Method for producing cold direct reduced iron

Abstract

Process for producing direct reduced iron (DRI), the process comprising the steps of: controlling the flow rate of unheated hydrogen entering a cooling section (5) such that Tpilot; tdrioutmax, wherein Tdrioutmax is a set maximum permissible temperature of the DRI leaving the cooling section (5); measuring the temperature Tcooltop of the cooling gas leaving the cooling section and increasing the flow rate FRcoolgas of the cooling gas introduced into the cooling section (5) until Tcooltop = Tcooltop min; adjusting the flow rate of the individually heated reducing gas introduced into the reduction section (4) and measuring the degree of metallization of the produced DRI, and measuring the furnace top gas temperature Ttopgas, and determining a minimum furnace top gas temperature Ttopgasmin lower than said minimum furnace top gas temperature Ttopgasmin, the degree of metallization lower than a minimum allowable value; and measuring a top gas temperature Ttopgas, and controlling at least one of a flow rate of the heated reducing gas and a temperature Tredgas to which the heated reducing gas is heated such that Ttopgasmin < = Ttopgass < = Topgasmax, where Topgasmax is a set maximum allowable temperature of the top gas.

Description

Technical Field

[0001] The present invention relates to a method for producing direct reduced iron (DRI), the method comprising the following steps: a) introducing iron ore into an ore inlet of a direct reduction shaft furnace; b) heating a reducing gas consisting essentially of hydrogen to a reducing gas temperature Tredgas, and introducing the heated reducing gas into a reduction section of the direct reduction shaft furnace to form a first component portion of the reducing gas for reducing the iron ore to hot DRI at a reduction temperature Tred, wherein Tredgas>Tred; c) introducing a cooling gas consisting essentially of hydrogen and having a temperature Tcoolgas into a cooling section of the direct reduction shaft furnace, the cooling section being located below the reduction section as seen in the flow direction of the DRI. The invention relates to a process for cooling a direct reduction shaft furnace and a direct reduction furnace comprising: a) cooling gas is introduced at an end of the cooling section opposite to an end of the cooling section adjacent to the reduction section, thereby cooling hot DRI having a temperature Tdritop when entering the cooling section to cold DRI, and heating the cooling gas to a temperature Tcooltop (by heat exchange with countercurrent DRI), wherein the cold DRI leaving the cooling section has a temperature Tdriout; d) allowing the hot cooling gas to enter the reduction section of the direct reduction shaft furnace to mix with the heated reducing gas and form a second component portion of the reducing gas; and e) discharging waste reducing gas from an upper portion of the reduction section of the direct reduction shaft furnace as top gas, wherein the top gas has a temperature Ttopgas. Background Art

[0002] With regard to the reduction of iron oxides to iron by means of hydrogen, the prior art (e.g. in CN111926135A) has proposed the use of hydrogen both as a reducing gas heated and introduced into the reduction section of the reduction shaft furnace and as a cooling gas introduced into a cooling section arranged downstream of the reduction section. The cooling gas is allowed to flow in a direction opposite to the flow direction of the direct reduced iron (DRI) through the cooling section and is allowed to flow upward into the reduction section, where it mixes with the externally heated reducing gas and assists the reduction process in the reduction section. Since the cooling gas has a significantly lower temperature than the DRI entering the cooling section from the reduction section, it will undergo heat exchange with the DRI. Thereby, the process becomes more energy efficient.

[0003] However, CN111926135A does not mention how to optimize the flow rates of cooling gas and heated reducing gas, respectively, to achieve an even more energy efficient process while still obtaining a product meeting certain quality standards.

[0004] Accordingly, the object of the present invention is to present a method with improved energy efficiency compared to the prior art as defined above and in the preamble of claim 1 of the present patent. In particular, the present invention aims to reduce the energy consumption associated with heating the reducing gas to be directly introduced into the reduction section (and not passing through the cooling section) by an external heater. Summary of the Invention

[0005] The object of the present invention is achieved by a method for producing direct reduced iron (DRI), which method comprises the following steps:

[0006] a) introducing iron ore into the ore inlet of a direct reduction shaft furnace;

[0007] b) heating a reducing gas consisting essentially of hydrogen to a reducing gas temperature Tredgas, and introducing the heated reducing gas into the reduction section of the direct reduction shaft furnace to form a first component part of the reducing gas that reduces the iron ore to hot DRI at a reduction temperature Tred, where Tredgas > Tred;

