System and process for producing compressed air

Abstract

The invention relates to a metallurgical or steel plant comprising a system (10; 110; 210; 310; 410; 510; 610; 710) for producing compressed air comprising one or more turbo-compressors (24; 124, 126; 224; C; 424; 526; 624; 724, 726) wherein the compressed air produced is divided into two streams, wherein the first stream (F1) feeds a utility and the other stream (F2) the turbine(s) (20; 120, 132; 220; 420; 532; 620; 720, 732) of the turbo-compressor after heating in a heating device comprising a heat exchanger (12; 112; 212; 312; 412; 512; 612; 712) configured to be fed by a thermo-vector fluid containing waste heat from said metallurgical / steel plant. The system further comprises a cooling device installed along said second duct (F1) with a second heat exchanger (18; 118; 218; 318; 418; 518; 618; 718) and possibly auxiliary devices, such as motors (642; 742A, 742B) or electric compressors (440; 540).

Description

[0001] TITLE: SYSTEM AND PROCESS FOR PRODUCING COMPRESSED AIR

[0002] Applicant: DANIELI & C. Officine Meccaniche S.p.A., Via Nazionale 41, 33042 Buttrio (UD), Italy

[0003] TECHNICAL FIELD

[0004] The present invention relates to a metallurgical or steel plant comprising a system for producing compressed air which includes the use of turbo-compressors and a corresponding process for the production of compressed air in a metallurgical or steel plant.

[0005] STATE OF THE ART

[0006] Many industrial activities, such as the production of metals or the alloys thereof, require the use of compressed air and provide considerable amounts of excess heat deriving from the various steps of the production process, which must be recovered, but more often is simply dissipated. Nowadays, compressed air has a considerable impact on industrial electrical consumption in the production of metals or alloys.

[0007] The demand for compressed air and hot air by the steel process, in a traditional plant, results in the consumption of electrical energy and fuel necessary for their production.

[0008] Greater efficiency in compressed air production is therefore desirable, leading to a significant reduction in electrical energy consumption and pollutant and greenhouse gas emissions, and thus to a more sustainable air compression process. Compressed air is produced in the state of the art by means of compressor devices which are classified into two groups: there are volumetric compressors and dynamic compressors (the latter are also called continuous flow compressors or turbo-compressors if coupled to a turbine). Dynamic compressors are suitable for high volumetric flow rates. The compressors can be driven by electric motors, internal combustion engines, such as piston motors and gas turbines or micro-turbines, or in any case other mechanical apparatuses, for example steam turbines or gas expanders. Documents US 2020 / 191048 Al and US 2020 / 095899 Al describe Brayton cycle air turbo compressor systems, the first with the purpose to increase the power of the system and the second with the purpose to extract heat energy from waste heat.

[0009] DISCLOSURE OF THE INVENTION

[0010] The object of the invention is to overcome the aforesaid drawbacks and to propose a metallurgical or steel plant and a relative process which produce pressure energy, i.e., compressed air, efficiently, economically and with a reduced environmental impact, usable in the metallurgical or steel process to decrease the energy demand of the auxiliary plants used in these processes. Further objects or advantages of the invention will become apparent from the following disclosure.

[0011] In a first aspect of the invention, the object is achieved by a metallurgical or steel plant comprising

[0012] (A) a system for producing compressed air comprising:

[0013] (a) a compression unit configured to receive air and to perform thereon a compressing work and then to emit it in compressed form;

[0014] (b) an expansion unit configured to receive compressed air and to generate thereon an expansion work and to then emit it in expanded form;

[0015] (c) at least one mechanical transmission device connecting said compression unit and said expansion unit, which is configured to receive said expansion work and to provide it to said compression unit as compression work;

[0016] (d) at least a first duct for compressed air, connecting said compression unit and said expansion unit;

[0017] (e) a heating device arranged between said compression unit and said expansion unit along said first duct for compressed air and configured to heat passing compressed air;

[0018] (f) at least a second duct for the transport of compressed air downstream of said compression unit, or a portion thereof, configured to deviate a portion of the compressed air produced during operation by said compression unit to a utility included in said metallurgical or steel plant, preferably a continuous casting machine or other system which may require compressed air, wherein said heating device comprises (e-1) a first heat exchanger configured to be fed by a thermo vector fluid containing waste heat from said metallurgical or steel plant; and wherein said system for producing compressed air further comprises

[0019] (g) a cooling device installed downstream of said compression unit along said second duct and comprising a second heat exchanger for extracting heat from said deviated compressed air;

[0020] (B) an apparatus producing a thermo vector fluid containing waste heat, preferably originating from furnaces, for example melting furnaces and reheating furnaces, and comprising a device for extracting said thermo vector fluid containing waste heat; wherein said device for extracting said thermo vector fluid containing waste heat is connected to said first heat exchanger.

