Extracted gas recovery in direct reduction processes

Abstract

The present disclosure relates to a process for producing sponge iron from iron ore, comprising the steps of: - charging iron ore into a direct reduction shaft (211), - introducing a hydrogen-rich reducing gas (215) into the direct reduction shaft to reduce the iron ore and produce sponge iron (209), - removing a top gas (216) from the direct reduction shaft, - splitting the top gas into a recycle stream (218) and an extract stream (256), - processing the extract stream through a separation unit (257) to provide a hydrogen-rich off-stream (258) and an inerts-rich off-stream (259), - introducing the recycle stream and the hydrogen-rich off-stream into the direct reduction shaft as constituent parts of the hydrogen-rich reducing gas. The present disclosure further relates to a system for producing sponge iron.

Description

[Technical Field]

[0001] 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] Steel is the world's most important engineering and construction material. It is difficult to find an object in the modern world that does not contain steel or that does not rely on steel for its production and / or transportation. As such, steel is intricately involved in nearly every aspect of our modern lives.

[0003] In 2018, total global production of crude steel was 1.81 billion tonnes, far exceeding any other metal, and is expected to reach 2.8 billion tonnes by 2050, of which 50% is expected to come from virgin iron sources. Steel is also the world's most recycled material at very high reuse grades due to the metal's ability to be used repeatedly after remelting, using electricity as the primary energy source.

[0004] Thus, steel is a foundation of modern society that will play an even more important role in the future.

[0005] Steel is produced mainly through three routes: i) An integrated process that uses virgin iron ore in a blast furnace (BF) to produce iron by reducing the iron oxide in the ore with carbon. The iron is further processed in the steelworks by oxygen blowing in a basic oxygen furnace (BOF) and subsequent refining to produce steel. This process is also commonly referred to as "oxygen steelmaking." ii) Scrap-based production using recycled steel melted in electric arc furnaces (EAFs) that use electricity as the primary energy source. This process is also commonly referred to as "electric steelmaking." iii) Direct reduction production based on virgin iron ore, which is reduced in a direct reduction (DR) process with carbonaceous reducing gases to produce sponge iron, which is subsequently melted together with scrap in an EAF to produce steel.

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

[0007] Although the above processes have been refined over decades and are approaching the theoretical minimum energy consumption, one fundamental problem remains unsolved: the reduction of iron ore using carbonaceous reductants produces CO2 as a by-product. In 2018, an average of 1.83 tons of CO2 was produced per ton of steel produced. The steel industry is one of the most CO2-producing industries, accounting for approximately 7% of global CO2 emissions. As long as carbonaceous reductants are used, excess CO2 generation is unavoidable in the steel production process.

[0008] The HYBRIT initiative was established to address this challenge. HYBRIT, short for HYdrogen BReakthrough Ironmaking Technology, is a joint venture between SSAB, LKAB and Vattenfall, partly funded by the Swedish Energy Agency, and aims to reduce CO2 emissions and decarbonise the steel industry.

[0009] At the heart of the HYBRIT concept is the direct reduction-based production of sponge iron from virgin iron ore. However, instead of using a carbonaceous reductant gas such as natural gas, as in current commercial direct reduction processes, HYBRIT proposes using hydrogen gas as the reductant, termed hydrogen direct reduction (H-DR). Hydrogen gas can be produced, for example, by water electrolysis using primarily fossil-free and / or renewable primary energy sources, as is the case for electricity production in Sweden. Thus, the key step of reducing iron ore can be accomplished without the need for fossil fuels as an input and with water instead of CO2 as a by-product.

[0010] When conducting a direct reduction process, even with the sealing of the direct reduction shaft inlet and / or outlet, it is almost inevitable that inert gases, such as nitrogen, will be introduced into the process gas. Due to their inert nature, inert gases are passively circulated in the process gas, and additional inert gas is always added to the process gas, for example, when iron ore is charged. However, other components of the process gas are typically either consumed by reaction (e.g., H2, CO, CH4) or removed from the circulation (e.g., HO). This means that unless measures are taken, the inert gases will gradually increase in proportion to the process gas, causing the process gas to gradually lose its "reducing strength." To avoid this situation, a portion of the process gas is typically extracted from the process gas circuit and burned in a process gas heater to maintain a suitable balance of inert components in the process gas. Such extraction may also be performed to control the pressure of the process gas circuit.

[0011] WO 2019 / 238720 describes a method for producing carburized sponge iron by hydrogen-based direct reduction. The reducing gas used is extracted as top gas. A first partial amount of the top gas is used as fuel gas to heat the reducing gas and / or carburizing gas. The size of this first partial amount of top gas is adjusted as a function of the content of nitrogen, carbon dioxide, carbon monoxide, and / or methane in the top gas. Summary of the Invention [Problem to be solved by the invention]

[0012] There remains a need for improved means for producing sponge iron using hydrogen-based direct reduction. [Means for solving the problem]

[0013] Summary of the Invention In conventional direct reduction processes, the reducing gas is typically derived from natural gas and the process gas heaters are typically fired with natural gas. Thus, the above-described extraction and combustion of process gas is economically justifiable, as it simply involves replacing a portion of the natural gas used to fire the process gas heaters with extracted gas derived from natural gas.

[0014] However, the present inventors have identified several drawbacks to such a means of ensuring a balanced inert component content in the process gas when the reducing gas is hydrogen-based. One drawback is that hydrogen typically burns at a higher local flame temperature, resulting in excessive NOx production. While controlling NOx emissions is feasible using emission abatement technologies, this adds complexity and expense to the process. Another drawback is that hydrogen is significantly more expensive than traditionally used reducing gases, especially when the hydrogen is produced by water electrolysis from renewable energy sources to avoid CO2 emissions and consumption of fossil fuels. Therefore, simply burning the extraction gas is economically detrimental, even if the resulting heat is utilized to heat the process gas.

