Arc furnace, and method for treating carbon dioxide using an arc furnace

Abstract

The present invention aims to reduce carbon dioxide emissions by treating it in an arc furnace, which is a metal melting furnace, and proposes a method for treating carbon dioxide that utilizes the decomposition of carbon dioxide by the arc of an arc furnace. [Solution] Using an arc furnace, which is a metal melting furnace, a mixed gas of carbon dioxide-containing gas and carbon powder is introduced into the arc furnace, and the mixed gas is brought into contact with the arc in the arc furnace to decompose the carbon dioxide into carbon monoxide and oxygen, and further react the carbon powder with the decomposition product oxygen to produce carbon monoxide. The decomposed carbon monoxide, unreacted oxygen, unreacted carbon powder, and undecomposed carbon dioxide are recovered by a recovery device, and the recovered undecomposed carbon dioxide is returned to the mixed gas as a carbon dioxide-containing gas for reuse.

Description

[Technical Field]

[0001] The present invention relates to an arc furnace and a method for treating carbon dioxide using an arc furnace. [Background Art]

[0002] In recent years, the global warming issue has become a global-scale problem, and countermeasures against global warming are being called for internationally. In particular, reducing the emission of carbon dioxide, a greenhouse gas that is a major contributing factor to global warming, has become an urgent issue.

[0003] For example, technologies called CCS (Carbon Capture Storage), which captures carbon dioxide and injects and stores it underground, and CCU (Carbon Capture Usage), which captures and reuses carbon dioxide, have been proposed. Representative CCU technologies include, for example, concrete fixation, conversion into carbon materials via molten salt electrolysis (utilization as a raw material for LIB negative electrode materials), methane synthesis from carbon dioxide such as methanation, and conversion into hydrocarbon fuels (methane, kerosene, etc.) using chemical reactions (Fischer-Tropsch process, etc.). In addition, direct utilization after capture (use as dry ice, or as carbon dioxide bubbles in carbonated beverages) has also been proposed.

[0004] The manufacturing industry accounts for 35% of carbon dioxide emissions in Japan, and 35% of that is attributable to the steel industry (Non-Patent Document 1). That is, the steel industry is an important industrial sector for achieving carbon neutrality across Japan, and as part of carbon neutralization efforts, attempts have been made to shift the molten iron production method from the blast furnace process, which emits large amounts of carbon dioxide, to arc furnaces (also called electric furnaces or e-furnaces), which emit less carbon dioxide.

[0005] On the other hand, as a proposal to use an arc furnace for treating carbon dioxide itself, a reforming method that reduces the amount of carbon dioxide generated by reforming carbon dioxide in the exhaust gas of an arc furnace with a reducing agent has been proposed (Patent Document 1).

[0006] Furthermore, although it does not utilize an arc furnace as a melting furnace, a method has been proposed in which carbon dioxide and carbon powder are reacted using the arc discharge energy between a pair of electrodes to convert carbon dioxide into carbon monoxide (Patent Document 2). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2012-31470 [Patent Document 2] Japanese Patent Publication No. 2005-289773 [Non-patent literature]

[0008] [Non-Patent Document 1] Ministry of Economy, Trade and Industry, Agency for Natural Resources and Energy website, "Global Initiatives Towards Decarbonization of the Steel Industry ~ What is Green Steel?", dated August 10, 2023. [Overview of the project] [Problems that the invention aims to solve]

[0009] The proposal described in Patent Document 1 focuses on reforming the exhaust gas of an arc furnace, and as part of this, it involves reforming the carbon dioxide in the exhaust gas with a reducing agent. However, the exhaust gas of an arc furnace does not contain a large amount of carbon dioxide to begin with; it is basically at the same concentration as the carbon dioxide in the atmosphere. Furthermore, since this does not treat the carbon dioxide in the arc furnace itself, but rather treats the carbon dioxide in the exhaust gas, there are limitations to its carbon dioxide reduction effect.

[0010] Patent Document 2 does not apply to an arc furnace as a melting furnace, but rather aims to improve the conversion efficiency when carbon dioxide and carbon powder are reacted to convert carbon dioxide into carbon monoxide. Therefore, it cannot be applied to an arc furnace.

[0011] Therefore, the present invention aims to reduce carbon dioxide emissions by treating carbon dioxide in an arc furnace used as a melting furnace, and proposes a method for treating carbon dioxide using carbon dioxide treatment technology within the arc furnace itself. [Means for solving the problem]

[0012] The inventors diligently conducted research and development to achieve the above objectives. As a result, they obtained the following findings.

[0013] [a] An arc furnace is a furnace that utilizes the phenomenon of high-density electric current flowing through a gas due to an arc discharge, and uses the thermal energy dissipated during the arc discharge to melt metals and other materials. It is known that the temperature around the arc during an arc discharge can reach nearly 5000°C. On the other hand, carbon dioxide (CO2) begins to decompose at high temperatures of 2000°C or higher, breaking down into CO (carbon monoxide) and O2 (oxygen), and it is known that theoretically 100% of it is decomposed above 5000°C. However, even if CO2 can be decomposed into CO and O2 in an arc furnace as a melting furnace, there is a concern that CO will burn when CO and O2 are recovered within the arc furnace, resulting in the resynthesis of CO2.

