Method and device for treating primary gas from a metallurgical vessel

Abstract

The invention relates to a method and a device for treating primary gas from a metallurgical vessel in order to effectively reduce NOx constituents. For this purpose, the primary gas is supplied with a reducing agent. Additionally, secondary gas is suctioned at an auxiliary suction point in the surroundings of the metallurgical vessel, and the primary gas is mixed with the colder secondary gas in order to produce a gas mixture. The aim of the invention is to allow the NOx constituents in the primary gas to be effectively reduced without the necessary measures for this purpose having an undesired reactive influence on the process within the metallurgical furnace and without requiring high investment costs for this purpose. According to the invention, this is achieved in that the primary gas is mixed with the secondary gas prior to supplying the reducing agent, namely such that the gas mixture is cooled to a temperature ranging from 400 °C to 600 °C prior to supplying the reducing agent, and the reducing agent is then supplied to the gas mixture, containing the primary gas.

Description

[0001] Method and apparatus for treating primary gas from a metallurgical vessel

[0002] The invention relates to a method and apparatus for treating primary gas from a metallurgical vessel for the effective reduction of NOx components. The metallurgical vessel according to the invention is, for example, an electric arc furnace, a reduction furnace, or an industrial furnace, each of which is suitable for the production of steel, iron alloys, or non-ferrous metal alloys.

[0003] In the prior art, it is common practice to first cool so-called primary gas, which is generated during the operation of a metallurgical vessel, especially a metallurgical furnace, and then mix it with so-called secondary gas, which is extracted from the surroundings of the metallurgical vessel, for example, via a roof hood, to form a gas mixture. The resulting gas mixture is then fed to a fabric filter, particularly to remove dust particles. The gas mixture is tested for its NOx content. If the NOx content is too high, the following methods for reducing it are known:

[0004] 1. It is possible to reduce the formation of NOx within the furnace, e.g., by using coal with a low nitrogen content, by minimizing the intake of ambient air into the furnace, or by covering the arc with foamed slag, etc. These measures do serve to reduce the NOx content, but not to the extent that is currently desired.

[0005] 2. SNCR (selective non-catalytic reduction): Injecting ammonia (NH3) or urea at a predefined temperature into an afterburner chamber to treat the primary gas. Typically, however, the temperature in the afterburner chamber is too low for effective NOx reduction. Reheating would then be required, which in turn would increase costs and result in additional undesirable CO2 emissions.

[0006] 3. SCR (Selective Catalytic Reduction): It is possible to retrofit a catalyst behind the fabric filter. However, the temperature there is often too low for a catalytic reaction. This would also require additional heating, with the undesirable side effects mentioned above. Added to this, of course, would be the investment costs for the catalyst.

[0007] International patent application WO 2018 / 104169 A1 discloses a method according to the preamble of patent claim 1 and a device according to the preamble of device claim 14. Specifically, the application discloses a metallurgical vessel in the form of an electric arc furnace that generates primary gas during operation. The primary gas is fed into an exhaust gas line via a manifold and a manifold gap for drawing in oxygen from the environment. Within the manifold, i.e., immediately after leaving the metallurgical vessel, urea is injected into the primary gas before the urea-enriched primary gas passes through an exhaust gas heater and is then enriched with said oxygen. The exhaust gas heater is designed to heat the primary gas stream with the injected urea to a temperature between 400°C and 1,000°C, preferably between 600°C and 800°C.

[0008] The treated exhaust gas is fed via the exhaust line to a coarse separator before being treated with water. After the water is added, the treated primary gas is mixed with secondary gas drawn from the environment of the metallurgical vessel. The resulting gas mixture is then filtered before being discharged into the environment.

[0009] In the process disclosed in international application WO 2018 / 104169 A1, the urea injection and the heating of the exhaust gas, as mentioned, take place in the immediate vicinity of the metallurgical vessel. This can result in the disadvantage that these measures can have undesirable effects on the process in the metallurgical vessel. Furthermore, the provision of the exhaust gas heating system is associated with high costs.

