System and method for direct reduction of iron oxide powder by hydrogen

Abstract

The invention relates to a system for direct reduction of an iron oxide powder by hydrogen, the system comprising a reduction reactor (3) arranged to (i) receive the iron oxide powder, hydrogen gas H2 and a heat input, and (ii) deliver pure iron and water in the form of steam. The heat input is provided by a solar tower (5a) comprising a solar receiver (9a) at the top thereof, designed to receive, in its focal point, solar radiation reflected by a plurality of heliostats (1) arranged around this solar tower (5a).

Description

SYSTEM AND METHOD FOR DIRECT HYDROGEN REDUCTION OF IRON OXIDE POWDER FIELD OF THE INVENTION

[0001] The present invention relates to a system for direct hydrogen reduction of iron oxide powder (Fe2O3 / Fe3O4 / FeO mixture). It also relates to a direct reduction process implemented in this system. STATE OF THE ART

[0002] Most current energy production systems use fuels that emit CO2 (natural gas, coal, fuel oil, etc.). In addition, the current increase in energy costs and the risk of shortages related to the energy dependencies of many countries around the world are a reason for the search for a green alternative energy for the generation of water vapor for industrial applications.

[0003] In this context, metallic fuels, as detailed in the article "Direct combustion of recyclable metal fuels for zero-carbon heat and power" in the journal Applied Energy in 2015, are a solution discussed to produce combustion without CO2 emissions, for all types of energy production applications. Metallic fuels (magnesium, aluminum, iron) have the advantage of generating, during their combustion, only solid metal oxides that are easily recoverable in a combustion system. These can then be recycled using renewable energy through an inert anode electrolysis process or zero CO2 thermal reduction by solar energy.Using hydrogen in this last reduction process as a reducing agent makes it possible to transfer the role of energy vector from this gas, which is very complex to store and transport, to very energy-dense iron powder, which is easy to store and transport and whose energy production systems are less expensive.

[0004] Direct reduction systems for iron oxides, traditionally used in metallurgy to produce "iron sponges", use coal or gas (from methane cracking) composed mainly of hydrogen and carbon monoxide as reducing agents. These processes produce high greenhouse gas emissions, and projects to produce iron from green hydrogen (GravitHy, SSAB, etc.) or electrolysis (Electra, Boston Metals, etc.) to decarbonize this activity are already emerging worldwide.

[0005] Research in the field of metal oxide reduction has also explored the supply of heat via solar energy and with gaseous (carbon monoxide, methane) or solid (biochar) reducing agents. Without a reducing agent, the reduction temperatures of some metal oxides are much too high to use solar concentration-type processes (solar tower power plants can reach between 500 and 1000°C, and the parabolic mirror at the PROMES laboratory in Odeillo can heat beyond 2300°C). Indeed, the reduction temperature is around 3400°C for magnesia (MgO) and 3700°C for alumina (Al2O3). With a reducing agent such as methane, the reduction temperature can drop to as low as 1500°C for both types of oxides (Y. Berro et al., Energy Conversion and Management, 2022).But the process used is a solar dish that concentrates a large amount of radiation onto a small volume in order to reach very high temperatures (over 2300°C). And it is a process that does not yet exist in this form on an industrial scale.

[0006] The aim of the present invention is to propose a new device and method for the direct reduction of iron oxide powder which makes it possible to produce an efficient energy vector and which can be deployed on an industrial scale.

[0007] This objective is achieved with a system for direct reduction by hydrogen of an iron oxide powder, comprising a reduction reactor arranged to (i) receive the iron oxide powder, hydrogen gas H2 and a heat supply and to (ii) deliver pure iron and water in vapor form, characterized in that the heat supply is provided by a solar tower comprising at its top a solar receiver designed to receive at its focus solar radiation reflected by a plurality of heliostats arranged around this solar tower.

[0008] In a first configuration of the invention, the reduction reactor comprises a fluidized bed arranged in the solar receiver and connected on the one hand to the outlet of an iron oxide reservoir and on the other hand to the inlet of a reservoir for receiving the pure iron resulting from the reduction of the iron oxides.

[0009] The reduction system may further advantageously comprise, downstream of the fluidized bed, means for condensing the water vapor and, downstream of said condenser means, means for separating, on the one hand, liquid water from the condenser means and, on the other hand, excess hydrogen which has not been recombined in the reduction reactor.

