FR2995600A1 - PROCESS FOR REDUCING CARBON DIOXIDE OF CARBON MONOXIDE BY VANADIUM OXIDES - Google Patents

Abstract

A method of treating a gas stream comprising CO (542, 642, 742), wherein CO is reduced by oxidation of vanadium oxides to produce a gaseous effluent comprising CO (543, 643). , 743). The vanadium oxides are advantageously regenerated in contact with a reducing compound (532, 632, 732) chosen from organic fuels of fossil origin, originating from biomass, or from synthesis, preferably methane. The invention also relates to a synthesis gas production process (533, 633, 733) in which vanadium oxides are reduced with at least one reducing compound (532, 632, 732) as described. above. According to the invention, these processes can be used to produce dihydrogen. The reactions between the vanadium oxides and the reducing compound or the CO are carried out at high temperature, preferably using a solar reactor.

Description

Field of the invention The field of the invention is that of the production of a gas comprising carbon monoxide from oxidation-reduction reactions using metal oxides and a carbon source. The invention particularly relates to a process for converting carbon dioxide to carbon monoxide by oxidation of vanadium oxides, and a process for producing synthesis gas by reducing vanadium oxides. General context Increasing the amount of carbon dioxide in the atmosphere, due in particular to the burning of fossil fuels, is now recognized as a factor of global warming. Many solutions have been proposed for capturing carbon dioxide from combustion fumes, especially with a view to sequestering it. However, the final sequestration of carbon dioxide presents many technical difficulties and generates a lot of concern because of the risks involved, especially those related to the sudden release into the atmosphere of large quantities of carbon dioxide stored for example under the sea or in aquifers, as has happened in the past with naturally occurring carbon dioxide. An alternative to the sequestration of captured carbon dioxide is to valorize it by converting it into a valuable chemical asset, that is to say a chemical whose chemical potential, for example expressed through the lower calorific value. PCI of the compound, is such that it can be used for the production of electricity, synthetic hydrocarbons, heat, etc. Thus, carbon dioxide can be converted to carbon or carbon monoxide, preferably avoiding the consumption of fossil energy so as not to produce more carbon dioxide than that which is converted. A great deal of research is being done in this direction, particularly in the field of solar thermochemistry, in particular for storing and transporting solar energy in a chemical form, for example in the form of dihydrogen, carbon monoxide, or a mixture of two (synthesis gas), obtained by thermal cracking. Carbon dioxide dissociation processes are thus known, using solar reactors developing very high temperatures, in which the carbon dioxide is dissociated in carbon monoxide by oxidation-reduction reactions with redox compounds, for example oxides. metal. The Fe304 / FeO redox couple is for example used in such a carbon monoxide CO 2 dissociation process (Steinfeld A. et al., "Design aspects of solar thermochemical engineering - a case study: two-step water-splitting cycle using the Fe3O4 / FeO redox system ", Solar Energy, Vol 65, No. 1, pp 43-53, 1999, and Eherensberger K. et al.," Production of carbon from carbon dioxide with iron oxides and high-temperature solar energy, Ind. Eng Chem Res, vol.36, pp645-648, 1997) In Steinfeld et al., The process of dissociation of CO2 has two main steps constituting a chemical loop: A first step is to decompose CO2 into carbon monoxide by oxidation of FeO wustite to Fe304 magnetite This is an exothermic reaction, conventionally carried out at a temperature of the order of 300 ° C. to 500 ° C. A second step consists in recycling the oxides of iron by an endothermic dissociation reaction of Fe304 magnetite in wusti Te FeO and oxygen at high temperature, in a solar reactor. The solar reactor generally comprises a window allowing concentrated solar radiation to enter through a solar concentrator, thus making it possible to reach very high temperatures capable of transforming magnetite into wustite, generally between 1800 ° C. and 2300 ° C. Intermediate steps are often necessary, for example a quenching step or separation of products from recycling to avoid recombination with wustite with oxygen during cooling. A major disadvantage of CO2 dissociation processes based on the use of iron oxides is the low conversion rate of CO2, especially related to the low reaction rates of these oxides, the degradation of redox compounds due to phenomena sintering, and the loss of redox compounds caused by uncontrolled volatilization. In addition, the quenching step or separation of the products from the iron oxide recycling step is a considerable loss of energy, which affects the efficiency of the process. Among the other redox compounds tested, the cerium oxides and the ZnO / Zn pair showed promising results. However, zinc oxide is a volatile oxide which is sublimated during the regeneration step, necessarily taking place at high temperature (about 2000 ° C.), which significantly complicates the process and imposes quenching steps and / or or separating expensive gas energy, and also poses safety and health problems. The processes of CO2 reduction by solar thermochemistry using cerium oxides are relatively expensive processes, in particular because of the high price of the raw material linked to a difficult supply. The recurring problems encountered with the various metal oxides bearing oxygen known for the dissociation of CO2 into carbon monoxide are notably linked to a temperature of thermal regeneration which is often very high, which requires the provision of considerable energy and which can pose problems of volatilization of the compounds, the stability of the oxides (thermal decomposition point too high, or melting point or sublimation too low), and the risks of agglomeration, in particular by sintering, related to a melting point too low of the metal oxide. To date, no satisfactory solution has been proposed, in particular in terms of CO2 conversion rate, simplicity of implementation and profitability, for the dissociation of CO2 into carbon monoxide on an industrial scale. OBJECTS AND SUMMARY OF THE INVENTION The present invention aims to provide a process for converting carbon dioxide to carbon monoxide which overcomes, at least in part, the disadvantages of the methods mentioned above. In general, the present invention aims to provide a process for converting carbon dioxide into carbon monoxide, which constitutes a valuable chemical active used for example for the production of hydrogen, hydrocarbons or basic chemicals, using a non-carbon dioxide energy source, for example a clean energy source such as solar energy. The present invention aims in particular to provide a process for converting at a very high rate of at least 50% and preferably close to a total conversion, carbon dioxide to carbon monoxide by a redox compound, for a economically profitable industrial exploitation.

The invention also aims to provide a process for reducing CO2 to carbon monoxide by a redox compound which is simple to implement on an industrial scale. In particular, the object of the invention is to enable the process to be carried out at temperatures compatible with the materials commonly used in the installations of the chemical industry such as furnaces and boilers using standard steels and refractories, as well as limiting the number of process steps, and avoid state changes, in particular the transition from a solid state to a liquid or gaseous state, of the redox compound used to reduce CO2. The invention also aims to provide a process for converting CO2 into carbon monoxide with a cheap and widely available redox compound.

