Metal ferrite oxygen carriers for chemical looping combustion of solid fuels

Abstract

The disclosure provides a metal ferrite oxygen carrier for the chemical looping combustion of solid carbonaceous fuels, such as coal, coke, coal and biomass char, and the like. The metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where MFe.sub.xO.sub.y is a chemical composition and M is one of Mg, Ca, Sr, Ba, Co, Mn, and combinations thereof. For example, MFe.sub.xO.sub.y may be one of MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, BaFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, MnFeO.sub.3, and combinations thereof. The MFe.sub.xO.sub.y is supported on an inert support. The inert support disperses the MFe.sub.xO.sub.y oxides to avoid agglomeration and improve performance stability. In an embodiment, the inert support comprises from about 5 wt. % to about 60 wt. % of the metal ferrite oxygen carrier and the MFe.sub.xO.sub.y comprises at least 30 wt. % of the metal ferrite oxygen carrier. The metal ferrite oxygen carriers disclosed display improved reduction rates over Fe.sub.2O.sub.3, and improved oxidation rates over CuO.

Claims

1. A method of combusting a solid carbonaceous fuel comprising: delivering a metal ferrite oxygen carrier to a fuel reactor, where the metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where M comprises; mixing the solid carbonaceous fuel and the metal ferrite oxygen carrier in the fuel reactor generating a reduced carrier, where the reduced carrier comprises an M component and an Fe.sub.cO.sub.d component where c>0 and d>0, where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, where 1.5x2.5 and 3.5y4.5, further where the M component comprises Ba and where the Fe.sub.cO.sub.d component comprises some portion of the Fe comprising the MFe.sub.xO.sub.y, where the Fe.sub.cO.sub.d component is FeO.sub.t, where 0t1.5, further where FeO.sub.t comprises Fe and maintaining the fuel reactor at a reducing temperature, where the reducing temperature is sufficient to reduce some portion of the metal ferrite oxygen carrier and oxidize some portion of the solid carbonaceous fuel, thereby combusting the solid carbonaceous fuel.

2. The method of claim 1 where the reducing temperature is from 800 C. to 1200 C.

3. The method of claim 2 where the MFe.sub.xO.sub.y is one of MFe.sub.uO.sub.v, MFe.sub.wO.sub.z, and combinations thereof, and where 1.5u2.5, 3.5v4.5, 1.5w2.5, and 2.5z3.5.

4. The method of claim 2 where the inert support comprises from about 5 wt. % to about 60 wt. % of the metal ferrite oxygen carrier and the MFe.sub.xO.sub.y comprises at least 30 wt. % of the metal ferrite oxygen carrier.

5. The method of claim 4 where 1.5u2.5 and 3.5v4.5.

6. The method of claim 4 where 1.5w2.5, and 2.5z3.5.

7. The method of claim 4 where the inert support comprises alumina.

8. The method of claim 1 further comprising injecting a gasification agent into the fuel reactor.

9. The method of claim 1 further comprising oxidizing the reduced carrier by contacting the reduced carrier and an oxidizing gas at an oxidizing temperature, where the oxidizing gas is comprised of oxygen, and where the oxidizing temperature is sufficient to generate an oxidizing reaction, where the reactants of the oxidizing reaction comprise some portion of the oxygen, some portion of the M component, and some portion of the Fe.sub.cO.sub.d component, and where the product of the oxidizing reaction is a re-oxidized carrier, where the re-oxidized carrier comprises MFe.sub.aO.sub.b on the inert support.

10. The method of claim 9 where the oxidizing temperature is from 800 C. to 1200 C.

11. The method of claim 10 where oxidizing the reduced carrier occurs in an oxidizing reactor, and further comprising: transferring the reduced carrier from the fuel reactor to the oxidizing reactor; supplying the oxidizing gas to the oxidizing reactor, thereby generating the re-oxidized carrier; transferring the re-oxidized carrier from the oxidizing reactor to the fuel reactor, and repeating the delivering step and the contacting step utilizing an additional quantity of the solid carbonaceous fuel as the solid carbonaceous fuel and the re-oxidized carrier as the metal ferrite oxygen carrier.

