GB2640843A

GB2640843A - Waste heat recovery arrangement for an electric arc furnace with forced circulation

Info

Publication number: GB2640843A
Application number: GB2406143.4A
Authority: GB
Inventors: Kumar Navlok; Kolhe Pushkar; Salim Alqaydi Maitha
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2024-05-02
Filing date: 2024-05-02
Publication date: 2025-11-12
Also published as: GB202406143D0; WO2025228753A1

Abstract

Apparatus for recovering waste heat from the exhaust gases of an electric arc furnace 3 has a forced circulation system comprising a pump 13 and at least one forced circulation duct 15 which is configured to absorb heat from exhaust gas from an electric arc furnace 3. Flow rate through the circulation duct(s) 15 is controlled by means of the pump 13 and can be adjusted depending on the temperature or quantity of exhaust gas. Figure 2 shows an apparatus comprising a first duct 15 for recovering thermal heat from exhaust gas and a second duct 15 for recovering heat generated by combusting the exhaust gas in a combustion chamber 23. The circulation system further comprises a steam buffer 19 for buffering steam. Steam from this buffer 19 can be fed either to a waste heat recovery unit 25 for recovery of heat from combusted exhaust gas or via a steam accumulator 35, header 37 to a heat engine 39 for generating electric power and the waste recovery unit 25.

Description

Waste heat recovery arrangement for an electric arc furnace with forced circulation The invention relates to a waste heat recovery arrangement for an electric arc furnace. The invention further relates to an electric arc furnace arrangement for producing steel, a steel mill and a method for recovering waste heat from an electric arc furnace.

More than one-third of the total energy supplied in a typical steel-making process using an electric arc furnace (EAF)is usually wasted. The process also consumes high amount of water which is required to cool down the EAF exhaust gas before it is released into atmosphere. The addition of water and/or cooling air dilution further adds to the volumetric flow of exhaust gases which needs to be handled by pollution control equipment such as bag filters (BF) or electrostatic precipitators (ESP) increasing the overall costs due to higher power consumption, in particular for commonly used induced draft (ID) fans.

The exhaust gases from an EAF flows along an exhaust gas flow path. This exhaust gas flow path typically extends through several water-cooled ducts, whereby the gas temperature is reduced. The exhaust gas flow path may further extend through a combustion or oxidation chamber where carbon monoxide (CO)and other combustible constituents are oxidized with additional air mixing. Finally, the gas is cooled down in a quench tower or the like by water spraying before it is transferred into a bag filter or the electrostatic precipitator. In certain plants, the cooling is achieved via both water cooling and air cooling (air dilution and/or indirect cooling). It is known to recover at least some heat in conventional waste heat recovery (WHR) solutions, using a waste heat recovery unit, in particular a heat exchanger, in the exhaust gas duct.

It is also known to recover waste heat from multiple EAFs in individual water jacketed ducts along with a common heat recovery unit where the saturated steam from a common accumulator will be superheated. In other known systems, waste heat from other sources like direct reduced iron (DRI) plant or furnaces of reheating mill are required to change the saturated steam from one or more heat recovery steam generator(s) (HRSG) to superheated condition before it can be taken into a steam turbine for power generation.

However, the overall known solutions still consume huge amounts of water and lose heat to the water cooling, thereby leading to an inefficient and unsustainable operation.

It is therefore an object of the invention to reduce heat losses when using an electric arc furnace.

This object is achieved by a waste heat recovery arrangement defined in claim 1. According to the invention, the waste heat recovery arrangement is provided with at least one forced circulation system comprising at least one pump element and at least one forced circulation liquid cooled duct connected to the at least one pump element, wherein the forced circulation duct is configured to absorb heat from exhaust gas of the electric arc furnace. For brevity's sake, the forced circulation liquid-cooled duct will be referred to as the "forced circulation duct" for the rest of this document.

For the electric arc furnace arrangement for producing steel, the object is achieved in that the electric arc furnace arrangement comprises an electric arc furnace and a waste heat recovery arrangement according to the invention that is connected to the electric arc furnace.

