Oxygen Carriers For Chemical Looping Research Articles

Iron-containing mixed-oxide composites as oxygen carriers for Chemical Looping with Oxygen Uncoupling (CLOU)

Chemical Looping with Oxygen Uncoupling (CLOU) offers a potentially effective approach for converting solid carbonaceous fuels with inherent carbon dioxide capture. It utilizes oxygen carriers that allow facile and reversible exchange of their lattice oxygen with external environment under varying oxygen partial pressures. The varying and often tunable thermodynamic properties of mixed oxides of first row transition metals make them potentially viable for CLOU applications. In this study, mixed iron–cobalt and iron–manganese oxides are synthesized and evaluated in terms of their ability to uncouple oxygen. The effects of adding a secondary perovskite phase on the uncoupling properties of these primary transition metal oxides are also investigated. The experimental results indicate that different cation compositions exhibit different oxygen uncoupling properties. The initial decomposition temperature of the oxygen carrier sample is found to generally decrease with increasing amount of Co or Mn. Addition of a secondary perovskite phase is found to significantly affect oxygen donation properties of the primary mixed metal oxides. For instance, CLOU properties of mixed Fe–Co oxides are enhanced by perovskite addition. In contrast, oxygen carrying capacity of mixed Fe–Mn oxides under an isothermal condition is negatively affected by perovskite addition. Redistribution of the transition metal cations between the primary and secondary oxide phases is likely to be responsible for such changes in their redox properties.

Investigation of Manganese–Iron Oxide Materials based on Manganese Ores as Oxygen Carriers for Chemical Looping with Oxygen Uncoupling (CLOU)

AbstractOxygen carrier materials were synthesized using five different types of manganese ores with addition of iron oxide (Fe2O3), and they were investigated for their reactivity and properties in chemical looping with oxygen uncoupling (CLOU). Two different Mn/Fe molar ratios (1:2 and 2:1) were examined. The samples were manufactured by using a lab‐scale extrusion method followed by sintering at 950 °C for 4 h. To measure their reactivity, the materials were exposed to gaseous CH4 and syngas (50:50 CO/H2) as well as solid fuel (petroleum coke), in a batch fluidized‐bed reactor. With CH4, the materials based on ores from Brazil and Gabon with a Mn/Fe molar ratio 2:1 showed the best reactivity. For reduction using syngas, samples based on Eastern European and South African ores provided higher amounts of oxygen equivalent, up to 3–5 % of their mass. To investigate the ability of the samples to release gas‐phase oxygen, solid fuel experiments using 0.1 g of petroleum coke in a bed of 10 g oxygen carrier were performed at 950 °C with Brazilian, Gabon, and Eastern European ores. In these solid fuel tests samples with a Mn/Fe molar ratio of 1:2 showed higher uncoupling rates. Based on X‐ray diffraction analysis, the primary phase detected in all fresh particles was the oxidized form of MnyFe1−yOx, that is, bixbyite [(Mn,Fe)2O3]. All of the particles showed sufficiently high crushing strength (>2 N) and a reasonable attrition resistance (attrition index <6 wt % h−1). This suggests that the materials could be suitable for practical applications. It is concluded that the oxygen carriers made by manganese ores have the potential to be used as oxygen carriers for chemical‐looping combustion and chemical looping with oxygen uncoupling.

Structure–property relationship of co-precipitated Cu-rich, Al2O3- or MgAl2O4-stabilized oxygen carriers for chemical looping with oxygen uncoupling (CLOU)