[0014] On the other hand, it is known from Ellingham diagrams (e.g., C.J. Osborn, 1950) and other sources that oxygen (O2) reacts with carbon monoxide (CO) before carbon monoxide (CO) when carbon powder (C) is present in the vicinity at temperatures above approximately 650°C, producing CO.

[0015] Therefore, the inventors conceived and developed a method to decompose carbon dioxide by applying the arc discharge of an arc furnace, bringing a mixed flow of carbon dioxide and carbon powder into contact with the arc, thereby decomposing carbon dioxide in the atmosphere into carbon monoxide and oxygen, and further reacting the decomposed oxygen with carbon to produce carbon monoxide. As a result, they found that 50-80% of the supplied carbon dioxide could be decomposed and recovered as carbon monoxide.

[0016] [b] Arc furnaces for metals such as steel and aluminum are normally operated in an air atmosphere, and active treatment of carbon dioxide has not been considered. Accordingly, the inventors have found that when introducing excess carbon dioxide into an arc furnace from the outside, by introducing carbon powder mixed therein as a carbon source and bringing the carbon powder into contact with the discharging arc, the introduced carbon dioxide can be efficiently decomposed.

[0017] [c] Even when carbon dioxide is decomposed by contact with an arc, as described above, 50 to 80% of the carbon dioxide is decomposed, and the remaining 20 to 50% is discharged as carbon dioxide. Furthermore, since the exhaust gas after decomposition contains harmful carbon monoxide (CO), it cannot be emitted directly to the atmosphere. Accordingly, the inventors have found that by separating carbon monoxide from the exhaust gas, further separating carbon dioxide from the exhaust gas and returning the separated carbon dioxide to the arc furnace, carbon dioxide can be decomposed efficiently.

[0018] The present invention has been made based on the above findings, and the gist thereof is as follows.

[0019] [1] An arc furnace for melting metal, comprising: a mixed gas supply cylinder that supplies a mixed gas of carbon powder mixed with carbon dioxide-containing gas into the arc furnace, wherein the mixed gas supply cylinder is arranged such that the mixed gas discharged from a discharge port of the mixed gas supply cylinder comes into contact with an arc in the arc furnace. [2] The arc furnace according to [1], comprising an exhaust gas treatment device that treats exhaust gas discharged from the arc furnace, wherein the exhaust gas treatment device comprises at least a carbon dioxide recovery device, and is configured to supply the carbon dioxide recovered by the carbon dioxide recovery device to the mixed gas. [3] The arc furnace according to [2], wherein the exhaust gas treatment device further comprises one or more selected from the group consisting of a carbon powder recovery device, an oxygen recovery device, and a carbon monoxide recovery device. [4] The arc furnace according to any one of [1] to [3], wherein the mixed gas supply cylindrical member is arranged so as to surround an electrode of the arc furnace, and a discharge port is arranged such that the mixed gas is discharged from around an end face of the electrode toward a central axis direction of the electrode. [5] The arc furnace according to any one of [1] to [3], wherein the mixed gas supply cylindrical member is an electrode of the arc furnace, a flow path for the mixed gas is arranged inside the electrode, and a discharge port for the mixed gas is arranged along a central axis of the electrode from an end face of the electrode. [6] Using an arc furnace for melting metal, A method for treating carbon dioxide using an arc furnace, characterized in that a mixed gas obtained by mixing carbon powder with a gas containing carbon dioxide is introduced into the arc furnace through a mixed gas supply cylindrical member, and the mixed gas is brought into contact with an arc in the arc furnace. [7] separating at least carbon dioxide from exhaust gas discharged from the arc furnace, The method for treating carbon dioxide using an arc furnace according to [6], wherein the separated carbon dioxide is supplied to the mixed gas to be introduced into the arc furnace. [8] The method for treating carbon dioxide using an arc furnace according to [7], wherein one or more of carbon powder, oxygen and carbon monoxide are further separated from the exhaust gas. [9] The method for treating carbon dioxide using an arc furnace according to any one of [6] to [8], wherein the mixed gas supply cylindrical member is arranged so as to surround an electrode of the arc furnace, and discharges the mixed gas toward a central axis direction of the electrode.

[10] The method for treating carbon dioxide using an arc furnace according to any one of [6] to [8], wherein the mixed gas supply cylindrical member is an electrode of the arc furnace, a flow path for the mixed gas is arranged along a central axis of the electrode, and the mixed gas is discharged toward a central axis direction of the electrode.

[11] The arc furnace according to any one of [1] to [5], wherein the metal is an iron alloy.

