Process for producing direct reduced iron

Abstract

1. A method for producing direct reduced iron, wherein iron oxide is reduced in a direct reduction furnace by a reducing gas, the direct reduction furnace comprising a reduction zone, a transition zone and a cooling zone, and a carbon-containing liquid is injected below the reduction zone.

Description

[Technical Field]

[0001] The present invention relates to a method for producing direct reduced iron (DRI) and a DRI production facility. [Background technology]

[0002] Currently, steel can be produced through two main manufacturing routes. The most commonly used production route today is by producing pig iron in a blast furnace using a reducing agent, primarily coke, to reduce iron oxide. This process consumes approximately 450-600 kg of coke per metric ton of pig iron, and the process releases large amounts of CO2, both in the production of coke from coal in the coking plant and in the production of pig iron.

[0003] The second main route involves the so-called "direct reduction process", among which are processes under the brands MIDREX, FINMET, ENERGIRON / HYL, COREX, FINEX, etc., in which sponge iron is produced from the direct reduction of an iron oxide support in the form of HDRI (high-temperature direct reduced iron), CDRI (low-temperature direct reduced iron) or HBI (high-temperature briquetted iron). Sponge iron in the form of HDRI, CDRI and HBI is usually further processed in an electric arc furnace.

[0004] Each direct reduction shaft with low-temperature DRI discharge has three zones: a reduction zone at the top, a transition zone in the middle, and a conical bottom cooling zone. In high-temperature DRI, this bottom section is primarily used for homogenizing the product before discharge.

[0005] Iron oxide reduction occurs in the upper section of the furnace at temperatures up to 950°C. Iron oxide ore and pellets, containing approximately 30% oxygen by weight, are charged directly to the top of the reduction shaft and allowed to descend by gravity through the reducing gas. This reducing gas enters the furnace at the bottom of the reduction zone and flows countercurrently to the charged iron oxide. Oxygen contained in the ore and pellets is removed by the gradual reduction of the iron oxide in a countercurrent reaction between the gas and the oxide. As the gas moves to the top of the furnace, the oxidant content of the gas increases.

[0006] Reducing gases generally include hydrogen and carbon monoxide (synthesis gas) and are obtained by catalytic reforming of natural gas. For example, in the so-called MIDREX process, first methane is converted to a reformer to produce synthesis gas or reducing gas by the following reaction: CH4+CO2→2CO+2H2 Iron oxide reacts with reducing gases, for example according to the following reaction: 3Fe2O3+CO / H2→2Fe3O4+CO2 / H2O Fe3O4+CO / H2→3FeO+CO2 / H2O FeO+CO / H2→Fe+CO2 / H20 At the end of the reduction zone, the ore is metallized.

[0007] A transition section is found below the reduction section, which is long enough to separate the reduction section from the cooling section, allowing independent control of both sections. In this section, carbonization of the metallization product occurs. Carbonization is a process that increases the carbon content of the metallization product inside the reduction furnace by the following reaction: 3Fe+CH4→Fe3C+2H2 (endothermic) 3Fe+2CO→Fe3C+CO2 (heat) 3Fe+CO+H2→Fe3C+H2O (heat)

[0008] Injection of natural gas into the transition zone uses the sensible heat of the metallization products in the transition zone to promote hydrocarbon cracking and carbon deposition. Due to the relatively low concentration of oxidants, natural gas in the transition zone is more likely to be cracked to H2 and carbon than reformed to H2 and CO. Natural gas cracking provides carbon for DRI carbonization while simultaneously adding a reducing agent (H2) to the gas to increase the gas reduction potential.

[0009] Gas injection is also performed in the cooling zone, typically consisting of recirculating the cooling gas and added natural gas. The addition of natural gas (NG) to the cooling gas allows operators to maintain a recirculating cooling gas circuit with a high methane content; otherwise, the main component in the cooling gas is nitrogen. Natural gas has a much higher heat capacity than N2: the cooling gas recirculation flow is 500-600 Nm3 / t with NG and 800 Nm3 / t without NG. While there is not much carbon deposition in the cooling zone, the upward flow of cooling gas to the higher furnace levels provides more hydrocarbons for cracking.

[0010] Considering the significant increase in atmospheric CO2 concentrations since the beginning of the last century and the subsequent greenhouse effect, it is essential to reduce CO2 emissions where CO2 is produced in large quantities, and therefore especially during DRI production.

[0011] One of the solutions currently being developed is to gradually increase the hydrogen content of the reducing gas, with the aim of reaching a pure hydrogen reducing gas. The following reduction reaction then occurs: Fe2O3+3H2=2Fe+3H2O Therefore, it releases harmless H2O instead of the greenhouse gas CO2.

