Methods and systems for producing direct reduced iron with efficient use of hydrogen gas

Abstract

Methods and systems for production of direct reduced iron (DRI) with a carbon content of from 0.1 %w / w to 5 %w / w using a reducing gas containing more than 80 vol% hydrogen gas with a high efficiency in a reduction system comprising a reducing gas circulation loop that comprises a shaft furnace, a gas heat exchanger, a gas cleaning and cooling equipment, a hydrogen gas separation unit, a reducing gas heater and the necessary piping and compressor or blower to cause said reducing gas to flow through said reducing gas circulation loop. The reducing gas stream that is fed to the shaft furnace is heated in three steps: a first step to about 130 °C is carried out with energy from a waste gas stream which in current processes is wasted, a second heating step to about 330 °C using a gas heat exchanger without fouling or clogging operational problems, with heat from exhaust gas exiting the shaft furnace, and a third heating step in an heater to a temperature between 800 °C 1100 °C.

Description

[0001] TITLE

[0002] METHODS AND SYSTEMS FOR PRODUCING DIRECT REDUCED IRON WITH EFFICIENT USE OF HYDROGEN GAS.

[0003] FIELD OF THE INVENTION

[0004] The invention relates to the field of steel making, more particularly to the field of producing direct reduced iron from iron-oxides-containing materials, also known as sponge iron or DRI, in moving-bed shaft furnaces using a reducing gas mostly composed of hydrogen and using thermal and chemical energy of exhaust gas exiting from the furnaces to at least partially raise the temperature of the reducing gas to the level adequate to carry out said iron oxides reduction.

[0005] BACKGROUND OF THE INVENTION

[0006] Decarbonization of the steel industry is leading to substitute fossil fuels such as coal, which is used in blastfurnaces, and liquid and gaseous hydrocarbons, such as natural gas, with hydrogen produced from renewable sources and / or with energy from renewable sources, leading to use hydrogen as the main reducing gas in direct reduction plants. The direct reduction of iron ores is mainly made in shaft furnaces with a reducing gas comprising H2 and CO in various proportions, derived from reformation, gasification or cracking of hydrocarbons Also, to further decrease CO2 emissions, it is proposed to carry out heating of reducing gases to the high temperatures required for reduction in gas heaters using electric energy instead of combustion of hydrocarbons.

[0007] WO 2023018787 discloses a direct reduction process where a reducing gas comprising more than about 80 vol% of hydrogen is fed to a reduction reactor, and where the temperature of the reducing gas is raised using electrical energy. WO 2023018787 shows a method for recycling spent reduction gas where the usual fired reducing gas heater is replaced with an electric heater and accumulation of non-condensable inert and oxidant gases comprised in the exhaust gas (top gas) from the shaft furnace is prevented from building-up as a result of said exhaust gas recycle by purging a portion of said exhaust gas and using it as fuel in a reformer or heater. Typically, about 1 / 3 of the top gas is purged and used as fuel and 2 / 3 of said top gas is recycled. WO 2023018787 teaches that the amount of top gas that can be recycled is increased by adding a hydrogen separator (membrane or PSA systems), but there is no suggestion that the hydrogen-lean gas from the hydrogen separator can be used to preheat the reducing gas stream that is fed to the reduction reactor or to preheat a carbon-containing gas to carburize the reduced iron. The reduction process of WO 2023018787 is energy-inefficient because heat of the exhaust gas is not recovered and therefore is lost when cooling said exhaust gas.

[0008] US 2024084410 also shows a direct reduction process where a reducing gas comprising more than about 80 vol% of hydrogen is fed to a reduction reactor, and where the temperature of the reducing gas is raisedusing electrical energy. A bleed-off stream of top gas is processed in a hydrogen separation unit producing a hydrogen-enriched fraction and an inert-enriched fraction. The hydrogen-enriched fraction is then recycled back to the direct reduction shaft. There is no teaching or suggestion in this patent application about using the inert-enriched fraction of the top gas to preheat the recycled top gas to avoid operational problems of fouling or clogging of said heat-exchanger due to accumulation and deposits of particles entrained in the top gas over the heat-transfer tubes of a gas heat exchanger that develop when the top gas is cooled below its dew point temperature and water is condensed inside said heat-transfer tubes. DRI is carburized by contacting with a carburizing gas in the lower part of the shaft furnace or in a separate vessel. The carburizing gas is withdrawn from the shaft furnace and hydrogen is separated therefrom in a second hydrogen separator. The hydrogen stream is added to the reducing gas circulating through the reduction zone of the shaft furnace and the hydrogen-lean gas containing carbon-bearing gases can be disposed of by combustion in a preheater. No more details are disclosed.