The steel mill according to the invention comprises a plurality of electric arc furnace arrangements according to the invention, wherein each electric arc furnace of an electric arc furnace arrangement is provided with a separate or dedicated waste heat recovery arrangement.

The object of the invention is further achieved by the method 5 according to claim 10. According to the inventive method for recovering waste heat from an electric arc furnace, exhaust gas from an electric arc furnace flows through at least one forced circulation duct of a forced circulation system, wherein heat from the exhaust gas is transferred into the 10 forced circulation system during the passage of the gas through the forced circulation duct, and wherein the recovered heat is supplied to a steam drum, an accumulator, or a heat exchanger by forced circulation.

Instead of using water-cooled exhaust gas ducts, the inventive solution uses forced circulation. The at least one forced circulation duct may act like an evaporator or an economizer module and recovers the heat from the exhaust gases.

The forced circulation system allows to handle the variation in exhaust gas quantity and temperature, thereby maintaining near constant temperature of exhaust gases at further positions downstream of the exhaust gas flow path, in particular at an inlet of an additional WHR unit, if such is present.

Furthermore, the inventive waste heat recovery arrangement can be installed on a single EAF plant. This is in contrast to known systems, in which a combination of multiple EAFs is used to overcome the cyclic nature of the exhaust gas flow and its temperature.

The forced circulation system can be used to adjust the 35 circulation ratio in response to a change in temperature and/or flow rate of the exhaust gas. Thus, combining a plurality of EAFs for a waste heat recovery system in order to compensate for the cyclic nature of the processes in the single EAFs may become obsolete.

An additional advantage of the invention is that the inventive solution recovers waste heat from exhaust gas discharged from the electric arc furnace as saturated steam rather than superheated steam.

Thus, the waste heat recovery arrangement can be installed on a single EAT plant without any additional waste heat source or fired boiler. In other words, an external or fired superheater for generating superheated steam is not required. This increases the overall heat recovery and power potential.

The inventive solution allows to reduce the gas volume to be handled by PCE and/or the fan size, since gas mixing from different sources and/or air dilution is not required. The solution also reduces the water consumption of the plant as the quantity of cooling water as well as spray water is reduced significantly.

In the inventive solution, the heat will be captured and used for power generation or as heating utility thereby reducing the metered energy consumption of a plant. This will improve the energy efficiency of the plant, reduce the carbon emission of the plant, reduce the water consumption of the plant as the quantity of cooling water as well as spray water is reduced significantly and reduce the ambient heating around the plant.

In order to implement the forced circulation, existing water jacketed ducts can be modified or replaced by forced circulation ducts. Capturing the heat by modifying or replacing existing water jacketed ducts may reduce the complexity of operation, investment required and offer efficient heat recovery opportunity.

In the following, further improvements of the invention and their advantages are described. The improvements can be combined independently with each other.

The advantages described in relation to the devices are equally applicable to the method according to the invention and vice versa. The features described in relation to the devices can be used for carrying out the method according to the invention. Preferably, the method according to the invention is carried out with a waste heat recovery arrangement according to the invention.

According to a first advantageous improvement of the waste heat recovery arrangement, at least one control unit for controlling a flow rate or circulation ratio using the at least one pump element may be provided. Thereby, the circulation ratio of the forced circulation system can be controlled.

The at least one control unit may comprise or may be connected to at least one temperature sensor for measuring the temperature of the exhaust gas. The flow rate or circulation ratio may thus be adjusted as a function of the exhaust gas temperature. The at least one control unit may help in handling a variation in exhaust gas quantity and temperature. Thus, maintaining a near constant temperature of exhaust gases can be facilitated, in particular at the inlet of a natural circulation based WHR unit.

According to another advantageous improvement of the waste heat recovery arrangement, the forced circulation system may further comprise at least one steam buffer for buffering steam. The at least one steam buffer may in particular be a steam drum or an accumulator. Using a heat buffer may improve the further handling of the heat, in particular in form of saturated steam in the steam buffer.

