Ironmaking method and associated plant

Abstract

An ironmaking method wherein hot metal and a blast furnace top gas are produced in a blast furnace, said method comprising the steps of capturing at least a part of the blast furnace top gas, separating carbon dioxide from the captured blast furnace top gas so as to produce a CO2-rich stream and a CO2-lean stream, injecting at least a part of the CO2- lean stream in the blast furnace, mixing the CO2-rich stream with a hydrogen makeup stream and subjecting the obtained CO2 / H2 gas mixture to a reverse water gas shift reaction in a reactor to produce a reducing stream comprising carbon monoxide CO and hydrogen H2, separating H2 from the reducing stream to produce a CO-rich stream and an H2-rich stream, mixing the H2-rich stream with the CO2-rich stream or with the CO2 / H2 gas mixture in the reactor and subjecting the obtained gas mixture to the reverse water gas shift reaction.

Description

Ironmaking method and associated plant

[0001] The invention is related to an ironmaking method and to the associated ironmaking plant.

[0002] In blast furnaces, the conversion of the iron-containing charge (sinter, pellets and iron ore) to cast iron, or hot metal, is conventionally carried out by reduction of the iron oxides by a reducing gas (in particular containing CO, H2 and N2), which is formed by partial combustion of coke and eventually auxiliary reducing agents at the tuyeres located in the bottom part of the blast furnace where air preheated to a temperature usually between 1000° C and 1300° C, called hot blast, is injected.

[0003] The auxiliary reducing agents that may be injected at the tuyeres to increase the productivity and reduce the costs may be coal in pulverized form, fuel oil, natural gas or reducing agents, combined with oxygen enrichment of the hot blast.

[0004] The gas recovered in the upper part of the blast furnace, called top gas, mainly consists of CO, CO2, H2 and N2 in respective proportions of 20-28%v, 17-25%v, 1-5%v and 48-55%v on dry basis. Despite partial use of this gas as fuel in other facilities of the steel plant (coke plant, blast heaters...), or ultimately at power plants to produce electricity, blast furnace remains a significant producer of CO2.

[0005] In view of the considerable increase in the concentration of CO2 in the atmosphere since the beginning of the last century and the subsequent greenhouse effect, it is essential to reduce emissions of CO2 where it is produced in a large quantity, and therefore in particular at blast furnaces.

[0006] For this purpose, during the last 50 years, the consumption of reducing agents in the blast furnace has been reduced by half so that, at present, in blast furnaces of conventional configuration, the consumption of carbon has reached a low limit linked to the laws of thermodynamics.

[0007] One solution considered to further reduce this carbon-based reductant consumption and thus to reduce the CO2 footprint of the blast furnace ironmaking route is to capture the top gas, remove CO2 and reinject the reducing part of it into the blast furnace shaft, which is a level above the usual tuyeres level at which the hot blast is injected. However, with this solution the reductant consumption reduction remains below 30% compared to the production in a conventional blast furnace (without top gas recycling) according to numerous calculations and trials performed. In terms of global CO2 footprint, it represents a reduction of less than 20% in volume of emitted CO2.

[0008] One solution is the use of hydrogen as reducing gas. A hydrogen rich stream is injected into the blast furnace in substitution of a part of coke as reducing agent. To have an impact on the CO2 footprint of the overall process this hydrogen must be green hydrogen or hydrogen recovered from the process itself.

[0009] Green hydrogen is not yet available in enough quantity to fulfil the needs and is subjected to fluctuations of supply as depending on renewable energies. Hydrogen separation techniques are not yet available on an industrial scale and their energy demand and operational costs are high.

[0010] There is thus a need for an ironmaking method allowing to significantly reduce the carbon-based reductant consumption in the blast furnace while limiting the overall carbon footprint of the process.