[0009] Although by using hydrogen for the chemical reduction of iron oxides producing water as by-product instead of carbon-containing gases, and an electric heater to raise the temperature of the reducing gas, the CO2 emissions of a direct reduction plant are significantly decreased, there is still a need of decreasing the overall consumption of hydrogen, and increasing the efficiency of the energy consumption in the production of directly reduced iron (DRI) by increasing heat recovery of the top gas with a consequent decrease of electrical (or thermal) energy and at the same time avoiding fouling problems in heatexchangers, with competitive capital and operation costs.

[0010] SUMMARY OF THE INVENTION

[0011] A first aspect of the present invention is a method for producing direct reduced iron with a carbon content of from 0.1 %w / w to 5 %w / w and at a temperature above 600 °C by reduction of iron oxides materials in a shaft furnace having a reduction zone and a carburizing zone, wherein reduction of iron oxides is carried out by

[0012] • feeding a reducing gas comprising more than 80 vol% hydrogen to said reduction zone,

[0013] • feeding a carburizing gas to the carburizing zone and causing spent carburizing gas to flow from said carburizing zone to said reduction zone,

[0014] • withdrawing an exhaust reducing gas stream from said reduction zone,

[0015] • transferring heat from said exhaust gas stream to the reducing gas in a gas heat exchanger, • cleaning and cooling said exhaust reducing gas stream and separating water therefrom to form a recycled reducing gas stream,

[0016] • separating hydrogen gas from a portion of said recycled reducing gas stream in a hydrogen separation unit forming a hydrogen-rich gas stream and a hydrogen-lean gas stream,combining the hydrogen-rich gas stream with said recycled reducing gas stream and a make-up gas stream comprising more than 80% hydrogen to form said reducing gas stream, wherein said method is characterized by raising the temperature of the reducing gas stream, prior to being fed to said reduction zone, in three heating steps:

[0017] • a first heating step to a temperature in the range from 100 °C to 140 °C with heat provided by combustion of at least a portion of said hydrogen-lean gas stream,

[0018] • a second heating step to a temperature in the range from 200 °C to 350 °C in said gas heat exchanger, and

[0019] • a third heating step to a temperature in the range from 800 °C to 1100 °C in a reducing gas heater.

[0020] According to an embodiment, the combustion of at least a portion of said hydrogen-lean gas stream is used to produce steam, and the first heating step is performed by transferring the heat of the steam to the reducing gas stream.

[0021] According to an embodiment, the hydrogen gas separation unit is a pressure-swing adsorption (PSA) unit, a molecular membrane unit, or a cryogenic unit.

[0022] According to an embodiment, the gas heat exchanger is a co-current gas heat exchanger or a countercurrent gas heat exchanger.

[0023] According to an embodiment, the reducing gas heater is an electric heater or a fuel combustion heater, or a combination of at least one electric heater and at least one fuel combustion heater. The fuel combustion heater can be operated with natural gas or at least a portion of said hydrogen-lean gas stream, or a combination thereof.

[0024] According to an embodiment, the exhaust reducing gas stream is cleaned and cooled in a cleaning and cooling equipment comprising a washing and cooling tower and a condensed water separation unit. According to an embodiment, the shaft furnace comprises an intermediate zone between the reduction zone and the carburizing zone, and the carburizing gas stream is fed to the intermediate zone. In an embodiment, the carburizing gas stream comprises natural gas, coke oven gas, methane, LPG or petroleum, syngas, lower alcohols, esters and ethers from fossil or renewable sources. Further, the carburizing gas stream is heated to a temperature in the range from 300 °C to 400 °C, prior to being introduced into said carburizing zone, using heat of combustion of natural gas or a portion of said hydrogen-lean gas stream, or a combination thereof.According to an embodiment, the carbon content of the direct reduced iron is between 0.5 %w / w and 1.5 %w / w.

[0025] According to an embodiment, the direct reduced iron discharged from the shaft furnace is fed into a direct reduced iron cooling vessel, separate from the shaft furnace, where the temperature of the direct reduced iron is reduced below 100 °C, preferably below 80 °C and more preferably below 50 °C by contact with a cooling gas stream. In an embodiment, the cooling gas stream comprises hydrogen.

[0026] According to an embodiment, the reducing gas stream exiting from the reducing gas heater can be deviated, in case of emergency when operational problems related to said shaft furnace occur, to an auxiliary cooling device and to said reducing gas circulation loop, so bypassing the shaft furnace.

[0027] According to an embodiment, the hydrogen-rich gas stream, the recycled reducing gas stream and the make-up gas stream comprising more than 80% hydrogen are combined to form said reducing gas stream before operating the three heating steps.

