EP1961828A1

EP1961828A1 - Blast furnace plant and method of operating heating stoves in a blast furnace plant

Info

Publication number: EP1961828A1
Application number: EP07300804A
Authority: EP
Inventors: Guy De Reals; Olivier Decayeux; Richard Dubettier-Grenier; Philippe Merino
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2007-02-21
Filing date: 2007-02-21
Publication date: 2008-08-27
Also published as: WO2008101812A1

Abstract

The invention relates to the operation of a hot stove for supplying hot blast to a blast furnace (22), whereby the hot stove is heated during an on-gas phase and is thereafter used to heat cold blast and to supply hot blast to a blast furnace (22) during its on-blast phase.

According to the invention, pressurized gas supplied by an air separation unit (23) is accumulated in buffer means (25) and the accumulated pressurized gas is used in the pressurization of the hot stove between the on-gas phase and the on-blast phase.

Description

The present invention relates to the operation of hot stoves in blast furnace plants and in particular to the pressurization of hot stoves.
In blast furnace plants, the blast gases, such as air or O₂ enriched air, are pressurized in one or more compressors, also known as blowers and hereafter referred to as the blast compressor, and the cold blast thus obtained is heated in a hot stove after which the hot blast is injected into the blast furnace. The pressure at which the hot blast is injected into the furnace generally varies between 1 and 7x10⁷Pa.
Hot stoves are operated in a regenerative regime whereby the hot stove goes through an on-gas phase during which the hot stove is heated and an on-blast phase during which the cold blast is heated by passing through the hot stove before being injected as hot blast into the blast furnace. Following an on-blast phase, the hot stove is reheated during a new on-gas phase in preparation for a further on-blast phase. The heating of the hot stove during the on-gas phase is usually realized by the combustion of blast furnace gas in the combustion chamber of the hot stove, typically at atmospheric pressure, with possible addition of natural gases and other steel gases such as converter gases, coke oven gases.
In order to provide a regular supply of hot blast to the blast furnace, a typical blast furnace plant will comprise a group of several hot stoves which are operated so that, at any time during the operation of the blast furnace, a sufficient number of hot stoves is in the on-blast phase for the supply of the required hot blast flow and to compensate for any hot stoves which are not in the on-blast phase, for example during reheating (on-gas phase), and which are therefore not supplying hot blast to the blast furnace. Such a group of hot stoves is known in the art as a group of alternating hot stoves.
Between an on-gas phase and a subsequent on-blast phase, the hot stove is subjected to a pressurization phase during which the hot stove is pressurized to the pressure required for the injection of the hot blast into the blast furnace. Likewise, between an on-blast phase and a subsequent on-gas phase, the hot stove is subjected to a depressurization phase.
The pressurization phase of a hot stove takes typically less than 10 minutes and occurs 1 to 3 times per hour in a blast furnace plant comprising a group of 3 or 4 hot stoves.
This pressurization phase of a hot stove requires an additional flow of pressurized gas which generally amounts to 5 to 15% of the cold blast flow of the blast furnace plant during normal operation (base level cold blast flow).
The additional gas flow required for hot stove pressurization is generally supplied by the blast compressor. In order to do so, the blast compressor must be designed for an overcapacity above the base level of cold blast so as to be able to supply, during hot-stove pressurization, not only the base level cold blast flow, but also the additional pressurization gas flow.
This results in the following problems:

higher construction costs of the blast compressor and thus of the overall blast furnace plant,
lower efficiency of the blast compressor, which, for most of the time during the operation of the plant operates at the cold blast base level and not at its optimum capacity,
reduced operational flexibility of the blast compressor, in particular for lower cold blast (base level) flow requirements.

