(178a) Reduction Kinetics of Coo-Nio/Al2o3 Oxygen Carrier for Chemical-Looping Combustion

In recent years, chemical-looping combustion (CLC) has been received growing interest as an energetically efficient combustion technology for the capture of CO2 and substantial reduction of NOx emissions. This is a two-stage process, consisting of two interconnected fluidized bed reactors, called fuel and air reactor. A solid oxygen carrier is circulated between the reactors, which acts as source of oxygen for combustion. In the fuel reactor, fuel burns with the framework oxygen of the solid carrier to produce CO2, H2O and the reduce state(s) of the oxygen carrier. The reduced oxygen carrier is re-oxidized in the air reactor with air and recycled back to the fuel reactor. Therefore, the flue gas stream never diluted with nitrogen and CO2 can easily be separated without further energy penalty. The possibility of NOx formation in this process is also negligible because the fuel burns in absence of air without flame. This communication, a part of the development of bimetallic oxygen carrier for CLC power generation process, reports the reduction mechanism and kinetics of Co-Ni/Al2O3 oxygen carrier methane as the reduction (fuel) gas. The kinetics experiments were performed in a novel fluidized bed CREC riser simulator under turbulent fluidized bed reaction conditions. The shrinking-core model was used for the kinetics determination assuming spherical grains of the oxygen carrier particles. The porosity analysis showed that the pore size of the carrier particle was slightly increased after reduction, which was also an indication of a possible shrinking core model since the molar volume of Ni (6.6 cm3/mol) is lower than that of NiO (7.54 cm3/mol). The rate-controlling steps of the gas-solid reactions in CLC process was established by observing the reaction rates at different set of experimental conditions as well as applying the theoretical calculations i.e. Sherwood number, Wiez-Prater criterion for external and internal mass transfer limitations respectively. Under studied reaction conditions the chemical reaction between methane molecule and the solid oxides has been shown to be controlling the reduction rate. The temperature variation experiments show that the reduction reaction is a strong function temperature, which further confirms the reaction controlling rate formulations. The reaction order was depended on both methane and oxygen carrier with values close to one and 0.4 for methane and oxygen carrier respectively. No effects of the gas products (H2O and CO2) on the reaction rate were detected under the studied reaction conditions. The activation energies calculated from the shrinking-core model was 29.1 ± 0.2 kJ/mol, which is consistent with the values, have been reported in literature.