Applications are known where metal-oxide oxygen carriers are utilized for the delivery of oxygen via reduction of the oxygen carrier. One such application which has been investigated extensively is chemical looping combustion. Chemical looping combustion systems generally utilize a fuel reactor, an air reactor, and a metal oxide oxygen carrier undergoing reduction in the fuel reactor and oxidation in the air reactor. The reduction in the fuel reactor is facilitated by close contact between a fuel and the oxygen carrier. The subsequent oxidation of the carrier in the air reactor is an exothermic process, and a stream of N2 is exhausted from the air reactor and carries the heat of oxidation to an attached power generation island.
Chemical looping combustion cycles provide potentially significant advantages. The enhanced reversibility of the two redox reactions offers improved efficiencies over traditional single stage combustions, where the release of a fuel's energy occurs in a highly irreversible manner. Further, with appropriate oxygen carriers, both redox reactions can occur at relatively low temperatures, allowing a power station to more closely approach an ideal work output without exposing components to excessive working temperatures. Additionally, and significantly, chemical looping combustion can serve as an effective carbon capture technique. Of the two flue gas streams generated, one is comprised of atmospheric N2 and residual O2, but sensibly free of CO2, while the second stream is comprised of CO2 and H2O, and contains almost all of the CO2 generated by the system. It is relatively uncomplicated to remove the water vapor, leading to a stream of almost pure CO2. For these reasons, chemical looping combustion systems have been extensively investigated. However, necessary characteristics of the oxygen carrier such as sufficient durability and reactivity have limited the success, particularly when the fuel utilized has been introduced to the fuel reactor as a solid such as carbon, coal, or biomass.
It is understood that high reactivity of the oxygen carrier is desired in chemical looping combustion systems in order to limit the solid inventories utilized in the various processes. Toward this end, Cu-based oxygen carriers have been extensively investigated for the combustion of both gaseous and solid fuels. Generally, Cu-based carriers possess a high reactivity for fuel combustion in chemical looping combustion systems, however their relatively low melting point has generated severe agglomeration problems in systems operating in the 600° C. to 1000° C. range. This is recognized as problematic in a reactor where solid particles are flowing, moving, and recirculating, and where particle agglomeration leads to reduced reactivity and a host of other potential operational problems. As a result, thus far, the agglomeration issue experienced with Cu-based carriers has limited their successful application in working environments despite the relatively high reactivities that would be otherwise afforded. For example, in an application using gaseous CH4 as fuel, investigators have reported that severe agglomeration issues appear in oxygen carriers having greater than 20 wt % supported CuO regardless of preparation method, and that generally less than 17 wt % supported CuO is recommended to provide for satisfactory performance. See de Diego et al., “Impregnated CuO/Al2O3 Oxygen Carriers for Chemical-Looping Combustion: Avoiding Fluidized Bed Agglomeration”, Energy & Fuels 19 (2005).
Because of these CuO limitations and because of relative availability, Fe2O3 as an oxygen carrier has also been extensively investigated. Fe2O3 generally has improved temperature stability over CuO, however the reactivity of Fe2O3 is significantly limited as compared to Cu-based oxygen carriers. Additionally, Fe2O3 requires relatively high temperatures as compared to CuO. These characteristics reduce overall system performance and increase the complexity of heat transfer requirements in a working system. It would be advantageous to formulate an oxygen carrier for use in chemical combustion systems where the higher reactivity of a Cu-based carrier could be utilized with mitigation of the agglomeration issues. It would be additionally advantageous if any such formulation preserved the temperature stability of Fe2O3 without the high temperature requirement of the Fe2O3 reducing reaction. Further, it would be advantageous if the oxygen carrier were effective for a chemical looping combustion process utilizing a solid carbonaceous fuel such as coal, coke, coal and biomass char, and the like.
