Utilisation of waste heat from slag

Abstract

The present invention relates to a device (10) for utilising the waste heat from slag (90) in a slag bed (20), wherein: the device (10) has a support structure (30) arranged next to the slag bed (20); the device (10) has a bearing structure (40) arranged on the support structure (30) and protruding over the slag bed (20); the device (10) has a planar heat exchanger oriented parallel to the surface; the planar heat exchanger is height-adjustable; the planar heat exchanger (50) is designed to absorb the radiant heat from the slag (90).

Description

[0001] Utilization of waste heat from slag

[0002] The invention relates to the use of waste heat from slag.

[0003] Slag is produced during metallurgical processes, such as pig iron production in blast furnaces, steel production in electric arc furnaces, LD converters, or DRI reduction furnaces (SAFs) / open slag bath furnaces (OSBFs), but also in other processes, particularly metalworking processes, for example, in foundries and non-ferrous metallurgy. Slag is generally a byproduct of the process, which can be used, for example, as an aggregate or mineral in other processes or products. Slag is typically formed at the high temperatures of the process, such as in blast furnaces or steel mills. Due to the high process temperature, usually exceeding 1,450 °C, the thermal energy of the slag is often very high in the processes that generate it, typically in the range of 1.5 to 2 GJ per ton of slag.Nevertheless, slag is nowadays mainly dumped into open slag heaps, where it is then cooled slowly by means of air and / or water cooling without any heat utilization or energy recovery. For further use, the cooled slag is usually crushed, freed of iron, and generally sieved and / or classified.

[0004] At the thyssenkrupp Steel Europe AG site in northern Duisburg alone, approximately 1.085 million tons of LD slag are produced annually during steel production. The slag is typically dumped at a temperature exceeding 1,450 °C. This corresponds to roughly 480 GWh of thermal energy, which is released unused into the environment at this single site every year. If this heat could be utilized, for example, by eliminating the need to burn natural gas for heat generation, an estimated 96,000 liters of CO2 could be saved annually at this site alone. Worldwide, the liquid blast furnace slag is predominantly transported to slag heaps in open slag buckets by rail or slag pot carrier and dumped there. The molten slag cools down and releases its heat unused into the environment. The liquid cools to its solidification point, solidifies, and changes into a solid state.The solid slag then cools down to approximately ambient temperature.

[0005] There are approaches that use the high temperature of the slag during cooling to heat the air and then use this air as a drying agent in further processes. However, the demand for and potential applications of hot air for drying purposes are limited, and the space required for such a drying system is very high.

[0006] Since the thermal energy is generated at a very high level—for example, steel slag is molten and usually poured into the slag heap at temperatures exceeding 1,450 °C—conversion into the highest possible energy form, such as its use as a high-quality heat source or for power generation, would be desirable. However, even a different form of utilization can lead to energy savings elsewhere.

[0007] The use of waste heat during the cooling of slag from iron and steel production is known from DE 10 2022 204 808 A1.

[0008] From WO 2013 / 186 664 A1 a device for the recovery of heat and exhaust gases from slag from the steel production cycle is known.

[0009] Heat recovery from steel slag is known from CN 117 897 505 A.

[0010] A device and a method for cooling slag are known from KR 2014 0003912 A.

[0011] A slag compression plant is known from US 5 397 104 A.

[0012] German patent DE 36 26 772 A1 discloses a method and a device for producing road construction materials and recovering heat from metallurgical slag. However, a problem with this process is that the slag is produced only discontinuously. Therefore, it cannot be directly combined with a continuous process. Another problem is that the slag is very hot at the beginning of the cooling process, but the temperature decreases exponentially towards the end. This results in rapid cooling at high temperatures and slow cooling at low temperatures. These extreme temperature fluctuations not only lead to complications in utilizing the heat but also represent a high mechanical stress due to the strong temperature changes.

[0013] Furthermore, it has not yet been resolved how a continuous energy release - for example at a constant operating temperature - is implemented when the temperature of the cooling slag decreases.

[0014] The object of the invention is to introduce an easily implementable concept for heat utilization in order to make at least a certain proportion of the heat usable in a simple way.

