# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 4 Discussion  
In this work, a manganese oxide-based material with TiO\({}_{2}\) addition has been produced by granulation and tested under conditions relevant for different Chemical Looping processes. Ti-doped manganese oxides with Ti/(Mn+Ti) molar ratios between 0 and 0.49 were previously identified as promising due to their capacity to release gaseous oxygen and regenerate in air under conditions where pure manganese oxide does not. In a previous work by the authors, these materials were prepared in the form of mixtures of the corresponding oxides followed by pelletization. The present work aimed to reproduce one of the identified materials (MnTi_20) through the granulation method, which allows for easier scale-up in a hypothetical commercial production of the oxygen carrier. Moreover, the preparation method can influence the reactivity of the particles; thus, this aspect was also evaluated. It has been possible to granulate MnTi_20 particles with adequate mechanical properties to be used in processes based on fluidized beds, such as the Chemical Looping processes. However, control of the calcination conditions of the material was revealed as necessary, especially the cooling conditions after calcination to ensure that most of the introduced TiO\({}_{2}\) is stabilized as MnTiO\({}_{3}\) and no free TiO\({}_{2}\) is found. Experimental evidence, both with pellet-produced and granulated particles, seems to indicate that the presence of this phase allows the Mn\({}_{3}\)O\({}_{4}\) phase to be reoxidized to Mn\({}_{2}\)O\({}_{3}\) under conditions in which pure manganese oxide cannot do. XRD analysis of the granulated particles showed a higher fraction of free TiO\({}_{2}\) and lower fraction of MnTiO\({}_{3}\) compared to those found in the corresponding particles prepared from pellets, which would also explain the differences in the oxygen transport capacity for the oxygen release observed between the two types of particles at the highest temperature tested, i.e., 940 \({}^{\circ}\)C.  
In order to be considered for further scale-up, the evolution of the mechanical properties of MnTi_20 along several redox cycles was studied for the first time. Long TGA tests (200 redox cycles) were performed, and in the experiments in the batch fluidized bed, the attrition rate of the granulated particles was estimated. In both types of experiments, an increase in the porosity of the samples was observed, leading to a decrease in the mechanical strength of the particles that may make them unsuitable for use in a fluidized bed. Despite the deterioration of their mechanical properties with the number of cycles, the granulated MnTi_20 particles maintained their reactivity, which was similar to or even better in some cases than Fe-doped manganese oxides previously reported in the literature. Another aspect to consider is the rate at which manganese-titanium mixed materials can release oxygen or react with typical combustion gases, i.e., H\({}_{2}\), CO, and CH\({}_{4}\). In this study, we have worked mainly in the 850-950 \({}^{\circ}\)C temperature window, which is a temperature range adequate for CLOU, TCES, and CLC processes based on oxygen carriers. It has been observed that although Mn\({}_{3}\)O\({}_{4}\) is capable of regenerating to Mn\({}_{2}\)O\({}_{3}\) under these conditions, the oxidation rate is slow compared to other non-manganese-basedmaterials, such as copper oxygen carriers. Depending on the Chemical Looping application to which this material is intended, this oxygen release rate should be improved, such as in CLOU or TCES applications, in order to guarantee high combustion efficiencies and the thermochemical storage capability, respectively, of the Mn-Ti-based materials. As it was mentioned in the introduction section, recent works propose the addition of MgO [19], SiO\({}_{2}\)[35], or even CeO\({}_{2}\)[36]. In future research with the MnTi_20 material, the effect of the addition of these oxides can be considered.