[0015] It would be advantageous to achieve a means of overcoming or at least mitigating at least some of the above-listed disadvantages. In particular, it would be desirable to enable a more economically viable sponge iron production process using hydrogen as the reducing gas. In order to better address one or more of these concerns, a process and system are provided having the features defined in the accompanying independent claims.

[0016] According to a first aspect, there is provided a process for producing sponge iron from iron ore according to the accompanying independent claim, the process comprising: - charging iron ore into a direct reduction shaft; - introducing a hydrogen-rich reducing gas into the direct reduction shaft to reduce iron ore and produce sponge iron; - removing top gas from the direct reduction shaft; - splitting the top gas into a recycle stream and a bleed-off stream; - processing the extract stream through a separation unit to provide a hydrogen-enriched off-stream and an inerts-enriched off-stream; - introducing the recycle stream and the hydrogen-enriched offstream into a direct reduction shaft as constituent portions of a hydrogen-rich reducing gas; Includes:

[0017] Such processes, like prior art methods, control the balance of inert components in the process gas by extracting a portion of the top gas. However, because the extracted hydrogen gas is subsequently separated from the inert gas (e.g., nitrogen) in the process, the hydrogen is not lost or wasted as heating fuel, etc. Instead, most of the extracted hydrogen is recovered and reused as a reducing gas. This significantly reduces the operating costs of such processes. Moreover, because most of the extracted hydrogen is no longer burned, the risk of excessive NOx emissions is significantly reduced or completely avoided.

[0018] An alternative solution to the identified shortcomings would be to treat the entire top gas stream to remove inert gases. Such a method would avoid the need to separate the top gas into a recycle stream and an extract stream. However, compared to the process of the present disclosure, such a solution would require the treatment of vast amounts of top gas, resulting in enormous increases in capital and operating costs. The process of the present disclosure achieves the benefits of controlling the balance of inert components in the process gas at significantly lower capital and operating costs compared to methods requiring the treatment of the entire top gas stream.

[0019] The seal gas may be introduced in conjunction with charging the iron ore into the direct reduction shaft, whereby the seal gas may form a constituent portion of the top gas. The seal gas may be nitrogen and / or carbon dioxide. Correspondingly, the inert gas may consist essentially of nitrogen and / or carbon dioxide.

[0020] The separation unit may be a cryogenic separation unit, a membrane separation unit, a pressure swing adsorption unit, or an amine CO scrubber. Several well-established gas separation means may be suitable for separating hydrogen from inert gases (e.g., nitrogen and / or carbon dioxide). For example, due to the large difference in boiling points between nitrogen (-195.8°C) and hydrogen (-252.9°C), cryogenic separation may be a preferred technique from a technical and / or economic standpoint.

[0021] The hydrogen-rich reducing gas may include hydrogen gas obtained by water electrolysis. For example, the process may further include introducing make-up gas directly into the reduction shaft as a constituent part of the hydrogen-rich reducing gas, where the make-up gas comprises, consists essentially of, or consists of hydrogen gas obtained by water electrolysis. The use of electrolytic hydrogen ensures that fewer fossil fuels are required to produce sponge iron. However, electrolytic hydrogen is currently more expensive than reducing gas derived from fossil sources. That is, the benefits of the process of the present disclosure are always more apparent when the reducing gas is derived at least in part from water electrolysis.

[0022] The process may further include introducing makeup gas directly into the reduction shaft as a constituent portion of the hydrogen-rich reducing gas, the makeup gas being essentially free of carbonaceous components. By ensuring that the makeup gas is essentially carbon-free, the resulting top gas may also be essentially carbon-free, and it is assumed that carbonaceous gases are not introduced into the top gas by other means, such as through the seal gas or through the leakage of carburizing gases into the top gas. If the top gas is essentially carbon-free, there is no need to treat the top gas to remove carbonaceous components, such as by amine absorption of carbon dioxide, and therefore the process and process equipment may be significantly simplified. Even if the top gas contains only a small portion of carbonaceous components, such as from carbon dioxide used as a seal gas, this may be controlled using extraction, still potentially avoiding the need to treat the entire top gas stream to remove carbonaceous components.

[0023] The process is - carburizing sponge iron using carburizing gas in a separate carburizing reactor or zone, resulting in carburized sponge iron and spent carburizing gas. It may further include:

[0024] Iron produced today in commercial blast furnaces or via direct reduction routes typically contains significant amounts of carbon (typically up to 5% by weight) due to carbon uptake during the reduction of iron ore. Besides its use as a reducing agent, carbon plays an additional important role in the steelmaking process, and it would be desirable to provide carburization of sponge iron for several reasons. Its presence in sponge iron lowers the melting point of the iron. During subsequent processing of the sponge iron, the exothermic dissociation of iron carbide and oxidation of carbon to CO provide heat for the process. Gas evolution in electric arc furnaces due to this CO production helps insulate the iron melt and provides a foamy slag that helps reduce wear on EAF electrodes. For at least these reasons, the presence of carbon in sponge iron can help reduce energy consumption during processing into steel.

[0025] By carburizing the sponge iron in a separate carburizing reactor or zone, one may still obtain carburized sponge iron while avoiding the presence of carbonaceous components in the top gas, which may significantly simplify process and system design by eliminating the need to treat the top gas to remove carbonaceous components, such as by amine absorption of carbon dioxide.