[12] A method for treating carbon dioxide using an arc furnace according to any one of items [6] to

[10] , wherein the aforementioned metal is an iron alloy. [Effects of the Invention]

[0020] This invention allows for the treatment of carbon dioxide using the arc furnace itself, contributing to a reduction in the amount of carbon dioxide in the environment. Furthermore, by recovering undecomposed carbon dioxide from the exhaust gas of the arc furnace and treating it again in the arc furnace, it is possible to efficiently reduce carbon dioxide in the environment. In addition, by separating CO (carbon monoxide), harmful substances are removed, and CO can be effectively utilized as fuel or reducing agent. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a conceptual diagram illustrating the arc furnace and carbon dioxide treatment method according to the present invention. [Figure 2] Figure 2 is a schematic diagram of an example of a mixed gas supply cylindrical structure, in which the mixed gas supply cylindrical structure is arranged to surround the electrodes. [Figure 3] Figure 3 is a schematic diagram of an example of a cylindrical mixed gas supply structure, specifically one in which a hollow electrode is used to serve as both the mixed gas supply structure and the electrode. [Modes for carrying out the invention]

[0022] An arc furnace and a method for treating carbon dioxide using an arc furnace according to the present invention will be described based on one embodiment thereof (hereinafter referred to as the present invention). It goes without saying that the present invention is not limited to this embodiment. Unless otherwise specified, carbon dioxide, carbon monoxide, and oxygen are in gaseous form.

[0023] It is known that the temperature of the arc generated in an arc furnace can reach approximately 5,000°C to 20,000°C. For example, arc heating commonly used in electric furnaces for steelmaking involves passing current between electrodes (graphite electrodes) installed at the top of the furnace and the steel scrap to be heated, which is charged into the furnace. This generates an arc between the electrodes and the material, and the thermal energy generated heats the material. This heating method, in which current flows directly through the material to be heated, is called direct arc heating. In a method in which no current flows through the material to be heated, but an arc is generated between electrodes placed outside the material, and the thermal energy of this arc heats the material, is called indirect arc heating. The arc furnace used in this invention may use either direct arc heating or indirect arc heating, and the heating method is not limited. In addition, direct heating methods include three-phase and single-phase systems, and indirect heating methods include vibratory and Rennerfelt types, but these types are not particularly limited.

[0024] The arc furnace constituting this invention is an arc furnace used as a melting furnace for melting metals. The metal to be melted is a metal used in the iron and steel industry and other metal industries (aluminum, copper, etc.). More preferably, it is an iron alloy (Fe-containing alloy), and even more preferably, it is steel (scrap material).

[0025] [Decomposition of carbon dioxide by arc, and production of carbon monoxide by reaction of carbon powder and oxygen] It is known that carbon dioxide (CO2) decomposes into carbon monoxide (CO) and oxygen (O2) at high temperatures. The decomposition of carbon dioxide begins at approximately 1500°C, and the decomposition progresses as the temperature rises, with 20% of carbon dioxide decomposing at approximately 2500°C, 50% at approximately 3000°C, 90% at approximately 4000°C, and almost 100% at approximately 5000°C. Carbon dioxide (CO2) → CO + 1 / 2O2

[0026] Since the arc temperature in the arc furnace exceeds 5000°C, it is easy to imagine that the temperature near the arc will also exceed 3000°C. This invention utilizes this temperature near the arc, which exceeds 3000°C, to decompose carbon dioxide into carbon monoxide and oxygen. Therefore, it is necessary to ensure a high frequency and duration of contact between carbon dioxide and the arc in order to efficiently raise the temperature of the carbon dioxide.

[0027] Although different in scale from arc furnaces, carbon dioxide arc welding also involves shielding the arc with carbon dioxide (CO2) while welding. It is known that 2-5% of the supplied carbon dioxide decomposes into CO and oxygen. The principle is similar to that of the present invention, and it is thought that the carbon dioxide is heated to a high temperature by the thermal energy of the arc, causing it to decompose. At this time, it can be inferred that rather than the entire supplied carbon dioxide gas being heated uniformly, a portion of the carbon dioxide was heated locally and decomposed.

[0028] It is known that oxygen (O2), when present with carbon powder (C) at temperatures above approximately 650°C, preferentially reacts with carbon (C) to produce carbon monoxide (CO) over carbon monoxide (CO). This is shown, for example, in the Ellingham diagram (e.g., CJOsborn, 1950). Reaction of oxygen and carbon at high temperatures (above 650°C): Carbon powder (C)+1 / 2O2→CO

[0029] When an arc discharge begins, the temperature inside the arc furnace rises to near the melting point of the material being heated. For example, if the material being heated is steel (steel products), its melting point is approximately 1500°C, so the temperature inside the arc furnace will be at least 1000°C to 1500°C. Therefore, the present invention utilizes this temperature inside the arc furnace to produce carbon monoxide by contacting carbon with oxygen inside the furnace. As mentioned above, the oxygen inside the furnace mainly consists of oxygen that is a product of the decomposition of carbon dioxide supplied from the outside at high temperatures in the environment near the arc.

