Reduction Properties of Physically Mixed Metallic Oxide Oxygen Carriers in Chemical Looping Combustion

A combined experimental and theoretical study was carried out to examine the reactivity of physically mixed metallic oxide oxygen carriers under conditions pertinent to chemical looping combustion. This was motivated by the notion that mixed metal oxides can resolve many of the shortcomings associated with the conventional oxygen carriers. Experiments were conducted on binary mixtures of Cu, Fe, and Ni oxides under reducing environments of H-2, CO, and CH4. Experiments were performed in a TGA (thermogravimetric analyzer) at atmospheric pressure over a range of temperatures between 500 and 950 degrees C. The concentration of the reducing gas (volume fraction of H-2, CO, and CH4) and the weight percentage of metal oxides were systematically varied between 20 and 70% volume fraction (VF) and 0-100% wt, respectively. For each mixture, the activation energy, reaction order, and pre-exponential factor were extracted from experimental measurements of the weight loss using the shrinking core model. Experimental observations suggested that for any given mixture, the conversion time (i.e., time corresponding to full conversion) was the geometric mean of the conversion times of the parent materials. On the basis of this observation, an in-depth theoretical analysis of the kinetic properties of mixed carriers was carried out which, in turn, led to the development of a simple methodology for determining the kinetic properties of mixed metal oxides from those of its parent materials. The methodology significantly reduces the need for extensive reactivity measurements when dealing with metal oxide mixtures. The validity of the methodology was verified through a series of comparisons with experimental data.