Chemical looping with oxygen uncoupling (CLOU) is a novel combustion technology that utilizes the decomposition reaction of an oxygen storage material to combust solid (hydro-)carbonaceous fuels. CuO is an ideal candidate for CLOU owing to its high oxygen uncoupling capacity (∼10wt.%) and its ability to release oxygen at temperatures relevant for chemical looping combustion (CLC), i.e. 850–1000°C. Here, we investigate the feasibility of Cu-rich, Al2O3 or MgAl2O4-stabilized oxygen carriers containing 70–90wt.% CuO for the CLOU process. Al2O3- and MgAl2O4-stabilized oxygen carriers were synthesized using a co-precipitation technique and calcined either at 800°C or 1000°C. The CLOU performance of the oxygen carriers was evaluated at 950°C in a fluidized bed reactor using a N2 atmosphere for the decomposition reaction and a mixture of 10.5vol.% O2 and 89.5vol.% N2 for the re-oxidation reaction. It was found that the calcination temperature and the surface area had no significant influence on the oxygen carrying capacity of the synthesized oxygen carriers. The oxygen uncoupling capacity of Al2O3-stabilized oxygen carriers was substantially below the theoretically expected values due to the formation of the mixed oxides CuAl2O4 and CuAlO2. Based on thermo-gravimetric measurements, a reaction pathway for the oxygen release and re-oxidation reactions of Al2O3-stabilized CuO was proposed. On the other hand, no interaction between MgAl2O4 and the active phase, i.e. CuO was observed. As a consequence, MgAl2O4-stabilized oxygen carriers possessed a high oxygen uncoupling capacity very close to the theoretical values. Owing to its stable and high oxygen release capacity, MgAl2O4-supported oxygen carriers containing 80wt.% CuO were identified as a promising material for CLOU.

Characterization and Evaluation of Prepared Fe2O3/Al2O3 Oxygen Carriers for Chemical Looping Process

Alumina supported Fe2O3 was prepared to evaluate its feasibility as an oxygen carrier for chemical looping using a thermogravimetric analyzer (TGA) and fixed bed reactor. Fe2O3/Al2O3 particles containing 60 wt% Fe2O3 and sintered at 1300°C demonstrated reasonable oxygen conversion. The reduction mechanism of Fe2O3/Al2O3 oxygen carriers was proposed based on the thermogravimetric analysis and XRD characterization. FeAl2O4 is formed and can serve as a supporting material as well as oxygen carrier in practical operations by using a moving bed reactor. The prepared Fe2O3/Al2O3 oxygen carriers demonstrated high CO2 yields in relation to syngas and methane combustion in the fixed bed reactor operated at temperatures higher than 500 and 850°C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe2O3/Al2O3 oxygen carriers for experiments conducted at temperatures above 800°C. The crush strength and TGA of Fe2O3/Al2O3 were examined, and the results showed that they had an appropriate crush strength and reasonable reactivity as oxygen carriers for use in a chemical looping process.

Feasibility Study of Fe-ti based Oxygen Carriers for Chemical Looping Combustion

The reactivity and high fuel conversion efficiency of metal oxide as oxygen carrier are two of the most important functions that can be research in order to make chemical looping combustion (CLC) a reality. In this study, oxygen carriers of Fe2O3, supported by titanium oxides were prepared to evaluate the feasibility as oxygen carriers for chemical looping by thermogravimetric analyzer (TGA), as well as gas conversion in a fixed bed reactor (FBR). Supported Fe2O3 oxygen carriers and sintered in air at 1100̊C is a suitable preparation parameter to demonstrate reasonable oxygen conversion. The reduction mechanism of supported Fe2O3 oxygen carriers were proposed in accordance with TGA and x-ray diffraction analysis (XRD) characterizations. Fe2TiO5 and inherent TiO2 as supporting material would be formed and carried on FBR in practical operation. The preparation of FT-03 (80% Fe2O3 and 20% TiO2) oxygen carrier demonstrated high CO2 yields regarding to syngas (10% H2 and 10% CO in N2) and methane (10% CH4 in N2) combustion in FBR, operated at temperatures higher than 500 and 900̊C, respectively. Moreover, great mechanical strength and reasonable reactivity of supported Fe2O3 oxygen carriers were also examined by texture analyzer with well-suited for used in CLC.

