Method for reducing the carbon footprint when operating a metallurgical plant for producing pig iron - Patents.com

Abstract

A method for reducing the carbon footprint when operating a metallurgical plant for producing pig iron (P), comprising the steps of: (a) preheating iron ore fines (A) to a temperature of more than 600° C. in a first electric preheater (10) based on the Joule effect and / or microwave heating to obtain preheated iron ore fines (B); (b) partially reducing the preheated iron ore fines (B) in one or more fluidized bed reactors (50) in the presence of a hot reducing gas (J) to obtain partially reduced iron (K, L); (c) feeding the partially reduced iron (K, L) to an immersed arc furnace (70) containing a bath of molten metal having an upper slag layer; and (d) further reducing and melting the partially reduced iron (K, L) in the immersed arc furnace (70) in the presence of a carbonaceous material (M) to obtain molten pig iron (P). and wherein in step (b) the hot reducing gas (J) comprises hydrogen (D), syngas (I), the off-gas (O) of the submerged arc furnace, other off-gas (H) from the metallurgical plant, or a mixture of two or more thereof, the syngas (I) being produced in one or more reforming reactors (40) from natural gas or biomethane (F), blast furnace gas (G), the off-gas (O) of the submerged arc furnace, other off-gas (H) from the metallurgical plant, or a mixture of two or more thereof, in the presence of air or oxygen-enriched air, steam or carbon dioxide (E), and wherein in step (b) the hot reducing gas (J) has a temperature of more than 550°C, and wherein in step (b) the partially reduced iron (K, L) has a degree of metallization of 55-75%, preferably 60-70%.

Description

[Technical field]

[0001] The present invention relates generally to a method for reducing the carbon footprint when operating a metallurgical plant for producing pig iron, and to a metallurgical plant for producing pig iron having a reduced carbon footprint. [Background technology]

[0002] The need and obligation to reduce global CO2 emissions is affecting one of the main responsible players: the steel industry. Global decarbonization is pushing steelmakers towards a transition towards a more sustainable production based on the maximization of so-called "green" sources, such as "green" electrical energy, as well as renewable reducing agents and fuels, as replacements for fossil fuels.

[0003] Hydrogen is considered to be a new key factor for the current CO2 reduction and especially for the future decarbonized steel production. To achieve the decarbonization targets, hydrogen should be produced without carbon dioxide emissions, which means, for example, production by electrolysis processes, supplied by electrical energy from renewable sources. In this way, "green" hydrogen is produced, completely free of carbon dioxide emissions. Nevertheless, green hydrogen production costs are currently high and, although a decrease is predicted in the coming years, this may undermine the feasibility of its application in the steelmaking sector even in future scenarios, due to the enormous energy and flow demands in the steelmaking process. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to provide a new route for more sustainable pig iron production from iron ore fines, in particular a method for pig iron production suitable for installation in metallurgical plants such as integrated steelworks, which should offer operating flexibility in the range of limiting carbon dioxide emissions to zero, such as to ensure a more gradual transition in reducing the carbon footprint, or at least to allow operation with a low carbon dioxide emission when certain renewable resources are temporarily unavailable. [Means for solving the problem]

[0005] In order to overcome the above mentioned problems, the present invention provides in a first aspect a method for reducing the carbon footprint of operating a metallurgical plant for producing pig iron, comprising: a) preheating the iron ore in a first electric preheater based on the Joule effect and / or microwave heating to a temperature of more than 600°C, preferably 700-900°C, in particular 750°C-850°C, for example about 800°C, to obtain preheated iron ore; b) partially reducing the preheated iron ore in one or more fluidized bed reactors in the presence of a hot reducing gas to obtain partially reduced iron; c) feeding partially reduced iron into a submerged arc furnace containing a bath of molten metal having an upper slag layer; d) further reducing and melting the partially reduced iron in a submerged arc furnace in the presence of carbonaceous material to obtain molten pig iron; The method of claim 1, wherein in step b) the hot reducing gas comprises hydrogen, syngas i.e. synthesis gas, off-gas of the fluidized bed reactor(s), off-gas of a submerged arc furnace, other (CO-containing) off-gas from a metallurgical plant, or a mixture thereof, said syngas being produced from natural gas or biomethane, blast furnace gas, off-gas of the submerged arc furnace itself or of another submerged arc furnace, other off-gas from a metallurgical plant, or a mixture of two or more thereof in one or more (catalytic or non-catalytic) reforming reactors in the presence of air or oxygen-enriched air, water vapor or carbon dioxide (depending on the reforming process used), and wherein in step b) the hot reducing gas has a temperature above 550° C. and wherein in step b) the partially reduced iron has a degree of metallization of 55-75%, preferably 60-70%.