[0030] By allowing these reactions (the decomposition of carbon dioxide and the synthesis of carbon monoxide from oxygen and carbon) to proceed simultaneously, the reaction shown in the following equation can ultimately be obtained. Decomposition reaction of carbon dioxide: CO2 → CO + 1 / 2O2 The reaction of oxygen and carbon to synthesize carbon monoxide: C + 1 / 2O2 → CO2 The reactions when these reactions occur simultaneously: Carbon dioxide (CO2) + carbon powder (C) → 2CO

[0031] [Mixed gas] A mixed gas is prepared by mixing carbon powder with a gas containing carbon dioxide (carbon dioxide-containing gas), and this is introduced into an arc furnace and brought into contact with the arc generated between the electrodes. Efficient contact of the mixed gas with the arc generated in the arc furnace can be achieved by using the following mixed gas supply cylindrical device. In experiments using a small test furnace, the inventors generated an arc equivalent to that of a steel arc furnace and brought a mixed gas, consisting of equal moles of carbon dioxide and carbon powder, into contact with the arc and measured the decomposition rate, confirming that 5-20% of the carbon dioxide was decomposed. It is speculated that this variation in the decomposition rate is due to the contactability between the carbon dioxide mixed gas and the arc. Furthermore, when the carbon mass balance was calculated from the amount of carbon powder and carbon dioxide introduced and the amount of carbon monoxide in the recovered exhaust gas, it was confirmed that 50-80% of the introduced carbon reacted with oxygen to become carbon monoxide.

[0032] The carbon dioxide content in the mixed gas is not particularly limited, but a higher concentration of carbon dioxide is preferable as it improves reactivity and generates more carbon monoxide. Preferably, the carbon dioxide content in all gas components constituting the mixed gas (i.e., the gas components of the carbon dioxide-containing gas) is 40 mol% or more, more preferably 45 mol% or more, or 50 mol% or more. The gas components other than carbon dioxide in the above-mentioned all gas components are not particularly limited, but examples include oxygen, nitrogen, carbon monoxide, etc., which constitute normal air (atmosphere). Ideally, the mixed gas should contain no gases other than carbon dioxide, and most preferably only carbon dioxide and carbon powder. The carbon powder content in the mixed gas is also not particularly limited, but the molar ratio of carbon dioxide to carbon powder (converted to carbon atoms) (moles of carbon equivalent in carbon dioxide / moles of carbon equivalent in carbon powder) is preferably 0.1 to 10. The lower limit of the molar ratio of carbon dioxide to carbon powder (converted to carbon atoms) is more preferably 0.2, 0.3, 0.4, 0.5, 0.7, or 0.9. The upper limit of the molar ratio of carbon dioxide to carbon powder (converted to carbon atoms) is preferably 5, 4, 3, 2.5, 2, or 1.4. Ideally, the molar ratio of carbon dioxide to carbon powder (converted to carbon atoms) should be 1.

[0033] The properties of the carbon powder are not particularly limited. Fine coal powder, graphite powder, powdered activated carbon, or carbon black can be used individually or in mixtures of two or more of these. These carbon powders may be commercially available. Considering mixing into carbon dioxide-containing gas, the transportability of the carbon powder in the gas, and the reactivity of the carbon powder with oxygen, the carbon powder should preferably be fine, specifically with a maximum particle size (diameter of the projected area) of 1000 μm or less. Preferably, it should be 700 μm or less, 500 μm or less, 300 μm or less, 100 μm or less, 50 μm or less, 20 μm or less, or 10 μm or less. In the case of carbon black alone, it may be 1 μm or less. The minimum particle size of the carbon powder is also not particularly limited, but since finer particles are more difficult to handle and less costly, it may be 0.01 μm or larger.

[0034] The method for mixing carbon dioxide-containing gas and carbon powder is not particularly limited. Existing methods for mixing powders into gas can be applied.

[0035] [Mixed gas supply cylindrical device] It is important to efficiently transfer the thermal energy of the arc to carbon dioxide. Therefore, when supplying carbon dioxide-containing gas into the arc furnace, it is preferable to supply the mixed gas near the arc using a mixed gas supply cylinder, so that the mixed gas and the arc come into contact. By using a mixed gas supply cylinder, it is possible to bring its outlet (e.g., lance outlet) close to the arc. The shape and arrangement of the mixed gas supply cylinder are not particularly limited. However, it is necessary to ensure sufficient contact time between the mixed gas and the arc.

[0036] For example, as shown schematically in Figure 2, the mixed gas supply cylindrical object 3 is positioned to surround the electrode 4 of the arc furnace, and the discharge port is positioned so that the mixed gas 35 is discharged from around the end face of the electrode in the direction of the electrode's central axis. In other words, the electrode 4 is positioned inside the mixed gas supply cylindrical object 3, and with this arrangement, the mixed gas 35 can be discharged (supplied) from around the end face of the electrode in the direction of the electrode's central axis, following the flow of the arc 5. This allows for adjustment of arc fluctuations (variations in which the arc flow deviates slightly from the electrode's central axis). The size of the discharge port diameter (inner diameter) of the mixed gas supply cylindrical object should be set appropriately according to the mixed gas flow. For example, it is good to set it to 1.5 to 2.5 times the electrode diameter. This ensures that even if the arc generated from the electrode fluctuates, the arc fluctuations remain within the range of the mixed gas flow, and the contact time between the mixed gas and the arc is ensured.