[0021] A system that may need compressed air within the metallurgical or steel plant may be, as mentioned above a continuous casting machine, but also a reheating furnace, a ladle or tundish preheating system, or a burner system.

[0022] The utility generally is not a part of the system for producing compressed air itself, but an independent utility. The utility in that case is external to the actual cycle of the compressed air production system and requires compressed air to perform its functions within metallurgical or steelmaking processes.

[0023] In other cases, the utility may correspond to the apparatus producing the thermo vector fluid, at the same time “consuming” compressed air produced by the system and supplying heat to the first heat exchanger. Hypothetical is also the supply of the compressor unit with fumes.

[0024] However, even in these cases, the utility uses compressed air not only to power the turbocharger system, but needs the compressed air to perform its actual function within the metallurgical or steel plant.

[0025] The thermo vector fluid may be of different nature. Hot liquids, vapours or fumes can be understood with the term thermo vector fluid within the scope of the present invention. In an advantageous embodiment of the invention, the thermo vector fluid containing waste heat is hot fumes, such as in particular the hot fumes resulting from heating processes in the metallurgical or steel sector and in a potential version, preferably after their de-dusting, the fumes exiting from an electric arc furnace and / or a ladle furnace during the melting or refining phase. The term "along the duct" includes variants in which the device is inserted inside or outside the duct. Other compressible fluids, such as mixtures of air with inert gases or inert gases, or mixtures of air with combustion fumes, are conceivable in place of air.

[0026] The heat extracted from the deviated compressed air can be used for various purposes in the metallurgical or steel plant, for example for district heating. For example, cooling can take place by means of air or water flows, but the use of diathermic oils is not excluded.

[0027] During the various production processes in the metallurgical and steel sector, a considerable amount of waste heat is generated (contained, for example, in fumes extracted from melting furnaces or heating furnaces), which can be exploited by means of the present energy recovery system to partially or totally replace the electric compressors for the production of compressed air and to reduce the energy consumption necessary to drive the compression unit, contributing to greater sustainability in the metallurgical and steel sector.

[0028] The combination between compression unit and expansion unit can advantageously be achieved by a turbo-compressor comprising a turbine (the "hot" side of the turbo-compressor) which is coupled to a compressor (the "cold" side thereof). The compressor sucks the air to be compressed and is driven through a mechanical transmission system by means of the expansion work obtained in the turbine in which, in turn, the compressed air received from the compressor expands. The compressed air produced by the compressor can be divided into two streams, one provides the compressed air for the dedicated utility and the other feeds, preferably in preheated form, the turbine which in turn drives the compressor.

[0029] In a preferred embodiment of the invention, the system can include on the first duct a connection to a compressed air source or an additional compressed air injector, which is preferably introduced into the system during its starting phase, and which is heated and sent to the expansion unit, so that by means of the expansion work generated in the latter the respective compressor can be rotated, putting the system into operation. Once the system is fully operational, the compressed air inlet from outside the circuit can be interrupted. In this regard, the connection to a compressed air source is preferably provided with a regulating valve.

[0030] In fact, when fully operational, the compression unit compresses the air drawn from the environment to the desired pressure: one part is tapped and sent to the respective utility inside the metallurgical or steel plant, while another part is sent to the first heat exchanger where, thanks to its feeding from a waste energy source (e.g., hot fumes), is heated. At this point, the heated compressed air enters the expansion unit, i.e., the turbine, generating mechanical power which, as seen, supports the rotation of a compressor, by means of a mechanical transmission device.

[0031] The system according to the invention for producing compressed air and for simultaneously recovering waste heat from a metallurgical or steel plant can be implemented in different configurations, according to the specific needs of the process, but the main configuration includes as components:

[0032] Two heat exchangers: o A first heat exchanger, the main one, which is inserted between the heat source to be exploited and a duct travelled by the compressed air in output from a compressor. The function of this heat exchanger is to recover the waste energy. o A second heat exchanger having the function of cooling compressed air, for example by means of water, oil, or gas (depending on economic convenience). This second heat exchanger within the scope of the present invention will also be called an aftercooler.

