Abstract:
A method and apparatus for increasing blast gas temperature in a shaft furnace by passing at least part of the blast gas through a plasma burner, and also supplying additional fuel to the furnace at the tuyere level.

Description:
This is a division of application Ser. No. 449,784, filed Mar. 11, 1974, now U.S. Pat. No. 3,970,290. 
    
    
     This invention relates to a method and an arrangement for rendering it possible to substantially increase the blast temperature for a shaft furnace, preferably a blast furnace, whereby the amount of desirable additives, e.g. oil, pulverized coal or water vapour on the tuyere level can be increased substantially and a considerable saving in metallurgical coke and an increase in output can be achieved. 
     It is known since a long time ago to decrease the coke consumption and increase the production in a blast furnace by increasing the blast temperature. At blast temperatures above 850° C, moreover, oil can be injected into the tuyeres and thereby contribute to an additional coke saving. The yield value for oil is for the first oil addition about 2 kg coke per kg oil. This value, however, decreases at increased oil addition and constant blast temperature to a value of about 1 kg coke per kg oil. A further increase in oil addition over a certain amount is not possible as this would result in too low a combustion temperature in front of the tuyeres and, besides, imply a lower output. 
     The aforesaid circumstances make it desirable to increase the blast temperature to the highest possible degree. In conventional arrangements for this purpose, however, e.g. in the so-called Cowper-apparatus 
     A. THE COKE AMOUNT IS SO LOW THAT IT IS NOT SUFFICIENT FOR THE REACTION CO 2  + C → 2 CO, which will be the case at a coke amount of 200 - 250 kg/t of pig iron. 
     B. THE COKE AMOUNT IS SO LOW THAT THE PERMEABILITY IN THE FURNACE WITHOUT APPENDICES IS SERIOUSLY DETERIORATED. According to experiments, this will occur at a coke amount of 200-300 kg/t of pig iron. 
     C. THE INJECTED OIL AMOUNT IS SO GREAT THAT IT CANNOT BE GASIFIED AND IS COMBUSTED PARTIALLY IN THE LIMITED AVAILABLE SPACE, I.E. IN THE CAVITY IN FRONT OF THE TUYERES. 
     According to these three cases, the proportionality between blast temperature, possible amount of injected oil and production increase can be assumed to remain substantially unchanged up to a blast temperature of 1600°-1800° C. At a further increase of blast temperature and oil amount the mode of operation of the furnace will be more like that of a sponge iron furnace and considerable difficulties should arise in the fusion zone. 
     In the present situation the most interesting feature in conjunction with increased blast temperature is increased oil injection. The higher blast temperature, however, renders it also possible to inject other fuels, such as coal or pulverized coke, oil slurry of coal or coke, natural gas, coke-oven gas etc. Other interesting additives on the tuyere level in connection with high blast temperatures are oxidic materials such as water, iron ore, flue gas substance and pre-reduced iron oxides as well as slag formers. 
     The method and arrangement according to the present invention render it possible to increase the temperature of the blast air for a shaft furnace to the desired value in a simple, economic and efficient way. 
     The invention is substantially characterized in that the blast gas entirely or partially is passed through a plasma. During the passage through the plasma, the gas temperature is increased. The plasma may preferably be generated in a so-called plasma burner, which per se is known for use in other connections. In a plasma burner the plasma is generated in the gas proper passing through the burner. The degree of efficiency of the plasma burner is 75% - 85% and relatively independent of the temperature. The temperature usually obtained in a gas leaving a plasma burner is between three thousand and four thousand degrees centigrade. Since the temperature of the blast air supplied to the tuyere can be controlled simply and efficiently by the plasma burner, a new control variable in the ironworks operation is obtained. At cold charge operation in a blast furnace, for example, the energy amount supplied through the tuyeres can be increased, whereby a substantially more rapid change in the energy balance of the blast furnace is obtained than it is possible to obtain by increasing the coke charging to the blast furnace. This latter method was normally used heretofore. The invention is described in greater detail with reference to the accompanying drawings as follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary elevation view, partially in section, of one embodiment of the invention; 
     FIG. 2 is a similar view of another embodiment of the invention; and 
     FIG. 3 is a similar view, partially schematic, showing a nozzle of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the invention is shown applied to a blast furnace 1 charged in the usual manner through an opening 2. The outgoing blast furnace gas is directed through the conduit 3 to a gas cleaner (not shown), from which the gas preferably is directed via the conduit 4 to a heat exchanger (not shown), for example a so-called Cowper-apparatus, and then is discharged through a chimney. The incoming, preferably preheated blast air is directed via the conduit 5 to a bustle pipe 6 disposed about the blast furnace shaft, from which pipe the air is directed into the blast furnace via a plurality of branches 7, 8 and tuyeres 9, 10. 
