Influence of fly ash components on the performance of recycled copper-based oxygen carriers for chemical looping combustion

Copper-based, silica-supported oxygen carriers were prepared with low concentrations of metal oxide impurities representing solid-fuel ash components that would co-leach when using nitric acid to recover copper oxide from spent fluidized bed materials in a solid-fuel-fired chemical looping system. Oxygen carriers were prepared to contain approximately 30 wt% CuO on a high surface area silica support with metal oxide impurity-to-CuO ratios of 1:100. Both single- and multi-impurity materials were considered with impurities being introduced as aqueous nitrates during synthesis. Many of the prepared oxygen carriers exhibited lower oxygen transport capacity than expected due to support-CuO interactions. Multi-impurity oxygen carriers displayed the best performance, even better than material containing only CuO, and the presence of alkali salts was determined to be important for enhancing oxygen transport capacity and reaction rates. All oxygen carrier materials were highly reactive, with oxidation and reduction achieving near completion within two minutes at 950 °C. This study concludes that the presence of impurities is not detrimental to oxygen carrier performance and a simple, low-cost oxygen carrier recycle process could be viable. With such a recycle process, it would be possible to reduce the make-up cost associated with the loss of oxygen carriers due to attrition, thus improving economics of chemical looping combustion.