Synergistic Effects of the Zr and Sm Co-doped Fe2O3/CeO2 Oxygen Carrier for Chemical Looping Hydrogen Generation
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CeO2 is a typical fluorite oxide with desirable lattice oxygen conductivity and can be applied as an active support for the Fe-based oxygen carrier in chemical looping hydrogen generation (CLHG). However, Fe2O3/CeO2 always suffers from low thermal stability and sintering. Doping foreign cations could be a proper way to improve its reactivity and cyclic stability. In this work, Zr4+ and Sm3+, with a smaller ionic radius and lower valence than Ce4+, respectively, were doped into Fe2O3/CeO2 using the co-precipitation method, and the doping effects on the reactivity and redox stability of Fe2O3/CeO2 in CLHG were investigated. Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 provided the best redox reactivity, and the purity of generated hydrogen for all oxygen carriers reached nearly 100% (the detection limit of CO/CO2 was 0.01% in volume). The reactivity followed the sequence Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 > Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.75Zr0.25O2 > Fe2O3/CeO2. However, the concentration of oxygen vacancy was ranked as Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.6Sm0.15Zr0.25O1.925 > Fe2O3/Ce0.75Zr0.25O2 > Fe2O3/CeO2. The Zr doping boosted the reactivity of Fe2O3/CeO2 mainly by enhancing its sintering resistance, whereas the Sm doping achieved it mainly by promoting the oxygen conductivity, whose ability to improve the thermal stability of Fe2O3/CeO2 was rather limited. Both Zr and Sm doping could suppress the outward migration of Fe cations in the particles, resulting in higher sintering resistance. Furthermore, Zr and Sm could dissolve into CeO2 for prepared Fe2O3/Ce0.6Sm0.15Zr0.25O1.925; however, the bleeding out of both dopants was observed with Sm0.5Zr0.5O1.75 formation, which could be detrimental to the redox stability of the oxygen carrier.Keywords:
Reactivity
Chemical-Looping Combustion
Ionic radius
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Chemical-looping combustion (CLC) is considered as one of innovative technologies for CO2 capture. In this paper, an oxygen carrier of (CoO+1.0% PtO2)/CoAl2O4 is synthesized for the mid-temperature chemical-looping combustion of di-methyl ether (DME). Experiments are implemented to investigate the reactivity, the carbon deposition behavior and the redox stability. Experimental results indicate that the looping material shows a good reactivity at reduction temperature of 673 K. Furthermore, we propose a cellular combustor reactor which has a potential of eliminating the gas-solid separation and avoiding the abrasion of solid looping material. Our study has provided the possibility for the integration of the mid-temperature solar thermal energy with the chemical-looping combustion of DME, and offering a new approach of developing the combustor reactor of the chemical-looping combustion.
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Reactivity
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Dimensional and phase changes of four candidate oxygen carrier materials for chemical looping combustion are investigated by dilatometry and high-temperature X-ray diffraction during four redox cycles. NiO/Ni2AlO4 does not exhibit significant dimensional changes during cycling, and it is shown that the support material also contributes to the oxygen carrying capacity. CaMn0.875Ti0.125O3 exhibited good chemical stability and small dimensional changes upon redox cycling. Cu0.95Fe1.05AlO4 showed a one-dimensional expansion of 9% after the experiments, and significant phase changes were seen. The complex set of reactions occurring during redox cycling of ilmenite (FeTiO3) was shown to be accompanied by dimensional changes, giving non-steady dimensional changes during the oxidation and reduction steps.
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This report summarizes results from the project, "Development of Enabling Technologies for Chemical Looping Combustion and Chemical Looping with Oxygen Uncoupling," which evaluated several aspects of dual fluidized bed chemical looping combustion and chemical looping with oxygen uncoupling (CLOU). The objective was to provide tools and enabling technologies to help advance fluidized bed chemical looping technology to pilot, demonstration and commercial scale. One focus area is oxygen carriers, which are key to chemical looping combustion. The copper oxygen carrier-coal ash system was systematically evaluated through a combination of thermodynamic modeling and lab-scale experiments, taking into consideration different oxygen carrier support materials and coal types. A method of mapping "safe" and "risk" zones for different combinations was established, and recommendations for target conditions are provided. In addition, a simple solution for limiting negative influence of some coal ashes, namely adding small amounts of calcium to the system, is proposed. In addition, a novel process for recovering and recycling copper from spent oxygen carriers is proposed. A new approach for design and operation of loop seals in a CLOU system was developed, and involves distinct gas injection points and a short horizontal section to help control the fate of the fluidizing gases. For the air-to-fuel reactor loop seal, the upstream side is fluidized with air, ideally input into the side rather than into the bottom, which prevents uncoupling (reduction) of the oxygen carrier before entering the fuel reactor. Also, alternative for separating oxygen carrier particles and gas in circulating fluidized bed-based chemical looping system is proposed. A new reduced reaction scheme for conversion of coal in copper-based CLOU was developed and implemented into CPFD Software's Barracuda VR package. The resulting model provides higher fidelity than the baseline, especially when it comes to minor reactions that are part of the overall combustion environment. For in-reactor heat extraction, it was determined in this project that the best way to do that in a dual circulating fluidized-bed system is through heat exchange low in the air reactor. The air reactor is the hotter of the two and for a fast-fluidized circulating bed, the lower, more dense section has a higher heat transfer coefficient and offers more consistent particle-wall contact since it doesn't have the splashing behavior the top of the bed does. It was determined that just the wall surface area in the lower quarter to third of the air reactor is sufficient to control temperatures in both reactors. Finally, consideration was given to a new concept for CLC, which involves a staged fuel reactor with a different type of oxygen carrier in each stage.
Chemical-Looping Combustion
Fluidized bed combustion
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A solid oxygen carrier is usually applied in a chemical looping conversion process to transfer oxygen from the gaseous oxygen source to the fuel, which can avoid the direct contact of these two reactants and hence decrease the energy penalty of separation. Among the solid oxygen carriers, iron-based oxygen carrier is an attractive option due to its inherent properties of low cost and environmentally-friendly. Several processes such as chemical looping combustion (CLC), chemical looping gasification (CLG), chemical looping reforming (CLR), and chemical looping hydrogen generation (CLHG) have been proposed and investigated based on the iron-based oxygen carrier. In this review, the relevant researches on the iron-based oxygen carrier are summarized, which include the characteristics of iron oxides, the preparations of the iron-based oxygen carrier based on the iron ores and some other low-cost iron contained materials, and their applications in the continuous operated chemical looping conversion processes. It is expected to provide a better understanding for the development and utilization of iron-based oxygen carrier in the practical chemical looping processes.
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Solid oxygen
Chemical energy
Energy carrier
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