[0006] In a second aspect, the invention proposes a metallurgical plant for producing pig iron with a reduced carbon footprint by implementing a method for reducing the carbon footprint when operating a metallurgical plant for producing pig iron, preferably according to the first aspect, the metallurgical plant comprising: - a first electric preheater configured to preheat the iron ore fines to a temperature of more than 600°C, preferably between 700 and 900°C, in particular between 750°C and 850°C, for example at about 800°C, based on Joule effect and / or microwave heating, to form a preheated iron ore fines; - one or more fluidized bed reactors configured for partially reducing the preheated iron ore fines to a degree of metallization of 55-75%, preferably 60-70%, in the presence of a hot reducing gas to partially reduced iron; a submerged electric arc furnace including a bath of molten metal having an upper slag layer configured to receive the partially reduced iron and to further reduce and melt the partially reduced iron in the presence of carbonaceous material to obtain molten pig iron, The metallurgical plant further comprises one or more (catalytic or non-catalytic) reforming reactors configured to produce syngas from a feed of one or more of natural gas or biomethane, a feed of blast furnace gas, submerged arc furnace off-gas and other off-gas from the metallurgical plant, or a mixture thereof, and a feed of air or oxygen-enriched air, steam or carbon dioxide (depending on the reforming process selected), and the metallurgical plant further comprises a feed of hydrogen and a high temperature reducing gas mixing device, the one or more (catalytic or non-catalytic) reforming reactors and a feed of hydrogen. and optionally a hot reducing gas mixing device in upstream fluid connection with one or more of said feeds of submerged arc furnace off-gas and other off-gases of the metallurgical plant, or one or more of the feeds of mixtures thereof, and in downstream fluid connection with the inlet of one or more fluidized bed reactors, said hot reducing gas mixing device configured to provide hot reducing gas at a temperature above 550° C. comprising hydrogen, syngas, off-gas of the fluidized bed reactor(s), off-gas of the submerged arc furnace, other off-gases of the metallurgical plant, or a mixture of two or more thereof. The hot reducing gas mixing device may be a dedicated mixing unit or may be only a combined feed of (preheated) hydrogen, syngas from a reforming reactor (catalytic or non-catalytic), off-gas of the fluidized bed reactor(s), off-gas of the submerged arc furnace, and other off-gases of the metallurgical plant.

[0007] In the context of the present invention, the other off-gases from metallurgical plants may be any available and suitable CO-containing off-gases, or a mixture of two or more thereof. They may be selected from one or more off-gases from coke oven plants, DRI (direct reduced iron) plants, basic oxygen furnaces, electric furnaces (other than the submerged arc furnace used in the present process), etc.

[0008] The core of the proposed process and metallurgical plant is therefore based on a partial (pre)reduction step in one or more fluidized bed reactors to a degree of metallization of 55-75%, preferably 60-70%, based exclusively on hot gaseous reducing agents, followed by an electrosmelting unit of the submerged electric arc furnace (SAF) type, where completion of the smelting and reduction takes place.

[0009] As a result, the present invention utilizes a combination of three discoveries: (1) that the kinetic curve of reduction of iron ore fines is very steep up to 70-75%, meaning that, for example, a degree of metallization of 75% can be reached within 20-30 minutes, while further metallization to 75%-95% takes more than 2 hours under the same conditions; (2) that this partial reduction can be obtained when carried out using only hot reducing gas as reducing agent, and furthermore can be based at least in part on off-gases available in the metallurgical plant, e.g. off-gases of the submerged electric arc furnace of the process itself or of other processes, and also other off-gases as detailed below; and (3) that this partial reduction can be achieved by using only hot reducing gas as reducing agent, and furthermore can be based at least in part on off-gases available in the metallurgical plant, e.g. off-gases of the submerged electric arc furnace of the process itself or of other processes, and also on other off-gases as detailed below. The present invention utilizes a combination of the fact that it can be obtained, at least in part, based on gas, biomethane, or mixtures thereof, and / or blast furnace gas, submerged arc furnace off-gas, other off-gas from metallurgical plants, or mixtures thereof, when they are converted into efficient reducing (syn)gases that are used in catalytic or non-catalytic reforming reactors either directly as such or in combination with variable proportions of hydrogen and / or other CO-rich available off-gas(es), and (4) that further processing in a submerged arc furnace in the presence of solid carbonaceous material acting as a further reducing agent allows the conversion of only partially reduced iron ore into pig iron.

[0010] According to the invention, the hot reducing gas of step b) comprises or consists of hydrogen, syngas, offgas of fluidized bed reactor(s), offgas of submerged arc furnace, other (CO-containing) offgas from metallurgical plants, or mixtures thereof. Preferably, said hot reducing gas comprises or consists of at least syngas, as defined in the present context, meaning syngas produced from natural gas or biomethane, blast furnace gas, offgas of fluidized bed reactor, offgas of submerged arc furnace, other offgas from metallurgical plants, or mixtures of two or more thereof, in one or more (catalytic or non-catalytic) reforming reactors, in the presence of air or oxygen-enriched air, water vapor or carbon dioxide (depending on the reforming process used). It optionally and advantageously comprises (additional) hydrogen, offgas of fluidized bed reactor(s), offgas of submerged arc furnace, other (CO-containing) offgas from metallurgical plants, or mixtures of two or more thereof. In an embodiment, the hot reducing gas comprises or consists of the syngas, (additional) hydrogen and at least one gas selected from fluidized bed reactor off-gas, submerged arc furnace off-gas and other (CO-containing) off-gas from metallurgical plants.

[0011] A preferred fluidized bed reactor for this purpose is a circulating type fluidized bed reactor, which provides a high slip velocity between the gas and the solids resulting in a high mass and heat transfer coefficient. Thus, the one or more fluidized bed reactors are preferably of the circulating type.

