## 3 Results and Discussion
### Reduction of Air-to-Fuel Equivalence Ratio  
To allow for a less restricted material selection and avoid solid inert addition, an alternative strategy to decouple oxygen and heat transport between the AR and FR is required. In order to achieve this, [PERSON] et al. (2018) suggested the deployment of a secondary system in which the OC is pre-reduced before entering the FR. This means that, as shown in Figure 3c, a partially reduced OC enters the FR (\(X_{s}<1\)), thus entailing a lower OC-to-fuel ratio (see Equation (12)). Instead of employing a secondary reactor to accomplish this, one can also operate the AR in a sub-stoichiometric fashion (\(\lambda<1\)), thereby preventing full re-oxidation of the OC in the AR. This means that in order to attain CLG conditions, the amount of air fed into the air reactor can be reduced, while retaining a constant OC circulation. As a consequence, the OC steadily reaches a lower degree of oxidation, hence lowering its oxygen release in the FR, until steady state is reached (more details see Appendix B). This approach has already been pursued in a 140 kW\({}_{\text{th}}\) chemical looping reforming unit, employing methane as a fuel ([PERSON], 2018). The suggested concept becomes more lucid when considering the simulation results shown in Figure 8. Clearly, the amount of fully reduced ilmenite leaving the air and fuel reactor increases when decreasing the air input into the AR for \(\phi<1\) (see Figure 8b). While the same is true for the solids leaving the FR for all presented CLG approaches, a strong increase in the FeTiO\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) content in the AR products is obtained when reducing \(\lambda\) below unity. This can be explained by the fact that the oxygen available in the air reactor is insufficient to fully re-oxidize the OC, signified through an O\({}_{2}\)-free product gas from the AR for \(\phi<1\) (see Figure 8a). Consequently, a pure stream of N\({}_{2}\) containing small concentrations of Argon and other minor compounds is produced in the AR ([PERSON], 2018). Since substantial quantities of OC are cycled through the system in a fully reduced state, they effectively act as an inert, meaning that they transfer sensible heat, but do not participate in the occurring chemical reactions through oxygen release and uptake. However, in practice the reduced OC could potentially function as a catalytic site for tar cracking and methane reforming and favor the formation of syngas ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018), thereby enhancing the process characteristics. Another advantage of the given approach is that an undiluted OC can be employed, which simplifies the required solid-gas and solid-solid (ash-OC-char) separation and the operation of the CLG unit with regard to the fluidization behavior. Moreover, the net heat duty of the process can be tailored promptly and easily through an adjustment of the air flow to the AR, allowing for quick responses to disturbances (e.g., variations in feedstock composition). The impact of the air-to-fuel equivalence ratio (\(\lambda\)) on \(\phi\) is shown in Figure 9a. In CLC mode (\(\lambda>1\)), where full OC oxidation is achieved in the AR (i.e., \(X_{s,AR}=1\)), \(\phi\) assumes a constant value, given by the amount of oxygen which is transported by a fully oxidized OC for a given circulation rate, regardless of the deployed air-to-fuel ratio (see Equation (12)). In contrast, lowering \(\lambda\) to values below unity to attain CLG operation means that \(\phi\) and \(\lambda\) are equal, as the oxygen transport to the FR is limited by the oxygen availability in the AR:  
\[\phi=\left\{ \begin{array}{cc}\lambda&\text{for }\lambda<1\\ \frac{R_{OC}\cdots R_{OC}}{m_{O,batch}}=const.&\text{for }\lambda\geq 1 \end{array} \right.