[0037] Alternatively, as schematically shown in Figure 3, the mixed gas supply cylindrical object 3 itself can be used as the electrode 4, a flow path for the mixed gas 35 can be placed inside the electrode 4, and a discharge port can be positioned to discharge (supply) the mixed gas 35 from the end face (arc generation side end face) of the electrode 4 (mixed gas supply cylindrical object 3) in the direction of the electrode's central axis. In other words, the mixed gas supply cylindrical object 3 itself can be used as the electrode 4 of the arc furnace, a flow path for the mixed gas 35 can be secured in the center of the electrode 4, the mixed gas 35 can be discharged from inside the arc generation side end face of the electrode (mixed gas supply cylindrical object), and the arc 5 can be generated around the discharge port. This increases the contact between the mixed gas and the arc, and allows the temperature of the mixed gas to be raised efficiently. For example, an opening can be made along the central axis of the electrode, the electrode itself can be made into a cylindrical object, and the mixed gas can be flowed through the opening. The diameter (inner diameter) of the opening can be appropriately set according to the flow rate of the mixed gas. For example, it is good to form an opening with a diameter of 0.1 to 0.5 times the electrode diameter. By doing this, the mixed gas passes through the generated arc, significantly improving the contact between the mixed gas and the arc.

[0038] Alternatively, for example, a cylindrical mixed gas supply device may be positioned between the electrode and the object being heated, with the mixed gas discharged perpendicular to the arc flow (electrode axis). Discharging the mixed gas toward the arc can increase the amount of mixed gas in contact with the arc. However, this would result in a structure where the cylindrical mixed gas supply device is inserted perpendicular to the electrode into the arc furnace, complicating the device structure. It is necessary to check whether such a structure is feasible based on the material being heated and the operating conditions of the arc furnace, and then design the device accordingly.

[0039] The material and shape of the cylindrical mixed gas supply object are not particularly limited. Since the temperature inside the arc furnace reaches about 1500°C due to the radiant heat from the arc and the radiant heat from the molten iron, it is desirable to have a structure that can withstand exposure to this high temperature. For example, it may be made of the same material as the electrode (graphite). Alternatively, it may be made of metal with an internal cooling structure and refractory material on the exterior. A lance structure used in steel converters may also be applied. The cooling structure is also not particularly limited. A double-tube structure with a water-cooled structure may be used. The cross-sectional shape may be circular, triangular, square, or polygonal. More preferably, a cylindrical shape that covers the electrode is recommended. Even when it also serves as an electrode, a cylindrical shape is preferable as it ensures a mixed gas flow path inside.

[0040] There are no particular limitations on the number of mixed gas supply tubular devices. For example, in a direct arc heating system with a three-phase system, a mixed gas supply tubular device may be placed for each electrode (three devices may be placed). Depending on the amount of carbon dioxide to be processed, a mixed gas supply tubular device may be placed only for a specific electrode. In these cases, it may be a standard cylindrical mixed gas supply device that encloses the electrode inside, or it may be a mixed gas supply tubular device with an integrated electrode.

[0041] When the temperature of the mixed gas in contact with the arc rises to approximately 1500°C or higher, the carbon dioxide in the mixed gas decomposes. The higher the temperature of the carbon dioxide-containing gas, the more the decomposition of carbon dioxide is accelerated. In addition, carbon that does not react with the oxygen produced by the decomposition of carbon dioxide falls onto the metal material being heated in the arc furnace. This fallen carbon powder reacts with the oxygen in the metal in the arc furnace to produce carbon monoxide, while carbon powder that does not react and is carried into the exhaust gas is removed by a bag filter installed before the exhaust gas treatment device.

[0042] [Carbon dioxide treatment device using an arc furnace] Figure 1 illustrates a carbon dioxide treatment apparatus 1 using an arc furnace 6, which is an example of one embodiment of the present invention. The carbon dioxide treatment apparatus 1 consists of an arc furnace 6 as a melting furnace, a mixed gas supply device 30 of carbon dioxide-containing gas and carbon powder, and an exhaust gas treatment apparatus 20.

[0043] [arc furnace] Inside the arc furnace 6, electrodes 4 and a cylindrical mixed gas supply object 3 are arranged. An arc is generated between the object to be heated (not shown) charged into the arc furnace and the electrodes 4, and the heat from this arc melts the object to be heated into molten metal 7. As mentioned above, the type of arc furnace is not particularly limited. However, in the case of an open-system arc furnace, the generated carbon monoxide burns with oxygen in the air flowing in from the atmosphere to produce carbon dioxide, so it is desirable to have a closed system that prevents air from flowing in from the atmosphere.

[0044] [Mixed gas supply device] The mixed gas supply device 30 supplies carbon dioxide-containing gas 33 from a carbon dioxide storage tank 31 (for example, a tank for temporarily storing carbon dioxide transported from another location), mixes it with pre-prepared carbon powder 32 in a mixer 34, and obtains a mixed gas 35 in which carbon powder 32 is mixed with carbon dioxide-containing gas 33. This mixed gas 35 is supplied into the arc furnace 6 through the mixed gas supply cylindrical object 3 of the arc furnace 6 so as to come into contact with the arc 5.

