Control of a plasma fired cupola

A safe and effected method of starting up operating and shutting down a plasma fired cupola. The starting up and shutting down procedures incorporated interlocks and warning signals that insure the safety of the operators and equipment and the controls utilize the temperature of the iron in the spout as a focal point to control the torch output by separately changing field current, arc current, and torch air supply, to compensate for changes in charge make-up, particulate feed, percentage of shroud air to shroud recirculating gases to produce different irons at various melt rates.

BACKGROUND OF THE INVENTION 
This invention relates to a method for controlling a cupola which receives 
a portion of its heat energy from a plasma torch. 
The cupola is a vertical cylindrical shaped furnace in which alternate 
layers of coke, metal scrap, and fluxing material such as lime stone are 
placed. The coke is burned and the heat thereby produced is combined with 
heat input of the plasma torch to melt the scrap to form iron. 
U.S. Pat. No. 4,530,101 describes a plasma fired cupola in which plasma 
torches are utilized to melt metal turnings in nozzles connected to the 
lower portion of the cupola. 
A related application entitled Plasma Fired Feed Nozzle filed May 8, 1985 
and assigned Ser. No. 047-811 describes how a plasma torch and feed nozzle 
are attached to the cupola and may be utilized to feed particulate 
material to the lower portion of the cupola. 
Another related application entitled A Plasma Fired Cupola filed May 8, 
1987 and assigned Ser. No. 047809 describes the operation of a cupola in 
which turnings and fine metal chips make up approximately 75% of the metal 
charge. 
An application entitled Replacement Of Coke In A Plasma Fired Cupola filed 
May 8, 1987 and assigned Ser. No. 047-808 describe the operation of a 
cupola in which coke is replaced with bituminous or authracite coal or 
utilizes pulverized coal to provide up to 25% of the carbon required to 
melt metal turnings and fine chips to form iron. In a cupola with a plasma 
torch and plasma torch feed nozzle, the melt rate chemistry and 
temperature of the iron produced are affected by the charge make-up, 
particulate feed through the feed nozzle, the blast air or recirculated 
gases and the output of the plasma torch which is effected by the field 
current, arc current and air supplied to the plasma torch. 
The object of this invention is to provide a reliable control system which 
allows a very wide range of operating conditions for producing a wide 
range of iron chemistry utilizing fine borings and chips as a major 
portion of the scrap being melted. 
SUMMARY OF THE INVENTION 
In general, a control system for a plasma fired cupola having a plasma 
torch disposed in a feed nozzle with an air supply, arc current supply and 
field current supply cooperating to raise and lower the heat output of the 
plasma torch, when utilized to make iron from scrap metal, wherein the 
iron is removed from the cupola via a spout disposed in the lower portion 
of the cupola comprises measuring the temperature of the molten iron in 
the spout and comparing the measured temperature of the molten iron with a 
predetermined temperature, selecting a desired melt rate and determining 
the plasma torch heat output required to produce this desired melt rate. 
To provide the desired heat output of the torch, ranges of air flow, arc 
current and field current are selected and within the selected ranges, arc 
current, air flow and field current supplied to the torch are individually 
changed until the measured temperature equals the predetermined 
temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings in detail and in particular FIG. 1, there is 
shown a cupola 1, which is a furnace having a base portion 3 and a 
vertical cylindrical housing 5 extending from the base 3. The base 3 and 
housing 5 are lined with fire brick 7 or other refractory material 
generally forming an unobstructed round open shaft 9 with an off gas 
conduit 10 connecting the upper end of the cupola to a stack (not shown). 
An opening 11 is disposed adjacent the upper end of the shaft 9 for 
placing a charge normally comprising coke, scrap iron or steel and a 
fluxing material in the cupola. 
Disposed adjacent the base portion 3 is a spout portion 13 having a skimmer 
15, which separates melted iron and slag which are separately drawn from 
the spout 13. 
A plasma torch nozzle 17 is disposed in fluid communication with the lower 
portion of the shaft 9, while only one plasma torch nozzle is shown, it is 
understood that any number may be utilized. The plasma torch nozzle 17 is 
described in detail in an application entitled Plasma Fired Feed Nozzle 
filed May 8, 1987 and assigned Ser. No. 047-811. 