[0010] The invention is based on the object of developing a known method and a known device for reducing nitrogen oxides in the primary gas of a metallurgical vessel, in particular a metallurgical furnace, in such a way that the NOx components can be reduced without the necessary measures having an undesirable influence on the process within the metallurgical furnace and without high investment costs being required.

[0011] This object is achieved in terms of process technology by the method claimed in patent claim 1. This method is characterized in that the mixing of the primary gas to form the gas mixture takes place in a controlled manner only with a branched-off portion of the secondary gas until the gas mixture has cooled to a temperature within the temperature range of 400°C to 600°C; and in that the reducing agent is added to the gas mixture containing the primary gas.

[0012] The present invention is based on the assumption that the primary gas, after exiting the metallurgical vessel, initially has a temperature too high to effectively reduce the nitrogen oxides. Therefore, the claimed cooling is provided according to the inventive method. This cooling is achieved particularly cost-effectively, namely by simply mixing the still-hot primary gas with the significantly cooler secondary gas drawn in from the environment. The secondary gas is supplied until the resulting gas mixture has a temperature within the claimed temperature range of 400°C to 600°C.

[0013] The term "claimed temperature from the temperature range..." can mean, on the one hand, that the temperature of the gas mixture is controlled (without feedback of the measured variable) or regulated (with feedback of the measured variable) to a specific temperature from the temperature range. On the other hand, this term can mean that the temperature of the gas mixture is allowed to fluctuate within the temperature range and that countermeasures are only taken if the lower range limit is undershot or the upper range limit is exceeded, in the sense of a two-point control.

[0014] The abbreviation Nox means nitrogen oxide.

[0015] The claimed mixing of the primary gas with the secondary gas can advantageously be realized particularly cost-effectively by simply implementing the necessary mixing device in the form of a junction of the exhaust line for the primary gas and the exhaust line for the branched portion of the secondary gas. A simple, suitably controlled bypass valve can preferably serve as the necessary actuator. According to the invention, the actuator is controlled in such a way that the portion of the sucked-in secondary gas that is fed to the primary gas is metered in such a way that the gas mixture has the desired temperature within the temperature range of 400°C to 600°C.

[0016] According to the invention, the injection of the reducing agent to reduce the nitrogen oxide content does not take place in the pure primary gas, but in the gas mixture of primary gas and secondary gas produced according to the invention.

[0017] The supply of the reducing agent to the gas mixture according to the invention takes place at such a great distance from the metallurgical vessel that any effect on the processes taking place in the metallurgical vessel can be excluded.

[0018] According to a first embodiment of the method according to the invention, the supply of the secondary gas to the warmer primary gas can be carried out either in the form of a control or in the form of a regulation. In a control process, the supplied partial quantity of the sucked-in secondary gas is adjusted so that the temperature of the gas mixture lies within the required temperature range. The control process does not provide feedback of the actual temperature of the gas mixture for monitoring purposes. This differs from a regulation process in which the actual temperature of the gas mixture is preferably measured continuously during an ongoing metallurgical process and compared with a predetermined target temperature from the temperature range of 400°C to 600°C.If an inadmissibly large deviation of the actual temperature from the desired target temperature is detected, the first partial quantity of the sucked-in secondary gas supplied to the gas mixture is varied accordingly.

[0019] Alternatively or additionally, the amount of primary gas supplied could also be varied using a primary gas control valve; however, this could potentially have an adverse effect on the process in the metallurgical vessel.

[0020] According to a further embodiment of the invention, the primary gas is first post-combusted in an afterburner chamber after leaving the metallurgical vessel but before mixing with the secondary gas, advantageously reducing, in particular, the carbon content. Especially when such post-combustion occurs, the temperature of the primary gas rises significantly above the temperature required for effective nitrogen oxide reduction by adding the reducing agent. In this case, the claimed cooling is necessary by admixing portions of the secondary gas with the primary gas.

[0021] However, particularly in the case of post-combustion, the temperature of the primary gas may be so high that cooling by admixing the secondary gas alone is insufficient. In this case, the post-combusted primary gas is pre-cooled in a cooling chamber to a temperature of 850°C to 750°C before being mixed with the secondary gas.

