 215,806.4 (without CW) 232,183.1

Table 5 Chemical exergy of input materials

Ch. Component n (kmol) Standard Ch. Exergy (kJ/kmol) Ch. Exergy (MJ) Scrap Iron

Fe 347.68 376,400 130,752.95 C 16.65 410,260 6,831.4 Si 2.63 854,600 2,251.67

Mn 2.4 482,300 1,158.83 P 0.39 875,800 339.3 S 0.37 609,600 228.17 Cr 1.62 544,300 879.32 Ni 0.41 232,700 95.13 Mo 0.23 730,300 167.47 Cu 0.63 134,200 84.47

TOTAL 372.71 5,250,460 142,788.71 Sponge Iron

Fe 2,715.88 376,400 1,022,258.91 C 264.67 410,260 108,585.18 P 22.5 875,800 19,707.9 S 18.56 609,600 11,313.54

MnO 10.3 119,400 1,230.41


Table 5 Chemical exergy of input materials (continued)

Ch. Component n (kmol) Standard Ch. Exergy (kJ/kmol) Ch. Exergy (MJ) SiO2 108.08 7,900 853.85

Al2O3 18.34 200,400 3,675.41 CaO 48.5 110,200 5,345.03 MgO 50.62 66,800 3,381.1

TOTAL 3,257.47 2,776,760 1,176,351.33 Cock

C 148.2 410,260 60,799.5 SiO2 1.33 7,900 10.52

Al2O3 0.59 200,400 117.93 H2O 2.22 900 2

Fe2O3 0.09 16,500 1.55 CaO 0.27 110,200 29.48

S 0.31 609,600 190.14 TOTAL 153.01 1,355,760 61,151.11

Electrode C 35.47 410,260 14,550.79 Si 0.02 854,600 19.58 Fe 0.01 376,400 4.34 Ca 0.02 712,400 11.44 Al 0.01 888,400 7.06

SiO2 0.004 7,900 0.03 Al2O3 0.002 200,400 0.42 Fe2O3 0.001 16,500 0.02 MgO 0.01 66,800 0.36

TOTAL 35.54 3,533,660 14,593.68 Lime

CaO 135.52 110,200 14,934.65 MgO 9.92 66,800 662.96

TOTAL 145.45 177,000 15,597.61 Dolomite

CaO 46.36 110,200 5,109.22 MgO 34.74 66,800 2,320.37

TOTAL 81.1 177,000 7,429.59 Oxygen

O2 355.57 3,970 1,411.6 TOTAL 355.57 3,970 1,411.6

OVERALL 4,400.8 1,419,323.6


Table 6 Chemical exergy of output materials

Ch. Component n (kmol) Standard Ch. Exergy (kJ/kmol) Ch. Exergy (MJ) Liquid Steel

Fe 2,920.91 376,400 1,099,429.56 C 11.46 410,260 4,701.59 Si 9.34 854,600 7,982.62

Mn 5.05 482,300 2,434.53 P 8.76 875,800 7,676.18 S 13.16 609,600 8,020.83 Cr 0.55 544,300 301.56 Ni 0.41 232,700 94.34 Mo 0.23 730,300 166.08 Cu 0.62 134,200 83.77

TOTAL 2,970.49 5,250,460 1,130,891.06 Steel in Slag

Fe 24.45 376,400 9,201.64 C 0.1 410,260 39.35 Si 0.08 854,600 66.81

Mn 0.04 482,300 20.38

P 0.07 875,800 64.25 S 0.11 609,600 67.13 Cr 0 544,300 2.52 Ni 0 232,700 0.79 Mo 0 730,300 1.39 Cu 0.01 134,200 0.7

TOTAL 24.86 5,250,460 9,464.95 Dust

Fe2O3 57.58 16,500 950.08 Al2O3 5.62 200,400 1,126.15 CaO 8.06 110,200 887.92 MnO 0.85 119,400 100.99 Cr2O3 0.07 36,500 2.5 SiO2 15.27 7,900 120.66

C 24.99 410,260 10,251.05 TOTAL 112.43 901,160 13,439.37

Slag CaO 216.64 110,200 23,873.5 FeO 2.84 127,000 360.98 SiO2 87.38 7,900 690.32


Table 6 Chemical exergy of output materials (continued)

Ch. Component n (kmol) Standard Ch. Exergy (kJ/kmol) Ch. Exergy (MJ) MnO 6.77 119,400 808.55 Fe2O3 0.05 16,500 0.88 Al2O3 13.32 200,400 2,668.4 Cr2O3 0.46 36,500 16.79 P2O5 7.03 319,540 2,245.09 CaS 5.98 844,600 5,048.61 MgO 95.28 66800 6,364.79

TOTAL 435.75 1,848,840 42,077.9 Stack Gas

CO 335.78 275,100 CO2 92.67 19,870 H2O 2.22 9,500

TOTAL 430.67 304,470 94,233.25* OVERALL 3,974.2 1,290,106.5

Note: This value is calculating by using equation (7).

