Reduction Rate Enhancements for Coal Direct Chemical Looping Combustion with an Iron Oxide Oxygen Carrier

Chemical looping combustion (CLC) has been suggested as an energetically efficient approach for coal combustion with CO2 sequestration. An iron oxide oxygen carrier is an attractive option because of its low cost and environmental compatibility. However, the low reactivity between iron oxide and coal is a challenge for its application. In this paper, the effects of the C/Fe2O3 molar ratio and alkali carbonate addition on the reduction rate of coal char with an Fe2O3 oxygen carrier and the feasibility of coal char direct CLC with an alkali-carbonate-impregnated Fe2O3 oxygen carrier were investigated using thermogravimetric analysis coupled with a mass spectrometer (TGA MS), an X-ray diffractometer (XRD), scanning electron microscopy (SEM), and carbon content tests. Results indicate that, on the premise of full consumption of C by an Fe2O3 oxygen carrier, higher C/Fe2O3 molar ratios are not only beneficial to the oxygen transport capacity but also to reduction kinetics. The kinetics enhancement by higher C/Fe2O3 molar ratios could be attributed to more iron oxide/carbon contacts, which improves the char gasification rate and, in turn, enhances the reduction rate. The reduction rate also increases with the increase of alkali carbonate addition, which could be ascribed to the synergistic effects of alkali carbonates and iron on char gasification. The catalytic activities of K2CO3, Na2CO3, and Li2CO3 decrease in the order of K2CO3 > Na2CO3 > Li2CO3. Overall, a high reduction rate of coal char with an Fe2O3 oxygen carrier can be achieved with the appropriate C/Fe2O3 molar ratio and alkali carbonate addition. Catalytic CLC by alkali carbonates appears to be an effective way to combust coal directly with an Fe2O3 oxygen carrier.