[0022] A further advantage of the claimed mixing of the primary gas with the secondary gas is that the negative pressure of the primary gas, which is higher than the negative pressure of the secondary gas, is equalized, and the resulting negative pressure of the gas mixture is lower than the negative pressure of the primary gas before mixing. This offers the advantage that the filters and blowers subsequently provided for treating the gas mixture can be designed accordingly for lower pressures and are therefore more cost-effective than devices designed for higher pressures.

[0023] Before mixing with the primary gas, the secondary gas typically has a temperature of 50°C to 80°C. Compared to the temperature of the primary gas before mixing, this secondary gas temperature is comparatively low. This has the advantage that the claimed mixing allows for effective cooling of the primary gas or the gas mixture resulting from the mixing to the desired temperature range.

[0024] According to a further embodiment, a hot gas cyclone can be provided between the cooling chamber and the claimed mixing device for mixing the primary gas with the secondary gas. The provision of the hot gas cyclone offers the advantage of cleaning the primary gas of coarse dust particles and simultaneously reducing the pressure of the primary gas.

[0025] To accelerate the reduction of nitrogen oxides by the added reducing agent, it is advantageous to pass the gas enriched with the reducing agent through a hot gas filter. The hot gas filter contains catalyst-coated plugs, with the coating acting as a catalyst for the desired NOx reduction and the plugs acting as dust collectors.

[0026] Finally, the purified and NOx-reduced gas mixture is drawn in at the outlet of the hot gas filter device by means of an induced draft fan and discharged through a chimney into the environment. Due to the reduced temperature and pressure of the gas mixture, the induced draft fan can be designed comparatively cost-effectively. The method according to the invention advantageously provides that the primary gas and the secondary gas are generally treated separately, in particular purified, before these gases are released into the environment via the chimney. Specifically, according to a further exemplary embodiment, the method according to the invention provides that a further portion of the sucked-in secondary gas not mixed with the primary gas is purified, in particular dust-reduced, in a filtering separator before it is discharged through the chimney.The filtering separator for the secondary gas can be designed more cost-effectively than the hot gas separator for the gas mixture because the pressure and temperature of the secondary gas to be cleaned are significantly lower than the pressure and temperature of the gas mixture to be cleaned by the hot gas filter device.

[0027] In both filters, ie in the filtering separator for the secondary gas as well as in the hot gas filter device for the gas mixture, activated carbon can advantageously be used to remove uranium and dioxins from the gases.

[0028] According to a further embodiment, at least a portion of the further subset of the purified, in particular dedusted, secondary gas is fed to the NOx-reduced and dedusted gas mixture, thus generating a residual gas mixture that has an even lower temperature and pressure than the gas mixture after leaving the hot gas filter. The lower pressure and temperature of the residual gas mixture, in turn, allow for a more cost-effective design of the induced draft fan.

[0029] This cooling of the gas mixture can also be achieved by means of a control or regulation system, as described above. A cost-effective additional bypass valve can also be used as the actuator.

[0030] In the process according to the invention, urea or ammonia is used in particular as a reducing agent to reduce the nitrogen oxide content in the primary gas. The above-mentioned object is further achieved by the device according to claim 14. The advantages of this device and the embodiments discussed in the further dependent device claims correspond to the advantages mentioned above with reference to the claimed process.

[0031] The invention is accompanied by a single figure which illustrates the device according to the invention.

[0032] The invention is described in detail below with reference to this figure with various embodiments.

[0033] Figure 1 illustrates the device according to the invention. It comprises all components arranged between a metallurgical vessel 1, in particular a metallurgical furnace, and a chimney 13. The metallurgical vessel 1 and the chimney 13 themselves are not part of the device according to the invention.

[0034] The device comprises an afterburner chamber 2 downstream of the metallurgical vessel 1, which may also provide a means of utilizing the waste heat generated therein. A cooling device 2.1 may be provided within or downstream of the afterburner chamber 2 for pre-cooling the primary gas to a temperature of typically 850°C to 750°C. Optionally, a hot gas cyclone 3 is connected downstream of the cooling chamber 2.1 for separating dust particles and reducing the pressure in the primary gas. A first actuator 4, for example, a primary gas control valve for regulating the volume flow of the primary gas in the exhaust line a for the primary gas, is connected downstream of the hot cyclone.