[0012] The submerged arc furnace (SAF) is a special type of electric (arc) furnace suitable for carrying out reduction processes. In the submerged arc furnace, the tip of the electrode is embedded in the slag, where the active electrical power is converted into thermal energy by the Joule effect and the reaction takes place. The load, consisting of lump ore and / or agglomerated fine ore and / or pre-reduced ore, flux and carbon carrier, descends and is heated according to the throughput of the furnace. Upon entering the reaction zone, the oxides with the lowest melting point liquefy. As the energy density increases towards the electrode, all oxides eventually melt. Thus, carbothermic reduction by solid carbon-containing materials takes place. Depending on the temperature control and the slag melting point, the metal oxides are reduced in order of the demand for electrical energy required for reduction by carbon. The slag forms a liquid layer, mainly made by gangue, through which the reduced metal droplets descend to form a metal bath at the bottom of the hearth. To ensure carbothermic reduction in the slag, the bath is carbon saturated, and thus the final product is hot pig iron, e.g., with a carbon content of 3-4%. In the context of the present invention, the term "submerged arc furnace" or "SAF" includes all the different possible electric arc furnace types, e.g., DC furnaces, AC furnaces, open bath furnaces, circular types, rectangular types, etc., optimized for a particular application.

[0013] As a result, the submerged arc furnace can be considered as a flexible electric smelter capable of carrying out reduction of metal oxides, input both as iron ore or pre-reduced iron (or direct reduced iron, DRI). Generally speaking, in iron production applications, it is desirable to have a pre-reduction step between the submerged arc furnaces to limit the electric energy consumption and improve the efficiency of the whole plant. Nevertheless, high DRI metallization is not required in the submerged arc furnace, unlike in the case of state-of-the-art electric arc furnaces (EAFs). This has allowed the inventors to find the optimal trade-off operating point of the present invention, with 55-75%, preferably 60-70%, of the metallization being the last part of the metallization carried out in a separated reactor, which is experienced as the most critical part due to availability issues (mainly due to sticking issues) and constraints that may impair the feasibility, such as reduced productivity, extended residence time, reduced efficiency, etc.

[0014] Moreover, the method and metallurgical plant described herein are particularly adapted to utilize renewable resources as energy sources and solid and gaseous reductants, such as "green" hydrogen, biochar, and "green" electricity. Moreover, the method can be flexibly and gradually converted to full green operation (zero carbon dioxide emissions) depending on the availability of resources, the hydrogen in the fluidized bed reactor can be produced (only) by electrolysis using renewable electricity ("green" H2), by fossil resources with the application of CO2 capture technology ("blue" H2), or by fossil resources ("gray" H2), fossil coal and / or biochar can be used in the submerged arc furnace, and off-gases of other metallurgical plants, such as integrated steelmaking gases, can be fed to the fluidized bed reactor. The method also allows mixed operation with (various proportions of) hydrogen, recycled CO2-containing metallurgical off-gas(s), and syngas, with particular aim to increase the flexibility of conversion to "green" operation depending on the availability and cost of the sources. In this regard, the proposed method can be operated flexibly, from completely avoiding carbon dioxide emissions to limited emissions, depending on the type and amount of energy resources used; the same metallurgical plant can be fed only with renewable energy sources, reducing gas, biomethane and solid reducing agents (carbonaceous materials), resulting in zero CO2 emissions, or can still be partially fed with fossil resources (e.g. grey / blue hydrogen, coal, natural gas, electricity from fossil fuels, etc.), resulting in a limited CO2 footprint, in any case less than the currently used iron production technologies.

[0015] Biomethane is a renewable energy source derived from agricultural biomass (dedicated crops, by-products, and agricultural and animal waste), agro-industrial (waste from the food process chain), and organic fraction municipal solid waste (OFMSW). Biomethane is obtained in two stages: through the production of raw biogas - mainly anaerobic digestion of biomass - and the subsequent removal of incompatible components (CO2), a process also known as "upgrading". Biomethane has a quality similar to fossil natural gas, with a methane concentration of more than 90%. The product of the present invention is therefore a "green" pig iron that can be produced completely CO2-free if only renewable sources are used as energy inputs, as well as solid and gaseous reducing agents.

[0016] As a result, in an advantageous embodiment, at least a part, preferably all, of the electrical energy required in the process or in the metallurgical plant is renewable electricity, in particular at least a part of the electrical energy required in the preheater(s) and in the submerged electric arc furnace is renewable electricity.

[0017] Alternatively or additionally, the carbonaceous material of step d) comprises (or consists of) biochar produced by biomass, optionally including up to 40% by weight of demolition wood, and / or up to 20% by weight of waste plastic. The carbonaceous material can be fed to the submerged arc furnace by itself and the partially reduced iron formed separately. However, it may be advantageous to feed the carbonaceous material at least partially to the submerged arc furnace in combination or mixture with the partially reduced iron. One particularly advantageous way of adding said carbonaceous material is described herein below.

[0018] Furthermore, the process can easily be configured to recycle a certain percentage of the integrated steel solids residues, either by adding it to the iron ore fines feed in step a) and / or by adding it to the pre-heated iron ore fines obtained in step a) and / or by adding it to the partially reduced iron obtained in step b), in accordance with the "circular economy" concept, with both additional environmental and economic benefits.