Inside Cover: Review of Oxygen Carriers for Chemical Looping with Oxygen Uncoupling (CLOU): Thermodynamics, Material Development, and Synthesis (Energy Technol. 11/2013)

Inside Cover: Review of Oxygen Carriers for Chemical Looping with Oxygen Uncoupling (CLOU): Thermodynamics, Material Development, and Synthesis (Energy Technol. 11/2013)

Review of Oxygen Carriers for Chemical Looping with Oxygen Uncoupling (CLOU): Thermodynamics, Material Development, and Synthesis

AbstractGlobal warming and climate change are most likely linked to the increasing concentration of the greenhouse gas carbon dioxide (CO2) in the atmosphere. Additionally, the consumption of fossil fuels is predicted further to increase in the coming decades, particularly due to the rapid development of populous countries such as Brazil, India, and China. Therefore, it is imperative to develop and implement processes that avoid the emission of anthropogenic CO2. One possible midterm solution is carbon‐dioxide capture and storage (CCS). In this context, the so‐called chemical‐looping combustion (CLC) process, that is, an emerging 3rd‐generation CCS technology, is particularly attractive due to its very low predicted CO2‐capture costs compared to the currently available technology (i.e., amine scrubbing). In CLC, lattice oxygen from a solid‐state oxygen carrier is used to combust a hydrocarbon fuel, which yields, after the condensation of steam, a pure stream of CO2 suitable for sequestration. To allow the application of CLC to solid fuels, chemical looping with oxygen uncoupling (CLOU) has been proposed. Here molecular oxygen is provided by using the decomposition reaction of the oxygen carrier, thus, effectively the solid fuel is combusted in an oxyfuel mode. Importantly, a cornerstone of the CLOU process is the development of suitable oxygen carriers. In the first part of the review we discuss the thermodynamic properties of various CLOU materials. Subsequently, recent advances in the development of novel oxygen carriers are summarized. In particular, we focus on the physical and chemical properties of the new materials and the synthesis strategies employed. The review is concluded with an outlook on the CLOU process.

Combined Cu/Mn Oxides as an Oxygen Carrier in Chemical Looping with Oxygen Uncoupling (CLOU)

This study investigates the O2 uncoupling properties of five different oxygen carrier particles, consisting of combined oxides of CuO and Mn3O4. The oxygen carriers were produced by freeze granulation followed by calcination at 950 °C for 6 h. Particles with 5, 10, 20, 31, and 61 wt % CuO were examined in both an inert (pure N2) atmosphere and in the presence of solid fuel (wood char) at 750 °C. At this relatively low temperature during fluidization with N2, the samples were capable of releasing gas-phase O2 in concentrations up to 1%. During reduction with wood char in 15 g of oxygen carriers, some materials could release gaseous O2 equal to 1.4% of their total mass. When the crushing strength and attrition index were measured with a customized jet cup, the mechanical stability of these samples was compared. These measurements showed that, in general, samples with a higher CuO content were more mechanically stable. On the basis of XRD analysis of the oxygen carriers, the major phase transitions were Mn2O3 ↔ Mn3O4 and combined spinel (Cu,Mn)3O4 ↔ CuMnO2. These transitions both provide a considerable amount of O2. It is concluded that the Cu–Mn–O system has considerable potential to be used as a oxygen carrier in chemical-looping applications at lower temperatures, perhaps interesting for biofuel combustion.

Using the Sol–Gel-Derived CuO/CuAl2O4 Oxygen Carrier in Chemical Looping with Oxygen Uncoupling for Three Typical Coals

Chemical looping with oxygen uncoupling (CLOU) without the coal gasification that limits the efficiency in ordinary chemical looping combustion (CLC) is a very promising approach for CO2 capture in the combustion of solid fuels. In the present work, the sol–gel-derived CuO/CuAl2O4 oxygen carrier was evaluated in terms of its ability to release gaseous oxygen in an oxygen-deficient atmosphere and its reactivity with coal in a laboratory-scaled fluidized-bed reactor at 850, 885, 900, 925, and 950 °C. Three typical Chinese coals of different coal ranks, GP coal (anthracite), FG coal (bituminous), and SL coal (lignite), were used as fuels in the CLOU experiments. The effects of the fuel-reactor temperature and coal rank on the carbon conversion rate, combustion efficiency, and CO2 yield were investigated. Nearly complete conversion of coals was attained. The conversion from carbon to CO2 can be enhanced if the temperature increases. Meanwhile, the combustion efficiency would be reduced because of the loss of ...