The applicant has shown that the use of vanadium oxides is, surprisingly, particularly well adapted to the reduction of carbon dioxide to carbon monoxide, especially in the context of solar thermochemistry, and allows a rate of particularly interesting conversion, up to almost total conversion of CO2 to carbon monoxide, at temperatures advantageously close to 1000 ° C ± 300 ° C. Thus, to achieve at least one of the above objectives, among others, the present invention provides, in a first aspect, a method of treating a gas stream comprising carbon dioxide comprising a step of reducing the carbon dioxide. carbon by oxidation of vanadium oxides to produce a gaseous effluent comprising at least carbon monoxide. According to one embodiment of the invention, the method further comprises a step of regeneration by reduction of the vanadium oxides with at least one reducing compound to reduce the oxidation state of said vanadium oxides and to produce a gaseous effluent comprising at least synthesis gas. Regeneration of the vanadium oxides is preferably carried out at a temperature of between 800 ° C. and 1400 ° C., preferably between 900 ° C. and 1300 ° C., and more preferably between 950 ° C. and 1150 ° C.

The reducing compound may be chosen from organic fuels of fossil origin, organic fuels derived from biomass, or synthetic organic fuels. Preferably, the reducing compound comprises methane. According to one embodiment of the invention, the reduction of carbon dioxide is carried out at a temperature of between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C., and more preferably between 950 ° C. and 1050 ° C. ° C. According to one embodiment of the invention, the molar ratio between the vanadium vanadium vanadium and the carbon dioxide during the carbon dioxide reduction step is greater than or equal to 1. According to one embodiment of the invention, In the invention, the reduction of carbon dioxide and the regeneration of vanadium oxides are carried out in a chemical loop in separate reactors by means of circulating fluidized beds. According to another embodiment, the reduction of carbon dioxide and the regeneration of the vanadium oxides are carried out alternately in at least one same reactor by means of a fixed bed or a non-circulating fluidized bed.

According to one embodiment of the invention, the vanadium oxides are heated in a solar reactor so as to provide the energy necessary for the regeneration of the vanadium oxides. According to a second aspect, the invention proposes a process for producing synthesis gas in which vanadium oxides are reduced using at least one reducing compound chosen from organic fuels of fossil origin, organic fuels originating from biomass, or synthetic organic fuels, preferably methane. Preferably, the reduction of the vanadium oxides is carried out at a temperature of between 800 ° C. and 1400 ° C., preferably between 900 ° C. and 1300 ° C., and more preferably between 950 ° C. and 1150 ° C. According to one embodiment of the invention, solar thermal energy is used to at least partially heat said vanadium oxides. According to another embodiment, the synthesis gas production process further comprises a step of recycling the vanadium oxides resulting from the reduction step with the aid of said at least one reducing compound, the recycling step consisting of oxidizing the vanadium oxides by contacting them with a gaseous stream comprising CO2, to increase the oxidation state of the vanadium oxides and produce a gaseous effluent comprising carbon monoxide. This recycling step may be carried out at a temperature of between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C., and more preferably between 950 ° C. and 1050 ° C. The invention finally proposes, according to a third aspect, a process for producing hydrogen comprising a treatment of a gaseous flow comprising carbon dioxide according to the invention or a production of synthesis gas according to the invention, in which: the gaseous effluent comprising at least carbon monoxide or the gaseous effluent comprising at least synthesis gas or a combination of the two effluents is sent to a cooling unit generating water vapor from liquid water to produce a cooled stream; the cooled stream is sent to a water conversion unit in which the carbon monoxide of the cooled stream reacts with the water vapor introduced into the water conversion unit to form a gaseous stream enriched with carbon and dihydrogen; and the gaseous stream enriched with carbon dioxide and dihydrogen is sent to a separation system for separating the hydrogen from said gaseous stream enriched with carbon dioxide and dihydrogen. Other objects and advantages of the invention will appear on reading the following description of particular embodiments of the invention, given by way of non-limiting examples, the description being made with reference to the appended figures described below.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a diagram representing the simulated thermodynamic equilibrium of different chemical species involved in the step of CO 2 reduction by CO vanadium oxides as a function of temperature, according to one embodiment of the invention. FIG. 2 is a diagram representing the simulated thermodynamic equilibrium of different species involved during the step of reducing CO 2 by vanadium oxides as a function of temperature, according to another embodiment of the invention .

Figure 3 is a diagram showing the simulated thermodynamic equilibrium of the various vanadium oxides as a function of temperature. FIG. 4 is a diagram representing the simulated thermodynamic equilibrium of different chemical species involved during the step of regeneration of vanadium oxides as a function of temperature, according to one embodiment of the invention.

Figures 5 to 7 schematically show different modes of implementation of the methods according to the invention. FIG. 8 schematically represents an example of integration of the processes according to the invention in a hydrogen production plant, in daytime operation. FIG. 9 represents an example of integration of the processes according to the invention in a hydrogen production installation, in night operation. Figure 10 schematically shows an alternative embodiment of the methods according to the invention in the context of hydrogen production, in daytime operation. In the figures, the same references designate identical or similar elements. DESCRIPTION OF THE INVENTION The object of the invention is to propose a process which makes it possible to convert carbon dioxide CO 2 into a valuable chemical reagent, such as carbon monoxide CO, by reaction with contact with vanadium oxides. using preferably the heat provided by the sun which is a renewable energy abundant and non-emitting CO2. CO2 reduction to carbon monoxide by vanadium oxides According to the invention, the CO2, which can come from a CO2 capture unit, is converted into CO during a CO2 reduction step by oxidation of carbon dioxide. vanadium oxides. This oxidation-reduction reaction between the CO 2 and vanadium oxides produces a gaseous effluent comprising at least CO. A small amount of unreacted CO2 may be present in the gaseous effluent produced. The CO2 reduction step in CO is carried out in a reactor heated by one or more heat sources. In the remainder of the description, reference will be made to the oxidation reactor to designate the reactor in which the vanadium oxides are oxidized during the CO2 reduction step, and to the reduction reactor to designate the reactor in which the oxides of vanadium are reduced by a reducing compound, and wherein the vanadium oxides can be regenerated during the CO 2 conversion process. It should be noted that in the present description the terms oxidation and reduction used with respect to vanadium oxides refer to the oxidation state of the vanadium oxides, which will also be referred to as the oxidation state OD. In particular, the degree of oxidation OD will be expressed according to the molar ratio of oxygen to vanadium ON. Thus, the main vanadium oxides used in the processes according to the invention are the following: - VO (D0 = 1); V203 (D 0 = 1.5); - V305 (D0 = 1.67); - VO2 (D0 = 2), which is the most oxidized form appearing in the system. By vanadium oxides is meant any vanadium oxide as described above, or any mixture of such oxides. The vanadium oxides introduced into the oxidation reactor to reduce CO2 to CO have, for the most part, a low oxidation state that allows CO2 to be decomposed into CO. The vanadium oxides used can come from natural sources such as ores, or from industrial waste, such as used catalysts used in the petroleum industry and containing vanadium oxides. In the latter case, the processes according to the invention advantageously make it possible to recycle such industrial waste. A source of vanadium oxides may, for example, consist of used catalysts used in H-Oil processes which aim to recover the residues resulting from heavy crudes by ebullated catalytic hydroconversion, and which may contain from 15 to 20% of vanadium oxides. In the H-Oil process, the treated feedstocks, generally vacuum distillates, generally contain metals, especially vanadium, in small amounts. These metals, during the contact between the charge and the catalyst, are deposited on the catalyst and accumulate gradually. In general, the concentration of metals on the catalyst in operation depends on the concentration of metals in the feeds.