[0015] This problem is solved by the device with the features specified in claim 1 and by the method with the features specified in claim 13. Advantageous embodiments are described in the dependent claims, the following description, and the drawings.

[0016] The device according to the invention serves to utilize the waste heat from slag in a slag bed. The source of the slag can vary considerably. It can be, for example, blast furnace slag, cupola furnace slag, steelworks slag, in particular converter slag, electric arc furnace slag, slag from a DRI melt reduction furnace or an open slag bath furnace, stainless steel slag, secondary metallurgical slag, or metallurgical slag. The direct integration of the device onto or above a slag bed means that existing processes do not need to be modified. Furthermore, the slag is solidified in a slag bed as before and can be processed further as before. This eliminates the immediate problem of removing the cooled and solidified slag. The device has a support structure arranged on one side next to the slag bed.A supporting structure can be a pillar, an arch, or the like. The supporting structure serves to stably mount the device and transfer the forces into the ground. The supporting structure is more or less vertical, or at least designed to transfer vertical forces, particularly the weight of the device. A triangular structure, for example, which itself has no vertical component but can still transfer vertical forces via its two legs, falls under the category of more or less vertical. The same applies to arches or corbelled vaults. Therefore, for the purposes of the invention, the Eiffel Tower could also be considered a vertical supporting structure. Such a structure can be found, for example, in high-voltage pylons, where a comparable, slightly pyramidal structure supports vertically transverse support structures for the actual power cables.The support structure is positioned on one side next to the slag bed, meaning that the structure only has a support structure on one side, which transfers forces downwards into the ground. This is advantageous because slag beds are usually arranged side by side in such a way that they have a kind of slope ("tipping edge") on one side. The rails or tracks for the slag pot carrier for transporting the slag run along the top of this slope, from where the slag is poured down into the slag bed. On the other side, opposite the slope, the slag bed is usually freely accessible at ground level, allowing heavy equipment, such as a wheel loader or excavator, to break up and remove the slag.Therefore, it is advantageous that the support structure is arranged only on one side, particularly at the top of the slope, as this ensures that access to the slag bed remains unobstructed for large equipment to remove the slag. The device features a load-bearing structure attached to the support structure and projecting over the slag bed, similar to those used, for example, in container crane systems. The load-bearing structure is arranged essentially transversely to the support structure. While the support structure is the essentially vertical structure, the load-bearing structure serves to support the components of the device that project over the slag bed. The load-bearing structure must therefore absorb the corresponding forces of all components arranged there, plus their own weight, and transfer them to the support structure, which then dissipates these forces into the ground. "Transverse" in the context of the invention does not necessarily mean perpendicular.A supporting structure can be horizontally arranged, but it can also have a triangular base shape, for example, to better transfer forces to the supporting structure. In the aforementioned example of high-voltage pylons, the supporting structures for the actual power cables also have a triangular base shape. The supporting structure and the load-bearing structure can be arranged, for example, in an L-shape (upside down), a T-shape, or a cross shape. The supporting structure is thus suspended above the slag bed and is only supported on one side by the supporting structure. The device has a surface heat exchanger aligned parallel to the surface of the slag bed. The essential feature of the invention is that only the lower side of the surface heat exchanger is horizontally aligned and thus parallel to the surface of the slag bed.The surface heat exchanger serves to absorb thermal radiation emanating from the slag bed and transfer it to a heat transfer fluid, which then flows through the interior of the surface heat exchanger. The surface heat exchanger is arranged laterally next to the support structure, which is located only on one side of the surface heat exchanger. The surface heat exchanger is positioned above the slag within the slag bed. The slag is not applied to or poured over the surface heat exchanger, nor is it otherwise in direct contact with it. The essential aspect of the invention is that there is no direct contact between the heat exchanger and the slag. This avoids the problems associated with removing the cooled and solidified slag, as is the case with the prior art.This method is also advantageous compared to heat transfer systems where heat is dissipated via a gas in direct contact with the slag. At the comparatively high temperatures of up to approximately 1,500 °C, the proportion of heat dissipated via radiation is higher than the direct heat conduction into the subsurface or through convection of the overlying gas. This form of energy transfer also has the advantage that the radiant output can be regulated by adjusting the distance, depending on the slag temperature during cooling. Therefore, the surface heat exchanger is height-adjustable. This allows for precise adjustment and thus a more uniform heat input. This is a fundamental advantage over previous systems, all of which must adhere much more closely to the exponential temperature profile.The surface heat exchanger is designed to absorb the radiant heat from the slag. This means, for example, that the underside of the surface heat exchanger is black to achieve the highest possible absorption. The emission maximum is roughly temperature-dependent, lying between 1 pm and 6 pm (Wien's displacement law); therefore, the absorption should be as high as possible, roughly between 0.3 pm and 100 pm, with a maximum absorption value in the range of 1 pm to 20 pm.

