METHOD OF OPERATING BLAST FURNACE

Abstract: According to a method of operating a blast furnace, pure oxygen (3), pulverized coal (11), and a temperature control gas (4) which substantially does not contain nitrogen are blown from tuyères (2). A preheating gas which substantially does not contain nitrogen is blown from an intermediate shaft level. A blast furnace gas which substantially does not contain nitrogen can be produced from a furnace top.

Description: The present invention relates to a method of operating a blast furnace capable of generating a blast furnace gas having a composition suitable as a synthetic chemical industrial gas. Most blast furnace gases generated in a conventional blast furnace are consumed in the steel works. However, the amount of gas consumed within such a plant has decreased in recent years in spite of the fact that the amount of blast furnace gas has increased due to the increase in the amount of pig iron manufactured and improvements in plant operation. Therefore, effective utilization of excess blast furnace gases has been a big problem. It is thus assumed that a large amount of CO gas contained in the blast furnace gas can serve as a synthetic chemical industrial gas such as a fuel methanol gas. Conventional blast furnace gas, however, contains a large amount of N₂ gas. In order to use the blast furnace gas as a synthetic chemical industrial gas, N₂ gas must be separated therefrom, resulting in high cost. Therefore, it is difficult to use the blast furnace gas as a synthetic chemical industrial gas on an industrial scale. Japanese Patent Publication No. 37-3356 describes a method of operating a blast furnace wherein oxygen containing proper amounts of CO₂ gas and H₂O steam in place of air is blown from blast furnace tuyères, and at the same time, a reduction gas essentially consisting of CO and H₂ separated from a B gas is blown, thereby setting the content of the reduction gas generated from the top of the furnace at 70%. This technique aims at decreasing a coke ratio but not at producing a synthetic chemical industrial gas. This prior-art patent does not describe blowing of a preheating gas from an intermediate shaft level of the blast furnace or blowing of pulverized coal from the tuyères. Japanses Patent Publication No. 52-32323 describes operations for blowing a top gas regenerated using fossil fuel together with oxygen-enriched gas from tuyères, and for blowing the regenerated top gas from an intermediate shaft level. This technique also aims at a decrease in the coke ratio but not at producing a synthetic chemical industrial gas. According to this technique, an oxygen-enriched gas is blown, not pure oxygen. Unless nitrogen is removed from the resultant blast furnace gas, it cannot be used as a synthetic chemical industrial gas. Japanese Patent Publication No. 50-22966 describes an operation wherein a nonoxidizing gas is blown at a temperature of 800°C or a temperature higher than that of a charge from a blowing position into a region where the charge temperature is 700°C or higher when a shaft furnace operation is performed using a preliminary reduced charge, thereby preheating the preliminary reduced charge and scrap. This technique also aims at decreasing the coke ratio, but not at producing a synthetic chemical industrial gas. Since pure oxygen is not blown, the blast furnace gas cannot be used as a synthetic chemical industrial gas unless nitrogen is removed therefrom. Japanese Patent Publication No. 51-8091 describes a technique for controlling oxygen and reduction gas contents to operate a blast furnace when an oxygen-enriched gas and a reduction gas are blown from tuyères. This technique, however, aims at improving the productivity of pig iron, but not at producing a synthetic chemical industrial gas. According to this technique, a preheating gas is not blown from an intermediate shaft level. Since pure oxygen is not blown in the blast furnace, nitrogen must be removed from the blast furnace gas if it is to be used as a synthetic chemical industrial gas. FR-A-2 486 962 discloses a method of operating a blast furnace comprising the steps of replacing the blast gas by a combination of recycled blast furnace gas, oxygen and finely divided carbon powder. The recycled gas is partially cleaned of carbon dioxide, it is preheated to 600-1300°C in order to control the temperature in the tuyère zone and the level of inert gases - in particular nitrogen - is kept low. An example shows a thus obtained blast furnace gas with 8% nitrogen and a recycled gas with 10%. The recycled gas may be injected at a higher level of the blast furnace, in the stack. It is a first object of the present invention to provide a method of operating a blast furnace wherein a blast furnace gas, free from nitrogen, can be produced as a synthetic chemical industrial gas while a stable production of pig iron by the blast furnace is maintained. It is a second object of the present invention to provide a method of operating a blast furnace wherein, even if pure oxygen is blown from tuyères, the theoretical flame temperature at the nose of tuyère is not excessively increased. It is a third object of the present invention to provide a method of operating a blast furnace wherein a lack of gas in the upper portion of the furnace can be compensated, even if pure oxygen is blown from the