Table 7 lists the energy balance of the different components in the EAF. Clearly, the electrical energy contributes the highest percentage of the input section, while liquid steel followed by heat loss contributes the highest percentages of the output section. In this table, heat loss was calculated by applying the first law (energy balance) to the energy input and output of the EAF. Table 8 lists the exergy balance of the different components in the EAF. In this table, the chemical exergy input and output are not shown; instead the difference is listed as the net chemical exergy in the input section. Table 7 Total energy balance of EAF without air infiltration

Component Energy (MJ) (%) Input

Scrap Iron 45.08 0.01 Sponge Iron 408.51 0.08 Coke 7.97 0.00 Electrode 1.55 0.00 Lime 31.18 0.01 Dolomite 16.46 0.00 Oxygen 52.22 0.01 Cooling Water 47,933.78 8.91 Electrical Energy 386,560.75 71.82 Net Chemical Energy 103,145.69 19.16 Total 538,204.1 100


Table 7 Total energy balance of EAF without air infiltration (continued)

Component Energy (MJ) (%) Output

Liquid Steel 236,588.29 43.96 Steel in Slag 1,980.12 0.37 Dust 17,998.41 3.34 Slag 43,733.15 8.13 Stack Gas 26,224.92 4.87 Cooling Water 95,867.56 17.81 Heat Lost 115,811.69 21.52 Total 538,204.1 100

Table 8 Total exergy balance of EAF without air infiltration

Component Exergy (MJ) (%) Input

Scrap Iron 0.37 0.00 Sponge Iron 3.39 0.00 Coke 0.07 0.00 Electrode 0.01 0.00 Lime 0.26 0.00 Dolomite 0.14 0.00 Oxygen 13.87 0.00 Cooling Water 786.81 0.15 Electrical Exergy 386,560.75 74.83 Net Chemical Exergy 129,217.10 25.01 Total 516,583.0 100

Output Liquid Steel 168,345.01 32.59 Steel in Slag 1,408.96 0.27 Dust 12,100.57 2.34 Slag 29,596.52 5.73 Stack Gas 17,652.07 3.42 Cooling Water 3,079.97 0.60 Exergy Losses 284,400.62 55.05 Total 516,583.0 100

Using data in Tables 7 and 8, the energy and exergy efficiencies of EAF without air infiltration were calculated as η=43.9% and φ=32.5%.

In this case the major sources of energy losses are heat transfer to the environment and the cooling water. The energy loss by stack gas is around 4.87% of total energy


output and the exergy loss is around 3.42% of total exergy output which are relatively lower than other sources. The major sources of exergy destruction are chemical reactions and heat transfer.

4.2 Energy and exergy analyses of EAF with air infiltration

As mentioned above, it is necessary to consider air infiltration into the EAF to obtain more realistic results. The air that infiltrates the EAF increases the heat loss through convection; therefore, more electrical energy is required to melt the iron. Furnace design data shows that the total mass flow rate of hot flue gas from the furnace is about 10.4 kg/s on average. In the case of EAF without air infiltration, mass balance of input and output materials (Tables 4 and 6) show that the mass flow rate of hot flue gas is 4.0 kg/s which represents the mass flow rate due to only combustion products. The difference between total mass flow rate of hot flue gas (10.4 kg/s) and 4.0 kg/s represents air infiltration mass flow rate which is 6.4 kg/s. Therefore, neglecting the effect of air infiltration in the analysis is not acceptable. Total mass balance of EAF with considering air infiltration is shown in Table 9. Also, Tables 10 and 11 list the total energy and exergy balances of the EAF considering the effects of air infiltration. Obviously, the electrical energy required is considerably increased. The output energy and exergy shares of hot stack gas also increase considerably to reach 18.3% and 12.2%, respectively. These values match the previous data published in the literature (Bisio, 1993).

