Steel production method and associated plant complex

Abstract

The present invention concerns a plant complex for steel production and a method for operating the plant complex.

Description

[0001] STEEL PRODUCTION METHOD AND ASSOCIATED PLANT COMPLEX

[0002] The present invention relates to a steel production method for producing a steel.

[0003] The invention also relates to a plant complex for the production of steel for implementing such a steel production method.

[0004] Hot metal is obtained in a blast furnace from iron ores, additives such as coke and other reducing agents such as coal, oil, gas, biomasses, recycled waste plastics or other substances containing carbon and / or hydrogen. CO, CO2, hydrogen and water vapour are inevitable products of the reduction reactions. Apart from the aforementioned constituents, a blast-furnace furnace top gas drawn off from the blast-furnace process often has a high content of nitrogen. The amount of gas and the composition of the blast-furnace top gas are dependent on the feedstock and the operating mode and are subjected to fluctuations. Typically, however, blast-furnace top gas contains 35 to 60% by volume N2, 20 to 30% by volume CO, 20 to 30% by volume CO2and 2 to 15% by volume H2. Around 30 to 40% of the blast-furnace top gas produced in the production of hot metal is generally used for heating up the hot air for the blast-furnace process in air heaters; the remaining amount of top gas may also be used externally in other areas of the works for heating purposes or for electricity generation.

[0005] In the converter, which is arranged downstream of the blast-furnace process, hot metal is converted into liquid steel. By blowing oxygen onto hot metal, amounts of unwanted components such as carbon, silicon, sulphur and phosphorus are reduced. Since the oxidation processes are exothermic, scrap is often added in amounts of up to 25% with respect to the hot metal to increase the metal yield. Furthermore, lime is added as slag forming agent to remove unwanted impurities form the hot metal. A converter gas that has a high content of CO and also contains nitrogen, hydrogen and CO2 is drawn off from the steel converter. A typical converter gas composition has from 50 to 70% by volume of CO, from 10 to 20% by volume of N2, about 15% by volume of CO2 and about 2% by volume of H2. The converter gas is either burned off or, in the case of modern steel mills, captured and passed on to be used for energy supply.

[0006] The plant complex may optionally be operated in combination with a coking plant. In this case, the plant complex described at the beginning additionally comprises a coke-oven plant, in which coal is converted into coke by a coking process. In the coking of coal into coke, a coke-oven gas occurs, containing high hydrogen content and considerable amounts of CH4. Typically, coke-oven gas contains 55 to 70% by volume H2, 20 to 30% by volume CH4, 5 to 10% by volume N2and 5 to 10% by volume CO. In addition, the coke-oven gas has fractions of CO2, NH3and H2S. In practice, the coke-oven gas is used in various areas of the works for heating purposes and in the power-generating process for electricity generation. In addition, it is known to use coke-oven gas together with blast-furnace top gas or with converter gas for producing syngases.

[0007] In an integrated metallurgical plant that is operated in combination with a coking plant, approximately 40 to 50% of the raw gases, or exhaust gas, that occur as blast-furnace top gas, converter gas and coke-oven gas are used for chemical or biotechnological engineering processes. Approximately 50 to 60% of the gases produced are fed to the power-generating plant and used for electricity generation. The electricity produced in the power-generating plant covers the electricity demand for the production of hot metal and liquid steel.

[0008] It is also known to use part of the exhaust gas to feed a biotechnological plant to produce a chemical product, such as alcohols, acetone or organic acids. To this end, part of the exhaust gas is for example fed to a pressure swing adsorption (PSA) system to be enriched, for example to comprise a higher content of CO, and the enriched gas is fed to the biotechnological plant.

[0009] The quantity of chemical product obtained from the biotechnological plant is therefore directly correlated to the enriched gas throughput of the PSA system, i.e to the quantity of enriched gas produced from the exhaust gas in the PSA system. There is therefore a need to improve the performances of the PSA system.

[0010] One of the aims of the invention is to satisfy this need by proposing a steel production method wherein the efficiency of the PSA system is improved. Another aim of the invention is to decrease the global CO2 footprint of the method.

