## 3 Reactivity of Oxygen Carrier with Different Reaction Times  
### Reactivity in a Single Cycle  
With the reaction temperature at 1223 K, the injection time of CO was changed, and the reaction characteristics in a single cycle of the lean iron ore under three different conditions were tested. In order to fully activate the iron ore, the tenth cycle was chosen as the study object in this section. The gas concentrations in test 1 to test 3 are shown in Figure 3.  
When the injection time of CO was 60 s, the maximum of CO\({}_{2}\) concentration reached 7.4%. However, there was also residual CO which could not be converted. The peak concentration of unconverted CO was 3.0%. When the CO injection times were 50 s and 40 s respectively, the maximums of CO\({}_{2}\) concentrations were 6.6% and 5.9% while the corresponding CO concentrations were 3.7% and 4.4%. During the oxidization period, once the pure air went into the reactor, the O\({}_{2}\) in the air reacted with the reduced oxygen carriers quickly. When the oxygen carrier was fully oxidized, the O\({}_{2}\) concentration climbed to 21%. It is clear in Figure 3 that the different injection times could cause the different reaction times for the reduction of oxygen carrier. As the injection time of CO was shortened, the CO concentration in the flue gas increased while the concentration of CO\({}_{2}\) decreased. The maximum values of the concentrations might be related with the reaction rate during the whole reduction process. The reaction rate was the comprehensive results of the kinetics, gas concentrations, partial pressure and so on. However, generally the conversion rate exhibits an increasing trend in the initial stage of a reaction. When the injection time was 40 s, the average reaction rate was less than that of the test 1. From this perspective, the CO\({}_{2}\) concentration in test 3 should be less than that of test 1 while the CO concentration in test 3 should be higher than that of test 1. These results matched well with the experimental data in Figure 3. Following, these results were used to analyze the effects of the CO injection time on the conversion of the CO.  
Figure 3: The gas concentrations at the outlet under different conditions: (**a**) Test 1; (**b**) Test 2; (**c**) Test 3.  
To calculate the conversion of CO, the gas volume of carbonaceous gas need to be acquired. During the experiments, 11% CO was adopted as the reduction gas. The total gas flow rate was 1 L/min, and thus the flow rate of pure CO was 0.11 L/min. In the flue gas, the total volume of each carbonaceous gas can be calculated by the following equation:  
\[V_{i}=\int_{0}^{t_{t}}c_{i}Q_{Vt}dt, \tag{3}\]  
where, \(V_{i}\) is the gas volume of \(i\); \(i\) denotes CO or CO\({}_{2}\); \(t_{t}\) is the total reaction time; \(c_{i}\) is the concentration of gas \(i\) and \(Q_{V_{t}}\) is the total flow of the flue gas.  
The conversion of CO can be obtained by Equation (4).  
\[X_{\text{CO}}=\frac{V_{\text{CO}_{2}}}{V_{\text{CO}_{2}}+V_{\text{CO}}}, \tag{4}\]  
In the test 1 to test 3, the volume integral results and conversion of CO are listed in Table 3.  
Previously, our group has designed and built a high-flux CFB unit for chemical looping combustion [39], in which the amount of oxygen carrier was excess to the demand amount of fuel. To simulate the large ratio of the oxygen carrier to fuel, the injected CO was less than the demand amount of the total conversion for oxygen carrier in this work. With the increase of injection time of CO, the conversion of CO became larger. Also, Table 3 illustrated that the increasing interval of CO conversion between test 2 and test 3 was much larger than that between test 2 and test 1, which demonstrated that the conversion rate declined against time.