Patent Application
20240191315
The present invention is related to a technique for melting a cold iron source using a heat source with reduced CO2 emissions to decrease greenhouse gas emissions in producing iron and steel products.
From the viewpoint of preventing global warming, techniques have been recently developed in the iron and steel industry to reduce CO2 gas emissions by decreasing fossil fuel consumption amounts. In existing integrated steelworks, molten pig iron is produced by reducing iron ore with carbon. The production of such molten pig iron requires approximately 500 kg of a carbon source per 1 t of molten pig iron to reduce the iron ore or the like. In contrast, when producing molten steel with a cold iron source such as iron scrap as a raw material, the carbon source necessary for reducing the iron ore is not required, while only energy with a sufficient heat quantity for melting the cold iron source is required. Accordingly, the CO2 emissions can be substantially reduced.
For an operation with a high content of a cold iron source, an electric furnace such as an arc furnace or an induction melting furnace is often used, where most of the heat for melting the cold iron source is provided by electric power. To decrease electric power unit consumption, the arc furnace is operated with adopting the following techniques: (1) an auxiliary burner is provided on a furnace wall or a slag removal port to promote the melting of the cold iron source in a cold spot and the like; (2) a so-called oxygen-enriched operation for providing oxidation heat for the iron is performed by feeding oxygen through an oxygen gas feeding lance; and (3) the heat transfer efficiency of an arc to molten steel is enhanced by feeding oxygen and carbon powder from an oxygen gas feeding lance and a carbon injection lance, respectively, causing slag foaming to cover the arc.
However, the oxygen-enriched operation has a problem of a decrease in yield accomplished by oxidation loss of iron. Moreover, most of the auxiliary burners use hydrocarbon as a fuel. The slag foaming operation generates CO2 gas by blowing the carbon powder into the slag. These techniques can decrease the electric power unit consumption but have a small effect of reducing CO2 emissions. In the first place, when the electric power in use is obtained from a fossil fuel, CO2 will be emitted from the use of the electric power as well. Therefore, to further reduce CO2 emissions, it is desirable to improve heat efficiency and increase use of energy sources with reduced CO2 emissions.
Even if we can obtain electric power with reduced CO2 emissions and replace the electric power we use with it in the future, the need for techniques to decrease the electric power unit consumption will remain unchanged. Thus, those techniques also need to aim to reduce the CO2 emissions. For example, hydrogen gas produced from renewable energy or other sources can be used as a fuel for an auxiliary burner or the like; however, hydrogen gas has a higher combustion rate and higher flame temperature compared to hydrocarbon gas and other gases, which poses a problem. The following are some known techniques to solve this problem.
For example, both Patent Literature 1 and Patent Literature 2 propose a technique for lowering the flame temperature by providing an additional combustion space upstream of a main combustion space to reduce the oxygen concentration therein. Further, Patent Literature 3 proposes a method for lowering the flame temperature by maintaining a distance between a hydrogen gas feeding flow path and a combustion air feeding flow path and thus preventing the hydrogen gas from mixing with combustion air to decrease the combustion rate.
Further, as a cold iron source to be melted, not only iron scrap but also solid reduced iron, which is produced by using a reducing agent with reduced CO2 emissions, is expected to be used in a larger amount.
The conventional techniques have the following problems.
The methods described in Patent Literatures 1 to 3 have a complicated and large facility structure, which brings about a problem of increased facility investment costs and maintenance costs if applied to a steelmaking electric furnace. Moreover, solid reduced iron typically contains gangue components, such as SiO2, Al2O3, derived from iron ore being a raw material thereof. Therefore, a large amount of slag might be produced at the time of melting, causing operation trouble such as slag solidification in a furnace.
The present invention is made in view of such a background and aims to provide an electric furnace that can obtain molten iron by melting a cold iron source using electric energy and a heat source with reduced CO2 emissions. The invention also proposes a steelmaking method that uses the electric furnace and heat source with reduced CO2 emissions to melt a cold iron source. The present invention further proposes a steelmaking method that can stably melt solid reduced iron as the cold iron source.
An electric furnace of the present invention can advantageously solve the abovementioned problems and obtain molten iron by melting a cold iron source using electric power. The electric furnace is characterized in that: the electric furnace is provided with a burner directed toward furnace contents; the burner comprises a powder-feeding pipe, a jet hole for jetting a fuel, and a jet hole for jetting combustion-supporting gas; a burner flame is formed by jetting hydrogen gas or a hydrogen-enriched gaseous fuel as the fuel; and an auxiliary material that is in a powder form or is processed into a powder form is jetted through the powder-feeding pipe, so that the auxiliary material passes through an inside of the burner flame.