[0011] To this end, the invention relates to a steel production method for producing a steel, comprising at least the following steps:

[0012] - a molten steel production step, wherein a molten steel is produced and at least one exhaust gas is generated,

[0013] - a gas conditioning step, wherein at least a part of the exhaust gas is fed to a pressure swing adsorption system to produce an enriched gas and a tail gas from the exhaust gas,

[0014] - a chemical product production step, wherein the enriched gas is fed to a biotechnological plant to produce at least one chemical product and a residual gas, the steel production method further comprising a mixed gas production step, wherein the tail gas and the residual gas are fed to an inlet of a venturi system, the residual gas being used as a driving force to generate a venturi effect in the venturi system and to withdraw the tail gas from the swing pressure adsorption system, a mixed gas comprising the tail gas and the residual gas being recuperated at a mixed gas outlet of the venturi system and said mixed gas being sent to a CO2 storage unit. Using the residual gas exiting the biotechnological plant as a driving force to generate a venturi effect improves the flushing of the vessels of the PSA system by reducing the pressure at the tail gas outlet. Indeed, the residual gas presents a high pressure and therefore contains a high amount of energy. By converting this energy into gas speed in the venturi system, the extraction of tail gas is considerably enhanced. This extraction allows improving the flushing of the vessels of the PSA system and thereby increases the enrichment of the enriched gas, which can for example have a higher CO content. The quantity of chemical product that can be obtained from the enriched gas in the biotechnological plant can in turn therefore be increased.

[0015] The steel production method according to the invention can further comprise the following features, considered alone or according to any technically feasible combination:

[0016] - the residual gas creates a partial vacuum at a tail gas outlet of the pressure swing adsorption system, said tail gas outlet opening into the inlet of the venturi system;

[0017] - the venturi system comprises a venturi duct extending between the venturi system inlet and the mixed gas outlet, said venturi duct comprising a throttling portion, wherein the section of venturi duct is reduced compared to the section of the rest of the venturi duct, the tail gas and the residual gas being circulated in said venturi duct during the mixed gas production step;

[0018] - the residual gas is fed to the inlet of the venturi system at a pressure from 2.5 barg to 3.5 barg;

[0019] - the mixed gas presents a pressure from 0.1 barg to 0.5 barg at the mixed gas outlet of the venturi system;

[0020] - it comprises an exhaust gas compression step, wherein the exhaust gas is compressed before being fed to the pressure swing adsorption system;

[0021] - the exhaust gas comprises CO and CO2, the gas conditioning step being arranged to produce a CO enriched gas comprising a higher content of CO compared to the exhaust gas,

[0022] - the biotechnological plant is a plant for the fermentation of the enriched gas, the chemical product produced by the biotechnological plant being chosen from alcohols such as methanol, ethanol, propanol, butanol, isobutanol or 2,3-butanediol ; acetone ; and organic acids such as acetic acid, butyric acid, isobutyric acid, propionic acid, hexanoic acid or caproic acid;

[0023] - the fermentation is carried out by one of more strains of carboxydotrophic bacteria which are preferably anaerobic ; preferably the carboxydotrophic bacterium is selected from Clostridium, Moorella and Carboxydothermus, most preferably the carboxydotrophic bacterium is Clostridium autoethanogenum such as a Clostridium autoethanogenum having the identifying characteristics of DSMZ deposit number 19630 or having the identifying characteristics of DSMZ deposit number DSMZ 10061 ;

[0024] - the molten steel production step comprises a hot metal production step, wherein a blast furnace produces hot metal and a blast furnace top gas, and a liquid steel production step, wherein a converter produces liquid steel and a converter gas, the exhaust gas comprising at least a part of said blast furnace top gas and / or at least a part of said converter gas;

[0025] - it further comprises a coking step, wherein coal is converted into coke and a coke-oven gas is produced, at least a part of the coke-oven gas being mixed with the exhaust gas and being fed to the pressure swing adsorption system in the gas conditioning step;

[0026] - it comprises a power-generating plant feeding step;

[0027] - the mixed gas is subjected to a liquefaction step before transport and storage; and / or

[0028] - the CO2 storage unit is a unit of geo-sequestration or a unit for solid storage by reaction of CO2 with metal oxides to produce stable carbonates.