[0045] [Exhaust gas treatment equipment] As mentioned above, when the mixed gas comes into contact with the arc 5, carbon dioxide is decomposed into carbon monoxide and oxygen, and the decomposition products, oxygen and carbon (carbon powder), react to produce carbon monoxide. The carbon monoxide produced by this reaction in the arc furnace 6, along with undecomposed carbon dioxide, unreacted oxygen and carbon powder, are discharged from the arc furnace as exhaust gas 10. Although exhaust gas 10 mainly consists of carbon monoxide, it is desirable to separate and recover the undecomposed and unreacted components for reuse. From the viewpoint of carbon dioxide decomposition treatment, the decomposition efficiency of carbon dioxide can be increased by installing at least a carbon dioxide recovery device 24 and returning the recovered carbon dioxide to the mixed gas as described later. For this reason, it is desirable to arrange at least a carbon dioxide recovery device 24 and one or more types of, for example, a carbon powder recovery device 21, an oxygen recovery device 22, and a carbon monoxide recovery device 23 in series as the exhaust gas treatment device 20.

[0046] [Carbon powder recovery device] The carbon powder recovery device 21 separates and recovers carbon powder from the exhaust gas 10. Since carbon powder is not only reusable as a carbon source but is also a combustible substance, it is desirable to recover it from the exhaust gas. The recovered carbon powder is sent to the mixed gas supply device 30 and can be reused as carbon powder 32, which is then mixed again into the mixed gas 35. Of course, it can also be used for other purposes as carbon powder.

[0047] [Carbon monoxide recovery device] The carbon monoxide recovery device 22 separates and recovers carbon monoxide from the exhaust gas 10. Since carbon monoxide is a harmful substance to living organisms, it is advisable to remove and separate it. Although carbon monoxide is a harmful substance, it can be reused as a reducing agent or fuel. For example, the recovered carbon monoxide 12 may be supplied through the furnace nozzle 9 as a stirring gas for the molten metal 7 in the arc furnace. It can also be reused as a chemical raw material or energy source. However, when carbon monoxide is reused as fuel, carbon dioxide is produced when it is burned, but the produced carbon dioxide may be injected back into the arc furnace and decomposed by the arc. For this reason, it is advisable to equip the exhaust gas treatment device with a carbon monoxide recovery device.

[0048] The mechanism for separating carbon monoxide in a carbon monoxide recovery device is not particularly limited, and any known mechanism (hereinafter referred to as "separation mechanism") can be employed. Examples include cryogenic separation, pressure fluctuation adsorption (PSA) method, membrane separation method, and activated carbon adsorption method.

[0049] The separated carbon monoxide can be supplied to the arc furnace and used for stirring to improve the heat transfer properties of the molten metal. In arc furnaces, it is known that carbon is added to the molten metal to stir it with carbon monoxide to improve its heat transfer properties, but the carbon monoxide gas recovered by the carbon monoxide recovery device can be used as a substitute for this stirring gas. In other words, it is possible to reduce the amount of carbon added to the molten metal in the arc furnace, which is preferable.

[0050] As an example of a specific mechanism, carbon monoxide separated and recovered by a carbon monoxide recovery device is supplied into the arc furnace through piping via a carbon monoxide supply nozzle. An example of the shape of the carbon monoxide supply nozzle is a CO supply cylindrical object (lance type). Carbon monoxide is supplied from the lower or bottom of the arc furnace and is supplied into the molten metal located in the lower or bottom of the arc furnace.

[0051] Furthermore, the separated and recovered carbon monoxide can be used as a chemical raw material or converted into hydrocarbon combustion. Examples of chemical raw materials include formic acid and methanol. An example of conversion into hydrocarbon combustion is the Fischer-Trobusch process. These uses may be carried out individually or in combination.

[0052] [Oxygen recovery device] The oxygen recovery device 23 separates and recovers oxygen from the exhaust gas 10. The exhaust gas 10 also contains O2 (oxygen) which is produced when carbon dioxide decomposes but does not react with carbon powder. By separating the oxygen, it can be effectively reused. Therefore, it is advisable to equip the exhaust gas treatment system with an oxygen recovery device. For example, the recovered oxygen 13 may be supplied through the furnace nozzle 8 to adjust the composition of the molten metal 7 in the arc furnace.

[0053] The mechanism for separating oxygen in the oxygen recovery device is not particularly limited, and any known mechanism (hereinafter referred to as "separation mechanism") can be employed. Examples include cryogenic separation, pressure fluctuation adsorption (PSA) method, membrane separation method, and activated carbon adsorption method.

[0054] The separated and recovered oxygen can be used, for example, to supply to an arc furnace to oxidize impurities in the molten metal, increasing its vapor pressure and allowing the oxidized impurities to be released as gas. In addition, it can be used for stirring to improve the heat transfer properties of the molten metal.