     In order to render it possible to increase the blast temperature beyond what is economically or technically possible by conventional methods, at least a part of the blast air is passed through a so-called plasma burner 11, which in the embodiment shown, is shunted to the conduit 5. The gas proportion passing through the plasma burner can be adjusted by a valve 12. A fresh air conduit 13 may possibly be connected directly to the plasma burner and a control valve be mounted in said conduit. Thereby the temperature and the amount of blast air to the blast furnace can be controlled accurately. A conduit 13a for introducing hydrocarbons, coke-oven gas, water or the like into the tuyeres is connected to the lower portion of the blast furnace. 
     The embodiment shown in FIG. 1 is adapted for use when the blast air is not to be heated to a temperature higher than about 1500° C. At blast temperatures above about 1500° C the plasma burner preferably is positioned in direct connection to the tuyere, for example as shown at the embodiment in FIG. 2, partly in order to reduce the heat stresses in the blast pipe system and partly to reduce the heat losses. FIG. 2 shows a part of the bottom portion of a blast furnace in connection to a tuyere 7, to which a branch 15 is drawn from a bustle pipe 16 of the same kind as shown in FIG. 1. Of the blast air from the bustle pipe a part is directed via a pipe 17 through a plasma burner 18 having its outlet disposed in the conduit 15 and directed inwards to the tuyere 14. Optionally, a fresh air conduit 17a may open into the conduit 17 in front of the plasma burner. A pipe 19 for the supply of, for example, hydrocarbons into the heated blast air, is inserted into the conduit 15 in front of the mouth of the plasma burner, with the hydrocarbon jet directed inwards to the tuyere. 
     As regards the oil injection, this can in principle be carried out in the same manner as it is carried at most of today&#39;s blast furnaces. An advantageous embodiment of a nozzle for injecting hydrocarbons, coke-oven gas, water or the like as well as heated air from a plasma burner into a blast furnace, is shown in FIG. 3. About the mouth of the blow pipe 20 from the plasma burner 20a, an annular nozzle 21 is provided, which includes a plurality of holes for injecting, for example, oil supplied through the conduit 22. Oil, heated air from the plasma burner, and blast air having not passed through the burner (arrows 24) are mixed in the tuyere. Blast air or blast gas is supplied from a source 20b by blast gas ducts 20c, and supply air or supply gas for the plasma burner is provided by the supply duct 20d. 
     As an example of operation-results possible to be achieved by the present invention, the following may be mentioned. A usual type of a blast furnace has a blast temperature of 900° C, a coke consumption of 600 kg per ton of pig iron, an oil consumption of 30 kg per ton of pig iron, and an output of 50 tons per hour. When the temperature of the blast air is increased by 500° to 1400° C by means of a plasma burner, additional 150 kg oil per ton of pig iron can be injected and thereby save 210 kg of coke per ton of pig iron. The degree of efficiency being assumed to be 80%, the energy consumption in the plasma burner will be 280 kWh per ton of pig iron. The increase in the output of the blast furnace in this conjunction will be 33%, i.e., about 17 tons per hour. 
     The invention, of course, can be applied also to furnaces other than blast furnaces, for example, shaft furnaces for the production of foundry pig iron (cupola furnaces), lime or high-alloy pig iron, primarily iron with high chromium or manganese content.