tuyères. It is a fourth object of the present invention to provide a method of operating a blast furnace wherein the amount of coke used can be reduced. The above objects are achieved by the method as claimed in claim 1. Optional features of the method are set out in the dependent claims. The invention provides that, pure oxygen is blown from tuyères. A blast furnace gas generated from the furnace top is converted to a gas substantially free from nitrogen. An increase in the theoretical flame temperature at the nose of tuyère upon blowing of pure oxygen from the tuyères can be prevented by blowing a temperature control gas (e.g., steam, water, carbon dioxide, and a blast furnace gas generated from the furnace top) from the tuyères. In addition, the lack of gas in the upper portion at the furnace upon blowing of pure oxygen from the tuyères can be prevented by blowing from an intermediate shaft level a preheating gas which substantially does not contain nitrogen and used for preheating a blast furnace charge, e.g., a gas obtained by combusting the blast furnace gas of the furnace top. Furthermore, pure oxygen is blown so that pulverized coal can be blown from the tuyères, thereby decreasing the amount of coke in the charge. The phrase "blast furnace gas which substantially does not contain nitrogen" includes a gas containing nitrogen (normally a concentration of 10% or less) which does not interfere with operation if it is used as a chemical gas. The phrase "preheating gas which substantially does not contain nitrogen" means a preheating gas containing an amount of nitrogen small enough to generate the blast furnace gas of the above composition. The term "pure oxygen" means oxygen of high purity containing an amount of nitrogen small enough to generate the blast furnace gas of the above composition. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic diagram for explaining an example of a method of operating a blast furnace according to the present invention; Fig. 2 is a graph showing the relationship between the concentration of oxygen blown from tuyères and the amount of pulverized coal; and Fig. 3 is a graph showing the relationship between the concentration of oxygen blown from the tuyères and the preheating gas amount. Example 1 Fig. 1 is a schematic diagram showing an example of a method of operating a blast furnace according to the present invention. A charge containing iron ore and coke as major constituents is charged into blast furnace 1 from a furnace top or receiving hopper. Pure oxygen 3, pulverized coal 11, H₂O (water or steam) 12, and a blast furnace gas as temperature control gas 4' are blown from tuyères 2. Preheating gas 5, which substantially does not contain nitrogen, is blown from an intermediate shaft level of the blast furnace to preheat the charge. Coke and pulverized coal are combusted with pure oxygen, iron ore is reduced and melted to produce pig iron and slag, and blast furnace gas 4 which substantially does not contain nitrogen is generated from the furnace top. Dust is removed from blast furnace gas 4 by dust collecter 7. The resultant gas, free from dust, is diverted to different destinations. A portion is supplied to combustion furnace 9, another portion is supplied as temperature control gas 4' to tuyères 2, another portion is utilized in the steelmaking plant, and the remaining portion is supplied to CO₂-separating plant 8. the resultant CO and H₂ gases are used as a synthetic chemical industrial gas. CO₂ gas from CO₂-separating plant 8 can be supplied as a temperature control gas to preheating gas generation combustion furnace 9 or tuyères 2. In the operation method described above, H₂O 12 and temperature control gas 4' are blown from tuyères 2 to prevent temperature rise at the nose of tuyère caused by blowing of pure oxygen. The blowing rate is controlled to set a theoretical flame temperature at the nose of tuyère to be 2,000 to 2,600°C. Pulverized coal blowing from tuyères 2 is used as a substitute for coke. According to the present invention, since pure oxygen is blown from tuyères 2, a large amount of pulverized coal can be blown. More specifically, when the concentration of oxygen blown from tuyères 2 is increased, the amount of pulverized coal is increased, as shown in Fig. 2, although the rate varies according to various conditions such as the type of pulverized coal. Upon an increase in concentration of oxygen blown from the tuyères, an amount of gas flowing through the furnace is decreased. For this reason, the gas must be replenished in the amount to compensate for shortage, as shown in Fig. 3. According to the present invention, blowing of pure oxygen from the tuyères and the preheating gas from the intermediate shaft level allows blowing of a large amount of pulverized coal, e.g., 400 kg/ton of pig iron, and preferably 100 to 400 kg/ton of pig iron. In other words, the amount of coke used in the operation can be greatly reduced. In order to control the amount of latent heat from the blast furnace, O₂ top gas from the tuyères and a blowing rate of H₂O are controlled to change a fuel ratio. Preheating gas 5 is used to increase a gas flow within the furnace and to preheat the charge in the furnace. Gas 5 can be generated by combusting the blast furnace gas in combustion furnace 9 with oxygen 3'. The blowing rate of preheating gas 5 is determined by