### \(H_{2}\)S and COS Concentration
#### 3.3.2 Variation of Temperature \(\vartheta\)  
Figure 12 shows the H\({}_{2}\)S concentration and Figure 13 the COS concentration at different temperatures for cases with the reference bed material, sewage sludge ash, but also for runs with limestone additive. Additionally, the H\({}_{2}\)S and COS concentrations in the thermochemical equilibrium, calculated by the software FactSage(r) as described Section 2.6 are shown. A strong dependence of the concentration of the two sulfur species on the temperature could be observed in the experiment as well as in the equilibrium calculations. At low temperatures high concentrations of up to 7653 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) for H\({}_{2}\)S and up to 215 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) for COS were measured with sewage sludge ash as bed material. With increasing temperatures the H\({}_{2}\)S and COS concentrations were decreasing until a minimum was reached before the concentration increased again at higher temperatures. This is related to the facts, that sewage sludge ash contains CaO which is can capture sulfur through reactions Equations (5) and (6) which are exothermic and therefore favored at lower temperatures. However the calcination reaction (Equation (4)) is favored at higher temperatures. The overlay of calcination and sulfur capture leads to the here seen temperature dependency. Figure 11: Measured and equilibrium H\({}_{2}\)S and COS concentrations on dry and N\({}_{2}\)-free basis at different oxygen ratios \(n_{\rm O2}\) and constant \(\vartheta\). For the case with limestone additive, a similar trend was observed but consequently lower concentrations were measured. The equilibrium has a similar trend as the experiments, but shows for all cases lower concentrations, which is in agreement with the fact that the equilibrium predicts optimal sulfur capture and thus the lowest possible concentration. The lowest measured concentration without limestone additive was 1855 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) at 778 \({}^{\circ}\)C and with limestone additive the lowest concentration was 624 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) at 770 \({}^{\circ}\)C. With the limestone additive the H\({}_{2}\)S concentration could thus be reduced by around factor 3 at these temperatures. The absolute equilibrium minimum is was calculated for 740 \({}^{\circ}\)C with 243 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\). For COS similar behavior was observed, at 770 \({}^{\circ}\)C-778 \({}^{\circ}\)C the concentration for sewage sludge ash bed is 40 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\), with limestone additive it is reduced to 13 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\), also by factor 3. The COS minimum equilibrium concentration was calculated for 740 \({}^{\circ}\)C with 6 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\). As said, with further increasing temperatures the measured and equilibrium H\({}_{2}\)S and COS concentrations are rising again. At the reference temperature of 840 \({}^{\circ}\)C, which is also a very common temperature to operate sewage sludge incinerators and gasifiers [62], slightly higher concentrations of H\({}_{2}\)S with 1873 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) and COS of 59 \(\times\) 10\({}^{-6}\) m\({}^{3}\) m\({}^{-3}\) were observed without additive. Again, the limestone additive brings a reduction. It was noticed that, with limestone additive, the COS concentrations can be brought very close to the equilibrium for all investigated temperatures above 750 \({}^{\circ}\)C, while for H\({}_{2}\)S, the concentrations with additive still have some distance to the equilibrium. In literature [34; 43], similar trends of H\({}_{2}\)S concentrations over temperature in the presence of CaO were also reported in experiments and equilibrium calculations. Figure 12: Measured and equilibrium H\({}_{2}\)S concentration on dry and N\({}_{2}\)-free basis at different temperatures \(\vartheta\).