## 3 Results and Discussion
### Optimizing CLG Efficiency  
In the previous section it was established that OC-to-fuel equivalence ratios smaller than unity are required in the FR. Moreover, it was demonstrated only when decoupling heat and oxygen transfer between the AR and FR, \(\phi<1\) and FR temperatures above 850 \({}^{\circ}\)C can be obtained for an autothermal CLG process. Thermodynamically speaking, it does not make a difference how this decoupling of heat and oxygen transport is attained, which is why the following considerations will focus on the CLG approach presented in Section 3.4, employing a reduction in the air-to-fuel equivalence ratio to achieve CLG behavior. When optimizing gasification processes, the trade-off between maximizing the carbon conversion in the gasifier and at the same time attaining high cold gas efficiencies is at the core of many optimization strategies. This is also the case in CLG, where \(\eta_{CGE}=1\) and complete char conversion is desired, yet not attainable. While large carbon capture efficiencies are obtained in cases where the char is gasified in the fuel reactor to a large extent, which is promoted by high FR temperatures [20, 37, 23], large steam/biomass ratios [20, 37], and high OC-to-fuel ratios (if sufficient char residence times are provided) [27, 52], cold gas efficiencies are maximized by the minimization of the oxidation of H\({}_{2}\) and hydrocarbons in the FR [35]. Although full oxidation of syngas in the FR should be limited to achieve large CGEs, formation of steam and CO\({}_{2}\) in the FR is required to a certain extent to obtain autothermal CLG conditions. The degree to which this formation of fully oxidized gas species is required is determined by the criterion of the CLG process being in heat balance (\(\dot{Q}_{net}=0\)). This means that the heat release attained through full feedstock oxidation has to balance the heat demand of pre-heating of all inlet streams to the given reactor temperatures, the heat of reaction for endothermic gasification reactions, and the heat losses of the CLG unit. This has also been shown in the previous sections where despite assuming chemical equilibrium (i.e., full feedstock conversion), cold gas efficiencies deviating strongly from unity were obtained for autothermal boundary conditions (see Figures 5, 7 and 9). Therefore, one approach to enhance the cold gas efficiency in CLG is a reduction in the inlet gas flows entering the air and fuel reactor. Since the air mass flow entering the AR is required to control \(\phi\), this leaves the steam mass flow entering the FR as a free variable which can be altered to enhance cold gas efficiencies. The effect of a reduction in the steam to biomass ratio on the net heat release of the process is shown in Figure 10a. It is visible that, with a decreasing steam to biomass ratio, the air-to-fuel equivalence ratio for which an autothermal process is attained decreases. Due to the direct correlation between the oxygen availability and cold gas efficiency in CLG (see Figure 10b), this also means that the CGE obtained for autothermal operation increases with decreasing steam/biomass ratio, so that the CGE is raised from 72.5 to 77.1%, when decreasing the steam/biomass ratio from 0.9 to 0.3. However,it is obvious that the reduction of the steam to biomass ratio would also entail a drop in carbon capture efficiencies of the process, as less steam is available for char gasification and the kinetic inhibition effect of syngas increases with decreasing steam concentrations (entailing larger syngas partial pressures) in the FR [8,12,67,68]. This becomes most obvious for a steam to biomass ratio of 0, for which char conversions in the FR would be diminutive in a real gasifier, due to the slow kinetics of heterogeneous solid-solid OC-feedback reactions [67, 68, 69]. As this drop in char conversion is not predicted by the equilibrium model, the negative effect on process efficiency with decreasing steam to biomass ratio cannot be evaluated in this study.