## 3 Results and Discussion
### Optimizing CLG Efficiency
However, sufficient steam availability clearly is a prerequisite in CLG, when targeting large char conversions and hence carbon capture efficiencies. Another possible measure to enhance CGEs are variations in the circulation rate of the OC, which is shown in Figure 10c,d. Clearly, larger solid circulation rates enhance the heat transport between the reactors and hence entail higher FR temperatures [16]. However, due to material attrition, solid loss, which necessitates continuous make-up feeding, also scales with the circulation rate. As shown in Figure 10d, the effect of this material loss on the process heat balance is comparatively small, thus its effect on the cold gas efficiency is low. However, the model predicts an increase in FR temperatures  
Figure 10: Net heat release and cold gas efficiency for CLC/CLG process as a function of the air to fuel equivalence ratio for different steam to biomass ratios (**a,b**), OC circulation rates (**c,d**), gas inlet temperatures (**e,f**), and air reactor temperatures (**g,h**). Circles mark the cold gas efficiency for autothermal CLG operation (\(\dot{m}_{\text{OC}}=const.\), so \(\phi=\lambda\) for \(\lambda<1\) and \(\phi=const.>1\) for \(\lambda>1\)). from 892 to 951 \({}^{\circ}\)C, when increasing the circulation rate from 6.3 to 10.6 t/h. This means that generally, large solid circulation rates are desired in CLG units, as large FR temperatures are beneficial for volatile and char conversion [20; 23; 37]. Yet, it has to be kept in mind that the solid circulation in dual fluidized bed systems requires solid entrainment from the fluidized bed riser, which can be increased through an increase in gas velocities (i.e., increase in steam/biomass ratio), smaller particle diameters or smaller reactor diameters [16]. Moreover, intensified solid circulation also increases the occurrence of a \"carbon slip\" to the AR, due to the lower residence times of the char particles in the FR [27; 28; 70]. This means that the OC circulation rate can only be varied within a given range. Increasing the inlet temperature of the steam and air entering the FR and AR respectively, thereby decreasing the heat demand for heating up of the gases inside the reactor, is a further strategy to boost cold gas efficiencies. As shown in Figure 10e, this approach allows for a reduction of the air-to-fuel equivalence ratio from 0.38 to 0.34 when increasing inlet temperatures from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C. Hence, maximizing inlet gas temperatures through heat recuperation is a key task in CLG in order to optimize the process efficiency, which is illustrated by the increase in the CGE from 68.3 to 76.5%, when increasing gas inlet temperatures from 300 to 600 \({}^{\circ}\)C (see Figure 10f). Due to the absence of corrosive compounds and the high process temperatures, the hot off-gases leaving the AR are ideal for steam generation and heat recuperation. On the other hand, special syngas coolers are being used to recuperate sensible heat from syngas streams for steam production [71; 72; 73], highlighting that efficient gas pre-heating using heat from process off-gases is possible in CLG. Furthermore, variations in the AR temperatures can be considered, in order to enhance CLG process efficiencies. Generally speaking, a reduction in average process temperatures is beneficial for the process heat release, as pre-heating demands for all educts (i.e., inlet gases & feedstock material) are being reduced as a consequence, thus allowing for intensified heat extraction for a given air-to-fuel ratio (see Figure 10g). As visible in Figure 10h, a slight increase in the CGE by 2.4 percentage points can be attained for autothermal CLG operation when lowering AR temperatures from 1050 to 1000 \({}^{\circ}\)C. Yet, it has to be kept in mind that in chemical looping processes, air and fuel reactor temperatures are coupled, which means that a drop in FR temperatures is an inevitable effect of reduced AR temperatures.