[0028] A second aspect of the present invention is a system for producing direct reduced iron with a carbon content of from 0.1 %w / w to 5 %w / w and at a temperature above 600 °C by reduction of iron oxides materials in a shaft furnace having a reduction zone and a carburizing zone,

[0029] • a first inlet to feed a reducing gas comprising more than 80 vol % hydrogen to said reduction zone, • a second inlet to feed a carburizing gas to the carburizing zone wherein the spent carburizing gas flows from said carburizing zone to said reduction zone,

[0030] • an outlet to withdraw an exhaust reducing gas stream from said reduction zone,

[0031] • a cleaning and cooling equipment configured to clean and cool said exhaust reducing gas stream and to separate water therefrom to form a recycled reducing gas stream,

[0032] • a hydrogen separation unit for separating hydrogen gas from a portion of said recycled reducing gas stream forming a hydrogen-rich gas stream and a hydrogen-lean gas stream,

[0033] • piping and accessories for combining the hydrogen-rich gas stream with said recycled reducing gas stream and a make-up gas stream comprising more than 80% hydrogen to form said reducing gas stream,

[0034] wherein said system is characterized by comprising:

[0035] • a first heat-transfer unit to raise the temperature of the reducing gas stream in the range from 100 °C to 140 °C with heat provided by combustion of a portion of said hydrogen-lean gas stream,• a second heat-transfer unit comprising a gas heat exchanger to raise the temperature of the reducing gas stream in the range from 200 °C to 350 °C with heat provided by exhaust reducing gas stream, and

[0036] • a third heat-transfer unit to raise the temperature of the reducing gas stream in the range from 800 °C to 1100 °C, wherein said three heat-transfer units are located upstream of said reduction zone.

[0037] In an embodiment, the first heat-transfer unit comprises a boiler to produce steam utilizing the heat of combustion of a portion of said hydrogen-lean gas stream and a gas heat exchanger to transfer the heat of said steam to the reducing gas stream.

[0038] In an embodiment, the hydrogen gas separation unit is a pressure-swing adsorption (PSA) unit, a molecular membrane unit, or a cryogenic unit.

[0039] In an embodiment, the third heat-transfer unit is an electric heater or a fuel combustion heater or a combination of at least one electric heater and at least one fuel combustion heater.

[0040] In an embodiment, the cleaning and cooling equipment comprises a washing and cooling tower and a condensed water separation unit.

[0041] In an embodiment, the shaft furnace comprises an intermediate zone between the reduction zone and the carburizing zone, and piping connecting a source of the carburizing gas stream to the carburizing zone and / or to the intermediate zone of the shaft furnace. In an embodiment, the carburizing gas stream comprises natural gas, coke oven gas, methane, LPG or petroleum, syngas, lower alcohols, esters and ethers from fossil or renewable sources. Further, the carburizing gas stream is heated to a temperature in the range from 300 °C to 400 °C, prior to being introduced into the carburizing zone and / or to the intermediate zone of the shaft furnace, using heat of combustion of natural gas or a portion of said hydrogen-lean gas stream, or a combination thereof.

[0042] In an embodiment, the carbon content of said direct reduced iron is between 0.5 and 1.5 %w / w.

[0043] In an embodiment, the system for producing direct reduced iron according to the invention further comprises piping connecting the reducing gas heater with a reducing gas cleaning and cooling equipment for selectively deviating, in case of emergency when operational problems related to said shaft furnace occur, the reducing gas stream exiting from the reducing gas heater bypassing the shaft furnace.

[0044] In an embodiment, the system for producing direct reduced iron according to the invention further comprises a direct reduced iron cooling vessel separate from said shaft furnace, where the temperature of the direct reduced iron discharged from the shaft furnace is reduced below 100 °C, preferably below80 °C and more preferably below 50 °C by contact with a cooling gas stream. In an embodiment, the cooling gas is a hydrogen gas.

[0045] BRIEF DESCRIPTION OF THE FIGURES

[0046] Figure 1 shows a schematic diagram of an embodiment of the invention as applied to a direct reduction plant where DRI is discharged from the shaft furnace and is conducted to a hot briquetting machine to produce hot briquetted iron (HBI).

[0047] Figure 2 shows a schematic diagram of an embodiment of the invention as applied to a direct reduction plant where DRI is discharged from the shaft furnace and cooled down in a DRI cooler.

[0048] DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] Current direct reduction processes produce DRI with variable carbon content by reacting iron-oxides-containing materials, iron ores in the form of pellets or lumps, with a reducing gas mostly comprising hydrogen and carbon monoxide in moving-bed shaft furnaces. The reducing gas is obtained by reforming a hydrocarbon with water and / or CO2 in catalytic reformers. There are also DRI plants without reformer where the reducing gas is formed within the shaft furnace from natural gas taking advantage of the catalytic effect of DRI.

