METHOD FOR OPERATING A BLAST FURNACE PLANT

Abstract

A method for operating a blast furnace plant that includes a blast furnace, at least one material hopper for charging raw materials to the blast furnace, having a upper seal valve and a lower seal valve, and at least one hot stove that produces hot blast for the blast furnace, the method including at least one charging cycle with the following steps: opening the upper seal valve, introducing raw materials into the material hopper, closing the upper seal valve, pressure equalization of the material hopper with blast furnace top pressure, and opening the lower seal valve to discharge raw materials into the blast furnace, wherein, in order to provide a cost-effective way to minimize the explosion danger during operation of a top charging system, an offgas from the at least one hot stove is transferred by a transfer system to the at least one material hopper and, before the lower seal valve is opened, the offgas is injected into the material hopper.

Claims

1. A method for operating a blast furnace plant that comprises a blast furnace, at least one material hopper for charging raw materials to the blast furnace, having an upper seal valve and a lower seal valve, and at least one hot stove that produces hot blast for the blast furnace, the method comprising at least one charging cycle with the following steps: opening the upper seal valve, introducing raw materials into the material hopper, closing the upper seal valve, pressure equalization of the material hopper with blast furnace top pressure, and opening the lower seal valve to discharge the raw materials into the blast furnace, wherein an offgas from the at least one hot stove is transferred by a transfer system to the at least one material hopper and the offgas is injected into the material hopper and the material hopper is pressurized to blast furnace top pressure before the lower seal valve is opened and the raw materials are discharged into the blast furnace.

2. The method according to claim 1, wherein the offgas is injected to constitute at least 70% v/v of a gas inside the material hopper when the lower seal valve is opened.

3. The method according to claim 1, wherein the offgas is injected so that the gas inside the material hopper has an O.sub.2 concentration of less than 4.5% v/v when the lower seal valve is opened.

4. The method according to claim 1, wherein after opening the lower seal valve, a gas inside the material hopper at least partially mixes with a blast furnace gas from the blast furnace having an H.sub.2 concentration of at least 5% v/v.

5. The method according to claim 1, wherein the offgas has an O.sub.2 concentration of less than 2% v/v.

6. The method according to claim 1, wherein after closing the upper seal valve and before opening the lower seal valve, an overpressure is generated inside the material hopper.

7. The method according to claim 1, wherein the offgas is at least partially injected before the raw materials are introduced into the material hopper.

8. The method according to claim 1, wherein the offgas is at least partially injected between the lower seal valve and a lower material gate.

9. The method according to claim 1, wherein the offgas is at least partially injected while the raw materials are introduced into the material hopper.

10. The method according to claim 1, wherein each hot stove alternatingly undergoes a heating phase, in which it is heated by a combustion that produces the offgas, and a blowing phase, in which it produces hot blast, and offgas is collected from the hot stove after the start of a heating phase and before the start of the following blowing phase.

11. The method according to claim 1, wherein the transfer system comprises a collecting pipe for each hot stove, a discharge pipe for each material hopper and an intermediate portion connecting each collecting pipe to each discharge pipe.

12. The method according to claim 1, wherein the offgas collected from a hot stove is cooled by a cooling device before it is injected into the material hopper.

13. The method according to claim 1, wherein the offgas is propelled through the transfer system by a blower unit.

14. The method according to claim 1, wherein the offgas is directed selectively to at least one of a plurality of material hoppers by a distribution valve unit.

15. A blast furnace plant, comprising: a blast furnace, at least one material hopper for charging raw materials to the blast furnace, having an upper seal valve and a lower seal valve, at least one hot stove adapted to produce hot blast for the blast furnace, and a transfer system adapted to transfer an offgas from the at least one hot stove to the at least one material hopper, wherein the blast furnace plant is adapted to inject the offgas into the material hopper and perform at least one charging cycle as defined in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Preferred embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

[0040] FIG. 1 is a schematic view of an inventive blast furnace plant; and

[0041] FIG. 2 is a schematic sectional view of a portion of the blast furnace plant from FIG. 1.

