## 2 Experimental
### Experimental Setup and Procedure  
The experimental setup used in this work was a batch-scale fixed bed, as shown in Figure 2. The reactor was made of quartz tube with inner diameter of 40 mm, and a porous board was placed at the medium of the reactor. The temperature in the reactor was precisely controlled by a K-thermocouple and fuzzy PID controller. The gases used in the experiment were supplied by the gas cylinders produced at Nanjing Shangyuan gas plant (Nanjing, China). The flow rates of the gases were controlled by the S49 32/MT mass flow controllers produced by Beijing HORIBA METRON Instruments Co. Ltd. (Beijing, China). Prior to each test, 50 g of the calcined lean iron ore was placed on the porous board and then the reactor was heated to 1223 K in N\({}_{2}\) atmosphere. When the temperature was kept stable, the redox \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Component** & **Fe2O3** & **SiO2** & **Al2O3** & **CaO** & **MgO** & **SO3** & **Others** \\ \hline Mass Fraction/\% & 44.16 & 18.93 & 2.98 & 17.09 & 7.43 & 2.35 & 7.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The X-ray fluorescence (XRF) analysis results of lean iron ore oxygen carrier (OC). Figure 1: Photo of the crushed oxygen carrier particles. cycles were carried out. The reduction process was initiated by injecting the 11% CO (diluted with N\({}_{2}\)) into the reactor. During different tests, the gas flow rate of the CO kept 1 L/min with the injection times of 60 s, 50 s and 40 s. After the reduction period, the 99.99% N\({}_{2}\) stream was introduced for purge period to avoid the effects of residual CO and CO\({}_{2}\) on the reaction. The gas flow rate was set as 1 L/min. Then the pure air was switched to replace the N\({}_{2}\) to oxidize the reduced oxygen carrier with the gas flow of 1 L/min as well. The oxidation of oxygen carrier lasted for 60 s to guarantee that the oxygen carrier was fully oxidized. After oxidation period, the N\({}_{2}\) was injected into the reactor for the second purge process to initiate the next cycle. The production gas leaving the reactor was filtered and dried. After cooling, the compositions of the flue gas were analyzed by the gas analyzer produced by Messgerate fur Rauchgase und Umweltschutz GmbH (MRU) in Neckarsulm Germany, whose sampling interval was set as 1 s during the whole reaction process. A single redox cycle included CO reduction (denoted as R), purge process using N\({}_{2}\) (denoted as P\({}_{1}\)), air oxidation (denoted as O), and the second purge process (denoted as P\({}_{2}\)). In the experiment, the first purge process was adopted to sweep out the residual unreacted CO and the production CO\({}_{2}\). The purpose of the second purge process was to sweep the residual air in the reactor and initial the next redox cycle. The CO injection time in test 1 was the longest in these three tests, which resulted in more carbonaceous gases in the reduction process. Thus the first purge time was also the longest as 180 s. The first purge process of test 2 and test 3 lasted for 120 s. The total time of test 3 was intended to be the same as that of test 2. The detailed experimental conditions are listed in Table 2. In order to investigate the reactivity stability of the oxygen carrier, 100 cycles were done in each test. \begin{table}
\begin{tabular}{c c c c} \hline
**Test Number** & **Test 1** & **Test 2** & **Test 3** \\ \hline Reducing gas & 11\% CO & 11\% CO & 11\% CO \\ Oxidization gas & Pure air & Pure air & Pure air \\ Sweep gas & 99.99\% N\({}_{2}\) & 99.99\% N\({}_{2}\) & 99.99\% N\({}_{2}\) \\ Reduction temperature (K) & 1223 & 1223 & 1223 \\ Oxidization temperature (K) & 1223 & 1223 & 1223 \\ Gas flow (L/min) & 1 & 1 & 1 \\ CO injection time (s) & 60 & 50 & 40 \\ The first purge time (s) & 180 & 120 & 120 \\ Air injection time (s) & 60 & 60 & 60 \\ The second purge time (s) & 180 & 70 & 80 \\ Time of a single cycle (s) & 480 & 300 & 300 \\ \hline \end{tabular}
\end{table} 
Table 2: Different experimental conditions. Figure 2: Bench-scale fixed bed reactor setup.