**1. Introduction**

The reduction of greenhouse gas emissions (GHGE) in order to reach the unilateral goals agreed upon in the UNFCCC Paris Agreement is one of the major challenges of civilization in the 21st century. While notable advances in the energy sector have been achieved in recent years [1,2], the de-carbonization of the transport sector, which is responsible for almost one quarter of the European GHGE emissions [3] and consumes 36% of the global final energy [1], signifies a key issue on the path to a closed carbon cycle. Especially the replacement of conventional fuels in the heavy freight transport and aviation industry, where electrification is currently not viable, remains a major hurdle. When considering the European Union's Renewable Energy Directive (RED II) [4], which set a target of a share of 14% renewable energy in the transport sector by 2030, while at the same time alleviating negative impacts on food availability and prices, it is clear that significant advances in renewable fuel generation are required.

The production of so-called advanced or second-generation biofuels through thermochemical conversion of biomass-based residues is an auspicious pathway to achieve these goals. Gasification is a mature thermochemical biomass conversion process, although its primary use is the generation of heat and electricity, while the synthesis of advanced biofuels through the gasification route has not been implemented in an industrial scale, ye<sup>t</sup> [5].

Commonly, biomass gasification is achieved through utilizing air or pure oxygen in the gasifier. Albeit, pure oxygen is typically used in gasification processes embedded in biomass-to-biofuel process chains, since a nitrogen-free, high calorific value syngas is required for fuel synthesis [6]. The provision of this oxygen requires an air separation unit (ASU), which is associated with high capital and operational costs, hence adversely a ffecting the energetic plant e fficiency and process economics [6,7]. Alternatively, steam [8–10] or carbon dioxide [10–12] can be deployed as the gasification medium. Yet, either of the two su ffers from slow gasification kinetics [6,13,14] and strong process endothermicity [6,15], limiting the process e fficiency. To circumvent this, the dual fluidized bed gasification (DFBG) technology achieves feedstock gasification in two connected reactors; a gasifier in which steam gasification of the deployed feedstock is attained, and a combustor in which the residual char is combusted facilitating full char conversion and the provision of heat, which is transported to the gasifier using an inert circulating bed material [16–18].

A similar gasification concept allowing for decent fuel conversions, without requiring an ASU is the chemical looping gasification (CLG) process, where biomass gasification is also carried out in two separate reactors (see Figure 1) [15,19–22]. Just as the related chemical looping combustion (CLC) process, CLG is realized using two coupled fluidized bed reactors, in order to attain good heat and mass transport characteristics [21,23,24]. Here, steam or carbon dioxide provide bed fluidization and gasification (see Equations (1) and (2)) of the feedstock in the fuel reactor (FR) [15,24]. Additional oxygen for the partial (see Equation (3)) or full (see Equations (4)–(6)) oxidation of gaseous hydrocarbon species, enhancing gasification kinetics and reducing the process endothermicity, is supplied through a circulating oxygen carrier (OC, MexOy) [19,21,24]. Furthermore, the homogeneous water gas shift (WGS) reaction (Equation (7)) takes place inside the gas phase.

$$\text{C} + \text{CO}\_2 \rightarrow \text{2CO} \tag{1}$$

$$\text{C} + H\_2\text{O} \rightarrow \text{CO} + H\_2\tag{2}$$

$$\rm{MexO\_3} + \rm{CH\_4} \rightarrow \rm{MexO\_{9^{-1}}} + \rm{2} \,\rm{H\_2} + \rm{CO} \tag{3}$$

$$4\text{Me}\_{\text{x}}\text{O}\_{\text{y}} + \text{CH}\_{4} \rightarrow 4\text{Me}\_{\text{x}}\text{O}\_{\text{y}-1} + 2\text{H}\_{2}\text{O} + \text{CO}\_{2} \tag{4}$$

$$\rm{Me}\_{x}O\_{y} + \rm{CO} \rightarrow \rm{Me}\_{x}O\_{y-1} + \rm{CO}\_{2} \tag{5}$$

$$\rm{Me}\_{x}O\_{y} + \rm{H}\_{2} \rightarrow \rm{Me}\_{x}O\_{y-1} + \rm{H}\_{2}O \tag{6}$$

$$\text{CO} + \text{H}\_2\text{O} \iff \text{H}\_2 + \text{CO}\_2 \tag{7}$$

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].

$$\rm{M}\_{x}O\_{y-1} + 0.5 \, O\_{2} \quad \rightarrow \, Me\_{x}O\_{y} \tag{8}$$

$$\text{C} + \text{O}\_2 \rightarrow \text{CO}\_2 \tag{9}$$

The latter reaction is generally undesired, as a high carbon conversion is targeted inside the FR, in order to maximize the carbon capture e fficiency of the process [23,26,27]. In literature, carbon capture e fficiencies 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 CO2.

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.

**Figure 1.** Schematic of chemical looping gasification (CLG) process.

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 CO2, 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 [32–35] 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 agen<sup>t</sup> 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 [40–43] 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. However, even more so than in CLC, CLG requires large heat transportation rates from the AR and FR due to the less pronounced occurrence of full oxidation reactions (Equations (4)–(6)), at the cost of highly endothermic partial oxidation reactions (Equation (3)) in the FR. This leads to a fundamental challenge in terms of process control, as both, heat and oxygen transfer between the two reactors, have to be controlled independently in order to attain an autothermal CLG process. Initial advances to reach this target were made by Ge et al. [37], diluting an active OC material with an inert, thus obtaining stable reactor temperatures for a lab-scale CLG unit. Yet, due to the significance of this inherent challenge, an in-depth analysis of this issue is required. Therefore, this work takes a holistic approach to this matter, employing process simulations in order to establish suitable process control measures to attain an autothermal CLG process. In the following, the developed process model will

be introduced in Section 2, before general process control and optimization strategies are presented and discussed in detail in Section 3. To round o ff these elucidations, the most crucial findings and an outlook on future research topics are given in Section 4 of this article.
