Download Operation of the electric arc furnace – With examples

OPERATION OF THE ELECTRIC
ARC FURNACE – WITH
EXAMPLES
Author: Luis Ricardo Jaccard
[email protected]
Introduction
• Certain issues related to the operation of electric arc furnaces
are well known but others, especially those relating to the
effects of electrical parameters on consumption (energy and
electrodes) or on the arc characteristics (stability and length) are
frequently cause of misunderstandings.
• The confusion usually arises from the propaganda of some
regulators manufacturers that assign to their equipment the
ability to increase power, reduce energy consumption, modify
lengths and/or stabilize the arcs, as if this were not unique
consequence of voltage, current and circuit reactance chosen by
those responsible for the operation.
2
Electrical parameters and energy consumption
Without doubt, choose the right operating parameters (voltage,
current and reactance) is essential for obtaining the required
active power and expected productivity with stable arc
(appropriate power factor) and with the least possible
consumption of electrodes, without excessive erosion of walls
and roof of the furnace, avoiding overloading the transformer or
disturbing excessively the electrical grid.
But, except in extreme cases, the electrical parameters of operation
have little or no influence on the amount of energy needed to
produce a given amount of steel.
3
Energy consumption
Energy consumption for producing one ton of steel depends on the size
of the furnace, the type of charge, melting and shut down times and
thermal losses in the environment, in the water cooled walls, in the
slag and in the hot heel. Also depends on the temperature of tapped
steel, on metal yield, and, among other factors, on the electrical
losses in the conductors.
Typical energy balance for a 100-ton furnace which operates with scrap
loaded with two baskets:
Energy in steel: 380 kWh/t
Input:
565 kWh/t
Energy in slag : 55 kWh/t
Energy in electrical losses: 20 kWh/t
Energy in gases and panels: 110 kWh/t
Output: 565 kWh/t
4
Consumption of electrical and chemical energy
In modern furnaces, power is supplied not only by electricity but also by the
chemistry (oxygen reacting with carbon). In the previous example, the furnace
demanded an amount of energy of 565 kWh per ton of product, but that
energy could be supplied with different rates of electricity and chemistry, for
example:
Total energy
Chemical energy
Electrical energy
565
105
460
565
145
420
565
185
380
This is the same furnace consuming the same amount of energy but with
different percentages of chemical energy. The electrical energy depends on
the amount of chemical energy.
5
Electrical losses
The electrical losses (I ². r) are those that occur in the conductors of the furnace
according to the current flow. Let's see the effect of these losses on the energy
consumption for an furnace that has a loss resistance of 0,4 mOhm, usually
operates with 1200 V, 60 kA and power factor 0,75. We start from the premise
that, for the normal type of charge of the furnace, the power consumption is
565 kWh/t, with 165 kWh/t for chemical energy.
V
(V)
I
(kA)
P
(MW)
Electrical
losses
power (MW)
Electric.
losses c.
(kWh/t)
Energy
consump.
(kWh/t)
Chemical
energy.
(kWh/t)
Electrical
energy
(kWh/t)
1200
60
93,5
4,3
18,4
565,0
165
400,0
1200
50
77,9
3,0
15,6
562,2
165
397,2
700
60
54,5
4,3
31,6
578,2
165
413,2
It is observed that by reducing current from 60 kA to 50 kA occurs a very small
reduction in energy consumption accompanied by a strong reduction of active
power (lower productivity). Reducing the voltage from 1200 V to 700 V obviously
causes a sharp drop in power with some increase in total energy consumption.
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Analysis of energy consumption
• The energy consumption of a furnace depends strongly on the type of load.
• When the scrap has low density or the furnace has low volume is sometimes
necessary to load the furnace with three or four baskets per heat. For loads made
with more than two baskets, each additional load, increases energy consumption
by about 25 kWh/t (for a 50 ton furnace). For our original example, if the load
were conducted in four stages, the total energy consumption (electric + chemical)
could be increased to 615 kWh/t (a bit less for a 100-ton furnace).