[0033] - At least one turbo-compressor: this machine consists of a compressor mechanically coupled to a turbine which, exploiting the expansion of compressed air at high temperature, drags the compressor and replaces (at least partially) the electric motor for the production of compressed air, currently in use in the state of the art.

[0034] In a preferred embodiment of the invention, the compression unit and the expansion unit have multi-stage compression and preferably also multi-stage expansion. The compression unit can be a multi-stage compression unit comprising a plurality of compressors connected in series and / or in parallel. The expansion unit can comprise a turbine or a plurality of turbines connected in series and / or in parallel. At least one of the compressors of the plurality of compressors is connected by means of a respective mechanical transmission device to one of the turbines forming a turbo-compressor.

[0035] Depending on the compressed air production pressure required, the turbo-compressor can be single stage (pressures up to about 3 - 5 barg) or double stage (pressures up to about 8 barg). A barg (bar gauge) is a unit of measurement which represents the difference between the pressure in bar in a space and the atmospheric pressure in bar. For example, if the atmospheric pressure were equal to 1 bar, where there is a pressure of 3 barg the absolute pressure would be 4 bar.

[0036] In a single-stage system, the compressor and turbine are usually of the low pressure type; whereas in a two-stage system, the first compressor and first turbine in the direction of air flow are usually at low pressure and the next compressor and next turbine at high pressure.

[0037] The starting of the system according to the invention is preferably pneumatic, thus requiring a compressed fluid, in this case compressed air, as described above. In fact, the compressed air introduced at least partially feeds the expansion unit which, with the expansion work, supports the operation of the compression unit until it reaches full operation. The size of the turbocompressor, the thermal inertia of the system and the inlet temperatures of the expansion unit affect the flow rate and consumption of compressed air needed for starting the system. In some cases it may be necessary to provide elements which contribute to carrying out part of the expansion work to support compression, to start the system more quickly in a moment when sufficient compressed air has not yet been produced. In this regard, in an advantageous variant of the invention, the system for producing compressed air further comprises

[0038] (h) an auxiliary device configured to start or facilitate the starting of the system, in particular of said one or more turbo-compressors.

[0039] A first possibility is to add to the single-stage or multi-stage configurations (for example double-stage) an auxiliary device at the inlet to the expansion unit which is capable of facilitating the starting of the system, lowering the flow rate and consumption of compressed air necessary for said starting. The auxiliary device is preferably selected from an electrical resistance, radiant tubes and a burner.

[0040] Especially in the cases where an electric compressor is present upstream or downstream of the turbo-compressor, the auxiliary device not only serves to start or facilitate the starting of the turbo-compressors, but also to raise the pressure level in input to the turbo-compressor expansion unit, so as to generate a greater expansion work.

[0041] A second possibility envisages that said auxiliary device is an electric compressor forming part of a plurality of compressors connected in series and / or in parallel. In other words, in a multistage system or in a system having a plurality of turbo-compressors, at least one of these is replaced by an electric compressor. The electric compressor can replace any turbo-compressor, so already the first of the compression units in the direction of air flow, as one of the following. Preferably, a high-pressure electric compressor replaces a high-pressure turbo-compressor and a low pressure electric compressor replaces a low pressure turbo-compressor.

[0042] A third possibility envisages a motor or generator coupled to the mechanical transmission connection, usually a shaft, of the turbo-compressor. For example, in the two-stage case two motors can be envisaged which are mechanically or electronically coupled. This generates the power needed to start the turbo-compressor and usually the motor will be decoupled when fully operational. In an embodiment of the invention, the auxiliary system is at least one motor installed along the mechanical transmission connection between a compressor and a turbine.

[0043] At full operations, however, the motor can also become a generator, subtracting part of the mechanical power from the transmission of the turbo-compressor to produce electrical energy. In the case of a compression unit comprising a plurality of compressors, an embodiment of the system according to the invention envisages that a cooling device is arranged between at least two compressors, for example an intercooler comprising a third heat exchanger for removing heat. The third exchanger serves to cool the compressed air in output from the previous stage of the compression unit to reduce the specific work of the following compression stage, with the output pressure set. This configuration is particularly suitable when the compressed air utility requires a pressure flow rate of up to about 8 barg.

[0044] Another preferred embodiment of the invention envisages, in particular in the single-stage version, the addition of an electrical booster upstream of the second heat exchanger whose purpose is to increase the pressure level on the branch of the system sending the compressed air to the utility.

[0045] The hot air resulting from the expansion unit is preferably conveyed by means of a pipe dedicated to a different utility, which makes use of hot expanded air in the metallurgical or steel plant, for example as combustion air in heating furnaces or in other types of burners.

