WO2009071805A2

WO2009071805A2 - Formulation of oxides, the production of same and use thereof as oxygen carrier in a process for the oxidation and/or deoxidation of a gas stream

Info

Publication number: WO2009071805A2
Application number: PCT/FR2008/052090
Authority: WO
Inventors: Sébastien Roux
Original assignee: Bio 3D Applications
Priority date: 2007-11-23
Filing date: 2008-11-19
Publication date: 2009-06-11
Also published as: FR2924035A1; FR2924035B1; EP2222597A2; WO2009071805A3

Abstract

Formulation of oxides intended to be used as oxygen carriers in a process for treating gas streams comprising several iterations of the following steps: reduction of at least one portion of said oxides in the presence of a first gas stream (FG1); and oxidation of at least one portion of said reduced oxides in the presence of a second gas stream (FG2) comprising oxygen elements; said formulation comprising a mixture of oxides composed: of at least a first oxide, known as a reactant, having a good reactivity with said first gas stream (FG1); and of a second oxide, known as a dopant, having a substantially constant reactivity with said first gas stream (FG1); the porosity of said mixture of oxides being favoured by: addition to said mixture of oxides of at least one compound, known as an adjuvant, and said formulation being obtained by drying the reactant+dopant+adjuvant mixture, at a drying temperature around 110°C. The invention also relates to a method of obtaining the formulation and to a process for treating gas streams using a formulation according to the invention.

Description

"Oxide formulation, its production and use as an oxygen carrier in a process of oxidation and / or deoxidation of a gas stream"

The present invention relates to a process for preparing an oxide formulation for use as an oxygen carrier in a gas flow process. It also relates to a formulation for use in a gas flow treatment process and a gas flow treatment method implementing such a formulation.

The field of the invention is the field of the treatment of gas flow in the presence of oxides used as oxygen carriers and more particularly the treatments comprising several iterations of a treatment cycle comprising the following steps: reduction of at least a portion of the oxides in the presence of a first gas stream, and

oxidizing at least a portion of the reduced oxides in the presence of a second gas stream comprising oxygen elements; Among the gas flow treatment processes, mention may be made of catalytic gas flow treatments or chemical loopback combustion known as "chemical looping combustion (CLC)".

CLC is a process developed by Richter et al. in 1983, which consists of the production of reversible combustion energy based on reduction and oxidation of oxides. This process represents a serious alternative for the production of clean energy. Indeed, this process allows the reduction of emissions of carbon dioxide, the main greenhouse gas, by its capture and sequestration. The combustion is then carried out thanks to the oxygen of an oxide, for example metal, instead of oxygen from atmospheric air, as in the case of a conventional combustion. This produces combustion products composed exclusively of carbon dioxide and water vapor on the one hand, and the metal used on the other hand. Carbon dioxide can be easily separated from water vapor by condensation, without energy, and can be trapped and sequestered. The metal obtained during combustion is regenerated by re-oxidation with oxygen from atmospheric air in a second reactor and recycled. The only gaseous effluent from the regeneration phase is slightly depleted air, released into the atmosphere.

In this process, the metal oxide used as oxygen carrier, must be very reactive and regenerable. It must have a good mechanical strength in order to limit the rupture of the particles, subjected during the repeated oxidation and reduction cycles to high temperatures as well as interactions between particles.

However, the oxides currently used do not meet all these conditions. The currently known oxides are classified into two groups:

a first group comprising oxides having a good reactivity such as NiO or Fe ₂ O ₃ , and

- A second group comprising oxides having a substantially constant reactivity such as AI ₂ O ₃ or CaO.

Work has been carried out to obtain oxide formulations in which the oxides of the first group are mixed with oxides of the second group. However, none of these formulations exhibited both good reactivity and good regenerability in a CLC process.

An object of the present invention is to provide a new oxide formulation having both good reactivity and good regenerability in a gas flow process, such as a CLC process or a catalysis process. The invention proposes to achieve this goal by an oxide formulation for use as an oxygen carrier in a gas flow treatment process comprising several process cycle iterations comprising the following steps:

- Reduction of at least a portion of the oxides in the presence of a first gas stream, and

oxidizing at least a portion of the reduced oxides in the presence of a second gas stream comprising oxygen elements; the formulation comprising:

a mixture of solid oxides composed of: at least one first oxide, called reactive, having a good reactivity with the first gaseous flow, and

At least one second oxide, called dopant, having a substantially constant reactivity during the oxidation and reduction steps; the porosity of the mixture of oxides being favored by adding to the mixture of oxides of at least one compound, said adjuvant.

