Recycling of hydrogen gas in direct reduction processes

Abstract

A system for producing sponge iron, the system comprising a direct reduction shaft (201) including a first inlet (202) for introducing iron ore into the shaft (201) and a first outlet (203) for removing sponge iron from the shaft (201), a source of reducing gas (206) connected to the shaft (201) through a gas line (207), a first compressor (208) provided in said gas line (207), and a primary circuit (209) for conducting at least a portion of a top gas therethrough, said primary circuit (209) being connected at one end to the shaft (201) and at another end to said first compressor (208). a primary circuit (209) joined to said gas line (207) downstream of said first compressor (208); a secondary circuit (210) for conducting at least a portion of gas removed from the gas conducted through the primary circuit (209), said secondary circuit (210) connected at one end to the primary circuit (209) and at another end to said gas line (207 upstream of said first compressor (208), said secondary circuit (210) including means (211) therein for reducing the pressure of said portion of gas conducted through the secondary circuit (210); and a first valve (212) for controlling the flow of said portion of gas into the secondary circuit (210).

Description

[Technical field]

[0001] Technical Field The present disclosure relates to a process for producing sponge iron from iron ore. The present disclosure further relates to a system for producing sponge iron. [Background technology]

[0002] Background technology Steel is the world's most important civil engineering and building material. It is difficult to find any object in the modern world that does not contain steel or that does not rely on steel for its manufacture and / or transportation. As such, steel is intricately involved in almost every aspect of our modern lives.

[0003] In 2018, the global production of crude steel was 1.81 billion tons, far more than any other metal. In 2050, it is expected to reach 2.8 billion tons, 50% of which is expected to come from virgin ferrous sources. Steel is also the world's most recycled material, with very high recycling grades due to the metal's ability to be used multiple times after remelting, using electricity as the primary energy source.

[0004] Steel is thus a cornerstone of modern society that will play an even greater role in the future.

[0005] Steel is produced mainly through three routes, namely i) Integrated production using virgin iron ore in a blast furnace (BF) where iron oxide in the ore is reduced by carbon to produce iron. The iron is further processed in the steel plant by blowing oxygen into a basic oxygen furnace (BOF) and then refining to produce steel. This process is also commonly referred to as "oxygen steelmaking". ii) Scrap-based production using recycled steel, which is melted in an electric arc furnace (EAF) using electricity as the primary source of energy. This process is also commonly referred to as "electric steelmaking". iii) Direct reduction production based on virgin iron ore, where virgin iron ore is reduced in a direct reduction (DR) process with carbonaceous reduction gas to produce sponge iron, which is subsequently smelted together with scrap in an EAF to produce steel.

[0006] The term crude iron is used herein to refer to all iron produced for further processing into steel, whether obtained from a blast furnace (i.e., pig iron) or direct reduction shaft (i.e., sponge iron).

[0007] The above mentioned processes have been refined over decades and are approaching the theoretical lowest energy consumption, but there is one fundamental challenge that has yet to be resolved: the reduction of iron ore using carbonaceous reductants produces CO as a by-product. 2 On average, every tonne of steel produced in 2018 produced 1.83 tonnes of CO 2 The steel industry produced the most CO 2 It is one of the industries that emits the most CO 2 Accounts for about 7% of emissions. Excessive CO 2 The occurrence is unavoidable in the steel making process as long as carbonaceous reducing agents are used.

[0008] To address this challenge, the HYBRIT initiative was established. HYBRIT (HYdrogen BReakthrough Ironmaking Technology) is a joint venture between SSAB, LKAB and Vattenfall, partly funded by the Swedish Energy Agency, and is run by the CO 2 The aim is to reduce emissions and decarbonize the steel industry.

[0009] Central to the HYBRIT concept is the production of sponge iron by direct reduction from virgin iron ore. But instead of using a carbonaceous reductant gas such as natural gas, as in current commercial direct reduction processes, HYBRIT proposes to use hydrogen gas as the reductant, and is called hydrogen direct reduction (H-DR). Hydrogen gas can be produced by water electrolysis, using primarily fossil-free and / or renewable primary energy sources, as is the case for example in electricity production in Sweden. Thus, the key step of reducing iron ore is CO2-free, without the need for fossil fuel inputs. 2 Instead, this can be achieved with water as a by-product.

[0010] Prior art uses reducing gas mostly composed of natural gas. Direct reduction plants usually include a shaft where reduction occurs. The shaft has an inlet at the top where iron ore pellets are introduced and an outlet at the bottom where sponge iron is removed from the shaft. There is also at least one inlet at the bottom of the shaft for introducing reducing gas into the shaft and at least one outlet at the top of the shaft for the outlet of top gas. Most of the top gas is composed of unreacted reducing gas, possibly mixed with inert gas used to seal the inlets and outlets for iron ore pellets and sponge iron, respectively. The traditional method of dealing with top gas is flaring the latter.