It is an object of the present invention to provide improved operation of a blast furnace plant, in particular with respect to the pressurization of hot stoves.
Accordingly, the present invention proposes the use of pressurized gas supplied by an air separation unit for the pressurization of a hot stove in a blast furnace plant, following an on-gas phase of the hot stove and prior to an on-blast phase of the hot stove, said pressurized gas being accumulated in buffer means before being introduced into the hot stove.
It is known to use an air separation unit for the oxygen enrichment of the blast gasses. The article "Optimising oxygen enrichment to blast furnaces using coal injection", published in Steel Times International, February/March 2003, pages 20 to 23, describes different configurations of such use.
In a typical air separation unit, air is compressed by means of the main air compressor (MAC) of the air separation unit. The compressed air leaving the MAC is usually cooled in a cooling unit prior to its entry into the purification unit. In the purification unit, the compressed air is purified by the removal of components, such as water, CO₂ and most hydrocarbons, which are less volatile than O₂ and N₂. The purified dessicated air is chilled in cooling means to a temperature suitable for its subsequent separation in the rectification unit. The rectification unit is typically a cryogenic distillation unit. The cooling means and the rectification unit are collectively known as the cold box of the air separation unit.
By using, in accordance with the invention, pressurized gas supplied by an air separation unit and accumulated in buffer means in the pressurization of a hot stove, the need for an overcapacity of the blast compressor for the supply of additional flow of pressurized gas required for the pressurization of the hot stove is reduced (when part of the additional flow of pressurized gas required for pressurization is thus supplied by the air separation unit) or even removed (when all of the additional flow of pressurized gas required for pressurization is supplied by the air separation unit), with a corresponding reduction of the construction costs. It also enables a more efficient use of the blast compressor, as the deviations from the optimum capacity of the blast compressor during hot stove pressurization are reduced or even eliminated. This also leads to increased flexibility of the blast furnace plant, as it allows for a wider choice of cold blast flow base levels within the operating range of the blast compressor.
The pressurized gas supplied by an air separation unit can be used to provide at least 30%, preferably at least 50%, more preferably at least 75% of the flow of pressurized gas required for the pressurization of the hot stove.
The invention also relates to a method of operating a hot stove for supplying hot blast to a blast furnace. The method includes heating the hot stove during an on-gas phase (step a); pressurizing the hot stove during a pressurization phase (step b), in which a flow of pressurized gas is ducted into the hot stove; using the stove in an on-blast phase (step c) to heat cold blast and to supply hot blast; and depressurizing the hot stove during a depressurization phase (step d). The depressurized hot stove is then ready to undergo a new on-gas phase (step a), etc. During step b a flow of pressurized gas is ducted into the hot stove for the pressurization of the stove. According to the invention, pressurized gas supplied by an air separation unit is accumulated in buffer means and the accumulated pressurized gas is used to provide at least part of the flow of pressurized gas in step b. As indicated above, the accumulated pressurized gas can be used to provide at least 30%, at least 50% or at least 75% of the flow of pressurized gas in step b. Preferably, all of the flow of pressurized gas in step b is supplied by the pressurized gas accumulated in the buffer means.
A gas supply from the air separation unit is preferably connected via a gas duct into the buffer means and from the buffer means into an inlet duct of the hot stove. Pressurized gas supplied by the air separation unit is transported to the buffer means via the gas duct. During step b, pressurized gas accumulated in the buffer means is supplied via the gas duct from the buffer means to the inlet duct of the hot stove.
Different buffer means are possible. The buffer means can, for example, include a section of the gas duct itself and/or one or more accumulator vessels.
The supply of accumulated pressurized gas from the buffer means to the hot stove during step b can be controlled by selectively opening and closing one or more valves positioned in the gas duct, for example positioned downstream of the buffer means.
The accumulation of pressurized gas from the air separation unit in the buffer means can likewise be controlled by selectively opening and closing one or more valves positioned in the gas duct, for example positioned upstream of the buffer means.
The supply over time of pressurized gas from the air separation unit into the buffer means can be conducted independently of the phase (on-gas, pressurization, on-blast, depressurization) of the hot stove at any given moment. The pressurized gas is preferably supplied from the air separation unit into the buffer means in a continuous manner.
When the hot stove comes on-blast following a pressurization phase, the flow of pressurized gas used for the pressurization of the hot stove may be injected into the blast furnace. The composition of the pressurized gas supplied by the air separation unit is preferably such that the flow of pressurized gas used in step b has an oxygen content substantially equal to the oxygen content of the cold blast entering the hot stove in the on-blast phase (step c). In this manner it is possible to have a constant oxygen content in the hot gasses flowing from the hot stove to the blast furnace at the beginning of the on-blast phase.
