Nanoscaled Oxygen Carrier Development For Chemical Looping Partial Oxidation Of Methane

Natural gas, which is mainly composed of methane, is an important industrial resource for syngas production. Nevertheless, the conventional methane to syngas conversion processes are highly energy intensive and suffer from low methane conversion. Chemical looping partial oxidation of methane is a promising alternative technology that consists two reactors connected by an oxygen carrier particle loop. This process has high syngas selectivity with higher energy efficiency, minimal environmental impact, and proper H2 to CO ratio for downstream operations. The major challenge for chemical looping processes is the development of high-performance oxygen carriers with high reactivity, recyclability, and recyclability. Our group employed novel iron-based oxygen carrier mixed with metal oxide support. The particles show extraordinary recyclability that sustained over 3000 redox cycles at 1000 oC without reactivity or mechanical deteriorates. Despite research efforts have been put into oxygen carrier development, there are still scopes for syngas yield and purification improvement, especially at low temperatures. In this dissertation, nanoscaled oxygen carriers are explored as oxygen carrier for chemical looping partial oxidation process. Nanoparticles have higher surface area and smaller size compared to bulk material, therefore increasing reaction rate by enhancing the surface reactivity and ionic diffusion. Mesoporous support was utilized to disperse and stabilize the nanoscaled oxygen carrier. The confinement effect of the mesopores of the support can separate the nanoparticles and prevent their aggregation during the reaction. SBA-15 supported iron oxides nanoparticles shows near 100% syngas selectivity and sustained over 75 redox cycles at 800 oC. Density functional theory calculations also indicate that nanoparticles have higher methane conversion rate and syngas selectivity. The effect of the mesoporous support structure on the reactivity of the oxygen carrier is also studied by both experiments and Dynamic Monte-Carlo simulations. With 2-D hexagonal cylindrical structure, SBA-15 can trap gas molecules and decrease the gas diffusion rate in the mesopores. While 3-D interconnected mesoporous support SBA-16 can decrease the congestion and trapping effect, therefore nanoparticles supported on SBA-16 show increased reaction rate compared with those on SBA-15. Dopant modification was also applied to nanoparticles. Nevertheless, doped iron oxide nanoparticles do not show extraordinary improvement in reactivity, stability, or selectivity, since the reactivity of the nanoparticles are not limited by surface active sites or ionic diffusivity. With the growing energy and environmental demands, chemical looping processes are facing more opportunities and challenges in oxygen carrier development. The research in this dissertation enlightens the chemical looping technology in the near future.