Method for achieving co2 savings in a blast furnace

Abstract

The invention relates to a method for operating a blast furnace (2), comprising: - introducing burden into the blast furnace; - injecting auxiliary fuel (20, 24) at a tuyere (8) of the blast furnace; - supplying hot blast air (12) to the blast furnace through the tuyere; wherein the auxiliary fuel comprises a carbon-lean fuel (24) having a low carbon- to-hydrogen weight ratio.

Description

METHOD FOR ACHIEVING CO2 SAVINGS IN A BLAST FURNACETechnical field

[0001] The invention relates to the field of blast furnaces, particularly to a method for operating blast furnaces.Background art

[0002] Blast furnaces, used in the steelmaking industry for iron production, are large vertical structures where raw materials such as iron ore and coke are introduced from the top. The reduction process begins at the top of the furnace, where these materials are progressively heated and reduced as they descend. An important stage occurs at the circular level near the base of the blast furnace, where tuyeres inject auxiliary fuels and hot blast air, supplied from a circumferential bustle pipe, to facilitate the reduction of iron oxides into metallic iron.

[0003] In state-of-the-art blast furnaces, an auxiliary fuel is injected into the tuyeres to reduce coke consumption. The auxiliary fuel, usually injected at the tuyere, contains carbon, hydrogen, and other components. It is partially oxidized by the hot blast to generate hot reducing gas, consisting of a mixture of carbon monoxide, hydrogen, and nitrogen. Carbon monoxide and hydrogen are the active molecules in the iron oxides reduction process.

[0004] Carbon is the preferred reducing agent in blast furnaces because its partial oxidation to carbon monoxide generates the necessary heat for the process. Pulverized coal injection (PCI) is particularly favored for this reason. However, this reliance on carbon-based fuels leads to substantial CO2emissions, which is a major environmental concern.

[0005] To address this, various combinations of fuels and techniques have been tested in prior art. These require, for instance, large-scale oxygen enrichment, which does not significantly reduce CO2 emissions and is often deemed too expensive.

[0006] Published patent document EP 2 733223 A1 discloses a method for operating a blast furnace which aims to increase combustion temperature and reduce the consumption of reducing agents, thereby lowering CO2emissions. This method involves using natural gas as a flammable reducing agent and pulverized coal as a solid reducing agent.

[0007] However, the solution disclosed in the document has room for improvement, particularly in further reducing carbon dioxide emissions while maintaining efficient combustion in the blast furnace.Summary of inventionTechnical problem

[0008] The goal of the present invention is to alleviate at least some of the problems noted in the prior art. In particular, the present invention aims at providing a solution for operating a blast furnace that achieves low carbon dioxide emissions.Summary

[0009] In accordance with a first aspect of the invention a method for operating a blast furnace is proposed, comprising:- introducing burden into the blast furnace;- injecting auxiliary fuel at a tuyere of the blast furnace;- supplying hot blast air to the blast furnace through the tuyere; wherein the auxiliary fuel comprises a carbon-lean fuel having a carbon-to- hydrogen weight ratio of less than 8, said method further comprising:- reheating the hot blast air by means of a plasma generator prior to its supply to the blast furnace.

[0010] Advantageously, the hot blast air may undergo two stages of heating: a first stage of heating up to a first temperature at the hot stoves prior to its supply to the blast furnace, and a second stage where it is further heated by means of the plasma generator.

[0011] The carbon-lean fuel may comprise at least one of the following: diesel, kerosene, natural gas, hydrogen-enriched natural gas, ethanol, methanol, coke oven gas, hydrogen, and ammonia.

[0012] Preferably, the plasma generator may be supplied with a feed gas comprising a mixture of oxygen and a neutral gas.

[0013] The neutral gas may preferably correspond to nitrogen.

[0014] The mixture of oxygen and neutral gas may preferably correspond to air, or a mixture of air and oxygen, or a mixture of air and nitrogen.

[0015] The method may further comprise injecting nitrogen into the hot blast air of the blast furnace upstream of the tuyere.