The dopant or the dopant of the oxide mixture makes it possible:

the stability of the oxide mixture, the reactivity of the oxide mixture, and

- the regenerability of the oxide mixture

The adjuvant promotes the porosity of the oxide mixture. It thus promotes the gas-oxide contact and the penetration of the first and second gas streams into the oxide mixture. The oxide mixture whose porosity is increased makes it possible to perform a gas flow treatment with better efficiency.

At least one of the first and second oxides may comprise a metal oxide.

The reagent may advantageously comprise at least one oxide selected from oxidizable elements such as: NiO, Fe ₂ O ₃ , CuO, CoO, etc. Advantageously, the reagent can essentially be composed of CuO.

Furthermore, the dopant may comprise at least one oxide chosen from elements such as: MnO ₂ , TiO ₂ , Al ₂ O ₃ , MgO, CaO, etc. Advantageously, the dopant can essentially be composed of CaO. The adjuvant may comprise a carbon compound promoting the porosity of the oxide mixture. It can essentially be composed of graphite. Graphite contains active carbon elements that promote the porosity of the oxide mixture and thus the contact of the gas with the solid oxides. The mass proportion of the adjuvant added to the mixture of oxides with respect to the mixture of oxides can be between 5 and 20%. Advantageously, this proportion can be 10%. The adjuvant is added to the mixture of oxides, during the process of obtaining the formulation, for promote the porosity of the obtained oxide formulation. In the formulation obtained, the proportion of adjuvant is very low, or even zero.

On the other hand, in the mixture of oxides the mass proportion: of the dopant can be between 35% and 45%, and the reagent can be between 65% and 55%.

Advantageously, these proportions may be around 40% for the dopant and around 60% for the reagent. Current tests show that the best results are obtained with 40% of dopant, mixed with 60% of reagent.

According to another aspect of the invention, there is provided a process for obtaining a formulation as described above, that is to say a formulation comprising oxides, intended to be used as oxygen carriers in a gas flow treatment process comprising several iterations of the following steps:

oxidizing at least a portion of the reduced oxides in the presence of a second gas stream comprising oxygen elements; said method is characterized in that it comprises the following steps:

a mixture of at least one first oxide, said reagent, having a good reactivity with said first gas stream with at least one second oxide, said dopant, having a substantially constant reactivity with the first gas stream, and adding to the mixture of oxides of a third compound, said adjuvant, promoting the porosity of the oxide mixture. The method according to the invention may further comprise homogenizing the reactive + dopant + adjuvant mixture by mixing this mixture. Moreover, the reactive + dopant + adjuvant mixture can be ground to obtain particles of desired size and then dispersed in water which advantageously can be distilled, so as to promote the homogenization of the mixture. This step is then completed by a dehydration step of the reactive + dopant + adjuvant mixture. The drying step may advantageously be followed by a step of calcining the reactive + dopant + adjuvant mixture in the presence of a third gas stream comprising oxygen elements for oxidizing the adjuvant.

The calcination step is advantageously carried out until the oxidation of the adjuvant disappears in the mixture.

The third stream may be chosen from gaseous oxides such as:

- air,

- air enriched with oxygen,

oxygen, or any oxidized compound.

Furthermore, the calcination of the reactive + doping + adjuvant mixture is carried out at a calcining temperature of between 550 and 1050 ^° C. Advantageously, this temperature is around 950 ^° C.

As mentioned above, the adjuvant may advantageously comprise a carbon compound, for example graphite.

In addition, the adjuvant can be added to the mixture of oxides in a mass proportion of between 5 and 20% relative to the mass of the oxide mixture.

Advantageously, the mass proportion of the adjuvant added is around 10% of the mass of the oxide mixture.

On the other hand, at least one of the first and second oxides may comprise an oxide.

The reagent may comprise at least one oxide chosen from oxidizable elements such as NiO, Fe ₂ O ₃ , CuO, CoO, etc. The reagent may, in particular, be essentially composed of CuO.

The dopant may be chosen from the elements such as: MnO ₂ , TiO ₂ , Al ₂ O ₃ , MgO, CaO, etc. The dopant may, in particular, be essentially composed of CaO.

