Characterization and evaluation of Fe_2O_3/Al_2O_3 oxygen carrier prepared by sol-gel combustion synthesis

摘要

The sol-gel combustion synthesis (SGCS) for oxygen carrier (OC) to be used in chemical looping combustion (CLC) was first designed and experimented in this work, which is a new method of OC synthesis by combining sol-gel technique and solution combustion synthesis. Cheap hydrated metal nitrates and urea were adopted as precursors to prepare Fe_2O_3/Al_2O_3 OC at the molar ratio to unity (Fe1 Al1), which was characterized through various means, including Fourier transforms infrared (FT1R) spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffractor (XRD), and N_2 isothermal adsorption/desorption method. FT1R analysis on the chemical structure of the dried gel of FelAll indicated that urea was partly hydrolyzed and the hydrated basic carbonate was formed by the combination of groups such as (Fe(_(1-y)Al_y)_(1-x)O_(1-3x), CO_3~(2-) and -0H-. By analyzing the staged products during SGCS, calcination was found as a necessary step to produce Fe_2O_3/Al_2O_3 OC with separate phases of α-Fe_2O_3 and α-Al_2O_3. Through TGA-DTA, the decomposition of the dried gel was found to undergo five stages. The analysis of the evolved gases from the gel decomposition using FTIR partially confirmed the staged decomposition and assisted a better understanding of the mechanism of SGCS. XRD identification further substantiated the necessity of calcination to synthesize Fe_2O_3/Al_2O_3 OC with separate phases of α-Fe_2O_3 and α-Al_2O_3, though it was not necessary for the synthesis of single phase α-Fe_2O_3 and α-Al_2O_3. Structural characterization performed on N_2 adsorption analyzer displayed that the pore shape of Fe1 Al1 particles was heterogeneous. Finally, H_2 temperature-programmed reduction (TPR) of FelAll products in TGA indicated that the reduction reaction of FelAll OC after calcination was a single step reaction from α-Fe_2O_3 to Fe, and calcination benefited to improve the transfer rate of the lattice oxygen from the OC to fuel H_2. Furthermore, four times of reduction and oxidization (redox) reaction by alternating with H_2 and air demonstrated the synthesized OC had good reactivity and sintering-resistance, much suitable to be used in the realistic CLC. Overall, the SGCS method was found superior to other existent methods to prepare Fe_2O_3/Al_2O_3 OC for CLC application.