[0016] The nitrogen may preferably be injected at a rate of up to 250Nm3 / t, and preferably 100Nm3 / t.

[0017] Preferably, the carbon-lean fuel may be injected at a rate ranging from 10kg / t to 200kg / t.

[0018] The plasma generator may preferably heat the hot blast air up to a temperature range of 1300°C to 2500°C before reaching the tuyere.

[0019] Preferably, the plasma generator may comprise an adjustable power ranging from 10OkWh / t to 500kWh / t to regulate the hot blast air temperature at the tuyere.

[0020] The carbon-lean fuel may preferably be used as the sole auxiliary fuel or in combination with pulverized coal injection.

[0021] In accordance with another aspect of the invention, a blast furnace suitable for achieving the steps of the method according to an aspect of the invention is provided. The blast furnace comprises:- a bustle pipe extending circumferentially around a furnace shell;- a tuyere connected to the bustle pipe via a tuyere stock to supply hot blast air into the blast furnace, wherein the blast furnace further comprises:- a plasma generator arranged upstream the tuyere, said plasma generator heating indirectly the hot blast with a plasma generated with a feed gas supplied through a feeding duct, said feed gas being distinct from the hot blast air of the bustle pipe and the tuyere stock, or said plasma generator comprising electrodes protruding in said tuyere stock and heating directly the hot blast air.

[0022] Preferably, the blast furnace comprises a supply pipe joining the tuyere downstream of the plasma generator and the bustle pipe, said supply pipebeing configured for injecting a carbon-lean fuel having a carbon-to- hydrogen weight ratio of less than 8 into the blast furnace.

[0023] The plasma generator comprising electrodes protruding in the tuyere stock may preferably be devoid of a feed gas supply.

[0024] Preferably, the plasma generator feed gas may comprise a temperature of at most 300°C. The plasma generator feed gas temperature may depend on the plasma generator technology. Preferably, the plasma generator’s feed gas of at most 300°C corresponds to a DC plasma torch.

[0025] The plasma generator may preferably be arranged at a distance of up to 3 meters from the tuyere along the hot blast air.

[0026] The blast furnace may further preferably comprise an injection conduit for pulverized coal injection, joining the tuyere along with the tuyere stock and the supply pipe.

[0027] The provision of aspects in accordance with the invention offers significant advantages in the iron oxides reduction process in blast furnaces. While hydrogen is a crucial molecule in reducing iron oxides, it does not oxidize in the raceway under the carbon-rich conditions of a blast furnace and, therefore, does not generate heat. Other fuels, such as ammonia (NH3) and methanol (CH3OH), require additional energy for cracking. Additionally, iron oxides reduction with carbon monoxide (CO) is exothermic, whereas reduction with hydrogen (H2) is endothermic. Consequently, auxiliary fuels with a high carbon-to-hydrogen (C / H) ratio, like pulverized coal, are typically preferred for injection into conventional blast furnaces.

[0028] Aspects of the present invention, however, enable a substantial reduction in CO2emissions by using a carbon-lean fuel with a low C / H ratio for at least part of the auxiliary fuel injected at the blast furnace tuyere. This is effectively achieved through the use of a plasma generator to reheat the hot blast air. As a result, the invention can achieve up to a 40% reduction in CO2emissions compared to conventional blast furnaces.

[0029] Furthermore, aspects of the invention ensure efficient combustion, allow to maintain a constant RAFT, raceway adiabatic flame temperature, without adding oxygen in the hot blast and thus preserves sufficient nitrogen in thegas to keep the top gas temperature in the correct range. Nitrogen is a pure heat carrier in the blast-furnace and does not participate in any chemical reaction.

