Chemical Looping Reforming of Coal Tar Vapor on the Surface of CaO-Modified Fe-Based Oxygen Carrier

In this work, Fe6Al4Ca1 composite oxygen carriers (OCs) were prepared using the impregnation method with Fe2O3 as the active component, Al2O3 as the inert support, and CaO as the modifier. The OC was used to reform and remove the gasifying coal tar in simulated high-temperature syngas. The effects of the molar ratio of oxygen carrier to tar (O/C), the reaction temperature, and the molar ratio of the added water vapors to carbon (S/C) on the chemical looping reforming (CLR) of coal tar were investigated in a series of experiments conducted in a fluidized bed reactor. The results showed that the conversion rate of CLR of coal tar vapor with Fe6Al4Ca1 as bed material was significantly higher than that with sand as bed material. The CLR optimum conditions of coal tar vapor were found to be as follows: reaction temperature of 900 °C, O/C molar ratio of 3, S/C molar ratio of 0.5, with a tar conversion rate of higher than 90%. Under these operating conditions, after 20 cycles, the Fe6Al4Ca1 composite OC showed better cyclic performance, whereas the average concentration of each syngas component was stable, among which the H2 concentration remained at about 50%. Furthermore, X-ray diffractometry, scanning electron microscopy, and specific surface area were used to characterize the reduced and oxidized OCs in multicycle. The results showed that comelting occurred between Fe2O3 and CaO, whereas some of the Ca entered the OCs in the form of calcium ferrite. Additionally, the decrease of OC activity after 15 cycles could be attributed to the loss of Fe2O3 in the composite OC and partial sintering on the surface. The coal tar vapor in syngas could be removed while undergoing CLR on the surface of iron–calcium OC, while the iron–calcium OC showed superior performance for the cracking of coal tar.