12. A method of combusting a solid carbonaceous fuel comprising: delivering a metal ferrite oxygen carrier to a fuel reactor, where the metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support where the inert support comprises from about 5 wt. % to about 60 wt. % of the metal ferrite oxygen carrier, the MFe.sub.xO.sub.y comprises at least 30 wt. % of the metal ferrite oxygen carrier, and where 0.5x1.5 and 2.5y3.5 and where 0.5a1.5 and 2.5b3.5, where M is one of Ba or Mn-and combinations thereof, and where the MFe.sub.xO.sub.y is one of MFe.sub.uO.sub.v, MFe.sub.wO.sub.z, and combinations thereof, where 1.5u2.5, 3.5v4.5, 1.5w2.5, and 2.5z3.5; mixing the solid carbonaceous fuel and the metal ferrite oxygen carrier in the fuel reactor A and maintaining the fuel reactor at a reducing temperature of from 800 C. to 1200 C. and generating a reduced carrier, where the reduced carrier comprises an M component and a Fe.sub.cO.sub.d component, where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where the Fe.sub.cO.sub.d component comprises some portion of the Fe comprising the MFe.sub.xO.sub.y, where c>0 and d0, thereby combusting the solid carbonaceous fuel; transferring the reduced carrier from the fuel reactor to an oxidizing reactor; oxidizing the reduced carrier by supplying an oxidizing gas to the oxidizing reactor, where the oxidizing gas is comprised of oxygen, and contacting the reduced carrier and the A oxidizing gas at an oxidizing temperature of from 800 C. to 1200 C. and generating an oxidizing reaction, where the reactants of the oxidizing reaction comprise some portion of the oxygen, some portion of the M component, and some portion of the Fe.sub.cO.sub.d component, and where the product of the oxidizing reaction is a re-oxidized carrier, where the re-oxidized carrier comprises MFe.sub.aO.sub.b on the inert support; and transporting the re-oxidized carrier from the oxidizing reactor to the fuel reactor; and repeating the delivering step, the contacting step, the transferring step, and the oxidizing step utilizing an additional quantity of the solid carbonaceous fuel as the solid carbonaceous fuel and the re-oxidized carrier as the metal ferrite oxygen carrier.

13. A method of combusting a solid carbonaceous fuel comprising: delivering a metal ferrite oxygen carrier to a fuel reactor, where the metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where M is Mn; mixing the solid carbonaceous fuel and the metal ferrite oxygen carrier in the fuel reactor generates a reduced carrier, where the reduced carrier comprises an M component and an Fe.sub.cO.sub.d component, where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where the Fe.sub.cO.sub.d component comprises some portion of the Fe comprising the MFe.sub.xO.sub.y, where c>0 and d0; oxidizing the reduced carrier by contacting the reduced carrier and an oxidizing gas at an oxidizing temperature, where the oxidizing gas is comprised of oxygen, and where the oxidizing A temperature is from 800 C. to 1200 C., sufficient to generate an oxidizing reaction, where the reactants of the oxidizing reaction comprise some portion of the oxygen, some portion of the M component, and some portion of the Fe.sub.cO.sub.d component, and where the product of the oxidizing reaction is a re-oxidized carrier, where the re-oxidized carrier comprises MFe.sub.aO.sub.b on the inert support, where 0.5x1.5 and 2.5y3.5 and where 0.5a1.5 and 2.5b3.5; and maintaining the fuel reactor at a reducing temperature, where the reducing temperature is sufficient to reduce some portion of the metal ferrite oxygen carrier and oxidize some portion of the solid carbonaceous fuel, thereby combusting the solid carbonaceous fuel.

14. The method of claim 13 where oxidizing the reduced carrier occurs in an oxidizing reactor, and further comprising: transferring the reduced carrier from the fuel reactor to the oxidizing reactor; supplying the oxidizing gas to the oxidizing reactor, thereby generating the re-oxidized carrier; transferring the re-oxidized carrier from the oxidizing reactor to the fuel reactor; and repeating the delivering step and the contacting step utilizing an additional quantity of the solid carbonaceous fuel as the solid carbonaceous fuel and the re-oxidized carrier as the metal ferrite oxygen carrier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a chemical looping combustion process utilizing an metal ferrite oxygen carrier.

(2) FIG. 2 illustrates XRD results for a metal ferrite oxygen carrier comprising MgFe.sub.2O.sub.4.

(3) FIG. 3 illustrates XRD results for a metal ferrite oxygen carrier comprising SrFe.sub.2O.sub.4.

(4) FIG. 4 illustrates XRD results for a metal ferrite oxygen carrier comprising CaFe.sub.2O.sub.4.

(5) FIG. 5 illustrates XRD results for a metal ferrite oxygen carrier comprising BaFe.sub.2O.sub.4.

(6) FIG. 6 illustrates XRD results for a metal ferrite oxygen carrier comprising CoFe.sub.2O.sub.4.

(7) FIG. 7 illustrates XRD results for a metal ferrite oxygen carrier comprising MnFeO.sub.3.

(8) FIG. 8 illustrates the reduction and oxidation rates of the metal ferrite oxygen carriers.

(9) FIG. 9 illustrates the reduction and oxidation rates of a metal ferrite oxygen carrier over multiple cycles.

(10) FIG. 10 illustrates the reduction temperatures of a metal ferrite oxygen carrier over multiple cycles.

