CaMn1−xBxO3−δ (B = Al, V, Fe, Co, and Ni) perovskite based oxygen carriers for chemical looping with oxygen uncoupling (CLOU)
Introduction
Concerns over global climate change resulting from anthropogenic CO2 emissions necessitate development of new power generation technologies with lower carbon footprints. Of the various carbon capture technologies, chemical looping combustion (CLC) has emerged as a promising option [1], [2]. The CLC processes aims at lowering the energy penalty for CO2 separation in combustion processes. This is achieved though cyclic redox reactions using interconnected fluidized bed or moving bed reactors [3], [4], [5], [6], [7], [8]. Solid oxygen carrier particles are used to carry oxygen and heat from air to the fuel without mixing these two components. As a result, a concentrated CO2 stream can be generated in the fuel reactor. The oxygen depleted carrier is then transported to a second reactor referred to as the air reactor. Here, air is used to regenerate the oxygen carrier while producing heat for power generation.
Development of low cost oxygen carriers with high activity for carbonaceous fuel conversions is highly desirable for chemical looping processes. While many oxygen carriers are effective for gaseous fuel combustion, they tend to be less active in converting solid fuels such as biomass or coal due to mass transfer and kinetic limitations. Enhancement of oxygen carrier activity toward solid carbon-based fuels is typically achieved through two methods. First, in-situ gasification uses CO2 or steam to gasify the solid fuel into syngas for oxygen carrier conversion [6], [7], [8], [9], [10], [11]. However, gasification of solid fuels with steam or CO2 is often slow and becomes a rate limiting step. A variety of oxygen carriers including oxides of Fe [8], [10], [12], [13], [14], Mn [13], [15], and Ni [16], [17] have been used in the conversion of solid fuels through the in-situ gasification approach. The second method is known as chemical looping with oxygen uncoupling (CLOU) [3], [18], [19]. In CLOU, the oxygen carrier is selected from metal oxides with significant equilibrium partial pressure of oxygen () at high temperature. As a result, the oxygen carrier is able to release a portion of its lattice oxygen into the gas phase. The gaseous oxygen then reacts with the solid fuel, leading to improved combustion kinetics. Typical oxygen carriers that exhibit CLOU properties include oxides and mixed oxides containing Cu, Mn, and/or Co [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. However, each oxide exhibits challenges, such as high cost and agglomeration of Cu oxides [19], [26], [27], [28], low decomposition temperature and toxicity of Co oxides [18], [23], and difficulty of regeneration of Mn oxides [15], [19], [21], [22], [23], [24], [25], [31], [32], [33], [34], [35], [36].
Recently, perovskite structured oxides have received increasing attention as oxygen carriers for redox applications [23], [31], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56]. Typically, perovskites take the form of ABO3−δ, where A is a large cation of either the alkali or rare earth metal, and B is a smaller cation of the transition metal group [57]. Perovskite structured oxides have been used both as supports and primary oxygen carriers. As a support, mixed ionic and electronic conductive (MIEC) perovskites such as La1−xSrxFeO3 (LSF) have shown to enhance the redox activity of iron oxides by nearly two orders of magnitude [51], [52], [53], [54]. Perovskite and perovskite supported iron oxides have also been explored as redox catalysts for syngas generation and water-splitting [42], [56], [57]. La containing perovskite supports have been shown to enhance the oxygen donation properties of Mn–Fe and Co–Fe oxides [23]. Up to 8.8% decrease in the initial decomposition temperature was reported for perovskite supported mixed metal oxides.
Besides its role as a support, perovskite oxygen carriers have been shown to be effective as standalone oxygen carriers in CLOU and CLC applications. CaMnO3 perovskites are of interest due to its low cost, availability, and CLOU properties [20], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [49], [58], [59], [60], [61], [62]. CaMnO3 based oxygen carriers have lower oxygen uncoupling capacities than copper oxides but can be synthesized from cheap manganese ores and Ca precursors [29], [61], while obtaining high solid fuel conversions [29], [31], [34], [36]. CaMnO3 faces the challenge of long term stability. CaMnO3 oxygen carriers undergo an irreversible phase change to spinel (CaMn2O4) and Ruddlesden–Popper (Ca2MnO4) phases [19], [20], [32], [59], [60]. In addition, CaMnO3 has also been shown to be susceptible to sulfur poisoning, forming CaSO4 [32], [36], [60]. Stabilization of the perovskite structure has been investigated through addition of secondary metal atoms into the A- and B-sites of the parent CaMnO3 [29], [31], [32], [33], [34], [35], [36], [37], [38], [60], [61], [63], [64], [65].