[0029] According to another aspect, the invention also relates to a plant complex for the production of steel for implementing an above-described steel production method, comprising at least: a molten steel production installation for producing a molten steel and an exhaust gas at an exhaust gas outlet, a pressure swing adsorption system for producing an enriched gas and a tail gas from at least a part of the exhaust gas, the pressure swing adsorption system comprising an inlet, in fluidic communication with the exhaust gas outlet, an enriched gas outlet and a tail gas outlet, a biotechnological plant for producing at least one chemical product and a residual gas from the enriched gas, the biotechnological plant comprising an inlet, in fluidic communication with the enriched gas outlet, and a residual gas outlet, a venturi system comprising an inlet in fluidic communication with the tail gas outlet and the residual gas outlet, and a mixed gas outlet, the tail gas and the residual gas circulating in the venturi system for producing a mixed gas at the mixed gas outlet, and a CO2 storage unit, to which the mixed gas is sent.

[0030] The plant complex according to the invention can further comprise the following features, considered alone or according to any technically feasible combination:

[0031] - the CO2 storage unit is a unit of geo-sequestration or a unit for solid storage by reaction of CO2 with metal oxides to produce stable carbonates.

[0032] Other aspects and advantages of the invention will appear upon reading the following description, given by way of example and made in reference to the appended drawings, wherein: - Fig. 1 is a diagram representing schematically a plant complex implementing a steel production method according to the invention, and

[0033] - Fig. 2 is a diagrammatical representation of a venturi system used in a steel production method according to the invention.

[0034] In reference to Fig. 1 , a plant complex for the production of steel implementing a steel production method is described.

[0035] The plant complex comprises mainly a molten steel production installation 2, a pressure swing adsorption system (PSA system) 4 and a biotechnological plant 6. According to particular embodiments, the plant complex also comprises a coke oven and / or a power-generating plant 10. These different installations are connected to each other by appropriate gas conveying means such that various gases can be transported between the different installations, as will be described in greater details subsequently.

[0036] The molten steel production installation 2 is arranged to produce a molten steel and at least one exhaust gas 12. To this end, the molten steel production installation 2 for example comprises a blast furnace 14, wherein hot metal such as molten iron 33 is obtained substantially from iron ore 29 and reducing agents 31 , in particular coke and coal. Reduction reactions cause the production of a blast-furnace top gas 16, which contains nitrogen, CO, CO2 and H2as the main constituents. The molten steel production installation 2 further comprises a converter 18 that is arranged downstream of the blast-furnace process, wherein hot metal is converted into liquid steel, i.e. molten steel. By blowing oxygen onto the liquid hot metal, amount of unwanted component, in particular carbon, silicon and phosphorus, are reduced. Scrap may be added in amounts of up to 25% with respect to the amount of hot metal. Furthermore, lime is added for forming slag. At the top of the converter, a converter gas 20 that has a very high proportion of CO is drawn off.

[0037] The exhaust gas 12 produced in the molten steel production installation 2 comprises at least a part of the blast furnace top gas 16 and / or at least a part of the converter gas 20. The exhaust gas 12 therefore comprises CO and CO2.

[0038] According to an embodiment, a part of the exhaust gas 12 can be used for operating the power-generating plant 10, as shown by arrow 11 of Fig. 1 . The power-generating plant 10 is typically designed as a steam-turbine power-generating plant (such as a traditional gasfired steamboiler and a steam turbine driven generator to generate the electrical energy) or as a gas-turbine power-generating plant or as a gas-turbine and steam-turbine powergenerating plant that is operated with a gas that comprises at least a partial amount of the blast-furnace top gas 16 that occurs in the production of hot metal in the blast furnace and / or a partial amount of the converter gas 20 that occurs in the converter steel works. According to the invention, at least a part of the exhaust gas 12 is fed to the PSA system 4 to produce an enriched gas 24 and a tail gas 26. To this end, an exhaust gas outlet of the molten steel production installation 2 is in fluidic communication with an inlet of the PSA system 4.

[0039] When a part of the exhaust gas 12 is used to operate the power-generating plant 10 and another part of the exhaust gas 12 is fed to the PSA system 4, the fluidic communication between the exhaust gas outlet and the PSA system inlet passes through a switchable gas diverter 28 for dividing the streams of exhaust gas that are fed to the power-generating plant 10 and to the PSA system 4. According to an embodiment, a mixing device 13 is for example provided upstream of the switchable gas diverter 28 in the direction flow, for mixing at least a part of the blast furnace top gas 16 and at least a part of the converter gas 20 to form the exhaust gas 12.

