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 production routes. The production route most commonly used today is by producing pig iron in a blast furnace using a reducing agent, mainly coke, to reduce the 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 them 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 cooling zone at the cone-shaped bottom. In high temperature discharge DRI, this bottom section is mainly used for homogenizing the product before discharge.

[0005] The reduction of the iron oxide takes place in the upper section of the furnace at temperatures up to 950°C. Iron oxide ore and pellets, containing about 30% oxygen by weight, are charged to the top of the direct reduction shaft and allowed to fall by gravity through the reducing gas, which enters the furnace at the bottom of the reduction zone and flows countercurrent to the charged iron oxide. The oxygen contained in the ore and pellets is removed in the gradual reduction of the iron oxide in a countercurrent reaction of the gas with the oxide. As the gas travels to the top of the furnace, the oxidant content of the gas is increased.

[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 drive hydrocarbon cracking and carbon deposition. Because of 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 generated in large quantities, and therefore especially during DRI production.

[0010] One of the solutions currently being developed is to gradually increase the hydrogen content into the reducing gas, with a view to reaching a pure hydrogen reducing gas. The following reduction reaction then occurs: Fe2O3+3H2=2Fe+3H2O Thus, 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. As explained above, this has an impact on the DRI product, which will have less and less carbon content.

[0012] The content of carbon in the DRI product is an important parameter in it and 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 consist of injecting mainly hydrocarbons, usually CH4, or coke oven gas into the shaft, but these gases contribute to an increase in the carbon footprint of the DRI process that is not consistent with a switch 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 is solved by the method according to the invention, in which iron oxide is reduced in a direct reduction furnace by a reducing gas, said iron oxide is first mixed with biochar to form a solid compound, and said solid compound is charged into said direct reduction furnace.

[0016] The method of the invention may also comprise the following optional features, taken separately or according to all possible technical combinations: -Biochar is produced by pyrolysis of biomass, the solid compound is a briquette and / or a pellet, the reducing gas comprises more than 50% hydrogen by volume; the reducing gas comprises greater than 99% hydrogen by volume; The reducing gas hydrogen is at least partially produced 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 reduction gas; The CO2-rich gas is subjected to a methanation step.

[0017] Other characteristics and advantages of the 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 attached drawings, in which: [Brief description 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 PREFERRED EMBODIMENTS

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

[0020] FIG. 1 shows the layout of a direct reduction plant making it possible to carry out the method according to the invention. In said method, the top of a direct reduction furnace (or shaft) 1 is charged with a compound 10 made of a mixture of iron oxide and biochar. Said compound may have any suitable shape that allows it to be loaded into the furnace, being preferentially charged in the form of briquettes and / or pellets. In a preferred embodiment, the compound 10 comprises 0.01 to 10% by weight of biochar. By biochar is meant the 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 lumber 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 metallization 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 a direct reduced iron that can be easily handled and maintains a good combustion potential for its future use.

[0023] The compounds 10 are reduced in the furnace 1 by reducing gas 11, which is injected into the furnace and flows countercurrent to the compounds 10. 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 H2. A stream of H2 40 may be provided to generate said reducing gas 11 by a dedicated H2 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, since it does not emit the CO2 produced.

[0026] In another embodiment, the 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, 1-4% N2. It has a temperature of 250-500 °C. When pure hydrogen is used as the reducing gas, the composition of said top reducing gas is rather composed of 40-80% v H2, 20-50% v H20 and some possible gas impurities coming from the shaft seal system or present in the hydrogen stream 40. When the amount of H2 in the reducing gas is varied and compound 10 is charged, the top gas 20 has an intermediate composition between the two previous cases.

[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 cleaning 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 a 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. A method for producing direct reduced iron by reducing iron oxide in a direct reduction furnace with a reducing gas, wherein the iron oxide is first mixed with biochar to form a solid composite, the solid composite containing 0.01 to 10% by weight of biochar, the solid composite being loaded into the direct reduction furnace, and the carbon content of the direct reduced iron being set to 0.5 to 3% by weight.

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

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

4. A method according to any one of claims 1 to 3, wherein the reducing gas comprises more than 50% hydrogen by volume.

5. A method according to any one of claims 1 to 3, wherein the reducing gas comprises more than 99% hydrogen by volume.

6. 6. The method according to claim 4 or 5, wherein the reducing gas hydrogen is at least partly produced by electrolysis.

7. 7. The method of claim 6, wherein the electrolysis is powered by renewable energy.

8. 8. The method according to claim 1, wherein a 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 CO2-rich gas and a H2-rich gas, and said H2-rich gas is at least partially used as the reducing gas.

9. 9. The method of claim 8, wherein the CO2-rich gas is subjected to a methanation step.

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