[0010] It may also comprise, upstream of the fluidized bed, hydrogen gas storage means provided to receive the excess hydrogen from the separating means, as well as downstream of these separating means, means for electrolyzing the liquid water from said separating means and delivering the hydrogen gas thus produced into the hydrogen storage means.

[0011] The solar tower can also include:

[0012] - a conduit for supplying iron oxides from an iron oxide tank to the fluidized bed,

[0013] - a conduit for supplying the reduction hydrogen from hydrogen storage means located near the base of the tower to the fluidized bed,

[0014] - a conduit for draining pure iron from the fluidized bed to an iron reservoir, said reservoirs of iron oxides and pure iron respectively being arranged near the base of said tower, and

[0015] - a conduit for evacuating water vapor from the fluidized bed.

[0016] The fluidized bed may also include a grid for fluidizing iron oxides and a high-temperature filter at the outlet of excess water vapor and hydrogen.

[0017] The solar receiver and the fluidized bed can be arranged so that said fluidized bed receives solar energy in a concentrated manner around its entire circumference.

[0018] In another configuration of a reduction system according to the invention, the reduction reactor may be arranged in whole or in part in a molten salt bath (arranged in the form of a molten salt exchanger / oxide reactor) arranged near the solar tower and thermally coupled to the solar receiver via a closed molten salt circuit.

[0019] The reduction reactor may further comprise means for conducting excess water vapor and hydrogen from the reduction reactor to means for separating the hydrogen from the water, said separating means being connected at the outlet on the one hand to water storage means and on the other hand to hydrogen gas storage means provided for supplying hydrogen to the reduction reactor.

[0020] The reduction system according to the invention may further comprise, downstream of the water storage means, electrolyser means provided for producing hydrogen gas and delivering it to the hydrogen storage means.

[0021] The electrolyser means can also be designed to receive an external water supply as an input.

[0022] The reduction system according to the invention may further comprise a unit for generating electrical energy by converting part of the heat present in the reduction reactor.

[0023] This electrical energy generation unit may comprise a water vapor circuit comprising a water vapor generator thermally coupled to the reduction reactor, a steam turbine mechanically coupled to an electric generator, and equipment for condensing water vapor from said turbine to deliver liquid water to the inlet of the water vapor generator.

[0024] According to another aspect of the invention, there is provided a method for direct reduction by hydrogen of an iron oxide powder, implemented in a reduction system according to the invention, comprising steps for:

[0025] - bringing iron oxide powder and hydrogen gas H2 into a reduction reactor designed to deliver pure iron and water in vapor form,

[0026] - supply heat to said reduction reactor,

[0027] characterized in that the heat supply is provided by a solar tower comprising at its top a solar receiver designed to receive at its focus solar radiation reflected by a plurality of heliostats arranged around this solar tower.

[0028] In a first configuration of the invention, the reduction of iron oxides is carried out at the top of the solar tower, in the form of a fluidized bed of iron oxides included in the solar receiver.

[0029] In another configuration of the invention, the reduction of iron oxides is carried out in a reduction plant installed near the solar tower, within a bath of molten salts thermally coupled to the solar receiver.

[0030] The reduction process according to the invention may further advantageously comprise a production of electrical energy from the heat present in the reduction reactor, as well as a production of hydrogen from the water vapor from the reduction reactor, to contribute to the supply of hydrogen required for the reduction of iron oxides.

[0031] The invention thus provides a system for direct reduction of iron oxides with hydrogen by solar means comprising a tower receiving solar radiation (at a receiver close to the top) from the heliostats (located around the tower), a device for fluidizing iron oxide powder in the heated volume of the tower receiver, thus promoting the gas-solid reaction mixture, a device for transporting the oxides and emptying the metal produced, a thermal recovery system at the exhaust to recover the calories during the condensation of the water vapors, and a device for recovering unreacted hydrogen using a phase separator (separation of liquid water and gaseous hydrogen).

