**Nomenclature**


#### **Appendix A. Boundary Conditions for CLG Process Model**

A summary of all model boundary conditions employed for the simulations presented in Section 3.2, Section 3.3 and Section 3.4 is given in Table A1.


**Table A1.** Boundary conditions for 1 MWth CLC/CLG process model for different CLG approaches.

\* CLG approach I: Reduction in OC circulation rate (see Section 3.2), CLG approach 2: Dilution with solid inert (see Section 3.3), CLG approach 3: Reduction of air inlet into AR (see Section 3.4).

#### **Appendix B. Shifting from CLC to CLG Operation through Variations in the Air-to-Fuel Equivalence Ratio**

As described in Section 3.4, the oxygen availability in the FR is solely dependent on the circulation rate of the OC and the oxygen transport capability of the OC material (*RO*), when operating the AR in air excess (λ > 1) in CLC, as the OC material is fully oxidized inside the AR. When subsequently reducing λ to values below unity from a steady state CLC operating point (see Figure A1a), the limited air availability in the AR leads to a transient phase during which the OC undergoes a continuous drop in the oxidation degree with each redox cycle, as more oxygen is consumed in the FR (combustion conditions) than is being supplied in the AR. As soon as the oxidation degree in the FR approaches 0, the oxygen availability in the subsequent redox cycle is determined by the oxygen supply in the AR. Hence, φ is equal to λ from this point onwards. As indicated in Figure A1a, this means that steady state CLG conditions are attained as a consequence. When on the other hand starting off with steady state CLG operation (λ < 1) before increasing λ beyond unity, the OC undergoes a transient phase during which its oxidation degree increases with each redox cycle, since more oxygen is supplied in the AR than is being consumed in the FR. As soon as the amount of oxygen transported by the OC is sufficient to fully oxidize the deployed feedstock, CLC conditions are attained. It has to be noted that this can be the case before steady state is reached (see Figure A1b). This means that despite the described discontinuity in the relation between λ and φ for λ = 1, a rapid switch in the OC-to-fuel ratio will not occur during operation, as the transition from CLC to CLG or vice versa will occur smoothly via a transient phase during which the oxidation degree of the OC adapts to the newly set boundary conditions.

**Figure A1.** Progression of the OC oxidation degree when shifting from CLC (λ > 1) to CLG (λ < 1) mode through variations of the air-to-fuel equivalence ratio. (**a**) Shift from CLC to CLG, (**b**) shift from CLG to CLG.

#### **Appendix C. Char Conversion in an Sub-Stoichiometrically Operated AR**

In order to establish how a mixture of unconverted char and a fully reduced OC behaves in an sub-stoichiometric oxygen containing atmosphere in the AR, a mixture of char (5 mole-%) and a reduced

OC (78 mole-% FeTiO3, 6 mole-% Fe2O3 and 11 mole-% TiO2) were reacted with different amounts of air in an RGIBBS reactor of varying temperature (900–1100 ◦C). The results for an AR temperature of 1000 ◦C are shown in Figure A2. It is visible that char conversion occurs prior to OC re-oxidation, as the char fraction is zero regardless of the deployed air-to-fuel ratio. Moreover, the chemical equilibrium predicts a further reduction of the OC in case the amount of oxygen contained in the inlet air is insufficient for char conversion. Certainly, this behavior can only be observed in case of sufficiently long reaction times (rarely given in a fluidized bed), since solid-solid reactions between OC and char particles are known to exhibit slow kinetics [67–69]. This means that when attempting full char conversion, the inlet air entering the AR has to be sufficient to provide full carbon combustion. When this is the case, it can be assumed that full char conversion is attained inside the AR. In terms of the CO content at the reactor outlet it can be seen that full CO conversion to CO2 is achieved regardless of the utilized air-to-fuel ratio, indicated by negligible concentrations of CO in the AR outlet (see Figure A2).

**Figure A2.** Solid and gas composition at chemical equilibrium for TAR = 1000 ◦C at varying air-to-fuel equivalence ratio λ (Inlet solid composition: 78 mole-% FeTiO3, 6 mole-% Fe2O3 and 11 mole-% TiO2).
