Process for the Production of Sponge Iron

Abstract

A process for the production of sponge iron and a system for the production of sponge iron. The process includes the steps of: producing electrolytic hydrogen and oxygen by electrolysis of water; producing methanol by reacting the electrolytic hydrogen with carbon dioxide; storing the methanol; reforming the methanol using water and/or oxygen to provide carbon dioxide and released hydrogen; providing the released hydrogen as a component portion of a reducing gas to a direct reduction shaft; and reducing iron ore in the direct reduction shaft using the reducing gas to produce the sponge iron.

Claims

1. A process for the production of sponge iron, the process comprising the steps: producing electrolytic hydrogen and oxygen by electrolysis of water; producing methanol by reacting the electrolytic hydrogen with carbon dioxide; storing the methanol; reforming the methanol using water and/or oxygen to provide carbon dioxide and released hydrogen; providing the released hydrogen as a component portion of a reducing gas to a direct reduction shaft; and reducing iron ore in the direct reduction shaft using the reducing gas to produce the sponge iron.

2. The process according to claim 1, further comprising the steps: producing syngas and oxygen by co-electrolysis of water and carbon dioxide; and providing the syngas as a component portion of the reducing gas.

3. The process according to claim 2, wherein the steps of co-electrolysis and reduction of iron ore are performed at a steady state, and wherein the steps of water electrolysis, methanol production and methanol reformation are performed dynamically.

4. The process according to claim 1, further comprising the step: preheating the reducing gas by heat exchange with a top gas from the direct reduction shaft.

5. The process according to claim 1, further comprising the step: preheating the reducing gas by passing through an oxy-fuel preheater, wherein the oxy-fuel preheater is heated by the combustion of biomass in oxygen, preferably the oxygen from the co-electrolysis step.

6. The process according to claim 1, further comprising the step: preheating the reducing gas by passing through an electric preheater.

7. The process according to claim 1, further comprising the steps: separating water and/or carbon dioxide from a top gas of the direct reduction shaft; and recycling the top gas as a component portion of the reducing gas.

8. The process according to claim 1, wherein the carbon dioxide used in producing methanol and/or used in co-electrolysis is carbon dioxide obtained in the step of catalytically reforming the methanol, and/or carbon dioxide obtained from the oxy-fuel preheater, and/or carbon dioxide separated from the top gas of the direct reduction shaft.

9. The process according to claim 1, wherein the methanol is reformed by endothermic steam reforming, exothermic partial oxidation, oxidative steam reforming, or autothermal reforming.

10. The process according to claim 1, wherein the reducing gas comprises from about 0 mol % to about 50 mol % carbon monoxide.

11. The process according to claim 1, wherein a proportion of the electrolytic hydrogen is used directly as a proportion of the reducing gas.

12. The process according to claim 1, wherein the process utilizes essentially fossil-free electricity, preferably essentially renewable electricity.

13. A system for the production of sponge iron, the system comprising: a first electrolyser arranged to produce hydrogen from the electrolysis of water; a methanol production unit arranged to produce methanol from hydrogen and carbon dioxide; a methanol storage unit; a methanol reformation unit arranged to reform methanol using water and/or oxygen to provide hydrogen and carbon dioxide; and a direct reduction shaft.

14. The system according to claim 13, further comprising a second electrolyser arranged to produce syngas from the co-electrolysis of water and carbon dioxide.

15. The system according to claim 13, further comprising an oxy-fuel preheater arranged to preheat a reducing gas for introduction into the direct reduction shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

[0049] FIG. 1 schematically illustrates the value-chain for a prior art steelmaking process according to the Hybrit concept;

[0050] FIG. 2a schematically illustrates an exemplifying embodiment of a system for the production of sponge iron;

[0051] FIG. 2b schematically illustrates another exemplifying embodiment of a system for the production of sponge iron;

[0052] FIG. 2c schematically illustrates a further exemplifying embodiment of a system for the production of sponge iron;

[0053] FIG. 3 is a flow chart schematically illustrating an exemplifying embodiment of a process for the production of sponge iron;

[0054] FIG. 4 schematically illustrates the maximum potential low temperature electrolyser overcapacity as a function of mol % CO in the reducing gas and wt % carbon in the sponge iron product; and

[0055] FIG. 5 schematically illustrates the energy balances for a range of reducing gas compositions for a sponge iron containing 1 wt % carbon (with the exception of H-DR, which gives a carbon-free sponge iron).

