DE112021007754T5 - Process for the production of directly reduced iron - Google Patents

Abstract

A process for producing direct reduced iron comprising reducing iron ore in a direct reduction furnace by a reducing gas, the reducing gas exiting the furnace through the top as upper reducing gas. The upper reducing gas is collected and at least partially subjected to a CO2 recovery step in which it is split into two streams, a CO2-rich and a CO2-lean stream. The CO2-rich stream is subjected to a hydrocarbon production step to produce a hydrocarbon product.

Description

The invention relates to a process for producing directly reduced iron.

Steel can currently be produced in two main ways. Today, the most common production method is to produce pig iron in a blast furnace using a reducing agent, mainly coke, to reduce iron oxides. This process uses approximately 450 to 600 kg of coke per tonne of pig iron; this process releases significant amounts of CO2 both in the production of coke from coal in a coking plant and in the production of the pig iron. The pig iron produced is then decarburised, for example in a converter or a blast furnace (BOF), to produce steel, which is then refined to obtain the correct composition. This is called the BF-BOF route.

The second important route is the so-called "direct reduction methods". These include processes under the brands MIDREX, FINMET, ENERGIRON/HYL, COREX, FINEX, etc., in which sponge iron in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron) or HBI (hot briquetted iron) is produced by direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI and HBI is usually further processed in electric arc furnaces.

In each direct reduction shaft there are 3 zones of cold DRI discharge: reduction zone at the top, transition/intermediate zone in the middle, cooling zone at the bottom of the cone. In hot DRI discharge this lower part is mainly used for homogenization of the product before discharge and to control the total solids content.

The reduction of iron oxides takes place in the upper part of the furnace at temperatures of up to 950 °C. Iron oxide ores and pellets with an oxygen content of about 30 percent by weight are introduced into the upper part of a direct reduction shaft and sink by gravity into a reducing gas. This reducing gas enters the furnace from the bottom of the reduction zone and flows countercurrently to the charged oxidized iron. The oxygen contained in ores and pellets is removed during the gradual reduction of iron oxides in a countercurrent reaction between gases and oxides. The oxidant content of the gas increases as the gas enters the upper part of the furnace.

The reducing gas generally consists of hydrogen and carbon monoxide (synthesis gas) and is obtained by catalytic reforming of natural gas. In the so-called MIDREX method, for example, methane is first converted in a reformer according to the following reaction to produce the synthesis gas or reducing gas: CH4 + CO2 = 2CO + 2H2 and the iron oxide reacts with the reducing gas, for example according to the following reactions: 3Fe2O3 + CO/H2 → 2Fe3O4 + CO2/H2O Fe3O4 + CO/H2 → 3FeO + CO2/H2O FeO + CO/H2 → Fe + CO2/H2O

At the end of the reduction zone the ore is metallized.

Below the reduction section there is a transition section long enough to separate the reduction section from the cooling section and allow independent control of both sections. In this section the carburization of the metallized product takes place. Carburization is the process of increasing the carbon content of the metallized product in the reduction furnace through the following reactions: 3Fe + CH4 → Fe3C + 2H2 3Fe + 2CO → Fe3C + CO2 3Fe + CO + H2 → Fe3C + H2O

Injecting natural gas into the transition zone utilizes the sensible heat of the metallized product in the transition zone to promote hydrocarbon cracking and carbon deposition. Due to the relatively low concentration of oxidants, natural gas from the transition zone is more likely to be cracked to H2 and carbon than reformed to H2 and CO. Hydrocarbon cracking provides carbon for DRI carburization while simultaneously adding a reductant (H2) to the gas, increasing the gas's reduction potential.

The gas feed also takes place in the cooling zone and usually consists of returned cooling gas and added natural gas. By adding natural gas (NG) to the cooling gas, the operator can maintain the cycle of the returned cooling gas with a high methane content. Otherwise, the predominant component of the cooling gas would be nitrogen. The heat capacity of natural gas is much larger than that of N2: the cooling gas return is 500-600 Nm3/t with NG and 800 Nm3/t without NG. Although not too much nitrogen will build up in the cooling zone, deposit a lot of carbon, but the upward flow of cooling gas to higher levels of the furnace will provide more hydrocarbon for cracking.

