# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 3 Results
### Granulated Particles: Comparison with Results from Particles Produced by Pelletization
The corresponding XRD patterns are also shown in Figure 4. As it was already found in similar previous experiments with the pellet-produced particles, the pyrophanite phase remained stable, and only Mn\({}_{3}\)O\({}_{4}\) was identified as a reduced species. This corroborates the hypothesis that only the redox pair Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) plays a role in the release of gaseous oxygen. At the lowest temperature (850 \({}^{\circ}\)C), both Mn\({}_{2}\)O\({}_{3}\) and Mn\({}_{3}\)O\({}_{4}\) were found together with MnTiO\({}_{3}\). However, at 940 \({}^{\circ}\)C the reduction degree was higher, and only Mn\({}_{3}\)O\({}_{4}\) and MnTiO\({}_{3}\) were identified in the reduced samples. Following these results, it was decided to select one Mn-Ti mixed oxide for further testing. The sample with Ti/(Mn+Ti) equal to 0.19 (20 wt.% TiO\({}_{2}\)) was selected, and from now on it will be denoted as MnTi\({}_{20}\). Table 2 summarizes the main properties of the MnTi\({}_{20}\) oxygen carrier produced by granulation that will be used in the present work. Figure 6: Oxygen uncoupling capability, \(R_{OC}^{ou}\), versus Ti/(Mn+Ti) ratio for manganese-titanium particles produced from pellets and granulated particles (results corresponding to the fifth cycle in TGA at 850 and 940 \({}^{\circ}\)C). Together with the oxygen transport capacity for the gaseous oxygen release (\(R_{\text{OC}}^{\omega}\)) of the oxygen carriers, another parameter to be considered regarding the use of these materials in CLOU/CLAS applications is the rate for this oxygen release. In order to compare the reactivity of different samples, the rate index parameter defined in Equation (4) was used. Table 3 compiles results of the normalized rate index at 950 \({}^{\circ}\)C obtained for the granulated Mn-Ti samples both in oxygen release cycles and in the reaction with the main combustion gases (H\({}_{2}\), CO, CH\({}_{4}\)) and with O\({}_{2}\). In the table, the corresponding values for the MnTi_20 particles obtained from pellets and granulated particles were included, as well as values for manganese-iron mixed oxides (Mn77 Fe and Mn55 FeTi7) and Mn-based oxygen carriers (MnSA, MnGBINE, Mn-ZrSG) previously prepared and evaluated by the authors. Mn77 Fe and Mn55 FeTi7 particles are Fe-doped manganese oxides [30; 31]. MnSA and MnGBINE are two manganese-based minerals with a high Mn\({}_{3}\)O\({}_{4}\) content [27]. Mn-ZrSG is a synthetic oxygen carrier prepared by incipient wetness impregnation of the manganese oxide on ZrO\({}_{2}\)[32]. As it can be observed in Table 3, only the Ti- or Fe-doped manganese oxides were capable of releasing oxygen and regenerating in air at 950 \({}^{\circ}\)C. Granulated MnTi_20 showed a similar rate index for oxygen release to that of the material obtained by peltetization and a better index for the subsequent oxidation. Moreover, the values of the rate index obtained for the main combustion gases are even slightly higher than those found for the material prepared from pellets. On the other hand, granulated MnTi_20 showed better values for the rate index in the oxygen release than some Mn-Fe mixed oxides [30; 31]. The values of the rate index with the main combustion gases were also lower for the Mn-Fe mixed oxides at similar temperatures (950 \({}^{\circ}\)C). Thus, MnTi_20 could be considered as an alternative to manganese-iron oxygen carriers. The rate indexes in Table 3 were also compared to those obtained with manganese oxide-based oxygen carriers but with mineral origin or prepared by the impregnation method. In the case of manganese-based minerals, it has been observed that in any case \begin{table}
\begin{tabular}{l l l} \hline  & **Calcined** & **Used (Batch FB)** \\ \hline Crushing strength (N) & 2.6 & 1 \\ Porosity (\%) & 31.2 & 34.4 \\ Skeletal density (kg/m\({}^{3}\)) & 4818 & 4739 \\ BET surface area (m\({}^{2}\)/g) & 0.6 & 0.7 \\ \hline \end{tabular}
\end{table} 
Table 2: Main properties of the MnTi_20 granulated particles.