DETAILED DESCRIPTION

[0056] The system for producing sponge iron according to the present invention comprises at least a first electrolyser, a methanol production unit, a methanol storage unit, a methanol reformation unit and a direct reduction shaft. The first electrolyser is arranged in fluid connection with the methanol production unit. The methanol production unit is arranged in fluid connection with the methanol storage unit. The methanol storage unit is arranged in fluid connection with the methanol reformation unit. The methanol reformation unit is arranged in fluid connection with the direct reduction shaft.

Water Electrolysis

[0057] The first electrolyser is arranged to produce hydrogen from the electrolysis of water, and may be any electrolyser known in the art and capable of performing such a reaction. Although the first electrolyser is referred to herein in singular, the quantities of hydrogen demanded by the present process will most likely necessitate a plurality of electrolyser units suitably arranged, e.g. arranged in parallel. The first electrolyser may be a low-temperature electrolyser, i.e. operating on liquid water, such as an alkaline electrolyser (AEL) or a proton exchange membrane electrolyser (PEMEL). AEL is at present the more established technology, but PEMEL may operate at higher current densities and may have better response times to variable loads. Alternatively, or in addition, the first electrolyser may comprise a high-temperature (steam) electrolyser, such as a solid oxide electrolyser (SOEL). SOEL may be more efficient than low-temperature technologies, particularly if a ready source of steam or heat is available.

Methanol Production

[0058] The methanol production unit is arranged to produce methanol from carbon dioxide and the electrolytic hydrogen from the first electrolysis unit. Methanol production proceeds according to the following reaction:

CO.sub.2+3 H.sub.2.fwdarw.CH.sub.3OH+H.sub.2O ΔH.sub.R=−49 kJ/mol   (1)

[0059] The methanol production unit typically utilizes one or more catalysts to catalyse the formation of methanol. For example, Cu/ZnO/Al.sub.2O.sub.3 catalysts are known to catalyst this reaction. Feed and recycle compressors may be used to provide a flow of reactants over the reactor bed(s). A distillation section, comprising one or more distillation columns, is typically arranged for separating the methanol and water products. As the reaction is exothermic, it may be desirable to cool the reactor beds using for example heat exchangers or quench streams in order to achieve suitable conversion per reactor pass. Typical reactor conditions are 210-280° C. at 40-80 bar. Suitable methanol production processes are described in Rikho-Struckmann et al. “Assessment of Methanol Synthesis Utilizing Exhaust CO2 for Chemical Storage of Electrical Energy”, Ind. & Eng. Chem. Res., 2010, 49(21), pp. 11073-11078, which is incorporated by reference herein.

Methanol Storage

[0060] The methanol storage unit is arranged to receive and store the methanol produced by the methanol production unit. The methanol storage unit may be any suitable means of storing such quantities of methanol as are applicable in the present invention. Methanol is a flammable liquid having a boiling point of 64.7° C., and has similar storage requirements as other volatile, flammable liquids such as gasoline. Suitable storage means include in aboveground storage tanks, such as floating roof storage tanks, or in underground storage tanks.

Methanol Reforming

[0061] The methanol reformation unit is arranged to reform the stored methanol to carbon dioxide and hydrogen when required. In order to distinguish the hydrogen produced by this reformation process from hydrogen produced from electrolysis or otherwise participating in the processes described herein, the hydrogen obtained from the methanol reformation step is termed “released hydrogen”. The release of H.sub.2 from methanol can be achieved via four reactions: endothermic decomposition (MD), exothermic partial oxidation (POX), endothermic steam reforming (SR) and oxidative steam reforming (OSR). However, as discussed below, endothermic decomposition is undesirable and partial oxidation has a number of drawbacks. Therefore, steam reforming and oxidative steam reforming are preferred.

[0062] Endothermic decomposition of methanol (MD) produces carbon monoxide and hydrogen by the following reaction:

CH.sub.3OH.fwdarw.CO+2 H.sub.2 ΔH.sub.R=−91 kJ/mol   (2)

[0063] If endothermic decomposition was to be used as the reforming reaction, this would impede dynamic operation of the electrolyser and reformer. This is because the product gas from endothermic decomposition comprises carbon monoxide as well as hydrogen. If the product gas is used “as is” in dynamic operation, this would lead to unacceptable fluctuation in the composition (H.sub.2:CO ratio) of the reducing gas to the direct reduction shaft. Therefore, the remaining alternatives are either to always use a fixed proportion of reformate in the reducing gas, or to remove CO prior to use as reducing gas, neither of which are attractive. Moreover, the high endothermicity of this reaction would make the reformation step energy-intensive to perform and complicates process design. Furthermore, replenishing the reducing gas utilizing such a reformation reaction would inevitably lead to an accumulation of carbon in the process unless the carbon is removed. Thus, such a process will inevitably lead to carbon emissions or require capture and storage of the CO.sub.2 produced. Finally, performing the endothermic decomposition in the absence of water may lead to excessive formation of byproducts such as methyl formate, methane and dimethyl ether. For at least these reasons, reforming methanol by endothermic decomposition is not favoured.