DETAILED DESCRIPTION

(11) The following description is provided to enable any person skilled in the art to use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide process for chemical looping combustion of a solid carbonaceous fuel utilizing a metal ferrite oxygen carrier which comprises MFe.sub.xO.sub.y on an inert support, where MFe.sub.xO.sub.y is a chemical composition and M is one of Mg, Ca, Sr, Ba, Co, Mn, and combinations thereof.

(12) The disclosure provides a metal ferrite oxygen carrier having improved durability and reactivity over metal oxides currently used in the chemical looping combustion of solid carbonaceous fuels, such as coal, coke, coal and biomass char, and the like. The metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where MFe.sub.xO.sub.y is a chemical composition and M is one of Mg, Ca, Sr, Ba, Co, Mn, and combinations thereof. In an embodiment, the MFe.sub.xO.sub.y is one of MFe.sub.uO.sub.v, MFe.sub.wO.sub.z, and combinations thereof, where 1.5u2.5, 3.5v4.5, 1.5w2.5, and 2.5z3.5. For example, MFe.sub.xO.sub.y may be one of MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, BaFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, MnFeO.sub.3, and combinations thereof. The metal ferrite oxygen carrier thereby comprises a metal ferrite (MFe.sub.2O.sub.4) with M selected from Group II elements Mg, Ca, Sr, and Ba and transition metal ferrites CoFe.sub.2O.sub.4 and MnFeO.sub.3.

(13) The group II metal ferrites showed better performance for solid fuel chemical looping combustion than that with transition metal ferrites. The group II elements may promote the oxygen release of Fe.sub.2O.sub.3 which cause higher reduction rate. The incorporation of group II elements in Fe.sub.2O3 decreased agglomeration of reduced state of Fe which lead to durable performance.

(14) The inert support disperses the MFe.sub.xO.sub.y oxides to avoid agglomeration and improve the performance stability of the metal ferrite oxygen carriers. The inert support material does not participate in the oxidation and reduction reactions of the MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier. In an embodiment, the inert support comprises from about 5 wt. % to about 60 wt. % of the metal ferrite oxygen carrier and the MFe.sub.xO.sub.y comprises at least 30 wt. % of the metal ferrite oxygen carrier. The metal ferrite oxygen carrier is effective for use as an oxygen carrier in chemical looping combustion applications for the combustion of solid carbonaceous fuels, as well as other applications where the transport of oxygen is facilitated through the reduction and subsequent re-oxidation of an oxygen carrier.

(15) A chemical looping combustion system within which the metal ferrite oxygen carrier disclosed here may be utilized is illustrated at FIG. 1. FIG. 1 illustrates a chemical combustion system generally at 100 and includes fuel reactor 101. Fuel reactor 101 receives a fuel flow of solid carbonaceous fuel at 102 and the metal ferrite oxygen carrier at 103, and provides mixing among the solid carbonaceous fuel and the metal ferrite oxygen carrier. Fuel reactor 101 is at a reducing temperature sufficient to reduce at least a portion of the metal ferrite oxygen carrier. In an embodiment, the reducing temperature is from about 800 C. to about 1200 C. In certain embodiments, fuel reactor 101 may also receive a flow of gasification agent at 109, such as steam, CO.sub.2, H.sub.2, oxygen, and/or air, or any other agent intended to generate gaseous products from the solid carbonaceous fuel within fuel reactor 101.

(16) Within fuel reactor 101, metal ferrite oxygen carrier interacts with carbon and possibly gaseous components from the solid carbonaceous fuel, and the MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier reduces to a reduced carrier comprising an M component and an Fe.sub.cO.sub.d component. The M component comprises some portion of the M comprising the MFe.sub.xO.sub.y. The Fe.sub.cO.sub.d component comprises some portion of the Fe comprising the MFe.sub.xO.sub.y, with c>0 and d0. For example, the Fe.sub.cO.sub.d component may be Fe or may be an iron oxide such as FeO, Fe2O3, and Fe3O4, among others. In an embodiment, the Fe.sub.cO.sub.d component is FeO.sub.t, where 0t1.5. For example, in an embodiment where the metal ferrite oxygen carrier is CaFe.sub.2O.sub.4 on the inert support, the CaFe.sub.2O.sub.4 interacts with carbon in fuel reactor 101 and generates a reduced carrier mainly comprising Fe, and FeO. In this embodiment, the M component CaFe.sub.2O.sub.5 generated by the reduction comprises some portion of the Ca comprising the CaFe.sub.2O.sub.4, and Fe and FeO comprise the Fe.sub.cO.sub.d component FeO.sub.t where 001.5. The MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier may additionally interact with CO, H.sub.2, and other gases which may be present from a gasification of the solid carbonaceous fuel. Following the reduction, an exhaust stream comprised of CO.sub.2 and possibly H.sub.2O may exit fuel reactor 101 at exhaust 104, and the reduced carrier may exit fuel reactor 101 at 105.

