## 3 Results and Discussion
### Reduction of OC Circulation  
One approach to obtain CLG behavior, which has been suggested by [PERSON] et al. (1998), is reducing the amount of OC cycled through the system (\(\dot{m}_{OC}\)), hence reducing \(\phi\). This approach can be deduced directly from Equation (12). Due to the resulting lower oxygen transport to the FR, syngas formation is favored, as less oxygen for full oxidation of the feedstock is provided by the OC. The simulation results for this approach are given in Figure 4. When considering the gas composition (Figure 4a) of the streams leaving the air and fuel reactor, various trends are visible. As expected, the syngas content in the gaseous FR products increases with decreasing OC circulation rate, which can directly be attributed to the lower oxygen/fuel ratio in the FR. Consequently, steam and CO\({}_{2}\) formation decrease. Yet, it has to be noted that substantial syngas concentrations are only attained for \(\phi\) < 1, which requires significant reductions in the OC circulation rate, when compared to CLC, where OC-to-fuel equivalence ratios as high as 8 ([PERSON] et al., 1998) and 25 ([PERSON] et al., 1998) are reported in literature for solid and gaseous fuels, respectively. For the gas concentrations leaving the AR, a strong impact of \(\phi\) on the effluent oxygen is visible. As the inlet air mass flow was not varied (\(\lambda\) = 1.2), this observation is clear, as less O\({}_{2}\) is removed from the gas stream due to the lower OC circulation for \(\phi\) < 1. Furthermore, the CO\({}_{2}\) content in the AR product is predicted to be insignificant, indicating a complete char conversion, which is expected in chemical equilibrium. When considering Figure 4b, showing the solid composition after the fuel and air reactor, it can be seen that the OC leaves the AR and FR in a fully oxidized (Fe\({}_{2}\)O\({}_{3}\) + TiO\({}_{2}\)) and reduced (FeTiO\({}_{3}\)) state, respectively for \(\phi\) < 1, whereas the OC is only partially reduced (indicated through the presence of Fe\({}_{3}\)O\({}_{4}\)) in the FR in case \(\phi\) exceeds unity. Hence, the fraction of FeTiO\({}_{3}\) leaving the FR strongly increases with decreasing OC circulation, signifying a higher degree of reduction of the OC, due to the lower oxygen availability. As expected one consequently obtains chemical looping combustion behavior (see Figure 3a) for oxygen-carrier-to-fuel equivalence ratios greater than unity (\(\phi\) > 1), whereas chemical looping gasification behavior (see Figure 3b) is attained for \(\phi\) < 1. Figure 4: Simulation results for CLG operation through reduced oxygen carrier (OC) circulation. Dry molar gas composition (**a**) and molar solid composition (**b**) as a function of \(\phi\) for varying OC circulation rates (\(\lambda\) = 1.2). Based on these findings, one can conclude that a successful shifting from CLC to CLG for a given air-to-fuel ratio can be attained through a reduction in the OC circulation, which can also be seen in Figure 5a, showing a linear dependence between the two parameters. This means that for a change of \(\phi\) from 1.0 to 0.5, the OC circulation rate has to be halved. However, lower solid circulation rates also result in a proportional decrease in the heat transport from the AR to the FR and hence a drop-off in FR temperatures [35; 58]. While a moderate decrease in fuel reactor temperatures with decreasing OC circulation rate is visible for \(\phi>1\), for which complete feedstock conversion is attained in the FR, this decrease becomes more prominent for \(\phi<1\), where gasification reactions in the FR are dominant, hence increasing the endothermicity of reactions occurring in the FR. Consequently, FR temperatures fall below 800 \({}^{\circ}\)C for \(\phi<0.5\), where the availability of circulating OC material for sensible heat transport between the FR and AR is halved, when compared to \(\phi=1\) and more importantly the syngas content in the FR products is significant (see Figure 4a).