[0011] However, when hydrogen is used primarily or exclusively as the reducing gas, flaring is a less attractive option from an energy efficiency perspective, since the production of hydrogen gas requires a significant amount of energy compared to natural gas. Furthermore, if the top gas contains nitrogen gas (which is typically used as a seal gas), flaring can result in NO X also be released, which is undesirable from an environmental point of view. Summary of the Invention [Problem to be solved by the invention]

[0012] It is therefore an object of the present invention to provide a process and system for the direct reduction of iron ore to sponge iron using primarily or exclusively hydrogen gas as the reducing gas, and means are provided for efficiently recycling unreacted hydrogen gas exiting the direct reduction shaft as part of the top gas. [Means for solving the problem]

[0013] Summary of the Invention The objects of the present invention are achieved by a process for producing sponge iron from iron ore, the process comprising the steps of: loading iron ore into a direct reduction shaft; introducing a hydrogen-rich reducing gas from a reducing gas source directly into the reduction shaft to reduce the iron ore and produce sponge iron; removing a top gas from the direct reduction shaft, the top gas including unreacted hydrogen gas; conducting at least a portion of the removed top gas into a primary circuit and mixing said portion with reducing gas from a reducing gas source at a point downstream of a first compressor provided in a gas line leading directly from the reducing gas source to the reduction shaft, and introducing the mixture directly into the reduction shaft; The method includes the steps of: removing a portion of the gas conducted therein from the primary circuit; and conducting the portion of the gas through a secondary circuit while reducing the pressure of the portion of the gas; and mixing the portion of the gas with reducing gas from a reducing gas source at a point in the gas line upstream of the first compressor.

[0014] The removal of a portion of the gas into the secondary circuit is usually done in response to the pressure in the primary circuit exceeding a certain level. Hydrogen is not lost or wasted, e.g. as heating fuel, instead most of the bleed-off hydrogen is recovered and reused as reducing gas. This lowers the operating costs of such a process. Moreover, since most of the bleed-off hydrogen is no longer combusted, excess NO X The risk of release of hydrogen is significantly reduced or completely avoided. In other words, the secondary circuit allows the pressure in the primary circuit to be controlled without flaring excess top gas containing expensive hydrogen gas from the system. The secondary circuit acts as a buffer and allows the amount of reducing gas conducted into the gas line from the reducing gas source to be reduced. According to one embodiment, under dry conditions, the reducing gas introduced into the direct reduction shaft contains more than 70 vol.% hydrogen. According to one embodiment, the reducing gas introduced into the shaft contains more than 80 vol.% hydrogen, and according to another embodiment, the reducing gas contains more than 90 vol.% hydrogen.

[0015] When the amount of top gas increases during operation, thereby increasing the pressure in the primary circuit, the excess hydrogen gas in the primary circuit is removed into the secondary circuit. The pressure in the primary circuit is accordingly controlled so as not to be too high relative to the pressure downstream of the first compressor. The excess hydrogen gas in the primary circuit is thus conducted back through the secondary circuit to the reducing gas line, so that venting or flaring the excess hydrogen gas in the primary circuit can be prevented. The reduction of the pressure in the secondary circuit is preferably achieved by means of a suitable valve, such as an expansion valve or a pressure reducing valve. When a pressure reducing valve is applied, power is preferably generated from the movement of the pressure reducing valve and is preferably used to generate hydrogen gas.

[0016] According to one embodiment, the first compressor is the final compressor stage in the gas line, which brings the pressure of the reducing gas from a reducing gas source in the gas line to its final pressure before it enters the direct reduction shaft.

[0017] According to one embodiment, the gas flow rate through the gas line into the direct reduction shaft is measured, and the reduction gas flow from the reduction gas source into the gas line is controlled based on the gas flow rate measured in the gas line. The total reduction gas flow rate through the gas line into the direct reduction shaft depends on the amount of iron ore introduced into the shaft and present in the shaft. If the reduction gas flow rate is too low, complete reduction of the iron ore in the direct reduction shaft is not achieved and the temperature in the shaft drops. If the flow rate is too high, an overpressure appears in the direct reduction shaft. According to one embodiment, the temperature in the shaft is measured, and the direct reduction gas flow rate (including gas from the primary circuit, the secondary circuit, and the gas from the reduction gas source) into the shaft is controlled based thereon. According to one embodiment, the pressure in the direct reduction shaft or in the primary circuit is measured, and the reduction gas flow rate into the direct reduction shaft is controlled based thereon. According to one embodiment, the reduction gas source includes at least one electrolysis device to produce hydrogen gas. According to one embodiment, the output of the electrolysis device is controlled as a means for controlling the reduction gas flow rate based on the temperature and pressure in the direct reduction shaft.

[0018] According to one embodiment, the removal of said portion of gas from the primary circuit into the secondary circuit depends on the gas pressure in the primary circuit.