In addition, the mixture of oxides can be produced with: a mass proportion of dopant of between 35 and 45%, and

a mass proportion of reagent of between 65 and 55%. Advantageously, in the mixture of oxides:

the mass proportion of the dopant is around 40%, and

the mass proportion of the reagent is around 60%. According to yet another aspect of the invention there is provided a gas flow treatment method implementing the oxide formulation according to the invention, said method comprising several iterations of a treatment cycle comprises the following steps:

- reducing at least a portion of said oxides in the presence of a first gas stream, and

- Oxidation of at least a portion of said reduced oxides in the presence of a second gas stream comprising oxygen elements.

The gas flow treatment method according to the invention may for example consist of a method of "chemical looping combustion" (CLC or looping by thermochemical combustion). In this case, the method according to the invention comprises, during the reduction step, a combustion of the first gas stream in the presence of the formulation which is in fact reduced. The treatment of the first gas stream is then a combustion.

In the case where the process according to the invention is a CLC process, the first gas stream is a gas stream having an energy value. The first gas stream may for example comprise: - natural gas,

methane (CH ₄ ),

- hydrogen (H ₂ ), or

- carbon monoxide (CO), or

any oxidizable compound The treatment method according to the invention implementing a formulation according to the invention makes it possible to produce a combustion of the first gas stream with a maximum and constant conversion rate, thus conferring on the process according to the invention characteristics of energy conversion that are not currently known in CLC processes. Thus, a method of producing energy by CLC implementing a formulation according to the invention makes it possible to produce energy with a high efficiency and substantially constant throughout the treatment cycles.

The gas flow treatment process may consist of a catalysis process. Catalysis can be carried out during the reduction step by oxidation of the first gaseous flow and / or during the oxidation step by reduction of the second gaseous flow.

The second gas stream may be composed of at least one gas stream comprising one or more oxygen atoms, for example:

- air enriched in O ₂ ,

- O ₂ ,

- elemental oxides, or

- any oxidized compound. This second gaseous flow makes it possible to provide the elements of oxygen necessary for the oxidation of the oxides present in the formulation.

The reaction temperature during the reduction and oxidation steps of the oxides carrying oxygen may be between 550 and 1000 ^° C. This temperature is advantageously located around 950 ^° C.

Other advantages and characteristics will appear on examining the detailed description of a non-limiting embodiment, and the attached drawings in which:

FIG. 1a is a schematic representation of the principle of the Chemical Looping Combustion (CLC) process;

FIG. 1b is a schematic representation of an energy production method implementing a CLC technology; FIG. 2 is a representation of a procedure of the process for obtaining an oxidation formulation according to the invention;

FIG. 3 is a representation of the efficiency of several oxide formulations as a function of the mass fraction of the dopant; FIG. 4 is a representation of the efficiency of several oxide formulations as a function of the calcination time of the reactive + dopant + adjuvant mixture;

FIG. 5 is a representation of the efficiency of several oxide formulations as a function of the calcination temperature of the reactive + dopant + adjuvant mixture; FIG. 6 is a representation of the evolution of the specific surface of the oxides of several formulations as a function of the calcination temperature;

FIG. 7 is a representation of the efficiency of several oxide formulations as a function of the doping oxide; and

FIG. 8 is a representation of the efficiency of several oxide formulations as a function of the reaction temperature during the oxidation and reduction steps.

The particular application example described below relates to a chemical loopback combustion process known as "Chemical Looping Combustion" (CLC).

Figure la is a schematic representation of the CLC principle. The CLC is a method of producing energy by the combustion, in a first reactor 11, of a first gas flow FG1, for example methane (CH ₄ ) in the presence of recyclable metal oxides MeO, allowing the separation of the effluents gaseous from this combustion. Indeed, compared to conventional combustion processes, this process uses oxygen O metal oxides MeO and not that of air. As a result, the only products of combustion are, on the one hand, CO ₂ and H ₂ O in the form of gaseous effluents, easily separable by simple condensation, and, on the other hand, the reduced metal Me by oxidation. of the first gas stream FG1 according to the reaction:

4MeO + CH ₄ -> 4Me + CO ₂ + 2H ₂ O (1.1)

The metal Me is recycled after its regeneration in a second reactor 12, also called the metal oxidation reactor Me, using the oxygen of a second gas stream FG2, which may for example be atmospheric air, according to the reaction:

4Me + 2O2 - * 4MeO (1.2)

When the second gas stream FG2 is atmospheric air, this reaction 1.2 produces as sole effluent slightly oxygen-depleted air having a significant heat capacity recoverable. The MeO metal oxides thus regenerated are then recycled and returned to the reactor 11, which is also called the oxide reduction reactor MeO, for a new oxidation and reduction cycle. Depending on the chemical nature of the metal oxide used, reaction 1.2 is always exothermic, while reaction 1.1 may be endothermic or exothermic. The total amount of heat provided by the two reactions is the same as that provided in a conventional combustion process. However, the thermodynamic considerations show that the loss of exergy in the CLC is much lower than in a conventional combustion process, which makes it possible to further increase the efficiency of the process.