[0040] When the plant complex comprises a coke oven, for example to produce coke to be used in the blast furnace 14, the coke-oven gas produced in the coke oven is for example mixed with the exhaust gas 12 such as to be fed to the PSA system 4 together with the exhaust gas 12.

[0041] According to the embodiment shown in Fig. 1 , before being fed to the PSA system 4, the exhaust gas 12, possibly mixed with the coke-oven gas, passes through a compression device 30, in which the exhaust gas 12 is compressed during a gas compression step. This compression of the exhaust gas 12 is for example performed such that the pressure of the exhaust gas 12, possibly mixed with the coke-oven gas, fed to an inlet of the PSA system 4 is comprised between 7.3 and 7.5 barg (bar gauge). Barg is the unit for the measurement of gauge pressure. Gauge pressure is measured against the ambient pressure. Therefore, it is equal to absolute pressure minus atmospheric pressure.

[0042] As described previously, the PSA system is arranged to produce an enriched gas 24, which is to be fed to the biotechnological plant 6. More particularly, the PSA system 4 aims to separate out and enrich the exhaust gas 12, and possibly mixed with the coke-oven gas, in CO. According to an embodiment, the enriched gas 24 is also enriched in H2. The PSA system 4 may further aim to remove from the gas any of the contaminants that are deleterious to the biotechnological plant 6. Said contaminant is especially HCN. Said contaminants are preferably sent to the tail gas 26. Preferably, the PSA comprises at least one pressure vessel, preferably at least six pressure vessels, for example eight vessels. Each pressure vessel comprises an adsorbent material. Once a vessel is saturated, it is flushed out towards a tail gas outlet and a split of the gas is achieved producing the enriched gas 24 and the tail gas 26. At an enriched gas outlet of the PSA system 4, the enriched gas 24 preferably comprises high CO and H2levels. In other words, the enriched gas 24 in particular comprises a higher content of CO compared to the exhaust gas 12.

[0043] On the other hand, the tail gas 26 has a high CO2content.

[0044] Average compositions of the different gases are provided in Table 1 .

[0045] The different gases have generally the following compositions and features of Table 1 :

[0046] Table 1

[0047] As will be described in greater details subsequently, the tail gas 26 exits the PSA system 4 via a tail gas outlet 34 (Fig. 2) and is fed to a venturi system 36.

[0048] The functioning of such a PSA system 4 is known per se and will not be described in greater detail herein.

[0049] The enriched gas outlet of the PSA system 4 is placed in fluidic communication with an inlet of the biotechnological plant 6 such that the enriched gas 24 is fed to the biotechnological plant 6 to produce at least one chemical product 38 and a residual gas 40.

[0050] Such a biotechnological plant 6 is for example a plant for the fermentation of the enriched gas 24. The enriched gas 24 is used biochemically by way of a fermentation process. Thanks to fermentation, the biotechnological plant 6 allows production of at least one chemical product 38. The chemical product 38 may be chosen from alcohols (such as methanol, ethanol, propanol, butanol, isobutanol or 2,3-butanediol), acetone and organic acids (such as acetic acid, butyric acid, isobutyric acid, propionic acid, hexanoic acid or caproic acid).

[0051] Preferably, the fermentation reaction is carried out by one of more strains of carboxydotrophic bacteria. By « carboxydotrophic bacteria », it is meant bacteria that have the ability to oxidize CO. Preferably, the carboxydotrophic bacteria are anaerobic. Preferably, the carboxydotrophic bacterium is selected from Clostridium, Moorella and Carboxydothermus.

[0052] Clostridium autothenogenum DSMZ (German Resource Centre for Biological Material) 19630, Clostridium autothenogenum DSMZ 10061 , Clostridium ragsdahlei ATCC no. BAA-622, Clostridium autoethanogenum, Moorella sp HUC22-1 , Moorella thermoaceticum, Moorella thermoautotrophica, Carboxydothermus, Clostridium formicoaceticum, Clostridium butyricum, Clostridium aceticum, Clostridium ljungdahlii, Clostridium ATCC 29797 and Clostridium carboxidivorans, in particular ATCC BAA-624.

[0053] Most preferably, the carboxydotrophic bacterium is Clostridium autoethanogenum. Preferably, the Clostridium autoethanogenum is a Clostridium autoethanogenum having the identifying characteristics of DSMZ deposit number 19630. In another embodiment, the Clostridium autoethanogenum is a Clostridium autoethanogenum having the identifying characteristics of DSMZ deposit number DSMZ 10061 .