[0011] This problem is solved by a method according to the invention, wherein hot metal and a blast furnace top gas are produced in a blast furnace, said method comprising the steps of capturing at least a part of the blast furnace top gas, separating carbon dioxide from the captured blast furnace top gas so as to produce a CO2-rich stream and a CO2- lean stream, injecting at least a part of the CO2-lean stream in the blast furnace, mixing the CO2-rich stream with a hydrogen makeup stream and subjecting the obtained CO2 / H2 gas mixture to a reverse water gas shift reaction in a reactor to produce a reducing stream comprising carbon monoxide CO and hydrogen H2, separating H2 from the reducing stream to produce a CO-rich stream and an H2-rich stream, mixing the H2-rich stream with the CO2-rich stream or with the CO2 / H2 gas mixture in the reactor and subjecting the obtained gas mixture to the reverse water gas shift reaction.

[0012] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: a gas containing more than 80%v of 02 is injected in the blast furnace at the tuyere level, at least a part of the CO-rich stream is mixed with the captured blast furnace top gas before the CO2 separation step, the H2 makeup gas stream is hydrogen produced by electrolysis of water., the gaseous stream subjected to the reverse water gas shift reaction is first heated at a temperature of at least 400°C, the gaseous stream subjected to the reverse water gas shift reaction is first heated at a temperature of at least 600°C, the gaseous stream is heated by electrical heating,the electrical heating is supplied by renewable energy, the gaseous stream subjected to the reverse water gas shift reaction has a H2 to CO2 molar ratio of at least 2, the gaseous stream subjected to the reverse water gas shift reaction has a H2 to CO2 molar ratio of at least 3, the CO2-lean stream is injected at the tuyere level of the blast furnace, at least a part of the CO2-lean stream is injected in the blast furnace at a temperature above 950°C, the heating of the CO2-lean gas stream is performed by electrical heating, the CO2-lean stream is injected at the shaft level of the blast furnace, at least a part of the CO2-lean stream is injected in the blast furnace at a temperature above 800°C.

[0013] The invention is also related to a plant allowing to implement a method according to anyone of the previous combinations.

[0014] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication, and which is in no way restrictive, with reference to the appended figures in which:Figure 1 illustrates an embodiment of a method according to the invention,Figure 2 illustrates another embodiment of a method according to the invention,Figure 3 is a graphic showing the expected CO2 conversion rate of the rWGS reaction according to the molar H2 / CO2 ratio for different temperatures of reaction.

[0015] First, it is noted that on the figures, the same references designate the same elements regardless of the figure on which they feature and regardless of the shape of these elements. Similarly, should elements not be specifically referenced in one of the figures, their references may be easily found by referring to another figure.

[0016] It is also noted that the figures represent mainly one embodiment of the object of the invention but other embodiments which correspond to the definition of the invention may exist. Elements in the figures are illustration and may not have been drawn to scale.

[0017] Figure 1 illustrates an ironmaking plant allowing to perform a method according to one embodiment of the invention. This plant comprises at least one blast furnace 1 wherein an iron-containing charge 4 such as sintered ore, pellets, iron ore is loaded together with a first carbon-based reductant 5 into the throat of the blast furnace 1 . This first-carbon basedreductant may be coke but is preferentially a non-fossil-based carbon reductant such as biochar or biocoal or waste plastics.

[0018] By biochar or biocoal it is meant a charcoal that is produced by pyrolysis of biomass in the absence of oxygen. Biomass is renewable organic material that comes from plants and animals. Biomass sources for energy include notably wood and wood processing wastes — firewood, wood pellets, and wood chips, lumber and furniture mill sawdust and waste, and black liquor from pulp and paper mills, agricultural crops and waste materials — corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues, biogenic materials in municipal solid waste, paper, cotton, and wool products, and food, yard, and wood wastes and animal manure and human sewage.

[0019] The iron-containing charge 4 is converted to hot metal 2 by reduction of the iron oxides. This reduction may be performed thanks to three inputs, first one being the loading of the first carbon-based reductant 5, second one being the injection of a blast 22 at a first level of injection 3A, also named tuyere level, and optionally the injection of a reducing gas at a second level of gas injection 3B located above this first level. For clarity’s sake, references 3A and 3B designate both the level of injection and the associated injection means at the considered level.

[0020] It is further noted that even if both gas injection levels 3A and 3B are illustrated as a pair of arrows in the figures it is only for illustration purposes and that these two gas injections are preferentially performed at each respective level around the whole circumference of the blast furnace 1.