The use of CLC-based technology for energy production provides 75% thermal efficiencies. Furthermore, the CLC process, which is based on a flameless combustion, does not allow the generation of pollutants such as for example NOx.

The disadvantage of the CLC lies in the fact that the materials used are subjected to high chemical and thermal stresses during each cycle of reduction and oxidation of the product. This can result in a decrease in the performance of the mediator or oxygen carrier after several cycles. To avoid this problem, the oxygen carriers must have a good reactivity in reduction and oxidation, a high selectivity to obtain the complete oxidation of the first gas flow FG1, a large capacity to exchange oxygen measured by the mass ratio of oxygen / total mass of solid, a very good regenerability, that is to say a good stability during the repeated reduction and oxidation cycles and a good mechanical resistance to the phenomena of attrition associated with the recirculation of the metal oxide. According to the invention, the formulated product remains stable.

Fig. 1b is a schematic representation of a power generation process employing CLC technology. The oxide formulation used comprises NiO. The flow FG1 is a dynamic flow (mechanical accelerator) such as CH ₄ methane. The temperature of the reactor 11, at the start of the reactions, is achieved by an organized precombustion or a preheating of the reactor 11 by an external device Cl. This device Cl carries out the combustion of a fuel in the presence of an oxidizer such as air or O ₂ . The thermal energy created by this combustion is used for the conditioning of the first gas flow FG1 in the device C1 which receives the first gas flow FG1. This preheating of the first gas stream FG1 is interrupted as soon as the reaction is initiated. The first gas stream FGL undergoes combustion in the reactor 11 according to the reaction: H + CH _ά 4MB → 4M + 2H + CO ₀ ₀ O (1.1 ') .DELTA.h ° = + 174 kJ / mol CH ₄

This combustion produces a combustion gas flow FC1 essentially composed of CO ₂ and H ₂ O and having a high thermal energy. Part of the thermal energy of the combustion gas stream FC1 is used to preheat the second gas stream FG2, which is air in the present example, in an exchanger E1.

The combustion gas stream FC1 then undergoes a step of capturing the CO ₂ present in this stream. This capture is carried out by cooling the combustion gas flow FC1 in an exchanger E3, in which liquid water H ₂ O (I) is vaporized by means of the thermal energy of the gas flow FCl. At the outlet of the exchanger E3, water vapor H ₂ O (g) is obtained at high temperature, and low temperature CO2 and water in the liquid state H ₂ O (I) coming from the flow gaseous combustion FCl. CO ₂ is separated from liquid water and sequestered. The high temperature water vapor is recovered in energy generation means such as a TAV steam turbine.

The cooling of the combustion gas flow FCl in the E3 exchanger in favor of water, and the exploitation of the water vapor obtained, allows the recovery of 250 g CO ₂ ZkWh of energy. A conventional combustion system associated with a process for separating CO ₂ from fumes that produces the same amount of CO ₂ ZkWh used today does not make it possible to recover half, ie a release of more than 125 CO ₂ ZkWh. energy used in addition to the colossal investment of the capture facility.

The second preheated gas stream FG2 is then injected into the reactor 12 and participates in the oxidation of the oxide formulation according to the following reaction:

O ₂ + 2Ni → 2NiO (1.2 ') ΔH ° = - 488 kJ / mol of O ₂

This oxidation reaction of the oxide formulation causes the oxygen depletion of the second gas stream FG2 and the regeneration of the oxide formulation. So we get out of the reactor 12, a reaction gas flow FR1 corresponding to the second gas stream depleted of oxygen. This reaction gas stream FR1 has a thermal energy. Part of this thermal energy is used, in an exchanger E2, to preheat the first gas flow FG1 and optionally the second gas flow FG2. The remainder of the thermal energy of the reaction gas stream FR1 is recovered in an exchanger E4 and used to produce water vapor H ₂ O (g) from liquid water H ₂ O (I). The water vapor obtained is then recovered in energy production means such as a TAV steam turbine. FIG. 2 is a representation of the various steps of a process for preparing a metal oxide formulation according to the invention comprising a first metal oxide, called a reactive oxide, having a good reactivity and a second metal oxide, called a dopant, whose the reactivity is constant throughout the reduction and oxidation cycles. The procedure comprises the following steps:

1. Mixing in the desired proportions of the reactive oxide and the doping oxide,

2. Addition in the desired proportions of an adjuvant, promoting the porosity of the mixture of oxides relative to the total mass of the preceding mixture. In the present example, the adjuvant is graphite,

3. Homogenization of the reaction mixture + dopant + adjuvant,

4. Dispersion in distilled water and homogenization by stirring, 5. Total dehydration in a ventilated oven,

6. Grinding the dried mixture,

7. calcination of the mixture at high temperature in an oven under an atmosphere of air,

8. Grinding the cooled mixture to obtain particles of the desired size.

In the present example, the reactive oxide is chosen from the following elemental oxides, for example: CuO, NiO, Fe ₂ O ₃ and CoO, and the doping oxide is chosen from the following oxides, for example: MgO, Al ₂ O ₃ , TiO ₂ and CaO. Indeed, tests show that most elemental oxides are very reactive but not very stable throughout the reduction and oxidation cycles while other elemental oxides have a stable reactivity, so the choice must be a balance between reactivity and stability, hence in the process according to the invention the choice CuO / CaO in the proportions specified above.

Furthermore, the addition of graphite in the oxide mixture makes it possible to promote the porosity of the solid formulation obtained. Indeed, at high temperature, graphite is oxidized to CO ₂ that is released by increasing the pore volume of the oxide mixture. Graphite also significantly improves the mechanical strength of the product.

As described in the text, any admixture that can be gasified with oxygen can be used, for example CO ₂ and CO ₂ carbon. The use of graphite as an adjuvant is not a limitation. Other mixtures of oxides or adjuvants, making it possible to obtain a porous mixture of oxides, may be used.

We will now describe the results obtained with the different formulations obtained by this method and the influence of the various factors on the results obtained.

Effect of mass fractions of reactive and doping oxides in the formulation

Attempts have been made to determine the optimum ratio of reactive oxide to doping oxide in order to obtain an interesting and constant reactivity during the cycles. Sixteen different formulations, prepared according to the procedure described above, with the following ratios 80/20, 60/40, 40/60, 20/80 were studied and characterized for five calcination temperatures and a duration of 6 h. The parameters measured are oxygen carrier conversion, specific surfaces, particle sizes and densities.

Figure 3 shows the results obtained for each of the formulations. From these results, we observe that the formulations based on 80% of reagent and 20% of dopant, have a very strong reactivity during the first cycles. But this performance is associated with a very low regeneration during cycles. The 20% of dopant seems insufficient to stabilize and thus keep the mechanical properties of the formulation. For formulations based on 60% reagent and 40% dopant, the reactivity is very strong during the first cycles. It is associated with a very good regeneration during the cycles. The 40% of dopant seems to stabilize and thus preserve the mechanical properties of the formulation. Finally, the formulations based on 40% and 20% of reagent and respectively 60% and 80% of dopant, have a medium or low reactivity during the first cycles. It is associated with good regeneration during cycles.

In conclusion, the formulations based on 60% of reagent and 40% of dopant are the most efficient.

Effect of calcination time

Trials were performed on four formulations to determine the optimal calcination time. The formulations studied are Fe ₂ O ₃ / CaO, NiO / CaO, CuO / CaO and CoO / CaO. These formulations were prepared according to the protocol described above. During these tests, three calcination times, namely 3h, 6h and 9h, were studied and characterized for five calcination temperatures for a mass fraction of 60/40.

Figure 4 shows the results obtained. The variation of the conversion of the first gas stream FG1 as a function of the calcining time for the four formulations studied at 950 ^° C. It can be seen that the reactivity of the formulations increases with the calcination times to stabilize after a period of 6 hours. This phenomenon is explained by a change in the state of the structure of the oxygen carriers. Indeed, the specific surface of the oxygen carrier develops with time. When the calcination is sufficient, the oxygen carrier develops a larger surface allowing a good gas / solid contact and thus a high reactivity. Effect of calcination temperature

Tests have been conducted on formulations comprising mixtures based on cobalt oxide CoO, iron oxide Fe ₂ O ₃ , nickel oxide NiO and copper oxide CuO formulated with 40% CaO lime. have been studied, in order to see the influence of the calcination temperature on the formulation. The formulations are calcined at several temperatures: 550 ^° C., 650 ^° C., 750 ^° C., 850 ^° C., 950 ^° C., 1050 ^° C. and 1150 ^° C. for a calcination time of 6 hours. Then, the obtained formulations are analyzed and characterized through different parameters which are essentially the conversion of the oxygen carrier and the specific surfaces.