[0035] Independent of the primary gas branch a just described, there is a secondary gas branch x, which begins at a secondary extraction point 8, for example a roof hood, with the help of which air - and thus oxygen - is extracted from the environment of the metallurgical vessel 1. This air is also referred to as secondary gas. The extraction takes place with the help of a secondary gas induced draft fan 10. Between the secondary extraction point 8 and the secondary gas induced draft fan 10, a filtering separator 9 for dedusting the secondary gas is provided in a line c for a further portion of the sucked-in secondary gas. With the help of the induced draft fan 10, any remaining secondary gas is released into the environment via the chimney 13.

[0036] In the drawing, a first branch b, also called a bypass or line for a portion (of the amount) of the sucked-in secondary gas, is provided for connecting the secondary gas line x to the primary gas line a. A first actuator 11, for example in the form of a first bypass flap, is installed in this first branch b. Where the first branch b and the primary gas line a meet, the required mixing of the primary gas and the branched-off portion of the secondary gas takes place. In this respect, this junction point is also referred to as a mixing device 14 within the meaning of the invention. A gas mixture is created from the said mixing of the branched-off portion of the sucked-in colder secondary gas with the warmer primary gas.The branched-off portion of the secondary gas is adjusted or regulated by means of the first actuator 11, which is controlled by a controller 18, such that the gas mixture resulting from the mixing in the line g has a temperature from the claimed temperature range of 400°C to 600°C.

[0037] The reducing agent is first added to this cooled gas mixture by means of a feed device 5 in order to reduce the nitrogen oxide content in the gas mixture. At the time the reducing agent is added, the gas mixture, as claimed, has a temperature between 400°C and 600°C. The gas mixture enriched with the reducing agent is then fed through line g to a hot gas filter device 6 with catalytically coated candles, in particular to accelerate NOx reduction and to separate dust from the gas mixture. After this filtering and catalysis of the gas mixture, it is drawn in through a line h by an induced draft fan 7.In order to further reduce the temperature of the gas mixture and preferably its pressure before it is released into the environment, a second branch, also called line e for a branched-off part of the further part of the sucked-in secondary gas, is optionally provided, which connects the secondary gas line d behind the filtering separator 9 with the line h for the gas mixture behind the hot gas filter 6. The connecting node represents a further mixing device 17. The mixing that takes place there creates a residual gas mixture in the line i. In the second branch e, analogous to the first branch b, a further actuator 12, for example again in the form of a bypass flap, is arranged. This further actuator 12 is also advantageously set or controlled with the help of the controller 18.The temperature of the remaining gas mixture is cooled to below 200°C, preferably below 100°C; see reference symbol C in the figure. At this point, the pressure level can also drop again due to the supplied secondary gas. The low pressure level and low temperature allow the induced draft fan 7 to be designed for lower pressures and lower temperatures, thus making it cost-effective.

[0038] The remaining gas mixture is released into the environment via the chimney at the outlet of the induced draught fan 7.

[0039] The key points of the method according to the invention are shown again below:

[0040] - Reliable NOx reduction without additional heating of the primary gas and without installation of a heat exchanger.

[0041] - Cleaning, i.e., in particular, dedusting, is carried out separately for the primary gas and the secondary gas. This is advisable because the primary gas has a significantly higher dust load than the secondary gas. Accordingly, the filter devices provided for this purpose, i.e., the hot gas filter device 6 and the fabric filter, i.e., the filtering separator 9, can be used differently and in a cost-optimized manner. Furthermore, the use of activated carbon can significantly reduce the proportion of dust hazardous to health. Separate treatment of the filtered dust from the primary gas and the secondary gas is advisable because the dust in the primary gas has a high iron content.

[0042] Joint disposal of the dust, as is necessary when primary gas and secondary gas are mixed, makes little sense due to the high iron content in the dust of the primary gas.

