## 5 Results and Discussion
### Calibration
The amount of heat loss is determined by the temperature difference between the boundary of the reactive bed and the vapor vessel, and by the heat loss coefficient. The temperature distribution of the outer vessel is plotted in purple in Figure 4. It furthermore contains the temperatures at the boundary of the reactive bed, \(T(y=y_{max})\), determined by a simulation with the numerical model at optimized parameters. The temperature differences between bed and vessel amount to more than 100 K at early times of reaction. At the end of the experiment, they still amount to 20 K. Due to the big temperature differences, \(1.34\times 10^{3}\) kJ of the overall released energy of reaction (\(2.24\times 10^{3}\) kJ) is lost through the reactor surface and only \(0.93\times 10^{3}\) kJ is transferred to the HTF channel. Temperature measurements in the HTF channel to verify these results are not available. The heat-transport mechanisms in form of heat loss, free convection, conduction, and radiation overlap. Free convection, however, appears only at the upper boundary of the symmetric reactor setup, as outlined in Section 2.2.2. If this mechanism dominates the heat loss, the temperatures in the reactive bed below and above the HTF channel are different and the symmetry condition is not fulfilled. In the experiment, all the thermocouples are installed in the top part of the reactive bed. Thus, the experiment does not provide any information on whether or not the two parts behave symmetrically. The reduced simulation domain provides a considerable simulation speed-up. The parameters k\({}_{R,1}\) and k\({}_{R,2}\) are linked via the reaction rate. Both a small value of k\({}_{R,1}\) and a large value of k\({}_{R,2}\) limit the reaction rate and lead to a flat slope in the temperature profiles. The optimal solution is found above the expected ranges for both parameters with k\({}_{R,1}=20.9\) and k\({}_{R,2}=5.58\). Whereas k\({}_{R,1}=20.9\) is only slightly larger than its upper value at 20, k\({}_{R,2}=5.58\) is clearly above its range. At initial states of conversion, k\({}_Experimental results and simulation results agree in this regard as shown in Figure 5d. The conversion in the experiment stagnates at 88% and the maximum conversion of the simulation reaches 95% after 3600 s of reaction. A large error was assigned to the experimental values at final stages in order to not exclude complete conversion. [PERSON] et al. [32] performed 33 cycles of charging and discharging with the same reactor filling. They reach maximal values of conversion between 90-95% for the duration of the experiments and state that the final 5-10% need significantly more time [31]. Additionally, they detect the formation of agglomerates. [PERSON] et al. [57] show, that the surrounding of the agglomerates form fracture-like preferential flow paths with higher permeability compared to the agglomerates. [PERSON] et al. [23] state for a directly operated fixed-bed reactor with CaO/Ca(OH)\({}_{2}\), the last 20% of solid bulk get converted slower as soon as agglomerates have been formed. In the reactive bed of the indirectly operated reactor concept, pressure gradients are the result of water consumption due to the chemical reaction. Differences in permeability in the agglomerates and their surroundings result in inhomogeneous pressure gradients. The reactive material close to the preferential flow paths is well supplied with reaction fluid and reacts fast. Less water reaches the solid material within the agglomerates. Here, in a self-enhancing mechanism, a smaller pressure gradient deteriorates the supply of water vapor even further and the reactive solid material reacts slowly. This retardation effect of the agglomerates becomes noticeable with increasing conversion, when the material close to the preferential flow paths has already reacted. The reduced heat production due to the lower reaction rate is overlapped by heat conduction and the slope of the temporal temperature profile is flattened.