(17) The reduced carrier exiting fuel reactor 101 at 105 may subsequently enter oxidation reactor 106. Oxidation reactor 106 further receives a flow of oxidizing gas such as air, and facilitates contact between the reduced carrier and the oxidizing gas, generating a re-oxidized carrier. The re-oxidized carrier is generated by an oxidizing reaction, where the reactants of the oxidizing reaction are a portion of the oxygen from the oxidizing gas, the M component comprising the reduced carrier, and the Fe.sub.cO.sub.d component comprising the reduced carrier. The product of the oxidizing reaction is the re-oxidized carrier, where the re-oxidized carrier comprises MFe.sub.aO.sub.b on the inert support. Generally, the MFe.sub.aO.sub.b comprising the re-oxidized carrier is substantially equivalent to the MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier. For example, when the metal ferrite oxygen carrier comprises CaFe.sub.2O.sub.4 on the inert support and the reduced carrier comprises CaFe.sub.2O.sub.5, Fe, and FeO, then the oxidation reaction generates a re-oxidized carrier comprising CaFe.sub.2O.sub.4 on the inert support. Oxidation reactor 106 is at an oxidation temperature sufficient to oxidize at least a portion of the reduced carrier. In an embodiment, the oxidizing temperature is from about 800 C. to about 1200 C.

(18) Within this disclosure, reducing or reduction as it applies to a metal ferrite oxygen carrier means the loss of oxygen from the MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier. For example, the reduction of a MFe.sub.xO.sub.y composition to FeO, Fe.sub.2O.sub.3, and/or Fe and an M component, where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, or alternatively, the reduction of a MFe.sub.xO.sub.y composition to a MFe.sub.xO.sub.y-1 composition. Oxidizing or oxidation as it applies to a metal ferrite oxygen carrier means a reaction with oxygen among the FeO, Fe.sub.2O.sub.3, and/or Fe and the M component generated by reduction of the MFe.sub.xO.sub.y, where the oxidation reaction produces the MFe.sub.xO.sub.y, or alternatively, a gain of oxygen by the MFe.sub.xO.sub.y-1 composition. Similarly, a reducing temperature is a temperature sufficient to generate reduction and an oxidizing temperature is a temperature sufficient to generate oxidation under other prevailing and germane existing conditions.

(19) Within this disclosure, solid carbonaceous fuel means a fuel comprising solid carbon, such as coal, coke, coal and biomass char, and the like. Under the reducing temperature of the fuel reactor and in some embodiments the influence of the gasification agent, the solid carbonaceous fuel may produce volatile gases and other compounds in the fuel reactor. In an embodiment, the solid carbonaceous fuel is at least 50 wt. % fixed carbon. In an additional embodiment, the solid carbonaceous fuel is at least 75 wt. % fixed carbon, and in a further embodiment, the solid carbonaceous fuel is at least 85 wt. % fixed carbon with a volatile matter content of less than 5 wt. %. In another embodiment where the solid carbonaceous fuel is substantially free of volatiles such as petcoke, char, and the like, the solid carbonaceous fuel is at least 90 wt. % fixed carbon. Fixed carbon and volatile matter contents may be determined by means known in the art. See e.g., ASTM StandardVol. 05.06 Gaseous Fuels, Coal and Coke, ASTM International (2013).

(20) Within this disclosure, mixing as it pertains to a metal ferrite oxygen carrier and a solid carbonaceous fuel means bringing the metal ferrite oxygen carrier and the solid carbonaceous fuel and/or some component thereof into sufficient proximity such that the MFe.sub.xO.sub.y comprising the metal ferrite oxygen carrier reduces to a reduced carrier comprising an M component and an Fe.sub.cO.sub.d component at the reducing temperature within the fuel reactor. In an embodiment, mixing means bringing the metal ferrite oxygen carrier and carbon comprising the solid carbonaceous fuel into sufficient proximity such that the metal ferrite oxygen carrier is reduced by a solid-solid reaction with the carbon comprising the solid carbonaceous fuel. See e.g., Siriwardane et al., Combustion and Flame 157 (2010).

(21) As stated and as is understood, the metal ferrite oxygen carrier comprising MFe.sub.xO.sub.y on the inert support may also be made up of additional components. In an embodiment, the MFe.sub.xO.sub.y on the inert support comprises at least 10 wt. % of the metal ferrite oxygen carrier. In another embodiment, the MFe.sub.xO.sub.y on the inert support comprises at least 25 wt. % of the metal ferrite oxygen carrier, and in a further embodiment, the MFe.sub.xO.sub.y on the inert support comprises at least 50 wt. % of the metal ferrite oxygen carrier. In an additional embodiment, an oxygen carrier comprises a plurality of reducing components where each component in the plurality undergoes a reduction reaction in contact with the solid carbonaceous fuel, and the metal ferrite oxygen carrier comprises at least 10 wt. %, at least 25 wt. %, or at least 50 wt. % of the plurality of reducing components.