Vanadium can be used in a mixture, the rest of the composition having a role of ballast, or a role of thermal buffer to limit temperature differences between equipment. Preferably, spent catalysts or natural ore treated to concentrate the vanadium oxides undergo grinding and sieving adapted to the operation of the unit, especially in the case of implementation in a fluidized bed. In this case, the median diameter of used catalysts is for example between 100 micrometers and 500 micrometers. In order to keep the vanadium oxides used catalysts in a low oxidation state in view of their use in the CO 2 conversion process according to the invention, especially with a view to their introduction into the oxidation reactor, it is preferable to storing the source of the vanadium oxides in a non-oxidizing environment, for example in the absence of air. If the source of the vanadium oxides is a natural ore, it is preferable to carry out a step of reducing the oxides prior to their introduction into the oxidation reactor, for example by introducing them into the reduction reactor where they react with a compound reducing agent, such as methane. Equations (I) to (III) describe the main reactions involved between CO2 and vanadium oxides in this step. 2 VO + CO2 -V203 + CO (I) 3 V203 + CO2 -> 2 V305 + CO (II V305 + CO2 -> 3 VO2 + CO (III) The total reaction is summarized according to the following equation (IV) : VO + CO2 CO + VO2 (IV) The other vanadium oxides correspond to less stable forms, which can be present in very small quantities in the system.This CO 2 treatment is carried out at high temperature.The reduction of CO2 by the oxides of vanadium is carried out at a temperature between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C., and even more preferably between 950 ° C. and 1050 ° C. Figure 1, which represents the thermodynamic equilibrium of vanadium oxides and CO2 as a function of temperature, for an initial equimolar mixture of VO and CO2, shows that from around 700 ° C, the conversion rate of CO 2 to carbon monoxide is significant, that is, more than 50%, and at this temperature there is no free carbon left from the CO2 degradation in the system Carbon in solid form produced at these temperatures, also commonly called coke, is indeed undesirable in the process, because it can hinder reactions, cause problems of agglomeration of particles, fouling reactors etc. These results come from calculations carried out by a thermochemical computation software, the HSC Chemistry software (version 6.1). The initial conditions used for the simulation shown in Figure 1 are as follows: a temperature of 100 ° C, a pressure of 1 bar, 1000 moles of CO2, 1000 moles of VO, and 100 moles of N2. The presence of nitrogen in the system is necessary for calculation by the software to avoid drifts. FIG. 2 similarly illustrates the thermodynamic equilibrium of vanadium oxides and of CO2 as a function of temperature, but with an initial mixture comprising twice as much VO as CO2 (2000 moles of VO per 1000 moles of CO2) . Nitrogen in a very small amount of nitrogen (1x10-2 mol) is taken into account in the initial state for the same reasons mentioned above. The results show a higher conversion rate of CO2 to CO than in the case of an initial equimolar vanadium / CO2 mixture. The CO 2 conversion to CO is greater than 50% above 700 ° C, and greater than about 90% above 850 ° C. It is almost total (within 5%) for a temperature equal to at least 950 ° C. Advantageously, the carbon is almost completely absent from the system from 900 ° C. Figures 1 and 2 show that the dominant vanadium oxides in the oxidation-reduction reactions involved with carbon dioxide are the following: V203, V305 and VO. Preferably, the molar ratio between vanadium and CO2 during the CO2 reduction step is greater than or equal to 1, preferably greater than or equal to 1.5, and even more preferably greater than 2. Excess vanadium contributes to improving the CO2 conversion rate to CO. The temperature range 700 ° C - 1300 ° C, preferably applied during this CO2 reduction step, and the preferred temperature ranges, therefore make it possible to obtain a conversion rate of CO 2 into CO 2 interesting, a free carbon content. minimal, or almost zero, while limiting the energy consumption required for the reaction. These temperature ranges also make it possible to use conventional installations, in particular from the point of view of the constituent materials of the reactors, which may be conventional refractory materials, which do not need to withstand very high temperatures. The application of these temperature ranges also makes it possible to limit the temperature differences between the step of reducing the CO 2 by the vanadium oxides and a step of regeneration of the said vanadium oxides, thus optimizing the energy requirements of the process. Thus, it is ultimately the cost of implementing the method that is minimized by the use of these temperature ranges. According to one embodiment of the invention, the oxidation reactor comprises an interface with the solar radiation to bring at least a portion of the energy 10 to said reactor. According to another embodiment, a solar reactor separate from the oxidation reactor is used to supply the energy necessary for the operation of the process, in particular the reduction of vanadium oxides which is endothermic. The CO 2 CO 2 reduction reaction by vanadium oxides is an exothermic reaction. Thus, the heat input provided at this stage is minimal. Reduction of vanadium oxides by at least one reducing compound The applicant has demonstrated that contacting a mixture of vanadium oxides comprising vanadium oxides in a high oxidation state with a reducing compound selected from organic fuels of fossil origin, organic fuels derived from biomass, or synthetic organic fuels, preferably methane, in particular under specific temperature conditions, makes it possible to regenerate said vanadium oxides as part of the process of the invention. CO2 reduction in CO, while producing syngas (CO + H2), without CO2 generation. Thus, the vanadium oxides from the CO2 reduction step are advantageously regenerated to be used again to reduce CO2. During this regeneration step, the mixture of vanadium oxides from the CO2 reduction stage mainly comprises vanadium oxides in a high oxidation state, such as V203 or V305, and to a lesser extent. measuring the V02. Such a mixture of vanadium oxides is reduced by a reducing compound in a reduction reactor by at least one reducing compound selected from organic fuels of fossil origin, organic fuels derived from biomass, or synthetic organic fuels. Gaseous hydrocarbons, such as methane, solid hydrocarbons, such as charcoal, or liquid hydrocarbons, such as a gas oil or a middle distillate, may for example be used as a reducing compound during this step. Methane is preferably used as a reducing compound, since it has the particular advantage of being widely available and inexpensive. In the case of coal, also inexpensive and abundant, it is in gaseous form during its reaction with the vanadium oxides, and can be gasified prior to its introduction into the reduction reactor, or introduced in solid and gasified form. in the reduction reactor.