Figure 5 shows the results obtained. In terms of reactivity, the best calcination temperature seems to be 950 ^° C. for the four CaO-based formulations. However, it marks a maximum limit of calcination temperature. Indeed, we find that the reactivity of the oxygen carriers decreases very rapidly beyond this calcination temperature. This phenomenon is explained by a change in the state of the structure and sintering of the oxygen carrier which affects the gas / solid contact and thus reduces the reactivity of the formulation. Indeed, Figure 6 shows the evolution of the specific surface as a function of the calcination temperature. The results show an increase in the specific surface area as a function of the temperature up to 950 ^° C., above this temperature the specific surface area decreases. This is explained by a non-negligible initial specific surface, then under the effect of the temperature this surface develops to reach a maximum at 950 ⁰ C. We also observe that the formulation based on cobalt oxide CoO presents the the lowest specific surface area and the nickel oxide NiO formulation has the highest specific surface area.

Influence of oxides

The optimization of the reactions of the oxygen carriers and the definition of the appropriate operating conditions constitute a very important part of the implementation of an industrial process. Formulated products have been studied in their reactions with methane for various temperatures and their reactivity was compared according to the binder used. As we have seen above, the results show that the formulations prepared with a mass fraction oxide mixture of 40% doping oxide and 60% reactive oxide, to which an adjuvant, graphite, is added. which promotes the porosity of the oxide mixture and calcined at 950 ^° C. for 6 hours, show the best results.

In order to see the influence of doping oxides on the reactivity and the regeneration of the obtained formulation, we traced the variation of the conversion rate of the formulations with and without doping oxide as a function of the successive cycles. Figure 7 shows the results obtained. We can see a significant improvement in the regeneration of the oxygen carriers formulated, whatever the dopant used. The combination of a reactive metal oxide and a doping metal oxide makes it possible to improve the conversion during the regeneration cycles of the oxygen carrier, by improving the mechanical strength of the particle during thermochemical cycles.

For example, for the formulated nickel oxide NiO, we obtain a strong conversion and an improvement of the regeneration in the majority of the cases. We observe for some of them in the first cycles a reorganization of the material that may require one to three cycles to find the final configuration. The two best formulations obtained with nickel are those obtained with lime CaO and titanium dioxide TiO ₂ . The selection of the doping oxide among the two presented will be based on the analysis of its cost. We can already recommend the use of lime with respect to titanium dioxide. The cobalt-based formulation has the same characteristics as those based on formulated nickel oxide.

On the other hand, high conversion and regeneration improvement is found for all iron oxide formulations. The formulation with alumina has some difficulties in regenerating from cycle 3 to cycle 7. The three best formulations with iron are those with CaO, TiO ₂ and MgO. The lime compared to the other two products offered, more expensive, seems to be the best choice. In addition, the oxygen transfer capacity of the mediator is very dependent on the reaction considered during its progress. For iron oxide-based oxygen carriers as a reactive mediator, several reactions are possible and correspond to the transformations Fe ₂ O ₃ - * Fe ₃ O ₄ , Fe ₂ O ₃ - ^ FeO or Fe ₂ O ₃ - ^ Fe. For the Fe ₂ O ₃ - Fe reaction, the transfer capacity is three times greater than for the Fe ₂ O ₃ - FeO ₂ reaction. It is also for the latter three times higher than for the reaction Fe ₂ O ₃ - ^ Fe ₃ O ₄ . The stability of the species of iron oxide depends on the nature of the first gas stream FG1 during its reduction but also its temperature. In view of these considerations, in the example of the reduction of the oxygen carrier with pure methane at 950 ^° C., we can assume that the mass variations on the reaction products result from the transformation Fe ₂ O ₃ - ^ Fe ₃ O ₄ . During the reduction of Fe ₂ O ₃ not formulated with CH ₄ , the iron oxide is first reduced to Fe ₂ O ₃ then to FeO to finally reach Fe. In addition, the presence of non-reactive products can modify the chemical changes inside the particles during the reaction.

On the other hand, we observe a high conversion and regeneration improvement for all CuO copper oxide formulated formulations. The formulation with the alumina has some difficulties to regenerate during the ten cycles. The three best formulations with copper are those with CaO, TiO ₂ and MgO. The lime compared to the other two products offered, more expensive, seems to be the best choice.