(22) Additionally, in an embodiment, the metal ferrite oxygen carrier is a plurality of oxygen carrier pellets where each oxygen carrier pellet in the plurality comprises the MFe.sub.xO.sub.y on the inert support. In an additional embodiment, a Sauter mean diameter of the plurality of oxygen carrier pellets is less than about 200 micron (m), preferably less than about 100 m. In a further embodiment, the solid carbonaceous fuel is a plurality of fuel pellets, and a Sauter mean diameter of the plurality of fuel pellets is less than about 200 micron (m), preferably less than about 100 m. The Sauter mean diameter may be determined by means known in the art such as sieving, microscopy, sedimentation, permeametry, laser diffraction, or other means, or as reported by a manufacturer of such as-described pellets or the operating instructions of machinery intended to produce such as-described pellets. See e.g., Martin Rhodes, Introduction to Particle Technology (2.sup.nd ed. 2008). The use of such sized pellets as described can promote solid-solid contact between the metal ferrite oxygen carrier and the solid carbonaceous fuel, enhancing the reaction mechanisms. See e.g., Siriwardane et al., Combustion and Flame 157 (2010). When a gasification agent is used for gasifying the solid fuel, the particle size of the carrier pellet may vary depending on the type of reactor bed used. In case of a fluid bed reactor, particle size may be 100-500 m, while in moving bed applications the particle size may be 1-5 mm.

(23) The oxidizing reaction occurring in oxidation reactor 106 is an exothermic reaction, and heat generated is carried from oxidizing reactor 106 by a gaseous flow exiting at 108. The gaseous flow exiting at 108 is comprised of the oxidizing gas less that oxygen utilized for the generation of the re-oxidized carrier, and may be sent to and utilized by a power generation cycle. For example, when the flow of oxidizing gas is air, the gaseous flow exiting at 108 is comprised of N.sub.2 and possibly some remaining O.sub.2, and other components. The re-oxidized carrier may be subsequently transported to fuel reactor 101 for use as the metal ferrite oxygen carrier in a cyclic operation.

(24) It is understood that FIG. 1 provides an exemplary application illustrating a chemical looping combustion process with a solid carbonaceous fuel such as coal, coke, coal and biomass char, and the like, however the specifics of the process illustrated are not intended to be limiting. Within this disclosure, it is only necessary that a metal ferrite oxygen carrier be delivered to a fuel reactor, where the metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, and where M is one of Mg, Ca, Sr, Ba, Co, Mn, and combinations thereof, and that the metal ferrite oxygen carrier contact a solid carbonaceous fuel at a reducing temperature sufficient to reduce some portion of the metal ferrite oxygen carrier and oxidize some portion of the solid carbonaceous fuel. An exemplary application is as the metal ferrite oxygen carrier in a chemical looping combustion process combusting a solid carbonaceous fuel or fuels such as coal, coke, coal and biomass char, and the like.

(25) As stated, the metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where MFe.sub.xO.sub.y is a chemical composition and M is one of Mg, Ca, Sr, Ba, Ni, Co, Mn, and combinations thereof. For example, MFe.sub.xO.sub.y may be one of MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, BaFe.sub.2O.sub.4, NiFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, MnFeO.sub.3, and combinations thereof. The inert support does not participate in the oxidation and reduction reactions of the MFe.sub.xO.sub.y. In an embodiment, the inert support is alumina (Al.sub.2O.sub.3).

(26) The performance of metal ferrite oxygen carriers comprising MgFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, and BaFe.sub.2O.sub.4 on an inert support of Al.sub.2O.sub.3 is illustrated at FIGS. 2-7, where the inert support comprises 10 wt. % of the metal ferrite oxygen carrier. At FIGS. 2-7, specific listed constituents generally indicate 2-theta values where peaks indicate the presence of the specific constituent on the 2-theta axis.

(27) X-ray diffraction (XRD) analyses were carried out using a Panalytical PW 3040 X-Pert Pro XRD system equipped with a 60 kV PW 3373/00 Cu LFF high-power ceramic tube with a Cu anode and a PW 3011/20 detector. The X-ray wavelength used was Cu KR-1 at 1.540 56 angstrom. The maximum goniometer resolution was 0.003 (2). System calibration was carried out using a polysilicon-pressed disk with the Si<111> referenced to 28.443 (2). Sample data were acquired at 40 kV and 45 mA in a line-focus mode using a standard PW3071/60 powder diffraction stage.