In this step, the vanadium oxides release oxygen which combines with the reducing compound to produce a gaseous effluent comprising at least synthesis gas (CO and H2). The gaseous effluent leaving the reduction reactor may also comprise other components initially present in the reducing compound in very small quantities, such as for example dinitrogen, CO2 or argon in the case of natural gas used. as a reducing compound. The invention therefore has the additional object of providing a method for producing synthesis gas in which vanadium oxides are reduced using at least one reducing compound chosen from organic fuels of fossil origin, organic fuels from biomass, or synthetic organic fuels, preferably comprising a hydrogen / carbon ratio H / C high, greater than or equal to 2, even more preferably greater than or equal to 3.6 (case of natural gas for example). Preferably, the reducing compound comprises methane. The main reactions used during this stage of reduction of vanadium oxides are illustrated in the case of methane by equations (V), (VI), and (VII) below. 3 VO2 + CH4 -V305 + CO + 2 H2 (V) 2 V305 + CH4 ----> 3 V203 + CO + 2 H2 (VI) V203 + CH4 -> 2VO + CO + 2H2 (VII) The total reaction can be summarized according to the following equation (VIII): VO2 + CH4 -> VO + CO + 2 H2 (VIII) In the case of methane, the synthesis gas production process according to the invention is similar to reforming by partial oxidation of methane in the presence of vanadium oxides which are reduced. Compared with conventional reforming of methane by steam-reforming or autothermal reforming, such a method has the particular advantage of producing a pure synthesis gas which therefore does not require a subsequent step of gas separation (to eliminate any NOx compounds , SOx).

The vanadium oxides advantageously remain in the solid state during the gas production process according to the invention, unlike conventional or autothermal reforming. This limits the effect of dilution and pollution of the synthesis gas. FIG. 3 represents the purely thermal equilibrium, calculated by the HSC Chemistry software, of the main vanadium oxides, for an initial system comprising 1x105 moles of vanadium oxide V305 and the following parameters: a temperature of 100 ° C., a pressure of 1 bar, 1x106 moles of nitrogen and 1x103 moles of CO2 in order to avoid drifts in the operation of the software. As shown in FIG. 3, the vanadium oxides require very high temperatures, of more than 2500 ° C., to recover a lower oxidation state. At these very high temperatures, most vanadium oxides are in liquid or gaseous form. The main vanadium oxides used in the processes according to the invention have a melting point of 1700 and 2000 ° C. The use of a reducing compound as described, for example methane, thus makes it possible to overcome the need to use very high temperatures to reduce vanadium oxides, which can pose problems in terms of feasibility. energy and economic cost, and complexity of the process, linked for example to the liquid or gaseous state of the vanadium oxides requiring devices for recovery / separation of products. Thus, during the step of reducing the vanadium oxides according to the invention, the vanadium oxides advantageously remain in the solid state, allowing an easy recovery thereof, with for example an implementation of the process in a circulating fluidized bed system well known in the prior art, and to avoid any problem of harmfulness related to volatilization. The reduction of the vanadium oxides with a reducing compound as described is an endothermic reaction, preferably carried out at a temperature of between 800 ° C. and 1400 ° C., preferably between 900 ° C. and 1300 ° C., and more preferably between 950 ° C. C and 1150 ° C. FIG. 4 represents the thermodynamic equilibrium, calculated by the HSC Chemistry software, of different chemical species involved during the stage of reduction of the vanadium oxides by methane as a function of the temperature. The initial mixture comprises 3x104 moles of CH4 and the vanadium oxides in the following quantities: 1.5x104 moles of VO, 1x103 moles of VO2, 7.4x104 moles of V203, and 1x104 moles of V305. The other initial parameters are a temperature of 100 ° C and a pressure of 1 bar. 1x10-2 moles of nitrogen are taken into account to avoid software drift. Figure 4 shows that from 800 ° C, the production of synthesis gas and the regeneration of vanadium oxides become interesting, with a rate of free carbon product which decreases. From 1050 ° C, there is advantageously no free carbon in the system, and the production of synthesis gas reaches a plateau. The temperature range 800 ° C-1400 ° C, and the preferred ranges, thus allow the production of synthesis gas at an interesting rate, with a minimum free carbon content, without significant production of water and CO2. In the synthesis gas production process according to the invention, the vanadium oxides which have been reduced by the reducing compound can be recycled in order to be used again to produce synthesis gas. This recycling step consists of the oxidation of the vanadium oxides by putting them in contact with an oxidizing compound, for example a gaseous flow comprising CO 2 as described above in connection with the CO 2 treatment process, to increase the oxidation state of said vanadium oxides and producing a gaseous effluent comprising carbon monoxide. The reduction of the vanadium oxides is carried out in a reduction reactor which is heated by one or more heat sources, for example by natural gas burners and / or by the heat provided by the fluidized vanadium oxides and heated by an interface. with concentrated solar radiation. According to one embodiment of the invention, the reduction reactor comprises an interface with the concentrated solar radiation. In another embodiment, a solar reactor separate from the reduction reactor is used to heat the vanadium oxides. Heat source used in the process Preferably, the energy source used to reach the reaction temperatures does not produce CO2. Solar energy is a good candidate for the process according to the invention. The use of solar reactors, heated by concentrated solar radiation by a solar concentrator, makes it possible to achieve the temperature ranges used in the present invention, while offering a minimal or even zero carbon footprint. Solar reactors of different types can be used, for example direct irradiation reactors in which the reagents present in the reactor are directly exposed to concentrated solar radiation, generally via a window, or indirectly irradiated reactors. comprising opaque walls capturing the heat of the concentrated solar radiation and transmitting it to the reagents contained in the reactor (Steinfeld and Palumbo, Solar Thermochemical Process Technology, Encyclopedia of Physical Science and Technology, RA Meyers Ed., Academic Press, Vol. 15., pp. 237-256, 2001). A solar reactor of direct irradiation type, such as for example described in US Pat. No. 4,290,779B or in the thesis of A. Bounaceu ("Interaction fluidized bed of solid particles-concentrated solar radiation for the development of a method of gas heating to more than 1000 K ", Thesis, Ecole Nationale Superieure des Mines de Paris, 2008), can for example be used in the context of an implementation of the fluidized bed process.

