Sustainable Oxygen Carriers for Chemical Looping Combustion with Oxygen Uncoupling and Methods for Their Manufacture

Abstract

An oxygen carrier (OC) for use in Chemical Looping technology with Oxygen Uncoupling (CLOU) for the combustion of carbonaceous fuels, in which commercial grade metal oxides selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof constitute a primary oxygen carrier component. The oxygen carrier contains, at least, a secondary oxygen carrier component which is comprised by low-value industrial materials which already contain metal oxides selected from the group consisting of Cu, Mn, Co, Fe, Ni oxides or mixtures thereof. The secondary oxygen carrier component has a minimum oxygen carrying capacity of 1 g of O.sub.2 per 100 g material in chemical looping reactions. Methods for the manufacture of the OC are also disclosed.

Claims

1-14. (canceled)

15. An oxygen carrier for use in chemical looping technology with oxygen uncoupling (CLOU) for the combustion of a carbonaceous fuel, comprising: a primary oxygen carrier component comprising a commercial grade metal oxide selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof; and a secondary oxygen carrier component comprising an industrial material which contain a metal oxide selected from the group consisting of Cu, Mn, Co, Fe and Ni oxides and mixtures thereof, wherein the secondary oxygen carrier component has an oxygen carrying capacity of no less than 1.0 g of O.sub.2/100 g material in chemical looping reactions.

16. The oxygen carrier according to claim 15, wherein said secondary oxygen carrier component is mixed with an amount of fuel ashes from fuel combustion, thereby facilitating production and enhancing mechanical stability of the oxygen carrier.

17. The oxygen carrier according to claim 15, wherein the carbonaceous fuel is selected from the group consisting of solid, liquid and gaseous carbonaceous fuels and mixtures thereof.

18. The oxygen carrier according to claim 15, wherein the carbonaceous fuel is predominantly solid fuels.

19. The oxygen carrier according to claim 15, wherein the primary oxygen carrier component is present in a concentration within the range of approximately 15-99% by weight.

20. The oxygen carrier according to claim 15, wherein the primary oxygen carrier component predominantly comprises oxides of Cu.

21. The oxygen carrier according to claim 15, wherein said secondary oxygen carrier component comprises waste material or a process-stream material generated from the production of ilmenite and comprises oxides selected from the group consisting of Fe, Mn, Cu, Co and Ni and mixtures thereof.

22. The oxygen carrier according to claim 15, wherein said secondary oxygen carrier component comprises waste material or a process-stream material from production of manganese-bearing materials and comprises oxides selected from the group consisting of Mn, Fe, Cu, Co and Ni and mixtures thereof.

23. The oxygen carrier according to claim 15, wherein said secondary oxygen carrier component comprises waste material or a process-stream material from production of cobalt-bearing materials and comprises oxides selected from the group consisting of Co, Mn, Fe, Cu and Ni and mixtures thereof.

24. The oxygen carrier according to claim 15, wherein said secondary oxygen carrier component comprises waste material or a process-stream material from production of nickel-bearing materials and comprises oxides selected from the group consisting of Ni, Co, Mn, Fe and Cu and mixtures thereof.

25. The oxygen carrier according to claim 15, wherein the oxygen carrier has an oxygen carrying capacity of no less than 1.2 g O.sub.2/100 g.

26. The oxygen carrier according to claim 15, wherein the oxygen carrier takes the form of particles prepared by agglomeration, compaction, palletization or spray drying and has a measured crushing strength of at least 3 N.

27. The oxygen carrier according to claim 15, wherein the primary oxygen carrier component is present in a concentration within the range of approximately 40-90% by weight.

28. The oxygen carrier according to claim 15, wherein the primary oxygen carrier component is present in a concentration within the range of approximately 60-80% by weight.

29. The oxygen carrier according to claim 15, wherein the oxygen carrier takes the form of particles prepared by agglomeration, compaction, palletization or spray drying and has a measured crushing strength of at least 5 N.