[0064] Partial oxidation of methanol (POX) proceeds by the following reaction:

CH.sub.3OH+½O.sub.2.fwdarw.CO.sub.2+2 H.sub.2 ΔH.sub.R=−192 kJ/mol   (3)

[0065] Partial oxidation suffers from the dual problems of low H.sub.2 yield and intermittent production of large amounts of heat, and therefore is not a preferred method of methanol reformation. However, continuous POX of small amounts of methanol could be attractive in order to raise the gas temperature prior to introduction into the reducing shaft, particularly if the reducing gas introduced to the direct reduction shaft is intended to comprise carbon-containing gas such as carbon monoxide anyway.

[0066] Steam reforming of methanol (SR) proceeds by the following reaction:

CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+3 H.sub.2 ΔH.sub.R=+49 kJ/mol   (4)

[0067] Oxidative steam reforming of methanol (OSR) is a combination of the SR and POX reactions, as illustrated by the following exemplifying stoichiometry:

4 CH.sub.3OH+3 H.sub.2O+½O.sub.2.fwdarw.4 CO.sub.2+11 H.sub.2 ΔH.sub.R=0 kJ/mol   (5)

[0068] For both SR and OSR, evaporation of the methanol and water feeds constitute the greater part of the heat demand for the process, and it is therefore highly advantageous if the feeds can be delivered in gaseous form to the reformer, using for example “waste” heat from other steps of the process. SR and OSR each have specific advantages when integrated into a methanol-based direct reduction process. OSR is heat-neutral for certain feed ratios, and is then termed autothermal reforming. SR demands approximately twice the heat of OSR operated in autothermal mode, but provides a higher yield of H.sub.2. As the hydrogen “lost” by reforming using OSR would be compensated for using hydrogen originally produced from electricity, the choice between SR and OSR will ultimately be decided on by the relative costs of heat and electricity. A further advantage of OSR is that the addition of O.sub.2 inside the reactor allows for very efficient heat transfer, allowing fast start-up times and rapid response to changes in reformer load.

[0069] Reforming may be performed using for example a Cu/ZnO/Al.sub.2O.sub.3 catalyst.

[0070] The carbon dioxide present in the reformed gas is preferably removed prior to introduction of the gas as a reducing gas into the direct reduction shaft. Removal of the carbon dioxide may be performed using techniques known in the art, such as pressure swing adsorption or membrane separation techniques.

Dynamic Operation

[0071] The electrolyser, methanol production, storage and reformation are intended to be operated dynamically in order to compensate for variations in the availability and/or cost of electricity.

[0072] In countries such as Sweden where renewable sources form a substantial share of the electricity supply, there is a significant correlation between abundance of renewable electricity and price, in some cases even resulting in negative electricity prices. This means that during times of abundant low-cost electricity supply, an excess of hydrogen may be produced by electrolysis and converted to methanol for storage. During times when electricity is scarce and/or expensive, the electrolyser may be run at a minimum load and hydrogen may in such a case be provided by the reformation of stored methanol. Due to this dynamic operation, electricity demand is steered to such times when supply is cheap and abundant, thus reducing the cost of the process and ensuring efficient use of transient electricity supplies, as renewable energy sources typically are. The electrolysers and stored methanol may also be used to provide further grid services independent of the production of sponge iron, such as load-levelling.

Reduction

[0073] The direct reduction shaft reactor may be of any kind commonly known in the art. By shaft reactor, it is meant a solid-gas countercurrent moving bed reactor, whereby a burden of iron ore pellets is introduced at an inlet at the top of the reactor and descends by gravity towards an outlet arranged at the bottom of the reactor. Reducing gas is introduced at a point lower than the inlet of the reactor and flows upwards counter to the moving bed of pellets ore in order to reduce the ore to metallized iron. Reduction is typically performed at temperatures of from about 900° C. to about 1100° C. The temperatures required are typically maintained by pre-heating of the process gases introduced into the reactor, for example using a preheater. Suitable preheaters are described herein. Further heating of the gases may be obtained after leaving the pre-heater and prior to introduction into the reactor by exothermic partial oxidation of the gases with oxygen or air. The reactor may have a cooling and discharge cone arranged at the bottom to allow the sponge iron to cool prior to discharge from the outlet.

[0074] The iron ore burden typically consists predominantly of iron ore pellets, although some lump iron ore may also be introduced. The iron ore pellets typically comprise mostly hematite, together with further additives or impurities such as gangue, fluxes and binders. However, the pellets may comprise some other metals and other ores such as magnetite. Iron ore pellets specified 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 reducing gas.