An off gas nozzle 19 is disposed in fluid communication with the shaft 9 
generally below the charge opening 11, however, its location is not 
critical. But it should generally be above the charge. A conduit or duct 
21 extends from the off gas nozzle 19 to a cyclone separator 23 utilized 
to remove particulate material from the off gas. There is a blower 25 
disposed in a conduit or duct 27 which provides fluid communication 
between the cyclone separator 23 and the plasma torch feed nozzle 17. 
A plasma torch 29 is disposed in the end of a plasma torch feed nozzle 17 
opposite the end opening into the cupola 1. Two particulate material bins 
31 are disposed above the plasma torch nozzle 17, each bin 31 has a screw 
auger 33 which feeds particulate material from the respective bin at a 
control rate to a conduit 35 which directs the particulate material to the 
plasma torch feed nozzle 17. 
A blast air conduit or duct 36 is disposed in fluid communication with the 
plasma torch nozzle and supplies combustion air for the coke. 
As shown in FIG. 1, a controller 51 is utilized to control the operation 
start-up and shutdown of the cupola either automatically or manually. The 
controller 51 receives a signal from an optical pyrometer 53 or other 
temperature sensing means indicative of the temperature of the molten 
metal in the spout 13. The temperature signal received from the pyrometer 
53 is compared with a predetermined temperature selected for the type of 
iron being produced and the use being made thereof. 
A desired melt rate is entered into the controller to establish a range of 
plasma torch air flow, arc current and field current for the plasma torch, 
which will produce a heat output in a range that will result in a desired 
melt rate. To determine the plasma torch heat output range, things such as 
the charge makeup, particulate feed, amount of blast air and amount of 
recirculated gases are also taken into account. 
A power supply 55 supplies and regulates the arc current and field current 
of the plasma torch and sends and receives signals related thereto to and 
from the controller 51. Upon receiving a signal from the controller 51 to 
raise or lower the output of the plasma torch, which raise or lower the 
temperature of the molten iron in the spout, the power supply first 
changes the arc current within predetermined limits. If additional changes 
are called for, the controller 51 then changes the air flow to the torch 
by operating a control valve 57 disposed in a conduit 58, which supplies 
air to the plasma torch. If these changes do not bring the temperature in 
line with the predetermined temperature, further changes are necessary. 
The controller 51 will then send a signal to the power supply 55 to change 
the field current, which sets a new range of arc current and air flow. The 
new range may overlap the previous range. The new arc current and air flow 
ranges cooperate to establish a new range of heat output for the plasma 
torch to further control the temperature of the molten iron in the spout. 
The controller 51 also controls the quantity of blast air being fed 
through the plasma feed nozzle 17 by operating a control valve 59 disposed 
in a blast air conduit 60 supplying blast or shroud air to the plasma feed 
nozzle 17. Recirculating gases are also fed as shroud gases via a control 
valve 61 disposed in the conduit 27. A cooling water sensor or control 
relay CR-2 is disposed in a cooling water conduit 64 which supplies 
cooling water to a cooling water jacket surrounding the plasma torch feed 
nozzle 17 and a cooling water sensor or control relay CR-3 is disposed in 
a cooling water conduit 66 which supplies cooling water to the plasma 
torch 29. The cooling water senors 63 and 65 provide signals to the 
controller 51 to indicate that the cooling water is flowing at a 
sufficient rate to protect the plasma torch and plasma torch feed nozzle. 
A nitrogen feed control valve 67 is disposed in a nitrogen supply conduit 
69 and the controller 51 automatically operates the nitrogen control valve 
67 to bleed nitrogen through the plasma torch 29 when the air supply via 
the conduit 58 is shut off. 
The control of blast air flow rate and or cycle gas flow rate, plasma torch 
air flow rate, torch field current is done through the controller. FIG. 2 
shows a portion of the relay logic of the controller and consists of four 
sections, power torch, arc ignitor and start control; field power control; 
torch and shroud air control; and torch power and ignitor automatic start 
and reset. The relay logic for the torch power supply and arc ignitor 
start control comprises a plurality of relays and contacts disposed 
between electrical lines L1 and L2 which represent 120 volt AC source. The 
circuit includes a control relay CR-1 connected in series with the 
normally open contacts CR-2 which are closed by the control relay CR-2 
(not shown) when there is cooling water flow to the plasma torch feed 
nozzle, normally open contacts CR-3 which are closed by the control relay 
CR-3 (not shown) when there is cooling water supplied to the plasma torch 
and field; normally open contacts CR-4 which are controlled by the control 
relay CR-4 (not shown) when the power supply permissive relays are 
energized, normally open contacts CR-5, which are closed by the control 
relays CR-5 (not shown) when air is flowing to the plasma torch and 
normally open contacts CR-6 which are controlled by the control relay CR-6 
(not shown) when the field is energized and the lines L1 and L2. 
