WO2025169099A1 - System and process for recovery of thermal energy from waste fumes - Google Patents

Abstract

The invention relates to a system (10; 110; 210) and a related process for recovering thermal energy from waste fumes from metallurgical/steel plants. The waste fumes are compressed in a compression unit (12; 112; 212) connected to an auxiliary group to start and/or keep the system running (16; 116; 216), preferably cooled after compression through heat exchangers (14, 18; 114, 118; 214, 218) and subsequently freed from CO₂ in a carbon dioxide capture plant (20; 120; 220). The so cleaned fumes are heated and enter an expansion unit (24; 124; 224). The heating of the fumes entering the expansion unit takes place with heat exchangers (22; 114, 118, 122; 222) whose conduits for conveying a heat-releasing fluid are fed directly by hot waste fumes or in the aforementioned cooling exchangers by the compressed hot fumes that pass through them.

Description

TITLE: SYSTEM AND PROCESS FOR RECOVERY OF THERMAL ENERGY FROM WASTE FUMES

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

TECHNICAL FIELD

The present invention relates to a system and a process for recovering thermal energy from waste fumes, in particular from fumes from metallurgical or steel plants.

BACKGROUND OF THE INVENTION

Metallurgical and steel plants produce large amounts of waste fumes rich in carbon dioxide and provide considerable amounts of heat in excess of what is used by the process itself, a heat that must be recovered and used or dissipated.

The capture of carbon dioxide from waste fumes is important for the protection of the environment and climate.

In metallurgical and steel plants, the process for capturing carbon dioxide (CO2) aims to reduce the environmental impact of these emissions, such as those originating in general from melting furnaces (for example EAF, acronym for electric arc furnace), from reheating furnaces (RHF) and from other processes where carbon dioxide is generated together with other gases. It should be noted that CO2 capture processes often require, in order to increase the capture efficiency, waste fumes compressed at a certain pressure which, in a traditional plant, results in the consumption of electricity in order to achieve this condition.

DISCLOSURE OF THE INVENTION

The invention aims to overcome the aforementioned drawbacks and to propose a system and a related process that are able to recover the waste heat contained in waste fumes from metallurgical or steel plants and to remove CO2, at least partially, from the fumes in an economical and simple way. Another object of the invention is to reduce the electrical consumption necessary to capture carbon dioxide from the fumes. Further objects or advantages of the invention will become apparent from the following description.

In a first aspect of the invention, the object is achieved by a system for recovering thermal energy from waste fumes from metallurgical or steel plants that comprises:

(a) a compression unit configured to receive and compress waste fumes;

(b) an expansion unit;

(c) a transmission member which connects said compression unit and said expansion unit and is configured for transmitting the mechanical energy obtained in the expansion unit at least partially to said compression unit, wherein the compression unit, the expansion unit and the transmission member form a turbo-compressor;

(d) an auxiliary group to start and/or keep the system running, e.g. a compressor, preferably of the electrical type, connected in series with said compression unit;

(e) a carbon dioxide capture plant arranged downstream of said compression unit and said auxiliary group to start and/or keep the system running;

(f) a first heat exchanger arranged upstream of said expansion unit comprising a conduit for conveying a heat-releasing fluid and a conduit for conveying a heat-receiving fluid; and

(f) pipes suitable for conveying a compressed fluid connecting the components listed under points (a), (b), (d), (e) and (f); wherein the conduit for conveying the heat-receiving fluid of said first heat exchanger feeds said expansion unit; and wherein the flow direction of said waste fumes proceeds through the compression unit and the auxiliary group to start and/or keep the system running to the expansion unit. In embodiments of the invention, the recovery system can comprise further compression units, which as will be described also perform the function of starting, as well as of keeping under pressure, arranged in succession, or expansion can take place in several stages and therefore by means of several expansion units. The expansion unit in general comprises a turbine. The number of heat exchangers can also be increased.

The combination of compression units and expansion units, mutually connected by means of a mechanical transmission device, configures a so-called turbo-compressor. The compression unit sucks the fumes to be compressed and is actuated by the mechanical transmission device, actuated in turn by means of the expansion work of the fumes that takes place in the turbine of the expansion unit.