[0021] Such production emits a blast furnace gas which is at least partly 10 recovered at the top of the furnace 1. As a matter of illustration, the top gas may comprise from 15 to 25%v of CO, from 20 to 30%v of CO2, from 2 to 32% of H2 and more than 30%v of N2. In a preferred embodiment where the blast is composed mainly of oxygen, the top gas may rather comprise from 40 and 50%v of CO, from 30 to 40%v of CO2, from 2 and 15% of H2 and less than 20%v of N2. The blast furnace gas which is not recovered is called export gas 9.

[0022] In all the text, % on dry basis have to be understood as calculation of the composition of the gas in which the presence of water (H2O) is neglected. Unless otherwise specified, all %v are % in volume on a dry basis.

[0023] In a preferred embodiment at least 80% in volume of the BFG is captured, preferably around 95%. Less than 95% in volume of the BFG is captured so that the remaining 5% of export gas 9 are used as a purge, notably to avoid N2 accumulation into the recycling loop as N2 is not extracted or consumed in any step of the process.

[0024] The blast furnace gas 10 is subjected to a separation step in a CO2 separation device 2 to produce a CO2-rich stream 12 and a 002-lean stream 11. The CO2 separation device 2 may be a chemical absorption unit, for example with use of amines, a Pressure Swing Adsorption device or PSA, a Vacuum Pressure Swing Adsorption device VPSA, a cryogenic unit, or a combination of those technologies. At the exit of the CO2 separation device 2, the CO2-rich stream 12 preferably comprises more than 85% in volume of CO2 while the 002-lean stream 11 preferably comprises less than 5% in volume of CO2, more preferably less than 3% in volume of CO2.

[0025] Before this separation step the recovered exhaust gas 10 may be first subjected to one or more pre-treatment steps such as a dedusting and / or a dewatering step and / or a desulfurization step.

[0026] The 002-lean stream 11 is at least partly injected into the blast furnace 1. This injection may be done at the first level of injection 3A and / or at the second level of injection 3B. The injection at the tuyere level 3A is done together with the blast 22.

[0027] The 002 separation step allows to re-inject a part of the recovered exhaust gas directly in the BF as reducing gas, thus reducing the need for carbon-based reductants while also reducing the amount of gas to be treated in subsequent steps.

[0028] The blast 22 may have a temperature upper or equal to 950°C, preferentially from 1000°C to 1300°C, and comprises preferably a second carbon-based reductant. This second-carbon based reductant is preferentially in pulverized form and may be coal, also called PCI (Pulverized Coal Injection) but is preferentially a non-fossil-based carbon reductant such as biochar or bio-coal according to previously given description or waste plastics.

[0029] In a preferred embodiment the blast 22 comprises from 35 to 70 Nm3 of oxygen per ton of hot metal to be produced. The remaining component of the hot blast is air. This oxygen is preferentially mixed to the air before heating. This hot blast allows the combustion of coke and the other carbon bearing reducing agents at the tuyeres, hence converting them into a reducing gas allowing iron ore reduction.

[0030] Nm3 is a unit of measurement of the quantity of gas which corresponds to the content of a volume of one cubic metre, for a gas under normal conditions of temperature and pressure (0 °C and 1 atm).

[0031] In another embodiment the blast 22 is composed of at least 75% in volume of oxygen and is injected at ambient temperature, usually around 25°C. This allows notably to reduce the amount of nitrogen injected into the furnace compared to classical hot blast injection, and thus the amount of nitrogen into the blast furnace top gas. This nitrogen does not react in any of the steps and thus tend to accumulate into the gas circuit and requires additional purge equipment. Moreover, thanks to the decrease of nitrogen in the top gas, after the oxidation and CO2 removal steps, only hydrogen and carbon monoxide with a very limited amount of nitrogen is obtained, making the recycling of this gas straight forward and highly profitable for the blast furnace operation.

[0032] The reducing gas 20 injected at the secondary level of injection 3B preferably has a temperature of at least 800°C, preferably at least 900°C.