[0017] Preferably, the surface heat exchanger is suspended beneath the supporting structure. A suspended arrangement offers the advantage of easy height adjustment. At the same time, the supporting structure itself can be designed to be rigid and therefore more stable. Alternatively, the entire supporting structure could, of course, be designed to be height-adjustable.

[0018] In a further embodiment of the invention, the device includes a control unit for tracking the surface heat exchanger. In the simplest case, this tracking can be regulated over time. The amount of slag is predetermined and constant by the size of the slag bucket or slag bed used, so that the cooling rate remains relatively constant despite fluctuating ambient conditions such as air temperature and humidity. However, active tracking can also be provided. Automatic tracking is preferred due to the long cooling periods.

[0019] In a further embodiment of the invention, the support structure extends to the side of the support structure opposite the slag bed. This results, for example, in a T- or cross-shaped structure consisting of a (essentially) vertical support structure and a (essentially) horizontal support structure, wherein the two sides of the support structure are preferably of different lengths. A storage device for the heat transfer fluid, which is heated in the surface heat exchanger, is arranged on the side opposite the slag bed. The surface heat exchanger and the storage device are fluidically connected via fluid lines to convey heated heat transfer fluid from the surface heat exchanger to the storage device and cold heat transfer fluid from the storage device to the surface heat exchanger.The storage device thus serves to further homogenize the temperature during the slag cooling process, enabling a more constant operating temperature and therefore better integration with heat-utilizing processes. Since the device is positioned on one side of the slag bed, the section of the support structure containing the surface heat exchanger extends along one side of the support structure. The storage device is then located on the second section of the support structure, opposite the first. Because the fluid itself is typically heavy, especially since a larger volume is required for homogenization, the storage device is usually significantly heavier than the surface heat exchanger, allowing the second section of the support structure to be considerably shorter.

[0020] In a further embodiment of the invention, the storage device is dimensioned such that the torque from the support structure on the side of the slag bed and the surface heat exchanger is approximately equal to the torque from the support structure on the side opposite the slag bed and the storage device. Since the surface heat exchanger is movable, a mathematically exact equality is not achievable anyway, especially since the heat transfer fluid undergoes a density change when heated. The only requirement is that the forces largely compensate each other, i.e., to at least 85%. For this purpose, the support structure is preferably shorter on the side opposite the slag bed, and the storage device is arranged as close as possible to the support structure. This allows for the largest possible storage device and thus the greatest possible uniformity of the temperature of the heat transfer fluid.

[0021] In a further embodiment of the invention, the storage device includes a conveying device for the heat transfer fluid. Integrating a conveying device into the storage device offers the design advantage of simplifying assembly. Furthermore, the weight is thus located on the side of the supporting structure opposite the surface heat exchanger and preferably significantly closer to the support structure, which minimizes the forces and optimally transfers them to the foundation.

[0022] In a further embodiment of the invention, the surface heat exchanger has a radiation-absorbing coating on its underside. This means that the surface heat exchanger is, for example, colored black on its underside to achieve the highest possible absorption. The emission maximum is roughly temperature-dependent, between 1 pm and 6 pm (Wien's displacement law), so the absorption should be as high as possible, roughly between 0.3 pm and 100 pm, with a maximum absorption in the range of 1 pm to 20 pm. Suitable coatings include, for example, standard black paint, and in particular carbon, such as carbon black. There are also white paints, such as oil paints, which, due to their integrated carbon black content, have comparable absorption coefficients to matte black paints: examples of this are special radiator paints.The advantage is that surface reflection is lower than with conventional paints and varnishes. Ideally, the entire surface would be covered with carbon nanotubes arranged vertically. However, the effort involved is disproportionate to the increased efficiency gained through improved absorption.