The method according to the present invention can also be used to operate a group of alternating hot stoves, whereby at least one and preferably each hot stove of the group is operated using said method. The present invention thus also provides a process for operating a group of alternating hot stoves supplying hot blast to a blast furnace. In said process, the hot stoves belonging to the group of alternating hot stoves are operated according to a method of operating a hot stove according to the invention as described herein.
The present invention also concerns a blast furnace plant comprising a blast furnace, a group of alternating hot stoves for supplying hot blast to the blast furnace and an air separation unit. The blast furnace plant further comprises means for pressurizing the hot stoves belonging to the group of alternating hot stoves following an on-gas phase of the hot stove and prior to an on-blast phase of the hot stove. The means for pressurizing the hot stoves comprise a gas duct for pressurized gas connecting the air separation unit to an inlet duct of the hot stove. The gas duct more specifically connects the air separation unit to buffer means, for accumulating pressurized gas supplied by the air separation unit in said buffer means, and further connects the buffer means to the inlet duct of the hot stove. In this manner, the pressurized gas accumulated in the buffer means can be introduced, via the gas duct, into the hot stove during the pressurization of the stove.
As explained above, the buffer unit can include a section of the gas duct and/or one or more accumulator vessels. The means for pressurizing the hot stoves preferably comprise one or more valves positioned in the gas duct, for example downstream of the buffer means, for controlling the flow of pressurized gas from the buffer means to the inlet duct of the hot stove. The means for pressurizing the hot stoves may also comprise one of more valves positioned in the gas duct, for example upstream of the buffer means, for controlling the flow of pressurized gas from the air separation unit to the buffer means.
In the context of the present invention (be it the use, the method of operating a hot stove, the process for operating a group of alternating hot stoves or the blast furnace plant of the invention), the pressurized gas supplied by the air separation can belong to different categories. Said categories correspond typically to different stages in the air separation process practiced in the air separation unit.
The pressurized gas supplied by the air separation unit can, for example, be humid air at a temperature ≥ 40°C. This humid air can be supplied by the MAC (main air compressor) of the air separation unit. In that case, the gas duct can be connected to the air separation unit at the outlet of the MAC and upstream of the cooling unit which precedes the purification unit, if such a cooling unit is present, or upstream of the purification unit, if no such cooling unit precedes the purification unit. In the present context, the term "humid air" means air which has not been subjected to a dessication step for the removal of water vapor as practiced in the purification unit of the air separation unit. Such humid air typically has a water content of 0.001 kg H₂O/Nm³ air or more. 1 Nm³ corresponds to the quantity of gas occupying a volume of 1 m³ at 0°C and 1.01325 bar.
When the MAC is a multistage compressor with alternating compression and cooling stages, the pressurized gas supplied by the air separation unit can also be humid air at a temperature ≥ 40°C, typically at a temperature ≥ 70°C and ≤ 150°C, obtained in the final compression stage of the multistage compressor prior to the compressor's final cooling stage.
The pressurized gas supplied by the air separation unit can also be humid air at a temperature < 40°, as can be provided by the cooling unit. In that case, the gas duct can be connected to the air separation unit downstream of the cooling unit and upstream of the purification unit.
Another possibility is dry air at a temperature < 40°C, as can be provided by the purification unit. The gas duct can then be connected to the air separation unit at an air outlet of the purification unit. In the present context, the term "dry air" means air which has been subjected to a dessication step as practiced in the purification unit of the air separation unit. Such dry air typically has a water content of less than 0.001 kg H₂O/Nm³ air.
The pressurized gas can also be dry air produced by a Dry Air Booster compressor (typically driven by electrical motor or steam turbine) of the air separation unit. Thereto, the gas duct can be connected to the outlet of the Dry Air Booster compressor.
A further possibility is pressurized dry air supplied by a booster of the air separation unit, which booster is driven by a cryogenic turbine of the unit, in which case the gas duct can be connected to the outlet of this booster.
As mentioned above, the pressurized gas accumulated in the buffer means preferably has a composition such that the gas present in the hot stove at the end of its pressurization phase has an oxygen content substantially equal to the oxygen content of the cold blast introduce into the hot stove during the subsequent on-blast phase. The oxygen content of the pressurized gas accumulated in the buffer means may be controlled by means foreseen thereto.
The invention can be used for different types of hot stoves. The hot stove may, for example, be a hot stack or a pebble heater.
The accumulated pressurized gas in the buffer means can also be used for other purposes. The accumulated pressurized gas may, for example, be used to provide instrument air for instruments of the blast furnace plant / of the air separation unit.
It is also possible, in accordance with the invention, to use the MAC of the air separation unit as a backup for the one or more blast compressors. Should the supply of cold blast by the one or more blast compressors fall below the minimum level required, for example following a malfunction of a blast compressor, at least part of the pressurized air supplied by the MAC can be added to the cold blast supply so as to ensure that at least the minimum level of cold blast is obtained.
Further details and advantages of the present invention are illustrated hereafter with reference to the following figures, wherein:

figure 1 is a graphic representation of a typical total gas flow into a group of 3 alternating hot stoves and the corresponding total hot blast flow from said group to the blast furnace (excluding any gas flows during the on-gas or heating phase of a hot stove, which gas flows do not form part of the cold or hot blast flows),
figure 2 is a schematic representation of an example of a blast furnace plant according to the present invention comprising a group of 3 alternating hot stoves and a pressurized gas buffer unit, and
figure 3 is a graphic representation of the evolution in time of the pressure in and the gas flows to and from the buffer unit of the installation of figure 2.

In a blast furnace plant having a group of, for example, three alternating hot stoves for the supply of hot blast to the hot furnace, the group of hot stoves is operated so that, at all times when the blast furnace is in use, two of the hot stoves are on-blast and supply hot blast to the furnace, while the third or spare hot stove is being prepared to come on-blast at the end of the on-blast phase of one of the two active hot stoves (which then in turn becomes the spare hot stove), thereby ensuring a constant flow of hot blast to the furnace.
As illustrated in figure 1, when a group of alternating hot stoves is operated so as to provide a constant flow of hot blast to a blast furnace, the flow of gasses into the group of hot stoves is significantly higher (peaks 10, 11) when the spare hot stove is in the pressurization phase due to the additional gas flow required for pressurization.
When, as known in the art, the blast compressor is used to provide the entire increase in gas flow into the hot stoves during pressurization, this blast compressor must be designed not only to supply the cold blast base level 12, which corresponds to the hot blast flow, but also to supply the peak level 13 of gas flow into the hot stoves during hot-stack pressurization. As mentioned above, this presents the disadvantages of a more costly blast compressor, which generally does not operate at its optimum capacity and which is more limited as to the range of cold blast base levels it can operate at and thus also limits the range of hot blast flows at which the blast furnace plant can operate.
The present invention reduces or even eliminates these disadvantages.
The blast furnace plant according to the invention illustrated in figure 2 comprises a group 20 of three alternating hot stoves of the cowper type. The compressed air for the cold blast is supplied by blast compressor 21 and sent to the group 20 of hot stoves, the hot blast leaving the stoves being sent to blast furnace 22.
The plant further comprises an air separation unit 23 which is connected to an inlet of group 20 via gas duct 24. A buffer vessel 25 is branched off from gas duct 24 between air separation unit 23 and the inlet of group 20.
The air separation unit 23 includes a main air compressor (MAC) 26. Compressor 26 is a multistage air compressor with alternating compression and cooling stages. At the exit of the final compression stage, the compressed air has a temperature of about 70°C to 150°C. In the final cooling stage, the temperature of this compressed air is lowered to about 15 to 30°C.
The compressed air then enters dessication unit 27, comprising an upstream cooling unit and a purification unit. The purified dessicated air leaving dessication unit 27 is then sent to cold box 28 for separation into fractions of different composition, typically including an oxygen-rich fraction.
Air separation unit 23 further includes Dry Air Booster compressor 29 which is used to pressurize dry air at a pressure which allows the vaporization of liquid pumped products (typically liquid oxygen, liquid nitrogen, liquid argon), and includes an air booster linked to a cryogenic turbine which enables to keep the cold box cold and possibly to produce liquid products.
During operation of the plant, pressurized gas is transported from air separation unit 23 to buffer vessel 25, where it is stored and accumulated.
When one of the hot stoves enters a pressurization phase, an amount of pressurized gas is transported via duct 24 from buffer vessel 25 to the inlet of group 20 to provide part of and preferably all of the additional gas flow required for the pressurization of the stove. When only part of the additional required gas flow is provided by buffer vessel 25, the increase in output of compressor 21 during peaks 10, 11 is reduced accordingly. When the buffer vessel supplies an amount pressurized gas corresponding to the additional gas flow required, the output of compressor 21 can remain unchanged when a hot stove enters the pressurization phase. It thus becomes possible to operate compressor 21 at its optimum capacity, regardless of whether a hot stove is being pressurized.
In the embodiment illustrated in figure 2, duct 24 transports the amount of pressurized gas from buffer vessel 25 to the supply line transporting compressed air for the cold blast from blast compressor 21 to the group of hot stoves 20.
According to a different embodiment, duct 24 transports the amount of pressurized gas from buffer vessel 25 to an inlet of group 20 which is separate from the supply line. In that case, the amount of pressurized gas can be transported from buffer vessel 25 via duct 24 and the separate inlet exclusively to the hot stove which has entered a pressurization phase.
According to the operating method illustrated in figure 3, a constant flow of pressurized gas from air separation unit 23 is supplied to buffer vessel 25. When none of the hot stoves are in the pressurization phase, no pressurized gas is transferred from buffer vessel 25 to the group of hot stoves and the pressure in buffer vessel 25 continues to rise. When, following an on-blast phase, one of the hot stoves enters a pressurization phase, pressurized gas is ducted from buffer vessel 25 to said hot stove for its pressurization, leading to a drop in pressure in the buffer vessel 25. When said hot stove reaches the end of its pressurization phase, the flow of pressurized gas from buffer vessel 25 to the hot stove is interrupted and the pressure in vessel 25 starts to rise again until one of the hot stoves again enters a pressurization phase, whereupon the process is repeated with respect to this hot stove.
If the pressure of the buffer is higher than the pressure of the cold blast supply during the on-blast phase, the pressurization time can be shortened with respect to the conventional configuration, involving gain of productivity.