[0046] The presence of a system for producing compressed air according to the invention in a metallurgical or steel plant, also thanks to exploiting waste heat, makes the plant at least partially autonomous in the supply of compressed air.

[0047] A second aspect of the invention relates to a process for producing compressed air, comprising the following steps:

[0048] (I) compression of air in a compression unit with the generation of compressed air;

[0049] (II) division of the compressed air into at least two portions; (III) cooling a first portion of compressed air and sending said first portion to a utility in a metallurgical or steel plant;

[0050] (IV) heating of a second portion of compressed air and sending the second portion of the heated, compressed air to an expansion unit, wherein the generated expansion work is used at least partially, preferably completely, to drive the compression unit; wherein

[0051] (i) said phase (IV) heating takes place in a first heat exchanger by recovering heat from said thermo vector fluid containing waste heat from said steel or metallurgical plant; and

[0052] (ii) the heat recovered in a second heat exchanger during said phase (III) cooling is used to heat utilities in said metallurgical or steel plant; and wherein, optionally:

[0053] - the compression unit comprises a plurality of compressors;

[0054] - between at least two compressors a third heat exchanger is arranged to cool the compressed air leaving the first compressor and entering the second compressor;

[0055] - a part of the energy contained in the air is not transformed into expansion work in said expansion unit, but is used to feed not the compression unit but a different utility.

[0056] Advantageously, the process is implemented in a system for producing compressed air according to the invention.

[0057] In a preferred embodiment of the process according to the invention, the starting of the compression unit and / or the expansion unit is facilitated by at least one of the following auxiliary devices:

[0058] • (a) an injector of supplemental compressed air upstream of the first heat exchanger, preferably provided with a control valve;

[0059] • (P) an auxiliary device, preferably selected from an electrical resistance, radiant tubes and a burner placed at the inlet of said expansion unit;

[0060] • (y) an electric compressor connected in series and / or in parallel with one or more compressors comprised in said compression unit;

[0061] • (6) a motor placed along a mechanical transmission connection between said compression unit and said expansion unit. At the exhaust of the expansion unit, the air flow (the plant waste) is still at a high temperature. In order to be able to further exploit this residual energy, it is possible in a variant of the invention to limit the expansion ratio to a certain extent so as to preserve in the outflow a part of the pressure energy supplied by the compression unit. Thereby, the possibility of pushing the hot air towards an application capable of exploiting the residual heat present downstream of the expansion is guaranteed, for example the burners in the heating furnaces through an appropriate piping system. The turbine can therefore also replace machines used for moving fluids, such as fans.

[0062] The implementation of the system for producing compressed air and the simultaneous recovery of heat in a metallurgical or steel plant offers several advantages:

[0063] - Reduction of operating costs for the production of metals or alloys such as steel, contributing to greater economic sustainability.

[0064] Increased energy efficiency: The recovery of thermal energy allows to exploit an otherwise dispersed resource, increasing the overall efficiency of the plant.

[0065] - Reduction of greenhouse gas emissions: by using the recovered thermal energy for the compression of a fluid, the system reduces the dependence on electrical energy. This results in a reduction in greenhouse gas emissions and contributes to the fight against climate change.

[0066] The features described for one aspect of the invention may be transferred mutatis mutandis to the other aspects of the invention. Transfer is implicit, as certain elements of the plant (e.g., the compression unit) correspond to respective process steps (e.g., compression) and vice versa. The purposes and advantages will be further highlighted in the disclosure of preferred examples of embodiments of the invention given by way of non-limiting example only.

[0067] Variant and further features of the invention are the subject matter of the dependent claims. The description of preferred embodiment examples of the system, the plant and of the process according to the invention is given, by way of example and not of limitation, with reference to the attached drawings. In particular, unless otherwise specified, the number, shape, size and materials of the system and of the individual components may vary, and equivalent elements may be applied without deviating from the inventive concept. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Fig. 1 depicts a basic diagram of a system for producing compressed air according to the state of the art.

[0069] Fig. 2 depicts a basic diagram of a first embodiment of a system for producing compressed air.

[0070] Fig. 3 depicts a basic diagram of a second embodiment of a system for producing compressed air in two stages.

[0071] Fig. 4 depicts a basic diagram of a third embodiment of a system for producing compressed air which integrates the system depicted in Figure 2 with an electric booster.

[0072] Fig. 5 depicts a basic diagram of a fourth embodiment of a system for producing compressed air which is provided with an auxiliary device for starting the system.