[0008] c) introducing a cooling gas consisting essentially of hydrogen and having a temperature Tcoolgas into the cooling section of the direct reduction shaft furnace, as seen in the flow direction of the DRI, said cooling section being located downstream of the reduction section, wherein the cooling gas is introduced at the end of the cooling section opposite to the end adjacent to the reduction section, so as to cool the hot DRI having a temperature Tdritop when entering the cooling section to cold DRI and heat the cooling gas to a temperature Tcooltop, where the cold DRI leaving the cooling section has a temperature Tdriout;

[0009] d) allowing the hot cooling gas to enter the reduction section of the direct reduction shaft furnace to mix with the heated reducing gas and form a second component part of the reducing gas; and

[0010] e) discharging waste reducing gas as top gas from the upper part of the reduction section of the direct reduction shaft furnace, said top gas having a temperature Ttopgas; the method is characterized in that it further comprises the following step:

[0011] f) measuring Tdriout and controlling the flow rate of the cooling gas entering the cooling section such that Tdriout < Tdrioutmax, where Tdrioutmax is the set maximum allowable temperature of the DRI leaving the cooling section,

[0012] g) measuring the temperature Tcooltop of the heated cooling gas and increasing the flow rate FRcoolgas of the cooling gas introduced into the cooling section until Tcooltop=Tcooltopmin, wherein Tcooltopmin is a predetermined minimum permissible temperature of the heated cooling gas and Tcooltopmin≤Tdritop,

[0013] h) regulating the flow rate FRredgas of heated reducing gas forming the first component part and determining the degree of metallization of the produced DRI and while doing so measuring the top gas temperature Ttopgas and determining a minimum top gas temperature Ttopgasmin below which the degree of metallization is below a minimum permissible value,

[0014] i) measuring the top gas temperature Ttopgas, and controlling at least one of the flow rate FRredgas of the heated reducing gas and the temperature Tredgas to which the heated reducing gas is heated before being introduced into the reduction section so that Ttopgasmin≤Ttopgas≤Topgasmax, wherein Topgasmax is the set maximum allowable temperature of the top gas.

[0015] Step g) results in optimization of the heat exchange between the cooling gas and the DRI and helps to enable a relatively low flow rate of externally heated reducing gas introduced into the reduction section, resulting in less energy consumption by one or more heaters for heating the first component portion of the reducing gas.

[0016] Step h) may be achieved by adjusting the flow rate of the first component part, measuring the corresponding top gas temperature, and performing laboratory measurements of the degree of metallization of the DRI corresponding to different flow levels and top gas temperatures, and determining at which top gas temperature the degree of metallization becomes unacceptable from a product quality point of view.

[0017] Steps h) and i) prevent the total flow of reducing gas through the reduction shaft from being excessive, thus also helping to reduce the flow of the first component portion of reducing gas, thus making the energy consumption through the heater for heating said first component portion less. There is a relationship between the top gas temperature and the final metallization degree of the final product, i.e. DRI. By studying this relationship, it is possible to determine a minimum top gas temperature Ttopgasmin, below which the metallization degree is below the minimum permissible value. For top gas temperatures above Ttopgasmin, an acceptable metallization degree is thus achieved. Step i) serves to achieve a sufficient top gas temperature, and thus a sufficient metallization degree.

[0018] Tdritop can be determined by indirect temperature measurement. The peak temperature of Tcooltop measured at a low cooling gas flow rate will be an indication of Tdritop.

[0019] According to one embodiment, Ttopgasmax = Topgasmin + 50 °C.

[0020] According to one embodiment, Ttopgasmax = Topgasmin + 25 °C.

[0021] According to one embodiment, Ttopgasmax = Topgasmin.

[0022] According to one embodiment, Tcooltopmin = Tdritop - 50 °C.

[0023] According to one embodiment, Tcooltopmin = Tdritop - 25 °C.

[0024] According to one embodiment, Tcooltopmin = Tdritop - 10 °C.

[0025] According to one embodiment, Tcooltopmin = Tdritop.

[0026] According to one embodiment, the minimum allowable value of the degree of metallization is 90 wt%, preferably 94 wt%, even more preferably 96 wt%, or even more preferably 98 wt%.

[0027] According to one embodiment, 850 °C < Tredgas < 1200 °C. According to another embodiment, 950 °C < Tredgas < 1100 °C.

