## 3 Results and Discussion  
### Degree of Torrefaction  
The HHV or the calorific value describes the potential energy content of a biomass. One of the major advantages of torrefaction is the elevated energy content per unit of mass of the torrefied yield. The degree of torrefaction can be used as an indicator to identify the relative energy gain in the torrefied product [21]. Nittkitron and Suthum [21] further illustrates that the value of degree of torrefaction exceeding unity shows a greater energy gain per unit mass. Figure 2 illustrates the degree of torrefaction with respect to torrefaction residence time at various torrefaction temperatures. From the graph, it is clear that with an increase in torrefaction residence time and temperature the degree of torrefaction increases. From Figure 2, it can be seen that increasing the torrefaction residence time and temperature significantly increased the HHV, which in turn increased the degree of torrefaction of the sewage sludge except for 350 \({}^{\circ}\)C. At 200-300 \({}^{\circ}\)C, there is an increase in the degree of torrefaction with an increase in torrefaction residence time. This increase might be as a result of the gain of the calorific value due to the removal of oxygen. In contrast, pyrolysis reaction may have occurred at a higher torrefaction temperature, resulting in the decrease in the degree of torrefaction at 350 \({}^{\circ}\)C with an increase in torrefaction residence time. A study by [PERSON] et al. [22] suggested that the effect of torrefaction temperature was greater than the effect of torrefaction residence time. Similarly, [PERSON] et al. [23] found that the torrefaction temperature was dominant over residence time until 275 \({}^{\circ}\)C, but at 300 \({}^{\circ}\)C, the torrefaction residence time had a significant effect on the torrefied yield. This increase might be as a result of the gain of the calorific value due to the removal of oxygen. However, at 350 \({}^{\circ}\)C, the rate of degree of torrefaction decreases on further increasing the torrefaction residence time. This may be attributed to the fact that sewage sludge is a non-lignocellulosic biomass and constitutes thermally degradable organic components, which degrades easily on elevated temperatures as a result of which there is deterioration in the degree of torrefaction. In addition, the increase was best gained between 200 and 300 \({}^{\circ}\)C, although the overall net gain of the degree of torrefaction was more pronounced for 250 \({}^{\circ}\)C. An increase in the calorific value with torrefaction temperatures were obtained by [PERSON] et al. [24], [PERSON] et al. [25], and [PERSON] et al. [26]. Other important parameters that contribute for the characterization of solid fuel are volatile matter and fixed carbon, which are also partly responsible for the alteration of calorific value [27]. As demonstrated in Figure 3a,b torrefaction has an opposite effect on these two operating parameters. With an increase in torrefaction temperature and residence time, it can be observed that the fixed carbon and ash increases whilst the volatile fraction decreases. The torrefied product is desired to have less volatile fraction and more fixed carbon as it amplifies the calorific value of the torrefied product. For 300 \({}^{\circ}\)C, there was a decrease of 13.77% in the volatile content, whereas an increase of 68.95% was observed for fixed carbon. Least changes were observed for 200 \({}^{\circ}\)C for both volatile fraction and fixed carbon. Although the highest value for fixed carbon is for 350 \({}^{\circ}\)C, the net gain in the value is minimal when compared to 250 and 300 \({}^{\circ}\)C. With an increase in torrefaction residence time and temperature,the ash content increased and the highest was for 350 \({}^{\circ}\)C. The increase in the ash content was primarily due to the mass loss during the torrefaction process. Similar results were obtained by [PERSON] et al. [28].