• Energy consumptions usually are referred to the tons of product and, if the metal
yield is low, specific consumption increases. If the furnace consumed 565 kWh
per ton of product with yield of 89%, scrap with a lower density that causes a
yield of 83% would increase the energy consumption to 606kWh/t.
• Also consider the effect that a dirty scrap causes in energy lost in the slag, by
requiring greater amounts of lime used.
• Higher final steel temperature is also an obvious cause of higher energy
consumption
• High tap to tap times, especially when resulting from high power off times, are
generally the main cause of high consumption.
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Active Power
• While the values ​of voltage, current and reactance do not cause a decisive effect
on the energy consumption of the furnace, they are essential to achieving the
desired productivity with a stable arc, low consumption of electrodes and
normal arc attack to the walls.
• To achieve a certain production of steel per hour, it will be required a certain
power: P (kW) = Cee (kWh/t) x G (t) / Power On (h), where Cee is the specific
consumption of electrical energy, G (t) is the weight of product per heat and
Power On is the operating time per heat.
• For a given process, as seen, the furnace requires a certain total amount of
energy that can be provided with greater or lesser proportion of chemical energy.
The furnace Power On time will always be proportional to the specific
consumption of electricity but will decrease when increasing the consumption of
chemical energy (see example).
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Power ON time
• Example:
Power On times for a 100 tons furnace with total power consumption of
565 kWh / t, using different chemical energy consumption values and
different active power values.
Total
Chemical Electrical
energy
energy
energy
(kWh/t) (kWh/t) (kWh/t)
Power ON
time for
80 MW
(minutes)
Power ON
time for
85 MW
(minutes)
Power ON
time for
90 MW
(minutes)
565
125
440
33,0
31,0
29,3
565
145
420
31,5
29,6
28,0
565
165
400
30,0
28,2
26,6
565
185
380
28,5
26,8
25,3
565
205
360
27,0
25,4
24,0
9
Arc voltage and arc lenght
• The arc voltage Va is equal to V x cos fi / 1.732, where V is the phase to phase
voltage and cos fi is the operational cosine fi at the infinite bus. Be a furnace
that operates with 75 MW active power and operational power factor of 0,75,
powered by a 100 MVA transformer with taps of 1300 V, 1100 V and 900 V:
Power
(MW)
Applied voltage
(V)
Current
(kA)
Arc voltage
(V)
75
1300
44,4
563
75
1100
52,5
476
75
900
64,1
390
• For these three alternatives productivity will be almost equal. The main
difference is that with higher voltages are needed lower currents (lower
consumption of electrodes) and, also, higher arc lengths (more heat radiation
to the walls). Arc length is proportional to arc voltage (usually 1 mm/V)
10
Reactance of the circuit
To keep the true power factor of 0,75 for the three alternatives of the previous
example certainly had to be modified the value of the reactance of the circuit for
each case.
From the equivalent circuit of the furnace can be deduced that the sine of the angle
 is directly proportional to the product of current and reactance and inversely
proportional to the applied voltage: sine  = I. X. 1.73 / V. For a power factor of
0,75, the sine must be equal to 0.66. The actual values ​(considering the effect of
harmonics) of the reactance for the example would be:
Power
(MW)
Applied voltage
(V)
Current
(kA)
Operational reactance
(mOhm)
75
1300
44,4
11,2
75
1100
52,5
8,0
75
900
64,1
5,3
For a given power factor, increasing the voltage must be increased the reactance.
11
Reactance and arc stability
Operational reactance Xop is greater than the sine wave reactance X due
to the effect of harmonics. “Xop / X” factor increases as the cosine
phi. As the current decreases, the cosine phi increases and the arc
voltage (Va  V . cos fi) also increases. But beyond a certain value, a
further decrease in current causes a very strong increase of the
reactance of the circuit, causing a decline in operational cos phi and
also in arc voltage. This point is known as stability limit.
