## 3 Results and Discussion
### Degree of Torrefaction
Improved fixed carbon value of torrefied biomass can be an advantage as it aids the heat of combustion improving the efficiency of the overall combustion applications [29]. In contrast, an increase in the ash content can have a negative impact on the usage of the torrefied product as a higher ash content is associated with fouling, slagging, and agglomeration of the bed [30]. A study by [PERSON] et al. [31] demonstrated a decrease of 38.88% in the volatile content, whereas [PERSON] [32] achieved only 7.8% decrease in the volatile content, the differences in the value was due to the types of the biomass that were used for the pretreatment process. The former was high due to the high volatile content that was present in the agricultural residue when compared to the latter, which considered woody biomass [33]. Figure 3: _Cont._  
Figure 2: The degree of torrefaction of torrefied sample as a function of torrefaction residence time. The molar ratios of hydrogen and oxygen with respect to the relative carbon content i.e., O/C and H/C ratios are substantial parameters for the characterization of fuel composition. From Figure 4, for both molar ratios, torrefaction at 200 \({}^{\circ}\)C did not have much of a significant effect, but with an increase in torrefaction temperatures from 250 to 300 \({}^{\circ}\)C presented a significant decrease. At 350 \({}^{\circ}\)C, for both H/C and O/C molar ratios, a steep decrease was observed. Whereas, a gradual decrease in the H/C molar ratio at 250 and 300 \({}^{\circ}\)C was achieved. For O/C at 300 \({}^{\circ}\)C, a slight increase was observed until 20 min residence time, after which the fall in the O/C value was noticeable. These alterations in the molar ratios are primarily due to the release of bound water and are also due to the fractional removal of oxygen by decarboxylation, and dehydration reactions [34]. This decrease consequently increased the gross calorific value, which in turn decreases the volatile content. In addition, with an increase in torrefaction temperature and residence time, the hydrogen and oxygen content of the torrefied product decreased, resulting in reduction of the H/C and O/C ratios. Therefore, it can be seen from the graph that the increase in torrefaction temperature and residence time provoked a reduction in the O/C and H/C molar ratios due to the loss of carbon in the form of carbon dioxide, light hydrocarbons, and water. Moreover, a similar reduction trend of O/C was observed by [PERSON] et al. [35] in their study for lignocellulosic biomass. Reduction in the oxygen and hydrogen content strengthens the fact that the hydroxyl groups deteriorate during torrefaction. Moreover, from an energy density perspective, C-C bond projects higher energy compared to C-O or C-H bonds as the relative increase in carbon content results in better calorific value. Increase in the relative carbon content and decrease of other elements, such as O, N, H, and S may also result in a decrease in the volatile fraction. Increase in torrefaction temperature also favors the generation of gases, such as carbon monoxide (CO), carbon-dioxide (CO\({}_{2}\)), thermal hydrocarbon (THC), and methane (CH\({}_{4}\)), which is demonstrated in Figure 5. The gas analysis was carried out independently in order to study the gases that are emitted during the torrefaction of sewage sludge. The gas emission was not studied for different residence time. Rather, a continuous emission test from 50 to 350 \({}^{\circ}\)C was conducted to study the emission properties during torrefaction. CO\({}_{2}\) and CO were released after 250 \({}^{\circ}\)C, whereas CH\({}_{4}\) was released approximately after 270 \({}^{\circ}\)C and only subtle traces of THC is generated until 240 \({}^{\circ}\)C. This increase in the CO\({}_{2}\) is associated with the decarboxylation of the acid group, as explained by [PERSON] and [PERSON] [36], whereas methane is produced due to the cracking and de-polymerization reactions [31].