WO2025021365A1 - Method for producing direct reduced iron - Google Patents

Abstract

The invention relates to a direct iron-ore reduction process comprising the steps of: producing a first gas stream of hydrogen and a second gas stream of nitrogen from liquid ammonia or an aqueous solution containing ammonia, in an ammonia electrolyser unit comprising at least one electrolysis cell; evacuating the first gas stream of hydrogen from the ammonia electrolyser unit; evacuating the second gas stream of nitrogen from the ammonia electrolyser unit; supplying iron ore to a direct-reduction reactor; supplying a hydrogen-containing reducing gas to the direct-reduction reactor, said hydrogen-containing reducing gas comprising at least part of the first gas stream of hydrogen; subjecting the iron ore to direct reduction with the hydrogen from the hydrogen-containing reducing gas in the direct-reduction reactor so as to obtain direct reduced iron, thereby generating a top gas; and evacuating the direct reduced iron from the direct-reduction reactor.

Description

METHOD FOR PRODUCING DIRECT REDUCED IRON FIELD OF THE INVENTION The invention relates to a method for producing direct reduced iron in a direct-reduction reactor for an iron- and steelmaking plant. BACKGROUND TO THE INVENTION Direct reduced iron (DRI) is produced from the direct reduction of iron ore conglomerates (mainly hematite, Fe2O3) in the form of lumps, pellets, or fines into iron by a reducing gas. Due to its structure with a very high specific surface area, direct-reduced iron is often also referred to as sponge iron. Direct reduction refers to a solid-state process which reduce iron oxides to metallic iron at temperatures below the melting point of iron. There are several processes for producing DRI known to the person skilled in the art. Known examples of direct-reduction processes include the MIDREX process, Tenova’s HYL process, Tenova’s HYL-I, the HYL-II and the HYL-III process, Posco’s HyREX process, and the HYBRIT process. A known process relates to a direct reduction plant (DRP) or DRI-reactor comprising a direct reduction shaft furnace having a reduction zone and a lower discharge zone from which direct reduced iron (DRI) in solid form is discharged at a regulated rate by means of a suitable discharge mechanism. Iron oxide conglomerates in the form of agglomerates, pellets, lumps or mixtures thereof are fed to the reduction furnace and descend by gravity through the reduction zone were DRI is formed by reaction of said iron oxides with a reducing gas stream at high temperature that is mainly composed of hydrogen and contains also carbon monoxide, carbon dioxide, methane, and nitrogen in those embodiments wherein a hydrocarbon such as natural gas or a syngas derived from coal is used as the source of the reducing gas. The reduction of iron oxides is carried out through the following net reactions:

The DRI in solid form is further processed directly on exit from the discharge zone, and optionally also after being compacted into briquettes, into a melt shop typically comprising one or more electric-arc furnaces (EAF) or submerged-arc furnaces (SAF; in the art also known as a reducing electrical furnace or REF). In most of these known direct-reduction processes, natural gas is reformed in a catalyst bed with steam and/or gaseous reduction products evacuated from the iron-reduction reactor to produce a reducing gas which is supplied to the iron-reduction reactor where it reacts with the iron oxides in the iron ore to generate reduced metallic iron. Partial oxidation processes which gasify liquid hydrocarbons, heavy residuals or coal have also been proposed to produce the reducing gas. In both cases, a reducing gas containing CO and H2 is obtained. Direct-reduction processes have thus far been of particular interest in regions which have access to suitable iron ores and inexpensive natural gas, non-coking coals and/or renewable energy sources, such as hydroelectric power. It is expected that non-coal based direct- reduction processes will gain in importance as the drive to reduce CO2 emissions in the iron- and steel-industry gets further momentum. However, there remains a need to further reduce the CO2 emissions of DRI production or, more generally, of DRI production and subsequent melting in EAF or SAF steel production. Patent document WO2023/036474-A1 discloses a process for the continuous direct reduction of iron ore in a direct-reduction unit comprising a direct-ore-reduction zone and an iron-collection zone, the process comprising the steps of: (a) supplying the iron ore and reducing gas containing one or more reducing