## 3 Results and Discussions
### Proximate Analysis  
In torrefaction, the decrease in the volatile fraction (VF) and the increase in fixed carbon (FC) are preferred as they enhance the calorific value and remove moisture from the biomass. Therefore, the VF of the biomass should decrease and the FC should increase after torrefaction. The drop in the VF increases with the increase in the torrefaction temperature because a part of the volatile is depolymerized [22]. The results of proximate analysis for torrefied biomass at varying torrefaction temperatures and residence times are illustrated in Figure 5a-c. As from the graph, the VF of the sewage sludge decreased by 3.29% for 200 \({}^{\circ}\)C. At higher temperatures, 250 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, the VF decreases by 7.35 and 7.82 wt %. Further increasing the temperature to 350 \({}^{\circ}\)C, the trend is similar with respect to temperature and residence time, but the absolute value is smaller, i.e., only a 5.82 wt % reduction was observed. For torrefaction temperatures between 250-300 \({}^{\circ}\)C, the fraction of FC reached a maximum value of 29.37 wt % and the ash fraction was approximately 9.2 wt %. Higher amounts of FC of torrefied biomass can produce a greater heat of combustion, which can be beneficial during combustion applications [20]. However, high ash content can be disadvantageous for usage of sewage sludge as an energy source. Additionally, ash management can be challenging as high amounts of ash can lead to fouling, slagging, and agglomeration of the bed [43]. [PERSON] et al. [44] demonstrated similar results where the volatile content decreased after an increment of torrefaction temperature from 230 to 270 \({}^{\circ}\)C. Furthermore, [PERSON] and [PERSON] [45] explains this decrease in the volatile matter is associated with the decomposition of biomass polymer as well as the release of light volatiles.  
[PERSON] et al. [15], in their studies, demonstrates that at torrefaction temperatures of 160 and 180 \({}^{\circ}\)C the changes in the biomass is due to dehydration reactions. It also states that further increasing the torrefaction temperature above 230 \({}^{\circ}\)C, devolatilization and carbonization take place, which are the principal reactions causing the alteration in the chemical composition of the torrefied biomass to that of the raw biomass. [PERSON] and [PERSON] [46] used two-step models in their studies and demonstrated that oxygenated species, such as water and acetic acid, are released in the first stage, and lactic acid and methanol were released in the second stage. The authors also demonstrated that an increase in the torrefaction temperature elevated the mass fraction of carbon and decreased the hydrogen and oxygen content. [PERSON] et al. [47], in their study, exhibited that the loss of VF is correlated to the thermal breakdown of carbohydrate fractions which amount to the accumulation buildup of ash after torrefaction. The study further illustrates that these alterations increase the FC content of the biomass, which complies well with the findings in this study.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Torrefaction Regimes** & **Temperature (\({}^{\circ}\)C)** & **TI [43]** & **TI (This Study)** \\ \hline Light & 200–235 & 0.93–0.95 & 0.96–0.99 \\ Medium & 235–275 & 0.95–0.97 & 0.97–1.02 \\ Severe & 275–300 & 0.97–1.0 & 0.97–1.0 \\  & 350 & & 1–0.96 \\ \hline \hline \end{tabular}
\end{table}
Table 2: TI in different regimes.