Development and performance of Cu-based oxygen carriers for chemical-looping combustion

Chemical-looping combustion (CLC) has the inherent property of separating the product CO2 from flue gases. Instead of air, it uses an oxygen carrier, usually in the form of a metal oxide, to provide oxygen for combustion. This paper focuses on the development and performance of a suitable Cu-based oxygen carrier for burning solid fuels using CLC. Carriers were made from CuO and Al2O3 (as a support) in three different ways: mechanical mixing, wet impregnation, and co-precipitation. The reactivity of these solids was assessed by measuring their ability to oxidize CO, when in a hot bed of sand fluidized by a mixture of CO and N2. After that, the Cu in the carrier was oxidized back to CuO by fluidizing the hot bed with air. These oxygen carriers were tested over many such cycles of reduction and oxidation. This work confirms that supporting CuO on Al2O3 enhances the ability of the resulting particles to withstand mechanical and thermal stresses in a fluidized bed. Also, only co-precipitation produces particles that have a high loading of copper and do not agglomerate at 800–900 °C. The performance of co-precipitated particles of CuO/Al2O3 at oxidizing CO to CO2 was significantly affected by the pH of the solution in which precipitation occurred: a high pH (9.7) gave particles that reacted completely and rapidly. After 18 cycles, such a co-precipitated carrier with 82.5 wt% CuO yielded all its oxygen when oxidizing CO. X-ray analysis showed that when heated, CuO reacted with Al2O3 to form CuAl2O4, which was fully reducible, so CuO experienced no loss in extent of reaction after forming this mixed oxide. An increase in operating temperature from 800 to 900 °C led to the CuO providing slightly less oxygen; this was because a little of the CuO decomposed to Cu2O between its reduction and oxidation, when the bed was fluidized by pure N2.