## 2 Modelling Methods
### Boundary Conditions  
For all subsequent simulations, the biomass input was selected in such a way, that the thermal load, \(P_{thr}\), of the chemical looping gasifier amounted to 1 MW. In terms of the circulating solid materials, the deployed oxygen carrier material is ilmenite, for which is has been established that the major redox stages are FeO + TiO\({}_{2}\), Fe\({}_{3}\)O\({}_{4}\), TiO\({}_{2}\) and Fe\({}_{2}\)TiO\({}_{5}\)[51]. These redox stages were modelled as FeTiO\({}_{3}\) (for FeO + TiO\({}_{2}\)), Fe\({}_{3}\)O\({}_{4}\), TiO\({}_{2}\), and Fe\({}_{2}\)O\({}_{3}\) + TiO\({}_{2}\) (for Fe\({}_{2}\)TiO\({}_{5}\)). Deeper redox stages (e.g., FeO) were also considered in the process model, yet were not found to be formed in notable amounts. The inert solid sand was modelled through pure SiO\({}_{2}\). The FR and AR are operated under atmospheric pressure. Moreover, the air reactor temperature was set to 1050 \({}^{\circ}\)C, if not stated otherwise. The fuel reactor temperature results from the energy balance of the process, requiring that both reactors are in heat balance (\(\dot{Q}_{FR}=0\), \(\dot{Q}_{AR}\geq 0\)). As the kinetic syngas inhibition of char gasification reactions [8, 12] is not considered in the RGIBBS equilibrium calculation, full char conversion is attained inside the FR for all temperatures considered in this study. Although this simplification signifies a deviation from reality, it does not impact the general inferences which will be elaborated on hereinafter. For the steam to biomass ratio in the FR a value of 0.9, reported for a 2-4 MW\({}_{\text{th}}\) chemical looping gasifier in literature [52], was selected if not stated otherwise. During CLC/CLG operation CO\({}_{2}\) is required for fuel feeding and inerting. This stream of CO\({}_{2}\), entering the fuel reactor, was selected in such a way that the CO\({}_{2}\) to biomass ratio amounts to 0.2, to take into account that the CO\({}_{2}\) input through the feeding section increases with increased thermal load. The two remaining process variables, the air mass flow entering the AR and the circulating oxygen carrier mass, were adjusted in such a way that autothermal CLG operation was achieved. A summary of all boundary conditions is given in Table 1 in Appendix A.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ultimate Analysis** & **wt-\%** & **Proximate Analysis** & **wt-\%** \\ \hline C (d.a.f.) & 50.8 & Moisture & 6.5 \\ H (d.a.f.) & 6 & Ash (d.b.) & 0.7 \\ N (d.a.f.) & 0.07 & Volatile matter (d.b.) & 85.1 \\ O (d.a.f.) & 43.2 & Fixed carbon (d.b.) & 14.2 \\ S (d.a.f.) & 0.008 & & \\ Cl (d.a.f.) & 0.006 & & \\ \hline Net calorific value [M]/kg] & 17.96 & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the Ultimate and Proximate analysis for industrial wood pellets.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Component** & **wt-\%** & **Component** & **wt-\%** \\ \hline ASH & 0.65 & H\({}_{2}\)O & 14.06 \\ CO & 55.20 & N\({}_{2}\) & 0.06 \\ C & 11.92 & CO\({}_{2}\) & 3.11 \\ CH\({}_{4}\) & 13.55 & H\({}_{2}\)S & 0.01 \\ H\({}_{2}\) & 1.43 & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mass yields [wt-%] for DECOMP Aspen Plzus® block for industrial wood pellets according to pyrolysis model of [PERSON] et al. [48] (T = 900 \({}^{\circ}\)C).