(28) FIG. 2 illustrates XRD patterns obtained before and following reduction of the MgFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 210 indicates the XRD pattern of the metal ferrite oxygen carrier comprising MgFe.sub.2O.sub.4 prior to reduction. As indicated, prior to reduction, trace 210 indicates the presence of MgFe.sub.2O.sub.4 (main) and Fe.sub.2O.sub.3, as evidenced by the peaks on trace 210 located at the 2-theta markers generally indicated by MgFe.sub.2O.sub.4 and Fe.sub.2O.sub.3. Trace 211 indicates the XRD pattern of the metal ferrite oxygen carrier comprising MgFe.sub.2O.sub.4 following reduction. Trace 211 indicates the presence of FeO (main) and Mg.sub.0.7Fe.sub.0.23Al.sub.1.97O.sub.4 as evidenced by the peaks on trace 211 located at the 2-theta markers generally indicated by FeO and Mg.sub.0.7Fe.sub.0.23Al.sub.1.97O.sub.4. FIG. 2 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Mg, reduced to a reduced carrier comprising an M component (Mg.sub.0.7Fe.sub.0.23Al.sub.1.97O.sub.4) and an Fe.sub.cO.sub.d component as FeO.sub.t (FeO), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where 0t1.5.

(29) FIG. 3 illustrates x-ray diffraction (XRD) patterns obtained before and following reduction of the SrFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 312 indicates the XRD pattern of the metal ferrite oxygen carrier comprising SrFe.sub.2O.sub.4 prior to reduction. As indicated, prior to reduction, trace 312 indicates the presence of SrFe.sub.2O.sub.4 (main) and Fe.sub.2O.sub.3, as evidenced by the peaks on trace 312 located at the 2-theta markers generally indicated by SrFe.sub.2O.sub.4 and Fe.sub.2O.sub.3. Trace 313 indicates the XRD pattern of the metal ferrite oxygen carrier comprising SrFe.sub.2O.sub.4 following reduction. Trace 313 indicates the presence of FeO (main) and SrAl.sub.2O.sub.4 as evidenced by the peaks on trace 313 located at the 2-theta markers generally indicated by FeO and SrAl.sub.2O.sub.4. FIG. 3 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Sr, reduced to a reduced carrier comprising an M component (SrAl.sub.2O.sub.4) and an Fe.sub.cO.sub.d component as FeO.sub.t (FeO), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where 0t1.5.

(30) FIG. 4 illustrates x-ray diffraction (XRD) patterns obtained before and following reduction of the CaFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 414 indicates the XRD pattern of the metal ferrite oxygen carrier comprising CaFe.sub.2O.sub.4 prior to reduction. As indicated, prior to reduction, trace 414 indicates the presence of CaFe.sub.2O.sub.4 (main) and Ca.sub.2Fe.sub.2O.sub.5, as evidenced by the peaks on trace 414 located at the 2-theta markers generally indicated by CaFe.sub.2O.sub.4 and Ca.sub.2Fe.sub.2O.sub.5. Trace 415 indicates the XRD pattern of the metal ferrite oxygen carrier comprising CaFe.sub.2O.sub.4 following reduction. Trace 415 indicates the presence of Fe.sub.0.902O (main), Fe, and Ca.sub.2Fe.sub.2O.sub.5 as evidenced by the peaks on trace 415 located at the 2-theta markers generally indicated by Fe.sub.0.902O, Fe, and Ca.sub.2Fe.sub.2O.sub.5. FIG. 4 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Ca, reduced to a reduced carrier comprising an M component (Ca.sub.2Fe.sub.2O.sub.5) and an Fe.sub.cO.sub.d component as FeO.sub.t (Fe.sub.0.902O and Fe), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where 0t1.5.

(31) FIG. 5 illustrates x-ray diffraction (XRD) patterns obtained before and following reduction of the BaFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 516 indicates the XRD pattern of the metal ferrite oxygen carrier comprising BaFe.sub.2O.sub.4 prior to reduction. As indicated, prior to reduction, trace 516 indicates the presence of BaFe.sub.2O.sub.4 (main) and Fe.sub.2O.sub.3, as evidenced by the peaks on trace 516 located at the 2-theta markers generally indicated by BaFe.sub.2O.sub.4 and Fe.sub.2O.sub.3. Trace 517 indicates the XRD pattern of the metal ferrite oxygen carrier comprising BaFe.sub.2O.sub.4 following reduction. Trace 517 indicates the presence of Fe, BaO, and Ba.sub.2Fe.sub.2O.sub.5 as evidenced by the peaks on trace 517 located at the 2-theta markers generally indicated by Fe, BaO, and Ba.sub.2Fe.sub.2O.sub.5. FIG. 5 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Ba, reduced to a reduced carrier comprising an M component (Ba.sub.2Fe.sub.2O.sub.5 and BaO) and an Fe.sub.cO.sub.d component as FeO.sub.t (Fe), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where 0t1.5.