[0023] In a further embodiment of the invention, a rail is arranged between the support structure and the slag bed. This arrangement lengthens the supporting structure and thus the lever arm, ultimately leading to higher forces in the support structure, which is disadvantageous. On the other hand, this ensures that the liquid slag is tipped away from the support structure, significantly reducing the risk of the slag, which can reach temperatures of up to approximately 1,500 °C, splashing against and damaging the support structure.

[0024] In another embodiment of the invention, the surface heat exchanger is connected to the support structure via at least three, preferably four, cable winches, allowing for height adjustment. This represents the simplest and very lightweight method for positioning. In a further alternative embodiment of the invention, the surface heat exchanger is connected to the support structure via a scissor lift mechanism. This represents a more stable method for positioning, which is less susceptible to wind and holds the surface heat exchanger more securely in position. The disadvantage here is the greater weight.

[0025] In a further embodiment of the invention, the surface heat exchanger has at least one sensor. The sensor is selected from the group consisting of: temperature sensor, infrared sensor, and distance sensor. While the temperature sensor and the infrared sensor are ideally suited to enable active tracking to maintain a heat flow that is as constant as possible, a distance sensor serves in particular to ensure that a sufficiently safe distance is always maintained. Before the slag is tipped from the slag bucket into the slag bed, a portion of the slag surface in the bucket has usually already solidified, so that the surface of the slag bed cannot be perfectly flat and perfectly reproducible after tipping. Therefore, active distance detection is advantageous.Therefore, it is particularly preferred to install either a temperature sensor and a distance sensor, or an infrared sensor and a distance sensor, or a temperature sensor, an infrared sensor and a distance sensor.

[0026] In a further embodiment of the invention, the support structure is arranged to be movable on a rail. This is advantageous for reducing the number of devices according to the invention when dealing with multiple slag beds. Above a temperature of, for example, 200 °C, the amount of energy to be recovered is very low. On the other hand, the slag bed must first cool down before the slag can be removed mechanically. Therefore, it can be advantageous for a device according to the invention to be arranged alternately, for example, over two or more slag beds arranged side by side. Due to its size and predefined position, a rail is therefore advantageous. Alternatively, a tracked chassis could be used, which offers greater flexibility and does not require rails. In a further embodiment of the invention, the surface heat exchanger has a first surface facing the slag bed.The slag bed has a second surface on its upper side. The first surface is one to two times larger than the second surface, or in other words, the surface heat exchanger is the same size as, or twice the size of, the slag bed. This allows a high proportion of the radiation to be captured without requiring unnecessarily large amounts of space between the slag beds.

[0027] In another embodiment of the invention, the device is connected to a local heating network, a district heating network, and / or an in-plant heating network. This can be considered a quick win in modern times, meaning simple and rapid implementation. Furthermore, the district heating network as a whole provides an additional buffer for temperature regulation. The only disadvantage is that, due to continuous metal production, slag is generated continuously throughout the year, while district heating consumption fluctuates considerably, particularly with the seasons, but also in the short term with the weather. Moreover, this low-calorific method of utilization allows for the highest possible utilization of the released heat, whereas with power generation via steam production in a surface heat exchanger, although higher-grade electrical energy can be generated up to a slag temperature of approximately 500 °C, the thermal energy below this temperature level is practically unusable.

[0028] In a further embodiment of the invention, the device according to the invention enables the combined generation of electricity (at high temperatures) and district heating (at lower temperatures resulting from the progressively decreasing slag temperature). This would provide the highest-value form of energy, analogous to combined heat and power (CHP).

[0029] In a further aspect, the invention relates to a method for operating a device according to the invention. The method comprises the following steps: a) tipping slag into the slag bed, b) lowering the surface heat exchanger over the slag to a first height for maximum temperature, c) lowering the surface heat exchanger as the slag cools, d) raising the surface heat exchanger, e) removing the cooled slag from the slag bed.