[0050] Referring to Figure 1, showing an exemplary embodiment of the invention, numeral 1 generally designates a DRI process plant comprising a moving-bed shaft furnace 10 comprising a reduction zone 12 and a carburizing zone 14. Iron oxides containing materials, for example, iron ores 15 in the form of lumps having particle size about 5 mm to 15 mm, or in the form of pellets with an iron content in the range typically from about 65 %w / w to 70 %w / w, are charged to the shaft furnace 10 through inlet 16 and descend through the reduction zone 12 where metallic iron is formed and thereafter through the carburizing zone 14. The residence time of the iron oxides containing materials is controlled by a discharge mechanism 18 which may take the form of a rotary valve (or star-type valve), or an oscillating rake or table which is operated at the desired discharge rate of DRI 20.

[0051] The reducing gas 22 fed to the reduction zone 12 is composed mainly of hydrogen gas so that the carbon-compounds emissions produced by the current DRI plants, operated with natural gas, are drastically decreased because the product of the reduction reactions of hydrogen is water which is condensed and separated from the exhausted reducing gas. In some embodiments, the composition of the reducing gas 22 in volume% is H2: 93.9; CO: 0.6; CO2: 0.3; CH4: 2.5; N2: 1.9; H2O: 0.8.

[0052] Since under the process conditions of the shaft furnace, the reduction reactions only reach a certain extent, the exhaust gas 24 withdrawn from the upper part of the reduction zone 12, also called top gas, is recycled to the shaft furnace in a reducing gas loop after passing through several steps of cleaning,cooling, dedusting and separation of some of the gases to avoid accumulation of inert gases, such as nitrogen and carbon compounds in said loop. In some embodiments, the composition of the exhaust gas 24 in volume% is H2: 60-80, H2O: 15-35, with minor amounts of CO, CO2, CH , and N2. To avoid accumulation of inert and carbon-bearing gases, the current practice is to purge a portion of the exhaust gas and use it as fuel or export it out of the reduction system for other uses. In some cases, the purged gas is combusted in a flare being finally expelled to the atmosphere.

[0053] In some embodiments of the invention, to have a better utilization of energy in the reduction system, most of the thermal and chemical energy available in the gas streams is used within the reduction system that otherwise would be wasted through the system flare or exported outside for other uses. To this end, part of the sensible heat of the exhaust gas 24 that exits the shaft furnace 10 at a temperature within about 300 °C to 450 °C is transferred to the reducing gas 26 that is fed to the shaft furnace 10 by means of a gas heat exchanger 28.

[0054] The gas heat exchanger 28 can be a co-current gas heat exchanger or a counter-current gas heat exchanger. In some embodiments, the heat transfer of the exhaust gas 24 to the reducing gas 26 is optimized by using a counter-current gas heat exchanger 28. Use of a gas heat exchanger, however, is made possible only if the reducing gas 26, is preheated to a temperature from 10 °C to 30 °C, preferably from 15 °C to 25 °C above the dew point of the exhaust gas stream 24 at the corresponding pressure level. In some embodiments, the reducing gas 26 is preheated to a temperature above 100 °C, preferably above 120 °C, and in other embodiments, above 130 °C, and less than 150 °C.

[0055] Preheating of the reducing gas 26 can be done in several ways, considering that the purpose of such preheating is to prevent the water in the exhaust gas stream 24 from condensing when its temperature descends while passing through the tubes of said gas heat exchanger 28 to avoid fouling or clogging problems of tubes in said gas heat exchanger 28.

[0056] According to the invention, preheating of reducing gas 26 is carried out by combustion of at least a portion of the hydrogen-lean gas stream 30 exiting from a hydrogen separation unit 32 which separates hydrogenrich gas stream 33 from exhaust gas 24 which hydrogen is introduced into the reducing gas loop.

[0057] In some embodiments, steam 34 is produced in a boiler 36 using heat of combustion of at least a portion of hydrogen-lean gas stream 30 and said steam 34 is used to heat the reducing gas 26 in a heatexchanger 40.

[0058] Exhaust gas stream 24, after cooling down in gas heat exchanger 28 to a temperature in the range from 200 °C to 250 °C, is treated in a cleaning and cooling unit 30 to remove water 42 produced by the reduction of iron oxides with hydrogen in the reduction zone 12.The cleaning and cooling unit 30 typically comprises several pieces of equipment not shown here for the sake of simplicity, such as a quench tower, a packed washing column to de-dust the gas, a gas-liquid separator, and a knock-out drum. Cooling and cleaning the exhaust gas 24 can be carried out in a manner known in the art.

[0059] The cold and clean exhaust gas stream 44 at a temperature between 30 °C and 50 °C, contains mainly hydrogen and minor amounts of inert gases such as N2 and carbonaceous gases such as CO, CO2 and CH4, derived from the carburizing gas that is fed to the carburizing zone of the shaft furnace 10 to incorporate carbon into the DRI 20. In some embodiments the composition of exhaust gas stream 44 in volume% is H2: 93.9; CO: 0.6; CO2: 0.3; CH4: 2.5; N2: 1.9; H2O: 0.8.