Solar energy is thus stored in the products that can be recovered from the processes according to the invention. The invention is not limited to the use of solar energy, and any other energy source preferably not generating carbon dioxide may be used, such as nuclear energy or wind energy.

The temperature rise of the reactors can be carried out initially by introducing a hot gaseous effluent into the reactors, for example a gas useful for the process such as CO2 or methane, or a neutral gas such as nitrogen, or by direct heating using conventional gas burner means. This phase makes it possible, for example, to reach a temperature of the order of 400-500 ° C., thus allowing the fluidization of the vanadium oxides, and possibly their circulation, according to the chosen embodiments, and exemplified in the method. following the description. The solar energy can then be used, for example, to bring the reaction zones to the appropriate temperatures for the CO 2 reduction and / or vanadium oxide reduction reactions.

Alternatively, only a source of energy that does not emit CO2, such as solar energy exploited via a solar reactor, is used to raise the temperature of the reactors until the reaction temperatures of the processes are reached. according to the invention. Examples of Implementation of the Methods According to the Invention FIG. 5 illustrates a mode of chemical loop implementation of the process for treating a gas stream comprising CO 2 and the process for producing synthesis gas according to the invention. According to this embodiment, the process comprises a step of reducing the CO 2 by oxidation oxidation of vanadium oxides, and a step of reducing said vanadium oxides by at least one reducing compound which can regenerate the vanadium oxides, thus be used again for the reduction of CO2, and during which is produced synthesis gas. The circulating fluidized bed technology is used in the installation 500 of the embodiment shown in FIG. 5, and in particular allows the continuous passage, via conduits (531, 541, 521), of the vanadium oxides of a oxidation reactor 540 in which is carried out the CO2 reduction step in CO at a reduction reactor 530 in which is carried out the step of regeneration of vanadium oxides. Throughout the process, the vanadium oxides are maintained in a fluid state by controlling the flow rates of the fluidizing gas at any point in the installation. The fluidization gas is preferably a gas useful for the process such as CO2 or methane, used respectively in the oxidation reactor 540 and the reduction reactor 530. The vanadium oxides are present in the process in the form of particles having characteristics specific to their use in circulating fluidized bed, namely, for example, a particle size adapted to fluidization, for example particles having a median diameter of between 100 micrometers and 500 micrometers, resistance to attrition, and a point high melting point. The particle size distribution can be obtained by grinding and sieving operations. The vanadium oxides are, for example, introduced in the form of a powder poured into the unit by pneumatic transport. In the oxidation reactor 540, vanadium oxides are introduced in a low oxidation state, and are oxidized at a temperature of preferably between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C. and more preferably still between 950 ° C. and 1050 ° C., for example at 1100 ° C., in contact with a gas stream 542 composed of CO2 introduced into the reactor 540. The CO 2 is thus converted into CO according to the total reaction described. in equation (IV), and a gaseous effluent 543 comprising CO exits the reactor 540. The exit gaseous effluent 543 may also comprise other gases, for example a small amount of unconverted CO2 or from a recombination with oxygen, or impurities contained in the initial stream 542, such as dinitrogen, water or dihydrogen. The vanadium oxides which have reacted with the CO2, and thus being in a greater oxidation state than at the time of their introduction into the reactor 540, are then transferred, via a conduit 521, into a reactor A solar concentrator 510 emits concentrated solar radiation to the solar reactor 520, in which the mixture of vanadium oxides is heated, so as to provide the energy required for the endothermic reaction to reduce the vanadium oxides by reducing compound in the reduction reactor 530. After their passage in the solar reactor 520, the vanadium oxides are indeed transferred via the conduit 531 into the reduction reactor 530. The vanadium oxides are then at a temperature greater than that reduction reactor 530. Oxidation-reduction reactions occur preferably at a temperature between 800 ° C and 1400 ° C, preferably between 950 ° C and 1300 ° C, and more preferably between 1050 ° C and 1150 ° C, for example at 1100 ° C, between the vanadium oxides from the reactor 540, being in a high oxidation state, and a reducing compound 532. These are endothermic reactions, which consume a part of the heat of the vanadium oxides provided for example by solar energy. These reactions thus allow the regeneration of said vanadium oxides, which can then be reinjected into the oxidation reactor 540 via line 541. The process thus forms a chemical loop. The reducing compound is preferably selected from organic fuels of fossil origin, organic fuels from biomass, or synthetic organic fuels. The methane is preferably used as a reducing gas 532. A gaseous effluent comprising at least synthesis gas 533, composed of CO and H 2, and formed according to the total reaction described in equation (VIII), leaves the reactor 530. The gaseous effluent 533 may also comprise other compounds in very small amounts, initially present in the reducing compound used. According to an alternative embodiment, the installation does not comprise a solar reactor separate from the reduction and oxidation reactors 540 540, and one of the reactors 530 or 540 comprises the solar reactor, preferably the reduction reactor 530 in wherein the endothermic reduction reaction of the vanadium oxides is produced by the reducing compound 532. The reduction reactor 530 can thus be a solar reactor comprising an interface with concentrated solar radiation from a solar concentrator 510, and be heated at least partially by concentrated solar radiation. Preferably, the pressure carried out during the process is close to atmospheric pressure. Advantageously, the contact time between the vanadium oxides and the reducing compound or CO 2 is between 1 second and 30 seconds, preferably between 1 second and 10 seconds. FIG. 6 schematizes another example of implementation of the process for treating a gas stream comprising CO2 and the process for producing synthesis gas according to the invention. According to this mode, the reduction of CO2 in CO and the regeneration of the vanadium oxides are carried out alternately in at least one and the same reactor 650. A fixed bed or non-circulating fluidized bed can be implemented. . The plant 600 includes a solar concentrator 610, a reactor 650, as well as a gas supply system and a gas outlet system including valves (631, 641, 634, 644). The solar concentrator 610 emits concentrated solar radiation at an interface with solar radiation, which may be part of the reactor 650, or be on a separate solar reactor (not shown). The vanadium oxides, having a low oxidation state, are introduced into the reactor 650 which is heated to a temperature sufficient to effect the conversion of CO2 to CO, that is to say at a temperature of between 700 ° C. and 1300 ° C, preferably between 850 ° C and 1150 ° C, and more preferably still between 950 ° C and 1050 ° C, for example at 1100 ° C. The temperature can be obtained by heating the reactor 650 with a conventional gas burner system and / or by introducing a hot gas stream and / or by the operation of the solar reactor. In the case where the solar reactor is distinct from the oxidation-reduction reactor 650, the temperature rise of the reactor 650 can be done, at least in part, by the introduction of the heated charge flow within the solar reactor. In a first step, a gas stream 642 comprising CO2 enters the reactor 650, by opening a valve 641 (valve 631 being closed), and is brought into contact with the mixture of vanadium oxides in a low oxidation state present in the reactor 650 in a fixed bed or in a fluidized bed. A gaseous effluent 643 consisting essentially of CO, resulting from the oxidation-reduction reactions between the vanadium oxides and the CO2 summarized by the total equation (IV), leaves the reactor 650 while the valve 644 is open (the valve 634 being closed). In a second step, the valves 641 and 644 are closed, and the valves 631 and 634 are open. A reducing compound 632, as described above, is introduced into the reactor 650, is brought into contact with the vanadium oxides from the first stage, and is in a high oxidation state within the reactor 650. of this step, the oxidation-reduction reactions are endothermic, and require a heat input, made by the vanadium oxides preferably heated by the solar reactor. The temperature during this second stage is between 800 ° C. and 1400 ° C., preferably between 950 ° C. and 1300 ° C., and more preferably between 1050 ° C. and 1150 ° C., for example at 1100 ° C. A gaseous effluent 633 comprising at least synthesis gas is then produced and leaves the reactor 650. The pressure applied may be constant, and is preferably between 10 bar and 100 bar, preferably between 15 bar and 25 bar. It is a cyclic process, alternating between the first step of CO 2 reduction in CO by vanadium oxides and the second step of reduction of vanadium oxides by a reducing compound, within the same reactor 650. 7 illustrates another embodiment of the methods according to the invention, comprising several installations similar to the installation 600 described with reference to FIG. 6, for example four installations (701, 702, 703, 704). According to this embodiment, each reactor (751, 752, 753, 754) undergoes a cycle successively comprising the first and second stages as described above in relation to FIG. 6, according to a given sequence of steps. Thus, the reactors 701 and 704 operate, for example, synchronously with respect to the first step of CO 2 reduction (operation shown in solid lines in FIG. 7), while at the same time the second step of FIG. Regeneration of the vanadium oxides is carried out in the reactors 702 and 703 (operation illustrated in dotted lines in FIG. 7). This embodiment allows a stable production of synthesis gas for subsequent use, for example Fischer-Tropsch unit. The operation of several reactors according to this embodiment also makes it possible to overcome any material problems affecting the production, such as the failure of a valve, and thus to maintain a reduced production. Destination of the products resulting from the processes according to the invention The products resulting from the processes according to the invention, that is to say the CO and / or the synthesis gas, can be used to produce dihydrogen. Hydrogen is indeed a high value-added product in various industries, particularly in the energy, refining and petrochemical sectors. The latter can for example be used as fuel in fuel cells, or to produce energy by combustion in gas turbines. It can also be used as a reagent in many refinery and petrochemical processes. FIGS. 8 to 10 are diagrams thus illustrating examples of integration of the CO 2 conversion process into CO according to the invention, comprising a step of regeneration of the vanadium oxides, in a hydrogen production plant. FIGS. 8 and 9 illustrate in particular an embodiment in which a chemical loop-type CO2 conversion plant 500, in every respect similar to that described with reference to FIG. 5, is integrated in a hydrogen production unit. . FIG. 8 illustrates in particular an example of the day-to-day operation of the hydrogen production unit, more specifically when solar energy is available, and FIG. 9 an example of night-time operation of the same unit (solar energy is not available anymore). The embodiment illustrated in Figure 9 may also operate during the day, in the absence of sun. With reference to FIG. 8, the gaseous effluent 543 comprising CO, produced in the oxidation reactor 540, and the gaseous effluent 533 comprising synthesis gas are sent to a steam production and overheating system 850. water 852 from 853 recycled water and 854 water make-up. This steam production allows the synthesis gas to be cooled, which is then compressed by a compression system 860. The gas is then sent in a water shift unit (shift) 870, then routed to a separator 880 to remove the water 853 from the synthesis gas. The synthesis gas 891 is sent to a molecular sieve separation system 890, comprising a CO2 separation system (not shown), the latter being partly recycled by routing to the reactor 540, and a second molecular sieve system ( not shown) separating pure hydrogen 892 from a purge 893. The purge 893 is a multi-component gas comprising mainly dihydrogen and CO2, and to a lesser extent CO and water. This purge is a valuable product that can be used to produce heat or electricity.