(32) FIG. 6 illustrates x-ray diffraction (XRD) patterns obtained before and following reduction of the CoFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 618 indicates the XRD pattern of the metal ferrite oxygen carrier comprising CoFe.sub.2O.sub.4 prior to reduction. As indicated, prior to reduction, trace 618 indicates the presence of CoFe.sub.2O.sub.4 (main) and Fe.sub.2O.sub.3, as evidenced by the peaks on trace 618 located at the 2-theta markers generally indicated by CoFe.sub.2O.sub.4 and Fe.sub.2O.sub.3. Trace 619 indicates the XRD pattern of the metal ferrite oxygen carrier comprising CoFe.sub.2O.sub.4 following reduction. Trace 619 indicates the presence of Co, CoFe.sub.t5.7, and Fe.sub.3O.sub.4 as evidenced by the peaks on trace 619 located at the 2-theta markers generally indicated by Co, CoFe.sub.15.7, and Fe.sub.3O.sub.4. FIG. 6 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Co, reduced to a reduced carrier comprising an M component (Co and CoFe.sub.15.7) and an Fe.sub.cO.sub.d component (Fe.sub.3O.sub.4), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where c>0 and d0.

(33) FIG. 7 illustrates x-ray diffraction (XRD) patterns obtained before and following reduction of the BaFe.sub.2O.sub.4 metal ferrite oxygen carrier. Trace 720 indicates the XRD pattern of the metal ferrite oxygen carrier comprising MnFeO.sub.3 prior to reduction. As indicated, prior to reduction, trace 720 indicates the presence of MnFeO.sub.3 (main) and Fe.sub.2O.sub.3, as evidenced by the peaks on trace 720 located at the 2-theta markers generally indicated by MnFeO.sub.3 and Fe.sub.2O.sub.3. Trace 721 indicates the XRD pattern of the metal ferrite oxygen carrier comprising MnFeO.sub.3 following reduction. Trace 721 indicates the presence of Fe, Fe.sub.0.95O, and MnO as evidenced by the peaks on trace 721 located at the 2-theta markers generally indicated by Fe, Fe.sub.0.95O, and MnO. FIG. 7 thus illustrates a metal ferrite oxygen carrier MFe.sub.xO.sub.y, where M is Mn, reduced to a reduced carrier comprising an M component (MnO) and an Fe.sub.cO.sub.d component as FeO.sub.t (Fe and Fe.sub.0.95O), where the M component comprises some portion of the M comprising the MFe.sub.xO.sub.y, and where 0t1.5.

(34) Comparison of reduction and oxidation rates are illustrated at FIG. 8 for the metal ferrite oxygen carriers comprising MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, BaFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, and MnFeO.sub.3 on an inert support of Al.sub.2O.sub.3, along with the reduction and oxidation rates for Fe.sub.2O.sub.3, CuO, CuFe.sub.2O.sub.4, and NiFe2O4 for comparison. The reduction and oxidation rates were obtained by TGA conducted in a thermogravimetric analyzer (Cahn Thermax 500) to investigate the redox properties of the metal ferrite oxygen carriers. Approximately 1000 mg of metal ferrite sample mixed with coal or carbon black was placed in a Quartz bucket equipped with Cahn Thermax 500. The mixture was heated in a quartz-bowl to 900 or 1000 C. at a heating rate of 5 C./min in N.sub.2 gas and a flow rate of 150 sccm. The sample was then maintained isothermal for the duration of the redox cycles. The reduction cycle generally consisted of 100% nitrogen at 150 sccm while the oxidation cycle generally consisted of air at 150 sccm.

