Chemical looping dechlorination through adsorbent-decorated Fe2O3/Al2O3 oxygen carriers

Abstract

Plastic waste incineration is the main source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which, as toxic materials, have aroused increasing attention all over the world. Based on the formation mechanisms of PCDD/Fs, the key approaches to eliminate PCDD/Fs are to maintain an oxygen-deficient atmosphere to suppress the de novo route of PCDD/Fs formation and to constrain the presence of free chlorine to prevent the formation of PCDD/Fs from the conversion of precursors (e.g., unchlorinated benzene and phenol). Chemical looping combustion (CLC), which utilizes active lattice oxygen provided by an oxygen carrier (OC) for fuel oxidation in the absence of O2 was, for the first time, proposed as a promising pattern for disposing plastic waste. To constrain the presence of free chlorine, the feasibility of dechlorination through adsorbent-decorated Fe-based OC during the CLC process was investigated through thermodynamic simulation, isothermal kinetic experiments in a thermogravimentric analyzer (TGA), and batch fluidized bed reactor experiments. For three CaO/K2O/Na2O adsorbents, CaO decoration for OC particles exhibits the highest dechlorination efficiency, however it results in the maximum activation energy in the TGA experiments and relatively more carbon deposition on the OC particles in these batch fluidized bed experiments. Subsequently, the effects of the CaO decoration method (wet impregnation, coprecipitation, and physical mixture), CaO loading and reaction temperature on the efficiencies of dechloridation and combustion were investigated in the batch fluidized bed reactor that simulates a real chemical looping process. The results indicated that the wet impregnation method demonstrates the best adsorbent decoration and that 5 wt% CaO loading reaches an optimized level of dechloridation. In addition, increasing the temperature can improve the dechlorination, but it also increases carbon deposition, leading to a lower combustion efficiency. Next, the dechloridation efficiency gradually descends, and correspondingly, the mole ratio between Ca and Fe on the OC particle surface exhibits a declining trend with an increase in the cycle number; these results were verified by ion chromatography and an environmental scanning electron microscope coupled with energy dispersive X-ray spectroscopy (ESEM-EDX), respectively. Notably, the ESEM-EDX result demonstrates the accumulation of Cl elements onto the surface of OC particles. Last, the reduced samples were washed using a dilute hydrochloric acid solution after 30 cycles, and the ESEM-EDX result demonstrates the removal of the Ca and Cl elements from the surface of these OC particles and the regeneration of these OC particles. Overall, these results provide sound evidence for the feasibility of chemical looping dechlorination through the use of adsorbent-decorated Fe2O3/Al2O3 OC particles.