[0073] Fig. 6A depicts a basic diagram of a fifth embodiment of a system for producing compressed air in which an electric compressor is provided downstream of the compressor of the turbo-compressor.

[0074] Fig. 6B depicts a basic diagram of a sixth embodiment of a system for producing compressed air in which an electric compressor is provided upstream of the compressor of the turbo-compressor.

[0075] Fig. 7A depicts a basic diagram of a seventh embodiment of a system for producing compressed air in one stage in which a motor coupled to the shaft of the turbocompressor is provided.

[0076] Fig. 7B depicts a basic diagram of an eighth embodiment of a system for producing compressed air in two stages in which a motor coupled to the shafts of the turbocompressors is provided.

[0077] DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

[0078] To clearly highlight the difference of the invention, in particular of the air compression system, with respect to the prior art, Fig. 1 depicts a basic diagram of a system S for producing compressed air according to the state of the art. The cold air, usually at room temperature, enters a traditional compressor C which is - by means of a shaft A - rotated by an electric motor M. The rotation work is transformed into compression work to generate compressed air which exits from the compressor C.

[0079] Fig. 2 depicts a basic diagram of a first embodiment of a system for producing compressed air. The system 10 can be implemented in different configurations, depending on the specific needs of the process. The main configuration includes heat exchangers and turbo-compressors. A main heat exchanger 12 is inserted between a heat source 14 to be exploited and a duct travelled by the compressed air in output from a compressor 16. The function of this exchanger 12 is to recover and use the waste energy from a metallurgical or steel plant (i.e., the heat source 14). A flow of cold air (for example at room temperature or even lower) enters the system 10 from the compressor 16, which is compressed and divided into two flows, of which the first Fi is conveyed to a second heat exchanger 18, for example an aftercooler having the function of cooling the compressed air, for example by means of water or air (depending on economic convenience), removing heat, while the second flow F2 is heated by means of the first heat exchanger 12 using a heat source 14. The air thus compressed and heated enters a turbine 20 which is mechanically connected by means of a coupling 22 to the compressor 16; the three elements together form what is called a turbo-compressor 24. The turbine 20, exploiting the expansion of compressed air at high temperature, drags the compressor 16, which replaces, at least partially, the compressors for the production of compressed air of the state of the art known to be powered by an electric motor (see Fig. 1). Hot air exploitable by other devices exits from the turbine 20 of the turbo-compressor 24. The turbo-compressor 24 preferably operates at low pressure, for example up to 3 - 5 bar(g). The system 10 includes on the first duct Fi a compressed air injector 17, which preferably introduces into the system 10, during the starting step thereof, compressed air that is heated in the first heat exchanger 12 and sent to the expansion unit 20, so that by means of the expansion work in the latter, the respective compressor 16 can be rotated, setting the system 10 into operation. Once the system 10 is fully operational, the compressed air inlet from outside the circuit can be interrupted. In this regard, a regulating valve 19 is provided.

[0080] Fig. 3 depicts a basic diagram of a second embodiment of a system for producing compressed air in two stages I and II. The main concept corresponds to that depicted in Figure 2. The system for producing compressed air 110 includes a main heat exchanger 112 which is positioned between a heat source 114 to be exploited and the duct for the transit of the compressed air in output from a compressor 128. The function of this heat exchanger 112 is to recover the waste energy of a metallurgical or steel plant (i.e., the heat source 114). Cold air (for example at room temperature or possibly even lower) enters the system 110 through a first compressor 116, is cooled in a third heat exchanger 134 (also called intercooler), further compressed in a second compressor 128 and divided into two flows of which the first Fi is conveyed in a second exchanger 118, for example an aftercooler which has the function of cooling the compressed air by water or air (depending on economic convenience) removing the heat therefrom, while the second flow F2 is heated by means of the first heat exchanger 112 through a heat source 114. The compressed air thus heated then enters a first turbine 132, this turbine 132 being mechanically connected by means of a coupling 130 to the second compressor 128; the three elements together determine a turbo-compressor 126. Subsequently, the air enters a second turbine 120 which is in turn connected by means of a mechanical transmission device 122 to the first compressor 116. The turbines 120, 132, exploiting the expansion of the compressed air at high temperature, drag the respective compressors 116, 128, which again replace, at least partially, the compression systems based on the drive by an electric motor usually known in the state of the art. Hot air exploitable by other devices exits from the turbine 120. Unlike Figure 2, the system 110 includes two turbo-compressors 124, 126. The turbo-compressor 124 is at low pressure (stage I) and the other 126 at another pressure (stage II). Furthermore, a further heat exchanger 134 has been added to cool the compressed air in output from the first compressor 116 (first stage I) in order to reduce the specific work of the second compressor 128 of the second stage II. This configuration is particularly suitable when the compressed air utility requires a pressure flow rate of up to about 8 barg. The system 110 includes, like the system 10 depicted in Figure 2, an additional compressed air injector 117 with a respective regulating valve 119.