Using the data in Tables 10 and 11, the energy and exergy efficiencies of EAF with air infiltration were calculated as η=41.7% and φ=30.8%. Table 9 Total mass balance of EAF considering air infiltration

Component m (kg) (%) Input

Scrap Iron 20,000 7.71 Sponge Iron 170,000 65.53 Coke 2,000 0.77 Electrode 429 0.16 Lime 8,000 3.08 Dolomite 4,000 1.54 Oxygen 11,377 4.39 Input Air 43,608 16.81 Cooling Water 1,146,107 – Total 259,414.4 (without CW) 100

Output Liquid Steel 164,609 63.45 Steel in Slag 1,378 0.53 Slag 24,788 9.56 Stack Gas 68,639 26.46 Cooling Water 1,146,107 – Total 259,414.4 (without CW) 100


Table 10 Total energy balance of EAF considering air infiltration

Component Energy (MJ) (%) Input

Scrap Iron 45.08 0.01 Sponge Iron 408.51 0.07 Coke 7.97 0.00 Electrode 1.55 0.00 Lime 31.18 0.01 Dolomite 16.46 0.00 Oxygen 52.22 0.01 Input Air 220.80 0.04 Cooling Water 47,933.78 8.44 Electrical Energy 415,800.00 73.25 Net Chemical Energy 103,145.69 18.17 Total 567,663.2 100

Output Liquid Steel 236,588.29 41.68 Steel in Slag 1,980.12 0.35 Slag 43,733.15 7.70 Stack Gas 104,067.10 18.33 Cooling Water 95,867.56 16.89 Heat Lost 85,427.02 15.05 Total 567,663.2 100

Table 11 Total exergy balance of EAF considering air infiltration


Scrap Iron 0.37 0.00 Sponge Iron 3.39 0.00 Coke 0.07 0.00 Electrode 0.01 0.00 Lime 0.26 0.00 Dolomite 0.14 0.00 Oxygen 13.87 0.00 Input Air 1.83 0.00 Cooling Water 786.71 0.14 Electrical Exergy 415,800.00 76.18 Net Chemical Exergy 129,217.10 23.68 Total 545,823.7 100


Table 11 Total exergy balance of EAF considering air infiltration (continued)

Component Exergy (MJ) (%) Output

Liquid Steel 168,345.01 30.84 Steel in Slag 1,408.96 0.26 Slag 29,596.52 5.42 Stack Gas 66,293.23 12.15 Cooling Water 3,079.97 0.56 Exergy Losses 277,100.06 50.77 Total 545,823.7 100

A comparison of these efficiencies with the values above shows that the energy and exergy efficiencies are reduced by air infiltration into the furnace, which thus indicates its negative effect on the system. These reduced efficiencies are due to a requirement for more electrical energy to heat the additional air that infiltrates the control volume and increases the energy waste at the exhaust duct.

Figure 5 shows the effects of various air infiltration rates on the EAF efficiencies. As the mass flow rate of the air infiltration increases, the energy and exergy efficiencies of the furnace are reduced. For example, by a 50% reduction of the air infiltration, about 2.7% of electrical energy input can be saved. Air infiltration can be reduced by better sealing of the openings in the furnace.

Figure 5 EAF efficiency versus infiltration air mass flow rate

4.3 Increasing the energy and exergy efficiencies of the EAF

Melting iron by EAF is a highly energy-consuming process in which the temperature of sponge iron and other input materials is increased from the ambient value to 1600°C. Since electrical energy is the most expensive form of energy used in the EAF, lowering its consumption is a major concern in these industries.

In the conventional EAF design, the hot stack gas passes through a series of water-cooled and air-cooled ducts to reach the radiant cooler and is finally exhausted to the


atmosphere by an ID fan. The flue gas is a rich source of thermal energy, because its high mass flow rate and high temperature. This could be used for sponge iron preheating, which in turn would reduce the electrical energy consumption in the furnace.

4.3.1 Sponge iron preheating The furnace stack gas cannot be directly used to preheat the sponge iron because of particle re-oxidation. Therefore, an intermediate gas must be used in the preheating process. Nitrogen is a fine candidate for this purpose because it does not react with sponge iron particles and is a readily available by-product of other processes in steel making plants (Hajidavalloo and Alagheband, 2008).

It is possible to place a heat exchanger in the exhaust duct of the furnace in which nitrogen absorb heat from the hot flue gas. The heated nitrogen would then enter the silo to preheat the sponge iron particles. Then, nitrogen could be recirculated to avoid its consumption. Figure 6 shows the schematics of the design for an actual EAF. The specification and performance of the heat exchanger are not a major concern in this paper but were discussed before (Hajidavalloo and Alagheband, 2008).

Figure 6 Layout of design for preheating of sponge iron using hot flue gas

4.3.2 Effect of sponge iron preheating on energy and exergy analysis As mentioned above, to increase the energy and exergy efficiencies of the system, the heat lost through stack gas can be recovered for preheating the sponge iron particles. In order to consider the effect of the preheating scheme on the efficiencies, the control volume is extended to include the elbow and exhaust duct as well as the EAF itself. This means that the cooling water for the elbow and duct is considered in the energy and exergy calculations.


Figure 7 shows the percentage of electrical energy saved versus the preheating temperature. For example, by preheating the sponge iron to 793 K, electrical energy consumption of the EAF would be reduced by about 21.4% which corresponds to about 89 GJ of electrical energy saved in each working period.