[0075] The reducing gas provided to the direct reduction shaft may comprise the released hydrogen from the methanol reformation step as a component part. The reducing gas may also comprise electrolytic hydrogen, that is to say hydrogen gas from the first electrolysis unit. As described below, the reducing gas may further comprise syngas obtained by co-electrolysis of water and carbon dioxide, as well as components of the top gas recycled from the direct reduction shaft. The reducing gas may be predominantly hydrogen gas. However, the reducing gas may comprise amounts of other gases such as steam, carbon monoxide and carbon dioxide, depending on the exact configuration of the system for production of sponge iron.

Carburization

[0076] Iron produced by present-day commercial blast furnace or direct reduction routes typically comprises significant amounts of carbon (typically up to 5 wt %), due to carbon incorporation during reduction of the iron ore. Besides its use as a reducing agent, carbon plays further important roles in the steelmaking process. Its presence in the crude iron from the BF or DR process, mainly in the form of iron carbide (Fe.sub.3C), lowers the melting point of the iron. During subsequent processing of the crude iron in an EAF or BOF, the exothermic dissociation of iron carbide and oxidation of carbon to CO supplies heat to the process. The gas evolution in the EAF due to this CO production provides a foamy slag that assists in thermally insulating the iron melt and helps diminish consumption of the EAF electrodes. For at least these reasons, the presence of carbon in the crude iron may assist in reducing energy consumption during processing to steel. The presence of carbon in the melt may also influence slag-metal reaction kinetics, and assist in purging dissolved gaseous elements from the metal. Moreover, the presence of carbon in direct reduced iron passivizes the sponge iron and enables simpler handling and transport. Finally, since the steel industry has a heritage and established practice with respect to carbon-containing iron, there may simply be a degree of reluctance among some steelmakers to adopt the use of carbon-free iron, regardless of any benefits.

[0077] For at least these reasons, it may be desirable to provide a crude sponge iron produced using substantially fossil-free means, but still containing carbon to an extent that it may be used as a drop-in replacement for present-day crude iron.

[0078] The presence of carbon monoxide in the reducing gas may provide a carburized product by the following reactions:

3 Fe+2 CO.fwdarw.Fe.sub.3C+CO.sub.2 ΔH.sub.R=−148.7 kJ/mol   (6)

3 Fe+CO+H.sub.2.fwdarw.Fe.sub.3C+H.sub.2O ΔH.sub.R=−105.0 kJ/mol   (7)

Fe+CO+H.sub.2.fwdarw.Fe.sub.3(C.sub.free)+H.sub.2O ΔH.sub.R=−135.6 kJ/mol   (8)

[0079] The presence of carbon monoxide in the reducing gas could conceivably be achieved by reforming methanol using endothermic decomposition to provide syngas, as described above, and using the resulting syngas as a component part of the reducing gas. However, as described above, the endothermic decomposition of methanol has a number of drawbacks. Moreover, since methanol reformation is intended to be used dynamically in order to compensate for electricity availability and/or price fluctuations, and since endothermic decomposition provides a fixed ratio of H.sub.2:CO of 2:1, using endothermic decomposition only would lead to the ratio of H.sub.2:CO in the reducing gas being determined by the dynamic operation of the reformer. The sponge iron resulting from such a process would have a variable quantity of incorporated carbon, which is not desirable.

[0080] The inventors of the present invention have identified that use of co-electrolysis of carbon dioxide and water, in combination with the process as described above, may be used to provide a carburized sponge iron. The co-electrolysis of carbon dioxide and water provides a product comprising a blend of carbon monoxide and hydrogen, herein termed syngas. Trace amounts of other gases, such as methane, may also be produced. The molar ratio of hydrogen and carbon monoxide can be tailored depending on the requirements of the syngas product. Co-electrolysis may be performed using a high-temperature solid oxide electrolyser (SOEL), which provides a more efficient use of electricity. For comparison, low-temperature electrolysis of water is estimated to require approximately 100 kWh per kmol (kilo-mole) hydrogen gas, whereas high-temperature co-electrolysis using a SOEL is estimated to require approximately 70 kWh per kmol. The reduction of iron oxide by hydrogen or carbon monoxide proceeds as follows:

3 H.sub.2+Fe.sub.2O.sub.3(s).fwdarw.2 Fe(s)+3 H.sub.2O ΔH.sub.R=+98.8 kJ/mol   (9)

3 CO+Fe.sub.2O.sub.3(s).fwdarw.2 Fe(s)+3 CO.sub.2 ΔH.sub.R=−24.8 kJ/mol   (10)

[0081] It can be seen that one mole carbon monoxide can reduce the same amount of iron oxide as one mole of hydrogen, although the reduction with hydrogen is endothermic whereas the reduction with carbon monoxide is exothermic. It should however be noted that having carbon monoxide in the reducing gas necessitates separation of carbon dioxide from the top gas of the reduction shaft in order to prevent accumulation. Such a separation is energy demanding. Overall however, using syngas from co-electrolysis of carbon dioxide and water is an energy-efficient means of obtaining a reduced and carburized sponge iron product.