[0028] According to one embodiment, 900 °C < Tred < 1000 °C. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the present invention will now be described in detail with reference to the drawings, in which:

[0030] Figure 1 is a schematic view of the components of an arrangement for directly reducing iron ore to sponge iron, and

[0031] Figure 2 is a flow chart showing an embodiment of the method according to the present invention. DETAILED DESCRIPTION

[0032] Referring to Figure 1 , an arrangement for directly reducing iron ore to sponge iron is presented to which the method according to the present invention can be applied.

[0033] The arrangement comprises a direct reduction shaft furnace 1 having an inlet 2 for introducing iron ore into the shaft furnace 1. An outlet 3 for discharging sponge iron in the form of pellets of direct reduced iron (DRI) is provided at the bottom of the shaft furnace 1. The direct reduction shaft furnace 1 comprises a reduction section 4, where reduction of the iron ore takes place, and a cooling section 5, where the DRI is cooled before leaving the shaft furnace 1 through the outlet 3. The direct reduction shaft furnace 1 is a vertical shaft furnace, wherein the reduction section 4 is arranged at the top of the cooling section 5, and wherein the inlet 2 for introducing iron ore is arranged at the top of the reduction section 4, and the outlet for discharging DRI is arranged at the bottom end of the cooling section 5. In other words, the cooling section 5 is located downstream of the reduction section 4 as seen in the flow direction of the DRI. As a result of this design and as will be disclosed later, cooling gas is introduced at the end of the cooling section 5 opposite to the end of the cooling section 5 adjacent to the reduction section 4.

[0034] The arrangement further comprises a hydrogen source 6 comprising an electrolyser arranged to produce hydrogen from water. A first gas line 7 extends from the hydrogen source 6 to a heater arrangement 8, which in a preferred embodiment comprises a plurality of electric heaters. The first gas line 7 further extends from the heater arrangement 8 to a reducing gas inlet 9, via which the hydrogen heated by the heater arrangement 8 and conducted by the first gas line 7 is introduced into the reducing section 4 of the reduction shaft 1.

[0035] The second gas line 10 extends from the hydrogen source 6 to a cooling gas inlet 11 through which unheated hydrogen from the hydrogen source 6 is introduced into the cooling section 5 of the direct reduction shaft furnace 1. The cooling gas inlet 11 is provided at the bottom of the cooling section 5.

[0036] A top gas outlet 12 is provided at the top of the direct reduction shaft furnace 1, through which the reduction gas from the reduction shaft furnace 1 is discharged from the shaft furnace 1. A third gas line 13 extends from the top gas outlet 12 to an arrangement 14 for top gas cleaning, which includes removing water and dust from the top gas, so that the cleaned top gas usually consists of hydrogen. The third gas line 13 further extends from the gas cleaning arrangement 14 and is connected to the first gas line 7 at a position upstream of the heater arrangement 8. Thereby, unconsumed hydrogen from the direct reduction shaft furnace 1 can be recycled.

[0037] The arrangement for direct reduction of iron ore further comprises a first control valve 15 arranged in the first gas line 7 for the purpose of controlling the flow of hydrogen to be heated in the heater arrangement 8 and introduced into the reduction section 4 via said heater arrangement 8. A second control valve 16 arranged for the purpose of controlling the flow of hydrogen in the second gas line 10 from the hydrogen source to the cooling section 5 is also provided.

[0038] The arrangement for direct reduction further comprises a first temperature sensor 17 configured to sense the temperature of the DRI leaving the cooling section 5. The first temperature sensor 17 is arranged at the outlet 3 of the cooling section 5. A second temperature sensor 18 is also provided which is configured to sense the temperature Tcooltop of the cooling gas at the top end of the cooling section 5. A third temperature sensor 19 is provided for the purpose of measuring the temperature Ttopgas of the top gas and a fourth temperature sensor 20 is provided for the purpose of measuring the temperature Tredgas of the hydrogen heated by the heater arrangement.

[0039] The arrangement for direct reduction of iron ore further comprises a control unit 21 configured to control the operation of the heater arrangement 8 , the first control valve 15 and the second control valve 16 based on inputs from the first, second, third and fourth temperature sensors 17 - 20 .