[0026] The process is - dividing the spent carburizing gas into a carburizing recycle stream and a carburizing extraction stream; - removing carbonaceous components from the carburized extract stream; - treating the carburized extract stream in a separation unit; The separation unit may be the same separation unit in which the top gas extract stream is processed, or it may be a different separation unit.

[0027] Extraction is also typically required in carburizing gas circuits to prevent the accumulation of inert gases. Moreover, performing separate reduction and carburizing steps ensures that most of the hydrogen present in the carburizing gas is not consumed, since little or no reduction occurs during the carburizing step. It is therefore desirable to recover this hydrogen and utilize it in the reduction step. Using the process steps described above, the hydrogen present in carburizing extraction can be recovered in the same way as in top gas extraction; that is, less electrolytic hydrogen may be required during the reduction step.

[0028] The inert-rich offstream can be processed in an auxiliary separation unit to provide an auxiliary hydrogen-enriched offstream. The auxiliary separation unit can be a membrane separation unit. For example, the (primary) separation unit can be a cryogenic separation unit, and the auxiliary separation unit can be a membrane separation unit. Due to process and economic constraints, the inert-rich offstream after the initial separation can still contain a significant amount of hydrogen, for example, up to 30% by volume (volume % determined at reference conditions of 1 atm and 0°C). By performing an auxiliary separation step, this hydrogen loss can be avoided. Membrane separation is particularly suitable for the auxiliary separation step because such a technique can be particularly effective for separating gas mixtures in which nitrogen predominates over hydrogen, such as those found in the inert-rich offstream.

[0029] According to a second aspect, there is provided a system for producing sponge iron according to the accompanying independent claim, the system comprising: a direct reduction shaft including a reducing gas inlet and a top gas outlet; a hydrogen gas source disposed in fluid communication with the reducing gas inlet; - an extraction valve disposed in fluid communication with the top gas outlet and configured to divide the top gas between the recirculation stream outlet and the extraction stream outlet; a separation unit disposed in fluid communication with the extract stream outlet and configured to separate the extract stream into a hydrogen-enriched stream and an inerts-enriched stream; Includes:

[0030] The advantages and features of the second aspect are to a large extent similar to those described above in relation to the first aspect, and the embodiments given in relation to the first aspect are generally applicable to the second aspect.

[0031] The separation unit may be a cryogenic separation unit, a membrane separation unit, a pressure swing adsorption unit, or an amine CO2 scrubber. Several well-established gas separation means may be suitable for separating hydrogen from inert gases (e.g., nitrogen and / or carbon dioxide). For example, due to the large difference in boiling points between nitrogen (-195.8°C) and hydrogen (-252.9°C), cryogenic separation may be a preferred technique from a technical and / or economic standpoint.

[0032] The hydrogen gas source can be a water electrolyzer unit. The use of electrolytic hydrogen ensures that less fossil fuels are required for the production of sponge iron. However, electrolytic hydrogen is currently more expensive than reducing gas derived from fossil sources. That is, the benefits of the disclosed process are always more apparent when the reducing gas is derived at least in part from water electrolysis.

[0033] The direct reduction shaft may include a reduction zone and a carburization zone. The direct reduction shaft may be positioned to prevent passage of gas from the carburization zone to the reduction zone. Thus, reduction and carburization may be carried out as separate, distinct stages in a common reactor having the reduction and carburization circuits.

[0034] Alternatively or additionally, the system may include a carburizing reactor, which allows for the carburizing to occur in a separate reactor, reducing the risk of accidental introduction of carburizing gases into the process gases of the reduction stage.

[0035] The carburizing zone or reactor may include a carburizing gas inlet and a spent carburizing gas outlet. In such a case, the system may include: - a carburizing gas source disposed in fluid communication with the carburizing gas inlet; - a carburizing extraction valve disposed in fluid communication with the spent carburizing gas outlet and configured to divide the spent carburizing gas between the carburizing recycle stream outlet and the carburizing extraction stream outlet; It may further include:

[0036] The carburized extract stream outlet may be placed in fluid connection with a separation unit, which may be the same separation unit in which the top gas extract stream is processed, or it may be a different separation unit.

[0037] Such an arrangement allows for the recovery of hydrogen from the carburizing extraction gases as described above, thus assisting in reducing consumption of electrolytic hydrogen.

[0038] The system may include one or more carbon separation units, and the carburizing extract stream outlet is disposed in fluid communication with the separation unit(s) via the one or more carbon separation units. The carbon separation unit removes unreacted carbonaceous components, such as hydrocarbons, CO, and / or CO2, from the carburizing extract stream before conveying the stream to the separation unit for separation into H2 and N2. The carbon separation unit may include, for example, a hydrocarbon separation unit (e.g., a cryogenic hydrocarbon separation unit) and / or a CO2 separation unit. However, according to some embodiments, the system does not include a CO2 separation unit. For example, if the sponge iron is sufficiently reduced before the start of the carburizing step and the carburizing gas does not contain significant amounts of oxygenic components, the carburizing extract gas will not contain significant amounts of CO2, and a CO2 separation unit may not be necessary.