[0030] The provision of aspects of the present invention does not require major changes in the blast-furnace layout and is quite easy to implement to the contrary of known top-gas recycling systems and methods, which require a large space to implement the top-gas treatment plant. Moreover, top-gas recycling requires a CO2 reforming or a CO2 removal plant, whereas in accordance with aspects of the present invention, the existing hot blast air is further heated to increase its energy.Brief description of the drawings

[0031] Embodiments of the invention will be described with reference to the following figures, which do not limit the scope of the invention, in which:

[0032] Figure 1 is a schematic representation of the steps of a method for operating a blast furnace according to the present invention;

[0033] Figure 2 is a schematic partial cross-section view of a blast furnace according to the invention, designed to execute the steps of the method outlined in Figure 1 ;

[0034] Figure 3 is a graph comparing CO2savings achieved using the method with oxygen enrichment according to prior art and the method of the invention across two different embodiments;

[0035] Figure 4 is a graph comparing CO2savings achieved using the method of the invention across three different embodiments: one where the carbon- lean fuel comprises ammonia, another where it comprises hydrogen, and a third comprising hydrogen and where nitrogen is used in the plasma generator’s feed gas;

[0036] Figure 5 is a graph depicting the evolution of the hot blast air temperature in relation to the CO2savings at varying hydrogen injection rates;

[0037] Figure 6 is a graph depicting the evolution of the hot blast air temperature in relation to the CO2savings at varying ammonia injection rates;

[0038] Figure 7 is a graph depicting the evolution of the hot blast air temperature in relation to the CO2savings at varying natural gas injection rates.

[0039] Figure 8 is a graph depicting values of the heat released during the conversion of auxiliary fuel to reducing gas in the blast furnace raceway. The heat is expressed in kilojoules per mole of reducing gas for different types of auxiliary fuels, including carbon-lean fuels according to the present invention.Detailed description of the drawings

[0040] The figures presented in this document are schematic representations intended for illustrative purposes only. They may not accurately reflect precise dimensions, proportions, or other specific details.

[0041] Figure 1 is a schematic representation of the steps of a method 100 for operating a blast furnace according to the present invention.

[0042] The method 100 comprises a first step S102 of introducing burden into the blast furnace. The burden can consist of at least ore and coke.

[0043] The blast furnace comprises a bustle pipe extending circumferentially around a furnace shell, and a plurality of tuyeres are connected to the bustle pipe by means of a tuyere stock in order to supply hot blast air into the blast furnace. At step S103, an auxiliary fuel is injected at one tuyere or a set of a plurality of tuyeres of the blast furnace. Advantageously, the present invention enables the injection of a carbon-lean fuel into the blast furnace. Said carbon-lean fuel comprises a carbon-to-hydrogen (C / H) weight ratio of less than 8 on dry basis.

[0044] The carbon-lean fuel can be derived from fossil sources or, preferably, from sustainable sources, it can be used either as the sole auxiliary fuel or in combination with pulverized coal injection. The carbon-lean fuel can partially substitute coke, can partially or entirely substitute pulverized coal, or any other fuel with a higher carbon-to-hydrogen (C / H) ratio. The pulverized coal and coke substitution is only limited by the maximum power of the plasma generator and the maximum hot blast temperature the tuyere stocks can accept.

[0045] The carbon-lean fuel comprises at least one of the following: diesel, kerosene, organic chemical compound (such as polyethylene “PE”), natural gas, ethanol (C2H5OH), methanol (CH3OH), coke oven gas (a mixture of H2, CH4, CO and N2), hydrogen (H2), and ammonia (NH3). Table 1 below shows the typical C / H weight ratio for each component in comparison with coke (having a C / H ratio of 85) and coal (C / H ratio in the range of 15-25) commonly injected in conventional blast furnaces.

[0046] Table 1 :

[0047] Preferably, the carbon-lean fuel is injected at a rate ranging from 10kg / t to 200kg / t. Table 2 below discloses for an efficient blast-furnace operation the preferred injection rates for each carbon-lean fuel type.

[0048] The preferred organic chemical compound is polyethylene; however, the invention is not limited to this compound alone, as any other organic chemical could be injected into the blast furnace, provided its C / H ratio is below 8.

[0049] Table 2:

[0050] More detailed examples will be given further in the present description, showing different embodiments with different injecting rates values.