[0040] The arrangement for direct reduction also includes such components as compressors (not shown) for generating suitable gas pressures in the corresponding gas pipelines, wherein the process pressure in the direct reduction shaft furnace can typically be in the range of 8 to 12 bars. A charging container 22 and a discharge container 23 are also provided, through which the iron ore is introduced into the reduction section 4 and the DRI is discharged from the cooling section 5 via the discharge container 23. When the charging container 22 and the discharge container 23 are opened toward the reduction section 4 and the cooling section 5, respectively, they can be pressurized with any suitable gas (e.g., nitrogen) to prevent the pressurized process gas (mainly hydrogen) in the reduction section 4 and the cooling section 5 from leaking to the atmosphere. According to one embodiment, the gas used to pressurize at least one of the charging container 22 and the discharge container 23, preferably both of them, mainly includes hydrogen. Thus, it is possible to prevent other gases other than hydrogen, such as nitrogen, from being introduced into the reduction section 4 and the cooling section 5.

[0041] The arrangement for direct reduction is configured to operate according to the following description of one embodiment of the method of the invention. Figure 2 The method comprises the following steps:

[0042] a) Iron ore is introduced into the iron ore inlet 2 of the direct reduction shaft furnace 1 .

[0043] b) Heating a reducing gas substantially composed of hydrogen to a reducing gas temperature Tredgas, and introducing the heated reducing gas into the reduction section 4 of the direct reduction shaft furnace 1 to form a first component part of the reducing gas that reduces iron ore to hot DRI at a reduction temperature Tred, where Tredgas > Tred. The reducing gas is initially taken from a hydrogen source 6, and when the reduction starts, part of it comes from the top gas that is cleaned in the cleaning arrangement 14 and is guided to the heater arrangement 8 via the third gas pipeline 7 and the first gas pipeline 7.

[0044] c) Introducing a cooling gas substantially composed of hydrogen and having a temperature Tcoolgas into the cooling section 5 of the direct reduction shaft furnace 1 via the second gas pipeline 10 and the inlet 11. Thus, due to the countercurrent flow of the DRI and the cooling gas through the cooling section 5, the hot DRI having a temperature Tdritop when entering the cooling section 5 is cooled to an outlet temperature Tdriout, and the cooling gas is heated to a temperature Tcooltop reached at the top of the cooling section 5 or at the bottom of the reduction section 4.

[0045] d) Allowing the hot cooling gas to enter the reduction section 4 of the direct reduction shaft furnace 1 to mix with the heated reducing gas conveyed via the first gas pipeline 7 and form a second component part of the reducing gas.

[0046] e) Discharging the top gas having a temperature Ttopgas from the upper part of the reduction section 4 of the direct reduction shaft furnace 1 through the top gas outlet 12.

[0047] f) Measuring Tdriout by means of the first temperature sensor 17, and controlling the flow rate FRcoolgas of the cooling gas entering the cooling section 4 by means of the control unit 21 and the second control valve 16 such that Tdriout < Tdrioutmax, where Tdrioutmax is the set maximum allowable temperature of the DRI leaving the cooling section 5.

[0048] g) Measuring the temperature Tcooltop of the heated cooling gas by means of the second temperature sensor 18 and increasing / regulating the flow rate of the cooling gas introduced into the cooling section until Tcooltop = Tcooltopmin, where Tcooltopmin is the predetermined minimum allowable temperature of the heated cooling gas and Tcooltopmin ≤ Tdritop. The control unit 21 thus controls the second control valve 16 based on the input from the second temperature sensor 18.

[0049] h) regulating the flow rate FRredgas of heated reducing gas forming the first component part and determining the degree of metallization of the produced DRI and while doing so measuring the top gas temperature Ttopgas by means of a third temperature sensor and determining a minimum top gas temperature Ttopgasmin below which the degree of metallization is below a minimum allowed value. The measurement of the degree of metallization is currently done in the laboratory, meaning that step h) can be considered an important step for determining a specific limit and once step h) has been used to determine this limit, minimization of the energy consumption by the heater arrangement is mainly accomplished by steps f), g) and i).

[0050] i) measuring the top gas temperature Ttopgas and controlling at least one of the flow rate FRredgas of the heated reducing gas and the temperature Tredgas to which the heated reducing gas is heated before being introduced into the reduction stage so that Ttopgasmin≤Ttopgas≤Topgasmax, wherein Topgasmax is a set maximum permissible temperature of the top gas. For this purpose, the control unit 21 controls the power of the first control valve 16 and the heater arrangement.

[0051] In the embodiments disclosed herein, Ttopgasmax = Topgasmin, which means that the top gas temperature is aimed to be as low as possible while still achieving satisfactory metallization.