30. The oxygen carrier according to claim 15, wherein the oxygen carrier takes the form of particles prepared by agglomeration, compaction, palletization or spray drying and has a measured crushing strength of at least 7 N.

31. The oxygen carrier according to claim 15, wherein the oxygen carrier has an oxygen carrying capacity of no less than 6 g O.sub.2/100 g.

32. The oxygen carrier according to claim 15, wherein the oxygen carrier has an oxygen carrying capacity of no less than 12 g O.sub.2/100 g.

33. A method of manufacturing an oxygen carrier for use in chemical looping technology with oxygen uncoupling (CLOU) for the combustion of carbonaceous fuel comprising a primary oxygen carrier component comprising a commercial grade metal oxide selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof, and a secondary oxygen carrier component comprising an industrial material which contain a metal oxide selected from the group consisting of Cu, Mn, Co, Fe and Ni oxides and mixtures thereof, wherein the secondary oxygen carrier component has an oxygen carrying capacity of no less than 1.0 g of O.sub.2/100 g material in chemical looping reactions, comprising the steps of: a. providing commercial grade metal oxides selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof as the primary oxygen carrier component; b. providing metal oxides selected from the group consisting of oxides of Cu, Mn, Fe, Co, and Ni from an industrial waste, tailing process stream or by-product as the secondary oxygen carrier component; c. mixing and subjecting the primary oxygen carrier component and secondary oxygen carrier component to conditions under which granule-forming agglomeration occurs to form granule; and d. thermally treating the granules by a process selected from the group consisting of cooling, drying and calcination and combinations thereof.

34. A method of manufacturing an oxygen carrier for use in chemical looping technology with oxygen uncoupling (CLOU) for the combustion of carbonaceous fuel comprising a primary oxygen carrier component comprising a commercial grade metal oxide selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof, and a secondary oxygen carrier component comprising an industrial material which contain a metal oxide selected from the group consisting of Cu, Mn, Co, Fe and Ni oxides and mixtures thereof, wherein the secondary oxygen carrier component has an oxygen carrying capacity of no less than 1.0 g of O.sub.2/100 g material in chemical looping reactions, comprising the steps of: a. providing commercial grade metal oxides selected from the group consisting of Cu, Mn, and Co oxides and mixtures thereof as the primary oxygen carrier component; b. providing metal oxides selected from the group consisting of oxides of Cu, Mn, Fe, Co, and Ni from an industrial waste, tailing process stream or by-product as the secondary oxygen carrier component; c. mixing the primary oxygen carrier component and secondary oxygen carrier component in a solution and subjecting the solution to conditions under which granule precipitation of added Cu, Mn, Co or mixtures thereof occurs, d. thermally treating the granules by a process selected from the group consisting of cooling, drying and calcination and combinations thereof.

Description

BRIEF DESCRIPTION OF THE FIGURES AND TABLES

[0092] FIG. 1 illustrates the schematic system of chemical looping process.

[0093] FIG. 2 illustrates generic interaction types between solid fuels and oxygen carrier occurring in fuel reactor of iG-CLC and CLOU technologies modes.

[0094] FIG. 3 illustrates evolution of O.sub.2 carrying capacity over CLC cycles.

[0095] FIG. 4 illustrates the evolution of gaseous O.sub.2 uptake/release capacity over CLOU cycles.

[0096] FIG. 5 illustrates long-term stability of O.sub.2 capacity over cycles; 42 redox cycles for CLOU and 90 redox cycles for CLC-CLOU.

[0097] FIG. 6 illustrates attrition resistance of up-scaled OC corresponding to Example 6 determined after 5 h and 24 h at room temperature and after 5 h at 800 C.

[0098] FIG. 7 illustrates interaction of OC with solid fuel at 925 C.; A-coal, B-biomass (wood chips).

[0099] The following embodiments provide the preferred preparation methods to obtain the sustainable and efficient OCs herein presented, by scalable methods for industrial implementation.

