Enhanced performance of red mud-based oxygen carriers by CuO for chemical looping combustion of methane

The development of inexpensive oxygen carriers is of great significance for the large-scale application of chemical looping combustion technology. Red mud, a solid waste of alumina industry, is a very promising candidate as oxygen carrier for this technology due to its suitable content of Fe2O3 and low cost. In this study, the red mud oxygen carrier is modified by CuO, which is used as a cost-effective oxygen carrier for chemical looping combustion of methane. The results show that CuO as an additive strongly improve the activity and redox stability of the red mud oxygen carriers. The sample with 20 wt% content of CuO represents the highest CH4 conversion (80%), CO2 selectivity (100%) and oxidation efficiency (2.9 mmol‧g−1‧min−1) in the multiple redox testing, which are only 22%, 81% and 1.1 mmol‧g−1‧min−1 for the raw red mud. Comprehensive characterizations indicate that two kinds of Cu species (free CuO and CuFe2O4) are detected in the CuO-modified red mud after calcination at 900 ℃. Coper oxides in the both oxides can be firstly reduced to metallic Cu during the reaction with methane. The reduce Cu species may acts as active sites for methane activation and oxygen releasing, which would improve the reaction between iron oxides in red mud and methane. The element mapping indicates that the Cu species are well dispersed on the oxygen carrier after reduction by methane, which is beneficial for enhancing the catalytic function of Cu species. After the long-term redox experiment, the particle size of Cu oxides increases, while the interlacing distribution between Cu and Fe oxides are promoted. This may improve the interaction between Cu and Fe oxides and help the CuO-modified red mud to maintain relatively high stability in the successive chemical looping combustion process.