The CO reacts with the water in the water conversion unit 870 according to the reaction (IX) below, well known by the English name of "water gas shift" and denoted WGS, or Dussan's reaction. which increases the hydrogen yield of the process of converting CO2 to CO in which synthesis gas is produced during the regeneration of the vanadium oxides. CO ± 112 0 -> 112 ± CO2 (IX) When the sun's energy is not available, what is referred to as "night mode" or nighttime operation in the present description, vanadium oxides can not be heated in the solar reactor to provide the energy necessary for their regeneration. It is then possible to keep the hydrogen production unit running in order to keep the plant 900 at operating temperatures and to minimize energy losses compared to daytime operation, as shown in FIG. 9 For this, the gas stream 942 entering the reactor 540 may be air, used to oxidize the vanadium oxides. In this case, there is no longer a transformation of CO2 into CO. The gaseous effluent 943 exiting the reactor 540 consists of hot air depleted of oxygen. The vanadium oxides in a high oxidation state are then transported and then heated in the reactor 520, in order to be brought to the reaction temperature of the reactor 530, in which the reducing compound, for example methane, is used to reduce vanadium oxides and produce syngas. To heat said vanadium oxides, it is possible to burn part of the production of hydrogen 892, as well as the entire purge 893 (not shown). FIG. 10 illustrates an operating variant of the day-running hydrogen production unit shown in FIG. 8. The hydrogen production unit comprises a plant 1000 similar to the plant 500 of FIG. the exception of a reactor 1040, positioned between the reactor 540 in which is carried out the conversion of CO2 to CO, and the reactor 520 in which the vanadium oxides are heated before their regeneration. It is an oxidation of air-assisted vanadium oxides, in which the vanadium oxides are partly oxidized in the reactor 540 in contact with the CO2 contained in the gas stream 542, and partly in the reactor 1040 at the contact with a gas stream 1042 containing oxygen, preferably air. Depleted hot air 1043 is produced and comes out of the reactor 1040. Thus, this embodiment makes it possible to minimize the solar energy input by the heat input produced during the exothermic reaction of the air with the oxides of vanadium, then preheated in the reactor 1040. The heat of the depleted air 1043 can be recovered to generate steam or for any other use.