For the regulator be able to maintain a certain current value is necessary
that a decrease in current causes an increase in arc voltage and,
therefore, for currents below the limit of stability the regulator fails
to maintain expected current value.
12
Stable and unstable arc
For 1000 V and reactance of 6 mOhm
550
When a decrease in current causes a decrease in arc
voltage (rather than an increase, as would be normal),
the regulator fails to maintain constant current, the
electrodes go up and down without stopping.
Electrode
500
Arc voltage (V)
450
400
350
UNSTABLE ARC
STABLE ARC
300
250
200
kA
150
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Stable and unstable arc
Explanation of the previous graphic:
To reduce the current is necessary to increase the arc length, raising the
electrodes. Under normal conditions, the decrease in current causes a decrease
in voltage drop and, consequently, the arc voltage increases. We have then a
stable condition, because the arc length was increased by the regulator and the
arc voltage also increased as the effect of the circuit. But when it is made an
attempt to regulate a current lower than the corresponding to the stability
limit, the distance between the electrodes and the load is greater than
necessary to maintain the arc with the available voltage and extinction occurs.
Due to the interruption of the current, the regulator lower the electrodes to
cause re-ignition. But the resulting current will be higher than expected and the
regulator raises the electrodes until a new arc extinction occurs. Thus, the
electrode will remain up and down without stopping. This phenomenon is
known as "pumping." In this case there is an incompatibility between the
selected voltage and current, that in addition to greatly reduction of the active
power, can cause vibrations in the arms and jerking of electrodes and water
cooled cables.
14
Long arc and unstable arc
It is quite common to associate an unstable arc with a long arc. But, in reality,
an arc can be very long and perfectly stable. The arc length is defined by
the arc voltage but stability is defined by the sinusoidal cosine phi which
can not exceed a certain value (approximately 0,85 for the period of
boring, 0,88 for melting and 0,90 for refining).
The values of the following table were calculated for the boring-down period
and for a sinusoidal reactance of 6 mOhm:
V
I (kA)
Cos fi sinus.
Cos fi real
Va
Arc stability
1200
65
0,826
0,670
464
Stable
1200
60
0,854
0,686
475
+/- Stable
1200
55
0,879
0,695
481
Unstable
1200
50
0,901
0,699
484
Much unstable
1200
45
0,921
0,698
483
Much unstable
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Long arc and short arc
• Clearly define if an arc is long or short is a relative matter. From a practical point
of view one could say that for a modern high-capacity furnace, an arc of
250 mm is a short arc, an arc of 450 mm is long and one of 650 mm is much
long. In fact, as was seen, it is possible to stabilize any length and constraints to
further increase the length of the arc are: the erosion of the walls and foamy
slag height.
• However, in relation to this topic there are also some misunderstanding. Some
manufacturers place on their electrode current controllers an option to "short
arc" or “long arc" which in reality corresponds to “higher current" or “lower
current". In fact, as can be seen in the previous table, a decrease in current
causes a very small increase in the arc voltage and below a certain point
(stability limit) destabilizes the arc. In addition, the power drops vertiginously.
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Electrode consumption
Electrode consumption (kg / t), for the same electricity consumption (kWh / t) is
proportional to the ratio between the current and arc voltage I / Va (tip
consumption) and a slightly more complex relationship corresponding to
consumption by oxidation but that basically shows a dependence on (H x D / I ²),
where H x D is the oxidation of the electrode surface (H is the height of oxidation
and D is the diameter of the electrode). Based on the formula we conclude that
the use of electrodes is inversely proportional to the arc voltage, as long as the
diameter of the electrode adapts to the current (with decreasing current, lower
electrode diameter). Not always a reduction in current causes a decrease in
electrode consumption because when the current is too low in relation to the
diameter of the electrodes a current reduction causes an increase in
consumption by oxidation higher than the reduction of the tip consumption. This
effect is heightened even more when the current falls below the stability limit
(see figure).