Arjmand et al. investigated Ca1−xLaxMn1−yMyO3 (where x = 0 or 0.1 and M = Mg, Ti, Fe, or Cu) oxygen carriers for CLOU properties between 900 and 1000 °C in a laboratory scale fluidized bed reactor [31]. It was determined that undoped CaMnO3 had the highest oxygen release capability (∼0.7% after 360 s). Doping of the A- and B-site was shown to decrease the oxygen release capability except when Cu was used. However, Cu-doped samples were prone to defluidization. Rydèn et al. studied a Ti-doped CaMnO3 oxygen carrier for CLOU [35]. The oxygen carrier exhibited oxygen uncoupling properties above 720 °C in a fluidized bed reactor. The maximum rate for oxygen uncoupling was observed to be 0.03 LN/min at 950 °C with a maximum concentration at 4 vol% when a mass of 200 g of the oxygen carrier is used. The oxygen carrier was also tested in a redox mode using methane as a fuel. Slight deactivation was observed. The deactivation was attributed to CaMn2O4 formation. Kallèn et al. used CaMn1−xMgxO3 oxygen carrier in a 10kWth gas-fired CLC unit [37]. The oxygen carrier was observed to release oxygen above 700 °C. No agglomeration was observed for 120 h of high temperature operations. The total attrition of fines (<45 μm) was determined to be less than 0.01%. Hallberg et al. investigated Mg, Ti, and Fe doped CaMnO3 oxygen carriers prepared by spray drying for CLOU properties and redox activity with methane [33]. The oxygen carriers had oxygen release of 0.3–0.5% by weight at 900 °C with a maximum release of nearly 1% by weight at 1000 °C. After 3 cycles of oxygen uncoupling and one methane cycle, the authors reported the formation of CaMn2O4 spinel phase in all tested oxygen carriers. CaMnO3 oxygen carriers have also been synthesized from combinations of manganese ores and Ca(OH)2 [29]. In reaction with methane, ores containing high concentrations of Al showed poor performance due to spinel formation between Al and Mn. The best performing oxygen carrier had an oxygen uncoupling capacity of 0.68 wt.%. Although a number of doped CaMnO3 oxygen carriers have shown promise for the CLOU process, detailed characterization of dopant effects on B-site doped CaMnO3 oxygen carriers and their long term performance has not been extensively studied.
In this study, we investigate the effect of B-site doping for CaMnO3 based oxygen carriers. Fe, Co, V, Ni, and Al are chosen as potential dopants. It was determined that oxygen release properties of the oxygen carriers vary with dopant type and concentrations. Of the various dopants chosen, Fe doped oxygen carrier exhibits long term stability and has low temperature CLOU properties around 650 °C. Fe dopants also induced noticeable α-oxygen release between 350 and 500 °C. The Fe-doped samples exhibit long term stability (100 cycles) of its uncoupling properties. As a result, Fe-doped samples observe more facile oxygen release for solid fuel conversions relative to undoped CaMnO3 oxygen carriers.
Section snippets
Oxygen carrier synthesis
Mixed oxides with a general formula of CaMn1−xBxO3 (B = V, Fe, Co, Ni, and Al and x ⩽ 0.5) are prepared using a citric acid sol–gel method. General procedure for the sol–gel samples includes dispersion of cation precursors Ca(NO3)2·4H2O (Sigma Aldrich), Fe(NO3)3·9H2O (98+%, Sigma Aldrich), Co(NO3)2·6H2O (ACS Reagent, Noah), Ni(NO3)2·6H2O (Sigma Aldrich), VCl3 (97%, Sigma Aldrich), Al(NO3)3·9H2O (ACS Reagent, Sigma Aldrich) and Mn(NO3)2·4H2O (Sigma Aldrich) in distilled water followed by addition of
Effect of dopant into the B-site
Effectiveness of dopant addition largely relies on the compatibility of the dopant and its parent structure. Ideally, dopants should be fully incorporated into the host structure to form a homogeneous solid solution. Fig. 1 illustrates the XRD spectra of the various B-site doped CaMnO3 oxygen carriers.
Of the dopants, Fe is effectively incorporated into the structure without any secondary phases. Ni, V, and Co doped CaMnO3 oxygen carriers are present with separate identified phases of NiO, V2O5,
Conclusion
The present study investigates the effect of B-site doping on oxygen carriers with a general composition of CaMnxM1−xO3 (M = Fe, V, CO, Ni, Al). The phase compatibility with CaMnO3 parent structure, oxygen carrying capacity, oxygen release temperature, phase stability, and CLOU performance of these B-site doped oxygen carriers are investigated. Secondary phases are formed for all dopants except for Fe. For V and Ni doped oxygen carriers, pure oxides of the dopant metals are formed along with a
Acknowledgements
Funding supports from the U.S. Department of Energy (Award Number FE001247) and Kenan Institute are greatly appreciated. We acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation.
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