[0081] Fig. 4 depicts a basic diagram of a third embodiment of a system for producing compressed air which integrates the system depicted in Figure 2 with an electric booster 236. The system 210 is substantially identical to that of Figure 2, with corresponding elements: a turbo-compressor 224 provided with a compressor 216 connected by means of a shaft 222 with a turbine 220, a first heat exchanger 212 fed by a heat source 214, and a second heat exchanger 218, possibly also comprising an injector as described above. The operations are also substantially identical between the system 10 of Figure 2 and the system 210 of Figure 3. In both cases it is a single stage system. The only difference lies in the addition of a booster or an electric compressor 236 which has the purpose of increasing the pressure level on the branch Fi that sends the compressed air to a utility.

[0082] Fig. 5 depicts a basic diagram of a fourth embodiment of a system for producing compressed air which is provided with an auxiliary device 338 for starting the system (which could also be present in the variants described above, as the booster described above could also be present). The system 310 is practically identical to that of Figures 2 to 4, comprising corresponding elements: a first heat exchanger 312 fed by a heat source 314 and a second heat exchanger 318 for cooling the compressed air of the branch Fi. Figure 5 does not specifically describe the compression-expansion system C which is generally indicated and can comprise one or more turbo-compressors (not depicted) and possibly also an intercooler. Unlike Figures 2 and 3, an auxiliary device 338 in input to the turbine line is seen in Figure 5. The auxiliary device 338 can be selected from an electrical resistance, radiant tubes, a burner, etc. and is capable of facilitating the starting of the system 310, lowering the flow rate and consumption of compressed air.

[0083] Fig. 6A depicts a basic diagram of a fifth embodiment of a system for producing compressed air in which an electric compressor 440 is provided downstream of the compressor 416 forming part of the turbo-compressor 424, while Fig. 6B depicts a basic diagram of a sixth embodiment of a system for producing compressed air in which an electric compressor 540 is provided upstream of the compressor 528 forming part of the turbo-compressor 526. The systems differ from the system 110 of Figure 2 only in that one of the turbo-compressors has been replaced by an electric compressor 440, 540. The latter can then be installed upstream or downstream of the compressor 416, 528 forming part of the turbo-compressors 424, 526. In the case of the "downstream" position of the compressor 416 forming part of the turbo-compressor 424, the two heat exchangers 412 and 418, the heat source 414, the third heat exchanger 434 between the two stages I and II are noted; and in the case of the "upstream" position of the compressor 540 forming part of the turbo-compressor 526, the two heat exchangers 512 and 518, the heat source 514, the third heat exchanger 534 between the two stages I and II are noted. In the case of Figure 6A there is a turbo-compressor 424 with a compressor 416 coupled by means of a shaft 422 to a turbine 420, and downstream of this compressor 416 an electric compressor 440; while in the case of figure 6B there is a turbo-compressor 526 with a compressor 528 coupled by means of a shaft 530 to a turbine 532, and upstream of this compressor 528 an electric compressor 540. The third heat exchanger 434, 534 is located between the compressor 416, 528 of the respective turbo-compressor 424, 526 and the electric compressor 440, 540.