Various CuO and Fe2O3 mixtures have been proposed in chemical looping combustion systems operating with gaseous fuels. See e.g. Yu et al., “Analysis of the sorbent energy transfer system (SETS) for power generation and CO2 capture”, Advances in Environmental Research 7 (2003); See also Wang et al., “Chemical looping combustion of coke oven gas by using Fe2O3/CuO with MgAl2O4 as oxygen carrier”, Energy Environ. Sci. 3 (2010). Yu et al. utilized a mixture of CuO and Fe2O3 in-situ to postulate a thermally neutral carrier and generated results based on thermodynamic calculations, treating the mixture's overall performance as the sum of heat contributions from individual CuO and Fe2O3 with expected performances. Such an approach fails to address the performance issues such as reactivity and agglomeration associated with CuO. Wang et al. combined CuO and Fe2O3 on an MgAl2O4 support and through calcination produced an oxygen carrier exhibiting a high concentration of iron oxide crystalline structures, with separated phases of CuO or Fe2O3. Agglomeration is improved, however sufficient reactivity required operation at the higher temperatures generally associated with Fe2O3 carriers. It would be advantageous to formulate an oxygen carrier combining CuO and Fe2O3 which exhibits CuO crystalline structure, in order to provide a uniform species for effective operation at the lower combustion peak temperatures of CuO carriers.
Further, evaluation of the CuO—Fe2O3 carriers discussed above and of oxygen carriers generally has been conducted using gaseous fuels, where the is fuel introduced into the fuel reactor as a reducing gas, and appropriate reaction between the oxygen carrier and the reducing gas becomes largely a function of facilitating germane thermodynamic conditions. However, in contrast to these gaseous fuel approaches, significant differences arise when utilizing a solid carbonaceous fuel such as coal, coke, coal and biomass char, and the like. The solid fuels generally enter as a solid composition and may undergo subsequent gasification, producing volatile gases, char, and other compounds. The particular oxygen carrier utilized in such an approach must be effective for oxygen donation under temperatures sufficient to generate char from the solid fuel and volatiles, and also be effective in facilitating the production of CO2 from the resulting char. Further, the thermodynamic favorability of interactions between the oxygen carrier and any resulting ash must be considered, in order to provide for the effective separation of the oxygen carrier over a cyclic process. Additionally, the combustion of solid fuel with an oxygen carrier is generally an endothermic process requiring some variety of heat transfer to the fuel reactor. As a result, an oxygen carrier exhibiting lower peak combustion temperatures would be preferred in order to mitigate any parasitic losses arising from the endothermic reaction.
It would be advantageous to provide an oxygen carrier where the higher reactivity of a Cu-based carrier could be combined with the temperature stability of Fe2O3 for a chemical looping process utilizing a solid carbonaceous fuel such as coal, coke, coal and biomass char, and the like. It would be further advantageous if interactions between the oxygen carrier and any resulting ash were thermodynamically unfavorable, in order to facilitate more effective separation over a cyclic process. It would be further advantageous lower peak combustion temperatures resulted, in order to mitigate parasitic losses during long term operation.
Accordingly, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping cycle such as chemical looping combustion where the higher reactivity of Cu-based carriers can be utilized with a mitigation of agglomeration.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping cycle where the temperature stability of Fe2O3 is largely preserved without the attendant high temperature requirement of Fe2O3 oxygen carriers.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping cycle which combines CuO and Fe2O3 in a manner providing for reduced agglomeration and increased durability.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping cycle which combines CuO and Fe2O3 in a manner providing for a reduced peak combustion temperature, in order to mitigate parasitic losses during long term operation.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping combustion process utilizing a solid carbonaceous fuel, where the oxygen carrier is effective for oxygen donation under temperatures sufficient to generate char and facilitate the production of CO2 from the resulting char.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping combustion process utilizing a solid carbonaceous fuel exhibiting the lower peak combustion temperatures of CuO, in order to mitigate any parasitic losses arising from the generally endothermic reaction experienced with solid fuels.
Further, it is an object of this disclosure to provide an oxygen carrier for use in a chemical looping combustion process utilizing a solid carbonaceous fuel where interactions between the oxygen carrier and the resulting ash thermodynamically unfavorable, in order to facilitate more effective separation over a cyclic process.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.