considering the amount of gas generated at a level below the blowing level such that a thermal flow ratio (solid/gas) preferably falls within the range of 0.8 to 1.0. If the thermal flow ratio is excessively low, a large amount of gas must be blown and its calories are wasted. However, if the thermal flow ratio is excessively high, a shortage of calories within the furnace occurs. The temperature in the furnace is then excessively decreased, and a failure to perform satisfactory gas reduction occurs. As a result, the furnace operation becomes unstable. The preheating gas temperature preferably falls within the range of 500 to 1,200°C. If the temperature is excessively low, chemical reduction cannot be sufficiently performed. However, if the temperature is excessively high, the solution loss increases. Therefore, the heat balance at the bottom of the furnace is disturbed, and the furnace operation becomes unstable. In addition, if iron ore reduction rate is high, the preheating gas temperature can be set to be low. However, if iron ore reduction rate is low, the preheating gas temperature can be set to be high. Therefore, without delaying the reduction reaction, the calories can be effectively utilized. The preheating gas temperature can be controlled by changing a ration of the blast furnace gas recycled from furnace top to O₂. According to the operation method described above, pure oxygen is blown and external N₂ gas is substantially not introduced to the system. Therefore, the blast furnace gas substantially does not contain N₂ gas, therefore N₂ need not be separated from the blast furnace gas. Only CO₂ gas is separated from the blast furnace gas to be used as a synthetic chemical industrial gas, as needed. Therefore, the cost of the gas can be greatly reduced. The temperature rise at the nose of tuyère or its vicinity upon blowing of pure oxygen can be prevented by blowing the blast furnace gas circulated from the furnnace top. In addition, the preheating gas blown from the intermediate shaft level prevents a shortage of gas flow, thereby stably operating the blast furnace. Furthermore, since pulverized coal is blown, the amount of coke used in the furnace can be greatly reduced, thereby reducing the operation cost. A required amount of blast furnace gas is subjected to CO₂ separation when it is used as a synthetic chemical industrial gas, thus further reducing the gas cost. The operation of the blast furnace according to the present invention will be described with reference to Fig. 1. Iron ore and coke (a coke ratio of 350 kg/T-HM or Ton-Hot Metal) were charged into a blast furnace (5,000 t-HM/d or Hot Metal/Day), and pure oxygen (349 Nm³/T), a top gas (165 Nm³/T), pulverized coal (300 kg/T-HM = 21 t/H), and steam (3 kg/T) for preventing variations in blast furnace gas composition were blown in the furnace. A preheating gas (1,000°C, 105 Nm³/T) was blown from the intermediate shaft portion of the blast furnace. In this case, the preheating gas was produced by combusting the top gas (105 Nm³/T) with oxygen (10 Nm³/T). The composition of the top gas produced by the blast furnace operation described above was 49% of CO, 33.5% of CO₂, 9.2% of H₂, 0.73% of H₂O, and 0.8% of N₂. The top gas thus substantially does not contain N₂ gas. The blast furnace gas was passed through the dust-collecter, and the gas without dust was diverted to different destinations. A portion (105 Nm³/T) was blown in the combustion furnace, another portion (165 Nm³/T) was blown from the tuyères, another portion (1,080 Nm³/T, 1,726 Kcaℓ/Nm³) was used in the steel works, and the remaining portion was subjected to CO₂ separation. The resultant CO and H₂ gases were used as a synthetic chemical industrial gas.

Claims: A method of operating a blast furnace, comprising the steps of: charging a charge including iron ore and coke from a furnace top into the blast furnace; blowing pure oxygen (3'), pulverized coal (11), and a temperature control gas (4') from tuyères, the temperature control gas being adapted to prevent a temperature rise at the nose of tuyère; blowing a preheating gas (5) which contains a concentration of 10 % or less of nitrogen from an intermediate shaft level to preheat the charge in the blast furnace; combusting coke with the pure oxygen to melt the iron ore and generating a blast furnace gas which substantially does not contain nitrogen; and separating CO₂ from at least a part of the generated blast furnace gas (8), in order to use the resultant CO and H₂ as a synthetic chemical industrial gas. A method according to claim 1, characterized in that the temperature control gas is a gas generated from a top of the blast furnace and is blown such that a theoretical flame temperature at the nose of tuyère falls within a range of 2,000 to 2,600°C. A method according to claim 1 or 2, characterized in that the preheating gas (5) has a temperature falling within a range of 500°C to 1,200°C, and an amount of the preheating gas is controlled such that a thermal flow ratio of solid to gas is set to be 0.8 to 1.0. A method according to any of the claims 1-3, characterized in that the pulverized coal is blown in an amount up to 400 kg/ton of pig iron.

Inventors: HOTTA, HIROHISA, MATSUURA, MASAHIRO, OONO, YOTARO, SAITO, HIROSHI
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1986
Main classification: C21B5
IPC codes: C21B5