[0012] However, this implies that the carbon content in the reducing gas decreases and at some point no more carbon is injected into the shaft, which, as explained above, affects the DRI product, which has an increasingly lower carbon content.

[0013] The carbon content in the DRI product is an important parameter in that it plays an important role in subsequent steps such as slag foaming in the electric arc furnace, but also helps to improve the transportability of the DRI product.

[0014] Solutions are already known to increase the carbon content of the product, and they consist mainly of hydrocarbons, usually CH4, or coke oven gas injected into the shaft. However, these gases contribute to an increase in the carbon footprint of the DRI process that is inconsistent with switching to pure H2 reduction. Summary of the Invention [Problem to be solved by the invention]

[0015] There is a need for a method that allows for increasing the carbon content in DRI products. There is also a need for a method that allows for further reducing the carbon footprint of the process. [Means for solving the problem]

[0016] This problem is solved by the method according to the invention, in which iron oxide is reduced by a reducing gas in a direct reduction reactor, said direct reduction reactor comprising a reduction zone, a transition zone and a cooling zone, and a carbon-containing liquid is injected below the reduction zone.

[0017] The method of the invention may also comprise the following optional features, considered separately or according to all possible technical combinations: The carbon-containing liquid is injected at least into the transition zone, the carbon-containing liquid is injected into at least the cooling zone; - a carbon-containing liquid is injected into the transition zone and the cooling zone; the carbon-containing liquid is a biofuel; the carbon-containing liquid is liquid alcohol; the carbon-containing liquid is a liquid hydrocarbon; the carbon-containing liquid is liquid ethanol; the reducing gas comprises more than 50% hydrogen by volume; the reducing gas comprises more than 99% hydrogen by volume; the reducing gas hydrogen is produced at least in part by electrolysis, -Electrolysis is powered by renewable energy, the top reduction gas is captured at the outlet of the direct reduction furnace and subjected to at least one separation step so as to be divided between a CO2-rich gas and an H2-rich gas, said H2-rich gas being at least partially used as the reducing gas; The CO2-rich gas is subjected to a hydrocarbon production step.

[0018] Other characteristics and advantages of the present invention will become apparent from the description of the invention given below by way of indication and which is in no way limiting, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 shows the layout of a direct reduction plant making it possible to carry out the method according to the invention. [Figure 2A] FIG. 1 shows curves simulating the increase in carbon content in DRI products when liquid ethanol or methanol is injected. [Figure 2B] FIG. 1 shows curves simulating the increase in carbon content in DRI products when liquid ethanol or methanol is injected. DETAILED DESCRIPTION OF THE INVENTION

[0020] Elements in the figures are illustrative and may not be drawn to scale.

[0021] FIG. 1 shows the layout of a direct reduction plant making it possible to carry out the method according to the invention.

[0022] The DRI production facility includes a DRI shaft 1 having, from top to bottom, an inlet 10 for iron ore moving through the shaft 1 by gravity, a reduction section located at the top of the shaft, a transition section located in the middle of the shaft, a cooling section located at the bottom, and an outlet from which direct reduced iron 12 is ultimately extracted.

[0023] In the process according to the invention, a direct reduction furnace (or shaft) 1 is charged at its top with iron oxide 10. This iron oxide 10 is reduced in the furnace 1 by reducing gas 11, which is injected into the furnace and flows countercurrently to the iron oxide. Reduced iron 12 exits the bottom of the furnace 1 for further processing, such as briquetting, before being used in a subsequent steelmaking step. After reducing the iron, the reducing gas exits at the top of the furnace as top reducing gas 20 (TRG).

[0024] Cooling gas 13 may be captured from the cooling zone of the furnace, subjected to a cleaning step in a cleaning device 30 such as a scrubber, compressed in a compressor 31 and then sent back to the cooling zone of the shaft 1 .

[0025] In the method according to the invention, carbon-containing liquid 40 is injected below the reduction zone in shaft 1. It may be injected into the transition zone as shown by stream 40A and / or into the cooling zone as shown by streams 40B and 40C. It may be injected alone 40B or in combination with cooling gas 13 40C.

[0026] Carbon-containing liquid means a liquid product containing carbon. It may be an alcohol, such as methanol or ethanol, or a hydrocarbon, such as methane. It may be of fossil or non-fossil origin, and in a preferred embodiment is a biofuel. Biofuel means a fuel produced by a process from biomass, rather than the very slow geological processes involved in the formation of fossil fuels such as petroleum. Biofuels can be produced from plants (i.e., energy crops) or from agricultural, commercial, domestic and / or industrial waste (if the waste has a biological origin). This biofuel can preferably be produced by the conversion of steelmaking gases.