Further, the electric furnace of the present invention is preferably a DC arc furnace, an AC arc furnace, or an induction melting furnace to solve the above problems.
A steelmaking method of the present invention can advantageously solve the abovementioned problems and obtain molten iron by melting a cold iron source using electric power in an electric furnace. The steelmaking method is characterized in that: the electric furnace is provided with a burner that comprises a jet hole for jetting a fuel and a jet hole for jetting combustion-supporting gas and that jets a flame through the jet holes toward an inside of the electric furnace; and for at least a part of duration of one-heat operation of the electric furnace, hydrogen gas or a hydrogen-enriched gaseous fuel is used as the fuel of the burner, and an auxiliary material that is in a powder form or is processed into a powder form is blown in to pass through an inside of the flame formed by the burner.
The steelmaking method of the present invention may provide more preferable solutions to the problems as follows:
According to the present invention, the powdery material is heated in the burner flame because it is fed through the burner flame, and serves as a heat transfer medium. Therefore, the combustion heat of the burner can be used for heating the cold iron source in the electric furnace with high efficiency, decreasing the amounts of the electric power used. Moreover, using hydrogen gas as the fuel can reduce the CO2 emissions. Further, the flame temperature is lowered because the sensible heat of the combustion gas of the burner is consumed to heat the powdery material in the burner flame, ensuring the durability of a burner nozzle and preventing furnace-body refractories of the electric furnace from being worn out.
In the present invention, the powdery lime heated by the burner flame is blown onto the slag resulting from the melting of solid reduced iron to heat the slag and lower the melting point of the slag. This can enhance the slag formation and prevent poor operation caused by slag solidification. In particular, when using an induction melting furnace as the electric furnace, the slag is not directly heated because an induction current is not generated thereon, and thus slag solidification is easily caused. The present invention significantly improves this problem.
Embodiments of the present invention will be described in details below. The drawings are schematic and may be different from reality. Moreover, the following embodiments illustrate examples of devices and methods for embodying technical concepts of the present invention and are not meant to specify configurations to those described below. In other words, the technical concepts of the present invention may be modified in various manners within the technical scope set forth in the claims.
The above embodiment uses AC arc furnace 101 having three electrodes as the electric furnace and also can use a DC arc furnace having an upper electrode and a lower electrode. When using the AC arc furnace 101, which has electrodes 5 and an arc in a central part of the furnace body 9, as the electric furnace, possible installation positions of the burner lance 1 are limited. As described later, the burner of the present embodiment can lower the temperature of the burner flame 7 by appropriately blowing the powdery auxiliary material 8, even when using the fuel of which the main component is hydrogen gas, achieving an operation without wearing out a water cooling panel on a furnace wall, refractories on the furnace floor, etc.
The above embodiment example is an integrally-formed burner lance configured with the powder-feeding pipe at the center and arranging the jet hole for jetting a fuel and the jet hole for jetting a combustion-supporting gas around the powder-feeding pipe; however, possible forms of the burner lance are not limited to this example. In other words, the burner according to the present invention may be provided with a powder-feeding pipe, a jet hole for jetting a fuel, and a jet hole for jetting a combustion-supporting gas, where hydrogen gas or a hydrogen-enriched gaseous fuel is jetted as the fuel to form a burner so that an auxiliary material jetted through the powder-feeding pipe passes through the inside of the burner flame. For example, it is acceptable to provide an integrally-formed lance with a jet hole for jetting a fuel and a jet hole for jetting a combustion-supporting gas and to separately arrange a powder-feeding pipe to be positioned adjacent to the lance, so that the auxiliary material jetted through the powder-feeding pipe passes through the inside of the burner flame.
In a steelmaking method according to another embodiment of the present invention, for example, the cold iron source 2 such as iron scrap or solid reduced iron is first charged from a bucket (not shown) into an electric furnace such as the AC arc furnace 101 shown in
The present inventors examined the heat transfer efficiency to the furnace contents and durability of the burner lance nozzle by using the electric furnace shown in
The powdery material may use: a slag forming agent that is the powder or powdered auxiliary material 8; dust; or the like. Efficient heating of the powdery material in the burner flame involves increasing the specific surface area, and thus the particle size is preferably approximately 100 μm or less. The auxiliary material with a larger particle size is preferably processed to have a particle size of approximately 100 μm or less. In this case, the particle size is expressed by the 50% passing rate on volume.
