Oxygen Uptake and Release Kinetics of Cu/Cu2O/CuO On Al2O3 and ZrO2-SiO2 As Materials for Chemical Looping Combustion

One key parameter in Chemical Looping Combustion (CLC) is the oxygen transport capacity of the CLC materials between the two process reactors; in practice, it may be limited by a combination of many factors, including the metal content of the inert binder, the oxygen uptake and release kinetics and the thermodynamic equilibrium under the process conditions in the two reactors, the respective residence times, the rate of contaminant deposition and removal (such as coke, sulfur, or ash deposits), and deterioration of the oxygen carrier. Such factors can have a significant impact on the economics and viability of a CLC power plant, because it is expected that many tons of CLC materials would be required in a practical CLC power plant (the required inventory of CLC material can be estimated to be in the range of 0.5 to 6 tons/MW power for typical CLC materials and process operation conditions).

With our ongoing program, we aim for a systematic study of those interconnected aspects, and for obtaining a more quantitative description of CLC from the perspective of gas-surface kinetics. As CLC materials, we have chosen copper oxide supported on two inert binders: 1. ZrO₂-SiO₂ (a high-performance material used in the field of catalysis), and 2. γ-Al₂O₃ for comparison (a more economic, generic binder previously described in literature); those materials are prepared by incipient wetness impregnation followed by a quantification of the prepared formulation by calibrated X-Ray fluorescence. The study is centered on TGA/DTA/DSC experiments, which allow for testing the oxygen uptake and release kinetics as a function of important process parameters; they will be amended with morphological characterizations (BET, Hg-porosimetry, electron microscopy, EDX elemental dispersion).

We will present experiments obtained in the temperature range from 450 to 1000 °C, using CLC materials with Cu loadings ranging from ~ 5 to 50 wt.-%. Our first results indicate that the ZrO₂-SiO₂ material (17 wt.-% Cu) can be cycled many times (more than 30 reduction/oxidation cycles  have been studied) with less than 2 % change of its oxygen uptake and release capacity. The experiments show that the material can be looped quantitatively between the fully oxidized state (CuO), and the fully reduced (metallic) state using methane as fuel at 750 °C; at higher temperatures (starting around 800 °C), the material begins to spontaneously decompose to Cu₂O when the air is removed, likely due to thermodynamic driving forces; thus, this temperature constitutes a significant upper boundary in order to maximize the oxygen transport capacity of the material in CLC. The observed oxygen uptake rates are 2.5x10^-8 mole O₂ sec^-1 (for 1 mg of this CLC material), and varied by less than 4 % over a temperature rate from 550 to 750 °C; this apparent lack of variation of the rate is probably due to extrinsic limitations of the oxygen uptake step – e.g. due to bulk or pore diffusion processes. On the other hand, oxygen release rates under methane exposure displays a strong temperature dependence; the reduction rate varies between 1.8x10^-8 mole O₂ sec^-1 at 750 °C, and 3.1x10^-9 mole O₂ sec^-1 at 550 °C (for 1 mg of the material, respectively). Moreover, reduction rates are lower than the oxygen uptake rates at the same temperatures (26 % slower at 750 °C, 79 % slower at 550 °C). These temperature findings indicate that oxygen diffusion in the lattice is not the limiting factor for the reduction rates. It will be discussed whether the observation of different reduction rates is linked to the kinetics of the fuel activation on the CLC materials. Finally, to begin to understand the impact of a more realistic fuel looping cycle, tests done with simulated gasified coal including steam and ppm-levels of H₂S will be shown.