[0100] In one of the embodiments, the OC is prepared by an agglomeration method, where the active support is enriched with Cu, Mn, Co oxides or mixtures thereof by mechanical mixing. In the agglomeration method, the mixture of solids in powder form (support and the adequate quantity of Cu, Mn, Co oxides or mixtures thereof). If necessary, the materials can be pre-dried to eliminate excess moisture that might hinder the agglomeration effect. The mixture is introduced in the agglomerator vessel, and dry-mixed using the rotation shafts. In this method, a binder can be used to enhance the agglomerates production yield and mechanical strength. As an example, polyethylene glycol (PEG) or polyvinyl alcohol (PVA) in an aqueous solution can be used as binder. After dry-mixing, the binder (e.g. water or an aqueous solution of PEG) is slowly added to the mixture at controlled flow. If necessary, the binder addition process can be stopped and resumed several times in order to optimize the final agglomerates size and mechanical properties. Once powder mixture is agglomerated, the agglomerates can be dried in air at ambient temperature, and then at a higher temperature (e.g. between 50 and 120 C.) to remove the humidity. Finally, they agglomerates are calcined. As a result, round-shaped agglomerates are obtained with this method. The OC thereby produced shows higher oxygen carrying capacity per total mass than the capacity ever reported for OCs with the same amount of added Cu, Mn, Co oxides or mixtures thereof over inert supports, as shown in FIG. 3, FIG. 5, and Table 1 and Table 3, as well as high and stable CLOU performance as shown in FIG. 4, FIG. 5 and Table 3.

[0101] In addition, the resulting OC shows high mechanical strength, with high crushing strength values, as shown in Table 2 and Table 3.

[0102] Furthermore, the preparation of OC according to this method can be scaled up for larger batches, with high oxygen capacity and good mechanical properties, as shown in FIG. 6 and FIG. 7.

[0103] In a preferred embodiment, the present invention provides an oxygen carrier prepared by the agglomeration method herein presented, wherein said oxygen carrier consists of agglomerates or particles wherein at least 80% of said particles have a size higher than 50 m. In a more preferred embodiment, the present invention provides an oxygen carrier, hereby said oxygen carrier consists of agglomerates or particles wherein at least 80% of said particles have a size higher than 100 m.

[0104] In another embodiment, the OC is prepared by a precipitation-coating method of Cu, Mn, Co oxides or mixtures thereof. The precipitation-coating method is based on the formation of CuO precipitate over the surface of support particles. In this method, a weighted amount of dried support is added to a certain volume of water and mixed to make a suspension. Weighted amount of Cu, Mn, Co oxides or mixtures thereof precursor salt (e.g. copper nitrate) is dissolved in another aliquot of water. The solution of the metal of metals salt is added dropwise to the suspension of support under continuous, vigorous stirring. A precipitation agent (e.g. NaOH aqueous solution) is added dropwise to the mixture with vigorous, continuous stirring, to modify the pH (e.g. until pH<10 in case of using copper nitrate as CuO precursor). The change of pH promotes the precipitation of the Cu, Mn, Co oxides or mixtures thereof over the particles of support present in the solution. After a certain time (e.g. 1-3 h of aging), the precipitate is filtered under vacuum, and washed several times with water until pH 7 and dried at a minimum of 50 C. for a minimum of one hour. The resulting material is calcined at a minimum of 500 C. The OC can be sieved down to the desired particle size distribution. Alternatively, the OC can be agglomerated, before or after calcination, according to the previous embodiment (i.e. the agglomeration method above described or similar). In another embodiment the particle size of the support is selected or modified accordingly to obtain higher or lower particle size of the final product.

[0105] In a preferred embodiment, the present invention provides an oxygen carrier prepared by the precipitation-coating method herein presented, wherein said oxygen carrier consists of agglomerates or particles wherein at least 80% of said particles have a size higher than 50 m. In a more preferred embodiment, the present invention provides an oxygen carrier; hereby said oxygen carrier consists of agglomerates or particles wherein at least 80% of said particles have a size higher than 100 m. As an example, several embodiments of the present invention are further demonstrated and described in the following proof of principle.

