Redox Characteristics of Fe-Ni/SiO2 Bimetallic Oxygen Carriers in CO under Conditions Pertinent to Chemical Looping Combustion

This study is concerned with the reduction/oxidation (redox) behavior of bimetallic oxygen carries of Ni and Fe under conditions pertinent to chemical looping combustion (CLC). Bimetallic oxygen carriers of Ni- and Fe-based metal oxides with different mixture ratios were developed through the incipient wet impregnation (IWP) method by employing SiO2 as the support. The redox properties of metal oxide mixtures were determined in a magnetic balanced thermogravimetric analyzer (TGA) over a range of temperatures between 500 and 950 degrees C at atmospheric pressure under alternating reduction and oxidation half cycles. The reduction half cycle was carried out in the presence of CO, CO2, and N-2 mixtures with different volume fractions of CO (typically 5, 10, and 70%), while the oxidation half cycle was conducted in normal air. Oxygen carriers exhibited temperature-dependent reactivity during both reduction and oxidation half cycles. The reactivity of monospecies Ni-based oxides on the silica support under the reducing environment of CO was found to be lower than that of Fe-based oxides. Furthermore, with the exception of 950 degrees C, reaction rates and the extent of reactions at high temperatures were generally found to be higher than those of low temperatures. The kinetic properties of all monospecies and bimetallic oxygen carriers were determined using the shrinking core model, from which the mixture kinetics of nickel and iron oxide on the SiO2 support were further correlated to the mixture ratio through a detailed theoretical analysis.