The auxiliary group to start and/or keep the system running preferably comprises at least one auxiliary device selected from the group consisting of an electric resistance, an air or steam injector, a fuel-feedable burner and an electric compressor. The fuel-feedable burner is preferably configured to heat the fumes directly or indirectly, for example by means of radiant pipes.

A heat exchanger has the task of transferring thermal energy from one medium or fluid to another. One of the media or fluids acts as a heat-releasing agent, the other as a heat receiver. The important aspect is that the two media or fluids are located in two separate conduits, one for conveying the heat-releasing fluid and the other one for the heat-receiving fluid. Fluids may have different states of aggregation. One of the two fluids may therefore, for example, be gaseous, the other liquid.

The operating principle of the recovery system illustrated above makes it possible to use the heat of the waste fumes to produce compressed fumes necessary in the CO2 capture plant and therefore to contribute to the extraction of carbon dioxide from the fumes with a considerable saving of electrical energy.

The heat exchangers present in the system also allow, for example, the residual heat from compression to be used as an additional energy source, as will be illustrated below. According to the invention, the residual heat generated by the compressors in the heating circuit is no longer dispersed, but used: for example, heat exchangers reintroduce a portion into the fume heating system to prepare them for expansion, with obvious environmental benefits.

Plants for capturing CO2 in various forms are known in the state of the art and the person skilled in the art identifies the one most suitable for their operating conditions with their common general knowledge. A detailed description of these systems can therefore be waived. In principle, the capture plant uses chemical reactions or the phenomenon of absorption to separate carbon dioxide from other components present in the fumes, for example through formation of potassium carbonate or carbamates. Alternatively, technologies for filtration of CO2 through membranes are known.

In a preferred embodiment of the invention, the system for recovering thermal energy from waste fumes further comprises: (g) a second heat exchanger; and optionally

(h) a third heat exchanger, wherein the second and third heat exchanger, if provided, are arranged respectively at the outlet of the compression unit or at the outlet of the auxiliary group to start and/or keep the system running, and wherein said second and said third heat exchanger each comprise a conduit for conveying a heat-releasing fluid and a conduit for conveying a heat-receiving fluid and wherein the conduits for conveying the heat-releasing fluid are configured to be crossed by the compressed fumes originating respectively from the compression unit and from the auxiliary group to start and/or keep the system running.

The cooling of the fumes, necessary for the purposes of their use in the carbon dioxide capture plant, with respective heat exchangers allows the heat removed to be used in other utilities or, as will be seen below, in the recovery system according to the invention.

In an advantageous configuration of the recovery system according to the invention, the system comprises both the second and the third heat exchangers which are arranged, respectively between said compression unit and the auxiliary group to start and/or keep the system running and downstream of the series formed by said compression unit and said auxiliary group to start and/or keep the system running, and upstream of said carbon dioxide capture plant.

In this configuration, the pipes advantageously connect the elements of the recovery system in the order indicated:

- the series formed by the compression unit and the auxiliary group to start and/or keep the system running and the conduits for conveying the heat-releasing fluid of the second and third heat exchangers;

- the carbon dioxide capture plant;

- the conduit for conveying the heat-receiving fluid of the first heat exchanger; and

- the expansion unit.

An alternatively advantageous connectivity of the aforementioned configuration provides that the pipes connecting the elements of the recovery system connect in the order indicated: - the series formed by the compression unit and the auxiliary group to start and/or keep the system running and the conduits for conveying the heat-releasing fluid of the second and third heat exchangers;

- the carbon dioxide capture plant;

- the conduit for conveying the heat-receiving fluid of the second heat exchanger;

- the conduit for conveying the heat-receiving fluid of the third heat exchanger;

- the conduit for conveying the heat-receiving fluid of the first heat exchanger and

- the expansion unit.

This configuration is particularly advantageous because it allows the heat produced during compression to be used directly within the recovery system to heat the fumes for the expansion unit. The first heat exchanger located downstream of the carbon dioxide capture plant in this case may contribute less to heating or even be eliminated altogether. A respective configuration is illustrated below.

