WO2013064870A1

WO2013064870A1 - Process for producing direct reduced iron (dri) with less co2 emissions to the atmosphere

Info

Publication number: WO2013064870A1
Application number: PCT/IB2011/054942
Authority: WO
Inventors: Pablo Enrique DUARTE-ESCAREÑO; Eugenio ZENDEJAS-MARTÍNEZ
Original assignee: Hyl Technologies, S.A. De C.V.
Priority date: 2011-11-04
Filing date: 2011-11-04
Publication date: 2013-05-10

Abstract

A process for producing direct reduced iron (DRI) with lower emissions of CO₂to the atmosphere, in a direct reduction system comprising a direct reduction reactor to which iron oxides are fed in the form of pellets, lumps or mixtures thereof, wherein said iron oxides are caused to react with a reducing gas mainly composed of hydrogen and carbon monoxide at high temperature, and wherein said reducing gas is derived from the reformation of a hydrocarbon-containing gas, and wherein a first portion of the reducing gas stream effluent from said reduction reactor, which contains H₂, CO, CO₂ and H₂O in various proportions, and which is cooled and cleaned in a cooler and which is combined with a gas containing hydrocarbons and said mixture passes through a catalytic reformer before being fed at high temperature to said reduction reactor. CO₂is removed from a portion of the gas stream effluent from said reduction reactor and the resultant gas stream is used as fuel in the reformer thus decreasing the amount of CO₂ emitted to the atmosphere. The gas stream containing less CO₂ but still containing CO to be used as fuel may further be reacted with water in a "shifter" reactor for converting said CO and H₂O to CO₂ and H₂. In this way more CO₂ may be removed from the fuel gas stream and therefore, the resultant fuel gas is mainly composed of hydrogen, further decreasing the amount of CO₂ emitted to the atmosphere.

Description

Title

Process for producing direct reduced iron (DRI) with less CO₂ emissions to the atmosphere.

Field of the invention

The present invention relates to processes and plants for the direct reduction of iron ores, and more particularly to a process for reducing solid particles containing iron oxides to metallic iron through the high temperature reaction of said oxides with a reducing gas mainly composed of hydrogen and carbon monoxide; wherein the carbon dioxide emissions, which are produced in said process (1) as a by-product of the reaction of iron oxides and (2) as a combustion product in the heat generation by the thermal equipment for producing said high-temperature reducing gas, are significantly decreased.

Background of the invention

DRI is a solid granular material produced by the reaction of particulate iron ores, mainly iron oxides in the form of lumps, pellets of concentrated ore, or mixtures thereof, with a reducing gas mainly composed of hydrogen and carbon monoxide, at a temperature in the range of 750°C to 1100°C. These systems commonly comprise vertical-shaft moving-bed reactors having a reduction zone in their upper part and a discharge zone in their lower part (which also may be used as a cooling zone for the DRI).

The reducing gas is commonly obtained by reformation of natural gas with steam and/or CO₂ in a catalytic reformer. The reducing gas can also be produced by reformation or partial combustion of other hydrocarbons, such as oil derivates and coal.

Typically, the high temperature reducing gas fed to the reactor is composed of hydrogen and carbon monoxide; which, after reacting with the particulate iron oxides, produce DRI with metallic and carbonized iron plus the by-products of water and carbon dioxide. Due to the restrictions of chemical equilibrium and to the kinetics of the reduction and carburization reactions shown below, among others, not all of the hydrogen and carbon monoxide reducing gases react with the iron oxides, and consequently, residual gas depleted in reducing potential is withdrawn as effluent from the upper part of the reduction zone, is cooled, and its remaining reduction potential is improved by the separation out of the oxidants, water and carbon dioxide, and is ultimately recycled back to the reduction zone as an improved and later reformation- enhanced high temperature reducing gas. Fe₂0₃ + 3H₂ -» 2Fe + 3H₂0

Fe₂0₃ + 3CO -» 2Fe + 3C0₂

3Fe + CH₄ -» Fe₃C + 2H₂

In a direct reduction plant of the type where the reducing gas is generated by reformation of hydrocarbons using the C0₂ and H₂0, present in the effluent gas from the reduction reactor, and where the resulting enhanced reducing gas is recycled back to the reactor after passing through the reformer; the main sources of C0₂ emissions are the flue gases exiting through the stack of the reformer (which are derived from combustion of natural gas or other suitable fuel to provide the heat necessary to carry out the reformation reactions and to increase the reducing gas to a temperature in the range between 750°C and 1 100°C). The reducing gas effluent from the reduction reactor comprises as major components H₂, CO, C0₂, H₂0, & CH₄ with minor amounts of N₂ and other inert gases.

