A method for producing direct reduced iron

Abstract

Abstract\nA direct reduction iron production method in which oxidized iron is reduced in a direct reduction furnace by a reducing gas, this oxidized iron is first mixed with biochar to form a solid component, and this solid component is loaded into said direct reduction furnace.

Description

A method for producing direct reduced iron

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

[002] Steel can currently be made by two main production routes. Today, the most common production method involves producing pig iron in a blast furnace using a reducing agent, mainly coke, to reduce the iron oxides. In this method, about 450 to 600 kg of coke is used per metric ton of pig iron. This method produces significant amounts of carbon dioxide, both from the production of coke from coal in a coking plant and from the production of pig iron.

[003] The second major method involves methods known as “direct reduction.” Among these methods are the brand-name methods such as MIDREX, FINMET, ENERGIRON / HYL, COREX, FINEX, and the like, in which sponge iron is produced in the form of HDRI (hot sponge iron), CDRI (cold sponge iron), or HBI (hot lumped iron) from the direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI, and HBI is usually subjected to additional processing in electric arc furnaces.

[004] In each cold-discharged DRI direct reduction shaft there are three zones: the upper reduction zone, the intermediate transition zone, and the cooling zone in the lower conical section. In hot-discharged DRI, this lower section is primarily used for product consolidation prior to discharge.

[005] The reduction of iron oxides takes place in the upper part of the furnace at temperatures up to 950°C. Iron oxide ores and pellets containing about 30% by weight of oxygen are loaded into the top of a direct reduction shaft and allowed to descend by gravity through a reducing gas. This reducing gas enters the furnace from the bottom of the reduction zone and flows countercurrently from the charged oxidized iron. The oxygen in the ores and pellets is removed from the iron oxides during the staged reduction by a countercurrent reaction between the gases and the oxide. The oxidizing content of the gas increases as it moves towards the top of the furnace.

[006] The reducing gas consists mainly of hydrogen and carbon monoxide (synthesis gas) and is obtained by catalytic reforming of natural gas. For example, in the MIDREX process, methane is first converted to a reformer according to the following reaction to produce synthesis gas or reduction gas: CH4 + CO2 -> 2CO + 2H2 And for example, iron oxide reacts with reducing gas according to the following reactions: 3Fe2O3 + CO / H2 -> 2Fe3O4+CO2 / H2O Fe3O4 + CO / H2 -> 3 FeO + CO2 / H2O FeO + CO / H2 -> Fe + CO2 / H20 At the end of the reduction zone, the ore is metallized.

[007] A transition section is located at the bottom of the reduction section. This section is long enough to separate the reduction section from the cooling section and to allow independent control of both sections. In this section, carburization of the metallized product occurs. Carburization is the process of increasing the carbon content of the metallized product within the reduction furnace through the following reactions: 3Fe + CH4 → Fe3C + 2H2 (heat sink) 3Fe + 2CO → Fe3C + CO2 (exothermic) 3Fe + CO + H2 → Fe3C + H2O (exothermic)

[008] Natural gas injection in the transition section is the use of sensible heat from the metallized product in the transition section to enhance hydrocarbon cracking and carbon deposition. Due to the relatively low concentration of oxidants, the transition section natural gas is more likely to be cracked into H2 and carbon than reformed into H2 and CO. Natural gas cracking provides carbon for DRI carburization and simultaneously adds a reducing agent (H2) to the gas, which increases the reductive capacity of the gas.

[009] Given the significant increase in atmospheric CO2 concentration compared to the beginning of the last century and the subsequent greenhouse effects, it is essential to reduce CO2 emissions at its mass production sites, and especially during DRI production.

[0010] One solution that is currently being developed is to progressively increase the hydrogen content into the reducing gas, ultimately resulting in a pure hydrogen reducing gas. The following reduction reaction then occurs: Fe2O3 + 3 H2 = 2 Fe + 3 H2O And thus, instead of the greenhouse gas CO2, harmless H2O is produced.

[0011] Of course, this means that the carbon content in the reducing gas will decrease and at a certain point, no more carbon will be injected into the shaft. As explained above, this has an effect on the DRI product, which will gradually have a lower carbon content.

[0012] The carbon content in the DRI product is a key component as it plays an important role in subsequent steps such as slag flooring in the electric arc furnace, but also helps improve the transportability of the DRI product.

[0013] There have been previous solutions to increase the carbon content of the product, but they have mainly been based on injecting hydrocarbons, usually CH4 or coke oven gas, into the shaft. However, these gases increase the carbon footprint of the DRI process, which is not compatible with the shift to reduction with pure H2.

[0014] There is a need for a method that allows for increasing the carbon content of the DRI product while at the same time reducing the carbon footprint of the process.

[0015] This problem is solved by a method according to the invention, in which oxidized iron is reduced in a direct reduction furnace by means of a reducing gas, this oxidized iron is first combined with biochar to form a solid compound, and this solid compound is then charged into the direct reduction furnace.

[0016] The inventive method may also include the following optional features, considered separately or in accordance with all technically possible combinations: - Biochar is produced by pyrolysis of biomass, - The solid composition is briquettes and / or pellets, - the reducing gas contains more than 50% hydrogen by volume, - The reducing gas contains more than 99% hydrogen by volume, - Hydrogen, the reducing gas, is produced at least in part by electrolysis, - Electrolysis is powered by renewable energy, - an upper reduction gas is captured at the outlet of the direct reduction furnace and subjected to at least one separate stage to be split between a CO2-rich gas and a H2-rich gas, this H2-rich gas being used at least partially as a reduction gas, - The CO2-rich gas is subjected to a methanation step.