Proof of Principle

[0106] All the materials and synthesis methods have been tested at laboratory scale, showing the technical feasibility of producing oxygen carriers with exceptional oxygen capacity and mechanical strength as above described.

[0107] Embodiments of the oxygen carriers herein disclosed are further demonstrated and described in the following description. The following examples and drawings will serve only to illustrate the technical viability of this invention and provide a useful description of the principles and conceptual aspects of this invention based on examples listed below, not limiting the invention to these particular embodiments.

EXAMPLES

[0108] Oxygen carrier particles with high mechanical strength and oxygen carrying capacity for laboratory scale were prepared using 2 different syntheses and processing methods; 1) agglomeration in a high shear mixer/granulator for powder granulation and fines hydration/pelletization (GMX) and 2) direct CuO precipitation-coating over support and the processing of the resulting fines by hydration/pelletization.

[0109] Both methods involve the use of low cost industrial materials as active support i.e. materials from industry that contain certain amounts of metal oxides readily active in CLOU or CLC chemical reactions, as, for example oxides of Mn, Cu, Fe, Ni and/or Co. The low value industrial material as received from the industrial process was crushed and sieved adequately to the needs of each test.

[0110] Thermogravimetric analyses (TGA) of all the samples were carried out to determine the reactivity of the OCs along redox cycles under different atmospheres. Two main properties were determined: 1. Oxygen uptake/release capacity, where molecular oxygen is released from the OC lattice only by the effect of temperature, so-called CLOU effect; and 2. Total oxygen carrying capacity, so called CLC-CLOU behavior, where the oxygen carrier is reacting with the gases from solid fuel pyrolysis and gasification (CLC), and, at the same time, molecular oxygen is also released by the CLOU effect, as schematically shown in FIG. 2.

Initially, the samples were tested under argon atmosphere for reduction and with synthetic air for the oxidation step along redox cycles at 925 C. Their oxygen uptake/release capacity was determined from the weight variation measured in the TGA. These measurements were carried out using a thermogravimetric analyzer Netzsch STA 449 F3 Jupiter TG-DSC. The total flow was 200 cm.sup.3/min both for reduction and oxidation. The time of a complete redox cycle was 20 min. Typically 20 mg of sample were placed in the Al.sub.2O.sub.3 crucible. The measurements were baseline corrected by the Proteus software package. Evolution of O.sub.2 uptake/release capacity over cycles CLOU is illustrated in FIG. 4. Later on, OCs were analyzed under mixtures of H.sub.2 (5 vol. %), CH.sub.4 (15 vol. %), H.sub.2O (35 vol. %), CO.sub.2 (25 vol. %), and N.sub.2 (20 vol. %) for the reduction step and synthetic air, for the oxidation step, flushed with 100 vol. % N.sub.2 in between with total flow 500 cm.sup.3/min and temperature 925 C. at all time. With this atmosphere, the gas composition expected around the particles in the fuel reactor of a CLC-CLOU system could be emulated and, therefore, it was determined the total oxygen carrying capacity of the OCs thereby tested. The evolution of total O.sub.2 carrying capacity over CLC-CLOU cycles is illustrated in FIG. 3. Long-term stability of O.sub.2 capacity over 42 redox cycles for CLOU and 90 redox cycles for CLC-CLOU was determined, as illustrated in FIG. 5.

[0111] The force needed to fracture a particle (i.e. crushing strength) was determined using a Digital Force Gauge SHIMPO FGV-10X apparatus. The mechanical strength was taken as the average value of at least 75 measurements undertaken on different particles of each sample randomly chosen.

[0112] Attrition resistance of up-scaled materials was determined using a test rig designed to simulate conditions in Chemical Looping Combustion reactor. 15 g of each sample was placed in a downcomer through the cyclone, the stand was mounted and compressed air was turned on with flow of 2.54 m.sup.3/h. This stream ensures that air speed reaches 100 m/s when going through contraction.