[0039] The system can include an auxiliary separation unit disposed in fluid communication with the inert component-enriched stream outlet of the separation unit. The auxiliary separation unit can be a membrane separation unit. For example, the (primary) separation unit can be a membrane separation unit or a cryogenic separation unit, and the auxiliary separation unit can be a membrane separation 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 fuller understanding of the present invention, and 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 represent like items in the various drawings. [Brief explanation of the drawings]

[0042] [Figure 1] The value chain of iron ore-based steelmaking related to the Hybrit concept is illustrated schematically. [Figure 2a] 1 illustrates generally one exemplary embodiment of a system suitable for carrying out the processes disclosed herein. [Figure 2b] 1 schematically illustrates another exemplary embodiment of a system suitable for carrying out the processes disclosed herein. [Figure 2c] 1 schematically illustrates a further exemplary embodiment of a system suitable for carrying out the processes disclosed herein. [Figure 3] 1 is a flow chart that generally illustrates certain exemplary embodiments of the processes disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0043] Detailed Description The present invention is based on the insight by the inventors that, for various reasons, prior art means of controlling the inert gas content and / or pressure in the process gas circuit by burning a portion of the top gas are undesirable when using hydrogen as a reducing gas. Compared to the combustion of conventional reducing gases (e.g., synthesis gas), the combustion of hydrogen results in the production of larger amounts of NOx. Moreover, because hydrogen is typically more expensive to produce compared to synthesis gas, the combustion of hydrogen is economically detrimental, even if it generates process heat. This is particularly true when the hydrogen-rich reducing gas is produced by relatively expensive means, such as water electrolysis.

[0044] The process of the present disclosure avoids such drawbacks by separating the extract gas into a hydrogen-enriched fraction and an inerts-enriched fraction, the latter of which is then recycled directly back to the reduction shaft.

[0045] definition The term process gas is used herein to refer to the gas mixture in a direct reduction process, regardless of the stage of the process. That is, process gas means the gas that is introduced into, passes through, leaves, and is recycled back to the direct reduction shaft. The process gas may be a reducing process gas if used in a reduction stage, or a carburizing process gas if the process requires a carburizing stage. More specific terms are used to refer to the process gas at various points in the process, or to refer to component gases that are added to the process gas to form part of the process gas.

[0046] 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, while the reducing component in the disclosed process is primarily or exclusively hydrogen. The reducing gas is introduced at a point below the inlet of the direct reduction shaft and flows upwardly against the moving bed of iron ore to reduce the ore.

[0047] Top gas is process gas removed from the top of the direct reduction shaft near the ore inlet. Top gas typically comprises a mixture of partially spent reducing gas, including oxidation products of the reducing components (e.g., HO), and inert components, such as seal gas, that were introduced into the process gas. After processing, the top gas can be recycled and returned to the direct reduction shaft as a component of the reducing gas.

[0048] The extraction is a stream separated from the top gas to control the content of the process gas, especially to prevent the accumulation of inert components in the process gas. Extraction can also be used to control the prevailing pressure in direct reduction systems.

[0049] Carburizing gas is a gas used in any carburizing stage to provide carburized (carbon-containing) sponge iron. Carburizing gas can be any gas known or expected in the art to provide carburization. Gas in this context refers to the gaseous substance at high temperatures prevailing in the carburizing reactor, but it can be liquid or solid at room temperature. Suitable carburizing gases include hydrocarbons such as methane, natural gas, LPG, or petroleum, or other carbonaceous materials such as synthesis gas, lower (C1-C6) alcohols, esters, and ethers. Carburizing gas can be of fossil origin, but is preferably derived in part or entirely from renewable sources to reduce net CO2 emissions.

[0050] The extract stream that is removed from the spent carburizing gas to prevent the buildup of inert components in the carburizing process gas is referred to as the carburizing extract stream.

[0051] Make-up gas is fresh gas added to the process gas to maintain reducing capacity. Typically, make-up gas is added to the recycled top gas before reintroduction into the direct reduction shaft. Therefore, the reducing gas typically includes the make-up gas together with the recycled top gas. The make-up gas and the recycled top gas can be mixed together before introduction into the direct reduction shaft, or they can be introduced into the shaft separately and mixed.

[0052] The seal gas is the gas that enters the direct reduction shaft from the ore charging equipment at the inlet of the direct reduction shaft. The outlet end of the direct reduction shaft can also be sealed with seal gas, so that seal gas can enter the DR shaft from the discharge equipment at the outlet of the direct reduction shaft. The seal gas is typically an inert gas to avoid the formation of explosive gas mixtures at the shaft inlet and outlet. An inert gas is a gas that does not form potentially flammable or explosive mixtures with either air or the process gas, i.e., a gas that cannot act as either an oxidizer or a fuel in a combustion reaction under the conditions prevailing in the process. The seal gas can consist essentially of nitrogen and / or carbon dioxide. Therefore, the inert gas removed from the extract can also consist essentially of nitrogen and / or carbon dioxide. While carbon dioxide is referred to herein as an inert gas, it should be noted that it can react with hydrogen in a water-gas shift reaction to provide carbon monoxide and steam under the conditions prevailing in the system.

[0053] reduction The direct reduction shaft can be of any type commonly known in the art. By shaft is meant a solid-gas countercurrent moving-bed reactor, in which the iron ore charge is introduced into an inlet at the top of the reactor and gravity-flows downward toward an outlet located at the bottom of the reactor. Reducing gas is introduced into the reactor at a point lower than the inlet and flows upward, countercurrent to the moving bed of ore, to reduce the ore to metallic iron. Reduction is typically carried out at temperatures between about 900°C and about 1100°C. The required temperature is typically maintained by preheating the process gases introduced into the reactor using a preheater, such as an electric preheater. Further heating of the gases can be achieved by exothermic partial oxidation of the gases with oxygen or air after leaving the preheater and before introduction into the reactor. Reduction can be carried out in the DR shaft at pressures between about 1 bar and about 10 bar, preferably between about 3 bar and about 8 bar. The reactor may have a cooling and discharge cone located at the bottom to allow cooling of the sponge iron before discharge from the outlet.