DETAILED DESCRIPTION

[0042] FIG. 1 shows a schematic representation of an inventive blast furnace plant 1 that is adapted for an inventive method. It comprises a blast furnace 10, the general operation of which is known in the art and therefore will not be explained here. Two material hoppers 20, one of which is shown schematically in FIG. 2, are disposed above the top of the blast furnace 10. Each material hopper 20 comprises an upper seal valve 21 for sealingly closing an upper opening, a lower seal valve 22 for sealingly closing a lower opening and a material gate 23 disposed above the lower seal valve 22. During operation, each of the material hoppers 20 receives the raw materials for the blast furnace 10. For instance, one material hopper 20 may receive iron ore, while the other material hopper 20 receives coke. The respective raw materials are temporarily stored in the material hopper 20 before it is discharged to the blast furnace 10.

[0043] Each material hopper 20 sequentially undergoes a plurality of charging cycles. At the start of each charging cycle, the lower seal valve 22 and the material gate 23 are closed and the upper seal valve 21 is opened. Then, raw materials can be filled into the material hopper 20 through the upper seal valve 21. When a predetermined quantity of raw materials has been filled in, the upper seal valve 21 is closed to provide an air-tight seal towards the outside of the material hopper 20. Then, the pressure inside the material hopper 20 is increased until an overpressure is reached that is above the pressure inside the blast furnace 10. After that, the lower seal valve 22 is opened, whereby gas exchange between the material hopper 20 the blast furnace 10 is enabled, since even in its closed position, the material gate 23 does not provide an air-tight seal. In order to discharge raw materials to the blast furnace 10, the material gate 23 is opened to a certain degree, whereby the material flow is controlled. Finally, when all raw materials have been discharged to the blast furnace 10, the material gate 23 and the lower seal valve 22 are closed. Then, after adjusting the pressure inside the material hopper 20 to ambient pressure, the upper seal valve 21 can be opened again and new raw materials can be filled in.

[0044] In an industrial setup, semi-clean BF gas is often injected in a first step in the material hopper, which brings it to a pressure of BF gas0.15 bar; this is called primary equalizing and afterwards hot blast stove off gas is injected to bring the hopper to BF top pressure (similar to secondary equalizing, normally done with nitrogen).

[0045] Generally speaking, some component of the blast furnace gas could form a combustible/flammable or explosive mixture with the oxygen in the ambient air. In particular, the blast furnace gas could comprise a considerable concentration of H.sub.2, e.g. at least 7% v/v, which combine with O.sub.2 to form a flammable mixture. In order to minimize or eliminate this problem, an offgas is injected into the material hopper 20 at certain stages of the charging cycle as will be discussed in the following. The offgas has an O.sub.2 concentration of less than 2% v/v and therefore can be largely regarded as an inert gas. The offgas is collected from a plurality of hot stoves 30, which are generally used to supply hot blast for the blast furnace 10. Each hot stove 30 alternatingly undergoes a heating phase, in which it is heated by a combustion that produces the offgas, and a blowing phase, in which it produces hot blast. The hot stove 30 is lined with checker bricks that temporarily store heat from the combustion. When the offgas inside the hot stove 30 is replaced with cold blast (i.e. air or another oxygen-containing gas with ambient temperature), heat is transferred to the cold blast, whereby hot blast is produced. A suitable fuel gas for the combustion can be introduced into the hot stove 30 by a supply pipe that is not shown for sake of simplicity. The same applies to a cold blast pipe for supplying cold blast and a hot blast pipe for transferring hot blast from the hot stove 30 to the blast furnace 10.

