Improvement of Oxygen Mobility with the Formation of Defects in the Crystal Structure of Red Mud as an Oxygen Carrier for Chemical Looping Combustion.

Fe2O3 is the major component of red mud, which is a by-produced after eluting aluminum from bauxite in the Bayer process, and can be used as an oxygen carrier. On the other hand, red mud is unsuitable for using oxygen in the crystal lattice because of its low surface area. In this study the red-mud sample was sulfidated at high temperatures to improve the lattice oxygen mobility by forming lattice defects in the iron oxide crystals. To form crystal defects on red mud, iron oxide was converted to iron sulfide with hydrogen sulfide, and then re-oxidized by air to remove the sulfur components. In these processes, it was possible to generate defects could be generated in the crystal structure. Crystal defects are formed by the difference in the molar volume of oxygen and sulfur bound to the metal in the oxidation-sulfidation process. The surface area of the defective red mud increased from approximately 25.9 m2/g to 122.1 m2/g, and the pore volume increased from 0.1714cc/g to 0.2803 cc/g. In addition, the formation of crystal defects increased the oxygen transfer capacity of red mud from 1.75% to 2.25% at 15 vol.% hydrogen. This means that the amount of oxygen transported during the reduction process could be enhanced approximately 1.29 fold.

Oxygen Transfer Capacity of Pseudobrookite Particles Derived from Ilmenite Mineral (x wt.%CuO/y wt.%red Mud-z wt.%ilmenite).

The minerals have a somewhat slower than other transition metals at critical reduction rates in their ability to deliver oxygen. Thus, single minerals alone do not exhibit a higher oxygen transfer capacity than metal oxide oxygen carriers. In this study, we try to solve the problem of single mineral ilmenite (FeTiO3) by combining it with Fe-based red mud and Cu oxide. When the ilmenite was used without calcination, the CH4-CO/air redox cycle showed rapid decayed. However, when ilmenite was calcined, the CH4-CO/air redox cycle became stable, and the oxygen transfer rate increased to 4.2%. This is because the FeTiO3 structure was converted to the pseudobrookite (Fe2TiO5) structure through the calcination process. That is, the Fe2+ ion in the ilmenite structure was converted into an Fe3+ ion. When 30 wt.% of red mud was added to the Fe ion, it reacted with the rutile-type titania mixed with pseudobrookite-typed Fe2TiO5, producing an almost perfect pseudobrookite crystal. This resulted in a slight increase in the capacity of oxygen transfer to 4.9%. When 15 wt.% of Cu oxide was added, the oxygen transfer capacity increased to 6.0%. This performance was indicated by the cyclic voltammetry curve that remained constant even after 200 cycles. Here, we argue that if low-cost minerals as a base material are used in appropriate amounts, the production of a lowest-cost oxygen carrier can be achieved.