[0082] The co-electrolysis may be performed at steady state in order to provide a constant supply of all of the carbon monoxide and a fixed proportion of the hydrogen of the reducing gas. The remaining hydrogen required for the reducing gas may be provided by dynamic operation of the water electrolyser and methanol reformer as described above. The reducing gas may thereby have a H.sub.2:CO molar ratio of from about 99:1 to about 60:40, such as from about 95:5 to about 70:30, or from about 90:10 to about 80:20.

Supply of Carbon Dioxide and Preheating

[0083] The present invention requires a source of carbon dioxide for operation: both in order to produce methanol, as well as for use in co-electrolysis if co-electrolysis is utilized. Although carbon dioxide introduced into the process may be captured and recycled, there may be some loss of carbon. Moreover, if a carburized sponge iron is to be produced then carbon will be lost by incorporation in the sponge iron. Therefore, there may be a need for a constant introduction of carbon into the process. This could be achieved by utilizing a supply of carbon dioxide from a high-concentration source. However, there is no guarantee that sufficiently large sources of biogenic carbon dioxide are available in sufficient geographic proximity to be utilizable. Using fossil carbon dioxide is not a feasible option if the goal of the process is to produce fossil-free steel. An alternative is to provide carbon and heat to the process by the combustion of biomass. To this end, the reducing gas may be preheated prior to introduction into the direct reduction shaft using oxy-fuel combustion of biomass. The principle of oxy-fuel combustion is simple: the biomass is combusted using essentially pure oxygen as the oxidant. The resulting flue stream consists essentially of carbon dioxide and steam. The steam may be removed by simple condensation, providing an essentially pure source of carbon dioxide. Conventionally, the provision of essentially pure oxygen is an economic impediment to the utilization of oxy-fuel combustion. However, in the present case there is a ready supply of oxygen available at low additional cost from the water electrolysis step, making oxy-fuel preheating of the reduction gas economically feasible. The heat produced by the oxy-fuel preheater may also be used elsewhere in the process, for example in order to generate a steam feed for the high-temperature co-electrolysis step, and/or to regenerate an amine used in a carbon-capture step.

[0084] Due to material limitations it may not be possible to fully heat the reducing gas to the desired temperature using oxy-fuel preheating only. Therefore, electric preheating of the reducing gases may be used as a complement or alternative to oxy-fuel preheating. Means for electrically heating process gases are known in the art.

[0085] It is conceived that preheating may be performed dynamically between the oxy-fuel preheater and electric preheater in order to ensure that oxy-fuel preheating is used only to the extent that the CO.sub.2 required for the process is produced. This may help minimise biomass consumption.

Carbon Capture and Storage

[0086] As previously noted, the carbon dioxide present in the gas from the methanol reformer may preferably be separated from the released hydrogen. This may be performed using techniques known in the art, such as pressure swing adsorption, amine absorption, or membrane separation techniques. Pressure swing adsorption is preferred.

[0087] If carbon monoxide is present in the reducing gas, then it will be necessary to separate carbon dioxide from the top gas of the direct reduction shaft in order to enable recycling of the top gas. Removal of carbon dioxide from the top gas is preferably achieved using techniques known in the art, such as pressure swing adsorption, amine absorption, or membrane separation techniques. Amine absorption is already widely utilized in carbon capture from flue gas and is the preferred technique.

[0088] The flue gas of the oxy-fuel preheater, if used, will consist essentially of steam and carbon dioxide. After condensation of the steam an essentially pure carbon dioxide stream will remain. However, if further purification of this carbon dioxide is required, it may be performed using techniques known in the art, such as pressure swing adsorption, amine absorption, or membrane separation techniques.

[0089] The various sources of carbon dioxide may be utilized directly in methanol production and/or co-electrolysis, or the carbon dioxide may be stored until such times as it is required. Means of storing carbon dioxide are known in the art.

