## 1 Introduction  
Biomass is a renewable energy resource that can either serve as an alternative or supplement to non-renewable energy sources such as coal for energy generation. It is classified as any organic matter derived from either plants or animals. A typical example includes wood, agricultural residue, energy crops, industrial waste and municipal waste. Municipal waste such as sewage sludge (SS) is classified as important biomass due to its composition of organic compounds of high calorific value. Sewage sludge is a byproduct of municipal wastewater treatment plants that are produced in large quantities worldwide. Its disposal through landfilling and landscaping poses a severe environmental challenge. Incineration on its own is very expensive, hence making these disposal techniques unsustainable. However, the use of SS as a solid fuel after the application of some pretreatment measures can be a sustainable management technique. Plant biomass such as sugarcane consists of three major polymer components, which include hemicellulose, cellulose and lignin. These components, usually termed lignocellulosic is the fibrous part of the plant that is not consumed by humans. Hence, making such plant biomass a good feedstock for energy production, as its use does not impact the food chain [1; 2; 3]. Biomass in its natural state has low energy density, high moisture content, low heating value, low combustion efficiency, and high volatiles compared to coal [1]. Nevertheless, these variations in biomass properties can be enhanced through torrefraction prior to its utilization for the production of heat and power. Torrefraction is a biomass pretreatment technique referred to in the literature as slow or mild pyrolysis carried out in an inert atmosphere and within a temperature span of 200-300 \({}^{\circ}\)C [4; 5]. Upon torrefraction, biomass fibrous structure is destroyed, and its moisture and oxygen content are removed through dehydration and carboxylation reaction. Thus, doubling the energy density, calorific value and increasing carbon percentage [6]. Previous studies have reported significant improvement in biomass fuel characteristics through torrefraction. For example, [PERSON] and [PERSON] [7] reported a reduction in the moisture content of pine chip and yellow pine from 6.69% and 7.94% to 3.30% and 3.11% after 30 min torrefraction at 225 \({}^{\circ}\)C. Equally, in [PERSON] et al. [6] study, the energy sorghum moisture content of 7.83% and sweet sorghum moisture content of 9.29% were both reduced to 2.05% and 5.51%, respectively, after torrefraction at a temperature of 250 \({}^{\circ}\)C and residence time of 30 min. In [PERSON] et al. [8] study, the analysis of torrefied sugarcane bagasse showed an increase in the higher heating value of about 30% and enrichment in carbon content, which was followed by a decline in the functionalities of oxygen and hydrogen in the produced char. Similarly, [PERSON] et al. [9] study showed an increase in calorific value of sugarcane bagasse from 17.90 MJ/kg to 20.29 MJ/kg after torrefraction of which translated in an increase in gasification conversion efficiency of about 10% when compared to raw sugarcane bagasse. [PERSON] et al. [10] studied the influence of temperature on torrefied sewage sludge and observed a rise in carbon/hydrogen ratio as torrefraction temperature increased. The study further suggested that decarboxylation reactions are bound to occur more at a longer residence time. [PERSON] et al. [11] study was focused on the enhancement of sewage sludge fuel properties through torrefraction. Their findings revealed that the mass and energy yield of the solid products were influenced by torrefraction temperature as well as residence time. In a recent study, torrefraction was employed as a valorization technique in improving the fuel properties of sewage sludge prior to gasification.