Alternatively or in addition to the heat buffer, the forced circulation system may further comprise at least one heat exchanger, in particular a closed loop heat exchanger. The heat exchanger may exchange heat between the forced circulation duct and a circuit comprising the steam drum. The heat exchanger may be used to heat up low temperature fluid coming from a heat engine or as an economizer.

In another variation, the heat exchanger may act as evaporator with a natural circulation of water on the secondary side, whereas the primary side will get heat from the forced circulation of liquid circulating through the liquid cooled ducts.

According to another advantageous improvement, the waste heat recovery arrangement may further comprise, in particular in addition to the at least one forced circulation system, at least one waste heat recovery unit, preferably formed by a heat exchanger, downstream of the at least one forced circulation duct. Thereby, the term "downstream" refers to the exhaust gas flow path.

In particular with the additional waste heat recovery unit downstream of the at least one forced circulation duct, no superheater is required. Instead, the hot exhaust gas after the forced circulation duct will be taken into the waste heat recovery unit (WHR) where additional heat recovery will be carried out, in particular by using evaporator and economizer modules. This solution will not only allow to cool down the exhaust gases to lowest possible limit, but will also reduce the costs of connecting several sources or EAFs to a common WHR unit and its associated operational complexities. The gas balancing and sequencing of operation of multiple EAF is avoided. Instead, the inventive solution allows to install an individual WHR system on an individual EAF outlet to provide operational flexibility. In the additional WHR unit, the exhaust gas may be cooled down to lowest allowable temperature considering SO, and/or other constituents of the exhaust gas.

The waste heat recovery arrangement according to the invention may further comprise a combustion or oxidation chamber that is provided with at least one forced circulation duct. In the combustion chamber, carbon monoxide (CO) in the exhaust gas may be oxidized to carbon dioxide (CO2). This exothermic reaction produces additional heat, which may be recovered by the at least one forced circulation duct. Other constituents in the exhaust gas may also be oxidized in the combustion chamber.

Advantageous embodiments of the invention are described below with reference to the figures. The embodiments are merely exemplary and their features can be combined independently of one another. Features that have the same structure or function are marked with the same reference signs.

In the figures, Figs. 1 to 4 show schematics of exemplary embodiments of waste heat recovery arrangements according to the invention; Fig. 5 schematically shows an exhaust gas flow path.

In the following, a first advantageous embodiment of a waste heat recovery arrangement 1 according to the invention is described with respect to Fig. 1.

Fig. 1 schematically shows a waste heat recovery arrangement 1. The waste heat recovery arrangement 1 is used for recovering waste heat of an electric arc furnace 3. The waste heat recovery arrangement 1 and the electric arc furnace 3 may together form an electric arc furnace arrangement 4.

The electric arc furnace 3 may be used for producing steel and may be part of a steel mill 5. The steel mill 5 may be provided with a plurality of electric arc furnace arrangements 4, whose waste heat recovery arrangements 1 may work independently of each other. A steel mill 5 may further comprise additional components, which are not shown in the schematic drawing of Fig. 1.

During steel production, the processes in the electric arc furnace 3 produce hot exhaust gases which are directed into a system of exhaust gas ducts 7. The flow path 9 of the exhaust gas is described later with respect to Fig. 5.

In order to efficiently recover the heat from the exhaust gases in the exhaust gas ducts 7, the waste heat recovery arrangement 1 is provided with a forced circulation system 11.

The forced circulation system 11 comprises a pump element 13 for driving a forced circulation of cooling liquid.

At least one of the exhaust gas ducts 7 is a forced circulation duct 15. Preferably all exhaust gas ducts 7 are forced circulation ducts 15.

The at least one forced circulation duct 15 may be an exhaust gas duct 7 that is made from or provided with channels 17 for the cooling liquid of the forced circulation system 11. The channels 17 may be made from pipes, in particular steel pipes. The channels 17 are connected to the pump element 13. Thus, cooling liquid can be transported through the channels 17 of the forced circulation duct 15 by the pumping power provided by the pump element 13. The channels 17 are only indicated as lines in the figures. The lines do not define certain directions or dimensions of the channels 17.

