# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 3 Results
### Stability with TGA Cycles  
The behavior of the Mn-Ti mixed oxides under several redox cycles was not studied in the previous work with particles obtained from pellets. Thus, in this work, the chemical and mechanical stability of granulated MnTi_20 was evaluated in the TGA in 200-redox-cycle tests at 950 \({}^{\circ}\)C. Figure 7 shows the solid conversion during reduction (Xr) versus time for the successive redox cycles performed. Figure 7A shows the corresponding results for N\({}_{2}\)-air cycles, and Figure 7B for hydrogen-diluted air cycles.  
During the first 50 cycles in Figure 7A, the solid conversion does not experience significant changes; however, from that cycle on, it seems that the complete regeneration of the oxygen carrier during oxidation is not reached, and therefore the reduction conversion in subsequent cycles progressively decreases. Figure 7B shows a slight activation of the sample in the reaction with hydrogen during the first 50 cycles. After this point, there are no significant changes in the reactivity of the material. After the 200-redox-cycle test shown in Figure 7B, an additional reactivity test was done with 15% H\({}_{2}\) and another with 15% CH\({}_{4}\) + 20% H\({}_{2}\)O at 950 \({}^{\circ}\)C in order to calculate the rate index of the used sample. Results are shown in Table 3. An increase in the rate index for both gases is observed, especially significant in the case of methane, which changes from 2.8 to 8.2%/min.  
The solid phases in the reacted TGA samples were also investigated after the 200-redox-cycle tests by XRD, showing again the unchanged permanence of the pyrophanite phase, both in reduced and oxidized samples. Nevertheless, changes in the crushing strength for the calcined and used material were detected. The crushing strength value decreased from 2.6 N in the calcined MnTi_20 to 0.4 N after 200 redox hydrogen-diluted air cycles. This is a low crushing strength value to operate in a fluidized bed system, since the crushing strength limit for a correct fluidization was set at 1 N. Figure 8 shows images  
Figure 7: Solid conversion during reduction (_Xr_) versus time through 200 redox cycles in TGA at T = 950 \({}^{\circ}\)C (**A**) Reduction = 100% N\({}_{2}\); Oxidation = air (**B**) Reduction = 2% H\({}_{2}\); Purge = 100 vol.% N\({}_{2}\); Oxidation = 10 vol.% O\({}_{2}\).  
obtained in a Scanning Electron Microscope (SEM) of both calcined particles (A and D) and used in the 200-redox-cycle tests in TGA (B-F).  
Images A to C show a general overview of the external morphology of the (A) calcined and (B-C) used particles. No significant differences are observed. However, differences can be appreciated in images (D to F), which correspond to particles embedded in a resin and then cut so that the internal morphology of the particles can be analyzed. Compared to the calcined particles (D), the images for the used particles (E and F) reveal an increase in the porosity, especially in the case of the samples reacted under hydrogen-diluted air cycles. This increase in porosity may be linked to the decrease in the crushing strength previously mentioned. It seems that the larger the reduction degree of the sample, the higher the increase in porosity. Thus, the degree of reduction in the particle should be controlled to avoid deterioration of the mechanical properties with the cycles.