Prospects of AI_2O_3 and MgAI_2O_4-Supported CuO Oxygen Carriers in Chemical-Looping Combustion (CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU)

摘要

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_2O_3 and MgAl_2O_4 for CLC and CLOU processes is investigated. The oxygen carriers were produced by freeze-granulation and calcined at 950 and 1050 ℃. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized bed at 900 and 925 ℃ under alternating reducing and oxidizing conditions. Tendencies toward 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 the case of AI_2O_3 as support, a CLC and a CLOU oxygen carrier were obtained depending on the calcination temperature. In addition, analyses of the CuO-AI_2O_3 phase equilibria system under oxidizing and reducing conditions have been carried out. At the investigated temperatures, it is inferred for the case of AI_2O_3 as support that part of the active phase (either CuO or CUAl_2O_4) is bound as CUAlO_2 during incomplete reduction with slow kinetics for reoxidation. However, when complete reduction is attained, the original active phase composition is rejuvenated upon oxidation. As a result, the use of CuAl_2O_4 is suggested for CLC processes from the point of agglomeration and attrition-free functioning of the oxygen carrier. In the case of MgAl_2O_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_2O_4-supported CuO is suggested as a suitable oxygen carrier for CLOU processes.