[0017] Other features and advantages of the invention will become apparent from the following description, which is intended to be illustrative and not restrictive. Reference is also made to the accompanying drawings, in which: - Figure 1 shows a view of a direct regeneration facility that allows a method according to the invention to be carried out. Elements of the images are illustrative and may not be to scale.

[0018] Figure 1 shows a view of a direct reduction plant which enables a method according to the invention to be carried out. In said method, a direct reduction furnace (or shaft) 1 is charged at the top with a composition 10 made of a mixture of oxidized iron and a biochar. This composition can have any suitable form which allows loading into the furnace and is preferably in the form of briquettes and / or pellets. In a preferred embodiment, the composition 10 comprises between 0.01 and 10% by weight of biochar. Biochar is a biochar produced by pyrolysis of biomass in the absence of oxygen.

[0019] Biomass is a renewable organic material of plant or animal origin. Biomass energy sources include wood and wood processing wastes—firewood, wood pellets and wood chips, lumber and furniture, sawdust and pulp mill waste and black liquor, agricultural products and waste materials—corn, soybeans, sugarcane, grass, woody plants and algae, and food and feed processing wastes, inorganic materials in municipal solid waste—paper, cotton and wool products and food, grass, and wood wastes, and animal and human wastes.

[0020] Composition 10 provides the iron oxides for reduction as well as the carbon source necessary for carburizing the metallized product. In a preferred embodiment, the carbon content of the direct reduced iron is between 0.5 and 3 wt.%, preferably between 1 and 2 wt.%, which allows for the achievement of a direct reduced iron that is easily transportable and retains a desirable combustibility for future use.

[0021] The composition 10 is reduced by a reducing gas 11 injected into the furnace and flowing countercurrently to the flow of the composition 10 in that furnace 1. Sponge iron 12 is removed from the bottom of the furnace 1 for further processing such as briquetting and before use in subsequent steelmaking steps. The reducing gas, after reducing the iron, is removed from the top of the furnace as top reduction gas 20 (TRG).

[0022] A cooling gas 13 can be trapped as it exits the cooling section of the furnace, subjected to a cleaning step in a cleaning device 30 such as a gas scrubber, compressed in a compressor 31 and then re-sent to the cooling section of the shaft 1.

[0023] In a preferred embodiment, the reducing gas 11 comprises at least 50%v hydrogen and more preferably more than 99%v H2. A H2 stream 40 can be provided for the production of the reducing gas 11 by a dedicated H2 production facility 9, such as a hydrolysis facility. The facility can be a water or steam hydrolysis facility. Preferably, the facility is operated using carbon dioxide neutral electricity, which includes electricity from renewable sources that is obtained as energy from renewable sources that are naturally regenerated within a time frame understandable to humans. These sources include, for example, sunlight, wind, rain, tides, waves and geothermal heat. In some embodiments, the use of electricity from nuclear sources can be utilized since this method of generating electricity does not produce carbon dioxide.

[0024] In another embodiment, the H2 stream 40 can be mixed with a portion of the upper reduction gas 20 to form the reducing gas 11. When used with natural gas, the upper reduction gas 20 typically comprises between 15 and 25%v of CO, between 12 and 20%v of CO2, between 35 and 55%v of H2, between 15 and 25%v of H2O, between 1 and 4% of N2, and has a temperature of between 250 and 500 degrees Celsius. When pure hydrogen is used for the reducing gas, the composition of that upper reduction gas will consist of between 40 and 80%v of H2, 20-50%v of H20, and some gaseous impurities from the shaft insulation system or present in the hydrogen stream 40. When the amount of H2 in the reducing gas is variable and composition 10 is loaded, the overhead gas 20 will have a composition intermediate between the two described above.

[0025] In one embodiment of the method according to the invention, the upper reduction gas 20, after a dust and moisture removal step in a cleaning device 5, such as a gas scrubber and a dehumidifier, is sent to a separation unit 6 and there it is divided into two streams 22, 23. The first stream 22 is a carbon dioxide-rich gas that can be trapped and used in various chemical processes. In a preferred embodiment, this carbon dioxide-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 means 7 is a heater.

[0026] The method according to the invention makes it possible to obtain a DRI product with sufficient carbon content without compromising the carbon footprint of the process.

[0027]

Claims

Claims 1. A method for producing direct reduction iron in which oxidized iron is reduced in a direct reduction furnace by a reducing gas, the oxidized iron is first mixed with biochar to form a solid composition, and the solid composition is charged into the direct reduction furnace, wherein the reducing gas comprises more than 99% by volume of hydrogen.

2. A method according to claim 1, wherein said biochar is produced by pyrolysis of biomass.

3. A method according to claim 1 or 2, wherein said solid composition is briquettes and / or pellets.

4. A method according to any one of claims 1 or 3, wherein the reducing gas hydrogen is produced at least in part by electrolysis.

5. A method according to claim 4, wherein said electrolysis is powered by renewable energy.

6. A method according to any preceding claim, wherein the upper reduction gas at the outlet of the direct reduction furnace is trapped and subjected to at least one separation step to separate it into a carbon dioxide-rich gas and an H2-rich gas, the H2-rich gas being at least partially utilized as the reduction gas.

7. The method according to claim 6, wherein said carbon dioxide-rich gas is subjected to a methanation step.

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