[0019] The reforming in the present invention can be carried out in any suitable reforming reactor or a combination of two or more reforming reactors, which may be of the same type or use different technologies, as known in the art of syngas production. The reforming reactor can be either a catalytic or non-catalytic reforming reactor, examples of such reactors are steam reforming reactors, such as catalytic steam reforming (CSR) reactors, dry reforming (DR) reactors, autothermal reforming (ATR) reactors, partial oxidation (POX) reactors, such as catalytic partial oxidation (CPO) reactors, membrane reforming (MR) reactors, or any combination of two or more different types of reactors.

[0020] Steam reforming (SMR) is a process that produces syngas by the reaction of hydrocarbons with water in the form of steam. The reaction can be represented by the following reaction: CH4+H2O⇔CO+3H2(1)

[0021] Dry reforming (DR), also known as carbon dioxide reforming, is a process that produces syngas from the reaction of a hydrocarbon, such as methane, with carbon dioxide, typically using a precious metal catalyst, such as Ni or a Ni alloy. The dry reforming reaction can be represented by the following reaction: CH4+CO2⇔2CO+2H2(2)

[0022] Autothermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. The reaction takes place in a single chamber where methane is partially oxidized. The reaction is exothermic. When ATR uses carbon dioxide, the H2:CO ratio produced is 1:1, and when ATR uses steam, the H2:CO ratio produced is 2.5:1. The outlet temperature of the syngas is 950-1100°C. In addition to reaction (1), ATR introduces the following reactions: CH4+0.5O2⇔CO+2H2(3)

[0023] Partial oxidation (POX) occurs when a sub-stoichiometric fuel-air mixture is partially combusted in a reformer to produce hydrogen-rich syngas. A distinction is made between thermal partial oxidation (TPO) and catalytic partial oxidation (CPO).

[0024] The process of catalytic partial oxidation (CPO) is also based on reaction (3), where oxygen may come from air, or oxygen-enriched air, or a combination of oxygen and nitrogen, and the reaction takes place by collisions of a few milliseconds, where the gaseous premixed reactants flow through an extremely hot catalytic surface. The originating rapid and selective chemical action is confined inside a thin solid-gas interphase region surrounding the catalyst particles. Here, the molecules spend a very short time at temperatures that typically vary between 600 and 1200 °C. A key issue for technological development is the possibility to avoid the propagation of the reaction to the gas phase, which must be kept at a "relatively low" temperature. This condition favors the formation of primary reaction products (i.e. CO and H2) that inhibit the chain reaction.

[0025] A membrane reforming (MR) reactor is a reactor in which oxygen separation, steam reforming (SR), and partial oxidation (POX) are combined in a single process.

[0026] In a preferred embodiment, hydrogen and / or blast furnace gas, submerged arc furnace off-gas, other off-gas from metallurgical plants, or mixtures thereof, are preheated to a temperature above 700° C. in one or more further (e.g. second, or second and third) electric pre-heater(s) based on Joule effect and / or microwave heating, and then fed to a fluidized bed reactor, preferably mixed with the (already hot) syngas from a catalytic or non-catalytic reforming reactor, said syngas being produced from natural gas, or biomethane, and optionally a certain proportion of blast furnace gas. In the case of the circulating fluidized bed reactor(s), their discharge is preferably reheated either in a separate electric heater, or advantageously in a second electric pre-heater, before being recycled.

[0027] In a further preferred embodiment, the iron ore fines have a particle size distribution in the range of 0.05 to 2 mm, advantageously in the range of 0.1 to 1 mm.

[0028] If necessary or desired, the method further comprises in step b) briquetting the partially reduced iron ore (at high temperature) to obtain briquette partially reduced iron, which is preferably hot-charged into a submerged arc furnace. As already briefly mentioned above, the carbonaceous material is advantageously fed to the submerged arc furnace at least partially (e.g. at least 60% by weight, e.g. at least 80% by weight, or even at least 90% by weight), preferably entirely, in combination or mixture with the partially reduced iron. Most preferably, at least a part, preferably entirely, of the carbonaceous material is first introduced into the briquette partially reduced iron during hot briquetting and then fed to the submerged arc furnace in step d). Thus, in such an embodiment, the carbonaceous material is briquette with the partially reduced iron to a reduced iron briquette (mixed with carbonaceous) which can be immediately used in step d). The carbonaceous material added to the briquettes and the carbonaceous material added separately may be different, such as coal and biochar. Blending a specific percentage of carbonaceous fine material with the DRI fines to produce DRI briquettes with a specific carbon content can be useful to optimize the electrosmelting process, including better control of the final C content of the hot metal. The remaining portion of carbonaceous material required for HBI smelting can be fed separately into the electrosmelter, as is done in state-of-the-art processes.

[0029] Indeed, in state-of-the-art electrosmelting processes for direct reduced iron (DRI) and high temperature briquetted iron (HBI) smelting, such as in electric arc furnaces or submerged arc furnaces, solid carbon is typically used to complete the iron oxide reduction, and solid coal is top-charged in the electric furnace along with other input raw materials in addition to the carbon content from the DRI.