[0032] The system according to the invention makes it possible to use the basic structure and design of a concentrated solar power plant tower that usually heats molten salts, and adapt it to heat instead iron oxide powder fluidized by hydrogen in order to reduce it to pure iron while emitting no greenhouse gases in the process (and to fully exploit the capacity of a solar tower power plant). This tower, whose initial function is the thermal storage of solar radiation in molten salts, is thus converted to an iron production function in order to allow storage of renewable energy over a longer period than by heating molten salts (whose thermal storage duration is of the order of 8 to 10 hours). This would also allow the transport of this energy, in the form of pure iron, over long distances (to areas with a low concentration of renewable energy but high energy demand).Local or non-local use of this recycled iron can be done using an iron combustion thermal power plant or other energy conversion processes.

[0033] Advantageously, in this same variant, the unreacted hydrogen is returned to the tank which supplies the iron oxide reduction plant.

[0034] In another variation of the direct solar reduction system for iron oxides, the water from the hydrogen oxidation reaction can be reused to produce hydrogen by in-situ green electrolysis.

[0035] In another variation of the direct solar iron oxide reduction system, a steam turbine power generation unit can be placed at the system's exhaust. This energy can be used to operate the plant or can be distributed to the electricity grid, thus improving the overall energy efficiency of the system.

[0036] Advantageously, in this same variant, the water resulting from the hydrogen oxidation reaction can be reused to produce hydrogen by in-situ green electrolysis.

[0037] Advantageously, the fluidized bed located in the hot zone of the solar tower receiver is supplied with iron oxides by a powder transport system which may consist of a worm screw or a blowing system (or any other suitable system). This operation is carried out when the fluidized bed has previously been emptied of the iron powder resulting from the reduction, thanks to a system for transporting the metal to its storage tank. These two operations are carried out successively after the reduction operation has converted all the iron oxide powder in the tank into pure iron. It is preferable to carry them out before starting the fluidization of the bed by hydrogen.

[0038] In the case of the reduction of iron oxides, based on a mixture of more than 99% Fe2O3 (as is the case in hematite or in oxide powder resulting from complete combustion of iron) whose reduction is the most energy-consuming (among the three iron oxides), the reaction takes place at a much lower temperature than the other two metals mentioned above (between 400 and 1000°C depending on the reducing agent chosen and the quality of the reaction mixture). This has the advantage of allowing the use of concentrated solar towers which are widespread throughout the world (Noor III in Ouarzazate, Ivanpah in California, etc.) and whose concentrated solar technology is already proven at high power (several hundred megawatts).Using hydrogen as a reducing agent at a sufficiently high temperature (between 700 and 900 °C) allows for a near-complete conversion of iron oxides to pure iron to be achieved more quickly, which would meet production targets on an industrial scale (Daniel Spreitzer et al. “Reduction of Iron Oxides with Hydrogen—A Review”). The main advantage of this process, with hydrogen as the reducing agent, is that it does not emit CO2 during the production of pure iron.

[0039] In another configuration of a system for direct reduction of iron oxides with hydrogen by solar means in which the reduction of the oxides is carried out in a power station located at the base of a solar tower or close to a solar tower, it comprises a tower receiving solar radiation (at a receiver close to the top) from the heliostats (located around the tower), to heat molten salts as conventionally. A device for fluidizing iron oxide powder heated by the molten salts is located in a reduction power station located close to the tower. This reduction power station is located and positioned in a similar manner to a thermal power station which conventionally converts the heat of the molten salts into electricity.The reduction system contains a device for transporting oxides and emptying the produced metal, a thermal recovery system at the exhaust to recover calories during the condensation of water vapors, and a device for recovering unreacted hydrogen using a phase separator (separation of liquid water and gaseous hydrogen).

[0040] Advantageously, in this same variant, the unreacted hydrogen is returned to the tank which supplies the iron oxide reduction plant.

[0041] Advantageously, in this same variant, the water resulting from the hydrogen oxidation reaction can be reused to produce hydrogen by in-situ green electrolysis.

[0042] Advantageously, the fluidized bed is supplied with iron oxides by a powder transport system which may consist of a worm screw or a blowing system (or any other suitable system). This operation is carried out when the fluidized bed has been previously emptied of the iron powder resulting from the reduction, or continuously, using a system for transporting the metal to its storage tank.

[0043] In another variant of the direct solar reduction device for iron oxides, the water from the hydrogen oxidation reaction can be reused to produce electricity in parallel by simultaneously heating the oxide reactor and a steam generator with molten salts.

