Thermochemically Stable Novel Oxygen Carriers Based on CaMn1–x–yTixFeyO3−δ for Chemical Looping

Abstract

The understanding and development of stable redox materials based on cheap and abundant elements, forming Ca–Mn–Ti–Fe–O-based perovskites, have been in focus for applications in renewable technologies such as chemical looping combustion and thermal energy storage. The present research focuses on developing stable materials to be utilized up to 1050 °C in a CLC process and has shown that the structure stability and oxygen transfer capacity can be achieved by tuning the content of different elements on B-sites of the perovskites. Various experiments, such as redox cycling under various fuels, temperatures, and pO2, were carried out to evaluate the oxygen transfer capacity, reaction rates under various fuels, etc. The redox stability at high temperatures was evaluated by redox cycles at 1050 °C followed by post SEM analyses on surface and depth profiling. The three developed materials can avoid phase change during redox due to the moderate oxygen transfer capacity of up to 5.6 wt % O2 for CaMn0.5Ti0.375Fe0.125O3−δ at 1050 °C, which is important for having stable particles. Cation diffusion was also investigated during redox cycling in the development of stable redox materials, and only a minor diffusion of Mn to the grain boundaries is seen in the least stable material. The findings show that perovskites with high stability can be obtained with more Ti on B-sites, termed as CaMn0.375Ti0.5Fe0.125O3−δ. The developed stable oxides, to some extent, have a reduced activity compared to the less stable composition with less Ti and more Mn, termed as CaMn0.5Ti0.375Fe0.125O3−δ, which possesses a higher oxygen release to inert ca. 1.1 wt % O2 compared to more stable CaMn0.375Ti0.5Fe0.125O3−δ that can release up to 0.8 wt % O2. Two of the materials have faster kinetics than ilmenite by a factor of 2 in H2.