Influence of temperature of thermochemical cycles

Optimum conditions of reduction reaction and oxidation temperatures were determined with oxygen carriers based on 40% lime through oxide conversion.

The major problem of reactive oxides lies, as indicated above, in their ability to regenerate. Indeed, we have observed a decrease in the conversion rate of these metal oxides as a function of the repeated cycles of reduction and oxidation because of their poor mechanical strength and thus their poor regeneration. To have a better regeneration of the reagent, their mechanical properties have been enhanced by the addition of doping metal oxides such as CaO, MgO, TiO2, and Al2O3.

In order to measure the impact of the oxidation and reduction temperature on the reactivity and the regeneration of the formulations, we traced the variation of the conversion rate of four formulations obtained by mixing a doping oxide which is CaO with a reactive oxide in the respective proportions by weight of 40 and 60%.

Figure 8 gives the results obtained. The mixture based on 60% of the oxides of nickel, iron, copper and cobalt are very reactive and retain a very high reactivity during the cycles thanks to the lime doping. Raising the reaction temperature increases the effectiveness of the formulation.

Thus, thanks to the process according to the invention, the CLC system allows: - a complete combustion, by transfer of the oxygen of the formulation

MeO for fuel, a flameless and smoke-free combustion, with a maximum CO ₂ concentration,

a recovery of all of this CO ₂ by simple thermal exchange of the gaseous effluent with the operating system, until the condensation of the steam, resulting from the combustion and contained in said effluent and the regeneration of the Me formulation without degradation of the oxy-carrier (air or gaseous oxide) and without pollution.

Of course, the invention is not limited to the examples which have just been described. Indeed, the formulation according to the invention can also be implemented by catalysis methods. The reactive and doping oxides used may be non-metallic oxides.

Claims

1) Formulation of oxides for use as oxygen carriers in a gas flow treatment process comprising several iterations of the following steps:

reducing at least a portion of said oxides in the presence of a first gaseous flow (FG1), and

oxidizing at least a portion of said reduced oxides in the presence of a second gas stream (FG2) comprising oxygen elements; said formulation comprising a mixture of oxides composed of:

at least one first oxide, said reagent, having a good reactivity with said first gas flow (FG1), and

at least one second oxide, called dopant, having a substantially constant reactivity with said first gas flow (FG1); the porosity of said mixture of oxides being favored by:

adding to said mixture of oxides at least one compound, said adjuvant, and said formulation being obtained by drying the reactive + doping + adjuvant mixture, at a drying temperature around 110 ^° C.

2) Formulation according to claim 1, characterized in that at least one of the first and second oxides comprises a metal oxide.

3) Formulation according to any one of claims 1 or 2, characterized in that the adjuvant comprises a carbon compound.

4) Formulation according to claim 2, characterized in that the adjuvant comprises essentially graphite.

5) Formulation according to any one of the preceding claims, characterized in that the mass proportion of the adjuvant relative to the oxide mixture is between 5 and 20%. 6) Formulation according to claim 5, characterized in that the mass proportion of the adjuvant relative to the oxide mixture is 10%

7) Formulation according to any one of the preceding claims, characterized in that in the mixture of oxides the mass proportion:

dopant is between 35% and 45%, and

the reagent is between 65% and 55%.

8) Formulation according to claim 7, characterized in that in the mixture of oxides the mass proportion of:

- dopant is 40%, and

- reagent is 60%.

9) Formulation according to any one of the preceding claims, characterized in that the reagent comprises at least one oxide selected from the following list: NiO, Fe ₂ O ₃ , CuO, CoO.

10) Formulation according to claim 9, characterized in that the reagent comprises essentially CuO.

11) Formulation according to any one of the preceding claims, characterized in that the dopant comprises at least one oxide selected from the following list: MnO ₂ , TiO ₂ , Al ₂ O ₃ , MgO, CaO.

12) Formulation according to claim 11, characterized in that the dopant essentially comprises CaO.

13) Process for obtaining a formulation comprising oxides, intended to be used as oxygen carriers in a gas flow treatment process comprising several iterations of the following steps:

reducing at least a portion of said oxides in the presence of a first gaseous flow (FG1), and oxidizing at least a portion of said reduced oxides in the presence of a second gas stream (FG2) comprising oxygen elements; said method comprising the following steps: - mixing of at least one first oxide, said reagent, having a good reactivity with said first gas stream (FG1) with at least one second oxide, said dopant, having a substantially constant reactivity with said first gaseous flow (FG2), and

adding to said mixture of oxides a third compound, said adjuvant, promoting the porosity of said mixture of oxides, characterized in that it further comprises a step of drying the reactive + doping + adjuvant mixture, at a temperature of drying around 110 ^° C.