[0044] The present invention allows the storage of solar energy by the production of iron which will serve as an energy vector by allowing delocalized use (example: reduction of oxides in the Maghreb, local production of energy and transport of part of the iron to Europe) and time-shifted use (iron can be stored for months without difficulty). The combustion of powdered iron is the means of producing energy without CO2 emissions for very varied applications (production of heat and electricity, mobility, etc.). The production of iron without greenhouse gases (by solar means) also makes it possible to decarbonize the metallurgy sector (steel production). DESCRIPTION OF FIGURES

[0045] Other features and advantages will appear on reading the following description of two particular embodiments and two non-limiting operating modes of the invention, given with reference to the figures in which:

[0046] - is a schematic representation of a first embodiment of the invention;

[0047] - is a schematic representation of a variant of the first embodiment of the invention presented in;

[0048] - is a schematic representation of a first mode of operation of the iron oxide reduction plant by solar concentration;

[0049] - is a schematic representation of a second mode of operation of the iron oxide reduction plant by solar concentration;

[0050] - is a schematic representation of a third mode of operation of the iron oxide reduction plant by solar concentration;

[0051] - is a schematic representation of a variant of the solar tower power plant presented in the, and;

[0052] - schematically represents another configuration of an iron oxide reduction system according to the invention, in which the molten salts directly heat a reduction plant near the solar tower;

[0053] - schematically represents a variant of this other configuration, including cogeneration of electrical energy.

[0054] DETAILED DESCRIPTION OF EXAMPLES OF ACHIEVEMENT

[0055] We will now describe, with reference to the, a first embodiment of the plant for the reduction of iron oxides using hydrogen by solar means.

[0056] Such a system is based on the hydrogen reduction of iron oxides that do not emit CO2 and allow the storage of solar energy in the iron produced for long periods and for possible transport over long distances. Thus, the system makes it possible to depollute both the metallurgical activity and that of energy production while allowing a circular economy where the metallic fuel (iron in this invention) is regenerated at each cycle of use thanks to the oxides recovered during combustion.

[0057] Hydrogen is transported from a tank 0 to an iron oxide powder fluidization system located in the solar receiver 9a of a tower 5a. Before this fluidization operation, a fluidized bed 3 is fed by an iron oxide tank 2 and a transport system (possible with an endless screw).

[0058] The iron oxide powder located in the receiver is heated to very high temperatures (above 500°C) using the concentration of solar radiation permitted by a field of heliostats 1 which track the sun individually and reflect it precisely in the direction of the receiver at the top of the solar tower 9a.

[0059] The hydrogen reduction reaction of iron oxides in a solar power plant tower is accompanied by the production of very high-temperature water vapor. This water vapor can be condensed into 5 to recover the calories from this change of state and improve the system's energy efficiency. The iron resulting from the reduction is collected in an iron tank 4.

[0060] The water vapor leaving the fluidized bed 3 is mixed with unreacted hydrogen. A phase separator 6 at the condenser outlet recovers the excess hydrogen for re-injection into tank 0. The liquid water is stored in a tank 7 following the separator.

[0061] Lapresents a variant of the first embodiment, illustrated in, an electrolyzer 8 producing hydrogen from green electricity, transforms the water from the storage tank 7 into hydrogen to reinject it into the tank 0. This makes it possible to limit the need for water or hydrogen supply to power the power plant (and avoid water stress). In a first operating mode of the invention illustrated in, the fluidized bed 3 is supplied with iron oxides from a tank 2 along the solar tower 5a (by a worm screw or blowing system or other powder transport system). The solar tower 5a further comprises a conduit with extraction 8a for evacuating the water vapor and hydrogen to a facility for treating this water vapor.

[0062] In a second mode of operation of the invention illustrated in, the hydrogen from the tank 0 is transported along the solar tower 5a and then passes through a fluidization grid (distributor) 6a in order to fluidize the iron oxide powder in the fluidized bed and promote mixing and therefore the reduction reaction. The heliostats 1 reflect the radiation towards the receiver 9a where the fluidized bed 3 is located in order to reach the reduction temperature of the iron oxides. The water vapor produced as well as the unreacted hydrogen pass through a filter (resistant to high temperatures) 7a in order to prevent the presence of particles from the fluidized bed at the exhaust. This filtration is made possible by limiting the pressure losses by an extraction 8a. The water vapor and the hydrogen are then transmitted to the condensation, separation and storage blocks.