[0060] The exhaust gas stream 44 is then split into two recycled gas streams, a first recycled gas stream 46, which is about 90 %v / v, and a second recycled gas stream 50, which is about 10 % v / v of the exhaust gas stream 44.

[0061] The first recycled gas stream 46 is compressed by compressor 48 and is recycled to the reduction zone 12 thus forming a reducing gas loop.

[0062] The second recycled gas stream 50 is fed to a hydrogen separation unit 32 where hydrogen is separated forming a hydrogen-rich gas stream 33 and a hydrogen-lean gas stream 30.

[0063] The hydrogen-rich gas stream 33, comprising more than 95% of hydrogen gas, is fed to the reducing gas loop. In some embodiments the composition of the hydrogen-rich gas stream 33 in volume% is H2: 98.8; CO: 0.04; CO2: 0.003; CH4: 0.12; N2: 0.5; H2O: 0.5.

[0064] The hydrogen-lean gas stream 30, comprising CO, CO2, CH4and N2, is used as fuel in the boiler 36, in the carburizing gas heater 64, and in the heater 54, when the heater 54 is fuel-combustion or comprises a fuel-combustion section with combustion burners. In some embodiments the composition of the hydrogen-lean gas stream 30 in volume% is H2: 24.1; CO: 8.9; CO2: 4.0; CH4: 36.1; N2: 21.6; H2O: 5.2. Optionally, the direct reduction plant can produce DRI 20 containing a high carbon content, in the range from about 3 %w / w to 5 %w / w, by feeding more natural gas 68 to the carburizing zone 14 than the amount needed to produce DRI 20 with a carbon content of 1.5 %w / w or less, and consequently, the hydrogenlean gas stream will contain more CO, CO2 and CH4. This hydrogen-lean gas stream 30 will then be used as fuel in boiler 36, heater 64 and the rest in the reducing gas heater 54 as fuel gas stream 59 when this heater 54 is fuel-combustion or comprises a fuel-combustion section with combustion burners.

[0065] A make-up hydrogen gas stream 52, which can be 100% pure hydrogen, or essentially hydrogen, e.g. more than about 98%, with minor impurities depending on the source thereof, is added to the reductiongas loop supplying the hydrogen consumed by the reduction reactions in the shaft furnace 10. The makeup hydrogen gas is preferably so-called green hydrogen, obtained using renewable energy, for example solar energy, wind or waterfalls, or from renewable sources, for example biomass, in order to decrease the carbon footprint of the reduction plant.

[0066] In some embodiments, the reducing gas stream 26 at a temperature below about 40 °C and comprising the recycled gas 46 in a proportion of about 86 % v / v, the hydrogen gas stream 33 in a proportion of about 8 % v / v and the make-up hydrogen gas stream 52 in a proportion of about 6 % v / v, is heated to a temperature from 800 °C to 1100 °C in three heating steps: a first heating step in the range from 100 °C to 150 °C, in heat exchanger 40 using steam 34 as a heat-transfer medium by combustion of said hydrogen-lean gas stream 30 in boiler 36, a second heating step in a co-current or counter-current gas heat exchanger 28 by heat transfer from the exhaust reducing gas 24 to a temperature in the range from 200 °C to 350 °C, and a third heating step in a reducing gas heater 54 to a temperature in the range from 800 °C to 1100 °C, forming the hot reducing gas 22 suitable for reduction or iron oxides.

[0067] In some embodiments, a portion 53 of about 7 % v / v of the reducing gas stream 26 is recycled to the reducing gas loop close to the suction point of the compressor 48 to increase the temperature of the cold reducing gas stream 46 to prevent water condensation and consequent corrosion problems at the compressor 48 and its associated equipment.

[0068] In some embodiments, the reducing gas heater 54 is a fuel-combustion heater operated with a fuel such as natural gas. Alternatively, in other embodiments, the fuel-combustion heater is operated with a portion of hydrogen-lean gas 30.

[0069] In other embodiments, the reducing gas heater 54 is an electric heater operated with electricity minimizing the carbon emissions of the reduction plant.

[0070] In other embodiments, the reducing gas heater may comprise a fuel-combustion heater and an electric heater, adding flexibility to the plant operation in case the availability of fuel or electricity is limited for technical or economic reasons, and at the same time decreasing at least partially the carbon emissions to the environment. For example, the reducing gas can be heated in the fuel-fired heater to a temperature between 600 °C and 700 °C to avoid carburization problems of the heat-exchange surfaces and in the electric heater to the temperature between 800 °C to 1100 °C suitable for the reduction of iron oxides. According to the invention, by heating the reducing gas 26 in the mentioned three steps, utilizing a counter-current heat exchanger 28, the consumption of electricity in the reducing gas heater 54 is about 530 Kwh / ton DRI, with a saving of about 36 Kwh / ton DRI due to the efficient use of the heat of the exhaustgas 24 in gas heat exchanger 28 and the heat of combustion of hydrogen-lean gas 30 in the boiler 36 and, optionally, in the reducing gas heater 54.