[0019] According to one embodiment, the process further comprises measuring the gas pressure in the primary circuit and conducting said portion of gas from the primary circuit into the secondary circuit in response to the measured pressure being equal to or greater than a first predetermined level. A pressure sensor, a controllable valve and a control unit for controlling the controllable valve based on information from the pressure sensor are thus used. In an alternative embodiment, a relief valve is used to bleed off said portion of the top gas into the secondary circuit in response to the pressure in the primary circuit exceeding the first predetermined level. A relief valve may be provided for permanent bleed-off of the top gas into the secondary circuit, independent of the pressure in the primary circuit.

[0020] According to one embodiment, the pressure in the primary circuit is regulated by removal of said part of the gas to the secondary circuit so as not to exceed said predetermined first level. As soon as the pressure level reaches said predetermined level, the control valve by which the gas flow from within the primary circuit into the secondary circuit is controlled is opened to such an extent that the pressure in the primary circuit does not increase further.

[0021] According to one embodiment, the primary circuit includes a second compressor provided downstream of a point along the primary circuit where said portion of the gas is removed to the secondary circuit, and said measurement of gas pressure is performed upstream of said second compressor. The second compressor is necessary to increase the gas pressure to a level above that downstream of the first compressor so that gas in the primary circuit can flow into the gas line and mix with the reducing gas in said gas line.

[0022] According to one embodiment, the gas pressure in the secondary circuit is reduced to a predetermined second level above the gas pressure level in the gas line upstream of said first compressor. The predetermined second level should be slightly higher than the pressure in the gas line upstream of the first compressor. An expansion valve or a pressure reducing valve may be used to reduce the pressure in the secondary circuit. According to one embodiment, said means are a pressure reducing valve, which includes a turbine and a means for converting the generated motion of the turbine into electrical power. For the further need to reduce the pressure in the secondary circuit, a vent valve may be provided in the secondary circuit. According to one embodiment, such a vent valve is provided upstream of the expansion valve or pressure reducing valve used to reduce the pressure and upstream of a control valve controlling the gas flow from the primary circuit into the secondary circuit. The vent valve may be a relief valve or an actuatable valve controlled by a control unit.

[0023] According to one embodiment, the top gas is subjected to a gas treatment step at a point along the first primary circuit between the point where the top gas is removed from the direct reduction shaft and the point where said portion of the gas is conducted into the secondary circuit.

[0024] According to one embodiment, the treatment step includes separation of inert gases from the portion of the top gas conducted through the primary circuit. The separation unit used for separation may be a cryogenic separation unit, a membrane separation unit, a pressure swing adsorption unit, or an amine CO 2 It may be a scrubber. Many well-established gas separation means may be suitable for separating hydrogen from inert gases (e.g. nitrogen and / or carbon dioxide). For example, cryogenic separation may be appropriate due to the large difference in boiling points between nitrogen (-195.8°C) and hydrogen (-252.9°C).

[0025] According to one embodiment, the treating step comprises separating water from the portion of the top gas conducted through the primary circuit. Preferably, the treating step also comprises removing dust from the top gas.

[0026] According to one embodiment, the treating step includes reducing the temperature of the top gas in a heat exchanger and using the heat from the top gas to heat another gas used in the process.

[0027] According to one embodiment, said other gas is a reducing gas which is introduced directly into the reduction shaft via said gas line.

[0028] The object of the present invention is also achieved by a system for producing sponge iron, the system comprising: A direct reduction shaft, a first inlet for introducing iron ore into the shaft; a first outlet for removing sponge iron from the shaft; a second inlet for introducing a reducing gas into the shaft; a direct reduction shaft including a second outlet for removing top gas from the shaft; a reducing gas source connected to the reducing gas inlet through a gas line; a first compressor provided in the gas line; a primary circuit for conducting at least a portion of the top gas therethrough, said primary circuit being connected at one end to the second outlet and at another end to the gas line downstream of the first compressor; a secondary circuit for conducting at least a portion of gas removed from gas conducted through the primary circuit, said secondary circuit being connected at one end to the primary circuit and at another end to said gas line upstream of said first compressor, said secondary circuit including means therein for reducing the pressure of said portion of gas conducted through the secondary circuit; and a first valve for controlling the flow of said portion of the gas into the secondary circuit.

[0029] According to one embodiment, the means for reducing the pressure comprises an expansion valve or a pressure reducing valve. According to one embodiment, said means is a pressure reducing valve, which comprises a turbine and means for converting the motion generated by the turbine into electrical power.

[0030] According to one embodiment, the system includes a controller for controlling the flow of reducing gas from a reducing gas source into the gas line based on the gas flow rate in the gas line. The measured gas flow rate in the gas line is the sum of reducing gas from the reducing gas source (which may also be referred to as make-up gas) and gas added thereto from the primary and secondary circuits. Thus, the measurement may consist of a single measurement downstream of the point where the primary circuit is connected to the gas line, or a combination of gas flow measurements in the gas line, the primary and secondary circuits.

