## 3 Reactivity of Oxygen Carrier with Different Reaction Times
### Investigation on Multi Cycles  
Figure 4 illustrates the CO\({}_{2}\) concentration variation with the number of cycles under different CO injection times of 60 s, 50 s and 40 s. In the multi-redox cycles, CO\({}_{2}\) concentration kept in a relatively stable range, which demonstrated that the lean iron ore had satisfying reaction stability as the oxygen carrier. When the injection times of CO were 60 s, 50 s and 40 s, the CO\({}_{2}\) concentrations were in the ranges of 6.7-7.5%, 6.4-6.7% and 5.9-6.1% separately. The time domain of each test was too long to exhibit the detailed variation in every single cycle. The maximum of the CO\({}_{2}\) concentration in Figure 4 could be employed to analyze the reactivity changes of the oxygen carrier. Thus, the following analyses were based on the maximum values of the CO\({}_{2}\) concentrations. \begin{table}
\begin{tabular}{c c c c} \hline
**Test** & \(\mathbf{V_{\text{CO}}/L}\) & \(\mathbf{V_{\text{CO}_{2}}/L}\) & **Conversion of CO** \\ \hline
1 & 0.021 & 0.089 & 80.9\% \\
2 & 0.025 & 0.067 & 73.9\% \\
3 & 0.028 & 0.045 & 61.6\% \\ \hline \end{tabular}
\end{table} 
Table 3: The conversion calculation of CO.  
Figure 4: \(Cont\). Compared with test 2 and test 3, the fluctuation interval of CO\({}_{2}\) concentration in test 1 was the greatest. However, it was common in three tests that the CO\({}_{2}\) concentration increased slightly in the initial several cycles, which demonstrated that the activity of the lean iron ore gradually increased and more CO was oxidized to CO\({}_{2}\). It exhibited that there was an activation process of the lean iron ore. The activation period and its mechanism have been found and investigated in previous literatures [41; 42; 43; 44]. This was attributed to the porosity increase of oxygen carrier particles in the initial cyclic reactions. As the cycle number increased, the CO\({}_{2}\) concentration maintained in a steady range and then decreased gradually. The reason for this phenomena was that the oxygen transport capacity of the lean iron ore maintained at a relatively stable stage in the following cycles. With the increase of the cycles, the porous structure of the natural lean iron ore was destroyed, which led to the decrease of the activity and oxygen transport capacity of the oxygen carrier. Incomplete reaction between the lean iron ore and CO was caused by reducing the exposure time of oxygen carrier in reducing atmosphere. In this case, the CO had not diffused into the inner core of oxygen carrier particles so that only the surface structure changed. Shortening the injection time of CO reduced the formation of FeO and/or Fe on the OC surface, which could avoid the secondary reaction between FeO/Fe and other compositions and could maintain the reactiveness of the lean iron ore. It should be noted that due to the breaks in a longtime redox, the total time exceeded the continuous operation time of 100 cycles. During the breaks, the reactor was in the N\({}_{2}\) atmosphere. Figures 5 and 6 illustrate the CO\({}_{2}\) and CO concentrations in the reduction periods of the 1 st, 50 th, and 100 th cycles separately. Results demonstrated that the concentration of CO increased gradually with the increase of the cycles while the concentration of CO\({}_{2}\) decreased accordingly in the reduction stage of the cyclic reaction. Figure 4: CO\({}_{2}\) concentration in the outlet gas in 100 cycles: (**a**) Test 1; (**b**) Test 2; (**c**) Test 3. The CO\({}_{2}\) concentration in test 1 decreased from 7.8% to 6.8% while it came down from 6.6% to 6.4% in test 2 and from 6.1% to 5.9% in test 3. The declines of CO\({}_{2}\) concentration demonstrated the degradation of the reactivity after multi cycles. Figure 5: The CO\({}_{2}\) concentration in the reduction period: **(a)** Test 1; **(b)** Test 2; **(c)** Test 3. Figure 6: The CO concentration in the reduction period: **(a)** Test 1; **(b)** Test 2; **(c)** Test 3. In these tests, the CO curve could reflect the reaction rate and the conversion of the oxygen carrier.