[0054] 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 primarily hematite along with further additives or impurities such as gangue, flux, binders, etc. However, the pellets may contain some other metals and other ores such as magnetite. Iron ore pellets specifically designed for direct reduction processes are commercially available, and such pellets may be used in the present process. Alternatively, the pellets may be specially adapted for a hydrogen-rich reduction step, as in the present process.

[0055] The reducing gas is hydrogen-rich. By reducing gas, we mean the sum of fresh makeup gas and recycled process gas introduced into the direct reduction shaft. By hydrogen-rich, we mean that the reducing gas entering the direct reduction shaft can contain or consist of more than 70% by volume of hydrogen gas, for example, more than 80% by volume of hydrogen gas, or more than 90% by volume of hydrogen gas (volume percentages are determined at reference conditions of 1 atm and 0°C). Preferably, the reduction is carried out as a separate stage. That is, carburization is not carried out, or if carburization is to be carried out, it is carried out separately from the reduction, i.e., in a separate reactor or in a separate, distinct zone of the direct reduction shaft. This significantly simplifies top gas processing, since it avoids the need for removal of carbonaceous components and the costs associated with such removal. In such cases, the makeup gas can consist essentially of or consist of hydrogen gas. Note that even if the makeup gas is exclusively hydrogen, some amount of carbon-containing gas may be present in the reducing gas. For example, if the outlet of the direct reduction shaft is coupled to the inlet of the carburization reactor, a relatively small amount of carbon-containing gas may accidentally seep from the carburization reactor into the direct reduction shaft. As another example, carbonates present in iron ore pellets may volatilize and appear as CO in the top gas of the DR shaft, resulting in a large amount of CO that can be recycled back to the DR shaft. Due to the predominance of hydrogen gas in the reducing gas circuit, some of the CO present may be converted to CO by the reverse water-gas shift reaction.

[0056] In some cases, it may be desirable to achieve some degree of carburization in conjunction with carrying out reduction as a single step. In such cases, the reducing gas may contain up to about 30% by volume, e.g., up to about 20% by volume, or up to about 10% by volume, of a carbon-containing gas (determined at reference conditions of 1 atm and 0°C). Suitable carbon-containing gases are disclosed below as carburizing gases.

[0057] The hydrogen gas may preferably be obtained at least in part by electrolysis of water. If water electrolysis is carried out using renewable energy, this allows for the provision of reducing gas from a renewable source. The electrolytic hydrogen can be conveyed directly from the electrolyzer to the DR shaft by a conduit, or the hydrogen can be stored as it is produced and conveyed to the DR shaft as needed.

[0058] As the top gas exits the direct reduction shaft, it will typically contain unreacted hydrogen, water (oxidation products of hydrogen), and inert gases. If carburization is performed together with reduction, the top gas may contain some carbonaceous components, such as methane, carbon monoxide, and carbon dioxide. As the top gas exits the direct reduction shaft, it may first be subjected to conditioning, such as dedusting to remove entrained solids and / or heat exchange to cool the top gas and heat the reducing gas. During heat exchange, water may be condensed from the top gas. Preferably, the top gas at this stage will consist essentially of hydrogen, inert gas, and residual water. However, if carbonaceous components are present in the top gas, such carbonaceous components may also be removed from the top gas, for example, by reforming and / or CO2 absorption.

[0059] After appropriate conditioning, the top gas is split into a recycle stream and an extract stream through an extract valve. The exact ratio of the extract stream to the recycle stream can vary, depending, for example, on the proportion of inert gas in the top gas, and can be varied throughout the process as needed. For example, the ratio of the top gas recycle stream to the extract stream (expressed as a volumetric flow rate) can be from about 99:1 to about 60:40, preferably from about 98:2 to about 80:20, and more preferably from about 96:4 to about 90:10. Typically, in prior art processes, the extract is disposed of by combustion. However, in the process of the present disclosure, the extract stream is instead separated into a hydrogen-enriched offstream and an inert-enriched offstream. This separation can be carried out using any method known in the art, including, but not limited to, cryogenic separation, membrane separation, pressure swing adsorption, and amine CO scrubbing. For example, cryogenic separation may be a suitable separation means when nitrogen is used as the seal gas due to the relatively large difference between the boiling points of nitrogen (-195.8°C) and hydrogen (-252.9°C). By separating only the extract without treating the entire conditioned top gas stream, inert component balance may be maintained and hydrogen losses may be reduced without the large capital and operating costs that would be associated with treating the entire top gas stream.

[0060] "Hydrogen-enriched" means that the offstream contains a higher percentage of hydrogen compared to the input extract stream. "Inerts-enriched" means that the offstream contains a higher percentage of inert gas compared to the input extract stream. The hydrogen-enriched offstream may contain at least 70% by volume hydrogen, e.g., at least 80% by volume hydrogen, at least 90% by volume hydrogen, or at least 95% by volume hydrogen. The inerts-enriched offstream may contain at least 50% by volume inert gas, e.g., at least 70% by volume inert gas.

[0061] The hydrogen-rich off-stream, together with the top gas recycle stream and make-up gas, is subsequently introduced directly into the reduction shaft as reducing gas, thus making economical use of hydrogen.

