## 2 Modelling Methods  
### Description of the Process Model  
The deployed Aspen Plus(tm) model, shown in Figure 2, is largely adopted from a previous study by [PERSON] et al. [25]. Here, the chemical reactions occurring in the AR and FR are modelled in two separate reactors, whereas gas-solid and solid-solid separation is achieved through cyclones and separators, respectively. In order to reduce model complexity, the AR and FR were modelled as equilibrium RGIBBS reactors in this work, as this simplification allows for a basic description of the most crucial phenomena required for process control and obviates the necessity of accurate kinetic data. To account for the solid circulation in chemical looping processes, a constant mass stream of solids continuously cycles through the system (OCR-TOAR/OCO-TOFR), after being added to the system after initiation of the simulation (INIT).  
For completeness and comprehensibility reasons, all components and streams are briefly described in the following:  
* Prior to any calculation, an initial solid mass flow is given into the system (INIT), to model the circulating solid OC mass. Instead of estimating the actual solid loss, the approach of [PERSON] et al. [25], setting the total OC loss (OCLOSS) to 1% of the circulating mass to achieve fast flowsheet conversion, was adopted. The same amount of fresh solids was constantly fed to the AR (MAKEUP), to achieve constant solid circulation.
* For both reactors, cyclones are employed to achieve solid-gas separation. The FR products are separated into a gas (FRGAS) and solid (SOLTOSEP) stream, via CYCL1 (separation efficiency 100%). Similarly, the AR products are separated into a gas (ARFLUE) and solid stream (OCO-TOFR) in CYLC2 (separation efficiency 100%).
* All streams entering the process are fed at ambient temperature (\(T_{0}\) = 25 \({}^{\circ}\)C), except for the stream STEAM, which is fed as saturated steam (120 \({}^{\circ}\)C).  
Figure 2: Flow sheet of the Aspen Plus(tm) CLG process model.  
* The steam and the air entering the FR/AR are preheated to a designated inlet temperature (\(T_{air,AR}\), \(T_{H2O,FR}\)). If not stated otherwise, the inlet temperature of both streams (STEAM, AIR) when entering the FR/AR is set to 400 \({}^{\circ}\)C.
* As Aspen Plus(tm) is not equipped to handle solid fuels, the biomass feedstock (FUEL) is fed to the decomposer (DECOMP), where it is decomposed into its pyrolysis products (DEVOLAD). The heat of pyrolysis (Q-DECOMP) is transferred to the fuel reactor. A detailed description of the decomposer block is given in Section 2.2.
* The pyrolysis products (DEVOLAD), the gasification agent (STEAMX), the OC recycled from the AR (OCO-TOF), and the CO\({}_{2}\) required for solid feeding and loop seal fluidization (SCREWFLU) are mixed (FRIN) before entering the fuel reactor.
* Subsequently, the educs entering the fuel reactor (FR) are converted into reaction products according to the chemical equilibrium at the given boundary conditions (T\({}_{\text{FR}}\), P\({}_{\text{FR}}\) = 1 atm).
* The solids leaving CYCL1 are separated into the OC fed to the AR (OCR-TOAR) and a stream containing carbon and ash (SOL) in the solids separation (SOLSEP). This separation signifies the removal of bed material (i.e., OC, ash and unconverted feedstock) from the FR via shuicing during operation. Additionally, a fraction of the oxygen carrier material is removed from the system (OCLOSS), to model OC losses via shuicing and attrition.
* The OC makeup stream (MAKEUP) and the inlet air (AIRX) are mixed (ARIN) before being fed to the AR.
* Inside the air reactor (AR) the reduced OC and the unreacted char react with the oxygen contained in the air according to the chemical equilibrium at the given boundary conditions (T\({}_{\text{AR}}\), P\({}_{\text{AR}}\) = 1 atm).