EXAMPLES

[0090] The invention will now be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

[0091] FIG. 1 schematically illustrates a prior art embodiment of the ore-based steelmaking value chain according to the Hybrit concept. The ore-based steelmaking value chain starts at the iron ore mine 101. After mining, iron ore 103 is concentrated and processed in a pelletizing plant 105, and iron ore pellets 107 are produced. 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 reductant and producing water 117 as the main by product. The hydrogen gas 115 is produced by electrolysis of water 117 in an electrolyser 119 using electricity 121 that is preferably primarily derived from fossil-free or renewable sources 122. The hydrogen gas 115 may be stored in a hydrogen storage 120 prior to introduction into the direct reduction shaft 111. The sponge iron 109 is melted using an electric arc furnace 123, optionally together with a proportion of scrap iron 125 or other iron source, to provide a melt 127. The melt 127 is subjected to further downstream secondary metallurgical processes 129, and steel 131 is produced. It is intended that the entire value-chain, from ore to steel is fossil-free and produces only low or zero carbon emissions.

[0092] FIG. 2a schematically illustrates an exemplifying embodiment of a system for the production of sponge iron according to the present disclosure. The system comprises an electrolysis unit 219 for the electrolysis of water. A proportion of the hydrogen produced by the electrolysis unit 219 is heated by passage through heat exchanger 229 and electric preheater 241 prior to introduction into direct reduction shaft 211 for reduction of iron ore 207 to sponge iron 209. The electrolysis unit 219 is operated dynamically. When electricity is cheap and/or abundant the electrolysis unit 219 may be used to produce an excess of hydrogen. The excess hydrogen may be conveyed to a methanol production unit 201 where it is converted to methanol prior to storage in methanol store 203. When electricity is expensive and/or in scant supply, the electrolysis unit 219 may instead be operated at a minimum load. To compensate for the shortfall of hydrogen produced by electrolysis, methanol from the methanol store 203 may be reformed in the methanol reforming unit 205 to provide hydrogen and carbon dioxide. These gases are separated in PSA unit 215. The carbon dioxide stream may be stored in CO.sub.2 store 217 ready for further use in producing methanol. The released hydrogen may be mixed with electrolytic hydrogen and introduced as reducing gas into the direct reduction shaft 211. In this manner, although the electrolysis unit 219, methanol production unit 201, methanol reforming unit 205, and related components may operate dynamically, the total amount of hydrogen supplied to the reducing gas circuit may be relatively constant, allowing steady-state operation of the direct reduction shaft 211 and related components. In the illustrated embodiments, the dynamic components are illustrated to the left of the shaded line and the steady-state components are illustrated to the right.

[0093] The top gas obtained from the direct reduction shaft 211 comprises mostly water and hydrogen. The water may be condensed and removed by passage of the top gas through heat exchanger 229, at the same time pre-heating the reducing gas. The water may be reused in the electrolyser 219 and/or in methanol reformation 205. After removal of water, the remaining top gas may be mixed with fresh hydrogen from the dynamic part of the system and introduced as reducing gas into the DR shaft as previously described.

[0094] No carbon dioxide is consumed in the process described, since any CO.sub.2 used in methanol production is subsequently released when the methanol is reformed to hydrogen and CO.sub.2. However, in order to account for loss of carbon in the processes, a CO.sub.2 source 218, such as an industrial flue stream, may be used in methanol production.

[0095] FIG. 2b schematically illustrates an exemplifying embodiment of a system comprising an alternative reducing gas pre-heating arrangement. An oxy-fuel preheater 223 is utilized to preheat the reducing gas prior to introduction into the direct reduction shaft 211. The reducing gas is passed through the oxy-fuel preheater 223 where it is heated by heat exchange with the combustion gases of the oxy-fuel preheater 223. The oxy-fuel preheater utilizes biomass 225 as fuel and pure oxygen as oxidant, thus providing a flue gas that consists essentially of water and CO.sub.2 only. Oxygen for the oxy-fuel combustion is supplied from the electrolysis unit 219, making the oxy-fuel process economical. Water is condensed from the flue gas in condenser 231 to provide an essentially pure source of CO.sub.2 which may be stored for subsequent use in methanol production. Thus, by utilizing an oxy-fuel preheater, the need for an external source of CO.sub.2 may be avoided. Although this embodiment is illustrated as lacking an electric preheater 241, such a preheater may be used in combination with an oxy-fuel preheater 223, preferably arranged downstream of the oxy-fuel preheater 223 to further heat gases leaving the oxy-fuel preheater 223.