The environmental regulations in many countries, concerning C0₂ emissions, are becoming stricter due to the current move to decrease the C0₂ greenhouse effect, and therefore, there are proposals addressed to selective removal of C0₂ to decrease the rate of its

accumulation in the atmosphere.

The present invention allows for selectively removing C0₂ from that portion of the gas stream effluent from the reduction reactor destined for use as fuel in the reformer. As a result, the amount of carbon expelled through the stack of the reformer is significantly decreased, because that gas stream portion contains mainly hydrogen. The hydrogen, after being burned, is transformed into water which is environmentally friendly. The separately removed carbon compounds are converted into C0₂, which can be utilized in other processes or can be sequestered and stored so as to avoid its emission to the atmosphere.

The objects of the present invention will generally be achieved by providing a method for direct reduction of iron ores comprising a reduction reactor, a reformer for the catalytic reformation of hydrocarbons, for example natural gas, with C0₂ and H₂0 present in the reducing gas stream recycled to said reactor, a cooler for cooling the reducing gas effluent from the reduction reactor, and a compressor for recycling a portion of the reducing gas effluent from said reactor.

Examples of processes of this kind for producing DRI are shown for example in U.S. patents Nos. 3,749,386 and 4,046,557.

After being cooled and washed, a portion of the effluent reducing gas is used as fuel in the burners of the reformer, and the C0₂ present therein as a product of the reduction reactions plus the CO2 produced by the combustion of CO and CH₄ flows to the atmosphere through the stack of the reformer. This portion of reducing gas which is withdrawn from the system as fuel advantageously also serves as the purge normally used to decrease the accumulation of inert gases in the reducing gas circuit of the process.

In a preferred embodiment of the invention, a significant decrease of the CO₂ emissions to the atmosphere by means of the conversion of CO to CO₂ , according to the following reaction shown below, which is then removed from the gas stream, giving as a result a stream of ¾ which is utilized as fuel in the burners of the reformer.

CO + H₂0 -» C0₂ + H₂

According to this preferred embodiment of the invention, the reducing gas stream removed from the reducing gas circuit of the process, and which will be utilized as fuel, is first passed through a CO conversion reactor ("shifter") and thereafter through a CO₂ removal unit, thus forming a fuel gas stream having hydrogen as its main component.

Objects of the invention

It is therefore an object of the present invention to provide a method and apparatus for producing DRI which adapts more easily with the environmental laws regulating the amount and type of industrial emissions to the atmosphere.

It is another object of the invention to provide a method and apparatus comprising a reduction reactor and a catalytic reformer of hydrocarbons with CO2 and H2O through which a portion of the reducing gas effluent from said reactor is recycled with a lesser amount of CO₂ emitted to the atmosphere.

It is a further object of the invention to provide a method and apparatus for the production of DRI wherein a portion of the fuel gas fed to said reformer is treated for removing CO₂ of the fuel gas utilized in the reformer.

It is still another object of the invention to provide a method and apparatus for the production of DRI wherein the CO₂ removed from the reduction system can be utilized in other processes with economic advantage.

It is also another object of the invention to provide a method and apparatus for the production of DRI wherein the CO₂ produced in the iron oxides reduction system can be removed and sequestered thus avoiding its emission to the atmosphere.

The objects of the present invention are achieved by providing a method for producing DRI in a direct reduction system comprising a reduction reactor to which iron ores in form of lumps or pellets or mixtures thereof, which react with a reducing gas mainly composed of hydrogen and carbon monoxide at high temperature, wherein said reducing gas is derived from the reformation of a hydrocarbon-containing gas, and wherein a first portion of the reducing gas effluent from said reduction reactor containing ¾, CO, CO₂, and ¾0 in varied proportions, is cleaned and dewatered in a gas cooler, and which is combined with hydrocarbons-containing gas, said gas mixture is recycled to a catalytic reformer before being fed at high temperature to said reduction reactor, wherein said method is characterized by reacting a second portion of said gas stream effluent from said reduction reactor with ¾0 for at least partially converting a portion of the CO and ¾0 into ¾ and CO2, remove CO2 from said second gas portion and utilizing the resulting gas stream as fuel in the reformer, whereby the amount of CO2 emitted to the atmosphere is decreased.