[0063] In a third mode of operation of the invention illustrated in, once the reduction of the iron oxides is almost complete, the iron is drained from the fluidized bed 3 to a storage tank 4 (by a worm or suction system or other powder transport system).

[0064] In a variant of the type of solar tower power plant presented in, and, the present invention in the context of a solar tower 5a whose receiver 9a receives in a concentrated manner the solar radiation reflected by a heliostat field 1 distributed over 360° around the tower to heat the entire fluidized bed 3 over its entire circumference. The excess steam and hydrogen are evacuated at 10, the iron oxides inlet is at 11, the hydrogen inlet at 12 and the iron evacuation at 13.

[0065] We will now describe, with reference to the, another configuration of a system 90 for reducing iron oxides in which a molten salt exchanger 915 is installed in a reduction plant (to heat the fluidized bed) located at the foot or near a solar tower 910 equipped at its top with a hearth 910 receiving solar radiation from a set of heliostats 92. The molten salt bath 915 is connected to a circuit 96 conducting cold salts from the bath via a cold salt reservoir 94 to the hearth 910 provided for heating this cold salt. The salt thus heated is returned to the molten salt bath (or salt / fluidized bed of oxides exchanger) 915 via a hot salt reservoir 93 and a hot salt injection circuit 95.

[0066] The molten salt exchanger 915 heats a reduction reactor 990 (from the inside and / or outside of the reactor) containing iron oxides and equipped with an inlet 97 for filling iron oxides and an outlet 98 for emptying the iron metal.

[0067] The reduction reactor 990 is equipped with a conduit 99 for injecting hydrogen H2 from a hydrogen tank 914 and a conduit 980 for the outlet of water H2O and excess hydrogen H2.

[0068] The outlet conduit 980 is connected to the inlet of a hydrogen / water separator 911 which comprises a hydrogen outlet connected via a conduit to a hydrogen tank 914 and a water outlet connected to a water tank 912.

[0069] The iron oxide reduction system 90 further comprises an electrolyzer 913 receiving water at the inlet from the water tank 912 and from an external water supply 916 and generating hydrogen at the outlet which is stored in the hydrogen tank 914.

[0070] In a variant of this other configuration, illustrated by the, the iron oxide reduction system 100 according to the invention further comprises an electrical generation system 930 comprising a steam generator 917 thermally coupled to the molten salts 915 (the molten salts are distributed in parallel or in series between the heating of the reactor and the generation of steam) and receiving water stored in a water tank 918. The steam from the steam generator 917 is admitted at the inlet of a turbine 919 driving an electrical generator 920 connected to a system of transformers 922 and delivering electrical energy to an electrical energy network. The turbine 919 is connected to an air-cooled condenser 921 which delivers water which is admitted at the inlet of the water tank 918.

[0071] Of course, the present invention is not limited to the exemplary embodiments which have just been described and other embodiments can be envisaged without departing from the scope of the present invention.

[0072] It is therefore possible to consider other methods of hydrogen production than electrolysis. In particular, water splitting technologies could be implemented.