[0030] Steps a) and e), with the intervening cooling of the slag, correspond to the current process and represent the only steps in which the slag is moved or processed. Therefore, the inventive method can be integrated into existing processes without any problems, without requiring any changes to the current procedures. Thus, any repercussions, for example, on iron and steel production, are ruled out. Even in the event of a failure of the device, for example, due to a defect, the slag processing would not be interrupted, but only the heat recovery. Therefore, the inventive method can be integrated into steel production without risk.

[0031] In step a), the surface heat exchanger is preferably in its highest position, allowing the slag to be easily tipped underneath it. The surface heat exchanger thus never comes into physical contact with the slag. Similarly, the lowering process in step c) is preferably always carried out with a gap to ensure that there is no contact between the surface heat exchanger and the slag.

[0032] In step b), a height of approximately 1 m has proven advantageous as the initial height. This height can vary, depending particularly on the type of absorbing coating and the heat transfer fluid velocity, for example between 2 m and 0.5 m. Therefore, in step b), the ideal initial height for achieving the heat flow is always selected for the specific device.

[0033] In step c), the temperature is then lowered to keep the heat flow as constant as possible. It must be taken into account that at a slag temperature below approximately 1350 °C, solidification occurs, releasing latent heat. Therefore, this temperature is maintained within the solidification range. The theoretical exponential temperature drop is thus halted at this point by the release of latent heat. For this reason, active temperature control, which adjusts to the current thermal radiation arriving at the surface heat exchanger, is preferred to ensure efficient temperature lowering.

[0034] In a further embodiment of the invention, step c) involves a continuous lowering process. The surface heat exchanger has a heat sensor. The distance is regulated to maintain a constant heat flow. The heat sensor can be, for example, a temperature sensor or an infrared sensor. This allows the heat flow to be kept largely constant.

[0035] In a further alternative embodiment of the invention, step c) involves a gradual lowering. This gradual lowering preferably occurs at predetermined time intervals. Since active control is not required, this is a comparatively simple control method.

[0036] In a further embodiment of the invention, the heat transfer fluid is selected from the list comprising thermal oil, water (including steam), and gas (for example, air, helium, argon, nitrogen, carbon dioxide). Water held under pressure is particularly preferred.

[0037] In a further embodiment of the invention, the heat transfer fluid is conveyed through the surface heat exchanger at a constant velocity. This also minimizes the auxiliary energy required to drive the pumps / blowers for the heat transfer fluid, since the drive power is proportional to the cube of the fluid's mass flow rate or the pump / blower speed.

[0038] The device and method according to the invention are explained in more detail below with reference to an exemplary embodiment shown in the drawings.

[0039] Fig. 1 Device

[0040] Fig. 2 Device with slag bucket

[0041] Fig. 3 Slag in the slag bed

[0042] Fig. 4 Surface heat exchanger at first level

[0043] Fig. 5 Surface heat exchanger below first level. Fig. 1 shows an exemplary and preferred device 10, which stands next to a slag bed 20 and projects over the slag bed 20. The support structure 30 stands vertically next to the slag bed 20, with a track 70 arranged between the support structure 30 and the slag bed 20 to transport slag 90 to the slag bed 20 using a slag bucket 80 and tip it into it. The device further comprises a horizontally arranged support structure 40 that projects over the slag bed 20. A surface heat exchanger 50, which is height-adjustable, is suspended from two of the shown cable winches 100 and two further winches in front of or behind the plane of the drawing. The underside of the heat exchanger is coated / painted in such a way that the emission coefficient of the radiation s is maximized in order to optimally absorb the radiant heat and to heat a heat transfer fluid flowing inside.A storage device 60 is arranged on the side of the support structure opposite the slag bed 20.

[0044] Fig. 2 shows the arrival of a slag bucket 80, from which the slag 90 is then tipped into the slag bed 20, as shown in Fig. 3. The surface heat exchanger 50 is then lowered to a first height at which the slag 90 has its highest temperature and thus emits the greatest radiant heat. This is shown in Fig. 4. As the slag 90 cools, the surface heat exchanger 50 is also lowered further, as shown in Fig. 5, in an attempt to keep the heat flow into the surface heat exchanger 50 constant.