[0113] Macro-TGA experiments with solid fuels were carried out in isothermal conditions. Volumetric flow of 100% CO.sub.2 was 0.040 m.sup.3/h. Sample was placed in the reactor when gas temperature inside the reactor reached 925 C. and kept inside until the mass stabilizationwhen no mass change was observed. The excess of oxygen available in the OC divided by the minimum or stoichiometric oxygen needed for the full combustion of the fuel for complete combustion () is 1.1 for coal and 1.3 for biomass.

The results of tests with solid fuel and attrition tests are shown in FIG. 6 which illustrates attrition resistance of up-scaled OC corresponding to Example 6. The attrition was determined at room temperature after 5 h and 24 h, and at 800 C. after 5 h. FIG. 7 illustrates interaction of OC with solid fuel at 925 C.; A-coal, B-biomass (wood chips) concern samples corresponding to Example 4 and 6 prepared in large quantities of 0.5 to 2 kg compared with Ilmenite concentrate (example of secondary OC of this invention).

[0114] The OC materials presented by examples in this invention showed crushing strength at least equal to 3 N. The OCs with highest crushing strength were Examples 5, 6 and 3, with values corresponding to 7.9, 6.7 and 6.3 N, as shown in Table 3.

[0115] The CLOU capacity after the 2.sup.nd redox cycle varied from 3 to 6 g O.sub.2/100 g OC for agglomerated samples enriched with CuO (Examples 1-6), and from 2 to 6 g O.sub.2/100 g for precipitated samples enriched with CuO (Examples 8-10), as shown in FIG. 4 and Table 3. Moreover, for the same examples, the CLC-CLOU capacity after 2.sup.nd redox cycle varied from 12 to 16 g O.sub.2/100 g OC and from 12 to 15 g O.sub.2/100 g correspondingly, as shown in FIG. 3 and Table 3. Thus, OCs proposed by the present invention and obtained by different preparation methods showed similar high activity towards both CLC and CLC-CLOU applications.

[0116] All the Examples corresponding to OCs enriched with CuO performed better in terms of CLC-CLOU behavior than any OC reported in literature, as shown in Table 1. OC enriched with Mn oxide presented at Example 7, also showed very promising results of high and long term CLC-CLOU activity, as shown in Table 3, FIG. 3 and FIG. 5.

[0117] Moreover, the attrition test results for up-scaled sample corresponding to Example 6, shown in FIG. 6, indicate high attrition resistance of this OC both at room temperature overtime as well as elevated temperature. This result is in agreement with the high crushing strength values of this OC.

[0118] Macro-TGA experiments with solid fuels were also performed for up scaled samples corresponding to Examples 4 and 6, and were compared with ilmenite concentrate sample. As it is shown in FIG. 7, all the enriched OCs prepared according to the present invention show better reactivity than ilmenite concentrate (an example of secondary OC of the present invention), both for coal and biomass combustion.

Examples Based on Mechanical Agglomeration Method:

[0119] In the agglomeration method, polyethylene glycol (PEG) aqueous suspension was used as an organic binder. The mixture of solids in powder form was dried at 100 C. for at least 2 h before the agglomeration tests. 100-200 g of powder was introduced in the 1 dm.sup.3 vessel of the agglomerator, and dry-mixed using rotation speed of 1500 rpm (mixer) and 3600 rpm (chopper). After 1 min of mixing, water or an aqueous solution of PEG was slowly added to the mixture using an integrated pump. After adding each 1 cm.sup.3 of solution, the binder addition was stopped and the vessel content was mixed for one extra minute with no dosing of liquid. Torque value was observed at all time of agglomeration. When a rapid increase of torque was detected, the agglomerator was stopped, and the total liquid volume used calculated. Round-shape agglomerates with sizes between 0.1 to 2 mm were obtained in the tests. Agglomerates were dried in air at ambient temperature and then overnight at 90 C. Finally, they were calcined. With this method, the obtained particles can be sieved to obtain the fraction of interest. In that case, smaller and bigger fractions can be separated, crashed if needed and reintroduced in the agglomeration unit for further processing. The measured oxygen carrying capacity on pure CLOU and on CLC-CLOU effects, and the crushing strength values are shown in Table 3.