Claims

System for direct reduction by hydrogen of an iron oxide powder, comprising a reduction reactor (3) arranged to (i) receive the iron oxide powder, hydrogen gas H2 and a heat supply and to (ii) deliver iron and water in vapor form, characterized in that the heat supply is provided by a solar tower (5a) comprising at its top a solar receiver (9a) designed to receive in its focus solar radiation reflected by a plurality of heliostats (1) arranged around this solar tower (5a). Reduction system according to the preceding claim, characterized in that the reduction reactor (3) comprises a fluidized bed arranged in the solar receiver (9a) and connected on the one hand to the outlet of an iron oxide reservoir (2) and on the other hand to the inlet of a reservoir (4) for receiving the pure iron resulting from the reduction of the iron oxides. Reduction system according to claim 1 or 2, characterized in that it further comprises, downstream of the fluidized bed, means (5) for condensing the water vapor and, downstream of said condenser means (5), means (6) for separating, on the one hand, liquid water from the condenser means (5) and, on the other hand, excess hydrogen which has not been recombined in the reduction reactor (3). Reduction system according to the preceding claim, characterized in that it further comprises, upstream of the fluidized bed, means (0) for storing hydrogen gas provided to receive the excess hydrogen from the separating means (6). Reduction system according to claim 3 or 4, characterized in that it further comprises, downstream of the separating means (6), means (8) for electrolyzing the liquid water from said separating means (6) and water storage means (7) in order to deliver the hydrogen gas thus produced into the hydrogen storage means (0). Reduction system according to any one of the preceding claims, characterized in that the solar tower (5a) further comprises: - a conduit for supplying iron oxides from an iron oxide reservoir (2) to the fluidized bed, - a conduit for supplying reducing hydrogen from hydrogen storage means located near the base of the tower (5a) to the fluidized bed (3), - a conduit for emptying pure iron from the fluidized bed (3) to an iron reservoir (4), said reservoirs (2, 4) respectively of iron oxides and pure iron being arranged near the base of said tower (5a), and - extraction means (8a) for evacuating water vapor from the fluidized bed (3). Reduction system according to any one of the preceding claims, characterized in that the fluidized bed (3) comprises a perforated plate (or other means of distributing the fluidization gas) (6a) for the fluidization of the iron oxides and a high temperature filter (7a) at the outlet of the water vapor and excess hydrogen. Reduction system according to any one of the preceding claims, characterized in that the solar receiver (9a) and the fluidized bed (3) are arranged so that said fluidized receives solar energy from the heliostats (1) in a concentrated manner over its entire circumference. Reduction system (90,100) according to any one of the preceding claims, characterized in that the reduction reactor (990) is arranged in whole or in part in a molten salt bath (915) arranged near the solar tower (910) and thermally coupled to the solar receiver via a closed circuit of molten salts (95,96). Reduction system (90, 100) according to any one of the preceding claims, characterized in that the reduction reactor (990) further comprises means (980) for conducting excess water vapor and hydrogen from the reduction reactor (990) to means (911) for separating the hydrogen from the water, said separating means (911) being connected at the outlet on the one hand to water storage means (912) and on the other hand to hydrogen gas storage means (914) provided for supplying hydrogen to the reduction reactor (990). Reduction system (90,100) according to the preceding claim, characterized in that it further comprises, downstream of the water storage means (912), electrolyzer means (913) provided for producing hydrogen gas and delivering it to the hydrogen storage means (914). Reduction system according to the preceding claim, characterized in that the electrolyser means (913) are further provided to receive an external water supply (916) as an input. Reduction system (100) according to any one of the preceding claims, characterized in that it further comprises an electrical energy generation unit (930) by converting part of the heat present in the molten salt bath (915). Reduction system according to the preceding claim, characterized in that the electrical energy generation unit (930) comprises a water vapor circuit comprising a water vapor generator (917) thermally coupled to the molten salt bath (915), a steam turbine (919) mechanically coupled to an electrical generator (920), and equipment (921) for condensing water vapor from said turbine (919) to deliver liquid water to the inlet of the water vapor generator (917). Method for direct reduction by hydrogen of an iron oxide powder, implemented in a reduction system according to any one of the preceding claims, comprising steps for:- bringing iron oxide powder and hydrogen gas H2 into a reduction reactor (3,990) provided to deliver pure iron and water in vapor form,- supplying heat to said reduction reactor (3,990),characterized in that the heat supply is provided by a solar tower (5a) comprising at its top a solar receiver (9a) provided to receive in its focus solar radiation reflected by a plurality of heliostats (1) arranged around this solar tower (5a). Reduction method according to the preceding claim, in which the reduction of iron oxides is carried out at the top of the solar tower (5a), in the form of a fluidized bed (3) of iron oxides included in the solar receiver (9a). Reduction method according to claim 15, in which the reduction of iron oxides is carried out in a reduction plant installed near the solar tower (910), within a bath of molten salts (915) thermally coupled to the solar receiver. Reduction method according to any one of claims 15 to 17, characterized in that it further comprises a production of electrical energy (930) from the heat present at the reduction reactor (990). Reduction process according to any one of claims 15 to 18, characterized in that it further comprises a production of hydrogen from the water vapor from the reduction reactor (3,990), to contribute to the supply of hydrogen required for the reduction of iron oxides.

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