agents to the direct-ore-reduction zone; (b) subjecting the iron ore to reduction with the one or more reducing agents inside the direct-ore- reduction zone so as to obtain direct-reduced iron; (c) transferring the direct-reduced iron obtained in step (b) from the direct-ore-reduction zone to the iron-collection zone; (d) supplying NH₃ to the iron-collection zone, where the NH3 is subjected to dissociation reaction 2NH₃→ N₂ + 3H₂ in catalytic contact with the direct-reduced iron so as to obtain nitrogen and hydrogen; and (e) transferring the nitrogen and hydrogen from the iron-collection zone to the direct-ore- reduction zone. However, the cracking of ammonia is a highly endothermic reaction. Injecting hot uncracked ammonia in the direct-reduction unit would reduce the temperature at the injection point, thereby slowing down the reaction between the reducing gas and the iron oxide containing charge. Patent document WO2023/036475-A1 discloses a direct iron-ore reduction process comprising the steps of: supplying a gas feed containing NH3 to a gas reformer, reforming the NH3 in the gas feed in the gas reformer by subjecting it to the dissociation reaction 2NH3 → N2 + 3H2 so as to obtain a hydrogen-containing reducing gas, evacuating the reducing gas from the gas reformer, supplying iron ore to a direct-reduction reactor, supplying at least part of the reducing gas to the direct-reduction reactor, subjecting the iron ore to direct reduction with the reducing gas in the direct-reduction reactor so as to obtain direct reduced iron, and evacuating the direct reduced iron from the direct-reduction reactor. Preferably the gas reformer comprises a nickel- or nickel-alloy-containing catalyst. The flue gas from the direct-reduction reactor, or a fraction thereof, may be used as fuel for heating the gas reformer. DESCRIPTION OF THE INVENTION It is an object of the invention to provide a continuous direct-reduction process with reduced CO2 emissions and whereby ammonia is used as a source of the hydrogen reducing gas for the direct reduction of iron ore. It is an object of the invention to provide a continuous direct-reduction process with improved use of hydrogen as reducing gas. These and other objects and further advantages are met or exceeded by the present invention according to claim 1 with preferred embodiments in the dependent claims. In order to achieve these objects, the present invention proposes, in a first aspect, a direct iron-ore reduction process comprising the steps of: - producing two separate gas streams, a first gas stream of hydrogen and a second gas stream of nitrogen, from liquid ammonia or an aqueous solution containing ammonia, in an ammonia electrolyser unit comprising at least one electrolysis cell; - evacuating the first gas stream of hydrogen from the ammonia electrolyser unit; - evacuating the second gas stream of nitrogen from the ammonia electrolyser unit; - supplying iron ore to a direct-reduction reactor; - supplying a hydrogen-containing reducing gas to the direct-reduction reactor, typically through one or more gas inlets, said hydrogen-containing reducing gas comprising at least part, and preferably in full, of the first gas stream of hydrogen; - subjecting the iron ore to direct reduction with the hydrogen from the hydrogen- containing reducing gas in the direct-reduction reactor so as to obtain direct reduced iron, thereby generating a top gas; - evacuating the direct reduced iron from the direct-reduction reactor; - treating the top gas in a top gas treatment system connected to the direct-reduction reactor; and - adding at least part of the top gas containing hydrogen, and preferably after being compressed, to the hydrogen-containing reducing gas. In accordance with the invention it has been found that the use of ammonia as hydrogen carrier for the direct reduction of iron ore process and whereby the hydrogen is obtained via the electrolysis of ammonia provides hydrogen gas with a very high purity of more than 99 mol.% hydrogen. The hydrogen gas stream can be used directly for the direct reduction of iron ore without using any nitrogen removal process step or dewatering step. The electrolysis process may be performed at ambient temperature. The ammonia electrolysis can be done using liquid ammonia in an ammonia electrolyser unit 20 comprising at least one electrolysis cell 25. The electrolysis process is preferable performed using an aqueous solution containing ammonia 21, and preferably in an alkaline electrolytic cell 25. At ambient temperature, the thermodynamic values are much in favour of the production of hydrogen coupled to the oxidation of ammonia compared to hydrogen production by electrolysis of water. An advantage of ammonia electrolysis is that the energy required is 0.06 V compared to 1.23 V for water electrolysis. Another advantage of this process is its ease of integration with renewable energy sources, i.e. electricity. An electrolysis cell can operate with renewable energy, e.g. wind and solar energy. The electrolysis of the ammonia aqueous solution 21 is carried out through the following net reactions: Anode reaction:

Cathode reaction: 6H₂O + 6e- → 3H₂ (g) + 6OH- (4) Overall reaction: 2NH₃(aq) → N₂(g) + 3H₂(g) (5) The reactions proceed in the alkaline aqueous solution with KOH as the supporting electrolyte. In an embodiment the aqueous solution containing ammonia 21 comprises 5 to 25 wt.% ammonia in a 0.5M to 7M aqueous KOH solution. The ammonia used may be of any origin. According to a preferred embodiment, the ammonia is produced using hydrogen with low-carbon footprint and/or is produced using renewable energy sources. The ammonia is thus advantageously produced using blue or, more preferably, green hydrogen. In an embodiment the electrolysis cell 25 comprises an electrocatalyst comprising an anode catalyst and an cathode catalyst to increase the kinetics of the reaction(s). Such catalysts are present on a support substrate or substrate, preferably on a substrate with a high surface area. In embodiments, the anode catalyst is nickel or platinum or platinum combined with iridium, ruthenium and/or nickel. In embodiments the cathode catalysts is platinum or nickel or nickel alloy. The substrate should be resistant to ammonia and be electrically conductive. In embodiments the substrate is made from nickel, aluminium, ferritic steel, stainless steel (e.g. type 316 or type 304 stainless steel), or titanium. In an embodiment the reducing gas 15 used in the process according to the invention is composed of at least 70 mol.% of hydrogen, and more preferably at least 80 mol.%. In an embodiment the reducing gas 15 is composed of at least 90 mol.% of hydrogen, preferably of at least 95 mol.% of hydrogen, more preferably of at least 97 mol.% of hydrogen, and most preferably of at least 99 mol.% of hydrogen. The higher the hydrogen content, the less carbon present in the iron ore is oxidized in the direct-reduction reactor 1 and thus the lower the CO2 footprint. The hydrogen-containing reducing gas 15 has preferably a nitrogen content of less than 20 mol.%, and preferably less than about 15 mol.%, and more preferably less than 10 mol.%, and most preferably less than 5 mol.%. The lower the nitrogen content the better as nitrogen has an adverse effect on the reduction efficiency of the iron ore in the reactor 1. In an embodiment following the evacuation from the ammonia electrolyser unit 20 the hydrogen-containing reducing gas 15 is compressed in a compressor 28 to bring it at the required pressure level for injection into the direct-reduction reactor 1. In an embodiment said gas is compressed to a pressure in a range of between about 2 and 20 bar, and preferably between about 5 and 9 bar. Preferably the first gas stream of hydrogen 11 from the ammonia electrolyser 20, and after being compressed, is heated, e.g. in heater 27, to a temperature in a range of 750^oC to 1100^oC, and preferably in a range of 750^oC to 980^oC, and more preferably to 900^oC to 980^oC, prior to supply into the direct-reduction reactor 1 as part of the hydrogen-containing reducing gas 15 having the same temperature. As part of the reduction reaction in the direct-reduction reactor 1 also a top gas 17 is formed comprising substantial amounts of recoverable hydrogen gas. In an embodiment the top gas 17 after dewatering comprises at least 50 mol.% of hydrogen, and preferably at least 60 mol.%, and more preferably at least 70 mol.%. The top gas 17 may be treated in a top gas treatment system 30 functionally connected to the direct-reduction reactor 1 and comprises a deduster unit 31. The optionally dedusted top gas 17a then travels to a heat recovery system where the temperature of the top gas is reduced in a controlled manner. The sensible energy Q recovered can be used for a variety of purposes. Preferably the cooled top gas 17a is dewatered to produce dewatered and cooled top gas stream 17b. In an embodiment at least part of said top gas 17a,17b is recirculated, preferably after being dewatered and dedusted, by adding it to the hydrogen-containing reducing gas 15 and subsequently supplying the gas stream to the direct-reduction reactor 1 through one or more gas inlets. Preferably the top gas stream 17b is compressed in a compressor unit 