In the forced circulation ducts 15, the heat from the exhaust 35 gases can be absorbed by the cooling liquid. The forced circulation ducts 15 may act like evaporators or economizer modules. In other words, the cooling liquid may change its phase and turn into steam, in particular saturated steam or may remain in liquid phase at higher temperature.

The waste heat recovery arrangement 1 preferably comprises a control unit 14 for the pump element 13. The control unit 14 preferably comprises at least one temperature sensor (not shown) or is connected to such. The temperature sensor is preferably configured and arranged to measure the temperature of the exhaust gas.

The control unit 14 can thus be used to control the flow rate or circulation ratio using the pump element 13 and thereby the circulation ratio of the forced circulation system 11. Preferably, the circulation ratio is adjusted in dependence of the temperature and/or quantity of the exhaust gas. Thereby, the waste heat recovery arrangement 1 can handle the variation in exhaust gas quantity and temperature and maintain near constant temperature of exhaust gases, in particular at an inlet of an additional WHR unit 25.

The forced circulation system 11 further comprises at least one steam buffer 19 for buffering steam produced in the forced circulation ducts 15. The steam buffer 19 is preferably a steam drum 21 or an accumulator.

The pump element 13 may be arranged at a position in the forced circulation system 11 that allows it to pump cooling liquid from the steam buffer 19 into the forced circulation ducts 15. From there, the heated cooling liquid, preferably in the form of saturated steam and water mixture, will be transported into the steam buffer 19.

Thus, a first cooling circuit 24 connects the pump element 13, the steam buffer 19 and the forced circulation ducts 15.

Thereby, a cool liquid section 26 extends from the steam buffer 19 via pump element 13 to the forced circulation ducts 15. In addition, a steam section 28 extends from the forced circulation ducts 15 to the steam buffer 19. The flow directions in sections 26, 28 of the first cooling circuit 24 is indicated by arrows in Fig. 1.

The waste heat recovery arrangement 1 comprises a combustion chamber 23 that is also provided with at least one forced circulation duct 15. Thereby, heat generated by exothermal processes in the combustion chamber 23 can be transferred into the forced circulation system 11 and thus be recovered.

In addition to the at least one forced circulation system 11, the waste heat recovery arrangement 1 further comprises at least one waste heat recovery unit 25 in the exhaust gas flow path 9, in particular a heat exchanger 27, downstream of the at least one forced circulation duct 15. Thereby, the term "downstream" refers to the exhaust gas flow path 9.

With help of the waste heat recovery unit 25, the exhaust gas can be cooled down to the lowest allowable temperature considering SO, and other constituents of the exhaust gas.

The waste heat recovery unit 25 may be connected to the steam buffer 19. Thus, steam generated in the waste heat recovery unit 25 can be collected in the steam buffer 19. The corresponding flow path is indicated by arrow 29 in Fig. 1.

Cooling liquid from the steam buffer 19 may be transported to the waste heat recovery unit 25. This is indicated by arrow 31 in Fig. 1.

The steam buffer 19 and the waste heat recovery unit 25 are part of a second cooling circuit 33, along which saturated steam from the steam buffer 19 can be transported to an accumulator 35. In the accumulator 35, saturated steam can be collected and stored.

From accumulator 35, the steam is transported via a steam header 37 to a heat engine 39, in particular a heat engine 39 that uses a thermodynamic cycle, in particular an organic rankine cycle (ORC), a steam rankine cycle (SRC) or a supercritical CO2 power cycle (5002).

In the heat engine 39, the waste heat from the electric arc 5 furnace 3 can be utilized, in particular for generating electric power.

Cooled liquid from the heat engine 39 is transported back to the waste heat recovery unit 25. Steam generated in the waste heat recovery unit 25 can be transported to the steam buffer 19. From steam buffer 19, steam is transported to the accumulator 35 again.