In a particularly advantageous embodiment of the invention, the system comprises in addition to the first heat exchanger a second heat exchanger comprising a conduit for conveying the heat-releasing fluid and a conduit for the heat-receiving fluid. In this case, the first heat exchanger is arranged downstream of the series formed by the compression unit and the auxiliary group to start and/or keep the system running and upstream of said carbon dioxide capture plant, while the second heat exchanger is arranged between said compression unit and said auxiliary group to start and/or keep the system running. Both heat exchangers are crossed, in the conduits for the heat-releasing fluid, by the compressed fumes exiting the compression elements and simultaneously, in the conduits for conveying the heat-receiving fluid, by the fumes exiting the CO2 capture plant, the latter are used to cool the compressed fumes and get heated at the same time. The first and second heat exchanger in this configuration are not only used to heat the fumes destined for the expansion unit, but also to cool the compressed fumes exiting the compression elements.

Cooling is useful in order to bring the fumes to the appropriate temperature for the correct operation of the carbon dioxide capture plant. In an alternative or supplementary case, it is still possible to provide directly in the carbon dioxide capture unit for a heat exchanger, such as a water/fume exchanger, to adjust the temperature of the fumes to the needs of the carbon dioxide capture plant. In fact, advantageously, in this preferred configuration, the pipes connect the elements of the system in the order indicated:

- the series formed by the compression unit and the auxiliary group to start and/or keep the system running and the conduits for conveying the heat -releasing fluid of the first and second heat exchangers;

- the carbon dioxide capture plant;

- the conduit for conveying the heat-receiving fluid of one of the two heat exchangers, preferably of the second heat exchanger;

- the conduit for conveying the heat-receiving fluid from the other of the two heat exchangers, preferably from the first heat exchanger; and

- the expansion unit.

This advantageous embodiment provides for the recovery of the compression thermal energy contained in the compressed fumes coming from the compression unit and the auxiliary group to start and/or keep the system running. In other words, the compressed fumes exchange heat with themselves in the heat exchangers used to cool the fumes exiting the compression unit and the auxiliary group to start and/or keep the system running and then use the removed heat for heating the fumes in preparation for expansion. This is an internal regeneration of the available heat. This configuration is particularly suitable when the fumes exiting the compression unit and the auxiliary group to start and/or keep the system running have a temperature close to that required at the turbine inlet.

In alternative embodiments of the invention, the auxiliary group to start and/or keep the system running is arranged, viewed in the direction of flow of the waste fumes, upstream or downstream of the compression unit. The position of the auxiliary group to start and/or keep the system running can vary without affecting the operation of the system. A position close to the carbon dioxide capture plant is preferable in order to adjust the correct pressure for the CO2 capture operation. In the recovery system according to the invention it is however possible to change the succession between the rotating machines or the compression devices. As said, said component may preferably be a compressor, even more preferably of an electrical type. However, other circuit starting systems are not excluded, such as for example the injection of compressed air or other compressed gas into the circuit by means of pressurised tanks, in which the entry of the gas into the circuit is allowed by a special regulation system. A second aspect of the invention relates to a metallurgical or steel plant which comprises:

(i) a system for recovering thermal energy from waste fumes according to the invention;

(ii) at least one apparatus producing hot waste fumes, preferably selected from melting furnaces and reheating furnaces, and comprising a device for extracting waste fumes; wherein said device for extracting hot fumes is connected to an inlet of said compression unit or of said auxiliary group to start and/or keep running the system and optionally, in the case of the recovery system that provides for a heat exchanger that does not use the heat coming from the exchangers used for cooling the compressed fumes, to the conduit for conveying the heatreleasing fluid of said first heat exchanger. A metallurgical or steel plant equipped with a system according to the invention could become practically autonomous in capturing CO2 as it can use internal energy sources, reducing the possible collection of electrical energy since said internal energy sources can derive from waste energy.