28, typically to a pressure in a range of about 2 to 20 bar, preferably in a range of about 5 to 9 bar. In an embodiment the top gas stream 17b is pre-heated or heated at least in part using sensible energy Q recovered using a heat recovery system 34 from the top gas 17a using a heat exchanger unit 35 prior to adding it to the hydrogen-containing reducing gas 15 generating a combined gas stream at elevated temperature for supply to the direct-reduction reactor 1, thereby avoiding the need for a separate heating source, e.g. using natural gas creating CO2 emissions, for preheating or heating at least said top gas stream 17b. Preferably the top gas stream 17b is heated to a temperature of at least about 450^oC, preferably to a temperature in a range of about 450^oC to 980^oC, and preferably of about 750^oC to 980^oC. To avoid contamination build-up of impurities in the top gas stream 17 at least part of the top gas may be purged as purge gas 45. As the top gas 17 comprises substantial amounts of hydrogen gas, in an embodiment at least part of the top gas 17 instead of being purged it is used as a fuel, optionally in combination with the addition of another fuel 40, e.g. natural gas, in a gas heater. In a preferred embodiment gas heater 27 heats the hydrogen-containing reducing gas 15. In an embodiment the direct-reduction reactor 1 is of the gravitational type, preferably a shaft furnace. In a preferred embodiment the direct-reduction reactor 1 is of the gravitational type and comprises a reduction zone, inside which the iron ore reduction processes occur, feeding means to feed iron ore agglomerate 2 to the reduction zone of said reactor, a reducing gas circuit being provided with injection means configured to feed the hydrogen-containing reducing gas stream 15 into the reduction reactor, a heater for the reducing gas, an aperture to extract the top gas 17, and a discharge zone to discharge the DRI 3 in solid form. The DRI 3 or sponge iron in solid form is evacuated from the direct-reduction reactor 1 and is further processed directly on exit from the discharge zone, and optionally also after being compacted into briquettes, in a melt shop typically comprising one or more electric-arc furnaces or submerged-arc furnaces (EAF, SAF, or REF). The furnaces have electrodes and a gas extraction duct to collect the hot gases that are produced during the charging, melting and refining of the DRI 3. Optionally also steel scrap is charged into the furnaces together with the DRI. The melt shop typically further comprises ladle furnaces for metallurgical processing like alloying and refining to produce molten steel or other molten iron containing products, and subsequently cast into slabs or coils, ready for rolling and further heat treatment. The invention will also be illustrated with reference to a non-limiting figure. Fig. 1 is a schematic diagram of the direct iron-ore reduction process according to the invention, whereby the iron ore 2 is introduced into a direct-reduction reactor 1 whereafter the iron oxides in the iron ore are reduced to direct-reduced iron 3 in solid form and is evacuated from the direct-reduction reactor 1. In the direct-reduction reactor 1, which may operate at pressures up to about 20 bar, hydrogen present in the reducing gas 15 acts as a reducing agent for the reduction to metallic iron of the iron oxides in the iron ore 2 by direct reduction. For the reduction process substantial amounts of hydrogen gas are being used. The hydrogen- containing reducing gas 15 is supplied to the direct-reduction reactor 1 through one or more gas inlets (not shown) and originates in this embodiment at least in part from the electrolysis of the ammonia aqueous solution 21 in an ammonia electrolyser unit 20. The hydrogen-containing reducing gas 15 may comprise certain amounts of supplementary gas(es) 16, e.g. natural gas (CH4) and/or oxygen gas to further increase the temperature of the gas stream to a temperature up to about 1100^oC. As part of the reduction reaction in the direct-reduction reactor 1 also a top gas 17 is formed comprising substantial amounts of recoverable hydrogen gas and energy. The top gas 17 may be treated in a top gas treatment system 30 functionally connected to the reactor 1 and may comprise a deduster unit 31, a heat recovery unit 34, a dewatering unit 33, and a compressor 28. In an embodiment the dedusted top gas stream 17a then travels to a heat recovery system 34 as part of the top gas treatment system 30, where the temperature of the top gas is reduced in a controlled manner, for example to a temperature between about 20^oC and 100^oC. The sensible energy Q