[0055] As an example of a specific mechanism, separated oxygen is supplied into the arc furnace from an oxygen supply nozzle. An example of the shape of the oxygen supply nozzle is an O2 supply cylindrical object (lance type). Oxygen is supplied from the lower part or bottom of the arc furnace and blown into contact with the molten metal located at the lower part or bottom of the arc furnace.

[0056] [Carbon dioxide capture device] The carbon dioxide recovery device 24 separates and recovers carbon dioxide from the exhaust gas 10. The exhaust gas also contains undecomposed carbon dioxide. To reduce and eliminate atmospheric emissions of carbon dioxide, it is advisable to separate and recover the carbon dioxide as well. The recovered carbon dioxide can be supplied to the carbon dioxide storage tank 31 and reused as carbon dioxide-containing gas 33 supplied to the arc furnace 6. It is then decomposed again by the arc. By circulating the undecomposed carbon dioxide in this way, it is possible to contribute to the reduction of carbon dioxide emissions. Therefore, it is advisable to equip the exhaust gas treatment device 20 with at least a carbon dioxide recovery device 24.

[0057] The mechanism for separating carbon dioxide in a carbon dioxide capture device is not particularly limited, and known mechanisms can be employed. Examples include chemical absorption using amines, cryogenic separation, pressure fluctuation adsorption (PSA), membrane separation, and activated carbon adsorption.

[0058] The separated and recovered carbon dioxide can be supplied back to the arc furnace and brought into contact with the arc to decompose into carbon monoxide and oxygen. In other words, by bringing unreacted carbon dioxide back into contact with the arc, the decomposition rate of carbon dioxide in the arc furnace can be improved.

[0059] As an example of a specific mechanism, the separated and recovered carbon dioxide is returned to a mixed gas supply cylinder and supplied into the arc furnace. The mixed gas supply cylinder for supplying the separated and recovered carbon dioxide may be the same as the mixed gas supply cylinder that initially supplied the mixed gas, or it may be a different cylinder.

[0060] As mentioned above, the order in which the carbon powder, carbon monoxide, oxygen, and carbon dioxide recovery devices are arranged is not limited. [Examples]

[0061] The effectiveness of the present invention was confirmed through tests using a small DC arc furnace. An example of this is described below. The arc furnace used was a 2L arc-type test furnace with a mirror-finish lower section made of SUS316L, internally lined with refractory material, and equipped with a cooling jacket. The metal to be molten was steel (scrap material), and it was charged into the arc furnace up to one-third of its total height from the bottom. A 5cm diameter graphite electrode was used, inserted from the top of the arc furnace and positioned 1cm above the top of the steel material.

[0062] A cylindrical container made of graphite with an inner diameter of 7 cm was used to supply the mixed gas containing carbon dioxide and carbon powder. The mixed gas supply container was positioned above the arc furnace, surrounding the outside of the electrodes (with a 1 cm gap between the mixed gas supply container and the electrodes), and with a gap between it and the steel material (see Figure 2). Furthermore, an electrode was prepared that also served as a mixed gas supply tube, with a hollow graphite electrode measuring 7 cm in outer diameter and 3 cm in inner diameter, and a central open tubular section serving as a flow path for the mixed gas (see Figure 3). Each electrode, equipped with one of these mixed gas supply tubular structures, was individually positioned in an arc furnace, and tests were conducted by actually flowing a mixed gas of carbon dioxide-containing gas and carbon powder through them.

[0063] The gas outlet was installed at the top of the arc furnace, and a 1 / 4-inch diameter pipe made of SUS316L was installed from the gas outlet, with a carbon powder recovery device, oxygen recovery device, carbon monoxide recovery device, and carbon dioxide recovery device arranged in that order.

[0064] A 1L (liter) SUS316L container with a built-in separation membrane (such as a bag filter) was installed as a carbon powder recovery device. The carbon powder recovered by the carbon powder recovery device is further connected to a cylindrical mixed gas supply device via piping.

[0065] A 1-liter (1L) SUS316L container with a built-in separation membrane was installed as an oxygen recovery device. The oxygen recovered by the oxygen recovery device was further connected via piping to an oxygen supply nozzle provided in the arc furnace.

[0066] A 1-liter (1L) SUS316L container with a built-in separation membrane was installed as a carbon monoxide recovery device. The carbon monoxide recovered by the carbon monoxide recovery device is further connected to a carbon monoxide supply nozzle provided in the arc furnace via piping.

[0067] A 1-liter (1L) SUS316L container with a built-in separation membrane was installed as a carbon dioxide capture device. The carbon dioxide capture device is equipped with an exhaust gas outlet. The carbon dioxide captured by the capture device is further connected to a mixed gas supply cylinder via piping.

[0068] In the test apparatus constructed in this manner, an arc was generated using a graphite electrode as the anode and steel as the cathode, and the arc was applied for 2 minutes after the entire amount of steel had melted. At that time, the gas mixture was adjusted to contain 50 mol% carbon dioxide and 50 mol% carbon powder (particle size 1 mm or less), and the supply rate of the gas mixture was adjusted to 0.1 L / min. The temperature inside the arc furnace at this time was approximately 1500°C, and the furnace pressure was atmospheric pressure.

