## 1 Introduction
\[C+CO_{2}\ \rightarrow\ 2\ CO \tag{1}\]  
\[C+H_{2}O\ \rightarrow\ CO+H_{2} \tag{2}\]  
\[Me_{x}O_{y}+CH_{4}\ \rightarrow\ Me_{x}O_{y-1}+2\ H_{2}+CO \tag{3}\]  
\[4 Me_{x}O_{y}+CH_{4}\ \rightarrow\ 4\ Me_{x}O_{y-1}+2\ H_{2}O+CO_{2} \tag{4}\]  
\[Me_{x}O_{y}+CO\ \rightarrow\ Me_{x}O_{y-1}+CO_{2} \tag{5}\]  
\[Me_{x}O_{y}+H_{2}\ \rightarrow\ Me_{x}O_{y-1}+H_{2}O \tag{6}\]  
\[CO+H_{2}O\ \leftrightarrow\ H_{2}+CO_{2} \tag{7}\]  
The required oxygen transport to the FR is facilitated through a repeated regeneration of the OC (see. Equation (8)) in the air reactor (AR) with oxygen contained in the inlet air [15; 20; 24]. Moreover, unconverted char is combusted in the air reactor (see. Equation (9)), leading to a full conversion of the deployed feedstock [23; 25]. \[Me_{x}O_{y-1}+0.5\ O_{2}\ \rightarrow\ Me_{x}O_{y} \tag{8}\]  
\[C+O_{2}\ \rightarrow\ CO_{2} \tag{9}\]  
The latter reaction is generally undesired, as a high carbon conversion is targeted inside the FR, in order to maximize the carbon capture efficiency of the process [23; 26; 27]. In literature, carbon capture efficiencies in the range of 90-99% are reported for CLC [26; 28; 29]. As approximately one third of the carbon contained in the feedstock is transferred into the valorized end-product (e.g., liquid Fischer-Tropsch fuels) in process chains employing CLG for syngas generation, this means that up to 65% of the carbon contained in the feedstock can be captured and stored, constituting negative emissions in case biogenic feedstocks are being employed. Yet, in reality figures falling short of this value can be expected, as a fraction of the feedstock carbon will be lost in the AR in the form of CO\({}_{2}\). Apart from the oxygen transport, the continuous solid circulation between the two reactors provides the required heat transport from the AR, in which the exothermic re-oxidation of the OC occurs, to the FR, where the endothermic gasification reactions take place [15; 19; 23], thus allowing for stable elevated reactor temperatures. CLG not only offers excellent characteristics in terms of feedstock flexibility [24], but is especially well suited for biomass-based feedstocks [30; 31], commonly exhibiting a reactive char and containing a large fraction of volatiles. This means that high char conversions can be achieved through the gasification reaction with steam or CO\({}_{2}\), while volatiles are converted to the desired syngas species through their partial oxidation on the OC surface (see Equation (3)). Furthermore, it is reported that iron containing materials [32; 33; 34; 35] can facilitate the cracking and oxidation of tars, which are known to be formed in significant amounts during biomass gasification [36]. While the role of the gasification agent is similar in CLC and CLG (i.e., char gasification), the oxygen carrier is meant to only partially oxidize the gaseous species in CLG, yielding a raw product gas with a high heating value [23; 37], instead of a heat release from the AR, which is used for heat and power generation in CLC [24; 38; 39]. This shift from CLC to CLG is achieved through lowering the oxygen-to-fuel equivalence ratio in the FR to values below unity. An autothermal CLG process, maximizing the chemical energy contained in the raw syngas without relying on external heating, is obtained when the net heat release from the process equals zero (neglecting heat losses). Although one might hence deduce that the transition from CLC to CLG is straightforward, there are major differences between the two processes. While large OC circulation rates are favorable in CLC, as they allow for a high oxygen availability in the FR, which favors fuel combustion [40; 41; 42; 43] and provide for a large heat transport from the AR to the FR [41; 44; 45], the former is not desired in CLG. Here, the oxygen availability in the FR has to be limited in order to prevent the full oxidation of the employed feedstock.