Control relay CR-20 is included in this portion of the circuit and is 
connected across lines L1 and L2 in series with the normally closed 
pushbutton switch PB-10 which is operated to interrupt the circuit. A 
clock CL is connected in parallel with the pushbutton switch PB-10 and 
control relay CR-20. Normally open contacts CR-101 and CR-20 and the 
normally open a contacts of pushbutton switch PB-9 are connected and 
normally open contacts CR-100 of the torch and shroud air permissive 
control relay and normally open contacts CR-1 of the torch power 
permissive relay CR-1. 
An arc ignitor high voltage power supply which initiates or ignites the arc 
in the plasma torch. Relay AI is connected across the lines L1 and L2 in 
series with the normally open contact CR-101 which is connected in 
parallel with the b contacts of the torch start switch PB-9. The field 
power control portion of the circuit includes control relay CR-7 which is 
connected across the lines L1 and L2 in series with normally open contacts 
CR-2, normally closed contacts CR-8 of the emergency stop switch, the 
normally closed and normally open contacts of the field pushbutton PB-7 
connected in parallel with the normally open contact of PB-7 are normally 
open contact CR-7 and the automatic manual switch S-7. The field contactor 
C is disposed in series with the normally open contacts CR-7 across the 
lines L1 and L2 and also form a portion of the field power control 
circuit. 
The torch and shroud air control portion of the circuit comprises a control 
relay CR-100 and a time delay relay TD-1 connected in parallel and across 
lines L1 and L2 in series with a normally closed contacts of test abort 
switch PB-12, normally open contacts CR-1 and the normally open a contacts 
of the test start switch PB-11. Connected in series with the contacts CR-1 
are the normally closed contacts TD-1 and connected in parallel with the a 
contacts of PB-11 are normally open contacts CR100. A warning horn H is 
connected in series with the normally open contacts of the tests warning 
horn switch PB-13 and connected to the lines L1 and L2. Connected in 
parallel with the contacts of PB-13 are the b contacts of the switch 
PB-11. 
A normal torch air solenoid valve SV-5003 is connected across line L1 and 
L2 in series with a torch air control switch S-1 and a cupola auxiliary 
equipment operational interlock switch S8 and a manual automatic mode 
switch S-10, which when in the manual mode, connects contacts S-1 and S-8 
directly to the solenoid SV-5003 and, when in the automatic mode, 
interposes normally open contacts CR-100 in series therewith. A standby 
torch solenoid valve SV-5002 is connected across the lines L1 and L2 in 
series with the normally closed contacts CR-100 and the normally open 
contacts of the torch air control switch S-1. A solenoid for the shroud 
air control valve S5000 is connected across lines L1 and L2 in series with 
a normally open contacts of the shroud air control switch S-5 and the 
cupola operator blast permissive switch S-65 and the manual automatic 
switch S-10. When S-10 is in the manual mode, S-5 and S-65 are directly 
connected in series with SV5000 and when in the automatic mode normally 
open contacts CR-100 are interposed therebetween. The torch power and arc 
ignitor automatic start and reset segment of the circuit comprises a 
control relay CR-101 connected in series with a normally closed contacts 
TD-2 and normally open contact TD-1 across lines L1 and L2. Connected in 
parallel with the contacts TD-2 and the control relay CR-101 is a time 
delay TD-2 and normally open contacts CR-100. 
The operation of the control system is as follows: The torch power and arc 
ignitor start control section consists of a pushbutton control station 
PB-9 and relay CR-20 with contacts to latch the relay CR-20. After torch 
air (CR-5) and cooling water flow (CR-2 and CR-3) are established and 
power alarm interlocks CR-4 are satisfied and field current CR-6 is on, 
the power supply permissive interlock CR-1 will close and provide power to 
the control pushbutton PB-9. Depressing the pushbutton switch PB-9 in 
manual mode will energize control relay CR-20 and latch contact associated 
therewith will seal in the relay. Simultaneously the torch power supply 
gating circuits will be activated, the arc ignitor and the plasma process 
elapsed time clock CL will be energized. In the automatic mode, the torch 
power and ignitor start control contact of PB-9 are paralleled by contacts 
from the control relay CR-101 of the torch power and ignitor reset 
circuit. 