[0045] Reference sign

[0046] 10 Device

[0047] 20 slag bed

[0048] 30 Support structure

[0049] 40 Supporting structure

[0050] 50 surface heat exchangers

[0051] 60 Storage device

[0052] 70 railway line

[0053] 80 slag buckets 90 slag

[0054] 100 winch

Claims

Patent claims 1. Device (10) for utilizing the waste heat from slag (90) in a slag bed (20), wherein the device (10) has a support structure (30) arranged on one side next to the slag bed (20), wherein the device (10) has a support structure (40) arranged on the support structure (30) and projecting over the slag bed (20), wherein the support structure (40) is arranged transversely to the support structure (30), wherein the device (10) has a surface heat exchanger (50) oriented parallel to the surface, wherein the surface heat exchanger (50) is arranged next to the support structure (30), wherein the surface heat exchanger (50) is arranged above the slag in the slag bed (20), wherein the surface heat exchanger (50) is height-adjustable, and wherein the surface heat exchanger (50) is designed to absorb the radiant heat of the slag (90).

2. Device (10) according to claim 1 , characterized in that the device (10) has a control device for tracking the surface heat exchanger (50).

3. Device (10) according to one of the preceding claims, characterized in that the support structure (40) extends to the side of the support structure (30) opposite the slag bed (20), wherein a storage device (60) is arranged on the side opposite the slag bed (20), wherein the surface heat exchanger (50) and the storage device (60) are fluidically connected via fluid lines.

4. Device (10) according to claim 3, characterized in that the The storage device (60) is dimensioned such that the torque from the support structure (40) on the side of the slag bed (20) and the Surface heat exchanger (50) equal to the torque from the supporting structure (40) on the side opposite the slag bed (20) and the storage device (60) is.

5. Device (10) according to one of claims 3 to 4, characterized in that the storage device (60) has a conveying device for the heat transfer fluid.

6. Device (10) according to one of the preceding claims, characterized in that the surface heat exchanger (50) has a radiation-absorbing coating on its underside.

7. Device (10) according to one of the preceding claims, characterized in that a rail (70) is arranged between the support structure (30) and the slag bed (20).

8. Device (10) according to one of the preceding claims, characterized in that the surface heat exchanger (50) is connected to the support structure (40) in a height-adjustable manner via at least three, preferably four, cable winches.

9. Device (10) according to one of the preceding claims, characterized in that the surface heat exchanger (50) has at least one sensor, wherein the sensor is selected from the group comprising: temperature sensor, infrared sensor, distance sensor.

10. Device (10) according to one of the preceding claims, characterized in that the support structure (30) is arranged to be movable on a rail.

11. Device (10) according to one of the preceding claims, characterized in that the surface heat exchanger (50) has a first surface facing the slag bed (20), wherein the slag bed (20) has a second surface on the top side, wherein the first surface is one to two times as large as the second surface.

12. Device (10) according to one of the preceding claims, characterized in that the device is connected to a local heating network, a district heating network and / or an in-house heating network.

13. Method for operating a device (10) according to one of the preceding claims, wherein the method comprises the following steps: a) tipping slag (90) into the slag bed (20), b) lowering the surface heat exchanger (50) over the slag to a first height for maximum temperature, c) lowering the surface heat exchanger (50) as the slag (90) cools, d) raising the surface heat exchanger (50), e) removing the cooled slag (90) from the slag bed (20).

14. Method according to claim 13, characterized in that in step c) a continuous lowering takes place, wherein the surface heat exchanger (50) has a heat sensor, wherein the distance is controlled to maintain a constant heat flow.

15. Method according to claim 13, characterized in that in step c) a stepwise lowering takes place, wherein the stepwise lowering preferably takes place after predetermined time intervals.

16. Method according to one of claims 13 to 15, characterized in that the heat transfer fluid is selected from the list comprising thermal oil, water, including steam, gases.

17. Method according to one of claims 13 to 16, characterized in that the heat transfer fluid is conveyed through the surface heat exchanger (50) at a constant speed.

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