## 3 Results and Discussion
### Reduction of Air-to-Fuel Equivalence Ratio
\tag{16}\]  
The discontinuity of this relation for \(\lambda=1\) can be explained by the fact that when surpassing this value, a transient shift from CLC (see Figure 3a) to CLG (see Figure 3c) behavior (or vice versa) occurs, which goes in hand with a continuous decrease (resp. increase) in the oxidation degree of the oxygen carrier, before steady state sets in (more details see Appendix B). Figure 8: Simulation results for CLG operation through reducing \(\lambda\). Dry molar gas composition (**a**) and molar solid composition (**b**) as a function of \(\phi\) (\(\dot{m}_{OC}=const.\)). In terms of FR temperatures, Figure 9b shows that the given approach leads to a successful retaining of FR temperatures above 900 \({}^{\circ}\)C, even for \(\phi\)-values as low as 0.4, due to the transportation of sensible heat by the OC. Moreover, the given approach yields more beneficial results in terms of the process heat balance, which can be seen in Figure 9c. Clearly, autothermal CLG operation is attained for \(\phi=0.37\), which means cold gas efficiencies exceeding 70% can be achieved (see Figure 9d). This is the case as in contrast to the previous approaches (see Sections 3.2 and 3.3), the AR is not operated in air excess during CLG operation, reducing the loss of sensible heat through the AR off-gases. This means that if one would reduce the air feed to the AR to the minimum extent required for full OC re-oxidation for the CLG approach employing inert dilution (see Section 3.3), enhanced cold gas efficiencies could be attained. Nonetheless, the given approach clearly shows advantages in terms of process control due to its flexibility, the possibility of freely selecting a suitable OC material (i.e., no specific limits on \(R_{O}\)), without having to consider material mixtures, and the availability of a catalytically active reduced OC material, instead of an inert solid, cycling through the system. Moreover, the chemical strain on the OC material is reduced as the change in oxidation degree for each redox cycle is lower, when compared to the former approaches, relying on full reduction and oxidation in the FR and AR, respectively (see Figure 3b,c), which should have beneficial effects on the OC lifetime. However, one issue that might arise due to the operation of the AR in an sub-stoichiometric fashion is related to the fact that during operation a fraction of the feedstock char leaves the FR unconverted and hence travels to the AR with the circulating OC material [23, 26, 27]. This so called \"carbon slip\"  
Figure 9: Simulation results for CLG operation through reducing \(\lambda\). OC-to-fuel ratio as a function of the air-to-fuel equivalence ratio \(\lambda\) (**a**). Fuel reactor temperature (**b**), relative net process heat (**c**), and cold gas efficiency (**d**) for different values of \(\lambda\) (\(\dot{m}_{OC}=const.\)). leads to competing reactions between the OC material and the residual char, in case the AR is operated with \(\lambda<1\). Yet, simulations show that in an oxygen deficient atmosphere carbon conversion is favored to OC re-oxidation in chemical equilibrium. Moreover, CO formation shows to be negligible (more details see Appendix C). Due to the fast kinetics of both char conversion and OC re-oxidation, it can be expected that equilibrium-like conditions are attained in the AR and hence all residual char is fully oxidized to CO\({}_{2}\) in the AR. This hypothesis is also supported by chemical looping experiments in small scale fixed bed reactors, during which it was established that in the beginning of the re-oxidation stage oxygen preferentially reacts with deposited carbon before re-oxidizing the OC [64, 63, 21]. Nonetheless, experiments showing that this is also the case in a continuously operated CLG unit and that CO formation is negligible are required to establish that full char conversion without substantial CO formation in the AR can be attained for this approach.