thus recovered may be used to heat or at least pre-heat top gas stream 17b using a heat exchanger unit 35 prior to adding said gas stream 17b to the hydrogen- containing reducing gas 11,15 generating a combined gas stream at elevated temperature for supply to the direct-reduction reactor 1. The cooled top gas 17a is preferably dewatered in dewatering unit 33 forming part of the top gas treatment system 30 to produce dewatered and cooled gas stream 17b. The dewatered and cooled top gas 17b may be compressed in a compressor 28 and at least in part may be purged as purge gas 45 to reduce the build-up of contamination of impurities in the gas circuit. When used as a purge gas 45 compression is not necessarily required and is optional. In an embodiment at least part of the dewatered and cooled top gas 17b is compressed in a compressor 28 and used as a fuel, optionally in combination with the addition of a further fuel(s) 40, e.g. natural gas, in gas heater 27 heating the hydrogen-containing reducing gas 15. In a preferred embodiment at least part of the dewatered and cooled top gas 17b is compressed in a compressor 28 and heated or pre-heated in heat exchanger 35 and added to the hydrogen-containing reducing gas 11,15 generating a combined gas stream at elevated temperature for supply to the direct-reduction reactor 1. The ammonia electrolyser unit 20 comprises one or more electrolysis cells 25 comprising each at least one anode and at least one cathode, for the electrolysis of the ammonia into a first gas stream of hydrogen 11 extracted from the ammonia electrolyser 20 via a cathode gas processing system (not shown) and into a second gas stream of nitrogen 12 extracted via an anode gas processing system (not shown). The second gas stream of nitrogen 12 can be used for example for the utility system used in the steelmaking process. The hydrogen in the first gas stream 11 is added at least in part, and preferably in full, to the hydrogen-containing reducing gas 15 supplied to the direct-reduction reactor 1 through one or more gas inlets (not shown). The gas stream is compressed in a compressor 28 prior to supply into the direct- reduction reactor 1. Following the addition of the top gas stream 17b the gas stream may be compressed in a further compressor 28a to the required operational pressure levels of the direct-reduction reactor 1. As an alternative, the first gas stream of hydrogen 11 and top gas stream 17b are combined and next compressed jointly in compressor 28 such that the further compressor 28a can be avoided. Subsequently the gas stream is heated in a heater unit 27 to a temperature in a range of 750^oC to 1100^oC, and preferably in a range of 750^oC to 980^oC, and more preferably to 900^oC to 980^oC, prior to supply into the direct-reduction reactor 1. The ammonia electrolyser 20 may comprise mass flow control system 22 and electrolyte management system 23 for controlling the concentration, e.g. via a pH regulator for monitoring and maintaining and maintaining the concentration of KOH within an operating range by adding KOH and an ammonia regulator for monitoring and maintaining the concentration of ammonia within an operating range by adding ammonia, and the level of the aqueous solution 21 containing ammonia, e.g. via a fluid level regulator to monitor and maintain the level of electrolyte within an operating range by adding water and KOH. The electrolyte management system 23 may comprise a water removal system (not shown) for removing excess water in the electrolyte. The ammonia electrolyser 20 may further comprise a cooling system (not shown) for regulating the temperature of the ammonia electrolyser. The ammonia electrolyser 20 further comprises a DC power supply system (not shown) to supply power to the electrolysis cell(s) 25 required for the electrolysis of ammonia. The above-discussion is intended to be merely illustrative of the present system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Any reference signs in the claims should not be construed as limiting the scope of the appended claims. List of reference numbers: 1. Direct-reduction reactor or DRI-reactor. 2. Iron ore. 3. Direct reduced iron. 11. First gas stream of hydrogen. 12. Second gas stream of nitrogen. 15. Hydrogen-containing reducing gas. 16. Supplementary gas. 17. Top gas. 20. Ammonia electrolyser unit. 21. Aqueous solution containing ammonia. 22. Mass flow controller(s). 23. Electrolyte management system. 25. Electrolysis cell(s). 27. Heater. 28. Compressor. 30. Top gas treatment system. 31. Deduster unit. 33. Dewatering unit. 34. Heat recovery unit. 35. Heat exchanger. 40. Fuel. 45. Purge gas.