[0043] - In the event that the catalytically coated ceramic filter elements or filter candles in the hot gas filter unit 6 and the fabric filter 9 are contaminated, for example, by high lead contamination, they can be advantageously replaced inexpensively. This is significantly more cost-effective than completely replacing the hot gas filter unit 6 or the fabric filter 9.

[0044] List of reference symbols

[0045] 1 metallurgical vessel

[0046] 2 afterburner chamber

[0047] 2.1 Cooling chamber

[0048] 3 hot gas cyclone

[0049] 4 Actuator

[0050] 5 Feed device for reducing agent

[0051] 6 Hot gas filter device

[0052] 7 induced draft fans

[0053] 8 Secondary extraction point, e.g., roof hood

[0054] 9 Fabric filter = filtering separator

[0055] 10 secondary gas induced draft fans

[0056] 11 , 12 Actuators, e.g. bypass valves

[0057] 13 Fireplace

[0058] 14 Mixing device, especially nodes in the gas pipeline network

[0059] 17 additional mixing devices

[0060] 18 Control a Line for primary gas = primary gas branch b Line for part of the sucked-in secondary gas c Line for further partial quantity of the sucked-in secondary gas d Line for purified part of the sucked-in secondary gas e Line for part of the further part of the sucked-in secondary gas f Line for remaining secondary gas g Line for gas mixture enriched with reducing agent h Line for purified gas mixture i Line for remaining gas mixture x Secondary gas line branch

[0061] A 400°C - 1,000

[0062] B 400°C - 600°C

[0063] C <100°C - 200°C

Claims

Patent claims:

1. Method for treating primary gas from a metallurgical vessel (1), comprising the following steps: Adding a reducing agent to at least the primary gas after it exits the vessel (1) to reduce NOx contained in the primary gas at a temperature of the primary gas in a temperature range above 400°C; Drawing in secondary gas at a secondary extraction point in the vicinity of the vessel (1), wherein the secondary gas is colder than the primary gas; and mixing the primary gas with the colder secondary gas to form a gas mixture; characterized in that the mixing of the primary gas to the gas mixture is carried out in a controlled manner only with a diverted portion of the secondary gas until the gas mixture has cooled to a temperature in the range of 400°C to 600°C; and that the reducing agent is added to the gas mixture containing the primary gas.

2. Method according to claim 1, characterized in that the mixing of the primary gas with the secondary gas to the desired temperature range is carried out in the form of a control or regulation, in that the partial quantity of secondary gas supplied to the primary gas as a cooling medium is set or varied by means of an actuator (11), e.g. a bypass valve, - in the case of regulation according to the difference between the temperature of the primary gas before mixing and the desired temperature range - such that the desired temperature is set for the gas mixture from the temperature range. A method according to claim 1 or 2, characterized in that the primary gas is first post-combusted in an afterburner chamber (2) after leaving the vessel (1) but before mixing with the secondary gas. A method according to claim 3, characterized in that the post-combusted primary gas is pre-cooled to a temperature of 850°C to 750°C in a cooling chamber (2.1) before mixing with the secondary gas. A method according to any one of the preceding claims, characterized in that the vacuum of the primary gas is greater than the vacuum of the secondary gas before mixing the primary gas with the secondary gas. A method according to any one of the preceding claims, characterized in that the secondary gas has a temperature of 50°C to 80°C before mixing with the primary gas.A method according to any one of claims 3 to 6, characterized in that the pre-cooled primary gas passes through a hot gas cyclone (3) for the separation of coarse dust particles and for reducing the pressure of the primary gas. A method according to any one of the preceding claims, characterized by. Accelerating the reduction by passing the [unclear text] with the Reducing agent enriched gas mixture from the primary gas and the Secondary gas at catalytically coated candles, wherein the coating acts as a catalyst and the candles as dust separators to clean the gas mixture; and The cleaned gas mixture, reduced by NOx, is drawn in using an induced draft fan (7) and the remaining gas mixture is discharged through a chimney (13).