Figure 7 Percentage of energy recovered from EAF versus sponge iron preheating temperature

Table 12 lists the total exergy balance of the EAF with the newly extended control volume and sponge iron preheating to 793 K. Considering all other variables almost constant, the electrical energy required is considerably reduced. The exergy efficiency of the EAF is increased to 36.8% from 30.8% and its energy efficiency is increased to 46.7% from 41.7%. Evidently, the preheating scheme improves the energy and exergy efficiencies considerably. Table 12 Total exergy balance for EAF with preheating sponge iron


Scrap Iron 0.37 0.00 Sponge Iron 3.39 0.00 Coke 0.07 0.00 Electrode 0.01 0.00 Lime 0.26 0.00 Dolomite 0.14 0.00 Oxygen 13.87 0.00

Furnace 786.71 0.17 Elbow 131.12 0.03 Cooling Water Duct 337.16 0.07

Nitrogen 0.16 0.00 Electrical Exergy 326,717.72 71.46 Net Chemical Exergy 129,217.10 28.22 Total 457,209.9 100


Table 12 Total exergy balance for EAF with preheating sponge iron (continued)

Component Exergy (MJ) (%) Output

Liquid Steel 168,345.01 36.82 Steel in Slag 1,408.96 0.31 Slag 29,596.52 6.47 Stack Gas 20,523.81 4.49

Furnace 3,079.97 0.67 Elbow 513.33 0.11 Cooling Water Duct 1,319.99 0.29

Nitrogen 31.14 0.01 Exergy Losses 232,391.18 50.83 Total 457,209.9 100

Figure 8 shows the effect of sponge iron preheating on the energy and exergy efficiencies of the EAF. As the sponge iron preheating temperature increases, the energy and exergy efficiencies increase.

Figure 8 Change of EAF efficiencies versus sponge iron preheating temperature

Apart from the energy and exergy improvements, the major benefit of the sponge iron preheating scheme is the accelerated production rate of the furnace due to a shorter tap-to-tap time which is the time between input and output materials. There are different estimations on the amount of time reduction in the melting process for a given increase in the charge temperature, because it is highly plant dependent and many local parameters should be taken into account to precisely predict the time reduction, as reported by Baily (2001). If it is assumed that tap-to-tap time reduction for every 100°C increase in charging material temperature is around 3 min., then for 500°C increase in charging temperature due to the preheating, tap-to-tap time reduction would be around 15 minutes. Since overall tap-to-tap time for each melting is around 110 min., then melting time


reduction would be around 13.6%, which means steel production may be increased around 13.6%. The production rate would be further increased if inlet gas temperature were increased.

5 Conclusions

Energy and exergy analyses were performed to evaluate the performance of an electric arc furnace. Energy and exergy shares of different input and output materials in the steelmaking process of the EAF were specified. It was found that combustion and heat transfer are two major sources of irreversibility in the EAF. Considering air infiltration into the EAF has a significance effect on the calculated energy and exergy efficiencies. The infiltration considerably reduces both the energy and the exergy efficiency. The study has shown that vast amounts of energy and exergy are wasted in the EAF industry which can be recovered by the application of a preheating scheme. The output hot flue gas contains 18.3% and 12.2% of the total energy and exergy inputs, respectively. Preheating the sponge iron particles by using waste heat from flue gas could decrease energy consumption and increase productivity. By adopting the preheating scheme, the energy and exergy efficiencies could increase by 5.0% and 6.0%, respectively.

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Nomenclature

CP constant pressure specific heat (kJ/kg-K)

I specific energy (kJ/kg)

Ex specific flow exergy (kJ/kg)

Ex exergy (MJ)

gE gravitational acceleration (m/s2)

h specific enthalpy (kJ/kg)

m Mass (kg)

n mole numbers (kmole)

P pressure

Q, q heat (MJ), specific heat (kJ/kg)

n molar ratio

S entropy (kJ/kg-K)

T temperature (K)

V flow rate (m/s)

W,w work (MJ), specific work (kJ/kg)

Y molar ratio of the gas constituent parts

Z height of flow (m)

η first law efficiency

φ exergy efficiency

Subscripts

act actual

ch chemical

ck coke

cw cooling water

dlm dolomite

dst dust

elk electrode

I for component i

in input

lim lime


loss loss

ls liquid steel

out output

oxy oxygen

phy physical

pot potential

rev reversible

scr scrap iron

sg stack gas

sl slag

sp sponge iron

st-sl steel in slag

0 property at environmental conditions

Superscripts

ch chemical

I irreversibility

kin kinetic

phy physical

pot potential

tm thermo-mechanical

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Exergy and energy analysis of an AC steel electric arc...

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