[0071] In some embodiments, DRI 20 is discharged from the shaft furnace 10 at high temperature, above 600 °C, preferably above 650 °C, and more preferably above 680 °C and is conducted to a hot briquetting machine 60 to produce briquettes, also known in the steel industry as hot briquetted iron (HBI) 62. HBI is produced in direct reduction plants to facilitate its handling, storage and transportation to remote points of use, when the DRI is not intended for local consumption.

[0072] In order to facilitate the subsequent processing of DRI to produce steel, which processing comprises melting or smelting and refining said DRI, it is important that the DRI contains carbon. Preferably, the carbon in the DRI is in the form of iron carbide FesC. In some embodiments, iron carbide is formed in the DRI by contacting said DRI with a hydrocarbon gas, usually natural gas which comprises methane and other hydrocarbons.

[0073] Since the carburizing reactions are endothermic and the DRI is intended to be briquetted at high temperature, the hydrocarbon gas 66 is heated in heater 64 to provide energy for said carburization and consequently to maintain the temperature of the DRI as high as possible.

[0074] In an embodiment illustrated in Figure 1, the hydrogen-lean gas 30 is split into two gas streams: gas stream 56 used as fuel in the boiler 34, and gas stream 58 which is used as fuel in gas heater 64 to raise the temperature of a hydrocarbon gas 66, for example natural gas, which is heated to a temperature in the range from 300 °C to 400 °C, preferably from 360 °C to 390 °C. Hot hydrocarbon gas 68 is fed in the carburizing zone 14, or in the intermediate zone, between the reduction zone 12 and the carburizing zone 14 to carburize DRI.

[0075] In some embodiments, a branch line 70, shown in dashed lines to indicate that it is not normally operating but only in transient emergency situations of abnormal operation of the direct reduction plant, for example when there are operational problems in the shaft furnace, provides a bypass route for hot reducing gas 22 exiting the heater 54 to be preliminarily cooled down in cooling unit 72 and then passed through the cooling unit 30, to prevent operation problems of the heater and the rest of components of the plant. Referring to Figure 2, an embodiment is shown where the DRI 20 discharged at high temperature from the shaft furnace 10 is cooled down in DRI cooler 74 to a temperature below about 100 °C, preferably below 80 °C instead of converting it to HBI. To this end, a cooling gas, for example hydrogen, is circulated in a cooling gas loop comprising said DRI cooler 74, and cooling gas cooling unit 76 where the cooling gas is cooled down and cleaned in a known manner. A cooling gas make-up stream 78 of a non-oxidant gas, for example hydrogen, at a temperature below about 40 °C is fed to the cooling loop to replace anycooling gas consumed. In some embodiments, the cooling gas 78 is hydrogen that exits the DRI cooler 74 at a temperature of about 220 °C, and in this case a surplus of said hydrogen 80 can be fed to the reducing gas loop upstream of compressor or blower 48.

[0076] The direct reduction method of the invention is capable of producing DRI with a carbon content from 0.1 %w / w to 5 %w / w if the desired product is cold DRI and from 0.1 %w / w to 1.5 %w / w if the product is hot DRI at a temperature above 600 °C adequate to be hot briquetted as HBI.

[0077] In some embodiments, since the DRI product is hot-briquetted, the temperature of the DRI at the discharge of the reactor must be above 680 °C. Carburizing DRI to have more than about 1.3 %w / w to 1.5 %w / w of carbon would lower the discharge temperature of DRI because the heat required by carburizing reactions is more than the heat that can be provided heating the carburizing gas.

[0078] In an exemplary simulation of an embodiment of the invention, the flow rate of the gas stream 50, which is the tail gas purged from the reduction system that in the prior art is discarded or used as fuel and comprises more than 80% hydrogen, according to the invention, is processed to separate hydrogen gas and take advantage of its hydrogen content for reduction of iron oxides.

[0079] In this example, the gas stream 50 comprises the following composition in volume%: H2: 93.1, CO: 0.7, CO2: 0.3, CH : 2.9, N2: 2.1, H20: 0.9 and after hydrogen gas separation is split into a hydrogen-rich stream 33 with the following composition in volume%: H2: 98.8, CH4: 0.2, N2: 0.5, H2O: 0.5 and a hydrogen-lean stream 30 comprising the following composition in volume%: H2: 24.1, CO: 9.0, CO2: 4.0, CH4: 36.1, N2: 21.6, H2O: 5.2. The flow rate of gas stream 33 is in the range from 90% to 95% of the flow rate of gas stream 50.