A third aspect of the invention concerns a process for recovering thermal energy from waste fumes from metallurgical or steel plants, comprising the following steps:

(I) compression of waste fumes in a compression unit connected in series to an auxiliary group to start and/or keep the system for the compression of the fumes running;

(II) cooling of the compressed fumes exiting the compression unit and an auxiliary group to start and/or keep the system running with heat exchangers;

(III) extraction of carbon dioxide from compressed and cooled fumes;

(IV) heating of the fumes at least partially freed from carbon dioxide with one or more heat exchangers;

(V) expansion of the heated fumes and the use of at least a portion of the expansion work to drive the compression unit.

The process uses the components described so far in its steps. Advantageously, the process is implemented in a recovery system according to the invention.

The heat exchanger for heating in step (IV) can advantageously be fed directly by waste fumes from the metallurgical or steel plant.

The fumes themselves provide the energy for their purification in the carbon dioxide capture plant, in the turbo-compressor that self-supports the system at least partially and, where appropriate, in the power supply of the first heat exchanger. In another variant of the process according to the invention, the fumes themselves produce thermal energy during their compression which in turn can be used to heat the fumes entering the expansion unit.

These variants, in fact, provide that in the process according to the invention, the fumes exiting step (III) are heated in step (IV):

- with a heat exchanger whose conduit for conveying heat-releasing fluids is fed with hot waste fumes originating from melting furnaces and/or reheating furnaces and/or

- with the heat exchangers of step (II) whose conduits for conveying the heat-receiving fluid are fed with the fumes exiting from step (III).

During the process for producing and treating steel or, more generally, metals and their alloys, a considerable amount of waste heat is generated, which can be exploited by means of the present recovery system according to the invention to partially replace the electric compressors, to exploit the heat of the compression of the fumes or contained in the fumes exiting from the metallurgical or steel furnaces, to prepare the fumes to the expansion unit, to use the cooled fumes in the CO2 capture plant, to cool the fumes exiting from the individual compression elements, and all this with the simultaneous purification of the fumes from CO2, according to the objects of the invention. This approach also allows to reduce electricity consumption, contributing to greater sustainability in the steel sector. According to the invention, the waste thermal energy is economically transformed into compression energy usable in CO2 capture processes.

The operation of the system takes place, for example, as follows: a first-stage compressor sucks the waste fumes from the upstream process and compresses them to an intermediate pressure. At this point, the fumes are conveyed into a heat exchanger to lower their temperature. An auxiliary group to start and/or keep the system running, in the preferential case where it is an electric compressor, further compresses the fumes to the desired pressure for the CO2 capture plant in a second stage. Subsequently, the fumes are cooled in a further heat exchanger and subsequently enter the capture plant, where part of the CO2 present in the fumes is captured and tapped by the system, while the other part is conveyed together with the remaining fumes in a downstream heat exchanger, in turn connected to the waste heat source and/or to the conduits for conveying the heat-receiving fluid from the heat exchangers for cooling the compressed fumes before entering the carbon dioxide capture plant. At this point, the hot compressed fumes (low in CO2) expand and generate the rotation of the impeller of the expansion unit which in turn, through the mechanical transmission element, supports the rotation of the first-stage compressor.

The remaining part of the energy that feeds the present recovery system and that exits the expansion unit will be available again in thermal form, but contained in a treated fluid, cleaner than the fumes from which heat is extracted.

The features described for one aspect of the invention can be transferred mutatis mutandis to the other aspects of the invention. The 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 waste heat recovery system for obtaining compression energy and CO2 capture shows an innovation in the steel and metallurgical sector. By also exploiting residual thermal energy, the CO2 capture plant can improve its energy efficiency, reduce greenhouse gas emissions, and promote greater environmental and operational sustainability.

The choice of the configuration of the recovery plant depends on the specific needs and operating conditions of the capture process, but also on the integration with the waste energy source which may be of a different nature: for example the hot fumes used directly and/or the heat developed during compression.

The invention achieves its intended objects and enjoys industrial applicability as it provides a system and process for recovering heat from waste fumes and simultaneously capturing CO2 from them, which has the following benefits:

• Reduced operating costs to capture CO2, contributing to greater economic sustainability.

• Increased energy efficiency: The recovery of thermal energy makes it possible to exploit an otherwise dispersed resource, increasing the overall efficiency of the plant.