[0096] FIG. 2c schematically illustrates an exemplifying embodiment of a system utilising both water electrolysis and water/CO.sub.2 co-electrolysis. As with previously described embodiments, this system can be conceptually divided into components operating under steady state, and components operating dynamically. A high-temperature co-electrolysis unit 227, such as a solid oxide electrolysis unit, is used for the co-electrolysis of CO.sub.2 and H.sub.2O to produce a mixture of CO and H.sub.2. The unit 227 operates under steady state conditions to provide a continuous quantity of syngas having a predetermined CO:H.sub.2 ratio to the reducing gas circuit. In the reducing gas circuit, the syngas is mixed with hydrogen gas from the dynamically operated part of the system, as well as reducing gas recycled from the top gas of the reduction shaft 211. This mixed reducing gas is conveyed through the oxy-fuel preheater 223. The reducing gas is further heated by passage through an electric preheater 241 prior to introduction into the direct reduction shaft 211. In the direct reduction shaft, iron ore 207 introduced as burden is conveyed downwards by gravity through the shaft. The iron ore 207 is reduced by the counter-current flow of reducing gas to sponge iron 209. Because the reducing gas in this embodiment comprises CO, the sponge iron 209 will be carburized to the appropriate degree, as determined by e.g. process conditions and the proportion of CO in the reducing gas. Top gas leaving the direct reduction shaft 211 will comprise steam, CO.sub.2, H.sub.2 and CO. Steam is removed from the top gas by heat exchange in heat exchanger 229, which also serves to pre heat a proportion of the reducing gases prior to introduction into the oxy-fuel preheater 223.

[0097] Carbon dioxide is removed from the top gas by passage through amine absorption unit 221, leaving a recycled top gas comprising essentially only CO and H.sub.2 and suitable for recycling to the reducing gas circuit.

[0098] The oxy-fuel preheater operates as described in conjunction with previous embodiments. Note that although the illustrated embodiment utilizes an oxy-fuel preheater 223 in combination with an electric preheater 241, an electric preheater 223 may not be necessary or desirable. The dynamic part of the system comprising inter alia the low-temperature water electrolysis unit 219, methanol production unit 201, methanol reforming unit 205, and related components, provides a relatively fixed proportion of hydrogen gas to the reducing gas circuit, as described in conjunction with previous embodiments.

[0099] In all illustrated embodiments a CO.sub.2 store 217 is provided. However, such a store is not strictly necessary, and the system may be free of such a store, meaning a carbon storage process step is also not required. This is because a CO.sub.2 source, either an extrinsic source 218 or the oxy-fuel preheater 223 may be used provide an excess of CO.sub.2 in the system at all times. Note however that if the reducing gas comprises carbon monoxide, separation of CO.sub.2 from the top gas will always be required in order to prevent accumulation of CO.sub.2 in the reducing gas, even if the separated CO.sub.2 is not subsequently stored.

[0100] FIG. 3 is a flow chart schematically illustrating a process for the production of sponge iron according to an exemplifying embodiment of the present disclosure. Step s301 denotes the start of the process. In step s303 electrolytic hydrogen and oxygen are produced by electrolysis of water. In step s305 methanol is produced by reacting the electrolytic hydrogen with carbon dioxide. In step s307 the methanol is stored. In step s309 the methanol is reformed using water and/or oxygen to provide carbon dioxide and released hydrogen. In step s311 the released hydrogen is provided as a component portion of a reducing gas to a direct reduction shaft. In step s313 iron ore is reduced in the direct reduction shaft using the reducing gas to produce the sponge iron. Step s315 denotes the end of the process.

Mass and Energy Balances

Method

[0101] The basic mass and energy balances of the suggested methanol-based direct reduction process as illustrated in FIG. 2c were modelled in order to evaluate its feasibility and performance. Water electrolysis unit 219 is taken to use alkaline electrolysis and co electrolysis unit 227 is taken to be a high-temperature solid oxide electrolysis unit. The calculations are performed on the basis of a process spanning both the direct reduction process and the subsequent electric arc furnace processing step, as the composition of the sponge iron output by the direct reduction significantly and directly impacts the energy requirements of the EAF step. Calculations were performed on reducing gas compositions ranging from pure hydrogen (i.e. no high-temperature coelectrolysis) to 70:30 molar ratio of hydrogen to carbon monoxide.

[0102] The reducing gas is assumed to always contain 5% CO.sub.2 and 5% of H.sub.2O (by mole) due to their incomplete separation from the top gas, at least for cases where the reducing gas is a mixture of hydrogen and carbon monoxide. To maintain the same gas flow in the pure hydrogen direct reduction case as in all other cases it is assumed that the reducing gas in this case contains 90% H.sub.2 and 10% H.sub.2O (by mole).

[0103] Iron ore pellets are taken to consist of Fe.sub.2O.sub.3 and 5% (by weight) of inert material. It was assumed that a DRI metallisation of 94% is achieved in all cases and that all remaining iron oxide is in the form of FeO. The production of steel is assumed to be 2 Mt per year with the plant in operation for 360 days per year in all considered cases.