Claims

CLAIMS 1. A direct iron-ore reduction process comprising the steps of: - producing two gas streams, a first gas stream of hydrogen (11) and a second gas stream of nitrogen (12), from liquid ammonia or an aqueous solution containing ammonia (21), in an ammonia electrolyser unit (20) comprising at least one electrolysis cell (25); - evacuating the first gas stream of hydrogen (11) from the ammonia electrolyser unit (20); - evacuating the second gas stream of nitrogen (12) from the ammonia electrolyser unit (20); - supplying iron ore (2) to a direct-reduction reactor (1); - supplying a hydrogen-containing reducing gas (15) to the direct-reduction reactor (1), the hydrogen-containing reducing gas (15) comprising at least part of the first gas stream of hydrogen (11); - subjecting the iron ore (2) to direct reduction with the hydrogen from the hydrogen- containing reducing gas (15) in the direct-reduction reactor (1) so as to obtain direct reduced iron (3), thereby generating a top gas (17); - evacuating the direct reduced iron (3) from the direct-reduction reactor (1); - treating the top gas (17) in a top gas treatment system (30) connected to the direct reduction reactor (1); - adding at least part of the top gas (17)containing hydrogen to the hydrogen-containing reducing gas (15). 2. Process according to claim 1, wherein the first gas stream of hydrogen (11) and the second gas stream of nitrogen (12) are produced from an aqueous solution containing ammonia (21) in an ammonia electrolyser unit (20) comprising at least one electrolysis cell (25), and the aqueous solution containing ammonia (21) comprises 5 to 25 wt.% ammonia in a 0.5M to 7M aqueous KOH solution. 3. Process according to claim 1, wherein the first gas stream of hydrogen (11) and the second gas stream of nitrogen (12) are produced from liquid ammonia in an ammonia electrolyser unit (20) comprising at least one electrolysis cell (25). 4. Process according to any one of claims 1 to 3, wherein the hydrogen-containing reducing gas (15) is heated to a temperature of at least 750^oC prior to being supplied to the direct- reduction reactor (1), preferably to a temperature of at least 900^oC, and more preferably to a temperature in a range of 900^oC to 980^oC. 5. Process according to any one of claims 1 to 4, wherein the hydrogen-containing reducing gas (15) is compressed prior to being supplied to the direct-reduction reactor (1), preferably to a pressure in a range of 2 to 20 bar. 6. Process according to any one of claims 1 to 5, wherein the hydrogen-containing reducing gas (15) supplied to the direct-reduction reactor (1) has a hydrogen content of at least 70 mol.%, preferably of at least 80 mol.%. 7. Process according to any one of claims 1 to 6, wherein the hydrogen-containing reducing gas (15) supplied to the direct-reduction reactor (1) has a nitrogen content of less than 20 mol.%, preferably of less than 15 mol.%, and more preferably of less than 10 mol.% 8. Process according to any one of claims 1 to 7, comprising dedusting of the top gas (17) using a dedusting unit (31) as part of the top gas treatment system (30) connected to the direct-reduction reactor (1). 9. Process according to any one of claims 1 to 8, further comprising recovering sensible heat from the top gas (17) using a heat recovery system (34) as part of the top gas treatment system (30) connected to the direct-reduction reactor (1). 10. Process according to any one of claims 1 to 9, further comprising dewatering of the top gas (17) using a dewatering system (33) as part of the top gas treatment system (30) connected to the direct-reduction reactor (1). 11. Process according to any one of claims 1 to 10, wherein the top gas (17) prior to adding it to the hydrogen-containing reducing gas (15) is heated to a temperature of at least 750^oC, preferably to a temperature in a range of 750^oC to 1100^oC, and more preferably in a range of 900^oC to 980^oC. 12. Process according to claim 11, wherein said top gas (17) is heated in a heat exchanger (35) at least in part using the sensible energy recovered by a heat recovery system (34) as part of the top gas treatment system (30) connected to the direct-reduction reactor (1). 13. Process according to any one of claims 1 to 12, wherein preferably after dewatering, the top gas (17) added to the hydrogen-containing reducing gas (15) has a hydrogen content of at least 50 mol.%, preferably of at least 60 mol.%, and more preferably of at least 70 mol.%.