Brief description of the drawings

Figure 1 is a schematic process diagram showing one embodiment of the present invention where the carbon content of the gas, used as fuel in a catalytic reformer comprised in the direct reduction process, is lowered.

Figure 2 is a schematic process diagram similar to that shown in Figure 1 , wherein the chemical absorption unit for removing CO₂ is substituted by a physical adsorption unit for CO₂ removal.

Figure 3 is a schematic process diagram of another embodiment of the invention wherein the gas effluent from the reactor and containing CO₂ and CO is reacted with steam for converting CO into CO₂ and H₂O into !¾, and then removing said CO₂ from said gas, thus producing a fuel gas mainly composed of H₂ whereby the CO₂ emission to the atmosphere is reduced.

Figure 4 is the schematic process diagram of Figure 3, wherein the CO₂ chemical absorption unit is substituted by a CO₂ physical adsorption unit.

Detailed description of the invention

In this specification some preferred embodiments of the invention are described with reference to the attached figures, which will help to better understand the spirit and scope of the invention. It will be understood that the description of the preferred embodiments is merely illustrative and not limitative and that the invention is defined by the scope of the claims. With reference to Figure 1 , numeral 10 generally designates a direct reduction reactor having a reduction zone 12, through which iron oxide particles 15 flow by gravity, at a regulated rate by means known in the art, fed at the upper part of said reduction zone 12 in the form of lumps, pellets or mixtures thereof. A reducing gas 86, mainly composed of hydrogen and carbon monoxide, at a temperature in the range between about 900°C and about 1 100°C, is introduced to the reduction zone 12 where it contacts the iron oxides and reduces said iron oxides to metallic iron (which can include carburized iron), producing a product known as direct reduce iron or DRI 18.

Due to the chemical equilibrium of reduction reactions with hydrogen and carbon monoxide, as well as of the reactions among the different gases, the reducing gas stream 20 effluent from the reduction zone 12 still contains hydrogen and carbon monoxide together with water and carbon dioxide (which are byproducts of the reduction reactions). For increasing the efficiency of the direct reduction process, the depleted reducing potential of this first gas stream 20 from the reactor is regenerated by removing water and carbon dioxide and recycling the resulting improved reducing gas (now having a higher concentration of hydrogen and carbon monoxide).

The gas stream effluent 20 exits reactor 10 at a temperature in the range from about 400°C to about 450°C depending on the conditions of temperature and pressure of the reduction zone, as well as on the reducibility of the iron ores.

According to one embodiment of the invention, the gas stream effluent 20 from reactor 10 passes through a heat exchanger 22 wherein sensible heat of the gas effluent 20 is transferred to water fed through pipe 21. The water is heated by the gas and is transformed into steam which exits heat exchanger 22 through outlet 23 to be used in the regeneration process of the solvent utilized in the CO2 absorption plant 38, or in other industrial processes. Alternately, such sensible heat of gas 20 may be used for pre-heating the recycled gas before being fed to reformer 72 or for pre-heating other gases in the plant.

The reducing gas effluent 20 from reactor 10 exits the heat exchanger 22 through pipe 24 and is further cooled by direct contact with water 31 in the cooler 30 where water produced by the reduction reactions is condensed and removed from the reducing gas received from pipe 24 and such condensate exits from the cooler 30 together with the outflow of water 31 through pipe 68.

The resulting improved reducing gas 32 from cooler 30 is split, with most flowing as a first portion of that improved second gas stream 32 through pipe 33 to compressor 44 and then passes through pipe 58 and is fed to reformer 72, after being combined with a hydrocarbon- containing make-up gas stream 64, for example natural gas, whereby the hydrocarbons present in the natural gas are reformed to hydrogen and carbon monoxide mainly according to the reactions:

CH₄ + H₂0 -» CO + 3H₂

CO + H₂0 -» C0₂ + H₂

The thus enhanced recycle reducing gas is fed as the third gas stream 86 to reactor 10 at a temperature between 750°C and 1 100°C and contacts the iron oxide-containing particles 15 in the reduction zone 12 and thereafter is withdrawn from said reduction zone 12 as the first gas stream 20 effluent, thus closing the reducing gas recycle circuit.