[0030] However, the inventors have found that carbonaceous material added to the partially reduced iron (before feeding into the electric arc furnace) is more efficient in the electrosmelting process than separately fed carbonaceous material such as externally fed coal / biochar. Indeed, the inventors have noted lower consumption, optimized process parameters and more flexible carbon content to be achieved in the final product. The inventors acknowledge that these advantages are due to the fact that the carbonaceous material mixed and / or briquetted with the partially reduced iron is in fine granular form and is homogeneously mixed with the partially reduced iron fines, making the use of coal in the smelting process more efficient, whereas with separate coal feeding, several undesirable phenomena have been observed, such as coal carryover with the off-gas, carbon burn-off and low reduction efficiency, all of which result in higher consumption and lower productivity.

[0031] One topic of particular interest is the C content in the metal product required depending on its use, and the C content provided during the initial steps of the process due to the context of the installation in an integrated steelworks. Indeed, a C content of more than 4 wt.% (typically 4.5 wt.%) is required in the produced hot metal, as well as in blast furnace hot metal, in order to be able to take advantage of the availability of existing downstream processes (e.g. blast furnaces). If this target is met, there is no need to install / modify existing downstream hot metal processing plants.

[0032] However, the C content of the partially reduced iron strongly depends on the direct reduction process, and in the case of gas-based direct reduction such as the present method, the specific C content in the partially reduced iron is essentially derived from the CO-containing reducing agent such as syngas. The use of a reducing gas with a lower CO content, and therefore a higher hydrogen content, than the reducing gas from the natural gas reforming process commonly used in the state of the art means that the carbon content in the partially reduced iron is lower, such as in the range of 0.1-3 wt.%, depending on the type of the specific reduction reactor. This also means that the complete replacement of the C-containing fuel and reducing agent with hydrogen will result in partially reduced iron with zero or almost zero carbon content.

[0033] Especially in the case of such zero or very low carbon content, direct coal input in the electric furnace is not optimal for the smelting process of step d), which leads to higher consumption, lower productivity and less flexibility in product properties (mainly referring to the carbon content in the metal product) as explained above. Moreover, this is likely to also increase the CO2 footprint of the entire electric smelting process.

[0034] In conclusion, the present invention aims to utilize the availability of CO2-enriched syngas, enhanced by enrichment with syngas, from catalytic or non-catalytic reforming of natural gas or biomethane, to produce "green" pig iron in an integrated steel or metallurgical plant, limiting the degree of reduction of iron ore fines, completing the reduction in a submerged electric arc furnace using different renewable energy sources, and selecting specific solutions to improve the economic feasibility of the application. The method also has the flexibility to operate entirely or partially with fossil fuels and reductants, depending on the specific local availability and costs, and a specific carbon dioxide footprint can be foreseen, which is limited compared to state-of-the-art routes, including the possibility of further reducing towards more abundant "green" resources, if available at a feasible cost. [Brief description of the drawings]

[0035] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] FIG. 1 is a schematic diagram of one embodiment of a metallurgical plant for producing pig iron with a reduced carbon footprint or a method for reducing the carbon footprint when operating a metallurgical plant for producing pig iron.

[0036] Further details and advantages of the invention will become apparent from the following detailed description of some non-limiting embodiments, with reference to the accompanying drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The plant is typically supplied with low grade iron ore fines having a particle size distribution in the range 0.05-5 mm, for example 0.1-1 mm, which may include pre-agglomerated ultrafine particles. In this context, the iron ore fines will typically have a particle size distribution in the range 1,500-3,500 kg / m 3 It is noteworthy that the iron ore fines contain hematite, goethite, and magnetite, with various iron contents, with a bulk density range of 1000 to 100000. Such iron ore fines are particularly well suited for the process disclosed herein, which involves partial reduction when fluidized with a reducing gas. If consolidated steel solid residues are added to the feedstock of step a), they preferably have a particle size similar to that of the iron ore fines. The iron ore fines A are first transported from a storage area to a first electric preheater 10. Preheating is carried out by an electric preheater based on the Joule effect, optionally coupled with a heat recovery system, or by microwave heating, utilizing available residual heat from an integrated steel mill or fluidized bed reactor syngas.

[0038] The preheated iron ore fines B are then conveyed via a suitable handling device for conveying fines, such as a chain conveyor or pneumatic conveying, to a fluidized bed input system and fed to the fluidized bed reactor 50. The fluidized bed reactor 50 is preferably of the circulating type, and the fluidized bed reactor discharge C is recirculated, preferably after being (re)heated in the second electric preheater 20, allowing increased flexibility in terms of heat exchange and residence time in terms of fine particle size distribution and optimal process efficiency.

[0039] Green, blue or grey hydrogen (or mixtures thereof) D can be used as reducing gas J in the fluidized bed reactor 50. By complete endothermic iron oxide reduction reaction with hydrogen D, other (recycled) metallurgical plant off-gas(s) H, syngas I or mixtures thereof J, the iron ore fines, but also preferably hydrogen and any other metallurgical plant off-gases, are preheated in one or more further preheaters 10, 20, 30 to a temperature of about 800° C. before being fed to the fluidized bed reactor 50. In a preferred embodiment, a second electric preheater 20 is provided to preheat the hydrogen D and the recycle effluent C of the fluidized bed reactor, optionally coupled with a heat recovery system from the available steelmaking gases, and a third electric preheater 30 is provided to preheat the submerged arc furnace off-gases and any other metallurgical plant off-gases (except blast furnace gas, which is fed to the catalytic or non-catalytic reforming reactor). This allows to reduce the consumption of hydrogen, used as fuel.