9. Method according to one of the preceding claims, characterized in that a further subset of the aspirated secondary gas, not mixed with the primary gas, is cleaned in a filtering separator (9), in particular by reducing dust, and subsequently a remaining part of the aspirated secondary gas is also discharged through the chimney (13).

10. Method according to claim 8 or 9, characterized in that activated carbon is also used in the purification of the gas mixture and / or the further subset of the secondary gas for the removal of furans and dioxins.

11. Method according to one of claims 8 to 10, characterized in that at least a part of the further subset of the purified, in particular dedusted, secondary gas is fed to the NOx-reduced and dedusted gas mixture in order to further cool the remaining gas mixture thus formed before it enters the induced draft fan (7) and the chimney (13) and preferably also to further reduce its pressure.

12. Method according to claim 11, characterized in that the cooling of the purified and NOx-reduced gas mixture is carried out by mixing it with the purified secondary gas in the form of a Control or regulation by adjusting or varying the portion of the secondary gas supplied to the gas mixture as a cooling medium using a further actuator (12), e.g., a further bypass valve, such that a desired temperature from a further desired temperature range is established for the remaining gas mixture – in the case of regulation according to the difference between the temperature of the gas mixture before mixing and a target temperature range. A method according to one of the preceding claims, characterized in that the reducing agent is urea or ammonia. A device for treating at least primary gas from a metallurgical vessel (1), comprising: a feed device (5) for supplying a reducing agent to at least the primary gas for the reduction of NOx in the primary gas at a primary gas temperature above 400°C;a secondary extraction point (8) with a downstream secondary gas induced draft fan (10) for drawing in the secondary gas from the vicinity of the metallurgical vessel (1) and for discharging a residual quantity of the secondary gas to a chimney (13); and a mixing device (14) is provided for mixing the primary gas with the drawn-in secondary gas to form a gas mixture; characterized in that the mixing device (14) is located upstream of the supply device (5) for the reducing agent; that a control device (18) with an actuator (11) is provided for controlling the mixing of the primary gas with only a portion of the drawn-in colder secondary gas to cool the gas mixture until the gas mixture has a temperature in the range of 400°C to 600°C; and; that the supply device (5) is designed to supply the reducing agent to the gas mixture at this temperature.

15. Device according to claim 14, characterized by an afterburning chamber (2) upstream of the mixing device (14) for afterburning the primary gas from the metallurgical vessel (1).

16. Device according to claim 15, characterized by a cooling chamber (2.1) downstream of the afterburning chamber (2), preferably in the form of a heat exchanger, for pre-cooling the afterburned primary gas to a temperature of 850°C to 750°C.

17. Device according to claim 16, characterized preferably by a hot gas cyclone downstream of the cooling chamber (2.1). (3) for separating coarse dust particles from the primary gas upstream of the mixing device (14).

18. Device according to one of claims 14 to 17, characterized by a hot gas filter device (6) in the form of a catalyst with catalytically coated candles downstream of the supply device (5) for the reducing agent for separating dust from the gas mixture on the candles and for accelerating the NOx reduction in the gas mixture when the gas mixture flows around the catalytic coating of the candles.

19. Device according to one of claims 14 to 18, characterized by an induced draft fan (7) for drawing in at least the dedusted and NOx-reduced gas mixture and for discharging a remaining gas mixture through the chimney (13).

0. Device according to one of claims 14 to 19, characterized by a fabric filter (9) for separating dust from the further portion of the aspirated secondary gas. Device according to claim 20, characterized by a further mixing device (17) for mixing the gas mixture filtered by the hot gas filter device (6) with a portion of the purified further portion of the secondary gas; and a further actuator (12) connected to the control unit (18), for example in the form of a further bypass valve, for controlling the portion of the purified further portion of the secondary gas supplied to the purified gas mixture in such a way that the further gas mixture resulting from the supply is cooled to a desired temperature before it is discharged through the chimney (13).Device according to one of claims 14 to 21, characterized in that the metallurgical vessel (1) is, for example, an electric arc furnace, a reduction furnace or an induction furnace, each of which is suitable for the production of steel, ferrous alloys or non-ferrous metal alloys.

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