# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 3 Results
### Batch Fluidized Bed Experiments
#### 3.3.2 Experiments with Combustion Gases  
During these tests, ten reduction-oxidation cycles were performed with each reducing gas at 950 \({}^{\circ}\)C. Figure 10 shows the gas product profile and solid conversion in the fifth cycle in the batch fluidized bed experiments with H\({}_{2}\), CO, and CH\({}_{4}\). Figure 10: Gas product profile and solid conversion (\(X\)s) in the fifth cycle in the batch fluidized bed experiments with (**A**) 50% H\({}_{2}\) + 20% H\({}_{2}\)O, (**B**) 50% CO + 20% CO\({}_{2}\), and (**C**) 12.5% CH\({}_{4}\) + 20% H\({}_{2}\)O. Oxidation in all cases with 10 vol.% O\({}_{2}\), T = 950 \({}^{\circ}\)C; Reduction time = 5 min. Figure 9: Oxygen concentration versus time during the third cycle at 850 and 950 \({}^{\circ}\)C with MnTi\({}_{20}\). Reactions with H\({}_{2}\) and CO in Figure 10A,B showed complete combustion of the gas in both cases, since no H\({}_{2}\) or negligible CO was detected at the batch reactor outlet. The solid conversion decreased during reduction. After that, it remained constant during the purge period and increased through MnTi_20 reoxidation using diluted air (10 vol.% O\({}_{2}\)). It can be observed that all of the oxygen introduced was consumed during the first stage of the oxidation reaction. During this step, the oxidation was limited by the flow of O\({}_{2}\) fed to the reactor. At the end of the oxidation period, the oxygen concentration rapidly increased to the inlet value. The highest value of solid conversion (_Xs_) was obtained in the case of the experiments with H\({}_{2}\) (~50%). Slightly lower conversion was reached in the case of CO (33.7%). The lowest solid conversion was reached in experiments with methane (22.6%), in Figure 10C. In this case, not all the methane was converted (35-40% methane conversion), but all the methane reacted was converted to CO\({}_{2}\) since no H\({}_{2}\) or CO were observed as combustion products during reduction. This fact suggests that neither methane reforming nor partial oxidation reactions with H\({}_{2}\)O or CO\({}_{2}\) were of significance. However, some thermal decomposition of methane could take place due to the presence of a slight amount of CO\({}_{2}\) at the beginning of the oxidation period. Carbon formation on oxygen carriers depends on the fuel used, the amount of oxygen added with the oxygen carrier, and the operating conditions of pressure and temperature. It is also interesting to evaluate the reaction rate of the MnTi_20 in the batch fluidized bed experiments through the calculation of the normalized rate index in the batch fluidized bed following Equations (6) and (7). Table 4 shows the values obtained for the different combustion gases, namely H\({}_{2}\), CO, and CH\({}_{4}\), together with values previously reported for manganese minerals (MnSA and MnHNE) and a manganese oxide oxygen carrier impregnated on ZrO\({}_{2}\) (Mn-ZrSG) [27; 32]. Values in Table 4 are calculated for the same materials and reacting gases than those shown in Table 3 but they are lower than in the TGA experiments. This fact can be attributed to mass transfer issues in the bubbles formed in the fluidized bed of the batch reactor. Reacting gases inside the bubbles have to diffuse to the emulsion, thus reducing the reaction rate with the oxygen carrier particles. The MnTi_20 oxygen carrier presents rate indexes in the same order of magnitude as the manganese minerals MnSA and MnGBINE. Values are similar for CO and CH\({}_{4}\), although a bit lower in the case of H\({}_{2}\). The impregnated material Mn-ZrSG presented a very high rate index for CH\({}_{4}\), more than 10 times that found for MnTi_20. A total of almost 60 h of continuous fluidization were performed with the MnTi_20 sample in the batch fluidized bed reactor. No fluidization or agglomeration problems were observed during this time. After the experimental campaign, the used sample was characterized.