17
Electrode consumption
18
Electrode consumption
Electrode consumption (kg/t), for 700 V yand1100 V, reactance 4 mOhm,
electrodes 600 mm, electrical energy consumption 415 kWh/t
5.2
4.9
4.7
4.4
Low impedance furnace
4.2
3.9
3.7
kg/t
3.4
3.2
2.9
700 V
2.7
43 MW
2.4
1100 V
2.2
1.9
95 MW
1.7
1.4
1.2
0.9
20
25
30
35
40
45
50
55
60
65
70
75
80
Corriente (kA)
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Flicker and harmonic distortion
Briefly, without going deeply into this important issue, we can say that
the levels of flicker and harmonic distortion are proportional to the
short- circuit power of the furnace Scf (reactive power when the
electrodes are in direct contact with metal). From an operational
perspective what can be done to decrease by approximately 20% the
levels of disturbance (flicker and harmonics), without reducing the active
power is to operate with low real power factor (about 0,63/0,65)
because with this condition the relation Scf / P is lower, and, also,
because the arc is more stable (see figure).
20
Flicker
“Actual short circuit power/maximum short circuit power”, for the same
active power, vs. real power factor
1.05
1.00
0.95
0.90
Lowest
flicker
0.85
0.80
0.75
0.36
0.39
0.42
0.45
0.48
0.51
0.54
0.57
0.60
0.63
0.66
0.69
0.72
0.75
0.78
0.81
Real power factor
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Summary
• For maximum productivity (t/h) is required to operate at maximum
value of active power that is compatible with the dimensions of the
furnace and with the characteristics of the transformer and power
circuit.
• The active power is the product "arc voltage x current“. For a given
active power, the higher the arc voltage, the greater the aggression of
the arc to the walls and, in general, the lower electrode consumption.
• The Power ON time is proportional to the specific consumption of
electrical energy and inversely proportional to the active power.
• A greater proportion of chemical energy reduces electrical energy
consumption and indirectly reduces the Power ON time.
22
Summary
• Large furnaces operated under normal conditions like, low downtime, scrap
charge density sufficient to allow a few number of charges and high metal yield
(0,89/0,90), also clean charge that reduces the amount of slag and pouring
temperature of 1620/1640 ° C usually require an amount of specific energy of
about 545 to 565 kWh per tonne of product (billets) or 485 to 508 kWh per
tonne of metal charge.
• A portion of the energy supplied is of chemical origin. The specific consumption
of electricity depends on the amount of chemical energy provided. If a
particular furnace operates with power consumption of 565 kWh/t and are
supplied 165 kWh/t of chemical energy, electrical energy consumption will be
565-165 = 400 kWh/t. It is common in steel plants to establish goals to reduce
just electrical consumption, when in fact, the aim should be to seek the
minimum total energy consumption (electrical + chemical).
23
Summary
• To get the regulator to maintain a certain current with stable arc, the circuit
must have a reactance value that depends on the voltage and current used
(sine phi sinusoidal = I . X. 1.73 / V must be sufficiently high for the cosine phi
does not exceed certain value, e.g. 0,85 for the period of boring down).
• The arc stability has no direct relation to the length of the arc, which depends
only on the arc voltage. An arc can be very long and stable, provided it has an
appropriate balance of voltage, current, and reactance.
• Sometimes it is implied that a long arc depends on low current operation. In
fact, the decrease in current causes a very small increase in arc voltage but from
a certain point causes arc instability.
24
Summary
• To operate with a certain active power (MW) and lower levels of disturbance of
the electrical network (flicker and harmonics) is convenient to operate with low
real power factor (0,60 to 0,65) but this requires a transformer with larger
apparent power (MVA).
• Operating electrical parameters (voltage, current, power, reactance, voltage arc)
generally do not have great influence on the energy consumption. The
exceptions are extreme cases, as the operation below the limit of stability
(current or reactance to low for the applied voltage) or the operation with too
low voltages and too high currents (high electrical losses).
25