# Study of the Applicability of Thermochemical Processes for Solid Recovered Fuel
## 3 Results
### Combustion  
The results from the mass balance for combustion are shown in Table 5 for the different SRF initial moistures analysed in combustion: 0, 2.5, 5, 7.5, and 10 wt% (the only variation concerning the moisture is for the mass of H\({}_{2}\)O in flue gas).  
Figure 6: Heat required for drying (MJ/kg of wet SRF vs. moisture remaining in the SRF after thermal drying).  
The energy balance results according to Equations (6)-(8) are shown in Table 6. The higher the moisture content in the SRF fed to the boiler, the lower the temperature of the gases leaving the furnace, since more water should be evaporated. The values obtained for burning-temperature densified SRF from screening waste are similar to the ones obtained in the combustion of other solid residues.  
The energy in the flue gases can be recovered, producing steam at 673 K and 50 bar, which can be used in the WWTP. Table 6 displays the amount of steam generated per kilogram of dry matter in SRF. Although a higher amount of steam is produced when burning totally dried SRF, higher energy is also required during the drying step.  
The final energy benefit after the combustion process (Table 6) can be calculated by subtracting the energy required for drying from the energy recovered in the heat exchanger to produce high-pressure steam at 50 bar and 673 K. The results showed that the initial moisture of the SRF fed to the boiler in the analysed range (0-10 wt%) slightly affects to the energy benefit, with the highest value being the one obtained for the SRF that was totally dried. Approximately 7 MJ could be recovered from the combustion of 1 kg of dry matter in SRF, which corresponds to 1.6 MJ per kg of raw SRF with 77.3 wt% moisture (before drying). This energy benefit could be even higher if the combustion gas stream after steam generation, still at 473 K, could be used for partially drying the SRF.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline Raw SRF moisture (\%) & (before drying) & & & 77.3 & \\ \hline \(\Delta h^{0}_{f,dry matter in SRF}\) (MJ/kg of dry matter in SRF) & & & \(-2.78\) & & \\ \hline Input SRF stream moisture (\%) & 0 & 2.5 & 5 & 7.5 & 10 \\ \hline \multicolumn{6}{c}{Energetic terms in the combustion energy balance} \\ \hline \(\Delta h_{input}\) (MJ/kg of dry matter in SRF) & \(-2.78\) & \(-3.18\) & \(-3.61\) & \(-4.06\) & \(-4.54\) \\ Heat losses (MJ/kg of dry matter in SRF) & 0.42 & 0.48 & 0.54 & 0.61 & 0.68 \\ \(\sum m_{i}\cdot\Delta h^{0}_{f,i}\) (MJ/kg of dry matter in SRF) & \(-26.1\) & \(-26.5\) & \(-26.8\) & \(-27.2\) & \(-27.6\) \\ \(\sum m_{i}\cdot Cp_{i}\) (MJ/kg of dry matter in SRF) & 0.024 & 0.024 & 0.025 & 0.025 & 0.025 \\ Output T (K) & 1237 & 1230 & 1222 & 1214 & 1206 \\ \hline \multicolumn{6}{c}{Heat exchanger: amount of steam produced} \\ \hline Mass of steam (kg/kg of dry matter in SRF) & 5.65 & 5.61 & 5.57 & 5.52 & 5.48 \\ \hline \multicolumn{6}{c}{Summary} \\ \hline Energy recovered in the heat exchanger to produce & 15.88 & 15.76 & 15.65 & 15.51 & 15.40 \\ high-pressure steam (MJ/kg of dry matter in SRF) & 8.85 & 8.81 & 8.77 & 8.68 & 8.63 \\ \multicolumn{6}{c}{Pring energy benefit (MJ/kg of dry matter in SRF)} & 7.03 & 6.95 & 6.88 & 6.83 & 6.77 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Energy balance results of SRF combustion.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline Raw SRF moisture (\%) & (before drying) & & & 77.3 & \\ \hline Input SRF stream moisture (\%) for combustion & 0 & 2.5 & 5 & 7.5 & 10 \\ \hline \multicolumn{6}{c}{Mass of combustion gases (kg/kg of dry matter in SRF)} \\ \hline CO\({}_{2}\) & 1.94 & 1.94 & 1.94 & 1.94 & 1.94 \\ H\({}_{2}\)O & 0.67 & 0.69 & 0.72 & 0.75 & 0.78 \\ NO & 0.06 & 0.06 & 0.06 & 0.06 & 0.06 \\ O\({}_{2}\) & 0.88 & 0.88 & 0.88 & 0.88 & 0.88 \\ N\({}_{2}\) & 8.67 & 8.67 & 8.67 & 8.67 & 8.67 \\ Total mass & 12.21 & 12.24 & 12.27 & 12.29 & 12.32 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Mass balance results of SRF combustion.