|1.||A method of preheating scrap that is to be charged into a smelting furnace particularly into an electric arc furnace (11) , the scrap being preheated by a circulating gas that by means of a heat exchanger (17) is heated by the exhaust fumes of the furnace, c h a r a c t e r i z e d in that a minor part of the circulating gas is continuously removed and burnt and replaced by clean gas, particularly air.|
|2.||A method according to claim 1 c h a r a c t e r i z e d in that the removed gas is burnt with additional fuel at a temperature exceeding 900 C.|
|3.||A method according to claim 2 c h a r a c t e r i z e d in that said temperature is at least 1100°C.|
|4.||A method according to any one of claims 13 c h a r a c t e r i z e d in that the burnt gas is utilized in the heat exchanger.|
|5.||A method according to any one of the proceeding claims c h a r a c t e r i z e d in that a second part of the circulating gas is continuously removed and burnt with additional fuel and the exhaust fumes are conveyed back to the circulating gas.|
|6.||A method according to claim 5 c h a r a c t e r i z e d in that said second part of the circulating gas is burnt at a temperature between 500 C and 800°C.|
|7.||A method according to any one of the preceeding claims, c h a r a c t e r i z e d in that subpressure is maintained in a skip (20) that contains the scrap that is being preheated.|
This invention relates to a method of preheating scrap that is to be charged into a smelting furnace, particularly into an electric arc furnace, the scrap being preheated by a circulating gas that by means of heat exchanger is heated by the exhaust fumes of the furnace.
It is desirable to preheat the scrap by various reasons. The most important reason might often be that the scrap should be dry when being charged since water on the scrap will result in an explosive development of gas. It is of course advantageous to utilize a part of the energy in the exhaust fumes for the preheating in order to reduce the energy consumption. The preheating of the scrap has also a positiv influence on the electrode consumption and on the yield and productivity. There are several different prior art methods of preheating scrap, some of which are described in US-A 4559629. No prior method, however, combines, as the method according to the invention, negligible exhaust of undesired substances whith comparatively low capital costs- and low energy consumption.
The invention will be described with reference to the accompanying drawing which is a flow chart of the exhaust fumes from an electric steel smelting furnace.
From the electric steel smelting furnace 11, the exhaust fumes are conveyed to an after burner 12 and through an exhaust tube 13 to a fan and filter unit 14. A valve 15 in the tube 13 is normally closed and the exhaust fumes are instead conveyed through a conduit 16 that is coupled in parallell with the valve 15. The conduit 16 includes the primary circuit of a heat exchanger 17.
Three scrap preheating skips 18, 19, 20 are coupled in parallel to the secondary circuit of the heat exchanger 17 by means of an inlet tube 22 to the exchanger and an outlet tube 21 from the exchanger so that a closed preheating circuit 17-22 is formed. In the Figure, the skip 20 is shown in operation whereas the skips 18, 19 are disconnected. In the skip 20, there is a scrap cage 51. In the closed
A conduit 24 branched from the conduit 21 leads to a burner 25 and the exhaust fumes from the burner 25 are conveyed to the main conduit 13 upstream of the heat exchanger 17. A conduit 26 branched from the conduit 16 leads to a burner 27, the exhaust conduit 28 of which leads to the conduit 21. Thus, the burner 27 is coupled in parallel with the skips 18-20. Alternatively, the two conduits 26, 28 can both be coupled to the conduit 21. In the system, there are two conduits 29, 30 with valves 31, 32 and there are also valves 33-49 and a flame jonisation detector 50 for gas analysis.
In normal operation, the valves of the system are in the illustrated positions. The exhaust fumes from the furnace 11 is sucked through the after burner 17, the conduit 16 and the filter and fan unit 14. The fan 23 circulates gas through the conduit 29, the conduit 22, the heat exchanger 17, the conduit 21 and the skip 20.
The exhaust flow from the furnace 11 through the primary circuit 16 of the heat exchanger 17 can be 80000 Nm 3/h the temperature upstream of the heat exchanger 17 can be 600-800 C and downstream of the heat exchanger 17 it can be 150-250 C. The secondary flow through the heat exchanger
3 17 can also be 80000 Nm /h and the -gas from the heat exchange in the conduit 21 can be at 500°C. The flow through the
3 conduit 24 can be 2000 Nm /h and by means of a supply of a fuel, e.g. gas, to the burner 25, the temperature of the exhaust from the burner 25 can be 1200 C. The flow through the burner 25 is normally between 1% and 10% of the flow through, the scrap and preferably between 2 and 5% thereof. The gas exhausted through the conduit 24 is replaced partly by the leakage air that is unavoidable particularly in the skips 18-20 and partly by air sucked in through the valve 41. Thus, it should be controlled that a subpressure in the skips 18-20 is maintained and that the pressure in the conduit 13 is lower than that in the conduit 24.
The flow through the conduit 26 can be 10000 Nm /h an by means of a supply of fuel, e.g. gas, to the burner 27, the temperature in the exhaust conduit 28 can be 650 C.
With the flame jonisation detector 50 the total conten of hydrocarbons is monitored. The content must not be so high that the gas becomes explosive. The hydrocarbons are burnt in the burner 27 at suitable temperature, normally between 500°C and 800°C, for example 650°C, and the flow through the burner 27 can be controlled by means of the val 43. The flow through the conduit 26 is normally between 10 % and 20 % of the flow through the scrap. The higher the oil content in the scrap, the higher the flow. The detector 50 is coupled to control the valve 43.
Chlorobensenes are not destroyed at the comparatively low temperature in the burner 27 but the temperature in the burner 25 should be such that they decompose therein. The temperature in the burner 25 should therefore exceed 900 C, preferably it should exceed 1100 C, it can for example be about 1200 C. The flow through the burner 25 is controlled by means of the valve 42. Usually additional air to the buners 25, 27 is not needed but the oxygen in the replacemen air sucked into the preheating circuit 17-22 as described will be sufficient for the combustion.
As can be understood from the description above, there is a combustion in the burner 27 at comparatively low temperature and comparatively large flow, and the exhaust fumes are returned to the closed circuit 17-22. There is also a combustion in the burner 25 at a substantially higher temperature and lower flow, and the exhaust fumes are conveyed off the closed circuit 17-22. Less than 10% or even less than 5% of the flow circulated through the skips
18, 20 is conveyed off the closed circuit.
Since the flows and temperatures in the burners 25, 27 can be controlled, a controlled and effective decomposition of the gaseous products from the preheating of the scrap can be achieved with a reasonable energy consumption. Since the exhaust fumes from the electric arc furnace do not pass through the scrap, no dust is deposited on the scrap and the
emission of dust will be negligable when the furnace is charged.
The invention is not limited to the above described examples but modifications can be made within the scope of the claims. The burner 27, for example, can be omitted if the flow through the burner 25 is increased to 15-30% for example 20-25%. As a result, however, the energy consumption will increase.
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