Evaluation of the Reactivity of Hematite Oxygen Carriers Modified Using Alkaline (Earth) Metals and Transition Metals for the Chemical Looping Conversion of Lignite

Abstract

Chemical looping (CL) technology is a novel technology for the clean and efficient use of energy. Oxygen carriers (OCs) are the cornerstone of CL technology. The development of low–cost, high–performance OCs is crucial for the application of CL conversion. Hematite, one of the natural Fe–based OCs, has several advantages (e.g., low cost and environmental friendliness), but its low reactivity limits its application in CL. The performance of hematite can be effectively improved by modifying some of its active components. This study explored the improvement of hematite reactivity by adding alkaline (earth) metals (K, Na, and Ca) and transition metals (Ni, Cu, and Mn). The crystal phases of the OCs were characterized using X-ray diffraction (XRD), and the results revealed that the addition of metals significantly changed the phase of the original hematite. The active solid solution of K–Fe–O and Na–Fe–O species exhibited strong catalytic activity to facilitate lignite char conversion. The addition of CaO promoted the devolatilization of lignite, while the formation of a solid CaFe2O4 solution with low reactivity inhibited the lattice oxygen release. The presence of CuO/CuFe2O4 in the Cu–modified sample could release a small amount of free O2 to promote volatile conversion. The high activity phases of NiO and NiFe2O4 in the Ni–modified OCs could improve the reaction activity of hematite. However, the MnFeO3 phase with low reaction activity was generated in the Mn–modified OC, decreasing the reaction rate of the Mn–modified OC with lignite char.