[0051] The method 100 comprises a step S104 of supplying hot blast air to the blast furnace through the tuyere or the set of tuyeres at which auxiliary fuel is injected. The supplied hot blast air arrives at the tuyere after a first heating stage at the hot stoves, arriving therefore at a temperature of around1150°C (±10%), however, such a temperature would not enable proper combustion for large volume of carbon-lean fuel in the blast furnace. In this regard, the method 100 further comprises a step S105 of further heating the hot blast air by means of a plasma generator, preferably a plasma torch, prior to its supply to the blast furnace.

[0052] The plasma generator preferably heats the hot blast air up to a temperature range of 1300°C to 2500°C before reaching the tuyere, and more preferably to a temperature ranging from 1500°C to 2000°C.

[0053] In an advantageous manner, the hot blast temperature at the tuyere is adjusted by changing the plasma generator power to obtain a RAFT (raceway adiabatic flame temperature) preferably between 1700°C and 2400°C and more preferably between 1800°C and 2200°C. This adjustment allows adaptation to each carbon-lean fuel, as each different component of the latter generates different volumes of reducing gas and heat during cracking and partial oxidation at the tuyere.

[0054] Preferably, the plasma generator comprises an adjustable power ranging from 100kWh / t to 500kWh / t to regulate the hot blast air temperature at the tuyere.

[0055] Table 3 below shows a comparison of a method according to prior art, where the auxiliary fuel injected corresponds solely to PCI, and two different embodiments of the present invention. In these embodiments, the auxiliary fuel corresponds to hydrogen, and to natural gas, which respectively enable to achieve 34% and 21 % reduction in carbon dioxide emissions compared to the state of the art.

[0056] Table 3:| C02saving | | | 34% | 21 % |*equivalent to 100Nm3 / t

[0057] It can be seen from Table 3 that the plasma generator power is adjusted between the two embodiments of the invention i.e., between the use of hydrogen and natural gas as the auxiliary fuel. For instance, when using hydrogen as the auxiliary fuel, the power needed was 344kWh / t HM (of hot metal), while 360kWh / t were needed when using natural gas as the auxiliary fuel.

[0058] Preferably, the plasma generator is supplied with a feed gas comprising a mixture of oxygen and a neutral gas. Said neutral gas preferably corresponds to nitrogen which can be injected at a rate of up to 250Nm3 / t (normal cubic meters per ton of produced hot metal), and more preferably around 100Nm3 / t.

[0059] Alternatively, the mixture of oxygen and neutral gas corresponds to air, or a mixture of air and oxygen, or a mixture of air and nitrogen.

[0060] The method 100 may also comprise injecting oxygen and / or inert gas in the hot blast air and / or in the plasma generator’s feed gas and / or directly at the tuyere to optimize the plasma generator efficiency and to control the volume of gas generated in front of the tuyere, also called raceway gas or tuyere gas. The volume and the composition of the tuyere gas is a parameter for controlling the top gas temperature of the blast furnace in the range of 75°C to 250°C and preferably in the range of 100°C to 180°C. Advantageously, at constant RAFT, the injection of additional nitrogen allows to increase the top gas temperature. The nitrogen injection at constant RAFT solves the issue of low top gas temperature encountered in prior art when using low C / H auxiliary fuel.

[0061] Figure 2 is a schematic partial cross-section view of a blast furnace 2 according to the invention, designed to execute the steps of the method 100 outlined in Figure 1 .

[0062] The blast furnace 2 comprises a bustle pipe 4 extending circumferentially around a furnace shell 6, and a tuyere 8 connected to the bustle pipe 4 via a tuyere stock 10 to supply hot blast air 12 into the blast furnace 2.

[0063] According to a first embodiment of the invention, the plasma generator 14 is arranged upstream of the tuyere 8, said generator 14 being supplied with a feed gas 16 through a feeding duct 18, said feed gas 16 being distinct from the hot blast air 12 of the bustle pipe 4 and the tuyere stock 10. Preferably, the plasma generator 14 according to the first embodiment of the invention corresponds to a DC (Direct Current) plasma torch, the functioning of which will not be described in detail in the context of the present invention for the sake of conciseness, as such a plasma torch is per se well known in the art.

