Mn/MgAl2O4 oxygen carriers for chemical looping combustion using coal: influence of the thermal treatment on the structure and reactivity

Mn-based solid oxygen carriers supported on MgAl2O4 were prepared by the microwave-assisted combustion method followed by wet impregnation of the active phase. The oxygen carriers were characterized by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy with energy-dispersive spectroscopy of the surface. The study aimed to evaluate the influence of the thermal treatment on the structure and reactivity of the oxygen carriers. Reactivity tests using coal as fuel were carried out in a thermobalance at temperatures of 750–850 °C to evaluate the behavior of the carriers. The results revealed that oxygen carriers, which were subjected to only a heat treatment after the impregnation step (MnC1), showed more reactive surfaces presenting a greater number of non-reactive and irreversible phase and thus displaying lower oxygen transport capacity as well as poorer reactivity. The carriers prepared by heat treating the support after the combustion reaction and again after the impregnation step (MnC2), showed higher oxygen transport capacity and greater reactivity with coal at 850 °C. The better reactivity is associated with the formation of Mg6MnO8 and Mn2O3 phases.