[0033] According to the invention, the CO2-rich stream 12 is mixed with an H2 makeup gas stream 50. The obtained CO2 / H2 mixture is then subjected to a reverse water gas shift reaction (rWGS) according to equation 1 in the reactor 3 to produce a reducing stream 13 comprising mainly carbon monoxide CO and hydrogen H2.Equation 1 CO2 + H2 CO + H20

[0034] The H2 make up gas stream 50 is preferably added in an amount allowing to fulfill a molar H21 CO2 ratio at least equal to 2, more preferably at least equal to 3 and even better at least equal to 4.

[0035] As illustrated in figure 3, the CO2 conversion rate of the rWGS reaction varies according to temperature and to the molar ratio of H2 to CO2. The curves were obtained using thermodynamical models, such as commercial software ThermoCalc®, FastSage® or ChemSims®. Under stoichiometric conditions (H2 I CO2 = 1), it would be necessary to operate at a temperature of at least 800 °C to get a conversion rate of CO2 equal or higher than 50 %. If the H2 I CO2 molar ratio is of 2, then the required temperature for the same conversion rate would be lower than 600 °C. In order to target CO2 conversion rates above 70%, the H2 / CO2 molar ratio has to be at least equal to 2, preferably at least equal to 3 to keep the required temperature lower than 1000°C.

[0036] The gas mixture subjected to the rWGS reaction may be first heated to a temperature upper than 400°C, preferably higher than 600°C. This heating is preferably done by electrical heating powered by CO2 neutral electricity. CO2 neutral electricity includes notably electricity from renewable sources which is defined as energy that is produced from renewable resources, which are naturally replenished on a human timescale,including sources like sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of electricity coming from nuclear sources can be used as it is not emitting CO2 to be produced. Increasing the temperature of the reaction allows shifting the reaction towards CO formation.

[0037] In a preferred embodiment the hydrogen is green hydrogen. Green hydrogen (GH or GH2) is hydrogen generated by renewable energy or from low-carbon power. This H2 stream may be provided by a dedicated H2 production plant, such as an electrolysis plant. It may be a water or steam electrolysis plant. Renewable energy must be understood as energy produced from renewable sources as previously defined.

[0038] The reducing stream 13 preferably comprises at least 70% in volume of CO + H2 on a dry basis. This reducing stream 13 is then subjected to a H2 separation step in a H2 gas separation unit 32 to produce a CO-rich stream 14 and an H2 rich stream 15. The CO-rich stream 14 preferably comprises more than 45% in volume of CO while the H2-rich stream 15 preferably comprises more than 95% in volume of H2.

[0039] The H2 gas separation unit 32 may be a H2-PSA or membranes.

[0040] The H2 rich stream 15 is then sent back to the reactor 3 to be mixed with the CO2- rich stream 12 or to the CO2 / H2 gas mixture and the obtained mixture is subjected to the reverse water gas shift reaction. The mixture of the H2 rich stream 15 and of the CO2-rich stream 12 may be done within the reactor 3 or upstream of it, as long as it is mixed before the reverse water gas shift reaction occurs.

[0041] This recycling of hydrogen allows to switch the reaction equilibrium towards CO production and thus to improve the efficiency of the process while reducing the need for fresh H2 coming from an external source. By external source it must be understood an H2 source which does not come for the steelmaking process itself, such as a dedicated H2 production plant or H2 purchase outside of the steelmaking plant.

[0042] The CO-rich stream 14 may be mixed with the captured blast furnace top gas 10 before the CO2 separation step in the reactor 2. It may also be valorized outside of the steelmaking plant.

[0043] Before the H2 separation step, the reducing stream 13 may go through a heat exchanger (not illustrated) to cool down the reducing stream 13 and capture the released heat. Said released heat may be transferred to one of the gases or mixture of gases sent to the reactor 3 for the rWGS reaction to increase their temperature. At the exit of the heat exchanger, the reducing stream 13 preferably has a temperature below 100°C, preferably below 50°C.

[0044] The method allows to recycle a part of the blast furnace gas (the CO2 lean stream) while either recycling or valorizing the second part through the production of the CO-rich gas. This reduces the CO2 emissions of the blast furnace without being limited by the availability of external hydrogen.