The field power control consists of a pushbutton control station PB-7 
automatic and manual selector switch S-7, field water flow sensor CR-2 and 
associated relays. When field water flow has been established and is 
greater than the low set point setting of the flow switch, the contacts of 
switch CR-2 will close. Energizing interlocking relay CR-2 (not shown) 
provides a permissive circuit to the field control station. With the 
automatic-manual switch in the manual position, the field on pushbutton 
PB-7 can be depressed energizing the control relay CR-7, and closing the 
field supply contacts CR-7 and latching the control relay CR-7. A 
permissive contact CR-2 is provided to the torch power supply alarm 
circuit to prevent operation of the plasma torch if sufficient cooling 
water is not available or to deenergize the torch power supply if cooling 
water is lost. In normal operation field current switch S-7 will be preset 
in this manual mode. When set in the automatic mode, if sufficient field 
coil cooling water has been established, the interlock relay CR-2 will be 
closed to energize the field control relay CR-7. The preset field current 
will be established. Increase or decrease of this field current is 
remotely controlled during normal operation without interruption of the 
process. 
The torch and shroud air control system is as follows: Assume that the air 
system is operational and that the air blast permissive switch S-65 has 
been closed to permit injection of air into the process reactor. Setting 
the operational mode switch S-10 to the manual position will permit 
presetting of the torch air flow at the standby and normal air flow 
conditions. Setting the operational mode switch S10 to automatic position 
will deenergize the shroud air and normal torch air solenoid valves 
SV-5000 and SV-5003 and energize the standby torch air solenoid valve 
SV-5002. Depressing the tests start pushbutton PB-11 will energize the 
torch and shroud air control relay CR-100, latch relay CR-100, energize 
the shroud air and torch normal air solenoid valves SV-5000 and SV-5003 
and deenergize the torch standby air solenoid valve SV-5002. Depressing 
the test start button PB-11 will also sound an audible warning horn 
indicating eminent energization of the plasma torch providing 
approximately a 35 second warning for personnel in the area of the cupola 
and energize the time delay device TD-1. The time delay also provides 
sufficient time for the torch and shroud air to stabilize at the optimum 
starting condition of the plasma torch and process system. At the 
completion of the timing cycle of approximately 35 seconds, the time delay 
relay TD-1 contacts change their state. The normally closed contact of 
TD-1 that parallels the permissive relay contact CR-1 will open. If a 
power supply is interrupted by CR-1 or if the test abort pushbutton PB-12 
is depressed the torch power supply and control relay CR-100 will be 
deenergized. When CR-100 is deenergized simultaneously the normal torch 
air and shroud air flow solenoid valves SV-5003 and 5000 will also 
de-energize and stand by air flow solenoid valve SV-5002 will be 
energized. De-energizing the time delay device TD-1 resets the timing 
device to 0 and unlatches the auto start control relay CR-101 and the 
torch power supply CR-20 and arc ignitor AI and is in the proper position 
to resume the delay sequence when the time delay device TD-1 is energized 
again. 
The ignitor control reset circuit is energized after the test start 
pushbutton PB-11 has been depressed and the time delay device TD-1 has 
completed the timing sequence, a normal open contact of TD-1 will close 
energizing control relay CR-101 and the timing device TD-2. Energizing of 
the control relay CR-1 will activate its contacts to energize the torch 
power supply control relay CR-20 and high voltage arc ignitor AI. The 
relay contacts of CR-101 parallel the torch start pushbutton control PB-9 
providing a manual or automatic start sequence for the system. At the 
completion of the timing cycle approximately 1/2 a second, the time delay 
device TD-2 normally closes contact in series with the control relay 
CR-101 will open and de-energize the arc ignitor AI and release a latch 
contact on the torch start circuit. Only momentary contact closure are 
required for igniting the arc. The control circuits hereinbefore described 
advantageously allow automatic and manual start up and operation of the 
cupola and diverse operation thereof to control the melting of scrap to 
make various grades of iron of different melt rates.