[0031] According to one embodiment, the control device includes a second valve for controlling the flow of reducing gas from a reducing gas source into the gas line, a gas flow meter for measuring gas flow through the gas line, and a control unit configured to control the second valve based on an input from the gas flow meter.

[0032] According to one embodiment, the first valve is configured to open to pass gas into the secondary circuit in response to gas pressure in the primary circuit exceeding a predetermined level.

[0033] According to one embodiment, the first valve is a controllable valve, and the system further comprises a pressure sensor disposed in the primary circuit and a control unit configured to control the controllable first valve based on an input received from the pressure sensor.

[0034] According to one embodiment, the primary circuit includes a second compressor provided downstream of a point along the primary circuit where the secondary circuit is connected to the primary circuit, and the pressure sensor is positioned upstream of said second compressor.

[0035] According to one embodiment, the primary circuit includes a device for treating top gas, said device including a device for separating inert gas from said portion of the top gas conducted through the primary circuit.

[0036] According to one embodiment, the primary circuit comprises a device for treating the top gas, said device comprising a device for separating water from said portion of the top gas conducted through the primary circuit. The device for treating the top gas preferably also comprises a device for removing top gas from the top gas.

[0037] According to one embodiment, the primary circuit comprises a device for treating the top gas, said device comprising a heat exchanger.

[0038] According to one embodiment, a heat exchanger is also connected to said gas line and configured to transfer heat from the top gas directly to the reducing gas introduced into the reduction shaft.

[0039] According to one embodiment, the reducing gas source comprises a water electrolysis unit.

[0040] Additional objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the following detailed description.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, together with further objects and advantages thereof, the detailed description set forth below should be read in conjunction with the accompanying drawings, in which like reference characters indicate like items in the various views, and in which: [Brief description of the drawings]

[0042] [Figure 1] A schematic diagram of the iron ore-based steelmaking value chain based on the HYBRIT concept. [Diagram 2] 1 illustrates generally an exemplary embodiment of a system suitable for carrying out the processes as disclosed herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Detailed Description definition The reducing gas is a gas capable of reducing iron ore to metallic iron. The reducing components in conventional direct reduction processes are typically hydrogen and carbon monoxide, whereas in the disclosed process the reducing component is primarily or exclusively hydrogen. The reducing gas is introduced into the direct reduction shaft at a point below the iron ore inlet and flows upwardly against the moving bed of iron ore to reduce the iron ore.

[0044] Top gas is a process gas that is removed from the top of the direct reduction shaft near the inlet of the iron ore. Top gas typically comprises a mixture of partly spent reducing gas, including oxidation products of the reducing components (e.g., HO), and inert components that have been introduced into the process gas, such as, for example, seal gas. After treatment, the top gas may be recycled back to the direct reduction shaft as a component of the reducing gas.

[0045] To prevent the build-up of inert components in the carburization process gas, a bleed-off stream removed from the spent carburization gas is called the carburization bleed-off stream.

[0046] The gas from the reducing gas source may be referred to as make-up gas. In the context of this application, make-up gas is added to the recycled top gas before being reintroduced into the direct reduction shaft. Thus, the reducing gas typically includes make-up gas together with the recycled top gas.

[0047] The seal gas is the gas that enters the direct reduction shaft from the ore charger at the inlet of the direct reduction (DR) shaft. The outlet end of the direct reduction shaft may also be sealed using seal gas, so that the seal gas may enter the DR shaft from the eductor at the outlet of the direct reduction shaft. The seal gas is typically an inert gas to avoid explosive gas mixtures from forming at the inlet and outlet of the shaft. An inert gas is a gas that does not potentially form flammable or explosive mixtures with either air or the process gas, i.e., a gas that may not act as an oxidizer or fuel in a combustion reaction under conditions prevailing in the process. The seal gas may consist essentially of nitrogen and / or carbon dioxide. It is noted that although carbon dioxide is referred to herein as an inert gas, it may react with hydrogen in a water-gas shift reaction to provide carbon monoxide and water vapor under conditions prevailing in the system.

[0048] reduction The direct reduction shaft may be of any type commonly known in the art. By shaft is meant a solid-gas countercurrent moving bed reactor, whereby the iron ore charge is introduced at an inlet at the top of the reactor and falls by gravity towards an outlet located at the bottom of the reactor. The reducing gas is introduced at a point lower than the inlet of the reactor and flows upwards against the moving bed of ore to reduce the ore to metallized iron. The reduction is typically carried out at a temperature of about 900°C to about 1100°C. The required temperature is typically maintained by preheating the process gases introduced into the reactor, using a preheater, for example an electric preheater. Further heating of the gases may be obtained after exiting the preheater and before introduction into the reactor by exothermic partial oxidation of the gases with oxygen or air. The reduction may be carried out in the DR shaft at a pressure of about 1 bar to about 10 bar, preferably about 3 bar to about 8 bar. The reactor may have a cooling and discharge cone located at the bottom to allow the sponge iron to cool before being discharged from the outlet.

