(91d) Chemical Looping Combustion of Coal Derived Synthesis Gas Over Copper-Based Oxygen Carrier

Chemical looping combustion (CLC) is an emerging technology for clean energy production from fossil and renewable fuels. In CLC, an oxygen carrier (typically a metal) is first oxidized with air. The hot metal oxide is then reduced in contact with a fuel in a second reactor, thus combusting the fuel. Finally, the reduced metal is transferred back to the oxidizer, closing the materials ?loop?. CLC thus allows for flame-less, NOx-free combustion without requiring expensive air separation. Most importantly, CLC produces sequestration-ready CO2-streams without significant energy penalty. However, CLC is currently suffering from insufficient stability of the oxygen carrier (particularly metal sintering), and slow metal re-oxidation kinetics.

We present results from a study in which CuO/bentonite and nanostructured Cu-Barium-Hexaaluminate (Cu-BHA) oxygen carriers were studied in chemical looping combustion for the gasification systems utilizing simulated synthesis gas. Global reaction rates of reduction and oxidation, as the function of reaction conversion, were calculated from 10-cycle oxidation/reduction tests utilizing thermogravimetric analysis at atmospheric pressure between 700 °C to 900 °C. It was found that the reduction reactions are always faster than oxidation reaction; reaction temperature and particle size do not significantly affect the reaction performance of CuO/bentonite. Results of the tapered element oscillating microbalance showed negative effect of pressure on the global rates of reduction-oxidation reactions at higher fractional conversions. X-ray photoelectron spectroscopy analysis of fresh and used CuO/bentonite revealed the presence of multiple oxidation states in the surface of oxidized sample and metallic Cu in the surface of reduced sample. X-ray diffraction patterns confirmed the presence of CuO in the bulk phase of oxidized sample. Scanning electron microscopy analysis showed significant morphology changes of reacted CuO/bentonite samples after the 10 oxidation-reduction cycles above 700oC in an atmospheric thermogravimetric analyzer.

The performance of this bentonite-based carrier was compared with that of a nanostructured Cu-BHA carrier. The Cu-BHA carrier was synthesized following a microemulsion-templated synthesis path, and characterized by BET, XRD, and TEM. It was then subjected to the same 10-cycle redox treatment as the bentonite carriers. We find that the nanosized Cu particles in the Cu-BHA are highly stable at the high-temperature conditions of the experiment due to a ?caging' effect by the mesoporous BHA matrix. A detailed analysis of the oxidation and reduction kinetics of the two carriers, based on a shrinking core model, showed that nanostructuring of the carrier does not lead to significant acceleration of the redox kinetics. This is in stark contrast to our earlier findings with Ni-based carriers, where nanostructuring was found to have a strong accelerating effect on the kinetics. The difference can be explained based on the (solid state) diffusion rates for the two oxides.

Overall, we find that both CuO/bentonite and Cu-BHA exhibit excellent reaction performance and thermal stability for CLC process at 700-900 oC.