Control system boosts productivity in steelmaking

Finnish high-tech company Real Time Systems has recently developed an advanced electrode control system that, compared with conventional systems, offers substantially higher productivity and reduced specific energy consumption in electric arc furnaces.

The first version of the system was tested in 2008 in a full scale trial during normal steel production on a Swedish arc furnace with an active power of 70 MW and currents of 60 kA.

“By examining the results we came to the conclusion that it would have been possible for the mill to produce about 45,000 tons more steel per year just by installing our system,” says Lassi Toivonen at Real Time Systems (RTS). “At the same time the energy consumption per steel ton was reduced by over 3 percent.”

Since then RTS has developed the system further. For example the new computation unit is significantly more powerful and can track the process quantities in real time.

“As the system always has reliable and reasonably stable estimates of process quantities, the electrodes are moved more smoothly, which leads to reduced fluctuation of current and power, and also gives the actuators a longer life cycle,” states Toivonen.

According to Toivonen, the new system was recently tested in a 120 MVA furnace of a Finnish steel mill with good results.

"We have developed the estimation methods for process quantities so that it’s now possible to obtain reliable electrode resistances, which has not been possible before,” he says.

“It also allowed us to implement an electrode resistance control that is superior over the conventional impedance control. This makes it possible to predict the mutual interaction of electrode resistances when the electrodes are moved and compensate for the resistance changes.”

Due to this there is no need for the endless and sometimes uncontrolled chain of subsequent adjustments that often take place in conventional electrode control.

“Normally, when you adjust one electrode, its effect on other electrodes must be compensated for. Then you have to re-adjust the first electrode again to compensate for the control actions made on the other electrodes and so on,” Toivonen says.

“As our system adjusts the electrodes as an entity, we are able to achieve more stable currents and power than it is possible with conventional electrode control systems.”

No upfront investment necessary

The business concept that RTS offers is without any risk and need of investment for the customer. The new control units incorporating advanced process quantity estimation are primarily offered as a service on gain share basis without any upfront payments.

“The payments are entirely based on the improved cash flow from energy savings and increased production case by case,” Toivonen says. “Savings can also come from reduced service costs and reduced costs for compensating flicker. We take care of maintenance and develop the production process together with the customer.”

The new and existing control systems are installed in parallel and the operator can choose which control to use at any time.

The new and existing control systems are installed in parallel and the operator can choose which control to use at any time. (Image courtesy RTS.)

RTS’s system is easy to install in parallel with the existing control system without need of production interruptions.

“The existing control can be instantly restored by turning a switch in the control room, which allows easy comparison of the existing traditional and the new system,” Toivonen says. “The interface to the automation system is made via the furnace’s existing interface and the operator uses the normal process display in the control room.”

The process

Scrap iron is an important source of raw material for the steel industry. The scrap is melted in an electric arc furnace by means of powerful electric arcs that are formed between the graphite electrodes and the charged scrap metal. At the bottom of the furnace there is a measuring electrode.

“The conventional control systems are based on the assumption that the measuring electrode is located in the electrical symmetry point,” says Toivonen. “But since all electrodes in reality interact and the charge is neither homogenous nor symmetrical, the assumption about the symmetry point is not valid. That’s why direct measurements fail.”

He points out that with conventional control system optimal results cannot be achieved since the arcs do not have equal, optimal lengths due to measurement problem mentioned.

“This leads to additional power losses subjected to furnace lining and exhaust gases,” Toivonen says. “With our system it is possible to accurately position the electrodes so that the arcs are always of equal length and thus highest possible portion of the energy is used for melting the scrap.”

According to Toivonen, the optimal position of electrodes and the stable process quantities employed in process control will lead to smaller thermal stress on the furnace lining, more stable currents and higher average power. “Since the power is more stable, the potential costs related to reactive power compensation are reduced as well”, he points out.

About Real Time Systems

Real Time Systems was established about 20 years ago by the inventors of the system. Initially the focus was on improved diagnostics in medical equipment but, as computational power increased, the focus was shifted towards industrial processes.

The major breakthrough took place in year 2008 when the RTS system was proven in a full scale trial. Based on the results, the Finnish listed high tech company Outotec Oyj decided to join RTS Oy by obtaining a minority position in the company.

Together with Outotec Oyj, RTS has further developed the technology, and is now ready for a global launch of the system.

The company is committed to continuously develop and improve the efficiency of the system together with the customers once the system is installed.

[Photo at the top courtesy Saironsteel.]

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