[0049] The iron ore charge typically consists primarily of iron ore pellets, although some lump iron ore may also be introduced. The iron ore pellets typically contain mostly hematite along with further additives or impurities such as gangue, fluxes and binders. However, the pellets may also contain some other metals and other ores, such as magnetite. Iron ore pellets designated for direct reduction processes are commercially available and such pellets may be used in the present process. Alternatively, the pellets may be specifically adapted for a hydrogen-rich reduction step such as in the present process.

[0050] The reducing gas is hydrogen-rich. By reducing gas is meant the sum of fresh make-up gas and the recycled portion of the top gas introduced into the direct reduction shaft. By hydrogen-rich is meant that the reducing gas entering the direct reduction shaft may be composed of more than 70 vol.% hydrogen gas, for example more than 80 vol.% hydrogen gas, or more than 90 vol.% hydrogen gas (vol.% determined at standard conditions of 1 atm and 0° C.). Preferably, the reduction is carried out as a separate stage, i.e. no carburization is carried out at all, or if carburization is carried out, it is carried out separately from the reduction, i.e. in a separate reactor or in a separate individual area of ​​the direct reduction shaft. This greatly simplifies the treatment of the top gas, since it avoids the need to remove carbonaceous components and the expenses associated with such removal. In such a case, the make-up gas may consist essentially of hydrogen gas or may consist of hydrogen gas. It should be noted that even if the make-up gas is exclusively hydrogen, a certain amount of carbon-containing gas may be present in the reducing gas. For example, if the sponge iron outlet of the direct reduction shaft is connected to the inlet of a carburizing reactor, a relatively small amount of carbon-containing gas may inadvertently seep from the carburizing reactor into the direct reduction shaft. As another example, carbonates present in the iron ore pellets may volatilize and produce CO in the top gas of the DR shaft. 2 may appear as large amounts of CO 2 may be recycled back into the DR shaft. Due to the predominance of hydrogen gas in the reducing gas circuit, any CO present 2 can be converted to CO via the reverse water-gas shift reaction.

[0051] In some cases, it may be desirable to obtain some degree of carburization in conjunction with performing the reduction as a single step. In such cases, the reducing gas may contain up to about 30 vol.% carbon-containing gas, such as up to about 20 vol.%, or up to about 10 vol.% (determined at standard conditions of 1 atm and 0° C.). Suitable carbon-containing gases are disclosed below as carburizing gases.

[0052] The hydrogen gas may preferably be obtained at least in part by electrolysis of water. The electrolysis of water may be performed using renewable energy, which in turn may provide the reducing gas from a renewable source. The electrolytic hydrogen may be transported by conduit directly from the electrolyzer to the DR shaft, or the hydrogen may be stored as it is produced and transported to the DR shaft as needed.

[0053] As it leaves the direct reduction shaft, the top gas typically contains unreacted hydrogen, water (oxidation products of hydrogen), and inert gases. If carburization is performed along with reduction, the top gas may also contain some carbonaceous components, e.g., methane, carbon monoxide, and carbon dioxide. As it leaves the direct reduction shaft, the top gas may first be conditioned, e.g., dedusted to remove entrained solids, and / or heat exchanged to cool the top gas and heat the reduction gas. During heat exchange, water may be condensed from the top gas. Preferably, the top gas at this stage consists essentially of hydrogen, inert gases, and residual water. However, if carbonaceous components are present in the top gas, such carbonaceous components may also be condensed from the top gas, e.g., reformed and / or CO 2 It may be removed by absorption.

[0054] Sponge iron The sponge iron product of the processes described herein is typically referred to as direct reduced iron (DRI). Depending on the process parameters, it may be provided as hot (HDRI) or cold (CDRI). Cold DRI may also be known as type (B) DRI. DRI may be prone to reoxidation and in some cases is pyrophoric. However, there are a number of known means of passivating DRI. One such passivation means commonly used to facilitate overseas shipping of the product is to press the hot DRI into briquettes. Such briquettes are commonly referred to as hot briquetted iron (HBI) and may also be known as type (A) DRI.

[0055] The sponge iron product obtained by the processes herein may be essentially fully metallized sponge iron, i.e., sponge iron having a degree of reduction (DoR) of greater than about 90%, such as greater than about 94% or greater than about 96%. The degree of reduction is defined as the amount of oxygen removed from the iron oxide, expressed as a percentage of the initial amount of oxygen present in the iron oxide. Due to reaction kinetics, it is often commercially undesirable to obtain sponge iron with a DoR of greater than about 96%, although such sponge iron may be produced if desired.