Optionally, a gas stream 84 containing molecular oxygen may be added to the hot third gas stream 86 of enhanced reducing gas in pipe 82 before being fed to reactor 10 for further raising its temperature, if it is considered necessary, whereby the productivity of the reduction process increases. This may be particularly useful if a hydrocarbon, for example natural gas, is also added to said enhanced reducing gas 86 before entering the reduction reactor for increasing the amount of reducing agents in the reducing gas comprising the hot third gas stream 86.

According to the present invention, the amount of carbon dioxide in the flue gas 49 from the burners of reformer 72 being emitted to the atmosphere through the stack 50 can be significantly decreased by rather than conventionally passing all the recycle gas, including the C0₂ resulting from the reduction reaction through the reformer 72, to instead remove C0₂ from a portion 52 of the reducing gas normally recycled (and preferably also converting CO in that same portion 52 to C0₂ before such removal), thus creating a fuel gas 74 for the reformer 72 having a high content of hydrogen with little C0₂.

Accordingly, more specifically, a second portion 52 of said second gas stream 32, already cooled and with a low content of water, is fed by compressor 34 through pipe 36 to a chemical absorption unit 38 and exits said unit 38 as a gas 74 with a low content of C0₂ and consequently a relatively high content of H₂ via pipe 46 for use as a fuel in the reformer 72 at an appropriate rate regulated by valve 51.

If the amount of fuel necessary for operating the reformer 72 is greater than the amount of gas 74 available form pipe 46, the fuel for the reformer 72 can supplemented with a gas stream 78, for example of natural gas or of other suitable fuel. There is also the possibility of transferring part of the gas with high hydrogen content through pipe 66 regulated by means of valve 68 for adjusting the composition of the reducing gas which is recycled to reactor 10, by injecting this gas before or after the reformer 72.

A suitable solvent is fed through pipe 41 to the CO2 chemical absorption tower 39 containing ethanolamines, and after passing through tower 39 is flowed back via pipe 43 to be regenerated in the desorption tower 40 and then recirculated once again back through pipe 41 into the absorption tower 39, thus removing CO2 42 in a manner known in the art. The CO2, thus removed, can be sold, used in other processes or sequestered, thus avoiding its emission to the atmosphere. To this end, a chemical absorption process using a solvent containing potassium carbonate can also be used.

The DRI 18 produced in reactor 10 may be discharged at a high temperature above about 500°C, to be charged to an electric arc furnace for steelmaking or to be hot briquetted, or may be cooled to a temperature lower than about 100°C to avoid its re-oxidation and discharged in contact to the atmosphere. The DRI may be cooled in the lower discharge portion 13 of the reactor 10 by contacting it with a cooling gas stream 84 circulating counter-currently with said DRI which gas is withdrawn at the upper part of the cooling zone 13 through pipe 86 and is then cooled and cleaned in cooler 88 by direct contact with water 90 and is then recycled by means of compressor 92. The cooling gas circuit has been shown in dotted lines to show its optional character.

With reference to Figure 2, wherein the same numerals designate similar elements shown in Figure 1, the CO2 removal system 138 is of the physical adsorption type, and may be of the kind where the adsorption and desorption are effected by Pressure Swing Adsorption or Vacuum Pressure Swing Adsorption (PSA or VPSA) known in the art, or also may be of the also known kind utilizing molecular membranes. A fourth gas stream 74 having a high content of hydrogen and a fifth gas stream 102 having a higher content of CO2 and CO are formed by said physical adsorption system 138. The fifth gas stream 102 is recycled to the reducing gas circuit such to pipe 33, and the fourth gas stream 74 is utilized as fuel in the reformer 72 to decrease the amount of CO2 49 emitted to the atmosphere, as has been described above with reference to Figure 1.

A further embodiment of the invention is shown in Figure 3, wherein the amount of CO2 49 produced in the reformer 72 is decreased furthermore by passing the second portion 52 of the second gas stream 32 which portion is to be used as fuel, and adding a suitable amount of steam 104 through a conversion reactor 106, commonly known as "shifter" for converting CO and H2O to H2 and CO2 in accordance with the following water shift reaction:

CO + H₂0 -» C0₂ + H₂ In this way, after removing CO2 in the absorption unit 58, the gas 74 has a minimum amount of carbon compounds and is almost wholly composed of hydrogen.

