# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 2 Materials and Methods
### Consumption of Biogas by the Recovery Boiler and Engines  
Figure 3a shows the operating scheme of the biogas valorisation section of the batch cycle, where live steam injections take place. The contribution of the thermal power from engine exhaust gases to the recovery boiler is also represented. Biogas is consumed by the boiler and engines. Thermal power from the exhaust gases of engines is recovered in the boiler. Figure 3b shows the period of no live steam injection; Figure 3c shows the average cycle. The heat recovery zone has a three-way valve that operates with a set-point defined by steam pressure. When live steam approaches the set-point pressure during the no-injection stage, this valve is closed and all gases are sent to the atmosphere. In these figures, the continuous lines indicate \"operation\" and dashed lines \"no operation\" of the equipment at the moment represented. The system, which is composed of digester and biogas engines, always operates continuously, whereas the gasometer-boiler system operates under batch mode. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Scenario 1** & **Scenario 2** & **Scenario 3** & **Scenario 4** & **Scenario 5** \\ \hline
**Digestion** & & & & & \\ Temperature (\({}^{\circ}\)C) & 37 & 41 & 41 & 37/41 \({}^{\text{a}}\) & 37 \({}^{\text{a}}\) \\ VSR of primary sludge & 50 & 50 &... &... &... \\ (\%) & 40 &... &... &... &... \\ (\%) &... &... & 60 &... &... \\ (\%) &... & 64 \({}^{\text{a}}\) & 64 \({}^{\text{a}}\) & 68 \({}^{\text{b}}\) & 70 \({}^{\text{a}}\) \\
**Dewatering** & & & & & \\ Concentration (\%) & 20.3 & 30 & 30 & 41 \({}^{\text{a}}\) & 45 \({}^{\text{a}}\) \\ (\%) & 7.5 & 9 & 9 & 11 \({}^{\text{c}}\) & 0 \({}^{\text{a}}\) \\ Temperature (\({}^{\circ}\)C) & 37 & 41 & 41 & 41 \({}^{\text{a}}\) & 100 \({}^{\text{a}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specific parameters of the WWTP for each scenario considered. Estimating the thermal power of the recovery mixed boiler (TP\({}_{\text{rb}}\), kW) is done using Equation (1), considering the demand for biogas and the heat recovered from engine exhaust gases. Equation (2) calculates the consumption of biogas by the engines and the recovery mixed boiler. Both equations consider the period of steam injection. The symbols \(<\) > are used to represent the average cycle for a specific variable. Any variable (X) of the batch cycle can be transformed into the average cycle by applying Equation (3), with this only being valid for boiler parameters, as the biogas produced by digesters and consumed by the engines is constant, whereas the conditions for the operation of gasometers is based on the stored amount of available biogas. Thus the net result is zero for each  
The energy balance of gasometers for the whole operating cycle is presented in Equation (5). The thermal power, which should be stored in the gasometers, corresponds with the periods of no steam injection. \[\text{TP}_{\text{biogas-su-gasom}}*\text{N}_{\text{reactor}}*\text{t}_{\text{inj} }=\text{TP}_{\text{biogas-st-gasom}}*\left(\text{t}_{\text{cycle}}-\text{N}_{ \text{reactor}}*\text{t}_{\text{inj}}\right) \tag{5}\]  
Figure 3: Representation of the operating point of the engines’ boiler system. (**a**) Steam injection. (**b**) No steam injection. (**c**) Average cycle. <TP>-gasom is the average thermal power of the gasometer. Equations (6) and (7) allow the estimation of the thermal output of the recovery boiler and the consumption of biogas for the average cycle. These equations are equivalent to Equations (1) and (2). Equation (7) allows the estimation of the thermal power of the biogas supplied by the gasometers during the steam injection period.