[0027] Once injected into the shaft, the carbon-containing liquid 40 is cracked by the heat released by the hot DRI, which produces reducing gases that carbonize the DRI product and increase its carbon content. Additionally, the enthalpy of vaporization further contributes to DRI cooling.

[0028] The injection of this liquid is carried out in order to increase the carbon content of the direct reduced iron to a range of 0.5-3 wt.%, preferably 1-2 wt.%, which makes it possible to obtain direct reduced iron that can be easily handled and that maintains a good combustion potential for its future use.

[0029] In a preferred embodiment, the reducing gas 11 comprises at least 50%v hydrogen, more preferentially more than 99%v H. An H stream 40 may be provided to produce said reducing gas 11 by a dedicated H generation plant 9, such as an electrolysis plant. It may be a water or steam electrolysis plant. It preferably operates using CO2-neutral electricity, which in particular includes electricity from renewable sources, defined as energy collected from renewable sources that are naturally replenished on human timescales, including sources such as sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of electricity from nuclear sources can be used, as the CO2 produced is not emitting.

[0030] In another embodiment, H2 stream 40 can be mixed with a portion of the overhead reducing gas 20 to form the reducing gas 11. When operating with natural gas, the overhead reducing gas 20 typically contains 15-25% v CO, 12-20% v CO2, 35-55% v H2, 15-25% v H2O, and 1-4% N2. It has a temperature of 250-500°C. When pure hydrogen is used as the reducing gas, the composition of the overhead reducing gas is rather 40-80% v H2, 20-50% v H2O, and some possible gas impurities from the shaft seal system or present in the hydrogen stream 40. When the amount of H2 in the reducing gas is varied and a carbon-containing liquid 40 is injected, the overhead gas 20 will have a composition between the two aforementioned cases.

[0031] In a further embodiment of the method according to the invention, the overhead reduced gas 20 after the dust and mist removal step in a washing device 5, such as a scrubber and demister, is sent to a separation unit 6 where it is split into two streams 22, 23. This separation unit 6 may be an absorber, an adsorption device, a cryogenic distillation device or a membrane. It may also be a combination of these different devices.

[0032] The first stream 22 is a CO2-rich gas that can be captured and used in different chemical processes. In a preferred embodiment, this CO2-rich gas 22 is subjected to a methanation step. The second stream 23 is an H2-rich gas that is sent to a preparation device 7 where it is mixed with other gases, optionally reformed, and heated to produce a reducing gas 11. In a preferred embodiment, the preparation device 7 is a heater.

[0033] All the different embodiments described above may be combined with each other.

[0034] Figures 2A and 2B are curves simulating the evolution of the weight percentage of carbon in the direct reduced iron product versus temperature when injecting 100 kg / ton of liquid ethanol DRI (Figure 2A) or 430 kg / ton of liquid methanol DRI (Figure 2B), respectively. In both cases, we find that it is possible to reach a carbon content in the solid product of approximately 2% by weight when the liquid is injected into the transition and / or cooling zones of the furnace. The advantage of ethanol is that it is less needed and more available compared to methanol. The simulations were performed using a thermodynamic model.

[0035] The process according to the invention makes it possible to obtain a DRI product with the required carbon content.

Claims

1. 1. A method for producing direct reduced iron, wherein iron oxide is reduced in a direct reduction furnace with a reducing gas, the reducing gas comprising greater than 99% hydrogen by volume, the direct reduction furnace comprising a reduction zone, a transition zone, and a cooling zone, and wherein a carbon-containing liquid is injected below the reduction zone.

2. The method of claim 1 , wherein the carbon-containing liquid is injected into at least the transition zone.

3. The method of claim 1 , wherein the carbon-containing liquid is injected into at least the cooling zone.

4. The method of claim 1 , wherein the carbon-containing liquid is injected into the transition zone and the cooling zone.

5. The method of claim 1 , wherein the carbon-containing liquid is a biofuel.

6. The method of any one of claims 1 to 5, wherein the carbon-containing liquid is a liquid alcohol.

7. The method of any one of claims 1 to 5, wherein the carbon-containing liquid is ethanol.

8. The method of any one of claims 1 to 5, wherein the carbon-containing liquid is a liquid hydrocarbon.

9. 6. The method according to any one of claims 1 to 5, wherein the reducing gas hydrogen is at least partly produced by electrolysis.

10. 10. The method of claim 9, wherein the electrolysis is powered by renewable energy.

11. 6. The method according to claim 1, wherein the top reducing gas is captured at the outlet of the direct reduction furnace and subjected to at least one separation step so as to be divided between a CO-rich gas and an H-rich gas, and the H-rich gas is at least partially used as the reducing gas.

12. 12. The method of claim 11, wherein the CO2-rich gas is subjected to a hydrocarbon production step.

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