As can be seen from the above reactions, although direct reduction has a better carbon footprint than the BF-BOF pathway, the direct reduction process still produces CO2.

A method is needed that enables further reduction of carbon emissions.

There is also a need for a method that allows increasing the carbon content in the DRI product without the need for an external carbon source. The carbon content in the DRI product is a key parameter as it plays an important role in the subsequent steps but also helps to improve the transportability of the DRI product.

This problem is solved by a process according to the invention, in which iron ore is reduced in a direct reduction furnace by a reducing gas, the reducing gas leaves the furnace through the upper part as upper reducing gas, this upper reducing gas is collected and at least partially subjected to a CO2 recovery step in which it is split into two streams, a CO2-rich stream and a CO2-lean stream, the CO2-rich stream being subjected to a hydrocarbon production step to produce a hydrocarbon product.

• - das Kohlenwasserstoffprodukt wird zumindest teilweise in den Direktreduktionsofen eingeleitet,
• - der CO2-arme Strom wird als Reduktionsgas wieder in den Ofen eingeleitet,
• - der CO2-reiche Strom enthält zwischen 80 und 100 Volumenprozent Kohlendioxid,
• - 1 bis 20 Volumenprozent des oberen Reduktionsgases werden dem Schritt der Kohlenwasserstoffproduktion unterzogen,
• - ein Wasserstoffstrom wird der Kohlenwasserstoffproduktion zugeführt, um mit dem CO2-reichen Strom zu reagieren,
• - das erzeugte Kohlenwasserstoffprodukt ist ein Gas, das mit dem Reduktionsgas gemischt wird, bevor es in den Ofen eingeleitet wird,
• - der erzeugte Kohlenwasserstoff ist eine Flüssigkeit,
• - der erzeugte Kohlenwasserstoff wird getrennt vom Reduktionsgas in die Übergangszone des Ofens eingeleitet,
• - der erzeugte Kohlenwasserstoff wird mit dem Kühlgas in die Kühlzone des Ofens eingeleitet,
• - die Kohlenwasserstoffkette umfasst 1 bis 5 Kohlenstoffe,
• - der Schritt der Kohlenwasserstoffproduktion ist ein Methanisierungsschritt,
• - der Methanisierungsschritt ist eine kalte Plasmareaktion,
• - das Reduktionsgas wird vor seiner Einspeisung in den Direktreduktionsofen in einem Reduktionsgasaufbereitungsschritt erhitzt, wobei der Reduktionsgasaufbereitungsschritt ein Aufbereitungsabgas abgibt, das zumindest teilweise dem Kohlenwasserstoffproduktionsschritt zugeführt wird.

The process according to the invention may also include the following optional features, which may be considered individually or in all possible technical combinations:

• - the hydrocarbon product is at least partially introduced into the direct reduction furnace,
• - the low-CO2 electricity is fed back into the furnace as reducing gas,
• - the CO2-rich stream contains between 80 and 100 percent carbon dioxide by volume,
• - 1 to 20 volume percent of the upper reducing gas is subjected to the hydrocarbon production step,
• - a hydrogen stream is fed into the hydrocarbon production to react with the CO2-rich stream,
• - the hydrocarbon product produced is a gas which is mixed with the reducing gas before being introduced into the furnace,
• - the hydrocarbon produced is a liquid,
• - the hydrocarbon produced is introduced into the transition zone of the furnace separately from the reducing gas,
• - the hydrocarbon produced is introduced into the cooling zone of the furnace with the cooling gas,
• - the hydrocarbon chain comprises 1 to 5 carbons,
• - the hydrocarbon production step is a methanation step,
• - the methanation step is a cold plasma reaction,
• - the reducing gas is heated in a reducing gas treatment step before being fed into the direct reduction furnace, the reducing gas treatment step releasing a treatment exhaust gas which is at least partially fed to the hydrocarbon production step.