The process diagram of the embodiment shown in Figure 4 is the same as that shown in Figure 3, comprising the "shifter" reactor with the difference that the CO2 removal system is of the type of physical adsorption instead of the type of chemical absorption. Like numerals designate like elements shown in both Figure 3 and Figure 4.

It will be evident to those skilled in the art that numerous modifications can be made to the embodiments of the invention herein described, as it may best be adapted to the circumstances of a particular application, without departing from the spirit and scope of the invention, which is defined by the attached claims.

Claims

What is claimed is:

1. A process for producing direct reduced iron by the direct reduction of iron ores to metallic iron, of the type wherein a CO and ¾ containing reducing gas at high temperature is caused to react with iron oxides in the form of pellets, lumps or mixtures thereof, in a reduction reactor, and wherein said reducing gas is produced by catalytic reformation of a gas containing hydrocarbons with oxidants, including CO2 and H_2; present in the a gas stream effluent from said reduction reactor; characterized by:

withdrawing a first gas stream of depleted reducing gas from said reduction reactor; cooling said first gas stream and removing water of said first gas stream to form a second gas stream of improved reducing gas;

feeding a first portion of said second gas stream to a catalytic reformer for converting the hydrocarbons, fed to the reformer, to yield makeup hydrogen and carbon monoxide to form a third gas stream of enhanced reducing gas;

feeding said third gas stream to said reduction reactor to produce said direct reduced iron;

feeding a second portion of said second gas stream to a carbon dioxide removal unit; forming thereby a fourth gas stream received from said unit having a higher

concentration of hydrogen and a lower concentration of carbon dioxide; and

utilizing said fourth gas stream as fuel in said reformer.

2. A process for producing direct reduced iron according to claim 1 , wherein said carbon dioxide removal unit functions by chemical absorption.

3. A process for producing direct reduced iron according to claim 1 , wherein said carbon dioxide removal unit functions by physical adsorption.

4. A process for producing direct reduced iron according to claim 2, wherein a solvent containing ethanolamines is used in said carbon dioxide removal unit.

5. A process for producing direct reduced iron according to claim 2, wherein a solvent containing potassium carbonate is used in said carbon dioxide removal unit.

6. A process for producing direct reduced iron according to claim 3, wherein a pressure swing CO2 removal system (PSA or VPSA) is used for carrying out said physical adsorption.

7. A process for producing direct reduced iron according to claim 3, wherein a molecular membrane system is used for carrying out said physical adsorption.

8. A process for producing direct reduced iron according to claim 1 , wherein at least a portion of the carbon monoxide present in the second portion of said second gas stream is converted by a water shift reaction to hydrogen and carbon dioxide and wherein carbon dioxide is afterwards removed from the shifted second portion to produce said fourth gas stream which is used as fuel in the reformer.

9. A process for producing direct reduced iron according to claim 8, wherein said carbon dioxide removal unit functions by chemical absorption.

10. A process for producing direct reduced iron according to claim 8, wherein said carbon dioxide removal unit functions by physical adsorption.

1 1. A process for producing direct reduced iron according to claim 9, wherein a solvent containing ethanolamines is used in said carbon dioxide removal unit.

12. A process for producing direct reduced iron according to claim 9, wherein a solvent containing potassium carbonate is used in said carbon dioxide removal unit.

13. A process for producing direct reduced iron according to claim 10, wherein a pressure swing CO2 removal system (PSA or VPSA) is used for carrying out said physical adsorption.

14. A process for producing direct reduced iron according to claim 10, wherein a molecular membrane system is used for carrying out said physical adsorption.

15. A process for producing direct reduced iron according to claim 6, wherein a fifth gas stream having a high content of CO2 and CO is formed by said pressure swing CO2 removal system which fifth gas stream is recycled to said reformer.

16. A process for producing direct reduced iron according to claim 15, wherein at least a portion of the carbon monoxide present in the second portion of said second gas stream is converted by a water shift reaction to hydrogen and carbon dioxide and wherein carbon dioxide is afterwards removed from the shifted second portion to produce said fourth gas stream which is used as fuel in the reformer.