[0104] As the degree of carburization that is achieved under different process conditions is uncertain, this is considered a variable in the calculations, with four cases of carburization (by weight): 0.0% (equivalent to H-DR when the concentration of CO in the reducing gas is zero), 0.5%, 1.0%, and 1.5%.

[0105] The electric arc furnace is assumed to be fed by hot DRI, resulting in an electricity demand that varies linearly with the DRI carburization between 760 kWh/t steel for carbon-free DRI and 520 kWh/t steel for DRI with a carburization of 2% (by weight).

Mass Balances

[0106] An important aspect of the suggested CH.sub.3OH-based direct reduction process is the carbon mass balance: the amount of CO.sub.2 delivered by the oxy-fuel combustion process and separated out from the top gas must be sufficient for the operation of the high-temperature electrolyser, assuming, as in the present calculations, that no extrinsic source of CO.sub.2 is utilized. This condition was found to be fulfilled for all considered reducing gas compositions. It was found that CO.sub.2 storage is not a necessary part of the direct reduction process utilizing oxy-fuel combustion as there is always an excess of CO.sub.2 in the system, even considering a typical oxy-fuel CO.sub.2 capture efficiency of 90%.

[0107] The excess of CO.sub.2 produced by oxy-fuel combustion and separated out from the top gas can be used to produce CH.sub.3OH and store H.sub.2. Using the calculated excess of CO.sub.2, it is estimated that for pure H-DR, the maximum relative overcapacity of low-temperature electrolysers is calculated to be 41%, increasing to 267% for the case of 30% (by mole) CO in the reducing gas and 1.5% (by weight) of carbon in produced DRI. These low-temperature electrolyser overcapacities would allow for maximum CH.sub.3OH production rates of between 611 and 938 t/d (116 to 177 t H.sub.2/d).

[0108] As the high-temperature electrolyser is delivering a mixture of CO and H.sub.2 (in a 1:1 molar ratio) and not pure CO, a greater amount of CO in the reducing gas will result in a smaller share of the total H.sub.2 being derived from the dynamic section of the process, i.e., the low-temperature electrolyser or CH.sub.3OH reformer. This is shown in FIG. 4, illustrating the share of total hydrogen delivered from the high-temperature electrolyser, as a function of mol % CO in the reducing gas and degree of carburization.

[0109] As can be seen, the share of the total H.sub.2 that is delivered from the high-temperature electrolyser increases rather rapidly with increasing reducing gas CO concentration. This means that as the proportion of CO is increased in the reducing gas, the possibility of dynamic operation of the system is decreased.

Energy Balances

[0110] The total calculated energy demand of the process for a range of reducing gas compositions is shown in FIG. 5. The carbon content of the DRI is 1 wt % in all cases except H-DR (0 mol % CO) where the carbon content of the DRI is 0 wt %. It is seen that the overall energy demand of the process decreases somewhat with increasing reducing gas CO concentration. It can be seen that the decrease in electricity demand is even greater, with a much more significant proportion of the energy demand being thermal energy in processes utilizing a CO-containing reducing gas. Therefore, the operational costs of utilizing CO-containing reducing gas may be even more favourable assuming that the cost of biomass is low relative to the electricity price, as is typically the case.

[0111] Another conclusion that can be drawn from FIG. 5 is that the total electricity demand of the steady state part of the direct reduction process, i.e., the high-temperature electrolyser, the EAF, and the electric pre-heating, increases with increasing reducing gas CO concentration. The result of this increase is that the minimum electricity demand of the process increases, thus deceasing the possibility of dynamic operation of the system. This increased minimum load may be disadvantageous during extended periods of high electricity prices. However, it should be noted that the increase in allowable low-temperature electrolyser overcapacity also increases, although not quite as rapidly, when increasing the reducing gas CO concentration. Therefore, the total decrease in electricity demand flexibility of the process (here meaning the difference between the maximum and minimum electricity demand) when going from H-DR to 30% (by mole) CO and 1.5% (by weight) DRI carburization is only around 39% (from 832 MW to 507 MW of variability).

[0112] Thus, to summarise the advantages of the various alternatives with regard to each other, a pure H-DR process may provide a greater possibility of dynamic operation as expressed in electricity demand flexibility, but the operational costs of a process utilizing a CO-containing reducing gas may be lower, especially in the case where the energy cost of biomass is low relative to the cost of electricity. A further advantage of the CO-containing reducing gas is that it provides a carburized sponge iron that may be used as a drop-in replacement for sponge irons produced by prior art fossil-based processes in subsequent EAF and secondary metallurgical processes.