[0052] In the embodiment disclosed herein, Tcooltopmin=Tdritop, which means that the heat of DRI is optimally utilized for the purpose of heating the cooling gas, thereby making it possible to reduce the energy consumption associated with heating the reducing gas to be directly introduced into the reduction section 4 by the heater arrangement 8.

[0053] In the disclosed embodiment, as a result of the reduction section 4 and the cooling section 5 being directly connected to each other, Tdritop =Tred.

[0054] In one embodiment, Tredgas is about 1050°C, and Tred is about 950°C.

[0055] In the disclosed embodiment, the minimum allowed level of metallization is 98 weight percent.

Claims

1. A method for producing direct reduced iron (DRI), the method comprising the following steps: a) Introducing iron ore into the iron ore inlet (2) of a direct reduction shaft furnace (1); b) Heating a reducing gas consisting essentially of hydrogen to a reducing gas temperature Tredgas, and introducing the heated reducing gas into the reduction section (4) of the direct reduction shaft furnace to form a first component part of the reducing gas that reduces the iron ore to hot DRI at a reduction temperature Tred, where Tredgas > Tred; c) Introducing a cooling gas consisting essentially of hydrogen and having a temperature Tcoolgas into the cooling section (5) of the direct reduction shaft furnace (1), which, as seen in the flow direction of the DRI, is located downstream of the reduction section (4), wherein the cooling gas is introduced at an end of the cooling section (5) opposite to the end adjacent to the reduction section (4), thereby cooling the hot DRI having a temperature Tdritop upon entering the cooling section (5) to cold DRI and heating the cooling gas to a temperature Tcooltop, where the cold DRI leaving the cooling section (5) has a temperature Tdriout; d) Allowing the hot cooling gas to enter the reduction section (4) of the direct reduction shaft furnace (1) to mix with the heated reducing gas and form a second component part of the reducing gas; and e) Discharging waste reducing gas as top gas from the upper part of the reduction section (4) of the direct reduction shaft furnace (1), the top gas having a temperature Ttopgas; the method is characterized in that it further comprises the following steps: f) Measuring Tdriout and controlling the flow rate FRcoolgas of the cooling gas entering the cooling section (5) such that Tdriout < Tdrioutmax, where Tdrioutmax is the set maximum allowable temperature of the DRI leaving the cooling section (5), g) Measuring the temperature Tcooltop of the heated cooling gas and increasing the flow rate FRcoolgas of the cooling gas introduced into the cooling section (5) until Tcooltop = Tcooltopmin, where Tcooltopmin is the predetermined minimum allowable temperature of the heated cooling gas and Tcooltopmin ≤ Tdritop, h) Adjusting the flow rate FRredgas of the heated reducing gas forming the first component part and determining the metallization degree of the produced DRI, and while doing so measuring the top gas temperature Ttopgas and determining the minimum top gas temperature Ttopgasmin, below which the metallization degree is lower than the minimum allowable value, i) measuring the top gas temperature Ttopgas, and controlling at least one of the flow rate of the heated reducing gas and the temperature Tredgas to which the heated reducing gas is heated before being introduced into the reduction section so that Ttopgasmin≤Ttopgas≤Topgasmax, wherein Topgasmax is the set maximum allowable temperature of the top gas.

2. The method according to claim 1, wherein Ttopgasmax=Topgasmin+50°C.

3. The method according to claim 1 or 2, wherein Ttopgasmax=Topgasmin+25°C.

4. The method according to claim 1 or 2, wherein Ttopgasmax=Topgasmin.

5. The method according to any one of claims 1 to 4, wherein Tcooltopmin = Tdritop - 50°C.

6. The method according to any one of claims 1 to 4, wherein Tcooltopmin = Tdritop - 25°C.

7. The method according to any one of claims 1 to 4, wherein Tcooltopmin = Tdritop - 10°C.

8. The method according to any one of claims 1 to 4, wherein Tcooltopmin=Tdritop.

9. The method according to any one of the preceding claims, wherein Tdritop =Tred.

10. The method according to any one of the preceding claims, wherein the minimum allowed value of the degree of metallization is 90 wt.-%, preferably 94 wt.-%, even more preferably 96 wt.-%, or even more preferably 98 wt.-%.

11. A method according to any one of the preceding claims, wherein 850°C <Tredgas<1200℃。 12. A method according to any one of the preceding claims, wherein 900°C <Tred<1000℃。

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