[0080] About 90% of gas stream 30, comprising combustible compounds H2, CO and CH4, is used as fuel stream 56 to produce steam 34 and about 10% of gas stream 30 is used as fuel gas stream 58 to preheat natural gas stream 66 in heater 64.

[0081] Nitrogen and any other inert gases contained in gas streams 56 and 58 are eliminated from the reduction system through the stacks of boiler 36 and heater 64.

[0082] As explained above in this example, the invention provides methods and systems for production of direct reduced iron with carbon content using hydrogen gas with a high efficiency in such use of hydrogen for its chemical value as reducing gas, by heating the reducing gas stream that is fed to the shaft furnace in three steps, where a first step from 38 °C to 132 °C is carried out with energy from a waste gas stream which in current processes is wasted, a second heating step from 132 °C to 338 °C using a co-current or counter-current gas heat exchanger, made possible by said first heating step to avoid fouling and plugging problems caused by deposits of dusts and fine particles entrained by the top gas exiting the shaft furnaceon the heat-exchange surfaces of the gas heat exchanger, when the temperature of the top gas is lower than its dew point, and a third heating step in an electric heater from 338 °C to 930 °C, where electricity consumption is in the order of 561 kWh / ton DRI and the heat transfer efficiency is 95%.

[0083] It will be understood that only some embodiments of the invention have been herein described for illustrative but not limiting purposes for a better comprehension of the spirit and scope thereof, and that many modifications can be made to the invention for adapting it to a particular application without departing of the scope of the invention as defined in the following claims.

Claims

CLAIMS1. Method for producing direct reduced iron with a carbon content of from 0.1 %w / w to 5 %w / w and at a temperature above 600 °C by reduction of iron oxides materials in a shaft furnace having a reduction zone and a carburizing zone, wherein reduction of iron oxides is carried out by• feeding a reducing gas comprising more than 80 vol% hydrogen to said reduction zone,• feeding a carburizing gas to the carburizing zone and causing spent carburizing gas to flow from said carburizing zone to said reduction zone,• withdrawing an exhaust reducing gas stream from said reduction zone,• transferring heat from said exhaust gas stream to the reducing gas in a gas heat exchanger, • cleaning and cooling said exhaust reducing gas stream and separating water therefrom to form a recycled reducing gas stream,• separating hydrogen gas from a portion of said recycled reducing gas stream in a hydrogen separation unit forming a hydrogen-rich gas stream and a hydrogen-lean gas stream,• combining the hydrogen-rich gas stream with said recycled reducing gas stream and a make-up gas stream comprising more than 80% hydrogen to form said reducing gas stream, wherein said method is characterized byraising the temperature of the reducing gas stream, prior to being fed to said reduction zone, in three heating steps:• a first heating step to a temperature in the range from 100 °C to 140 °C with heat provided by combustion of at least a portion of said hydrogen-lean gas stream,• a second heating step to a temperature in the range from 200 °C to 350 °C in said gas heat exchanger, and• a third heating step to a temperature in the range from 800 °C to 1100 °C in a reducing gas heater.

2. A method for producing direct reduced iron according to claim 1, wherein said combustion of at least a portion of said hydrogen-lean gas stream is used to produce steam and heat of said steam is transferred to the reducing gas stream.

3. A method for producing direct reduced iron according to claim 1 , wherein said hydrogen gas separation unit is a pressure-swing adsorption (PSA) unit, a molecular membrane unit, or a cryogenic unit.

4. A method for producing direct reduced iron according to claim 1 , wherein said gas heat exchanger is a co-current gas heat exchanger or a counter-current gas heat exchanger.

5. A method for producing direct reduced iron according to claim 1, wherein said reducing gas heater is an electric heater or a fuel combustion heater.

6. A method for producing direct reduced iron according to claim 5, wherein said fuel combustion heater is operated with at least a portion of said hydrogen-lean gas stream.

7. A method for producing direct reduced iron according to claim 1, wherein said reducing gas heater is a combination of at least one electric heater and at least one fuel combustion heater.

8. A method for producing direct reduced iron according to claim 1, wherein said exhaust reducing gas stream is cleaned and cooled in a cleaning and cooling equipment comprising a washing and cooling tower and a condensed water separation unit.

9. A method for producing direct reduced iron according to claim 1, further comprising feeding said carburizing gas stream to an intermediate zone of said shaft furnace below the reduction zone.

10. A method for producing direct reduced iron according to claim 1, wherein the carbon content of said direct reduced iron is between 0.5 %w / w and 1.5 %w / w.

11. A method for producing direct reduced iron according to claim 1 , wherein said carburizing gas stream comprises natural gas, coke oven gas, methane, LPG or petroleum, syngas, lower alcohols, esters and ethers from fossil or renewable sources.

12. A method for producing direct reduced iron according to claim 1, wherein said carburizing gas stream is heated to a temperature in the range from 300 °C to 400 °C, prior to being introduced into said carburizing zone, using heat of combustion of a portion of said hydrogen-lean gas stream.