Example 1

[0120] A preparation of an oxygen carrier involves agglomeration of 48 g of CuO, 72 g of Mn sinter (with an approximate content of 60 wt. % of Mn in oxide form) using 13.2 g of 15 wt. % aqueous solution of polyethylene glycol 4000. Dried agglomerates are calcined for 2 h at 820 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 820 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 40 wt. % of CuO (primary OC) and 60 wt. % of Mn sinter (secondary OC) agglomerates as final product.

Example 2

[0121] A preparation of an oxygen carrier according to the experimental conditions described in Example 1, wherein the quantities of CuO, manganese sinter and the binder are as follows: 60 g of CuO, 40 g of Mn sinter using 11.8 g of 15 wt. % aqueous solution of polyethylene glycol 4000. Thereby obtaining 60 wt. % of CuO (primary OC) and 40 wt. % of Mn sinter (secondary OC) agglomerates as final product.

Example 3

[0122] A preparation of an oxygen carrier according to the experimental conditions described in Example 1 wherein the quantities of CuO, manganese sinter and the binder are as follows: 80 g of CuO, 20 g of Mn sinter using 12.6 g of 15 wt. % aqueous solution of polyethylene glycol 4000. Thereby obtaining 80 wt. % of CuO (primary OC) and 20 wt. % of Mn sinter (secondary OC) agglomerates as final product.

Example 4

[0123] A preparation of an oxygen carrier involves agglomeration 90 g of CuO, 30 g of ilmenite concentrate (with an approximate content of 35 wt. % of Fe in oxide form) and 30 g of fly-ash (from Sobieski coal) using 22 g of 15 wt. % aqueous solution of polyethylenglycol 4000. Dried agglomerates are calcined for 2 h at 1100 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 1100 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 60 wt. % of CuO (primary OC), 20 wt. % of Ilmenite (secondary OC) and 20 wt. % of fly-ash (binder) agglomerates as a final product.

Example 5

[0124] A preparation of an oxygen carrier involves agglomeration 32 g of CuO, 48 g of Mn-containing tailing (with a content lower than 60 wt. % Mn in oxide state) using 9.4 g of 15 wt. % aqueous solution of polyethylene glycol. Dried agglomerates are calcined for 2 h at 820 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 820 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 40 wt. % of CuO (primary OC) and 60 wt. % of Mn-containing tailing (secondary OC) agglomerates as final product.

Example 6

[0125] A preparation of an oxygen carrier according to the experimental conditions described in Example 5, wherein the quantities of CuO, Mn-containing tailing and the binder are as follows: 90 g of CuO, 60 g of Mn-containing tailing using 17.1 g of 15 wt. % aqueous solution of polyethylene glycol 4000. Thereby obtaining 60 wt. % of CuO (primary OC) and 40 wt. % of Mn-containing tailing (secondary OC) agglomerates as final product.

Example 7

[0126] A preparation of an oxygen carrier involves agglomeration 60 g of MnO.sub.2, 40 g of Mn-containing tailing (with a content lower than 60 wt. % Mn in oxide state) using 21 g of 15 wt. % aqueous solution of polyethylene glycol. Dried agglomerates are calcined for 2 h at 820 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 820 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 60 wt. % of Mn.sub.2O.sub.3 (primary OC) and 40 wt. % of Mn-containing tailing (secondary OC) agglomerates as final product.

Examples Based on Precipitation-Coating Method

[0127] The precipitation-coating method is based on the generation of CuO precipitate over the surface of active support particles. A weighted amount of dried support (particle size <100 m) is added to deionized water and mixed to make a suspension using magnetic stirrer (600 rpm, RT, 10 min). Weighted amount of copper precursor salt is dissolved in deionized water and mixed (800 rpm, RT, 10 min). Solution of copper salt is added dropwise to the aqueous suspension of support (15-20 drops/min) under continuous, vigorous mechanical stirring. Precipitation agent is a 2 mol/dm.sup.3 NaOH aqueous solution. It is added dropwise to the precursor and support mixture until pH value is equal to 10 (15-20 drops/min), with vigorous, continuous stirring. After 1-3 h of aging, precipitate is filtered under vacuum, washed several times with water to pH value 7 and dried at 90 C. overnight. The material is calcined at temperatures varying between 820 and 1100 C. Agglomeration of precipitate can be another step before or after calcination. Several embodiments of the invention were prepared and are reported below.