Examination of Perovskite Structure CaMnO3-δwith MgO Addition as Oxygen Carrier for Chemical Looping with Oxygen Uncoupling Using Methane and Syngas

Perovskite structure oxygen carriers with the general formula CaMnxMg1-xO3-δwere spray-dried and examined in a batch fluidized bed reactor. The CLOU behavior, reactivity towards methane, and syngas were investigated at temperature 900°C to 1050°C. All particles showed CLOU behavior at these temperatures. For experiments with methane, a bed mass corresponding to 57 kg/MW was used in the reactor, and the average CH4to CO2conversion was above 97% for most materials. Full syngas conversion was achieved for all materials utilizing a bed mass corresponding to 178 kg/MW. SEM/EDX and XRD confirmed the presence of MgO in the fresh and used samples, indicating that the Mg cation is not incorporated into the perovskite structure and the active compound is likely pure CaMnO3-δ. The very high reactivity with fuel gases, comparable to that of baseline oxygen carriers of NiO, makes these perovskite particles highly interesting for commercial CLC application. Contrary to NiO, oxygen carriers based on CaMnO3-δhave no thermodynamic limitations for methane oxidation to CO2and H2O, not to mention that the materials are environmentally friendly and can utilize much cheaper raw materials for production. The physical properties, crystalline phases, and morphology information were also determined in this work.

Investigation of Different Mn–Fe Oxides as Oxygen Carrier for Chemical-Looping with Oxygen Uncoupling (CLOU)

The appropriate oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) should be thermodynamically capable of being oxidized in the air reactor and also release gaseous O2 in the fuel reactor at appropriate temperatures and oxygen partial pressures. It should also be mechanically durable, cheap, and environmentally friendly. Iron–manganese oxides appear to be especially promising due to favorable thermodynamics. In this work, combined metal oxides of iron and manganese were investigated for the CLOU process. Particles with different ratios of Mn/Fe were produced using spray drying. The particles were calcined at 950 and 1100 °C for 4 h and then tested with respect to parameters important for CLOU. The crushing strength for these materials was between 0.1 to 1.7 N, depending on their composition and sintering temperature. The ability of the iron–manganese oxide particles to release oxygen in the gas phase was examined by decomposition of the material in a stream of N2. Moreover, the reaction with both methane and synthesis gas (50/50% CO/H2) was examined in a batch fluidized bed reactor. Here, the particles were alternately oxidized with 5% O2 and reduced in N2 or with fuel at 850 °C, 900 and 950 °C. From the results, it can be concluded that during the nitrogen period, the oxygen carriers with Mn3O4 content in the range from 20 wt % to 40 wt % release oxygen at 900 °C, whereas the materials with higher manganese content show no oxygen release. This is because they could not be oxidized to bixbyite. By decreasing the temperature from 900 to 850 °C, it was possible to oxidize oxygen carriers with manganese oxide content of 50 wt % and higher, and consequently, oxygen release during the nitrogen period was seen for these materials. This is in agreement with the phase diagram for this system. The reaction rate with methane follows the oxygen release trend very well. At the higher reaction temperature, 950 °C, oxygen carriers with manganese content in the range from 25% to 33% show the best gas conversion of methane. At 850 °C, on the other hand, high methane conversion is seen for particles with high manganese content. In fact, several particles had almost full conversion of methane to CO2 and H2O at 850 °C using a bed mass in the batch reactor corresponding to 70 kg oxygen carrier/MW.