[0064] According to a second embodiment of the invention, the plasma generator 14’ preferably comprises electrodes (not shown) protruding in the hot blast stream, the latter operates without a feed gas, thus, said plasma generator 14’ is devoid of a feed gas duct. In this configuration, nitrogen can be injected directly before the hot stoves or anywhere upstream the tuyere 8. The plasma generator 14’ according to the second embodiment can be a TAP torch (Transferred Arc Plasma Torch) or a NTAP torch (Non- Transferred Arc Plasma Torch), the functioning of which will not be described in detail in the context of the present invention for the sake of conciseness, as such plasma torches are per se well known in the art.

[0065] The blast furnace 2 may include multiple plasma generators 14, 14’, such as multiple plasma generators 14, 14’ for each tuyere 8 or a single plasma generator 14, 14’ for multiple tuyeres 8 (all or part of the total number of tuyeres).

[0066] Preferably, in the second embodiment of the invention, the feeding duct 18 is totally separated from the bustle pipe 4 and from the tuyere stock 10. In this configuration, the feed gas 16 can have a lower temperature compared to the hot blast air. Preferably, the plasma generator feed gas comprises a temperature of at most 300°C, and more preferably comprised between 20°C (ambient temperature) and 200°C.

[0067] Preferably the plasma generator 14, 14’ is arranged at the tuyere 8 or at a distance of up to 3 meters from said tuyere 8 along the hot blast air flow direction. The implementation in the bustle pipe 4 and / or upstream thebustle pipe 4 would require an advanced refractory lining to accommodate to the new hot blast temperature conditions.

[0068] The blast furnace 2 preferably comprises an injection conduit 20 for pulverized coal injection 22 joining the tuyere 8.

[0069] The carbon-lean fuel 24 having C / H weight ratio of less than 8 is injected into the blast furnace preferably by means of a supply pipe 26 protruding in the tuyere 8 downstream of the plasma generator 14, 14’ and the bustle pipe 4. This arrangement enables the mixing of the carbon-lean fuel 24 with the further heated hot blast air (at 1300-2500°C) in the tuyere 8, which ensures an efficient combustion of the carbon-lean fuel 24 in the blast furnace 2.

[0070] Figure 3 is a graph 200 comparing CO2savings achieved using a method according to prior art and the method of the invention across two different embodiments.

[0071] The method according to prior art comprises the use of natural gas and oxygen enrichment, without an additional heating step of the hot blast air before the tuyere. Table 4 below shows different values of CO2(in kg / t) savings achieved by varying the injection rate of natural gas.

[0072] Table 4:

[0073] The method according to the invention utilizes natural gas as the auxiliary fuel and employs a plasma torch with a feed gas containing air and 100Nm3 / t of nitrogen in one embodiment, while another embodiment omits entirely the nitrogen addition. Table 5 below shows different values of CO2savings achieved by varying the injection rate of natural gas for both embodiments according to the invention.

[0074] Table 5:

[0075] The graph 200 illustrates that increasing the natural gas rate significantly reduces CO2emissions, with a more substantial reduction achieved when using 100Nm3 / t of nitrogen. In fact, CO2saving difference in Table 5 corresponds to the delta between the CO2savings achieved with and without N2.

[0076] In contrast, the prior art methods using oxygen enrichment do not achieve the same level of CO2savings as the method described in the present invention.

[0077] Figure 4 is a graph 300 comparing CO2savings achieved using the method of the invention across three different embodiments: one where the carbon- lean fuel comprises ammonia, another where it comprises hydrogen, and a third where nitrogen is used in the plasma generator’s feed gas.

[0078] Table 6 shows different values of CO2savings achieved by varying the injection rate of the carbon-lean fuel consisting of ammonia, without the use of nitrogen in the feed gas. Nitrogen addition is no help because the cracking of ammonia in the raceway already generates nitrogen. The conversion of ammonia rate to equivalent hydrogen rate is done by multiplying by a factor (3 / 17) representing the hydrogen weight ratio in ammonia. Ammonia cracking reaction: 2 NH3^> N2+3 H2.