Example 8

[0128] A preparation of an oxygen carrier by the CuO precipitation-coating method involves suspending 8 g of Mn-containing tailing (with a content lower than 60 wt. % Mn in oxide state) in 75 cm.sup.3 of deionized water. At the same time, 36.24 g of copper (II) nitrate trihydrate Cu(NO.sub.3).sub.2.3H.sub.2O is dissolved in deionized water. Solution of copper salt (2 mol/dm.sup.3) is dropped to a suspension of support (15-20 drops/min) under continuous, vigorous mechanical stirring. NaOH aqueous solution is dropped to the mixture of precursor and support until pH value >10 (15-20 drops/min), with vigorous, continuous stirring. After 2 h of aging, precipitate is filtered under vacuum, washed 4 times with deionized water to pH value 7 and dried at 90 C. overnight. Dry precipitate is calcined for 2 h at 820 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 820 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 60 wt. % of CuO (primary OC) and 40 wt. % of Mn-containing tailing (secondary OC) powder as final product.

Example 9

[0129] A preparation of an oxygen carrier by the CuO precipitation-coating method involves suspending 8 g of ilmenite concentrate (with an approximate content of 35 wt. % of Fe in oxide form) in 75 cm.sup.3 of deionized water. Afterwards, preparation of the oxygen carrier is performed according to the experimental conditions described in Example 8. Washed and dry precipitate is calcined for 2 h at 1100 C. using a Heraeus-Saga Petroleum furnace with static air flow and the following temperature profile: starting temperature 90 C., heating at 10 C./min up to 1100 C. during 2 hours, then cooling down to 90 C. at 15 C./min, thereby obtaining 60 wt. % of CuO (primary OC) and 40 wt. % of ilmenite concentrate (secondary OC) powder as final product.

Example 10

[0130] A preparation of an oxygen carrier by the CuO precipitation-coating method involves suspending 8 g of Mn sinter (with an approximate content of 60 wt % of Mn in oxide form) in 75 cm.sup.3 of deionized water. Afterwards, preparation of the oxygen carrier is performed according to the experimental conditions described in Example 8, thereby obtaining 60 wt. % of CuO (primary OC) and 40 wt. % of Mn sinter (secondary OC) powder as final product.

[0131] In summary, OCs prepared by this invention have shown significantly higher O.sub.2 carrying capacity for CLC-CLOU than the maximum theoretical capacity ever reached by OCs stabilized with synthetic non-active supports. Table 1 compares the molecular oxygen release capacity and total oxygen carrying capacity for OCs containing different amount of CuO as an active phase for reported values and provided in the present invention. Ilmenite concentrate was selected as an example of the secondary OC of the present invention for comparison. Results of the total oxygen carrying capacity of ilmenite concentrate are presented in FIG. 3, FIG. 5 and Table 3.

[0132] Results of the molecular oxygen release capacity, total oxygen carrying capacity and crushing strength for Examples of this invention and an example of the secondary OC (ilmenite concentrate) are summarized in Table 3. It is preferred that the oxygen carrier has a minimum oxygen carrying capacity higher than 6 g O.sub.2/100 g OC, and more preferred higher than 12 g O.sub.2/100 g OC, this later value being achieved in Examples 1-6 and 8-10, cf. table 3.

[0133] Another advantage of preparing OCs by the present invention is the high mechanical strength of the resulting materials, compared to previously reported OCs which combine CuO with synthetic supports in different compositions, as shown in Table 2. The last but not less important advantage of OCs provided by this invention comparing to known materials is their potential for producing cost-effective OCs, based on low-value industrial streams and by using simple and scalable production methods.

REFERENCES

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