Development of Sinter-Resistant Core-Shell LaMn_xFe_(1-x)O3@mSiO2 Oxygen Carriers for Chemical Looping Combustion

摘要

This work investigates the possibility of using LaMn_(.07)Fe_(0.3)O_(3.15)@mSiO2 as oxygen carriers for chemical looping combustion (CLC), CLC is a new combustion technique with inherent separation of CO2 from atmospheric N2. LaMn_(0.7)Fe_(0.3)O_(3.15)@SiO2 core—shell materials were prepared by coating a layer of mesostructured silica around the agglomerated perovsldte particles. The oxygen carriers were characterized using different methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 sorption, hydrogen temperature-programmed reduction (H2-TPR), and temperature-programmed desorption of oxygen (TPD-O2). The reactivity and stability of the carrier materials were tested in a special reactor, allowing for short contact time between the fluidized carrier and the reactive gas [Chemical Reactor Engineering Centre (CREC) fluidized riser simulator]. Multiple reduction—oxidation cycles were performed. TEM images of the carriers showed that a perfect mesoporous silica layer was formed around samples with 4, 32, and 55 nm in thickness. The oxygen carriers having a core—shell structure showed higher reactivity and stability during 10 repeated redox cycles compared to the LaMn_(0.7)Fe_(0.3)O_(3.15) core. This could be due to a protective role of the silica shell against sintering of the particles during repeated cycles under CLC conditions. The agglomeration of the particles, which occurred at high temperatures during CLC cycles, is more controllable in the core—shell-structured carriers, as confirmed by SEM images. XRD patterns confirmed that the crystal structure of all perovsldtes remained unchanged after multiple redox cycles. Methane conversion and partial conversion to CO2 were observed to increase with the contact time between methane and the carrier. Indeed, more oxygen from the carrier surface, grain boundaries, and even from the bulk lattice was released to react with methane. Upon rising the contact time, less CO was formed, which is desirable for CLC application. Increasing the reaction temperature and methane partial pressure lead to enhanced conversions of CH4 under CLC conditions.