The production of dihydrogen according to the invention is not limited to the examples illustrated in FIGS. 8 to 10. Thus, for example, the production of dihydrogen can be carried out using the synthesis gas alone, produced by the reduction of vanadium oxides by a reducing compound chosen from organic fuels of fossil origin, organic fuels derived from biomass, or synthetic organic fuels, preferably methane. The production of dihydrogen can also be carried out solely from the gaseous effluent comprising at least CO produced during the reduction of CO2 by the vanadium oxides. The synthesis gas can also be used as a feed for other chemical transformation processes, for example the Fischer-Tropsch process for synthesizing liquid hydrocarbons with long hydrocarbon chains, which can then be used as basic elements for the production of fuels. Examples The examples are based on numerical simulations with Aspen HYSYS V7.2 software from ASPENTECH. EXAMPLE 1 Process for CO2 Transformation and Production of Synthetic Loop Gas This example is based on the chemical loop embodiment described with reference to FIG. 5. A solar concentrator 510 sends concentrated solar radiation to a solar reactor 520 in which 4200 tonnes / hour of vanadium oxides enter at 1100 ° C and exit at 1200 ° C. The power required to achieve this increase in temperature is 109 MW, which corresponds to about 19 hectares of mirrors. The heated oxides are then sent via a conduit 531 to the reactor 530, into which 20.1 tonnes / hour of methane 532 are introduced. At the outlet of this reactor, a gaseous effluent comprising a stoichiometric mixture (CO + 2H 2) is sent to users, at a rate of 40.4 tonnes / hour. Since the methane reduction reaction of the vanadium oxides is endothermic, the reactor 530 is at a temperature of about 1100 ° C. The metal oxides in a low oxidation state are then sent via the conduit 541 to the reactor 540, in which 55.8 tons / hour of a gaseous effluent 542 containing CO2 is introduced. At the outlet of the reactor 540, a gaseous effluent 543 containing CO, as well as a very small amount of CO2 (approximately 4.6 mol%), is produced at a rate of 36.5 tonnes / hour. The vanadium oxides in a larger oxidation state leave the reactor 540, via the conduit 521, and are returned to the solar reactor 520.

Example 2: Hydrogen Production in Day Mode Incorporating a Chemical Loop Process for Carbon Dioxide Transformation and Synthesis Gas Production This example is based on the embodiment described with reference to FIG. 8.

In this embodiment, it is possible to produce 100,000 m3 / h of hydrogen under normal conditions of temperature and pressure, in day configuration. This second example integrates the flow rates described in example 1 in relation with the installation 500 of FIG. 8. The overall flow 851 is generated by the pooling of the effluents 533 and 543. The flow rate of the flow 851 is equal to the the sum of effluent flows 533 and 543, respectively 40.4 tonnes / h and 36.5 tonnes / h, representing a flow rate of 76.9 tonnes / h. Flow 851 comprises 48.8 mol% of dihydrogen, 50 mol% of CO, and 1.2 mol% of CO2. The molecular weight of the flow 851 is 15.5 and the temperature of 1108 ° C. at a pressure close to atmospheric pressure. The high amount of CO makes it possible, by means of the conversion reaction with water (IX), to produce additional hydrogen. The high temperature makes it possible to generate and superheat the steam necessary for this reaction with a steam generation system close to what is generally done at the outlet of the gas turbines or in a power plant. The very high molecular weight of the 851 flux relative to that of hydrogen makes it easy to compress the mixture. In the case for example of the use of a centrifugal compressor, the compression ratio is even greater than the molecular weight is large. It is thus possible to overcome a recurring problem of processes generating pure hydrogen at atmospheric pressure: the compression of the hydrogen necessary to bring the gas back to the usual pressures of use in hydrogen networks, of the order of 20 bar which is very difficult and very expensive to achieve. The flow 851 is cooled in the system 850, generating the water vapor 852, then is compressed in the compression system 860. The energy required for the compression at the compression unit 860 is of the order 16 MW. The stream 851 is then sent with the superheated steam 852 to the water conversion unit 870, including shift reactors and heat exchangers. In this unit 870, substantially all of the CO is converted and the cooled mixture is sent to the separator 880, which will return the condensed water 853 to the steam generating system 850. A makeup water 854 is required because a significant portion of the water has been converted to hydrogen using CO. The synthesis gas 891, consisting essentially of hydrogen and CO2 is sent to the molecular sieve separation system 890, comprising a first system which will separate almost pure low pressure CO 2 894, which may be recycled in part in the process. reactor 540. The rest of the synthesis gas is purified in unit 890 by a second system separating pure hydrogen 892 at high pressure, produced at a flow rate of 9 tonnes / h, from a low pressure purge 893. Purge 893 has a flow rate of 24 tonnes / h, and is composed of 41.5 mol% of dihydrogen, 2.2 mol% of H 2 O, 3 mol% of CO, and 53.3 mol% of CO2.

This purge 893 is energy-efficient and can be sent to an installation to produce heat or electricity. In this example the CO2 produced 894 has a flow rate of 88 tonnes / h, which is close to what would release in the atmosphere a normal steam reforming unit for the same production of hydrogen. In the case of the invention, the CO2 released into the atmosphere is only 32 tons / h, since 56 tons / h are recycled in the reactor 540, resulting in a 63% reduction in emissions. EXAMPLE 3 Production of Hydrogen in Non-Solar Mode Incorporating a Chemical-Cooled Process for Converting CO2 and Producing Synthesis Gas This example is based on the embodiment described with reference to FIG. 9.