• Reduction of greenhouse gas emissions: using the recovered thermal energy for the compression of the fumes, the plant reduces dependence on electricity and thus also secondary emissions, as well as direct emissions from the plant.

Said objects and advantages will be further highlighted in the description of preferred examples of embodiments of the invention given by way of non-limiting example only

Variants 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

Fig. 1 shows a principle diagram of a first embodiment of a system for recovering thermal energy from waste fumes from metallurgical or steel plants that shows the basic configuration.

Fig. 2 shows a principle diagram of a second embodiment of a system for recovering thermal energy from waste fumes from metallurgical or steel plants.

Fig. 3 shows a principle diagram of a third embodiment of a system for recovering thermal energy from waste fumes from metallurgical or steel plants.

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

Fig. 1 shows a principle diagram of a first embodiment of a system 10 for recovering thermal energy from waste fumes from metallurgical or steel plants that shows the basic configuration that can then be implemented in different configurations, according to the specific needs of the process. The main configuration includes:

- Heat exchangers, including a main heat exchanger 22 that is inserted between a heat source (not depicted) from which to recover waste energy and the fumes exiting a CO2 capture plant 20. The function of this heat exchanger 22 is to recover waste energy. In the absence of waste heat or its integration, the heat source could be an active source, such as for example an electrical resistance, a fuel-feedable burner that heats the fumes directly or indirectly through radiant tubes, a steam injector. In addition, intermediate heat exchangers 14 and 18 are provided. These heat exchangers 14, 18 have the function of cooling the fumes following compression by, for example, cooling water;

- a turbo-compressor 28: this machine comprises a compressor 12 coupled by means of a mechanical transmission member 26 to an expansion unit 24 with a turbine;

- an auxiliary group to start and/or keep the system running 16, preferably consisting of a compressor preferably of an electric type: it is therefore in the preferential case a second-stage compressor that is driven by an electric motor. This component has the function of bringing the fumes to the pressure desired by the CO2 capture plant 20, in addition to assisting at the start and in self-support of the turbo-compressor 28.

The recovery system works as follows: the fumes to be treated enter the first compressor 12 which is connected via a transmission member 26 to the expansion unit 24. These three elements together form the turbo-compressor 28. The fumes compressed in the first stage are cooled in a first heat exchanger 14 to subsequently be further compressed in the auxiliary group to start and/or keep the system running 16, which can be as seen a preferably electric compressor, but other systems are not excluded. Upon exiting therefrom, the further compressed fumes are cooled in a second heat exchanger 18 and then pass through a CO2 capture plant 20 which allows the at least partial extraction of carbon dioxide from the fumes which can then be, for example, chemically fixed in the form of carbonates or bicarbonates. At the exit of the CO2 capture plant 20, the treated fumes pass through another heat exchanger 22 to be heated and finally reach the turbine in the expansion unit 24 which, collecting expansion work, drives the first compressor 12. The fumes exiting the expansion unit 24 are cleaner, in terms of CO2, than the fumes entering the compression unit 12.

Fig. 2 shows a principle diagram of a second embodiment of a system 110 for recovering thermal energy from waste fumes from metallurgical or steel plants. The fumes to be treated enter the compression unit 112 and are compressed. The heat generated by the compression is removed in a heat exchanger 114. The fumes thus cooled are further compressed in a preferably electric compressor 116 and then further cooled in a heat exchanger 118. The fumes compressed and cooled in two stages reach the plant 120 for the capture of CO2 which is removed. The fumes deprived of at least a part of the CO2 are not heated in an exchanger fed by a heat source (heat exchanger 22 not present), but by the flow of compressed hot fumes passing through the two heat exchangers 114, 118 mentioned above, as the purified fumes pass through said two heat exchangers 114, 118. A pipe a carries the carbon dioxide-free fumes into the exchanger 114, then with pipe b into the exchanger 118 and from there with the pipe c into the turbine of the expansion unit 124. Advantageously, the direction of travel of the cold fumes first crosses the exchanger at a lower temperature and then the one with higher temperatures to efficiently exchange heat. In these configurations the recovered waste heat is reduced, while in a variant of the embodiment example, the section c may comprise another heat exchanger 122 fed by an external heat source, for example hot waste fumes from a melting furnace or a reheating furnace.

