Oxygen release from manganese ores relevant for chemical looping with oxygen uncoupling conditions

The use of manganese ore as oxygen carrier has recently gained interest, primarily due to the combination of low cost and moderate to high reactivity. The possibility of an oxygen uncoupling reaction enhancing reactivity may be an additional advantage. In this paper, six manganese ores (Gloria, Tshipi, Gui Zhou, Metmin, Sibelco Calcined and SinAus) were evaluated with respect to their ability to release oxygen with a devolatilized wood fuel (char) using a small batch fluidized bed reactor. The char was used in order to establish the rate of the oxygen release i.e. oxygen uncoupling. The manganese ore samples were heat treated for 24 h at 950 °C and sieved to a size fraction of 125-180 µm prior to experiments. The oxygen release was evaluated at four temperatures (850, 900, 950 and 1000 °C). Moreover, comparisons were made with the reactivity of these ores with gaseous fuels made in previous works. All six investigated ores showed a small O2 release (CLOU effect), which increased with temperature. Still, the oxygen uncoupling effect is likely not the dominating reaction when using gaseous fuels, as the overall oxygen transfer via CLOU is considerably smaller compared to the oxygen transfer when gaseous components react directly with the particle. For example, at 1000 °C the Gloria ore had the highest mass-based reduction rate, dω/dt, was 0.047 wt%/s for methane and 0.36 wt%/s for syngas, which should be compared with the much lower dω/dt of 0.0073 wt%/s for char, which is a measure of the CLOU effect. The best performing ores were Gloria and Tshipi, which showed the highest uncoupling capacity and rate of oxygen release. It is interesting to note that these two ores had significantly higher Ca and Mg fractions compared to the other four ores with lower reactivity. This suggests that combined Mn-Ca phases may be active for the oxygen release reaction. The ores were analyzed with XRD and SEM-EDX to establish which phases were present in the ores. It was found that the bulk of the ores are very heterogeneous, while the individual particles are much more homogeneous in composition.