The cold iron source 2 preferably uses iron scrap and/or solid reduced iron reduced by using a reducing agent with less CO2 emissions. Some solid reduced iron contains SiO2 and Al2O3 by approximately 10 to 20 mass % as gangue components derived from iron ore, depending on its brand. When the solid reduced iron is melted, these substances form slag 4, which appears on the bath surface of the molten iron 3. The slag has a composition with a high melting point as it is, and thus would easily be solidified and adhere to the furnace wall, which may interfere with the operation. Especially, when using the induction melting furnace 102, the slag is not heated by induction, which is like to cause slag solidification.
It is preferable to use lime as the powdery auxiliary material 8 to be heated by the burner and fed, because it can control the basicity (i.e., the CaO/SiO2 ratio in mass) of the slag to be approximately 1.0. Thus, the melting point of the slag can be lowered to prevent the solidification of the slag. The slag is heated by the heated powdery material, which achieves the advantageous effect of promoting slag formation. After the slag formation, slag may be removed or poured out during the melting or before tapping by tilting the furnace body of the arc furnace or the induction melting furnace.
Further, any type of electric furnace is applicable as long as it can melt the cold iron source with electric energy to obtain molten iron. For example, the electric furnace includes, in addition to the AC or DC arc furnace, a submerged arc furnace which applies heat by submerging a Soederberg self-baking electrode or the like in the slag can be applicable; an indirect-type resistance furnace which heats an object to be heated with radiation from a heat-generating body provided in the furnace, convection in the furnace, and/or a conductive heat transfer; and a plasma arc melting furnace.
The molten iron 3 melted in the present embodiment has a composition equivalent to the metal composition of the iron scrap or the solid reduced iron as a material and usually comprises molten steel with a relatively low C content. To adjust the composition, it is possible, in the same electric furnace where the melting was performed, to perform other treatments comprising adding alloy, conducting finishing decarburization treatment and dephosphorization treatment with oxygen-blowing refining. It is also possible to perform a secondary refining process comprising molten steel desulfurization and vacuum degassing after tapping. Subsequently, a semi-finished product such as a cast slab is obtained by casting step such as continuous casting and other processes.
A cold iron source melting operation test was performed using the AC arc furnace 101 with the same configuration as that shown in
The burner lance 1, which was provided with a fuel-feeding line and an oxygen-feeding line, was installed in the furnace body. The tip end part 10 of the burner lance 1 had the same multiple-pipe structure as that shown in
After the electric current started to be applied, when the melting of the first-charged cold iron source progressed to lower the height of the charged materials in the furnace such that there was space in the upper part of the furnace, the burner lance 1 was descended to apply heat from the burner flame 7 as well. Powdery lime was fed into the electric furnace using argon gas as conveyance gas at a feeding speed of 100 kg/min. The fuel gas 16 was jetted at different flow rates depending on its gas type used: 5 Nm3/min for methane gas, 16 Nm3/min for hydrogen gas, and 10.5 Nm3/min for hydrogen-methane gas mixture. Oxygen gas was fed as the combustion-supporting gas 17 for the combustion of the fuel gas 16 in each case.
When the first-charged cold iron source further melted and reached a flat bath state, a state where any unmelted portion of the cold iron source was soaked in the molten metal, slag was removed through the slag removal port. Then, the electric current and the burner were turned off to open the furnace lid and charge the cold iron source for the second time. After the second charge of the cold iron source, the electric current was resumed to perform the operation in the same manner as after the first charge. Thus, molten steel at 1650° C. was eventually obtained and tapped into a ladle.
A comparison was made on the wear statuses of the burner lance nozzle, the electric power unit consumption, the presence/absence of slag solidification, and CO2 emissions for each treatment condition. Table 1 shows the treatment conditions and results. The nozzle wear factor is an index indicating a relative wear amount when a wear amount in the case of using methane as the fuel gas was set to 1.0. The electric power unit consumption is an index calculated by dividing the electric power consumption under each treatment condition by the electric power consumption in treatment No. 1, which represents a conventional example. The slag solidification was visually observed and determined. The CO2 emissions index is an index calculated by dividing the CO2 emissions under each treatment condition by the CO2 emissions in treatment No. 1, which represents a conventional example. In this comparison, the fossil fuels consumed to obtain the electric power were also taken into consideration to calculate the CO2 emissions.