The installation illustrated in FIG. 8, the operation of which is exemplified in example 2, operates during the day in solar mode. At night, it is important to maintain the installation temperature, and interesting to maintain even reduced production. To do this, when the solar energy is no longer available, we stop sending CO2 into the reactor 540, and instead inject 108 tons / h of air 942 to continue to oxidize the vanadium oxides. This oxidation is more extensive than the oxidation of vanadium oxides with CO2, and the reaction is then more strongly exothermic. Thus, less heat is required in the reactor 520 to heat the vanadium oxides prior to their reduction in the reactor 530. The power required to heat the oxides in the reactor 520 is 21 MW instead of the 109 MW from the solar energy required in day mode operation. This heat can be obtained by burning on the hot vanadium oxides of the purge 893 and a small part of the production of 892 hydrogen, of the order of 130 kg / h. EXAMPLE 4 Production of Hydrogen in Day Mode Incorporating a Chemical CO2 Transformation Process and Synthetic Gas Production, and Air-Assisted This example is based on the embodiment described with reference to FIG. 10. A solar concentrator 510 sends concentrated solar radiation to a solar reactor 520, in which 3700 tonnes / hour of vanadium oxides enter at 1150 ° C and exit at 1200 ° C. The power required to achieve this increase in temperature is 72 MW, which corresponds to about 12.5 hectares of mirrors. The heated oxides are then sent to the oxidation reactor 530 into which is introduced via the conduit 531 23.8 tons / hour of methane 532. At the outlet of this reactor, a gaseous effluent 533 comprising a stoichiometric mixture (CO + 2 H2) is sent to users at a rate of 47.8 tons / hour. Since the methane reduction reaction of the vanadium oxides is endothermic, the reactor 530 is at a temperature of about 1100 ° C. The metal oxides in a low oxidation state are then sent via line 541 to reactor 540, where 36.8 tonnes / hour of a gas stream 542 containing mainly CO2 is introduced. At the outlet of the reactor 540, a gaseous effluent 543 containing CO, as well as a very small amount of CO2 (4.6%), is produced at a flow rate of 25.9 tonnes / hour. The vanadium oxides in a higher oxidation state leave the reactor 540, and are returned via the conduit 1041 to the reactor 1040, in which 10.8 tons / h of air 1042 are sent. The vanadium oxides are carried at a higher oxidation state, and the exothermic reaction brings the temperature of the reactor 1040 to 1150 ° C. The vanadium oxides are then sent to the solar concentrator 520 via line 521. The gaseous effluent 1043 is composed of air that is depleted at a very high temperature, which can be used to generate water vapor, or to feed a circuit. heat, or produce electricity using for example a Stirling engine.

Claims (16)

REVENDICATIONS1. A process for treating a gaseous flow comprising carbon dioxide (542, 642, 742) comprising: a step of reducing carbon dioxide by oxidation of vanadium oxides to produce a gaseous effluent comprising at least carbon monoxide ( 543, 643, 743). The process according to claim 1, further comprising a regeneration step of reducing said vanadium oxides by at least one reducing compound (532, 632, 732) to reduce the oxidation state of said vanadium oxides and to produce a a gaseous effluent comprising at least synthesis gas (533, 633, 733). 3. Method according to claim 2, wherein the regeneration of the vanadium oxides is carried out at a temperature between 800 ° C and 1400 ° C, preferably between 900 ° C and 1300 ° C, and more preferably between 950 ° C and 1150 ° C. ° C. 4. Method according to one of claims 2 and 3, wherein the reducing compound 15 (532, 632, 732) is selected from organic fuels of fossil origin, organic fuels from biomass, or organic fuels of synthesis. The process of claim 4, wherein the reducing compound (532, 632, 732) comprises methane. 20 6. Process according to any one of the preceding claims, in which the reduction of carbon dioxide is carried out at a temperature of between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C., and more preferably between 950 ° C. and 950 ° C. ° C and 1050 ° C. 7. A process as claimed in any one of the preceding claims wherein the vanadium vanadium molar ratio to carbon dioxide in the carbon dioxide reduction step is greater than or equal to 1. The process according to any one of claims 2 to 7, wherein the reduction of carbon dioxide and the regeneration of the vanadium oxides are carried out in a chemical loop in separate reactors (530, 540) using circulating fluidized beds. 9. Process according to any one of claims 2 to 7, in which the reduction of carbon dioxide and the regeneration of the vanadium oxides are carried out alternately in at least one and the same reactor (650, 750, 751, 752, 753, 754) by means of a fixed bed or a non-circulating fluidized bed. 10. Method according to one of claims 2 to 9, wherein said vanadium oxides are heated in a solar reactor (520) so as to provide the energy required for the regeneration of said vanadium oxides. 11. Process for the production of synthesis gas (533, 633, 733) in which vanadium oxides are reduced with at least one reducing compound (532, 632, 732) chosen from organic fuels of origin fossil fuel, organic fuels derived from biomass, or synthetic organic fuels, preferably methane. 12. Process for producing synthesis gas (533, 633, 733) according to claim 11, in which the reduction of the vanadium oxides is carried out at a temperature of between 800 ° C. and 1400 ° C., preferably between 900 ° C. C and 1300 ° C, and more preferably between 950 ° C and 1150 ° C. 13. Process for producing syngas (533, 633, 733) according to claim 12, wherein solar thermal energy is used to at least partially heat said vanadium oxides. 14. A process for producing synthesis gas (533, 633, 733) according to one of claims 11 to 13, further comprising a step of recycling the vanadium oxides from the reduction step with the aid of said at least one reducing compound, said recycling step consisting in the oxidation of said vanadium oxides by placing them in contact with a gas stream comprising CO2 (542, 642, 742), to increase the oxidation state of said vanadium oxides; and producing a gaseous effluent comprising carbon monoxide (543, 643, 743). 15. Process for producing synthesis gas according to claim 14, in which the recycling step is carried out at a temperature of between 700 ° C. and 1300 ° C., preferably between 850 ° C. and 1150 ° C., and more preferentially between 950 ° C and 1050 ° C. A process for producing hydrogen comprising treating a gas stream comprising carbon dioxide (542, 642, 724) according to one of claims 1 to 10 or a production of synthesis gas (533, 633, 733). ) according to one of claims 11 to 15, wherein: the gaseous effluent comprising at least carbon monoxide (543, 643, 743) or the gaseous effluent comprising at least synthesis gas (533, 633, 733); or a combination of said effluents (851) is supplied to a cooling unit (850) generating water vapor (852) from liquid water (853, 854) to produce a cooled stream; the cooled stream is sent to a water conversion unit (870) in which the carbon monoxide of the cooled stream reacts with the water vapor (852) introduced into the water conversion unit (870) to form a gas stream enriched with carbon dioxide and dihydrogen; and the gaseous stream enriched with carbon dioxide and dihydrogen is sent to a separation system (890) for separating the dihydrogen (892) from said gaseous stream enriched with carbon dioxide and dihydrogen.