## 1 Introduction
[17] conducted torrefaction of five different types of agricultural residues with respect to torrefaction temperature and residence time and concluded that torrefaction escalated the physical and thermochemical properties of agricultural residues compared to raw biomass in terms of energy density and calorific value. Numerous works focuses on torrefaction of lignocellulosic biomass [16; 18; 19; 20; 21], while the torrefaction of non-lignocellulosic biomass has been studied to a lesser degree [22; 23; 24]. A study by [PERSON] et al. [22] demonstrated that, despite no, or a low amount of, hemicellulose present in sewage sludge, the energy density increased with torrefaction. In addition, the study also concluded that the effect of torrefaction temperature was more pronounced than the residence time. [PERSON] et al. [24] compared sewage sludge and leucaena torrefaction using a single-mode microwave oven, which established a higher calorific value, fuel ratio, and low atomic H/C and O/C ratios. However, they suggested the use of co-torrefaction of sewage sludge with other biomass for better torrefaction performance. [PERSON] et al. [25], in their research, utilized a blend of sewage sludge and waste wood torrefaction and highlighted the benefits of co-torrefaction including, but not limited to, constructive influence on gasification and pyrolysis. Study of torrefaction of sewage sludge remains limited and has been torrefied in muffle furnaces [22], fixed bed reactors [23], auger reactors [26], and fluidized bed reactors [4]. According to [PERSON] et al. [26] muffle furnace and fixed bed is less feasible for industrial scale. Fluidized beds exhibit excellent gas-solid heat and mass transfer and has been used for combustion [27; 28], gasification [29], and pyrolysis [30]. Despite the leverage fluidized beds portray, to our knowledge, only a few studies has been carried out for torrefaction using this technology [4; 7; 31]. In a fluidized bed reactor, biomass is mixed well and a uniform temperature distribution is maintained, which results in a uniform product quality, which is generally difficult in the case of other reactors [22]. This study, therefore, attempts to investigate the torrefaction of sewage sludge using a fluidized bed in order to take maximum benefit of the swift heat and mass transfer. This paper also aims to assess the qualitative analysis of torrefied biomass with respect to their calorific value, O/C and H/C molar ratios, gas analysis, and proximate and ultimate analysis. Chemical exergy was also investigated. In addition, various correlations for predicting the higher heating value (HHV) from different studies were considered and a comparative study of experimental HHV with predicted HHV from different correlations was carried out. An attempt to study the optimum torrefraction temperature of sewage sludge in a fluidized bed was made as it has the long-term benefit of commercialization. Overall, this work focuses on the torrefraction of sewage sludge in a laboratory-scale fluidized bed, the characterization of torrefied sewage sludge properties, and also the identification of the optimized torrefraction temperature range for sewage sludge.