[0062] The inerts-enriched offstream is disposed of in an appropriate manner. If the inerts-enriched offstream contains significant amounts of hydrogen, it may be subjected to auxiliary separation to recover additional hydrogen before disposal. This auxiliary separation may be carried out using any method known in the art, including, but not limited to, cryogenic separation, membrane separation, and pressure swing adsorption. Because membrane separation can be particularly effective in separating mixtures in which nitrogen predominates over hydrogen, membrane separation is the preferred means of auxiliary separation when the seal gas contains nitrogen.

[0063] Carburizing In some cases, it may be desirable to produce carburized sponge iron. In such cases, carburization can be carried out as a separate stage in the process. Separate stages mean that the reducing process gases and the carburizing process gases can be handled separately and no unintentional mixing occurs between the two process gas circuits. This is most easily accomplished by carrying out the carburizing in separate reactors, but can also be achieved by carrying out the carburizing in separate, distinct carburizing zones in a direct reduction shaft, as long as appropriate measures are taken to avoid mixing of gases between the reducing and carburizing zones.

[0064] If a separate carburizing reactor is used, such reactor may preferably be a shaft reactor. As mentioned above, shaft refers to a solid-gas countercurrent moving bed reactor. In this case, sponge iron is introduced at the inlet of the reactor, and carburizing gas flows countercurrently to the moving sponge iron bed to carburize and optionally further reduce the sponge iron. Carburized sponge iron is obtained at the outlet of the reactor.

[0065] Alternatively, the carburizing reactor may be a conveyor unit or a batch reactor, however, a continuous reactor such as a carburizing shaft is preferred.

[0066] The DR shaft and carburizing reactor may be coupled such that the outlet of the DR shaft is directly coupled to the inlet of the carburizing reactor, so long as provisions are provided to prevent carburizing gas from penetrating into the DR shaft to any significant extent. Such provisions may include a pressure differential between the reactors that prevents carburizing gas from penetrating into the direct reduction shaft and / or a lock or exhaust device that provides a physical barrier to gas transport into the direct reduction shaft. Alternatively, the DR shaft and carburizing unit may be coupled by a shaft or chute, or may utilize additional means for transporting the sponge iron intermediate, such as one or more transport crucibles.

[0067] The carburizing gas can be any gas known or expected in the art to provide carburization. Gas in this context refers to the gaseous substance at high temperatures prevailing in the carburizing reactor, but it can be liquid or solid at room temperature. Suitable carburizing gases include hydrocarbons such as methane, natural gas, LPG, or petroleum, or other carbonaceous materials such as syngas, lower (C1-C6) alcohols, esters, and ethers. The carburizing gas can be of fossil origin, but is preferably obtained partially or entirely from renewable sources to reduce net CO2 emissions. Renewable refers to a resource that is naturally replenished on a human timescale. The high utilization of carbon present in the carburizing gas allows for the use of renewable carburizing gases, despite their relative scarcity and high cost compared to fossil equivalents. Suitable renewable carburizing gases include biomethane, biogas, gas obtained from the pyrolysis or partial combustion of biomass, lower alcohols or ethers such as methanol, DME, or ethanol derived from renewable feedstocks, or combinations thereof. Sulfur-containing carburizing gases may be used since sulfur is known to prevent graphite nucleation and passivate the carburized sponge iron product.

[0068] The carburization stage can be configured to proceed to provide a sponge iron product with any desired carbon content. The desired carbon content can typically be in the range of about 1% to about 3% by weight. This can be adjusted by rational selection of carburization process parameters, including, but not limited to, reactor residence time, reaction temperature, reaction pressure, carburizing gas flow rate, and carburizing gas composition. The required temperature is typically maintained by preheating the process gas introduced into the reactor using a preheater, such as an electric preheater. Further heating of the gas can be obtained by exothermic partial oxidation of the gas with oxygen or air after leaving the preheater and before introduction into the reactor. However, if a hot sponge iron intermediate is introduced into the carburization reactor as feed and a cold sponge iron product is desired, neither a preheater nor partial oxidation may be required. The carburization reactor can have a cooling and discharge cone located at the bottom to allow cooling of the carburized sponge iron before discharge from the outlet.

[0069] Spent carburizing gas can be treated to remove undesirable components and recycled back to the carburizing reactor and / or reduction reactor. For example, hydrogen can be separated from the carburizing off-gas and either stored for use as a reducing gas or delivered directly to the DR shaft. Such separation can be performed, for example, using membrane separation technology or pressure swing adsorption. The off-gas can be subjected to a reforming step to reform any CO2 formed during carburizing into CO2. Alternatively, any CO2 formed during carburizing can be captured and either stored (CCS), reformed, released, or utilized for other purposes (CCU). Any water and / or dust in the carburizing gas can also be removed. Residual gas, primarily consisting of unreacted carburizing gas and CO2, can be recycled back to the carburizing reactor.

[0070] To maintain the proper balance of inert gases in the carburizing process gas, it may be necessary to provide a carburized extract. In such cases, this extract may be subjected to separation in the same manner as the reduced extract to recover more hydrogen for use as a reducing gas. The carburized extract may be processed in the same separation unit as the reduced extract, or in a separate separation unit.

[0071] The carburization and reduction steps can be integrated in various ways to improve utilization of resources used in the process. For example, hydrogen formed in the carburization step can be used in the reduction step described above, or CO formed in the carburization step can be reformed to CO for further carburization. Off-gas from the carburization step and / or top gas from the reduction step can be fed through one or more heat exchangers to preheat the gases introduced into the reactor.