[0054] During the fermentation, typically the enriched gas 24 which enters into the biotechnological plant 6 is rich in CO and, possibly, H2. Indeed, the richer the feed gas is in CO and H2, the more product can be produced in the biotechnological plant.

[0055] The fermentation may be carried out in any suitable bioreactor. The term "bioreactor" includes a fermentation device consisting of one or more vessels and / or towers or piping arrangements, which can include the Continuous Stirred Tank Reactor (CSTR), Immobilized Cell Reactor (ICR), Trickle Bed Reactor (TBR), Bubble Column, Gas Lift Fermenter, Membrane Reactor such as Hollow Fibre Membrane Bioreactor (HFMBR), Static Mixer, or other vessel or other device suitable for gas-liquid contact. In some embodiments of the invention, the bioreactor may comprise a first growth reactor in which the bacteria are cultured, and a second fermentation reactor, to which fermentation broth from the growth reactor is fed and in which most of the fermentation product is produced.

[0056] The fermentation should desirably be carried out under appropriate conditions for the desired fermentation to occur (preferably CO-to-alcohol). Reaction conditions that should be considered include pressure, temperature, gas flow rate, liquid flow rate, media pH, media redox potential, agitation rate (if using a continuous stirred tank reactor), inoculum level, maximum gas substrate concentrations to ensure that CO in the liquid phase does not become limiting, and maximum product concentrations to avoid product inhibition.

[0057] The optimum reaction conditions will depend partly on the particular carboxydotrophic bacteria used. However, in general, it may be preferable that the fermentation be performed at a pressure higher than ambient pressure. Operating at increased pressures allows a significant increase in the rate of CO transfer from the gas phase to the liquid phase where it can be taken up by the carboxydotrophic bacteria as a carbon source for the production of alcohol.

[0058] The chemical product(s) 38 of the fermentation reaction can be recovered using known methods. However, briefly and by way of example only, alcohol such as ethanol may be recovered from the fermentation broth by methods such as fractional distillation or evaporation, and extractive distillation. Distillation of ethanol from a fermentation broth yields an azeotropic mixture of ethanol and water (i.e. 95% ethanol and 5% water). Anhydrous ethanol can subsequently be obtained through the use of molecular sieve ethanol dehydration technology, which is also well-known in the art.

[0059] The residual gas 40 produced by the biotechnological plant 6 is used as a driving force promoting the venturi effect in the venturi system 36. Indeed, when leaving the biotechnological plant 6, the residual gas 40 is at a high pressure, for example substantially comprised between 4 barg and 4.5 barg, and this high pressure is an efficient energy source for the venturi system 36 in order to improve the flushing of the vessels the PSA system 4.

[0060] According to an embodiment, the residual gas 40 passes through a scrubber (not shown) to catch the alcohol contained in the residual gas 40 before being fed to the venturi system 36. The pressure of the residual gas 40 after the scrubber is for example substantially comprised from 2.5 barg to 3.5 barg, which makes it particularly efficient to be used as a driving force promoting the venturi effect in the venturi system.

[0061] To this end, a residual gas outlet of the biotechnological plant 6 is in fluidic communication with a residual gas inlet 42 (Fig. 2) of the venturi system 36. The residual gas 40 at the residual gas inlet 42 creates a partial vacuum at the tail gas outlet 34 of the PSA system 4 which opens into the inlet of the venturi system 36. The venturi system 36 comprises a venturi duct 44 extending between the venturi system inlet, wherein the residual gas inlet 42 and the tail gas outlet 34 open, and a mixed gas outlet 46, where a mixed gas 48, resulting from the mixing of the residual gas 40 and the tail gas 26 in the venturi duct 44, is emitted. The venturi duct 44 comprises, between the venturi system inlet and the mixed gas outlet 46, a throttling portion 50 wherein the section of venturi duct is reduced compared to the section of the rest of the venturi duct 44. More particularly, the section of the venturi duct 44 is gradually reduced from the venturi system inlet to the throttling portion 50 and is gradually increased from the throttling portion 50 to the mixed gas outlet 46 such that the energy of the residual gas 40 is converted into speed in the venturi duct 44 thanks to the venturi effect.