[0069] The exhaust gas from the arc furnace outlet and the exhaust gas after the carbon dioxide recovery device were recovered and their composition was analyzed by gas chromatography to measure the amount of carbon dioxide in the exhaust gas. The carbon mass balance was calculated from the amount of carbon dioxide in the input mixed gas, the weight of carbon powder, and the amount of carbon monoxide in the recovered exhaust gas, and the decomposition rate of the supplied carbon dioxide was evaluated. As a result, it was confirmed that in both the case of a cylindrical mixed gas supply structure surrounding the electrode (see Figure 2) and the case of a hollow electrode that also serves as a cylindrical mixed gas supply structure (see Figure 3), 5-20% of the input carbon dioxide was decomposed when carbon dioxide in the exhaust gas was not recovered or reused. Furthermore, when carbon dioxide in the exhaust gas was recovered and reused, similar results were obtained for both electrodes, and it was confirmed that 50-80% of the supplied carbon dioxide (CO2) was decomposed into CO (carbon monoxide) once a stable region was reached. [Industrial applicability]

[0070] This invention can be used in arc furnaces. That is, it can be used in industries that utilize arc furnaces, such as the iron and steel industry and the metal industry (aluminum, copper, etc.). [Explanation of Symbols]

[0071] 1 Arc Furnace 2. Carbon dioxide gas (CO2 gas) and carbon powder 3. Mixed gas supply cylindrical object 4 Graphite electrodes 5 Arc 6. Arc Furnace Body 7. Molten metal 8. Carbon monoxide supply nozzle 9. Oxygen supply nozzle 10 Arc furnace exhaust gas 11. Carbon powder 12. Oxygen gas (recovered gas) 13. Carbon monoxide gas (recovered gas) 14. Carbon dioxide gas (recovered gas) 20. Exhaust gas recovery system (exhaust gas recovery system) 21 Carbon powder recovery device 22. Oxygen recovery device 23. Carbon monoxide recovery system 24 Carbon dioxide capture device 30. Carbon powder mixing device (carbon powder mixing device) 31 Carbon dioxide storage devices 32 Carbon powder 33. Carbon dioxide-containing gases 34 Mixing equipment 35 Mixed gas

Claims

1. An arc furnace for melting metals, An arc furnace characterized by comprising a cylindrical mixed gas supply object that supplies a mixed gas, which is a mixture of carbon dioxide and carbon powder, into the arc furnace, wherein the cylindrical mixed gas supply object is positioned such that the mixed gas discharged from the outlet of the cylindrical mixed gas supply object comes into contact with the arc inside the arc furnace.

2. The arc furnace has an exhaust gas treatment device for treating the exhaust gas discharged from the arc furnace, The exhaust gas treatment device has at least a carbon dioxide recovery device, The arc furnace according to claim 1, configured to supply carbon dioxide recovered by the carbon dioxide recovery device to the mixed gas.

3. The arc furnace according to claim 2, wherein the exhaust gas treatment device further comprises one or more of the following: a carbon powder recovery device, an oxygen recovery device, and a carbon monoxide recovery device.

4. The arc furnace according to any one of claims 1 to 3, wherein the cylindrical mixed gas supply object is arranged to surround the electrodes of the arc furnace, and the discharge port is arranged so that the mixed gas is discharged from around the end face of the electrodes in the direction of the central axis of the electrodes.

5. The arc furnace according to any one of claims 1 to 3, wherein the cylindrical mixed gas supply object is an electrode of the arc furnace, a flow path for the mixed gas is arranged inside the electrode, and a discharge port for the mixed gas is arranged from the end face of the electrode along the central axis of the electrode.

6. Using an arc furnace to melt metal, A method for treating carbon dioxide using an arc furnace, characterized by introducing a mixed gas, which is a mixture of carbon powder and a gas containing carbon dioxide, into the arc furnace through a cylindrical mixed gas supply device, and bringing the mixed gas into contact with an arc in the arc furnace.

7. At least carbon dioxide is separated from the exhaust gas discharged from the arc furnace, A method for treating carbon dioxide using an arc furnace according to claim 6, wherein the separated carbon dioxide is supplied to the mixed gas introduced into the arc furnace.

8. A method for treating carbon dioxide using an arc furnace according to claim 7, further comprising separating one or more of carbon powder, oxygen, and carbon monoxide from the exhaust gas.

9. A method for treating carbon dioxide using an arc furnace according to any one of claims 6 to 8, wherein the cylindrical mixed gas supply object is arranged to surround the electrodes of the arc furnace and the mixed gas is discharged in the direction of the central axis of the electrodes.

10. A method for treating carbon dioxide using an arc furnace according to any one of claims 6 to 8, wherein the cylindrical mixed gas supply object is an electrode of the arc furnace, a flow path for the mixed gas is arranged along the central axis of the electrode, and the mixed gas is discharged in the direction of the central axis of the electrode.

11. The arc furnace according to any one of claims 1 to 3, wherein the metal is an iron alloy.

12. A method for treating carbon dioxide using an arc furnace according to any one of claims 6 to 8, wherein the metal is an iron alloy.

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