[0040] The hydrogen D fed to the fluidized bed reactor 50 can be partially replaced by syngas I and other (CO-containing) off-gas(s) H of the metallurgical plant. Said syngas, rich in carbon monoxide and containing a certain amount of hydrogen, is produced in a catalytic or non-catalytic reforming reactor or reformer 40 and is fed by natural gas and / or biomethane F, blast furnace gas G, submerged arc furnace off-gas O and / or other off-gas H from the metallurgical plant and, depending on the reforming technology used, air or oxygen-enriched air (autothermal reforming, or (catalytic) partial oxidation), steam (autothermal reforming, or steam reforming) or carbon dioxide (dry reforming) E (see reactions (1) to (3) above). In other words, if submerged arc furnace off-gas O and / or other off-gas H from the metallurgical plant are used, they can be used as is or with prior reforming or both. The main advantage of this "recycling" is the reduction of hydrogen consumption, making use of the availability of CO2-rich gases with limited calorific value, such as blast furnace gas, which can be used more efficiently in the reduction process than in energy production. Furthermore, the use of CO2-containing syngas in the fluidized bed reactor 50 benefits the process due to the exothermic CO2 combustion reaction with heat release and the specific carbon content remaining in the partially reduced iron K or L, resulting in a reduced consumption of carbonaceous material M, such as coal / biochar in the submerged arc furnace 70, a more efficient reduction process in the submerged arc furnace 70, and limited reoxidation phenomena in handling the hot partially reduced iron K or L. The carbonaceous material M may also contain further additives, such as slag formers.

[0041] The partially reduced iron K in the form of fine powder, for example with a degree of pre-reduction metallization limited to about 60-70%, is discharged and transported from the reactor in an inert atmosphere (for example nitrogen or argon) to avoid reoxidation phenomena. The partially reduced iron K fine powder is then fed directly to the submerged arc furnace 70 or is preferably hot briquettered in the hot briquetting unit 60 to improve their mechanical properties before being handled in the downstream electric arc furnace input system. The choice of hot partially reduced iron input to the submerged arc furnace as fine powder or briquette depends on the specific project conditions (for example raw material characteristics, utilities, prices, etc.) and affects the design and performance of the submerged arc furnace. If required by the hot briquetting process (depending on the specific equipment type), the hot partially reduced iron fine powder discharged from the fluidized bed reactor at a temperature of 600-650 ° C can be heated, for example, via a third electric heater (for example based on the Joule effect concept or microwave heating) up to 700-750 ° C. In an advantageous embodiment, at least a portion of the carbonaceous material may be combined or mixed with the partially reduced iron and fed to the electric arc furnace. It is particularly advantageous to introduce at least a portion of the carbonaceous material into the briquette reduced iron. Indeed, the concept of high temperature briquetting of the partially reduced fines with a certain amount of carbonaceous material, such as coal, is advantageous for optimizing the smelting process. This advantageous solution, which helps to facilitate proper feeding to the electric furnace, compared to high temperature briquetting of the partially reduced iron fines without carbonaceous material, may further include: - Installation of additional handling equipment, such as inert atmosphere, for handling carbonaceous materials (such as coal) and mixing with the partially reduced iron; - preferably modified hot briquetting machine designs (e.g. size, pressure, etc.) suitable for processing different input feeds; Optionally, a carbonaceous material pre-heating device (e.g., 200°C to 400°C) if required by the high temperature briquetting process depending on the specific partially reduced iron fines and carbonaceous material characteristics (mainly temperature, degree of metallization of the partially reduced iron, amount of carbonaceous material, etc.).

[0042] Briquetting of the carbonaceous material and partially reduced iron in this manner allows the mixture of carbonaceous material and partially reduced iron fines to be homogenized and compressed, limiting the loss in efficiency of external carbonaceous material charged to the electric smelter, mainly due to coal carryover, burn-off, and coarser particle size.

[0043] The briquetting system (and, optionally, upstream and downstream thereof) is preferably configured to function under an inert atmosphere to avoid undesirable re-oxidation of the partially reduced iron.

[0044] The partially reduced iron, in the form of fines K or briquettes L (whether or not containing carbonaceous material), is then hot-charged at about 700° C. into an electric smelter type 70 submerged arc furnace, where completion of reduction and smelting takes place with the carbonaceous material M (contained in the briquettes L and / or added separately).

[0045] For completely carbon dioxide-free pig iron production, in the proposed invention biochar is used as carbonaceous material M (reductant) in the submerged arc furnace 70 (added separately and / or as part of the briquette L) instead of conventionally used fossil coals such as anthracite or coke. Biochar can be produced by a biomass torrefaction process, which finally includes a certain percentage of demolition wood (up to 40%) and waste plastics (up to 20%). The properties of biochar depend on the type of input biomass and torrefaction process and are in any case suitable for use in the submerged arc furnace 70.

