## 1 Introduction  
Biomass as a renewable energy source is recognized globally and is available in a generous amount on the earth, which can be transformed into biofuels or energy, utilizing various thermal, physical, or biological processes. Not only the curbing depletion of fossil fuel but negative environmental impacts associated with it, such as greenhouse gases, acid rain, and deterioration in climate has caught the interest of exploiting the renewable energy like biomass as fuel [1]. A study by [PERSON] et al. [2] has revealed integrating biomass into combined cooling and heating power (CCHP) for providing energy for buildings. Utilizing biomass or waste can lessen the environmental issues due to high carbon dioxide emissions, which are chiefly produced by fossil fuels. Hence, clean and renewable energy sources are in high demand [3] and sewage sludge is acknowledged as a low-cost material for biomass combustion [4] yet, these wastes are disposed into landfills or the ocean due to economic reasons. As the sewage sludge constitutes high amount of organic content, sewage sludge surely can be a promising biomass resource for energy recovery. Utilizing sewage sludge to generate heat through incineration and combustion can be a good alternative but the emission of heavy metals has led to various disagreements [5]. Due to low hemicellulose and cellulose, and a high ash content of sewage sludge, the combustion behavior is entirely different from that of lignocellulosic biomass [6]. For these reasons, the quality of raw sludge needs improvement to utilize it to obtain useful forms of energy. Energy recovery therefore plays a vital role whilst considering the management of sewage sludge. Various comparative studies have been carried out for the analysis of sewage sludge for energy recovery using various alternatives, such as Life-cycle assessment (LCA) [7] and SWOT (strengths, weaknesses, opportunities, and threats) [8].  
For decades, thermochemical technologies, such as combustion, gasification, pyrolysis, and others, have been employed for biomass conversion [9]. A promising alternative is provided by torrefaction,which alters sludge into coal-like solid fuel particles. During torrefraction, the biomass is heated to a temperature range of 200-350 \({}^{\circ}\)C in the absence of oxygen. During the process, loss of moisture occurs with a partial loss of the volatile (approximately 20%) as a result of which there is alteration in the characteristics of original raw biomass [10]. With the removal of the light fraction volatiles, the heating value of the torrefied biomass gradually increases [10]. This thermal treatment converts oxygen to carbon monoxide and carbon dioxide, which in turn reduces the O/C ratio but elevates the energy density and hydrophobicity of sludge [11]. Torrefraction lessens the moisture content of the biomass, which is an imperative benefit that inhibits biological degradation [12], increases the energy density [13], and enhances the combustion efficiency [14]. Despite these advantages study of non-lignocellulosic biomass is still limited. Various reactors have been used to carry torrefraction, such as muffle furnace [15], fixed bed reactors [16], auger reactor [17], and fluidized bed reactors [18]. To the best of our knowledge, downdraft fixed bed has not yet been used for the torrefraction of biomass although the downdraft fixed bed reactors have been utilized for gasification of biomass [19]. A study by [PERSON] et al. [19] reveals the benefit of using downdraft fixed bed over updraft and cross-draft bed reactor for gasification. This study, therefore, explores the benefits of using downdraft fixed bed reactor for torrefraction under different operating parameters. Higher heating value (HHV) or calorific value, different molar ratios, chemical exergy, torrefraction index, and severity factor were used to evaluate the optimum temperature range for torrefraction of sewage sludge using fixed bed downdraft reactor.