[0084] Fig. 7A depicts a basic diagram of a seventh embodiment of a system for producing compressed air in a single stage in which a motor / generator 642 is provided coupled to the shaft 622 of the turbo-compressor 624; while Fig. 7B depicts a basic diagram of an eighth embodiment of a system for producing compressed air in two stages in which a motor / generator 742A, 742B is provided coupled to the shafts 722, 730 of the turbo-compressors 724, 726, depending on its use, useful for starting the system or for electricity production. The single-stage system 610 is substantially identical to that of Fig. 2, with corresponding elements: a turbo-compressor 624 with a compressor 616 connected with a shaft 622 to a turbine 620, a first heat exchanger 612 fed by a heat source 614, and a second heat exchanger 618. The operations are also substantially identical between the system 10 of Figure 2 and the system 610 of Figure 7A. In both cases it is a single stage system. The only difference is the addition of a motor / generator 642 coupled to the shaft 622 of the turbo-compressor 624. The system for producing compressed air 710 has a main heat exchanger 712 which is inserted between a heat source 714 to be exploited and compressed air in output from a compressor 728. The function of this heat exchanger 712 is to recover the waste energy of a metallurgical or steel plant (i.e., the heat source 714). In the system 710, cold air enters the compressor 716, is heated in the third heat exchanger 734, further compressed in the compressor 728 and divided into two streams of which the first Fi feeds a second heat exchanger 718, an aftercooler, which has the function of cooling the compressed air by means of water or air (depending on economic convenience) and removing heat, while the second stream F2 is heated in the first heat exchanger 712 by the heat provided by a heat source 714. The compressed air thus heated enters a first turbine 732 and subsequently a second turbine 720 which are mechanically coupled by means of a corresponding shaft 722, 730 to the respective compressor 716, 728. The turbines 720, 732, exploiting the expansion of the compressed air at high temperature, drag the compressors 716 and 728, respectively. Hot air exploitable by other devices exits from the turbine 720. There are therefore two turbocompressors 724, 726: one at low pressure (that of stage I) and the other at high pressure (stage II). Furthermore, an additional heat exchanger 734 is added to cool the compressed air in output from the first compressor 716 (first stage I) to reduce the specific work of the second stage II compressor 728. In the two-stage case of Figure 7B, two motors 742A, 742B are therefore provided which are mechanically or electronically coupled. This generates the power needed to start the system and will be decoupled at full operations. In the drawings, the compression units are formed respectively by a single compressor (16, 216, 416, 616) or by a combination of compressors (116 + 128; 416 + 440; 540 + 528; 716 + 728), as the expansion units are formed by a single turbine (20; 220; 420; 532; 620) or by a combination of turbines (120 + 132; 720 + 732).