[0062] The pressure of the mixed gas 48 at the mixed gas outlet 46 is also reduced thanks to the venturi effect, which improves the flushing of the vessels of the PSA system 4. The pressure of the mixed gas 48 is for example substantially comprised between 0.1 barg and 0.5 barg at the mixed gas outlet 46 of the venturi system for an exhaust gas 12 having a pressure substantially comprised from 7.3 barg to 7.5 barg and a residual gas 40 having a pressure from 2.5 barg to 3.5 barg, as described previously. With such an improved flushing of the vessels of the PSA system 4, the content of CO, and possibly H2, of the enriched gas 24 is increased, which in turn results in a higher production of chemical product(s) 38 in the biotechnological plant 6.

[0063] According to the invention, the mixed gas 48 is sent to a CO2 storage unit 15, as shown in Fig. 1 .

[0064] By “CO2 storage unit”, it is meant a plant or structure able to store or sequester CO2. CO2 storage units include gaseous or liquid storage in deep geological formations (geosequestration), and solid storage by reaction of CO2with metal oxides to produce stable carbonates. Storage capacity, containment efficiency and injectivity are the three factors that require major pre-assessment to decide the feasibility of CO2 storage in a candidate geological formation. Geo-sequestration involves injecting CO2, generally in supercritical form, into underground geological formations. The alternative uses of oil fields, gas fields, saline formations, unmineable coal seams and saline-filled basalt formations are also encompassed. The mixed gas 48 is preferably subjected to a purification and / or a transformation step (not represented) before its storage. In a preferred embodiment the mixed gas 48 is liquefied to be transported. It may then be stored offshore.

[0065] Table 2 shows a comparison of the composition of the enriched gas 24 at the inlet of the biotechnological plant 6 when the tail gas 26 passes or not through a venturi system 36 as described previously:

[0066] Table 2

[0067] As can be seen from this table, the venturi system 36 fed with the tail gas 26 from the PSA system 4 and the residual gas 40 from the biotechnological plant 6 therefore increases the CO and H2content of the enriched gas 24 while reducing its CO2content compared to an enriched gas obtained at the outlet of a PSA system wherein the tail gas is not fed to a venturi system.

[0068] As indicated previously, such an enriched gas 24 improves the production of chemical product(s) 38 in the biotechnological plant 6. More particularly, with an enriched gas composition of Table 2, the productivity of the biotechnological plant 6 is increased by substantially 2.5%. This may represent, for a yearly production, several thousands of tons of chemical products additionally produced and valorized.

[0069] As the CO and H2 content of the enriched gas 24 is increased, it also means that their content in the mixed gas 48 is decreased, thus increasing the CO2 content of the mixed gas 48 and allowing it to be used in CO2 sequestration technologies that typically require a minimum amount of CO2 to be technically and economically viable.

[0070] Many different arrangements of the described invention are possible without departing from the spirit and scope of the present invention. Embodiments of the present invention are described herein with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the disclosed improvements without departing from the scope of the present invention.

[0071] Further, it will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims. Not all steps listed in the various figures and description need to be carried out in the specific order described. The description should not be restricted to the specific described embodiments.

Claims

CLAIMS1. Steel production method for producing a steel, comprising at least the following steps:- a molten steel production step, wherein a molten steel is produced and at least one exhaust gas (12) is generated,- a gas conditioning step, wherein at least a part of the exhaust gas (12) is fed to a pressure swing adsorption system (4) to produce an enriched gas (24) and a tail gas (26) from the exhaust gas (12),- a chemical product production step, wherein the enriched gas (24) is fed to a biotechnological plant (6) to produce at least one chemical product (38) and a residual gas (40), the steel production method further comprising a mixed gas production step, wherein the tail gas (26) and the residual gas (40) are fed to an inlet of a venturi system (36), the residual gas (40) being used as a driving force to generate a venturi effect in the venturi system (4) and to withdraw the tail gas (26) from the swing pressure adsorption system (4), a mixed gas (48) comprising the tail gas (26) and the residual gas (40) being recuperated at a mixed gas outlet (46) of the venturi system (36) and said mixed gas (48) being sent to a CO2 storage unit.

2. Steel production method according to claim 1 , wherein the residual gas (40) creates a partial vacuum at a tail gas outlet (34) of the pressure swing adsorption system (4), said tail gas outlet (34) opening into the inlet of the venturi system (36).