The invention also relates to a direct reduction plant for carrying out a process according to the invention, which comprises a hydrocarbon production unit.

• - den Aufbau einer Direktreduktionsanlage zur Durchführung eines erfindungsgemäßen Verfahrens zeigt

Further characteristics and advantages of the invention will become clear from the following non-limiting description of the invention with reference to the accompanying figures, in which:

• - shows the structure of a direct reduction plant for carrying out a process according to the invention

Elements shown in the images are for illustrative purposes and may not be drawn to scale.

shows the structure of a direct reduction plant for carrying out a process according to the invention. The direct reduction furnace (or shaft) 1 is charged at its upper end with oxidized iron 10 in the form of ore or pellets. The iron 10 is reduced in the furnace 1 by a reducing gas 11 which is introduced into the furnace and flows countercurrently to the oxidized iron. The reduced iron 12 leaves the bottom of the furnace 1 for further processing, e.g. for briquetting, before being used in the subsequent steps of steel production. After the iron reduction, the reducing gas exits at the upper end of the furnace as upper reducing gas 20 (TRG).

The upper reducing gas 20 usually consists of 15 to 25%v CO, 12 to 20%v CO2, 35 to 55 percent H2, 15 to 25%v H2O, 1 to 4 percent N2. It has a temperature of 250 to 500 °C.

A cooling gas 13 is withdrawn from the cooling zone of the furnace, subjected to a cleaning step in a cleaning device 30, e.g. a scrubber, compressed in a compressor 31 and then returned to the cooling zone of the shaft 1.

According to the invention, the top reducing gas 20, preferably after dedusting and demisting in a cleaning device 5, such as a scrubber and a demister, is fed to a CO2 recovery device 8 in which the CO2 from the top reducing gas is concentrated and divided into a CO2-poor stream 21 and a CO2-rich stream 22. The first stream 21, which is poor in CO2, is fed to a processing device 7 where it is mixed with other gases, optionally reformed and heated to produce the reducing gas 11. In a preferred embodiment, the processing device 7 is a reformer. The processing device 7 emits a processing exhaust gas 27, also called exhaust gas.

The CO2 recovery device can be an absorption device, an adsorption device, a cryogenic distillation device, membranes, or a combination of these different devices.

The second stream 22, which is rich in CO2 and preferably represents 1 to 20%v of the upper reducing gas 20, is sent to a hydrocarbon production device 6 to undergo a hydrocarbon production step. This second stream may contain between 80 and 100%v of CO2. In the hydrocarbon production device 6, the CO2 is first converted into carbon monoxide CO. This may be done, for example, by a hydrogenation step if hydrogen is present in sufficient quantity to produce CO according to the following reaction: CO2 + H2 → CO + H2O

This reaction is called the reverse water gas shift reaction (RGWS). This reaction is carried out in the presence of a catalyst such as ZnAl2O4 or Fe2O3/Cr2O3.

It can also be achieved by a thermochemical conversion such as the Boudouard reaction or methane reforming, by an electrochemical conversion or by a plasma technology.

The CO produced in this way is then converted into hydrocarbons by Fischer-Tropsch reactions: (2n+1)H2 + nCO → C _n H _2n+2 +nH2O

Where n is an integer greater than or equal to 1 and preferably ranges from 1 to 5.

The skilled person knows how to select the right catalyst and process conditions to carry out the desired Fischer-Tropsch reaction and produce the desired hydrocarbon.

In a preferred embodiment, CO2 and H2 contained in the CO2-rich stream 22 react to form methane CH4 according to the following reaction: CO2 + 4H2 → CH4 + 2H2O

In this embodiment, the hydrocarbon production device is a methanation device 6, such as catalytic reactors, bioreactors, cold plasma/DBD reactors or warm plasma reactors.