[0079] Table 6:

[0080] Table 7 below presents various CO2savings achieved by adjusting the hydrogen injection rate, both with and without the use of nitrogen in the feed gas.

[0081] Table ?:

[0082] The graph 300 shows that combining 100Nm3 / t of nitrogen with hydrogen results in greater CO2savings than using hydrogen alone. The additional nitrogen leaves the raceway at the raceway adiabatic flame temperature which was kept constant for all the calculated cases. For the blast-furnace, it is like a pure additional heat input which reduces the fuel consumption and thus brings CO2 savings. The ammonia injected in the blast furnace decomposes by cracking in the raceway to a gas mixture having the weight composition (1 H2, 14 / 3 N2). Because of the presence of nitrogen in the ammonia molecule the CO2 savings with ammonia are higher than the equivalent quantity of pure hydrogen. Surprisingly, on the graph 300, the hydrogen and nitrogen line meets the ammonia line at the value 27 kg.eqH2Indeed the weight of 100Nm3of nitrogen is 125kg and ammonia generates 125kg of nitrogen with 27 kg of hydrogen.

[0083] Figures 5 to 7 depict graphs 400, 500, and 600, showing the evolution of hot blast air temperature in relation to CO2savings at varying injection rates of hydrogen, ammonia, and natural gas, respectively.

[0084] With reference to figure 5, the CO2savings values are derived from Table 7 for the embodiment that uses hydrogen without nitrogen.

[0085] It can be concluded that hydrogen does not require extremely high hot blast temperatures; temperatures below 2000°C are sufficient to achieve a CO2reduction of over 500kg / t.

[0086] With reference to figure 6, the CO2savings values are derived from Table 6 for the embodiment using ammonia.

[0087] The graph 500 shows that when the ammonia injection rate exceeds 100 kg / t, the hot blast air temperature must be increased significantly to generate proportional CO2savings.

[0088] With reference to figure 7, the CO2savings values are derived from Table 5 for the embodiment which uses natural gas without nitrogen.

[0089] The observation in graph 600 is similar to that for ammonia, indicating that the hot blast air temperature must be significantly increased to generate proportional CO2savings.

[0090] Figure 8 is a graph depicting values of the heat released during the conversion of auxiliary fuels to reducing gas with air in standard conditions. The heat is expressed in kilojoules per mole of reducing gas for different types of auxiliary fuels, including carbon-lean fuels according to the present invention.

[0091] Three major categories A, B and C can be distinguished from the graph of figure 8, including a first category A of known PCIs (pulverized coal injection) from prior art, having a carbon-to-hydrogen weight ratio superior to 8. Each PCI exhibits a high reducing gas energy, which is the reason why its use in blast furnaces is widespread.

[0092] A second category B of carbon-lean fuels can be observed, each fuel type has a C / H ratio of less than 8. They overall exhibit a convenient reducing gas energy. Some of said fuels are used in the state of the art blast furnaces, but the injection rate is limited in order to keep efficient operation conditions of the blast furnace. However, while CO2emissions remain high.

[0093] We also observe a third category, C, of carbon-lean fuels that exhibit negative reducing gas energy, rendering them unsuitable for tuyere injection in conventional blast furnaces. Some tests of hydrogen injection in blastfurnace tuyere were reported but none of them was performed for a period longer than few hours.

[0094] However, with the present invention, particularly through the effective use of the plasma generator to energize the hot blast air, all carbon-lean fuels from the second and third categories (B and C) can be utilized either alone or in combination with PCI, and advantageously achieving significant injection rate and CO2reductions.

[0095] It should be highlighted that all disclosed embodiments in this patent document are combinable, each comprising synergistic features aiming to collectively address the technical problem associated with blast furnace emissions. The skilled person in the art possesses the capability to combine various embodiments outlined within the patent document to achieve desired outcomes or address specific challenges.