Claims

CLAIMS1) A metallurgical or steel plant comprising(A) a system (10; 110; 210; 310; 410; 510; 610; 710) for producing compressed air comprising:(a) a compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) configured to receive air and to perform thereon a compressing work and then to emit it in compressed form;(b) an expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732) configured to receive compressed air and to generate thereon an expansion work and to then emit it in expanded form;(c) at least one mechanical transmission device (22; 122, 130; 222; 422; 530; 622; 722, 730) connecting said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) and said expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732), which is configured to receive said expansion work and to provide it to said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) as compression work;(d) at least a first duct (F2) for compressed air, connecting said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) and said expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732);(e) a heating device arranged between said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) and said expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732) along said first duct (F2) for compressed air and configured to heat passing compressed air;(f) at least a second duct (Fi) for the transport of compressed air downstream of said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728), or a portion thereof, configured to deviate a portion of the compressed air produced during operation by said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) to a utility included in said metallurgical or steel plant; wherein said heating device comprises(e-1) a first heat exchanger (12; 112; 212; 312; 412; 512; 612; 712) configured to be fed by a thermo vector fluid containing waste heat from said metallurgical or steel plant;and wherein said system for producing compressed air (10; 110; 210; 310; 410; 510; 610; 710) further comprises(g) a cooling device installed downstream of said compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 728) along said second duct (Fi) and comprising a second heat exchanger (18; 118; 218; 318; 418; 518; 618; 718) for extracting heat from said deviated compressed ai; and(B) an apparatus (14; 114; 214; 314; 414; 514; 614; 714) producing a thermo vector fluid containing waste heat preferably originating from furnaces, for example melting furnaces and reheating furnaces, and comprising a device for extracting said thermo vector fluid containing waste heat; wherein said device for extracting said thermo vector fluid containing waste heat is connected to said first heat exchanger (12; 112; 212; 312; 412; 512; 612; 712).2) The metallurgical or steel plant according to claim 1, characterized in that- said compression unit (116, 128; 416, 440; 540, 528; 716, 728) is a multi-stage compression unit comprising a plurality of compressors connected in series and / or in parallel;- said expansion unit (120, 132; 420; 532; 720, 732) comprises a turbine or a plurality of turbines connected in series and / or in parallel; wherein at least one of the compressors (116, 128; 416; 528; 716, 728) of said plurality of compressors is connected by means of a respective mechanical transmission device (122, 130; 422; 530; 722, 730) to one of said turbines (120, 132; 420; 532; 620; 720, 732) forming a turbocompressor (124, 126; 424; 526; 726, 724).3) The metallurgical or steel plant according to claim 1 or 2, characterized by further comprising(h) an auxiliary device (338; 440; 540; 642; 742A, 742B) configured to start or to facilitate the starting of the system (310; 410; 510; 610; 710), in particular of said one or more turbo-compressors (424; 526; 624; 724, 726).4) The metallurgical or steel plant according to claim 3, characterized in that said auxiliary device (338) is arranged at the inlet of said expansion unit and is preferably selected from an electrical resistance, radiant tubes and a burner.5) The metallurgical or steel plant according to claim 3, characterized in that said auxiliary device is an injector (17; 117) of supplemental compressed air arranged upstream of the first heat exchanger (12; 112), and is preferably provided with a regulating valve (19; 119).6) The metallurgical or steel plant according to claims 2 and 3, characterized in that said auxiliary device (440; 540) is an electric compressor forming part of said plurality of compressors (416, 440; 540, 528) connected in series and / or in parallel.7) The metallurgical or steel plant according to claims 2 and 3, characterized in that said auxiliary device (642; 742A, 742B) is at least a motor or generator installed along the mechanical transmission connection between a compressor (616; 716, 728) and a turbine (620; 720, 732).8) The metallurgical or steel plant to any one of claims 2 to 7, characterized in that a cooling device comprising a third heat exchanger (134; 434; 534; 734) for heat removal is arranged between at least two compressors (116, 128; 416, 440; 540, 528; 716, 728).9) The metallurgical or steel plant according to any one of the preceding claims, characterized in that said expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732) is configured to perform a partial expansion and to send at least a portion of the hot air to a utility, such as a reheating furnace or a burner, due to the residual amount of energy contained therein.10) The metallurgical or steel plant according to any one of the preceding claims, characterized in that said utility is a continuous casting machine.11) The metallurgical or steel plant according to any one of the preceding claims, characterized in that said utility is selected from the group consisting of a reheating furnace, a ladle or tundish preheating system and a burner system.12) The metallurgical or steel plant according to claim 11, characterized in that said utility corresponds to the apparatus (14; 114; 214; 314; 414; 514; 614; 714) producing the thermo vector fluid.13) The metallurgical or steel plant according to claim 8, characterized in that said utility requires a pressure flow rate of up to about 8 barg.14) The metallurgical or steel plant according to any one of the preceding claims, characterized in that said second heat exchanger (18; 118; 218; 318; 418; 518; 618; 718) feeds a district heating.15) A process for producing compressed air comprising the following steps:(I) compression of air in a compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) with the generation of compressed air;(II) division of the compressed air into at least two portions (Fi, F2);(III) cooling a first portion of compressed air (Fi) and sending said first portion (Fi) to a utility in a metallurgical or steel plant;(IV) heating of a second portion of compressed air (F2) and sending the second portion of the, heated, compressed air (F2) to an expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732) wherein the generated expansion work is used at least partially, preferably completely, to drive the compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728); wherein(i) said phase (IV) heating takes place in a first heat exchanger (12; 112; 212; 312; 412; 512; 612; 712) by recovering heat from said thermo vector fluid containing waste heat from said steel or metallurgical plant; and(ii) the heat recovered in a second heat exchanger (18; 118; 218; 318; 418; 518; 618; 718) during said phase (III) cooling is used to heat utilities in said metallurgical or steel plant; and wherein, optionally:- the compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) comprises a plurality of compressors;- between at least two compressors (116, 128; 416, 440; 540, 528; 716, 728) a third heat exchanger (134; 434; 534; 734) is arranged to cool the compressed air leaving the first compressor (116; 416; 540; 716) and entering the second compressor (128; 440; 528; 728)- a part of the energy contained in the air is not transformed into expansion work in said expansion unit (20; 120, 132; 220; 420; 532; 620; 720, 732), but is used to feed not the compression unit (16; 116, 128; 216; 416, 440; 540, 528; 616; 716, 728) but a different utility.16) The process for producing compressed air according to claim 15, characterized in that the starting of said compression unit (528; 616; 716, 728) and / or of said expansion unit (420) is facilitated by at least one of the following auxiliary devices(а) an injector (17; 117) of supplemental compressed air upstream of the first heat exchanger (12; 112), preferably provided with a control valve (19; 119);(P) an auxiliary device (338), preferably selected from an electrical resistance, radiant tubes and a burner, placed at the inlet of said expansion unit;(y) an electric compressor (440; 540) connected in series and / or in parallel with one or more compressors (416; 528) comprised in said compression unit;(б) a motor or generator (642; 742A, 742B) placed along a mechanical transmission connection between said compression unit (616; 716, 728) and said expansion unit (620; 720, 732).

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