For transporting the cooling liquid in the second cooling 15 cycle 33, a pump element 41 may be provided.

The general flow direction of saturated steam in the second cooling cycle 33 is indicated by arrow 43 in Fig.1. The general flow direction of cooling liquid is indicated by 20 arrow 45 in Fig. 1.

With the design of the waste heat recovery arrangement 1 according to the invention, there is no need for a superheater. As already mentioned in the beginning, the electric arc furnace arrangement 4 comprising the electric arc furnace 3 and the waste heat recovery arrangement 1 is capable of working independently from other electric arc furnaces 3 and/or other waste heat recovery arrangements 1. Combining several waste heat recovery arrangements 1 together in order to compensate for variations in exhaust gas temperature and/or quantity is not necessary. Variations can be compensated due to using forced circulation and a steam buffer 19.

In Fig. 1, the exhaust gas flow path 9 is depicted as extending from the forced circulation ducts 15 to the waste heat recovery unit 25. However, at least a portion of the exhaust gas may be directed from the forced circulation ducts to a gas conditioning system 47. This is described in detail later with respect to Fig. 5.

In the following, further advantageous embodiments of the invention are described with respect to Figs. 2 to 4. For the sake of brevity, only the differences to the embodiment described with respect to Fig. 1 are described in detail.

The embodiment shown in Fig. 2 uses only a single pump element 13. This can be achieved by connecting the first and second cooling circuits 24 and 33 to a common cooling circuit 49 that uses the pumping power of pump element 13.

Cooling liquid from the heat engine 39 is, as also in the first embodiment, directed to the waste heat recovery unit 25 to collect heat from the exhaust gas therein. However, the preheated cooling liquid is not directed to the steam buffer 19, but to the forced circulation ducts 15 instead. From the forced circulation ducts 15, the cooling liquid, now in the form of saturated steam or steam-water mixture, is directed to the steam buffer 19.

However, a portion of the cooling liquid in the waste heat recovering unit 25 may flow further in the waste heat recovering unit 25 to take up more heat and form saturated steam. Afterwards, the saturated steam is also directed into the steam buffer 19.

From the steam buffer 19, the saturated steam is, as also in 30 the first embodiment, directed into the accumulator 35 and from the accumulator 35 via the steam header 37 to the heat engine 39.

In the third exemplary embodiment of the waste heat recovery 35 arrangement 1 depicted in Fig. 3, the first cooling circuit 24 does not include the steam buffer 19.

Instead, a heat exchanger 51 is provided, said heat exchanger 51 having a primary side and a secondary side.

The primary side is connected to the first cooling circuit 24. The secondary side is connected via a closed cooling circuit 53 to the steam buffer 19. The flow of cooling liquid in the closed cooling circuit 53 is preferably driven by natural circulation.

The fourth embodiment of Fig. 4 combines the features of the second and third embodiments.

The first cooling circuit 24 of the fourth embodiment is similar to the first cooling circuit 24 of the third 15 embodiment.

The second cooling circuit 33 of the fourth embodiment is in so far similar to the second embodiment in that the cold cooling liquid coming from the heat engine 39 is not directed into the steam buffer 19 after passing through the waste heat recovery unit 25. Instead, the cooling liquid, which was preheated during its passage through the waste heat recovery unit 25, is directed to the second side of the heat exchanger 51.

The heat exchanger 51 may thus heat up the low temperature cooling liquid coming from the heat engine 39. Subsequently to the passage through the heat exchanger, the cooling liquid, now preferably in form of saturated steam, is directed to the steam buffer 19.

In the following, the exhaust gas flow path 9 is briefly described with respect to Fig. 5.

A first strand 54 of the exhaust gas flow path 9 extends through the forced circulation ducts 15 to an inlet 55 the waste heat recovery unit 25 and may also include the combustion chamber 23.

From an outlet 57 of the waste heat recovery unit 25, the exhaust gas flow path 9 leads through a pollution control system 59 to a flue-gas stack 61 from which the exhaust gases 5 is released into the atmosphere.