[0056] If carburization is performed, sponge iron having any desired carbon content may be produced by the processes described herein from about 0 to about 7% by weight. However, it is typically desirable for the sponge iron to have a carbon content of about 0.5 to about 5% by weight carbon, preferably about 1 to about 4% by weight, for example about 3% by weight, for further processing, although this may depend on the ratio of sponge iron to scrap used in the subsequent EAF processing step.

[0057] Embodiment The present invention will now be described in more detail with reference to certain exemplary embodiments and drawings. However, the present invention is not limited to the exemplary embodiments discussed herein and / or shown in the drawings, but may vary within the scope of the appended claims. Furthermore, the drawings should not be considered as drawn to scale, since some features may be exaggerated in order to more clearly illustrate certain features.

[0058] FIG. 1 illustrates a schematic of an iron ore-based steelmaking value chain according to the HYBRIT concept. The iron ore-based steelmaking value chain starts with an iron ore mine 101. After mining, iron ore 103 is concentrated and processed in a pelleting plant 105 to produce iron ore pellets 107. These pellets, together with any lump ore used in the process, are converted to sponge iron 109 by reduction in a direct reduction shaft 111 using hydrogen gas 115 as the main reducing agent and producing water 117a as the main by-product. The sponge iron 109 may optionally be carburized either in the direct reduction shaft 111 or in a separate carburization reactor (not shown). Hydrogen gas 115 is produced by electrolysis 117b of water in an electrolyzer 119 using electricity 121, preferably derived primarily from fossil-free or renewable resources 122. The hydrogen gas 115 may be stored in a hydrogen storage 120 before being introduced into the direct reduction shaft 111. The sponge iron 109 is melted using an electric arc furnace 123, optionally together with a percentage of scrap iron 125 or other iron source, to provide a smelt 127. The smelt 127 undergoes further downstream secondary metallurgical processes 129 to produce steel 131. The entire value chain from ore to steel is intended to be fossil free and with only low or no carbon emissions.

[0059] FIG. 2 illustrates generally an exemplary embodiment of a system suitable for carrying out the processes as disclosed herein.

[0060] The system presented in Figure 2 includes a direct reduction (DR) shaft 201. The DR shaft includes a first inlet 202 for introducing iron ore into the DR shaft and a first outlet 203 for removing sponge iron from the DR shaft. The DR shaft 201 further includes a plurality of second inlets 204 for introducing reducing gas into the shaft and at least one second outlet 205 for removing top gas from the DR shaft. It should be understood that there may be many second inlets 204, but for simplicity, only one is shown in the figure.

[0061] The system further comprises a reducing gas source 206 connected with the reducing gas inlet 204 through a gas line 207. The reducing gas source 206 may comprise a hydrogen production unit, typically comprising a water electrolysis unit. The reducing gas from the reducing gas source may therefore comprise almost only hydrogen gas. The reducing gas from the reducing gas source 206 has a rather low pressure of around 1.25 bar and needs to be compressed before being introduced into the DR shaft 201. The pressure in the DR shaft will be in the range of 8-10 bar during operation of the DR shaft. The system therefore further comprises a first compressor 208 provided in the gas line 207, configured to increase the pressure of the reducing gas to around 8 bar. For simplicity, only one compressor 208 is shown in the drawings. However, it should be understood that said compressor may consist of multiple compressors in line, if considered expedient.

[0062] The system further includes a primary circuit 209 for conducting at least a portion of the top gas therethrough, the primary circuit 209 being connected at one end to the second gas outlet 205 and at another end to the gas line 207 downstream of the first compressor 208.

[0063] A secondary circuit 210 is also provided for conducting at least a portion of the gas removed from the gas conducted through the primary circuit 209. The secondary circuit 210 is connected at one end to the primary circuit 209 and at another end to said gas line 207 upstream of the first compressor 208. The secondary circuit 210 further includes a means 211 therein for reducing the pressure of said portion of the gas conducted through the secondary circuit 210, and a first valve 212 for controlling the flow of said portion of the gas into the secondary circuit 210. In the embodiment shown, the means 211 for reducing the pressure in the secondary circuit 210 includes a pressure reducing valve from which energy is transferred from gas to motion and further to power, which may be recycled in the system, such as for operating an electrolyzer in the hydrogen gas source 206. A vent valve 221 is also provided in the secondary circuit 210, which is preferably a relief valve used to vent gas in case of an emergency, for example if the pressure reducing valve fails and there is elevated pressure in the secondary circuit 210. A further controllable valve (not shown) may also be provided to control the venting of the secondary circuit 210.

[0064] The secondary circuit 210 allows for the pressure in the primary circuit 209 to be controlled without flaring excess top gas containing expensive hydrogen gas from the system. The secondary circuit 210 acts as a buffer and allows for the amount of reducing gas conducted into the gas line 207 from the reducing gas source.

