Use of Coal as Fuel for Chemical-Looping Combustion with Ni-Based Oxygen Carrier

Chemical-looping combustion is an indirect combustion technology with inherent separation of the greenhouse gas CO2. The feasibility of using NiO as an oxygen carrier during chemical-looping combustion of coal has been investigated experimentally at 800-960 degrees C in the present work. The experiments were carried out in a fluidized bed, where the steam acted as the gasification-fluidization medium. Coal gasification and the reaction of oxygen carrier with the water gas take place simultaneously in the reactor. The oxygen carrier particles exhibit high reactivity above 900 degrees C, and the dry basis concentration of CO2 in the exit gas of the reactor is nearly 95%. The flue gas composition as a function of the reactor temperature and cyclic reduction number is discussed. At 800-960 degrees C, the dry basis concentration of CO2 in the flue gas presents a monotonously increasing trend, whereas the dry basis concentration of CO, H-2, and CH4 decreases monotonously. The concentrations of CO2, CO, H-2, and CH4 in the flue gas as a function of cyclic reduction number present a para-curve characteristic at 900 degrees C. With the increase of cyclic reduction number, the dry basis concentration of CO2 decreases remarkably, while the dry basis concentrations of CO, H-2, and CH4 increase rapidly. Moreover, the peak value of H-2 concentration is less than that of CO. The performance of the NiO-based oxygen carriers was also evaluated using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues of oxygen carrier. The results indicate that NiO is one of the suitable oxygen carriers for chemical-looping combustion of coal.