Fig. 3 shows a principle diagram of a third embodiment of a system 210 for recovering thermal energy from waste fumes from metallurgical or steel plants corresponding to that of figure 1 with an auxiliary group to start and/or keep the system running 216 which is preferably an electric compressor, a compression unit 212, three heat exchangers 214, 218, 222, a carbon dioxide capture plant 220, an expansion unit 224 which is connected via a transmission member 226 to the compression unit 212 in which the last mentioned three elements form a turbocompressor 228. Unlike figure 1, the turbo-compressor 228 and the preferably electric compressor 216 switched positions. It is not excluded that figure 3 can also incorporate the elements described in figure 2.

Claims

1) A system (10; 110; 210) for recovering thermal energy from waste fumes from metallurgical or steel plants comprising:

(a) a compression unit (12; 112; 212) configured to receive and compress waste fumes;

(b) an expansion unit (24; 124; 224);

(c) a transmission member (26; 126; 226) which connects said compression unit (12; 112; 212) and said expansion unit (24; 124; 224) and is configured for transmitting the mechanical energy obtained in said expansion unit (24; 124; 224), at least partially, to said compression unit (12; 112; 212), wherein the compression unit (12; 112; 212), the expansion unit (24; 124; 224) and the transmission member (26; 126; 226) form a turbocompressor (28; 128; 228);

(d) an auxiliary group to start and/or keep the system running (16; 116; 216), e.g., a compressor, preferably of the electrical type, connected in series with said compression unit (12; 112; 212);

(e) a carbon dioxide capture plant (20; 120; 220) arranged downstream of said compression unit (12; 112; 212) and of said auxiliary group to start and/or keep the system running (16; 116; 216);

(f) a first heat exchanger (22; 122, 118; 222) arranged upstream of said expansion unit (24; 124; 224) comprising a conduit for conveying a heat-releasing fluid and a conduit for conveying a heat-receiving fluid; and

(f) pipes suitable for conveying a compressed fluid connecting the components listed under points (a), (b), (d), (e) and (f); wherein the heat-receiving fluid conduit of said first heat exchanger (22; 122, 118; 222) feeds said expansion unit (24; 124; 224); and wherein the flow direction of said waste fumes proceeds through the compression unit (12; 112; 212) and the auxiliary group to start and/or keep the system running (16; 116; 216) to the expansion unit (24; 124; 224).

2) The system (10; 110; 210) for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 1, characterized in that it further comprises: (g) a second heat exchanger (14; 114; 214); and optionally

(h) a third heat exchanger (18; 118; 218), wherein the second heat exchanger (14; 114; 214) and the third heat exchanger (18; 118; 218), if provided, are arranged respectively at the outlet of the compression unit (12; 112; 212) or at the outlet of the auxiliary group to start and/or keep the system running (16; 116; 216), and wherein said second (14; 114; 214) and said third heat exchanger (18; 118; 218) each comprise a conduit for conveying a heat-releasing fluid and a conduit for conveying a heat-receiving fluid, and wherein the conduits for conveying the heat-releasing fluid are configured to be crossed by the compressed fumes deriving respectively from the compression unit (12; 112; 212) and from the auxiliary group to start and/or keep the system running (16; 116; 216).

3) The system (10; 110; 210) for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 2, characterized by comprising both the second (14; 114; 214) and the third heat exchanger (18; 118; 218) which are arranged, respectively, between said compression unit (12; 112; 212) and said auxiliary group to start and/or keep the system running (16; 116; 216), and downstream of the series formed by said compression unit (12; 112; 212) and said auxiliary group to start and/or keep the system running (16; 116; 216), and upstream of said carbon dioxide capture plant (20; 120; 220).