Should the H2 content in the upper reducing gas and thus in the second stream 22 be insufficient for the hydrocarbon production reaction, an additional H2 stream 40 can be supplied to the hydrocarbon production unit 6. This H2 stream can be provided by a special H2 production plant 9, such as an electrolysis plant. It can be a water or steam electrolysis plant. It is preferably operated with _CO2 neutral electricity, which in particular comprises electricity from renewable sources, i.e. energy obtained from renewable resources that renew themselves naturally within a human time span, including sources such as sunlight, wind, rain, tides, waves and geothermal energy. In some cases, electricity from nuclear sources can be used since its production does not emit _CO2 .

This H2 stream 40 can also be added to the reducing gas 11.

The exhaust gas 27 can also be fed to the hydrocarbon production unit 6.

In a preferred embodiment, the hydrocarbon product 23 leaving the hydrocarbon production device 6 is reintroduced into the furnace 1.

In a first embodiment, represented by stream 24, this hydrocarbon product 23 is a gas which is mixed with the reducing gas in the processing device.

In a second embodiment, represented by stream 25, it is either introduced into the furnace together with the reducing gas or introduced independently into the transition zone of the furnace. In a third embodiment, represented by stream 26, it is either introduced into the furnace together with the cooling gas 13 or introduced independently into the cooling zone of the furnace. The hydrocarbon product 23 can be in gaseous and/or liquid form. All of these embodiments can be combined with one another.

In all embodiments, the hydrocarbon product serves as a carbon supplier for the DRI product. In a preferred embodiment, the carbon content of the direct reduced iron is set at 0.5 to 3 wt.%, preferably 1 to 2 wt.%, whereby a direct reduced iron can be obtained that is easy to handle and retains a good combustion potential for its future use. The amount of gas supplied to the hydrocarbon production device can be controlled according to the amount of carbon required in the DRI product.

The process according to the invention makes it possible to improve the carbon footprint of the direct reduction process by capturing and utilizing the emitted CO2. This also avoids the need for an external source to increase the carbon content in the DRI product.

Claims (15)

A process for producing directly reduced iron comprising reducing iron ore in a direct reduction furnace by a reducing gas, the reducing gas exiting the furnace through the upper part as upper reducing gas, the upper reducing gas being collected and at least partially subjected to a CO2 recovery step during which it is split into two streams, a CO2-rich stream and a CO2-lean stream, the CO2-rich stream being subjected to a hydrocarbon production step to produce a hydrocarbon product. Procedure according to Claim 1 , in which the hydrocarbon product is then at least partially introduced into the direct reduction furnace. Procedure according to Claim 1 , in which the low-CO2 stream is fed back into the furnace as reducing gas. Procedure according to Claim 1 or 2 , with the CO2-rich stream containing between 80 and 100 percent carbon dioxide by volume. A process according to any preceding claim, wherein 1 to 20 volume percent of the upper reducing gas is subjected to the hydrocarbon production step. A process according to any preceding claim, wherein a hydrogen stream is fed to the hydrocarbon production step to react with the CO2-rich stream. A process according to any preceding claim, wherein the hydrocarbon product produced is a gas which is mixed with the reducing gas prior to its introduction into the furnace. A process according to any preceding claim, wherein the hydrocarbon produced is a liquid. A process according to any preceding claim, wherein the hydrocarbon produced is introduced into the transition zone of the furnace separately from the reducing gas. A process according to any preceding claim, wherein the hydrocarbon produced is introduced into the cooling zone of the furnace. A process according to any one of the preceding claims, wherein the hydrocarbon chain contains 1 to 5 carbons. A process according to any preceding claim, wherein the hydrocarbon production step is a methanation step. Procedure according to Claim 11 , where the methanation step is a cold plasma step. A process according to any preceding claim, wherein the reducing gas is heated in a reducing gas conditioning step prior to being fed to the direct reduction furnace, the reducing gas conditioning step emitting a conditioning exhaust gas which is at least partially fed to the hydrocarbon production step. Direct reduction plant for carrying out a process according to one of the preceding Claims, wherein the direct reduction plant comprises a hydrocarbon production unit.

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