[0065] The system further includes a control device for controlling the flow of reducing gas from the reducing gas source into the gas line 207. If the reducing gas source 206 includes a water electrolysis device, such a control system includes a control unit 215 configured to control the output of the water electrolysis device. If the reducing gas source 206 includes a hydrogen gas reservoir or a hydrogen gas pipeline from which hydrogen gas is taken, the control device includes a second valve 213 for controlling the flow of reducing gas from the reducing gas source 206 into the gas line 207. In either case, the system should include a gas flow meter 214 for measuring the gas flow through the gas line 207, and a control unit 215 configured to control the water electrolysis device or to control said second valve 213 based on an input from the gas flow meter 214. The gas flow meter 214 is located downstream of the point where the primary circuit 209 is connected to the gas line 207. If control is performed by controlling only the output of the water electrolysis device, the second valve 213 may be omitted.

[0066] The control device also includes a temperature sensor 216 for measuring a temperature indicative of the temperature inside or at the outlet of the DR shaft 201. The temperature in the DR shaft indicates how the reduction of the iron ore is progressing. Accordingly, an incomplete reduction due to the absence of reducing gas will cause the temperature inside the DR shaft to decrease, thereby revealing such a deficiency and thus being used as an input to the control unit 215. Based on the temperature input, the control unit 215 is thus configured to control the gas flow rate from the hydrogen gas source into the gas line 207 and to increase the flow rate in response to a temperature below a predetermined level.

[0067] The temperature sensor 216 may be located inside the DR shaft, or for example in the gas outlet 205, in which case the top gas exiting the DR shaft can be inferred to have a temperature indicative of the temperature inside the DR shaft 201.

[0068] The first valve 212 is a controllable valve, and the system further includes a pressure sensor 217 disposed in the primary circuit 209. The control unit 215 is configured to control said controllable first valve 212 based on an input received from the pressure sensor 217. The primary circuit 209 includes a second compressor 218 provided at a point along the primary circuit 209 where the secondary circuit 210 is connected to the primary circuit 209, and the pressure sensor 217 is located upstream of said second compressor 218. The control unit 215 is configured to open the first valve 212 in response to the pressure in the primary circuit 209 exceeding a predetermined level. Alternatively, the first valve may be a relief valve that is set to automatically open when the pressure in the primary circuit 209 exceeds said predetermined level. The means 211 for reducing the gas pressure in the secondary circuit are designed to reduce the pressure to a pressure slightly above the gas pressure in the gas line 207 upstream of the first compressor 208, for example to a pressure of about 1.5 bar.

[0069] The primary circuit 209 further comprises a device 219 for treating the top gas, said device 219 comprising a device (not shown in detail) for separating inert gas from the portion of the top gas conducted through the primary circuit 209. The treatment device 219 also comprises a device (not shown in detail) for separating water and dust from said portion of the top gas conducted through the primary circuit 209. The treatment device 219 also comprises a heat exchanger (not shown in detail) for exchanging heat between the top gas and the reducing gas flowing through the gas line 207. One or more separate heaters 220 for heating the reducing gas in the gas line 207 may also be provided.

[0070] The system described above with reference to Figure 2 can recycle hydrogen gas in the event of pressure buildup in the primary circuit instead of flaring it. The control unit 215 is configured to control the flow of reducing gas from the reducing gas source 206 into the gas line 207 based on input from the disclosed sensors. If the reducing gas source 206 is a water electrolyzer, the control unit 215 may be configured to control the output of the electrolyzer based on input from said sensors and to effectively take advantage of recycling the reducing gas via the secondary circuit 210.

Claims

1. 1. A process for producing sponge iron from iron ore comprising the steps of: loading iron ore into a direct reduction shaft (201); introducing a hydrogen-rich reducing gas from a reducing gas source (206) into the direct reduction shaft (201) to reduce the iron ore and produce sponge iron; removing top gas from the direct reduction shaft (201), the top gas including unreacted hydrogen gas; conducting at least a portion of the removed top gas in a primary circuit (209) and mixing said portion with reducing gas from said reducing gas source (206) at a point downstream of a first compressor (208) provided in a gas line (207) leading from said reducing gas source (206) to said direct reduction shaft (201) and introducing said mixture into said direct reduction shaft (201); removing a portion of the gas conducted therein from the primary circuit (209) and conducting the portion of the gas through a secondary circuit (210) while reducing a pressure of the portion of the gas, and mixing the portion of the gas with reducing gas from the reducing gas source (206) at a point in the gas line (207) upstream of the first compressor (208).

2. 2. The process of claim 1, wherein a gas flow rate through the gas line (207) and into the direct reduction shaft (201) is measured, and a reduction gas flow from the reduction gas source (206) into the gas line (207) is controlled based on the gas flow rate measured in the gas line (207).

3. The process of claim 1 or 2, wherein the removal of the portion of gas from the primary circuit (209) to the secondary circuit (210) is dependent on the gas pressure in the primary circuit (209).