[0046] The submerged arc furnace 70 can also recycle a certain percentage of the solid residues of the integrated steel mill as solid waste injection N, e.g. dust and sludge from blast furnaces or basic oxygen furnaces, mill scale, dedusting dust, etc. Recycling of solid residues improves the feasibility and environmental benefits of the application of the present invention by avoiding landfills and recovering the iron, carbon, and zinc content of the solid waste. Residual flow rates, up to 5% of the total submerged arc furnace input, can be injected directly into the furnace metal bath in the form of dry dust (moisture <3%) with a particle size of 100% <250 microns. Wet and / or coarse residues need to be pretreated with dryers and / or mills before electric submerged arc furnace injection, while low moisture and fine dusts (e.g. stockhouse dust, BOF dust) can be injected directly without pretreatment. In the case of solid waste injection flow rates exceeding 5% of the total submerged electric arc furnace input material, additional waste can be added at the top in the form of dry pellets or low-temperature briquettes after suitable low-temperature agglomeration treatments accompanied by mixing, pelleting or briquetting, and drying processes. For example, in the case of carbon-containing solid residues such as blast furnace sludge and dust, no additional biochar is required for the reduction of the waste iron ore, and an overall saving in biochar (or coal) consumption can be obtained.

[0047] The flexibility of operation of submerged electric arc furnace type electric smelters allows them to accept partially reduced iron briquettes of less than optimal quality, as well as a certain amount of partially reduced iron briquette fines resulting from the screening of hot briquettes, which improves the availability of a high-temperature briquetting process that completely or partially avoids the internal recirculation of fines.

[0048] The hot reducing gas J fed to the fluidized bed 50 may be a mixture of different proportions of hydrogen D, CO2-enriched submerged arc furnace off-gas O, and / or other recycled metallurgical plant off-gas(s) H, and syngas I produced in a catalytic or non-catalytic reforming reactor / reformer 40 from natural gas or biomethane F, blast furnace gas G, submerged arc furnace off-gas O, other metallurgical plant off-gas H, and air or oxygen-enriched air, steam or carbon dioxide E. The output of this catalytic or non-catalytic reforming reactor 40 is a syngas I suitable for use as reducing gas J in the fluidized bed reactor, for example by replacing a certain amount of hydrogen or other recycled waste gas. This option may have a large OpEx advantage by replacing a certain amount of hydrogen D with syngas I produced by natural gas or biomethane F, and blast furnace gas G, submerged arc furnace off-gas O, and / or other off-gas from the metallurgical plant H.

[0049] The proposed process and metallurgical plant has a modular size, with each fluidized bed reactor 50 being capable of reaching a maximum production of, for example, 550 kty DRI and each submerged electric arc furnace 70 being capable of reaching a maximum size of 1.5 Mtpy hot pig iron P.

[0050] The hot cast iron P can then be cast as cast iron Q in a casting unit 80 . [Explanation of symbols]

[0051] Legend: 10 First electric preheater 20 Second electric preheater 30 Third electric preheater 40 Reforming reactor 50 Fluidized Bed Reactor 60 High-Temperature Briquetting Unit 70 Immersed Arc Furnace 80 Casting Unit A Iron ore fines B Preheated iron ore fines C Fluidized bed reactor discharge D Hydrogen E Air or oxygen-enriched air, water vapor or carbon dioxide F Natural gas or biomethane G Blast furnace gas H Other metallurgical plant off-gas(s) I Syngas J (high temperature) reducing gas K Partially reduced iron (fine powder) L Partially reduced iron (briquette) M Carbonaceous materials (and additives) N Solid waste injection O Submerged arc furnace off-gas P Hot metal / molten pig iron Q Casting pig iron

Claims

1. 1. A method for reducing the carbon footprint of operating a metallurgical plant for producing pig iron (P), comprising: a) preheating iron ore fines (A) in a first electric preheater (10) based on the Joule effect and / or microwave heating to a temperature of more than 600°C to obtain preheated iron ore fines (B); b) partially reducing the preheated iron ore fines (B) in one or more fluidized bed reactors (50) in the presence of only hot reducing gas (J) as a reducing agent to obtain partially reduced iron (K, L); c) feeding the partially reduced iron (K, L) into an immersion arc furnace (70) containing a bath of molten metal having an upper slag layer; d) further reducing and melting the partially reduced iron (K, L) in the presence of a carbonaceous material (M) in the submerged arc furnace (70) to obtain molten pig iron (P), a method for producing the partially reduced iron (K, L) in step b) from one or more reforming reactors (40) in the presence of air or oxygen-enriched air, steam, or carbon dioxide (E); wherein in step b), the high-temperature reducing gas (J) comprises hydrogen (D), syngas (I), the off-gas (O) of the submerged arc furnace, other off-gas (H) from the metallurgical plant, or a mixture of two or more thereof; and wherein the syngas (I) is produced from natural gas or biomethane (F), blast furnace gas (G), the off-gas (O) of the submerged arc furnace, other off-gas (H) from the metallurgical plant, or a mixture of two or more thereof in the presence of air or oxygen-enriched air, steam, or carbon dioxide (E); and wherein in step b), the high-temperature reducing gas (J) has a temperature of more than 550°C; and wherein in step b), the partially reduced iron (K, L) has a degree of metallization of 55 to 75%.