(35) FIG. 8 illustrates reduction rates as Fe.sub.2O.sub.3 reduction rate 823, CuO reduction rate 825, MgFe.sub.2O.sub.4 reduction rate 827, CaFe.sub.2O.sub.4 reduction rate 829, SrFe.sub.2O.sub.4 reduction rate 831, BaFe.sub.2O.sub.4 reduction rate 833, CuFe.sub.2O.sub.4 reduction rate 835, CoFe.sub.2O.sub.4 reduction rate 837, NiFe.sub.2O.sub.4 reduction rate 837, and MnFeO.sub.3 reduction rate 841 As illustrated, MgFe.sub.2O.sub.4 reduction rate 827, CaFe.sub.2O.sub.4 reduction rate 829, SrFe.sub.2O.sub.4 reduction rate 831, BaFe.sub.2O.sub.4 reduction rate 833, CoFe.sub.2O.sub.4 reduction rate 837, and MnFeO.sub.3 reduction rate 841 display improved reduction rates over Fe.sub.2O.sub.3 reduction rate 823, and reduction rates which exceed or are comparable with CuFe.sub.2O.sub.4 reduction rate 835 and NiFe.sub.2O.sub.4 reduction rate 837. Similarly, FIG. 8 illustrates oxidation rates as Fe.sub.2O.sub.3 oxidation rate 822, CuO oxidation rate 824, MgFe.sub.2O.sub.4 oxidation rate 826, CaFe.sub.2O.sub.4 oxidation rate 828, SrFe.sub.2O.sub.4 oxidation rate 830, BaFe.sub.2O.sub.4 oxidation rate 832, CuFe.sub.2O.sub.4 oxidation rate 834, CoFe.sub.2O.sub.4 oxidation rate 836, NiFe.sub.2O.sub.4 oxidation rate 838, and MnFeO.sub.3 oxidation rate 840. As indicated, MgFe.sub.2O.sub.4 oxidation rate 826, CaFe.sub.2O.sub.4 oxidation rate 828, SrFe.sub.2O.sub.4 oxidation rate 830, BaFe.sub.2O.sub.4 oxidation rate 832, CoFe.sub.2O.sub.4 oxidation rate 836, and MnFeO.sub.3 oxidation rate 840 display improved oxidation rates over CuO oxidation rate 824. Additionally, Group II metal ferrites MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, and BaFe.sub.2O.sub.4 display improved reduction and oxidation rates over the transition metal ferrites CuFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, NiFe.sub.2O.sub.4, and MnFeO.sub.3. In particular, the metal ferrite oxygen carrier comprising BaFe.sub.2O.sub.4 displays the best reduction and oxidation rates among the metal ferrite oxygen carriers of this disclosure, and provides a reduction rate comparable to that of CuO. Additionally, the metal ferrite oxygen carrier comprising BaFe.sub.2O.sub.4 may be operated up to 1000 C. without agglomeration.

(36) FIG. 9 illustrates the TGA test data during cyclic tests of carbon CLC with 10% Al.sub.2O.sub.3/BaFe.sub.2O.sub.4 synthesized by microwave method. BaFe.sub.2O.sub.4 showed stable reduction rates during cyclic tests while oxidation rates improved. The reaction temperature slightly increased with increasing cycles. At FIG. 9, reduction rates 952, 954, 956, 958, 960, 962, 964, 966, and 968 correspond to cycles 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 respectively, while oxidation rates 953, 955, 957, 959, 961, 963, 965, 967, and 969 correspond to cycles 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 respectively. FIG. 10 illustrates reduction temperatures for the cycles 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 as reduction temperatures 1070, 1071, 1072, 1073, 1074, 1075, 1076, 1077, 1078, and 1079 respectively.

(37) In an embodiment, the metal ferrite oxygen carriers of this disclosure are synthesized using a microwave method. In microwave (Anton Paar Synthos 3000) method, metal nitrates or metal acetate were used as precursor of oxygen carriers. Metal nitrates or metal acetates were dissolved in the diethylene glycol and the solution was heated up to 200 C.-250 C. in the microwave reactor for 30-45 min. The resulting solid precipitate was washed with DI water and separated by centrifugation. The material was dried in an oven at 100 C. overnight and calcined in air at 600-1000 C. for 6 h.

(38) In another embodiment, the metal ferrite oxygen carriers of this disclosure are synthesized using a solid reaction method. Solid Reaction Method was evaluated as a preparation method since it is a more cost effective than microwave method. In Solid Reaction method, metal nitrates were mixed with citric acid to enhance bonding and prevent aggregation at high temperature. The mixture was heated to 1000 C. at a ramping rate of 3 C./min in air and kept at 1000 C. for 6 h.

(39) Thus, the disclosure provides a metal ferrite oxygen carrier having improved durability and reactivity over metal oxides currently used in the chemical looping combustion of solid carbonaceous fuels, such as coal, coke, coal and biomass char, and the like. The metal ferrite oxygen carrier comprises MFe.sub.xO.sub.y on an inert support, where MFe.sub.xO.sub.y is a chemical composition and M is one of Mg, Ca, Sr, Ba, Co, Mn, and combinations thereof. The metal ferrite oxygen carrier thereby comprises a metal ferrite (MFe.sub.2O.sub.4) with M selected from Group II elements Mg, Ca, Sr, and Ba and transition metal ferrites CoFe.sub.2O.sub.4 and MnFeO.sub.3. The metal ferrite oxygen carriers disclosed display improved reduction rates over Fe.sub.2O.sub.3, and improved oxidation rates over CuO. Additionally, Group II metal ferrites MgFe.sub.2O.sub.4, CaFe.sub.2O.sub.4, SrFe.sub.2O.sub.4, and BaFe.sub.2O.sub.4 display improved reduction and oxidation rates over the transition metal ferrites CuFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, NiFe.sub.2O.sub.4, and MnFeO.sub.3.

(40) It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention and it is not intended to be exhaustive or limit the invention to the precise form disclosed. Numerous modifications and alternative arrangements may be devised by those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the claims appended hereto.

(41) In addition, the previously described versions of the present invention have many advantages, including but not limited to those described above. However, the invention does not require that all advantages and aspects be incorporated into every embodiment of the present invention.

(42) All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.