Claims

Claims1 . Method (100) for operating a blast furnace (2), comprising:- introducing (S102) burden into the blast furnace (2);- injecting (S103) auxiliary fuel (20, 24) at a tuyere (8) of the blast furnace (2);- supplying (S104) hot blast air (12) to the blast furnace (2) through the tuyere (8); characterized in that the auxiliary fuel (20, 24) comprises a carbon-lean fuel (24) having a carbon-to- hydrogen weight ratio of less than 8, said method (100) further comprising:- reheating (S105) the hot blast air (12) by means of a plasma generator (14; 14’) prior to its supply to the blast furnace (2).

2. Method (100) according to claim 1 , wherein the carbon-lean fuel (24) comprises at least one of the following: diesel, organic chemical compound, kerosene, natural gas, hydrogen-enriched natural gas, ethanol, methanol, coke oven gas, hydrogen, and ammonia.

3. Method (100) according to any of claims 1 and 2, wherein the plasma generator (14) is supplied with a feed gas (16) comprising a mixture of oxygen and a neutral gas.

4. Method (100) according to claim 3, wherein the neutral gas corresponds to nitrogen.

5. Method (100) according to claim 3, wherein the mixture of oxygen and neutral gas corresponds to air, or a mixture of air and oxygen, or a mixture of air and nitrogen.

6. Method (100) according to any of claims 1 to 5, further comprising injecting nitrogen into the hot blast air (12) of the blast furnace (2) upstream of the tuyere (8).

7. Method (100) according to claim 6, wherein the nitrogen is injected at a rate of up to 250Nm3 / t, and preferably 100Nm3 / t.

8. Method (100) according to any of claims 1 to 7, wherein the carbon-lean fuel (24) is injected at a rate ranging from 10kg / t to 200kg / t.

9. Method (100) according to any of claims 1 to 8, wherein the plasma generator (14; 14’) heats the hot blast air (12) up to a temperature range of 1300°C to 2500°C before reaching the tuyere (8).

10. Method (100) according to any of claims 1 to 9, wherein the plasma generator (14; 14’) comprises an adjustable power ranging from 100kWh / t to 500kWh / t to regulate the hot blast air (12) temperature at the tuyere (8).

11. Method (100) according to any of claims 1 to 10, wherein the carbon-lean fuel (24) is used as the sole auxiliary fuel (20, 24) or in combination with pulverized coal injection (20).

12. Blast furnace (2) suitable for achieving the steps of the method (100) according to any of claims 1 to 11 , comprising:- a bustle pipe (4) extending circumferentially around a furnace shell (6);- a tuyere (8) connected to the bustle pipe (4) via a tuyere stock (10) to supply hot blast air (12) into the blast furnace (2), characterized in that the blast furnace (2) further comprises:- a plasma generator (14; 14’) arranged upstream the tuyere (8), said plasma generator (14) heating indirectly the hot blast with a plasma generated with a feed gas (16) supplied through a feeding duct (18), said feed gas being distinct from the hot blast air (12) of the bustle pipe (4) and the tuyere stock (10), or said plasma generator (14’) comprising electrodes protruding in said tuyere stock (10) and heating directly the hot blast air (12).

13. Blast furnace (2) according to claim 12, wherein said blast furnace (2) comprises a supply pipe (26) joining the tuyere (8) downstream of the plasma generator (14; 14’) and the bustle pipe (4), said supply pipe (26) being configured for injecting a carbon-lean fuel (24) having a carbon-to-hydrogen weight ratio of less than 8 into the blast furnace (2).1814. Blast furnace (2) according to any of claims 12 and 13, wherein the plasma generator (14’) comprising electrodes protruding in the tuyere stock (10) is devoid of a feed gas supply.

15. Blast furnace (2) according to any of claims 12 and 13, wherein the plasma generator (14) feed gas (16) comprises a temperature of at most 300°C.

16. Blast furnace (2) according to any of claims 12 to 15, wherein the plasma generator (14; 14’) is arranged at a distance of up to 3 meters from the tuyere (8) along the hot blast air (12).

17. Blast furnace (2) according to any of claims 12 to 16 and according to claim 13, further comprising an injection conduit (22) for pulverized coal injection (20), joining the tuyere (8) along with the tuyere stock (10) and the supply pipe (24).

Images