[0046] The offgas is partially transferred through an offgas pipe 31 to a chimney 33, from where it is released to the environment. Some of the internal heat of the offgas, which may initially have a temperature between 300 C. and 400 C., is recuperated by a first heat exchanger 32 in the offgas pipe 31. Another portion of the offgas is a transferred to the material hoppers 20 by a transfer system 40, which is described in detail in the following. A collecting pipe 41 originates from each of the hot stoves 30. The gas flow through the respective collecting pipe 41 can be controlled by a control valve 42. The control valve 42 is operated so that offgas is only collected from the hot stove 30 when it is in a heating phase, whereas no gas is collected from the hot stove 30 when it is in a blowing phase, thereby avoiding introduction of oxygen-rich gas is into the transfer system 40. Each collecting pipe 41 is connected to an intermediate portion 50 of the transfer system 40, or more specifically, to an intermediate pipe 51. In a second heat exchanger 52, the internal heat of the offgas is recuperated and it is cooled down to a temperature of e.g. 45. Downstream of the second heat exchanger 52, the offgas reaches a reservoir 53, which is used to smooth the gas flow and condensate a part of the residue moisture in the gas, where it can be temporarily stored. Then, the intermediate pipe 51 reaches a blower section 54, where it branches into three blower pipes 55. Each blower pipe 55 comprises a blower 56, by which the offgas is propelled through the transfer system 40. The output of each blower 56 can be adjusted in order to adapt the flow rate of the offgas.

[0047] The intermediate portion 50 is connected to two discharge pipes 60, each of which is connected to one of the material hoppers 20. Each discharge pipe 60 comprises a flow meter 61 by which the gas flow can be monitored. In particular, information from the gas flow meter 61 can be used to adequately control the blowers 56. A distribution valve unit 62 comprises two control valves 63, namely one in each discharge pipe 60. The distribution valve unit 62 adjusts the gas flow and may in particular block the gas flow through one discharge pipe 60 in a situation where no offgas needs to be injected into the corresponding material hopper 20. Furthermore, each distribution line 60 comprises a check valve 64, a relief pipe 67 and two shut-off valves 65, 66 disposed upstream and downstream of the relief pipe 67. E.g. for maintenance purposes, gas can be released from the discharge pipe 60 through the relief pipe 67. The shut-off valves 65, 66 can be used to isolate one part of the transfer system 40 before the relief pipe 67 is opened.

[0048] As shown in the sectional view of FIG. 2, the discharge pipe 60 enters the material hopper 20 between the lower seal valve 22 and the material gate 23. When all raw materials have been discharged to the blast furnace 10, the lower seal valve 22 and the material gate 23 are closed as described above. However, the material gate 23 does not provide an air-tight seal with respect to the rest of the material hopper 20. Now, before the upper seal valve 21 is opened, offgas is injected through the discharge pipe 60, so that possibly remaining H.sub.2 from the blast furnace gas is expelled from the lower part of the material hopper 20. It is possible to continue injection until at least 90% v/v of the gas inside the material hopper 20 has been replaced by offgas. The gas previously contained in the material hopper 20 can be released through a relief valve (not shown). Then, raw materials can be introduced through the upper seal valve 21 as described above. At this point, the H.sub.2 concentration inside the material hopper 20 is negligible, wherefore no explosive mixture can form. However, ambient air is introduced together with the raw materials, thereby introducing considerable amounts of O.sub.2 into the material hopper 20. This could potentially pose an explosion risk when the lower seal valve 22 is reopened. This risk can be avoided in various ways. For instance, after the raw materials have been introduced, offgas injection through the discharge pipe 60 can be continued in order to displace O.sub.2 at least from the lower part of the material hopper. Alternatively or additionally, an additional discharge pipe 70 may be provided, which is disposed to inject offgas into an upper portion of the material hopper 20 near the upper seal valve 21. Through this discharge pipe 70, offgas can be injected while the raw materials are introduced. Thus, the ambient air around the raw materials will be significantly diluted. Thus, the O.sub.2 concentration in the material hopper can be reduced to e.g. less than 5% v/v.