2. 10. The method of claim 1, wherein the one or more fluidized bed reactors (50) are of the circulating type.

3. 2. The method of claim 1, wherein the hydrogen (D) is preheated in a second electric preheater (20), and the off-gas (O) of the immersed arc furnace and other off-gases (H) from the metallurgical plant are preheated in a third electric preheater (30), both second and third preheaters being independently based on Joule effect or microwave heating to temperatures above 700°C.

4. 4. The method according to any one of claims 1 to 3, wherein the carbonaceous material (M) in step d) comprises or consists of biochar produced by biomass.

5. The method of claim 4, wherein the carbonaceous material (M) in step d) comprises up to 40% by weight of dismantled wood and up to 20% by weight of waste plastic.

6. The method according to any one of claims 1 to 3, wherein the iron ore fines (A) have a particle size distribution in the range of 0.1 to 1 mm.

7. 4. The method according to claim 1, wherein step b) further comprises hot briquetting the partially reduced iron ore fines (K) to obtain briquette partially reduced iron (L).

8. 8. The method according to claim 7, wherein the carbonaceous material (M) is at least partially introduced into the briquette partially reduced iron (L) during hot briquetting and fed into a submerged electric arc furnace (70) in step d).

9. 4. The method according to any one of claims 1 to 3, wherein the other off-gases (H) of the metallurgical plant comprise one or more of off-gases from a coke oven plant, a direct reduced iron plant, and a basic oxygen furnace.

10. 4. The method according to any one of claims 1 to 3, wherein all the electrical energy required in the preheater(s) and the immersed arc furnace is renewable electricity.

11. A metallurgical plant for producing pig iron (P) with a reduced carbon footprint, comprising: a first electric preheater (10) configured to preheat the iron ore fines (A) to a preheated iron ore fines (B) at a temperature above 600°C based on the Joule effect or microwave heating; one or more fluidized bed reactors (50) configured to partially reduce said preheated iron ore fines (B) to a degree of metallization of 55-75% in the presence of only hot reducing gas (J) as reducing agent to partially reduced iron (K, L); an immersed arc furnace (70) containing a bath of molten metal with an upper slag layer, configured to receive the partially reduced iron (K, L) and to further reduce and melt the partially reduced iron (K, L) in the presence of carbonaceous material (M) to obtain molten pig iron (P), The metallurgical plant further comprises one or more reforming reactors (40), which are configured to produce syngas (I) from a feed of one or more of natural gas or biomethane feed (F), a feed of blast furnace gas (G), the off-gas (O) of the submerged arc furnace and other off-gas (H) from the metallurgical plant, or a mixture of two or more thereof, and a feed of air or oxygen-enriched air, steam or carbon dioxide (E), and the metallurgical plant further comprises a feed of hydrogen (D) and a high-temperature reducing gas mixing device, which is configured to mix the feed of hydrogen (D) with the one or more reforming reactors (40) and the feed of hydrogen (D). and a high-temperature reducing gas mixing device fluidly connected upstream with one or more of the feeds of one or more of the off-gas (O) of the submerged arc furnace and other off-gas (H) of the metallurgical plant, or a mixture of two or more thereof, and fluidly connected downstream with an inlet of the one or more fluidized bed reactors (50), wherein the high-temperature reducing gas mixing device is configured to supply a high-temperature reducing gas (J) at a temperature above 550°C comprising hydrogen (D), syngas (I), the off-gas (O) of the submerged arc furnace, other off-gas (H) of the metallurgical plant, or a mixture of two or more thereof.

12. 12. The metallurgical plant of claim 11, wherein the one or more fluidized bed reactors (50) are of the circulating type.

13. 12. The metallurgical plant of claim 11, comprising: a second electric preheater (20) based on Joule effect or microwave heating fluidly connected between the hydrogen feed (D) and the hot reducing gas mixing device; a third electric preheater (30) based on Joule effect or microwave heating fluidly connected between the one or more off-gas supplies (O) of the immersed arc furnace and other off-gases (H) of the metallurgical plant; and the hot reducing gas mixing device, wherein the second and third electric preheaters (20, 30) are configured to preheat associated off-gas(s) and syngas to a temperature above 700°C.

14. 14. The metallurgical plant according to any one of claims 11 to 13, wherein the carbonaceous material (M) is provided from a source comprising or consisting of biochar produced by biomass.

15. A metallurgical plant as described in claim 14, wherein the carbonaceous material (M) comprises up to 40% by weight of dismantled wood and up to 20% by weight of waste plastic.

16. 14. The metallurgical plant of any one of claims 11 to 13, further comprising a hot briquetting unit configured to briquette the partially reduced iron ore fines (K) into briquetted partially reduced iron (L).

17. The metallurgical plant according to any one of claims 11 to 13, wherein the metallurgical plant comprises one or more of a coke oven plant, a direct reduced iron plant, a blast furnace, and a basic oxygen furnace, and supplies the other off-gas (H) of the metallurgical plant.

18. 14. The metallurgical plant of any one of claims 11 to 13, wherein all electrical energy required in the preheater(s) and the submerged electric arc furnace is renewable electricity.

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