Transport of the exhaust gas can be improved by at least one induced draft fan 63 in the exhaust gas flow path 9.

In the alternative, or additionally to the first strand 54, the exhaust gas may flow through a second strand 56 of the exhaust gas flow path 9. The second strand 56 extends through the gas conditioning system 4? towards the pollution control system 59. The second strand 56 thus bypasses the waste heat recovery system 25 in case of any failures in WHR unit 25 or if required by the process.

For controlling the flow of the exhaust gas, in particular to control the portions of exhaust gas, which enter the first strand 54 and/or second strand 56, a plurality of dampers 65 can be present at several positions of the exhaust gas flow path 9.

Reference numerals 1 waste heat recovery arrangement 3 electric arc furnace 4 electric arc furnace arrangement steel mill 7 exhaust gas ducts 9 exhaust gas flow path 11 forced circulation system 13 pump element 14 control unit forced circulation duct 17 channel 19 steam buffer 21 steam drum 23 combustion chamber 24 first cooling circuit waste heat recovery unit 26 cool liquid section 27 heat exchanger 28 steam section 29, 31 arrows 33 second cooling circuit accumulator 37 steam header 39 heat engine 41 pump element 43, 45 arrows 47 gas conditioning system 49 common cooling circuit 51 heat exchanger 53 closed cooling circuit 54 first strand of exhaust gas flow path inlet of waste heat recovery unit 56 second strand of exhaust gas flow path 57 outlet of waste heat recovery unit 59 pollution control system

Claims

Patent claims 1. Waste heat recovery arrangement (1) for an electric arc furnace (3), wherein the waste heat recovery arrangement (1) is provided with at least one forced circulation system (11) comprising at least one pump element (13) and at least one forced circulation duct (15) connected to the at least one pump element (13), and wherein the forced circulation duct (15) is configured to absorb heat from exhaust gas of the electric arc furnace (3).
2. Waste heat recovery arrangement (1) according to claim 1, comprising at least one control unit (14) for controlling flow rate or circulation ratio using the at least one pump 15 element (13).
3. Waste heat recovery arrangement (1) according to claim 1 or 2, wherein the forced circulation system (11) further comprises at least one steam buffer (19) for buffering steam.
4. Waste heat recovery arrangement (1) according to claim 3, wherein the at least one steam buffer (19) is a steam drum (21) or an accumulator (35).
5. Waste heat recovery arrangement (1) according to any one of claims 1 to 4, wherein the forced circulation system (11) further comprises at least one, in particular indirect, heat exchanger (51).
6. Waste heat recovery arrangement (1) according to any one of claims 1 to 5, further comprising at least one waste heat recovery unit (25) downstream of the at least one forced circulation duct (15).
7. Waste heat recovery arrangement (1) according to any one of claims 1 to 6, further comprising a combustion chamber (23) that is provided with at least one forced circulation duct (15).
8. Electric arc furnace arrangement (4) for producing steel, comprising an electric arc furnace (3) and a waste heat recovery arrangement (1) according to any one of the aforementioned claims connected to the electric arc furnace (3).
9. Steel mill (5) comprising a plurality of electric arc furnace arrangements (4) according to claim 8, wherein each electric arc furnace (3) of an electric arc furnace arrangement (4) is provided with a separate waste heat recovery arrangement (1).
10. Method for recovering waste heat from an electric arc furnace (3), wherein exhaust gas from an electric arc furnace (3) flows through at least one forced circulation duct (15) of a forced circulation system (11), wherein heat from the exhaust gas is transferred into the forced circulation system (11) during the passage of the gas through the forced circulation duct (15), and wherein the recovered heat is supplied to a steam drum (21), or an accumulator (35), or a heat exchanger (51), by forced circulation.
11. Method for recovering waste heat from an electric arc furnace (3) according to claim 10, wherein remaining heat in the exhaust gas after passage through the forced circulation duct (15) is recovered by a waste heat recovery unit (25) downstream of the forced circulation duct (15).