3. Steel production method according to claim 1 or 2, wherein the venturi system (36) comprises a venturi duct (44) extending between the venturi system inlet and the mixed gas outlet (46), said venturi duct (44) comprising a throttling portion (50) wherein the section of venturi duct (44) is reduced compared to the section of the rest of the venturi duct (44), the tail gas (26) and the residual gas (40) being circulated in said venturi duct (44) during the mixed gas production step.

4. Steel production method according to any one of claims 1 to 3, wherein the residual gas (40) is fed to the inlet of the venturi system (36) at a pressure from 2.5 barg to 3.5 barg.

5. Steel production method according to any one of claims 1 to 4, wherein the mixed gas (48) presents a pressure from 0.1 barg to 0.5 barg at the mixed gas outlet (46) of the venturi system (36).

6. Steel production method according to any one of claims 1 to 5, comprising an exhaust gas compression step, wherein the exhaust gas (12) is compressed before being fed to the pressure swing adsorption system (36).

7. Steel production method according to any one of claims 1 to 6, wherein the exhaust gas (12) comprises CO and CO2, the gas conditioning step being arranged to produce a CO enriched gas (24) comprising a higher content of CO compared to the exhaust gas (12).

8. Steel production method according to any one of claims 1 to 7, wherein the biotechnological plant (6) is a plant for the fermentation of the enriched gas (24), the chemical product (38) produced by the biotechnological plant (6) being chosen from alcohols such as methanol, ethanol, propanol, butanol, isobutanol or 2,3-butanediol ; acetone ; and organic acids such as acetic acid, butyric acid, isobutyric acid, propionic acid, hexanoic acid or caproic acid.

9. Steel production method according to claim 8, wherein the fermentation is carried out by one of more strains of carboxydotrophic bacteria which are preferably anaerobic ; preferably the carboxydotrophic bacterium is selected from Clostridium, Moorella and Carboxydothermus, most preferably the carboxydotrophic bacterium is Clostridium autoethanogenum such as a Clostridium autoethanogenum having the identifying characteristics of DSMZ deposit number 19630 or having the identifying characteristics of DSMZ deposit number DSMZ 10061 .

10. Steel production method according to any one of claims 1 to 9, wherein the molten steel production step comprises a hot metal production step, wherein a blast furnace (14) produces hot metal and a blast furnace top gas (16), and a liquid steel production step, wherein a converter (18) produces liquid steel and a converter gas (20), the exhaust gas (12) comprising at least a part of said blast furnace top gas (16) and / or at least a part of said converter gas (20).1 1. Steel production method according to any one of claims 1 to 10, further comprising a coking step, wherein coal is converted into coke and a coke-oven gas isproduced, at least a part of the coke-oven gas being mixed with the exhaust gas (12) and being fed to the pressure swing adsorption system (4) in the gas conditioning step.

12. Steel production method according to any one of claims 1 to 11 , comprising a power-generating plant (10) feeding step.

13. Steel production method according to any one of claims 1 to 12, wherein the mixed gas (48) is subjected to a liquefaction step before transport and storage.

14. Plant complex for the production of steel for implementing a steel production method according to any one of claims 1 to 13, comprising at least:- a molten steel production installation (2) for producing a molten steel and an exhaust gas (12) at an exhaust gas outlet,- a pressure swing adsorption system (4) for producing an enriched gas (24) and a tail gas (26) from at least a part of the exhaust gas (12), the pressure swing adsorption system (4) comprising an inlet, in fluidic communication with the exhaust gas outlet, an enriched gas outlet and a tail gas outlet (34),- a biotechnological plant (6) for producing at least one chemical product (38) and a residual gas (40) from the enriched gas (24), the biotechnological plant (6) comprising an inlet, in fluidic communication with the enriched gas outlet, and a residual gas outlet,- a venturi system (36) comprising an inlet in fluidic communication with the tail gas outlet (34) and the residual gas outlet, and a mixed gas outlet (46), the tail gas (24) and the residual gas (40) circulating in the venturi system (36) for producing a mixed gas (48) at the mixed gas outlet (46), and- a CO2 storage unit (15), to which the mixed gas (48) is sent.

15. Steel production method according to any one of claims 1 to 13 or plant complex according to claim 14, wherein the CO2 storage unit is a unit of geo-sequestration or a unit for solid storage by reaction of CO2 with metal oxides to produce stable carbonates.

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