# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 2 Materials and Methods
### Batch Fluidized Bed Reactor  
The oxygen release behavior of the oxygen carriers and the reactivity with the main combustion gases, (CO, H\({}_{2}\), and CH\({}_{4}\)), were investigated in a batch fluidized bed reactor at different temperatures from 850 to 950 \({}^{\circ}\)C (see Figure 3). A detailed description of the batch fluidized bed reactor can be found elsewhere [27].  
Initially, 300 g of the oxygen carrier were loaded in the batch fluidized reactor. The sample was heated in air to the desired temperature, and then successive redox cycles were performed. In the oxygen release experiments, pure nitrogen and air were used during reduction and oxidation periods at 850 \({}^{\circ}\)C and 950 \({}^{\circ}\)C, respectively. In the experiments with combustion gases, the reaction temperature was set in all cases to 950 \({}^{\circ}\)C. The reacting mixture used during the reducing periods was 50% H\({}_{2}\) + 20% H\({}_{2}\)O, 50% CO + 20% CO\({}_{2}\), and 12.5% CH\({}_{4}\) + 20% H\({}_{2}\)O, respectively, while 10 vol.% O\({}_{2}\) was used in all cases during oxidation. The gas velocity in the experiments was set to 12-15 cm/s. Reduction was limited to 2 or 5 min while complete oxidation was always reached. To avoid mixing atmospheres, pure nitrogen was used as a purge between reduction/oxidation periods during 2 min. A total of 10 reduction-oxidation cycles were carried out with each gas. Gas  
Figure 3: Scheme of the batch fluidized bed.  
product distribution as well as solid conversion during reduction and oxidation steps were obtained for both types of experiments. The oxygen carrier conversion (\(X_{\text{s}}\)) was defined as:  
\[X_{\text{s}}=\frac{\left|m-m_{r}\right|}{m_{oxid-m_{r}}} \tag{5}\]  
Details of the calculation of oxygen carrier conversion in the batch fluidized bed can be found elsewhere [27]. From the results, it is also possible to calculate a normalized rate index for the experiments with different gases in the batch fluidized bed reactor: The normalized rate index for the batch experiments is defined as:  
\[Rate\ Index\left(\frac{\%}{min}\right)=60\cdot 100\cdot R_{\text{OC}}\cdot\left( \frac{dX_{i}}{dt}\right)\bigg{|}_{norm}=60\cdot 100\cdot\frac{P_{ref}}{p_{m}} \tag{6}\]  
where \(P_{ref}\) is the reference partial pressure (0.15 atm), which would approximately correspond to a full conversion of the gas, \(P_{m}\) refers to the average partial pressure of the gaseous fuel in the reactor being calculated by the following equation through an integration process, assuming a first-order reaction [27].  
\[p_{m}=\frac{\left(p_{in}-p_{out}\right)\cdot p_{in}}{\left(p_{out}-p_{in} \right)\cdot e_{g}+\left(p_{in}+e_{g}\cdot p_{out}\right)\cdot\left(1+e_{g} \right)\ln\left(\frac{p_{in}+e_{g}\cdot P_{out}}{\left(1+e_{g}\right)\cdot P_{ out}}\right)} \tag{7}\]  
where \(p_{in}\) and \(p_{out}\) represent the partial pressures of the gaseous fuel at the inlet and outlet of the batch fluidized bed reactor, respectively, and \(e_{g}\) is the expansion factor, i.e., \(e_{g}=0\) for CO and H\({}_{2}\) and \(e_{g}=0.25\) for reduction with 12.5% CH\({}_{4}\).  
The test in the batch fluidized bed also allowed for the analysis of the fluidization behavior with respect to attrition and the agglomeration phenomena of the oxygen carriers. The reactor was equipped with two pressure taps to continuously check the differential pressure between the inlet and outlet streams of the batch reactor and therefore detect agglomeration. In the downstream of the reactor, there were two parallel hot filters to recover elutriated solid during the consecutive redox cycles. The attrition rate was obtained by the following equation:  
\[v_{attrtion}=\frac{\left(m_{elut}/\ m_{\text{OC}}\right)\times 100}{t}\times 3600 \quad\left[\%/\ h\right] \tag{8}\]  
where \(v_{attrtion}\) is the attrition rate, \(m_{elut}\) is the oxygen carrier elutriated from the batch fluidized bed reactor, which is recovered in hot parallel filters and pipes, and \(m_{\infty}\) is the total mass of oxygen carrier in the experiments.