[0072] 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 can be provided as hot (HDRI) or cold (CDRI). Cold DRI may also be known as Type (B) DRI. DRI can be prone to reoxidation and, in some cases, is pyrophoric. However, several known means exist for passivating DRI. One such passivation means, commonly used to facilitate overseas transportation of the product, is to press hot DRI into briquettes. Such briquettes are commonly referred to as hot briquetted iron (HBI), also known as Type (A) DRI.

[0073] The sponge iron product obtained by the process herein can be essentially fully metallized sponge iron, i.e., sponge iron having a degree of reduction (DoR) of greater than about 90%, e.g., 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. While obtaining sponge iron with a DoR of greater than about 96% is often not commercially advantageous due to reaction kinetics, such sponge iron can be produced if desired.

[0074] When carburization is performed, sponge iron having any desired carbon content from about 0 to about 7 weight percent can be produced by the processes described herein. Typically, however, for further processing, it is desirable for the sponge iron to have a carbon content of about 0.5 to about 5 weight percent, preferably about 1 to about 4 weight percent, e.g., about 3 weight percent, although this may depend on the ratio of interfacial iron to scrap used in the subsequent EAF processing step.

[0075] 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 to be drawn to scale, as some features may be exaggerated to more clearly illustrate certain features.

[0076] FIG. 1 illustrates a schematic diagram of the iron ore-based steelmaking value chain for the Hybrit concept. The iron ore-based steelmaking value chain begins at an iron ore mine 101. After mining, iron ore 103 is concentrated and processed in a pelletizing plant 105 to produce iron ore pellets 107. These pellets, along with any agglomerated 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 primary reductant and producing water 117a as the primary by-product. The sponge iron 109 can be optionally carburized in either the direct reduction shaft 111 or a separate carburization reactor (not illustrated). Hydrogen gas 115 is preferably produced by water electrolysis 117b in an electrolyzer 119 using electricity 121 derived primarily from fossil-free or renewable sources 122. The hydrogen gas 115 can be stored in a hydrogen storage tank 120 prior to introduction into the direct reduction shaft 111. The sponge iron 109 is melted, optionally together with a portion of scrap iron 125 or other iron source, using an electric arc furnace 123 to provide a smelt 127. The smelt 127 is subjected to further downstream secondary metallurgical processes 129 to produce steel 131. The entire value chain from ore to steel can be fossil-free and is intended to produce negligible or no carbon emissions.

[0077] FIG. 2a schematically illustrates one exemplary embodiment of a system suitable for carrying out the processes disclosed herein.

[0078] Iron ore 207 is introduced into direct reduction shaft 211. As the ore 207 passes through shaft 211, it is progressively reduced to sponge iron 209 by reducing gas 215. Top gas 216, i.e., partially spent reducing gas, exits direct reduction shaft 211 and passes through heat exchanger 251, which is used to preheat the reducing gas 215. Water is condensed from the top gas 216 through heat exchanger 251. The top gas 216 is then purified in purification unit 253 to remove further impurities such as dust and residual water. After purification, the top gas 216 passes through extraction valve 254 to separate the top gas into recycled top gas stream 218 and extract stream 256. Extract stream 256 passes through separation unit 257 to separate hydrogen-enriched offstream 258 and hydrogen-enriched offstream 259. inactive ingredients The hydrogen-enriched off-stream 258 is recombined with the recycled top gas 218, passed through a compressor 255, and combined with the make-up gas 219 to form the reducing gas 215. The reducing gas 215 passes through a heat exchanger 251 and a preheater 241 to be heated to an appropriate temperature before introduction into the direct reduction shaft 211. The preheater 241 may utilize combustion, such as the combustion of a biofuel, or may utilize electrical gas heating. The temperature of the reducing gas 215 may be further increased by partial oxidation before introduction into the direct reduction shaft 211.

[0079] It should be noted that although the extraction valve 254 and the subsequent reintroduction point of the hydrogen-enriched off-stream 258 into the recycled top gas 218 are both illustrated as being upstream of the compressor, one or both of these points may be downstream of the compressor.

[0080] FIG. 2b schematically illustrates a system similar to that of FIG. 2a, except that a direct reduction shaft 211 includes a carburizing zone to obtain carburized sponge iron 209. The sponge iron 209 is carburized with carburizing gas 214 in countercurrent flow, such that carburized sponge iron 209 is obtained at the discharge outlet of the direct reduction shaft 211. Spent carburizing gas 248 leaving the carburizing zone passes through a purification unit 260, a hydrogen separation unit 261 (the separated hydrogen is used as reducing gas 215), and a CO2 absorption unit 263. The spent carburizing gas 248 then passes through an extraction valve 264, which splits the spent carburizing gas 248 into a recycle stream 267 and an extraction stream 266. The extraction stream 266 is disposed of in a conventional manner, such as by combustion in a preheater. The recycle stream 267 is combined with fresh carburizing gas 212 to provide carburizing gas 214. Fresh carburizing gas 212 is supplied from a carburizing gas source 245, such as a biomass gasifier. The carburizing gas 214 passes through a compressor 265 and optionally a preheater 247 before being introduced into the carburizing zone of the direct reduction shaft 211. The temperature of the carburizing gas entering the direct reduction shaft 211 can be further increased by partial oxidation. In such a case, a supply of oxygen (not shown) would be disposed between the preheater 247 and the shaft 211.

[0081] Figure 2c schematically illustrates a system similar to that of Figure 2b. However, in the system of Figure 2c, instead of a separate carburizing zone in direct reduction shaft 211, carburizing is carried out in a separate reactor, illustrated as carburizing shaft 213. Additionally, carburized extract stream 266 is processed in separation unit 269 to provide a further hydrogen-enriched stream 270 that is utilized as a component portion of reducing gas 215, and a further inerts-enriched stream 271 that can be disposed of in a conventional manner.

