## 3 Results and Discussion  
### Attaining CLG Behavior  
Generally, shifting from a combustion to a gasification process is achieved through lowering the air/oxygen-to-fuel ratio of the process, thereby decreasing the ratio of fully to partially oxidized gas species leaving the process and hence increasing the heating value of the product gas [6, 53, 54]. Here, the critical parameter is the so called air-to-fuel equivalence ratio given by the ratio of oxygen fed to the AR, \(\dot{m}_{O,AR}\), and the oxygen required for full feedstock combustion, \(\dot{m}_{O,stoich}\):  
\[\lambda=\frac{\dot{m}_{O,AR}}{\dot{m}_{O,stoich}}. \tag{10}\]  
According to this definition, (close to) full combustion of the feedstock is attained for air-to-fuel equivalence ratios larger than unity (\(\lambda>1\)), while gasification processes require sub-stoichiometric oxygen feeding (i.e., \(\lambda<1\)). Due to the dissection of the gasification/combustion reaction into two separate reactors in chemical looping processes, there is no direct contact between the air entering the AR and the fuel entering the FR. Hence, the application of an alternative parameter, the oxygen-carrier-to-fuel equivalence ratio, \(\phi^{\prime}\), relating the amount of oxygen carried by the OC to the FR to the oxygen required for stoichiometric combustion, has been suggested [43]:  
\[\phi^{\prime}=\frac{R_{\text{OC}}\cdot\dot{m}_{OC}}{\dot{m}_{O,stoich}}. \tag{11}\]  
Here, \(R_{\text{OC}}\) denotes the oxygen transport capacity of the given oxygen carrier material. While this parameter accurately relates the two quantities for CLC, where the OC always leaves the AR in a (close to) fully oxidized state, this is not necessarily the case in CLG. Therefore, a slightly altered oxygen-carrier-to-fuel equivalence ratio, \(\phi\), considering the possibility of a partially reduced OC leaving the AR, has been proposed for gasification applications [35]:  
\[\phi=\frac{R_{\text{OC}}\cdot\dot{m}_{OC}\cdot X_{s,AR}}{\dot{m}_{O,stoich}}, \tag{12}\]  
where \(X_{s,AR}\) signifies the oxidation degree of the oxygen carrier at the AR outlet, given by [24, 35]:  
\[X_{s,AR}=\frac{m_{OC,AR}-m_{OC,rd}}{R_{\text{OC}}\cdot m_{OC,ax}}. \tag{13}\]  
Here, \(m_{OC,rd}\) and \(m_{OC,ax}\) are the mass of an OC sample in a fully reduced and oxidized state respectively, while \(m_{OC,AR}\) is the mass of the OC sample leaving the AR. For ilmenite the fully reduced oxygen carrier is approximated by FeTiO\({}_{3}\), the fully oxidized state is approximated by Fe\({}_{2}\)O\({}_{3}\) + 2 TiO\({}_{2}\), and Fe\({}_{3}\)O\({}_{4}\) + 3 TiO\({}_{2}\) denotes an intermediate redox state (\(X_{s}=0.67\)). In order to assess how \(\lambda\) and \(\phi\) have to be adjusted in order to obtain an efficient CLG process, one should first assess the general impact of these two parameters on the process. Due to the relative fast kinetics of the OC re-oxidation [55, 56, 57], the oxygen carrier is often assumed to leave the AR in a (close to) fully oxidized state for \(\lambda>1\) in chemical looping processes. In contrast, sub-stoichiometric air-to-fuel equivalence ratios (\(\lambda<1\)) only lead to a partial re-oxidation of the OC in the AR. Following the same logic, the OC material can be assumed to leave the FR in a (close to) fully reduced state in case \(\phi<1\), whereas partial reduction is attained for \(\phi>1\). From these deductions, it becomes clear that \"standard\" CLC operation is attained for \(\lambda>1\) and \(\phi>1\), [42, 43]. Here, a highly oxidized OC leaves the AR, before being partially reduced in the FR, which is illustrated in Figure 3a. When targeting pronounced syngas formation, the oxygen release in the FR has to be limited, so that full feedstock oxidation is prevented [35,52]. The most obvious avenue that can be pursued to achieve this is lowering \(\phi\) below unity. When doing so, the employed air-to-fuel equivalence ratio \(\lambda\) determines how much oxygen is transported between the two reactors per gram of OC.