[0045] Another embodiment of a method according to the invention is illustrated in Figure 2. All the elements of the embodiment illustrated in figure 1 have the same reference in figure 2 and all options described in the embodiment of figure 1 may be combined with this embodiment when technically possible.

[0046] In this embodiment, the CO-rich stream 14 may be divided into two CO-rich streams 14A and 14B. The first CO-rich stream 14A may be mixed, as in the embodiment of Figure 1 , with the BFG 10 before the CO2 separation step in the CO2 separation device 2. The second CO-rich stream 14B may be sent to a secondary CO2 separation unit 33 to produce a secondary CO2-rich stream 42 and a secondary CO2-lean stream 41. The secondary CO2-rich stream 42 preferably comprises more than 90% in volume of CO2, the secondary CO2-lean stream preferably comprises less than 3% in volume of CO2.

[0047] The secondary CO2-rich stream 42 may be sent back to the reactor 3 to enrich the entry gas in CO2 and improve the efficiency of the rWGS Reaction. As it as a high content of CO2 it may also be used for carbon storage technologies, which require a high purity gas.

[0048] The secondary CO2-lean stream 41 may be reinjected into the blast furnace 1 , at the shaft level 3B and / or at the tuyere level 3A. As this secondary lean-stream has a low content in CO2 this avoids the transformation of CO2 into CO within the furnace which is an endothermic reaction. The heat requirements of the blast furnace are thus decreased.

[0049] This allows recovering most of the reducing gases contained in the blast furnace gas 10 and recycling them back to the blast furnace, thus reducing CO2 emissions. Thanks to the rWGS reaction the CO2 formed during the reduction of iron charge into the blast furnace is converted to CO which can be used in the blast furnace for its reducing power. By recycling this CO the need of partial combustion of external carbon sources, such as coke, necessary for iron oxides reduction is decreased, thus decreasing the overall CO2 footprint of the blast furnace ironmaking process.Example

[0050] A simulation was performed with the use of a blast furnace operating method according to prior art (A1 and A2) and with a method according to the invention (B to D).

[0051] Operational conditions are illustrated in table 1.Table 1 - Operational Conditions* According to prior art , thm = ton of hot metal

[0052] Prior art method A1 corresponds to a standard BF operating method with hot blast air injection and without any top gas recycling, A2 corresponds to a method wherein the hot blast is 100% 02 and top gas is recovered, subjected to a CO2 separation step and the CO2 depleted gas is recycled to the blast furnace at both shaft and tuyere level.

[0053] Methods B to D are according to the layout of figure 1.

[0054] Calculations were done using the commercial thermodynamical model Thermocalc®. For the purpose of the calculation the target was to reach a raceway adiabatic flame temperature (RAFT) of 2230°C. The RAFT is a commonly known parameter for the person skilled in the art of blast furnace operations, definition can notably be found in Coal Handbook, volume 2 - Towards Cleaner Coal Utilization 2ndEdition.

[0055] Among the calculated results are the amount of coke that needs to be charged in the throat of the blast furnace in kg / ton of hot metal (thm), the flow rate of gas recycled to the blast furnace at both tuyere and shaft levels, the proportion in volume of said recycled gas injected at the shaft level 3B and the quantity of external H2 that was needed.Table 2 - ResultsAccording to prior art, thm = ton of hot metal

[0056] The composition of the recovered top gas 10 and of the CO2-lean gas 11 were also calculated and are given respectively in table 3 and table 4.Table 3 - Recovered gas compositionTable 3 Table 4 - CO2-lean gas compositionTable 4

[0057] Then based on the previous calculations it was possible to estimate the amount of CO2 recycled to the blast furnace, the amount of carbon savings considering the amount of PCI and coke charged into the BF and the global CO2 emissions of each process. Results are illustrated in table 5.Table 5 - CO2 impactTable 5

[0058] The amount of CO2 recycled at BF is the sum of the CO2 re-injected to the BF through the injection of the CO2-lean gas 11 and of the CO2 converted to CO by the rWGS reaction.