13. A method for producing direct reduced iron according to claim 1 , further comprising feeding said direct reduced iron discharged from the shaft furnace into a direct reduced iron cooling vessel, separate from the shaft furnace, where the temperature of said direct reduced iron is brought down by contact with acooling gas stream to bring down the temperature of said direct reduced iron below 100 °C, preferably below 80 °C and more preferably below 50 °C.

14. A method for producing direct reduced iron according to claim 13, wherein the cooling gas is hydrogen.

15. A method for producing direct reduced iron according to claim 1, further comprising selectively deviating the reducing gas stream exiting from the reducing gas heater to an auxiliary cooling device and to said reducing gas circulation loop, bypassing said shaft furnace, if operational problems related to said shaft furnace occur.

16. A system for producing direct reduced iron with a carbon content of from 0.1 %w / w to 5 %w / w and at a temperature above 600 °C by reduction of iron oxides materials in a shaft furnace having a reduction zone and a carburizing zone,• a first inlet to feed a reducing gas comprising more than 80 vol % hydrogen to said reduction zone, • a second inlet to feed a carburizing gas to the carburizing zone wherein the spent carburizing gas flows from said carburizing zone to said reduction zone,• an outlet to withdraw an exhaust reducing gas stream from said reduction zone,• a cleaning and cooling equipment configured to clean and cool said exhaust reducing gas stream and to separate water therefrom to form a recycled reducing gas stream,• a hydrogen separation unit for separating hydrogen gas from a portion of said recycled reducing gas stream forming a hydrogen-rich gas stream and a hydrogen-lean gas stream,• piping and accessories for combining the hydrogen-rich gas stream with said recycled reducing gas stream and a make-up gas stream comprising more than 80% hydrogen to form said reducing gas stream,wherein said system is characterized by comprising:• a first heat-transfer unit to raise the temperature of the reducing gas stream in the range from 100 °C to 140 °C with heat provided by combustion of a portion of said hydrogen-lean gas stream, • a second heat-transfer unit comprising a gas heat exchanger to raise the temperature of the reducing gas stream in the range from 200 °C to 350 °C with heat provided by exhaust reducing gas stream, and• a third heat-transfer unit to raise the temperature of the reducing gas stream in the range from 800 °C to 1100 °C, wherein said three heat-transfer units are located upstream of said reduction zone.

17. A system for producing direct reduced iron according to claim 16, wherein said first heat-transfer unit comprises a boiler to produce steam utilizing the heat of combustion of a portion of said hydrogen-lean gas stream and a gas heat exchanger to transfer the heat of said steam to the reducing gas stream.

18. A system for producing direct reduced iron according to claim 16, wherein said hydrogen gas separation unit is a pressure-swing adsorption (PSA) unit, a molecular membrane unit, or a cryogenic unit.

19. A system for producing direct reduced iron according to claim 16, wherein said third heat-transfer unit is an electric heater or a fuel combustion heater.

20. A system for producing direct reduced iron according to claim 16, wherein said third heat-transfer unit comprises at least one electric heater and at least one fuel combustion heater.

21. A system for producing direct reduced iron according to claim 16, wherein said cleaning and cooling equipment comprises a washing and cooling tower and a condensed water separation unit.

22. A system for producing direct reduced iron according to claim 16, further comprising piping connecting a source of a carburizing gas and the carburizing zone and / or an intermediate zone of said shaft furnace below the reduction zone to produce direct reduced iron with carbon.

23. A system for producing direct reduced iron according to claim 16, wherein the carbon content of said direct reduced iron is between 0.5 and 1.5 %w / w.

24. A system for producing direct reduced iron according to claim 16, wherein said carburizing gas is selected form the group consisting of natural gas, coke oven gas, methane, LPG or petroleum, syngas, lower alcohols, esters and ethers from fossil or renewable sources.

25. A system for producing direct reduced iron according to claim 16, further comprising a heater for heating said carburizing gas to a temperature in the range from 300 °C to 400 °C using heat of combustion of at least a portion of said hydrogen-lean gas stream.

26. A system for producing direct reduced iron according to claim 16, further comprising piping connecting said reducing gas heater with a reducing gas cleaning and cooling equipment for selectively deviating the reducing gas stream exiting from said heater bypassing said shaft furnace, if operational problems related to said shaft furnace occur.

27. A system for producing direct reduced iron according to claim 16, further comprising a direct reduced iron cooling vessel separate from said shaft furnace, where the temperature of said direct reduced iron discharged from the shaft furnace is brought down by contact with a cooling gas stream to bring down the temperature of said direct reduced iron below 100 °C, preferably below 80 °C and more preferably below 50 °C.

28. A system for producing direct reduced iron according to claim 27, wherein said cooling gas is a hydrogen gas.

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