The concept of chemical looping reactions has been widely applied in chemical industries, for example, the production of hydrogen peroxide (H2O2) from hydrogen and oxygen using 9,10-anthraquinone as the looping intermediate. Fundamental research on chemical looping reactions has also been applied to energy systems, for example, the splitting of water (H2O) to produce oxygen and hydrogen using ZnO as the looping intermediate. Fossil fuel chemical looping applications had been used commercially with the steam-iron process for coal from the 1900s to the 1940s and had been demonstrated at a pilot scale with the carbon dioxide acceptor process in the 1960s and 1970s. There are presently no chemical looping processes using fossil fuels in commercial operation. A key factor that hampered the continued use of these earlier processes for fossil energy operation was the inadequacy of the reactivity and recyclability of the looping particles. This factor led to higher costs for product generation using the chemical looping processes, compared to the other processes that use particularly petroleum or natural gas as feedstock. With CO2 emission control now being considered as a requirement, interest in chemical looping technology has resurfaced. In particular, chemical looping processes are appealing because of their unique ability to generate a sequestration-ready CO, stream while yielding high energy conversion efficiency. Renewed fundamental and applied research since the early 1980s has emphasized on improvements over earlier shortcomings. New techniques have been developed for direct possessing of coal or other solid carbonaceous feedstock in chemical looping reactors. Significant progress demonstrated by the operation of several small pilot scale units worldwide indicates that the chemical looping technology may become commercially viable in the future for processing carbonaceous fuels. This perspective article describes the fundamental and applied aspects of modern chemical looping technology that utilizes fossil fuel as feedstock. It discusses chemical looping reaction thermodynamics, looping particle selection, reactor design, and process configurations. It highlights both the chemical looping combustion and the chemical looping gasification processes that are at various stages of the development. Opportunities and challenges for chemical looping process commercialization are also illustrated.