[0059] The C savings were calculated using A1 process as reference. They are based on the amount of coke and PCI used in each process, considering only carbon content of those materials. The considered coke has a content in carbon of 85.632% by weight while the coal comprises 81.04% by weight of carbon.

[0060] The CO2 emissions are calculated considering the CO2 content of the exhaust gas from which the amount of CO2 recycled to the BF is subtracted. They do not take into account the CO2 indirect reduction linked to the decrease of coke and coal charged into the BF.

[0061] In the three simulations according to the invention (B to D), the amount of external H2 supply increases with the temperature of injection of the reducing gas at the tuyere because, as the temperature increased, more CO2 is converted into CO by the rWGS reaction and thus more H2 is consumed by the reaction. But in the meantime, more CO is introduced in the blast furnace which reduces the needs of coke and thus the global CO2 emissions.

[0062] As can be seen in the results, the method according to the invention allows further reduction of the amount of coke and PCI charged into the blast furnace compared to prior art methods. It also allows reducing the amount of CO2 emitted. With the method according to the invention it is thus possible to reduce the amount of CO2 emissions of the blast furnace.

Claims

CLAIMS1 ) An ironmaking method comprising the production of hot metal and of a blast furnace top gas (10) in a blast furnace (1 ), said method comprising the steps of: a. Capturing at least a part of the blast furnace top gas (10), b. Separating carbon dioxide from the captured blast furnace top gas (10) so as to produce a CO2-rich stream (12) and a CO2-lean stream (11 ), c. Injecting at least a part of the CO2-lean stream (11 ) in the blast furnace (1 ), d. Mixing the CO2-rich stream (12) with a hydrogen makeup stream (50) and subjecting the obtained CO2 / H2 gas mixture to a reverse water gas shift reaction in a reactor (3) to produce a reducing stream (13) comprising carbon monoxide CO and hydrogen H2, e. Separating H2 from the reducing stream to produce a CO-rich stream (14) and an H2-rich stream (15), f. Mixing the H2-rich stream (15) with the CO2-rich stream (12) or with the CO2 / H2 gas mixture in the reactor (3) and subjecting the obtained gas mixture to the reverse water gas shift reaction.2) A method according to claim 1 wherein a gas (22) containing more than 80% in volume of 02 is injected in the blast furnace at the tuyere level (3A).3) A method according to claim 1 or 2 wherein at least a part of the CO-rich stream (14) is mixed with the captured blast furnace top gas (10) before the CO2 separation step.4) A method according to any one of claims 1 or 2 wherein the H2 makeup gas stream (50) is hydrogen produced by electrolysis of water.5) A method according to any one of the claims 1 to 4 wherein the gaseous stream subjected to the reverse water gas shift reaction is first heated at a temperature of at least 400°C.6) A method according to any one of the claims 1 to 5 wherein the gaseous stream subjected to the reverse water gas shift reaction is first heated at a temperature of at least 600°C.7) A method according to any one of claims 5 or 6 wherein the gaseous stream subjected to the reverse water gas shift reaction is heated by electrical heating.8) A method according to claim 7 wherein said electrical heating is powered by CO2 neutral electricity.9) A method according to any one of the claims 1 to 8 wherein the gaseous stream subjected to the reverse water gas shift reaction has a H2 to CO2 molar ratio of at least 2.10) A method according to any one of the claims 1 to 9 wherein the gaseous stream subjected to the reverse water gas shift reaction has a H2 to CO2 molar ratio of at least 3.11 ) A method according to any one of the claims 1 to 10 wherein the CO2-lean stream (11 ) is injected at the tuyere level (3A) of the blast furnace (1 ).12) A method according to claim 11 wherein at least a part of the CO2-lean stream (11 ) is injected in the blast furnace (1 ) at a temperature above 950°C.13) A method according to claim 12 wherein the heating of the CO2-lean stream (11 ) is performed by electrical heating.14) A method according to any one of the claims 1 to 13 wherein the CO2-lean stream (11 ) is injected at the shaft level (3B) of the blast furnace (1 ).15) A method according to claim 14 wherein at least a part of the CO2-lean stream (11 ) is injected in the blast furnace (1 ) at a temperature above 800°C.16) A plant allowing to implement a method according to any one of the claims 1