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 iron oxide is first mixed with biochar to form a solid compound, and the solid compound is charged to the direct reduction furnace.

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 oxidant, the 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] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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]

[0014] What is needed is a method that allows for increasing the carbon content in DRI products while reducing the carbon footprint of the process. [Means for solving the problem]

[0015] This problem occurs when iron oxide is reduced in a direct reduction furnace by a reducing gas, and the iron oxide is first mixed with biochar to form a solid compound forming the solid compound is charged into the direct reduction furnace.

[0016] The method of the invention may also comprise the following optional features, considered separately or according to all possible technical combinations: -Biochar is produced by pyrolysis of biomass, -solid compound is a briquette and / or pellet, 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 in which it is 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 methanation step.

[0017] 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]

[0018] [Figure 1] FIG. 1 shows the layout of a direct reduction plant making it possible to carry out the method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Elements in the figures are illustrative and may not be drawn to scale.

[0020] Figure 1 shows the layout of a direct reduction plant that makes it possible to carry out the method according to the invention, in which, at the top of the direct reduction furnace (or shaft) 1, a furnace made of a mixture of iron oxide and biochar is placed. compound 10 is loaded. compound may have any suitable shape that allows it to be loaded into the furnace, and is preferentially charged in the form of briquettes and / or pellets. In a preferred embodiment, compound 10 contains 0.01 to 10% by weight of biochar. Biochar refers to charcoal produced by pyrolysis of biomass in the absence of oxygen.

[0021] Biomass is renewable organic material derived from plants and animals. Biomass sources for energy include wood and wood processing wastes—firewood, wood pellets and wood chips, sawdust and waste from sawmills and furniture factories, and black liquor from pulp and paper mills; agricultural crops and wastes—corn, soybeans, sugarcane, 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 waste and animal manure; and human sewage.

[0022] compound 10 provides both the iron oxide to be reduced and the carbon source necessary to carburize the metallized product. In a preferred embodiment, the carbon content of the direct reduced iron is set to 0.5-3 wt. %, preferably 1-2 wt. %, which makes it possible to obtain direct reduced iron that can be easily handled and maintains good combustion potential for its future use.

[0023] The aforementioned compound 10 is poured into the furnace, and compound The iron is reduced in the furnace 1 by reducing gas 11 flowing countercurrently to the furnace 10. The 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 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.

[0026] In another embodiment, H2 stream 40 may be mixed with a portion of the top reducing gas 20 to form the reducing gas 11. When operating with natural gas, the top 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 top 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. The amount of H2 in the reducing gas may vary, compound When 10 is charged, the top gas 20 has an intermediate composition between the two cases described above.

[0027] In one embodiment of the method according to the invention, the overhead reducing 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. 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 the reducing gas 11. In a preferred embodiment, the preparation device 7 is a heater.

[0028] The method according to the invention makes it possible to obtain a DRI product with a sufficient carbon content without compromising the CO2 footprint of the process.

Claims

1. 1. A method for producing direct reduced iron by reducing iron oxide in a direct reduction furnace with a reducing gas, wherein the reducing gas comprises more than 99% hydrogen by volume, the iron oxide is first mixed with biochar to form a solid composite, the solid composite comprising 0.01 to 10% biochar by weight, the solid composite being charged into the direct reduction furnace, and a carbon content of the direct reduced iron is set to 0.5 to 3% by weight.

2. 10. The method of claim 1, wherein the biochar is produced by pyrolysis of biomass.

3. 3. The method according to claim 1 or 2, wherein the solid composite is a briquette and / or a pellet.

4. The method of claim 1 , wherein the reducing gas, hydrogen, is at least partially produced by electrolysis.

5. 5. The method of claim 4, wherein the electrolysis is powered by renewable energy.

6. 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.

7. 7. The method of claim 6, wherein the CO2-rich gas is subjected to a methanation step.

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