Claims

Use of pressurized gas supplied by an air separation unit (23) for pressurizing a hot stove in a blast furnace plant following an on-gas phase and prior to an on-blast phase of the hot stove, the pressurized gas being accumulated in buffer means (25) before being introduced into the hot stove.
A method of operating a hot stove for supplying hot blast to a blast furnace (22), the method including:
a) heating the hot stove during an on-gas phase;

b) pressurizing the hot stove during a pressurization phase, in which a flow of pressurized gas is ducted into the hot stove;

c) using the hot stove in an on-blast phase to heat cold blast and to supply hot blast;

d) depressurizing the hot stove during a depressurization phase; the method being characterized in that it includes accumulating pressurized gas supplied by an air separation unit (23) in buffer means (25) and using the accumulated pressurized gas to provide at least part of the flow of pressurized gas in step (b).
A method according to claim 2, a gas supply from the air separation unit (23) being connected via a gas duct (24) into the buffer means (25) and from the buffer means (25) into an inlet duct of the hot stove, the method including supplying accumulated pressurized gas via the gas duct (24) from the buffer means (25) to the inlet duct of the hot stove during step (b).
A method according to claim 3, including using a section of the gas duct itself as buffer means.
A method according to claim 3 or 4, including using one or more accumulator vessels as buffer means.
A method according to any one of claims 3 to 5, including controlling the supply of accumulated pressurized gas from the buffer means (25) to the inlet duct of the hot stove by selectively opening and closing one or more valves positioned in the gas duct.
A method according to any one of claims 3 to 6, including controlling the accumulation of pressurized gas from the air separation unit (23) in the buffer means (25) by operating one or more valves positioned in the gas duct.
A method according to any one of claims 2 to 7, including continuously supplying pressurized gas from the air separation unit (23) into the buffer means (25).
A method according to any one of claims 2 to 8, including accumulating in the buffer means (25) pressurized gas from the air separation unit (23) having a composition such that the flow of pressurized gas ducted into the hot stove in step (b) has an oxygen content substantially equal to the oxygen content of the cold blast in step (c).
A method according to any one of claims 2 to 9, including using the accumulated pressurized gas to provide instrument air.
A process for operating a group of alternating hot stoves (20) supplying hot blast to a blast furnace (22), the process including using a method according to any one of claims 2 to 10 to operate the hot stoves belonging to the group of hot stoves.
A blast furnace plant comprising
- a blast furnace (22);

- a group of alternating hot stoves (20) for supplying hot blast to the blast furnace; and

- an air separation unit (23);

- characterized in that the plant further comprises an installation for pressurizing the hot stoves belonging to the group of alternating hot stoves (20) following an on-gas phase of the hot stove and prior to an on-blast phase of the hot stove, the installation comprising a gas duct (24) for pressurized gas connecting the air separation unit to an inlet duct of the hot stove, characterized in that the gas duct (24)
(a) connects the air separation unit (23) to buffer means (25) for accumulating pressurized gas in said buffer means and

(b) connects the buffer means (25) to the inlet duct of the hot stove.
A blast furnace plant according to claim 12, wherein the buffer means include a section of the gas duct.
A blast furnace plant according to claim 12 or 13 wherein the buffer means include one or more accumulator vessels.
A blast furnace plant according to any one of claims 12 to 14, comprising one or more valves positioned in the gas duct for controlling the flow of accumulated pressurized gas from the buffer means (25) to the inlet duct of the hot stove.
A blast furnace plant according to any one of claims 12 to 15, comprising one or more valves positioned in the gas duct for controlling the flow of pressurized gas from the air separation unit (23) to the buffer means (25).