4. 3. The process of claim 1 or 2, comprising the steps of measuring the gas pressure in the primary circuit (209) and, in response to the measured pressure being equal to or greater than a predetermined first level, conducting the portion of gas from the primary circuit (209) into the secondary circuit (210).

5. 5. The process of claim 4, wherein the pressure in the primary circuit (209) is regulated by removal of the portion of gas to the secondary circuit (210) so as not to exceed the predetermined first level.

6. 5. The process of claim 4, wherein the primary circuit (209) includes a second compressor (218) provided downstream of a point along the primary circuit (209) at which the portion of gas is removed to the secondary circuit (210), and the measurement of the gas pressure is performed upstream of the second compressor (218).

7. 3. The process of claim 1 or 2, wherein the gas pressure in the secondary circuit (210) is reduced to a predetermined second level above a gas pressure level in the gas line (207) upstream of the first compressor (208).

8. 3. The process of claim 1 or 2, wherein the top gas is subjected to a gas treatment step at a point along the first primary circuit (209) between the point where the top gas is removed from the direct reduction shaft (201) and the point where the portion of the gas is conducted into the secondary circuit (210).

9. 9. The process of claim 8, wherein the treating step includes separation of inert gas from the portion of the top gas conducted through the primary circuit (209).

10. 9. The process of claim 8, wherein the treating step includes separating water from the portion of the top gas conducted through the primary circuit (209).

11. 9. The process of claim 8, wherein the treating step includes reducing the temperature of the top gas in a heat exchanger and using the heat from the top gas to heat another gas used in the process.

12. 12. The process of claim 11, wherein the other gas is a reducing gas introduced into the direct reduction shaft (201) via the gas line (207).

13. 1. A system for producing sponge iron, comprising: A direct reduction shaft (201), a first inlet (202) for introducing iron ore into said shaft (201); a first outlet (203) for removing sponge iron from said shaft (201); A second inlet (204) for introducing a reducing gas into said shaft (201); a direct reduction shaft (201) including a second outlet (205) for removing top gas from said shaft (201); a reducing gas source (206) connected to the reducing gas inlet (204) through a gas line (207); a first compressor (208) provided in said gas line (207); a primary circuit (209) for conducting at least a portion of the top gas therethrough, the primary circuit (209) being connected at one end to the second outlet (205) and at another end to the gas line (207) downstream of the first compressor (208); a secondary circuit (210) for conducting at least a portion of gas removed from the gas conducted through said primary circuit (209), said secondary circuit (210) being connected at one end to said primary circuit (209) and at another end to said gas line (207) upstream of said first compressor (208), said secondary circuit (210) including means (211) therein for reducing the pressure of said portion of gas conducted through said secondary circuit (210); a first valve (212) for controlling the flow of the portion of gas into the secondary circuit (210).

14. 14. The system of claim 13, further comprising a controller for controlling a flow of reducing gas from the reducing gas source into the gas line based on the gas flow rate in the gas line.

15. 15. The system of claim 13 or 14, wherein the control device includes a second valve (213) for controlling a flow of reducing gas from the reducing gas source (206) into the gas line (207), a gas flow meter (214) for measuring gas flow through the gas line (207), and a control unit (215) configured to control the second valve (213) based on an input from the gas flow meter (214).

16. 15. The system of claim 13 or 14, wherein the first valve (212) is configured to open to pass gas into the secondary circuit (210) in response to the gas pressure in the primary circuit (209) exceeding a predetermined level.

17. 15. The system of claim 13 or 14, wherein the first valve (212) is a controllable valve, the system further comprising a pressure sensor (217) disposed in the primary circuit (209) and a control unit (215) configured to control the controllable first valve (212) based on an input received from the pressure sensor (217).

18. 18. The system of claim 17, wherein the primary circuit (209) includes a second compressor (218) provided downstream of a point along the primary circuit (209) where the secondary circuit (210) is connected to the primary circuit (209), and the pressure sensor (217) is positioned upstream of the second compressor (218).

19. 15. The system of claim 13 or 14, wherein the primary circuit (209) includes a device (219) for treating the top gas, the device (219) including a device for separating an inert gas from the portion of the top gas conducted through the primary circuit (209).

20. 15. The system of claim 13 or 14, wherein the primary circuit (209) includes a device (219) for treating the top gas, the device including a device for separating water from the portion of the top gas conducted through the primary circuit (209).

21. 15. The system of claim 13 or 14, wherein the primary circuit (209) comprises a device (219) for treating the top gas, the device (219) comprising a heat exchanger.

22. 22. The system of claim 21, wherein the heat exchanger is also connected to the gas line (207) and configured to transfer heat from the top gas to the reducing gas introduced into the direct reduction shaft (201).

23. The system of claim 13 or 14, wherein the reducing gas source (206) comprises a water electrolysis unit.

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