No abstract

No abstract

No abstract

No abstract

No abstract

A screening of Fe-and Mn-based ores and industrial products was made in order to identify suitable lowcost materials that could be used as oxygen carriers in chemical-looping combustion (CLC). A laboratory fluidized bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used. Fifteen grams of each material with a particle size of 125-180 µm was exposed to a flow of 450 mL n /min of either methane or syngas (50% CO, 50% H 2 ) during reduction. During the oxidizing phase to a flow of 1000 mL n /min, 5% O 2 in nitrogen was used. All materials had a high reactivity with syngas. Some materials such as the Mn-based Colormax and the Fe-based Glo ¨dskal had also a high reactivity with methane making them possible candidates for CLC with gaseous fuel. Some of the materials, especially the Mn-based ones, showed poor mechanical stability and poor fluidizing properties. Roughly half of the Fe-based materials, but only one of the Mn-based materials, had properties that could make them suitable as oxygen carriers in a CLC system for solid fuels.

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are attractive solutions for efficient combustion with direct separation of carbon dioxide. In this work, the feasibility of CuO supported on Al 2 O 3 and MgAl 2 O 4 for CLC and CLOU processes are investigated. The oxygen carriers were produced by freeze-granulation and calcined at 950 and 1050°C. The chemical-looping characteristics were evaluated in a laboratoryscale fluidized bed at 900 and 925°C under alternating reducing and oxidizing conditions. Tendencies towards agglomeration, defluidization and loss of active phase were analyzed by changing the experimental process variables, such as reaction time, temperature and reducing and inert environments. Complete conversion of methane was obtained for all oxygen carriers investigated in this work. Three out of four oxygen carriers also featured the rapid release of oxygen in an inert environment (CLOU). In case of Al 2 O 3 as support, a CLC and a CLOU oxygen carrier were obtained depending on the calcination temperature. In addition, analyses of the CuO-Al 2 O 3 phase equilibria system under oxidizing and reducing conditions has been carried out. At the investigated temperatures, it is inferred for the case of Al 2 O 3 as support that part of the active phase (either CuO or CuAl 2 O 4) is bound as CuAlO 2 during incomplete reduction with slow kinetics for re-oxidation. However, when complete reduction is attained, the original active phase composition is rejuvenated upon oxidation. As a result, the use of CuAl 2 O 4 is suggested for CLC processes from the point of agglomeration and attrition-free functioning of the oxygen carrier. In case of MgAl 2 O 4 as support, the oxygen carrier exhibited a stable oxygen releasing behavior due to the existence of relatively intact CuO. Together with the absence of agglomeration and major morphological changes, the use of MgAl 2 O 4-supported CuO is suggested as a suitable oxygen carrier for CLOU processes.

-Oxygen-carrier materials for chemical-looping with oxygen uncoupling (CLOU) must be capable to take up and release gas-phase O 2 at conditions relevant for generation of heat and power. In principle, the capability of a certain material to do so is determined by its thermodynamic properties. This paper provides an overview of the possibility to design feasible oxygen carrier materials from combined oxides, i.e. oxides with crystal structures that include several different cations. Relevant literature is reviewed and the thermodynamic properties and key characteristics of a few selected combined oxide systems are calculated and compared to experimental data. The general challenges and opportunities of the combined oxide concept are discussed. The focus is on materials with manganese as one of its components and the following families of compounds and solid solutions have been considered: (Mn