Chlorine-resistant Ca–Cu and Ba–Cu oxygen carriers in chemical looping combustion of coal and biomass

In chemical looping combustion (CLC) of chlorine-rich coal or biomass, oxygen carriers (OCs) and heat exchange equipment can be severely corroded by chlorine-containing species. Thus, calcium (Ca) and barium (Ba)-based additives were incorporated into copper (Cu)-based OCs via a sol–gel method to construct composite OCs. For the first time, these OCs were applied to achieve high-efficiency chlorine fixation and resistance during solid-fuel CLC. The effects of Ca/Ba-doping on gaseous chlorine fixation and combustion performance were systematically investigated. The experimental results demonstrated that alkaline earth metals in Ca–Cu and Ba–Cu OCs preferentially reacted with HCl, forming stable chlorides, increasing lattice oxygen activity, and promoting gas–solid reactions. Additionally, Ca/Ba doping moderately improved coal and biomass char gasification. Notably, the Ca–Cu OC exhibited superior stability, whereas the combustion efficiency of the Ba–Cu OC was highly temperature-dependent. Increasing the oxygen-to-fuel ratio did not alter the proportion of chlorine converted to HCl. Further analysis revealed that Ca/Ba doping increased the OC specific surface area and pore volume, increasing the contact area with fuels and stabilizing CLC performance. These findings provide insights into the chlorine-capturing mechanisms of alkaline earth metals, advancing the development of chlorine-resistant OCs for efficient CLC applications.