Under the conditions (Treatment Nos. 2 to 4) in which the furnace contents were heated by the burner flame alone, the burner combustion heat was not effectively transferred, resulting in the electric power unit consumption being substantially equal to the conventional example (Treatment 1) where no burner was used. Under the condition s (Treatment Nos. 3 and 4) in which the hydrogen-containing gas was used as the fuel without feeding the powder, the burner lance nozzle was substantially worn out. Under the conditions (Treatment Nos. 5 to 7) in which powdery lime was blown in through the burner flame, the combustion heat of the burner was transferred to the furnace contents with high efficiency, significantly reducing the electric power unit consumption. Under the conditions (Treatment Nos. 6 and 7) that used the hydrogen-containing gas, the CO2 emissions was substantially reduced as compared with the conditions (Treatment No. 5) that used methane as the burner fuel, preventing the burner nozzle from being worn out.
In the present examples using the arc furnace, slag solidification was not observed in any condition.
A cold iron source melting operation test was performed by using the induction melting furnace 102 with the same configuration as that shown in
The burner lance 1 provided with a fuel-feeding line and an oxygen-feeding line was installed in the furnace body. The tip end part 10 of the burner lance 1 had a similar structure to the multiple-pipe structure shown in
After the electric current started to be applied, when the melting of the first-charged cold iron source progressed to lower the height of the charged materials in the furnace such that there was space in the upper part of the furnace, the burner lance 1 was descended to apply heat from the burner flame 7 as well. Powdery lime was fed into the electric furnace using argon gas as conveyance gas at a feeding speed of 100 kg/min. The fuel gas 16 was jetted at different flow rates depending on its gas type used: 5 Nm3/min for methane gas, 16 Nm3/min for hydrogen gas, and 10.5 Nm3/min for hydrogen-methane gas mixture. Oxygen gas was fed as the combustion-supporting gas 17 for the combustion of the fuel gas 16 in each case.
When the first-charged cold iron source further melted and reached a flat bath state, where any unmelted portion of the cold iron source was soaked in the molten metal, the electric current and the burner were turned off to open the furnace lid and remove the slag by tilting the furnace body. After the completion of the slag removal, the furnace body was resumed to the original vertical posture to charge the cold iron source for the second time, and then the application of the electric current was resumed. After the second charge of the cold iron source, the same operation as after the first charge was performed. Thus, molten steel at 1650° ° C. was eventually obtained and tapped into a ladle.
A comparison was made on the wear statuses of the burner lance nozzle, the electric power unit consumption, the presence/absence of slag solidification, and CO2 emissions for each treatment condition. Table 2 shows the treatment conditions and results. The evaluation indexes are the same as those used in Example 1 were used. Treatment No. 8 served as a conventional example to be compared.
Under the conditions (Treatment Nos. 9 to 11) in which the furnace contents were heated by the burner flame alone, the burner combustion heat was not effectively transferred, resulting in the electric power unit consumption being substantially equal to the conventional example (Treatment 8) where no burner was used. Under the conditions (Treatment Nos. 10 and 11) in which the hydrogen-containing gas was used as the fuel without feeding the powder, the burner lance nozzle was substantially worn out. Further, the operation was hindered by slag solidification in the furnace.
Under the conditions (Treatment Nos. 12 to 14) where the powdery lime was added via the burner flame, because the combustion heat of the burner was transferred to the furnace contents with high efficiency, it was possible to significantly decrease the electric power consumption rates. Further, as compared with the level (Treatment No. 12) using methane as the burner fuel, the situations under the conditions (Treatment Nos. 13 and 14) using the hydrogen-containing gas were able to significantly reduce the CO2 emissions and were also able to prevent the burner nozzle from being worn out. In addition, it was also possible to prevent solidification of the slag.
The electric furnace and the steelmaking method of the present invention can increase the heat transfer efficiency, and melt the cold iron source by using the heat source with reduced CO2 emissions. This can reduce electric power unit consumption and decrease the environmental impact, which are industrially useful. Further, the present invention is applicable to other processes in a refining furnace that requires both heat source with reduced CO2 emissions and the addition of a powdery auxiliary material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-078969 | May 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/017372 | 4/8/2022 | WO |