4) The system (10; 210) for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 3, characterized in that said pipes connect the following elements in the order indicated

- the series formed by the compression unit (12; 212) and the auxiliary group to start and/or keep the system running (16; 216) and the conduits to convey the heat-releasing fluid of the second (14; 214) and third heat exchanger (18; 218);

- the carbon dioxide capture plant (20; 220);

- the conduit for conveying the heat-receiving fluid of the first heat exchanger (22; 222); and

- the expansion unit (24; 224). 5) The system (110) for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 3, characterized in that said pipes connect the following elements in the order indicated:

- the series formed by the compression unit (112) and the auxiliary group to start and/or keep the system running (116) and the conduits to convey the heat-releasing fluid of the second (114) and the third heat exchanger (118);

- the carbon dioxide capture plant (120);

- the conduit for conveying the heat-receiving fluid of the second heat exchanger (114);

- the conduit for conveying the heat-receiving fluid of the third heat exchanger (118);

- the conduit for conveying the heat-receiving fluid of the first heat exchanger (122); and

- the expansion unit (124).

6) The system (110) for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 1, characterized by comprising a second heat exchanger (114) comprising a conduit for conveying the heat-releasing fluid and a conduit for the heat-receiving fluid, and in that said first heat exchanger (118) is arranged downstream of the series formed by the compression unit (112) and the auxiliary group to start and/or keep running the system (116) and upstream of said carbon dioxide capture plant (120), and that said second heat exchanger (114) is arranged between said compression unit (112) and said auxiliary group to start and/or keep running the system (116), and by the fact that said pipes connect the following elements in the order indicated

- the series formed by said compression unit (112) and the auxiliary group to start and/or keep running the system (116) and the conduits for conveying the heat-releasing fluid of the first (118) and the second heat exchanger (114);

- the carbon dioxide capture plant (120);

- the conduit for conveying the heat-receiving fluid of one of the two heat exchangers, preferably of the second heat exchanger (114);

- the conduit for conveying the heat-receiving fluid of the other of the two heat exchangers, preferably of the first heat exchanger (118); and

- the expansion unit (124). 7) The system (10; 110; 210) for recovering thermal energy from waste fumes from metallurgical or steel plants according to any one of the preceding claims, characterized in that said auxiliary group to start and/or keep running the system (16; 116; 216) is arranged, viewed in the direction of flow of the waste fumes, upstream or downstream of said compression unit (12; 112; 212).

8) A metallurgical or steel plant comprising:

(i) a system (10; 110; 210) for recovering thermal energy from waste fumes originating from said metallurgical or steel plant according to any one of the preceding claims;

(ii) at least one apparatus producing hot waste fumes, preferably selected from melting furnaces and reheating furnaces, and comprising a device for extracting waste fumes; wherein said device for extracting waste fumes is connected to an inlet of said compression unit (12; 112; 212) or of said auxiliary group to start and/or keep running the system (16; 116; 216), and optionally, in the case of the system according to any one of the preceding claims 1 to 5 to the conduit for conveying the heat-releasing fluid of said first heat exchanger (22; 122; 222).

9) A process for recovering thermal energy from waste fumes from metallurgical or steel plants, comprising the following steps:

(I) compression of waste fumes in a compression unit (12; 112; 212) connected in series to an auxiliary group to start and/or keep the system running (16; 116; 216);

(II) cooling of the compressed fumes exiting the compression unit (12; 112; 212) and the auxiliary group to start and/or keep the system running (16; 116; 216) with heat exchangers (14, 18; 114, 118; 214, 218);

(III) extraction of carbon dioxide from the compressed and cooled fumes;

(IV) heating of the fumes freed at least partially from carbon dioxide with one or more heat exchangers (22; 114, 118, 122; 222)

(V) expansion of the heated fumes and the use of at least part of the expansion work to drive the compression unit (12; 112; 212). 10) The process for recovering thermal energy from waste fumes from metallurgical or steel plants according to claim 9, characterized in that the fumes exiting step (III) are heated in step (IV)

- by means of a heat exchanger (22; 122; 222) whose conduit for the heat-releasing fluids is fed with hot waste fumes originating from melting furnaces and/or reheating furnaces and/or

- with the heat exchangers (14, 18; 114, 118; 214, 218) of phase (II) whose conduits for conveying the heat-receiving fluid are fed with the fumes exiting from step (III).