14) Method according to claim 13, characterized in that it further comprises a homogenization of the reaction mixture + dopant + adjuvant by stirring said mixture.

15) Process according to any one of claims 13 or 14, characterized in that it comprises a dispersion in distilled water of the reaction mixture + dopant + adjuvant.

16) Method according to any one of claims 13 to 15, characterized in that it further comprises at least one step of grinding the reaction mixture + dopant + adjuvant to obtain particles to the desired size.

17) Process according to any one of claims 13 to 16, characterized in that it further comprises a step of calcination of the reaction mixture + dopant + adjuvant in the presence of a third gaseous flow comprising oxygen elements, said calcination step being conducted until disappearance of the adjuvant in said mixture.

18) Process according to claim 17, characterized in that the third stream consists of a gas stream chosen from the following list: - air,

- air enriched with oxygen,

- oxygen,

- carbon monoxide, - carbon dioxide, or

- any oxidized compound.

19) Method according to any one of claims 17 or 18, characterized in that the calcination of the reaction mixture + doping + adjuvant is carried out at a calcination temperature of between 650 and 1050 ^° C.

20) Process according to any one of claims 17 or 18, characterized in that the calcination temperature is 950 ⁰ C.

21) Method according to any one of claims 13 to 20, characterized in that the adjuvant comprises a carbon compound.

22) Method according to claim 21, characterized in that the adjuvant comprises graphite.

23) Method according to any one of claims 13 to 22, characterized in that the adjuvant is added in a mass proportion of between 5 and 20% of the mass of the oxide mixture.

24) Method according to claim 23, characterized in that the adjuvant is added in a mass proportion of 10% of the mass of the oxide mixture.

25) Method according to any one of claims 13 to 24, characterized in that at least one of the first and second oxide comprises a metal oxide.

26) Process according to any one of claims 13 to 25, characterized in that the mixture of oxides is produced with: a mass proportion of dopant of between 35 and 45%, and

a mass proportion of reagent of between 65 and 55%.

27) Method according to claim 26, characterized in that the mixture of the oxides is produced with:

a mass proportion of dopant of 40%, and

a mass proportion of reagent of 60%.

28) Method according to any one of claims 13 to 27, characterized in that the reagent comprises at least one oxide selected from the following list: NiO, Fe ₂ O ₃ , CuO, CoO.

29) Method according to any one of claims 13 to 27, characterized in that the dopant comprises at least one oxide selected from the following list: MnO ₂ , TiO ₂ , Al ₂ O ₃ , MgO, CaO.

30) Method according to claim 29, characterized in that the dopant essentially comprises CaO.

31) Process for treating a gas stream using the oxide formulation according to any one of the preceding claims, said method comprising several iterations of the following steps:

- Reducing at least a portion of said oxides in the presence of a first gas stream (FG1), and - oxidizing at least a portion of said reduced oxides in the presence of a second gas stream (FG2) comprising elements of oxygen.

32) A method according to claim 31, characterized in that it comprises, during the reduction step, a combustion of the first gas stream (FGl) in the presence of the formulation, said method consisting of a method of "chemical looping combustion "(Loop by Chemical Combustion or Combustion by Thermochemical Looping"). 33) Method according to claim 32, characterized in that the first gas stream (FGl) is selected from the following list:

- natural gas,

methane (CH ₄ ), hydrogen (H ₂ ),

- carbon monoxide (CO), or

any oxidizable compound.

34) Method according to any one of claims 31 to 33, characterized in that the second gas stream (FG2) is composed of at least one gas stream chosen from the following list:

- air,

- air enriched in O ₂ ,

- O ₂ , - CO ₂ , or

- any oxidized compound.

35) Process according to any one of claims 31 to 34, characterized in that it consists of a catalysis process, said catalysis being carried out: - during the oxidation reduction step of the first gas stream (FGl), and or

during the reduction oxidation step of the second gas stream (FG2).

36. Process according to any one of claims 31 to 35, characterized in that the reaction temperature during the reduction and oxidation steps is between 550 and 1000 ^° C.

37. Process according to claim 36, characterized in that the reaction temperature during the reduction and oxidation stages is around 950 ^° C.