[0082] FIG. 3 is a flow chart that schematically illustrates an exemplary embodiment of the process disclosed herein. Step s301 represents the start of the process. In step s303, iron ore is charged into the direct reduction shaft. In step s305, a hydrogen-rich reducing gas is introduced into the direct reduction shaft to reduce the iron ore and produce sponge iron. In step s307, a top gas is removed from the direct reduction shaft. In step s309, the top gas is split into a recycle stream and an extract stream. In step s311, the extract stream is processed through a separation unit to provide a hydrogen-rich offstream and an inerts-enriched offstream. In step s313, the recycle stream and the hydrogen-rich offstream are introduced as component portions of the hydrogen-rich reducing gas into the direct reduction shaft. Step s315 represents the end of the process.

Claims

1. 1. A process for producing sponge iron from iron ore, comprising: - charging (s303) iron ore (207) into the direct reduction shaft (211); - introducing (s305) a hydrogen-rich reducing gas (215) containing more than 80% by volume of hydrogen gas into the direct reduction shaft to reduce the iron ore and produce sponge iron (209); - removing (s307) top gas (216) from said direct reduction shaft; - subjecting said top gas to heat exchange in order to cool said top gas and to heat said hydrogen-rich reducing gas; - splitting (s309) said top gas into a recycle stream (218) and an extraction stream (256); - processing (s311) said extract stream through a separation unit (257) to provide a hydrogen-enriched offstream (258) and an inerts-enriched offstream (259); - introducing the recycle stream and the hydrogen-enriched offstream into the direct reduction shaft as constituent parts of the hydrogen-rich reducing gas (s311); A process involving:

2. 10. The process of claim 1, wherein the separation unit (257) is a cryogenic separation unit, a membrane separation unit, or a pressure swing adsorption unit.

3. 3. The process of claim 1 or 2, further comprising introducing a make-up gas (219) into the direct reduction shaft as a constituent portion of the hydrogen-rich reducing gas, the make-up gas comprising hydrogen gas obtained by water electrolysis.

4. 4. The process of claim 1, further comprising introducing a make-up gas (219) into the direct reduction shaft as a constituent portion of the hydrogen-rich reducing gas, the make-up gas being free of carbonaceous components.

5. - carburizing said sponge iron using carburizing gas in a separate carburizing reactor (213) or zone, resulting in carburized sponge iron and spent carburizing gas (248); - splitting said spent carburizing gas into a carburizing recycle stream (267) and a carburizing extraction stream (266); - removing carbonaceous components from said carburized extract stream; - treating said carburized extract stream in a separation unit; The process of any one of claims 1 to 4, further comprising:

6. The process of any one of claims 1 to 5, wherein the inerts-enriched offstream is treated in an auxiliary separation unit to provide an auxiliary hydrogen-enriched offstream.

7. The auxiliary separation unit is a membrane separation unit. The process of claim 6.

8. 1. A system for producing sponge iron, comprising: a direct reduction shaft (211) comprising a reducing gas inlet and a top gas outlet, wherein a hydrogen-rich reducing gas comprising more than 80% by volume of hydrogen gas is introduced into the direct reduction shaft through the reducing gas inlet; a make-up gas source (220), said make-up gas consisting of hydrogen gas (219), said make-up gas source being disposed in fluid communication with said reducing gas inlet; a heat exchanger (251) for preheating the hydrogen-rich reducing gas, the heat exchanger (251) being arranged in fluid connection with the top gas outlet so that the top gas passes through the heat exchanger which cools the top gas and heats the hydrogen-rich reducing gas; an extraction valve (254) disposed in fluid communication with said top gas outlet and arranged to divide the top gas between a recycle stream outlet and an extraction stream outlet; a separation unit (257) disposed in fluid connection with said extract stream outlet and configured to separate the extract stream into a hydrogen-enriched stream and an inerts-enriched stream; A system including:

9. The system of claim 8 , wherein the separation unit is a cryogenic separation unit, a membrane separation unit, or a pressure swing adsorption unit.

10. 10. The system of claim 8 or 9, wherein the make-up gas source is a water electrolyzer unit.

11. 11. The system of claim 8, wherein the direct reduction shaft comprises a reduction zone and a carburizing zone, the direct reduction shaft being positioned to prevent passage of gas from the carburizing zone to the reduction zone.

12. The system of any one of claims 8 to 10, further comprising a carburization reactor (213).

13. The carburizing zone or reactor includes a carburizing gas inlet and a spent carburizing gas outlet, and the system includes: a carburizing gas source (245) placed in fluid connection with said carburizing gas inlet; a carburizing extraction valve (264) disposed in fluid communication with said spent carburizing gas outlet and arranged to divide the spent carburizing gas between a carburizing recycle stream outlet and a carburizing extraction stream outlet; wherein the carburized extract stream outlet is disposed in fluid communication with a separation unit. A system according to any one of claims 8 to 12.

14. 14. The system of claim 13, further comprising one or more carbon separation units, wherein the carburized extract stream outlet is disposed in fluid communication with the separation units via the one or more carbon separation units.

15. CO ２ The system according to any one of claims 8 to 14, which does not include a separation unit.

16. 16. The system of any one of claims 8 to 15, further comprising an auxiliary separation unit disposed in fluid connection with the inert component-enriched stream outlet of the separation unit, wherein the auxiliary separation unit is a membrane separation unit.

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